Analysis of Preparation Process and Advantages of High Transparency Optical LSR Lens

Introduction

High transparency optical liquid silicone rubber (LSR) lenses have emerged as a critical alternative to traditional glass and thermoplastic optical components in consumer electronics, automotive lighting, medical devices, and AR/VR systems. Unlike polycarbonate (PC) and polymethyl methacrylate (PMMA), optical LSR exhibits unique properties including 92%+ visible light transmittance, -40°C to 200°C operating temperature stability, inherent UV resistance, and low birefringence, making it ideal for high-performance optical applications requiring long-term reliability in harsh environments.

The global market for optical LSR lenses is projected to grow at a CAGR of 12.7% from 2024 to 2030, driven by rising demand for miniaturized, lightweight optical modules in smart wearables and adaptive automotive lighting. However, the preparation of high-transparency LSR lenses requires strict control over raw material formulation, molding process parameters, and post-treatment to avoid defects such as bubbles, haze, and refractive index inhomogeneity. This article systematically analyzes the material formulation system, precision injection molding process, post-treatment and quality control methods of high-transparency optical LSR lenses, and quantifies their performance advantages compared to traditional optical materials, providing technical reference for mass production and application expansion of LSR optical components.

Material Formulation Design for High Transparency Optical LSR

The transparency and optical uniformity of LSR lenses are fundamentally determined by the raw material formulation, which requires precise control of polymer matrix composition, crosslinking systems, and additive content to minimize light scattering and absorption.

Base Polymer Selection and Molecular Weight Regulation

The core component of high-transparency optical LSR is vinyl-terminated polydimethylsiloxane (V-PDMS), whose molecular weight distribution and vinyl content directly affect crosslinking density, mechanical properties, and optical performance. To achieve high transparency, the base polymer must have a narrow molecular weight distribution (polydispersity index < 1.2) to avoid microphase separation between polymer chains of different lengths, which would cause light scattering and increase haze.

Table 1 shows the performance comparison of V-PDMS with different vinyl contents for optical LSR applications:

As shown in Table 1, 0.15 mmol/g vinyl content offers the optimal balance: crosslinking density is sufficient to ensure structural stability of the lens, while the refractive index uniformity is high, achieving peak transmittance of 92.1%. For applications requiring higher refractive index (1.43~1.45), phenyl-modified V-PDMS is used, where the introduction of phenyl groups increases the polarizability of the polymer chain, raising the refractive index without significantly reducing transmittance. However, phenyl content must be controlled below 15 mol% to avoid excessive chain rigidity that increases residual stress during molding.

Crosslinking System and Additive Optimization

The crosslinking system of optical LSR adopts a platinum-catalyzed hydrosilylation reaction between hydrogen-containing PDMS (H-PDMS) and vinyl groups on the base polymer, which avoids the byproducts generated by peroxide curing systems, thus eliminating internal voids and haze caused by volatile small molecules. The molar ratio of Si-H groups in H-PDMS to Si-Vi groups in V-PDMS (H/Vi ratio) is a key parameter: when the ratio is 1.1~1.3:1, crosslinking is complete, residual Si-H or Si-Vi groups are minimized, and the risk of post-curing yellowing is reduced.

For additives, optical LSR formulations require strict restriction of non-reactive components to avoid light scattering:

1. Platinum catalyst dosage: Controlled at 5~10 ppm, higher dosages will cause the cured LSR to turn yellow under long-term UV exposure, while lower dosages will lead to incomplete curing and reduced mechanical strength.
2. Inhibitor: Uses ethynyl cyclohexanol with a dosage of 0.02~0.05 phr to adjust the pot life of the LSR mixture to 48~72 hours at 25°C, ensuring no premature crosslinking occurs during material transportation and injection molding feeding.
3. UV stabilizer: For outdoor applications, 0.1~0.3 phr of hindered amine light stabilizer (HALS) modified with silicone groups is added, which has good compatibility with the PDMS matrix, improving UV aging resistance without reducing transmittance. After 1000 hours of UVB (313nm) irradiation at 50°C, the transmittance of LSR with modified HALS decreases by only 0.8%, compared to 3.2% for unmodified LSR.

Importantly, no reinforcing fillers such as fumed silica are added to high-transparency optical LSR formulations, as the refractive index difference between silica (1.46) and PDMS (1.41) will cause significant light scattering, increasing haze to above 5% even with nanoscale silica. Instead, crosslinking density is adjusted to meet the hardness and modulus requirements of the lens, ensuring optical performance is prioritized.

Precision Injection Molding Process for High Transparency LSR Lenses

LSR injection molding is a thermoset processing method, which is fundamentally different from thermoplastic lens molding, requiring precise control of material metering, mold temperature, injection speed, and curing time to ensure optical uniformity and surface accuracy of the lens.

Molding Equipment and Mold Design Requirements

Specialized LSR injection molding machines are required for optical lens production, equipped with a closed-loop proportional control metering system to ensure the mixing ratio of the two-component LSR (A component: V-PDMS + platinum catalyst; B component: V-PDMS + H-PDMS + inhibitor) is accurate to ±0.1%, as mixing ratio errors exceeding 0.5% will lead to incomplete curing and refractive index deviations of up to 0.002, resulting in focal length offset and imaging distortion.

The mold for optical LSR lenses has the following core design points:

1. Mold material: Uses S136 stainless steel with a hardness of HRC 52~55, and the cavity surface is polished to a roughness Ra < 5 nm, ensuring the surface of the molded lens reaches optical grade without secondary polishing. For lenses requiring micro-nano structures (such as Fresnel lenses for AR), the mold cavity is processed by ultra-precision diamond turning, with a profile accuracy of ±0.2 μm.
2. Temperature control system: Adopts independent oil heating for the moving and fixed molds, with temperature control accuracy of ±0.5°C. The mold temperature is usually set between 130°C and 170°C: lower temperatures lead to incomplete curing and longer cycle times, while higher temperatures cause the LSR to cure rapidly at the gate, resulting in insufficient filling and bubble defects.
3. Gating system: Uses a cold runner system with a needle valve gate, which can seal the runner after injection is completed, avoiding LSR curing in the runner and reducing material waste. The gate diameter is designed to be 0.3~0.8 mm, and the shear rate at the gate is controlled below 10⁴ s⁻¹ to prevent shear-induced molecular chain orientation, which would cause birefringence in the cured lens.
4. Ventilation system: The cavity is equipped with ventilation grooves with a depth of 0.02~0.03 mm and a width of 5~10 mm, ensuring that air trapped during injection can be discharged in time. For lenses with a thickness exceeding 5 mm, a vacuum exhaust system is added to reduce the cavity pressure to below 10 mbar before injection, completely eliminating bubble defects.

Process Parameter Optimization and Defect Control

The key process parameters for high-transparency LSR lens molding include injection speed, holding pressure, curing time, and demolding temperature, and their optimization is shown in Table 2 for a 2 mm thick, 10 mm diameter plano-convex LSR lens:

Common optical defects in LSR lens molding and their control methods are as follows:

1. Bubbles: Caused by trapped air or incomplete degassing of raw materials. Control methods include pre-degassing the two-component LSR under vacuum for 15 minutes before feeding, reducing the injection speed in the filling stage, and maintaining a holding pressure for 5~10 seconds after filling to dissolve tiny bubbles into the LSR matrix.
2. Haze: Caused by microphase separation, residual small molecules, or surface roughness. Control methods include using LSR formulations with narrow molecular weight distribution, ensuring the H/Vi ratio is between 1.1 and 1.3, and regularly polishing the mold cavity to maintain surface roughness below 5 nm.
3. Birefringence: Caused by molecular chain orientation during injection and residual stress after curing. Control methods include reducing the injection speed to lower shear force, increasing the mold temperature to prolong the relaxation time of molecular chains before crosslinking, and adding a post-curing step to eliminate residual stress.

For high-precision lenses used in AR/VR systems, the birefringence value must be controlled below 10 nm/cm, which can be achieved by adding a 2-second delay between the end of injection and the start of curing, allowing oriented molecular chains to fully relax before crosslinking is completed.

Post-Treatment and Optical Performance Testing of LSR Lenses

Even with optimized molding processes, cured LSR lenses usually have a small amount of residual stress and unreacted small molecules, requiring post-treatment to further improve optical stability, followed by strict performance testing to meet application requirements.

Post-Treatment Process for Performance Stabilization

The main post-treatment processes for high-transparency optical LSR lenses include post-curing, surface modification, and edge trimming:

1. Post-curing: The lenses are placed in a forced air oven for 2~4 hours at 150°C, which can make the unreacted Si-H and Si-Vi groups fully crosslinked, remove residual inhibitors and low molecular weight oligomers, and eliminate internal residual stress. For lenses used in medical devices, post-curing is performed at 121°C for 1 hour under high-pressure steam, which not only stabilizes optical performance but also meets sterilization requirements. After post-curing, the residual stress of the lens is reduced by more than 80%, and the birefringence value is reduced from 30~50 nm/cm to below 10 nm/cm, while the transmittance is increased by 0.3~0.5% due to the removal of volatile small molecules.
2. Surface modification: Although LSR has good hydrophobicity (water contact angle of ~110°), its surface is prone to dust adsorption due to low surface energy, which reduces light transmittance during use. For applications requiring anti-fouling and anti-reflection properties, plasma-enhanced chemical vapor deposition (PECVD) is used to coat a 100~200 nm thick SiO₂/TiO₂ anti-reflection (AR) coating on the lens surface. The AR coating can reduce the surface reflectance of LSR from 3.2% per side to less than 0.5% per side, increasing the total transmittance of a 2 mm thick lens from 92.1% to more than 94%, and improving the surface hardness from Shore A 50 to pencil hardness 3H, enhancing scratch resistance.
3. Edge trimming: For lenses molded with a gate, the gate residue is trimmed by a CNC laser cutting system with a positioning accuracy of ±1 μm, avoiding edge chipping or stress concentration caused by mechanical cutting. The trimmed edge roughness is controlled below Ra 0.2 μm, meeting the assembly requirements of optical modules.

Optical Performance Testing and Quality Control

The performance of high-transparency optical LSR lenses must be tested against strict standards, with key testing items and acceptance criteria shown in Table 3:

In mass production, an automatic optical inspection (AOI) system is used for 100% inspection of lenses, which can detect surface defects such as scratches, bubbles, and impurities with a minimum size of 1 μm, ensuring the product yield is above 95%. For high-end AR/VR lenses, additional focal length and modulation transfer function (MTF) testing is performed, requiring the MTF value at 50 lp/mm to be ≥0.8, meeting the requirements of high-resolution imaging.

Performance Advantages and Application Scenarios of High Transparency Optical LSR Lenses

Compared with traditional optical materials (glass, PC, PMMA), high-transparency LSR lenses have unique comprehensive performance advantages, enabling their rapid penetration in multiple high-growth application fields.

Comparative Performance Analysis with Traditional Optical Materials

Table 4 quantifies the performance comparison between optical LSR and traditional optical materials:

As shown in Table 4, LSR has obvious advantages in temperature stability, UV resistance, and density: its operating temperature range is 80°C higher than PC and 120°C higher than PMMA, making it suitable for high-temperature environments such as automotive headlights and outdoor lighting; its 85%+ UV transmittance makes it ideal for UV disinfection equipment optical components, where PC and PMMA are completely opaque and have poor UV aging resistance; and its density is 20% lower than PC and 18% lower than PMMA, which helps reduce the weight of wearable devices such as AR glasses.

Compared with glass, LSR has obvious advantages in mass production efficiency and impact resistance: the molding cycle of LSR is only 1/10 of that of glass grinding, and the material cost is 30~50% lower for complex aspherical lenses; LSR lenses will not break when impacted, which improves the safety of automotive lighting and medical device applications. The only disadvantage of LSR compared to glass is the relatively narrow refractive index range, but the development of high-refractive index LSR (up to 1