Commonly Used Chemical Materials for Pigment Coating

Coating Additives 

In addition to pigments and binders, pigment coatings also comprise various auxiliary materials (additives). While these additives cannot alter the fundamental properties of coated paper and are not mandatory in all coating formulations, they play a crucial role in enhancing coating operations and improving product quality in pigment-coated paper production. Types include dispersants, defoaming agents, water-resistant agents, flow modifiers, preservatives, fluorescent dyes (fluorescent whitening agents), softeners, and lubricants, among others.

Dispersant 

Coating pigments are typically hydrophilic due to the mutual attraction of complex charge layers on particle surfaces. For instance, kaolin clay is naturally acidic, causing particles to flocculate together. To prepare coatings, high shear force is necessary to disrupt the flocculation structure physically. Simultaneously, particle attraction must be inhibited to prevent further flocculation, achieving stable dispersion for coating. Dispersants fulfill this fluidity requirement.

Dispersants primarily fall into two categories of compounds exerting a dispersing effect on coating pigments. One category includes natural polymers or their derivatives possessing protective colloid properties, which also act as dispersants and adhesives in paints due to their adhesive force. Examples include casein, carboxymethyl cellulose (CMC), hydroxyethyl starch, starch phosphate, and Arabic gum resin. The other category comprises conventional dispersants like polyphosphate polymer electrolytes and surfactants, which adsorb onto particle surfaces to maintain a stable charge state, simultaneously reacting with certain polyvalent metal ions hindering pigment dispersion.

The dispersant dosage typically ranges around 0.5%, exceeding the amount required for pigment particles. Pre-dispersed pigments, where dispersants are added during manufacturing, only require the necessary amount during paint preparation.

Defoaming Agent 

Foaming poses a longstanding challenge in water-based paints. Synthetic latex, heavily utilized in paints, exacerbates foam formation, necessitating defoaming agents for foam elimination. These agents generally fall into two categories: foam suppressors added during paint preparation to inhibit foam formation and foam breakers added when foam arises. For instance, silicone oil suppresses foam formation but cannot eliminate existing foam, while alcohols like octanol break down foam but do not prevent foaming.

Foam breakers typically comprise low-volatility, highly diffusible oily substances or water-soluble surfactants. Examples include octanol, cyclohexanol, ethylene glycol, and non-ionic surfactants. Oily substances used as foam breakers may lead to oil spotting on coating surfaces and should be minimized. Regardless of type, dosage should be minimized, and equipment utilized to mitigate foam generation.

Water-Resistant Agent 

Substances capable of chemically bonding with hydrophilic groups in cellulose, starch, PVA, and other hydrophilic materials, converting them into water-resistant substances, are termed water-resistant agents (those solely covering hydrophilic groups without chemical reaction are termed water repellents). Water-resistant agents do not confer waterproof properties to paper.

Water-resistant agents are categorized into wet strength enhancers and insolubilizing agents based on application. Insolubilizing agents render water-based pigment coatings water-resistant. Starch and PVA used as binders in printing coated paper coatings are hydrophilic polymers requiring water resistance for offset printing.

Types of water-resistant agents include urea-formaldehyde resin, melamine-formaldehyde resin, polyamide-polyamine-epichlorohydrin resin, polyethyleneimine, ketone-aldehyde resin, dialdehyde starch, glyoxal, various cross-linking agents, and inorganic salts (e.g., ammonium zirconium carbonate, zinc sulfate). While melamine-formaldehyde resin can be used for starch binders, its propensity for formaldehyde odor during manufacture and use diminishes its popularity. Glyoxal is effective but may yellow due to reactions with certain components.

Flow Modifier 

In coating processes, inadequate coating fluidity within the coating machine's appropriate range precludes achieving required coating amounts and optimal coating surfaces. Viscosity reducers are employed to lower paint viscosity and improve fluidity, while tackifiers are used when paint viscosity is too low for coating.

Viscosity reducers include dicyandiamide and urea, reducing coatings' viscosity containing casein, starch, etc., while maintaining viscosity stability during storage. Dicyandiamide is particularly effective, with polyvinylpyrrolidone offering similar efficacy.