Investigating Paper Curl by Sheet Splitting

First the theory of paper curl and the literature of curl related paper testing is briefly reviewed. Based on the theory quantitative models for the influence of fiber orientation two sidedness and paper hygroexpansivity on curl are given. It has to be pointed out that fiber orientation affects CD curl and diagonal curl in different ways: CD curl is promoted by FO anisotropy two-sidedness but diagonal curl is promoted by FO angle two-sidedness. In the second part of the article industrial copy paper with curl problems is analyzed using the model equations. Fiber orientation across the z-direction of the copy papers is measured using a sheet splitting method and hygroexpansivity is determined using an ultrasonic sheet tester. Curl behavior of the paper is well described by the model equations. In particular it can be explained, why the CD curl and the diagonal curl in the examined papers show great differences. Thus the outlined analysis procedure, i.e. measurement of z-directional FO measurement and hygroexpansivity, may help to evaluate interrelations between fiber orientation and curl of industrial papers with curl problems.