NBSIR 83-2750

Prediction of the Long Term

Stability of Polyester- Based

Recording Media

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polvmer Science and Standards Division

Washington, DC 20234

Second Annual Report

August 1 983

National Archives

Washington, DC 20408

NBSIR 83-2750

v • PREDICTION OF THE LONG TERM

STABILITY OF POLYESTER-BASED

RECORDING MEDIA

NATIONAL BUREAU. OF STANDARDS

LIBRARY SEP 1 5 1983

ocjVckCO

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

Second Annual Report

August 1 983

Prepared for National Archives

Washington, DC 20408

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

National Bureau of Standards

Washington, D.C. 20234

Abstract

The stability of po ly lethy lene te rephtha la te J is being studied in

order to predict its long term behavior as the base for film and magnetic

tape. This report contains results of the second year's work. Film base,

photographic film, and electrographic film are being aged at several

temperatures and relative humidities, RH. Acid contents, mechanical properties, and molecular weight have been measured at intervals. The rate of

increase in acid content, k, provides a useful measure of the aeyraoation

rate. Data for films aged between 115 and 55 C at 100 % RH obey the

equation In k = 39.3 - (14000/T), where k is in %/day and T is the absolute temperature. The water content is included in k, which is approximately proportional to RH. Lifetimes of the films appear to ba equal to

69.3/k days. Extrapolation to 25 and 20 C gives 400 and 900 years for the

Lifetime at 100 % RH. A more sensitive analytical method than now used

will be required to make useful measurements of acid content at 35 C, even

after samples have been aged for five years. Two films have always failed

before the others. The short-lived films might well be stored with others,

since failure of the former would warn of the approaching failure of the

Latter

Magnetic tapes are also being aged, primarily to study the binder

that holds the magnetic oxide to the substrate. Binder on five brands of

tape that were aged at 60 C became crosslinked at RH of zero and 11 %,

changed little at 30 % RH, but degraded at 100 % RH. The binder on tapes

aged at 85 C and 100 % RH first degraded severeLy and then appeared to

crosslink slightly. Badly degraded binder was not gummy but was easily

detached from the substrate. Model binder aged at 85 C and 100 % RH

eventually became brittle.

1
Key words: degradation; hydrolysis; lifetime; magnetic tape; photographic

film; po ly lethy lene terephtha late); stability

1 .0 Introduct i on

Data of many kinds are recorded and stored on photographic film and

magnetic tape. Both media contain an active element in a dispersing medium

bonaed to a substrate consisting of po ly (ethy lene terephtha late) , PET. A

PET-based electrographic film is now available wnich can be revised. The

lifetime of the PET under common storage conditions is very long but not

known. The lifetime of the dispersing medium for the iron oxide particles

of magnetic tapes is also uncertain but is expected to be less than that

of PET because the medium is based on polyester polyurethane, which is

known to be hydrolytically unstable. For these reasons, the National

Arcnives is sponsoring an environmental aging study of PET and magnetic

tapes at the National Bureau of Standards. This is the second annual

report in what is expected to be a study lasting five years.

The results with PET in the first year were consistent with the

literature. Hydrolytic degradation was the most important process. About

100 days aging at B5 C and 100 % relative humidity, RH, made PET weak and

brittle and reduced its molecular weight from aoout 24000 to 12000. Aging

for up to about 150 days at 0, 25, and 50 % RH at B5 C and at 100 % RH at

55 and 35 C caused almost negligible changes in tensile strength, elongation, and molecular weight. About one chain scission per molecule or 5 X

10"5 m oi scissions per gram brought about embrittlement, but the imprecision in these quantities was about 50 %. Thus, although PET appearea extremely sensitive to changes in molecular weight, such measurements were not a practical way of measuring the very small amounts of degradation

necessary to assess changes at 55 and 35 C or those unaer ambient conditions .Acid content is another possible measure of degradation, since each

hydrolysis reaction generates an acid group when it breaks the polymer

chain. We had procured a potenti ometri c titrator,in oraer to measure acid

content, but had not had an opportunity to use it before the first report.

Polyester polyurethanes were known to be susceptible to hydrolysis,

so this was expectea to be the reaction that determined tape lifetime. Our

own studies with magnetic tape had not been started at the time of writing

the first report. It had been reported that the degradation of the polyester polyurethane binder, which holds the magnetic particles, was Limited

2
by the equilibrium between hydrolysis and esterification and it was con- cluded that tapes stored at 18 C and 40 % RH should retain acceptable

properties indefinitely because of this [1]. We equilibrated polyesters

and two polyester polyurethanes at various RH and concluded that hydrolytic equilibrium did not prevent serious Loss of properties [2]. However,

tape binder differs from the polymers that we studied in that it is highly

crosslinked and so may behave differently.

In the second year of work agings have been continued and additional

data of the same kind obtained. Acid content was evaluated as a means of

measuring degradation of PET. Commercial magnetic tapes were aged and are being evaluated as in reference [1].

2.0 Experi menta l

PET samples included coated and uncoated electrographic film, exposed

and developed photographic film, and uncoated film base, all commercially

available. Most of these materials were provided by the National Archives

but we purchased the film base. We were also given amorphous PET. Some of

this was crystallized and some was stretched uniaxially without crystallizing it. Experiments with the latter two samples should give us some

knowledge of the effect of crystallinity and orientation on hydrolysis.

Samples of PET were aged in closed containers above water or solutions of LiCL at temperatures of 85, 70, 55, and 35 C and RH of 100, 50,

and 25 %. At intervals tensile strength and elongation were measured on a tensile testing machine and molecular weight was measured by gel permeation chromatography * as described in the previous report. Other agings

were done in boiling water.

Acid content was measured by potentiometri c titration with tetrabutyl

ammonium hydroxide in ethanoL as base. The polymer was dissolved in a mixture of o-cresol and chloroform. The titrator adds increments of base when

the rate of change in the voltage has decreased to some preset value. The

end point is reached when the difference between the voltages at successive increments first exceeds and then drops below some preset value. The

measurement is precise to about 2 X 10"® mol acid. This method was used

because it is being considered as a standard for the PET base of photographic film certified for achival use. Ten rolls each of six brands of magnetic tape were purchased. Rolls

contain 750 feet of tape except for one set of 2000 foot rolls. All tapes

3
are 1/2 inch wide and are certified for 6250 bpi. Tapes were the most

expensive that the suppliers offered except for the 2000 foot rolls, for

which the more expensive and less expensive graaes are both oeing tested.

The tapes are being aged at temperatures of 65, 60, and 37 C and RH of 0,

11 , 30, and 100 %.

Model tape binoer was made in order to explore effects in the absence

of the magnetic particles. A commercial polyester polyurethane was dissolved in tetrahydrofuran. Enough toluene di isocyanate was added to react

with each urethane group. The tetrahydrofuran was evaporated and the

mixture was heated at 100 C far five days. The resulting crcsslinked film

was extracted with tetrahydrofuran. The sol content was Less than 1 %.

Samples were aged at 100, 50, and 25 % RH at 85 C.

Magnetic iron oxide was procurred and added to a tetrahydrofuran

solution of the same polyester polyurethane as above. The tetrahydrofuran

was vaporized and the sample was aged at 100 % RH and 85 C for 14 days.

Dimethyl formamide was added and the iron oxide was removed by centrifugation. The acid content was then determined.

The weight fraction of binder layer on commercial tapes was determined by immersing about 1 .6 g of tape in acetone and then scraping the

softened binder and associated oxide off the PET substrate. Then the

acetone was evaporated and the dried scrapings weighed. The organic component was then burned off and the residue was weighed, giving the amount of

iron oxide. The organic content of the binder layer was taken as the

difference between these weights. Soluble portions of the tape were

determined by uncoiling about 50 feet into a 4 liter beaker. This was

extracted with two 750 cra^ portions of tetrahydrofuran for a total time of

1 hour. The sol fraction was determined by drying and weighing the tape

and by evaporating the filtered solvent to dryness and weighing the residue. Final weighings were made after 60 hours of evacuation. The tapes are

quite hygroscopic and weighings were maae in closed containers.

The adhesion of the binder layer was tested by applying 3/8 inch-wide

sticky tape to the binder layer of unextracted tape and peeling it off in

a testing machine. The force required went through an initiaL maximum ana

then decreased and became approximately constant during most of the peel.

The latter force was taken as the peel strength. The noise level auring

the peel was about 1/4 to 1/2 as large as the value taken, so the peel

strength is not very precisely known. Despite this it should be significant because it varies widely between brands of tape and even between

4
rolls of the same brand, in the case of one brand. Peel strengths on otner

brands did not change from roll to roll.

Generally, five 50-foot lengths of a single tape were aged in a closed jar at each temperature and humidity mentioned above. Solutions

saturated with LiCl and CaClg gave 11 and 30 % RH. Molecular Sieves were

used to get 0 % RH. Individual lengths of tape were removed at intervals

and the sol content and peel strength were measured as described above.

Tensile strength and elongation were also measured.

3.0 Results

3.1 PET.

3.1.1 Preliminary Acid Measurements. The American National Standards Institute Vesicular / Toned Image Task Force ran a round robin using two

methods of measuring acid content in PET. We participated in this in order

to gain experience with the titrator and the technique. Five samples of

PET were analyzed in duplicate by the two methods. All laboratories preferred the method described in the section above. The range of acid con- tents of the samples was 0.29 to 0.50 X equiv./g and our values were

within that range. A second purpose of the round robin was to measure the

effect of the accelerated aging described in ANSI PHI .41-1981 on acid

content. All Laboratories found no significant effect.

Our laboratory offered to try to find conditions that would offer a more meaningful accelerated aging test. Samples of PET polymers were aged

in boiling water and portions were removed at intervals. Tensile strength,

TS, elongation, E, and acid content, [A], were measured after the aging

was ended. The polymers used were the film base that we used previously

and the samples from the round robin that had the highest and lowest acid

contents, designated here as E and F. The results obtained are listed in

Table I. Values are generally means of two or more determinations.

Acid content increases with aging time. After 17 days the differences

between acid contents of the three samples had increased by factors of

between three and four. Tensile strength and elongation decrease with

aging. About one third of the film base removed as one piece at 14 days

broke when it was creased. The remainder, also one piece, did not break

when creased. Acid content ,tensi Le strength, and elongation all indicate

significant differences between the two portions. The reason for this

5
behavior is unknown.

Titrations are believed reproducible to about 2 X IQ"6 e quiv. acid.

They were done in duplicate except for sample E at 17 days. The average

difference between duplicate determi nati cns was 2.2 X 10"6 equiv. 12.4 X

IQ”6 equiv. std. dev.). Portions of polymer used usually contained about

10”4 equiv. of acid, so the relative imprecision is about two percent.

Hydrolysis of PET is catalyzed by acid and is autocata ly ti c because

of the acid generated [3). The reaction is:

k

HgQ + -{C0H4CQ262^46 26 ^” “CgH^COgH + HOCgH^OgC— 11)

A

Here A represents an acid group and k the rate constant. The water and

ester contents change little during the portion of reaction of interest so they can be included in the rate constant. The differential equation

connecting [A] and time, t, is (d[A)/dt) = k [A), which integrates to

ln([AJ/[A Q ]j = kt, where the subscript indicates the initial concentration. Fig 1 is a plot of Ln([A)/[A 0

)) versus time. The slope is approximately the same for the three polymers, 8.0 % / day. This represents the

rate of increase in acid content.

An initially greater acid content results in greater increases in

acid content during aging. Thus the technique makes differences between

polymers more obvious. From these data it appears that polymer F originally had a higher acid content than E. Elongation of film base changes very suddenly between 10 and 14 days

aging. One might think of using a ten day aging period in boiling water as

an accelerated test and making the criterion of failure be that the acid

content not exceed 10“^equiv/g at the end of that period.

3.1.2 PET at. 85. C Acid content, number average molecular weight [M

n ),tensile strength, and elongation for the unaged PET materials are in Table

II. The initial range of acid content is 0.31 to 0.48 X IQ"4 equiv/g.

Aging at 85 C at 100, 50, and 25 5o RH increases acid content and reduces

tensile strength and elongation, eventually embrittling the material. Data

in Table III show these effects for the film base. Samples of aged polymers were small and the imprecision in [A] is about 10 %. "Y" or "N" in

the column headed "Crease" indicates whether or not the samples could be

6
creased without breaking. Elongation must be very low for failure to occur

since samples could be creased that had as little as 5 % elongation.

Microfilms A and 3, encapsulating film, and coated electrographic

film D behaved about the same as film base. Uncoated electrographic film C

became brittle much earlier. It failed the crease test at 61 days at 100 %

RH, 81 days at 50 So RH, and 341 days at 25 % RH. Coated electrographic

film C failed at intermediate times: 81, 146,and 650 days at 100, 50, and

25 % RH, respectively. It is uncertain whether the slightly higher initial

acid content of the C films caused their early failure, because there were

also initial differences in elongation and heat of fusion, commented on in

the the first report. Presumably these differences are due to variations

in orientation and crystallinity, which could affect the behavior in

hostile environments.

Changes in M

n offer an alternative to acid measurements as a means of

calculating the rate constant, k. Each hydrolytic scission increases the

number of acid groups and the number of polymer molecules by one, as shown

in reaction 1. Thus the change in the concentration of acid groups, [A] -

[A

Q

] f is the same as the change in the concentration of polymer molecules,

so that [A3 = [A Q

3 +A(M n” 1) and In {([A Q

J + AtMn"1

3 J/t A 0

3 3 = kt.

Values of k were calculated from this expression and from ln[[A3/[A

Q

3 = kt

by assuming a linear variation of the logarithmic quantities with time and

using the method of least squares to calculate the slopes, aesignatea k M

and k^> res pectively. They are Listed in Table IV. The imprecision indicated is the standard error; where none is given only two points were used

to calculate the slope. The averages of the values in each column are

Listed at the bottom of the table with their standard deviations. Neither

k seems to depend on the particular film used but values of k A are

somewhat larger than k^. Probably this is due to some systematic error in

the molecular weights, which were measured by GPC. For practical purposes

kf/j and k^ are treated as the same quantity, k.

3 .1 .3 PET a t 115. 70 . 55 . and 35 C at 1 00 %RH A sample left from last year

that was aged for 13 days at 115 C and 100 % RH in a sealed glass tube has

an acid content of 7.35 X equiv/g. The value of k^ is 22.6 %/day.

Data obtained with film base and microfilm 8 at 70 C are in Table V.

The acid contents of these samples have not yet been determined. Values of

k

jyj are 0.25 and 0.19 %/day for film base and microfilm B, respectively.

Most of the samples aged at 55 C and 100 %RH have shown no signifi7
cant changes in ^nsnia strength, elongation, or molecular weight in

almost two years. However, the elongation of the film base of unccated C was only 63 %, down significantly from 180*. There has also been a signifies; -..wf.^e in the acid number of the film base. Duplicate determinations on samples which each had about 0.8 X IQ-4 equiv. of acid gave acid

contents of 0.44 and 0.43 X 1 0-4 equiv/g after being aged fdr 586 days.

The average value of k^ is 0.037 %/ day.

No detectable change occurred in the PET in samples aged for two

years at 35 C and 100 *RH but the cuatings on these films have deteriorated even at 35 C and 25 * RH. The gelatin Layer of the microfilms has

become sticky and even disappeared immediately above the LiCl solution.

Electrographic films D and coated C have discolored. We have no samples of

these with images and so can not assess the effect on the legibility.

3 .1 .4 Li fet

i

me o f PET Fig 2 is an Arrhenius plot of the rate constants at

100 % RH. The least squares line is shown. The slope of this line gives an activation energy of 117 kj/mol or 28 kcal/mol. This value agrees well

with 27 kcal/mol that we gave last year and also with the value 29

kcal/mol found by McMahon et al. [4],

The glass transition temperature of PET is about 70 C, so finding

that the rate at 55 C falls on the same Arrhenius line as rates above the

glass temperature implies that it is valid to extrapolate to still lower

temperatures. This was done to give rates at 100 * RH at 35, 25, and 20 C. The Lifetime, t^ f can be estimated from these rates as follows. Data in

Tables I and III indicate that the acid content approximately doubles

before the tensile strength and elongation deteriorate seriously. Consequently, the lifetime was taken as (In 2)/k, where k is the fractional

rate of change in the acid content. Values of t L at the three temperatures

above are about 9D, 400, and 900 years. Data in Table IV indicate that

lifetimes at 5D % RH at the same temperatures should be more than twice as long. The lifetimes agree with those estimated by Adelstein ana Me Crea

[5].

It is gratifying that these estimated times are so long but it should

be noted that the result is greatly dependent on the value at 55 0. The

acid content at 55 C should double before this study ends, which will

permit more accurate measurement of k. It is desirable to get a measurement at 35 C to reduce the extrapolation range.The change in acid content

at 35 C should be about 2 * after five years, which will be barely signi8
ficant with the current methods.

3.1.5 Future p Ians w i th PET The agings will be continued and the acid

content at 55 C will be determined with better precision. It is desirable

to measure the acid content with about 10 fold greater precision than now

being done, in order to measure k at 35 C. Increasing the sample size

would help to some extent, but it is already large and using much more

would exhaust even our largest samples fairly quickly. An infrared method

has been described [6] that involves exposing PET to sulfur tetraf luori be.

This converts acid groups to acid flouride groups. These absorb far enough

from the ester carbonyl absorption to appear as a separate peak. We intend

to test this method using aged samples with known amounts of acid.

Oriented amorphous PET is being aged at 70 C and 100 % RH. Unfortunately, aging it at 85 C causes crystallization, which we wish to avoid.

Crystalline PET is also being aged. Results from both samples should be

available in about one year. They should give information about the relative importance of amorphous and crystalline regions in the degradation

process

.

9
3 2. Magnetic Tapes

3.2.1 Background Magnetic tapes consist of Fe 2

Q 2 particles dispersed in a polyester polyurethane-based binder that is bended to a PET substrate.

Conversations with manufacturers who supply products to tape producers

indicate that magnetic tapes are made in the following manner. The oxide

is coated with a wetting agent, which may be butcxy ethyl stearate or ethyl stearate. A commercial polyester polyurethane is dissolved in tetrahydrdfuran.To this are added the oxide and a multifunctional polymeric

isocyanate The suspension is applied td the substrate and the combinatidn

is then heated to remove solvent and bring about reaction between the

urethane and isocyanate groups to form allophanate linkages:

-NCO + -NHC0

2 NHCON CO 2“ 12]

Catalysts are used td speed up the reaction and a coupling compound may be

used to make the binaer adhere to the substrate. The binder is highly

crosslinked because of the allophanate reaction.

There are about 10 aliphatic ester linkages between urethane Links in

the original polyester polyurethane. These are more subject to hydrolysis

than the aromatic ester linkages of PET and most polyester polyurethanes

degrade to soft non-rubbery materials in 10-15 days at B5 C and 100 % RH.

The hydrolysis obeys the same kinetic equations as the PET hydrolysis [7].

Lifetimes under ambient conditions are much longer, but in warm moist

environments polyester polyurethanes have been known to fail in three

years.

Magnetic tapes usually Last longer than this, perhaps because their

environment is not very humid, but andther factor seems to be involved.

Cuddihy [8] shdwed that the acetone-extractable fraction of tape binoer on tapes aged for up to 85 days at 36-75 C increased at 100 and 30 % RH but

decreased at 11 and 0 % RH. He attributed the decrease to esterification,

the reverse of reaction [1], and predicted that tapes cauld be stable

indefinitely because of the equilibrium between esterification ano hydrolysis. Decreasing the temperature at constant RH increased the esterification tendency, leading Bertram and Cuddihy to recommend 18 C and 40 % RH

as ideal storage conditions [1]. They found that tapes could be used

without problems if the content of extractable was less than twice the

initial value. Working with soluble polyesters and pdlyester polyure1 0
thanes, we found that acid content at 35 C and 25 %RH changed in 300 days

from 3.5 to 3.3 X 10“4 equiv/g, implying that equilibrium was near or that

the time to reach equilibrium was very long [2]. The molecular weight

should equal [A]”1 g/mol so it could be as low as 2700. Speculations were

made that the high cross link density of tape binder encouraged esterification or caused the binder to remain as gel despite having many chain

ends [2].

Cuddihy worked with only about 1.5 g of tape in his samples and the

weight fraction of acetone extractable material ranged from 10 to 30 mg.

His data are for one unidentified brand of tape although he notes that

other tapes behaved similarly. The use of acetone as an extractant raised

the possibility that molecular weight limited the weight of extractables,

because acetone does not completely dissolve the polyester polyurethane

used to make tape binder. Our measurements with this polyester polyurethane gave 7000 and 28000 for the molecular weights of the acetone-soluble

portion and the whole polymer, respectively. For these reasons it was

decided to repeat the Cuddihy procedures using tetrahydrofuran to extract

10-15 g samples of several brands of tape.

3.2.2 Ini tia l Tape Characteri sties Table VI lists the weight percentages

of the binder layer (oxide plus associated binder and other organic components] and the sol content of each unaged brand of tape. The peel

strength is also given. The last two columns give the weight percentage of

organic material in the binder Layer and the portion of this organic

material that is extractable. These two quantities are dependent on the

weight of iron oxides and may be inaccurate if the oxides changed when the

organic component was burned off. Tape 1 has a back coat that contains

carbon. Soluble components of this are included as "Sol" and it was assumed that the coating was 50 % carbon in calculating the value in the

last column. Tape 3 contained an acetone-soluble white crystalline material that appeared as a residue surrounding the fragments of binder Layer

when the acetone had evaporated. This was not observed with the other

tapes. The Sol was determined from the weight loss of the tape except with

tape 4. The binder layer fell off this tape as soon as it was immersed in

tetrahydrofuran so the sol content was determined by filtering the mixture

and evaporating the solvent.

Double listings under Sol give results of duplicate determinations

and give some feeling for the imprecision of the measurements. These

11
duplicates were averaged in calculating the scl as a percentage df the

crganic component of the binder layer. The amounts of sol and the peel

strengths vary widely from tape to tape. A greater than sign means that

the binder layer did not peel off. Peel strength of tape 5 varied from

reel to reel, accounting for the wide limits listed. Portions taken at 400

foot intervals on one reel each of strongly adhering and weakly adhering

binder revealed no samples from which binder vms stripped from the former

reel and no stripping force greater than 40 N/m from the latter reel.

Measurements have not yet been performed on tape 6, which is the more

expensive of the two grades of tape purchased from one supplier. Tape 5 is

the less expensive of these two grades. The supplier of tapes 5 and 6 is

not a manufacturer, but has tapes made to his specifications ano sells

them under his own label. The other tapes bear manufacturer's labels and

were purchased from authorized retailers.

3.2.3 Tapes at 85 C and 100 & RfcL Table VII gives results obtained by aging

four brands of tape at 35 C and 100 % RH. Tensile strength and elongation

were seriously affected after 46 days. Tapes 1 and 5 broke during

handling. Acid contents of these tapes will be determined. The sol content

goes through a maximum. The peel strengths were low but were not measured

because the tapes were rather brittle. None of the binders became soft or gummy during aging. Instead they tended to flake off, and it was necessary

to determine sol fractions by evaporating the tetrahydrofuran. Ordinary

polyester polyurethanes would be very soft after 46 days under these

condi ti ons

.

3.2.4 Tapes a

t

6J] C Table VIII shows the effect of aging the tapes for 35

days at 60 C at 0, 11, 30, and 100 % RH. Values for the unaged tapes are

included. The sol contents show the same trend as Cuodihy's data: decreased sol at 0 and 11 %RH and increased sol at 100 % RH. Aging at 30 %

RH caused little change in the sol content. Peel strength decreased to

about 2/3 of its original value in tape 1 and to 1/10 of its original

value in the other tapes at 100 % RH. Aging at lower RH caused smaller

changes in peel strength. Significant Loss of peel strength occurs at 30 %

RH, which might limit tape lifetime. None of the binders appears to have

softened during aging.

Peel strengths have been measured at other times during these agings.

Most of the loss of strength occurs in the first 40 days.
Tensile strength and elongation have not yet changed significantly in

any sample at 60 C.

3.2.5 Mode l Studies The model tape binder did not soften when aged for two

months at 85 C at any RH. Instead samples became stiffer and that aged at

100 % RH is actually brittle. These results appear consistent with the

observation that binder layer on magnetic tapes aged at 60 and 85 C did

not soften. It may be that there are reactions in addition to hydrolysis

and esterification and that some currently unknown reaction brings about

crosslinking

.

The sample of the polyester polyurethane that was mixed with magnetic

iron oxide and aged at 85 C and 100 % RH formed acid faster than polyester

polyurethane without the iron oxide. Thus it appears that tape binder

ought to be destabilized by the heavy Loading of iron oxide.

3.2.6 Future p Ians with magnetic tapes The agings will be continued. Results from the agings at 37 C will be obtained within a few months. It is

planned to put data on tapes from each supplier and store them at 50 and

100 % RH at 20-25 C. These will be read at intervals. More experiments ars planned with model binder polymers.

4.0 Pi scussi on The findings with PET indicate that the material should be quite

stable at ambient conditions provided that the process by which PET is

made and the rather specific combination of orientation and crystallinity

now given the polymer are not changed. If changes were to be made, some

knowledge of their effect on stability would be useful.

One could make use of the fact that coated and uncoated films C lose

tensiLe strength and elongation faster than the other materials. Samples

of film C could be stored under the same conditions as films used to store

archival information. Our results indicate that uncoated C would fail

first, followed by coated C. There would be ample time before the failure

of the films containing archival information for it to be copied on fresh

materia l

.

The changes in the coatings under relatively mild conditions are

somewhat disturbing. The films are exposed to laboratory light for about

1/3 of the time. Deterioration of the coatings is usually worst immediately above the Liquid interface. Photographic gelatin has been used for a

13
very Long time and it was our impression that it would be stable at 35 C

at low RH.

The studies with tapes are not far enough along for us to have come

to any firm conclusions. However, we suspect that other reactions than

hydrolysis and esterification play a significant part in the process.

5.0 Summary

PET has been aged for about two years. Samples aged at 100 % RH at 85

and 70 C deteriorated relatively rapidly. One particularly Labile sample

shows loss of tensile strength and elongation after aging at 55 C and 100%

RH. Lower RH results in much slower degraaation but all PET samples at 25

% RH and 85 C lost most of their tensile strength ana elongation witnin

two years. Tensile strength and elongation decreased as acid content

increased. An increase in acid content of about 0.4 X 10“^ equiv/g from an initial value of about 0.3 X 10~^ can be tolerated. PET became quite

brittle when the acid content exceeded 1Q“4 equiv/g. The relation between

acid content and time is [A] = [

0

] exp kt. The primary difficulty with

using acid content to monitor PET degradation is that the required sample

size is inconveniently large. An effort is being made to aaaress this

problem

Various magnetic tapes are being aged at different temperatures ana humidities. The tapes are rather different initially, in so far as extractables and peel strength are concerned. Sol contents increase at 100 % RH

but decrease at low RH. The binder layer behaves differently from the

polyester polyurethane from which it is derived since the former does not seem to soften during aging. The reasons for the difference are unknown.

14
References

1J H. N. Bertram and E. F. Cuddihy, I. E. E. E. Trans. Mag. 18., 993,

(1982)

2) D. W. Brown, R. E. Lowry, and L. E. Smith, submitted to J. App. Polymer

Sci. and Polymer Preprints, 23, #2, 214, 119823.

3) H. Zimmerman and N. T. Kim, Polymer Sci. and Eng., 20, 680, 11980).

4) W. Me Mahon, H. A. Birdsall, G. R. Johnson, and C. T. Camilli, Chem.

Eng. Data 4, (13, 57, (1959J.

5) P. Z. Adelstein and J. L. Me Crea, J. App. Photographic Eng., 7, 160,

(1981)

6) D. J. Carlsson and S. M. Milnera, J. App. Polymer Sci. ,27., 1589,

(19823 .

7) D. W. Brown, R. E. Lowry, and L. E. Smith, Macromolecules, 13., 248,

(1980)

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

1 5

TABLE I

Acid Content, Tensile Strength, and Elongation at Break of PET Aged in Boiling Water

Time

4 [A] TS,

MPa'

E

Days 10 equiv/g %

Film Base (4 mil thick)

0 0.36 215 142

3 0o45 192 130

5« 9 0.55 179 135

9c 7 0.73 150 103

14c0 1c 05 116 40

14.0 1.15 71 2

17.0 1.41 Weak Brittl

E (7 mil thick)

0 0.29 162 90

3 0.36 173 113

5.9 0.45 171 97

17 1.13 95 4

F (4 mil thick)

0 0o50

17 1.84 weak brittle

] 1 MPa = 145 PSI.
Table II

Properties of Unaged PET^

. ' 10

4

[A] 10

‘ 4 M

n

• TS

" E eqoiv / g g/mol

n ~ MPa %

Film Base 0.35 1.77 2042

144

Microfilm A 0.31 2.15 200 126

Microfilm B 0.32 1.69 193 122

Encapsulating Film 0.37 1.59 177 80

Uncoated Electrographi c Film C 0.41 1.44 116 185

Coated Electrographic Film C 0.48 1.54 167 105

Coated Electrographic Film D 0.34 1.68 201 99

^4 mil films except D which is 4.8 mil

2

1 MPa = 145 PSI.

\
TABLE III

PET Film Base at 85 C v•

Time RH 10

4

[A] 10~ 4M- 10

5 a(M ? TS - E' Crease

Days % equiv / g g/mol n mol/g n MPa % Y/N

Unaged -> 0.35 1.77 0 204 144 Y

61 100 1.15 3.0 170 113 Y

76 100 0.64 1.13 3.2 Y

81 100 0.85 1.07 3.7 108 18 Y

97 100 1.25 0.85 6.1 114 5 Y

108 100 0o8Q 6.9 weak brittle N

178 100 5.05 0.55 12.5 weak brittle N 97 50 1.19 2.7 158 80 Y 146 50 1.41 1.4 150 100 Y

340 50 1.66 0.68 9.1 weak brittle N

154 25 1.77 0 190 142 Y 341 25 1.34 1.8 155 107 Y

650 25 136 78 Y
Table

IV.

HYDROLYSIS

RATE

CONSTANTS

IN

PET

FROM

ACID

CONTENT

(k

fl )

and

MOLECULAR

WEIGHT

(kM )

at

85

<_> o

rc CM o i— co o <— o o o • • • • • o o o o o Oo on • +1 +1 +1 +1 +1 +1 s: O CM O CO '

CO

lo r— CM r— r— CM f—

CM

OO

OO

O • o

CO cm c— «3-

O r-^ o o o o o r— o o o o o o o o o d +1 +1 +1 +1 +1 +1 +1 +i CO 1— CM cn CT1 CO CO CO rn CO 'Cj- CM CM CO CM CO CO oc • • o o o o o o o o oLO cr> o LO cs co co CO o • c ^ CO ir> lo o • • • • • • +i o o o o o o CM

LO• o

>)

TJ

cu CO CO i— CM

O 00 CO

S- r— o o i— co o r—

03 • • • • • • • o o o o o o o 21 +i +1 +1 +1 +1 +1 +i CM «=i- cr> -5i- «3- o CM ~0 co r— O LO I— CO r— O c s: • • rO o r— r- O r— O r— r—

rc

<c QC

CO LO 1

LO o O CM CM

4-

O • • • • O o o o o r— +1 +1 +1 +!

CO

+-> CO co o «d- i— cm CO

•r— CM CO CO CM LO

O CO c • • ro r— r— r— r— r— r— r— r—

«=C CQ E

CJ i (U E E •r~ CO r— i— TO O a Ll_ L_ fO -r- -r- a» cu

CU CO 4- 4- +-> T3 •a . cn i, o o ns <u CU OL fO S S- S- o 4-» 4-> n3 S- r— r— O O U n3 ra a a> o •r— *<- -r- C O o c >

Q_ LU 2 s 3 O

O LU <
Table V

PET at 70 C and 100% RH

Material Time/days

10" M

n /g/mol TS/MPa E/%

Film Base 0 1.77 204 144

136 1.41 183 124

312 117 39

Microfilm B 0 1.69 193 122

136 1.46 132 75

312 no 23

(1) 136 1.46 131 65

(1) 31? 128 56

(1) Microfilm B aged after removal of the gelatin layer.
Table VI

Characteristics of Unaged Magnetic Tape

Tape Binder Sol Peel . Strength

Organic Sol in

Layer (BL) Content in BL Org BL

wt% wt% N/ri wt% wt%

1 2 20^ 0.8, 0.9 325 30

3

9.5

4

2 23 1.4 > 800 24 25

3 32 2.1, 2.6 150 25 30

4 24 4.8 > 800 29 69

5 28 0.7, 0.6 35-> 450 26 9.7

lN/m = 1.02 g/cm

2

Mot including the back coat with carbon black that is 6% of the tape weight

3Oxide binder only

4Assuming half the back coat is carbon
Table VII

Effect of Aging Magnetic Tape at

85 C and 100% RH

Time TS E Sol Sol in

Days MPa % % Org BL

Tape 1

0 230 139 0.8 9.5

18 4o 7 56

46 130 5

Tape 2

4.3 bl

0 290 118 1.4 25

18 3.0 55

46 143 40 2.4 44

Tape 3

0 230 103 2.4 30

18 2.4 30

46 130 32 1.8 23

Tape 5

0 220 130 0.7 9.7

18 5.0 69

46 115 4 4.1 57
Table VIII

Effect of Aging Magnetic Tapes at

60 C C for 85 Days

R.H. Sol Sol in Peel

Org BL Strength

% wt% wt% N/m

Tape 1

Unaged 0.8 9.5 325

0 0.5 6.0 350

11 0.6 7.1

30 0.5 6.0 250

100 4.6 55 200

Tape 2

Unaged 1.4 25 >800

0 0.8 15 >600

11 1.0 18 >650

30 1.3 24 100

100 2.7 49 50

Tape 3

Unaged 2.4 30 150

0 1.4 18 140

n 1.8 23

30 2.3 29 60

100 5.4 68 15

Tape 4

Unaged 4.8 68 >800

0 4.0 56 >700

11 3.5 49

30 4.4 62 >750

100 5.0 77 25

Tape 5

Unaged 0.7 9.7 >450

0 0.4 5.5 >500

n 0.3 4.2

30 0.7 9.7 >400

100 5.3 74 12
i

Fig. 1 Ln ( [a] / [aq

1 ) vs Aging time in boiling water.

, Film base; O , polymer E; , polymer F; —, Ln ( [

A ]/ [a] ) = 0.080 t- 0.017
120 100 80 60 40 20

103

/T

Fig. 2 Arrhenius Plot for PET at 100% RH. was used

when it was available; otherwise, k M

was used. —, Ln k = - (14000/T) +39.3
NBS-114A (rev. 2 -ac) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 83-2750 August 1983

4. TITLE AND SUBTITLE

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

5. AUTHOR(S)

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

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

l/l/82-l/f/83

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

National Archives

8th and Pennsylvania N.W.

Washington, DC 20004

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached. The stability of po Ly [ethy lene terephtha late] is being studied in oroer to predict

its long terra behavior as the base for film and magnetic tape. This report contains

results of the second year's work. Film base, photographic film, and electrographic

film are being aged at several temperatures and relative humidities, RH. Acid contents, mechanical properties, and molecular weight have been measured at intervals. The

rate of increase in acid content, k, provides a useful measure of the degradation rate.

Data for films aged between 115 and 55 C at 100 % RH obey the equation In k = 39.3 -

(14000/T), where k is in %/day and T is the absolute temperature. The water content is

included in k, which is approximately proportional to RH. Lifetimes of the films appear

to be equal to 69.3/k days. Extrapolation to 25 and 20 C gives 400 and 900 years for

the Lifetime at 100 % RH. A more sensitive analytical method than now used will be

required to make useful measurements of acid content at 35 C, even after samples have

been aged for five years. Two films have always failed before the others. The ShortLived films might well be stored with others, since failure of the former would warn of

the approaching failure of the Latter.

Magnetic tapes are also being aged, primarily to study the binder that holds the

magnetic oxide to the substrate. Binder on five brands of tape that were aged at 60 C

became crosslinked at RH of zero and 11 %, changed little at 30 % RH, but degraded at i 100 % RH. The binder on tapes aged at 85 C and 100 % RH first degraded severely and

then appeared to crosslink slightly. Badly degraded binder was not gummy but was easily’

detached from the substrate. Model binder aged at 85 C and 100 % RH eventually became i brittle

Key words: degradation; hydrolysis; Lifetime;

[ethylene terephtha late); stability

UNAVAILABILITY

py] Unlimited

| | For Official Distribution, Do Not Release to NTIS

magnetic tape; photographic film; poly14. NO. OF

PRINTED PAGES

27

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Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

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