‘Green’ composites using cross-linked soy flour and flax yarns

Abstract

Environment-friendly, fully biodegradable, ‘green’ composites based on plant based fibers and resins are increasingly being developed for various applications as replacements for non-degradable materials derived from petroleum that are currently being used. Unlike petroleum, plant based proteins, starches and fibers are yearly renewable. In addition, these green composites may be easily composted after their life, completing nature's carbon cycle. In this study, soy flour (SF) was modified by cross-linking it with glutaraldehyde (GA). The cross-linked soy flour (CSF) polymer was characterized for its tensile and thermal properties. The effect of glycerol on the mechanical properties of the soy flour was characterized and optimized. CSF polymer showed improved tensile properties and thermal stability, compared to unmodified SF resin, for use as a resin to fabricate composites. Unidirectional green composites using flax yarn and CSF resin were fabricated and characterized for their tensile and flexural properties. The composite specimens exhibited fracture stress and Young's modulus of 259.5 MPa and 3.71 GPa, respectively, and flexural strength of 174 MPa, in the longitudinal direction. These properties seem to be sufficient for considering these green composites for indoor structural applications.

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Introduction

Most of the conventional plastics used today are derived from petroleum. At the current consumption rate, the worldwide petroleum reserves have been estimated to last only for the next 50 years or less.¹ More than 60 billion pounds of plastic waste, mostly petroleum based, is discarded each year in the United States of America alone.¹ Most of this waste ends up in landfills. Biodegradable polymers produced from renewable resources such as plants, animals and microbes through biochemical reactions offer a convenient and environment friendly solution to the problem of plastic waste. Various biodegradable polymers such as starch, wheat gluten, zein, polyhydroxybutyrate-co-valerate (PHBV), whey protein, soy protein etc. have been explored for applications in composites.^2–7 Though wheat gluten and zein have been reported to form high strength films,^5,6 they are more expensive than soy protein.⁸ Soy protein offers several advantages over synthetic polymers.⁹ First, the soy protein purification process is benign and environment friendly. Second, these proteins can form ductile and viscous polymers that can be used as resins. Third, the cost of raw materials is low. Fourth, soybean is an annual crop and is abundantly available worldwide. In the 1930s, Henry Ford pioneered the use of soy protein for plastics and fibers.¹⁰ Ford used soy plastic for various car parts such as gear shift knobs, horn buttons and window frames in an effort to produce an “all agricultural” car.¹⁰ However, after the outbreak of World War II, most efforts and materials were reallocated to the war and research in soy plastics was abandoned. Also, the plastic molding technology of today was not available at that time. At present, because of the increased environmental awareness and stringent environmental laws, most manufacturers are looking towards ‘greener’ and more environment friendly alternatives for conventional polymers and composites.¹¹

Soybean products are available commercially as defatted soy flour (SF), soy protein isolate (SPI) and soy protein concentrate (SPC). Chemically, SPI contains 90% protein and 4% carbohydrates, SPC contains 70% protein and 18% carbohydrates, while SF, which requires less purification, contains about 55% protein and 32% carbohydrate and is the least expensive variety. Soy protein is globular, reactive and often water soluble, as compared to helical or planar, non-reactive and water resistant synthetic polymers.¹² Soy protein consists of various polar and reactive amino acids such as cystine, arginine, lysine, hystidine, etc. which can be used for cross-linking it and improving the tensile and thermal properties. In this research, glutaraldehyde (GA) has been used as a crosslinking agent to modify SF. Several researchers have shown that GA reacts with the amine groups in protein, particularly in an alkaline pH, to form intermolecular cross-links.^13–15 Although GA can readily react with the amine groups, there has been no consensus on the reaction of proteins with GA. Different mechanisms have been proposed for explaining the reaction between GA and proteins. Blass et al.¹⁵ have shown that monomeric GA binds irreversibly to proteins. Habeeb and Hiramoto¹⁴ reported that GA reacted extensively with the α-amino group of glycine and the α- and ε-amino groups of lysine, while it only partially reacted with the α-amino groups of histidine and tyrosine. They also showed that the cross-linking of GA with proteins produced two species, monomers and aggregates. The monomers were a small component and had a sedimentation coefficient similar to the native protein. The aggregates were the products of intermolecular cross-linking and showed a three-fold increase in the sedimentation coefficient as compared to the native protein. Richard and Knowles¹³ suggested that GA reacts with protein through an aldol condensation followed by a cross-linking reaction. They proposed that the smallest cross-link that can be formed between the protein molecule and GA would have five carbon atoms in a chain between two nitrogen atoms (from amine groups) similar to a double Schiff base formation.

Chabba and Netravali¹⁶ cross-linked SPC using GA to form a suitable resin system. As a result of the cross-linking, the strength of the SPC resin increased significantly while the moisture absorption decreased. A typical structure of soy protein cross-linked with GA is shown in Fig. 1.¹⁶ It was, however, difficult to assess the average degree of cross-linking because of the complexity of the chemistry. The cross-linking was judged based on the improved tensile strength and decreased fracture strain and moisture absorption after the GA modification as well as the significant increase in the viscosity immediately following the addition of GA to the SPC solution. Chabba and Netravali¹⁷ also blended the cross-linked SPC with poly(vinyl alcohol) to improve its toughness. Other modifications of soy proteins have also been tried. Lodha and Netravali¹⁸ used stearic acid to modify SPI. The mechanical and thermal properties of the resin were shown to improve with the stearic acid content. One of the main reasons for this improvement was the hydrophobic nature of the stearic acid which made the SPI resin less moisture absorbent.¹⁸ In another modification, Lodha and Netravali¹⁹ used Phytagel^® to modify SPI and form an interpenetrating network (IPN) like structure. Because of the cross-linked IPN like structure the strength of the resin increased almost ten-fold to 60 MPa and the modulus increased nine-fold to about 900 MPa. Tummala et al.²⁰ have modified SF with polyester amide to fabricate composites using hemp fibers.

Fig. 1 Schematic of the glutaraldehyde (GA) cross-linked soy flour structure.

Natural ‘lignocellulosic’ (plant based) fibers, e.g. ramie, flax, hemp, kenaf, bamboo, etc., are increasingly being used as reinforcement in both biodegradable and non-biodegradable polymers, to make them ‘greener’.^11,21,22 Natural fibers offer several advantages over synthetic fibers, e.g. glass, graphite and aramids, in terms of low cost, low density, low energy consumption and no skin irritation.^23–26 Also, they are CO₂ neutral, biodegradable, compostable, and yearly renewable. In addition, bast fibers, derived from the stems of plants, also show good thermal as well as acoustic insulation properties because of their hollow nature.^23–26

Mohanty et al.²² fabricated biodegradable composites using twisted jute yarn and Biopol^TM and showed a 150% increase in the tensile and impact strength of composites as compared to pure resin. Chabba and Netravali^16,17 fabricated unidirectional flax yarn as well as flax fabric reinforced green composites using GA cross-linked SPC as well as PVA blended resins. Both composites showed good tensile and flexural properties. Lodha and Netravali²⁷ used stearic acid modified SPI and ramie fibers to fabricate unidirectional green composites. Nam²⁸ also used ramie fibers to reinforce SPC. Both these efforts yielded composites with excellent mechanical properties. In the present research, spun flax yarns were used for fabricating composites. Use of twisted yarns in composites allows the fabrication of continuous length unidirectional composites (composite structures in which the fibers/yarns are aligned in one preferred direction within the polymer matrix/resin). In such composites, the maximum strength is observed in the longitudinal direction of the yarns.²⁹ The parallel laying of yarns (or fibers) is easy if the yarns (or fibers) are not short length and are continuous. Since flax fibers come in short lengths, 1 m or less, the only way to have them in continuous form is to spin (twist) them into yarns. Spun yarns also have some fiber ends protruding out, commonly referred to as yarn hairiness. In composites, hairiness may lead to better mechanical interlocking between fibers and the resin as the protruding fibers interlock the yarns or get deeply embedded into the resin, thus improving the fiber/resin interfacial bonding. Another advantage of using twisted yarns in composite manufacturing is the increased surface roughness of yarns compared to fibers, which further increases the fiber/resin interfacial strength due to increased mechanical interlocking.

In the present research, fully biodegradable, sustainable and environment friendly ‘green’ composites have been fabricated using cross-linked soy flour (CSF) resin and flax yarns. The tensile and flexural properties of the CSF resin and composites have been characterized. The tensile and flexural properties of the green composites have been compared with glass reinforced poly(propylene) composites.

Experimental

Materials

Soy flour was provided by Archer Daniels Midland Company, IL. Analytical grade glycerol and sodium hydroxide were obtained from Fisher Scientific, PA. Glutaraldehyde (GA), 25 wt% solution in water was obtained from Aldrich Chemical Company, Milwaukee, WI. All chemicals were used as received, without any further treatment. Flax yarn, in bleached form, was provided by Sachdeva Fabrics Pvt. Ltd., New Delhi, India.

Processing

To process SF powder into a suitable resin for fabricating ‘green’ composites, it was mixed with distilled and deionized water in a beaker in 1 ∶ 9 ratio (by weight) and the desired amount of glycerol was added as a plasticizer. Without the plasticizer, SF was found to be brittle, weak and difficult to process into useful films. To study the effect of plasticizer (glycerol) content on the tensile properties of resin, different glycerol contents, 0%, 5%, 10% and 15% (w/w of SF) were used. The SF solution was homogenized using a magnetic stirrer for 15 minutes and then the pH of the mixture was adjusted to 11 ± 0.1, using 1N NaOH solution. The SF solution was again stirred for 15 minutes and then the beaker was transferred to a water bath maintained at 70 °C. The solution was ‘pre-cured’ by stirring for 27 minutes in a water bath at 70 °C, after which GA was added to the mixture and stirred for 3 more minutes at the same temperature. The final concentration of GA solution was 40% on an SF weight basis. This stage is referred to as pre-cured resin. With the fast cross-linking reaction, the viscosity of the mixture increased rapidly. The pre-cured solution was cast on Teflon^® coated glass plates and dried at room temperature for 48 hours to obtain the resin sheets. Finally, the dried soy flour sheets were cured by hot pressing in a Carver hydraulic hot press, model 3891-4PROA00, at 120 °C for 25 minutes under a force of 178 kN. The cured, GA cross-linked SF polymer sheets were conditioned at standard ASTM atmosphere (65% r.h. and 21 °C) for 3 days before characterization of various properties. Based on the results, SF was cross-linked with 40% GA solution and 0% glycerol for composite preparation. This composition is henceforth referred to as CSF resin.

Analysis

Tensile properties of the cross-linked SF polymer sheets were characterized in accordance with ASTM D 882-97. Conditioned polymer sheets were cut into rectangular specimens of 110 mm × 20 mm dimensions. Three thickness measurements were carried out along the length of each specimen, and the average of these values was used for calculating the fracture stress and Young's modulus. The tests were performed on an Instron tensile tester, model 1122, at a strain rate of 1 min⁻¹ and a gauge length of 50 mm. Thermo-gravimetric analysis of conditioned CSF polymer sheets was carried out using TA Instruments, Thermo-Gravimetric Analyzer (TGA), model 2050. The specimens were scanned in a nitrogen atmosphere from 25 °C to 400 °C at a ramp rate of 10 °C min⁻¹.

Flax yarn was characterized for its tensile properties such as Young's modulus and fracture stress and strain according to ASTM D 2256-97. Yarn specimens were conditioned at standard ASTM atmosphere for 5 days, prior to characterizing their properties. Individual yarn specimens were tested on an Instron testing machine, model 1122. A 100 mm length of yarn was used to determine the yarn diameter using an optical microscope. The same specimens were used for measuring tensile properties. Tests were performed using a gauge length of 50 mm and at a strain rate of 1 min⁻¹. Flax yarn linear density (mass per unit length, also referred to as yarn count) was measured according to ASTM D 1059-97.

Unidirectional composites were prepared using flax yarn and CSF resin. To prepare composites, flax yarn was wrapped tightly around a rectangular metal frame to achieve sufficient stress needed to maintain parallel alignment of the yarn strands. The entire frame was immersed in the pre-cured CSF resin for 30 minutes to allow the resin to impregnate the flax yarns as completely as possible. The pre-cured resin had the consistency of a thick paste. The frame was then taken out and specimens were oven dried at 35 °C for approximately 24 hours. The dried (uncured) composite specimens were removed from the frame and hot pressed (cured) and conditioned following the same procedure used for the CSF resin. The cured composites had approximately 60% yarn content, by weight.

The composite specimens were characterized for their tensile and flexural properties in the longitudinal direction in accordance with ASTM D 3039/D 3039M-00 and ASTM D 790-99, respectively. Tensile test specimens had a width of 10 mm and were tested at a gauge length of 20 mm. The flexural test specimens had a width of 10 mm and the tests were carried out using a span length of 25 mm. Tensile and flexural tests were performed at a crosshead speed of 20 mm min⁻¹ (strain rate of 1 min⁻¹) and 2 mm min⁻¹, respectively.

Results and discussion

Fig. 2 shows the effect of glycerol content on the tensile properties of cross-linked SF resin containing 40% GA (10% solid GA content). As expected, increasing glycerol content from 0% to 15% increased the fracture strain from 3.6% to 31.7%. It also reduced the fracture stress and from 12.6 MPa to 6.7 MPa and Young's modulus from 525.6 MPa to 101.7 MPa. These results indicate that glycerol acts as a good plasticizer for CSF, leading to reduced brittleness and increased plasticity. Glycerol has been reported to increase the flexibility and extensibility of soy protein plastics by reducing the interaction between protein molecules.^{3,16–19,30,31} Based on the tensile properties, 40% GA solution and 0% glycerol (CSF) was selected as the optimum blend concentration for use in composites. Although brittle, the CSF resin fracture strain was comparable to the flax yarn fracture strain, while the modulus and strength values were significantly higher, hence better tensile and flexural properties of the composites could be expected. In addition, glycerol has been shown to slightly reduce the fiber–resin interfacial bonding by acting as a lubricant between the fiber and the resin, thus affecting the composite properties.³² Without the cross-linking of SF with GA, it was difficult to fabricate composites with no glycerol.

Fig. 2 Effect of glycerol on the tensile properties of modified soy flour resin.

Fig. 3 shows the typical TGA thermograms for the SF resin containing 0% glycerol and CSF resin, in a nitrogen atmosphere. The TGA thermogram for the CSF resin consistently shows less weight loss at all temperatures compared to SF resin, indicating a higher thermal stability than SF resin. The improved thermal stability of CSF resin is attributed to the cross-linking between GA and SF.^16,17 Also, it is clear from the plot that the CSF resin is very stable up to 120 °C, the processing temperature for yarn-reinforced composites. The weight loss up to 120 °C can be attributed mostly to the loss of moisture from the specimen.

Fig. 3 TGA thermograms for SF and CSF resins.

The average linear density of the flax yarn was found to be 13.91 Ne (English count) or 382 denier. Tensile properties such as fracture stress, Young's modulus and fracture strain of flax yarn are summarized in Table 1. As can be seen from these data, there is a large variability in the fracture stress and Young's modulus values, which can be attributed to the variability in the flax fiber yarns and the variation of yarn diameter along its length. Most plant based fibers and their yarns show such high variation compared to synthetic fibers which tend to be much more uniform. The yarns used in this study were also brittle because of the high twist level.

Table 1 Comparison of tensile properties of flax yarn and composites

	Fracture stress/MPa	Young's modulus/GPa	Fracture strain (%)
a Figures in parentheses are CV%.
Flax yarn	360.2 (20.3)^a	4.8 (16.6)	13.0 (16.2)
Flax yarn reinforced CSF composites in longitudinal direction	259.5 (10.8)^a	3.7 (23.7)	10.7 (5.6)

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Table 1 also summarizes the tensile properties of unidirectional yarn composites in the longitudinal direction. Fig. 4 shows a typical load vs. displacement plot for the tensile test of flax yarn reinforced CSF composites in the longitudinal direction. As can be seen from Fig. 4, the composite specimens attain their highest load and then fail catastrophically. In this case, the flax yarn reinforced composites increased the tensile strength by over 2000% (259.5 MPa) as compared to the CSF resin (12.6 MPa). The Young's modulus increased by over 700% to 3.71 GPa compared to 525.6 MPa for the resin. The strength of the flax yarn and good bonding between the flax yarns and CSF resin contribute to the high tensile properties. As mentioned earlier, about 32% of the SF is carbohydrate. This helps improve the fiber–resin interfacial bonding because of the similar chemistry containing polar glucose units. Simple rule of mixture²⁶ was used for predicting the longitudinal fracture stress and Young's modulus for flax yarn reinforced CSF composites. The fracture stress and Young's modulus values for composites calculated using rule of mixture were 221.9 MPa and 2.4 GPa, respectively, while the experimental values for the composite fracture stress and Young's modulus were 259.5 MPa and 3.71 GPa, respectively. These values are within the experimental variation.

Fig. 4 Typical load vs. displacement plot for a tensile test of flax yarn reinforced CSF composites in longitudinal direction.

Chabba and Netravali¹⁷ reported the fracture stress and Young's modulus values of 126.1 MPa and 2.24 GPa, respectively, for flax yarn reinforced cross-linked SPC composites. The fiber weight fraction in that case was around 45% compared to 60% used in this study. They attributed lower tensile values to poor yarn orientation due to relaxation and voids in the composites. Nam and Netravali⁴ have reported a strength of 271 MPa for unidirectional composites made using ramie fibers and SPC resin. Lodha and Netravali²⁷ have reported strengths of 267 MPa for ramie fiber reinforced composites made using stearic acid modified SPI resin. Flax yarn reinforced Phytagel^® modified SPI resin composites were also reported by Lodha and Netravali.¹⁹ In this case the composite strength was around 220 MPa. The results obtained in this study are within the range of properties reported by others. However, the CSF resin should be less expensive compared to modified SPI and SPC resins. These strengths are lower than the pre-stressed glass–polypropylene (PP) composites which had tensile strength and Young's modulus values of 774 MPa and 27 GPa, respectively.³³ However, the glass–PP composites had a density of 1.48 g cm⁻³ compared to about 1.3 g cm⁻³ for the green composites in this study. Also, it should be possible to improve the green composite properties further, using a similar pre-stressing technique.

Table 2 summarizes the flexural properties, at yield, of unidirectional yarn composites in the longitudinal direction. Fig. 5 shows a typical load displacement plot for a flexural test of flax yarn reinforced CSF composites in the longitudinal direction. Chabba and Netravali¹⁷ reported the flexural stress and flexural modulus values of 86.1 MPa and 1.18 GPa, respectively, for flax yarn reinforced cross-linked SPC resin composites. Nam²⁸ fabricated unidirectional composites using ramie fiber and SPC resin. She reported flexural stress and flexural modulus values of 225 MPa and 12.3 GPa, respectively, in the longitudinal direction. Lodha and Netravali²⁷ have reported flexural stress and modulus values of 185 MPa and 13.9 GPa, respectively, for unidirectional composites made using ramie fiber and stearic acid modified SPI resin. Thus the flax yarn reinforced CSF composites fabricated in this research show flexural strength and modulus values comparable to fiber-reinforced composites using more expensive versions of soy protein. The flexural properties of green composites, however, are lower than 529 MPa and 27 GPa, for flexural strength and modulus, respectively, values obtained for the pre-stressed glass–PP composites.³³ However, in another case where the fibers were not pre-stressed, the flexural strength and modulus of glass–PP composites were 265 MPa and 11.7 GPa, which are very comparable to the values for green composites obtained in this study.³⁴

Fig. 5 Typical load vs. displacement plot for a flexural test of flax yarn reinforced CSF composites in longitudinal direction.

Table 2 Flexural properties of flax yarn reinforced CSF composites in longitudinal direction

Flexural strength/MPa	Flexural modulus/GPa	Fracture strain (%)
a Figures in parentheses are CV%.
174.5 (16.9)^a	10.44 (27.4)^a	2.6 (15.4)^a

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Various SPI based resins have been characterized for their degradation in compost medium.³⁵ All were found to degrade within six months. The CSF used in this study, having similar chemistry to SPI and modified SPI resins, is also believed to be fully degradable making the flax yarn reinforced composites to be truly ‘green’.

Conclusions

SF was successfully crosslinked with GA. Cross-linked SF (CSF) resin showed improved thermal stability and processability for manufacturing green composites. Flax yarn and CSF resin were used to fabricate fully biodegradable, environment friendly green composites. Flax yarn reinforced CSF composites, with 60% fiber weight fraction, exhibited improvements of over 700% in fracture stress and over 2000% in Young's modulus in the longitudinal direction as compared to the CSF resin. These composites can be used in secondary and, in some cases, primary structures in indoor applications.

Acknowledgements

The authors would like to thank National Textile Center (NTC) and College of Human Ecology, Cornell University, who provided the financial support for this research. One of the authors, Glenn Mathews, would like to acknowledge the support offered by Cornell Center for Materials Research (CCMR) through NSF/REU program. The authors would also like to thank Archer Daniels Midland (ADM) Company, IL, USA and Sachdeva Fabrics World, New Delhi, India, for providing soy flour and flax yarn, respectively.

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