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The curauá challenge: optimizing natural fibres
The curauá challenge: optimizing natural fibres Curauá fibre is one of many natural fibres suggested as a suitable replacement for less ‘green’ glass and carbon fibre reinforcements – especially in the automotive sector. Scientists from SABIC in Brazil looked at how its use can be optimized with polyamide 6 and polypropylene.
Eco-friendly, sustainable bio-composites from vegetal (plant-derived) fibre and crop-derived plastics (bio-plastics) are gaining popularity in recent years, not only as a solution to growing environmental problems, but also as a way of reducing the need for petroleum. These biopolymers have already moved into mainstream use, and composites based on renewable feedstock may soon be competing with commodity plastics. These bio-composites come from renewable sources that, in principle, are inexhaustible; they are biodegradable, which is an important characteristic for components that must be disposed of at the end of their useful life; they are recyclable and can be easily converted into thermal energy through combustion without leaving residue, causing less pollution and may gain additional carbon credit. Using natural fibres as plastic reinforcement is not new. At the beginning of the 20th century, phenyl-formaldehyde and melamine-formaldehyde resins compounded with paper or cotton were used for electric insulation. Applications in the automotive industry were already found in the 60s, when coconut fibres were used to manufacture car seats, and polypropylene (PP) composites with wood flour, moulded by compression, were applied as substrates for car interiors. In the 90s, polyethylene (PE) composites with natural fibres were used as a substitute for wood in deck boards, fencing and industrial flooring. Due to the many benefits that bio-based materials offer, the automotive industry
Figure 1: Curauá plant.
Plastics Additives & Compounding November/December 2009
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ISSN1464-391X/09 © 2009 Elsevier Ltd. All rights reserved.
Natural fibres is looking seriously at these types of products. Today, European car manufacturers are using natural fibre composites in internal parts such as door panels, trunk liners and instrumental panels [1]. DaimlerChrysler is looking into the use of flax, coconut and abaca fibres in their Mercedes vehicles, while Ford has been researching using biomaterials such as soy-based foam for seating [2]. More recently, polyvinyl chloride (PVC) composites with wood flour have been used for window and door frames, thanks to their balance of thermal resistance and stiffness [3]. In this decade, interest in natural fibres as a reinforcing material for thermoplastics has increased significantly, and not only in the automotive industry. Applications have grown very fast in structural components and also as finished parts.
Testing the fibre Due to the limited thermal stability of natural fibres, only thermoplastics that soften at temperatures below 200°C, such as polyolefins and PVC, are commonly used in these composites. The base resin and the fibre are chosen in accordance with their typical properties, application requirements, availability and cost. The fibres normally used have a length ranging from 3-8 mm and their concentration can vary from 20-70% of the composite. In this example, we studied curauá fibre composites up to 30% loading with nylon 6 (PA6) polypropylene (PP) and high-density polypropylene (HDPE). The main component of natural fibres is cellulose. The elementary unit of the cellulose macromolecule is anhydro-d-glucose, which contains three hydroxyl groups (-OH). These hydroxyls form hydrogen bonds inside the molecules itself (intramolecular) and between other cellulose molecules (intermolecular), as well as with hydroxyl groups from the air. Therefore, all natural fibres are hydrophilic, that is, they absorb water in the range of 8-12.6% [4]. Another important characteristic of natural fibres is their degree of polymerization. The molecules of each fibre differ in this aspect and, consequently, the fibres
Table 1: Composition of the curauá fibre. Composition Cellulose Hemicellulose Holocellulose Lignin Ash content Moisture
are a complex mixture of the homologous series (C6H10O5)n. Fibres from the base of the plants normally have a greater degree of polymerization – approximately 10,000. The fibrils of cellulose macromolecules form spirals along the fibre axis and the strength and stiffness of hemp, ramie and jute are correlated with the angle between the axis and the fibril of the fibres. The smaller this angle is, the higher the mechanical properties [4]. Cellulose contains different natural substances, the most important of which are lignin and different waxes. Observations made with a scanning electronic microscope (SEM) reveal that the natural fibres are composed of a bundle of simple fibrils covered and bonded together by the lignin [5]. Thus, lignin influences the structure, properties and morphology of the natural fibres. The waxes are responsible for the wettability and adhesion characteristics of the fibres and they can be removed by extraction with organic solvents [4].
Using curauá fibre Curauá (Ananas erectifolius) is a plant from the Bromeliad (pineapple) family, cultivated in the Brazilian Amazon region [6, 7]. The leaves from the curauá plant reach up to 1-5 m in length and 4 cm in width and are straight and flat, with thorns throughout their width (see Figure 1). When the plants are about eight months old, the leaves yield about 8% fibre on a dry weight basis. The fibre content is about 5-8%, and the balance is mucilage, which can be used as animal food or as organic fertilizer. In Brazil, these fibres are mainly made
13
(%) 73.6 9.9 83.5 7.5 0.9 7.9
into hammocks, fishing lines, ropes and nets by the indigenous population. The curauá used in industrial manufacture is obtained from farms in Amazon region. The fibres are processed (extracted, washed and dried in open air) and milled to obtain workable forms of fibres. The fibres extracted from its leaves have high mechanical strength when compared to other fibres like sisal, jute and flax. Its composition is shown in Table 1 [8]. Figure 2 shows the tensile modulus of different natural fibres compared to that of glass fibre. If specific modulus (relative to density) is considered, curauá stands out relative to other fibres, which enable composites with curauá fibre up to a 15% weight reduction [9].
Fibre/matrix compatibility The properties of composites depend on the individual components and on their interfacial compatibility. The adhesion between fibre and matrix is obtained by mechanical anchoring of the fibre ends into the matrix. In many cases, the absorption of moisture by untreated fibres or poor wettability can cause insufficient interfacial adhesion to the polymer matrix. The lack of interfacial interactions leads to internal strains, porosity and environmental degradation. The wettability of the fibre depends on the polymer viscosity and the surface tension of both materials. The surface tension of the polymer must be as low as possible, at least lower than that of the fibre. Therefore, modification of the fibre and/or polymer matrix is a key area for obtaining good composite properties [4].
Plastics Additives & Compounding November/December 2009
Natural fibres
Figure 2: Comparison of the tensile modulus (nominal and specific) of natural fibres with glass fibre.
Surface modification There are several different methods for the modification of surface energy of the fibre and the polymer. Physical methods like stretching, calendaring and the production of hybrid yarns change the structural and surface properties of the fibre and thereby influence the mechanical bonding in the matrix. Another physical method used is electric discharge (plasma or corona), which can increase the roughness of the fibres and activate its surface oxidation, leading to an increase in aldehyde groups. It can also generate crosslinking and free radicals [4]. Oxidizing agents such as sodium or calcium hypochlorite and hydrogen peroxides can be used to remove dust and oil from the natural fibres. Alkaline treatment with NaOH, known as mercerization, extracts lignin and hemicellulose, increasing the tensile strength and the elongation at break of the composite. In this process, fibres were immersed in a 10 wt% solution of NaOH for 1-2 hours, followed by continuous washing and drying at 100°C [5]. It is also possible to improve the compatibility of the fibres with the polymer matrix through chemical methods that introduce a third material (coupling agent), with intermediate properties of the two initial ones. There are several mechanisms of coupling in materials: they can eliminate the weak boundary layers; they can produce a flexible and tough layer; they can form a highly
crosslinked interphase region with an intermediate modulus; they can improve wettability between the polymer and the substrate; they can create covalent bonds with both materials; and/or they can alter the acidity of the substrate surface [4]. The surface energy of the fibres is closely related to their hydrophilic character. Thus, modifying natural fibres with stearic acid make fibres more hydrophobic and improves their dispersion in PP. Silane coupling agents may add hydrophilic properties to the interface, especially when amino-functional silanes are used as primers for reactive polymers. The organofunctional group of the silane reacts with the polymer, through copolymerization and/or the formation of an interpenetrating network. This curing reaction of a silane treated substrate enhances the wetting by the resin [4]. Another effective method of natural fibre modification is graft copolymerization, initiated by free radicals of the cellulose molecule. In this case, the cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. As a result, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with suitable solutions such as acrylonitrile or methyl methacrylate. The resulting copolymer has the characteristics of both cellulose fibres and grafted polymer. For example, the treatment of natural
fibres with hot PP/maleic anhydride copolymers provides covalent bonds across the interface. After this treatment, the surface energy of the fibres is increased to a level much closer to the surface energy of the matrix. This gives improved wettability and, consequently, higher interfacial adhesion. The PP chain allows segmental crystallization and cohesive coupling between the modified fibre and the PP matrix [4]. Curauá fibre interacts in a different way in different resin systems. In PA-6, the fibres show good bonding to the polymer matrix, with or without drying. The PA-6 composites made using curauá fibre subject to surface modification with N2 plasma and caustic soda treatments show only slight improvement of physical properties compared to similar products made using untreated fibres. This shows that virgin curauá fibres undergo significant adhesion to the PA-6 matrix and surface modification is not needed. This may be due to good adhesion of fibres to the nylon 6 matrix through hydrogen bonding between cellulose OH groups and nylon amide groups. In contrast, for both HDPE and PP, the chemical coupling agents required significant fibre interaction. In the presence of polyethylene grafted with maleic anhydride, HDPE and PP showed good adhesion with improved properties. It is also noted that untreated curauá fibre undergoes bonding to PA-6 matrix that is better than that of treated fibre in HDPE or PE.
Table 2: Comparative values of 20% curauá filled PX07444 with unfilled PA-6 and PA-6 reinforced with 20% of glass fibre (GF) or talc (MF). Property Density (g/cm3) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%) Izod impact strength (KJ/m2) Flexural strength (MPa) HDT @ 1.82 MPa (˚C)
Plastics Additives & Compounding November/December 2009
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PA6 PX07444 PA6+20%GF PA6+20%MF 1.14 1.18 1.27 1.27 63 83 101 73 1.4 5.5 6.5 6.5 >60 3 3 6 10 9 9 9 95 115 160 115 57 186 194 110
Natural fibres Successful compounding The majority of composites with natural fibres and thermoplastics are produced by extrusion. Some processors use twin screw extruders to produce granulated composites; others use single screw extruders, while others use the latter to manufacture final products directly, like profiles or sheets. The fibre moisture can be removed in a previous process, by drying in an air-circulating oven (8 hours at 80°C), or n the extruder itself, if there is a vacuum outlet downstream in the barrel. During the extrusion, there is one factor that needs to be considered: at temperatures above 160°C and in the presence of oxygen, the fibres can undergo thermal oxidation resulting in darkening, and, at higher temperatures, mechanical degradation will occur. From this point of view, the use of twin screw extruders is preferred, because of the possibility of controlling the energy introduction via screw configuration and processing conditions in such a way that a lower process temperature can be reliably ensured. The residence time of the fibres at the polymer melting temperature, without the presence of oxygen is short – between 10 and 20 seconds. In this way, excellent mixing and dispersing can be achieved, especially when co-rotating, intermeshing screws are used. The best screw configuration is obtained experimentally, but profiles with deep flight depth should be preferred, because of the large volume occupied by the fibres (a bulk density of around 0.15 kg/dm3) [10]. In preparing curauá thermoplastic composites, thermoplastic resin is premixed with additives and fed into the first housing of the extruder via gravimetric metering. The fibre is added at the extruder downstream by a lateral feeder directly into the zone in which the polymer is already molten. Better mixing and higher throughput is achieved by feeding a pre-mix of fibre/resin masterbatch rather than naked fibres. The air entrained with the fibres can escape via a back venting system in the unit. In this process, the fibres are homogeneously distributed in the molten resin so that each fibre is completely wet by the polymer. Subsequent vacuum venting over a length of
Figure 3: Comparison of the specific tensile strength and modulus (relative to density) of 20% curauá fibre filled PX07444 with 20% of glass fibre (GF) or talc (MF) reinforced PA-6.
4D ensures that the residual moisture and volatiles are withdrawn. A machine like this, with a diameter of 76 mm, can produce up to 1000 kg/h of PP or HDPE composite with a ratio from 40-70% of natural fibres [8]. Using a twin-screw extruder therefore provides a higher shear and a good dispersion of the fibre in the matrix. There is also evidence for extensive fibrillation of the fibres.
Composite properties The addition of natural fibres to thermoplastics maintains, at least, the properties of the unfilled polymers. Generally they also increase the rigidity of the composite, which, in turn, makes it more brittle. However, this lower toughness is still higher than composites with mineral fillers, such as talc. UV protection is a cause for concern, because these composites tend to fade with time and need to be well stabilized and pigmented to reduce this effect. In addition to size and content, the type of fibre utilized has a great influence on the properties of the composite. Harder fibres, for example, generally increase rigidity and strength, but cause a darker colouration in the final product [10]. A product portfolio has been developed based on curauá fibres in PA-6 thermoplastic composites with fibre loading up to 40%. These products show better flexural strength and tensile modulus
15
compared to unfilled PA-6 coupled with lower density providing good weight to strength advantage. Table 2 shows a comparison of PA-6 reinforced with 20% of curauá fibre PX07444 with unfilled PA-6, and PA-6 reinforced with traditional fillers. As can be observed, the tensile and flexural properties of PX07444 are better than those of unfilled PA-6, but lower than PA-6 reinforced with glass fibre. In relation to PA-6 reinforced with 20% of talc, PX07444 shows higher tensile strength, lower tensile modulus and equal notched Izod impact strength. Its density is lower than PA-6 reinforced with talc or glass fibre, and its HDT is higher than unfilled PA-6 or PA-6 reinforced with talc, and is comparable to PA-6 reinforced with glass fibre. Figure 3 shows a comparison of the specific (divided by density) tensile strength and modulus of each product already mentioned. Taking this factor into consideration, one can note that the results of PX07444 are much closer to those of PA-6 reinforced with glass fibre, enabling it to replace the latter in specific applications, such as automobile interior components or electric tools housing, for example.
Automotive applications The nylon 6 Curauá fibre filled product described was evaluated in the automo-
Plastics Additives & Compounding November/December 2009
Natural fibres
Figure 4: Automotive sun visor bracket mounting parts made using 20% Curaua fibre filled nylon 6 [11].
tive sun visor brackets and frame parts as shown in Figure 4. The moulded parts had very good surface aesthetics coupled with excellent mechanical properties, when moulded using the following parameters: • Drying conditions: 6h at 100°C • Melt temperature: 220-245°C • Mould temperature: 70°C • Injection pressure: 55 bar • Holding pressure: 50 bar • Cycle time: 24 seconds for the pin and 30 seconds for the frame.
Need for green composites Curauá fibre reinforced plastic composites provide excellent weight to strength properties when compared to mineral filled products. The product properties track closely with that of glass fibre filled products but it may be necessary to improve its fibre-polymer interactions to obtain direct offset products. The natural fibre plastic composites open up many opportunities not only in automotive but also in many other industrial applications where there is a need for ‘green’ plastic composites. The development of composites with natural fibres is just beginning. Although the ecological aspects of these fibres are their main consideration for industrial applications, including automotive segment, current and future research works should focus on how to increase the performance of
these composites to make them function as well as traditional products. The continuous development of fibre treatments, new compounding techniques, the fibre-matrix adhesion and the use of engineering thermoplastics as base resin will bring significant improvement for the properties of these materials. This improved performance will make its use as a replacement of glass fibre and mineral fillers much more feasible in a large number of applications. Currently sold only in Brazil, these natural fibre composites are being evaluated for production and sale in other regions.
References 1) Zah, R, Hischier, R, Leao, A, L, Braun, J., J. Cleaner Production., 15, pp 1032- 1040 (2002) and references therein. 2) Juska, C. “2006 Automotive Plastics Report Card”. A Report by the Ecology Center. Nov 2006. www.ecocenter.org 3) C Clemons, “Wood-plastic composites in the United States: the interfacing of two industries”, J. Forest Products, 52, p.10-18 (2002). 4) A K Bledzki, S Reihmane, J Gassan, “Properties and Modification Methods for Vegetable Fibres for Natural Fibre Composites”, J. Appl. Polym. Sci., 59, p.1329-1336 (1996). 5) A Gomes, K Goda, J Ohgi “Effects of alkali treatment to reinforcement on tensile properties of curauá fibre
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green composites” JSME International Journal Series A 47 p.541-546 (2004). 6) Silva, R. V., Aquino, E. M. F., J. Reinforced Plastics and Composites, 27(1), pp 103-112 7) Tomczak, F., Satyanarayana, K. G., Sydenstricker, T. H. D., Composites: Part A, 38, pp 227-2236 (2007). 8) I Kleba, J Zabold, “Poliuretano com fibras naturais ganha espaço na indústria automotiva”, Plástico Industrial, p.88-99 (11/2004) 9) A Leão, R Rowell, N Tavares, “Applications of natural fibres in automotive industry in Brazil – thermoforming process”, in: Science and Technology Polymers and Advanced Materials, P N Prasad ed., Plenum Press, New York (1998) 10) H Frisk, D Schwendemann, “Compounding Wood Fibres with Plastics”, Kunststoffe Plast Europe, 4, p.1-5 (2004) 11) Pematec Triangel do Brasil Ltda. www.pematec.com.br Contacts: Jay Amarasekera, Paulo Aparecido dos Santos, Joao Carlos Giriolli, Glauco Moraes SABIC innovative Plastics www.sabic.com Barbara Mano, Marco-A. De Paoli Instituto de Quimica, UNICAMP, Campinas, Brazil. Part of this work was first presented at the 8th-Annual SPE Automotive Composites Conference & Exhibition, Troy, Michigan, 16-18 September, 2008.
Plastics Additives & Compounding November/December 2009
Eco-friendly, sustainable bio-composites from vegetal (plant-derived) fibre and crop-derived plastics (bio-plastics) are gaining popularity in recent years, not only as a solution to growing environmental problems, but also as a way of reducing the need for petroleum. These biopolymers have already moved into mainstream use, and composites based on renewable feedstock may soon be competing with commodity plastics. These bio-composites come from renewable sources that, in principle, are inexhaustible; they are biodegradable, which is an important characteristic for components that must be disposed of at the end of their useful life; they are recyclable and can be easily converted into thermal energy through combustion without leaving residue, causing less pollution and may gain additional carbon credit. Using natural fibres as plastic reinforcement is not new. At the beginning of the 20th century, phenyl-formaldehyde and melamine-formaldehyde resins compounded with paper or cotton were used for electric insulation. Applications in the automotive industry were already found in the 60s, when coconut fibres were used to manufacture car seats, and polypropylene (PP) composites with wood flour, moulded by compression, were applied as substrates for car interiors. In the 90s, polyethylene (PE) composites with natural fibres were used as a substitute for wood in deck boards, fencing and industrial flooring. Due to the many benefits that bio-based materials offer, the automotive industry
Figure 1: Curauá plant.
Plastics Additives & Compounding November/December 2009
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ISSN1464-391X/09 © 2009 Elsevier Ltd. All rights reserved.
Natural fibres is looking seriously at these types of products. Today, European car manufacturers are using natural fibre composites in internal parts such as door panels, trunk liners and instrumental panels [1]. DaimlerChrysler is looking into the use of flax, coconut and abaca fibres in their Mercedes vehicles, while Ford has been researching using biomaterials such as soy-based foam for seating [2]. More recently, polyvinyl chloride (PVC) composites with wood flour have been used for window and door frames, thanks to their balance of thermal resistance and stiffness [3]. In this decade, interest in natural fibres as a reinforcing material for thermoplastics has increased significantly, and not only in the automotive industry. Applications have grown very fast in structural components and also as finished parts.
Testing the fibre Due to the limited thermal stability of natural fibres, only thermoplastics that soften at temperatures below 200°C, such as polyolefins and PVC, are commonly used in these composites. The base resin and the fibre are chosen in accordance with their typical properties, application requirements, availability and cost. The fibres normally used have a length ranging from 3-8 mm and their concentration can vary from 20-70% of the composite. In this example, we studied curauá fibre composites up to 30% loading with nylon 6 (PA6) polypropylene (PP) and high-density polypropylene (HDPE). The main component of natural fibres is cellulose. The elementary unit of the cellulose macromolecule is anhydro-d-glucose, which contains three hydroxyl groups (-OH). These hydroxyls form hydrogen bonds inside the molecules itself (intramolecular) and between other cellulose molecules (intermolecular), as well as with hydroxyl groups from the air. Therefore, all natural fibres are hydrophilic, that is, they absorb water in the range of 8-12.6% [4]. Another important characteristic of natural fibres is their degree of polymerization. The molecules of each fibre differ in this aspect and, consequently, the fibres
Table 1: Composition of the curauá fibre. Composition Cellulose Hemicellulose Holocellulose Lignin Ash content Moisture
are a complex mixture of the homologous series (C6H10O5)n. Fibres from the base of the plants normally have a greater degree of polymerization – approximately 10,000. The fibrils of cellulose macromolecules form spirals along the fibre axis and the strength and stiffness of hemp, ramie and jute are correlated with the angle between the axis and the fibril of the fibres. The smaller this angle is, the higher the mechanical properties [4]. Cellulose contains different natural substances, the most important of which are lignin and different waxes. Observations made with a scanning electronic microscope (SEM) reveal that the natural fibres are composed of a bundle of simple fibrils covered and bonded together by the lignin [5]. Thus, lignin influences the structure, properties and morphology of the natural fibres. The waxes are responsible for the wettability and adhesion characteristics of the fibres and they can be removed by extraction with organic solvents [4].
Using curauá fibre Curauá (Ananas erectifolius) is a plant from the Bromeliad (pineapple) family, cultivated in the Brazilian Amazon region [6, 7]. The leaves from the curauá plant reach up to 1-5 m in length and 4 cm in width and are straight and flat, with thorns throughout their width (see Figure 1). When the plants are about eight months old, the leaves yield about 8% fibre on a dry weight basis. The fibre content is about 5-8%, and the balance is mucilage, which can be used as animal food or as organic fertilizer. In Brazil, these fibres are mainly made
13
(%) 73.6 9.9 83.5 7.5 0.9 7.9
into hammocks, fishing lines, ropes and nets by the indigenous population. The curauá used in industrial manufacture is obtained from farms in Amazon region. The fibres are processed (extracted, washed and dried in open air) and milled to obtain workable forms of fibres. The fibres extracted from its leaves have high mechanical strength when compared to other fibres like sisal, jute and flax. Its composition is shown in Table 1 [8]. Figure 2 shows the tensile modulus of different natural fibres compared to that of glass fibre. If specific modulus (relative to density) is considered, curauá stands out relative to other fibres, which enable composites with curauá fibre up to a 15% weight reduction [9].
Fibre/matrix compatibility The properties of composites depend on the individual components and on their interfacial compatibility. The adhesion between fibre and matrix is obtained by mechanical anchoring of the fibre ends into the matrix. In many cases, the absorption of moisture by untreated fibres or poor wettability can cause insufficient interfacial adhesion to the polymer matrix. The lack of interfacial interactions leads to internal strains, porosity and environmental degradation. The wettability of the fibre depends on the polymer viscosity and the surface tension of both materials. The surface tension of the polymer must be as low as possible, at least lower than that of the fibre. Therefore, modification of the fibre and/or polymer matrix is a key area for obtaining good composite properties [4].
Plastics Additives & Compounding November/December 2009
Natural fibres
Figure 2: Comparison of the tensile modulus (nominal and specific) of natural fibres with glass fibre.
Surface modification There are several different methods for the modification of surface energy of the fibre and the polymer. Physical methods like stretching, calendaring and the production of hybrid yarns change the structural and surface properties of the fibre and thereby influence the mechanical bonding in the matrix. Another physical method used is electric discharge (plasma or corona), which can increase the roughness of the fibres and activate its surface oxidation, leading to an increase in aldehyde groups. It can also generate crosslinking and free radicals [4]. Oxidizing agents such as sodium or calcium hypochlorite and hydrogen peroxides can be used to remove dust and oil from the natural fibres. Alkaline treatment with NaOH, known as mercerization, extracts lignin and hemicellulose, increasing the tensile strength and the elongation at break of the composite. In this process, fibres were immersed in a 10 wt% solution of NaOH for 1-2 hours, followed by continuous washing and drying at 100°C [5]. It is also possible to improve the compatibility of the fibres with the polymer matrix through chemical methods that introduce a third material (coupling agent), with intermediate properties of the two initial ones. There are several mechanisms of coupling in materials: they can eliminate the weak boundary layers; they can produce a flexible and tough layer; they can form a highly
crosslinked interphase region with an intermediate modulus; they can improve wettability between the polymer and the substrate; they can create covalent bonds with both materials; and/or they can alter the acidity of the substrate surface [4]. The surface energy of the fibres is closely related to their hydrophilic character. Thus, modifying natural fibres with stearic acid make fibres more hydrophobic and improves their dispersion in PP. Silane coupling agents may add hydrophilic properties to the interface, especially when amino-functional silanes are used as primers for reactive polymers. The organofunctional group of the silane reacts with the polymer, through copolymerization and/or the formation of an interpenetrating network. This curing reaction of a silane treated substrate enhances the wetting by the resin [4]. Another effective method of natural fibre modification is graft copolymerization, initiated by free radicals of the cellulose molecule. In this case, the cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. As a result, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with suitable solutions such as acrylonitrile or methyl methacrylate. The resulting copolymer has the characteristics of both cellulose fibres and grafted polymer. For example, the treatment of natural
fibres with hot PP/maleic anhydride copolymers provides covalent bonds across the interface. After this treatment, the surface energy of the fibres is increased to a level much closer to the surface energy of the matrix. This gives improved wettability and, consequently, higher interfacial adhesion. The PP chain allows segmental crystallization and cohesive coupling between the modified fibre and the PP matrix [4]. Curauá fibre interacts in a different way in different resin systems. In PA-6, the fibres show good bonding to the polymer matrix, with or without drying. The PA-6 composites made using curauá fibre subject to surface modification with N2 plasma and caustic soda treatments show only slight improvement of physical properties compared to similar products made using untreated fibres. This shows that virgin curauá fibres undergo significant adhesion to the PA-6 matrix and surface modification is not needed. This may be due to good adhesion of fibres to the nylon 6 matrix through hydrogen bonding between cellulose OH groups and nylon amide groups. In contrast, for both HDPE and PP, the chemical coupling agents required significant fibre interaction. In the presence of polyethylene grafted with maleic anhydride, HDPE and PP showed good adhesion with improved properties. It is also noted that untreated curauá fibre undergoes bonding to PA-6 matrix that is better than that of treated fibre in HDPE or PE.
Table 2: Comparative values of 20% curauá filled PX07444 with unfilled PA-6 and PA-6 reinforced with 20% of glass fibre (GF) or talc (MF). Property Density (g/cm3) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%) Izod impact strength (KJ/m2) Flexural strength (MPa) HDT @ 1.82 MPa (˚C)
Plastics Additives & Compounding November/December 2009
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PA6 PX07444 PA6+20%GF PA6+20%MF 1.14 1.18 1.27 1.27 63 83 101 73 1.4 5.5 6.5 6.5 >60 3 3 6 10 9 9 9 95 115 160 115 57 186 194 110
Natural fibres Successful compounding The majority of composites with natural fibres and thermoplastics are produced by extrusion. Some processors use twin screw extruders to produce granulated composites; others use single screw extruders, while others use the latter to manufacture final products directly, like profiles or sheets. The fibre moisture can be removed in a previous process, by drying in an air-circulating oven (8 hours at 80°C), or n the extruder itself, if there is a vacuum outlet downstream in the barrel. During the extrusion, there is one factor that needs to be considered: at temperatures above 160°C and in the presence of oxygen, the fibres can undergo thermal oxidation resulting in darkening, and, at higher temperatures, mechanical degradation will occur. From this point of view, the use of twin screw extruders is preferred, because of the possibility of controlling the energy introduction via screw configuration and processing conditions in such a way that a lower process temperature can be reliably ensured. The residence time of the fibres at the polymer melting temperature, without the presence of oxygen is short – between 10 and 20 seconds. In this way, excellent mixing and dispersing can be achieved, especially when co-rotating, intermeshing screws are used. The best screw configuration is obtained experimentally, but profiles with deep flight depth should be preferred, because of the large volume occupied by the fibres (a bulk density of around 0.15 kg/dm3) [10]. In preparing curauá thermoplastic composites, thermoplastic resin is premixed with additives and fed into the first housing of the extruder via gravimetric metering. The fibre is added at the extruder downstream by a lateral feeder directly into the zone in which the polymer is already molten. Better mixing and higher throughput is achieved by feeding a pre-mix of fibre/resin masterbatch rather than naked fibres. The air entrained with the fibres can escape via a back venting system in the unit. In this process, the fibres are homogeneously distributed in the molten resin so that each fibre is completely wet by the polymer. Subsequent vacuum venting over a length of
Figure 3: Comparison of the specific tensile strength and modulus (relative to density) of 20% curauá fibre filled PX07444 with 20% of glass fibre (GF) or talc (MF) reinforced PA-6.
4D ensures that the residual moisture and volatiles are withdrawn. A machine like this, with a diameter of 76 mm, can produce up to 1000 kg/h of PP or HDPE composite with a ratio from 40-70% of natural fibres [8]. Using a twin-screw extruder therefore provides a higher shear and a good dispersion of the fibre in the matrix. There is also evidence for extensive fibrillation of the fibres.
Composite properties The addition of natural fibres to thermoplastics maintains, at least, the properties of the unfilled polymers. Generally they also increase the rigidity of the composite, which, in turn, makes it more brittle. However, this lower toughness is still higher than composites with mineral fillers, such as talc. UV protection is a cause for concern, because these composites tend to fade with time and need to be well stabilized and pigmented to reduce this effect. In addition to size and content, the type of fibre utilized has a great influence on the properties of the composite. Harder fibres, for example, generally increase rigidity and strength, but cause a darker colouration in the final product [10]. A product portfolio has been developed based on curauá fibres in PA-6 thermoplastic composites with fibre loading up to 40%. These products show better flexural strength and tensile modulus
15
compared to unfilled PA-6 coupled with lower density providing good weight to strength advantage. Table 2 shows a comparison of PA-6 reinforced with 20% of curauá fibre PX07444 with unfilled PA-6, and PA-6 reinforced with traditional fillers. As can be observed, the tensile and flexural properties of PX07444 are better than those of unfilled PA-6, but lower than PA-6 reinforced with glass fibre. In relation to PA-6 reinforced with 20% of talc, PX07444 shows higher tensile strength, lower tensile modulus and equal notched Izod impact strength. Its density is lower than PA-6 reinforced with talc or glass fibre, and its HDT is higher than unfilled PA-6 or PA-6 reinforced with talc, and is comparable to PA-6 reinforced with glass fibre. Figure 3 shows a comparison of the specific (divided by density) tensile strength and modulus of each product already mentioned. Taking this factor into consideration, one can note that the results of PX07444 are much closer to those of PA-6 reinforced with glass fibre, enabling it to replace the latter in specific applications, such as automobile interior components or electric tools housing, for example.
Automotive applications The nylon 6 Curauá fibre filled product described was evaluated in the automo-
Plastics Additives & Compounding November/December 2009
Natural fibres
Figure 4: Automotive sun visor bracket mounting parts made using 20% Curaua fibre filled nylon 6 [11].
tive sun visor brackets and frame parts as shown in Figure 4. The moulded parts had very good surface aesthetics coupled with excellent mechanical properties, when moulded using the following parameters: • Drying conditions: 6h at 100°C • Melt temperature: 220-245°C • Mould temperature: 70°C • Injection pressure: 55 bar • Holding pressure: 50 bar • Cycle time: 24 seconds for the pin and 30 seconds for the frame.
Need for green composites Curauá fibre reinforced plastic composites provide excellent weight to strength properties when compared to mineral filled products. The product properties track closely with that of glass fibre filled products but it may be necessary to improve its fibre-polymer interactions to obtain direct offset products. The natural fibre plastic composites open up many opportunities not only in automotive but also in many other industrial applications where there is a need for ‘green’ plastic composites. The development of composites with natural fibres is just beginning. Although the ecological aspects of these fibres are their main consideration for industrial applications, including automotive segment, current and future research works should focus on how to increase the performance of
these composites to make them function as well as traditional products. The continuous development of fibre treatments, new compounding techniques, the fibre-matrix adhesion and the use of engineering thermoplastics as base resin will bring significant improvement for the properties of these materials. This improved performance will make its use as a replacement of glass fibre and mineral fillers much more feasible in a large number of applications. Currently sold only in Brazil, these natural fibre composites are being evaluated for production and sale in other regions.
References 1) Zah, R, Hischier, R, Leao, A, L, Braun, J., J. Cleaner Production., 15, pp 1032- 1040 (2002) and references therein. 2) Juska, C. “2006 Automotive Plastics Report Card”. A Report by the Ecology Center. Nov 2006. www.ecocenter.org 3) C Clemons, “Wood-plastic composites in the United States: the interfacing of two industries”, J. Forest Products, 52, p.10-18 (2002). 4) A K Bledzki, S Reihmane, J Gassan, “Properties and Modification Methods for Vegetable Fibres for Natural Fibre Composites”, J. Appl. Polym. Sci., 59, p.1329-1336 (1996). 5) A Gomes, K Goda, J Ohgi “Effects of alkali treatment to reinforcement on tensile properties of curauá fibre
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green composites” JSME International Journal Series A 47 p.541-546 (2004). 6) Silva, R. V., Aquino, E. M. F., J. Reinforced Plastics and Composites, 27(1), pp 103-112 7) Tomczak, F., Satyanarayana, K. G., Sydenstricker, T. H. D., Composites: Part A, 38, pp 227-2236 (2007). 8) I Kleba, J Zabold, “Poliuretano com fibras naturais ganha espaço na indústria automotiva”, Plástico Industrial, p.88-99 (11/2004) 9) A Leão, R Rowell, N Tavares, “Applications of natural fibres in automotive industry in Brazil – thermoforming process”, in: Science and Technology Polymers and Advanced Materials, P N Prasad ed., Plenum Press, New York (1998) 10) H Frisk, D Schwendemann, “Compounding Wood Fibres with Plastics”, Kunststoffe Plast Europe, 4, p.1-5 (2004) 11) Pematec Triangel do Brasil Ltda. www.pematec.com.br Contacts: Jay Amarasekera, Paulo Aparecido dos Santos, Joao Carlos Giriolli, Glauco Moraes SABIC innovative Plastics www.sabic.com Barbara Mano, Marco-A. De Paoli Instituto de Quimica, UNICAMP, Campinas, Brazil. Part of this work was first presented at the 8th-Annual SPE Automotive Composites Conference & Exhibition, Troy, Michigan, 16-18 September, 2008.
Plastics Additives & Compounding November/December 2009