Pulp fiber-based composites with plasticized starch via high-shear mixing

Abstract

Fiber-based materials are difficult to process at elevated temperatures as they do not flow as easily as polymers do. The aim of the present work is to obtain a lignocellulose-based thermoformable material with as high fiber content as possible, preferably above 60 wt%. Starch has been demonstrated to bring thermoprocessability when added in large amounts to cellulose fibers. However, the aim of the present work is to obtain a fibre-starch composite with a high fibre content, 60-70 wt%, by mixing bleached Kraft pulp, starch, and plasticizers (glycerol or sorbitol) in a two-step mixing process. First, the pulp was pre-mixed with water, starch, and plasticizer at 90 °C and stirred until reaching a dry content of ca. 90 wt%, where after the pre-mix was further processed using high-shear mixing. Scanning electron microscopy analyses confirmed the successful gelatinization of starch and efficient fiber dispersion after mixing. Differential scanning calorimetry revealed thermal transitions in the samples with the highest sorbitol content, 20 wt%.

Sheets were successfully produced from two formulations via compression molding of the high-shear mixed materials: pulp fibers (60 wt%), starch (20 wt%), and glycerol or sorbitol (20 wt%), respectively. Sheets were pressed at 150 or 200°C. Tensile test revealed that the sorbitol-containing samples had higher strength but lower ductility compared to the glycerol-containing samples. The glycerol-containing samples processed at 200°C exhibited significantly lower strength and stiffness than samples prepared at 150°C, likely due to evaporation of glycerol and phase separation. The most favorable results showed a more than sixfold increase in stiffness and strength compared to pure Kraft pulp.

Structural weaknesses in the composites were primarily observed at interfaces where starch agglomerates lacked fiber reinforcement. Nevertheless, the method shows strong promise for producing thermoformable lignocellulose-based composites with up to 60 wt% fiber through the incorporation of starch and plasticizer—without compromising mechanical performance. Further optimization of the thermoprocessing conditions is necessary to enhance material strength and uniformity.