Etch primers

Etch primer pretreatment This is suitable for overcoating with most coatings but is sensitive to moisture during application. It must not be applied as a thin, transparent coating (typically, 10 pm) or inter-coat adhesion loss can take place. 

Zinc or aluminum metal spray Sprayed metal coatings are porous and should be sealed after application by applying a sealer coat (i.e. a thin coat such as an etch primer) or a thinned version of the final coating system. Oil-based systems should not be used. Metal spray coatings can have excellent durability without overcoating with paint and, particularly for aggressive conditions it is preferable to leave them with sealer only. 

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

Etch primers are widely used. They are mostly based on polyvinyl butyral and contain chromates and phosphoric acid. They are said to act both as primers and as etching solutions because it is believed that the chromates and phosphoric acid form an inorganic him, which provides adhesion, while oxidised polyvinyl butyral provides an organic him. For direct application to new galvanised steel, the best known primers are based on calcium orthoplumbate pigment and metallic lead, but these are now less used for environmental reasons. Zinc-dust paints and zinc-phosphate pigmented paints are also used, but the trend is to use pretreatments to assure good adhesion. 

A characteristic of the group (a) of resins is that they air-dry solely by solvent evaporation and remain permanently solvent soluble. This fact, combined with the need to use strong solvents, makes brush application very difficult, but sprayed coats can be applied at intervals of one hour. A full vinyl system such as (o) possesses excellent chemical and water resistance. Many members of group (o) have very poor adhesion to metal, and have therefore been exploited as strip lacquers for temporary protection. Excellent adhesion is, however, obtained by initial application of an etching primer the best known of such primers comprises polyvinyl butyral, zinc tetroxy-chromate and phosphoric acid. 

After major surface contaminants have been removed, e.g. by wet abrasive blasting of hot-rolled structural steel, application of a thin coat of an etch primer greatly reduces the incidence of underfilm corrosion, presumably by eleminating localised areas of poor adhesion. Phosphate pretreatments followed by effective rinsing have a similar effect over cold reduced sheets. 

For a complete panel replacement, the refinisher starts with a panel preprimed in the appropriate stoving primer. For spot repairs or larger repairs without replacement of metal, there will be areas which have to be rubbed through to clean metal. Any indentations then have to be filled with a stopper or spray filler, probably based on unsaturated polyester resins and styrene, with cure initiated by mixing in an organic peroxide. After sanding, remaining bare metal areas are sprayed with a two-pack etch primer. 

Etch primers partially fulfil the roles of both pretreatment and primer. They contain phosphoric acid for surface passivation and are based on polyvinyl butyral ... 

While etch primers, also known as pretreatment primers and wash primers, can be regarded as priming paints which promote their own adhesion by etching the metal surface, they may also be regarded as phosphate/chromate etching treatments which leave an organic residue on the surface to form the basis of the subsequent paint scheme. A detailed account of the etch primers has been given by Coleman . 

The standard etch primer (WP-1, DEF-1408) consists of two solutions, one containing polyvinyl butyral resin and zinc tetroxychromate in ethyl alcohol with n-butanol, and the second containing phosphoric acid and ethyl alcohol. It is essential that a small critical amount of water be present in the latter. The two solutions are mixed in appropriate ratio for use the mixture deteriorates and should be discarded when more than 8 h old. Single-pack etch primers of reasonable shelf life are available but contain less phosphoric acid than the above and are not considered to be so effective. 

The reactions which take place when the mixed etch primer is applied to a metal are complex. Part of the phosphoric acid reacts with the zinc tetroxychromate pigment to form chromic acid, zinc phospliates and zinc chromates of lower basicity. The phosphoric acid also attacks the metal surface and forms on it a thin chromate-sealed phosphate film. Chromic acid is reduced by the alcohols in the presence of phosphoric acid to form chromium phosphate and aldehydes. It is believed that part of the chromium phosphate then reacts with the resin to form an insoluble complex. Excess zinc tetroxy chromate, and perhaps some more soluble less basic zinc chromes, remain to function as normal chromate pigments, i.e. to impart chromate to water penetrating the film during exposure. Although the primer film is hard... 

Etch priming is widely used on aluminium alloy, and is particularly effective on cadmium and zinc. The adhesion to stainless steel and titanium is good. It has also been used quite widely on bare steel and on magnesium alloy, but on these metals its performance is not, in the opinion of some investigators, always quite reliable. For best protection the etch primer coating is followed with a full paint scheme. 

Polyvinyl butyral etch primer 10 13 5 min 1 h Adhesion primer for ferrous and nonferrous metals... 

Wash or etch primers based on poly(vinyl butyral) and phenolic resins pigmented with zinc... 

Self-etching primers are blends that can both etch and prime the surface in one step. They contain at least one acidic monomer, in order to allow them to undertake the etching process. They also eliminate the need to rinse away the solubilized products to the etching step. Instead, these substances become incorporated into the primer layer. This has the advantage of reducing the technique sensitivity associated with the process of bonding [4]. However, there is some evidence that the resulting adhesive bonds are weaker than those with substances that do not incorporate any mineral components of the smear layer [24]. 

The acidity of self-etching primers allows them not only to penetrate the smear layer and incorporate its component, but also to interact with the underlying intact dentine. In doing so, it forms a hybrid layer of the type previously identified with three-layer bonding systems [20,21], and which is essential in promoting adhesion. This acidity varies between formulations, and can be classified as either strong or weak, depending on the acid involved in the blend and also its concentration [25]. 

Weak self-etch primers (sometimes described as mild ) have pH values around 2, and as a result only partially attack the surface of the dentine and hence only solubilize a fraction of the available mineral phase. The hybrid layer formed with such primers is therefore relatively rich in mineral content, with bonding occurring by the interaction of carboxylic or phosphoric acid functional groups with the dentine surface. 

On the other hand, strong self-etch primers have a pH of around 1, and hence contain strong acids based on phosphoric acid groups only. They interact more aggressively with the smear layer of the freshly cut tooth, removing more of it from the surface. Though these substances nonetheless include the components of the mineral phase, the resulting bonded surface closely resembles that created by the etch-and-rinse technique. 

M. Ogata, N. Harada, S. Yamaguchi, M. Nakajima, P.N. Periera, J. Tagami, Effect of different burs on bond strengths of self-etching primer bonding systems. Open Dent. 26 (2001)375-382. 

H. Koibuchi, N. Yashuda, N. Nakabayashi, Bonding to dentin with a self-etching primer the effect of smear layers. Dent. Mater. 17 (2001) 122-126. 

F.R. Tay, H. Sano, R. Carvalho, E.L. Pashley, D.H. Pashley, An ultrastructural study of the influence of acidity of self-etching primers and smear layer thickness on bonding to intact dentin, J. Adhes. Dent. 2 (2000) 83-98. 

B. Van Meerbeek, Y. Yoshida, J. De Munck, S. Inoue, M. Vargas, P. Lambrechts, G. Vanherle, in J. Tagami (Ed.), Self-etching Primer Current Status and Its Evolution, Proceedings of the International Symposium 01 in Tokyo, Dental Materials Department, Kuraray Medical Inc., Tokyo, 2003, pp. 41-55. 

Primer - applied to the dean surface and designed to adhere strongly and prevent corrosion. These can be chromate or phosphate coatings, as previously described, or etch-primer solutions and anticorrosive primers, containing zinc phosphate or iron oxide, and can be single or two pack. Some waterborne primers are available. 

Uses Pigment for etch primer paints Manuf./Distrib. Landers-Segal Color http //www.pigments. com... 

During a repair, the damaged area is sanded down to bare metal and filled as necessary. An etch primer is often used to enable the VR coating to adhere to the surface of the metal. This is a very low solids coating which is applied at a low film thickness. Etch primers typically contain phosphoric acid and zinc chromate (although chromates are becoming unpopular for toxicity reasons), a binder (such as polyvinyl butyral) and a solvent system (usually based on ethanol, butanol and water). 

Trawinskl (135) USA aerospace / St cold chem etch primers... 

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