Ultra-high modulus polyethylene. 1 Effect of drawing temperature

Drawing behaviour and the properties of ultra-drawn high density polyethylene have been investigated as a function of the drawing temperature. An optimum temperature has been found for each type of polyethylene, at which the best drawing behaviour is found. It appears that the temperature range for effective drawing (leading to a high draw ratio and high Young's modulus) depends on the molecular weight and its distribution. The temperature range of the effective drawing is shifted towards higher temperatures for polyethylene exhibiting broader molecular weight distribution and higher weightaverage molecular weight. Ultra-high modulus and transport samples have been obtained by drawing high density polyethylene with broad molecular weight distribution ($\text{M}\text{?}{}_{\mathrm{w}}\text{M}\text{?}{}_{\mathrm{n}}\text{～ 20}$ and $\text{M}\text{?}{}_{\mathrm{w}}\text{～ 200 000}$) at higher drawing temperatures. It has been found that in the range of drawing temperatures 80–105°C the modulus of this polyethylene is higher for samples drawn at higher temperatures. Transparent samples with draw ratios of 35–40 and with Young's moduli of 600–650 kbar (at room temperature) have been obtained by drawing the polyethylene at 100°–105°C. We conclude that the high molecular fraction in the polyethylene, forming tie molecules in the drawn material, is responsible for the high modulus, while the low molecular weight fraction facilitates alignment of the long chains and retards the internal voiding (whitening) to a very high draw ratio during drawing at the higher temperatures.     

Ultra-high strength polyethylene filaments by solution spinning/drawing. 3. Influence of drawing temperature

The influence of the temperature on the drawing behaviour of gel-fibres, which were obtained by spinning of a 2% w/w solution of high molecular weight polyethylene (M_w = 1.1 × 10⁶) in decalin, was studied in the range from 70 to 143°C. It was found that the drawing temperature, like the presence of solvent in the gel-fibres, affected the maximum attainable draw ratio, but did not influence the effectiveness of the hot drawing below the melting point of the polymer.     

Ultra-high strength polyethylene by hot drawing of surface growth fibers

Summary Hot drawing at 150°C has been applied to high molecular weight polyethylene fibers produced by flow induced crystallization in a Couette apparatus, referred to as the surface growth technique. A distinct improvement of the tensile properties of the fibers was noticed upon drawing. A tensile strength at break of 4.7 GPa was reached. Drawability is discussed in relation to fiber morphology. The shish-kebab like structure of the surface growth fiber was transformed into a morphology consisting of smooth fibrils upon drawing.     

Ultra-high strength and ultra-high modulus fibers from polyethylene

Summary Using the method of hot drawing developed earlier, an attempt has been made to obtain ultra-high modulus and ultrahigh strength PE filaments from original filaments produced by the surface growth technique. The average tensile strength of the drawn fibers reaches 5.5 GPa and the value of modulus measured in a dead loading creep experiment is estimated to be 44 GPa. 13 % of the drawn specimens had extremely high tensile strength close to theoretical estimates. The great scatter of the tensile strength data is attributed to the kink-band formation in the specimens due to their bending during preparation or during drawing.     

Very high modulus high‐density polyethylene and high‐modulus polypropylene obtained by solid‐state roll drawing

This article describes results obtained with a process developed for rolling and drawing simultaneously polymer profiles in the solid state. Solid‐state roll drawing has the advantage of being continuous, which allows relatively high production rates and the generation of high deformation ratios with some degree of biaxial orientation. The roll‐drawing process allows the extent of biaxial orientation to be controlled by the adjustment of the tension and compression loads applied to the polymers, in particular semicrystalline thermoplastics. Some experimental results obtained with a four‐station roll‐drawing apparatus are presented, particularly on high‐density polyethylene (HDPE) and polypropylene. The effect of process parameters, such as the gap between the rolls and tension, are discussed. Aspects discussed also include relaxation; structure development in terms of orientation and crystallinity as a function of draw ratio (λ); λ as a function of process parameters; and finally, mechanical and thermal properties as a function of λ. Moduli as high as 25 GPa in the longitudinal direction and about 4 GPa in the transverse direction were obtained with successively rolled, initially thick, HDPE profiles. © 2006 Government of Canada. Exclusive worldwide publication right in the article have been transferred to Wiley Periodicals, Inc. J Appl Polym Sci 102: 3391–3399, 2006     

Melt transcrystallization of polyethylene on high modulus polyethylene fibers

A polytheylene composite was prepared and tested. It was consisted of a high-density polyethylene (HDPE) matrix and uniaxial gel-spun high-modulus PE fiber. Aided by the similarity between matrix and fiber, transcrystallization of HDPE melt on the PE fiber surface was generated. Nucleating agents were not employed. The transcrystalline growth of HDPE on the PE fiber surface was found to consist of an inner and an outer zone. The inner zone, 2–3 μm thick, is composed of HDPE crystals nucleated on the PE fiber surface. Photomicrographs showed a well-defined region of row-nucleated HDPE on the surface of PE fiber. This means the fibrils of HDPE were found to grow out from the PE fiber axis and HDPE crystallites are oriented in planes perpendicular to the PE fiber axis. The fiber in the composite induced the transcrystalline growth of HDPE on the PE fiber surface at higher temperature than on cooling the melt. For 36 wt% fiber, the increase was 2.5°C, also resulting in ～ 10% more crystals. Crystallization of a composite with 50 wt% fiber at 124°C involved two steps: The first a fast transcrystallization of HDPE on the PE fiber surface followed by the bulk crystallization of the HDPE.     

Preparation of high modulus polyethylene sheet by the roller-drawing method

The high density polyethylene (HDPE) sheets were drawn through a pair of heated rollers. The process, referred to as roller drawing, was found to be useful for producing high modulus and high strength HDPE sheets. The higher draw ratio could be obtained for the HDPE sheet with lower molecular weight and narrower molecular weight distribution. The Young's modulus and the breaking strength reached 43 GPa and 0.67 GPa, respectively, at the highest draw ratio. The measurements of wide-angle X-ray diffraction (WAXD) pole figures revealed that the crystallographic a-, b-, and c-axes were oriented to the normal direction (ND), the traverse direction (TD), and the drawing direction (DD), respectively. The small-angle X-ray scattering (SAXS) of the roller-drawn HDPE sheets with draw ratio higher than 7 exhibited two intensity maxima on the meridian, suggesting the presence of the two-phase structure in which crystalline and amorphous regions are stacked alternately along DD. The relationship between mechanical properties and microstructure was discussed on the basis of the concept of the formation of amorphous tie molecules in the interfibrillar and intercrystallite regions.     

Adhesion between high strength and high modulus polyethylene fibers by use of polyethylene gel as an adhesive

The use of polyethylene gels to adhere high strength and high modulus polyethylene fibers together has been investigated. It was found that the joint strength obtained using high density polyethylene (HDPE) gel in decalin or tetralin depended on the heating temperature and time of the gel. When heated at 100°C for 30 min, the gel showed such a high joint strength that polyethylene fibers were torn, and this should be enough for practical uses. The results of the present study demonstrate that utilization of the polyethylene gel as an adhesive is an extremely effective method when polyethylene fibers are used in composite materials.     

Spinning of high molecular weight polyethylene solution and subsequent drawing in a temperature gradient

Summary The spinning and drawing of high molecular weight polyethylene solution forming ultra-high strength fibres has been studied and occurrances on the molecular level are discussed. The fibre properties were found to be strongly influenced by the drawing stress and the temperature conditions.     

High modulus/high strength polyethylene obtained by high-pressure injection molding

Increasing the nominal injection pressure up to 500 MP a improves the mechanical properties (modulus and strength) of injection-molded high-molecular-weight high-density polyethylene substantially. By a proper combination of barrel and mold temperature, the modulus of the molded parts (test bars) may be increased at least eight times compared to parts molded at 100 MPa. This improvement is partly due to the formation of high-strength crystalline modifications of the polyethylene induced by flow and pressure. The extent to which these structure modifications occur in the samples molded at various conditions has been determined by thermal analysis (DTA). When increasing the thickness of the samples, a sharp reduction of the modulus and strength was observed, even though the concentration of the crystalline high-strength phase was higher. An explanation of this effect in terms of the relaxation of the tie-molecules connecting the crystallites is suggested.     

The structure of highly oriented high modulus polyethylene

A comparative study of the structure of uniaxially-oriented bulk samples of linear PE of various molecular weights over a wide range of draw ratios has been carried out by means of small and wide angle X-ray analysis, NMR-spectroscopy and DSC. The structure of oriented high molecular weight PE samples, prepared by drawing single crystal mats to draw ratios up to 200, have also been studied. A large number of unfolded tie-molecules have been found to exist in the trans-conformation in PE amorphous regions. These tie-molecules, which connect the adjacent crystallites along the axis of orientation, and are not large crystals, account for the high mechanical properties of these samples. It has been shown that in the course of the orientation process, chain unfolding in the amorphous regions is accompanied by the formation of continuous linear systems consisting of several coherent crystallites and tie-molecules. These are coherently aligned along the macromolecular axis.     

Creep and recovery of ultra high modulus polyethylene

The creep and recovery behaviour of ultra high modulus polyethylene has been studied over the temperature range 20–70°C. Four types of material were examined; low molecular weight and intermediate molecular weight homopolymer, an ethylene hexene-1 copolymer, and a sample prepared by γ-irradiation of isotropic low molecular weight polymer prior to drawing. With the exception of the low molecular weight homopolymer, the materials showed an apparent critical stress below which there was no detectable permanent creep. It is proposed that the behaviour of all the materials can be described to a good approximation by a simple model, where two activated processes are coupled in parallel. Tentative structural explanations of the results are also given.     

Die drawing technology of high molecular weight polyethylene

A comprehensive investigation of the die drawing technology of a high molecular weight polythylene (HMWPE) rod has been carried out. The effect of draw temperature, draw speed, nominal draw ratio, and exit diameter of the dies has been studied. The oriented HMWPE products were characterized mainly by the determination of the three-point bend modulus and the tensile strength. The tensile strength and the modulus of the drawn HMWPE rod could reach 700 MPa and 18 GPa, respectively. In addition, it was found that forced cooling at the die exit was essential when drawing billets with large section areas. © 1993 John Wiley & Sons, Inc.     

Surface morphology and nanostructure of high modulus polyethylene fiber

Summary Surface morphology and nanostructure of the ultrahigh molecular weight polyethylene fiber Spectra 900 were examined by atomic force microscopy under water, with height and lateral force registration. Non-uniform surface morphology, where the bundles of microfibres are covered by interwoven randomly oriented fibrils (hairs) and twisted ribbons, was registered in the large scale images. The nanofibril elements detected in the bundles and in the hairs are 5–8 nm in width. Periodical stripes found by lateral force measurement on individual fibrils (long period) were assigned to sequences of crystalline and less-ordered regions. The repeat distance of the long period varies from place to place being in the range of 10 – 20 nm. The complexity of the polyethylene fiber structure is supported by visualizing large crystallites, which were found in the ribbons splitfrom the fiber surfaces.Herrn Prof. Dr. Ing. E. H. Hansjörg Sinn zu seinem 65. Geburtstag herzlichst gewidmet     

Cavitation during tensile drawing of annealed high density polyethylene

Annealing of semicrystalline polymers usually leads to refinement and thickening of crystals. It appears that also cavitation is affected during tensile drawing. In the uniaxially drawn high density polyethylene massive cavitation was detected by X-ray scattering in the samples previously annealed at 125 °C. The number of voids depends on the annealing time, while their size and orientation depends on the local strain. Cavitation resulted in 30% increase in volume for the annealed samples, strained to 4-5. Cavitation and volume increase were not observed for small and intermediate strains if polyethylene samples were not annealed. The decrease in the drawing rate results in the reduction of cavitation and void stability - at the low strain rate voids were detected during tensile drawing, though they disappeared after unloading the sample.     

Study of the production of ultra-high modulus polyoxymethylene by tensile drawing at high temperatures

The tensile drawing behaviour of polyoxymethylene has been studied, with particular reference to the production of ultra-high modulus oriented material. The influence of molecular weight and draw temperature, and the comparative effectiveness of single-stage and two-stage drawing processes have also been examined. In addition to the molecular weight range it appears that both draw temperature and drawing rate must be specified within very narrow limits, if ultra-high modulus material is to be produced. It is tentatively suggested that this is because effective high draw requires a suitable coincidence of rate processes involving both the crystalline and non-crystalline regions of the polymer.     

Surface modification of ultra high modulus polyethylene fibers by an atmospheric pressure plasma jet

To improve their adhesion properties, ultra high modulus polyethylene (UHMPE) fibers were treated by an atmospheric pressure helium plasma jet (APPJ), which was operated at radio frequency (13.56 MHz). The surface properties of the fibers were investigated by X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and contact angle measurement. The surface dyeability improvement after plasma treatments was investigated using laser scanning confocal microscopy (LSCM). The adhesion strengths of the fibers with epoxy were evaluated by microbond tests. In addition, the influence of operational parameters of the plasma treatment including power input and treatment temperature was studied. XPS analysis showed a significant increase in the surface oxygen content. LSCM results showed that the plasma treatments greatly increased fluorescence dye concentrations on the surface and higher diffusion rate to the fiber center. The tensile strength of UHMPE fiber either remained unchanged or decreased by 10–13.6% after plasma treatment. The contact angle exhibited a characteristic increase in wettability, due to the polar groups introduced by plasma treatment. The microbond test showed that the interfacial shear strengths (IFSS) increase significantly (57–139%) after plasma treatment for all groups and the optimum activation is obtained at 100°C and 5 W power input. SEM analysis showed roughened surfaces after the plasma treatments. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

高强高模聚乙烯纤维产业化的现状及思考

介绍了高强高模聚乙烯纤维产业化的国内外发展现状、产品性能和应用技术,特别研究了有关国外公司产业化的发展战略,指出了国内高强高模聚乙烯纤维产业与国外的差距。对进一步做大做强我国高强高模聚乙烯纤维产业提出了看法:关键在于真正重视技术进步和研发创新,敢于交流和借鉴国内外先进技术,以企业为主体实行产、学、研三结合,推进技术进步和创新,整合行业资源,组建有实力的大企业进行应用技术研究和市场拓展开发。