Popular Science | Classification and Characteristics of Papermaking Dyes

During paper manufacturing, various colorants are commonly applied to endow paper with the desired shade. To produce paper of target color, colorants are either incorporated into pulp or coated onto the paper surface; this procedure is known as paper dyeing or toning. This article elaborates on the fundamental properties and application characteristics of typical dyes widely adopted in the papermaking industry.

At present, dyes used conventionally in papermaking fall into three main categories: direct dyes, basic dyes and acid dyes. In addition, fluorescent whitening agents for whitening purposes and functional dyes, which have gained growing application in recent years, are also included.

1.Direct Dyes

Direct dyes refer to a class of water-soluble dyes containing water-soluble groups (sulfonic or carboxylic groups), alongside hydroxyl or amino groups capable of forming hydrogen bonds. They feature substantive affinity to fibers and can dye fibers directly without mordants. Such dyes are extensively used for dyeing cellulosic fibers, protein fibers and blended textiles.

Boasting strong substantivity toward cellulose fibers, direct dyes enable direct fiber dyeing. Inconsistent mixing with pulp will lead to uneven dyeing and speckles. They are generally unsuitable for groundwood pulp dyeing: high lignin content in groundwood results in poor dye adsorption and consequently pale dyed shades.

Certain vivid blue direct dyes can serve as paper whitening agents under specific circumstances, with Direct Violet as a typical example. Common commercial grades include Direct Brilliant Blue, Direct Sky Blue, Direct Reddish Brown, Direct Scarlet, Direct Freezing Yellow, Direct Dark Brown MM and Direct Black EX.

2.Basic Dyes

Basic dyes dissociate into cations in aqueous solution or form cationic salts upon acidification; they are organic compounds with basic amino functional groups. Soluble in water, basic dyes deliver outstanding tinting strength and bright hues, ranking among the most popular options in papermaking. However, they suffer poor light and heat fastness as well as inferior resistance to acids, alkalis and chloride ions, making finished paper prone to fading.

They display high affinity toward unbleached pulp and groundwood pulp, achieving satisfactory dyeing outcomes without rosin or alum addition, yet show weak substantivity for bleached pulp that necessitates mordant supplementation.

Application temperature should be controlled below 70°C to avoid insoluble color lake formation and paper speckles; the optimal dyeing pH ranges from 4.5 to 6.5.

Extra caution is required when dyeing mixed stock of bleached pulp and groundwood pulp to prevent uneven coloration: two feasible approaches are pre-dyeing bleached pulp prior to groundwood incorporation, or adding rosin and alum to mixed pulp before dye dosing.

Basic dyes bind to pulp fibers via precipitation, usually yielding brighter shades than the intrinsic color of the original dye powder.

3.Acid Dyes

Acid dyes are formulated for dyeing under acidic aqueous conditions. Most carry sulfonic acid groups while a small portion contain carboxylic groups. By chemical structure, they are classified into azo, anthraquinone, triarylmethane and nitro dyes; by application method, they are sorted into strong-acid, weak-acid, acid mordant and acid complex dyes.

Highly soluble in water, acid dyes are barely affected by water hardness and pulp temperature variations. Compared with basic dyes, they have weaker tinting power and less brilliant shades but superior light and thermal stability, alongside moderate resistance to acids, alkalis and chloride ions.

Possessing exceptional affinity to protein fibers, they are primarily used for leather dyeing. No substantivity toward lignin renders them free of speckle risks on mixed pulp.

Temperature exerts minimal influence on acid dye performance, with the optimum dyeing pH fixed between 4.5 and 5.0.

Typical papermaking-grade acid dyes include Acid Light Yellow G, Acid Anthraquinone Blue, Acid Sky Blue, Acid Scarlet G, Acid Scarlet GR and Acid Golden Yellow G.

4.White Dyes – Fluorescent Whitening Agents (FWAs)

Fluorescent whitening agents, also named white dyes, are derivatives or salts of diaminostilbene. Their molecular structures generally consist of sulfamate groups for fluorescence excitation, aromatic/aliphatic amine moieties for UV absorption, and cyanuric chloride groups to improve binding durability.

FWAs brighten bleached pulp and boost paper whiteness via an optical effect instead of actual pulp bleaching. Their whitening performance is negligible or ineffective for low-brightness pulp (brightness below 65%) or high-lignin pulp grades. Stilbene-based FWAs (disulfonate, tetrasulfonate and hexasulfonate types) dominate paper brightening applications.

5.Other Dye Varieties

Beyond the above categories, the papermaking industry also applies vat dyes, mordant dyes, azoic dyes, sulfur dyes, reactive dyes and disperse dyes. With the booming usage of pressure-sensitive and thermosensitive recording paper in recent decades, functional dyes represented by black-forming pressure/thermochromic dyes have attracted widespread industrial attention.