Gulf Construction Online - Correct sealants crucial for perfect facades

Facades

Correct sealants crucial for perfect facades

STUART WAKEHAM, sales director of Tremco illbruck*, Middle East and Africa region, discusses the importance of choosing the correct sealant material, particularly in building complex facades.

01 December 2013

TODAY’S facades incorporate a wide range of materials, from glass and aluminium to natural stone and concrete to exterior insulation and finishing systems (EIFS), glass-reinforced concrete (GRC), glass-reinforced plastic (GRP) and even coated fabrics – clearly all very different materials with different characteristics. Some are porous, others non-porous, all will absorb heat to varying degrees and all have different coefficients of linear expansion.

However, the success of a façade as the key part of the building envelope relies heavily upon effective sealing between the various components. Therefore, the use of the correct type of sealant material is critical. In making this selection, a number of questions need to be answered such as:

• What is the joint width?
• What is the anticipated joint movement?
• What are the substrates?
• Is the joint internal or external, and if external is it exposed to ultraviolet (UV) rays?
• What is the anticipated service temperature?
• Does the joint need to be breathable?
• Does it have to be fire rated?
• Are acoustics a consideration?
• Is colour or appearance important?
• Will the joint sealant be in contact with other sealant/gasket materials?

Only once all of these questions have been addressed can the appropriate material be selected – but what are the choices?

Putting chemistry to one side for a moment, there are broadly speaking three commonly used types of sealing material – wet sealants, dry sealants and membranes.

Wet sealants

These are what we typically think of as “sealants”. They include silicones, polyurethanes, acrylics, butyls and now also hybrids.

Silicones are non-organic and have great UV resistance and exhibit tremendous adhesion to non-porous surfaces. They have extremely high working temperatures and so are ideal for weather sealing between glass and aluminium, and also when used either as the structural bond and secondary seal in a structurally glazed curtain-wall. However, silicones don’t bond particularly well with porous substrates; primers are often required and they can stain sensitive substrates such as natural stone. Also, silicone doesn’t have great resistance to moisture vapour transmission.

Polyurethanes have been around for some time now. They are not quite as good as a silicone in terms of UV resistance or high service temperatures but do bond well with porous substrates and do not stain, making them suitable for stone and concrete facades. They are also the best performing sealants in wide joints (100 mm-plus).

Acrylics can be solvented but are now nearly all water based. They are not ideal for external use and are mainly used internally as gap fillers, suitable for joints up to 20 mm wide. They are fast curing, paintable and possess excellent acoustic properties due to their relative density.

Butyls are more commonly used in tape form but available as a wet sealant. They are ideal as a bedding material or for sealing metal to metal butt joints as they have tremendous resistance to moisture vapour transmission (MVT). They are also very dense and are ideal acoustic seals. Butyls are available in both “setting” and “non-setting” formulation.

Hybrids are the latest technology driven by the need for greener products, particularly in Europe. This has led to the production of some remarkably high-performance materials that exhibit good adhesion to all substrates and will not stain sensitive stones or plastics. They are based on polyether polymers, as used in polyurethane sealants, but terminated with silane polymers instead of the non-environment-friendly isocyanates. These compounds can be engineered to vary from extremely low modulus, having very high movement capabilities, to very high modulus, making them exceptionally strong adhesives.

Dry sealants

This category of sealants are often described as tapes but the fact they are “pre-formed” generally means that they are cleaner and quicker to apply than wet sealants and no cure-time is involved. The majority of tapes used in facades are based on butyl and form very effective air and moisture seals. They are mainly applied under compression (that is, behind a pressure plate or similar), some containing a hard core or shim which controls the degree by which the tape can be compressed.

The other type of dry sealant, which is widely used in Europe, is the pre-compressed polyurethane (PU) foam tape. This tape is supplied in a highly compressed condition and is bonded to one side of the joint. Within 30 minutes it expands to fill the joint, though it remains under 60 per cent compression, meaning that it still has a huge capacity to accommodate the movement of the joint. It is totally rain and dust proof, fire rated, acoustically rated and cannot fail adhesively, or cohesively. It is clean and quick to apply with little, if any, wastage. It is environment-friendly and will not stain the substrate. The other major difference it has over a wet sealant is that it is breathable, thereby negating condensation risk.

Membranes

This final category deals with methods of closing large gaps, generally between the curtain-wall or façade frame and the structure. Commonly used air-seals include EPDM (ethylene propylene diene monomer) membranes, which are strong and flexible and highly effective, but it is imperative that the correct adhesive be used to bond the membrane to the substrate. Butyl membranes are also used particularly as a vapour check. Finally, where very wide, very high movement joints are concerned, a silicone membrane is also used as a “bridge” or “bandage” joint seal. This is normally on aluminium substrates and the silicone sheet is easily bonded in the appropriate silicone sealant.

Conclusion

The number of materials and methods available for sealing facades joints has evolved with the complexity of façade design. When it comes to sealants, one size certainly does not fit all. Using the wrong sealant can lead to compatibility issues, poor adhesion, and a premature failure of the seal which lead to air and water ingress and a whole host of related problems. The relative cost of the sealant as a percentage of the façade is very small so it pays to make the right choice.

* Tremco illbruck is a European manufacturer and service provider of high-performance sealing, bonding, flooring, waterproofing, and passive fire protection for the construction and manufacturing industries.