Achieving an airtight seal

Sealant is applied around the edge of the glazing, to the back of the recessed spacer bar, to achieve an airtight seal. There are a number of sealed unit edge sealants available with Polysulphide, Hotmelt, Polyurethane and Silicone being the mainstay of the industry. These edge sealants are known as the secondary sealants. They are usually combined with a spacer bar edge sealant, known as PIB – polyisobutylene. PIB is known as the primary sealant.

2-part sealant

Polysulphide, Polyurethane and Silicone are commonly used. These types of sealant are made up of a base material (1st part) and a hardening material (2nd part). They are mixed together in a measured ratio and chemically cure to a solid compound. Once they have cured – usually about 2-3 hours for a solid finish – the material is set and cannot be reused or altered. 2-part sealant has a chemical bond to the glass. This means that there is a cross link of molecules between the sealant and surface of the glass. This sealant cannot be pulled off the glass and will tear leaving a residue on the surface.


This is widely used as it is low maintenance in terms of machinery.


This is typically less expensive but due to its reactive nature, it results in high machinery costs. It is generally used by massive volume sealed unit manufacturers and has many other industrial uses making the price low.


Silicone is used by commercial unit manufacturers where the edge seal may be exposed to the UV light from the sun. It is UV resistant. All the other sealants described will be broken down by UV and result in failure. Silicone however, is the worst performing sealant in terms of moisture ingress and gas loss. Thus the PIB has to be exact and consistent. Many tall glass buildings – smart hotels and banks etc, will have glazing made using silicone.


This is a single component rubber butyl compound and is a thermoplastic material. This means it can be heated up and cooled down and will flow when hot and be solid when cool. There is no change in the sealant structure during this process. Hotmelt has a mechanical bond to the glass. This means it ‘grabs’ onto the glass surface but doesn’t react with the glass. If the correct force is applied, the sealant can be pulled off the glass.

Polyisobutylene PIB is applied to the edge of the spacer tube and does two things:

1. Holds the two panes of glass together whilst the glass travels down the production line.
2. When applied correctly, provides the primary seal between the glass and the spacer frame, sealing the cavity space from the external atmosphere and reducing gas loss to a minimum.

When PIB is used, the sealed unit is known as a dual sealed unit. There are two seals of protection. This is recognised as a superior unit to ‘old’ single sealed units. .

At Glasscraft, we manufacture both Polysulphide and Hotmelt together with PIB to give the best performing sealed units available.