lEPORT NO. DOT/MTB/OHMO-76/4 f(J 0S}^ ~IG>-

PERFORMANCE OF PLASTIC PACKAGING FOR

HAZARDOUS MATERIALS TRANSPORTATION

PART I. MECHANICAL PROPERTIES

JOHN M. CRISSMAN, CHARLES M. GUTTMAN, AND

LOUIS J. ZAPAS

Crissman, J. M., Guttrnan, G. M., Zapas, L.

J., Performance of plastic packaging for

hazardous materials transportation. Part

1 . Mechanical properties. Report Ko.

DOT/MTB/omo-76/4, 51 pages (U.S.

Department of Transportation, Materials

Transportation Bureau, Office of Hazardous

Materials Operations, Washington, DC

1976 ).

311

1 1 19 u I I ^ iw Kj m J 9 puDiic rnrougn

the National Technical Information Service,

Springfield, Virginia 22161 .

Prepared for

U.S. DEPARTMENT OF TRANSPORTATION

MATERIALS TRANSPORTATION BUREAU

OFFICE OF HAZARDOUS MATERIALS OPERATIONS

WASHINGTON, D.C. 20500
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.
lechnical keporf Documentation Page

1 . f< epor t No. Dar/HTB/OHMO-76/4

2. Government Accejsion Nc

4. Title ond Subtitle

PERFOFMAl^JCE OF PIASTIC PAC3<AGn']G FOR HAZARDOUS

MATERIALS TRANSPORTATION; PART I MECHANICAL

PROPERTIES

7. Author's) John M. Crissman, Charles M. Guttman & Louis J. Zapas

3. Reciptent's Co^olog No.

5. Report Date October 1976

6. Performing Orgonizotion Code NBSIR 76-1168 (R)

8. Perfo rming Orgoni zotion Report No.

9, Performing Orgonizotion Nome ond Address National Bureau of Standards

U. S. Department of Canmerce

Washington, D. C. 20234

10. Work Unit No. (TRAIS)

1 1 . Controct or Gront Ho. DOT AS-50074

12. Sponsoring Agency Nome ond Address

U. S. Department of Transportation

Materials Transportation Bureau

Office of Hazardous Materials Operations — Washington. De--CL—2ii59[D

15. Supptementory Notes

13. Type of Report ond Period Covered Final

June 1975 - June 1976

14. Sponsoring Agency Code MTH-1

16.

Abstrocf

Ihis report, prepared for the U. S. D^)artment of Transportation, oontains

background information useful in evaluating the performance of plastic

packagings for hazardous materials transportation, insofar as mechanical

prcpierties are ooncemed. Current DoT regulations and test methods are

reviewed, as well as testing procedures from other organizations and

oountries. Also included are reconmendations to modify crurrent DcT

regulatiais to make test methods more quantitative. Finally, experimental

data are presented which represent the initial stage of a study it is

hoped will ultimately lead to the establishment of criteria upon vdiich

the long time behavior of plastic containers can be predicted based on

short time tests.

17.

Key Words Hazardous materials; mechanical properties; plastic packagings; polyethylene

methods of test; transportation.

19. Security Clas;»if. (of this report) Unclassified

18.

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

Information Service, Springfield, VA

22151.

20, Security Classif. (of this page)

Uhclassified

21. No. of P ages 45

22. Price

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

This Report is Part I of a series prepared for the Office of Hazardous Materials

Operations under Research Contract No. DOT AS-50074, Part I is concerned with mechanical

properties aspects of the performance of plastic packagings for the transport of hazardous

materials. The authors gratefully acknowledge the assistance and cooperation of the

following organizations: the Office of Hazardous Materials Operations Staff, in particular

Mr. Mario Gigliotti, for his guidance throughout this project; 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 Company.
0§ih9im'

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TABLE OF CONTENTS

PAGE

1. INTRODUCTION 1

2. POLYETHYLENE - THE PLASTIC FOR CONTAINERS 2

3. GENERAL CHARACTERISTICS OF POLYETHYLENE 4

4. DEVELOPMENT OF STANDARDS AND SPECIFICATIONS FOR PLASTIC CONTAINERS 11

5. REVIEW OF PRESENT DOT SPECIFICATIONS AND TESTS AND OTHER TESTS FOR

POLYETHYLENE CONTAINERS 12

6. DISCUSSION AND TENTATIVE RECOMMENDATIONS 16

7. LABORATORY STUDIES OF MECHANICAL PROPERTIES LEADING TO THE FAILURE OF

POLYETHYLENE 23

APPENDIX A - ADDITIVITY OF DAMAGE UNDER CONDITIONS OF A CONSTANT RATE OF LOADING . . 39

APPENDIX B - A COMMENT ON VISCOELASTIC PROPERTIES 43
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LIST OF FIGURES

Figure 3.1 Idealized Stress-Strain Curve for a Typical Crystalline Olefin

Polymer 8

Figure 3.2 Stress-Strain Curve Showing the Dependence of Stress on Density

and Rate of Strain 8

Figure 3.3 Effect of Temperature and Rate of Loading on Tensile Stress at the

Yield Point for 0.3-M.I., 0.950 Density Copolymer 9

Figure 7.1 Log Time-to-Break versus Static Load for Linear Polyethylene

Sample A (M^ - 160,000) 27

Figure 7.2 Log Time-to-Break versus Static Load for Linear Polyethylene

Sample B (M^ = 90,000). Open symbols - in air, • presoaked

in motor oil at 60 °C for 24 hours, 0 - presoaked in hexane at 23 °C for 24 hours , 28

Figure 7.3 Log Time-to-Break versus Static Load for Linear Polyethylene

Sample C (M^ = 50,000) 29

Figure 7.4 Log Time-to-Break versus Static Load for Specimens From Two Coimiercial

Polyethylene Containers 31

Figure 7.5 Comparison of Log Time to Break versus Static Load for Sample A and two Commercial Polyethylene Container Materials 32

Figure 7.6 Appearance of Specimens from Sample A after Failure 34

Figure 7,7 Appearance of Specimens from Sample B after Failure: (a) 296K, and

(b) 330 and 350K 35

Figure 7.8 Schematic Representation of a Possible Stress History applied to a Plastic Specimen 36

Figure A-1 Schematic Representation of a Constant Rate of Loading Experiment in

which a Preload has been Applied for a time t 40

Figure A-2 Prediction of Log Time-to-Break for a Static Load from Constant Rate

of Loading Experiments 41
LIST OF TABLES

Table 2.1 List of Plastic Specifications Containers Found in 49 CFR 3

Table 3.1 Relationship of Mechanical Properties of Polyethylene to Variations in Melt Index and Molecular Weight 6

Table 3.2 Physical Properties of Typical Polyethylene Resins 7

Table 5.1 List of Resin Specifications and Container Tests for DOT Approved

Containers T 4

Table 5.2 Partial List of Allowed Ladings for Plastic Containers 17

Table 7.1 Sample Characteristics Pertinent to the Time-to-Break Study 26
1. INTRODUCTION

The hazardous materials regulations embodied In the "Code of Federal Regulations, Title

49 - Transportation, Parts 100 to 199" (Title 49 CFR) are published and maintained by the

Office of Hazardous Materials Operations (OHMO). Within these regulations are specifications

for plastic containers used in the transportation of hazardous materials. These specifications require, among other things, that the plastic containers be compatible with the lading

and that the plastic containers should not be permeable to the lading to an extent which

could create a hazard. There are, however, no standards on which one can base decisions

as to whether a plastic container, selected for a particular use, is or is not in conformance

with either aspect of the regulations. A similar situation prevails in the specifications

for reuse of plastic containers.

In addition to its role in protecting the public interest through the formulation of

specifications and regulations, the OHMO must frequently rule on requests for exemptions

which allow the shipment of hazardous materials in packages which have not yet come within

the purview of the regulatory system. A significant number of these exemptions are for use of plastic containers. Decisions on these containers are difficult to render in the absence

of data on their material properties as they relate to the intended use.

In the light of the National Bureau of Standards (NBS) legislative mandate for

"...development of methods for testing materials, mechanisms, and structures...." and for

"...cooperation with other Government agencies and with private organizations in the

establishment of standard practices, incorporated in codes and specifications," the NBS is

uniquely suited to assist the OHMO in the solution of the problems outlined above. The

position of the NBS, as a party accountable only to the public interest, aids materially

in maintaining equity in the process of formulating specifications and regulations by

diminishing the possibility of conflicts of interest. The role of the NBS in obtaining

and evaluating critical data on materials properties is well-established.

In fiscal year 1976 under Department of Transportation (DOT) contract AS-50074, the NBS

Polymers Division initiated studies of mechanical properties affecting plastic container

reuse and of permeation properties of ladings through plastics. This effort included a survey of the technical literature, an analysis of the tests given in 49 CFR, a survey of

other test methods, and laboratory studies on damage in polyethylene (PE).

The findings of all these investigations are presented in detail in this final report

for Contract DOT AS-50074. Briefly stated, our findings are that; (1) performance criteria

regarding permeation of lading materials should be developed for use by DOT, and (2)

additional performance criteria should be implemented in the area of mechanical properties.

The latter criteria should address the issues of failure due to stress cracking and failure

due to long-time, low-level applied stresses encountered in the shipping and storage of

the container.

All of the above areas, permeation performance, stress crack resistance, and long-term

mechanical integrity, are particularly relevant in situations where the containers are expected to be reused. The trend toward reusable containers is a result of the widespread

use of Specification 34 (paragraph 178.19 of 49 CFR) as the basis for exemptions.

There have been a number of developments with regard to plastic containers in the

packaging industry in recent years which have subjected the regulatory system to increasing

pressure. Container fabricators have found ways to make sturdy, chemically resistant,

industrial containers in a wide range of styles and sizes. Resin producers have developed

materials which result in improved impact resistance, stress-crack resistance, and barrier

performance. Shippers have found that significant economies can be realized by use of the

lighter weight plastic containers. The net result has been that plastic containers have

been removed from the sphere where they were regarded as useful only for situations in which

metal or glass containers did not provide adequate protection, and now compete in many areas

both on economic and performance grounds with other types of containers.

1
From the standpoint of the OHMO these innovations have been reflected in the increasing

numbers of requests: (1) to expand the scope of exemptions to include new commodities,

(2) to determine new specifications covering new container configurations, or (3) to change existing specifications to encompass modifications in container design.

In part, these pressures arise from the nature of the Hazardous Materials Regulations

themselves. Within the Regulations (49 CFR) , the following two lists appear: (1) a list

of commodities, and (2) a list of container specifications. Packages are selected from the

container list on a commodity- by- commodity basis. There has long been a recognition that

such a system can be Und is) unwieldy and that a performance-oriented approach should be

developed [Ip. The Naval Surface Weapons Command (NSWC) has been attempting to devise

such an approach with respect to barrels, drums, and packs [2].

Plastic containers present special problems of their own and the OHMO has determined

that there is a need for studies of plastic materials properties which can be used as input

in formulating performance-oriented specifications.

In this report, three potential approaches are considered as possible solutions to the

problems of developing adequate specifications for plastic containers used in shipping

hazardous materials. First, the important physical parameters relating to the mechanical

or permeation failures of plastic containers will be discussed. Second, from a general

knowledge and understanding of the permeability of polymeric materials and related failure

modes, the validity and usefulness of the present DOT tests, as well as the usefulness of

tests from other organizations, ASTM, ISO, etc,, will be analyzed. Finally, areas where new

tests are necessary will be suggested and baseline data necessary to the establishment of

such tests will be presented.

Laboratory studies to date in the NBS Polymers Division have been concerned primarily

with the creep failure, in extension, of polyethylene over a wide range of stress levels.

Experimental and theoretical work have demonstrated that the assumption of additivity of

damage as a result of various stress and temperature histories is valid for polyethylene in

a normal environment (absence of a stress-cracking agent). Using the assumption of additivity of damage as well as data on tlme-to-break versus applied stress at different tem- peratures, specimen lifetimes have been predicted for several different simple stress histories.

2. POLYETHYLENE - THE PLASTIC FOR CONTAINERS

In confronting the materials science questions which bear on shipping containers, the

first step in determining what materials are being used was to examine the materials properties given in the specifications found in 49 CFR. Table 2.1 is a list of all the specifications listed in paragraph 173 of 49 CFR which deals with plastic containers. Table 2.1

does not include all of the references found in paragraph 173 of 49 CFR for special uses of polyethylene in non-specification packages for use with specific ladings. When allowance

is made for the frequency of use of these specifications, it becomes clear that polyethylene

is the principle material used in fabricating nonmetal lie containers for the shipment of

hazardous materials.

As will be shown later, "polyethylene" is not one substance but covers many different

materials under one name. In an attempt to further delineate the limits of the NBS investigations, inquiries were directed to the OHMO, resin suppliers, container manufacturers,

and shippers to ascertain which forms of PE are in common use in the manufacture of con- tainers for hazardous materials.

This informal survey revealed a consensus that the most critical applications were those

involving free-standing containers, and, of the various resins in use, high-molecular-weight

polyethylene was the most important resin.

^Numbers in square brackets refer to literature references at the end of Part I.

2
TABLE 2.1

LIST OF PLASTIC SPECIFICATION CONTAINERS

FOUND IN 49 CFR

SPEC PARA DESCRIPTION OVPK' CAP^

IH 178.13 Polyethylene carboys in low

CARBON STEEL OR OTHER EQUALLY EFFICIENT METAL CRATES

YES 13

43A 178.18 Rubber drums NO 30

34 178.19 Reusable molded polyethylene CONTAINER FOR USE WITHOUT OVERPACK. Removable head

NOT AUTHORIZED.

NO 30

2T 178.21 Polyethylene container YES 13

2U 178.24 Molded or thermoformed polyethylene CONTAINERS HAVING

RATED CAPACITY OVER ONE GALLON. Removable head containers OR containers fabricated

FROM FILM NOT AUTHORIZED.

YES 55

2E 178. 24A Inside polyethylene bottle YES 5

2TL 178.24 Polyethylene container YES 14

2S 178.35 Polyethylene container YES 55

2SL 178. 35A Molded or thermoformed YES 55

44P 178.241 All-plastic bag NO 81

^A "yes" means that this container is meant to be used with another

AS an overpack.

^The number given is the maximum permitted value exclusive of age.

UNIT

GAL

GAL

GAL

GAL

GAL

QTS

GAL

GAL

GAL

LBS

CONTAINER

3
A further review of the plastics industry trade journals led to the same conclusion.

For example, in the December 1975 Modem Plaatias (page 10), it was estimated that 11,000

to 12,000 metric tons of high molecular weight polyethylene (HMWPE) would be used in 1976

in the production of 30 to 55 gallon drums to be used in all forms of transport. It was predicted that by the mid-1980‘s, 200,000 metric tons of HMWPE would be consumed annually

by this market with plastic drums taking over 50 percent of the total drum market.

According to the Plaetiae World of February 16, 1976, HMWPE shipping containers and

utility pails are taking 30 percent of the market away from steel in 1975-76.

It should be noted that evolving technology is rapidly placing new materials into

contention as container resins.

3. GENERAL CHARACTERISTICS OF POLYETHYLENE

This report deals with tests and experiments on polyethylene because of the preeminence

of this polymer in the plastics packaging industry. A brief description of the significant

properties of polyethylene which control its mechanical and permeation properties will be

given here. A number of terms which are widely used in the plastic container industry will

be introduced and defined.

Polyethylene is obtained by the polymerization, or linking together, of numerous

ethylene, CH 2 = CH 2 molecules. Polyethylene may vary according to: (1) average size of

molecules or molecular weight, (2) distribution of size of molecules within the product,

and (3) extent of branching of these molecules. (Branching describes the incorporation of

short aliphatic side chains onto the long polyethylene molecules.) All of these characteristics affect the physical and chemical properties of the solid polymer.

3.1 Characterization of the Polymer

The determination of the molecular weight, its distribution and the degree of branching

is usually categorized as the characterization of the polymer. Molecular weight and

molecular weight distribution are usually determined by solution properties of the polymer. Light scattering, osmometry, ultra centrifugation, and dilute solution viscosity are some classical methods to obtain various measures of molecular weight. Gel Permeation Chromatography (GPC) is a more recent method to determine molecular weight and molecular weight

distribution. All of these tests for molecular weight, except for GPC, are rather tedious

and expensive to perform and are, therefore, not widely used in industry.

For these reasons, industry has turned to simpler methods of characterizing polyethylene

resins. ASTM Method of Test D-1248-74^, Standard Specification for Polyethylene Plastics

Molding and Extrusion Materials, provides for five categories of polyethylene distinguished

by the flow rates of polymer melts as measured by ASTM D-1238-70, Test for Measuring Flow

Rates of Thermoplastics by Extrusion Plastometer. Results of this test are usually stated

in terms of the melt index.

The flow rate obtained with the extrusion plastometer is not a fundamental polymer

property and should not be so regarded. It is an empirically defined parameter critically

influenced by the physical properties and molecular structure of the polymer and the conditions of measurement. The rheological, or flow, characteristics of polymer melts depend upon

a number of variables. In general, however, the higher the molecular weight, the lower the

melt index. Thus, one may use the melt index as a comparative measure of molecular weight.

The melt index is useful to polymer resin manufacturers as a method of controlling

material uniformity. While the data from this test are not directly translatable into

relative end-use processing characteristics, the melt index is nonetheless strongly indicative of relative "flowabil ity" of various kinds and grades of PE. However, the melt index

^A1 1 ASTM tests and specifications referred to in this Report can be found in the ASTM

Annual Book of ASTM Standarde, Parts 35 and 36 (1976).

4
of one manufacturer's resin may not be equivalent to that of another's with respect to its

processing characteristics. The "property" measured by this test is basically the melt

viscosity.

It should be pointed out at this time that the melt index of material taken from the

final product may not be the same as that of the base resin, that is the melt index may be

modified by the processing itself. For example, a polyethylene resin held at a high temperature (in the melt) for a substantial time may undergo degradation, thus increasing the melt

index.

The mechanical properties important to use of the polymer in container applications are generally correlated to the melt index. In Table 3.1 some of these properties and their

relation to variations in melt index and molecular weight are shown.

ASTM D-1248-74 also provides for four types of polyethylene, as distinguished by their

density as determined by ASTM D-1505, Test for Density of Plastics by the Density Gradient

Technique. In specification D-1248-74:

Type I is polyethylene in a density range of 0.910 to 0.925 g/cm^. This is termed "lowdensity" or "branched" polyethylene, or LDPE.

Type II is polyethylene in a density range of 0.926 to 0.940 g/cm^. This is termed

"medium-density" or "linear" polyethylene or HOPE.

Type III is polyethylene in a density range of 0.941 to 0.959 g/cm^. This is termed

"high-density" or "linear" polyethylene or HOPE.

Type IV is a polyethylene of density 0.960 g/cm^ or greater. This type is termed

"high-density" also.

In general, density is a measure of both degree of branching and degree of crystallinity. Type I polyethylene is highly branched and contains moderate crystallinity while

Type IV polyethylene has little branching and is highly crystalline.

"Degree of Crystallinity" is here used to indicate that the molecular ordering in

polyethylene can vary from a random configuration analogous to a bowl of spaghetti to an ordering where nearly all the volume is filled with crystallites. The extent to which the

polymer exhibits ordering (crystallinity) is mainly influenced by molecular weight, thermal

history, and degree of branching.

A number of other properties roughly correlate with density. In Table 3.2 variation in

several properties among different polyethylene types are shown. The chief differences among

these types of polyethylene are in rigidity, heat resistance, and resistance to loading.

Generally, increased density results in greater stiffness, strength, and heat resistance,

while resulting in lower impact strength and resilience.

The physical and mechanical properties of polyethylene are not determined entirely by

the manufacturer of the resin. For a fixed molecular weight plastic, the process associated

with forming the resin into a container can result in modifications to the physical and

mechanical properties of the containers as a result of the stresses and temperatures applied

during processing.

For example, a container rapidly cooled from the melt will have a somewhat lower density

and crystallinity than one slowly-cooled. At the same time, quenching, or rapid cooling, may

lead to much greater "residual" stress than will slow cooling. On the other hand, a polymer

melt held at a too high temperature for some length of time can undergo degradation of the

resin.

5
TABLE 3.1

RELATIONSHIP OF f^ECHANICAL PROPERTIES OF POLYETHYLENE TO

VARIATIONS IN MELT INDEX AND MOLECULAR WEIGHT

Property

As Average Molecular

Weight Increases

(Melt Index Decreases)

As Molecular Weight

Distribution Broadens

Melt Viscosity Increases -

Tensile Strength at Rupture Increases No Significant Change

Elongation at Rupture Increases No Significant Change

Resistance to Creep Increases Increases

Impact Strength Increases -

Resistance to Low

Temperature Brittleness Increases Increases

Environmental Stress

Cracking Resistance Increases Increases

Softening Temperature Increases

6
TABLE

3.2

PHYSICAL

PROPERTIES

OF

TYPICAL

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f— LU h- < > LL CD < < LD < ll. U- _l Q. t 1 —

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I— CD LU z X CO h- o UJ

u LU CD cn (X o cn _j 1—

1—1 X Q. S

UJ cn h- _J

X

X 1- CX 1— cx cn CO

N 4 Q_ cn co LjJ _i Q < on cx < z O LU X O CJ o LU o X

Od (X LU

Q SI 1—4 1— z c_> LU

Cl. h- _l

O > cn (—1 Q

co >—1 SI —I 1 _l z

cn cn < I 1 z LU s < LU 1- O cx cx 1— H- < LU LU z LU t- X

LU DC z 1— < s LU 1— z *— o CJ

cx

—I —1 U- > Li- z LU UJ CD 1 7
3.2 Mechanical Properties of Polyethylene

A variety of mechanical tests and measurements may be made on polyethylene. However,

since the main thrust of this report with respect to mechanical properties is in the area of

mechanical failure of the material, those properties and tests which lead to some understanding of mechanical failure will be emphasized.

A common mechanical measurement made on materials is the elastic modulus or "stiffness."

For all materials there is a region where doubling the applied stress doubles the strain and

visa versa. This is referred to as the linear region of the material. In the linear

region the' ratio of the applied stress to strain is called the elastic modulus.

For polyethylene, the linear region occurs only for strains of 10"'^ or smaller. By way

of comparison, in the case of rubber this region occurs for strains of 10"i or smaller,

whereas in some metals it occurs for strains of 10"^ or less. At higher strain, stress and

strain are not connected by a linear relationship; then the description of the mechanical

properties become more complex. Figure 3.1 is a typical stress strain curve for a polymer.

The modulus is commonly taken to be the initial slope of the curve.

Other features derived from stress-strain curves are the yield point, tensile strength,

and elongation. They represent, respectively, the maximum elastic strength, the ultimate

strength, and the amount that the polyethylene can be drawn before it breaks. These mechanical properties are especially important in practical applications.

Fig. 3.1. Idealized Stress-Strain Curve

for a Typical Crystalline Olefin Polymer

Most mechanical properties depend on the state of the resin. For example, the tensile

strength decreases with increased temperature. Increased density results in an increase

in tensile strength, as does an increase in molecular weight. Furthermore, these properties

depend on the rate at which the material is strained. Figure 3.2 is a stress-strain curve

for two representative grades of polyethylene at two different strain rates. The yield

point, tensile strength, and elongation depend not only on the density of the material, but

also on the rate at which the experiment was done.

Fig. 3.2. Stress-Strain Curve Showing

Dependence of Stress on Rate of Strain

and Density

8
The variation of the yield point of one particular sample as a function of rate of

strain and temperature is shown In figure 3.3. From these examples, it is clear there are strongly time-dependent phenomena influencing the mechanical properties of polyethylene. As will be shown later (Section 7), at high levels of stress the failure times (time-to-break

or time-to-fai 1 ) for polyethylene A can be much larger than for polyethylene B, but at a low

level of stress the reverse may be true.

Fig. 3.3. Effect of Temperature

and Rate of Loading on Tensile

Stress at Yield Point for 0.3-

M.I. 0.950 Density Copolymer

Two Other often discussed material properties related to failure of the polymer are impact strength and brittleness temperature. Impact strength is a measure of the polymer's

ability to withstand high stresses for short times without failure. Impact strength Is affected by molecular weight and, to a lesser extent molecular weight distribution. As molecular weight increases, impact strength increases. A commonly used measure of the impact

strength is the IZOD impact test (ASTM 256-72a).

The IZOD impact test indicates the energy required to break notched specimens under

standard conditions. It is calculated at ft-lb/in of notch and is usually calculated on the basis of a 1 inch specimen, although the specimen used may be thinner in the lateral

direction. The IZOD value is useful in comparing various grades of a plastic. However, some materials are notch-sensitive and derive stress concentrations from the notching operation.

Furthermore, the test is open to much criticism as it shows different results depending on whether or not the sample had been notched. For example, in the notched IZOD impact test

conducted over a wide temperature range, low-and high-density polyethylenes react differently

[3]. Between 0 °F and 20 °F, low-density polyethylene goes through a sharp transition point

where the impact strength increases from less than one ft-lb/in of notch to 18 ft-lb/in.

On the other hand, high-density polyethylene shows a gradual increase in impact strength

as the temperature is raised. Data on unnotched specimens are quite different. The

transition point for low-density polyethylene does not appear and the two curves nearly

superpose.

The brittleness temperature is of some use in judging the relative merits of various

materials for low- temperature flexing or impact. It can be measured by an ASTM test (ASTM

D-746-73). As is pointed out in paragraph 3 of this test, it is specifically relevant only

for the materials and conditions specified in the test, and the values cannot be directly

applied to other shapes and conditions. The brittleness temperature does not put any

lower limit on service temperature for end products, but this property is sometimes used

in specifications.

The effect of different environments on the mechanical properties of polyethylene has

not been discussed so far. Materials which swell, soften, or which chemically react with

the polymer will change its failure properties. In addition, other ladings which show no appreciable chemical or physical effect on the polymer in the absence of stress can, under

9
the influence of internal or applied stresses, accelerate the failure. These materials are categorized under the general heading of stress-cracking agents. Stress-cracking is the

appearance of cracks, internal or external, and thus, failure of the polymer at times earlier

(or lower stresses) than the polymer would have failed in an inert environment.

Depending on the manner of its initiation, the resulting failure may be recognized as environmental, thermal, fatigue, solvent, or oxidative. Elevation of temperature usually

heightens stress-cracking activity. At the same time, environmental stress-cracking resistance generally improves as the melt index decreases. Broadening of the molecular-weight

distribution with low molecular weight material (<30,000) decreases stress-cracking re- sistance, whereas broadening with high molecular weight material can increase stresscracking resistance. However, during processing at high rates of shear the high molecular

weight components can be broken down so that it is important for stress-cracking tests to use test specimens obtained from the final product.

ASTM-2561-70 and ASTM-1 693-70 provide various tests to determine the effects of

environmental stress-cracking agents on blow molded polyethylene containers.

There are also other forms of stress-induced failure involving no chemical changes.

For example, in solvent-cracking the liquids which induce failure apparently do so by

weakening the surface layers of the material through solvation and swelling. Another

example is thermal stress-cracking which is brought about through internal physical changes

as a result of heating prestressed polyethylene; it is to be distinguished from thermal

embrittlement which can occur in unstressed material. Thermal stress-cracking is a weakness

primarily of the higher density polyolefins. For polyethyl enes , it is enhanced by increasing

the melt index. Molecular-weight distribution also remains important, particularly in the more highly crystalline polymers. Static fatigue, or brittle creep failure, is closely

related to thermal stress-cracking. ASTM-2951-71 provides a test method for thermal stresscrack resistance of Types III and IV polyethylene plastics.

Other types of stress failure involve chemical alteration of the polymer. Beyond the

initiation step, which consists of an irreversible oxidative damage, oxidative stresscracking is mechanically indistinguishable from the others. Catalyst residues in the

polymer, or certain additives, can be oxidation promoters. Therefore, antioxidants must be

added in order to minimize this type of failure.

3.3

Chemical Resistance

Polyethylenes have excellent chemical resistance to many chemicals and solvents. At ambient temperature, they are resistent to acids and alkalies except for oxidizing acids

such as nitric, chlorosulfonic, and fuming sulfuric. They are unaffected by hydrofluoric

acid. The polyethylenes are generally insoluble in organic solvents at temperatures below

50 °C (122 °F). However, at higher temperature they are soluble to varying degrees in hydrocarbons and halogenated hydrocarbons. They are appreciably affected by chlorinated solvents,

aliphatic and aromatic hydrocarbons, certain esters, and oils. Polyethylene can be dissolved

at temperatures greater than 71 °C (160 °F) in toluene, xylene, amyl acetate, trichloroethylene

petroleum ether, paraffin, turpentine and lubricating oils.

3.4

Permeation

Polyethylene is permeable to gases, vapor, and liquids. The permeation rates depend

not only on the nature of the permeant, but also on the physical state of the polymer.

Material permeating into the polymer may change the polymer's physical and mechanical

properties.

3.5

Surface Cleanability

Polyethylene, being generally chemically inert and essentially non-polar, shows low

adhesion, i.e., little tendency to bond to materials. Thus one might expect it to be easily

cleaned. However, the characteristic of being permeable to many materials means that although the surface may be "clean," substances absorbed in the container walls may be re- movable only with difficulty.

10
4. DEVELOPMENT OF STANDARDS AND SRECIFICATIONS FOR PLASTIC CONTAINERS

The problem of setting up specifications and tests to insure safe transport of hazardous materials in plastics containers is a complex one. For normal transportation use, it

is known that there are plastic materials which should not be used for packaging because

packages made from these materials are too fragile, too hard to handle, too expensive, would

not afford the required protection, or some combination of these. In general, people who

work in the packaging industry are able to predict whether a given package will be satisfactory or not on an empirical basis derived from past experience, knowledge of materials

properties, and economic factors. Furthermore, if a new package fails it is simply pulled

off the market with no major adverse consequences to the public.

When dealing with packages for hazardous materials ladings, however, more sophisticated

methods must be brought to bear because the risks associated with a trial and error approach

are generally regarded as unacceptable. Furthermore, setting down specifications for each

lading and each container on a case-by-case approach is rapidly becoming impractical in the

face of the proliferation in the number of commodities and package types moving in commerce. Under these circumstances, what is needed is a set of criteria based on standards which a reasonable person can interpret and apply to assess the level of risk associated with a particular combination of package and lading.

Standards or regulations are generally either specification-oriented or performanceoriented, and a clear distinction exists between them. The specification-oriented approach

is typified by many building codes. There are a myriad of details as to construction

materials, and configuration which go into constructing a building to meet the specifications. It is not uncommon, under this system, for an innovative, improved solution to a problem to sit in limbo until codes are amended to permit the use of the new approach.

After enough such modifications have been made, however, the code may no longer be interpretable by everyone who needs to comply with it.

In performance-oriented approach, the person who wishes to demonstrate compliance with

a set of regulations needs only to submit test data in support of his contention that his

product meets or exceeds certain levels of performance. As long as the given product meets

these criteria, there is no need to specify the color, or weight of the product, nor the

material from which it is made unless, of course, compatibility with a lading is to be a problem. It is possible and most often, desirable to combine the two approaches in order

to arrive at the most equitable set of regulations.

To insure that a set of tests and specifications will minimize the number of failures,

it is necessary to consider all situations which can lead to failure of the container. In so doing, methods must be established whereby the in-use history of the container can be

approximated by a sequence of laboratory tests. Listed below are a number of factors which

appear governing the use of plastic containers. Solely for convenience sake, they have been

divided into two categories--those dealing with the container itself and those dealing

with the interaction of the lading and container. In reality, the distinction between the

two may not be clear-cut because of interdependencies.

1) Container Use Conditions

a) Temperature history

The maximum and minimum temperatures the container will experience in service and

the maximum time a container is likely to be subjected to the maximum temperature must be

determined. The effect of cycling temperature on the material properties will also be an important factor. For example, it is shown in Section 7 that a 34 °C rise in temperature

can, for identical stress histories, cause a 1000-fold decrease in time-to-fail for polyethylene under the conditions of an applied uniaxial stress.

b) Stress history

Both the maximum stress and the time the container is at maximum stress should be

established. Again, the effect of various stress histories, including cycling the stress,

will be important. For example, the question arises as to whether a given container is more

11
likely to fail as a result of a high stress occurring for a short time, such as might be

experienced in dropping, or fail as the result of a low stress sustained over a long time

period, as may occur in stacking and/or vapor pressure increase of the lading if the

container is unvented. Again, it will be demonstrated in Section 7 that the results can

be greatly different depending upon the polymer resin type.

c) Useful lifetime

Containers designated for one trip use (not reusable) should be designated with the

above mentioned factors in mind. However, if the container is to be reused, additional

factors such as the maximum number of trips and/or maximum length of service must be determined as well. Stated another way, what is the maximum allowable period in service before

retesting is required? These times or periods shall be referred to as the lifetime or useful lifetime.

2 ) Effect of Lading

a) Chemical attack

Some ladings may soften, swell, or react chemically with polyethylene. Two questions

which arise in this connection are: (i) should ladings known to interact with polyethylene

be allowed to be shipped in polyethylene containers (for example nitric acid or mixtures

containing nitric acid), and (ii) if so, how does one determine that such an interaction has

taken place?

b) Stress-cracking

A variety of chemicals are known to stress crack polyethyl enes to varying degrees.

What is needed in this case is to determine which ladings are stress cracking agents and

what level of stress-cracking agent is acceptable to be shipped and for how long or often.

Thus, a test to determine if a lading is to be judged a stress-cracking agent and, if so, a scale of severity needs to be established.

c) Permeation

Many liquids and gases permeate polyethylene, some may go through the polymer

leaving it unchanged. Others may change its mechanical or other physical properties.

Tests need to be established to determine the permeability of the polymer to its lading,

and to what degree a change in mechanical or other physical properties of the container due

to permeant is acceptable.

Special problems may exist with respect to containers designated as reusable.

Ladings from an earlier use which have permeated the container may at a later time diffuse

back out into a subsequent lading, thus contaminating it.

d) Product alteration

Similar to c) , the lading may be contaminated by the extracting from the polymer of

additives (antioxidants, etc.).

5. REVIEW OF PRESENT DOT SPECIFICATIONS AND TESTS AND OTHER TESTS FOR

POLYETHYLENE CONTAINERS

The present DOT specifications and tests for polyethylene containers are contained in

Part 49 of the Code of Federal Regulations (49 CFR) . These address specifications on material properties of the resins used to manufacture the containers as well as qualification tests on actual containers. In this section, these tests will be reviewed in the light

of the discussions in Section 4. Comments on the current requirements used by DOT for

acceptance of plastic containers used under exemptions will also be given.

12
Also reviewed in this section are tests and standards from sources other than 49 CFR.

An effort has been made to broadly cover the 'standards literature. In so doing, the

following publications have been searched.

1) Amerioan National Standarde Catalogue, 1975 catalog and 1976 supplement.

2) World index of Plaatio Standards, NBS Special Publication No. 352 (December

1971).

3) An Index of U.S. Voluntary Engineering Standards, NBS Special Publication No. 329.

4) An Index of U.S. Voluntary Engineering Standards, Supplement 1 and 2, NBS

Special Publication No. 329/1 and 329/2.

5) Transportation of Dangerous Goods (1970), and Supplement (1973), Part II. Recommendation prepared by United Nations Committee of Experts on the transport

of dangerous goods.

This survey has uncovered standards from sources such as International Standards Organization (ISO), ASTM, Department of Defense (DOD), Japanese Standards Association, and

British Standards Association, etc.

In order to clarify the kinds of materials and what one is dealing with, all of the

specifications in Part 178 of Title 49 CFR for free-standing polyethylene containers or

for plastic containers intended for use with overpack are listed in Table 2.1. Table 5.1 lists the materials properties required of the resins used to manufacture the containers

and also summarizes which tests are required on which container. Table 5.2 gives a partial

list of allowed ladings for plastic containers. Those familiar with DOT tests will recognize many similarities to tests from other sources. However, a number of interesting

differences occur and some of these will be mentioned at the appropriate point.

5.1 Comments on Resin Specifications

1) Melt Index (ASTM 1238-62T)

The melt-index test is primarily useful to polymer manufacturers as a method of

controlling material uniformity. While the data from this test are not directly translatable into relative end-use processing characteristics, the melt-index value is none- theless strongly indicative of relative "flowabil ity" of various kinds and grades of

polyethylene. The "property" measured by this test is basically the melt viscosity, or

"rate of shear." In general, the materials that are more resistant to flow are those of

higher molecular weight so that the melt index is useful in determining the approximate

molecular weight.

However, polymers, such as polyethylene, in the amorphous state (melts) and above a molecular weight of about 30,000 show a non-newtonian behavior when they are sheared; that

is to say, the viscosity depends on the rate of shear. In the limit of zero shear, the

viscosity depends on the weight average molecular weight to the 3.4 power. At higher

rates of shear, the non-newtonian viscosity depends not only on the molecular weight, but

also on the molecular weight distribution. Although the designers of this test tried to simulate a viscometer which extrudes the material at low rates of shear, the rates attained

are mostly in the non-newtonian region. Furthermore, the short capillary used in the test

introduces difficulties due to end effects, which depend highly on the "elasticity" of the

material which, in turn, depends on the molecular weight distribution. As a result, one can obtain the same melt index from materials which have different weight average molecular

weights. Thus, this test is acceptable only to the extent that one is comparing materials

which have, similar molecular weight distributions. Since most resins produced commercially

have approximately log-normal molecular weight distributions, resins having the same density, the same melt index, the same tensile strength, and same percent elongation

should not show significant differences in their general performance.

13
TABLE S.l

LIST OF RESIN SPECIFICATIONS AND CONTAINER TLS1S FOR DOT APPROVED CONTAINERS

Rcsin Specification Drop Test

CIFICATION

Number

Melt

Index

Dens

Min.

ITYMax.

Tensile

Strength

(psi)

LlONGAT ION

(%)

Height Temp. Place Vibration

Test

Compression

Test Hydrostat

Test

IH 2.5‘ N.S.^ N. S. N. s. N.s. 9

“ F ANY No No No R3A n.a. N.A. N.A. N.s. N.S. 6 N.s. CHIME no No Yes

9 N.S. closure

39 <1.2 .991 .965 1500 75 9 N.S. WEAKEST YES Yes Yes

Ej ' F ANY

2T <2.6‘ .910' .925' 1500 900 6

“ F ANY Yes No No

9

° F ANY

9 N.s. ANY

2U <2.6‘ .910' .925' 1500 900 9 N.S. ANY Yes No No

(Type III P . E . MAY 9

• F ANY

BE USED FOR <6 gal)

2E Blow molding GRADE 9 N.S. WEAKEST No No No capable of holding 9

° F ANY

SHAPE WHEN STANDING

empty AND OPEN. 2TL <2.6‘ .910' .925' 1500 900 e

• F ANY Yes No No

9 N.s. WEAKEST

9

“ F ANY

2S 2.6' .910' .925' 1500 900' 6

' F ANY Yes No No

9

' F ANY

9 N.s. ANY 2SL <2.6‘ .910' .925' 1500' 900 6

“ F ANY Yes No No

9 N.s. ANY

9

° F ANY 99P < .6 2100’ 350’ 6 N.s. 6 DROPS No No No

‘Denotes test method acceptable to Bureau of Explosives.

=’Not specified.

^Denotes elongation and tensile strength tested according to AST^i 882-73; also, drop test according to

ASTFi 1709 62T, and tear test as measured according to ASTM 01922-611.

14
2) Tensile Strength and Percent Elongation (ASTM D638)

Tensile properties are an indication of strength in a material. The force necessary

to pull the specimen apart is determined along with prebreak stretch. The elastic modulus

("modulus of elasticity" or "tensile modulus") is the ratio of stress to strain in the

limit of very small strain. The tensile strength is a useful value because parts should

be designed to accommodate stresses to a degree well below this value.

For some applications where almost rubbery elasticity is desirable, a high ultimate

elongation may be an asset. For rigid parts on the other hand, there is little benefit in

the fact that they can be highly stretched.

There is benefit in moderate elongation, however, since this quality permits absorbing

rapid impact and shock. The total area under a stress-strain curve is a measure of overall

toughness. A material of very high tensile strength and little elongation would be brittle.

The above test may distinguish between two materials of the same melt-index and

density but different molecular weight distributions. This may not be true of materials

with bimodal distributions, or different densities.

3) Density (ASTM D1505-63T)

The density of the resin is a rough measure of the crystallinity and the branching of

the polymer (see table in Section 3). As such, it gives another measure of the polymer's

ability to perform.

5.2 Review of Container Tests

For ease of identification we shall give names to the tests.

1) Drop Tests (tests similar to 178.19-7 (a) and (b))^

The drop tests as presently used appear to be tests for failures which occur during

handling. DOT regulation 178.19-7 (b) requires that a minimum of three containers taken

at random from each continuous production lot be drop tested according to paragraph

178.19-7 (a) (2). There is no requirement that a single container shall be expected to withstand more than one test.

By way of comparison the SPI test for drop-impact resistance of plastic containers

(Technical Bulletin PBD-4-1968) specifies that a minimum of twenty (20) randomly selected

containers are required for testing, and the Japanese Industrial Standard for blow-molded

containers from polyethylene (JIS Z0202-1962) requires that each container for special use (hazardous materials) be dropped 10 times. The Japanese test further specifies the order

of drops in such a way that each edging is subjected to a direct impact. In each of these

tests including the DOT test, the test temperature and drop height are not substantially

different.

The ASTM Drop Test (ASTM D2463-74) contains a variety of procedures for making drop

tests on containers. The static Drop Height Method (procedure A) is similar to the SPI

test for drop impact in that it requires a minimum of 20 containers to be dropped.

Apparently, ASTM and SPI feel that 20 containers represent an appropriate sample size.

In procedure B, a concept not incorporated into DOT tests is introduced. This is the idea

of a test to failure. In this procedure (referred to as the "Bruceton Staircase Method"),

the mean failure height of the test specimens is determined. It consists of dropping a set of test specimens from various heights, the drop height being raised or lowered depending on the result obtained on the specimen most recently tested. That is, if the previous

^Citations of this type refer to paragraphs in Code of Federal Eegulations , Title 49,

Transportation, Parts 100 to 199. The revision date applicable to citations in this

report is October 1, 1975.

15
specimen failed, the drop height is lowered; if the previous specimen did not fail, the

drop height is raised. Such a procedure provides a more quantitative estimate of the

strength of the container under drop conditions and allows for two types of containers to be quantitatively compared under drop conditions.

2) Vibration Test (tests similar to 178.19-7(c)(l ))

This test appears to be directed at testing container closures. Since tests for

closures are beyond the scope of this report they will not be discussed further.

3) Compression Test (tests similar to 178.19-7 (c)(2))

The Compression Test presumably is a means to determine the effects related to stacking

or storage of containers for relatively short periods. The DOT test 178.19-7 (c)(2)

specifies (as does the Japanese Industrial Standard Test Z 1706-1966 (7.4)) that the

container must be maintained in the load-bearing condition (compression) for 48 hours at room temperature. On the other hand, the U.N. Committee of Experts recommends that the

compression test should be carried out at 40 °C (104 °F) for a period of 28 days.

The SPI Column Crush Test (Technical Bulletin PBD-3-1968) is essentially a short time

test insofar as the container is deformed in compression at a rate of one inch per minute,

and it is not clear what relevance, if any, this test bears to stacking or storage over

any realistic time period.

Compression tests such as the DOT, SPI, and Japanese Industrial Standard Tests mentioned

above are very difficult to evaluate since it is not clear just how the information gained

from the test relates to in-use behavior. For example, since the container is filled only

to 98 percent of its capacity, the specified load may not be sufficient to cause an increase in

the hydrostatic pressure on the water. The result in this case will be different than if an increase in hydrostatic pressure does occur.

4) The Hydrostatic Pressure Test (similar to 178.19-7 (a)(3))

Presumably, this is a test to determine the effectiveness of the closure and presence

of cracks in the container. Tests at room temperature for such a short time can give no indication of efficacy for a long term stress situation.

5) Stress-Cracking Test

An important area which most testing procedures and regulations, including DOT regulations, do not address in any meaningful way is stress-cracking. The Japanese Industrial

Standard does include a stress-cracking test which is similar to the ASTM test ASTM D2561-

70 (procedure B). This is one instance where most test procedures for plastic containers

are deficient, particularly where reuse is concerned.

5.3 Compatabil i ty Requirements

As required by 49 CFR, only certain ladings may be carried in specified containers.

Table 5.2 provides lists by container type of the various ladings which may be carried in

different allowed DOT specification containers. The range of materials allowed is enormous

and varied. Corrosive liquids, oxidizing materials, and organic peroxides are most evident.

For these, it is possible to determine their compatibility with polyethylene as long as the

industrial grades of the materials do not vary. However, a large group of allowed materials

do not have a clear chemical composition. They are: Class B poisonous liquids, nos (not

otherwise specified) cleaning compounds; liquid organic peroxides nos; acids, nos; flammable

liquids, not specifically provided for. There are a variety of materials which are additives,

which in themselves may not be dangerous but may be incompatable with the polyethylene.

The various additives may result in early failure, thus allowing leakage of the lading.

The only requirement is that use of containers is "authorized only for materials that will not react with polyethylene and result in container failure." Further, there are no tests

or criteria for container reactivity or failure in these terms. This would seem to be a serious deficiency in 49 CFR.

16
TABLE 5.2

PARTIAL LIST OF ALLOWED LADINGS FOR PLASTIC CONTAINERS

SPEC 34 - REUSABLE MOLDED POLYETHYLENE FOR USE

WITHOUT OVERPACK

Acids or other corrosive liquids nos 173.245

Hydrochloric (muriatic) acid, etc. 173.263

Sulfuric acid 173.272

Hypochlorite solutions 173.277

Hydrofluosilicic acid 173.265

Hydrogen peroxide solution in water 173.266

SPEC 2S - POLYETHYLENE DRUMS

Flammable liquids nos 173.119

Alcohol 173.125

Acids or other corrosive liquids 173.245

Class B> poisonous liquids, nos 173.346

Arsenic acid 173.348

Carbolic acid (phenol) liquid 173.346

Dinitrobenzol 173.371

Hydrogen peroxide solution in water 173.266

Flammable liquids not specifically provided for 173.119

Hydrochloric (muriatic) acid, hydrochloric (muriatic) acid mixtures,

HYDROCHLORIC (mURIATIC) ACID SOLUTION, INHIBITED SODIUM CHLORITE

SOLUTION, AND CLEANING COMPOUNDS, LIQUID, CONTAINING HYDROCHLORIC

(muriatic) acid 173.263

Hydrofluosilicic acid 173.265

Hypochlorite solutions 173.277

Propellant explosives (solid) for cannon, small arms,

ROCKETS, guided MISSILES, OR OTHER DEVICES, AND

PROPELLANT EXPLOSIVES (lIQUID) 173.93

Hydrobromic acid 173.262

Hydrofluoric acid 173.264

Hydrogen peroxide solution in water 173.266

Sulfuric acid 173.272

Etching acid liquid, nos 173.299

Tris-(I-aziridinyl) phosphine oxide 173.299a

Radioactive materials 173.393

Liquid organic peroxides, nos, and liquid organic peroxide

SOLUTION NOS 173.221

Benzene phosphorus dichloride and benzene phosphorus thiodichloride 173.250a

17
SPEC 2SL - MOLDED OR THERMOFORMED POLYETHYLENE CONTAINER

Flammable liquids 173.119

Peracetic acid 173.223

Acids or corrosive liquids 173. 2A5

Hydrochloric acid 173.263

Hydrofluoric acid 173.264

Hydrofluosilicic acid 173.265

Sulfuric acid 173.272

Hydrogen peroxide solution in water 173.266

Hydrochloric (muriatic) acid, hydrochloric (muriatic) acid mixtures,

HYDROCHLORIC (mURIATIC) AICD SOLUTION, INHIBITED SODIUM CHLORITE

SOLUTION AND CLEANING COMPOUNDS, LIQUID, CONTAINING HYDROCHLORIC

(MURIATIC) ACID 173.263

Hypochlorite solutions 173.277

Hydrofluoric acid 173.264

Hydrofluosilicic acid 173.265

Hydrogen peroxide solution in water 173.266

Allyl chloroformate, benzyl chloroformate, etc. 173.288

Etching acid liquid, nos 173.299

Tris-(I-aziridinyl) phosphine oxide 173.299a

Radioactive materials 173.393

Liquid organic peroxides, nos, and liquid organic peroxide

SOLUTIONS, NOS 173.221

Benzene phosphorus dichloride and benzene phosphorus THIODICHLORIDE 173.250a

SPEC 21P - FIBER DRUM

Flammable liquids nos 173.119

Acids or other corrosive liquids nos 173.245

Compounds, cleaning liquid 173.256

Hydrochloric (muriatic) acid, etc. 173.263

Hydrofluosilicic acid 173.265

Hydrogen peroxide solution in water 173.266

Sulfuric acid

Low SPECIFIC RADIOACTIVE MATERIALS

173.272

Radioactive materials 173.393

18
SPEC 2U - MOLDED OR THERMOFORMED

POLYETHYLENE CONTAINER

Class poisonous liquids^ nos 173. 3A6

Carbolic acid (phenol) liquid 173. 3A9

Dinitrophenol solutions 173.362a

Dinitrobenzol 173.371

Acids or other corrosive liquids not specifically provided for 173.245

Compounds, cleaning liquid 173.256

Hydrochloric (muriatic) acid, hydrochloric (muriatic) acid, mixtures

HYDROCHLORIC (mURIATIC) ACID SOLUTION, INHIBITED SODIUM CHLORITE

SOLUTION AND CLEANING COMPOUNDS, LIQUID, CONTAINING HYDROCHLORIC

(muriatic) acid 173.263

Hydrofluosilicic acid ' 173.265

Alcohol 173.125

Liquid organic peroxides, nos and liquid organic peroxide

SOLUTIONS, NOS OTHER THAN ACETYL PEROXIDE SOLUTION, ACETYL

BENZOYL PEROXIDE SOLUTION, CUMENE HYDROPEROXIDE, DICUMYL PEROXIDE,

HYDROGEN PEROXIDE, PERACETIC ACID, AND TERTIARY BUTYLI SOPROPYL

BENZENE HYDROPEROXIDE 173.221

Acids or other corrosive liquids, nos 173.245

Sulfuric acid 173.272

Hypochlorite solutions 173.277

Poisonous liquids, nos 173.346

Flammable liquids, nos 173.119

Acetyl peroxide and acetyl benzoyl peroxide solution 173.222

Tris-(I-aziridinyl) phosphine oxide 173.299a

Hydrogen peroxide solution in water 173.266

Liquid organic peroxides, nos, and liquid organic peroxide

SOLUTIONS, NOS 173.221

Benzene phosphorus dichloride and benzene phosphorus thiodichloride 173.250a

SPEC 6D - CYLINDRICAL STEEL OVERPACK, STRAIGHT

SIDED FOR INSIDE PLASTIC CONTAINERS

Propellant explosives (liquid) for cannon, small arms, etc. 173.93

Flammable liquids, nos 173.119

Alcohol 173.125

Liquid organic peroxides, nos and liquid organic peroxide solutions

NOS OTHER ACETYL PEROXIDE SOLUTION, ETC. 173.211

Acids or other corrosive liquids, nos 173.245

Electrolyte (acid) or corrosive battery fluid 173.257

Hydrobromic acid 173.262

19
Hydrochloric (muriatic) acid, hydrochloric (muriatic) acid

MIXTURES. HYDROCHLORIC (mURIATIC) ACID SOLUTION. INHIBITED. ETC. 173.263

Hydrofluoric acid 173.264

Hydrofluosilicic acid 173.265

Hydrogen peroxide solution in water 173.266

Sulfuric acid 173.272

Hypochlorite solutions 173.277

Poisonous liquids, nos 173.346

Tris-(I-aziridinyl) phosphine oxide 173.299a

Compounds, cleaning liquid 173.256

Radioactive materials 173.393

Benzene phosphorus dichloride and benzene phosphorus thiodichloride 173.250a

SPEC 37^1 - CYLINDERICAL STEEL OVERPACK STRAIGHT

SIDED FOR INSIDE PLASTIC CONTAINER

Flammable liquids, nos 173.119

Liquid organic peroxides, nos. and liquid solutions etc. 173.221

Peracetic acid 173.223

Acids or other corrosive liquids, nos 173.245

Electrolyte (acid) or corrosive battery fluid 173.257

Hydrochloric (muriatic) acid, hydrochloric (muriatic) acid mixtures,

hydrochloric (muriatic) acid solution, inhibited, etc. 173.263

Hydrofluoric acid i 173.264

Hydrofluosilicic acid 173.265

Hydrogen peroxide solution in water 173.266

Sulfuric acid 173.272

Etching acid liquid, nos 173.299

Tris-(I-aziridinyl) phosphine oxide 173.299a

Radioactive materials 173.393

Benzene phosphorous dichloride and benzene phosphorus thiodichloride 173.250a

20
6. DISCUSSION AND TENTATIVE RECOMMENDATIONS

The first four sections of this report were concerned primarily with delineating the more important resin characteristics and physical properties of polyethylene as they

relate to the manufacture of plastic containers. Section 5 presented a review of several

of the present DOT specifications and test methods as well as test procedures used by

other countries and organizations such as ASTM, SPI, and the U.N. Committee of Experts.

This survey, along with studies in our laboratory, have led to the conclusion that inadequacies exist in the present DOT specifications and test methods. Viewed overall the

recommendations of the U.N. Committee of Experts on the transport of dangerous goods and

the Japanese Industrial Standard for blow-molded containers of polyethylene appear to be more comprehensive than do the DOT regulations.

The present section is intended to provide preliminary recommendations whereby present

DOT specifications and test methods can be modified to yield more quantitative results

with greater predictive value. In Section 7, work will be described which may possibly

aid in the establishment of test methods relevant to the reuse of containers. However,

further experimental and theoretical work is necessary in the areas of mechanical properties

in order to determine the validity of the assumptions made. Such work may require experimentation on actual containers under a variety of conditions simulating transport.

6.1 Resin and Materials Properties Tests

In general, DOT regulations presently require that materials properties be run on the

resin prior to processing. Although these properties are important from a practical point

of view, they are not as important as the material properties of the finished container.

For example, if one assumes that melt index is an important material property affecting

the performance of a finished container (see the discussion of melt index under Section

3), then the fabricator should be required to keep the melt index below some acceptable

limit throughout his process to the finished container. Changes in the melt index are accompanied by changes in tensile properties, crystallinity, and molecular orientation

which may have an adverse effect on container performance. In the case of steel cylinders

for compressed gases, the regulations already require data from materials tests on finished

containers. It should be possible to devise similar criteria for plastic containers.

Furthermore, in the absence of such data it is not possible to correlate failures in the

field with suspected defects in container design or industrial forming processes.

6.2 Test on Actual Containers

It should be apparent from the discussion in Section 5 that most of the test method

procedures concerned with the mechanical integrity of the container could be improved by

the incorporation of the idea of a test to failure. For example, the present DOT Drop

Test for Specification 34 containers is based on a pass-fail criteria at fixed levels of

performance determined from statistical samples of doubtful meaning. The importance of

this statement is underscored by the fact that failures in drop tests have provided most

of the clear indications that some manufacturers have been experiencing difficulty in

making containers that meet specifications. ASTM test D-2463-74 provides a clear example

of how the drop test procedures can be modified to provide a more quantitative result with

greater statistical validity. An important point to be made here is that it is not sufficient to base acceptance upon a pass-fail criteria determined at only one drop height.

More extensive test data should be required which provide information about the breadth of

the pass-fail distribution curve.

Similar statements apply to the vibration test, hydrostatic pressure test, and static

compression test. The importance of running the hydrostatic pressure test to failure will

be of particular value in determining the weak point of a new container design. In the case of the compression test, current DOT tests do not require that the applied stress

necessarily be large enough to cause yield. Such data should be required to insure that

safe stress levels, well below the yield point, will not be exceeded during normal transportation or storage usage.

21
Although modifications to these or any of the other mechanical tests to yield more quantitative measures of the failure in these modes would be valuable in showing differences

in effectiveness of container manufacturers' container design, none of these mechanical

tests face the questions of long-time failure in polyethylene containers.

As was discussed in Section 3, polyethylene is known to fail as a result of the

application of low stresses for long times. This is a well known phenomenon and is documented in Section 7. Thus, it is possible that a container which passes the drop test (a

short-time, high-stress test) may fail upon being shipped for many days under conditions

which will lead to an increase in vapor pressure of the material. This problem is more

serious in the case of a container to be reused. A container may experience several drops

as well as long-time, low-stress situations. The cumulative damage caused by these varied

incidents could lead to failure during the next shipping. Just which test will assure

that failure is unlikely to occur is currently under investigation.

None of the tests surveyed from other sources face this problem. There exist few, if

any, retest procedures. Perhaps this is primarily a reflection of the fact that most

container specifications, which have been surveyed, do not specifically address reuse. The

one test which possibly comes closest to considering the question of long time failure is

the stacking test, specified by the U.N. Committee of Experts on the transport of dangerous

goods. In their test, the container must be capable of withstanding a predetermined applied

stress for a period of 28 days at 40 °C (104 °F).

From the discussion above, it seems that two kinds of lifetime tests are needed. One

type is needed to determine whether or not a new container will be able to withstand the

various temperatures and stress histories which are likely to be experienced under extreme

shipping conditions. A second category of tests, or retests, is needed to determine

whether a container intended for reuse can be used for another extended period of use. Both types of tests would be expected to give an estimate of the lifetime of the container.

Two sets of data are necessary to determine how these tests may be established.

First, estimates of the stress-temperature history the container sees during a year's use need to be established. Secondly, it is necessary to determine how failure of a polymer

depends on its previous stress-temperature history. For example, are many high-stress,

short-time occurrences (resulting from drops) more likely to cause failures than a longterm, low-stress situation resulting from stacking?

The first question falls within the realm of study of the transportation process

itself. The second question can only be answered by a study of the failure mechanisms of

the polymer. Studies with this intent in mind have been initiated and the results to date

are reported on in Section 7. In fact the experimental studies reported in Section 7,

although, as yet, far from complete, have led to suggestions for some general tests on lifetime of containers. The general scheme of the tests would be:

(1) In order to make meaningful lifetime tests for a container, measurements should

be made on polymer specimens obtained from an actual container. To do this,

pieces should be cut from an actual container and cut into tensile specimens.

The specimens are then subjected to high temperatures (i.e., 77 °C) for a number

of different tensile loads. The time to break as a function of tensile load is

then determined. From these data and baseline data on polyethylene, similar to that provided in Section 7, the lifetime of the container can be estimated.

However, it must be recognized that the data supplied in Section 7 do not

represent the whole picture, since the data were obtained for uniaxial stresses

only. In actual use, materials nearly always experience biaxial, or multiaxial

stresses which may significantly alter the lifetime behavior.

(2) A test-to-failure with known hydrostatic pressure should be applied to a container

of new design or made by a new process or new processor. The time-to-fail and

the weak spot of the container design is thus determined. From the total con- tainer data and material data from (1) above, a better estimate of the lifetime

of containers, made in the same way, can be obtained.

22
In this manner, one would have an estimation of failure times for a given container

design from the second test discussed above and a control on the effect of materials or processing changes from the first test.

6.3 Tests Relating to Interaction of the Container and the Lading

There is little or nothing in the present DOT requirements concerning the interactions

of the container with Its lading. For containers made of polymeric materials, this is a major problem as was indicated in Section 3. The DOT requirement 49 CFR only requires

that the lading and the material of the container be compatible. It was pointed out in Section 5, there is not a sequence of tests which define compatibility. In the case of

exemptions, compatibility is better defined in terms of a sequence of tests and observations.

This sequence of tests tries to establish quantitatively permeation loss, stress-cracking,

and chemical attack, but even here quantitative requirements are not required. More quantitative tests appear to be necessary. In the following, several approaches to quantitative

requirements will be proposed.

1) Stress-Cracking Tests

a) Lifetime tests in the presence of lading materials

The following proposed tests depend on, as yet, unconfirmed assumptions. The usefulness

of the tests as measures of lifetimes (especially for reuse) in the presence of lading

depends on research proposed to be done.

All containers of new design, made by a new processor, or made from a new polyethylene

should be tested to failure at high temperatures and applied pressure with a known stresscracking agent, for example, Igepal solution. This test could replace test number 2 discussed in the previous subsection. The time-to-fail and weak point of the container,

with a known stress-cracking agent, should be determined.

From time-to-fail with a known stress-cracking agent, container lifetime with different

ladings can be estimated. This would be done in the following manner: a scale of the

stress-cracking ability would be established for the various ladings by determining the

time-to-break versus stress curves for these various ladings materials on laboratory

specimens of polyethylene. The time-to-break curves would be scaled to the time-to-break

curves on specimens stress-cracked with the known stress-cracking agent. From these data,

the lifetime of the container with the lading could be estimated.

The validity of the scaling procedure depends on the assumption that the time-tobreak curves of various ladings are of the same functional form. Research aimed at improving

the experimental underpinnings of these assumptions is proposed for next year.

b) Reuse criteria

Any of the above tests to determine the lifetime of the container could be used for

retests to determine available further lifetime of the containers after an extended period

of use. The success of such a procedure depends on the assumption that damage due to stress cracking and/or permeation induced moduli changes is additive for different ladings.

It is proposed (in future work) to search for additivity or superposition criteria.

7. LABORATORY STUDIES OF MECHANICAL PROPERTIES LEADING TO THE FAILURE OF POLYETHYLENE

In this section the laboratory work carried out to date in the NBS Polymers Division

pertaining to mechanical properties will be summarized. The present studies were undertaken

with the goals of: (1) determining certain baseline data relevant to the failure of polyethylene under adverse and nonadverse conditions, and (2) exploring concepts which might

prove useful in establishing procedures whereby plastic containers can be tested for

reuse. The point should be made that in order to accomplish (2) a knowledge of (1) is

required. What one is particularly looking for in (1) are insights which will lead to the

estimation of long-term behavior based on relatively short-time tests.

23
7.1 General Introduction

The main problem associated with the reuse of plastic packaging for hazardous materials

ladings lies In predicting the lifetime of the container. In actual use, a container may

be subjected to a wide range of mechanical and environmental histories. Its lifetime will

depend not only upon the present state of stress (or strain) and environment, but on the

past stress (or strain) history and environment as well. By environment, we mean both

physical and chemical, including temperature and type of lading. As described in earlier

sections, when different resins from which the containers are made are compared, the life

time will further depend on molecular factors such as chemical composition, molecular

weight distribution, impurities, and architectural make-up (morphology). Another variable

which can influence lifetime Is that of the processing itself. It is entirely possible

for two manufacturers of a similar type of container, using the same polymer resin, to produce containers of markedly different mechanical properties, including lifetime, due

simply to differences In the operating procedures of processing.

It is known further from earlier work in our laboratory that even the simplest equation capable of describing the mechanical behavior of polymers is complicated due to the

dependence of the material's "memory" on previous strain history. It is also known that

the memory is dependent on temperature, swelling agents, and oxidative degradation.

Because the stress applied on a material at any time depends on all these factors in a rather complex way, the status of a container in reuse cannot be specified without a knowledge of its previous in-use history.

To date, the information available in the open literature concerning ultimate properties

of polymeric materials is fragmentary, and no study concerned with lifetime has addressed

itself to all of the important factors. The initial task requires considerable groundwork in the form of experiments to determine how polymeric materials behave under conditions

involving even simple histories. Such experiments can provide the basis upon which an understanding of the more complex histories likely to be encountered in actual container

performance can be built.

Of concern here are several rather specific aspects of the overall problem and the

present work represents an attempt to establish some of the more important experimental

and theoretical parameters involved. Section 7, which is divided into four subsections

and two appendices will deal exclusively with polyethylene since the great majority of

commercial plastics containers in use today are fabricated from polyethylene.

1 . Experimental Procedures

2. Time-to-Break Study

(i) Linder conditions of static loading to air

(ii) Under adverse chemical environment

3. Description of Failed Samples

4. Assumption of additivity of damage

Appendix A - Constant Rate of Loading Experiments

Appendix B - Comment on Viscoeleastic Properties

1) Experimental Procedures

a) Samples

Linear polyethylenes (PE) of three different molecular weights were used to obtain

time-to-break data. Some of the pertinent sample characteristics are summarized in Table

7.1. The two polyethylenes labeled A and B were from a commercial source and had weight

average molecular weights, as determined by light scattering, of 160,000 and 90,000, respectively. Both polymers contained lonol and dilaurylthiodipropionate as thermal stabilizers

24
and had relatively little branching. However, It should be pointed out that both resins

had experienced rather long shelf lives prior to molding and it is not clear to what

extent the thermal stabilizers remained operative. Sample C, which had a weight average

molecular weight of 50,000 as determined by Gel Permeation Chromatography, was the NBS

Standard Reference Material (SRM) 1475. Additionally, samples were cut from two commercial

PE containers, namely a cider and bleach bottle.

For the time-to-break study, molded specimens of all three molecular weights were

prepared as follows. Since each resin was in the form of pellets, a flat sheet 15 x 15 x 0.15 cm was molded first. The pellets were melted in a heated press at the temperature

specified in Table 7.1 and a pressure of approximately 3.8 x 10^ pascals (556 psi) then

applied. Each sheet was allowed to cool under pressure in the press to a temperature

below 80 °C before being removed. Each sheet was then remolded in a "picture-frame" type

mold under the same conditions outlined above to obtain a more bubble free and thinner

sheet 15 X 5 X 0.1 cm in dimensions. In order to relax each sheet at a temperature higher

than any to be employed in the breaking experiment, each sheet was heated in an oven at 115 °C in air for a period of 72 hours.

Individual specimens were cut from each sheet by first cutting the sheet into 0.635

cm wide strips which were then recut with a die into the shape of a "dog-bone." The final

gauge length of the thin section was approximately 3 cm long and 0.32 cm in width. For purposes of area determination, each specimen was measured along its entire gauge length

with a dial gauge in order to establish the minimum thickness. For the molded specimens,

the maximum variation in thickness for a given sample was in all cases less than 0.01 cm. (10 percent)

.

In the case of the commercial container materials, the bottles were blow-molded, and

variations in thickness over the length of the specimen were in some cases as great as 30

percent. In all cases, the areas used for calculation of the load per unit area were the

minimum measured values.

The densities quoted in Table 7.1 were determined by hydrostatic weighing in boiled

distilled water. For samples A, B, and C, the values shown represent averages based on measurement of specimens from four different sheets of each molecular weight, whereas for

the two commercial containers, the values are based on only a single determination. In the case of samples A, B, and C, the largest deviation from the average was 0.001 g/cm^, which

based on analysis of the measurement technique also represents the estimated error.

7.2 Apparatus

For the time-to-break study, the specimens were clamped in aluminum clamps designed so that they hung vertically with little or no bending of the specimen while under load.

Under the conditions of static loading, the time-to-break varied from 1 second to approximately 145 days. In cases where the specimen was subjected to a constant rate of loading,

a light weight plastic bucket was attached to the lower clamp and water allowed to flow in at a predetermined constant rate until the specimen broke. Such an arrangement is convenient

in that the bucket could either be preloaded statically for a predetermined period of time

before superimposing a constant rate of loading, or not, depending upon the experiment.

This method also has the added advantage that the specimen always broke with little, if

any, cold-drawing.

In addition to the experiments at room temperature, data were also collected at 330 K (57 °C), 350 K (77 °C), and 370 K (97 °C). In these instances, the breaking experiment

was done in an oven which could be controlled to within 0.03 K during the time of loading.

As will become evident later from the results, a small change in temperature can be of

major importance in influencing the time-to-break for polyethylene.

2) Time-to-Break Study

a) Time-to-break as a function of temperature for constant loads

Time-to-break data (tB in seconds) as a function of stress (expressed as kgf/cm^) are shown in figures 7.1, 7.2, and 7.3 for the three different molecular weight samples of

polyethylene. Equivalent results for the two commercial containers are presented in

figure 7.4.

25
TABLE 7.1

SAMPLE CHARACTERISTICS PERTINENT TO TIME-TO-BREAK STUDY

Sample Approximate

MOLECULAR

WEIGHT

Molding

TEMPERATURE

°c

Annealing

TEMPERATURE

“C

Density

AFTER ANNEALING

g/cm^

A 160,000 175 115 0.969

B 90,000 160 115 0.973

C 50,000 160 115 0.971

Commercial

CIDER

CONTAINER

Unknown Untreated 0.9A6

Commercial

BLEACH

CONTAINER

Unknown Untreated 0.957

26
time

to

break

(s)

Log. Time- to-Break Versus Static Load for Polyethylene Sample A

(M^ = 160,000).

7.1
time

to

break

(s)

7.2 Log. Time- to-Break Versus Static Load for Polyethylene Sample B

(M^ = 90,000). Open Symbols - in air, 0 - presoaked in motor

oil at 60°C for 24 hours, 0 - presoaked in hexane at 23°C for

24 hours.

28
time

to

break

(s)

7.3 Log. Time-to-Break Versus Static Load for Polyethylene Sample C

(M^ = 50,000).

29
For sample A (Mw = 160,000), it is to be observed from figure 7.1 that at room tem- perature (296 K) the log te is essentially a linear function of load over nearly six decades

of time. Also, there is little scatter of the data points, particularly at the long

times. In equation form the data can be expressed reasonably well by

In tg = In t^ - ciog (1

)

where to and a are material properties and a is the applied stress expressed in kgf/cm^.

This equation is of the same form as that developed by Coleman [4] to describe the creep

failure of nylon yarns. Interestingly enough, the linear line shown in figure 7.1 if

extrapolated to zero load, yields a to from equation (1) of 2.4 x 10® seconds. It is

known that at small loads, the curve will eventually turn upward since it is unlikely that

a relaxed specimen would actually break under its own weight in a span of only 6.5 years.

However, it must be recognized that it is possible for a polymeric container to fail in a finite time even if stored empty, due to the presence of internal stresses which require

very long times to relax. It is also possible that at the very long times, environmental

factors such as the presence of oxygen may become important contributions to early failure

under stress.

For the two lower molecular weight samples B and C (figs. 7.2 and 7.3), similar

remarks apply to the room temperature results. Note, however, that with decreased molecular

weight the scatter becomes progressively worse at the long times.

Since temperature is known to be an important factor in determining lifetime, all

three PE resins were examined at several elevated temperatures. Just how dramatic the

effect is can be seen from the results for sample A (fig. 7.1). For a stress of 75

kgf/cm^, tB dropped nearly 5.5 decades in time from 296 K to 370 K (97 °C). At 150 kgf/cm^,

an increase in temperature of only 34 °C decreased tB by a factor of more than 600. Since

plastic containers in actual use may be subjected to temperatures in this range, temperature

must be considered of primary importance especially when hazardous materials are involved.

Another salient feature of the data is that, except for sample B, a pronounced curvature

upward appears at the higher temperatures with decreased loading. It is interesting to find that at the higher temperatures the scatter in the data points becomes significantly

less than is the case at room temperature. In the case of sample B, the slope of the log

tB vs load curve does increase with increased temperature, but there is little or no curvature of the line indicated.

Several major features stand out when the results in figures 7.1 to 7.3 are compared

overall. The first is that at large stress (>250 kgf/cm^), the 296 K line is no longer

linear but curves downward. This statement applies to sample A, but only at loads greater

than those shown in figure 7.1. The second feature is that, while there does appear to be

an effect of molecular weight, it is not especially important except at long times. That

is, at room temperature the slope of the log tB vs load time tends to decrease with decreased molecular weight, but only at long times (>10® seconds) does the effect become

prominent. For example, at 75 kgf/cm^ the tB for sample C is about one decade less than

that for sample A. However, it cannot be stated with certainty whether the observed

behavior is a result of molecular weight, or is a manifestation of density or crystallinity

differences, the possibility that more than one breaking mechanism is operative, or environmental factors.

Time-to-break data for specimens from the two commercial cider and bleach bottles are shown in figure 7.4. No substantive information is available concerning the polyethylene

resin used to make either container. However, judging by their intended use it can be sur- mised that the molecular weight is likely to be high and that they contain some branched

material. In addition, the bleach bottle contains white dye. Since both were blow-molded,

some orientation may also be presented. In the present study, all of the specimens were cut

vertically and it is not yet established whether or not specimens cut horizontally along a ci rcumference, or diagonally, will have a measurably different time-to-break behavior.

The feature of primary interest is the much greater slope over the entire load range

than is observed for the other three PE polymers. This feature has the interesting conse- quence that the curves cross over. Whereas the commercial containers fail much earlier at

30
time

to

break

(s)

7.4 Log. Time-to-Break Versus Static Load for Two Coitinercial

Polyethylene Containers.

31
7.5 Comparison of Log. Time- to-Break Versus Static Load for Sample A

and Two Commercial Polyethylene Containers.

32
large loads, they become superior in terms of time-to-break at the small loads. This

behavior is demonstrated in figure 7.5 where >the lines shown are those taken from figures

7.1 and 7.4. For example, at 296 K and a stress of 225 kgf/cm^ the commercial bottle

specimens broke in a time 100 times shorter than did sample A, yet at a stress of 100

kgf/cm2 they are a decade or more longer in time-to-break. Based on the 350 K data, one would conclude that at stresses of less than about 60 kgf/cm^, the commercial bottle would

require an exceedingly long time-to-break in air.

Another interesting result is that both types of commercial bottles behave identically at room temperature. Whether this behavior is accidental or the two resins are basically the same is not known.

7.3 Environmental Factors

It is well known that environmental factors play an important role in deciding the

ultimate properties of materials. Although environmental factors have not yet been examined

in any depth, several possible stress-cracking agents have been examined in order to demonstrate the effect in this report. Specimens from sample B were soaked in motor oil

at 60 °C for 24 hours and in hexane at 23 °C for 24 hours. The results are shown in figure

7.2. Only three specimens soaked in motor oil were tested, one at each of the temperatures

used in the previous study. As is evident from figure 7.2, all three points essentially

lie within the experimental scatter, and therefore the motor oil had little or no effect

for the very limited range of test conditions carried out.

On the other hand, hexane, which is known to be a swelling agent for PE, has a profound

influence on the time-to-break of this polyethylene even at room temperature. The timeto-break dropped by about a factor of two for specimens soaked in hexane for only one day.

Note that for the specimens tested at 330 K (fig. 7.2), the effect appears to diminish as the load is decreased. This effect probably is a reflection that at elevated temperature,

and since the specimen during the test was not submerged in hexane, the hexane was driven

out of the specimen because of the increased length of the experiment.

It muat be emphasized that all of the data shown here were obtained for specimens

exposed to an environment prior to loading. Were the specimens to be in a stressed state

during initial exposure, the effect no doubt will be far more dramatic.

7.4 Specimen Appearance after Failure

Another aspect of the time-to-break study concerns the types of failure exhibited by

the various samples of polyethylene. Included in figures 7.6 and 7.7 are photographs

taken of a number of failed specimens which are representative of samples A and B. The

behavior of sample C was essentially the same as that for sample B. Shown to the left of

each specimen is the tensile load applied and to the right the time-to-break. Since

several of the specimens did not actually break but cold-drew until the weight was stopped

by the framework, the question arose as to what constituted breaking. As a criterion to determine the time-to-break, the rather simplistic definition was adopted in this study

that failure occurred whenever a well defined neck appeared, or if no neck formed when the

sample broke by crack growth. In cases where necking occurred, the specimen generally drew

and/or broke in a time very short compared to the total time-to-break.

For sample A (MW = 160,000), it can be seen in figure 7.6 that at 296 K neck formation

occurred over the entire tensile load range. Following neck formation at the very large

loads, little, if any, cold-drawing occurred before the sample broke. As the applied load

decreased, the tendency to cold-draw after necking increased until at intermediate timesto-break the specimens sometimes did not break but cold-draw to large extension ratios.

Note that in some instances more than one neck formed. More recent data indicate that at even longer times the specimens show little or no tendency to neck and/or cold draw, but

break cleanly. At the higher temperatures, the tendency to form a neck and then cold-draw

becomes even more pronounced. However, at the highest temperature 370 K and for very small

loads, the reverse was true as can be seen in figure 7.6 for the specimen stressed to 40

kgf/cm2. Perhaps the most interesting specimen shown is the one at the bottom in figure

7.6 which purposely was unloaded before it could break. Long thin strands of polymer can

be seen connecting the separated material, an indication that crazing occurred rather than

simply crack growth.

33
LINEAR PE 160000

296K

TIME TO

BREAK

(sec)

LOAD

Ka/ctrr:

288

256

215

195

150

I 07

300

1140

6 700

72000

3000

330K

175

150

125

100

I 35

490

3360

LOAD

(Kg^rrft

I ME TO

BREAK

(sec)

20

232

750

2300

350

125

06

370K

I 680

5400

8 700

7.6 Appearance of Specimens from Sample A after Failure,

34
35
LINEAR PE 90000)

TIME TO

BREAK 330K (sec.)

18

22

32

79

2 46

494

830

2,471

3,000

6,200

TIME TO

BREAK 350K (sec.)

7.7 (b) 330 and 350K

Mfm ^ Mm 1 25

/ \ - J 75

730

1,6 8 0

1 4,020

1 LOAD

(Kg/tm^)

185

180 /23S

175

165

138

125

1 1 3 1

1 00

90

83 1 i ii

1 loaqI

(Kg/cmr)

113

100 1 mm < 75

68 1 ^ 53 1 • w#

(

36
In marked contrast to sample A are the specimens from sample B (MW = 90,000) which at room temperature showed no sign of necking prior to breaking (see figure 7.7a). Over the

entire load range, the fracture surfaces are relatively smooth as in brittle fracture. It

is important to point out that prior to the actual breaking, in many instances severe damage in the form of a whitened appearance occurred throughout the interior of the entire

gauge length. Such a result is usually indicative of massive internal and/or surface

defects such as crazing and demonstrates that the failure represents a bulk phenomenon and

does not arise simply from a single surface defect.

As for sample A, the tendency to neck and cold-draw becomes enhanced at the higher

temperatures for sample B (see fig. 7.7b). The specimens tested at 330 K show most

clearly the trend observed overall. At high loadings, necking occurred followed by little

or no cold-drawing before fracture. As the load decreased, cold-drawing occurred more readily until finally at sufficiently small loads the trend was reversed. At very small

loads, necking no longer occurred and the fracture was more of a brittle type. The effect

of molecular weight appears to be primarily one of enhancing the extent of cold-drawing as the molecular weight increases. Sample C (MW = 50,000) showed even less tendency to colddraw at the higher temperatures than did sample B. The results overall suggest that more

than one failure mechanism is possible depending upon the load conditions and temperature.

One useful concept which relates directly to lifetime and lifetime prediction is the

idea of additivity, or accumulation, of damage. That is to say, damage which a material

experiences as a result of a particular stress history is not recovered, but accumulates

in time to some point at which the material fails. In mathematical terms, this "criterion"

can be expressed by an equation of the type

where o(c) is the stress applied at time C and is the time-to-break for a constant

load o. When f(t) reaches unity (f(t) =1), the material fails.

In simple form an example which will serve to demonstrate the idea is given in the

figure 7.8 below.

7.5 Assumption of Additivity of Damage

t.B

0

cr A

Atn

Ato

At

to n

Figure 7.8 Schematic Representation of a Possible

Stress History Applied to a Plastic Specimen.

37
Suppose a material up to a time to has a history free of stresses. At time t^, it is

subjected to a stress oq for a period Ato- At time ti , it experiences a new stress oi for a period Ati, and so on until at time tp a stress is imposed for a period Atp.

The cumulative time of damage or fraction time to failure, is then

At At,

f(t + At ) = -—7 y + 7—7 r +

Note that in the time intervals where the stresses are zero there is no contribution to the damage. This means that In the interval from t»0 to t=ti , the total damage f(ti) is

simply

"^0

However, the same damage would occur if a smaller stress a was applied for the entire

time interval from t=0 to t=ti- This leads to the concept of equivalency of stress

histories in which case the same total damage will result.

From experiments in our laboratory (see Section 7.2) carried out isothermally for

several different temperatures, and the concept of time-temperature superposition, it is

believed that the additivity of damage criterion will also work for stress histories where

the temperature varies. If, in bur previous example, the stress history depends on temperature as well as time so that at Ato there is an associated 0 q (temperature, then the

general form of the expression from the fractional time to failure becomes

At, At,

^^^n ^^n^

^B^°o’®o^ ^B^°l ’®1

Atn

where now Cn(a.,e.) is then time-to-break at constant load a. at a temperature e.. oil 1 1

If f(tn + Atp) represents a typical stress and temperature history for one trip of a container, then for reuse

represents the maximum number of additional trips that can be expected for the container.

However, even for wel 1 -control 1 ed samples the scatter in time-to-break can be a factor of

two, so that the number of safe trips should be less than N/2. An additional safety

factor may be desirable as well. If f(t,e) for one trip is known, an equivalent stress

and temperature for a short-duration test can be determined to give 2f(t,e). Then for

reuse, several containers can be selected at random and subjected to the short-duration

test. If they pass, the containers can be used for another trip. Naturally before the

above test is applied sample containers will have to pass the drop test, or leak test, if

necessary.

Now in order for the above test to be applicable, the times 5B(aj,e\j), where i runs from 0 to n, for the container and the average damage per trip f(tp + At^) must be known;,

that is to say, data of the type presented In Section 7.2. Fulmer [5] has shown that a scaling factor can be obtained between an actual container and laboratory specimens.

Since the scaling depends on the shape, manufacturing conditions, and resin, the scaling

factor must be determined not only when changes of design are involved, but also when a different resin is used. The average damage per trip, f(t,e), can be estimated by modeling

the various conditions of an average trip.

It must be remembered that an important assumption made above is that of a constant

chemical environment, or to be more precise, under various stress and temperature histories in air. It remains to be determined whether the additivity of damage criterion

remains valid in the presence of other chemical environments. Further research is cur- rently underway in our laboratory to resolve this question.

38
With regard to the validity of the damage criterion itself, even in the absence of an adverse chemical environment, several experiments have been carried out involving relatively simple stress histories. The results of these experiments, which suggest that for

the simple stress histories employed, the concept is valid, are summarized in Appendix A.

Finally in Appendix B are included comments pertinent to viscoelastic behavior in general. The comments are intended to show that it is possible from single- and multistep stress relaxation experiments to predict the stress for any other uniaxial strain

history.

39
APPENDIX A

ADDITIVITY OF DAMAGE UNDER CONDITIONS OF A CONSTANT RATE OF LOADING

As pointed out in Section 7.3, a useful concept which relates directly to lifetime

and lifetime prediction is the idea of additivity of damage. That is to say, damage which

a material experiences as a result of a particular strain (or stress) history is not re- covered, but accumulates in time to some point at which the material will fail. In equation form, this criterion can be expressed as

where cb[o(?)] is the time-to-break for stress o(c). Such a relationship will be valid

only when the environmental conditions are held fixed. In principle, knowing the proper

form of 0 ( 5 ), it should be possible to predict the time-to-break under a complex history

of loading.

In the simple case under consideration here, that of static loading, it is known that

the time-to-break (ts) for sample A can be represented over most of the time scale covered

in figure 7.1 by

where to and a are material properties and ao is the applied load. Suppose now one wishes

to predict from the static data the time-to-bifeak for a similar specimen subjected to a constant rate of loading experiment. Then a(c) = oq + m? where m is the rate of loading

expressed as kg/cm^-s and oq is the preload, if any, in kg/cm^. In the present case, oq

represents a preload of the plastic bucket (15.7 kg/cm^) applied at time t=0. Equation

(A-1) then becomes

Here tg is the time-to-break at constant rate of loading. It is known from the data for

sample A at 296 K (fig. 7.1) that a = 0.054 cm^ kgf"^ and tg = 2.4 x 10® seconds. For m

values of 0.1 and 1.0 kgf/cm^-s, equation (A-2) yields values for tg of 2620 and 304

seconds, respectively. Constant rate of loading experiments have been done for a number

of specimens from sample A at rates varying from 0.02 to 0.7 kgf/cm^-s (room temperature).

If plotted on log-log paper, the log "tg versus log m data yield a straight line. For

rates of 0.1 and 1.0 kgf/cm^-s, the experimental values for ^ are 2450 and 290 seconds,

respectively. Therefore, the calculated values for ^g based upon static loading conditions

are within 6 percent, or less of those determined from the constant rate of loading experiments. The agreement is remarkably good and is well within the experimental scatter.

It is also possible to go the other way, that is to be able to predict the time-tobreak for static loading conditions (tg) from constant rate of loading data ('tg). In this

latter case, the appropriate equation, as derived from equation (A-1), becomes

(A-1)

t = t e

0

which integrates to yield

In (A-2)

(A-3)

40
To be successful in this case requires evaluation of

dln^g

din m

’

which for sample A at both room temperature and 350 K, represents the slope of straight

lines, at least over the range of m covered. Values of tp, as calculated from equation

(A-3), are shown in figure A. 2 for a number of specimens broken under a constant rate of

loading. The solid lines represent the same lines shown in figure 7.1. Again, the

agreement is remarkably good at both temperatures.

In principle, one might anticipate that this method can be extended to predict failure

at long times based upon shorter time tests. For example, in the case where a preload oi

is applied for a time ti and a constant rate of loading history then superposed at time ti,

equation (A-1) takes the form

where now

tg = time to break for a static stress of aj

'to = time to break from application of a constant rate of loading

D ^

= time to break for a static load of (oi + Og)

Schematically these quantities are shown in figure A-1 below.

CT

A.l Schematic Representation of a Constant Rate of

Loading Experiment in which a Preload Has Been

Applied for Time t^.

An attempt has been made to check the validity of equation (A-4) by preloading specimens

of sample A for specified times at stress in the 100 to 125 kgf/cm^ range. The t 5's calculated from equation (A-4) tend to scatter over about one decade, falling generally on the

low time side of the straight line shown in figure A-2. One problem with this method is that

for preloaded samples the log-log plot tg versus m does not necessarily yield a straight line

so that the error in determining din tg/d1n m can be large.
Tims

To

Brook

(seconds)

A. 2 Prediction of Time-to-Fail for a Static Load Based on Constant

Rate of Loading Experiments. Solid Lines - Taken from Figure

7.1; Open Circles - Predicted Values for Time-to-Fail under

Static Loading Conditions Based on Constant Rate of Loading

Experiments

.

4 2
In conclusion, it is found that for simple loading histories such as static loading or constant rate of loading (preloading for zero time), there is excellent agreement between the

values of to calculated from static loading data and those determined experimentally from

constant rate of loading results. Therefore, it would seem that the assumption of additivity of damage is a valid concept insofar as the simple histories investigated here are concerned.
APPENDIX B

A COMMENT ON VISCOELASTIC PROPERTIES

It was mentioned earlier that the mechanical properties of PE, and polymers in general,

are time-dependent as well as strain and strain (or stress) history-dependent. Fracture

mechanics as developed for metals cannot be used, especially for long time fatigue cycling.

Some years ago, a thermodynamic theory for describing the mechanical behavior of materials

known as simple elastic fluids was developed in our laboratory. This theory derived by

Bernstein, Kearsley and Zapas^ and known as the BKZ theory, was later modified to describe

the behavior of semicrystalline polymers.

In a simple version, one can translate the theory as follows: the stress at any time,

t, is given as a summation of contributions from the configuration at time t and all past

configurations at time x, weighted with the elapsed time t - t. This type of mathematical

formulation is known as a theory of materials with a fading memory. In the modified theory

the elapsed time depends on the previous strain history.

As an example, consider simple extension. The tensile stress a(t) is given by the following equation

where e(i) is the strain at time x, x(x) = 1 + e(x) and H* (o, o) is the partial derivative

of H with respect to the second argument. The function 4>(e(t), e(c), t-c) is such that it

equals unity when e(?) is equal to e(t). In a single step stress relaxation experiment

where x(x) = 1 for -«><x<0 and x(x) for x>0, equation (B-1) yields:

where H(l, t) = 0, as derived from the BKZ theory.

From single step stress relaxation experiments, the behavior of H with respect to the

first argument can be determined. Then from multi-step stress relaxation experiments, the

dependence upon the second argument can be found. By making 4> more specific, one can minimize the experiments needed. One can then predict the stress in any other uniaxial strain

history.

(B-1)

o(t) = H(x(t),t) (B-2)

“^B. Bernstein, E. A. Kearsley, and L. J. Zapas, Trans. Soc. Rheol . I, 391 (1963).
BIBLIOGRAPHY

[1] Code of Federal Regulations 49 CFR, Transportation Parts 100 to 199 (1974), U.S. Govern

ment Printing Office, Washington, D.C. 20402.

[2] ASTM Annual Book of ASTM Standarda, Parts 35 and 36 (1976), ASTM Headquarters, 1916

Race St. Philadelphia, Pa. 19103.

[3] Deformation and Fracture of High Polymere, edited by Kaush, 381-395, Plenum Press

(1973).

[4] American National Standards Catalogue, 1975 Catalogue and 1976 Supplement.

[5] World Index of Plastics Standards, NBS Special Publications No. 352 (December 1971).

[6] An Index of U.S. Voluntary Engineering Standards, NBS Special Publication No. 329.

[7] An Index of U.S. Voluntary Engineering Standards, Supplement 1 and 2, NBS Special

Publication No. 329/1 and Z2V2.

[8] Transportation of Dangerous Goods, (1970), Supplement (1973) Part II. Recommendation

prepared by United Nations Committee of Experts on the Transportation of Dangerous

Goods.

[9] International Standard (ISO) 2234, 2244, 2247 and 2248 (1972); 2872-2876 (1973).

[10] Society of the Plastics Industry, Technical Bulletin PBD-3,4,5- 1968 and PBD-8-1969.

[11] Japanese Industrial Standard, JIS-Z0202-1962, Z0205-1961, Z0212-1961, and Z1706-1966.

[12] Australian Standard Specification, AS K163-1967, Plastics Containers for the Transporta

tion of Liquids.

[13] British Standard 3897:1965, Plastics (Polyethylene) Containers.

[14] Israel Standard 755/T(250 x 4)1, Polyethylene Containers.
REFERENCES

[1] Rothwell, G. J., ASTME Vectors 1969/2.

[2] Naval Surface Weapons Center (WO) Report No. TES-20-74-7 (1974).

[3] Handbook of Vlaetioa and Elaatomere

,

C. A. Harper, Editor-in-Chief (McGraw-Hill Inc

1975).

[4] Coleman, B. D. , J. Poly Sci. 447 (1956).

[5] Fulmer, G. E., SPE Journal. No. 10, 108 (1968).

46