In fundamental terms the practice of trapping air between two sheets of glass (i.e. Double Glazing) would appear to be quite simple, this however is not as easy to achieve as would appear and there are a number of pitfalls. The very first attempt to produce double glazing was tried in St Helens (Lancs) and this consisted of sealing round the perimeter of double glazing using melted lead. This trial method was proved to be unsatisfactory in terms of economics and weight.
A very enterprising would be manufacturer of double glazing structures in Hull tried to use a one part acid cured silicone sealant as the edge seal in double glazing, this was shown to be unsatisfactory in terms of the amount of moisture vapour that passed through the sealant and condensed between the sheets of glass. This obviously defeated the object of the exercise. Alternative methods using one part polysulphides and one part polyurethanes were also tried but failed for the same reason. Once again great disappointment in Hull Double Glazing!
A breakthrough was found using the relatively new product of 2 part epoxy-polysulphides. These proved to have a lower moisture vapour transmission rate than the Hull inspired one part sealants however they still allowed more moisture through than was desirable, this was overcome by incorporating a powdered molecular sieve compound in the spacer bar that was used to keep the two sheets of glass apart in the double glazing unit. A very neat solution!
The race was now on; manufacturers of units devoted to the sole use of installers of double glazing from Hull to Liverpool and from John O`Groats to Lands End were all trying to develop new double glazing sealants. The first contender to try and oust epoxy polysulphides were 2 part Polyurethanes, a second serious contender were manufacturers of 2 part polysulphides. Eventually Polysulphides and polyurethanes dominated the market.
The main reason for this domination was that polyurethanes and polysulphides are flexible whereas epoxy polyurethanes are not, as a result if you live north of a line from Hull to Liverpool the winters are much colder and since epoxy polysulphides are inflexible they are subject to cold cracking at the lower winter temperatures. This is obviously very undesirable as the double glazing unit will fail.
Product development was involved and eventually usage of 2 part sealants was abandoned in favour of single part hot melt sealants. These were much easier to apply, were cheaper and did not suffer the drawback of being 2 part products.
Derek Pearson