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What are the optical properties of polylactic acid fibers
source：www.kingcharmgroup.com | Release time：2026-01-051. Core optical performance indicators and their manifestations
1. Light transmittance and haze
The amorphous state of polylactic acid fibers exhibits excellent light transmittance, with visible light transmittance reaching 85%–92%, which is close to that of polyester (PET) fibers. It also has low haze (typically <5%), resulting in minimal scattering of light and a high-transparency texture.
If the crystallinity of the fiber increases (such as after heat setting treatment), the refractive index difference between the crystalline and amorphous regions will lead to light scattering, resulting in a decrease in light transmittance to 60%–80%, a corresponding increase in haze, and a change in fiber appearance from transparent to translucent or milky white.
2. Refractive index
The refractive index of polylactic acid fibers exhibits anisotropy: the refractive index parallel to the fiber axis (n∥) is approximately 1.57–1.59, while the refractive index perpendicular to the axis (n⊥) is approximately 1.49–1.51. The birefringence (Δn = n∥-n⊥) is about 0.08–0.09, which is lower than that of nylon 6 fibers (Δn ≈ 0.10) and slightly higher than that of polypropylene (PP) fibers.
The difference in refractive index arises from the orientation and arrangement of molecular chains during the fiber drawing process. The higher the draw ratio, the greater the degree of molecular orientation, and consequently, the birefringence index increases.
3. Glossiness
The surface glossiness of polylactic acid (PLA) fibers can be controlled: the untreated fiber surface is smooth, exhibiting a soft semi-dull to glossy effect, with a glossiness value of approximately 40–60 GU (Gloss Units); by adding a matting agent (such as titanium dioxide), dull-type PLA fibers can be prepared, with the glossiness reduced to 10–30 GU.
Compared to natural fibers such as cotton and linen, polylactic acid fibers exhibit a more uniform and soft luster, devoid of a "waxy texture", combining the crispness of synthetic fibers with the warmth of natural fibers.
4. UV light resistance stability
Polylactic acid fibers are sensitive to ultraviolet (UV) radiation. UV light with wavelengths ranging from 280 to 360 nm can damage the ester bonds in the molecular chain, leading to yellowing, embrittlement, and a decrease in light transmittance of the fibers.
If ultraviolet absorbers (such as benzotriazoles) or antioxidants are added during the fiber preparation process, the UV resistance can be significantly improved. After 500 hours of UV aging, the yellowing index (ΔYI) can be controlled within 5, and the decrease in light transmittance is less than 10%.
II. Key Factors Affecting Optical Performance
1. Crystallinity and orientation
Crystallinity is the core factor affecting light transmittance: when crystallinity is less than 10%, the fiber exhibits high transparency; when crystallinity exceeds 30%, the scattering effect in the crystalline region intensifies, rendering the fiber translucent.
The degree of orientation determines the birefringence: when the stretching ratio is increased from 1 to 5, the degree of molecular orientation is significantly enhanced, and the birefringence can be increased from 0.02 to 0.09, resulting in more pronounced optical anisotropy in the fiber.
2. Processing technology
Spinning temperature: Too high a temperature (over 230°C) can lead to thermal degradation of polylactic acid, resulting in the production of small molecular impurities, which can decrease the light transmittance of the fibers and increase haze. Too low a temperature can lead to poor melt flowability, making the fiber surface prone to defects and affecting the uniformity of gloss.
Heat setting treatment: High-temperature heat setting promotes fiber crystallization, reducing light transmittance, but it can enhance gloss stability and prevent gloss fluctuations caused by crystallization changes during subsequent use.
3. Additives
Adding nucleating agents (such as talcum powder and polyethylene glycol) can refine the grain size, reduce light scattering, and maintain high light transmittance while improving crystallinity.
The addition of colorants or functional additives (such as antibacterial agents) can reduce light transmittance, with the specific impact depending on the particle size and dispersibility of the additives.
III. Typical Applications Driven by Optical Performance
1. Textile and apparel industry
Highly transparent and glossy polylactic acid fibers can be used to make lightweight and breathable shirts and dresses, with a soft and non-dazzling luster, combining environmental friendliness with aesthetics. Matte polylactic acid fibers can replace polyester fibers for making underwear and sportswear, providing a skin-friendly touch and a low-key luster.
2. Medical materials field
Highly transparent polylactic acid fibers can be used to prepare medical sutures and tissue engineering scaffolds. Their good light transmittance facilitates doctors' observation of suture positions during surgery. At the same time, their low birefringence property can avoid interference with optical detection equipment (such as endoscopes).
3. Packaging and decoration field
Nonwoven fabrics and films made from polylactic acid fibers are highly transparent and biodegradable, making them suitable for food packaging and flower packaging, balancing visual effects and environmental protection requirements. By adjusting the glossiness, they can also be used to produce decorative shimmering fibers and matte fabrics.
IV. Performance Shortcomings and Optimization Directions
1. Shortcomings
Poor UV resistance, prone to yellowing and aging after long-term outdoor use;
An increase in crystallinity significantly reduces light transmittance, limiting its application in highly transparent products.
2. Optimization direction
Blending modification: Blend spinning polylactic acid with UV-resistant polymers (such as polycarbonate and polyurethane) to enhance UV resistance;
Surface coating: A transparent UV-resistant coating is applied to the surface of the fiber, which does not affect light transmittance but blocks UV light;
Correct crystal control: By employing a low-temperature stretching and stepwise heat-setting process, the crystallinity of the fibers is controlled to be within 10%–20%, balancing light transmittance and mechanical properties.
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