Cause and process control of sedimentation delamino in emulsion paint

Causes of latex paint sedimentation and delamination

The reason why the latex paint will settle is that the pigment and filler particles in the latex paint will fall under the action of gravity, and the van der Waals between the particles will promote the aggregation of the particles, making the particles larger, the gravity effect will be significant, and the settlement will be intensified. According to Stoke's law, the sedimentation velocity of pigments and fillers is proportional to the grinding fineness, proportional to the pigment density, and decreases as the viscosity of the system increases. Although the difference between the density of pigments and fillers and the density of the liquid medium can be adjusted to increase the viscosity of the liquid phase to reduce the sedimentation velocity, the actual precipitation system is more complicated, and the interaction between pigments, pigments, emulsions, wetting and dispersing agents, thickeners, etc. Influence, especially the flocculation of the system greatly increases the sedimentation velocity. Common factors that cause system flocculation include: incompatibility of raw materials, such as mismatch between dispersant and emulsion, thickener, mismatch between hydroxyethyl cellulose and associative thickener, and mismatch of associative thickener. Adding too much or too little, especially the emulsion in latex paint is a thermodynamically unstable system, which is easily affected by various external factors and flocculates and coalesces. Therefore, only by taking necessary measures during the production of latex paint can the sedimentation of latex paint in storage be slowed down or eliminated.

Control of Settling and Delamination of Latex Paint 

1. Process control

Film-forming aids are substances that help emulsions form films. They are generally oily, only a small amount is soluble in water, and a large amount exists in latex particles. Excessive use of film-forming aids will demulsify the latex particles. Therefore, it is necessary to add a certain amount of surfactant to form micelles in the liquid phase to accommodate the film-forming aids. At the same time, the surfactant will occupy the gaps on the surface of the polymer that are not surrounded by the emulsifier, so that the film-forming aid cannot enter the latex smoothly in large quantities, avoiding latex demulsification and preventing the generation of gel. 

The rheology modifier will adsorb with the pigment and filler particles to cause flocculation, which can be improved by diluting with water. Dilute the rheology modifier with water, try to make the latex particles not or less prone to local latex demulsification and flocculation due to too high concentration of the local rheology modifier. Due to the adsorption of the rheology modifier and the latex particles, the flow of the liquid phase is hindered and the rheology of the fluid is changed. If the local concentration is too high, they will pull the latex particles close together, causing mutual extrusion and demulsification of the latex particles. In order to reduce the gel particles in the latex paint, the following methods can be adopted: during the emulsion feeding process, the agitator should be kept at a medium-low speed to avoid excessive shearing and cause the emulsion to break; add a certain amount of surfactant before the paint is mixed , to protect the latex particles from demulsification; the thickener should be diluted with water and added (some polyurethane thickeners are diluted with alcohol ether solvents). 

2. Selection of thickener

When hydroxyethyl cellulose (HEC) thickener is used, the phenomenon of water separation in the upper layer sometimes occurs, because the adsorption of cellulose molecules is weak, and it is very easy to be replaced by the wetting agent in the paint, the surfactant in the emulsion, etc. , free in the aqueous phase. The molecular weight and addition amount of hydroxyethyl cellulose should be selected according to the system. The hydrodynamic diameter of the thickener coil should not exceed the average layer thickness of the continuous phase surrounding the dispersed phase. If the molecular weight is too large, the binder and pigment particles will flocculate; The amount of thickener added should be lower than the critical concentration C* (according to the Mark Millevel equation C*=2.5/"η") to avoid excessive overlapping or entanglement; the molecular weight of HEC is higher, the amount added is too much and at the same time The use of highly hydrophobic associative thickeners can easily cause rapid flocculation and water separation of latex paint.

3. Selection of wetting and dispersing agent

The main function of the dispersant is to moisten the surface of the pigment and filler, assist mechanical shearing to grind the pigment and filler from the secondary structure to the primary structure (primary structure), and stabilize the dispersion. Its mechanism of action: the ionic dispersant is adsorbed on the surface of the pigment and filler to form an electric double layer, and the dispersion system is stabilized by the repulsion of the same-sex charges. In addition, ionic and non-ionic dispersants will also form a hydrophilic adsorption layer on the surface of pigments and fillers, stabilizing the dispersion system due to the steric hindrance effect.

Inorganic wetting and dispersing agents include polyphosphates, such as sodium hexametaphosphate, sodium tripolyphosphate, sodium tetrametaphosphate, etc., which are cheap and have good dispersion effects, but have a certain impact on the stability of the emulsion; organic wetting Wet dispersants include anionic, cationic and non-ionic surfactants, which are characterized by good dispersion effect but more foam; the other type is polymer resin dispersants (such as polyacrylates), which are characterized by Moderate price, less dosage, good dispersion effect, less foam, etc.

4. Selection of anti-settling fillers

Kaolin is soft, has excellent suspension, and is easily dispersed in water. Most of the kaolin has a hexagonal flaky structure, and some contain amorphous flaky powder, which has viscoplasticity when wetted, low oil content, prevents pigment precipitation, and can also improve the hiding power and viscosity stability of latex paint.

5. Selection of thixotropic agent

When there is no external force or the external force is small, the pigment and filler particles maintain a loose network structure in the latex paint, and such a structure will be destroyed by slight agitation. After being destroyed, it can return to the original high-viscosity gel state after resting, so that the pigment and filler particles remain suspended. This thixotropic latex paint has experienced a change of sol, gel, and sol from storage to construction to film formation. process, thereby avoiding the sedimentation phenomenon in the storage of latex paint. This thixotropic latex paint can be obtained by selecting a thixotropic agent.

6. Design of Pigment Volume Concentration (PVC)

PVC has a great influence on the settlement of latex paint. Under the condition that the total solid volume content is good, PVC increases, and the leveling and anti-settling properties of latex paint are significantly worse; PVC decreases, and the leveling and anti-settling properties Obviously, the production cost will increase accordingly. Comprehensive price factors, choose PVC as 42% to 50%.

Therefore, in the production process of latex paint, the calcium ion stability, mechanical stability and compatibility with thickeners and pigments and fillers of the emulsion should be fully considered; the selection of effective thixotropic dispersants to prepare thixotropic latex paint can avoid latex paint. Settling during storage; adjusting the formula and selecting a suitable thickener to make the viscosity of latex paint more reasonable under high, medium and low shear conditions; using kaolin as a body filler can effectively prevent the filler from settling and maintain viscosity stability.