Analysis and Application of Technical Requirements of Desiccant for Insulating Glass.

^{--Comparing China's current industry standards with those of European and American countries}

With the development of the times, the desiccant industry for insulating glass has continued to develop and innovate, from the original silica gel material to the 4A molecular sieve adopted in the 1960s, to the improved mixture of 3A and 13X molecular sieve, and the mixture of 3A molecular sieve and silica gel, and the mixture of 3A molecular sieve and attapulgite has undergone more than 60 years of iterative changes. With the continuous expansion of the scope of application and performance requirements of desiccants for insulating glass, to enable glass deep-processing enterprises to effectively control the quality of desiccants and change the disorderly and unlabeled state of production and publicity in the desiccant industry, it is necessary to use desiccant for insulating glass In-depth analysis of various technical indicators of desiccant, so that deep processing enterprises can clearly understand the impact of desiccant performance on insulating glass, to better apply it to the production of insulating glass to promote the quality of insulating glass and the healthy development of the industry.

There are two current standards in China, namely the national standard "3A Molecular Sieve" and the industry standard "Desiccant for Insulating Glass". Judging from the corresponding requirements of the national standard and the industry standard, many technical indicators and detection methods are roughly the same. However, in the Chinese industry standard, desiccants for insulating glass are divided into Class A and Class B. Class A is 3A molecular sieve, and Class B is desiccant. With attapulgite clay as the main material, there are differences in the requirements of some technical indicators, mainly in static water adsorption, the water content of packaging products, gas desorption, temperature rise, powder falling, crushing resistance, loss on ignition and detection methods.

1. Analysis and application of technical requirements

1.1 Adsorption performance

The static water adsorption capacity is used to measure the moisture adsorption of the desiccant in the relatively closed insulating glass inner cavity system and is the basic data of the physical properties of the desiccant.

The specific application is reflected in that the super strong static water adsorption capacity can ensure that the moisture in the inner cavity of the insulating glass is fully absorbed, and the moisture that penetrates the inner cavity through the sealant every year during the service period of the insulating glass (15 years) is fully absorbed, thereby ensuring The inner cavity of the insulating glass is dry, and the lower dew point is continuously controlled. Therefore, the static water adsorption performance is a very important indicator, which is directly related to the energy-saving effect and service life of the insulating glass. In addition, to achieve the corresponding performance of insulating glass, a desiccant with a continuously controlled low-temperature dew point is required. Because the thermal conductivity of glass is 0.77W/mK, the thermal conductivity of air is 0.028W/mK, and the water molecules in the inner cavity are the main factors affecting the conduction and convective transfer of energy in the insulating glass, the improvement of the insulating glass inner cavity The degree of dryness is an important factor to ensure and improve the performance of insulating glass. Therefore, according to the use requirements of insulating glass for energy saving and heat preservation and the dew point requirements of minus 40°C to 60°C in the national standard, it is necessary to specify the technical requirements and measurement methods for static water adsorption at a relative humidity (RH) of 10% and a temperature of 25°C, such a low-humidity test environment is closer to the actual working environment of insulating glass.

The static nitrogen adsorption capacity is a measure of the adsorption capacity of the desiccant for nitrogen in a relatively closed insulating glass cavity system. The specific application is reflected in the ultra-low nitrogen adsorption capacity, which ensures that the insulating glass will not be distorted and deformed by the change of internal and external cavity pressure under different temperature changes, resulting in a glass burst. The nitrogen content in the inner cavity of the insulating glass exceeds 70%, and the nitrogen adsorption is too high, which will cause an imbalance of the inner and outer cavity pressure of the insulating glass. For example, 250mL of 4A molecular sieve can release 700mL of gas when it rises from normal temperature to 70℃. The 3A molecular sieve can exclude all other molecules during the water adsorption process due to its small pore size, and the amount of nitrogen absorbed per gram will not exceed 2mg.
The moisture content of the package is the amount of water that the desiccant has absorbed before use. The specific application is reflected in the ultra-low packaging water content, which means that the less pre-absorbed water the desiccant has during storage and transportation, the greater the remaining adsorption capacity, the stronger its dew point control ability, and the better the quality and performance of the desiccant. The total adsorption capacity of the desiccant is certain, and the desiccant absorbs too much water in the process of production, turnover, packaging, etc., which will affect the use effect. The moisture content of the package is directly related to the performance of the product when it leaves the factory and is closely related to the actual water absorption performance of the product when it is used.

The water absorption rate is a measure of how quickly the desiccant absorbs water. The specific application is reflected in the lower water absorption rate, which can better meet the time control requirements of insulating glass production, filling, and assembling. The desiccant starts to absorb water when it leaves the sealed environment of the package, and the production procedure of insulating glass requires that it should be completed within 45 minutes from filling to assembling, so a lower water absorption rate is required to leave more water absorption capacity for insulating glass. Molecular sieves still have a fairly high adsorption capacity even at very low partial pressures or concentrations. The adsorption of water by molecular sieve is a physical adsorption phenomenon, and no chemical changes are caused during the adsorption process.

Temperature rise refers to the instantaneous temperature rise caused by the desiccant absorbing water and releasing heat. The specific application is reflected in the fact that the desiccant has to go through multiple links such as packaging, transportation, and storage from the factory to the use. There is a problem in a certain link, which causes the desiccant to be exposed to the air and start to absorb moisture in the air, causing the product to fail. Therefore, the temperature rise is a quick and effective way to detect whether the product is invalid before the desiccant is used. Many factors affect the temperature rise, including: 

1. Different types of desiccant, different temperatures, and sound; 
2. The smaller the particle size of the molecular sieve, the higher the temperature rise; 
3. The better the thermal insulation of the test container, the higher the temperature; 
4. The material and shape of the container are different. 
5. Water temperature affects the temperature rise; 
6. The temperature rise is affected by the position of the thermometer test point; 
7. The temperature rise is affected by the sensitivity of the thermometer; 
8. The amount of molecular sieve and water is different (even if the ratio is the same).

Therefore, for 3A molecular sieves, under the same test method, the higher the instantaneous temperature rise, the stronger the water adsorption performance of the molecular sieve.

The gas desorption amount refers to the amount of degassing when a certain volume of desiccant is heated to 70°C. The specific application is reflected in the fact that glass deep-processing enterprises can quickly, simply, and effectively identify whether the desiccant’s air absorption exceeds the standard before use. important. 3A molecular sieve hardly adsorbs nitrogen. To restrict the desiccant products for insulating glass currently on the market and avoid false ones, counterfeit and inferior products, etc., it is necessary to increase the technical requirements and measurement methods of gas desorption capacity, to identify whether the product is a 3A molecular sieve. 

Table 1 Comparison of some technical indicators

1.2 Mechanical Strength Properties

The falling powder degree/dust amount refers to the percentage of the mass of floating ash and dust attached to the surface of the granular desiccant to the mass of the sample. The specific application is reflected in the fact that the desiccant produces a large amount of floating dust and dust during the filling process. These dusts not only pollute the working environment but also deposit on the machine pipelines and valves.

It affects the automatic development performance and sealing performance of pneumatic pipeline valves and affects the aesthetics and light transmission performance of insulating glass.In the field of insulating glass, the dust problem is not caused by the quality of the desiccant, but by the excessive pressure of the filling machine (over 0.6MPa) during the desiccant filling operation, burrs on the inner wall of the spacer, excessive air holes, and leakage of pin joints. and many other reasons. Therefore, it is suggested that after filling, it is best to wipe the aluminum strip with a dry degreasing white cloth, choose a reasonable air pressure filling, pay attention to the drilling bit, and do not damage the desiccant particles when inflating. The resistance to crushing is the maximum uniform pressure that the desiccant can withstand while keeping the shape of the particles intact. The specific application is reflected in the fact that the desiccant has high mechanical strength, which can avoid breaking due to collisions, impacts, etc., resulting in fine debris and dust, polluting the glass or blocking the pipeline, and affecting the appearance of the glass. The desiccant is subject to many different forms of stress, such as wear and collision caused by the contact between the particles and the container wall during packaging and transportation; after being loaded into the equipment, the gravity load of the upper material and the impact force generated by the airflow during operation, etc. The crush resistance of some spherical desiccants increases with the particle size, such as 0.9mm~1.5mmN14N, and 1.5mm~2.0mmN20N, while 0.5mm~0.9mm is not required due to the small particle size.1.3 Industrial Design Parameters.

Bulk density refers to the mass of desiccant in unit bulk volume. The specific application is reflected in that it can be used to check whether the desiccant filling is uniform and compact, and the qualified bulk density can ensure that the filling amount of the desiccant is controlled within the most economical and reasonable range. The bulk density of molecular sieves reflects the relationship between their pore structure, chemical composition, and crystal phase composition, and is also an important indicator in the cost control of desiccant filling. According to different purposes, it can be divided into two types: tapped bulk density and loose bulk density. The tapped bulk density is mainly carried out with the help of a vibrator. Molecular sieves are added to the measuring cylinder at a fixed rate and vibrated at the same time to finally obtain the bulk density. The loose bulk density is mainly used by free fall motion.

Particle size refers to the mass percentage of desiccant within the specified particle size range to the total amount. The specific application is reflected in that the desiccant should have a reasonable particle size distribution so that it can be easily applied to insulating glass spacers of different specifications. Desiccant products often have different size requirements according to the differences in production equipment and products of deep-processing enterprises, so the products have a certain particle size distribution. For example, conventional filling equipment uses 0.5mm~0.9mm spherical products. To improve the strength of spherical molecular sieves, it is usually achieved by increasing the particle size of molecular sieves. If the particle size is too large or too uniform, the particle gap ratio of molecular sieves will increase during filling. 

^{Figure 1 The Insulating Glass Molecular Sieve Desiccant 1}

2. Discussion and analysis

The national standard "3A Molecular Sieve" was implemented on April 1, 2018. The revision of this standard mainly refers to the relevant requirements for molecular sieve desiccants in the European standard EN1279: 2014 "Building Insulating Glass", so that the Chinese national standard is in line with the European Union standard.

The industry standard "Desiccant for Insulating Glass" was implemented on July 1, 2012. When this standard is revised, it mainly refers to the relevant requirements of the old version of the national standard "3A Molecular Sieve" for spherical 3A molecular sieves for insulating glass, and also adds a new category: Type B desiccant o But it deep drying ability in low humidity environment is weak, the drying ability of silica gel and activated alumina is very different, and the gap is even bigger than 3A molecular sieve. Most Type B desiccants on the market are doped with calcium chloride. Calcium chloride, as a common desiccant, becomes the main active ingredient of this type of desiccant when mixed with it.

National standards such as France, the United States, and Australia have specific requirements for the category of desiccant for insulating glass. The CEKAL certification of insulating glass in France requires the use of 3A molecular sieves and puts forward corresponding technical index requirements for the products. The United States stipulates in the IGCC®/IGMA® certification requirements that there are three types of desiccants: molecular sieve, silica gel, and molecular sieve/silica gel mixture. The requirements of the Australian standard are similar to those of the United States, except that there is an additional type of molecular sieve sealant combination (DM). Although the European standard does not make clear restrictions on the categories of desiccants, it puts forward requirements on the physical and chemical characteristics and performance of desiccants, and requires desiccant manufacturers to provide XRF and XRD pattern analysis of desiccants, and use XRD patterns to determine the crystal structure; Use the XRF spectrum to qualitatively and quantitatively analyze the elements, and finally convert them into oxides for analysis; to achieve the role of qualitative desiccant components and categories, which is also a commonly used analysis method in the molecular sieve industry. On the whole, although there are differences in the material categories of desiccant in relevant standards at home and abroad, they all tend to choose molecular sieves.

3. Epilogue

With the in-depth research on desiccant for insulating glass and the continuous development of the domestic desiccant industry, the technical performance and market demand for desiccant for insulating glass are constantly changing, and the production scale of enterprises is also expanding rapidly. At the same time, due to the development of production technology, the quality of products has generally improved. Therefore, some technical indicators stipulated in the industry standard "Desiccant for Insulating Glass" implemented in 2012 are no longer suitable for the current insulating glass, and cannot play a role in supervising the quality of the industry's products, which is not conducive to the development of the entire industry.

In addition, in terms of product technical requirements, the technical requirements in the current industry standard are no longer in line with the current national standard. Therefore, it is necessary to revise the industry standard as soon as possible. By revising the product technical indicators in the standard and revising the relevant testing methods to strengthen the supervision of the quality of desiccant products, standardize the testing of product quality and its application in various fields, and protect the rights and interests of legal enterprises. Promote the continuous development of the technical level of the entire hollow glass and desiccant industry.

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