Impact resistance and secondary transitions

Instrumented Charpy impact measurements were made on polycarbonate from liquid nitrogen temperature to room temperature. The polymer had a transition from brittle fracture to ductile failure at ?130°C. Scanning electron micrographs of the fracture surfaces did not correlate with the secondary transition or test temperature. A Fourier analysis of the impact pulse covers a wide range of frequencies, but the dominant frequencies at room temperature are below 200 Hz. Time–temperature superposition shows that the secondary transition occurs over a broad frequency range centered at 7 MHz at room temperature. Impact strength and the secondary transition (at impact frequency) both have a maximum value around ?75°C.     

The influence of cooling rate on the adhesion of polyethylene coatings applied to metals as a hot melt

The quenching from the melt of polyethylene coatings on steel, polished copper and black-oxidized copper increased their adhesion. The fracture surfaces examined in a scanning electron microscope showed the quenched polymer had a more fibrous texture. Quenching lowered the crystallinity and yield strength of the polymer but greatly increased its elongation at break requiring a greater energy for fracture. The tear strength also was higher. The increased adhesion is attributed to the changed mechanical properties of the quenched polymer, particularly to its increased fracture energy.     

Preparation of zeolite filled glassy polymer membranes

The incorporation of zeolite particles in the micrometer range into polymeric matrices was investigated as a way to improve the gas separation properties of the polymer materials used in the form of membranes. The adhesion between the polymer phase and the external surface of the particles appeared to be a major problem in the preparation of such membranes when the polymer is in the glassy state at room temperature. Various methods were investigated to improve the internal membrane structure, that is, surface modification of the zeolite external surface, preparation above the glass-transition temperature, and heat treatment. Improved structures were obtained as observed by scanning electron microscopy, but the influence on the gas separation properties was not in agreement with the observed structural improvements. © 1994 John Wiley & Sons, Inc.     

Low-temperature graft copolymerization of 1-vinyl imidazole on low-density polyethylene films with simultaneous lamination of copper foils

Surface modification of argon plasma-pretreated low-density polyethylene (LDPE) films by graft copolymerization with 1-vinyl imidazole (VIDz) and with concurrent lamination of copper foils at room temperature and at an elevated temperature were carried out. The adhesion strengths were reported as lap shear adhesion strengths and T-peel strengths. The surfaces of the graft copolymerized films and the mechanically delaminated LDPE and Cu surfaces were characterized by X-ray photoelectron spectroscopy (XPS). It was found that plasma pretreatment of LDPE alone, and in the absence of VIDz, could give rise to strong lap shear adhesion between the polymer and copper. Significant T-peel strengths, however, were obtained only for LDPE/Cu laminates obtained from the simultaneous graft copolymerization and lamination technique. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1977–1983, 1998     

Short fiber-reinforced thermoplastic elastomers from blends of natural rubber and polyethylene

Thermoplastic elastomer blends of natural rubber (NR) with high density polyethylene (HDPE) and with low density polyethylene (LDPE) were reinforced with short silk fiber. Processing characteristics such as torque and temperature developed during mixing and the effect of processing parameters such as nip gap and number of passes in the mill necessary to secure maximum orientation of the fibers in the blends were studied. A small nip gap and a single pass in the mill were found to give best results. Of the different mixing sequences studied, the sequence where short fibers followed by rubber were added to the molten thermoplastic was found to give a uniform dispersion of fibers. Fiber breakage and the change in aspect ratio of the fibers after mixing were also examined. It was observed that, as a direct consequence of the mixing sequence, each fiber was coated with a layer of thermoplastic. Although the properties improved on the addition of the dry bonding system of silica–resorcinol–hexamethylenetetramine, the comparatively long curing time required for full development of adhesion between the fibers and the matrix proved to be a major disadvantage associated with the incorporation of the bonding system. The tensile and tear properties were substantially enhanced, but the ultimate elongation decreased sharply with increasing loading of short fibers in the blends. The effect of fiber orientation and the development of anisotropy in the properties was also noted. Scanning electron microscopy (SEM) studies of the benzene-extracted surfaces of the NR/HDPE (high density polyethylene) blends substantiated the theory of fibers behaving like “mechanical anchors” between the rubber and thermoplastic phase. The effect of fiber loading on the tear and tensile properties of the blends of NR/LDPE with varying blend ratios was studied. Most pronounced improvement in the properties on the addition of short fibers was observed in the high rubber blends. As the plastic content in the blends increased, the short fibers were found to have a lesser influence on the properties. SEM photomicrographs of the tensile and tear fracture surfaces indicated the fiber orientations and the effect of orientation, fiber loading, and blend ratios on the nature of fracture.     

The role of crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) is a semi-crystalline polymer, which has been employed in a range of engineering applications due to its extremely low coefficient of friction, resistance to corrosion, and excellent electrical insulation properties. Despite failure-sensitive applications such as surgical implants, aerospace components, motor seals, and barriers for hazardous chemicals, the mechanisms of crack propagation in PTFE have received limited coverage in the literature. Moreover, PTFE exhibits complex crystalline phase behavior that includes four well-characterized phases with both local and long range order. Three crystalline structures (phases II, IV, and I) are observed at atmospheric pressure with transitions between them occurring at 19 and 30 °C. This observation provides a unique opportunity for investigation of the effects of a polymers crystalline phase on fracture and microstructure evolution. Moreover, due to the presence of three unique ambient pressure phases near room temperature, it is essential to develop an understanding of the effects of temperature-induced phase transitions on fracture mechanisms of PTFE to prevent failure over the normal range of operating temperatures. In this work, we present values for the J-integral fracture toughness of PTFE for a range of temperatures and loading rates employing the single specimen normalization technique. Crack propagation in PTFE is found to be strongly phase dependent with a brittle-to-ductile transition in the crack propagation behavior associated with the two room temperature phase transitions. Increases in fracture toughness are shown to result from the onset of stable fibril formation bridging the crack plane and increased plastic deformation. The stability of drawing fibrils is primarily determined by temperature and crystalline phase with additional dependence on loading rate and microstructure anisotropy. [LAUR-05-0004]     

Fracture energies and tensile strength of an EPDM/PP thermoplastic elastomer

Measurements of the tear strength of EPDM/PP thermoplastic elastomers (EPDM/PP TPEs, Santoprene 201‐87) were carried out at various rates and temperatures. In addition, a cutting technique developed recently was adopted to measure the fracture energy in a process where a well‐controlled geometry of the crack tip was obtained. Results show that the EPDM/PP TPEs possess a relatively high tear strength of 10.40 ± 0.94 kJ/m² at room temperature. Furthermore, good tear strength is still preserved, about 1.87 ± 0.38 kJ/m², at 150°C, where some PP crystals are melted and start to flow. In contrast, the intrinsic strength of EPDM/PP TPEs determined from a cutting test is varied slightly, 700–1000 J/m², over a wide range of temperatures and rates. A comparison of the fracture energy measured by tearing and cutting tests is provided and discussed. The energy density per unit volume of EPDM/PP TPEs determined from the cutting test is 9.7 GJ/m³, which is about twice larger than that for the rupture of C C bonds at room temperature. It is suggested that plastic yielding is a more effective process to enhance the toughness than is simply viscoelastic motion. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1033–1044, 2000     

Binary and ternary particulate composites. II. Tensile behavior over a wide range of strain rates

Tensile properties of both a binary material, i.e., polystyrene (PS) reinforced by 15 vol % of glass beads, and ternary composites, i.e., showing either a maleated styrene/ethylene-co-butylene/styrene copolymer or a styrene-co-methacrylic acid copolymer (SAMA) adduct at the PS/glass-beads interface, are analyzed at room temperature and over a wide range of strain rates. Because the poor quality of the adhesion at the PS/glass-beads interface, the fracture toughness of these binary composites is strongly reduced, whatever the strain rate. The presence of the rubbery interlayer does not change the deformation mechanisms of the composite and the work to break is not significantly enhanced. This results from the poor compatibility between PS and the rubbery interphase leading to the debonding of coated glass beads. The good adhesion quality at the interfaces between phases in the ternary composite showing the SAMA adduct, i.e., SAMA/glass-beads and PS/SAMA interfaces, hinders the decohesion phenomenon. This results in an improvement in both the transfer load and the maximum strength. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1041–1046, 1997     

Effects of reactive reinforced interface on the morphology and tensile properties of amorphous polyamide–SAN blends

The effects of reactive reinforced interface on the morphology and tensile properties of amorphous polyamide (a-PA) and styrene-acrylonitrile (SAN) copolymer blend have been investigated using styrene maleic anhydride (SMA) copolymer as a reactive compatibilizer. The anhydride groups of SMA copolymer can react with the amine groups of polyamide and form in situ graft copolymers at the a-PA–SAN interfaces during the blend preparation. The interfacial adhesion strength of the reactive reinforced interface was evaluated quantitatively using an asymmetric double cantilever beam fracture test as a function of SMA copolymer content using a model adhesive joint. The interfacial adhesion strength was found to increase with the content of SMA copolymer and then level off. The morphological observations of a-PA–SAN (80/20 w/w) blends showed that the finer dispersion of the SAN domains with rather narrow distribution was obtained by the addition of SMA copolymer into the blends. The trend of morphology change was not in accord with that of the interfacial adhesion strength with respect to the content of SMA copolymer. However, the results of tensile properties showed very similar behavior to the case of the interfacial adhesion strength with respect to SMA content; that is, there was an optimum level of the reactive compatibilizer beyond which the interfacial adhesion strength and tensile strength did not change significantly. These results clearly reveal that tensile properties of polymer blend are highly dependent on the interfacial adhesion strength. Furthermore, it is suggested that the asymmetric double cantilever beam fracture test using a model interface is a useful method to quantify the adhesion strength between the phases in real polymer blends. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1925–1933, 1998     

Mechanical Properties of Mullite

The mechanical properties of alkoxy-derived, high-purity, translucent, theoretically dense mullite (3AI₂O₃.2SiO₂) were investigated over the temperature range room temperature to 1500°C. Large agglomerates were found to contribute to the formation of porosity nests which act as strength-controlling flaws at room temperature as well as at high temperatures. Despite the slow crack growth above 1300°C, a slight increase in fracture stress and a large increase in K_Ic were observed up to 1500°C. These increases are explained by the dominance of energy dissipation through plastic relaxation in the plastic zone over grain-boundary sliding due to the presence of the glassy phase.     

Biodegradable PPC/(PVA-TPU) ternary blend blown films with enhanced mechanical properties

Biodegradable films of poly(propylene carbonate)/poly(vinyl alcohol)-thermoplastic polyurethane [PPC/(PVA-TPU)] ternary blends were successfully prepared by melting blending method. The mechanical properties of poly(propylene carbonate) blown film were greatly improved by blending PPC with PVA-TPU. In order to afford the melt processing of PVA, the PVA-TPU binary blend was firstly prepared using thermoplastic polyurethane as a polymeric plasticizer. The rheological behavior, mechanical properties and morphology of these blends were studied. Considering its melt viscosity and thermally processing temperature, the PVA-50%TPU, as a modifier, was blended with PPC to prepare PPC/(PVA-TPU) ternary blend. SEM observation revealed a basic one-phase morphological structure with very good interfacial adhesion between the extremely blurred PPC and PVA-TPU two components. Meanwhile, the miscibility of the ternary components was verified by only one glass-transition temperature obtained from DMA tests. The tensile strength and tear strength of PPC/(PVA-TPU) blown films were determined at different temperatures. The results demonstrate that the mechanical properties of PPC/(PVA-TPU) films were enhanced dramatically at low temperature when compared with neat PPC. At room temperature, PPC/30 %(PVA-50%TPU) blown film exhibited a tensile strength of 26 MPa, and an elongation at break of 484.0 %. Its tear strength in the take-up direction is 124.1 kN/m, and the one in machine direction is 141.9 kN/m. At a low temperature of 0 °C, PPC/30 %(PVA-50%TPU) exhibited a tensile strength of 40.7 MPa and tear strength of 107 kN/m, which are 153 % and 142 % of those of neat PPC respectively. The blending of PPC with the PVA plasticized with TPU provides a practical way to extend the application of the new biodegradable polymer of PPC in the area of blown films.     

Antioxidants and stabilizers. IL. 1,3,5-trimethyl-2,4,6-tris (1-tert-butylperoxy-3,5-ditert-butylcyclohexa-2,5-diene-4-onylmethyl)benzene: Formation,properties and effect on the oxidation of tetralin and isotactic polypropylene

The transformation of the antioxidant 1,3,5-trimethyl-2,4,6-tris(3,5-ditert-butyl-4-hydroxybenzyl)benzene (I) by the oxidation with tert-butyl hydroperoxide in the presence of Co(II)acetylacetonate was investigated. The reaction in tert-butyl alcohol gives rise to product II of partial oxidation, while in benzene cyclohexadienone III is formed. The oxidation products were defined, and their thermal properties and behaviour in light were determined. The oxidative transformation of trisphenol I leads to a loss of its antioxidative activity. The oxidation products II and III at the temperature of their formation do not exhibit an unfavourable effect on the process of thermal oxidation. However, after heating of the hydrocarbon substrate containing them to a temperature close to their decomposition point they act as initiators. They undergo photochemical transformation already at room temperature and give colour to the polymer.     

Application of a gripping system to test a uniaxial graphite fiber-reinforced composite (PMR 15/celion 6000) in tension at 316°C

A gripping system has been developed to test uniaxial, 0° orientation PMR 15/Celion 6000 composites at elevated temperatures. The method involves compression of grit-blasted laminate between grit-blasted metal to give a non-slipping interface for load transfer. Tensile testing at both 316°C and room temperature indicated that deformation was elastic to fracture and that the variation in tensile properties for one laminate is the same as that for several panels. In addition, the tensile properties for uniaxial PMR 15/Celion 6000 are identical at 316°C and room temperature. For nominally 51 volume percent fiber, the elastic modulus is 119.6 GPa, the fracture stress is 1370 MPa, and the strain to fracture is about 1.15 percent. In addition, data are presented which indicate that the gripping system can be used for long term, elevated temperature testing of composite materials.     

Structure-property relations in polypropylene mica composites

Polypropylene composite samples with concentrations of mica of 10, 20, 30, 40, 45, 50, and 60 wt percent were prepared under identical conditions by injection molding. The influence of mica concentration on the structure and mechanical properties of the composites was investigated using a polarizing microscope, scanning electron microscope, differential scanning calorimetry, and stress-strain characteristics. Flexural and impact strengths show an initial rise and then gradual fall with increasing mica concentration. An optimum of these properties occurred at ∼20 wt percent mica. Tensile modulus and heat distortion temperature show a continuous rise; whereas the percent elongation decreased with mica content. These changes in properties are discussed in terms of a skin-core morphology, transcrystallinity, crystallinity, fracture surface morphology, flake orientation, and interfacial adhesion in these composites. This study helps in optimizing the mica concentration for injection molded composites.     

Mechanical properties and failure mode of thermoplastic elastomers from natural rubber/poly(methyl methacrylate)/natural rubber-g-poly(methyl methacrylate) blends

The mechanical properties and fracture behavior of natural rubber/poly-(methyl methacrylate) blends were investigated as a function of composition, graft copolymer concentration, and mixing conditions. The mechanical properties and failure behavior vary with the blend ratio, graft copolymer concentration, and mixing conditions. Various two-phase models were used to fit the experimental mechanical properties. Mechanical properties such as stress–strain behavior, tensile strength, tensile modulus, tear strength, and Izod impact strength were evaluated as a function of compatibilizer concentration. The domain size of the dispersed phase decreases with graft copolymer concentration followed by a leveling off at higher concentration. The mechanical properties attain a maximum value at the leveling point, which is an indication of interfacial saturation and the attainment of maximum interfacial adhesion between the homopolymers. Tensile and tear fracture surfaces were examined by scanning electron microscopy. The detachment of the dispersed domains from the matrix is an indication of no adhesion between the two phases in the case of uncompatibilized blends. Microfibrils between the matrix and the dispersed phase indicate a sign of interfacial adhesion between the phases in the case of compatibilized blends. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:1245–1255, 1997     

Resorbable materials of poly(L-lactide). II. Fibers spun from solutions of poly(L-lactide) in good solvents

Fibers of poly(L -lactide) (PLLA) with a tensile strength up to 1.2 GPa and Young's modulus in the range of 12–15 GPA were obtained by a hot drawing of fibers spun from solution of PLLA in good solvents such as dichloromethane and trichloromethane. The tensile strength of fibers was strongly dependent on the molecular weight of PLLA and on polymer concentrations in the spinning solution. Changing of the polymer concentration in the spinning solution gives rise to formation of fibers with different shape and porosity. Fibers spun from 10–20% solutions at room temperature exhibit a regular structurization, due to the melt fracture. These fibers had knot strengths up to 0.6 GPa, whereas fibers with a smooth surface spun from more dilute solutions had weaker square knots up to 0.3 GPa.     

Influence of temperature on crack-bridging ability of coatings for concrete

The crack-bridging properties of different organic coatings have been studied over a range of thicknesses and applied temperatures. In each case the data allowed to determine linear relationships between crack-bridging ability (CBA) and coating thickness. In addition, a direct correlation between crack bridging ability and elongation at break of the unsupported films has been found, once verified that adhesion properties do not change over temperature.     

The effect of molecular weight distribution on polyethylene film properties

Polymer molecular weight heterogeneity affects the rheological properties of polymer melts such as melt viscosity, fracture and die swell. These rheological properties affect the conversion of the polymer from the bulk resin state to its final usable form. In this particular study, the effect of molecular weight distribution on polyethylene blown film characteristics was studied. The effect of the molecular weight heterogeneity on the rheological characteristics of the polymer in the molten state and its effect on the film properties is presented. The properties studied included film gloss, haze, tear resistance and film impact strength. This study shows that broadening the molecular weight distribution increases haze and reduces film gloss. Further, it was shown that a linear relationship exists between film gloss and external haze. Both values are measures of surface irregularities in the film which are affected by the drawing characteristics of the polymer. A broader molecular weight distribution results in increased impact strength as measured by the Dart Drop Impact Test. This is, it is believed, a result of the increase in long chain branching of the higher molecular weight fractions of the polymer which cause a higher degree of molecular weight entanglement at the branch sites. In contrast the tear strength is reduced as the molecular weight distribution broadens because of the low molecular weight fraction in the broad spectrum material which tend to decrease resistance to tear.     

The brittle fracture of amorphous thermoplastic polymers

The brittle fracture properties of polyphenylene oxide, polysulfone, polycarbonate, and poly(methyl methacrylate) thermoplastic polymers were investigated over a wide range of temperatures. Fracture energy measurements were made using double edge-notched tensile samples. Tensile strength, tensile strain, and initial elastic modulus were measured for calculation of the fracture energy and further analysis of the polymer behavior. It was found that mechanical transitions in the tensile properties corresponded reasonably well with transitions in the fracture energy in the temperature range investigated. Fracture surface photographs permitted visual analysis of the fracture process. It was found that the roughest fracture surface corresponded to the maximum in the fracture energy for a given polymer. A theory for prediction of polymer tensile yield strain is presented, based on the volume dilation concept. The implications of this theory are discussed in terms of the crack tip flow process leading to brittle fracture.     

莫来石基复合材料研究进展

莫来石(3Al2O3· 2SiO2)陶瓷具有抗蠕变能力高、热膨胀系数和导热系数低、抗腐蚀性和抗热震性优异的特点,是一种很有前景的高温结构材料.它最显著的优势是强度和韧性随着温度升高不仅不会下降反而有所提高.然而,莫来石的室温力学性能较差,限制了它的广泛应用.针对如何提高莫来石的室温强度和韧性,目前的研究集中于利用第二相进行改性,取得了一定成果.本文介绍了非连续相(颗粒、晶须、短切纤维)和连续纤维增强莫来石基复合材料的研究现状,包括制备工艺、微观结构和主要性能,指出了存在的问题和今后的发展方向.