The primers refer to the set of paints that are applied as first coat of paint system and are intended to:
Protect the surface from corrosion.
Facilitate adherence to the following layers of paint.
Corrosion protection of primers is achieved by two effects:
Addition of anti-corrosion agents
After applying the primer acts as a barrier between the corrosive environment and the workpiece, for that the primer must be completely waterproof and will not allow the passage or entrance of corrosive agents such as oxygen and hydrogen in different ways which may occur (water, sulfates, nitrates, acid rain ...)
On the other hand if we add to the primer specific fillers or pigments specific, greatly improved anticorrosive properties of the paint, the zinc powder is an anticorrosive pigment most use in case of penetration of corrosive agent, it reacts with zinc (anode sacrifice) and not with the material to be protected. Zinc phosphates are the most used in Europe, while the borates are the most used in America.
Primers is the first layer of paint applied over the surface, therefore primers has to be anchored perfectly on the surface and be compatible with the following layers or coating materials, in order to fulfill this requirement primers must :
Have filling power / viscosity in order to penetrate and fill the valleys and ridges of the micro-roughness which is on surface.
Be compatible with the surface and subsequent layers of paints, primers perform functions of compatibilizing agents.
Cleaning the surface of potential contaminants and to activate by adhesion promoter functions in order to ensure a perfect anchorage.
Currently in the market there are a wide range of primers which are designed to fulfill some or all of the above functions, we can classify the primers:
Primers for plastics
Wash primer and Shop primer
Wash primers are those which clean the surface of possible contaminants and promote adhesion between the futures layers of paint and substrate, these primers are based on the phosphated process (phosphoric acid) to clean and promote adhesion.
Shop primers are those that promote adhesion between the substrate and subsequent coats of paint, on numerous occasions these primers have anticorrosive agents or pigments that increase the short-term corrosion protection.
Wash and shop are primers which generally have a vinyl backbone, require a low thickness of paint (between 5 and 8 microns for Wash primer and between 20 and 25 for shop primer) so that the drying time is very short, but the disadvantage of corrosion protection is short term and are not compatible with polyester backbone fillers.
Filler primers are used when the surface has been shoot blasted, sanded with a high abrasive grain or any means which has generated a certain level of roughness, the main objective of this type of primer is to fill the valleys and ridges that on the surface to ensure the strength and anchorage of the first layer which defines the adhesion of subsequent layers of paint, on the other hand this type of primers have anticorrosive agents that promote the long term corrosion protection properties.
Filler primers are generally based on epoxy chemistry also exist and are used with polyurethane chemistry backbone, unlike the Shop and Wash primers drying time is increased and required more thickness, but are primers more compatible with the majority of the paints and greatly enhance the anticorrosive properties of the substrate.
The main function of sealing primers is to isolate one layer from another being compatible between the two, generally used when a surface has been apply putty and is desired to isolate the other topcoats, normally exist in the market sanding or wet on wet primers, the choice of sealer primer is defined by the work done and the final aesthetic you want to achieve.
Primer for plastic
Primers for plastics are intended to promote compatibility and adhesion between the plastic and future layers of paint, usually this kind of primers do not have anti-corrosion agents due the nature of the plastic material based on polymer science, which is why also known as adhesion-promoting primer for plastics.
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