Low price – at what cost?: ‘it is impossible to determine performance from appearance’

Synopsis

The proliferation of ‘low price’ offers for molecular sieves of Chinese origin, presents the European Insulating Glass manufacturer with some tempting opportunities to reduce cost. Experience has shown however that this can be at the expense of unit quality, with severe gas desorption and low water capacity leading to reduced unit lifetime, and a bead form that can negatively impact the efficiency of automatic filling equipment. This article reviews the fundamental requirements of a molecular sieve to help the sealed unit manufacturer make an informed decision on fitness for use.

Molecular sieve consumption in Europe was widely recognised to be at record levels in 2004. Oil and gas producers, buoyed by the escalating price of a barrel of oil, increased change out frequency of molecular sieve dehydration and purification units. At the same time, insulating glass producers were kept busy feeding the insatiable demand created by expansion in the construction sector of the emerging European countries. In this highly charged environment, demand for molecular sieve occasionally exceeded supply and prices moved up. IG manufacturers, driven by the popular mantra to ‘drive cost down’ have clearly been tempted to take advantage of the increasing availability of products from previously untested sources – but at what cost to quality?

It is difficult to escape the impact China is making on the Western world. Indeed at all levels of business, much time is being devoted to trying to reconcile the enormous opportunity presented by such a rapidly growing economy with the real threats such expansion presents to our traditional manufacturing base. This is not the subject for discussion today but our recent experience suggests that in the rush to benefit from the low price offers for some Chinese molecular sieves, sealed unit performance is, quite literally, being ‘thrown out of the window’.

Take the case of a reputable window manufacturer who, in good faith, had awarded part of its molecular sieve requirement to a distributor of Chinese sourced product. At face value, the material looked OK: beige, more or less spherical beads, about the right size and with no obvious dust. However after some month’s usage, the manufacturer experienced unaccountable unit failure during testing to their National Quality Standard. Samples of product were analysed and found to be 4–Angstrom molecular sieve, not the 3 Angstrom material the manufacturer believed he was buying. Herein lies the problem – it is impossible to determine product performance from its visual appearance. A cursory inspection of the product specification sheet can mislead rather than inform, with test parameters missing or at best ambiguous. With regard to the 4A molecular sieve problem mentioned above, those of you who have been in the industry some years will recall these products lost favour in the 1980s due to excessive ad- and de-sorption of air within the unit cavity. This leads to distortion of reflected image, sealant/glass bond stress leading to greater moisture ingress and, on low aspect ratio insulating glass units, cracking of the glass itself at low temperature.

For all manufacturers concerned with upholding product performance in the face of inescapable cost increase, it is worth taking time to go back to basics and review the influence that desiccant choice has, not just on sealed unit performance, but also unit manufacturing and environmental health & safety.

Back to basics

What is a Molecular Sieve?

Molecular sieves are synthetic, crystalline metal alumino-silicates. These crystalline structures have three-dimensional pore systems with openings that are precisely defined. These uniform openings allow molecules smaller in diameter to be adsorbed whilst excluding larger molecules. This sieving phenomenon led to the name ‘Molecular Sieve’ (see Fig. 1).

The pore openings in any one type of molecular sieve are precisely defined and the most common types have pore openings of roughly:

3Å, 4Å or 10Å

(1Å = 1 Angström = 1 x 10-10m).

For insulating glass units, the molecular sieve must be selective towards water adsorption having a pore size that allows the entry of water molecules but must suppress the pick up of nitrogen, oxygen or other common filling gases such as argon, krypton or SF6. This can only be achieved by using a 3A type molecular sieve as depicted in the following tables 1 and 2.

Table 1: Critical molecule diameters

Table 2: Adsorptivity of water, air, argon and SF6 by various desiccants

Glass deflection and sealant stress

As discovered by our insulating glass manufacturer, use of molecular sieves with a pore opening larger than 3 Å is inappropriate, because these materials adsorb large amounts of gas molecules especially at cold temperatures. At higher temperatures these adsorbed gases will be released. Consequently, the temperature-controlled intake and release of these gases will lead to negative and positive pressures inside the double pane window. The consequences of such pressure changes are increased inward or outward deflections of the panes in the unit, resulting in:

Glass deflection;
Glass stress and breakage;
Reduced insulating performance;
Sealant stress resulting in the loss of seal integrity;
Reduced unit life.

A similar, albeit less pronounced effect, will be observed if inappropriate binders are used.

The amount of air desorbed by a molecular sieve at elevated temperature is easily determined and the industry benchmark for a good 3A molecular sieve can be considered no more than 25ml air per 250 ml of molecular sieve (measured at 70 deg C). This figure can rise to as high as 600ml air for a 4A molecular sieve, which if used in an insulating glass unit, would produce significant pumping effects and consequently higher glass stress. This will produce distorted reflected images, reduce the lifetime of the insulating glass unit and under extreme conditions can lead to glass breakage. The following calculation demonstrates the deflection behaviour of IG units, one filled with a gas desorption-free 3A molecular sieve, the other filled with a gas-releasing 4A molecular sieve (Figs 4, 5, and 6 on the opposite page.) The calculations refer to the following assumed data for the IG units:

Window size (length x width):

50 cm x 35 cm;
Pane distance: 16 mm
Pane thickness: 4 mm. Temperature/Humidity/Air Pressure during manufacture: 25°C/50% RH/1013 mbar
Temperature and Air Pressure at place of installation: 25°C/50% RH/1013 mbar
Type of desiccant: MS 3A and MS 4A, respectively
Amount of desiccant: 80g

From these diagrams it can be seen that both pane deflection and glass stress rise with increasing temperature differences between the insulating glass production site and the end user’s installation site. In case of 4A molecular sieve, the risk of glass breakage and the stress on both the sealant and on the glass panes is much more pronounced than for 3A molecular sieve.

High quality insulating glass units are expected to have a service life of approximately 25 years. This life span will be reduced when the IG unit is exposed to increased pane deflection. Consequently, the use of high quality 3A molecular sieve is essential for the performance of the insulating glass unit.

Unit Lifetime

To maximise unit lifetime it is necessary to maximise the available water capacity of the molecular sieve. This is a function of Total Volatile content (moisture preloading) and Equilibrium Water Capacity. The temperature rise test is well accepted in the industry, but only as a measure of level of activity, not an absolute measure of adsorptive capacity. Tests show that adsorption capacity can vary by as much as 40% from a range of molecular sieve sources, yet all achieve the minimum specified temperature rise.

Spacer Profile Filling

Beaded molecular sieves provide convenient and quick filling of spacer bar profile. The action of filling does however create the potential for dust creation and consequent impact on:

1. Visual inspection – dust is unsightly on glass or spacer surfaces;

2. Sealant/glass adhesion can be impaired leading to potential premature unit failure;

3. Automatic spacer filling systems are susceptible to malfunction in the presence of excessive dust;

4. Excessive dust has Health & Safety implications for the insulating glass manufacturing workforce.

Dust can be minimised by molecular sieve manufacturers through appropriate binder selection and state of the art bead forming and calcination technology. It should be recognised that binders are as a rule inorganic products, with the potential to contain hazardous fibres and concentrations of quartz.

Bead size distribution must be controlled within a tight specification to ensure problem free operation of some automatic fillers. An excessive proportion over 0.9mm in diameter is known to block filling injectors.

Bulk density of the molecular sieve determines the weight of product that occupies the spacer bar volume and should be considered in the economical calculation.

Fitness for Use

With all the above considerations it may be a surprise to some readers that products having only 60% of the adsorption capacity of a high quality molecular sieve and/or adsorption characteristics which may lead to glass breakage and deflection are tolerated at all for use in insulating glass in the European market. Possibly, fitness for use is taken to be compliance with the EN1279 European Standard for insulating glass units. A reasonable proposition you may say, but this only addresses the adsorptive capacity of the product: it does not call for low glass deflection, minimal dust generation, and appropriate adsorption kinetics and bead integrity.

Ultimately of course, the decision to procure any product from a new and perhaps undisclosed manufacturing source rests with the user. However, the user should insist that the supplier confirms the provenance of the product offered. The supplier must also be able to demonstrate that such product meets all the requirements needed to preserve the hard earned reputation of a quality insulating glass unit manufacturer.