Study On Preparation And Properties Of Pressure Sensitive Adhesives Based On Natural Rubber

Natural rubber serves as the skeleton material of traditional transdermal patches. These patches cannot be directly melt-processed due to the high molecular weight of natural rubber. Thus, they are usually produced by solution coating technology. This technology not only pollutes the environment but also has low production efficiency, which hardly meets the requirement of producing modern transdermal patches. In this paper, natural rubber was initially subjected to degradation by an economical, efficient and controllable method. Subsequently, living anionic polymerization and graft-onto coupling reaction were utilized to synthesize thermoplastic natural rubber (NR-g-PS) with one backbone of tailored natural rubber and branching chains of polystyrene. Finally, NR-g-PS serves as the skeleton material to prepare natural rubber-based adhesives by melt-processing technology. The main contents of this study are as follows:Firstly, based on mechanisms of epoxidation (in situ) and oxidative degradation of natural rubber, protein-free natural rubber was epoxidized with HCOOH/H2O2 in situ and degraded with periodic acid, respectively. With the optimization of periodic acid and epoxy groups, natural rubber was tailored to meet the requirement of synthesizing thermoplastic natural rubber. The experiments were carried out in an ice water bath to lower the possibilities of side reactions. GPC results showed that epoxidation degree and periodic acid are two key factors of determining the degradability of natural rubber. These two factors could be utilized to tailor the target molecular weights by adjusting their ratios.'H-NMR results indicated that degradation occurs in epoxy groups, and the content of functional end groups is associated to the depth of degradation.Secondly, anion polymerization and graft onto technique were utilized to prepare NR-g-PS with degraded natural rubber as the backbone and polystyrene side chains. GPC and 1 H-NMR results suggested that polystyrene was grafted onto natural rubber successfully. DSC detected subtle glass temperature of polystyrene, indicating that NR-g-PS was thermoplastic. Rheological results detected obvious glass transition temperature of polystyrene branches. The storage moduli of grafted natural rubber were higher than that of degraded ones when temperature was below the Tg of polystyrene.Finally, based on polymer blending theory, thermoplastic NR-g-PS was blended with C5 resin to prepare adhesives for modern transdermal patches by adjusting their ratio. Adhesion performances showed that their optimal ratio depends on the epoxidation degree of the backbone. This is probably because the compatibility between grafted natural rubber and C5 are influenced by epoxidation degree.