Polymer stabilizer
[1][2] All of these degrade the polymer at a chemical level, via chain scission, uncontrolled recombination and cross-linking, which adversely affects many key properties such as strength, malleability, appearance and colour.
They allow plastic items to be produced faster and with fewer defects, extend their useful lifespan, and facilitate their recycling.
Environmentally friendly stabilizers for bioplastics should be made from bio-based materials, e.g. epoxidized soybean oil, and cause hardly any odor or VOC emissions.
In 2023, almost half of all polymer stabilizers sold worldwide were based on calcium, followed by lead (25.1 %), tin (15.4 %), liquid mixed metals (LMM) and other types.
[9][10] The overall process is exceedingly complex and will vary between polymers[11] but the first few steps are shown below in general: Due to its rapid reaction with oxygen the scavenging of the initial alkyl radical (R•) is difficult and can only be achieved using specialised antioxidants[12] the majority of primary antioxidants react instead with the longer lasting peroxy radicals (ROO•).
The most important commercial stabilizers for this are hindered phenols such as BHT or analogues thereof and secondary aromatic amines such as alkylated-diphenylamine.
Amines are typically more effective, but cause pronounced discoloration, which is often undesirable (i.e., in food packaging, clothing).
[14] The quinone methides of these can rearrange once to give a hydroxycinnamate, regenerating the phenolic antioxidant group and allowing further radicals to be scavenged.
Organosulfur compounds are also efficient hydroperoxide decomposers, with thioethers being particularly effective against long-term thermal aging, they are ultimately oxidise up to sulfoxides and sulfones.
This is naturally present in the air at very low concentrations but is exceedingly reactive, particularly towards unsaturated polymers such as rubber, where it causes ozone cracking.
They achieve this by having a low ionization energy which allows them to react with ozone via electron transfer, this converts them into radical cations that are stabilized by aromaticity.
Phenolic benzotriazoles (e.g. UV-360, UV-328) and hydroxyphenyl-triazines (e.g. Bemotrizinol) are used to stabilise polycarbonates and acrylics,[25] oxanilides are used for polyamides and polyurethanes, while benzophenones are used for PVC.
The acids or bases in the PPS matrix can disrupt the performance of the conventional UV absorbers such as HPBT.
[26] Photo-oxidation can begin with the absorption of light by a chromophore within the polymer (which may be a dye or impurity) causing it to enter an excited state.
Quenchers are able to absorb energy from excited molecules via a Förster mechanism and then dissipate it harmlessly as either heat or lower frequency fluorescent light.
Even though HALS are extremely effective in polyolefins, polyethylene and polyurethane, they are ineffective in polyvinyl chloride (PVC).
Safer modern alternatives include metallic soaps such as calcium stearate, as well as barium and zinc compounds, along with various synergists.
[33] Degradation resulting from microorganisms (biodegradation) involves its own class of special bio-stabilizers and biocides (e.g. isothiazolinones).
[35] These possess high electron affinities, which allow them to trap and neutralize charge carriers that can cause dielectric breakdown of the insulation.
