Industrial Adhesive Bonding

Adhesives hold parts together in cars, aircraft, appliances and electronics, sometimes alongside fasteners and sometimes in place of them. A good bond can be stronger than the material around it. A bad bond can look exactly the same until it lets go. Nearly everything that decides which one you get happens before the parts come together: how clean the surface is, how well the adhesive was mixed, and whether the joint was held still long enough. Because you usually cannot see a weak bond, the process has to be followed exactly every time.

What You Will Be Able to Do

• Match common industrial adhesives to the kind of job they do.
• Read the terms on a technical data sheet, including pot life and fixture time.
• Prepare a surface so the adhesive can grip.
• Mix a two-part adhesive correctly, and explain what a wrong ratio does.
• Fixture a joint through cure, check the result and work safely with these chemicals.

Why Bond Instead of Fasten

A bolt or rivet puts all of its load on a small spot and needs a hole, which weakens the part. Adhesive spreads the load across the whole joint. It also seals out water, joins different materials such as metal to plastic, and keeps thin parts from being marked by fasteners.

There are tradeoffs. Bonding takes careful preparation and cure time, joints are hard to take apart, and it is difficult to inspect a finished joint. Some adhesives lose strength with heat or weather. That is why engineers select the adhesive and the process, and why you should not substitute.

Common Adhesive Types

Epoxy cure times range from 5 minutes to 24 hours to set, depending on the formula. Full strength typically takes 24 to 72 hours. RTV comes in several temperature ratings, and some types give off acid as they cure, which can corrode electronics. CA is not suitable for joints that flex.

Reading the Data Sheet

Every adhesive has a technical data sheet (TDS). These terms tell you how much time you have.

Check the expiration date before you open anything. Expired adhesive may not cure fully, and you may not find that out until the joint fails. Store it as the sheet says, because some products need a refrigerator and many have to warm up to room temperature before use.

Surface Preparation

Surface preparation is the most important step in bonding. Adhesive has to touch the actual material. Oil, dust, oxide and mold release all get in the way, and the adhesive grips the contamination instead. Even skin oil from a fingerprint can weaken a bond, so wear clean nitrile gloves when you handle prepared surfaces.

1. Remove contamination. Wipe the surfaces with isopropyl alcohol (IPA) or the solvent the adhesive maker recommends. Use a clean, lint-free cloth. Wipe in one direction and turn the cloth to a clean area, so you do not push dirt back and forth.
2. Abrade if the process calls for it. Light sanding, often with 120 to 180 grit, gives the adhesive more surface to grip and removes oxide. Solvent-clean first, so you do not grind oil into the surface.
3. Clean again. Wipe away sanding dust with a fresh cloth and let the solvent flash off.
4. Apply primer if it is specified. Some adhesive systems need a primer to activate the surface. Follow its open time, the window between priming and bonding.
5. Bond promptly. A clean surface does not stay clean. Dust, handling and oxide build up quickly, so bond within the time the process allows.

If you are not sure a metal surface is clean, a water-break test can help. Water sprayed on a clean surface stays in an even sheet. If it beads or breaks up, something oily is still there.

Some plastics are hard to bond. Polyethylene, polypropylene and PTFE have low surface energy, which means adhesives bead up and do not wet them. These usually need a special primer, or flame or plasma treatment. Do not assume a wipe will be enough.

Mixing Two-Part Adhesives

A two-part adhesive is a chemical reaction, and a reaction needs the right amounts. It is not like adding extra sugar. With the wrong ratio, some of the resin or hardener is left over with nothing to react with. That leftover material stays in the joint and leaves it soft, tacky, brittle or with soft spots, and it may never harden fully. Adding more hardener does not speed it up or make it stronger. You usually cannot fix a bad mix. The joint has to be taken apart, cleaned and redone.

Check whether the ratio is by weight or by volume. The two are not the same, because resin and hardener have different densities.

Worked example: a sheet lists 100 to 45 by weight, and you need to use 200 grams of resin.

• Scale factor: 200 / 100 = 2.
• Hardener: 45 x 2 = 90 grams.
• Total mix: 290 grams.

If you are mixing by hand:

1. Check the batch dates, and let the material come to the temperature the sheet asks for.
2. Weigh or measure both parts on a calibrated scale or in marked cups.
3. Mix until the color is even and has no streaks. Keep going for the time the sheet gives.
4. Scrape the sides and bottom of the cup. Unmixed material hides there and turns into soft spots.
5. Note the time. Your pot life starts now.
6. Mix only what you can use within the pot life.

With a dual cartridge and static mixing nozzle, the gun does the measuring. Run out a small bead into waste until the color is even, because the first bit can be off ratio. Replace the nozzle if the adhesive has sat in it long enough to start curing.

Warning: Curing epoxy gives off heat, and a large batch in a cup can heat itself until it smokes, foams or burns you. Mix small batches, and spread out any leftovers in a thin layer to cure. Do not put a hot cup in a trash can.

Applying Adhesive

• Apply the amount and pattern in the instruction. Too little starves the joint. Too much makes a mess and can lengthen the cure.
• For structural bonds, apply to both surfaces unless the data sheet says otherwise.
• Keep within the open time. Do not assemble once the adhesive has started to gel, because it will not wet the other surface.
• Close the joint in one motion, without sliding it around more than the process allows.
• Keep the bond line thickness to spec. Some adhesives include small glass beads to set it. Too thin can starve the joint, and too thick is weaker.
• A small even line of squeeze-out shows the joint is full.

Wipe off excess before it cures, or trim it afterward. Uncured epoxy usually cleans up with acetone or the solvent your process lists. Cured epoxy has to come off mechanically. Acetone is flammable, so use it in small amounts, away from ignition sources.

Fixturing and Cure

Fixturing means holding parts in the right position, with no movement, until the adhesive is strong enough. Movement during cure breaks the adhesive as it forms, and the joint looks fine but is weak.

1. Use the clamps, jigs or tape the process specifies.
2. Apply even pressure, enough to hold the parts together but not so much that you squeeze out all the adhesive.
3. Leave the joint alone until it reaches fixture time, and do not load it until the process allows.
4. Record the time and the batch numbers if your process calls for it.

Temperature drives cure. Warmth speeds it up, and cold slows it down, sometimes a lot. In a cold shop a joint may not reach full strength in the time on the sheet, and below the minimum temperature some products may not cure properly at all. Moisture-cure products need humidity too. Follow the data sheet, and use oven cure only at the listed temperature and time.

Checking the Bond

You cannot see bond strength, so plants control the process and test samples. Common checks are a test coupon made along with the parts and pulled to failure, a check of squeeze-out and voids, and a review of batch and cure records. How a sample breaks tells you a lot.

A clean peel is a warning sign that points back to cleaning or primer.

Working Safely With Adhesives

Read the Safety Data Sheet (SDS) for each product before you use it. It lists the hazards and the right gloves, ventilation and first aid. Follow your plant's rules for storage and disposal.

• Skin: Epoxies and acrylics can sensitize you. That means that after repeated skin contact, you can develop an allergy that lasts, and even small exposures can then set it off. Wear the gloves the SDS calls for. Clean skin with soap and water, not solvent.
• Eyes: Wear safety glasses. Keep an eyewash handy.
• Fumes: Use the ventilation provided, especially with acrylics and solvents. Some urethanes contain isocyanates, which can cause asthma. If a respirator is needed, your plant's respirator program covers that.
• Fire: Many cleaning solvents are flammable. Keep them closed and away from ignition sources.
• Superglue: CA bonds skin in seconds. Do not pull it apart. Soak it in warm soapy water and peel gently, and get medical help for eyes.

Warning: Do not wear cotton or wool gloves with cyanoacrylate. The glue reacts with those fibers and gets hot enough to cause burns. Use nitrile gloves.

Troubleshooting

Common Mistakes

Check Your Understanding

1. A coupon peels cleanly off one aluminum surface. The adhesive was mixed correctly. What do you look at first?

Surface prep. A clean peel means the adhesive never gripped the metal. Check the solvent wipe, the abrasion, handling and how long the surface sat before bonding.

2. A two-part epoxy lists 2:1 by volume. In a hurry, someone mixes equal amounts. What happens?

The mix has too much of one part left with nothing to react with. It may stay soft or tacky, with low strength. It will not fix itself. Take it apart, clean and redo the joint.

3. Your data sheet gives a pot life of 20 minutes. You need 45 minutes to finish a batch of parts. What do you do?

Mix smaller batches, each one you can use within 20 minutes. Do not try to stretch a batch that has started to thicken.

4. A bonded assembly passed fixture time, and the next station wants to load it onto a test rig. What do you check?

Fixture time only means it can be handled. Check whether the process allows loading before full cure, which can take 24 hours or more.

5. You need to bond a polypropylene bracket, and a coworker says a good IPA wipe will do it. Is that right?

No. Polypropylene has low surface energy, so adhesive does not wet it well. It usually needs a special primer or a surface treatment, as the process calls for.

Key Takeaways

• Surface preparation decides most of a bond's strength. Clean with IPA or the listed solvent, and use a lint-free cloth.
• Two-part adhesives need the exact ratio, a full mix and a batch you can use within pot life.
• A wrong ratio leaves a soft, tacky or brittle joint that usually has to be redone.
• Hold the joint still through fixture time, and do not load it until the process allows.
• Warmth speeds cure, and cold slows it.
• Read the SDS, protect your skin, and never use cotton gloves with cyanoacrylate.