Environmental Engineering Reference
In-Depth Information
O
Cell OH
+
R
Cell OCR
O
+
HCl
Cl
Figure 6.15
Synthesis of cellulose organic esters by reaction with fatty acid chlorides.
and include the use of partial vacuum [17] or dry nitrogen gas flow [18] to take
HCl out of the reactor.
Nevertheless, the use of fatty acids (>C6) is still possible but only with the use
of a co-reagent that forms
in situ
new stronger entities. Among others, we can cite
trifluoroacetic anhydride [19] and
N,N
-dicyclohexylcarbodiimide [20].
Cellulose acetates
are by far the most important organic ester. Diacetate
(DS = 2.4) is produced either in filament form for fibres or in powder for melt
processing. Diacetate filaments are obtained by dissolution in acetone, extrusion
through a spin and then evaporation of the solvent. The obtained fibres are used in
textiles (called simply 'acetate') and in cigarette filters (tow). Triacetate (DS = 2.9)
finds application in brilliant easy-to-dye textiles.
The industrial production of cellulose diacetate utilises acetic anhydride and
sulphuric acid as catalyst. Reaction is conducted at a low temperature and cellu-
lose starts to dissolve in the acetylation bath as the reaction progresses. The reac-
tion is conducted until practically complete acetylation. The homogeneous
solution obtained is then hydrolysed to reduce DS to a value lower than 2.4.
Precipitation in diluted acetic acid, washing with water and final drying yields
cellulose acetate flakes.
The dramatic reduction of hydrogen bonding renders cellulose acetates thermo-
plastic. However, their softening points are very close to their decomposition tem-
perature (around 300°C). As a result, external plasticisers (e.g. phthalates) are
often used to process cellulose diacetate in plastic applications (eyewear, screw-
driver handles, etc).
Cellulose mixed esters
(acetate-butyrate, acetate propionate) are used in the
same plastic applications. The irregularity introduced by two different substitu-
ents diminishes the glass transition to around 120°C. Moreover, the combination
of a short side-chain such as acetate and a long chain from a fatty acid (in a 2.4:1
ratio respectively) results in very interesting biomaterials; they are as hydrophobic
as simple fatty esters but with better mechanical properties (Figure 6.16) and
higher glass transition (Figure 6.17). Their biodegradability is also increased due
to changes in crystallinity.
Cellulose esters of long-chain carboxylic acids (up to C
20
) are interesting ther-
moplastic materials. An exhaustive description of all the synthesis methods has
recently been reported [21]. These derivatives present softening points between
70 and 250°C depending on the DS and the number of carbon atoms in the acyl
chain [22]. The higher the DS value, the lower the softening point. Nevertheless,
the difficulty in obtaining high DS values without using toxic chemicals is a
