Selected topics in wood flour- polypropylene composites: Thermal, mechanical and time dependent response

Abstract

The need for materials for specific purposes with environment-friendly characteristics is growing, due to limited natural resources and increasing environmental regulations (1-3). Demands for saving fossil resources and for relieving the strain on disposal sites and atmosphere must be taken into account when developing and using new materials (4). For a wide variety of applications in the field of low and medium mechanical loads, renewable raw materials are increasingly used as strengthening components for innovative products, as an alternative to the use of established fibers (glass fiber and carbon fiber) (5). Moreover, the use of lignocellulosic materials in thermoplastic composites may contribute to reduce the waste of vegetal biomass (6). Recently, Guillet (7) showed that plastics have the lowest energy costs of all comparable materials and cause less environmental pollution in their production and fabrication. The technology of photodegradable plastics and experimental studies of the biodegradation of conventional and photodegradable polyethylene (PE), polypropylene (PP), and poly(ethylene terephthalate) may lead to thermoplastic/cellulosic filler composites even more environment friendly, since these reinforcing fillers derive from renewable sources, are biodegradable, and can reduce squandering of many valuable cellulosic fibers (7). Wood fiber reinforced plastic profiles are growing rapidly in nonstructural wood replacement applications like decking. Suitable matrix materials for natural fiber reinforced polymers are resin systems, thermoplastic starch, and polyolefins (PE and PP), while polypropylene provides most of the advantages with regard to economic (price), ecological (recycling behavior), and technical requirements (higher thermal stability than polyethylene) (7). Moreover, among commodity thermoplastics, polypropylene possesses outstanding properties like low density, good flex life, sterilizability, good surface hardness, very good abrasion resistance, and excellent electrical insulation properties. There are environmental and economical advantages in producing wood flour (WF) thermoplastic composites. Commonly, the main purpose for the addition of cellulose-based fillers to thermoplastics is to reduce costs per unit volume and improve stiffness (8). Low price cellulose-based fibers such as wood flour, wood fibers, and cellulose fibers have high stiffness and low density, and are recyclable and nonabrasive. Contrary to PP, cellulosic fibers are highly hygroscopic, due to the presence of polar groups on its different components. In particular, wood fibers have a natural tendency to absorb moisture, due to their overall structure made of cellulose fibers in an amorphous matrix of hemicellulose and lignin, all three components containing numerous hydroxyl groups that are strongly hydrophilic (9,10). Reinforced PP combines fiber composite mechanical properties with thermoplastic versatility of manufacturing allowing its use in several industrial sectors characterized by mass production: cars, sports, domestic appliances, and so on. The excellent processability is a key characteristic of short fiber/particulate-reinforced PP; thus, the utilization of almost the same plastic processing/molding methods developed for unreinforced polymers is adequate. Moreover, wood fibers are less abrasive to molds and mixing equipments than mineral fillers. Recent studies have demonstrated the feasibility of developing microcellular structures in polymer?wood fiber composites. The wood fiber reinforced polypropylene microfoams, a new trend in wood fiber strengthened plastic racks made by injection molding process, are now being studied. Bledzki and Faruk (11) examined in a very recent work, the influence of the different types of chemical foaming agents, wood fiber types and contents, as well as the use of a compatibilizer on the physicomechanical properties of wood fiber?PP microfoamed composites. The renaissance of the cellulose fiber composites, however, has been greatly hindered by the available forms of the cellulose fibers. Rather than providing continuous cellulose filaments, which would be the essence of the reinforcement, present techniques produce fibers or fabrics consisting of discontinuous waved cellulose fibers (12). The situation makes the wood flour a competitive substitute for the cellulose fibers, especially when extrusion compounding and injection molding are adopted as processing methods. This is clearly so, when polypropylene is concerned as the matrix polymer (13). The promise of using a cellulosic reinforcement, in combination with the possibility of plastics recycling, has made the subject of polypropylene-wood fiber or flour blends one of the most widely studied systems in this research area.