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Ultra durable powder coatings for automotive applications
Ultra durable powder coatings for automotive applications
Keyword: Powder coatings
Powder coatings have a long association with the automobile industry. The original applications took advantage of the protective stone chip and corrosion qualities of powder coatings. Example end uses included springs, radiators, axles, engine blocks and steel bumpers.
Powder coatings have developed to keep pace with these increasing demands and are capable of performing to levels greater than those associated with liquid paints. An illustration of this is the use of powder coatings on body shells. Using advanced manufacturing techniques, such as Akzo Nobel's patented Particle Management Technology, it is possible to produce a powder primer surfacer with excellent flow and levelling. Autosurfacer has been supplied to Scania for use on truck cabinet body shells. This type of powder coating is capable of producing excellent flow and levelling. This high performance can be quantified using the Byk wavescan measurement system. Using this measurement system a number of parameters can be derived. The long wave value is commonly taken to be representative of flow by comparison to perception by the human eye. The lower the value returned, the better the flow and levelling of the cured film. Scania monitor the wavescan for all of the cabinets they produce – using Autosurfacer Scania achieve typical longwave values of six, at a powder film build of 70m3.
Another area in which performance demands have increased substantially is for ultra durable coatings. This paper describes one system which has risen to the challenge.
For the purposes of the following discussions exterior durability is defined as the ability of a coating to withstand degradation by ultra-violet radiation.
Exterior durability can be measured by both natural and accelerated exposures. Natural weathering always takes precedence over accelerated test methods. Tropical exposure sites, such as Florida, Pretoria and Allunga, are favoured because the levels of ultra-violet radiation and ambient temperature are high and so provide a more aggressive test environment.
Historically automotive specifications have called out for 12 months' Florida exposure. Recently OEMs such as Volkswagen and General Motors have been demanding higher performance for longer exposures. One of the strongest specifications is GM4367M (General Motors, 1994a) which ultimately calls out for 60 months' Florida exposure, with a minimum gloss retention value of 65 per cent. It was this specification which was employed to measure the performance of the ultra durable powder system Autobody 5000.
GM4367M requires the use of 5° South Florida black box exposure. In this test the panels from the "roof" of the box simulate the hood or roof of an automobile. This technique simulates the heat entrapment of an automobile body and test panels' peak temperatures in excess of 80°C have been recorded during August. This elevated temperature accelerates any chemical or degradation processes and is therefore more severe than the traditional open backed method.
The Autobody 5000 were developed with this demanding specification as the target. Once the panels were on exposure it became apparent that the ultra durable powder paint was clearly going to exceed the GM4367M requirements. Table I summarises the performance of the original gloss and matt black system versus the demands of GM4367M, a unified weathering specification covering all classes of exterior durable coatings.
Table I Performance of Autobody 5000 versus GM4367M
The true significance of this performance becomes apparent when it is considered that the ultra durable powder exceeds all weathering performance requirements, including those for "state-of-the-art" body colour/colour coat.
Given that the Autobody 5000 gloss and matt were performing so strongly it was decided to extend the exposure period beyond the specification, by a further 24 months, to a total exposure of 84 months.
Figure 1 clearly shows that onset of degradation, as characterised by loss of gloss, does not occur until 72 months for the matt and 78 months for the gloss. The ultra durable system exhibits at least a fivefold improvement in performance by comparison to the standard durability. It should be remembered that the conventional system is perfectly suitable for many exterior applications and meets the requirements of the majority of current automotive specifications.
Figure 1 Demonstrates the outstanding weathering resistance of the ultra durable Autobody 5000 powder coating
UV degradation not only causes an erosion of the paint film surface, characterised by loss of gloss, but in extreme cases can also lead to a loss of paint film integrity.
Post-Florida testing was performed every 12 months, for the first 60 months of exposure. This ensures that film integrity, and hence on-vehicle performance, have not been compromised by the extended weathering (see Table II).
Even after 60 months' exposure there has been no degradation in film integrity, particularly the stone chip resistance (General Motors, 1994b). Since the original series were first submitted for Florida exposure several other systems, colour matched to OEM standards, have also been exposed for five years in Florida. In all cases the colour matched system including clears have lived up to the high standards set by the original gloss and matt coatings.
Table II Damage resistance post-Florida testing of Autobody 5000 gloss and matt
Ultra durability track record
Autobody 5000 enjoys a comprehensive track record. The ultra durable powder coating meets the most demanding automotive specifications and is employed on several vehicles. These include the Audi A3, Volkswagen Golf Combi, Toyota Avensis and Jaguar XJ saloon. Jaguar chose Autobody 5000 because of its proven Florida weathering resistance. However, the visual appearance of the coating was an important factor for Jaguar who wanted to maintain the quality of paint finish that could be achieved with a liquid paint system. In order to achieve the necessary visual appearance Akzo Nobel employed its proprietary Particle Management Technology to achieve the highest quality finish possible for a powder coating. This resulted in a low gloss powder coating without the "sparkle effect" traditionally associated with two component matt systems.
Autobody 5000 has been evaluated versus some of the most demanding specifications in the field and emerged successful. For example, this system is the sole technology listed as approved versus Ford's worldwide powder exterior specifications (Ford, 1999).
In the latest developments Autobody 5000 has won approvals in the USA from the "big 3" – Ford, GM and Daimler Chrysler.
Graeme A. BellTechnology Manager – Automotive, Akzo Nobel
Bell, G.A. (1999), "Complete solutions to powder primer surfacers for automotive applications", Paint & Ink International, 20 November/December, p. 20.GM4367M (1994), "Exterior top coat qualifications procedure", General Motors Corporation, Warren, MI 48090-9010, USA.GM9508P (1994), "Chip resistance of coating", General Motors Corporation, Warren, MI 48090 90107, USA."Worldwide and European approved source list", Ford, 13131 Lyndon, Detroit, MI 48227, USA, October.