Fast Elasticity Properties of Paper Web, 1999 Papermakers Conference Proceedings

Matti Kurki, Pasi Kekko, Pekka Martikainen--The increase in machine speeds is challenging paper machine designers throughout the paper making line. Increased velocities (up to 2000 m/min) are lifting also the main velocity-dependent centripetal and aerodynamical forces affecting the web to a new level. This phenomenon is emphasized in places where open draws are still used. In these locations the paper web has to ultimately carry all of those forces.

During the paper making process the web will go through several very fast strains. Standard paper testing methods cannot describe all the necessary features connected to fast elasticity behavior of the web. Fast straining of the paper is intensively connected to time-dependent behavior of the paper. This rheological behavior can be divided into two major behavior types, relaxation and creeping. Both of these features are closely related to the breaking strength of the paper.

To study these essential properties a new type of testing equipment has been developed. Both fast relaxation and dynamical strength behavior of the paper can be studied with this Impact equipment. Testing includes standard-sized paper strip which can be strained with a velocity of 1.0 m/s. By adjusting the degree of straining the test can be either non-destructive (relaxation test) or destructive (dynamical strength).

The analyses performed in carefully prepared moisture and temperature conditions with newsprint paper show that, in the relaxation test, both maximum tension and relaxation tension are correlating with each other extremely well in many measuring cases. However, when changing the degree of strain applied to the strip, the maximum tension will increase almost linearly with the strain, but the relaxation tension starts to saturate to certain constant tension level despite the degree of strain increase.

The results obtained from the dynamical strength tests revealed a bilinear stress-strain dependence varying with paper grade and process variables. From this curve it is easy to separate almost linear “primary” and “additional” portions which have their own special behavior. The ”primary” strain has been found to be almost constant, i.e. not depending on any furnish preparing or processing operations in paper making.

The “additional” strain portion has been noticed to vary depending on the paper processing. For example, an increase of the relative velocity difference in the first draw between the press and dryer sections reduces the ”additional” strain available.

In conclusion, this new measuring method seems to give a new perspective to the analysis of paper material. It is also providing important knowledge in developing of the paper web less susceptible to runnability disturbances caused by high-speed web handling.