1.0 Introduction
The silica gel used in the production of photovoltaic modules is a special organic silica for photovoltaics. It is necessary to seal the laminate assembly with silica gel and bond the junction box. One-component room-temperature vulcanized silicone rubber is formed by condensation and cross-linking to form elastomers. After half a century of development, different curing systems and factions have been formed. Their technologies and applications are not the same or different. The basic structural unit of organic silicone products is composed of silicon-oxygen (Si-O) chains, and the side chains are connected to other various organic groups through silicon atoms.
2.0 silicone type and comparison
The development of organic silicones has gone through several important stages: deacidification, desorption, traditional dealcoholation, and new dealcoholization. At present, the mainstream single-component silica gel on the market is a deterpenoid type, and there are few de-alcohol-type products.
One-component RTV silicone rubber generally consists of the following components:
Base rubber (or reactive base polymer) - hydroxyl-terminated polydimethylorganosiloxane.
Crosslinkers - polyfunctional silane compounds containing hydrolyzable groups;
Catalysts - diorganotin dicarboxylates, diorganotin di(p-diketonates), titanates and their chelates;
· Reinforcing agent (filler) - silica, calcium carbonate, diatomaceous earth, etc.
Plasticizers - non-reactive dimethyl silicone oil, MDT silicone oil, paraffin mixture, etc.;
· Others - thixotropic agents, tackifiers, moisture scavengers, mildewproofing agents, heat-resistant additives, flame retardants, etc.
3.0 Yellowing mechanism of silica gel
1. Environment induced yellowing
The environmental factors that cause yellowing of silica gel are mainly temperature. At high temperature, the internal molecules of silica gel decompose, specifically, the NO3- ions decompose into nitrogen-containing structures such as NO2, which makes the silica appear yellow in appearance. Another explanation is that the silica gel molecules decompose into groups capable of absorbing colored light, making the silica yellow in appearance.
2. Component induced yellowing
The catalyst used for the dealcoholic silica gel is a titanium complex (titanate). The titanium complex itself is a yellow liquid. The catalyst accelerates the reaction speed during the curing process and does not react itself. Therefore, the catalyst still exists in the product after the reaction is over. The liquid catalyst is present in the product and will liberate downwards. The free catalyst turns yellow. This yellow color is the color of the catalyst. It will only affect the color of the appearance of the product. It will not affect the electrical, physical and chemical properties of the cured silicone.
4.0 Summary
In summary, the most common reason for yellowing of silica gel is the change in the valence state of N. The N-containing group absorbs colored light in the light, making the appearance of silica gel yellow. (The author WeChat public account: Photovoltaic experience network)
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