Meaning
Chemical bonding occurs when high-energy radiation or peroxides strip hydrogen atoms from polymer backbones to create reactive sites. This alkyl radical crosslinking process links individual chains into a three-dimensional network. Such modifications improve heat resistance in polyolefins used for high-temperature pipe or wire insulation.
Reaction Mechanism
Initiators decompose to form highly reactive species that abstract hydrogen from the carbon-hydrogen bonds along the resin chain. Once the alkyl radical crosslinking begins, the resulting carbon-centered radicals on adjacent chains pair up to form covalent carbon-carbon bonds. This transition from a thermoplastic to a thermoset state occurs without the need for additional monomers or complex catalysts.
Material Property
Resistance to chemical attack and environmental stress cracking increases as the gel content of the material rises. Excessive alkyl radical crosslinking can lead to brittleness or reduced elongation at break in the finished part. Designers must manage the dosage of ionizing radiation or chemical additives to reach the target molecular weight distribution without degrading the polymer base.
High degrees of bonding can also make the material difficult to recycle because it no longer melts under heat.
Processing Boundary
Viscosity rises rapidly during the formation of the network, which complicates traditional injection moulding if the reaction triggers too early. Stable alkyl radical crosslinking requires precise temperature control in the melt stream to prevent premature gelation in the barrel. Extrusion processes often utilize a post-moulding curing step to ensure the final geometry remains stable during the chemical transformation.
