Combining the pH measurement with the solids
fraction measurements (Fig. 6), we are then able to
establish the solids fraction when crosslinking is
favored. The figure shows that the pH decreases below
7 when a solids content of approximately 68 wt% is
achieved. Depending on the particular packing of the
particles, this solids content would correspond to the
point of contact between particles. (For mono-size
hard particles, the maximum solids fraction at close
packing is 74 vol%.) The gas chromatography data
clearly showed that the chemical reaction rate increases
substantially below a pH of 7. Therefore,
crosslinking in the film is favorable only near and after
the point of close packing. Polymer interdiffusion
likewise cannot occur until particles have made
physical contact, of course. Thus, interdiffusion and