Biocomposites from Renewable Resources

Keywords

Biopolyester • Polylactic acid • Poly(3-hydroxybutyrate-co-3-

hydroxyhexanoate) • Copolyester • Interfacial adhesion • Compatibilizer

The biopolyester poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) was used as matrix, and flax fibers were used as reinforcement to develop composites. Since P (3HB -co-3HHx) copolymer is hydrophobic and flax fibers are hydrophilic, the fibers were either acetylated or grafted with polyester to improve adhesion between the fibers and the matrix. In addition, longer length flax fibers were used in matrix form, and short flax fibers were mechanically mixed into the matrix and compres­sion molded into composites. Figure 70.1 shows the strength and modulus of the composites obtained using various configurations of the fibers. As seen from the figure, the addition of the fibers, especially the long fibers, substantially increased the strength and modulus of the composites. However, the strength of the short fiber-reinforced composites (black bars) does not show a major change with the modification of the fibers, but the modulus of the composites obtained using acetylated fibers (A) was considerably higher than the composites without modifi­cation (U) and those grafted with polyester (P) [07Zin]. In addition to flax fibers, several other fibers such as abaca and jute have been used as reinforcement with PHBV as the matrix. Some of the properties of the composites developed using PHBV as matrix and natural fibers as reinforcement are provided in Table 70.1. PHBV matrix was also reinforced with 30 or 40 % bamboo fiber having a length of 5 cm and diameter between 10 and 100 pm [08Sin]. No significant increase in properties was observed when the fiber content was increased from 30 to 40 %, but the tensile modulus had increased by 175 % after incorporating the fibers compared to the neat polymer. The improvement in tensile modulus was close to the theoreti­cal possible increase that was calculated using Christensen’s equations. A number of voids and clusters of fibers were observed in the fractured surfaces that were responsible for the relatively low impact and tensile strength [08Sin]. SEM images

Fig. 70.1 Properties of biocomposites developed using PHBV as matrix and various biofibers as reinforcement [12Far]. Reproduced with permission from Elsevier

Table 70.1 Properties of PHBV composites reinforced with various natural and regenerated cellulose fibers [07Zin] Reinforcement Strength (MPa) Elongation (%) Modulus (GPa) None 27.3 ± 0.3 7.0 ± 1.1 2.1 ± 0.07 Man-made cellulose 41.7 ± 3.8 2.3 ± 1.0 4.4 ± 0.3 Abaca 28.0 ± 1.3 0.9 ± 0.1 4.4 ± 0.06 Jute 35.2 ± 1.3 0.8 ± 0.0 7.0 ± 0.26

in Fig. 70.2 show that at 40 % fiber loading, there are excessive fiber and consider­able low levels of matrix that lead to poor binding and therefore relatively poor properties. Table 70.2 lists some of the properties of the bamboo-PHBV composites at the two different loading levels studied.

Bledzki and Jaszkiewicz have compared the properties of PLA and PHBV composites reinforced with various natural fibers [10Ble]. Natural fibers such as abaca and flax and man-made cellulose (rayon) were used as reinforcement (30 %) with various synthetic polymers as the matrix. The PHBV matrix was blended with 27 % poly(butylene adipate-co-butylene terephthalate) to improve processability. PP matrix used for comparison was grafted with 5 wt.% of maleic anhydride. Table 70.3 provides a comparison of the properties of the composites developed using various matrix and reinforcements. As seen from the table, the addition of the reinforcing fibers considerably increases the modulus and strength but decreases the elongation. Among the different fibers studied, jute provides higher improvement in tensile properties than abaca or cellulose. SEM analysis showed that PHBV had poor interaction with the fibers and therefore had relatively inferior properties.

PHBV and PLA fibers were combined with various natural fibers (abaca, jute, and flax) in a 70:30 ratio and used to develop composites in comparison to similar composites made using traditional PP, and the composite properties were analyzed using dynamic mechanical analysis [13Ada]. Table 70.4 provides some of the properties of the composites developed. The addition of the fibers increases the tensile and storage modulus for all the composites studied. Jute fibers seem to provide better properties, and PHBV composites had better tensile and storage

Fig. 70.2 SEM images of bamboo fiber (40 %)-reinforced PHBV composites show low levels of matrix due to the high fiber content and therefore poor properties [08Sin]. Reproduced with permission from Elsevier. (a) shows the distribution of the fibers in the matrix and the higher — magnification image in (b) shows the voids between the fibers and matrix indicating poor compatibility

Table 70.2 Effect of increasing concentration of bamboo fibers on the tensile, flexural, and impact resistance properties of PHBV composites [08Sin] % Bamboo Tensile Flexural Impact strength (J/m2) Heat deflection temp (°C) Strength (MPa) Modulus (GPa) Strength (MPa) Modulus (GPa) 0 21.4 1.02 30.27 1.28 34.3 114 30 18.9 7.71 30.6 2.56 24.3 120 40 16.7 2.8 29.6 3.26 23.8 123

properties than PLA. In another study, unmodified PHBV and PHBV treated with maleic anhydride were reinforced with kenaf fibers, and composites developed through compression molding at 170 °C. It was reported that kenaf fibers increased the strength from about 12 to 18 MPa. However, poor interaction and binding were

Table 70.3 Tensile properties of composites made from PHBV, PLA, and PP as matrix and cellulose, abaca, and jute as reinforcement [10Ble] Matrix Fiber Tensile modulus (GPa) Tensile strength (MPa) Tensile elongation (%) PP — 1.5 ± 0.0 29.2 ± 0.4 >50 Cellulose 3.7 ± 0.1 71.6 ± 2.7 3.5 ± 0.5 Abaca 4.9 ± 0.1 42.0 ± 0.5 1.7 ± 0.2 Jute 5.8 ± 0.5 47.9 ± 2.7 1.4 ± 0.1 PHBV/ Ecoflex — 2.1 ± 0.1 27.3 ± 0.3 2.1 ± 0.1 Cellulose 4.4 ± 0.1 41.7 ± 3.8 4.4 ± 0.3 Abaca 4.4 ± 0.1 28.0 ± 1.3 4.4 ± 0.1 Jute 7.0 ± 0.3 35.2 ± 1.3 7.0 ± 0.3 PLA — 3.4 ± 0.2 63.5 ± 0.4 3.4 ± 0.2 Cellulose 5.8 ± 0.1 92.0 ± 4.7 5.8 ± 0.2 Abaca 8.0 ± 0.3 74.0 ± 0.7 8.0 ± 0.3 Jute 9.6 ± 0.3 81.9 ± 2.9 9.6 ± 0.4

Table 70.4 Properties of PP, PHBV, and PLA fibers reinforced with various cellulose fibers [13Ada] Matrix Fiber Tensile modulus (GPa) Storage modulus at 1 Hz (GPa) 23 °C 50 °C 80 ° C PP — 1.5 ± 0.0 1.9 1.1 0.5 Cellulose 3.7 ± 0.1 3.2 2.4 1.7 Abaca 4.9 ± 0.1 5.5 4.2 2.7 Flax 4.8 ± 0.1 4.8 3.6 2.3 Jute 5.8 ± 0.5 5.4 4.2 2.8 PHBV/Ecoflex — 2.1 ± 0.1 2.9 1.9 0.9 Cellulose 4.4 ± 0.1 4.7 3.7 2.3 Abaca 4.4 ± 0.1 4.4 3.4 2.1 Flax 5.3 ± 0.2 5.7 4.5 2.8 Jute 7.0 ± 0.1 6.7 5.4 3.6 PLA — 3.4 ± 0.2 3.2 3.0 0.01 Cellulose 5.8 ± 0.1 5.0 4.7 0.4 Abaca 8.0 ± 0.3 5.7 5.4 0.1 Flax 8.0 ± 0.6 5.9 5.5 0.2 Jute 9.6 ± 03 6.2 5.9 0.2

observed between the kenaf and PHBV, but the addition of the compatibilizer increased adhesion and provided good tensile properties to the composites [07Ave].

Abaca fibers were also used to reinforce poly(butylene succinate) (PBS), polyestercarbonate (PEC), or poly(lactic acid) (PLA) as matrix. Fibers used had diameters of 0.2 mm and lengths of 5 mm and were chemically modified through esterification, alkali treatment, and cyanoethylation [03Shi]. An increase in modu­lus was observed after the addition of abaca fibers irrespective of the treatment of

Fig. 70.3 Optical images of untreated (a), alkali-treated (b), and silane-coupled Sterculia urens fabrics (c) [13Jay]. Reproduced with permission from Elsevier

Table 70.5 Comparison of the properties of PLA reinforced with unmodified and modified Sterculia urens fabrics [13Jay] Maximum stress (MPa) Young’s modulus (MPa) Elongation at break (%) Fabric No Yes No Yes No Yes Untreated 10.0 11.0 641 870 2.0 1.8 Alkali treated 18.9 20.3 2,019 2,693 2.4 2.3 PLA/untreated biocomposites 58.2 62.1 3,181 4,121 3.7 4.2 PLA/alkali treated biocomposites 69.1 78.4 4,208 5,905 5.0 5.4

the fibers or the type of matrix used. The addition of fibers into matrices such as PLA which had higher strength did not show a major increase in properties of the composites [03Shi].

PLA has also been reinforced with a natural fabric Sterculia urens and the properties of the reinforced composites were studied [13Jay]. Figure 70.3 shows the image of a natural Sterculia urens fabric (thickness of 0.16 mm), alkali-treated fabric, and a fabric coupled with a silane coupling agent. The fabrics were blended (20 %) with PLA and compression molded into films at 180 °C. Tensile properties of the unreinforced and PLA reinforced composites with various fabrics are shown in Table 70.5. From the table, it can be inferred that the addition of the fabric increases the tensile properties of the composites, and further, alkali-treated and silane-coupled fabrics provide better properties to the composites due to improved compatibility [13Jay]. Hybrid PLA biocomposites were prepared using kenaf fibers and cornhusk flour as the reinforcement [14Kwo]. The ability to predict the influence of aspect ratio on the properties of the composites developed using injection molding was studied. It was found that the actual and predicted value of composite properties did not have good correlation. Initial values of aspect ratio determined before extrusion were suggested to provide a better estimate of the properties [14Kwo].

Bamboo fiber pulp with cellulose content of 96 % was treated with alkali or silane coupled with commercial coupling agent KH560 and used to reinforce virgin or maleated PLA [14Lu]. FTIR studies confirmed coupling of the silane groups, but no major changes in crystallinity or crystal structure were observed. Table 70.6

Table 70.6 Changes in the properties of PLLA composites reinforced with 2 % of modified and unmodified bamboo fibers [14Lu] Matrix/reinforcement Strength (MPa) Elongation (%) Modulus (GPa) Toughness (kJ/m2) Pure PLLA 61 ± 3.2 15 ± 1.3 1.7 ± 0.1 4.3 ± 0.3 PLLA/bamboo 56 ± 2.1 6 ± 1.1 1.9 ± 0.2 4.2 ± 0.2 MA-PLLA/bamboo 69 ± 3.3 12 ± 2.1 2.3 ± 0.2 6.1 ± 0.3 PLLA/alkali bamboo 67 ± 2.2 13 ± 2.3 2.4 ± 0.2 6.8 ± 0.4 PLLA/silane-coupled bamboo 72 ± 3.4 11 ± 2.1 2.6 ± 0.2 4.9 ± 0.2

Fig. 70.4 Schematic of the possible mechanism of interaction between the matrix and reinforce­ment after chemical modifications [14Lu]. Reproduced with permission from Elsevier

shows the changes in the properties of PLA composites reinforced with 2 % of untreated and chemically modified bamboo fibers [14Lu]. As seen from the table, marginal increase in the tensile properties was observed after reinforcing with the raw bamboo fibers. However, considerable increase in the toughness was observed when maleated PLLA or alkali-treated bamboo was used. The silane-coupled bamboo fibers provide considerable increase in strength, but the other properties did not show major changes [14Lu]. Figure 70.4 shows the possible mechanism for the improvement in composite properties after treatment of the bamboo fibers [14Lu].

Fig. 70.5 Fracture surface of the wheat gluten composites reinforced with three (1.5, 2.5, and 5 % left, middle and right images, respectively) levels of fibers [12Sek]. Reproduced with permission from Elsevier

In most studies, biopolymers such as PLA and PHBV have been reinforced with cellulose fibers. In a unique deviation from this approach, PLA was reinforced with silk fibers [10Zha]. Bombyx mori silk fibers were processed into length of about 5 mm and melt compounded with PLA in weight ratios ranging from 1 to 7 %. Compounded samples were later injection molded to form composites. Dynamic mechanical analysis showed that storage modulus increased with the addition of the fibers. Good interfacial bonding was observed between PLA and the silk fibers which were supposed to have a plasticizing effect [10Zha]. Degradation of the composites using enzymes showed that the PLA matrix degraded much faster than silk fibers and that weight loss of the composites increased with increasing amounts of silk due to degradation of the sericin peptides [10Zha]. Instead of using virgin silk fibers, silk waste fibers were used to reinforce wheat gluten plasticized with 10 % glycerol [12Sek]. Before using as reinforcement, the silk fibers were treated with 1 % NaOH for 1 h to remove sericin, and the fibers were later cut into lengths of about 1 mm. The treated silk fibers and wheat gluten were mixed together and cast to form sheets which were later compression molded at 120 °C, 2 MPa for 20 min to form the composites [12Sek]. Glutaraldehyde was also added as a cross­linking agent to improve the properties of the composites. Tensile strength of the composites increased from 17 to 28 MPa with the addition of 5 % of silk fibers. Similarly, modulus increased from 811 to 1,605 MPa, whereas elongation decreased from 13.9 to 3.4 %. SEM images (Fig. 70.5) showed narrow interaction between the fibers and matrix at different loadings.

In another study, waste silk fibers processed into various lengths were used as reinforcement for poly(butylene) succinate (PBS) composites. Silk waste obtained after processing was cut into 25.4,12.7, 6.4, and 3.2 mm and reinforced into PBS in 20, 30, 40, and 50 % by weight. Composites were fabricated by compression molding at 135 °C for 10 min at a pressure of 6.9 MPa [06Han]. Tensile strength and modulus of the composites were found to increase from 35 to 42 MPa and 0.5 to 1.3 GPa, respectively, when the proportion of the fibers was increased from 0 to 40 % [06Han]. Further increase in the fiber content to 50 % resulted in a decrease in tensile properties. Similar phenomenon was observed when the length of the fibers was decreased. However, flexural strength and modulus did not show a significant

Fig. 70.6 Biodegradation (%) of the thermoplastic matrices and the fiber-reinforced composites with (a) and without (b) the compatibilizer [08Lov]. Reproduced with permission from Elsevier

change when the length of the fibers was changed [06Han]. Dynamic thermal analysis also showed that the storage modulus and tan S peak height were affected to a larger extent by the amount of fibers rather than the length of the fibers.

Although several researchers claim to have developed biodegradable composites using reinforcement and matrix from renewable resources, actual biodegradability tests or the degradation of the composites in various environments has rarely been reported. In one study, the biodegradation of poly(lactic acid) biocomposites reinforced with coir and starch was tested. The biodegradation test was done according to ISO standard 14855. Samples were placed in a bioreactor and the carbon dioxide evolved during the biodegradation was measured. Figure 70.6 shows the % degradation of the individual components and the composites devel­oped after various incubation times [08Lov]. After 90 days of incubation, the thermoplastic starch has a much higher level of degradation. The composites, however, did not show major differences in degradation. During biodegradation, a biofilm is formed on the surface, and the bacteria and fungi on this biofilm accelerate the degradation of the composites. Significant erosion of the surface of the composites (Figs. 70.7 and 70.8) was observed after 21 days, and complete erosion of the matrix was seen after 70 days of incubation [08Lov].

Nanofiber composites were also developed using eucalyptus kraft pulp and PHBV. Nanofibrous cellulose was obtained by chemical treatment, and fibers had diameters of 5-10 nm in aqueous gels, but the fibers were found to agglomerate and had typical diameters of 1 pm and were used as reinforcement in the composites. PHBV dispersed in distilled water was mixed with nanofibrillated cellulose (2.5­10 %) and injection molded into specimens at 180 °C [13Sri]. The addition of NFC into the PHBV matrix increased the tensile strength as seen in Fig. 70.9. However, ultimate tensile strength did not show a major difference, but modulus of the

Fig. 70.7 SEM images demonstrating the changes in the surface of the fibers due to degradation in laboratory composting conditions [08Lov]. Standard matrix (a), matrix after 21 days of degradation (b), fiber after 21 days of degradation (c), composite after 70 (d) and 90 (e) days of degradation. Reproduced with permission Elsevier

composites increased and elongation decreased substantially as seen in Table 70.7. Further, DMA also showed an increased modulus with increasing PHBV content. Crystallization and glass transition temperatures were also found to increase, but the inclusion of PHBV decreased the thermal resistance.

Fig. 70.8 Digital picture of the composite sample before (a) and after (b) degradation for 90 days [08Lov]. Near-complete removal of the matrix can be seen from the figure on the right. Reproduced with permission from Elsevier

Fig. 70.9 Stress-strain behavior of PHBV composites reinforced with three levels of nanofibrillated cellulose [13Sri]. Reproduced with permission from Elsevier

Table 70.7 Tensile properties of nanocellulose-reinforced PHBV composites at three different levels of nanocellulose contents [13Sri] Sample Tensile strength (MPa) Strain at break Modulus (MPa) PHBV 31.7 ± 0.3 0.088 ± 0.01 1,682 ± 36 PHBV + 2.5 % NFC 32.1 ± 1.0 0.067 ± 0.01 2,065 ± 143 PHBV + 5 % NFC 34.4 ± 0.3 0.055 ± 0.004 2,601 ± 49 PHBV + 10 % NFC 34.3 ± 0.4 0.039 ± 0.002 3,196 ± 87

References

[03Shi] Shibata, M., Ozawa, K., Teramoto, N., Yosomiya, R., Takeishi, H.: Macromol. Mater. Eng. 288, 35 (2003)

[06Han] Han, S. O., Lee, S. M., Park, W. H., Cho, D.: J. Appl. Polym. Sci. 100, 4972 (2006)

[07Ave] Avella, M., Bogoeva-Gaceva, G., Buzarovska, A., Errico, M. E., Gentile, G., Grozdanov, A.: J. Appl. Polym. Sci. 104, 3192 (2007)

[07Zin] Zini, E., Focarete, M. L., Noda, I., Scandola, M.: Compos. Sci. Technol. 67, 2085 (2007)

[08Lov] Lovino, R., Zullo, R., Rao, M. A., Cassar, L., Gianfreda, L.: Polym. Deg. Stab. 93, 147 (2008)

[08Sin] Singh, S., Mohanty, A. K., Sugie, T., Takai, Y., Hamada, H.: Compos. Part A 39, 875 (2008)

[10Ble] Bledzki, A. K., Jaszkiewicz, A.: Compos. Sci. Technol. 70, 1687 (2010)

[10Zha] Zhao, Y., Cheung, H., Lau, K., Xu, C., Zhao, D., Li, H.: Polym. Deg. Stab. 95, 1978

(2010)

[12Far] Faruk, O., Bledzki, A. K., Fink, H., Sain, M.: Prog. Polym. Sci. 37(11), 1552 (2012)

[12Sek] Sekhar, M. C., Veerapratap, S., Song, J. I., Luo, N., Zhang, J., Rajulu, V. A., Rao, C. K.: Mater. Lett. 77, 86 (2012)

[13Ada] Adam, J., Korneliusz, B. A., Agnieszka, M.: J. Appl. Polym. Sci. 130, 3175 (2013)

[13Jay] Jayaramudu, J., Reddy, G. S.M., Varaprasad, K., Sadiku, E. R., Ray, S. S., VaradaRajulu, A.: Carbohydr. Polym. 94, 822 (2013)

[13Sri] Srithep, Y., Ellingham, T., Peng, J., Sabo, R., Clemons, C., Turng, L., Pilla, S.: Polym. Deg. Stab. 98, 1439 (2013)

[14Kwo] Kwon, H., Sunthornvarabhas, J., Park, J., Lee, J., Kim, H., Piyachomkwan, K., Sriroth, K., Cho, D.: Compos. Part B 56, 232 (2014)

[14Lu] Lu, T., Liu, S., Jiang, M., Xu, X., Wang, Y., Wang, Z., Gou, J., Hui, D., Zhou, Z.: Compos. Part B 62, 191 (2014)