We often see that some design units point out in the technical specification of anti-corrosion coating that polyurethane finish or acrylic polyurethane finish should be used as the finish paint, and then the paint purchasing unit makes an inquiry from the paint manufacturers according to this name in the inquiry list, but the prices quoted by the manufacturers are different, or even several times different. Why? This is because there are many varieties of polyurethane finish, and the product cost and performance are very different.
Polyurethane finish refers to the coating containing carbamate bond in the paint film. There are single component and two-component varieties. Single component is rarely used now (that is, it is only used as primer). The two-component polyurethane finish is commonly used for steel structure surface coating. One component is hydroxyl resin component, and the other component is isocyanate based resin component.
The two-component polyurethane paint can be divided into many kinds according to the difference of its hydroxyl component and isocyanate based component, and the performance is very different.
The hydroxyl resins mainly used to make polyurethane coatings are as follows, and their properties are briefly introduced as follows:
varieties | performance |
Hydroxyacrylic resin (according to the acrylic monomer and hydroxyl content of synthetic resin, it can be divided into many varieties and specifications) | one The finished paint has good weather resistance (light and color retention). two The better the weather resistance, the higher the price of the resin. |
Saturated polyester | one The outstanding performance is that it has good wettability for pigments and is easy to make paint with high gloss and fullness, which is better than hydroxy acrylic resin. two Compared with hydroxy acrylic resin, the reaction rate with isocyanate curing agent is slower, and the drying of paint is slower. three A good polyester resin can also be used to make a finish with high weather resistance, but it is more expensive than hydroxy acrylic resin, and the raw materials are not as popular as hydroxy acrylic resin. |
Hydroxy alkyd resin (some also called oil-free polyester resin, saturated polyester resin, etc.) | one The weather resistance of the finished paint is general (with yellowing), and it is usually used to make wood paint or primer. two The price is not high |
Others include epoxy resin, polyether resin, etc | It is not suitable for making weather resistant polyurethane finish. Polyether resin is usually used to make elastic polyurethane paint |
The isocyanate resins used to make polyurethane coatings are as follows, and their properties are briefly introduced as follows:
varieties | performance |
Aromatic isocyanate resin, such as Bayer Corporate L-75 There are many domestic products of the same kind. | one Due to the presence of benzene ring in the molecular structure, the polyurethane paint used for production is prone to yellowing and discoloration under light; two Low price |
Aliphatic isocyanate resin dimer, such as Bayer Corporate N-75 。 | one The finished paint has good light and color retention performance, without yellowing and discoloration . 。 2． high price |
Aliphatic isocyanate trimer, such as Bayer Corporate N-3390 Etc. | one The finished paint has excellent light and color retention performance, and is usually used to make automotive repair paint, high-performance mechanical coatings, etc. two . High price |
According to the combination of hydroxyl component and isocyanate component in the above raw materials (of course, some of them are meaningless), supplemented by other different raw materials (such as pigments, etc.), and different uses, many kinds of polyurethane finish can be made, with great differences in cost and performance.
Then, how to distinguish the performance of polyurethane finish?
Here, taking polyurethane for industrial protection and polyurethane for construction machinery familiar to Xiao Bian as examples, the following product standards are available for reference:
— HG/T 2454-2014 Solvent based polyurethane coating (two-component)
— SSPC-PAINT SPECIFICATION NO. 36 Two component weather resistant aliphatic polyurethane finish
— JIS K5657 Polyurethane resin coating for steel structure
Focus on the following factors:
1、 Solids
It is divided into volume solids and weight solids, which represent the effective ingredients of paint. Especially volume solids. According to the volume solids and specific gravity, the theoretical weight of paint at a certain dry film thickness can be calculated, and then the theoretical cost can be calculated with the price of kg.
Solids can be divided into factory state and construction state. In the case of the same factory state solids, the lower the viscosity of the paint, the less diluent may need to be added during construction, which can save diluent costs and reduce VOC Emission pollution. That is, high solid and low viscosity, which is the development direction of polyurethane finish.
The test methods and standards for solids include:
GB/T 1725-2007 Paints, Varnishes and Plastics Determination of nonvolatile matter content
GB/T 6751-1986 Paints and varnishes Determination of volatile and non-volatile matter
GB/T 9272-2007 Paints and varnishes Determination of non-volatile matter volume fraction of coatings by measuring dry coating density
ISO 3251-2008 Colored paint Varnish and plastic - Determination of nonvolatile matter content
ISO 3233 -1999 Paints and Varnishes - Determination of non-volatile matter per unit volume by measuring dry film density
2、 Weatherability
It refers to the light and color retention ability of polyurethane finish. This ability is related to the hydroxyl component, isocyanate component, pigment, auxiliary agent, etc. of polyurethane finish. The standards for testing weather resistance include accelerated light aging test and exposure test under natural climate conditions. Usually, this indicator is difficult for users, and the detection cost is also high.
The criteria for testing weather resistance are:
GB/T14522-2008 Test method for artificial weathering of plastics, coatings and rubber materials used in mechanical industrial products - Fluorescent ultraviolet lamp
GB/T 1865-1997 Paints and varnishes Artificial weathering and artificial radiation exposure (filtered xenon arc radiation)
ISO 11507-2007Paints and varnishes - Exposure of coatings to artificial weathering - Exposureto fluorescent UV and water
ISO 11341-2004Paints and varnishes - Artificial weathering and exposure to artificialradiation - Exposure to filtered xenon-arc radiation
3、 Decorative performance
For some mechanical and automotive products with high decorative requirements, the decorative and retention properties of polyurethane finish are also very important. This includes gloss, fullness, freshness, hardness, etc.
4、 Environmental protection
include VOC Content, harmful heavy metal content, free isocyanate content, etc.
5、 Other performance
Such as covering power, adhesion, impact resistance, abrasion resistance, chemical resistance, etc. These are not the main factors affecting prices.
