Osmotic blistering

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. 

Fig. 5 Mechanism of osmotic blistering due to soluble impurities (adapted from Ref [79]). (Reprinted with permission.)...

Blistering by osmotic processes, due to soluble impurities at the film-substrate interface When a coating is applied on a surface contaminated with soluble salts, osmotic blistering takes place. If water is in contact with the painted surface, water will permeate, promoted by the osmotic pressure generated by the difference in the... 

The treatment of existing osmotic blisters requires careful planning and preparation. If the hygroscopic/hydrophilic materials responsible are not completely removed from the laminate, then a recurrence of the problem is highly probable. The Osmosis Manual [41] provides a clear explanation of the procedures for rehabilitation of a blistered laminate. The manual has also been published in summary form in the open literature [39]. 

Of the four mechanisms, osmotic blister formation is the most important. Past experience has indicated that 70% of all paint coating failures are the result of poor or inadequate surface preparation prior to application of the coating. Types of osmotically active surfaces at Aim-substrate interfaces are impurities such as sand particles from incomplete washing after wet sanding, water soluble salt residues from phosphating, or surface nests in a rust layer. It is also possible to entrap hydrophilic solvents to create blisters. 

Osmotic Blister Repair Hawkeye Industries Duratec Vinyl Ester Primer 297... 

A clear, user friendly, UV stabilised gelcoat offering excellent flexibility and osmotic blister resistance based on specially formulated resins. Available in white and colours, when some of the values quoted here may... 

Of the four mechanisms, osmotic blister formation is the most important. Past experience has indicated that 70% of all paint coating failures are the... 

Blistering due to phase separation during film formation A special type of osmotic blistering can occur when the formulation includes two solvents. The phase separation process occurs at a later stage in film formation and may occur at the coating/ substrate interface. 

Clustering of water at the metal-polymer interface and the subsequent formation of an electrochemical double layer can occur only if the adhesion between metal and coating is weaker than the bond between metal and water or pol5mier and water. For a proper evaluation, however, a distinction must be made between wet adhesion and osmotic blistering. 

Growth of n(n osmotic) blister followed by perforation and blocking of die defect... 

EIS can also be used to measure and characterize defects in coating. This because a defect will also have a specific impedance response. Not very defect is, however, visible in measurements and care must therefore be taken in the interpretation of the data. To show this, two essentially identical panels were tested and will be referred to as cases A and B. On both panels a 100- im-thick industrial coating was applied over a salt film that was present on the metal surface. As a result of this, both panels showed delamination and osmotic blistering as soon as they were exposed to an electrolyte. 

Inorganic non-chromate replacements are less elfective and more soluble than chromates their utility in higher concentration leads to flushing and osmotic blistering. 

Osmotic blistering of organic coatings as a function of the solubility of corrosion inhibitor pigments [20]... 

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