Incorporating Functional Fillers into Silicone Elastomer Systems

Optimal formulation and processes can maximize filled silicone material performance.

April 1, 2008

NuSil Technology

Silicones can be developed into fluids, gels, adhesives, elastomers, and resins designed with unique properties that make them ideal for specific application uses in the defense and aerospace industries. Silicones are most widely known for their ability to maintain elastomeric properties in extreme conditions, but an additional benefit of these materials is the ability to incorporate large amounts of fillers that can impart properties such as electrical and thermal conductivity, and radar-absorbing characteristics. Silicone materials appear in a wide variety of material compositions, and this broad range of material compositions makes silicone a viable option to endless numbers of optic applications.

Many powder fillers are currently added to silicone systems to achieve different properties (see table). Some of the most significant properties that fillers can add include thermal and electrical conductivity, thermal stability, color, and strength to the elastomer system. The chemical and physical properties of siloxane polymers allows for the incorporation of various fillers. The bond angles of the silicon-oxygen bonds create large amounts of free volume in silicone elastomers. This free volume, associated with the high compressibility found in silicones, bodes well for dispersions of fillers in silicone.

Reinforcement fillers are the most common fillers added to silicone elastomer systems used to improve mechanical properties, as filler particles reinforce an elastomer by reducing the mobility of the siloxane chains. The uniform distribution and the particle surface area available to make contact with the siloxane chains have the most influence on a reinforced elastomer's physical properties.

Functional fillers impart properties uncharacteristic of polysiloxane elastomers like color, thermal conductivity, or electrical conductivity. The table shows a brief list of both reinforcing and functional fillers.

The use of functionally filled silicone elastomers for radar-absorbing materials is well known. Radar absorption is achievable with the use of either magnetic fillers or electrically conductive fillers in silicones. Both mechanisms act to reduce energy received from the transmitter/receiver antennae, although through different means. For instance, magnetic fillers such as MnZn, NiZn, and MgZn ferrites absorb and dissipate energy via magnetic hysteresis, while the addition of lossy fillers — such as carbon, graphite, metals, iron oxide, and titanium oxide — dissipate energy through ohmic loss. Filler loadings for these types of fillers range from 5 percent to 20 percent by weight.

Many considerations must be taken into account when formulating a filled silicone system to achieve the desired properties. The use-based considerations for the engineer are as follows:

Physical Properties — Targeting the desired physical properties is the first objective to consider. While it is mentioned above that functional fillers can add specific properties to an elastomer system, the addition of too much filler may adversely affect physical properties like tensile, tear, elongation, and adhesion.
Homogeneous Mixtures — For mixtures of functional fillers to perform their desired function, the ideal displacement of filler will be homogeneous throughout the mixture. Consistency is the key in making a dispersion of any powder. Inadequate mixing causes clumping, which can lead to accelerated filler settling and unnecessary variability in properties. Uniform mixing is dependent on the shearing capacity of the dispersion equipment, the length of shearing time, the viscosity of the liquid, and the particle size and density of the powder.

Unlike reinforcing fillers, functional fillers typically are not soluble and have little to no reactivity with polysiloxane polymers, resulting in separation over time. This separation can significantly alter the intended property. For instance, electrical conductivity through silicone elastomers depends on point-to-point contact of conductive fillers, and settling of those fillers may create an electrically insulating layer between the surface of the elastomer and the conductive filler. Airless mixing can reverse this separation in the uncured state but it is irreversible once the material begins to cure.

Processing — Silicone materials containing a high level of functional fillers are typically high-viscosity products, which can be limiting for applications like coating but ideal for applications like form-in-place gasketing and groove filling. One option for expanding the use of high-viscosity products is the addition of solvents that lower the system viscosity for coating applications. It is important to note that lower viscosity systems fillers separate more rapidly than high-viscosity systems.
Weight — The few metal fillers listed above have a high density and can add to the weight of the material, which can be problematic for aerospace applications. One development strategy may be to add a minimal amount of filler to provide the necessary radar-absorbing properties. If weight becomes a problem, microballons may be necessary to reduce material density.

Silicone material's properties and adaptability make it a candidate for use in a broad range of applications in the defense and aerospace industries. Several considerations must be taken into account when developing a material for specific applications. Incorporating functional fillers offers significant property benefits; however, factors such as processing and weight of modified silicones must be considered. In summary, the optimal formulation and processes can maximize filled silicone material performance.

This article was written by Brian W. Burkitt, Senior Technical Salesman for Engineering Materials, and Stephen Bruner, Marketing Director, at NuSil Technology. For more information, click here .