I. Definition and Classification of Waterborne Coatings

1. Definition

Water-based coatings are heterogeneous systems formed by using water as the dispersion medium and resin as the dispersed phase.

2. Classification of Water-Based Coatings

II. Development of Waterborne Coatings

1. Development of Water-Based Coatings

(1) It is reported that China’s total paint production reached 7.55 million tons, of which water-based paints accounted for 3.21 million tons, representing approximately 43% of the total output.

(2) In 2010, China’s annual consumption of wood-coating paints was approximately over 800,000 tons, of which solvent-based coatings accounted for 98% and waterborne wood-coating paints accounted for only about 2%.

(3) In 2011, water-based coatings already accounted for more than 50% of China’s coatings market.

(4) According to the situation at paint production and sales conferences over the past few years, the promotion of waterborne coatings in China has not been as successful as expected.

(5) Hot industry, cold market

(6) Promoting water-based coatings from Chinese brands proves particularly challenging.

2. Reasons for the market downturn

(1) There is a gap between the quality and application of domestic brands and those of foreign brands.

(2) High costs prevent prices from coming down.

(3) Compared to traditional solvent-based coatings, there is a certain gap in terms of functionality.

(4) Chinese people’s environmental awareness

(5) Insufficient policy support力度.

3. The Three Major Development Directions of Waterborne Coatings

(1) Toward water-based formulations: low VOC, low toxicity, excellent durability, water resistance, and fullness.

(2) Toward functionalization: fire resistance, mold prevention, and corrosion resistance.

(3) Development toward high performance and high-end features: weather resistance, anti-fouling properties, and chemical resistance.

III. Characteristics of Water-Based Coatings

1. Characteristics of water-based coatings

(1) Water-based coatings are multiphase systems.

(2) Water-soluble resins are produced by introducing hydrophilic groups into solution resins, followed by neutralization, and finally dispersing the resulting product in water.

(3) Generally, a hydrophilic solvent—a cosolvent—needs to be added.

(4) The solid content of waterborne coatings is typically lower than that of solvent-based coatings.

2. Composition of water-based coatings

The composition of water-based coatings includes water-based resin, pigments and fillers, additives, neutralizing agents, and water.

3. Advantages of water-based coatings

(1) Using water as the solvent saves substantial resources; eliminates fire hazards during construction; reduces atmospheric pollution; and employs only small amounts of low-toxicity alcohol ether organic solvents, thereby improving working conditions. Typically, conventional water-based coatings contain organic solvents accounting for 10% to 15% of the total coating composition; however, current cathodic electrophoretic coatings have reduced their organic solvent content to below 1.2%, yielding significant benefits in terms of pollution reduction and resource conservation.

(2) Water-based coatings can be directly applied and constructed onto wet surfaces and in humid environments. They exhibit excellent compatibility with various substrate materials and boast strong coating adhesion. Coating application tools can be easily cleaned with water, significantly reducing the consumption of cleaning solvents. Electrophoretic coatings are uniform and smooth, with excellent leveling properties. They can coat even interior cavities, weld seams, sharp edges, and corners to achieve a consistent film thickness, providing outstanding protective performance. Electrophoretic coatings offer the best corrosion resistance; thick-film cathodic electrophoretic coatings can withstand salt-spray tests for up to 1,200 hours.

4. Issues with water-based coatings

(1) Water-based coatings have high requirements for cleanliness during the application process and on the surface of the substrate. Due to water’s high surface tension, contaminants can easily cause shrinkage holes in the coating film. Moreover, water-based coatings exhibit poor dispersion stability under strong mechanical forces; when the flow rate in the delivery pipeline changes abruptly, dispersed particles can be compressed into solid particles, resulting in a speckled appearance on the coating film.

(2) Water-based coatings are highly corrosive to coating equipment, necessitating the use of corrosion-resistant linings or stainless steel materials, which significantly increases equipment costs.

(3) Bake-on waterborne coatings have relatively strict requirements for construction environment conditions (temperature and humidity), which increases the investment in temperature- and humidity-control equipment and also raises energy consumption. Water has a high latent heat of vaporization, leading to significant energy consumption during the baking process.

(4) Water-based coatings suffer from poor water resistance, which leads to instability in both the coating itself and the bath solution, as well as poor water resistance of the resulting film. The medium used in water-based coatings is typically slightly alkaline (pH 7.5–pH 8.5), causing the ester bonds in the resin to easily hydrolyze, thereby degrading the molecular chains and compromising the stability of both the coating and the bath solution, as well as the performance of the coated film.

IV. Mechanism of Water Solubility in Waterborne Coatings

1. The reason synthetic resins can dissolve in water is that their polymer molecular chains contain a certain number of strongly hydrophilic functional groups, such as carboxyl groups, amino groups, hydroxyl groups, ether groups, and amide groups.

2. However, when these polar groups are mixed with water, they mostly form only emulsions. Their corresponding salts, on the other hand, can partially dissolve in water; thus, most water-soluble resins achieve water solubility by being neutralized into salt forms.

V. Synthesis Principles of Water-Soluble Resins

1. The synthesis principle of water-soluble resins

(1) Introducing amine groups (or carboxyl groups) into the resin molecular chain and neutralizing them with an acid (or base) to form cationic (or anionic) resins enables these resins to dissolve in water or disperse in water in a solubilized state.

(2) A certain number of strongly hydrophilic functional groups, such as carboxyl, hydroxyl, amino, ether, and amide groups, are introduced into the resin molecular chains, enabling self-emulsification and dispersion in water.

(3) Emulsion polymerization with the addition of an emulsifier, or forced emulsification of the resin to form a water-dispersible emulsion. Sometimes, several methods are used simultaneously to enhance water dispersibility and the stability of the water-dispersible liquid.

(4) Although water-based coatings use water as the dispersion medium, a small amount of alcohol ether organic co-solvent must be added to enhance the water solubility of the resin, adjust the viscosity of the water-based coating, and improve the leveling properties of the coating film.

2. Cosolvent (Co-solvent)

(1) The function of a cosolvent is to increase the solubility of the resin in water, while also helping to adjust the viscosity of the resin solution, enhance the stability of the paint liquid, and improve the leveling properties and appearance of the paint film. When comparing the same resin, different cosolvents will yield varying degrees of cosolvent effect.

(2) Among water-soluble alcohols, alcohols with longer carbon chains exhibit better co-solvent effects than those with shorter carbon chains; alcohols containing ether groups show superior co-solvent performance compared to those without ether groups. Therefore, butanol is superior to ethanol, and butyl solvent is even better than butanol. The typical addition amount of a co-solvent is less than 30% of the resin weight.

(3) Commonly used cosolvents include ethanol, isopropyl alcohol, n-butanol, tert-butanol, ethyl solvex, butyl solvex, and sec-butanol, among others.

VI. Applications of Water-Based Coatings

1. Applications of water-based architectural coatings

Water-based architectural coatings

The main varieties include waterborne polyurethane coatings and water-soluble acrylic coatings, among others.

Applications for interior walls: Polyvinyl acetate-based coatings are the primary type of interior wall paints, largely replacing the 106 and 107 architectural coatings that previously relied on polyvinyl alcohol as a binder due to their poor water resistance.

Applications for exterior walls: Primarily based on styrene-acrylic emulsions (Germany and Japan), while in China, acrylic emulsions are predominantly used. Currently, styrene-acrylic emulsions are widely employed in the preparation of interior wall coatings.

2. Application on wood wall panels

Solvent-based coatings:

When solvent-based coatings are applied to wood wall panels, they tend to develop bubbles, leading to coating failure. The primary reason is that the film formed by solvent-based coatings has very low permeability to water vapor. As the moisture absorbed by the wood is heated and evaporated by sunlight, it causes bubbles to form in the coating film.

Latex paint:

Latex paints are not cross-linked, and their coatings have excellent permeability to water vapor; therefore, latex paints rarely exhibit bubbling.

Latex paints exhibit better wrinkle resistance than solvent-based paints; however, latex paints have poorer stain resistance compared to solvent-based paints. The requirements for breathability and stain resistance represent a conflicting trade-off for latex resins. Further research is needed to improve the performance of latex resins and to develop more rational latex paint formulations that enhance the stain resistance of waterborne coatings!

3. Water-based industrial coatings

The main varieties include automotive coatings, marine coatings, container coatings, furniture coatings, and railway vehicle coatings, among others.

(1) Water-based automotive coatings

Water-based automotive coatings include water-based primers, water-based mid-coats, water-based topcoats, and water-based clear coats.

(2) Automotive mid-coat paint

The mid-coat, also known as the second-layer primer, serves to reinforce and smooth out uneven areas. The main types of mid-coats include two-component water-soluble acrylic systems.

(3) Automotive topcoat

The topcoat for automobiles is an intermediate coating containing colored pigments and flake aluminum pigments to produce color and shimmer effects. Water-based topcoats mainly include two types: water-soluble acrylic amino esters and water-soluble polyesters.

(4) Automotive primer

90% of automotive primers use cathodic electrodeposition coatings. The fifth-generation cathodic electrodeposition coatings feature high electrodeposition efficiency, thick coating films, and excellent edge coverage, resulting in significantly enhanced salt resistance of the resulting coating film.

(5) Automotive clear coat

A two-component coating system composed of a water-soluble methanol-etherified melamine-formaldehyde resin and a waterborne alkyd resin, formulated in proportion to provide a clear topcoat with weather-resistant properties.

(6) Cathodic electrophoretic coating

It is also used in the light industrial sector for metal furniture and home appliances; in the construction industry for aluminum doors and windows, copper fittings, and the like; and in the small hardware industry for various artificial jewelry and ornaments.

4. Water-based wood coatings

Primarily based on waterborne polyurethane, it is modified by one or more of the following: acrylic, silicone, and organic fluorine.

5. Water-based metallic anti-corrosion coating

6. Water-based plastic coatings

Water-based plastic paint is a special-purpose craft paint that is non-toxic and odorless, and can be diluted with water. It comes in three finishes: matte, semi-gloss, and glossy. The clear coat has a light color and high transparency, while the colored coats offer vibrant and rich hues. This paint boasts excellent product quality, easy application, high solids content, and low cost per square meter. The resulting coating film is full-bodied and durable, featuring a wide range of colors, resistance to yellowing, strong adhesion, and outstanding physical properties that combine both decorative appeal and protective performance. It is primarily used for coating and protecting various rigid plastic surfaces, including ABS, PE, PP, and PVC foamed profiles.