Blocked decomposed film
As a film decomposes the acids formed create an environment in the film base polymer that causes the plasticisers that are physically attached to the chains to detach and migrate to the surface of the film base. Some of the plasticisers will move through the emulsion, creating small rupture marks. In extreme cases these will cause large holes to appear in the emulsion. Examples of these holes in the upper left and right corners are shown in Fig 12.16.
While the acid levels are building the gelatin is also being affected, the decrease in pH creating a positively charged surface. Plasticisers are attracted to positive charges and will bond to the charges on the surface of the gelatin, emulsion and backing layer. The plasticisers also form intermolecular bonds that eventually form a solid layer that is bonded to each of the gelatin layers. This turns the film into a solid blocked mass.
Unblocking decomposed film
Since it is the plasticisers that are creating the adhesion to unblock the film, a solvent that dissolves the plasticisers is required. Selection of a suitable solvent is critical to prevent minimal leaching of the plasticisers that still remain in the film base.
Ethanol is a very effective solvent but will also readily leach any remaining plasticisers from the base. Ethanol also reacts with the acid (acetic and nitric – see note below) with one of the by-products being water. The water produced will substantially soften the emulsion gelatin. To reduce the amount of water produced an approximately two per cent mixture of ethanol and 3M HFE 71-DA is used, the exact concentration needed is established on a case-by-case basis.
To apply the solvent, use a small syringe to drop a small amount of solution at the interface between the layers of film as it is unwound.
The procedure is slow and painstaking and requires a deal of care to prevent damage to the emulsion, which becomes very soft. Fortunately the dissolution of the plasticiser is a fast reaction.
WARNING: ethanol reacts exothermically with nitric acid and generates toxic nitrogen oxides. For this reason ethanol should not be used for unblocking nitrate film.