Consequently, the resistance of this hdpe against crack growth has been lowest. Since variance of any of these factors can change the environmental stresscrack resistance of the container, the test results are representative only of a given test performed under defined conditions in. Properties of lldpe are different from those of conventional ldpe and hdpe. Stress crack resistance sometimes referred to as slow crack growth resistance. Pe 100 creep rupture strength, pe crack resistance and. Investigations on environmental stress cracking resistance. Standard test method for environmental stresscrack. Tends to become brittle at lower temperature ranges. Slow crack resistance in polyethylene has been reported extensively in the literature 19,19. Effects of recycled hdpe and nanoclay on stress cracking.
These observations clearly indicate that the hdped pipe is generally more resistant to scg fracture compared to the hdpea pipe. It is well known from the pipe and cable insulation industries that pe can stress crack over long periods of time. Mdpe exhibited excellent scg and stress crack resistance, and dominated pe pipe manufacture for 30 years until the development of bimodal hdpe pe100. Of the elastomers investigated in this study, enjay butyl 007 a. Environmental stress cracking resistance of polyethylene. Pdf stress cracking resistance of hdpe geomembranes with. Some copolymers of ldpe are available with improved stresscrack resistance.
Hdpe is characterized by a linear polymer chain with few branches, and contains smaller amounts of comonomers such as butene, hexene or octene. Hdpe grades are used in a variety of processing techniques such as extrusion, extrusion blow molding, injection and rotational molding. Hdpe corrugated pipe service life corrugated stress. Some polymers, such as polypropylene, are not affected by slow crack growth. Environmental stress crack resistance poly processing. Unstressed polyethylene at room and normal service temperatures up to 60c 140f is highly. Environmental stress crack resistance of hdpe abg geosynthetics. Environmental stress crack resistance of polyethylene introduction to environmental stress cracking and escr over the past decade, highdensity polyethylene hdpe materials have improved significantly and now meet more stringent performance standards. Minimum required strength mrs this provides longterm strength and creep resistance. Experimental study of stress cracking in high density. Polypropylene has a significantly higher stress crack resistance and is therefore not susceptible to cracking at the same magnitudes at which hdpe may crack.
Environmental stress crack resistance of polyethylene ineos. This stress relaxation allows testing to run without failure for very long periods 1,500 hours. Strain hardening modulus as a measure of environmental. Astm d5397072012, standard test method for evaluation of stress crack resistance of polyolefin geomembranes using notched constant tensile load test. The bent strip test is a constant strain test, but polyethylene relaxes when strained. Modeling of the fracture mechanism of hdpe subjected to. Presto testing instruments manufacturers of environmental stress cracking resistance etc. Food grade material fda, food grade material is available upon request. Geomembranes made from high density polyethylene hope have a high percent crystallinity and are therefore of concern with regard to stress cracking. Challenges still remain, however, with respect to the stress crack resistance scr of hdpe pipe over its design life when recycled hdpe is blended with the pristine resin. It is one of the most common causes of unexpected brittle failure in thermoplastic. Fair resistance of bl4 against crack growth is due to higher molecular weight of this sample and the presence of long chains having short chain branches.
Xlpe has 5 times the impact and tensile strength of hdpe. Different polymers exhibit varying degrees of escr. Hdpe, like steel, is a semicrystalline material, with no crystals in the melted state. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. Basic understanding of stress cracking in hdpe products. This video describes astm d5397 standard test method for evaluation of stress crack resistance of polyolefin geomembranes using notched constant tension load test, as it. The environmental stress crack resistance of semicrystalline polymers such as polyethylene pe is less severe than in amorphous polymers 1. The 100year stress crack resistance scr of corrugated hdpe pipes was evaluated using a 600 mm diameter pipe. Crazes or white spots are commonly found as a precursor to this type of fracture in pe. However, exposing pe to stresscrack agents results in unexpected brittle fracture 24. There are several gri tests currently included in this standard. The stress cracking is more severe with detergent, bleach, motor oil and some industrial chemical packaging.
Further, for any given stress within the brittle fracture regime, the failure time for hdped is considerably longer. Superior corrosion resistance high stress crack resistance moisture resistance hdpe wear and abrasion resistant stress crack resistant high tensile strength impervious to most chemicals excellent energy absorption. Some grades of hdpe have very good resistance against esc, while some have marginal resilience. Dk plastics midwest plastic sheet extruder and theromformer. Environmental stress cracking may account for around 1530% of all plastic component failures in service. The notched constant ligament stress ncls test astm 2163 was used for the pipe liner assessment. The changes in the stress crack resistance, scr measured by the single point notched constant tensile load. Establishing 100year service life for hdpe drainage pipe michael pluimer technical and engineering manager, plastics pipe institute. High stress crack resistance borsafe pe100 for the toughest conditions. Environmental stress crack resistance of polyethylene.
One of the fastest growing plastics today is linear lowdensity polyethylene lldpe, used mainly in film applications but also suitable for injection, rotational and blow molding. Highdensity polyethylene hdpe resins are well suited for a range of extrusion film applications such as grocery, garbage and deepfreezer bags 10 to 25 microns and injection, blown and rotational molding processes. While crystallinity provides strength, it also can lead to cracking. Over the last forty years polyethylene pipe has seen a sharp rise in popularity amongst utility companies due to its ease of use and good track record.
Hence, an important design factor for pe pipes became commonly known as environmental stress cracking resistance escr, which led to the development of mdpe. Effect of molecular structure parameters on crystallinity. Grace hsuan, jy zhang and wk wong department of civil, architectural and environmental engineering, drexel university, philadelphia, usa. Environmental stress crack resistance escr is a commonly used test to characterize cracking failure of highdensity polyethylene in applications such as wires, cables, blow molded containers, and other rigid packaging applications. Understanding environmental stress crack resistance. Enhancing the environmental stress crack resistance of hdpe. Environmental stress crack resistance of polyethylene today, this test is specified less frequently because it is not as aggressive towards modern resins. Moldedin stress can be another contributor to stresscracking. With some resins the addition of rubber doubles the escr while with others a fiftyfold improvement can be effected. Environmental stress crack resistance escr is a vital property required in plastics for its longevity. The addition of a surfaceactive chemical, an environmental effect, accelerates this type of failure, thus this mechanism is termed environmental stress cracking. Exxonmobil hdpe product grades combines toughness and stiffness with resistance to environmental stress cracking at elevated temperatures and warping.
About half the time it correlates with more stresscrack resistance and half with less resistance. Puncture resistance stress crack resistance carbon black dispersion in the viewing and subsequent quantitative interpretation of astm d 5596 only near spherical agglomerates shall be included in the assessment. Most studies on the environmental stress cracking of polyethylene focus on the. Astm international, west conshohocken, pa, 2012, koerner, r. A new index test method, the notched constant tensile load nctl test, for evaluating high. Environmental stress cracking an overview sciencedirect topics. Evaluate the longterm stress crack resistance of corrugated hdpe pipes y. Environmental stress crack resistance of polyethylene ten rectangularshaped specimens are cut from a molded plaque prepared with standard methods. Experimental study of stress cracking in high density polyethylene pipes a thesis submitted to the faculty of drexel university by jingyu zhang.
Hdpe geomembranes are prone to stress cracking when subject to tensile stress less than their shortterm mechanical yield strength 75, 60. According to astm d 883, stress crack is defined as an external or internal crack in a plastic caused by tensile stresses less than its shorttime mechanical strength. This test method is intended to apply only to hdpe of a limited melt index 0. Excellent impact strength, stress crack resistance, and barrier characteristic. It was demonstrated that a significant improvement in the stress cracking resistance of hdpe based materials can be achieved through blending with other types of polyethylene. High stress crack resistance borsafe pe100 for the. A slightly less crystalline hdpe will have better crack resistance, and density is a direct measure of crystallinity. Resistance to this mode of failure can be best gauged through constanttensileload testing, while taking into account the differences in. According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused by tensile stresses less than its shortterm mechanical strength. Xlpe has 20 times the environmental stress crack resistance of hdpe. Effect of aging on the stress crack resistance of an hdpe. The specimens are bent and inserted into a c shaped bracket, creating a stress in the. The amount of crystalline content in the container depends on the rate of cooling.
Vistamaxxperformance polymers can be used to improve the environmental stress crack resistance escr of high density polyethylene hdpe. In the case of base clearance, which is a function of temperature, that temperature also has a role in relieving residual stress from the injection process. Environmental stress crack resistance lyondellbasell. The scr tests were performed on the finished pipe at the liner and junction locations. Hdped occurs at much longer times compared to that of hdpea. Environment stress cracking of hdpe products is quite common due to its semi crystalline nature which gets attacked due to chemicals present in the environment. Technical note stress crack resistance for geomembrane products the stress crack resistance scr of polyethylene pe geomembranes has been the subject of study since the introduction of the materials over 20 years ago. Technical note stress crack resistance for geomembrane. The nature of polyethylene pipe failure plasticstoday. Can be used for hot fill opportunities beyond the limits of hdpe.
Furthermore, the system contains carbon black as filler. The stress crack resistance scr of polyethylene pe geomembranes has been the subject of study since the introduction of the materials over 20 years ago. Environmental stress cracking is one of the most common causes of unexpected brittle failure of thermoplastic polymers known at present. In studying the effect of rubber on the environmental stress crack resistance escr of low and high density polyethylene, the choice of base resin is important. A controlled notch is cut horizontally across each specimen, which serves as a crack initiation point. Astm f26 18 standard test method for notched, constant. The ability of a polymer to resist slow crack growth or environmental stress cracking is known as escr. High density polyethylene hdpe sheet can be susceptible to a phenomenon known as environmental stress cracking esc. Thinner sometimes correlates with more, and sometimes with less, stresscrack resistance. A new accelerated test method for stress crack resistance. The stress crack resistance of some geosynthetics, such as high density polyethylene geomembranes whichare made from semicrystalline polymers, must be properly evaluated to justify their long.
1191 1099 1395 836 781 471 1337 44 1523 1371 1078 1518 1586 1310 364 612 240 697 1432 1418 252 767 1497 1321 1472 340 1420 1217 173 242 1025 188 95 1351 1476 766