Cross-linked polyethylene has an altered linear structure, in the form of an interwoven three-dimensional network, similar to that of thermosetting plastics.
Crosslinking methods
There are three methods for crosslinking low and high density polyethylene. The methods are as follows: Organic peroxide, Insertion of a silane and High energy radiation.
Organic peroxide
This technique requires low initial processing temperatures to prevent premature decomposition of the peroxide (usually dicumyl peroxide); subsequently, the secondary application of high temperature and pressure is intended to initiate crosslinking and ensure the stability of the product geometry during crosslinking.
The degree of crosslinking depends basically on the level of peroxide decomposition and the structure of the polymer, in this sense, the conditions that increase crosslinking are: high levels of active oxygen in the peroxide, molecules with a high degree of branching, low steric hindrance of the side chains in the main molecule, medium molecular weight and low density in the polyethylene.
This explains why low density polyethylene is more easily crosslinked by this method than high density polyethylene.
Inserted silane
Two components are used in this technology: the ethylene copolymer with vinyl-silane groups and the crosslinking catalyst masterbatch; the latter is used in proportions of about 5%.
The copolymer comes from the chemical insertion of the vinyl-silane functional group into the polymer chains.
Crosslinking is subsequent to the transformation process due to the presence of moisture, which initiates crosslinking and generally the article is exposed in one of three systems: a steam room, immersion in hot water and storage with humidity and ambient conditions, which reports crosslinking after a few weeks.
The rate of crosslinking depends on several factors: humidity, temperature, article construction, ethylene vinyl silane copolymer characteristics, catalyst and compound additives.
High-energy radiation
This method consists of bombarding the product in its final form with high-energy radiation from an electron or isotope accelerator. This method allows the depth of the crosslinked layer to be graduated and does not require special control over the processing temperature.The restriction of this technology is the high capital required for the acquisition of the necessary equipment and facilities.