Rub-out test
The rub-out test (dt .: Ausreibprüfung ), rare Rub-up test (dt. Aufreibprüfung ), is one of the most widely used tests of coatings .
principle
In the case of varnishes and emulsion paints, solvents or water as well as other volatile substances escape from the still damp film after application . Since the composition of the lacquer changes constantly during the drying phase, flocculation (re-agglomeration) of the pigments contained can occur. Also, a can segregation of pigments of different density occur within the paint layer, such as titanium dioxide and copper phthalocyanine . In the worst case, hexagonal cells, the so-called Bénard cells , are formed, which are caused by currents within the paint layer.
Additional shear forces are introduced into the system by rubbing. In this way, the agglomerates can be broken up again (deagglomerated) or the segregation canceled. The rubbed surface can therefore have a significantly different appearance (usually stronger in color ) than the non-rubbed surface.
The stronger / weaker in color the rubbed surface appears compared to the unrubbed surface, the stronger the rub-out effect. If the rubbed surface appears stronger in color than the unrubbed surface, this is referred to as a positive rub-out effect , and the rubbed surface appears weaker in color from a negative rub-out effect . Ideally, no difference in color strength and shade can be seen.
execution
The rub-out test exists in numerous variants, as it was never specified in a normative manner, but is nevertheless carried out very frequently by almost every paint factory. The only exception are pure powder coating manufacturers , since powder coating is only liquid in the oven .
What all variants have in common is that they rub on the freshly applied surface.
The main variations of the rub-out test are:
- Circular friction surface or straight friction surface (parallel to an edge of the test object)
- Rub with your bare finger, with laboratory gloves or with a brush
- Friction time up to a certain limiting viscosity ( pull-out ) or a defined test time
- Visual evaluation or colorimetric comparison of hue and / or color strength (for the latter, the so-called rub-out number (RBZ) can also be specified)
- Additional application of a thick paint strip on the edge of the test item to differentiate between flocculation and segregation
- Testing mixed with white or testing in full tone (only when testing pigments or pigment preparations , is not necessary when testing finished colors)
- Establishing a specification: does not make sense, but is tried again and again
- Testing against a defined (product) standard or without a standard as an individual test
- Test over a white background, over glass or between two glass plates
criticism
There are many criticisms of the rub-out test, but this test is one of the most important in the paint sector. This is mainly due to the quick and easy implementation and the lack of a more precise and equally simple test. In addition, the informative value is sufficient for the purpose of quick paint testing. Due to the large number of variants, results from different laboratories are rarely comparable. In detail, the following points lead to the differences and are therefore usually determined within a company.
Depending on the paint system, rubbing with your bare finger can be unhealthy in the long run. On the other hand, there is the fact that the viscosity limit is almost imperceptible with laboratory gloves. In the case of the (unusual) test with a brush, the method should be more standardized, but the main effect is more time-consuming cleaning. In all cases, the result depends heavily on the examiner and his / her daily form.
A defined test time is used to standardize the total energy input during the test. The problem is that the breaking up of the flocculates is only effective if it is also stabilized by the advanced drying process. A fixed test time takes into account neither differences between the testers, nor different drying times due to the system. The "pulling on" of the lacquer is "checked" by the finger, so finding an end point independently of the examiner is extremely difficult.
The colorimetric evaluation should lead to a more defined evaluation. It must be noted here that the layer thickness is almost always changed by rubbing and therefore it cannot be ensured that the substrate is not visible under the rubbed surface. Even with a higher layer thickness of the rubbed surface, the non-rubbed surface could still have a certain transparency. This falsifies the colorimetric results. With visual evaluation, this problem can be narrowed down, but then the entire scale is arbitrary and dependent on the examiner.
When testing between glass plates, the hiding power and not the color strength development is tested. The disadvantage here is that on the one hand the simplicity of the test is lost and on the other hand the universality. Whether a pigment sample becomes more transparent or opaque on further rubbing depends essentially on whether the mean particle size is above or below the particle size for maximum hiding power.
special cases
Ruby
Originally, rub-out or rub-up only referred to the performance of the test, i.e. the rubbing out itself. However, the name has also become established as a description of the result. The term rub-in refers to the opposite effect.
The term rub-in is misleading because this is also a rub-out effect . What can be observed here is that it is not the colored pigment that flocculates, but the white pigment. As a result, the white pigment is deflocculated when rubbing and the rubbed surface appears weaker in color than the unrubbed surface. This creates the impression that friction causes the colored pigment to flocculate, which of course is not the case.
The effect of the decrease in color strength is correctly referred to as the negative rub-out effect .
Impact assessment
The rub-out test is also used to test the compatibility of pigment preparations that can be used as intended in various target systems. The pigment preparation is incorporated into various paint systems representative of the application areas of the preparation and a rub-out test is carried out in each case. So a kind of fingerprint can be created for compatibility. In the ideal case, the preparation is compatible in every system, so there are no differences between the rubbed and unrubbed surface.
Individual evidence
- ↑ U. Zorll: Römpp Lexikon der Lacke und Druckfarben. Thieme Verlag, Stuttgart 1998.
- ↑ T. Brock, M. Groteklaes, P. Mischke: Textbook of paint technology. Vincentz Network, 2000, p. 178.
- ↑ H. Kittel: Textbook of paints and coatings. Volume 5: Pigments, Fillers and Colorimetry. Hirzel Verlag; Stuttgart / Leipzig 2003, p. 330.
- ↑ A. Goldschmidt, H.-J. Streitberger: BASF Handbook Painting Technology. Vincentz Network, 2002, pp. 147, 223, 325.
- ↑ a b T. Brock, M. Groteklaes, P. Mischke: Textbook of paint technology. Vincentz Network, 2000, p. 360.
- ↑ A. Goldschmidt, H.-J. Streitberger: BASF Handbook Painting Technology. Vincentz Network, 2002, p. 325.
- ↑ a b Columbian Chemicals Company: Raven Blacks - Paint and coatings application guide. Marletta, USA, p. 14.
- ↑ a b A. Reiter: Investigation of the compatibility of aqueous binder-free pigment preparations in different types of binders. 2007, p. 11.