Two-part adhesive that comes unstuck: the problem comes before bonding

“I bonded the bracket with the two-part adhesive and it came away clean.” Clean means that all the adhesive stayed on one side and the other surface is shiny. The adhesive worked, it never bonded: and that depends on what happened before you applied it.

Preparing the surface is half the job

A structural adhesive bonds to whatever it finds. If it finds oil, oxide, dust or a release agent, it sticks to that. The sequence is always the same:

• Degrease with a suitable solvent and a clean cloth.
• Abrade smooth surfaces with an abrasive: the adhesive has more surface to grip.
• Degrease again and remove the dust.
• Do not touch with your fingers: the grease from your skin is enough to ruin the bond.

Where the manufacturer provides for it, a primer makes the difference on metals and plastics.

Epoxy, polyurethane, acrylic

Epoxies are rigid and very strong, excellent on metal, stone and concrete, and also for filling. Polyurethanes are more elastic, suited to when the two materials expand differently. Acrylics and methacrylates harden quickly and are a little more forgiving about the preparation of some plastics and metals. In the collection you will find the Unifix Allbond and Maxbond lines, the Faren two-part adhesives and the Bossong pourable epoxy resins.

Plastics that do not bond like the others

Polyethylene and polypropylene (jerrycans, crates, many pipes) have a surface that almost no adhesive will stick to. You need specific adhesives for PE/PP, such as Maxbond PEPP, and even then the preparation is the one specified by the manufacturer. If you do not know what plastic it is, look at the recycling symbol moulded on the part.

Mixing and timing

The ratio between the two components is what it is, and that is that: adding more hardener does not speed things up, it ruins the adhesive. With double cartridges you use the static mixer and discard the first bead, until the colour is uniform.

Then there are three times to tell apart: the open time within which to join the parts, the time after which they can be handled and that of the final strength. Cold lengthens all of them. Loading the part after it has “gone off” but before the final strength is reached is the most frequent mistake.

A joint done properly

An adhesive layer that is too thin, obtained by clamping the parts until it is all squeezed out, holds less than a regular layer. Structural adhesives work well in shear, when the surfaces slide over one another, and badly in peel, when you try to lift a corner like a plaster. If the part is going to be pulled off like a plaster, it is worth rethinking the bonding or adding a mechanical fixing.

What I need to know

Tell me the two materials to be joined, what stress the joint will have to withstand, what temperature it works at and in what environment, and how much time you have before loading it. I will suggest the right adhesive, the primer if needed and the right gun for the cartridges.

We stock these

A few items from the range we keep. If you cannot find the size or the model you need, just ask: what you see online is only part of what we can source for you.

Two-part adhesive Maxbond Speed Hightec UnifixUnifix Two-part adhesive Maxbond Hightec UnifixUnifix Two-part adhesive FarenFaren Two-part adhesive hightec Maxbond Speed UnifixUnifix Two-part adhesive hightec Maxbond Display 12 pcs UnifixUnifix Two-part adhesive hightec Maxbond Speed Display 12 pcs UnifixUnifix

Browse the whole category: Structural Adhesives