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.