Coupling Agents for Filled and Reinforced Plastics: 4 Types, Silanes, Titanates, Zirconates and Maleated Polyolefins

A coupling agent is a molecule that bonds chemically to an inorganic or natural filler surface at one end and to the polymer at the other, so that stress passes across the interface instead of stopping at it, and plastics use 4 types of them at levels between 0.2 and 5 wt%. Without that bridge, a glass fibre or a mineral particle is a hole in the matrix rather than a reinforcement, so which chemistry bonds to which surface?

Four chemistries answer that question. Silanes hydrolyse onto siliceous surfaces and are the route for glass fibre, silica, wollastonite and kaolin. Titanates react with surface protons instead, which is how calcium carbonate and carbon black are treated. Zirconates are the neighbouring organometallic chemistry, offered on the same substrates as the titanates. Maleated polyolefins work from the opposite direction, sitting in the matrix rather than on the filler, and they are the coupling route for glass-, talc- and wood-filled polypropylene and polyethylene. Coupling agents are one of the 43 families of plastic additives and belong to the property-modifier group, beside plasticizers, impact modifiers and compatibilizers.

This reference sets the boundary of the family against compatibilizers, adhesion promoters, sizings and dispersants, explains the 3 bonding mechanisms, describes the 4 types with their identified grades and CAS numbers, matches each filler and polymer to a coupling route, gives dosage by application route and filler particle size, states what coupling changes in a filled compound, lists the test standards that measure it, sets out the EU food-contact and REACH position of every chemistry named, identifies the producers, and closes with the 8 coupling-agent substances held in our substance database.

Table 1. The 4 types of coupling agents at a glance. The Type column carries the one link to each chemistry on this page.

Levels are study and supplier ranges for the named systems, not a general recommendation, and the source of each row is named in its own type section below. Rows 2 and 3 gain their own pages as the pages on titanate coupling agents and zirconate coupling agents publish.

What Is a Coupling Agent in Polymers?#

A coupling agent in a polymer is a bifunctional molecule or a functionalised polymer that forms a chemical link between an inorganic or natural surface, such as glass fibre, silica or wood flour, and the polymer matrix around it. The molecule has two ends with fixed roles: a hydrolysable or reactive group that finds the substrate, and an organofunctional group that reacts with, or entangles in, the resin. Because the bridge is chemical rather than physical, the filled compound behaves as one material, and the adhesion it creates at the interface enters the strength of the compound as a quantified term in the interfacial interaction model published by B. Pukanszky in Composites 21 (1990) 255-262.

So how much of an additive is a coupling agent, when it ends up bound to the filler rather than dissolved in the polymer? It is dosed against filler surface area, because the BET specific surface area of the mineral drives coupling-agent demand, which is why every dosage rule on this page names a route before it names a number. Two neighbouring functions are not coupling. A coupling agent is not a crosslinker: the same vinyl silane grafted onto the polymer and moisture-cured is silane crosslinking, a different function on a different page. A coupling agent is not a dispersant either, although one titanate performs both jobs at once.

What is the difference between a coupling agent and a compatibilizer?#

A coupling agent bonds a polymer to an inorganic or natural surface, while a compatibilizer bonds one polymer to another, and the two answers to "which interface" are what separate the families. A coupling agent acts at the polymer-filler or polymer-fibre interface of a filled or reinforced compound. A compatibilizer acts at the polymer-polymer interface of an immiscible blend, where the problem is domain size and interfacial tension rather than stress transfer to a rigid particle. The chemistries follow the interface: organosilanes, titanates and zirconates have no role in a polymer blend, because a blend offers no inorganic surface to condense onto.

Table 2. Coupling agent and compatibilizer compared.

The overlap is real, and it is one product rather than two. Maleic anhydride grafted polypropylene (CAS 25722-45-6) couples glass fibre, talc, mica, wood and natural fibres in polypropylene, and the same grade serves as a reactive compatibilizer for PP/PA blends and mixed recyclate, so both words name the same material. Blend pairs and loadings are on compatibilizers, which answers the same comparison from the polymer-polymer side.

Is a coupling agent the same as an adhesion promoter, a sizing or a dispersant?#

"Adhesion promoter" is the same chemistry under another name, "sizing" is the form in which a silane reaches a glass fibre rather than a chemistry of its own, and a dispersant does a different job. 3-Aminopropyltriethoxysilane, for example, is recorded in our substance database as a coupling agent and adhesion promoter in one line, the second word applying when the substrate is a coating or a surface rather than a filler inside a compound. A sizing is a delivery form: the aqueous coating applied to glass fibre as the filaments leave the bushing, in which the silane is one component, at a total sizing content on the fibre of 0.5-2.0 wt%, as described on glass fiber reinforced plastics. A dispersant has a different target, because wetting and de-agglomeration of pigment and filler particles are the job of dispersing agents for plastics and masterbatch, not the creation of a chemical bond.

A primer is the coatings word for the same deposition step, so in a compound the silane is either pre-applied to the filler or added to the mix, and nothing is primed on the polymer side. The titanate KR-TTS shows how far the categories overlap: it is recorded as a titanate coupling agent and dispersant, because the isostearate chains it leaves on the surface improve dispersion while the titanium end does the coupling.

How Do Coupling Agents Work at the Filler Interface?#

Coupling agents reach the filler surface in 3 different ways: silanes hydrolyse and condense onto surface hydroxyl groups, titanates and zirconates react with surface protons instead, and maleated polyolefins react from the matrix side without ever being applied to the filler. Each mechanism sets its own substrate range, its own application route and its own dosage basis, so the mechanism is the first thing to establish for any filled or reinforced system.

How does a silane bond to an inorganic surface?#

A silane coupling agent bonds to an inorganic surface in 4 steps: the alkoxy groups hydrolyse to silanols, the silanols condense with the hydroxyl groups on the surface to form Si-O-M bonds, the remaining silanols condense with each other into a polysiloxane interphase, and the organofunctional end reacts with or entangles in the polymer. E. P. Plueddemann of Dow Corning set out this sequence in the standard text of the field, Silane Coupling Agents (Plenum, 2nd edition, 1991), and it holds for every organofunctional type. The 4 steps run in the order below.

1. Hydrolyse the alkoxy groups. Water converts each Si-OR group into Si-OH and releases the corresponding alcohol, methanol from a trimethoxysilane and ethanol from a triethoxysilane. The alkoxy group sets how fast this step runs.
2. Condense onto the surface. The silanols react with the M-OH groups of the mineral or the glass to form the Si-O-M oxane bonds that anchor the molecule.
3. Self-condense into an interphase. Surplus silanols condense with each other into a polysiloxane layer around the particle or fibre, so the treatment is a thin phase rather than isolated molecules.
4. Bond to the polymer. The organofunctional group reacts with a matching group in the resin, or, where no reaction is possible, entangles with the matrix.

3-Aminopropyltriethoxysilane is the worked example held in our source library: its ethoxy groups hydrolyse to silanols, the silanols condense with surface M-OH to form Si-O-M bonds and a siloxane interphase, and the primary amine then reacts with or hydrogen-bonds to the end groups of polyamide, epoxy, phenolic and PBT resins. The oxane bond is reversible in water, which makes bond count the durability variable: B. Arkles of Gelest reports that dipodal silanes, carrying two silicon atoms and six hydrolysable groups per molecule, give up to 100,000 times greater hydrolysis resistance than a conventional monosilane.

How do titanates work without surface hydroxyls?#

A titanate coupling agent reacts with the protons on a filler surface rather than with hydroxyl groups, which is why it works on calcium carbonate and carbon black, the two substrates on which silanes perform poorly. In the monoalkoxy titanate KR-TTS the isopropoxy group is the reactive end, and it needs only a surface proton to anchor the molecule, so the requirement for an abundant population of surface M-OH groups disappears. Salvatore J. Monte of Kenrich Petrochemicals developed the titanate and zirconate coupling agents on this principle and entered the Plastics Hall of Fame in 2021.

What remains on the surface after the reaction matters as much as the bond itself. The three isostearate chains of KR-TTS stay attached and are hydrophobic, so they lower melt viscosity and improve dispersion of the treated filler, which is why the same product is sold as a coupling agent and a dispersant, and why a highly filled polyolefin or PVC compound buys stress transfer and easier flow from one addition. The proton mechanism also explains the substrate split that runs through the rest of this page: a carbonate surface offers protons but few hydroxyls, while a siliceous surface offers both.

How do maleated polyolefins couple from the matrix side?#

A maleated polyolefin never touches the filler before compounding: it is added to the matrix, and its grafted anhydride groups react with the silanol groups of glass or the hydroxyl groups of cellulose once the melt wets the surface. The succinic anhydride groups grafted onto the polypropylene chain also react with the NH2 end groups of polyamides, while the polypropylene backbone co-crystallises and entangles with the polypropylene matrix, so stress passes from the filler through the graft into the bulk polymer.

How the graft is made limits what it can be. Grafting is peroxide-initiated radical grafting in reactive extrusion: peroxide radicals abstract hydrogen from the polypropylene chain, maleic anhydride adds to the macroradicals, and beta-scission of polypropylene competes with the grafting reaction and intensifies as peroxide content rises. Maleic anhydride does not homopolymerise at 180-190 °C, and molar mass falls as the anhydride level rises to about 1 wt% and then plateaus, which is why commercial grades cluster near that anhydride content and why melt flow rate rises with grafting. Grafting conditions and grade data are on maleic anhydride grafted polypropylene (PP-g-MAH).

4 Types of Coupling Agents#

The 4 types of coupling agents used in plastics are silanes, titanates, zirconates and maleated polyolefins, and they divide on one question: whether the molecule is applied to the filler or added to the polymer. The first three are surface chemistries applied to the mineral or the fibre, by pretreatment or in a size bath, and the fourth is a functionalised polymer metered into the compound like a resin. Glass-fibre sizing is not a fifth type but a route, because the active chemistry inside a size is a silane.

1. Silane coupling agents#

Silane coupling agents are organofunctional trialkoxysilanes, four of which are identified substances in this reference: the amino silane APTES, the epoxy silane GLYMO, the methacryl silane MEMO and the vinyl silane VTMS. Their shared structure is Y-(CH2)n-Si(OR)3, where the three alkoxy groups build the bond to the inorganic surface and the organofunctional group Y decides which resins the molecule can serve. Silanes carry the most complete data set of the 4 classes, from loading rules by particle size to five entries in the EU Union list.

The four identified grades cover the working range of the class. 3-Aminopropyltriethoxysilane (APTES, CAS 919-30-2, EC 213-048-4, C9H23NO3Si, 221.37 g/mol) is sold as Silquest A-1100, Dynasylan AMEO, Z-6011 and KH-550, and it treats glass fibre and mineral fillers for polyamide, PBT, polycarbonate, phenolics and epoxies. 3-Glycidoxypropyltrimethoxysilane (GLYMO, CAS 2530-83-8, EC 219-784-2, C9H20O5Si, 236.34 g/mol), sold as KH-560, Dynasylan GLYMO, Z-6040, KBM-403 and Silquest A-187, treats glass and mineral fillers in epoxies, polyesters and polyurethane and is the sizing component for glass fibre in PET, PBT and polycarbonate. 3-Methacryloxypropyltrimethoxysilane (MEMO, CAS 2530-85-0, EC 219-785-8, C10H20O5Si, 248.35 g/mol), sold as KH-570, Dynasylan MEMO and Silquest A-174, serves free-radical-cured systems. Vinyltrimethoxysilane (VTMS, CAS 2768-02-7, EC 220-449-8, C5H12O3Si, 148.23 g/mol) couples fillers inside crosslinked systems, although its primary industrial use is silane crosslinking rather than coupling. Blended into a compound rather than pre-applied to the filler, silanes are used at 0.2-1.0 wt% of the total mix.

Which organofunctional group suits which polymer?#

The organofunctional end decides the polymer: amino silanes for polyamide, PBT, polycarbonate, phenolics and epoxies, epoxy silanes for epoxies, polyesters and polyurethane, and methacryl silanes for unsaturated polyester and other free-radical-cured systems. The rule follows directly from step 4 of the bonding sequence, because the organofunctional group has to find a reaction partner in the resin.

Table 3. Organofunctional silane groups and the resins they serve.

A formulator looking for a silane for an epoxy resin therefore has two candidates, the epoxy silane GLYMO and the amino silane APTES, each of which carries a group that opens or reacts with an epoxide. Polyolefins are the exception that shapes the rest of this page: polypropylene and polyethylene offer no reactive group at all, so no organofunctional end has anything to bond to, and a glass- or talc-filled polypropylene is coupled with a maleated polyolefin in the matrix instead.

2. Titanate coupling agents#

Titanate coupling agents are organotitanates that bond to filler surfaces through proton reaction, and the one fully identified grade in this reference is isopropyl triisostearoyl titanate (KR-TTS, CAS 61417-49-0), the standard treatment for calcium carbonate in polyolefins and PVC. The class exists because the silane mechanism fails on carbonate surfaces, and a titanate answers that failure with a different reaction rather than with a higher silane dose.

KR-TTS carries the EC number 262-774-8, the formula C57H112O7Ti and a molar mass of 957.4 g/mol, and Kenrich Petrochemicals sells it as Ken-React KR TTS, as the liquid concentrate CAPS KR TTS/L and as the powder form CAPOW KR TTS/H. It is a monoalkoxy organotitanate, it functions as a coupling agent and a dispersant for calcium carbonate and other fillers in polyolefins and PVC, and its recorded alternatives are a stearic acid coating, the neoalkoxy titanates of the LICA series, the zirconates of the NZ series and, on siliceous fillers, the silanes. Under REACH it is registered with 5 active Article 10 dossiers, the first dated 25 May 2018, and it was not on the Candidate List when we checked on 22 September 2026. Its food-contact position is the sharpest difference from the silanes: KR-TTS is not found in Annex I of Regulation (EU) No 10/2011, so it is not authorised for food-contact plastics in the EU, and our source library holds no US food-contact clearance for it. Its notified GHS classification is H315, H319 and H335.

3. Zirconate coupling agents#

Zirconate coupling agents are organozirconates of the NZ series, offered by the same producer as the titanates and for the same substrates, and this reference holds no verified grade data for them yet. Our records name the zirconate NZ series only where it appears as a recorded alternative to the titanate KR-TTS on mineral fillers, and they hold no CAS number, no dosage, no food-contact entry and no REACH status for any individual zirconate grade. Aluminates are the third organometallic class named in the coupling literature, and this site holds no verified data on them either. Both gaps are stated rather than filled, and the zirconate page carries grade data only once primary records exist behind it.

4. Maleated polyolefins (PP-g-MAH and PE-g-MAH)#

Maleated polyolefins are polypropylene or polyethylene chains carrying about 1 % grafted maleic anhydride, and they are the standard coupling route for glass-, talc- and wood-filled polyolefins, where no silane has anything to react with in the matrix. The class is identified by one CAS number for the polypropylene version, 25722-45-6, and it travels under the synonyms PP-g-MA, MAH-g-PP, MAPP and maleated polypropylene, with the polyethylene version written PE-g-MAH. Its recorded function is twofold: coupling agent for glass fibre, talc, mica, wood and natural fibres in polypropylene, and reactive compatibilizer for PP/PA blends and recyclate.

Two product families carry the class in the market. The SI Group grades are listed on Polybond, which covers POLYBOND 3200, 3000, 3150 and 3002 as maleated polypropylenes and POLYBOND 3029 as a maleated HDPE; POLYBOND 3200 is supplied as off-white pellets carrying 0.8-1.2 % maleic anhydride at a density of 0.91 g/cm3 at 23 °C. The Dow grades are the FUSABOND series, with E-series grades for polyethylene, N-series grades for polyethylene copolymers and P-series grades for polypropylene, including E205 and E204 for HDPE and E226 for LLDPE.

Levels are set per system rather than per family. Studies of polypropylene with short glass fibre use 0.5-2 wt% of the compound, one hybrid-fibre study used 5 wt%, wood-plastic composites use 1-3 wt%, and 5 % of POLYBOND 3150 or 3002 in recycled polypropylene containing nylon raised notched and reverse Izod impact more than threefold (Plastics Technology). Because these products are polymers, they are exempt from REACH registration under Article 2(9), so their EU food-contact status runs through their monomers rather than through a substance entry: maleic anhydride is FCM 234 with a group 3 restriction and an SML(T) of 30 mg/kg expressed as maleic acid, and propylene is FCM 275 with no SML.

Which Coupling Agent Suits Each Filler and Polymer?#

The filler decides the chemistry and the polymer decides the route: a silane works on silica, glass, wollastonite and kaolin because those surfaces carry hydroxyl groups, while calcium carbonate and carbon black need a titanate or a stearic acid coating instead. The second half of that rule matters as much as the first, because a silane-treatable filler in a polyolefin still ends up coupled by a maleated polyolefin: the treated surface has nothing to bond to in the matrix. Particle size, aspect ratio and surface area of each mineral are compared on fillers for plastics, and they set the dose once the chemistry is fixed.

Table 4. Which coupling route suits each filler and fibre.

One practical question decides who does the work. A mineral producer may deliver the filler already treated, in which case the treatment step itself, from the mineral producer's side, is filler surface treatment, and the compounder buys a coated grade rather than a drum of silane. Otherwise the compounder applies the chemistry in house, and the choice between pretreatment, integral blend and sizing sets both the equipment and the level.

Why silanes do not work on calcium carbonate#

Silanes need surface hydroxyl groups to condense with, and calcium carbonate does not supply them, which is why chalk and marble sit at the bottom of every silane substrate ranking together with gypsum, barytes, graphite and carbon black. Gelest records that list of poor substrates directly, and the limit is chemical rather than economic, so raising the silane level on a carbonate does not convert it into a coupled system. Grades and loadings of calcium carbonate in plastics are unaffected by this: the mineral remains the highest-tonnage filler in the industry, and it simply takes a different surface chemistry.

Two alternatives cover the gap. A titanate reacts with the surface protons that the carbonate does offer, which is the coupling answer, and the cheaper of the two routes is a stearic acid coating, which makes the particle hydrophobic and improves dispersion without creating a covalent bridge to the polymer. Our source library holds no verified coating level for stearic acid on calcium carbonate, so this page gives none, and the choice between the two routes turns on whether the compound needs stress transfer or only easier flow and lower water pick-up.

Which coupling agent do wood-plastic composites and natural fibres need?#

Wood-plastic composites need a maleated polyolefin, because wood flour is polar cellulose dispersed in a non-polar polyethylene or polypropylene matrix and nothing else in the compound will bond the two. The anhydride reacts with the hydroxyl groups of cellulose while the polyolefin backbone entangles with the matrix, and the level used in wood-plastic composites is 1-3 wt% of the compound. Grade choice and loading are on coupling agents for wood-plastic and natural fibre composites, which carries the WPC-specific selection.

The processing window separates WPC from mineral-filled compounding. A wood-plastic composite runs about 28 °C below the unfilled resin and the matrix stays at or below about 200 °C, because cellulose degrades above that temperature, so the coupling agent works inside a narrow thermal band. A published recycled-HDPE profile runs 34 % recycled HDPE and 60 % sawdust, with a coupling wax, stearic acid and pigment. A 2026 study of a glass-fibre and wood-filled polypropylene composite, published in Materials, found the optimum maleated polypropylene loading to be 1 wt% for flexural strength, which rose by 24.7 %, and 3 wt% for tensile strength, modulus and impact, so one level does not optimise every property at once. The full WPC package, with lubricants, UV stabilizers and biocides, is on additives for wood-plastic composites.

Food-contact law restricts the filler before it restricts the coupling agent. Cellulose is authorised under Regulation (EU) No 10/2011 as FCM 553, both as an additive and as a monomer; lignocellulose is FCM 595, authorised only as a monomer or starting substance; and untreated wood flour and fibres, formerly FCM 96, were deleted by Regulation (EU) 2023/1442, with articles first placed on the market before 1 February 2025 saleable until stocks are exhausted. Flax, hemp and jute behave like wood flour at the interface; see natural fiber composites.

Do coupling agents help recycled and mixed plastics?#

In recycled and mixed plastics the same products appear, but they are usually doing compatibilizer work: the interface that limits a mixed recyclate is polymer against polymer, not polymer against filler. That distinction changes which number applies, because the loadings reported for recyclate are blend loadings rather than coupling doses, and that job belongs to compatibilizers for recycled plastics.

Two sourced systems set the scale. POLYBOND 3150 and POLYBOND 3002 at 5 % in recycled polypropylene containing nylon raised notched and reverse Izod impact more than threefold, as reported by Plastics Technology. Kenrich's Ken-React CAPS KPR 12, a titanate supplied with a catalyst, is used in mixed post-industrial resin at 1.5-1.75 %, and the supplier reports 20 % faster processing at a 9 % lower temperature, which is a supplier claim rather than an independently verified result. Beyond these two systems our source library holds no figure for coupling agents in filled recyclate, and the rest of the restabilization package is on additives for recycled plastics.

How to select a coupling agent in 7 steps#

Select a coupling agent in 7 steps: identify the filler surface, identify the polymer, choose the application route, set the level from particle size rather than from resin weight, check the processing window, screen the regulatory status and confirm the gain by testing. The order matters, because each step removes candidates that the next step would otherwise have to test.

1. Name the filler or fibre and establish whether its surface carries hydroxyl groups. Siliceous surfaces take a silane; calcium carbonate, gypsum, barytes, graphite and carbon black do not.
2. Name the polymer and find the organofunctional group that will react with it. A polyolefin carries none, so it needs matrix-side coupling with a maleated polyolefin.
3. Choose the route: filler pretreatment, integral blend, aqueous sizing on glass fibre, or matrix addition of a functionalised polymer.
4. Set the level from the filler particle size, or from the study range for the system, rather than from a percentage of the resin, because demand follows surface area.
5. Check the processing window. Grafted polyolefins raise melt flow rate, integral silane blends need vacuum removal of the alcohol by-product, and wood-filled compounds should stay at or below about 200 °C.
6. Screen the food-contact and REACH status of the exact chemistry in every target market, because authorisations differ per substance and per use.
7. Confirm by testing tensile and flexural strength, impact and, for natural-fibre systems, water uptake, against an untreated control at the same verified filler loading.

The general method behind these steps is on how to select plastic additives, which applies the same sequence across the other additive families.

How Much Coupling Agent Is Needed? Dosage by Route and Particle Size#

There is no single coupling-agent dosage, because the level follows the route: 0.2 to 1.0 wt% of the mix for a silane blended into the compound, 0.75 to 1.5 % on the filler for a pretreatment, 0.5 to 2.0 % in a glass-fibre size bath and 0.5 to 5 wt% of the compound for a maleated polyolefin. Every figure below therefore names its basis, and the bases are not interchangeable.

Table 5. Coupling agent level by route, filler particle size and system.

The particle-size rule is a surface-area rule in disguise, which is why a sub-micron filler needs at least 1.5 % silane on the filler while a filler coarser than 100 µm needs 0.1 % or less: finer particles present more surface per gram and therefore more sites to cover. Where the exact grade is known, the loading can be calculated rather than read from a table, by dividing the filler's BET specific surface area in m2/g by the silane's specific wetting surface, which is about 208 to 528 m2/g for common silanes. Unit rules and the conversion to wt%, ppm and let-down ratio are set out separately, and none of these levels converts cleanly into phr, because the basis changes from row to row. Each loading rule is written against particle size (D50, top cut) or against a named system, never against resin weight alone.

Filler pretreatment, integral blend or glass-fibre sizing?#

A silane reaches the filler by one of 3 routes, and each sets its own level and its own equipment: pretreatment of the filler before compounding, an integral blend sprayed onto the pre-blend, or an aqueous size applied to glass fibre at the bushing. The three are listed below with the conditions recorded for each.

• Filler pretreatment. Aqueous-alcohol deposition uses 95 % ethanol with 5 % water, adjusted to pH 4.5-5.5 with acetic acid, at a silane concentration of 2 %, with 5 minutes of hydrolysis before the filler is added. The loading then follows the particle-size rows above.
• Integral blend. The silane is sprayed onto a pre-blend of resin and filler and the mixture is melt-compounded, at 0.2-1.0 wt% of the total mix. This is the route when pretreated filler is unavailable or uneconomic, and it answers the question of when a silane coupling agent is added: at the pre-blend stage, immediately before compounding.
• Glass-fibre sizing. An aqueous bath carries 0.5-2.0 % silane at pH 5.5 and the size is cured at 110-120 °C for 20-30 minutes, leaving 0.5-2.0 wt% of total size on the fibre. A size is a system rather than a single chemical, and the full size recipe is on coupling agents for glass fiber.

The by-product decides the equipment. Hydrolysis releases methanol or ethanol into the melt, so an integral blend is devolatilised under vacuum, and vacuum devolatilization is part of the plastic compounding step. That requirement is the practical reason integral blends are capped near 1.0 wt%: more silane means more alcohol to strip in the same residence time, while a maleated polyolefin, which releases nothing, is metered in like a resin.

What Does a Coupling Agent Change in a Filled Compound?#

A coupling agent changes 4 things in a filled compound: stress transfer and therefore tensile and flexural strength, impact behaviour, water uptake in natural-fibre systems, and melt viscosity and dispersion. Each recorded value below belongs to a named system rather than to the family as a whole, because a coupling agent has no general performance figure.

Table 6. Recorded effects of coupling agents, by system and source type.

How much of a filled compound's strength comes from the interface can be quantified rather than asserted. The interfacial interaction model published by B. Pukanszky in Composites 21 (1990) 255-262 is the standard treatment, and it separates the contribution of the filler itself from the contribution of the interface between filler and matrix. That separation is why a coupling-agent trial is always run against an untreated control at an identical, verified filler loading.

How Is Coupling Performance Tested?#

Coupling is measured indirectly, through the properties it changes: tensile and flexural strength to ISO 527 and ASTM D638, notched impact to ASTM D256 or ISO 179, melt flow rate to ISO 1133, and ash content to confirm that two compared compounds really carry the same filler loading. No standard measures interfacial adhesion inside a compound directly, so a coupling trial is a property plan, and all methods are indexed under testing plastic additives.

Table 7. Test methods that show whether a coupling agent is working.

Flexural strength moved most in the wood-filled polypropylene study cited above, so it belongs in any coupling trial, and water absorption belongs in any natural-fibre trial. The reference values for the reinforcement come from the fibre rather than from the interface: E-glass has a density of 2.58 g/cm3, a tensile strength of 3445 MPa and a modulus of 76.0 GPa, the ceiling against which a coupled compound is judged.

How Are Coupling Agents Regulated?#

Coupling agents are regulated in 3 layers: REACH registration and the Candidate List for the silanes and the titanate, the polymer exemption of Article 2(9) for the maleated polyolefins, and Regulation (EU) No 10/2011 for anything that ends up in a food-contact plastic. The three layers do not overlap neatly, and a product that is unproblematic under one can be unusable under another, which is why the food-contact check belongs inside the selection sequence rather than at the end of a development project. Every instrument is summarised in plastic additive regulations.

EU food contact: the Union list entries of Regulation (EU) No 10/2011#

Every silane coupling agent that is authorised for food-contact plastics in the EU is listed in Regulation (EU) No 10/2011 as a monomer or starting substance with a restricted use, not as an additive: APTES under FCM 377, MEMO under FCM 788, GLYMO under FCM 1068 and vinyltrimethoxysilane under FCM 453. Each entry restricts the use as well as the migration, so the authorisation is narrower than the phrase "listed in Annex I" suggests.

Table 8. Union list entries relevant to coupling agents.

Union list values on this page were verified against the consolidated version of Regulation (EU) No 10/2011 of 16 March 2025, and later amendments have not been checked. The titanate KR-TTS is not found in Annex I at all, so it holds no food-contact authorisation in the EU.

"Surface treatment only" is a use restriction with teeth. GLYMO is the narrowest case: it is authorised only inside a glass-fibre sizing, only for fibres embedded in five named low-diffusivity plastics, and only where residues are not detectable at 0.01 mg/kg for the substance and 0.06 mg/kg for each reaction product, so the shorthand "GLYMO is food-contact approved" is wrong in three ways. MEMO may be used only to treat inorganic fillers, and APTES carries both a migration limit and a residue limit on the treated filler. How the Union list and its use restrictions work is explained on EU 10/2011.

REACH, SVHC status and the US FDA routes#

All four silanes in this reference and the titanate KR-TTS are REACH registered, and none of them was on the Candidate List when this page was last checked on 22 September 2026. The registration counts recorded per substance are 26 active dossiers for APTES, whose lead dossier was first registered on 9 September 2010, 28 for GLYMO, 21 for MEMO and 24 for VTMS, each first registered on 26 October 2010, and 5 for KR-TTS, first registered on 25 May 2018. The maleated polyolefins sit outside this layer entirely, because polymers are exempt from registration under Article 2(9) of Regulation (EC) No 1907/2006.

One vinyl silane in the wider family is restricted, which makes the Candidate List a per-substance check rather than a class judgement. Tris(2-methoxyethoxy)vinylsilane (CAS 1067-53-4, EC 213-934-0), used in rubbers, plastics and sealants, was included on the Candidate List on 17 January 2022 as toxic for reproduction under Article 57(c), and it appears among the Appendix 6 additions to REACH Annex XVII entries 28 to 30 made by Regulation (EU) 2021/2204. Every listed additive is tracked on the SVHC Candidate List. Classification is a separate question from authorisation, and it is where the handling drawbacks of the silanes appear: APTES carries harmonised CLP index 612-108-00-0, Acute Tox. 4 H302 and Skin Corr. 1B H314, and is a volatile amine with a fishy odour; VTMS carries index 014-049-00-0, Skin Sens. 1B H317; maleic anhydride carries index 607-096-00-9, including Resp. Sens. 1 H334 and Skin Sens. 1A H317; GLYMO, MEMO and KR-TTS have no harmonised entry.

The US routes run through adhesives and coatings rather than through a coupling-agent listing. 21 CFR 175.105 lists gamma-aminopropyltrimethoxysilane, N-beta-aminoethyl-gamma-aminopropyltrimethoxysilane, polymerised butyl titanate, glycidyl methacrylate and oxazoline for adhesives, and 21 CFR 175.300 allows N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane at up to 1.3 % by weight of resin in repeated-use coatings. Our source library holds no generic 21 CFR listing for PP-g-MAH, so food-contact status for a maleated polypropylene is grade-specific; 21 CFR 177.1520 covers ethylene-maleic anhydride copolymers at up to 2 % anhydride and 21 CFR 177.1350 covers maleic-anhydride-grafted EVA at up to 2 %. Every 21 CFR section is mapped on FDA food contact rules.

Who Makes Coupling Agents? Silane, Titanate and Functional-Polyolefin Suppliers#

Coupling agents come from 3 supplier groups: the silane producers Evonik, Momentive, Dow, Shin-Etsu and Wacker, the organometallic specialist Kenrich Petrochemicals, and the functional-polyolefin producers SI Group and Dow. The three groups sell different products in different forms, so one buyer rarely sources the whole family from one company, and company profiles are in the directory of plastic additive manufacturers and suppliers.

Table 9. Coupling agent suppliers by chemistry and brand line.

The buyer's directory is compatibilizer and coupling agent suppliers, which covers both families from the sourcing side. Grade codes cross-reference across suppliers, so KH-550, Silquest A-1100, Dynasylan AMEO and Z-6011 are the same substance, APTES, under four names, while KH-560 is GLYMO and KH-570 is MEMO. Published market figures for coupling agents as a family are not held in our source library, so this page gives none; segment data for the whole additive market are on plastic additives market.

Complete List of Coupling Agent Substances (8 Pages)#

The table lists the 8 coupling-agent substances that have their own pages on this site, with CAS number, coupling type, function, main substrates and EU food-contact entry. The order follows the 4 types used throughout this page.

Table 10. The 8 coupling-agent substances with their own pages.

Each row opens its own substance page as that page publishes, and every other substance is in the plastic additives database.

What Else Is Called a Coupling Agent? Ultrasound Gel, Dental Silanes and Peptide Reagents#

Three other industries use "coupling agent" for something else: dentistry, where a silane primer bonds composite resin to a ceramic surface, ultrasound, where a gel couples the transducer to the skin, and peptide synthesis, where a coupling reagent joins two amino acids. The dental sense is the closest relative, because it is the same silane chemistry acting on a glass or ceramic surface, in a different industry. The ultrasound sense shares nothing with the others but the word, since an acoustic coupling gel transmits sound between two surfaces and forms no chemical bond. The peptide sense is a family of condensation reagents used in organic synthesis.

Mechanical and computing senses complete the picture, named here only so that a reader who arrived on the wrong page can leave it quickly: a coupling in engineering is a hardware item joining two shafts or two pipes, and coupling in computer science describes dependency between software modules. None of these senses is covered on this site, which is restricted to additives used in plastics.

Coupling agents in rubber and other neighbouring industries#

The same silane chemistry runs through rubber, coatings, adhesives and sealants, and the best-known silane handbooks are written from the coatings and elastomer side rather than from the compounder's side. That framing explains a recurring mismatch in the literature, because a deposition procedure written for a coated substrate assumes a surface to be primed, while a compounder needs a level on a filler or in a mix. Rubber compounding uses silane coupling between silica and the elastomer network, and the grades used there are not covered on this site, because rubber formulation lies outside its border. This reference names the neighbouring use and stops there.

Coupling agent FAQs#

Three questions recur across the search results for this family, and each has a short answer that follows from the material above.

What are KH-550, KH-560 and KH-570?#

They are the Chinese grade codes for the three most common silane coupling agents: KH-550 is 3-aminopropyltriethoxysilane (CAS 919-30-2), sold elsewhere as Silquest A-1100 and Dynasylan AMEO; KH-560 is the epoxy silane GLYMO (CAS 2530-83-8), also sold as Silquest A-187, Z-6040 and KBM-403; and KH-570 is the methacryl silane MEMO (CAS 2530-85-0), also sold as Silquest A-174 and Dynasylan MEMO. The code identifies the substance rather than the purity or the specification, so a purchase specification still names the CAS number.

Does every filler need a coupling agent?#

No: a coupling agent pays for itself when the filler carries load, and a cost-reducing filler in a non-structural part often does not need one, which is why calcium carbonate is more often stearic-acid coated than coupled. The decision follows the job of the filler. Where the mineral is there to displace resin volume, a surface treatment that improves dispersion and flow is enough; where the mineral or the fibre is there to carry stress, the interface has to transfer it, and only a coupling agent does that.

Can a coupling agent replace a compatibilizer in recycled plastics?#

A maleated polyolefin does both jobs, which is why it appears under both names, but the two interfaces are different: at 5 % in recycled polypropylene with nylon, Polybond grades are acting on the polymer-polymer interface, not on a filler surface. A silane, a titanate or a zirconate cannot make that substitution, because a mixed recyclate presents no inorganic surface for them to bond to, so the answer is product-specific rather than family-wide.