For wood and plant source materials, the pre-treatments are
similar and consist of techniques that are usually used in pulp
and paper industry. In practice, lignin impedes separation of wood
into its component fibres, so delignification is a necessary preparation
step to produce NCC. Excellent descriptions of pulping and
bleaching processes are available, for instance in the review by
Siquera, Bras, et al. (2010), and the book by Smook (1992). Here
we may briefly say that the processes mainly consist of a chemical
treatment (pulping) of biomass previously chipped to depolymerize
and eventually solubilize lignin and hemicelluloses, and
a subsequent bleaching with oxidizing agents such as oxygen or
NaClO2.
The steam explosion process is another efficient pre-treatment
method for converting lignocellulosic biomass with the final aim of
separating nanofibers (Cherian et al., 2010; Naik et al., 2010). Steam
explosion pre-treatment technologies have been an active area of
research for the past two decades, especially because the resulting
feedstock is much more amenable to enzymatic hydrolysis (Hayes,
2009). In this process the biomass sample is first milled and then
subjected to high pressure steam for short time (20 s to 20 min) at a
temperature 200–270 ◦C and a pressure of 14–16 bar. The pressure
in digester is then dropped quickly by opening the steam and the
material is exposed to normal atmospheric pressure to cause explosion
which break down lignocellulosic structure. Steam explosion
causes the hemicelluloses and lignin from the wood to be decomposed
and converted into low molecular weight fraction which can
be recovered by extraction. Therefore most of water soluble fraction
of hemicelluloses can be removed by water extraction. At the
same time, a part of the low molecular weight fraction of lignin is
also extracted. Other chemical treatments are necessary to remove
all lignin content. All these steps allow elimination of lignin and
hemicelluloses, while leaving cellulose moieties intact if optimal
conditions are respected. The effectiveness of the steam explosion
is dependent on the biomass feedstock and, for instance, the process
is less effective for softwood than for hardwood