The application and requirements of polyisobutylene sealant for insulating glass

Polyisobutylene (PIB) hot-melt sealant for insulating glass (commonly known as butyl sealant) is a viscoplastic material whose function is to ensure the framing and bonding molding, and provide a sealing barrier for the product. JC/T914 is a product standard formulated following national conditions. So far, there is no similar foreign standard for reference. It has played an important role in product quality control and adapting to the production of insulating glass. With the rapid growth of the output of insulating glass, butyl sealant has entered large-scale production, and the application technology has been significantly improved, creating conditions for improving the scientific and applicability of product standards. This article focuses on analyzing the application, composition, physical properties, and interrelation of butyl sealant, discussing product technical requirements, and put forward some suggestions for reference.

^{The polyisobutylene sealant for insulating glass 1}

1. The main composition of sealant

The process performance and sealing performance of sealant depends on the product formulation composition. The base glue of the butyl sealant is liquid polyisobutylene (PIB), which is a viscous fluid with a molecular weight of 40,000 to 90,000, and the dosage is generally more than 40%. PIB is an amorphous carbon-carbon backbone homopolymer. It does not contain double bonds and never solidifies. It has chemical inertness similar to paraffin hydrocarbons. It gives butyl sealants excellent chemical corrosion resistance, weather resistance, and Heat and immersion aging, extremely low permeability to air, moisture, and other gases, very low surface energy makes it wet and sticky to difficult-to-adhesive materials such as PE, EPDM, and extremely low glass transition temperature (-60°C) Plastic flow is repeatedly deformed by an external force, but because PIB is soluble in hydrocarbon solvents, butyl sealant is not resistant to oil. The molecular structure of polyisobutylene is as follows:

^{The polyisobutylene molecular structure 1}

^{The butyl rubber molecular structure 1}

^{The chlorinated butyl rubber structure 1}

Butyl sealant components need to add tackifiers, lubricants, softeners, and fillers. Tackifiers used to increase the initial bonding force, generally, rosin, coumarone resin, disproportionated rosin resin, etc., are mostly added in the form of solid powder; lubricants and softeners are often used to adjust product operation technology, generally using heat -Ultraviolet rays irradiate non-volatile oxidized polyethylene, etc. to ensure the internal quality of the insulating glass; most of the fillers are carbon black and calcium carbonate powder. The plasticity of the system is improved by hot melt blending and ultraviolet rays are shielded, which is beneficial to durable sealing.

It is worth noting that the supply of butyl rubber or chlorinated butyl rubber is non-fluid, and solvent oil is often added to improve the processability when mixing these ingredients. The inner surface of the hollow glass may be fogged under heat-ultraviolet radiation, which affects the product. Durable and transparent.

2. The application function of butyl sealant in the frame and composite of insulating glass　　 

The insulating glass structure forming process starts with butyl sealant inserting spacers to form a spacer frame, hot-coating a specified size of butyl rubber on the specified surface of the spacer frame, and then pressing the two pieces of glass on the laminator and filling the outer sealant. The molded and bonded insulating glass is placed on the stand. In this stage, the bonding and positioning of the spacer-glass composite sheet depend on the butyl sealant. The basic structure is shown in Figure 1.

^{Figure 1 The insulating glass group frame sticking structure}

The butyl sealant is supplied as a cylindrical compound, which can be directly loaded into the hot-pressing extruder, and extruded in a molten viscous fluid state under high pressure after exhausting. The extrusion speed is synchronized with the movement speed of the spacer. The hand-held operation can ensure that the adhesive tape is regular and continuous. The current common problems and their causes have the following analysis:

1) The butyl glue squeezed into the side surface of the spacer bar appears irregular burrs. The reason may be that the glue nozzle is partially blocked by foreign matter or has burrs. The glue nozzle should be checked frequently and the outlet should be cleaned up if necessary. Do trimming and smoothing.

2) There are white lines (bonding gaps) in the visible adhesive strip after the glass is joined. The reason may be that the injection speed is not matched, or the air is not exhausted after the machine is loaded, or the supplied butyl rubber contains air.

3) The extruded rubber strip is broken. The reason may be that the hot pressing temperature is too low, or the viscous fluidity of the glue is insufficient, or the temperature is too high to cause excessive melting and flow, or there are bubbles in the glue;

4) Poor adhesion between the spacer and butyl sealant may be caused by interface oil and dust, or poor initial adhesion of butyl rubber, or poor infiltration due to the mismatch between the conveyor belt speed and the extrusion speed of the spacer, or the pressure roller Improper location, causing bonding dislocation.

^{The insulating glass edge sealing structure 1}

3. The butyl sealant sealing function in insulating glass applications　　

Single-channel sealed insulating glass has a very short service life, which shows the importance of inner layer sealing. The moisture vapor transmission rate of butyl sealant is ≤1 g/m² •d, the edge bonding width (permeation channel width) is about 7mm, and the bonding thickness (permeation path length) is 0.3~0.6mm (Figure 2). The moisture penetration rate depends on the moisture permeability of the butyl sealant and the bonding thickness. If calculated according to the bonding thickness of 0.6mm, the theoretical life of the saturated moisture absorption of the insulating glass is more than 30 years due to the moisture penetration. Many factors are affecting the early failure of insulating glass in buildings. The effect of high temperature and high humidity is a significant factor. High humidity increases water vapor diffusion and osmotic pressure. High temperature increases the internal air pressure of insulating glass, which causes the panel to flex and deform and stretch the glue joint, that is, enlargement. The penetration area accelerates the penetration of moisture. It is necessary to determine the quantitative relationship through experiments.

In recent years, the phenomenon of oil seepage, oil flow, and even black rubber block of butyl sealant in the inner layer of insulating glass in buildings is caused by accidents (Figure 2 and Figure 3), which are caused by the hydrocarbon oil or plasticizing in the outer sealant. The penetration of the butyl sealant will cause the butyl sealant to dissolve, precipitate and even ulcerate. It should be avoided that the external rubber is mixed with oil or plasticizer that dissolves polyisobutylene, and the butyl sealant should also be prevented from being mixed with oil, such as detected by ultraviolet fogging test and thermal weight loss test.

4. Technical performance analysis of butyl sealant　

4.1 Current standard requirements

The standard specifies the minimum performance requirements of the product, including appearance, density, penetration, shear strength, water vapor transmission rate, thermal weight loss, ultraviolet fogging, low-temperature flexibility, etc. To deeply analyze the performance related to product consistency and function, Table 1 lists the main performance test results of the 6 commonly used brands (including imported) products in the market.

^{The main technical requirements of hot melt butyl rubber for insulating glass and the performance of some products 1}

1) Appearance-no visible particles, gels, and air inclusions are required. The insulating glass production line requires a sealant to be a uniform and delicate thermoplastic colloid to ensure continuous and uninterrupted extrusion of the rubber strip.

2) Density-calculation basis for insulating glass production materials and relatively characterize the consistency of product composition. The density of each product can be different, but the density of each product should be stable, and its limit index is ±0.05g/cm³ .

3) Penetration-characterize the plasticity and high-temperature rheology of the product at room temperature. A high temperature (130℃) penetration range of 230～330 (1/10mm) is required. Too low may affect the extrusion construction and bonding quality, and too high may cause the flow to affect the regularity of the extruded rubber; normal temperature (25°C) is required The penetration range is 30～50 (1/10mm). Exceeding the range may affect the initial adhesion size of the insulating glass. Figure 3 shows the penetration change curve of 6 products at a temperature of 25℃-150℃. It can be seen that their theology is very similar, and the fluctuation range of the penetration test value is very small, concentrated at 43～48 (1/10mm) at 25℃. 130℃ is 225～254 (1/10mm). This good condition is conducive to the online production of insulating glass, but it is worth discussing whether to adjust the index requirements according to this level.

4) Thermal weight loss characterizes the volatile low-molecular-weight content and thermal stability of the product at high temperatures, which has a certain meaning for controlling the oil filling of the product formulation and preventing the inner surface of the hollow glass from atomizing. The standard requires that the quality change of the product under the condition of 130℃×50his ≤0.5%. The results in Table 1 show that the thermal weight loss of the product is less than 0.1%, indicating that the product has very little volatile low-molecular-weight content. It is a restricted product. Whether the project can be drafted index is ≤0.1%.

5) Low-temperature flexibility characterizes the possibility of product embrittlement at low temperature, embrittlement cracking is not conducive to sealing. The standard requires the product to remain flexible at -45°C. Polyisobutylene has a very low glass transition temperature (-60°C), giving butyl sealants excellent low-temperature performance. Changes in composition or the addition of other polymers may deteriorate low-temperature performance.

6) Water vapor transmission rate characterizes the product's ability to block water vapor penetration. The transmission rate is required to be ≤1.10 g/m² •d, which is an important indicator to ensure the sealing life of the insulating glass. The results in Table 1 show that the products meet the requirements. Whether to improve the durability of insulating glass sealing, it is worth discussing whether to draw up more stringent indicators.

7) Ultraviolet radiation fogging characterizes the possibility of organic condensation of products in the hollow glass under ultraviolet radiation. As the inner sealing layer of insulating glass, the non-fogging of butyl sealant is the basis to ensure that the insulating glass is resistant to ultraviolet radiation. Although the ultraviolet radiation fogging test is an inspection item for insulating glass, the inspection of butyl sealant is crucial Important, it is conducive to the detection of oily or solvent-containing products in advance.

8) Shear strength characterizes the adhesion performance of the product to the substrate (glass, aluminum, stainless steel, etc.), and is of significance for the evaluation of the applicability of the butyl sealant in the insulating glass laminate and transfer process. It is worth noting that the physical form of this product is a viscoplastic body, and the bonding strength is sensitive to temperature and extremely sensitive to the force-acting speed. The standard stipulates that the testing tensile speed is 50mm/min, that is, the force acting speed is faster, and the hollow glass is transferred on the production line. During the process, the product is required to have a certain degree of stickiness, that is, to ensure that the hollow glass does not displace or disintegrate during the bonding operation process, so the testing strength should choose an appropriate loading speed. As for the strength of the second sealant after curing, it is not important. However, it is worth noting that under the effect of thermal deformation and displacement in the application of insulating glass, the elongation performance of the butyl sealant under low-speed stretching can ensure the continuous and complete rheology of the sealing layer under deformation conditions and ensure the sealing of the insulating glass.

4.2 application-related requirements　　 

Related applications and requirements worthy of consideration, such as extrudability, plasticity, stickiness, and elongation at break, have the following analysis:

1) Extrudability-characterize the processability of the sealant suitable for hot extrusion applications. The test method is to extrude the product from the nozzle of the specified caliber under a certain temperature and pressure, measure the extrusion volume per unit time, and determine the applicability of the product in the extrusion construction of the production line. The test results of product extrudability ranged from 6.94 to 7.96 (ml/the 30s) (Table 2). The test method of this project is not standardized.

2) Plasticity characterizes the plastic deformation ability of the product under constant pressure. The general standardized test method for quality control of plastic sealing putty is adopted, that is, the sample is placed between the pressure plates of the plasticity tester, and the static load is applied with a weight of 5kg. The time for the sample thickness to be compressed from 20mm to 10mm is measured, and the test is measured The shrinkage of the thickness of the sample at 10 min after unloading. This project is of significance for evaluating the plastic deformation performance of the product composite sheet under compression, and the test method is scientific and practical. The results of the product plasticity test are shown in Table 2.

^{The extrudability and plasticity test results 1}

3) Adhesiveness—Evaluate the adhesive ability of the product to adapt to the bonding and forming process of insulating glass. In the hollow glass bonding and molding process, when the bonding components are transported, transferred, and stacked, the butyl sealant will bear the weight of a single piece of glass, and the adhesiveness should ensure that the two pieces of glass will not be misaligned or separated within a short period of time. The test adopts the test method of pressure-sensitive tape, adopts the glass-metal plate to bond the test piece, hangs on one side on the support of the stickiness tester, and measures the time of slippage or slippage. Specific technical indicators can be drawn up with reference to the production conditions verification test.

4) Elongation at break characterizes the maximum elongation deformation when the product breaks in tension. The industry believes that "the maximum elongation rate of sealed butyl rubber is only 10%. If the elongation rate of the second sealant is very large, it will damage the first seal and affect the airtightness of the insulating glass." It can be seen that the elongation at break is related to the sealing performance. , The elongation at break index should be drawn up. The test measured the stress-strain curve of 4 products when they were stretched (speed 50mm/min) (Figure 5). The elongation at break of butyl sealant all exceeded 400%, that is, the glue layer can be kept under tension and deformation within this range. Continuity, will not break and destroy.

^{The Tensile stress curve of insulating glass butyl sealant 1}

5. Discussion　　 

The thermoplastic butyl sealant based on viscous fluid polymer has rheological properties imparted by viscous fluid. At present, there is little research on the deformation characteristics of its slit glue and its relationship with moisture penetration. The hollow glass flexes under the action of external force or internal pressure to expand the thickness of the glue seam, and at the same time change the vertical dimension of the butyl sealant, the viscous fluid seems to shrink into the narrow seam under the action of the opening effect. If the effect of surface adhesion is not considered, the thickness is doubled (from 0.6mm to 1.2mm) based on Poisson's ratio of 0.5, and the bonding width (length of penetration path) should be retracted inward by 50% (from 7mm) 3.5mm), it is necessary to test and study the influence of this change on the penetration rate to provide a technical basis for reducing the durability of the insulating glass seal.

^{Figure 6 The follow-up insulating glass spacer 1}　

In addition, butyl sealant is three-sided bonding, and the stress-strain model is different from two-sided bonding. In order to reduce the influence of unfavorable deformation on the sealing performance of insulating glass, can the bonding structure be changed, such as the development of follow-up spacers, flexible spacers, etc. (Figure 1b, c), the spacer side and the sealant can bend with the glass The deformation is synchronized in the same direction to reduce the change in the thickness and length of the sealing channel (Figure 6).