3.2. Properties of cellulose films
Fig. 5 (left) shows the stress–strain curves of the cellulose films. The tensile strength (b) and elongation at break (εb) of the films prepared from the water coagulant at various temperatures are listed in Table 2. The RC35 film coagulated at 35 ◦C exhibited the better mechanical properties and the b values achieved to 139 MPa, much larger than that of the RC0 cellulose film coagulated with the acidic. The other cellulose films of RC25, RC30, RC40, and RC45 displayed tensile strength similar to RC0. Usually, optical transmittance (Tr) reflects the homogeneity, and is related to
the structure of the materials. The Tr values at 800 nm of the RC films are summarized in Table 2. The regenerated cellulose films exhibited a good optical transmittance at 800 nm on the whole, indicating a homogeneous structure. Obviously the Tr values of the
films (RC25–RC45) were lower than that coagulated with the sulfuric acid. This could be explained that there were many bigger holes in and on the surface of the films coagulated in water, compared with that in acid. These holes made the light to lose, leading to the reducing of the Tr value. In addition, the film was contracted more
easily in the sulfuric acid aqueous system, leading to the relatively smaller pore size of the RC0 film. However, if the drawing orientation was performed, the porosity would be reduced, leading to the improvements of the transmittance and the mechanical properties of the films. The mechanical properties of the cellulose films with different
thickness are shown in Fig. 5 (right). It was indicated that with a decrease of the thickness of the cellulose films, the tensile strength increased on the whole. This further proved that the coagulation and regeneration were mainly influenced by the diffusion speed of the nonsolvent. Thinner film was regenerated more completely, leading to higher tensile strength. Water is low-cost and non-polluted coagulant, where no other chemical agents were brought to the system. So the waste water from the producing of the regenerated cellulose films is easy to be recycled. Therefore, this coagulant is “green”, and it is very important in the industry of novel cellulose materials.