Bio-Based Materials - Introduction to Chemicals from Biomass - page 222

6.3.1.2 Regeneration of Cellulose without Chemical Modification

The processes and products described in this section use solvents to turn cellulose
into a solution that can be filtered and extruded. No derivatisation of the cellulose
hydroxyl groups occurs; precipitation therefore takes place instead of chemical
regeneration.
Among the most ancient solvents for cellulose we find cuprammonium
hydroxide and cupriethylenediamine, but only the former was used industri-
ally to yield the cupro fibre. Later, other cellulose solvents were developed
such as the N -methylmorpholine- N -oxide (NMMO) in water and the systems
of lithium chloride with an aprotic polar solvent (dimethyl acetamide, dime-
thyl formamide and dimethyl sulfoxide). Only the former has been successful
however, yielding the lyocell fibre. More recently, ionic liquids capable of
dissolving cellulose have been developed. The most widely known is 1- N -butyl-
3-methylimidazolium chloride (BMIMCl). However, no significant industrial
development of yarn fabrication with this process has been reported so far. The
reasons why some of these solvents have not found industrial outlets include
poor mechanical properties of the fibre (too rigid or low elasticity) and diffi-
culties in the control of all the process parameters and in the recycling of the
solvent.
All the cellulose regeneration processes, with or without chemical modifica-
tion, cause cellulose molecules to organise in a different crystalline form called
cellulose II and sometimes cellulose IV (especially in high-wet-modulus- or
HWM-modal fibres).
It is worth noting that the mercerisation process, introduced in the nineteenth
century, also allows a cellulose II structure to be obtained, but without dissolution
of the fibres and therefore with no reshaping. Cotton fibres are soaked in a con-
centrated (19%) NaOH solution then washed. Mercerised cotton is softer to touch
and demonstrates more brilliance than natural cotton.

6.3.1.3 Microcrystalline Cellulose

Microcrystalline cellulose (MCC) is obtained by a controlled acid treatment
intended to destroy the molecular bonding in the amorphous zones of cellulose.
Typically, HCl or H 2 SO 4 are used at 110°C for 15 min over native cellulose or
regenerated cellulose. Colloidal gels are therefore obtained showing thixotropy.
MCC is used in the preparation of pharmaceutical compressed tablets due to its
binding and disintegration properties.
In the ice-cream industry, MCC is used to avoid the formation of large ice crys-
tals, increasing smoothness. MCC is also used as a rheological modifier in water-
based paintings and toothpaste. In chemistry laboratories, MCC is also used as a
support for chromatography (column, thin layer, etc.).