Flax (Linum usitatissimum L.) fibre reinforced polymer composite materials: A review on preparation, properties and prospects

Accepted Manuscript Flax (Linum usitatissimum L.) fibre reinforced polymer composite materials: A review on preparation, properties and prospects M. Ramesh PII: DOI: Reference:

S0079-6425(18)30118-X https://doi.org/10.1016/j.pmatsci.2018.12.004 JPMS 546

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Progress in Materials Science

Received Date: Revised Date: Accepted Date:

5 December 2017 12 December 2018 21 December 2018

Please cite this article as: Ramesh, M., Flax (Linum usitatissimum L.) fibre reinforced polymer composite materials: A review on preparation, properties and prospects, Progress in Materials Science (2018), doi: https://doi.org/ 10.1016/j.pmatsci.2018.12.004

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Flax (Linum usitatissimum L.) fibre reinforced polymer composite materials: A review on preparation, properties and prospects M. Ramesh Department of Mechanical Engineering, KIT-Kalaignarkarunanidhi Institute of Technology, Coimbatore - 641402, Tamil Nadu, India. E-mail: [email protected] Mobile : +91-9444103890.

Abstract History is often marked by the materials and technology that reflect human capability and understanding. The increasing needs on bio-degradable and eco-friendly nature for consumer goods, the usage of bio-based materials is on demand. Flax has attracted attention since Stone Age as one of a few plants from which highly valued products are made. Fibres from flax plants are cost-effective, bio-degradable and exhibited good mechanical properties. This review focuses on the preparation, properties and prospects of flax fibres and its composites. The plant growth, harvesting, fibre structure, properties, effect of chemical treatments, influence of various factors such as fibre length, diameter, fibre location, influence of fibre orientation on properties of composites, and processing-structure-property relationships have been reviewed. Then the effect of fibre configuration, manufacturing processes, fibre quantity, matrix selection, ecological effects, mechanical properties, life cycle assessment, failure studies and fibre/matrix interface parameters on the characteristics of flax fibre composites have also been analyzed. Many open issues and ideas for further improvement are also analyzed, and more emphases are given for the development of environment-friendly bio-inspired material. Focusing on the summary of the review and the effects of the flax fibres on the finished products are also presented in this review.

Keywords: Flax fibre; bio-degradability; polymer composites; eco-friendly; physical properties; life cycle assessment. Abbreviations AE-Acoustic emission; ATR-Attenuated total reflectance; CO2-Carbon dioxide; DMA-Dynamic mechanical analysis; DMTA-Dynamic mechanical thermal analysis; DSC-Differential scanning calorimetry; DTG-Differential thermo-gravimetry; EDTA-Ethylene diamine tetra-acetic acid; FFRCs-Flax fibre reinforced composites; FTIR-Fourier transform infrared spectroscopy; HDPEHigh density polyethylene; HMTA-Hexamethylene tetramine; HRR-Heat release rate; ICMInjection compression moulding; IFSS-Interfacial shear strength; ILSS-Interlaminar shear strength; IR-Infra-red; KBr- Potassium bromide; KOH-Potassium hydroxide; LCA-Life cycle assessment; LCM-Liquid composite moulding; LPET-Low melting polyethylene terephthalate; MAPP-Maleic anhydride polypropylene; N2-Nitrogen; NaClO2-Sodium chlorite; NaOH-Sodium hydroxide; NIR-Near infra-red spectroscopy; O2-oxygen; PCB-Printed circuit boards; PCWPrimary cell wall; PEG-Poly ethylene glycol; PFRCs-Plant fibre reinforced composites; PPPolypropylene; PVA-Poly vinyl acetate; RTM-Resin transfer moulding; SCW-Secondary cell wall; SEM-Scanning electron microscopy; SP-Snap point; TDP-Thiodiphenol; TEMTransmission electron microscope; TGA-Thermo-gravimetric analysis; TPS-Transient plane source; UTM-Universal testing machine; VARTM-Vacuum assisted resin transfer molding; VEVinyl ester; XPS-X ray photoelectron; XRD-X ray diffraction. 1.

Introduction The combination of two or more materials with distinct characteristics reinforced

properly, for fabricating a new material is called composite material [1]. These materials were introduced because they have the potential to be manipulated to have combinations of properties

that cannot be obtained by conventional materials [2-4]. The composite materials fabricated for aircraft parts many years back, identified its importance in all engineering applications such as manufacturing of sports goods, medical instruments, armed equipment, automotive parts, marine equipment, etc. [5-11]. The reinforcement of conventional fibres for processing composites creates many issues regarding bio-degradation, energy consumption and its impact on the environment. Aside from the high energy consumption during processing, silica sand is the primary raw material for glass fibre generation which also have an impact on the environmental [12-14]. In this regard, several types of research were carried out in the past to evaluate the effective use of plant fibre reinforcement for composites [15-19]. The new regulations on the recycling of materials and ecological effects forced scientists to develop new bio-based materials derived from renewable resources [19]. This leads to the production and development of a wide variety of materials based on renewable and natural resources [20-39]. Also, because of photosynthesis, the cultivation of plant fibres reduces the amount of carbon dioxide (CO2) and increases the amount of oxygen (O2) in the atmosphere, thereby reducing the impact of the green-house effect [40, 41]. Nowadays, plant-based fibres are replacing synthetic fibres as reinforcement in polymer matrix materials for structural and other engineering industrial applications and attracted interest due to severe ecological effects and need for fabricating sustainable products [16, 42-46]. A report has provided that about 4.3x106 kg of plant fibres was being utilized by the automobile industries every year as reinforcing agents in composites [47]. The composites reinforced with bio-degradable and renewable materials posses good mechanical properties [48]. Number of scientists and researchers have started investigating the usage of plant fibres as reinforcing materials in composites [44, 49-53]. The fibres and fillers from plants like abaca, areca catechu

husk, banana, bamboo, cabana, coir, cotton, date palm, flax, fruit bunch, hemp, kenaf, jute, oil palm, onion skin, orange peel, pampa grass, pineapple leaf, ramie, reed, rice bran, rice husk, sisal, snake grass, sugar cane bagasse, sorghum grain, soy protein, and henequen have been used as the reinforcing material in composites [21, 24, 44, 49, 54-99]. These fibres have little advancement over synthetic fibres, such as less cost, good strength, ease of handling, low fossilfuel energy requirements and can offer good mechanical properties [46, 49, 100-108]. In comparison with conventional fibres, they are available in abundance, renewable, biodegradable, and lightweight materials [109-115]. They have superior physical properties, thermal insulation properties, dimensional stability, resistance to abrasion, and cause less skin and respiratory irritations [49, 104-106, 116-125]. The studies on the bio-based composite materials are in the increasing trend, and its impact on the surrounding environment during its growth and processing must be taken into account. The natural fibres and bio-polymers are mixed thoroughly; create a new material which is known as bio-composite materials or eco-composites [37, 48, 126-129]. The materials derived from renewable resources are eco-friendly, easily compostable and energy saving materials [130133]. The tremendous growth and increasing interest in the field of bio-based materials is pinpointing its wide range of applications in near future as the next generation structural materials. The plant fibre reinforced composites (PFRCs) have recently gained attraction in automotive industries [134-138]. The plant fibres are the most preferable because they offer several advantages economically and ecologically over conventional fibres as reinforcing materials [16, 139]. A detailed review of several plant fibres and its bio-composites was carried out by many scientists and researchers previously [20, 45, 49, 112, 140-169]. In this review, the focus has been given on the preparation and properties of flax fibres and its composite materials.

The organization of the paper is as follows: Section 2 describes the flax plant cultivation, harvesting, fibre extraction, structure and composition, fibre properties, fibre surface modification, processing-structure-property relationships, etc. Section 3 presents the flax fibre reinforced composites (FFRCs) and its processing methods. Section 4 explains the different properties of FFRCs in detail under different types of loading. In Section 5, the morphological properties of these composites are enumerated. The environmental impact of flax fibres is assessed in Section 6. Section 7 lists the industrial applications of FFRCs. The future research issues’ pertaining to these composites is explained in Section 8. Finally, Section 9 concludes the paper and summarizes the findings of this review. 2.

Flax plants

2.1

Cultivation and harvesting Flax (Linum usitatissimum L.) belongs to the family Linaceae, which consists of 13

genera and 300 species and this is the only one species with agricultural importance of its family (usitatissimum means most useful). Flax was grown and discovered in Egyptian graves before 5000 BC which is the first fibre extracted from plants, and reinforced into matrices and one of the most widely utilized bio-fibres [170]. Flax grows in moderate climates, and currently, the main flax producing countries are China, France, and Belarus [171]. Canada is the major cultivator and exporter of flax since 1994. In 2005-06, Canada produced about 1.035 million tonnes and currently ships 60% of its exports to the European Union, 30% to the United States, and 4% to Japan [172]. Other leading cultivators of flax are France, Belgium, and the Netherlands, with nearly 130,000 acres under cultivation annually. In 2007, the European Union produced 122,000 tonnes of flax fibres [173]. The economic processing and the yield of flax fibres can still be improved to ensure capacities and qualities that are acceptable to set up

powerful process plants successfully [174, 175]. There is a chance for cost-cutting by using the common farming equipment in place of special purpose equipment [176]. When the moisture content was low in the straw, then it was baled by a number of round balers. The balers produced bales having a width of 1.2 m, diameter of 1.5 m and weighing 250 kg each. The quantity of flax cultivated from the year 2000 to 2015 was around 6000 tonnes, with a minimum of around 4000 tonnes and a maximum of around 8000 tonnes [177]. It is one among the ancient cultivated plants, cultivating in moderate weather zones for its seeds and fibres [178]. The crop is grown in two forms: linseed, which has thicker and shorter barks with many sub-branches; which is taller, with thin stems sparingly branched and bearing fewer capsules [179]. Flax is cultivated many years for its fibre, oil and is a crop with economic value; flax is a target for profound research and introduction of valuable traits such as herbicide resistance, tolerance to biotic and abiotic stress, improved oil and fibre quality [180]. Flax is an erect annual plant that grows 0.5-1.25 m tall with a stem diameter of 16-32 mm. Bio-stimulators used in flax cultivation show higher efficiency on poorer soils, where abiotic stresses of the environment cause higher losses than on good soils. The study conducted in 2010 and 2011 on two types of soils was to compare the efficacy of acetylsalicylic and salicylic acids in the cultivation of fibrous flax. The salicylic acid showed no effect on the amount of flax yield during experiments [181]. The flax plant had attained yellow ripeness on its outer stem, and it has been cut immediately. The cutting period depends on the sowing time and the number of days between sowing and yellow color ripeness [176, 177]. An examination on the sowing time is reported as approximately April end, with March end as the earliest and mid of May as the last sowing time [182]. As per the data available with Swedish field experiments, the approximate production of

flax straw including seed is about 68000 kg/ha [183, 184]. The time taken from the initial yellow ripeness to complete ripeness is around 10-15 days, during normal climatic conditions [176, 185]. Moreover, the day’s average temperature has a significant influence on the maturing rate. This indicates that the maturity stages are around 160 to 240 days. Water deficit in the environment inhibits the majority of processes crucial for the growth and development of plants, decreasing its yield and quality [186]. Gradually, as the water deficit increases the following processes become hindered: cell growth, protein synthesis, the activity of nitrate reductase, the closing of stomata, photosynthesis rate, and protoplasmic circulation. The elongation of the flax fibre is finished above the so-called snap point, where the bark of stem which contains quality fibres changes its characteristics. The position of the snap point is normally 6-8 cm from the top of the plant, which is well pronounced during the rapid growth of the plant, moves the stem upward, following plant growth, and vanishes during flowering. The fibres, combining the secondary wall layers in the middle portion with the primary wall at the cell tip, were never observed at the cross-sections of flax plants. The elongation of the individual fibre takes place within 2-4 days at a growth rate of 1-2 cm per day [187]. The developmental stage of the flax is presented in Fig. 1 [188]. Fig. 1 Localization of different developmental stages of flax (A-coordinated growth; B, Cintrusive growth; D, E-thickening of cell wall; and SP-snap point) [188] The major problems associated with flax plants during the growing and harvesting is the quantity and quality of fibre and this may vary significantly between different seasons as a consequence of changes in weather conditions. The highest quantity and quality yields are obtained in drizzle weather, clouded sky, with a moderate air temperature between 18-20°C [189-191]. To fulfill the requirements of the industry, the usage of flax fibres and its products, a

reliable supply at a nominal cost must be ensured. This can be achieved by using sophisticated machines and equipments with high capabilities, or by using temporary stores that maintain the fibre quantities harvested during different seasons. This increases the reliability of supply, but it requires higher production and maintenance costs. Therefore, it is vital that the capability of equipment should be optimal in continuation to the supply of required raw material [176]. 2.2

Flax fibre extraction

2.2.1 Retting process Retting is a process, which takes place during the rainy season, after the harvesting of the plants and therefore the stems have been harvested when it is green [192-195]. The retting is also defined as the process in which fibres are extracted from the plant by a variety of microorganisms. Retting process has a significant effect in enhancing the surface characteristics of fibres [196, 197]. The indoor retting and drying are not feasible economically during winter and spring seasons [198, 199]. The common retting process which is called dry retting means that the fibres are extracted from the plants by the frosts [198]. The harvested plants were spread out in an open field and left exposed for at least 40 days [175]. The plants were turned sides every 2 weeks. During this time, bacteria and fungi grew on the plants and degraded the cell-wall polysaccharide and middle lamella, thereby releasing fibres from the stem matrix [200, 201]. Then the fibres are separated from the residues of the plants by the process called scutching [202]. The chemical modifications can complete this separation process to enhance the quality of the fibre and its surface roughness [203]. The chemical and physical characteristics of flax fibres can be affected by the retting process [204-206]. The other method is used to extract fibres from the woody portion is dew or enzyme retting. The plant stem is broken by soaking it with water or treating the stems micro-

biologically or by steam explosion [46, 175, 207]. Dew retting is carried out by spreading the stems on the ground after harvesting and threshing of the seed capsules. Flax straw on the field is influenced by exposure to dew, rain, wind, and sunlight. Dew retting consists the breakdown of the pectin compounds by micro-organisms starting from the cambium layer, which binds the cortex to the central woody core, following phloem parenchyma, which cements the bundles to the thick-walled epidermis. During the last stage of retting, the bundles are split lengthwise into fibres [175]. During dew retting, the stems are dampened by dew at night, and then dried during the day. The weather condition suitable for dew retting is three weeks after harvesting, but during this period the dew-retting process was not complete [208]. Water and dew-retting depend upon colonization and partial plant bio-degradation by micro-organisms in the retting consortium and are influenced by environmental conditions, thus making retting difficult to control [209]. Indigenous fungi and bacteria partially decompose the pectinaceous and matrix substances to separate fibres from the plant. Enzyme retting depends upon processing conditions using pectinase rich enzymes and chelators to separate the fibres from the stem [175]. 2.2.2 Decortication process The decortication process is a mechanical process whereby fibres are extracted from flax straw. Decortication consists of several stages and can be based on either of two techniques (i) a hammer mill method whereby straw is broken by a striking action and (ii) a roller system, which involves crushing the straw [175, 210]. The process which is called mechanical decortications is used to isolate the coarse fibre from the stem of the plants by producing the technical fibres [211, 212]. The removal of woody portion from the fibres, become a problem in the decorticating process [213, 214]. A roller-based system deflects the flax stem over a curved surface causing resultant tensile stress. The inclined plane decorticator comprises a profiled rubber-timing belt

fitted to rigid support with an inclination of 10%. The roller consists of a profiled gear wheel with a matching profile to the belt and approximate weight 2.65 kg. The roller is positioned 50 cm above a straw placed on the inclined plane and released, it has attained a speed of 1 m/sec. Straws were cut into 10 cm length from the appropriate section of the stem and conditioned for 48 h. Each straw was weighed, and the diameter was measured. The straw was placed on the inclined plane, and the roller was released over the straw. The straw was then held in a unidirectional airflow to remove shive by aligning the straw axis with the direction of the air stream for 3 s. The straw was then rotated through 180° and held for a further 3 s to remove loosely held shive. Remaining shive was removed manually, and fibre peel was retained and weighed [215]. 2.3 Flax fibres Flax fibres are also known as technical fibres consist of a number of ultimate or single cell fibres that are bundled together. The flax fibres create high interest for many applications due to its excellent properties and low density, ease of handling as well as its accessibility [203, 216-226]. Flax is one of the strongest fibres considered from the available plant fibres [227]. A single flax plant produces around 20,000 fibres, each fibre yielding an average weight of 0.4 g and the average length is about 30 cm [188]. The production of flax fibre uses little energy and needs little fertilizer and pesticides, so its cultivation offers an environmental pause which can be used to maintain soil quality and bio-diversity within crop rotations [40]. Flax fibres, like most other agricultural products, are heterogeneous in nature; thus drying it involves complex heat and mass transfer between the surrounding air and the fibre [228]. The structure of the flax fibre from stem to microfibril which is reproduced from [110] is presented in Fig. 2. Fig. 2 The structure of a flax fibre [110]

2.4 Structure and composition 2.4.1 Structure Flax is a cellulosic fibre, but its structure is more crystalline, making it crisper, stronger stiffer, and easily wrinkled. Flax fibre is a strong, staple, relatively short, non-continuous, irregular polygonal cross-section and has a hollow structure [229]. The primary cell is only 0.2 μm thick which is called the outer wall [230]. The flax fibre is about 1 m long with the average diameter of 12-16 μm and contains around 40 fibres in its cross-section [173, 221, 222]. They are over-lapping for a particular length and are joined by pectins and hemi-celluloses, which are the mixture of polysaccharides. The flax fibres are polyhedral in cross-section with approximately 57° angles to increase the packing in the fibre [230]. The flax fibres are heterogeneous and different constituents come into consideration when that has been produced in the form of continuous reinforcement [203, 220, 231-237]. The development of fibres in stem is presented in Fig. 3 [238]. The hand cut cross-section in the middle of the stem, stained with carmalum showing the fibre organization within bundles at plant maturity is shown in Fig. 3 (a) and the cross section of a fibre cell observed by using a transmission electron microscope is presented in Fig. 3 (b). Dense layers within the secondary wall stained with ruthenium red consist of polymers with high charge density. Fig. 3 Development of fibres in flax stem (a) cross-section in the middle of the stem, (b) microscopic image of flax fibre cell cross section (PCW-primary cell wall, SCW-secondary cell wall) [238] The polygonal shape of the flax fibres with 5-7 sides is shown in Fig. 4 [202]. From the figure, it is observed that the longitudinal view of fibre shows a non-constant transverse direction. The dimensions of the fibre along the longitudinal and transverse directions are of 4-77

mm and 5-76 μm respectively. The microstructure of fibre is extremely complex due to its heterogeneous nature along its length and the different materials present in variable proportions. The cellulose content in the secondary cell wall is laid down in the direction of orientation, the highly crystalline microfibrils which are combined by the hemi-cellulose or pectin matrix [220]. Whereas, the complex structure of flax fibres can be compared to composites which is composed of cellulose fibrils within the hemi-cellulose matrix [220, 239]. Fig. 4 Polygonal shape of flax fibres [202] Fig. 5 (a) shows optical images of the cross-section of flax fibres. From the image it is observed that the individual fibre is having more polygonal than circular cross-section. Moreover, the bundles are clustered in the form of one central fibre surrounded by neighboring fibres. Each fibre has a hexagonal cross-section, and the lumen is insignificant [240]. The flax stem cross section is presented in Fig. 5 (b). The section is from near the base of an immature, linseed type cultivar, and has been stained with toluidine blue. The thick-walled bast fibres remain largely unstained and are arranged in bundles, near the periphery of the stem. In this immature stem, the still-growing secondary xylem already dominates the cross-sectional area [241, 242]. Fig. 5 Flax (a) fibre cross-section and (b) stem cross-section Flax fibre contains cellulose, pectin, lignin, hemi-cellulose, waxes, fats, mineral salts, and natural coloring matters [175, 205, 243]. The cellulose meso-fibrils mainly composed with pectin and hemi-cellulose are embedded in an amorphous polysaccharide matrix. The hemi-cellulose having branched structure similar to that of cellulose acts as a matrix [224], it may also act as a coupling agent [220, 238]; it has varying nature based on it is the primary or secondary layers [234, 244, 245]. There are two different types of pectin present in flax fibres [238]; at the

adjoining of the primary layers, which form large macro-molecules and confirm adhesion between the cells, and the fibres in the secondary layers which are part of the matrix covers the meso-fibrils [246]. Homo-galacturonanes and rhamno-galacturonanes are the most common pectin in flax fibre [238, 247-249]. The flax fibres contain between 6-10% of water by weight of which acts as a cell wall plasticizer [250]. The composite structure of flax is presented in Fig. 6 [203]. The stem of the plant is composed, from the outer part to the inner stem, of bark, phloem, xylem and a central void. The cross-section contains 10-40 fibres which are connected through pectin. The primary cell wall coats the secondary cell wall which is capable for the load carrying capacity of the fibre and encloses the lumen. Each layer is composed of micro-fibrils of cellulose which run parallel from one to another and form a micro-fibrillar angle along the fibre direction; this angle is minimum in the secondary cell wall. A micro-fibril is mainly constituted of cellulose links embedded in an amorphous matrix which comprises of hemi-celluloses and pectins [203]. Fig. 6 Composite structure of flax [203] 2.4.2 Composition of flax fibre The main constituents of flax are cellulose, hemi-cellulose, lignin, pectin, and wax, in varying proportions. In this cellulose, hemi-cellulose and lignin are the major constituents which decide the mechanical strengths of the fibres. The cellulose is the strongest and stiffest constituent of the fibre. The cellulose has a semi-crystalline polysaccharide with a huge amount of hydroxyl group, giving hydrophilic characteristic to fibre. The cellulose represents between 65-75% of the total weight of flax fibres [251]. The compositions of flax fibres reported by various researchers are listed in Table 1. Table 1 Chemical composition of flax fibres

The thermal and bio-degradations, and moisture absorption of flax fibre take place because of hemi-cellulose [100]. Lignin is a hydro-carbon that acts as a binding agent in the fibre. Pectin is a polysaccharide, acts as a partition wall between hemi-cellulose and cellulose which also act as a coupling agent [258, 259]. The lignin is aromatic, amorphous, highly complex, which is the polymers of phenylpropane units has the lower moisture absorption [260]. The wettability and adhesion characteristics of flax fibres can be highly affected by the waxy substances. The flax fibre consisting of cellulose fibrils is spirally wound in a matrix of amorphous hemi-cellulose and lignin. They are at 10-11° orientation according to the axis of the fibre because of that a twisted structure is possible [44, 230, 252, 261]. 2.5 Flax fibre surface modification/treatments The plant fibres have the poor interface quality with the matrix. The strength of materials depends on the interfacial bonding between the fibre and the matrix [262-265]. The fibre surface modification gives advantages such as preventing the moisture absorption, cleaning the fibre surface and enhancement of surface smoothness which leads to improve the interfacial adhesion between fibre and matrix which results in significant improvement in the properties [266]. The surface modification also improves the fibre separation and some undesirable effects, such as reduction in mechanical strengths, change in structure through swelling, degradation on the fibre surface can occur. Therefore surface modification is very important when the fibres are used for industrial purposes [267-270]. Treatments under high temperature and pressure allow decreasing hydrophilic capacity, swelling, and shrinkage [271-273]. To maximize the performance of the flax fibres and its composites, different fibre surface treatments have been investigated in many studies [44, 274-280]. The efficient stress transfer from the matrix to the fibre can increase the strength due to the excellent interfacial bonding between the fibres and the matrix. To improve

this, the physical and chemical surface modifications are frequently done [57, 102, 109, 202, 252, 281-289]. Physical modifications such as plasma treatment [44, 45, 102, 290-293], corona treatment [45, 294], electron beam irradiation [295], and autoclave treatment [102] have been done to increase the compatibility between fibre and resin. The chemical [44, 45, 294, 296-300] surface treatments such as dewaxing [301], alkali treatment [45, 62, 109, 281, 302-308], reaction with acid compounds [196, 304, 309-311], silane treatment [283, 312-319], acetylation treatment [320-328], cyanoethylation [329], stearic acid [330, 331], maleic anhydride [288, 332], maleic anhydride polypropylene (MAPP) [125, 289, 333-337], acetic acid/anhydride [109, 306, 307], isocyanates [62, 338, 339], triazine [340], benzoylation [341, 342], organo-silanes [288, 322, 343], grafting [344] and graft co-polymerization [345] have been performed to improve the properties. The coupling agents contain reactive chemical groups that can react with the fibre as well as resin [345]. 2.5.1 Alkaline treatment It is one of the very usual and popular treatments used on flax fibres [346, 347]. In this process, the fibres are immersed in an alkaline solution, resulting in change of fibre properties. Actually, this treatment has been carried out to enhance the affinity of fibre and its appearance [348], and also the alkalization has a positive trend on the strengths of the material made of these surface modified fibres. It is also used as a pre-treatment in other types of chemical modifications for flax fibres [109, 202, 349, 350]. One of the advantageous of alkaline treatment is that alkalis, such as NaOH and KOH, can help the crystalline lattice of flax fibre transform from one form of cellulose into another form, which is more stable and readily penetrable by ions [202, 349]. This process removes a particular amount of lignin, wax, and oil which covers

the outer surface of the fibre and de-polymerizes the structure of the native cellulose. During the process, the hemi-cellulose is removed; the inter-fibrillar region has low density and makes the fibrils more capable of re-arranging themselves along the elongation direction [351]. The fibre surface was modified with alkali solution for 30 min, followed by immersing in an acrylic acid solution for an hour, and thoroughly washed with purified water, then drying in a hot oven at 70°C for one day [352]. The reaction of acrylic acid with -OH group of the fibre is presented in Eq. (1) [353]. Fibre-OH + CH2=CH–COOH NaoH Fibre–O–CH2–CH2–COOH.

(1)

The flax fibres were washed with detergent and warm tap water and then soaked in hot water for 30 min. The fibres were then soaked in a 5% NaOH solution for 30 min to activate the hydroxyl groups for effective reaction with silane in the succeeding treatment. After which, the fibres were soaked for 30 min in a 60:40 ethanol/water mixture with vinyl-tri-methoxysilane, cleaned with pure water and then dried for 24 h at 50°C [354]. The fibres were soaked in an alkali solution for 2 h at 25°C, and then thoroughly washed with clean tap water and neutralized with an acetic acid solution for 30 min. Then the fibres were washed with hot water for 30 min and again washed with clean running water. Then, at last, the fibres were dried in the atmospheric air and then in a hot oven above 100°C for 6 h [245, 282, 355]. The alkali treated flax fibres were immersed into acrylic acid at 50°C and acetic anhydride at room temperature for an hour, and then dried in a hot oven at 80°C for 24 h [356]. The fibres were immersed in the alkaline solution with different concentrations ranging from 1-15% at atmospheric temperature for 2 h. Then the fibres were washed with acidified water, followed by clean tap water until cleaned thoroughly. Lastly, the fibres were dried in an air circulating oven at 90°C for 8 h [107]. 2.5.2 MAPP treatment

The surface treatments on flax fibres were conducted by using a coupling agent, MAPP [357]. MAPP is a very popular bonding material for PFRCs and very effective in increasing the strength of the materials [125, 323, 337, 345, 358, 359-361]. By adding a small amount of MAPP is to improve the strength of materials considerably. Even though MAPP is an excellent bonding liquid for PFRCs this may be costly or may not be environment friendly. MAPP has proved quite an enhancement in mechanical properties, when they are mixed with plant fibres before mixing with resin [45]. 2.5.3 Zein treatment The zein solution with different concentrations varying from 2-6% was prepared with ethanol and water mixture in the proportion of 80:20. The flax fibres were cut into pieces and soaked in this mixture for 2 h with continuous stirring. Then the fibres were taken out and dried in the atmospheric air in a hot oven at 110°C. Finally, the treated fibres were reinforced by using a proper matrix to fabricate the composites for testing [362]. 2.5.4 Autoclave treatment An autoclave treatment was carried out on the flax fibres. The upgrading process used for the flax was developed [219]. The treatment consists of steaming the fibre at a temperature above 130°C for 30 min in autoclave equipment. Then it is heated at a temperature around 130°C for 2 h which is followed by drying [102]. 2.5.5 Plasma treatment The plasma treatment technique is one of the important, very popular surface treatment methods for flax fibres [363]. It gives many advantages when compared to the traditional treatment processes. This is a more economical and environmentally friendly process because it does not require water and chemicals. The huge advantage of this process is to reduce the

polluting oxidants [364]. A cold plasma treatment was carried out on the fibres. The gas used for the process was helium, and the processing method is as follows: the flax fibre was put inside the reaction chamber, then the system was pumped up to a pressure of 10-4 Pa, and the gas was introduced in a controlled flow. Knowing that radicals usually react with O2 of ambient air, the optimal parameters for plasma treatment were deduced from the minimum value of the water contact angle [365]. The flax fibre was treated in discharge mode, and the helium-plasma conditions were chosen for a treatment time of 5 min and for an incident power of 50 W [102]. The fibre surfaces were modified by argon and atmospheric pressure plasma systems under different plasma powers. The bonding of argon treated fibre is superior to those of air treated and untreated fibres. Moreover, higher plasma causes greater interfacial cohesion, which was confirmed by fibre pull out tests [366]. Flax fibres are originally covered with a lot of residual lignin, dust and silica particles [367]. Fibres are submerged in 5% NaOH solution for an hour then washed and dried overnight at 100°C [368]. 2.5.6 Styrene treatment The treatment was done in a 50 cm3 solvent, which is the mixture of toluene and styrene is in the ratio of 45:5 and 0.12 mg benzyl-peroxide for 3 h at atmospheric temperature and then dried for 3 h at 80°C. To optimize the compatibility between the fibres and the matrix, no washing was done after the treatment [369]. 2.5.7 Silane treatment The treatment of fibres with c-metha-cryloxy-propyltrimethoxysilane was done by using ethanol and water is in the ratio of 80:20 for 24 h under nitrogen (N2) atmosphere and then heated at 100°C for 4 h. Then, the fibres were washed by using the same solution at room temperature [369]. The coating bath consisted of a solution of the silicone in toluene. The flax

fibre was stored for 40 s in the elastomer solution with a maximum concentration of 20 wt %. During the removal of the toluene by heating, cross-linking of the elastomer occurs around the fibre. At this stage, the coating reactive function has enabled to react with the fibre surface to build a strong interfacial adhesion between the fibre layers [370]. 2.5.8 Enzymic treatment A successful biological method developed in the 1980s in the treatment of flax fibres is using enzymes. Sharma [371] developed an enzyme that could degrade non-cellulosic polysaccharides on dew-retted flax fibres in 1987. A mixture of Pectional AC and Ultrazym, and enzymes extracted from cultures of Ceraceomyces sublaevis was used in the treatment, and the enzyme-modified roving produced good quality fibres when compared with the untreated one [372]. Flaxzyme, a mixed-enzyme commercial product of Novozymes from Aspergillums, was developed later. Then, an improved method scouring of flax roving with the aid of the enzymes was introduced in 2003. The advantages of scouring with the enzymes were tested under mild reaction conditions, in comparison with traditional chemical scouring. The results showed that the enzyme scouring, followed by a wet spinning, yielded a high quality yarn [373]. The enzyme treatment was carried out on a shaker at pH 7.0 and 45°C, as well as at different time duration from 1 to 48 h. After the enzymatic treatment, an additional bleaching procedure with 1% H2O2 at 25°C for 1 h was applied. Separately, a sample bleached with 1% H2O2 at 80°C for 1 h was prepared and was considered as a target reference for the bleached flax fibres. All samples were treated with H2O2 without shaking [374]. 2.5.9 Thiodiphenol (TDP) treatment Initially, the fibres were treated with the dihydric phenol, and de-waxed with acetone for 24 h. This was done to clean the impurities on the surface which may form hydrogen bonds with

TDP. Then, the fibres were dried in an oven at 100°C for 3 h to eliminate the residual solvent and moisture. The particular amount of dihydric phenol was dissolved in 5 ml of dioxane, and 3 g of cleaned fibres were soaked into the solution. The solvent was evaporated at room temperature. This procedure was applied to maintain a uniform distribution of dihydric phenol on the fibre surface. Then the fibres were dried in an air circulating oven at 50°C for 3 h to remove the residues completely and were used for composite fabrication [290]. 2.5.10 Thermal treatment After the addition of distilled water to obtain a half dilution of the resin mixture, mixing was performed for 2 min at 140 rpm. For thermal treatment, a household microwave was used during 1 min/100 ml of the mixture, and then 2 min mixing at 140 rpm was performed. For dilution and thermal treatment combination, the mixture has undergone the following steps: dilution, mixing, thermal treatment, and again mixing. Whatever the process used to obtain a mixture, the aggregate coating was performed with a flax fibre mass equal to the mixture mass before dilution and mixing under the conditions described before. Then coated fibres were dried at 50°C until constant mass before composite processing [375]. 2.5.11 Poly ethylene glycol (PEG) or Poly vinyl acetate (PVA) treatment A solution was prepared by using either PEG or PVA dissolved in water at a concentration level of 100 g/l, with aluminium sulphate at 10 g/l, citric acid at 10 g/l and Triton X100 detergent at 1 g/l. Then the fibre, was immersed in the solution with the ratio of 11:1, and then heated above 120°C in a furnace for 15 min. When the required temperature was attained, it was kept for 5-20 min at atmospheric temperature and then cleaned with running water for 1 h [376]. 2.5.12 Bleaching process

Unless otherwise stated, the flax fibres were impregnated in a solution containing sodium chlorite (NaClO2), hexamethylene tetramine (HMTA) activator and non-ionic wetting agent using a material to solution ratio of 1:50. This process was carried out at temperatures between 50-90°C for different time periods. After the bleaching process, the fibres were thoroughly washed with water and finally dried at ambient conditions. The bleaching reaction was monitored by determining the decomposed NaClO 2. The chemical and physical characteristics of the surface modified fibres were evaluated by measuring the carboxyl and carbonyl contents, whiteness index, percentage loss in fibre weight and tensile strength [377]. 2.6. Properties of flax fibres The properties of the plant-based fibres are highly dependent on harvesting, which is influenced by growing condition, geographical location, climatic condition, soil quality, weather circumstances, etc. [40, 378]. The properties of flax fibres depend on the crystal structure, cellulose content, level of crystallization and polymerization, micro-fibrils angle, the porosity and the size of the lumen. Fibre strength reduces with the moisture content, and the stiffness diminishes with increasing uptake of water in the fibre pores since it reduces the cohesion of the fibrils [40]. Flax is a low density material when compared to glass fibre, but it has a competent strength [100]. Many factors like structure, experimental conditions, test speed, etc., have effects on the properties of flax fibres. The parameters, concerning relative humidity [216], fibre length [219, 230], different chemical treatments [219, 379], fibre location in the stem [203, 234], microstructure of the fibres [380], water treatment and drying cycle treatment [381], fibre defects [250], plants with different varieties [381-383], fibre diameter [382] and gauge length [203, 234, 379, 380, 382] influence the strength of the flax fibres. The strength of the fibres varies from 100-2200 MPa, and the failure strain is in between 0.6-3.7 [203, 217-222, 224].

2.6.1 Fibre diameter and length measurements The dimensions of the fibres were measured after taking number of measurements and to obtain their average values concerning diameter and length. The fibres were conditioned at 20°C for 24 h with the average relative humidity of 65%. The fibres were then spread out on a slide and covered with a second slide. The flax fibre samples were pressed at an elevated temperature between the jaws of the press to get thin sheets. Then, the fibres were scanned with the help of a microscope and the average values were measured using image analysis software [384, 385]. The fibre dimensions were analyzed to find out the influence of bonding agent and fibre quantity. At least 300 measurements were taken on each sample, and average values were used to find the dimensions [289]. 2.6.2 Fibre fineness measurement The fineness of fibre was measured with the help of the airflow method. The fibres were teased and formed into a ball before being placed into the cylinder for air permeability measurement. Fineness or linear density of fibres was obtained employing the gravimetric method. Fibre fineness was related with the degree of splitting of flax fibres. Flax rovings were garneted into loose fibres by hands and then processed at 20°C for 24 h with the relative humidity of 65%. Flax fibres were cut off by using specimen cutter, and weight of fibres was measured. Then the fibres were counted, and fineness was measured [386]. The samples were measured 10 times each, and the average permeability value converted into fineness of fibre [376, 387, 388]. 2.6.3 Young’s modulus calculation The Young’s modulus can be calculated either by using Halpin-Tsai equation or by the application of a law of mixtures and the properties of the components. At the same time, it is

important to account the components having no role which can reach 15% of the fibre and comparable with porosity (V0). Easily, voids can be considered frequently by adapting a matrix modulus E’m and Poisson ratio v’m using a rule of mixtures [389]. If the volume fractions Vf, Vm, and Vo are known (Vf + Vm + Vo) then the matrix properties are: (2) (3) is the volume fraction of the matrix. The compiled elastic properties of the main components of a flax fibre are given in Table 2 [390-396]. The stiffness of the cellulose fibrils is around 30 times greater than that of the hemi-cellulose and lignin, this influences the Young’s modulus drastically. The density of cellulose is similar to that of the density of flax fibres, the mass fraction will be considered equal to the volume fraction [397]. Table 2 Elastic properties of main components of the flax fibre 2.6.4 Measurement of surface tension The measurement of surface tension was done using the contact angle measurement process. It is an accurate way to evaluate the surface energy and wettability. There are 3 fluids such as purified water, glycerol and di-iodo-methane taken as reference. The values were measured and analyzed at the surrounding temperature around 30°C. A deposited 3 ml drop with a micro-syringe was imaged using a camera. The contact angle was calculated from a contact angle meter [107]. 2.6.5 Tensile properties Tensile properties of flax fibres are essential when considering as reinforcing material in polymer composites. The tensile deformation of flax fibre is influenced by the specimens, even when these fibres are cultivated in the same location and the test parameters considered are

identical. Generally, in the stem, the fibres are stronger and stiffer; at the mid-span and the tip, the fibres have moderate properties. Flax fibres extracted from different locations in the stem also affect the tensile strength, since fibres at different locations have different chemical compositions and porosity [203]. The plant fibres are comparatively short and as there are crosssection variations along its length and 10 mm gauge length were taken. Then the fibre was fixed in a 2 N capacity load cell equipped testing machine and loaded at a constant cross-head speed of 1 mm/min until fracture [383]. This test has been carried out on Instron 5567 machine as per ASTM D2256 [220] and NFT 25-704 [383] standards. The length of the sample is 150 mm and handled in a manner to avoid any change in a twist of the samples. The test was repeated 10 times at the atmospheric temperature, and the average values were reported. The fibre loaded in the testing setup is presented in Fig. 7(a) [42]. The failure of fibre is a combination of fibre slippage and breakage which is shown in Fig. 7(b) [42]. Fig. 7 Tensile testing of flax fibre (a) before loading and (b) after fracture [42] The tensile strength of flax fibres was carried out with the help of a testing setup attached with a load beam of 20 N and at a cross-head speed of 0.5 mm/min. The fibres were in the jaws, by using paper to avoid breakage, and the gauge length was taken as 25 mm [376, 388]. The stress vs. strain curve of the flax fibre is given in Fig. 8 [379]. The response curve can be divided into three parts. In the first part, the strain varies from 0-0.3%, this deformation associated with a loading of the fibre, through the deformation of each cell wall. In the second part, the strain ranges from 0.3-0.8%, and the non-linear behavior was interpreted as an elasto-visco-plastic deformation since the alignment of the cellulosic micro-fibrils with the tensile axis led to the rearrangement of the amorphous parts of the wall and the final rupture. The last part is thought to correspond to the elastic response of the aligned micro-fibrils to the applied tensile strain [224].

Fig. 8 Typical tensile stress vs. strain plots of flax fibres [379] The tensile properties of flax fibres based on their position in the plant are listed in Table 3 [203, 234, 380]. From the table, it is observed that these properties are highly dependent on the location of the fibre in the plant. Except fibre location, the fibre diameter also has significant effect on the tensile strength. Besides, the methods of extraction of fibres also lead to the scattering of the fibre properties. Before the test, the fibre is pasted on a frame, and it is fixed on the testing equipment loaded with a 2N load cell. Then the fibre is subjected at a cross-head speed of 1 mm/min up to fracture. At least 30 fibres have been tested at each zone of the plant [203]. From the test, it is observed that the tensile strength of elementary fibre is around 1500 MPa and matured fibre is 800 MPa [227]. The modulus of flax fibres depends on the fibre diameter, which ranges from 39-78GPa for fibres of diameter 35-5 μm [406, 407]. The variation is probably related to the variation in relative lumen size between fibres of various diameters [408]. The tensile properties of flax fibres by various researchers are compiled and presented in Table 4. Table 3 Tensile strength of flax fibres based on their position in the plant Table 4 Compiled properties of flax fibres reported by various researchers 2.6.6 Moisture absorption behavior The strength of plant fibres increases with moisture content and decreases with increase in temperature [239]. The influence of moisture on the properties of plant fibres showed that the strength of the crystalline regions was not changed by moisture increase [390]. The moisture absorption of plant fibres badly affected, the bonding with resin leading to pre-mature ageing by degradation and decrease in strength [409-411]. The moisture absorption of flax fibre initially followed a linear relationship with the Fickian diffusion and slowed down as the moisture

content approached its equilibrium [412]. The cellulose molecule of the flax fibre contains hydroxyl groups that form water molecules. This water molecule is absorbed directly onto these hydrophilic groups and become fixed to the fibre molecule structure. As more water molecules are absorbed, they may be attracted to other hydrophilic groups or form further layers on top of the water molecules already absorbed. As a rule of thumb, flax fibre can absorb more than 10% of moisture in less than an hour at room temperature on the relative humidity of 90%. 2.6.7 Single fibre fragmentation The samples for this test were fabricated on the hot stage by reinforcing the fibres within two polymer layers. The samples were cut and experimented by using a micro-tensometer with a cross-head velocity of 1 mm/min [101]. Then the fibres were manually separated from the bundles and both the ends of the fibre were pasted onto a frame as per the ASTM D 3379-75 standards. The length of the fibre outside the frame was fixed in between 5-20 mm [221]. During fixing, the samples were handled only by paper tabs and the working area of the fibre was not disturbed [369]. 2.6.8 Single fibre micro-droplet test Flax fibres are often split into several branches at different locations along their length. Short sections of fibre without splits were selected and pre-dried in a oven for 24 h at 80°C, then conditioned to the required relative humidity in experiments at a constant temperature of 20°C for 24 h using the climate control chamber. A single fibre was held to a paper window frame by wood adhesive while being kept taut by a negligibly small tension. Two small droplets of resin were applied to the fibre in the middle of the frame using the tip of a stainless steel filament. The single fibre samples were cured under 20°C and the relative humidity required by the experiments in the climate chamber. The fibre was then carefully cut at the middle to produce

specimens. The specimen was tested on a tensile tester at an elongation rate of 2.5 mm/min, and the force vs. displacement curve was recorded [412]. 2.6.9 Fourier transform infra-red (FTIR) spectroscopy The fibre surface characteristics were examined using FTIR and roughness tests. To evaluate the composition and molecular structure of fibres, they were analyzed using IR spectrometry. The spectra were measured at room temperature using a FTIR spectrometer. The data were collected over a spectral range from 50 to 4000 cm-1. In the mid IR part of this range, samples were prepared in a potassium bromide (KBr) pellet [201]. 2.6.10 Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy The influence of argon and plasma modification on flax fibre was investigated by ATRFTIR analysis. ATR-FTIR spectrometer was used in the absorption mode in the range of 4004000 cm-1 [366]. The effect of different treatments showed considerable differences in the IR spectra according to the type of treatment is shown in Fig. 9 [202]. From the figure, it is observed that both raw and surface modified fibres show the absorption level of the carbonyl stretching at 1728 cm-1, but the absorbing capacity of surface modified fibres is markedly higher than a raw one. The corresponding IR absorbing band near 1730 cm-1 should have disappeared because of the surface modification. The occurrence of IR at 1728 cm-1 on the surface of fibres must be correlated to the presence ester group grafted on it, which offered a chemical basis to the observed properties [202]. Fig. 9 ATR-FTIR spectroscopy analysis of flax fibres [202] 2.6.11 X-ray photoelectron spectroscopy (XPS) analysis XPS indicated that the plasma-treated fibre had higher O2 concentration and higher O2/carbon ratio than the untreated and argon plasma treated fibres [366]. The XPS spectra were

recorded by using a mono-chromatic Al-Kα radiation source, a base pressure of 10-9-10-10 torr in the vacuum chamber, and electro-static energy analyzer. This spectrum was collected in constant analyzer energy mode with pass energy of 96 eV for elemental quantification purposes. The concentrations of different chemical states of carbon at the peak were obtained by fitting the curves with Gauss-Lorentz functions [366]. 2.6.12 Thermo-gravimetric analysis (TGA) TGA is a method which has been used to find the quality of fibre from its characteristics [175]. TGA was carried out to find the weight changes of the fibres with respect to time [383]. To measure the amount of water, temperature above 100°C was chosen by drying until a constant weight was reached [413]. The typical TGA graph of the flax fibre is presented in Fig. 10 [229]. The 5-6% weight loss up to 120°C can be attributed due to the loss of moisture in the fibres. It can be seen from the graph, that flax fibres are thermally stable up to 250°C and start to degrade when the temperature crossed 300°C. From the figure, it is further observed that the fibres lost its weight by 3/4th of its actual value at 380°C. No degradation was expected at a lower temperature, since the fibres were prepared at around 100-120°C [229]. Fig. 10 Typical TGA thermogram of flax yarn [229] 2.6.13 Differential scanning calorimetry (DSC) analysis DSC was carried out with the help of the calorimeter by applying different heating and cooling cycles. The specimen was fabricated on the hot stage, and then parts containing the transcrystallization region were cut under a microscope. The specimen weight ranges from 5-12 mg [101]. The thermal scanning was applied including the thermal history of the matrix [414]. The specimen was heated up to 200°C to examine the crystallization and melting enthalpies. The degree of crystallinity was calculated with the help of Eq. (4).

Degree of crystallinity

(4)

Where ∆Hc and ∆Hm are the crystallization and melting enthalpies. ∆H100% crystalline = 93.7 J/g is the theoretical melting enthalpy of 100% crystalline matrix [250]. The different cooling rates have been used to determine the crystallization and glass transition temperatures. 2.6.14 X-ray diffraction (XRD) analysis The XRD analysis is carried out on a diffractometer with a copper anode working at 40 kV and 40 mA and loaded with a scintillation detector and a mono-chromator [101]. The radial scattering energy distribution was obtained from a 2D detector images. As a measure of crystallinity, an index Xc was calculated by subtracting the scattering intensity at an angle of 33°, where no crystalline contribution to scattering is assumed, from the maximum scattering intensity found in the intensity distribution, and dividing this difference by the maximum intensity value [415]. 2.6.15 Thermal degradation The thermal degradation of flax fibres was investigated at temperatures of about 200°C [416-419]. It is observed that, the thermal degradation is not having any effect for the first few minutes and at lower temperatures. It is proved that the raw flax fibre retains its actual strength after exposure at 170°C for 120 min, whereas at 210°C the strength decreases almost by 50% in the same time interval [417]. Mieck et al. [420] reported that the significant damage of the fibres is evident after an exhibit of few minutes at a temperature above 240°C. It is proved that flax fibre retains its original strength when stored in a convection oven at around 200°C for half an hour [421]. 2.6.16 Differential thermo-gravimetry (DTG) analysis

The fibres were cut into a length between 0.5 and 1 mm. Three samples of weight in between 3.0-3.2 mg were taken. The samples underwent pyrolysis in a balance and furnace unit at 30-600ºC temperature range with an average heating rate of 20ºC/min in an open atmosphere. The loss of weight over the temperature range and its derivative were calculated by using the software [376, 388]. 2.6.17 Surface morphology of flax fibres The images of the flax fibre were scanned by using electron microscopes of models JEOL-T220 [101], and FEI Quanta FEG 250 [366] operating between 2.5-5 kV. A thin layer of gold coating of 40 nm thickness was sprayed on the fibres by using a vacuum unit [101, 366]. Fig. 11 shows the flax fibre surface (a) without alkalinization, (b) with alkalinization, (c) green flax surface, (d) dew retted flax surface, and (e) duralin flax surface [368]. It is observed that the dew-retted fibre has a rough surface while duralin and green fibres have a relatively smooth surface. The irregular surface of the dew retted fibre may be the reason for the forming of uniform and thick layer than the other fibres. Fig.11 (f) shows the SEM micrograph of untreated flax fibre, while Fig. 11 (g) shows the fibre after plasma treatment. The surface of the treated fibres seems to be cleaned-up. The surface topography highlights the reduction in surface impurities, which were present in the untreated fibres. Flax fibre is a hexagonal cross-section and often described either as a long spindle or as a shuttle [220]. The faces and edges of the fibre are shown in Fig. 11 (h). Fig. 11 Flax fibre surface (a) untreated, (b) alkali treated, (c) green flax surface, (d) dew retted flax surface, (e) duralin flax surface, (f) untreated, (g) plasma treated, and (h) hexagonal shape

The compiled SEM images of raw and chemically treated flax fibres are depicted in Fig. 12 [220, 379, 388, 422, 423]. Fig. 12 (a) reveals the untreated flax fibre and Fig. 12 (b) shows the surface of lipid acylated flax fibre. One can easily observe the surface roughness of the acylated fibre from this image. Fig. 12 (c) shows the surface of protein coating treated flax fibre. It can be noticed that the roughness is lower than lipid acylated flax fibre because of protein coating [422]. Fig. 12 (d) gives samples of deformed areas, like kink bands, called nodes or dislocations [220]. There seems to be some separation and cleaning of the fibres, with enzymatic treatment only, although some fibre separation has taken place and a significant amount of debris attached to the fibre surface is still present which is visible in Fig. 12 (e) [379, 388]. The major separation seems to have been brought about by the ethylene diamine tetra-acetic acid (EDTA) treatment which is shown in Fig. 12 (f) [379, 388]. The surface of flax fibres before and after enzymatic treatment is presented in Fig. 12 (g and h) [423]. Fig. 12 SEM images of flax fibres (a) untreated, (b) lipid acylated, (c) protein coated, (d) fibre defects, (e) enzymic treated, (f) EDTA treated, (g) and (h) before and after enzymatic modification SEM images of raw and chemically modified flax fibres are presented in Fig. 13 [202, 386]. The assembly of fibres and the surface of a unique fibre is presented in Fig. 13 (a-b). From these images, it can be observed a kink band is the main defect in flax fibres. In Fig. 13 (c) the structure of a flax fibre in the uni-axial direction is presented. The de-natured fibre surface which has been subjected to alkali treatment shows the direction of the macro-fibrils is illustrated in Fig. 13 (d). The formic acid surface treated flax fibre is given in Fig. 13 (e). The macro-fibrils are also observed, but the roughness is low for only NaOH treatment and high for NaOH and acetic anhydride treatment [202]. Fig. 13 (f) shows the untreated flax fibre, Fig. 13 (g) scoured

flax fibre at pH of 9.0, temperature of 35°C, and scouring time of 12 h; and Fig. 13 (h) scoured flax at the suggested optimal synthesis conditions at pH of 9.069, temperature of 43.19°C, and scouring time of 13.78 h [386]. Fig. 13 SEM images of flax fibres (a) raw fibre, (b) defected raw fibre, (c) treated with hot alcoholic NaOH, (d) hot alcoholic NaOH and cold acetic anhydride treated, (e) formic acid treated, (f) untreated, (g) scoured at general condition, and (h) scoured at optimal synthesis condition The surface of the flax fibres has a striated structure oriented in the longitudinal direction which is seen in the images presented in Fig. 14 (a and d). But in Fig. 14 (b, c, e, and f) the striated structure is clear, and can be seen that consist of macro-fibrils arranged in parallel next to each other. From Fig. 14 (c and f), it is estimated that the transverse dimension of these macrofibrils is to be in the range of 100-250 nm. Short fibrils are protruding from the surface of the fibres which is shown in Fig. 14 (b and c). The transverse dimension of these fibrils is in the range of 30-50 nm. It seems like these fibrils are part of the longitudinally oriented macro-fibrils at the fibre surface, and then, at some locations, short lengths of these fibrils are sticking out from the macro-fibrils. Within the region of kink bands, at some locations, the macro-fibrils at the surface are separated from each other to form an opening inside the cell wall [424]. Fig. 14 Surface morphology of flax fibres (a)-(c) a single fibre, and (d)-(f) a bundle of fibres Fig. 15 shows some typical images obtained on flax fibre cross-section using electron and optical microscopy [209]. The non-circular cross-section of the flax fibres is clearly illustrated in the micrographs. It can be seen that these fibres consist of an assembly of many elementary fibres. The fibres are located around the stem of the plant, and tend to contain between 10-40 elementary fibres. The elementary fibres have a polygonal cross-section, allowing them to fit

closely together. Each elementary fibre contains multiple concentric cell walls, with a void in the middle known as the lumen. It is clear from these images that the structure of flax fibre reveals many potential weak interfaces [209]. The images further reveal that the flax fibre has been damaged during sample preparation and has failed at a number of internal interfaces [404]. It is assumed that the flax fibre has a circular cross-section, not exactly true but AFM pictures have shown that it is a close approximation which is shown in Fig. 16 [414]. Fig. 15 SEM images of cross-section of flax stem (a) un-retted, (b) viscozyme retted, (c) flax fibre cross-section and (d) fibre cross-section optical [209] Fig. 16 AFM pictures after nano-indentation of flax fibre cross-section [414] 2.6.18 Processing-structure-property relationships The structure of flax fibres is highly complex because of the hierarchical arrangement and the different composition of materials present with variable proportions [220]. To identify the effect of the chemical composition of flax fibers on various physical properties and to compare them with those of the similar plant fibers. The separation and characterization of specific components, which are important steps for understanding their relative arrangement and roles in the wall, are very difficult because of the complex three-dimensional organization of the cell wall [425]. It is evident that attempts should be made to find new openings for this important fibre crop, and that a good understanding of its structure and properties would be of assistance, since it is recognized that any changes in structure may significantly affect the properties [426]. Flax fibres composed of cellulose, hemicelluloses, and lignin together with other constituents, have structures which make the stem of the plant strong and stiff [427]. Flax is a cellulosic fibre which has a natural wax coating on its surface and also has a hollow lumen as the main feature of its structure [428]. The surface wax imparts hydrophobic

and oleophilic properties. The presence of wax in fibre surface enhances the bonding between the surfaces through van der Waals forces, because of hydrocarbons are present in these waxes [429]. Initially, various methods were adopted to determine the components of flax fibre, viz., fats and waxes, water solubles, pectin, hemicellulose, lignin, and cellulose [430]. Finally, a method given by Soutar and Bryden [431], which was later revised by Gamer [432], was chosen for the determination of these components. From the review, it is found that the variations in properties among the fibres are for several reasons such as their source of origin, the method of retting, the nature of impurities present, chemical, and the mechanical processing methods performed on them. 3. FFRCs The presently used composites are fabricated by using conventional fibres and synthetic polymers which are the derivatives of petroleum, an un-sustainable fossil fuel. These are not easily degradable and therefore, cause severe problems during disposal at the end of their life and cannot be recycled or reused. Therefore, bio-degradable fibres reinforced with bio-based matrix materials have captured the attention of researchers in recent years [330, 354, 433-435]. Natural fibre reinforced composites gains attraction due to their availability, carbon neutrality regarding the environment, applicability to the available processing machines and good mechanical properties [100, 313, 436-440]. By embedding natural fibres into a bio-resin made out of essence from natural resources new materials have been developed is called bio-composites [37, 64, 83, 84, 298, 398, 441-459]. FFRCs with both thermoset [340, 407, 460-467] and thermoplastic [217, 233, 370, 468-471] matrices have been developed by several researchers. The possible ways to use bio-based resins from renewable resources [53, 117, 449, 472] and waste from linen yarn production [473-476] have also been considered. Especially flax fibre reinforced with

polypropylene [101, 104, 110, 288, 289, 477-479], polyethylene [480, 481], polyester [57, 411, 482], polyolefins [299, 483, 484], polystyrene [485, 486], and epoxy [460, 487] composites have been developed and then, the interfacial relationship between the flax fibres and the bio-based matrices were studied [109, 414, 488, 489]. 3.1 Processing methods Nowadays, it is very difficult to develop bio-based green composites with good properties regarding strength, bio-degradability and environmentally friendly characteristics [490]. Many research articles on processing of FFRCs have been published over the years and concluded that the major issues are to achieve strength, eco-friendliness, property variability and low cost [491]. Flax fibres as reinforcing material in composites are not only considered in the form of mono-filament configuration [492, 493]. Mono-filament fibres are further processed into mats [109, 401, 480, 494], rovings [37, 495], yarns [496], and fabrics [19, 42, 497-499] in composites. To date, different manufacturing methods have been developed to produce FFRCs, such as hand lay-up [19], compression molding [250, 480, 492, 493], film stacking [109], vacuum infusion [42, 497], filament winding [496], manual winding [497], resin transfer molding [401, 495], injection molding [492], pultrusion [37, 500], etc. When selecting a fabrication method, the parameters including the targeted strength, the properties of raw materials and manufacturing cost all should be considered [501]. The size of a composite is treated as a dominating factor for composite fabrication. For preliminary evaluation of composites with small size, injection and compression moldings are preferred as a consequence of their simplicity and fast processing period. For structures with large size, open molding and auto-clave processes are preferable. 3.1.1 Filament winding

It is the most suitable and convenient process for manufacturing composites where the fibres normally are in the form of yarns [501]. The fabrication of composites reinforced with flax fibres consisting of two steps: fibre/matrix production by filament winding and then the composite fabrication by vacuum bagging and compression molding. The fibre and the resin were mixed by a machine to make fibre/matrix assemblies. After making fibre/matrix assemblies, it was dried for 16 h in a chamber to remove the moisture. Then the assemblies were processed into composites by using a press. The assembly was then transferred to the heating chamber at 200°C, subsequently conveyed to the press after melting of the matrix in the vacuum chamber. Finally, the assembly was consolidated with a compressive force of 200 kN at 30°C for 1 min into a composite laminate [502]. 3.1.2 Pultrusion Pultrusion process is mainly used in a particular sector of the composite manufacturers. It is mainly used for producing long and uniform cross-section parts. It offers many advantages of continuous processing of composites when compared to the other composite manufacturing processes. This process has been exclusively developed for thermosetting matrices, however, thermoplastic matrices show rapid growth, and the efforts have been made during the recent years. This process has certain advantages such as good chemical resistance, high mechanical strength, excellent re-cyclability, high-temperature resistance and their ability to be post-shaped [503-505]. 3.1.3 Injection molding In injection molding, fibres are usually chopped into short according to the critical fibre length in which the stress should be fully transformed from the matrix to the fibre, and the fibre can be loaded fully assuming a good interfacial adhesion is achieved; the amount of the mixture

can be pre-designed. The resin was cured at 60°C for 48 h before the process. Then the fibres were extruded at different fibre ratios of 20% and 30%. The compounding was done in an extruder at 20 rpm and with the temperature range 175-185°C. Then it was dried for 48 h at 60°C in a vacuum chamber. The injection molding was then carried out and all the parameters were kept constant. The temperature profile was kept in between 165-180°C in the nozzle. The temperature is close to those reported earlier and to cause degradation of flax fibres. The injection pressure was kept 190 bar at 30°C for 95 s [417]. The temperature maintained during the process is 30°C [384]. 3.1.4 Compression molding This technique is a combination of pressing as well as autoclave method. The fibres are usually in the forms of chopped fibres and mat. The curing pressure is maintaining 1.8 MPa for obtaining the least amount of voids in the composites. The curing temperature is 140°C for 1 h to get 100% curing of the resin [506]. The fibres were cut into small pieces and then dried in hot air oven around 100°C for over 7 h. The fibres were placed between the resin mats and wrapped in Teflon sheets and molded by using aluminium plates. Then, this mould is placed between the two platens of compression molding machine and cured at 210°C for 20 min under constant pressure, followed by cooling under the same pressure for few min [362, 494]. The accelerator was mixed with the resin and the mixture was out gazed under vacuum. Then, the initiator solution was added to the mixture. The composites were manufactured by pouring the resin on the fibres in a mould. Then the samples were kept under the press for 24 h at room temperature to get the required size [369]. The specimens were processed in a mould that fitted in a mettler so that, the resin could be melted. The diameter of fibres was measured by using microscopy and kept in between 100-

160 μm. Then the fibres were stretched across the groove of the mould and positioned in place with the help of the adhesives. The resin films were shaped into the correct size to fit the mould and stacked inside the mould, covered by slips before inserting into the hot stage. The heating and cooling conditions for the different composites were similar to those used for testing [488]. The fibres were cut and stacked for compression molding. The two heated platens on the hot press were raised to 60°C before the fibres were placed between them. The platens were then closed to apply a 50 kN constant force to the sample and the temperature was raised to 190°C, which was maintained for 20 min before the heating elements were switched off to let the sample cool down [498]. 3.1.5 Hand lay-up technique It is a labour intensive method which is easy to deal with, and cost-effective, it is widely used in civil infrastructure to retrofit and strengthen structure with fibre reinforced composites. The fibres were cut to 250 x 250 mm size and layers stacked in sequence. The laminates are fabricated using the press plates with the fibre volume fraction of 23 and 34%. The amount of resin used to impregnate the fibre is maintained, to obtain a complete impregnation. The laminates are cured in a hot-platen press for 2 h at 50°C with a pressure of 3 bar [507]. 3.1.6 Wet lay-up method FFRCs were prepared by using wet lay-up process. The resin and hardener were mixed in the ratio of 4:1 and mechanically stirred at room temperature for 15 min. The resultant mixture was de-gassed in a vacuum chamber to eliminate air bubbles. The de-gassed resin/hardener formulation was then used to impregnate flax fibres which were then stacked in a cross-ply configuration to achieve a required thick laminate. The panel was cured overnight at ambient

conditions. The FFRCs were prepared by uniformly applying the resin/hardener formulation onto fibre surfaces before hand-laying impregnated flax fibres [508]. 3.1.7 Resin transfer molding (RTM) The RTM and vacuum infusion enables the production of composites with high volume fraction and high strength-to-weight ratio [42]. The fibre preforms normally in the form of fabric and mat. In general, there is no limitation on the dimensions of composites with RTM and vacuum infusion processes, which is very important for real-time industrial applications. 3.1.8 Stacking technique Stacking technique consists of heating and applying pressure on a stack of composite layers for a particular period. The fibres were cut into a defined length and the mats are prepared. Then the fibres have to be randomly oriented to have isotropic properties in the composite laminate. The thin resin films are prepared by heating and applying pressure on the pellets. The flax mats are cut to exact shape with similar size. The composites are prepared from a layer of resin films reinforced with fibre mats. Then the stack is placed on a hydraulic press equipped with heating jaws. Finally, the assembly is heated, pressed and cooled to room temperature under uniform pressure [15]. The stacking process was done at 200°C for 3 min under 10 MPa pressure. The temperature was maintained with the help of the rheological test, where the viscosity is in between 190-200°C [509]. After pressing of stacked composite laminate, the mould was cooled by the clean tap water to reduce temperature to 50°C. The stacking technique is presented in Fig. 17 [509]. Fig. 17 Stacking technique of FFRCs fabrication [509] Accordingly, the resin film and a layer of flax fibres were placed on one another alternately. The flax fibres were arranged in a crossed-ply manner. The laminates of crossed-ply

architecture are traditionally used in the composite field to assess their mechanical properties. Quasi-unidirectional lay-up of flax fibre was achieved by combing and fixing their end sections with adhesive tapes [510]. The laminates were made by infusing a stack of catalyzed resin. Then the mats were placed between the glass sheets, cured for one day at ambient temperature [376]. 3.1.9 Commingling Commingling is one of the efficient ways of producing good quality and economically viable composite products. The composites can be produced with different type of intermediate products such as film stacking, powder impregnation, and different types of yarns and fabric. Commingled yarn can be used to achieve a superior fibre and matrix impregnation in the preform stage compared to the other intermediate product. Since, it is highly expensive to produce commingled yarn, another preform such as uncommingled yarn can be used [511]. The main advantages of this processing technique are the possibility to produce the complex shaped parts, the requirement of less applied pressure and time for impregnation which happened due to the polymer flow distance reduced by totally covered commingled matrix and leads to reduced production cost [512]. 3.1.10 Compound production The FFRCs were fabricated in a kneader, equipped with rotors at 185°C. The compatibilizer and resin were added to the kneader and kneaded for 2 min to get a constant torque. Then the fibres were added with the mixture and it was kneaded, confirming that the fibres were added, kneading was done for 4 min. This leads the compounds to a total kneading time of 10-12 min, and the temperature increased to a maximum of 210°C during kneading [110]. 3.1.11 Vacuum bagging technique

The composite structures were made with a single lamination using the vacuum bagging technique. This method involves an initial hand lay-up phase and then the polymerization of the matrix in a bag in which low pressure is reached by a vacuum pump. Vacuum bag technology brings some advantages to the final characteristics of composite laminate compared to hand layup technology. All the laminates were dried at an ambient temperature for 24 h and then postcured for 8 h at 60°C [17]. 3.1.12 Vacuum infusion process The composite panels have been produced using a vacuum infusion process. For this purpose, a stack of dry reinforcement plies has been laminated over a glass mould. After the lamination stage, the layout has been completed with the flow media and the infusion network, and finally, the mould has been sealed with the vacuum bag. The curing stage of the resin, after the infusion is concluded, has been carried out at normal ambient surrounding atmosphere, while the post-curing is done using a hot oven in two stages: at 60°C for 3 h and subsequently at 80°C for 4 h. The in-mould pressure measured during the infusion stages was equal to 20 mbar [513]. 3.1.13 Vacuum assisted resin transfer molding (VARTM) process FFRC specimens were fabricated by using VARTM process. In this process, the fibre layers were reinforced in the warp and weft directions were kept on a mold. The injection and venting lines were made on the opposite edges of the specimen. The whole setup was vacuum bagged and sealed, and the pressure was kept at 95 KPa. The resin was used together with a hardener. The untreated and chemically treated fibres were used for composite fabrication. It was found that the chemical treatment leads to a reduction of the fibre content due to loss of fibres and impurities during the chemical treatment of fibres [355, 514]. 3.1.14 Liquid composite molding (LCM) process

The resin transfer molding (RTM), injection compression molding (ICM) and resin infusion (also known as VRTM) are the applications of the LCM process, has been growing rapidly nowadays. In the LCM process, the injection of resin has done before and/or after compaction of the reinforcing fibres. As a result of the compaction of the reinforcing fibres, the pressure generated during the process, need to be determined. The mould filling stages of LCM process requires relationships between fibre content, permeability, and the compaction effect of the reinforcing fibres. In this process, the reinforcing fibres are generally in the form of mats or woven textiles, fabrics, or rovings. In the LCM process, the fibres are typically tied together in tows, separated by large gaps which facilitate the flow of resin during the saturation process. The resin must flow around and into the yarns and wet the fibres [515]. 3.1.15 Extrusion process Extrusion is a process of melting the polymer, thoroughly mixing the flax fibres within the molten polyethylene, and conveying the mixture through a die to produce extrudates. The effect of extrusion parameters on extrudate characteristics has been studied by a number of researchers [516-518]. Some studies have focused on the effects of extrusion on tensile strength and moisture intake of the product. It was noted that high extrusion temperatures increased tensile strength and moisture content, while screw speed had a significant effect on the tensile properties. This process ensures that the flax fibres are uniformly dispersed within the melt, consequently resulting in composites with better properties. However, certain extrusion parameters must be considered to optimize the effect of extrusion on extrudate properties. Screw speed must be increased to minimize residence time and maximize output. However, when a product is heat sensitive, the screw speed is limited by the maximum shear rate that the product can experience without degradation [354].

4 Properties of FFRCs 4.1 Mechanical properties The strength of the PFRCs is mainly based on their composition, fibre structure, cellulose content, place of origin, planting condition, climatic condition, and the extraction methods and storage [43]. It seems that for plant fibres the highest mechanical strengths were obtained along the direction of fibre orientation with the fibres laid down as straight as possible [307, 333, 462, 519-522]. To improve the mechanical properties of the composites, nano-particles need to be added [229]. Fibre tenacity was measured by using fibre bundles cut into a particular length. The bundles were weighed to calculate the density, which was in between 300-400 mg/m. The measurement of tensile modulus was done by using extensometer [57]. The mechanical characteristics highly depend on the fibre alignment with respect to the direction of loading [523]. Scida et al. [524] has assessed the influence of ageing on properties of FFRCs and the effect of knitted structure on the properties and fracture mechanisms has been studied by Kannan et al. [511]. From the studies, it is concluded that the parallel lamination of plain FFRCs has the highest flexural strength and impact value. It is further noted that, the highest flexural strain value is obtained for the rib-knitted FFRCs. The comparison of physical, tensile and fatigue properties of flax, glass and flax/glass composites are presented in Table 5 [525, 526]. Table 5 Comparison of physical, tensile and fatigue properties 4.1.1 Tensile properties This test is fixing the sample in a machining setup and applying it to the force until failure, and this force is recorded as a function of the increase in gauge length [527]. There are several experiments conducted to find out the tensile strength of the FFRCs. Many studies deal with fibres embedded in a synthetic resin [42, 333, 520, 528] but few studies were on the

processing of fully bio-degradable composites by incorporating with natural resin [307, 519, 521]. The main problem for PFRCs is to achieve high tensile strength, coupled with the low variability in properties and comparatively low cost [171]. The tensile test samples were prepared as per the following standards, viz; ASTM D3039 [17, 356, 498, 506, 509, 513, 524, 529], ASTM D638 [354, 355, 494, 507, 530], ASTM D882 [531], ASTM D3379-75 [532], ISO 527 [15, 107, 110, 383, 384, 502], GB1447-83 [533], and DIN EN 61 [534]. The tests were conducted by using the different models of universal testing machine (UTM) [498, 502, 506, 509], Instron of models 5543 [535], 4301 [102, 107], 3382 [19, 513, 524], 4411 [495], 2630111[495], 3369 [494, 507, 530, 531, 533], 1195 [376, 388], 4507 [536], 5567 [355, 356], 4206 [289, 537], MTS Synergie RT1000 [15, 383], Zwick Z010 [110, 424], and Zwick Roell [17, 415, 534] by various researchers. Samples were prepared with the dimensions of 250 x 25 x 2 [17, 509], 180 x 20 x 2 [19, 502], 45 x 18 x 2 [535] and 80 x 10 x 4 mm [534]. The tensile test was carried out with the cross-head speeds ranging 1-5 mm/min and the setup is equipped with a load cell of 5-25 kN [17, 19, 102, 107, 110, 289, 354, 355, 356, 376, 383, 388, 415, 494, 495, 502, 503, 506, 524, 530, 531, 535, 537, 538]. Many researchers have done the experiment at 20±5°C in temperature and 65% in average RH. In many experiments, five samples were tested and the average values are used for detailed analysis. The reported tensile strengths of FFRCs are presented in Table 6. Table 6 Reported tensile strengths of the FFRCs The typical tensile load vs. strain curves of the FFRCs is presented in Fig. 18 (a) [355]. From the figure, it is observed that the nonlinear behavior in the first stage, starts from the origin point to the maximum loading point at which the fibres are broken. The shape of curves is concave, which is opposite to that of fibres; because the reinforcement for both the fibres and the

knitting yarn withstand tensile loads. The second stage is from the maximum loading point to the total breakage of the fibre. In this stage, a drastic drop of the tensile strength is observed, because the tensile loads are only withstood by the yarns. The results indicated that the chemical treatment could lead to the reduction of the tensile strength. The treatment of fibre has a very obvious positive effect on the tensile properties of the FFRCs which is shown in Fig. 18 (b) [355]. It is observed that the effects on the tensile properties after the treatment are almost the same. However, the improvement of the tensile strength is much greater than that of the modulus. Fig. 18 Typical (a) load vs. strain and (b) stress vs. strain curves of FFRCs [355] 4.1.2 Static compression testing Compression experiments were performed according to ASTM D695 [507, 530], ASTM D7336M-12 [18], and ASTM D3410 [529] standards by using a universal MTS-type tester [507, 530], Instron 5567 [18] and H25 K-H UTM [529] equipment. The geometry was a rectangular specimen with a length/width ratio equal to two. Compressive force and displacements were recorded, and testing was halted prior to total compression of the test specimen [507, 530]. The cross-head speed used was 10 mm/min. A variable displacement transducer was used to record the displacement [18, 540]. The strain at the rate of 10 mm/min was applied, and the compression range was 7.5 mm. The readings were set at zero initially, and the load was applied till the complete failure of the sample [529]. 4.1.3 Flexural properties The combination of the tensile and compressive strengths is known as flexural strength, which varies from the inter-laminar shear strength. In this test, various mechanisms such as tension, compression, and shearing take place simultaneously [494]. The three-point testing

method is the most common and popular especially for composite materials. The elongation of the specimen is measured in the transverse direction which is perpendicular to the axis. The process which includes mounting the test sample in the machine and apply a force up to fracture [527]. Flexural test of composites was done in UTM of different models such as Trapezium X (AG-100 KNX) [483], Zwick 1445 [110, 541], Instron 5500R [412], Instron 3365 [17], Instron 5567 [355], Monsanto T10 [542] and LLOYD 30K [513] by various researchers. This test has been carried out according to ASTM D790 [17, 355, 356, 412, 494, 511, 513, 542] and ISO 178 [110] standards. The flexural strength was examined by using 4 x 10 x 80 and 2 x 15 x 40 mm size specimens at a span length of 32 mm with a cross-head velocity of 1-6 mm/min. The samples were tested at room temperature and the setup is equipped with a load cell of 5 kN. The tests are conducted under a standard air condition of 25±5°C and 65±5% RH. The flexural modulus, which is the ratio of stress to strain at any point on the stress vs. strain curve, is calculated using Eq. (5) [538]. (5) Where Ef = flexural modulus of elasticity, L = support span, b = width of the beam, h = thickness of beam, and m = slope. The stress vs. deflection curves of FFRCs is shown in Fig. 19 [355]. It is shown that the improvement on the properties of the composite after the treatment is same. Whereas, the modulus is higher when compared to the strength. This is due to the testing condition, and the delamination is the main failure mode of the composites. By avoiding the slippage effect between the reinforcing fibres and matrix plays a very important role in increasing the modulus. Thus, there is a significant improvement in flexural modulus of the composite after the NaOH treatment. The first stage starts from the initial point to the breaking of the flax and resin and

stress of the composites. The next stage starts after the breaking of the yarns and resin. The knitting yarn withstands and the flexural load is clearly visible in the figure. Therefore, the flexural stresses of the untreated and chemically treated composites do not have much difference [355]. Fig. 19 Typical stress vs. deflection curves of FFRCs [355] 4.1.4 Impact properties The ability of a material to absorb energy under sudden load at different velocities is called impact strength. This strength is directly related to its toughness, but the impact energy is used for comparison and does not give toughness of the composites. The fibres play a major role in the impact resistance of the material as they interact with the crack formation in the matrix and act as a transferring medium [494]. These failure modes are strongly dependent on the fibre type, resin type, lay-up, thickness, loading velocity and projectile type. For low-velocity impact tests, the usage of pendulums like the ones in the charpy test [543], drop towers or drop weights [544] has become standard. Impact test was carried out by using izod impact tester QC 639 [511], and Instron dynatup [494]. The sample dimensions are 63.5 x 12.7 x 2 and 4 x 10 x 80 mm as per ASTM D256 standards [493, 494, 511]. The span length is 60-80 mm, drop mass is 6-7 kg, and the drop rate of 3.29 m/s have been used. During the test, the energy absorbed by the specimen was measured, and stored for subsequent analysis. This energy is used for relative comparison and does not give the toughness of the material [289]. Impact tests with unnotched samples were done at atmospheric temperature, with the help of the testing setup. The clamps were fixed at 71 mm and the velocity was 3.7 m/s [110, 534, 541]. 4.1.5 Interfacial/laminar shear strength (IFSS/ILSS)

The level of adhesion in the fibre/matrix interface is measured by way of IFSS. The test methods applied for the experimental determination of IFSS include fibre fragmentation [197, 545-548], fibre pull-out [279, 488, 549-552], and micro-bead [202, 553, 554] tests. Considering the importance of the fibre and matrix interfacial strength, there are numerous experiments conducted to increase the strength of the material by improving the interfacial adhesion and the fibre wettability [44, 109, 332, 333, 555]. The assumption taken is which does not hold in specimen, that there is a constant shear stress at the fibre/matrix interface. But in a real case, nonlinear shear stress is exhibited at the regions near the fibre ends [556]. The method for IFSS estimation has been proposed using the pulled out fibre length distribution of FFRCs [557]. The method assumes Weibull two-parameter distribution of fibre tensile strength. However, the strength-length scaling of fibres has been shown to deviate from that stipulated value by the Weibull distribution in some cases [221, 558]. The IFSS of FFRCs was carried out with the help of the micro-bead method [87, 559, 560]. The samples were dried in an air circulating oven for an hour at 35°C and allowed curing for 5 min at 120°C. The fibre diameter (d) and its length (l) embedded in the matrix was measured using a microscope. The position of the micro-bead was adjusted under the micro-vice plates. The end of the fibre was fixed in the vice and applied force at a cross-head speed of 5 mm/min. The shear force (F), required was measured, and the IFSS value was determined by using the Eq. (6) [229]. IFSS (

(6)

Short beam shear tests were performed based on ASTM D2344 to measure the ILSS of the composites. The velocity was 1 mm/min, and the span-to-depth ratio was 4:1. The size of the specimen is 12 x 6 x 32 mm [506, 356]. Improvements on ILSS depended on the fibre bridging

which was mainly caused by the flax yarn structure and rough surface of flax fibre. The ILSS can be calculated by using Eq. (7) [513]. ILSS = 0.75 x PMax/bh

(7)

where PMax = maximum load, b = width of the specimen and h = measured thickness. The ILSS of FFRCs reported by various researchers is compiled and given in Table 7. Table 7 Reported ILSS of FFRCs 4.1.6 Fatigue testing The fatigue tests were carried out according to ISO 13003 [499] and DIN 50 100 [370] standards. To avoid excessive self-heating, the loading frequency of 5 Hz was identified by an experimental program [499]. Tension-tension fatigue tests were conducted under load amplitude control mode with a loading ratio of R=0.1. For each of the five stress levels ranging from 0.4 to 0.8, by increments of 0.1, five replicate tests were performed for all types of specimens. Tests were stopped at failure or after 2x106 cycles, whichever came first. Tests were conducted on a servo-hydraulic MTS 809 machine with a capability of 100 kN, with a thermal chamber maintaining a constant temperature of 23°C, and without humidity control [378]. The tensiontension load increasing tests with 5x103 cycles per maximum stress level starting from 5 MPa were driven in tension mode, and the samples of size 130 x 25 x 4 mm were taken for each test. Testing frequency and stress ratio were chosen to be 10 Hz and 0.1 respectively, with maximum internal heating of the samples not to exceed 7°C before damage initiation [370]. 4.1.7 Water/moisture absorption properties PFRCs can absorb many times more water than synthetic fibre composites [19]. Number of researchers has done experiment on water uptake characteristics of PFRCs [561-576]. There have been numerous studies on water damage of PFRCs [19, 100, 102, 524, 575, 577-582]. The

aging mechanisms in PFRCs in water have mostly been studied at the macro-scale [233, 566, 583-586]. The moisture absorption test was carried out according to ASTM D570 [354, 531] and NF EN12571 [490] standards. The test specimens were cut in the form of bars of 76.2 mm long and 25.4 mm wide. The fibre/matrix interface is often cited as a privileged region for degradation, resulting in a decrease in composite properties. Moisture can seriously damage the fibre/matrix interface, leading to deterioration of stress transfer efficiencies from the matrix to reinforcing fibres [582, 587, 588]. The degradation process starts with the swelling of fibres that develops stress at the interface and causes micro-cracking of the matrix around the fibres [581, 589, 590]. The cracks exacerbate water absorption and its attack on the interface of fibre and matrix. The absorbed water starts to establish molecular hydrogen bonding with the fibres and thereby decreases the bonding between the fibres and the matrix. These lead to the de-bonding between the fibre and matrix. The adverse effect of moisture on PFRCs in end-use is well known, and a lot of efforts have been focused on water absorption models and decrease of moisture on the mechanical performance of these composites [45, 100, 142-144, 282, 381, 577, 579, 582, 590-601]. Swelling of the surface by moisture absorption can lead to small cracks in the specimen and affect the properties [122]. Hence, moisture absorption is one of the very important issues that need to be rectified for a better performance of plant fibres and its composites; in fact there is a reduction in properties [19, 196, 233, 602, 603]. The moisture intake of composites can be estimated by using gravimetric analysis. The weight of the moisture absorbed specimens was measured in accordance with the immersion time. The moisture intake behavior of the composites was evaluated after various periods, by using Eq. (8). (8)

Where Mt = relative moisture intake, W0 = weight of specimen before immersion and Wt = weight of specimen at immersion time t. Moisture absorption characteristics were examined on specimens in an automated gravimetric vapour absorption system by using an electronic micro-balance. This has been conducted in a thermo-regulated water bath at 25°C. The content of water at equilibrium is used to build the absorption iso-therm [604]. The mechanism for water damage is shown in Fig. 20 [551]. If the PFRC is submerged into water, the fibres absorb water and swelling. The absorbed water causes stress in the matrix, represented in Fig. 20(a). The stress relieved by molecular relaxation processes in the matrix is shown in Fig. 20(b). The fibres were shrinking when the composite is dried, but the matrix does not contract to its original shape because molecular relaxation processes have distorted it. The shrunken fibre came out from the resin created a gap which is shown in Fig. 20(c). Fig. 20 Water damage mechanisms (a) wet fibre expansion, (b) composite after stress has been released, and (c) fibre contraction during drying [551] The water absorption specimens were cured in a hot oven at 50°C for 24 h, followed by cooling in a desiccator, and then the samples were weighed. Conditioned specimens were then subjected to the 24 h immersion procedure as specified in the standard. After the 24 h immersion, the samples were again weighed [354, 531]. A paper towel was used to wipe each specimen free of surface moisture before weighing [515]. The weight of the specimens is measured at regular time intervals. The weight was measured by using an analytical balance and the percentage of weight gain was calculated from the values [490]. The hydric interface in matrix reinforced with treated flax-fibres and the influence of sea water ageing on the properties of FFRCs were studied [369, 584]. The moisture absorption percentage for the samples ageing at 20°C and 40°C, as a

function of square root of ageing time divided by thickness is presented in Fig. 21 [524]. The figure shows the moisture absorption behavior predicted from Fick’s second law of diffusion. Fig. 21 Water absorption behavior of FFRCs [524] The flax shaves were cured to reach constant weight for a week before moisture intake experiments have been carried out. The moisture intake capacity and the saturation time of the specimen were measured by immersing in distilled water. In general, the measured quantity of shaves was placed in a sieve, and weighed before hand, immersed in a known volume of water. Then measure the weight of the sieve and its contents were measured regularly until achieving constant value [490]. The results of water absorption for the FFRCs showed that twelve times as high that of the synthetic fibre reinforced composites. The tensile modulus and the failure strain of FFRCs are severely affected by water ageing whereas only the tensile stress is reduced regarding the glass fibre composites [19]. The influence of moisture intake of FFRCs investigated by immersing in distilled water at an atmospheric temperature, found that it is increased with the increasing of fibre content [289]. The effect of sea water aging on properties of the composites was investigated and found that the damage at the fibre/matrix interface was one of the main degradation mechanisms. The interfacial bonding strength depends on several parameters, including the surface energy, chemistry, and roughness of the fibre and these parameters can be modified by treatments [202, 481, 549]. 4.1.8 Permeability measurements Permeability measurements of fibre reinforcements in composites were carried out from 1980s by various researchers [605-609]. These measurements on PFRCs are very new, and the exposure on the measurements is relatively low. From the experimental study, it is confirmed that large diameter fibres possess lower permeability when compared to medium and small

diameter fibres and this would be caused by the presence of more lightly packed and dislocated fibres [515]. If the diameter of the fibre is small, they were densely packed and more open channels were available for the flow of resin. The effect of fibre diameter on its tensile strength, its resin permeability and the resulting composite properties have been investigated [610]. The important factor affecting the permeability of a fibre is the swelling, when it is exposed to moisture [611, 612]. This is because of hydrophilic nature of plant fibres as well as low molecular weight compounds present in resins, could produce fibre swelling [612]. The fibre swelling reduces the size of flow paths and thus affecting the flow of resin and result, it reduces the reinforcement permeability. The permeability of a flax preform was measured, and the results were compared with the glass fibres for similar fibre contents [613]. The permeability values are similar and the swelling effect in work is different but oil is used as test fluid which seems to prevent any fibres swelling as suggested by other authors [515, 611, 612]. It is found that the permeability of PFRCs is lower than that of glass fibre composites for similar fibre volume contents [515, 611, 612, 614] while in other cases it is similar or even higher values [613, 615]. Permeability tests on the FFRCs were performed with the help of the resin infusion equipment composed of a vacuum bag mounted on an injection point. Two mold plates are required to make accurate measurements; otherwise, a correction factor is used in the equipment [616]. The tests were performed under a constant 20 in-Hg vacuum. The experimental setup is shown in Fig. 22 [617]. A vacuum bag was deposited over the plate with an inlet port located in the mid of the reinforcement [617]. The injection time was noted and images were captured at different time intervals for permeability calculations. The equivalent permeability (Ke) was calculated using the Eq. (9) [608].

μ

(9)

where Rx,e, ΔP, t, φ, μ and Rx0,e are the radius of the flow in the x direction, pressure gradient, elapsed time, porosity of the reinforcing material, viscosity of the resin and the equivalent inlet radius respectively. By plotting the dimensionless factor F with respect to time, the slope of F versus t can be measured and the equivalent permeability Ke directly obtained from Eq. (9). From Ke, the permeabilities in the x and y directions (Kxx and Kyy) can be calculated. Fig. 22 Permeability test setup [617] The permeability values obtained from the experiments were compared with the manual calculation. To avoid the edge effect, the first location in both directions was taken away from the edge of the insert. Also, considering the slower impregnation of the reinforcements, the tests duration was kept below half an hour to minimize the influence of resin flow. The minimum curing time of the resin being used is one hour, keeping the testing within half an hour was considered to avoid change in the resin viscosity [617]. 4.1.9 Acoustic emission (AE) AE is an efficient technique which represents the generation of ultrasonic waves due to damage propagation within the material under loading [618, 619]. The sound absorption coefficient is examined as per ISO 10534-2 and ASTM E1050-98 standards by using two microphone transfer function method [490]. The damage development occurring in materials have been analyzed, by using the data acquisition system, a sampling rate of 5 MHz and a 40 dB pre-amplification was used to record AE data. The acoustic signals recorded during tests were analyzed based on amplitude and signal duration. The percentage of the number of hits versus amplitude and versus signal duration for samples is shown in Fig. 23. From the figure, it is observed that all specimens have a maximum percentage of the number of rounds centered around 40-45 dB [524].

Fig. 23 AE signal distribution of FFRCs based on (a) amplitude and (b) duration [524] Flax fibres are mixed with the matrix at different fibre loadings ranging from 0-50%. The feasibility of using the ultrasonic longitudinal sound wave in the definition of the fibre content and distribution in PFRCs has been studied [368]. Rectangular plates of 80 x 220 x 4 mm size are injection molded. The effect of fibre content is correlated with the sound speed of the longitudinal wave as well as the wave attenuation. The speed is calculated using the simple time of flight method. The attenuation coefficient of the material is characterized by the aid of the scan. The longitudinal wave speed is found to increase by 1% every 7% increase in fibre weight. The speed is relatively higher than the measured speeds for the same samples using the contact ultrasonic method. The attenuation coefficient is found to increase in a parabolic trend with the fibre content. 4.1.10 Rheological experiments Rheological tests have been conducted by using a rheometer (Model: Gemini 2000, Geometry: Parallel plate, Make: Bohlin Instruments) at the temperature range 190-200°C. The diameter of the parallel plates maintained was 20 mm, and the gap between the plates was set as 1.7 mm. The viscosity was determined using the shear rate gradient from 0.01-100 s-1 and the zero viscosity value was evaluated by using the Carreau-Yasuda model [620]. 4.1.11 Rockwell hardness test Rockwell hardness test was performed on square shaped specimens of surface area equal to 645 mm2, based on the recommended dimensions in the ASTM standards. This test was carried out at room temperature using a Rockwell hardness tester which was loaded with a steel sphere at a load of 100 kg. The samples were visually tested for parallel flat surfaces to eliminate deflection caused by poor contact and the anvil of the hardness testing machine. A large steel

ball was chosen to distribute the load more evenly, since fibres may influence the penetration of the indenter and could result in variations in data. The total time taken from the initial indentation, to load dwell time, and finally to the recording of the measurement was 10 s, which was controlled entirely by the hardness tester. For each sample, 15 readings were made directly from the testing device, and the mean value of these readings was reported as the hardness of each specimen [538]. 4.1.12 Wear test Wear testing was done as per ASTM D 3702 standards on DUCOM machine (Model: TR-20LE) designed as per ISO-9001. The surfaces of the friction plate were cleaned by using emery paper at a speed rate of 4 m/s. Initially, the weight of the samples was measured and then fixed in the wear-testing machine. First the machine was set to zero wear, and then the speed was varied by giving definite trends at 4 m/s. Further testing was done at 4 m/s with varying loads of 0.2, 0.5, 1, 1.5 and 2.0 kg. The machine was allowed to run for 15 min for each load and the readings were noted. The sample was taken out after 15 min, and the final weight was noted hence the actual weight loss due to wear was recorded [529]. 4.1.13 Fourier transform infrared (FTIR) spectra analysis An FTIR spectrum of FFRCs was obtained by using the spectrometers such as spectrum 100 FTIR [531] and Nicolet FTIR [356, 535, 621]. The spectrum is in the range from 4000 to 600 cm-1 using 8-10 mg of composite powder. The composite powder was prepared manually in pestle and mortar [531]. The absorption spectra were recorded by using the microscope. This allows the evanescent wave to probe depths ranging from 1-3 µm and depending on the wavelength [621]. FTIR spectra were obtained for untreated, NaOH, acetic anhydride, and acrylic acid treated samples. To perform FTIR testing, a mass of approximately 8 g was taken,

pressed at different fibre content for 70 min and also from one non-pressed pre-preg. All were grinded and screened through a 125 μm sieve, then compressed with spectrometer grade KBr into micro-disks, using 3 mg of product for 160 mg of KBr [39]. 4.1.14 Fracture toughness-compact tension Compact tension specimens were machined from the laminates in accordance with the dimensions given in BS 7448. The thickness of the laminates was approximately 3.8 mm, and the width was set at 60 mm. The initial portion of the notch was machined with a diamond saw, and a starter crack was then introduced at the root of the notch [379]. The ratio of crack length to width was maintained between 0.45-0.55. The specimens were machined such that the notch ran either be parallel or perpendicular to the warp yarns of the surface plies. The specimens were loaded to failure in a UTM. The tests were performed under elongation control at a constant cross-head velocity of 0.4 mm/min. Crack growth in the samples was followed by the digital camera, with images being captured at selected points along the load line-displacement curve [497]. 4.2 Thermal properties The thermo-physical behaviors are very important for several applications like heat exchangers and heat dissipation based materials in electronic products. From the published results, it is observed that the PFRCs have drawbacks in heat dissipation characteristics. To estimate the moisture content and other volatile components present in the materials, the thermal analysis has been performed. This is very important, because moisture and volatile components have a deteriorating effect on the characteristics of composites [622]. To study the heat transfer ability of materials the thermal conductivity, diffusivity and specific heat are the important properties [362]. The ability of the material to transport heat is called thermal conductivity, and

the thermal diffusivity is a measure of its ability to adjust the temperature with the surroundings [623, 624]. 4.2.1 Thermo-gravimetric analysis (TGA) TGA of FFRCs was carried out on a SDT Q600 apparatus [415], thermo-analyzer TA4000 [625] and TGA Q50 series apparatus [507]. The test sample was placed in the sample holder and subjected to temperature scans from room temperature to 600°C at a rate of 5-10°C min-1 and air consumption of 200 ml/min. Curves of weight loss were computed using software [415, 625]. TGA measurements were carried out on 10-15 mg sample placed in a metallic pan, heated from 20-700°C at a rate of 20°C/min in a N2 atmosphere with a flow rate of 60 ml/min to avoid unwanted oxidation [507]. TGA of approximately 5 mg dried films was conducted at a rate of 10°C min-1 between room temperature and 700°C in N2 atmosphere on a TG-IR interface. Wide angle XRD patterns were obtained using an Anton Paar SAXS with line collimation equipped with Cu-Kα radiation at a wavelength of 0.1542 nm, accelerating voltage of 45 kV, and the electric flow of 40 mA. The scanning range was from 2° to 79° with a rate of 0.01°Cs -1 [531]. From the position of a peak, the corresponding spacing was computed from the Bragg’s diffraction Eq. 2d sin θ = nλ

(10)

where, n=order of reflection, λ=wavelength of radiation, θ=angle of reflection and d=interlamellar spacing. 4.2.2 Dynamic mechanical analysis (DMA) The method which is used for estimating the mechanical properties of composites based on its visco-elasticity is called DMA. It measures the deformation of a material by a sinusoidal or other periodic stress. The DMA depend on the quantity of fibre, fibre orientation, the addition of

additives such as fillers, compatibilizer, etc. DMA is essential for establishing a wide range of temperature dependent material data [494]. The analyzer of models, SEIKO DMS 6100 [507], Perkin-Elmer DMA 8000 [494], Tritec 2000 DMA [415], and Perkin-Elmer DMA7 [290] were used to determine the visco-elastic behavior of FFRCs in bending mode over a temperature range of 0-300°C according to ASTM D5023 standards. The composites were cut into samples having dimensions of 40 x 10 x 2.6 mm. The experiment was conducted at different frequencies. The storage modulus (E’), loss modulus (E’’) and damping factor (tan δ) of the specimen were measured [494, 507]. For all the thermal analyses tests, the samples were cut to the required sizes, freeze-dried and then stored in desiccators under N2. The measurements consisted of temperature ramps from 25-300°C using a heating rate of 3°C min-1. These were performed under a test frequency of 1 Hz and within the linear visco-elastic range of the samples as first determined from strain ramps [415]. The range of constant static and dynamic stress used was 2800 and 2000 kPa, at a frequency of 10 Hz for all samples. The scans were taken at a rate of 2°C/min from -50-100°C. An average 2-3 scans replicates were presented, and the glass transition temperature (Tg) was obtained from the E’’ curve [290]. 4.2.3 Dynamic mechanical thermal analysis (DMTA) DMTA was carried out on a dynamic mechanical analyzer (Model: DMA8000, Make: Perkin Elmer) with dual cantilever at a frequency rate of 1 Hz. The films tested were 50 x 10 mm in dimensions, and the test temperature ranged from 25-150°C, with a rate of 2°C per min. The thickness of the specimens was 0.50 mm. The α-relaxation temperature was determined as the peak value of the loss angle tangent [531]. 4.2.4 Thermal conductivity

Thermal conductivity experiments were conducted on the specimen of size 10 x 10 x 10 cm. The samples were dried in an air circulating oven at 50°C, and weight of the samples were measured at 24 h time intervals until the loss in weight not to exceed 1%. In order to maintain the proper contact between the transient plane source (TPS) sensor and the specimen, the surfaces of the samples were polished [490]. The size of the sample must satisfy the condition of an infinite medium, which indicates how far the heating pulse has propagated into the sample during the transient time, is less than the distance from the heater to the nearest boundary of the sample [626]. A TPS element was kept in between the samples. A chucking device was used to ensure good thermal contact between the TPS sensor and the sample. In the TPS technique, the source of heat is a hot disc made out of a bifilar spiral, which also serves as a sensor of the temperature increase in the samples. 4.2.5 Thermal diffusivity Thermal diffusivity is one of the important thermal properties of heat transfer [627]. The thermal diffusivity of PFRCs at the processing temperature is higher than the melting temperature. The thermal diffusivity of composites is determined in two ways viz., the direct measurement and estimation by using the ratio of thermal conductivity to volumetric specific heat. This experiment was conducted at room temperature or lower than that of the melting temperature of the material [628-630]. The thermal diffusivity (α) is a function of its thermal conductivity (k), specific heat capacity (Cp), and density (ρ) which can be evaluated by using Eq. (11) [352]. The standard deviation of thermal diffusivity (SDα) was estimated by using Eq. (12) [631]. (11)

(12) where SDk=standard deviation of thermal conductivity, SDρ=standard deviation of density, and SDCρ=standard deviation of specific heat. The obtained values showed that fibre quantity has a significant influence on this property. It decreases with fibre content, which states that the material containing flax fibre will require a longer time to be heated or cooled than the matrix. The thermal diffusivity decreases very slowly with the temperature, due to the fact that the density of the composites was assumed constant within the temperature range [352]. 4.2.6 Differential scanning calorimetry (DSC) studies DSC experiments have been carried out by using calorimeters of types Perkin Elmer Pyris 1 [15, 290, 362, 384, 531], and TA-DSC2010 [57]. The metallic plates with holes have been used and the mass of the specimen is 10 mg. In order to remove the thermal history, the samples were heated up to 230°C for 5 min. The accuracy of temperatures measured from a peak extreme is about ±0.5°C. The non-isothermal crystallization and melting temperatures are determined from the crystallization peak at the heating or cooling rate of ±20°C/min. The melting temperatures were obtained from the peaks maxima measured at the same heating rate. The melting enthalpies were evaluated by using constant integration limits [15, 384]. The melting and crystallization characteristics of FFRCs were observed in a N2 atmosphere at a heating rate of 10°C/min. The crystallization temperature (Tc), melting temperature (Tf), heat of fusion (ΔHf), and percentage of crystallinity ( χc) were obtained from DSC studies [362]. The crystallinity was calculated using Eq. (13). (13)

Where

taken from crystalline iso-static resin and w=matrix weight fraction. The

sample was heated upto 150°C to delete existing thermal history, then quenched to -100°C, and then heated at 20°C min-1 from -100 to 150°C [57]. 4.2.7 Fire reaction properties The testing method for the fire performance is the cone calorimeter method which is conducted in accordance to IS0 5660 standards [632, 633]. This is the most advanced apparatus among all bench-scale reaction-to-fire test instruments developed during the last few years. The main property, which is determined during tests, is heat release rate (HRR)-the basic factor for fire modeling. The HRR is determined by measuring O2 consumption derived from O2 concentration and flow rate in the combustion product stream [632, 634]. This test method is based on the heat of combustion that is proportional to the amount of O2 required, which releases 13.1 MJ energy per kg of O2 consumed, irrespectively of the type of material tested [632, 634]. It is of importance that materials can be tested in two positions-horizontal or vertical in a wide range of heat flux intensities from 5-100 kW/m2 [508, 633]. Composites incorporating bioderived constituents pose increased fire risks when used in engineering applications threatened by fire [635]. Kandola and Kandare [636] reviewed wide-ranging fire retardation strategies shown to significantly improve both the fire reaction and fire resistance of synthetic fibre composites. Various fire protection strategies may be adapted to improve the fire performance and fire structural survivability of composites by incorporating highly-flammable plant-derived constituents. Several authors successfully demonstrated improvements in fire reaction properties of polymer composites incorporating fire retardant additives [636-648]. However, in most cases, fire safety ratings are achieved at elevated fire retardant loading concentrations. Inadvertently,

high fire retardant loadings lead to the degradation in ambient-temperature mechanical properties of fire-retarded fibre composites [649]. Fire retardancy properties of FFRCs were evaluated by Kandere et al. [508]. The HRR measured for fire-protected composites were significantly lower than those measured for its unprotected counterparts. Through thickness temperature profiles across the composites revealed the effectiveness of the fire retardant veil in minimizing thermal damage of underlying substrate composites. Test specimens of size 90 x 90 mm and nominal thickness were underwent to radiant heating from a calorimeter operating at the same heat flux rate. The heat exposed surfaces of the test specimens were positioned 25 mm away from the cone heater. HRR profiles of non-insulated and fire-protected FFRCs were measured, and the HRR-time data for laminates with and without fire protection is shown in Fig. 24 (a) [508]. Average heat rate emission data for composites with and without fire protection is presented in Fig. 24 (b) [508]. Fig. 24 (a) HRR and (b) heat rate emission of FFRCs [508] 5. Morphological properties 5.1 Scanning electron microscopy (SEM) and image analysis To determine the degree of interfacial bonding between the fibres and the matrix, the surface morphology of fractured composites was examined [201, 535]. The SEM analysis of the fracture surfaces is very important to observe the surface texture of fibres, the way of crack initiation and the failure mode of the composites [494]. The samples subjected to SEM evaluation were coated with bal-tec/chromium/gold/graphite/platinum/palladium to avoid electro-static charging as well as clear visibility of the fractured surfaces [53, 102, 107, 201, 290, 370, 414, 490, 494, 497, 507, 515, 530, 531, 534, 537, 538]. The micrographs were taken by using different instruments such as Philips FEG XL 30 [290, 490, 494], LEO 1530 FEG [102],

Jeol JSM-6031F [15], Jeol JSM-5510 [537], Hitachi S-520 [497], Supra FE-SEM Zeiss [502], JSM 6460LV [414], CAM SCAN 4 DV [370], Philips XL40 [513], CS 24 [534], Hitachi-S3400N [507, 530], Jeol JSM 35 CF [535], FEI Quanta-200 [376, 388, 531], Jeol JSM-5800LV [201], Jeol JSM-6380 [538], Jeol JSM-6700F [515], LEO 1530 FEG [107] by various researchers. The samples for SEM characterization were prepared by freezing them in liquid N2 before fracturing [290, 531]. To observe the surface morphology of the composites under nonisothermal conditions the optical microscopy with cross polarizers was used [290]. The fractured surfaces of untreated, zein treated FFRCs are presented in Fig. 25 (a-c). The cavities present in the composites are clearly visible in Fig. 25 (a). This indicated that the bonding between the fibres and the matrix is poor, and when it is subjected to load, the fibres to be pulled out from the matrix easily. The presence of many short and the broken fibres that came out from the matrix are clearly visible in Fig. 25 (b). This showed that the adhesion between the fibres and the matrix is significantly improved, and when a load is applied, the breakages of fibres takes place but they do not fully come out from the matrix. The presence of resin coated fibre ends indicating cohesion and bonding between the fibres and matrix is shown in Fig. 25 (c) [494]. In plasma treated FFRCs, the bonding between the fibres and matrix is comparatively stronger which is shown in Fig. 25 (d). This is the fact that plasma treatment should improve the adhesion, but weaken the flax fibres [102]. The SEM images of maleic anhydride and silane treated FFRCs are shown in Fig. 25 (e and f). The clean fibre surfaces visible in the images are indicating poor wettability of fibres and weak bonding. The NaOH and MAPP treated FFRCs are presented in Fig. 25 (g and h) [537].

Fig. 25 SEM images of fractured surfaces of FFRCs (a) untreated, (b) 2% zein treated, (c) 4% zein treated, (d) plasma treated, (e) MA treated, (f) silane treated, (g) MAPP treated and (h) NaOH treated SEM analysis of raw and modified flax fibres indicated that the matrix-fibre interface was better defined with treated flax, as shown in Fig. 26 [650]. The treatment included soxhlet extraction of the waxes and mercerization. The washed fibre, cross linked-grafted composites had higher storage moduli than the un-washed one, which indicated good bonding between the washed flax fibres and the resin. It was found that the composites which contained washed flax fibres showed matrix crystallization at higher temperatures than the composites containing unwashed flax fibres. Un-washed FFRCs had a higher water absorption rate compared with the other composites. Polyacrylate modified flax composites absorbed less water than both washed and un-washed flax composites without polyacrylate. Fig. 26 SEM images at fibre-matrix interfaces of (a) unwashed flax, (b) washed flax, (c) unwashed flax and (d) washed flax [650] The presence of the binding material on the surface of flax fibres is shown in Fig. 27 (a). This showed that the binder does not get into honey-comb structure, so it does not take the place of air, and the FFRC should present thermal, and noise absorption are shown in Fig. 27 (b). Fig. 27 (c) shows evidence for the shear failure of the fibre in the composite along the micro-fibril direction [383]. The de-bonding of fibre between the primary and secondary cell walls is clearly observed in Fig 27 (d) [383]. Fig. 27 (e) shows the composite reinforced with un-treated flax fibres which is characterized by poor adhesion between the fibres and the matrix and that produce a relatively clean surface over the pull-out fibres due to a greater extent of de-bonding

[651]. The un-treated FFRC is presented in Fig. 27 (f) [537]. Fibre pullout from the matrix is clearly shown in this image. Fig. 27 SEM images of FFRCs (a) based on flax-shaves, (b) flax-shaves structure, (c) after tensile failure, (d) fracture surface, (e) tensile surface fracture and (f) fibre embedded in matrix SEM micrographs shown in Fig. 28 (a-b), revealed similar morphology of fracture surfaces for both matrices suggesting a comparable level of adhesion [652]. SEM characterization of FFRCs shows the brittle fracture at a macroscopic level with a significant presence of pull-out with weak interface and presence of fibrillation shown in Fig. 28 (c and d) [513]. For hybrid composites including plant fibres, flax/hemp/basalt hybrid composite laminates show brittle behavior and tensile damage at hemp/flax fibres layers interface and the presence of extensive pullout visible in Fig. 28 (e and f) [513]. The broken surface of an enzyme treated FFRC is presented in Fig. 28 (g). From the image it is seen that the fibres appear to be present mainly as ultimates, well-separated fibres are clearly visible. Fig. 28 (h) shows a typical failure surface of an EDTA treated FFRCs. It appears to be a very strong interfacial relationship between the EDTA modified fibres and the matrix is evidenced by the lack of fibre pull out when compared with the composites reinforced with the enzyme treated fibres [346]. Fig. 28 SEM images of the fractured surfaces of FFRCs Fig. 29 (a) shows the method of fracture of FFRCs [203]. The effect of the compatibilizer is shown in Fig. 29 (b and c). The fibres pullout from its sockets is clearly visible, whereas the fibres with compatibilizer are fully coated and embedded in the matrix. The compatibilizer well adheres to the primary cell is shown in Fig. 29 (b). It is observed that the bonding between the fibre and matrix is good, and there is no change in the fibre surface before the composite

fabrication. The fibres are exposed during the fabrication process and found that the influence of fibre surface modification before fabrication should be minimal. The crack has not passed through the interphase between fibre and matrix, or transverse to the direction of fibre orientation is shown in Fig. 29 (d and e). The secondary cell wall of fibre came out, whereas the primary cell wall still adhered with the matrix. The arrow points at a fibril have separated from the small fibril bundle [110]. The fractured surface of FFRC is presented in Fig. 29 (f) [220]. Fig. 29 SEM images of the fractured surfaces of FFRCs (a, f) untreated and (b-e) MAPP treated The flax fibres processed by transcrystallization process under isothermal conditions are shown in Fig. 30. The spherulites were formed and the formation followed by the transcrystalline process is shown in Fig. 30 (a). A transcrystalline layer formed completely after 7 min, can be seen from Fig. 30 (b), and is quite uniform [101]. No transcrystalline layer can be seen around the fibres, even though the bulk spherulites are well grown as in Fig. 30 (c and d). The new spherulites started to grow around the fibre slowly, after 5 min of isothermal crystallization [101]. The transcrystallization process for flax fibre at 145°C is shown in Fig. 30 (e). From the image, it can be seen that the transcrystallization layer formed is somewhat big in size and uniform. To know the maximum possible temperature for transcrystallization process, an isothermal crystallization was performed at 155°C. It is found that, the thick layer has been formed when the crystallization was very slow and the maximum thickness reached around 540 mm after 42 h, and it was visible even with the naked eye. In Fig. 30 (f) the transcrystallization layer after 42 h of crystallization at 155°C is shown. It can be observed very easily a well defined transcrystallization layer, and the boundaries between the trans-crystals are also visible [101].

Fig. 30 SEM images of isothermal crystallization of flax at (a) 137°C (2 min), (b) 137°C (7 min), (c, d) 137°C (4 min), (e) 145°C (120 min) and (f) 155°C (42 h) [101] 5.2 Optical microscopy analysis Optical microscopy showed that the agglomerated untreated fibres and voids in the composite, for the untreated flax and hot water washed flax in Fig. 31 [356]. There are lots of cuticles and pectin to bond fibres together. These contaminations may also have contributed to the unclear bands of the spectrum of the untreated flax. Hot water washing only can remove partial of substances on the surface of flax, so the agglomeration of flax still existed. Acrylic acid treated fibres before being processed into composite laminates appeared to be the least entangled, which can also be seen in the micrograph of the acetic anhydride treated fibres. The diameter of the fibre bundle of acetic anhydride treatment and acrylic acid treatment showed smaller than that of hot water washed flax fibre. It indicates the acetic anhydride treatment and the acrylic acid treatment remove more non-cellulosic chemical and separate the bundles. Separating fibres into fibrils exposed larger contact area to the matrix and hence the composite properties are increased by increase an interfacial strength between fibre and matrix. From the micrographs, different surface treatments have noticeable differences. The acetic acid treated fibres appear to be more elongated and deformed than the other fibres. While improving the interfacial strength, the treatment can reduce the load bearing capability of the fibre, if the fibre was damaged or modified by the treatment [356]. Fig. 31 Optical images of FFRCs (a) untreated, (b) hot water treated, (c) acetic anhydride treated and (d) acrylic acid treated [356] 5.3 Transmission electron microscopy (TEM) analysis

The flax fibre thickness of the middle lamella was estimated in between 200-800 nm from TEM images obtained on fibre cross-sections which is presented in Fig. 32 [48]. The sample folds are represented in the dark areas on the images. The triple points which correspond to the contact of more than two fibres were not taken into account to reach this estimation. The area of contact between two fibres were used to estimate the inter-phase thickness, followed by a mean thickness of 500 nm was used to calculate the shear modulus [48]. Fig. 32 TEM images of cross-sections of flax fibres at different magnifications [48] 5.4 Polarization microscopy analysis To observe the structure of the fibre and to correlate the results with other measurements polarization microscopy is essential. The results of flax composite observations for different cooling rates are shown in Fig. 33 [414]. The air cooled composites exhibit amorphous structure, with rare spherulites as shown in Fig. 33 (a). The intermediate cooling results at 10°C/min, few more spherulites with larger size is visible in Fig. 33 (b). The image presented in Fig. 33 (c) shows the slow cooled samples with a cooling rate of 1°C/min have a crystalline structure with large spherulites [414]. Besides matrix modification, the cooling rate also affects the morphology of fibre surface at the interface. It is noticed that the growing transcrystallization region around the flax fibre, has a larger number of spherulites with slow cooling is seen from Fig. 33 (b and c). The formation pattern and its influence on properties depend on the fibre/matrix interface [101, 414, 653, 654]. Both thermal stress and mechanical anchorage in the fibre/matrix interface has been increased by the anisotropy of the transcrystalline zone [655-658]. The rough, porous surfaces of fibres have influence on mechanical interlocking [659]. Fig. 33 Polarization micrographs of FFRCs (a) air cooling, (b) 10°C/min and (c) 10.1°C/min [414]

5.5 Confocal microscopy analysis The cross-sections of fibres in the composites, manufactured from the three different diameter flax fibres were captured by confocal microscopy and presented in Fig. 34 [515]. The images of the samples reinforced with larger diameter flax fibres are presented in Fig. 34 (a and b). From the figure, it is observed that the fibre cell walls appear as circular in shape, and gathered in bundles of many cells. The images of medium diameter FFRCs are depicted in Fig 34 (c and d) and smaller diameter fibres are shown in Fig. 34 (e and f). The voids are appearing in black and the micro-cracks within the matrix were extremely rare in these images. From the images, it is further observed that the resin penetrated into the fibres, and lumens of few fibre cells. The images with higher magnification showed that there is an excellent adhesion at the fibres/matrix interface [515]. Fig. 34 Confocal microscopy images of FFRCs [515] 6.Environmental impact assessment of flax fibres Nowadays, the usage of materials protecting the environment is growing rapidly [490]. The plants lose a high amount of leaves that are transported and accumulated as landfill. This accumulation requires a specific waste disposal with negative consequences to the environment. Therefore, the valorization of plant fibre as reinforcement in composites would serve both for mechanical and waste recycling purposes [660]. The need to achieve environmental sustainability in engineering has spurred the development and creation of new engineering materials incorporating bio-derived reinforcements [145, 661-665]. With increasing environmental awareness, the application of flax fibres, is growing rapidly due to market demands for green products. This calls for the investigation of the environmental performance of FFRCs. To this end, life cycle assessment (LCA) can be applied as a standardized method to

quantify environmental impacts [666]. This analysis is used for determining the overall environmental effects related to products/processes, which determines the mass and energy flows during the processing of material. The assessment is done according to ISO 14040-14043 standards [667]. From the above literature, to reduce the environmental impacts on the production of FFRCs, several agricultural operations can be considered by using the proper method in ground preparation, using organic fertilizer and biological methods to control pets. In addition, considering the production of aligned fibre reinforcement without the need for energy-intensive spinning operation is a key point to reduce energy consumption. The environmental friendly characteristics of these composites are still questionable because of their processing requirements, which consume more energy. Hence, detailed LCA is required in fabricating of composites with excellent properties. Diener and Siehler [131] used LCA to deal with underfloor panels made from glass and flax fibre mats. A study showed that the flax reinforced panel scores better for all environmental conditions. The main impact categories, such as global warming, acidification, eutrophication, ozone precursors, toxicity air, toxicity water, nonrenewable energy, the environmental impact is reduced close to 20%, in the remaining cases (i.e., ozone depletion, waste, and resources) the reduction of impacts is higher. These reductions of environmental impacts reflect the fact that the manufacture of flax fibre mats required 80% less energy than glass fibre mats, the total energy savings for the entire component are smaller, since the overall environmental impact is dominated by resin input. Van der Werf and Turunen [668] quantified major environmental impacts associated with the production of flax and hemp fibres by using LCA. It was found that the impacts of the hemp and the flax were similar, except for the pesticide use and water use during processing. Later,

Garcia et al. [669] studied a methodology to quantify the potential environmental impact associated with the production of flax and hemp fibres. In addition, two flow indicators were considered: energy and pesticide use. System boundaries were covered from soil management up to straw processing and transportation of fibre bales to mill. Production of all inputs for each system (fertilizers, pesticides, seeds, energy carriers) and their supply was also included, as well as machinery production, use and maintenance. It was found that the production of hemp fibre reported higher values for all the impact categories analyzed. On the contrary, flow indicators were more intensive in the flax scenario due to irrigation and pesticide consumption, as that observed by Van der Werf and Turunen [668]. Deng et al. [670] revealed that the overall weighted environmental scores of printed circuit boards (PCB) from FFRCs are significantly lower than the conventional composites, especially in impact categories of climate change (60%), human toxicity (40%), fossil resources depletion (55%), photo-chemical oxidant formation (45%) and fresh water eutrophication (58%), indicating bio-based materials as PCB substrate offer promising perspectives for final replacement of the conventional materials. It is noted that flax fibres consume little energy during its production compared to the same quantity of glass fibre [131]. Flax fibres also have the potential to store CO2 during its growth resulting in emission reductions [671]. Duigou et al. [672] evaluated the environmental impact of flax-based composites production using simplified LCA following the ISO 14044 standards. The study indicated that the FFRCs are very attractive regarding environmental impacts. Further improvements in mechanical strength are necessary if they are to be used in transport application compared to glass fibre reinforced composites. 7 Industrial applications of FFRCs

The composites from renewable resources are helped to obsolete non-renewable waste, reduce raw material usage, and lessen green-house gas emissions [673]. Independently of weather conditions and yields, however, the industrial application of plant fibres requires making high quality fibres continuously available in large quantities [178]. The development of PFRCs for commercial applications should enable the presence of reusable, recyclable and eco-friendly materials while achieving equivalent or better properties as those of synthetic fibre composites [290]. Further advantage is that the lightweight of PFRC can reduce the total weight of automobile or aircraft and thus reducing fuel consumption during utilization, which is a great contribution to the environment [103, 674]. Over the past several years, an attempt has been made to prepare materials for automotive, construction, electronics, and other products by using plant-based fibres as reinforcements [675]. When compared to the synthetic fibres, plant fibres possess excellent sound absorbing properties, more resistive and have better energy utilization. In automotive parts, composite materials not only reduce the mass of the component but also lower the energy needed for production [146]. The PFRCs can be designed for door panels, head rests, parcel shelves, roof upholstery to reduce the environmental impact, structural weight, and manufacturing costs. Using these materials energy efficiency can be increased and construction waste would be reduced providing a solution to infrastructure needs and promoting sustainability concepts [676]. Flax is one of the important plant fibres used for the structural applications because of its higher strength and stiffness compared to other fibres [110, 502]. The use of flax for cloth production dates back at least to ancient Egyptian times. Flax fibres are used as reinforcing materials for manufacturing of several interior parts of an automobile [113]. The models of a roof structure and beams have also been processed by using flax fibres [677, 678]. The PFRCs

are appropriate materials in the automobile and construction industries [121, 679]. PFRCs are used for light weight applications, packing goods, medical engineering, household and general appliances [231, 255, 680-684]. In fact, they are already used for many components, as it has been demonstrated by various automotive manufacturers, e.g., Daimler Chrysler, BMW and others [541]. By commercial application, over 95% of PFRCs are being used for non-structural automotive interior components [171, 685-688]. The composites made from flax fibres in the forms of panels, tubes, and sandwich plates have been used to replace the wooden fittings, fixtures, furniture, and noise insulating panels [43]. Several automobile components such as the door panels, window shelves, roof liner, instrument panels, glove box, arm rests, seat backs, spare wheel covers, noise absorber panels and package trays are now being manufactured using PFRCs [16, 103, 111, 136, 672]. Other than automotive applications, these composites are being considered for applications in (i) construction and infrastructure (such as beams, roof panels, bridges) [171, 686, 689-694], (ii) sports and leisure goods manufacturing (for boat hulls, canoes, bicycle frames, tennis rackets) [182, 183, 185, 188, 193, 194], (iii) making of furniture and consumer goods (such as packaging, cases, chairs, tables, helmets, ironing boards) [171, 685, 687, 688, 690-694], and (iv) fabrication of pipes and tanks (for water drainage and transportation) [44, 171, 687, 689, 694, 695]. Flax fibres, along with hemp fibres, were used in composites for a Henry Ford car in 1941, claiming that they show impact strength ten times greater than steel. Flax is used in composites to produce lightweight parts of cars [44]. The automotive panels for Mercedes and rear parcel shelf panels in the 2000 Chevrolet Impala are manufactured by using FFRCs [45]. Flax is used for making cords for hafting stone tools, weaving baskets, or sewing garments around Dzudzuana Cave up to 30,000 years ago [696]. Flax was used in Neolithic cultures as a

source of both fibre and possibly oil, as evidenced by the discovery of linen textiles in prehistoric sites including the 9,000 year old Nahal Hemar Cave in the Judean desert, Israel, and by the presence of flax seeds among occupational debris at Tell Ramad (ca. 7,000 BCE) near Damascus, Syria [697, 698]. The flax tow is used for composites processes like filament winding, braiding or pultrusion, allowing the possibility to achieve prismatic hollow beams used for structural applications [536, 699]. The feasibility of FFRC as concrete confinement has been investigated [10]. From the investigation, it is observed that the flax has increased the strength and structural ductility significantly, and increased compressive strength up to 54 MPa, compared with the conventional concrete of 25 MPa. The pre-fabricated FFRC is used as a lightweight member for concrete to reduce the construction time and protects the encased concrete from a potentially harsh environment, e.g., de-icing salts and other chemicals. Further studies of FFRCs as different structural elements are in progress, i.e., flax composite tube encased concrete as bridge pier [9, 11] and flax mat/balsa bio-sandwich in transport application [665, 672]. Flax based linseed oil is used as industrial oil in the production of paint, varnish, ink, linoleum, and as a concrete sealant [700]. 8 Future prospects The development of renewable and bio-degradable materials turns attention towards sustainable growth for a positive environment. The fibres derived from the plants, used for several years, make an abundant source of natural resources and have become increasingly popular among researchers and users that are highly health conscious and environment-friendly [701, 702]. The recycling of composites is on the right track, but challenges still have to be taken-up in order to finally make it for commercial applications [703]. The recent advances in

flax fibre technology could complement the conventional breeding methods, towards the establishment of new, perspective varieties with specific design in less time and labor consuming manner. The fact, that in general flax is a source for industrial and non-food products facilitates the approval and public acceptance of genetically manipulated flax cultivars for commercial use. This review revealed that there is more research needed not only on the planting, maturing, harvesting, drying and retting of flax fibres, but also on equipment used for this purposes. The large-scale harvesting and handling of flax is lacking due to the lack of reliable equipment, it is clear that we can reduce the harvest and handling costs considerably which increases total income. A critical issue is that the characteristics of FFRCs depend on the quality of the fibre and the adhesion between the fibre and the matrix. Surface modifications of the fibre/matrix and use of additives can improve the properties of composites. The life cycle of these composites should be tailored to meet specific requirements. Future work on LCA of flax fibre and its composites should focus on the bio-degradable polymers. However, the high cost of some bio-degradable polymers is a major drawback when considered for industrial applications. The main problem which needs to be overcome for commercial utilization of FFRCs is the durability. Durability relates to resistance to deterioration resulting from external as well as internal causes. The lack of data related to the durability of FFRCs is one major challenge that need to be discussed before suggesting this material for industrial applications. Future work on flax composites should be focused on understanding the environmental assessment, further improving the mechanical properties and moisture resistance. From the literature, it is found that the proper selection of fibre surface treatment can improve the properties of composites; therefore much attention is needed in surface modification of flax fibres. Additionally, novel fibre extraction processes and

composite manufacturing methods should be further developed. The moisture removal will also affect the properties of the fibres and hence the interfacial adhesion between the fibre and matrix should be improved. This needs more attention and requires more elaborative research in the future. Further investigation might also be conducted on morphological studies at the nano-scale in order to assess the surface roughness required for improving the properties. The potential measurements on polarity changes of flax fibres also require further investigation. 9 Summary and conclusion Research on FFRCs is creating hype among researchers due to its acceptable strengths and environmentally friendly nature. An elaborated review on planting, harvesting, extraction methods, fibre surface treatment process, composite fabrication process, properties, LCA and applications of FFRCs is presented from the published results. This main aim of this literature is to motivate the researchers to look at the flax fibre as a reinforcing material in polymer matrix composites and to replace the harmful synthetic fibres. As a summary of this review, the major findings could be listed below:  The cultivating soil trafficability had a certain influence, especially on the time between baling and storage. It is expected that the soil trafficability limit, has relatively high influence on cultivation of flax fibres.  The selection of fibres is very important, to control retting and related defects, and to regulate fibre demand and supply cycles.  The transport distances of fibres are as short as possible for easy operation, handling, storage and cost reduction.  To fully utilize the properties of flax fibres, it is important to understand its characteristics before reinforced into matrix.

 The chemical surface modification of the fibre is to enhance the properties of the material. The main motive of this process is to improve the interfacial adhesion between the fibres and matrices without reduction in the fibre performances.  The chemical structure of the fibre must be controlled during chemical treatment in order to separate the fibres by dissolving the hemi-cellulose and pectin matrix.  Fibre pull-out tests will be conducted to evaluate the interfacial bonding between fibres/matrix, by controlling the processing conditions, the interfacial adhesion between the fibre/matrix can be improved.  The flax fibre is expected to be circular cross section with a constant diameter but, from the stress analysis and SEM studies, it is confirmed that the cross section is polygonal shape and the diameter is not constant.  The Young’s modulus of the flax fibre decreases with increase in the fibre diameter along the longitudinal direction of the fibre.  The hygro-thermal ageing affects the tensile strength of the FFRCs, and a significant reduction in stiffness is in the moisture absorbed composites.  Helium plasma and auto-clave treatments improve both the stiffness and moisture resistance of FFRCs.  The thermal behavior of the FFRCs depends on fibre quantity, glass transition temperature, degree of crystallinity, etc.  Rheological behavior of FFRCs decreases during the processing and this emphasizes the degradation of the matrix layer during recycling of composites.  The thermal conductivity and diffusivity of surface modified FFRCs decreases when compared to the untreated composites.

 The fibre surface morphology is greatly affected by surface roughness and it is affecting the transcrystalline layer and the ability of the fibre to induce into the matrix.  All the properties discussed in this review are encouraging the idea of replacing synthetic fibres by flax fibres.  It is concluded that the flax fibres are cost-effective materials with potential to replace synthetic fibres as a reinforcing agent in fibre reinforced composites.  From the review it is confirmed that the FFRCs have the potential to be the future generation engineering materials for several applications such as infrastructure, automotive sector and manufacturing of consumer goods. Acknowledgements The author is verymuch grateful to all the publishers (e.g. Elsevier, Springer, SAGE, Taylor & Francis, Wiley, etc.) and the authors of the publications cited in this review. The author would like to thank Dr. C. Deepa, KIT-Kalaignarkarunanidhi Institute of Technology for her support in preparing this review article. The author extends his thanks to Dr. S. Nithya, KITKalaignarkarunanidhi Institute of Technology for her support in proofreading of this article. References 1.

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Vitae

Prof. M. Ramesh awarded PhD degree in the area of biocomposite materials from Jawaharlal Nehru Technological University Anantapur, Andhra Pradesh, India. He received his Master of Engineering degree in Computer Aided Design (CAD) from University of Madras, Tamil Nadu, India. He received his Bachelor of Engineering degree in Mechanical Engineering from Bharathiyar University, Coimbatore, India. Presently, he is working as Professor & Vice-Principal at KIT-Kalaignarkarunanidhi Institute of Technology, Coimbatore, India. His research interests include bio-materials, polymer nano-composites, hybrid composites and energy efficient materials. More details can be found at (https://scholar.google.co.in/citations?user=YzVpt7AAAAAJ&hl=en).

Fig. 1 Localization of different developmental stages of flax (A-coordinated growth; B, Cintrusive growth; D, E-thickening of cell wall; and SP-snap point) [188]

Fig. 2 The structure of a flax fibre [110]

(b)

(a)

Fig. 3 Development of fibres in flax stems (a) cross-section in the middle of the stem, (b) microscopic image of flax fibre cell cross section (PCW-primary cell wall, SCW-secondary cell wall) [238]

(a)

(b)

Fig. 4 Polygonal shape of flax fibres [202]

(a)

(b)

Fig. 5 Flax (a) fibre cross-section and (b) stem cross-section

Fig. 6 Composite structure of flax [203]

(a)

(b)

Fig. 7 Tensile testing of flax fibre (a) before loading and (b) after fracture [42]

Fig. 8 Typical tensile stress vs. strain plots of flax fibres [379]

Fig. 9 ATR-FTIR spectroscopy analysis of flax fibres [202]

Fig. 10 Typical TGA thermogram of flax yarn [229]

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 11 Flax fibre surface (a) untreated, (b) alkali treated, (c) green flax surface, (d) dew retted flax surface, (e) Duralin flax surface, (f) untreated, (g) plasma treated, and (h) hexagonal shape

(a)

(b)

(c)

(d)

(e)

(f)

(h)

(g)

Fig. 12 SEM images of flax fibres (a) untreated, (b) lipid acylated, (c) protein coated, (d) fibre defects, (e) enzymic treated, (f) EDTA treated, (g) and (h) before and after enzymatic modification

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 13 SEM images of flax fibres (a) raw fibre, (b) defected raw fibre, (c) treated with hot alcoholic NaOH, (d) hot alcoholic NaOH and cold acetic anhydride treated, (e) formic acid treated, (f) untreated, (g) scoured at general condition, and (h) scoured at optimal synthesis condition

(a)

(d)

(b)

(e)

(c)

(f)

Fig. 14 Surface morphology of flax fibres (a)-(c) a single fibre, and (d)-(f) a bundle of fibres

(a)

(b)

(c)

(d)

Fig. 15 SEM images of cross-section of flax stem (a) un-retted, (b) viscozyme retted, (c) flax fibre cross-section and (d) fibre cross-section optical [209] (a)

(b)

Fig. 16 AFM pictures after nano-indentation of flax fibre cross-section [414]

Fig. 17 Stacking technique of FFRCs fabrication [509]

(a)

(b)

Fig. 18 Typical (a) load vs. strain and (b) stress vs. strain curves of FFRCs [355]

Fig. 19 Typical stress vs. deflection curves of FFRCs [355]

Fig. 20 Water damage mechanisms (a) wet fibre expansion, (b) composite after stress has been released, and (c) fibre contraction during drying [551]

Fig. 21 Water absorption behavior of FFRCs [524]

(a)

(b) Fig. 22 Permeability test setup [617]

(a)

(b)

Fig. 23 AE signal distribution of FFRCs based on (a) amplitude and (b) duration [524] (a)

(b)

Fig. 24 (a) HRR and (b) heat rate emission of FFRCs [508]

(a)

(b)

(c)

(e)

(g)

(d)

(f)

(h)

Fig. 25 SEM images of fractured surfaces of FFRCs (a) untreated, (b) 2% zein treated, (c) 4% zein treated, (d) plasma treated, (e) MA treated, (f) silane treated, (g) MAPP treated and (h) NaOH treated

(a)

(b)

(c)

(d)

Fig. 26 SEM images at fibre-matrix interfaces of (a) unwashed flax, (b) washed flax, (c) unwashed flax and (d) washed flax [650]

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 27 SEM images of FFRCs (a) based on flax-shaves, (b) flax-shaves structure, (c) after tensile failure, (d) fracture surface, (e) tensile surface fracture and (f) fibre embedded in matrix

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 28 SEM images of the fractured surfaces of FFRCs

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 29 SEM images of the fractured surfaces of FFRCs (a, f) untreated and (b-e) MAPP treated

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 30 SEM images of isothermal crystallization of flax at (a) 137°C (2 min), (b) 137°C (7 min), (c, d) 137°C (4 min), (e) 145°C (120 min) and (f) 155°C (42 h) [101]

(a)

(b)

(c)

(d)

Fig. 31 Optical images of FFRCs (a) untreated, (b) hot water treated, (c) acetic anhydride treated and (d) acrylic acid treated [356]

(a)

(b)

(c)

(e)

(d)

(f)

Fig. 32 TEM images of cross-sections of flax fibres at different magnifications [48]

(a)

(b)

(c)

Fig. 33 Polarization micrographs of FFRCs (a) air cooling, (b) 10°C/min and (c) 10.1°C/min [414]

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 34 Confocal microscopy images of FFRCs [515]

Table 1 Chemical composition of flax fibres Cellulose

Hemi-

Lignin (%)

(%)

cellulose (%)

64.1

16.7

2.0

67

11

73.8

Pectin (%)

Wax

Moisture content Ref.

(%)

(wt.%)

1.8

1.5

10.0

[252]

2.0

-

-

-

[253]

13.7

2.9

-

-

7.9

[254]

65

-

2.5

-

-

-

[255]

65-80

-

25-30

20.5-34

<5

-

[256]

71-75

18.6-20.6

2.2

2.2

1.7

10.0

[257]

Table 2 Elastic properties of main components of the flax fibre Property

Range

Cellulose EL (MPa)

120000-138000

ET (MPa)

27200

GLT (MPa)

4400

vLT

0.1

L/d

50000

Hemicelloulose EL (MPa)

8000

EL (MPa)

4000

GLT (MPa)

2000

vLT

0.2

Lignin E (MPa)

4000

G (MPa)

1500

V

0.33

Table 3 Tensile strength of flax fibres based on their position in the plant Fibre

Diameter

Young’s

Tensile strength

Ultimate

variety

(µm)

modulus (GPa)

(MPa)

strain (%)

Herms

19.0±3.5

59.1±17.5

1129±390

1.9±0.4

Middle

19.6±6.7

68.2±35.8

1454±835

2.3±0.6

Bottom

20.1±4.1

46.9±15.8

755±384

1.6±0.5

21.5±5.3

51±22

753±353

1.8±0.7

Middle

21.3±6.3

57±29

865±413

1.8±0.7

Bottom

21.3±6.3

51±26

783±347

2.0±0.9

-

19.3±5.5

63±36

1250±700

2.3±1.1

Location

Top

Top

Agatha

Table 4 Compiled properties of flax fibres reported by various researchers Diameter

Relative

Tensile

Elastic modulus Strain at failure Ref.

(µm)

density(g/cm3)

strength (MPa)

(GPa)

(%)

12-600

1.4-1.5

343-2000

27.6-103

1.2-3.3

[379]

10-60

1.52

840

100

1.8

[230]

10-60

1.52

1500

50

-

[232]

76±16

-

470±165

37±15

1.4±0.5

[398]

17.8±5.8

1.53

1339±486

58±15

3.27±0.4

[220]

-

-

621±295

51.7±18.2

1.33±0.56

[217]

-

-

600-2000

12-85

1-4

[397, 399]

-

1.4

800-1500

60-80

1.2-1.6

[400]

-

1.4-1.5

600-1100

45-100

1.5-2.4

[401, 402]

12-34

-

1100

89±35

-

[221]

12.9±3.3

-

1111±544

71.7±23.3

1.7±0.6

[221]

15.8±4.1

-

733±271

49.5±3.2

1.7±0.6

[221]

15.6±2.3

-

741±400

45.6±16.7

1.7±0.6

[221]

21.2±6.6

-

863±447

48.0±20.3

2.1±0.8

[221]

13.7±3.7

-

899±461

55.5±20.9

1.7±0.6

[382]

15.8±4.5

-

808±442

51.1±15.0

1.6±0.4

[403]

-

-

365-1060

36.8-61.9

0.94-2.13

[404]

15±0.6

1.53

1380±419

71±25

2.1±0.8

[405]

Table 5 Comparison of physical, tensile and fatigue properties

Physical

Tensile

Fatigue

Properties

Unit

Flax

Glass

Flax/glass

Fibre volume content

%

30.9

42.8

-

Density

-3

gm

1.31

1.79

0.732

Composite stiffness

GPa

23.4

36.9

0.634

Composite specific stiffness

GPa/gcm-3

17.9

20.6

0.869

Effective fibre stiffness

GPa

67.6

81.6

0.828

Composite strength

MPa

277

826

0.335

Composite specific strength

MPa/gcm-3

213

461

0.462

Effective fibre strength

MPa

883

1920

0.460

Composite failure strength

%

1.70

1.90

0.895

Single cycle strength

MPa

236

567

0.416

Fatigue strength at 106 cycles

MPa

115

204

0.564

Table 6 Reported tensile strengths of the FFRCs Fibre

Flax

Matrix

Low

Tensile

Young’s

Processing

strength (MPa)

modulus (GPa)

method

33

Winding

[502]

27-28

Winding

[477]

density 343

Ref.

polyethylene (LDPE) Falx

Polypropylene 250-321 (PP)

Flax

Epoxy

279

39

RTM

[401]

Flax

Epoxy

160

15

Hand lay-up

[539]

Flax

Epoxy

133

28

Autoclave

[464]

Flax

Epoxy

127

17

Compression

[203]

molding

Table 7 Reported ILSS of FFRCs Matrix

ILSS (MPa)

Ref.

PP

6-13

[479]

LDPE

6

[481]

High density polyethylene

18

[481]

Polyester

6-16

[202, 414]

Epoxy

16

[414]

Poly-hydroxybutyrate

5-22

[488]

Poly-lactic acid

5-22

[414, 488]