赢创德固赛 EVONIK DEGUSSA

解决方案&产品

九、有机硅

Silicone Rubber

Silicones are high-molecular compounds of silicon, oxygen and organic groups. The direct synthesis that results by converting organo-halides with silicon and silicon alloys in the production of organo-silicon compounds is also known as the ROCHOW synthesis.

The initial product is Dimethyl-polysiloxane. Cross-linking happens as a result of the vulcanization e.g. initiated by peroxides. Silicone rubber, because of its high heat stability and low temperature flexibility, is suitable for versatile applications in temperature ranges from -60°C to 250°C.

To achieve high mechanical strength, a concentration of 30 - 40% of the desired AEROSIL^® products needs to be added.

1补强剂

Reinforcement

AEROSIL^® fumed silica Reinforces Elastomers

Using AEROSIL^® products as a reinforcing filler in elastomers produces a considerable improvement in their mechanical properties, such as tensile strength, elongation at break and tear resistance. AEROSIL^® fumed silica also helps to control the influence of temperature on mechanical properties. The level of improvement depends on the type of polymer and on the type and concentration of AEROSIL^® products used.

Typical application areas are:

§  HTV Silicone Rubber

  Extruded Parts
AEROSIL^® 200, AEROSIL^® 300, AEROSIL^® 380, AEROSIL^® R 104, AEROSIL^® R 106, AEROSIL^® R 812 S

§  RTV-1-Silicone Rubber

  Sealants
AEROSIL^® 130, AEROSIL^® 150, AEROSIL^® 200, AEROSIL^® R 972, AEROSIL^® R 974, AEROSIL^® R 104, AEROSIL^® R 106, AEROSIL^® R 812 S

§  RTV-2-Silicone Rubber

  Molding Compounds
AEROSIL^® 90, AEROSIL^® 130, AEROSIL^® 150, AEROSIL^® 200, AEROSIL^® 300, AEROSIL^® R 812 S, AEROSIL^® R 8200

§  Liquid Silicone Rubber

  Extruded Parts

  Injection Moldings
AEROSIL^® 90, AEROSIL^® 130, AEROSIL^® 150, AEROSIL^® 200, AEROSIL^® 300, AEROSIL^® R 812 S, AEROSIL^® R 8200

§  Synthetic Rubber

  Conveyor Belts

  Cable Insulation

  Rollers

  Sealants

§  Fluorinated Rubber

  Sealants
AEROSIL^® R 972, AEROSIL^® R 974, AEROSIL^® R 8200

2增稠与触变

Silicone Thickening & Thixotropy

AEROSIL^® fumed silica as a Thickener and Thixotrope in Silicone Rubber

Silicone rubber compounds containing silica are rarely processed to finished products directly after manufacture. Generally, silicone manufactures do not make finished products, but instead provide other branches of industry with their compounds for further processing.

The manufacturers of silicone compounds must ensure that the compounds can be further processed even after lengthy periods of storage or transportation. Bearing this mind, the rheological stability, which is influenced to a great extent by the reinforcing silica used, gains special significance.

Fumed silicas in particular provide thickening and thixotropic properties which are a disadvantage in most of the silicone rubber applications and must therefore be overcome.

When AEROSIL^® fumed silica is dispersed in polymers to compound silicone rubber, the silanol groups on the surface of different particles can interact by hydrogen bond with each other and with the silicone polymers to form connecting bridges. A three dimensional structure develops, which has a thickening effect. This structure can be broken down again by subjecting the system to mechanical stress, either through stirring (mixer), kneading (kneader) or milling (two or triple roll mill). The extent of the break down depends on the type and duration of the mechanical stress. The thickened system thereby regains it's mobility. When in a state of rest, the AEROSIL^® fumed silica particles link up again and the original viscosity is restored. This effect is known as thixotropy and named crepe hardening effect in the silicone rubber industry. It can be shown that a system thickened with AEROSIL^® products requires a certain minimum amount of energy to enable it to flow, i.e. it has a so called "yield point". Surface modified, hydrophobic AEROSIL^® fumed silica shows less thickening than hydrophilic AEROSIL^® fumed silica grades in silicone rubber. The viscosity values of silicone rubber compounds containing AEROSIL^® products increase with an increasing surface area of the silica used. On the other hand hydrophilic AEROSIL^® fumed silica has a less pronounced thickening effect in polar and semi-polar liquids. In these systems, hydrophobic AEROSIL^® fumed silica often shows a remarkable rheological performance, particulary where liquid mixtures or solutions are concerned.

AEROSIL^® fumed silica's thickening and thixotropic effects are largely dependent on the intensity and efficiency of the dispersion. Choosing the most suitable method and dispersion equipment depends on the consistency of the silicone rubber system. It is sometimes advisable to produce a concentrate from part of the silicone polymer or part of the formulation with the total quantity of AEROSIL^® fumed silica (thickphase) and then to disperse this with high shear. The remaining polymer or remaining part of the formulation should then be added to reduce the AEROSIL^® fumed silica content to the required level (redilution).

In principle the thickening effect of AEROSIL^® fumed silica increases with decreasing primary particle size. The effect is, therefore, relatively limited with AEROSIL^® OX 50 (primary particle size of 40 nm), but is strongly pronounced with AEROSIL^® 300 (primary particel size of 7 nm). However, as particles become increasingly fine, they require higher shear levels to achieve the optimum thickening effect.

AEROSIL^® fumed silica for rheology control is widely used in silicone rubber (silicone sealants), coatings, plastics, printing inks, adhesives, lubricants, creams, ointments and in toothpaste.

Typical application areas are:

§  HTV Silicone Rubber

  Extruded Parts, moulded parts
AEROSIL^® 200, AEROSIL^® 300, AEROSIL^® 380, AEROSIL^® R 104, AEROSIL^® R 106, AEROSIL^® R 812, AEROSIL^® R 812 S

§  RTV-1-Silicone Rubber

  Sealants
AEROSIL^® 130, AEROSIL^{< sup> 150, AEROSIL® 200, AEROSIL® R 972, AEROSIL® R 974, AEROSIL® R 104, AEROSIL® R 106, AEROSIL® R 812, AEROSIL® R 812 S}

§  RTV-2-Silicone Rubber

  Moulding Compounds
AEROSIL^® 90, AEROSIL^® 130, AEROSIL^® 150, AEROSIL^® 200, AEROSIL^® 300, AEROSIL^® R 812, AEROSIL^® R 812 S, AEROSIL^® R 8200

§  Liquid Silicone Rubber

  Extruded Parts

  Injection Mouldings
AEROSIL^® 200, AEROSIL^® 300, AEROSIL^® R 812, AEROSIL^® R 812 S, AEROSIL^® R 8200

3热稳定剂

Silicon Heat Stability

AEROXIDE^® TiO₂ P 25 and AEROXIDE^® TiO₂ PF 2

Effective Heat Stabilizers for Silicone Rubber

Silicone rubber is an elastomer whose physical and mechanical properties change only very slightly upon heating. The reason for the high heat resistance is the stable Si-O-Si framework. Substantial degradation of the polymers starts taking place at temperatures above 200 °C. The changes at high temperatures can essentially take two paths:

§  Atmospheric oxygen leads to degradation of the organic groups. This oxidation first results in embrittlement or gelling and then degradation of the polymer chain to form volatile products.

§  In the absence of atmospheric oxygen, depolymerization leads to low molecular weight siloxanes , particulary in the presence of acids and bases which act as catalysts.

Only at temperatures above 450 °C pyrolytic degradation of the silicone rubber occurs forming silica, carbon dioxide and water.

Due to the oxidizing properties of the metal ions, the addition of the iron-titanium mixed oxide AEROXIDE^® TiO₂ PF 2 or AEROXIDE^® TiO₂ P 25 stabilizes the silicone rubber against heat. The metal oxides can acquire electrons and thus render free radicals formed in the thermal stressing of the polymer harmless. Uncontrolled polymerization and oxidation reactions are reduced to a minimum.

These finely divided metal oxides from Degussa are produced by the known AEROSIL^® process. In silicone rubber, AEROXIDE^® TiO₂ P 25 improves the heat stability which also improves the flame-retarding properties. AEROXIDE^® TiO₂ PF 2, also a fumed titanium dioxide, is a further development of AEROXIDE^® TiO₂ P 25. Particularly in addition-crosslinking silicone rubber systems such as Liquid Silicone Rubber (LSR) and RTV-2C Silicone Rubber, TiO₂ AEROXIDE^® TiO₂ PF 2 can achieve a significant improvement in the heat stabilization , compared with TiO₂ AEROXIDE^® TiO₂ P 25.

The influence of AEROXIDE^® TiO₂ P 25 and AEROXIDE^® TiO₂ PF 2 on the thermal stability of different silicone rubber systems (HTV, LSR, RTV-1C, RTV-2C) was studied by Degussa's applied technology department as a function of change in physical and mechanical properties of the temperature treated vulcanizates. A comparison with non stabilized silicone rubber and silicone rubber containing other heat stabilizers like pure Iron Oxide was also done by increasing the storage temperature from 250°C to 275 °C and even up to 300°C using a convection oven.The heat stabilizers in each case have been incorporated into a standard formulation of the corresponding silicone rubber system. As parameters relevant to the performance of the rubbers, the loss in weight, the Shore A hardness, and the rebound resilience of the samples were determined. A difference between P 25 and PF 2 concerning the weight loss cannot be seen by this investigation. Therefore the elongation at break determined on the original samples and compared with the results of samples, which were exposed to the deteriorating influence of air at specified elevated temperatures according to ASTM D 573-99, e.g. in the case of HTV silicone rubber after 7 days at 275°C, the change in elongation at break of PF 2 stabilized silicone rubber was less than 50 %. On the other hand, only 3 days P 25 stabilized rubber shows a change in elongation of more than 50 %.