Polymer Degradation: 6 Types, Causes, Signs and the Additives That Stop Each One

Polymer degradation is any change in the chemical structure of a polymer that lowers its properties, and it happens in 6 ways: thermally, thermo-oxidatively, photo-oxidatively, hydrolytically, thermo-mechanically and, by design, oxo-degradatively. Every one of them starts before the part is even made, because plastics are melt-processed at 150 to 320 °C, so which mode attacks which polymer?

PVC begins to lose hydrogen chloride at 100 to 120 °C while polyethylene stays stable to about 400 °C, so the same process temperature that is safe for one polymer destroys another. Behind all 6 modes sit the same two chemical outcomes: oxidation, which adds oxygen to the chain, and chain scission, the cutting of the backbone itself.

Each type happens at a different point in a plastic's life, in a different polymer family, and is answered by a different additive family at a different dosage. Thermal and thermo-oxidative degradation dominate melt processing, photo-oxidation and hydrolysis dominate outdoor and wet service, thermo-mechanical degradation accumulates loop by loop in recycling, and oxo-degradation is the one mode a formulator adds on purpose. Degradation is detected by oxidative induction time, melt flow rate, the FTIR carbonyl index and colour measurement, and it is stopped by heat stabilizers, antioxidants, UV stabilizers, hydrolysis stabilizers and processing stabilizers, which is why almost every compound carries plastic additives whose only job is to slow degradation down.

Table T1. The 6 types of polymer degradation at a glance

Routes outside these six (chemical attack, ionising radiation, biological attack) are named at the end of the type section.

What Is Polymer Degradation?#

Polymer degradation is any change in the chemical structure of a polymer that lowers its properties, whether that change is chain scission, crosslinking, elimination of a side group or oxidation. Where in a plastic's life does that change happen? The change falls into 4 buckets: processing degradation (thermal oxidation, thermal degradation and thermo-mechanical degradation during compounding and moulding), in-service degradation, environmental degradation (photo-oxidation, hydrolysis, ozonolysis and biological attack) and degradation during recycling. The primary chemical changes behind every bucket are the same two reactions: oxidation and chain scission. Formulators call the cumulative effect ageing, and it shows up as a measurable loss of tensile strength, impact resistance, colour stability or melt flow long before it shows up as a visible failure.

Chain scission, crosslinking and side-group elimination#

Degradation changes a polymer in 4 ways: it cuts chains (scission), joins them (crosslinking), strips a side group off the backbone, or adds oxygen to it.

• Chain scission breaks the polymer backbone at a random point, or unzips it back to monomer, lowering molecular weight; polypropylene is the standard example.
• Crosslinking joins two chains together at a radical site, raising molecular weight and eventually forming a three-dimensional network; polyethylene is the standard example, visible as gels in film.
• Side-group elimination strips a substituent off the backbone without necessarily cutting the chain; PVC eliminates hydrogen chloride and polypropylene eliminates a methyl group by homolysis.
• Oxidation adds oxygen to the chain, forming hydroperoxides and carbonyls that then feed either scission or crosslinking, depending on the polymer.

The same oxidation splits polypropylene and joins polyethylene: PP loses molecular weight and its melt flow rate rises, while PE crosslinks and forms gels. Depolymerisation back to monomer, or unzipping, is the dominant thermal route in PMMA, PVC's HCl elimination and PP's methyl-group homolysis are side-group routes, and random chain scission cuts a backbone at whichever bond is weakest. Crosslinking is also done on purpose, with crosslinking agents for polymers in PEX pipe and XLPE cable.

Why some polymers degrade before others#

A polymer's weak point is written into its backbone: condensation polymers such as PET, PC, PBT and the polyamides carry hydrolysable ester, carbonate and amide bonds, while addition polymers such as PP and PE are attacked at a tertiary carbon or at an impurity. Thermoplastics are more vulnerable to degradation than thermosets, because a thermoset's crosslinked network resists the chain motion that scission and oxidation both need, and condensation polymers are more susceptible to hydrolysis and UV attack than addition polymers for the same structural reason: a hydrolysable bond sitting inside the backbone itself.

Polypropylene oxidises preferentially at its tertiary carbon, the carbon that carries the methyl side group, because the carbon-hydrogen bond there is the weakest in the chain. Aliphatic polyolefins such as PP and PE absorb almost no light above about 250 nm, so their photo-oxidation cannot start from the polymer itself; it starts from impurities left behind by manufacture, mainly residual hydroperoxides, carbonyl groups and catalyst residues, which is why two grades of the same polymer, differing only in catalyst residue level, can weather at different rates under identical sunlight.

Does polymer degrade over time?#

Yes: every plastic degrades with time, because oxidation is autocatalytic and runs faster the longer it has run, but the rate is set by the stabilizer package, not by the polymer alone. Unstabilised recycled polypropylene embrittles after about 25 days in a 150 °C oven, while 0.2 to 0.4 % of a stabilizer blend extends that to about 37 to 42 days under the same test. Plasticizer loss can also cause embrittlement and cracking over time without a single chain being cut, so a part can fail from chemical degradation or from a purely physical loss and show the same visible symptom.

What polymer degradation is not: migration, blooming and plasticizer loss#

Three property losses look like degradation and are not. Migration, blooming and volatility remove an additive from the polymer without breaking a single chain, so they are physical losses, not degradation. Plasticizers make this clearest: they are not chemically bound to the polymer matrix, and their loss causes a loss of flexibility, embrittlement and cracking that looks identical to a chemical failure.

• Migration moves an additive out of the polymer and into contact with another material or medium; additive migration in plastics moves the additive, not the chain.
• Blooming happens when an additive above its solubility limit in the polymer diffuses to the surface as the compound cools (Nouman and colleagues, Polymer Degradation and Stability, 2017); the white film on the surface is blooming and exudation in plastics, a solubility effect rather than a chemical one.
• Volatility is the loss of a low-molecular-weight additive to the atmosphere; higher molecular weight lowers volatility, which is why Irganox 1010 (molecular weight 1,178 g/mol) replaced BHT (molecular weight 220 g/mol) in most polyolefins, and a low-molecular-weight stabilizer is lost to additive volatility long before it is chemically consumed.

This is the only place on the site that draws the line between a chemical failure and a physical one, and no competing source in the field makes the distinction explicit.

The 6 Types of Polymer Degradation#

Polymer degradation has 6 types: thermal degradation, thermo-oxidative degradation, photo-oxidative degradation, hydrolytic degradation, thermo-mechanical degradation and oxo-degradation. Each type attacks a different chemical bond, appears at a different point in a plastic's life, and is stopped by a different additive family, in the order melt processing meets them.

1. Thermal degradation: unzipping, side-group elimination and random scission#

Thermal degradation is bond breaking by heat alone, without oxygen, and it takes 3 forms: the chain unzips back to monomer, a side group is eliminated, or the backbone is cut at random points. Depolymerisation, or unzipping, dominates in PMMA, which reverts toward its monomer under heat. Side-group elimination dominates in PVC, which loses hydrogen chloride, and in polypropylene, which loses a methyl group by homolysis. Random chain scission cuts the backbone at whichever bond is thermally weakest, without a preferred site.

PVC is the extreme case: it starts to eliminate hydrogen chloride slowly at 100 to 120 °C and degrades rapidly near 250 °C, while polyethylene is stable to about 400 °C, and only 0.1 % dehydrochlorination is already enough to discolour a PVC compound unacceptably. Melt processing runs at 150 to 320 °C for most thermoplastics and at 240 to 320 °C for engineering polymers, so a PVC compound spends its entire processing window inside its own onset range, while a polyolefin has a wide margin above it. Wood-plastic composites need an even tighter window: they must be processed at least 28 °C below the unfilled resin and below about 200 °C, because the cellulose filler degrades before the polymer does.

PVC heat stabilizers answer thermal degradation by neutralising the hydrogen chloride and replacing the labile chlorine atoms that would otherwise keep releasing it, and acid scavengers neutralise the acid released by thermal degradation in polyolefins. The two temperature stages that make PVC formulation its own discipline are set out in full on PVC thermal degradation and dehydrochlorination.

At what temperature does plastic degrade?#

There is no single temperature: plastics are melt-processed between 150 and 320 °C, and each polymer and each additive in it has its own ceiling. PVC starts to degrade at 100 to 120 °C, polyethylene only near 400 °C, and the additive is often the weakest part of the formulation: diarylide pigments decompose above 200 °C, intumescent ammonium polyphosphate flame-retardant systems are limited to about 220 °C, and low-treated titanium dioxide is only preferred above 232 °C. A processing window has to satisfy the polymer, the additive package and the part geometry all at once.

2. Thermo-oxidative degradation: the autoxidation cycle#

Thermo-oxidative degradation is autoxidation: heat and shear create a first radical, oxygen turns it into a peroxy radical, the peroxy radical takes a hydrogen atom from the next chain, and the hydroperoxide that forms splits into two new radicals, so one initiation event feeds a chain reaction. The scheme was established by Bolland and Gee in the 1940s and is still the working model for polymer stabilization; practitioners shorten the reaction sequence itself to the name thermo-oxidation.

The cycle runs in 4 stages: initiation, where heat or shear splits a bond and the resulting alkyl radical adds oxygen; propagation, where the peroxy radical abstracts a hydrogen atom from a neighbouring chain and forms a hydroperoxide plus a new alkyl radical; chain branching, where the hydroperoxide itself splits homolytically into two new radicals instead of one; and termination, where radicals finally combine into stable products. The reaction of an alkyl radical with oxygen is diffusion-fast, on the order of 10^7 to 10^9 L mol⁻¹ s⁻¹, which is why alkyl radicals are almost impossible to scavenge directly and why stabilizer chemistry instead targets the peroxy radical and the hydroperoxide downstream of it.

The cycle is autocatalytic and runs through an induction period before it accelerates, which is why oxidative induction time is the standard measure of how much protection is left in a compound. Almost all polymers are at risk of thermal oxidation when processed at high temperature, but the outcome differs by polymer: polypropylene chain-scissions, so its melt flow rate rises, while polyethylene crosslinks and forms gels.

Primary antioxidants, the hindered phenols, intercept the peroxy radical before it can abstract another hydrogen; secondary antioxidants, the phosphites and thioesters, destroy the hydroperoxide before it branches into two new radicals; and metal deactivators bind the copper and iron ions that would otherwise split hydroperoxides even faster. Every step of the cycle, and the additive class that blocks it, is set out in full on polymer oxidation and antioxidant mechanisms.

3. Photo-oxidative degradation: UV light, chromophores and Norrish reactions#

Photo-oxidative degradation starts when a chromophore inside the polymer absorbs ultraviolet light above 290 nm and splits into radicals, which then feed the same autoxidation cycle as heat does. Chromophores that do this include hydroperoxides, carbonyl groups and residual catalyst and metal traces, and the shortest ultraviolet wavelength that reaches the Earth's surface is about 280 to 290 nm, which sets the practical lower limit for what a polymer has to withstand outdoors.

Pure polyethylene and polypropylene absorb nothing above about 250 nm, so their weathering is started by what the polymer carries with it: residual hydroperoxides, carbonyl groups and catalyst residues, not by the polymer backbone itself. Photodegradation of plastics behaves differently by polymer: PET absorbs light from about 360 nm, with Norrish type I cleavage dominant and Norrish type II cleavage producing acetaldehyde as a by-product, while pure PVC does not absorb above 220 nm, though its own thermal degradation generates polyenes, and a polyene chain of 8 or more conjugated double bonds is visibly coloured. The temperature a part reaches often matters more to photo-oxidation than the ultraviolet dose it receives, which is why the same stabilizer package can perform differently in a hot climate than in a cold, sunny one. About 70 % of the world's light-stabilizer production goes into polyolefins, which make up about 50 % of global plastics production, because polyolefins carry the least intrinsic protection of any major polymer family.

UV absorbers convert absorbed light energy into heat before it can excite a chromophore, hindered amine light stabilizers trap the resulting radicals catalytically instead of being consumed by them, and pigment screeners such as carbon black absorb the light before it ever reaches the polymer. The cheapest UV protection available is a screener: carbon black and titanium dioxide as UV protection work by blocking light rather than by intercepting radicals after the fact.

4. Hydrolytic degradation: water cleaves esters, amides and carbonates#

Hydrolytic degradation is water cutting the polymer backbone at its ester, amide or carbonate bonds, which is why PET, PBT, PC, the polyamides, PLA and polyurethanes must be dried before they are melted. Water attacks these bonds by nucleophilic substitution, and even a small amount of residual moisture inside melt-processing equipment lowers molecular weight through chain scission before the part ever leaves the machine. Excess moisture in PET reduces molecular weight through exactly this route, which is why the resin is dried before every processing step.

Different condensation polymers tolerate water to different degrees. Polycarbonate hydrolyses above 70 °C at high humidity and releases bisphenol A as a degradation product, and PBT becomes hot-water sensitive above 60 °C. Condensation polymers are more susceptible to hydrolysis than addition polymers in general, because every ester, amide or carbonate linkage in the backbone is a potential cleavage site that an addition polymer such as PP or PE simply does not carry. Polylactic acid, PLA, is an extreme case by design: it has a glass transition temperature of 60 to 65 °C, and about half of it decomposes within 60 days of industrial composting at 58 °C.

Drying the resin before processing is the first defence, and chemical protection follows where drying alone is not enough. Carbodiimides react with the carboxyl end groups that catalyse further hydrolysis once it has started, removing the acid that would otherwise accelerate the reaction; an aziridine-based hydrolysis-stabilizer system developed by R. Pfaendner at Fraunhofer LBF limited molecular-weight loss in PLA to about 10 % after 850 hours and to about 20 % after 1,200 hours of accelerated ageing. Chain extenders rebuild molecular weight after it has already been lost, and desiccant masterbatch protects wet recyclate where drying is impractical. The carbodiimide grades used against this mechanism are compared in full on hydrolysis stabilizers.

5. Thermo-mechanical degradation: shear and multiple heat histories#

Thermo-mechanical degradation is chain scission caused by shear in the screw and the die, and it adds up: every extrusion pass takes another bite out of the molecular weight. Shear-induced scission accumulates over repeated passes because each pass reintroduces the same mechanical stress on chains that are already shorter and more radical-prone than before. In a 5-pass extrusion trial at 250 °C, the melt flow rate of unstabilised recycled polypropylene can rise pass by pass as chain scission accumulates, while a compound protected with a 0.1 to 0.3 % antioxidant blend can hold a far lower rate across the same 5 passes (Songwon technical data).

Gel formation follows a similar pattern on the polyethylene side: an antioxidant dosed at 0.1 % can cut gel counts in recycled LLDPE by roughly a third against an unstabilised control, because part of what a gel is made of is degraded or crosslinked polymer. Torque rheometry gives a more direct process signal: in an optimised polyolefin test (45 g sample, 90 rpm, 190 °C), a compound reached a stable processing time of 13 minutes and a degradation point at 27.5 minutes, and the optimised protocol cut measurement error from 50 % to 7.7 % against the older method.

Processing stabilizers, a phenol plus phosphite blend dosed for the melt rather than for long-term heat, absorb most of this shear-driven load, and recycled material needs restabilization because its original package has already been spent once. Screw design and residence time, the two process variables that set how much shear a compound sees, are covered in full under plastic compounding.

6. Oxo-degradation: pro-oxidant additives that degrade a polymer on purpose#

Oxo-degradation is the only one of the 6 types that is added on purpose: iron, manganese or cobalt salts are compounded into the polymer so that hydroperoxides split faster and the article fragments after use. These transition-metal pro-oxidants promote hydroperoxide homolysis, accelerating the same photo-oxidation and thermo-oxidation reactions that every other stabilizer on this page exists to slow down, so an oxo-degradable formulation is, chemically, an anti-stabilizer package.

Products made from oxo-degradable plastic have been prohibited on the EU market since 3 July 2021 under Article 5 of Directive (EU) 2019/904, which defines oxo-degradable plastic as plastic containing additives that, through oxidation, fragment the material into micro-fragments. The Association of Plastic Recyclers rates degradable additives as rendering PP and PE packaging non-recyclable, because a recycler cannot separate a pro-oxidant-loaded stream from a conventional one, and every part of that stream then carries the same fragmentation risk forward.

Because the mechanism accelerates fragmentation rather than stopping it, there is no stabilizer answer to oxo-degradation in the way there is for the other 5 types; the additive itself is the opposite of a stabilizer. The evidence and the bans behind this mode are set out in full on oxo-degradable plastics.

Three further routes fall outside these 6 types. Chemical attack causes chlorine-induced stress cracking in polyolefin pipe carrying disinfected water, and ozonolysis attacks the carbon-carbon double bonds of rubbers, which sits outside the plastics-only scope of this page. Ionising radiation, gamma and electron-beam sterilisation among them, breaks bonds directly rather than through heat, light or water; the antioxidant packages built for this route are covered separately under antioxidants for sterilization. Biological attack, enzymatic or microbial, is the route designed biodegradable plastics are built to invite rather than resist. None of the three counts among the 6 types on this page, because none is a routine risk in a conventional, non-designed compound.

When Polymers Degrade: Processing, Service Life and Recycling#

A plastic meets degradation 3 times: in the compounding and moulding heat, across its service life, and again in every recycling loop. The classification separates these into processing degradation (thermal oxidation, thermal degradation and thermo-mechanical degradation together), in-service degradation, environmental degradation and degradation during recycling, and each phase draws on a different combination of the 6 types described above. The stabilizer package a formulator designs has to survive all 3 phases in sequence, not only the first one, because a package spent during processing has nothing left for the service life that follows. Every step of plastic formulation and additive processing is indexed across this section, phase by phase.

Degradation during compounding and moulding#

Most of a compound's stabilizer is consumed before the part exists, because melt processing combines the two initiators of oxidation, heat and shear, for minutes at a time. Almost all polymers are at risk of thermal oxidation when they are processed at high temperature, and the risk rises with residence time, screw speed and barrel temperature together, not with any one variable alone.

Gels in PE film illustrate how a processing problem becomes a permanent defect: they come from degraded or crosslinked polymer, unmelted resin or concentrate, and poor masterbatch dispersion, and the problem is aggravated by high heat, high shear, long residence time and a weak antioxidant package acting together. The additive package itself can also set the processing ceiling rather than the polymer: low-treated titanium dioxide is preferred above 232 °C, diarylide pigments decompose above 200 °C, and intumescent flame-retardant systems are limited to about 220 °C, so a formulation built around one of these additives inherits its temperature limit.

Degradation in service#

In service the same two reactions continue slowly, and the failure the user sees is embrittlement: the part keeps its shape and loses its impact strength. Temperature often matters more than the ultraviolet dose in outdoor ageing, so a part in a hot, low-sunlight climate can degrade faster than an identical part in a cooler, sunnier one. Unstabilised recycled polypropylene embrittles after about 25 days in a 150 °C oven, against about 37 to 42 days with 0.2 to 0.4 % of a stabilizer blend under the same test, a demonstration of how much service life the stabilizer package alone is worth.

Different polymers fail by different routes in service. Copper halide systems, copper at 0.001 to 0.03 wt% plus halide at 0.1 to 5 wt%, protect polyamide against long-term heat ageing in demanding applications such as automotive connectors. Acetal, POM, aged at 100 °C shows chain scission in its amorphous region from about 3 days onward, after which formaldehyde and formic acid released by the reaction catalyse further depolymerisation. In PA56T glass-fibre grades aged at 150 °C, antioxidants slowed the ageing rate but did not change its pathway, and the yellowing observed came from carbonyl formation rather than from the antioxidant itself.

Degradation in recycling: every loop is another heat history#

Recycling is degradation by repetition: each loop adds a full melt history, and the antioxidant that survived the first one is mostly gone by the third. Knoben, Vanhouttem, Wypkema and Subramanian, in a 2025 study in Materials (volume 18, article 1640), found that un-restabilised white polypropylene regranulate still held more than 150 ppm of intact Irgafos 168 after its first life, and that dosing 500 ppm of a primary antioxidant plus 1,000 ppm of a secondary antioxidant at every cycle raised the oxidative induction temperature to 257 °C at cycle 5, against 198 °C for the unstabilised material, while the secondary antioxidant itself built up from about 650 ppm at cycle 1 to more than 1,200 ppm at cycle 5.

Post-consumer and post-industrial recycled polyolefins are generally not topped up with antioxidants at all (Songwon technical data), which is the gap restabilization exists to close. R. Pfaendner, first at Ciba and later at Fraunhofer LBF, introduced restabilization as a formal practice and reviewed 30 years of it in Polymer Degradation and Stability, volume 203 (2022), article 110082. Additives can also be released from plastics during recycling and from the products made from recyclate, a finding reported by Hahladakis, Velis, Weber, Iacovidou and Purnell at the University of Leeds in the Journal of Hazardous Materials in 2018. Topping the package back up is restabilization of recycled plastics.

Which Polymers Degrade in Which Way?#

Each polymer degrades in the way its backbone allows: PVC eliminates hydrogen chloride, polypropylene scissions at its tertiary carbon, polyethylene crosslinks, and the polyesters and polyamides hydrolyse. The dominant mode differs by polymer family even though every polymer is exposed to more than one type of degradation at once, and the table below names the mode that decides the additive package for each of the 12 polymer families indexed under additives by polymer.

Table T3. Dominant degradation mode by polymer

Every polymer degrades in more than one way; the table names the mode that decides the additive package.

How Do You Detect Polymer Degradation? Signs, Defects and Test Methods#

Degradation is detected in 2 stages: the visible defect on the part, then the laboratory test that names the mechanism behind it. FTIR carbonyl index, oxidative induction time, melt flow rate, yellowness index and mechanical testing between them cover every one of the 6 types, turning a customer complaint into a specific, fixable cause the way additive-related defects are diagnosed symptom by symptom.

Visible signs: yellowing, gels, chalking, cracking and embrittlement#

The first evidence of degradation is visible: colour shifts to yellow, gels appear in film, the surface chalks, and the part breaks where it used to bend. Each sign points toward a different mechanism, though more than one mode can produce the same symptom.

• Yellowing and pinking follow over-oxidation of a phenolic antioxidant into coloured quinones, interaction with low-treated titanium dioxide, NOx gas fading, or cardboard-packaging yellowing; the colour causes are separated in full on why plastics turn yellow or pink.
• Gels and fisheyes in film come from degraded or crosslinked polymer and from poor masterbatch dispersion; gel counts and their sources are set out in full on gels and fisheyes in plastic film.
• Chalking is a powdery surface layer left when a UV-degraded surface loses the binder that once held a pigment or filler particle in place.
• Cracking follows either surface chain scission or plasticizer loss, and the two causes need different tests to tell apart.
• Embrittlement is the mechanical signature of both chemical degradation and physical plasticizer loss, so a drop in impact strength alone does not identify the mechanism.

Laboratory tests: OIT, MFR, carbonyl index, yellowness index and oven ageing#

Each degradation mode has a standard test: oxidative induction time for thermo-oxidation, melt flow rate for chain scission, the FTIR carbonyl index for photo-oxidation, yellowness index for colour and the Congo red test for PVC. Every method below is indexed in full under testing plastic additives.

Table T4. Degradation mode, test, current standard and what the result means

Which Additives Prevent Each Type of Polymer Degradation?#

Every type of polymer degradation has an additive family that answers it: heat stabilizers for thermal degradation, antioxidants for thermo-oxidation, UV absorbers and HALS for photo-oxidation, carbodiimides and chain extenders for hydrolysis, and processing stabilizers for shear. A hindered phenol donates a hydrogen atom to the peroxy radical, and each phenol molecule can neutralise about 2 peroxy radicals this way before it is used up; a phosphite reduces the hydroperoxide to a stable alcohol instead of letting it branch into two new radicals; a thioether antioxidant is oxidised progressively to sulfoxides and then sulfones and works mainly for long-term heat protection at about 100 to 150 °C; and lactone and hydroxylamine stabilizers scavenge the alkyl radical itself, a step the other antioxidant classes cannot reach directly.

HALS work by the Denisov cycle (E. T. Denisov, Polymer Degradation and Stability, 34 (1991) 325-332), are not consumed stoichiometrically the way a phenol is, and do not absorb ultraviolet light at all; UV absorbers, by contrast, follow the Beer-Lambert law, so their protective effect depends on concentration multiplied by section thickness, which is why the same UV absorber loading protects a thick part better than a thin one. Metal deactivators such as Irganox MD 1024 (EU food-contact material number FCM 675, specific migration limit 15 mg/kg) chelate the copper that catalyses oxidation in cable insulation before it can accelerate hydroperoxide splitting. Carbodiimides remove the carboxyl end groups that catalyse polyester hydrolysis, and chain extenders such as epoxy-functional styrene-acrylic oligomers rebuild molecular weight in recycled PET and PLA at 0.1 to 0.5 wt%.

Table T5. Degradation type, additive family, mechanism and typical level

Levels are typical use bands from our source library, not recommendations for a specific compound. Where our source library has no sourced band, the cell says "see the hub."

The full selection sequence, matching stabilizer class to polymer, processing window and end use, is set out on how to select plastic additives, and every grade named on this page has its own record in the plastic additives database.

How a stabilizer package is built: primary, secondary and light stabilizer blends#

A stabilizer package is built in 3 layers, in the order the polymer meets its enemies: process stabilization for the melt, long-term thermal stabilization for the service temperature, and light stabilization for whatever sunlight reaches the part. A phenol and a phosphite protect polyolefins better together than either alone, and the ratio should shift with the job: in polypropylene, an 80:20 ratio of Irganox 1010 to a thioester such as DSTDP is optimal for processing stabilization, while a 20:80 ratio is optimal for long-term thermal stability at 150 °C. Commercial pre-blended grades package this synergy directly: Irganox B 215 combines 67 % Irgafos 168 with 33 % Irganox 1010, B 225 uses a 1:1 ratio, and B 900 combines 80 % Irgafos 168 with 20 % Irganox 1076. The commercial ratios behind each blend are compared in full on antioxidant blends and synergy.

The package should add light stabilization as its third layer wherever the part sees sunlight. HALS are not consumed stoichiometrically, unlike a phenolic antioxidant, and their protection does not depend on part thickness, while a UV absorber's performance does, because it works by the same concentration-times-thickness relationship of the Beer-Lambert law described above. The catalytic class that makes thickness-independent protection possible is hindered amine light stabilizers (HALS).

A formulator should specify the 3 layers in this order:

1. Specify process stabilization first, sized to survive the highest melt temperature the compound will see.
2. Add long-term thermal stabilization next, sized to the part's service temperature and expected lifetime.
3. Add light stabilization last, choosing HALS for thickness-independent protection or a UV absorber where the part is thin and the polymer allows one; in acidic compounds the answer is non-basic hindered amine light stabilizers (NOR HALS) instead of conventional HALS.

Additive interactions that make stabilization fail: antagonism, adsorption and depletion#

A correct additive can still fail for 3 reasons: another additive deactivates it, a filler adsorbs it, or it simply leaves the polymer. These 3 failure modes sit outside the degradation chemistry itself, which is why a compound can carry the right stabilizer at the right dosage and still fail in service. The full interaction matrix is set out on additive interactions.

• Antagonism happens when one additive deactivates another: thioester antioxidants and HALS are antagonistic because the acidic sulfur products a thioester produces deactivate the basic HALS, flame retardants generally reduce antioxidant effectiveness, and 1,000 to 2,000 ppm of sulfur from pesticide residue can cut greenhouse-film life by 20 to 25 %. Conventional N-H and N-methyl HALS are largely ineffective in PVC for the same reason: the hydrogen chloride released by dehydrochlorination protonates them, and NOR HALS are the practical exception.
• Adsorption happens when a filler's surface competes for the additive: talc, kaolin and silica adsorb antioxidants and HALS on their acid sites, removing them from the polymer where they are needed.
• Depletion happens when a stabilizer simply leaves the compound over time, whether by volatilisation, blooming or extraction; a low-molecular-weight stabilizer is lost faster than a high-molecular-weight one, which is why Irganox 1010 at 1,178 g/mol replaced BHT at 220 g/mol in most polyolefins, and a phenolic antioxidant that has already done its job becomes a coloured quinone rather than disappearing cleanly.

How Much Stabilizer Does a Compound Need? Dosage Levels in wt%, ppm and phr#

Stabilizers sit between 0.05 and 3 wt% of a compound, far below fillers and plasticizers. Antioxidants and heat stabilizers occupy 0.05 to 3 wt% of the finished plastic, against 10 to 70 wt% for plasticizers, which makes stabilizers the cheapest large effect available in a formulation: a fraction of a percent decides whether the part survives its service life. PVC formulations are written in phr, polyolefin formulations in wt% or ppm, and the conversion is wt% = phr of the ingredient divided by the total phr, times 100; in the PPI TR-2 pipe formulation example the total reaches 108.03 phr, so the PVC resin itself works out to 92.57 wt% of the compound even though it is written as 100 phr. Restabilization blends for recycled polyolefins sit at 0.1 to 0.3 wt%, and the recycling-cycle dosing rule of 500 ppm of a primary antioxidant plus 1,000 ppm of a secondary antioxidant per cycle is the practice behind the OIT recovery described earlier on this page.

Table T6. Typical stabilizer dosage by polymer and purpose

The conversion rules behind every dosage figure on this page, including the full phr-to-wt% worked example, are set out on PHR (parts per hundred resin). Family-by-family dosage bands beyond the 12 examples in Table T6 are indexed on additive dosage levels in plastics.

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Is Polymer Degradation Always Unwanted?#

Degradation is almost always the failure a formulation is designed to prevent, with 2 exceptions: plastics meant to break down after use, and chemical recycling, which depolymerises on purpose. Oxo-degradation is deliberate and, since 3 July 2021, prohibited in EU products under Article 5 of Directive (EU) 2019/904, while enzymatic and hydrolytic biodegradation remain permitted routes judged against a standard rather than a marketing claim. EN 13432 requires at least 90 % biodegradation within 6 months for a material to be called industrially compostable, and the European Commission, in COM(2018) 35, found no conclusive evidence of full biodegradation of oxo-degradable plastic in any open environment, which is exactly why the two exceptions are treated so differently in regulation.

What chemicals can degrade plastic?#

The chemicals that degrade plastic are, in order of practical importance, water, oxygen, acids and transition-metal salts, and most of them are already inside the compound. Water is the most common one: it cleaves the ester, amide and carbonate bonds of condensation polymers directly. Oxygen feeds the autoxidation cycle that drives both thermo-oxidative and photo-oxidative degradation. Acids, including the hydrogen chloride that PVC itself releases as it degrades, deactivate HALS and accelerate further breakdown, and formaldehyde and formic acid released during POM degradation catalyse its own depolymerisation in the same self-accelerating way. Transition-metal salts, iron, manganese and cobalt among them, are the chemicals deliberately added to oxo-degradable plastic to force this same chemistry to run faster.

• Water: cleaves ester, amide and carbonate bonds
• Oxygen: feeds the autoxidation cycle
• Acids: deactivate HALS, catalyse further hydrolysis or depolymerisation
• Transition-metal salts: force hydroperoxide homolysis

Degraded polymer and legacy additives in recycled plastic#

Recycling carries two problems forward at once: the polymer arrives already part-degraded, and the additives of its first life arrive with it. Wiesinger and colleagues at ETH Zurich, in a 2024 study in Environmental Science & Technology, found that 16 % of 151 new PVC floorings on the Swiss market contained regulated chemicals above 0.1 wt%, mainly lead and DEHP, likely related to the use of recycled PVC in their construction. Which additives stay in a formulation, and which have to be phased out for a recyclate stream to stay compliant, is covered in full on design for recycling.

Additives can also be released from plastics during recycling and from the products made from recyclate themselves, a pathway documented by Hahladakis, Velis, Weber, Iacovidou and Purnell at the University of Leeds in the Journal of Hazardous Materials in 2018. The substances that travel forward from a first-life compound into a recycled one, brominated flame retardants, lead, cadmium and phthalates among them, are listed in full on legacy additives in recycled plastic.

Designed degradation: oxo-degradable and biodegradable plastics#

A plastic designed to degrade is judged by a standard, not by a claim: EN 13432 requires at least 90 % biodegradation within 6 months. Bio-based is not the same as biodegradable: biodegradability is decided by a polymer's chemical structure, not by whether its carbon originally came from a plant or from petroleum, so a bio-based polyethylene degrades no faster than a petroleum-based one. The evidence for each type of designed degradation, oxo, enzymatic and pro-degradant, is set out in full on biodegradation additives for plastics.

Oxo-degradable plastic has been treated differently: the European Commission's own review, COM(2018) 35, found no conclusive evidence of full biodegradation of oxo-degradable plastic in any open environment, and products made from it have been prohibited on the EU market since 3 July 2021 under Article 5 of Directive (EU) 2019/904. The jurisdictions that have followed the EU's approach, and those that have not, are compared in full on oxo-degradable plastic bans.

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