Introduction to HTV Silicone Rubber

Tires- One type of HTV product (Cited from: https://pixabay.com/zh/photos/winter-tires-tire-service-tire-4664205/)

Cover Photo cited from: https://pixabay.com/zh/photos/rubber-ducks-bath-ducks-bathing-fun-3412065/

In some articles we have written in the past, we have briefly introduced silicone rubber could be divided to HTV (High Temperature Vulcanized) Silicone Rubber and RTV (Room Temperature Vulcanized) Silicone Rubber. Today, we will mainly focus on introducing HTV silicone rubbers.

The manufacturing of HTV silicone rubbers generally includes three phases: The first is to use silicone intermediates to be the raw material to form linear polysiloxane which has high molecular quantity (around 0.4 million to 0.8 million) (also called raw rubber), which is the most important type of silicone rubber among organic silicone products; the second is to use raw rubber as the bone materials and to add some other additives to mix them and to make mixed rubber; the third is to process the mixed rubber by moulding, injection and other physical ways to vulcanize the mixed rubbers to other elastic products under high temperature.

HTV products usually have excellent electrical isolation, water resistance, wet resistance, hitting resistance and have biological inertia and breathability. It could be mainly used for aerospace, tablets, electronic devices, sailing, metals, mechanics, automotive and healthcare. It could be used to make sealing circles, pads, tubes and cable wires in different shapes and it could also be used to make organs, blood tubes, breathable films and rubber molds.

Categories of HTV Silicone Rubbers

HTV silicone rubber is one of the earliest-applied silicone rubbers and now it has many categories.

The first type is dimethyl polysiloxane rubber; it is the oldest type of silicone rubber which is to use siloxane as the main chain with its all side groups are made of methyl. It could keep excellent elasticity during -60℃ and 250℃. It is due to its low vulcanized activity and bad artificial effectiveness, now they are basically all replaced by methyl vinyl silicone rubbers unless in some textile coating industries.

The second type is methyl vinyl polysiloxane rubber. It is based on methyl one, which is to lead some vinyl to the side chain which is full of methyl. The proposition of vinyl is usually around 0.1% ~ 0.3%. The change of some functional groups has enhanced the vulcanized activity of silicone rubber largely, this improves the physically mechanic properties of vulcanized rubbers and the corresponding elasticity to lower the compressed permanent shaping. Therefore, it has extremely expanded HTV’s application areas. It is due to the excellent properties, matured manufacturing processes and just very few increased cost of manufacturing of methyl vinyl silicone rubbers; therefore it is manufactured for the maximum amount, applied for maximum industries, has most categories among all HTV products. Besides the common type rubber materials, many other types of silicone rubbers which are for special uses are also needed to be compatible with methyl vinyl silicone rubbers.

The third type is the methyl vinyl phenyl polysiloxane rubber or called phenyl silicone rubber. It is similar with the first two types, it is to bring some phenyl to the molecular chain of vinyl silicone rubbers. When the phenyl mol proposition is around 0.05 to 0.10, these silicone rubbers are called low phenyl silicone rubbers. At this time, rubber’s hardening temperature reaches the bottom of -115℃ which allows itself to have the best cold resistance. When the proposition reaches between 0.15 and 0.25, it will be called medium phenyl silicone rubber. It has the feature of easily inflammable. When it is over 0.3, we usually call it as high phenyl silicone rubber which has good resistance of radiations. Phenyl silicone rubber is usually required on cold resistance, high-energy radiation resistance and heating isolation. Medium and high phenyl silicone oil is hard to be processes and have worse physically mechanical properties thus they are limited in manufacturing and applications.

The forth type is the methyl vinyl r-trifluoropropyl polysiloxane rubber or called fluoroailicone rubber. Its main feature is that it has excellent oil resistance and solvent resistance which is way better than vinyl silicone rubber. Its temperature resistance is a little worse than vinyl silicone oil. The working temperature is between -50 and 250℃. It has better stability in room temperature or high temperature.

Phenylene polysiloxane rubber and phenylatylene silicone rubber are new types of silicone rubbers. They are designed for adapting nuclear engines and navigation technologies. Their properties are higher tensile strengths, γ ray resistance and excellent heating resistance. They also have great enhancements in compressing reshaping resistance under high temperature but their cold resistances are weaker than low phenyl silicone rubber.

Nitril silicone rubber has the similar properties with fluoroailicone rubber. It has oil resistance, solvent resistance and excellent could resistance. However, it is due to it may have hydrolysis during polymerization conditions; therefore it has bad repeatability of raw rubbers which leads to the obstacles of applications and developments.

Boron silicone rubber is another new type of silicone rubber. It has high thermal aging resistance and could be used under 400℃ for long-term period. It could work continuously for hours between 420 and 480℃. It could also keep elasticity in -54℃. Therefore, it is very suitable to be used as sealing materials for high-speed airplanes and spaceships. The U.S. has the brand numbers for boron series silicone rubbers in the end of 20^th century 60s, however, they are rarely reported after 70s which may be due to the difficult composition processes and they are poisoned and expensive.

Summary

It is true that HTV silicone rubbers have been developed for a long time and thus has been divided or developed more subcategories which are specifically designed to be applied in certain fields or industries. As we introduced in the last blog, the corresponding technologies are now also very varied and providing the possibility to develop new types of silicone rubbers with better & more effective physical & chemical properties.