Stabilizer for polymers

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. 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 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 react very rapidly with molecular oxygen (rate constants ≈ 107–109 mol–1 s–1) to give peroxy radicals, these 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: Due to its rapid reaction with oxygen the scavenging of the initial alkyl radical (R•) is exceedingly difficult and can only be achieved using specialised antioxidants baring reactive groups, such as acryloyls, 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, which converts them into organic hydroperoxides (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: The end products of these reactions are typically quinones, which may also impart unwanted colour. Modern phenolic antioxidants often 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 quinones 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 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: 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: