Optical Liquid Silicone Rubber (LSR)

Gentle could be defined as electromagnetic radiation which has different frequencies and wavelength. The spectrum that may be picked up by the retina of a human eye is named seen gentle [1]. Supplies through which light can refracted, mirrored, transmitted, dispersed, polarized, detected and reworked are called optical materials [2].

Optical Supplies

The number of optical supplies has expanded lately. Prior to now, glass and other ceramic materials have been the few supplies obtainable that provided the very 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-vary association [3]. The atoms in glass are organized randomly, and because of this construction, glass is transparent (Fig.1) [2]. Conversely, metals have an organized structure and therefore will not be clear. In glass, photons (the elementary particles that type the light) are able to move via glass with out interacting with any atom [2]. Because of this structure, it has low mechanical properties and high density (between 2.Three and 6.3 g/cm3) [1], which can be thought-about an obstacle.

Fig. 1 Chemical construction of glass

Optically Transparent Polymers

Some polymers have innate properties much like glass, but they have low physical properties. Examples of some of these polymers embrace the thermoplastic materials Polycarbonate (Pc), and Polymethyl Methacrylate (PMMA) which might be processed utilizing injection molding; and Epoxy resins (EPI), thermoset supplies which are compression molded [4]. These thermoplastic and thermoset materials have benefits, together with prime quality surfaces reflecting the mold surface, they are simply processed, they usually are available in a variety of grades with a variety of properties. There are disadvantages as effectively, together with 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 primary optically clear silicone rubber developed in the 1950s [5]. The chemical constructions of liquid silicone rubber and glass have elements in frequent. Like glass, LSR can also be formed by Si and O (Fig. 2), nevertheless the additional radicals in its construction is what makes silicone rubbers opaque or translucent by nature. Though frequent in some regards, the mechanical and bodily 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 onerous as glass (roughly 90 Shore A). Its density is also a plus, it ranges between 1.1 and 2.3 g/cm3, significantly decrease than glass [7].

Thermal Performance

Most functions specifying optical materials will likely be in high temperature environments. Because of LSR’s good thermal stability, optically clear Liquid Silicone Rubber performs nicely and maintains its transparency with out decreasing over time [7]. Thermoset epoxy resins for instance do not carry out properly, their clarity decreases and can flip black when subjected to 200°C for 200 hours. LSRs offer benefits over polycarbonates as well, the optical LSR material will maintain homogeneous light distribution over a range of wavelengths, whereas when polycarbonate is used at particular wavelengths, it is going to turn yellow [3].

Fig. 2 Chemical structure of silicone rubber [3]

Chemical Structure and Additives

Optical Liquid Silicone Rubber, without any additives and with completely different molecular weights, have been shown in material literature to contain Phenyl, Methyl and Trifluoropropyl teams in its chemical structure [8]. So how does one-part optical silicone rubber differ from other liquid silicone rubbers? Through the synthesis, the polymer repeat unit is modified and creates a short-vary construction similar to glass. The drawback of this modification is its readability will decrease attributable to thermal aging, similar to the carbon-based polymers. Desk 1 presents the refractive index of these optical silicone rubbers as compared with glass [3].

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

Although one-half optical silicone rubber can be used, optical silicones with two-part addition curing are extra commonly used and most popular. In two-part LSRs, the vulcanization is activated using a platinum catalyst [9]. Additionally, particular silicone rubbers catalyzed using 2,5-dimethyl-2, 5-di(t-butylperoxy) hexane is taken into account optimum due its absence of by-products [4]. To improve the clarity of silicone rubber, it is necessary to change its chemical structure, typically using additives. The primary goal of the additives is to alter the refractive index until it is identical or very much like glass’s refractive index. The mostly used additive is the silicon dioxide (silica) which has a rod-form morphology and a median particle size of approximately 15 nm. Aside from the first objective being to improve the mechanical properties, this additive can even improve the optical properties as effectively [10]. The addition of extra-advantageous silica or wet-process hydrophobic silica impacts the morphology so it retains its readability even at excessive temperatures, and also improves processing.

Optical Readability Measurement

The readability of a material might be evaluated using totally different properties: proportion of light transmission, refractive index, percentage of haze (a measure of the diffused and transmitted gentle), Abbe number (measurement of the material’s light dispersion), and yellowness index, (calculated primarily based on color adjustments in the material caused by pure or synthetic radiation). In assessing the optical quality of Liquid Silicone Rubber (LSR) vs. glass based mostly on these metrics, optical LSRs are superior in all except haze and yellowness index, the place glass has higher values. Total, the optical properties of a modified optical LSR, are generally better than Computer and PMMA polymers, and glass as effectively [3].


Optical Liquid Silicone Rubbers are more and more replacing glass in lighting functions, significantly in bulbs. Glass used in most of these applications, with excessive power consumption the high temperature on the glass surface is the primary cause for the quick life of the product [11]. In 1962, mild-emitting diode (LED) had been introduced. The LED is a two-lead semiconductor mild supply that’s activated when a voltage is applied that releases vitality in the type of photons (mild). The use of optical LSRs in these functions affords increased product life, because of the material’s thermal stability even in excessive temperature even over an extended period of time [1]. Figure three shows the comparability between the glass bulb and the LED lightning system with a Liquid Silicone Rubber enclosure.

Determine 3. Glass gentle bulb and optical silicone rubber LED bulb [3]

Different merchandise the place silicone rubber can be utilized embody white reflectors and diffusers utilized in lightning, digital or automotive purposes. As a refractive or TIR (Total Inner Reflection) lens, with the appropriated design, the sunshine loss during reflection is eradicated fully. Optical LSRs are also used efficiently in imagery and scanning applications, in bar-code scanners, spectrometers and particle counters [3]. In medical applications, optical Liquid Silicone Rubber might be found in tubing, endoscopy components, catheters and lenses. In new, emerging telecommunications fields, optical LSRs are utilized in microlens arrays, and diffractive optical components; in electrical functions they can be present in solar assortment merchandise and as fibers in photonics [8]. For photonics purposes, ultraviolet (UV) gentle is used to cure the silicone rubber; the publicity of the fabric to an acceptable wavelength permits the technology of waveguide’s patterns that can be used efficiently in lithography.

Along with purposes for optical silicone rubber with seen spectrums of mild, there are also applications in different ranges of light. For instance, functions with UV-visible / Near Infrared range used for data transmission [11]. Relying 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 can be essential for attaining desirable properties in the ultimate product. Similar to optical carbon-based polymers, silicone rubbers might be injection molded, compression molded or forged [6]. LSR’s brief processing time, no material waste as a result of elimination of sprues and runners, and its initial low viscosity, allows for the production of tight tolerance complicated products. In addition, wear on the machine, and mold parts happens at a really gradual charge, and really low birefringence is generated which may be a problem for some optical purposes. Throughout the cooling course of (after processing), inside stresses aren’t created when processing LSR, even in thick walls, providing elevated mechanical and dimensional stability ultimately merchandise [13]. From the rheological standpoint, optical Liquid Silicone Rubber’s inherent viscosity is independent of the shear price, and is decrease in contrast to standard LSR, positively affecting the processing, for instance the movement may be controlled and predicted easily. Figure 4 shows the comparison in habits between commonplace LSR and optical Liquid Silicone Rubber.

Determine 4. Comparison of conduct between commonplace and optical silicone rubber [14]

Though LSRs have many processing advantages, there are also some considerations to take under consideration when processing. The primary consideration entails the composition of the fabric, because most optical LSRs are comprised of two-elements, mixing is required. It is crucial that the fabric is homogenous and combined effectively earlier than processing [14]. Also variables corresponding to shrinkage, half geometry, and the floor end of the mold can negatively have an effect on the optical properties and ought to be addressed. Because of its low viscosity, there’s a tendency for flash to be generated within the product, for that reason, extra design concerns are essential to keep away from or reduce flashing.

LSR injection molders, for instance SIMTEC Silicone Parts, who’re thought to be consultants in Liquid Silicone Rubber technology, utilize their information, experience, and superior manufacturing to optimize the production of top of the range LSR, Two-Shot, and Multi-Shot LSR components.


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