Search

Anisotropy resulting from fiber orientation distribution is an important factor in paper mechanical strength and dimensional stability. An optical method designed to measure this property in paper and other dielectric sheet materials, using far-infrared waves, is presented. Paper acts as a dichroic polarizer in a narrow band around 100 cm-1, and, according to correlations with other characterization techniques, the amplitude of this polarizing effect is related to fiber orientation anisotropy. After a description of this technique and the hardware involved, several results and applications of this type of measurement are presented and discussed.

This report describes experiments aimed at demonstrating the potential of the submillimetre laser in the field of nondestructive material testing. Most of the research has concentrated on paper properties, such as sheet density, moisture content, and fibre orientation, but other materials have also been tested. The results show that this type of laser is very well suited to the measurement of various physical properties of dielectric sheet materials.

Optical thin film methods were used to determine the refractive index and absorption coefficient of paper at 70.5,96.5, and 118.8 μm. Transmittances and reflectances were measured of paper, two quartz and two TPX plates, as well as of the paper pressed between the quartz and TPX plates. Measurements were performed at angles of incidence between 0 and 50°. The optical constants found provided a reasonable fit between the experimental points and the calculated performance. The small residual departures between the two sets of data are probably due to inhomogeneities within the paper.

The use of submillimetre (SMM) waves for non destructive testing of paper has already been demonstrated. This report presents the basic results wich led to the use of SMM waves to monitor the fibre orientation anisotropy in paper. The measurement of moisture content and basis weight is also covered. An experiment to test the effect of moisture content on the polarizing properties of paper is presented. Measures were made for three wavelengths (70, 96 and 118 microns) and for moisture content covering the region 0 to 40 %. The effect of bound water to cellulose surface will be introduced to explain the results.