There are mainly four types of shop primers: polyvinyl butyral shop primer (PVB), epoxy zinc-rich shop primer, epoxy iron red shop primer and inorganic zinc silicate shop primer. There are also other types of shop primers, such as acrylic iron red shop primers, which are not much.
(1) Polyvinyl alcohol shop primer Polyvinyl butyral (PVB) shop primer, also known as phosphating primer and etching primer, is an improved version of the traditional phosphating primer. Developed for the adhesion of light metals during the World Wars. The mechanism of these shop primers is the same as those chemical inhibitors, such as manganese phosphate and zinc phosphate. These phosphates form a heat-resistant microcrystalline protective coating with a rough surface, thus increasing the surface contact area and creating a good substrate for subsequent coatings. This treatment also improves the cold-rolled characteristics of steel-worked profiles. Inefficient chromate inhibitors form chromate and ferrous oxide on ferrous metals, but these inhibitors have poor outdoor weathering properties. Adding corrosion-inhibiting pigments, iron oxide red and phenolic resin can improve the weather resistance of PVB shop primer. PVB is polymerized from reactive polyvinyl alcohol and butyral. PVB shop primers have one-component products and two- component products, but the curing agent of both products is phosphoric acid, the main agent of the original phosphating primer. In one-component products, phosphoric acid is synthesized with the paint during production.
The anti-rust performance of polyvinyl alcohol shop primer is quite good, but its chemical resistance is limited , because of the presence of phenolic resin, which will saponify when it encounters alkali, and its solvent resistance is also low. Another problem is incompatibility with cathodically protected surfaces, such as hulls, compartments, etc., due to the generation of alkaline ions by cathodic reactions. Generally speaking, PVB shop primer cannot be used as a part of the immersion system . The existence of chromate and acrylic aldehyde have certain health hazards during high-temperature operations, such as electric welding. The current applications of this shop primer have been limited.
(2) Epoxy zinc-rich shop primer Epoxy zinc-rich primer began to be used in the 1960s , mainly due to the conflict between modern automation equipment and high-performance antirust requirements . The early epoxy zinc-rich shop primer paint film contains high-strength zinc powder, which is as high as 85% (mass ratio), and the zinc powder particles are in contact with each other to produce electrochemical protection, and the anti-rust performance is quite good.
High zinc content, while enhancing rust protection, also poses problems. First of all, after the steel is treated with zinc-rich epoxy, it can withstand the atmosphere without visible corrosion, and small mechanical damage will not cause problems, because the anodic effect of zinc powder can seal the damage point and stop further corrosion. However, high zinc content has an adverse effect on welding and cutting speed. According to the technology and equipment used, in many cases, it is necessary to remove the primer on the welding and cutting parts to ensure high quality and operational safety. Obviously, this takes time and money. Zinc salts (white rust) formed by zinc-rich shop primers are water soluble and they cause osmotic blistering of subsequent coatings.
The current epoxy zinc-rich paint tends to be low-zinc type, and its anti-rust performance is worse than that of zinc-rich type, but the working conditions for welders are correspondingly better. This type of low-zinc epoxy shop primer is reddish-gray, because the reduced zinc powder is replaced by iron oxide red, talcum powder or mica powder. The water content of the mixed solvent used in the epoxy zinc-rich shop primer must be strictly controlled, otherwise the paint will easily swell during storage.
(3) Epoxy iron red shop primer Since the zinc oxide mist produced by the zinc-containing shop primer during candle welding and cutting is harmful from the perspective of labor protection, zinc-free shop primer can be used when the construction period is short Paint, its anti-rust performance is relatively low. The epoxy iron red shop primer was thus produced. It uses iron oxide red as the main anti-rust pigment, and sometimes aluminum powder is added to enhance its anti-rust performance. No high zinc powder content, improved welding and cutting performance, good weather resistance, but slightly worse than zinc-rich shop primer. It has good solvent resistance and chemical stability, which is better than zinc rich shop primer. Epoxy iron red shop primer is not selective to topcoats.
The drying time of epoxy iron red shop primer is slightly longer, and the workability is poor in low temperature and high humidity environment. Due to the mechanical damage caused during handling and stacking, the amount of work required in the secondary surface treatment is relatively large. Cutting and welding quality can be affected by the carbon dioxide produced when the primer burns away. In the process of steel plate assembly, it is greatly affected by pyrotechnic work, and it is easy to be burned. For these reasons, some people do not like to use epoxy iron red shop primer. The dry film thickness of the epoxy iron red shop primer during construction should not exceed 25µm, otherwise it is easy to cause the adhesion to decrease. After the steel plate is polished and sandblasted, the remaining primer may be mistaken for rust, so special attention should be paid to the cleanliness of the surface treatment.
(4) Inorganic zinc silicate shop primer Inorganic zinc silicate shop primer is made of ethyl orthosilicate as the base material, with zinc powder, and other pigments and fillers, solvents, additives and so on. The curing and film formation of inorganic zinc silicate workshop primer depends on the absorption of moisture in the air by ethyl orthosilicate, hydrolysis and polycondensation, and then reacts with zinc powder and steel surface active iron to produce zinc-silicate-iron compound salt and firmly adhere on the steel surface. It has strong anti-rust performance, and the high-zinc-containing inorganic zinc silicate shop primer can prevent rust for 9~14 months. Inorganic zinc silicate shop primer has outstanding properties such as excellent mechanical properties, good heat resistance, and less damage to the surface during thermal processing, and has become the main shop primer at present.
Since zinc powder will produce zinc fog during cutting and welding, which is not good for human health, medium zinc-containing and low-zinc shop primers are mostly used at present. In the shop primer with medium zinc content, the mass ratio of zinc powder is about 40%, and the volume ratio is 13%~15%; the mass ratio of zinc powder in low zinc shop primer is 24%~30%, and the volume ratio is 8 % % ~ 12%. Iron oxide red is added to some low-zinc inorganic zinc silicate shop primers, so the paint film will be grayish red. Reducing the zinc powder content can improve the welding quality and the speed of cutting welding.
At present, the development direction of inorganic zinc silicate shop primer is to reduce the content of zinc powder, reduce the generation of zinc salt, improve its heat resistance to 800 °C, reduce heat damage and the impact on the back of the steel plate, and at the same time Increase cutting and welding speed. Because the volume solids of shop primers are low, usually only about 25%, and the voc content is too high, it is necessary to vigorously develop water-soluble inorganic zinc silicate shop primers. However, since shop primers are automatically constructed by assembly lines, all current assembly lines are designed according to the construction characteristics of solvent-based shop primers. Affected by this, water-soluble shop primers cannot be widely used.