Perspective coating technology
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First. The concept and significance of film
The film, that is, the film, is a processing technique in which a plastic film is coated with a binder, and the paper print is heated and pressurized to bond it together to form a paper-plastic product. The film-coated printed matter has a thin and transparent plastic film on the surface, and the surface is smoother and brighter, thereby improving the gloss and fastness of the printed matter. The graphic color is more vivid, three-dimensional, and waterproof. Anti-fouling, abrasion resistance, folding resistance, chemical corrosion resistance, etc.
Second. Introduction of film classification
There are three types of filming processes: dry, wet, and oily. According to the raw materials and equipment used, it can be divided into coating film and pre-coated film process.
1. Coating film
That is, the coating film process is first applied with a binder on the film, and then hot pressed, but the production process is prone to fire.
2. Pre-coated film
Because the laminating equipment does not need adhesive heating and drying system, the laminating process greatly simplifies the laminating process, and the operation is very convenient. It can be used with the start-up, and has great production flexibility, no solvent smell, no environmental pollution, and more importantly, it can be completely Avoiding bubbles, delamination and other failures, the finished product has extremely high transparency, and has broad application prospects and promotion value.
The pre-coated film structure is composed of a substrate and a glue layer. The substrate is usually a PET film and a BOPP film. From the viewpoint of material cost and processing technology, most of the pre-coated film substrates are BOPP film, and the thickness is 12-20 micrometers. The thickness is 5-15 microns. According to different processing equipment and process conditions, different thicknesses of glue layer are selected, and the glue layer is divided into hot melt adhesive and organic polymer low temperature resin. The difference between the two is that the hot melt adhesive is made by blending and modifying several materials of the main adhesive resin, the tackifier and the modifier, and the organic polymer resin is a single polymer low temperature copolymer.
Due to limitations in technology, production equipment, and raw materials, pre-coated films (mostly hot melt adhesives) produced by domestic equipment still have quality defects. Since the production process is a process similar to that of the coating film, the colloid is dissolved in an organic solvent, and the rubber is roll coated on the substrate film by a concave anilox roll, so that the solvent is not volatilized sufficiently, and an odor is generated during operation. After the film is coated, the solvent evaporates and the surface is easy to foam. At the same time, since the hot melt adhesive is blended by several kinds of polymer materials, the temperature control is not good, which will cause degradation and cross-linking of the high molecular polymer in the hot melt adhesive, and the surface of the film is poor. Some pre-coated film colloids easily adhere to the outer surface of the substrate, resulting in poor winding. Since the adhesive layer is thin and the surface is not activated, defects such as insufficient adhesion to the printed matter are likely to occur. Since the hot melt adhesive is composed of several kinds of materials, the transparency after coating is significantly inferior to that of the low temperature pure resin type precoat film.
3. Water-based film
It is a method of processing the surface of printed matter to improve the gloss and strength of the surface of the printed matter and enhance the aesthetic effect. To measure the quality of the film, it is also necessary to examine its brightness, strength, and adhesion. The water-based wet film is favored by customers for its high strength, easy recycling and non-polluting properties.
4. Curved draping film
It is a brand-new transfer technology that can cope with complex workpieces and is suitable for different materials. It also greatly increases the added value of products due to vivid patterns and vivid colors. This technology is a special chemical treatment film, after printing a certain color pattern, it is sent to the surface of the water, using water pressure to write the pattern evenly on the surface of the product, while the coated film is automatically dissolved, washed and baked. After drying, a layer of transparent protective coating is applied, and the product has a completely different visual effect.
The coating process can be divided into cold (cold pressure) coating, hot (heating) coating and liquid coating according to the heat, pressure and material. The thermal coating technology refers to heating and activating the pre-coated adhesive on the film, by which the film is bonded to the printed product; and the cold film is pressed directly to bond the film to the printed product by the adhesive. Together. Both methods can produce various special films such as UV protective film, waterproof or anti-friction film. Recently, there has also been a new technology called "liquid film". Simply use a brush, spray gun, roller or special laminator to apply a film directly on the surface of the product. Although the technical operation is simple and the price is low, it is still not mature.
Three. Material selection of film
Commonly used plastic films are: polyvinyl chloride (PVC), polypropylene (BOPP) and polyester (PET) films. Among them, BOPP film (15-20pm) is flexible, non-toxic, high transparency and low price, and is an ideal composite material in the film coating process.
Depending on the properties of the film itself and the purpose of use, the thickness of the film is preferably between 0.01 and 0.02 mm. It must be treated by corona or other methods. The surface tension of the treated surface should be 4 Pa for better wettability and adhesion. The corona treatment surface should be uniform and the transparency should be as high as possible to ensure the best definition of the covered print. The transmittance of PET is generally 88-90%, and the transmittance of other films is usually between 92-93%. The film needs to have good light resistance, is not easy to change color under long-time illumination, and the geometric size is stable.
Since the film is in contact with a solvent, a binder, an ink, etc., it must have a certain chemical stability. The appearance of the film surface should be flat, without unevenness and wrinkles, and the film is required to have no bubbles, shrinkage holes, pinholes and pitting. Due to the limited adjustment ability of the laminating machine, it is also required that the rewinding is neat and the two sides are elastic and uniform to ensure uniform coating. Of course, the cost is low.
Four. Composite strength
Refers to the bond fastness between the plastic film and the paper (board) print in the film product. The size depends on the adhesion between the plastic film, the paper (board) print and the adhesive (or hot melt adhesive). When laminating, the adhesion is mainly derived from the mechanical bonding force and physico-chemical bonding force between the plastic film, the paper (plate) printing article and the adhesive (or hot melt adhesive).
The key to film fastness is the affinity of the glue to the print (ink and substrate). As a film coating industry, printing inks are not selectable, that is, only the film that is suitable for the ink market is the only choice, so as a pre-coating film, only the performance of all inks can be regarded as high quality. The composite strength of the coated product is affected by the process parameters in the film production process, such as the temperature, pressure, speed of the coating process, the coating condition of the adhesive, the film tension and environmental factors. At the same time, it is affected by the printing process, such as the surface state of the printed product formed after printing, such as the thickness of the ink layer, the area and distribution of the image, the characteristics of the ink, the use of the thinner, the use of dusting, and the drying of the ink layer.
Five. Factors affecting the quality of the film
1. Printed ink layer condition
Refers to paper properties, ink properties, ink layer thickness, graphic area and printed graphic density. When the printed ink layer is thick and the image area is large, many paper fiber capillary pores are blocked, which hinders the penetration and diffusion of the adhesive, making the printed matter and the plastic film difficult to adhere, and delamination, foaming and the like are prone to occur.
2. Ambient humidity
Between the adhesive and the plastic film and the printed matter, the water content changes as the relative humidity of the air changes. Humidity-sensitive prints can cause internal stresses due to dimensional changes. For example, the longitudinal expansion ratio of the printed matter is 0.5%, and the longitudinal heat shrinkage of the BOPP film is 4%. If the water absorption of the printed matter is too large and the elongation occurs, and the internal stress is caused between the heat shrinkage of the film, the film product may be curled, wrinkled, and bonded. Not strong. In addition, in a high humidity environment, the equilibrium moisture value of the printed matter will also change, and a large amount of moisture absorbed from the air medium will be released from the surface during the hot pressing compounding process, stagnating at the bonding interface, and partially forming a non-adhesive bond. phenomenon. Moreover, the change in the equilibrium moisture value of the printed matter (moisture from the air medium or dehumidification into the air medium) occurs mostly at the edge of the printed matter, so that it forms a ruffled edge or a tight edge, and is not easily formed well with the film in the hot press composite. The wrinkles are caused by the bonding, so that the production cannot be carried out smoothly.
3. Temperature control
Heating can soften the film and bond the paper to the plastic film. However, the temperature is too high, the film is easy to break, which seriously affects the quality of the product. Therefore, measures such as fan cooling and turning off the heating wire should be adopted.
Six. The green development trend of plastic film
The coated paper becomes a white pollution because it cannot be recycled, and toxic substances such as toluene and scorpion water volatilize during the coating process, which impairs people's health. Today, advocating "protecting the environment and protecting resources", the plastic film coating process has been eliminated in Europe and the United States, and European and American countries have even refused to import various packaging materials with plastic film. Although there are water-based plastic film coating processes and hot-melt pre-coating film dry plastic compounding processes, the former still fails to solve the problem that the film printing waste cannot be degraded and cannot be recycled. The latter requires a large amount of heat in the production process and is discarded. The material needs to be separated in paper and plastic at 60 ° C, and the cost is high.
With the improvement of environmental awareness, new scientific and technological achievements will eliminate the old process products and make the film process change qualitatively. "Non-toxic, harmless, green and environmentally friendly" naturally becomes an inevitable development trend of the film coating process. In addition, the glazing process is therefore optimistic. Glazing is a method of coating, spraying or printing to add a colorless transparent coating on the surface of the printed matter, that is, glazing oil. After calendering and drying, the gloss of the surface of the printed product is increased, and the stiffness is also improved. After the glazing, the printed waste can be recycled or decomposed by itself, and does not pollute the environment. Therefore, glazing is a processing technology with great development potential, and it is possible to gradually occupy the plastic coating process market. Although the dilute machines used in oil glazing and alcohol glazing on the market are toluene and alcohol, respectively, which are harmful to the human body and unfavorable to environmental protection, water glazing has gradually become the mainstream of glazing process. The water-based UV glazing process that has been developed so far is an ideal environmentally friendly glazing process and the development direction of the glazing process in the future.