Categories & Properties of Silicone Rubber Products

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In the early few articles, we have briefly introduced the general information about silicone rubbers. Today we will go further into some more detailed part of the whole silicone rubber category.

Silicone rubber is considered one of the most important organic silicone products. The materials which are to produce silicone rubbers are linear polysiloxane or called siliconegums, reinforcing fillers, agents and modified additives. By mixing all the materials then we could get the rubber compound which will be vulcanized in some certain conditions to become vulcanizates.

The chemical structure of siliconegum is similar with silicone oil which is displayed below:

More Categories of Silicone Rubbers

In the formula, R’ is hadrocarbyl o or hydroxyl, R usually refers to the methyl but may be also some other groups like ethyl or phenyl to improve and enhance some aspects. For example, to use phenyls to alternative some parts of methyls could lead to the improvements on softness and anti-radiation when silicone rubbers are in low temperature; to add a few vinyls which could be used for enhancing the vulcanization process; trifluoropropyl’s existence could allow silicone rubbers to have excellent features like oil resistance. Therefore, it is according to the linked organic groups, silicone rubbers could be divided into dimethyl silicone rubber, methyl vinyl silicone rubbers, methyl phenyl silicone oils, fluorosilicone rubber and many other types.

It is based on the different vulcanization methods & conditions, silicone rubbers could be divided into HTV (high temperature valcanising) or RTV (room temperature valcanising). If we categorize silicone rubbers by product forms and mixture methods, HTV could be divided into HCR (high consistency rubber) and LSR (liquid silicone rubber). HTV is organic polysiloxane who has high molecular quantity which is usually between 0.4 and 0.8 million; RTVs have lower molecular quantity which are between 30 thousand and 60 thousand. In the two edges of molecular chain will have one or two functional groups and these groups will be reacted in some certain conditions to form three-dimensional crossing structure. High-viscosity TTV now takes around 80% of world’s total silicone rubber market shares. LSR and RTV have taken 10% of market share, respectively. There is one new type of silicone rubber which is made by solidification of illumination takes only a very small percentage of today’s silicone rubber products market.

Properties of Silicone Rubbers

Silicone rubbers have advantages that natural rubbers and other composite rubbers do not have, and these leading pros are determined by the Si-O key in its main chain.

Silicone rubbers have very high heating stability and outstanding low-temperature property which allows silicone rubbers to keep softness, resilience, surface hardness and mechanical properties. Besides these, they also have excellent isolation of electricity, weather ability and breathability. It is also clear and non-smelly.

Resistance to Extreme Temperatures

The most famous property of silicone rubber is the heating resistance. They could be used in a long-term period in a condition with air and 180 Celsius degrees. If we choose appropriate fillers and high-temperature agents, the applicable temperature could reach at a maximum of 375 Celsius degree. By the corresponding research, normal silicone rubbers could be used more than 20 years in 120 Celsius degrees and 5 years in 150 Celsius degrees.

The heat aging of silicone rubbers are mainly influenced by oxidation of side chains and the break of the siloxane key of main chain. On one hand, the heating resistance is decided by the side chain. On the other hand, the breaking of main chain called also be called sealing heating resistance, it is different from the aging by oxidation, it’s closed related to the main chain’s softness and end groups and the existence of water, acid and alkaline have evident promotions on the breaking of chains.

The physical strength of silicone rubber is a little lower than other organic silicone rubbers. However, it has the strongest strength when in high temperature. For instance, silicone rubbers will only have a permanent shaping rate at 7~10% when in 150 Celsius degree and continuously pressing. This low-permanent-shaping is the good material to O-rings, pads and other relevant sealing products.

The cold resistance of silicone rubbers and the elasticity in cold environment is not increased by adding plasticizers but to change the molecular structure of polysiloxane. To add some phenyls to the molecules of polymers could improve the low-temperature viscosity of silicone rubbers.

It is due to the top-level heating resistance, it is widely applied in the sealing pads of engines of aircrafts, steam and electrical vehicles.

It is also ideal shock absorption material.

Electricity Properties

Silicone rubber has good electricity properties. The dielectric strength and isolation are not influenced much by temperatures or frequencies. In a wide temperature range, the dielectric strength will maintain status quo; in a bigger frequency range, the dielectric constant will not be changed much. Silicone rubbers are not easy to be ignited and burned, even if the silicone rubber is on fire, the produced silicone dioxide is also isolative. Therefore, it is used as safe and reliable surface material of wire cables.

Resistance to Chemicals and Weathers

Generally, silicone rubbers have good resistance to chemicals, solvents or oils. Solvents will soft the silicone oils. However, once the solvents are volatilized, most of the original properties of silicone rubbers will recover.

Silicone oils are also resistant to acid, alkaline and salts which have low impurity. If we immerse silicone rubber in the 10% H2SO4 in 7 days with common temperature, the change of its weight and volume will be both less than 1% and the mechanical strength will not be changed basically.

Silicone oils are also resistant to oxygen, ozone and ultraviolet rays. Therefore, the silicone rubbers will not be cracked when being used or placed in outdoors for a long-term period. The general opinion of silicone rubber thinks that the period could be longer than 20 years.

Breathability

Polydimethylsiloxane’s helix structure & its large space allow it has fantastic breathability. In the room temperature, the transparency rate to air, oxygen and other gases are 30-50 times over natural rubbers.

Summary

This time we just introduce the further categories of silicone rubbers and the details of some good features silicone rubbers have.

If we refer to more specific subcategories of silicone rubbers, more valuable features could be found easily.