PLA Research & information: general biocomposite biodegradability bionanocomposites by-products cellulose cellulosic fibers densification density durability fiber size flax flexural properties gas permeability green composites hot-pressing process mat porosity mat thermal conductivity mechanical properties modulus of elasticity modulus of rupture moisture absorption nanofiber cellulose non-dietetic uses opacity panels starch thermal conductivity thermoplastic composites