Research progress of the hottest waterborne coatin

Research progress of water-based coatings

water based coatings represent the main development direction of low pollution coatings. In order to continuously improve its performance and expand its application range, a lot of research has been carried out on waterborne coatings at home and abroad for nearly half a century, among which soap free lotion polymerization, room temperature crosslinking, UV curing and the mixing of waterborne resins are the research hotspots in this field at present, and will become the key technology for the development of waterborne coatings

1. Soap free lotion polymerization technology with high solid content

the emulsifier used in conventional lotion polymerization remains in the lotion, resulting in the water resistance of the film; Poor stain resistance and gloss, and easy to produce foam in the process of use. Therefore, both at home and abroad have been committed to developing soap free lotion polymerization technology. In order to synthesize a stable soap free lotion with high solid content that can meet the requirements of coatings, a lot of research has been carried out at home and abroad, among which there are three promising methods

water soluble monomers used for copolymerization include carboxylic acids, amides, hydroxyls, sulfonic acid monomers and some cationic monomers. In the process of polymerization, the comonomer is bound to the surface of colloidal particles due to its strong hydrophilicity, forming a hydrophilic film and producing a three-dimensional stability effect. Ionic monomers also generate charges on the surface of colloidal particles, and maintain the stability of lotion through electrostatic repulsion. The type, dosage, feeding method and neutralization degree of carboxyl monomers all affect the polymerization and the stability of lotion. The water solubility of the monomer is too large, and it is easy to homopolymerize in the aqueous phase; On the contrary, it is easy to be buried in colloidal particles, which is not conducive to soap free polymerization. Soap free lotion with high solid content can be prepared by using appropriate water-soluble monomers. Soap free lotion with solid content of more than 40% can be prepared by two-step polymerization with carboxyl monomers. The first step is to polymerize at low pH to make seed lotion; The second step is to increase the pH value of the seed lotion, ionize the carboxyl group on the polymer, and form highly charged latex particles as a place for further polymerization. A stable soap free lotion with high solid content can also be prepared by combining water-soluble hydroxyl monomers and ionic initiators. Tang Guangliang and others prepared stable soap free lotion with solid content of up to 60% by using alkenyl glycerol ether sulfonate and 3-allyloxy-2-hydroxypropyl sulfonate

there are many kinds of polymerizable emulsifiers that have been reported, mainly including derivatives of allyl alcohol, derivatives of styrene until the sample is destroyed, derivatives of maleic acid, derivatives of acrylamide, derivatives of (meth) acrylic acid and its esters, etc. In order to bond the polymerizable emulsifier to the surface of latex particles and produce good stability, the polymerizable emulsifier should have appropriate polymerization activity and hydrophilicity. If the polymerization activity is too high, it will copolymerize with other monomers and be buried in the particles at the early stage of the polymerization process; If the activity is low, it will copolymerize with other monomers at the later stage of the polymerization process, so it is not easy to be buried in the particles and located on the surface of the particles; But the polymerization activity is too small, it is difficult to bond to latex particles. Polymerizable emulsifiers with too little hydrophilicity are also easy to be buried in the particles, but too much may polymerize in the aqueous phase to form water-soluble polymers. The position of polymerizable group has a great influence on the polymerization process and the stability of lotion. Those with double bonds at the hydrophobic end are the easiest to polymerize, while those at the hydrophilic end are difficult to polymerize due to the repulsion between the hydrophilic ends. A stable soap free lotion with a solid content of more than 50% can be prepared by using a suitable polymerizable emulsifier

using macromolecular emulsifier, the mobility of macromolecular emulsifier is far lower than that of small molecule emulsifier, and it may also graft with latex particles in the polymerization process. Therefore, using macromolecular emulsifier can overcome the shortcomings of small molecule emulsifier, which is easy to migrate and foam. Macromolecular emulsifiers used in lotion polymerization include block copolymers, graft copolymers, etc. Stable soap free lotion with solid content of 30% - 40% can be prepared by using macromolecular emulsifier. Some foreign companies have begun to use soap free lotion polymerization technology to produce high-performance coatings, and most of them use polymerizable emulsifiers. However, due to the low polymerization activity of polymerizable emulsifiers, a considerable part of them still play the role of small molecular emulsifiers, so the problems existing in conventional lotion polymerization cannot be completely solved. In addition, the price of this kind of emulsifier is too high and its adaptability is poor, which also affects their promotion and use

2. The room temperature crosslinking technology of water-based coatings can significantly improve the performance of the film, which is an important means to develop water-based industrial coatings. The crosslinking of waterborne coatings can be divided into baking crosslinking and room temperature crosslinking. Baking crosslinking requires energy consumption and is inconvenient to use. Therefore, people pay more attention to the study of room temperature crosslinking. In recent years, many new room temperature crosslinking systems have been developed abroad. The author believes that the following are the main ones with industrial application prospects

crosslinking system based on Michael addition reaction in recent years, room temperature crosslinking system based on Michael addition reaction has developed rapidly. The reaction requires alkali metal alkoxides or strong organic bases (such as tetrabutylammonium hydroxide) as catalysts. Rohm Haas' catalyst synthesized from tertiary amine and epoxy compound also has good catalytic effect. Low molecular weight trimethylolpropane triacrylate, acrylic polymer containing acryloyl group and unsaturated polyester synthesized from maleic acid, methylene succinic acid, maleimide and their derivatives can be used as receptors for active hydrogen. Acrylate containing acetyl group, malonic acid polyester, polysulfide, polyhydrazine and polyamine protected in the form of keto imine can be used as active hydrogen donors. The system can be quickly crosslinked at room temperature, and the formed film has good strength, water resistance and scrub resistance

the crosslinking system of carbonyl and hydrazide groups can realize the crosslinking and curing of coatings at room temperature by using the dehydration reaction between carbonyl containing polymers and hydrazide groups under acid catalytic conditions. Crosslinking agents containing hydrazide groups can be either small molecular compounds (such as dihydrazide adipate) or polymers containing hydrazide. Polymers containing carbonyl groups are generally obtained by copolymerization of monomers containing inverted metallographic microscope uses and acting carbonyl groups, such as diacetone acrylamide and acetylacetoxyethyl methacrylate (AAEM) with other monomers. Mitsubishi Yuka Badische company used acrolein to synthesize a one component room temperature curing lotion coating through lotion polymerization and hydrazidation. Carbonyl containing polymers can also be crosslinked at room temperature with compounds containing polyamine urea. In this system, ammonia is generally used to adjust the lotion to be alkaline and sealed for storage. When used, ammonia volatilizes with water, the system is acidic, and the crosslinking reaction can occur

crosslinking system of acetyl and polyamine compounds Room Temperature Curing Waterborne Coatings with good solvent resistance, water resistance, adhesion resistance and other advantages can be prepared by using the reaction of polymers containing acetyl functional groups and polyamines. Polymers containing acetyl functional groups are obtained by copolymerization of AAEM with other monomers. The monomer has the characteristics of low toxicity and easy copolymerization. Adding ammonia or volatile amine makes the pH of the system> 9, and the acetyl group can be quickly transformed into a stable enamine, which can prevent the hydrolysis and crosslinking of acetyl group during storage and improve the performance and storage stability of the product. At present, the main problem of room temperature crosslinking of waterborne coatings is that the activity of crosslinking reaction is low at room temperature, so the crosslinking density of the film is small, so the hardness and water resistance of the film still cannot reach the level of solvent based paint

3. UV curable crosslinked UV curable coatings generally use reactive diluent monomers and other organic volatile components (VOCs). These components are not only harmful to the environment and human health, but also difficult to react completely in the UV curing process, affecting the performance of the film. The curing time of water-based coatings at room temperature is generally long and incomplete. Therefore, UV curable waterborne coatings have been studied abroad since the late 1980s. Using water instead of reactive diluent, on the one hand, it eliminates the pollution and stimulation caused by the use of VOCs in UV curable coatings, on the other hand, it also provides a new curing method for water-based coatings. The composition of UV curable waterborne coatings generally includes waterborne unsaturated functional resins, photoinitiators and various additives. The water-based resin containing unsaturated functional groups is cured by free radical polymerization under UV irradiation under the action of photoinitiator. Unsaturated resins are mostly obtained by unsaturated functionalization of common resins. For example, polyurethane acrylates are generally made of (meth) acrylic acid- β- Hydroxyethyl (propylene) ester is obtained by reacting with isocyanate group at the end of polyurethane. Polyester acrylate is generally obtained by hydroxyl group at the end of polyester and acrylate. (3) check whether the power supply voltage connected to the pipe ring stiffness tester is normal or by reacting polyester carboxyl group with glycidyl methacrylate. Unsaturated polyester and epoxy resin can be directly used in UV curable waterborne coatings. The above resins can be water-soluble, lotion or hydrosol. The photoinitiator used shall be highly compatible with the aqueous resin. In developed countries such as Europe and the United States, some companies have launched UV curable water-based paint products, which are mainly used as cover varnish and ink for plastics. However, in general, UV curable waterborne coatings are still in the exploratory stage, and the development of efficient photoinitiators is the main topic to be carried out

4. Mixing technology of water-borne resin

mixing of hard and soft lotion for more than 10 years, the method of using hard and soft lotion to improve the hardness, anti adhesion, anti fouling of the film and reduce the amount of coalescer has attracted the interest of foreign scholars. The hard and soft lotion is used to mix, the soft lotion ensures the film formation at low temperature, and the hard lotion gives the film hardness and anti adhesion. Foreign studies have shown that in order to obtain a good coating, the mass ratio of soft lotion to hard lotion should be greater than 1, and the most ideal is 60:40. Under this condition, the minimum film forming temperature of the mixed lotion is close to that of the soft lotion, but the hardness and anti adhesion of the film are significantly improved. The gloss of the film formed by the mixed lotion is generally worse than that of the single lotion. In order to obtain a film with high gloss, the refractive index of the two lotion should be close. The particle size of hard latex is smaller than that of soft latex. The gloss of the film is also related to the COOH on the surface of hard latex particles and the coverage of surfactants

the mixing of lotion and water-soluble resin and the addition of water-soluble polymer or oligomer can improve the film-forming characteristics of lotion and improve the compactness and gloss of the film

water soluble polymers have the effect of highly intensive technology of new materials:

① water soluble polymers with many hydrophilic groups and small molecular weight have good dispersibility to pigments

② water soluble polymers with surface activity can improve the stability of lotion

③ fill the gap between latex particles to improve the compactness of the film. In order to give full play to the synergistic effect of water-soluble polymer and lotion and increase the amount of water-soluble polymer, the added water-soluble polymer must contain active functional groups and cross-linking reaction can occur in the process of film formation

mixing of different types of resins mixing various types of resins can learn from each other. Using simple mixing, due to the poor compatibility between polymers, there is phase separation, which can not achieve the desired effect. In order to solve this problem, many studies have been carried out around chemical grafting and hybrid polymerization. Hybrid polymerization can be considered as the polymerization of another polymer in the presence of one polymer. About acrylic acid in the presence of water dispersible polyurethane