Effect of basalt fibre hybridisation and sizing removal on mechanical and thermal properties of hemp fibre reinforced HDPE composites

Abstract

Despite the advantages offered by natural fibre-based thermoplastic composites in terms of environmental impact and cost, their mechanical performance is generally lower than that of synthetic counterparts. Hybridisation with mineral fibres (basalt) can broaden the industrial applications of natural fibre reinforced composites. The present study focused on the performance of injection-moulded short basalt fibre, hemp fibre and hemp/basalt fibre hybrid high density polyethylene (HDPE) composites. Effects of a maleated coupling agent on the thermal and mechanical properties of the resulting composites were evaluated as a function of the fibre mass fraction. Hybridisation of hemp fibres with basalt fibres was found to significantly increase the mechanical properties and the crystallinity of hemp-fibre reinforced composites thus suggesting that short hemp/basalt fibre hybrid HDPE composites are promising candidates for semi-structural applications. Additionally, a sizing removal procedure mimicking the conditions experienced in an end-of-life composite thermal recycling process was defined and discussed in terms of residual mechanical properties of basalt/HDPE composites.

Introduction

It is well known that materials have permitted the development of mankind since the very beginning and no technological advancement is possible without suitable materials able to support it. Besides materials and their specific properties (thermal, mechanical, electrical and so forth), other fundamental factors that need to be taken into account include the environmental aspects of their production, use, disposal at end of life in an attempt to reduce adverse impact. It is worth mentioning how, over the years, our dependence gradually shifted from a reliance on renewable materials to one that depends on materials that use resources not easily replaceable [1]. Nowadays this trend is reversed due to a greater awareness of global resource depletion which triggered the exploration of new ways to use materials as environmental prerogatives became more and more pressing. In this framework, the use of lightweight, low cost natural fibres offers the potential to replace glass fillers in several applications [2]. For the last two decades, thermoplastic as well as thermoset-based natural fibre reinforced composites (NFCs) have experienced a tremendous growth especially in the automotive industry for door panels, seat backs, headliners, package trays, dashboards, and interior parts [3], [4]. According to a recent market report released by Lucintel [5], the global natural fibre composite materials market is forecast to grow at a Compound Annual Growth Rate (CAGR) of 8.2% from 2015 to 2020. Interest in bio-based materials, and specifically, natural fibre reinforced composites, coincides not only with legislation that has been introduced in large markets such as the European Union (Directive 2000/53/EC) but with the priority of many major automakers’ interest in global sustainability. A typical vehicle is a very complex environment, characterised by over 40,000 parts, 1000 different materials and 10,000 chemical substances. 75% is typically represented by metals and 17% by plastics, elastomers and textiles. As stated in Ford’s 17th annual report on sustainability progress, the environmental, economic and performance benefits of using plant-based materials may include reductions in carbon dioxide (CO₂) emissions, vehicle weight and petroleum consumption; lower manufacturing energy use and costs; reduced pressure on natural resources; keeping waste out of landfill; and the creation of new markets and revenue streams for farmers [6]. Unfortunately, most of these composite materials have received much commercial success in the non-structural (cosmetic) and rarely in semistructural applications mainly because of some technical challenges, such as moisture stability, fibre/polymer interphase compatibility, and consistent, repeatable fibre sources which are still the subject of intense research across the world. An approach currently used both in research and practical applications is hybridisation, as a means of combining bioresources with conventional materials thus obtaining products with a suitable balance of properties and cost [7], [8]. From a literature survey, natural fibres are usually hybridised with glass fibres and improved thermal stability, mechanical properties and resistance against water absorption are reported [8]. In this context, the aim of this study is to develop high performance composites based on high density polyethylene using short hemp fibres and basalt fibres as reinforcement, to be used in semistructural applications. Basalt fibres are produced from mineral origin and are rapidly gaining interest as a replacement of E-glass fibres in composite applications due to improved mechanical properties and better thermal resistance that could in principle allow better prospects to survive end of life recycling processes [9]. Both hemp and basalt fibres have been widely investigated as reinforcement in polymer matrices [9], [10] but their combination in short fibre reinforced HDPE composites has not been addressed. In vehicles, polymeric materials can be used in interior and exterior parts and among different polymers, polyolefins (polypropylene and high density polyethylene) have experienced a tremendous growth in the automotive industry due to their excellent cost–performance ratio, low density, great weathering and abrasion resistance, and good chemical resistance. Recently Lu and Oza [11] investigated hemp fibre reinforced composites based on recycled high density polyethylene (rHDPE) and virgin high density polyethylene (vHDPE). In order to increase fibre/matrix compatibility, hemp fibres were treated with 5 wt% NaOH. The results showed that the mechanical properties of hemp–rHDPE composites were compatible with, or even higher than those of hemp–vHDPE composites. The same authors in [12] studied the thermal stability of HDPE/hemp composites where the fibres were subjected to two different chemical treatments, with NaOH and a silane solution (triethoxyvinylsilane). Both silane and NaOH treatment were reported to improve the thermal stability of the resulting composites, but silane treated hemp-HDPE composites exhibited superior thermal stability due to a better interphase. As regards basalt fibres, several works have confirmed their potentialities to replace glass fibres [13], [14], [15], [16], [17], [18], [19], but from these studies clearly emerged the need to increase, also for basalt fibres, the interfacial adhesion with the polymeric matrix. In the present study the effect of a compatibilising agent, namely maleic anhydride grafted high density polyethylene (MAPE), on the mechanical and thermal properties of hemp, basalt and their hybrid composites has been studied. In addition, the level of interfacial adhesion of two commercially available basalt fibres with different sizings has been assessed through single fibre fragmentation tests (SFFT) in order to select the optimal basalt fibre for HDPE reinforcement. In an attempt to shed light on possible synergic effects between basalt fibre sizing and maleic anhydride grafted high density polyethylene coupling agent, some composite formulations were also manufactured with basalt fibres previously subjected to a sizing removal process based on a combination of acetone treatment and high temperature exposure. These results are reported for the first time for a HDPE/basalt system.