Effect of mechanical activation pretreatment on the properties of sugarcane bagasse/poly(vinyl chloride) composites

Abstract

This current work is concerned with the pretreatment of sugarcane bagasse (SCB) by mechanical activation (MA) using a self-designed stirring ball mill and surface modification of SCB using aluminate coupling agent (ACA). The untreated and differently treated SCBs were used to produce composites with poly(vinyl chloride) (PVC) as polymer matrix. The activation grade (A_g) measurement and Fourier transform infrared (FTIR) analysis of SCB showed that MA enhanced the condensation reaction between ACA and hydroxyl groups of the SCB fibres, which obviously increased the hydrophobicity of SCB. It was found that the mechanical properties of both the PVC composites reinforced by SCB with and without ACA modification increased with increasing milling time (t_M). Scanning electron microscopy (SEM) analysis showed that MA pretreatment significantly improved the dispersion of SCB in the composites and interfacial adhesion between SCB and PVC matrix, resulting in better mechanical properties of the composites.

Introduction

Natural fibres/polymer composites (NFPC), defined as polymer matrix reinforced with wood or other natural fibres, are principally produced from commodity thermoplastics such as polyethylene (PE), polypropylene (PP), poly(lactic acid) (PLA) and poly(vinyl chloride) (PVC) [1], [2]. Due to the advantages of low cost, good mechanical and thermal properties, low density, water resistance, availability of renewable natural resources and biodegradability, NFPC are widely used in various purposes such as construction materials, decorative parts, aerospace components, and vehicles’ compartments [2], [3]. With the worldwide shortage of trees, increased wood costs and competition of wood resources from traditional wood sectors, developing environmentally friendly natural fibre sources for plastic composites is being actively pursued. Currently, the use of agricultural residues (including stalks of most cereal crops, bagasse, rice husks, and other wastes) for the production of composites has been at the centre of attention [4], [5], [6].

Sugarcane bagasse (SCB), an agro-industrial residue produced in sugar and alcohol industry, is considered as one of the largest natural fibre resources because of its high cellulose content, high yield and annual regeneration capacity. At present, the main conventional uses of SCB are for energy in the sugar factories through burning, electricity generation, pulp and paper production. There are also some other applications of SCB, such as the productions of ethanol and protein-enriched cattle feed. But these technologies are complex or immature, and the processes have been demonstrated to be uneconomical, which limit their extensive uses in industries. Moreover, the remaining SCB still lead to adverse impacts on environment [7], [8], [9]. Thus, utilization of SCB in composites can more efficiently use this bioresource. Like other lignocellulosic fibres, SCB is approximately composed of 50% cellulose, 25% hemicellulose and 25% lignin, and they associate with each other by hydrogen bonds and some other covalent bonds. Much of the cellulose in SCB is in a crystalline structure, and the microfibril bundles of cellulose are weakly bound through hydrogen bonding. Lignin, an amorphous heteropolymer, gives the SCB structural support, impermeability, and resistance against oxidative stress and microbial attack. Hemicellulose, also an amorphous polymer, serves as a connection between the cellulose and lignin fibres and gives more rigidity to the whole cellulose–hemicellulose–lignin network [10], [11]. The elementary unit of a cellulose macromolecule is anhydro-d-glucose, which contains three hydroxyl (OH) groups. These hydroxyl groups form hydrogen bonds inside the macromolecule itself (intramolecular) and between other cellulose macromolecules (intermolecular). So, all natural fibres are polar and hydrophilic in nature [3]. On the other hand, the polymer matrices are largely non-polar and hydrophobic. Therefore, both the natural fibres and thermoplastic matrices have incompatible surfaces, hindering the stress transfer at the interface. The incompatibility of the hydrophilic natural fibres and hydrophobic thermoplastic matrix leads to poor interfacial adhesion, which then results in a composite material with poor physical and mechanical properties [12], [13]. Furthermore, the natural fibres have tendency to form aggregates during processing and low resistance to moisture, which provide poor dispersion of the fibres in matrix and a potential reduction as reinforcement [14]. For overcoming the drawbacks (incompatibility and aggregation) presented in natural fibres composites, various methods have been used for surface modification of the fibres, such as alkaline treatment, esterification, etherification, graft copolymerization, coupling agent treatment, and their combination treatments [15], [16], [17], [18]. Among these methods, coupling agent treatment has been frequently applied to improve the properties of natural fibres reinforced composites [19], [20], [21]. However, the highly-ordered and recalcitrant structure of lignocellulosic fibres and particularly the crystallinity of cellulose make the fibres resist assault of other reagents [11]. In order to enhance the chemical reactivity of natural fibres and make them more accessible to other reagents, the pretreatment of lignocellulosic materials is essential to break the tight cellulose–hemicellulose–lignin network, disrupt the crystalline structure and increase the amorphization of cellulose [22]. At present, the main pretreatment methods for lignocellulosic fibres include physical pretreatments (comminution and extrusion), chemical pretreatments (acid pretreatment, alkaline pretreatment, organosolv pretreatment, etc.), physico-chemical pretreatments (steam explosion, hot-compressed water pretreatment, ammonia fibre explosion, wet oxidation, carbon dioxide pretreatment, etc.), and their combination pretreatments [11], [23], [24]. Every method has its own advantages and disadvantages, so it is necessary to apply suitable pretreatment technologies based on the properties of different lignocellulosic materials and their subsequent uses.

Nowadays, more and more innovative technologies are made to develop a simple, effective and environmentally friendly process for the pretreatment of lignocellulosic materials. Mechanical activation (MA), usually carried out by high-energy milling, refers to the use of friction, collision, impingement, shear or other mechanical actions to change the crystalline structures and properties of the solids [25]. When suffered from stress during mechanical milling, the size reduction and structural disorder (evaluated by solid amorphization) of the solids are accompanied by chemical bonds distorting and bond length extending. If the imposed stress is beyond the chemical bonding energy, the bonds of solids can be effectively broken up, along with the collapse of their crystalline structure. As a result, the activated radicals and functional groups, which may easily react with other reagents, are produced during this process [26]. In addition, a part of mechanical energy could be converted into internal energy of the milled solids during MA processing and thus enhanced the chemical reactivity of solids [27]. Compared with other methods, MA pretreatment is a relatively simple and environmentally friendly procedure attributing to the operations without the use of solvents, intermediate fusion, and so on. In our previous studies, MA has been successfully used for the pretreatment of SCB. The results showed that MA could significantly change the crystalline structures and physicochemical properties of SCB and thus enhance its chemical reactivity [28], [29], [30]. Hopefully, MA pretreatment can enhance the dispersion of SCB in thermoplastic matrix due to the size reduction and structural disorder, and the surface modification of SCB using coupling agent and thus improve the compatibility between SCB and thermoplastics. In the current work, both untreated and differently treated SCBs were used to produce NFPC with PVC as matrix. The effect of MA pretreatment on surface modification of SCB using coupling agent, and the properties of PVC composites reinforced by SCBs with and without surface modification were investigated in detail.