Additives to Improve the Optical Properties of Liquid Silicone Rubber (LSR)

It was in the 1950s when Liquid Silicone Rubbers (LSR) was first used as an optical material.  Lighting applications were the first widespread use of optical silicone rubber of its transparency, and medical applications soon followed. Because of the chemical structure of LSRs (Silicon and Oxygen atoms), which is naturally opaque, with a translucent milky color, its clarity must be improved for optical grades. Fortunately, Liquid Silicone Rubber can be altered by incorporating components in its structure [1]. The chemical structure can be modified the during synthesis in various ways, by: 1) including groups to create a short-range arrangement; or 2) employing external materials called additives to create a new phase that increases the transparency. Depending on the application, there are various optical properties that can be improved, such as haze, gloss, clarity or refractive index [2]. When the optical properties are modified, the mechanical and chemical properties may also be affected, therefore it is important to find an acceptable balance between optical specifications and its function in its end-use application.

For the purpose of this paper, we will focus the use of additives to LSRs to improve its optical qualities. An additive can be defined as “a substance which is incorporated into the silicone to achieve a technical effect in the finished product, and is intended to be an essential part of the finished article” [3]. The optical properties are modified directly by substances called nucleating agents or clarifiers, optical brighteners, and pigments or colorants. The extent of the additive property improvement depends of the additive concentration, its morphology, and the degree of dispersion [4].

Nucleating agents improve the crystallinity in zones where crosslinking does not occur. In addition to transparency, these agents also improve processing. Silica is one of the nucleating agents that can be used. Silica has a refraction index similar to transparent polymers [5] and also improves the mechanical properties of the final product [6]. These types of additives are reinforcing and extending fillers, they increase a specific property and are extended through the formulation. The silica particles form an entanglement with the LSR macromolecules, creating a wide space between the chains. This improves the material strength and enhances the light transmission through the material, even in thick parts. The more silica there is in the Liquid Silicone Rubber, the higher the transparency but only to a certain point, saturation of the matrix creates opaqueness [5]. When using silica, it should be cautioned that with an increase in surface area, there is a tendency to create clusters (agglomerates) which are hard to mix. Failure to disperse the agglomerates well, will create milky spots in the final product.

Arrangement of silica between silicone rubber molecules

Figure 1. Arrangement of silica between silicone rubber molecules [5]

Other additives such as optical brighteners will improve the whiteness of the product. The material absorbs UV radiation and emits blue light. Examples of optical brighteners are benzoxazoles and coumarin [3]. Pigments and colorants also can help to improve the optical properties of the silicone because, due to the size of the pigment, only a few microns in size, the light crosses without interacting with any of the material’s atoms. The use of these nanoparticle additives has been increasing. The shape of the particle also affects the optical behavior of the additives. The particles can be incorporated into the LSR during polymerization, during mixing, or during metering in the injection molding machine. The shape of the additive particle is used to classify the additive, for example: spheres, tubes, wires, fibers or platelets [7].

Let’s take a closer look at how these additives improve the optical properties of Liquid Silicone Rubber. When light hits the material, particles less than 25 nm in size do not scatter the light. Depending on the chemical structure of the silicone, the optimal size can be calculated using Rayleigh scattering [8]. There is also a theory that air molecules, with a similar size and distribution as an additive, can be used, however this theory has not been tested yet [5]. Interestingly, the influence of additives improvement of Liquid Silicone Rubber’s optical properties is directly related to the mixing rate used during compounding. This can have both positive and negative effects. For example, if a high mixing rate is used to improve dispersion of the additive, the haze decreases (a clarity attribute), however the yellowness increases due to the increased surface area exposed to oxygen [9].

Arrangement of silica between silicone rubber molecules

Figure 1. Function of optical additives in Liquid Silicone Rubber [9]

Although additives can help improve the optical properties of silicone rubber, the silicone rubber itself must possess a high enough refractive index or the additives will not be efficient. If the base material is a linear Polysiloxane, even with optimal amount of silica, the clarity will be low. For that reason, it is also necessary to modify the Liquid Silicone Rubber. This can be achieved by creating copolymers with a complex chemical structure that are inherently amorphous. For example, Polisiloxane-Imide has better refractive index values. Also, the presence of aromatic or aliphatic cyclic groups integrated into the silicone backbone can improve the optical properties [1].

Before and after the production of the LSR products, several tests should be done to confirm if the optical properties specified can be met. Gloss, haze, birefringence, and ultraviolet or visible light absorption and transmission are examples of properties tested [2]. These types of tests are normally done using spectroscopes. There are also standards related to the analysis of different properties. ASTM D542-14 outlines the test methodology for determining the index of refraction of transparent plastics. Keep in mind, the index of refraction is related to the wavelength of the light used to measure it. ASTM D1003-13 analyzes the haze and luminous transmittance of transparent plastics. ASTM D1746-15 provides the test for determining the transparency of plastic sheets. ISO standards include, ISO 489 which employs the Becke Line method to measure the refractive index of polymers; ISO 13468-1 determines the total luminous transmittance of transparent materials; and ISO CD 14782 tests the haze of transparent materials.

References

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