Experimental investigation of the effect of residual stresses on rapid crack propagation in polyethylene (PE100) pipes

Pipe extruded from polyethylene of strength class PE100 was tested using the ISO 13477 Small Scale Steady State “S4” method, to investigate the effect of frozen‐in stress on rapid crack propagation (RCP). It was found that the lower the residual stress, the lower the S4 critical temperature T_cS4 for RCP. Different experimental thermal treatments were used to independently modify residual stresses and crystallinity, to study the separate effects on RCP. It was found that the effect of crystallinity was less significant than that of residual stresses. It has previously been suggested that the residual stress influence on T_cS4 is determined by the balance of two mechanisms: additional stored strain energy prior to fracture helps to drive the crack, while the closing moment after fracture helps to close the flaring pipe wall. Tests on annealed specimens suggested that the first effect dominates, especially soon after crack initiation. However, the observed effect seems too large to be explained by this mechanism, and we suggest that the observed benefit from annealing may be better explained by a change in crack front shape. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

The dependence of rapid crack propagation in polyethylene pipes on the plane stress fracture energy of the resin

The critical temperature [CT] for rapid crack propagation [RCP] was measured in 11 polyethylene [PE] 200 mm diameter gas pipes each with different resins. The plane stress fracture energy [PSFE] in thin Charpy impact specimens of the resin was found to correlate with the CT. The higher the PSFE, the lower the CT. This result was related to the observation that the PSFE decreases as the temperature decreases. It was found that the impact energy of specimens from compression molded pipe that was remelted at 180°C gave a better correlation with the CT than specimens that were machined from the inner wall of the pipe. Consequently, it is now possible to predict the CT of a pipe by measuring the PSFE of the resin without making the pipe.     

Thermostimulative shape memory effect of linear low‐density polyethylene/polypropylene (LLDPE/PP) blends compatibilized by crosslinked LLDPE/PP blend (LLDPE–PP)

A novel linear low‐density polyethylene (LLDPE)/polypropylene (PP) thermostimulative shape memory blends were prepared by melt blending with moderate crosslinked LLDPE/PP blend (LLDPE–PP) as compatibilizer. In this shape memory polymer (SMP) blends, dispersed PP acted as fixed phase whereas continuous LLDPE phase acted as reversible or switch phase. LLDPE–PP improved the compatibility of LLDPE/PP blends as shown in scanning electron microscopic photos. Dynamic mechanical analysis test showed that the melt strengths of the blends were enhanced with increasing LLDPE–PP content. A shape memory mechanism for this type of SMP system was then concluded. It was found that when the blend ratio of LLDPE/PP/LLDPE–PP was 87/13/6, the blend exhibited the best shape memory effect at stretch ratio of 80%, stretch rate of 25 mm/min, and recovery temperature of 135°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

The effect of a polypropylene skin on the structure and properties of PE/PP dual layer pressure pipe

A new and growing family of polyethylene (PE)‐based pressure pipes have a polypropylene (PP) skin. The effect of the PP skin on the structure and properties of the core PE pipe was investigated by comparing the skinned pipe with an uncoated pipe made from the same PE material and with the same dimensions. The annealing effect introduced by the skin changed the PE core pipe density profile across the wall thickness, increasing density in the PE core pipe near to its outer surface. The density at the bore of the coated and the uncoated pipe was similar. The melting temperature and enthalpy of melting data from DSC agreed with the density profile results. The melting temperature of PE core pipe material close to the PP skin increased with increasing skin thickness. Residual stress assessment indicated that, as the PP skin thickness increased, the PE core pipe had a lower level of overall residual stress in the hoop direction. Long‐term hydrostatic strength (LTHS) tests were carried out and showed a higher strength for the coated pipe than the uncoated one. The observed structural changes have been used to explain the relative strength of these two PE pipes. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Nanoclay effect on transport properties of thermoplastic polyurethane/polypropylene (TPU/PP) blends

In this paper the diffusion of water through the nanoclay filled TPU/PP blends was investigated at various temperatures. The effect of blend ratio, compatibilisation and nanoclay addition on the transport properties was studied in detail. Special attention has been given to study the mechanism of diffusion. Thermodynamics and Arrhenius parameters were evaluated from the diffusion data. The various parameters such as diffusion coefficient (D), permeation coefficient (P), sorption coefficients (S) were evaluated at different diffusion conditions. Attempts were made to correlate the observed morphology of the blends with transport properties. Addition of PP into TPU decreases water sorption of the system due to the decrease in overall polarity of the system and further reduced by addition of compatibiliser and nanoclay into the system. Compared to the ether-TPU based blend nanocomposites, the ester-TPU blends show better compatibility as confirmed by analysis.     

Compatibilization of polyethylene/aluminum hydroxide (PE/ATH) and polyethylene/magnesium hydroxide (PE/MH) composites with functionalized polyethylenes

Study was made of the compatibilization of polyethylene/aluminum hydroxide (PE/ATH) and polyethylene/magnesium hydroxide (PE/MH) composites with composition 60/40 wt%. Compatibilizers were hydroxyl or carboxylic acid functionalized copolymers prepared in our laboratory with metallocene catalysts and commercial butyl acrylate, maleic anhydride, epoxy, and acrylic acid functionalized polyethylenes. Comparison was made with stearic acid treatment of the composites. The effect of polymeric compatibilizers on the properties of the composites was studied by tensile and impact tests, dynamic mechanical analysis, differential scanning calorimetry, morphology studies, and flammability tests. Adhesion of the PE/ATH and PE/MH composites was significantly improved, as seen in morphology studies in the changed fracture mechanism. The improved adhesion was particularly reflected in the mechanical properties: an improvement in both stiffness and toughness was achieved with use of the functionalized copolymers prepared with metallocene catalysts. According to cone calorimetry, flame retardancy properties of the composites did not deteriorate upon addition of compatibilizers.     

我国聚丙烯市场研究

阐述了我国聚丙烯市场的供应、消费状况及其市场结构变化，分析了聚丙烯市场的价格变化规律，综合论述了我国聚丙烯工业在加入WTO所面临的机遇与挑战，提出了我国聚丙烯工业的发展建议。     

An investigation of the interfacial adhesion between reclaimed newspaper and recycled polypropylene composites through the investigation of their mechanical properties

The effects of different parameters (e.g. method of defibration of newspaper; size of the fiber; type and concentrations of coupling agents, impact modifiers, and fire retardants) on the mechanical properties of old newspaper fiber-filled recycled polypropylene (PP) have been evaluated. Statistical methods were applied to determine the effects of variables on the tensile strength, Young's modulus, tensile toughness, and impact strength. Statistical analysis revealed that the composites based on newspaper fiber prepared by mechanical defibration showed a better tensile strength compared with those prepared by the steam explosion process, while the latter type of fiber outperformed the former as far as the other mechanical properties are concerned. In the presence of maleic anhydride grafted PP (MPP) and the initiator dicumyl peroxide (DCPO), the tensile strength of the composites increased with increasing fiber loading and fiber size. Moreover, both the tensile strength and the modulus increased with an increase in the concentrations of MPP and DCPO, even in the presence of an impact modifier and fire retardants for surface-modified newspaper-filled composites. Since the tensile strength of short fiber reinforced composites is strongly dependent of the degree of adhesion between the fiber and matrix, the experimental results suggest that either one of these (MPP and DCPO) or both act as the interfacial bonding agent to develop a strong interphase between old newspaper fiber and recycled PP.     

Determination of interfacial tension between PA/PP and LCP/PP by IFR method and effect of compatibilization on interfacial tension

The dynamic imbedded fiber retraction (IFR) method was used to measure the interfacial tension between two molten polymers. The aim was to evaluate the applicability of this method for polypropylene (PP) used as matrix, and two polyamide (PA6, PA66) and thermotropic main-chain liquid crystalline polymer fibers. The effect on the interfacial properties of modifying the PP matrix with compatibilizers was studied as well. The IFR method was found to be suitable for evaluating the interfacial properties of these polymer blends. The measured interfacial tensions correlated well with the morphology and mechanical properties of the blends and values calculated from the harmonic mean equation. Although the measured interfacial tensions were generally lower than the theoretical ones, the order of the values for the different polymer pairs was similar. © 1995 John Wiley & Sons, Inc.     

The effect of interfacial adhesion on the impact strength of immiscible PP/PETG blends compatibilized with triblock copolymers

In this work, recycled Poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) plastic (PETG) was used to enhance the properties and lower the cost of polypropylene(PP). In order to adjust the interfacial adhesion, three triblock copolymers having the same styrene block at two ends but different block in the middle, were used a the compatibilizers, namely, styrene–ethylene/butylene–styrene (SEBS), styrene–butadiene–styrene (SBS), styrene–isoprene–styrene (SIS). The ratio of PP to PETG was fixed at 70/30 and the relationship between interfacial adhesion and mechanical properties was investigated. The addition of SIS caused a considerable increase in Izod impact toughness, but only slightly improved toughness was observed for blends compatibilized with SEBS. The effect of SBS on improving the impact toughness lied in between that of SIS and SEBS. SEM micrographs showed that PETG forms a fibrillar-like structure for all the uncompatibilized and compatibilized blends, and the blends compatibilized with SBS have smallest domain size, the blends compatibilized with SEBS have largest domain size, while the ones compatibilized with SIS show a moderate domain size. Results from melt rheometry and SEM observation together with work of interfacial adhesion, indicated a strongest interfacial adhesion in blends compatibilized with SBS, poorest in blends compatibilized with SEBS, and moderate in blends compatibilized with SIS. It is very interesting to found that the much improved impact strength was not observed in the blends with the strongest interfacial adhesion but achieved in the blend with moderate interfacial adhesion. Investigation on the impact fractured surface revealed an easier debonding of fibril from matrix and consequently drawn out of matrix in blends compatibilized by SIS with moderate interfacial adhesion, which was considered as the main reason for the much improved impact toughness in this system.     

Effect of residual stresses to the crack path in alumina/zirconia laminates

Laminates with alternating layers are well known from nature. The strongly bonded alumina/zirconia (Al₂O₃/ZrO₂) layers can combine high fracture resistance with high strength and stiffness when properly tailored. The presence of compressive residual stresses formed in Al₂O₃ layers can suppress and deflect cracks propagating through the layers. The crack path is governed by both the elastic properties and the internal stress field of individual layers. The laminates with various layer-thickness ratios ranging from 0.1 to 3 were used to investigate the effect of residual stresses and influence of crack formation pattern on the crack path development. The indentation surface cracks observed in various alumina-zirconia laminates exhibit the same crack deflection independently on the level of internal stresses. The crack deflection observed on the fracture surfaces of bending specimens was related to the indentations cracks. The complicated crack path was explained experimentally by 3D reconstruction with the support of numerical simulations.     

Influence of interfacial agents on the physicochemical characteristics of binary polyethylene/polyamide 6 and ternary polyethylene/polypropylene/polyamide 6 blends

Binary low-density polythylene/polyamide 6 and ternary low-density polyethylene/polypropylene/polyamide 6 blends were prepared by melt mixing, without and with the addition of two different commercial products [poly(ethylere-co-buthylacrylate-co-maleic anhydride) and poly(ethylene-co-vinylacetate) grafted with maleic anhydride] used as interfacial modifiers. More precisely, the polypropylene was a propylene/ethylene random copolymer, containg 6% by weight of ethylene. The polyamide 6/interfacial agent and polyethylene/ interfacial agent systems were also considered. Differential scanning calorimetry, microscopic observations—together with chemical etchings—and mechanical tests supported the occurrence of strong interactions at the interface, especially when using the buthyl acrylate-based agent. The compatibilizing effect of the interfacial agents was also analyzed in the light of interfacial tension determinations. Eventually, low-density polyethylene modifications induced by compatibilization were studied carrying out WAXD analysis. © 1996 John Wiley & Sons, Inc.     

The effect of interfacial microstructure on the interfacial strength of glass fiber/polypropylene resin composites

In this study, a new method to form resin droplets on fibers has been developed, and samples for the single fiber pull-out test were prepared using this method. The effects of microstructure of polypropylene (PP) resin and the microstructure of interface between the glass fiber and PP resin on the interfacial strength have been investigated. In addition, the influence of the microstructure of the interface on the interfacial strength of glass fiberreinforced PP composites have been discussed. It has been found that in the pull-out test, the transcrystallinity formed at the interface between the glass fiber and PP resin improved the interfacial strength when no spherulites developed in the PP matrix. On the other hand, it has been found that when the spherulites were well developed in the PP matrix, the transcrystallinity formed at the interface reduced the interfacial strength. Finally, rapid cooling has been shown to improve the interfacial strength between the fiber and resin in the crystalline polymer matrix composites. © 1994 John Wiley & Sons, Inc.     

Effects of crystallinity and supermolecular formations on the interfacial shear strength and adhesion in GF/PP composites

Aim of this study was to screen the morphological effects on the interfacial shear strength (i) in glass fibre (GF) reinforced isotactic polypropylene (iPP) model composites. i was determined by a modified single fibre pull-out technique. It was established that the relation between i (5–6 MPa) and the yield stress of the iPP (y30 MPa) is at about 1:6 and that the i values were not influenced by the mophological superstructure set under isothermal crystallization conditions. Increased i was only observed when specimens were produced non-isothermally, by quenching (i9 MPa). This improvement could not be related to thermal shrinkage stresses. The enhancement in i was attributed to better wetting and improved adhesion due to the enlarged amorphous PP (aPP)-phase. A schematic adhesion model considering the wetting behaviour of aPP and iPP was proposed.     

原位形成FPP偶联Al(OH)3/PP中的界面相互作用研究

制备低含量Al(OH)3填充PP复合材料[Al(OH)3/PP]，研究原位形成的官能团化聚丙烯（FPP）在Al(OH)3/PP中的结晶、熔融行为，结晶形态，以及与各组分间的相互作用。研究认为，在复合材料中存在Al(OH)3与FPP间的化学作用，FPP与PP的相容与共结晶作用，Al(OH)3表面异相诱导成核作用，FPP对Al(OH)3表面异相诱导成核作用的活化作用等，从而改善了Al(OH)3/PP的物理与力学性能。     

The effect of temperature on the interfacial tension of polycarbonate/polyethylene blends

Controlling interfacial tension during the processing of blends is critical to developing morphologies that will yield consistent and acceptable mechanical properties. Determination of the interfacial tension is important in developing a predictive understanding of the effects of processing conditions on the morphology and the physical properties of multicomponent systems. The focus of the research reported herein was to investigate the temperature dependence of the interfacial tension for blends composed of polycarbonate (PC) and polyethylene (PE). The effects of temperature were characterized by direct measurements of the interfacial tension using the imbedded fiber retraction (IFR) method. The interfacial tensions of PC/PE were measured at 210, 220, 230, and 240°C. The temperature dependence of the interfacial tension was found to be ?0.018 ± 0.006 dyn/cm-°C. In general, the interfacial tension, evaluated for low-viscosity simple fluids, is commonly a weak function of temperature (on the order of 0.01 dyn/cm-°C). The results found in this study are in accord with those findings.     

The influence of compatibilizers on nitrile-butadiene rubber and polypropylene (NBR/PP) blends

Four kinds of compatibilizers—chlorinated polyethylene (normal CPE), highly chlorinated CPE, maleic anhydride grafted with polypropylene (MP), and chlorinated polypropylene (CPP)—were used to study the influence of compatibilizing agents on the properties of nitrile-butadiene rubber and polypropylene (NBR/PP) blends, a kind of thermoplastic elastomer (TPE). The results show that the most proper amount of normal CPE, highly chlorinated CPE, MP, and CPP are 9, 8, 7, and 6 wt%, respectively, in the NBR/PP blends. The CPP was the best compatibilizer for NBR/PP blends among the four. NBR/PP blends obtained excellent properties of thermoplastic materials and can be molded with the general processing technologies for thermoplastics, such as injection, extrusion, blow molding, and the like. The mechanical properties are similar to that of Geolast, produced by Monsanto Company, and exceed the Chinese national criterion (GB7527-87). The tensile strength was 13.8 MPa; the elongation at break was 290%; and the compression set was 32%. After the blends were immersed in oil for 70 h, the tensile strength was 10.4 MPa, and the degree of oil absorption was 12%. The compatibility of the blends was consistent with the morphology from transmission electron microscopy (TEM).     

Overload effect on the fatigue crack propagation of PC/ABS alloy

The effect of single overload within an otherwise constant amplitude loading sequence on the fatigue crack propagation (FCP) behavior of the alloy of polycarbonate and acrylonitrile-butadiene-styrene (PC/ABS) is experimentally investigated in this paper. An improved compliance method is employed to measure the fatigue crack length of the specimen. Optical and scanning electron microscopes are used to observe the features of crack surface and the process of crack tip deformation. The overload waveform has slight effect, while the overload ratio has great effect on the crack growth retardation. A small crack increment is produced during overloading. The crack growth rate reduces quickly, and then increases gradually until it reaches the steady crack growth rate level when the loading recovers to normal constant amplitude fatigue loads. Porous or dimple features govern the fatigue crack surfaces.     

Effects of crystallinity of polypropylene (PP) on the mechanical properties of PP/styrene-ethylene-butylene-styrene-g-maleic anhydride (SEBS-g-MA)/teak wood flour (TWF) composites

Polymer Bulletin - Tensile and impact properties of PP/SEBS-g-MA/WF composites up to wood flour volume fraction 0.31 are evaluated. Tensile modulus and strength increased while elongation-at-break...