The Effect of Varying the Amount of Short Hemp Fibers on Mechanical and Thermal Properties of Wood–Plastic Composites from Biobased Polyethylene Processed by Injection Molding

Abstract

Biobased HDPE (bioHDPE) was melt-compounded with different percentages (2.5 to 40.0 wt.%) of short hemp fibers (HF) as a natural reinforcement to obtain environmentally friendly wood plastic composites (WPC). These WPC were melt-compounded using a twin-screw extrusion and shaped into standard samples by injection molding. To improve the poor compatibility between the high non-polar BioHDPE matrix and the highly hydrophilic lignocellulosic fibers, a malleated copolymer, namely, polyethylene-graft-maleic anhydride (PE-g-MA), was used. The addition of short hemp fibers provided a remarkable increase in the stiffness that, in combination with PE-g-MA, led to good mechanical performance. In particular, 40 wt.% HF drastically increased the Young’s modulus and impact strength of BioHDPE, reaching values of 5275 MPa and 3.6 kJ/m2, respectively, which are very interesting values compared to neat bioHDPE of 826 MPa and 2.0 kJ/m2. These results were corroborated by dynamic mechanical thermal analysis (DMTA) results, which revealed a clear increasing tendency on stiffness with increasing the fiber loading over the whole temperature range. The crystal structure was not altered by the introduction of the natural fibers as could be seen in the XRD patterns in which mainly the heights of the main peaks changed, and only small peaks associated with the presence of the fiber appeared. Analysis of the thermal properties of the composites showed that no differences in melting temperature occurred and the non-isothermal crystallization process was satisfactorily described from the combined Avrami and Ozawa model. As for the thermal degradation, the introduction of HF resulted in the polymer degradation taking place at a higher temperature. As for the change in color of the injected samples, it was observed that the increase in fiber generated a clear modification in the final shades of the pieces, reaching colors very similar to dark woods for percentages higher than 20% HF. Finally, the incorporation of an increasing percentage of fibers also increased water absorption due to its lignocellulosic nature in a linear way, which drastically improved the polarity of the composite

Mechanical and thermal properties of recycled polyethylene/surface treated hemp fiber bio‐composites

“…The higher the filler content, the higher the Cellulose and, therefore, the higher the water absorption 90 . The increase in the amount of water absorption in the raw hemp fiber with an additive rate of 50% is attributed to the lignocellulosic structure of the hemp fiber 18 . The lignocellulosic structure has highly polar compounds, and therefore, the water affinity of hemp fiber is high 82 .…”

“…90 The increase in the amount of water absorption in the raw hemp fiber with an additive rate of 50% is attributed to the lignocellulosic structure of the hemp fiber. 18 The lignocellulosic structure has highly polar compounds, and therefore, the water affinity of hemp fiber is high. 82 Since the extraction process removed hydrophilic compounds from the raw hemp fiber, a 47.5% decrease was observed in the water absorption capacity of the rPE-CNF composites when the highest additive rates were evaluated compared to the raw hemp-reinforced composite (Figure 11A,B).…”

“…In these studies, mostly mechanical properties such as impact, shear stress, and tensile stress was analyzed and investigate with the different binders. 15,16 The studies of various binders has been reported such as biodegradable polymers, 17 polyethylene, 18 epoxies, 19,20 polypropylene, [21][22][23] polybutylene adipate-co-terephthalate, 24 polyamide PA 11, 25 and so forth. In most of the studies, different compatibilizers and plasticizers were used to improve the mechanical properties of binders.…”

Green Composites from Partially Bio-Based Poly(butylene succinate-co-adipate)-PBSA and Short Hemp Fibers with Itaconic Acid-Derived Compatibilizers and Plasticizers

“…Regarding to crystallinity, it can be observed how the degree of crystallinity of the neat BioPBSA and the composite with 30 wt.% of hemp fibers does not vary, with a value around 18.5% in both cases. According to the data reported by Dolçà et al this is due to the amount of fiber introduced in the matrix polymer, increasing the crystallinity values as the fiber percentage increases [ 67 ]. With the addition of DBI to the BioPBSA/HEMP composite, the degree of crystallinity decreases slightly, leading to a combined effect of, to a lesser degree, plasticizing, predominating the compatibilization effect, increasing the polarity between the BioPBSA and hemp chains, resulting in a restriction in the mobility of the chains, with percentages of 16% for both formulations.…”

“…Generally, the introduction of natural fibers allows wood–plastic composites (WPC) to be obtained due to the intrinsic color of the lignocellulosic filler. In this case, the incorporation of short hemp fibers gives the composites a characteristic brown-like color [ 67 ].…”