Preparation of aqueous dispersions of fluorinated hydroxyacrylic acid

High-solids solvent-based fluorinated acrylic acid resin was first prepared from fluorinated acrylic acid monomer, butyl methacrylate (BMA), butyl acrylate (BA) hydroxyethyl methacrylate (HEMA), and acrylic acid (AA) monomers, and then the aqueous dispersion of fluorinated hydroxyacrylic acid was prepared by amine neutralization and finally dispersion with water. The effects of relative molecular mass, reaction temperature, neutralization degree, solvent amount, acid value, and amount of fluorinated monomer (fluorine content) on the resin preparation were discussed.
　　INTRODUCTION
　　Two-component acrylic-polyurethane coatings play an important role in national production and have a wide range of applications. FEVE coatings are widely used for the protection of outdoor steel structures, large marine bridges and other equipment due to their excellent outdoor durability, but the VOC content of the prepared coatings is high, which causes a large pollution to the atmosphere during the construction of the film. Most of the water-based fluorine resins on the market are fluorocarbon emulsions prepared by emulsion polymerization technology, which have the disadvantage of low gloss and are mainly used in the field of architectural coatings. Acrylic resins are widely used because of their high gloss, good color retention, good alkali resistance and low price. In this paper, we studied the copolymerization of fluoroacrylate monomer in acrylic resin to produce fluoroacrylic resin aqueous dispersion, which can maintain the characteristics of ordinary acrylic resin and have the characteristics of fluorine resin such as weathering and staining resistance, and the VOC content of the resin is much lower than that of solvent-based FEVE resin, which can produce 2K-PU coating and one-component amino baking paint. The application range is very wide.
　　1. Experimental part
　　1.1
　　Raw material
　　Monomer: (meth)trifluoroethyl acrylate, butyl methacrylate (MBA), hydroxyethyl methacrylate (HEMA), butyl acrylate (BA), acrylic acid (AA): industrial products, Jilin Petrochemical; initiator: tert-butyl peroxybenzoate (TBPB), di-tert-butyl peroxide (DTBP): industrial products, neutralizer: N,N-dimethylethanolamine (DMAE). Industrial, solvent: ethylene glycol butyl ether (BCS), S-100# solvent: Industrial, curing agent: aqueous isocyanate Bayhydur XP-2655: Industrial,.
　　1.2
　　Resin synthesis
　　Put the solvent into a four-mouth flask, raise the temperature to the specified temperature, mix a part of trifluoroethyl (meth)acrylate, MBA, HEMA, BA, AA, and initiator, and then add them into the four-mouth flask drop by drop at a uniform rate for 3~4h; then add the remaining mixture of trifluoroethyl (meth)acrylate, MBA, HEMA, BA, AA, and initiator drop by drop for 1~2h. After the drops are finished, hold for 1 h, and then add one more part of initiator to eliminate the residual list. After holding for 2 h, the temperature was lowered to 90 ℃, and the neutralizer DMAE was added, and then deionized water was added for dispersion to obtain the aqueous dispersion of fluorinated hydroxyacrylic acid resin.
　　1.3
　　Performance testing and characterization
　　(1) Appearance: visual inspection.
　　(2) Relative molecular mass and its distribution: the relative relative molecular mass of the aqueous dispersion was characterized by using GPC of JINDUN CHEMICAL.
　　(3) Non-volatile component of aqueous dispersion: measured in accordance with GB/T 1725-2007.
　　(4) Viscosity of water dispersion: measured in accordance with GB/T 2794-1995; NDJ-II rotational viscometer, using 3# rotor, 30 r/min.
　　(5) storage stability: 50 mL of the sample into 80 mL of transparent plastic bottles, sealed into (50 ± 2) ℃ thermostatic oven, regular observation of the sample appearance; (6) plate making conditions: the sample containing the water dispersion viscosity: the sample is measured in accordance with GB/T 27/1995; NDJ-II rotary viscometer, using 3 # rotor, 30 r/min; (7) the sample is measured in accordance with GB/T 27/1995.
　　(6) plate making conditions: fluorinated acrylic dispersion and water dispersible isocyanate curing agent were mixed in proportion, sprayed on tinplate, baked at 80 ℃ for 60 min after surface drying, and the dry film thickness was about (30±5) μm.
　　2. Results and Discussion
　　2.1
　　Effect of relative molecular mass on dispersion
　　The size of the relative molecular mass of the resin directly affects the performance of the final coating film. The smaller the relative molecular mass of the acrylic resin, the higher the prepared solids, the lower the solvent content, and the richer the appearance of the coating film. However, the smaller the relative molecular mass of the hydroxy acrylic resin, the better it is. After the relative molecular mass reaches a certain level, some of the resin chains may not contain hydroxyl groups and cannot participate in cross-linking and curing at a later stage, and exist in the form of plasticizer in the paint film. This kind of small molecule resin will have the tendency to migrate to the surface during the use of the paint film, causing the film to lose light and deteriorate water resistance, which affects the performance of the paint film. Similarly, to ensure that there are enough carboxyl groups in each resin chain to provide later water dispersion, it is necessary to ensure that the resin has a relatively suitable relative molecular mass. However, too large a relative molecular mass can also cause problems such as large polymerization viscosity, coarse particles in the dispersion process, poor water dispersion, and reduced resin appearance and storage stability. Therefore, to ensure that the aqueous dispersion has a good appearance and excellent coating performance, the relative molecular mass of the resin and its distribution should be strictly controlled. The influence of relative molecular mass on the appearance and viscosity of aqueous dispersion is shown in
　　Note: The solids of resin is 45%, same as below.
　　It can be seen that as the relative molecular mass of the resin increases, the viscosity of the dispersion tends to rise, while the appearance of the dispersion changes from milky white to translucent blue phase and then gradually becomes milky white. Because the relative molecular mass of resin is small to a certain extent, the content of hydrophilic groups on part of resin chain is low, which is not enough to provide resin water solubility (dispersibility), when adding water for dispersion, the part of resin is suspended in the system as water-insoluble particles, and the particles are coarse, and the appearance of resin is milky white, so the viscosity of the system is very low. As the relative molecular mass of the resin increases, the content of hydrophilic groups on the resin chain increases and the resin has better hydrophilicity, the appearance of the resin becomes slightly transparent with blue phase, the particle size of the dispersion is smaller, and the viscosity gradually increases. And with the further increase of relative molecular mass, although the hydrophilic group on the molecular chain is increasing, at the same time the length of hydrophobic chain segment increases more (the proportion of hydrophobic components is large), which leads to the hydrophilic chain segment is difficult to envelop the hydrophobic chain segment all inside the particle, the adjacent hydrophobic chain segment molecules form aggregates due to hydrogen bonding, the volume becomes larger, and the molecules become entangled and cause the viscosity to increase. According to the experimental results, the relative molecular mass of the resin is 3000~6000 g/mol, and the dispersion has a good appearance.
　　2.2
　　Effect of reaction temperature on the dispersion
　　The effect of reaction temperature on the viscosity of the dispersion: the viscosity of the resin gradually decreases as the reaction temperature increases. The increase of reaction temperature means that the decomposition rate of initiator is accelerated, and it also increases the termination rate of chain growth radical and the rate of chain transfer, so the increase of reaction temperature can reduce the relative molecular mass of resin, so that the viscosity of resin can be reduced.
　　As can be seen, when the reaction temperature is at 160°C, the initiator dosage decreases from 4% to 2%, and the appearance of the resin becomes slightly translucent with blue phase again, so the dosage of initiator is reduced to ensure that the relative molecular mass of the resin is within a reasonable range, which can reduce the risk of deterioration of the resin weatherability due to excessive dosage of initiator.
　　2.3
　　Influence of neutralization degree on dispersion viscosity
　　In order to realize the water solubility of resin, hydrophilic groups such as hydroxyl (-OH) and carboxyl (-COOH) are usually introduced in polymer chains, and the -COOH on polymer chains are neutralized by organic amines to make the resin hydrophilic. The amount of organic amine has a great influence on the viscosity of the resin
　　It can be seen that with the increase of the neutralization degree, the resin viscosity shows a slow rise first, and then a sharp increase. This is because with the increase of organic amine addition, the carboxyl groups on the resin chain are neutralized more and more, and the number of hydrophilic chain segments of the resin gradually increases, and the chain segments initially encapsulated inside the dispersion particles gradually spread in the aqueous phase and form aggregates, and the number of aggregates increases, and the friction between them also increases, resulting in the system viscosity gradually increases. From the viscosity results, the system neutralization degree control at 70%-90% is more appropriate.
　　2.4
　　The effect of solvent content on the dispersion
　　The resin is made by solvent free radical polymerization, so the solvent content plays a decisive role on the relative molecular mass and viscosity of the resin. When the amount of solvent is low, the concentration of monomer is too high during polymerization, which will cause the relative molecular mass to increase, and the viscosity of the system becomes large at a later stage, which will cause the automatic acceleration of free radical polymerization, resulting in a surge of resin viscosity. Variation of dispersion viscosity with different solvent amounts.
　　From the viscosity curve, we can see that when the solvent amount is between 7% and 10%, the viscosity of the dispersion is low, and then the viscosity rises with the increase of solvent amount; but when it exceeds 15%, the viscosity shows a rapid decline. This is because, the solvent amount is less than 7%, the concentration of monomer is higher in the synthesis process, and the relative molecular mass of resin becomes larger causing the viscosity of the system to become larger. After the solvent amount exceeds 10%, more and more co-solvents help the resin to dissolve in water better, and the molecular chain gradually spreads out, which makes the movement of the molecular chain difficult due to the hydrogen bonding between molecules and increases the viscosity of the resin. As the amount of solvent increases further, the amount of water becomes less and less under the condition of maintaining certain solids, and the resin dissolves into the solvent more and more, which makes the viscosity of the system decrease rapidly because the solvent used is a good solvent with strong solvency power.
　　2.5
　　The effect of carboxyl monomer addition on dispersion
　　The aqueous resin relies on the ammonium salting of hydrophilic monomer to obtain hydrophilicity, so the amount of carboxyl group (-COOH) as hydrophilic functional group plays a decisive role in the hydrophilicity of the resin. Too little amount of carboxyl group will affect the stability of resin, while too much amount of carboxyl group will cause the viscosity of the system to increase, resulting in low construction solids, sagging, low film thickness and other disadvantages, and most importantly, it will affect the water resistance of resin. A variety of resins with carboxyl content were prepared for the test, and their effects on resin performance are shown.
　　It can be seen that with the increase of carboxyl content, the viscosity of resin rises rapidly, and when the carboxyl content reaches a certain degree, the hydrophilicity of resin becomes better and better, and gradually becomes water-soluble instead of water-dispersible. Through the experiment, we found that the appearance and viscosity of dispersion are more suitable with the addition of carboxyl monomer between 2.5% and 3.5%, and the storage stability is also very good.
　　2.6
　　Effect of fluorine content on resin properties
　　The dispersions were prepared with trifluoroethyl acrylate monomer, which is widely available and suitable in price, to ensure a high market competitiveness in the subsequent promotion of the product. The different fluorine content will have a large impact on the appearance and properties of the dispersions. The fluorine content of trifluoroethyl acrylate monomer was 37%, and the dispersions with different fluorine contents were prepared separately, and the properties of the resulting dispersions are shown.
　　It can be seen that with the increase of fluorine monomer, the appearance of the resin gradually changed from milky white to semi-translucent blue phase, and then to milky blue phase. This indicates that the particle size of the dispersion changes from large to small and then gradually becomes larger, while the storage stability also shows the same trend. This is due to the high water and oil repellency of the fluorinated monomer. When a small amount of fluorinated monomer copolymerization is used, a certain number of fluorocarbon groups are arranged on the side chain of the resin, and the system becomes "water-in-oil", the high water repellency of the fluorocarbon groups makes the emulsion particles better "agglomerate" and get smaller particle size. This results in a more transparent appearance of the dispersion. However, when the amount of fluorinated monomers reaches a certain level, firstly, due to the competitive polymerization rate of fluorinated monomers, the monomer conversion rate decreases, and some fluorinated monomers remain in the system, causing the dispersion to deteriorate in appearance and stability; secondly, the increasing number of fluorinated carbon groups on the resin chain and their increasing repulsion will greatly affect the hydrophobicity of hydrophilic groups on the resin chain, resulting in a decrease in the hydrophilicity of the resin, an increase in the particle size of the emulsion, a deterioration in appearance and storage stability. The hydrophilicity of the resin decreases, the particle size of the emulsion increases, the appearance deteriorates and the storage stability decreases. From the consideration of technical index and cost factors, it is more appropriate to control the fluorine content at 8%~13%.
　　2.7
　　Basic performance test of coating film
　　Waterborne two-component acrylic polyurethane coating film was prepared by using waterborne isocyanate curing agent and fluorinated acrylic dispersion. The reference formula is shown.
　　The varnish was prepared according to the above formula, and the performance was tested after complete drying as required. The results are shown.
　　From the data, it can be concluded that the fluorinated acrylic dispersion has a higher gloss than conventional FEVE coatings and can be better used in areas where weathering and decorative properties are required.
　　3.Conclusion
　　In this study, the influence of multiple factors on the preparation and stability of fluorinated acrylic dispersions has been investigated, and the parameters that enable the preparation of stable fluorinated acrylic dispersions have been determined, which are of some guidance for such resins. The fluorinated acrylic dispersions prepared in this study have excellent properties such as gloss, appearance, weathering resistance, etc., which are of good practical significance.