Research on PU coating process
Research on PU coating process
This article mainly introduces the research on the production process of polyurethane PU coating, effectively solving the “skin hanging” phenomenon of coating slurry during the production process; improving the fluidity of slurry, solving problems such as high thickness coating and knife distance selection to meet the needs of Customer demand.
PU refers to polyurethane, and the PU coating is waterproof and breathable.
PU: Polyurethane coating glue (Polyurethane is referred to as PU). The full name of polyurethane is polyurethane. It is a polymer compound containing -NHCOO- unit in its molecular structure. This unit is formed by the reaction of isocyanate group and hydroxyl group. The reaction formula is as follows :—N=C=O+HOˉ→—NH-COOˉIn the 1970s, German Otto Bayer first synthesized PU. Around 1950, PU appeared in Europe as a textile finishing agent, but most of them were solvent-based products for dry coating finishing. In the 1960s, due to the increase in people’s environmental awareness and the introduction of government environmental regulations, water-based PU coatings came into being. After the 1970s, water-based PU coatings developed rapidly, and PU-coated fabrics have been widely used. Since the 1980s, breakthroughs have occurred in PU research and application technology. Compared with foreign countries, domestic research on PU textile finishing agents is relatively late. Polyurethane coating agent is the main type of development today. Its advantages are: the coating is soft and elastic; the coating has good strength and can be used for very thin coatings; the coating is porous and has moisture permeability and ventilation properties; wear resistance , resistant to moisture and dry cleaning. Its shortcomings are: higher cost; poor weather resistance; hydrolysis when exposed to water, heat, and alkali. PU coating agents are classified according to composition: polyester polyurethane; polyether polyurethane; aromatic isocyanate polyurethane; aliphatic isocyanate polyurethane. According to the medium used, it is divided into solvent type and water type. Solvent-based PU coating Solvent-based PU has good strength and water resistance, but is highly toxic and easy to burn. In terms of components, it is also divided into two-group classification and single-group classification. The two-component product consists of a prepolymer and a cross-linking agent. The prepolymer is a prepolymer with hydroxyl groups at the end produced by the reaction of isocyanate and oligomeric polyol. Cross-linking agents are compounds containing multiple (three or more) isocyanate groups. During coating finishing, the prepolymer reacts with the cross-linking agent to form a thermosetting network film, giving the textile excellent properties. Such products include Crisvon7367SL from Dainippon Ink Company. The one-component product has a linear structure, with both amorphous and crystalline areas, and is thermoplastic. The one-component product is made of isocyanate-terminated (-NCO) prepolymer through chain extension. Products such as domestic AR-1401 and foreign products include Dainippon Ink Company’s Crisvon2016EL and 2116EL. Bayer’s Impranil series. Most solvent-based PU coating glues use DMF or a mixture of toluene and isopropyl alcohol as the solvent. In order to achieve waterproof and moisture-permeable effects, solvent-based coating finishing agents generally use wet coating processes to process fabrics. Water-based PU coating water-based type is divided into two types: water-soluble and water-dispersed. Water-based PU is used for fabric coating and finishing, with large quantities and wide coverage, such as Japan’s Superflex series and Germany’s Imperanil water-dispersed series. Water-based PU has good film-forming properties and good waterproof properties. ElastronCT-7 and C-52 are both water-based PU, used for waterproof and moisture-permeable coatings. Water-dispersed PU can be made into nonionic, anionic and cationic dispersions. Water-dispersed PU is sensitive to acid and alkali. In the presence of acid, anionic PU will condense, while cationic PU is not resistant to alkali. Water-based PU coating glue was initially manufactured using the phase inversion emulsification method, but the product contained emulsifiers, which adversely affected adhesion, toughness and water resistance. In order to solve this problem, in the late 1980s, self-emulsifying water-based coating glues gradually increased. The main principle is to introduce an appropriate amount of hydrophilic groups into the polymer chain and spontaneously disperse under certain conditions to form an emulsion. Water-based PU coating glue is usually used for dry coating. In order to improve the water resistance, softness and durability of the coated product, front and rear waterproofing should be carried out. PU macromolecules contain a large number of polar groups, and the intermolecular force is very strong, resulting in excellent film-forming properties. It can form a tough and durable film on fabrics, has good water repellency, and also has certain moisture permeability. The reason is: on the one hand, the polar groups or hydrophilic groups in PU, such as -OH, -NHCOO-, -SO3H, -COOH, etc. act as “chemical step stones” [9] to cause water vapor molecules to move along the The ladder migrates from the high humidity side to the low humidity side; on the other hand, PU is composed of soft segments and hard segments, forming amorphous regions and crystallized regions in the structure respectively, because the molecular chains in the amorphous region are relatively loose.
Polyurethane PU water-based coating has become one of the new methods for textile finishing today. Because this product has many functions, high performance and great changes, it occupies an important position in the domestic and foreign textile fabric markets. Because it is completely water-based instead of solvent-based, it not only changes the working environment of operators, but also eliminates safety hazards and environmental issues. Polyurethane
Coatings can form multifunctional and high value-added textiles, and can be widely used in various fields such as clothing, decoration, industry, and military industry, and their prospects are very broad.
The research of this project was listed as the “National New Product Plan” of the National Science and Technology Commission in 1995. This product was rated as “Excellent New Product of Jiangsu Province” in 1996. More than 300,000 meters of this product were trial-produced and processed into garments. Mainly sold in the United States, Japan, and Southeast Asia, and has been well received by customers.
1. Main issues
1.1 Solve the problem of slurry “hanging skin” in production.
The polymer of water-dispersed polyurethane is particles.In this state, a higher temperature must be used to produce a continuous film with better performance.
During the production and use process, this material will produce local cross-links with changes in time and climate. After the cross-linking change occurs, a thick layer of skin forms on the surface of the slurry, commonly known as “hanging skin”. These crusting slurries are brought to the cloth surface during coating to form color (spots), which directly affects the quality of the finished product. After careful observation and analysis, it was found that under certain conditions, the slurry first produced self-crosslinking and then increased in viscosity, followed by “hanging skin”. After many selection experiments, we finally decided to add a moisturizing agent to the slurry to delay the “skin hanging” of the slurry to meet the needs of production. However, we have conducted many tests to determine the appropriate amount of moisturizer
1.3 Solve the problem of high thickness coating.
Coating processing techniques include dry method, wet method, hot melt method, transfer method, adhesive method, etc. Among them, the dry method is most used. The so-called dry method is to evenly apply the coating slurry on the base fabric with an applicator. , the solvent or water is volatilized by heating (such as drying or baking), and the coating agent forms a thin film on the surface of the fabric. Our factory adopts a dry process.
The multifunctional coating machine in our factory is imported from the Austrian J.Zimer Company. This equipment is used for both roller scraper type and garden mesh. The scraper on the roller has an adjustable blade distance. In order to adapt to the scraping products with different requirements, we conducted the following tests on the blade distance.
(1) Hard angle and hard knife method:
Choose a hard knife with a hard angle, that is, the distance between the knife edge and the center of the lining roller is zero, that is, hob scraping. The main advantage of applying this knife distance is that the coating layer of the scraping slurry on the base fabric is thick and soft, and can be easily After two coats of scraping, the coating film thickness can reach more than 5 wires. Such a thickness is absolutely unreachable on ordinary coating equipment. However, the temperature of the drying room after scraping with this scraper should not be too high, otherwise it will reduce Peeling fastness, when the temperature is 130℃ and the vehicle speed is 15m/min, a hard-edged knife can solve the problem of high-thickness coating and scraping.
(2) Floating knife method:
The floating knife means that the distance between the knife edge and the center of the lining roller is adjusted to 20 to 30 degrees. The floating knife scraping is generally suitable for coating and scraping on the base fabric to form a slightly thinner film. After two coats of scraping, the thickness is about 3 filaments. ; The main advantage of the floating knife coating is that it can improve the uniformity and coverage of the slurry coating. The temperature of the drying room after the floating knife coating can be appropriately increased to about 150°C, and the vehicle speed is 20 to 25 meters/minute, which is relatively high. Suitable for large-scale production. It is also more convenient for workers to clean the lining roller.
1.3 Solve the problem of semi-waterproof quality of base fabric.
Polyurethane PU coated textiles require high water resistance, which is difficult to achieve by coating processing alone. For this reason, semi-waterproof finishing must be carried out on the base fabric before scraping to achieve the water resistance requirements of the finished product and improve the quality of the finished product. Feel.
Our factory currently has two types of equipment that can perform waterproof finishing: needle plate or cloth clip type setting machines. When these two types of equipment perform waterproof finishing, the needle eye spacing and clip heads produced by both sides of the base fabric after passing through the needle clip or cloth clip are Wrinkle marks are easy to produce edge wrinkles during scratching, and are difficult to eliminate and directly affect the scratching. For this reason, we designed and modified a simple semi-waterproof finishing equipment based on the actual situation – a finishing car with a rolling groove. On this equipment After semi-waterproofing, the two sides of the base fabric not only do not produce crepe marks, but are also very flat, providing a good quality base fabric for the subsequent coating and scraping process.
2. Conclusion:
By solving the problem of “hanging skin” on the conjunctiva of polyurethane PU coating slurry, we improve the fluidity of the slurry, solve the semi-waterproof quality of the base fabric and overcome the problem of high-thickness coating and scratching. The production process of this variety has been basically perfected. The products developed have successfully completed 300,000 meters, and the quality of the finished products is about 95%. The internal quality is in line with the standards, and all indicators have met the requirements of customers. After the products are made into garments, they are exported to the United States, Southeast Asia and other countries, and are welcomed by customers.
3. Conclusion:
As a new generation of textiles with high added value, polyurethane PU coated fabrics should have a series of functions such as flame retardant, heat insulation, reflective, antistatic, etc. These are subject to further research and discussion. We will stabilize this On the basis of product quality, we strive to develop more functional coated textiles to further increase and improve the performance of coated products, further expand the application fields, meet the needs of domestic and foreign textile markets, and enable enterprises to obtain better benefits. 30tuSZ
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