Paper surface and thermal, electrical and friction characteristics
This section covers the macroscopic surface properties of paper and the physical properties that are directly related to its surface geometry. Emphasis is given to empirical characterisation methods and papermaking effects. The relationship between surface properties and the fibre network structure is given limited attention, although the undulations of the paper surface are certainly related to local grammage variations, sheet consolidation, calendering and, in general, the pores under the surface.
The roughness of the paper surface is important particularly in printing papers and graphical boards, which are often coated to improve their printing properties. The required amount of coating colour depends on the roughness of the base paper. In the following discussion, the study of paper roughness focuses on characterisation methods. Physical mechanisms, such as the effects of local compressibility, are also examined to some extent.
Several physical phenomena are directly dependent on the surface geometry. Two such phenomena are the electrical properties and friction of paper. Surface chemistry seems to be the primary factor that determines the friction of paper. At first glance, it may be surprising that the surface topology of paper is not important.
The porous nature of the three-dimensional structure of paper makes it complicated to define the surface of paper. A two-dimensional height map measured by profilometry gives an apparent surface but hides the fact that this apparently well-defined object includes overhangs: moving along the surface, in reality one often would end up under a fibre that covers a pore directly connected to the surface.
These above-mentioned matters are discussed more in detail in the following articles.