Polymer stabilizer

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Polymer stabilizers (British English: polymer stabilisers) 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 (PVC), as the production and processing of this type of plastic would not be possible without stabilizing chemicals. [1] 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. [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.

Contents

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 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, [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

Antioxidants inhibit autoxidation 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 or glass transition temperature (~200-300 °C). Under these conditions reactions with oxygen occur much more rapidly. Once initiated, autoxidation can be autocatalytic. [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 stabilizers polypropylene and unsaturated polymers such as rubber will slowly degrade at room temperature where as polystyrene 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 (radical scavengers)

Pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate): A primary antioxidant consisting of sterically hindered phenols with para-propionate groups. PAO-2 100.svg
Pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate): A primary antioxidant consisting of sterically hindered phenols with para-propionate groups.

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 radicals (R•). 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 (rate constants ≈ 107–109 mol–1 s–1) to give peroxy radicals, [8] which in turn abstract hydrogen from a fresh section of polymer in a chain 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:

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 [12] 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 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). 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. [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 of phloretic acid). [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. [15] [16] Ultimately however, primary antioxidants are sacrificial and once they are fully consumed the polymer will begin to degrade.

Secondary antioxidants (hydroperoxides scavengers)

Tris(2,4-di-tert-butylphenyl)phosphite, a phosphite widely used as a secondary antioxidant in polymers. BigPhosphite31570-04-4.png
Tris(2,4-di-tert-butylphenyl)phosphite, a phosphite widely used as a secondary antioxidant in polymers.

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: [6]

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. [17] These will convert polymer hydroperoxides to alcohols, becoming oxidized to organophosphates in the process: [18] [19]

ROOH + P(OR')3 → OP(OR')3 + ROH

Transesterification can then take place, in which the hydroxylated polymer is exchanged for a phenol: [20]

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. [21]

Antiozonant

6PPD is a p-phenylenediamine based antiozonant widely used in tires 6PPD skeletal.svg
6PPD is a p-phenylenediamine based antiozonant widely used in tires

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 (PPD) 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. [22] [23] Some of these products can then be scavenged by antioxidants.

Light stabilizers

Bisoctrizole: A benzotriazole-phenol based UV absorber Bisoctrizole.svg
Bisoctrizole: A benzotriazole-phenol based UV absorber

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. [24]

UV absorbers

UV 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 (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.

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. [26]

Quenchers

A nickel-phenoxide quencher. CAS number: 14516-71-3 UV-1084 100.svg
A nickel-phenoxide quencher. CAS number: 14516-71-3

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. [27] Nickel quenchers tend to be used in agricultural plastics such as plastic mulch.

Hindered amine light stabilizers

Example structure of a HAL LMW-HA(L)S-1 100.svg
Example structure of a HAL

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. [28]

Other Classes

Polymers are susceptible to degradation by a variety of pathways beyond oxygen and light.

Acid Scavengers

Acid scavengers, also referred to as antacids, neutralize acidic impurities, [29] especially those that release HCl. 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 include metallic soaps, such as calcium stearate and zinc stearate, mineral agents, such as hydrotalcite and hydrocalumite, and basic metal oxides, such as calcium oxide, zinc oxide or magnesium oxide.

Metal deactivators

Metal ions, such as those of Ti, Al and Cu, 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 of Ziegler-Natta catalysts in polypropylene. In these instances metal deactivators may be added to improve stability. Deactivators work by chelation to form an inactive coordination complex with the metal ion. Salen-type compounds are common.

Thermal stabilizers

Thermal (or heat) stabilizers are used almost exclusively in PVC. At temperatures above 70 °C the unstabilized material is susceptible to degradation with loss of HCl. Once this dehydrochlorination starts it is autocatalytic, with rising acidity accelerating degradation. A wide range of agents have been used to prevent this, with many of the early agents such as lead stearate, organotins and cadmium complexes being highly toxic. Safer modern alternatives include metallic soaps such as calcium stearate, as well as barium and zinc compounds, along with various synergists. [31] Addition levels vary typically from 2% to 4%.

Flame retardants

Flame retardants are a broad range of compounds that improve fire resistance of polymers. Examples include brominated compounds along with aluminium hydroxide, antimony trioxide, and various organophosphates. [5] [32] Flame retardants are known to reduce the effectiveness of antioxidants. [33]

Biocides

Degradation resulting from microorganisms (biodegradation) involves its own class of special bio-stabilizers and biocides (e.g. isothiazolinones).

Voltage stabilizers

These additives are added to polymers used as sheathing for electrical cables, most commonly PEX. [34] Compounds include benzil and thioxanthone derivatives. [35] These possess high electron affinities, which allow them to trap and neutralize charge carriers that can cause dielectric breakdown of the insulation. [36]

See also

Other additives

Related Research Articles

Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants are frequently added to industrial products, such as polymers, fuels, and lubricants, to extend their usable lifetimes. Foods are also treated with antioxidants to forestall spoilage, in particular the rancidification of oils and fats. In cells, antioxidants such as glutathione, mycothiol, or bacillithiol, and enzyme systems like superoxide dismutase, can prevent damage from oxidative stress.

Rancidification is the process of complete or incomplete autoxidation or hydrolysis of fats and oils when exposed to air, light, moisture, or bacterial action, producing short-chain aldehydes, ketones and free fatty acids.

<span class="mw-page-title-main">Polymer degradation</span> Alteration in the polymer properties under the influence of environmental factors

Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling. The rate of this degradation varies significantly; biodegradation can take decades, whereas some industrial processes can completely decompose a polymer in hours.

<span class="mw-page-title-main">Polyvinyl fluoride</span> Chemical compound

Polyvinyl fluoride (PVF) or –(CH2CHF)n– is a polymer material mainly used in the flammability-lowering coatings of airplane interiors and photovoltaic module backsheets. It is also used in raincoats and metal sheeting. Polyvinyl fluoride is a thermoplastic fluoropolymer with a repeating vinyl fluoride unit, and it is structurally very similar to polyvinyl chloride.

<span class="mw-page-title-main">Organic peroxides</span> Organic compounds of the form R–O–O–R’

In organic chemistry, organic peroxides are organic compounds containing the peroxide functional group. If the R′ is hydrogen, the compounds are called hydroperoxides, which are discussed in that article. The O−O bond of peroxides easily breaks, producing free radicals of the form RO•. Thus, organic peroxides are useful as initiators for some types of polymerization, such as the acrylic, unsaturated polyester, and vinyl ester resins used in glass-reinforced plastics. MEKP and benzoyl peroxide are commonly used for this purpose. However, the same property also means that organic peroxides can explosively combust. Organic peroxides, like their inorganic counterparts, are often powerful bleaching agents.

<span class="mw-page-title-main">Hot-melt adhesive</span> Glue applied by heating

Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.

In industrial chemistry, a stabilizer or stabiliser is a chemical that is used to prevent degradation. Above all, heat and light stabilizers are added to plastic and rubber materials because they ensure safe processing and protect products against aging and weathering. In particular polyvinyl chloride would not be possible without stabilizers.

Autoxidation refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organic compounds in air at ambient temperatures. Many common phenomena can be attributed to autoxidation, such as food going rancid, the 'drying' of varnishes and paints, and the perishing of rubber. It is also an important concept in both industrial chemistry and biology. Autoxidation is therefore a fairly broad term and can encompass examples of photooxygenation and catalytic oxidation.

<span class="mw-page-title-main">Photodegradation</span> Alteration of materials by light

Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it destroys paintings and other artifacts. It is, however, partly responsible for remineralization of biomass and is used intentionally in some disinfection technologies. Photodegradation does not apply to how materials may be aged or degraded via infrared light or heat, but does include degradation in all of the ultraviolet light wavebands.

<span class="mw-page-title-main">Hydroperoxide</span> Class of chemical compounds

Hydroperoxides or peroxols are compounds of the form ROOH, where R stands for any group, typically organic, which contain the hydroperoxy functional group. Hydroperoxide also refers to the hydroperoxide anion and its salts, and the neutral hydroperoxyl radical (•OOH) consist of an unbond hydroperoxy group. When R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. Organic hydroperoxides can either intentionally or unintentionally initiate explosive polymerisation in materials with unsaturated chemical bonds.

<span class="mw-page-title-main">Dakin oxidation</span> Organic redox reaction that converts hydroxyphenyl aldehydes or ketones into benzenediols

The Dakin oxidation (or Dakin reaction) is an organic redox reaction in which an ortho- or para-hydroxylated phenyl aldehyde (2-hydroxybenzaldehyde or 4-hydroxybenzaldehyde) or ketone reacts with hydrogen peroxide (H2O2) in base to form a benzenediol and a carboxylate. Overall, the carbonyl group is oxidised, whereas the H2O2 is reduced.

<span class="mw-page-title-main">Hindered amine light stabilizers</span>

Hindered amine light stabilizers (HALS) are chemical compounds containing an amine functional group that are used as stabilizers in plastics and polymers. These compounds are typically derivatives of tetramethylpiperidine and are primarily used to protect the polymers from the effects of photo-oxidation; as opposed to other forms of polymer degradation such as ozonolysis. They are also increasingly being used as thermal stabilizers, particularly for low and moderate level of heat, however during the high temperature processing of polymers they remain less effective than traditional phenolic antioxidants.

In polymers, such as plastics, thermal degradation refers to a type of polymer degradation where damaging chemical changes take place at elevated temperatures, without the simultaneous involvement of other compounds such as oxygen. Simply put, even in the absence of air, polymers will begin to degrade if heated high enough. It is distinct from thermal-oxidation, which can usually take place at less elevated temperatures.

<span class="mw-page-title-main">Photo-oxidation of polymers</span>

In polymer chemistry photo-oxidation is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidation causes the polymer chains to break, resulting in the material becoming increasingly brittle. This leads to mechanical failure and, at an advanced stage, the formation of microplastics. In textiles the process is called phototendering.

Biodegradable additives are additives that enhance the biodegradation of polymers by allowing microorganisms to utilize the carbon within the polymer chain as a source of energy. Biodegradable additives attract microorganisms to the polymer through quorum sensing after biofilm creation on the plastic product. Additives are generally in masterbatch formation that use carrier resins such as polyethylene (PE), polypropylene (PP), polystyrene (PS) or polyethylene terephthalate (PET).

Poly(ethylene adipate) or PEA is an aliphatic polyester. It is most commonly synthesized from a polycondensation reaction between ethylene glycol and adipic acid. PEA has been studied as it is biodegradable through a variety of mechanisms and also fairly inexpensive compared to other polymers. Its lower molecular weight compared to many polymers aids in its biodegradability.

<i>N</i>-Isopropyl-<i>N</i>-phenyl-1,4-phenylenediamine Chemical compound

N-Isopropyl-N′-phenyl-1,4-phenylenediamine (often abbreviated IPPD) is an organic compound commonly used as an antiozonant in rubbers. Like other p-phenylenediamine-based antiozonants it works by virtue of its low ionization energy, which allows it to react with ozone faster than ozone will react with rubber. This reaction converts it to the corresponding aminoxyl radical (R2N–O•), with the ozone being converted to a hydroperoxyl radical (HOO•), these species can then be scavenged by other antioxidant polymer stabilizers.

In polymer chemistry, polymerisation inhibitors are chemical compounds added to monomers to prevent their self-polymerisation. Unsaturated monomers such as acrylates, vinyl chloride, butadiene and styrene require inhibitors for both processing and safe transport and storage. Many monomers are purified industrially by distillation, which can lead to thermally-initiated polymerisation. Styrene, for example, is distilled at temperatures above 100 °C whereupon it undergoes thermal polymerisation at a rate of ~2% per hour. This polymerisation is undesirable, as it can foul the fractionating tower; it is also typically exothermic, which can lead to a runaway reaction and potential explosion if left unchecked. Once initiated, polymerisation is typically radical in mechanism and as such many polymerisation inhibitors act as radical scavengers.

Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate is a hindered phenolic antioxidant commonly used as a polymer stabiliser.

Tris(3,5-di-<i>tert</i>-butyl-4-hydroxybenzyl) isocyanurate Chemical compound

Tris(3,5-di-tert-butyl-4-hydroxybenzyl) isocyanurate is a chemical compound used as a polymer stabilizer in plastics. Like other hindered phenols it acts as a primary antioxidant. More than 1000 tonnes per year are used in the EU.

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  33. ↑ Pfaendner, Rudolf (December 2013). "(Photo)oxidative degradation and stabilization of flame retarded polymers". Polymer Degradation and Stability. 98 (12): 2430–2435. doi:10.1016/j.polymdegradstab.2013.07.005.
  34. ↑ Bjurström, Anton; Edin, Hans; Hillborg, Henrik; Nilsson, Fritjof; Olsson, Richard T.; Pierre, Max; Unge, Mikael; Hedenqvist, Mikael S. (July 2024). "A Review of Polyolefin‐Insulation Materials in High Voltage Transmission; From Electronic Structures to Final Products". Advanced Materials. doi: 10.1002/adma.202401464 .
  35. ↑ Wutzel, Harald; Jarvid, Markus; Bjuggren, Jonas M.; Johansson, Anette; Englund, Villgot; Gubanski, Stanislaw; Andersson, Mats R. (February 2015). "Thioxanthone derivatives as stabilizers against electrical breakdown in cross-linked polyethylene for high voltage cable applications". Polymer Degradation and Stability. 112: 63–69. doi:10.1016/j.polymdegradstab.2014.12.002.
  36. ↑ Jarvid, Markus; Johansson, Anette; Englund, Villgot; Lundin, Angelica; Gubanski, Stanislaw; Müller, Christian; Andersson, Mats R. (2015). "High electron affinity: a guiding criterion for voltage stabilizer design". Journal of Materials Chemistry A. 3 (14): 7273–7286. doi:10.1039/C4TA04956J.
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