What are the characteristics of painted board primers and how to achieve high speed production requirements

At present, the most commonly used epoxy primers for color coated panels are bisphenol A based e-06 or e-03 epoxy resins. The main difference from its earlier formulation is that it has A lower titanium dioxide content and USES A safer solvent that works better but is more expensive.

With the continuous progress of technology, the speed of the color coated board production line is increasing, the curing time of the coating, that is, the residence time in the curing furnace has a tendency to reduce the requirements, because the speed of the production line depends on the curing time of the coating.If the use of an extended furnace body or increase in heat source to reduce the curing time of the primer, it will increase the cost of the color coating unit, not economically.Therefore, the reaction rate of the primer must be improved. In addition, the film tenderness after leaving the curing furnace should also be considered.When the primer passes through the subsequent guide roll, the undercured coating is torn off and falls onto the roll. To overcome this problem, a urea-formaldehyde resin is added to the epoxy primer.The reduction in flexibility was not significant, and it was not cost-effective to use the more expensive melamine-formaldehyde resin instead of the urea-formaldehyde resin to combine with the epoxy.

In order to reduce the cost of raw materials, the primer now contains less titanium dioxide than before.In order to improve the performance of primers at higher production speeds, the viscosity of primers should be reduced, but this increases the problem of pigment settling, especially strontium chromate.Reducing the distribution of paint in the top layer of an opaque primer creates a dark color primer, making it more difficult to mix bright tones.To overcome this problem, anti-settling agents need to be added to the formulation, including hydrophobic pyrolysis silicon dioxide, which not only helps prevent settling, but also helps with the anti-rust properties of epoxy primers.

When an epoxy primer is used with a polyvinylidene finish, the finish must be matched with a pigment.Special care should be taken because fluoropolymers with 70% content in the base material will pass through uv rays.Therefore, the paint must be carefully mixed with pigment to block the transmission of ultraviolet rays, otherwise it will reach the primer and cause powder, resulting in subsequent interlayer adhesion problems.Special primers based on epoxy and phenolic resins have been prepared for application of polyvinylidene topcoats to improve the performance of primers in "acid rain" environments.

For color coated substrates, in addition to hot dip galvanized steel, there are some alternative substrates, such as zinc-aluminum alloy substrates, which require the addition of strontium chromate in the primer formula to maintain good corrosion resistance, especially in the post-processing of the exposed cutting edge around.

Strontium chromate, however, is now considered potentially hazardous to health.Although the roller coating method of the coil coating eliminates the spray paint fog, there is still a potential hazard to the coating manufacturing that must be processed to contain the metal, and the disposal of the coil coating substrate at the end of its service life is also worth considering.Much work has been done to develop ion-exchange silicon dioxide as an alternative to chromate, and field tests are needed to verify its effectiveness.A sol-gel coating, developed in the 1970s, is now in use in a number of areas, including aviation primers, where it is used mainly as an alternative to chromate pigments, and will also be used in coil primers.This coating has good adhesion with a wide range of substrates such as metal, forming a very dense and firm gas tight film, with high microhardness, impact resistance, flexibility, corrosion resistance and wear resistance.It usually consists of alkoxy silicone or periodic table Ⅲ, Ⅳ, Ⅴ some elements of the alkoxy compounds or other subgroup element alkoxy compounds.Inorganic salt, containing this component of the film forming material dissolved in some aliphatic alcohol solvents, under the action of acid catalyst can be hydrolyzed polymerization into transparent colloidal dispersion, with fillers and additives can be solidified into oxide coating, and can be used including roller coating and other methods of construction.

Back to the epoxy primer, although it has excellent corrosion resistance and adhesion, it lacks flexibility.Since polyester primers and polyurethane primers are known for their flexibility, this modified coating is required for primers of some special-purpose color coated board varieties.For example, like home appliances color steel plate on the flexibility of the high demand, so usually used polyester primer coating.

Now, many of the domestic and international color coating units are commonly used in the so-called "universal primer."It is economical, has excellent adhesion to the substrate, has good interlaminar adhesion to various types of coatings, and can meet the requirements of processing and forming.