A deformation induced order-disorder transition in isotactic polypropylene

In this study, isotactic polypropylene (i-PP) has been deformed by uniaxial compression to draw ratios up to 16×, and at draw temperatures from 30°C to 140°C. An order-disorder transition in the crystals is observed at draw temperatures well above the stability limit, 70°C, reported for the disordered phase; Furthermore, this disordered phase (called smectic) is found to induce ductility and improve the efficiency of draw. The deformation induced smectic phase has been characterized using WAXS, DSC, and on-line compression load versus draw ratio measurements. In consequence, a set of process conditions are offered to optimize draw. A mechanism for plastic deformation of i-PP is also suggested, using the smectic phase as a probe.     

Some rheological aspects of fracture in thermoforming

This paper focuses on several aspects of drawability, including the interactions between material parameters, operating temperatures, and frictional properties of the material. The deep draw process for a molten plastic sheet can be described by a simplified model using the characteristics of the normal stress as a lumped parameter. A rapid evaluation of the draw ratio and mold closing speed can be obtained by systematically drawing a series of three draw ratios. The fracture mode should then be examined to determine the appropriateness of molding temperatures. Therefore the maximum attainable draw ratio can be calculated from the elongational viscosity data. To support the analysis, data is provided on the formation of a cup with polystyrene sheets and using Maxwell extensional model as an example.     

The effects of roll drawing on the structure and properties of oriented poly(ethylene terephthalate)

The effect of processing conditions on the structure and properties of roll drawn poly(ethylene terephthalate) (PET) was examined. It was found that, when roll drawing amorphous PET at temperatures just above the glass transition, only very low draw ratios were obtained. This is probably because there were no crystallites to lock in the applied extension. Roll drawing at high temperatures, above 130°C, where there was significant thermal crystallization, produced film of high strength. At temperatures between 130°C and 190°C, the properties were almost independent of processing temperature. Mechanical tests performed on roll drawn samples, processed in this temperature range, showed that the initial modulus and the yield stress increased linearly with draw ratio. The yield strain decreased with draw ratio up to λ = 4.0, and then became almost constant. The processing temperature that produced samples with the greatest strength was 170°C. This was because the highest draw ratios were obtained at this temperature while maintaining constant width deformation. At low draw ratios, the crystallinity increased with the processing, whereas at higher draw ratios, it was independent of temperature. This constant level of crystalline fraction may have produced the constant failure strain that was observed at high draw ratios. The orientation functions were similarly unaffected by the processing temperature, although birefringence measurements did suggest that lower processing temperatures may have produced higher levels of orientation. The orientation of the trans conformers was independent of the temperature, but the overall content was increased at higher processing temperatures.     

The influence of drawing temperature on mechanical properties and organization of melt spun polyethylene solid-state drawn in the pseudo-affine regime

Mechanical properties of high density polyethylene (HDPE) solid-state drawn with fixed draw ratio at different temperatures in a fiber/tape spin line were investigated. All drawing experiments were performed in the pseudo-affine regime, i.e. no effective relaxation of the molecules occurs during drawing. For such conditions, the Young's modulus is uniquely determined by the applied draw ratio. The general appearance of the stress-strain behavior of drawn HDPE, and in particular its yield strength, however, is strongly influenced by the stretching temperature applied. For a fixed draw ratio, a significant drop in yield stress can be observed with decreasing drawing temperature. Characterization of structure and organization of the solid-state drawn HDPE was performed using various analytical techniques, such as wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). It is proposed that solid-state drawing at temperatures above the α-relaxation temperature results in relative large crystals so that corresponding tapes show a high yield point. Drawing at low temperatures below the α-relaxation temperature of PE, however, causes formation of small or imperfect crystals that can be destructed at low stress (low yield point), which is a preferable start situation for a second solid-state drawing step in a multiple drawing process.     

操作条件对正渗透分离性能的影响

正渗透是以渗透压差为驱动力的新型膜分离过程。采用水流分布较佳的膜池结构,研究了膜朝向、流动方式对正渗透水通量性能的影响,结果表明PRO模式（当膜的活性层朝向驱动液时）的水通量明显高于FO模式（当膜的活性层朝向原料液时）,但其衰减程度较大;在溶液浓度差相同的条件下,逆流操作更利于水通量的提高。针对FO模式和逆流条件,探讨了溶液温度对水通量和反向盐通量的影响,结果表明：膜两侧溶液温度同步升高时,正渗透过程的水通量和反向盐通量均增加,且水通量的增加幅度大于反向盐通量;单侧增加溶液的温度时,驱动液侧温度升高对水通量性能的提升效果优于原料液侧。综合考虑过程能耗和系统性能,认为单独升高驱动液温度更具实用价值。     

Crystalline-state extrusion of low density polyethylenes

Three low density polyethylenes, one long branched (A) and two linear (B and C), have been solid-state-extruded at several constant temperatures from ambient to 80°C and to draw ratios ? 8. The initial densities and melt indices of A, B, and C are 0.920, 0.920, and 0.935 g/cm³, and 1.9, 0.8, and 1.2, respectively. Melt-crystallized cylindrical billets were extruded through conical dies in an Instron Capillary Rheometer. The linear polymers were found to draw by extrusion more readily than the branched; all three strain-harden. Density, birefringence, tensile, and thermal properties have been evaluated as functions of extrusion temperature and draw ratio. Despite a measured loss via die swell, substantial orientation takes place during solid-state extrusion as evidenced by increases in transparency, birefringence, and tensile modulus (up to 4.5 times that of the original isotropic polymer). Depending on the polymer and the draw temperature, density does go through a minimum or shows a monotonic increase with draw by extrusion. A minimum in modulus is also observed at low draw and at all draw temperatures for all three polymers. The highest tensile moduli achieved are 0.73, 0.46, and 1.5 GPa for A, B, and C, respectively, at their highest draw ratio. The melting point for polymer B decreases with extrusion draw ratio, whereas it remains constant after a small initial drop, for the two others. For all three low density polyethylenes, birefringence increases rapidly with extrusion draw and then levels off at high draw. The birefringence limit is similar for A and B, i.e., 0.046 ± 0.004, but higher for C, i.e., 0.068 ± 0.009. This work extends beyond others in that it studies the effect of short as well as long branches in solid-state extrusion by comparing the linear and long branched LDPE polymers and LDPE with prior evaluations of HDPE.     

Necking of high-density polyethylene

Necking draw of high-density polyethylene is studied at draw rates of 2.5 × 10^?2 to 25 mm/min and at temperatures of ?40° to 80°C. Effect of temperature and draw rate on necking stress is interpreted in terms of viscoelastic flow of amorphous phase accompanying orientation of crystallites. It is proved that reducibility of draw rate and temperature holds and that the reduction factor obeys approximately the Williams-Landel-Ferry equation. Necking stress at an extremely low draw rate, critical necking stress, is discussed in terms of the phase equilibrium under stress between two states before and after microfracture of crystallites. The theory, with some approximations, leads to the equation by Iida in which the critical necking stress is expressed by fusion parameters. The thermodynamic behavior of isothermal necking is discussed and a phenomenologic criterion for necking is presented.     

Free radical activity in gamma‐irradiated polyethylene film,drawn tape and ultra‐high‐modulus fibres determined by grafting performance

Grafting rates of gaseous butadiene to a range of morphological forms of gamma‐irradiated polyethylene, including ultra‐high‐modulus fibres (UHMPE), have been measured in order to determine the availability of active free radicals over time at various temperatures. Blank experiments on unirradiated samples showed that monomer diffusion is not rate‐controlling with film and natural draw ratio tapes, but is likely to be a major factor in the control of grafting rates in UHMPE fibres. Grafting rates from monomer loss/time experiments with irradiated samples indicate that grafting is always in competition with free radical self‐annihilation, the extent being influenced by temperature, dose and morphology, including prior sample annealing. At lower temperatures, graft‐active radicals are produced over long periods of time, eg close to linear grafting rates were monitored over 20 hours for PE tape at 0 °C (50 kGy) and for gel‐spun UHMPE at 40 °C (100 kGy). At higher temperatures, grafting rates steadily decrease with time. Grafting rates are almost independent of irradiation dose in the early stages, however, the dose has an increasing positive influence as the reaction proceeds. At any given temperature and irradiation dose, the rates decrease in the series undrawn film; natural draw ratio tape; high draw ratio gel‐spun fibre; high draw ratio melt‐spun fibre. An analogy is drawn between these results and the optimum conditions required for improving the creep properties of PE tape and UHMPE fibres by acetylene‐sensitized irradiation crosslinking. Copyright © 2005 Society of Chemical Industry     

自伸长纤维拉伸工艺条件的研究

采用涤纶特制部分取向丝 ,对其采用不同的拉伸温度和拉伸比 ,分析不同拉伸条件对拉伸丝干热伸长率的影响 ,优选拉伸条件为 :拉伸温度高于玻璃化温度 2 2 .5℃、拉伸比 1.4 5,可以得到干热伸长率达 12 .6 3%的自伸长纤维。并且在工业化试验设备上制得干热伸长率为 14% ,沸水伸长率为 7%的自伸长纤维。     

Postspinning draw of polymeric fibers: Multiscale micromechanical model for a solid polymer under finite deformation and strain‐induced crystallization

A polymeric fiber postspinning draw model is developed. The fiber is stretched between the take‐up roll and the draw roll and then relaxed between the draw roll and the relax roll. The behavior of the polymeric material is described by a cooperative elastic–viscoplastic model for a wide range of temperatures and strain rates. The profiles of the fiber velocity, stress, strain rate, and temperature between the different rolls are simulated via the coupling of the cooperative model with the mass, momentum, and energy equations and the boundary conditions. Simulations are conducted with the finite‐element method. The computed results show an increase in the fiber stress between the take‐up roll and the draw roll due to the molecular orientation and the increase in the crystallization percentage. The sliding distance of the fiber on the draw roll is related to the draw ratio and fiber stiffness. A dramatic drop in the fiber strain rate on the draw roll leads to relaxation of the intermolecular resistance followed by a freeze of the fiber structure when the strain rate vanishes to zero on the draw roll and between the draw roll and the relax roll. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2259–2266, 2006     

Effects of orientation on the penetration,crazing, and dissolution of polystyrene by n-hexane

Extruded, drawn, and quenched samples of polystyrene were subsequently immersed in liquid n-hexane at temperatures between 35° and 55°C for various time intervals. Samples were removed from the immersion bath, quenched and fractured. Subsequent microscopic examination of the cross-sections revealed a distinct boundary between a crazed outer shell and an essentially unpenetrated central core. The time dependence of the depth of penetration of the advancing craze front was measured at various temperatures for several draw ratios. The initial rate of penetration increased monotonically with draw ratio (orientation) and the advance of the penetrant front was completely controlled by diffusion for drawn samples at 55°C. More complex kinetics, involving relaxations at the moving boundary, describe the penetration at lower temperatures; a slight systematic variation in the relative contribution of diffusion and relaxation was observed with increasing draw ratio. An activation energy of 23.7 k cal/g-mole characterized the temperature dependence of the initial penetration rate, independent of sample orientation. Gravimetric swelling experiments were confounded by sample dissolution in the case of the oriented samples. Intriguing swelling patterns, including discernable differences between the pronounced edge effects in the draw and tranverse directions, were apparent. Conversely, diffusion transverse to the orientation direction was accelerated by the orientation resulting in an increasing component of relaxation control in the penetration experiments and increased rates of dissolution in the oriented samples.     

Effects of zone drawing on the structure of metallocene polyethylene

The influence of zone drawing on bulk properties and structure of metallocene polyethylene (m‐PE) is reported. Two different m‐PE materials were subjected to tensile stresses above the yield point by zone drawing in the temperature range from 50 to 100°C. Drawn materials were characterized by using small‐ and wide‐angle X‐ray scattering (SAXS, WAXS), molecular retraction, and small‐angle light scattering (SALS). Structural changes were studied as a function of drawing temperature, engineering stress, and draw ratio. WAXS showed strong crystalline orientation in drawn samples, and only the orthorhombic crystal modification was observed. SAXS showed lamellar orientation in drawn samples. At low drawing temperatures of 50 or 60°C, draw ratio increased as a step function of stress. There is a stress barrier, which must be exceeded before high‐draw ratios can be achieved at these temperatures. At drawing temperatures of 70°C or above, the barrier stress is low enough that draw ratio increases nearly linearly as a function of stress. Below the stress barrier, spherulitic structure is observed by small‐angle light scattering (SALS). Elongation occurs via deformation of the interspherulitic amorphous phase. Molecular retraction was low for these samples, indicating mostly plastic deformation of the amorphous material. Above the stress barrier, SALS showed that spherulites are destroyed. Elongation occurs via deformation of the intraspherulitic amorphous phase. Molecular retraction for these samples was high, indicating elastic deformation of the amorphous material. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3492–3504, 2001     

Techniques for the evaluation of the drawing behavior of experimental fibers

Equipment and techniques are described which have been developed for determining the drawing behavior of fibers from novel polymers which are available only in very small quantities. Emphasis is placed upon information obtained from the stress–strain diagrams of the fibers during drawing. A parameter, the reinforcement factor, is described which reflects the degree of orientation induced by drawing short lengths of fiber in a discontinuous manner. This parameter exhibits maxima at various combinations of temperature and draw rate, and the relationship between the corresponding temperatures and draw rates is shown to be logarithmic. The parameter is used to predict temperatures and draw rates which will give fibers having a high degree of orientation and good tensile properties when drawn continuously. The application of these techniques to give acceptable fibers from poly(m-phenylene adamantane-1,3-dicarboxamide) is described. This material is of interest as one possessing a high degree of thermal stability, and the best fibers produced to date have a tenacity of 4.0 g per denier, an initial modulus of 60 g per denier, and a break extension of 16.0%.     

55 dtex/24 f高收缩涤纶直纺FDY的开发

采用常规PTA及EPTA为原料进行熔体直接纺涤纶,选择热辊式FDY工艺路线,通过降低第一热 辊温度(H1)、第二热辊温度(H2)和拉伸倍数(DR),可生产沸水收缩率在13％-50％的高收缩纤维。当 H_186℃,H290℃,DR2.0时,生产的FDY沸水收缩率高达50％以上,且其它物性指标可满足后加工要求。     

Orientation and crystallization in poly(ethylene terephthalate) during drawing at high temperatures and strain rates

We review some recent research developments on structure development during drawing of poly(ethylene terephthalate) film, and we report a study of constant-load drawing of amorphous PET film at temperatures of 120°C and 132°C, including the effects of redrawing high-temperature drawn film at lower temperature. To permit constant-load drawing at high temperature without inducing crystallization in the undrawn specimen, a drawing instrument was built that permits very rapid heating of the sample, and its operation is described. The initial stage of drawing at high temperatures is characterized by polymer flow where, owing to high rates of molecular relaxation, neither molecular orientation nor crystallization occurs. Strain-rate increases sharply in the course of the deformation, reducing the time available for relaxation, and the chains start to orient at a draw ratio that depends on temperature. Orientation rapidly reaches a saturation level, which is lower at the higher draw temperature. Crystallization onset seems to lag only slightly behind orientation onset because the critical orientation for inducing crystallization is very low at these temperatures. It appears that there is time for crystallization to proceed to pseudo-equilibrium values corresponding to a particular orientation level, which differs from previous results obtained from constant-force drawing at lower temperatures, and possible reasons for this are discussed. In two-stage drawing, where film drawn at 132°C was redrawn along the same axis at 100°C, high draw ratios were obtained despite the high strain rates, and the levels of noncrystalline orientation and crystallinity were similar to the levels expected from single stage drawing at 100°C.     

Two-stage drawing of poly(ethylene 2,6-naphthalate)

Initially amorphous and semicrystalline films of poly(ethylene 2,6-naphthalate) with different molecular weights were drawn by two-stage drawing, that is, coextrusion at low temperatures (25–160°C) followed by tensile drawing at high temperatures (200–245°C). Both films could be drawn up to a draw ratio of 8–10 by this method under controlled conditions. The tensile modulus and strength of drawn samples were greatly affected by the draw temperature for the first stage, predrawn morphology, and molecular weight. The remarkable effects of these variables on the tensile properties are closely related to the difference in the resultant amorphous chain orientation of the samples, reflecting the disentanglements and chain slippage during drawing, and the dissipation of chain orientation after processing.     

Microstructural effects on hot drawing syndiotactic styrene P‐methyl styrene copolymer

Crystalline syndiotactic styrene/p‐methyl styrene copolymer (SPMS) has been oriented by tensile drawing at various temperatures between the glass transition and crystalline melting point. The microstructural changes resulting from drawing have been studied using differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). WIth increasing draw temperature, both melting temperature and crystalline dimensions of the oriented samples increase. The heat of fusion increases with increasing draw temperature up to ～200°C. It also increases with draw ratio and draw rate, while the crystalline width increases only with draw ratio. THe amorphous fraction shows a clear glass transition, the temperature of which (T_g) increases with draw ratio. However, T_g decreases somewhat with increasing draw temperature. This is interpreted in terms of the stretching of the randomly coiled amorphous phase molecules.     

Studies on α to β phase transformations in mechanically deformed PVDF films

PVDF cast films were drawn at different temperatures to different draw ratios at constant draw rate to understand the mechanism of α to β phase transformation during mechanical deformation. WAXD and FTIR studies were carried out to determine the formation and content of β phase in the drawn films. Lower stretch temperatures gave higher fractions of β phase. The cast PVDF films were also drawn at suitable temperatures below the PVDF ambient melting point to the draw ratio of 6.4. The highest fraction of β phase obtained in these ultra drawn films was 0.98. SALS studies carried out for films at different stretch ratios show the change in spherulitic structure with the stretching parameters and give information for the understanding of phase transformation during stretching of PVDF films. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of processing history on the morphology and properties of polypropylene/thermotropic liquid crystalline polymer blends

Preparation, morphology, and mechanical properties were studied of blends of a thermotropic liquid crystalline polymer (TLCP) with two different grades of polypropylene, one with and one without overlap in processing temperatures, using two different blending methods. The highly viscous grade (PP-1) was of sufficient thermal stability to be blended with the TLCP (Vectra A950) in a single-screw extruder with an Egan mixing section on the screw. The low viscous grade (PP-2) could not be processed at the same temperature as the TLCP because of degradation. Its blends were, therefore, prepared by a special coextrusion technique, i.e. feeding the two components from two separate extruders to a Ross static mixer. In both methods drawing of the extrudate is necessary to obtain satisfactory mechanical properties. The PP-1/TLCP blends had to be extruded twice in order to obtain proper mixing. The morphology of these blends ranges from a pronounced skin-core morphology at low extrudate draw ratio (DR = 3) to a high-aspect ratio fiber/matrix morphology at high draw ratio (DR = 15). The morphology of the PP-2/TLCP blends was always a high-aspect ratio fiber/matrix morphology even at low draw ratios. The TLCP fibers were generated in this coextrusion process under conditions where the viscosity of the dispersed phase was higher than the viscosity of the matrix. Breakup experiments demonstrate that fibers of a thickness of approximately 1 μm disintegrate into droplets within a few seconds at temperatures above the melting point of the TLCP. This is probably the cause of the skin-core morphology obtained with single-screw extrusion. Tensile modulus and strength of all blends increase with extrudate draw ratio. The deformation of the TLCP phase in the drawn blends is less than affine, probably because of slip between the phases. The moduli of the PP-1/TLCP blends as a function of the draw ratio can be described well by a modified Halpin-Tsai equation taking into account both changes in aspect ratio and molecular orientation of the TLCP fibers. The level of reinforcement in the PP-2/TLCP blends is lower than expected, probably because of the low temperature of drawing. This demonstrates a limitation of the coextrusion process: blending at temperatures that are too low reduces mechanical properties.     

Structure formation in stretched sheets of poly(butylene terephthalate)

Crystalline and amorphous sheets of poly(butylene terephthalate) (PBT) were drawn in the temperature range of 20–150°C. The molecular orientation and the relative amount of α- and β-form crystals in the stretched sheets were studied by wide-angle X-ray diffraction (WAXD) and density measurements. When crystalline PBT sheets are drawn at lower temperatures, α-form crystals are partially transformed into β-form crystals. Both α- and β-form crystals are formed by drawing amorphous PBT sheets. The relative amount of α- and β-form crystals is much more sensitive to drawing temperature than to draw ratio. The α-form crystallinity is higher at higher drawing temperature and increases slightly with increasing draw ratio. The second moments of orientation functions of α- and β-form crystals increase with increasing draw ratio, and the increase of the orientation function is suppressed at higher draw ratio. The orientation function of α-form crystals is higher than that of β-form crystals in a same sample.