Essential characteristics of water-based industrial coatings

Essential characteristics of water-based industrial coatings

Basic characteristics of water

Non-combustible, non-toxic, non-polluting

Economical, sustainable resources

Freezing at 0℃, boiling at 100℃

Strong polarity, dissolves and disperses ionic substances

Dissolve a small amount of oxygen and other gases

Versatility in neutral, acidic or alkaline environments

High chemical reactivity, hydrolysis and corrosion

Biological necessity

The essence of water

Two elements, three atoms;

The four hybrid orbitals come from the oxygen 2s, 2px, 2py and 2pz atomic orbitals, pointing outwards from the oxygen nucleus to the corners of the tetrahedron;

The asymmetric charge distribution becomes a polar molecule (α=114°29′, β=104°27′);

Intermolecular hydrogen bonds, the hydrogen nucleus interacts with the lone pair of negative charges of neighboring water molecules to produce a hydrogen bond with an energy of about one-tenth of the molecular O-H bond.

Aqueous solution structure

Liquid water is a highly organized substance.

The detailed arrangement of molecules in liquid water is indeterminate at com" href="">coatingol.

Derived from the structure of ice, some extra molecules occupy the interstitial space and exchange with molecules in the hydrogen bond network.

Water has one of the highest known dielectric constants, 78 at 25°C.
The higher the dielectric constant of the medium, the lower the energy required to separate opposite charges, which makes water one of the factors that make water an effective solvent for ionic substances.

The principle of water-based organic resin

The principle of water-based technology: polymer molecules are coated with hydrophilic substances/chemically grafted with hydrophilic groups, (or after neutralization) they can be diluted with water.

External emulsification: Emulsifier (ionic or nonionic) is added to the water phase to form micelles, which are formed by emulsion polymerization to form latex; or hydrophilic substances are added, which are obtained by reverse emulsification.
Usually called emulsion.

Self-emulsification: grafting hydrophilic groups (-COOH, -CH2-CH2-O-, -NH2, etc.
) on polymer molecules, (or after neutralization) can be diluted with water.
Usually called a dispersion.

Macro morphology of water-based resin

Macro form (emulsion, slightly transparent dispersion, transparent solution)

Most water-soluble resins are not really soluble in water (except polyvinyl alcohol, cellulose, polyacrylic acid), but form a relatively stable polymer colloidal dispersion solution after the amine salt solution of an organic solvent is diluted with water.

Microscopic morphology of water-based resin

Heterogeneous system: latex particles, polymer aggregate particles

Free hydrophilic substances, neutralizers, co-solvents, etc.

Water-based resin dilution viscosity change-water-based dispersion, water-based resin

Viscosity peaks exist during the dilution of water-based resin

There is no viscosity peak in the aqueous dispersion, and the viscosity changes greatly compared with the resin solution

Water-based resin dilution viscosity change -water-based resin

Water-based resin viscosity is also related to molecular weight, polarity and amount of co-solvent.

Distribution of solvents in water-based resins

The solvent is unevenly distributed in the water phase and the polymer phase; it affects the viscosity of the system, the aggregation of particles into a film, and the evaporation and stability of water

Drying process of water-based industrial coatings

Volatilization of water and organic solvents

Strong interaction deviates from Raoult's law

Relative humidity affects water volatilization, and has little effect on solvents

Azeotropic effect

The heat capacity and heat of vaporization of water are abnormally large

(Boiling point, butyl acetate (126℃) volatilizes faster than n-butanol (118℃))

Wet stage

The volatilization rate of the mixed solvent is controlled by factors such as vapor pressure, surface temperature, relative humidity, air velocity, and surface area to volume ratio.

Dry stage

The volatilization rate of the mixed solvent is determined by the diffusion rate of the solvent molecules.

Denseness of water-based paint film

The film formation process involves diffusion between particles and molecules

Heterogeneous film (relative to solvent-based coatings)

Structure and composition of water-based paint film

Heterogeneous dense coating film; there are enriched areas of different substances; hydrophilic substances/groups still partially remain in the coating film.

Corrosion of water-based industrial coatings

Flash rust concept and evaluation

ISO12944-4 defines flash rusting as'slight rust formation on a preparedsteel surface soon afterpreparation'

ASTMD 610-85 (1989) Standard Test Method for Evaluating the Degree of Rust on Painted Steel Surface

Influencing factors of paint "flash rust"

Water-based polymer type (important factor)

Relative humidity during film formation (very important factor)

System pH (factor of low importance)

Metal type (important factor) at the metal weld (submerged arc welding)>cast iron>carbon steel

A classic example of water-based industrial coating application -electrophoretic coating

It is suitable for comprehensive coating of complex shapes, uniform film formation, thin film, high performance, and automation.