Marion Noel12, Emmanuel Fredon1, Eric Mougel1, Eric Masson2 and Daniel Masson1
1LERMAB INRA 1093, UHP Nancy I, ENSTIB,
In order to design a new, efficient and competitive material, no process is better than combining several components, and their own performance. Such composite materials are designed to meet specific technical expectations. They mostly consist in an organic, ceramic or metallic reinforced matrix, where reinforcement can be long or divided fibres, divided particles, filling dust… Reinforcement is expected to confer to the material its mechanical properties. The matrix binds the reinforcement parts together and guaranties the forces transmission.
Within the composite materials widely manufactured in mass industrial production, an attractive and well-known, tremendously emerging composite category is the WPC, or Wood Plastic Composites category. This material is composed of a polymeric matrix, preferably thermoplast resin as polyvinylchloride or polyethylene, even if thermoset resins as epoxies or phenolics can be, or have been, used (phenol-formaldehyde mixed with wood flour known as Bakelite, for instance). The reinforcement has to be a natural compound as wood or possibly other fibrous biomass. The addition of fillers leads to two major improvements. First the material is lightened, as fillers density is lower than the matrix density. Second, the mechanical properties are expected to be improved.
Literature content is abundant as the combinations potential is huge. Most of times, the admixtures encounter a poor interfacial adhesion between natural hydrophilic polar fillers and synthetic hydrophobic nonpolar matrix. To reach a better interfacial cohesion, chemical treatment of the surface of reinforcing components has to be carried out to allow creating bridge of chemical bonds between matrix and reinforcement. Bledzki & Gassan, 1999 and George et al., 2001 provide a list of possible cellulosic fibers treatments from esterification or acetylation of cellulose to treatment with isocyanates or silanes or use of triazine or organosilanes as coupling agents.
The use of cellulosic fibres instead of synthetic reinforcements allows to reduce production cost but also to involve natural compounds within the material conception. In the same guideline, biobased polymers have been more recently considered as matrices. Those biopolymers come from bioresources and/or can be degraded at the end of their life cycle. Bio-
sourced bio-polymers can come from animals (chitin, protein, …), microorganisms (poly(lactic acid), poly(hydroxyalkanoates), …), or plants (starch, cellulose, lignin, …). Fossile bio-polymers are mostly composed of polycaprolactone, poly(lactic acid), poly(glycolic acid), polyvinylalcohol and polyesteramide. Bio-polymers are competitive with standard polymers in terms of performance, even if their producing cost and involved energy consumption are still high.
In the environmental point of view, entirely biobased composites are obviously promising. Reinforcement of biopolymer matrices with natural fibres or fillers has been reported. Fink & Ganster, 2006 witnessed a good compatibility between cellulosic fibres with poly(lactic acid) without coupling, while polyethylene, polypropylene and polystyrene had to be grafted or copolymerised with maleic acid anhydride. Gatenholm et al., 1992 noticed also an excellent dispersibility of cellulose fibres in polyhydroxybutyrate (PHB) matrix compared with synthetic matrices.
Bio-based, natural materials are often sensitive to their environment. It is the case of wood, used since centuries in the building, furniture, artistic fields for its excellent properties. Constituted of cellulose reinforcement in a ligneous matrix, wood is itself a natural composite. But its high hydrophilic character and sensitivity to biological attacks are its most limiting disadvantages. Indeed, wood exposed outdoor has to be protected either by surface coatings or preservation chemicals. Coatings have the advantage to be more or less innocuous to the environment, but a mechanical crack in the coating layer leads to unavoidable and irreversible wood degradation. Chemical treatments penetrate the wood structure in a higher thickness, but a major concern is the releasing of preservative chemical in the environment. That’s why grafting or in-situ polymerization of chemical compounds is expected. Rowell, 2005 described the chemical modification of wood. As hydroxyl groups in wood are responsible of the hydrophilic property, their esterification with acid anhydrides is a possibility to reduce moisture affinity with wood. Different epoxydes or isocyanates are also susceptible to react with wood. Wood impregnation by polyethylene glycol, polyglycerols, formaldehyde resins, styrens or methyl methacrylate have been widely reported (Ibach & Rowell, 2001; Zhang et al., 2006).
A different way to produce an entirely biobased composite is to reinforce a natural matrix, as wood, with natural or biodegradable polymers, as polymerized lactic acid. This association has not been much reported to date (Noel et al., 2009). The affinity between both materials should lead to interesting chemical reaction providing wood protection. Grafting of carboxylic end groups of poly(lactic acid) onto hydroxyl groups of wood, and in-situ polymerization should avoid any chemical release in the environment. What is looked for is an increase in wood dimensional stability and biological resistance. Simultaneously, the density increase due to the treatment is expected to induce gainful property variation.