At the suggestion of Page and Tydeman, we have examined the structure of freely dried handsheets, using the scanning electron microscope . This was done because of the unique advantages of this instrument for this type of examination and in the hope that an independent test of their theory could be made.
In the present work, unbleached kraft pulp was beaten to 450 CSF and 60 g/m2 handsheets were formed in a British sheet machine under, standard conditions. The sheets were pressed at 50 lb/in2 between plates and blotters. After pressing, one sheet was dried on its plate with the usual restraining ring in place. A second sheet was peeled from the plate, floated on mercury so that it was free to shrink and air dried at room temperature.
The surfaces of these two sheets were then examined in the scanning electron microscope, using the technique described elsewhere at this symposium (pp . 101-108). The method of examination was to choose several fields at random and take low magnification pictures of these. In each of these low magnification fields, several right angle fibre crossings were selected and photographed at higher magnification. These were then used to estimate the extent of compression in fibres over bonded areas.
Visual examination of the pictures gave the following results. Three independent observers agreed that 12 out of 16 fibre crossings in the freely dried handsheet exhibited microcompressions compared with only 3 out of 12 in the plate-dried handsheet. In addition, the severity of the compressions was obviously greater in the former. For both handsheets, doubtful cases were listed as having no compressions.
In this short note, it is impossible to illustrate completely the extent and variety of the microcompressions observed. Only eight pictures of the freely dried handsheet have been chosen to illustrate some of the results (Fig. 1-8).