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About primer and base coat: Is there a difference? .... Yes!

In day-to-day business, both terms are used synonymously. They often stand for pure adhesion promoters and for bonding agents.

Primers and base coats, however, perform different tasks in coating and bonding processes.
They are often different in terms of material and not comparable in application.

The adhesion of coatings and adhesives to other surfaces is the result of intermolecular interactions between the surface and the coating material, known as van der Waals forces. They depend on the chemical properties, texture and condition of the surface such as roughness, smoothness, porosity.

As an example: the adhesive tensile strength of an Epoxy resin on frosted glass (sandblasted) is better by a factor of 5 than that of a comparative sample on a smooth glass surface.

Primers improve adhesion to critical substrates enormously. many cases, a permanent bond between coating or adhesive and substrate is only possible with the right primer.

Ideally, they act as a chemical coupler and enable bonds between the substrate and coating or adhesive with enormous strength

However, such primers are usually not binders in the classical sense and do not form films!

Ideally, the effect of a primer/adhesion promoter is based on a hermaphroditic function that enables reactions with the substrate and binder of the coating materials. For example, with a special primer – from the ARCAN Spezial Primer line – it is easily possible to increase the adhesion of the Epoxy resin to the smooth glass in the above mentioned example by more than a factor of 10!

Primers are always specifically acting products. There can be no universally acting types suitable for every substrate.

The function of a base coat, on the other hand, is different.

Specifically adapted to the substrates, base coats strengthen and harden the substrate, bind fine dust on the surface, fill pores, block and isolate moisture, pollutants, etc..

They are also called: inlet ground or deep primer. 

These terms already describe the application in the approach. In most cases, these products are composed of binders chemically similar to (or at least compatible with) the subsequent coatings. Sufficiently thin and contain no or only small amounts of fillers and pigments.

An important and often insufficiently considered function of a base coating is pore closure. Sufficient filling of the pores prevents the formation of pinholes or pinholes in the surface of a coating, caused by burst bubbles of air that migrate from the pores through the fresh film to the surface. 

On porous, absorbent substrates without a primer or with a primer that is too lean (i.e. low in binders), part of the binder of a coating, migrates into the pore structure of the substrate. Solids remain, because of the size of the particles (a filter effect of the pores). WIPO Patent Application WO/2015/183719
At the interface between coating and substrate, this leads to binder depletion of the coating. Typical consequential damages are adhesion problems. Properly formulated base coatings prevent this damage.

The terms primer/adhesion promoter on the one hand and base coat on the other hand stand for products with different properties and for different applications. 
They should therefore also be used unambiguously to avoid misunderstandings and errors.



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