The effect of molecular weight on the physical and mechanical properties of ultra-drawn high density polyethylene

A study has been made on the effects of molecular weight on the physical and mechanical properties of cold-extruded high density polyethylene. Prior data indicate that such ultra-drawn strands contain a significant fraction of extended-chain crystals. Four samples, spanning the molecular weight range of 59,000 to 147,000, were cold-extruded under the same conditions and were examined with respect to their melting point, degree of crystallinity, linear expansion coefficient, Young' modulus, strain to break, and tensile strength. The degree of crystallinity, linear expansion coefficient, and modulus did not change significantly with molecular weight. The melting point, strain to break, and tensile strength do increase with increasing molecular weight. This leads to the conclusion that the amount of extended-chain crystals is invariant with molecular weight. Higher molecular weight polymers are seen as providing a greater number of the chains, thus giving the fiber a higher tensile strength.     

Investigation on the thermal behaviors and mechanical properties of ultrahigh molecular weight polyethylene (UHMW-PE) fibers

Fibers of ultrahigh molecular weight polyethylene (UHMW-PE) were prepared with the gel fiber drawing method, and the solvent and extraction solvent used were a general kerosene and gasoline, respectively. The thermal behaviors and mechanical properties of the fiber were studied using thermal analysis, a wide-angle X-ray diffractiometer, density, the sound orientation factor, as well as mechanical property measurement. The results showed that the morphology of macromolecular chains was changed from the folded state to an extendedcain structure with increasing of the drawing ratio. In addition, the crystal form of the fiber also changed. These changes were more evident while the drawing ratio exceeded 20. The tensile strength, similar to the modulus of the fibers, increases with an increasing draw ratio in the range that we researched, whereas the sonic velocity orientation factor and the degree of crystallinity increase slowly when the draw ratio is over 30. © 1996 John Wiley & Sons, Inc.     

The effect of initial morphology on the mechanical properties of ultra-high molecular weight polyethylene

Studies of the physical and mechanical properties of melt crystallized ultra-high molecular weight polyethylene morphologies from melts with different thermal histories indicate that their properties depend on the degree of fusion of the powder particles during their processing and can be enhanced by heating the polymer above 220°C. The degree of cohesion of the powder particles and their initial morphology also have a significant effect on the deformability of the polymer in the solid state forming methods used to prepare high modulus and strength products.     

Influence of organophilic montmorillonite and polypropylene on the rheological behaviors and mechanical properties of ultrahigh molecular weight polyethylene

The effects of organophilic montmorillonite (OM)/poly(ethylene glycol) (PEG) hybrids and polypropylene (PP) on the phase morphology, rheological behaviors, and mechanical properties of ultrahigh molecular weight polyethylene (UHMWPE) were investigated. The presence of the OM/PEG hybrids and PP in UHMWPE was found that it was able to lead to a significant reduction of melt viscosity and enhancement in tensile strength, and elongation at break of UHMWPE. A quantitative analysis indicated a larger affinity of the OM to the PEG than to PP or UHMWPE in the composites, suggesting that OM was intercalated by PEG. This was proposed to be responsible for the reduction of viscosity. Polarizing optical microscopy analysis, on the other hand, indicated that the dispersed OM, which acted as a nucleating agent, lowered the spherulite dimension and increased the spherulite number, resulting in high tensile strength and elongation at break. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Processing of ultrahigh molecular weight polyethylene

The extent of recrystallization of nascent UHMWPE powder is easily measured by calorimetry. Melting and recrystallization of nascent UHMWPE at 140°C can be suppressed by compression molding. Crystals of UHMWPE prepared from dilute solution show a peak melting temperature of 140°C and exhibit crystallinity up to 75.5% depending on crystallization temperature. Large changes in crystallinity result from drawing single crystal mats or compression-molded films.     

Melting of ultrahigh molecular weight polyethylene

On heating in DSC, samples of UHMWPE show a single, fairly sharp, melting endotherm which may be increased to a peak temperature of 147°C and 77% crystallinity by annealing at elevated temperatures. An irreversible conversion of nascent to folded crystals, between 134 and 142°C, was observed by heating nascent UHMWPE powder in the calorimeter. In the presence of n-hexatriacontane, the melting endotherm of UHMWPE was depressed and broadened and the conversion of nascent to melt-crystallized polyethylene facilitated on heating. A melt-crystallized mixture of ordinary linear polyethylene (HDPE) and UHMWPE was not resolved on remelting. After annealing this mixture for 12 h at 130°C, HDPE was fractionated and the melting of UHMWPE was sharpened. Crystals of UHMWPE, prepared from dilute solution in xylene, show a single sharp melting endotherm and high crystallinity, but the melting peak is reduced in temperature compared to nascent crystallized powder.     

Effects of sunshine UV irradiation on the tensile properties and structure of ultrahigh molecular weight polyethylene fiber

This study investigated sunlight‐simulated ultraviolet (UV) beam irradiation on the tensile properties and structure of ultrahigh molecular weight polyethylene (UHMWPE) fibers. The tensile results showed that after 300 h sunlight UV irradiation, the tensile properties of the UHMWPE fibers were obviously degraded. Investigation of morphology revealed that the crystallinity was slightly increased, whereas the overall orientation and molecular weight of the fibers were decreased. SEM observations indicated that the degradation process was nonuniform throughout the fiber and a change from a ductile to a brittle fracture mechanism was found after UV irradiation. DMA results showed two β‐relaxations and one α‐relaxation in the original single filament, and UV irradiation led to the increased intensity of the high‐temperature β‐relaxation and the lowered position of the low‐temperature β‐relaxation. This indicated that irradiation‐induced molecular scission and branching were located primarily in the amorphous and the interface areas of the fiber. Changes in the thermal behavior were also examined by DSC. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2757–2763, 2003     

超高分子量聚乙烯树脂的溶胀性的检测分析

通过上海化工研究院设计的在线检测装置对超高分子量聚乙烯(UHMWPE)溶胀过程中的温度、黏度进行测试,形成了UHMWPE树脂溶胀工艺的检测方法,对黏均分子量分别为3.261×106(A),3.833×106(B),3.455×106(C)3种不同树脂的溶胀温度、溶胀时间、溶胀比等参数进行了分析。结果表明:A,B,C树脂的溶胀温度分别为121,120,119℃;对溶胀釜加热系统设定统一温度,釜内温度达120℃时,A,B,C树脂的溶胀时间分别为60,30,3 min,溶胀比分别为2.37,2.51,2.31。     

The effect of powder particle fusion on the mechanical properties of ultra-high molecular weight polyethylene

Rheo-optical and mechanical property studies with compression molded ultra-high molecular weight polyethylene specimens at different temperatures indicate that their mechanical performance is dependent on the degree of fusion of the powder particles during compression and can be enhanced by heating the polymer powder at temperatures above 220°C. Although the mechanical performance of the compression molded specimens can be improved further by solid-state drawing at a draw ratio 5, the anisotropic morphologies from molded specimen above 220°C have higher initial slope of stress to elongation, strength to break, and an outstanding elastic recovery in compreision to the compression molded specimens at 180°C.     

Roller drawing of ultrahigh molecular weight polyethylene

The roller drawing of ultrahigh molecular weight polyethylene (UHMW-PE) sheets were carried out in the roller temperature T_r range of 100–140°C. In addition to the roller drawing in the solid state (T_r = 100°C), we attempted to crystallize the molten UHMW-PE sheet under the roller-drawing process (T_r = 100–140°C). The tensile and dynamic viscoelastic properties, the molecular orientation, and the microstructure of the roller-drawn UHMW-PE sheets were investigated. The mechanical properties of UHMW-PE sheets were much improved by crystallization during the roller drawing process at T_r = 140°C. The sheets roller-drawn at T_r = 135 and 140°C exhibited c-axis orientation to the draw direction and (100) alignment in the sheet plane. However, at T_r = 100°C the elastic motion of the amorphous chains induces the twinnings of lattice, which enhances the transition to the (110) alignment in the sheet plane. The dynamic storage modulus below γ-dispersion temperature showed good correlation with crystallinity and orientation functions, while taut tie molecules and thick crystallites play an important role in the storage modulus above γ-dipersion temperature.     

The effect of ultra-high molecular weight polyethylene fiber on the mechanical properties of acrylic bone cement

The effects of the addition of ultra-high molecular weight polyethylene fiber (UHMWPE) on the mechanical properties of standard surgical Simplex-P radiopaque bone cement have been investigated. It was found that the tensile strength and tensile modulus were apparently not improved by the incorporation of UHMWPE in the acrylic bone cement. The results of bending strength and bending modulus indicated that a reinforcing effect is obtained at UHMWPE contents as low as 1 wt%, and then levelled off with increasing UHMWPE contents. When the UHMWPE contents as low as 2 wt%, the values of compressive strength and modulus seemed approximate the same; whereas the values of compressive strength and modulus decreased with increasing UHMWPE contents. From the results of dynamic mechanical analysis (DMA), the values of dynamic storage modulus of bone cement increased at UHMWPE fiber as low as 2 wt%, but beyond that UHMWPE content the value of the dynamic storage modulus decreased with increasing UHMWPE contents. The same results were also found for the dynamic loss modulus. When methyl methacrylate was grafted onto UHMWPE by plasma and UV irradiation treatment, it was found that by adding the treated UHMWPE fiber in acrylic bone cement had a significant reinforcing effect on the mechanical properties of bone cement.     

Oxidation of gamma‐irradiated ultrahigh molecular weight polyethylene

Ultrahigh molecular weight polyethylene (UHMWPE), the current polymer of choice in orthopedic prosthetic devices, is typically sterilized by exposure to Co‐60 gamma irradiation prior to packaging for long‐term storage. However, the exposure to Co‐60 irradiation generates free radicals along the polymer chain that can participate in a series of reactions commencing with the oxidation of the free radicals to form reactive peroxy radicals. This study was undertaken to identify the role of hydroperoxide species in shelf‐aged and accelerated aged UHMWPE samples by using a nitric oxide derivatization technique. It is shown that the concentration of hydroperoxides did not change appreciably with shelf aging. However, during accelerated aging the hydroperoxide concentration increased to a plateau and then decreased, suggesting its role as an intermediate in the process. By contrast, the concentrations of carbonyl species continued to increase during shelf aging and accelerated aging. The effects of several packaging materials on the oxidation characteristics were also investigated. A vacuum foil package is shown to be effective in preventing oxidation to a significant extent during accelerated aging. However, accelerated aging after removal from the foil pack resulted in oxidative degradation. Extended vacuum to remove dissolved oxygen and a 5‐week room‐temperature healing process in the foil pack were shown to be ineffective in reducing oxidative degradation. It also was shown that increased moisture content in the aging environment did not affect the degradation process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2525–2542, 2000     

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.     

Effects of stearic acid on the tensile properties of ultrahigh molecular weight polyethylene fibers

For the purpose of the development of ultrahigh molecular weight polyethylene (UHMWPE) fibers with improved tensile properties, the stearic acid (SA) was added to the gel spinning of UHMWPE and acted as a lubricant film. SA addition was intended to be 0.2, 0.4, 0.6, 0.8, and 1.0 wt% of UHMWPE for forming the SA modified UHMWPE fibers. The tensile properties, thermal properties, crystallization properties, and orientation properties of the prepared UHMWPE fibers were systematically investigated. Results show that there is a more significant tensile property for UHMWPE fibers as SA addition is 0.6 wt%. Their tensile strength and tensile modulus reach 32.86 and 1580.89 cN/dtex, which are raised to an extent of 12.0% and 7.7%, respectively, compared with UHMWPE fibers alone. Moreover, the thermal properties, crystallization properties, and orientation properties of the prepared UHMWPE fibers are enhanced observably when the SA addition is 0.6 wt%.     

Influence of copolymer fraction composition in ultrahigh molecular weight polyethylene blends with ethylene/1‐hexene copolymers on material physical and tensile properties

The reactor blends (RBs) with bimodal molecular weight distribution on the base of ultrahigh molecular weight polyethylene (UHMWPE) and low molecular weight random ethylene/1‐hexene copolymers (CEH) were synthesized by two‐step processes including ethylene polymerization followed by ethylene/1‐hexene copolymerization over rac‐(CH₃)₂Si(Ind)₂ZrCl₂/methylaluminoxane catalyst. The four series of blends differed in a composition of copolymer fraction that was varied in a wide range (from 3.0 to 37.0 mol % of 1‐hexene). The differential scanning calorimetric study shows the double melting behavior of the net semicrystalline CEHs, which can be attributed to intramolecular heterogeneity in chain branch distribution. The introduction of CEHs leads to the modification of nascent RB crystalline and amorphous phases. Physical and tensile properties as well as melting indexes of the materials depend not only on the percentage of copolymer fraction that varied from 6.9 to 35.8 wt % but also on its composition. The increase of copolymer fraction with high content of 1‐hexene (≥11.0 mol %) in the blends leads to the change of the character of stress–strain curves; the materials behave as elastomers. Controlled regulation of copolymer fraction characteristics in the synthesis yields RBs combining the enough high strength, good plastic properties with enhanced melting indexes as compared with the net UHMWPE. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40151.     

Effect of zone drawing accompanied with crosslinking on the structure and properties of ultrahigh molecular weight polyethylene gel film

To enhance the thermal properties of ultrahigh molecular weight (UHMW) (viscosity-average molecular weight of 6 × 10⁶) polyethylene (PE) gel film, this was crosslinked by dicumyl peroxide (DCP) during a high-temperature zone drawing, which is effective to orient film. Through a series of experiments, it turned out that crosslinking actualized by an optimum amount of DCP and high-temperature zone drawing technique caused significant changes in the structure and properties of UHMW PE gel film. That is, crosslinking increased storage modulus of UHMW PE gel film at 25°C, resulting in improving thermal properties of the film. On the contrary, the crosslinking effect played a hindering role in raising the draw ratio of UHMW PE gel film. Maximum storage modulus of 165 GPa at 25°C could be obtained at the draw ratio of 324 of uncrosslinked homo-PE gel film. In the case of crosslinked PE gel film, the highest storage modulus at 25°C reached 65 GPa at maximum draw ratio of 150. Crosslinked film exhibited high modulus, even at 190°C, to some extent, while uncrosslinked homo-PE gel film was molten completely at 150°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1583–1590, 1997     

Improvement of the mechanical properties of an ultrahigh molecular weight polyethylene fiber/epoxy composite by corona‐discharge treatment

The interfacial shear strength of an ultrahigh molecular weight (UHMW) polyethylene (PE) fiber/epoxy‐resin system was greatly improved by the corona‐discharge treatment of the fiber. The UHMW PE‐fiber/epoxy‐resin composite was prepared with corona‐discharge‐treated UHMW PE fiber. The mechanical properties of the composite sheet were determined by tensile testing. The tensile strength of the composite was also very much improved. However, the tensile strength of the composite was about one‐half of the theoretical strength. This result was due to the molecular degradation of the PE‐fiber surface caused by surface modification. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1162–1168, 2001     

Morphology and mechanical properties of injection‐molded ultrahigh molecular weight polyethylene/polypropylene blends and comparison with compression molding

The mechanical properties and morphology of UHMWPE/PP(80/20) blend molded by injection and compression‐molding were investigated comparatively. The results showed that the injection‐molded part had obviously higher Young's modulus and yield strength, and much lower elongation at break and impact strength, than compression‐molded one. A skin‐core structure was formed during injection molding in which UHMWPE particles elongated highly in the skin and the orientation was much weakened in the core. In the compression‐molded part, the phase morphology was isotropic from the skin to the core section. The difference in consolidation degree between two molded parts that the compression molded part consolidated better than the injection one was also clearly shown. In addition, compositional analysis revealed that there was more PP in the skin than core for the injection‐molded part, whereas opposite case occurred to the compression‐molded one. All these factors together accounted for the different behavior in mechanical properties for two molded parts. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Viscous behavior of ultrahigh molecular weight polyethylene solution

The viscous behavior of the decalin solution of ultrahigh molecular weight polyethylene (UHMEPE) was studied. The influence of the concentration of polymer as well as the temperature was investigated. The flow curve can be described by the power-law model. The dependence of the viscosity on the temperature can be described by the Arrhenius–Frenkel–Eyring equation. The dependence of viscosity on the concentration can also be described by a power-law correlation. The addition of aluminum stearate increased the activation energy of flow of the solution. The viscosity of UHMWPE solution was decreased at lower concentration and increased at higher concentration of UHMWPE. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:289–293, 1997