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A very useful device that is almost unknown in the United States, but has long been used in Japan is the FDOM (free damped oscillation method, also called pendulum-damping). This Japanese instrument (Figure 1) is based on the same principle as pendulum or rocker hardness tests (Persoz or Konig hardness). In this type of hardness measurement, a pendulum (Figure 2) rocks back and forth in contact with a coating surface. A soft surface absorbs energy, causing the rocking motion to die out quickly, whereas a hard surface absorbs relatively little energy and the rocking takes a long time to die out. In the FDOM, the pendulum is in contact with the test surface via a cylinder (cured coatings) or knife edge (wet coatings). The platen on which the painted coupon is placed has a heating element and the temperature can be programmed to simulate a bake schedule or increase at a certain rate. The motion is initiated automatically and the rate at which motion dies out (damping) and the period of motion, P, are both measured. These parameters tell us about the mechanical properties of the specimen. Damping peaks identify glass transition temperatures as can be seen in Figure 3, which shows results of two runs on separate cured specimens of a 2K isocyanate-acrylic polyol automotive clearcoat over a basecoat. Based on the peaks, the [T.sub.g] was 84[degrees]C. Figures 4 and 5 illustrate the cure behavior of a similar 2K paint over a waterborne basecoat and over bare steel. After an induction period, crosslinking began, which resulted in an increase in damping (the slope of the line gives an estimate of the relative rate of cure). The period of motion decreased as the modulus of the material increased (in proportion to 1/[P.sup.2]). There was peculiar behavior in the damping plot in Figure 5 from about 12 minutes on. The reason for this was not clear, but may have been an artifact of the rapid cure. The material certainly was quickly becoming harder and stiffer, as shown by the plot of the period of motion. The difference in the cure rates of the coating over the two substrates pointed to a slowing of topcoat cure by components of the basecoat.






The FDOM technique is well-suited for following changes in coatings during cure and on exposure, for comparing cure behavior over different primers or substrates, and for connecting wrinkling and other defects with differences in cure rates between coating layers. Drying and bake simulations can be used to optimize formulations, balance topcoat and undercoat cure, and better understand cure processes. Measurements can be done on small painted coupons (wet or cured) cut from panels or even sheet metal from a car hood, appliance, or other object that has a coating defect or failure. The period and damping of the rocking motion on the surface of the coating are measured repeatedly with time at a given temperature or as the temperature of the coupon is increased.

Unfortunately, very few publications on this device have been in English. See JCT CoatingsTech, 1 (3), 22 (2004), for a brief discussion of its use in analyzing [T.sub.g] change on weathering. The technique will become better known if ISO adopts the two-part ([T.sub.g] and onset of crosslinking) pendulum-damping test method being developed by Working Group 31 of ISO Technical Committee 35/SC 9.

By Clifford K. Schoff, Schoff Associates
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Title Annotation:COATINGS CLINIC
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Aug 1, 2010
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