New polymeric-coupling agent for polypropylene/wood-flour composites

Abstract

The suitability of using polypropylene modified with an organosilane as a coupling agent for polypropylene/wood-flour composites was investigated. The tensile properties, the water-absorption behavior, the thermal degradation properties and the morphology of the composites were analyzed and compared with those of non-coupled composites and of composites coupled with polypropylene modified with maleic anhydride. The coupling agents were prepared in the laboratory and it was verified that the silane showed higher reactivity towards PP chains. The results indicated that the silane-modified polymer increased the interfacial adhesion between the fibers and the polymer matrix and this effect was better than that obtained for the maleated-polypropylene-coupled composites.

Introduction

Wood-fiber thermoplastics composites have received considerable attention from industry in recent years. The growing commercial importance of these materials has expanded efforts to understand their structure–properties relations and for exploring new methodologies for their production [1].

Wood fibers are attractive fillers for thermoplastic polymers, mainly because of their low cost, low-density and high-specific properties. They are biodegradable and non-abrasive during processing, improving the stiffness and the strength of thermoplastics [2]. Because of their wide availability, natural fibers offer a real alternative to the reinforcing fibers presently available [3]. However, the high level of moisture absorption of the wood fibers and the poor adhesion with hydrophobic polymeric matrices (such as PP) can lead to debonding with age and to lowering mechanical properties. Several studies showed that fiber–polymer bonding can be improved by the use of coupling agents [4], [5], [6], [7]. In some cases it was verified that the use of coupling agents also served to moderate and somewhat mitigate moisture movement through the composite, thus improving the mechanical properties of the materials [8], [9]. However, very limited data are available on the relationship between coupling treatment, surface wettability and interfacial bonding strength of wood and apolar polymers.

The low degradation temperature of natural fibers is also a limitation found when considering their use as fillers for thermoplastic polymers. Natural fibers are composed of a variety of chemical substances that present different degradation profiles. The three most important ones are cellulose, hemicellulose and lignin [10]. The degradation of these and other components during processing may produce a detrimental effect on the mechanical properties of the composites, both by changing the structure of the fiber as by producing volatile compounds that create microvoids across the interfaces. On the other hand, the use of coupling agents has also been shown to influence the degradation temperature of the fibers. As an example, Marcovich and co-workers [10] verified that wood-flour treatment with maleic anhydride as well as addition of PP modified with maleic anhydride increased the degradation temperature of PP/wood-fiber composites.

Several methods of improving adhesion in natural fiber/polyolefin composites have been described in the literature. Some methods are based on fiber modification (physical or chemical [7]) and others are based on the addition of a coupling agent for interfacial adhesion improvement [7], [11], [12], [13]. Sanadi et al. [12] reported that small amounts of PP modified with maleic anhydride added to the composites significantly increased the mechanical properties.

Coupling agents based on alkoxysilanes are frequently employed for cellulosic fibers. Hydrolysis of the alcoxyl groups with subsequent reaction with hydroxyl groups from cellulose can provide chemical bonding with the fibers [7], [14]. However, the bonding of the silane-coupling agents with non-polar matrices is often weak and/or unexplained. Some authors proposed that the hydrophobic part of the organosilane molecules can interact with carbonyl groups formed by oxidation of the non-polar matrices at high temperatures [15].

Recently, the literature reported the utilization of functionalized polymers as coupling agents for natural fiber/polyolefin composites [6], [13], [16]. Pracella et al. [13] observed improved fiber dispersion and higher interfacial adhesion when polyolefins grafted with glycidyl methacrylate were added to PP/hemp composites. Lee et al. [16] verified higher tensile strength and lower creep deflection for PP/wood-flour composites coupled with maleated polypropylene. Among others, the effect of acrylic acid grafted PE on the interfacial tension between PE and wood filler was studied by Wang et al. [6].

In the present study, we analyzed the suitability of using PP modified with vinyltriethoxysilane (PPVTES) as coupling agent for PP/wood-flour composites. The results were compared with those obtained for similar composites coupled with PP modified with maleic anhydride (PPMA). A systematic investigation on the effect of fiber loading and coupling agent type and concentration was undertaken to obtain optimum mechanical strength and low water-absorption level. The thermal stability of the composites was analyzed employing TGA/DTG, whereas the fiber–matrix morphology was studied through scanning electron microscope (SEM).