When do synthetic woven fabrics require bleaching?

These situations can be categorized into two main categories: one is a problem with the fiber itself (it becomes less white); the other is that the desired whiteness exceeds the fiber's natural beauty.

The following are specific situations in which bleaching is necessary:

I. Chemical yellowing of the fiber itself

This is the most common type of situation. Although native synthetic fibers are pure white, their chemical structure may be damaged by external factors during subsequent processing, storage, or use, resulting in yellowing.

Thermal Oxidative Yellowing

Cause: Synthetic fibers such as polyester and nylon can undergo thermal oxidative degradation during high-temperature processing (such as excessively high heat setting temperatures or prolonged periods) or prolonged exposure to high temperatures, leading to molecular chain breakage or the formation of chromophores, resulting in yellowing.

Treatment: Fabrics that have yellowed require reduction bleaching. A reducing agent (such as sodium dithionite, also known as hydrosulfite) is used to reduce the chromophores to a colorless substance, thereby eliminating the yellowing. This process is often referred to in the industry as "stripping" or "color correction."

Photooxidative Yellowing

Cause: Chemical fibers (especially nylon and polypropylene) can age under prolonged exposure to ultraviolet light, resulting in yellowing.

Treatment: Reductive bleaching is also necessary to correct this condition.

Yellowing Caused by Additives

Cause: Certain additives, such as UV inhibitors and antioxidants, added to fibers may exhibit a slight yellowish tint, either on their own or after exposure to heat.

Treatment: If this yellow tint affects the whiteness of the final product, bleaching is necessary to remove it.

II. Special Cases with Extreme Whiteness Requirements

When "commercial white" does not meet the required standards, post-treatment is necessary to improve it.

Pursuing Ultra-High Whiteness and Color

Scenario: Some high-end brands have extremely stringent requirements for white fabrics, aiming for a pure, vibrant, and fluorescent white finish (such as high-end wedding dresses, medical protective clothing, and luxury brand white coats).

Treatment: Using fluorescent brighteners alone is often insufficient to neutralize the inherent yellowish tint of the fabric. In these cases, a combined "bleaching + brightening" process is employed. Process: First, use reductive bleaching (such as hydrosulfite) to "prime" the fabric to a more neutral, pure white. Then, perform fluorescent whitening. This treatment achieves a whiteness far exceeding that achieved by simple whitening, with a higher whiteness value and a more vibrant and eye-catching color.

Dyeing Extremely Light or Vivid Colors

Situation: When dyeing extremely light or vivid colors (such as pale pink, ice blue, or bright yellow), any slight deviation in the base color can significantly affect the color's purity.

Treatment: To ensure dyeing accuracy and vividness, the fabric must first be bleached to ensure an "absolutely pure canvas" and eliminate any interference from the base color with the extremely pale color.

III. Dealing with Stains or Impurities

Removing Stubborn Impurities

Situation: Although synthetic fibers are inherently free of impurities, they may acquire stubborn organic stains during processing that are difficult to remove with standard scouring agents.

Treatment: Oxidative bleaching (such as hydrogen peroxide) can break down these stains due to its powerful oxidizing power. However, this operation carries a high risk and requires careful control to avoid damaging the fiber.