Q: What should I measure in my coatings?

Edgar Gutoff, Consulting Chemical Engineer 617/734-7081 ebgutof@coe.neu.edu -- Converting Magazine, 3/1/2001

A: Measurements made on a coating depend on the end-use of the coating. However, in all coatings the dry coverage, usually expressed as weight per unit area (g/m2, mg/ft2, oz/ream, etc.), is important. Sometimes coverage is expressed as dry thickness, in mils or mm. Normally a minimum coverage is needed to obtain the desired properties, and excess coverage is just wasted material. In some coatings the coverage uniformity is very important, that is, the coverage should not vary by more than a certain percentage from location to location. Both the cross-web and down-web variability should be measured. In all cases a well thought out sampling scheme based on the principles of statistical process control should be used. In some cases the customer may specify the sampling scheme needed to qualify the coating for acceptability.

There is no one measurement technique suitable for measuring the coverage in all coatings. In some cases a circular disk can be punched out of the coated web, weighed, the coating dissolved off, and the sample dried and reweighed, with the coating weight found by difference. Dissolving off the coating may or may not be a simple task. If the web has been shown to be very uniform the second weighing may not be necessary, and the average weight of a punched out disk of uncoated web may be used. One should never assume that the web is uniform without adequate testing.

In other cases the thickness of the coated web may be measured by a micrometer or similar instrument, the coating dissolved off and the sample dried and remeasured, to obtain the dry coating thickness by difference. This is normally not as accurate as the weighting technique, but may be adequate in some cases. Again, if the web has been shown to be sufficiently uniform in thickness, the second measurement may not be necessary.

In some cases the active ingredient in the coating may be measured by transmission spectroscopy at a given wavelength, or by FTIR (Fourier Transform infrared analysis). FTIR examines all the wavelengths at once, and uses a mathematical technique to obtain the desired absorption vs. wavelength.

These techniques are mainly for organic functional groups. The web itself must not absorb the wavelengths of interest. Other techniques such as x-ray fluorescence may be used to measure the coverage of metals at least as heavy as magnesium.

On-line measurements of coverage frequently use beta gauges. Beta rays, which are electrons, are absorbed by matter in proportion to the mass. Two beta gauges, one before the coating stand and one at the dry end of the machine, tied into a computer to compensate for the time lag, will give a readout of the dry coverage. If both units scan across the width synchronously, with suitable software both cross-web and down-web variability can be measured.

Most other measurements depend on the specific coating. For pressure-sensitive adhesive tapes, tack and bond strength are important. Gloss and magnetic properties are important in magnetic tapes. Optical clarity is needed in certain coatings. Medical test strips have their own specific requirements. Color photographic films must reproduce colors accurately and have the desired photographic sensitivity. General testing data available from ASTM and TAPPI can describe testing methods for your particular coating situation.

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