**2.2. Thermal properties**

at different filler loadings, temperatures, and crosshead speeds. The obtained results indicated that tensile strength of the composites decreased with increasing filler contents in the absence of compatibilizing agent, whereas in the presence of compatibilizing agents, these mechanical properties were significantly increased [15]. Another report showed that the bioflour-filled [rice husk flour (RHF), wood flour (WF)] maleic anhydride grafted polypropylene (MAPP) composites have good mechanical properties compared with pure polypropylene (PP) composites. The enhancement of mechanical properties was strongly dependent on the amount of MA graft (%) and the MAPP molecular weight, which is shown in **Figure 4** [16].

**Figure 3.** Flexural strength of the green composites with (a) natural fillers (bamboo flour and coffee grounds) and (b)

MDI.

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The most interesting study proved the manufacturing effect on mechanical properties of lignocellulosic material-filled polypropylene biocomposites. The obtained results of tensile strength and modulus of the biocomposites significantly improved with a fabricated twinscrew extruding system compared with a single-screw extruding system [17]. The mechanical properties of the biodegradable polymers and PBS-WF, PBS-BF biocomposites were analyzed with increasing hydrolysis time at 50°C and 90% relative humidity (RH). The resultant properties of these polymers and biocomposites show decrement with the increasing hydrolysis time, due to the easy hydrolytic degradation of the ester linkage of the biodegradable polymers. However, when the antihydrolysis agent trimethylolpropane triacrylate (TMPTA) was treated with PBS, tensile strength was significantly increased with the increasing hydrolysis time as compared to the nontreated PBS. The same results were observed for the PBS-based biocomposites [18]. The addition of paper sludge to thermoplastic polymer composites significantly improved the tensile properties with increasing mixing ratios, and tensile strength Fiber-reinforced polymer composites are often used as structural components that are exposed to extremely high or low heats. These applications include the following:


In this section, we explore the thermal properties of different natural fiber-reinforced biocomposites.

Lee et al. [21] studied the polymerization of aniline on bacterial cellulose and characterization of bacterial cellulose/polyaniline nanocomposite films. In this study, the thermal stability of the composites is described by thermogravimetric analysis (TGA). The spectacular effect indicated that the pure bacterial cellulose has good thermal stability compared with combination of bacterial cellulose and polyaniline composites. The weight loss occurred in two stages for composites. First stage was obtained at 200°C, due to the combination result of bacterial cellulose and the side chain or impurities of polyaniline. The obtained result at this stage indicates the change in macromolecule of cellulose in smaller one. For these reasons, the weight reduction pertains at low temperatures. On the other hand, due to thermaloxidative degradation of the main polyaniline chain, it establishes the second stage of weight loss at around 300°C [21]. In most of the automotive, military, aerospace applications, thermal expansion and coefficient of thermal expansion (CTE) are determined by thermomechanical analysis (TMA). Kim et al. observed this effect on natural flour-filled biodegradable polymer composites. The TMA method for determining CTE is useful for understanding the dimensional changes of biocomposite materials as well as the thermal stresses caused by increasing temperature. The effect of porous, inorganic filler treated and nontreated PBS-WF biocomposites as a function of inorganic filler type reveals that the slight decrement of thermal expansion and CTE value of PBS-WF hybrid composites was observed with the addition of 3 wt% porous inorganic filler in the biocomposites. This indicates the prevention of thermal expansion at high temperature due to the addition of lower thermal expansion porous inorganic fillers in the biocomposites. A similar effect was also observed in CTE by Kim et al. [22]. Pineapple skin (P) bioflour, nondestarched, and destarched (C) cassava root bioflours were used for the preparation of polypropylene (PP) and poly(butylene succinate) (PBS) biocomposites. TGA analysis reveals that the thermal stability of the composites decreased, as compared with pure PP and PBS materials, due to the lower degradation temperature of the bioflours (261–351°C). Differential scanning calorimetry (DSC) analysis indicated that bioflours improved nucleation and crystallinity [12]. The compatibilizing effect was also considered for improving the interface between the matrix and filler. This can also affect the thermal properties of the final product. Yang et al. [15] observed the thermal analysis of lignocellulosic material-filled PP biocomposites. With the increasing filler content, there was no change in glass transition temperature (*Tg* ) and melting temperature (*Tm*) of the biocomposites. This indicates that there is no interface between the matrix and natural filler. However, there was significant change found when the compatibilizing agent was introduced between the matrix and natural filler. The storage modulus of the biocomposite increases with the increasing filler (RHF, WF) content and it is higher than neat PP [23]. The similar effect was also observed with addition of the compatibilizing agent, which is shown in **Figure 5**.

For the fabrication of thermosetting polycardanol biocomposite, the surface was treated with jute fibers using GPS and 3-aminopropyltriethoxy silane (APS). The fiber treatment with GPS and APS were improved the interfacial adhesion between jute fibers and polycardanol resin, compared with untreated jute fiber. This result indicates that the thermal stability and thermomechanical stability also improved [24]. The influence of a zeolite type on thermal properties of natural flour-filled PP composites revealed that the addition of the zeolite content to the PP-RHF and PP-WF composites had peculiar behavior on thermal stability. With the increasing content of natural and synthetic zeolite contents, the thermal stability and degradation temperature also increase. In the presence of 3 and 5% of natural and synthetic zeolite at PP-RHF and PP-WF composites, the thermal stability for 5% mass loss was in the range of 303–329°C and 322–331°C, respectively. However, the thermal stability and degradation temperature was not significantly changed with the increasing natural zeolite content. Interestingly, this property significantly improved in the presence of quartz and the formation of metal oxides in the pozzolan content on the PP and natural flour surface. These results suggest that the addition of inorganic porous materials to reinforcing fillers enhanced the thermal

reduction pertains at low temperatures. On the other hand, due to thermaloxidative degradation of the main polyaniline chain, it establishes the second stage of weight loss at around 300°C [21]. In most of the automotive, military, aerospace applications, thermal expansion and coefficient of thermal expansion (CTE) are determined by thermomechanical analysis (TMA). Kim et al. observed this effect on natural flour-filled biodegradable polymer composites. The TMA method for determining CTE is useful for understanding the dimensional changes of biocomposite materials as well as the thermal stresses caused by increasing temperature. The effect of porous, inorganic filler treated and nontreated PBS-WF biocomposites as a function of inorganic filler type reveals that the slight decrement of thermal expansion and CTE value of PBS-WF hybrid composites was observed with the addition of 3 wt% porous inorganic filler in the biocomposites. This indicates the prevention of thermal expansion at high temperature due to the addition of lower thermal expansion porous inorganic fillers in the biocomposites. A similar effect was also observed in CTE by Kim et al. [22]. Pineapple skin (P) bioflour, nondestarched, and destarched (C) cassava root bioflours were used for the preparation of polypropylene (PP) and poly(butylene succinate) (PBS) biocomposites. TGA analysis reveals that the thermal stability of the composites decreased, as compared with pure PP and PBS materials, due to the lower degradation temperature of the bioflours (261–351°C). Differential scanning calorimetry (DSC) analysis indicated that bioflours improved nucleation and crystallinity [12]. The compatibilizing effect was also considered for improving the interface between the matrix and filler. This can also affect the thermal properties of the final product. Yang et al. [15] observed the thermal analysis of lignocellulosic material-filled PP biocomposites. With the increasing filler content, there was no change in glass transition

) and melting temperature (*Tm*) of the biocomposites. This indicates that there

is no interface between the matrix and natural filler. However, there was significant change found when the compatibilizing agent was introduced between the matrix and natural filler. The storage modulus of the biocomposite increases with the increasing filler (RHF, WF) content and it is higher than neat PP [23]. The similar effect was also observed with addition of

For the fabrication of thermosetting polycardanol biocomposite, the surface was treated with jute fibers using GPS and 3-aminopropyltriethoxy silane (APS). The fiber treatment with GPS and APS were improved the interfacial adhesion between jute fibers and polycardanol resin, compared with untreated jute fiber. This result indicates that the thermal stability and thermomechanical stability also improved [24]. The influence of a zeolite type on thermal properties of natural flour-filled PP composites revealed that the addition of the zeolite content to the PP-RHF and PP-WF composites had peculiar behavior on thermal stability. With the increasing content of natural and synthetic zeolite contents, the thermal stability and degradation temperature also increase. In the presence of 3 and 5% of natural and synthetic zeolite at PP-RHF and PP-WF composites, the thermal stability for 5% mass loss was in the range of 303–329°C and 322–331°C, respectively. However, the thermal stability and degradation temperature was not significantly changed with the increasing natural zeolite content. Interestingly, this property significantly improved in the presence of quartz and the formation of metal oxides in the pozzolan content on the PP and natural flour surface. These results suggest that the addition of inorganic porous materials to reinforcing fillers enhanced the thermal

the compatibilizing agent, which is shown in **Figure 5**.

temperature (*Tg*

184 Composites from Renewable and Sustainable Materials

**Figure 5.** Storage modulus of biocomposites from −80 to 100°C as a function of temperature. (a) RHF filled biocomposite; (b) WF filled biocomposite.

stability of the hybrid composites [25]. In the same manner, the thermal stability and thermal expansion study was performed for bioflour-filled PP biocomposites with different pozzolan contents. With the increasing pozzolan content, at 5% mass loss the thermal stability of the biocomposite increased. On the other hand, the CTE and thermal expansion of the biocomposites decreased with the increasing pozzolan content. There is no significant change in glass transition temperature (*Tg* ), melting temperature (*Tm*), and percentage of crystallinity (*Xc* ) of the biocomposites. However, the enhancement of interfacial adhesion was observed in maleic anhydride-grafted PP (MAPP)-treated biocomposites, which showed higher thermal stability, thermal expansion, and *Xc* compared with nontreated biocomposites even at 1% pozzolan content [26]. Another study revealed the effect of the addition of two different compatibilizing agents on thermal properties, maleic anhydride (MA)-grafted polypropylene (MAPP) and MA-grafted polyethylene (MAPE) to bioflour-filled polypropylene (PP) and low-density polyethylene (LDPE) composites. With the increasing MAPP and MAPE content, the thermal stability, storage modulus (*E*′), tan *δ*max peak temperature (glass transition temperature: *T*g ), crystallinity (*Xc* ), and loss modulus (*E*″max) peak temperature (β relaxation) were slightly increased except melting temperature (*Tm*). The improvement in these properties encountered due to good interfacial adhesion between the bioflour and PP matrix in the presence of compatibilizing agent treatment [27]. It is well-known fact that the composite systems must have good thermal stability and thermal expansion properties, which affect the quality of the final products. For example, during the summer, dashboard is affected by the high temperature inside vehicle. On the other hand, the thermal stability of these composite systems is very important because these materials must withstand against heat during the fire. To study this effect, Yang et al. [15] studied the effect of lignocellulosic filler on thermoplastic polyester polymer biocomposites. In TGA study, the thermal stability slightly decreased and the ash content increased with increasing the filler loading. This result revealed that there is no interfacing between the filler and matrix. To improve the interface adhesion, a suitable compatibilizer was used for improving the thermal properties. In TMA analysis, the thermal analysis, the thermal expansion of the composites was found to decrease with increasing the filler and incorporating compatibilizing agent. This result elucidates that the thermal expansion of the composite materials was prevented by using the lignocellulosic filler under different atmospheric conditions. These results are shown in **Figures 6** and **7** [28].

Another study on thermal properties of rice husk flour (RHF)-filled polypropylene (PP) and high-density polyethylene (HDPE) composites revealed that the thermal stability of PP and HDPE was higher than RHF. Moreover, with the increasing RHF content, the thermal stability decreases and the ash content increases. On the other hand, the activation energy of the RHFfilled PP composites increased slowly in the initial stage and thereafter remained almost constant, whereas that of the RHF-filled HDPE composites decreased between 30 and 40 mass% of RHF contents. This is due to the interfacial adhesion and dispersion of RHF in the PP and HDPE matrix [29].

#### **2.3. Biodegradation properties**

In this modern society, petroleum-based synthetic polymers are widely used for many applications, such as polyolefin in packaging, bottle, and molding products. Globally, the annual disposal of petrochemical plastics reached nearly 150 million tons, which creates serious environment problems, especially with the continuously increasing production and consumption

**Figure 6.** TG curves of RHF-filled LDPE composites at a heating rate of 40°C min–1 in an N2 atmosphere.

effect, Yang et al. [15] studied the effect of lignocellulosic filler on thermoplastic polyester polymer biocomposites. In TGA study, the thermal stability slightly decreased and the ash content increased with increasing the filler loading. This result revealed that there is no interfacing between the filler and matrix. To improve the interface adhesion, a suitable compatibilizer was used for improving the thermal properties. In TMA analysis, the thermal analysis, the thermal expansion of the composites was found to decrease with increasing the filler and incorporating compatibilizing agent. This result elucidates that the thermal expansion of the composite materials was prevented by using the lignocellulosic filler under different atmo-

Another study on thermal properties of rice husk flour (RHF)-filled polypropylene (PP) and high-density polyethylene (HDPE) composites revealed that the thermal stability of PP and HDPE was higher than RHF. Moreover, with the increasing RHF content, the thermal stability decreases and the ash content increases. On the other hand, the activation energy of the RHFfilled PP composites increased slowly in the initial stage and thereafter remained almost constant, whereas that of the RHF-filled HDPE composites decreased between 30 and 40 mass% of RHF contents. This is due to the interfacial adhesion and dispersion of RHF in the PP and

In this modern society, petroleum-based synthetic polymers are widely used for many applications, such as polyolefin in packaging, bottle, and molding products. Globally, the annual disposal of petrochemical plastics reached nearly 150 million tons, which creates serious environment problems, especially with the continuously increasing production and consumption

spheric conditions. These results are shown in **Figures 6** and **7** [28].

**Figure 6.** TG curves of RHF-filled LDPE composites at a heating rate of 40°C min–1 in an N2

atmosphere.

HDPE matrix [29].

**2.3. Biodegradation properties**

186 Composites from Renewable and Sustainable Materials

**Figure 7.** Thermal expansion of the RHF-PP and WF-PP composites: (a) RHF-PP composites, (b) WF-PP composites.

of these materials. Moreover, these plastic wastes capable of resistance of microbial attack, this caused undesired pollutant in soil, rivers and marine. Nowadays eco-friendly biodegradable polymers receive great attention in order to replace the consumption of petroleum-based plastic materials. These biodegradable polymer materials have potential to complete degradation into natural ecosystems such as active sludge, natural soil, lake, and marine. On the other hand, these eco-friendly polymers are capable chemical transformation by the action of biological enzymes or microorganisms. Several researchers are reported that the biodegradability of the biocomposites was most important factor for many composites.

Kim et al. [30] investigated the biodegradability of PBS and bioflour, which is a poly(butylene succinate) (PBS) biocomposite filled with rice-husk flour (RHF) reinforcing in natural and aerobic compost soil. This result indicated the percentage of weight loss of HDPE, PBS, and biocomposites, which is shown in **Figure 8**. The percentage weight loss of biocomposites decreased rapidly with increasing the RHF content. This indicates that the cellulosic materials were easily attacked by microorganisms and enhanced by the degradation capability of the composites compared with PBS and HDPE matrix. On the other hand, the significant comparison identified the percentage weight loss of the biocomposites in a natural and compost

**Figure 8.** Percentage weight loss of HDPE, PBS, and biocomposites for 80 days.

**Figure 9.** Comparison of the percentage weight loss of PBS and biocomposites at 40 wt% filler loading in natural and compost soil.

soil environment with 40 wt% filler loading. Herein, the faster degradation rate was identified in a compost soil burial test compared with a natural soil burial test over 80 days. Due to the composting environment in the chamber, the enhanced biodegradation rate of biocomposites is shown in **Figure 9** [30].

A similar study investigated the biodegradability of agro-filled PBS biocomposites and their weight loss percentage. Biodegradation generally caused by microorganisms involves hydrolytic depolymerization of cellulose materials to lower molecular weight compounds, yielding monomeric glucose units. In addition, major deterioration of cellulose and woodbased lignocellulosic materials is caused by microorganisms. **Figure 10** shows the effect of filler particle size on weight loss of the agro-flour-filled PBS biocomposites at 40 wt% filler loading. The significant effect of size variation of filler particle size on weight loss can be seen. Generally, the smaller particle size possesses a higher surface area that makes better contact with the PBS matrix, this indeed the weight loss of the larger particle size (80–100 mesh) filled-PBS biocomposites was slightly greater than that of the smaller particle size (200 mesh) filled PBS biocomposites.

Iovino et al. [31] investigated the biodegradation of poly(lactic acid)/starch/coir biocomposites under controlled composting conditions. The composite formed by reinforced thermoplastic starch (TPS) and short natural fiber (coir) with poly(lactic acid) (PLA), with and without the incorporation of maleic anhydride (MA) as a coupling agent. The biodegradation test was carried on materials of TPS and matrix (containing 75% of PLA and 25% of TPS). The result of the incubation period reveals that the TPS matrix showed a higher level of biodegradation (higher amounts of evolved CO<sup>2</sup> ) than PLA, this might be arise due to attack of microorganisms on TPS. The fibers seemed to play a secondary role in the process as confirmed by the slight differences in carbon dioxide produced. The compatibilized composite revealed a lower percentage of evolved CO<sup>2</sup> than the uncompatibilized one [31]. Similarly, the degradation of sago-starch-filled linear low-density polyethylene (LLDPE) composites under a soil burial test was observed the presence of holes on samples due to microbial activity. Moreover, the loss in properties (tensile strength, elongation at break and weight loss) of the composites was identified. After 12 months of soil burial, the tensile strength and elongation at break of the composites decreased. Weight loss of the composites changed from 0.6% during the first month to 2% in the 12 month [32]. Pradhan et al. [33] studied the compostability and

soil environment with 40 wt% filler loading. Herein, the faster degradation rate was identified in a compost soil burial test compared with a natural soil burial test over 80 days. Due to the composting environment in the chamber, the enhanced biodegradation rate of biocomposites

**Figure 9.** Comparison of the percentage weight loss of PBS and biocomposites at 40 wt% filler loading in natural and

**Figure 8.** Percentage weight loss of HDPE, PBS, and biocomposites for 80 days.

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A similar study investigated the biodegradability of agro-filled PBS biocomposites and their weight loss percentage. Biodegradation generally caused by microorganisms involves

is shown in **Figure 9** [30].

compost soil.

**Figure 10.** Comparison of the percentage weight loss of agro-flour-filled PBS biocomposites in natural soil at 40 wt% filler loading.

biodegradability of PLA-wheat straw and PLA-soy straw-based green composites. The result of this study elucidates that under aerobic composting the soy and wheat straw degraded rapidly over 70% within 45 days. The similar result obtained in the process of composites degradation irrespective of the biomass used, this rate of degradation was higher than that of pure PLA. Indeed, the faster rate degradation in composites may be due to the presence of degradable natural biomass in composites and due to reduced average molecular weight of PLA [33]. Lu et al. observed the biodegradation behavior of PLA/distiller's dried grains with soluble (DDGS) composites. These materials consist of bio-based and strong potential for industrial applications. The composites were made by adding 20% DDGS to the 80% of PLA and biodegradation experiments were conducted in soil under landscape conditions. The result of this experiment shows that during 24 weeks of degradation time the weight loss of the composites was 10.5%, while the weight loss of pure PLA was only 0.1% during the same time interval. With increasing the degradation time, the surface cracks and voids caused by erosion and loss of polymer chain length were clearly observed as shown in **Figure 11** [34].

The untreated and treated with acetic anhydride-treated (AA-) abaca fibers were reinforced with aliphatic polyesters (poly (ϵ-caprolactone) (PCL), poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), PBS, and PLA). The biodegradability of obtained composites was studied by the soil-burial test. The result of the test reveals that the presence of abaca fiber or AA-abaca did not show any effect of weight loss on PCL composites, because PCL itself has a relatively high biodegradability. However, the addition of abaca fibers was shown to accelerate the weight loss of PBS and PHBV composites. Moreover, no weight loss was observed in pure PLA and PLA/AA-abaca composites, but PLA/untreated abaca composites showed 10% weight loss after 60 days due to degradation of fiber by microbial activity [35]. Yussuf et al. [36] investigated the biodegradability difference between PLA/kenaf fibers (KF) and PLA/rice husk flour (RHF) composites by natural soil burial test. The result of this test elucidates that the biodegradability of these composites slightly increased and reached 1.2 and 0.8% for PLA-KF and PLA-RHF, respectively, for a period of 90 days. Moreover, this percentage change in biodegradability of composites is higher as compared to pure PLA, because microorganisms are easily attacked in the presence of natural fibers [36]. Another study emphasized the effect

**Figure 11.** SEM micrographs of PLA/DDGS 80/20 composites after 0, 8, 16, and 24 weeks. Biodegradation time dependence of weight loss of pure PLA and PLA/DDGS (80/20) composites in soil medium [34].

of starch on biodegradability of PLA and poly (hydroxyester-ether) (PHEE) composite bars in soil for 1 year. Due to the fast attack of microorganisms on starch, the rates of weight loss increased in the order pure PLA (~0%/year) < starch/PLA (0–15%/year) < starch/PHEE/ PLA (4–50%/year) and increased with increasing starch and PHEE contents [37]. Alimuzzaman et al. [38] studied the biodegradability of nonwoven flax fiber-reinforced PLA biocomposites under soil burial test during 120 days. The obtained result of the test emphasized that the percentage of weight loss of the PLA, flax fibers, and biocomposites increases with increasing the soil burial time for all samples. The weight loss of PLA and flax fibers after 120 days is found to be 3.08 and 91.41%, respectively. This indicates that the flax fibers show more biodegradability than PLA. The similar result obtained in composites due to presence of flax fibers. Moreover, the weight loss also increases with increasing the content of flax fibers to the pure PLA. During the soil burial test, the presence of various microorganisms and water in the soil can attack flax composites easily. This induces the fiber degradation and resulted degradation of composites [38].
