Modification of polylactide by reactive blending with polyhydroxybutyrate oligomers formed by thermal recycling through E1cB-elimination pathway
Graphical abstract
Introduction
The growing concern over the devastating effects of continuous plastic waste disposal in ecological environments in recent years has heightened public and government awareness of the need to reverse this trend. Single-use packagings made of petroleum-based polymers account for a sizable portion of plastic wastes [1]. To this end, laws and regulations are becoming more focused on plastic waste management in the packaging sector. According to the European Action Plan for the Circular Economy, by 2030, all polymeric packaging on the EU market should be reusable or recyclable [2]. In the highly demanding sector of single-use packaging, replacing conventional plastics with biodegradable polymers made from renewable resources has become a popular alternative solution [3]. Most bio-based biodegradable polymers decompose quickly in favorable industrial composting or landfill conditions, leaving a very low carbon footprint [4]. Despite its environmental benefits, biodegradability is associated with a loss of energy and material resources. As a result, without a viable recycling strategy in place, composting or landfill disposal of the relatively expensive bio-based biodegradable polymers may not be a viable solution. Biodegradable plastics are technically recyclable, but they are currently treated as an impurity in conventional plastic's recycling systems [5]. However, as their market share grows, it is expected that the recycling of certain biodegradable plastics will become more common in the future. Among a few commercially available biodegradable polymers, special attention has been given by the scientific community to polylactide (PLA) and polyhydroxybutyrate (PHB). PLA and PHB are bio-based linear aliphatic polyesters with high biocompatibility and low toxicity, making them promising alternatives to petroleum-based polymers [6], [7]. Despite numerous advantages, these polymers suffer from major drawbacks, hindering their widespread application.
PHB is a stiff and brittle material with a high degree of crystallinity and low thermal stability [8]. PHB's re-processability is severely limited by its susceptibility to thermal degradation [9]. Thermal degradation of PHB predominantly occurs through cis-elimination reaction, a non-radical, random intramolecular chain scission involving a six-membered ring transition state. The cis-elimination reaction generates crotonic acid (CA) and unsaturated crotonate-ended oligomers while decreasing the molecular weight monotonically [10], [11]. Unfortunately, due to the non-radical nature of the cis-elimination mechanism, conventional antioxidants and thermal stabilizers are unable to effectively prevent PHB thermal degradation [12]. Several studies have attempted to selectively thermally recycle PHB into intermediate substances for various applications, such as the synthesis of poly(crotonic acid) [13]. Parodi and coworkers [14] recently developed a novel thermolytic distillation process (60 min, 170 °C, 150 mbar) to selectively depolymerize PHB to crotonic acid (up to 92% CA recovery for pure PHB). In another study, a microwave-assisted thermal degradation process was developed to recycle PHB copolymers into crotonate-ended oligomers[15].
PLA, on the other hand, has inherent brittleness and low melt strength. Several approaches to improve PLA's mechanical properties have been proposed, including copolymerization, blending with other polymers, and the use of plasticizers [6]. Among these, one of the most common methods for modifying PLA toughness is the use of oligomeric plasticizers. So far, many researchers have attempted to modify the mechanical properties of PLA by introducing synthetic oligomeric plasticizers, particularly polyethylene glycol derivates [16], [17], [18], [19], [20]. Several studies, however, have recently investigated other renewable and sustainable alternatives to synthetic modifiers. Chaochanchaikul et al. studied the role of ozonated soybean oil (OSBO) as a bio-based plasticizer in improving the toughness of PLA [21]. To improve the compatibility of OSBO with PLA, an ozonolysis reaction was used to increase the ester groups and create hydroxyl units in the oil chain. Despite the aforementioned modification, OSBO was ineffective as a plasticizer, with no significant improvement in tensile elongation or decrease in glass transition temperature (Tg). Traces of partial phase separation from the PLA matrix were also visible at 15 wt% of OSBO. Greco et al. improved PLA ductility by using cardanol oil as a bio-based plasticizer [22]. Cardanol oil is a phenolic lipid produced by vacuum distilling cashew nut shell liquid. To improve compatibility, the cardanol oil was also acetylated and further epoxidized. The addition of 20% epoxidized cardanol acetate reduced the Tg of PLA to 22.3 °C, indicating good miscibility and plasticization efficiency. Tensile elongation, on the other hand, increased to 281%, while elastic modulus decreased dramatically to 353 MPa. Perez-Nakai et al. used maleic anhydride to chemically modify hemp seed oil (MHO) and Brazil nut seed oil (MBNO) to create bio-based plasticizers for improving the toughness of PLA [23]. Both seed oils performed well in terms of plasticization. At 7.5 phr concentration, MBNO and MHO increased the tensile elongation of pure PLA from 7.4% to 52% and 42%, respectively. The decrease in Tg, on the other hand, was negligible (∼61 °C for pure PLA to ∼ 58 °C at 7.5phr of oils). Elastic and tensile modulus were also decreased, which was attributed to a reduction in molecular interactions caused by the addition of plasticizers. Quiles-Carrillo et al. investigated the effect of maleic anhydride-modified hemp seed oil (MHO) on PLA toughness in another study. They discovered that traces of partial phase separation could be detected at relatively low oil concentrations (5 wt%), implying that introducing maleic anhydride on the oil structure has limited capability to improve plasticizer miscibility. They also discovered that adding MHO increases the tensile strength and modulus of PLA, which was ascribed to chain-extension or branching reactions caused by the reaction of MHO maleic anhydride groups with PLA terminal hydroxyl groups [24].
Unlike the above-mentioned bio-based plasticizers, the chemical structure of PHB is similar to that of PLA, except for one additional –CH2- group in the repeating unit backbone. Furthermore, it is well established that low molecular weight PHB is highly miscible with PLA without further modification [25], [26]. To that end, crotonate-ended PHB oligomers could have interesting applications as bio-based oligomeric plasticizers for PLA toughness modification. Adopting PHB oligomers in the above-mentioned application would be a step toward a sustainable closed-loop plastic economy, in which renewable materials are recycled and reused without generating waste. Hakkarainen et al. [27] made the very first attempts for PLA plasticization with PHB oligomers. They followed a two-step extrusion process in which PHB was thermally recycled in a vertical twin-screw extruder before being melt blended with PLA in the presence of a free radical initiator. The reactive extrusion process with the radical initiator was carried out to covalently attach the unsaturated crotonate end groups of PHB to PLA in order to control the migration tendency of oligomers. They observed excellent plasticization efficiency by incorporating 20 wt% PHB oligomers as the blend's tensile elongation increased to 538%. Despite the aforementioned progress, some issues remained unresolved: (1) PHB oligomers (Mn ≈ 1600 g/mol) were obtained by thermal recycling at 220 °C for 45 min, which was a relatively long process; (2) recycling processes were carried through the cis-elimination reaction, which resulted in a mixture of crotonate-ended oligomers and crotonic acid rather than the selective formation of oligomers; and (3) the obtained recycling product had a dark brown color, which would affect the color of blend.
In the present study, crotonate-ended PHB oligomers were synthesized using a two-step recycling approach. PHB was thermally recycled in the first step through an E1cB-elimination reaction in the presence of potassium carbonate salt (see scheme 1). As a result, PHB could be selectively recycled to crotonate-ended oligomers at lower temperatures (190 °C) and for much shorter durations. In the second step, the obtained products were then neutralized with a dilute HCl solution to improve their thermal stability via a protonation process. PHB oligomers were then reactive melt blended with PLA in an internal mixer in the presence of a peroxide initiator, and the mechanical and crystallization properties were compared with pure PLA and simple blends containing no peroxide initiator. Concerning the deteriorating effect of plasticizers on PLA melt stability, we attempted to investigate the impact of the reactive blending process on mitigating the negative effects of low molecular weight oligomers on PLA melt rheology and extensional viscosity. We demonstrated that, in addition to being a good plasticizer, PHB oligomers could be an excellent rheology modifier, improving the shear-thinning and rheological strain-hardening behavior of PLA. Finally, we proposed possible reactions that could occur during the reactive blending process based on all of our observations.
Section snippets
Materials
Bacterial poly(3-hydroxybutyrate) (PHB) powder with Mw = 223,000 g/mol, PDI = 1.23, and purity of more than 98% was supplied by BIOMER, Krailling, Germany. Polylactide (PLA) with Mn = 128450 g/mol, PDI = 1.7 and D isomer = 1.4% was supplied by Natureworks LLC, USA (grade 4032D). Potassium carbonate (K2CO3) and hydrochloric acid (HCl) were supplied by Merk and Mojalali companies, respectively. 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane (Luperox 101), a radical initiator with a half-life of
Results and discussion
Fig. 1 depicts the physical appearance of recycled products at various time intervals. It can be seen that the samples were formed into brittle waxy clusters with a yellowish-white appearance. The recycling duration had no discernible effect on the overall color of the samples; however, rPHB9 exhibited some light brown sticky traces (marked with red arrows), indicating over-degradation. In contrast to the previous study [27], where recycled products of similar molecular weight had a dark brown
Conclusion
PHB was thermally recycled into crotonate-ended oligomers, which could be used as a functional additive to improve the toughness and rheological behavior of PLA. Thermal recycling was carried out through the fast and highly efficient pathway of the E1cB-elimination, resulting in oligomers of 1170 g/mol after 6 min of treatment at 190 °C. The oligomers were subsequently protonated with HCl to neutralize active carboxylate end groups and remove metal impurities. TGA thermograms revealed that the
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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