Paint runoff from vertical surfaces is an undesirable feature which, if present, causes not only unsightly drops or curtains. It is also heterogeneous thickness of the coating, which causes local less thickness, as well as local thickening, which, due to uneven drying and its thickness after drying, causes inhomogeneous strength of the coating.
In order to prevent such phenomena, sagging resistance tests should be performed already at the stage of the laboratory selection of rheological modifiers. One of the best evaluation methods is ASTM D4400 which is called the Anti-Sag Index. You can often find test results in articles that are not fully interpreted well, and the test itself was probably not fully carried out in accordance with the standard. As the ASTM D4400 standard is on the agenda in our daily work, I decided to present in a nutshell the rules for performing the test and interpreting the results in full compliance with ASTM D4400.
Under what conditions do wet coatings run off? Usually this is due to a poor rheological profile, in particular due to the phenomenon of thixotropy, which is not a desirable property of the paint on vertical surfaces. The paint is subjected to high shear forces during application, whether by spraying, brushing or roller. Before the drip test, it must also be subjected to sufficient forces to make the test reliable. Therefore, in ASTM D4400, the test is performed not on paint straight out of the can, but on pre-shearing paint.
What is pre-shearing. It is the preparation of the paint for the test, reducing its viscosity to the level that can be expected with practical application methods. Such a procedure before applying the paint for the sag-resistance test will lower its viscosity and make the test more reliable than in the case of paint that stood before the test at rest. The standardized pre-shearing process prepares the paints in the same way, which in the case of repeatability of tests is of great importance.
The ASTM D4400 standard indicates two pre-shearing methods: for non-aqueous paints and for aqueous paints. Solvent paints are prepared by mixing up to 1 pint in a container with a circular mixing paddle of the specified diameter for 1 minute at 1300 – 3600 rpm. For water-based paints, a syringe is used. A hose (syrringe extension tubing) with a suitable internal and external diameter is used for filling, and a needle for a certain size is used to discharge the paint
By forcing the paint through the needle, it loses its viscosity, followed by application with the applicator with graded gaps (Leneta Anti-Sag Meter). Drawdown of Anti-Sag Meter is preferably carried out with an automatic applicator of 150 mm/sec. The entire process in our laboratory is fully compliant with ASTM D4400 requirements
Immediately after the drawdown, the card is placed vertically and allowed to dry. We usually perform 2-5 determinations to obtain the repeatability required by the standard.
Different Anti-Sag Meters are used for testing, depending on the type of paint and the desired gap heights. ASM-4 (medium) with gaps from 4 mils (100 µm) to 24 mils (600 µm) is used for architectural paints.
After the strips have dried, an evaluation is made. The evaluation usually causes a lot of problems and I have seen many strange ways of interpreting the results. The ASTM D4400 standard calculates the result in the form of the Anti-Sag Index.
Therefore, it is incorrect to give the result as the thickness of the joint where there was no drainage. It is a complete mistake to give the result as wet film thickness which corresponds to the height of the gap! This is an elementary error. The gap height is never the same as the wet film thickness. This is because, firstly, the applied strip, due to the viscosity of the paint and the surface tension, never equals the height of the gap leading to it, and secondly, each strip is affected by gravity, which causes thickening of the stripes in the direction of their flow, which is not visible to the naked eye.
The assessment of the Anti-Sag Index is made on the basis of the highest slit that gives a non-drip strip, which is subtracted from the next strip multiplied by the Addendum Fraction, which is the way the next strip starts sag. The result is therefore a slot with an additional computed index of the next lane and an interpretation of how it has connected to the next strip.
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