Duplex Systems  Selection, Execution, and Application -

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Epoxy resin coatings with high mechanical resistance and good resistance to chemicals are

preferably used inside buildings because of their typical chalking under UV exposure, under

natural weathering conditions normally together with UV-resistant top coats. 

When using coating materials based on water-dispersed acrylic resins, coatings with very good adhesion and permanent elasticity will be obtained. These coating materials contain essentially water as volatile matter so that no or only low emissions of solvents will occur. The film formation takes place by evaporation of the water and coalescence (fusion of the polymer particles) of the dispersed binder and is not reversible. 

6 Adhesion of the paint coating on the zinc coating An unobjectionable adhesion of the paint coating on the zinc coating is precondition for duplex systems of high durability. If the state of the art is consequently observed by all parties, outstanding results will be achieved. In the same way as zinc coating and paint coating can supplement each other in an optimum manner when using suitable coating materials and complying with the technical rules, they can, on the other hand, also adversely influence each other distinctly. Most of the defects due to insufficient adhesion, which occur again and again, can be avoided very simply by adaptation of the surface of the zinc coating to the subsequent paint system. This can take place by: z Selection or modification of the coating material for a given (defined) zinc coating, or z Preparation of the required zinc surface by process engineering or surface preparation for the intended paint system. In particular in the case of hot-dip galvanizing it is appropriate to test the adhesion of the paint coatings on the steel components or testplates with representative zinc coating surface. This test is an essential part when selecting suitable coating materials.

z Ammonia solution wash with wetting agent (ANW): This method is using an aqueous solution of ammonia (NH4OH). A mixture of about 1 ¼ litre of ammonia solution, 10 % by mass, and 10 litres of water, with addition of 1 to 2 “crown corks” of wetting agent has been proved suitable. The surface to be cleaned is to be grinded with corundum inbedded plastics fleece and the above mixture until a grey foam is formed which should be allowed to react for about 10 minutes. Afterwards it is to be thoroughly rinsed with fresh water. Caution: Legislation regarding waste water is to be considered. For protection of the skin rubber gloves during the grinding work are appropriate; in the case of interior work good ventilation is to be provided. In no case steel wool shall be used for grinding.

8 Test methods for paint coatings 8.1 General Impairment of adhesion of paint coatings of duplex systems may normally occur after stress by humidity, e. g. during natural weathering, by condensation etc.. Testing the adhesion of paint coatings exclusively in dry or unstressed condition is therefore meaningful at most in the case of interior applications without condensation conditions. On site a paint coating on a hot-dip galvanized surface (zinc-coated article) can be tested as follows: z Film thickness in accordance with EN ISO 2808 in connection with EN ISO 2360 z Adhesion in accordance with EN ISO 2409 and EN 24624 

Basic tests / Testing in the laboratory Besides the tests listed in 8.1 further tests are necessary. In EN ISO 12944-6 „Corrosion protection of steel structures by protective paint systems“ the adhesion of paint systems on zinc-coated steel is tested after they have been aged in accordance with EN ISO 6270. The time period of ageing is at least 240 h and is prolonged for high corrosive stress and high durability to 720 h. During the ageing the painted side of the test panels is exposed to condensed water vapour from a water bath heated to 40 °C, whilst the reverse side of the panels is in contact with a standard atmosphere of 23 °C. Due to the resulting temperature difference condensation at the painted side and water transfer within the paint coating into the direction of the phase boundary paint coating/zinc coating takes place which in turn leads to internal stresses in the paint coating. The adhesion is evaluated in accordance with EN ISO 12944-6 at standard atmosphere 24 h after the ageing. The test is passed if no more than classification 1 results in the cross-cut test and no blisters, cracks, flaking or rust has become visible. For certain fields of application the above tests are not sufficient. Therefore, further test methods (e.g. ISO 7253 and EN ISO 3231), in particular for atmospheric stresses > C3, can be agreed. 

11 Particular notes 11.1 Determination of dry film thickness In accordance with EN ISO 12944-5 the procedure for checking nominal dry film thicknesses (instruments, calibration, any allowance for the contribution of the surface roughness to the result) is to be agreed between the parties concerned . In accordance with the specifications in EN ISO 1461 the thickness of the zinc coating has to be checked prior to the paint work. Film thickness values for individual coats as well as for paint coatings to be applied in the shop and on site should principally be specified as nominal film thicknesses in accordance with EN ISO 12944-5. Unless otherwise indicated in the technical literature of the paint manufacturer, „...individual dry film thicknesses of less than 80 % of the nominal dry film thickness are not acceptable. Individual dry film thicknesses between 80 % and 100 % are acceptable, provided that the overall average (mean) is equal to or greater than the nominal dry film thickness and no other agreement has been made” (ISO 12944-5, sub-clause 5.4). For the maximum dry film thickness the information in the technical data sheet of the paint manufacturer applies. If no information is given in the technical data sheet of the paint manufacturer, the maximum dry film thickness should not be greater than 3 times the nominal film thickness. “In the case of excessive dry film thickness, expert agreement shall be found between the parties. For products or systems which have a critical maximum dry film thickness or, in special cases, information given in the manufacturer’s technical data sheet shall be observed.”(ISO 12944-5, sub-clause 5.4). The thickness of paint coatings on zinc is to be determined on representative areas, based on a statistically sufficient number of individual measurements and using measuring instruments working by the eddy current method described in EN ISO 2808 in connection with EN 2360. If individual measurements are below the acceptable range of the nominal dry film thickness, additional measurements are to be carried out in order to limit the area concerned with regard to the extent of any additional work required. It is not permitted to compensate the nominal film thickness of the paint coating with excessive thicknesses of the zinc coating, whilst maintaining the total thickness of the whole system (zinc coating + paint coating). 11.2 Reference areas Reference areas are suitable areas on the structure used to establish an acceptable standard for the work, to check that data provided by a manufacturer or contractor is correct and to enable the performance of the coating to be assessed at a later time. If reference areas are intended to be used for guarantee purposes, this is to be agreed between the parties concerned together with the criteria to be used in their evaluation. 

They are to be prepared in representative locations of the structure and should also include welds, bolt connections, edges, corners and other areas of the structure at which an increased corrosive stress is to be expected. The size and number of reference areas shall be in a reasonable proportion to the type of the complete structure, both technically and economically. Detailed information regarding execution, supervision and documentation of reference areas is given in EN ISO 12944-7 and 8 (Annex B).