As the viscosity of water-based resin is very low, it cannot meet the needs of storage and construction performance of the coating, so it is necessary to use suitable thickener to adjust the viscosity of water-based coating to the correct state.
There are many varieties of thickeners. When selecting thickeners, in addition to their thickening efficiency and control of coating rheology, some other factors should be considered to make the coating have the best construction performance, the best coating film appearance and the longest service life.
The selection of the thickener species is mainly based on the need and the actual situation of the formulation.
When selecting and using thickeners, these are important.
1. High molecular weight HEC has a greater degree of entanglement compared to low molecular weight and exhibits greater thickening efficiency during storage. And when the shear rate increases, the winding state is destroyed, the greater the shear rate, the smaller the effect of molecular weight on viscosity. This thickening mechanism has nothing to do with the base material, pigments and additives used, only need to choose the right molecular weight of cellulose and adjust the concentration of thickener can get the right viscosity, and thus widely used.
2.HEUR thickener is a viscous aqueous solution with diol or diol ether as co-solvent, with solid content of 20%~40%. The role of the co-solvent is to inhibit the adhesion, otherwise such thickeners are in the gel state at the same concentration. At the same time, the presence of solvent can avoid the product from freezing, but it must be warmed up in winter before use.
3. Low-solid, low-viscosity products are easy to dispose of and can be transported and stored in bulk. Therefore, some HEUR thickeners have different solid content of the same product supply. The co-solvent content of low viscosity thickeners is higher, and the mid-shear viscosity of the paint will be slightly lower when used, which can be offset by reducing the co-solvent added elsewhere in the formulation.
4. Under suitable mixing conditions, low-viscosity HEUR can be added directly to latex paints. When using high viscosity products, the thickener needs to be diluted with a mixture of water and co-solvent before it can be added. If you add water to dilute the thickener directly, it will reduce the concentration of the original co-solvent in the product, which will increase the adhesion and cause the viscosity to rise.
5. Adding thickener to the mixing tank should be steady and slow, and should be put along the wall tank. The speed of adding should not be so fast that the thickener stays on the surface of the liquid, but should be dragged into the liquid and swirled down around the stirring shaft, otherwise the thickener will not be mixed well or the thickener will be overly thickened or flocculated due to the high local concentration.
6. HEUR thickener is added to the paint mixing tank after other liquid components and before emulsion, so as to ensure maximum gloss.
7. HASE thickeners are added directly to the paint in the form of an emulsion in the manufacture of emulsion paints without prior dilution or pre-neutralization. It can be added as the last component in the mixing phase, in the pigment dispersion phase, or as the first component in the mixing.
8. Since HASE is a high acid emulsion, after adding, if there is alkali in the emulsion paint, it will compete for this alkali. Therefore, it is required to add the HASE thickener emulsion slowly and steadily, and stir well, otherwise, it will make the pigment dispersion system or emulsion binder local instability, and the latter is stabilized by the neutralized surface group.
9. Alkali can be added before or after the thickening agent is added. The advantage of adding before is to ensure that no local instability of the pigment dispersion or emulsion binder will be caused by the thickener grabbing alkali from the surface of the pigment or binder. The advantage of adding the alkali afterwards is that the thickener particles are well dispersed before they are swollen or dissolved by the alkali, preventing local thickening or agglomeration, depending on the formulation, equipment and manufacturing procedure. The safest method is to dilute the HASE thickener with water first and then neutralize it with alkali in advance.
10. HASE thickener starts to swell at a pH of about 6, and the thickening efficiency comes into full play at a pH of 7 to 8. Adjusting the pH of latex paint to above 8 can keep the pH of latex paint from decreasing below 8, thus ensuring the stability of viscosity.
Post time: Aug-05-2022