Meaning
Photolytic reaction pathways describe how carbonyl compounds in polymers decompose after absorbing ultraviolet radiation. This norrish cleavage occurs in two distinct pathways, designated type one and type two, which both lead to the breakdown of the polymer backbone. Polyethylene and ketones within the chain structure are particularly susceptible to this mode of degradation when exposed to outdoor sunlight.
The reaction reduces the molecular weight of the plastic, leading to loss of impact strength and eventual embrittlement. This chemical pathway represents a main vulnerability of untreated polyolefins in agricultural and automotive applications.
Scission Pathway
Radicals generated by the type one reaction initiate further degradation cycles by reacting with atmospheric oxygen. The type two reaction proceeds through an intramolecular rearrangement to produce alkene and ketone end-groups without generating free radicals. Both paths reduce the structural integrity of the moulded component over time.
Degradation Consequence
Mechanical failure of outdoor plastic enclosures often stems from this light-induced molecular breakdown. Parts lose their elongation at break and develop surface cracks under low loads. Moulders incorporate carbon black or benzotriazole light stabilizers to block the damaging ultraviolet wavelengths.
Molecular Weight
Chemical analysis of aged parts reveals a broad distribution of shorter polymer chains due to ongoing scission events. Melt flow rate measurements of degraded scrap show a marked increase in flowability because of the lower molecular weight. This change makes the regrind material unsuitable for high-load applications.
