Curtain walling

Sealants are an essential part of modern construction and have a crucial role to play in the design of weathertight joints. ASTIM defines a sealant in building terms as 'a material which has the adhesive and cohesive properties needed to form a seal'. Joints can fail as a result of poor design, workmanship or the incorrect choice of sealant for the job.

Broadly, there are several basic types of sealant failure:

• adhesion failure;
• cohesion failure;
• delamination failure; and
• reversion.

Adhesion failure is a loss of bond between the sealant and the surface to which it is applied. The failure mode is characterised by a visible separation along the line of the interface - this is termed interfacial adhesion failure. Most sealants require a primer or at least clean surfaces free of contamination by water, frost oil or dust. Omitting the primer and or failing to clean the contact surfaces prior to application may cause adhesion failure.

Cohesion failure is characterised by tears or splits within the body of the sealant, while the bonding faces remain intact. In these cases that the sealant has probably been overstressed either as a result of poor joint design or a loss of flexibility. Joint profile is important - it is generally accepted that for a sealed joint to work it should be bonded on 2 faces only. However, the thickness of the joint is also relevant. A thinner joint will possess more flexibility than a thicker one, but equally will be more vulnerable to the effects of exposure to UV light and the ensuing degradation.

The profile of the sealant joint influences the performance of the sealant. For example, the triangular fillet profiles that are often found around windows perform badly in comparison to rectangular joints, particularly if they are concave. (Defects in buildings, HMSO, London, 1989)

Sealants are either designated by categories of ±12.5%, ±50% or +100%/-50% of initial movement capability. Joint movement on a percentage basis is also affected by the size of the joint. For example, a 6mm movement on a 13mm wide joint will be twice as much on a percentage basis as a 6mm movement on a 25mm wide joint. Therefore, narrower joints are more prone to cohesive failure of the sealant. Many polyurethane sealants will stiffen and show a loss in movement capability over time, making them vulnerable to cohesion failure.

Delamination failure occurs when the sealant overstresses a coating or material to which it is applied. For example, the bond between a mastic seal and a paint film may be perfect, but joint movement produces a stress that is too great for the paint/substrate bond to accommodate. Polyurethane sealants also have the inherent property of increasing 2- to 3-fold in sealant modulus in cold temperatures, making them stiffer and less flexible, thereby increasing joint stress. (EIFS Restoration Guide, Dow Corning Corporation, USA, 2002)

Similar effects can occur when the substrate becomes softened as a result of trapped moisture. Polyurethane backing rods can absorb moisture and hold it in contact with certain vulnerable materials, such as acrylic coatings applied to external rendering systems. Therefore, external acrylic coatings should be stopped short of a joint before the application of sealant. Such failures are termed interphasal failures.

Reversion failures and deterioration

Reversion is a term used to describe deterioration in the elastomeric properties of a sealant. (Some authorities term this as a loss of rheological properties.) Reversion is a difficult subject as there are mixed scientific opinions as to its cause. Essentially, reversion is the conversion of a sealant from a cured to an uncured state, usually initiated by high temperatures, UV exposure and water vapour. Experimentation has revealed conflicting evidence as to the process. The colour of the sealant may also play a part as a result of the energy absorbing capabilities of the different pigments.

Sealant reversion can be tested fairly easily by taking a sample of the material and rolling it into a ball. If the sealant holds its new shape, it has lost its rheological properties and can be termed reverted.

Another form of reversion, again easily identified, is the reversion of silicone sealant back to a powder form. Typically, this can be seen to affect the thin joints in stone cladding and is probably the result of high temperature and moisture.

Sealants such as multi-component polyurethanes are organic materials that degrade in UV light. The degradation results in loss of flexibility and movement capability, ultimately ending in reversion.

Hardening of the sealants over time can cause a greater stress to be exerted at the bonding face with each extension and compression cycle. If the stress exceeds the allowable stress in the background material, localised adhesion failure can occur.