Chain Extenders for Polymers: 6 Types for PET, rPET, PLA and PBT, Dosage and Selection

Chain extenders are multifunctional reactive additives that join the broken ends of condensation polymers such as PET, PLA, PBT and polyamide back together, so a recyclate regains the molar mass, intrinsic viscosity and melt strength it lost during processing, and this reference sorts them into 6 chemistries. They are used at roughly 0.1 to 1.5 wt% of the polymer, which raises the practical question every compounder running recyclate has to answer: which of the 6 chemistries belongs in which polyester, and at what level before the melt gels?

Chain extension changes five measurable things. Molar mass and intrinsic viscosity rise, which is what an epoxy-functional oligomer of the Joncryl ADR type does in recycled PET. Melt flow rate falls. Melt strength and strain hardening rise where the extender is multifunctional enough to build long-chain branching, the job of pyromellitic dianhydride and of the 9-epoxy-group oligomers. Processability in extrusion foaming, blow moulding, film blowing and thermoforming follows from that melt strength, and the share of recyclate a converter can run goes up.

Chain extenders are one of the 43 families of plastic additives, filed with the property modifiers next to impact modifiers and compatibilizers. This page covers the mechanism and the end groups involved, the 6 chemistries with their grades and CAS numbers, what chain extension changes, dosage in wt% with the gel limit that caps it, which chemistry suits PET, PLA, PBT, polyamide and PC recyclate, a 6-step selection route, the reaction window, the four test methods, the EU and US food-contact position for recyclate, the producers, and all 5 chain-extender substances in the directory.

The 6 chemistries differ in the end group they attack, the number of reactive groups each molecule carries and the level at which the literature uses them, as the table sets out.

Levels are research or tested values from the cited studies, not supplier recommendations. Where our source library holds no sourced level, the cell says so.

What Is a Chain Extender in Plastics?#

A chain extender is a plastic additive with two or more reactive groups that attach to the carboxyl, hydroxyl or amine ends of a condensation polymer and link shortened chains into one longer or branched molecule. The reaction raises molar mass, intrinsic viscosity, melt viscosity and melt strength in a single extrusion pass, which is why the family is also sold as molecular-weight builders, IV boosters and branching agents.

It does not work in polyolefins such as polyethylene and polypropylene or in PVC, because those chains carry no reactive end groups to attach to. It is also not a crosslinker: the target is a soluble, processable melt rather than an insoluble network. In the site taxonomy, chain extenders sit in the property-modifier group, one of the 43 additive families, alongside impact modifiers, compatibilizers and coupling agents.

Why do PET, PLA, PBT and polyamides lose molar mass in the melt?#

Polyesters and polyamides degrade in the melt by hydrolysis: residual water cleaves the ester or amide bond, and every cleavage leaves a new carboxyl end group that catalyses the next one, so intrinsic viscosity falls further with each pass through the extruder. Four routes run in parallel in a recycling extruder.

• Hydrolysis of the ester bond, driven by residual moisture in the flake, autocatalytic because each scission adds a carboxyl group.
• Thermal scission at melt temperature, which in PET also releases acetaldehyde, detectable as an off-taste in bottled water at 10 to 20 ppb.
• Thermo-oxidative scission, which needs only traces of oxygen and is the reason a recyclate also needs restabilization.
• Contamination, above all PVC fragments and residual moisture in rPET, both of which raise the degradation rate of the batch.

What the loss costs is set by the application. Indicative intrinsic-viscosity ranges for PET grades run from 0.40 to 0.70 dL/g for textile fibre, 0.60 to 0.70 dL/g for BoPET film, 0.70 to 0.78 dL/g for general-purpose bottles, 0.78 to 0.85 dL/g for carbonated soft-drink bottles and 1.00 to 2.00 dL/g for monofilament and engineering grades.

Chain extender, hydrolysis stabilizer or compatibilizer: what is the difference?#

A chain extender rebuilds chains that are already broken, a hydrolysis stabilizer scavenges the water and acid that would break them, and a compatibilizer works at the interface between two polymers that do not mix. The three families act on three different problems, and a formulation running post-consumer polyester often needs all three at once.

Timing separates the first two: a chain extender repairs damage that has happened, while a hydrolysis stabilizer is preventive and its dose follows the expected service life. Carbodiimides sit in both boxes, because the same N=C=N group that removes free acid also couples two chains, and the additives that scavenge the water and acid instead are compared on hydrolysis stabilizers.

Function separates the third. A compatibilizer changes an interface between two immiscible polymers rather than the length of one chain, and the epoxy-functional oligomers do both: the interfacial route belongs to the compatibilizers, the chain-end route to this page.

Is a chain extender the same as a branching agent?#

Partly: an extender with two reactive groups only rejoins chains end to end, while an extender with four or more, such as pyromellitic dianhydride or an epoxy oligomer carrying about nine epoxy groups, also builds long-chain branches. Long-chain branching is what raises melt strength and strain hardening, the two properties that extrusion foaming, blow moulding, film blowing and thermoforming depend on. A bifunctional oxazoline raises molar mass without giving any of that, so the two labels overlap only above functionality 2, and the other routes to melt strength are set out on melt strength enhancers.

How Do Chain Extenders Work?#

A chain extender works in three steps: it disperses in the melt, its reactive groups open on the chain ends they match, and the chains it has joined form either a longer linear molecule or a branched star. Each chemistry opens differently: an epoxide ring opens on a carboxyl end quickly and on a hydroxyl end more slowly, an anhydride ring opens on a hydroxyl end and leaves a new carboxyl group behind, and an oxazoline ring forms an ester-amide link without releasing anything at all. The architecture that results decides whether the melt merely thickens or starts to strain-harden.

Which end groups a chain extender reacts with: carboxyl, hydroxyl and amine#

Which end group a chain extender attacks decides which polymer it suits: epoxides and oxazolines react with carboxyl ends, anhydrides and isocyanates with hydroxyl ends, and carbodiimides with the carboxyl groups that hydrolysis has just created. Polyamides carry amine ends as well as carboxyl ends, which is why epoxy-functional oligomers react faster in polyamide than in polyester.

The by-product matters as much as the reaction. The anhydride ring of PMDA forms an ester link and a new carboxyl group with no volatile released, while its hydroxyl reactivity is low, which is why formulators combine it with an epoxide. The oxazoline ring of 1,3-PBO releases no by-product at all, and because the molecule is bifunctional the coupling is linear and the acid number of the polyester falls.

Linear extension and long-chain branching: where melt strength comes from#

Melt strength comes from branches, not from length alone: a bifunctional extender such as 1,3-PBO only rejoins two chains end to end, while an epoxy oligomer with about nine epoxy groups per molecule ties up to nine chains to one backbone and creates the long-chain branching that makes a melt strain-harden. Functionality is therefore the first number on a datasheet worth reading.

Three architectures follow from three functionality bands.

• Linear coupling, from bifunctional extenders such as 1,3-PBO and the monomeric carbodiimide, which raises molar mass and viscosity.
• Star and comb architecture, from tetrafunctional PMDA and from the epoxy oligomers, where Joncryl ADR-4368 carries a weight-average molar mass of 6,800 g/mol, an epoxy equivalent weight of 285 g/mol and about 9 epoxy groups per molecule.
• Hyperbranched structures, reached when a tetrafunctional or higher extender reacts repeatedly, where strain hardening appears and where gels begin.

Grade choice shifts the same axis. Joncryl ADR-4400 has a weight-average molar mass of 7,100 g/mol with a number-average functionality of about 5 and a weight-average of about 14, while ADR-4468 reaches about 9 and about 24, so it branches more strongly at the same weight percentage.

Gel formation: what happens when a chain extender is overdosed#

Gels are the limit that sets the maximum dose: Karl and co-workers at SINTEF and the University of Hannover measured gel content at or below 2 percent when recycled PET was extended with 0.5 to 1.5 wt% Joncryl ADR-4400 at 280 °C, and about 32 percent at 3 wt%, in their 2024 study in Industrial and Engineering Chemistry Research. A gel is a crosslinked particle that no longer flows, showing up as specks in film, as a filter-pressure rise in fibre spinning and as lost ductility.

Three measured points describe the window in that work.

• 0.5 wt% Joncryl ADR-4400 in rPET at 280 °C: gel content at or below 2 %.
• 1.5 wt%: gel content still at or below 2 %, with the largest inherent-viscosity and extrusion-force gains of the series.
• 3.0 wt%: gel content about 32 %, the level at which the study measured gelation rather than a usable dose.

Residence time compounds dose, because overdosing or a long residence in the barrel produces gel particles even inside the nominal range, and the extraction method behind the figure is described on gel content.

The reaction window: temperature, dwell time and conversion#

A chain extender needs both temperature and time: a study on PA 10.10 published in Polymers in 2023 measured 99 percent conversion of two Joncryl grades at 200 °C after 120 seconds or at 280 °C after 30 seconds, while BASF recommends about 260 °C with at least 120 seconds of dwell. Conversion below that leaves unreacted oligomer in the part, which migrates and wastes the dose.

Three facts define the window in practice.

• Conversion: 99 % for Joncryl ADR-4400 and ADR-4468 at 200 °C in 120 s, or at 280 °C in 30 s.
• Supplier statement: BASF gives about 260 °C and a dwell time of at least 120 s.
• Drying: a laboratory protocol dried rPET flakes at 140 °C for 17 h and the Joncryl at 120 °C for 3 h.

Moisture works against the reaction twice over, because water hydrolyses the polymer during the same pass and hydrolysed PMDA promotes degradation instead of reversing it. Altınbay, Nofar and co-workers reported in International Journal of Biological Macromolecules in 2025 that aged PBAT reacts hardly at all. Barrel profile, dwell time and dispersion are covered on plastic compounding.

6 Types of Chain Extenders for Plastics#

The 6 types of chain extenders for plastics are epoxy-functional oligomers, dianhydrides, oxazolines, carbodiimides, diisocyanates, and a group of other reactive types that covers phosphites, bis-caprolactams and aziridines. Epoxy-functional oligomers of the Joncryl ADR type carry most of the commercial volume in recycled polyester, and every other chemistry is chosen for a specific reason: no by-product, a second function, or a legacy position in a formulation that has not been requalified. The order below is commercial prominence in plastics recycling, and every table on this page follows it.

1. Epoxy-functional oligomers: the Joncryl ADR type#

Epoxy-functional oligomers are styrene-acrylic chains carrying pendant glycidyl groups that react with carboxyl ends quickly and hydroxyl ends more slowly, which makes them the default chain extender for recycled PET, PLA, PBT and polyamide. BASF supplies the reference line as Joncryl ADR, with the grades ADR-4368 (C, CS and F), ADR-4368-S, ADR-4400 and ADR-4468. ADR-4368 has a weight-average molar mass of 6,800 g/mol, an epoxy equivalent weight of 285 g/mol and about 9 epoxy groups per molecule, with a glass transition temperature of 54 °C and a density of 1.08 g/cm³ in a PLA/PA11 study in Materials.

Dosage in recycled PET runs from 0.5 to 1.5 wt% while gel content stays at or below 2 %, and the same oligomer acts as a reactive compatibilizer in PLA/PBAT and PLA/PA11 blends. The residual monomer glycidyl methacrylate is EU food-contact material number 220 with a specific migration limit of 0.02 mg/kg, carries the harmonised CLP classification Carc. 1B H350, Muta. 2 H341 and Repr. 1B H360F, and was added to the California Proposition 65 list for cancer on 27 January 2023. BASF has published no food-contact clearance for the ADR grades in the sources checked, and our source library could not confirm that any grade maps to EU entry FCM 857. The ADR grades are compared one by one on epoxy chain extenders.

2. Dianhydrides: PMDA#

Dianhydrides are aromatic tetrafunctional anhydrides, above all pyromellitic dianhydride (CAS 89-32-7), whose four anhydride positions open on hydroxyl end groups and build up to four-arm stars without releasing a volatile by-product. PMDA is benzene-1,2,4,5-tetracarboxylic dianhydride, EC 201-898-9, C10H2O6, 218.12 g/mol, melting at 286 to 287 °C with a density of 1.68 g/cm³. Cusano, Grizzuti and co-workers at Naples used 0.2, 0.25 and 0.3 wt% in PET in their 2023 extensional-rheology and strain-hardening work in Materials.

Two properties decide where PMDA is used. Its hydroxyl reactivity is low, which is why it is often combined with an epoxide rather than used alone, and it is hygroscopic, so hydrolysed PMDA promotes degradation instead of reversing it and dry handling is a condition of use rather than a precaution. Worker safety is the third consideration: PMDA carries the harmonised CLP entry with index number 607-098-00-X, classified Eye Dam. 1 H318, Resp. Sens. 1 H334 and Skin Sens. 1 H317, with occupational asthma reported, which makes dust control a design requirement wherever the powder is dosed.

3. Oxazolines: bisoxazoline (1,3-PBO)#

Oxazolines are bifunctional ring compounds, the commercial example being 2,2'-(1,3-phenylene)bis(2-oxazoline) (CAS 34052-90-9), whose rings open on carboxyl end groups to form ester-amide links and release nothing at all. The substance is EC 421-510-3, C12H12N2O2, 216.24 g/mol, and is supplied as a dry powder. Because it is bifunctional the coupling is linear, and because it consumes acid end groups the acid number of the polyester falls, which improves hydrolytic stability as a second effect.

1,3-PBO is used in PET, rPET, PBT and PLA and is often combined with anhydrides or diisocyanates when branching is wanted as well as coupling, with the alternative compound 2,2'-bis(2-oxazoline) carrying CAS 36697-72-0. Our source library holds no sourced dosage range for 1,3-PBO in PET or rPET, so no level is given here. It is a REACH-notified new substance with 3 active Article 10 full dossiers, it is not on the Candidate List according to the ECHA CHEM lookup of 22 September 2026, it is not listed in Annex I of Regulation (EU) No 10/2011 in the consolidation of 16 March 2025, and it has no harmonised CLP entry.

4. Carbodiimides: hydrolysis stabilizer and mild chain extender#

Carbodiimides are sterically hindered compounds whose N=C=N group reacts with the carboxyl ends that hydrolysis has just created, so they stabilize a polyester against further hydrolysis and extend its chains at the same time. The reaction forms an N-acylurea in place and removes the acid that catalyses the next scission, which is why this site files the class under hydrolysis stabilizers and treats chain extension as its second function.

The monomeric grade is bis(2,6-diisopropylphenyl)carbodiimide, CAS 2162-74-5, sold by LANXESS as Stabaxol I, and it is EU food-contact material number 438 as a monomer or starting substance only, with a specific migration limit of 0.05 mg/kg expressed as the sum of the carbodiimide and its hydrolysis product 2,6-diisopropylaniline. It is REACH registered with 3 active Article 10 dossiers, the first dated 14 February 2013; the hazard statements H302, H360(F) and H372 are notified by suppliers, with no harmonised CLP entry as of September 2026. Polymeric grades run as Stabaxol P, P LF, P 100, P 110 and P 200 from LANXESS and CARBODILITE HMV-15CA and HMV-5CA-LC from Nisshinbo, and Stabaxol P-100 has been used at 2.0 wt% with 0.75 wt% of an epoxy chain extender in a PLA packaging blend. The monomeric and polymeric grades are compared on carbodiimide hydrolysis stabilizers.

5. Diisocyanates and blocked isocyanates#

Diisocyanates are bifunctional isocyanates such as hexamethylene diisocyanate that react with both hydroxyl and carboxyl chain ends, which makes the coupling fast but also makes the residual monomer the limiting factor in food contact. Functionality is 2, so the architecture is linear, and blocked isocyanates exist to delay that reactivity until a defined temperature is reached in the barrel.

Regulation is the reason this chemistry stays marginal in food-contact polyester. Regulation (EU) No 10/2011 lists hexamethylene diisocyanate as food-contact material number 372, CAS 822-06-0, as a monomer, with a limit of 1 mg/kg in the final product expressed as isocyanate under group restriction 17. Our source library holds no sourced chain-extension dosage for diisocyanates in thermoplastic polyesters, and the dosing practice used for polyurethane synthesis does not transfer, because there the same molecule is a building block of the polymer rather than an additive to a finished one.

6. Other reactive types: phosphites, bis-caprolactams and aziridines#

The remaining chain extenders are niche or legacy chemistries: triphenyl phosphite, carbonyl biscaprolactam, aziridines such as TTMAP, and the trifunctional epoxide TGIC, which industry has largely abandoned since its listing as a substance of very high concern. They appear in the literature mostly in combination, for instance PMDA together with TGIC or with an aziridine to build long-chain branching in PLA, rather than as stand-alone products with published dose ranges.

Two of the group carry a clear status. TGIC is trifunctional glycidyl isocyanurate, it reacts with carboxyl end groups, and it has been on the SVHC Candidate List since 18 June 2012 as a mutagen under Article 57(b) of REACH, which is why the styrene-acrylic epoxy oligomers replaced it. TGDDM, a tetrafunctional epoxy branching agent, is cited in the literature without a commercial position in recycling. A bio-based route also sits here: Luo and co-workers reported in International Journal of Biological Macromolecules in 2024 that 1 phr of epoxidized cardanol oleate raised the molar mass of PLA from 15.3 to 17.1 × 10⁴ g/mol.

What Does a Chain Extender Change? IV, Melt Strength, MFR and Colour#

Chain extension changes five measurable properties: intrinsic viscosity and molar mass rise, melt flow rate falls, melt strength and strain hardening rise, elongation and toughness can improve, and the share of recyclate a converter can run goes up. The first two move in opposite directions by definition, which is why a laboratory confirms the reaction by measuring both.

Numbers exist for each direction. Karl and co-workers found larger inherent-viscosity and extrusion-force gains in recycled PET with Joncryl than with PMDA, and Matumba, Mohanty and Misra reported in Polymers in 2023 that toughness rose at 0.5 wt% Joncryl in PLA blends.

Toughness is where the family reaches its limit. A chain extender alone does not restore impact strength: in recycled PET with glass fibre, Monti and co-workers found in Polymers in 2021 that an ethylene copolymer or an ionomer was needed to lift notched Izod from 5.2 to about 8.1 kJ/m², a gain of 55 % measured to ISO 180 at 23 °C, and toughness in recyclate is therefore the job of the impact modifiers. Colour is not a documented effect of this family in our source library, so no yellowness-index claim is made here.

How Much Chain Extender Does a Polymer Need? Dosage in wt%#

Chain extenders work between about 0.1 and 1.5 wt% of the polymer, and the upper end is set by gel formation rather than by the molar mass the formulator wants. Every level below is the value published with its result, classified as a tested study value, a supplier statement, or an absence in our source library.

Levels are the values published with the cited result, not supplier recommendations. Percentages are by weight of the polymer unless the source states otherwise.

Read the PLA band as 0.1 to 1.0 wt% depending on the blend: 0.1 to 0.5 wt% in PLA/PCL and PLA/PEG, 0.5 wt% in a PLA/PBAT cast film and about 1.0 wt% at interfacial saturation in PLA/PA11.

Units follow the polymer, not the additive. Polyesters and polyamides are dosed in wt% of the polymer, 1 wt% equals 10,000 ppm, and phr is the parts-per-hundred-resin convention of PVC and rubber rather than of polyester, with the conversions set out on PHR (parts per hundred resin). Levels for every other additive family are on additive dosage levels in plastics.

Which Chain Extender for Which Polymer?#

Chain extenders are used in four polymer groups, all of them condensation polymers: PET and recycled PET, PLA and other bio-based polyesters, PBT, and the polyamides, with polycarbonate recyclate as a fifth case for the epoxy oligomers. Polyolefins and PVC are outside the family altogether, because chain extension needs reactive end groups and those polymers have none.

Chain extenders for PET and recycled PET#

Recycled PET is the largest use of chain extenders, because mechanical recycling lowers intrinsic viscosity below the level a bottle, a tray or a fibre needs, and an epoxy-functional oligomer rebuilds it in the extruder rather than in a solid-state reactor. Solid-state polycondensation remains the competing route and rebuilds molar mass without an additive, at the cost of a separate process step.

Two chemistries dominate the published comparisons. Karl and co-workers measured larger inherent-viscosity and extrusion-force gains with Joncryl ADR-4400 than with PMDA in the same rPET, at 0.5 to 1.5 wt% with gel content at or below 2 %, while the strain-hardening literature from Naples works with PMDA at 0.2 to 0.3 wt%. Flake quality, IV targets and line settings are on chain extenders for PET and rPET.

Feedstock quality changes the arithmetic. Karl and co-workers also found that the extender reacts first with the low-molar-mass contaminants and oligomers in the recyclate and only then with the PET chains, so a dirtier flake consumes part of the dose before any chain is rebuilt. A chain extender does not replace restabilization either; the antioxidant side of the formulation is covered on restabilization of recycled plastics, and the recyclate package as a whole is on additives for recycled plastics.

A PET package also carries reheat additives, toners, acetaldehyde scavengers and oxygen barriers, and the full picture sits on additives for PET resin.

Chain extenders for PLA and bio-based polyesters#

PLA is the second use of chain extenders, and the dose is narrow: a cast PLA/PBAT film was stabilized with 0.5 wt% Joncryl ADR while 0.9 wt% was already an excess, and in a PLA/PA11 80/20 blend the interfacial reaction saturated at about 1.0 wt%. Matumba, Mohanty and Misra add the third band, 0.1 to 0.5 wt% in PLA/PCL and PLA/PEG blends with toughness rising at 0.5 wt%.

Temperature discipline is tighter than in PET, because PLA has a glass transition temperature of 60 to 65 °C and melts between 130 and 180 °C, so the barrel has less headroom above the melting point in which to reach conversion. Altınbay, Nofar and co-workers reported that aged PBAT reacts hardly at all, which matters for compostable film blends built on recycled PBAT. PLA and PLA/PBAT film data are on chain extenders for PLA.

Crystallinity is the side effect to watch. Polycarbodiimide and aziridine suppress PLA crystallinity to below 5 %, which conflicts with a nucleating agent added for heat resistance in the same compound. The rest of the PLA package, which is chain extenders, plasticizers, nucleating agents and hydrolysis stabilizers, is on additives for PLA.

Chain extenders for PBT, polyamides and PC recyclate#

PBT, the polyamides and polycarbonate recyclate use the same epoxy-functional oligomers as PET, with polyamide differing in one respect: its amine end groups react as well as its carboxyl ends, so conversion is fast. The PA 10.10 work in Polymers in 2023 measured 99 % conversion at 200 °C in 120 seconds or at 280 °C in 30 seconds, with BASF recommending about 260 °C and at least 120 seconds of dwell, across levels of 0.25 to 1.25 wt%.

Polyamide also has dedicated products. Brüggemann supplies BRUGGOLEN M1251 and M1253 for linear chain extension in polyamides and BRUGGOLEN M1417 as a chain breaker for upcycling high-viscosity waste, and no dosage for those grades is in our source library, so only the function is stated here. Joncryl ADR-4368-S has been used as a chain extender in recycled PA6/PP with carbon fibre, and the polyamide package, including the copper heat stabilizers, is on additives for nylon (polyamide).

Carbodiimides reach the same polymers from the hydrolysis-stabilizer side: linear coupling by carbodiimide has been described for recycled polyamide textiles in ACS Omega in 2026, which matters because textile waste arrives with a low and variable molar mass. PBT and polycarbonate recyclate follow the PET logic, with no sourced level for either in our source library.

How Do You Select a Chain Extender? 6 Criteria#

Selecting a chain extender starts with the end group, not with the supplier: identify whether the polymer carries carboxyl, hydroxyl or amine ends, then match the functionality to the rheology the process needs, then set the level below the gel threshold. The 6 steps below run in that order, and each one eliminates candidates before the next applies.

1. Identify the end group. Carboxyl ends point to an epoxide, an oxazoline or a carbodiimide, hydroxyl ends to an anhydride or an isocyanate, amine ends in polyamide to an epoxide.
2. Match the functionality to the target rheology. Functionality 2 raises molar mass and viscosity, functionality 4 or more adds the long-chain branching that foaming, thermoforming and blow molding need.
3. Set the level below the gel threshold. Take the published gel data as the ceiling, not the molar-mass target, and treat 3 wt% Joncryl in rPET as the level at which gelation was measured.
4. Check the reaction window against the machine. Compare the 99 % conversion conditions and the recommended 260 °C with at least 120 s dwell against the real barrel profile and throughput.
5. Check the food-contact route. Confirm the EU status of the substance and of its residual monomer before the recyclate reaches packaging, because a process authorisation does not authorise the additive.
6. Check the interaction with the rest of the package. Test against nucleating agents, impact modifiers, restabilization and the contaminant load of the feedstock.

Recyclability belongs in the same decision, because a chain extender stays in the polymer and enters the next recycling loop with it, and how additive choices affect sorting and reprocessing is on design for recycling. The general framework for any additive decision is on how to select plastic additives.

Where to add it: dry blend, side feeder or masterbatch#

Where the chain extender enters the extruder decides how much of it reacts: a powder fed into the throat sees the full barrel and the full dwell time, while a side-fed addition downstream may fall short of the 120 seconds that 99 percent conversion needed at 200 °C. Feed position is therefore a process parameter with the same weight as temperature.

Three routes are in industrial use. A dry blend of flake and powder is the simplest and the least reproducible, because a low-density powder segregates from dense flake in the hopper. Gravimetric feeding into the throat or through a side feeder gives a controlled ratio, and feed ports, side feeders and screw design are covered on twin-screw compounding. Handling constraints belong to the chemistry: PMDA needs dry handling, and a published laboratory protocol dried rPET flakes at 140 °C for 17 h and the Joncryl at 120 °C for 3 h before compounding.

Most chain extenders reach the machine as an additive masterbatch rather than as a neat powder. Avient supplies the CESA-Extend line for that route and Sukano supplies equivalent carriers, which removes the dust exposure of a reactive powder and moves the metering problem to a pellet feeder the line already has.

Which additives and contaminants conflict with a chain extender?#

A chain extender competes with everything else in the melt that carries a reactive group: Karl and co-workers found that Joncryl reacted first with the low-molar-mass contaminants in recycled PET and only then with the PET chains, so a dirtier flake consumes part of the dose before any chain is rebuilt. Four antagonisms recur.

• Contaminant load, including oligomers and low-molar-mass species in post-consumer flake, which consumes dose without rebuilding molar mass.
• Moisture and PVC fragments in rPET, which raise the degradation rate during the same pass, and hydrolysed PMDA, which promotes degradation instead of reversing it.
• Long residence time in the barrel, which converts a nominally safe level into gel particles.
• Nucleating agents in PLA, whose effect is undercut where polycarbodiimide and aziridine suppress crystallinity to below 5 %.

Overdosing carries a compliance cost as well as a rheology cost, because the residual glycidyl methacrylate of the epoxy oligomers is limited to 0.02 mg/kg in food contact and carries a harmonised CMR classification. The degradation routes a chain extender reverses are set out on polymer degradation, and gels, black specks and viscosity drift are collected on troubleshooting additive-related defects.

How Is Chain Extension Tested?#

Chain extension is verified with four measurements: intrinsic viscosity, melt flow rate, melt strength and gel content, and the first two move in opposite directions when the reaction has worked. A rising melt volume-flow rate in an undried PET or polyamide sample means hydrolysis rather than extension, which is why drying before the test is part of the method rather than a preparation detail.

Interpretation needs the application target rather than an absolute value, because the intrinsic-viscosity ranges quoted earlier are indicative grade bands and not requirements. All methods are indexed under testing plastic additives.

How Are Chain Extenders Regulated?#

Chain extenders are regulated on three routes at once: as substances under REACH and CLP, as food-contact substances under Regulation (EU) No 10/2011, and as process inputs to recycling under Regulation (EU) 2022/1616. Which route bites hardest depends on the molecule size, because the oligomeric extenders and the small molecules are treated differently at the first step.

Polymers are exempt from registration under Article 2(9) of REACH while their monomers are not, which is why the styrene-acrylic oligomers are not registered as such and their residual glycidyl methacrylate is, and the polymer exemption is explained on REACH and plastic additives. Small molecules such as PMDA, 1,3-PBO and the monomeric carbodiimide carry their own dossiers, their own classifications and their own food-contact entries.

Food contact and recyclate: EU 10/2011, Regulation (EU) 2022/1616 and the PPWR targets#

No chain extender may be added to food-contact recyclate unless the substance itself is authorised under Regulation (EU) No 10/2011, which is the rule Regulation (EU) 2022/1616 applies to every recycling process it recognises. Regulation (EU) 2022/1616 was adopted on 15 September 2022, published in the Official Journal on 20 September 2022 and entered into force on 10 October 2022, replacing Regulation (EC) No 282/2008; among its suitable technologies is post-consumer mechanical PET recycling with a maximum of 5 % non-food input, whose output may not be used in microwave or oven applications. The Union list and the additive-versus-monomer distinction are explained on EU 10/2011, and the process-authorisation rules are on recycled plastics regulations.

Every value in the table below is a specific migration limits (SML) entry from Annex I of Regulation (EU) No 10/2011, checked against the consolidation of 16 March 2025, with ECHA CHEM checks dated 22 September 2026.

The US position is an absence rather than a clearance. No 21 CFR listing and no Food Contact Notification was found for the epoxy-functional oligomers, for PMDA as an additive or for 1,3-PBO in a plastics context in the sources checked in September 2026, and how 21 CFR parts 174 to 178 fit together is on FDA food contact rules.

Demand comes from the recycled-content law rather than from the additive law. The PPWR, Regulation (EU) 2025/40, was published in the Official Journal on 22 January 2025 and applies from 12 August 2026; from 1 January 2030 it requires minimum post-consumer recycled content of 30 % in contact-sensitive PET packaging, 10 % in other contact-sensitive plastic packaging, 30 % in single-use plastic beverage bottles and 35 % in other plastic packaging, rising in 2040 to 50 %, 25 %, 65 % and 65 %, as listed on the EU Packaging and Packaging Waste Regulation (PPWR) page. The packaging view is on additives for food packaging.

REACH, CLP and worker safety: the TGIC case#

Only one chain extender has ever been listed as a substance of very high concern: TGIC, the trifunctional glycidyl isocyanurate, entered the REACH Candidate List on 18 June 2012 as a mutagen under Article 57(b) and has since been replaced by the styrene-acrylic epoxy oligomers. Neither PMDA, 1,3-PBO nor the monomeric carbodiimide is on the Candidate List according to the ECHA CHEM lookups of 22 September 2026.

Registration status differs by molecule size rather than by function. PMDA holds 14 active Article 10 dossiers with a first registration of 22 January 2018, 1,3-PBO is a notified new substance with 3 active Article 10 full dossiers, and bis(2,6-diisopropylphenyl)carbodiimide holds 3 dossiers with a first registration of 14 February 2013.

Worker exposure is the sharpest issue in this family. PMDA carries the harmonised classification Resp. Sens. 1 H334 with Skin Sens. 1 H317 and Eye Dam. 1 H318 under Annex VI index 607-098-00-X, with occupational asthma reported, which makes enclosed dosing and dust extraction a design requirement, and how an Annex VI entry is assigned is on CLP classification of plastic additives. Glycidyl methacrylate, the residual monomer of the epoxy oligomers, was added to California Proposition 65 on 27 January 2023, while the hazard statements of the monomeric carbodiimide are supplier notifications without a harmonised entry.

Who Makes Chain Extenders? Suppliers and Market#

Four companies supply most of the chain extenders used in plastics: BASF with the Joncryl ADR grades, LANXESS with the Stabaxol carbodiimides, Brüggemann with the BRUGGOLEN polyamide grades, and Avient with the CESA-Extend masterbatch line it acquired from Clariant in 2020. Sukano, Nisshinbo and the Chinese styrene-acrylic producers complete the table.

Corporate history explains two of those rows. Avient was formed as PolyOne on 31 August 2000 and acquired the Clariant masterbatch business in 2020 for about USD 1.6 bn, which is how the CESA line changed owner, and BASF completed its acquisition of Ciba on 9 April 2009. No producer is named here for PMDA, because the candidate names in the source material are unverified. Where these producers sit in the wider additive industry is on largest plastic additive companies.

Market data for this family does not exist in our source library. No market size, growth rate or price for chain extenders has been verified against a primary source, so none is published here. Reactive additives are bought from the same houses listed under compatibilizer and coupling agent suppliers.

Complete List of Chain Extenders and Related Reactive Additives: 5 Substances#

Five substances in the plastic additives database are used to extend polymer chains: the Joncryl ADR oligomers, pyromellitic dianhydride, bisoxazoline, and the monomeric and polymeric carbodiimides that also serve as hydrolysis stabilizers. Two of the five are filed under hydrolysis stabilizers, because that is their primary function and chain extension is the second.

Every other additive family is in the plastic additives database, where each of these five substances has its own record with identity, dosage and regulatory status.

How Do Chain Extenders Differ from Polyurethane Chain Extenders, Crosslinkers and Compatibilizers?#

The word "chain extender" covers three different product classes, and only one of them is a plastic additive: the reactive additives on this page, the diols and diamines that build the hard segment of a polyurethane, and the metal links sold to lengthen a necklace. The table separates the neighbouring additive families from each other by what each one changes and where it acts.

Polyurethane chain extenders: BDO, HQEE and diamines are raw materials, not additives#

Polyurethane chain extenders are short diols and diamines such as 1,4-butanediol (CAS 110-63-4), HQEE (CAS 104-38-1) and DETDA (CAS 68479-98-1) that build the hard segment of the polymer as it is synthesised, so they are raw materials and not additives added to a finished polymer. In polyurethane terminology a chain extender has a functionality of 2 and a crosslinker a functionality of 3 or more, which is the same functionality logic used on this page for a different purpose.

This site classes monomers and curatives as out of scope, so no dosage, supplier or selection advice for the polyurethane class appears here. One regulatory data point marks the distance between the two fields: MOCA, a polyurethane curative, is on the REACH Authorisation List with a sunset date of 22 November 2017, which is an authorisation question for polymer synthesis rather than an additive question for a thermoplastic.

"Chain extender" outside plastics: jewellery and chain hardware#

Outside plastics, a chain extender is a short length of metal chain that lengthens a necklace, a handbag strap or a ceiling-fan pull, which is why this page always writes "chain extenders for polymers". In US search the bare term resolves to those products, as the Google Autocomplete and SERP checks of 22 September 2026 show, and the plastics sense needs "for polymers", "for plastics" or "for PET" to separate it.

Frequently asked questions about chain extenders#

The 4 questions below are the ones compounders and recyclers ask most often about chain extenders: what the additive does, how the reaction works, whether Joncryl ADR is a chain extender or a compatibilizer, and whether more molar mass always means a better part.

What does a chain extender do?#

A chain extender rejoins polymer chains that processing has broken, which raises molar mass, intrinsic viscosity, melt viscosity and melt strength and lets a converter run a higher share of recyclate. The chains break during melt processing through hydrolysis, thermal and thermo-oxidative scission, and the additive reverses the rheological consequence of that damage in one extrusion pass. Repairing intrinsic viscosity in recycled PET is the commercial core of the family.

How do chain extenders work?#

Chain extenders work by reaction, not by blending: their epoxide, anhydride, oxazoline, carbodiimide or isocyanate groups open on the carboxyl, hydroxyl or amine ends of the polymer chain and form a covalent link between two or more chains. An epoxide reacts with carboxyl ends fast and with hydroxyl ends more slowly, an anhydride ring opens on a hydroxyl end and leaves a new carboxyl group, and an oxazoline ring forms an ester-amide link with no by-product at all. Functionality above 2 turns the link into a branch.

Is Joncryl ADR a chain extender or a compatibilizer?#

Both: the same epoxy groups that rejoin polyester chains also react across the interface of two immiscible polymers, and in a PLA/PA11 80/20 blend that interfacial reaction saturates at about 1.0 wt%. About 9 epoxy groups per molecule in ADR-4368 make the two functions inseparable, since a molecule that can tie nine chain ends together does not distinguish between two ends of the same polymer and one end of each of two polymers. Which function dominates is decided by the blend, not by the grade.

Does a higher molar mass always give a better part?#

No: above the gel threshold the melt carries crosslinked particles instead of longer chains, and even below it a higher molar mass restores rheology rather than toughness, which in recycled PET still needs an impact modifier. Karl and co-workers measured about 32 % gel at 3 wt% in rPET, and Monti and co-workers found that notched Izod in glass-filled recycled PET rose from 5.2 to about 8.1 kJ/m² only when an ethylene copolymer or ionomer was added. The chain scission the extender reverses is defined in the glossary, and the falling melt flow rate can move the material outside the machine's window.