There are two properties that are predominantly responsible for giving the contact adhesives their outstanding status. They are the only adhesives that deliver quantifiable immediacy in bonding - an equivalent effect can otherwise only be achieved through mechanical bracing. Their range of adhesion is also extremely broad, so they can often be used as problem solvers for difficult bonding jobs. Most products today are still solvent-based, which is no longer in keeping with the times for reasons of environmental and worker health. This article summarises the current state of technology and the corresponding situation with environmental concerns and worker protection.

Applications of contact adhesives

As substrates for contact adhesives, regardless of whether it’s solvent- or water-based, virtually all standard building materials like screeds, levelling compounds, plasters, and installation or metal boards are suitable. The surface in question should be as smooth and flat as possible. Smooth dense substrates like metal boards, old floors, or finishes must be thoroughly sanded and vacuumed. Chalk- or dust-producing surfaces such as calcium sulphate bonded screeds and levelling compounds or chipboard must be primed at the very least, and better still, levelled. For floor coverings or profiles that are adhered around edges or corners, there will be a permanent high tensile stress effect caused by strain. Typical cases include flutes, the outer edges of skirting, or staircases. Contact adhesives can display their particular strengths here.

The immediate bonding strength causes floor coverings and profiles to retain their shape despite the sometimes high tensions from the outset, the high final strength guarantees extremely long-lasting bonding. A prerequisite is obviously subfloor preparation as described in the following, performed by a professional.

Although not necessarily typical, bonding in the area is also done to a considerable extent with contact adhesives when, for example, smaller floor covering areas are intended to be bonded directly adjoining a profile or a stair step, or if resilient floor coverings need to be adhered in lifts. The requirement for extensive bonding of floor coverings can also exist for walls. These are often impact walls in sports facilities, but also floor coverings that are adhered to the wall in schools or hospitals.

Since the load must also be absorbed by the dead weight of the floor covering when wall mounting adhesive and extreme point loads can occur due to “ball impact,” an especially high performance capacity in the adhesive is essential for long-lasting bonding functionality. For a long time, impact wall bonding, to name an example, was a domain of solvent-based contact adhesives due to the extremely high requirements. Now, however, these requirements can also be met by water-based systems - against the backdrop of large-area application and the concomitant significant solvent load in sealed rooms, a hugely welcome development!

Environment and worker protection

Solvent-based contact adhesives are a highly technically useful and, in application, very reliable group of adhesives. Their comparatively high solvent content of about 70-80%, however, holds two considerable risks from a viewpoint considering worker health: For one thing, those processing and using the spaces are impacted negatively in terms of health by inhaling solvents; for another, highly flammable solvent/air mixtures can form and and constitute a high risk of explosion and fire.

A thoroughly realistic scenario that is unfortunately confirmed again and again by corresponding accidents. In accordance with best environmental practice, solvent-based products ultimately might no longer be used at all. The manufacturers of installation materials continue to make major efforts to further reduce emissions from aqueous dispersion-based adhesives. The continued use of solvent-based products is counter-productive to this - it is simply outdated!

The two aspects of the solvent discussion mentioned have surely contributed significantly to the fact that use of solvent-based contact adhesives has already dwindled noticeably in recent years. The corresponding statistics from the industrial adhesives association provide graphic evidence of this.

The biggest proportion of substitutes might account for alternative installation techniques, like using dry adhesives or applying the hot-melt bonding method. Aqueous dispersion-based contact adhesives, due to technical limitations, were previously only acceptable for a small number of installers. After introducing the latest generation of dispersion-based contact adhesives with the performance data described above, however, the argument of limited technical capability might not be effective anymore.

This changed situation has also been acknowledged by the government and taken into consideration in the revised version of the Technical Rules for Hazardous Substances 610, 2011 version. Under point 3.2, it says: "Floor coverings can be installed on all substrates using ... solvent-free dispersion-based adhesives (GISCODE D1), SMP adhesives (GISCODE RS 10) or solvent-free PU adhesives (GISCODE RU 0.5 and RU 1).” This formulation leaves nothing to be desired in terms of clarity. The “loophole” often abused in the past regarding “technical necessity” from the TRGS 610, 1999 version, is no longer there. Every user must be aware that the worker safety regulations no longer allow the use of solvent-based adhesives - with all that this entails!

Operating principles of contact adhesives

Contact adhesives are applied to the parts being bonded on both sides, for example with skirting boards, it is applied on the back side of the board and on the wall area that is being bonded. They then flash off until the solvents or dispersants are completely evaporated. Then the “contact bonding time” begins. This is the time in which the dried adhesive film on the floor covering continues to merge upon contact with the second, also dried film on the substrate. The contact bonding time depends on the product, and generally takes between 2 and 12 hours.

The double-sided application is a disadvantage in comparison to single-sided adhesives (pressure-sensitive or wet adhesives), due to the additional operation. But it provides advantages for the installer too: The floor covering can, for example, be covered and stored temporarily at almost any time. The actual bonding, then, occurs once the coated floor covering has had contact with the coated substrate while aligning and pressing down.

The binding agent in the building contact adhesive is usually polychloroprene, which is also marketed under the names Neopren or Neoprene (R). Aside from that, there are also polyurethane-based products that are practically unknown in the building industry. During bonding, the two dried adhesive areas are pressed against each other with high pressure for a short time. This causes crystallisation to occur inside the adhesive layers and high forces of adhesion combine with the immediate bonding strength described above. This process is known as filming. Afterwards, the adhesive strength continues to grow; final strength is reached after one to three days. The diagram in image 5 illustrates this procedure with reference to the force/path diagram typical for contact adhesives. The volatile increase in force at the beginning of the curve is responsible for the immediate strength of this group of adhesives.

Often the user doesn’t even know that the immediate strength confers a unique position, because other types of adhesive such as single-sided adhesives often have a perceptible initial bond, or tack, too. An indication of this can be seen in the form of threads forming, seen by many installers as evidence of quality. However, the tack can only hold the floor covering on the floor for a limited time if this is under tension. Bonding is not permanent with these adhesives until the adhesive strength has slowly built up during the setting phase. The final strength reached after this ensures that the functionality of the floor covering is guaranteed in spite of occurring stress. Adhesive tapes or dry adhesives, often used in skirting areas as alternatives to contact adhesives, behave in a similar way. As pressure sensitive adhesives, they have a high initial bond. For floor coverings with long-term tensile forces, such as occur in tight radii on stair noising, they can reach their boundaries in function. Since they are completely harmless for environmental and worker health reasons, they should be used where technically possible as alternatives to solvent-based contact adhesives.

Performance capacity of contact adhesives

In the past, solvent-based contact adhesives were superior to the few dispersion products on the market due to the combination of their most important technical properties like immediate bonding, final strength, and plasticiser resistance. This dominance has been overridden with the latest generation of water-based neoprene contact adhesives, as the following comparison of important technical data concerning approved solvent-based products with their water-based successors makes clear. The immediate strength in both product types is comparable. The foundation for assessment in this is the absence of suitable measuring methods, evaluation of the installer when using difficult floor coverings such as stubborn needle punch floor coverings, on tricky substrates like staircases with tight radii.

The final strength of the dispersion-based adhesive, measured as shear resistance under PVC, is about 30% higher than that of the solvent-based adhesive (image 7). With corresponding peel resistance, the water-based adhesive surpasses its predecessor, and by almost twice over.