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Sensing the pressure: perusing the plethora of possibilities with PS

Sensing the pressure: perusing the plethora of possibilities with PS
Martin Cummins

Published by Martin Cummins - UK Technical Support Manager, Bostik

Martin has been involved in the industry for over 20 years. He’s passionate about flooring in all its guises, with his particular expertise being in subfloor preparation products and adhesives. Martin is an active member of the CFA manufacturers committee and he sits on the CFA council. He aims to keep abreast of all the changes in the industry and feeds this knowledge back to installers to make sure when floors are laid they perform to their maximum potential.

The terminology ‘PS’ raises its head more within flooring these days. But what is PS, what are the benefits, and are there any pitfalls? I’d like to give my perspective on the topic, from a technical point of view, while also understanding the contractors’ desires and needs. As always, feel free to contact me to challenge or discuss in further detail.

In the flooring world, PS doesn’t stand for Post Script as in ‘PS I love you’. Neither does it stand for PlayStation as in PS4. What it refers to is Pressure Sensitive. Regular readers of my articles will know of my discomfort with acronyms or initials as they can easily lead to nonchalance and misunderstanding. DVD, for example, doesn’t mean Digital Video Disc, AD doesn’t mean After Death, and I’m sure lots of you can come up with other examples.

In the flooring world, PS doesn’t stand for Post Script as in ‘PS I love you’. Neither does it stand for PlayStation as in PS4. What it refers to is Pressure Sensitive.

So, this led me to ask a group what PS was and what it meant. Almost everyone got the Pressure Sensitive aspect of the initials but describing what this referred to was another matter. The answers all came from logical thought processes yet ranged from an adhesive that will suffer if you apply pressure to it (surely, therefore, unsuitable on a floor), an adhesive that will become stronger the more pressure you put on it (is there not a limit?), to an adhesive that won’t work unless you apply pressure all the time. Somewhere amongst this is, I believe, the reality of what PS means in flooring terms, and why these types of adhesives are singled out as different or special compared to general flooring adhesives.

To understand the difference between typical adhesives and PS types we need to understand the principles of adhesion. Adhesion is a characteristic that holds or bonds two materials together. The general aspects are the ability to bond and give a resistance to shear (pulling materials apart in the place that they are bonded: i.e. on a floor, it will be the movement along the floor itself), and a resistance to peel (pulling materials apart at 90° to one another: i.e. pulling the floor covering vertically off the floor).

The general aspects [of adhesion] are the ability to bond and give a resistance to shear, and a resistance to peel.

A high number isn’t always what you will want from an adhesive. For example, a tackifier needs to have very low peel so you can lift the tiles, or you may want an underlay to come up with the carpet rather than tear and remain on the floor. There are standards that floor coverings should be tested to and they must meet particular bond strengths to be classified as permanent bonds. If you attended the virtual CFJ LIVE Expo earlier this year, you might have seen Bostik’s demonstrations of these tests with an explanation of what they mean.

When using a standard wet-stick adhesive, the reliance on adhesion comes from ‘wetting’ out both the floor and the floor covering. The resins and polymers within the adhesive are still in dispersion when the floor covering is rollered down into place and they transfer onto the back of the flooring. As the adhesive dries over hours, days or weeks, this increases the bond strength to achieve the required strengths in shear and peel. Reactive adhesives work in a similar manner but, rather than dry out, they either cure within themselves or utilise atmospheric moisture.

With all the above adhesives you will be recommended a waiting time before laying your flooring. This is to ensure sufficient moisture has evaporated to give the initial grab to hold the flooring. There will also be a maximum working time to ensure you get your floor covering into place, otherwise the adhesive will not offer the required characteristics. Exceed this working time and the adhesive will either be ‘dead’, offering little in the way of adhesion to the floor covering, or it will not meet the minimum bond strength requirements. You have to work within the capabilities of the adhesives.

Exceed [the] working time and the adhesive will either be ‘dead’ … or it will not meet the minimum bond strength requirements.

Some adhesives do have long working times – some even longer than 60 minutes – and will often claim to have PS characteristics. They give a big benefit if you are doing intricate patterns, or if you have very low subfloor absorbency, so can’t risk trapping moisture from the adhesive between the floor covering and the subfloor.

However, PS characteristics and true PS are, to me, very different claims. A true PS adhesive is one that can be left to fully dry and, provided it doesn’t receive dust or any other contaminations, it can be left as a dried film for days or weeks, while still providing the same level of adhesion. It gives a very strong surface tack and, as the name implies, it will enhance the bond as pressure is applied, which – strangely enough – is what happens to floor coverings as they are trafficked. Note that you can also use PS adhesives as wet-stick products, but they don’t generally offer any of the benefits of grab, taking down curl or resisting slippage when used in wet applications.

In very general terms, the materials within adhesives such as fillers and resins don’t retain their tackiness, whereas the polymers do. True PS adhesives usually have little structure to them, which can make them quite ‘runny’ on trowelling out, but a benefit of this is that you can use pre-wetted rollers to flatten out ridges and it increases the drying rate. This is excellent for thin floor coverings where the grin through of trowel marks can be unsightly at the very least.

A tacky dry film enables the contractor to work directly onto a non-absorbent subfloor when applying impervious materials or ones with low porosity. This could be, for example, an application direct to DPMs, directly onto acoustic and thermal underlays, or even onto previous floor coverings. It also allows easy placement of tiled products without the slipping and sliding or the potential of glue oozing between tiles. All these are massive benefits to the contractor. But here’s the rub…

Often, [dry-stick PS adhesives] will not achieve the minimum bond strength that has been quoted as required for a permanent bond adhesive.

Dry-stick application greatly reduces the bond strength, particularly in shear and shear creep between floor coverings and substrates. Often, they will not achieve the minimum bond strength that has been quoted as required for a permanent bond adhesive. In practice, this means that, should the floor covering itself have a tendency to move, then it is unlikely that a dry-stick PS adhesive is going to offer any great resistance. Such situations could include areas subject to significant thermal change where the floor coverings will try to expand and shrink. The more unstable or sensitive the floor covering is then the more likely this is to occur. HT adhesives are still the answer in these applications, particularly with LVT products, which have significant strength under shrinkage and expansion. Also, rubber is such a flexible and strong product (often referred to as a ‘live’ covering) that I would not consider a dry-stick PS for this application either.

There is a place for PS adhesives but, even though they may offer benefits for the contractor, they are not always the best answer technically and many applications will fail if relying purely on PS adhesive bonds. As always, check the datasheets, contact the adhesive and floor covering manufacturer, and ensure before you start that the PS is the answer. There is a plethora of perfectly suitable adhesives in the market, all there for a range of reasons, and there will always be one that is suited to all your needs for any particular application.