Applications such as LED lighting, optical devices, and display modules have extremely high requirements for the transmittance and weather resistance of encapsulating materials. However, some users report that encapsulants develop yellowing, transmittance decline, and refractive index reduction after prolonged use or light exposure, directly affecting luminous efficiency and product lifespan—with problems particularly pronounced in applications such as outdoor LED displays and automotive headlights.

The essence of encapsulant yellowing is chemical structural change in the material under the action of light, heat, and oxygen. Specific causes include: molecular chains of conventional condensation-cure silicone encapsulants contain unsaturated bonds or impurities that readily undergo oxidation reactions under UV irradiation to generate chromophores; residual catalysts, solvents, or low-molecular-weight impurities in the encapsulant accelerate decomposition under light and high temperature; in high-temperature, high-humidity environments, moisture penetration accelerates the material aging process; and some high-refractive-index encapsulants add sulfur-containing or aromatic-ring-containing modifiers to increase refractive index, and these groups themselves have poor UV resistance.

From a material selection perspective, addition-cure liquid silicone rubber (LSR) systems are recommended as the priority. Addition-cure LSR cures through hydrosilylation, contains no unsaturated bonds in its molecular structure, and releases no small-molecule byproducts during curing, giving it significantly superior yellowing resistance compared to condensation-cure products. For LED encapsulation requiring high refractive index (n≥1.50), the product's UV resistance grade must be confirmed, and dedicated UV-resistant modified formulations should be selected, typically incorporating UV absorbers and hindered amine light stabilizers.

From a validation testing perspective, encapsulants should undergo reliability testing before use. Conventional test conditions are 85°C/85%RH dual-85 testing for 1,000 hours, or UV aging testing (UVB-313 lamps, 0.76 W/m²@340 nm) for 500 hours. Only products with transmittance decline not exceeding 5% and yellowing index (YI) change not exceeding 3 after testing are suitable for outdoor or high-luminous-flux scenarios. For demanding applications such as automotive headlights and outdoor displays, encapsulant products certified to IEC 61215 (photovoltaic modules) or AEC-Q200 (automotive electronics) are recommended.

In addition, the encapsulation process also affects weather resistance. After potting or dispensing, complete curing must be ensured before putting into service—residual reactive groups in incompletely cured encapsulant will accelerate later-stage yellowing. During storage, encapsulants should be kept sealed and protected from light to avoid prolonged exposure to air causing moisture absorption or light-induced pre-reaction.

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