59 NBS MONOGRAPH 5

Preservation of Documents by Lamination
THE NATIONAL BUREAU OF STANDARDS

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UNITED STATES DEPARTMENT OF COMMERCE • Frederick H. Mueller, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Preservation of Documents by Lamination

William K. Wilson and B. W. Forshee

National Bureau of Standards Monograph 5

Issued October 30, 1959

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington 25, D.C. - Price 20 cents

Contents

Pago

1. Introduction 1 2. Properties of an ideal laminating film 1 3. Properties of cellulose acetate laminating film 2

3.1. Stability of cellulose acetate films 2

3.2. Plasticizer loss of cellulose acetate films 4

a. General 4

b. Materials and methods of test 5

c. Results and discussion 5

3.3. Physical properties of cellulose acetate films 7

a. General 7

b. Materials and methods of test 8

c. Results and discussion 9

4. Degradation of film and paper during lamination 11

5. Tissue reinforcement 12

6. Deacidification 14

7. Special laminating methods 15

8. Preliminary study of films other than cellulose acetate 16

9. Specifications for archival laminating film formulated from cellulose acetate 1

9.1. General 17

9.2. Test methods 17

9.3. Improvements in specifications IS

9.4. Problems in procurement 19

10. References 19

in

Preservation of Documents by Lamination

William K. Wilson and B. W. Forshee

The chemical stability and physical properties of cellulose acetate film used to preserve and restore old or damaged documents by lamination were investigated. Pretreatment of documents with alkaline media before lamination is desirable if the paper contains an

appreciable amount of acid. The lamination process does not degrade cellulose acetate film to a measurable extent. Acid-free papers are not degraded during lamination, but papers

containing acid are degraded in proportion to the amount of acid present. Tissue added to the laminate increases the tensile strength and internal tear resistance, but decreases edge-tear

resistance as compared to that of film alone extending beyond the paper. The loss of plasticizer with time from a cellulose acetate laminating film does not impair

the properties of the laminate. Composition and performance specifications for a cellulose acetate laminating film suitable for archival use are presented. The properties of polyethylene and polyethylene terephthalate films of interest in connection with their possible use for the protection of documents are discussed.

1. Introduction

The lamination of documents with cellulose acetate film as a means of preservation and

restoration has been practiced for more than 20

years [l]. 1 The National Bureau of Standards was asked by the National Archives, Army Map

Service, Library of Congress, and Virginia State

Library to investigate several aspects of the

preservation of documents by lamination. The

principal objective of this program, which was

supported by these agencies, was to develop

information necessary to establish specifications

for cellulose acetate film of commercially practicable quality that would have the maximum

stability for lamination.

2. Properties of an Ideal Laminating Film

An ideal laminating film could be described as having the following properties:

(1) It should be very flexible, and able to withstand all of the flexing or folding that is re- quired of a paper during use.

(2) It should be considerably stronger than the

paper it protects. Most papers that might be

found in repositories exhibit an elongation of

about 2 percent upon rupture. Therefore, in

order to protect the body of the document, the

film should show satisfactory strength at _ an

elongation of less than 2 percent. As a practical

consideration, the film should be strong enough

to make reinforcement with tissue unnecessary

and, if possible, to eliminate the use of cloth on

! maps as a part of such laminates.

(3) It should have considerable elongation

beyond the yield point on a stress-strain curve, as a high edge tear is usually associated with

high elongation. By leaving a narrow margin

of film around the edges of the laminate, a strain at the edge would result in elongation of the film,

' Figures Id brackets indicate the literature references at the end of this monograph.

but not in damage to the paper.

(4) It should be resistant to abrasion.

(5) It should be resistant to degradation. The

film should be inherently stable and resistant to any environmental conditions that it could

reasonably be expected to encounter.

(6) It should be possible to attach the film to the paper with a minimum of effort, preferably at room temperature, either in a small shop without

special equipment or in a large shop using machines for greater production.

(7) It should be possible to remove the film

from the paper, if this should become necessary,

with a minimum of effort and without damage

to the paper.

(8) The film should be transparent to light

throughout the visible spectrum and preferably

into the infrared and ultraviolet.

At present there is no known film that completely meets all these requirements, although a properly formulated cellulose acetate film repre- sents a good compromise. Resistance to abrasion

and transmission of light were not included in the

work reported in this paper.

1
3. Properties of Cellulose Acetate Laminating Film

3.1. Stability of Cellulose Acetate Films

The work on the degradation of cellulose acetate

film conducted in the course of this investigation

has been published elsewhere [2], and only a sum- mary of the most important findings will be given

here. Several commercial films and laboratory

films were degraded at elevated temperatures in

oxygen or nitrogen, and the reaction was followed

by determining the intrinsic viscosity of speci- mens at intervals. Intrinsic fluidity was calculated from intrinsic viscosity, and the slope of the

plot of intrinsic fluidity against time was taken

as one criterion of degradation. The numerical

value of intrinsic fluidity increases with degradation. Two cellulose acetate films were selected for

intensive study—a plasticized film and an experimental unplasticized film. The change in intrinsic

fluidity of the plasticized film with time was

studied at temperatures ranging from 60° to 177° C, and the unplasticized film was studied at temperatures ranging from 110° to 176° C. Degradation was principally oxidative in nature, and

moisture did not appreciably affect the degradation under the experimental conditions that were

used.

Typical plots for the degradation of a cellulose acetate film in moist and dry oxygen are given in figure 1. A slow initial reaction is followed by a

TIME, DAYS Figure 1. Degradation of commercial cellulose acetate film No. 4 (1-mil thick) in moist and dry oxygen at 110° C.

very fast secondary reaction. The slow reaction

is the more important in the study of films for archival use, as a film has usually lost most of its usefulness at the end of the slow reaction period.

Considerable information can be obtained by

plotting the logarithm of the degradation rate

against the reciprocal of the absolute temperature.

Such a plot is shown in figure 2 for the initial degradation of these two films in dry oxygen at various temperatures; the change in intrinsic

fluidity (AF) per hour is plotted against the

reciprocal of the absolute temperature. The

straight lines indicate that the same reaction is occurring at each temperature but at a different

rate. The slope of the line is a measure of the

energy required to activate the reaction. It is obvious that the unplasticized film is the more

stable of the two films.

D PLASTICIZED FILM • UNPLASTICIZED FILM

Figtjrb 2. Arrhenius plots for the initial degradation

reaction of two cellulose acetate films.

2
On the basis of the data obtained on tbese two

films over a wide temperature range, a single

temperature was selected at which to study the

degradation rate of other commercial and experimental films. The change in intrinsic fluidity of

| these films with time was determined after aging

at 124° C in flowing oxygen moistened with water

vapor at 30° C. Less stable films are degraded

in one or two days at this temperature while more stable films may last two or three months.

The selection of the concentration of water vapor was arbitrary, as water vapor did not appear to have any great influence on the rate of degradat ion. Data on the degradation of 11 commercial

cellulose acetate films in moist oxygen at 124° C

are plotted in figures 3,4, and 5. Films numbers

0

~" 20 40 60 80 100 TIME, hr

Figure 3. Degradation of commercial cellulose acetate films

in moist oxygen at 124° C.

2.0 |

1 1

1 o

0 5 10 15 20

TIME , OAVS

Figure 4. Degradation of commercial cellulose acetate films

(1-mil thick) in mmst oxygen at 124° C.

0 20 40 60 80 100

TIME, OAYS

Figure 5. Degradation of commercial cellulose acetate films in moist oxygen at 124° C.

10 and 14 are extruded films, while the remainder

are cast films. These films show very great

differences in rate of degradation. Numbers 9

and 10 were not aged long enough to determine

the shape of the aging curve. The remainder of

the films, with the exception of numbers 12 and 13,

show the typical initial slow reaction or initiation

period followed by a more rapid degradation. A

rapid initial reaction followed by a slower reaction

is exhibited by numbers 12 and 13. The degradation rates of several experimental

cellulose acetate films are shown in figures 6, 7,

and 8 as part of a study of the effect of typical

plasticizers on stability. Each plasticized film

contains 20-percent plasticizer, based on the

weight of the film. It is obvious that plasticizers can greatly influence the stability of cellulose acetate. In agreement with published data [3], tri- pbenyl phosphate acts as a stabilizer. As shown

in figure 8, ether esters can cause serious degradation.

i 1' A

' 2D

• / / 8

rf

15 V

r-O

1o

CO / b /

|.o / / * • /

• NO PLASTICIZER

' o / O 20% DIETHYL PHTHALATE —•

"T5 & 20% DIMETHYL PHTHALATE

0.5

1

20% TRIPHENYL PHOSPHATE

1 1

I

1 I I 0 10 20 30 40 50

TIME, DAYS

Figure 6. Degradation of experimental cellulose acetate films (1-mil thick) in moist oxygen at 124° C; flake A with

various plasticizers.

3
1.5

0 10 20 30 40 50

TIME, DAYS

Figure 7. Degradation of experimental cellulose acetate films (l-mil thick) in moist oxygen at 124° C; flake A with

various plasticizers.

T

A 20% TRIBUTOXYETHYL PHOSPHATE —I 1 1 I 0 5 10 15 20 25

TIME, hr Figure 8. Degradation of experimental cellulose acetate films {l-mil thick) in moist oxygen at 124° C; flake A with

various plasticizers.

Data on the effect of acid and of acid acceptors on the degradation of experimental cellulose acetate films are plotted in figure 9. Magnesium acetate acts as an acid acceptor and suppresses the

degrading effect of sulfuric acid in the film. Acid

can be present in a cellulose acetate film as an

impurity from the manufacturing process as well

as from atmospheric contamination.

1.5

z 1.0

1 1

1

1

• FILM WITHOUT ADDITIVES 0 FILM WITH 0.01% H2S04 a FILM WITH 0,01% HjSO^ PLUS 05% MAGNESIUM ACETATE

1 • 1 1 1 0 10 20 30 40 50

TIME, DAYS

Figure 9. Degradation of experimental unplasticized cellulose acetate films (l-mil thick) in moist oxygen at 124° C;

flake D with acid and acid acceptor.

Data on the degradation of an experimental film

formulated especially for stability are plotted in figure 10. The stability of this film is far greater

than that of any of the commercial or other ex- perimental films for which data have been obtained. For maximum stability, a cellulose acetate film should contain plasticizers that do not

contribute to degradation, an antioxidant, and an

acid acceptor.

0 25 50 75 100 125 TIME, DAYS Figure 10. Degradation of an experimental cellulose acetate ! film (l-mil thick) in moist oxygen at 124° C. The samples are duplicates with the composition given in table 6,

1 footnote b.

I

3.2. Plasticizer Loss of Cellulose Acetate Films I

a. General

The loss of plasticizer from plastic articles under

service conditions is a very important consideration in formulation, and each article must be considered with respect to its particular end use. For

many purposes a plastic has lost much or all of its utility if only a small percentage of the plasticizer

is lost. For others, plasticizer loss is not so important.

An archival film must contain enough of an

active solvent plasticizer to permit lamination to paper at a temperature low enough to keep degradation of the film and the paper at a low level. Since many documents repaired in this way are 1 to be stored for several hundred years, it is important to have information on rate of plasticizer

loss up to as near 100-percent loss as possible. It

is also desirable to know something about the 1 change in physical properties of the film after

extensive loss of plasticizer.

Several laboratory tests have been devised to simulate the plasticizer loss that a plastic article

might suffer in service. These have been adequately discussed elsewhere [4 through 8]. Several investigators have inquired into the

mechanism of plasticizer loss. In a series of

articles on migration of plasticizer in vinyl resins

[9 through 12], the problem has been classified as "surface resistance controlling" and "vinyl resist- ance controlling," or diffusion controlling. The

conditions of use determine whether plasticizer

loss is surface controlling or diffusion controlling.

Plasticizer loss from an archival film, which is approximately 1 mil thick, would be "surface

controlling" if laminated documents were stored

loosely with good ventilation. If several laminates

4
were stored together, as in a book that had been

laminated and rebound, plasticizer loss would be

' ' diffusion controlling. '

b. Materials and Methods of Test The films used for weight loss study at various

temperatures were prepared from a cellulose acetate flake with a degree of substitution of 2.3 and

a viscosity (ASTM D 871-48, formula A) of 41

sec. Each film was formulated to contain 20.5-

percent plasticizer. The casting solvent was a mixture of 90-percent acetone and 10-percent

methyl alcohol by volume. Three films were

prepared, one containing dimethyl phthalate, one

with diethyl phthalate, and the other with tri- phenyl phosphate.

The experimental film containing triphenyl

phosphate and two commercial films, numbers 4

and 7, were used in the study of the change in

physical properties due to plasticizer loss in activated charcoal. The amount of plasticizer in the

two commercial films was determined by the

method of Whitnack and Gantz [13] using 50: 50

absolute ethyl alcohol and /i-hexane in a semimicro

Soxhlet extractor. Film number 4 contained 26-

percent plasticizer, and film number 7 contained

29 percent plasticizer.

An ordinary oven with forced circulation was

used at various temperatures for study of rate of

plasticizer loss. The same oven was used for the

weight loss experiments in activated charcoal at 50° C. The film specimens were placed in a photographic tray and separated from each other

with layers of charcoal. All physical tests were

performed at 23° C and 50-percent relative

humidity.

c. Results and Discussion

Weight loss curves were obtained on the films

containing dimethyl phthalate and diethyl phthalate, respectively, at 75°, 90°, 110°, and 125° C.

Weight loss curves on the film containing triphenyl

phosphate were obtained at 110°, 125°, and 140°

C. Typical curves for dimethyl phthalate and

, triphenyl phosphate are shown in figure 1 1 . These

i curves all show a high rate of plasticizer loss in the

early part of the curve, but an appreciable amount

of the plasticizer is very tenaciously retained.

Dimethvl phthalate and diethyl phthalate are

active solvent plasticizers for cellulose acetate; triphenyl phosphate is not. In order to obtain more information on the

nature of the weight loss curves, the data were

! analyzed using the time-ratio tables developed by

Flynn [14]. From data on the degree of advancement of a rate process (plasticizer loss in this case)

the exponent n in the rate equation dx/dt=Jc(l-x) n may be estimated. In this case x is the percent

plasticizer disappearing in time t, k is a constant,

and n is an exponent whose value is determined

from the data. This analysis showed a definite

difference between triphenyl phosphate and the

0 10 20 30 40

TIME , DAYS

Figure 11. Typical plasticizer loss curves for cellulose acetate films (1-mil thick) formulated from flake C and

containing dimethyl phthalate or triphenyl phosphate.

two active solvent plasticizers. The data on the

loss of dimethyl and diethyl phthalate give a value

of n of about 5. Triphenyl phosphate gives a much higher value of n (about 10) in the early part

of the plasticizer loss curve. By the time half of

the plasticizer is gone, the value of n is about 3, and remains mostly between 2 and 3 up to about

80-percent plasticizer loss. It appears that the two active solvent plasticizers would be retained very tenaciously by

cellulose acetate in the last stages of plasticizer

loss. For a film that is to be kept indefinitely, the use of triphenyl phosphate probably woidd offer

little advantage over dimethyl phthalate with

respect to weight loss. The rate of plasticizer loss is plotted against

1/T on semilog paper in figures 12, 13, and 14. The slopes of the tangents to the plasticizer loss curves were taken as the rate at 50-percent loss. A measure of the rate of plasticizer loss between

0- and 25-percent loss was obtained from the slope

of the secant between 0 and 25 percent. There

are distinct differences in the activation energies

of plasticizer loss at the two levels. Initially the

activation energy, E, is of the order of 20 kcal for

all three films. At the 50-percent level, the activation energy for the active solvent plasticizers is a little less than 30 kcal; for triphenyl phosphate

it is about 40 kcal. These values for activation energy are only

approximate and must be considered as orders of

magnitude. However, it is clear that the energy

involved is more than one would expect from a

simple evaporation process. Also, plasticizer loss becomes more temperature-dependent as the

plasticizer is lost. These two facts make it clear that plasticizer loss from a cellulose acetate

archival film probably could not be expected to go

to completion.

519560—59 2 5
50 I

1 1 1 r

lOV'K

Figure 12. Arrhenius plots for the loss of dimethyl phthalate

from a cellulose acetate film (1-mil thick) formulated from

flake C. An unplasticized cellulose acetate film 1 mil

thick has sufficient flexibility for archival purposes.

However, as a plasticizer is required initially to achieve lamination, the properties of films that

have lost an appreciable amount of plasticizer

should be examined. Unfortunately, heating a

film at a temperature high enough to evaporate

the plasticizer within a reasonable length of time

also may cause physical changes in the film. In

order to avoid this as much as possible, three films were heated at 50° C in activated charcoal. The

latter absorbs the plasticizer and greatly acceler- ates the plasticizer loss. Data on weight loss and

dimensional change are given in table 1, and the

data on dimensional change are plotted in figure 15. The plot of shrinkage against weight loss in

figure 1 5 gives a fairly straight line for the three

films. It appears that shrinkage is a function of

weight loss, regardless of the type of plasticizer.

20

5

0.05

0.02

1 1

1

\

fl \ SECANT, 0 TO 25% LOSS \ E = 23 kcal

-

O

-

\ °

a \

-

-

SLOPE AT 50% LOSS \

E-29 kcal \

-

-

°\ -

-

-

1 1 1 2.5 2.6 2.7 2.8 2.9

I0 3/*K

Figure 13. Arrhenius plots for the loss of diethyl phthalate

j from a cellulose acetate film (1-mil thick) formulated from

flake C.

The weight loss values for films numbers 4 and

7 in table 1 represent about half of the plasticizer

originally in the films. As the rate of plasticizer

loss becomes very low at this point, the shrinkage

values for unlaminated films would not exceed

those in table 1 for many years. As the paper

and tissue, if present, of laminated documents

would tend to hold the cellulose acetate film in place and prevent full shrinkage, and as the process would occur over a period of many years, enough "creep" of the film should occur to relieve

any strain due to potential shrinkage. Therefore,

shrinkage should not be a problem.

Data on the shrinkage of laminated specimens

of bond paper after aging in activated charcoal at 50° C are given in table 2. It is obvious that

tissue reduces the shrinkage to a remarkable degree. There is considerable difference in shrinkage between the long and the short directions of

the specimens. This is probably due to differences

6
1.0

J« 0.5

G 0.2

t-m

<t_j

a.o 0.1

0.05

0.02

0.01

SECANT, 0 TO 25% LOSS

E= 16 kcal

2.3

SLOPE AT 50 % LOSS

E = 42 kcal

Table 1. Weight loss and shrinkage of cellulose acetate films after aging in activated charcoal at 50° C

2.7

Figuhe 14. Arrhenius plots for the loss of triphenyl phosphate from a cellulose acetate film (1-mil thick) formulated from flake C.

o COMMERCIAL FILM NO. 4 • " " NO. 7

4 EXPERIMENTAL FILM; FLAKE A WITH 20% TRIPHENYL PHOSPHATE

WEIGHT LOSS, %

Commercial Commercial Experifilm No. 4 film No. 7 mental

film »

Weight loss: After 5 days percent.. 6.5 10.0 1.5 After 16 days ...do.... 8.9 10.8 1.9 After 46 days ...do 10.7 12.3 3.6 Shrinkage:

After 5 days_. ...do.... 1.63 2.29 .48 After 16 days do.— 1.95 2. 27 .51 After 46 days do- 2.69 2.76 .96

"Made with cellulose acetate flake A, described in table 4, and containing

20-percent triphenyl phosphate, based on weight of film.

Table 2. Shrinkage of laminated specimens, with and

without tissue, after aging in activated charcoal a

Laminate tested

Bond paper laminated with cellulose acetate film No. 7 only. Paper edge to edge: Cross direction Machine direction Film edge to edge: b Cross direction Machine direction Bond paper laminated with cellulose acetate film No. 7 and tissue. Paper edge to edge: Cross direction Machine direction Film and tissue edge to edge: b Cross direction Machine direction

Percent shrinkage

after aging in ac- tivated charcoal

at 50° C for:

Figure 15. The shrinkage of plasticized cellulose acetate films (1-mil thick) as a function of weight loss.

• Data were obtained by James Gear, National Archives and Records

Service. Specimens were approximately 14 X 23 cm.

>> The film, or film and tissue, of the laminated specimens extended about

0.3 cm beyond the edge of the paper.

in the machine and cross directions of the paper.

Stress-strain data and folding endurance data

have been obtained on the films heated in charcoal and are given in table 3. None of the properties of interest in an archival film has changed

enough to be considered significant. The experimental film plasticized with triphenyl phosphate

showed a significant decrease in elongation at break and energy to rupture. The former is not

particularly important, but it is tied up with the

latter, which, it is felt, is important in determining

edge tear. However, this formulation is not

representative of archival films.

3.3. Physical Properties of Cellulose Acetate

Films

a. General

The physical properties of a cellulose acetate laminating film are fairly closety defined by the

plasticizer requirement. An unplasticized film

has a very high softening temperature, and it is
Table 3. Physical properties of cellulose acetate films before and after heating in charcoal for 46 days at 60°C

Film designation Tensile

at break

Elongation

at break

Yield

strength

Elongation

at yield point Energy

to rupture

Folding

endurance, MIT, 1-kg tension

kg/15 mm kg/15 mm Double Film No. 7: strip % strip % g-cm/m folds Control.- 2.

1

26 2.0 3.6 5.01 1,444

TTptitprl in pharpnHl 2.

3

23 2 2 3 7 1, 863 Film No. 4: Control 2.0 21 1.9 3.9 3. 85 1,047 Heated in charcoal _ 2.0 23 2.0 3.7 4.26 1,105 Experimental film: » Control.. 2.3 15 2.3 3.3 3.30 1, 136 Heated in charcoal.. 2.3 5.5 2.4 3.2 1. 11 899

a Made with cellulose acetate (lake A described in tabic 4 and containing 20-percent triphenyl phosphate, based on weight of film.

necessary to formulate the cellulose acetate with an active solvent plasticizer in order to reduce the

softening temperature enough to allow lamination without damage to the film or the document.

A typical load-elongation curve for plasticized

cellulose acetate is shown in figure 16. The load

is the pull in kilograms on a strip of film in an automatic recording tensile testing machine, and

the elongation is the amount of stretch as the

pulling force is applied. The first part of the curve up to the yield point mostly represents

elastic behavior, and if the load were removed,

the film would essentially go back to its original

length. If the load is removed beyond the yield

point and before the breaking strength is reached,

the specimen will have a permanent set and will not go back to its original length.

The elongation of plasticized cellulose acetate

film is approximately 3 percent at the yield point.

The extent to which paper may be elongated

without rupture varies with the paper and with

the direction of stretch, but is usually between 1 and 4 percent. Therefore, the elongation of the

paper and the elongation of the film that would be

useful in protecting the body of the document are

fairly well matched.

The area under the load-elongation curve is a

function of the energy required to rupture the

ELONGATION AT RUPTURE -

TENSILE STRENGTH

AT RUPTURE

specimen, and is a measure of the toughness of the

film. Before full advantage could be taken of its toughness, the film would have acquired a permanent set and the paper would have been torn. Many properties of films, such as edge tear, toughness, tensile strength and elongation, depend on the condition of the edge of the film. A small

defect or notch can result in considerable impairment of film properties. Plastic films in general

have a high edge tear in comparison with internal

tear, which is usually very low.

b. Materials and Methods of Test

Physical test data were obtained on several

experimental films and commercial films. With

one exception, the experimental films were plasticized with 20-percent dimethyl phthalate, and

were cast from a solution of 16-g cellulose acetate and 4-g plasticizer in 180-ml acetone and 20-ml

methyl alcohol. Flake B was dissolved in 180-ml

methylene chloride and 20-ml methyl alcohol.

All films were conditioned at 23° C and 50-percent

relative humidity for at least two days before

testing.

Edge tear was determined according to ASTM

method D 1004 on specimens cut with a special

die, and is the force in kilograms required to start a tear in the specimen.

Folding endurance data were obtained with an MIT tester at 1-kg tension according to TAPPI

Standard T 423.

Tensile load-elongation data 2 were obtained

with a constant-rate-of-straining machine with an

initial jaw separation of 4 in. The rate of motion

of the crosshead was 2 in./min.

^ A modification of ASTM D 903, Peel or Stripping Strength of Adhesives [16], was used

for the evaluation of lamination efficiency. A

sheet of commercial cellulose acetate laminating

film was laminated between two sheets of a strong,

hard, bleached sulfate paper 3 with the paper

ELONGATION, % Figure 16. Typical load-elongation curve for plasticized

cellulose acetate film (1-mil thick).

2 IData were obtained by Russell J. Capott. P sjMade in the paper mill at the National Bureau of Standards, mill run

No.il882.

8
extending about 10 cm beyond the film at one end

of the sandwich. Strips 15-mm wide were cut from this sandwich, and one end of each strip of

paper was placed in the clamps of an automatic

recording tensile testing machine. The load

value at the beginning of failure was always the

highest, failure occurring between the film and

one strip of paper which then tore and feathered out to rupture.

c. Results and Discussion

Physical test data on several experimental films

and commercial films are given in table 4. The

degree of substitution of the cellulose acetate samples varies from 2.31 to 2.68 and the viscosity

varies over a wide range, but most of the physical

properties fall within fairly narrow limits. The

commercial cellulose acetate films have a higher

elongation at break than the experimental films

and a much higher energy to rupture.

Some of the experimental films gave considerably lower folding endurance values than did the

commercial films. The unplasticized experimental film exhibited an unusually high folding

endurance, indicating that cellulose acetate films

1-mil thick do not depend on plasticizers for

flexibility. Unplasticized cellulose acetate films

3-mils thick may exhibit a folding endurance of several hundred double-folds [15]. In order to obtain an estimate of the intrinsic

fluidity at which a cellulose acetate film is no

longer useful, specimens of commercial film number 4 were aged in oxygen for 8, 16, 24, and 40

hr, and in nitrogen for 24 and 40 hr. Intrinsic

fluidity values and load-elongation data are given in table 5 and typical load-elongation curves are given in figure 17. There are no signifTable 'I. Physical properties a of cellulose acetate, polyethylene, and polyethylene lerephthalate films

Degree of Tensile Elonga- Elonga- Folding Edge tear Material tested substitu- Viscosity strength tion at Yield tion at Energy to endurance, (ASTM D Thickness

tion at break break strength yield rupture MIT, 1-kg tension1004)

Experimental film b made with cellulose acetate

Flake A 2.44 = 51

kg/15-mm

strip2.0 %

6.2

kgll5-mm

strip2.0 %

3.1 g-cm/103 1.03

Double

folds511 Q

205 mil1.5 B 2.68 d 165 2.5 5.4 2.5 3.1 1.18 919 105 1.5 C 2. 31 «41 2.4 6.4 2.4 3.2 1.34 506 90 1.5 D 2.41 «69 2.5 8.3 2.6 3.2 1.90 806 135 1.5 D* >_.. 2. 41 «69 3.0 4.3 3.0 3.2 1.10 1,393 1.1 E. 2.3 7.4 2.5 3.2 1.56 732 115 1.5 2.2 6.5 2.2 3.1 1.20 922 85 1.5 O 2.49 °23 2.0 6.2 2.

1

3.0 1. 15 1, 148 160 1. 2-1.

5

H 2. 41 «30 2.5 7.3 2.6 3.3 1.70 750 115 1.5

I 2.40 "44 2.4 5.9 2.5 3.2 1.30 846 170 1.2 J 2.41 "62 2.4 6.8 2.5 3.2 1.49 1, 297 90 1.2

Commercial films

Cellulose acetate: Xo. 4 1.9 19.6 1.9 3.4 3.44 1,007 75 0.9 Xo. 7 „ 1.8 20.9 1.6 3.4 3. 33 1,419 65 .9 No. 8 2.0 15.8 2.0 3.4 2.94 1, 638 70 .9

Polyethylene terephthalate 2.5 50.2 1.9 3.4 11.4 ' >200, 000 300 .5

0.9 165 0.5 11.4 13.1 w 170 1.5

" Data were obtained with 10 specimens cut in the long direction of the film. d ASTM D 871, solvent C, manufacturer's data.

» Ail expei imental films contained 20 percent by weight of dimethyl phthal- « ASTM method D 871, solvent B, manufacturer's data.

1 ate, except D* which was unplasticized. f Test discontinued after 200,000 folds,

i « ASTM method D 871, solvent A, manufacturer's data. e Specimens elongated too much to obtain fold data.

Table 5. Correlation of intrinsic fluidity with physical properties of cellulose acetate film No. 4

Tensile Elongation Yield- Elongation Energy Intrinsic b Treatment of film at break" at break" strength" at yield" to rupture" fluidity

kg/15-mm kg/15-mm %

g-cm/103 strip % strip Untreated 1.8 11.2 1.8 3.5 1.86 0.59

Nitrogen at 124° C:

0.98 .63 24 hr _-. 1.4 6.5 1.4 3.0

40 hr 1.7 13.9 1.6 3.0 2. 10 .58 Oxygen at 124° C:

1.5 3.2 1.68 8 hr 1.5 11.9 .66

. 75 16 hr 1.5 4.0 1.5 2.9 0. 51 24 hr 0.9 1.0 .06 1.12

•Average for 20 specimens.

b Average for 3 or 4 specimens. 9
0 I I I L

I

1 1 r

24 hr IN OXYGEN AT 124 °C

I. F.= 1.12

0 4 8 12 16 ELONGATION ,%

Figure 17. Tensile load-elongation curves for cellulose acetate film No. 4 (1-mil thick) at various intrinsic

fluidities (I.F.).

icant changes in the films heated in nitrogen.

A significant increase in intrinsic fluidity has

occurred after 16 hr in oxygen, and the elongation

at break and the energy values have decreased

greatly. After 24 hr in oxygen the intrinsic

fluidity values average 1.12 and the load-elongation data indicate that the film is practically

useless. From the data given above, it is obvious that

as the intrinsic fluidity of a plasticized cellulose acetate film increases, the elongation at break and

the energy to rupture deteriorate first. When the

value for elongation at break has decreased to the point where it becomes equal to the elongation

at the yield point, the tensile strength starts to deteriorate. From an inspection of the data in table 5 and the load-elongation curves in figure 17,

the intrinsic fluidity of a cellulose acetate film

for archival use should not be greater than 0.66. j \ To evaluate lamination efficiency, samples were 1 It laminated at 400 lb/in. 2 at various temperatures' on a flatbed press, and the force to start peeling] i the paper from the film was determined. The

data are plotted in figure 18. As one would expect,

the strength of the bond increases with temperature of lamination.

4501 1 1

1

I *

200* L_l 1

1 1—

225 250 275 300 TEMPERATURE OF LAMINATION ,°F Figure 18. Strength of bond between paper and a commercial cellulose acetate laminating film versus temperature

of lamination. In order to evaluate various films for lamination, it is desirable to be able to determine or to

calculate the softening temperatux-e of the film. Softening temperature is difficult to determine or I to specify, as the values depend on the method of

measurement. In one method the film is heated on a copper block at a rate of 15° C per minute,

and tbe temperature at which the film becomes I limp is defined as the softening temperature [18]. In the method of Moelter and Schweizer [19] a creep test is used in which the film is held under

tension during heating at a constant rate of 2° C

1 per minute. Graphical treatment of the creep

data gives the softening temperature. Moelter ' and Schweizer obtained data on several plasticizers, calculated the softening point depression

constant for several types of plasticizer, and dej veloped the equation t=t0e~

kn from the experimental data. In this equation t is the softening

temperature in deg C of a film containing n moles

of plasticizer, t0 is the softening temperature of

10
the acetate flake, and k is the softening point and the data are given in table 6. These data

depression constant for the plasticizer. suggest that a softening temperature near 114° C

Using this equation, the softening temperatures would be desirable, as the first three films laminate

- of four of the more promising films were calculated, satisfactorily while the last one does not.

4. Degradation of Film and Paper During Lamination

The degradation of film and paper during the

lamination process was studied on three laminating

presses in the Washington area. Specimens of

film of proper weight for the viscosity test were

placed in folds of aluminum foil and run through

the presses. Two commercial films showed no

measurable change in viscosity even after 20

passes in a press. Samples of rag paper were run through the

presses in the same manner as the films, except

that the aluminum foil was omitted. The paper

had a pB. of 4.9, cold extraction. Some of this

paper was treated with alum to give a pl\ of 3.9, cold extraction. Viscosity in cuprammonium

hydroxide solution was determined before and

after the specimens had been run through the

presses, and the degree of polymerization (DP)

calculated from the viscosity data. The data are given in table 7.

Table 6. Calculated softening temperatures of typical

cellulose acetate laminating films a

Film designation Softening temperature

"C

Commercial film No. 4 113.5 Commercial film No. 7 117 Experimental film No. 2 •> 114 Experimental film No. 3

0 127.5

» Softening temperatures were calculated using the equation: t=Urkn (see p. 10).

* Composition of film No. 2: 73 percent cellulose acetate flake E,

5 percent triphenyl phosphate,

20 percent dimethyl phthalate,

0.5 percent magnesium acetate tetrahydrate,

1.0 percent resorcinol monobenzoate,

0.5 percent di-tert-butyl p-cresol.

c Composition same as No. 2 except that the film contains 15 percent di- methyl phthalate and 78 percent cellulose acetate. Films Nos. 2 and 3 were

supplied by W. D. Paist of Celanese Corporation of America.

I The untreated specimens do not show an appreciable change after one pass in any of the presses. There are definite changes after 20 passes, both in

DP and in color. The color changes show good

correlation with the changes in DP.

The specimens treated with alum to give a pH

of 3.9 show drastic deterioration even after one

pass. The DP tests were made a few weeks after

the papers were treated, and it is obvious that some degradation occurred at room temperature.

Enough acid was released during the short time in

the press to degrade the cellulose to its leveling-off

degree of polymerization. A graph illustrating

the latter is shown in figure 19.

Table 7. Degradation of rag paper in laminating presses

Type of press

Number

of passes

in

press

Paper 1119B > pH 4.9 Paper 1119B with alum

added, pH 3.9

Degree

of poly- merization b

Color Degree

of poly- merization b

Color

Flatbed

Cylindrical..

Cylindrical..

1 or

5

10

20

1020

15

1020

720 White 509 287 White.

Slight yellowing. Yellow and

brittle; many

breaks. Brown; very

brittle. Very dark; very

brittle. Almost black. Black.

235

235

306

235

650 Slight yellowing.

Slight yellowing. Yellowing

598 Distinct

yellowing.

273

168 Almost black.Slight Black.

yellowing. Yellowing

538 Distinct

yellowing.

a Paper 1119B was made in the NBS paper mill from a mixture of new

and old rags; it contains 1.5-percent rosin and 1.5-percent alum.

b The degree of polymerization is the number of anhydroglucose units per

cellulose molecule.

2000

1600

0 I

I TIME Figube 19. Leveling-off degree of polymerization of a cotton

cellulose. The time scale depends on the temperature and

the acid concentration.

11
Cellulose is known to consist of crystalline and

amorphous areas. Although the amorphous areas

are accessible to water and acid, and are easily

hydrolyzed, the crystalline areas are not penetrated by water. The hydrolyzing, or depolymerizing, effect of acid is very rapid in the amorphous areas, but, as the acid presumably can act only on the surface of the crystals, the action on them is very slow. The DP value that prevails

when the fast reaction in the amorphous areas is

over and only the slow reaction with the crystals is occurring is termed the leveling-off DP. The

leveling-off DP of cotton is about 200 to 250.

Wood pulps may be higher or lower, depending on

the source, and regenerated celluloses usually run from 15 to 60. The data obtained show that papers containing

acid can be seriously degraded during lamination.

Lamination does not degrade film or papers that

contain little or no acid.

5. Tissue B

Tissue has been used for years to strengthen

laminated documents. In order to determine as quantitatively as possible the actual contribution

of tissue to the properties of a laminate, several

specimens were laminated according to a predetermined pattern. Paper, film, and tissue are all anisotropic in nature; that is, the physical properties depend on the direction of measurement.

However, as the film is only mildly anisotropic,

the film direction was kept constant. The papers used (table 8) were selected because:

(1) the acid content was too small to cause degradation during lamination, and (2) the papers were

very weak, especially paper 1216, and any advantage of tissue reinforcement should be more apparent. Commercial film No. 7 was used in these

experiments.

Table 8. Composition and properties of papers a used in

the study of tissue reinforcement

Composition and properties Paper

1214 Paper

1216

Fiber Old rag None

1.1 0.5 90.3

6.2

3.8

524 267

Old rag

28.2 None

None

92.7

9.5

3.3

28

25

Filler t> percent.. Alum __ __ _ do. Rosin

_.

_ do Alpha cellulose. __ do... pH, cold extraction. Thickness. mil Folding endurance, MIT, 0.5 kg: Machine direction. _ ... .double folds.. Cross direction ... . . do.

» These papers were made in NBS paper mill.

b Precipitated calcium carbonate. The laminations were performed by personnel

at two different agencies, one of which uses a steam-heated flat-bed hydraulic press and the

other a cylindrical press with electrically heated

preheater platens. Laminations on the hydraulic

press were done at a pressure of 300 lb/in. 2 with

steam applied to the press for 135 sec; the steam

pressure rose to about 74 lb (152° C) in about 1 min. The pressure used on the cylindrical type

press was not measured. The lamination sandwich was kept in the preheater 20 to 30 sec. before

it was passed through the unheated pressure

rollers. From previous measurements, it is esti- mated that the temperature of the preheater was

approximately 182° C.

A modification of the Finch edge-tear device j (TAPPI standard T 470) was developed for the

evaluation of edge tear. The Finch device is used in the lower jaw of a tensile machine, and

concentrates the stress on each side of a test strip of paper. The modification concentrates the stress - at only one edge of the specimen. The Finch

attachment and the modification are shown in 1 figure 20. The lower end of the specimen is clamped behind a }i-irx. thick metal angle that is about 22}<!° from the horizontal. The tearing edge was dressed to a highly polished half-round. The

dimensions are purely arbitrary as any edge-tear

test is a modified tensile test with a high stress I concentration at the edge of the test specimen.

Because of elongation of the specimen, an edgetear value will depend on the angle of the tearing

device, as the angle determines the stress concentration at the edge. A zero angle would corre- spond to the usual tensile test. Data on the effect of width of extension of film

or of film plus tissue on edge tear, using the

modified Finch attachment, are given in table 9. The width of extension of film on the paper and

film laminate is not critical above 1 mm. If the

film does not extend beyond the edge of the paper,

the edge tear is considerably lower. The width

of extension of film plus tissue on the laminate of I paper, film, and tissue is not critical. It is obvious

that tissue decreases the edge tear of a laminate.

The values in the last column are consistent with

the value for the edge tear of film and tissue. It ; must be emphasized thai the laminates containing

tissue have a higher edge tear than the paper alone.

Laboratory data are not available on the abrasion resistance of documents laminated with and

without tissue. Resistance to edge tear does not

necessarily correlate with resistance to abrasion at the edge of the laminate.

Data on tensile strength, edge tear, internal

tear, and elongation at break of paper, film, tissue,

and various combinations of film with paper and i tissue are given in table 10. The data show few \ significant interactions. There is no significant

( interaction between the values for film and paper i and the laminates of film and paper. However,

considerable interaction occurs when tissue is added

to the laminate.

12
UPPER CLAMP

Figure 20. Sketches of (a) the Finch device for determining

the edge tear of paper and of (b) the modification developed

for testing laminated papers.

Table 9. Edge tear of laminates B using the modified Finch

method. Effect of extension of film or film and tissue on

specimens made with paper 1216 and cellulose acetate film No. 7

Width of trim

2 sheets

film No. 7 laminated Paper

only

Tissue and

film lami- nated

Paper and

film lami- nated

Paper,

film, and

tissue laminated mm kg

2. 10 kg

0. 14 kg kg kg

0.41 1.04

2. 72

2. 36

3. 18

3.13

3. 08

2. 31

0.50

.41

.41 .41

.38

.41

.41

» Laminated on a flatbed press.

Table 10. Physical properties of various combinations of paper, film and tissue

Direction » Tensile

strength

kg!IB mm

strip

Edge tear, modified Internal tear Elongation

at break

aper Finch

g

g %

«, Film only

t

->

2.0 1.51040

'<*) 3.0

3.0

14.0 28.5

Two sheets of film laminated

tt

o p

3.7

d 0

1.5 F

2100 c

(*) F9.9 c17.2 F

25.8 C

1.9

Tissue only

T

->

2.1 (0

(0

(0

12.0 (02.3 (0

Film and tissue laminated e

ft-> T-»

10.0 3.4

6.8

9.2

2.0

6.6

415 425

445

315

410 315

29.0

56.0 40.2

34.0

57.0 44.0

3.7

17.4 3.3

3.3

1.2 3.3

Paper 1214 only

T

-»

5.4 3.4295

265

41.7 45.2 2.5

3.7

Film and paper 1214, laminated s

t

-> II

7.4 5.68.4

6.4 20502800

1000 86052.

1

52.4 CO

(«) 3.4

7.1 2.96.1

Film, tissue, and paper 1214, laminated

T

T

->T

->

-»

TT—

—

—

ft T->

15.0 8.7

6.6

11.7 13.2

10.1

16.1 9.5

6.9

12.9

13.7

10.5

500 420

415

365

395

540

410 365

365

410

365 365

88.0

77.6

68.8 81.8

93

75

112.0 82.0

73.0

90.0

10086

3.7

3.7

7.0 3.4

3.7

3.6

3.8

2.6

4.7

3.5

3.8

3.7

Paper 1216 only

T

->

2.1. 7

9 135180

26.8 29.6

2

3 2

5

Film and paper 1216, laminated «

T

-» ft

5.4 4.98.4

6.4 35503150

1000 860 37.9

38.3 CO(') 5.4 10.3 3.25.5

Film, tissue, and paper 1216, laminated «

?—

>

T

~>->

tt—

t-»

tt T->

13.2 6.8

6.0 9.3

12.1 8.9

12.7 6.4

5.5 10.2

12.4 10.5

475

410

540 455

455

540

410 315

315

365

365

365

84.6

64.0

68.0 80.6

88

78

86.0

68.0 66.0 81.0

89

79

3.7

3.7

7.0 3.5

3.7 3.7

3.8

2.6

4.8 3.3

3.7

3.3

» Arrows indicate machine direction and cross direction of the film, tissue or paper; 1 arrow is shown for each sheet. Vertical arrow indicates machine

direction, horizontal arrow indicates cross direction.

i> These data were not obtained.

« Laminated on a flatbed press.

d Laminated on a cylindrical type press.

« Insufficient sample.

' Tissue too weak to test.

« The film or the film and tissue extended 2-mm beyond edge of paper.

13
The values for edge tear using the modification

of the Finch device are not additive. The film

has a high edge tear, but when laminated to

tissue, the edge tear of the laminate drops to a

fraction of that of the film. The data for internal tear show an interesting

interaction in that the sums of the values for tear

of the individual sheets add up to considerably

less than the values for laminated sheets of film,

paper and tissue. As indicated by the data tabulated in table 11, the amount of interaction depends on whether one uses the tear value of a

single sheet of film, multiplied by two, or whether

one uses the value of two laminated sheets of film. The laminates prepared on the cylindrical type

press show a higher numerical value for internal

tear than those prepared on the flatbed press, whereas laminates prepared on the flatbed press show a higher edge tear. Data on elongation at break follow, in general, an expected pattern. The elongation values of

the laminates usually lie between that of the film

and of the paper, tissue, or paper and tissue. The values for the laminates usually lie very close to those of the paper or tissue. When the paper

or tissue ruptures, this apparently serves as a

starting point for a tear in the film. For the same reason, the edge tear of a laminate is much

less than that of the film and only slightly higher

than that of the paper.

Table 11. Internal tear of laminates compared with

internal tear of component sheets of the laminates

Laminate components

Machine direction

of paper and tissue Cross direction of paper and tissue

F" O Fa Cb

Film, 2 sheets laminated

99.9

41.7

24.0

9 17.2 41.7

24.0

9

9.9 45.2

0

917.2 45.2

0 Paper 1214 TissueX2 Sum 75.6 82.9 55.1 62.4 FilmX2 (1 sheet=3 0 g) 6.0 41.7

24.0

6.0

45.2

0 Paper 1214 TissueX2 Sum 71.7 51.2 Experimental values for paper 1214, film, tissue 88.0

9.9 26.8

24.0

112.0

17.2 26.8

24.0

68.8

9.9

29.6 0

73.0

17.2 29.6

0

Film, 2 sheets laminated Paper 1216 TissueX2 Sum.. " 60.7 68.0 39.5 46.8 FilmX2 (1 sheet=3 0 g) 6.0

26.8

24.0

6.0 29.6 0 Paper 1216... - TissueX2 -

Sum 56.8 35.6 Experimental values for paper 1216, film, tissue 84.6 86.0 68.0 66.0

• Laminated on a flatbed press.

b Laminated on a cylindrical type press.

6. Deacidification

The term deacidification as used in this paper

refers to the Barrow method of neutralizing the

acid in paper by means of consecutive washes

with saturated solutions of calcium hydroxide

and calcium bicarbonate [20]. Two practically neutral papers, containing no

sizing, filler, or alum, were selected for this work

a cotton rag paper and a soda sulfite wood-pulp

paper. Specimens of these papers were treated

with 0.02 N sulfuric acid, other specimens were

treated with 1 -percent alum solution, and still others were washed with distilled water. The

data from the physical tests of the water-washed

specimens were used as controls to compare with

data from the physical testing of the acid- and

alum-treated papers. Some specimens were deacidified, and some of the specimens were aged at 124° C in oxygen for 24 hr with and without

prior deacidification. In addition, some of the

specimens were laminated and subjected to ac- celerated aging in order to evaluate the effect of

acid in the paper and of deacidification on the

film. The data on the properties of the papers tested

in the investigation of deacidification are summarized in table 12. It is obvious that deacidification prevents the degradation that otherwise

occurs during accelerated aging.

Data on the degradation of film laminated to the

acid-treated and deacidified papers are plotted in

figure 21. One line is drawn through the data from

the specimens of film laminated to alum-treated

paper and the specimens laminated to acid-treated

paper. Also, a single line appears to represent

• LAMINATED TO WATER-WASHED PAPER

A LAMINATED TO ALUM-TREATED PAPER

TIME , OAYS

Figure 21. Change in intrinsic fluidity with time in moist oxygen at 124° C of specimens of commercial cellulose acetate film No. 7 laminated to specimens of paper 732.

14
Table 12. Effect of deacidification on the accelerated aging of papers

Treatment

Not aged Aged in oxygen at 124° G for 24 hr Sample

pH, cold

extract Tensile DP Brightness » pH, cold

extract Tensile DP Brightness »

100% cotton paper 732

Not deacidified

1.

2.3.

4.

None

Water-washed.

1 percent alum

0.02 WHjSOj..

kg/15-mm kg/15-mm

strip % strip 6.2 8.7 478 69.6 6.5 8.0 455

5.9 7.7 477 71.4 6.4 7.8 460

4.5 8.3 513 71.7 4.0 (») 310 3.7 6.4 402 72.9 3.7 (") 317

66.6

64.6

56.0

19.8

Deacidified

L None 9.7 8.4 611 71.9 8.0 478 67.3

2.... .. Water-washed.. 9.3 7.8 487 73.5 7.8 473 66.8

3... 1 percent alum 8.9 7.7 512 72.3 8 0 483 70.6

4 0.02 N H2 SO< 9. 2 7. 5 487 74. 6 7. 7 465 67/9

Soda-sulfite paper 1133

Not deacidified

None 5.1 7.7 635 55.2 5.5 7.3 616 51.5 Water-washed. 5.7 7. 5 624 57.8 5.4 7.2 658 52.8

1 percent alum . 4.4 7.5 582 54.4 4.3 2.6 353 48.2

0.02 N H2S04 3.7 6.0 417 68.6 3.6 ( b

) 356 23.4

Deacidified

None 9.0 7.4 7.5

7.5

7.4

630 625

612

599

57.5

59.3 57.5 60.5

7.2

7.3

7.1 6.9

692 519

65.2

55.5

56.3

Water-washed.

1 percent alum ... 627

0.02 N H2S04 540

• Reflectance values obtained using the Hunter multipurpose reflectometer with blue filter. b Very weak and brittle.

the data from the films laminated to deacidified papers. It is obvious that acid in the paper

accelerates the degradation of the film. Also,

deacidification appears to make the film somewhat

more stable. Data on the effect of washing and of deacidification od the physical properties of paper from old

books are given in table 13. Specimens were deacidified in the usual way, and other specimens were treated with water instead of the deacidification solutions. The deacidification process did not

cause any significant change in tensile strength or folding endurance.

Table 13. Effect of washing and of deacidification on the physical properties of old all-rag papers

Tensile strength, kg/15-mm strip Folding endurance, MIT, 0.5-kg tension, double folds

Machine Cross Machin-e Cross Pages from book dated 1S40: direction direction direction direction 2.9 1.9 20 13 Washed with water 2.6 1.7 31 16

2.9 2.0 31 18

Pages from book dated 1869: 3.9 2.6 2 2 Washed with water 3.7 2.6 4 4 Deacidified — 3.8 2.4 3 3

7. Special Laminating Methods

Although lamination of archival documents with

cellulose acetate film is usually accomplished by

means of heat and pressure, Kathpalia has described a solvent lamination technique that does not

require special equipment [21]. A sandwich, consisting of tissue, film, document, film, and tissue is assembled on a flat surface. Using a nonlinting

cloth, acetone is applied to the sandwich in the

center and wiped toward the edges. The sandwich

is then turned over and the process repeated.

Best results are obtained if the laminate is placed

in a letter press or hydraulic press for a few hours.

Gear [22] has compared some physical properties

of laminates prepared by the Kathpalia method

with those of laminates prepared using a steamheated flatbed press. These data are reproduced

in table 14. It is obvious that the solvent technique compares favorably with the machine

method using heat and pressure. The solvent

lamination technique should be attractive to an

operator who cannot justify the cost of a laminating

press.

15
Table 14. Physical properties of paper, film, and tissue laminated by solvent a and machine b techniques

Direction Tensilestrength

(.kg/15-mm

strip)

Elongation (%) Edge tear°(frt Internal

tear (g)

Paper Film Tis- sueSol- vent Machine

Sol- vent MachineSol- vent MachineSol- vent Machine

* d

—>

tt

tt :£tt

6.3

12.7

6.8

12.1

3.2

5.1

3.7

3.7

410

450

410

450

105

82

64

88

t

-> T

->

t

2.7

1.9 2.0

1.5

2.1

2.2

3.5

14.0 28.5

2.3 Too weak

140 180 1,050 Specimens not

obtained Too weak to

test

26.8

29.6 3.0

3.0

12.0

to test

6 Kathpalia method of solvent lamination.

b Steam-heated flatbed press; same conditions]as described infection 5.

• Modified Finch method.

d Machine direction.

e Cross direction.

8. Preliminary Study of Films

Polyethylene and polyethylene terephthalate

films are probably the only films other than

the cellulose ester type which can presently be

considered as possible laminating films. Polyethylene appears promising, especially some

of the newer types of polyethylene and a chemically similar polymer, polypropylene. This group

of polymers merits further study. The loadelongation data in table 4 and figure 22 show some

obvious advantages and disadvantages. The yield

strength is very low, and the strength at 2- to 3- percent elongation, which is the value that is imi r

Ol 1 I L_

O 50 100 150 ELONGATION, %

Figure 22. Typical tensile load-elongation curves for

cellulose acetate, polyethylene and polyethylene terephthalate.

Acetone is somewhat toxic, and the maximum ' i allowable concentration in the atmosphere is

' generally considered to be about 500 volumes of acetone vapor per million volumes of air [23]. > Acetone is flammable and mixtures of acetone

vapor with air, between 2.5 and 12.8 percent, I are explosive [24]. Therefore adequate safety ; precautions should be maintained.

Another approach which may be useful for special purposes is lamination of a nonwoven

fabric, using an adhesive, to one side of the

document. Tribolet [25] has suggested this

' technique for mounting documents for display , purposes and to add strength where only limited

reinforcement is required. This should add suffi- j cient strength to allow careful handling, and the a document would still be available to be "seen i and touched" in its original state. The stability

of the adhesives and of the nonwoven fabrics ; is a factor that would have to be considered in j the preservation of documents by this technique. 3

Other than Cellulose Acetate

portant in protection of the paper, is still lower.

This is probably sufficient for ordinary service, but

a much milder impact would be sufficient to dam- f age the laminate than one prepared with cellulose acetate. The edge tear of polyethylene film is comparable to that of cellulose acetate film. Polyethylene terephthalate has the best physical

properties for archival purposes of any film studied. . However, the selection of an adhesive with good

aging qualities for Use with it would require further work.

The load-elongation data in table 4 and figure 22

show that a 0.5-mil polyethylene terephthalate

film is very similar in behavior to 1.0-mil cellulose acetate film up to the yield point. These two films

have similar capacities to protect the body of a document. Polyethylene terephthalate has a much higher energy to rupture and a higher measured edge tear. Therefore, the edge tear in serv-

( ice should be very satisfactory. It could be used

in thinner films to equal the performance of eel- ' lulose acetate plus tissue, or in thicker films to substitute for cloth reinforcement.

A problem with films such as polyethylene and

polyethylene terephthalate is the difficulty of re- moval from a document, should this become nec-

: essary. Neither film is soluble at room tempera- 1 ture in any known solvent. Although removal is , possible, the probability of damage to the document is much greater than with cellule se acetate. One approach that would make for easier removal 1

is the use of a nonwoven fabric with an adhesive, ; as discussed in section 7. A nonwoven fabric

made from polyethylene terephthalate fiber would

add considerable strength to a document with little increase in thickness, assuming that the proper

;

16
adhesive were available. The open texture of the non-woven fabric would allow access of a solvent to the adhesive, if removal of the fabric became

necessary.

Three NBS directories laminated with polyethylene terephthalate, polyethylene, and cellulose acetate films showed no significant differences in their conditions after about 6 months of use.

9. Specifications for Archival Laminating Film Formulated from Cellulose Acetate

9.1. General

The basic elements necessary for a performance

specification and for a composition specification are given in tables 15 and 16. It should be noted

that the accelerated aging test will require several

weeks for completion. Therefore the establishment of a list of acceptable materials under commercial designations, with tests of representative

lots at intervals rather than the testing of each

procurement lot, may be required.

Table 15. Suggested -performance requirements for a

laminating film formulated from cellulose acetate

Tensile strength at break, kilograms per 15-mm width, minimum

Tensile strength at yield point, kilograms per 15-mm width, minimum

Elongation at break, minimum

Elongation at yield point %—

Folding endurance, MIT machine at 1-kg tension, double

folds, minimum

Thickness mil.. Heat softening temperature: Moelter-Schwelzer ° C._

Copper-block method ° C__

Accelerated aging for 40 days in moist oxygen at 124° C: Rate of change of intrinsic fluidity with time in days,, maximum . Time to reach an intrinsic fluidity of 1.0, minimum,. days.

Viscosity of cellulose acetate, centipoises, minimum:

Cast film Extruded film

1.8

1.8

15.0

3.5 ±0.5

1,000

0.88 ±10%

114.5 ±2.5

123.5 ±2.5

0.02 40

10060

Table 16. Suggested composition requirements for a cast laminating film formulated from cellulose acetate

Tolerances in composition requirements should be in accordance with

good commercial practice, and as agreed upon between the buyer and the manufacturer. Tolerances for some items have been suggested by industry

representatives and are given here. Accelerated aging » and thickness requirements should be included to- gether with the following composition requirements.

Cellulose acetate — Degree of substitution, 2.4 ±0.1.

Viscosity, centipoises, minimum, 100. Bound sulfate, percent, maximum, 0.05^.

Plasticizer Triphenyl phosphate, 0 5 percent. Dimethyl phthalate. d 20 percent. Acid acceptor, minimum

Magnesium acetate, epoxy resin acid acceptor, or any other material agreed upon between the buyer and the seller.

Ultraviolet absorber, minimum

Resorcinol monobenzoate, or any other ultraviolet absorber agreed upon between the buyer and the seller. The material shall be one which has been proven satisfactory in actual use, preferably

in weathering applications.

Antioxidant, 8 minimum ... Octylphenol (p-l,l,3,3-tetramethylbutylphenol), (p-cyclohexyl- phenol), nonylphenol, or any other agreed upon between the buyer and the seller.

%by

weight

73

0.5

1.0

0.5

» The accelerated aging requirements for a film for which the composition

is specified could be upgraded. The maximum rate of change of intrinsic fluidity could be 0.01, and the minimum time to reach an intrinsic fluidity of

1.0 could be 90 days.

b Very pure cellulose acetates with bound sulfate contents around 0.002 percent are available, but the price is approximately 10 cents more per pound. The acid acceptor Is added to neutralize the bound sulfate and prevent degradation.

« The triphenyl phosphate could be omitted. In this case the formulation should contain 25 percent dimethyl phthalate instead of 20 percent. However, triphenyl phosphate Increases the hardness and durability of the film.

<• Diethyl phthalate could be substituted for dimethyl phthalate, but the

latter is the better choice.

• These antioxidants have been recommended by a manufacturer of cellu- lose acetate film.

9.2. Test Methods

Tensile strength and elongation shall be determined by ASTM method D 882-56T [26], Method

A, with a rate of motion of the crosshead of 2

m./min. Special care shall be taken to ensure the

first-class condition of the specimen cutter; a

special double-edge film cutter is recommended.

The yield point shall be located as indicated in

II section 3.3 of this publication. A general discussion of stress-strain phenomena is given in refer- ences [27] and [28]. Folding endurance shall be determined by method

T 423 m-50 of the Technical Association of the

Pulp and Paper Industry, using tb e MIT machine

at 1-kg tension. The considerations mentioned

above with regard to the specimen cutter shall apply.

Thickness shall be determined by TAPPI

method T 411 m-44, using an instrument with an

accuracy of 0.0002 in.

Heat softening temperature shall be determined

by the method of Moelter and Schweizer [19], or by placing the specimen on a polished copper

block and heating at the rate of 15° C/min. until

the specimen becomes limp [18]. Bound sulfate content shall be determined by

the method of Malm and Tanghe [29]. Intrinsic fluidity shall be determined as follows:

Weigh the cellulose acetate with an accuracy of

±0.2 mg making allowance for the plasticizer.

Dissolve the cellulose acetate in enough of the

solvent (9 parts of methylene chloride and 1 part

of methyl alcohol by weight), precisely measured,

to give a solution about one percent in cellulose acetate by weight. Transfer the solution quantitatively to an Ubbelohde suspended-level viscometer and measure the time of flow at 20 ±0.1° C.

Dilute the cellulose acetate solution with viscosity

solvent, precisely measured, to about ){ and %

percent cellulose acetate, measuring the time of

17
flow after each dilution. Finally, measure the

time of flow of pure solvent and calculate the

specific viscosity for each dilution from the equation:

Viscosity (solution) -viscosity (solvent)

Viscosity (solvent)

-dt , ,, t .,, , specific viscosity riot the logarithm of — •

:—- against concentration °

concentration and extrapolate to zero concentration to obtain the intrinsic viscosity. The intrinsic fluidity is the reciprocal of intrinsic viscosity.

Any viscometer with a range of 0.5 to 2.5 cp

should be satisfactory. An Ubbelohde suspendedlevel is preferable, with a large reservoir at the

bottom of the viscometer, making it possible to dilute the solution in the viscometer. The instrument may be calibrated using standard oils obtained from NBS. Directions for calibrating viscometers are given in reference [30]. Viscosity of cellulose acetate. Enough film, making allowance for the plasticizer, shall be taken to give 6 ±0.01 g of cellulose acetate. This shall be

dissolved in 100 ± 0.1 ml of a mixture of 95 ±0.1 g

of acetone and 5 ±0.01 g of water. The viscosity

shall be measured at 23° C by the ASTM ball-drop

method, D 1343 [31]. Degree of substitution shall be determined by

ASTM method D 871 [32]. This method determines the acetic acid or acetyl content, from which

the degree of substitution may be calculated.

Plasticizer content shall be determined by the

method of Whitnack and Gantz [13]. A value for

total plasticizer is sufficient. Accelerated aging. The film shall be aged at 124° ± 0.2° C in flowing oxygen (50 ±5 ml/min.)

moistened with water vapor at 30° C. Several

acceptable ways of doing this are described here.

An oil bath is satisfactory for maintaining the

specimens at 124° C. Although somewhat ex- pensive, a silicone oil, that can be used up to about

180° C, is very stable and gives excellent results. Other oils could be used, but none is as satisfactory as the silicone fluid. Either a stainless steel or

glass container for the oil is satisfactory. A con- venient arrangement is to fit a stainless steel beaker into an electrical heating mantle in an aluminum housing and supply enough electrical energy from a variable transformer to the mantle

to maintain the oil a few degrees below 125° C.

A small electrical tape heater between the mantle

and the beaker (or a small immersion heater in the

oil) can be used to supply heat intermittently from

a relay to maintain the bath at about 125° C in order to keep the temperature of the specimens at 124° C ±0.2. It is desirable to measure the

temperature inside the reaction vessel with a thermocouple. A good quality mercury regulator

is necessary to control the current to the small

tape heater or immersion heater. As this regulator can carry only a very small current, it is J connected to a relay through which current is

\ supplied to the tape heater.

The temperature of the oil bath should be

measured with a thermometer that can be read , with an accuracy of 0.1° C. A small continuous

duty stirrer is necessary to maintain a uniform

temperature in the bath.

A reflux type of constant temperature apparatus,

using a liquid that boils at the desired temperature, l may be used. Although not as accurate or as

flexible as an oil bath, the investment in time and

money for setting up the equipment is relatively

small.

Many types of reaction vessels will serve the

purpose. One type of reaction vessel is made by

sealing a coarse fritted-glass funnel to a standard i taper joint. The fritted glass serves as a plat- 6 form for the specimens, which are loosely rolled up

in glass tubes. The rate of flow of oxygen is not ; critical unless an antioxidant is present, in which

case a high rate of flow causes the antioxidant to I disappear from the film at a high rate. Therefore, j in order to compare the degradation rates of films , that may contain antioxidants, it is necessary to l use comparable rates of oxygen flow, and special

reaction vessels holding only one specimen.

The oxygen flow may be regulated by a twostage reducing valve for one reaction vessel, but

for several reaction vessels it is necessary to provide a copper or brass pipe manifold with good

quality needle valves. By keeping a definite . pressure on the manifold, e.g., 10 lb, the flow in each needle valve can be adjusted without disj turbing the adjustment of any other valve. The I flow rate may be measured with sufficient accuracy

by displacement of water in a graduated cylinder.

In order to obtain a value for rate of change of j intrinsic fluidity with time, intrinsic fluidity is plotted as the ordinate and time in days is plotted I as the abscissa. The rate of degradation is the

slope of this plot. Some judgment must be used \ in drawing a line through the experimental points, as there are many cases where a single straight ' line cannot be drawn. Sometimes there is an

induction period of several days during which ' little or no degradation occurs. This is characteristic of manj7' formulations, and a film containing } an antioxidant exhibits this behavior to a marked ' degree. Therefore, one straight line would repre- sent the induction period and have a low slope or a

slope of 0. Another straight line would be

drawn through the points obtained beyond the

induction period.

9.3. Improvements in Specifications

As indicated above, it is desirable that the film

contain an acid acceptor, e.g., in the form of cal- cium or magnesium salts to neutralize the bound

sulfate residue in the cellulose acetate. Most of

the bound sulfate is removed after acetylation,

but it is not practicable to remove the last traces. The determination of the amount of acid acceptor

18
should be a simple matter of water extraction and

electrometric titration, but a method has not

been developed.

The determination of the amount of ultraviolet

absorbers and antioxidants has received little attention in the technical literature, and the es- tablishment of suitable methods would require

considerable research. Therefore, the inclusion

of such stabilizers as a part of a specification for an archival film can only be an expression of

desire on the part of the purchaser, as procurement

lots could not be analyzed with confidence for

compliance with specifications.

Presumably, an ultraviolet absorber would dis- appear from a film after several years. Assuming

that a laminated document would receive the

most use during the first few years after lamination, the absorber should minimize degradation

due to light during this period. However, as it probably would not be present for the full life of the

film, it is important that the film have good

stability irrespective of the presence of stabilizers. It has been shown in this investigation that

antioxidants prevent oxidative degradation, at

least for a time, of films which would otherwise

be very unstable. The extent of this protection

depends on how long antioxidants remain in the

film. Not enough information is available to be

able to say whether a film containing a stabilizer

and a plasticizer that contributes to oxidative

instability could be detected with certainty by

means of aging in flowing oxygen at 124° C.

The composition specification calls for two plasticizers, triphenyl phosphate and dimethyl phthalate. The dimethyl phthalate is an active solvent

plasticizer and is necessary for lowering the softening point of the film. Diethyl phthalate could

be substituted for dimethyl phthalate. It is not

quite as effective in lowering the softening point,

but it is not quite as volatile, and dimensional

stability of films containing it is better than those

containing dimethyl phthalate. The choice is not

particularly critical. Triphenyl phosphate has been included because

films containing it show good aging properties in

oxygen at 124° C. If a film contains a good antioxidant, it is probable that the triphenyl phosphate

would not be necessary. On the other hand, triphenyl phosphate does enhance the physical properties of the film. Experience in the procurement of archival film

using these suggested specifications should result in improvement in the specifications. One possibility would be to combine the best parts, as shown by experience, of the performance and

the composition specifications into one specification. It is suggested that the specifications always

be kept as flexible as possible in order to take

advantage of new developments.

9.4. Problems in Procurement

A cellulose acetate film may be made either by

casting or by extruding. In making a cast film,

the ingredients are dissolved in an appropriate

solvent and the film is cast by causing the solution

to flow onto a large wheel or moving belt from

which the solvent is evaporated. This is a continuous process. Although the principle is very

simple, and films can be cast in the laboratory

using inexpensive equipment, the continuous largescale casting of commercial films is not easily

adaptable to formulation changes. The manufacture of special formulations by casting is not at- tractive to film producers unless the tonnage of

film required is large enough to warrant the high

cost of starting and stopping the complicated

system associated with film casting.

Manufacturers probably would be willing to make an extruded film in a special formulation,

as extrusion is a batch operation instead of a continuous operation. However, the ingredients must

be mixed hot in a milling machine, and this causes some degradation. An extruded film is seldom as perfect in appearance as a cast film. This is considered to be of negligible importance, however,

as these imperfections are not likely to be noticeable after lamination. An extruded film probably

would be a little cheaper than a cast film.

The authors acknowledge with thanks the as- sistance and cooperation of the various sponsors

throughout this project. The able assistance of

Mr. Wilbur Poole, who was on detail from National

Archives for more than a year, is gratefully

acknowledged.

10. References

[1] B. W. Scribner, Protection of documents with cellu- lose acetate sheeting, NBS Misc. Publ. M168, 1941.

[2] W. K. Wilson and B. W. Forshee, SPE Journal 15, 146 (Feb. 1959).

[3] G. C. DeCroes and J. W. Tamblyn, Protection of

cellulose esters against breakdown by heat and

light, Modern Plastics 29, 127 (April 1952).

[4] ASTM D 1203-55, Volatile loss from plastic materials,

ASTM Book of Standards, pt 9, p. 476 (1958).

[5] ASTM D 786-49, Standard specifications for cellu- lose acetate plastic sheets, ASTM Book of Standards, pt 9, p. 136 (1958).

[6] ASTM D 756-56, Resistance of plastics to accelerated

service conditions, ASTM Book of Standards, pt 9, p. 441 (1958).

[7] ASTM E 95-58T, Tentative specifications for cell- type oven with controlled rates of ventilation,

ASTM Book of Standards, pt 9, p. 1867 (1958). 19
[8] A. K. Doolittle, The technology of solvents and plasticizers, Chapt. 15 (John Wiley & Sons, Inc., New

York, N.Y., 1954).

[9] H. M. Quackenbos, Jr., Plasticizers in vinyl chloride

resins, Ind. Eng. Chem. 46, 1335 (1954).

[10] M. C. Reed, H. F. Klemm, and E. F. Schulz, Plasticizers in vinyl chloride resins, Ind. Eng. Chem.

46, 1344 (1954).

[11] L. H. Wartman and W. J. Frissell, Plasticizer prob- lems in vinyl resin studies, Plastics Technol. 2, 583

(Sept. 1956).

[12] W. J. Frissell, Volatility of vinyl plasticizers, Ind. Eng. Chem. 48, 1096 (1956).

[13] G. C. Whitnack and E. Gantz, Extraction and deter- mination of plasticizers from cellulose acetate plastics, Anal. Chem. 24, 1060 (1952).

14] J. H. Flynn, Rapid determination of the order of chemical reactions by time-ratio tables (in preparation) . [15] Hercules Powder Co., Cellulose acetate properties

and uses, 1954 edition.

[16] ASTM D 903-49 Peel or stripping strength of adhesives, ASTM Book of Standards, pt 7, p. 1182

(1955).

[17] T. J. Carter and J. R. Kanagy, A flex tension test for leather, J. Am. Leather Chemists' Assoc. 49, 23 (1954).

[18] Celanese Corp. of America, Celanese acetate sheeting

and film.

[19] G. M. Moelter and E. Schweizer, Heat softening of

cellulose acetate, Ind. Eng. Chem. 41, 684 (1949).

[20] W. J. Barrow, Manuscripts and Documents—their deterioration and restoration (University of Virginia Press, Charlottesville, Va., 1955) ; Restoration

methods, Am. Archivist 6, 151 (1943).

[21] Y. Kathpalia, Hand lamination with cellulose acetate, Am. Archivist 21, 271, No. 3 (July 1958).

[22] J. L. Gear, Comments on Mr. Kathpalia's article, Am. Archivist 21, 275, No. 3 (July 1958).

[23] The Merck Index, p. 8 (Merck and Co., Inc., Rahway,

N.J., 1952).

[24] M. B. Jacobs, The analytical chemistry of industrial poisons, hazards and solvents, p. 618 (Interscience

Publishers, Inc., New York, N.Y., 1941).

[25] H. W. Tribolet, private communication.

[26] ASTM D 882-56, Methods of test for tensile properties of thin plastic sheets and films, ASTM Book of Standards, pt 9, p. 335 (1958).

[27] A. X. Schmidt and C. A. Marlies, Principles of high

polymer theory and practice (McGraw-Hill Book

Co., Inc., New York, N.Y., 1948).

[28] ASTM D 638-58T, Test for tensile properties of

plastics ASTM Book of Standards, pt 9, p. 329

(1958).

[29] C. J. Malm and L. J. Tanghe, Determination of small amounts of combined sulfur, Ind. Eng.

Chem., Anal. Ed. 14, 940 (1942).

[30] M. R. Cannon and M. R. Fenske, Viscosity measurement, Ind. Eng. Chem., Anal. Ed. 10, 297 (1938).

[31] ASTM D 1343-56, Viscosity of cellulose derivatives by ball drop method, ASTM Book of Standards,

pt 4, p. 801 (1955).

[32] ASTM D 871-56, Methods of testing cellulose acetate, ASTM Book of Standards, pt 4, p. 776 (1955).

Washington, D.C., July 9, 1959.

20 U.S. GOVERNMENT PRINTING OFFICE: 1959
THE NATIONAL BUREAU OF STANDARDS

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Colorado, is suggested in the following listing of the divisions and sections engaged in technical work. In general, each section carries out specialized research, development, and engineering in the field indicated by its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front cover. WASHINGTON, D.C.

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Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

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