Characterization of spatial coating layer formation - definition of a representative sample size

Statistically representative analysis of the coating layer is important if the coating structures of differently coated sheets are to be compared. State of the art measuring techniques used to evaluate parameters of coating structures are limited in their sample size. The investigation of paper cross sections with 2D analysis methods is usually in the range of some millimeters of sample length. 3D measuring techniques are limited to a sample area of a few square millimeters. Therefore a representative analysis of coating structures demands the definition of a required sample size and number of samples. This paper presents the results of a repeatability experiment designed to evaluate the necessary sample size and the number of samples for a representative evaluation of coating layer structures. The experiment was carried out with a serial sectioning technique on a 135 g/m² woodfree coated paper. The discussion regarding the required sample size shows that an area of 5 mm ² represents the properties of local coating thickness distributions. Furthermore, it is shown that - having a sample area of 5 mm² - 3-5 samples have to be analyzed to obtain representative and stable results for mean, standard deviation and skewness of coating thickness and base sheet caliper distributions.