Selection theory of wetting agent for waterborne industrial coating substrate

Summary

It focuses on introducing the multi-phase mechanism of the star-shaped substrate wetting agent from the surface thermodynamic formula.

Keywords: star polyether, substrate wetting agent, multiphase action mechanism, foam suppression, substrate wetting control, viscosity destructive

Due to the high surface tension of water, the surface tension of water-based coatings with water as the main carrier is often relatively high.

Adding a suitable substrate wetting agent to water-based industrial coatings is a key technology in the formulation of water-based industrial coatings .

Waterborne coating substrate wetting agent and its multiphase wetting theory

Substrate surface wetting means that the gas on the surface of the industrial substrate is replaced by the water-based coating

The heterogeneous theory of the substrate wetting agent for water-based industrial coatings refers to the use of substrate wetting agents not only to reduce the surface tension of the water-based coating to control the surface wetting of the industrial substrate, but also to control the surface wetting of the industrial substrate through the addition of the substrate wetting agent.

Usually, the conditions for judging the wetting state of the substrate include the surface thermodynamic formula and the Youngs equilibrium state equation

Surface thermodynamic formula (process judgment spontaneously under dynamic conditions)

The dynamic wetting of the surface of the industrial substrate is divided into three processes: the industrial substrate is wetted, soaked, and spread by the water-based paint, and meets the following conditions:

Wetting △ G1/A=Ysl-Ylg-Ysg<0;

Wetting △ G2/A=Ysl-Ysg<0;

Spread △ G3/A=Ysl+Ylg-Ysg<0;

The same industrial substrate wetting system:

△ G3> △ G2> △ G1

△ G3<0 (spreading and wetting dynamically and spontaneously)

Spreading wetting is the highest standard requirement for wetting
.

The process of automatic expansion of water-based coatings into a film on the surface of industrial substrates is called the Gibbs free energy change per unit area of ​​spreading △ G(T, p)/A=Ysl+Ylg-Ysg defines the spreading coefficient S=- △ G/A

△ G<0 spread

△ G<0 do not spread

S>0 spread

S<0 do not spread

Youngs Wetting Equilibrium State Theory Equation (Static Equilibrium Wetting State Judgment)

Youngs wetting equilibrium state theory equation (under the condition that P pressure, T temperature, and N composition remain unchanged)

Ysg-Ysl=Ylg cosθ; cosθ=(Ysg-Ysl)/Ylg

Water-based industrial coatings judge the wetting state by the stable equilibrium state θ>=90 Industrial substrates cannot be wetted by the coating; θ<90 Industrial substrates can be wetted by the coating
.

Youngs equilibrium state equation shows that under the condition of relatively fixed Ysg (gas-solid surface tension), the equilibrium wetting state is not only related to the value of Ylg (liquid-gas surface tension) but also related to Ysl (solid-liquid surface tension)
.

Comprehensive judgment conditions for the spreading and wetting of industrial substrates by water-based coatings

When the coating coats the substrate, there is a dynamic process under the action of external energy, and the substrate wetting agent (surfactant) will undergo dynamic system rearrangement before forming stable micelles
.
Ylg (gas/paint (dynamic)), Ysl (substrate/paint (dynamic)) are all changing, and the spreading conditions Ylg (gas/paint) + Ysl (substrate/paint)-Ysg (gas/substrate) <0, dynamic spreading and wetting is mainly carried out spontaneously under the condition of conforming to the surface thermodynamic equation
.
The Youngs equation is only suitable for judging the wetting state of equilibrium state
.

The surface thermodynamic formula and Youngs equation are used as the objective judgment criteria for the wetting of the coating substrate, and meet the following criteria:

1) Surface thermodynamic formula: S spreading and wetting =-△ G/A-Ysg (substrate)

-Ylg (paint) -Ysl (substrate/paint)>0

2) Youngs equation: cosθ=(Ysg-Ysl)/Ylg θ<90

Ysl solid-liquid surface tension (difficult to measure), Ysg gas-solid surface tension (calculated), Ylg liquid-gas surface tension (measurable)

Changing Ysl can also improve the wetting and spreading, and different substrate wetting agents can also affect the size of Ysl, which depends on the influence of complex factors such as the force between the coating and the substrate and physical adsorption
.

Multi-phase action mode for perfect spreading and wetting

1.
Reduce the surface tension Ylg of the coating by adding a substrate wetting agent

2.
Change the surface tension Ysl through the substrate wetting agent

According to the comprehensive theoretical derivation of the surface thermodynamic equation and the equilibrium state Youngs equation, whether it is thermodynamic spontaneous spreading wetting or equilibrium state wetting, it is necessary to meet the theoretical requirements of multiphase interaction
.

Influence of substrate wetting agent on waterborne industrial coating system

Experiments have proved that the lower the Ylg of the surface tension in the water, the lower the Ylg of the coating is not necessarily the lowest
.

The addition of the substrate wetting agent will cause intermolecular interaction with other components of the coating to cause changes in the potential energy of the molecules in the coating, and the value of Ylg will also change differently
.

The relationship between the wetting ability of the water-based industrial coating substrate and the wetting ability of the substrate and Ysl

Polyether silicone 0.
1% aqueous solution Ylg=20.
6×10-3 Nm-1

PAG33 0.
1% aqueous solution Ylg=30.
5×10-3 Nm-1

Acetylene glycol polyether 465 0.
1% aqueous solution Ylg=28.
5×10-3 Nm-1

In the figure, the PAG33 water-based paint with relatively high Ylg wetting and spreading is more perfect.
Experiments show that the wetting ability of Ylg in the substrate of water-based industrial coatings is not the only key factor
.
You can't just emphasize Ylg and ignore the influence of other factors such as Ysl
.

The influence of substrate wetting agent on system foam

When the intermolecular force F repulsion at the hydrophilic end remains unchanged or decreases, the stronger the intermolecular force (gravitation) at the hydrophobic end (Fprime)
.
The less likely the foam is to form, and the easier it is to be squeezed to form foam suppression properties
.
Star-shaped polyether reduces the stability of the foam by reducing the molecular action of the hydrophilic end and increasing the action of the hydrophobic end
.

Performance comparison of Xipusen star substrate wetting agent

Viscosity destructiveness: refers to the greater viscosity reduction of the entire water-based coating viscosity system, which will increase the difficulty of later thickening.
Too much thickener will cause cost and resistance
.

Description: 1.
Star-shaped polyether PAG37 has a strong advantage over acetylene diether ether in terms of foam control requirements and substrate wetting; 2.
Star-shaped polyether PAG33 has strong substrate wetting, recoatability, and matting properties.
Compared with polyether silicones (monomers and gemini) in terms of viscosity destructiveness, etc.
, it has strong advantages
.

Concluding remarks

The water-based industrial coating substrate wetting agent developed by Sipsen Chemical based on star polyethers can not only reduce the surface tension of water-based coatings, but also improve the surface force (surface tension) between the coating and industrial substrates, so as to achieve water-based The perfect control of the wetting of the industrial substrate by the coating
.
The star-shaped polyether substrate wetting product has unique characteristics such as high-efficiency substrate wetting ability, defoaming ability, low viscosity reduction (can reduce the amount of system thickener), no effect on recoatability, and no matting
.
With the development of environmentally friendly water-based industrial coatings, the star-shaped substrate wetting agent can perfectly meet the technical requirements for wetting control of various substrates in the process of green water-based industrial coatings
.