Water-based coatings: Raw material selection · Formulation design · Production process

Author: Zhang Xinghua
Editor: Wang Chun
Publisher:
Publish Date: 2000-01-01
Features:
Fragment: The first classification method is based on chemical synthesis, such as vinyl acetate copolymer, styrene-acrylic copolymer, pure acrylic polymer, and butadiene-styrene copolymer dispersion, which are the main components of coatings. However, there are too many varieties of acrylic and methacrylic monomers, making copolymers very complex. The second classification method is based on the type and charge of the dispersant (emulsifier, protective colloid) used. Since dispersants remain in the coating film for a period after curing, the amount of dispersant affects the water stability and anti-corrosion performance of the film. Additionally, the size and uniformity (monodispersed or polydispersed) of polymer particles in the dispersion are also important. Dispersed particles should be kept as small and uniform as possible (microdispersed), or a polydispersed system should be precisely formulated so that the polymer can flow without interference during film formation (coalescence). For dispersions, an important indicator is low film-forming temperature (MFFT), which predicts the possibility of film formation at any temperature. It is closely related to the synthesis of the polymer and its glass transition temperature. To lower MFFT, coalescing agents and film-forming additives can be added to the dispersion (see Section 3 of Chapter 4). Dispersions have many advantages, such as maintaining operable viscosity even at high solid content, requiring very little solvent, and rapid physical drying. However, they also have some limitations, such as difficulty in achieving high gloss and high anti-corrosion performance for coatings. Additionally, rapid drying cannot be achieved when used as spray or deposition coatings. New developments in coatings are gradually overcoming these issues. Moreover, in practice, multiple binders can be combined to achieve optimal specific properties.
Section 5: Emulsions
The emulsions referred to here are polymers that are further dispersed into a water phase, with their relative molecular weight in the range of oligomers and relatively large particle size. Most resin emulsions are stabilized by non-bonded emulsifiers (mostly anionic or cationic). A typical composition of resin emulsions is as follows: resin 60 parts, emulsifier 6 parts, water (biocide, defoamer) 34 parts. When selecting emulsifiers, the HLB (hydrophilic-lipophilic balance) value is very important and must be compatible with the resin. The cost of emulsifiers for resins is relatively high. Therefore, the larger particle size of emulsion resins and the higher amount of emulsifier used (about 10% of the resin) often significantly limit their application. The gloss and flow properties are often not compatible with corresponding coatings. They are highly recommended to be combined with acrylic dispersions. Therefore, in practice, significant efforts have been made to improve the aforementioned properties. For example, attempts have been made to coordinate the synthesis or drying of emulsifiers with resins. There are now examples of emulsions without emulsifiers. This gradually blurs the boundary between emulsions and colloids. In addition to the resin emulsions mentioned above, there are also very promising acrylic secondary dispersions with significant market potential. Currently, acrylics can be used to produce a variety of water-dilutable industrial coatings, from anti-corrosion primers to topcoats.