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R.H. of the air with which the film base is in equilibrium. Therefore, these factors must be held constant if truly comparative values are to be obtained.
Physical Characteristics
The most important physical characteristic of film base is that connected with swelling and shrinkage. In discussing this question, a clear distinction must be made between two forms :
(a) Temporary, or reversible shrinkage.
(b) Permanent, or irreversible shrinkage.
Temporary stretch or shrinkage may be caused through variations in the moisture content of the film, or through changes in temperature :—
(i) Moisture : Film base swells when it absorbs moisture and shrinks when it gives it up. Consequently, the dimensions of a film will vary according to the R.H. of the air with which the base is in equilibrium. The degree of change in dimension along the film ranges between 0.05% and 0.12% per 10% change in R.H.
(ii) Heat : Like most other materials, film base expands if heated and shrinks if cooled. The change is of the order of 0.05% per 10° F. at normal temperatures.
Since in average handling conditions humidity varies over a wider range than temperature, and the change per 1% R.H. is usually greater than that per degree F., the effect of moisture on dimensional changes predominates. Because of this, film frequently is found to shrink when exposed to a warm
10 15 20
TIME. DAYS
g. 2. Retention of tear strength of film base at 100° C.
10 15 20
TIME. DAYS
Fig. 3. Retention of flexibility of film base at 100° C.
atmosphere, because raising the temperature is usuallv accompanied by a fall in R.H.
Permanent shrinkage is caused principally through evaporation of residual solvent and plasticiser from the base. It is commercially impossible to remove all traces of solvent in manufacture, so that some evaporation later is inevitable. Shrinkage characteristics are, however, purposely controlled to^ome extent by the amount of residual solvent left in the base.
Another important characteristic is the viscosity degradation of the base. If a film base deteriorates chemically on keeping, then when a sample is dissolved in a suitable solvent the solution will have a lowered viscosity as compared with one prepared with a fresh film sample. This viscosity degradation is a measure of the chemical stability of the base.
Accelerated Aging
The mechanical and physical characteristics just described are used as a yardstick in assessing the quality of a film base : but obviously it is of great importance to know, not only how freshly made base measures up, but also