A lumen-loaded pulp is one containing filler which is confined to the lumen surfaces of the fibres. Prerequisites for obtaining such a pulp are filler particles which are small enough to pass through the pit apertures and chemical conditions favourable to a good bond between the particles and the lumen surface.
The first stage in the preparation of lumen-loaded pulp is agitation of the fibres in a concentrated suspension of filler. At high levels of agitation, entry of the filler particles into the lumens appears to be very rapid and the rate of the uptake of particles by the lumen surface is predictable from a Langmuir-type, adsorption-desorption mechanism. At prolonged times of agitation, a plateau level of loading is achieved which approximates to single particle coverage of those sites on the lumen surface capable of holding particles in the applied turbulent field. The second stage in the preparation is that of a wash which removes all filler particles not bound to lumen surfaces.
If subsequent to washing, the loaded fibres are subjected to high turbulence in water, some filler is lost relatively rapidly but the residual filler is much more resistant to removal. The more weakly-held filler can be kept to a minimum by the use of high levels of shear throughout preparation.