We often fall into thinking – if a little adhesion is good, more is better. This isn’t the case with many applications. Adhesives and sealants are available in a variety of formulations to provide the desired amount of strength when needed, and means for de-bonding when needed. Examples include removable threadlockers and very flexible form in place gaskets which are easily removed without damaging softer metals. In other applications, consider increasing or reducing the bond area to dial in the strength needed.
Most structural adhesives have been developed to form the highest strength bonds possible and to be as resistant to chemicals and heat as possible. Removing the adhesive is quite an arduous task, and parts can become damaged in the process.
When considering bonding with an adhesive, also consider if the assembly will ever need to be disassembled for repair. See some basic tips on adhesive de-bonding here Adhesive removal
In cases where the need to dismantle a bonded joint was unexpected, the head-scratching begins… How do you break the adhesive joint without destroying the parts? This is a tough question which is dependent on a number of factors:
These are available in a range of strengths. The low strength products can easily be undone with tools such as spanners or wrenches. For high strength permanent threadlocking adhesives, using heavy duty tools can result in shearing the bolt you are trying to undo. If you have used a threadlocker on a large diameter bolt and/or it has a long engagement, even if you have used a low strength “dismantleable” product, it may be very difficult to undo due to the large bond area.
Placing a flat-bladed screwdriver or similar implement and hitting it with a hammer in an effort to prize the components apart should be enough to pop the gasket. Anaerobic adhesives are very good in tensile shear or compression but weak under peel or cleavage stress.
Normally, these are low strength products that can be undone with a suitably sized wrench. Like the threadlockers, if used on large diameter or long engagement pipes, these could prove more difficult to undo.
Retaining compounds are actually for the permanent bonding of bearings, housings, shafts, keyways, and other concentric joints. They are typically high-strength and impossible to remove without heat or chemicals.
Heating the bond area with a blow-torch, or placing the item in an oven to heat up will help weaken the adhesive. Attempt adhesive de-bonding while the parts are as hot as possible (once they cool down, the original strength will return!). You will require oven or foundry gloves to hold the parts.
Once components have been successfully dissembled, clean up before re-bonding. A wire brush, wire wool, wet and dry paper are all good for removing cured anaerobic (which often appears as a white-colored powdery solid). Wipe down with acetone.
Stubborn cured lumps will come off after soaking in an aggressive solvent such as acetone or methylene chloride. Parts that refuse to come apart can be soaked in such solvents overnight, and the disassembly process is attempted the next morning. Remember to ensure no solvent residue is present on parts and the solvent bucket has been removed when using the blowtorch…
Always store the solvent in tins with the lid on in a flame proof cabinet.
These recommendations assume all component parts are metal.
Instant Adhesives: Removing cyanoacrylate can be difficult. Becuase they are often used to bond plastics and rubber, which will not withstand high temperatures or aggressive solvents. These adhesives are generally fairly brittle. Therefore, pulling parts apart with a peeling motion will make the bond easier to break. If possible, heat the parts to above 80°C (the point at which most cyanoacrylates lose a lot of strength) and then attempt to pull apart.
If parts are metal and not delicate, you can use more extreme heat or solvent soaking in acetone or methylene chloride.
Soak skin bonded with cyanoacrylate in hot soapy water. Pry stuck fingers apart by rolling a pencil gently between the fingers. Do not use solvent on your hands as it will de-fat the skin. Soapy water is not only good for de-bonding adhesive from skin. If your components can take a nice long bath, they will de-bond over time. To shorten the time, use hot water.
These types of high strength adhesives can be tricky to de-bond. Certain products have both high shear and peel strengths so trying to peel parts apart may not work. Check the maximum operating temperature of the adhesive. Assess if you can heat the component parts above this temperature to attempt disassembly. Most 2-part epoxies, acrylics, and PUs will start to degrade permanently at 200°C. Single part epoxies degrade at higher temperatures. will need to be taken even higher. Use methylene chloride to remove the cured adhesive. However, if you have a large or complex joint, it will only “eat” into the edges very slowly.
Glass
Obviously, glass cannot be peeled, whacked, or levered, of course! Similarly, the heating method of adhesive de-bonding can be problematic with glass. For example, if the substrates are glass to metal, the differential thermal expansion and contraction could cause glass cracking. However, glass to glass, you could heat it to the point the adhesive degrades permanently (>200°C). For removal of bonds on glass and metal, soak in solvent as per other adhesive types.
Plastics
Solvents will attack some plastics, such as polycarbonate or acrylic. Even if you manage to get the components apart, removing cured adhesive will be a problem. Check the water absorption rates with the manufacturer; some products will absorb water. Boiling the parts in water may allow the adhesive to absorb enough water to soften it. Remove the adhesive while it is still wet. The strength will return upon drying.
Professionals know there is a specialist market for adhesives that will need to be de-bonded at some stage, and there are a number of innovative methods:
As long as we don’t end up in cars that fall apart when someone sticks a magnetic sign on the bodywork….!
Until these ideas become working solutions, consider the possibility of removing adhesives prior to selecting a product. Then review the three methods of adhesive de-bonding; Dissolving, Thermal, and Physical stress. Generally, a combination of all three of these with the right amount of time can free the bond.
For further help and advice, please contact Permabond.
