Enhancing crystallization and mechanical properties of poly(lactic acid)/milled glass fiber composites via self-assembled nanoscale interfacial structures

Abstract

In this work, renewable poly(lactic acid) (PLA) was mixed with recyclable milled glass fiber (MGF) to improve its crystallization and mechanical properties, which is also good for building a circular economy. To improve the interface interaction between PLA and MGF, silane coupling agent was first adopted as surface modifier, then silica nanoparticles (SiO₂) with different size (7 nm and 30 nm) were introduced onto the MGF surface via self-assembly strategy (MGF@SiO₂). It is found that MGF@SiO₂ could enhance the crystallization of PLA, even result in a formation of transcrystallization. Moreover, the interfacial interaction could be greatly enhanced via introducing SiO₂ (especially 30 nm SiO₂) onto the fiber surface, which could form mechanical locking. As a result, an obvious improvement of mechanical properties has been achieved. This new approach offers an effective way to adjust the interface structure and the interface interaction, thus helping tailor the performance of polymer/fiber composites.

Introduction

Due to the environment pressure and sustainable requirement, developing renewable and environment-friendly resource and propelling resource recycling become increasingly important. To replace petroleum based plastics, poly(lactic acid) (PLA), a bio-based and biodegradability polymer, has drawn worldwide attention, particularly since achieving the commercial production. The unique advantages, such as easy processability, excellent biocompatibility and attractive cost-performance ratio, help it become the most widely used bioplastic [1], [2]. However, its applications in some fields are still hampered by its inherent drawbacks like relatively high cost, inherent brittleness, low crystallization rate and not high enough strength and modulus [1], [3], [4], [5]. For the past decades, it was found that adding filler is a simple and effective way to enhance the performance of PLA. Plenty of researchers have focused on filling PLA with plant-derived fiber [6], [7], [8], [9]. However, there is a relative shortage of information concerning filling PLA with mineral fibers [10], [11], which possess higher mechanical properties, controllable size and water non-absorption compared with plant-derived fibers [6]. This is of great significance to the application of PLA in the fields of engineering plastics as the environmentally friendly substitutive material of traditional petroleum based plastics, e.g. polyolefin and polyamide.

Silicate glass fiber has been widely accepted as a reinforcement in polymer-based composites for commercial applications including automobile, wind turbine blades, transportation pipes, printed circuit board substrate, chemical storage tanks and construction materials, etc. According to the report from the China Fiberglass Industry Association in 2016, it was estimated that more than 3 million tons of glass fibers were produced all over China in 2015. They are accompanied by a large amount of production waste and scrap waste. A 2005 report by Hemmings showed that solid waste accounting for 10–20 wt% of total production is produced and disposed by landfill in a typical glass fiber manufacturing facility [12]. In response, milled glass fiber (MGF) with cylindrical shape, as one of the solutions, is produced by grinding the residues from continuous glass fiber into small lengths [13]. Similar MGF can also be obtained from recycled plastics such as printed circuit board substrate [14], [15]. In general, the ratios of the length to diameter of MGF lie between 10 and 20. As a component of polymer composites, MGF can be used to improve mechanical properties, wear resistance, thermal properties, flame retardance, formability and dimensional stability of various polymers, such as epoxy resin, poly(butylene terephthalate), polyethylene, polyurethane, polypropylene and polyurea [15], [16], [17], [18], [19], [20], [21], [22].

Apart from the matrix and the filler properties, variables such as fiber length, fiber length distribution, fiber dispersion, and the interaction between fiber and matrix in particular have significant influence on the properties of glass fiber reinforced polymer composites [23], [24], [25], [26]. It is also considered that the length of MGF is shorter than that of critical fiber which is required for the appearance of the filling effect on strength; that is to say, fiber breakage will not happen [16], [27], [28]. Therefore, the interfacial adhesion between fiber and matrix is especially important in determining the strength and the toughness of MGF loaded composites [29]. Silane coupling agents, which possess both silicon ends and organic groups, have been widely used as fiber surface modifier in industrial production to achieve effective interfacial bonding [30], [31], [32]. Grafting polymer on the surface of MGF is also an important technology to improve the interfacial adhesion, but the operation condition is very strict [19], [33], [34], [35].

Another increasingly intriguing approach is to construct nanostructure on the fiber surface by nanoparticles, especially by spherical nanoparticles. This method could lead to the change of fiber surface topography, enlargement of the filler specific surface area and enhancement of the dispersibility of nanoparticles [36], [37], [38], [39], [40], [41]. Zhang Ying-Chen et al. [36] tried to use helium plasma to improve the interaction between carbon fiber and silica nanoparticle (SiO₂). Others added SiO₂ to the sizing agents for carbon fiber, basalt fiber and glass fiber, and found that it is an effective way in improving the mechanical properties of the fibers and the composites [37], [39], [40]. However, the content of SiO₂ is limited and the particles are wrapped in a layer of sizing agents, which reduce the specific surface area of the particles. Our earlier studies have shown that the glass fiber encapsulated by graphene oxide via self-assembly strategy exhibits excellent nucleating ability for PLA and is beneficial to increase the mechanical properties of the composites [11], [42]. Self-assembly strategy is a simple and promising approach for building controlled nanoscale architectures.

Herein, in this work, SiO₂ was chosen as the MGF modifier via self-assembly strategy to enhance the interfacial interaction between PLA and MGF. SiO₂ has many advantages including affluent raw material sources, wide range of sizes and cost effectiveness. More importantly, SiO₂ with ultrahigh specific surface area has many highly reactive hydroxyl groups on the surface and thus it can be easily modified. SiO₂ coated MGF (MGF@SiO₂) was fabricated via self-assembly strategy with the help of silane coupling agents. Our goal is two-folds. One is to investigate the impact of SiO₂ coating on the heterogeneous nucleation effect of MGF for PLA. The other is to explore the influence of SiO₂ on the mechanical properties of PLA/MGF composites. This work aims to develop a new strategy to improve the interfacial interaction between PLA and MGF and thus improve the performance of the prepared composites.