Aerospace Structural Adhesives: Film, Paste and Specs
Aerospace structural adhesives are film or paste epoxy systems bought as a qualified system of adhesive plus primer, and the bond is only as good as the surface prep and cure. MMM-A-132B is the federal specification for metal to metal airframe adhesives, SAE AMS-A-25463 replaced the military film spec for metal sandwich, and ASTM D3933 covers phosphoric acid anodize prep.
Updated
Film and paste adhesives and where they are used
Film adhesive is the default for large, high quality bonds cured under heat and pressure. Paste adhesive is the default for repairs, local bonds and parts that cannot go in an autoclave or press. MMM-A-132B recognizes both as forms of the same adhesive class: Form F is film and Form P is paste.
| Form | What it is | Where it is used | Watch for |
|---|---|---|---|
| Film, supported | Adhesive on a carrier cloth, sold by weight per square foot | Skin to doubler, skin to stringer and metal to honeycomb bonds | Freezer storage, out-time at room temperature, cure pressure |
| Film, unsupported | Adhesive film with no carrier | Bonds where bondline thickness and weight are tightly controlled | Handling damage, thickness control without a carrier |
| Paste, two-part | Base and hardener mixed at a fixed ratio | Repairs, inserts, local fittings, room temperature or low temperature cure | Mix ratio, pot life, bondline control |
| Paste, one-part | Premixed, heat cured | Production bonds where mixing on the floor is not wanted | Cold storage and thaw before use |
A film's thickness in mils and weight in pounds per square foot to the nearest 0.001 pound must be stated under MMM-A-132B, and films of one composition made in more than one weight range are tested separately. Buy the weight the process spec calls for, not the nearest one available.
The specifications that govern structural adhesives
Older drawings call out federal or military adhesive specs. Newer ones usually call out a prime's own material specification that names approved products. Both are valid; the PO has to match the drawing.
| Spec | What it covers | Status as checked |
|---|---|---|
| MMM-A-132B | Heat resistant adhesives for bonding primary and secondary structural and external metallic airframe parts exposed to -67 to 500 °F | Dated 1 April 1994. Inactivated for new design by Notice 1 (19 April 2001), then reactivated for new or existing design by Notice 2 (14 February 2005). Shown as active. |
| MIL-A-25463B | Film adhesive for bonding metal facings to metal cores and components of sandwich panels, up to 500 °F | Cancelled, superseded by SAE AMS-A-25463 |
| ASTM D3933 | Standard practice for preparation of aluminum surfaces for structural adhesive bonding by phosphoric acid anodizing | ASTM standard, first published in 1980 according to MIL-HDBK-691B |
| ASTM D1002 | Lap shear strength of single lap joint, metal to metal adhesive specimens in tension | ASTM standard |
| ASTM D3165 | Strength properties of adhesives in shear by tension loading of laminated assemblies | ASTM standard |
| MIL-HDBK-691B | Military handbook on adhesive bonding, dated 12 March 1987 | Guidance only, useful for background on prep and test methods |
We could not open the current SAE AMS adhesive specs or ASTM texts directly, so the table gives their scope and titles from the sources listed, not their revision letters. Put the revision your drawing cites on the PO.
Have a part like this?Send it in under a minute. A person replies within 1 business day.How an adhesive callout is built: MMM-A-132B decoded
MMM-A-132B classifies every adhesive by type, class, form and cure group, and a full callout names all four. The classification comes straight from paragraph 1.2 of the specification.
- Type sets the service temperature. Type I is for long time exposure from -67 to 180 °F. Type II is for long time exposure from -67 to 300 °F. Type III adds short time exposure from 300 to 500 °F. Type IV is for long time exposure from -67 to 500 °F.
- Class applies to Type I only and sets toughness screening. Class 1 is high T-peel and blister detection, Class 2 normal T-peel and blister detection, Class 3 no T-peel and no blister detection.
- Form is F for film or P for paste.
- Cure group sets the cure temperature. Group 1 cures at 100 °F or less, Group 2 over 100 to 200 °F, Group 3 over 200 to 300 °F, Group 4 over 300 °F.
So a callout of MMM-A-132, Type I, Class 2, Form F, Group 3 asks for a film adhesive rated to 180 °F long term, tested for normal T-peel and blister detection, that cures between 200 and 300 °F. MIL-HDBK-691B shows a sandwich material spec calling for resin conforming to MMM-A-132, Type 1, Class 2, which is the same pattern in use.
Surface preparation and primers
Most bond failures are surface failures. MIL-HDBK-691B states that chemical treatments are the most effective preparation of aluminum for adhesion where long term environmental exposure is required.
- FPL etch. The sulfuric acid and sodium dichromate etch developed at the Forest Products Laboratory is described in the handbook as proven, controllable and reliable for structural bonding of aluminum.
- P2 etch. The handbook reports that P2 etch gives bonds at least equal to FPL and is acceptable as a deoxidizer before phosphoric acid anodizing.
- Phosphoric acid anodize (PAA). Described in the handbook as one of the better methods for long term durability under adverse conditions, with the procedure given in ASTM D3933.
- Non-chromate treatments. Programs replacing chromate processes use other preparations, including sol-gel treatments, under their own process specifications. We could not confirm a single public governing spec, so state the process specification your design authority approved.
The primer is part of the adhesive system. MMM-A-132B says primers may be furnished with the adhesive, the manufacturer must specify the primer, the primer is identified on the qualified products list with the adhesive, and approval applies only to the formulation that was qualified. Substituting a different primer means you are no longer using the qualified system. Surface finish details for parts that will be bonded are on surface finishing.
Cure cycles
A structural adhesive is only qualified for the cure the manufacturer documented. MMM-A-132B requires the manufacturer to give the complete surface prep procedure, mixing instructions, application procedure, maximum allowable open and closed assembly time with temperature and humidity controls, and the time, temperature and pressure for each segment of the cure cycle with maximum and minimum limits. Ask for that data with the material, and make sure the bonding shop's process spec sits inside those limits.
Two practical points. First, the cure group on the PO has to match the equipment: a Group 4 film needs an oven, press or autoclave capable of more than 300 °F with pressure, and a Group 1 paste does not. Second, thermocouple records of the actual part, not just the oven setpoint, are what show the bondline saw the specified cycle. Bonded composite work is covered in more depth on composites fabrication.
Freezer storage and out-time
Film and one-part paste adhesives are stored cold, and the clock starts at manufacture. MMM-A-132B requires exterior containers to be marked with the date of manufacture by month and year, not by code, an expiration date, and a storage note telling the receiver the maximum storage temperature, and it states that film adhesives shall be expedited to cold storage. The manufacturer must also state its recommended storage life. MIL-STD-129 adds that for selected Federal Supply Class 8040 adhesive items, shelf life marking may run from the date of shipment or the date of manufacture, as applicable.
Out-time is the accumulated time the material spends above its storage temperature, including thawing, kitting and lay-up. Each manufacturer publishes its own out-time limit, and bonding shops log every withdrawal against the roll or kit. Two habits prevent most problems: let a sealed roll reach room temperature before opening it so moisture does not condense on the film, and record out-time on the roll itself rather than in a separate log that can drift. How the shelf life clock and dating work across all chemicals is on shelf life control.
Lap shear testing
Lap shear is the standard acceptance test for metal bonding because it is simple and sensitive to surface prep. ASTM D1002 covers single lap joint metal to metal specimens in tension, and ASTM D3165 covers shear by tension loading of laminated assemblies, which better represents thick adherends.
MMM-A-132B shows what a disciplined test looks like. Specimens are gripped so the load line passes through the center of the bond, load is applied at 1,200 to 1,400 psi per minute, six specimens are tested, and the average and minimum must meet the table value. Each result records the failure mode, whether cohesive failure within the adhesive, adhesion failure where adhesive peels from the metal, or contact failure from incomplete contact, and the adhesive thickness measured to 0.0001 in. Elevated and low temperature tests repeat this at temperature, down to -67 °F.
For production, ask the bonding shop to process lap shear coupons with each batch of parts, prepared and cured with the parts. A failure mode of adhesion failure on a coupon points to surface prep, even if the strength passes.
Ordering checklist for structural adhesives
- Specification and revision, with type, class, form and cure group, or the prime material spec and approved product.
- The qualified primer that goes with the adhesive, ordered as a matched system.
- Film weight and thickness, roll width and length, or kit size for paste.
- Manufacturer, batch or roll number, date of manufacture and expiration date on every unit.
- Minimum remaining shelf life at receipt.
- Shipping condition: frozen or refrigerated with dry ice or coolant and a temperature indicator.
- Certificate of conformance and, where your program requires it, the manufacturer's batch test report. See certificates of conformance and material certs and traceability.
- Any prime process spec for surface prep and cure that the bonding shop must follow.
Counterfeit and quality risks, and how to receive adhesive
The realistic risks are expired material, rolls that sat warm in transit, broken system pairing of adhesive and primer, and partial rolls of unknown history. Expired film may still look and feel normal.
- Reject any shipment of cold stored adhesive that arrives without coolant or with a tripped temperature indicator until the manufacturer or your engineering clears it.
- Match batch and roll numbers on the label to the certificate.
- Do not accept rewound or split rolls unless the out-time history comes with them.
- Confirm the primer batch and adhesive batch are both listed on the paperwork.
- Put the material straight into cold storage, and log it in before anyone opens it.
See receiving inspection and counterfeit parts avoidance for the wider controls.
How we source structural adhesives
We do not stock adhesive or publish a catalog. We source the qualified adhesive and its primer through authorized distributors or the manufacturer, flow your spec, revision, form, cure group and shelf life requirements down on the order, specify the cold chain, and check the batch, dates and certificate before shipment. We can combine adhesive with the bonded details, the machined fittings and the fasteners on one PO, or buy the whole bonded assembly from a qualified bonding shop. Discontinued legacy products go through our obsolete parts process, and any substitute goes to your engineering for approval.
Related pages: aerospace sealants, greases and lubricants, and the chemicals category.
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Questions
Is MMM-A-132 still an active specification?
Yes, as of our check. MMM-A-132B is dated 1 April 1994. Notice 1 in April 2001 made it inactive for new design, but Notice 2 on 14 February 2005 reactivated it for new or existing design acquisition, and EverySpec shows it as active. Many newer programs call out a prime material specification instead, so follow whatever your drawing invokes.
What do the type, class and cure group in MMM-A-132 mean?
Type sets service temperature: Type I to 180 °F, Type II to 300 °F, Type III to 300 °F with short exposures to 500 °F, and Type IV to 500 °F long term. Class applies to Type I and sets T-peel and blister screening. Form is F for film or P for paste. Cure group sets cure temperature, from Group 1 at 100 °F or less to Group 4 above 300 °F.
Why does the primer have to match the adhesive?
Under MMM-A-132B the primer is identified on the qualified products list together with the adhesive and becomes an integral part of that adhesive system, and approval applies only to the formulation that was qualified. Using a different primer means the bond is no longer the qualified system, and its strength and durability data may not apply. Order the adhesive and primer as a pair.
What is phosphoric acid anodizing for bonding?
Phosphoric acid anodizing, or PAA, is an electrolytic surface treatment for aluminum before structural bonding. MIL-HDBK-691B describes it as one of the better methods for long term durability under adverse conditions, and the procedure is given in ASTM D3933. It is usually preceded by a deoxidizing etch. The bonding shop's process spec controls the details, so name it on the order.
What is adhesive out-time?
Out-time is the total time a cold stored adhesive spends above its storage temperature, from thawing through kitting and lay-up. Each manufacturer publishes a limit, and exceeding it can reduce flow and bond strength. Shops log every withdrawal against the roll or kit. When buying partial rolls or kits, require the out-time history with the material or do not accept it.
Which lap shear test should I ask for?
ASTM D1002 is the common single lap test for metal to metal bonds, and ASTM D3165 covers laminated assemblies loaded in shear. Ask the bonding shop to process coupons with each batch of parts, cured with the parts, and to report strength and failure mode. Adhesion failure, where adhesive peels cleanly off the metal, usually points to a surface preparation problem.
Related
Sources
- MMM-A-132B, Adhesives, Heat Resistant, Airframe Structural, Metal to Metal (EverySpec)
- MMM-A-132B Notice 1, inactivation for new design, 2001 (EverySpec)
- MMM-A-132B Notice 2, reactivation, 2005 (EverySpec)
- MIL-A-25463B, Adhesive, Film Form, Metallic Structural Sandwich, superseded by SAE AMS-A-25463 (EverySpec)
- MIL-HDBK-691B, Adhesive Bonding (EverySpec)
- MIL-A-25463B Notice 2 (EverySpec)
- MIL-STD-129R Change 2, Military Marking for Shipment and Storage (EverySpec)
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