Crosslinked Polyethylene

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It is known that polyethylene is one of the most popular polymers ever used(approxiamtely 65-70%). And it's range of usage is very wide. But there are some limitations to use it on some applications. For example, to use it as a pipe forhot water supply. Because of its low physical and mechanical properties, polyethylene itself cannot be used to produce pipes for hot water supply. In order to increase its physical and mechanical properties the term "crosslinking" should be provided. So what does the "crosslinking" mean? The term "crosslinking" is used for bonds between the macromolecules of the polyethylene. As it's known that the macromolecules of polyethylene are linear. Because of this linearity the polyethylene shows low physical and mechanical properties. To increase physical and mechanical properties these linear macromolecules should be crosslinked (there should become bonds between linear macromolecules). After crosslinking the macromolecules form bigger macromolecule , each of them has bonds between each other. The structure of crosslinked state is shown below :-----------(1) -----------------------(1) -------------------(1)| | | | | | | | | | | | | | | | | | | | .. .......------------------------(1) -----------------------(1) | | | | | |.....................................................1 - linear macromolecules| - bondsBecause of crosslinking the motion of macromolecules are limited relatively to each other.This means that crosslinking increases thermostability. And not only but other properties too. Here below is shown the change in properties after crosslinking:Density - D (decreases)thermostability - I (increases)Plasticity - DStability to low temperatures- IStability to UV light - IChemical stability - I

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But, how to get crosslinked polyethylene? How to make polyethylene to become crosslinked?There are 2 methods of making the polyethylene to become crosslinked:+ physical+ chemicalPhysical method is based on crosslinking the polyethylene with radioactive rays. These radioactive rays passed through polyethylene macromolecule form free radicals . After that these free radicals form bonds between macromolecules - making them crosslinked. There is also another type of physical method. This type of crosslinking is based on passing through polyethylene electrons with a high velocity. Chemical method is based on crosslinking the polyethylene with organic peroxides, silanes and other crosslinking agents.