PET polyester film is a biaxially oriented film made from polyethylene terephthalate polymer of the polyester family. Optical grade PET polyester film has high transmittance, low haze, high brightness, no yellowing, good adhesion, good flatness, no bright and dark lines, high temperature and ultraviolet radiation resistance, good stiffness, anti-burning, not easy to break, etc., and has been widely used in recent years. Especially in the field of LCD production, it is an indispensable raw and auxiliary material, for example, for LCD, at least 7~8 PET type optical films with different functions can be used (2 light diffusion films, 1 brightening film, 2 phase difference films, 1 anti-glare film, 1 shielding conductive film). These functional PET-type optical films are mainly obtained by applying various functional coatings to the surface of PET polyester films. Therefore, the development of various functional coatings for PET-type optical films in the field of liquid crystal display is one of the important directions of optical film research.

　　Generally, the substrates of optical films should be flat and defect-free, with high gloss and light transmission, low haze, and good coating performance. However, if the substrate is flat and defect-free, there are fewer active points of contact between the coating and the surface of the film substrate, which makes it difficult for the coating liquid to penetrate and form adsorption with it; if the substrate has high gloss and light transmission, the surface roughness of the substrate is low, which leads to fewer bonding points between the coating liquid and the surface of the film substrate. Meanwhile, the characteristics of PET polyester film itself, such as the highly oriented crystalline surface of PET film, make it difficult for the coating molecules to penetrate into the PET film, which also makes it difficult to form a good interfacial bond with the coating. Therefore, in the preparation of PET optical film, it is another important direction for optical film research to solve the problem of interfacial bonding between the substrate film and the coating.

　　Research Status of PET Optical Film Coating Technology

　　The coating technology of PET optical film is the key technology of optical film. According to Coating Online, according to the role played by the coating, the coating can be divided into three categories, namely, protective coating, decorative coating and functional coating. By applying different coatings on PET polyester films, different functional films can be produced. For example, the surface of PET polyester film is coated with anti-scratch and anti-static (106~108Ω) coatings to make screen protection film for cell phone or computer screen surface protection; PET polyester film is coated with coatings containing optical particles or glass beads to make optical diffusion film, which can be used for LCD panel backlight module to effectively eliminate light and dark interlacing or dot phenomenon and enhance light brightness. It can be used for LCD panel backlight module, which can effectively eliminate light and dark streaks or dot phenomenon, enhance light brightness, so that the light emitted from the light guide plate can be softened, and provide uniform surface light source for LCD panel; coating resin system with particle composition on the surface of PET polyester film can be made into anti-glare film, which can be used for LCD panel, with the effect of preventing screen trailing by using the scattering of reflected light and internal scattering generated by the refractive index difference between hard coated resin and particles, etc. The technologies related to coating are mainly coating formulation technology, coating curing technology and coating process technology.

　　1.1

　　PET-type optical film coating formulation technology

　　PET-type optical film coatings for LCD field are mainly silicone, polyurethane and acrylate coatings or their modified derivatives according to the main resin used.

　　01

　　Silicone coatings

　　Silicone materials are generally composed of silicone oxygen links as the basic structural unit, and the side chains can be connected with various other organic groups through silicon atoms. This special composition and molecular structure makes it combine the characteristics of organic materials with the functions of inorganic materials. Silicone coatings are often used as transparent wear-resistant coatings because of their good light transmission. This kind of coating has sufficient flexibility, and has good bonding performance with the base material; there is no double bond in the main chain, therefore, it is not easy to be decomposed by ultraviolet light and ozone, even if exposed to outdoor or high ozone concentration environment for a long time, it will not crack and viscous creep, and the physical and electrical properties are basically unchanged; resin type silicone coating material will not yellow even under strong ultraviolet light, and its Color can remain unchanged for many years, so many scientists have carried out research in this area. According to the coating online, if a variety of nanoparticles are introduced into the coating as an enhancer, the wear resistance and denseness of the coating can be further improved.

　　The methacryloxypropyltrimethoxysilane (KH570) hydrolysis polymerization product as the main film-forming substance, the introduction of ethyl orthosilicate (TEOS) hydrolysis product silica sol as inorganic reinforcement, adjust the pH value of the two mixed solutions, the use of the co-condensation reaction between the two hydroxyl groups on the surface of the substrate to prepare organic / inorganic composite transparent wear-resistant coatings.

　　The performance of the organic/silicone anti-fog and wear-resistant coatings was investigated using γ-epoxy propyl trimethoxysilane (KH-560, industrial grade), ethyl orthosilicate (TEOS, CP grade) and diethanolamine as the main raw materials. The results showed that the fogging, abrasion resistance, transparency, adhesion and rupture resistance of the obtained coatings were excellent. The surface is homogeneous and flat, the pencil hardness can reach 3H~4H, and the contact angle with water is about 10°.

　　A hard coating containing a fluorine-siloxane graft polymer and UV-curable resin is applied to the transparent film substrate. The weight ratio of fluorine-siloxane graft polymer to UV-curable resin is preferably0.05:100~5.00:100, and the hard coating is treated with alkali saponification. The hardness of the pencil is tested to 2H~8H. The coating can be used as a base coat for anti-reflective films when a high refractive index silicone is selected.

　　Nitric acid is added to the combination of sodium acetate, tetraethoxysilane, methyltriethoxysilane, isopropanol and ethanol and a catalytic reaction is carried out for 2h to obtain silica sol, which can be used as an abrasion and scratch resistant coating.

　　02

　　Polyurethane coating

　　Polyurethane coating molecules contain not only a considerable number of polyurethane bonds, but also urea bonds, ether bonds, ester bonds, and urethane bonds, and the cured coating film has good abrasion resistance, corrosion resistance, chemical resistance, temperature resistance, and elasticity.

　　A coating can be made by dissolving dipping elastic powder with xylene, then adding butyl ester, diacetone alcohol, cyclohexanone, matting powder, hand feeling elastic paste and anti-settling agent, then adding elastic resin and leveling agent, and after coating, and conducting performance test. The results show that the coating has good adhesion, fine and smooth handfeel, strong scratch resistance and excellent corrosion resistance. By adding different particles to the coating, a diffusion film or a brightening film can be prepared.

　　By dehydrating polyether diol at high temperature and then adding TDI and catalyst, a polyurethane prepolymer can be obtained; then adding dimethyl silicone oil, epoxy resin, chain extender, crosslinker, plasticizer, defoamer and filler in turn, a polyurethane coating modified with silicone and epoxy resin can be obtained. The coating film of this coating has high mechanical strength, good adhesion, low water absorption, good thermal stability and acid and alkali resistance.

　　The UV-curable acrylate polyurethane was prepared by introducing acrylate groups into the side chains of polyurethane using hydroxyethyl acrylate (HEA) and homemade dihydroxyacrylate as raw materials. The coating film has better heat resistance, solvent resistance, pendulum hardness and mechanical properties than the polyurethane coating film without acrylate monomer.

　　Diffusion films can be prepared by applying a resin coating to a substrate such as PET film. The coating uses a polyurethane resin to which one or more organic or inorganic particles with a single particle size distribution are added as diffusion particles with a particle size of1to 50 μm, and modified polysiloxane (polysiloxane) or polycarboxylic acid (polycarboxylic acid) is used to improve the dispersion effect.

　　03

　　Acrylic coatings

　　Acrylic coatings for optical films are generally UV-curable (UV-curable coatings for short). UV curing coating materials (Ultraviolet Curing Coatings) as an energy efficient and environmentally friendly modern green coatings, has been maintaining a rapid growth momentum. Compared with the traditional natural drying or heat curing coating, UV curing coating has the advantages of high crosslink density, fast curing speed, energy saving, excellent coating film performance, wide range of substrates, etc. Another advantage is that it does not contain volatile solvents, thus helping to eliminate VOC (Volatile Organic Compound) pollution of the environment, and also saves It also saves a lot of organic solvents and reduces production costs, making it an important curing process for optical film production. In recent years, there have been many related studies at home and abroad that acrylic resins have good wetting and dispersing properties for inorganic nano-metal oxide and organic polymer particles, in addition to high crosslinking density.

　　1.2

　　Coating process technology

　　Unlike ordinary protective films or tapes, optical films must be of uniform thickness, and the surface must be dust-free and less crystalline, which places higher demands on the coating technology and requires a more uniform film thickness. Therefore, the precision coating process has been gradually developed and improved. Optical film coatings are traditionally prepared by vacuum vapor deposition, chemical deposition, plasma polymerization, etc. These methods are difficult to achieve mass production of rolled film substrates; in modern coating processes, the commonly used coating methods are dip coating, roll coating, slope flow extrusion coating, and drop curtain coating. These coating methods are each suitable for different properties of the material and coating thickness, and have their own advantages and disadvantages. Such as dip coating of the coating amount is not easy to control, by the characteristics of the material and temperature; roller coating of the coating thickness is easy to control, the coating amount is easier to control, uniformity is better, but by the material characteristics, adapt to the material viscosity range is narrow; slope flow extrusion coating although the coating amount is easy to control, but adapt to the material viscosity range is narrow, the coating amount is too small is not easy to coat, and can not be coated to the edge, resulting in Substrate waste; drop curtain coating is a pre-volume coating method, its operation is easy, longitudinal and transverse coating thickness are very uniform, coating no scratch, stripes, orange peel pattern, but drop curtain is easily affected by factors such as vehicle speed, therefore, stable drop curtain is a big problem.

　　The thickness of the currently used coating has a wide range of variation, some coatings are very thin, and the surface requires uniformity and smoothness. In order to increase the production capacity, the coating speed should be high, so some research institutes have started to study micro gravure roll coating and strip slit type coating. Micro gravure roller coating has the advantages of easy operation, wide range of coating volume, substrate saving, wide range of thickness adaptation of substrates, good coating appearance presentability, smooth and glossy surface, etc. Slit coating is a pre-volume coating method with uniform coating, large size coating and ultra-thin layer coating. These two coating methods are the new precision coating process technology developed after the precision coating process technology such as primary multi-layer slope flow extrusion coating and drop curtain coating, which is gradually applied in flat panel display, optoelectronic products, lithium batteries and other related product preparation.

　　Micro gravure coating method was first proposed by Japan&39;s Fuji, which mainly coated high-end functional films, and had used micro gravure coating method in its anti-reflective film preparation process, others such as Konica Minolta, Hitachi Maxell, etc. also used micro gravure coating equipment in related products.

　　The strip seam coating method was also first developed and applied by Fuji in Japan. Fuji has prepared an anti-reflective film with a 4-layer structure, i.e. substrate/hard layer/medium refractive index layer/high refractive index layer/low refractive index layer, preferably with a slit coating method. Huang Shang-Hong has studied the design of the slit coating nozzle cavity and discussed the role of each component of the slit coating nozzle, pointing out the key factors for designing and manufacturing the slit coating nozzle, such as the construction of the distribution cavity to make reasonable pressure distribution, reduce vortex, and adapt to the slit, setting the sub-cavity as needed, and making the material retention time in the coating nozzle the same.

　　The micro gravure coating system and slit coating system is a precision coating system involving elastic fluid dynamics, which needs to be coated in an environment with high requirements for coating cleanliness.

　　1.3

　　PET-type optical film coating curing technology

　　The curing methods for the preparation of optical films can be divided into thermal curing and UV curing. Thermal curing is the more traditional and common curing method. Generally speaking, thermal curing is not easy to produce serious volume shrinkage due to rolled edges, thus producing shrinkage stress and affecting the coating strength. However, thermal curing requires a long time cycle, labor-intensive, for solvent-based coating solution is also prone to environmental pollution problems. UV curing is summarized as having "5E" characteristics, namely, high efficiency (Efficient), wide adaptability (Enabling), economic (Economical), energy saving (Energy Saving), environmentally friendly (Environmental Friendly). The disadvantages of UV curing are: free radical type light curing coating curing process is easily inhibited by oxygen, the surface curing is not good; UV wavelengths are different, the depth of irradiation is not the same, so it will make the internal coating is not fully cured, so that the coating and the substrate interface at the bonding strength is not enough.

　　At this stage, some people often use the combination of heat curing and light curing dual curing method, that is, light curing to make the system quickly set or to achieve "surface dry", and then use heat curing to make the "shadow" part or the bottom part of the curing completely, to achieve the system "For solvent-based coating solution, it can also be placed at high or medium temperature for a few moments to make the solvent evaporate and then placed under UV light, thus avoiding solvent residue in the coating system and affecting its adhesive strength or other properties, and shortening the curing time and improving efficiency.

　　Study on improving the bonding performance of PET polyester film/coating interface

　　Untreated PET polyester films are difficult for coatings to bond tightly with. Therefore, PET polyester film/coating interface bonding technology has become the primary problem for domestic and foreign scholars to study the functional application of coated PET polyester film. Not much research has been done in this area, and from the only reports available, it appears that the main focus is on the following aspects.

　　2.1

　　Matching coating materials for PET polyester films

　　The main component of PET polyester films is polyethylene terephthalate polymer, whose chemical structure is

　　Picture

　　The chemical formula shows that there are two hydroxyl- OH at the ends of the macromolecular structure and an aromatic ring in the middle, which are connected to each other by ester bonds. When we choose the raw material for the coating, we can introduce into the coating solution containing -COOH, -SO3H, -POH, -NHC=O, -C=O, -NHR -, -OH, -SH, etc. These functional groups can easily react with polar groups such as hydroxyl groups and ester bonds in PET molecules to improve the interfacial bonding strength.

　　Almost all surface coating techniques are based on the prerequisite that the coating solution can be wettable on the substrate surface for its bonding. According to the Sell- Neumann equation, when the interfacial tension of the film substrate/coating system tends to 0, the contact angle of the system interface will also tend to0, when the interfacial bond strength is at its highest. Therefore, in the actual operation process, the interfacial tension of the coating solution is adjusted by selecting the components of the coating solution and adjusting the ratio of each component so that the surface tensions can match each other.

　　2.2

　　Surface treatment of PET polyester films

　　The purpose of polymer surface modification is to change the appearance of polymer surface morphology, supramolecular structure, defects or stress state, chemical composition, etc., remove dirt, increase surface area, remove weak boundary layer, so as to improve the mechanical, chemical, mechanical, optical, electrical and other properties of polymer surface, enhance the function of polymers and expand their applications.

　　PET polyester film has low surface tension and low affinity to the coating solution. Moreover, PET polyester film has high crystallinity and is not easily swollen by the coating solution after coating and intermolecular diffusion occurs, thus the adhesion is relatively poor. Therefore, the surface treatment of PET film substrate can improve its surface tension, reduce its surface crystallinity and improve its surface bonding performance. The common treatment methods for PET polyester films at home and abroad include chemical oxidation, photochemical treatment, plasma treatment, corona discharge treatment and undercoating treatment. Photochemical treatment and corona treatment are time-sensitive and expensive; primer treatment is not affected by changes in temperature and humidity and is suitable for product line production.

　　A study has investigated the effects of acetone cleaning surface treatment, chemical treatment and Co60 irradiation treatment on the surface of PET polyester film on the content of major elements, adhesive peel strength and water absorption. The results showed that the durability of the bonded joints of the polyester film treated with acetone cleaning was lower than that of the chemical treatment, while the durability of the bonded joints of the polyester film irradiated with Co60 was the best.

　　The surface grafting of PET polyester films with acrylic acid was modified by UV light using benzophenone (BP) as photoinitiator. After the grafting reaction, carboxyl groups (-COOH) were introduced to the surface of the PET polyester film. The results of the surface contact angle measurement showed that the contact angle of the PET film surface decreased gradually with the increase of the grafting rate.

　　An aqueous coating solution for optical polyester films containing 1.5% wt polyester resin,1.0% wt melamine resin, 5.0% wt acrylic resin and a small amount of surfactant was used on the PET polyester film. After coating, the polyester film obtained has high transparency, low haze, excellent adhesion and slipperiness, and is suitable for application on substrates for optical applications such as diffusion films and brightening films.

　　By coating PET polyester film with a coating solution containing acrylic adhesive, silicon wetting agent, colloidal silica, and melamine hardener, the resulting PET film has excellent adhesion strength to the post-treatment processing layer and is more suitable for optical film applications.

　　According to coating online, there are many other factors that affect the interfacial bond strength, such as interfacial residual stress, coating thickness, curing temperature, curing time and post-curing.

　　Prospect

　　(1) With the increasing demand for functional and environmentally friendly coatings for liquid crystal displays, PET-type coatings for optical films will develop into high-performance, multi-functional and environmentally friendly coatings, which is an important basis for the research of various high-performance optical films.

　　(2) The curing method of PET optical film coating is developing from the original single curing method to double curing method, such as light - heat curing, heat - light curing, light - moisture curing, light - oxygen curing and other curing methods are gradually studied and applied, free radical - cation and other mixed light curing system also has research value.

　　(3) Develop new surface treatment technologies for optical PET films to improve the bonding performance of PET film/coating interface.

　　(4) Continue to seek new precision coating technology to adapt to the competitive market model of low cost, high quality and mass production, which is an inevitable requirement for future research and development.