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Appendix 4
Appendix 4: Deterioration Pathway of Cellulose Acetate
Degradation of Cellulose Acetate
There are two main mechanisms involved in the degradation of
cellulose acetate: hydrolysis and oxidation. Hydrolysis may be of the cellulose
acetate polymer or of the plasticiser (Glastrup, 1995). Degradation may derive
from the nature of the film, from processing, or from storage conditions, and
may be affected by humidity, temperature, pollutants, water, light, fungal
growth, insects, chemicals, and physical damage (ISO 5466: 1992 (E)).
Hydrolysis is the major decomposition reaction, and involves
the removal of side groups from the polymer backbone. This deacetylation may be
assisted by the presence of acids, catalysts and high temperatures. Hydrolysis
may be followed by hydrolytic or oxidative scission of the glycosidic linkages
in the cellulose chain, resulting in shrinkage and weakening of the base. As
the polymer degrades, it loses its ability to retain the plasticiser, which may
migrate to the surface of the film. Volatile residual solvents may escape,
particularly as the base decomposes. Loss of plasticiser and residual solvent
will result in shrinkage, and the gelatin emulsion may separate from the base
in some areas.
The Vinegar Syndrome
Deacetylation of cellulose acetate will occur in the
presence of water. The acetyl side groups will react with hydrogen from the
water molecule, forming acetic acid, which may diffuse to the surface and
escape. Cellulose acetate can contain approximately 6 % water by weight, and if
excessive desiccation takes place the base may shrink. There is therefore
necessarily some water present at all times, permitting hydrolysis. Elevated
relative humidity and temperatures will accelerate the reaction, as will acids.
The acetic acid formed as a by product of degradation may take part in
catalysing further deacetylation, and the reaction becomes autocatalytic.
Degradation has been described as having a slow induction
period, wherein the rate of hydrolysis is governed primarily by temperature and
relative humidity, and an autocatalytic point, past which the reactions proceed
increasingly rapidly and independently of heat and moisture (they are
self-fuelling) (Reilly, 1993). This autocatalytic point is considered to be at
0.5 free acidity, and marks the onset of what is known as the vinegar syndrome.
The cellulose acetate will only begin to smell of vinegar at this stage; films
with the vinegar odour are therefore already in quite an advanced stage of
degradation.
The reaction rate has been represented diagrammatically as
follows:

Free acidity is not the same as pH. It is a measure of the
amount of free acid in a system, and is found by titration (Roff and Scott,
1971). It is considered to be more sensitive and reliable than pH measurements.
Deacetylation and the formation of acetic acid is an
exothermic reaction, and the heat produced may also help to spur the process.
Hydrolysis may be represented as:

There are many factors that may influence the rate and
degree of degradation of cellulose acetate. These include:
- The presence of iron particles (which may act as a
catalyst).
- The quantity / type of plasticiser present in the base.
- Pollutants.
- The presence of nitrogen dioxide from degrading cellulose
nitrate.
- The degree of substitution (DS) of the polymer.
- The presence / thickness of a gelatin emulsion.
Symptoms of Deterioration
As previously stated, there are few signs of cellulose
acetate degradation during the induction period. Once past the autocatalytic
point, when the film is already in quite an advanced stage of deterioration,
the more noticeable symptoms will begin to appear. These may include:
- embrittlement (through chain scission),
- shrinkage (through chain scission / loss of
solvents and plasticiser),
- formation of crystals / bubbles on the surface
(through migration of the plasticiser), and
- the appearance of a pink / blue colour (through
reversion of anti-halation dyes in the gelatin backing layer to their coloured
form, by reaction with acetic acid)
(Reilly, 1993).
Classification of deterioration
Cellulose acetate degradation has been classed
into five stages:
- Level 1: Good physical condition. Slight vinegar
smell.
- Level 2: Good physical condition. Strong vinegar
smell
- Level 3: Edge warping (slight ripples) apparent.
Some embrittlement possible.
- Level 4: Strong physical distortions.
Plasticiser on surface (bubbles / crystals). Some embrittlement.
- Level 5: Emulsion begins to separate from the
base (channeling or buckling). Plasticiser crystals. Considerable
embrittlement.
(Nishimura, 1995)
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