Is there any high-temperature resistant paint on the plastic?

Because of its light weight, easy processing, corrosion resistance and rich resources, plastics have widely replaced traditional materials, such as metal, wood, leather and so on, and generally appear in our daily life. However, plastic products will have defects such as uneven color, monotonous color and mottling, which are easy to age, brittle, scratch, stain and static electricity. Coatings can avoid the above defects and problems, and the research and development of plastic coatings has been widely concerned by coating workers. However, because plastic is a low surface energy substance, its surface coating is much more difficult than steel, wood, architecture and so on, and adhesion has become a major obstacle to the development of plastic coatings. It is necessary to choose different formulations of coatings according to different plastic substrates in order to produce better adhesion. With the improvement of people's living standards and the strengthening of environmental awareness, plastic coatings, like other coatings, are developing towards functionalization and environmental protection.

1 plastic surface adhesion theory

Plastic surface has low free energy and low polarity. Its surface free energy is lower than 10-5J/cm2, so the surface is difficult to attach. Many theories have been put forward about the adhesion between coatings and plastic substrates: diffusion theory, solubility parameter theory, electrostatic theory, chemical bonding theory, adsorption and mechanical occlusion theory. Among them, the diffusion theory has been widely recognized, which holds that the adhesion of coatings is closely related to the ability of coatings to diffuse into plastic matrix. In fact, various theories are not contradictory. Once the coating penetrates into the plastic substrate, the electrostatic and mechanical engagement between the permeate and the plastic substrate will obviously enhance the adhesion between them. If the coating wants to penetrate into the plastic substrate, it must first be able to wet the substrate well, and it can be spread on the substrate by adjusting the proportion of each component in the coating. At this time, the coating and the plastic substrate are bonded together by chemical force. If the solubility parameters of film-forming resin and plastic substrate are properly selected, a mixed layer can be formed between paint film and plastic surface, and then the electrostatic and mechanical biting force between paint and plastic can play a role. Although the mixed layer helps adhesion, it depends on the slight dissolution of plastic by the solvent in the paint. If the paint is not properly grasped, the surface of plastic substrate will be uneven, the paint film will wrinkle, and the leveling property will be poor, which will affect the appearance. Therefore, the solubility parameters of the solvent used should be as far away from the solubility of plastics as possible. Because plastics and coatings are complex systems with physical and chemical heterogeneity, there are internal stresses, such as shrinkage stress, thermal stress and deformation stress, which will adversely affect the adhesion of coatings. Therefore, it is necessary to add appropriate additives and select appropriate coating system to eliminate them. Different plastic substrates should also choose the appropriate coating system according to the principle of similar structure and polarity.

2 Research progress of coatings for different plastic substrates

At present, the main types of plastics are polyolefin, PS, ABS, PC, PMMA, polyamide and polyether plastics. Among them, polyolefin, ABS and PS plastics are the most widely used. Because the structure and polarity of different plastic substrates are very different, different plastic substrates should adopt different coating systems, as shown in Table 1.

Table 1 Different plastic substrates choose different coating systems according to their structures and polarities.

Table 1

2. 1 polyolefin plastic coating

Polyolefin plastics mainly include polypropylene plastics (PP) and polyethylene plastics (PE). Polyolefin plastic substrate has high crystallinity, strong solvent resistance, low surface polarity and surface energy. Besides suitable coating system, suitable surface treatment is also needed. Chlorinated polyolefin coatings and fluorocarbon resin coatings with similar molecular structure and solubility parameters to plastic substrates can provide good adhesion and are the first choice for this kind of plastic coatings. Epoxy resin, polyurethane and two-component acrylic coatings can also be used. There are two kinds of coatings for polyolefin plastics: one is the coating containing chlorinated polyolefin (CPO). CPO introduces the polar group "chlorine" into polyolefin, which can be mixed with some coating resins to wet the plastic surface and improve the adhesion. . The other is a coating without CPO. In order to improve the adhesion between coating and substrate, some special functional groups need to be introduced.

2.2ABS plastic coating

ABS is a polymer composed of acrylonitrile, butadiene and styrene. By changing the proportion of the three components, adopting different combinations and different molecular weights of polymers, ABS resins with various properties and different specifications can be manufactured. At present, the monomer content ranges from 20% to 30%; B accounts for 6% ~ 30%; S accounts for 45%~70%. It can be seen that ABS and PS are the same family. However, due to its advantages of strong rigidity, high hardness, good toughness, good surface and good formability, its application scope far exceeds PS. But relatively speaking, its cost is higher than PS, which restricts its development. Because ABS contains styrene monomer, the coating suitable for PS plastic coating is also suitable for ABS coating. Because ABS contains polar monomer acrylonitrile, it has higher surface tension than other plastic products and is easier to coat. It can choose a wide range of coatings, such as volatile coatings, epoxy alkyd nitro coatings, polyurethane oily coatings and two-component conversion coatings, such as acrylic polyurethane coatings. According to the needs, these coatings can be made into light, semi-light, various colors of metal texture and soft rubber texture coatings. Thermoplastic acrylic resin was developed as the base material of ABS plastic coating, and the corresponding varnish and white paint were prepared, which had good physical properties. Low-hydroxyl acrylic resin and mixed etherified melamine formaldehyde resin were crosslinked and cured at low temperature under acid catalysis, forming a one-component fast-drying high-gloss plastic coating with strong adhesion. Using acrylic resin as base material and nitrocellulose to prepare room-temperature drying metallic flash paint, compared with acrylic amino flash paint, it does not need high-temperature baking to form a film, and the construction is very convenient. Then the surface coatings for ABS plastics were made from thermoplastic acrylic resin, cellulose acetate butyrate (CAB), different additives and other raw materials, which also had metallic texture. Recently, Japan has also developed a metal-like water-based coating for plastic coating, which consists of polymer water-dispersible resin, metal pigment, coloring pigment, film-forming additive, water-based coating additive and water. It can be used to coat various plastic surfaces, especially ABS and PS.

2.3PS plastic coating

Because of its good formability, beautiful appearance and excellent comprehensive mechanical properties, PS and ABS modified by it dominate the plastic shell market of electrical appliances. For cost reasons, PS is more acceptable to manufacturers than ABS. However, the application of pure PS is limited because of its brittleness, easy fragmentation and poor heat resistance. HIPS, that is, high impact polystyrene, obtained by modifying polystyrene with rubber-based polymer, has been widely used in household appliance shells and is increasingly replacing ABS because of solving the above shortcomings. HIPS is an amorphous, colorless and transparent plastic, which contains polar molecules and has good adhesion to coatings. Applicable systems include epoxy coating, thermoplastic acrylate resin coating, thermosetting acrylate-polyurethane resin coating, acrylic acid modified alkyd resin coating, alkyd resin modified polystyrene coating, modified cellulose coating and so on. Liang Zengtian developed a plastic coating for HIPS based on acrylate resin, and modified the acrylate resin coating with nitrocellulose, introducing some polar groups, such as carboxyl and cyano, to improve the adhesion of the coating, and achieved good results. Fang Xiumin and others modified nitrolacquer with acrylic resin to prepare acrylic modified nitrolacquer with excellent performance, which can also be used for surface coating of HIPS plastics. Because the domestic color TV industry has developed from low-end color TV to high-end color TV, its coatings have also developed from single-layer thermoplastic coatings to high-gloss coatings. In this case, Janson Yang et al. developed a high gloss coating of HIPS plastic for TV housing. The coating is composed of thermoplastic acrylic primer and polyurethane topcoat, and is prepared by 2C2B process. For the requirements of environmental protection, waterborne coatings and UV-curable coatings have been developed. Cui Jinfeng and others developed a comb-like polyamide prepolymer for ultraviolet curing plastics. A prepolymer with polyamide as the main chain and ethyl acrylate carbamate as the side chain was prepared by the addition reaction of low molecular weight polyamide resin and ethyl acrylate carbamate-based monoisocyanate, which can meet the application requirements of HIPS plastics. Igami, Kiyotaka et al. developed a polymer composition for light-resistant coating of plastics. The composition consists of ultraviolet-absorbing polyester polyol and an ultraviolet-absorbing polymer prepared from acrylic polymer or polyurethane. When coated on HIPS substrate, the adhesion can be improved after drying, and the light resistance is good under wetting conditions.

3 Development trend of coatings for plastics

With the rapid development of household appliances, mobile phones, automobiles and other industries, as well as people's new requirements for external surface decoration and protection, coatings for plastics are developing in the direction of multi-function and environmental protection.

3. 1 Research progress of functional plastic coatings

3. 1. 1 fireproof plastic coating

Fire retardant coatings are also called flame retardant coatings. Plastic itself is a polymer material and belongs to flammable substances. Adding effective flame retardant into plastics and coatings can achieve the purpose of fire prevention. AlbertoFina et al. added some aluminum-containing isobutyl silsesquioxane (Al-POSS) as flame retardant to PP, which had obvious flame retardant effect compared with PP without Al-POSS. Wang Huajin et al. developed an ultra-thin fireproof coating with excellent fireproof performance for the surface of plastic products with modified highly chlorinated polyethylene resin as film former, pentaerythritol as charring agent and ammonium polyphosphate as charring catalyst. Wang Min and others added amide and guanidine to the resin to develop a fire retardant coating. When heated, amide and guanidine components decompose to produce a large number of non-combustible gases such as NH3 and CO2, which blow the molten components into an expanded foam layer. This kind of expansion layer has strong isolation function (heat insulation and gas insulation), which protects the substrate and makes it difficult to reach the combustion condition, so it can effectively prevent fire. China patent 1060859A also introduces a polyurethane plastic fire retardant coating made of formaldehyde, urea, sodium hydroxide, melamine, dicyandiamide, pentaerythritol, phosphoric acid, triethanolamine and hydrochloric acid.

3. 1.2 conductive plastic coating

Conductive coating is a new type of coating produced in the late 1950s. The main difference with other coatings is the addition of conductive filler. With the widespread use of household appliances and mobile phones. Conductive coating can prevent electromagnetic interference, radio frequency interference, radio noise interference and corona interference. Some conductive coatings can also play an antistatic role, so people pay more and more attention to them. GE plastics co., ltd has developed a conductive resin NorilGTX674PC, which is suitable for powder coating substrates. The resin is made of polyamide and modified polyphenylene ether, and contains conductive filler, which can play a conductive role. Japan Shendong Coating Company recently developed a water-based conductive coating for electromagnetic interference (EMI) shielding, which is mainly used to coat the inner surface of plastic shell. Trimite Co., Ltd. of Axbridge recently introduced two new electromagnetic shielding coatings, J 190 and J 19 1, which can meet the requirements of anti-EMI. These two kinds of coatings have excellent hiding power, and are used for mixed substrates such as ABS, PVC and modified polyphenylene ether, with high adhesion and conductivity, which can greatly reduce the cost and improve the application efficiency.

3. 1.3 antistatic plastic coating

When the surfaces of nonmetallic parts such as plastics are rubbed or hit by airflow and liquid, static electricity is easy to be generated and accumulated. When static electricity accumulates to a certain extent, it will discharge, which will lead to the failure and scrapping of various precision instruments and precision electronic components, and even lead to the fire or explosion of flammable and explosive materials, resulting in huge loss of life and property. In addition, the static electricity accumulated on the surface of plastic products is difficult to purify due to serious dust absorption, thus affecting the appearance of products and their application in ultra-clean environments (such as operating rooms, computer rooms, precision instruments, etc.). In order to avoid the generation and accumulation of static electricity on the plastic surface, the current external treatment method is to coat anti-static paint on the plastic surface. Chen et al. used acrylate as base material and added conductive materials such as graphite to make the volume resistivity of paint film104 ~109 Ω cm. However, most antistatic coatings made of conductive materials have poor adhesion and oil resistance, ugly color and generally high cost. Because some conductive coatings can be used to prevent static electricity, they can also be directly coated with conductive coatings to form an anti-static layer to prevent static electricity. If the conductive polyaniline composite nano-material is directly sprayed or brushed on the plastic substrate, it not only plays an anti-static role, but also has high adhesion and low cost. Antistatic agent solution can also be sprayed on the plastic surface, such as Germany Dressler and others spraying surfactant solution (containing ≥ 20% organic solution) on the surface of molded plastic parts, which has antistatic effect.

3. 1.4 Wear-resistant and scratch-resistant plastic coating

Plastic products, such as mobile phones, tape recorders, televisions, etc. Often in contact with human hands, its coating is easy to wear, which requires improving the wear resistance and scratch resistance of the coating. As early as 1940, DuPont conducted related research. At first, it was to improve the existing coating varieties, but the effect was not obvious. Later, by using organopolysiloxane systems such as "polycarbonate MR-5000" and "TuffakCM-2", the wear resistance of the coating film was improved. Later generations have done a lot of work on this basis, making silicone coatings the mainstream of wear-resistant and scratch-resistant coatings. Subsequently, fluorocarbon resin was also studied. Its hard coating has excellent scratch resistance and wear resistance. In recent years, together with silicone resin, it has been widely used in the coating of plastic products. Shanghai Institute of Organic Chemistry has developed a fluoroplastic coating containing tetrafluoroethylene and ethylene terpolymer, which has good wear resistance. At the same time, UV curable coatings for plastics are also under development. Compared with silane system, they have the advantages of fast curing speed, convenient operation, low cost and no environmental pollution. American patent. 385,964 introduces a coating for ultraviolet radiation curing plastics, which can be used for PC plastic surface coating. In recent years, people have explored a new type of coating, that is, organic-inorganic composite coating, mainly by adding inorganic silicon dioxide into the coating. Zhang et al. modified the emulsion with core-shell emulsion polymerization process and functional monomers to prepare styrene-acrylic emulsion with core/shell structure, and then compounded it with nano-silica to obtain high-performance waterborne nano-composite plastic coatings. The hardness of the coating reaches more than 3 hours, and the scratch resistance and wear resistance are greatly improved. Nakaya, Hidekazu and others used sol-gel technology to react siloxane oligomer with phenyl-containing alkoxysilane to prepare polysiloxane-based materials and phenyl-containing organic polymers uniformly dispersed in the substrate, and also developed an organic-inorganic composite hard coating for plastic substrates, which has good scratch resistance.

3. 1.5 Other functional plastic coatings

Other plastic coatings with special functions, such as anti-corrosion coatings and anti-condensation coatings, have also been studied in recent years. For example, there are reports in the United States and Germany about polyaniline anticorrosive plastic coating, which can also be applied to most metal equipment, and has anticorrosive function, extending the life of materials by 10 times. Lu, Pang Jia and others have studied a kind of coating for anti-condensation plastics. After coating and drying, the coating can form an insoluble and heat-sealable anti-condensation coating on polymer films.

3.2 Development of Waterborne Plastic Coatings

Due to the requirements of environmental protection and resources, in recent 10 years, people have demanded to limit the use of high VOC and toxic coatings, and the European Union even formulated a standard to completely ban the production, sale and use of organic solvents in 2004. It has promoted the development of coatings industry to low-toxic environmental protection coatings represented by water-based coatings, high-solid coatings, light-cured coatings and powder coatings, among which water-based coatings are the most important. If China wants to connect with the world, it must also work hard on water-based, pollution-free and low-cost plastic coatings. As early as the end of 1980s and the beginning of 1990s, developed countries such as Europe and America started the research and development of water-borne plastic coatings, and so far they have a large number of patents. The research and development of waterborne coatings in China started late, and there are few reports about waterborne coatings for plastics. A waterborne acrylic resin for plastics was developed in Zhang Jinglin, and the corresponding silver-white, phthalocyanine blue and iron red coatings were prepared. The performance reached various standards. Polymer Chemistry R&D Center of Chengdu Institute of Organic Chemistry, China Academy of Sciences has done a lot of work in the theoretical basic research of key technologies of waterborne plastic coatings and the pre-research of waterborne plastics, and has taken the lead in preparing polymer nano-aqueous dispersions by soap-free emulsion polymerization technology in China. The prepared polymer nanoparticles have a particle size of 30-80 nanometers and are evenly distributed. Chengdu Cohen Fine Chemical Co., Ltd. uses the unique new technology of synthesizing polymer nano-materials to prepare water-borne polymer nano-dispersion resin, and with corresponding fillers and additives, the project of "Green Nano-waterborne Plastic Coatings" has entered the pilot and industrialization stage. The main performance has reached the advanced level of similar products abroad, filling the gap in China, and the product and preparation technology are at the leading level in China. At present, it has begun to cooperate with Changhong Group Electric Appliance Co., Ltd. to develop Changhong TV shell coating.

4 conclusion

In recent years, with the enhancement of people's awareness of environmental protection, the development of water-based plastic coatings will certainly meet the requirements of the times and eventually replace solvent-based coatings. However, at present, the adhesion of water-based plastic coatings to plastics is very poor, far less than that of solvent-based coatings, which restricts its development. How to solve this problem will be the primary task of paint researchers. In order to solve this problem, many researchers began to explore a new synthetic process to prepare modified composite waterborne resin to replace the traditional synthetic process and single resin.