Polylactide/basalt fiber composites with tailorable mechanical properties: Effect of surface treatment of fibers and annealing
Introduction
Polylactide (PLA) is the most widely studied and utilized biodegradable and renewable aliphatic polyester, with potential to replace conventional petrochemical-based polymers. However, PLA shows dissatisfactory outcomes in many fields, despite its eco-friendly characteristics and sustainability. This limits its further applications. Several technologies [1] have therefore emerged in recent years with an emphasis on achieving mechanical and chemical properties equivalent or superior to conventional polymers. Among those most commonly used chemical or physical modification ways, hybridization with micro- or nano-sized particles has been revealed to be a simple but very effective one to improve final properties or to control hierarchical structures of PLA [2], [3]. Many kinds of particles with different dimensions have been used as the filler successively to produce bio-based PLA composites with improved properties or even with unexpected performance. Among them, natural or man-made fiber materials with micro-sized 1-D structure, including glass fibers (silicates) [3], [4] and carbon fibers/nanotubes (carbonaceous particles) [5], [6], [7], [8], as well as natural fibers (cellulose material) [9], [10], [11], [12], etc., have attracted much interest. This is because high aspect ratios of fibers favor improving their adhesion with PLA matrix and the formation of denser network structure. Thus, those fiber materials can be used as the reinforcements and even as the good toughener to PLA [9], [10].
Recently, basalt fiber (BF) has attracted increasing interest in the fiber filled polymer composite fields as the reinforcement because of its satisfactory mechanical modulus and strength, as well as desirable temperature resistance and good chemical resistance [13]. Actually BF has been used in the military applications since World War II in last century. As one important member in silicate fiber family, BF shows higher strength and elastic modulus as compared to glass fiber (GF). Its impact strength and abrasion resistance are also better than GF [14]. The key point is that its unit cost is lower than GF, especially far cheaper than their carbon counterparts such as carbon fiber (CF). Thus, BF becomes a good alternative increasingly to reinforce the common polyolefins [15], [16], [17], engineering plastics [18], [19], [20], [21] and thermoset resin [22], [23], aiming at fabricating new composites with high performance, or extending the applications of those traditional polymer materials.
Besides those good mechanical and thermal properties, BF is also non-toxic and natural [13]. It is a mineral fiber, but can be considered also as natural because it can be found in nature as the solidification of molten lava, which is virtually present everywhere on the globe [24]. It is also harmless to the human health, which has been experimentally proved [25]. These eco-friendly features make it a good alternative to reinforce biodegradable aliphatic polyesters to prepare green composites [26]. There are already a few reports on the BF reinforced PLA composites [24], [25], [26], [27], [28], [29], [30], [31]. Kim et al. [27] performed a pioneer work on PLA/BF composites. They treated BF with plasma polymerization and found an evident improvement of phase adhesion in the composites, which resulted in 45% increase of the mechanical strength. Similar reinforcement of pristine BF was also found by Tábi’s group [26]. Besides, they reported that the long BF reinforced PLA was superior to the short BF reinforced one [24], and the presence of BF effectively reduced the strain and increase time to failure of the composites during creep load, which might open the possibilities for the BF filled PLA composites to be used in the long-term constantly loaded structural or engineering applications [28].
Moreover, BF was proved to be chemically and biologically inert and hence PLA/BF composites could be used even in medical implants [29] (actually BF is degradable (to the mineral content of the soil) although its degradation process takes much longer time compared to PLA [24]). This indicates that PLA/BF composites could still be used as the biocomposites. Besides, filling a biodegradable polymer with a non-biodegradable fiber still makes sense. PLA is not only used because it is biodegradable but also because it is renewable. The polymer has been reinforced with natural fibers in a large number of studies. However, natural fibers may not be strong enough for all applications, and as a consequence, BFs could be considered.
Therefore, using BF as the reinforcement of PLA is expected not only to provide a new type of green composites, but also to fabricate environmentally friendly yet economically justifiable structural and functional materials. Although the structure and properties of PLA/BF composite systems have already been explored, some issues are still worthy of deep study. For instance, the relations between fiber-matrix interface structure and final properties of composites, and between fiber surface structure and bulk properties of PLA, are not yet very clear. In this work, PLA composites with silicane coupling agent-treated BF and pristine one were prepared for the interfacial structure and property study. The surface properties of BF were detected by the fiber wetting technology. The mechanical strengths of the samples containing BFs with and without treatment were then explored. Finally, the relations between interfacial structure and annealing histories were further established through morphological methods and fiber pull-out experiments. The aim of this work is to provide useful information on the fabrication of PLA/BF composites with tailorable structure and properties.
Section snippets
Material preparation
PLA (2002D) was purchased from NatureWorks Co. Ltd., USA. It has the D content of 4.25 wt% and the residual monomer content of 0.3 wt%, with number average molecular weight of about 80,000 g mol−1. Basalt fibers with the average diameters of 7–15 μm and the average lengths of 5 mm (the aspect ratio is about 300–520) were purchased from Zhejiang Shijin basalt fiber Co. Ltd., P. R. China. It is a pristine fiber material with the elastic modulus of 91–110 GPa and the density of 2.63–2.65 g cm−3. The
Reinforcement of pristine basalt fiber in composite systems
The presence of pristine BF has evident reinforcement to PLA, as can be seen in Fig. 1. The tensile strength and modulus increase with increasing BF loadings. As the loadings increase to 20 wt%, the strength and modulus increase from 56.08 to 72.98 MPa by about 30%, and from 2.58 to 3.80 GPa by about 47%, respectively. These reinforcing levels are higher than those reported by Tábi and coworkers [26], but lower than those reported by Liu et al. [30], which are mainly attributed to the different
Conclusions
Pristine BF has evident reinforcement to PLA. 20 wt% BF can increase the strength and modulus by about 30% and 47%, respectively. The increased system modulus can be well described by Wu modified Halpin-Tsai models. The surface treatment of BF with silane improves its affinity to PLA matrix, reducing the interfacial energy in composite systems, and leading to evident increases of both the tensile and impact strengths. The composites show higher level of unrecoverable strain at break, although
Acknowledgements
Financial support from the National Natural Science Foundation of China (51573156) and the Innovation Program for Undergraduates of Jiangsu Province (201611117020Z) is gratefully acknowledged.
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