Photographic Science and Engineering. Vol. 3, No. 1, 1959
Presented at the Annual Conference, Rochester, N.Y., 9 October 1958 Received 12 September 1958.
Photographic films made with the more dimensionally stable supports, such as polystyrene, undergo small but significant size changes caused by the gelatin emulsion and gelatin backing layers. Some of the physical properties of these gelatin layers, measured on specimens stripped from a special base, are described. It is shown how the thickness, elastic modulus, humidity expansion, hysteresis, moisture relaxation, and stress relaxation of the gelatin layers affect the dimensional stability of the supported film. The modulus of the emulsion decreases on photographic processing and is very dependent on relative humidity. An unusual type of "reversed", dimensional hysteresis is described which is peculiar to gelatin-coated films on moisture-resistant supports. This effect is shown to have a major influence on the dimensional change of the film during processing.
New types of photographic film support made from various synthetic high polymers, such as polyvinyl chloride, polystyrene and polyesters, have come into general use in recent years. This has been so primarily in the graphic arts field, because of superior moisture resistance and improved dimensional ;stability. These materials permit closer registration in color work and more accurate reproduction of scale drawings without the difficulties involved in using glass plates. No photographic film is absolutely stable in dimensions, and the synthetic high polymer films will expand or shrink a few hundredths of a percent under some conditions. This can be troublesome if uncontrolled, particularly with large films. For example, a size change of only 0.02% is equal to 0.008 in. in a 40-in. sheet, which is more than can be tolerated in some applications. As these new films attain wider use where dimensional stability is critical, it becomes increasingly important to understand the nature of these small size changes and the manner in which then can be minimized
It has been pointed out previously that gelatin emulsions increase the humidity coefficient of linear expansion and also the permanent shrinkage of photographic film.' This is because the gelatin exerts a lateral compressive force on the base when dry and literally squeezes it into a smaller size. The effect of the emulsion and base thickness on dimensional stability was illustrated by Centa,2 and a theoretical relationship was derived by Umberger.3 Hysteresis has also been mentioned as a phenomenon responsible for small dimensional changes in film.1 With cellulose ester film supports the dimensional effects caused by gelatin layers are of secondary importance to the base shrinkage, but with the more moisture-resistant high polymer supports used in critical applications, such factors must be considered.
It is the purpose of this paper to describe some of the physical properties of gelatin emulsions and gelatin non-curl backing layers which affect the dimensional stability of low-shrink photographic films. Although polystyrene support was used in this investigation the conclusions reached concerning the effect of gelatin apply as well to films on other love-shrink supports Part I deals with the properties of the unsupported gelatin layers alone, and Part II deals with their effects on the dimensional behavior of the complete film.
Two different types of graphic arcs films on poly styrene base were used for this investigation:
|Nominal base thickness mils,||Nominal emulsion thickness, mils|
|Kodalith Ortho PB Film|
|Kodak Commercial PB Film||10||0.70|
Both Kodalith and Commercial films have a dye gelatin backing layer for antihalation and curl control) which has approximately the same thickness as the respective emulsions. Kodalith emulsion is quite thin, hard, and has a high silver gelatin ratio, whereas Commercial emulsion is relatively thick, soft, and has a low silver gelatin ratio. These differences are due to photographic requirements, but they also effect the physical characteristics of the emulsions and the dimensional stability of the films.
During the course of this work, Type 2 Kodalith emulsion was replaced by Type 3 to .provide photographic improvements; therefore, some data were obtained on each. However, both types have essentially the same physical properties except that Type 3 is slightly thinner.
Regular production lots of film were used wherever possible. In order to determine the effect of gelatin thickness on some film properties, it was necessary to make special coatings. In most work on unsupported gelatin layers reported in the literature, specimens for study were obtained by making hand coatings about 5 mils thick on glass plates. It has been our experience that this technique yields gelatin films having significantly different properties from those of normal photographic film coatings which are only 1/5 to 1/20 of this thickness. This difficulty was overcome in the present work by coating the emulsion or backing on a social film base having a very weak adhesive layer from which the gelatin coating could be readily stripped after drying. In the case of Kodalith emulsion and backing, the thickness was increased to 0.6 mils to facilitate handling of the stripped layer.
All special coatings were made using standard gelatin emulsion or backing and were applied on a pilot plant coating machine which was operated as closely as possible to commercial conditions. The coatings were then stored at 70°F and 50% RH until used.
The dimensional change measurements were made on 35mm x 12-in. specimens by means of a pin gage. In the case of unsupported gelatin layers, it was necessary, because of their extreme thinness, to reinforce the ends of the specimens with adhesive tape before perforating the holes to fit the gage pins. For measuring these materials the gage tension was reduced from the normal 150 grams to 50 grams by counterbalancing. For unsupported gelatin layers, five specimens were measured and the results averaged; for supported films the results for three specimens were averaged The accuracy of the dimensional change measurements is considered to be ±0.03% of the dimension for unsupported gelatin layers and ±0.005% for supported film.
Fig 1. Equilibrium moisture sorption of various unsupported gelatin film layers at 70°F
Since thickness measurements on the very thin gelatin layers are critical, special precautions were taken. Unsupported gelatin layers were measured with a dial micrometer to 0.00002 in. and the average of five measurements on individual samples recorded. Supported gelatin layers were measured by means of a microscope on sections of film cut with a microtome.
All conditioning of specimens was done in constant humidity rooms controlled to ±1.0% RH and ± 0.5°F. Sufficient time was allowed for equilibrium, usually two hours. Occasionally the conditioning time had to be extended overnight because of practical considerations.
As mentioned before the effect of gelatin layers on the dimensional stability of photographic film depends on the compressive forces they exert on the base. This is determine primarily by the thickness, humidity expansion and modulus elasticity of each layer. Several other factors are also involved and an understanding of the subject is aided by first discussing moisture sorption relationships and the phenomena of moisture relaxation and hysteresis.
The moisture sorption of gelatin at various relative humidities is a determining factor in its mechanical and dimensional properties. Moisture sorption curves for the various film layers used in the present study are shown in Fig. 1. The difference in equilibrium moisture content between the two gelatin emulsions and the gelatin backing layer is due to the difference in composition, principally the amount of silver halide. The very high moisture sorption of all the gelatin layers compared with polystyrene base, which has a maximum moisture take-up of less than 0.1% is one of the reasons for the adverse effect which gelatin has on the dimensional stability of photographic film. However, it will be shown later that the dimensional and mechanical properties of gelatin layers are affected by other factors as well as the moisture content.
Gelatin layers exhibit a phenomenon which may be described as moisture relaxation. This is a contributing factor in the small dimensional changes which occur in low-shrink photographic films.
Some years ago, C. D. Tate4 observed that the curl of a single-coated photographic film permanently increased when conditioned for an hour or two at high relative humidity (above 60% RH). This could be explained most readily by shrinkage of the gelatin emulsion. It was soon established that the gelatin did indeed shrink under these condition, and since no change in weight occurred, shrinkage was not due to the loss of any volatile material.
Fig. 2. Relaxation of unsupported Kodalith emulsion as a function of time after raising the relative humidity from an initial condition of 50% RH. Temperature, 70°F. (Broken lines indicate return to 50% RH.)
Figure 2 shows the change in length of unsupported Kodalith emulsion specimens, as a function of logarithmic time after raising the relative humidity from an initial condition of 50% RH. The gelatin first lengthens as moisture is sorbed at the higher relative humidity; it then shrinks, first rapidly, and then at a decreasing rate over a period of several hours. The higher the relative humidity, the sooner the maximum length increase occurs, And the greater is the subsequent rate of shrinkage. When the samples are returned to the starting relative humidity (50% RH), a net shrinkage is observed. This shrinkage amounts to about 0.2%, for the sample conditioned at 60% RH, 0.4% for the sample conditioned at 70% RH, and 2.4% for the sample conditioned at 80% RH.
Fig. 3. Dimensional change of unsupported Kodalith emulsion through the first sorption and desorption cycle at 70°F. Conditioning time at each humidity, 2 hours.
Figure 3 illustrates the dimensional changes which occur on the first humidity cycle when an unsupported emulsion is conditioned at a succession of relative humidities starting at 10% RH. A closed hysteresis loop is not obtained because of the relaxation shrinkage which unavoidably occurs at the high conditioning. relative humidities. The relaxed gelatin also exhibits a larger humidity coefficient than the unrelaxed gelatin.
The relaxation shrinkage exhibited by dried gelatin layers at high relative humidities is analogous to that observed by Leaderman5 when a cellulosic filament is stretched while -wet and dried under tension. When rewet the filament shrinks. Leaderman describes this phenomenon as "swelling recovery" and points out the similarity to "thermo-recovery" or relaxation shrinkage at increased temperature. This occurs when high polymers, such as rubber or thermoplastic resin are stretched while hot, cooled under tension, and then reheated after removal of the external stress. In both cases, the polymer molecules are stretched and then "frozen" in position either by drying or cooling When "unfrozen" at a later time by moisture or heat the molecules are free to contract to their preferred position.
Fig. 4. Moisture hysteresis loop for unsupported Kodalith emulsion at 70°F.
When gelatin layers coated on film base are dried, most of the lateral shrinkage is restricted by the base and this may be considered equivalent to stretching the gelatin. In other words a structure is established during the drying of the gelatin in which the molecules remain extended.6,7 When stripped from the temporary base and rehumidified the internal viscosity of the gelatin is lowered sufficiently for the extended molecules to contract, and the specimen shrinks.
Gelatin, like paper and other moisture absorbing materials, exhibits hysteresis when subjected to a sorption-desorption humidity cycle.8 That is, the amount of moisture which the gelatin contains at equilibrium is greater when a given relative humidity is approached from above than when it is approached from below. A moisture hysteresis loop for unsupported Kodalith emulsion is shown in Fig. 4. Moisture hysteresis has the appearance of a lag in the attainment of equilibrium but conditioning times appreciably longer than the two hours used in the present experiment do not eliminate it. Various theories to account for moisture hysteresis in high polymers are discussed in the literature.9
Fig. 5. Dimensional hysteresis loop for unsupported Kodalith emulsion at 70°F after relaxation at 80% RH.
A similar hysteresis curve is obtained if the change in length of an unsupported gelatin emulsion, after having been relaxed at a high humidity, is followed through a humidity cycle, as shown in Fig. 5. The emulsion layer is longer on the desorption part of the cycle than on the sorption side. This parallels the change in moisture content (Fig. 4). The dimensional hysteresis curve for the gelatin backing used on Kodalith Ortho PB Film (not shown) is virtually identical with that of the emulsion. This behavior will be referred to again in a later section.
Humidity Coefficients of Expansion of Unsupported Gelatin Layers at 70°f:
|Humidity coefficients of expansion, 10 5 in./in 1% RH|
|Material||Thickness, mils||Before relaxation||After relaxation at 80% RH|
|Kodalith Ortho Type 3 Emulsion|
|Kodak Commercial Emulsion|
The gelatin layers were stripped from the base at 50% RH, and the humidity coefficient determined from the linear expansion between 10% and 70% RH, except for unprocessed Commercial emulsion which was determined between 10%; and 60% RH because of relaxation shrinkage above 60% R H.
The humidity coefficients of linear expansion of unsupported emulsions and gelatin backings may be determined from the change in length between two specified relative humidities. It is customary to determine humidity coefficients from the sorption part of the cycle, and this was done to obtain the data given in Table I. The humidity coefficients of the three gelatin materials studied are similar. Processing and re-drying results in a higher humidity coefficient. Relaxation of the unprocessed gelatin at high relative humidity produces the same effect.
The humidity coefficient of expansion of an unsupported gelatin emulsion or backing is several hundred times that for polystyrene base. This is an important factor in the dimensional stability, of photographic film.
Effect of Photographic Processing on the Young's Modulus of Unsupported Gelatin Layers Compared with Film Support at 70°F-10% RH
|Young's Modulus, 105 psi|
|Material||Unprocessed||Processed black||Processed clear|
|Kodalith Ortho Type 3 Emulsion||11.0||10.3||8.0|
|Kodak Commercial Emulsion||10.5||9.3||8.2|
Tests made on Instron tensile machine according to A.S.T.M. Method D882-56T, Method A. Rate of straining, 0.1 in./in./min.
The Young's moduli of the unsupported emulsion and backing layers were determined with an Instron tensile machine at several relative humidities at 70°F. The moduli at 10% RH are given in Table II for three gelatin layers and for polystyrene support. At this low relative humidity the gelatin layers have a much higher modulus than the base, and the emulsions have a higher modulus than the gelatin backing. When processed to give zero silver density (clear), the emulsion modulus decreases to the same level as that of the gelatin backing. When processed to maximum density (black), the Young's modulus is reduced, but not as much as when processed clear. The original silver halide in the emulsion, and to a lesser extent the reduced silver left after processing black, imparts increased stiffness to the gelatin. This phenomenon is analogous to the increase in modulus of plastics upon the addition of inert fillers.10
Fig. 6. Effect of relative humidity on the Young's modulus of unprocessed gelatin layers compared with polystyrene base. Rate of strain, 0.1 in./in./min at 70°F.
The effect of relative humidity on the moduli of the two emulsions and the gelatin backing (unprocessed), compared with polystyrene support, is shown in Fig. 6. The modulus of the polystyrene base is independent of humidity; whereas the moduli of the gelatin layers decrease very rapidly above 60%, RH. This effect of humidity on the modulus of the emulsion is directly related to its moisture sorption (Fig. 1) and plays an important part in the dimensional stability of the film.
The influence of the gelatin layers on the dimensional characteristics of film is described in the following sections, from the standpoint of relative humidity, processing, and aging effects. Thermal expansion. has not been included because it is primarily a support property. Dimensional hysteresis due to moisture discussed first because this is essential to an understanding of humidity coefficients :and processing dimensional changes.
Fig. 7. Dimensional hysteresis loop for unprocessed Kodalith O PB Film, Type 2, at 70°F.
A. Start of first sorption cycle.
B. Shrinkage due to moisture relaxation of gelatin.
C. Point of mechanical equilibrium between gelatin and base.
D. Expansion due to stress relaxation of gelatin.
A dimensional hysteresis curve for Kodalith Ortho PB Film is shown in Fig. 7 and illustrates several curious phenomena. The sorption curve in the first humidity cycle exhibits a break between 70% and 80% RH. This is caused by the moisture-induced relaxation shrinkage of the two gelatin layers and the sharp decrease in elastic modulus as described earlier The magnitude of the relaxation shrinkage of the supported film at 80% RH, of course, is much less than that of the unsupported emulsion (Fig. 3). On the second humidity cycle a closed hysteresis loop is obtained . In the case of Commercial PB Film (data not shown), the break in the first sorption curve occurs between 60% and 70% (RH. This demonstrates how errors can occur if humidity coefficients are calculated arbitrarily over certain ranges in relative humidity.
The second peculiarity of the dimensional hysteresis curves shown in Fig. 7 is that the sorption curve falls above the desorption curve. This is opposite to the normal moisture hysteresis (Fig. 4) and also opposite to the dimensional hysteresis obtained with the unsupported emulsion after relaxation (Fig. 5).
Fig. 8. Change in tensile stress with time at constant strain during the desorption and sorption of moisture in an unsupported, processed Kodalith emulsion at 70°F. Numbers are percent relative humidity. Measurements made on Instron tensile machine.
An attempt has been made to explain this "reversed" dimensional hysteresis exhibited by gelatin-coated films on moisture-resistant supports. Although high polymers exhibit a stress-strain hysteresis,11 he direction is the opposite from that needed to explain this phenomenon. The magnitude of the base compression at 10% RH (about 0.1%) is very small compared to the emulsion stretch (about 2%). This indicates that the gelatin is the more likely, cause of this behavior.
Fig. 9. The tensile stress exerted by an unsupported, processed Kodalith emulsion at constant strain during desorption and sorption of moisture at 70°F. (Replot of data from Fig. 8.) A-Initial stress of 25 psi applied by tensile machine at 60% RH.
An experiment was devised in which a specimen of unsupported processed Kodalith emulsion was placed in the jaws of the Instron tensile machine at 70°F-60% R H and subjected to a small tensile stress (25 psi). The relative humidity was then gradually lowered to 10%[?] RH, in steps of 10% RH, allowing 20 min at each step, and then gradually returned to 60% RH In the same way. The jaws of the machine remained stationary during the experiment and the change in stress was recorded automatically through the strain gage. The internal stress in the gelatin during desorption and sorption is plotted against time in Fig. 8, and against relative humidity in Fig. 9. It is clear from these curves that the stress in the gelatin increases in a fairly linear manner as the humidity decreases to about 20% RH. At this humidity or lower, the stress is high enough that stress relaxation with time becomes apparent. On the sorption half of the cycle, the stress in the gelatin decreases very much faster than it increased during desorption. This produces a reversed hysteresis loop which is perfectly analogous to the reversed dimensional hysteresis loon of the supported film (Fig 10).
Fig. 10. The shrinkage of processed Kodalith Ortho PB Film upon desorption and sorption of moisture at 70°F. (Replot of data from second cycle of Fig. 7.)
Figures 8 and 9 clearly show that the reversed hysteresis effect is a property associated with the gelatin under tension. It is also apparent that stress relaxation occurs in the gelatin when under tension at low relative humidity. It is likely that greater stress relaxation took place in this experiment than was observed because two opposing effects occur simultaneously when the relative humidity is lowered, i.e., increase in tension and increase in relaxation. Thus a considerable part of this phenomenon can be explained by stress relaxation in the gelatin, although some other factor may also be involved. This reversed dimensional hysteresis has been found with all gelatin-coated polystyrene or other moisture-resistant high polymer supports which were studied. However, the dimensional hysteresis for cellulose-acetate-base films is generally normal in that the sorption curve falls below the desorption curve. This is undoubtedly because acetate base itself absorbs moisture and exhibits a normal hysteresis, which masks the effect of the gelatin.
Fig. 11. Effect of various humidity cycles on the dimensional change of processed Kodalith Ortho PB Film, Type 2, at 70°F.
The practical significance of hysteresis in gelatin-coated films will also depend on the range of relative humidity- to which the film is subjected. Figure 11 illustrates various dimensional hysteresis loops in which the spread between sorption and desorption curves varies with the humidity range.
The hysteresis curves for supported film described in the preceding section illustrate the problem of determining the humidity coefficients of linear expansion. The dimension versus humidity curves are not truly linear; their slopes vary with the previous moisture history, and relaxation of the gelatin interferes above 70% RH (sometimes above 60% RH). Arbitrarily we have chosen to calculate humidity coefficients from the sorption curve between 10% and 60% or 70% RH. Ideally both humidities should be approached from the same side, but in the present work it was more convenient to approach 10%,) RH from above and 60% or 70% RH from below. Although this makes a small difference in the result, the data are all comparable.
Average Humidity Coefficients of Expansion of Several Polystyrene Base Films at .70°F
|Average measured thickness, mils||Gelatin to-base thickness ratio||Humidity coefficient of expansion 10 5in./in./1% RH|
|Kodalith Ortho PB Film, Type 2 (5-mil base)|
|Kodalith Ortho PB Film, Type 3 (5 mil base)|
|Kodalith Ortho PB Film, Type 3 (10-mil base)|
|Processed black||0.24||0.22||0 046||0.60|
|Kodak Commercial PB Film, (10 mil base)|
Coefficients calculated between 10% and 70%; RH except for unprocessed Commercial PB Film which was, calculated between 10% and 60% RH because of relaxation shrinkage above 60% RH.
The humidity coefficients of linear expansion of several production polystyrene base films determined in this way are given in Table III These data show that the thicker base and the thinner emulsion give the lower humidity coefficient. The coefficients also decrease on processing, and decrease Tightly more when the film is processed clear than when it is processed black. This is due to the decrease in modulus (Table II) and thickness of the emulsion upon processing.
Fig. 12. Effect of gelatin/base thickness ratio on the humidity expansion of experimentally coated Kodalith PB Films at 70°F.
The effect of Kodalith emulsion thickness and backing thickness on the humidity coefficient was explored over a wide range in a series of experimental coatings on polystyrene base. As noted by Umberger,3 the humidity coefficient of photographic film, to a first approximation, is directly proportional to the gelatin thickness and inversely proportional to the base thickness. The data for Kodalith film are plotted in this way in Fig. 12 for unprocessed and processed film and do approximate a linear relationship. The slopes of these curves correlate with the moduli of the emulsion. (For comparison purposes it may be noted that production Kodalith Ortho PB Film has a gelatin; base thickness ratio close to 0.1. )
An equation relating the humidity coefficient of the film to the thickness, modulus, and humidity coefficient of the emulsion and base layers has been derived by Umberger.3 Umberger did not determine the humidity coefficient of gelatin or emulsion experimentally but calculated this value from measurements on supported films by means of his equation, obtaining 1.4 X 10-4 in./in./1% RH. Experimental values of this constant determined in the present work for several types of unsupported emulsion and gelatin backings are close to 4 X 10-4 in./in./1% RH (Table I), which is about three times Umberger's value.
This discrepancy is due to the conditions under which the humidity coefficients were measured; other conditions can give results much closer to those calculated by Umberger. Apparently, the effective humidity coefficient of the emulsion when attached to the base is appreciably less than the values determined on the unsupported emulsion for Table I. Evidence for this has been obtained by measuring the instantaneous shrinkage which occurs in the emulsion when stripped from a temporary base after equilibration at low relative humidity. This shrinkage is considerably less than the humidity contraction of the unsupported emulsion.
The Umberger equation does not take into account the fact that both the emulsion and base components of photographic film are not perfectly elastic but exhibit stress relaxation and creep under load. For this reason, modulus values used in the equation should be determined at much lower rates of strain than those normally used in tensile testing. At a rate of strain corresponding to that which occurs in film when subjected to humidity changes, unsupported gelatin layers have a modulus of about half the magnitude of those shown in Table II.
Recent data indicate that film haying gelatin on one side only has a higher humidity coefficient than film having the same gelatin thickness divided between the two sides. No explanation can be offered for this at the Present time.
Fig. 13. Dimensional change of Kodalith Ortho PB Film, Type 2, followed through a humidity cycle before processing and again after processing to illustrate how the processing dimensional change depends on the conditioning procedure. The identifying letters correspond to the test procedures listed in Table IV. Solid lines indicate unprocessed (raw) film; broken lines indicate the same film after processing.
Regardless of these factors, the Umberger equation is very useful in relating the pertinent variables and predicting their effects on the humidity coefficient of expansion of photographic film.
It has been customary- in this laboratory for a number of years to measure the processing dimensional change of photographic film by conditioning samples at 50% RH for measurement before processing and again after processing. The conditioning humidity was always approached from below to eliminate any hysteresis effect. This test procedure generally, gives a processing dimensional change for polystyrene base film close to zero. Investigation showed that if the film is subjected to a different moisture history prior to conditioning at 50% RH either before or after processing, a slightly different value for the processing; dimensional change is obtained. It was apparent that a preconditioning procedure designed to eliminate hysteresis in laboratory tests may be misleading to the film user who takes no such precautions.
The dimensions of an emulsion-coated film sample were followed through a complete humidity cycle before processing and again after processing, and these results are shown in Fig. 13. The hysteresis cycle for the processed film falls above that for the unprocessed film. This is because the modulus and thickness of the decrease as the result of processing, and this lowers the humidity coefficient of expansion of the film (see Tables II and III. It is clear from Fig. 13 that the processing dimensional change, as measured at any given relative humidity such as 50%, will depend on whether equilibrium is approached from above or below, both before and after processing. In other words, it depends on the previous moisture history of the film, regardless of the fact that it is conditioned at 50% RH for both measurements.
Effect of Various Preconditioning Procedures on
the Average Processing Dimensional Change of Several Polystyrene
(All measurements made after reconditioning at 70°F-50% RH)
|Processing dimensional change, %|
|Procedure A||Procedure B||Procedure C|
|Preconditioning RH of unprocessed film, %||10||10||80|
|Preconditioning RH of processed film, %,||10||*||10|
|Film Type||Base thickness, mils||Gelatin-to-base thickness ratio|
|Kodalith Ortho PB; Film, Type 2||5||0.121||+0 01 to-0.01||-0.040||+0.030|
|Kodalith Ortho PB Film, Type 3||5||0.100||+0 01 to-0.01||-0.035||+0 025|
|Kodalith Ortho PB Film, Type 3||10||0 048[?]||+0 01 to-0.01||-0.015||+0.015|
|Kodak Commercial PB Film||10||0.137[?]||+0 01 to-0.01||-0 050||+0.020|
* Dried down from a wet condition after processing/
Fig. 14. Effect of gelatin/base thickness ratio on the processing dimensional change of experimentally coated Kodalith PB Films. Letters refer to conditioning procedures in Table IV.
Four different combinations of preconditioning history before and after processing are possible:
|Preconditioning Relative Humidity|
|Before processing||After processing|
Three of these combinations are illustrated in Fig. 13 and Table IV. Procedure A gives a processing dimensional change very close to zero; Procedure B gives a small negative value; and Procedure C gives a small positive value. The values for processing dimensional change also vary with the conditioning humidity. It is thus apparent that in commercial practice the processing dimensional change can vary with the weather and the drying procedure. Overdrying after processing or low ambient relative humidities will tend to give a positive processing dimensional change.
Fig. 15. Effect of emulsion/base thickness ratio on the aging shrinkage of processed, experimental Commercial PB Films. All measurements made after reconditioning to 70°F-50% RH and calculated from original measurement made before processing.
The effect of emulsion thickness on the processing dimensional change was investigated with some special coatings of Kodalith emulsion and gelatin backing on 5-mil polystyrene base (Fig. 14). A sufficient range of thickness was used in order to exaggerate the effect. The processing dimensional chap increases with the gelatin 'base thickness ratio, particularly when determined by Procedure C of Table IV.
Film base, like all thermoplastics, flows or creep when subjected to stress for any length of time. It obvious, therefore, that the lateral compressive force exerted by the emulsion and gelatin backing on the base will cause a permanent shrinkage. It is also clear that the amount of this shrinkage will increase with the following factors:
The first two of these factors increase the compressive stress on the base and the third decreases the modulus of the base, i.e., its resistance to deformation. Some of these effects are illustrated in Fig. 15 which shows that aging shrinkage increases appreciably with gelatin base thickness ratio when the film is stored at 120°F-20%, RH. However, no such effect is apparent in even a year at 78°F-60% RH, because at this condition the gelatin exerts very little compressive force on the base. (Of course, any shrinkage caused by the emulsion and backing is over and above that which the base alone may show under the same storage conditions.) Although this type of shrinkage is very small with polystyrene-base films under normal storage conditions, Fig. 15 emphasizes the desirability of avoiding storage at very low relative humidities or elevated temperatures in cases where accurate size holding is required over a long period of time.
It has been shown that the dimensional change characteristics of moisture-resistant high polymer base films are highly dependent on the emulsion and gelatin backing employed. The elastic modulus, humidity expansion, and thickness of each layer affect the dimensional response of the film to relative humidity changes to photographic processing, and to aging effects. Several dimensional change phenomena which occur in the film can be explained in terms of hysteresis, moisture relaxation, and stress relaxation of the gelatin layers. An understanding of the phenomena described should assist in controlling or selecting conditions which will minimize the dimensional changes of film in use.
The authors wish to thank J. T. Parker, Manufacturing Experiments Division, and E., J. Wiegand, Film Testing Division, Eastman Kodak Company, for their assistance in providing some of the experimental data.
1. Calhoun, J.M., Photogrammetric Eng., 13: 163-221(1947).
2. Centa, J. M., "Effect of Base and Emulsion Thickness on Dimensional Stability of Graphic Art. Films," Proceedings, Eighth Annual Technical Meeting, Technical Association of the Graphic Arts, May 7-9, 1956, pp. 75-79.
3. Umberger. J.Q., Phot. Sci. and Eng. 1: 69-73, (1957).
4. Tate, C.D., unpublished report, Manufacturing Experiments Division, Eastman Kodak Company, April 26, 1937.
5. Leaderman H., "Elastic and Creep Properties of Filamentous Materials and Other High Polymers," Textile Foundation, Inc., Washington, D. C., 1943, pp. 98,128.
6. Bradbury, H. and Martin, C., Proc. Roy. Soc., Series A, 214: 183-192 (1952).
7. Jopling, D.W., J. Appl. Chem. (London), 6: 79-84, (1956).
8. Sheppard, S. E., Houck, R. C., and Dittmar, C. J. Phys. Chem., 44: 185-207 (1940).
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10. Lozano B.J., and Yorgiadis, A., "The Behavior of Plastics Under Repeated Stress," Symposium on Plastics, American Society for Testing Materials, Philadelphia, 1944, p. 86.
11. Schmidt, A. X., and Mathes, C., 1., Principles of High Polymer Theory and Practice, McGraw-Hill, Inc., New fork, 194F, p. 274.
* Since the phenomenon described here is opposite to direction normal moisture hysteresis there may be some confusion in calling it hysteresis. However, no better term is known and for the purpose of this paper, dimensional hysteresis will be used to mean a failure the film to return to the same dimensions when a given relative humidity is approached from opposite directions.