Environmental Engineering Reference
In-Depth Information
6.3.2
Cellulose Derivatives
6.3.2.1
Cellulose Inorganic Esters
Cellulose nitrates are the most important inorganic esters of cellulose. Depending
on the degree of substitution (DS), that is, the average number of hydroxyl groups
modified in a unitary glucose unit, the cellulose nitrates go from a resistant but
inflammable polymer for film and photographic applications (celluloid grade,
DS = 1.9) through a polymer for lacquers (DS = 2.05-2.35) to a powerful explo-
sive that burns spontaneously in air (gun cotton, DS = 2.7).
The industrial production of vegetal parchment usually sold as paper for the
bakery industry involves the formation of another cellulose inorganic ester: cel-
lulose sulphates. As part of this process, a continuous sheet of paper is immersed
for a few seconds in a concentrated sulphuric (65-75%) acid bath maintained at
low temperature. Sulphates are formed and acid hydrolysis of the cellulose fibres
starts. The sheet is then passed through several rinsing baths to hydrolyse the sul-
phates and to eliminate the sulphuric acid (Figure 6.14). A continuous matrix of
gelatinised cellulose is formed on both sides of the paper, which protects the inner
mat of cellulose fibres. The latter ensures rigidity and mechanical resistance,
whereas the continuous matrix shows extraordinary hydrophobic and lipophobic
properties as well as high resistance to temperature. As a result, kitchen parch-
ment is resistant to humidity and to greasy food and it is perfectly adapted for
cooking.
6.3.2.2
Cellulose Organic Esters
The synthesis of cellulose organic esters can be accomplished in many ways. The
acylation of the hydroxyl groups of cellulose require strong agents such as acid
chlorides (Figure 6.15) or acid anhydrides. The former are preferred for long fatty
chains but it is necessary to use a strong base such as pyridine to neutralise the
formed HCl, which can cause extensive degradation of the biopolymer. Other
systems without pyridine have been proposed to limit the degradation of cellulose
Vegetal
parchment
Paper
Washing spray
1
2
3 …10
11 2…
19
20
Sulphurisation
oarse washing
Deep
washing
Additives
bath
Drying
Figure 6.14
Process for the production of vegetal parchment.
