High Consistency Rubber or HCR, Silicone Elastomers are materials that function as high-performance silicone seals. They can be used for a range of applications, especially within the automotive and aerospace industries.
HCR Silicone Elastomers are ideal because they have excellent heat resistance while still maintaining the properties of a low-temperature material. They also provide very good outgassing test results because they are typically post-cured.
While most Liquid Silicone Rubber (LSR) and Room-Temperature-Vulcanizing (RTV) materials can be found readily in a pre-compounded system, the HCR elastomers are generally formulated outside of the main polymer suppliers and need to be mixed together with other ingredients to create a usable seal and gasket material. Here is a breakdown of how HCR Silicone Elastomers are made:
Typical Formulation
Ingredients in phr (parts per hundred parts of rubber):
- Polymer 100
- Filler 0-50
- Heat Stabilizer 1-2
- Peroxide 1-3
- Pigment 2-4
Polymer
Polymers are the bulk of the formula for Silicone Elastomers. The variety of base materials on the market means that a lot of compounding has already gone into these materials before they ever meet a custom mixer.
These polymers come in a range of product lines that will cover the wide range of material hardness. The main type to be used is a Vinyl Methyl Silicone, or VMQ, material. This is a Silicone Elastomer that has been modified to have vinyl groups on the polymer backbone. This leads to additional crosslinking and allows the silicone to be easily cured as it adds double bond sites to the material.
The higher the level of vinyl, the more crosslinking that will occur within the material. Controlling the level of vinyl helps polymer manufacturers to design in the degree of cure they want in an HCR material.
Specialty grades of HCR polymers also include a Fluoro Vinyl Methyl Silicone (FVMS). This Silicone Elastomer benefits from the addition of Fluorinated groups to the polymer backbone. This will lead to increased resistance to chemicals such as aviation fuels and gasoline. PVMQ’s (or Phenyl Vinyl Methyl Silicones) are modified to include phenyl groups. These groups allow this silicone elastomer to be exposed to much lower temperatures, down to -100°C and still be flexible.
Filler
The next major component of an HCR material are any added fillers. Just like in organic rubber, the purpose of these fillers is to add tensile and tear strength to the elastomer, as well as to raise the durometer or hardness of the material.
Fume- treated silica will typically be used because it has a very small particle size, treating the silica in this way helps it to better disperse in the elastomer. This is also less of a health hazard to use for a mixer since a mined silicate is typically sharp and requires the use of dust masks.
Unlike organic rubbers, fillers are not always needed in a custom formulation. Since many product lines have a range of semi-finished options available, they can usually be blended together to achieve the desired fusion of properties and final hardness, rather than just adding fillers. This means that fillers can be added more sparingly to target a specific property, improving tear resistance, for example. Unlike an organic rubber, carbon black is generally avoided as a filler except in rare circumstances.
Heat Stabilizer
Just as it sounds, a heat stabilizer is the ingredient that helps to protect the polymer backbone from being debilitated by heat. These are usually some form of a metal oxide, such as Magnesium Oxide.
Many seal materials for silicone are colored either a red or a brown shade because these use the Red Iron Oxide pigment. Using a higher amount of pigmentation than necessary can help act as a heat stabilizer for the material. The drawback of doing so is that this can increase the difficulty in processing the raw silicone elastomer.
Peroxide
When it comes to curing HCR material, a peroxide is typically the curing agent of choice. One benefit of peroxide use is that it is always used up during the curing process. The reason for this is that the peroxide bond gets cleaved apart as the energy created from the polymers exposure to high temperatures creates crosslinks.
The crosslinks are made as this process attacks the vinyl double bonds, and incorporates the peroxide so none is left behind. This again aids to more heat resistant properties, since no catalyst material is left behind to attack the crosslinks and weaken the material under extreme conditions.
Pigments
Unlike organic materials, the color that most silicone elastomers are available in ranges from a milky clear to an off-white or cream color. Given that carbon black tends to be avoided as a filler material, there is always a vast range of color options available. The most common pigment is the Red Iron Oxide pigment, but just about any standard color option is available and can also be blended together to create custom colors. High Consistency Rubber or HCR, Silicone Elastomers are materials that function as high-performance silicone seals. They can be used for a range of applications, especially within the automotive and aerospace industries.
HCR Silicone Elastomers are ideal because they have excellent heat resistance while still maintaining the properties of a low-temperature material. They also provide very good outgassing test results because they are typically post-cured.
While most Liquid Silicone Rubber (LSR) and Room-Temperature-Vulcanizing (RTV) materials can be found readily in a pre-compounded system, the HCR elastomers are generally formulated outside of the main polymer suppliers and need to be mixed together with other ingredients to create a usable seal and gasket material. Here is a breakdown of how HCR Silicone Elastomers are made:
Specialty Ingredients
Other specialty ingredients can be added to the materials as the need for supplementary features arises or depending on the specificity of the situation. An example of this would be internal mold releases, which help silicone elastomers become easier to mold and process. Another ingredient added could be metal particles, which allow silicone to be used in electronics, or simply to shield electronics from interference.