Introduction to coating knowledge
Introduction to coating knowledge
1. Introduction to coating agents
There are many ways to classify coating glues. The main categories according to chemical structure are:
1. Polyacrylate (PA); 2. Polyurethane (PU); 3. Polyvinyl chloride (PVC); 4. Silicone; 5. Synthetic rubber (such as polychloroprene, etc.).
In addition, there are polytetrafluoroethylene, polyamide, polyester, polyethylene, polypropylene and protein types. At present, the main applications are polyacrylates and polyurethanes.
According to the medium used, it is divided into two types: solvent type and water-based type.
Oil-based coating glue is made by using oil-based solvents (toluene, DMF, MEK, etc.) as the solvent to dissolve acrylic acid or polyurethane. Its advantages are good film-forming properties and good fastness. PU can be pasted with hot air glue and is a commonly used coating glue. The disadvantage is that due to the use of oily solvents, there is a lack of environmental protection. Especially now that the price of oil has increased, the price of oily solvents has been rising. Most coating glues use relatively poor oily solvents due to their high cost, resulting in The environmental protection standards are not up to standard. In addition, with the further improvement of environmental standards in various countries, the environmental protection requirements for coating glue are getting higher and higher.
Water-based coating glue is made by using water as a solvent to emulsify acrylic or polyurethane resin through special emulsification equipment. Its biggest advantages are: many varieties, environmental protection, and low price. Nowadays, many coatings are made of water-soluble glue, such as: general water-soluble PA, water-soluble PU, film glue, oily waxy feeling, and scratch color slurry and other functional slurries. Water solubility, with its functionality, environmental protection, and low price, will be the direction of coatings in the future. The disadvantages are poor film-forming properties, poor washing fastness, and PU cannot be pasted with hot air glue.
According to different coating processes and baking conditions, there are dry coating glue and wet coating glue, low-temperature cross-linked coating glue and high-temperature cross-linked coating glue point. Dry and low-temperature cross-linked coating adhesives are the future development trend of coated fabrics due to their simple coating process, low baking temperature, labor saving and energy saving.
Polyacrylate coating glue (Polyacrylate abbreviated as PA) acrylic
Performance: Good sunlight and weather fastness, not easy to yellow; good transparency and co-compatibility, which is beneficial to the production of colored coating products; good washing resistance; strong adhesion; low cost. Poor elasticity and easy to wrinkle; poor surface finish; difficult to adjust the feel.
Polyurethane coating glue (Polyurethane, PU for short)
Polyurethane, the full name of polyurethane, is a polymer compound containing -NHCOO- units in its molecular structure. This unit is formed by the reaction of isocyanate groups and hydroxyl groups. 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; it is wear-resistant, moisture-resistant, and dry-cleaning resistant.
The shortcomings are:
The cost is higher; the weather resistance is poor; it will be hydrolyzed when exposed to water, heat and alkali.
PU coating agent classification
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 glue
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. 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 glue
The water-based type is divided into two types: water-soluble and water-dispersible. Water-based PU is used for fabric coating and finishing, with large quantities and wide coverage, and has good waterproof properties. Water-based PU coating glue is usually used for dry coating. To improve the durability of coating productsFor flexibility, softness and durability, front and back waterproof finishing should be carried out.
From the perspective of waterproof coating, although PVC is good at waterproofing, it will become hard and brittle in winter, prone to creases or breaks, and is heavier than PU coating. PU coating can not only overcome the shortcomings of PU, but is also very waterproof. Silicone coating is similar to PU, but has better ductility, and can achieve the effect of multiple coatings of PU when the coating thickness is very small.
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