Silicone fluids have important applications in many modern fields including the automotive, electrical, and medical industries. Their advantages include high temperature and chemical resistance, optical transparency, and good electrical properties. Certain application areas and processing steps require a reliable rheological model.
Compounds based on silicone rubbers are designed to process well in molding equipment, especially liquid silicone. Compounds are available in a variety of flow ranges from very stiff for compression molding to very soft for transfer molding. Viscosity is the most well-known expression of rheology and it is used to monitor formulation stability over time and to optimize the sensory and performance attributes.
The rheological properties of silicone rubbers are of importance because they are major determinants in the handling characteristics and adaptations to soft and hard behaviors during processing.
Their plasticities are normally measured by the Mesa Spiral Flow. This test provides an indication of the relative ability of the compound to flow and fill molds. Torque rheometer measurements are sometimes utilized to characterize both flow and curing properties.
Because viscosity is a property that is easy to observe, it is frequently used to monitor formulation stability over time. Rheology has become increasingly important because an understanding of its parameters has allowed formulators to optimize the sensory and performance attributes of the products.
The viscosity of the polymer depends on the molecular weight of the material which, in turn, is a function of the degree of polymerization.
That means, the greater the degree of polymerization, the higher the molecular weight, and the higher the molecular weight, the longer the polymer. The longer the polymer is, the higher the viscosity. And the higher the viscosity, the more slowly the polymer will flow.
The addition of functional groups on the siloxane backbone can modify the rheology behavior. Varying degrees of substitution can result in wax-like silicones. The viscosity can be increased by increasing the chain length, adding Si-O units or by employing a crosslinker.
In general, silicone rubber materials exhibit Newtonian rheological behaviors when they have low molecular weights and they are independent of the shear rate applied during processing. But when the molecular weight increases, the polymer becomes entangled and exhibits a viscoelastic response and a decrease in viscosity, called shear-thinning. Depending on the different additives present in the silicone compound, they can be thixotropic or dilatant, depending on the final application and process used in them. The viscosity has a relatively small effect on silicone rubbers’ chemical properties, but it affects the flow behavior and solubility.
The rheological properties can be measured using different techniques. For example, a cone and plate viscometer measures flow characteristics.
Since silicone rubbers are also cured, their behavior is studied by chemorheology, which describes the rheological behavior of crosslinked polymers during the chemical reaction of vulcanization or curing. The change in the silicone rubber flow behavior is determined using rheological measurements during the crosslinking reaction. The results can be used in the evaluation of the material, the design of the process, and the simulation of the process conditions. Here, one of the critical points that can be found is the gel point. At this point, the polymer no longer flows so the silicone must be formed before reaching this point. The increase in temperature of those silicones creates two different effects in the viscosity: First, there is a decrease in the viscosity, but when the curing reaction begins the viscosity becomes higher due to the increase in the silicone’s molecular weight, created by the crosslinks. Rotational rheometers have proven devices to monitor this behavior, because they can capture the transition from solid to liquid.