Types and development trends of commonly used water-based coatings

Water-based paint is a kind of paint with water as diluent. 70% -90% of the composition is water that is not harmful to human body and the environment. No organic solvent (such as benzene, ketone, ester, ether, etc.) is needed during the production process. Substances.) And formaldehyde, lead, chromium and other harmful substances. Water-based paint uses water as a dispersion medium. During the construction, water volatilizes and returns to nature in a vaporized state, without causing harm to human bodies and the environment.
Solvent-based coatings are a new polymer material composed of a mixture of high-molecular substances and ingredients, and can be applied to the surface of a substrate to form a continuous adherent coating film. Volatile organic compounds (VOC) in coatings are emitted into the air in the form of gaseous molecules during their production, construction, drying, curing and film formation, which not only causes a certain degree of environmental pollution, but also poses a threat to human health. In order to protect the environment and the safety of human life on earth, countries around the world have formulated laws and regulations on environmental protection to limit the content and emissions of VOCs in paints. China has formulated mandatory safety standards and emission limits for harmful substances such as volatile organic compounds, the sum of toluene and xylene, and soluble cadmium. In recent years, China has further improved the laws and regulations related to environmental protection, and has established a relatively complete “National System Standard for the Development and Safety of China’s Coatings for Low Pollution”, which includes standards for testing methods for harmful substances in coatings (such as VOC difference measurement method, VOC Gas chromatography and VOC in latex paints, etc.) and product limits for hazardous substances in coatings (such as the limit of hazardous substances in solvent-based wood coatings and the limits of hazardous substances in interior and exterior walls). One of the important sources of VOC is the main constituents of coatings—dispersion media and film-forming materials. In view of this shortcoming of organic solvent-based coatings, the progress and development of water-based resin production technology is urgent, and water-based coatings are gradually replacing organic solvent-based coatings. The general trend is bright. According to statistics, at present: Guangdong, Jiangsu, Shaanxi, Hebei, Hubei, Tianjin, Beijing and other provinces have promoted the “oil to water” policy. Water-based paints include water-soluble paints, water-reducible paints, and water-dispersible paints (latex paints). Water-soluble coatings use water-soluble resins as film-forming materials, represented by polyvinyl alcohol and its various modified products. In addition, there are also water-soluble alkyd resins, water-soluble epoxy resins, and inorganic polymer water-based resins.

Several commonly used water-based coatings
(1) Epoxy resin
Epoxy resin has excellent physical and chemical properties, such as good adhesion, excellent chemical resistance and solvent resistance, high hardness, excellent corrosion resistance and thermal stability. Therefore, it has always attracted people’s attention. Water-based epoxy resins can be divided into water-based epoxy resins and water-soluble epoxy resins. There are three main preparation methods: direct emulsification, reverse conversion and self-emulsification. In addition, some people use cellulose single crystal nano-material mixed modification method to make water-based epoxy coatings with good flexibility and hardness.
Water-based epoxy coatings are formed by the cross-linking of epoxy resin and curing agent. The properties of curing agent have a great influence on the physical and chemical properties of the coating film. Therefore, the study of modification of water-based curing agents has become a hot topic at home and abroad. In recent years, in addition to a lot of research on water-based curing agents, a lot of research work has been done on nano-material-modified water-based epoxy coatings. Using nano-SiO2, nano-TiO2 and other nano-materials in water-based epoxy coatings to make water-based nano-modified epoxy coatings, which can make its adhesion, scrub resistance, weather resistance, hardness, flexibility, aging resistance and other properties Greatly improved. Preparation and performance of composite modified nano-SiO2 / epoxy coating. First, the nano-SiO2 is mixed with a solvent, and then a coupling agent and a hyperdispersant are added according to a certain ratio to obtain a modified nano-SiO2. Then, the modified nano-SiO2 and E-44 epoxy resin are mixed to prepare a nano-SiO2 / epoxy composite coating. The results show that when the amount of modified nano-SiO2 is 2%, it can be effectively compounded with the resin base material, and the thermal decomposition temperature can reach 296 ° C, which is 50 ° C higher than the pure resin base material, and the mechanical properties of the paint film are also very Greatly improved.
(2) Polyurethane
Polyurethane contains strong polar isocyanate groups (—NCO), —OH and urea groups, etc., and the polyurethane molecules can form hydrogen bonds, the presence of van der Waals force and high cohesion, and it has a good adhesion to polar plastic surfaces. force. For non-polar plastics (such as PE and PP), in addition to treating the surface of the plastic, polyurethane resins can also be grafted with segments with similar chemical properties, surface tension, and solubility parameters as these non-polar resins.
According to the appearance of water-based polyurethane, it is divided into polyurethane aqueous solution, polyurethane aqueous dispersion and polyurethane emulsion. The most practical applications are polyurethane emulsions and dispersions, which are generally referred to as water-based polyurethanes or polyurethane emulsions. According to the composition, there are single and two components. The single component belongs to the thermoplastic resin. The polymer does not crosslink during the film formation process, which is convenient for construction. The two-component water-based polyurethane coating consists of active-NCO curing agent components and It is composed of an aqueous polyol containing reactive hydrogen (hydroxyl) that can react with -NCO. The two are mixed evenly before construction, and a cross-linking reaction occurs during film formation, and the coating film performance is good.
One-component waterborne polyurethane coating
One-component waterborne polyurethane coating is the most common and most commonly used among waterborne polyurethane coatings. It has the advantages of high elongation at break (up to 800%), proper strength (20MPa) and drying at room temperature. Traditional one-component waterborne polyurethane coatings generally have a relatively low molecular weight or a low degree of crosslinking. In order to further improve the mechanical properties and chemical resistance of one-component waterborne polyurethane coatings, the performance of coatings can be improved by introducing reactive groups for cross-linking or using composite modified binders, such as polyols, polyisocyanates, and Polyfunctional compounds such as polyamines are used to synthesize aqueous polyurethane dispersions with a crosslinked structure; internal crosslinking agents are added, such as carbodiimide, methylimine, and azacyclopropane compounds; heat-activated crosslinking and auto-oxidative crosslinking Wait. In combination with epoxy resin, the higher branching degree of epoxy resin is introduced into the polyurethane main chain, which can improve the adhesion, drying rate, hardness and water resistance of the emulsion coating film.
Two-component waterborne polyurethane coating
In the early 1990s, Jacobs and others successfully developed a polyisocyanate curing agent that can disperse in water, thereby bringing the two-component waterborne polyurethane coating into the practical research stage. It has low film forming temperature, strong adhesion, good abrasion resistance, High hardness, excellent chemical resistance and weather resistance. In order to obtain practical coatings with uniform appearance and intrinsic quality, two-component polyurethane aqueous dispersion coatings should meet the following requirements: (1) The polyol system should have emulsifying ability, so as to ensure that the polyurethane curing agent (particularly (Non-hydrophilic modified curing agent) emulsified, has a dispersion function, so that the particle size of the dispersion is as small as possible in order to better mix and diffuse in water; (2) the viscosity of the curing agent should be as small as possible, thereby reducing organic solvents The amount used does not even need an organic solvent, and at the same time, it can ensure good mixing with the hydroxyl-containing component. If it is applied to the coating of automotive interior parts, in view of the better adhesion of one-component water-based polyurethane, one-component water-based polyurethane can be used as a primer and a middle coat, and two-component water-based polyurethane is used as a top coat and a finishing paint.
(3) Acrylate
Acrylate emulsions have promoted the rapid development of acrylic emulsion coatings due to their excellent weather resistance, light resistance and low odor. The use of core-shell structure can well solve the Tg and minimum film forming temperature (MFT) of emulsions. Contradictions. At present, according to the different ways of adding shell monomers, the methods of acrylate emulsion polymerization can be divided into batch method, equilibrium swelling method, semi-continuous method and continuous method. The factors that affect the performance of acrylate emulsions include emulsifiers, monomer weight ratio of core layer / shell layer, and shell layer Tg. At present, there are various methods for modifying water-based acrylate coatings, such as polyurethane modification, silicone modification, fluorine-containing monomer modification, and nano-material modification.
(4) Alkyd resin
Alkyd resin is an important coating resin. It has abundant monomer sources, low prices, many varieties, large formula changes, convenient chemical modification, and good performance. The development of water-based alkyd resins has undergone two stages of external emulsification and internal emulsification. Currently, internal-water emulsification methods are mainly used to synthesize aqueous alkyd resin dispersions. The so-called internal emulsification method is to neutralize a carboxyl group or an amino group in a polymer with an appropriate base or acid, respectively, so that the polymer can be dispersed in water. Although the water-based alkyd resin coating has good brushing performance and lubricating performance, it also has the disadvantages of slow drying of the coating film, low hardness, poor water resistance and corrosion resistance, and poor outdoor weather resistance. Modification is required to meet these properties Claim. At present, the modification of water-based alkyd resin mainly includes physical modification and chemical modification. Among them, acrylic resin, silicone resin and styrene have the most significant effect. Self-drying water-based acrylic acid modified alkyd resin. In addition to the addition of water-based acrylic acid, a neutralizing agent, a drier, and a co-solvent are also added to the alkyd emulsion. The modified water-based alkyd resin coating has good color retention, gloss retention, weather resistance, durability, corrosion resistance, fast-drying and high hardness, etc., which overcomes the poor storage stability and drying of conventional water-based alkyd coatings. The slow speed, early hardness, poor water resistance and poor solvent resistance have widened the application fields of water-based alkyd coatings.

Compared with the conventional water-based alkyd resin, the water-soluble alkyd resin modified by sulfonate has a greatly improved water solubility, and the amount of co-solvent is reduced. The main chain of the water-based alkyd resin contains an ester bond formed by the polymerization of a polyhydric alcohol and a polybasic acid. The ester bond is easily broken by the attack of an acid and a base (neutralizing agent). Therefore, water-soluble The sulfonate prepolymer can greatly improve the water solubility of the resin. Only a small amount of amine neutralizer is added to obtain good water solubility, which greatly reduces the attack of the neutralizer on the ester bond. In addition to the greatly improved water solubility, the modified water-based alkyd resin also has excellent flexibility, adhesion and impact resistance.
Polyurethane coatings are a variety of coatings with excellent comprehensive properties. Therefore, it is desired to introduce the excellent properties of polyurethane into alkyd resin, and use it to improve the physical and mechanical properties, weather resistance and chemical resistance of alkyd resin. In fact, a class of coatings with a high yield and performance between solvent-based alkyd and two-component polyurethanes has been formed, so-called urethane oils or one-component polyurethane coatings, while the hydration of urethane oils is Current research hotspots.

Outlook
Compared with solvent-based coatings, the VOC content of water-based coatings using water as a diluent is greatly reduced, which is good for the environment and harmless to the health of the constructors and users. In the future, vigorously carrying out basic research to reduce the construction difficulty and improve the use performance of water-based coatings is an urgent task for water-based coatings to expand the application field and popularize on a large scale. At the same time, strengthening the work of policy guidance, promotion of technology, change of ideas, and elimination of public prejudice and concerns about water-based coatings will also promote the application and development of water-based coatings. In short, under the guidance of the development of the coating industry: pollution-free, resource-saving, energy-saving and economic, efficiency, ecology, energy and other principles, water-based coatings will have greater development.