Film Formation Mechanism of Varnish

In the packaging and printing industry, in order to improve the high added value of packaging and printing products, finishing techniques such as post-press coating and surface coating (spraying or printing) are applied to the surfaces of the products after leveling and drying. Thin and uniform transparent bright layer, the mechanism is the use of the characteristics of the printed screen features of the main body of light and printed surface properties of the surface reflected light with its qualitative upgrade to make up for the lack of packaging and printing products in the early stage. At present, there are many factors involved in coating and finishing processes, such as packaging and printing substrates, inks, varnish, raw materials (including adhesion, viscosity, surface tension, etc.), and the environmental conditions of the coating production process (mainly high temperature, High-humidity in the rainy season may cause off-skin of the base oil and surface oil on the printed product, or even a haze after the surface is glazing, and cause the product to be scrapped, causing an imbalance in the finishing technology. The cause of this failure is:

1, the purpose of polishing:

The purpose of producing and using base oil and surface (gloss) oil is to obtain a coating of a coating film that is required for compounding. The film formation process directly affects whether the coating oil can fully exert its intended effect and whether it can be polished. The economical, durable, and convenient performance of oil in finishing. Film formation of the coating oil includes two processes of coating (spraying, coating, and printing) with a coating of the coating oil on the surface of the package-printing substrate to form a continuous (solid) continuous varnish film. The current popular way of finishing glazing: first base oil (alcohol soluble or water-soluble), and then on the UV oil. Its finishing process covers almost all paper products and even flexible packaging products. Of course, during the process of painting the varnish, the mechanical and optical properties of the varnish film layer can be determined. For the film-forming properties of two different solidification forms and properties of base oil and surface oil, how to select correct and reasonable operation production process techniques, under the condition of ensuring quality, it does not affect the quality of the post-processing process, but also It is the ultimate goal to get the best finishing art effect.

2, the film formation mechanism:

The base oil and surface oil sold on the market belong to the liquid (state)-linked glazing oil. When the glazing oil is on the surface of packaging printing products, only the first step of coating glazing is completed. Because the "wet film" of the liquid varnish becomes a dry film, it is a morphological change process that the varnish must undergo, that is, it gradually changes from a liquid (state) that can flow to a liquid (state) that does not flow easily. This series of changes is also a change in fluidity or viscosity as defined by the academic community. The liquid base oil or varnish has a viscosity of about 0.03-0.07 Pa.s during the finishing process. Therefore, the viscosity of the "wet film" started to be obtained after the liquid varnish is coated on the surface of the packaged printed product is very low. In order to be a “dry film” with certain mechanical properties, that is, when people often refer to the “full dry” stage of bottom oil or surface oil, the viscosity must be at least 10 Mpa.s or more. From the “wet film” to the “dry film” viscosity changes, it can be assumed that the drying or curing process of any liquid (state) base oil or light (surface) oil must undergo changes in the viscosity of the product. The process, which is a solid (state) base oil or light (surface) oil, due to its inherently powdery nature, is not immediately possible after the finish is adhered to the surface of the coated printed product. The process of changing into a solid (state) continuous optical oil film can be completed only when a certain condition is reached.

3, resulting in light oil film atomization factors:

Normally, the varnish will not be atomized. For example, after the user places a layer of polyolefin film on the surface of the cigarette box, when the humidity is high, especially when the temperature is high, the primer is applied. The surface oil is often caused by a very short aerosol, which causes poor adhesion of the varnish, and the polyolefin film coated on the surface of the varnish will have a thick and uniform mist-like vapor, which in turn reduces or leads to the commodity. The brand's endoplasmic changes.

The liquid varnish is applied to the surface of the packaged printing article to form a thin layer of flowable liquid which generally becomes a "wet film." It is based on different mechanisms, through different ways (infrared drying box UV light curing drying) into a solid (state) continuous "dry film." This will give the desired coating varnish layer. This process of changing from "wet film" to "dry film" is often referred to as "drying" or "curing." This is the core stage of the film formation process after finishing the base oil and surface (light) oils, which we often call the drying or curing drying process. This is due to the change in the morphology of the liquid/that is, the aggregation of the dispersed particles into a continuous light oil film to achieve the film formation process. Regardless of the base oil, varnish or solid base oil or varnish, the drying speed of the process often referred to as “drying” or “curing” is composed of the structure of the varnish itself and the film forming conditions (including temperature, (Humidity, film thickness, etc.) and the material of the printed packaged product, especially the moisture content, determine the characteristics.

Different forms and compositions of base oil and varnish have their own film-forming mechanism. The film formation mechanism is determined by the nature of the film-forming substances used in the finishing materials. For example, the film formation of the base oil can be defined as the physical film-forming method. The solvent or dispersion medium volatilizes to form a film, and the formation of a UV surface oil can be defined as a chemical film formation method, that is, a film formation process of a polymer that is dried by ultraviolet irradiation to cause a polymerization reaction of a varnish. The former is often divided into two types: solvent type or dispersion type, but the base oil must pass through a form during the film formation process, such as non-convertible base oil, solvent or dispersion in the coated “wet film”. As the medium evaporates into the atmosphere, the viscosity gradually increases to a certain degree to form a solid (ie, the entire process of forming the base oil film is completed). If the conversion of the base oil in the solvent or dispersion medium, and then chemically film, this film formation method is the reverse process of production of liquid bottom oil or dispersion base oil.

In short, the drying speed and drying degree of the primer coating film are directly related to the volatilization ability of the solvent or dispersion medium used, as well as the diffusion degree of the solvent (including water) in the oil film, and the molecular structure, molecular weight, and the like of the film forming material. Glass temperature related. For example, most of the base oils used as the main body of thermoplastic acrylic resin are formed by solvent volatilization. Inevitably, in the liquid state, the liquid is changed to a solid state, and in addition to packaging and printing the substrate, a certain amount of solvent remains. In addition, the polymer particles are agglomerated to form a film, and the method for forming the film of the base oil relies on the fact that the polymer particles as a film forming material cohere into each other under a certain condition to form a continuous solid state oil film layer. This is a film forming method that disperses the shape of the bottom oil. Dispersion base oils containing a volatile dispersion medium, such as base oils of emulsions, non-water-dispersible base oils, and organosols, etc., aggregate to form a continuous glossy film layer when the dispersion medium volatilizes.

If it is a converted film-forming material, it is chemically formed into a primer film. At present, the film formation of “water-soluble base oil” also depends on the aggregation of polymer particles as the main film-forming method. The plastic sol contains non-volatile dispersion medium, and its film formation is accompanied by swelling of the polymer particles in the dispersion medium. Coagulation and film formation. Under normal conditions, the solid bottom oil is formed into a film by heat fusion and agglomeration of the polymer particles under heating conditions, and the thermosetting resin must be chemically reacted before being formed into a film. The latter UV light (surface) oil is generally referred to as a chemical film forming method. Chemical film formation can be divided into two types: chain-locking polymerization and film-forming polymerization.

At present, the UV varnish used in packaging and printing products, in addition to the formation of oxidized polymers, generally adopts two forms of initiation polymerization and energy-initiated polymerization (all referred to as photosensitive resins, ie photosensitive resin of varnish system under ultraviolet irradiation. A varnish film formed by the polymerization reaction is called ultraviolet curing varnish). Polymerization is initiated by polymerizing a typical unsaturated polyester resin liquid varnish with an initiator. The mechanism is to use the material to contain unsaturated groups and to form a macromolecular mask when the initiator delimits to generate a chain reaction.

Often favored by packaging and printing and finishing technology is the energy to initiate the polymerization form, the main body of which uses inexpensive epoxy acrylic resin and so on. This type of use of the covalent bond contained in the material forms a gloss mask by energy-initiated polymerization. This is due to the fact that covalent bond splitting requires more energy, so one of the main forms of radiative energy induced by the use of ultraviolet light was chosen. For this reason people are often called light-cured UV varnish. In the presence of a photosensitizer, radical addition of the film-forming substance of varnish proceeds very rapidly and can be cured in minutes or even seconds. There is also the use of electron radiation film forming optical oil called electronic curing varnish. Because the monomer or polymer containing a covalent bond can be directly excited to generate free radicals, the addition reaction can be completed and cured in a few seconds. Therefore, the electron beam curing varnish is one of the energy-initiated polymerization forms in the film forming process in the packaging and printing products.

Looking at the base oil and light (face) materials and film forming mechanisms already described above, we will find that the varnish system adopts easily emulsified resin as the light oil film forming material material, during the production, storage and use of varnish. As the temperature rises or the humidity is high, the water vapor in the goods and packaging will continuously condense on the inner surface of the top layer of polyethylene film, generating a lot of fine water droplets, forming an opaque misty white water film. The light transmittance is greatly reduced. As the temperature in the film rises, the emulsified acidic moisture causes the instability of the coating oil and releases the white mist-like gas, which affects the coating of the UV oil material after printing. Surface properties. In serious cases, it caused users to reduce the price of packaging prints, and even be sued for claims.

4, to prevent coating ink and polyolefin aerosol methods and methods:

In the face of aerosol failures, it should be clearly stated that aerosols are caused by the defects of the varnish film and the polyolefin film, and may even extend to the packaging printing substrates containing excessive moisture. Based on experience, the solution is:

First, the anti-fogging agent is added to the coating oil or polyolefin film system to make full use of the surface activity of the anti-fogging agent to make the above materials hydrophilic, and the resulting fine water droplets gradually develop on the surface of the varnish film or the polyolefin film. Condensation into large water droplets, so that water droplets flow down the film or back to the packaging printing products, or even within the goods, so as to restore the transparency of the film, the coating oil, polyolefin film through the role of anti-fogging agent - (1) Use of hydrophilic polymer materials; (2) Cross-linking aids; (3) Surfactants absorb moisture from them. Not only can prevent and inhibit the severe failure of packaging and printing products from white frost, but also can greatly increase the water resistance and friction resistance of the resin in coating oil and polyolefin systems. It is recommended to add a small amount of nucleating agent to improve the transparency of the film while the soft-packaging polyolefin film material is added with an anti-fogging agent.

Second, (1) the user should store the packaged goods in a ventilated and dry warehouse as much as possible; (2) design a coating that can evaporate gradient balance and flow smoothly according to the different volatilization rates of the solvent (or water solvent). Oil products or ink products, so that the residual solvent of the coating oil is reduced to the minimum; (3) In the coating oil system, the wax material can be released by blocking the moisture release; (4) The heat-insulating, heat-resistant mica powder Or the silicone resin is introduced into the coating oil system to reduce the stability of the packaged printing substrate when the temperature rises; (5) To select the packaging printing substrate that has a low moisture content or a small PH value. (6) Add a catalyst (such as stannous oxide) to the varnish system to increase the transparent hardness value of the varnish film.

Source: China Ink Technology Network