The effect of calendering on the mechanical properties of paper-based, self-reinforcing composites

In this study, self-reinforcing composites (SRCs) were produced via the partial dissolution route with a NaOH/urea solvent from paper made of softwood sulphite dissolving and abaca pulp. Solvent welding leads to increased tensile strength due to gluing the fibres together with the dissolved portion of cellulose but does not densify the material completely. The resulting porosity makes it difficult to compare the obtained materials with other composites and gives the potential for optimizing the SRCs. Calendering, however, is a well-known and easy method to reduce the thickness of paper and therefore reduce the porosity, but the influence of calendering on the mechanical properties has not been widely studied for paper or for SRCs at this stage. The change of morphology and mechanical properties was investigated by calendering the untreated paper and the SRCs with nip pressures from 10 up to 200 kN/m and then comparing scanning electron microscopy (SEM), X-ray diffraction, tensile strength and short-crush resistance. The SEM imaging indicated that calendering indeed densifies the paper and the SRCs by increasing the nip pressures, but tensile strength measurements showed that the strength of paper and SRCs increases for low nip pressures but significantly decreases for high nip pressures despite better densification. Furthermore, it was found that the elastic modulus can be increased by calendering, and short-crush resistance is not influenced by calendering at all.