Behavior of the Flexural Strength of Hemp/Polypropylene Composites: Evaluation of the Intrinsic Flexural Strength of Untreated Hemp Strands

Abstract

The growing demand for plant fiber-reinforced composites offers new opportunities to compete against glass fiber (GF)-reinforced composites, but their performance must be assessed, revised, and improved as much as possible. This work reports on the production and the flexural strength of composites from polypropylene (PP) and hemp strands (20–50 wt.%), using maleic anhydride-grafted PP (MAPP) as a compatibilizer. A computational assessment of the reaction between cellulose and MAPP suggested the formation of only one ester bond per maleic anhydride unit as the most stable product. We determined the most favorable MAPP dosage to be 0.06 g per gram of fiber. The maximum enhancement in flexural strength that was attained with this proportion of MAPP was 148%, corresponding to the maximum fiber load. The modified rule of mixtures and the assumption of similar coupling factors for tensile and flexural strength allowed us to estimate the intrinsic flexural strength of hemp strands as 953 ± 116 MPa. While falling short of the values for sized GF (2415 MPa), the reinforcement efficiency parameter of the natural fibers (0.209) was found to be higher than that of GF (0.045).

Methodologies to Evaluate the Micromechanics Flexural Strength Properties of Natural-Fiber-Reinforced Composites: The Case of Abaca-Fiber-Reinforced Bio Polyethylene Composites

“…MAPE-lignin CB may take place either through the esterification of exposed hydroxyl (-OH) groups, transesterification of methoxy (-OCH 3 ) groups, or both. In any case, as estimated in a previous work of ours for MAPP and polypropylene [46], the simultaneous bonding of the two reactive oxygen atoms of each anhydride moiety is thermodynamically hindered. Instead, the unreacted end becomes able to establish hydrogen bonds (HBs), which, due to the lower density of hydroxyl groups in lignin, are expected to be more relevant in the case of cellulose (Figure 1).…”

“…The contribution of the fibers: the modified rule of mixtures is applied in order to determine the contribution of the phases to the composite’s strength [ 33 , 46 ]: …”

“…where the flexural strength of the composite and the reinforcement are represented by σ C t and σ F f , respectively, and the tensile strength of the composite and the reinforcement are represented by σ C t and σ F f , respectively. The contribution of the fibers: the modified rule of mixtures is applied in order to determine the contribution of the phases to the composite's strength [33,46]:…”

Plant Fibers as Composite Reinforcements for Biomedical Applications

“…The reinforcement amount in the composite can vary depending on the type, fiber size, and desired properties. For instance, the optimal percentage of reinforcement can be between 20 and 50% of the composite [103]. Singha et al prepared d unsaturated polyesters (UPE) reinforced using Grewia optiva fibers and determined the percentage to be 30% of the fiber loading to obtain optimal properties, such as tensile, flexural, and compressive strength [104].…”

Mechanical Performances Analysis and Prediction of Short Plant Fiber-Reinforced PLA Composites

“…Compared to synthetic fibers, plant fibers have the advantages of convenience, low cost, high specific strength, renewability, and biodegradability [ 7 ]. Plant fiber-reinforced PLA composites (PFRP), which align with the current trend of environmental protection and energy conservation, have become a research hotspot in the field of green composites [ 7 , 8 , 9 ].…”