origin and transformation

Silicones come from a unique chemical compound, one of the most common on earth: silicone dioxide or silica noted SiO2 en chemistry. Silica or silicone dioxide is currently related to quartz or glass sand.

In order to obtain a workable raw material, few steps are necessary. After isolating silicon chemical element noted Si in chemistry, various chemical reactions such as synthesis and distillation generate more or less long chains of polysiloxane, more commonly called polymers, the basic component of silicones.

The groups R and R ', that constitute the length of the chains, may be functional groups, that are reactive or not. These functional groups will give rise to different technical properties. The most common functional groups being:

• reactive groups: vinyl, hydrogen silane and silanol;
• non-reactive groups: methyl, phenyl and fluoro.

Polymers come in two main forms

"Liquid": in this case, the number of m and n repetitions that are present in the "Structure of polysiloxanes" image are called "small": thus, we obtain short chains. The polymer viscosity will be low and we will have obtained a LSR (Liquid Silicone Rubber).

"Compact" : in this case, the number of m and n repetitions are "big", so we obtain long chains. The viscosity of the polymer will be high, thus obtaining a HCR (High Consistency Silicone Rubber).

In order to improve properties of polysiloxane mixing (or polymers), active and passive loads can be added. These can be pigments for coloring, and/or loads to add mechanical properties or to improve curing and transformation. Pigment or color batches may be certified for non-toxicity, or for food, medical, implantable applications...

Catalyst type and curing mode

Catalyst: a substance that triggers a reaction
Vulcanization or Curing: Change of state; Passage from raw material to a processed product.

The changes of state, or curing, of silicone mixtures, may be done in two ways :

by heat input, or High Temperature Vulcanisation (HTV) or Hot Vulcanizable Elastomer

or with the relative humidity of the ambient air, or Room Temperature Vulcanisation (RTV) or Cold Vulcanizable Rubber. RTV silicones are used for sticking and assembly (single-component) or for mold creation (two-component).

Two principal catalysts are used in operating mode :

Peroxides : offering high performance properties to fatigability, they are used for standard applications;

Platinum : since it does not contain chlorine, platinum can be used without post cooking and is particularly used for integration into bases for food, medical and surgical purposes.

Peroxided silicones are HTV and are only present in HCR. Peroxide decomposition generates free radicals thus allowing a vinyl functional group to link up with a methyl functional group. This operation binds all the molecular chains together. In order to avoid the appearance of a white film on the product, post baking for 4 hours at 200 ° C is necessary. Peroxide silicones present better mechanical properties, a longer lifespan, and lower costs than platinum silicones.

The platinum-catalyzed silicones are HTV or RTV and may be present either in the form of HCR or LSR. Platinum silicones are very often presented in two parts: The A part, containing the catalyst and the vinyl polysiloxanes; and the B part, which contains the hydrogenosilanes and inhibitor. When the two parts are mixed, the inhibitor, which is present in greater or lesser quantities, retains the action of the platinum catalyst. The heat releases the inhibitor and allows the platinum to assemble the functional groups, thus linking the molecular chains together.

Intrinsic properties depending on kinds of functional groups

The so-called rubbery property of silicones makes it possible to present standardized hardness indices of 0 to 100 Shore A, and the most effective breakage elongations associated with tear-off behaviors. Silicones also have natural adhesion capabilities and are hydrophobic.

The open cells of silicones allow active and passive charge integration, increasing thermal properties, vapor and flame resistance, a decreasing adhesion coefficient, economic charges, and so on.

Based on the intended use and characteristics needed, we can use our experience to help you choose the best silicone, taking both hardness and functional group into account, and perform the necessary tests.

VMQ Silicones (siloxane composed with vinyl groups) are widely used. They can be used from -50 to +200°C with hardness from 7 to 90 +/- 5 shA, catalyzed with platinum or peroxide.

PVMQ Silicones (siloxane composed with phenyl and vinyl functional groups) allow product use at low temperatures to -110°C.

Fluoronated FVMQ Silicones (siloxane composed by fluoronated and vinyl functional groups) have the same characteristics as VMQ, but they react better with oil, solvents and hydrocarbons.

Silicones can meet various standards, varying in color and degree of hardness.

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