Technical Resources Area
Technical Resources Area |
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It cannot be stressed too highly that correct metal preparation and pre-treatment are fundamental to all successful metal finishing operations, including plastic coating. In order to achieve the optimum protective or decorative coating performance, all of the surfaces of the metal parts to be coated must be free of rust, grease, mill scale and any other contamination because these might inhibit the adhesion of the coating. This in turn can give rise to a premature failure of the coating.
Several metal pre-treatment systems exist that are used prior to plastic coating applications. The most commonly used methods are:-
2.2.1 Degreasing.
2.2.2 Mechanical pre-treatment.
2.2.3 Chemical pre-treatment of steel.
2.2.4 Pre-treatment of Zinc plated or galvanised steel.
2.2.5 Pre-treatment of cast iron.
2.2.6 Pre-treatment of aluminium
More than one pre-treatment process may be necessary for some metal components. This will depend on the design of the part, the application for which it is intended and the coating performance specification that is agreed for the job. Typically, mechanical pre-treatment, such as grit blasting, can follow degreasing.
The first stage of any metal work pre-treatment is the removal of oil and greases from the metal surface. This is essential if the subsequent pre-treatment processes are not to be contaminated and if the initial and long-term adhesion is to be optimal.
Degreasing can be done using either solvents or aqueous solutions.
The solvent degrease process is by immersion of the items into the hot vapour of a chlorinated solvent. The vapours condense on the cold metal and so wash off the soluble oils and greases, This process will not remove stearates, which may have been used in the cold drawing process to produce metal sheets and in tube and pipe forming processes. This solvent degrease process is usually used prior to mechanical pre-treatment.
The aqueous degreasing systems use either alkali or detergents or a combination of these. The items are immersed in a hot bath of the solution. The solution reacts with the contaminants and dissolves them from the surface. It is usual to follow this by washing in hot water prior to further aqueous chemical pre-treatments.
The most commonly used method of mechanical pre-treatment for items that require to be plastic coated is grit blasting. Sometimes this method is called shot blasting. The objective of grit blasting is to clean the surface of the metal substrate and to leave a slightly roughened surface, which allows the coating to adhere securely. If any grease or oil is present on the metal surface, grit blasting alone is not sufficient as a pre-treatment. Any such components should be degreased prior to grit blasting.
Various blasting media are available. The decision of which type to use will depend on the metalwork that is to be cleaned. As a general rule, round shaped media should not be used. It is better to use sharp edged media wherever possible. These will not only abrade the surface better, but they will create a profile on the metal substrate containing undercuts. These will generally enhance the level of adhesion achieved by the coating to the substrate. Ensure that the grit media remain clean. Dirty or oily grit can deposit and leave dirt on the metalwork being processed. Chilled iron shot leaves a deposit of graphite on the metal surface and should not be used since this can interfere with the bond between the coating and the metal.
For the majority of Plascoat PPA 571 external coating applications, the following pre-treatment guideline is reasonable. The metal should have a visual surface cleanliness of between Swedish Standard Sa 2.5 and 3, in accordance with ISO 8501 - 1, 1988, and a surface profile of 30 microns or 10% of the required film thickness which ever is the lowest.
The important aspect of chemical pre-treatment is that it leaves a thin film corrosion resistant barrier layer on the metal surface. If this film is not damaged, it will aid the long-term adhesion of the plastic coating.
Always apply chemical pre-treatments in accordance with the manufacturer's instructions. Particular attention should be paid to details of the processing times and temperatures, as well as to the recommended chemical solution strengths. Such a pre-treatment system will include usually an appropriate degreasing stage.
Several chemical pre-treatment systems are available for use with plastic coating. These include zinc phosphate, iron phosphate and chromate. Care must always be taken when plastic coating any metalwork that has been chemically pre-treated, because the resulting thin film layer is temperature sensitive and may be adversely affected by excessive oven preheat temperatures. Zinc phosphate gives the best corrosion resistance but may be damaged by the high pre-heat temperatures normally required for fluid bed applications. Therefore Plascoat recommend the use of iron phosphate for fluid bed applied PPA 571 and the use of Zinc phosphate for electrostatically applied PPA 571ES. Your pre-treatment chemical supplier will be able to offer more details information for your specific requirement. For suggested pre-treatment systems see "PPA 571 - Recommended Pre-treatment for Galvanised Steel".
It is often difficult to achieve good coating adhesion on zinc, galvanised and other plated surfaces. It is therefore recommended that a test coating be conducted on such surfaces in order to ensure that the metal surface is suitable for coating.
When Plascoat PPA 571 is applied over a zinc or galvanised substrate, the following guidelines may be helpful, in order to achieve a satisfactory coating:
Plascoat recommend a mechanical pre-treatment using a non-ferrous medium such as fine copper slag, J Blast or Carborundum powder. A particle size of 0.2 to 0.5 mm would be suitable. Do not use angular iron grit, which may cause premature failure due to bimetallic corrosion. The blasting pressure should not exceed 0.3 Mpa (40 psi) and we would recommend a nozzle diameter of about 6 mm and a nozzle to work piece distance of 300 mm.
If a chemical pre-treatment were felt to be necessary then we would recommend an acid pickle followed by phosphating or chromating. See "PPA571 - Recommended Pre-treatment of Galvanised Steel".
Where hot dip galvanising is being used under PPA 571ES it is essential to pre-bake the metal at 30°C above the coating temperature for about 30 minutes to remove volatiles.
There are several different qualities and types of cast iron and therefore the required pre-treatment will vary. Therefore this information is for guidance only. The main problems associated with any powder coating onto cast iron are poor adhesion and pinholes due to gassing from the pores in the cast iron. These problems are overcome by pre-baking the items at 30°C above the pre-heat temperature for one to two hours. When cool the items are grit blasted and any large holes filled in using fire cement. During blasting graphite powder dust may be released. This must be removed using an air blast and suitable dust extraction system. The items can then be preheated and coated in the usual way.
2.2.6 Pre-treatment of Aluminium (Aluminum)
PPA 571 bonds extremely well to clean aluminium and salt spray tests (ISO 7253)have consistently shown that there is no loss of adhesion on scribed plates even after 1000 hours. However many aluminium parts, especially aluminium extrusions, will have lubricating soaps on the surface. These must be removed before coating. To do this we recommend an acid cleaner such as Chemetall's Pyroclean 208. The contacts for Chemetall can be found at www.chemetall.com.
Of mechanical pre-treatment is the only pre-treatment option then we would recommend a non-metallic grit such as aluminium oxide. This is because metal grits may leave metal particles embedded in the surface leading, later, to bi-metallic corrosion.
2.2.7 Plascoat PPA 571ES over Oxsilan pre-treatements
See: Plascoat PPA 571ES over Oxsilan pre-treatements
05/02/2008
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