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A HISTORICAL PERSPECTIVE OF HDPE PIPING MATERIAL DEVELOPMENT

William I Adams WL Plastics Corp

Cedar City, Utah, USA

Over more than a century, polyethylene and polyethylene piping materials have been discovered, rediscovered, commercialized and developed. With each new development, field experience and research helped identify and characterize polyethylene material performance, in most cases, leading to improved durability and performance. This paper provides an overview of polyethylene structure, a historical view of significant developments and research, and how ongoing research has provided for continuing development and improvement of polyethylene piping performance.

Polyethylene was first commercialized in the late 1930’s, roughly 70 years ago. Since that time, the polyethylene family of materials has developed into two categories that can generally be described as non-durable and durable. The predominant use for polyethylene is for non-durable articles such as packaging, toys, bottles, jugs, containers, and the like. Non-durable articles are intended for limited service, and are commonly discarded after use. Non-durable articles can be recovered from the waste stream and recycled into new non-durable articles.

Durable articles are a much smaller part of the overall polyethylene industry. These include products such as sheet liners for waste reservoirs and hazardous waste landfills, and piping systems. The structure of polyethylene is designed for durability with emphasis on withstanding long-term applied stress without failure.

The general structure of solid polyethylene is that of a semi-crystalline polymer. Density is directly related to crystallinity; a more crystalline polyethylene has higher density. Molecular weight relates to the length of the polymer chain; that is, a longer polymer chain has higher molecular weight. Related to molecular weight is molecular weight distribution, which is the statistical distribution of molecule length (weight).

A third structural parameter is side chain branching, which is the lateral attachment (grafting) of short copolymer molecules along the length of the primary molecular chain.

Homopolymer polyethylene is produced as a single repeating polymeric unit without copolymer side chain branching. Copolymer polyethylene that is typically used for durable articles is a grafted copolymer, which means that a second, usually shorter polymeric unit is grafted at intervals onto the side of the main polymer chain.

Polyethylene for non-durable articles has a low cost raw material structure for high production rates. Generally, these materials are lower density and lower molecular weight.

Polyethylene for durable products is generally a grafted copolymer having higher molecular weight and higher density. Its structure results in higher cost as a raw material, but its higher crystallinity and structural design for long-term durability provide mechanical properties that are better suited for piping and applications that require long-term resistance to failure from applied stress.

An understanding of the structure of polyethylene provides

an understanding of how it responds to applied stress. Polyethylene is a thermoplastic polymer; that is, it is composed of long chain molecules. When heated (melted), the polymer can be shaped and then cooled to a solid that holds the shape. Thermoplastics can be reheated and reshaped numerous times.

Polyethylene piping materials are grafted copolymers; that is, short copolymer molecules are attached to the primary molecule backbone at intervals along the molecule chain.

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As polyethylene cools from the melt, molecules fold into ordered crystals and into randomly entangled structure surrounding the crystals. Polyethylene piping materials incorporate a significant fraction of higher molecular weight (longer) molecules that are generally too long to fold into a single crystal. When these longer molecules cross from crystal to crystal, they tie the crystalline structure together, and are called “tie molecules”.

The semi-crystalline structure of solid polyethylene gives rise to visco-elastic behavior. When stress (load) is applied to polyethylene, the response is dependent on the magnitude and duration of the load. A high magnitude, short duration load typically elicits a stiff elastomer response. If this load is of high magnitude, the material will yield, elongate significantly (sometimes over 700%), and then break in the yielded area. High magnitude loads below yield elicit a rubber-like response.

A review of Figure 4 helps explain high, short duration load response. When yielding occurs, the bond strength between tie molecules and crystals is greater than the intra-crystalline shear strength; that is, when the material yields, crystals shear apart. At loads below yield, crystals remain intact and inter-crystalline randomly entangled tie molecules provide resilience, a rubber-like elastic response.

As sustained load is reduced to a level below the elastic limit, permanent deformation (creep or cold flow) occurs. If a creep-level load is removed, some deformation will be recoverable (primary creep) and some unrecoverable (secondary creep). Creep is not to be confused with the deformation of a viscous liquid (such as grease) where deformation is completely unrecoverable. Visco-elastic creep is non-linear; that is, a sustained creep-level load will produce deformation however, the rate of deformation decreases over time and eventually plateaus.

If we reduce load below a creep threshold, creep does not occur; a different mechanism comes into play. Under relatively low sustained loads, the boundary between crystals and tie molecules becomes vulnerable to stress concentration (stress intensification) and eventually the material fails as tie molecules shear at the crystal boundary. This failure mechanism is called stress cracking or slow crack growth

(SCG). It is characterized by little or no evidence of ductile elongation at the failure, and intermittent (stop-start) crack propagation.

Because pressure piping design loads are well below creep-level stresses, the resistance of polyethylene piping materials to SCG is a key determinant of long-term performance in pressure piping applications. The balance of this paper is devoted to this phenomenon and developments in testing, material and processing that improve resistance to SCG failure.

DF, design factor ESC, environmental stress-cracking ESCR, environmental stress-cracking resistance HDB, hydrostatic design basis HDPE, high density polyethylene HDS, hydrostatic design stress LTHS, long-term hydrostatic strength SCG, slow crack growth

Discovery dates vary somewhat depending on the reference, but the principals are the same. Around 1898-1899, German chemist Hans von Pechmann accidentally synthesized a white, waxy polymeric substance that was characterized around 1900 by his colleagues Eugen Bamberger and Frederich Tschirner as “polymethylene”, which is chemically identical to polyethylene(1)(2).

Polyethylene was accidentally rediscovered in Britain by Imperial Chemical Industries (ICI) chemists Eric Fawcett and Reginald Gibson around 1933. ICI’s chemist Michael Perrin developed a manufacturing process for “polythene” in 1935 and further developed the material into a commercially viable material in 1939. Polyethylene was first used as a wire and cable insulator for transoceanic telephone cables, and in WWII by the Allies as a cable insulator for airborne radar(3). Carl Ship Marvel, an American chemist and consultant for E. I du Pont de Nemours & Co., also rediscovered polyethylene in 1930, but du Pont did not commercialize the material at that time.

In 1951, American chemists Robert Banks and John Hogan of Phillips Petroleum discovered that chromium trioxide based catalysts promoted polymerization at milder temperatures and pressures. This was followed in 1953 by Nobel prize winning chemists Karl Ziegler of Germany and Giulio Natta of Italy who developed catalyst systems based on titanium halides and organoaluminum compounds for polymerization at even milder conditions.

By turn of the decade in 1960, commercially viable polyethylene materials were available for a wide range of applications including pipe and tubing. One of the first “tubing” applications greatly benefited Phillips Petroleum who


had experienced some difficulty controlling production from their new polymerization process. They filled warehouses with what we now kindly call “wide-spec” material, but were saved by Wham-O inventors Richard Knerr and Robert “Spud” Merlin who used the material to make the Hula-Hoop(4)(5).

Although the Hula-Hoop is a toy, exceptional chemical resistance, light weight, flexibility and ease of fabrication offered bright promise over traditional metal and non-ferrous piping. Polyethylene materials are easily extruded into tubes and molded into fittings to produce piping products for serious applications such as transporting municipal and industrial fluids including compressed gases, natural gas, oil, raw and reclaimed water, drinking water, sewage, wastewater, chemical solutions, slurries, and nuclear reactor cooling water.

Durability is defined largely by the application. At one end of the applications spectrum, non-durable packaging materials are typically used once and then discarded, and may even be compounded with additives to promote degradation in landfill conditions. In any event, the useful life of non-durable packaging materials is short, perhaps a few months. Pipe however, is at the opposite end of the scale. Typical durable piping systems are significant capital investments that are expected to maintain strength and containment for a design service life that may be many decades.

Criteria for durability (resistance to failure) are determined by the specific application. For example, leakage from pressure gas distribution piping is not acceptable; therefore, long-term leak resistance is the critical criteria for natural gas service. On the other hand, resistance to structural loads is the predominant criteria for a non-pressure sewer or culvert pipe; that is, the application can tolerate some deterioration provided that sufficient structural capacity is retained.

As with most applications, field experience identifies areas for improvement and research provides the means for understanding field experience and developing materials that provide improved long-term durability in piping applications. In the 1960’s and 1970’s, various PE materials were used for piping applications. It was initially believed that tensile properties as used for metals were sufficient for design. Unfortunately, field experience quickly revealed that stress design based on short-term tensile properties was wholly inadequate as a predictor of performance under long-term applied stress. Mechanical responses such as creep were revealed, and standards such as ASTM D2837 Standard Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials(6) were developed to better estimate design stress limits for long-term pressure piping applications. Standards such as ASTM D2837 analyze long-term test data to arrive at acceptable long-term stresses for

thermoplastic pressure piping materials including polyethylene.

The ASTM D 2837 method applies a statistical regression analysis to a hoop-stress/time to failure data set developed by subjecting pipe specimens to various internal pressure stresses. ASTM D2837 sets dataset requirements by specifying the number of failures and failure times ranging from 100 to 10,000 hours. A regression analysis is applied to the data to project a hoop stress level that would result in failures at 100,000 hours. This stress level is termed the Long-Term Hydrostatic Strength, or LTHS. The LTHS is then compared to categorized LTHS value ranges and assigned a Hydrostatic Design Basis, or HDB. When applied to piping applications, the HDB is reduced by a design factor (DF) to provide the hydrostatic design stress (HDS) that is used for pressure design. The DF provides for variations in materials, fabrication, installation, operation, and unknown conditions of the application. For many thermoplastic polymers, ASTM D2837 has proved to be an excellent predictive methodology.

However, piping made from early polyethylene polymers exhibited signs of premature aging even though design stresses were well within predicted design limits. In particular, some early gas pipe materials operating at 3 to 1 and higher design factors prematurely developed leaks at stress-cracks, a most unacceptable condition for gas pipe (Figure 5). In response, industry resources were mobilized to develop an understanding of the stress-cracking phenomenon(7), to develop testing methods that characterized resistance to stress-cracking, and to develop more robust, stress-crack resistant materials.

Initially, resistance to stress cracking was evaluated using conventional environmental stress cracking (ESC) tests such as ASTM D1693 Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics(8) where a coupon of polyethylene material is notched, bent, and immersed in a chemical stress cracking solution at elevated temperature. Failure


is determined by observing the test specimens for visible cracks on the test coupon surface. Figures 6 and 7 illustrate ASTM D1693 test specimen and testing configuration.

A related test, ASTM F1248 Standard Test Method for

Determination of Environmental Stress Crack Resistance (ESCR) of Polyethylene Pipe(9) was developed to determine ESC resistance for specimens taken from pipe rather than specimens prepared from a compression molded resin plaque. Figures 8, 9 and 10 illustrate the test specimen and testing configuration.

Although ESC tests such as ASTM D1693 and ASTM F1248 offer a relative “feel” for stress cracking resistance of polyethylene, ESC tests do not produce data that supports

performance predictive methodology and because a chemical accelerant is used, do not model field cracking such as that shown in Figure 5 very well. ESC tests provide qualitative information relative to the development of surface cracks, but there is no known way to correlate ESC resistance to allowable field stress as determined by stress-rupture testing and analysis. In part this is because under high applied stress and especially at elevated temperature, polyethylene materials will creep thus reducing the stress level over time. Bent specimen tests such as those in ASTM D1693 and ASTM F1248 are susceptible to creep that limits test usefulness to several hundred hours. Polyethylene materials that are susceptible to ESC chemical attack in a few hundred hours or less can be characterized and compared to other polyethylene materials having comparable ESC resistance. But polyethylene materials that resist chemical attack for more than a thousand hours are very likely to never fail ESC tests; that is, ESC tests cease to be useful for polyethylene materials that have good resistance to ESC failure.

Further, the field cracking environment is vastly different from the chemically accelerated ESC test environment. In Figure 5, the cracked pipes are not severely deformed, and installed gas pipes are not immersed in hot stress cracking solutions. For these reasons, ASTM D1693 is no longer applicable to polyethylene pressure piping materials, and ASTM F1248 has been withdrawn by ASTM. It is no longer an active standard.

Under ASTM D2837 methodology, a viable regression analysis of stress-rupture test data requires that a consistent pipe specimen failure mode. For many thermoplastic materials the failure mode is consistently ductile; that is, elongation (deformation) of the pipe wall and then failure in the elongated area. For these materials, there is no indication that the long-term field failure mode will vary from the ASTM D2837 test failure mode. Accordingly, there is reasonable certainty that the LTHS and the resulting HDB and HDS predicted by the regression analysis will be valid for the future time predicted by the stress-rupture methodology.

When unanticipated field problems such as premature cracking failures in gas pipe occur, we want to find out why sooner rather than later. But as we have seen, stress regression testing takes up to 10,000 hours, about 14 months and it doesn’t necessarily model field conditions. We know that reactions are accelerated by increasing stress, by increasing temperature, by varying stress and temperature (fatigue), by applying an aggressive chemical environment, or by combinations thereof. So an obvious way to shorten the overall time for regression analysis is to apply known acceleration techniques to obtain test data in a shorter time.

When ASTM D2837 sustained pressure testing is conducted at higher temperatures and relatively low stresses, a different failure mode in semi-crystalline polyethylene materials is revealed. The new failure mode is characterized by no discernable ductile elongation; that is, brittle-like cracks


that grew through the pipe wall. Research showed that the brittle-like failures produced by elevated temperature stress rupture tests were the same as the field stress-cracks.

When the ASTM D2837 regression analysis is applied to data that is limited to brittle type failures, we find that the slope of the regression line is significantly steeper compared to the slope in the ductile failure region. Figure 11 illustrates stress-rupture curves for a thermoplastic material that has both ductile and brittle failure modes. Ductile failures are described by the flatter, left section of the curve, and brittle failures by the steeper right section. The normal ASTM D2837 regression analysis extends the ductile failure portion of the curve to the prediction time (100,000 hours), but for materials that may have a secondary brittle failure mode, extending the ductile failure curve may greatly over-predict the design stress for the material.

Research into polyethylene’s long-term brittle failure mode prompted changes to ASTM D2837 and the development of ISO 9080 Plastics piping and ducting systems - Determination of the long-term hydrostatic strength of thermoplastics materials in pipe form by extrapolation(10). The ASTM D2837 regression analysis equation assumes and relies on a common failure mode1

1 ASTM D 2837 and ISO 9080 methodologies are different. ASTM D 2837 has simpler mathematics, but its focus on the ductile failure mode does not always fully describe material performance. Validation ensures that brittle failures do not occur within the predicted ductile failure timeframe. ISO 9080’s more complex mathematics use a computer program to fully describe the ductile and brittle performance of the material. ISO 9080 includes brittle failures within the prediction timeframe, but lowers the predicted allowable stress accordingly. Both methods are equally valid, and the same material can be evaluated with both methods, but the different mathematics do not allow interchanging or converting between long-term strength values.

. For this reason, a “validation” requirement for polyethylene materials was added. Validation uses additional elevated temperature sustained

pressure testing and analysis to prove that the ductile-brittle transition occurs after the predictive intercept point. ISO 9080 uses a more complex regression analysis equation to describe both ductile and brittle portions of the curve. ISO 9080 became an ISO standard in 2003.

Research successfully identified the long-term brittle cracking phenomenon and now turned to developing materials that were more resistant to brittle cracking.

Once the brittle failure mode had been identified, characterized and reproduced in the laboratory, materials having reduced susceptibility to stress-cracking failure were needed. Tests that revealed brittle failure in polyethylene materials more quickly than sustained pressure tests were needed. We see from Figure 11 that at 80°C (176°F), several hundred hours are required to achieve brittle failures. Higher temperatures must be used cautiously because the material begins to significantly lose crystallinity. As discussed in the introduction, stress cracking occurs at the crystalline amorphous boundary so crystalline structure is required for SCG evaluation.

Looking back to Figure 5, we see that another feature of the field cracks is that cracks emanate from points of stress concentration, for example the junction of the saddle fitting base to the main pipe. Experts in fracture mechanics determined that a sharp notch cut across (normal to) the plane of applied tensile stress could provide appropriate stress intensification and further reduce the time to failure. Thus we have the fundamental characteristics for what we now call slow crack growth (SCG) resistance tests; e.g., elevated temperature, constant tensile load, and a sharp notch to intensify stress and serve as an initiation site. Research created a variety of SCG tests, several of which are illustrated below.

An early SCG resistance test was the notched c-ring section test developed by the Battelle Memorial Institute under Gas Research Institute contract(11) (Figure 12).

In this test, a pipe ring specimen is cut into c-shaped segments, razor notched across the pipe ID, and then a bending load similar to a beam flexure test is applied.

Another early SCG resistance test was the constant tensile load (CTL) test developed by Battelle Memorial Laboratories


and Argonne National Laboratories also under Gas Research Institute contract.(12)(13). In the CTL test, a pipe ring is razor notched inside and out across the diameter, split disks are fitted into the ring ID, and then a tensile load is applied to separate the split disks (Figure 13). This applies a hoop-like tensile stress across the notched ligament in the pipe wall. CTL tests were conducted in ambient and elevated temperatures, and in various environments such as air, water, and chemical stress cracking solutions.

Of course, research into the brittle cracking phenomena of polyethylene pipe was not exclusively a North American effort. Polyethylene pipe is used worldwide for gas distribution. Research in Japan yielded a full notch creep test (FNCT) where a tensile bar is razor notched on four sides and then subjected to a constant tensile load at elevated temperature(14) (Figure 14).

An important finding from notched c-ring, CTL and FNCT research was that scientists were able to correlate brittle SCG failures produced using these tests to stress rupture testing and analysis per ASTM D2837 and ISO 9080. Depending upon the test and the test environment, brittle, SCG failure results could be accelerated by multiples of up to 100 times.

Around the same time, materials scientists at the University of Pennsylvania under Gas Research Institute contract developed another notched bar specimen test for SCG resistance nicknamed PENT, an acronym for PEnnsylvania Notch Test(18). The PENT specimen is similar to the Japanese FNCT specimen except that there is a razor notch on one side, and coplanar side grooves on two adjacent sides (Figure 15). The specimen is subjected to a constant tensile load at elevated temperatures to induce plane strain fracture.

The PENT SCG test was developed into an ASTM standard and published in 1997 as ASTM F1473 Standard Test Method for Notch Tensile Test to Measure the Resistance to Slow Crack Growth of Polyethylene Pipes and Resins(15).

Additional research by the Geosynthetic Research Institute at Drexel University funded by the U.S. Environmental Protection Agency led to the development of a SCG resistance test for geotextile membranes(16), which are polyethylene sheets used as pond and landfill liners to contain hazardous waste and prevent the migration of hazardous effluent into underground aquifers. Like cracks in some polyethylene gas pipe, some polyethylene geotextile liners had prematurely developed brittle cracks and similar to pipe, ESC tests proved to be inadequate predictors of field performance.

The notched constant tensile load (NCTL) test developed by the Geosynthetic Research Institute uses a smaller, thinner specimen compared to FNCT and PENT tests. NCTL uses a microtensile specimen with a razor notch (Figure 16).


A benefit derived from small specimens is that many can be tested in a small space, and quite a lot of data obtained in a relatively short period of time. Included in an NCTL study report(16) is an interesting chart that illustrates the effects of testing the same material in various media at the same temperature (Figure 17).

For the Figure 17 data, the same polyethylene material was tested in a solution of 10% Igepal CO630 and 90% tap water, 100% tap water, and in air. (Igepal CO630 is a strong detergent, a chemical that is known to cause stress cracking in polyethylene.) Of interest is that the slopes of the brittle curves for the 10% Igepal and water are virtually identical, but the onset of brittle failure is considerably accelerated in the 10% Igepal solution. Further, the onset of brittle failure in tap water and air is about the same, but the slope of the brittle failure curve in air is remarkably steeper compared to the brittle failure slopes in water and 10% Igepal solution. It would appear that the elevated temperature air media has an additional effect on the brittle failure of PE materials that does not occur in the other media.

The NCTL test was developed into an ASTM standard and published as ASTM D5397 Standard Test Method for

Evaluation of Stress Crack Resistance of Polyolefin Geomembranes Using Notched Constant Tensile Load Test(17) in 1993.

The availability of SCG resistance tests that could rapidly and accurately evaluate polymer modifications provided tools for developing polyethylene polymers having improved resistance to brittle, SCG failure. Two SCG tests in particular have developed into important tools for developing SCG resistant polyethylene materials. In North America, PENT SCG testing is extensively used for pressure piping materials, and the NCTL test has been further developed for use with non-pressure corrugated pipe.

SCG research and development with polyethylene materials has been correlated to stress-rupture testing, thus establishing a measure for field resistance to brittle SCG field failure. Data from SCG tests is applied to compare SCG resistance performance of polyethylene copolymers, to evaluate the effects of branch density and location in the molecular structure(18) and to establish physical requirements for SCG resistance of PE materials(19). More recently, SCG resistance has led to the development of dual-reactor technology for producing bi-modal polyethylene materials that show great promise as the next evolution in high-performance polyethylene materials(20).

Aside from pressure pipe, the SCG resistance work from the Geosynthetic Research Institute did not go unnoticed by the corrugated polyethylene pipe industry. Corrugated pipes employ a profile wall construction that minimizes material usage, but provides resistance to deformation from applied external loads.

The discussion so far has generally focused on the durability of pressure pipes where resistance to leakage is the predominant requirement for durability. With non-pressure pipes, however, the predominant criteria is the ability to resist structural loads, that is, some deterioration may be tolerated provided the ability to sustain structural loads such as loads from soil embedment and live loads from traffic above the pipe is retained.

Of particular interest for corrugated pipes is the ability of the NCTL test to evaluate thin sections of PE materials for


SCG resistance. In fact, the NCTL test has been adapted to evaluate corrugated PE pipe materials as the notched constant ligament stress (NCLS) test, which was developed into ASTM F2136 Standard Test Method for Notched, Constant Ligament-Stress (NCLS) Test to Determine Slow-Crack-Growth Resistance of HDPE Resins or HDPE Corrugated Pipe(21) and published in 2001. The NCLS specimen is identical to the NCTL specimen, but the test parameters are different. The corrugated PE pipe industry is now developing product standards that include NCLS requirements for PE materials used in corrugated pipe manufacture.

Research into polyethylene structure revealed the effects of comonomers, comonomer placement and molecular weight on resistance to SCG. Initially, it was revealed that higher order comonomers improved resistance to SCG (See Figure 2). Increasing comonomer branch length from ethyl to butyl to hexyl to octyl improved SCG resistance by orders of magnitude.

Research also identified that in conventional polyethylene manufacture, comonomers generally attached themselves to a lower molecular weight fraction; that is, to shorter molecules. In Figure 19(20), the bell curve shows molecular weight distribution, or the statistical distribution of molecule length. A “unimodal” distribution is characterized by a bell curve with a single peak. When comonomer distribution is shown with the distribution curve, it can be seen that more comonomer branching occurs with shorter rather than longer molecules. Shorter, more highly branched molecules interfere with crystal development because it is more difficult for a branched molecule to fold tightly to its neighbor. However, reduced crystallinity provided greater tie molecule density for the same overall molecular weight distribution thereby improving SCG resistance.

Reduced density depresses general mechanical properties most of which are beneficial for piping. If a way could be found to invert comonomer distribution, improvements in both

mechanical properties and long-term resistance to SCG could be realized.

Polymer scientists realized that a blend of near homopolymer polyethylene and very high molecular weight polyethylene with a high level of copolymerization would do the trick. This unique “bimodal” structure is illustrated in Figure 20(20).

Bimodal structure improves crystallinity and SCG resistance. A review of Figure 3 shows why this should be so. The lower molecular weight fraction can fold tightly into larger crystals to improve general mechanical performance. The additional higher molecular weight fraction (Figure 20 second peak) increases tie molecule density to better secure the structure together and improve SCG resistance.

Significant gains in overall fracture toughness across the entire use range from subfreezing to elevated temperatures have been realized. Compared previous generation piping materials, elevated temperature strength is improved. SCG resistance has been increased by orders of magnitude. Where PENT SCG resistance has been tens of hours, the new structure resists SCG failure for thousands of hours. SCG resistance is so greatly improved that the potential for field stress cracking is virtually removed from consideration.

Polyethylene has existed for over a century, first as a laboratory curiosity, and then as a material for commercial products. Roughly five decades ago, tubular products were produced from polyethylene creating a new worldwide polyethylene pipe industry. The need for more durable PE pipe materials grew from early field experience with brittle crack field failures in sensitive gas pipe applications. Research identified and characterized the phenomena as slow crack growth. SCG evaluation methods were developed, used to understand SCG resistance in polyethylene materials and to develop a new polyethylene structure for high performance PE piping products that virtually eliminates SCG as a long-term performance concern.


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9. ASTM F 1248, "Standard Test Method for Determination of Environmental Stress Crack Resistance (ESCR) of Polyethylene Pipe", ASTM International.

10. ISO 9080 “Plastics piping and ducting systems -- Determination of the long-term hydrostatic strength of thermoplastics materials in pipe form by extrapolation

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15. ASTM F 1473, “Standard Test Method for Notch Tensile Test to Measure the Resistance to Slow Crack Growth of Polyethylene Pipes and Resins”, ASTM International.

16. R. M. Koerner, Y. Hsuan, A. E. Lord, Geosynthetic Research Institute Final Report, “Stress Cracking Behavior of High Density Polyethylene Geomembranes and Its Minimization”, U. S. Environmental Protection Agency, Cooperative Agreement No. CR-815692, July 31, 1992.

17. ASTM D 5397, “Standard Test Method for Evaluation of Stress Crack Resistance of Polyolefin Geomembranes Using Notched Constant Tensile Load Test”, ASTM International.

18. N. Brown and X. Lu, “A Quality Control Test for Predicting the Lifetime of Polyethylene Pipes, and Fittings and Resins”, Plastics Pipes VIII Conference, Koningshof, The Netherlands, September 21-24, 1992.

19. Y. Huang, N Brown, “The Dependence of Butyl Branch Density on Slow Crack Growthin Polyethylene: Kinetics”, Journal of Polymer Science: Part B: Polymer Physics, Vol. 28, 2007-2021 (1990).

20. S. Joseph, “How do bimodal polyethylene resins provide improved pipe properties?”, Lyondell webinar, October 31, 2005

21. ASTM F 2136, “Standard Test Method for Notched, Constant Ligament-Stress (NCLS) Test to Determine Slow-Crack-Growth Resistance of HDPE Resins or HDPE Corrugated Pipe”, ASTM International.


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