There is a need for a Standard Specification for Coating Weight Uniformity in the Converting Industry. One of the key requirements for an effective coated product or coating process development is to have standardized names and a common definition for all terms. This will insure that when the product and process terms are used everybody understands their meaning. There are standard definitions of typical terms such as degrees Fahrenheit, BTU, foot, inches, acetone, and pounds etc. so that when values are used their meaning is understood by everyone.
However in the converting industry, there are several terms that are widely used for which there are no agreed-to conventions for reporting values. Thus, there can be a variety of meanings and interpretations for the term. A good example of this is the reported coating weight percent uniformity for a specific product or coating method. There are no indications of how the values are obtained and the calculation method that is used to express results. The reported value of coated product may not give an accurate value for the uniformity within or a roll or the variation through out a long production. If the product performance depends on the coating weight the customer may see variations in product performance. Also, the differences in uniformity values for a coating method may depend on how the values are obtained and are not true indicators of its performance.
Typically, coating weight uniformity is obtained by measurements on the wide stock roll from the coating line. Because of the width of the coated roll, 30 to 300 inches, several measurements are made across the width of the roll to give the transverse direction coating weight profile. There are a variety of statistical functions that can be used to describe the uniformity values for a specific roll, so that roll-to-roll uniformity and coating applicator performance can be compared. The possible statistical functions are:
• The Coefficient of variation,1 sigma/average expressed as %
• 2 sigma/average expressed as %
• 3 sigma/average expressed as %
• (high-low)/average expressed as %
Therefore, if the calculation method and the number of measured profiles used in the calculation are not know; it is difficult to compare roll to roll t variability in a production campaign and the capability of a coating applicator.
A good standard specification would be to use, (high-low)/average expressed as %, as the standard calculation method, since it accurately describes variation that a customer would encounter. In addition, a minimum number of measured profiles and their location in roll should be required.