Relationship between Components & Properties of Silicone Rubbers

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Silicone rubber is the organic polysiloxane which is in the form of long-chain with a high macromolecular quantity (over 148000). Its chemical structure is similar with silicone oil.

Regardless any type of silicone rubber, none of them will have heat effect phenomenon. High-temperature vulcanized silicone rubber is organic siloxane which has high macromolecular quantity which is usually between 400000 and 800000; room temperature vulcanized silicone rubber which has a lower macromolecular quantity between 30000 and 50000. On the two sides of molecular chain they usually carry one or two functional groups respectively. Under certain circumstances, the water in air or proper agents may react with those functional groups to form high macromolecular crosslinking structure – silicone rubber.

Silicone rubber has many advantages which natural rubbers and other composited rubbers have which is determined by properties of –Si-O- key that forms the molecular chain of silicone rubbers.

Heating & Cold Resistance

It is due to the Si-O-Si key is the basic key form of silicone rubber polymer macromolecules, the silicon atoms are mainly linked to methyl, and to lead some unsaturated functional groups onto the side chains. Thus, the intermolecular force is small and the molecules are in the form of helix. The methyls are arranged on the outer side and could switch free, thus the polysiloxane could show better cold and heating resistance.

To be more specific, the root reason why it has strong heating resistance is the big energy of chemical key. For example, if we insert some functional groups like phenyls onto the side functional groups, the corresponding heating resistance could reach 400℃. However, if the silicone elastomers contain around 5% KOH, it will be resolved at 125℃, Therefore, when we are making organic silicon coating agents, we must remove any left agents or other impurities to ensure the coating adhesive could be used for a longer time between 250 ~ 300℃.

As for the cold resistance, it is due to the molecular structure of silicone rubber is helix and soft, and polysiloxane has low vitrification temperature and embrittlement temperature. It has a lower brittleness point than natural rubbers or other composited rubbers thus it has inner low-temperature elasticity.  The vitrification temperature of low phenyl silicone rubber is -120℃ and its vulcanized adhesive still has better elasticity between -100 ~-70℃, and it could keep elasticity in -90℃.

Compression & Deformation Resistance

In high and low temperature, silicone rubber could keep excellent elasticity and good compression & deformation resistance. Especially during the heating condition, the capacity to restore shapes is very important.

The degree of permanent deformation of silicone rubber is stable between -60 ~ 250℃. Although normal composited rubber has a lower permanent deformation degree in room temperature. However, as the temperature changes, composited silicone rubber’s permanent deformation will be bigger.

Generally, organic silicone rubber needs to be secondary vulcanized especially when making low-pressure compression permanent deformation products. On the other hand, it is necessary to select the most proper vulcanizing agents.

Electrical Properties

Silicone rubber has excellent electrical properties. Silicone rubber’s arc resistance is better than common rubbers. Though it may have some vinyl but it could be basically saturated rubbers. The common opinion is to think that it has lower carbon proposition to compare with other organic rubbers and it does not use carbon black as the fillers. Thus, after the discharge, there will be basically no carbons thus the inflaming opportunity is way smaller. As the alternative, there will be isolated silica to be produced which is very stable under high pressures.

Weather Resistance

The main chain of silicone rubber is –Si(R)2-O-Si(R)2-, this key is hard to be oxidized by oxygen, ultraviolet rays and ozone. Its Si-O key is longer than C-C key which has a better weather resistance and irradiation resistance. Therefore, silicone rubber will not be aged when being used outdoors for a long time and the corresponding mechanical and electrical properties will basically have no changes. The general opinion is that silicone rubber could be used outdoors for over 20 years.

Mechanical Resistance

Even though the molecular quantity of silicone rubber reaches between 500000 and 700000, its softness is still way better than other organic rubbers. This is due to its methyl is circling around Si- axis and the hydrogen atom contains a big percentage. Therefore, the distance between neighbored molecules is bigger; the intermolecular force is smaller which leads to a very low tensile strength which is lower than 1MPa and an extensibility of 50% ~ 80%.

Chemical Resistance

The effects of solvents on silicone rubber are to expand and to soften, and once the solvent is volatilized most of properties of silicone rubber will be restored. For example, silicone rubber has very good durability of some extreme solvents and edible oil. It will only expand little and basically has no changes in mechanical properties.

Steam Resistance

Steams could also reduce the properties of silicone rubber. The main reason is the breaching reaction of its main chain. It is in order to improve the steam resistance of silicone rubber; there are some ways to fulfill the goal. However, it is important to avoid from using silicone rubber in the steam which has over 2MPa pressure.

Biological Inertia

The Si atom is not only linked to CH3, it could also be linked to amino or methoxy. This allows silicone rubber has both of inorganic Si-O structure and organic Si-O structure to make it become half inorganic macromolecular product. It is from biological perspective; organic silicone material is one of the known most inorganic compounds and contains the inertia which others do not have. It is the material which cannot be metabolized by all microbes or all biological processes.

Summary

As we could see clearly, silicone rubber does have many advantages and most of the excellent properties are awarded by itself- to be more specific, its unique chemical structure or chemical keys.