Experimental

The heartwood section of Populus tremuloides was chosen as a substrate for the

impregnation study. A cylindrical probe with a radius a = 25 mm and a length

Z = 150 mm was prepared. The axis of the cylindrical sample is parallel to the

fiber axis representing the longitudinal direction. The radial surface represents

the radial and tangential surfaces of the wood sample.

The bare surfaces were exposed in the impregnator to a solution of 2.3 mol L–1

NaOH for 60 and 15 min for radial and axial impregnation, respectively, under a

variety of conditions (temperature 25, 50, 75 or 100 °C; pressure: 200, 790, 1480

and 2179 kPa). The experimental set-up and the conditions are described in detail

elsewhere [56,57]. The concentration profile of Na+ ions, the diffusing species,

was determined by measuring the local X-ray intensity on the sliced sample. After

completion of the impregnation, the samples were frozen and sliced at marked

locations using a fine power saw. The marked samples were then subjected to

scanning electron microscopy analysis. The X-ray intensity of the sodium element

at the sample edge was defined as reference intensity, I0. The X-ray intensity is

directly proportional to the concentration of the Na+ ion. Thus, the X-ray intensity

ratio, I/I0, translates to the concentration ratio, c/c0. Equations (68) and (69) can

be solved using I/I0.

154 4 Chemical Pulping Processes