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Silicone rubber formula design?

Release time:

2019-05-14 12:30

Source:

silicone rubber formula design?
 
Silicone rubberFormulation design: compared with the general general rubber, all three major types of silicone rubber are relatively simple, so is the heat vulcanization type. In addition to raw rubber, the compounding agent mainly includes reinforcing agent, vulcanizing agent and some special additives. Generally, only 5 to 6 components can be used to form a practical formula. Silicone rubber formulation design should take into account the following points.

(1) Silicone rubber is a raw rubber with high saturation, which usually cannot be vulcanized with sulfur, but is vulcanized with heat. Thermal vulcanization is an organic peroxide as a vulcanizing agent, so the compound must not contain active substances that can interact with peroxide decomposition products, otherwise it will affect the vulcanization.

(2) Silicone rubber products are generally used at high temperatures, and their compounding agents should remain stable at high temperatures. For this reason, inorganic oxides are usually used as reinforcing agents.

(3) Silicone rubber is easy to cause the cleavage and rearrangement of siloxane bonds under the action of trace amounts of polar chemical reagents such as acid or alkali, resulting in a decrease in the heat resistance of silicone rubber. Therefore, in the selection of compounding agents must take into account their acidity and alkalinity, while also taking into account the acidity of peroxide decomposition products, so as not to affect the performance of vulcanized rubber.
 
(I) the choice of raw rubber design formula should be based on the performance of the product and the use of conditions, the selection of raw rubber with different characteristics. For general silicone rubber products require the use of temperature in the range of -70 ℃ ~ 250 ℃, vinyl silicone rubber can be used; when the product's use temperature requirements are high (-90~300 ℃), low phenyl silicone rubber can be used; When the product requires high and low temperature resistance and fuel or solvent resistance, fluorosilicone rubber should be used.
 
(II) vulcanizing agent for heat vulcanized silicone rubber vulcanizing agent mainly organic peroxides, aliphatic azo compounds, inorganic compounds, high-energy rays, etc., of which the most commonly used is organic peroxide. This is because organic peroxides are generally relatively stable at room temperature, but can rapidly decompose to produce free radicals at higher vulcanization temperatures, thereby crosslinking silicone rubber.
 
Peroxides can be divided into two categories according to their activity. One is a general-purpose type, that is, high activity, can play a vulcanization effect on a variety of silicone rubber; the other is a vinyl-specific type, that is, low activity, only for vinyl-containing silicone rubber vulcanization.
 
In addition to the above-mentioned general differences between the two classes of peroxides, each peroxide has its own characteristics. The vulcanizing agent BP is the most commonly used vulcanizing agent for molded products. It has fast vulcanization speed and high production efficiency, but it is not suitable for the production of thick products. Curing agent DCBP because of its product is not volatile, vulcanization without pressure will produce bubbles, especially suitable for hot air vulcanization products, but its decomposition temperature is low, easy to cause scorching, rubber storage time is short. Curing agent BP and DCBP are crystalline powder, explosive, for safe operation and suitable for dispersion, usually using their dispersion in silicone oil or silicone rubber paste, the general content of 50%. The boiling point of the vulcanizing agent is 110 ° C. The vulcanizing agent volatilizes when the compound is stored at room temperature, and is preferably used in the form of a molecular sieve as a carrier.
 
The vulcanizing agent DTBP does not react with air or carbon black and can be used to make conductive rubber and products with difficult molding operations. The curing agent DBPMH is similar to DTBP, but it is not volatile at room temperature, and its decomposition products are very volatile, which can shorten the two-stage curing time. The vulcanizing agent DCP is not volatile at room temperature and has the characteristics of vinyl special type. At the same time, the volatility of the decomposition product is also low, and it can be used for vulcanization in occasions with small external pressure. The vulcanizing agent TBPB is used in the manufacture of sponge products.
 
The amount of peroxide used is influenced by a number of factors. For example, raw rubber varieties, filler type and dosage, processing technology, etc. In general, as long as the desired crosslinking can be achieved, the curing agent should be as little as possible. However, the actual dosage is much higher than the theoretical dosage, because the influence of various processing factors must be taken into account, such as uneven mixing, peroxide loss during rubber storage, and the inhibition of air and other compounding agents during vulcanization. For vinyl silicone rubber molded products rubber, the common ranges of various peroxides are as follows: vulcanizing agent BP0.5 ~ 1; vulcanizing agent DCBP1 ~ 2; vulcanizing agent DTBP1 ~ 2; vulcanizing agent DCP0.5 ~ 1; vulcanizing agent DBPMH0.5 ~ 1; vulcanizing agent TBPB0.5 ~ 1. As the vinyl content increases, the amount of peroxide should be reduced.
 
The amount of peroxylun in the mucilage, extruded product and adhesive compound should be higher than that in the molding compound. In some cases, the use of two peroxides can reduce the amount of vulcanizing agent, reduce the curing temperature and improve the curing effect.
The formulation design process is not limited to laboratory testing, but includes the following research stages.

1. Research and analyze the formula used in the production of similar products and similar products.
 
2, develop a basic formula, and on this basis to develop a continuous improvement formula.
 
3, according to the determined plan, in the laboratory conditions to develop an improved formula of the rubber, long test, select the best formula, as the next step of the trial formula.
 
4, in the production or intermediate production conditions for the pilot test, the preparation of the rubber process (mixing, extrusion, calendering, etc.) and physical performance test.
 
5, trial production, make trial products, and test in accordance with standards and technology. According to the test data obtained from the above-mentioned various test stages, it can help to select the final production formula.
If the requirements cannot be met, the experimental study shall be continued until the indicators required by the call are obtained.

Heat resistant silicone rubber formula design: high temperature resistant silicone rubber high temperature resistant (300~350 ℃) silicone rubber mainly phenyl silicone rubber, phenyl ether silicone rubber, silicon boron rubber and silicon nitrogen rubber.
 
Because they are difficult to synthesize, the price is high, and some varieties (such as silicon boron rubber) are highly toxic, so they are difficult to apply in large quantities. The general commercial silicone rubber is methyl vinyl silicone rubber, which needs to be added with heat stabilizer (the most effective is ferric oxide), and the maximum use temperature is 250 ℃.
 
We have tested a variety of heat stabilizers and adopted the method of compounding and special treatment. Silicone rubber vulcanizate can only maintain elasticity and sealing effect for a short time at 300 ℃. Commercial oil-resistant silicone rubber-fluorosilicone rubber can only be resistant to 180 ℃. The Institute of Chemistry of the Chinese Academy of Sciences has achieved great success in heat-resistant silicone rubber, which is summarized as follows.
 
1 350 ℃ high temperature resistant silicone rubber. Methyl phenyl silicon nitrogen rubber synthesized by the Institute of Chemistry is used as a heat-resistant additive for commercial vinyl silicone rubber to eliminate the degradation of the main chain. The heat-resistant level is increased to 350 ℃, and the county has good physical properties. Effect of Silicone Nitrogen Rubber on Properties of Silicone Rubber Addition of Silicone Nitrogen Rubber (parts by mass) 0 20 Hardness (Shore Type A), Degree 45 41 Fracture Strength? M Pa 619 812 elongation at break,% 260 320 performance change after hot air aging (350 ℃× 24 h): hardness, degree +17+7 strength at break,% - 5316 - 2312 elongation at break,% - 50 - 3112 thermal loss,% 33 8 2 250 ℃ resistant fluorosilicone rubber is improved by adding antioxidants, anti-main chain degradation agents (silicone nitrogen rubber) and improving the cross-linking system (cross-linking with chemically synthesized polysilazane cross-linking agent KH2CL) to improve the thermal stability of fluorosilicone rubber.
 
However, when designing the formulation of silicone rubber, the following points should be considered:
 
(1) Silicone rubber is a raw rubber with high saturation, which usually cannot be vulcanized with sulfur, but is vulcanized with heat. Thermal vulcanization is an organic peroxide as a vulcanizing agent, so the compound must not contain active substances that can interact with peroxide decomposition products (such as channel carbon black, certain organic accelerators and antioxidants, etc.), otherwise it will affect the vulcanization.
 
(2) Silicone rubber products are generally used at high temperatures, and their compounding agents should remain stable at high temperatures. For this reason, inorganic oxides are usually used as reinforcing agents.
 
(3) Silicone rubber is easy to cause the cleavage and rearrangement of siloxane bonds under the action of a small amount of polar chemical reagents such as acid or alkali, resulting in a decrease in the heat resistance of silicone rubber. Therefore, in the selection of compounding agents must take into account their acidity and alkalinity, and should also take into account the acidity of peroxide decomposition products, so as not to affect the performance of vulcanized rubber.

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