Speck (varnish)
Specks are common defects in paintwork . While the typical appearance of the specks can be easily recognized, the causes are diverse.
The so-called gel particles , which can be distinguished from the specks, but are often counted among the specks, show a similar error pattern . Gel particles are soft, flexible ("rubbery"), whereas specks are hard.
Appearance
Specks are small, undesirable bumps in the paint film. They are to be distinguished from craters , which represent undesirable depressions in the paint surface. The diameter of an individual speck corresponds roughly to the layer thickness of the defective paint, i.e. usually 20 to 100 µm. The diameter of the particles causing the specks must correspond to the layer thickness of the paint in order to be visible in the paint surface. When investigating the cause, the correct layer thickness must therefore be observed, e.g. B. in painting or an analytical sieving. Specks can occur both selectively and frequently.
causes
Specks can have different causes. Specks can arise from the inclusion of foreign bodies in the paint film. B. can get into the film through contamination of the paint booth. However, specks can also arise from particles belonging to the paint formulation, for example if the pigments are not sufficiently dispersed or gel particles are too large in the binder .
Wrong wording
Specks can result from the use of the wrong raw materials, which agglomerate in the formulation. The situation is similar when the necessary raw materials (e.g. defoamers ) are omitted or the pigments are inadequately stabilized , which can then reagglomerate (flocculation).
Process errors in paint production
The wrong sequence when weighing raw materials or mixing up raw materials can lead to agglomeration or reaction of paint components that would not have occurred in the finished paint. Incorrect process parameters and / or manufacturing conditions can also lead to specks. The dispersion temperature should be emphasized here, since too high a dispersion temperature can lead to gelling of the binder, whereas pigment agglomerates remain in the system if the dispersion temperature is too low. Both lead to specks.
Process error in the paint application
When processing the paint, the ambient conditions are also important. These can lead to specks and other defects, even if the paint itself is okay. In particular, the temperature and the humidity , but also the distance between the spray gun and the surface, should be mentioned here. If these parameters are not matched to the respective paint, pre-cured material can get into the paint layer that is applied at the same time. Craters can also be caused by spray mist that has already dried and falls into the drying paint. This means very finely divided paint material that does not reach the workpiece during painting and remains in the room air. The paint contaminates itself, so to speak, with this defect pattern.
storage
When storing paints, especially powder paints , the paint material can pre-react depending on the storage temperature and duration. In the process, larger, gelled particles are formed which can no longer be incorporated into the paint film during application.
analysis
Whether the cause of the specks is to be found in production or in the formulation can be determined by microscopic examination of the speck itself, but also of the filtrate from an analytical sieve. Optionally, infrared spectroscopy can be performed to identify the contaminating material.
Troubleshooting
The contaminated paint material can be partially processed by filtration . This turns out to be difficult, especially with the more elastic gel ponds, so that deep filtering is necessary here. Defective coatings usually have to be sanded and painted over; in rare cases, polishing is sufficient.
As a preventive measure, a cleanliness test of the paint material can be carried out before application. The constant control of storage is also used for prevention, so that no paint material can be stored for too long.
Individual evidence
- ↑ a b c d H. Römpp; Römpp Lexicon; Lacquers and printing inks; 1998; Thieme; Stuttgart; ISBN 9783137760016
- ↑ a b c d e f g R. Somborn; Flawless: disturbing particles; Paint and varnish; 05/2006; Page 55f
- ↑ a b c C. Schoff; Coatings Clinic: Seeds; JCT Coatings Tech; 09/2005; Page 68
- ↑ R. Somborn; Flawless: spray mist disturbs the paint layer; Paint and varnish; 01/2006; Page 47f