Cellulose Based Composites (New Green Nanomaterials) || Environment-Friendly “Green” Resins and Advanced Green Composites

137

Section IICellulose-Fiber-Based Composites

Cellulose Based Composites: New Green Nanomaterials, First Edition.Edited by Juan P. Hinestroza and Anil N. Netravali.c© 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

139

7Environment-Friendly ‘‘Green’’ Resins and Advanced GreenCompositesXiaosong Huang and Anil N. Netravali

7.1Introduction

Polymers and fiber-reinforced polymeric composites have been used extensively inapplications ranging from aerospace to autos and from circuit boards to sportinggoods. This is due to their good mechanical and physical properties, lower density,relatively low cost, and possibilities for engineering desired properties. The currenttrend is to use high-strength composites for housing and civil structures. Whilethese composites are designed with long-term durability in mind, disposing offthese materials after their useful life has become a critical issue because of theirnondegradable nature. In addition, as composites are made using two dissimilarmaterials, they are difficult to recycle or reuse. This is particularly true in thecase of thermoset resins, which, once cross-linked, cannot be processed further.While a small percentage of composites are either incinerated or recycled, over90% of the composites end up in landfills at the end of their life. In addition,petroleum, from which these polymers and fibers are derived, is a nonsustainablecommodity. It is estimated that at the current rate of consumption, we will runout of petroleum well before the end of this century. One way to help solveboth environmental and waste issues caused by petroleum-based polymers andcomposites is to replace them with plant-based, fully biodegradable, and sustainablematerials [1–4].

Different plant-based materials such as starch, protein, and cellulose, in variousforms, have been studied as replacements for the traditional plastics and composites[1, 3, 5–10]. Several modifications have been carried out to improve the mechanicaland physical properties of these materials [1–10]. Some of the modified materialshave appropriate properties and have shown potential to replace the nondegradablepolymers for certain applications. At the end of their life, these materials can beeasily disposed of or composted without harming nature.

Soy bean is an annual crop and makes an ideal source for proteins or long-chainpolypeptide molecules. Soy protein contains 18 different amino acid residues andis water soluble at certain pH [11, 12]. The active polar groups, such as hydroxyl,carboxyl, and amine, provide the ability for chemical modifications of the soy

Cellulose Based Composites: New Green Nanomaterials, First Edition.Edited by Juan P. Hinestroza and Anil N. Netravali.c© 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

140 7 Environment-Friendly ‘‘Green’’ Resins and Advanced Green Composites

protein molecules. Soy proteins are commercially available in three forms: defattedsoy flour (SF), soy protein concentrate (SPC), and soy protein isolate (SPI). WhileSF contains about 55% protein and 35% carbohydrates, SPC contains 68–72%protein and 19–21% carbohydrate and SPI contains about 92% protein [11]. Owingto the hydroxyl, carboxyl, and amine groups in soy protein, the protein moleculescan easily form intramolecular and intermolecular hydrogen bonding. In addition,the sulfhydryl groups in cysteine also form S–S cross-links. But the films madeof pure soy protein are brittle and do not have high fracture stress and fracturetoughness required for most applications. Their moisture sensitivity is also toohigh. As a result, modifications are needed before the soy protein resins can beused extensively.

Agar agar (agar) has been used as a gelling agent in food for a long time. Agarmainly consists of two components, agarose and agaropectin [13–15]. Agarosehas a higher molecular weight and is believed to be the main gelling componentof agar. Agarose has a linear structure with alternating (1→3)-β-d-galactose and(1→4)-α-3,6-anhydro-l-galactose [14, 15]. These two basic galactose units are shownin Figure 7.1a and b, respectively. Owing to the large number of hydroxyl groupsin agar, hydrogen bonds can be formed between molecules at multiple locations,resulting in a gelled structure. Agar gels show high fracture toughness and can beused for the modification of soy protein materials [16, 17].

Polymeric nanocomposites, with nanoparticles dispersed, have been studiedextensively because of their capability to improve mechanical, physical, thermal,and barrier properties with very low nanoparticle loading of 1–5% by weight [18–22].For such composites, montmorillonite (MMT) and other clay nanoparticles havebeen used by many researchers [23–27]. MMT is one of the most commonly usedclay for nanocomposites and has a structure of stacked sheets. While one dimensionof these sheets (thickness) is about1 nm, the other two dimensions could be morethan 1000 nm. This results in individual sheets with high aspect ratios whenexfoliated. If the individual nanoclay sheets are dispersed properly, its nanosizecan provide a significant amount of interface between the clay and the resin withonly a small weight percentage of the clay. This results in excellent mechanicaland physical properties of the nanocomposites [23–27]. In addition, the clay sheetsare impermeable to water and gas and thus can provide the nanocomposites better

OH

H

O

H

H

OH OHHO

OHOH

O

H

(a) (b)

O

O

H

H

H

H

O

Figure 7.1 (a) β-D-Galactose unit and (b) α-3,6-anhydro-L-galactose.

7.1 Introduction 141

barrier properties against liquids and gases. Also, because clays are thermally stableup to very high temperatures, their addition can also improve the thermal stabilityof the polymer.

Plant-based fibers have been used extensively to replace the commonly used non-degradable fibers as the reinforcement for either petroleum-based or biodegradableresins. Plant-based fibers offer several advantages compared to the traditionalsynthetic fibers. Besides being biodegradable and fully sustainable, some of thesefibers have excellent mechanical properties. For example, ramie fibers have beenshown to possess Young’s modulus of more than 100 GPa [28, 29]. Some fibers suchas flax show fracture stress values of more than 1 GPa [28, 29]. Most of the fibersalso provide high toughness because of the relatively high fracture strain of thosefibers. Plant-based fibers are fibrillar in nature, that is, composed of fibrils that arebound together by lignin and hemicellulose [30]. During fracturing under tensionor shear, fibrils debond from lignin. The fibril separation can be observed in manyfibers. The debonding is associated with large amount of energy absorption andcontributes partially to the high toughness of these fibers. Owing to the hollow andcellular structure, bast fibers can also provide good acoustic and thermal insulatingproperties [30, 31]. The major constituents of the plant-based fibers are cellulose,hemicellulose, lignin, pectin, and waxes [30]. However, cellulose, which exists in ahighly orientated state, is mainly responsible for the high mechanical properties ofthese fibers. The three hydroxyl groups in the glucose repeating units in celluloseallow strong intermolecular hydrogen bonding [30]. The linear cellulose moleculescan crystallize, and the crystal content, orientation, and the microfibrillar anglewith the fiber axis together determine the mechanical properties of the fiber. Inaddition, most plant-based fibers have low density in the range of 1.5 g cm−3,resulting in high specific strength and stiffness.

Regenerated continuous cellulose fibers such as viscose rayon have been usedfor ease of composite fabrication [32–36]. These fibers are weaker than naturalfibers and are not useful as reinforcement in composites. However, Boerstoel et al.[32] and Northolt et al. [33] have been successful in preparing a liquid crystalline(LC) solution by dissolving cellulose in highly concentrated phosphoric acid andwet spinning it into high-strength cellulose (LC-cellulose) fibers. The wet-spinningprocess with air gap used for cellulose is similar to the one used to spin Kevlar®fibers and produces high molecular orientation and crystallinity. Such LC-cellulosefibers were reported to have high Young’s modulus of about 44 GPa, shear modulusof 3.8 GPa, and high fracture stress of more than 1.7 GPa. Being continuous fibers,they are easy to incorporate into unidirectional composites.

The current work focused on modifying the SPC-based resin with modifiedwith agar and nanoclay to improve the mechanical and physical properties. Themodified resin was then used to fabricate fully biodegradable ‘‘green’’ compositesusing linen yarns with moderate mechanical properties as well as high-strengthadvanced green composites using LC-cellulose fibers. The properties of both resinsand composites were characterized.


7.2Experimental

7.2.1Materials

SPC powder was provided by Archer Daniels Midland Company, Decatur, IL,USA, under the brand name Arcon® S. Cloisite® Na+ nanoclay was obtainedfrom Southern Clay Products, Inc., Gonzales, TX, USA. Agar was obtained fromAcros Organics, Morris Plains, NJ, USA. Analytical grade NaOH and glycerol werepurchased from Fisher Scientific, Pittsburg, PA, USA. Before using, NaOH wasfirst dissolved into distilled and deionized water to obtain a 1 M solution. Agarand glycerol were used directly without further purification. The clay nanoparticleswere dispersed in distilled water using ultrasonication and high- speed magneticstirring to form a fully exfoliated clay dispersion. Larger clay agglomerates inthe suspension were removed by a stabilization process (a week). The clear claydispersion was used for SPC modification. LC-cellulose fibers were kindly providedby Dr H. Boerstoel, Teijin Twaron BV, The Netherlands. Linen yarns (bleached)were obtained from Sachdeva Fabrics Pvt. Ltd., New Delhi, India.

7.2.2Preparation of the Modified SPC

Agar was dissolved in distilled and deionized water in a water bath kept at 90 ◦Cusing magnetic stirrer for about 1 h. At the same time, SPC powder was dispersedin distilled and deionized water in a weight ratio of 1 : 12 at room temperature.Once agar and SPC were dispersed separately in water, they were mixed togetherat 85 ◦C. Specimens with different amounts of agar (10, 20, 30, 40, 50, and 60parts based on weight, SPC is 100 parts) were prepared. Once the mixture ofSPC and agar became uniform, the desired amount of glycerol was added as aplasticizer to optimize the process to make the specimen films. The mixture wasprocessed in a water bath at 85 ◦C for 30 min to dissociate soy protein aggregatesand open soy protein molecules so as to entangle agar and soy protein moleculeswith each other. The mixture was then transferred onto a poly(tetrafluoroethylene)(PTFE)-coated mold for drying. The drying process was conducted in an air-circulating oven at 35 ◦C for 24 h. The dried resin films were hot pressed (cured)using Carver hydraulic hot press (model 3891-4PROA00) at 8 MPa and 120 ◦C for25 min.

The processing steps to prepare the nanoclay-modified resin were similar to theprevious process for agar-modified SPC resin preparation. The purified nanoclaydispersion was added directly into the agar and SPC mixture 3 min before it wastaken out from the water bath kept at 85 ◦C. The resin was then poured ontoPTFE-coated mold and dried at 35 ◦C for 24 h. The same process for specimencuring (hot pressing) described above was then followed.

7.2 Experimental 143

All resin films were conditioned for 3 days at 21 ◦C and 65% relative humidityas per ASTM (American Society for Testing and Materials) before characterizingtheir mechanical properties.

To fabricate linen yarn or LC-cellulose-fiber-reinforced composites, linen yarnsor LC-cellulose fibers were first wound tightly around an aluminum frame to obtainthe unidirectionally aligned LC-cellulose fibers or linen yarns. The desired precuredresin solution was then poured onto the aligned fibers or yarns separately. Thefibers or yarns were soaked in the resin for a couple of hours to ensure maximumresin penetration. Drying at 40 ◦C (36 h, on an average) was followed to get a singlelayer of composite laminate (prepregs). Four such prepregs were stacked togetherin the same direction and a small amount of precured modified soy protein resinwas placed between them. The laminated composite was then dried again at 40 ◦Cfor 8 h. The dried composite sheet was cured in the hot press at a pressure of about8 MPa and 120 ◦C for 25 min. Cured composite specimens were conditioned for5 days at 21 ◦C and 65% relative humidity before testing. The composites had afiber or yarn weight fraction of about 48%.

7.2.3Specimen Characterization

Both resin films and linen yarn or LC-cellulose-fiber-reinforced composites werecharacterized for their tensile properties using an Instron universal tensile tester(model 5566). Tensile properties such as specimen tensile failure stress, tensilefailure strain, Young’s modulus, and toughness were obtained according to ASTM D882-97. The specimen toughness was calculated as the area below the stress–straincurves, which gave the energy consumed to break the specimen. Rectangulartest specimens (strips) had thicknesses of around 0.14 mm for resin films and0.4–0.6 mm for composites. Both resin film and composite test specimens had awidth of 10 mm. The gauge length for these tests was 50 mm and the strain ratewas 0.5 min−1. At least five specimens were tested for each resin and composite toobtain average values.

Wide-angle X-ray diffraction (XRD) was used to evaluate the nanoparticle disper-sion. The specimen films were scanned from 1◦ to 40◦ at 2◦ min−1 employing theCu-Kα X-ray radiation with a wavelength of 1.5405 A. The d-spacing informationwas recorded.

The conditioned yarns and fibers were also characterized for their tensileproperties using the Instron universal testing machine, model 5566 according toASTM D2256-02. Tests were performed using a gauge length of 50 mm and astrain rate of 0.5 min−1. Twenty specimens were tested to obtain the average values.The diameter of yarn and fiber was measured using an optical light microscope(Olympus, model BX51).

Three-point bending tests were performed to characterize the flexural propertiesof the composites in accordance with ASTM D 790-02. The flexural tests were alsocarried out on the Instron universal tensile tester at a strain rate of 0.01 mm (mmmin)−1. The loading nose and supporters had a radius of 3.2 mm. The flexural


properties, such as the flexural modulus and flexural stress, in the fiber/yarn(longitudinal) direction were characterized.

Scanning electron microscope (SEM) was used to characterize the fracturesurface of the composites in tensile mode. The SEM used was a Leica, model 440X.The specimens were sputtered with gold–palladium for the SEM characterizationof the composite fracture surfaces.

7.3Results and Discussion

7.3.1Mechanical Properties of the Modified Resins

The mechanical properties and moisture content of the soy protein resin filmswith different amounts of agar are presented in Table 7.1. These resin specimenscontained 10 parts of glycerol as a plasticizer. It is clear from the data in Table 7.1that the incorporation of agar increased the Young’s modulus, fracture stress, andtoughness, whereas the fracture strain of the SPC resin remained nearly constant.For specimens containing 50 parts of agar, the Young’s modulus increased from722 MPa for SPC without agar to more than 2 GPa. At the same time, the fracturestress increased by about 250% to 50.8 MPa from about 21 MPa for SPC withoutagar. As mentioned earlier, agar is a polysaccharide containing different galactoseunits. Once dissolved in appropriate amount of water, agar cross-links with itselfthrough hydrogen bonds forming a gel. When mixed with SPC, an interpenetratingnetwork (IPN)-like structure is expected because SPC is also capable of forming aself-cross-linked structure through hydrogen bonding and disulfide bonds [7, 11,12, 37]. At the same time, strong hydrogen bonds and the possible covalent esterand/or ether bonds can also be formed during precuring and curing steps provideda strong interaction between the agar and SPC networks. Such IPN-like structureformed by blending soy protein with gellan has also resulted in significant increase

Table 7.1 Mechanical properties of SPC resin modified with varying amounts of agar.

Agar/SPC/glycerolcontent (by weight)

Failure stress(MPa)

Failurestrain (%)

Young’smodulus (MPa)

Toughness(MPa)

0/100/10 20.7 (7.8)a 11.6 (8.2) 684 (6.3) 3.1 (8.8)10/100/10 30.8 (6.9) 12.1 (7.5) 1081 (7.1) 3.4 (8.2)20/100/10 34.9 (10.2) 13.7 (9.9) 1386 (8.2) 3.7 (10.3)30/100/10 42.8 (9.8) 12.9 (9.1) 1537 (8.3) 4.5 (10.0)40/100/10 48.9 (9.3) 11.7 (8.9) 1680 (7.9) 4.6 (9.7)50/100/10 50.8 (8.6) 12.6 (8.9) 2033 (7.7) 5.1 (10.2)60/100/10 53.4 (10.4) 11.9 (11.0) 2274 (8.9) 4.7 (11.7)

aValues in parentheses are CV% (coefficient of variation of the mean).

7.3 Results and Discussion 145

in mechanical properties [7, 37]. The increased cross-link density in the IPN-likeagar–SPC resin contributes to the increased specimen modulus. Toughness of themodified SPC resin increased from 3.1 to 5.1 MPa with the incorporation of 50parts of agar as seen from Table 7.1. This significant increase in toughness was adirect result of the increased modulus and fracture stress while almost no changein fracture strain. This can also be explained by the increased cross-link densitythrough either chemical bonds or physical interlocking in the agar-modified resinand the efficient stress transfer between agar and SPC molecules. Gellan has beenshown to be more efficient than agar in increasing the Young’s modulus of SPCand SPI [7, 37]. Adding 40 parts of gellan to SPI containing 30 parts of glycerolincreased the Young’s modulus from 98.7 to 388.7 MPa and from 201 to 717 MPafor SPC containing 20 parts of glycerol [7, 37]. This was the result of the strongerinter- and intramolecular interactions among gellan molecules because of the ionicbonds formed in gellan in addition to hydrogen bonds formed with SPI and SPC.However, agar–SPC blends have shown sufficient tensile properties with muchlower cost compared to the gellan–soy protein blends.

Nanoclay was used to further improve the Young’s modulus of the agar-modifiedSPC film. The effect of nanoclay loading on the mechanical properties and moisturecontent on SPC resins is shown in Table 7.2. Compared with agar modification,nanoclay was more efficient in increasing the modulus than fracture stress ascould be expected. With nanoclay loading of seven parts, the modulus increasedto 3 GPa (about 50%) compared to specimen with no clay (2 GPa) and the fracturestress increased from 51 to 65 MPa. Exfoliated clay sheets have high aspect ratio,thickness of about 1 nm, and the other two dimensions in several hundred tothousand nanometers. The exfoliated clay sheets supply a large amount of surfacearea. As both clay and soy protein are hydrophilic, the interaction between clayparticles and soy protein is expected to be high, which can hinder the motion of theSPC protein chains and thus improves the stiffness of the resulting resin. However,because of the reduced freedom of motion for soy protein molecules, the resin alsoshowed brittle behavior with a reduced fracture strain. The toughness of the clay-modified film decreased because of the lower fracture strain. Other researchers have

Table 7.2 Mechanical properties of modified SPC resins containing various nanoclayloadings.

Nanoclay/Agar/SPC/glycerolcontent (by weight)

Failure stress(MPa)

Failurestrain (%)

Young’smodulus (MPa)

Toughness(MPa)

0/50/100/10 50.8 (9.4)a 12.6 (10.4) 2033 (8.2) 5.1 (10.2)1/50/100/10 53.5 (8.8) 12.3 (9.8) 2190 (8.1) 4.8 (11.4)3/50/100/10 56.0 (7.9) 11.2 (8.6) 2401 (7.3) 4.5 (9.3)5/50/100/10 61.3 (8.3) 9.2 (8.5) 2778 (7.4) 4.1 (9.7)7/50/100/10 65.3 (9.1) 7.5 (9.7) 3023 (8.0) 3.3 (8.9)

aValues in parentheses are CV%.


0

2000

4000

6000

1 6 11 16 21 26

Degree (°)

CP

S

Native clay

Agar/SPC/glycerol = 50/100/10

Nanoclay/agar/SPC/glycerol = 2/50/100/10



Figure 7.2 XRD scans of native nanoclay particles and modified SPC with varying amountsof nanoclay.

added nanoclay sheets to different biodegradable polymer systems and obtainedsimilar property enhancement [7, 10, 38, 39].

Figure 7.2 shows the XRD scans of native clay sheets and clay-modified SPCspecimens containing 50 parts of agar and 10 parts of glycerol. Clay sheets innative state showed a peak at around 9◦, which indicates a layered structure. Oncethe clay particles were exfoliated and dispersed in the resin, the XRD scans of theresins showed no evidence of the peak, which indicated the exfoliated nanoclayhas been fully dispersed. With higher loading of nanoclay in the resin, the claysheets formed some larger aggregates, as evidenced by the peaks at low incidenceangles (about 3◦). As both nanoclay sheets and soy protein have polar nature,the interaction between them was expected to be strong, which can stabilize theexfoliated clay sheets in the protein matrix.

7.3.2Characterization of Linen Yarns and LC-Cellulose Fibers

The topographies of the LC-cellulose fibers and bleached linen fibers and yarns wereobserved using optical microscope and are shown in Figure 7.3. The LC-cellulosefibers showed a very uniform diameter of around 12 μm with a smooth surface asseen in Figure 7.3a. Bleached linen yarns were also used in the fabrication of greencomposites. The linen fiber is a staple fiber and the fibers used in this study hada mean diameter of 15 μm (as seen in Figure 7.3b). The linen fiber showed highdiameter variation along the fiber length, as in the case with most other plant-basedfibers [40, 41]. As seen in Figure 7.3c, the yarn used in this study had a meanyarn diameter of 150 μm. The yarn was also produced with a high twist. A highertwist can provide frictional force to hold the fibers together to obtain a higher yarnfracture stress. However, any twist above a certain limit lowers the modulus andstrength of the yarn as the fibers become more and more oblique to the yarn axis.


−421 −211 211 421u

100 μm

100 μm

−316

−421 −211 211 421u

−158

0

158

316

−316

−158

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316

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(c)

200 μm

−421 −211 211 421u

−316

−158

0

158

316

Figure 7.3 (a) LC-cellulose fibers; (b) linen fibers separated from the linen yarns; and (c)linen yarns.

Table 7.3 Tensile properties of the linen yarns and LC-cellulose fibers.

Strain (%) Stress (MPa) Modulus (GPa)

Linen yarn 7.03 (9.3)a 319 (9.2) 6.8 (8.4)LC-cellulose fiber 11.46 (8.9) 1685 (10.1) 39.9 (7.9)


The tensile properties of the linen yarns and the LC-cellulose fibers are listedin Table 7.3. The high twist linen yarn showed a fairly high tensile failure stressof 319 MPa and Young’s modulus of 6.8 GPa. Although the fracture stress andmodulus of a yarn are generally lower than those of the corresponding fiber,yarns show a higher fracture strain because of the twist. Part of this strain comesfrom straightening of the fibers parallel to the yarn axis when stretched. As plant-based fibers are short, they need to be spun into continuous yarns to provide the


convenience of fabricating unidirectional composites. The LC-cellulose fibers hadexcellent mechanical properties with tensile failure stress of about 1.7 GPa andYoung’s modulus of 39.9 GPa. It has been reported that the LC-cellulose fiberadopts a cellulose II structure with a high molecular orientation and crystallinity[32, 33]. The Young’s modulus of the LC-cellulose fiber is comparable to otherhigh-strength fibers, such as glass fiber. Both high fracture stress and high Young’smodulus are a result of the high crystallinity and orientation of cellulose moleculesin the fiber. The conditioned LC-cellulose fiber showed a high fracture strain ofabout 11%, which is much higher compared to most of the natural and syntheticfibers used to fabricate composites. This indicates that relatively high energy willbe consumed to break the LC-cellulose fibers because they also possess excellentmodulus. The composites made using these fibers can be expected to have hightoughness and fracture strength.

7.3.3Characterization of Linen Yarns and LC-Cellulose-Fiber-Reinforced Composites

The two resins used to fabricate the composites were (i) SPC with 10 parts ofglycerol (SPC resin) and (ii) SPC with 10 parts of glycerol, 50 parts of agar, and 5parts of nanoclay (modified SPC resin). Unidirectional composites were fabricatedseparately using these two resins and linen yarns or LC-cellulose fibers.

Table 7.4 presents the tensile and flexural properties of the linen-yarn-reinforcedgreen composites for both SPC and the modified SPC resin. For comparison,theoretical values for Young’s modulus of the composites, calculated using therule of mixture, are also presented in Table 7.4. The yarn volume fraction wascalculated to be 44% in these composites by assuming a fiber density of 1.5 g cm−3.Composites with both resins had fairly high mechanical properties with failurestress of more than 200 MPa and flexural stress of about 90 MPa. The compositesmade using the modified SPC resin resulted in higher Young’s modulus of about4.3 GPa and higher failure stress of about 235 MPa compared to about 3.3 GPa andabout 205 MPa, respectively, for the composites made using the unmodified SPCresin. This increase could be attributed to the significantly higher resin stiffness aswell as better fiber/resin bonding. In most cases, the properties of the composite

Table 7.4 Tensile and flexural properties of the composites made using linen yarns and SPCand modified SPC resins.

Composition of the resin Failurestress(MPa)

Failurestrain(%)

Young’smodulus

(GPa)

Flexuralmodulus

(GPa)

Flexuralstress(MPa)

Young’s modulus(theoretical) (GPa)

SPC/glycerol= 100/10 204.9 (7.8)a 6.4 (8.5) 3.3 (7.9) 4.9 (8.7) 88 (10.1) 3.40Nanoclay/Agar /SPC/Glycerol= 50/5/100/10

235.4 (8.6) 6.1 (9.7) 4.3 (8.2) 5.5 (9.4) 101 (9.9) 4.61



are determined by the reinforcing fiber. However, the resins in this study, especiallythe modified resin, had Young’s modulus of about 50% of that of the linen yarns.As a result, resin properties also contributed to the composite properties. Theother reason for the higher modulus could be the higher interfacial adhesion inthe case of the modified SPC resin. Better fiber/resin interfacial adhesion can beobserved from the SEM photoimages shown in Figure 7.4 and as discussed later.On the basis of the local load sharing model for fiber-reinforced composites, theload on a broken fiber can be mitigated and redistributed among the neighboringunbroken fibers through the resin shear and fiber/resin interface [8, 42]. As a result,high fiber/resin interfacial adhesion allows efficient transfer of load among fibers

200 μm 200 μmEHT=10.00 kV WD= 17 mm

Photo No. =7344 Detector= SE1Mag= 63 X

100 μmEHT=10.00 kV WD= 17 mm

Photo No. =7350 Detector= SE1Mag= 106 X

(a) (b)

(c)

Figure 7.4 SEM photoimages of the fracture surfaces of the composites made using(a) linen yarns and (b) SPC with SPC/glycerol= 100/10; and (c) modified SPC withnanoclay/agar/SPC/glycerol= 5/50/100/10.


resulting in enhanced composite properties, especially the failure stress. Interfacialproperty was especially important in the yarn-reinforced composites, because theresin in this case also behaved as a binder to hold together the short fibers withinthe linen yarns. During the drying of the modified resin, higher shrinkage wasobserved. This shrinkage significantly increases the resin grip over the fibers toprovide better mechanical interlocking. As a result, the interfacial shear strengthwith the modified resin was expected to be higher. In addition, the better tensileproperties of the composite using the nanoclay-modified SPC could also be due tohaving less defects in the composite. It has been reported that clay can increasethe surface tension of water similar to inorganic salts [43]. Clay could also increasethe surface tension of the SPC dispersion in water. This hypothesis is based onthe observations that the air bubbles trapped in the precured clay-modified SPCdispersion were low in number compared to the bubbles observed in unmodifiedSPC dispersion. The increased surface tension of the resin can help wetting of theyarns as well as its penetration into the yarn structure. In addition, the smallernumber of air bubbles in the resin can reduce the voids formed in the composite,which act as defects. Chabba et al. [1] prepared yarn-reinforced composites usingglutaraldehyde-cross-linked soy flour resin and flax yarns. They reported failurestress of 259.5 MPa and Young’s modulus of 3.7 GPa. The lower failure stress inthis study was due to the lower mechanical properties of the linen yarn, whereasthe higher modulus in this study was most likely due to the much higher stiffnessof the resin used.

In both cases, the measured Young’s modulus values of the composites werelower than the calculated values based on the rule of mixture. This is believed to bea result of the poor resin penetration. As noted earlier, the linen yarns used in thisstudy had high twist. The tight structure of the yarn made it difficult for the resinto penetrate because of its high viscosity. The voids (air) between fibers within ayarn were trapped and behaved as defects in the composites, which reduced thestress as well as Young’s modulus of the composites. Also, the shrinkage of theresin during drying and curing processes could change the alignment of yarns. Thereduced degree of yarn alignment can also significantly affect composite modulusand tensile failure stress.

Figure 7.4 shows SEM photoimages of the fracture surfaces of the linen-yarn-reinforced composites. Figure 7.4a and b shows the tensile fracture surfaces of thecomposites made using SPC and the modified SPC resin, respectively. As discussedpreviously, the interfacial properties in both composites, in general, were expectedto be good because of the hydrophilic nature of the cellulose in the linen yarn andthe polar amino acid residues in the soy protein resins. This was confirmed by theresin residues on the yarn surface even after the breaking of the composites asshown in Figure 7.4a,b. Composites made using the modified SPC seemed to havebetter interfacial adhesion with the yarn because more yarn surfaces were coveredby resin residues as in Figure 7.4b. However, a close look at the fibers inside theyarn showed fiber pull-out as observed from Figure 7.4c, indicating a less thandesirable interfacial shear strength or, more correctly, lack of resin penetration.Although resin residues were clearly observed on the surface of some of the fibers,


most of the fiber surface was not covered by the resin. This confirms the poorresin penetration into the yarns. Lack of resin penetration was due to both the highyarn twist, which packs the fibers close together without any space in between forthe resin to penetrate, and the high viscosity of the resin. As the resin was notbonded to all the fibers in the yarn and particularly could not fully penetrate, voidsexisted between fibers inside the yarn. As stated earlier, poor resin penetrationaffects the mechanical properties of the final composites in two ways. Firstly, thestress transfer from broken fibers to the neighboring intact ones is not efficient,and secondly, the voids act as defects. Both these factors contribute to the lowermodulus of the composites compared with the theoretical values calculated usingthe rule of mixture. It can be seen from Table 7.4 that the difference between thetheoretical values and those from the experiments were not far apart. However, thedifference is larger in the case of the modified resin, which has a higher viscosity.

The flexural properties of the linen-yarn-reinforced composites were charac-terized using three-point bending tests. Composites made using SPC and themodified SPC resins had flexural moduli of 4.9 and 5.5 GPa, respectively, as indi-cated in Table 7.4. Both composites showed flexural stresses of around 90 MPa.As explained earlier for the tensile properties, the higher flexural modulus forthe composites made using the modified SPC resin was due to both the highermechanical properties of the resin and the better fiber/resin interfacial adhesion.Some delamination was observed in case of both composites during the three-pointbending test. This is attributed to the low resin penetration into the yarns, the shearforce during the bending test could easily delaminate the composite layers and,therefore, the composites became less stiff. It is clear from these observations thatbetter mechanical properties of the composites could be obtained if low twist yarnswere used. Less tight packing of fibers in the yarn will allow the resin to penetrateeasily in between the fibers and thus improves the composite properties.

The LC-cellulose-fiber-reinforced composites were also characterized for theirtensile and flexural properties, and the results are presented in Table 7.5. Forcomparison, theoretical values for Young’s modulus, calculated using the ruleof mixture, are also presented in Table 7.5. The fiber volume fractions in bothcomposites were calculated to be around 45% by assuming a fiber density of

Table 7.5 Tensile and flexural properties of the composites made using LC-cellulose fibersand SPC and modified SPC resins.

Composition Failurestress(MPa)

Failurestrain (%)

Young’smodulus

(GPa)

Flexuralmodulus

(GPa)

Flexuralstress(MPa)

Young’smodulus

(theoretical)(GPa)

SPC/glycerol= 100/10 571 (7.7)a 9.63 (8.1) 12.42 (6.9) 24.9 (9.0) 231 (8.9) 18.0Nanoclay/Agar/SPC/Glycerol= 50/5/100/10

616 (9.2) 9.18 (10.3) 13.72 (8.4) 26.8 (9.6) 252 (10.7) 19.2



1.5 g cm−3. The composites made using the SPC resin had a tensile failure stressof 571 MPa and a Young’s modulus of 12.4 GPa. For composite specimens madeusing the modified SPC resin, the tensile failure stress and Young’s moduluswere 616 MPa and 13.7 GPa, respectively, an increase of about 8–10% over theSPC resin composites. These composites showed much higher tensile propertiesthan the linen-yarn-reinforced composites, as could be expected, mainly becauseof the significantly higher tensile properties of the LC-cellulose fiber. As in thecase of linen fibers with three hydroxyl groups on each glucose monomeric unit incellulose, the LC-cellulose fibers are expected to have excellent hydrogen bondingwith soy protein resins resulting in good interfacial properties in the composites.However, these fibers had smooth surfaces that prevent mechanical bonding. Thisallows the stress to be readily transferred from the broken fibers to the unbrokenfibers and thus contributes to the high failure stress of the composite as explainedin the case of linen yarn composites. Nam and Netravali [40, 41] reported a failurestress of 271 MPa and Young’s modulus of 4.9 GPa in the longitudinal directionfor ramie fiber/SPC unidirectional composites. These values are comparableto the linen-yarn-based composites. And much higher tensile properties of theLC-cellulose-based composites in this study reflect the higher tensile propertiesof the LC-cellulose fiber. In addition, tension was applied on the LC-cellulosefibers to ensure good fiber alignment in the composite fabrication process. Thebetter fiber alignment also contributed to the higher mechanical properties of theLC-cellulose-fiber-reinforced composites.

The higher mechanical properties of the composites with the modified SPC, asmentioned earlier, are a result of better resin mechanical properties and fiber/resininterfacial adhesion. As stated earlier, these composites had 48% fiber by weight(about 46% by volume). If the volume fraction was increased to 60% the LC-cellulose fiber composites would have tensile failure stress of about 750 and800 MPa and Young’s modulus values of 16.2 and 17.9 GPa for the SPC and themodified SPC resins, respectively. The failure stresses in longitudinal direction forthe two composites are much higher than most of the petroleum-based plastics andcomparable to soft steel and E-glass-fiber-reinforced composites [44, 45]. It shouldbe pointed out that these composites also have a high failure strain comparedto most advanced composites that use fibers such as graphite and Kevlar®. As aresult, these composites possess excellent fracture toughness making them usefulin many applications where toughness is critical. Also, most steel varieties havestrength between 300 and 500 MPa, lower than the LC-cellulose-based composites.However, steel density is above 7.8 g cm−3 compared to the 1.4 g cm−3 density ofthe LC-cellulose-reinforced green composites. As a result, these green compositesare five to six times stronger than steel on ‘‘per weight’’ basis and can be termed as‘‘advanced green composites.’’

The LC-cellulose composites also showed excellent flexural properties. These arepresented in Table 7.5. The composites made using modified SPC resin showeda flexural modulus of 26.8 GPa and a flexural stress of 252 MPa. The flexuralproperties of composites made using the SPC resin were slightly lower and thedifference was not statistically significant. Compared to the linen-yarn-reinforced

7.4 Conclusions 153

100 μm100μm

(a) (b)

Figure 7.5 SEM photoimages of the fracture surfaces of the composites made usingLC-cellulose fibers and (a) SPC with SPC/glycerol= 100/10 and (b) modified SPC withnanoclay/agar/SPC/glycerol= 5/50/100/10.

composites, the flexural stress values of the LC-cellulose-reinforced composites wastwo times high and flexural modulus was more than four times, which was a directresult of the much higher tensile properties of the LC-cellulose fibers comparedto the linen yarns. The continuous and strong LC-cellulose fibers provide excellentflexural properties for these green composites.

Figure 7.5 presents SEM photoimages of the fracture surfaces of the LC-cellulose composites fractured in tensile mode. Similar to the linen-yarn-reinforcedcomposites, a certain degree of the fiber pull-out can be seen. LC-cellulose fibersused in this study were in the form of a loose bundle. Resin penetration in thiscase was not as poor as in the case of linen yarns. However, the LC-cellulose fibershad a uniform diameter and a smooth surface, which resulted in lower mechanicalinterlocking and relatively low friction between the fibers and the resin resultingin a low interfacial interaction. The low load transfer efficiency is one importantreason for the lower mechanical properties compared with the theoretical values(Table 7.4) calculated using the rule of mixture.

7.4Conclusions

The soy protein resin modified with agar showed significant increase in bothfracture stress and modulus. Nanoclay was very effective in further increasing theresin modulus. With five parts of nanoclay loading, the Young’s modulus and thefracture stress of the resin increased from 2.0 to about 2.8 GPa and from 50.8 toabout 61.3 MPa, respectively. The SPC resin modified with both agar and nanoclaywas used for green composite fabrication. Unidirectional composites made usinglinen yarns or LC-cellulose fibers and the modified SPC resin exhibited excellenttensile and flexural properties in the longitudinal direction. The tensile failure


stress of the LC-cellulose-reinforced composite was 616 MPa and the Young’smodulus was 13.7 GPa. The flexural modulus and flexural stress of the LC-cellulose-reinforced composite along the fiber direction was 26.8 GPa and 252 MPa,respectively. These properties could be improved significantly by increasing thefiber volume content. These biodegradable composites made from renewableresources provide equal or better properties than many petroleum-based polymersand polymeric composites. While the composites made using linen yarns maybe used in many indoor applications where moderate mechanical properties aresufficient, the advanced green composites made using LC-cellulose fibers may beused in structural applications, and because of their high toughness, they may alsobe used in ballistic applications.

Acknowledgments

The authors gratefully acknowledge the College of Human Ecology at CornellUniversity, the Cornell Center for Materials Research, National Textile Center, andthe National Science Foundation for providing funding and testing facilities. Theauthors thank Professor Dotsevi Sogah and Dr Xiaoping Chen in the Departmentof Chemistry and Chemical Biology at the Cornell University for giving valuablesuggestions for nanoclay particle process. The authors also thank Dr H. Boerstoel,Teijin Twaron BV, The Netherlands, for providing the LC-cellulose fibers.

Abbreviations

LC liquid crystallineMMT montmorillonitePTFE poly(tetrafluoroethylene)SEM scanning electron microscopeSF defatted soy flourSPC soy protein concentrateSPI soy protein isolateXRD X-ray diffraction

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