A polyurethane adhesive bonds through the polyaddition of polyisocyanates with polyols into urethane links, and cures either with ambient moisture, with a mixed polyol component or after drying and heat activation of a dispersion. Among industrial adhesive chemicals, its raw materials fall into four classes: isocyanates or isocyanate-terminated prepolymers, polyols, catalysts, and optional solvents, plasticizers and fillers.
Four cure routes carry this chemistry into production: one-part moisture-curing adhesives, two-part adhesives mixed from a polyol and an isocyanate component, waterborne polyurethane dispersion (PUD) adhesives and reactive polyurethane hot melts (PUR). Solvent-borne thermoplastic polyurethane (TPU) adhesives are a physically setting variant that crosslinks once an isocyanate hardener is added.
The sections below give four published starting formulas with their results, the isocyanate (NCO) content of each isocyanate family, five ASTM methods with current editions, the CLP Annex VI lines of the main monomers and the 21 CFR 175.105 routes for polyurethane resins. They also cover REACH entry 74 training, required since 2023-08-24, and the EU exposure limit of 6 µg NCO/m³, then match each cure route to the requirement it serves.
Sources and checks: identities and hazard classifications are read at ECHA and the issuing regulator, standards at the body that publishes them, and grade values at the data sheet named next to them; methodology and fact-checking explains each step.
What Is a Polyurethane Adhesive?#
A polyurethane adhesive is a reactive adhesive whose isocyanate (NCO) groups react with polyol hydroxyl groups to form urethane links, while excess NCO groups react with water to form urea links. The polymer therefore forms in the bond line itself, building its molecular weight only after application. Every route on this page runs on this chemistry; what changes is which side carries the free NCO groups and what supplies the partner, a polyol, moisture or both.
PU adhesive, urethane adhesive, PU glue and polyurethane-based adhesive all name the same chemistry. Each ends as a crosslinked or semi-crystalline polyurethane network in the joint.
How Polyurethane Adhesives Cure: Urethane and Urea Links#
Polyurethane adhesives cure by two addition reactions of the NCO group: with hydroxyl groups to urethane links and with water to urea links. The urethane reaction (NCO + OH) builds the network of a two-part adhesive when the polyol and isocyanate components meet, and the same reaction makes an NCO-terminated prepolymer from excess diisocyanate and a polyol before a one-part adhesive is filled.
Moisture cure is the second reaction: NCO reacts with water to an unstable carbamic acid, which splits into an amine and carbon dioxide (CO2), and the amine then reacts with a further NCO group to a urea link. This chemical reaction is how adhesives cure in one-part polyurethane grades, as opposed to evaporation, cooling or radiation; the four routes are compared under how adhesives cure.
Catalysts steer the balance between the two reactions. Tin and bismuth carboxylates catalyze the NCO-OH (gel) reaction by coordination, tertiary amines catalyze both reactions, and morpholine amines such as 2,2'-dimorpholinodiethyl ether (DMDEE) favor the NCO-water reaction while keeping a one-component adhesive stable in storage.
Chemical Cure and Physical Set in Polyurethane Systems#
Three polyurethane systems set physically before they cure chemically: solvent-borne TPU adhesives, reactive hot melts and waterborne dispersions. A solvent-borne TPU adhesive, based on a high-molecular-weight crystallizing polyester urethane, sets as the solvent leaves and the polymer crystallizes. A PUR hot melt sets as its polyester segments crystallize on cooling, which gives green strength before any chemical cure.
A PUD adhesive film is dried first, then heat-activated so that its crystalline polyester soft segment melts, joined under pressure and left to recrystallize. In the probe tack measurements of Dormish (Bayer MaterialScience, Adhesives & Sealants Industry, 2005-11-01), tack sets in at about 50 °C (122 °F) and peaks at about 60 °C (140 °F), typical product values of the Dispercoll dispersions tested.
Chemical crosslinking follows the physical set in all three systems. Residual hydroxyl groups of the TPU react with the added polyisocyanate, free NCO groups of the PUR hot melt react with moisture to urea, and the water-emulsifiable polyisocyanate in a PUD adhesive crosslinks the film, which adds heat, plasticizer, fat and oil resistance.
What Are Polyurethane Adhesives Made Of?#
Polyurethane adhesives are made of an isocyanate or NCO-terminated prepolymer, polyols and a catalyst, plus optional solvents, plasticizers and fillers: the isocyanate is the reactive binder, the polyol the soft segment, the catalyst the cure-rate control. The table lists each raw-material class, from methylene diphenyl diisocyanate (MDI), polymeric MDI (pMDI), toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) to fillers, with its family page, member substances and one typical value with basis and source.
| Function | Family hub | Substances | Typical value (basis, source) |
|---|---|---|---|
| Isocyanate | isocyanates and polyols | MDI and pMDI, TDI, HDI and aliphatic polyisocyanates, IPDI | NCO 33.6 wt % (pure 4,4'-MDI), 30.7 to 33.0 wt % (pMDI), about 48 wt % (TDI), 49.7 wt % or more (HDI), 37.5 wt % or more (IPDI); typical producer specifications (Covestro 2021 and 2025, BASF 2019) |
| Reactive binder (one-part, PUR) | same hub | polyurethane prepolymers | 2.1 to 24.5 wt % NCO (MDI prepolymers), 3.3 to 6.8 wt % NCO (TDI prepolymers); typical product values, Covestro |
| Polyol | same hub | polyether polyols, polyester polyols | polypropylene glycol (PPG) OH value about 14 to 263 mg KOH/g (Covestro guide 2025); polycaprolactone (PCL) diols 2,000 to 4,000 g/mol in reactive hot melts (Ingevity, ASC 2023-12-27) |
| Physically setting base | base polymers for adhesives | thermoplastic polyurethanes, polyurethane dispersions | anionic PUD at about 50 % solids in a published starting formula (Dormish 2005) |
| Catalyst | functional additives for adhesives | catalysts for polyurethane adhesives | reactive amine 0.005 to 0.1 wt % of formulation (claim range, US6635722B2) |
| Solvent | solvents in adhesives | ethyl acetate, acetone, MEK | viscosity reduction; TDI adducts supplied at 35 to 75 % in ethyl acetate (Covestro guide 2025) |
| Plasticizer | plasticizers for adhesives | none named in the source | low-temperature flexibility; share not stated |
| Filler | fillers for adhesives | calcium carbonate, carbon black | chalk-filled two-part laboratory reference, density 1.46 g/cm³ (Peczek et al., Materials 2025); carbon black share not stated |
Isocyanate Components of Polyurethane Adhesives#
Polyurethane adhesives use four isocyanate families: aromatic methylene diphenyl diisocyanate and toluene diisocyanate, and aliphatic hexamethylene diisocyanate and isophorone diisocyanate. In the Covestro brochure of 2021 and guide of 2025, pure 4,4'-MDI has an NCO equivalent weight of 125 g/eq and a functionality of 2.0, pMDI grades reach a functionality of 2.2 to 3.2 at 25 to 700 mPa·s (cP) at 25 °C (77 °F), and TDI adducts and isocyanurates are supplied at 7.0 to 13.3 wt % NCO. TDI has a much higher vapor pressure than MDI, so adhesives use it mostly as these low-volatility trimethylolpropane (TMP) adducts or isocyanurates rather than as the monomer.
Aliphatic isocyanates give light-stable bonds. Aliphatic NCO groups do not hydrolyze to aromatic amines, so HDI, IPDI and dicyclohexylmethane-4,4'-diisocyanate (H12MDI) are chosen where a bond line must not yellow. Oligomerization to isocyanurates and biurets raises functionality and lowers vapor pressure; HDI enters two-part adhesives as a polyisocyanate crosslinker and waterborne adhesives as a hydrophilic polyisocyanate crosslinker. IPDI carries one primary and one secondary NCO group of different reactivity.
Polyols in Polyurethane Adhesives: Polyether and Polyester#
Polyols form the soft segment of a polyurethane adhesive: polyether polyols give flexibility, and crystalline polyester polyols give rigidity and green strength. Polyether polyols include PPG, glycerol propoxylate (a PPG triol) and polytetramethylene ether glycol (PTMEG), whose low glass transition temperature keeps the cured bond flexible in the cold. The secondary hydroxyl groups of PPG react more slowly than the primary groups of ethylene oxide-tipped grades, and amine-started polyols raise the reactivity of a two-part polyol component.
Polyester polyols include adipate polyester polyols, PCL diols and polycarbonate diols. Their crystallization on cooling gives the green strength of reactive hot melts and heat-activated adhesives, and their ester groups give polar adhesion. Ester groups hydrolyze more readily than ether groups, so a polyester-rich formula carries a higher hydrolysis risk than a polyether-rich one; the reactive hot melt of the architecture table blends both types 1:1.
NCO-Terminated Prepolymers: The Reactive Binder#
An NCO-terminated prepolymer is the reaction product of a polyol with excess diisocyanate, and its free NCO end groups cure later with moisture or with a second component. The prepolymer is the binder of one-part moisture-curing adhesives and PUR hot melts. Covestro's prepolymer brochure lists MDI prepolymers at 220 to 33,000 mPa·s (cP) at 23 °C (73.4 °F) with a functionality of 2.0 to 4.0, and gives residual monomer below 0.5 % for standard grades and below 0.1 % for "ultra" grades (typical product values).
Residual monomer decides three things: the vapor hazard of the adhesive, its SDS classification and whether REACH entry 74 training applies. A prepolymer has no CLP Annex VI entry of its own; its label follows the free diisocyanate it still contains, and the mixture carries the isocyanate statement EUH204.
Catalysts, Solvents and Fillers in Polyurethane Adhesives#
Catalysts, solvents and fillers adjust the cure rate, viscosity and density of a polyurethane adhesive without forming its backbone. The catalysts in use are dibutyltin dilaurate (DBTDL), bismuth carboxylates such as bismuth neodecanoate, 1,4-diazabicyclo[2.2.2]octane (DABCO, also called TEDA) and DMDEE. The claims of US patent 6,635,722 B2 set a reactive amine catalyst at 0.005 to 0.1 wt % of formulation, a range the raw-material table carries with its basis.
Ethyl acetate, acetone and methyl ethyl ketone (MEK) lower viscosity in solvent-borne grades, and plasticizers add low-temperature flexibility; the sources name no share for either. Chalk (calcium carbonate) appears as the filler of a laboratory two-part reference by Peczek and co-workers (Materials, 2025), whose mix ratio, open time and cure schedule sit in the architecture table below.
Polyurethane Adhesive Formulation#
Polyurethane adhesive formulation sets the ratio of isocyanate groups to isocyanate-reactive groups, which the two-part patent EP3020744A1 places at 0.8 to 1.2, with its two components mixed 100:25 by weight in the example. Water in polyols and fillers counts as isocyanate-reactive as well, so moisture carried in by raw materials shifts the ratio and releases CO2. The index arithmetic of metering belongs to two-part polyurethane adhesives, listed in the cure-route table.
Formulation Architecture by Cure Route#
The cure route decides which side carries the NCO groups, as the table shows for five routes from patent claims, producer ranges and laboratory examples.
| Route | Isocyanate side | Polyol or base side | Catalyst | Other |
|---|---|---|---|---|
| One-part moisture cure | MDI or TDI NCO prepolymer (Covestro range in the raw-material table) | polyether and polyester polyols built into the prepolymer | DMDEE or reactive amine (claim range in the raw-material table) | none |
| Two-part | pMDI or MDI prepolymer; HDI isocyanurate for light-stable bonds | polyether/polyester polyol blend, amine-started polyol | tin, bismuth or amine; amount sets pot life | chalk filler, A:B 100:20, open time 70 min, cure 7 days at 23 °C and 50 % RH (laboratory, Peczek et al., Materials 2025) |
| Waterborne PUD | water-emulsifiable polyisocyanate | anionic PUD | not stated | thickener (parts in the starting-formula table) |
| Reactive PU hot melt | 4,4'-MDI at NCO/OH 1.5 (study) | poly(1,4-butanediol adipate) Mw 2,826 and PPG Mw 425, 1:1 | not stated | reacted at 90 °C (194 °F) under nitrogen (Carbonell Blasco et al., Polymers 2022) |
| Solvent-borne TPU | added polyisocyanate hardener | high-molecular-weight crystallizing polyester TPU | not stated | ethyl acetate, acetone or MEK |
Patent claims, producer ranges and laboratory examples; not recommendations.
Read the table from the isocyanate column: in the one-part and hot melt routes all NCO groups sit in a prepolymer made before filling, while in the two-part and PUD routes the isocyanate stays separate until mixing. That split decides storage, because a one-part container holds a moisture-reactive prepolymer and has to stay dry.
Four Published Starting Formulas#
Four published starting formulas, one per cure route, come from a journal, two patents and a trade-journal article, and the table gives each composition with the result its source reports.
| Route | Source | Composition | Reported result |
|---|---|---|---|
| PU prepolymer (reactive hot melt type) | TAPPI Journal 79(11), 1996 | 3,700 MW adipate polyester diol + pure 4,4'-MDI to 2 % NCO (index 1.9), no catalyst | viscosity at 121 °C (249.8 °F): about 4,500 cP with pure MDI, 3,500 cP with 50 % 2,4'-MDI, 6,500 cP with carbodiimide-modified MDI |
| One-part moisture-curing prepolymer | Henkel, US6635722B2 (patent example) | polyether + polyester polyols 1,770 g, vacuum-mixed 30 min at 116 °C (240 °F); MDI 230 g, 30 min at 110 to 121 °C (230 to 250 °F); reactive amine catalyst 0.02 wt % | 50-mil (1.27 mm) film tensile at 4 h: 12.2 MPa (1,767 psi) with the reactive amine against 1.75 MPa (254 psi) for the control |
| Two-part | 3M Innovative Properties, EP3020744A1 (published 2016-05-18) | polyol component C1 : polyisocyanate component C2 = 100:25 by weight (example) | ratio of NCO groups to isocyanate-reactive groups 0.8 to 1.2, preferably 0.9 to 1.1 (patent range) |
| Waterborne PUD | Bayer MaterialScience (Dormish), Adhesives & Sealants Industry, 2005-11-01 | anionic PUD (about 50 % solids) 100 parts; thickener 0 to 2 parts; water-emulsifiable polyisocyanate 3 to 5 parts | no tack at 45 °C (113 °F) for the second dispersion tested (typical product value) |
Grams and parts as published; no wt % is derived from them.
The TAPPI Journal prepolymer of 1996 shows how far isomer choice alone moves viscosity: the polyol, the NCO content and the index stay fixed, and only the MDI type changes. The US6635722B2 example isolates one catalyst at one dosage and measures its effect on early strength four hours after application, on a free film rather than on a bonded joint.
All four are patent and journal examples, not recommendations: a formulator re-derives the ratio from the NCO content and hydroxyl number of the raw materials actually bought.
How Each Component Moves Viscosity, Cure Speed and Flexibility#
Each component moves one property of a polyurethane adhesive in a set direction, as the table summarizes from the sources above.
| Component | Raising its share or switching to it |
|---|---|
| Polyether polyol | more flexibility at low temperature |
| Crystalline polyester polyol | more green strength and rigidity; higher hydrolysis sensitivity |
| 2,4'-rich MDI instead of pure 4,4'-MDI | lower prepolymer viscosity and residual monomer |
| Carbodiimide-modified MDI | higher prepolymer viscosity (TAPPI 1996) |
| Aliphatic isocyanate (HDI, IPDI) | light-stable, non-yellowing bond |
| More catalyst (two-part) | shorter pot life |
| Reactive amine catalyst (one-part) | faster moisture cure |
| Solvent or plasticizer | lower viscosity, low-temperature flexibility |
The table sets out the central trade-off of polyurethane formulation: the polyester share that gives fast handling strength also raises hydrolysis sensitivity, and the catalyst level that speeds cure also shortens the working time of a two-part mix. Isomer choice is the one lever in the table that lowers viscosity without a solvent, as the 2,4'-rich MDI row shows.
Each request is passed straight to independent companies of that trade, and no company pays AdhesiveAdditives.com to be named or to receive requests; the editorial policy explains how.
Properties of Polyurethane Adhesives#
Polyurethane adhesive properties follow from the isocyanate and the polyol: aliphatic diisocyanates give light-stable, non-yellowing bonds, polyether soft segments give flexibility, and crystalline polyester segments give green strength. The values below are patent, journal and producer values with their basis, not class values for all polyurethane adhesives.
Flexibility, Light Stability and Prepolymer Viscosity#
Flexible polyurethane adhesives get their flexibility from polyether soft segments, and light-stable ones get their color stability from aliphatic isocyanates. A flexible polyurethane adhesive keeps its low-temperature flexibility because polyether chains such as PPG and PTMEG stay mobile in the cured network. For light-stable two-part bonds, the Covestro guide of 2025 lists HDI polyisocyanates at 19.5 to 23.5 wt % NCO and 150 to 35,000 mPa·s (cP) at 23 °C (73.4 °F), with an average functionality from about 2.5 to above 5 (typical product values).
Prepolymer viscosity depends on the MDI isomer. In the TAPPI Journal 1996 prepolymer at 2 % NCO, viscosity at 121 °C (249.8 °F) is about 4,500 cP with pure 4,4'-MDI, falls to about 3,500 cP with 50 % 2,4'-MDI and rises to about 6,500 cP with carbodiimide-modified MDI. A lower prepolymer viscosity eases application at the same temperature.
Cure Speed and Moisture Sensitivity#
One-part polyurethane adhesives cure at a rate set by humidity and temperature, while two-part grades cure at a rate set by their reactants and catalyst amount. One-part cure depends on the air and the substrate because ambient or substrate moisture is the reaction partner. CO2 from the water reaction causes bubbles in thick bond lines, and the BASF TDI Handbook (2019) notes that small amounts of water can produce enough CO2 to rupture a sealed container.
Two-part pot life depends on the reactant type and the catalyst amount. The catalyst also moves early strength in one-part systems: in a US6635722B2 example, 0.02 wt % reactive amine raised the 4-hour film tensile strength from 1.75 MPa (254 psi) to 12.2 MPa (1,767 psi).
Which Substrates Do Polyurethane Adhesives Bond?#
Polyurethane adhesives bond seven substrate groups: wood, metals, plastics, rubber, leather, films and foils, and composites such as sheet molding compound (SMC). The seven groups and the routes that bond them are listed below.
- Wood: one-part moisture-curing and two-part grades.
- Metals: two-part grades, and one-part grades over a primer.
- Plastics: two-part grades.
- Rubber: a recorded polyurethane substrate.
- Leather: crosslinked solvent-borne TPU and waterborne PUD bonds.
- Films and foils: two-part laminating adhesives.
- Composites such as SMC: two-part grades.
Polyurethane adhesives do bond rubber. Polyethylene and polypropylene are the exception: as low-energy plastics they resist bonding until their surface energy is raised, a mechanism explained under why low-energy plastics resist bonding.
One-part grades suit wood and other porous substrates because those substrates supply the moisture for cure, and they bond metals over a primer. Polyurethane parts themselves fall under plastics.
Processing Windows of Polyurethane Adhesives#
Polyurethane adhesives are processed in one of four windows: one-part grades cure with ambient moisture, two-part grades must be applied within their pot life, PUD films are heat-activated before joining, and PUR hot melts are applied molten. The PUD window is the one stated most precisely in numbers. In the Dormish study (Adhesives & Sealants Industry, 2005), the dried film is flash-activated by infrared to about 55 to 65 °C (131 to 149 °F), the joint is pressed for 15 s at 1.4 MPa (200 psi) in a polyvinyl chloride (PVC) peel test, and crystallization half-tack times run from 27 to 80 s after activation.
The one-part window starts in the factory: in the US6635722B2 example the polyols are vacuum-mixed and the MDI is reacted for 30 minutes at 110 to 121 °C (230 to 250 °F) before the prepolymer is filled, and after application cure follows the moisture available at the bond line. A two-part window is the pot life of the mixed batch, which the catalyst amount sets, and a PUR hot melt is applied molten and cures with moisture once it has crystallized.
How Are Polyurethane Adhesives Tested?#
Polyurethane adhesives are tested on two levels: the raw materials for NCO content and hydroxyl number, and the bonded joint for lap shear and T-peel strength, with ASTM D2572-19, D5155-25, D4274-23 and D1002-10(2019) the current editions. The table gives each property with its standard, edition and status as listed by ASTM on 2026-11-02.
| Property | Standard and edition | Status | Method page |
|---|---|---|---|
| NCO content of urethane prepolymers | ASTM D2572-19 | active | NCO content testing |
| NCO content of aromatic isocyanates | ASTM D5155-25 | active | NCO content testing (same page) |
| Hydroxyl number of polyols | ASTM D4274-23 | active | NCO content testing (same page) |
| Lap shear strength | ASTM D1002-10(2019) | active | lap shear testing |
| T-peel strength | named here without an edition | not stated | peel testing |
| Structural wood bonds, exterior | ASTM D2559-12a(2024) | active | none |
| Pot life and open time | named here without an edition | not stated | pot life and open time |
NCO content is the incoming check on a prepolymer before use, because the mix ratio and the cure both depend on it; hydroxyl number does the same for the polyol side. A lap shear or peel value is comparable only between data sheets that name the same method and conditions.
Regulation of Polyurethane Adhesives#
Three rule sets govern polyurethane adhesives: REACH Annex XVII entry 74 training for diisocyanates at 0.1 % by weight or more, CLP labeling with EUH204, and food-packaging listings such as 21 CFR 175.105. The table gives the CLP Annex VI line of each main raw material with its index number; raw materials without an entry read "No Annex VI entry", and the product SDS governs the mixture.
| Substance | CAS (EC) | Hazard line |
|---|---|---|
| 4,4'-MDI | 101-68-8 (EC 202-966-0) | Annex VI 615-005-00-9: H351, H332, H335, H373, H315, H319, H334, H317 |
| pMDI | 9016-87-9 (ECHA list number 618-498-9) | No Annex VI entry |
| TDI | 26471-62-5 (EC 247-722-4) | Annex VI 615-006-00-4: H351, H330, H319, H335, H315, H334, H317, H412 |
| HDI | 822-06-0 (EC 212-485-8) | Annex VI 615-011-00-1: H331, H319, H335, H315, H334, H317 |
| IPDI | 4098-71-9 (EC 223-861-6) | Annex VI 615-008-00-5: H331, H335, H315, H319, H334, H317, H411 until 2027-01-31; from 2027-02-01 (ATP23, Regulation (EU) 2025/1222): H330, H314, H318, H334, H317, H411 |
| DBTDL | 77-58-7 (EC 201-039-8) | Annex VI 050-030-00-3: H341, H360FD, H372 |
| Ethyl acetate | 141-78-6 (EC 205-500-4) | Annex VI 607-022-00-5: H225, H319, H336, EUH066 |
| Polyether polyols (PPG), NCO prepolymers | 25322-69-4 (PPG, polymer, no EC number); prepolymers product-specific | No Annex VI entry |
Polyurethane Adhesives Under REACH Entry 74 and CLP#
Polyurethane adhesives containing diisocyanates at 0.1 % by weight or more, individually and in combination, may be used industrially or professionally only after documented training under REACH Annex XVII entry 74 (Regulation (EU) 2020/1149). The training obligation applies since 2023-08-24, suppliers have had to state it on the packaging since 2022-02-24, and the training is renewed at least every five years. The rule is a training requirement, not a ban on diisocyanates.
Residual monomer decides whether a finished adhesive is at or above the threshold, which is why the monomer content of the prepolymer matters for compliance as much as for hygiene. Who has to be trained, by whom and with which records is set out under REACH entry 74 training.
Every mixture containing isocyanates, monomers, oligomers and prepolymers included, carries EUH204 "Contains isocyanates. May produce an allergic reaction." under CLP Annex II section 2.4 (Regulation (EC) No 1272/2008, consolidated text of 2026-07-01). Workplace air is limited by Directive (EU) 2024/869 to 6 µg NCO/m³ as an 8-hour time-weighted average and 12 µg NCO/m³ short-term, with transitional values of 10 and 20 µg NCO/m³ until 2028-12-31 and transposition due by 2026-04-09.
Food-Packaging Polyurethane Adhesives: 21 CFR 175.105 and EU Rules#
In the US, a polyurethane adhesive in food packaging is built from components listed in 21 CFR 175.105 and used within its conditions, and its container label reads "food-packaging adhesive". The adhesive is either separated from the food by a functional barrier or used within two limits: with dry food, in a quantity within good manufacturing practice; with fatty and aqueous food, at no more than the trace amount at seams and at the edge exposure between packaging laminates, with seams and laminates staying firmly bonded (eCFR, read 2026-10-30). The section clears components under conditions of use; it does not approve a finished adhesive.
Paragraph (c)(5) names the polyurethane routes: resins from diisocyanates with listed polyols or polyesters, a route from TDI or H12MDI (CAS 5124-30-1) and a route from TMXDI (CAS 2778-42-9). Toluene 2,4-diisocyanate and toluene 2,6-diisocyanate are listed by name, PPG (minimum molecular weight 150) and PTMEG are listed polyols, and DBTDL and stannous 2-ethylhexanoate are listed only as catalysts for polyurethane resins; bismuth catalysts, DABCO and DMDEE are not named. Listing conditions and migration testing for food-contact adhesives in each jurisdiction are compared under food-contact adhesives.
In the EU, Regulation (EC) No 1935/2004 is the framework for all food-contact materials. For plastic food-contact materials, Regulation (EU) No 10/2011 Annex II, as amended by Regulation (EU) 2020/1245, requires each listed primary aromatic amine (PAA) to be not detectable at a detection limit of 0.002 mg/kg of food or food simulant, with the sum of other PAAs not above 0.01 mg/kg. Aromatic polyurethane laminating adhesives are therefore fully cured before the laminate meets food.
How to Choose a Polyurethane Adhesive#
Choose a polyurethane adhesive by cure route: one-part for porous or air-exposed bond lines, two-part for humidity-independent cure, waterborne PUD for heat-activated bonds and PUR hot melt for immediate green strength. Substrate, light exposure and regulation then narrow the choice within the route: a light-stable bond line points to an aliphatic crosslinker, and a US food-packaging use points to components listed in 21 CFR 175.105.
Load-bearing metal and composite joints belong to structural adhesives, where polyurethane competes with epoxy and acrylic chemistries; joint design for structural polyurethane bonding is covered under structural polyurethane bonding. Where diisocyanates are to be avoided altogether, silyl-modified polymers cure with moisture without an isocyanate: their alkoxysilyl groups hydrolyze and the silanols condense to siloxane links.
The table maps each requirement to a route and the evidence behind it.
| Requirement | Route | Evidence |
|---|---|---|
| Cure independent of humidity | two-part | polyol and isocyanate react at a set ratio; pot life set by catalyst amount |
| One component, porous substrates | one-part moisture cure | cures with ambient or substrate moisture |
| Heat-activated bond (footwear, lamination) | waterborne PUD | dried, heat-activated and pressed (window in the processing section) |
| Handling strength at once | reactive PU hot melt | crystallization first, then moisture cure |
| Light-stable bond line | two-part with HDI or IPDI crosslinker | aliphatic NCO does not hydrolyze to aromatic amines |
| No diisocyanate training | silyl-modified (isocyanate-free) | entry 74 applies only to diisocyanates |
| US food packaging | any route built from 21 CFR 175.105(c)(5) components | conditions of use and label |
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Limits of Polyurethane Adhesives#
Polyurethane adhesives have five limits: CO2 bubbling, off-ratio cure, crystallized isocyanate, primary aromatic amine migration and sensitizer labeling. The five limits and their causes are listed below.
- CO2 bubbling: moisture reacts with NCO and releases CO2, which foams thick bond lines.
- Off-ratio cure: a wrong mix ratio or incomplete mixing leaves a soft, tacky or brittle bond.
- Crystallized isocyanate: TDI 80/20 freezes at 14 °C (57.2 °F) according to the BASF TDI Handbook (2019), 4,4'-MDI melts at 37 °C (99 °F, NIOSH), and frozen TDI can stratify by isomer.
- PAA migration: aromatic laminating adhesives that are not fully cured release primary aromatic amines into food laminates.
- Sensitizer labeling and training: EUH204 on the label and entry 74 training for industrial and professional users.
The first four limits are properties of the isocyanate side, controlled through dry storage, metered and complete mixing, temperature control and full cure; the fifth is a labeling and training duty.
The sensitizer statement also separates polyurethane from epoxy adhesives: isocyanate mixtures carry EUH204, while epoxy mixtures with constituents of molecular weight 700 or less carry EUH205. Strength, flexibility, cure and use of the two chemistries are compared under epoxy vs polyurethane adhesives.
Types of Polyurethane Adhesive: Cure Routes Compared#
Polyurethane adhesives divide by cure route into one-part moisture-curing, two-part and waterborne PUD systems, with reactive PU hot melts as a fourth route, solvent-borne TPU as a physically setting variant and silyl-modified polymers as the isocyanate-free alternative. The table compares the routes by isocyanate side, set and cure, and main substrates or uses.
| Type | Isocyanate side | Set and cure | Substrates or uses |
|---|---|---|---|
| moisture-curing one-part polyurethane adhesives | MDI or TDI NCO prepolymer | ambient or substrate moisture to urea, CO2 released | wood, porous substrates, metals with primer |
| two-part polyurethane adhesives | pMDI, MDI prepolymer, HDI polyisocyanate | polyol and isocyanate mixed at a set NCO index | metals, SMC and composites, plastics, wood |
| waterborne PUD adhesives | water-emulsifiable polyisocyanate, 3 to 5 parts per 100 parts PUD | dry, heat-activate, recrystallize, crosslink | footwear and lamination |
| reactive polyurethane hot melts | MDI prepolymer | crystallization, then moisture cure | bookbinding and assembly |
| Solvent-borne TPU | added polyisocyanate hardener | solvent loss and crystallization, then crosslinking | footwear |
| silyl-modified polymer adhesives (isocyanate-free alternative) | none | moisture hydrolysis and condensation of silyl groups | bonds where diisocyanates are avoided |
The first four rows are the polyurethane routes proper; the silyl-modified row is a different chemistry that takes the place of polyurethane where diisocyanate training is to be avoided.
Polyurethane Adhesives in Industry and Research#
Beyond formulation, polyurethane adhesives raise three practical questions: where industry uses them, what their data sheets state and which bio-based raw materials have reached them. Among the types of adhesives by chemistry, polyurethane is the reactive chemistry whose cure partner can be moisture, a polyol or both.
Where Are Industrial Polyurethane Adhesives Used?#
Industrial polyurethane adhesives are used for glazing, for bookbinding and assembly with PUR hot melts, for battery and laminating bonds with two-part grades, and for crosslinked footwear bonds. Three application pages cover the uses with the most formulation detail.
- Footwear: footwear adhesives based on crosslinked solvent-borne TPU and waterborne PUD.
- Packaging: flexible packaging laminating adhesives for film and foil laminates.
- Wood: wood adhesives in industry, with one-part and two-part polyurethane grades.
Automotive urethane adhesives in this industrial sense are glazing adhesives and two-part battery adhesives.
What Should a Polyurethane Adhesive Data Sheet State?#
A polyurethane adhesive data sheet should state its values as typical product values, each with the test method and temperature behind it. A viscosity without a temperature, or an NCO content without a method, cannot be compared with another supplier's figure.
Compare adhesive manufacturers and suppliers on the same six fields, listed below; how to compare and source them is covered under adhesive manufacturers and suppliers.
- NCO content, with the ASTM method used.
- Viscosity, with the temperature of measurement.
- Mix ratio by weight, for two-part grades.
- Pot life or open time, with the test conditions.
- Cure schedule, with temperature and relative humidity.
- EUH204 statement and the REACH entry 74 training statement.
Bio-Based Polyols and Isocyanates in Polyurethane Adhesives#
Bio-based polyester polyols and a bio-based aliphatic isocyanate are both documented for polyurethane adhesives. In a laboratory PUR hot melt by Carbonell Blasco and co-workers at INESCOP (Polymers, 2022), bio-based polyester polyols replaced 25 to 75 % of the PPG in a poly(1,4-butanediol adipate) and PPG reference adhesive. On the isocyanate side, pentamethylene diisocyanate (PDI) isocyanurate carries about 68 % bio-based carbon (typical product value, Covestro guide 2025).