Irradiation crosslinking of PVC
Irradiation crosslinking is one of the earliest PVC crosslinking methods, and it is also the most widely used crosslinking method. The United States, Japan and other countries have used this method to produce radiation-crosslinked PVC insulated wires. Ordinary PVC materials are not cross-linked under irradiation, and dehydrochlorination and degradation reactions mainly occur, and conjugated double bonds are generated to discolor the product. In 1959, Pinner and Miller first discovered that polyfunctional unsaturated monomers can strengthen the cross-linking reaction under PVC irradiation, thereby making PVC irradiation cross-linking possible. The added polyfunctional unsaturated monomers mainly include trimethylolpropane trimethacrylate (TMPTMA), trimethylolpropane triacrylate (TMPTA), triallyl isocyanurate (TAIC), triene Propyl cyanurate (TAC), tetraethylene glycol dimethacrylate (TEGDM), tetraethylene glycol diacrylate (TEG-DA), tripropylene glycol diacrylate (TPGDA), dipropylene glycol diacrylate (DPGDA) etc.
Over the years, a large number of studies have gradually revealed the reaction principle and structural changes in PVC irradiation crosslinking, and have been able to control the structure and performance of irradiation crosslinked PVC products, so that the irradiation crosslinking technology of PVC has become increasingly mature.
PVC irradiation cross-linking generally takes Co60-γ rays or high-energy electron (EB) rays as the irradiation source, multifunctional unsaturated monomer as the cross-linking agent, and the cross-linking reaction is a free radical reaction. PVC is exposed to C- The Cl bond is broken to form a free radical active center. The polyfunctional unsaturated monomer preferentially generates free radicals and self-polymerizes upon being triggered by irradiation. At the same time, it is grafted onto PVC long chain free radicals. Joint agent) γ-PVC.
VK SHARMA and others used electron beam (EB) to irradiate cross-linked soft PVC, and studied the effects of three cross-linking agents-TMPTA, TEGDM and TEGDA on the cross-linking rate and thermal stability of soft PVC. (TBLS) as the stabilizer of the system. The results show that 5% TMPTA has the best cross-linking effect. When the gel mass fraction is 60%, its tensile strength reaches 23.5MPa, which is about 7% higher than when it is not cross-linked. At the same time, the volume of cross-linked soft PVC The resistivity and decomposition temperature can also be significantly improved.
Ratnam et al. Adopted the same irradiation cross-linking method, using TMPTA to cross-link hard PVC, and TBLS as the stabilizer of the system. The gel content and tensile strength of hard PVC were studied when the irradiation dose was 20-200 kGy. The relationship between the hardness and the Tg at the irradiation dose of 100kGy were also determined, and the FTIR analysis confirmed that the method of electron beam irradiation can effectively avoid the occurrence of degradation reactions. The study found that when the irradiation dose was 100 kGy, the gel mass fraction reached 85%. At this time, the Tg of the cross-linked hard PVC was increased by 2.5 ° C compared with the uncross-linked sample. At the same time, the research on the performance of radiation cross-linked hard PVC shows that the tensile strength and hardness of the hard PVC sample cross-linked with the appropriate amount of cross-linking agent (4%) have been significantly improved when the gel mass fraction reaches At 80%, its tensile strength reaches the maximum value of 55MPa, which is 30% higher than when it is not cross-linked. At this time, the hardness of hard PVC also increased by about 13%, and with the increase in the gel mass fraction showed an increasing trend.
Radiation cross-linking of PVC is a very complex reaction, mainly including PVC cross-linking, degradation, HCl removal, etc. The influence of various factors on the cross-linking of PVC irradiation is achieved by affecting the competitive relationship between the three. The process of PVC irradiation crosslinking reaction is affected by many factors: irradiation dose, irradiation temperature, reaction atmosphere, crosslinking agent, plasticizer, filler and processing aid. Compared with the chemical cross-linking method, the irradiation cross-linking method has many advantages and is widely used in the wire and cable industry.
Irradiated crosslinked PVC products have excellent performance, high production efficiency, energy saving, and no environmental pollution. With people's attention to environmental issues and the advancement of irradiation technology, PVC irradiation cross-linking technology will surely attract more and more people's attention.