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Gelatin in the emulsion is the principle component of the binder layer. At various times attempts have been made to substitute other substances for gelatin, but none have been able to match gelatin’s overall performance.
Gelatin does not occur in nature. It is a commercial product derived from the principle protein of the skin, bones and sinews of animals. The production of photographic gelatin requires that the inorganic components are removed first. This is done by soaking in dilute hydrochloric acid. The demineralised product, called ossein, is washed and treated with dilute alkali to neutralise it.
The next stage is a long soak in strong alkali (lime processing). During this stage the helical structure is broken up to produce long coiled chains of gelatin. The gelatin produced by this alkali ossein process exhibits its minimum solubility in the acid range somewhere around pH 4.8.
Photographic gelatin has many physical properties that make it very suitable for use in motion picture film.
Should any change occur in these physical properties then the integrity of the film’s content can be compromised.
From a conservation point of view it is the chemical properties of gelatin that are most significant. Gelatin is an interesting substance in that it can behave as an acid or a base, it is amphoteric. It can form a strong structure that is quite chemically stable and can even act as a buffer to reduce the affect of pollutants on the image forming materials.
The stable structure of gelatin as it is found in a photographic emulsion under 'normal’ conditions is that of long tightly coiled chains. In this state the gelatin is at its iso electric point (IEP) and the pH is around 4.8. At the IEP the coils have an equal balance of basic amino groups and acidic carboxylic groups. Consequently there is no repulsion of like charges along the coils.
fig 4.1 Swelling curve for a lime processed gelatin
Shepard et al, 1942
As the pH swings away from the IEP towards a more acidic condition, such as will be found as film decomposes, the amino groups change to become positively charged (+), if a similar swing towards a more basic condition occurs the carboxylic group changes to become more negatively charged (-). As the charge on the coils changes to either a net positive or net negative condition the coils will repel each other and uncoil slightly. This causes the gelatin to increase in solubility and significantly swell, Fig 4.1 & 4.2.
Although swelling is primarily dependant upon pH some ionic and nonionic solutes can interact to promote swelling, notably SO4-2 (sulfates).
Swelling of gelatin will take place evenly in all directions depending upon the adhesion of the layer to a rigid support. This has an effect by constraining the swelling to the direction perpendicular to the support.
The change in pH caused by decomposition has a significant effect on the emulsion gelatin. The increased solubility and general softening of the emulsion layer increases the risk of damage to the image and reduces the options available for treatment.
Gelatin is amorphous — it has a non defined structure. The slight moisture content, even in 'dry’ gelatin, gives the flexibility needed for motion picture use. If there is an increase in the moisture content at a given temperature the gelatin will become 'softer’ with a greater tendancy to flow. The point at which this occurs is called the 'glass transition point’(Tg). There is a direct relationship between temperature and moisture content, the higher the moisture content the lower the temperature at which Tg is reached and vice versa.
Glass transition is not the same as melting. Melting occurs in crystalline substances. Melting happens when the polymer chains fall out of their crystal structures and become a disordered liquid. The glass transition phenomenon only happens to amorphous polymers.