Measurement of Local Variation of Ink Tack on Coated Papers, 1998 Coating Conference Proceedings

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Backtrap mottle (BTM) is believed to develop because of non-uniform tack development during multi-color offset printing. A novel method and device for studying the tack variation at the millimeter scale are described in this paper. The device measures the force required to pull an inked probe off the paper surface and quantifies the rate of tack buildup of the ink and its local variation during the process of ink setting. Two probe sizes are reported here.

An empirical equation, Y=b1*[1-exp (-b2*t)]+b3, fits the initial experimental tack force “Y” as a function of time “t.” The coefficients b1, b2, and b3 represent the maximum of tack increase caused by the ink/paper interaction, the initial rate of tack buildup of the ink on paper and the initial tack when ink contacts the paper, respectively. Coated papers with different degrees of backtrap mottle are differentiated using the proposed method. The point-to-point variation in the initial rate of tack buildup (b2) with a 1.1 mm probe is found to correlate well with the subjective rating of backtrap mottle. Other standard laboratory tests do not correlate with mottle problems. The method and device are a new way to screen papers immediately after production for print quality problems.