Twin-screw extrusion impact on natural fibre morphology and material properties in poly(lactic acid) based biocomposites

Natural fibres from miscanthus and bamboo were added to poly(lactic acid) by twin-screw extrusion. The influence of extruder screw speed and of total feeding rate was studied first on fibre morphology and then on mechanical and thermal properties of injected biocomposites. Increasing the screw speed from 100 to 300 rpm such as increasing the feeding rate in the same time up to 40 kg/h helped to preserve fibre length. Indeed, if shear rate was increased with higher screw speeds, residence time in the extruder and blend viscosity were reduced. However, such conditions doubled electrical energy spent by produced matter weight without significant effect on material properties. The comparison of four bamboo grades with various fibre sizes enlightened that fibre breakages were more consequent when longer fibres were added in the extruder. Longer fibres were beneficial for material mechanical properties by increasing flexural strength, while short fibres restrained material deformation under heat by promoting crystallinity and hindering more chain mobility

Thermo-mechanical processing of sugar beet pulp. I. Twin-screw extrusion process

Sugar beet pulp (SBP) is the raffinate of sugar extraction. Composed of empty vegetal cells, three quarters of it consist of polysaccharides. As it is cheap and produced in great quantities SBP is a potential raw material for industrial applications other than cattle feeding. Twin-screw extrusion modified its structure and destructuring level depended on the specific mechanical energy provided (SME). By gradually increasing this energy, the rate of soluble matter increased, cell structure was progressively destroyed and SBP rheological behaviour was modified. For an SME of 745 W h kg-1, SBP examined through a scanning electron microscope showed a structure similar to that of a composite formed by a continued matrix consisting mainly of pectin and hemicelluloses filled with cellulose microfibres. Plasticized SBP was then formed by injection-molding. Thus treated, SBP becomes a cheap alternative to the use of thermoplastic starch for the production of biodegradable materials

Thermo-mechanical processing of sugar beet pulp. II. Thermal and rheological properties of thermoplastic SBP

Thermoplastic properties of extruded sugar beet pulp (SBP) are closely linked to water–polymer relationships. DSC analyses of water evaporation, water fusion and biopolymer relaxation according to SBP moisture content gave a more accurate estimation of SBP hydration steps than classical water adsorption isotherm. Three moisture contents (8%, 26%, 41% db) were then defined as limits of the different hydration behaviours. Melt viscosity measurements of SBP showed its shear thinning behaviour and the measured apparent viscosity is in the same range as thermoplastic starch viscosity. Using only water as plasticizer, to one temperature corresponded one moisture content to reach the optimal flowing properties: 110°C/35%, 120°C/25% and 130°C/20%. This "plasticization" temperature has been estimated through DSC measurements of large amounts of moistened SBP in pressure-resistant pans. This new kind of DSC measurement revealed a new sample mass dependent second order transition between 140 and 180°C, which temperature is linearly linked to the sample mass

Synthesis and properties of composites of starch and chemically modified natural rubber

A means is developed for forming polysaccharide-based composites with useful material properties through use of unmodified and chemically modified natural rubber latex (NRL). Starch was used as a model for polysaccharides. The NRL was modified by grafting with dimethylaminoethyl methacrylate (DMAEMA) to form a latex with cationic water-soluble polymeric “hairs” of polyDMAEMA, which should form hydrogen bonds with starch. Starch solutions, containing 20% glycerol as a film-forming aid, and the modified NRL were mixed and films allowed to form. The unmodified latex acted only as filler in the starch films, but with modified NRL, the mechanical properties of the films were significantly altered. The elastic modulus was greatly decreased and strain at break greatly increased. The glass transition temperature increased from –48°C to –32°C, suggesting significant compatibilization. Freeze-fracture TEM micrographs indicate strong interactions between the surface of the modified NRL and starch. The polyDMAEMA chains are more hydrophilic than the starch, and the addition of grafted latex results in a 20° drop of the water contact angle of the formed film, and a 25% increase of the water absorption compared to the native starch; with unmodified NRL, the opposite effect was observed

Thermo-mechanical processing of sugar beet pulp. III. Study of extruded films improvement with various plasticizers and cross-linkers

Thermoplastic sugar beet pulp (thermo-mechanical processing was discussed in previous studies) was formed into film strips by extrusion. Film tensile properties are discussed according to the molecular structure of external plasticizer. Sorbitol, fructose and adipic acid have a marked antiplasticizing effect, while urea and xylitol gave higher ultimate tensile stress than glycerol for a comparable strain at break. Xylitol can be considered as the best plasticizer with UTS and EL of, respectively, 4.9 MPa and 11.3% and water absorption (85%RH, 25°C) was less than 25%. Glycidyl methacrylate was directly used in the extrusion process as cross-linker. In high humidity atmosphere (97%RH, 25°C), film water absorption was then kept under 40% while the tensile properties were improved of 50% and with a 30 minute UV post-treatment the mass gain in absorption was even less than 30% after 5 days