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What's the difference between ... S1 and S2?
by David Wilson, Head of Technical Services , Building Adhesives Limited

David Rowley

With more than 40,000 calls annually to its free technical advice line, BAL's experts hear questions on every aspect of tiling. One current ‘hot topic' concerns the difference between S1 and S2 adhesive classifications. The question is usually a variation upon, "Is an S1 adhesive better than an S2 adhesive?"

The degree of flexibility in an adhesive matters when there is likely to be lateral movement from pulling (tension), pushing (compression), bending (flexing) or twisting (torsion) during the tiling's lifetime. This can happen to some degree almost anywhere. There are many possible causes of differing movements including vibrations, drying shrinkage, ambient humidity or temperature fluctuations, the amount of static and dynamic loading on the floor and deflection stresses. The most common substrate providing movement in tiling though are timber floors.

S1 and S2 classifications are optional, additional classifications for cementitious adhesives defined in BS EN 12002:2002. These classifications basically relate to how an adhesive will deform in order to accommodate a limited amount of movement after tile fixing (known as transverse deformation).

An adhesive allowing transverse deformation of less than 2.5 mm in the specified test is not regarded as deformable. Those permitting deformation above 2.5mm, but less than 5mm are classified as S1. Adhesives that are highly deformable and allow transverse deformation of 5mm or more are classified as S2. It is possible to modify cementitious adhesives that are not S1 classified with a suitable liquid or powder polymer additive to improve the formulation to that of an S1 adhesive, but achieving S2 typically requires a higher level of polymer addition i.e. a separate liquid polymer component and/or a modified powder e.g. with rubber.

It is worth remembering that there are two types of deformability. Currently the transverse deformation tests do not measure flexural ability; therefore not taking into account products with "elastomeric properties". An adhesive offering ‘plastic' deformation, such as S1 or S2 adhesives will allow movement, but will not return to its original state. Elastomeric adhesives have rubber-like elasticity and therefore offer ‘elastomeric' deformation. Elastomeric adhesives therefore are so flexible they will allow movement and return to its original "neutral state" when there is relaxation of the movement stresses; therefore offering much greater deformation. As not all deformable adhesives are elastomeric, this is worth checking.

With that in mind, a single-part, highly polymer-modified cementitious adhesive is generally capable of attaining an S1 classification. It will undergo plastic deformation in its early life but fully cured it has increased adhesion and compressive strengths. This makes it suitable for solid structures such as in-situ concrete, concrete blockwork, etc, where initial setting movement and drying shrinkage are the main sources of stress. An S2 adhesive will maintain some deformation properties when fully cured and therefore be more suitable for installations subject to frequent or permanent vibration such as timber. Where there are areas of ongoing movement and deformation is likely to be quite large, products that contain elastomeric fillers and are modified with acrylic polymer admixture, such as BAL Fastflex, should be used as they are highly deformable, elastomeric and exceed the S2 requirement by a factor of three.

So, is an S1 adhesive ‘better' than an S2 adhesive? BAL has manufactured S1 and S2 adhesives for some years, as well as products which greatly exceed the minimum S2 requirements. This is because varying deformability and flexural strengths are needed for differing tiling applications. Not all S2 adhesives are the same. For timber floors and installations subject to frequent or permanent vibration the adhesive needs to be more deformable and ultimately and logically an S2 adhesive with elastomeric properties would then be the ‘better' choice.

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