have higher resistance to acid attack (Habibi et al., 2010). Typical
procedures currently employed consist of several steps, as shown
in scheme in Fig. 1. It must be said that the very limited commercial
availability of NCC is mainly due to the time consuming production
process and the low yield.
Typical procedures for the production of NCC consist of the following
steps:
(1) Strong acid hydrolysis of pure cellulosic material under strictly
controlled conditions of temperature, time, agitation, and with
control of other conditions such as nature and concentration of
the acid and the acid to cellulose ratio;
(2) Dilution with water to stop reaction and repeated washing with
successive centrifugation;
(3) Extensive dialysis against distilled water to fully remove free
acid molecules;
(4) Mechanical treatment, usually sonication, to disperse the
nanocrystals as a uniform stable suspension;
(5) Eventual concentration and drying of the suspension to yield
solid NCC.
Perhaps one of the most detailed reports on sulphuric acid
hydrolysis of cellulose was provided by Dong et al. (1998): it
includes investigation on the effects of temperature, reaction time,
and sonication on the properties of resulting nanocrystals. In particular
they showed how as the reaction time increased from 10 to
240 min at 45 ◦C the surface charge of crystallite generally increased
while the length of the crystallites decreased and approximately
levelled off.