Optical Liquid Silicone Rubber (LSR)

Light can be defined as electromagnetic radiation which has different frequencies and wavelength. The spectrum that can be picked up by the retina of a human eye is called visible light [1]. Materials through which light can refracted, reflected, transmitted, dispersed, polarized, detected and transformed are called optical materials [2].

OPTICAL MATERIALS

The number of optical materials has expanded recently. In the past, glass and other ceramic materials were the few materials available that offered the best refractive index values. If we look at glass specifically, the chemical structure of glass is formed by Silicon (Si) and Oxygen (O) with low-range arrangement [3]. The atoms in glass are arranged randomly, and because of this structure, glass is transparent (Fig.1) [2]. Conversely, metals have an organized structure and therefore are not transparent. In glass, photons (the elementary particles that form the light) are able to pass through glass without interacting with any atom [2].  Because of this structure, it has low mechanical properties and high density (between 2.3 and 6.3 g/cm3) [1], which can be considered a disadvantage.

Fig. 1 Chemical structure of glass

OPTICALLY TRANSPARENT POLYMERS

Some polymers have innate properties similar to glass, but they have low physical properties. Examples of some of these polymers include the thermoplastic materials Polycarbonate (PC), and Polymethyl Methacrylate (PMMA) that are processed using injection molding; and Epoxy resins (EPI), thermoset materials that are compression molded [4]. These thermoplastic and thermoset materials have advantages, including high quality surfaces reflecting the mold surface, they are easily processed, and they are available in a variety of grades with a wide range of properties. There are disadvantages as well, including thermal stability − their thermal properties are low compared to glass [3].

OPTICAL LIQUID SILICONE RUBBER (LSR)

Optically clear grades of LSR polymers offer advantages over both glass and thermoplastic and thermoset optical polymers. Optical silicone rubber has been around awhile, with the first optically clear silicone rubber developed in the 1950s [5]. The chemical structures of liquid silicone rubber and glass have elements in common. Like glass, LSR is also formed by Si and O (Fig. 2), however the additional radicals in its structure is what makes silicone rubbers opaque or translucent by nature. Although common in some regards, the mechanical and physical properties of Liquid Silicone Rubber are superior to glass and carbon-based polymers [6]. In relation to hardness, LSRs can be as flexible as 5 Shore A, or as hard as glass (approximately 90 Shore A). Its density is also a plus, it ranges between 1.1 and 2.3 g/cm3, significantly lower than glass [7].

THERMAL PERFORMANCE

Most applications specifying optical materials will be in high temperature environments. Because of LSR’s good thermal stability, optically clear Liquid Silicone Rubber performs well and maintains its transparency without decreasing over time [7]. Thermoset epoxy resins for example do not perform well, their clarity decreases and will turn black when subjected to 200°C for 200 hours. LSRs offer advantages over polycarbonates as well, the optical LSR material will maintain homogeneous light distribution over a range of wavelengths, whereas when polycarbonate is used at specific wavelengths, it will turn yellow [3].

Fig. 2 Chemical structure of silicone rubber [3]

CHEMICAL STRUCTURE AND ADDITIVES

Optical Liquid Silicone Rubber, without any additives and with different molecular weights, have been shown in material literature to contain Phenyl, Methyl and Trifluoropropyl groups in its chemical structure [8]. So how does one-part optical silicone rubber differ from other liquid silicone rubbers? During the synthesis, the polymer repeat unit is modified and creates a short-range structure similar to glass. The disadvantage of this modification is its clarity will decrease due to thermal aging, similar to the carbon-based polymers. Table 1 presents the refractive index of these optical silicone rubbers in comparison with glass [3].

Table 1. Refractive indexes of different silicone rubbers in comparison with glass [3]

Although one-part optical silicone rubber can be used, optical silicones with two-part addition curing are more commonly used and preferred. In two-part LSRs, the vulcanization is activated using a platinum catalyst [9]. Also, special silicone rubbers catalyzed using 2,5-dimethyl-2, 5-di(t-butylperoxy) hexane is considered optimum due its absence of by-products [4]. To improve the clarity of silicone rubber, it is necessary to modify its chemical structure, typically using additives. The main objective of the additives is to change the refractive index until it is the same or very similar to glass’s refractive index. The most commonly used additive is the silicon dioxide (silica) which has a rod-shape morphology and an average particle size of approximately 15 nm. Apart from the primary objective being to improve the mechanical properties, this additive can also improve the optical properties as well [10]. The addition of extra-fine silica or wet-process hydrophobic silica affects the morphology so it retains its clarity even at high temperatures, and also improves processing.

OPTICAL CLARITY MEASUREMENT

The clarity of a material can be evaluated using different properties: percentage of light transmission, refractive index, percentage of haze (a measure of the diffused and transmitted light), Abbe number (measurement of the material’s light dispersion), and yellowness index, (calculated based on color changes in the material caused by natural or artificial radiation). In assessing the optical quality of Liquid Silicone Rubber (LSR) vs. glass based on these metrics, optical LSRs are superior in all except haze and yellowness index, where glass has better values. Overall, the optical properties of a modified optical LSR, are generally better than PC and PMMA polymers, and glass as well [3].

APPLICATIONS

Optical Liquid Silicone Rubbers are increasingly replacing glass in lighting applications, particularly in bulbs. Glass used in these types of applications, with high energy consumption the high temperature on the glass surface is the primary cause for the short life of the product [11]. In 1962, light-emitting diode (LED) were introduced.  The LED is a two-lead semiconductor light source that is activated when a voltage is applied that releases energy in the form of photons (light). The use of optical LSRs in these applications offers increased product life, because of the material’s thermal stability even in high temperature even over an extended period of time [1]. Figure 3 shows the comparison between the glass bulb and the LED lightning system with a Liquid Silicone Rubber enclosure.

Figure 3. Glass light bulb and optical silicone rubber LED bulb [3]

Other products where silicone rubber can be used include white reflectors and diffusers used in lightning, electronic or automotive applications. As a refractive or TIR (Total Internal Reflection) lens, with the appropriated design, the light loss during reflection is eliminated completely. Optical LSRs are also used successfully in imagery and scanning applications, in bar-code scanners, spectrometers and particle counters [3]. In medical applications, optical Liquid Silicone Rubber can be found in tubing, endoscopy components, catheters and lenses. In new, emerging telecommunications fields, optical LSRs are used in microlens arrays, and diffractive optical elements; in electrical applications they can be found in solar collection products and as fibers in photonics [8]. For photonics applications, ultraviolet (UV) light is used to cure the silicone rubber; the exposure of the material to an appropriate wavelength permits the generation of waveguide’s patterns that can be used successfully in lithography.

In addition to applications for optical silicone rubber with visible spectrums of light, there are also applications in other ranges of light. For example, applications with UV-visible / Near Infrared range used for data transmission [11]. Depending on the chemical composition of the grade and additives used, LSR wavelengths can range between 850 nm and 1300-1600 nm.

Processing Liquid Silicone Rubber

The processing of optical Liquid Silicone Rubber is also critical for attaining desirable properties in the final product. Similar to optical carbon-based polymers, silicone rubbers can be injection molded, compression molded or cast [6]. LSR’s short processing time, no material waste due to the elimination of sprues and runners, and its initial low viscosity, allows for the production of tight tolerance complex products. In addition, wear on the machine, and mold components occurs at a very slow rate, and very low birefringence is generated which can be a problem for some optical applications. During the cooling process (after processing), internal stresses are not created when processing LSR, even in thick walls, providing increased mechanical and dimensional stability in the end products [13]. From the rheological point of view, optical Liquid Silicone Rubber’s inherent viscosity is independent of the shear rate, and is lower compared to standard LSR, positively affecting the processing, for example the flow can be controlled and predicted easily. Figure 4 shows the comparison in behavior between standard LSR and optical Liquid Silicone Rubber.

Figure 4. Comparison of behavior between standard and optical silicone rubber [14]

Although LSRs have many processing advantages, there are also some considerations to take into account when processing. The first consideration involves the composition of the material, because most optical LSRs are comprised of two-parts, mixing is required. It is important that the material is homogenous and mixed well before processing [14]. Also variables such as shrinkage, part geometry, and the surface finish of the mold can negatively affect the optical properties and should be addressed. Due to its low viscosity, there is a tendency for flash to be generated in the product, for that reason, additional design considerations are necessary to avoid or minimize flashing.

LSR injection molders, for example SIMTEC Silicone Parts, who are regarded as experts in Liquid Silicone Rubber technology, utilize their knowledge, experience, and advanced manufacturing to optimize the production of high quality LSR, Two-Shot, and Multi-Shot LSR components.

References

1. https://www.merriam-webster.com.
2. https://www.researchgate.net/publication/279946258_Optical_Materials
3. https://ww.dow-corning.com
4. M. Altman, J.D. Lytle. Optical design techniques for polymer optics. S.P.I.E. Proceeding, Vol. 237, 380-385, 1980.
5. G. Gu, Q.L. Zhou. Preparation of high strength and optically transparent silicone rubber. European Polymer Journal, Vol 34, No. 11, 1727-1733, 1998
6. https://www.wacker.com
7. https://omnexus.specialchem.com/centers/liquid-silicone-rubber
8. https://www.momentive.com
9. https://www.wacker.com
10. Jones, W. Ando, J. Chojnowski. Silicon-containing polymers, Kluwer Academic Publishers, Dordrech, 2000
11. https://www.quantumsilicones.com
12. https://www.nusil.com
13. E. Mark. Some interesting things about Polysiloxanes. Accounts of Chemical Research, Vol. 37, No. 12, 946-953, 2004
14. https://www.latticematerials.com