Reference

NBS

Publications

MBSIR 82-2530

tion of the Long Term

Stability of Polyester-Based

Recording Media

AlllQb ESSMS3

U 5. DEPARTMENT OF COMMERCE

Narionai Bureau of Standards

Center for Materials Science

Poiynrer Science and Standards Division

Washington, DC 20234

Firs: Annual Report

June 1 982

Prepared for

ai Archives

igton, DC 20408

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19(i2

HATlONAL BUREAU. Of STASDARD3

UBRAiRlf NBSIR 82-2530

PREDICTION OF THE LONG TERM

STABILITY OF POLYESTER-BASED

RECORDING MEDIA

MAR

I )'/. a /

D. W. Brown, R. E. Lowry, and L. E. Smith

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polymer Science and Standards Division

Washington, DC 20234

First Annual Report

June 1 982

Prepared for National Archives

Washington, DC 20408

H 1983

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

PREDICTION OF THE LONG TERM STABILITY OF POLYESTER-BASED RECORDING MEDIA

by

Daniel W. Brown, Robert E. Lowry and Leslie E. Smith

Polymer Science and Standards Division

National Bureau of Standards

Washington, D. C 20234

ABSTRACT

The stability of poly(ethyleneterephthalate) is being studied in order

to predict its long term behavior as the base of the film and tape used to

record archival information. This report contains results of the first year's

work. Film base, with and without photographic and other coatings, was aged

at several temperatures and humidities. Mechanical and calorimetric properties

and molecular weights were measured at intervals. Degradation was relatively

rapid at temperatures of 115, 100, and 85 °C at 100% relative humidity. The

-fi scission rate at 85 °C was about 10" mol scission/g-day and the activation

energy was 113 kJ/mol. Rates decreased strongly with relative humidity, becoming

negligible in dry air and nitrogen. At 55 °C and 100% relative humidity

degradation was not significant in 163 days. Samples were seriously embrittled

-4

by the introduction of about one scission per molecule—about 10 mol scission/g.

In its present state, gel permeation chromatography probably is not precise

enough to be a good degradation monitor, since one would like to know when

degradation had exceeded a fraction of the damaging level. Differential scanning

calorimetry showed little change in melting behavior of samples aged at 55 and

85 °C. Glassy samples crystallized during aging at 85 °C but not at 55 °C.

Magnetic tapes based on poly(ethylene terephthalate) usually have a

polyester polyurethane binder that holds the magnetic particles. The

2

binder is thought to be more sensitive to hydrolysis than the tape base.

Results obtained with a thermoplastic polyester polyurethane imply that

there may be an equilibrium extent of degradation for any storage con-

.5

dition. At 20 °C and 50% relative humidity, about 70 x 10 mol scissions/g

might occur. The corresponding molecular weight is only 1400 and it is

expected that binder of this molecular weight would be too soft.

It is anticipated that the study will go on for four more years. Agings

at 35 °C at several humidities are going on and samples will be examined at

intervals. Attempts will be made to develop more sensitive methods to detect

degradation.

Key Words: degradation; film base; hydrolysis; photographic film; polyester;

poly{ethylene terephthalate) ; recording media; stability

1 .0 Introduction

Many of the cultural, historic, and commercial records are now kept

in forms other than that of the traditional paper books or documents. Tapes

for audio, video, or digitally encoded information as well as microfilm are

increasingly popular storage media because of their higher storage density and

their compatibility with automated data access and retrieval systems. Newer

electrographic imaging systems combine the advantages of microfilm with the

ability to update pages, thereby totally eliminating the need for paper records

even for active files.

Poly(ethylene terephthalate) (PET) is the supporting base in much of the

photographic film and most of the magnetic tape that is used to record

archival information. The lifetime of PET is unknown under conditions of archival

storage. Similarly, polyester polyurethane binder, used to hold magnetic

particles on magnetic tapes, has an unknown lifetime.
3

Over twenty years ago, similar considerations prompted the Library of Congress,

with support from the Rockefeller Foundation, to sponsor a study of the archival

qualities of phonograph discs and the types of magnetic tapes available at that

time. The report of that study, "Preservation and Storage of Sound Recordings"

by A. G. Pickett and M. M. Lemcoe, contains a discussion of the probable modes of

deterioration of those recording media and recommendations for starage conditions

that still seem reasonable today. The present study was begun in 1981 under

the sponsorship of the National Archives. It begins a five-year program aimed

at producing measurement methods and data for predicting the lifetimes of

polyester-based materials under archival storage conditions. It is hoped that

the results will be general enough to be applicable to the products which will

be available during the next ten to twenty years and is therefore not merely an

assessment of the stability of specific current products. This report describes

the results of the first year of the study.

2.0 Background

q q

II It 2.1 PET . This polymer has the chemical structure, (CH

2

CH

2

O C CgH^CO)^.

Industrial production has been described recently [1]. Currently the

polymer is made from dimethyl terephthalate by catalyzed ester exchange

with ethylene glycol. Two exchange steps are involved. In the first,

bis (hydroxyethyl ) terephathal ate is formed at about 150-200 '’C and methanol

is distilled off. Reaction catalysts are acetates of cobalt, magnesium or zinc.

PET is formed in the second step by heating the first product to 280 °C with

antimony trioxide. Ethylene glycol is fonned, which is removed under vacuum,

thereby favoring polymer formation. At 280 °C, residual catalyst from the first

step would convert some ethylene glycol to bis- (hydroxyethyl ) ether. This could
4

enter the polymer through ester exchange and affect its properties. To prevent

this, phosphoric acid or triphenyl phosphate is added along with the antimony

trioxide. These complex the acetate catalysts and render them innocuous.

Recently high purity terephthalic acid has become available in large

quantities and may become the starting material of choice. In tnis process

bis-(hydroxyethyl ) terephthalate would be made directly from terephthalic

acid and ethylene glycol, using titanium alkoxides or dialkyl tin oxides or

carboxylates. With these present, conversion of bis (hydroxyethyl ) terephthalate

to PET does not require antimony trioxide, although some may be used.

Storage qualities of the finished product may be affected by the process

used. Specifically, since catalysts must act on fop/vard and reverse reactions,

residual esterfication catalyst will promote hydrolysis. If the catalyst is

destroyed by side reactions (hydrolysis, for example) the residual activity

may not be great. In a study of the hydrolysis of poly (butylene adipate)

glycol we found that it hydrolyzed with a smaller rate constant if it nad

been prepared without catalyst.

A side reaction that occurs during polymerization of PET may also

be important to its subsequent storage qualities. This is the self disproportionation of the polymer chain to form vinyl (-- CH = CH^) and acid

It (HO C CgH^ CO--) end groups. Hydrolysis of esters is known to be acid

catalyzed so manufacturers strive to keep the acid content low. Acid concentrations of 15-50 y eq/g are typical of commercial polymers [2],

The number (M^) and weight (M^) average molecular weights of commercial

PET are about 20,000 and 45,000 g/mol respectively. About 1.3-1. 8 wt. %
5

of cyclic compounds-dimers, trimer, tetramer, and pentamer, are present.

The most abundant is a cyclic trimer [2,3]. Conceivably, these compounds may

migrate out of the polymer in prolonged or high temperature agings, affecting

physical properties and the measured M^.

PET is weak, brittle, and opaque unless subjected to a complex

thermomechanical treatment [2] to be described. Polymer is heated. 'above

the melting temperature, T^ (about 250 °C), extruded, and quenched. Then it is

heated to about 135 °C (the glass temperature, T^, is about 70 °C) and stretched

biaxially. Subsequently the polymer is heated to about 150 °C, under restraint,

and seme crystal 1 ization occurs. It is then cooled and may be "tensilized" by

stretching it unidi rectional ly. The result is a clear film with a tensile

strength as high as 200 MPa (30,000 psi) and an elongation at break of about

150%. Although the film is quite clear, the amount of crystallinity is

appreciable--from 30 to 50% is estimated by calorimetry.

Glassy PET, prepared by simply melting and quenching the polymer,

is clear but is weaker and less deformable than fully treated film. Therefore,

the tremendous improvement in mechanical properties brought about by the

thermcmechanical treatment is due to residual orientation and stress in the

film. The presence of residual stress is shown by the two fold shrinkage in

length and width that occurs if a film is heated quickly to 285 °C.

Temperatures in previous aging studies of PET have generally been

above T^ in order to keep aging times from becoming unreasonably long. Archival

storage temperatures are normally of the order of 20 °C or below [4].
6

Thus a rather rather long extrapolation below is required when applying

previous data to the temperature region of interest. There is no convincing

evidence that such extrapolation is valid. Indeed, one anticipates some

discontinuity at T^ because of changes in mobility. Thus, above T^ one

would expect that a scission in an amorphous portion of a polymer chain would

be followed by rapid relaxation of any strained configurations that portion

had. Considerably below T^ one would expect the broken chain would maintain

its strained configuration. Presumably there should be differences in physical

characteristics that reflect such contrasting behavior that would not be

taken into account by simple extrapolation through T^.

There have been several aging studies involving PET. We will not attempt

to review all but will describe the most important ones. The reaction of primary

interest is ester hydrolysis, which breaks the polymer chain generating acid and

alcohol groups, i.e. H 2

O + -CO

2

- -CO

2 H + H0-.

One of the most thorough of the previous studies is that by McMahon

et al. [5]. The hydrolysis of PET was studied at temperatures between

130 and 50 °C at relative humidities, RH, of 0, 20, 50, 75, 95, and 100%.

Samples consisted of 0.5 and 10.0 mil-thick film, the former more oriented

than the latter, and 1100 denier yarn. The maximum time was 300 days. The

amount of chain scission, calculated as the fraction of ester groups hydrolyzed,

was deduced from intrinsic viscosities. Measurements were also made of tensile

strength, elongation and density. Infra-red spectra were studied.

Plots of the amount of chain scission versus time showed upward curvature.

However, this became appreciable only after samples had become brittle so rate

constanti. for chain scission were taken as the initial slopes. For the 0.5 mil

sheet these were approximately proportional to RH at constant temperature and

had an activation energy of about 110 k J/mole, independent of RH. Rate

constants for the 10- mil sheet increased more rapidly than relative humidity

but were 1/4 to 1/2 those of the 0.5 mil sheet. The activation energy was
7

about 120 kJ/moK It was assumed that'diffusion limited the degradation

rate of the 10 mil sheet. The yarn, which had threads thinner than 0.5

mil, was considerably more stable than either sheet. Possibly orientation

and/or crystal 1 inity contributed to these rate differences.

The 0.5-mil and 10-mil films were judged to have failed when 0.7

and 0.5% of the ester groups had been hydrolyzed, respecti vely , which

amounts to about one ester group broken per molecule. Extrapolation by the

Arrhenius equation gave lifetimes of about 500 and 800 years, respectively,

for these films at 25 °C, 50% RH.

The lowest temperature at which work was reported is 60 °C. In 300

days at 100% relative humidity the intrinsic viscosity had decreased from

0.53 to 0.45 dL/g. The decrease was approximately uniform with time.

The value of T^ in such a long experiment may be less than 70 °C, so it is

uncertain if this degradation is below T^.

Arguments were given [5] to the effect that degradation due to thermal

or oxidative scission is about three orders of magnitude less than

hydrolytic scission at 100% RH. The data for films at 90 °C in dry air

initially showed a small decrease in intrinsic viscosity followed by an

increase to about the initial value.

During degradation of the films there were increases in density,

attributed to increases in crystall ini ty, and also increases in absorbance

at 3425 cm""', attributed to incorporation of -OH groups in the polymer.

A more recent paper described hydrolysis at 100 °C, 100% RH [6].

Carboxylic acid contents were measured at various times for samples

having different initial acid contents. These data show clearly that

degradation is autocatalytic, the rate increasing proportionally to

initial acid content and exponentially with time. Since acid contents of the

samples studied by McMahon et al. [5] were not given, it is possible that some

of the differences between samples were due to differing initial acid contents.
8

The above studies did not involve film bearing photographic emulsions,

which might affect the degradation. Adel stein and McCrea [7,8] have aged

photographic film and also uncoated film base.

Samples were sheets of base, 4 mil thick in the case of PET and

also sheets of exposed and developed photographic film, on the same

thickness of base. Most of these were preconditioned at 21 °C. 5C% RH

and then sealed in 2.5 cm diameter glass tubes, about 100 sheets of one

kind to each tube. The volume of the tube was made small to minimize

evaporation of moisture from the samples. The samples were then placed

in ovens at temperatures between 71 and 125 °C for various times. Properties measured included intrinsic viscosity, tensile strength and elongation, emulsion melting and image loss, and brittleness on rapidly bending

the film. The last two are qualitative tests.

The results for the first three properties were assumed to obey

first order kinetics and rate constants were caltulated as the fraction

of each property lost per unit time. These rate constants were not usea

directly in extrapolation but instead were used to calculate times to 20%

viscosity loss, 10% loss in tensile strength, and 10% loss in elongation at

break. Times to emulsion melting and image loss and onset of brittleness

were also judged. These times were extrapolated by the Arrhenius equation

to 21 °C. The extrapolated times range from 1000 to 2000 years. Relative

humidities at each temperature are unknown and must be different, so the

reliability of the extrapolation appears questionable.

Longer experiments were done in which samples were aged at 25 °C - 60% RH,

38 °C -78% RH, and 50 °C - 20% RH for times up to 24 years. After 24 years at

25 °C, 60% RH decreases in intrinsic viscosity and elongation were 3% and

22% respecti vely; tensile strength appeared to increase slightly, about 5%.

The extrapolation described above predicts that in 24 years losses of these

three properties will be 0.1, 0.2, and 0.4% respectively at 25 °C. Thus the
9

extrapolation described above predicts rather different changes in properties

than were observed.

PET base with a photographic emulsion layer degraded about three

times faster than the uncoated support. This was attributed to production

of ammonia from the gelatin. Ammonia is known to attack PET quite severely [2].

However, to us it seems that the effect might also be due to humidity differences,

since gelatin which sorbs water strongly at 21 °C, might desorb it at oven

temperatures. Relative humidities would be greater in these tubes. At high

temperature the film with the cellulose triacetate base appears somewhat more

stable than the film with the PET base. Below 100 °C this situation is reversed.

2.2 Magnetic Tape . Most magnetic tapes consist of a thin (0.25-1.50 mil) PET

substrate with a front coating containing a magnetic material, commonly y ^£ 20 ^ or Cr02. The magnetic material is embedded in a polymeric binder. Common

binders are based on polyester polyurethanes. Sometimes there is a carbon

coating on the back, which is also embedded in a polymeric binder. The carb'on

coating dissipates electrostatic charge.

The polyester polyurethane binders used on tapes are highly crosslinked materials with complex structures. They can be expected to have

5-10 times as many ester groups as urethane groups. Aliphatic esters

are used; these are more susceptable to hydrolytic degradation than the

PET substrate. Thus the degradation of interest in magnetic tapes is

primarily that of the binder. On degradation this softens; adjacent

layers of tape may stick together or the binder may stick to the recording

heads. Ultimately the tape becomes unreadable.

The hydrolytic degradation of soluble aliphatic polyester polyurethanes

is quite well understood [9,10]. The reaction proceeds by the equation:

H

2 O + -CO

2

- -C02H(A) + HO- (1)
10

Here an ester link, -C02“, reacts with water, breaking the polymer chain

and giving an alcohol and a carboxylic acid (A), which catalyzes the reaction,

_2 Typically there are about 10 mol ester groups for one gram of polymer.

If 5 - 10% of these are hydrolyzed the polyurethane molecular weight

becomes about 1000 - 2000 g/mol . This is far too low to have the mechanical

properties of a polymer. Thus severe property degradation can occur

without much change in the ester concentration. For any sample, the

water content is determined by the relative humidity and the temperature and

is not much affected by the extent of degradation. Consequently, ester and

water concentrations can be included in the rate constant, k. Scissions

in ,mol/g are equal to the change in the reciprocal of the number average

molecular weight, A This equals the change in acid content. Since

the reaction is acid catalyzed the applicable differential equations are:

d[A] ^ ^ ^^^n

= k[A] ( 2 ) dt dt

where t is time and k is the fractional rate of increase in acid concentration.

After integration:

[A] =

= [Ag] (e'^^-1) (4)

_C

where [A^] is the initial acid content, about 10 eq/g.

For polyester polyurethanes based on polycaprol actones and polybutylene

adipates, k was approximaiely proportional to relative humidity and, at

100% RH, increased with temperature with an activation energy of about 75

kJ/mol [101.

Carbodi imides are sometimes used as hydrolysis stabilizers in

polyester polyurethanes because they react with acid, reducing reaction 1 to

negligible proportions. However, there is a slow uncatalyzed hydrolysis,

ordinarily negligible, which continually generates acid [11]. This reacts

with carbodi imide, eventually consuming it. Subsequently, reaction 1 again

becomes significant and destroys the polymer.
11

Esterification, the reverse of hydrolysis, may be significant at

low relative humidities. If k"* is the rate constant for the reverse of

reaction (1) then the rate of ester formation will be k [A] [HO-]. The

acid concentration is squared because acid was a catalyst for the forward

reaction and so must be one for the back reaction. If hydrolysis has occurred

to a moderate extent, [A] and the [H0-] will be about equal, since [A^] is small

and acid and alcohol form at equal rates. Thus a sample undergoing degradation

at low RH ought to reach an equilibrium described by the equations;

k[A] = k '[A]^ or k = k "'[A]^ (5)

At 35°C, 100% RH, unstabilized, soluble polyester polyurethanes

lasted about 400 - 500 days before becoming very weak. Highly crosslinked materials would be expected to last longer because more scissions

would be required to reduce the molecular weight to a critical value.

Such samples have been examined. Cross-linked reticulated foams, taken

from airplane fuel tanks, have been studied by preaging them until

soluble and then aging them for additional periods to measure the rate

of increase in acid content [12]. The rate data were extrapolated backward

in time to get the acid contents before aging. These correlated well with

mechanical properties and the times (5-8 years) the materials had been in

service. The actual rate constants were about equal to those of soluble

polyurethanes.

Two studies have been concerned with the hydrolytic stability of

magnetic tape binder. E. F. Cuddi hy measured the weight of acetonesoluble components and evaluated the adhesion between binders and PET

[13]. Agings were done in air and in nitrogen at relative humidities of

0, 11, 30, and 100% at temperatures of 36, 48, 61, and 75 °C. Samples

that had been hermetically sealed at relative humidities of 30, 60, and
12

100% at each temperature were also aged. The weight of the acetone

soluble materials decreased at 0 and 11% RH and in all the hermetically

sealed samples, indicating that crosslinking occurred. The other samples

showed increases in sol fraction. These increases were attributed to

scissions caused by hydrolysis. The crosslinking was attributed to the

reverse process, esterification.

The adhesive strength between binder and PET was estimated by

rubbing the tape with a swab that had been soaked in tetrahydrofuran and

measuring the time required to dislodge the binder layer. Values were

14±2 seconds for unaged tape. They ranged down to as little as one

second for a sample aged 14 weeks at 75 °C, 100% RH and up to 30 seconds

for samples aged 14 weeks in dry air or nitrogen at 75 °C. Thus the

results parallel the sol fraction observations.

The crosslinking effect found for hermetically sealed samples is

puzzling since the samples were supposedly equilibrated at humidities

high enough to cause degradation. Thus, unless the binder initially had

considerable acid and alcohol, one would expect that the sol fraction

would increase and the humidity decrease until hydrolysis and esterification went on at equal rates. Relative humidity was measured in

several experiments and found to increase. As noted above, sol fractions

decreased. Possibly the iron oxide particles contain some Fe(0H)2, which

reacted with carboxylic acid to give water and an iron carboxylate.

Cuddihy concluded that at a relative humidity of 24% at 21 °C the

binder should be stable, neither crosslinking nor degrading.

The second study of tape stability was undertaken for the U.S. Air Force- [14].

The effect of humid aging was only part of the study; much of the effort went

into mechanical characteristics of tape winding and their effect on long term

tape readability.
13

To study the effect of humidity, reading error rates were measured

before and after aging tapes at: 55 °C, 100% RH; 20 °C, 50% RH; and 20 °C

in a hemetically sealed container (conditions at closure were not stated).

Tapes were examined at three month intervals for 12-15 months. Low

density and high density multi track systems were used. At 20 °Z it was

concluded that the error rate increased somewhat but remained small,

6 7 about one error in 10 to 10 bits. However there were some unreadable

blocks and tracks, usually on the outer part of the reel or on -the

outer edge of the tape, that were eliminated from consideration. At 55 '"C

there was severe binder degradation, to the point where binder flaked

off the tape if it was stretched. Reading difficulties started after

six months of aging and became worse. However, with the multi track system,

information could be recovered after 15 months aging at 55 °C, 100% RH by

combining results from several tracks.

The authors of the report were aware of the Cuddihy work and

attempted to use it to calculate a relative humidity-temperature line

along which there would be no hydrolysis. However, they set up the

equilibrium as k = k"'[A], which appears contrary to the law of mass

action. Consequently, their recommended storage conditions, 18 °Q at 40%

RH, are suspect in our estimation. Our own experience with a variety of

polyester polyurethane elastomers suggests that considerable degradation

would occur under such conditions, but we have no experience yet with

tape binder.

3.0 Present Approach

3.1 PET . Samples of several materials are being aged at 25, 50, and 100%

relative humidity at 35, 55, and 85 °C. In addition to two exposed and

developed photographic films, there are samples of two electrographic

systems, some uncoated PET film base, and NBS 1470, a PET film having
14

known permeation characteristics. Samples for each condition are in one

large test tube closed with a rubber stopper. The samples are suspended

on a frame above the LiCl humidifying solution. In addition, samples of

the leader are being aged in sealed tubes under dry air, dry nitrogen,

and nitrogen at 100% RH. The coating has been removed from some of the

commercial samples in order to get results with and without coatings.

At 35°C some samples contain test patterns in order to see effects on

the gelatin layer.

Samples are removed at intervals, tested for brittleness by creasing,

and their molecular v^eights are determined by gel permeation chromatography

.

Tensile strengths^ TS, and elongation, E, are measured in many cases. A

differential scanning calorimeter (DSC) is used to observe glass and melting

behavior and measure aH^, the heat of fusion. A potentiometric titrator has

been received and acid contents will be determined. Some infrared measurements

were made and more are planned.

Two sets of smaller scale experiments are being run. These involve

glassy samples and samples preaged at 85 °C, 100% RH. These are being

aged only at 100% RH, the last set only at 55 and 35 °C.

Properties of unaged samples are in Table I. The first three materials

(all photographic products) and coated electrographic film D have similar

properties. Other materials have somewhat lower values of and TS and

higher values of aH^. Values between 113 and 144 J/g have been estimated

[15] for 100% crystalline PET, so our samples appear to be 30-50% crystalline

by calorimetry.

Uncoated electrographic filT> C has particulary high values of aH^

and E and a low value of TS. It "necks" in the tensile tester suggesting

that it was not prestretched as much as the other materials.
15

The lower molecular weights of the glassy polymer indicate that

it degraded during preparation. When heated in the calorimeter it

exhibits a strong glass transition, then crystallizes, and finally melts.

This behavior is shown later. Pieces of the glassy polymer were small and

contained bubbles. Consequently, their tensile properties were not measured.

3.2 Tapes . To date we have not aged magnetic tapes but have been studying

equilibrium characteristics of esterfication at different humidities and

temperatures. Materials studied include a polyester polyurethane elastomer

and several polyester diols used to make polyester polyurethanes.

4.0 Resul ts

4.1

^

4.1.1 Mechanical Properties . Table II lists the number of days of aging

after which samples cracked when creased, a qualitative test designated

“Finger Brittleness." These times depend on humidity and temperature.

No failures have yet been observed in samples aged at 85 °C, 25% RH or at

55 °C. At 35 °C, 100% RH, approximately 100 days cause failure of all samples.

Electrographic film C was embrittled more rapidly than the others.

Figures 1 and 2 show elongation at break as a function of aging

time at 85 °C. The figures show effects of surrounding gas, relative humidity,

and removal of any coating. The lines indicate the general trends of the data.

At 100% RH almost all elongation is lost between 80 and 100 days, whether in

air or in nitrogen, whether coated or not. Specimens of uncoated and coated

electrographic film C, aged at 100% RH, were not run in the tensile tester.

The former was too brittle to handle after 61 days, at which time only a

small piece of the latter was taken. After 81 days the latter was too brittle

to handle. At 50, 25, and 0% RH all samples show progress! vely less loss of

elongation.
16

Figures 3 and 4 show tensile strength as a function of aging time

at 85 °C. Decreases on aging are not nearly as severe as found for

elongation. The influence of humidity and the lack of any consistent

difference associated with the presence of a coating are qualitatively

as found with elongation.

4.1.2 Chain Scissions in PET . Molecular weights were measured by use of gel

permeation chromatography using four microstyragel columns having pore

sizes 10 , 10 , 10 , and 10 nm. The solvent was hexafluoroisbpropanol

containing 0.01 M sodium trifluoroacetate. This salt tends to prevent

adsorption of polymer on the packing. Calibration was performed by

using published chromatograms and intrinsic viscosities of PET in

hexafluorisopropanol [16] to calculate the Mark HouWink equation between

intrinsic viscosity and molecular weight at the chromatogram peak. This

relationship was assumed valid for our polymers. Then we determined the

intrinsic viscosities and chromatographed several of our degraded and

undergraded samples. Use of the Mark Houwink equation gave molecular

weight at the peak as a function of retention volume on our column set.

This calibration technique is being used on a tentative basis.

Efforts were also made to develop a calibration with a better theoretical basis. Narrow fractions of polymethyl -methacrylate were purchased

and a universal calibration was developed (polystyrene is insoluble in

hexafluroi sopropanol and so could not be used). When applied to PET this

calibration gave values between 3000 and 200 for undegraded and degraded

samples and thus was seriously in error. The reason for the failure is

unknown. Also, in our preliminary work we found that metastable solutions

of PET could be formed by dissolving it first in hexafl uoroi sopropanol and

then diluting 30 fold with chloroform. These solutions were stable for
7-10 days and so could be chromatographed. The mixed solvent, being effectively

chloroform, dissolved polystyrene fractions, which were chromatographed and

the results made the basis of a universal calibration. When applied to PET

the result was as above, unbelievably low molecular weights.

Several chromatograms of degraded and undegraded PET in hexafluoro3 isopropanol are in Figure 5. The secondary maximum at about 37 cm was

found with all materials. The progressive shift of the peak maxima to

higher retention volumes as aging time increases means that the molecular

weight decreases on aging. Aging at 100 °C and 115 for short times

caused severe degradation. There are small peaks at higher retention volumes

3

than shown here, 40 - 60 cm . These are observed if polymer-free solvent

is injected and so are ignored.

The molecular weights are calculated from the chromatograms by

assuming that the height above the baseline is proportional to the concentration of polymer, whose molecular weight is determined from the

retention volume at which that height occurs. Therefore, the weight

fraction of polymer in a certain molecular weight range is proportional

to that fraction of the total area under the chromatogram between certain

retention volumes. A digital computer is used to make the necessary

sunmations and calculate M,, and M„. w n One character! Stic of the results is that is quite sensitive to

location of the baseline. This is shown in Fig. 6, which gives M,, and

for two baseline locations. The location of the baseline is really

somewhat subjective, particularly in the low molecular weight region

because of the solvent impurities and because there is probably some

adsorption of the polymer on the column packing. The larger standard

deviation found for M than M in table I for film base reflects this situation.

n w
18

In calculating scissions, is the quantity of interest because

represents mol polymer per gram. Therefore is the increase in

mol polymer per gram. This is equal to the mol of scissions per gram,

since each scission adds one to the number of polymer molecules. Probably

-1 -5

a(M^ ) is uncertain by about 2 x 10 mol scission per gram.

Figures 7 and 8 are plots of versus time at 85 °C. At 100%

RH a linear extrapolation of a(M^^"^) at longer times to zero time falls below

the origin, suggesting that the scission rates increase with time; the

line is drawn so as to emphasize this trend. The scatter is great and

it is thought there are no significant differences in scission rates

between samples. This is true even of coated and uncoated electrographic

films C despite their early mechanical failures, compared with those of

_5

the other samples. At 100 days there is aoout 6 x 10 mol scission/g,

indicating that vaKies of have been reduced from about 17000 to 8000.

Thus, there has been about one scission per original molecule at failure,

in agreement with the work of McMahon et al . [5]. Degradation is much

less at 50, 25, and 0% RH.

Figure 9 shows scissions formed in the glassy polymer at 100% RH at

85 °C and also at 55 °C. At 85 the scission rate is about four times as

large as with the other samples. At 55 °C the number of scissions is

negligible.

Table III shows results from a variety of samples aged at 55 °C,

100% RH for 163 days. No property decreases very much and some increase

slightly. Consequently, we have as yet no evidence of degradation at 55 °C.

Single aging experiments were done in sealed tubes at higher temperatures

than 85 °C. At 115 °C there were 26 x lO'" mol scissions/g in 13 days and

at 100 °C there were 22 x 10'^ mol scissions/g in 39 days. At 85 °C at 100% RH
19

-5

there were 11 x 10 mol scission/g in 123 days. Overall rates are plotted

in Fig. 10. The activation energy is about 113 k J/mol or 27 kcal/mol in

reasonable agreement with 29 kcal/mol found by McMahon et al . [5]. Their

rates at 85 100% RH are about 2 x 10 ^ and 10"^ mol scissions/g, day

for 0.5 mil and 10 mil film, respectively. Our value for the same conditions

_6

is 0.9 X 10" mol scissions/g, day.

^.1.3 Calorimetry with PET . Some scans between 45 and 180 °C from the

differential scanning calorimeter are in fig. 11. For each curve the starting

ordinate value is arbitrary and the slope is made approximately zero. The

curves for the unaged commercial samples show deflections at about 75 °C that

are indicative of glass temperatures , although they are very weak. That for

unaged film base is typical of most of our materials. The deflection for the

uncoated electrographic film C is the largest for any of the commercial

samples. Aged samples do not show a normal T^ but their curves appear to

have weak melting peaks about 30-45 degrees above the aging temperature.

Interpretation will not be attempted now because the effects of time, humidity,

and extent of degradation are unknown.

Curves, for the same polymer and also for some aged NBS 1470 between

180 and 280 ®C are in Fig .12; the ordinate sensitivity is one tenth that

in Fig. 11. Curves 1 and 2 cover the range found for unaged polymers

except for the glassy one. The temperatures at the peak maxima, T max,

and the heats of fusion are listed in' Table IV. Long heating does not affect

these quantities much at 55 °C and 85 °C. The changes with samples

aged at 100 °C and 115 probably are significant.

Fig. 13 and 14 show calorimeter scans for the glassy material. The

ordinate scales are both made the same as in Fig. 12 in order to keep

the crystal! ization peaks on scale. The transition at T^ for unaged glassy

material is much stronger than for the other unaged materials. Aging at
20

85 °C permits crystallization, which greatly reduces the size of the

glass transition and eliminates crystallization during the scan. Aging

at 55 °C did not cause crystallization but it did shift the glass transition temperature. It is encouraging that crystal 1 ization did not occur

since this process might destroy the orientation of commercial films.

4.2 Polyester Polyurethanes . Cuddihy's work suggests that hydrolysis of

polyester polyurethane tape binder may be equilibrium limited ^t low

relative humidity [13]. Binder insolubility and the presence of iron oxide

prevent reliable kinetic analysis so we made a preliminary study of equilibrium

using soluble polyester polyurethane and polyester diols. In this study

hydrolysis was followed by measuring the acid content as a function of time.

A thermoplastic polycaprolactone polyurethane elastomer with an initial

-5

acid content of 10 eq/g was aged at 85 °C, 100% RH until the acid content

-5 was 25.5 X 10 eq/g. Portions of this material were then re-aged at 85 °C

at relative humidites of 27, 12, and 0%. The variation in acid content during

the re-aging experiments is shown in Fig. 15. At 27 and 12% RH the equil-

-5

ibrium acid contents are 60 and 35 x 10 eq/g, respecti vely. Samples

were also aged at 8% RH at 55 °C and at 11% RH at 35 °C. Equilibrium acid

_5

contents were 33 and 28 xlO eq/g respecti vely. Thus equilibrium acid

contents change little with temperature at the same relative humidity.

Polyester diols were also equilibrated at 85 °C and nominal relative

humidities of 95, 50, and 25%. At 85 °C these materials are liquids with

viscosities of about 30 Pa.s (300 cp). [A] in eq/g of poly(butylene

_3 adipate) and poly(caprolactone) diols at equilibrium were 3.9 x 10

and 3.2 x 10'^ at 95% RH, 1.4 x 10'^ and 1.2 x 10'^ at 50 % RH, and 0.6 x 10"^

[poly(butyleneadipate)] at 25% RH. The equilibration is now being studied

at 55 °C and it appears that the equilibria [A] are not much different

from those at 85 °C at the same relative humidity.
21

Rate constants for the hydrolysis are known from previous work [10].

Therefore, rate constants for the back reaction can be calculated, by

equation 5 for the polyurethane and by its modification for the polyester

diols. Modification is required because the polyester diols initially contain

_3

about 10 eq/g of -OH groups; consequently [A] and [-0H] are not equal.

Secondly, at high relative humidity the ester concentration is changed by

hydrolysis. The equation is:

= k '[A] ([-OH^] + [A]) (6)

Here [E^] and [-OH^] are initial concentrations of ester and -OH groups

and E is the equilibrium concentration of ester. At equilibrium, [A] >>[A^],

so that the [-0H] is nearly equal to ([-OH ]

-! [A]). [E^] and [-0H ] are

u u u -2 -3

known character! sties, being 10 and 2.1 x 10 eq/g, respectively, for

-3 -3

poly(butyleneadipate) diol and 8.77 x 10 and 10 eq/g, respectively, for

the poly(caprolactone) diol.

At 0% RH there is no hydrolysis so k^ for the polyurethane was calculated

from the kinetic data for this condition in Fig. 15. If third order kinetics

1 r -1-^ are assumed then k is half the slope of [A] " versus time. The three points

fit third order kinetics rather well (Fig. 15, x's and dashed line).

The values of k^ are listed in Table V. As humidity increases k^

decreases for all three materials. The polyester diols have smaller k^

values than the polyester polyurethane.

The k"* at about 10% RH for the polyurethane were extrapolated to 20 °C

3 2 2 by the Arrhenius relation to obtain 1.2 x 10 g /eq x day. The

corresponding value of k, obtained by extrapolating data in reference [10],

is 1.2 X 10"^ day""*. If these rate constants are substituted in eq. (5)

_5

the equilibrium acid content is calculated to be 32 x 10 eq/g, about as

observed at 85 - 35 °C at this humidity. If one neglects any change in k^

with relative humidity and takes k as being proportional to relative

humidity then the equilibrium acid content will vary as the square root
22

-5 -5

of the relative humidity ratios, giving 50 x 10 and 70 x 10 eq/g at

25 and 50% RH at 20 °C. The reticulated foams studied earlier had lost

_5 substantially all their tensile strength at acid contents of 22 x 10 and

-5 -5

36 X 10 eq/g. About 65:4 of the strength was retained at 9 x 10 eq acid/g

Alternatively, one can neglect equilibrium and use k to calculate

the time to reach some limiting acid content from the initial acid content,

-5 -4 -d

which is probably about 10 eq/g. Values of k are 3 x 10 and 6 x 10 ’ day

at 25 and 50% RH at 20 °C. If the limiting acid content is 15‘x 10'^ eq/g

then, by solving eq. (3) for t, one obtains times equal to about 25 and 12

years respectively for the above conditions.

These calculations were all based on data from a thermoplastic

polyester polyurethane and it is not known how closely they will apoly

to tape binders. The ideas should be valid for any aliphatic polyester

polyurethane and modifications exist if stabilizers are present [11].

5.0 Pi scussion

5.1 PET. The degradation observed is consistent with previous work and supports the idea that the most likely cause of degradation under ordinary

conditions will be hydrolysis. It is somewhat surprising that serious

loss in physical properties occurs with only one scission per molecule.

However the molecular weight of the polymer is quite low sc one scission

per molecule may bring below a critical value. In addition, the

highly oriented nature of the material may furnish an explanation. One

picture of a strained semi crystal 1 ine polymer is that much of the stress

is carried by strained amorphous portions of chains that tie crystalline

regions together [17]. Another concept is that mechanically strained

bonds are sensitive to chemical attack [18]. If one combines these

ideas then the attack would tend to be localized at bonds that are

critical for mechanical strength.
23

The results with the glassy material appear somewhat in conflict

with these ideas, since nominally unoriented, glassy polymer degraded

faster than the oriented PET. However, the DSC results indicate that

the sample crystallized during aging. This might have had some effect

because the degradation that occurred during heating prior to forming

the glass probably introduced acid groups, which would be expected to

have a catalytic effect on hydrolysis. Being on chain ends, they would

probably be located preferential ly in non-crystal 1 ine regions,'

In the coming year the agings at 55 °C and 35 °C will be continued

and others will be started at 70 °C. Acid contents will be measured by

use of a potentiometric titrator, which was delivered recently.

Some agings will be also be started in the presence of small concentrations

of sulfur dioxide and nitrogen oxides. These may be envi ronmental ly

significant for the degradation since the concentration of acid pollutants

is likely to increase as lower grade fuels become more common.

Finding that severe mechanical degradation requires very little

chemical degradation has made us rather skeptical of the value of gel

chromatography as a method of monitoring samples, unless the precision

of the measurements can be improved by a factor of ten or more. Infrared

methods will be investigated more thoroughly in an effort to find a

sensitive means for detecting degradation. Other methods to be considered

are transmission of sound and light [19].

5.2 Magnetic Tape . Two problems involved in studying degradation of tape

binder are the presence of crosslinks and iron oxide. The latter may

represent 80% of the coating. Methods employed will be like those used

to study crosslinked foams--pre-aging to render the binder soluble, additional

aging, and then determination of the acid contents. Filtration or centrifugation

will be used to remove the iron oxide particles. Infrared spectroscopy will

also be used to get evidence for reaction between binder and iron oxide.
Experiments will be carried out at several temperatures and humidities. The

hydrolysis of tape binder is expected to be much more rapid than that of PET.

REFERENCES

1) G. W. Parshall in "Homogeneous Catalysis: The Applications and

Chemistry of Catalysis by Soluble Transition Metal Complexes,"

John Wiley and Sons, N.Y. 1980, p. 208.

2) C. J. Heffelfinger and K. L. Knox in "The Science and Technology

of Polymer Films," O.J. Sweeting, Ed. Wiley-Interscience,' N.Y.

1971, Vol. 2 p. 587-633.

3) S. Shiono, J. Polymer Sci , Polymer Chem Ed, 1_7_, 4123 (1979).

4) ANSI PHI. 43-1979.

5) W. McMahon, H. A. Birdsall, G. R. Johnson and C. T. Camilli,

Chem. Eng. Data 4(1), 57 (1959).

6) H. Zimmermann, Second International Symposium on Degradation and

Stabilization of Polymers, Dubronvinik, Yugoslavia, Oct. 4-6, 1978,

paper 02.

7) P. Z. Adelstein and J. L. McCrea, Photographic Sci. and Eng. 9_, 395 (1965).

8) P. Z. Adelstein and J. L. McCrea, J. Appl . Photographic Eng. _^1 160 (1981).

9) C. W. Schol lenberger and F. D. Stewart, J. El astopl ast. , 28 (1971).

10) D. W. Brown, R. E. Lowry, and L. E. Smith, Macromolecules, T3, 248 (1980).

11) D. W. Brown, R. E. Lowry, and L. E. Smith, Macromolecules, 15, 453 (1982).

12) D. W. Brown, R. E. Lowry, and L. E. Smith, Polymer Preprints 223 (1981 ).

13) E. F. Cuddihy, I.E.E.E. Trans Mag, May 16 , 558 (1980).

14) N. Bertram and A. Eshel , Reoording Media Archival Attributes

(Magnetic) to USAF, AFSC Rome Development Center, N.Y. 13441,

Report F 30602-78-C-0181 , Nov 1979 p. 75-111.

15) J. Brandrup and E. H. Imergut, Polymer Handbook, J. Wiley, 197b. p. V 73.

16) E. E. Drott in "Liquid Chromatography of Polymers and Related

Materials, " J. Cazes, Ed. Maroel Dekker Inc., 1977 p. 41.

17) A. Peterlin, J. Polymer Sci. Polymer Symp. 297 (1971 ).

18) V. A. Kargin and G. L. Slonimisky in "Encyclopedia of Polymer

Science and Technology," H. F. Mark and N. G. Gaylord, Ed.,

Interscience, 1968, Vol. 8, p. 509.

19) R. J. Samuels in "The Science and Technology of Polymer Films,"

O.J. Sweeting Ed, John Wiley and Sons Inc. 1968 , Vol. 1, p. 255.
m ;

‘jj4C3U5'e ffcaUjrg

. fe.

' _

'ji OEJi..,

' I' lfct >V*4 ''V^nivi^ 1^9^,-#

i,'/ii ^rff^rii-^O' ,T' t,^ i>'"i. no2nfiat> ,d'.» f^»2j[)*t^8 .A“ .j^ . fiqrts?^

fttt ••%';. ’>

; % ur m

'

fit

,8^|f''i-^.4-' -'A •>• te>vf^wv(Ji/y ,arnrv/)6n4u(} Q*? fis'fef fdjBia • • ^ ' ^,7

'#»•«. »4' *AK' *it*%»^‘

T ' A, j JIHI "^^:':lm

’ -naib'T'.U'i-at'S^^'r ' i-'.t^ ’'iAntiVi! ' <Sc-d'WI^.' ";”. -•39-4U ^ nsjf;. ,ve5^ *

S.-1. .;'. 'fScJ "it' M Vnw?^iEw'

' 4

^ <iT'f * "« ^0 'p' ^ >i

.... 1 - , t i|7 - ;.r ^ 'V, :.;^^^^||||^
Table I. Properties of Unaged PEt"*

q/mol

10 -\

q/mol

TS,

M Pa

E,

°L

AH^,

USl

Uncoated Film Base 4. o 1+ o 1.77 ±0.34 204 ±5^ 144 ±8 47 ±3

Microfilm A 5,04 2.15 200 ±14 126 ±13 48

Microfilm B 4.80 1.69 193 ±11 122 ±11 48

Encapsulating Film 4.16 1.59 177 ±22 80 ±15 56

Uncoated El ectrographic Film C 4.20 1.44 116 ±7' 185 ±22 59

Coated Electrographic Film C 4.07 1.54 167 ±5 105 ±7 55

Coated Electrographic Film 4.60 1.68 201 ±10 99 ±16 47

PET Film NBS 1470^ 4.50 1.45 150 ±11 96 ±29 54

PET, NBS 1470 Glassy Polymer^ 2.60 0.88 -- -32

56

'Samoles 0.01 cm (4 mil) thick except as indicated. Uncertainties are standard deviations of three or more measurements.

^1 M PA = 145 psi

^4.8 mil

"^1 mil

5 Made by heating and quenching NBS 1470. About 10 mil thick.
Table II. Days Aging of PET at 85 °C to Induce Finger Brittleness

Material

Fi Im Base

Microfilm A

Microfilm B Encapsulating Film

Uncoated El ectrographic Film C Coated Electrographic Film C Coated Electrographic Film D PET Film NBS 1470

100% RH 50% RH

105 >146

97 >146

97 >146

105 >146

61 81

31 146

>81 >84

90 --

Table III. Percentage Change in Properties of PE1 Aged at

100% RH, for 163 days

Material M M TS E w n Film Base 0 10 -8 -8

Microfilm A -3 -2 -3 +10

Microfilm B -2 28 6 6

Uncoated Electrographic Film C -7 15 -9 12

Coated El ectrographic Fi Im C 3 20 -9 -2

Encapsulating Film -1 23 6 _6

Arithmetic Mean 2.5 15 -5.3 9
Table IV. Effect of Aging on and aH^

Material Aging Conditions T °C

‘Max’

^ AH|T, ' !£ days %RH

Film Base Unaged 251 ±0^ 47 ±3

Film Base 55 163 100 252 49

Film Base 85 154 25 252 46

Film Base 85 105 100 253 52

Uncoated Electrograohic

Film C Unaged 260 59

Uncoated Electrographic i Film C 55 163 100 257 65

Uncoated Electrographic

Film C 85 154 25 257 62

NB5 1470 Unaged 255 54

NES 1470 55 90 100 254 52

NBS 1470 85 39 100 255 48

NBS 1470 85 60 100 253 53

NBS 1470 85 90 100 255 59

NBS 1470 100 39 100 249 64

NBS 1470 115 13 100 243 62

^Uncertainties are standard deviations of three measurements

.

Table V. Rate Constants for Esterfication

Material Temp./°C RH/% lO-V/g^

Poly(caprol actone) polyurethane 85 0 4.1

Poly(caprolactone) polyurethane 85 12 1.9

Poly(caprolactone) polyurethane 85 27 1.4

Poly(caprolactone) polyurethane 55 8 0.17

Poly(caprolactone) polyurethane 35 11 0.06

Poly(caprol actone) diol 85 50 0.47

Poly(caprolactone) diol 85 95 0.13

Poly(butylcneadipate) diol 85 25 0.42

Pcly(butyleneadipate) diol 85 50 0.26

Poly(butyleneadipate) diol 85 95 0.07

200
200 o

inoo

c/5 ><G

O

in • O

cx: c_j ij 07

iT

Elon^tion

at

break

versus

aging

time

at

85

°C.

Q,•

uncoated

electrographic

film

LW,lI,I|

coated

electrograpiiic

film

C;A,

NBS

1470;

^encapsulating

film.

Shading

rl/ci r

-i

*'»

r.

-i

/-I 1^
25%

RH

3.^^’

strength

versus

aging

time

at

85

"C.

0,

0,

(I, •

film

base

mi

crofi

1m • A>

Jki"Ic

ro fiI

III 8;

o.<>

coated e

lectrographic

film

0.

Shadings

arvJ

flags

as

in

figure
200 CO

O 0-

03><G

errs lZ.

'I.

Tensile

strengtli

versus

aging

time

at

85

‘^'C.

(<,

I,

uncoated

electrographic

film

C;

Cii,Ci,0tt

coated

electrographic

film

C;

A<

NBS

1470;

^encapsulating

film.

Shading

code
-3

aj

'zri -o

71>1

.-a

0)> —

0

o

o './I ^ >1a

-iJ -a a

*3 'O

aj

T3 3 -3 O

5

*3 lD

OJ31 • «

.t3 ./> 3 >, 3 -a -3o '3 3

Jl 'Jl = >1 3 3

i- -3 O O

3

5 3

i- -3

3 '.3 o

1/1 >,3

3 3

3L 3

_n c\j 3^
39.6

I

24.75

37.6

Fi^-

b.

Effect

of

chroma

to

cjrdni

baseline

on

and

Film

Base

aged

85

°C,

100%

RH,

81

days.

Final

volume

.39.6

cm'^,

area

-

347,

14

=

33400,

-

10700.

Final

volume

37.6

cni^,

area

=

343

, M

=

33600,

M

=

11900.

^
6/SUOISSiOS |OUJ '(j^_^iAJ)V gOL
100%

RH o

in oE

C7> C

•I o

IT3 SZ

U 00

<13 =

CT).,- O 4-

<4O

O)

OI o

O)

<13 OUc

Z3

CT)

<13

</1 Q.

<T3 U

OJ

C/5 ><Q

«»

C3 -- O o oo

UO CQ

CO 2:

4>J

<T3 ^

3;

CD4-

•<-

U

CT-!-

<13 -c:

</) f13 3 i. . i/l OOi

s- o

OJ >. O) > +J s_ U 3

</) 0) CT> O <11 —

1/1 "O

C

on OJ

—

O <13 </! t/1 O 13 U

6/suojssjos |ouj
10

A(Mp

mol

scissions/g

0 50 100

DAYS

Chain scissions in glassy PET made from NBS 1470. 0, 35 =C, lOOii RH; I, 5 100 lO
in

10

k.

mol/

g

.

DAY

10^/T

Fiq ,10. Arrhenius plot of scission rate at 100% RH.

2.8
o

11.

1)1

ft(?reritidl

scantiino

(.a 1

nr

im(^

ter

curves.

1.

unaged

film

base;

?,

imaged

uncnated

e

lec

trographi

c

film

C;

3,

film

base

aged

85

'’C,

lOOX

Rll,

105

days,

4,

film

base

aged

85

‘'C,

25%

Kll,

154

days;

5

,

film

base

aged

55

‘"C,

100%

Idl,

163

days.

C_3 3

iZ

13

Uitferentidl

scaiminy

calorimeter

curves

for

glassy

polymer.

1)

Unayed;

2)

Ayed

85

lUOX

RH.

61

days;

3)

Aged

85

"C,

lUOZ

RM,

103

days;

Ayed

55

"C,

100%

Rll,

103

days.
o

CO

CNi

o

CO

CN

a

o

o

CO

^ 1/1 r— ra

cr- ul

OifferentUl

scannincj

calorinfiter

curves

for

glassy

polymer.

Same

curve

designations
70

DAYS

Re-aging of

Re-aging 35 ’C,

oreaged polycapvc^actone polyurethane e'as

RH as indicated. [A]: Q, 21% RH; Ji, 12'. tomer.

RH; 0,

Rreaci ng

Ot RH; X,

35 =C, IGC^ Ri [A "2] (gasnec I i ne .

NBS-114A ( _ oO)

4. TITLE AND SUBTITLE

Prediction of the Long Term Stability of Polyester-Based Reconiing Media

5. AUTHOR(S)

Daniel W. Brown, Robert E. Lowry, and Leslie E. Smith

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

national bureau of STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR 82-2530 563 June 1982

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

10.

SUPPLEMENTARY NOTES

I '] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

The stability of poly(ethylene terephthalate) is being studied in order to predict

its long term behavior as the base of the film and tape used to record archival information. This report contains results of the first year's work. Film base, with and

without photographic and other coatings, was aged at several temperatures and humidities

Mechanical and calorimetric properties and molecular weights were measured at intervals.

Degradation was relatively rapid at temperatures of 115, 100, and 85®C at lOOft relative

humidity. The scission rate at 85°C was about 10“® mol scission/g-day and the activation

k J/mol. Rates decreased strongly with relative humidity, becoming negair and nitrogen. At 55°C and 100% relative humidity degradation was not

163 days. Samples are seriously embrittled by the introduction of about

one scission per molecule--about 10"^ mol scission /g. Differential scanning calorimetrjyshowed little change in melting behavior of semi crystal 1 ine samples aged at 55 and 85°C

Glassy samples crystallized during aging at 85°C but not at 55°C.

Magnetic tapes based on poly(ethylene terephthatate) usually have a polyester polyurethane binder that holds the magnetic particles. The binder is thought to be more sen sitive to hydrolysis than the tape base. Results obtained with a thermoplastic polyeste

polyurethane imply that there may be an equilibrium extent of degradation for any storagje

condition. It is anticipated that the study will go on for four more years.

energy was 113

ligible in dry

significant in

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s; degradation; film base; hydrolysis; photographic film; polyester; poly(ethylene terephthalate); recording media; stability

13.

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