NBS

PUBLICATIONS

NAT L INST. OF STAND & TECH R.I.C.

A1110H OSntM

NBSIR 84-2988

Prediction of the Long Term

Stability of Polyester-Based

Recording Media

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

U.S. DEPARTMENT OF COMMERCE

National Bureau o? Standards

Center for Materials Science

Polymer Science and Standards Division

Gaithersburg, MD 20899

Progress Report

December 1984

wed for icnal Archives

shington, DC 20408

library

NBSIR 84-2988

PREDICTION OF THE LONG TERM

STABILITY OF POLYESTER-BASED

RECORDING MEDIA

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

Gaithersburg, MD 20899

Progress Report

December 1984

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

Gaithersburg, Md . , 20899

Abstract

Aging studies with po Ly (ethy lene terephtha la te ] film base indicate the

Lifetime is equal to about 1000 years if the material is stored at 20-25°C

and 50% relative humidity. Concentration changes of acid and alcohol groups

that occur as a result of aging have been measured by infrared anaLysis.

Rate constants calculated by this method agree reasonably well with those

calculated from acid contents determined by titration.

Crosslinked polyester polyurethanes were prepared as models of the

binder of magnetic tape. Aging studies with these materials indicate that

they hydrolyze more slowly than ordinary polyester polyurethanes. Samples

aged at B5°C at 100, 50, and 25% relative humidity eventually deteriorated

greatly in a physical sense. Magnetic tapes were aged and measurements made of the sol content of

the binder and its adhesion to the polyester base. The latter quantity

appears to be a more valuable indicator of tape condition than sol content.

Values of binder adhesion of six brands of magnetic tape initially varied

between 800 and 35 N/m (or g/cm] . Binder adhesion in aged tapes was less

the higher the temperature and humidity of aging. A tape transport had

difficulty processing tape with values of binder adhesion as low as 10 N/m.

There was no problem at 35 N/m. It is anticipated that the lifetime of

magnetic tapes can be predicted by measurements of binder adhesion.

1 .0 Introduction

A five-year environmental aging study aimed at establishing the lifetimes of magnetic tapes and the po Ly [ethy lene terephthalate) [PET] base of

photographic and electrographic film is being sponsored at the National

Bureau of Standards by the National Archives. This is the third report on this work.

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

1
magnetic tape. Both media contain an active element in a dispersing medium

bonded to a substrate of PET. Electrographic film also has a PET base. The

lifetime of PET under mild storage conditions is very long but unknown. The

iron oxide of magnetic tape is dispersed in polyester polyurethane, a material known to be hydrolytically unstable. Consequently, there is concern about the keeping qualities of film base and oxide binder.

EerLier results in this study agreed with previous work in that

hydroLysis appeared to be the most important degradation process. The

chemical reaction in both PET and polyurethane is hydroLysis of the ester

Li nkage

k

H

2

0 + -CO 2“ —* -OH + -C0

2

H [1]

The products of this reaction are an acid and an alcohol; in addition, the

polymer chain is broken Leading to a reduction in the molecular weight. The

previous report showed that the increase in the acid content of PET was approximately equal to the increase in the number of polymer molecules,

consistent with reaction 1 being the onLy scission process. The same result

had been found earlier with polyester polyurethanes[1 ]

Reaction 1 is catalyzed by the acid formed so the hydroLysis rate

accelerates exponentially at the smalL extents of reaction sufficient to

seriously reduce the mechanical properties. Applicable equations are:

[A] = [A

q

] exp(kt) (2)

1/M - 1/M

0 = [AQ ](e kt - 1) (3)

Here [A], k, t, and M are acid content, rate constant, time, and molecular

weight, respectively. Subscripts indicate use of initial values. The rate

constant, k, is pseudo first order because the ester and water concentrations do not change significantly. Physically, k is the fractional rate of

increase in the acid content. Values of k are approximately proportional to

the relative humidity, RH. Units used for [A] are mol or eq acid/g polymer;

these are equal quantities. The quantity (1/M - 1 /M Q

] represents the mol

scissions/g polymer.

The rate constant was calculated from rearranged forms of Eq. 2 and 3,

k=[ Ln([A]/[A

Q

] )]/t and k= (ln[ (M

_1 -M

0

_1 + [A

Q

] )/[A

Q

] ] }/t, respectively. Values

were obtained with PET at temperatures between 115 and 55°C at 100% RH and

at 85°C at 100, 50, and 25% RH. The set at 100% RH followed the Arrhenius

equation closely. Correlation of acid content with tensile strength and

elongation at break indicated that doubling the acid content Left tolerable

physical properties in the films but tripling it caused serious embrittle2
merit. Consequently, Eq. 2 was rearranged to give t explicitly and lifetime

was assumed to be equal to (lOOln 2)/k or 69.3/k days if k is in %/day.

Lifetimes obtained by extrapolation of k to 25 and 20°C were 400 and 900

yea r 6 , respectively, at 100 % RH. Values at 50% RH are twice as long if k

is proportional to RH.

The weakest point in the above analysis is the long extrapolation to

ambient temperature. Most of the experimental k are at temperatures above

the glass temperature of PET. about 70°C. The only value of k at 55°C was

based on a value of [A]/[A

Q

3 equal to 1.24. This k was on the Arrhenius

line so we extrapolated linearly below the glass temperature. Values of k

based on Larger [A]/[A

Q

] and also at Lower temperatures are desired to

better validate this extrapolation. Changes in [A] are very slow at temperatures of 55 and 35°C. Samples aged at 55°C are expected to double in

acid content in five years; acid contents of those aged at 35°C may increase by only two percent.

Work on PET in this reporting period has been Limited to accumulating

data on samples aged at 70, 55, and 35°C at 25, 50, and 100% RH and developing an infrared analysis based on the increase in concentrations of acid

and alcohol groups during hydrolysis.

The polyurethane binder on magnetic tape was expected to obey equations with the same form as those above. Analysis of the reaction is

complicated by the large amount of iron oxide suspended in the binder, the

presence of lubricants and wetting agents, and the fact that the binder is

crosslinked. Additionally, there is evidence that binder becomes more

crosslinked at 0 and 15% RH although it degrades at 30 and 100% RH [ 2 ] . In

the last reporting period we observed this kind of behavior in six kinds of

magnetic tape, although our data suggest slow crosslinking even at 30% RH.

Cuddihy attributed cross Linking to reaction of acid and ester, i.e., the

reverse of hydrolysis and predicted that an equilibrium would be reached

that would resuLt in more or Less permanently stabLe tape[2j. Later Bertram

and Cuddihy settled on 18°C and 40% RH as the ideal condition for

storage[3]. However, soluble polyester po Lyurethanes do not appear to

stabilize at molecular weights sufficiently high to have tolerabLe physical

properties even at 15% RH L 4 ] . Thus there is some uncertainty about what is

happening to the binder when magnetic tapes are aged at Low RH. Additionally, there is the possibility of the binder becoming detached from the PET

backing even if it does not degrade. The binding strength did not deteriorate in tape sampLes aged at Low RH but did at 100% RH[2] and it is uncer3
tain at what RH significant Loss in binding strength will occur.

In this reporting period magnetic tapes were aged at several temperatures and RH. Sol fraction and binding force between the magnetic Layer

and the PET base were measured. Portions of aged tape were tested on a tape

transport. Model binder was prepared and aged.

2.0 Exper i menta L PET samples include coated and uncoated electrographi c film, exposed

and developed photographic film, and uncoated film base. The National

Archives provided all but the uncoated film base, which was purchased.

Samples were aged at 70, 55, and 35°C at 25, 50, and 100% RH above solutions of Lithium chloride. Tensile strength and elongation at break were measured at intervals on a tensile testing machine. Acid content was measured by titration with tetrabutyl ammonium hydroxide. More details about

these methods are given in earlier reports.

Model binders with different crosslink densities were prepared by

dissolving a commercial polyester polyurethane in tetrahydrofuran, adding

the desired amount of toluene di isocyanate, evaporating the solvent, and

curing the film at 100°C under nitrogen for one week. Samples of these

films were aged at 25, 50, and 100% RH at 85, 60, and 35°C. Infrared

spectra were taken and tensile strength and elongation were measured.

q Swelling ratio was measured by placing 50 mg. samples in 20 cm of tetrahydrofuran for about 60 hours, decanting the solvent, letting residual

droplets evaporate, and weighing the swollen polymer in the closed container. Sorbed solvent was removed under vacuum at 100°C to give dry gel.

Densities of solvent and dry polymer were used to calculate the volume

swelling ratio by assuming volumes were additive. The sol fraction was calculated by subtracting the gel fraction from one. Some films became

completely soluble after aging. Acid contents of these were determined by

titration in dimethyl formamide with tetrabutyl ammonium hydroxide.

Six kinds of commercial magnetic tapes were aged at 85, 60, and 35°C

at 0, 11, 30, and 100% RH. Aged samples were extracted with tetrahydrofuran

to measure the sol content. Binder adhesion was measured by applying sticky

tape to the binder Layer and using a tensile testing machine to separate

the binder Layer from the PET base. A tape transport was procured and used

to write and read data to and from 50 foot Lengths of the aged tapes. The

program stopped the test after 25 successful write, read cycles or one unsuccessful one. An unsuccessful attempt was recorded only after several

4
trials, as determined by the transport characteristics.

Infrared spectre were obtained in a commercial Fourier Transform

-1

Infrared, FTIR , machine. Signals from 1000 scans at 8 cm resolution or from 100 scans at 2 cm

-1 resolution were averaged to give the spectra.

3.0 Resu Its

3.1 Ell

3.1.1 Aoinq at 70.55. and 35% Table I lists tensile strength, T, elongation at break, E, acid content, and k for several kinds of PET after different amounts of aging at 55°C and 100 % RH. There are sma l L but probably

significant changes in tensile strength and elongation in most of the

samples. Uncoated electrographic film C Lost almost all of its ability to

e longa te

The acid contents are based on titrations unless use of FTIR is noted.

The latter technique is described beLow but the method is not in its final

form so we have not made measurements on a L L the films. Acid content has

increased significantly for all samples tested. Values of k were calculated

from the listed acid contents. Uncoated e Lectrographi c film C appears to

have a higher k than the other films. Unaged, this material had a higher

crystalline content than the other films in addition to Lower tensile

strength and higher elongation.

Tensile strength and elongation did not change significantly when

samples were aged at 25 and 50% RH at 55°C or at 35°C at 100% RH for three

years. Acid contents have not been measured on these samples.

The Long term stability of the various coatings is more questionable

than the stability of the PET. Coatings on C and D have changed color even

at 35°C. Films aged at 55°C have coatings that are more easily detached

than before aging. Some of the changes may be due to Light or oxidation,

the effects of which will be investigated in the coming period. The photographic films Lost the emulsion Layer, but only within five cm. of the

surface of the LiCl solution used to control the humidity. Positive results

were obtained when the denuded areas were tested for Cl

-

. Apparently there was inadvertent transfer of salt solution to the film. The gelatin of the

emulsion layer must have been affected by the salt solution, which has a high ionic strength.

Amorphous PET was aged at 70°C and 100%RH to determine if it accumu5
Lated acid at a different rate than semi crysta L Li ne commercial films.

Unfortunately, it crystallized during aging so we have ended this effort.

3.1.2 Infra red Studies Figures 1 and 2 show FTIR spectra of dry PET

—1 —1

between 3800 and 3100 cm using 2 and 8 cm resolution, respectively.

Specimens were three layers of film base each 0.004 in. thick that had been

aged in boiling water, as described in the previous report, for the number

of days indicated on the plots. Elongation at break and acid content of the

film, as measured by titration, are listed in Table II along with differ-

—1 ences between the absorbance at 3717 cm and absorbances at 3542 and 3256

-1 cm The FTIR instrument is continually purged with dry air. Sorbed moisture in the films absorbs in the region plotted so spectra changed appreciably for about six hours as the films dried out. but did not change

significantly thereafter. There is ample room in the purged space of the

instrument for many samples. Opening the purged space briefly to transfer

dried samples to and from the sample holder did not noticeably change the

spectra. The procedure adopted was to put samples in the purged space the

day before they were to be run so that they had more than 16 hours of

drying time.

Spectra at both resolutions show increased absorption between about

—1 „ 3600 and 3100 cm as the aging time increases. The whole level of absorption seems to increase although a peak at 3542 cm becomes more prominent.

There are small, regularly spaced peaks in all the spectra that are due to

interference between reflections from the front and back of the films.

Increasing the resolution sharpens these peaks but does not change the

absorption in other ways.

Acid and alcohol groups in PET absorb at 3256 and 3542 cm , respectively[5]. Addleman and Zichy have applied infra-red spectroscopy to measure acid and alcohol group concentrations in PETL6]. Their samples had not

been artificially aged and acid contents ranged only from 0.49 to 0.59 X

10 ^ mol/g. Their criterion of dryness was that the sample be more trans-

'I parent at 3663 than at 3717 cm . Our samples dried as described above met

this condition. Their method of analysis is considerably more complicated

than what we are currently using because they had to allow for differences

in crystallinity and orientation, both of which affected the absorption

coefficients. Our comparisons are between original and aged material which

appear to have the same crystalline content, as measured by differential

6
scanning calorimeter. There are no dimensional changes, which would be

expected to occur if orientation changed. It is desirable to orient samples

consistently in the instrument beam to minimize effects due to dichroism,

but these seem to be only a few per cent in our instrument.

—1

Our method is to use the absorbance at 3717 cm as the baseline above

—1 which to measure absorbance at 3542 and 3256 cm . Table II contains

absorbance differences that are means of those found in 8 and 2 cm. spectra. Fig. 3 correlates the increases in acid content with the increase

in absorbance difference. These are reasonably linear correlations. The

initial acid content for most films is 0.35 X 10"^ mol/g so doubling it

should increase the difference in absorbance above that of the base line by

0.2 in the acid region and 0.3 in the alcohol region. There is ample

sensitivity to measure that difference so the lines of Fig. 3 could be used

to anticipate failure or calculate the rate constant for the increase in

acid content. The acid contents with i subscripts in Table I are means of

the two values calculated from the Lines of Fig. 3.

Probably the interfering reflections contribute importantly to the

scatter of the data about the lines in Fig. 3. Roughening the samples

reduces the reflections but introduces uncertain differences in thickness.

One method suggested by Addelman and Zichy[6], which we have not tried as

yet, is to coat each film with a dry hydrocarbon.

3 2. Studies wi th Mode l Binder[7]

3.2.1 Cha racteri sti cs of unaqed materia Is The binder layer of magnetic tape

consists of iron oxide particles dispersed in a crosslinked polyester

polyurethane. This layer is prepared by dissolving a polyester polyurethane, adding iron oxide and crosslinking agent, applying the slurry to PET

film, and heating the film to evaporate solvent and cause crosslinking. The

iron oxide is precoated with a wetting agent and a catalyst is used to

speed the reaction. A polyisocyanate is used as the crosslinking agent. It

reacts with the urethane groups to form allophanate linkages:

OH 0 10H

81 8 81

-OCN- + -NCO -OCNCN- (4)

The soluble polyester polyurethane is a block copolymer with the

structure

7
OH HO OH HO

II

I III II I | ii -IR-0CNC6

H4CH 2C

6H4NC0 (CH 2

) 40CNC6

H4CH 2 C

bH4

NC0 )- [5)

The group, R, is a polyester, frequently po Ly (buty Lene adipate), with a molecular weight of about 1000; it is the soft segment. The rest of structure 5 has a molecular weight of about 600; it is the hard segment and it

contains the urethane groups. There are about 15 segments of each type in an average polymer molecule. Hard segments associate to some extent and

make the bulk polyurethane behave as if it were crosslinked. The allophanate reaction introduces chemical bonds between hard segments, which may be on different molecules, and so generates an infinite network known as gel.

Not every molecule need be part of the gel.

Our model binder Lacks iron oxide, wetting agent, and catalyst and it uses a mixture of toluene di isocyanates, 20% ortho and 30% para, instead of

a polyisocyanate. Nevertheless, it should respond to hydrolysis much Like

binder because the hydrolysis occurs in the ester linkages, which are the

same in binder and model.

Polymers that have been crosslinked past the gel point are incompletely soluble. They are evaluated by measuring the fraction of material

that is soluble (sol,s) and the volume fraction of gel in polymer swollen

by solvent. We used these methods and also infra-red spectroscopy to measure the change in acid content. Scission of the ester linkage during

degradation creates additional sol, increases the extent of swelling, and

increases the acid content, which can not be measured by titration unless

the polymer is nearly all sol.

One crosslinked unit per weight average molecule gives gelation[3].

Our starting polymer has number and weight average molecular weights of

about 25000 and 50000 respectively, as measured by gel chromatography. Thus

—5

only 2 X 10 mol crosslinked units/g polymer is required for gelation.

Much more crosslinking is possible because each hard segment contains four

urethane groups, so there are about 60 possible crosslinking sites per

molecule. A network of hard segments is expected if one urethane group in

four is crosslinked. This second network should not be degraded by hydrolysis of the ester groups.

Table III Lists some of the initial properties of the model binders.

Quantities from left to right are: I/U, the ratio of isocyanate to urethane

groups mixed together; s, the sol fraction; v, the volume fraction of

polymer in a gel swollen with tetrahydrofuran ;

, the number of crosslinked

uni ts/polymer molecule; tensile strength; elongation at break; and ^Abs/L,

8
—-1 the difference in absorbances at 3520 and 3750 cm divided by the thickness of the film.

The material with I/U equal to zero is starting polymer. Amounts of

diisocyanate much in excess of those theoretically required for gelation

were used for the models. Sol fraction decreased and the volume fraction of

polymer in swollen gel increased as I/U was increased. Values of Y were

calculated from s and v, by equations of Charlesby[9] and FLorylTO]

,

respectively. The Charlesby equation was used when I/U was equal to 0.14

and the Flory equation was used with the other two model binders. Sol

fractions in these two polymers appeared to be zero, which prevents use of

the Charlesby equation. The Flory equation will not allow for crosslinks in

the sol, so it was not used when I/U was 0.14. Theoretical values of , assuming complete reaction of isocyanate with urethane, are 9,35, and 63

crosslinked uni ts/po Lymer molecule, at I/U equal to 0.14, 0.55, and 1.10,

respectively. The poor agreement with the experimental value at low I/U may

be due to crosslinks within one polymer molecule and Loss of isocyanate

either by evaporation or reaction with i nadventi ti ous water. Agreement is

better at I/U equal to 0.55 and 1.1 but our aging studies suggest the Flory

equation gives too high a crosslink density in this system.

Tensile strength and elongation were measured using rectangular specimens about 2 X 0.3 X 0.01 cm. These had a tendency to tear if the crosslink

density was high. The variance is about 25% at I/U equal 0J55 ana 1.1 and

about 10% at I/U equal 0 and 0.14.

Water contents at 35 and 85°C and 100% RH were nearly the same in the

soluble polyester polyurethane and in model binders with I/U equal to 0.14

and 1 .1 . Infra-red spectra of aged and unaged dry films between 3800 and 3320

cm are in Fig. 4. Lines 1-3 are for unaged film. The large peak that is

—1

off the absorbance scale at 3350 cm is due to NH stretching. This peak is

broader in model binder than in polyester polyurethane. We did not find the

acid OH stretching region at 3250 cm useful because the NH band was so intense. A useful correlation was developed using the alcohol band at 3520

cm . The spectra of unaged films with high crosslink density show smaller

absorbance differences between 3520 and 3750 cm . The values of £Abs/l in

TabLe III quantify this result. It is reasonable because reaction between

isocyanate and alcohol is more rapid than the allophanate reaction.

A small sharp peak at 2280 cm , a region where isocyanate absorbs,

was present in spectra of model binders made with I/U equal to 0.55 and

9
1.1. This peak disappeared during aging, presumably as a result of reaction

between isocyanate and water. Spectra of the polyester polyurethane and

the model binder with I/U equaL to 0.14 lacked this peak.

A question of importance because of the autocata lyti c effect of acid,

is whether the isocyanate reacts with carboxylic acid? This is present in —C the starting polyurethane at a concentration of about 10 mol/g. Schollenberger and Stewartlll] found no reaction between acid and isocyanate during

urethane preparation. Here reaction would compete with the allophanate

reaction, which is slow. We aged the polyurethane until it had an acid

content of 6 X 10 ^ mol/g and then treated it with enough monoisocyanate

[used so the product would be soluble) to react with all the urethane,

alcohol, and acid present. After reaction for one week at 100°C the acid

content was only 0.25 X 10

-^ mol/g. Apparently acid can be destroyed by

i socyana te

3.2.2 Aging effects at 85°C in. mode l binders Pieces of the film made with

I/U equal to 1.1 were aged in sealed glass tubes above well dried Molecular

Sieves in vacuum, dry nitrogen, and dry air. Samples were aLso aged in wet

air at 100% RH and above water vapor in the absence of air. Three exposure

times were used: 18.7, 38.7, and 56.7 days. None of the sampLes that were

aged dry swelled less in solvent than it did before aging. Pieces of film

made with I/U equal to 0.14 were aged for 10.4 days in dry air and in wet

air at 100% RH. The sol fraction of the film aged dry was the same as before aging. Thus our conclusion is that model binder does not crosslink

when aged dry. This is a distinctly different result than obtained with

magnetic tapes.

The samples aged wet became quite highly colored during aging whereas

those aged dry changed very slightly. Fig. 5 illustrates this effect in the

films made with I/U equal to 1.1. The unaged specimens and those aged dry

barely show in Fig. 5 because they are nearLy colorless. SampLes aged in vacuum are Like those aged in dry Ng. Samples aged wet were at 100% RH

except as noted. Discoloration is worse in wet air than in air-free water

vapor. There have been studies of the discoloration of polyurethanes during

aging[12,13] but the authors did not investigate the effect of moisture.

SampLes aged for Long periods at high humidity became hard and brittle. Their acid contents actually were Less than those of sampLes aged for

shorter times. Our speculation was that the molecules had degraded so much

that polyester evaporated and dissolved in the aqueous phase, thereby
raising the glass temperature of the remaining polymer and decreasing its

acid content. Samples were submitted for nitrogen analysis; results are in

Table IV. The increases in nitrogen content support the hypothesis. Crosslinking and lower RH appear to reduce the effect.

Fig. 6 shows how tensile strength and elongation change with time in

samples aged at 100% RH. The Lines connect sequential points for each value

of I/U. Values for the crosslinked samples usually increase initially. It

seems reasonable that scissions in a tightly crosslinked polymer initially

increase extensibility. The effect on tensile strength probably is due to

decreased sensitivity to edge imperfections in a more extensible polymer.

Additional aging causes a rapid decrease in both T and E at times that

increase with I/U.

Fig. 7 shows values of T and E after aging at 25% RH. Longer times

were required to give the same effects than at 100% RH. The elongations of

the two more denseLy crosslinked samples remain Large. Less extensive

degradation at 25% than at 100% RH probably gave products that remained in

the polymers and softened them. Polymers aged for the Longest times are probably too soft to be satisfactory binder even at high I/U.

Agings were also done at 50% RH with results intermediate between

those at 25 and 100% RH. ALL samples eventually broke at low elongation.

Fig. 8 shows the sol fraction of the two more denseLy crosslinked

samples after aging at 25, 50, and 100% RH. The material with I/U equal to

0.55 eventually becomes completely soluble at all RH whereas the other

material appears to reach a final sol fraction equal to 0.6. The weight

fraction of hard segment plus added toluene diisocyanate is 0.42 when I/U

is 1.1. Accordingly, we feel that the material that does not dissolve is

primarily crosslinked hard segment, although some hard segment and some

ester are in sol and gel respectively. The fact that the polymer with I/U

equal to 0.55 did not form permanent gel, even though it had twice the

isocyanate sufficient to do so, suggests that at least half the isocyanate

was wasted. Crosslinks between urethane groups within the same hard segment

or molecule could account for this. An additional implication is that the

values o in Table III for materials with I/U equal to 0.55 and 1.1 are much too high. The reasons for the failure of the Flory equation may be

connected with the preferred association of hard segments in polyester

polyurethanes. We suspect that chains coming from hard segment blocks

interfere with one another and so do not swell to normal equilibrium.

Values of the sol fraction can be used to calculate the number of
scissions. We did this using the X of Teble III 17] but now think the

results were wrong because the initial crosslink densities were too high.

_3 Mol scissions/g reached values as high as 4 X 10 even at 253) RH. This is

much higher than found in polyester polyurethanes at the same RH[4]. More

reasonable values are obtained if X is about 10 for unaged model binder

with I/U equal to 0.55. To get this would require that the sol fraction be

0.01, which is too small to be significant. The corresponding density of

crosslinked units, V/M

0

, is 4 X 10~4 mol/g if M

Q

is 25000 g/mol. The

polymer shouLd become completely soluble during hydrolysis when H is 0.5,

since it has a most probable distribution. The concentration of crosslinked

units will be unchanged if only the ester groups hydrolyze. Therefore the

molecular weight will be 0.5/14 X 10

-<^] or 1250 when complete solubilization occurs. There should be one acid group per molecule in the degraded

material, making [A] equal to 8 X 10~^ mol/g.

Acid content can be measured by titration of soluble poLymers. Values

obtained by titration of crosslinked, partly soluble poLymers are not

expected to be valid because the base will not get into the polymer. Infrared spectroscopy appears to be useful in measuring the increase in acid

content of such materials.

Lines 4-6 in Fig 4 are infra-red spectra of aged polymers. Comparisons

with the spectra of the corresponding unaged poLymers indicate the aged

samples have a Larger absorbance at 3520 cm , in the alcohol OH stretching

region, and at 3717 cm . The absorbance of each sample between 3700 and

6500 cm is substantially constant but is higher for the aged films. The cause of this raised absorbance plateau is uncertain but we have treated

the flat region as the baseline and measured AAbs/l as for the unaged

poLymers. Titrations for acid were performed on soluble poLymers. Acid

contents were obtained with unaged starting polymer, I/U=0, and aged poLymers with I/U=0, 0.14, and 0.55. As many alcohol groups should form as acid

groups, according to reaction 1. Thus one shouLd expect the increase in

acid content wilL be proportional to the increase in the absorbance difference ,i .e., .AAAbs/L. Furthermore, if the originaL acid content is much

less than the acid content after aging, as seems reasonable, then the acid

contents shouLd be proportional to the increase in absorbance difference.

This is shown to be approximately true in Fig. 9. The line in this figure

was used to calculate acid contents of polymers that were not completely

soluble

Fig. 10 is a plot of acid content vs. time at 85°C and 100% RH. Acid
contents were determined by titretion or by use of Fig. 9. Lines ere celculeted by Eq. 2, using velues of L

Q

3 end k Listed on the plots. The

intent is to fit only the early points. Figures 11 and 12 contain similar

results from samples aged at 85°C and 50 and 25% RH, respectively. Note

that at 25% RH the last [A] determined for the polymer with I/U equal to

0.55 is about 7 X 10

-4

mol/g. The conclusions drawn from these plots are

that [A

q

] and k are Less for the densely crosslinked films.

Presumably reaction between acid and isocyanate reduces [A

Q

]. Reasons

for the reduction in k are speculative, but it is suggested that the rate

constant for esterification, i.e. the reverse of reaction 1, is effectively

greater in a densely crosslinked gel than in an uncrossli nked polymer

because the acid and alcohol can not separate normaLLy because of physical

constraints. The apparent result would be to decrease k, the rate constant

for hydrolysis.

Preliminary tests of this hypothesis have been made by pre-aging model

binder and re-aging it at 0% RH. Under this condition the acid content

should decrease and hydrolysis should be absent. Reaction is just the

reverse of reaction 1. The differential equation connecting [A] and time

i s [4]

-Id [ A 3/dt ) = k' [ A 3 2C L H

0

] + [A]) (6)

Here [H

Q

] is the alcohol content before the initial aging and k' is the

rate constant for the back reaction. Acid concentration is squared because

acid catalyzes hydrolysis and so must catalyze esterification.

Results in Table V were obtained by integrating Eq 6 and solving the

resulting equation for k', using [A

Q

] as the acid content at the start of

the re-aging period and [A] and t as acid content and time after re- aging[4]. Thus each k' is calculated from two points, [

q 3,0 and [A3 , t . The

[H 3, which are required, were calculated as folLows. The acid content for

—5

uncross Li nked polymer was determined by titration and found to be 10

eq./g. The molecular weight is 20,000-25,000, so there are about 10 4 mol

of end groups/g polymer. Only 10% of these are acid groups so the concentration of alcohol groups was taken as 10 4 mol/g. It was then assumed that

AAbs/L, in Table III, was proportional to the alcohol content so [H

Q

] for

unaged crosslinked film is 10 4 times the ratio of the ( Abs/U's.

The final column Lists the k

1 . Results probably are not very accurate

so the first five listings are regarded as not differing significantly. Thu

Last three listings indicate that k

1 is larger for the most densely crosslinked material. However, the effect seems to be less the greater the
initial [A

Q

], i.e. the greater the extent of pre-aging. The implication is

that hydrolysis will eventually reduce k' to its value for uncrosslinked

polymer. The second listing for k' uses 10~^ mol/g for [H

Q

], which we regard as known only approximately. Larger [H

Q

] does not eliminate the

larger k' of polymer with I/U equal to 1.1.

Previously it was found that k' in uncrosslinked polyester diols and

polyurethanes depended on RH[4]. Consequently, the values of k' in Table V

probably should not be used at any other RH than zero. Model binders are being aged at 60 and 35 °C. Preliminary results at

60 °C are like those at 85 °C but a five fold greater time is required to

accomplish the same result. No significant degradation had occurred at 35

°C.

3 .3 Maoneti c tapes

3.3.1 Unaqed tapes Characteristics of unaged tapes are listed in Table VI.

The values are nearly the same as those in the Last report. Changes were

made as additional measurements affected the average results. Tape 1 has

binder with carbon on the surface opposite the one with magnetic oxide. The

other tapes do not have this coating. Several rolls of Tape 5 had very low

adhesive force between binder and PET base so the group has been divided

into 5A and 5B.

The weight of the binder layer was determined by immersing tape in

acetone, scraping off the swollen Layer, and weighing the PET base. The

organic content of the binder Layer was determined by burning off the

binder in air. It is possible that some reduction of the iron oxide may

have occurred during this operation, which wouLd result in an unknown

error. The quantities in the coLumn headed "Sol in Org BL" are probably

somewhat more than the sol content of the polymer that makes up the binder

layer because of the extraction of additives, such as Lubricant, catalyst

fragments, and oxide wetting agent.

The acid content of the sol from Tape 4 was measured by titration and

—5

found to be 8 X 10 mol/g. A Lower value was expected because polyester

polyurethanes generally are made with acid contents approximately equal to

10 mol/g. Lower molecular weight components, which have more chain ends

and so more acid, tend to remain as sol during crosslinking. This makes the

acid content of the sol different from that of the whole polymer but even

-5

if there is no acid in the gel the overall value is about 5 X 10 . Acid

14
contents of sol from other tapes were not measured because they are smaller

fractions of the organic component of the binder layer.

The binder Layer can be removed from the PET base by immersing tape in

the proper solvent mixture for about 60 seconds. A mixture containing three

volume percent water in hexaf luoroi sopropanoL works for all except Tape 4.

The binder floats off and curLs up but has enough strength so it can be

drawn flat onto a 10 mil-thick Teflon sheet, that is also immersed. Hexaf luoroi sopropanoL is sorbed by the skin and is a poisonous vapor, so care must be exercised during these operations. Binder on Tape 4 is very fragile

when highly swollen, because of its Low crosslink density. A mixture of 10

volume percent water in acetone swelLs the binder Layer of Tape 4 adequately. A small amount of material is actually dissolved during the removal

operations. The use of mixed solvent and non solvent reduces this to a relatively small fraction of the total sol.

Fig. 13 compares the infra-red spectra of binder layer from Tape 1 and

the soluble polyester polyurethane used to make our model binder. The

spectra are very simiLar, except for the scattering due to the oxide particles, which results in a decreasing baseline, and absorption due to the

—I magnetic oxide at about 600 cm . There is also a broad, weak peak at 2200

—4 cm in the spectrum of the binder Layer that is not in that of the polyurethane . Fig. 14 compares spectra of binder layers from six kinds of tape. All

spectra are very simiLar but there are differences in the weak absorptions

between 2600 and 1800 cm . The source of these absorptions is unknown.

Isocyanates and carbodi imi des have relatively sharp absorptions at 2270 and

—'I 2170 cm respectively. Isocyanate hydrolyzes to amine at room temperature, so it is unlikely to be present even in unaged tapes. Carbodiimide is

used to stabilize polyester polyurethanes and might be an additive, but a narrower peak should result. We had hoped to study the degradation of the

binder layer by using the absorption due to alcohol groups, as in model

binder, but these absorptions are so weak compared with the scattering that

the approach does not appear promising. Signal to noise was much improved

by making 10,000 scans, which requires about two hours for each spectrum

but even then the desired absorbance difference was not known precisely.

Additionally, our aging data suggest that adhesion of binder layer to PET

base may be the limiting factor in tape life.

3.3.2 Aged magnetic ta pes Tapes 1-5A were aged in vacuum, dry nitrogen, dry
air, wet air [100% RH), and air-free water vapor at 85° C. Aging under dry

conditions caused a reduction in the sol content, presumably due to crosslinking. Wet air and water vapor caused approximately equal increases in

soL content.

An aging temperature of 85° C appeared more severe than necessary or desirable, since our interest was in changes at ambient temperature. Most

of our results now involve agings at 60 and 35° C. Fig. 15 shows sol contents after aging at SC° C and 100% RH. Most of

the tapes reach 5% sol in about 100 days and then give slightly less sol on additional aging. A sol content of 5% corresponds to about 60-70% of the

organic component of the binder layer being soLuble. Tape 2 reaches only 3%

sol, which corresponds to about 55% soluble binder. Even Tape 4 binder,

which is 66% soLuble to start with, never becomes more than 70% soluble.

These results resemble behavior of the most highly crosslinked model

binder

SoL fractions of tapes aged at 60°C and 30, 15, and 0% RH decreased

with time, implying that the binder crosslinked. Sol extracted from unaged

Tape 4 gelled when aged at 85°C and 0% RH, confirming that crosslinking

occurred

SoL contents of Tapes 1,2, 3, 4, and 5A are 1.8, 2.4, 4.0, 5.4, and

2.3%, respectively, after about 250 days aging at 35°C and 100% RH. The

infra-red spectra of the extracted sol are Like the spectrum of the polyes-

—A ter polyurethane in Fig. 13 except for a small sharp peak at 2250 cm in

the former spectra. Attempts to determine the acid contents of sol ex- tracted from aged tapes by titration have not been successful because color

changes during titration preclude use of indicators and no sudden change occurs in electrode potential during potenti ometri c titration.

Deteriorating adhesive force between binder and PET base could cause failure of the tape. Our method of measuring this force is to apply a 20-30

cm length of 3/8-in wide embossing tape to the center of the oxide Layer of

the 1/2 in-wide magnetic tape and then separate the two tapes in a tensile

testing machine. A high crosshead speed helps start the separation of

binder layer and PET; 20 and 40 in/min were used. The force is initially

high but decreases quickly and is within about 25% of some average value

during most of the pull. There is considerable oscillation of the pen

during the pull because the separation occurs as a series of stops and

starts. Fig. 16 shows the recorder trace from three such measurements.

Unaged Tapes 2, 4, and 5A do not lose binder layer during these
experiments. The other unaged tapes Lose binder, sometimes by failure

between binder Layer and PET (adhesive] and sometimes by failure within the

binder Layer (cohesive]. Fig. 17 shows the behavior patterns in unaged

tapes that do Lose binder during the measurement. Most fai Lures are mainly

adhesive or mainly cohesive but not exclusively either. Sometimes horizontal or vertical strips of binder Layer are Left on the PET. There have been cases in which the type of residue has changed within one puLl. Fig. 17

contains an example of this. No convincing rationalization of these phenomena has occurred to us. The measured adhesive or cohesive force decreases during aging in a way that depends on temperature and RH. Binder Layer fell off all tapes

aged at 60 °C and 100% RH for one year. These tapes were obviously not

useful. Fig. 18 and 19 show the time dependence of binder adhesion (or

cohesion) after aging at 60 °C and 30 and 15% RH, respectively. Great

decreases from the initial values occur. Very Low values, about 10 N/m, are reached for Tapes 3 and 6. Aging at 15% RH makes the sol content decrease,

so changes in sol content probably will not be reliable indicators of tape

reliabi lity

Values of binder adhesion exceed 80 N/m for all tapes after nearly 500

days at 35 °C and 100% RH. Aging at Lower RH for about one year leaves

sti l L higher va Lues.

Experiments with aged tapes indicate that write and read problems

occur when the binder adhesion value has decreased to about 10 N/m. The

programmed write-read sequence, described in the Experimental section,

failed after 14 passes of Tape 3, aged until the binder layer adhesive

force was 10 N/m. There was binder Layer on the heads and when it was

removed additional write-read passes were made. Failure again occurred

before 25 passes of each type were made. Other aged tapes would run through

25 passes. These results imply that Large decreases in binder adhesion can occur before tapes become useless. Consequently, binder adhesion can be

used to monitor tape condition.

4.0 Discussion

4.1 PET All resuLts to date indicate that the type of PET now being made

will Last several centuries under the mild conditions anticipated for

storage of archival materials. The extrapolation through the glass temperature is still a source of uncertainty and will remain so until some
determination, probably of the acid content, gives the rate constant at a lower temperature than 55 °C.

It is emphasized that the excellent physical properties of PET may

depend on a rather specific combination of orientation and crystallinity.

Any changes might result in a material that has different sensitivity to

hydrolysis. Results obtained with the electrographic films C may illustrate

this. These films fail physically before the other films and have significantly different crystallinity than the other films, as described in the

first report.

The infra-red technique described above can be used to monitor degradation, if that becomes desirable. It is necessary to remove any film

coating from an area sufficiently Large to transmit the beam. The twc-fold

increase in acid content is a reasonable failure criterion, excepting films

C, so long as films of the present type are in use. Changes in film characteristics would require that the failure criterion be reestablished.

4.2 Mode l Binder The model binders behaved in a manner qualitatively anticipated from results obtained earLier with soluble polyester polyurethanes.

There is autocata Lysi s due to the acid catalyzed hydrolysis. However, times

to reach specific acid contents are longer because of the Lower initial

acid content and, less conclusively, also because of the Lower effective

rate constants for hydrolysis, Fig. 10 - 12 and Table V. An acid content of

8 X 10 4 mol/g was obtained after Long aging of the film with I/U equal to

0.55 at 85 °C and 25% RH. This is about the same as obtained at equilibrium

with the soluble polyester polyurethane used to make the model binder[4L

Highly crosslinked model binders became soft and weak at 85 °C even at 25%

RH but did not appear to crystallize when cooled to room temperature as did

the soluble polyester polyurethanes. Degraded model binder contains allophanate branches which may prevent crystallization. The presence of a network that does not hydrolyze in the most highly crosslinked model did

not prevent severe deterioration of the physical properties. Thus the aging

results to date indicate that crosslinked polyester polyurethanes do not

have sufficiently good physical properties at equilibrium to remain permanently acceptable as binder layer for tape. The main uncertainty in this

conclusion is due to the lack of a complete set of data at other temperatures. Such data are being obtained.

4.3 Maqneti c tapes Tape binder appears to crosslink at humidities at which
crosslinked polyester polyurethanes degrade. One explanation might be that

initial acid and alcohol concentrations in the binder are sufficiently high

to drive the reaction toward crosslinking. We can make an estimate of what

is required to do this. The relation governing equilibrium i s [ 4 ]

k[E 3 [W ] = k’[A] [H] [73

Here [ E 3 and [ W 3 are ester and water concentrations and the other quantities are as defined above. Therefore, [ A 3 [ H 3 at equilibrium is k[E][W3/k'.

Values of [ E 3 , k[W], and k' are in Tables I and IV of reference[43 . From

—7 P P them the product [A 3 [ H 3 is calculated to be 8.2 X 10 mol/g at equilibrium at 85°C and 25% RH. Consequently, [ H 3 equals 8.2 X 10 ^ if [A3 equals

10 mol/g. A ten-fold greater acid concentration, which is twice the value

found for the binder of Tape 4, would require proportionately fewer alcohol

groups. Such concentrations seem impossibly large; pure butane diol, for

_p example, has a concentration of alcohol groups equal to about 2 X 10

mol/g. Therefore, we think some process other than acid and alcohol reacting to give ester is required to explain the crosslinking observed by

Cuddihy at 0 and 15% R H [ 2 3 and by us at 0, 15, and 30% RH.

Our original speculation was that the iron oxide accounted for crosslinking. However, data in the previous report showed that a sample of the

kind of iron oxide used in tape manufacture actua-lly increased the hydrolysis rate of the polyester polyurethane.

Finding that extracted sol absorbs at 2250 cm but that polyester

polyurethane does not, suggests that an additive which might cause the

crosslinking is present in the extract. Absorption between 2200 and 2300

cm is not very common. Isocyanate and carbodiimide are two possibilities

and either could cause crosslinking if multifunctional. However, isocyanates react with atmospheric moisture and the only po lyca rbodi i mi de we have

—'l experience with absorbs at 2170 cm . Therefore, such compounds seem to be

ruled out.

The cause of the crosslinking is still not known.

Apparently tape Lifetime is limited more by binder adhesion than by

the sol content of the binder. Bertram and Cuddihy reported that tapes with

more than 1.5% sol, based on the total tape weight, became difficult to

use[33. Tapes 3, 4, and 6 have more sol than that before aging and they

perform well. Furthermore, the sol content of Tape 4 does not change much

on hydrolysis. Thus sol content does not appear to be a useful quantity for

Lifetime prediction. Binder adhesion has a large range, from more than 300

to about 2 N/m, with failure occurring somewhere between 10 and 50 N/m. The
measured values are not very precise but the method probably can be improved. Some error in the prediction is tolerable because tape is marginally useful even after considerable Loss of adhesion has occurred.

Our method of estimating tape Lifetime will involve measuring binder

adhesion after aging at different temperatures and humidities and correlating the measurements with the ability of the tape transport to process

the tape. A decrease to some Level of binder adhesion will be defined as ending tape Life. Rates of decrease and tape lifetimes will be known as functions of temperature and humidity. Extrapolation by means of the

Arrhenius relation will give values under the desired conditions. This

approach is empirical but it is probably the best that can be done, given

the variation in tape characteristics.

5.0 Future work

Aging of PET at 55 and 35°C will continue. The main effort will be to

improve the infra-red method. Some work will be started to better understand the deterioration of the coatings.

Aging of model binder will continue at 60 and 35°C. The main concern will be whether variations with temperature and humidity extrapolate so as

to change our conclusion that crosslinked polyester polyurethanes will

eventually hydrolyze to useless materials at ambient conditions.

An attempt will be made to estimate the Lifetime of magnetic tapes as outlined above. Experiments will be performed with extracts from tapes to

try to understand the crosslinking mechanism.

6 .0 Conclusions

PET films as currently made for photographic and electrographic use should Last more than 1000 years if stored at 20-25°C and 50% RH. The

lifetimes of the coatings, which bear the information, do not appear to be

that long and are unknown.

Magnetic tape Lifetime appears to be a function of binder adhesion. A

program has been suggested to base lifetime prediction of magnetic tapes on variations of binder adhesion with temperature and humidity.

Experiments with crosslinked polyester polyurethanes indicates that

these materials will not stabilize at a tolerable extent of hydrolysis even

at Low humidity. This conclusion is based on results at onLy one temperature, 85°C. Experiments are continuing at other temperatures with

these materials.

20
References

1. D. W. Brown, R. E. Lowry, and L. E. Smith, Macromolecules, 1

, 248

( 1980 ).

2. E. F. Cuddihy , I.E.E.E. Trans. Magn. J_6, 558 (1980).

3. H. N. Bertram and E. F. Cuddihy, I.E.E.E. Trans. Magn. J_8, 993

( 1982 ).

4. D. W. Brown, R. E. Lowry, and L. E. Smith, J. App. Polymer Sci. 28 , 3779 (1983).

5. D. Patterson and I. M. Ward, Trans. Far. Soc . 53_, 291 (1957).

6. R. L. Addleman and K. Zicky, Polymer J_3_, 391 (1972).

7. D. W. Brown, R. E. Lowry, and L. E. Smith, American Chemical Society

Meeting, Phil., Pa, Aug. 26-31, 1984, Preprint, Division of Polymeric

Materials Science and Engineering 51 , 155 (1984).

8. P. J. Flory, Principles of Polymer Chemistry, Cornell University

Press, Ithaca, NY, 1953, p. 358.

9. A. Charlesby, J. Polymer Sci. 1

, 513 (1953)*

10. P. J. Flory, Principles of Polymer Chemistry, Cornell University

Press, Ithaca, NY, 1 953 * p. 379.

11. C. S. Schollenberger and F. D. Stewart, J. Elastoplast. 3_» 28

(1971 ).

12. H. C. Beachell and C. P. Ngoc Son, J. App. Polymer Sci., 7_, 2217

(1963).

13. N. S. Allen and J. F. McKellar, J. App. Polymer Sci., 28_, 1441

(1976).

14. American National Standards Institute Specifications for Photographic

Film for Archival Records, Silver-Gelatin Type, on Polyester Base, PH

1 .41-1976.

Table I

PET Films at 55 °C 100$ RH

Material Time T

Days MPa

Film base 0 204

586 190

1105 1 65

Microfilm A 0 200

660

s 163

660 167

1105

s 166

1105 1 65

Microfilm B 0 193

660

s 175

660 172

1105 s

171

1105 187

Encapsulating 0 177

Film 660 145

1105 151

Uncoated 0 1 1 6

Electrographic C 660 110

1105 110

Coated Electro- 0 1 67

graphic C 660 1 50

1105 140

Coated Electro- 0 201

graphic D 1 040 177

Subscripts

i, From FTIR measurements; s, strip

E 1 O' [A] k

% eq/g $/day

144 0.35

^ nQ O ll)' 0.037

1 06 0.54

i 0. 03 9

126

100

96

90

85

0.31

1 22

1 1 6

102

88

99

0.32

80

83

0.37

73 0. 57j_ 0 . 03 9

185

1 42

0.41

5 0.68

i 0.046 i

105

78

68

0.48

99

70

0.34

of emulsion layer before aging.
Table II

Acid Content, Elongation at Break and Absorbance Difference

in PET Aged in Boiling Water

)

Time

Days

Elongation

!

[A]

1

24 eq/g

AAbs2 AAbs^

0 1 42 0.36 0.33 0.27

3 1 30 0.45 0.45 0.31

5.9 135 0.55 0.54 0.36

9.7 103 0.73 0.62 0.49

1 4.0 40 1.05 0.79 0.60

17.0 Brittle 1 .41 0.98 0.76

Superscripts

(1) From spectra of three stacked films, each 0.004 inch thick.

(2) AAbs^ = Absorbance at 3542 cm’^ minus that at 3717 cm””'. (3) AAbS 2

= Absorbance at 3256 cm” 1 minus that at 3717 cm” 1

.
Table III

Initial Properties of Polyurethane Films

I/U s V Y T E AAbs/2,

mol/ratio fr fr X 2,/pol M Pa at cm” 1

0 1 - 0 63 1200 6.8

0.14 0.79 0.054 0.6 73 900 5.8

.55 -0 0.38 42 45 370 0.045

1.1 -0 0.47 71 40 130 0.15
Table IV

Elemental Composition Change at 85 °C

I/M Time RH 10 5 [A] N

mol/ratio Days % mol/g %

0 0 - 1 3-6

0 48 100 80 5.6

1 .1 0 - <1 6 .

1 .1 57 100 53 7.1

1 .1 113 50 52 6 . 0
Table V

Recombination of Acid and Alcohol at 85 °C in Dry Polyurethane

(60 days except as indicated)

-(d[A]/dt) = k'[A] 2 ([H0 ]+[A])

I/U 10 6 H

o 1°Ta

o

10 4 A 10 5 k'

moi iqiio - mol/ g g / mo l , a

0 100 5.1 1 .9* 1 .3

0 100 5.1 1 .It 1 .1 0 100 2.3 1 .1 3.1

0.14 86 1 .9 1 .2 2.2, 2.1

0.55 0.67 2.5 1 .75 1.4, 0.9

1 .1 2.2 6.6 1.1 6.6, 4.2

1 .1 2.2 1 .25 0.59 18, 8.2

1 .1 2.2 0.62 0.29 73, 22^

+ ) 68 d, *) 219 d, -*) 2nd value uses H

Q

= 10

-i

* mol/g
Table VI

rhwi J.aracter istics of Unaged Magnetic Tape

Tape gi prigy1 SOI Binder Organic Sol

Layer (BL) r* ~ r v 1Jn Uo 0 Adhesion in BL Org

wt %

T -T W lx /3 N/m * wt $ wt$

1 20

2

0.7 325 303 8

4 2 23 1 -3 >800 24 24

3 32 2.4 150 25 30

4 24 4.6 >800 29 66

5A 28 0.7 >450 26 9.6

5B 1 .0 35

6 2.6 1 60

1

N/m = 1.02 g/cm

2 Not including the back coat that is 6% of the tape weight.

3 Oxide binder only

Assuming half the back coat is carbon.
RBSOFIBfINCE:

3800 9^oo iioo i5oo iToo iioo"

WAVENUMBERS

FIG 1 INFRA-RED SPECTRA OF PET AFTER AGING IN BOILING UATER . 2-CH*' RESOLUTION. CURVES LABELED UITH AGING TINE IN PAYS

.
QO

3eoo 3^00 iioo s£qq bToo iioo iioo

WAVENUMBERS

_

FIS. 2 INFRA-RED SPECTRA OF PET AFTER AGINS IN BOILINS WATER

8-CM* RESOLUTION. CURVES LABELED WITH AGING TIHE IN DAYS,
FIG. 3 CHANGE IN CA1 VS CHANGE IN 4ABS
FIG. 4 FTIR SPECTRA OF URETHANE FILMS.

UNAGED :).I/U=0, i =0.01 2 CM; 2. I/U=0. 14, 1=0.013 CM;

3. I/U=0.55, 1=0.015 CM. AGED AT 85

e C: 4. 1/0=0.14,

1=0.01Q CM. 25% RH. 50 DAYS; 5. I/UO.55, 1=0.015 CM,

25% RH, 59 DAYS; 6. MULTIPLY ABSORBANCE BY 2, I/U=

0.55, 1=0.015 CM, 50% RH, 87 DAYS.
FIG. 5

Color Development in Crosslinked Polyurethane,

(I/U) = 1.1, at 85 °C

Unaged

Dry N2 Dry Air Wet Air Vapor Days
RELATIVE

T

AND

t

FIG. 6 T AMD E RELATIVE TO INITIAL VALUES

VS. DAYS AT 85 C AND tOOX RH

T, UNSHADED SYMBOLS AND ; E, SHADED SYMBOLS AND ;

-Ok y A I/U=0; + , -* I/U=0 .14; 0,3 I/U=

0.55; O, 4I/U=1 . 1 .
FIG. 7

T AND E RELATIVE TO INITIAL VALUES

VS. DAYS AT 85 C AND 2S% RH

T , UNSHADED, ; E, SHADED, ; A

,

A , I/U=0;+,*, I/U-0.14;

, a, i/u-o.ss; 0,4, i/u=i.i
SOL

FRACTION

DAYS

FIG . 8 SOL FRACTION VS TIME AT 85 C

A, I/U-0.SS; O, IAJ-I.l

100* RH , sax RH , 252 RH •.
Oli(»/l°m)

(y)

A (A Aba/O.cm

-

Fiy. 9 Acid content vs. increase inAlabsorbenca/f i Ira thickness)

during aging.

I/U=0:X,unao8d; * ,85C, 25X RH; dL 60C, IQOiflH. I/U=0 . 14 : ,^,2; 85C, 100, 50, 25S RHjCJ, 6QC, 100URH. I/U=0.55:

85C, 100, 5 OS* RH. Line: C A ] =0.01 3X Al ^Abs/ L )
2

FIG. 10 ACID CONTENT VS DAYS AT

85 C AND 130 A RH

4,I/U=0; + , 1/11=0. 14;

O, I/U=0 . 55; Q,I/U=I .

LINES, A=A 0 EXPC£'6
DAYS

I I ACID CONTENT VS DAYS AT

85 C AND 50X RH

A.I/UO; +,I/U=0.I4;

,I/U=0.55; 0,I/U=l .

LINES, A=A. EXPCfetO
MOL/G

FIG. 12 ACID CONTENT VS DAYS AT

85 C AND 2SX RH

A.I/U=0; +,I/U=0.I4;

D,I/U=0.55; 0,I/U=l . ] LINES, A=A 0 EXPC/k £>
TH

FIG

13

INFRA-RED

SPECTRA

OF

BINDER

LAYER

AND

POLYURETHANE
ABSORBANCE

.60

.85

1.10

1.35

1.80

1.85

2.10

2.35

2.60 FIG. 14 INFRA-RED SPECTRA OF BINDER

LAYER FROM MAGNETIC TAPES

6

WAVENUMBERS
FIG

IS

TAPE

EXTRACT

AFTER

AGING

AT

60

C

AND

I00X

RH

03iovyiX3 y. in

DAYS
111 !': I ! 1 1 TNM

FIG. 16 RECORDER PLOTS DURING STRIPPING

OF BINDER LAYER FROM TAPES I, 5A, 5B

TAPE 1, FULL SCALE 500G;

TAPE 5A, 1KG FS; TAPE 5B, 200G FS
FIG. 17

STRIPPING PATTERNS OF BINDER LAYER

TAPE

1, UNAGED

3, UNAGED

SB, UNAGED

6, UNAGED

4, 60 C,30X RH, 52S DAYS

N/M 100
FIG

18

BINDER

ADHESION

AFTER

AGING

AT

60

C

AND

30

%

RH

©

©

(0

© © © © © © © © ©

© © © © © © © ©

00 <0 1/3 <0 N *—

&3JL3W/N

DAYS
FIG

19

BINDER

ADHESION

AFTER

AGING

AT

68

C

AND

1GX

RH

<3

CO

<D

a CO CO <S <3 CO

CO CO o <S CO

c00 CSJ

y313W/N

DAYS

NBS-114A REV. 2-8CJ U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No.

NBSIR 84-2988

3. Publication Date December 1984

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)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contracb'Grant No. Letter of 8/24/83

8. Type of Report & Period Covered

Progress 8/83-12/84

9.

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

National Archives

Washington, DC 20408

10.

SUPPLEMENTARY NOTES

_ Document describes a computer program; SF-185, FlPS Software Summary, is attached.

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

_bib/ io eraohv nr literature survev. mention it here! ! Aging studies with po Ly [ethy Lene terephtha Late] film base indicate the Lifetime is equaL to about 1000 years if the materiaL is stored at 20-25°C and 50%

reLative humidity. Concentration changes of acid and aLcohoL groups that occur as

a resuLt of aging have been measured by infrared anaLysis. Rate constants caLcuLated by this method agree reasonabLy weLL with those caLcuLated from acid contents determined by titration.

CrossLinked poLyester po Lyurethanes were prepared as modeLs of the binder of

' magnetic tape. Aging studies with these materiaLs indicate that they hydroLyze

more sLowLy than ordinary poLyester po Lyurethanes. SampLes aged at 85°C at 100,

50, and 25% reLative humidity eventuaLLy deteriorated greatLy in a physicaL sense. Magnetic tapes were aged and measurements made of the soL content of the

binder and its adhesion to the poLyester base. The Latter quantity appears to be a more vaLuabLe indicator of tape condition than soL content. VaLues of binder

adhesion of six brands of magnetic tape initiaLLy varied between 800 and 35 N/m

[or g/cm]. Binder adhesion in aged tapes was Less the higher the temperature and

humidity of aging. A tape transport had difficuLty processing tape with vaLues of

binder adhesion as Low as 10 N/m. There was no probLem at 35 N/m. It is anticipated that the Lifetime of magnetic tapes can be predicted by measurements of

binder adhesi on.

12. KEY WORDS (Six to twelve enTrTes; alphabet! car order; capitalize only proper names; ana separate <e/ r, , Hydrolysis; magnetic tape; polyester; poly (ethylene terephthalate) ; polyurethane

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

| y I Uni imited 49

For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Offce, Aashmgion, D.C. 20402. IS. Price

</> 00 cn

I yi Order From National Technical Information Service (NTIS), Springfield. VA. 2216 1

J