Abstract
Sourced from agricultural waste, hemp hurds are a low-cost renewable material with high stiffness; however, despite their potential to be used as low-cost filler in natural fiber reinforced polymer biocomposites, they are often discarded. In this study, the potential to add value to hemp hurds by incorporating them into poly(lactic acid) (PLA) biopolymer to form bio-based materials for packaging applications is investigated. However, as with many plant fibers, the inherent hydrophilicity of hemp hurds leads to inferior filler-matrix interfacial interactions, compromising the mechanical properties of the resulting biocomposites. In this study, two chemical treatments, alkaline (NaOH) and alkaline/peroxide (NaOH/H2O2) were employed to treat hemp hurds to improve their miscibility with poly(lactic acid) (PLA) for the formation of biocomposites. The effects of reinforcement content (5, 10, and 15 wt. %), chemical treatments (purely alkaline vs. alkaline/peroxide) and treatment cycles (1 and 3 cycles) on the mechanical and thermal properties of the biocomposites were investigated. The biocomposites of treated hemp hurd powder exhibited enhanced thermal stability in the temperature range commonly used to process PLA (130–180 °C). The biocomposites containing 15 wt. % hemp hurd powder prepared using a single-cycle alkaline/peroxide treatment (PLA/15APHH1) exhibited a Young’s modulus of 2674 MPa, which is 70% higher than that of neat PLA and 9.3% higher than that of biocomposites comprised of PLA containing the same wt. % of untreated hemp hurd powder (PLA/15UHH). Furthermore, the tensile strength of the PLA/15APHH1 biocomposite was found to be 62.6 MPa, which was 6.5% lower than that of neat PLA and 23% higher than that of the PLA/15UHH sample. The results suggest that the fabricated PLA/hemp hurd powder biocomposites have great potential to be utilized in green and sustainable packaging applications.
Highlights
Environmental issues associated with both plastic waste and depletion of petroleum resources have been a major driving force for the production and utilization of biodegradable biopolymers
The characteristic peaks are located at the following wavenumbers: 3335 cm−1 (OH stretching in cellulose, hemicellulose) [16], 2913 cm−1 (CH stretching in cellulose) [15], 1734 cm−1 (C=O stretching of acetyl groups in hemicellulose) [15,40], 1239 cm−1 (C-O stretching of acetyl groups in lignin) [40,41,42], 1158 cm−1 [43], 1100–1000 cm−1 (C-O and C-O-C stretching vibrations of cellulose) [40,42,44,45], 896 cm−1 [43]
The main changes in the spectra observed after alkaline and alkaline/peroxide treatment were the notable decrease in the intensity of the peaks at 1734 and 1239 cm−1 corresponding to hemicellulose and lignin, respectively, suggesting the removal of these groups
Summary
Environmental issues associated with both plastic waste and depletion of petroleum resources have been a major driving force for the production and utilization of biodegradable biopolymers. Poly(lactic acid) (PLA) is attractive for a variety of applications owing to its biocompatibility [1], renewability [2], and promising mechanical properties [3,4] Despite these key advantages, PLA has had only limited adoption in fields such as packaging, the automotive industry, and construction, mainly because of some crucial drawbacks including brittleness, low thermal stability, and relatively high production cost. Like other natural fibers, hemp hurds contain noncellulosic components such as hemicellulose, lignin, pectin, and wax; these components tend to have low thermal stability and high hydrophilicity, which may result in poor filler-matrix interfacial interaction as well as inferior processibility during the formation of biocomposites [14,15,16]. Surface modification of natural fibers through chemical treatments (e.g., alkalization [17], silanization [18], acetylation [19]), physical treatments (e.g., plasma [20,21] and thermal steam explosion [22]), and biological treatments (e.g., enzyme [23]) are typically performed before biocomposite processing
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