The stress–strain curves (Fig. 2) of TPS, which had been immediately enveloped after thermoplastic processing and stored for 1 week in the plastic bags before the testing, showed that for TPS with low fiber contents (0 and 5%) represented the typical pattern of rubbery starch plastic materials reported previously (Van Soest & Knooren, 1997). The plots were essentially linear at low strain and curved towards the strain axis at higher strains. It was clear that with the increase in fiber contents, the height of the rubber plateau was increased although the length was shortened.
A comparison of the tensile strength, elongation, Youngs modulus and Energy Break for the fiber reinforced TPS with several different fiber contents to those of the pure TPS matrix (Fig. 3). showed that for increasing fiber content, the initial tensile strength was trebled up to 15.16 MPa, while the application of the fiber made the elongation fall from 105 to 19%. The Youngs modulus of TPS behaved analogous to the tensile strength as a consequent of introducing fibers.
Energy Break, visually expressed as the areas below the stress–strain curves of TPS (in Fig. 2) had the similar behavior with the elongation. A considerable increase of tensile strength indicated that TPS was suited as the matrix for natural cellulose fibers. This was due to the remarkable intrinsic adhesion of the fiber–matrix interface caused by the chemical similarity of starch and the cellulose fiber.TPS with different fiber contents were conditioned at different RHs. Changes in the environmental humidity and storage time greatly affected the water contents of TPS, which, in turn, induced large changes in the tensile strength (as shown in Fig. 4). The materials gradually lost mechanical strength with increase of water contents. The greater the fiber contents, the more the tensile strength of TPS was for almost the whole range of water contents. This evolution could be linked to strong fiber–TPS matrix interaction between the two carbohydrate products. The existence of such interaction, related to the fiber contents, had already been confirmed by Ave´rous et al. (2001).
The Reinforcement effect increased with the fiber contents at the same water contents. However, this reinforcement effect
was gradually weakened the increase of water content; since water could separately form hydrogen bond with starch and
fiber, and then substitute original interaction between starch and fiber. At the high water contents (O20%) the fiber contents would have no effect on the tensile strength, and at even higher water contents (O30%) both the fiber and water
contents would have no effect on the tensile strength. The elongation of TPS with the different fiber contents changed with increasing water contents similarly (Fig. 4),i.e. the elongation of all samples decreased when the water contents deviated from a certain value. The more the fiber content was, the less the change of the curve. With increase
of fiber contents, TPS basically had a reducing elongation over the whole range of water contents. For any individual TPS with different fiber content, water content could not obviously affect the elongation at high water contents (O25%).