SO₂–ethanol–water (SEW) and kraft pulping of giant milkweed (Calotropis procera) for cellulose acetate film production

SO₂–ethanol–water (SEW) fractionation and kraft pulping were applied to produce pulp from stem chips of Calotropis procera. The two pulps with different purities of cellulose were used for the production of cellulose acetate (CA) films. The motivation of this study was to investigate the possibility to produce high-quality cellulose CA films from low-grade pulp. The results showed that SEW pulping was more efficient in the selective removal of lignin and hemicelluloses from Calotropis procera wood as compared to kraft pulping. There, the pulping intensity was enhanced by increasing the H-factor to reach low lignin and hexenuronic acid (HexA) concentrations. However, a relatively high amount of alkali-insoluble xylan remained in the bleached kraft pulp and also the HexA content could not be fully removed during ECF-bleaching. In contrast, xylan was readily removed during SEW pulping and only a small amount of HexA (8.6 meq/kg) was observed in the pulp. High weight gains of about 55 and 63% were obtained during acetylation of kraft pulp and SEW pulp, respectively indicating a high degree of substitution (DS). The CA films prepared from both kraft and SEW pulps revealed higher transparency (88.0 and 88.5%), tensile stress (109 and 146 MPa) and modulus (4.6 and 5.6 GPa) as reported by previous studies. The results confirmed that CA films can be produced from SEW or kraft pulps through iodine catalysis showing similar properties than those obtained from high-purity dissolving pulps.