Adsorption
Adsorption is an interface phenomenon, which is the phenomenon in which atoms and molecules of one substance attach to the surface of another substance. The phenomenon that the ink is fixed on the paper surface during the printing process is an adsorption phenomenon. Absorption is also an interface phenomenon. Absorption occurs if the atoms and molecules of one substance penetrate the interface uniformly into the atoms or molecules of another substance. Attachment and absorption are called attachment
Adhesion is the fastness of the ink to the printing surface. If the adhesion is poor and the adhesion is not good, there is no certain fastness, and it is not resistant to friction, impact, and fatigue, which means that there is no printing.
How to improve ink adhesion to substrates
To improve the adhesion of ink to substrates, we should start from the following two aspects, namely the printability of the ink and the printability of the substrate.
Ink suitability control
01The resin in the ink matches the substrate
Resin is the central material of the ink binder, which directly affects the fastness to fixation. The resin for the binder in printed metal substrate inks should be epoxy resin. Because it has epoxy groups and has reactivity, it has a very strong adhesion after crosslinking polymerization Strong. To print polyethylene plastic, the connecting material should be polyethylene; to print polypropylene, use polypropylene as the connecting material, because they have the same polarity and molecular weight; to print polyurethane materials, use polyurethane resin in the ink as the connecting material. After printing, the film is tough and wear-resistant, and has good adhesion.
02 Resin solvent and substrate solubility parameters should be similar
The resin, solvent, and substrate in the ink can be printed on the substrate only when the solubility parameters are close. In this way, the solvent will swell on the surface of the substrate, and the colorant in the ink will enter the interior of the substrate across the interface to form a firm adhesion.
03 Add the right additives
Adding a coupling agent: The coupling agent can promote the coupling of the resin and the polymer printing material in the ink, thereby improving the fixing effect of the ink.
Adding cross-linking agent: The role of the cross-linking agent is mainly to form the cross-linking of the adhesive, reduce the adhesive film-forming temperature, and improve the ink fixing effect. At the same time, the hardness, water resistance, solvent resistance and drying speed of the ink film are also Greatly improve. Steaming or baking must be performed at the same time to further improve the fixing effect and improve the washing fastness.
However, in recent years, a new type of adhesive that can cross-link itself has appeared, but it must be cross-linked at a higher temperature. However, the temperature is too high, which will make the fabric brittle, so it is now developing towards low-temperature self-crosslinking.
Surface treatment of substrate
01 Plastic and plastic film processing
Because of its convenient use, plastic film has bright color, strong and durable after finishing processing such as glazing and laminating. It is widely used in the packaging industry and is a beautiful landscape of the packaging industry. However, the surface of plastics is very different due to the different properties of molecular structure, density, degree of crystallization, and polar groups. Even the same plastic has high, medium and low density differences. In addition, the surface of the plastic is smooth, and a stabilizer is added in the production. It has certain acid and alkali resistance and oxygen resistance, so the adsorption of the ink is very poor, affecting the firmness and abrasion resistance. Surface treatment.
The principle of treatment is to change the polarity of the plastic surface. The plastic surface is non-polar. After treatment, the plastic surface has polar groups. This polar group can be connected to the polar groups in the ink binder, so that the ink is firmly attached to the plastic surface. Common processing methods are:
A. Corona discharge treatment
The structure of the corona treatment device is a high-voltage AC motor, an output transformer, and two electrodes. When processing, let the plastic film pass between the two electrodes, and the high-voltage electricity will highly ionize the oxygen in the air to generate ozone, which will activate the film surface, generate electric fire discharge, generate polar groups, and increase the molecular polarity. Surface tension is improved, dust is eliminated at the same time, pits that are invisible to the naked eye are generated, the surface is roughened, and the adsorption of the ink is enhanced, which is a current common application method.
B. Flame treatment
The principle of this method is: let the plastic film quickly pass through the oxidizing flame, bombard the burrs invisible to the naked eye, and greatly improve the adhesion to the ink.
This method focuses on “fast”, otherwise it will “burn out” the surface, reduce the adhesion of the ink, and cause the flame-treated oxide film to fall off with the ink. The temperature used during processing should be lower than the thermal deformation temperature of the plastic film. As commonly used for packaging polyethylene, the heat deformation temperature is 60 to 80 ° C, and the heat deformation temperature of polyethylene is 100 to 110 ° C.
C. Plasma treatment
Under the action of a strong electric field, high temperature, and laser light, a neutral atom or molecule will lose electrons and ionize as ions. This kind of ions contains equal positive and negative charges, so it is called plasma.
The principle of processing is: the RF generator emits laser energy at a higher voltage, which generates a characteristic glow discharge phenomenon, so that the passing gas material generates many excited states of electrons, ions and atoms, and they then bombard the plastic surface and make the plastic surface The active group changes or forms new groups or free radicals, and generates deposition, which achieves the purpose of chemical and physical modification of the polymer surface, generates polarity, and will dock with the polar group in the ink binder. As a result, the ink adhesion is enhanced.
D. Chemicals and solvents
Treat the plastic surface with an oxidant to oxidize the surface of the plastic film to form hydrophilic groups or other functional groups. It will be docked with the polar groups in the ink to enhance the adsorption of the ink.
For thicker films, solvents can be used. Commonly used solvents are surfactants or chlorine-based solvents, such as dichloromethane, pentachloroethane, trichloroethylene, etc., which can change the wettability of the surface of plastic films and damage the stability of plastic films before manufacturing. Additives (plasticizers, antioxidants, etc.).
Drug treatment method: coating the surface of the plastic film with chemical drugs such as potassium permanganate chlorosulfonic acid and cycloalkylchromic acid, so that the surface can be chemically corroded to improve the wetting ability of the ink.
E. Static elimination treatment
Because the plastic film has good insulation properties, the resistance is very large, it is easy to carry static electricity, and it is difficult to remove the plastic. Before printing, the plastic must be destaticized to eliminate static electricity and remove dust and foreign matter, thereby enhancing the ink’s adsorption. The method of removing static electricity is mainly to remove static electricity with a static removing agent.
Silicon-based antistatic agent: First remove the grease and moisture with alcohol such as methanol or ethanol, and then repeatedly apply the antistatic agent with a brush or roller or immerse the plastic film in the antistatic agent. It is treated at a temperature of 60 to 80 ° C for 3 hours, dried after coating, and left for 5 hours after drying to print.
Surfactant destaticizer: The principle of this method to remove static electricity is different from that of silicon-based antistatic agents. It is mainly achieved by increasing the conductivity and reducing the surface resistance. The method can also be applied by roll coating and dipping, and silicon. Solvent treatment is similar.
The above processing methods are for most plastics, but there is a small part of plastics with polar groups on the surface, rough surface and low density. For example, polystyrene and polyvinyl chloride can be printed directly without treatment.
02 Prepress processing of metal printing materials
With the development of society, the application of metal materials is becoming more and more widespread, such as metal signs, three-piece cans, two-piece cans and other containers.
The metal surface is coated with a layer of anti-rust oil and process lubricant during transportation and storage, and the metal is relatively lively. It is exposed to the air for a long time and absorbs moisture and oxygen in the air to form an oxide film. Oil and oxide film, it will repel and dewetting the ink. Therefore, the metal surface must be treated before printing.
1) Degreasing treatment
Because fats and oils are divided into saponified oils (such as vegetable oils) and non-saponified oils (such as petroleum jelly, paraffin, and motor oil), the method of degreasing is different.
a. Desaponified oil
The principle of removing saponified oil is to use the saponification or emulsification of fat to make it fall off the metal surface. For aluminum plate degreasing, 3 ~ 4% sodium hydroxide reacts with metal surface oils and fats, and 5% trisodium phosphate is used to make saponification of oils into small droplets dispersed in water. The water is 89 ~ 92% at 60 Degreasing at a temperature of ~ 70 ° C for 30 ~ 60 seconds will release the grease from the metal surface.
Chemical degreasing for steel plate, formula: 50g sodium hydroxide, 30g sodium carbonate, 30g trisodium phosphate, 5g sodium silicate, 800g water, temperature 50 ~ 80 ℃, time 5 ~ 7 minutes. Role: Saponification reaction between sodium hydroxide and oil, sodium carbonate to control the reaction rate: trisodium phosphate and sodium silicate emulsify, enhance the activity of oil stains and disperse in water.
b. Organic solvent degreasing
Can remove non-saponified oil, such as lubricating oil, paraffin, petroleum jelly, commonly used organic solvents are gasoline, kerosene, toluene, propanol, cycloethanol, and some chlorinated alkanes, alkanes and so on. The method is to soak the metal substrate in a container containing an organic solvent. This method has a good degreasing effect on asphalt blocks and dry solid grease.
c. Electrochemical degreasing
It is to hang the printing material on the cathode or anode in alkaline electrolysis solution. The chemical action of the electrode reduces the surface tension of the solution interface. During the electrolysis, hydrogen and oxygen bubbles are precipitated on the electrode, which has a strong tearing effect on the ink, which reduces the adhesion of the grease and detaches from the metal surface. This method completely removes oil. Works well
2) Remove the oxide film
Metals absorb moisture in the air to form an oxide film ZnO, Al2O3, FeO, Fe2O3, and Zn (OH) 3. These naturally-formed oxide layers have a loose structure and easily react with acids and bases, making it difficult for ink to adhere. Therefore, a dilute solution of sulfuric acid or hydrochloric acid can be used to coat the surface of the metal oxide layer. In order to separate the oxide layer from the metal surface.
03 glass surface treatment
1) Glass surface condition and printing is moderate
The main component of glass is SiO2, silicon is mainly inside the glass, and oxygen atoms are on the glass surface. This structure has a high surface energy and is easy to interact with other substances. If it encounters air, it will combine with hydrogen in the air to form pro The water gene, OH (hydroxyl), a hydrophilic group floats on the glass surface, and the adhesion of the ink is difficult. There are alkali ions on the glass surface, forming Na-O bonds. This type of bond can easily break the bond between air and water, making it difficult for ink to adhere. Therefore, the glass surface must be treated before printing.
2) Glass surface treatment method
a. The fat-sensitive treatment is performed by coating various silicone coupling agents on the surface of the glass to form fat-sensitive groups to improve the affinity of the glass for the ink. One method is to apply a 0.5 to 1% coupling agent ethanol solution on the glass surface, which forms a good affinity after hydrolysis; another method is to add 1 to 5% of the coupling agent in the ink before printing Spreads on the glass surface by itself.
b. Degreasing treatment, remove grease to improve wettability, wash and degrease with acetone, methyl ethyl ketone, or degrease with dichloroethylene steam.
c. Strong acid treatment: Through strong acid treatment, remove alkali ions and improve ink adhesion. Preparation of soaking solution; one part of chromic acid, 100 parts of concentrated sulfuric acid, 4 parts of distilled water. The glass is immersed in the solution for 15-20 minutes, then washed with distilled water, and dried at 80-93 ° C for 20-30 minutes.
d. Physical treatment, light sandblasting with fine powder abrasive or water grinding with water sanding can remove surface attachments. Improve ink affinity.