Ball-milling Promoted Chemical Bonding between Cellulose and Plastics - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 220

Such interactions are promoted most by addition of a specific amount (30 wt%)
of water to amorphous cellulose. However, no mechanism has been resolved to
explain this unique crystalline transformation into cellulose II.
Another characteristic of amorphous cellulose prepared by ball milling of
native cellulose is related to promoted aggregation between powders by hot-
pressing. Hot-pressing of amorphous cellulose powders with a controlled
amount of water added (W c )ofca. 3-6 wt% yields a transparent plate, if
molding temperature and pressure are arranged according to the amount of
water. For example, the addition of 3.8 wt% water yields a most transparent
plate at 150 1C and 200 MPa; a small increased addition of 5.8 wt% water at a
decreased temperature of 120 1C and the same pressure has the same effect.
With W c smaller than 3 wt%, a resultant plate remains white and fragile,
while it becomes slightly brown with W c ¼ ca. 6 wt% and contracts with
cracks when W c is greater than 10 wt%. With W c smaller than 15 wt%, cel-
lulose exhibits recrystallization to cellulose I during hot-pressing.
Figure 9.12 shows crystallinity indexes (CI) 19 of the surface part of cellulose
plates at 200 MPa against molding temperature. With W c ¼ 0 wt% the CI
hardly changes in the range 25-180 1C, indicating that cellulose in the plate
is still amorphous. With W c ¼ 3.8 wt%, which gives a most transparent plate
at 150 1C and 200 MPa, the CI increases with milling temperature, particu-
larly at greater than 120 1C, suggesting that formation of the transparent
plate is accompanied by recrystallization. For W c ¼ 14.8 wt%, which often
results in plates with cracks, recrystallization occurs even at the lower tem-
perature, and the CI levels off at temperatures greater than 150 1C, probably
due to pyrolysis of cellulose molecules, considering the marked decrease in
the DP at the same temperature. 23 Note that the averaged CI of the whole
plate, which is prepared with W c ¼ 3.8 wt% at 150 1C and 200 MPa, is 24%,
which is smaller than the 33% shown in Figure 9.12.
If amorphous powders of cellulose are hydrothermally treated at 120 1C for
10 min, the resultant powders do not yield a transparent plate under any hot-
press conditions. The CI value of the powders is ca. 40%, suggesting that a
network of hydrogen bonds is formed principally within each powder, but
not between powders. Therefore, a mechanism for producing a transparent
plate by hot-pressing is based on formation of hydrogen bonds between
cellulose powders during recrystallization to cellulose I. Water molecules
adsorbed in cellulose are bound primarily to hydroxyl groups of cellulose
chains in amorphous regions; breaking hydrogen bonds originally formed
there, and consequently activating them, elevates the mobility of cellulose
chains. Accordingly, formation of a new network of hydrogen bonds ac-
companied by removal of water molecules under the hot-pressing treatment
causes rearrangement and recrystallization of cellulose molecules particu-
larly near the surface of a plate more than in the inner region where water
cannot be fully removed.
The two characteristics of amorphous cellulose demonstrate that native
cellulose can be activated by ball milling. As described below, such
amorphous cellulose can react even with maleic anhydride in the solid state.