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The History of High Density Polyethylene (HDPE)

Here is a collection of information and research for those searching for historical development process of and evolution of todays HDPE.

www.pca.state.mn.us

inventors.about.com

k-mac-plastics.com

portal.acs.org

www.goodfellowusa.com

www.plasticpipe.org

HISTORY AND PHYSICAL CHEMISTRY OF HDPE

At the very close of the 19th century, German chemist Hans von Pechmann noted
a precipitate while working with a form of methane in ether. In 1900, German
chemists Eugen Bamberger and Friedrich Tschirner identified this compound as
polymethylene, a very close cousin to polyethylene. Thirty years later, a high-density
residue was created by an American chemist at E.I. du Pont de Nemours & Company,
Inc., Carl Shipp Marvel, by subjecting ethylene to a large amount of pressure.
Working with ethylene at high pressures, British chemists Eric Fawcett and Reginald
Gibson created a solid form of polyethylene in 1935. Its first commercial application
came during World War II, when the British used it to insulate radar cables. In 1953,
Karl Ziegler of the Kaiser Wilhelm Institute (renamed the Max Planck Institute) and
Erhard Holzkamp invented high-density polyethylene (HDPE). The process included
the use of catalysts and low pressure, which is the basis for the formulation of many
varieties of polyethylene compounds. Two years later, in 1955, HDPE was produced
as pipe. For his successful invention of HDPE, Ziegler was awarded the 1963 Nobel
Prize for Chemistry.
Today, plastic materials used for pipes are classed under thermosetting or
thermoplastic resins. Plastic highway drainage pipes belong almost entirely to the
thermoplastic group (most commonly, high-density polyethylene (HDPE), PVC
and ABS). They exhibit attributes of toughness, flexibility, chemical resistance and
non-conducting electrical properties. Thermoplastic highway drainage pipes have
been used for highway drainage since the early 1970s. Since then, growing out of
applications for agricultural drainage, more HDPE drainage pipes have been installed
than all other plastic pipes combined. They are being used for storm sewers,
perforated underdrains, storm drains, slope drains, cross drains and culverts.
Physical Chemistry and Mechanical Properties of HDPE
High-density polyethylene (HDPE) (0.941 < density < 0.965) is a thermoplastic
material composed of carbon and hydrogen atoms joined together forming high
molecular weight products as shown in Figure 1-1c. Methane gas (Figure 1-1a) is
converted into ethylene (Figure 1-1b), then, with the application of heat and pressure,
into polyethylene (Figure 1-1c). The polymer chain may be 500,000 to 1,000,000
carbon units long. Short and/or long side chain molecules exist with the polymer’s
long main chain molecules. The longer the main chain, the greater the number of

atoms, and consequently, the greater the molecular weight. The molecular weight,
the molecular weight distribution and the amount of branching determine many
of the mechanical and chemical properties of the end product.
Other common polyethylene (PE) materials are medium-density polyethylene (MDPE)
(0.926 < density < 0.940) used for low-pressure gas pipelines; low-density polyethylene
(LDPE) (0.910 < density < 0.925), typical for small-diameter water-distribution
pipes: Linear low-density polyethylene (LLDPE), which retains much of the strength
of HDPE and the flexibility of LDPE, has application for drainage pipes. Less
common PE materials are ultra-high molecular weight polyethylene (UHMWPE)
(density > 0.965) and very low density polyethylene (VLDPE) (density < 0.910).
Other thermoplastic materials used for drainage pipes are polyvinyl chloride (PVC),
polypropylene (PP), polybutylene (PB) and acrylonitrile-butadiene-styrene (ABS).
The property characteristics of polyethylene depend upon the arrangement of the
molecular chains. The molecular chains, shown schematically in Figure 1-1c, are
three-dimensional and lie in wavy planes. Not shown, but branching off the main
chains, are side chains of varying lengths. The number, size and type of these side
chains determine, in large part, the properties of density, stiffness, tensile strength,
flexibility, hardness, brittleness, elongation, creep characteristics and melt viscosity
that are the results of the manufacturing effort and can occur during service
performance of polyethylene pipe.
fig
Figure 1-1c: Polyethylene Molecular Chain

CHAPTER 1: HISTORY AND PHYSICAL CHEMISTRY OF HDPE

Polyethylene is characterized as a semi-crystalline polymer, made up of crystalline
regions and amorphous regions. Crystalline regions are those of highly ordered,
neatly folded, layered (in parallel) and densely packed molecular chains. These occur
only when chains branching off the sides of the primary chains are small in number.
Within crystalline regions, molecules have properties that are locally (within each
crystal) directionally dependent. Where tangled molecular chains branching off
the molecular trunk chains interfere with or inhibit the close and layered packing
of the trunks, the random resulting arrangement is of lesser density, and termed
amorphous. An abundance of closely packed polymer chains results in a tough
material of moderate stiffness.
High-density polyethylene resin has a greater proportion of crystalline regions than
low-density polyethylene. The size and size distribution of crystalline regions are
determinants of the tensile strength and environmental stress crack resistance of the
end product. HDPE, with fewer branches than MDPE or LDPE, has a greater
proportion of crystals, which results in greater density and greater strength (see
Figure 1-2). LDPE has a structure with both long and short molecular branches.
With a lesser proportion of crystals than HDPE, it has greater flexibility but less
strength. LLDPE structurally differs from LDPE in that the molecular trunk has
shorter branches, which serve to inhibit the polymer chains becoming too closely
packed. Hypothetically, a completely crystalline polyethylene would be too brittle
to be functional and a completely amorphous polyethylene would be waxlike, much
like paraffin. Upon heating, the ordered crystalline structure regresses to the
disordered amorphous state; with cooling, the partially crystalline structure is
recovered. This attribute permits thermal welding of polyethylene to polyethylene.
The melting point of polyethylene is defined as that temperature at which the plastic
transitions to a completely amorphous state. In HDPE and other thermoplastic
materials, the molecular chains are not cross-linked and such plastics will melt
with the application of a sufficient amount of heat. With the application of heat,
thermoplastic resins may be shaped, formed, molded or extruded. Thermosetting
resins are composed of chemically cross-linked molecular chains, which set at the
time the plastic is first formed; these resins will not melt, but rather disintegrate
at a temperature lower than its melting point, when sufficient heat is added.

fig2

During processing, elevated temperatures and energy associated with forming and
shaping the polyethylene cause random orientations of molecules within the molten
material to directionally align in the extruding orifice. At room temperatures, the
ordered arrangement of the layered crystalline polyethylene molecules is maintained.
Tie molecules link the crystalline and amorphous regions. When the capacities of the
polymer chains are overwhelmed by tension, the polymer flows (alters its shape).
Tensile forces (stresses) then initiate brittle fracture, evidenced by cracking. In HDPE
this may occur at very high strain rates.
Once a crack is initiated, tensile forces (stresses), which were contained prior to the
event of cracking, are released. These released tensile forces (stresses) are captured by
the material at the leading tips of the crack, thereby greatly increasing the intensity
of the stress field and the likelihood of continued cracking at that point and all
points forward. The terms stress riser and stress intensity factor are used to identify
and quantify the increase in the stress field at the tips of a crack. If these regions
contain and adequately respond to this increased burden, then the cracks will not
propagate; if they do not, crack propagation will result. This characterizes the
mechanism of slow crack growth. Stress risers are proportional to the measure of
stress. Cracks will not propagate in a stress-free environment or where the level
of stress at the tip of a crack is at a sufficiently low threshold. When the tip of a
propagating crack leaves a crystal, it enters the disordered, non-layered, more loosely
packed, tangled molecules of the amorphous region where the energy associated with
the stress field is partially dissipated as the tangled mass of molecules adjusts in time
to the sustained forces.

When polyethylene is pulled at low strain rates, in those areas where stretching has
taken place, elongated rearrangement of the mass will be irreversible when molecular
chains begin to slip by one another. Ultimate tensile strength occurs when the bonds
between the molecular chains are fractured. The energy that would otherwise be
stored in the system and which would otherwise be available to restore the region to
its original geometry, is dissipated and unrecoverable with the event of the fracture.
The new arrangement of molecules alters the stress/strain response of the remaining
region. With increasing load and fewer bonds to resist, the material is less stiff and
therefore takes less force to cause a unit of deformation. This phenomenon is noted
on a stress-strain curve by an ever-decreasing slope as the curve bends increasingly to
the right as the process continues. This is what defines strain softening, a characteristic
of polyethylene and all materials that yield under increasing load. (The curved stressstrain
curve of Figure 1-3 is an example of a strain softening material.) With sustained
loads, the continuing deformation is defined as plastic flow. If, at some point in the
deformation process the deformation is maintained, the loads and resulting internal
stresses relax. This process of adjustment is called stress relaxation.
Mechanical Properties and Cell Classifications
HDPE is a non-linear viscoelastic material with time-dependent properties.
A thermoplastic pipe, serving as only one component of a pipe/soil composite
structure, benefits by its attribute of stress relaxation wherein stresses (forces) are
shed and transferred to the soil. Predictability of performance of a pipe in service
(stress, strain and deformation responses, stability) requires knowledge of the
mechanical properties of the HDPE resin and knowledge of the profile geometry.
ASTM D 3350 resin cell classifications provide the means for identification,
close characterization and specification of material properties for polyethylene.
Manufacturers of HDPE drainage pipes may choose higher cell classifications than
the minimums required by these specifications in order to optimize competing
economic and performance constraints of production, handling and service.
Density, molecular weight and molecular weight distribution dominate the resin
properties that influence the manufacture of the polyethylene pipe and the subsequent
performance of the pipe. Table 1-1 lists cell classification properties and the ASTM
specification governing the laboratory procedure that defines and determines each.
(Note that melt index (MI) is inversely related to molecular weight.) Note that cell
classifications for density and molecular weight are included in Table 1-1; molecular
weight distribution (MWD) is not.

fig3

 

Molecular Weight Distribution
The distribution of different sized molecules in a polyethylene polymer typically
follows the bell shaped normal distribution curve described by the Gaussian
probability theory. As with other populations, the bell shaped curve can reflect
distributions ranging from narrow to broad. A polymer containing a broad range
of chain lengths is said to have a broad molecular weight distribution (MWD).
Resins with this type of distribution have good Environmental Stress Crack
Resistance (ESCR), good impact resistance and good processability.
A polymer with a narrow MWD contains molecules that are nearly the same
in molecular weight. It will crystallize at a faster, more uniform rate. This results
in a product that will hold its shape.
Polymers can also have a bimodal shaped distribution curve which, as the name
suggests, seem to depict a blend of two different polymer populations, each with
its particular average and distribution. Resins having a bimodal MWD contain
both very short and very long polyethylene molecules, giving the resin excellent
physical properties while maintaining good processability.
MWD is dependent upon the type of process used to manufacture the particular
polyethylene resin. For polymers of the same density and average molecular weight,
their melt flow rates are relatively independent of MWD. Therefore, resins that have
the same density and melt index (MI) can have very different molecular weight
distributions. The effects of density, molecular weight and molecular weight
distribution on physical properties are summarized in Table 1-2.

fig4

Density
The density of polyethylene is a measure of the proportion of crystals within
its mass. Crystals, a result of the layering and close packing of polyethylene
molecules, are denser than the tangled, disordered arrangement of molecules in the
amorphous regions. Copolymers are often used to create and control the formation
of side branches. Homopolymers, with densities of 0.960 and above, are produced
without copolymers and experience very little branching. To reduce the density,
butene, hexene or octene are added to make a copolymer. Butene will add branches
two carbon units long; hexene, four carbon units long; and octene, six carbon units
long. The greater the length of the branched carbon chains, the lower the final
density. ASTM D 3350 classifies polyethylene by density as follows: high-density
polyethylene (HDPE) (0.941 < density < 0.965), low-density polyethylene (LDPE)
(0.910 < density < 0.925), medium-density polyethylene (MDPE) (0.926 < density <

0.940). Less commonly employed PE materials are homopolymers (density > 0.965)
and very low density polyethylene (VLDPE) (density < 0.910). Flexural stiffness and
tensile strength increase with density; the result is increasing brittleness, and decreasing
toughness and stress crack resistance.
Melt Index
The melt flow rate measures the viscosity of the polyethylene resin in its molten
state. It is a parameter related to the average molecular weight of the resin chains
of polymer extruded through a standard size orifice under specified conditions of
pressure and temperature in a ten-minute period of time. The greater the lengths
of molecules, the greater the molecular weight and the greater the difficulty in
extruding the resin through the standard orifice. The result: resins of greater viscosity
as measured by a lower melt flow rate. When the test is conducted with pressure
delivered by a standard load caused by a 47.6 lb (21.6 kg) weight at a temperature
of 374°F (190°C ), the resulting melt flow rate is termed the melt index (MI). The
greater the viscosity, the lower the melt index value.
A lower MI (higher average molecular weight) is predictive of greater tensile strength,
toughness and greater stress crack resistance. However, the lower the MI, the greater
the energy required, at any extrusion temperature, to extrude polyethylene resin.
The average molecular weight, as measured by the MI, does not identify the range
of chain lengths within the molecules; the molecular weight distribution (MWD)
does. Polyethylene polymers of the same MI and the same density may have very
different properties if the molecular weight distributions (MWD) are different.
A polymer with a narrow MWD will crystallize more rapidly and with greater
uniformity, resulting in less warpage and greater fidelity to the intended geometry.
A polymer with broad MWD may have better stress crack resistance, impact
resistance and ease of processing.
Flexural Modulus
The flexural modulus (Ef ) is a material stiffness that is, in part, predictive of a
structure or a structural element’s resistance to bending under the application of
loads. When combined with the geometric stiffness (a function of the moment of
inertia and other geometric properties), the composite stiffness is termed the bending
stiffness. The greater the bending stiffness, the greater the bending resistance and,
other things being equal, the greater the bending stresses. For flexible pipe, the
material modulus (E) is a composite of the material’s flexural stiffness (Ef) and ring
compression stiffness (Ec). Current design practice assumes equivalence for working
values Ef and Ec.

Non-linear stress/strain curves of HDPE, and the modular values derived therefrom,
are sensitive to rates of load application and are generally ‘linear’ up to approximately
2% strain. Stress and strain are determined at the point of maximum bending on a
simply supported test beam caused by a centrally applied load. The slope of the line
drawn between points of zero strain and 2% strain on a stress/strain curve typically
defines the flexural modulus. Because of the stress relaxation attribute of HDPE, the
less rapid the loading and the longer the duration of load application, the flatter the
early slope of the stress/strain curve and the lower the estimate of flexural modulus;
hence the need for a carefully defined (see ASTM D 790) rate of load application.
(See Figure 1-3.)

fig5

 

For HDPE pipes, the minimum pipe stiffness requirements set by specification
determines, in part, the amount of material required, the cost of which dominates
the cost of the finished pipe delivered to the job site. The characteristics of the
stress/strain curve and the associated values of stress, strain and pipe stiffness
are sensitive to the rates of application of load and displacement.
Stiffness requirements for pipes of any material may be met by material adjustments
to the modulus of elasticity, geometric adjustments to the moment of inertia, or both.
Profile pipe walls, easily shaped in HDPE by extrusion and/or vacuum forming, are
designed to increase the wall’s moment of inertia above that which would be the case
for a solid wall pipe of the same material content, thereby enabling an optimization
of cross-sectional area. The flexural modulus increases with density for a given melt
index. See Table 1.2 for the effects of changes in density and melt index on the more
general properties of HDPE.

Tensile Strength
The point at which a stress causes a material to deform beyond its elastic region
(permanent deformation) is called the tensile strength at yield. When stressed
below the yield point, an elastic material recovers all the energy that went into its
deformation. Recovery is possible for polyethylene when the crystals are subjected
to low strain levels and maintain their integrity. A formulation of greater density
(higher fraction of crystals, lower melt index) is predictive of greater tensile strength
and increasing brittleness.
The force required to break the test sample is called the ultimate strength or the
tensile strength at break. The strength is calculated by dividing the force (at yield
or break) by the original cross-sectional area. ASTM D 638, Standard Test Method for
Tensile Properties of Plastics, is used to determine the tensile properties of polyethylene
pipe resins. Test specimens are usually shaped as a flat “dog bone”, but specimens can
also be rod-shaped or tubular per ASTM D 638. During the tensile test, polyethylene,
which is a ductile material, exhibits a cold drawing phenomenon once the yield
strength is exceeded. The test sample develops a “neck down” region where the molecules
begin to align themselves in the direction of the applied load. This strain-induced
orientation causes the material to become stiffer in the axial direction while the
transverse direction (90° to the axial direction) strength is lower. The stretching or
elongation for materials such as polyethylene can be ten times the original gauge
length of the sample (1000% elongation). Failure occurs when the molecules reach
their breaking strain or when test sample defects, such as edge nicks, begin to grow
and cause failure. Fibrillation, the stretching and tearing of the polymer structure,
usually occurs just prior to rupture.
Tensile or compressive elastic deformations of a test specimen along a longitudinal
axis excite respective inward or outward deformations parallel to a transverse axis
normal to the first. Poission’s ratio is the ratio of lateral strain to longitudinal strain.
When tested according to ASTM E 132, Standard Test Method for Poisson’s Ratio at
Room Temperature, Poisson’s ratio for polyethylene is between 0.40 and 0.45.
Environmental Stress Crack Resisitance (ESCR)
Under certain conditions of temperature and stress in the presence of certain
chemicals, polyethylene may begin to crack sooner than it would at the same
temperature and stress in the absence of those chemicals. This phenomenon
is called environmental stress cracking (ESC).

Stress cracking agents for polyethylene tend to be polar materials such as alcohols,
detergents (wetting agents), halogens and aromatics. The property of a material
to resist ESC is called environmental stress crack resistance, or simply ESCR. The
mechanism is not fully understood, but failures from ESC tend to be due to the
development of cracks in areas of tensile stress which slowly grow and propagate
over time. Stress cracking may be avoided by using appropriate resin formulations
of stress crack resistant materials; appropriate geometric designs and manufacturing
controls that prevent the occurrence of severe stress risers; and by limiting stresses
and strains during pipe installation.
There are over 40 different ESCR test methods used to determine the chemical
resistance of various materials. The standard test currently used in the polyethylene
industry is the bent-strip test. It is also called the “Bell Test,” since it was developed
during the 1950’s for wire and cable coatings for the telephone industry. ASTM
D 1693, Standard Test Method for Environmental Stress Cracking of Ethylene Plastics,
describes the test method used to determine the ESCR value for polyethylene. Ten
small compression-molded specimens are notched and bent and then placed into a
holder. The holder is immersed into a tube of a surfactant, typically one such as
Igepal CO-630 at 212°F (100°C) and 100% concentration, and the time to failure is
noted. The results are reported using the notation Fxx, where xx is the percentage of
samples that have failed. For example, the statement F20=500 hours means that 20%
of the samples have failed within 0 to 500 hours.
This test was developed when the time to failure was less than 10 hours. Excellent
stress crack resistance of modern resins, coupled with stress relaxation in the pre-bent
samples results in a test method wherein few failures occur. The efficacy of the test
diminishes after a few hundred hours. This test is currently used mainly as a quality
assurance test rather than providing definitive rankings of pipe performance.
Notched Constant Ligament Stress (NCLS)
Disadvantages of the ESCR test method are overcome with the Notched Constant
Tensile Load (NCTL) test as described in ASTM D 5397. Because ASTM D 5397 is
intended for geosynthetic materials using membranes as the specimen, a new test
method was developed for piping materials – the Notched Constant Ligament Stress
(NCLS) test. In this test method, HDPE resin is compression molded into a plaque.
Dumbbell samples are machined from the plaque and notched in the midsection.
Samples are placed in an elevated temperature bath containing a wetting agent for
acceleration. The sample is then subjected to a constant ligament stress until a brittle
failure occurs from slow crack growth. This is now an ASTM test method, F 2136.

HDPE – A Material of Choice
Metal, plastic, concrete and clay make up most of the materials used for the
manufacture of drainage pipes. Metal pipes may be steel, ductile iron or aluminum;
concrete pipes may be steel-reinforced, earth-reinforced, non-reinforced, precast or
cast-in-place; and plastic pipes may be of thermosetting resins (e.g., glass-reinforced
epoxy or polyurethane) or thermoplastic resins (e.g., HDPE, PVC, polypropylene
or ABS (acrylonitrile-butadiene-styrene)). The material longest in use is vitrified clay;
the newest materials are plastic. Some pipes are built with a combination of materials;
corrugated steel pipes lined and/or coated and/or paved (inverts) with plastic,
bituminous or concrete materials. Durability (mostly, resistance to chemical and
electro-chemical corrosion and abrasion), surety of structural performance over
time, integrity of joints, surety of hydraulic performance (as pipe ages), ease of
construction, availability and life cycle costs dominate the choice of pipe material(s).
Highway drainage facilities are often subject to hostile effluents and embedment soils.
Concrete pipe is subject to chemical attack when in the environments of low pH
(acids) and/or soluble salts (sulfates and chlorides) in drainage waters and neighboring
soils. Sulfates, mainly those of sodium, calcium, potassium and magnesium, are found
in many locations in the states of the northern Great Plains, in the alkali soils of western
and southwestern arid regions, and in seawater. Uncoated (or otherwise unprotected)
galvanized steel pipes are degraded in environments of low pH and low resistivity of
soil or water. Permissible levels of pH vary by jurisdiction; a range of soil or water of
6.0 < pH < 9.5 is generally accepted. Unlike pipes of concrete, steel, aluminum and
iron, thermoplastic and vitrified clay pipes do not corrode or otherwise degrade in
these environments; expensive maintenance is not required. Unlike metal pipes and
steel reinforcement of concrete pipes, thermoplastic and vitrified clay pipes are nonconductors;
cathodic protection is not required to prevent degradation due to galvanic
corrosion at locations of low soil resistivity or in the vicinity of stray electrical direct
currents. Polyethylene is often used to line and encase metal pipes thereby offering
barrier protection from aggressive soils or stray electrical currents leading to galvanic
corrosion. HDPE offers a range of 1.5 < pH < 14.
Accidental highway spillage of high concentrations of some organically based
chemicals, such as crude oils and their derivatives (solvents, gasoline, kerosene)
or concentrated acids and bases, may cause swelling and softening of thermoplastic
materials if sustained over long periods (measured in months). Of the four most
common drainage pipes of thermoplastic materials (ABS, PVC, polypropylene,
and HDPE), resistance to these aggressive chemicals is in the order noted; ABS
the least resistive, HDPE is the most resistive.

Polyester and epoxy thermosetting resin pipes, reinforced with continuous windings
of glass filaments, primarily intended for sanitary sewers, were found to be corrosive
in the presence of available hydrogen ion (present in acids and water). Penetration
to the glass/resin interface may result in debonding of the glass reinforcement and
wicking along the glass/resin interface. Thermosetting resin pipes reinforced with
randomly oriented chopped fibers of short lengths have succeeded these pipes.
The chemical inertness of HDPE and the flexible “trampoline” response of the long
chain molecules of HDPE result in a highly corrosion-resistant material. HDPE pipe
is most often favored for transporting slurries containing highly abrasive mine tailings.
Abrasion of metal, bituminous and concrete protective coatings of metal and concrete
pipes (a function of the square of the flow velocity) leave these pipes vulnerable to
accelerated erosion after penetration to the bare pipe material.
For the same conditions of embedment, the more flexible the pipe the lesser
the proportion of overburden load attracted to the pipe. The attribute of stress
relaxation of HDPE pipes (and thermoplastic pipes in general), which is greater than
any relaxation of the embedding soil, assures that overburden loads and stresses within
the pipe walls will decrease with time. The result is that a significant proportion of
loads initially resisted by a flexible pipe will be transferred to the soil of the pipe/soil
composite structure; the opposite is true for rigid pipes. Furthermore, the ability of
buried flexible pipes to alter their shapes from circles to ellipses is exactly what
transforms much of what would be bending stresses (which include tensile stresses)
into membrane ring compression stresses. For the same conditions of embedment,
rigid pipes (which lack the ability to comply with alteration of shape) respond with
greater tensile stresses than flexible pipes and, in the case of concrete pipes, require
steel reinforcement to manage these tensile stresses. HDPE pipes, properly embedded
in a competent soil mass, result in a formidable soil/pipe composite structure that is
almost entirely in the favored ring compression.
Favorable and commonly accepted roughness values of Manning’s ‘n’ of 0.010 – 0.013
make smooth-lined corrugated HDPE a favorable choice for the transport of drainage
waters. Velocity of flow is insensitive to changes in pipe shape due to service loads.
The non-stick surface of HDPE resists scaling and pitting, and therefore does not
require a design with a less favorable Manning’s ‘n’ to accommodate future conditions.

Additional Considerations

Crack Resistance:
Weak molecular bonds, perpendicular to the densely packed layered molecules of
polyethylene crystals, tie adjacent molecules. In response to tensile stresses, cracks may
form and propagate parallel to these layers by rupturing these weak bonds. Less dense
and disordered arrangements of molecules in amorphous regions are more resistant to
crack propagation than the layered molecules in crystals. For polyethylene resins of
the same molecular weight, the lesser the density, the greater the resistance to stress
cracking. The greater the proportion of crystals, the greater the density and brittleness
of the resin. Density alone, however, is an inadequate predictor of stress crack resistance.
All common materials, extruded or otherwise shaped or formed at elevated
processing temperatures, shrink during cooling. Residual stresses, which result,
combine with those stresses resisting externally applied loads. In processes where
stretching after forming takes place result in mechanical properties parallel to the
direction of stretch different than those oriented perpendicular to the direction of
stretch. At low rates of strain, should cracking of these orthotropic materials occur,
they are likely to be parallel to the direction of stretch.
A more general purpose of ASTM D 1693, the test for ESCR, is prediction of
the performance of ethylene resins subjected to environments such as soaps,
wetting agents, oils, detergents or other materials likely to be stored or marketed
in containers. This test is likely to assure proper material formulation (inclusive
of post-consumer recycled resins) and to minimize contaminant inclusions.
Compression
The response of a buried flexible pipe is dominated by compression. Note in Table
1-1 there is no cell classification for compression. For purposes of design and for
small strains (less than 2%), the compression modulus is taken to be of equal
magnitude as the elastic tensile modulus. At greater stress levels, compression strain
is less than the tensile strain. HDPE in compression does not tear or crack; stability
for thin elements is a design consideration.

Bibliography
AASHTO M294 – Materials Specification for Corrugated Polyethylene Pipe
AASHTO Section 18 – Standard Specifications for Highway Bridges
ASTM D 695 – Standard Test Method for Compressive Properties of Rigid Plastics
ASTM D 3350 – Standard Specifications for Polyethylene Plastic Pipe and Fittings
Gabriel, L.H. and Moran, E.T., Service Life of Drainage Pipe, Synthesis of
Highway Practice 254, Transportation Research Board, 1998.
Gabriel, L.H., Bennett, O.N., and Schneier, B., Polyethylene Pipe Specifications,
NCHRP Project 20-7, Task 68, Transportation Research Board, Washington D.C.,
October 1995.
Gabriel, L.H., When Plastic Pipe Responds – Relax and Enjoy, Proceedings of the
Third Conference on Structural Performance of Pipes, Ed., Mitchell, Sargand
and White, Ohio University, 1998.
Kampbell, N.E, Kozman, D.P. and Goddard, J.B., Changes in Hydraulic Capacity
of Corrugated HDPE Pipe With Time, Proceedings of the Third Conference on
Structural Performance of Pipes, Ed., Mitchell, Sargand and White, Ohio University,
1998.
Koerner, Hsuan, Lord, Stress Cracking Behavior of High Density Polyethylene
Geomembranes and Its Minimization, Geosynthetic Research Institute, Drexel
University, July 1992.
Kuhlman, C.J., Weed, D.N., and Campbell, F.S., Accelerated Fracture Mechanics
Evaluation of Slow Crack Potential in Corrugated Polyethylene Pipes, Southwest
Research Institute, San Antonio, Texas, February 1995.
Plastics Pipe Institute (PPI), Engineering Properties of Polyethylene, The Society for
the Plastics Industry, Inc., 1993
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