REPORT NO. DOT/MTB/OHMO-80/3

r

NBS

PUBLICATIONS

PERFORMANCE OF PLASTIC PACKAGING FOR

HAZARDOUS MATERIALS TRANSPORTATION

RT VI. MASS LOSS AND EFFECTIVE CARBON ATOM NUMBER MEASUREMENT

J. C. PHILLIPS

APRIL 1980

FINAL REPORT

Document is available to the U.S. public through

the Nationel Technical Information Service,

Springfield, Virginia 22161

Prepared for

U.S. DEPARTMENT OF TRANSPORTATION

MATERIALS TRANSPORTATION BUREAU

OFFICE OF HAZARDOUS MATERIALS REGULATIONS

WASHINGTON. D.C. 20590

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PERFORMANCE OF PLASTIC PACKAGING FOR HAZARDOUS MATERIALS TRANSPORTATION

Part VI: Mass Loss and Effective Carbon Atom Number Measurements

by

Joseph C. Phillips

Polymer Science and Standards Division

National Measurement Laboratory

National Bureau of Standards

Washington, D. C. 20234

For

U.S. Dept, of Transportation

Materials Transportation Bureau

Office of Hazardous Materials Regulation

Washington, D. C. 20590

Research Contract DOT AS-50074

April 1980
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Table of Contents

1. Introduction

2. Materials

A. Ladings

B. Polyethylene Bottles

3. Experimental

4. Results and Discussion

A. Mass Loss

B. Effective Carbon Atom Number

5. Conclusion

6

7

7

7

7

8

8

9

2

1
Abstract

This report describes the mass loss through polyethylene bottles of

two n-alkyl series (n-alcohols and n-carboxylic esters) and a group of

miscellaneous compounds. The experiments were performed at 50 °C using

both low density and high density polyethylene bottles. The backbone

chain length, N^, for each series of n-alkyl 's was extended to 16 in

order to characterize more fully the dependence of loss rate from PE on

chain length of permeant.

The results suggest that the effective carbon atom number for a

given permeant may be determined from a single density resin bottle

using the n-alkane series as the standard lading. Once the effective

carbon atom number has been determined under chosen conditions, the

permeability performance of the permeant for another resin density may

be predicted with reasonable accuracy. These results seem to complement

the "permachor" method to minimizing the number of tests required for

the determination of the performance of full-scale containers.

2
Preface

This report is part VI of a series prepared for the Office of

Hazardous Materials Operations under Research Contract DOT AS-50074.

This work deals with the establishment of criteria for predicting

permeation performance of various kinds of ladings in polyethylene

bottles. This study verifies, at least for the permeants and containers

studied herein, the validity of the effective carbon atom number as a

parameter for transferring results of permeation measurements among

various shipping containers made from various PE resin. The present

work extends the concepts presented in Part IV of this series.

3
List of Tables

1. Polyethylene (PE) Bottles Used for Mass Loss Measurements

2. Loss Rate at t = 50 °C for various classes of compounds using low

density (CPE) and high density (LPE) bottles.

3. Average Effective Carbon Atom Number for Various Classes of Compounds

(Based on Skeletal Chain Length).

4. Predicted Loss Rate at 50 °C for High Density Bottle From Loss Rate

of Low Density Bottle.

4
List of Figures

1

.

Mass-Loss Data, n-Hexane

2. Mass-Loss Data, n-Alcohols (c - c ) 1 1+

3. Mass-Loss Data, n-Alcohols (Decyl and Dodecyl

)

4. Mass-Loss Data, n-Cetyl Alcohol

5. Mass-Loss Data, n-Carboxylic Esters (Ethyl, Butyl, and Amyl Acetate)

6. Mass-Loss Data, n-Carboxylic Esters (Methyl Heptanoate and Methyl

Tetradeconate)

7. Mass-Loss Data, 1 ,2-Dichloroethane, Acetone, Methyl

and Acetontrile [Low Density PE]

Ethyl Ketone,

8. Mass-Loss Data, 1 ,2-Dichlorethane, Acetone, Methyl

Acetontrile [High Density PE]

Ethyl Ketone, and

9. Loss-Rate Data, Low Density PE

10. Loss-Rate Data, High Density PE

11 . Loss-Rate Data Versus Effective Carbon Atom Number

12. Loss-Rate Data Versus Density of PE

5
Introduction

The permeation rate of vapors and gases through a given barrier

material depends on parameters such as thickness, density of the material,

and the compatibility of the permeant and the barrier [1]. Various

attempts at correlating permeability with these barrier parameters have

proven successful in many instances and have also made possible the prediction of the transmission performance of a given barrier and the

permeant [2,3]. In a previous report to DOT (Part IV, DOT AS-50074), an

effective carbon atom number was introduced as a parameter for predicting

permeation performance of n-alkanes in PE bottles [4]. This present

report has attempted to establish the validity of the effective carbon

atom number by: (1) studying other homologous series; and (2) looking

at a wider range of densities of the PE resin.

This work investigated the loss rate for an n-alcohol series

(N^ =1 to 16) and an n-carboxylic ester series (N^ = 5 to 16) in PE

bottles of two different densities (p~ .92 g/cm and p~ .94 cm) at 50 °C.

The effective carbon atom number was determined for each permeant and

found to be relatively insensitive to the density of both PE resin

bottles [4]. By using the average effective carbon atom number, the

loss rate in the high density PE bottle was predicted for a group of

miscellaneous compounds (1 ,2- dichloroethane; acetone; methyl ethyl ketone;

acetonitrile) from the loss rate measurements in the low density PE

bottle with rather good agreement to the experimental data. These

results tend to support the statistical linear model for transmission

rate as outlined in part IV of the permeation report DOT AS-50074.

6
Material

s

A. Ladings

The n-alcohols (N^ = 1 to 16), n-carboxylic esters (N^ = 5 to 16),

and a group of miscellaneous compounds used in this study are listed in

Table II. All samples were either A.C.S. certified, reagent or technical

grade and were used without any further purification.

B. Polyethylene Bottles

Table I lists some information on the PE bottles used in this study.

The low density (CPE, p~ .92 g/cm ) and high density (LPE, p

~ .94 g/cm )

bottles were manufacturered by the Nalge Company. The actual densities,

as determined by a flotation method using ethanol and distilled water

at 25 °C, were found to be p

= .9235 g/cm^ and p = .9453 g/cm^. Some

high density PE bottles made by Phillips Petroleum Company were found

to have a density of p = .9430. The mean wall thickness of the CPE and

LPE bottles were, respecti vely , .1062 cm and .0950 cm and the "Phillips"

bottles had a mean thickness of .0671 cm. The CPE bottles as shown in

Table I showed the greatest variation in thickness over its wall surfaces

while the "Phillips" bottle had the least variation. Such density and

thickness fluctuations for a given bottle contributes to the uncertainty

in the mass loss measurement.

Experimental

Mass loss measurements were performed in a safety-designed air

circulating oven maintained at 50.0 °C ± 0.4 °C. Marked positions within

the oven were calibrated for temperature variations and during the experiment, the oven temperature was monitored by a thermistor probe. The zero

time of the experiment was taken as the initial placement of the bottles

7
(filled with -100 m^of sample) into the oven [4]. At subsequent times,

the bottles were removed and weighed on a top-loading balance sensitive

to about .01 g. From the change in mass versus time, the loss rate for

each sample was determined.

Results and Discussion

A. Mass Loss

Typical curves for mass loss measurements are shown in Figs. 1-8.

All the curves seem to posses the same general mass-time behavior, i.e.,

as the time increases beyond the induction period, the curves tend to

asymptotical ly approach linear behavior. From permeation theory [5]

such behavior may be represented as

where Am is the amount of permeant lost after time t, [3Am/9t]

is the steady-state rate of loss, ^ is a parameter which depends on the

molecular transport process and the initial experimental conditions and

x

is the time-lag which is proportional to the thickness of the container

wall, Z, and inversely proportional to the diffusivity of the permeant,

D, i.e., T = Z^/6D.

The mass-time data for this study have been analyzed only in terms

of the steady-state rate, Q, and is summarized in Table II and Figs. 9

and 10. As can be seen in Fig.l , Figs. 3-6, and in Table II, an increase

in PE density tends to diminish the amount and rate of permeant passing

through the container walls. If the diffusion process is truly Fickian,

then the steady-state rate should be inversely proportional to the

( 1 )

effective thickness for a constant area [4]. The results in Fig.l and

8
Table I and II for the "Phillips" bottle do indeed suggest an inverse

dependence of Q on £, but a small difference in density (as will be shown

later) could also appreciably increase the loss rate.

The results in Table II and Figs. 9 and 10 further suggests a dependence of loss rate on chain length as expected. Here the chain length

is defined as the backbone or skeletal chain length, which includes

all atoms in the primary chain [6] exclusive of atoms at the chain end

that constitute a chemical group (e.g., CN, NH^, OH, etc.). The loss

rate, §, may be empirically defined [3] as:

Q = $Qe“^p/RT (2)

where Q is a constant, E is the apparent activation energy, R is the

\J p gas constant, and T is the absolute temperature. Since the apparent

activation energy generally increases as the chain length increases,

the loss rate according to Eq.(2) should decrease. For chain lengths

below some critical value, (N^~5) the loss rate may not be a monotonic

function due to polarity effects [3] as shown in Figs. 9 and 10 for

the n-alcohol and n-carboxylic ester series. Two members of the ketone

series (acetone and MEK) also seem to show polarity effects for the early

members of the series. The results in Figs. 9 and 10 also suggest that

as the chain length increases, the loss rate for each series tend to

approach that of the n-alkanes.

B. Effective Carbon Atom Number

The effective carbon atom number, n^, is just a shift parameter

that characterizes the loss rate of a given permeant to that of some

standard or reference permeant [4]. This concept is analogous to that

9
of the "permachor" approach [3]. In order to account for all atoms in

the primary chain, the backbone or skeletal chain length, N^, is used

in this study rather than only the number of carbon atoms in the chain.

The choice for the former appear to be small and roughly constant.

Figure 9 for low density PE may be used to illustrate the detennination of n^ for a given permeant. For example, the effective carbon atom

number for n-amyl acetate is found by shifting at constant loss rate

to the n-alkane curve (note dotted line in Fig. 9). The abscissa at the

intersection of the two curves is defined as n . Hence, the determinae tion of n^ may be given mathematically as:

He = log {Qf^/Q^)/^ (3)

where Q. is the loss rate for a given permeant, Q and k are parameters

r\ 9 U

that may be determined from the loss rate equation for the n-alkanes,

i . e .

,

log Q = log + kN^ (4)

By using Eqs.(3) and (4), the effective carbon atom number was calculated

from the results in Figs. 9 and 10. In Fig. 11 a semi-log plot of loss

rate and n^ for both PE density bottles does indeed yield parallel lines

within experimental error. The average n^ values for both PE density

bottles are assembled in Table III with a maximum deviation of about

10% which occurred in the alcohol series. The lines in Fig. 11 may be

represented as:

log = F(pp,T) + k^n^ (5)

where F is a function of PE density and temperature and k^ is apparently

independent of PE polymer density. By using Eq.(5) and loss rate results

10
for the miscellaneous compounds in Table II for the low density PE bottles,

the loss rate for the high density PE bottle was predicted as shown in

Table IV. The results are in rather good agreement with the experimental

values and tend to reaffirm the validity of the effective carbon atom

number [4]. Similar calculations may also be done for the other compounds

with comparable agreement. Figure 12 further illustrates the fact that

for > 5 the nature of the steady-state diffusion process in the PE

polymer matrix is somewhat similar for each series of permeants.

If Eq.(5) is now combined with the results of Fig. 11, the loss rate of

the permeant at constant temperature becomes:

log in = F(pp,T) + kpHp = log ino * C(Pp - Ppp) + (6)

where C is a constant, and are, respectively , the density of PE

for the reference or standard state and

p^

is some other PE density of

interest. Thus at constant temperature, the loss rate for a given permeant may be estimated by Eq.(6) from a knowledge of only n^ and

The latter quantity is determined from the n-alkanes in the standard

state. As discussed above, the effective carbon atom number of a given

permeant may be experimental ly determined from some standard permeant

in a given polymer resin.

11
Concl usion

This work has presented data at constant temperature on mass loss

and loss rate for n-alcohols and n-carboxylic esters and a group of

miscellaneous compounds of two different densities in PE resin bottles.

From the data an effective carbon atom number, n^, for each permeant

was determined and found to be roughly independent of the density of

the PE polymer matrix. By measuring loss rate of the n-alkanes as standard permeants in PE bottles of a given density, the loss rate of other

permeants may be predicted at some other PE density with reasonable

accuracy by using n^ values.

This work further suggests that a standard measurement system of

bottles (constant wall thickness, density, type, etc.) and permeants

(e.g., n-alkanes) could be chosen for obtaining quantitative mass loss

information. Effective carbon atom numbers derived from such measurements could be compiled into a common data pool [4] and made accessible

to the concerned community (manufacturers , shippers, users, etc.) for

the purpose of ascertaining permeation performance. An assemblage of

information of this kind could also aid rulemaking efforts in attempting

to normalize such diversified community interests [4].

This investigation has not addressed the question of the dependence

of loss rate on temperature. Such studies are highly desirable for acer- taining the activation energies as a function of chain length which enters

directly into Eq.(2). Another question of equal importance is the dependence of loss rate on container wall thickness, Limited results

reported here do indicate an inverse dependence on but more extensive

and well-defined experiments need to be done in order to predict the permeation performance of full-scale containers with some degree of confidence.

12
List of References

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

Orville J. Sweeting, John Wiley & Sons, Inc., New York, N.Y. pp. 1-131

(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 53-133, 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. Barnes, J. D. and Martin, G. M. , Performance of Plastic Packaging for

HM Transporation Part IV. Permeation, Report No. DOT/MTB/OHMR-8012.

Available from National Technical Information Service, Springfield,

VA 22151.

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

Diffusion in Polymers , J. Crank and G. S. Park, Eds., (Academic

Press, New York, 1968.)

6. Bondi, A., "Physical Properties of Molecular Crystals, Liquids and

Glasses", John Wiley &'Sons, Inc., New York, NY (1968).

13
TABLE I

Polyethylene (PE) Bottles Used For Mass Loss Measurements

Density , g/cm3 Average Vol ume TYPE Stated Found Thickness, cm Capacity, m£

CPE

(Low Density)^ .92 .9235 ± .0009 .1062 ± .0190 125

LPE

(High Density)^ .94 .9453 ± .0012 .0950 ± .0068 125

"Phillips"

(High Density)*^ - .9430 ± .0030 .0671 ± .0028 125

^Nalge Company

^Phillips Petroleum Company

14
Loss

Rate

att=

50

°C

for

Various

Classes

of

Compounds

(AO)o

ca

>1

4-> •r—<AC

O) Q

-ac

fO

Q_

<L_5

>1

+J

cn

c:

O) Q

05c

fO

is

the

backbone

of

skeletal

chain

length.
16J
TABLE IV

Predicted Loss Rate at 50 °C For

High Density Bottle From Loss Rate of

Low Density Bottle (Using Fig. 11)

h Miscellaneous

Compounds

Qk

Loss Rate, (g/hr)

[High Density]

Actual Predicted

1 Acetonitrile .00210 .0013

2 1 ,2-Dichloroethane .0470 .041

3 Acetone .00450 .0041

4 Methyl Ethyl Ketone (MEK) .00820 .0077

17
o

(N

O

00

o

o

o

CO

o o o

CM T6 ‘SSO"| SSB|/\|

Time,

Hours
CSJ

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o

00

o

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6 ‘sso~| sse|/\|

Time,

Hours
o

CM

o

00

o

CM

o

CO

o

d

Time,

Hours
CPE

Bottle

LPE

Bottle

C7^

6 ‘sso| sseiAi

120

180

240

Time,

Hours
LT)

O)

o

CO

o

d

120

180

240

Time,

Hours
6 ‘ssoi sse|/\|I

120

180

240

Time,

Hours
I

o o o

CO

^

CM

6 'sso| sse|/\|

o

o

LO

o

100

150

200

Time,

Hours
00

Lo o in o

o

CD

o

cn

6 ‘sso~| sse|/\|

120

180

Time,

Hours
(jq/6) ‘ Vo 6oi

Chain

Length,
®

1,2-Dichloroethane

o

(jq/6) ‘ Vq Bon

cn

Chain

Length,
1 1 o

CM CO ^

(jM/6)

‘ Vq 6oi

Effective

Carbon

Atom

Number,
I

I