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Research Progress Of Unsaturated Polyester Resin Coatings

Jun 05, 2020

Abstract: Unsaturated polyester resin (UPR) coatings are one of the earliest developed varieties of coatings. UPR as a film-forming resin for coatings not only has excellent coating performance and low cost, so UPR coatings are widely used in the coating industry. The article reviews the research progress of various existing UPR coatings. The UPR coatings are classified into air-drying UPR coatings, anti-fouling UPR coatings, fire-resistant UPR coatings, anti-corrosion UPR coatings, insulating UPR coatings, and low (zero) volatile organic compound (VOC) emission UPR coatings. The characteristics and development status of various coatings are expounded separately, the mechanism of coatings is briefly described, and the future development trend and research direction of UPR coatings are prospected.

 Keywords: unsaturated polyester resin; modification; coating

Unsaturated polyester resin (UPR) has high hardness, high fullness, good weather resistance, water resistance, oil resistance, high gloss, excellent electrical insulation, light weight and low price. However, the advantages of universal and low cost are one of the most important film-forming resins for coatings. UPR has poor air-drying, poor fire resistance, anticorrosion and antifouling properties, poor heat resistance and insulation, and large amounts of volatile organic solvents. Environmental and other shortcomings, with the continuous expansion of the production field and the increasing awareness of environmental protection, UPR coatings have been unable to meet the needs of social development. In recent decades, the research on the modification of UPR coatings has been the focus of attention, and a series of UPR coatings have been produced, which are widely used in the coating of household appliances and furniture, the corrosion of metal components, and road marking. System, anti-fouling of ships, insulation of electronic equipment, and fire-retardant equipment.

This study introduces the characteristics and development status of various existing UPR coatings, and discusses the future development trend and research direction of UPR coatings.

1 Air-drying UPR coating

Due to the problem of surface oxygen inhibition when UPR coating is cured at room temperature, the surface of the coating film is usually sticky, and the performance is poor, which cannot meet the coating requirements. Therefore, overcoming the surface oxygen inhibition has become one of the urgent problems to be solved by UPR coatings. Air-drying coatings are a type of coatings developed to overcome the problem of UPR surface oxygen inhibition.

 1.1 UPR surface oxygen inhibition mechanism

The mechanism of UPR surface oxygen inhibition can be expressed as: At room temperature, oxygen reacts with the primary radicals in the resin system to form inactive peroxy radicals first, as shown in formula (1).

MX·+O2→MX—O—O·Formula (1)

Peroxy radical itself or disproportionation termination or coupling termination with other radicals, sometimes also add a small amount of monomers to form a copolymer of low relative molecular weight, so that the UPR cross-linking and curing can not continue, macroscopically manifested as a coating film The surface is sticky.

1.2 Methods to improve the air-drying of UPR coatings

 In order to overcome the problem of oxygen inhibition on the surface of UPR coatings, research in this area has never been interrupted. At present, there are methods such as film covering method, paraffin covering method, adding cellulose acetate butyrate method and introducing air-drying groups in unsaturated polyester (UP) molecules.

The earliest used is the film covering method and paraffin covering method. The method is to cover the polyester film on the coating film to isolate the air, or add a film when preparing the coating, so that the coating forms a thin wax film when the coating is cured, float on a small amount of high-melting paraffin, and the surface of the coating film to isolate the air . However, the former construction operation is cumbersome, and the addition of the coating structure paraffin will make it difficult to use the components with complex paint shapes; when the latter is used, the film transparency and interlayer adhesion are reduced, and even the disadvantages of shrinkage holes due to local surface tension are reduced. . In order to overcome the above shortcomings, a method of cellulose acetate butyrate (CAB) to isolate air was developed. The coating system with CAB added will improve not only the coating film but also other properties of the coating film. Normally, the drying performance at room temperature is better, and the viscosity is lower. During operation, the first used CAB should have a high content of butyric acid groups. CAB is added to the UPR at 150°C. After dissolution, styrene is added to dilute it to make a coating.

The synthesis process of the DCPD UPR for coatings and the effect of the selection of raw materials on the synthetic resin at the same time on the performance of the resin, paint testing. DCPD is a by-product C5 fraction produced by petroleum cracking to produce ethylene. It is produced by dehydrogenation, de-weighting and refining. It has a wide range of sources and a relatively low price. DCPD has the molecular structure of allyl ether, and has a similar air-drying modification mechanism as allyl ether glycidyl ether. It can be used as a synthetic raw material for UP to prepare air-drying UP, which can be used to prepare air-drying UPR coating. Tests have shown that the comprehensive performance of UPR for coatings prepared by DCPD is relatively good, and can basically meet the requirements of resins for low and medium-grade furniture paints.

However, there are also problems with this method, such as the coating film is relatively brittle after curing, the greening is serious, and the thick coating is easy to whitish. In this regard, reports of oil-modified DCPD-type UPR have been produced. Wan Shiguan and others used soybean oil or castor oil to modify the UPR for DCPD type coatings and conducted paint blending performance tests. The test results show that the use of oil greatly improves the flexibility and greening resistance of the coating film. At the same time, the thick coating film does not blush and has good transparency.

2 Antifouling UPR coating

After the marine life is attached to the surface of the ship, it will not only increase the quality of the ship, reduce the speed, but also reduce the maneuverability. Fuel consumption has increased; it will also cause increased corrosion of ships, desalination equipment and underwater facilities. In order to reduce the hazards of marine life adhesion and prevent marine life from fouling ships and offshore installations, antifouling coatings are regarded as an economical and efficient way. UPR itself does not have antifouling properties, and generally requires the addition of antifouling additives to achieve the purpose of antifouling.

 In 1958, Montermosodi once proposed an acrylic resin antifouling paint containing tetrabutyltin. Since then, antifouling paints using organotin compounds as poisons have been widely used. However, organotin compounds have teratogenicity, and the inheritance and survival of other organisms. The use of them in the anti-fouling modification of UPR coatings has developed a new type of non-toxic anti-fouling agent. At present, the light antifouling coating has become the main direction of UPR coating antifouling modification.

Kaarnakari et al. invented the UPR gel paint with stain resistance. The paint can be used as a marine paint. It can play a role in 5 parts of the ship that is submerged under water for a long time. . The paint contains antifouling agent, 1 n-octyl-4-isothiazolinone-3, cyclopropyl-N'-(1,-dimethylethyl 3,4(yl)-6-(methylthio )-1,5-triazine-2,-diamine and N'[Fluoro-dichloro-methyl)thio]phthalimide or mixtures thereof are non-toxic antifouling agents. Tests show that the coating not only has good antifouling properties but also has excellent surface properties, making it an ideal coating for marine antifouling.

3 Fireproof UPR coating

UPR has a high content of carbon and hydrogen elements, with a limiting oxygen index of 19.5 to 21.5. It is a flammable material, and its UPR coating with its main film-forming material has poor fire insulation performance.

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Fireproofing of UPR coatings With the increasing demands on the fireproofing performance of coatings, flame retardant modification has become a hot topic at home and abroad. After decades of development, today's UPR fire-retardant coatings have become the main varieties in the field of fire-retardant coatings, and new technologies that more meet the needs of society are emerging.

UPR coating flame retardant modification methods are divided into two kinds of reaction method and addition method. The reaction method is to perform molecular modification on UP: use synthetic monomers with flame retardant elements to prepare UP, and then mix them uniformly with active monomers to make flame retardant UPR. For example, tetrabromophthalic anhydride, chlorobridge anhydride, etc. can react with the dibasic acid in UPR to introduce flame retardant elements; epichlorohydrin, etc. can be used for flame retardant modification of glycol. The use of phosphorus-containing (or simultaneously vinyl phosphate, etc., halogen-containing) reactive flame retardants such as dimethyl methyl phosphate can also produce flame retardant UPR. Adding method is to add flame retardant (such as aluminum hydroxide, etc.) in the ordinary UPR to achieve flame retardant effect. Flame retardant additives can be divided into inorganic additives and organic additives, such as aluminum hydroxide (ATH) as inorganic flame retardant additives. ATH can be used as both a smoke suppressant and a filler, and its application is very common. Among the organic additives, bromine- or chlorine-containing flame retardants are mostly used, among which bromine-containing flame retardants are more common. Typical bromine flame retardants are decabromodiphenyl ether (DBDPO), octabromodiphenyl ether (OBDPO), tetrabromodiphenyl ether (TBDPO), etc. The use of halogen-based flame retardant additives, the amount of addition is small, the flame retardant effect is significant, and the application is very wide [11].

However, the use of halogen as a flame retardant, especially some bromophenyl ether flame retardants, will produce a large amount of toxic gases in the process of severe combustion of human health and the environment, which is a serious hazard. In recent years, people have gradually been questioned. On January 27, 2003, the European Union officially announced the WEEE and RoHS directives, which restricted the use of some halogen-containing substances. Since then, halogen-free flame retardancy, especially phosphorus and nitrogen flame retardant research has become a fire resistance UPR. The main research direction of coatings. The research hotspot at home and abroad, Gu et al. [12] used unsaturated polyester-epoxy resin composite as the matrix resin, and fireproofing aids used ammonium polyphosphate (APP), polymelamine (Mel) and pentaerythritol tri (PER). And supplemented with fillers and other additives to make intumescent fireproof coatings. The experimental results show that when the thickness of the coating is 2.0mm, the fire resistance limit time exceeds 210min, and the fire performance is excellent.

4 Anti-corrosion UPR coating

Ordinary UPR has poor chemical resistance. The corrosion of UPR can be divided into physical corrosion and chemical corrosion: physical corrosion is due to the polar groups such as hydroxyl and carboxyl groups in the UP molecular structure, which makes the cured product easily adsorb and absorb with water or polar organic solvents, causing swelling The chemical corrosion is caused by the hydrolysis reaction of the ester group in the UP molecule, which causes the degradation of the cured product and the anticorrosive modification of the coating. In order to improve the corrosion resistance of UPR coatings, it is essential to have anti-corrosion UPR coatings.

In order to improve the corrosion resistance of UPR coatings, in recent decades, people have obtained many UPR varieties with corrosion resistance by changing the molecular structure of UP, so that the paint film itself has good corrosion resistance, such as the use of relative molecular weight with ether oxygen These methods can replace the ester bond, improve the curing of large glycol or dibasic acid to synthesize UP, crosslink density of the substance, increase the relative molecular mass of UP, and improve the molecular structure of UP. These methods are the symmetry of UPR anticorrosive coating. The development of UPR's corrosion resistance laid the foundation. Ayman et al. used maleic rosin anhydride, maleic anhydride, isophthalic acid, adipic acid, propylene glycol and diethylene glycol as reaction raw materials to prepare anti-corrosive UP by in-situ polymerization. A ratio of 2.5:1 is used to make UPR for anticorrosive coatings. Since the resin prepared with maleic rosin anhydride as a raw material has a high cross-linking density after curing, it can prevent the invasion of corrosive substances, so the coating has excellent anticorrosive properties. In addition, the evaluation of the chemical corrosion resistance of the resin system based on the steel corrosion resistance shows that it can be used for oil pipelines and coatings after 500h corrosion in the salt spray test, and it still has good adhesion and coating of containers.

In fact, the anti-corrosion effect of the general coating comes from two aspects: one is to prevent corrosive substances from invading the substrate through the paint film, and the other is to prevent the occurrence of corrosion by the blocking effect of the anti-corrosion pigment and filler. If UPR is mixed with suitable pigments (especially flake inert pigments), anticorrosive coatings with better performance can be made. At present, there have been reports of anti-corrosion by adding glass flakes to the paint. The thickness of the glass flakes is generally between 2 and 5 μm, and dozens of layers can be arranged in the coating, so that the diffusion of the corrosive medium becomes a complex and tortuous infiltration diffusion path in the coating, which is quite difficult and difficult to penetrate into the substrate . At the same time, the glass flakes will divide the coating and cure it, and the shrinkage of the coating film is small, which greatly reduces the shrinkage stress of the coating into many small spaces. The residual stress of each contact surface is reduced, and the additional reduction of the coating film is added. Work hard.

5 Insulating UPR coating

With the development of electrical equipment in the direction of large capacity, high performance, small size, safety and reliability, many problems need to be solved by heat-resistant insulating materials, which can not only improve the reliability of electrical equipment operation and extend the service life, but also reduce the insulation Thickness increases capacity and ensures the stability of product quality. At present, the research on the heat-resistant insulation modification of insulating UPR coatings mainly focuses on the modification of the matrix resin, generally by introducing heat-resistant groups to improve the heat resistance of the resin, such as heat-resistant anhydride modified UPR, silicone Resin modified UPR, imine modified UPR, etc. By using these methods, the heat-resistant insulation stability of the insulating paint can be significantly improved.

Chen Yaxin et al. synthesized an acid anhydride containing heat-resistant groups and used it in epoxy resin/UP system to prepare insulating impregnating varnish. Since the heat-resistant groups participate in the cross-linking film-forming reaction of the coating, the heat resistance of the coating is significantly improved. It has been tested that the insulating paint can reach the heat resistance level of F level, and can fully meet the insulation treatment requirements of F level indication, motor and electrical appliances. Pravat et al. developed a silicone resin modified UPR insulating paint and evaluated its heat-resistant insulating properties. Because silicone resin has excellent thermal oxidation stability and outstanding electrical insulation, it can maintain good insulation performance in a wide temperature and frequency range. It can be used for heat-resistant insulation modification of UPR insulating paint The effect is remarkable. Studies have shown that the temperature resistance index of the insulation paint impregnating the coil can reach 208 ℃, which can be used in heat-resistant insulation occasions of 200 ℃ grade. Lu Juncai et al. obtained a new type of unsaturated polyester imide solvent-free impregnating lacquer with high heat resistance and high temperature resistance index in the ordinary polyester sub-adhesive and the introduction of heat-resistant imine epoxy in the amine lacquer. The measured value of the adhesive force of the polyester imide paint at 180℃ reached 55N. The 180 ℃ secant line shows that the French temperature resistance index is 182.7 ℃, and the insulation level reaches H level.

6.1 Waterborne UPR coating

Water-based UPR coating is one of the important varieties of environmental protection coatings. Compared with solvent-based coatings, it has the advantages of low VOC content, no odor, no burning and low toxicity compared to materials. Its main component is water-based UP. Aqueous UP is an ionic copolyester that introduces ionic structural units into the molecular chain. The presence of ionic groups also makes it have excellent hygroscopic properties and ions not only impart UP with water solubility and conductivity characteristics. Commonly used water-based modifiers are volatile amines, sulfonates, etc. These monomers can be used alone or in combination. The salt groups produced or introduced by them can impart water solubility or water dispersibility to the resin. Controlling the different acid value or neutralization degree of UP can provide different water solubility and make different dispersion systems, such as solution type, colloid type, emulsion type, etc.

However, water-based UPR coatings also have defects, such as poor water resistance and solvent resistance, low hardness, poor gloss and fullness, and slow drying speed, which limits their further development. In recent years, a lot of research work has been carried out.

Satpute et al. reacted propylene glycol (PG) with maleic anhydride (MA) and phthalic anhydride (PA) in different proportions to prepare 6 different UPs with carboxyl end groups and a relative molecular weight of about 3000. MA and PG are used separately to prepare UP. Emulsifying these UPs with ammonia as a neutralizer in water can obtain a stable emulsion with excellent coating film performance MA. After testing, it is found that the UPR emulsion with high content has the characteristics of high hardness, good flexibility, excellent water resistance, chemical resistance, and solvent resistance. The surface drying time of the water-based paint prepared with this emulsion is within 1h. Shi Zhichao et al. used a suitable amount of acrylic monomers to initiate solution graft polymerization of UP under the initiation of benzoyl peroxide (BPO). The acrylic/UP hybrid was used to prepare an aqueous dispersion coating, and the acrylics were studied. The effect of monomer water dispersion, BPO, triethylamine and co-solvent dosage on product performance. The results showed that the composition of BPO is 2.2% when the grafting rate is high, about 20%. The modified UPR paint film drying time is greatly shortened, the hardness can reach 4H ~ 5H, suitable for aqueous dispersion paints dried at room temperature. JankowskiP et al. prepared a UV-curable water-soluble resin containing water-absorbing sulfonic acid groups by copolymerization, and emulsified it to prepare a UV-curable water-based UPR coating. Sulfonate uses sodium 3-hydroxy-1-propane sulfonate or sodium dihydroxy propane sulfonate. The different content in the water-based coating allows the coating film to obtain a hardness of 125 to 312 (persoz pendulum hardness tester) and Good adhesion to glass or metal substrates. Research shows that this kind of coating can fully meet the requirements of glass, wood and metal coating.

6.2 Powder UPR coating

Powder coating is also a kind of environment-friendly coating, which has the advantages of no solvent, zero VOC emissions, high coating efficiency, good film performance, easy control of film thickness, relatively safe production and coating operations. However, powder coatings also have the disadvantages of high production costs, difficulty in obtaining thin coating films, the inability to share paint replacement equipment and general coatings, cumbersome color and variety procedures, and high baking temperatures. These shortcomings limit its further development, especially its construction baking temperature is relatively high, generally can only be used for metal and other people have developed UV light solid material coating for this problem. In recent years, chemical powder coatings. This curing method greatly reduces the curing temperature of the coating and the surface flatness of the coating film is effectively improved, which can fully meet the coating requirements of heat-sensitive materials such as wood, plastics, and MDF.

UPR type photocurable resin developed early and has a large sales volume. It is made of UV and photocurable powder coating not only overcomes the problem of high baking temperature of powder coating, but also is environmentally friendly and non-polluting. Research is on the rise. Xu Jie et al. synthesized two different structures of UPR, amorphous and semi-crystalline, and prepared a two-component UV-curable powder coating. Analysis and testing show that the impact resistance and adhesion of the coating film after the semi-crystalline UPR is added to the coating formulation are obviously good after curing. AlcónN and other improvements, using UPR as the main film-forming material of the coating, strontium aluminate as a luminescent pigment, -hydroxy-2-4methyl-1-[-(2-hydroxyethoxy)phenyl]-1-acetone It is a photoinitiator, and is matched with a suitable defoamer and leveling agent to produce a UV-curable powder coating. The effects of different levels of luminescent pigments and the preparation process of the coating on the properties of the coating film were studied. The results show that the lower the luminescent pigment content, the shorter the decay time of the coating film; when the coating is prepared, if the extrusion speed is not properly controlled, it will cause the coating color to develop. Black; good performance when the thickness of the coating film is greater than 100 μm.

7 Conclusion

UPR coatings are a type of thermosetting resin coatings with excellent performance. After the UPR coating modification research has achieved certain achievements, several decades of development, and there are still new products and technologies that continue to be produced to make its use range further. expand. Therefore, there is reason to believe that the development prospects of UPR coatings are very broad. With the gradual strengthening of environmental protection awareness at home and abroad and the increase in environmental protection in various countries, environmental protection coatings such as water-based UPR coatings and UPR powder coatings should be the main direction of future development, and their further functionalization, refinement and high performance should be Its main development trend.