Crater (paint)
A frequently occurring surface defect in paintwork is called a crater , previously also known as fish eyes . The causes for the formation of craters can be different with the same appearance. If craters appear on workpieces that have been coated in a production plant, the only thing left to correct the error is usually to remove the paint and then repaint the workpieces. Craters often cannot be detected on a painted surface before the problem occurs and, for this reason, often only appear in the coating plant, but not in the paint factory. The reason for this can be different substrates or local contamination or incompatibilities in painting systems
Appearance
Craters are circular depressions in the paint film that reach down to the substrate. They can occur singly or in clusters, have an annular bulge or not.
causes
Craters can result from an unfavorable solvent composition , the formation of separation products, due to the entry of air and / or traces of moisture into the paint material or the release of gases from the substrate. Another possible cause is the impact of substances with low surface tension on the paint film that has not yet hardened, whereby particles of the paint itself can also be responsible for the craters. The paint does not evenly wet the surface or the dirt particles, which leads to the problem described.
formulation
Craters arise, for example, from the spontaneous, local release of volatile constituents in a late phase of crosslinking when the viscosity is already relatively high and the film cannot flow completely. Incompatibilities can arise when one or more paint components have a significantly lower / higher surface tension than the rest of the coating. Errors in the paint composition can also only arise later, during paint production. Causes here can be, for example, inadequate raw material batches, incorrect weights or the exchange of raw materials, which leads to incompatibilities within the system.
contamination
Contamination can result from the contamination of individual paint components, but also from the absorption of foreign substances from the surrounding area (e.g. hoses, production hall). Contamination is one of the most serious problems that can arise in a coating company, as the origin of the contaminating material is usually very difficult to identify, but on the other hand, even small amounts can lead to major damage ( silicone oil , mineral oil ). There are frequent reports about the formation of craters through cosmetic articles from employees in paint production or in paint shops.
Incorrect pretreatment
If the object surface has not been properly pretreated, impurities and foreign matter will remain on the surface of the object to be painted. These can be oils or greases that were used for temporary corrosion protection, but also substances from surface pretreatment, such as. B. Abrasives .
analysis
A microscopic examination is usually carried out to isolate the cause. This is used to determine whether the observed defect is really a crater, specks or outgassing ( pinpricks ). If craters and the associated costs occur, more expensive analyzes are sometimes used to get to the bottom of the cause of the error. In this case, attempts are usually made to determine the chemical composition of the pollution through analyzes using ToF-SIMS .
Troubleshooting
If craters occur when creating a paint formulation, several possible solutions can be considered. For example, additives are offered against the formation of craters , which essentially lower the surface tension of the paint by enriching it on the surface. Faulty paint material that has already been completed cannot usually be processed and must therefore be disposed of. Coated workpieces must be sanded down and repainted.
For the reasons mentioned, prevention plays a very important role in avoiding craters. This essentially means cleanliness in the paint production and paint shop, good pretreatment of the workpiece before painting, control of the viscosity of the paint and the use of wetting agents and solvents with low surface tension.
Individual evidence
- ↑ a b c C. Schoff; Coatings Clinic: Crater (ing); JCT Coatings Tech; 02/2006; Page 72
- ↑ a b c H. Römpp; Römpp Lexicon; Lacquers and printing inks; Page 335f 1998; Thieme; Stuttgart; ISBN 9783137760016
- ↑ a b c d e R. Somborn; Flawless: Crater horror scenario; Paint and varnish 06/2005; Page 44f
- ↑ C. Schoff; Coatings Clinic: Pops and Popping; JCT Coatings Tech; 06/2004; Page 68