Silicone Liquid Rubber
An informative presentation of the silicone liquid rubber curing incorporating the degree of cure is the Time-Temperature-Transformation (TTT) diagram, developed by Enns and Gillham. It can be used to relate the material properties of thermosets, such as Silicone Liquid Rubber (LSR), as a function of time and processing temperature. The diagram visually presents various lines representing constant degrees of cure. The curve labeled c=,c-g. represents the gel point and c=1 represents the fully cured resin.
Both curves have corresponding glass transition temperatures, T-g1 and T-g gel, for the glass transition temperature of the fully cured resin and at its gel point, respectively. The glass transition temperature of the uncured resin, T-g0, and an S-shaped curve labeled “vitrification line,” are also depicted. The vitrification line represents the boundary at which the glass transition temperature becomes the processing temperature. Hence, to the left of the vitrification curve, the curing process is controlled by a very slow diffusion process.
The TTT-diagram shows an arbitrary process temperature. The material being processed reaches the gel point at t=t-gel, and the vitrification line at t=t-g. At this point, the material has reached a degree of cure of c-1, and the glass transition temperature of the resin is equal to the processing temperature. The material continues to cure very slowly (in other words it is diffusion controlled) until it reaches a degree of cure just below c-1.
The various regions in the diagram delineate the material properties of thermosets. The region labeled “viscous liquid” is the one where the resin is found from the beginning of processing until the gel point has been reached. The flow and deformation that occurs during processing or shaping must occur within this region. The region labeled “char” must be avoided during processing, since at high processing temperatures, the polymer will eventually undergo thermal degradation.
At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts and components, we are exclusively focused and specialized in the production of Silicone Liquid Rubber (LSR) and 2-Shot (LSR/Thermoplastic) components.
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