Additives in coatings, though used in small quantities, significantly enhance coating performance and have become an indispensable component. Common additives for waterborne coatings include film-forming aids, thickeners, dispersants, wetting agents, defoamers, plasticizers, and mildew fungicides. Thickeners, as rheological additives, not only increase coating viscosity to prevent sagging during application but also endow coatings with excellent mechanical properties and storage stability. For waterborne coatings with low viscosity, they are a crucial class of additives.
Currently, numerous thickener varieties are available on the market, mainly categorized into four types: inorganic thickeners, cellulose-based thickeners, polyacrylate thickeners, and associative polyurethane thickeners.
- Cellulose-based thickeners: With a long history of use and diverse varieties (e.g., methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose), they were once the mainstream. HEC is the most commonly used.
- Polyacrylate thickeners: Primarily fall into two types: water-soluble polyacrylates and emulsion thickeners made from homopolymers or copolymers of acrylic acid/methacrylic acid. The latter, acidic in nature, must be neutralized with alkali or ammonia to pH 8–9 for thickening, also known as alkali-swellable acrylic thickeners.
- Polyurethane thickeners: These are newly developed associative thickeners in recent years.
- Inorganic thickeners: These are gel minerals that absorb water, swell, and form thixotropic gels, mainly including bentonite, attapulgite, and aluminum silicate, with bentonite being the most widely used.
The thickening mechanism lies in the hydrogen bonding between the hydrophobic main chain of cellulose and surrounding water molecules, which increases the fluid volume of the polymer, reduces the free movement space of particles, and thus enhances system viscosity. Viscosity can also be increased through entanglement of molecular chains, exhibiting high viscosity under static or low-shear conditions and low viscosity under high shear. This is because at static or low shear rates, cellulose molecular chains are in a disordered state, giving the system high viscosity; at high shear rates, the molecules align orderly in the flow direction and slide easily, leading to viscosity reduction.
When dissolved in water, the carboxylate ions in polyacrylate thickeners repel each other via electrostatic forces, extending the molecular chains from helical to rod-like structures, thereby increasing the viscosity of the aqueous phase. Additionally, they form a network structure by bridging between latex particles and pigments, further enhancing system viscosity.
A.J. Reuvers studied the thickening mechanism in detail. These thickeners have hydrophilic and hydrophobic groups in their molecular structure, exhibiting surfactant properties. When the aqueous solution concentration exceeds a certain threshold, micelles form, and the association of micelles with polymer particles creates a network structure, increasing system viscosity. Moreover, each molecule carries multiple micelles, reducing water mobility and enhancing aqueous phase viscosity. These thickeners not only affect coating rheology but also interact with adjacent latex particles; excessively strong interactions may cause latex delamination.
Bentonite, a layered silicate, swells into flocculent substances after absorbing water, showing good suspension and dispersibility. It combines with an appropriate amount of water to form a colloid, releasing charged particles in water to increase system viscosity.
- Advantages: High thickening efficiency (especially for the aqueous phase), wide application range with few restrictions on coating formulations, and broad pH adaptability.
- Disadvantages: Poor leveling, excessive splashing during roller coating, poor stability, and susceptibility to microbial degradation. Their high static/low-shear viscosity and low high-shear viscosity prevent sagging after coating but compromise leveling. Studies show that increasing the relative molecular mass of thickeners enhances splashing. As they "fix water" to thicken with minimal adsorption on pigments and latex particles, their volume expansion crowds pigments and latex particles, prone to flocculation and instability. Being natural polymers, they are vulnerable to microbial attack.
- Advantages: Strong thickening ability, good leveling, and excellent biological stability.
- Disadvantages: Sensitivity to pH and poor water resistance.
- Advantages: The associative structure is disrupted under shear force, reducing viscosity, which recovers when shear force disappears, preventing sagging during application. The viscosity recovery has a certain lag, beneficial for film leveling. With a lower relative molecular mass (thousands to tens of thousands) than the first two types (hundreds of thousands to millions), they do not promote splashing. The hydrophilic and hydrophobic groups in their molecules enhance water resistance by strongly adhering to the coating matrix. As latex particles participate in association, no flocculation occurs, resulting in smooth films with high gloss.
- Considerations: Their micelle-based thickening mechanism means components affecting micelles in the coating formulation will influence thickening performance. When using these thickeners, fully consider factors affecting thickening properties, and avoid arbitrary changes to emulsion, defoamer, dispersant, or film-forming aid.
- Advantages: Strong thickening, good thixotropy, wide pH adaptability, and high stability.
- Disadvantages: As inorganic powders, bentonite has high light absorption, significantly reducing film surface gloss (similar to matting agents). Thus, control the dosage in glossy latex coatings. Nanotechnology has nano-sized inorganic particles, endowing inorganic thickeners with new properties.
- Thick coatings: Choose high-viscosity HEC with strong thixotropy or carboxyl-containing polyethylene thickeners with Bingham flow.
- Flat latex coatings with high/medium pigment volume concentration (PVC): Use a combination of high-viscosity and low-viscosity HEC, or acrylic emulsion thickeners.
- Glossy latex coatings: Select thickeners that do not affect film gloss, such as Allied Colloids' VG 2, Rohm and Hass' acrylic emulsion thickener TT-935, and polyurethane thickeners Exp300 or QR-708.
- Blending thickeners: Mixing two thickeners with different rheological characteristics (e.g., 612 & 621N, 1550 & 2000 for balancing high-medium-low shear viscosity, or HASE & HN-HEC) often outperforms single use. For styrene-acrylic and pure acrylic emulsions in mid-to-high-end flat/glossy latex paints, blend HEUR (612, 1550) with high medium-low shear thickening efficiency and HEC/HASE for vinyl acetate copolymers (e.g., vinyl-acrylic, vinyl-tertiary emulsions) with larger particle sizes in medium-low PVC latex paints.
- Cellulose-based thickeners: L.M. Zhang comprehensively introduced synthesis methods and hydrophobic modification, discussing dissolution characteristics, viscosity behavior, and surface activity in aqueous solutions. Jiang Qibin et al. developed a new modified associative cellulose thickener by grafting associative bonds onto HEC, featuring lower molecular weight, reduced splashing, and 缔合作用 with pigments/fillers to form a 3D network for improved stability.
- Polyacrylate thickeners: Modifying water-soluble polyacrylate thickeners enhances performance. For example, Diamond Shamrock's SN-Thickener series (SN-603, SN-607, SN-612) are urethane-modified polyether acrylic thickeners.
- Hydrophobic thickeners: With higher requirements for water resistance and application performance, hydrophobic thickeners like Allied Colloids' Rheovis CR (alkali-activated acrylic emulsion thickener with reactive surfactants on molecular branches) have evolved. The long hydrophobic end groups form micelles in water, transforming thickening from linear to network structures.
- Associative acrylic thickeners: Luo Hong et al. found they outperform vinyl acetate-acrylic and non-associative thickeners in application, hiding power, leveling, anti-sagging, electrolyte resistance, and post-thickening. The chemical structure of reactive surfactants significantly impacts rheology: longer hydrophilic groups reduce thickener dosage, dodecyl hydrophobic groups optimize electrolyte resistance, and polyether-type reactive surfactants without hydrophobic groups show poor stability.
- Interaction with surfactants: J. Edward Glass et al. showed that both associative thickeners and surfactants act by adsorbing to particle surfaces. Excessive surfactants displace thickeners from latex particle surfaces into the continuous phase, inhibiting association and degrading leveling, gloss, and hiding power.
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