UV Protection Methods for Polarizers
Enhancing UV Resistance (100nm ~ 400nm) in Polarizers
Enhancing UV resistance (100nm ~ 400nm) is essential to prevent the polarizer layer from fading, degrading, yellowing, or failing due to ultraviolet exposure. Additionally, UV can damage the molecular structure of liquid crystals, leading to reduced performance. Once liquid crystal materials degrade, color deviation or yellow spots may occur, affecting display uniformity, image quality, and lifespan.
This time, we will focus on how to improve the UV resistance of polarizers.
Understanding Polarizer Structure and Process Flow
To understand how to enhance the UV resistance of a polarizer, we first need a basic understanding of its structure and manufacturing process.
General Structure of a Polarizer:
- Surface Coating:
- Optional additional treatments.
- Functional coatings applied directly to the surface of the TAC film.
- Examples: Anti-scratch coatings (used in most polarizers), AR (Anti-Reflective), AF (Anti-Fingerprint), AS (Anti-Smudge), UV-blocking.
- Functional Film (Surface Protection Layer):
- Optional additional film bonded to the TAC surface for enhanced performance.
- Examples: Brightness enhancement film, UV protection, AF/AR/AS coatings, anti-static film.
- TAC Film (Triacetyl Cellulose) – Upper TAC:
- Provides physical protection and structural support.
- Protects the polarizer layer from scratches, moisture, oxidation, etc.
- Offers mechanical strength and flexibility.
- PVA Polarizer Layer – Polyvinyl Alcohol:
- The core functional layer.
- After stretching, PVA molecules align with dye molecules, which selectively absorb light waves in a specific direction, thus achieving polarization.
- TAC Film – Lower TAC:
- Symmetrical structure enhances mechanical strength.
- Protects the underside of the polarizer layer.
- Pressure-Sensitive Adhesive (PSA) or Optical Clear Adhesive (OCA):
- Bonds the polarizer to the LCD or touch panel glass.
- Must remain transparent, bubble-free, and resistant to yellowing.
- Release Film or Protective Film
From this structure, it’s clear that the PVA polarizer layer is the most critical component. Therefore, to better understand how to enhance UV resistance, we must also understand the manufacturing process of the PVA polarizer layer.
PVA Polarizer Layer Manufacturing Steps
- Dyeing:
PVA is a water-soluble polymer. It is immersed in a solution containing iodine or organic dyes, which adhere to the film to form the polarizing material. - Stretching (Orientation):
The dyed PVA film is stretched in one direction to align the dye molecules. These molecules selectively absorb light waves aligned with their direction, creating linearly polarized light. The transmitted light is perpendicular to the orientation of the dye. - Crosslinking Treatment (to enhance water resistance and stability):
The stretched film is treated with chemical crosslinking agents such as boric acid and sodium ions to increase mechanical strength and moisture resistance. - Drying:
The film is dried to remove excess moisture and stabilize the polarization effect.
Complete Production Flow of a Polarizer
Raw materials → Dyeing → Stretching → Crosslinking → Drying → Lamination → Cutting → Inspection → Packaging
Strategies to Enhance UV Resistance
Several structural and process-related measures can be taken to improve UV resistance:
-
Surface Coating:
- Apply UV-curable coatings containing UV protection components or nano UV shielding materials.
- Provides a durable surface protection layer.
-
Functional Protection Film:
- Add UV barrier films (e.g., PET + UV coating) to further block short-wave UV rays.
-
Upper and Lower TAC Films:
- Use TAC films with UV absorbers to absorb UV rays (especially in the 280–400nm range), preventing penetration and protecting the internal PVA layer.
- UV-absorbing or UV-reflecting coatings may also be applied to the TAC surface, especially in high-end applications.
-
PVA Layer – Dyeing and Stretching:
Stretching Process:
- During polarizer manufacturing, PVA films are dyed, then stretched to align dye molecules.
- Unstretched PVA resembles tangled threads (random molecular chains), while stretched films have orderly aligned chains—like neatly combed hair—which control light direction.
How Stretching Enhances UV Resistance:
- Molecular Orientation Enhances UV Absorption (Indirect UV Protection):
- Stretched PVA has highly ordered molecular chains, offering better light stability.
- If UV-absorbing dyes are used (e.g., iodine-based), stretching aligns them for more effective UV absorption.
- Even without dye, stretched PVA shows slightly enhanced UV absorption.
- Improved Structural Stability and UV Aging Resistance:
- Stretching increases crystallinity and tightens molecular structure, improving UV durability.
- Crucial for long-term exposure scenarios (e.g., outdoor sunglasses, automotive displays).
- Reduced Free Radical Pathways:
- UV light excites unsaturated bonds in polymers, forming free radicals that cause degradation.
- Ordered molecular structures reduce free radical diffusion, improving UV resilience.
Dyeing Process:
- PVA alone absorbs little visible light—even after stretching—so polarization efficiency is low.
- Dyes (like iodine) absorb light in specific directions and align with PVA chains during stretching to create high polarization efficiency (high transmission + high polarization).
- Using UV-resistant, highly photostable dyes (e.g., special anthraquinone or azo dyes) prevents fading and extends lifespan.
- Dye concentration must be controlled to avoid UV-accelerated degradation from overdosing.
Especially in LCDs, where high polarization performance is required, stretched dye films remain the most cost-effective and high-performance solution.
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Adhesive Layer (PSA/OCA):
- Used to bond the two TAC films and the PVA layer.
- Must be optically clear and durable.
- Use optical adhesives with UV inhibitors to prevent yellowing, bubbling, or failure under UV.
- Uniform coating/lamination without bubbles is critical to avoid UV leakage paths.
Summary: Key Points on UV Resistance in Polarizers
- UV radiation is the main external factor causing polarizer aging, especially for the PVA layer and dye structure. It leads to fading, breakage, contrast loss, and reduced display/module lifespan.
- UV resistance design involves both materials and structure:
- Material level: Use TAC films, optical adhesives, and surface coatings with UV absorbers—mainstream absorbers include benzotriazole and triazine compounds.
- Structural level: Stack UV-cut films or use UV coatings for enhanced shielding—especially critical for outdoor or in-vehicle applications with high UV exposure.
- Working principle of UV absorbers:
- Convert UV energy into heat or harmless long-wave radiation, protecting the core PVA and dye layers from degradation.
- UV cut performance is reflected in how well the entire structure limits UV (<400nm) transmission.
- Surface protection designs also play a key role:
- Modern polarizers usually include a hard anti-scratch coating (HC).
- Additional functional films (AR, anti-smudge, UV-blocking) may be added based on application needs vs. cost.
- System-level perspective:
- Polarizers are just one part of the UV protection system.
- Liquid crystals, light guide structures, and full module sealing are equally important.
- UV resistance should be considered at the system design stage—for example, adding front-cover UV filter glass, using UV-resistant liquid crystal materials, etc.
Conclusion:
UV resistance is not just the task of a single material, but a multi-layered, multi-material system design strategy that directly affects the long-term stability and reliability of LCD displays.