Sealing materials for photovoltaic modules

The sealing material of photovoltaic modules mainly refers to silicone sealants such as paste-like room temperature vulcanized silicone rubber (RTV).
Silicone rubber has excellent heat resistance, cold resistance, UV resistance and atmospheric aging resistance, and can be used for a long time at temperatures as low as -60°C and as high as +250°C. After being cured with a certain formula, the silicone rubber can be maintained in harsh environments such as sun, rain and snow for 25 years without cracking, brittleness and high strength. Therefore, silicone rubber is used for sealing and bonding of photovoltaic modules. best material.

According to the product packaging form, silicone rubber can be divided into two categories: one-component and two-component. One-component liquid silicone rubber is made by uniformly mixing polysiloxane, cross-linking agent, filler, catalyst and other additives under the condition of isolating moisture. After contacting with moisture in the air, the silicone rubber is cross-linked and cured to play a sealing role.

One-component silicone rubber has the advantages of convenient use and relatively low equipment investment cost, and is currently a commonly used sealant for bonding photovoltaic module frames and junction boxes. Two-component silicone rubber is a type of silicone sealant in which the base material and the vulcanizing agent are packaged separately and mixed in proportion when used. Compared with single-component silicone rubber, two-component silicone rubber has relatively lower requirements on the temperature and humidity of the curing environment, and the curing speed is fast, which can greatly shorten the handling and packing time. However, the cost of two-component silicone rubber products is relatively high. It needs to be equipped with two-component sizing equipment. The equipment investment is relatively high, and the control requirements for the sizing ratio are high. Therefore, generally only the sizing amount and volume are relatively large. It is used when the curing time is required. Generally, the junction box potting glue and the back bracket adhesive of the film/double glass module mostly use two-component silicone rubber.

On photovoltaic modules, silicone rubber is often used as frame bonding sealant, junction box bonding sealant, junction box potting sealant, film/double glass module back beam bonding structural adhesive, etc. The sealant for the frame and the junction box mainly considers the bonding and sealing performance, and generally the same silicone rubber can be used; the junction box potting glue must not only be able to seal and bond but also meet the electrical insulation performance requirements; and the back beam glue is mainly used for bonding It is required to have very strong bonding properties, and silicone construction sealants (commonly known as silicone glue) are generally used. Here are introduced according to the three categories of sealing silicone rubber, potting silicone rubber, and silicone rubber.

  1. Sealing silicone rubber

The aluminum frame and glass of photovoltaic modules are materials with high hardness. If the two are assembled in direct contact, the glass will be easily damaged, so a buffer layer needs to be added between them; Due to the effects of temperature changes, wind, rain, snow, ice, salt spray, moisture, etc., the module frame and laminate must be firmly bonded and sealed to ensure long-term reliable operation of photovoltaic modules. As the key material connecting the frame and the laminate, the sealing silicone rubber can fully fill the gap between the laminate and the aluminum frame. The requirements of bonding and sealing, but also greatly improve the bearing capacity and resistance to deformation of photovoltaic modules.

After the photovoltaic module is framed and packaged, the junction box can be bonded to the backplane with silicone rubber. In order to ensure the reliable connection between the junction box and the module, the performance of the silicone rubber is required to meet the following requirements:
(1) It has a certain thixotropy, does not flow, and is not easy to cause pollution;
(2) The viscosity is moderate, easy to use, it can be applied manually or automatically dispensed with equipment; 3) The curing speed is moderate, with reasonable operation time and less glue cleaning time, to meet the requirements of production tact:
(4) Excellent bonding matching and bonding strength, good bonding matching for aluminum profiles, glass, photovoltaic backplanes, and junction boxes;
(5) Excellent mechanical load resistance and good thermal deformation compensation ability;
(6) Excellent electrical insulation properties and flame retardant properties;
(7) Long-term and reliable environmental aging resistance, resistance to ultraviolet radiation, resistance to rain and contamination, resistance to hail impact, and can resist thermal expansion and contraction caused by changes in ambient temperature.

The one-component silicone rubber used for photovoltaic module frame sealing and junction box bonding mainly includes two types: deketone and dealcohol. Deketone fatty silicone sealants include Tianshan 1527, Dow Corning PV-8101, etc., and dealcoholized silicone rubber sealants include Dow Corning 7091, Tianshan 1581, etc. Among them, Tianshan 1527 is widely used by Chinese module manufacturers for its excellent performance and stable quality. Its market share once exceeded 50%. When using it, it should be noted that some dealcoholized silica gel will react with some EVA outdoors. The yellowing phenomenon of the EVA at the edge of the module is caused, mainly because the sealant uses a highly active curing accelerator, which reacts with the UV absorber in the EVA to produce a colored integration.

In recent years, in order to meet the increasing production cycle requirements and shorten the handling and packing time, photovoltaic enterprises have also begun to use some two-component silicone rubber, double-sided tapes, etc. for photovoltaic module frame sealing and junction box bonding. For double-sided tapes, most of the polyethylene foam tapes currently used in the market are composed of foamed polyethylene and acrylate pressure-sensitive adhesive coatings. The bonding effect is achieved by the pressure-sensitive adhesive coating. The foam substrate can give the tape some compressibility.

Compared with silicone sealing, the sealing method of foam tape does not require post-curing, and can be moved and stacked after installation, which not only saves space, but also greatly improves production efficiency. However, foam tape has poor high temperature resistance and shear strength. On the low side, the cohesive destructive force is much lower than that of silica gel. Therefore, the same component design, the component using tape, its load resistance is much lower than the component using silicone rubber, so you need to pay attention when choosing. At present, polyethylene foam tapes are mainly produced and supplied by large foreign companies such as Roman, tesa and Saint-Gobain, and the cost is also slightly higher. At present, from the perspective of the entire market, single-component silicone rubber still occupies the mainstream position, reaching more than 85%.

  1. Potting silicone rubber

The silicone rubber used for junction box potting is usually two-component silicone rubber, and the junction box potting glue mainly plays the role of sealing, insulation and heat dissipation. After the junction box is potted, the internal oxygen can be replaced by glue, which can reduce the oxidation and corrosion rate of the metal terminals inside the junction box, and at the same time prevent the water vapor from contacting the charged body, avoid short circuit of components, and improve the protection of the junction box; , The solar cell module junction box is equipped with bypass diodes. When part of the cells of the module are blocked, the current passes through the bypass diodes, causing the temperature of the diodes to rise sharply. It can effectively avoid fire caused by overheating of the junction box and prolong the service life of the diode.

The specific performance requirements for the junction box potting compound are as follows:
(1) Good liquidity. Since the junction box includes electronic components such as diodes, the structure is complex. In order to fill all the gaps in the junction box, the junction box potting glue is required to have good fluidity;
(2) The operable time and gel time meet the requirements of different production processes. According to the production process requirements, the potting glue is usually required to have a long operating time to ensure that the junction box is completely filled; at the same time, it has a short gel time after potting, so that the potting glue can be cured to a certain extent, Turnover will not flow or fall, which meets the requirements of component handling and requires that the potting glue can continue to cure in the junction box under the condition of isolation of air.

(3) Excellent insulation. The potting compound needs to have excellent insulation properties when it contacts electronic components such as diodes and copper sheets;
(4) High thermal conductivity. The bypass diode in the junction box will generate a lot of heat when it is working. If the heat cannot be dissipated in time, the bypass diode is at risk of burning;
(5) High flame retardancy;
(6) Excellent aging resistance.

  1. Silicone glue

With the increasing market demand for photovoltaic power generation, the development of photovoltaic module products that can be easily installed has become one of the goals pursued by companies in the industry. Among them, the installation of photovoltaic modules can effectively improve the installation efficiency of photovoltaic modules. Silicone glue is generally used for bonding the back beam and components, which not only can meet the long-term aging resistance, but also has excellent bonding performance. Because the modules will be affected by external factors such as light, temperature changes, wind, rain, snow, ice, salt spray, moisture and other external factors, the silicone adhesive plays a very important role in the strength and safety of the modules and systems. .

Silicone sealants can be divided into one-component and two-component sealants according to the curing method when used. Silicone glue has a 40-year history of use in the construction field, and the curtain wall glass using silicone glue has a 20-year service life. Under the premise of fully understanding the reliability and failure mechanism of silicone glue, and by strictly controlling the quality of the glue and the construction environment, the application of silicone glue in the photovoltaic field can fully meet the reliability requirements. The method of evaluating the silicone adhesive for back beam bracket bonding is based on the initial mechanical stress, the performance after high and low temperature aging, the performance after fatigue test, and the aging after salt spray, acid spray, immersion cleaning agent solution, and ultraviolet radiation. For comparison of the performance of the materials, the decay rate of mechanical properties after aging is required to be lower than 25%, and the debonding area is not higher than 10%. The specific performance requirements of the silicone adhesive for back beam bracket bonding are as follows:

(1) It has a certain thixotropy, does not flow, and is not easy to cause pollution; (2) The two-component silicone adhesive is evenly mixed in proportion, and no bubbles are generated;
(3) The curing speed is moderate, with reasonable operation time and short glue cleaning time, which can meet the requirements of production rhythm;
(4) Excellent bonding matching and bonding strength, and the bonding performance of various substrates meets the requirements of the load performance of components;
(5) Excellent mechanical load resistance and good thermal deformation compensation ability;
(6) Excellent flame retardant properties;
(7) Long-term and reliable environmental aging resistance, there is no danger of damage due to internal or external effects (such as water, moisture, sunlight exposure, temperature changes, etc.).

  1. Performance requirements of silicone rubber sealants

In order to realize the standardized application of silicone rubber sealants in the photovoltaic industry, improve the technical level of enterprises, and ensure the reliable quality of photovoltaic modules, the National Adhesive Standardization Committee organized well-known adhesive and photovoltaic module companies in the industry to interpret 1EC61215, IEC61730, U1.1703, etc. International photovoltaic standards, especially the requirements for safe use, mechanical bearing, environmental aging, etc., and in-depth understanding of the overall requirements for photovoltaic modules and materials. By analyzing the technical requirements of different glue points for photovoltaic modules, the glues for photovoltaic modules are classified, and five types of glues for frame sealing, junction box bonding, junction box potting, bus bar sealing, and film module bracket bonding are finally determined. It also formulated technical standards and formed a complete national standard of GB/T 29595-2013 “Silicone Rubber Sealants for Ground Photovoltaic Module Sealing Materials”. The standard was released and implemented on December 1, 2013.

For the corresponding relationship between the flame retardant grade of silicone rubber sealant and HAI, CTI index, please refer to the corresponding relationship between different flame retardant grades and HA1, CTI index in Table 1.

Figure 1 - Corresponding relationship of HAI/CTI indexes of different flame retardant grades
Figure 1 – Corresponding relationship of HAI/CTI indexes of different flame retardant grades

Read more: Evolution of PV module technology

Related Posts