Publication: Biodegradable poly (hydroxybutyrate-co-hydroxyhexanoate)/poly (butylene succinate) blends and their kenaf fibre composites
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Date
2024-09-01
Authors
Mohd Zharif, Ahmad Thirmizir
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Abstract
The increasing demand for sustainable materials and concerns over environmental issues related to synthetic polymers have increased interest in biodegradable polymers for various applications. However, enhancing the mechanical properties of biodegradable polymer blends, in this case poly(hydroxybutyrate-co hydroxyhexanoate)/poly(butylene succinate) (PHBHH/PBS) blends, remains a significant challenge, especially when lignocellulosic fibres like water-retted kenaf bast fibre (WKF) are used as reinforcement. This study aims to produce a PHBHH/PBS blend with optimal strength and toughness as a matrix for biodegradable composites derived from WKF. The PHBHH/PBS blends and their WKF composites were produced via melt blending in the presence of maleated compatibilisers, dicumyl peroxide (DCP), or both. Maleated PHBHH (PHBHHgMA) and PBS (PBSgMA) compatibilisers were synthesised via melt grafting in the presence of DCP as the free radical initiator. The addition of the compatibilisers improved the tensile strengths of the 80PHBHH/20PBS and 50PHBHH/50PBS blends, with the highest increments of 14 % and 20 % achieved at maleic anhydride (MA) concentrations of 3 and 5 phr for PHBHHgMA (3PHBHHgMA) and PBSgMA (5PBSgMA), respectively. However, improvements in toughness (Kc and Gc) were not improved. To enhance the toughness, DCP (at 0.75 wt. %) was introduced, resulting in the partially crosslinked 5PBSgMA and 3PHBHHgMA compatibilised 80PHBHH/20PBS and 50PHBHH/50PBS blends exhibiting superior toughness, with increases in Kc by 9 – 23 % and in Gc by 41 – 68 %, as well as higher tensile strength (15 – 23 %) compared to their uncrosslinked counterparts. In fact, the partially crosslinked compatibilised 50PHBHH/50PBS blends demonstrated superior tensile strength and toughness (Kc and Gc) compared to their 80PHBHH/20PBS blend counterparts due to the higher proportion of flexible and tough PBS fraction. Additionally, WKF with an optimum loading of 30 wt. % was added into the neat, compatibilised, and partially crosslinked compatibilised 50PHBHH/50PBS blends to further improve their mechanical properties. The most remarkable improvements in tensile strength (⁓ 60 %), and toughness, Kc (⁓ 22 %) and Gc (⁓ 52 %) were attained by the partially crosslinked 5PBSgMA compatibilised 35PHBHH/35PBS/30WKF blend composite due to the synergistic effect of maleated PBS compatibilisation and DCP-initiated free radical crosslinking reactions. The 30-day water immersion test demonstrated that the addition of hydrophilic WKF significantly increased the water absorption (⁓ 10-fold) and exacerbated the deterioration of the 50PHBHH/50PBS blends. Similarly, the 6-month soil burial test also revealed that the presence of WKF significantly increased the biodegradation rate of the blends. However, the introduction of maleated compatibilisers and combinations of maleated compatibilisers and DCP significantly mitigated these effects in the 35PHBHH/35PBS/30WKF composites. Overall, the combined use of maleated compatibilisers, particularly 5PBSgMA and DCP crosslinking agent not only significantly improves the mechanical properties of the composites but is also effective in enhancing their durability against water deterioration and soil biodegradation, particularly in the early stages (2 – 4 months) of burial, compared to the use of maleated compatibiliser alone.