Application of essential work of fracture concept to toughness characterization of high‐density polyethylene

Deformation and fracture toughness of high‐density polyethylene (HDPE) in plane‐stress tension was studied using the concept of essential work of fracture (EWF). Strain range for necking was determined from uniaxial tensile test, and was used to explain the deformation transition for 2‐staged crack growth in double‐edge‐notched tensile test. Through work‐partitioning, EWF values for HDPE were determined for each stage of the crack growth. Appropriateness of these EWF values to represent the material toughness is discussed. The study concludes that the EWF values for ductile polymers like HDPE may not be constant, but vary with the deformation behaviour involved in the crack growth process. POLYM. ENG. SCI., 47:1327–1337, 2007. © 2007 Society of Plastics Engineers     

Evaluation of the tear properties of polyethylene blown films using the essential work of fracture concept

In the case of very thin materials such as blown films, the applied stress state in front of the crack tip is normally a plane stress condition, and the deformation around the crack tip due to the remote stress is very large. However, current standard test methods for quantifying the fracture toughness of thin films, such as the Elmendorf tear test, cannot explain or represent the tear characteristics accurately. The common way of interpreting the test results from the Elmendorf tear test is to develop an empirical correlation and then compare the average values. In this paper, essential work of fracture (EWF) tests for five commercial polyethylene (PE) blown films have been conducted, and the fundamentals of their tear properties based on fracture mechanics have been studied. The results from the EWF test are interpreted based on two important parameters, i.e., the essential work of fracture (W_e) and the non-essential work of fracture (W_p). Further, the relationship between these parameters and the current standard Elmendorf tear test is shown.     

Selected physical characteristics of polystyrene/high density polyethylene composites prepared from virgin and recycled materials

Mixtures of polystyrene and high density polyethylene were injection molded from recycled and virgin polymers to generate cocontinuous structures. The mechanical properties of these blends were evaluated to assess their conformance to rule of mixtures behavior in general and to identify areas of synergy or incompatibility in specific. Flexural and tensile data for recycled blends showed that generally the properties are not additive, except in a cocontinuous region of composition near 35/65 PS/HDPE that has been identified previously for recycled materials. Analysis of crystallinity in the HDPE phase of these blends by differential scanning calorimetry indicates a marked reduction in the level of HDPE crystallinity at the 35/65 PS/HDPE composition. Similar blends of virgin PS/HDPE polymer do not show the differing regions of incompatibility and synergy illustrated by the recycled materials, but rather show approximate conformance to the rule of mixtures. Furthermore, the virgin blends show virtually no crystallinity suppression and a more pronounced T_g shift in the polystyrene compared to recycled materials. Detailed characterization of the recycled materials in terms of polymer and particulate impurities should improve understanding of these differences and perhaps provide direction for obtaining enhanced synergistic behavior in virgin polymer blends. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Enhanced thermal stability and flame retarding properties of recycled polyethylene based wood composites via addition of polyethylene/nanoclay masterbatch

Abstract

Barrier and mechanical properties of wood powder composites based on recycled polyethylene (RPE) were modified using a commercial nanoclay masterbatch. X-ray diffraction, dynamic rheology and thermogravimetric analysis measurements showed that nanoclay from the selected masterbatch was well dispersed and formed a percolation network in both virgin and RPEs. The resulting nanocomposites promoted the thermal stability of matrix significantly. Modification efficiency of nanoclay, however, was evidently influenced by the type of matrix, where the strongest effect was achieved in a low viscosity virgin high density PE. The masterbatch was incorporated into an industrial formula designed extrusion quality RPE/wood flour composite. Processing procedures, mainly compounding cycles, and material composition, mainly clay content and type of coupling agent, were optimised. Two extrusion cycles led to higher uniformity of resulting composites than one cycle. Addition of a coupling agent, which has medium viscosity and plenty functional groups, led to enhanced tensile strength. The twice compounded composites were well stiffened and strengthened via combination of 6 wt-% clay and medium viscosity coupling agent. All composites without the addition of nanoclay burned faster after ignition and dripped much earlier and more compared to the composites containing nanoclay even with as small amount as 3 wt-% and being compounded once. The material with 6 wt-% clay showed the best sample integrity and burned slowest of all the tested composites. Furthermore, no dripping during combustion was seen for this material. This study shows that the incorporation of nanoclay using the selected masterbatch can effectively improve the flame retarding properties of RPE based wood composites.     

Preparation of highly filled wood flour/recycled high density polyethylene composites by in situ reactive extrusion

Highly filled wood flour/recycled high density polyethylene (WF/RHDPE) composites were directly prepared by in situ reactive extrusion using a twin‐screw/single‐screw extruder system. The effects of dicumyl peroxide (DCP) content on extrusion pressure, rheological behavior, mechanical properties, fractured surface morphology of the composites, and melting temperature of RHDPE in the composites were investigated. The extrusion pressure and torque of WF/RHDPE composite melt increased with DCP content. Mechanical property tests and scanning electron microscopy analysis results confirmed that the interfacial interaction of the composites was improved by in situ reaction. The composites show lower melting peak temperature (T_m) than RHDPE. The cooling in profile extrusion shortened the crystallization time, resulting in decrease of crystalline order of RHDPE in the composites. There are no noticeable changes of T_m values with increasing DCP content. Comparative study on composites with maleic anhydride grafted polyethylene as compatibilizer demonstrated that mechanochemical treatment with DCP and maleic anhydride was an effective method to improve interfacial adhesion for WF/RHDPE composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

High strength polyethylene fibers from high density polyethylene/organoclay composites

High strength fibers were prepared from high density polyethylene (HDPE)/organically modified montmorillonite (OMMT) composites. X‐ray diffraction study revealed that the composites were of conventional or phase‐separated type. As‐spun composite fibers were found to have higher drawability than as‐spun HDPE fiber. As a result of an increased drawability, fibers with much higher mechanical properties were obtained. The highest modulus and tensile strength obtained in the present study were 38 GPa and 1.7 GPa, respectively. Study of internal morphology suggests that the role of OMMT is to suppress a defect formation and allows the fiber to be drawn to higher draw ratio. Analysis of the mechanical properties of the fibers using a Griffith type relationship suggested that the fibers have much smaller defects and the predicted attainable strength for the fiber is much higher than that previously predicted for melt‐spun and hot drawn fiber. POLYM. ENG. SCI., 47:943–950, 2007. © 2007 Society of Plastics Engineers     

Rheological characterization of polyethylene/polyester recycled tire fibers/ground tire rubber composites

The rheological behavior of polyester recycled tire fibers (RTF) mixed with ground tire rubber and linear low density polyethylene (LLDPE) with and without styrene–ethylene–butylene–styrene grafted maleic anhydride (SEBS‐g‐MA) as a compatibilizer was investigated in the melt (small amplitude oscillatory shear) and solid (dynamic mechanical analysis) states. In particular, the effect of RTF content (10, 25, and 50 wt %), extrusion screw speed (110, 180, and 250 rpm), and temperature profiles (extrusion and injection molding) was studied. In general, it was found that the rheological properties in the melt state (modulus and viscosity) of the uncompatibilized samples increased with RTF content, but higher values were obtained when SEBS‐g‐MA was added due to better interfacial coupling. Although similar results were obtained in the solid state, it was shown that melt rheology can better explain the variations as the measurements are more sensitive to the interface quality since the matrix contribution is less important. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46563.     

Application of the essential work of fracture concept to high temperature deformation in polyoxymethylene

Essential Work of Fracture (EWF) tests have been carried out on double edge notched samples machined from injection molded sheets of commercial grades of polyoxymethylene homopolymer with different molecular weight averages. Most of the measurements were made at 100⁰C and over a range of test speeds in which polyoxymethylene is anticipated to undergo a macroscopic ductile‐brittle transition with decreasing strain rate. The results reflect both the existence and the molecular weight dependence of this transition, and are argued to be valid in terms of the European Structural Integrity Society's EWF draft test protocol under certain test conditions. However, it is shown that the applicability of the test method used here becomes highly questionable for test speeds in the immediate vicinity of the transition, owing to the influence of the initial ligament length on the crack tip deformation mechanisms.     

Properties of recycled high density polyethylene from milk bottles

The effect of virgin, high-density polyethylene (HDPE)/recycled HDPE composition on the physical properties of the blends was investigated. The recycled HDPE was obtained from a postconsumer cycle of milk bottles. It was found that elongation at break was the mechanical property mostly affected by the content of recycled HDPE. Overall, however, the recycled HDPE from milk bottles was found to be a material with useful properties not largely different from those of virgin resin and thus could be used, at an appropriate concentration in virgin HDPE, for different applications.     

Determination of plane‐strain fracture toughness of polyethylene copolymer based on the concept of essential work of fracture

Toughness and deformation capacity of six polyethylene copolymers in plane‐strain fracture were characterized using the concept of essential work of fracture (EWF). Two types of regression analysis were considered: one based on the traditional, total work of fracture, while the other on the energy partition to extract the portion that is relevant to the plane‐strain fracture. In particular, the latter analysis excludes energy that is for the final stretch of the surface flanks and produces toughness values that are smaller than those determined based on the total work of fracture. The study found that two types of analysis rank the copolymers in different orders. Based on the results from the energy‐partition approach, the article discusses the influence of material characteristics (molecular weight, branch concentration, density, etc.) on the plane‐strain fracture toughness of the polyethylene copolymers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Glass fiber/wood flour modified high density polyethylene composites

Composites of high density polyethylene (HDPE) with the reinforcements of glass fiber (GF) and wood flour (WF) have been studied in this work. High‐density polyethylene‐grafted maleic hydride (HDPE‐g‐MAH) was used as a compatibilizer. In particular, the effect of GF, WF, and HDPE‐g‐MAH on the overall properties of GF/WF/HDPE composites (GWPCs in short form) was systematically studied. The results indicate that HDPE‐g‐MAH as a compatibilizer can effectively promote the interfacial adhesion between GF/WF and HDPE. By the incorporations of GF/WF, the heat deflection temperature can reach above 120°C, and the water absorption can be below 0.7%, also the tensile strength, flexural strength, and impact strength of GWPCs can surpass 55.2 Mpa, 69.4 Mpa, and 11.1 KJ/m², respectively. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of nanoclay on positive temperature coefficient to resistivity characteristics of high density polyethylene/silver‐coated glass bead composites

Positive temperature coefficient to resistivity characteristics of high density polyethylene (HDPE)/silver (Ag)‐coated glass bead (45 wt%) composites, without and with nanoclay, has been investigated with reference to HDPE/carbon black (CB) (10 wt%) composites. Plot of resistivity versus temperature of HDPE/CB (10 wt%) composites showed a sudden rise in resistivity (PTC trip) at ≈128°C, close to the melting temperature (T_m) of HDPE. However, for HDPE/Ag coated glass bead (45 wt%) composites, the PTC trip temperature (≈88°C) appeared well below the T_m of HDPE. Addition of 1 phr clay in the composites resulted in an increase in PTC trip temperature of HDPE/Ag‐coated glass bead (45 wt%) composites, whereas no significant effect of clay on PTC trip temperature was evident in HDPE/CB/clay composites. We proposed that the PTC trip temperature in HDPE/Ag‐coated glass bead composites was governed by the difference in coefficient of thermal expansion of HDPE and Ag‐coated glass beads. The room temperature resistivity and PTC trip temperature of HDPE/Ag‐coated glass bead (45 wt%) composites were found to be very stable on thermal cycling. Dynamic mechanical analyzer results showed higher storage modulus of HDPE/Ag‐coated glass bead (45 wt%) composites compared with the HDPE/CB (10 wt%) composites. Thermal stability of HDPE/Ag‐coated glass bead (45 wt%) composites was also improved compared with that of HDPE/CB (10 wt%) composites. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Composites from compounding wood fibers with recycled high density polyethylene

The development of composites consisting of wood fibers and recycled plastics offers not only an opportunity to utilize an abundant natural resource but also a means to alleviate the serious plastics disposal problem. In this work, aspen fibers are incorporated into recycled high density polyethylene with a co-rotating inter-meshing twin-screw extruder to study processing-property relationships. Tensile, impact, and flexural strengths are measured as functions of fiber concentration. The effects of fiber pretreatment, screw configuration, and compounding temperature on the properties of composites are discussed.     

Foam extrusion of high density polyethylene/wood‐flour composites using chemical foaming agents

A one‐way analysis of variance and thermal analysis were performed in this study to examine the influences of the contents, types (exothermic vs. endothermic), and forms (pure vs. masterbatch) of chemical foaming agents (CFAs), as well as the use of coupling agents, on the density reduction (or void fraction) and cell morphology of extrusion‐foamed neat high density polyethylene (HDPE) and HDPE/wood‐flour composites. The CFA types and forms did not affect the void fractions of both the neat HDPE and HDPE/wood‐flour composites. However, a gas containment limit was observed for neat HDPE foams whereas the average cell size achieved in the HDPE/wood‐flour composite foams remained insensitive to the CFA contents, irrespective of the foaming agent types. The experimental results indicated that the use of coupling agent in the formulation was required to achieve HDPE/wood‐flour composite foams with high void fraction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3139–3150, 2003     

Fracture characterization of low‐density polyethylenes by the essential work of fracture: Changes induced by thermal treatments and testing temperature

The tensile deformation and fracture behavior of commercially available low‐density polyethylene (LDPE) films, having different molecular characteristics, was studied. Submitting samples to specific thermal histories controlled the morphological structure of these semicrystalline polymers. Phase‐structure analysis of the resulting materials was performed by DMA and DSC analyses. The plane‐stress essential work of fracture methodology was chosen because the materials used had failed after complete necking of the remaining ligament. Significant differences in behavior, induced by thermal treatments, were found for the tensile yield stress and the specific nonessential work of fracture, but not in the specific essential work of fracture. The results show that the mechanical properties and fracture behavior depend not only on the crystallinity levels and molecular weight characteristics of the samples, but also upon the degree of structural continuity. The β‐relaxation process, associated with the crystal‐amorphous interphase, strongly influences the fracture behavior at testing temperatures chosen below the β‐relaxation temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 781–796, 1999     

Effect of matrix molecular weight on the dispersion of nanoclay in unmodified high density polyethylene

The effect that polymer molecular weight has on the dispersion of relatively polar montmorillonite (MMT) in nonpolar, unmodified high density polyethylene (HDPE) was examined. Polymer layered silicate (PLS) nanocomposites were prepared via melt compounding in a single screw extruder using three unmodified HDPE matrices of differing molecular weight and organically modified MMT (organoclay) in concentrations ranging from 2 to 8 wt%. The weight average molecular weights (M _W) of the HDPE matrices used ranged from 87,000 to 460,000 g/mol. X‐ray diffraction (XRD), tensile testing, dynamic mechanical thermal analysis (DMTA), and dynamic rheometry were performed on these nanocomposites. Nanocomposites generated from the high molecular weight (HMW) HDPE matrix exhibited increased intercalation of the MMT as shown by XRD and greater improvements in the Young's modulus when compared with nanocomposites generated from the low (LMW) and middle molecular weight (MMW) matrices. DMTA measurements carried out in torsion showed that the increase in shear modulus of the HMW nanocomposites was not as great as that of the LMW and MMW counterparts as observed from a lower percentage enhancement in the storage modulus (G′) and estimated heat distortion temperature (HDT). This was attributed to the higher degree of mechanical anisotropy in the HMW nanocomposites. POLYM. COMPOS., 28:499–511, 2007. © 2007 Society of Plastics Engineers     

Plane strain fracture toughness testing of high density polyethylene

The concepts of linear elastic fracture mechanics (LEFM) are applied to three grades of high density polyethylene in an attempt to determine their fracture behavior in terms of a linear elastic fracture toughness, K_c. The effect of specimen size (thickness and width), crack length and the mode of loading on K_c has been investigated in order to determine the plane strain fracture toughness, K_Ic, of these materials. The effect of temperature (between +23 and ?180°C) on their fracture behavior has also been investigated and compared in terms of their plane strain fracture toughness values.     

Scanning electron micrographs of high density polyethylene fracture surfaces

Three high density polythylenes with widely differing molecular weights have been subjected to a fast fracture process and studied by scanning electron micrography. Two polymers in the low to medium molecular weight range show evidence of substantial melting during facture, in support of some previous work. On the other hand the ultra high molecular weight material seems to deform on a more massive scale without recognisable relics of a melting process.     

HDPE/竹粉复合材料的界面演变及流变特性

采用扫描电子显微镜-X射线能谱仪跟踪硅烷偶联剂中Si元素在高密度聚乙烯(HDPE)基竹塑共混体系界面处的分布,并结合旋转流变仪研究了共混体系加工过程中的界面演变过程。发现随着加工时间的延长,界面处Si元素的含量相对增加,体系的动态模量、黏度升高。表明偶联剂连接的竹粉与HDPE分子链间的相互作用增强,界面层厚度增加。共混体系的动态流变测试结果表明,竹粉填充体系在低频末端区的线性黏弹行为显著不同于HDPE基体,表现出"类固体"特性,动态流变测试对偶联剂的加入所引起的体系的黏弹行为及结构变化响应较敏感。Cole-Cole曲线可反映竹粉粒子网络结构及竹粉与HDPE基体界面相关的松弛信息,体现体系界面性质及竹粉与分子链间网络结构的变化。     

The essential fracture work concept for toughness measurement of ductile polymers

The essential work of fracture method is explored. The method was used to determine the fracture toughness of a series of toughened polymer blends and the crack resistance of a thin ductile polymer film, which could not be tested using the J-integral method. A comparison between J-integral and the specific essential work of fracture was carried out to test the equivalence of the two methods. The effects of geometry on the essential work of fracture and the plane-stress/plane-strain transition were studied. It has been shown that the specific essential work of fracture is a material constant, independent of sample geometry, and equivalent to the critical J-integral. The plane-stress/plane-strain transition is found to depend on the nature of the material tested. The sample thickness requirement for valid plane-strain specific essential work of fracture is discussed, and it is proposed that the size requirement for the plane-strain specific essential work of fracture may be less rigorous than that for plane-strain J_IC measurement.