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REPORT NO. D0T/MTB/0HM0-76/5

PERFORMANCE OF PLASTIC PACKAGING FOR HAZARDOUS MATERIALS

TRANSPORTATION PART II. PERMEATION

JOHN D. BARNES AND GORDON M. MARTIN

Earner. J. D. , Martin., G. M. , Performance

of plastic packaging for hazardous

materials transportation. Part II.

Permeation, Report No. DOT/MTB/OHMO- 76/5 ,

29 pages (National Technical Information

f Service, Springfield, VA Oct. 1976). ,

311 /'?. X$S //? -?(, ?

j

rj2, l-fi#

OCTOBER 1978

FINAL REPORT

DOCUMENT IS AVAILABLE TO THE PUBLIC THROUGH THE

NATIONAL TECHNICAL INFORMATION SERVICE

SPRINGFIELD . VIRGINIA 22151

PREPARED FOR

U.S. DEPARTMENT OF TRANSPORTATION

MATERIALS TRANSPORTATION BUREAU

OFFICE OF HAZARDOUS MATERIALS OPERATIONS

WASHINGTON, D.C. 20590
NOTICE

This document is disseminated under the sponsorship of the

Department of Transportation in the interest of information

exchange. The United States Government assumes no liability

for its contents or use thereof.
Technical Report Documentation Page

1 . Report No. D0T/MTB/0HM0- 76/52. Government Accession No. 3. Recipient's Cotolog No.

4. Title and Subti tl

PERFORMANCE OF PLASTIC PACKAGINGS FOR HAZARDOUS

MATERIALS TRANSPORTATION

Part II - Permeation

5. Report Date October 1976

6. Performing Organization Code

8. Performing Organization Report No. NBSIR 76-1163 (R)

7. Authors)

John D. Barnes and Gordon M. Martin

9. Performing Organization Name and Address National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

10. Work Unit No. (TRAIS)

3110526

11. Contract or Grant No. DOT AS-50074

13. Type of Report and Period Covered Final

June 1975 - June 1976

12. Sponsoring Agency Name and Address U. S. Dept, of Transportation

Materials Transportation Bureau

Office of Hazardous Materials Operations

Washington, D. C. 20590

14. Sponsoring Agency Code MTH-1

15. Supplementary Notes

16. Abstract Permeation as a mode of failure for plastics packagings is discussed.

The materials properties which determine permeation performance are defined.

Measurement methods aimed at determining values for the materials properties

are surveyed.

A "matrix? scheme is introduced for evaluating the risks associated with

the permeation failure of a package containing a hazardous materials lading.

Permeation factors influencing reuse of plastic containers are described.

Laboratory data from an evaluation of a simple method of test for estimating

the intrinsic property of a lading to permeate polyethylene is presented.

17. Key Words 18. Distribution Statement

Hazardous materials; transportation;

plastics packagings; polyethylene;

permeation; methods of test.

Documentation is available to the U. S. public through the National Technical

Information Service, Springfield, VA

22151.

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21- No. of Pages 22. Price Unclassified Unclassified 26

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
PREFACE

This report is Part II of a series prepared for the Office of Hazardous Materials

Operations under research contract no. DOT AS-50074. It is presupposed that the reader is

cognizant of the background material presented in Part I. This part will be concerned

exclusively with the permeation aspects of the performance of plastic packagings for the

transport of hazardous materials. The authors wish to acknowledge the assistance and

cooperation of the following organizations: The Office of Hazardous Materials Operations

Staff, particularly Mr. Mario Gigliotti for his guidance to the work from the inception;

members of the Plastic Drum Institute of the Society of the Plastics Industry; members of

the Manufacturing Chemists Association; and Mr. P. E. Campbell of the Phillips Petroleum

Co. These organizations contributed valuable results from their technical experience.
CONTENTS

INTRODUCTION ’ 1

The Nature of Permeation 1

Measurement Methods for Permeation Performance 11

The Relationship Between Performance Data and Levels

of Hazard 13

Permeation and Reuse 18

Conclusions and Recommendations 19

APPENDIX - Permeability of Four-Ounce Bottles to

Selected Ladings 20

References 27

ILLUSTRATIONS

Figure Title Page

1 Permeation Failure in a Four-Ounce Polyethylene Bottle 2

2 Normalized Permeability Rate of Benzene Through

Polyethylenes of Different Densities 5

3 Time Dependence of Permeation Flux 10

4 Method of Weighing Bottles 22

5 Permeation of 0-Dichlorobenzene 26

TABLES

Table Title Page

1 General Guidelines to Compatibility of Chemicals Reagents

with Polyethylene 4

2 Units for the Transport of Matter Through Polymer Barriers 7

3 Geometrical Parameters for Various Container Shapes 8

4 Lading Characteristics by Category 14
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Tests such as the one whose result is shown in Fig. 1 offer a good simple route to the identification of materials which tend to> permeate polyethylene. The procedure used

is described in ASTM Test Method D 2684-73 "Standard Recommended Practice for Determining

Permeability of Thermoplastic Containers [2]". The experiments carried out at NBS are described in an Appendix to this report.

Briefly stated, the method consists of filling a blow molded plastic bottle with the

substance in question, sealing the bottle tightly, weighing the whole package, and placing

it in an oven at a controlled temperature. The weight change of the bottle and its

contents is measured periodically and plotted as a function of time. The steady-state

rate of weight loss (or gain) can be obtained from the plot.

The results of the experiment clearly depend on:

1. The molecular nature of the lading.

2. The characteristics of the plastic in the bottle.

3. The characteristics of the bottle.

4. The temperature and pressure at which the experiment is carried out.

In a more subtle fashion one might expect the results to depend upon:

1. The amount of lading in the bottle at the start of the experiment.

2. Whether or not the bottle had been previously used.

The packaging industry carries out tests similar to ASTM D 2684-73. Resin or container

manufacturers and large-scale shippers are generally the only companies equipped for such

tests. In such a testing environment there is little standardization of conditions and

results are usually stated in simple pass-fail terms. Each container manufacturer, resin

producer, or shipper has tended to develop his own list of ladings and packages which are "incompatible" with each other. There are also recommendations from resin manufacturers

which state that certain classes of chemicals may give problems when packaged in containers

made from certain resins.

There are useful general guidelines which can be extracted from this body of experience.

Table 1 summarizes the guidelines used by a major producer of resins. This table demonstrates the importance of the molecular structure of the lading. Table 7 gives some additional results from the NBS experiments.

It is generally agreed that the density of a given polyethylene has an important

effect on permeation rates [3,4]. This effect is due to the semicrystal 1 ine nature of

polyethylene, an interpretation based on the view that permeation takes place most readily

in the disordered matrix in which the crystallites are embedded. The higher the density,

the higher the crystal 1 inity, the smaller the "amorphous" content and, therefore the lower

the permeation rate. This effect is not one of direct proportional i ty as can be seen from

Fig. 2 [4]. In addition there are a variety of ways of achieving a given density and the curve in Fig. 2 should not be viewed as being a universal one for the system benzenepolyethylene.

In comparing the results of different permeation tests it is important to know

something about the molecular properties, especially the density, of the resin used to make the containers.

However, the fact that resin manufacturers characterize their products in terms of

density should not, as mentioned earlier, be taken to mean that any product fabricated

from that resin will have the specified density. Table 8 shows variations in density

from one part of the bottle to another. A density which is higher in the thick part of

the container than in the thin part may be attributable to slower cooling in those parts.

Similar data from a bottle made from a different resin, also given in Table 8 illustrates

that the variations among resins are generally greater than those within a given piece.

3
GENERAL

GUIDELINES

TO

COMPATIBILITY

OF

CHEMICAL

REAGENTS

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POLYETHYLENE

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RATE

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DENSITY, GMS./CC

Figure 2 Permeation Rate as a Function of Density: Benzene and Polyethylene [4].

5
At the beginning of this report the permeation rate was defined as the amount of

matter transported across a unit area of a barrier in unit time and it is this form in

which the results are given in Table 1 and Table 7. The purpose of doing so was, of course, to minimize the effect of container geometry. There are other ways to express the

results of permeation tests. These are summarized in Table 2 and the relationships among

them will be examined here briefly.

Define 0 as the steady-state rate of loss (or gain) of material from a container with

a capacity Q0

. Define F as the permeation flux. If A is the surface area of the container

and if the surface is uniform all over the container, then,

Q

= FA; (1)

i.e., the rate of loss of product is directly proportional to the area of the container.

0 is also the rate at which vapors will accumulate in an enclosed space. Naturally, as vapors accumulate on the low concentration side of the barrier the driving force for

permeation will be diminished thus F, G, and Q will decrease as the vapor concentration

approaches equilibrium. However, hazard levels due to flammable or poisonous vapor concentrations

are generally well below such equilibrium values and the effect of F and Q decreasing with

time can be ignored in this context.

It is interesting to note that Q generally exhibits no more than a weak dependence on the amount of lading in the container. Experimental verification of this observation is provided by Fig. 5 which shows that the slope of the weight loss curve does not change

even though the bottle is only half full at the end of the experiment. In order to reduce the total amount of a product lost through permeation it is, therefore, desirable

to keep the surface area of the container as small as possible.

2/3 For a container of arbitrary shape with volume V, the Area, A, is given by A=<j>V F(r).

<j) is a geometrical constant depending on the particular shape which is chosen (cube,

sphere, cylinder, etc.). The aspect ratio, r, is defined as the ratio between two principal

dimensions of the container (for example, the diameter and the height in a cylinder).

Table 3 lists <j> and F(r) for a variety of container shapes. It is easy to see that a spherical container will permeate the least for a given combination of lading and resin.

It is also easy to show that, if it is required to transport a fixed quantity of

material which permeates and all other things are equal, maximum protection against permeation

failure is gained by packaging the commmodity in the minimum number of containers.

Another important determinant of the rate of loss from a container is the wall

thickness. If G is now defined as the transmission rate for unit wall thickness and the

container has a wall thickness t then, Q

= GA/£. The parameter G defined in this way is

a characteristic of the container material and the lading taken in combination. G can be

solved for by computing

( 2 )

and can be presumed to have some universal meaning only to the extent that the material is homogeneous. Note that the dimensions of G as defined in this way are: amount of substance

per unit area per unit time times unit length. In industrial practice values of G are quoted for a standard unit of length, say 1 mm, and values of G for barriers of different

thickness are obtained by dividing G for the standard thickness by a, the desired thickness.

The phrase "amount of substance" deserves some comment at this point. For gases the quantity

of substances is usually expressed as cubic centimeters of gas at standard temperature and

pressure (cc(STP)). For liquids one might use grams as the measure of "amount of substance".

The SI system of units provides for amount of substance to be expressed in "moles" and

that is how Tables 1 and 7 are expressed.

6
TABLE' 2

UNITS FOR THE TRANSPORT OF MATTER THROUGH POLYMER BARRIERS

Quantity Symbol Units Functional Depends On

Mass Transfer 6 Moles/Time Size and shape

OF CONTAINER.

VALUE OF F

Permeation Flux F Moles/Time/Area f_q f“A Thickness of

WALLS. *}

VALUE OF G

Transmission Rate G Moles/Time/Area

a STANDARD

THICKNESS

G=F* Pressure

DIFFERENCE.

VALUE OF P

Permeability

Coefficient

P Moles/Time/Area/

Pressure a STANDARD

THICKNESS

P=_G_

prp2

Nature of lading.

NATURE OF RESIN

TEMPERATURE

7
GEOMETRICAL

PARAMETERS

FOR

VARIOUS

CONTAINER

SHAPES

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to a barrier of standard thickness. '

Certain of the processes used to form polyethylene containers virtually guarantee

that homogeneity will not be maintained. For this reason care must be taken in using

eq. (2) to reduce results from different containers to a common footing.

As mentioned earlier chemical activity differences across the barrier provide the

driving force for permeation. In the case of gases the pressure and in the case of

liquids the vapor pressure are appropriate measures of activity.

This leads to the definition of yet another type of coefficient, called the permeability coefficient, and given the symbol P. The definition of P is;

where p] and P 2 represent the (vapor) pressures on each side of the barrier. To the

extent that the material is homogeneous P can be regarded as a parameter which is characteristic of the particular combination of lading and resin under consideration. It

is the most desirable way to express the results of permeation measurements.

It is common industrial practice to express the results of permeation tests in terms

of the percentage of the original mass which is lost over some period of time (for example,

1 year). This usage measures the economic impact of the permeation loss on the consumer,

who is getting less product than he believes he is getting. Permeation results expressed in

percentages are, however, irrelevant to assessing the hazards due to the release of hazardous

materials through permeation. In this latter instance it must be remembered that, to a first approximation, material permeates out of a plastic container at the same rate whether

the container is full or nearly empty.

Those who claim that filling a container half full (or less) is a way to "accelerate"

a permeation test are obviously victims of the fallacy of expressing test results in percentage terms.

The discussion up to this point has treated permeation as a steady-state phenomenon.

It has been assumed that one is primarily interested in the regime represented by the

straight-line portion of responses such as those depicted in Fig. 5. Indeed, such features

are of primary importance when one is dealing with relatively thin-walled containers.

Fig. 3 depicts the time dependence of permeation flux through the walls of a container

which has not been exposed to the lading for t < 0. To be noted is the fact that there is an initial period in which there is only a very small flux of material. This is followed by

a rapid rise in the flux which is followed, in turn, by a leveling off at the steadystate value. The time to reach a value of 1/2 the steady-state flux, t-]^, will be

called the "induction time" in what follows. Other authors [6] refer torelated times as the "time lag".

The induction time may be on the order of weeks for some combinations of ladings

and containers. To a first approximation the induction time varies directly as the

square of the wall thickness and inversely as the "diffusion coefficient" D. D is a parameter similar to P in that it depends upon the combination of lading, resin and

temperature.

This characteristic induction period is one aspect of the protection which a container affords. Of course, once the container has been exposed to the lading the

induction characteristics are altered. For this reason containers which are being

reused should be regarded as not having an induction time.

Up to this point the discussion has centered around the transport of matter through

the container walls. Another important problem is the uptake of material into the

container walls. If C is used to denote the concentration (in moles/unit volume) of

permeant in the container material after an infinitely long exposure, S the solubility.

9
Figure 3 Time Dependence of Permeation Flux. F^ is the Flux Value Obtained After

an Infinite Time.

10
and p is, as before, the (vapor) pressure of the permeant then a relationship C=Sp

(known as Henry’s Law) is frequently valid. Lf the concentration is measured as a function of time one finds that the time-scale of the experiment is governed by the

induction time discussed earlier. Desorption of material from the container walls is simply the reverse of absorption (or uptake) and is, therefore governed by the same

consi derations.

It is easy to show [6] that the permeability, P, is related to the diffusion coefficient,

D, and the solubility, S, by;

P = DS (4)

This relation emphasizes that, for permeation to take place a lading must both

diffuse into and be absorbed by the container material. Measurements of D and S require

relatively sophisticated experimental methods. For this reason, eq. (4) does not appear

to be a very good route to the estimation of permeability coefficients.

There are a number of calculations related to container performance which can be

carried out once values of D and S are known. Persons who wish to estimate permeation

performance would, of course, have to be familiar with these ideas. The interested

reader will find these topics dealt with in detail in the books by Crank [7] and Crank and

Park [6].

The evaluation of permeation performance involves answering two questions: 1) What

is the tendency of a given lading to permeate polyethylene? and 2) How well does a container of a particular design resist permeation?

Fortunately, the answer to the first question is largely independent of the answer to

the second. In more concrete terms this means that a ranking scheme in which a measured

quantity, say P, is used to rank permeants with respect to a given resin is unlikely to be

altered by changing resins. If one assumes that the number of ladings of interest is N and that the number of container designs is equal to M then under the "uncoupling" assumption most of the information needed to characterize a N x M combinations can be obtained

from a number of measurements proportional to N + M. Neither N nor M have to grow very

large before important economic advantages can be realized.

The results obtained in studies leading to the "Permachor" method of estimating

permeation in polyethylene [8] demonstrate that such an approach may be feasible.

The outline of permeation phenomena presented above has necessarily been very sketchy.

Such topics as non-Fickian behavior and the concentration dependence of D and S have not

been discussed although they may be very important for certain hazardous materials. The

attempt which has been made has been to provide a basis for a quantitative approach to permeation performance.

MEASUREMENT METHODS FOR PERMEATION PERFORMANCE

The previous discussion has outlined some quantitative features of permeation performance and some parameters which influence such performance have been identified. The

question to be addressed in this section is "How can one actually measure permeation

phenomena in the laboratory?".

Conceptually, the problem is very simple. One needs 1) a source of permeant 2) a means of supporting the barrier material and 3) a means of measuring the amount of permeant

which has passed through the barrier in a given time interval. A certain amount of auxiliary

equipment for maintaining temperatures and protecting personnel will also be required.

Actual measurement systems can cost from a few thousand to many tens of thousands of

dollars. A simple system could be set up using equipment and procedures similar to those

given in the Appendix. NBS experience indicates that values of P accurate to 10% or better can be estimated from such experiments. Further testing is presently being conducted

with the goal of establishing standard conditions for such a test. For convenience, this

test will be referred to as "the bottle test".

1

1
The term "bottle" is here used to indicate any free-standing object fabricated (except

possibly for the closure) from the resin of interest. In the event that the specimens of

barrier material which are available do not lend themselves to such treatment the "cup"

test used for Water Vapor Transmission Rate (ASTM E 96) [2] could be used with suitable

modification. This test is also a mass-loss test.

While the bottle test or the cup test yield quite satisfactory values for P they do

not, in general, give good values for t-j /2 * This 1S because the balances which are used

to weigh heavy objects lack sensitivity to small mass changes.

To gain the required sensitivity one must go away from mass loss as the quantity to

be measured and use a detector which signals the appearance of minute amounts of permeant

on the outside of the "bottle".

In discussing detection systems one can divide the available range into two parts:

1) those sensitive to total quantity of material and 2) those sensitive only to selected

components. Perhaps the simplest form of the former type of detector is represented by

the Dow Cell (ASTM D 1434) [2] used to measure gas transmission rates of film materials.

Modern adaptations of the Dow Cell afford a high degree of sensitivity [9] and further

research is underway to determine how useful it can be for organic vapors as permeants.

One example of the more specialized type of detection system is the Modern Controls

OXTRAN [ 1 0] 2 system in which permeating gas is passed through a fuel cell and the current

resulting from the presence of 0^ is measured. This system is in wide use in the packaging

industry.

Other approaches use chromatographic or photometric [11] detection systems which can

be "tuned" to the compound of interest. This latter feature is of interest when dealing

with mixtures. Such systems have not yet become commercially significant. Up to this

point the emphasis has been on the direct measurement of the permeability coefficient P. It should be noted, in passing, that P can also be calculated from measured values of D and S in eq. 4. Measurements of sorption and desorption kinetics provide an alternative

route to the estimation of D and S [6]. The sorption and desorption methods may become

the methods of choice when only small quantities of polymer are available or it is desired to limit the amount of the lading which one is working with.

Underlying any measurement system is the concept of standards. Data must be obtained

and reported in a fashion such that comparisons with other data are meaningful.

In the case of permeation data this means that the quoted values should be obtained

using resins (and fabrication procedures) which are carefully standardized and carefully

characterized. This would allow for a quick and easy comparison of one permeant as being more of a problem than another.

In order to accomplish this degree of standardization it is desirable for some one party to obtain a large supply of well-characterized resin similar to those used in actual

containers and, using carefully controlled processes, fabricate standard specimens, possibly

in the form of bottles, from this resin. Careful characterization of a sample of the

resulting specimens could be done so that users would be able to check their own test

facilities and procedures. The remainder of the supply of specimens would be made available

to persons who need to make permeation tests as they need them. The suggestion that the

standard specimen be available in the form of bottles is based on the idea that tests such

as ASTM D 2684 offer the most readily available route to a widely-used, standards based

system of performance measurement for permeation.

2 The identification of products of specific manufacturers is intended only in specifying

experimental conditions. Such identification does not, in any way imply endorsement by

the National Bureau of Standards.
The supply should be large enough to permit measurements using the original batch of

bottles over a long period of time. Due to the wide range of resin compositions in industrial use it may be necessary to provide 'standard specimens made from more than one

kind of resin. Tests aimed at establishing specifications for standard bottles are currently in progress.

The above survey of test methodology is intended to indicate that the principles of

permeation methods are widely understood. The number of methods in common use is small

and instrumental techniques tend to be rather specialized.

A need does exist to develop flexible, accurate instrumental methods for use in an industrial environment. A necessary corollary to the development of such methods is the

development of standards for use in calibrating the instruments and characterizing the

measurement systems. The ASTM, SPI and other elements of the consensus standards system

should be persuaded to adopt a wider variety of standard methods of test for permeation of

substances other than gases. These same bodies should also be encouraged to incorporate

permeation tests into their specifications for resins.

Another important area of measurement concern is in the non-destructive evaluation of

containers for reuse. Here the need is to identify chemical species which are absorbed in

the container walls. Such species may, in time, desorb and contaminate a lading with

disastrous effect.

From an instrumentation viewpoint the problem is one of spectroscopy or chromatography.

Special fittings and vacuum systems could be devised which would sweep vapors from a 55-

gallon drum into a trap. The substance in the trap could be vaporized for subsequent

analysis by gas chromatography or infrared spectroscopy. The objection to these methods

that they require skilled personnel is a very valid one and there may not be sufficient

interest to justify the considerable expense.

The question as to whether a particular lading will be absorbed in the container

walls and how long it will take to desorb can be answered quantitatively by using values

of D and S from permeation experiments and qualitatively by observing that any substance

with a reasonable value of P is bound to be absorbed.

In this brief discussion of measurement methods the emphasis has been on the evaluation

of P(or G) as materials properties. In principle once P or G is known for a given combination

of resin and lading the performance of a container made from the given resin can be calculated

by straightforward engineering procedures. The next report in this series will give some

examples of how this is done.

More subtle problems arise when one tries to calculate what G would be in a certain

resin (say resin Y) when its value in another resin (say resin R) is given. There is a reasonable likelihood that such measurements can be made reliably enough for transportation

purposes. There is work in progress which is evaluating literature proposals on the

subject [8].

THE RELATIONSHIP BETWEEN PERFORMANCE DATA AND LEVELS OF HAZARD

In the earlier parts of this chapter a simple method for evaluating the tendency of

a lading to permeate polyethylene, the "bottle test", was described. It was further

indicated that the relative permeation performance of particular container designs could

be evaluated using a relatively small number of tests with standardized ladings on actual

shipping containers. In this section the two aspects of the problem will be combined and

a demonstration will be made of how such results can be used to establish levels of

hazards. Decisions as to whether such hazards are or are not acceptable in transportation

and what steps must be taken to regulate them must be of necessity, be left to OHMO.

In assessing levels of hazard the first aspect must be the characteristics of the

lading. Table 4 shows how ladings may be grouped into several (in this case 5) classes

(categories). This grouping is dependent on two factors: 1) how rapidly the lading

permeates polyethylene and 2) how much of it can be tolerated outside the container in an enclosed space. The first aspect is directly related to the G value. The second aspect

is established from a knowledge of the toxicity or other hazards of the material. OSHA,

FDA, or EPA may well be involved in the setting of such levels.
TABLE A

LADING CHARACTERISTICS BY CATEGORY

Category No. Description

0 Permeation through a standardized

REFERENCE CONTAINER IS TOO LOW TO

BE DETECTABLE. REASONABLE QUANTITIES

OF THE MATERIAL CAN BE TOLERATED

OUTSIDE THE CONTAINER.

1 Permeation rate is measurable.

Scaling laws indicate that a soundly

ENGINEERED SHIPPING CONTAINER WILL

KEEP RELEASE AT A MINIMUM,

2 Permeation results in release which

could be annoying. Some care should

BE TAKEN IN SELECTION OF A SPECIFICATION PACKAGE,

3 Permeation leads to releases which

COULD CAUSE PROBLEMS. EXCEPTIONAL

ENGINEERING AND HANDLING REQUIREMENT

CAN BE IMPOSED WHICH BRING RISKS

WITHIN ACCEPTABLE BOUNDS,

4 A PLASTIC CONTAINER WHICH WOULD KEEP

RELEASE OF THIS MATERIAL WITHIN

ACCEPTABLE LIMITS IS NOT AVAILABLE.

14
The following scenario illustrates the measurements and calculations which must be

carried out in assigning a lading to a particular category (category 2 is this example).

These tests are similar in spirit to those used to classify substances as flammable

(e.g., 49 CFR 173.119). Measurements are made using a standardized reference container at

60 °C. The rate of weight loss in the steady-state is obtained. The permeation characteristics

of the subject lading are found to be intermediate between two of the standard ladings

used to compare actual shipping containers. Based on this data an estimate of the amount

of material which will be released into a standard trailer body during a 48 hour period

from a legal-limit load comprised of 30 gallon Spec. 34 drums is formed. If the resulting

concentration is less than 1/10 of the level set by OSHA for a 20 minute exposure to the

vapor but is greater than 1/1000 of the OSHA level then the lading shall be assigned to Category 2.

The above is merely an illustrative scenario and not a specific proposal. A great

deal of work must be done before any such proposal can be made. The ranking in Table 4 gives a rough indication of the degree of risk associated with failure because of release

through permeation. The higher the ranking, the greater the risk. In this respect the

ranking system parallels that established by NFPA for evaluating the flammability, health

and reactivity hazards of materials [12].

The reactivity question was omitted from Table 4 in order to keep the table simple.

In any final analysis the risk associated with a given lading becoming contaminated as a result of permeation of oxygen or water (to name only two common contaminants) into the

package must be made a part of the ranking scheme.

Table 4 is qualitative in nature and is intended only to convey the range of situations

which exist with respect to ladings which have been or might be contemplated for transportation

in plastic packagings.

The other side of the question, the performance of actual shipping containers is addressed in Table 5. The title of the table uses the word "package" to convey the idea

that the container must be tested together with its overpack where appropriate.

The tests which are used to establish which category a design should fall into are carried out using certain standardized ladings. The higher the category that a container

is assigned to on the scale of Table 5, the more stringent are the performance requirements

imposed on it. Category 4 containers might involve such special engineering treatments as surface sulfonation [13] or layers of material with improved barrier properties [14].

Category 3 might include some of the better containers covered by the present Specification 34.

Table 5, like Table 4, is only intended to give some idea of the range of choices

which are available. Taking each aspect by itself there is no way to assess the hazards

presented. A substance which presents serious problems in terms of permeation can be

shipped safely if a properly designed container is used. On the other hand, a container

with low performance should perhaps not be used to ship commodities which pose even minimal

hazards.

These ideas are summarized in Table 6. This table is in "matrix" form with the

lading categories labeling the columns and the container categories labeling the rows. At

the intersection of each row and each column is a number which categorizes the level of

risk presented by the particular combination of package and lading. Generally speaking

the higher the level of risk derived for a given combination of package and lading using

this "matrix" procedure the more drastic are the consequences of the failure of the

combination.
TABLE 5

PACKAGE CHARACTERISTICS BY CATEGORY

Category No,

0

Description

Permeation tests with standard

LADINGS INDICATE THAT THIS CONTAINER

IS A POOR BARRIER,

Permeation tests indicate that this

CONTAINER WILL MAINTAIN RELEASE

WITHIN ACCEPTABLE LIMITS FOR A LOWHAZARD MATERIAL,

Such packages represent good sound

INDUSTRIAL PRACTICE, PERMEATION

TESTS SHOW THAT RELEASE IS WITHIN

ACCEPTABLE LIMITS FOR ALL BUT THE

MOST TROUBLESOME LADINGS,

Such packages represent the best

LEVEL OF NORMAL INDUSTRIAL PRACTICE.

Resin selection, fabrication

PROCEDURES, QUALITY ASSURANCE

PROCEDURES, ARE ALL ABOVE THE NORM.

Special engineering steps have been

TAKEN TO LIMIT PERMEATION. SUCH

STEPS MAY ONLY HELP FOR ONE CLASS OF

ladings, Container is intended for

"Special Situations", or "State of

the Art"

i 6

PROBLEMS,
TABLE F

RISK LEVEL AS A FUNCTION OF LAD I NO AND PACKAGE

Lading

Package 0 1 2 3 4

0 0 5 5 5 5

1 0 3 3 5 5

2 0 2 2 4 5

3 0 1 2 4 5

4 0 1 2 3 5

Category

0

Description

No PERMEATION HAZARD IS PRESENTED BY SUCH A COMB I NAT

OF LADING AND PACKAGE.

1 Ordinary care in handling and storage should be sufficient

TO GUARD AGAINST RELEASE BY PERMEATION.

2 The set of measures which are ordinarily taken for

MATERIALS WHICH ARE KNOWN TO BE HAZARDOUS SHOULD BE

EXERCISED.

3 The potential exists for serious risk to life or property

IN A LIMITED NUMBER OF SPECIAL SITUATIONS. CARE SHOULD

BE TAKEN TO AVOID THESE.

4 The potential for risk to life and property is relatively

high. Choices with regard to transportation of the

COMMODITY ARE LIMITED. SPECIAL VIGILANCE MUST BE

MAINTAINED.

5 Such situations present unacceptable risks.
Once again this table is intended only to illustrate a range of available alternatives. It is easy to appreciate that there may be a large number of borderline cases where a particular combination should be moved up or down one notch. This system as outlined does provide for advances in technology which introduce improved products. It also affords a relatively straightforward guide for making the decisions which must sometimes

be made when new information about a particular combination of package and lading becomes

available as, for example, when a substance is discovered to have a previously unknown

toxicity.

PERMEATION AND REUSE

The previous section of this chapter was a demonstration of one possible way to incorporate performance data into a system for assessing hazards. One aspect of the

problem which was not considered was reuse.

From the permeation point of view the most obvious difficulties are caused when a substance which has been absorbed in the container walls from one episode of filling,

shipping, and storage is desorbed to contaminate a (possibly different) lading in a subsequent episode. It is fair to say that this problem will exist with any substance

which permeates polyethylene to any extent at all. Whether the condition poses a hazard

or not is a function of the properties of the contaminated lading and the use to which it

is to be put. If, for example, the lading is a food product (as some hazardous materials

are) then any substance which desorbs into it from the walls of the container may fall

within the purview of the Food and Drug Administration.

The entire question of reuse is centered on 49 CFR 178.19 (Specification 34) which

provides for an all-plastic container which is reusable but which does not limit the

conditions under which the container may be reused. The guidelines for reuse laid down in

49 CFR 173.28 and 49 CFR 173.24, since they are strictly qualitative, are difficult to comply with. It is possible to propose methods for non-destructive evaluation of residual

permeants in containers. Such methods could be based on chromatographic or spectroscopic

detection. The dairy industry is presently using a system based on a flame-ionization

detection principle for examining milk containers which have been returned for refilling

[15].

The number of possible combinations of ladings and contaminants is so large that a case-by-case examination of the problem is impossible. Some general guidelines can, however, be offered.

First of all the risk resulting from contamination of the lading must be taken into

account when a reusable container is being considered. This could well result in the

assignment of a lading to a higher risk category than it would otherwise receive. Secondly,

the container performance level may have to be downgraded when a container which may have

been exposed to a different lading is to be reused to ship a lading which is sensitive to contamination. The "matrix" procedure outlined in Table 6, should result in a higher

level of risk being assigned to such reuse situations.

If shipping containers are placed in dedicated service, i.e., used to ship only one

lading and returned to the shipper for refilling, the risk factor arising from contamination

of the lading may be reduced. The risk factor is bound to increase even when a given

container is reused in dedicated service for the simple reason that no one can be absolutely

certain that the container will not be diverted to some other use through ignorance or inadvertence. It should also be remembered that there are cases where the induction time

effect provides an appreciable part of the protection against permeation. In such cases a reused container is bound to provide less protection than a fresh one. On this basis the

conclusion that reuse of plastic containers for hazardous materials offers significantly

increased risks appears to be inescapable. These increases in risk are associated, in large measure, with the permeation properties of polyethylene. Thus, a screening system

for evaluating the tendency of ladings to permeate polyethylene would serve to identify

ladings which would cause problems in reusable containers. Similarly performance criteria

which would evaluate container designs with respect to their qualities as barriers to permeation would also serve to identify designs which would be relatively more satisfactory

in a reuse environment.

18
It should, however, be pointed out that the performance criteria used in setting risk

levels in the preceding section are largely based on the materials properties P or G. In the evaluation of absorption and desorption phenomena the diffusion coefficient D is responsible, together with the wall thickness i, for regulating the time-scale of the

processes. Therefore, cleaning methods which would seek to make contaminated containers

available for reuse must be based on somehow increasing the value of D. This can be

accomplished by raising the temperature, softening the polyethylene using solvents, or converting the absorbed species to one with a larger value of D. In the latter two approaches steps must be taken to remove the agent used for the purposes mentioned. Any

such reconditioning procedures would necessarily include characterization of the amount

(and possibly the chemical nature) of the permeant remaining. As pointed out earlier in

this report inexpensive equipment usable by unskilled personnel is not currently available.

The number of substances which permeate polyethylene is large enough to render any

container which has been in reuse for a significant period of time practically uncharacterizable as to the various permeants which may be absorbed in its walls. Furthermore, there

is no evidence that procedures can be developed which can materially aid in the removal of

absorbed materials from the walls of a container without compromising the container.

These findings raise serious doubts as to whether indiscriminate reuse of plastic

shipping containers such as is encouraged by the existing Specification 34 and the exemptions based thereon is in the public interest.

CONCLUSIONS AND RECOMMENDATIONS

This report has sought to demonstrate that a performance measurement system using

very simple ideas and techniques can be devised to evaluate hazards arising from permeation

failure of plastic shipping containers.

As a first step in this process, permeation failures, which are somewhat peculiar to plastic containers, must be identified. Some examples of failures produced under laboratory

conditions were described. As the next step in the process of constructing a performance

measurement system, a simple method of test, the "bottle test", which has already been

developed by others for similar purposes was described. This presentation was followed by

a brief outline of some other ways to measure permeation. The question of how to estimate

the extent of risk arising from permeation was then addressed by introducing a "matrix"

scheme which combines the characteristics of the lading and the container, determined

separately, into a ranking which represents the level of hazard.

Once a performance measurement system has been constructed it should be incorporated

into the regulatory system. CFR 49 does not at present, address itself to the problem of

permeation in such a way as to permit the use of measurement results.

The ladings presently being contemplated for shipment in all-plastic containers under

existing exemptions include flammable organic solvents whose permeation rate with respect

to polyethylene can be shown to create a hazard. This alone is enough to demonstrate that

the approach presently being used to evaluate these risks is seriously deficient.

Any system which is devised should call for the reporting of quantitative results

obtained under carefully controlled and standardized conditions. Under such a system

there would be less need to periodically "rediscover the wheel" and decisions could be

taken more quickly and in greater confidence that they reflect field experience.

There is a need for standards at each step of the process. Further work, which is currently in progress, is expected to provide the needed standards. In addition to the

work aimed at providing standards studies of the permeation behavior of mixtures and of

the effect of permeants which alter the materials properties of polyethylene should be

made. The present work which has been concerned with individual reagents addresses only

the tip of the iceberg.

19
APPENDIX

PERMEABILITY OF FOUR-OUNCE POLYETHYLENE BOTTLES TO SELECTED LADINGS

INTRODUCTION

The purpose of these measurements was to assess the suitability of ASTM D 2684-73

(Standard Recommended Practice for Determining Permeability of Thermoplastic Containers)

or suitable modifications thereof as a means of screening ladings for their level of

activity as permeants in polyethylene. The preliminary measurements reported herein were

intended first of all to reveal the range of variation to be expected in the transmission

rate, G. A secondary purpose was to determine if suitable results could be obtained with

inexpensive equipment.

EXPERIMENTAL PROCEDURE

Four-ounce blow-molded polyethylene bottles with screw-on caps were tested with

various ladings. Since explosion proof equipment was not available, no ladings with low

flash points were used. A wire loop was attached around the lower neck of each bottle as shown in Fig. 1. The bottles could be weighed empty and also with the wire loop in place.

The bottles were filled about three- fourth 's full in a fume hood. One hundred to two

hundred grams of reagent were required for each bottle. The caps were screwed on with a ring of silicone rubber adhesive inside to form a gas tight seal (except that an epoxy

seal was used on the bottles containing hydrofluoric and hydrochloric acid).

After allowing the sealent to cure for an hour or two the total weights were taken

and the bottles were placed inside an oven constant temperature chamber with forced air

circulation operating at the desired temperature. Two empty bottles were also placed in

the oven as a control. A pipe was connected from the vent on the incubator to the laboratory air exhaust system to keep the oven under slightly reduced pressure, so that the

permeating vapors would not spread throughout the laboratory. The bottles were set in

glass crystallization dishes to protect the heating coil at the bottom of the oven in case

of bottle failure.

Tests were run at temperatures of 40, 50 and 60 °C. Each lading was run at two

different temperatures depending upon its permeation rate. (See Table 7). The first

effect was a buildup of pressure inside the bottle, but as the material permeated a reduced pressure resulted.

The bottles were transferred one at a time to a second smaller oven operated at the test temperature for weighing. A trip balance on top had a wire hook passing through a tube into the chamber (see Fig. 4). Where hazardous materials which permeated at a rapid

rate were involved, safety glasses, rubber gloves, and a respirator were worn during the

transferring operation. Weights (to the nearest tenth of a gram) and the times were

recorded for periods of a week or less up to a month or more depending upon the rate of

permeation.

This experimental method did not conform to that set forth in the ASTM procedure in

certain details:

(1) Rather than taking the bottles out and allowing them to cool before weighing,

they were weighed at the test temperature in a second oven. In this way,

weights could be taken more frequently without influencing the test time.

(2) A less sensitive balance reading only to tenths of a gram was used. This

corresponds to 0.1% for 100 grams of solution and should be sufficiently accurate

for this purpose.

(3) The bottle caps were sealed with a silicone rubber adhesive rather than a plastic to plastic heat seal.

20
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TRIP

BALANCE
(4) The laboratory temperature was maintained at 22.5 to 0.5 °C, but no provision

was made for controlling the relative humidity to 50 ± 5%.

(5) Each bottle was set in a glass crystallization dish rather than having one tray below all the bottles. This is a minor modification.

(6) Two bottles, rather than three, were tested for each condition since it was desired to survey a number of materials.

Two different types of bottles were tested. Dimensional measurements were made on two bottles of each type with a vernier calipers. Then the bottles were cut up and the

thickness measured with a micrometer. The densities of pieces cut from different areas were measured by hydrostatic weighing (ASTM D 792, Method A-l). These values are summarized

in Table 8.

As discussed in this report and shown in Table 2, the mass transfer rate Q is related

to the transmission rate G by:

<>- e i(A-l)

where A is the area and 2 is the thickness. The bottles used in this experiment did not,

however, possess a uniform thickness. Thus A and 2 in eq. (A-l) must be treated as "effective

area" and "effective thickness". The density, while not strictly uniform throughout the

walls of the bottle, was sufficiently uniform to permit the assumption that G is everywhere

the same. Under this assumption Q could be written as:

Q

= G (A-2)

where B, S. T, N. and C stand for bottom, side, taper, neck and closure, respectively.

Each of these components could be treated as a simple geometrical shape. The bottom and

closure as discs, the side and neck as cylinders and the taper as the frustum of a cone. The individual ratios (A/2.) may be added to get an equivalent value for the entire bottle:

For a type #1 bottle

2 155.3 cm /mm and for a type #2 bottle

(A-3) —^E 2 = 178.7 cm /mm.

t The value of the equivalent area for a bottle depends upon what value is used for the

equivalent thickness. Since almost 90% of the mass transfer takes place through the

sides, it seems reasonable to use the average thickness of the sides to calculate an equivalent area. For a type #1 bottle with sides 0.876 mm thick this equivalent area is

136 cm^. p or a type #2 bottle with sides 0.698 mm thick the equivalent area is 125 cm2

. In ASTM D 2684-73 the area of a standard container is given as 154 cm2

. The thickness is not specified but is to be calculated from the area and weight of the bottle and the

density of the material in it.

23
TABLE 8

PHYSICAL CHARACTERISTICS OF BOTTLES

Type £1 Type #2

Avg. Wt. Std, Dev. Avg. Wt. Std. Dev.

g g g g

Bottle Alone 16.56 0.14 12.37 .03

Bottle & Cap 28.95 0.11 25.78 .06

Dimensions in cm (see Note 1)

O.D. Neck "d" 2.1 2.0

O.D. Bottle "D" A, 70 A. 51

Overall height "L" 10.9 10.3

Height to base neck "H" 9.2 8.9

Height to taper "h" 8.1 7.7

Thickness in mm (see Note 2)

Bottle #35 Bottle #36 Bottle "A" Bottle "B"

Below Neck 2,57 1.70 1.73

Side 0.89 .86 0.71 0.69

Bottom 2.54 2.59 1.61 1.70

Density in g per CM''* (see Note 3)

Below Neck 0.9190 0,9190 0.9A37 0.9A29

Side 0,9171 0,9169 0.9A06 0.9A05

Bottom 0.9192 0.9179 0.9A3A 0.9AA6

(1) These values are accurate to + ,05 cm.

(2) Errors in thickness values are Less than 0,050 mm.

(3) Density values are correct to + .000A.

24
RESULTS

Type #1 bottles were made of a low density semi-transparent polyethylene. They were

slightly larger and had thicker walls than type #2 bottles which were made of a high

density polyethylene. In each case the bottle was thicker at the neck and tapering portion

below the neck and at the bottom than it was on the sides. For both types of bottles the

density of the thicker sections was greater than that of the thinner ones. This is probably

due to the fact that the thicker areas cool more slowly after molding and have more time to crystallize than the thinner regions.

After an induction period which was too short to observe in most cases, a plot of the

observed weight against time gave a straight line within experimental error (see Fig. 5).

From the slope of a straight line fitted to the experimental points by eye, the rate of

weight loss could be determined. The results are shown in Table 7. Dividing the rate of

weight loss in grams per day by the molecular weight gives the mass transfer rate Q in

moles per day. When Q is divided by the equivalent area of the bottle and multiplied by

the thickness corresponding to this equivalent area, the transmission rate G, which is also

shown in the table, is obtained.

The lowest permeation rates among the chemicals tested were hydrofluoric acid, hydrochloric acid, and butyl stearate, while the highest rates were for dichlorobenzene and

trichlorobenzene. The permeation rate increased with the temperature and there is some indication that the variation in rate from bottle to bottle was greater for larger values

of G.

CONCLUSIONS

The experiments whose results are summarized in Table 7 were limited, by safety

considerations, to a few ladings whose risks, particularly in terms of flammability were

judged to be acceptable.

The high permeation rates for trichlorobenzene and o-dichlorobenzene are roughly in

agreement with the guidelines in Table 1. The same is true of the values for hydrochloric

and hydrofluoric acid.

The fact that tetradecane shows quite an appreciable permeation rate in spite of its

great chain length indicated that saturated hydrocarbons such as are found in petroleum

products would have quite high permeation rates. More detailed studies, aimed at evaluating the usefulness of the "permachar" [8] correlation are presently underway.

The reagents chosen for these experiments are of rather limited industrial interest

when compared with others such as benzene, xylene, toluene, and cyclohexane. These latter

substances will have to be tested when a more satisfactory oven becomes available.

Many more measurements will be needed to establish the amount of variability to be

expected from the bottles and the best methods for making a satisfactory seal. The tem- peratures used in this experiment were within those considered to be applicable for brief

exposures during transportation. In order to make more realistic calculations which would

predict packaging performance over longer time periods it is necessary to extend the

measurements to temperatures in the range of 20 °C.

25
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26
REFERENCES

[1] References of this type refer to paragraphs in "Code of Federal Regulations,

Title 49, Transportation Parts, 100 to 199". The revision date appropriate

to this report is October 1975.

[2] "1976 Annual Book of ASTM Standards Parts 35 and 36", Pub. 1976 by American

Society for Testing and Materials, Phila. Pa.

[3] Michaels, A. S., et al . , Journal of Applied Physics, 35, 3165 (1974).

[4] Campbell, P. E., "Plastics in Drug Packaging" talk presented at 19th National

Meeting of the Academy of Pharmaceutical Sciences", Nov. 16-20, 1975.

[5] Stull, D. R., "Vapor Pressure of Pure Substances: Organic Compounds", Industrial

and Engineering Chemistry, 39, 517-540 (1947).

[6] Crank, J. and Park, G. S., "Methods of Measurement", Chapter 1 in book, "Diffusion

in Polymers", J. Crank and G. S. Park, pp. 1-37 (Academic Press, New York, 1968).

[7] Crank, J., "The Mathematics of Diffusion", book, Clarendon Press, Oxford (1974).

[8] Salame, M., "The Prediction of Liquid Permeation in Polyethylene and Related

Polymers", SPE Transactions, 1_> 153-163, (1961 ).

[9] Pye, D. G., Hoehn, H. H., and Panar, M., "Measurement of Gas Permeability of

Polymers. I. Permeabilities in Constant Volume/Variable Pressure Apparatus",

Journal of Applied Polymer Science, 20, 1921-1931 (1976).

[10] Wood, R. C., private communication.

[11] ASTM F 372-73, "Standard Method of Test for Water Vapor Transmission of Flexible

Barrier Materials Using An Infrared Detection Technique", 1976 Annual Book of ASTM

Standards, Part 21 pp. 367-372. Pub 1976 American Society for Testing and Materials,

Phila, Pa.

[12] Recommended System for the Identification of the Fire Hazards of Materials

NFPA No. 7047, 1969 Edition. Pub. 1973 by National Fire Protection Association,

Boston, Mass.

[13] Thomas, L. S., et al., "The Effective Use of Surface Sulfonation as a Barrier

System for HDPE Gas Tanks", paper presented at Society at Plastics Engineers

regional meeting in Houston, Texas, March 11-12, 1975.

[14] Speas, C. A., "Packaging Food Products in Plastic Containers", from Chemistry of

Food Packaging, Advances in Chemistry Series No. 135, Pub. by American Chemical

Society (1974).

[15] Walker, D., unpublished correspondence.

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