Desiccants - Why are Molecular Sieves used in Insulating Glass Units?
Molecular sieves are used in sealed insulating glass units for one reason: to prevent fogging. Fogging or condensation, on inner glass surfaces can be caused by water and solvent vapours inside sealed dual pane windows.
To begin with, moisture is present in the air that is trapped within the unit during assembly. Furthermore, after the unit is sealed, moisture vapour continues to enter through the sealant throughout the entire life of the window. The rate of moisture vapour transmission (MVTR) depends on sealant quality, assembly procedures, and outside temperatures and humidities. Solvent vapours are generated by some sealants during curing and at this stage some of them will migrate into the interior air space.
Window fogging occurs when the moisture (or solvent) vapour entrapped between the two panes of the insulating glass unit condenses on the glass surface of the unit. This happens when the temperature of the glass surface is lower than the dew point of the air between the two panes. Water or solvent dew point is the air temperature at which moisture or solvent vapour begins to visibly condense on the glass surfaces inside the unit. The lower the vapour concentration, the lower the dew point and less chance of fogging.
It is evident that to prevent fogging, the dew point of the entrapped air must be kept well below any temperature the window unit may encounter. To accomplish this, the moisture (and solvent) vapour concentration inside the dual pane window must be controlled.
Molecular sieves have the highest adsorption capacity for moisture of all commercially available adsorbents and, if required, they also can remove solvent vapours. When moisture (or solvent) vapours from the air space between the two window panes are adsorbed on the molecular sieves, the moisture (or solvent) vapour concentration in the air space is reduced and the moisture (or solvent) dew point of the air drops to a lower value and no condensation can occur until the glass temperature goes below that.
It is possible for windows to fog at any outside temperature between the daytime high and night-time low. Therefore, to prevent fogging, the dew point of the entrapped air must be kept well below any temperature the window might encounter.
In most dual pane windows deflection and stress are minimal and not noticeable. Although molecular sieves play a minor role in deflection and stress, one should select a type of molecular sieve that minimises air adsorption, and thus deflection, while maximising water vapour adsorption. Therefore, low air adsorbing molecular sieves will provide maximum protection under severe climatic conditions and should be used when manufacturing insulating dual pane sealed units using a hot melt sealant.
UOP’s adsorbents business is the worlds largest producer and supplier of molecular sieve adsorbents, with production facilities in the United States, Italy, Japan, China, and Germany.
All of these manufacturing sites have obtained the ISO-9002 standard, this guarantees that UOP molecular sieve products are manufactured to uniform high quality standards world-wide.
Quality Control
In order to warrant constant high quality, UOP production laboratories carry out systematic control of the following parameters:
- Loss on ignition (LOI), test to determine the residual moisture level.
- Water capacity, test giving the amount of water adsorbed by the molecular sieves at 25 degrees c & 60% & 10% RH.
- Bead distribution, to ensure the beads have the correct size distribution.
- Dust level, to avoid shipping material which contains dust.
- Crush strength, to check physical strength of beads.
- Attrition resistance, to check the amount of dust formed during handling.
- Density test, to secure correct volumetric weight.
- Delta T, to have a guideline for the exothermic heat & corresponding available water capacity.
- Air desorbsion, to ensure right material in order to minimise defection.
