The film of fibrous cellulose triacetate which was made by the vapour phase acetylation, is brittle due to its molecular symmetry and crystallinity.
In order to improve this defect, its molecular symmetry must be broken, with this object in view, the properly pretreated cellulose was mix-acylated by the vapour of acetic and propionic or butyric anhydride mixtures of various mol ratios using ZnCl
2 catalyser and the reactions were investigated kinetically.
Given reaction extents (mol fraction) of the mixed acylation as
x,
y, the applied rate equation becomes:
In the acetylation-propionation or acetylation-butyration, the total rate constant
k is the sum of the anhydride mol-fractional means of the respective independent rate constants, but acetylation rate tends to become smaller and propionation or butyration rate larger than the mol-fractional mean. This was worked out also from the activation energies.
The films of the soluble mixed triesters, containing more than 20 mol% of propionyl group or more than 10 mol% of butyryl group are not brittle.
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