Polyethylene, polypropylene plastic screen printing ink

First, plastic profile

The organic high molecular polymer can be molded into a shape under the conditions of heat and pressure, and a material that can maintain a fixed shape under normal temperature and normal pressure is called a plastic. The main components of plastics are synthetic resins. Synthetic resins are synthetic products obtained by artificially synthesizing organic polymers. Their properties are similar to those of natural resins. In order to change certain properties, some auxiliary materials, such as fillers, can be added. Plasticizers, stabilizers, and colorants, etc., are obtained by uniformly adhering synthetic resin components together. Plastics can be divided into two types, thermoplastics and thermosetting plastics, depending on their performance after being heated.

Commonly used polyethylene and polypropylene are two kinds of general plastics, mainly urea-formaldehyde plastics, due to their large output, low price, and a wide range of applications.

1. Polyethylene (PE) plastic

Polyethylene plastics are used in many ways and can be made into a variety of finished products through extrusion, injection, and other molding processes. The molecular chain of polyethylene is a long chain of methylene groups containing a certain amount of pendant groups. According to the chemical principle, the longer the number of side groups in the polymer chain, the lower the crystallinity of the polymer, that is, the higher the density of the polyethylene, the more disadvantageous the surface coating is. In general, polyethylene is insoluble in many solvents, but when the temperature exceeds 70°C, the polyethylene can be dissolved in a small amount in solvents such as toluene and amyl acetate. This property of polyethylene provides conditions for surface finishing. We can soak the plastic by heating the solvent to make its surface swell and destroy part of the crystallization to increase the adhesion of the printing ink ink film on the plastic surface. A mixture of concentrated nitric acid, chromic acid, fuming sulfuric acid and sulfuric acid can slowly act on polyethylene at room temperature; a mixture of sulfuric acid and nitric acid at 90 to 100°C can rapidly destroy polyethylene; use potassium dichromate, sulfuric acid, and distilled water The mixed liquid, soaked in polyethylene plastic at 70-75°C, is beneficial to enhance the adhesion of the ink film on the plastic surface.

2. Polypropylene (PP) plastic

For many years, polypropylene has been one of the important varieties for plastics development. It is suitable for all molding methods. It can be used for various pipes, building materials, water pipes, various boxes, films, and fibers. Industrial polypropylene has a high degree of crystallinity and its isotactic index is as high as 89% to 95%. At the same time, it also contains a small amount of macromolecular chains of amorphous atactic structure and low crystalline stereoblock structure. When the melting point of the plastic is in the range of 170 to 175°C, the glass transition temperature of the isotactic polymer is -13 to 0°C, while the glass transition temperature of the syndiotactic polymer is 0°C. Concentrated nitric acid and fuming sulfuric acid are capable of attacking polypropylene. At higher temperatures, polypropylene is soluble in aromatics and oxygenated hydrocarbons, but it can only swell at room temperature. Acids and esters also have a certain degree of dissolution, and as the content of isotactic polymers increases, the solvent resistance will increase accordingly.

In general, polyethylene and polypropylene are high-molecular materials with high crystallinity and low surface tension and non-polar molecular structures. There is no polar group such as carbonyl group, carboxyl group, hydroxyl group in their molecular structure, however, the surface coating of polypropylene can be easier compared with polyethylene.

For polyolefin materials, in order to ensure the adhesion of the ink film, the plastic surface must be treated. One method is to increase the surface tension of the plastic solids by using ultraviolet light to increase the molecular weight of the low molecular layer of the plastic surface in the presence of a photosensitizer and increase the tension on the solid surface. Another method is to oxidize the polyolefin plastic surface molecules to generate polar groups such as carboxyl, hydroxyl, and carbonyl groups on the surface to increase the critical surface tension of the plastic surface and improve the adhesion of inks and coatings to plastic surfaces. Performance, can be used flame, chromic acid mixture, discharge oxidation, ozone oxidation and other treatment methods.

Second, polyethylene, polypropylene plastic printing ink

Plastic printing inks are also referred to as paints or printing materials. The following is a brief description of the performance requirements of plastic prints for inks and the process requirements of screen printing operations for inks:

1. Performance requirements of inks for plastic prints

1 printing ink ink film appearance smooth light;

2 The ink should have good adhesion on the plastic surface;

3 ink ink film should have greater hardness and wear resistance;

4 ink ink film should have a certain degree of water resistance and hot water resistance;

5 ink film should have a certain degree of alkali resistance, acid resistance, and can resist the erosion of alcohol and other organic solvents;

6 ink ink film has a certain degree of weather resistance, the impact of small changes in temperature and humidity.

2. Screen printing ink process requirements

1Thickening is allowed in the printing process, but the use period should exceed 4h; 2The ink used in the screen printing is easy to operate, and the squeegee operation is convenient. Plates are not drawn when off-grid, non-stick webs; the prints are clear and leveling is good.

3 Wire mesh and other utensils are easy to clean.

In the actual production work, we should choose the appropriate PE, PP plastic screen printing ink based on the above performance requirements and process requirements. Although phenolic, alkyd and other coatings are more convenient to operate, their adhesion, hardness, and abrasion resistance cannot meet the requirements. Nitrocellulose paint, vinyl chloride paint, acrylic paint and other properties are good, but it was found through screen printing test that due to the quick evaporation of solvent, it is necessary to frequently add solvent in the process of scraping coating, also when the printing plate is off-grid. The wire drawing phenomenon will occur, the film will not be easy to level, and it will be easy to block the mesh during the scratch coating process, so it is not suitable for PE and PP plastic screen printing.

Since plastic prints are not allowed to be baked, only room temperature catalytically curable inks are allowed. Easy-to-use room-temperature catalytically curable coatings include polyurethane (urethane) coatings, acid-cured amino coatings, and amine-cured epoxy coatings. Polyurethane coatings can easily react with moisture (water) in the air. Therefore, the paint is not allowed to be exposed on the plate for a long time; the acid catalyst in the acid-curing coating has a corrosive effect on the screen, and the acid-curing paint has a comparative advantage. Concentrated formaldehyde odor cannot be manually drawn by hand; amine-cured epoxy inks can be formulated as high solids, low viscosity coatings because the resin has a certain amount of hydrocarbyl and epoxy groups, and can also react with the treated plastic surface. Hydrocarbon groups and carbonyl groups form chemical bonds that can increase the fixation fastness of plastic surfaces. Of course, epoxy coatings also have deficiencies, but properly considered from the formula design, or get a better choice of ink with certain chemical and physical properties, amine curing epoxy coating is a suitable polyolefin plastic printing coating.

Epoxy resin and different pigments can be processed into different colors of ink, according to the requirements of product performance, appropriate use of filler can also be made of different gloss ink. In order to improve the ink properties, additives such as driers, leveling agents, defoamers and the like may also be suitably used.

Source: Ink Information