Polymer stabilizers (British English:polymer stabilisers) are chemical additives which may be added topolymeric materials to inhibit or retard their degradation. Mainly they protectplastic andrubber products against heat, oxidation, and UV light. The biggest quantity of stabilizers is used forpolyvinyl chloride (PVC), as the production and processing of this type of plastic would not be possible without stabilizing chemicals.[1] Commonpolymer degradation processes includeoxidation,UV-damage,thermal degradation,ozonolysis, combinations thereof such asphoto-oxidation, as well as reactions withcatalyst residues, dyes, or impurities.[1][2] All of these degrade the polymer at a chemical level, viachain scission, uncontrolled recombination andcross-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.[1] However they also continue to stabilisewaste 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 asfood contact approval are also present. Environmentally friendly stabilizers forbioplastics should be made from bio-based materials, e.g. epoxidizedsoybean oil, and cause hardly any odor orVOC emissions. A wide range of stabilizers is therefore needed.
The market for antioxidant stabilisers alone was estimated at US$1.69 billion for 2017,[3] 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 (25.1 %), tin (15.4 %), liquid mixed metals (LMM) and other types.[4]
Antioxidants inhibitautoxidation that occurs when polymers reacts with atmospheric oxygen.[5] 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 (e.g.injection molding) require them to be above their melting point orglass transition temperature (~200-300 °C). Under these conditions reactions with oxygen occur much more rapidly. Once initiated, autoxidation can beautocatalytic.[6] 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 stabilizerspolypropylene and unsaturated polymers such asrubber will slowly degrade at room temperature where aspolystyrene can be stable even at high temperatures.[7] Antioxidants are of great importance during the process stage, with long-term stability at ambient temperature increasingly being supplied by hindered amine light stabilizers (HALs). Antioxidants are often referred to as being primary or secondary depending on their mechanism of action.
Primary antioxidants (also known as chain-breaking antioxidants) act asradical scavengers and remove peroxy radicals (ROO•), as well as to a lesser extent alkoxy radicals (RO•),hydroxyl radicals (HO•) and alkyl radicals (R•). Oxidation begins with the formation of alkyl radicals, which are formed when the high temperatures and highshear stress experienced during processing snaps the polymer chains in ahomolytic manner. These alkyl radicals react very rapidly with molecular oxygen (rate constants ≈107–109 mol–1 s–1) to give peroxy radicals,[8] which in turnabstract hydrogen from a fresh section of polymer in achain propagation step to give new alkyl radicals.[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•). Hydrogen abstraction is usually therate 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 organichydroperoxide (ROOH). The most important commercial stabilizers for this are hinderedphenols such asBHT oranalogues 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). The overall reaction with phenols is shown below:
The end products of these reactions are typicallyquinone methides, which may also impart unwanted colour.[13] Modern phenolic antioxidants have complex molecular structures, often including a propionate-group at the para position of the phenol (i.e. they are ortho-alkylated analogues ofphloretic acid).[14] The quinone methides of these can rearrange once to give ahydroxycinnamate, regenerating the phenolic antioxidant group and allowing further radicals to be scavenged.[15][16] Ultimately however, primary antioxidants are sacrificial and once they are fully consumed the polymer will begin to degrade.
Secondary antioxidants act to removeorganic hydroperoxides (ROOH) formed by the action of primary antioxidants. Hydroperoxides are less reactive than radical species but can initiate fresh radical reactions:[6]
As they are less chemically active they require a more reactive antioxidant. The most commonly employed class arephosphite esters, often of hindered phenols e.g.Tris(2,4-di-tert-butylphenyl)phosphite.[17] These will convert polymer hydroperoxides to alcohols, becoming oxidized toorganophosphates in the process:[18][19]
Transesterification can then take place, in which the hydroxylated polymer is exchanged for a phenol:[20]
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 efficienthydroperoxide decomposers, withthioethers being particularly effective against long-term thermal aging, they are ultimately oxidise up tosulfoxides andsulfones.[21]
Antiozonants prevent or slow down the degradation of material caused byozone. This is naturally present in the air at very low concentrations but is exceedingly reactive, particularly towards unsaturated polymers such as rubber, where it causesozone cracking. The mechanism ofozonolysis is different from other forms of oxidation and hence requires its own class of antioxidant stabilizers.These are primarily derivatives ofp-phenylenediamine (PPD) and work by reacting withozone faster than it can react with vulnerable functional groups in the polymer (typicallyalkene groups). They achieve this by having a lowionization energy which allows them to react with ozone via electron transfer, this converts them into radical cations that are stabilized byaromaticity. Such species remain reactive and will react further, giving products such as1,4-benzoquinone, phenylenediamine-dimers andaminoxyl radicals.[22][23] Some of these products can then be scavenged by antioxidants.
Light stabilizer are used to inhibit polymer photo-oxidation, which is the combined result of the action of light and oxygen. Likeautoxidation 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.[24]
UV susceptibility varies significantly between different polymers. Certainpolycarbonates,polyesters andpolyurethanes are highly susceptible, degrading via aPhoto-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 (e.g.UV-360,UV-328) and hydroxyphenyl-triazines (e.g.Bemotrizinol) are used to stabilisepolycarbonates andacrylics,[25] oxanilides are used forpolyamides and polyurethanes, whilebenzophenones are used forPVC.
Strongly light-absorbingPPS 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.[26]
Photo-oxidation can begin with the absorption of light by achromophore within the polymer (which may be a dye or impurity) causing it to enter anexcited state. This can then react with ambient oxygen, converting it into highly reactivesinglet oxygen.Quenchers are able to absorb energy from excited molecules via aFörster 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.[27] Nickel quenchers tend to be used inagricultural plastics such asplastic mulch.
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The ability of hindered amine light stabilizers (HALS or HAS) to scavenge radicals produced by weathering, may be explained by the formation ofaminoxyl radicals through a process known as the Denisov Cycle. The aminoxyl radical (N-O•) 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 inpolyolefins,polyethylene andpolyurethane, they are ineffective inpolyvinyl chloride (PVC). It is thought that their ability to form nitroxyl radicals is disrupted. HALS act as a base and become neutralized byhydrochloric acid (HCl) 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.[28]
Polymers are susceptible to degradation by a variety of pathways beyond oxygen and light.
Acid scavengers, also referred to as antacids, neutralize acidic impurities,[29] especially those that releaseHCl.PVC is susceptible to acid-catalyzed degradation, the HCl being derived from the polymer itself.Ziegler–Natta catalysts and halogenated flame retardants also serve as sources of acids. Common acid scavengers includemetallic soaps, such ascalcium stearate andzinc stearate, mineral agents, such ashydrotalcite andhydrocalumite, and basic metal oxides, such ascalcium oxide,zinc oxide ormagnesium oxide.
Metal ions, such as those ofTi,Al andCu, can accelerate the degradation of polymers.[30] This is of particular concern where polymers are in direct contact with metal, such as in wiring and cable. More generally, the metal catalysts used to form the polymer may simply become encapsulated within it during production, this is typically true ofZiegler-Natta catalysts inpolypropylene. In these instancesmetal deactivators may be added to improve stability. Deactivators work bychelation to form an inactivecoordination complex with the metal ion.Salen-type compounds are common.
Thermal (or heat) stabilizers are used almost exclusively inPVC. At temperatures above 70 °C the unstabilized material is susceptible to degradation with loss of HCl.Once this dehydrochlorination starts it isautocatalytic, with rising acidity accelerating degradation. A wide range of agents have been used to prevent this, with many of the early agents such aslead stearate,organotins andcadmium complexes being highly toxic. Safer modern alternatives includemetallic soaps such ascalcium stearate, as well as barium and zinc compounds, along with various synergists.[31] Addition levels vary typically from 2% to 4%.
Flame retardants are a broad range of compounds that improve fire resistance of polymers. Examples includebrominated compounds along withaluminium hydroxide,antimony trioxide, and variousorganophosphates.[5][32] Flame retardants are known to reduce the effectiveness of antioxidants.[33]
Degradation resulting from microorganisms (biodegradation) involves its own class of special bio-stabilizers andbiocides (e.g.isothiazolinones).
These additives are added to polymers used as sheathing forelectrical cables, most commonlyPEX.[34] Compounds includebenzil andthioxanthone derivatives.[35] These possess high electron affinities, which allow them to trap and neutralizecharge carriers that can causedielectric breakdown of the insulation.[36]
