The
effect of various solvents on the de-cross-linking reaction
of cross-linked polyethylene under subcritical and supercritical conditions
was studied. The gel content of the de-cross-linked polyethylene decreased
from 60 to 0.8–2.5% at 380 °C within 5 min in all of the
solvents tested in this study. When water, ethanol, and 2-propanol
were used, the M
w of the de-cross-linked
polyethylenes decreased from 349,000 to 200,000–227,000 g/mol,
and much lower M
w values of 70,000–90,000
g/mol were observed when acetone and diisopropyl ether were used.
The de-cross-linked polyethylenes treated in the different solvents
had a very similar chemical structure to raw polyethylene. According
to a kinetic study using a first-order reaction model, the de-cross-linking
rate in each solvent increased in the order 2-propanol < ethanol
< n-hexane < water < methanol ≪ diisopropyl
ether ≪ acetone at 350 °C.
The physical and rheological properties of thermoplasticized irradiation-crosslinked polyethylene foam using supercritical methanol treatment were investigated by GPC, FTIR, DSC, WAXS, DMTA and UDS. The polyethylene foam was selectively decrosslinked into thermoplasticized polyethylene in an appropriate supercritical methanol condition without any undesirable side reactions such as oxidation and disproportionation. The thermoplasticization was promoted with increasing reaction temperature to reach completion above 380 o C. The supercritical reaction condition affected the crystallization behavior, and mechanical and rheological properties of the decrosslinked polyethylene foam, but not its crystallographic structure or crystallinity.
The crystallization, and mechanical and rheological properties of decrosslinked-crosslinked-highdensity polyethylenes using supercritical methanol were investigated by DSC, WAXS, DMTA, and UDS. Crosslinked high-density polyethylenes were successfully decrosslinked in a supercritical methanol condition. The residual gel content of the decrosslinked samples decreased with the reaction temperature. The crystallization behavior, mechanical, and rheological properties of the decrosslinked samples were influenced considerably by the gel content. As the gel content increased, the network gel structure restricted the chain mobility of polymer molecules in the melt state and hindered their crystallization. Thus, the nonterminal yield behavior in the melt state was enhanced and the crystallinity decreased. The dynamic elastic modulus of the decrosslinked sample in solid state increased with the increase in the crystallinity.
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