U. S. DEPARTMENT OF COMMERCE

DANIEL C. ROPER, Secretary NATIONAL BUREAU OF STANDARDS

LYMAN J. BRIGGS, Director

NATIONAL BUREAU OF STANDARDS MISCELLANEOUS PUBLICATION M158

EVALUATION OF MOTION-PICTURE FILM

FOR PERMANENT RECORDS

By

JOHN R. HILL and CHARLES G. WEBER

Issued July 16, 1937

UNITED STATES

GOVERNMENT PRINTING OFFICE WASHINGTON : 1937

For sale by the Superintendent of Documents, Washington, D. C, Price 5 cents

EVALUATION OF MOTION-PICTURE FILM FOR

PERMANENT RECORDS

By John R. Hill and Charles G. Weber

ABSTRACT

In previous publications, the results have been reported of studies on the

stability of motion-picture film and its care in libraries. To assist the user of film in securing a material of high quality, this paper gives detailed test methods

for determining folding endurance, pH, copper number, and relative viscosity. It is suggested that for record purposes film having a base of cellulose acetate that is free from cellulose nitrate be required. The cellulose acetate film should have the following resistance to an aging test consisting in heating 72 hours at

100° C: Retention of folding endurance, not less than 75 percent; decrease in relative viscosity, not over 3.0 percent; increase in copper number, not more

than 0.2; a change in pH of not over 0.5. Developed film should be free from

sodium thiosulphate (hypo).

CONTENTS

Page

I. Introduction 1 II. Testing 2

1. Accelerated aging 2

2. Folding endurance 2

3. pH value 3

4. Copper number 3

5. Relative viscosity 3

6. Conditioning 4

7. Residual hypo 4

8. Cellulose nitrate 4

III. Properties of film for permanent records 5

I. INTRODUCTION

Motion-picture film of the cellulose acetate type appears to be

destined for very wide use for record purposes. Its use facilitates the

rapid and inexpensive copying of books and manuscripts. Records on this medium require only a fraction of the storage space required

for the same on paper, and accelerated aging tests at the National

Bureau of Standards 1 have shown the acetate film, if well made and

properly developed and fixed, to be as stable as paper used for permanent records. The preservation of acetate film has been described

in a previous publication.2 The purpose of this paper is to assist the

user in selecting film for record purposes, by describing in detail tests that are applicable, and by suggesting limiting values of quality.

» J. Research NBS 17, 871 (1936) RP950.

* J. Research NBS 17, 753 (1936) RP942.

15163 7—37 1
2 Miscellaneous Publications, National Bureau of Standards

II. TESTING

The stability of film was determined by measuring the effects of

accelerated aging on some of its properties. The properties tested were flexibility, which is the physical property most closely related

to use requirements, was measured by a folding test; acidity, which

is a measure of the harmful acid present, was expresed as pH value;

the copper number, a measure of the degraded cellulose present, was

determined; and the relative viscosity in acetone solution was measured. A lowering of viscosity is considered an indication of breakdown in the molecular structure of a cellulose compound. These

tests were made on samples before and after accelerating aging, and the

results compared to find the changes occurring.

1. ACCELERATED AGING

The aging treatment used at the Bureau consists in oven-aging at 100° C in dry air for a period of 72 hours. This test of stability was

chosen because it was expected that film, having a cellulose compound

as a base, might behave similarly to paper, for which the results of such

heat treatment are in accord with the general experience in the use of

papers. The results of the accelerated aging of papers also agree

well with those obtained by exposure to sunlight,3 exposure to air at more nearly normal temperatures,3 and a 4-year period of natural

aging.4

2. FOLDING ENDURANCE

For determination of the folding endurance of film, the Bureau uses a folding tester of the Pfund type 5 shown in figure 1 . It consists essentially of a fixed jaw A, and a movable jaw B, which are opened and

closed by the screws C. In operation, a test specimen is clamped in

place with the jaws at the maximum distance apart, and folded by

moving jaw B up against jaw A, then back against plate D. The folds are caused to form alternately on opposite sides of the jaws by directing

the film with a blunt-pointed instrument. The testing length is controlled by the position of plate D which regulates the maximum dis- tance between the jaws. The number of folds sustained by the film

before breaking is recorded as the folding endurance. The instrument

used tests specimens 15 mm wide, and these strips should be cut approximately 50 mm long. The 15 mm width is taken from the

center of the film by trimming off the edges.

Although operated manually, this tester gave more uniform results

than were obtained with the testers commonly used for paper. Diffi- culty was encountered in the use of the latter because slight distortion

of the film on aging caused edge cracks when folded over the jaw edges.

The Pfund instrument folds without tension, forming a small loop not

in contact with any metal edge. Table 1 shows some results obtained

by different operators over a period of several months on the same

brand of film. Each value is an average for 10 strips of film. The

testing length was 19 mm.

3 R. H. Rasch, Accelerated aging test for paper, BS J. Research 7, 465 (1931) RP352.

4 R. H. Rasch, and B. W. Scribner, Comparison of natural aging of paper with accelerated aging, BS J. Research 11, 727 (1933) RP620.

5 This instrument was developed by A. H. Pfund, Johns Hopkins University, for the Du Pont Film

Manufacturing Corporation.
NBS Miscellaneous Publication Ml 58

Figure 1.

Pfund type of folding endurance tester used for determining the

flexibility of motion-picture films.

Weber] Permanent Record Films 3

Table 1.—Test data on cellulose acetate film, showing reproducibility of results with Pfund tester

Series Number of single folds (average of 10 strips of film)

Operator A Operator B Operator C

1 . 17

19

18

18

1818

1720

21 2

3 Average ... 18 1.71

18 1.68

19 Standard deviation of individual values --. 2. 05

3. pH VALUE

The pH value of film was determined in the following manner. A

1-g sample of film, including both base and emulsion, is transferred

to an Erlenmeyer flask and 100 ml of acetone, containing 10 percent

by volume of water, is added. After complete dispersion of the film

base, the pH of the solution is determined by means of a glass elec- trode. A convenient apparatus for this purpose has been described

by Wallace 6 for the measurement of the pH of leather extracts. The water and acetone should be purified by distillation, and the

combined solvent should have a pH of 7 ±0.4. Duplicate determinations for the film should agree within 0.1 pH unit.

4. COPPER NUMBER

For use in the determination of copper number, the film is made

into a thin solution by dissolving 1 g of the film base in 20 g of acetone. The strips of gelatin remaining are removed by decantation. Approximately 45 ml of the acetone solution is poured in a fine stream into a liter of water, heated to approximately 85° C. The water is stirred

vigorously during the addition to precipitate the cellulose acetate in a convenient form. The precipitated cellulose acetate is filtered and

washed five or six times with ether to remove water and plasticizer.

After drying 1 hour at 65° C, samples of the precipitated acetate are weighed for the copper-number determination. The determination

is made exactly as prescribed for paper in the standard methods of

the Technical Association of the Pulp and Paper Industry.7 The test is based upon the quantitative reduction of an alkaline

copper solution by the degraded celluloses in the sample. The copper

number is the weight in grams of copper reduced from the cupric to the cuprous state, by 100 g of the sample.

5. RELATIVE VISCOSITY

A solution, consisting of approximately 1 percent of cellulose ester dissolved in acetone, is used in determining the relative viscosity.

This solution is made from a sample of film (base and emulsion)

weighing 1.000 g. The sample is cut into pieces small enough to drop

into a 100-ml flask, and the flask is partially filled with acetone. After

solution of the film base is complete and the emulsion has settled to

6 J. Research NBS 15, 5 (1935) RP805.

Tech. Assn. Pulp and Paper Ind. T430m (1936). Copies can be secured from the Association, 122 East

42d St., New York, N. Y., for 25 cents each.
4 Miscellaneous Publications, National Bureau of Standards

the bottom, the flask is immersed in a bath maintained at 30 ±0.02°

C until temperature equilibrium is reached, the solution diluted to 100 ml with acetone, and 5 ml is then transferred to an Ostwald pipet

which is also immersed in the constant-temperature bath. The time

of flow of the solution through the capillary of the pipet is measured

with a stop watch which can be read to at least one-fifth second.

The time of flow is also measured for the pure solvent. Not less than

three or four determinations are made for each solution, and the values

obtained should agree within two or three tenths of a second. The

relative viscosity is then calculated as the ratio of the time of flow of

the solution to the time of flow of the solvent.

6. CONDITIONING

Since cellulose esters and gelatin are hygroscopic substances, the

physical properties of the film will be affected by the relative humidity

of the surrounding atmosphere. Consequently, in order to obtain as uniform results as possible, all samples of film are conditioned 24

hours at 65 percent relative humidity and 70° F before testing them,

and all weighings and physical tests are made under those conditions,

which are standard for paper and textiles. In the determination of folding endurance, the strips of film re- served for control tests are kept in an atmosphere with relative humidity below 65 percent until the heated strips are ready for test, then all strips are conditioned and tested together. When this procedure is followed, both control and heated strips approach equilibrium

from the same direction, and hence have comparable moisture contents.

7. RESIDUAL HYPO

The method used for detecting the presence of sodium thiosulphate

in films is that recommended by Crabtree and Ross.8 This test consists in placing a strip of film in a solution of mercuric chloride in a

test tube or glass cylinder and observing any turbidity which develops

in the solution. If sodium thiosulphate is present it reduces mercuric

ion, and an insoluble mercurous compound is formed which causes the

turbidity. If no thiosu.phate or other reducing agent is present, the

solution remains clear although the silver image is bleached white.

The test solution contains 25 g of mercuric chloride, 25 g of potassium

bromide, and water to make 1 liter. A 1.5-inch length of 3 5-mm

motion-picture film, cut into small pieces and immersed in 10 ml

of solution, was found to be a satisfactory amount for this test. The

solution and film are allowed to stand 15 minutes, with occasional

stirring, in order to give sufficient time for complete reaction.

8. CELLULOSE NITRATE

Traces of cellulose nitrate, the presence of which is undesirable, can be detected by means of a diphenylamine indicator. The indi- cator is a solution of diphenylamine which may be easily prepared by

mixing 20 ml of concentrated sulphuric acid, 10 ml of glacial acetic

acid, and 0.3 g of diphenylamine. A few drops of the indicator on the

uncoated surface of the film results in a blue coloration if any nitrate

is present.

s J. Soc. Motion Picture Engrs. 14, 419 (1930).
Weber] Permanent Record Films 5

III. PROPERTIES OF FILM FOR PERMANENT RECORDS

Table 2 contains some data obtained on cellulose acetate films from

three different sources. These films are considered to have excellent

stability.

Table 2.

Properties of cellulose acetate film

Sample

Folding endurance Acidity Copper number Relative viscosity

at 30° C

Original Loss on

aging 2 pH value

of original Change

after aging 2

g of Cu

per 100 g

of Cellulose ace- tate orig- inal

Change

after aging 2 Original Change

after aging 2

12

3

Single

folds i 17.4 20.7

17.7

Percent

18

18

21

6.2

5.6

6.5

-0.3 -.1 +.2

2. 77

3. 95 2.72

+0. 07 +.03

-.01

2. 65 5 2. 65 7 2.78 s

Percent -1.

-2.1 -1.3

i Average values for 20 strips.

* Aging period 72 hours. From a consideration of these results, it appears that cellulose acetate film having the following properties will be suitable for records

of permanent value.

1. A folding endurance of not less than 16 single folds, with not less than 75 percent retention of folding endurance after heating 72 hours

at 100° C.

2. A pH value of not less than 5.5, with a decrease of not more than

0.5 after heating 72 hours at 100° C.

3. An increase in copper number after heating 72 hours at 100° C

of not more than 0.2.

4. A decrease in relative viscosity after heating 72 hours at 100° C

of not more than 3.0 percent.

5. Developed film shall be free from sodium thiosulphate (hypo)

as indicated by the mercuric chloride test, and the base should be free from cellulose nitrate as indicated by the diphenylamine test. Washington, May 11, 1937.

o