Perspective Laminating Technology
One. The concept and significance of lamination
Laminating, that is, sticking film, is the processing technology of coating plastic film with adhesive, and heating and pressurizing the paper print to make it bond together to form a paper-plastic product. The film-covered printed matter has a thin and transparent plastic film on the surface, which makes the surface smoother and brighter, thereby improving the gloss and fastness of the printed matter. The graphics and text are brighter in color and rich in three-dimensional sense. Antifouling, abrasion resistance, folding resistance, chemical corrosion resistance, etc.
two. Lamination classification introduction
According to the process of coating, it is divided into three types: dry, wet and oily. According to different raw materials and equipment used, it can be divided into coating film and pre-coating film process.
1. Coated film
That is to say, in the coating film process operation, the adhesive is first coated on the film, and then hot pressed, but the production process is prone to fire.
2. Pre-coated film
In this process, the laminating equipment does not require a binder heating and drying system, which greatly simplifies the laminating process, and the operation is very convenient. It can be turned on whenever it is used. The production flexibility is large. At the same time, there is no solvent odor, no environmental pollution, and more importantly, it can Avoid bubbles, delamination and other failures, the transparency of its finished products is extremely high, with broad application prospects and promotion value.
The structure of the pre-coated film is composed of a substrate and an adhesive layer. The substrate is usually PET and BOPP film. From the perspective of material cost and processing technology, most pre-coated film substrates use BOPP film with a thickness of 12-20 microns. The adhesive layer The thickness is 5-15 microns. According to different processing equipment and process conditions, different thicknesses of adhesive layer are selected. The adhesive layer is divided into hot melt adhesive and organic polymer low temperature resin. The difference between the two is that hot melt adhesives are made by blending and modifying several materials such as main tackifier resin, tackifier and modifier, while the organic polymer resin is a single polymer low-temperature copolymer.
Due to technology, production equipment, and raw material restrictions, the pre-coated films (mostly hot-melt adhesives) produced by domestic equipment still have quality defects. Because there are production processes that rely on a process similar to that of coating the film, the colloid is dissolved with an organic solvent, and the rubber roller is coated on the substrate film with a concave anilox roller, so there is insufficient solvent evaporation, and an odor occurs during operation After coating, the solvent evaporates and the surface is prone to blistering. At the same time, because the hot melt adhesive is blended with several kinds of polymer materials, during use, poor temperature control will cause the degradation and cross-linking of the polymer in the hot melt adhesive, making the coating surface bad. Some pre-coated film colloids are easy to adhere to the 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 made of several materials, the transparency after coating is significantly worse than that of low-temperature pure resin pre-coating film.
3. Water-based coating
It is a method of processing the surface of the 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 coating, the brightness, strength, and adhesion of the coating should also be investigated. Water-based wet film is favored by customers for its high strength, easy recycling and no pollution.
4. Curved film
It is a brand-new transfer technology that can cope with complex-shaped workpieces and is suitable for different materials. It also greatly increases the added value of the product due to the vivid patterns and vivid colors. This technology is a film with special chemical treatment. After printing a certain color pattern, it is sent to the surface of the water. Using the effect of water pressure, the pattern is evenly written on the surface of the product. The coated film is automatically dissolved, cleaned and dried. After drying, a layer of transparent protective coating is applied, and the product has presented a completely different visual effect.
The lamination process can be divided into cold (cold pressing) lamination, hot (hot active) lamination, and liquid lamination according to cold and heat, pressure conditions and materials. Hot laminating technology refers to heating and activating the pre-coated adhesive on the film, through which the film and the printed product are bonded together; and cold laminating film is directly pressed by the adhesive to bond the film and the printed product Together. These two methods can make various special films such as ultraviolet protective film, waterproof or anti-friction film. Recently, there is also an emerging technology called "liquid film" method. The transparent liquid is brushed directly on the surface of the product with a brush, spray gun, roller or special laminating machine. Although the technical operation is simple and the price is low, it is not yet mature.
three. Material selection
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 cheap, which is an ideal composite material in the coating process.
According to the performance of the film itself and the purpose of use, the thickness of the film should be between 0.01-0.02mm, which needs to be treated by corona or other methods. The surface tension of the treated surface should reach 4Pa in order to have better wettability and adhesion performance. The corona treatment surface should be uniform, the higher the transparency, the better, to ensure that the printed matter covered has the best clarity. The light transmittance of PET is generally 88-90%, and the light transmittance of other films is usually between 92-93%. The film needs to have good light resistance, it is not easy to change color under long-term light irradiation, and the geometric size should be stable.
Since the film is in contact with solvents, adhesives, inks, etc., it has certain chemical stability. The appearance of the film surface should be smooth, without unevenness and wrinkles, and the film should be free of bubbles, shrinkage holes, pinholes and pits. Due to the limited adjustment ability of the laminating machine, it is also required to rewind neatly and have the same tightness on both sides to ensure even glue application. Of course, the cost is lower.
four. Compound strength
Refers to the adhesion fastness between plastic film and paper (board) printed products in laminated products. Its size depends on the adhesion between plastic film, paper (board) printed products and adhesive (or hot melt). When laminating, the adhesive force mainly comes from the mechanical bonding force and physical and chemical bonding force between the plastic film, paper (board) printed matter and the adhesive (or hot melt adhesive).
The key to the fastness of the film is the affinity of the glue with the printed matter (ink and substrate). As the coating industry, printing inks are not selectable, that is, only coatings suitable for the ink market are 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 production process of the coated film, such as the temperature, pressure, speed, coating condition of the adhesive, film tension and environmental factors of the coated film process. At the same time, it is also 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 graphics, the characteristics of the ink, the use of diluent, powder spray, and the drying of the ink layer.
Fives. Factors affecting the quality of the film
1. Printed ink layer status
Refers to the nature of paper, ink performance, ink layer thickness, graphic area and printed graphic density, etc. When the printed ink layer is thick and the graphic area is large, it will close many paper fiber capillary holes, hinder the penetration and diffusion of the adhesive, make the printed product and the plastic film difficult to bond, and prone to delamination, blistering and other failures.
2. environment humidity
The water content changes between the adhesive and the plastic film and printed matter as the relative humidity of the air changes. Humidity-sensitive prints will generate internal stress due to dimensional changes. For example, the longitudinal expansion rate of the printed product is 0.5%, and the longitudinal thermal shrinkage of the BOPP film is 4%. If the printed product absorbs too much water and stretches, it will cause internal stress between the film and the heat shrinkage, which will cause the film product to curl, wrinkle, and fail to adhere. prison. In addition, in an environment with high humidity, the equilibrium moisture value of the printed matter will also change. A large amount of moisture absorbed from the air medium will be released from the surface during the hot-pressing process, stagnate at the bonding interface, and partially form a non-adhesive phenomenon. Moreover, the change in the equilibrium moisture value of the printed matter (absorption of moisture from the air medium or humidification of the air medium) occurs mostly at the edge of the printed matter, making it form a lotus leaf edge or a tight edge, and it is not easy to form with the film in the hot press compound Good adhesion results in wrinkles, which prevents the production from proceeding smoothly.
3. temperature control
Heating can soften the film and make the paper adhere to the plastic film. However, the temperature is too high, the film is easy to break, which seriously affects the quality of the product, so measures such as fan cooling and closing the heating wire should be taken.
six. Green development trend of plastic film
The coated paper becomes a kind of white pollution because it cannot be recycled, and toxic substances such as toluene and water are evaporated during the coating process, which harms people's health. Today, advocating "protecting the environment and protecting resources", the plastic film process has been phased out in Europe and the United States and other countries. The European and American countries have even refused to import plastic packaging. Although there are water-based plastic film coating process and hot melt pre-applied dry plastic composite process, the former still fails to solve the problem that the waste of film printed matter cannot be degraded and cannot be recycled. The latter requires a lot of heat in the production process and is discarded The material needs to be separated in paper and plastic in water at 60 ℃, and the cost is relatively high.
With the improvement of environmental protection awareness, the new technological achievements will eliminate the products of the old process. It is the qualitative change of the coating process. "Non-toxic, harmless and green" naturally becomes the inevitable development trend of the coating process. In addition, the glazing process is therefore optimistic. Varnishing is to add a layer of colorless transparent paint on the surface of the printed product by coating, spraying or printing, that is, glazing oil. After calendering and drying, the gloss of the surface of the printed product is increased, and its stiffness is also improved. The printed waste after glazing can be recycled or decomposed by itself without polluting the environment. Therefore, glazing is a processing technology with great development potential, which may gradually occupy the plastic film process market. Although the thinners used for oily and alcoholic glazing on the market are toluene and alcohol, which are also harmful to the human body and harmful to the environment, waterborne glazing has gradually become the mainstream of the glazing process. The waterborne UV glazing process that has been developed so far is an ideal environmentally friendly glazing process, and is also the future development direction of the glazing process.
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