Influence of Paper Content and Matrix Selection on the Porosity, Crystallinity and Water Uptake of Thermoplastic Paper Composites

Pulp and paper fibers have favorable reinforcement properties, such as constant fiber quality, while being widely available at low cost. However, they also have severe drawbacks related to water uptake and material degradation. In this work, paper was used for sustainable reinforcement in thermoplastic composite laminates, which were characterized by thermal analysis and moisture uptake tests. The porosity of the composite samples with varying paper content and matrix material was determined by thermogravimetric analysis and related to their water uptake. Polypropylene and polyamide 12 matrix composites had the lowest porosities, between 2 and 6 vol%, and thus comparatively low water uptake. Additionally, the influence of moisture uptake on the mechanical properties of the composites was investigated by tensile testing, which resulted in a significant decrease in modulus and strength in the wet state. After drying, however, 70–90% of the original tensile properties were regained. Additionally, a nucleating effect of the paper surface could be observed, which led to a linear increase in matrix crystallinity with increasing paper content.