Fabrication of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) biocomposite reinforced wood fiber modified with mixed coupling agents CS-201 and KH550
Introduction
With the development of society and economic, nondegradable foil resources pose serious damages to ecological environment, and it can be envisaged that the bio-based and biodegradable materials such as cellulose, chitin, will receive considerable attentions (Zhang et al., 2012). However, due to inherent shortcomings, such as high cost and poor thermal stability, the industrial application of biopolymer is still limited compared with traditional oil-based polymers (Torres-Tello et al., 2017). Poly-hydroxyalkanoates (PHA) produced by carbon sources are generally considered as potential alternative for its biocompatible and biodegradable (Li et al., 2019b). Because of the high cost, the practical application of PHA is limited (Yin et al., 2019). Among various commercially-available biopolymers, poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB), as one of the PHAs family, is recognized as the leading example to prepare biopolymers for its high overall performance (Li et al., 2019a). P34HB shows the characteristic conversion from hard crystalline material to amorphous elastomers according to the 4HB content (Zhang et al., 2012). As the 4HB increases, P34HB displays properties of semi-crystallized plastics. However, the extensive application of P34HB are restricted by the poor crystallization behavior, narrow thermal processing window (An and Ma, 2017).
Wood fiber (WF) as an environmental friendly material has received attention in various respects due to the properties of low price, flexibility during processing, renewable and degradable, and can be recycled in nature without causing environmental pollution. (Kazemi-Najafi et al., 2012). Wood plastic composite has been widely studied, such as Poly(lactic acid) wood veneer sandwich composites and wood waste fibers modified poly(3-hydroxybutyrate) (PHB) (Luedtke et al., 2019; Panaitescu et al., 2020). Poplar has the advantages of fast growth and high yield. It is one of the most important deciduous tree species on earth. The available poplar wood can be used in our work with P34HB to reduce costs. However, the weak interfacial compatibility limits the practical application of the wood-polymer composites because of the inherently hydrophilic of WF and hydrophobic of polymer matrices (Han et al., 2012; Rao et al., 2018).
To improve the interfacial adhesion and mechanical performance of WF and P34HB, various methods have been reported to reduce the hydrophilicity of wood fibers, such as coupling agents (maleated polypropylene), compatibility agents (maleated polyethylene) (Buitrago-Suescún and Monroy, 2018; Wang et al., 2019). A coupling agent can enhance the interfacial adhesion of the biocomposite by making chemical bridge between wood filler and P34HB matrix. Silanes have been widely used in inorganic and mineral filler reinforced polymer composites as an efficient coupling agent (Xie et al., 2010). Titanate coupling agents have been added to some material surfaces (Ding et al., 2020; Fu et al., 2020). Compared to the single coupling agent, the composites treated with mixed coupling agents were discovered to be an effective way for improving mechanical properties because of the synergistic modification of the mixed coupling agents. Zhou et al. treated the composite with the combination of maleic anhydride (MAPE) and bis(triethoxysilylpropyl)tetrasulfide (Si69) coupling agents and the composite showed excellent performance for chemical interactions and mechanical interlocking of the mixed coupling agents. (Zhou et al., 2019).
Based on the above considerations, titanate coupling agent CS-201 and silane coupling agent KH550 as mixed coupling agents, wood fibers were modified to eliminate the polarity on the surface. A poly(3-hydroxybutyrate-co-4-hydroxybutyrate) biocomposite reinforced modified wood fibers (P34HB/WF) were processed by hot pressing method after blending. It was found that the thermal and mechanical properties of the biocomposite were significantly improved by the mixed coupling agents compared with single coupling agent (Table 3). The synergistic effect of the mixed coupling agents on P34HB/WF biocomposite was investigated in detail. The optimal contents of the mixed coupling agent were further evaluated by microstructures, thermal, mechanical and crystallization of P34HB/WF biocomposite. This work exhibits a pathway to design economic and sustainable biocomposite with high strength.
Section snippets
Materials
P34HB (12 mol% 4HB) was sourced from Tianjin Green Bio Material Co., Ltd. Wood fibers (Populus tomentosa, 80 mesh) was provided by Lichang Wood Industry Co., Ltd. titanate coupling agent (CS-201) and silane coupling agent (KH550) were acquired from Nanjing Chuangshi chemical auxiliary Co., Ltd. Glacial acetic acid and anhydrous ethanol were obtained from Tianjin Chemical Reagent No. 3 Factory.
Preparation of P34HB/WF biocomposite
Wood fibers were placed at 103℃ for 12 h in drying oven and P34HB were dried at 80℃ for 6 h in vacuum
SEM analysis of P34HB/WF biocomposite
Fig. 1 displayed the fractured surfaces of P34HB/WF biocomposite. Pull-out was seen from un-modified WF and P34HB matrix (Fig. 1a), and there was a clear boundary between WF and P34HB matrix because of the weak surface adhesion (Han et al., 2012). The gap was formed between WF and P34HB (Fig. 1b) when the content of mixed coupling agents was 0.5 %, but there is no fiber pull-out. P34HB/WF biocomposite with 1% mixed coupling agents (Fig. 1c) displayed a smoother surface and WF was tightly
Conclusions
In this work, the P34HB/WF biocomposite modified with the mixed coupling agents were prepared by hot pressing process. The mixed coupling agents showed a good synergistic effect on the interfacial compatibility of P34HB/WF biocomposite. The thermal stability of P34HB/WF biocomposite with mixed coupling agents was enhanced. The P34HB and WF were connected together by molecular chains through the mixed coupling agents. The E' of the P34HB/WF biocomposite was negatively related to the temperature.
CRediT authorship contribution statement
Xutong Liu: Writing - original draft, Writing - review & editing, Methodology, Formal analysis. Xiaojun Ma: Resources. Lizi Zhu: Writing - original draft, Formal analysis, Visualization, Project administration. Lizhi Zhu: Writing - original draft, Formal analysis, Visualization, Project administration.
Declaration of Competing Interest
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.
Acknowledgments
This research has been financially supported by Fundamental Research Funds for the Tianjin Universities (2019ZD039), and Natural Science Foundation of Tianjin city (18JCYBJC90100).
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