When it comes to PSAs, always follow the flooring manufacturer’s installation guide and the adhesive manufacturer’s technical data sheet and you won’t go far wrong, says Steve Thornton.

PRESSURE-sensitive adhesives (PSAs) have been around for quite some time. They’re non-reactive, acrylic-based adhesives. Once they’re applied, they start to dry – but how do you know exactly what part of the drying process is the right time to fit the flooring you’re intending to install?

The attraction to pressure sensitive adhesives when they entered the market was owing to the perception that you’d be able to spread over very large areas, install the flooring at your own pace and the adhesive will always be receptive. However, this isn’t the case.

While a PSA that’s fully dry will retain a very tacky surface for a long time, most installations require that there’s a transfer of adhesive to the floorcovering that’s being installed. The transfer of the adhesive – meaning the adhesive has covered the floor on which it was applied and the backing of the flooring to be installed – is what provides the ultimate bond strength that the installation will achieve.

First, it’s beneficial to understand exactly how the bonding process takes place. There’s a high volume of a liquid component in acrylic-based flooring adhesives – that liquid being water. Water is the medium that carries all the important bits that make the adhesive work. However, while the adhesive retains water, it is unable to gain its bond strength.

When you apply the adhesive to a subfloor, there’s generally two directions for the moisture to escape. The first is by evaporation – simply put, the moisture disappears into the air. The second is by absorption, it soaks into the subfloor and at the same time it takes the important bits with it – this is what gives the adhesive a ‘grab’ onto the floor.

To achieve a high bond with a floorcovering, contact is needed between the backing of the floorcovering and the adhesive before it is too dry.

Because PSA remains very tacky when it’s fully dry, it can help reduce the effects of late placement – the point at which an adhesive is no longer receptive to the application of a floorcovering.

The tacky surface is so sticky it will actually hold the flooring down quite easily. Because of this, the boundaries and limitations of the adhesive have been pushed, and pushed, then pushed a bit further. Throughout my career as a floorlayer, I’ve seen people spread PSA to very large rooms, then simply go home, come back the next day, and start to fit the flooring! Just to clarify, this isn’t the right thing to do!

So, how do you tell when the correct time has elapsed to place your floorcovering? All adhesives will have a technical data sheet to refer to, and this is obviously the first port of call. This will provide the basic information for airing times. However, these times can vary massively when the adhesive is applied in different conditions or on different substrates.

The airing times on absorbent bases in warm conditions will be much shorter, whereas the airing time in cold, damp conditions on non-absorbent bases could be extended significantly.

In brief, when adhering flooring that has an absorbent backing, only a short airing time is needed, as the backing requires a wet application of adhesive to soak into. Resilient flooring and tiles need the adhesive to start forming a light skin before placement – this shows that enough water has dispersed from the adhesive, yet the adhesive is still liquid enough to achieve a good transfer to the backing of the floorcovering.

When we come to fit moisture sensitive flooring onto truly non-absorbent substrates, such as terrazzo or metal decks, then applying the adhesive and allowing it to fully dry and yet retain good adhesion is a real benefit. Trapping any moisture between resilient flooring and a truly non-absorbent base will, over time, cause problems. Bubbles may appear and the flooring may work its way loose. In the case of metal decks or panels, the presence of the moisture may cause the panels to rust and discolour the material.

Although it’s theoretically possible to allow a PSA to completely dry to a tacky film and then install flooring, the installation environment would have to be maintained at very constant and stable temperatures and conditions. In this scenario, as there’s no transfer of adhesive, there wouldn’t be enough resistance from the adhesive to withstand any movement owing to changes in the temperature or environment that cause the flooring to expand or contract.

It’s true to say that when used correctly, PSA is a very strong and versatile adhesive. PSAs are generally low viscosity adhesives, and this allows them to be utilised in ways that other adhesives can’t.

They’re very easy to apply with rollers, for example. Once applied to the floor, the adhesive can be rolled while it’s still wet; this relieves the surface of any trowel ridges. This is particularly beneficial with very thin floorcoverings as it eliminates the risk of any adhesive ridges shadowing through the material.

Its ability to be applied by roller also has benefits when there’s a requirement to apply the adhesive vertically. Walls can be applied quickly and easily where needed.

For me, the benefits of a pressure sensitive adhesive are its versatility and its ease of use – but this can also be its downside. It can be used for many different things in many different ways, but not always correctly.

Always follow the flooring manufacturer’s installation guide and the adhesive manufacturer’s technical data sheet and you won’t go far wrong.
stephen.thornton@bostik.com
www.bostik-profloor.co.uk
Steve Thornton is technical manager – wall and floor, Bostik