How to prepare a plastic surface for fastening

We have already outlined in this blog how important it is to identify the exact type of plastic material you are working with, so that you can consequently use the best adhesive that particular plastic, knowing that each surface has different characteristics and peculiarities. There are polymers with more or less surface energy, with different treatments having been applied to them, and so on. Once you have chosen the most effective adhesive – be it a high-performance double-sided tape with, a resealable fastening solution, a spray adhesive for plastic or, for example, a hot melt adhesive – before moving on to the actual gluing it is best to make sure you are dealing with a ready and properly treated surface: So let’s have a look at how to prepare a plastic surface for fastening with adhesives, outlining steps such as cleaning, abrasion, surface activation, and using a specific primer.

Why cleaning the plastic surface for fastening is crucial

Cleaning is always essential, because adhesive must have the closest possible contact with the plastic material. The reason is simple: adhesive products, whether they are special glues or high-performance double-sided adhesive tapes, work thanks to the electrostatic forces which operate at the molecular level. So, the proximity between the molecules of the adhesive and the molecules of the plastic surface is fundamental; everything present on the plastic surface, up to the smallest pollutant, becomes a barrier that prevents the fastening of that specific area. In the case, for example, of powder molecules, the latter will be glued to a possible adhesive, rather than the molecules of the plastic surface below.

What products should I use to clean plastic surfaces before using adhesives?

One might believe that typical solvent products, such as ketones and acetones, which, for example, are often used in heavy industry applications, to clean metal surfaces, are suitable for preparing plastic surface for fastening by means of gluing with professional adhesives. In the case of plastics, however, these products must be avoided: quite simply, normal solvents exert their destructive action on polymers as well.

Clean the plastic with isopropyl alcohol

The product suitable for cleaning plastic before gluing it with professional adhesives is isopropyl alcohol, a particular type of less aggressive solvent, which does not threaten the material as classic solvents do. It should be noted, however that, in some situations, isopropyl alcohol may also cause damage. This is the case of plastic surfaces which have been subject traction when the alcohol is applied: isopropyl alcohol could make its wat into very thin stress cracks, compromising the tightness of the material itself. The advice is, therefore, to always avoid applying any stress to the objects to be glued, to dilute isopropyl alcohol, and not to leave the latter on the plastic surface for a period of time longer than what is strictly necessary.

Preparing plastic surfaces for fastening: abrasion

Sometimes, to prepare plastic surfaces for fastening it can be very advantageous to insert an additional step to cleaning alone, i.e., an abrasive pre-treatment. The advantages are many, starting from the fact that, when using the correct professional abrasive (which must have an extremely fine grain, and must be used without putting any pressure on the object), it is possible to increase the roughness of the material, and therefore, microscopically significantly increase the total fastening area available for the adhesive to latch on (sometimes the available surface even doubles). The abrasion must be done with great care, to create a slight surface satin finishing; at this point it will be possible to clean it with isopropyl alcohol, as outlined above, and then dry it with a dry cloth. The surface is now ready to be glued with industrial plastic adhesives.

Preparing plastic surfaces for fastening: activation of the surface

There are several possible activities to activate the plastic surface before gluing it with professional adhesives. Among them are plasma or flame treatment, both processes which guarantee a more powerful surface fastening. The overall result is better than classic abrasion pre-treatment, but it is obviously a more expensive solution, which can be taken into account when the amount of plastic surfaces fastening is high.

Treating plastic surfaces before using adhesives: the primer

Another treatment that in some cases may be suitable or even mandatory is that of the primer, i.e., a substance inserted as an intermediate layer between the plastic surface and the adhesive. Even the primer to be used, it must be noted, has to be chosen carefully: it must be a product which can work well both with the chosen plastic adhesive, and with the plastic surface itself, as well as the particular substrate on which you are working. Examples of primers for fastening plastics are the Universal UV Primer for 3M VHB Tapes, to be used on substrates which tend to be difficult, such as polypropylene, ABS, PET/PBT mixtures; and the Primer 3M VHB 94, which can also be used on polyethylene.