Condensed silicone rubber and molded silicone rubber have many differences in product packaging form, use mode 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 used in the form of single-component packaging, or can be used in the form of two-component packaging. Since the additive silicone rubber is a temperature-sensitive product, it is necessary to separate the catalyst and the crosslinker, usually in the form of two-component packaging to avoid reactions during storage. Of course, there are also single-component silicone rubber products on the market, which are mainly divided into two types: one is low-temperature storage, that is, it needs to be stored and transported under freezing/refrigeration conditions, which is more inconvenient to use; The other is room temperature storage, this product uses a special structure of catalysts and inhibitors, at room temperature does not have reactive activity, the catalyst activity is released at high temperature to achieve cross-linked curing, but the cost is high, the curing temperature is relatively high.


2, curing speed
Due to the difference of curing mechanism, the curing speed of condensed and additive silicone rubber is different. Condensed silicone rubber (mainly refers to a single component) is to react from the surface with the moisture in the air, gradually vulcanized from the surface to the inside, deep curing is slower, usually takes 24h or even a week to completely vulcanized. Molded silicone rubber can be vulcanized at both room temperature and high temperature. At room temperature, the internal and external vulcanization of the compound is more consistent, and there is no obvious priority. Under high temperature conditions, the curing speed has a very wide adjustable temperature, and the adhesive can achieve rapid vulcanization within 5min or even 1min under baking conditions. It should be noted that under heating conditions, the curing speed of molded silicone rubber has a certain relationship with the sizing thickness, the heat transfer of the substrate, and the blast effect of the baking equipment, and the specific application is analyzed.


3, body strength
Whether condensed or formed silicone rubber, the mechanical properties can be improved by reinforcing fillers such as nano-calcium carbonate and fumed white carbon black, among which the fumed white carbon black has the most obvious reinforcing effect. Due to the differences in the system, condensed silicone rubber usually uses nano-calcium carbonate as the main reinforcement filler, and the addition amount of fumed silica is limited, so the tensile strength is low, generally not more than 3MPa, while the addition of molded silicone rubber generally uses fumed silica as the reinforcement filler, and the addition amount can be increased to more than 30% by the treatment of hamethyl disilazane and other modifiers. Tensile strength can reach more than 10MPa, far more than condensed silicone rubber.


4. Adhesion
Condensed silicone rubber by adding silane coupling agent, can achieve the bonding of a variety of materials, so the current in the construction, home appliances, lighting, rail transit, photovoltaic and other fields of bonding fixed sealant most of the use of condensed silicone rubber, it is based on its good bonding and bonding universality. However, the adhesion of additive silicone rubber is far less than that of condensed silicone rubber, which is related to the low polarity of its molecular chain and the polar groups that can react with the coupling agent are not contained in the molecular chain. The adhesion of additive silicone rubber is generally improved in the following three ways: (1) the surface treatment of the substrate with a primer; (2) Add a tackifier with a special structure, and usually require high temperature baking to achieve good bonding.


In addition, there are differences in reaction byproducts and catalyst poisoning between additive and condensed silicone rubber. Condensed silicone rubber usually releases small molecules such as acetic acid, alcohol or ketone oxime during the curing process, which has a certain odor, while the additive silicone rubber has no reaction byproducts and has a low odor. Because the platinum catalyst added to the molded silicone rubber is prone to poisoning and inactivation, it cannot contact organic matter containing N, P, S and other elements and ionic chemicals containing Sn, Pb, Hg, Bi, As and other heavy metals, condensed silicone rubber does not have this limitation.