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Comparison of PC, PMMA Lenses and Silicone Optical Lenses

Jul. 20, 2021

Yejia Optical Technology (Guangdong) Corporation, as a group company located in Dongguan China for 12 years. Owned two division, optical and silicone division.

We-silicone division has running mass production of optical LSR lens.


Material, equipment and workshop.

Materials: Dow MS-1001, MS-4002, Doneson DSA-7070, Momentive LSR7180, Wacker LR 7601.

Tooling: Self-developed valve needle cold runner lens mold.

Equipment: Germany Arburg Injection Machine

Workshop: Class 100K dust-free clean workshop


The properties and strength of LSR lens

Transmittance- lenses should try spread as much lights as possible-if light is absorbed, the light transmission efficiency will be affected. High transparency is the basic requirements for lens material. PC transmittance is around 87%, due to the poor optical performance, it has been eliminated by the optical products industry that requires high transparency.


PMMA transmittance is about 92%, the most widely used optical material. While LSR optical silicone transmittance can reach to 98%, it can greatly reduce the impact of color distortion, reflectivity and clarity on visual perception caused by transparency during use of the optical lens.


Residual stress

The residual stress inside the molded product can easily cause lens deformation and cracking, leading to the birefringence phenomenon of the optical products, which directly affects the optical performance, and the safety is also difficult to guarantee.


The molding process of PC and PMMA thermoplastic compounds is a physical change, the linear molecular chain is under the influence of the injection flow speed and the temperature change off the molding gradient, the stretched and compressed molecular chain is quickly cooled and shaped, and it’s in an unstable arrangement state. It cannot be restored to the original free and stable state. the two-component liquid silicone LSR is formed by thermochemical reaction. It is injected into the high-temperature mold at low temperature.

When the material reaches a certain high temperature, the cross-linking reaction between the molecular chains will gradually occur. There is basically no cross-linking reaction between the molecular chains during the entire curing and molding process. Residual stress occurs.


RI Refractive Index

When light enters another medium from one medium, when its propagation direction is not perpendicular to the interface between the two media, the propagation direction at the interface changes.

The refractive index is related to the wavelength of light. The refractive index of the lens is smaller when the wavelength is longer, and the refractive index of the lens is larger when the wavelength is shorter. When the light is refracted through the lens, the wavelength of each color light is different, and the degree of refraction is also different. After refraction, it decomposes into dispersion. The refractive index of the lens material is large, and the degree of refraction after the light passes through is large. 


Usually, the dispersion is serious, which means the ghost is serious. Dispersion is an important indicator to measure the sharpness of lens imaging. The greater the dispersion, the worse the sharpness of its imaging. In terms of refractive index, PC is about 1.526, PMMA is about 1.592, and glass is about 1.523. They all have a certain degree of dispersion, which is easy to cause visual fatigue. Wacker optical silica gel is 1.41, which can maintain a stable low refractive index in high and low temperature environments and a variety of wavelengths of light, with a low degree of dispersion, and high-definition imaging effects can be obtained.


Contractility

PC and PMMA are thermoplastic materials that can be repeatedly heated and softened and cooled to harden. During the molding process, the product shrinks unevenly due to changes in local temperature and thickness, which leads to shrinkage stress in the lens product. When the product is used, the environmental temperature change affects it More obvious.

LSR silicone material is a thermosetting material, and there is no significant change after heating after molding. The molding is thermal reaction molding, and the shrinkage during the molding process is balanced. The injection molding process can be used to achieve mass production of molded parts with fine structures.

Density

Another major advantage of silicone elastomer is their low density. The silicone parts made from it are much lighter than glass parts of the same size, and have a slightly lower density than PC and PMMA. The relative density of PC is generally about 1.2g/cm3, PMMA is generally 1.18g/cm3, and LSR optical silica gel is 1.12g/cm3. LSR products can reduce the weight-to-volume ratio of the finished product under certain conditions of guaranteed function, which is conducive to the lighter weight of the product.


Thermal Stability

The thermal deformation temperature of PMMA is about 90 degrees, and that of PC is about 130 degrees. Higher temperatures will cause lens deformation and dimensional instability, which will affect the life of the product. The organic silicon polymer chain is composed of Si-O bonds, the bond energy is 422~494KJ/mol, and the CC bond energy of the plastic polymer chain is 360 KJ/mol. The higher bond energy determines its high temperature resistance. At the same time, Si The unique flexible structure of the -O main chain allows it to maintain good flexibility at low temperatures. Silicone optical lenses have good thermal stability and can maintain long-lasting elasticity at low temperatures without using plasticizers. They can be used at -50~200°C for a long time, maintaining dimensional stability, and having excellent mechanical and optical properties.


Weather resistance, Radiation resistance

PC and PMMA will gradually turn yellow when exposed to heat, artificial or natural light sources, or ultraviolet radiation, which will seriously affect their use. Silica gel is extremely resistant to oxygen or ozone. They will not be affected by long-term exposure to visible light and ultraviolet radiation. Unlike organic plastics, they will not turn yellow. WACKER’s new silicone elastomers It can maintain crystal-like transparency even under long-term load. In addition, the silicone elastomer does not absorb water, has excellent hydrophobicity, and has better anti-fingerprint and anti-fouling ability than PC or PMMA, which also brings convenience to users.

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