The effect of plasticizers
But that’s not the whole story. Photographic film base incorporates a plasticizer at around 15% by weight of the film base and that’s, typically, a phthalate ester combination with triphenyl phosphate. In many severely degraded films, white crystalline deposits can be seen on the surface of the film material itself. We ran chemical tests and discovered that the crystalline deposit was plasticizer material. What influence might the plasticizer have? It must be playing some part, whether it be active or merely facile, because it’s changing - coming out of the support material itself.
Most phthalate testers (which include dimethyl, diethyl and dibutyl) have lots of CH2 linkages that are particularly unstable and susceptible to breakdown by the presence of oxygen. In going from dimethyl to diethyl to dibutyl, the amount of oxygen taken up increases. We believe that, under the influence of oxygen, those systems then break down to produce what we call peroxides and acid. Triphenyl phosphate doesn’t possess any of those units and so apparently doesn’t absorb any oxygen and doesn’t produce any acid or peroxide.
When polymers break down, when they interact with oxygen, they produce peroxides, which are notoriously unstable, and break down to form radicals. Now a chemical bond is composed of two electrons and is relatively stable, but when a chemical bond is broken to produce radicals, the radicals have only one electron - and electrons, like people, don’t like being alone. They’d rather be paired up. So any radical on its own will seek another electron; if the electron is paired up in another bond, it will grab hold of it and break the bond. This will produce more radicals, and the whole thing takes off and produces more and more radicals. The result is more and more broken chemical bonds. The presence of peroxides in a polymer system is, therefore, particularly important.
We decided to artificially age polymer systems - movie film - with and without plasticizer material, and look at the viscosity retention as one parameter relating to that breakdown. In the presence of plasticizer, the film seemed be in much better condition and retained its viscosity to a much higher degree than when no plasticizer was present. We wondered if the plasticizers somehow contributed to the stability of the polymer itself. The plasticizer was obviously being lost from the polymer, so we looked at percentage plasticizer loss in a given time, at a given temperature, under conditions of high relative humidity and acetic acid conditions. In the presence of acetic acid, a far greater proportion of plasticizer was lost from the film.
The plasticizer is designed so that its chemical structure is specifically compatible with the original base material. We believe that, as the base material changes its chemical composition (loses acetate groups, the chain breaks down, etc), the plasticizer is no longer compatible with that system and is pushed to the surface, where it crystallizes out. So, plasticizers don’t contribute to the instability of the material, rather they are lost because the material itself is breaking down.
