Introduction to Modified Silicone Rubbers

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Silicone rubber is one type of macromolecular elastic material whose molecular main key consists of repeat Si-O chains which contain both of inorganic and organic properties. It has high dissociation energy and bound tightly. Therefore, silicone rubber has features like cold and heating resistance, oxidation resistance, odor resistance, excellent electrical isolation and radiation resistance. It is due to the molecules of polysiloxane is in the form of helix and the organic functional groups face the outside and they are easy to be circled around the Si-O axis, its internal cohesive energy density is low and molecular softness is good. Therefore, its vitrification temperature is low and has excellent low temperature resistance. It has low surface tensile strength and performs humidity resistance and good breathability. The side functional groups which are linked to raw rubber’s silicon atoms are mainly inertia hydrocarbon groups. Therefore they perform special biological inertia, non-poison, non-smelly and excellent mildew resistance. The main shortages of silicone rubber are acid resistance, oil resistance, solvent resistance and steam resistance is worse than other organic composited rubbers in room temperature. Especially the alkaline resistance is worse. During the processing procedures, it is necessary to adopt high-specification strengthening fillers and the cost of rubber is high thus the application fields of silicone rubber are restricted.

It is in order to main the properties of polysiloxane and to improve its weaknesses, we could modify silicone rubbers. The modification methods mainly include physical ways and chemical ways. The chemical ways mainly refers to the chemical grafting (to lead organic polymers or chains into polysiloxane and to change the molecular structure or focus status) and polymerization (to use new monomer to polymer with polysiloxane) to modify silicone rubbers. However, these methods are limited in improving the physical and mechanical properties of silicone rubbers. Physical modifications include to mix polysiloxane with other macro polymers and to fill other materials to modify. The mixture modification is beneficial to supply the insufficient of single component and filling modification could enhance physical and mechanical properties of macro polymers from some degrees and to lower the raw material cost or  to grant new functions on materials. Both of mixture and filling modifications are flexible and simple.

As for the types of mixture modifications, the main goal of rubber mixture is to improve the shortages occurred on present’s properties and to improve the using properties and processing properties of rubbers. For example, for the natural rubber, it is due to it has excellent integrated mechanical properties and processing efficiency, it is widely applied. However, its thermal aging resistance, odor aging resistance, oil resistance and chemical resistance is bad.

Most composited rubbers have worse processing abilities and non-ideal mechanical properties which bring difficulties to production. When to mix these composited rubbers with natural rubbers, their properties could supply each other and especially they could improve the processing capacity of composited rubber products.

The second type is to mix rubbers with composited resins. They are not only to satisfy the modification requirements of composited rubber products but also an important path to realize the modification of rubbers. Composited resins have strengths on properties like they have high strength, excellent thermal aging resistance and good chemical resistance. These are the properties which are urgently required by some composited rubbers. When to mix rubbers with a few composited resins, some properties of rubbers could be improved and to increase the industrial value of these rubbers to expand the applied fields.

The third type is to mix special composited rubber with general rubbers or other special composited rubbers. It could not only to maintain the using ability of rubbers effectively, but also to lower the production cost to improve the using rate of special rubbers. For example, to mix fluorine rubbers with acrylate rubbers, to mix silicone rubbers with EPDM rubbers and so on.

However, it is hard to fulfill the industrial demands if just to mix two types of rubbers who have way different properties and molecular structures. These are needed to be tested by more explorations and selections from mechanism and practices.

As for the types of filling modifications, the mineral powders are the strengthening fillers and their reaction with silicone rubbers is one of the basic paths for fillers to strengthen the silicone rubbers. Many surfaces of ores contain a rich number of Lewis, Bronsted sour points. By making full use of the properties of powders, we could use a series of physical or chemical methods to process them to make these powders have the capacity to combine with silicone rubbers to increase the mechanical properties of the manufactured products.

Many aspects of mineral powders will influence the properties of the strengthened silicone rubbers. First of all, it is the relation between powderability and strengthening ability. When the particle radius reaches the level of nm, its surface physical and chemical properties will be strengthened and the micro combination with other materials will also be affected and finally leads to the macro changes on properties.

Secondly, it is the influence of modification on surface properties of mineral powders on strengthening properties. When the powders are modified by coupling agents, the organic groups are covering the surface which will increase the mechanical properties of vulcanized rubbers to be improved.

Finally, it is the influence of composited mineral powders on strengthening properties. The strengthening properties of composited powders are better than single powder, and dual-modified powder’s strengthening property is better than single-modified powder.

Summary

It is similar with silicone oil, silicone rubber has also been modified much and thus a series of modified silicone rubber has been applied in many different industries and fields. As one of the traditional ways to enhance chemical’s properties, modification is always one of the first ways for scientists or researchers to consider being as the first attempt to test to improve corresponding physical or chemical properties in chemistry.