flax fabric green composites

Accepted Manuscript Title: Improved Flame-Retardant and Tensile Properties of Thermoplastic Starch/Flax Fabric Green Composites Authors: M.N. Prabhakar, Atta ur Rehman Shah, Jung-Il Song PII: DOI: Reference:

S0144-8617(17)30292-8 http://dx.doi.org/doi:10.1016/j.carbpol.2017.03.036 CARP 12125

To appear in: Received date: Revised date: Accepted date:

1-10-2016 4-3-2017 11-3-2017

Please cite this article as: Prabhakar, MN., Rehman Shah, Atta ur., & Song, Jung-Il., Improved Flame-Retardant and Tensile Properties of Thermoplastic Starch/Flax Fabric Green Composites.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.03.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved Flame-Retardant and Tensile Properties of Thermoplastic Starch/Flax Fabric Green Composites M. N. Prabhakar1, Atta ur Rehman Shah2, Jung-Il Song1* 1 Department of Mechanical Engineering, Changwon National University, Changwon, Korea 2 Department of Mechanical Engineering, HITEC University, Taxila cantt, Pakistan *

Corresponding author: [email protected]

Highlights: 

Flame retardant green composites were developed from plasticized starch and APP.

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Mechano-ball milling process was employed to prepare plasticized starch powder.

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The properties of the composites were investigated as a function of APP.

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The composites exhibit superior flame retardant and mechanical properties.

Abstract This article highlights the development of biodegradable flame-retardant composites using a compression technique on low-cost starch, flax fabric (FF) and ammonium polyphosphate (APP) raw materials. The starch was plasticized into thermoplastic starch through a mechano-ball milling process and composites were developed by reinforcing the FF and incorporating varying amounts of APP. The effects of APP on the flammability and thermal properties of the composites were studied. Limited oxygen index and horizontal-burning tests exhibited significant sustainability of the composites toward flame and direct flame self-extinguishment. It was observed that at higher temperatures, APP leads to formation of thermally stable char. The flame retardant properties of the composites were speculated to be due to the protective compact crosslinked network (P-O-P and P-O-C) of the char. The reported effects of APP include improvement in mechanical and biodegradation properties. This investigation provides the design of novel flame-retardant green composites with excellent properties. 1

Keywords: starch; flax fabric; ammonium polyphosphate; flame retardancy.

1. Introduction Non-eco plastic waste generated from non-biodegradable polymers results in severe hazards to society. Therefore, the present plastic scenario necessitates biodegradable polymers based on renewable resources (Endres, & Siebert-Raths, 2012; Ghosh Dastidar, & Netravali, 2012; Acioli-Moura, & Sun, 2008; Raghavendra, Jayaramudu, Varaprasad, Sadiku, Sinha Ray, Mohana Raju, 2013; Prabhakar, Rehaman Shah Atta Ur and Song Jung-Il, 2015). A main resource for renewable biodegradable polymers is the plant kingdom, which offers natural, abundant, non-toxic and non-exhaustible biodegradable polymers (Curvelo, de Carvalho, & Agnelli, 2001). Starch is a renowned biodegradable polymer that is widely utilized to develop bio-plastics for biodegradable films or biodegradable composites (Glenn, Orts, & Nobes, 2001; Raghavendra, Jung, kim, Seo, 2016). Chemically, starch is a polysaccharide consisting of amylose and branched amylopectin (-D-glucose links with 1,4 and1,6- branch) (French,1984; Whistler & Daniel, 1984). The structural features of starch are not suitable to bring plasticity and mechanical properties to starch-based products (Alissandratos, & Halling, 2012; Jimenez, Jose Fabra, Talens, & Chiralt, 2012). Hence, starch is modified to plasticized starch prior to use by gelatinization and heating with the additives such as water, glycerol/sorbitol to obtain processed thermoplastic starch (TPS) (Cyras, Manfredi, Ton-That, & Vazquez, 2008). Further, to overcome the poor mechanical properties, reinforcement techniques are used (Kuciel, & Liber-Knec, 2009; Castillo, López, López, Zaritzky, & García, 2013; Cerclé, Sarazin, & Favis, 2013). In most studies, reinforcement with natural fibres/fabrics was adopted by the field specialists to elevate the biodegradability of the starch-based composites (Agnantopoulou, Tserki, Marras, Philippou, & Panayiotou, 2012; Wang, Thompson, & Liu, 2012; Soykeabkaew, Laosat, Ngaokla, Yodsuwan, & Tunkasiri, 2012; Ayadi, & Dole, 2011; Benezet, Stanojlovic-Davidovic, Bergeret, Ferry, & Crespy, 2012). Further, the use of fabric is highly preferred as its reinforcement provides uniform strength throughout the developed composite. 2

Flax fabric (FF) is one of the cheapest and strongest (high tensile and low elongation) fabrics that is made from natural flax fibres (Dittenber, & Ganga Rao, 2012). The fibres of FF are made of cellulose (Mohanty, Misra, Drazal, 2005). It possesses low density with low water absorption capacity (̴ 7%) compared with other natural fibres and it has vibration absorbing and ultraviolet ray blocking properties (Bos, Van Den Oever, & Peters, 2002; Eichhorn, Baillie, & Zafeiropoulos, et al., 2001). Due to these significant properties, fabric-based composites have been developed with good mechanical properties for many applications such as marine and automotive use and in wind turbine blades (Libo & Nawawi, 2015). In recent years, flame-retardant composites have gained importance (Koronis, Silva, & Fontul, 2013). Biodegradable composites based on Starch or FF mainly finds its application in packaging (containers, wraps, disposable eating utensils, loose fill, antistatic) and agricultural sectors (mulch, controlled released devices) (María Guadalupe Lomelí-Ramírez, Arturo Javier Barrios-Guzmán, Salvador García-Enriquez, José de Jesús Rivera-Prado, & Ricardo ManríquezGonzález, 2014; Gregory, William Orts, Syed Imam, Bor-sen Chiou, & Delilah Wood, 2014). However, flame-retardant composites based on starch have not been focussed much. Despite the low cost, the flame retardant properties of starch and FF are poor. An appropriate flame retardant incorporation would essentially impart flame retardant properties to the composites. In recent years, halogen-free flame retardants gained significance due to various environmental concerns (Chen, Xu, Rao, Huang, Wang, Chen, & Wang, 2014). Hence, research has typically been focused on halogen-free flame retardants. Ammonium polyphosphate (APP) is one such flame retardant and intumescent compound that falls in the class of the focused subject. The flame retardancy properties of APP are due to the presence of flame-retardant constituents (Phosphorus and Nitrogen) in its structural composition. In addition, the advantage of APP is that after disposal, the major constituents of APP, Phosphorus and Nitrogen, combine with soil and bring fertility to the land. Thus, APP was used in the current investigation. In the present investigation, high-strength biodegradable flame-retardant composites were developed using the raw materials of low-cost starch, FF and APP. Preparation of TPS as a powder form through mechano-ball mill technique by utilizing fabric reinforcement in the presence of APP to manufacture bio fire retardant composites is a unique concept in this study. The mechanical,

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biodegradability, and flame-retardant properties of the composites were studied as a function of the fabric and flame-retardant additive. The details of this investigation are presented with an emphasis on the flame retardancy properties of the composites.

2. Materials and Methods 2.1. Materials Corn starch (72% amylopectin and 28% amylose) was obtained from Samyang Corporation Ltd, Seoul, South Korea. Glycerol (ACS reagent, ≥ 99.5%) was purchased from Sigma-Aldrich and used as received. Ammonium polyphosphate with an average degree of polymerization n>1000 was supplied by Yee Young Cerachem Ltd., Korea. Bi-directional balanced flax fabric (reinforced fabric size: 200 ×200×0.39 mm) from Lineo (Belgium) (density 1.45 g/cm3) was oven-dried at 50 oC for 12 h prior to processing and use as a reinforcement. 1.1 Preparation of Thermoplastic starch Thermoplastic starch (TPS) was prepared from native corn starch and glycerin solution. The required glycerin solution was initially prepared by adding glycerin (45 mL) to 30 mL of hot water (80 oC) under stirring. The solution was allowed to mix for 10 min until homogeneous. With the aid of the glycerin solution and 150 g of corn starch, TPS powder was thoroughly prepared and introduced into ball milling (Dongwon Scientific co., Korea) containing teflon-coated metal balls. The weight ratio of Teflon-coated metal balls and TPS powder was fixed at 3:1. The TPS powder was mechanically agitated at a rotation speed of 200 RPM for 2 h to obtain plasticized starch, thermoplastic starch (TPS). The TPS was maintained overnight in an air-tight polyethylene bag to enhance flow properties in order to intensify glycerine penetration into the starch granules through diffusion (Forssell, Mikkila, Moates, & Parker 1997; Maryam Sabetzadeh, Rouhollah Bagheri, Mahmood Masoomi, 2012). During the plasticization of starch, water gelatinizes starch and glycerin improves process-ability and reduces embrittlement by inhibiting the retrogradation process after processing. 1.2 Preparation of matrix 4

The TPS-ammonium polyphosphate (TPS-A) matrix was prepared by adding ammonium polyphosphate (3,6 and 9 wt%) to the prepared TPS and mixing thoroughly, followed by 1 h ball milling to obtain various wt% of TPS-A mixture. The TPS-A mixture obtained was used as a matrix for making composite in subsequent steps.

1.3 Preparation of TF composites The thermoplastic starch-flax fabric composites (TF composites) were fabricated by compression moulding as described in the literature (Herrera-Franco, & Valadez-González, 2004). In brief, a pre-formulated amount of TPS was carefully distributed on the mould of a compression moulding in a Wetech laboratory press (hot press tester) (Wetech Co., Ltd., Korea). The mixture was hot pressed at 160 oC for 30 min under a pressure of 7 MPa and cooled to ambient temperature to obtain 200×200×2 mm sheets. The machine was preheated at 140 oC for 10 min prior to hot pressing. Similarly, the FF/TPS composites were prepared by placing three layers of FF (9 wt%) in between proportionately distributed TPS matrix, as MFMFMFM (M=Matrix; F=Fabric) (TF). A hand lay-up roller (Elder & Song Manufacturer; USA) was used to uniformly distribute the matrix. Similarly, TFA composites containing 3, 6 and 9 wt% of APP were fabricated using FF and TA and designated TF-3A, TF-6A and TF-9A, respectively. The procedure for preparation of TPS and fabrication of TFA composites is shown in Fig. 1. 2.4. Characterization 2.4.1. Fourier Transform Infrared (FTIR) analysis The FTIR spectra of thermoplastic starch and the flax fabric/thermoplastic starch composites were recorded from wavenumber 400-4000 cm-1 using computerized Fourier transform infrared spectroscopy (FTIR) (Model: FT-IR-6300, JASCO International Co., Ltd., Japan) under dry air at ambient temperatures using the KBR disk method. For analysis, the samples were 5

completely dried in an oven (Vacuum-oven, Model: SH VDO-30NG, Samheung Energy Co., Ltd., Korea) at 60 oC for 5 h. FTIR spectra were recorded from 400 to 4000 cm-1 with 32 scans in each case at a resolution of 4 cm-1. 2.4.2. Field emission scanning electron microscopy (FESEM) To study the surface morphology of the composites, tensile fractured specimen surface and char residues obtained after the horizontal burning test were adhered onto a cylindrical aluminium stub using double-sided tape. Later, the samples were sputter-coated with platinum using a Polaron SC 7640 instrument under a flow of argon and placed into the FESEM chamber to observe surface morphology with a field emission scanning electron microscope (FESEM) (Model: CZ/MIRA 1 LMH/ H.S., TESCAN Inc., Czech Republic) at an accelerated voltage of 05-30 kV. 2.4.3. Tensile testing The tensile strength and modulus of composite specimens (150×15×2.6 mm) were measured using testing machine (100 kN load cell) (Model: MTS 810, MTS Systems Corporation, Korea (branch), USA) per ASTM D6389 standards. The tensile tests for all composite specimens were carried out at room temperature with a gauge length of 120 mm and a crosshead speed of 2 mm/min. 2.4.4. Horizontal burning test The horizontal burning test was used to evaluate the effect of fire retardants on the flammability of the TPS composites according to UL-94 standards (Yoshihiko Arao, Sakae Nakamura, yuta Tomita, Kyouhei Takakuwa, Toshikazu Umemura, & Tatsuya Tanaka, 2014). In brief, the sample was held horizontally and a flame fuelled by organic solvent (butane) was applied to one end of the sample for 30 sec. The height and angle of a flame with respect to the vertical direction were 10 mm and 450, respectively. The time taken for the flame to reach from the first reference mark (20 mm from the end) to second reference mark (80 mm from the end) was measured. Prior to the test, the specimens were dried at 80 oC for 24 h. The tests were conducted at least three times for each composition. 2.4.4.1. Limited oxygen index (LOI) 6

The flame retardancy of all samples was characterized by limiting oxygen index (LOI). The LOI was measured at room temperature using an oxygen index instrument (Model: FT-LOI404, FT-LOI-504, FESTEC International Co. Ltd., Korea) according to ASTM D2863-97 with 100 mm×6.5 mm×3 mm3 specimens (Ayfer Donmez Cavdar, Fatih Mengeloğlu, & Kadir Karakus, 2015). The LOI value is defined as the minimal oxygen concentration (O2) in the oxygen/nitrogen mixture that either maintains flame combustion of the material for 3 minutes or consumes a length of 5 cm of the sample, with the sample placed in a vertical position (the top of the test sample is inflamed with a burner). The LOI is expressed as:

2.4.5. Thermogravimetric analysis (TGA) The thermal stability and decomposition behaviour of constituents of the composites were analysed using a thermogravimetric analyser (Model: TGA Q600/SDT, TA instruments, USA) at a heating rate of 10 oC/min under both inert (N2) and air atmospheres and the scanning scope ranged from 30 oC to 800 oC. The residue was evaluated as the specimen’s residual weight at 800 o

C.

2.4.6. Biodegradation as a result of burial in soil Forty×8×2 mm samples of the TPS composites were buried approximately 10 cm deep in a mixture of 50% sand and 50% soil (Gaurav Kale, Rafael Auras, Sher Paul Singh, & Ramani Narayan, 2007). The temperature was maintained at 30 ± 2 oC. The water content of the soil and sand mixture was kept between 30–40% by adding 400 mL water to each 1250 g of mixture every 3 days to supply the nutrients for the microbes along with overcoming the dryness of the soil (Kaith, Jindal, Jana & Maiti, 2010). The degradation of the samples was determined at predetermined intervals (1, 2, 4, and 6 weeks) by carefully removing the sample from the soil and washing with distilled water to remove soil from the sample. The sample was wiped with tissue and dried at 80 oC for 24 h in a vacuum oven. The weight loss of the sample over time was used to indicate the degradation rate of the soil burial test. The weight loss was calculated using the

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following equation, where (Mi) is the initial sample mass and (Mf) is the final sample mass after drying.

3. RESULTS AND DISCUSSION In this work, highly flame retardant and biodegradable composites were prepared from ST (as TPS), APP, and FF. Unlike other general melt processes such as extrusion, injection or batch mixer rheometer, the TPS was prepared by a physical mechano-ball milling process as described in the experimental section. After the ST, along with glycerol and water, was subjected to ball milling, it was left undisturbed overnight in an air-tight polyethylene bag to enhance flow properties. The plasticization of starch occurs, in which water gelatinizes starch and glycerin improves process-ability and reduces embrittlement by inhibiting the retrogradation process (Forssell, Mikkila, Moates, & Parker, 1997). Ultimately, TPS is obtained. This approach is a relatively inexpensive, easy and convenient method. In addition, TPS can be stored after preparation and used at any time. This is the most advantageous feature of the prepared TPS in our method. The prepared TPS was confirmed by FTIR and TGA studies. 3.1. Analysis of TPS An FTIR analysis was conducted to examine the formation of TPS. Fig. 2(A) shows the spectra of starch, glycerol, and TPS. From the FTIR analysis, it was observed that the peaks obtained for all samples were similar to those reported in the literature. The FTIR peaks of corn starch were at 3425 cm-1 (O-H bands), 2923 cm-1 (C-H stretching vibrations of aliphatic groups), 1640 cm-1 (absorbed water), 1152 cm-1 (C-C and C-O stretching) and 1008 cm-1 (C-O-H bending vibration) (Heartwin, Pratik Bhandari, Milford, 2010). Glycerol showed strong bands at 916 (C-C vibrations), 988 (C-C vibrations), 1018, 1103 (C-O stretching of glycerol structure), and 3270 cm1

(C-H and O-H stretching) (Madhumita, Rout, Mohanty, & Nayak, 2013). However, TPS

exhibited an FTIR spectrum representing a combined pattern of ST and glycerol. The interaction of glycerol with ST can be concluded by comparing the O-H stretching spectral peak of ST and glycerol. The O-H stretching peak observed for ST at 3425 cm-1 was shifted to 3281 cm-1. The

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shift in low frequency in the case of TPS is indicative of the interaction of glycerol with ST through hydrogen bonds (Ma, Chang, Yu & Stumborg, 2009). From the TPS spectra, it is evident that glycerol could interact with ST through molecular interactions (Maria Guadalupe Lomeli-Ramirez, Satyanarayana, Ricardo Manriquez-Gonzalez, SetsuoIwakiri, Graciela Bolzon de Muniz, & Thais Sydenstricker Flores-Sahagun, 2014). Hence, glycerol acted as a plasticizer in TPS production (Jansson, & Thuvander, 2004). Similar observations for plasticized starch have been documented in the literature (Lomelí-Ramírez, Barrios-Guzmán, García-Enriquez, Rivera-Prado, & Manríquez-González, 2014). The plasticized nature of TPS was additionally revealed from DSC and is reported on in the Supporting Information, SI (Fig. S1). The pure starch exhibited the glass transition temperature (Tg) at 227 oC TPS exhibited it at approximately 42 oC. TPS exhibits flow properties and behaves as a thermoplastic material at its transition phase due to its higher Tg. The flow properties (plasticity) of TPS are due to hydrogen bonding of starch with low molecular weight molecules, water and glycerol (Päivi Myllärinen, Riitta Partanen, Jukka Seppälä, Pirkko Forssell, 2002). 3.2. Analysis of the composites Prepared TPS powder, FF, and APP were utilized for preparation of the composites. The formation of composites was confirmed using FTIR and FESEM-EDS techniques. 3.2.1. FTIR analysis FTIR was performed to identify the bonding interaction of the individual components of the composite. Fig. 2(B, C & D) shows the FTIR spectra of FF, TF, APP and TFA composites. The FF showed peaks at 3323 cm-1 (hydroxyl group of cellulose (O-H)); 1721 cm-1 ((C=O) acetyl ester of hemicellulose and carbonyl aldehyde of lignin); 2913 and 2857 cm-1 ((C-H) methyl groups); 1028, 1009, and 908 cm-1 ((C-O) glucopyranose rings), and 1604 cm-1 (water) (GardeaHernandez, Ibarra-Gomez, Flores-Gallardo, Hernandez-Escobar, Perez-Romo, & ZaragozaContreras, 2008). However, the TF composite showed similar FTIR spectra to those of TPS; most of the characteristic peaks of lignocellulosic FF appeared at almost the same position as those of ST but the O-H peak (hydrogen bonded hydroxyl) was observed to broaden and was slightly shifted to 3264 cm -1 in TF composites (3289 cm-1 in TPS), which confirms the intermolecular 9

hydrogen bonding between TPS and FF (Amnuay Wattanakornsiri & Sampan Tongnunui, 2014). Further, the typical FTIR peaks of APP were observed at 3200 ((N-H)) and 1430 cm-1 ((N-H)) of NH4+ and at 1260 ((P=O)), 1077 (P-O-P) and 892 cm-1 (P-OH) (Kaewtatip, & Thongmee, 2012). For the TFA composite, the combination of peaks of ST, FF and APP were clearly observed. Strong absorption peaks at 1232 and 862 cm-1 refer to the stretching vibration of P=O and P-O-P of APP; the vibration peak at 3278 cm-1 refers to the N-H group. The hydroxyl groups of ST have been partially coated with phosphate groups by phosphorylation from phosphoric acid forming phosphorus ester (intensive peak at 789 cm-1) (Liodakis, Fetsis, & Agiovlasitis, 2009). In addition, the absorption peak in Fig. 2(C&D), at 1232 cm-1 (P=O) rises systematically by slightly suppressing the intensity of C-O-C at 1008 cm-1 with increasing wt% of APP (Carosio, Fontaine, Alongi, & Bourbigot, 2015). Based on the FTIR results, it was evident that all the peaks of the individual components (TPS, FF, and APP) are observed in the TFA composite, indicating the composite nature. This composite nature was additionally supported by FESEM-EDS data. 3.2.2. FESEM-EDS analysis The FESEM and EDS spectra of TF and TFA are shown in Fig. 3. In the case of surface FESEM images, TF exhibited a smooth surface morphology overall, indicating that all the plasticised starch molecules (TPS powder) melted completely at the compression moulding temperature (160 oC) even with enforcement of FF. However, different/dense surface morphology was observed for TFA composites due to APP. No significant distinguishable differences as a function of APP concentration were noticed. Hence, irrespective of APP concentration, all the TFA composites (TFA-3/6/9) showed similar surface morphologies. The APP micro-pa1rticles (indicated with arrows) clearly visible over the surface and appeared to be covered by TPS matrix. (Seyed Fakhreddin Hosseini, Masoud Rezaei, Mojgan Zandi, & Farhid Farahmandghavi, 2015; Aruna Subasinghe, & Debes Bhattacharyya, 2014). In the case of the fracture FESEM images, as expected, all the TFA composites revealed the reinforced FF fibre in their matrix. However, a clear difference in the appearance can be noticed in both TF and TFA. The fractured surfaces of the TF composite appeared even whereas the fractured surfaces of the TFA composite appeared coarse due to APP particles. FF (fibre) fracture 10

failure was more predominant than fibre-matrix interfacial failure due to improved interfacial bonding as explained in the FTIR analysis and elongation behaviour explained in Table1. Hence, the fracture of fibres was noticed without fibre pull-out. This is reliable evidence of the strong interfacial interaction between FF and TPS in TF composites (Teixeira, Curvelo, Corrêa, Marconcini, Glenn, Mattoso, 2012). The EDS spectra of the composites were as expected. The peaks of C (carbon) and O (oxygen) for the TF composite were expected to originate from the individual components’ elemental composition, i.e., starch and FF. In the case of TFA composites, peaks of P (phosphorus) and N (nitrogen) were obtained in addition to C (carbon) and O (oxygen). The appearance of a peak corresponding to P is indicative of the presence of uniformly distributed APP in the TPS matrix. Overall, through FTIR and FESEM-EDS it was possible to conclude that composites were successfully formed from TPS, APP and FF. 3.3. Evaluation of composite properties The main goal of this investigation is to develop flame-retardant bio-composites with improved strength. To evaluate these properties, mechanical, horizontal burning, and biodegradable tests were performed. 3.3.1. Mechanical properties The effect of reinforced FF and influence of APP filler on the mechanical properties of the TPS composites were investigated using a tensile test. Table 1 shows the results of mechanical properties (tensile strength and modulus) tests of the processed TPS, FF, TF and TFA composites. From the table, it is evident that the order of tensile strength was: FF (44.4 MPa)>TFA-6 (18.19)>TFA-3(17.7)>TFA-9(16.89)>TF(16.45)> processed TPS (2.92 MPa). The processed TPS had the lowest tensile strength and the FF had the highest tensile strength. It was concluded that the FF reinforcement increased the tensile properties. This might be due to the strong interfacial bonding that developed between FF and the matrix (Yokesahachart & Yoksan 2011). Existence of the bonding interaction was supported by FTIR and FESEM images. The incorporation of APP further enhanced the tensile properties of the composites. All the APP-loaded composites showed 11

increased tensile strength from that of the TF composite. This might be due to good compatibility of APP with TPS (evidenced from both the surface and fracture FESEM images) (Bootklad, & Kaewtatip, 2015). Among APP-filled composites, the observed trend was: TFA-6 (18.19)> TFA3 (17.7)> TFA-9 (16.89). Contrary to general expectation, TFA9 had the lowest tensile strength of the APP composites. Because the distribution of higher concentration of APP may cause possible agglomeration, which may affect the APP bonding with the other constituents of the composites. Hence, under high filler loading hindering the dispersion and thereby reducing homogeneity (Aruna Subasinghe, & Debes Bhattacharyya, 2014). Similarly, the order of the tensile modulus was: TF (0.6)>TFA-9 (0.59)>TFA-6 (0.55)>TFA-3 (0.52)>FF (0.32)> processed TPS (0.16). The processed TPS had the lowest tensile modulus and the TF had the highest tensile modulus. The FF reinforcement increased the tensile modulus because FF possesses a high modulus (0.32 GPa) due to its high percentage of crystalline molecular orientation cellulose (70%) (MReza Foruzanmehr, Pascal Y. Vuillaume, Saïd Elkoun, Mathieu Robert, 2016) and strong interfacial interaction with the TPS matrix as explained in the FESEM and FTIR studies. It was observed that the incorporation of APP increased the modulus properties. This might be due to the rigid APP particles or attributed to the increase in the stiffness of ductile TPS composites. Among APP-filled composites, the observed trend was: TFA-9 (0.59)>TFA-6 (0.55)>TFA-3 (0.52). Overall, it can be concluded from the tensile test data that the TFA composites have high tensile properties that are comparable to some synthetic polymers (polyethylene, HDPE) (Carlos, Aryane, Iseli, & Derval, 2015). Therefore, these composites can replace conventional polymers in various applications.

3.3.2. Biodegradation Biodegradation is an important characteristic property by which a material can be classified as biodegradable. The biodegradation of TPS, TF composites, and TFA composites was measured as a function of weight loss over time (weeks) and is reported in (SI: Table. S2).

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Starch, a carbohydrate, performs the role of food source for microbes to attack and degrade (Knud Erik Bach Knudsen, 2015). Hence, biodegradation was observed in all specimens. From Table 3, it is clear that the biodegradation followed the order: TPS>TFA9>TFA6>TFA3>TF. It is well known that biodegradation depends on the effective action of microbes over the substrate. Moisture is a factor that greatly facilitates the growth and effective action of microbes (Dongling Qiao, Hongsheng Liu, Long Yu, Xianyang Bao, George P. Simon, Eustathios Petinakis, & Ling Chen, 2016). The hydrophilic groups of starch facilitate the absorption of moisture in their vicinity and offer microbes to act. Thus, TPS was degraded at a faster rate. In the case of TF, though the hydrophilic groups of FF (cellulose) facilitate moister absorption, the insolubility and poor swellability (absorption ability just 7%) of crystalline cellulose (70%) resist the relative microbial action, resulting in a lower degradation rate than that of TPS. These results were in agreement with those reported by Ibrahim et al. (Hamdy Ibrahima, Mahmoud Faragb, Hassan Megaheda, & Sherif Mehanny, 2014). Speculation might be appropriate because the rate of TF degradation was almost same as that of TPS for the first week and then decreased. This is possibly due to microbe attack on the surface of the composite, comprised of TPS, rather than the internal FF. However, TFA composites showed a degradation trend between those of TPS and TF. The lower degradation of TFA than that of TPS might be due to the effect of APP incorporation, which restricted the microbes’ attack of the starch (Xiaofei, M., Jiugao, Y., & John F. Kennedy, 2005). However, the degradation tended to increase with proportionate increase in APP concentration (TFA9>TFA6>TFA3). This trend was contrary to our general expectation. The observed phenomenon was attributed to leaching of APP, which leads to weight loss of the composites (Katalin Bocz, Beáta Szolnoki, Maria W£Adyka-Przybylak, Krzysztof Bujnowicz, György Harakály, Brigitta Bodzay, Emese Zimonyi, Andrea Toldy, & György Marosi, 2013). Thus, composites with quantitatively higher amounts of APP showed more weight loss than their counterparts, and vice-versa. The phenomenon of leaching of APP is well documented in the literature (Riyuan, Guangfu, Weilei, Wenxiang, Liang, Zhaoping, Xinzhu, & Jianqiu, 2015). 3.3.3. Thermal and oxidative thermal analysis The aim of thermogravimetric analysis (TGA) is to obtain fundamental information on the feasible range of temperature up to which the composites can be used safely. Hence, the thermal

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properties were studied by subjecting the composites under both inert (nitrogen) and oxidative (air) atmospheres. The TG and DTG curves of TPS, TF, and TFA-3/6/9 composites are shown in Fig. 4 (A and A1) and their Tonset and Tmax are tabulated in Table 2. First, thermal degradation was evaluated in a nitrogen environment (no oxidation). The thermal degradation of TF proceeds in two steps of weight loss between 270- 400 °C that are the result of competitive decomposition of both starch and FF to volatile products (mainly levoglucosan, furan, and furan derivatives) to give a thermally stable aromatic char. The thermal degradation resulted in a final residue of 8.1% for TF. TFA-3/6/9 also exhibited a similar pattern of two-step degradation. The overlap of their curves indicates their similar behaviour. The degradation curves were typically observed at lower temperatures than that of TF. The presence of the APP is liable for a strong anticipation in the typical degradation process (see Tonset and Tmax values in Table 2). APP releases acidic phosphoric acid and favours the decomposition of TFA at lower temperatures (Iskender Ozsoy, Askin Demirkol, Adhullah Mimaroglu, Huseyin Unal, & Zafer Demir, 2015). It is considered a positive phenomenon because the released phosphoric acid reacts with the possible hydroxyl groups of starch/FF and forms esters, thereby resulting in formation of thermally insulating char for TFA (Camino, Luda, & Costa, 1995). As a consequence of this effect, with the increase of APP concentration, a proportional increase in the final residue was noted for TFA composites (Table 2). Similar observations were noted by Carosio et al. (Carosio, Fontaine, Alongi, & Bourbigot, 2015). The thermo-oxidative stability of the composites was estimated by TGA and DTG analyses in air. As can be observed from Fig. 4 (B and B1), the thermal oxidation of TF that occurs in two steps in the 270 to 400 °C range is due to the decomposition of starch and FF to both volatiles and aliphatic char (16 % at 400 oC) (Tao Zhang, Hongqiang Yan, LieShen, Zhengping Fang, Xianming Zhang, Jiajun Wang, & Baoyue Zhang, 2014). This char is then oxidized to CO and CO2 during the second step (450-550 oC) leaving almost no residue (Jingjing, QianRen, Wenge, & Wentao, 2014). As already observed in TGA in nitrogen, TFA composites displayed an anticipation in the degradation due to the presence of APP. The effects of APP for the build-up of an insulating char

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were observed and are evident from the DTG curves, which were typically observed at lower temperatures than TF. In addition, the Tonset and Tmax of TFA were strongly supported. 3.3.4. Evolution of fire retardant properties (a) Horizontal burning test The ability to withstand burning during acute fire accidents is a characteristic typical of material classified as flame retardant material. To evaluate the flame retardancy properties of the composites, a horizontal burning test was performed and the burning time and rate results are shown in Table 2. It is clear from the table that the burning time and the rate of burning followed the order: TFA composites> TF> TPS. TPS exhibited a low burning time and high burning rate due to its flammable characteristics (Francys, Daniel, Gregory, Jose, & Luiz, 2013). However, TF showed a slight improvement in the flame retardant properties due to incorporation of lignocellulosic FF which influenced the higher mass residue instead of the thermoplastic starch matrix (Yumi Ohta, Yukishige Kondo, Kazuo Kawada, Toshio Teranaka & Norio Yoshino, 2008). Interesting results were observed in the case of TFA composites as none of the composites were burnt as shown the inserted narrow digital pictures in Fig.5 (horizontal burning test specimens before and after the test). An effective char at the point of contact of flame was noted which can prevent the heat transfer and flame spread during combustion. Similar results were reported by Bocz et al. (Katalin Bocz, Beáta Szolnoki, Maria WLAdyka-Przybylak, Krzysztof Bujnowicz, György Harakály, Brigitta Bodzay, Emese Zimonyi, Andrea Toldy, & György Marosi, 2013). It was speculated that the APP has both a phosphorous intumescent acid source (phosphoric acid) and blowing source (NH3), which help form large amounts of thermally stable carbonaceous residue (char), creating an inert atmosphere and thereby preventing the catch of fire and imparting flame-retardant properties to the composites (Reti, Casetta, Duquesne, Bourbigot, & Delobel, 2008). (b) LOI The flame retardancy of TPS composites further evaluated using the LOI for sustainability with flame in the presence of oxygen and the results are shown in Table 2. TPS majorly composed of starch had an LOI of 21%. However, TF composed of both starch and cellulose (basic 15

component of FF) had almost the same LOI (22%). This is possibly due to the similar chemical constitutions of TF and TPS (Battegazzore, Bocchini, & Frache, 2016). However, TFA composites exhibited higher LOI values than both TPS and TF, indicating their superior flame-retardant properties. Undoubtedly, the enhanced LOI values were due to the incorporation of APP. For all of the TFA composites, LOI analysis was initially performed in the living environmental conditions (i.e., 21% oxygen). However, due to the super flame retardancy of the developed TFA composites, the samples could not burn and remained stable (LOI, photos). Thus, LOI was performed with an increased oxygen level. Irrespective of the APP percentage (3/6/9 %), the minimum level of oxygen that was able to initiate burning was 41%. This is much higher than the oxygen levels in living environments. Based on the LOI test, it was concluded that the TFA could effectively function as fireretardant composites. These composites can be safely used even when oxygen levels are as high as approximately 40%. 3.3.4.1. Mechanism The mechanism of TFA with flame is highly critical. Based on the available literature and by correlating with the experimental TGA, FT-IR and FESEM data of char residue, a plausible mechanism was considered and is shown in Scheme 1. The mechanism of fire retardancy of TFA composites was assumed due to the formation of stable crosslinked P-O- P and P-O- C bonds, and is speculated to occur in the following steps: Step 1: In the initial stages of burning, the unstable structural groups of TFA decompose to ammonia, water and phosphoric acid. The release of ammonia and water has been confirmed by many authors (Zhaolu, Dinghua, & Rongjie, Yang, 2016; Liu, Chen, & Yu, 2010). In addition, ammonia-characteristic TGA weight loss was observed at >260 oC and the FT-IR spectra at approximately 3250 cm-1 that refer to the decrease in the N-H bond of NH4+ due to release of ammonia in our experiments is strong evidence for this step (Fig. 5). Step 2: Later, the esterification reaction occurs between the phosphoric acid generated in step 1 with TPS/FF through dehydration, resulting in formation of phosphate ester and evaluation of

16

water. The formation of ester in step 2 has been confirmed by many authors (Camino, Costa, & Trossarelli, 1984; Song, Fang, Tong, & Xu, 2009). Step 3: This ester might react with starch or itself, which might subsequently crosslink to a threedimensional network structure of P-O-P and P-O-C bonds. Due to these bonds, the char of TFA remained continuous, compact and coherent, as evidenced from FESEM images of horizontal test samples (Fig. 4). The existence of crosslinked P-O-P and P-O-C bonds was confirmed by FTIR spectrum, which showed that the burnt composites had different FTIR spectra (SI: Fig. S3) than that of unburnt TFA composite, with stretching vibrations at 800 and 1000 cm-1 corresponding to P-O-P and P-O-C bonds, respectively (Cheng-Liang Deng, shuang-Lan du, Jing Zhao, Zhen-Qi Shen, Cong Deng, & Yu-Zhong Wang, 2014). The advantage of this step is that the crosslinked P-O-P and P-O-C bonds are stable and form at the surface of the composite facing the flame exposure, consequently protecting the underlying materials. As a part of this, stabilization is induced to the remaining APP and, thus, more APP would be available for phosphorylation (with FF/TPS) and char formation in the subsequent steps if flame is propagated (Hongjiao, Hong, Bo, Liqiao, & Bingshe, 2011). 4. CONCLUSIONS Biodegradable flame-retardant composites were developed via compression technique utilizing low-cost starch, flax fabric (FF) and ammonium polyphosphate (APP). The starch was modified to plasticized starch by a mechano-ball milling process and composites were developed by reinforcing FF (9%) and incorporating varying percentages of APP (3, 6 and 9%). All the composites exhibited remarkable sustainability to fire and direct flame self-extinguishment as evident from LOI and HBT test results, which were greater than 41 (LOI) and a V-O rating (HBT). The significant flame-retardant properties were due to formation of a high proportion of stable char, approximately 30%-40%, that could prevent the underlying composites from combusting during burning. Mechanical properties indicated that the strength of the TFA composites could be increased from that of TPS with the incorporation of APP. However, the increased trend could be observed up to 6% APP only. TFA composites showed a smaller degradation trend than that of TPS. Further, degradation tended to increase with proportionate increase in APP concentration due to leaching of APP. Overall, the results of this study contribute significant information on the 17

development of safer and environmentally friendly, biodegradable, flame-retardant composites with improved mechanical properties. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Government of the Republic of Korea (Ministry of Science, ICT and Future Planning (MSIP)) (No. 2013R1A2A2A01017108 and 2011-0030058).

18

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Figure 1. The schematic representation of the procedure for preparation of TPS and fabrication of TFA composites.

25

Figure 2. FTIR spectra of TPS composites (A) starch, glycerin & TPS, (B) FF, TF, APP & TF9A, (C) Intensity surface absorption spectra and (D) 3d projection of restricted IR region of TFA composites.

26

Figure 3. Top view FESEM images (first column: upper Surfaces; second column: tensile fracture surfaces) and EDX spectra (third column) of APP loaded TF composites: (A) TF, (B) TF-3A, (C) TF-6A and (D) TF-9A.

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Figure 4. Thermogravimetric and derivative weight curves under N2 (A&A1) and air (B&B1) of TF, TF-3A, TF-6A and TF-9A composites at a heating rate of 10 oC.

28

Figure 5. Top view FESEM images (first column: upper Surfaces before burning; second column: upper surfaces of after burning (horizontal burning test)) and EDX spectra (third column) of APP loaded TF composites: (a) TF (b) TF-3A, (c) TF-6A and (d) TF-9A.

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Scheme 1. Plausible flame retardant mechanism of APP in TFA composite system.

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Table 1. Results of Tensile properties of TPS composites. Composition

(MPa)

Tensile modulus (GPa)

Elongation at break (%)

TPS TF TF-3A TF-6A TF-9A

2.92 16.45 17.7 18.19 16.89

0.16 0.60 0.52 0.55 0.59

11.406 5.586 5.957 6.148 6.495

31

Table 2. TGA, HBT and LOI analysis data of TPS composites.

Thermogravimetric Data Tonset Composition

10% 0

[ C]

Tmax 75% [0C]

Residue at 800 0C [%]

T onset 0 10% [ C]

Nitrogen

Horizontal flammability Data

Tmax 75% [0C]

Residue at 400 0C [%]

Burning time (Sec)

Air

Burning rate (mm/min)

After glow

LOI

LOI (%)

Air

Ratin g

O2+N2

TPS

235

318

7.7

264

323

16.14

328

0.228

Yes

21

-

TF

224

326

8.1

244

331

16.28

425

0.176

Yes

22.3

-

TF-3A

210

514

25.68

230

471

36.27

-

-

No

41

V-0

TF-6A

221

762

29.45

224

501

38.31

-

-

No

45.5

V-0

TF-9A

218

800

31.62

228

530

41.79

-

-

No

47

V-0

32