Stabilizers are a class of chemical additives commonly added to polymeric materials, such as plastics, to inhibit or retard their degradation. Polymers can be subject to various degradation processes, including oxidation, UV-damage, thermal degradation, ozonolysis, combinations thereof such as photo-oxidation, as well as reactions with catalyst residues or other impurities. These processes all degrade the polymer on a chemical level, leading to chain scission that can adversely affect its mechanical properties such as strength and malleability, as well as its appearance and colour.
A vast number of chemically distinct polymers exist, with their degradation pathways varying according to their chemical structure, as such an equally wide range of stabilizers exists. The market for antioxidant stabilisers alone was estimated at US$1.69 billion for 2017, with the total market for all stabilizers expected to reach US$5.5 billion by 2025.
Most carbon-based materials deteriorate over time due to the effects of light, atmospheric oxygen and other environmental factors. In plastics this can cause discoloration and embrittlement, which can reduce the useful lifespan of the object. Stabilizers are used to prevent this by inhibiting the various degradation processes such as oxidation, chain scission and uncontrolled recombinations and cross-linking. Typically a number of different stabilizers will be needed in combination, each inhibiting a particular degradation pathway. The effectiveness of the stabilizers depends on solubility, distribution within the plastic, ability to stabilize, and rate of loss during processing and use. The effect on the viscosity on the plastic is also an important concern for processing.
Antioxidants inhibit autoxidation that occurs when polymers reacts with atmospheric oxygen. For some compounds this can happen 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 or glass transition temperature (~200-300 °C) and under these conditions reactions with oxygen occur much more rapidly. Once initiated, autoxidation proceeds via a free radical chain reaction which 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 (HALs). Antioxidants are often referred to as being primary or secondary depending on their mechanism of action.
Primary antioxidants (radical scavengers)Edit
Primary antioxidants (also known as chain-breaking antioxidants) act as radical scavengers and remove peroxy radicals (ROO•), as well as to a lesser extent alkoxy radicals (RO•), hydroxyl radicals (HO•) and alkyl radials (R•). Oxidation begins with the formation of alkyl radials, 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 (rate constants ≈ 107–109 mol–1 s–1) to give peroxy radicals, which in turn abstract hydrogen from a fresh section of polymer in a chain propagation step to give new alkyl radials. The overall process is exceedingly complex and will vary between polymers but the first few steps are shown below in general:
- R-R → 2 R•
- R• + O2 → ROO•
- ROO• + RH → ROOH + R•
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 the majority of primary antioxidants react instead with the longer lasting peroxy radicals (ROO•). 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 (ROOH). The most important commercial stabilzers 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). The overall reaction with phenols is shown below:
- ROO• + ArOH → ROOH + ArO•
- ArO• → nonradical products
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 (i.e. they are ortho-alkylated analogues of phloretic acid). 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 being to degrade.
Secondary antioxidants (hydroperoxides scavengers)Edit
Secondary antioxidants act to remove organic hydroperoxides (ROOH) formed by the action of primary antioxidants. Hydroperoxides are less reactive than radical species but can initiate fresh radical reactions:
- ROOH + RH → RO• + R• + H2O
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. Tris(2,4-di-tert-butylphenyl)phosphite. These will convert polymer hydroperoxides to alcohols, becoming oxidized to organophosphates in the process:
- ROOH + P(OR')3 → OP(OR')3 + ROH
Transesterification can then take place, in which the hydroxylated polymer is exchanged for a phenol:
- ROH + OP(OR')3 → R'OH + OP(OR')2OR
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.
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 based on p-phenylenediamine and work by reacting with ozone faster than it can react with vulnerable functional groups in the polymer (typically alkene groups). 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 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.
The UV absorbers dissipate the absorbed light energy from UV rays as heat by reversible intramolecular proton transfer. This reduces the absorption of UV rays by the polymer matrix and hence reduces the rate of weathering. Typical UV-absorbers are benzotriazoles and hydroxyphenyltriazines for polycarbonates and acrylics, oxanilides for polyamides and benzophenones 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.
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. 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.
Hindered amine light stabilizersEdit
The ability of hindered amine light stabilizers (HALS or HAS) 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 (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 in polyolefins, polyethylene and polyurethane, they are ineffective in polyvinyl chloride (PVC). It is thought that their ability to form nitroxyl radicals is disrupted. HALS act as a base and become neutralized by hydrochloric 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.
In addition to the effects of light and oxygen polymers may also be degraded by various other environmental factors
Acid scavengers, also referred to as antacids, neutralize acidic impurities, especially those that can act as a source of HCl. They are important stabilizers in many types of polymer, particularly PVC, as well as those produced using Ziegler–Natta catalysts, or that use brominated or chlorinated flame retardants. Common examples include metallic soaps, such as calcium stearate, mineral agents like hydrotalcite and hydrocalumite, and basic metal oxides such as zinc oxide.
Heat (or thermal) stabilizers are mostly used for PVC, as unstabilized material is particularly prone to thermal degradation. These agents minimize loss of HCl, a degradation process that starts above 70 °C. Once dehydrochlorination starts, it is autocatalytic. Many diverse agents have been used including, traditionally, derivatives of heavy metals (lead, cadmium). Increasingly, metallic soaps (metal "salts" of fatty acids) are favored, species such as calcium stearate. Addition levels vary typically from 2% to 4%. The choice of the best heat stabilizer depends on its cost effectiveness in the end use application, performance specification requirements, processing technology and regulatory approvals.
Flame retardants are a broad range of compounds added to polymers to improve fire resistance.
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