Radiation Effect Of Ethylene-vinyl Alcohol Copolymers (EVOH)

Ethylene-vinyl alcohol copolymers (EVOH) are one of the three high gas-barrier materials. They are used widely in various fields such as good packing, gasoline tanks, or other materials. EVOH are expected to work as one of the soft materials for no emission of poisonous gas upon incineration and chlorine-free. However EVOH are hygroscopic and the absorbed water lowers their ability to inhibit oxygen diffusion, which is undesirable in food packaging. Introducing crosslinking bonds between polymer molecules by chemical or radiation treaments is a powerful method to reduce the hygroscopicty. So the aim of this paper is studying EVOH from following aspects for the first time: 1) the radiation effect of different structural EVOH; 2) how to enhance the crosslinking efficiency of EVOH and the best condition for crosslinking; 3) the variation of characterization after crosslinking.EVOH are constructed by random sequencing radiation crosslinking ethylene (E) and degradation vinyl alcohol (VA) monomeric units. The two different reactions will keep their own properties in the process of radiation. So the radiation effect of EVOH is closely related to the structure. It is concluded that the more VA content is, the higher gelation dose is. As the VA content is less than 11mol%, the gelation dose is lower than 400kGy. The retardant effet of degradation units on crosslinking reaction can be ignored, and the crosslinking mechanism is similar to that of pure HDPE. At this time, EVOH are easier to be irradiated crosslinking. As the VA content is 56mol%, the gelation dose increases to 800kGy.Even the VA content is as high as 68mol%, gel network can still be formed at 1800kGy. The degradation reaction is dominant in the process of radiation and the crosslinking mechanism is varies. The formation of gel may be due to the conjured double bonds assigned to carbonyl groups. As the VA content is in the range of 56-68mol%, EVOH are"radiation resistance"copolymers. Due to the outstanding gas barrier of EVOH with high VA content, following studyings focus on enhancing the crosslinking efficiency of"radiation resistance"EVOH. From the gel fraction of EVOH-56 and EVOH-68 (the VA content is 56mol% and 68mol% repectively), it is found that the most effective agent for irradiation crossling is trially isocyanurate (TAIC). The optimum content of TAIC is around 5wt%, and the optimum irradiation dose is 200kGy. At this condition, 65.6% gel is formed for EVOH-56 while the gelation dose of pure EVOH-56 is 800kGy. Similar to the radiation effect of pure EVOH, the more VA content is, the more difficultily chemical bonds could be formed. At the optimum radiation condition, only 37.4% gel was formed for EVOH-68/5wt% TAIC. From the results of mechanical test at high temperature of irradiated EVOH/TAIC blend, it is found that the tensile strength and the elastic modulus increases and the elongation at break decreases. This result indicates that the heat stability of EVOH could be improved after crosslinking. And the water absorption is also found to decrease.Increasing the temperature above the glass transition temperature of EVOH could enhance its radiation crosslinking efficiency. As the temperature increases to 130℃,the gel content of EVOH-56 could reach 56.6% while that of EVOH-68 is 66.7%. At the same radiation temperature, the tensile strengths of EVOH decrease with increasing of irradiation dose; the elongation at break also decrease with increasing of irradiation dose at first, and almost keep a constant after 100kGy.In EVOH/glycerin blend, we obtained higher gel than that in pure EVOH when the irradiation dose exceed 800kGy, and the gel content increased with the absorbed dose. But it can't enhance radiation crosslinking efficiency of EVOH as water. The gel content is 26.7% at most in EVOH/30%glycerin irradiated at 1200kGy.