Preparation And Hydrolytic Crosslinking Of Vinyl Chloride Copolymers Containing Silane

Chemical crosslinking is an effective way for poly(vinyl chloride) (PVC) to improve the heat resistance, the dimensional stability at high temperature and the elasticity of soft product. Concerning the serious PVC degradation in the peroxide or radiation crosslinking process, and the poor processing property of crosslinked PVC resin synthesized by the free-radical crosslinking copolymerization, the silane crosslinking of PVC was adapted in this thesis. The effects of preparation methods, silane types and usage, the polymerization, processing and hydrolytic conditions on the structure of PVC, and the structure-property relationship of crosslinked PVC were investigated.Silane A and B were used as crosslinking agents in the grafting and hydrolytic crosslinking reactions of PVC. It was found that the crosslinking efficiency of silane A was higher than silane B. The gel content of silane-grafted PVC increased with the increased silane usage. It was also found that the degradation and discolouration occurred after the grafting of silane onto PVC.Vinylsilane C, D and E were respectively used as crosslinking agents in the synthesis of silane containing PVC resin by suspension copolymerization. The effects of silane types and usage, pH buffer and the storage on the gel content of PVC resins, and the crosslinking behavior of silane containing PVC after processing and hydrolytic crosslinking were investigated. It was found that the gel content of resin increased with the increased silane usage. PVC resins with very low gel content and with good processability could be prepared through adding pH buffer during the polymerization process, and through choosing vinylsilane monomer with suitable hydrolytic activity. The gel content of PVC resin kept no variation basically with the increased period of storage, indicating that the resin had good storage stability. VC/vinylsilane copolymers were partially crosslinked after the processing and the gel content increased with the increased silane content in the resin. Since VC/D copolymer degraded obviously during processing, and silane E had highest hydrolytic activity, leading to the fast crosslinking, silane C was chosen as the ideal crosslinking monomer for PVC. When silane C usage increased, the mole fraction of silane C in VC/C copolymer increased, while the molecular weight and polydispersity of resinhad no obvious change.VC/C copolymers were used in further processing and hydrolytic crosslinking. The effects of silane C usage, silane B usage, catalyst types and usage, processing and crosslinking conditions on the gel content of VC/C copolymer were mainly investigated. It was found that the gel content of VC/C copolymer increased with the increased silane C usage and the catalyst usage. Silane B added during processing could reduce the gel content of the copolymer. As the fusion temperature and time, the shear rate, the pressing temperature and time increased, the gel content of the copolymer increased. The catalytic efficiency of dibutyltin dilaurate (DBTL) was higher than that of tribasic lead sulfate (TBLS). The hydrolytic crosslinking rate increased as the crosslinking temperature increased and the gel content increased with the increase of crosslinking time. In order to decrease the crosslinking during the processing of VC/C copolymer, silane C usage, the catalyst type and usage, and silane B should be well controlled. When silane C usage increased, the crosslinking density of gel increased, the molecular weight of sol and the molecular weight between the crosslinking sites decreased, and the polydispersity of sol became narrower.The gel content of crosslinked PVC would affect the mechanical properties at high temperature and heat deformation resistance. It was found that the tensile strength and elongation at break of PVC with low gel content increased with the increase of the gel content. When the gel content of PVC was too high, the tensile strength and elongation were decreased. The initial deformation temperature increased, the deformation decreased and the dimensional stability improved as the gel content of PVC increased.