Mark Hickox of Thermoseal on choosing the right desiccant. en 1279 Desiccant selection

With the end of the transition period for EN 1279 edging ever closer and the onset of periodic testing to satisfy the on-going testing requirements, it is worth making a closer examination of the types of desiccant available for use in the manufacture of insulating glass units.

With ever-spiralling energy costs, many companies are looking towards making savings whenever and wherever they can. There seems to have been an explosion of new desiccants on the market recently, often coming with significantly cheaper prices to tempt the beleaguered manufacturer.

Three areas to consider

There are three main technical areas to consider when selecting a desiccant, these being low deflection (gas adsorption/ desorption), initial moisture content and total moisture capacity.

Desiccants used nowadays are generally straight 3A molecular sieves which only adsorb moisture with no capacity to adsorb other volatiles. Whilst this is acceptable for most types of unit and components, it is important that the desiccant has good low deflection performance especially when used in conjunction with hot melt systems. Excessive deflection can cause not only unsightly distortion once fitted, but also cold cracking or even implosion.

It is one thing saying that a desiccant has low deflection, but the more enlightened supplier should be able to demonstrate good low deflection performance. A simple layman's test for this is a methanol delta-t test. The main victim of cold cracking is the decorative panel and a few replacements this winter would soon put pay to any monetary savings made on the desiccant!

Initial moisture content and total moisture capacity are the invisible elements of a unit's ability to perform. It is generally accepted that most top quality desiccants on the market have an initial moisture content of approximately 1% or less and a total moisture capacity of a approximately 20%. Some newer grades on sale have an initial moisture content of over 2%, a total moisture capacity as low as 16% and bead sizes up to 50% larger than current products in use.

This combination could theoretically reduce unit life by up to one third unless more desiccant is used than before in the spacer bar frame.

It could be argued that these units should still survive the normal guarantee period of 5 years and therefore in theory, there is no problem.

Periodic Testing

Of greater concern could be the effect on the periodic testing required by EN 1279 Part 6. CEN Solutions, one of the leading bodies on interpreting moisture penetration index results to EN 1279 offered the following statement.

‘Desiccant selection could become increasingly important in maintaining an unblemished record in the on-going periodic testing required by EN 1279.

‘Whilst it is normal procedure for the Initial Type Test to EN 1279 Part 2 to use a new box of desiccant, the periodic testing is a test of the manufacturer's daily production environment with whatever components are available at the time being used.

‘If a desiccant has higher initial moisture content to begin with, then the longer the box has been opened, the higher this will become.

‘Add to this the initial moisture adsorbed from the unit cavity, and then this figure could become excessively high.

‘If the EN 1279 standard is taken literally, then an initial moisture content of over 3% on desiccant taken from a unit frame before sealing should result in rejection of the desiccant. At present, the accepted method of testing is the delta T temperature rise method which gives no indication of moisture content.

‘Whilst there is a requirement in EN 1279 for desiccant manufacturers to provide data for correlating the percentage moisture content with whatever test method is used i.e. delta T rise, this is very slow in coming to the public domain, even though it has been requested.

‘In the meantime, figures of over 3% initial moisture content are being measured in certain units for periodic testing but we cannot categorically say that this was greater than 3% before the unit was sealed. Therefore, the test is allowed to continue. If the desiccant has a lower than normal total moisture capacity, then it could drastically affect the calculation of the moisture penetration index, in some cases leading to failure of the periodic test. All this could happen irrespective of the quality of the sealing operation.

Rules of interchangeability

‘Finally, please bear in mind that if the rules of inter-changeability are applied to the letter, companies who have already tested to Part 2 with a desiccant having a total moisture content of 20%+, cannot automatically change to an inferior grade i.e. a desiccant with a lower total moisture content without undergoing a full recalculation of their Initial Type Test results so that it can be proved that the new product would have passed if used in this original test.

‘It would be interesting to hear from the Notified Body Test Houses on whether this is being done as a matter of course and whether regular checks are undertaken on the TMC of desiccants currently in use.'

Other factors not considered in this article are excessive bead size, higher bulk density and increased dust, any of which have a detrimental effect on the performance of insulating glass units and in the case of dust, serious health and safety issues.

Thermoseal are suppliers of Eurosiv and UOP desiccant to suit all types of insulated glass production. Our entire ranges of desiccant have been approved by industry test houses and we have been recognised as UOP Distributor of the Year for molecular sieve on three occasions.