Silicone rubber can be divided into blended silicone rubber and liquid silicone rubber according to the form of the product. Among them, liquid silicone rubber is a rubber material with self leveling or thixotropic properties, which is composed of linear polyorganosiloxane with medium polymerization degree as the basic polymer filler and various additives. After vulcanization, it forms an elastomer. Liquid silicone rubber can be divided into two types based on its cross-linking and vulcanization mechanism: condensed silicone rubber and additive silicone rubber.
The curing mechanism of the two types of liquid silicone rubber is completely different. Condensed silicone rubber utilizes the moisture curing reaction mechanism, where the hydroxyl group of 107 rubber and silane crosslinking agent undergo condensation reaction under the conditions of organic tin/organic titanium catalyst and moisture to achieve crosslinking curing; Additive silicone rubber utilizes the mechanism of silicon hydrogen addition reaction, which involves the addition reaction of ethylene containing polysiloxane and Si-H containing polysiloxane on a platinum catalyst, followed by heating (or room temperature) to achieve cross-linking and curing.
Due to the different curing mechanisms, there are many differences between condensed silicone rubber and additive silicone rubber in terms of product packaging form, usage, and performance (Figure 2), such as curing speed, body strength, adhesion, etc., but each has its own advantages.
1. Packaging form. Condensed silicone rubber can be packaged in either single component or dual component packaging. Additive silicone rubber is a temperature sensitive product that requires separation of the catalyst and crosslinking agent. It is usually packaged in a two-component form to avoid reactions during storage.
2. Curing speed. Due to differences in curing mechanisms, there is a significant difference in curing speed between condensed and additive silicone rubber. Condensed silicone rubber (mainly referring to single component) reacts from the surface with the help of moisture in the air, gradually vulcanizing from the surface to the inside. Deep curing is slower and usually takes 24 hours or even a week to fully vulcanize. Additive silicone rubber can be vulcanized at both room temperature and high temperature. Under room temperature conditions, the internal and external vulcanization of the rubber material is relatively consistent, without a clear distinction in order.
3. Body strength. Both condensed and additive silicone rubber can be reinforced with fillers such as nano calcium carbonate and gas-phase white carbon black to enhance their mechanical properties, with gas-phase white carbon black having the most significant reinforcement effect.
4. Adhesion. Condensed silicone rubber can achieve adhesion to various materials by adding silane coupling agents. Therefore, the majority of adhesive and fixing sealants in the fields of construction, home appliances, lighting, rail transit, photovoltaics, etc. currently use condensed silicone rubber, which is based on its good adhesion and wide adhesion. However, the adhesive properties of additive silicone rubber are far inferior to those of condensed silicone rubber, which is related to the low polarity of its molecular chains and the absence of polar groups that can react with coupling agents.
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