Determination of Cure Kinetics of Liquid Silicone Rubber Using DSC Data

Multiple scanning rates were used to gain insight into the effect of time and temperature on the curing reaction of LSR and HCR systems. Properties of peak temperature, heat of reaction, and extent of reaction are obtained.

Cure Kinetics of Silicone Rubber

Most silicone rubbers are two-component systems. In the case of the Liquid Silicone Rubber (LSR), component A contains both a platinum catalyst and an alcohol inhibitor while component B contains methylhydro-gensiloxane for cross-linking. High Consistency Rubbers (HCR) contain aroyl-peroxides and alkyl-peroxides as the catalyst.

The cure kinetics is a complex phenomena and includes several steps. During the process, macromolecules are linked with chemical bonds and a cross-linked network is obtained. These reactions are exothermic and irreversible. Numerous experimental techniques have been used to analyze the cure kinetics, but Differential Scanning Calorimetry (DSC) is the most popular.

Cure Kinetics Experiment

The DSC technique measures the instantaneous heat, Q, from a reactive sample as a function of temperature. It can be used directly, accurately and quickly to study the rate of curing from the reaction heat.

Here, the difference in the amount of heat required to increase the temperature of a sample and reference (an empty crucible) is measured as a function of temperature.

cure kinetics experiment

 

To characterize the reaction, the samples of LSR and HCR are placed in sealed aluminum pans. The mass samples ranged from 10 mg to 30 mg. The total reaction heat was measured by a dynamic scan from 20°C to 150°C using heating rates of 1.0, 2.5, 5.0, and 10°C/min.

liquid silicone vs hard silicone

 

Repeatability of curing was obtained for each liquid silicone rubber and heating rate, but not for the HCR. Solid silicone rubbers present lower repeatability than liquid silicone rubber because the material is pre-vulcanized up to a certain degree before storing.

The DSC experiments provide the cure evolution, dc/dt; the kinetics are then modeled with a transformation equation and the parameters, for such expression, are obtained numerically. The kinetic analysis of silicone cure involves a proper selection for the kinetic model that fits the experimentally observed behavior (for example, the Kamal-Sourour model).

kamal sourour model

 

The data of heating rate and the temperature at which the maximum rate of reaction occurs was plotted and fitted to a linear model. The activation energy of each silicone rubber was calculated with the data from the four dynamic scanning rates tested. The data obtained from the DSC scans was used to determine the kinetics constants for the Kamal-Sourour kinetic model. This process was done by fitting the instantaneous curing rate and the percentage of curing at each specific temperature into a model that describes the reaction. The technique uses one or more dynamic DSC scans to determine a set of kinetic parameters that model the curing process.

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