A structural adhesive is a reactive thermoset (an epoxy, a toughened methacrylate or a two-part polyurethane) that forms a bond able to bear a structural load. Among industrial adhesive chemicals it is built from four raw-material groups: a resin or monomer, a hardener or initiator, a toughener and a filler.
Epoxies, methacrylates (structural acrylics) and two-part polyurethanes cover the class, and the chemistry table at the end of the main content compares their seven types.
A published one-part structural epoxy cured 28 min at 180 °C (356 °F) reaches 20.6 MPa lap shear on CR340 steel, and 20 wt % of a polyurethane toughener lifts its impact wedge-peel energy from 4.1 J to 25.8 J. Diethylenetriamine needs 10.9 parts per 100 parts of a resin with an epoxide equivalent weight of 189. One aerospace paste is rated to 177 °C (350 °F) with a glass transition at 78 °C. Liquid bisphenol A epoxy resin carries CLP Annex VI entry 603-074-00-8, N,N-dimethyl-p-toluidine has been Carc. 1B since 2025-09-01, and diisocyanates at 0.1 % by weight or more need trained users under REACH Annex XVII entry 74.
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 Structural Adhesive?#
A structural adhesive is an adhesive whose bond bears a structural load, which in industrial practice means a crosslinked epoxy, methacrylate or polyurethane network rather than a physically setting polymer. The producer Permabond gives the same definition in a 2018 article: an adhesive that forms a bond which bears a structural load. The definition names a function, not a strength number.
Grade values show the level such bonds reach. The crash-durable epoxy TEROSON EP 5089 exceeds 20 MPa (2,900 psi) tensile lap shear on 0.75 mm zinc-coated steel by DIN EN 1465 after 25 min at 175 °C (347 °F) (typical product value, Henkel TDS, December 2022). That figure describes one grade, substrate and cure schedule, not a requirement that makes an adhesive structural.
How Structural Adhesives Cure and Carry Load#
Structural adhesives carry load because each one cures by chemical reaction into a crosslinked thermoset network that does not melt again. In epoxies, oxirane rings open in step-growth addition with amines, polyamides, amidoamines, thiols, anhydrides or latent hardeners such as dicyandiamide, or homopolymerize under an ionic catalyst. The cured network carries hydroxyl and ether groups, and those polar groups bond to the oxide surfaces of aluminum and steel.
Methacrylates cure by strongly exothermic free-radical redox polymerization at room temperature. Two-part polyurethanes cure when the polyol meets the polyisocyanate at a set NCO index. Polyaddition and chain polymerization are the chemical-reaction routes described under how adhesives cure.
Toughening completes the load path: a dispersed phase of rubber, core-shell particles or nanoparticles adds peel and impact resistance to the brittle network.
Structural, Semi-Structural and Non-Structural Adhesives#
Structural, semi-structural and non-structural adhesives differ by the load class of the joint, one classification axis next to setting mechanism, polymer nature and delivery form. ASTM covers the axis through two guides, D4800-94(2023) for classifying and specifying adhesives and D6465-99(2024) for selecting aerospace and general-purpose adhesives. Wood adhesive standards carry the same split: EN 204:2016 and EN 12765:2016 classify adhesives for non-structural applications, and ASTM D2559-12a(2024) covers structural wood adhesives for exterior exposure. This page attaches strength values to grades and test methods, never to the class.
What Are Structural Adhesives Made Of?#
Structural adhesives are made of a reactive resin or monomer, a hardener or initiator, a toughener and fillers: epoxy with an amine or latent hardener, methyl methacrylate with a redox initiator, or a polyol with a polyisocyanate. The table below compares the eight raw-material functions, the substances that fill them and the published share of each with its basis.
| Function | Family hub | Substances | Share (basis, source) |
|---|---|---|---|
| Epoxy resin | thermoset adhesive resins | bisphenol A epoxy resin, bisphenol F epoxy resin, epoxy novolac resin, epoxy reactive diluents | resin = 100 parts, the basis of every phr value; diluent at most about 20 % of the resin portion (HELOXY Modifier 8, Westlake TDS, read 2026-11-02) |
| Epoxy hardener | curing agents and hardeners | aliphatic amine hardeners, polyamide hardeners, dicyandiamide, epoxy accelerators | phr = AHEW × 100 / EEW; polyamide 50 to 70 phr with an EEW 190 resin (Versamid 140, BASF TDS May 2014); latent agents 5 to 10 phr (Pasatta, Polymer Innovation Blog, 2022) |
| Methacrylate monomer | monomers for reactive adhesives | methyl methacrylate | Part A shares not stated on this page |
| Redox initiator | functional additives for adhesives | benzoyl peroxide, amine accelerators (DMPT) | peroxide 0.5 to 6 wt % of Part A plus Part B on a 100 % peroxide basis, 2.5 wt % most preferred (US patent 6,433,091 B1) |
| Isocyanate and polyol | isocyanates and polyols | MDI, polyurethane prepolymers, polyether polyols, polyester polyols | pMDI functionality 2.2 to 3.2 (Covestro brochure, 2021); polyol side : isocyanate side 100:25 by weight (EP3020744A1) |
| Toughener | (same additives hub) | epoxy tougheners (CTBN, PTPU, core-shell rubber) | CTBN or PTPU 5 to 30 wt % studied (Chae et al. 2020); core-shell rubber 2 to 6 wt % (Glaskova-Kuzmina et al. 2022) |
| Filler | fillers for adhesives | calcium carbonate | 14 g in a 62.51 g batch, 22 wt % (calculated from Chae et al. 2020) |
| Adhesion promoter | adhesion promoters for adhesives | HEMA phosphate (2-hydroxyethyl methacrylate phosphate) | adhesion gains from 0.5 wt % (Evonik VISIOMER HEMA-P statement, 2025) |
Epoxy Resins and Hardeners in Structural Adhesives#
Liquid bisphenol A epoxy resin is the 100-part base of a structural epoxy, and every hardener and modifier is dosed against it. Bisphenol F resin lowers viscosity, and epoxy novolac co-resins with a functionality of 2.5 to 3.8 raise the glass transition temperature (Tg). Glycidyl ether reactive diluents enter the network too, capped at about 20 % of the resin portion in the HELOXY Modifier 8 data sheet from Westlake.
Room-temperature hardeners are aliphatic amines and polyamides, dosed near stoichiometry. Heat-cure structural epoxies use latent dicyandiamide: unaccelerated in bisphenol A diglycidyl ether, it peaks at about 180 °C (356 °F) by DSC at 20 °C/min, with virtually no cure at 130 °C (266 °F) (US patent 4,459,398 A). A urea accelerator brings the cure down to about 150 °C (302 °F) (Pasatta, Polymer Innovation Blog, 2022).
Methacrylate Monomers and Redox Initiators#
Structural methacrylates are built on methyl methacrylate (MMA) monomer, with chlorosulfonated polyethylene (CSPE) as toughener and sulfonyl chloride redox partner, and methacrylic acid as an adhesion monomer.
Two redox pairs start the cure at room temperature. Dibenzoyl peroxide (BPO) forms radicals with a tertiary aromatic amine such as N,N-dimethyl-p-toluidine (DMPT), and CSPE reacts with a dihydropyridine (DHP) reductant. US patent 6,433,091 B1 supplies benzoyl peroxide as a paste in Part B. Borane initiators extend the chemistry to low-surface-energy plastics (Aronovich and Boinovich, 2021).
Isocyanates and Polyols for Structural Polyurethanes#
Structural two-part polyurethanes pair a polyol component with polymeric methylene diphenyl diisocyanate (pMDI) or an MDI prepolymer, mixed at a set NCO index. The polyol side blends polyether and polyester polyols, an amine-started polyol raises reactivity, and hexamethylene diisocyanate (HDI) isocyanurate serves as a light-stable aliphatic crosslinker.
Catalysts decide the working window: tin, bismuth or amine catalysts set the cure rate, and their type and amount set the pot life of the mixed adhesive (peer-reviewed review, PMC11050924).
Tougheners, Fillers and Adhesion Promoters in Structural Adhesives#
Tougheners in structural epoxies are carboxyl-terminated butadiene-acrylonitrile rubber (CTBN), phenol-terminated polyurethane (PTPU) and core-shell rubber (CSR). Chae et al. (Polymers, 2020) studied CTBN and PTPU at 5 to 30 wt %, and Glaskova-Kuzmina et al. (Polymers, 2022) give 2 to 6 wt % for core-shell rubber, which arrives as a masterbatch in bisphenol A diglycidyl ether (DGEBA) and so adds epoxide groups to the blend.
Calcium carbonate is the filler of the published formula below. In methacrylates, 2-hydroxyethyl methacrylate phosphate (HEMA phosphate) improves adhesion to polar metal surfaces from 0.5 wt %, according to a 2025 product statement by Evonik.
Structural Adhesive Formulation#
Structural adhesive formulation balances one reactive resin against its hardener at a fixed stoichiometry, then adds toughener and filler; the epoxy resin is the 100-part base and the amine hardener follows phr = AHEW × 100 / EEW. AHEW is the amine hydrogen equivalent weight and EEW the epoxide equivalent weight, both in grams per equivalent. Methacrylates split the redox pair across Part A and Part B, and polyurethanes meet at the NCO balance the mix ratio fixes.
Composition by Structural Chemistry#
The table compares six structural chemistries by resin, hardener or initiator, toughener and filler, and mix ratio, each with its published source.
| Chemistry | Resin or monomer | Hardener or initiator | Toughener and filler | Mix ratio | Source |
|---|---|---|---|---|---|
| One-part heat-cure epoxy | DGEBA 48 wt % plus CSR masterbatch 24 wt % | dicyandiamide 5 wt %, urea accelerator 1 wt % | core-shell rubber about 10 wt %; calcium carbonate 22 wt % | one part | Chae et al. 2020 (wt % calculated from the published grams) |
| Two-part epoxy, amine | liquid epoxy EEW 189 = 100 parts | DETA 10.9 phr (stoichiometric, calculated from AHEW 20.6 g/eq) | not stated | per phr | stoichiometry |
| Two-part epoxy, polyamide | liquid epoxy EEW 190 = 100 parts | Versamid 140, 50 to 70 phr | not stated | per phr | BASF TDS May 2014 |
| Two-part aerospace epoxy paste | Part A | Part B | not stated | 100:17 by weight (Part B about 15 wt %, calculated) | LOCTITE EA 9394 AERO, Henkel TDS Rev. 7/2015 |
| Methacrylate (MMA) | MMA Part A | redox activator side | elastomer (CSPE) | 10:1 by volume | LOCTITE AA H8000, H8003 (Henkel TDS Sep 2022, Jan 2020); Plexus MA320 (ITW TDS 10/2017) |
| Two-part polyurethane | polyol side (C1) | polyisocyanate side (C2) | not stated | 100:25 by weight (C2 = 20 wt %, calculated); NCO to reactive groups 0.8 to 1.2 | EP3020744A1 (published 2016-05-18) |
The rows mix a paper formula, data sheet ratios and a patent example, and none is a recommendation. Each row fixes its basis, wt % of the mix, phr of resin or a ratio by weight or volume, and values on different bases are never compared without conversion. No tackifier, wax or oil appears in any row, which separates reactive structural chemistries from physically setting thermoplastic adhesives.
A Published One-Part Structural Epoxy Formula#
One complete structural epoxy formula is published with every part: the crash-type control of Chae et al., Polymers 2020, 12, 1549, from the Korea Institute of Industrial Technology (KITECH).
| Component (grade as published) | Function | Grams (as published) | wt % (calculated, rounded) |
|---|---|---|---|
| DGEBA, YD-128 (EEW 187) | base resin | 30 | 48 |
| Core-shell rubber masterbatch, Kaneka MX-154 (40 wt % CSR in DGEBA, EEW 301) | toughener and resin | 15 | 24 |
| Dicyandiamide, Dyhard 100 (AHEW 21.02) | latent hardener | 3.12 | 5 |
| N,N-dimethyl-N'-phenylurea | accelerator | 0.39 | 1 |
| Calcium carbonate, Omyacarb 10 | filler | 14 | 22 |
| Sum | 62.51 | 100 | |
| Results (paper values) | cure 28 min at 180 °C (356 °F) | lap shear 20.6 MPa, CR340 steel | impact wedge peel 4.1 J (ISO 11343, 2 m/s) |
Core-shell rubber makes up 6 g of the masterbatch, about 10 wt % of the paste, and the total DGEBA from both resin parts is 39 g, about 62 wt % (calculated). The formula also carries 1 wt % dimer acid-modified epoxy whose basis the paper does not state, so it is left out of the wt % column. The dicyandiamide works out at about 7 parts per 100 parts of the epoxy-bearing components (3.12 of 45, calculated), inside the 5 to 10 phr given for latent agents (Pasatta, 2022).
The lap shear result was measured in the ISO 4587:2003 geometry used by the authors. Both results are paper values for one laboratory formula, published as an example and not as a recommendation for a product.
Mix Ratio and Stoichiometry of Two-Part Structural Adhesives#
Two-part structural adhesives are mixed at the ratio their chemistry fixes: amine stoichiometry for epoxies, a weight or volume ratio on the data sheet, and the NCO balance for polyurethanes. Diethylenetriamine (DETA, 103.17 g/mol) carries 5 active hydrogens, so its AHEW is 20.6 g/eq and it needs 10.9 phr with a resin of EEW 189 or 190 (calculated). The aerospace paste LOCTITE EA 9394 AERO is weighed at 100:17 rather than metered by volume, as its Henkel data sheet advises for structural use.
Polyurethanes balance isocyanate groups against isocyanate-reactive groups. The patent EP3020744A1 sets the ratio of NCO groups to isocyanate-reactive groups at 0.8 to 1.2, preferably 0.9 to 1.1, and water in polyols and fillers counts as reactive equivalents in that balance.
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 Structural Adhesives#
Structural adhesives are characterized by shear strength, peel and impact resistance, modulus and elongation, and the temperature at which a strength criterion is still met; each value belongs to a named grade, test method and cure schedule. The two ends of the class differ in stiffness and strain: the crash epoxy TEROSON EP 5089 has a modulus of 1.6 GPa, a tensile strength of 35 MPa and 10 % elongation (Henkel TDS, December 2022), while the methacrylate Plexus MA320 strains 60 to 100 % to failure by ASTM D638 (ITW TDS 10/2017).
How Toughening Raises Peel and Impact Resistance#
Toughening raises peel and impact resistance because a dispersed rubbery phase absorbs fracture energy that the brittle epoxy network cannot. In Chae et al. (Polymers, 2020), adding 20 wt % PTPU to the one-part control raised impact wedge-peel energy about 6.3 times to 25.8 J (calculated) and lap shear on CR340 steel to 32.3 MPa. The best CTBN-modified paste of the series reached 5.8 J, less than a quarter of the PTPU result.
Toughening costs heat resistance: Tg fell by about 4 °C per 5 wt % CTBN and about 8 °C per 5 wt % PTPU in the same study. Crash-durable formulations belong to the toughened epoxy adhesives in the chemistry table below.
Service Temperature of Structural Adhesives#
Service temperature is not the glass transition temperature: a data sheet rating names the temperature at which a strength criterion still holds. The aerospace paste LOCTITE EA 9394 AERO is rated to 177 °C (350 °F), defined in its Henkel data sheet as the temperature that retains 6.9 MPa (1,000 psi) lap shear by ASTM D1002, while its Tg is 78 °C dry and 68 °C wet. Confusing the service temperature and Tg of this paste misplaces its limit by 99 °C.
The table compares the heat data printed on four structural grade data sheets.
| Grade (producer, data sheet date) | Chemistry | Service rating | Tg or hot strength |
|---|---|---|---|
| TEROSON EP 5089 (Henkel, Dec 2022) | one-part toughened epoxy | -40 to 90 °C (-40 to 194 °F) | not compared |
| TEROSON EP 5055 (Henkel, Nov 2024) | two-part epoxy | to 80 °C (176 °F), 120 °C (248 °F) short term | not compared |
| LOCTITE EA 9394 AERO (Henkel, Rev. 7/2015) | two-part epoxy paste | see text | lap shear 11.0 MPa at 149 °C (300.2 °F), PAA 2024-T3 (ASTM D1002) |
| LOCTITE AA H8000 (Henkel, Sep 2022) | methacrylate | not compared | Tg 71 °C (ISO 11359-2, as cited on the data sheet) |
The methacrylate and paste Tg values sit 7 °C apart, yet only the paste carries a rating defined by retained lap shear. Moisture uptake lowers hot-wet properties, and the paste's wet Tg sits 10 °C below its dry Tg.
Which Substrates Do Structural Adhesives Bond?#
Structural adhesives bond five substrate groups: bare and zinc-coated steel, aluminum alloys, fiber composites, titanium, and engineering plastics, the last with methacrylates. The five groups are listed below.
- Steel: bare, hot-dip galvanized (HDG) and electrogalvanized (EGS) body-shop sheet.
- Aluminum alloys: 2024-T3 and 7075 in aerospace.
- Composites: carbon- and glass-fiber-reinforced plastics (CFRP, GFRP), sheet molding compound (SMC) and honeycomb cores.
- Titanium: aerospace joints.
- Engineering plastics: ABS, PVC and polycarbonate with methacrylates; polyolefins only with borane-initiated systems.
Surface condition decides the chemistry. Body-shop epoxies such as TEROSON EP 5089 go onto oily sheet carrying no more than 3 g/m² of oil (Henkel TDS, December 2022). The Henkel and ITW data sheets of LOCTITE AA H8000, H8003 and Plexus MA320 state that these methacrylates need little or no surface preparation on metals and plastics.
Aerospace aluminum gets the most demanding preparation. ASTM D3933-98(2025) is the guide for preparing aluminum surfaces for structural adhesive bonding by phosphoric acid anodizing, and bond primers protect the anodized or sol-gel-treated surface before epoxy film or paste bonding. Corona, plasma, flame, primer and abrasion methods are compared under surface preparation for bonding.
Structural Adhesive Cure Schedules and Working Time#
Structural adhesives cure in three regimes: one-part epoxies in an oven, two-part epoxies at room temperature over hours to days, and methacrylates at room temperature to fixture within minutes. The table lists the published schedules of five grades, each at the temperature its data sheet names.
| Grade (producer, data sheet date) | Chemistry and mix | Application or working time | Cure or fixture |
|---|---|---|---|
| TEROSON EP 5089 (Henkel, Dec 2022) | one-part toughened epoxy | applied at 35 to 55 °C nozzle temperature, 250 Pa·s at 45 °C | 15 min at 175 °C (347 °F), at least 10 min at 150 °C (302 °F) metal temperature |
| TEROSON EP 5055 (Henkel, Nov 2024) | two-part epoxy, 1:1 by volume | 80 min at 23 °C (73.4 °F) | 48 h at 23 °C, 1 h at 60 °C (140 °F) or 30 min at 80 °C (176 °F) |
| LOCTITE EA 9394 AERO (Henkel, Rev. 7/2015) | two-part epoxy paste, 100:17 by weight | pot life 90 min (450 g, 25 °C) | 3 to 5 days at 25 °C (77 °F) or 1 h at 66 °C (150.8 °F) |
| LOCTITE AA H8003 (Henkel, Jan 2020) | methacrylate, 10:1 by volume | 7 min at 23 °C | fixture 5 to 15 min at 23 °C |
| Plexus MA320 (ITW, 10/2017) | methacrylate, 10:1 by volume | 8 to 12 min at 23 °C | fixture 25 to 30 min at 23 °C |
Pot life means little without its mixed mass; ISO 10364:2024 (fourth edition) is the current pot-life method for structural adhesives. Exotherm sets the batch limit: the EA 9394 AERO data sheet forbids mixing more than 450 g, and LOCTITE AA H8000 peaks at 128 °C (262.4 °F) after 29 min in a 10 g mass (Henkel TDS, September 2022). That paste's pot life is measured by ASTM D2471-99, withdrawn in 2008 with no replacement and still cited on data sheets.
How Are Structural Adhesives Tested?#
Structural adhesives are tested for tensile lap shear, peel, impact wedge-peel cleavage, fatigue and durability under humidity and load, and the failure pattern is recorded on each specimen. The table lists each property with its standard, edition and status.
| Property | Standard and edition | Status | Method page |
|---|---|---|---|
| Tensile lap shear, metal to metal | ASTM D1002-10(2019); ISO 4587:2003; EN 1465:2009 | active; current; current | lap shear test |
| Tensile strength, bar and rod specimens | ASTM D2095-96(2023) | active | on the lap shear page |
| T-peel | ASTM D1876-08(2023); ISO 11339:2022 | active; current | peel strength testing |
| Floating roller peel | ASTM D3167-10(2025) | active | on the peel page |
| Impact wedge-peel cleavage | ISO 11343:2019 | current | impact wedge peel test |
| Cleavage fracture strength | ASTM D3433-99(2020) | active | on the fracture page |
| Fatigue in tensile shear | ISO 9664:1993 (EN ISO 9664:2000) | current | no method page |
| Durability under humidity, temperature and load | ISO 14615:1997 | current | adhesive durability testing |
| Failure pattern designation | ISO 10365:2022 (second edition, code letters) | current | adhesive failure modes |
| Pot life, mixed mass stated | ISO 10364:2024 (fourth edition) | current | pot life |
Lap shear is a comparative value, not design data: the failing load over the overlap area is an apparent shear strength, because the single-lap joint also carries peel stress, and ASTM D4896-01(2024) guides the use of such results. The 2022 edition of ISO 10365, which replaced the 1992 edition, added two failure-pattern codes: failure with stress whitening of adhesive (SWCF) and debonding due to bondline corrosion (COR).
Regulation of Structural Adhesives#
Three EU rule sets shape structural adhesives: CLP Annex VI lines and the EUH205 statement for epoxies, the Annex VI lines of methacrylate monomers and initiators, and REACH Annex XVII entry 74 for two-part polyurethanes with diisocyanates. The table gives the hazard line of eight main structural raw materials under CLP (Regulation (EC) No 1272/2008, consolidated text of 2026-07-01).
| Substance | CAS | Hazard line |
|---|---|---|
| Liquid bisphenol A epoxy resin (number average MW 700 or less) | 25068-38-6 (EC 500-033-5) | Annex VI 603-074-00-8: Skin Irrit. 2 H315, Eye Irrit. 2 H319, Skin Sens. 1 H317, Aquatic Chronic 2 H411 |
| Bisphenol A diglycidyl ether (BADGE) | 1675-54-3 (EC 216-823-5) | Annex VI 603-073-00-2: H315, H319, H317 |
| Bisphenol F epoxy resin | 9003-36-5 | No Annex VI entry |
| 4,4'-Methylene diphenyl diisocyanate (4,4'-MDI) | 101-68-8 (EC 202-966-0) | Annex VI 615-005-00-9: Carc. 2 H351, Acute Tox. 4 H332, STOT SE 3 H335, STOT RE 2 H373, Skin Irrit. 2 H315, Eye Irrit. 2 H319, Resp. Sens. 1 H334, Skin Sens. 1 H317 |
| Methyl methacrylate | 80-62-6 (EC 201-297-1) | Annex VI 607-035-00-6: H225, H315, H317, H335 |
| Methacrylic acid | 79-41-4 (EC 201-204-4) | Annex VI 607-088-00-5: H302, H312, H314; STOT SE 3 H335 at C ≥ 1 % (specific concentration limit) |
| Dibenzoyl peroxide (BPO) | 94-36-0 (EC 202-327-6) | Annex VI 617-008-00-0 (ATP22, applicable 2026-05-01): Org. Perox. B H241, Eye Irrit. 2 H319, Skin Sens. 1 H317, Aquatic Acute 1 H400, Aquatic Chronic 1 H410 |
| N,N-Dimethyl-p-toluidine (DMPT) | 99-97-8 (EC 202-805-4) | Annex VI 612-296-00-4 (ATP21, applicable 2025-09-01): Carc. 1B H350, Acute Tox. 3 H301, Acute Tox. 4 H332, STOT RE 2 H373, Aquatic Chronic 3 H412 |
A raw material without an Annex VI entry, such as bisphenol F epoxy resin, is classified by its suppliers, and the safety data sheet of the actual product beats any generic line.
Hazard Labels of Structural Epoxy and Methacrylate Adhesives#
Structural epoxy labels carry skin sensitization from the resin, and only the oligomeric resin adds the aquatic class H411 that BADGE lacks. Mixtures containing epoxy constituents with an average molecular weight of 700 or less also carry the supplemental statement EUH205 (CLP Annex II, section 2.5).
Methacrylate labels add flammability and irritation through MMA. Directive 2009/161/EU sets an EU indicative occupational exposure limit for MMA of 50 ppm over 8 h and 100 ppm short-term. Label wording and the adhesive-specific EUH phrases are covered under hazard labeling for adhesives.
The redox initiators changed class most recently: N,N-dimethyl-p-toluidine became Carc. 1B with ATP21 on 2025-09-01 and has been on the California Proposition 65 list for cancer since 2014-05-02 (OEHHA list of 2026-07-31), and dibenzoyl peroxide gained its ATP22 line on 2026-05-01.
Two-Part Polyurethanes and the Diisocyanate Rule#
Two-part structural polyurethanes take their hazard line from the isocyanate side, where 4,4'-MDI carries Resp. Sens. 1 H334 and polymeric MDI (CAS 9016-87-9) has no EC number and no Annex VI entry. Isocyanate mixtures carry EUH204, "Contains isocyanates. May produce an allergic reaction." (CLP Annex II, section 2.4).
REACH Annex XVII entry 74 (Regulation (EU) 2020/1149) allows industrial or professional use of diisocyanates at 0.1 % by weight or more, individually and in combination, since 2023-08-24 only after documented training, renewed at least every five years. Packaging has carried the training statement since 2022-02-24, and the training obligations for adhesive users are covered under diisocyanates in adhesives.
How to Choose a Structural Adhesive#
Choose a structural adhesive by substrate and surface condition first, then by the cure you can run (oven, room temperature, minutes or days), the service temperature and the impact load. The table matches six requirements to a structural chemistry, with grade evidence from dated data sheets.
| Requirement | Structural chemistry | Evidence: grade (producer, data sheet date) |
|---|---|---|
| Crash and impact loads on oily steel, oven available | one-part toughened epoxy | TEROSON EP 5089 (Henkel, Dec 2022): lap shear above 20 MPa (2,900 psi), impact peel above 25 N/mm (method not stated), 15 min at 175 °C |
| Room-temperature cure for repair or assembly | two-part epoxy | TEROSON EP 5055 (Henkel, Nov 2024): 21 MPa (3,100 psi) on sanded cold-rolled steel by DIN EN 1465 after 7 days at 23 °C, 80 min working time |
| High service temperature, aerospace paste | two-part epoxy paste | LOCTITE EA 9394 AERO (Henkel, Rev. 7/2015): 28.9 MPa (4,200 psi) at 25 °C and 8.3 MPa (1,200 psi) at 177 °C on PAA 2024-T3 (ASTM D1002) |
| Minutes to fixture, little surface preparation | methacrylate (MMA) | LOCTITE AA H8003 (Henkel, Jan 2020): fixture 5 to 15 min, at least 20.5 N/mm² (3,000 psi) on bare 2024-T3 aluminum; LOCTITE AA H8000 (Henkel, Sep 2022): at least 19.31 N/mm² (2,800 psi) on grit-blasted steel after 24 h (product specifications) |
| Elongation and gap filling | methacrylate (MMA) | Plexus MA320 (ITW, 10/2017): strain to failure 60 to 100 % (ASTM D638), lap shear 10.3 to 13.8 MPa at 0.75 mm gap (ASTM D1002) |
| Choosing between epoxy and MMA | both | epoxy vs acrylic structural adhesives |
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.
Oily steel with an e-coat oven points to a one-part toughened epoxy, and a line that cannot heat points to a two-part epoxy or a methacrylate. The evidence column holds values of single grades; the data sheet of the grade finally chosen replaces it.
Limits of Structural Adhesives#
Five limits of structural adhesives are listed below, each with its process or service cause.
- Undercure: thick or shadowed flanges that miss the oven schedule stay below full cure and below their design Tg.
- Wash-off: a one-part body-shop epoxy with too low a viscosity washes out in the pretreatment line before the oven.
- Exotherm: large mixed masses of methacrylates and two-part epoxies overheat, so data sheets cap the batch mass.
- Galvanic corrosion: a conductive adhesive that bridges dissimilar metals lets the less noble metal corrode.
- Hot-wet loss: moisture uptake lowers hot-wet properties, so the wet Tg sits below the dry Tg.
Structural Adhesive Types: Epoxy, Acrylic and Polyurethane Compared#
Structural adhesives come in three chemistries, epoxy, methacrylate (structural acrylic) and two-part polyurethane, and epoxy splits further into one-part heat-cure, two-part room-temperature and toughened grades. The table compares the seven types by cure, mix and a dated grade example.
| Type | Cure | Mix | Grade example (producer, date) |
|---|---|---|---|
| epoxy adhesives | polyaddition with amine, anhydride or latent hardener | one or two parts | see the epoxy rows below |
| one-part heat-cure epoxy adhesives | latent dicyandiamide, oven | one part | TEROSON EP 5089, 15 min at 175 °C (Henkel TDS Dec 2022) |
| two-part epoxy adhesives | amine or polyamide at room temperature | 1:1 by volume (EP 5055); 100:17 by weight (EA 9394 AERO) | TEROSON EP 5055 (Henkel TDS Nov 2024); LOCTITE EA 9394 AERO (Henkel TDS Rev. 7/2015) |
| toughened epoxy adhesives | epoxy cure with a dispersed rubbery phase | one or two parts | Chae et al. 2020 control and PTPU series (paper) |
| structural acrylic and MMA adhesives | radical redox at room temperature | 10:1 by volume | LOCTITE AA H8000, H8003 (Henkel TDS Sep 2022, Jan 2020); Plexus MA320 (ITW TDS 10/2017) |
| polyurethane adhesives | polyol plus polyisocyanate | two parts | no grade value on this page |
| two-part polyurethane adhesives | NCO balance set by the mix | 100:25 by weight in EP3020744A1 | patent example |
Epoxy carries most of the grade evidence, while methacrylates show the fastest fixture and the highest strain to failure. The polyurethane rows carry a patent mix ratio instead of a grade value, so a polyurethane choice rests on the supplier's data sheet.
Structural Adhesives in Industry#
Structural adhesives serve car bodies, aircraft structures and electric-vehicle battery packs, where the chemistry choices above meet production lines. The wider family of types of adhesives by chemistry places these reactive grades next to hot melt, pressure-sensitive and water-based adhesives.
Where Are Structural Adhesives Used?#
Structural epoxy and its sister chemistries are used in three industries, listed below.
- Automotive: body-shop epoxies and panel-repair epoxies, covered under automotive adhesives.
- Aerospace: pre-catalyzed epoxy film adhesives cured at 121 °C (250 °F) or 177 °C (350 °F), for example 90 min at 121 °C for EA 9628 (Henkel aerospace guide, 2024), covered under aerospace and composite adhesives.
- Electric vehicles: structural and thermally conductive two-part polyurethane, epoxy or acrylic adhesives bonding cells to cooling plates and housings, covered under EV battery adhesives.
Film adhesives are never two-part products, and battery silicone gap fillers form no structural bond.
Structural Bonding in Place of Welds and Rivets#
Structural bonding enters car-body joints because a one-part epoxy fits the existing line: it goes onto oily steel, survives the pretreatment washers and cures in the e-coat oven with the body. TEROSON EP 5089 showed no loss of adhesion and no corrosion after 1,000 h of salt spray at 35 °C (95 °F) in 5 % NaCl (Henkel TDS, December 2022). A strength comparison between bonded, welded and riveted joints needs joint-level test data and is not drawn on this page.
What Should a Structural Adhesive Data Sheet State?#
A structural adhesive data sheet should state each value as a typical product value or a specification, with substrate, surface preparation, cure schedule and test method.
Comparing suppliers works only on identical fields, and the comparison of adhesive manufacturers and suppliers starts from the six fields listed below.
- Lap shear with substrate, preparation, method and cure.
- Peel and impact values with their methods.
- Tg with its method.
- Service temperature with its definition.
- Mix ratio, and pot life with the mixed mass.
- Hazard lines and statements such as EUH205 or EUH204.