Antioxidants for polymers: how they help prevent thermal degradation.

Antioxidants for polymers are additives that interrupt the oxidation reactions heat sets off in plastics, and they prevent thermal degradation by stopping free radicals from multiplying and by breaking down the hydroperoxides that would otherwise create new ones. When a polymer is melted and sheared during extrusion or moulding, or held at elevated temperature in service, its chains break and form radicals that react with oxygen in a self-feeding chain reaction. Left unchecked, this changes the melt viscosity, discolours the product, makes it brittle and shortens its life.

Two families do the protecting: primary antioxidants, mostly hindered phenols, which scavenge radicals, and secondary antioxidants, mainly phosphites and thioesters, which decompose hydroperoxides. They are used together because each covers a step the other cannot. This guide from Chelora Chemicals explains how thermal degradation happens, how each type of antioxidant works, how they differ in processing and long-term protection, how to choose a package, and how it is tested. For the other half of a polymer stabiliser package, see our guide to UV absorbers vs HALS.

Primary vs Secondary Antioxidants at a Glance

What Is Thermal Degradation of Polymers?

Thermal degradation of polymers is the chemical breakdown that occurs when heat, shear and oxygen attack polymer chains, causing chain scission or cross-linking and a loss of properties. It happens most intensely during processing and continues slowly during service.

In processing, the high temperatures and shear stress in an extruder or injection-moulding machine snap polymer chains and create the first radicals, and any oxygen present makes the damage worse. In service, products such as pipes, automotive under-bonnet parts, cable insulation and appliance housings are exposed to warm, oxygen-containing air for years. The visible signs are yellowing, a change in melt flow, surface cracking and embrittlement, while the underlying damage is a drop in molecular weight or a cross-linked, brittle network. Polyolefins such as polypropylene and polyethylene are especially vulnerable because their hydrocarbon chains are readily oxidised, which is why almost every grade sold for demanding use carries an antioxidant package.

How Does Polymer Oxidation Work?

Polymer oxidation is a radical chain reaction known as autoxidation, in which an alkyl radical picks up oxygen, forms a peroxy radical, abstracts hydrogen from another chain, and creates a hydroperoxide that later splits into more radicals.

The cycle starts when heat and shear create alkyl radicals (R•). These react with oxygen to give peroxy radicals (ROO•), which pull a hydrogen atom from a neighbouring polymer chain, forming a hydroperoxide (ROOH) and a fresh alkyl radical that restarts the cycle. Hydroperoxides are the dangerous intermediate, because on heating they split into two new radicals, which makes the process accelerate, a behaviour called autocatalysis. Traces of metals such as copper can speed up that splitting, which is why contamination matters. The overview of this mechanism and the antioxidant classes is summarised in the polymer stabilizer reference. The two antioxidant families target two different points in this cycle: one catches the radicals, the other removes the hydroperoxides.

What Are Primary Antioxidants and How Do They Work?

Primary antioxidants are radical scavengers, most commonly sterically hindered phenols, that donate a hydrogen atom to a reactive radical and so stop the oxidation chain.

A hindered phenol gives up the hydrogen atom of its hydroxyl group to a peroxy radical, turning it into a stable hydroperoxide-type product and leaving a phenoxyl radical. The bulky groups next to the hydroxyl, usually tert-butyl groups, make that phenoxyl radical stable and unreactive, so it does not restart the chain and can even capture a second radical. Because they act immediately on radicals, primary antioxidants are valued for protecting polymers during extrusion and for long-term heat ageing. They are, however, sacrificial: each molecule is consumed as it works, so protection lasts until the antioxidant is used up. They also cannot break down hydroperoxides, so some hydroperoxide always builds up and needs a second line of defence. Low molecular weight phenols can also volatilise or migrate out of the polymer, which is why higher molecular weight grades are chosen for thin parts, high temperatures and long service lives.

What Are Secondary Antioxidants and How Do They Work?

Secondary antioxidants are hydroperoxide decomposers, mainly organic phosphites and sulfur-based thioesters, that convert hydroperoxides into stable, non-radical products before they can split into new radicals.

Phosphites react with hydroperoxides and reduce them to alcohols, being oxidised to phosphates in the process, which protects the polymer during melt processing and helps keep colour low. A hindered aryl phosphite such as tris(2,4-di-tert-butylphenyl)phosphite (CAS 31570-04-4) is a widely used example, and some phosphites also release a phenolic antioxidant as they react, so they are sometimes described as multi-functional. Thioesters, such as dilauryl thiodipropionate (CAS 123-28-4), decompose hydroperoxides more slowly and are used mainly to support long-term heat ageing, though they can add odour. Secondary antioxidants do not trap radicals themselves, so they are never used alone, and some phosphites are sensitive to moisture, so storage matters. A recent review of phosphite chemistry is available in this open-access paper.

Why Do Primary and Secondary Antioxidants Work Better Together?

They work better together because they stop two different steps of the same cycle, so the secondary antioxidant removes the hydroperoxides that the primary antioxidant cannot, and the primary antioxidant is consumed more slowly as a result.

Published work and industry practice both describe a strong synergy between hindered phenols and phosphites, with the phosphite protecting the phenol during processing and extending its working life. That synergy is not unlimited or universal: some studies report little added benefit when particular combinations are tried, so the pairing and ratio must be verified. The practical result of a good combination is lower discolouration, a more stable melt flow after repeated processing and a longer period before the polymer starts to degrade in service.

What Is the Difference Between Processing and Long-Term Stabilisation?

Processing stabilisation protects the polymer during the short, hot, high-shear steps of compounding and moulding, while long-term stabilisation protects the finished article during years of use at lower temperatures.

Which Polymers Need Antioxidants?

Nearly all commercial polymers need some antioxidant protection, but the type and amount depend on how easily the polymer oxidises and how it is processed and used.

How Do You Choose an Antioxidant Package?

Choose an antioxidant package by working through the polymer, the processing temperature, the service conditions and the regulatory needs in order, then confirm the shortlist by testing.

1. Define the polymer and process. Note the resin, the melt temperature, residence time and shear, since these set how much processing protection is needed and how volatile an additive can be.
2. Define the service life and temperature. Set the target life and the continuous use temperature, because long-term protection needs more robust, lower-volatility primary antioxidants.
3. Choose the antioxidant types. Select a hindered phenol and, for most polyolefins and engineering plastics, add a phosphite, with a thioester only if long-term heat ageing requires it.
4. Choose molecular weight and form. Use higher molecular weight grades where volatility, migration or extraction is a risk, and consider blends or masterbatches for easier, more accurate dosing.
5. Check interactions and regulations. Look at pigments, fillers, flame retardants and metal contamination that can consume or interfere with antioxidants, and confirm food-contact, drinking-water or toy regulations for the finished article.
6. Set loading and verify by testing. Start from supplier guidance, commonly a few hundredths to a few tenths of a percent per component, then confirm with oxidation induction time, multi-pass extrusion and oven ageing.

How Are Polymer Antioxidants Tested?

Polymer antioxidants are tested by measuring how long a sample resists oxidation, how stable its melt flow stays after repeated processing, how it ages in an oven, and how its colour changes.

Oxidation induction time (OIT) is measured by differential scanning calorimetry, commonly following ASTM D3895: a small sample is heated to a set temperature, oxygen is introduced, and the time until oxidation begins is recorded, with a longer time showing better stabilisation. Melt flow index measured after several passes through an extruder shows how well the package protects the polymer during processing, since a stable melt flow means little chain scission or cross-linking. Long-term oven ageing holds samples at elevated temperature for weeks or months and tracks the time until embrittlement or loss of properties, and colour or yellowness measurements show whether the package causes or prevents discolouration. Because a short OIT test cannot fully predict years of service, long-term ageing remains the benchmark for demanding applications.

How Did Polymer Antioxidants Develop?

Polymer antioxidants developed from antioxidants first used in rubber and fuels, and became essential to plastics as polyolefins grew in volume in the second half of the twentieth century.

Early antioxidants, including aromatic amines for rubber and simple hindered phenols such as BHT, showed that radical scavengers could slow oxidation. As polypropylene and polyethylene scaled up commercially, which are very easily oxidised at processing temperatures, the industry developed higher molecular weight, lower-volatility hindered phenols with better permanence, and organophosphites as processing stabilisers. The combination of a phenol and a phosphite became the standard polyolefin package, and later developments brought hydrolytically more stable phosphites, multi-functional molecules that combine phenol and phosphite functions, polymer-bound antioxidants for low migration, and blends supplied as one-pack masterbatches. Today antioxidant packages are designed together with light stabilisers and processing aids to suit the polymer, the process and the expected service life.

Glossary of Terms

Primary antioxidant

A radical scavenger, usually a hindered phenol, that stops the oxidation chain.

Secondary antioxidant

A hydroperoxide decomposer, such as a phosphite or thioester, that prevents new radicals forming.

Hindered phenol

A phenol with bulky tert-butyl groups that make its radical stable and unreactive.

Phosphite

An organophosphorus compound that reduces hydroperoxides to alcohols and protects colour in processing.

Thioester

A sulfur-based secondary antioxidant used to support long-term heat ageing.

Autoxidation

The radical chain reaction in which a polymer reacts with oxygen and keeps feeding itself.

Hydroperoxide (ROOH)

The dangerous intermediate that splits on heating into two new radicals.

Autocatalysis

The acceleration that results when oxidation products create more radicals.

Chain scission

The breaking of polymer chains, lowering molecular weight and strength.

Oxidation induction time

A DSC measure of how long a sample resists oxidation in oxygen at a set temperature.

Melt flow index

A flow measurement taken after repeated extrusion to show processing stability.

Loading

The concentration at which an additive is dosed, as a percentage of polymer weight.