I. Selection of different coating methods for different optical film products
1. Coating method for products with microstructured optical films
(1) Prism film is also called brightness enhancement film (BEF). The coating feature is that the prism film coating thickness is: substrate: PET film
38 ~ 250 μm, the coating method is roller coating, and the best choice is three-roll coating.
10~20um ，技术规范给出的厚度误差为±5%，减去基材厚度误差的1%后，还剩下4%，厚度误差小（<2um），一般采用刮刀辊涂布，涂布厚度调节范围大，适应涂布厚度10~30um，涂料粘度20~100s的各类胶水的涂布。 (2) Diffusion film (including upper diffusion film and lower diffusion film). The coating thickness of diffusion film is usually 10-20um. The thickness error given by the technical specifications is ± 5%. After subtracting 1% of the thickness error of the substrate, The remaining 4%, the thickness error is small (<2um), generally using a doctor blade coating, coating thickness adjustment range is large, suitable for coating thickness of 10 ~ 30um, coating viscosity of 20 ~ 100s for the coating of various types of glue. Metering roller coating, wire roller and anilox roller coating, suitable for coating all types of glue with a thickness of about 10um and a viscosity of less than 20s. It is best to use metering roll coating and slit coating.
2. Coating method of optical film products for flat coating
(1 ) Hardened film: LCD anti-reflection film AR, LR (anti-glare AG), scratch-resistant and wear-resistant layer on the surface of IMD mobile phone casing. Coating characteristics: small and uniform coating amount. 0.5-50μm之间的厚度是正常的，在1-20μm和2-10μm之间是比较合适的，但最好是在3到7μm之间。 The thickness of the hard coat layer is normal between 0.5-50 μm, and it is more suitable between 1-20 μm and 2-10 μm, but preferably between 3 and 7 μm. Coating can be selected: micro gravure, slit coating, comma scraper, etc. The best coating method is to adapt to the glue widely, and is equipped with two coating heads, micro gravure and slit coating.
Second, Songde company hardened film (anti-scratch film) high precision coating machine technology introduction
Features applied to IMD anti-scratch film: It has good scratch resistance and high hardness requirements. If it is expressed by pencil hardness, the general optical film is 3H, and the enhanced optical film requires 4H ~ 5H hardness.
The coating machine design process flow follows:
(1) Unwinding of double-station PET optical film substrate → automatic roll change cutting → surface corona treatment → correction → traction and tension separation → micro gravure coating head (used when the amount of glue is small and the viscosity is low) → narrow Slit coating head (used when the amount of glue is large and the viscosity is high) → vacuum suction roller traction → 21M hot air drying → UV curing → correction → compound with protective film → inkjet printing → rewinding Protective film unwinding → Correction →
(2) After the coating of the hardened layer is completed, apply silica gel on the back → compound with release film
Coating form: MICRO GRAVURE + SLOT DIE
Coating substrate: PET: 50u-188u
Coating speed: 10 ~ 50m / min (depending on process conditions)
Mechanical speed: 80 m / min
Maximum substrate width: 1100mm
Unwinding diameter: Φ800 mm
Rewinding maximum diameter: Φ800 mm
Tension range: 5 ~ 50Kg / full width
段，长21m Maximum oven temperature: 180 ℃ 3m x 7 sections, length 21m
Inner diameter of substrate core: 3 ″, 6 ″
Intelligent control twelve-axis vector variable frequency drive
Machine power: about 110kw
Drying heat source: electric heating, heating energy consumption 630,000 kcal
Machine weight: about 50 T
1.Micro Roller System
Micro gravure coating technology is suitable for solvent-based medium and low viscosity coating liquids.
Viscosity range: 10 ~ 800 cps
Coating amount: 5 ~ 80 g / m²
Coating thickness 1 ~ 40um Wet coating thickness, occasionally can be coated to 0.5 ~ 80um
Coating substrate: PET 1.6 ~ 350 um.
Line speed: 5 ~ 40 mpm
Micro gravure coating can provide users with a variety of adjustment methods. The position of the two guide rollers determines the contact angle between the microgravure and the substrate, and the size of the wrap angle and the line contact are adjustable.
Comparison of micro gravure coating method and traditional gravure coating method :
Anilox roll diameter
20 ~ 50mm very small line contact with substrate
20mm diameter at 300mm width, 50mm diameter at 1600mm width
125 ~ 250mm
No pressure roller, belongs to kiss coating method
With pressure roller
In forward coating, part of the coating liquid in the cells is transferred to the substrate, and part of the coating liquid remains in the cells.
The scraper is soft and light pressure,
1 ~ 40um wet coating thickness
Wet thickness 1 ~ 50um
1 ~ 1500mps
The micro gravure coating method is a new technology developed in order to meet the requirements of a small and uniform coating volume. It is simple, reliable, and repeatable.
The surface of the gravure roll is engraved with patterns or grooves. High-precision bearings are installed at both ends of the gravure roll. The roller surface is partially immersed in the feed tank. One end is connected to the servo motor through an elastic coupling. The motor drives the gravure roll to rotate from the liquid tank. After coating, the material on the gravure roller pattern step is wiped clean after being quantified by a flexible steel doctor blade. A certain volume of coating liquid remains in the groove. After the contact, the coating liquid forms a wetting line at the contact point. The coating liquid wets the substrate under the action of surface tension. The coating liquid in the groove is taken out due to the viscous pulling force, and then transferred to the surface of the substrate and then wetted. Wet and spread to achieve uniform thin-layer coating.
Standard gravure (anilox roller) coating can be divided into forward coating and reverse coating, but both must use a rubber pressure roller with a diameter approximately the same as that of the anilox roller. The coated substrate passes between the anilox roll and the pressure roll. The problems of wrinkling and disconnection are often encountered. In addition, the contact surface of the pressure roller will also bring some mechanical, hydraulic and pressure problems, which will affect the coating quality.
The small diameter also makes the contact area between the microgravure and the substrate small. Different diameters will obviously form different wrap angles. Generally, coating problems caused by larger contact surfaces are much larger than small contact surfaces.
In traditional gravure coating, there is a coating liquid accumulation on the substrate in-coating point and out-coating point, and these drops will cause the coating cycle to be disordered. The larger the diameter of the traditional anilox roller, the larger the interference dripping, and the greater the adverse effect on the coating. Coupled with the pressure roller, the pressure roller may appear inclined, or other mechanical problems, it is more difficult to guarantee the coating quality.
The microgravure plate basically eliminates the adverse effects caused by the pressure roller because there is no pressure roller, and the problem of dripping is basically eliminated because the diameter of the roller is small. The dripping amount at the entry point and the exit point is rarely stable.
Elastohydrodynamic Contact System
When the physical system involves both fluid friction caused by fluid flow and elastic stress generated by deformable solids. And this deformed solid becomes the boundary of fluid flow. This system of interaction is called elastohydrodynamic systems (EHD).
In the gravure coating method, the substrate is equivalent to a deformable solid, and the coating liquid is the fluid therein. The micro gravure coating method can achieve a highly uniform thin layer coating. Because the micro gravure coating method uses an elastic hydrodynamic contact system, the coating gap is no longer affected by the back roll processing accuracy, and the coating gap is mainly composed of the substrate. The tension is determined, and the gap can be changed with the change of the surface of the substrate to achieve contour coating.
Common concepts include:
(1) Wetting line, (2) wrap angle, (3) upstream and downstream boundary, (4) speed ratio s, (5) transfer rate.
Micro gravure coated meniscus
In the research of gravure coating process, N. Kapur et al. Found that other conditions are unchanged:
H = 1.34R （μv / σ） 2/3
Among them: H—wet coating thickness; R—bead meniscus radius; μ—viscosity; v—fluid velocity; (σ) surface tension. Blok and van Rossum (1953) covered the film with a roller at 180 ° and used oil as a lubricant. The gap between the film and the roller was H = 0.43R (6 μv / N) 2/3.
Among them: R—roller radius; N—film tension; μ—viscosity; v—roller speed (linear speed). Although this formula does not directly provide the thickness of the liquid film, H as the gap between the film and the roller is an important factor affecting the thickness of the coating. We verified through this test that the coating tension is an important factor affecting the thickness of the coating. Based on the above two formulas, it is not difficult to draw the following conclusions: h∝R (μv / σN) 2/3
Among them: h—wet coating thickness; R—gravure roll radius; N—film tension; μ—viscosity; v—roller speed (linear speed); σ—surface tension of coating liquid. This formula comprehensively summarizes the influencing factors of the thickness of the micro gravure coating, and the influence relationship of each factor on the thickness, which has guiding significance for testing and production.
Micro gravure coating process
Micro gravure coating technology is already a relatively mature coating technology abroad. Its operation control basically uses automatic electrical control. The general control process includes: scraper advancement and retreat, scraper depression and lift, cantilever depression and lift, The kiss roller is pressed down and lifted, and the coating roller speed (or speed ratio s) is adjusted. The manual mechanical adjustment part on the coating head includes: blade angle, pressure, and wrap angle adjustment. Once these mechanical adjustments are determined, they will generally not change. Only minor adjustments will be made when there are apparent defects in the coating. In addition, the drag control part is basically the same as the other methods, but the proposed control accuracy will be higher, which is related to the performance requirements of the product. Generally, the uniformity requirements of micro gravure coated products are high.
Coating thickness control
Micro gravure coating is not a precise predetermined amount of coating, and is different from slope, curtains and slits, and its coating amount should be determined according to the material and coating process conditions.
First, the coating amount of micro gravure coating is mainly determined by the mesh volume of the gravure roll. The influential factors of the gravure roll itself on the coating liquid transfer rate are: the number of anilox roll lines, the ratio of the opening to the screen wall, the shape of the cell, the depth of the cell, the surface finish of the cell, and the angle of the line, which are directly related to the production of the gravure roll. .
The relationship between the material removal rate from the cavity and the speed S of the gravure roll line and speed.
The relationship between the percentage of the coating liquid taken out from the mesh and the ratio S
It can be seen that in the case of a fixed S ratio (between 1 and 1.5), the coating thickness can also be increased by appropriately increasing the wrap angle, but this adjustment is minor, and the wrap angle is too large, it is easy to generate eddy currents, thereby trapping air. Or particles cause drawing ills.
Relationship between coating thickness and ratio S
h∝R (μv / σN) 2/3 This formula comprehensively summarizes the influencing factors of micro gravure coating thickness and the influence of each factor on the thickness. Other conditions are certain. The coating thickness is proportional to the viscosity and is proportional to the surface tension. Inversely. The μv / σ in the formula is just a measure of the capillary number, so as the capillary number increases and the coating thickness increases. Increasing the coating tension N in the formula will reduce the coating gap and eventually thin the coating. Therefore, in the actual production and test process, it is necessary to constantly explore the coating of each coating roller for different characteristics of materials and coating process parameters. Layer thickness, accumulated experience, in order to select the gravure roller that meets the thickness requirements in a short time.
The coating conditions in the micro gravure coating method are such that the number of screen lines on the surface of the entire gravure roll is 50 to 800 lines per inch, and 100 to 300 lines per inch is very suitable. The engraving depth of the network cable is possible from 1 to 600 μm, and more suitable from 5 to 200 μm. The rotation speed of the microgravure roll is possible between 3 and 800 rpm, and it is best between 5 and 200 rpm. The substrate transfer speed is possible between 0.5 and 100 meters per minute, and preferably between 1 and 50 meters per minute.
Machining accuracy required by microgravure: tolerance of behavior
The relationship between the number of microgravure lines and the amount of glue:
Number of lines
Wet coating thickness μ *
Number of lines
Wet coating thickness μ *
Note: * The coating amount is determined by the speed difference of the micro gravure speed ÷ linear speed.
Defects of the micro gravure coating method
The application of the micro gravure coating method in the field of high-precision thin-layer coating is unmatched by most coating methods, and it is currently the most widely used in the thin-layer coating field, and its position cannot be shaken. But the micro gravure coating method also has its own disadvantages:
(1) The so-called pre-determined coating is relative, not the exact pre-determined coating. There are many factors affecting the thickness of the coating. To achieve the precise pre-determined coating of a certain product, a gravure roll made by a fixed manufacturer is required To ensure the stability of the material characteristics and the coating parameters to be determined through repeated tests;
(2) The gravure roll should be replaced for different coating thicknesses, and the gravure roll itself has a service life;
(3) The coating speed is generally not more than 40m / min, and the production efficiency is not high.
Micro-gravure coating head and purification room that Songde has installed
2.Slot Die System
Slit coating is a more efficient and more controlled coating method than roller coating and other traditional coating methods.
The slit coating technology has the following characteristics:
1) Can be applied with a thickness of 1 ~ 100µ; 10 ~ 100gsm (wet coating amount); linear speed: about 300mpm; for optical film coating, the coating speed of extrusion coating is about 30m / min is more suitable .
2) Can apply discontinuous mode.
3) Can be applied in a large area at one time without staining caused by different edge thickness.
4) Easy to clean and disassemble.
5) Suitable for a wide range of viscosity: 500 ~ 5,000 cps.
Slot die, die positioning device, vacuum box, vacuum supply system, stream delivery system. The height of the positioning device is fixed, and the angle between the die head and the back roller is adjustable. It can be adjusted within 90 ° or 10 ° parallel to the back roller. Linear slide guides and pneumatic devices are provided in three positions to ensure the precision and accuracy of die movement.
An indicator is installed at the end of the positioning device to ensure the accuracy and repeatability of the measurement of the positioning device. The indicator can measure the thickness difference of 2.5 micrometers in order to precisely determine the current coating position.
Our comparative analysis of SLOT-DIE horizontal and vertical coatings found that due to the difference in coating defects and the coating mechanism, vertical coating is more effective in low viscosity operations, but high viscosity. The horizontal coating is relatively stable at the time, and the demarcation point between the two is about 75 mPa · s.
The angle formed by the lip of the slit die and the conveying direction of the film on the back roll is good at about 30 ° or about 90 °.
Effects of vacuum evacuation
How does the vacuum pumping method affect horizontal or vertical coating? We found that it can effectively reduce the thickness of the coating, especially on the horizontal coating.
The stainless steel flow delivery system is placed on a custom movable vehicle, including: screw pumps, explosion-proof motors, controllers, filtration systems, pressure indicators, valves and link hoses, and pressure-stable fluid delivery systems to ensure Uniform coating.
3 , drying technology
Features of hardened film coating oven:
1. Since the coating of the hardened film is basically a single-sided coating method, a solvent coating solution is used, and a hot air drying oven is usually used for drying. An ordinary roller convection oven is sufficient, and an air-floating oven structure is not required. Can save costs.
2. Oven length: 18 ~ 24m, 3 meters per section.
3. The production method to meet the cleanliness requirements: the overall stainless steel oven; the connection part is fully welded and passivated; there are no right angles, all are rounded, which is easy to clean.
Dry the coated solution with a dry air stream in the first drying zone. The maximum wind speed on the surface of the coating film is about 1m / s, and it is more suitable at about 2m / s and about 30m / s. It is best at about 3m / s or about 20m / s.
When the wind speed is less than 1m / s, the difference in drying rate in the width direction of the substrate will become obvious, which will cause drying unevenness, prolong the drying time, and ultimately reduce production efficiency. When the wind speed is too high, the generated wind pressure will blow away the coated solution.
There is no particular restriction on the direction of the dry air flow. The minimum value of wind speed is more suitable at 0.1m / s.
The temperature of the dry air stream can be between room temperature and 200 ° C, and it is more suitable between room temperature and 150 ° C, and the best temperature is between room temperature and 100 ° C.
The second drying stage is performed in the second drying zone, and its temperature is about 50 ° C higher than that in the first drying zone. If this temperature difference is about 60 ° C, it is more suitable, and about 70 ° C is best.
The drying temperature is preferably equal to or lower than the temperature at which the coating solvent starts to evaporate. For example, commercial optical initiators can be used with UV-curable resins. When dried under hot air at 120 ° C, their content will evaporate by several tens of percentage points in a few minutes; mono- or difunctional acrylates When the monomer is dried in a hot air at 100 ° C, its content will also drop sharply. In the above-mentioned cases, the temperature is preferably controlled to be equal to or lower than the temperature at which each coating component starts to evaporate.
4 , curing technology
Cured by UV electrodeless lamp. After passing through the drying zone, the coating film will enter the curing zone, which cures the coating film by ionizing radiation or heating. Ion radiation is not limited as long as the compound can be activated by ultraviolet rays, electron beams, and r rays to crosslink and cure. From the perspective of ease of operation and obtaining high energy, ultraviolet light is most suitable.
5 , coating tension control method
Micro gravure and slit coating unique tension control method: the vacuum suction roller is used as the driving traction roller position, instead of the traction roller / press roller, the position is located after coating and before drying .
Vacuum rolls have suitable roll surfaces to suit a variety of substrates. For example: completely meshless surface, stainless steel (chrome steel) mesh; rubber surface layer of different surface materials, which also has adsorption holes drilled on it; plasma ceramic surface layer. For optical film coating, a vacuum suction roll with a stainless steel mesh surface should be selected to avoid the formation of imprints on the wet coating caused by the deformation of the substrate due to the suction holes, which damages the uniformity of the coating.