Choosing the right primary sealant and applying it correctly

Insulating Glass Unit (IGU) sealants have enjoyed a great deal of attention in recent years, largely due to the performance requirements of EN 1279-2. Most of this has focussed on the secondary or outer seal – but what about primary sealants?

To fully understand the important role that a good primary sealant can play in unit construction, it is useful to look at what makes a good IGU sealant system in general.

The sealant system in an IGU has to perform three main functions:

1. Provide adequate adhesion, i.e. hold the unit together;

2. Prevent moisture penetration;

3. Prevent gas loss.

Whilst Hot Melt Butyl outer sealants perform all three of the above tasks, so that if required they can be used as a single seal, two-component secondary sealants such as polysulfides cannot perform these tasks alone. The reason for this is that while they provide the strength to hold the unit together, they do not provide adequate moisture or gas barriers. Units constructed using only a Polysulfide sealant, therefore, will not meet the required standards of EN 1279–2. In this situation, primary sealant is essential.

Primary sealants explained

Primary sealants are based on Polyisobutylene (PIB) and are warm applied to the sides of the spacer tube at around 120°C to 150°C. PIB is commonly supplied in 7kg slugs that fit into the heating chamber of the application machine.

Generally after pressing, the width of the bead when viewed through the glass should be around 3-4mm

Unlike secondary two-part sealants, PIB primary sealants have a low moisture vapour transmission rate and low gas permeability (particularly to Argon and SF6 etc), and therefore do form an effective moisture and gas barrier in IGU construction.

Primary sealants must also have excellent adhesion to the glass and spacer as they play a crucial role in holding the unit together during the assembly process; this is particularly important where units are manufactured on vertical lines.

Application Importance

Together the PIB primary sealant and secondary two-part Polysulfide form a partnership that meets all three of the essential functions outlined earlier – but only if correctly applied (see Image A). It is very important that the application of the PIB, when used in conjunction with a two-part outer sealant, is even and continuous around the entire spacer frame, including the corners. Primary sealants may well be effective moisture barriers but they cannot perform this role effectively if gaps are present (see Image B) that allow an easy path for moisture ingress or gas loss.

Having the right application equipment is also essential. In order to correctly apply a primary sealant, a PIB application machine and press are required. These allow for the correct amount of PIB to be applied to each side of the spacer tube and then pressed to form an even continuous bead of the correct width.

Application of the correct amount of PIB should be monitored at least on a daily basis. Too little could result in gaps or weaknesses and possibly lead to early failure, whilst an excess of PIB could result in sealant being visible inside the unit cavity.

Generally after pressing, the width of the bead when viewed through the glass should be around 3-4mm, a width usually achieved by application of between 2-4 grammes per linear metre on each side of the spacer, depending on the profile. Correct application should be checked regularly as part of factory production and control procedure by, for example, weighing a one square metre frame before and after application.

In conclusion, while selecting a high performance secondary seal is important, particularly in meeting EN 1279-2 requirements, this is worth little without a good quality primary sealant. To construct a dual seal IGU that will pass legislative requirements and provide an optimum lifespan, choosing the right primary sealant and applying it correctly (particularly if using a two-component secondary sealant) is not only a consideration, it’s a must.