SILICONE

The vulcanisation of HTV silicones using organic peroxides is a radical crosslinking process that typically takes place at temperatures between 120°C and 200°C. Organic peroxides, such as dicumyl peroxide, bis(2,4-dichlorobenzoyl) peroxide or 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, thermally decompose generating highly reactive free radicals. These radicals abstract hydrogen atoms from the methyl groups of the polysiloxane chain, forming macro-radicals that react with each other, giving rise to Si–C–C–Si crosslinks.

The degree of crosslinking depends on factors such as peroxide concentration, temperature and vulcanisation time, directly affecting the mechanical and thermal properties of the final material. During the process, volatile by-products are generated (such as alcohols or ketones), which is why post-curing at high temperatures is often needed to remove them and improve the silicone’s stability.

This method is widely used for applications requiring high thermo-mechanical performance and ageing resistance.

The vulcanisation of HTV silicones using platinum systems occurs through a catalytic hydrosilylation mechanism, leading to the formation of a three-dimensional network without generating volatile by-products. In this process, vinyl groups (–CH=CH₂) present along the polysiloxane chain react with silicon hydride groups (–Si–H) in the presence of platinum-based catalysts. The reaction is highly selective and proceeds through a catalytic cycle in which platinum coordinates the vinyl double bond and facilitates the addition of the Si–H group, forming Si–C–C–Si type crosslinks.

The absence of by-products results in higher purity of the final material, making these systems particularly suitable for medical, food and electronic applications. Furthermore, crosslinking occurs at lower temperatures and with less thermal degradation, producing elastomers with more uniform mechanical properties and better dielectric characteristics.

During the vulcanisation process, silicone transitions from a plastic, deformable state to an elastic state.

In the case of HTV silicones crosslinked with organic peroxides, the transition from a plastic (or flow-dominated viscoelastic) state to an elastic (crosslinked rubber) state is governed by the evolution of crosslink density during the radical reaction. Initially, the material consists of linear or weakly branched polysiloxane chains with high segmental mobility, which under stress show plastic behaviour with significant permanent deformation. As temperature rises, the peroxides decompose, generating radicals that progressively induce the formation of crosslinks between chains: the material thus passes through the so-called gel point, at which an infinite three-dimensional network forms, marking the critical transition from fluid to elastic behaviour.

From a rheological standpoint, this transition is observed as the intersection between the storage modulus (S’) and the loss modulus (S”): before the gel point, S” dominates (viscous behaviour), while afterwards S’ dominates (elastic behaviour). Microscopically, network growth drastically reduces chain mobility, preventing relative sliding so that deformation energy is stored elastically rather than dissipated. The material thus takes on the typical entropic response of elastomers, in which deformation is reversible thanks to the chains returning to statistically more probable configurations.

It is important to note that the transition is not instantaneous but depends on the kinetics of peroxide decomposition and radical diffusion; furthermore, parameters such as molecular weight distribution, filler content and crosslinker concentration influence the position of the gel point and the sharpness of the transition. After the gel point, further crosslinking reactions increase crosslink density, boosting elastic modulus, mechanical strength and thermal stability, until the fully vulcanised state is reached.

Even in the case of platinum-catalysed silicones, the transition from plastic to elastic state is observed as the gel point is reached, but the kinetics are generally more controllable than with peroxide systems, thanks to the ability to modulate catalytic activity using inhibitors (e.g. acetylenes or specific vinyl compounds).

Through our in-house laboratory, the rheometric behaviour of every compound is carefully monitored using a state-of-the-art MDR rheometer connected to our production database containing the reference values and permitted tolerances for each of the 700 formulations developed to date.