Transparent composite films prepared from chemically modified cellulose fibers

Abstract

Mixtures of partially carboxymethylated softwood kraft pulp fibers (CMF) and carboxymethylcellulose (CMC) are made into CMF/CMC “composite” films without mechanical fibrillation or high temperature pressing. Drying conditions largely influence the physical appearance of the pulp fibers upon carboxymethylation, as an efficient and complete structural collapse of the swollen fibers is required to achieve transparency. Ethanol dried CMF/CMC mixtures produce white, fluffy fibers with increased water absorbency. Such improvement in water absorbency is most likely due to the presence of structurally deformed CMF which provide intrinsic water pockets along with the hydrophilic nature of CMC. On the other hand, thin films with different transparency can be prepared when CMF/CMC mixtures are dried at 50 °C from aqueous suspensions. Physical and optical properties of the CMF/CMC films are mainly dependent on the degree of substitution (DS), as well as pH. With an increase in the DS, tensile strength, Young’s modulus, density, transparency, water absorbency, and gas barrier property are all increased. Tensile strength and Young’s modulus of CMF/CMC films are as high as 165 MPa and 13 GPa, respectively. The oxygen transmission rate of CMF/CMC film at DS 0.45 is as low as 0.0161 cm3 μm m−2 day−1 kPa−1 at 0 % relative humidity. In contrast to CMF/CMC in sodium form (Na-CMF/CMC), CMF/CMC in hydrogen form (H-CMF/CMC) make films with improved wet strength without sacrificing transparency. Transparent CMF/CMC films can be used in areas such as disappearing packaging, wound dressing, and moisture sensing.