Fluorescent whitening agents, functioning as fluorescent white dyes, utilize fluorescence to impart a sense of whiteness to the human visual system. Similar to dyes employed in coloring various fibers, these agents have the capacity to dye a variety of fibers, including paper, cotton, linen, and viscose fibers, especially on cellulose fibers. Discovered in the 1920s, these agents officially entered the market in 1939, boasting a history of over 80 years. Optical brighteners, with numerous varieties, find widespread applications across diverse industries, with the paper industry being the largest consumer.

1. Fundamental Principles of Fluorescent Whitening

Fluorescent whitening agents, possessing a unique structure with conjugated double bonds and good smoothness, act as colorless dyes. When exposed to sunlight, they absorb ultraviolet rays (wavelength of 300~400nm), activate molecules, and re-form in the ground state. This process converts part of the ultraviolet energy into lower-energy visible light, reflecting blue-violet light (420 to 500 nm) on paper. This effectively counteracts the yellowish tint resulting from excessive reflection of yellow light in paper fibers, enhancing paper whiteness and creating a bright, dazzling effect. The effectiveness of these agents varies with changes in incident light sources, particularly thriving under high UV content.

Different types of fluorescent whitening agents exhibit varying fluorescence colors, determined by the wavelength range of UV light they absorb. Varieties absorbing below 335nm exhibit reddish fluorescence, while those absorbing above 365nm exhibit greenish fluorescence.

2. Varieties Employed in the Paper Industry

Pulp fibers, typically yellow or off-white due to lignin absorption, experience limited improvement in whiteness through traditional dyeing or bleaching methods. Adding fluorescent whitening agents to papermaking, through methods like pulp whitening, surface sizing, or surface coating, can significantly enhance paper whiteness. These agents must meet specific requirements for compatibility, pH resistance, and affinity to paper fibers. Commonly used varieties in the papermaking industry include stilbene triazine types, such as disulfonic acid, tetrasulfonic acid, and hexasulfonic acid.

3. Application in Papermaking Processes

• Pulp Whitening: Adding fluorescent whitening agents directly to pulp for whitening.
• Surface Sizing: Incorporating agents into surface sizing glue to enhance whiteness.
• Surface Coating: Applying agents to coating solutions during paper coating processes.

Factors affecting pulp whiteness include feeding sequence, pH value, and freeness. Sizing and coating processes further impact paper whiteness, with considerations for feeding sequence, cationic additives, salts, packing, dyeing time, dry/wet strength agents, pH value, and glue amount.

4. Coating and Whiteness

Coating processes involve applying a paint layer containing fluorescent whitening agents to enhance paper smoothness, whiteness, and waterproof properties. Suitable agents include tetrasulfonic acid and hexasulfonic acid derivatives, such as PA-202 and liquid fluorescent agents PA-206, PA-207.

In summary, the application of fluorescent whitening agents in paper manufacturing serves as a crucial aspect for enhancing paper whiteness and achieving desired visual effects. The careful selection and integration of these agents in various papermaking processes contribute to the production of high-quality, visually appealing paper products.