High Density Polyethylene Resins Research Articles

Mechanical properties and molecular structures of virgin and recycled HDPE polymers used in gravity sewer systems

The widespread use of plastics in the conditioning, packaging and building material sectors generates an enormous amount of industrial waste which could be recycled for wastewater pipes and fittings. Nevertheless, current manufacturing standards in the piping industry recommend against the use of post-consumer recycled materials—a policy based on inadequate understanding of the properties and long-term mechanical performance of recycled materials. The present study compared the material characteristics of virgin and recycled high-density polyethylene (HDPE) plastics commonly found in the piping industry. Mechanical testing, oxidative induction time (OIT), melt flow index (MFI) and thermal analysis were used in conjunction with X-ray fluorescence (μ-XRF), size exclusion chromatography and 13C solid-state NMR to evaluate mechanical behavior and molecular structure as well as contaminant or filler contents. This study provides evidence for the degradation processes impact that can occur when post-industrial and post-consumer polymers are recycled. However, the study identified two measures to improve the material qualities of post-consumer recycled HDPE: 1) reducing the amount of contaminants or, alternatively, improving their compatibility with HDPE resins, and 2) improving current sorting and recycling processes to increase the amount of tie molecules in HDPE materials.

Tensile properties of polyformaldehyde blends and nanocomposites

Abstract The effects of the compositions and tensile rates on the tensile properties of the polyformaldehyde (POM)/ethylene-vinyl acetate copolymer (EVA)/high-density polyethylene (HDPE) blends and POM/EVA/HDPE composites filled with nanometer calcium carbonate were investigated by means of a tensile test machine at room temperature. The results showed that the Young’s modulus, tensile yield strength, and tensile elongation at break of the blends and the composites decreased nonlinearly with increase in the HDPE weight fraction; it should be attributed to the low stiffness and strength of the HDPE resin; the difference in the Young’s modulus and tensile yield strength between the blends and the composites was insignificant under the same experimental conditions. Both the Young’s modulus and tensile yield strength of the blends and the composites increased nonlinearly with increase in the tensile rate in the case of low tensile rate level. Moreover, the predictions of the Young’s modulus and tensile strength were roughly close to the measured data from the blends.

Mechanical Properties of Fumed Silica / HDPE Composites

Fumed silica /High density Polyethylene (HDPE) composites were prepared via melt mixing using a two –screw co-rotating extruder. To improve the dispersion degree of the fumed silica, the γ-aminopropyl triethoxy silane (KH-550) was used as silane coupling agent. The composites which obstained different fumed silica content were prepared by melt-mixing process. Next composites mechanical properties were measured by the crystal polarized light microscopy (PLM), Shore hardness, the tensile testing. The results indicate that fumed silica can enhance HDPE resin and the mechanical properties of composites are optimum when fumed silica content is 3.0 wt%.

Radiation induced emulsion grafting of glycidyl methacrylate onto high density polyethylene: A kinetic study

Emulsion graft polymerisation of glycidyl methacrylate monomer onto high density polyethylene (HDPE) resin was studied. The radicals required to initiate the reaction were produced in HDPE resin by irradiation with the electron beam accelerator at ambient and vacuum conditions. The grafting reaction of GMA with irradiated HDPE was initiated by emulsion method using Tween 20 as a surfactant and water as a solvent under controlled conditions. The kinetic behavior of grafting reaction was investigated with respect to the degree of grafting under various parameters i.e. monomer concentration, radiation dose, grafting temperature and surfactant concentration. The degree of grafting was found to be a function of the investigated parameters. Particularly, surfactant concentration in the emulsion was found to be crucial for determining the micelle size and eventually the degree of grafting. The order of dependence of the initial rate of grafting on the monomer concentration and the irradiation dose was found to be 0.94 and 0.60, respectively. The initial rate of grafting was also found to increase with the increase in the grafting temperature varied in the range of 308-333 K. The optimum reaction parameters required to obtain desired and reproducible degrees of grafting in the resin could be identified. Moreover, the emulsion radiation grafting method was found to offer essential advantages over the conventional grafting with solvents including low radiation dose, less monomer concentration and cost effectiveness.

The effect of microstructure on the slow crack growth resistance in polyethylene resins

The slow crack growth (SCG) in high density polyethylene (HDPE) is a phenomenon dominated by crazing. In this work, the crazing was analyzed from a microstructural point of view. PENT (Pennsylvania Edge Notched Tensile) test was chosen to study the evolution of the craze with time for different resins from PE‐80 up to PE‐100 grades. Two different geometries, the standard and an alternative named CDNT (Circumferentially Deep Notched Tensile), were employed. Failure times were correlated with intercrystalline parameters like tie molecules and the molecular weight between entanglements. Experimental results showed good correlations using both direct SCG test (standard PENT and CDNT geometries). Finally, the strain hardening modulus was correlated with PENT failure times. The results disclosed an outstanding correlation for several polyethylene grades from blow molding up to PE‐80, PE‐100, and higher resistant to crack grades. These results permitted an easy‐classifying and ranking method as much to the old polyethylene grades as to the new generation of HDPE resins with a very high SCG resistance. POLYM. ENG. SCI., 55:1018–1023, 2015. © 2014 Society of Plastics Engineers

Numerical prediction of nonlinear rheology of branched polymer melts

In a recent short communication [Read, D. J. et al., Science 333, 1871 (2011)], we showed that a computational scheme can describe the nonlinear flow properties for a series of industrial low-density polyethylene (LDPE) resins starting from the molecular architecture. The molecular architecture itself is determined by fitting parameters of a reaction kinetics model to average structural information obtained from gel-permeation chromatography and light scattering. Flow responses of these molecules in transient uniaxial extension and shear are calculated by mapping the stretch and orientation dynamics of the segments within the molecules to effective pom-pom modes. In this paper, we provide the details of the computational scheme and present additional results on a LDPE and a high-density polyethylene resin to illustrate the dependence of segmental maximum stretch variables on the flow rate.

PET 생수병 내 휘발성 물질의 동정 및 이행량 분석

본 연구에서는 PET preform, 성형된 PET병, 그리고 PET병 생수에서 HS-SPME GC/MS를 통해 휘발성 물질을 동정하고 HDPE cap으로부터 생수에 이행된 2,4-DTBP의 이행량을 분석하였다. PET preform과 완성된 PET에서 nonanal과 decanal이 확인되었다. 이 물질들은 이취의 원인 물질로서 preform보다 PET병에서 농도가 더 높게 나타났으며, PET병에 대해 온도 증가에 따른 휘발성 물질을 분석한 결과 온도가 증가할수록 물질의 농도가 증가하였다. 특히 nonanal, vinyl benzoate, 그리고 decanal과 같이 온도에 민감한 휘발성 물질이 크게 증가하였다. 그리고 생수병의 HDPE cap에서 항산화제인 Irgafos 168을 정량한 결과 <TEX>$206{\pm}20.1{\mu}g/g$</TEX>의 농도를 나타내었다. 2,4-DTBP는 항산화제인 Irgafos 168의 분해 물질이므로 생수에서 검출된 2,4-DTBP는 cap에서 생수로 이행된 휘발성 물질로 확인되었으며 이행량은 <TEX>$4.8{\pm}0.2{\mu}g/L$</TEX>로 확인되었다. 따라서 PET병 생수의 관능학적 품질을 높이고 소비자의 안전을 위하여 PET 및 HDPE 제조 공정에서 가능한 온도를 낮게 제어하는 기술이 요구되며, 생수 제품의 유통 및 소비자 취급 시 적절한 온도 유지와 올바른 제품 보관을 통하여 저분자의 이취 원인 물질들을 줄이는 것이 중요할 것으로 판단된다. This study was carried out to identify the volatile organic compounds (VOCs) in polyethylene terephthalate (PET) bottles and to determine the extent to which VOCs migrate into mineral water during the bottling process and storage. A greater amount of nonanal and decanal was generated from the PET bottles than from the PET preforms. Benzene, ethylbenzene, nonanal, and vinyl benzoate were identified from the PET bottles when the incubation temperature of the headspace solid-phase microextraction (HS-SPME) sampler was set to 60, 80, and <TEX>$100^{\circ}C$</TEX>. As the incubation temperature increased, the concentrations of nonanal, vinyl benzoate, and decanal increased significantly. When the high-density polyethylene (HDPE) PET bottle caps were extracted with dichloromethane, the level of Irgafos 168 was found to be <TEX>$206{\pm}20.1\mu}g/g$</TEX>. The concentration of 2,4-di-tert-butylphenol in water was <TEX>$4.80{\pm}0.2{\mu}g/L$</TEX>. Therefore, it is necessary to avoid exposing PET and HDPE resins to high temperatures during the manufacturing process and storage of bottled water.

Reaction mechanism for oxidation and degradation of high density polyethylene in chlorinated water

Polyethylene is increasingly used for potable water pipe even though polyethylene is susceptible to oxidative degradation. Accelerated aging conditions with chlorinated water solutions which minimized variations in solution chemistry were used in a 160-day (3840 h) immersion study of high density polyethylene (HDPE) pipe and HDPE resin. Samples were periodically characterized for changes in visual appearance and surface chemistry using infrared spectroscopy. Surface carbonyl bonds were detected for both HDPE pipe and HDPE resin samples. Experiments with isotopic 18O2 gas demonstrated that molecular oxygen is partly responsible for formation of the carbonyl oxygen. Both HDPE pipe and HDPE resin samples were demonstrated to form 4-chloro-2-methylbutan-2-ol and 2,3-dichloro-2-methylbutane as novel HDPE breakdown products which leached into chlorinated water. From these data, reactive species involved in the breakdown of HDPE pipe have been identified. A proposed mechanism for the breakdown of HDPE is Cl• or HO• initiated H-abstraction to produce a carbon-center radical that reacts with triplet oxygen to form a peroxyl, which upon decomposition yields a carbonyl and further propagates the formation of radicals.

Effect of Molecular Structure on the Short-Term and Long-Term Mechanical Behavior of High-Density Polyethylene

Environmental stress cracking (ESC) of polyethylene (PE) in structural applications can result in catastrophic failure without any visible warning. Polyethylene with excellent short-term mechanical strength can have poor long-term performance because of ESC. Because PE structures can have an expected service life of 50 years or more, the lack of understanding regarding their long-term mechanical performance is a major concern. For practical reasons, it is not possible to test PE for years before use. Currently, most mechanical tests for PE only run for a few hours. Therefore, it is important to understand the connections between short-term and long-term mechanical properties. Short-term mechanical behavior, such as creep over 8 h, is studied in conjunction with ESC for six high-density PE resins. In addition, the molecular structure influences on creep of PE are extensively investigated. Complex creep behavior is explained from the point of combined effects of molecular properties. It was found that high-density polyethylene (HDPE) resins with large creep strain usually exhibit high environmental stress cracking resistance. However, the short-term creep behavior of HDPE was affected by molecular weight, molecular weight distribution, and short-chain branching content of the material.

Effect of Low Molecular Weight Ethylene Homopolymer on Structure and Properties of Bimodal High Density Polyethylene

The effect of low molecular weight (LMW) ethylene homoploymer on the structure and properties of bimodal high density polyethylene (HDPE) was studied by blending two commercial bimodal HDPE resins (tandem reactor) with two types of LWM ethylene homopolymer. The molecule weight and molecule weight distribution, crystsallinity, rheological and mechanical properties of composites were characterized by high temperature gel permeation chromatography (HT-GPC), differential scanning calorimetry (DSC), capillary rheometer respectively. For composites of bimodal HDPE and LMW ethylene homopolymer, the polydispersity index (PDI) and crystallinity can be effectively increased when reduce the molecular weight of first reactor in tandem reactor. The viscosity of composites decreases at high shear rate. Increasing of LMW component has a positive effect on processing properties and tensile strength, while makes composites brittle.

Influence factors of internal structure and interfacial compatibility of UHMWPE fiber/SEBS resin composites: Processing parameters, structure of fiber and nature of resin

This study investigates in detail the effects of processing parameters (pressure and temperature) on the internal structure and interfacial compatibility of unidirectional (UD) sheets of ultrahigh molecular weight polyethylene (UHMWPE) fiber/styrene–ethylene–butylene–styrene (SEBS) resin composites by dynamic mechanical thermal analysis (DMTA) and T-peel tests. The variation of storage moduli of the composites with the processing pressure indicate that almost all of the resin phase had changed into the interfacial phase; the tanδ curves also testified that the ethylene–butylene (EB) segment of SEBS matrix was entangled with the molecular chains of polyethylene (PE) fibers. Moreover, the result of the T-peel tests of composites indicated that the pressure produces more significant changes than the temperature for both monofilm and no-film UD composites. Besides, the latter had higher adhesion strengths under the same processing conditions due to increased adhesion between the fiber and the resin. Next, high-density polyethylene (HDPE) modified SEBS resin was used as the matrix of the composites. Their interfacial adhesion strengths were found to increase slightly and the DMTA curves indicate that the crystallinity of the modified matrix increased with the content of HDPE resin. All of these benefit the improvement of the protection property of the composites.

Crystallization kinetics modeling of high density and linear low density polyethylene resins

AbstractThis paper describes isothermal and nonisothermal crystallization kinetics of a Ziegler‐Natta catalyzed high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins. Standard techniques such as differential scanning calorimetry (DSC) and light depolarization microscopy (LDM) techniques were used to measure isothermal kinetics at low supercoolings. DSC was also used to measure nonisothermal crystallization kinetics at low cooling rates. Extrapolation of isothermal crystallization half‐times of Z‐N catalyzed LLDPE resin using the isothermal half‐time analysis led to erroneous predictions, possibly due to Z‐N LLDPE consisting of a mixture of molecules having different amounts of short chain branching (comonomer). However, predicted reciprocal half‐times at high supercoolings, using isothermal half‐time analysis and using nonlinear regression of nonisothermal crystallization kinetics measured at low cooling rates using the differential Nakamura model, of the HDPE were similar to measured reciprocal half times at high supercoolings of a similar HDPE by Patki and Phillips. It is also shown that the differential Nakamura model can be effectively used to model nonisothermal crystallization kinetics of HDPE resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Viscoelastic, thermal, and morphological analysis of HDPE/EVA/CaCO3 ternary blends

High density polyethylene (HDPE), calcium carbonate (CaCO3), and ethylene vinyl acetate (EVA) ternary reinforced blends were prepared by melt blend technique using a twin screw extruder. The thermal properties of these prepared ternary blends were investigated by differential scanning calorimetry. The effect of EVA loading on the melting temperature (Tm) and the crystallization temperature (TC) was evaluated. It was found that the expected heterogeneous nucleating effect of CaCO3 was hindered due to the presence of EVA. The melt viscosities of the ternary reinforced blends were affected by the % loading of CaCO3, EVA, and vinyl acetate content. Viscoelastic analysis showed that there is a reduction of the storage modulus (G′) with increasing of EVA loading as compared to neat HDPE resin or to HDPE/CACO3 blends only. The morphology of the composites was characterized by scanning electron microscopy (SEM). The dispersion and interfacial interaction between CaCO3 with EVA and HDPE matrix were also investigated by SEM. We observed two main types of phase structures; encapsulation of the CaCO3 by EVA and separate dispersion of the phases. Other properties of ternary HDPE/CaCO3/EVA reinforced blends were investigated as well using thermal, rheological, and viscoelastic techniques.

Effect of sparse long‐chain branching on the step‐strain behavior of a series of well‐defined polyethylenes

AbstractThe effect of sparse long chain branching, LCB, on the shear step‐strain relaxation modulus is analyzed using a series of eight high‐density polyethylene (HDPE) resins. Strains of 1 to 1250% are imposed on materials with LCB content ranging from zero to 3.33 LCB per 10,000 carbon atoms. All materials are observed to obey time–strain separation beyond some characteristic time, τk. The presence of LCB is observed to increase the value of τk relative to the linear resin. The behavior of the relaxation modulus at times shorter than τk is investigated by an analysis of the enhancement seen in the linear relaxation modulus, G0(t), as a function of strain and LCB content. This enhancement is seen to (1) increase with increasing strain in all resins, (2) be significantly larger in the sparsely branched HDPE resins relative to the linear HDPE resin, and (3) increase in magnitude with increasing LCB content. The shape and smoothness of the damping function is also investigated. The finite rise time to impose the desired strain is compared to the Rouse relaxation time of linear HDPE resins studied. Sparse LCB is found to increase the magnitude of the relaxation modulus at short times relative to the linear resin. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Effect of Sparse Long-chain Branching on the Film-casting Behavior for a Series of Well-defined HDPEs

Abstract The effect of sparse long chain branching, LCB, on the film-casting process is analyzed using a molecular model for the rheological behavior for a series of well-characterized, high-density polyethylene (HDPE) resins and a low-density polyethylene (LDPE) resin at various drawdown ratios. A full rheological characterization in both shear and shear-free flows is also presented. At low drawdown ratios, the LDPE exhibited the least degree of necking at distances less than the HDPE frostline. The sparsely-branched HDPE resins films had similar final film-widths that were larger than those of the linear HDPE. As the drawdown ratio was increased, film width profiles separated based on branching level. Small amounts of LCB were found to reduce the amount of necking at intermediate drawdown ratios. At higher drawdown ratios, the sparsely-branched HDPE resins of lower LCB content had film-widths that mimicked that of the linear HDPE, while the sparsely-branched HDPE resins of higher LCB content retained a larger film width. Molecular structural analysis via the Pom-Pom constitutive model suggested that branching that was distributed across a larger range of backbone lengths served to improve resistance to necking. As the drawdown ratio increased, the length of the backbones dominating the response decreased, so that the linear chains were controlling the necking behavior of the sparsely-branched resins of lower LCB content while remaining in the branched regime for higher LCB content HDPEs. Other processing variables such as shear viscosity magnitude, extrudate swell, and non-isothermal processing conditions were eliminated as contributing factors to the differences in the film width profile.

Rheological and molecular properties of organic peroxide induced long chain branching of recycled and virgin high density polyethylene resin

AbstractCommercial blow molding grade recycled high density polyethylene (R‐HDPE) and blow molding grade virgin high density polyethylene (V‐HDPE) were reactively extruded with various compositions (0.00–0.15% wt/wt) of different peroxides in a twin screw extruder. The aim was to produce the extended chain mechanism of a blow molding grade HDPE photopolymer—a polymer resin comprising a significant part of the post consumer recycled plastic stream in Australia. In shear rheological tests, the modified material exhibited an increase in viscoelastic properties and complex viscosity compared to unmodified counterparts. Higher extent of viscoelastic properties enhancement was observed with 1, 3 1, 4 BIS (tert‐butylperoxyisopropyl) Benzene (OP2). This could be attributed to the higher degree of branching. The weight average molecular weight of the all modified materials and its molecular weight distribution (MWD) widened with peroxide modification. These results also support that formation of branching dominates the modification process at molecular level. Increase in branching index (g′) with increase in peroxide composition also confirmed higher degree of branching. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Note: Method for overcoming ductile failure in Münstedt-type extensional rheometers

The purpose of this note is to introduce an experimental technique to overcome ductile failure in uniaxial extensional flow and show a simple illustration of its effectiveness. Ductile failure prevents the rheological measurement of transient stress growth at higher strains for certain strain-hardening materials. This reduces the accuracy of nonlinear parameters for constitutive equations fit from transient stress growth data, as well as their effectiveness in modeling extensionally driven processes such as film casting. We propose an experimental technique to overcome ductile failure called encapsulation in which the material that undergoes ductile failure is surrounded by a resin that readily deforms homogeneously at higher strains. The materials are arranged in concentric cylinders and are deformed in parallel. A simple parallel model is shown to calculate the viscosity of the core material and is tested for its robustness in a linear low-density polyethylene/low-density polyethylene system in which the viscosities of each resin are known at all strains, strain rates, and temperatures investigated. Agreement is found at each strain rate investigated across all strains studied. The method is then extended to measure the viscosity of a sparsely branched high-density polyethylene (HDPE) system that undergoes ductile failure at higher strain rates. The technique shows good agreement with experimental results up to the onset of ductile failure in the pure resin and predicts viscosity growth of the HDPE resin well beyond the critical strain for the onset of failure seen in experimental measurements of the pure HDPE. Repeatability of the technique is also illustrated.

Characterization of the Microstructure of Bimodal HDPE Resin

In this work, two industrial bimodal high density polyethylene resins, resin A and resin B, having similar molecular weight (Mw), molecular weight distribution (MWD, and short-chain branching (SCB) content but different mechanical properties, were fractionated through cross-fractionation. The fractions were further characterized by GPC, 13C NMR, DSC and FT-IR techniques. These two resins were firstly fractionated into two fractions, i.e., high-temperature and low-temperature fractions, via preparative solution crystallization fractionation. Resin A with much better mechanical properties contains more high-temperature fractions with longer crystallizable sequences. The SCB content in the low-temperature fraction of resin A is lower than that of resin B. Both low-temperature fractions were then further fractionated using solvent gradient fractionation (SGF). The characterization of SGF fractions indicates that most of the branches fall into the high molecular weight chains in both low-temperature fractions. However, the high molecular weight chains in the low-temperature fraction of resin A contain less SCB than that of resin B.

A Tensile Strain Hardening Test Indicator of Environmental Stress Cracking Resistance

Environmental stress cracking resistance (ESCR) is an important indicator of performance for high density polyethylene (HDPE) in structural and polymer pipe applications. The commonly used test for determining ESCR of HDPE can be time consuming and rather imprecise. A tensile strain hardening test was recently proposed to offer a faster way to characterize ESCR of polyethylene. In this paper, a practical approach is adopted whereby the test is extended to room temperature and shown to relate reliably to the ESCR of HDPE. Several HDPE resins (including pipe‐grade resins) are analyzed at strain rates of 0.5 mm/min and 7 mm/min to compare the effect of strain rate. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of HDPE in a reliable fashion. In our study the more direct measure of “hardening stiffness” is used to compare resins instead of strain hardening modulus. Because no true stress‐strain measurement is needed, this is a much simpler test method than other methods previously suggested. In addition, the use of the natural drawing ratio (NDR) as ESCR ranking indicator is examined. Results show that NDR can also be employed as a strain rate‐independent indicator of ESCR of HDPE. The test proposed herein is practical, simple and precise, and hence a more reliable indicator of ESCR performance of HDPE.

Application of the essential work of fracture method in ranking the performance in service of high-density polyethylene resins employed in pressure pipes

High-density polyethylene resins have increasingly been used in the production of pipes for water- and gas-pressurized distribution systems and are expected to remain in service for several years, but they eventually fail prematurely by creep fracture. Usual standard methods used to rank resins in terms of their resistance to fracture are expensive and non-practical for quality control purposes, justifying the search for alternative methods. Essential work of fracture (EWF) method provides a relatively simple procedure to characterize the fracture behavior of ductile polymers, such as polyethylene resins. In the present work, six resins were analyzed using the EWF methodology. The results show that the plastic work dissipation factor, βwp, is the most reliable parameter to evaluate the performance. Attention must be given to specimen preparation that might result in excessive dispersion in the results, especially for the essential work of fracture we.