1.0 Introduction
The performance of organic silica gel is directly affected by its formulation. The types and proportions of various additives in the formula will affect the quality of silica gel. Some mature brands of silica gel, its products can still maintain excellent performance for many years, and has a close relationship with its mature formula.
Additives in silica gel generally include several parts:
2.0 crosslinker
The role of the cross-linking agent is to cross-link with the silica gel base material to generate a three-dimensional structure and adsorb onto the adhered object.
Silicone rubber has many types of cross-linking agents. The chemical structure of the cross-linking agent is one of the main factors affecting the performance of the silicone rubber; different cross-linking agents, different by-products are released during the cross-linking process, single-component RTV silicone rubber Usually classified according to this.
The amount of crosslinker has a great influence on the performance of silicone rubber. With the increase of the amount of cross-linking agent, the tensile strength of silicone rubber increases; when the amount of cross-linking agent increases, the tensile strength will gradually reach the maximum value; then there is a downward trend.
Therefore, after the cross-linking agent is added, cross-linking reaction occurs between the cross-linking agent and the macromolecular chain of the silicone rubber. The cross-linking agent serves as a cross-linking point of the macromolecular chain. When the cross-linking agent increases, the cross-linking density of the silicone rubber molecular chain increases. Improve, so that the silicone rubber molecules form a better three-dimensional network structure, with good tensile strength. However, when the amount of the cross-linking agent is too large, the density of the linking point of the silicone rubber molecular chain and the cross-linking agent increases, and the molecular chain is limited, so that the flexibility of the silicone rubber is decreased, and the elasticity of the silicone rubber is decreased. When external force acts on the silicone rubber, stress distribution will occur due to uneven distribution of stress inside the silicone rubber, and damage will easily occur during stretching, resulting in a decrease in the tensile strength of the silicone rubber.
3.0 Catalyst
In practice, in order to speed up the crosslinking reaction, it is usually necessary to add a catalyst. The vulcanization of the dealcoholic RTV-1 silicone rubber is carried out gradually from the surface to the depth. The amount of catalyst has a certain influence on the dry time of the silicone rubber. The catalyst itself does not participate in the reaction and only accelerates the cross-linking reaction. After the cross-linking reaction is completed, the catalyst will remain in the silica gel.
With the increase of the amount of catalyst, the dry time of the dealcoholic RTV-1 silicone rubber is significantly faster. Silica gels with different curing systems also have different catalyst components. Silica gels for deacidification, deoxalization, and other curing systems generally use organic tin (Lewis acid) as a catalyst, and most dealcoholic silica gels use titanates. Titanate as a light yellow liquid, the silicone will remain in the colloid after curing, with time, the titanate will gradually precipitate out to the colloidal surface, the colloidal light yellow, this phenomenon will not affect Silicone performance.
4.0 reinforcing agent
The mechanical properties of the elastomer formed by the cross-linking of the silicone rubber directly with the cross-linking agent are poor and cannot be used for building sealing. Therefore, it is necessary to add reinforcing fillers to improve the mechanical properties of the sealant.
With the increase of the amount of silica added, the tensile strength of silicone rubber is continuously increased, and the tensile strength is reduced when the maximum value is reached. The molecular chains of silica are similar to those of silicone rubber macromolecule chains and can form hydrogen bonds with the silicone rubber macromolecular chains, and thus can adsorb the macromolecular chains of silicone rubber to form physical adsorption sites. With the increase of the amount of white carbon black added, the adsorption point of silica and silicone rubber molecular chains increases, and the adsorption also increases, and the mechanical strength of silicone rubber increases. When the silica content is too high, it will partially concentrate in the silicone rubber, and the adsorption of the aggregated silica and the silicone rubber will decrease, resulting in a decrease in the reinforcing effect and an increase in the brittleness. When subjected to an external force, the tensile strength of the silicone rubber will decline. trend.
In addition, the two materials commonly used in reinforcing agents are white carbon black and calcium carbonate, which have different reinforcing mechanisms in silicone rubber. The molecular structure of silica is similar to the main chain structure of silicone rubber macromolecules, and its surface contains a large amount of S i-OH group energy, forming hydrogen bonds with SiO bonds in the silicone rubber molecular chain, and white carbon as a physical Cross-linking points to achieve enhanced purpose. The active calcium carbonate is stearic acid-treated light calcium carbonate. After the active calcium carbonate is dispersed in the silicone rubber, the long-chain molecules of stearic acid on the surface of the particles are intertwined with each other to form a loose particle surface network structure. It plays a reinforcing role on silicone rubber. Silica enhances better than calcium carbonate but costs more.
5.0 Plasticizers
Any material added to a polymer system that increases the plasticity of the polymer system (plasticity is a material characteristic of permanent deformation of the material at a given load) can be called a plasticizer.
The main role of the plasticizer is to weaken the secondary bond between polymer molecules, ie van der Waals force, thereby increasing the mobility of the polymer chain and reducing the crystallinity of the polymer chain, ie increasing the plasticity of the polymer. It shows that the hardness, modulus, softening temperature and embrittlement temperature of the polymer decrease, while the elongation, flexibility and flexibility increase.
6.0 Others
6.1 Thixotropic agents
When the grease is sheared, the consistency becomes smaller, and when the shear is stopped, the property of increasing the consistency is thixotropy. Thixotropy is also known as shaking. It is a reversible sol phenomenon where the gel body is subjected to mechanical forces such as oscillation and compression. It was discovered by H.M.F. Freundlich 1928. The slender particles are connected by weak chemical bonds to form a network structure and are easily destroyed by external forces. At present, four major categories are commonly used: meteorological silica, organic bentonite, hydrogenated castor oil, and polyamide wax. However, these four types are quite different in their use. The effect of the thixotropy of paint on the order from large to small is : Polyamide wax, silica, organic bentonite, hydrogenated castor oil.
6.2 Tackifiers
Despite the stickiness of natural rubber, the adhesive strength of adhesives produced by rubber alone is insufficient for many applications. Most commercial synthetic elastomers lack stickiness, either on their own or on other surfaces. Therefore, in order to increase their viscosity, a resin thickener is added. Large quantities of latex and solvent systems, especially when used as contact adhesives, require the use of specific types of tackifying resins.
Other components include moisture scavenger, anti-mold agent, and flame retardant.
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