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Home > TECHNOLOGY > Technical Exchange > The Influences on IG lifespan by Various Desiccants

The Influences on IG lifespan by Various Desiccants

    Source:未知 Author:NATERGY ENERGY Release time:2016-03-22

Author: Angfeng Liu     Shandong Natergy Energy Technology Co., Ltd.

Abstract   
Many factors affect the lifespan of insulating glass; desiccant is the most essential one. Hereby we compared the corrosive and non-corrosive desiccant, shallow and deep adsorptive desiccant, and specially analyzed the key parameters of various molecular sieves, which would influence the life span of IG unit. Point out that the IG life-span should be 50 years above with proper processing and auxiliary materials.

Key words
insulating glass desiccant  insulating glass lifespan  corrosive   molecular sieve

1. Introduction
Currently the biggest problem with IG industry is that there is above 80% insulating glass with short lifespan, their average lifespan is no more than 10 years. So most insulating glass panes whose lifespan is terminated need to be replaced within seven or eight years. If the IG pane is not replaced in due time, the IG unit not only lose its energy-saving function, but also accelerate repeats of vapor and condense of moisture inside IG unit, Furthrmore it will accelerate heat transfer, cause more energy loss. Finally in order to avoid the huge loss, ensure the interest of consumers, realize the energy-saving goal, it is necessary to discuss the factors affecting the lifespan of insulating glass, thus to improve the lifespan of insulating glass. Desiccant, as the core component of insulating glass, will affect the lifespan of insulating glass considerably.

2.  Influences which are imposed by different types of desiccant on the lifespan of IG

2.1 Influences which are imposed by unstable shapes of desiccant on the lifespan of IG
As we know, IG unit need the desiccant which should have the chemical and physical stability and deep adsorptive capacity.
Generally all solid desiccants can be devided into two types. The first type is their physical properties will change after adsorbing water, the second type is that their physical properties will not change after adsorbing water .
It is chemical reaction that the desiccant whose physical properties changes after adsorbing water, the substance after reaction will show some corrosiveness. When adsorbing water, they will be formed as strong acid, alkali; or become ketoglutarate crystallization, or even solution.
Even the desiccant whose physical appearance will not change also can be devided into two conditions. One kind is chemical reaction, the other is physical adsorption. After chemical reaction, there will be inner crystallization, acid or even alkali, but there is neither change in physical appearance or corrosiveness.
Under certain temperature, the desiccant will reach eqilibrium status with outside environment, if there is any change in temperature, the desiccant will further adsorb or release water in order to reach a new adsorption balance. Under general situation, it will release water when temperature rise, it will adsorb water when temperature decrease. In IG unit, when temperature changes, the desiccant will also release or adsorb water accordingly. However, there will be big change of adsorption balance for medium or shallow adsorption capacity type desiccant,for strong adsorption desiccant, the adsorption balance is not so obvious as temperature changes.
The desiccant whose adsorption balance always affected by temperature changes will be powdered and moved after adsorption of water. The desiccant with long term physical appearance changing will be fragile or scaling. However, in IG unit, the pieces or dust of desiccant will expand slowly to the aluminum spacer, or to the surface of sealant or worse on the surface of glass pane through the hole of aluminum spacer. Under such circumstances, the desiccant will erode IG spacer and sealant, even the surface of functional glass pane.

Table 1 Characteristics of common desiccant

desiccant desiccant
physical properties change
desiccant
without physical properties change
  lime calcium chloride phosphorus oxide unsintered clay silica gel active aluminum oxide, sinter clay molecular sieve bead
transfer characteristic transfer transfer transfer not not not not not
corrosive characteristic strong strong weak none none none none none
crystallized characteristic grain brittleness crystalization brittleness crystalization grain none
shape glue-stick
none
shape glue-stick
none
shape glue-stick
none
shape glue-stick
deep adsorption capacity middle middle strong weak middle middle weak strong


















The corrosion theory can be explained in this way:
Aluminum is amphoteric metal, it can react with both acid and alkali. Its oxide has also this kind of characteristics. Meantime the micro ions will accelerate the oxidation process. Although the catalytic action is very slow for dry gas, the ion concentration in little water will destroy the oxidation film irrevocable.
 
If the desiccant is calcium chloride,
2H2O+CaCl2=2H++2OH-+Ca2++2Cl-

Oxidation film of aluminum spacer destroyed is in the following way,
 
6H++Al2O3=3H2O+2Al3+                                                            
Al3++ 3OH-=Al(OH)3                                                             
 
After Oxidation film is destroyed,
 
2 Al+6OH-=2Al(OH)3+6e                                                           
6 H++3O2 = 6 OH- -6e                                                             
4 Al+6OH-+6H++3O2=4Al(OH)3

So, as time goes, the oxide film on aluminum spacer will be totally destroyed and the new dense
oxide film can not form because of the ions. Then aluminum spacer will be corroded gradually and salt marsh will appear, finally gap and holes will form to cause IG unit invalid.
This acid desiccant contact with the sealant throughout the year, which will easily make butyl volatilize and have rainbow phenomenon on the glass surface. It will speed up sealant’s ageing and oxidation to cause IG tightness fail.
So as time goes, the Oxidation film of aluminum spacer will be completely destroyed and also the new dense protective film can not be formed due to the exist of ions. The Aluminum spacer will be slowly eroded and then salinification is formed. Finally lead to the sealing failure of IG unit.
And on the other side, due to the continuous contact between the sealant and various type of acidic or Alkaline desiccant, the aging process of sealant will double.
In conclusion, there are two deadly disadvantages for desiccant with physical changes:
One is the crush caused by transfer, the other is extensive corrosiveness as time goes by. So the desiccant with physical appearance change can not be used in IG unit.

2.2 Influences which are imposed by variant adsorptive depth desiccant on the lifespan of IG
Below, you will find the adsorption capacity of variant desiccant in different relative humidity under 25 centigrade.


In the case of low relative humidity, molecular sieve adsorption capacity is far greater than the silica and alumina.
Because the current widely accepted standard for insulating glass desiccant is to test the adsorption capacity in the air environment at 25℃ with 10% relative humidity, at which the absolute humidity can be expressed as -8℃ dew point. In other words, the desiccant inspection standards, is to test the adsorption capacity when dew point is below -8 ℃. From the figure we can see when the relative humidity is 10%, the adsorption capacity of molecular sieve is about 4 times than silica or alumina, so from the figure above, we choose zeolite molecular sieve as the best desiccant for insulating glass.
How is the control property of molecular sieve over dew point on earth?

Figure 2 the balance of absorbent capacity molecular sieve in extremely dry state

This picture is pure zeolite powder under condition of very low relative humidity, the adsorption capacity, the actual use made of the 3A molecular sieve through the ion-exchange and molding processing, the actual adsorption value for the figure 65% of value.
The figure shows that when moisture in air reaches 1ppm, the molecular sieve can still be adsorb  5% of self-weight water, when reaches 100ppm, molecular sieve can adsorb to the self-weight of 17%, when reaches 1000ppm (equivalent to 0.1% of the water), the balance of molecular sieve adsorption capacity tends to saturate. At this time dew point is about -40 ℃ in the air.
So, in theory, zeolite affinity for water molecules is very large, only at very low relative humidity in the circumstances, the adsorption capacity of molecular sieve can be tangible impacted, that is, molecular sieve has strong ability to maintain a low dew point. Normally after the insulating glass filled with molecular sieve, dew point can not be measured by usual device. And in normal use of more than ten years, dew point is very rare measured.
Therefore, molecular sieve is a must option for insulating glass desiccant.
According to data, angle insulating glass permeability rate is the annual per square meter- 0.27g. The following table according to the analysis of data, angle insulating glass dew point of -40 ℃can keep 48 years, according to the standard calculation of dew point of -10 ℃, insulating glass in use up to 50 years of life is no problem.

Dew point 100g zeolite under different humidity equilibrium adsorption capacity 1m x 1m double-channel sealed insulating glass built-in life, when 100 grams of molecular sieve  
Insulating glass-annual water permeability Insulating glass dew Life
Gusseted style Angle style Gusseted style Angle style
-40 13 g 0.8g 0.27g 16.25 year 48 year
-10 16 g 0.8g 0.27g 20 year 59 year
5 18 g 0.8g 0.27g 22.5 year 67 year










Table 2 insulating glass dew life with molecular sieve installed

2.3 Influences which are imposed by respiration desiccant on the lifespan of IG
As the crystallization of molecular sieve is an alumina-silicate network, it has very uniform pore and its pore size is perfect for small molecules to go through. Due to molecular sieve specification model is different, the size of its channels is also different.
For insulating glass desiccant, only water molecules are required to be absorbed into its pores, rather than allow air and other molecules. Water molecules is 2.8 Å (1 micron = 10,000 angstrom) , in the air nitrogen, oxygen, argon gas is about 4Å. So only 3A type zeolite can be used, any other type of molecular sieves are not suitable.
4A type zeolite pore 4.3 Å, so type 4A molecular sieve can absorb nitrogen and oxygen in the air. Composition in the air, in addition to water outside, 4A molecular sieve is more biased in favor of the larger polar nitrogen adsorption, so the detection of inspiratory amount of molecular sieve suction is mainly to detect the amount of nitrogen breathing subject.(The current type of molecular sieve 3A made from 4A molecular sieve after ion-exchange. therefore 4A molecular sieve is always cheaper than the 3A molecular sieve, which is often used to counterfeit zeolite 3A. )
For non-molecular sieves desiccant, because they do not have a similar crystal structure of zeolite-like network, so they have no high affinity to water, let alone inspiratory capacity problems.
How is the respiration affect the lifespan of insulating glass?
Inspiratory capacity of 4A molecular sieve is about 7ml per gram or so, while 3A molecular sieve per gram will be less than 0.5ml. The nitrogen adsorbed by 4A molecular sieve is very sensitive to small changes in temperature at usual temperature. For example, 250ml 4A zeolite from room temperature arises to 70 ℃ can emit more than 700ml gases. The 3A molecular sieve could be less than the amount 50ml.

In the processing of insulating glass, we generally fill molecular sieves at room temperature conditions. If the molecular sieve is 4A type, when the temperature decreases, the molecular sieve will adsorb a large amount of air, resulting in lower pressure in the glass; on the contrary, when the temperature rises, the internal zeolite molecular sieve will release a large number of air , so that the pressure in the glass increase.
4A molecular sieve containing hollow glass pieces together, because of the change of day and night, alternating with the seasons, 4A molecular sieve due to temperature changes in the breathing deflated, like breathing, glass will continue to bear the pressure of inward and outward. This is sometimes inward and sometimes outward pressure significantly reduced the lifespan of insulating glass, especially the kind of hidden frame glass, two pieces of glass are completely sealed by adhesive bonding strength of its internal air pressure from the seal capacity which is  very limited, glass seal can fail at any time.
Experience has shown that, using 4A molecular sieve as insulating glass desiccant, its life is only a quarter of 3A molecular sieve, more dangerous is this: If you are using 4A molecular sieves when insulating glass is too thin or no steel, glass may break suddenly explode, or the entire piece of glass falls. If this glass is installed in high buildings, it will cause accidents.
Then, what is the inspiratory amount of molecular sieve on earth? One way widely used in foreign
countries: Use 250ml molecular sieve, heat them from the usual temperature to 70 ℃, put them in constant temperature for four hours, using water method to see how much gas discharged, generally require displacement in 50ml below.

2.4 History of desiccant
Insulating glass is widely used in the United States earlier time, when the molecular sieves desiccant are not used but the silica gel. Silica gel desiccant was still found to occur condensation phenomena in cold areas, life expectancy is shorter. So people use 4A type as the desiccant in sixties with a short time, and soon found the question of 4A molecular sieves, modified, then used the mixture of molecular sieves 3A and 13X, expecting 13X molecular sieve to adsorb the organic in insulating glass. Though 13X is less, still there is respiration. Because the silica gel also has a good adsorption capacity of organic matter, but there is no respiration, so we changed to a mixture of 3A molecular sieve and silica gel to absorb water and volatile organic chemicals. With the glue process improvement, fewer and fewer volatile organic compounds, at the same time, we discovered attapulgite also has a good adsorption of organic matter, so no blending on a molecular sieve silica gel powder, but rather a mixture of zeolite and attapulgite.

3. Influences on IG lifespan with different parameters
3.1 Gas adsorption
Some instructions only mention adsorption capacity without testing conditions, acturally, that is meaningless.
For example, compared with the molecular sieve, adsorption capacity of silica gel is not too bad, but its deep absorption capacity is very poor, for example, C-type silica gel under the conditions of 30 ℃ and relative humidity 90%, can adsorb 3 times of its own weight, and under the same conditions, the molecular sieve can not reach that. However, such a high adsorption capacity does not make any sense for IG, because once the temperature decrease, IG unit will be filled with water vapor. So only at relative humidity of 10%, the adsorption capacity can be seen as the core indicator. At other high relative humidity, the adsorption capacity is meaningless to measure the dew point control property.

3.2 Ignition loss
For ignition loss, the initial focus is to control the moisture residence, meantime, it can also ensure the relative unreactiveness of desiccant. If the initial moisture residence is too high, the desiccant may be over-absorbent. So the desiccant dew point control greatly reduces, the loading of this desiccant initial dew point will rise, which will lead to a corresponding reduction in the life of IG.
In addition, the high ignition loss may also be another problem. The desiccant may be contain chloride, or easily decomposed inorganic, such as nitrite, acid or alkali substances, or contain volatile organic compounds. So when ignition loss is too high, the volatile chemical substances and other rusty material may be contained, which will affect the life span of IG. And all that has been analyzed in the first chapter.

3.3 Meaning and testing methods of delta T value
Delta T value, strictly speaking, is not a technical indicator, but a commercial target.
After absorbing water, temperature will rise is an important feature, but not the unique feature of molecular sieve. If the delta T value is high, may be it is molecular sieve, calcium chloride, calcium oxide or a strongly acidic strongly alkaline desiccant. You can test it in aqueous solution to determine whether it is corrosive. Even if we exclude the other corrosive desiccant, we still can not distinguish between molecular sieve 3A and 4A. There must be a gas adsorption testing instrument to test the 3A or 4A.
Therefore, it is meaningless to use delta T value method alone to determine the quality of insulating glass desiccant, and in order to know more, we must test the pH value of aqueous solution and irritant characteristics (corrosive) and gas desorption to determine the quality of a desiccant. In the absence of corrosive and qualified gas discharge, the higher delta T value is, the better the quality is, the longer of IG lifespan.
But the delta T value test must follow scientific methods, unscientific methods can’t be used to measure the quality of desiccant.

First of all, you can not use volume measurement to measure delta T value, because desiccant adsorption capacity is calculated according to its weight, and delta T value is an indirect test method, using a certain volume of desiccant and a certain amount of water mixed to test delta T value, the higher the desiccant density is, the higher delta T value is. Because in a certain volume, the higher the proportion, the more the weight, so if molecular sieve adsorption capacity is certain, the higher density is, the higher delta T value is. The lower density is, the lower delta T value is. Therefore, delta T value test should be based on weight method.
Secondly, water can not be added before the molecular sieve, because molecular sieve density is smaller than water density, the smaller the molecular sieve, the longer it will float on the water, then the molecular sieve will fall down at a slower speed, and that will lengthen the cooling time. On the other hand, if pull water into molecular sieve, the cooling time is rather concentrated, so if water added into the same molecular sieve, the lower density is, the lower delta T value is. Therefore, you should put water into the molecular sieve to test the delta T value.

Again, delta T value also relates to the following factors:
1. The smaller desiccant particle size is, the higher the delta T value is;
2. Delta T value of molecular sieve 4A is higher than 3A;
3. The better insulation of test container used, the higher the delta T value will be;
4. The material and shape of containers affect the delta T value results;
5. Water temperature affects the delta T value;
6. Test location does not affect the delta T value;
7. The sensitivity of thermometer affects the delta T value;
8. The amount of water and desiccant affects delta T value (even proportion is certain);
 
Finally, the result of delta T value concern to the testing skill, so if you want the delta T value effective, you should use the right and unified testing methods. Otherwise, little difference of the resultst is meaningless. 
 
In order to make the measurement with a reference value, you must ensure that:
1. All materials are molecular sieve 3A;
2. Closer partical size;
3. Use the same device;
4. Use the right approach for testing.
 
Only in such cases, the higher the delta T value is, the better the desiccant is, the more conducive to prolong the lifespan of IG.

4. Severity of the lifespan of IG and urgency of choosing qualified IG desiccant
IG just large-scale popularized and people haven’t notice the hazards of short lifespan of IG. Suppose if the lifespan of this IG is seven years, this situation will appear around 2013. The whole society will pay a big price to their disregard of inferior IG. To improve the lifespan of IG has been a looming energy-saving and strategic issue.
If we can not resolve this issue, people will gradually lose heart because of the inferior quality of IG. This refusal of using IG will be more and more serious, which will lead the development of IG industry to be an embarrassing situation. This situation will severely restrict the development of the IG industry, especially every IG manufacturing company.
Therefore, to ensure the lifespan of IG is of great significance. And to choose good quality desiccant is of great importance.