Paint, blistering

Zinc Oxide. We found that as little as 5 lb of zinc oxide per 100 gal (1 gal = 3.785 L) of paint was sufiBcient to impart acceptable early blister resistance to the paint when tested in the rain or in the tap water fog box (Table I). Higher zinc oxide levels showed no further blister-resistance improvement in this paint. Blistering in the deionized water fog box was so severe that even the HEC-thickened control had a blister rating of 4D. (The note to Table I explains the rating method used.) However, a noticeable effect occurred at zinc oxide levels of 5 lb/100 gal of paint and higher. The zinc oxide combined with the ammonia in the paint and formed a zinc ammonium complex that may ionically cross-link the carboxyl groups in the HASE thickener and other paint constituents, and thereby reduce the overall water sensitivity of the paint. 

Table II compares the zinc oxide paint results from all three sources of water. The more stringent deionized water fog box showed that even the HEC-thickened paint blisters badly, as does the HEUR paint, which shows no blistering on exterior exposures. The zinc cross-linked HASE-thickened paints blistered much more in the deionized water fog box than they did in the tap water fog box or in the rain. No significant differences were noted between 5 and 25 lb of zinc oxide per 100 gal of paint.

Globules of white fire smashed into the stable doors, breaking apart into complex webs which probed the woodwork with the tenacity of a ghoul s fingers. Glossy black paint blistered and vaporised, the planks began to blaze furiously. 

Chem. Descrip. Acrylic prepolymer emulsion Uses Acrylic for interior flat latex paints, primer sealers, exterior masonry, exterior wood stains, caulks, sealants, and mastics Features Prevents paint blistering/peeling soon after applic./before the paint film is leached of its water exiraclables provides alkali resist., pig-... 

Other possible interfacial degradative mechanisms include the build up of osmotic pressure at the oxide/adhesive interface(6) (akin to the phenomenon of paint blistering by osmotic gradients), disbonding by alkali produced by the cathodic reaction in metallic corrosion(107), and the imposition of stress leading to bond stress-corrosion cracking( 108-110). 

In circumstances where corrosion protection is required, chromate-based rinses are commonly applied to other conversion-coated surfaces to improve corrosion performance in applications such as pre-paint. Typically, corrosion resistance is improved by 100% with a phosphate/chromate rinse compared to that with the phosphate alone. This is reflected in measurably improved adhesion performance and reduced paint blistering. 

The visual approach is often used in the atmospheric testing of painted, metallic-coated sheet. Extent of paint blistering, paint undercutting, and appearance of corrosion products at edges, scribes, and formed areas are typically observed and recorded. 

Figure 9.17 Filiform corrosion and paint blistering accelerated by deicing salt.

A time to reach blister temperature for paint removal was used to determine the relative speed of paint removal. In each case, the paint consisted of an alkyd resin binder on painted metal and wood surfaces. A calibrated thermocouple attached to a unit with a digital temperature scale was placed directly on the surface, and the temperature was recorded when the paint blistered Table 11.12 lists the results. This test showed that a metal substrate requires more cleaning time (+5-72 sec) and a higher temperature (+77-82 °F (+22-28 °C)) than less thermally conductive substrate, such as wood. 

FIG. A-61 A paint blister. (Source Altair RIters International Limited.)... 

Because filiform corrosion prqngates on structural areas that are either clad or coated with organic paint, this type of material deterioration can read extensively before it is detected by airoaft maintenance personnel. In typical cases, filiform corrosion develops around fastener holes on airfimne sheet structures. Paint blistering around the rivet holes is a characteristic feature of this type of corrosion. Documented case histories of filiform corrosion of various components include the following. 

Fuselage Skins. On a Boeiiig 707 aircraft operated by a major commercial airline, filiform corrosion occurred on fuselage skins along rows of fasteners (Ref 27). Paint blistering is produced on airfhune aluminum sheet-metal structures when this form of attack occurs (Ref 27). 

Special vinyl acetate copolymer paints have been developed with gready improved resistance to blistering or peeling when immersed in water. This property allows better cleaning and use in very humid environments. These lattices exhibit the water resistance of higher priced acryUc resins (150). VAc, vinyl chloride—ethylene terpolymers have been developed which provide the exterior resistance properties of vinyl chloride with the dexibiUty of the ethylene for exterior paint vehicles (151). 

To rinse, wash the surface in clear, continuous running water to remove all traces of soluble salts which may cause blistering on the surface. The tank can be similar to the phosphating lank. It may, however, be coated with an anti-corrosive paint to extend its life. 

Air pockets or bubbles are left on the surface of all concrete. Good vibration and placing techniques will reduce their number but not eliminate them. Many air pockets have a small opening on the surface in relation to their size. Paints will not penetrate into such holes, with the result that air or solvent is trapped and subsequent expansion will cause the coating to blister. In addition, some air pockets are covered with a thin layer of cement that also has no strength and will cause loss of adhesion. 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

The texture and drying properties of the paint are important. If it is too thin it will leave the peaks of the matt coating uncovered and if too thick, gas will be trapped in the valleys giving a tendency to blister. In view of the... 

It is necessary to choose the type of paint with care. If it skins-over too quickly in the oven the gases cannot escape and blistering results, but if setting is deferred for too long the paint will flow to the base of the article. Some experimentation is advisable and the conditions once established should be retained. Some users treat the sprayed coating with a passivator of 200 g sodium dichromate in 1 litre of 6 7o sulphuric acid and dry it before enamelling. 

A wide variety of paint systems may, however, be used over the pretreatment primers. Applying a coat of inhibitive primer such as zinc chromate after the pretreatment primer is less advantageous than applying an additional coat of finishing paint indeed, unless the inhibitive primer is carefully formulated, it may contribute problems of intercoat adhesion and blistering. 

Paint failure is related to climatic conditions, and the weather prevailing during application of the paint and during subsequent exposure will determine the life of the paint system. This applies, of course, particularly to outdoor work. In unfavourable weather conditions, cracking and blistering can be promoted as a consequence of the expansion of the products of corrosion, and in the case of iron and steel this can lead to under-rusting. 

Hot steam and severe condensation acting on a film surface exert a very destructive effect, comparable with that of a paint remover they are particularly liable to cause swelling. Dry steam, in contrast to condensed steam, does not cause corrosionLess severe attack by water vapour can cause blistering, which can be of two types intercoat blisters between paint films, and blisters through the complete film system. Only the latter leads to corrosion of the underlying material. 

Paint films exposed to condensation often fail unexpectedly by very early blistering between primer and finishing coat, usually associated with soluble salts trapped under the relatively impermeable finishing coat. Relatively more permeable latex-based paints are less prone to this failure. 

Even small traces of certain corrosion stimulants, notably soluble chlorides and sulphates, can maintain a continuing corrosion process under a paint film because the salts accelerate the initial dissolution of ferrous iron (and other metal ions) but are not immobilised in the hydrated oxide corrosion products. Filiform corrosion is the most spectacular example of this phenomenon, but progressive spread, preceded by blistering, is also observed from scratches or other breaks in a coating, for example during salt spray tests. 

Blistering the formation of dome-shaped projections or blisters in paints or varnish films by local loss of adhesion and lifting of the film from the underlying surface. Such blisters may contain liquid, vapour, gas or crystals. 

After phosphating, thorough rinsing with water is necessary in order to remove soluble salts which would otherwise tend to promote blistering under a paint him. Care should also be taken to ensure that the water supply itself is sufficiently free from harmful salts. Experience has shown that a water supply is potentially injurious if it exceeds any one of the three following limits ... 

In certain cases rinsing may be dispensed with after non-accelerated phosphate treatment, but blistering of paint due to local concentration of solution in seams and crevices may occur. Rinsing is generally applied, regardless of the type of phosphate process employed... 

Table 15.13. This specification follows good industrial practice, with additional safeguards in rinsing to remove residues to treatment solutions. Nonaccelerated treatments must be followed by a single rinse which may contain chromate accelerated treatments must be followed by three rinses—cold water, hot water and a final chromate rinse. Table 15.14 shows the salt-spray test requirements for phosphate coatings with various finishes without formation of rust the paints and lacquer have the additional requirement that no rust shall be visible beyond 0-2 in (5 mm) from the deliberate scratches and no blistering, lifting or flaking beyond 0-05 in (1-27 mm) from the original boundaries of the scratches.

Adherence tests This group of techniques involves the testing of the metal-to-paint adherence. These techniques are covered by descriptions such as prohesion , blister, pull off (BS 3900 Part 10 1979(1989)) and crosscut (BS 3900 Part 6 1976(1989)). Detailed descriptions of these techniques will be found in the appropriate references. 

Figures 19.62 and 19.63 were taken at the time of investigation into a failure of 1001 of steel which should have been coated with a nearly maintenance-free system. Zinc metal spray and four coats of paint were specified but, for reasons of economy, special measures for inspection were not taken. Within months of erection areas were flaking off where millscale had not been removed (Fig. 19.62) and other areas were blistering. Figure...

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