Polymer stabilizer
Polymer stabilizers are chemical additives which may be added to polymeric materials to inhibit or retard their degradation. Mainly they protect plastic and rubber products against heat, oxidation, and UV light. The biggest quantity of stabilizers is used for polyvinyl chloride, as the production and processing of this type of plastic would not be possible without stabilizing chemicals.
Common polymer degradation processes include oxidation, UV-damage, thermal degradation, ozonolysis, combinations thereof such as photo-oxidation, as well as reactions with catalyst residues, dyes, or impurities.
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.
Stabilizers are used at all stages of the polymer life-cycle. They allow plastic items to be produced faster and with fewer defects, extend their useful lifespan, and facilitate their recycling. However they also continue to stabilise waste plastic, causing it to remain in the environment for longer.
Many different types of plastic exist and each may be vulnerable to several types of degradation, which usually results in several different stabilisers being used in combination. Even for objects made from the same type of plastic, different applications may have different stabilisation requirements. Regulatory considerations, such as food contact approval are also present. 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. A wide range of stabilizers is therefore needed.
The market for antioxidant stabilisers alone was estimated at US$1.69 billion for 2017, with the total market for all polymer stabilizers expected to reach US$6.5 billion by 2033. In 2023, almost half of all polymer stabilizers sold worldwide were based on calcium, followed by lead, tin, liquid mixed metals and other types.
Antioxidants
Antioxidants inhibit autoxidation that occurs when polymers reacts with atmospheric oxygen. Aerobic degradation occurs gradually at room temperature, but almost all polymers are at risk of thermal-oxidation when they are processed at high temperatures. The molding or casting of plastics require them to be above their melting point or glass transition temperature. Under these conditions reactions with oxygen occur much more rapidly. Once initiated, autoxidation can be autocatalytic. As such, even though efforts are usually made to reduce oxygen levels, total exclusion is often not achievable and even exceedingly low concentrations of oxygen can be sufficient to initiate degradation. Sensitivity to oxidation varies significantly depending on the polymer in question; without stabilizers polypropylene and unsaturated polymers such as rubber will slowly degrade at room temperature where as polystyrene can be stable even at high temperatures. Antioxidants are of great importance during the process stage, with long-term stability at ambient temperature increasingly being supplied by hindered amine light stabilizers. Antioxidants are often referred to as being primary or secondary depending on their mechanism of action.Primary antioxidants (radical scavengers)
Primary antioxidants act as radical scavengers and remove peroxy radicals, as well as to a lesser extent alkoxy radicals, hydroxyl radicals and alkyl radicals. Oxidation begins with the formation of alkyl radicals, which are formed when the high temperatures and high shear stress experienced during processing snaps the polymer chains in a homolytic manner. These alkyl radicals react very rapidly with molecular oxygen is difficult and can only be achieved using specialised antioxidants the majority of primary antioxidants react instead with the longer lasting peroxy radicals. Hydrogen abstraction is usually the rate determining step in the polymer degradation and the peroxy radicals can be scavenged by hydrogen donation from an alternative source, namely the primary antioxidant. This converts them into an organic hydroperoxide. 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. The overall reaction with phenols is shown below:The end products of these reactions are typically quinone methides, which may also impart unwanted colour. Modern phenolic antioxidants have complex molecular structures, often including a propionate-group at the para position of the phenol. The quinone methides of these can rearrange once to give a hydroxycinnamate, regenerating the phenolic antioxidant group and allowing further radicals to be scavenged. Ultimately however, primary antioxidants are sacrificial and once they are fully consumed the polymer will begin to degrade.
Secondary antioxidants (hydroperoxides scavengers)
Secondary antioxidants act to remove organic hydroperoxides formed by the action of primary antioxidants. Hydroperoxides are less reactive than radical species but can initiate fresh radical reactions:As they are less chemically active they require a more reactive antioxidant. The most commonly employed class are phosphite esters, often of hindered phenols e.g. Trisphosphite. These will convert polymer hydroperoxides to alcohols, becoming oxidized to organophosphates in the process:
Transesterification can then take place, in which the hydroxylated polymer is exchanged for a phenol:
This exchange further stabilizes the polymer by releasing a primary antioxidant, because of this phosphites are sometimes considered multi-functional antioxidants as they can combine both types of activity.
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.
Antiozonant
Antiozonants prevent or slow down the degradation of material caused by ozone. 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. The mechanism of ozonolysis is different from other forms of oxidation and hence requires its own class of antioxidant stabilizers.These are primarily derivatives of p-phenylenediamine and work by reacting with ozone faster than it can react with vulnerable functional groups in the polymer. 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. Such species remain reactive and will react further, giving products such as 1,4-benzoquinone, phenylenediamine-dimers and aminoxyl radicals. Some of these products can then be scavenged by antioxidants.
Light stabilizers
Light stabilizer are used to inhibit polymer photo-oxidation, which is the combined result of the action of light and oxygen. Like autoxidation this is a free radical process, hence the antioxidants described above are effective inhibiting agents, however additional classes of additives are also beneficial, such as UV absorbers, quenchers of excited states and HALS.UV absorbers
susceptibility varies significantly between different polymers. Certain polycarbonates, polyesters and polyurethanes are highly susceptible, degrading via a Photo-Fries rearrangement. UV stabilisers absorb and dissipate the energy from UV rays as heat, typically by reversible intramolecular proton transfer. This reduces the absorption of UV rays by the polymer matrix and hence reduces the rate of weathering. Phenolic benzotriazoles and hydroxyphenyl-triazines are used to stabilise polycarbonates and acrylics, oxanilides are used for polyamides and polyurethanes, while benzophenones are used for PVC.Strongly light-absorbing PPS is difficult to stabilize. Even antioxidants fail in this electron-rich polymer. The acids or bases in the PPS matrix can disrupt the performance of the conventional UV absorbers such as HPBT. PTHPBT, which is a modification of HPBT are shown to be effective, even in these conditions.
Quenchers
Photo-oxidation can begin with the absorption of light by a chromophore within the polymer causing it to enter an excited state. This can then react with ambient oxygen, converting it into highly reactive singlet oxygen. 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. Singlet oxygen can be quenched by metal chelates, with nickel phenolates being a common example. Nickel quenchers tend to be used in agricultural plastics such as plastic mulch.Hindered amine light stabilizers
The ability of hindered amine light stabilizers to scavenge radicals produced by weathering, may be explained by the formation of aminoxyl radicals through a process known as the Denisov Cycle. The aminoxyl radical combines with free radicals in polymers:N-O• + R• → N-O-R
Although they are traditionally considered as light stabilizers, they can also stabilize thermal degradation.
Even though HALS are extremely effective in polyolefins, polyethylene and polyurethane, they are ineffective in polyvinyl chloride. It is thought that their ability to form nitroxyl radicals is disrupted. HALS act as a base and become neutralized by hydrochloric acid that is released by photooxidation of PVC. The exception is the recently developed NOR HALS, which is not a strong base and is not deactivated by HCl.