Compatibilizers for Polymer Blends: 10 Blend Pairs, Dosage and Selection

Compatibilizers for polymer blends are block, graft or reactive copolymers that locate at the interface between two immiscible polymers, where they lower the interfacial tension, stop the dispersed droplets from merging again and carry stress from one phase to the other. They are dosed at roughly 2 to 10 wt% of the blend, and in an isotactic-polypropylene-rich blend 5 wt% of a propylene-based olefin block copolymer was enough to shrink the minor-phase domains measurably; so which chemistry belongs in which pair?

The answer follows the chain ends of the two polymers, a polyamide asking for an anhydride and a polyester for an epoxide, which is why compatibilizers sit in the property-modifier group of plastic additives next to impact modifiers and coupling agents rather than among the stabilizers. Compatibiliser is the UK spelling.

This page covers why two polymers almost never mix, what a compatibilizer changes at the interface, which chemistry suits each of 10 named blend pairs from PP/PE to mixed recyclate, how much a blend needs, how it is reacted in a twin-screw extruder, how the result is measured, and how the polymer escapes REACH registration while its monomers carry the limits.

Key figures

• 2-10 wt% of the blend: the typical compatibilizer dose
• 10 named blend pairs covered on this page, from PP/PE to mixed recyclate
• 5 wt% propylene-based olefin block copolymer: enough to reduce minor-phase domain size in an iPP-rich blend (peer-reviewed, 2025)
• 30 mg/kg SML(T) as maleic acid (FCM 234) and 0.02 mg/kg SML for glycidyl methacrylate (FCM 220): the two monomer limits that decide food-contact use

Why Do Polymer Blends Need a Compatibilizer?#

Polymer blends need a compatibilizer because two different polymers almost never mix: joining long chains gains so little entropy that the free energy of mixing stays positive, and the blend separates into a matrix with a coarse dispersed phase held together by a weak interface. That separation is the thermodynamic ground state of the pair, not a mixing fault better equipment can cure, so the only thing left to engineer is the boundary.

A compatibilizer is the polymeric additive that repairs that interface, and the full class, its 5 chemical types and its complete substance list sit on the hub for compatibilizers. If mixing two polymers almost never works, what exactly makes the pair immiscible in the first place?

Why are most polymer pairs immiscible?#

Most polymer pairs are immiscible because mixing gains entropy in proportion to the number of independent molecules, and a long chain counts as one molecule however many repeat units it carries, so the entropy term almost vanishes and any positive enthalpy of mixing keeps the free energy of mixing above zero. Two low-molar-mass liquids of similar chemistry mix because the combinatorial entropy of mixing overcomes a small unfavourable enthalpy; tie the same molecules into chains of 5,000 repeat units and the entropy term collapses, leaving the enthalpy in charge. Only a specific favourable interaction, hydrogen bonding or a strong dipole match, pushes it negative and gives a single phase.

The formal criterion for miscibility is the Flory-Huggins interaction parameter, with the difference in solubility parameter between the two polymers as its practical proxy. Both are comparative tools rather than pass marks: no validated threshold value of that parameter for polymer-polymer pairs is established in this reference, so no cut-off is printed. The screening question is therefore how coarse a pair's morphology becomes without help.

What happens in a polymer blend without a compatibilizer?#

A polymer blend without a compatibilizer shows 5 symptoms, and all 5 come from the same interface: coarse domains, coalescence, a weak phase boundary, low impact strength and elongation, and delamination in the finished part. The 5 symptoms are listed below in process order.

• Coarse dispersed domains, because high interfacial tension resists droplet break-up in melt mixing
• Coalescence of those domains as soon as shear stops, so the morphology drifts downstream
• A weak interface across which stress does not transfer to the dispersed phase
• Low notched impact strength and low elongation at break, the properties that collapse first
• Delamination at the surface of the moulded part, where the phases simply separate

No numeric domain-size ceiling for an uncompatibilized blend is established in this reference, and none is quoted: the useful measurement is the difference between the compatibilized and uncompatibilized versions of one formulation.

What is the difference between miscibility and compatibility?#

Miscibility means the two polymers form a single phase, while compatibility means they stay in two phases but adhere well enough at the boundary to behave as one material. A compatibilizer therefore does not make a blend miscible: it makes a two-phase blend behave as if it were, by repairing the interface rather than removing it. Every property gain on this page is a two-phase gain.

How Does a Compatibilizer Change the Morphology of a Polymer Blend?#

A compatibilizer changes 3 things at the boundary between the two polymers: it lowers the interfacial tension so the minor phase breaks into finer droplets, it suppresses coalescence so those droplets stay apart, and it raises interfacial adhesion so stress passes from one phase to the other. All three effects need the molecule in the right place, and only a block or graft copolymer whose segments are miscible with each phase ends up at the interface instead of dissolved in one of them.

Lower interfacial tension and a finer dispersed phase#

Interfacial tension falls because the compatibilizer lays a copolymer across the boundary with one segment dissolved in each phase, and the lower the tension, the smaller the droplet that the shear field in the extruder can still break. Droplet break-up is a competition between the viscous stress the screw applies and the interfacial tension pulling a droplet back into a sphere, so lowering the tension lets the same extruder, at the same screw speed and melt temperature, produce a finer dispersion.

In an isotactic-polypropylene-rich blend with LDPE or HDPE, a 2025 peer-reviewed study found that 5 wt% of a propylene-based olefin block copolymer of the INTUNE type reduced the size of the minor-phase domains. No before-and-after domain size in microns is established in this reference for that system, so the finding is a reduction rather than a target dimension.

Suppressed coalescence and a morphology that survives processing#

Coalescence stops because the copolymer layer on each droplet keeps the next droplet at a distance, a steric stabilisation that holds the dispersion together once the shear that created it is gone. Two uncoated droplets that touch in a quiescent melt merge, because merging removes interfacial area and lowers the free energy of the system; a copolymer brush on each surface has to be compressed first, and that compression costs energy.

The downstream consequence is why the effect matters commercially. A fine morphology produced in the compounder is worth nothing if it coarsens again in the barrel and hot runner of a moulding machine, where the melt sits with little shear for seconds at a time. Suppressed coalescence is what makes the morphology of the part resemble that of the pellet, and the instability of an uncompatibilized blend is why its properties scatter between moulding conditions. This reference records no coarsening rate for any pair.

Interfacial adhesion and stress transfer across the phase boundary#

Interfacial adhesion rises because the compatibilizer's segments entangle with, or react into, the chains of both phases, so a load applied to the matrix is carried across the boundary instead of opening it. Entanglement is the physical version of that anchoring and a covalent graft the chemical version, and both make the dispersed particle deform with the matrix rather than debond from it.

Plastics Technology reports that 2 to 5 % of a polyolefin elastomer, typically 3 %, raised the impact strength and elongation of a 70/30 HDPE/PP recyclate about threefold while the modulus was kept. That trade-press figure shows which properties respond to adhesion: stiffness is a bulk property of the two phases and barely moves, while impact strength and elongation at break depend on whether a crack can run along the interface. Impact strength responds first for the same reason rubber toughening works at all, through crazing, rubber-particle cavitation and matrix shear yielding, the three mechanisms set out in Bucknall's 2007 review.

Reactive and non-reactive compatibilization of polymer blends#

Non-reactive compatibilization adds a copolymer that is already finished and only has to find the interface, while reactive compatibilization adds a functional polymer that builds the graft copolymer in the extruder by reacting with the chain ends of one phase. Reactive compatibilization is the industrial standard wherever one phase carries reactive chain ends, which covers PA/PP, PA/ABS, PC/PBT and the polyester-polyolefin blends. The table compares the two routes on the 7 criteria that decide between them.

Grades named in this table are examples from our source library, not recommendations. All 5 chemical types are compared on the compatibilizer hub.

Which Compatibilizer Suits Which Polymer Blend? 10 Blend Pairs#

The compatibilizer follows the chain ends of the two polymers: a polyamide brings amine end groups and asks for an anhydride, a polyester brings carboxyl and hydroxyl end groups and asks for an epoxide, and two polyolefins bring nothing reactive at all, so they need a block copolymer that is simply miscible with both. This page covers 10 blend pairs, in the order of the table below, and that order repeats in every list, image and schema block.

Published dosages exist for some of those pairs and not for others: every number below is the value published with the cited result, not a recommendation, and the three pairs for which our source library holds no loading say so in their own cell. The table pairs each of the 10 blends with the chemistry that matches its chain ends, the dosage as published and the type of source behind it.

Dosages are the values published with the cited result, not recommendations. Percentages are by weight of the blend unless the source states otherwise. Where our source library holds no dosage, the cell says so.

Request quotes for a blend compatibilizer: name the two polymers, the ratio, the target property and the volume, through the plastic additive supplier finder. Producers and brand lines are listed in the directory of compatibilizer and coupling agent suppliers. Formulators comparing chemistries first can download the Compatibilizer Selection Chart: Blend Pair, Type and Dosage (PDF), with email, role and company.

1. PP/PE and other polyolefin blends#

PP/PE blends need a non-reactive block copolymer because neither polypropylene nor polyethylene carries a reactive chain end, so there is nothing for an anhydride or an epoxide to attack and the compatibilizer has to work by segment miscibility alone. Polyolefin pairs are the highest-volume case in this family, because post-consumer streams arrive as PP and PE together.

Three sourced options cover the pair. An ethylene-octene polyolefin elastomer (POE) is the best-documented: Plastics Technology reports it at 2 to 5 %, typically 3 %, in a 70/30 HDPE/PP recyclate, where impact strength and elongation rose about threefold with the modulus kept, on an ENGAGE 8100 type grade of 0.870 g/cm3. A propylene-based elastomer of the Vistamaxx type carries a supplier starting point of 5 % in PE/PP recyclate, and a propylene-based olefin block copolymer at 5 wt% reduced minor-phase domain size in the 2025 study cited above. The rest of the package travelling with the PP phase is on additives for polypropylene.

EPDM (CAS 25038-36-2) is the fourth option, an effective PP/PE compatibilizer in its own right, with the maleated version EPDM-g-MAH used for HDPE/PP and LDPE/PP. The hub owns the general question of a compatibilizer for PP and PE; the guide for the other phase is additives for polyethylene.

2. PP/PS and PE/PS blends#

PP/PS and PE/PS blends use a styrenic block copolymer such as SEBS (CAS 66070-58-4), whose polystyrene end blocks dissolve in the polystyrene phase while its saturated ethylene-butylene midblock dissolves in the polyolefin, so the molecule is pinned across the interface by its own architecture. That architecture is why the block copolymer locates at polyolefin/polystyrene boundaries and lowers interfacial tension without reacting, while the saturated midblock adds a rubbery phase stable to UV and ozone. The maleic anhydride grafted version of the same backbone is reactive and belongs to polyamide and polyester blends instead.

No loading for a virgin PP/PS or PE/PS blend is established in our source library, and none is invented here. The only sourced levels for this chemistry are the recyclate grades at 3 to 10 %, supplier data for post-consumer streams rather than a designed styrenic blend, so a virgin PP/PS formulation starts from a trial. The styrenic side is covered under additives for polystyrene.

3. PA/PP and PA/PE blends#

PA/PP and PA/PE blends use a maleic anhydride grafted polyolefin, because the anhydride reacts with the amine end groups of the polyamide to form imide links while the polyolefin backbone stays entangled in the polyolefin phase. Reactive compatibilization is the industrial standard for this pair, and PP-g-MAH (CAS 25722-45-6) is the highest-volume product that delivers it, with POLYBOND 3200 specified at 0.8 to 1.2 % grafted maleic anhydride.

The grafting chemistry, the anhydride content of commercial grades and the storage behaviour of maleic anhydride grafted polymers are on their own node, so this section states only what the blend needs from them.

Plastics Technology reports that 5 % of a maleated polypropylene raised the notched and reverse Izod impact strength of a recycled PP and polyamide blend about threefold. The grades behind that result, Polybond 3150 and 3002, are the reference data point for the pair, and the trade-press origin is part of it: a reported compound result, not a specification.

Where the polyamide is the matrix rather than the dispersed phase, the grafted backbone becomes an elastomer, with maleic anhydride grafted EPR, EPDM, POE and SEBS serving that role. The full package for the polyamide phase is on additives for nylon.

4. PA/ABS blends and toughened polyamide#

PA/ABS blends use maleic anhydride grafted elastomers, and reactive compatibilization is the industrial standard for them: the anhydride anchors in the polyamide through the same imide reaction while the elastomer backbone bridges to the styrene-acrylonitrile phase. Without that bridge the phases separate on cooling and a notched bar breaks at the boundary.

A 2024 study of a PA6/PP/ABS/SEBS blend found that 10 wt% of a maleic-anhydride-and-styrene grafted polyolefin elastomer together with 10 wt% of a grafted SEBS raised impact strength by 823 % against the uncompatibilized blend. That is one result on one four-component blend at a combined 20 wt% of grafted elastomer, and it is not a property of PA/ABS blends as a class. The styrene-acrylonitrile side is covered under additives for ABS, SAN and ASA.

Ionomer resins are the second route for a polyamide matrix, working through ionic clusters rather than a covalent graft: ethylene-methacrylic acid chemistry (CAS 25608-26-8 sodium salt, 28516-43-0 zinc salt) gives melt strength and the polarity that lets acid and salt groups reach polyamide end groups.

5. PET/PE and PET/PP blends#

PET/PE and PET/PP blends use an epoxy-functional ethylene terpolymer, because the glycidyl methacrylate epoxide opens against the carboxyl and hydroxyl chain ends of the polyester and ties the elastomer to the PET chain. The epoxide is right here for the same reason the anhydride is right for a polyamide: it matches the chain end the polymer presents.

The epoxide chemistry behind these grades is on the node for glycidyl methacrylate (GMA) functional compatibilizers. Two compositions are recorded: LOTADER AX8900 (E-MA-GMA, CAS 51541-08-3) carries 24 wt% methyl acrylate and 8 wt% GMA, while ELVALOY PTW (E-nBA-GMA) is built from 66.75 wt% ethylene, 28 wt% n-butyl acrylate and 5.25 wt% GMA.

Two sourced results frame the pair. A study of PET/HDPE found that 10 wt% ELVALOY PTW increased the strain at break, and the same grade toughens CPET trays. Monti and colleagues, writing in Polymers in 2021, measured notched Izod rising from 5.2 to about 8.1 kJ/m2, a gain of 55 %, to ISO 180 at 23 °C, in a glass-fibre-filled recycled PET compound modified with ethylene copolymers or an ethylene-methacrylic acid ionomer, a composite rather than a plain PET/PE blend. Both grades belong to one chemistry, the ethylene acrylate terpolymer.

For post-consumer polyolefin and PET streams the non-reactive route is used instead, with SEBS grades at 3 to 5 % in supplier data. The rest of the PET formulation is on additives for PET resin; the recyclate case is blend pair 10.

6. PBT, PC/PBT and PC/PET blends#

PBT, PC/PBT and PC/PET blends use the same epoxy-functional ethylene terpolymer as the PET blends, and reactive compatibilization is the industrial standard for them, because both phases present ester chemistry that an epoxide can reach. LOTADER AX8900 appears on the supplier application list for PBT, PET, PPS, PC/PBT and PC/PET, which is the commercial record of exactly that match.

No published loading for these blends is established in our source library, and that gap is stated rather than filled: the page names the chemistry and the grade, and sends the formulator to a trial series and the current supplier datasheet for a starting level. Transesterification between two polyesters is real in PC/PBT and PC/PET compounding, and it is named as a phenomenon only, because no rate, catalyst or stabiliser for it is recorded here.

The same grade carries two labels, impact modifier for PPS, PBT and PET and compatibilizer for polyester-polyolefin blends, so one datasheet covers both jobs. The complete PBT package is on additives for PBT.

7. PC/ABS blends#

PC/ABS blends appear on the supplier application list for epoxy-functional ethylene terpolymers, while the toughening route recorded for polycarbonate and PC/ABS runs through core-shell modifiers based on methyl methacrylate-butadiene-styrene or acrylic chemistry. No dosage for PC/ABS is established in our source library for either route.

This pair sits on the border between compatibilization and impact modification. PC and ABS are already sold as a commercial alloy, so the additive question in practice is toughening, or repairing a recyclate stream, rather than first-time compatibilization. A 2025 study on PC/ABS recycling compatibilization is referenced in our dossier without extractable data.

Grade-level data for silicone-acrylic core-shell modifiers is unverified here, so the chemistry class is named and no grade is. The toughening and stabilization package for this alloy is on additives for polycarbonate and PC/ABS, and the core-shell question belongs to the impact-modifier family below the contextual border.

8. PLA/PBAT and PLA/PA11 blends#

PLA/PBAT and PLA/PA11 blends use a multifunctional epoxide oligomer at well under 1 wt%, and the reason for the low level is saturation: once every reachable chain end at the interface has reacted, more oligomer stops helping and starts building gels. Biopolymer blends are the one case in this family dosed in fractions of a per cent.

Three peer-reviewed levels are recorded, each in its own system. In a PLA/PA11 80/20 blend the interfacial reaction saturates at about 1.0 wt%, with 0.5 to 3 wt% tested. In a PLA/PBAT cast film, 0.5 wt% gave stable extrusion while 0.9 wt% was an excess. Across PLA blends generally, 0.1 to 0.5 wt% has been studied, with toughness rising at 0.5 wt%.

The reason is functionality. Joncryl ADR 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 in the ADR-4368 grade, so one oligomer couples several chains at once, which is why it is also sold as a chain extender. No food-contact or REACH statement about it appears here, because its polymer and notification status are unverified.

Nucleation, plasticization and chain extension of the PLA phase are on additives for PLA; the chain-extension sense of the molecule is treated below the contextual border.

9. PE/EVOH and PE/PA barrier structures#

A PE/EVOH or PE/PA barrier structure uses a maleic anhydride grafted polyethylene as its tie layer, and the same anhydride that bonds the layers in the film is what compatibilizes them when the film is reground. The anhydride reacts with the hydroxyl groups of EVOH and the amine ends of polyamide, which is the only reason a polyolefin adheres to either.

The Association of Plastic Recyclers rates a co-extruded PE film with up to 10 wt% EVOH as Preferred when the tie layer carries more than 0.1 wt% maleic anhydride, and limits adhesive-laminated structures to 5 wt% EVOH. Those are design-guide thresholds for a structure, not dosages: 0.1 wt% is a minimum anhydride content in the tie resin and 10 wt% a ceiling on the barrier layer. The tie-resin families used here, Fusabond E and N, are anhydride-modified polyethylenes.

The full recycling case for compatibilizers for multilayer film recycling sits on its own node.

10. Mixed recyclate blends: PP/PE and polyolefin/PET from post-consumer streams#

Mixed recyclate is a polymer blend by accident, because post-consumer polypropylene and polyethylene cannot be separated economically and a polyolefin bale almost always carries some PET, so the recycler compounds a blend whether or not the blend was designed. Every chemistry above appears here, chosen by whichever chain ends the contamination brings.

Supplier data covers three streams with the CirKular+ styrenic grades. In a PCR PP/PET blend at 72/18, C1000 at 3 to 5 % (10 % in the cited example) raised notched Izod by 50 % and yield strength by 30 %. In PCR HDPE/PET, C1010 at 3 to 5 % (5 % in the example) raised notched Izod by 70 % and yield by 40 %. In PCR PP, C2000 is used at 3 to 10 % (9 % in the example). A separate catalytic route, a neoalkoxy titanate with a catalyst at 1.5 to 1.75 % in mixed post-industrial resin, carries a supplier claim of 20 % faster processing at a 9 % lower temperature.

Contamination levels, line practice and the recycled-content rules are on compatibilizers for recycled plastics. The whole recyclate package is on additives for recycled plastics.

How Much Compatibilizer Does a Polymer Blend Need?#

A polymer blend needs about 2 to 10 wt% of a compatibilizer, and the number is set by how much interface has to be covered, which means by how much minor phase there is and how fine the dispersion has to be. Sourced blend-pair values run from 0.5 wt% in a PLA/PBAT cast film to a combined 20 wt% in one four-component polyamide study, so the band describes the centre of the distribution, not its edges.

Typical additive dosage levels in plastics are compared family by family, which puts this band next to the parts-per-million levels of a stabilizer. Four factors drive the level inside it.

• The volume fraction of the minor phase, which sets the interfacial area to be covered
• The target domain size, since a finer dispersion means more interface per unit volume
• The chemistry of the route, because a reactive compatibilizer saturates once the reachable chain ends have reacted
• The property being bought, since impact and elongation respond at lower levels than modulus or barrier behaviour

Both ends carry a counter-example. In a PLA/PA11 80/20 blend the interfacial reaction saturated at about 1.0 wt%, so oligomer beyond that point contributed nothing; the 2024 PA6/PP/ABS/SEBS study used two grafted elastomers at 10 wt% plus 10 wt%. Neither is a recommendation, and the starting point should be the sourced value for the nearest pair above, confirmed in a trial.

A 20 wt% masterbatch let down at 25 kg per tonne gives 0.5 wt% of active compatibilizer in the blend (0.20 x 2.5 % = 0.5 %). In that example the active level, not the pellet addition rate, is what the interface responds to.

Percentages on this page are by weight of the whole blend. The PHR (parts per hundred resin) convention used for PVC is not used for blends, which have no single base resin to count against, and a supplier percentage quoted without a base is reproduced as given.

How Is a Compatibilizer Added to a Polymer Blend?#

A compatibilizer is added in the melt, in the same step that makes the blend, because the copolymer has to reach an interface that only exists once both polymers are molten and sheared together. That step is ordinary melt plastic compounding, with one difference: on the reactive route the extruder is also a reactor, because the graft copolymer is built in the barrel rather than poured in from a bag. The geometry is the standard compounding geometry in this reference, a co-rotating twin-screw extruder with a vented two-stage screw at a length-to-diameter ratio of about 36:1.

Reactive extrusion: screw design, residence time and melt temperature#

Reactive extrusion needs melt temperature, shear and residence time together, because the interfacial reaction competes with chain scission and only the fraction of the compatibilizer that reaches an interface in time does any work. The window is set by the resin pair and the grade: no residence time, melt temperature or screw speed for blend compatibilization is established in our source library.

The flow consequence comes from the grafting chemistry. In a peroxide-grafted polyolefin, radicals abstract hydrogen from the chain and the anhydride adds to the resulting macroradical, while beta-scission of the backbone competes with grafting and intensifies as the peroxide level rises; the anhydride does not homopolymerize at 180 to 190 °C, and molar mass falls with anhydride content up to about 1 wt% before it plateaus. A grafted compatibilizer therefore arrives with a lower molar mass than its base resin, which raises the melt flow rate and shifts the processing window as well as the morphology. Feeding position and screw design are covered under twin-screw compounding of additives.

Compatibilizer masterbatch and let-down#

Most compatibilizers reach the blend either as neat pellets fed by their own gravimetric feeder or as a masterbatch let down into the main stream, and the choice is a dosing-accuracy decision rather than a chemical one. Neat addition is normal at whole-percentage levels, where a gravimetric feeder holds the tolerance; a concentrate earns its place at the fractional levels used in biopolymer blends, where weighing 0.5 wt% accurately on a line is the harder problem.

The carrier deserves more attention in this family than in most. A masterbatch is a concentrated mixture of additives extruded together in a carrier matrix, and for a compatibilizer that carrier is itself a polymer that joins the blend, so a carrier chosen without regard to the pair adds a third phase to a system whose purpose is to manage two. Carrier selection and let-down ratios are covered under masterbatch, and no typical active content for a compatibilizer masterbatch is established in our source library.

How Is the Compatibilization of a Polymer Blend Measured?#

Compatibilization is measured on the compounded blend rather than on the additive, and it shows up first in notched impact strength and elongation at break and second in the size of the dispersed domains. Each of these methods is described under testing plastic additives. The three interfacial effects map onto three measurements: domain size onto morphology, coalescence onto the stability of that morphology after moulding, and adhesion onto mechanical properties. The table lists the 7 measurements that track compatibilization, with the standard, the unit and what a compatibilizer changes in each.

ISO and ASTM impact results are not interconvertible: ISO 180 reports kJ/m2 and ASTM D256 reports J/m.

Impact strength, tensile strength and elongation at break#

Notched impact strength and elongation at break are the two properties that a compatibilizer changes most, which is why every sourced result in the blend-pair table above is reported as an Izod or an elongation figure. Both measure what happens at a crack tip, which in an uncompatibilized blend finds a ready-made path along the phase boundary. Yield strength moves less: it rose by 30 % and 40 % in the two sourced recyclate cases.

The unit trap is the first thing to check on any quoted result. ISO 180 reports notched Izod in kJ/m2 while ASTM D256 reports it in J/m on a 63.5 x 12.7 x 3.2 mm specimen, so a number quoted without its standard cannot be compared with another. Both notched methods are set out on impact strength (Izod, Charpy). Elongation at break is measured as part of tensile testing of plastics, to ISO 527-1 and ISO 527-2 or ASTM D638-22. No pass or fail criterion for a compatibilized blend is established here, so the end point comes from the product specification.

Phase morphology: domain size and interfacial adhesion#

Phase morphology is read as the size of the minor-phase domains and the state of the boundary between them, and the method is microscopy, for which this reference records no standard. That absence matters, because a morphology claim is only as good as its described preparation: the cryo-fracture or etching step, the magnification and the number of fields counted decide what the number means. The 2025 olefin block copolymer result above is a domain-size result of this kind.

The length scale comes from the neighbouring rubber-toughening literature. High-impact polystyrene works best with rubber particles of 2 to 5 µm, and Wu, writing in Polymer in 1985, put the critical interparticle distance for PA66 rubber blends at 0.3 µm, a criterion that describes the matrix ligament between particles rather than the particles and is independent of particle size and volume fraction. Those are impact-modifier values, cited only to give a sense of scale, and neither is a target for a compatibilized blend.

Melt flow rate, intrinsic viscosity and melt strength#

A compatibilizer moves the flow of the compound in two opposite directions: a peroxide-grafted polyolefin carries a lower molar mass and raises the melt flow rate, while a multifunctional epoxide oligomer couples chains and lowers it. Knowing which direction applies is part of specifying the grade, since the compound has to survive the moulding window as well as the impact test.

Both show up in the melt flow rate, measured to ISO 1133-1 or ASTM D1238-26 with polypropylene at 230 °C and 2.16 kg and polyethylene at 190 °C and 2.16 kg, an empirical index rather than a fundamental property and therefore only comparable between compounds run at the same condition. For a polyester phase, intrinsic viscosity to ASTM D4603 or ISO 1628-5 is the better measure, since it tracks molar mass directly. Melt strength, by Rheotens or capillary rheometry to ISO 11443, matters where the compound is extruded into film or thermoformed, and it rises with an ionomer or a multifunctional epoxide.

Which Rules Apply to Compatibilizers in Polymer Blends?#

Compatibilizers are regulated through their monomers and through the polymer sections of the food-contact rules, not as additives in their own right, because almost every compatibilizer in this reference is itself a polymer. That single fact shapes the whole regulatory file: no registration number to quote for the pellet, no specific migration limit written against it and no hazard classification attached to it, while the monomers behind it carry all three.

REACH: the polymer exemption and the monomers behind it#

Under Article 2(9) of Regulation (EC) No 1907/2006 a polymer does not have to be registered, so the compatibilizer itself carries no REACH registration number while the monomers it was made from do. The exemption covers the polymeric modifiers used across this family, chlorinated polyethylene, MBS, SEBS, polyolefin elastomer, EPDM, ionomers, PP-g-MAH and E-MA-GMA among them. Exempt from registration as a polymer is not the same as unregulated: the monomers are registered separately, and the hazard and food-contact files run through them.

The hazard sits with the monomer in every case. Glycidyl methacrylate carries a harmonised classification under CLP Annex VI, index 607-123-00-4, as Carc. 1B H350, Muta. 2 H341 and Repr. 1B H360F, and it was listed under California Proposition 65 for cancer on 27 January 2023. Maleic anhydride carries index 607-096-00-9, with Acute Tox. 4, Skin Corr. 1B, Resp. Sens. 1 H334, Skin Sens. 1A and STOT RE 1. Those entries belong to the residual monomer, not to the polymer sold as a compatibilizer. Neither monomer is on the REACH SVHC Candidate List, which stands at 253 entries with its last update on 4 February 2026. How the polymer exemption fits the rest of REACH and plastic additives is explained there.

Food contact: EU 10/2011 monomer limits and the US 21 CFR polymer sections#

In the EU, the food-contact status of a compatibilizer is decided by the Annex I entries of its monomers under Regulation (EU) No 10/2011, while in the US it is decided by whether the polymer falls inside one of the polymer sections of 21 CFR part 177. Neither route approves the additive: a monomer is on the Union list with a limit, or a polymer is cleared under a numbered section. The table gives both routes for every monomer a compatibilizer brings with it.

US percentages here are composition or use limits set by the regulation, not dosages. Check the food type and condition of use in the cited section before relying on them.

Two gaps matter. PP-g-MAH has no generic 21 CFR listing here, so its US status is grade-specific and runs through a notification or a supplier letter, and the multifunctional epoxide oligomer used in PLA blends has no confirmed EU or US status at all. The generic SML of 60 mg/kg, the overall migration limit of 10 mg/dm2 (60 mg/kg for infant food) and the 0.01 mg/kg functional-barrier limit are explained on EU 10/2011.

Recycled material adds a second EU instrument: recycled plastics for food contact fall under Regulation (EU) 2022/1616, and any additive used in the recycling step must itself be authorised under Regulation (EU) No 10/2011, so a compatibilizer in a food-grade recyclate is assessed twice. Food types and conditions of use are decoded under FDA food contact rules. The demand comes from the EU Packaging and Packaging Waste Regulation (PPWR): Regulation (EU) 2025/40 applies from 12 August 2026 and its Article 7 sets recycled-content minima from 1 January 2030, among them 30 % for contact-sensitive PET packaging and 35 % for other plastic packaging.

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How Do Compatibilized Blends Differ from Impact-Modified, Filled and Chain-Extended Compounds?#

The four families differ by the interface they work on: a compatibilizer repairs a polymer-polymer interface, a coupling agent repairs a polymer-filler interface, an impact modifier adds a dispersed rubber phase, and a chain extender has no interface at all because it works on chain ends. That one distinction resolves most of the confusion in supplier literature, where the same drum of maleated polyolefin appears in three catalogues under three names: the product can be identical, and the interface it is bought for is what the name describes.

Impact modifiers: the same grades, a different job#

The same grade often appears in two catalogues: an epoxy-functional ethylene terpolymer is sold as an impact modifier for PPS, PBT and PET and as a compatibilizer for polyester-polyolefin blends, and a maleic-anhydride-grafted SEBS as both for polyamides. The job differs: as an impact modifier the elastomer is the dispersed phase that cavitates and lets the matrix yield, and as a compatibilizer it is the layer between two phases already there. The toughening job, and the grades sold for it, are on impact modifiers.

Coupling agents: the polymer-filler interface#

A coupling agent works between a polymer and an inorganic or natural surface such as glass, talc or wood flour, and most coupling agents are low-molar-mass reactive molecules dosed at 0.2 to 1.0 wt% of the mix rather than polymers dosed in whole percent. The overlap has one cause: a maleic-anhydride-grafted polyolefin reacts with filler hydroxyls and polyamide amine ends, so the same drum sells as a coupling agent at roughly 1 to 5 wt% and a compatibilizer at 2 to 10 wt%. Silanes, titanates and the maleated polyolefins in their coupling role are on coupling agents, and the wood-flour case is on coupling agents for wood-plastic and natural fibre composites.

Chain extenders: molar mass instead of an interface#

A chain extender works on chain ends rather than an interface: it couples the carboxyl, hydroxyl and amine ends of a condensation polymer to one multifunctional molecule and raises molar mass, melt viscosity and melt strength, typically at 0.1 to 1.5 wt%. A multifunctional epoxide oligomer does both jobs at once, which is why the PLA blends above are dosed at well under 1 wt% while the polyolefin blends need several per cent. The molar-mass job is covered under chain extenders for polymers.

Compatibilizers in recycled polymer blends#

Recycled polymer blends are where most compatibilizer volume now goes, and they raise one question a virgin blend does not: the compatibilizer stays in the material, so a 2026 review by Scholz and colleagues describes it as a legacy additive in the next recycling stream. That is an argument for designing the structure so it needs less of it, which is the subject of design for recycling.

Frequently asked questions about compatibilizers for polymer blends#

What is a polymer blend?#

A polymer blend is a material made by melt-mixing two or more polymers, which for almost every pair means a matrix with a second polymer dispersed in it rather than a single phase. Its defining feature is an internal polymer-polymer interface, and that interface is what a compatibilizer is for.

What is the difference between a polymer blend and a polymer composite?#

A polymer blend disperses one polymer in another, while a polymer composite disperses an inorganic or natural reinforcement such as glass fibre, talc or wood flour. The interface decides the additive: a blend takes a compatibilizer at whole percentages of the blend, a composite a coupling agent at 0.2 to 1.0 wt% of the mix.

Can you give an example of a miscible polymer blend?#

Miscible polymer pairs are rare, and this reference does not certify any specific pair as miscible: the entropy of mixing of long chains is so small that almost every pair separates into two phases. A blend does not have to be miscible to perform, it has to be compatible, which is what the interface work on this page achieves.

Is a compatibilizer the same as a coupling agent?#

No: a compatibilizer works at the interface between two polymers and a coupling agent between a polymer and a filler or fibre, even though the same maleic-anhydride-grafted polyolefin is sold under both names. The product can be identical; the interface is not.

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