Transport of Acetic Acid in

Polyethylene

U S DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Polymer Science and Standards Division

Washington, DC 20234

July 1 983

{

-QC100

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pared for source Recovery Program

33-2/16

1933

NBSIR 83-2716

TRANSPORT OF ACETIC ACID IN

POLYETHYLENE

NATIONAL BT/P.EAOI OF STANDARDS

library

AUG

r\CrV CX-C q c_- vOO

l«l*3

Joseph C. Phillips

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Polymer Science and Standards Division

Washington, DC 20234

July 1983

Prepared for Resource Recovery Program

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

TRANSPORT OF ACETIC ACID IN POLYETHYLENE

BY

Joseph C. Phillips

Polymer Science and Standards Division

National Measurement Laboratory

National Bureau of Standards

Washington, D. C. 20234

ABSTRACT

This report describes the mass loss and sorption/desorption of Acetic

Acid (HAc) in polyethylene of different densities at various temperatures.

Mass loss for a mixture of HAc and of three concentrations were also done

at the highest temperature used.

Experimental data for mass loss were obtained at temperatures of

40 °C, 60 °C, and 78 °C for a conventional density bottle (CPE, p~.92 g/cm

3

)

and a linear polyethylene density bottle (LPE, p. .94 g/cm ). At t = 78 °C a

mixture, of HAc^O consisting of 100% HAc, 75% HAc, and 25% HAc by volume

was used as permeant in the CPE and LPE bottles. From the mass loss data,

the loss rate and diffusivity were determined.

Data for sorption/desorption were obtained at temperatures of

approximately 30 °C, 50 °C, and 70 °C at a vapor pressure of p = 17 mm Hg

(2.3 kPa). The low density polyethylene film (LDPE) had a density of

3 -3

P..S06 g/cm and a thickness of 10.16 x 10 cm. From the sorption and

desorption measurements, diffusivity, solubility, and permeability were

determined.

Table of Contents

1. Introduction

2. Materials

A. Permeants

B. Polyethylene

3. Experimental

A. Mass Loss

B. Sorption./ Desorption

4. Results and Discussion

A. Mass Loss

B. Sorption/Desorption

5.

Conclusion

2

Introduction

Polymeric materials of low permeability are increasingly used as liners

for storage and disposal of hazardous and municipal wastes. The primary

function of these barrier materials is to prevent leachate and liquid

components in the waste from leaking from an impoundment and polluting

groundwater. For permanent storage, such as in municipal landfills, the

liner should be an effective barrier for an extended period of time. In

the absence of actual experience on the long term performance of polymeric

liners there exists the need for predictive methods based on accelerated test

methodologies for estimating service life.

Liner materials may be subject to a variety of chemical substances that

may adversely affect the performance of the liner by chemical degradation,

swelling the polymer, and environmental stress cracking. Each of these

factors can degrade mechanical properties and increase transport of substances

through the liner. Mechanical properties of the liner could be degraded to

such an extent that holes are developed permitting rapid transport of the

impounded liquid or leachate.

The overall goal of the present program is to identify factors which

limit the long term performance of barrier materials and to develop test

protocols for evaluation of candidate materials.

In the present phase of the work the compatibility of liner materials

with simple organic acids is investigated. The series acetic, butyric and

isobutyric acids have been identified 'as the major organic constituents of

leachate from municipal landfills with the most prevalent species being acetic

acid. The initial phase of the investigation is concerned with the compatibility

of polyethyl enes with acetic acid. The second phase will address the activity

of acetic acid as an environemntal stress cracking agent and the compatibility

of polyethylenes of the kind used as a liner materials with other organic acids.

3

The transport of vapors and gases through a given material is dependent on

the density, thickness, and the compatibility of the material and the vapor or

gas [1], The transmission performance of some barrier materials may be predicted

to reasonable accuracy in certain instances [2,3]. In other cases, additional

experimental data may be necessary. This report is concerned primarily with the

transport of acetic acid through polyethylene at various temperatures and may be

useful in characterizing certain materials as prospective liners for landfills.

The mass loss and sorption/desorption of acetic acid (HAc) in polyethylene

bottles and LDPE (low density polyethylene) film have been investigated. Mass

loss data were obtained as a function of temperature in LDPE (low density) and HDPE

(high density) bottles. At the highest temperature used mass loss was recorded for

various mixtures of HAc/L^O. The sorption/desorption data were also obtained as a

function of temperature at a single pressure using a LDPE film.

The results from mass loss indicate that the activation energy of permeation

is independent of density over the range investigated. The diffusivity from sorption

data at p = 17 mm Hg (2.3 kPa) as a function of temperature shows deviations from

diffusivity determined from mass loss; this suggests a concentration dependence

which increases from vapor to liquid.

Material*

A. Permeants

Reagent glacial acetic acid (MCB Manufacturing Chemists, Inc.; Cincinnati,

OH) and distilled water were the liquids used. The acetic acid was used without

further purification.

*Certain commercial equipment, instruments, or materials are identified in this

report in order to adequately specify the experimental procedure. In no case

does such identification imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the material or equipment identified

is necessarily the best available for the purpose.

4

Polyethylene

For mass loss measurements, conventional 125 ml polyethylene bottles

(p=. 92 g/cm ; also designated as CPE) and linear polyethylene bottles

3

( p= . 94 g/cm ; also designated as LPE) were used [4]. The actual densities

as determined by a floatation method using ethanol /distilled water at 25 °C [4],

3 3

were found to be, respectively, p=.9235 g/cm and p=.9430 g/cm . The mean wall

thickness of the CPE and LPE bottles were, respectively, .1062 and .0950 cm [4].

The sorption/desorption experiments were performed on low density PE film

(p=.906 g/cm ; Abar Plastics, Inc., Beltsville, MD) with a thickness of .102 mm.

The crystallinity determined from density measurements was 38.5% [comparatively

,

the crystallinities for the CPE and LPE bottles were, respectively, 47.3% and

> 59.5%].

Experimental

A. Mass Loss

Experiments were performed at 40°C, 60°C and 78°C in a safety-designed air

circulating oven. The sample bottles containing 100 ml of HAC or HAc /H2 0 mixtures

were placed on prescribed spaces within the oven that were calibrated with a .quartz

thermometer for temperature variations. After placing samples in the oven, weight

determinations were done at subsequent times on a top-loading balance sensitive to

about - 01 g . The loss rate for each bottle was determined from the change in mass

versus time.

B. Sorption/Desorption

These experiments were done. at 30°C, 50°C, and 70°C using a quartz spring

sensitive to 2.7 mm/mg. Before performing each experiment, all samples were

conditioned in vacuum. At a given time, the suspended rectangular sample (weight

-49 mg) was allowed to come in contact with acetic acid vapor at a pressure p = 17 mm

Hg (2.3 kPa). From a plot of the reduced mass uptake [M(t)/M(°°) = the mass uptake

at tine t divided by the equilibrium mass uptake] versus the square root of time,

the diffusivity was determined; the solubility was obtained from the equilibrium

mass uptake [M(*0].

5

Results and Discussion

A. Mass Loss

Figures 1 and 2 show mass loss for the LDPE and HOPE bottles for

temperatures of 40 °C and 60 °C, respectively. At t = 78 °C mass loss

measurements were performed for various concentrations of acetic acid in

distilled water as shown in Figure 3 for the LDPE bottles and in Figure 4

for HOPE bottles. The concentrations of acetic acid in volume percent were

100%, 75%, 50% and 25%. All of the curves as shown possess the same general

mass loss-time behavior, i.e., beyond the induction period, the curves approach

linear behavior asymptotically. From permeation theory [5], such behavior

may be represented as

Am = 0 (t-q(t/x)t) (1)

where Am is the mass loss at time t, Q is the steady-state rate of

loss, describes the molecular transport process and initial experimental

conditions, and x is the time-lag which is proportional to the thickness of

the container wall, z 3 and inversely proportional to the diffusivity of the

permeant, D, i.e.,

x = Jl 2 /6D (2)

The parameter q may be calculated [6] if certain assumptions are made concerning

the initial state of the polymer, the Fickian nature of the transport process,

and the concentration independence of the diffusion constant.

The steady-state loss rate determined from the mass loss data is shown in Fig. 5

for the CPE and LPE bottles as a function of temperature. The log Q versus

reciprocal temperature plots are linear and parallel. This indicates that the

activation energy, Ep, for permeation is approximately constant for both CPE

and LPE bottles, i.e.,

Q = Q

0

e'

E P

/RT

(3)

The value for Ep over the temperature range shown in Fig. 5 was found to be 17.3

Kcal/mole. This value compares rather well with Ep calculated from the data of
*
6

Salame [3] for acetic acid in PE. The quantity 0o may be estimated for CPE

and LPE by extrapolating the data in Fig. 5 to 1/T = 0. By inserting the

results of Eq. (3) into Eq. (1), the amount of permeant lost as a function of

time may also be determined.

The uncertainty in the calculation of D using Eq. (2) depends dramatically

on the uncertainty in time lag t and the thickness £. The former quantity may

vary from a few percent to well over 100% depending on whether the equilibrium

loss rate had been attained over the time range of measurement; the latter

quantity contributes an uncertainty that varies as the square of the thickness

and may be substantial for very thin and/or inhomogeneous walls [e.g., the

error in i ranged from ±7% for the LPE bottles to ±18% for the CPE bottles].

In order to test the influence of a second component on the loss rate of

acetic acid measurements were made at various concentrations in distilled water.

The results in terms of volume fraction of acetic acid, V, are shown in Figure 6.

• • A plot of log Q versus V was non-linear while a plot of log Q versus Va was

1 inear, i .e.

,

log Q = log Q

q + Va

(4)

where a is a constant and was found to be roughly 2.5. The rationale for the

behavior exhibited by Eq. (4) has not been explored at this time. It may be

noteworthy to mention that a simple additivity concept [6] using the mole

fractions of components 1 and 2 doesn't seem to apply to the HAC /H 20 mixtures

for the concentration range used here. Apparently one can not neglect the

different kinds of interactions that exist in this system, e.g., polymer/liquid

[PE/HAC , PE/H2 0, PE/HAc :H 2 0] and liquid/liquid (HA

c /H 2 0, HAC /HAC , H

2 0/H 2 0].

B. Sorption/Desorption

The sorption and desorption of acetic acid vapor at p = 17 mm (2.3 kPa)

for t = 30 °C, 50 °C, and 70 °C are shown in Figures 7, 8, and 9 respectively.

The behavior at very early times could be Fickian while the increasing differences

between sorption and desorption curves suggest a concentration-dependent diffusion

constant. The limiting diffusivity constant, D, was calculated from the reduced macc nntaL'o a c f 71 . i . p

.

7

M(t)/MH = 2/1 (4D/tt)

1/2

t

1/2 (5)

where M(t) is the mass uptake at time t and M(») is the equilibrium mass

uptake at t-*». The equilibrium mass uptake and concentration at 30°C, 50°C

and 70°C are, respectively, M(°°)=0.344 mg, c=7.02 x 10” 2 mg/mg; M(<=°)=0.122 mg,

c=2.49x!0~ 2 mg/mg; and M(=°)=0.047 mg, c=8.28 x 10*^ mg/mg. The solubilities at

p=17 mm Hg were calculated from the concentrations, i.e., S=c/p where c=M(»)/M

f

is the ratio of the equilibrium mass uptake to the mass of the film (Mf~49 mg].

Due to spring oscillation, data for times earlier than shown was difficult to

obtain. The desorption and sorption data appears to be as expected at all three

1 /2

temperatures, i.e., the slope of the reduced mass uptake versus t tends to

increase with temperature which indicates a corresponding increase in diffusivity.

The diffusivity and solubility determined from sorption as a function of

temperature are shown in Figure 10. The apparent linear relationships suggest

that the activation energy of diffusion, E Q

, and the heat of solution, aH, are

« roughly constant over the temperature range investigated, i.e.,

-E

d

/RT

D = D„e

0

( 6 )

or

S " Soe -ah/rt

The values determined for Eq and aH were, respectively, 11.1 Kcal/mole and -11.3

Kcal/mole. The heat of solution, aH, suggest that the heat of condensation is

neaative and greater in maanitude than the heat of mixing, i.e., AH=H ,+H . .

The term H

concl depends on the Lennard-Jones potential parameter, e/k, which is

quite dependent on permeant size and polarity [8]. The term H . is a function

II I I A of the mixing parameter x [8]. In addition to the two terms mentioned above, AH

also has a contribution due to any crystalline melting that may occur over the

temperature range of interest.

The calculated permeability [P=SD] from Fig. 10 shows little temperature

deoendence for sorption which seems to be in agreement with that determined

from E , i.e., P=P 0 e~

E P^

RT where E

p may be estimated from E

Q

and :.H [E

p =AH+ED

].

8

If one assumes that Eq~

H

concj , then it is apparent that Hm

- makes a small

almost negligible contribution to the overall activation energy of permeation

for HAC in the sorption experiment using thin films of LDPE. Also shown in

Fig. 10 are calculated values of the diffusivity determined from the mass loss

data utilizing Eq. (2); the density and thickness were those of LDPE films

used for the sorption/desorption measurements. The difference in the

sorption and mass loss results seems to suggest that the diffusivity is

concentration-dependent. This explanation seems to be reasonable since the

sorption measurements were performed at a low vapor pressure (p=17 mm Hg)

while mass loss measurements were done with liquid. To further check the

diffusivity results in Fig. 10, it would be desirable to obtain mass loss from

a lower density bottle and to perform sorption/desorption measurements as a

function of pressure.

Conclusion

This preliminary investigation has presented data at various temperatures

on mass loss and sorption/desorption of acetic acid in PE bottles and LDPE

films. From the mass loss measurements, loss rate and diffusivity were

determined; from the sorption/desorption data, diffusivity and solubility were

calculated. The temperature dependence of these properties indicate that

activation energies of permeation and diffusion and the heat of solution are

roughly constant over the temperature ranges investigated. Differences in

sorption and desorption results and the comparison of diffusivity calculated

from mass loss measurements also suggest that diffusivity is concentrationdependent.

9

This investigation also attempted to address the question of mixtures.

Even though HAc/h^O solutions were studied only at one temperature, the effect

of adding to HAc can dramatically affect the loss rate of HAc. It would be

highly desirable to study this mixture and other mixtures of different acids

in the homologous series as a function of temperature. Such a study would be of

great benefit in helping to characterize a given plastic as a potential liner

material

.

References

1. The Science and Technology of Polymer Films, Vol. II, edited by

Orville J. Sweeting, John Wiley & Sons, Inc., New York, NY pp. 1-31 (1971).

2. Bent, H. A. and Pinsky, J., Theoretical Investigation of the Mechanism

of Transfer of Materials Through Polyethylene, Wright Air Development

Center Technical Report 54-133, Part 2 (1955).

3. Salame, M., An Empirical Method for the Prediction of Liquid Permeation

in Polyethylene and Related Polymers, SPE Transaction, 1, No. 4, pp. 1-11

(1961).

4. Phillips, J. C., "Performance of Plastic Packaging for Hazardous Materials

Transportation: Part VI, Mass Loss and Effective Carbon Atom Number

Measurement", Report No. D0T/MTB/0HM0-80/3, U. S. Department of Transportation,

1980.

5. Crank, J. and Parks, G. S. , Methods of Measurement, Chapter 1 in Diffusion

in Polymers , J. Crank and G. S. Parks, Eds., (Academic Press, New York, 1968).

6. Barnes, J. D., Martin, G. M. , and McCrackin, F. L., performance of plastic

packaging for Hazardous Materials Transportation: Part IV, Standardizing

permeation Measurements, "D0T/AS-50074. Available from National Technical

Information Service, Springfield, VA. 22151.

7. Phillips, J. C., Peterlin, A., and Waters, P. F. , J. Polym. Sci., Poly. Phy.

Ed., 19, 789 (1981).

8. "The Science and Technology of Polymer Films", Vol. II, edited by Orville

J. Sweeting, John Wiley & Sons, Inc., New York, N.Y., P.31 (1971).

List of Figures

1. Mass-Loss Data, PE/Acetic Acid (t=40 °C).

2. Mass-Loss Data, PE/Acetic Acid (t=60 °C).

3. Mass-Loss Data, LDPE/Water-Acetic Acid (t=78 °C).

4o Mass-Loss Data, HDPE/Water-Acetic Acid (t=78 °C).

5. Loss-Rate Data versus temperature, LDPE and HDPE.

6. Loss-Rate Data of Acetic Acid/Water versus volume percent

Acetic Acid (t=78 °C).

7. Sorption/Desorption Data, LDPE/Acetic Acid (t=30 °C; p = 17 mm Hg).

8. Sorption/Desorption Data, LDPE/Acetic Acid (t=50 °C; p

= 17 nm Hg).

9. Sorption/Desorption Data, LDPE/Acetic Acid (t=70 °C; p = 17 mm Hg).

10. Diffusivity and Solubility Data from Sorption versus temperature, LDPE

(Acetic Acid).

6 ‘MV

Time,

Hrs.

t

o

CO

o

CO

o

o

04

o

6 ‘MV

04

&C •H

P*4

Time,

Hrs.

HAc/H

20

78°C

[CPE

Bottles]

(6) MV ‘sson ssetu

Time,

Hrs.

(6) MV ‘ssoi sse|/\i

160

180

200

220

Q

x

1

02

(g/hr)

1/T

Q

(g/hr)

V, Volume Fraction (HAc)

Fig. 6

LDPE/HAc

o>

00

CD

LO

00

CM

<J) CO N CD Lfi

a CO CM

W>

•H

(oo)iAI/(i)l/M-|. JO (oo)|/m/(})IAI

ZA

(U!U1)

o 00

o

00

N.

CO

LO

00

CM

o

cc •H

P=<

(oo)|/m/0)IAI-1- JO Miai/Wiai

t

1/2

(min

)

1/2

LDPE/HAc

(oo)l/M/(l)!AI-l- JO (oo)|/\|/(l)|/\|

t’/z

(min)

1

/*

D

x

10

8

cm 2

/sec

LDPE/HAc

(Sorption)

100

10

0 Diffusivity

a Solubility

* Calculated diffusivity from

mass loss data

(using thickness and

density of film used

for sorption)

P = 17 mm Hg

1 i= 10.16 x IQ-3 cm

1.0

.00280 .00300 .00320 .00340

10

1.0

.1

1/T Fig. 10

S

x

10

4

mg/mg/mm

NBS-114A ire«v. 2 -ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2716

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

July 1983

4. TITLE AND SUBTITLE

TRANSPORT OF ACETIC ACID IN POLYETHYLENE

5. AUTHOR(S)

J. C. Phillips

S. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

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

10.

SUPPLEMENTARY NOTES

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

11.

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

bi bl iography or literature survey, mention it here)

This report describes the mass loss and sorption/desorption of Acetic Acid (HAc)

in polyethylene of different densities at various temperatures. Mass loss for a mixture of HAc and H20 of three concentrations were also done at the highest

temperature used.

Experimental data for mass loss were obtained at temperatures of approximately

40°C, 60°C, and 78°C for a low density bottle (LDPE, p~.92 g/cm3

) and a high density

bottle (HDPE, p~.94 g/cm3 ) . At t=78°C a mixture of HAc/H2 0 consisting of 100% HAc,

75% HAc, and 25% HAc by volume was used as permeant in the LDPE and HDPE bottles.

From the mass loss data, the loss rate and diffusivity were determined.

O Data for sorption/desorption were obtained at temperatures of approximately 30 C. 50°C, and 70°C at a vapor pressure of p~17mm Hg. The polyethylene film had a density

of p~ . 906 g/cm3 and a thickness of 4 mil (10 . 16xl0_3 cm) . From the sorption and

desorption measurements, diffusivity, solubility, and permeability were determined.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Acetic Acid, desorption, diffusion, mass loss, polyethylene, sorption.

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