Journal of Film Preservation - FIAF

macro-molecular chain scission takes place. In cellulases action a
decrease in the molecular weigh (MW) of the polymers has been
observed. Hence, the initial MW value of 50.000 (DS=1.7) was reduced
through the biodegradation process to 8.500.
Figure 10: Cellulose triacetate unit with its carbon
numbers
47 Journal of Film Preservation / 67 / 2004
This mechanism is in
agreement with
common
microbiological
knowledge. There
should be at least
two neighbouring
unsubstituted
glucose molecules
before
biodegradation by
cellulases can start.
The probability of
their existence decreases as the degree of acetylation increases, and
experimental results suggest that biodegradation increases as the
degree of acetylation decreases. Samios 21 and co-workers have
published a mathematical model to quantify such probability.
Sakai22 and co-workers, Moriyoshi23 and co-workers, and Nelsonxxiv and co-workers have described the biodegradability of cellulose
acetates. The organisms used in these studies were Pseudomonas and
Neisseria isolated from soils. Both bacteria can assimilate cellulose
acetates using them as a carbon source. The results are coincident with
those obtained employing the fungi Aspergillus. This confirms that the
esterases involved in the early steps of cellulose acetate degradation
are similar, and facilitate the further hydrolysis of the main chains of
cellulose acetate by cellulolytic enzymes.
Even though the studies focussed on the degradation of cellulose
acetates, to this day little is known about the mechanism responsible
for enzymatic degradation of cellulose acetate plastics. Therefore,
further detailed studies on degrading micro-organisms and enzymes
are necessary to clarified the mechanisms of cellulose acetates and,
particularly, of photographic plastics materials.
Biodegradation of Gelatine
Gelatine structure
Evidence of gelatine use appears to go back at least 5,000 years. Its
name derives from the Latin word “gelatus” which means firm or
frozen. Probably, gelatine was discovered as a clear liquid by cooking
bones or skins, which, when dried has adhesive properties. Gelatines
are high molecular weight polypeptides derived from collagen.
The molecular structure of collagen is formed by three helical peptide
chains coupled in a parallel association giving rise a rod-like structure
(1.4 x 300nm). This structure is shown in figure 11.
Collagen is transformed into gelatine by thermal
de-naturation, and some breaking of peptide
bonds always takes place. Gelatines can be
manufactured reproducibly with only slight
degradation. Different types of gelatines25, 26,
Figure 11: Collagen
structure