Plastic deformation in linear polyethylene

A detailed study has been made of drawn linear polyethylene using vibrational spectroscopy. The proportion of a monoclinic crystal modification and its dependence on draw ratio is determined by draw temperature, rate and medium (air or water). This is explained by the role of mechanical energy increasing local temperatures within the samples and the corresponding loss of heat to the environment. Monoclinic material acts as a sensitive molecular level thermometer and indicates that temperatures in the vicinity of the melting point can be generated by drawing.     

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。     

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     

Processing of thermotropic liquid crystalline polymers and their blends—analysis of melt-spinning TLCP fibers

Isothermal melt-spinning of two thermotropic liquid crystalline polymers (TLCPs), a wholly aromatic copolyester KU-9211 (also named K161 from Bayer AG) and an aliphatic containing TLCP, PET/PHB60 (Tennessee Eastman), was studied to analyze the effect of processing conditions on fiber properties. Fibers were melt-spun from a capillary rheometer equipped with an isothermal chamber in which cross-flowed air was used as the cooling medium. The processing variables studied included the extrusion temperature, the extrusion rate, the cooling conditions, and the draw ratio. As-spun fibers were characterized by measuring storage moduli and molecular orientation parameters as a function of draw ratio under various processing conditions. Among the processing variables studied, the draw ratio was the primary factor in determining both the fiber modulus and the molecular orientation. The extrusion rate did not appear to affect the fiber properties within the range studied. The properties of K161 fibers were also dependent on the extrusion and cooling temperatures, while PET/PHB60 fibers were rather insensitive to the processing temperatures within data scatter and temperatures studied. A composite model based on a rigid-rod rotation mechanism and the deformation of nematic domains in an elongational flow field was used to model the experimental results and was compared with other theories available. Conformance of data to the composite model was obtained by use of a single temperature dependent parameter n, suggesting that the rigid-rod rotation mechanism could be used to predict the orientation development of TLCPs. The Halpin-Tsai equations and the orthotropic equation for angular dependence were used to describe the elastic properties of the TLCP fibers.     

Modeling the large‐strain constitutive behavior of polycarbonate under isothermal and anisothermal conditions

The tensile behavior of polycarbonate was studied at large strains below the glass‐transition temperature. Experiments were carried out at a series of constant temperatures and also under conditions of falling temperatures. The specimens necked with a natural draw ratio of approximately 2, and the study was mainly focused on the necked material. Isothermal experiments revealed an elastic mechanism that initiated beyond the natural draw ratio. A model consisting of an Eyring process and two Gaussian elastic mechanisms was found to be applicable to both the isothermal and anisothermal stress‐relaxation and stress–strain results. The same model also produced reasonable estimates of the stresses generated during the necking process. In addition, a simple relationship between the isothermal and anisothermal stress relaxation was demonstrated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2105–2116, 2005     

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.     

Phenomenological model for the unified studies of different physical properties of polymers

A phenomenological model has been proposed to understand the development of strong anisotropy in physical properties of semicrystalline polymers on uniaxial drawing. The proposed model introduces temperature as an explicit parameter in the orientation function. The model contains two free parameters for each polymer which can be evaluated by fitting the experimental data on birefringence versus draw ratio at a particular temperature. The calculated values of elastic moduli, thermal conductivity and expansibility along and perpendicular to the drawing direction over the entire range of draw ratio and a wide range of temperature are in good agreement with experimental values.     

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     

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.     

Biaxial orientation of poly(vinyl chloride) compounds Part 2 –Structure–property relationships and their time dependency

Abstract

X-ray diffraction and thermomechanical analysis have been used, respectively, to examine structural order and shrinkage behaviour for oriented samples of rigid and flexible poly(vinyl chloride) (PVC). Results were compared with previously measured tensile properties and structure–property relationships explored. X-ray diffraction showed that drawing produces planar crystallite orientation in PVC sheets. If drawing and subsequent annealing conditions are held constant, but draw ratio is varied, there is good correlation between structural order measured by X-ray diffraction and tensile strength. Increased annealing time and temperature improve crystallite order and dimensional stability, while tensile strength is unchanged. The greatest enhancement in tensile strength is achieved by stretching PVC towards its maximum draw ratio at 90°C, but optimum thermal stability of the oriented structure is achieved when higher annealing temperatures are used. Room temperature recovery is observed for flexible PVC when the material has a glass transition temperature below ambient. This can be delayed by increased annealing time and temperature, and by increased draw ratio.     

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。     

Melting behavior of polypropylene fibers studied by differential scanning calorimetry

Polypropylene fibers produced in a compact-spinning process were studied by differential scanning calorimetry (DSC). With unrestrained fibers, the onset of melting increases with decreasing draw ratio, increasing M_w/M_n, decreasing extrusion temperature, increasing annealing ratio, and increasing draw-down ratio. These trends are discussed in terms of restraints and reorganization. The onset of melting is shifted to lower temperatures as the heating rate increases for all combinations of material and processing parameters, indicating suppressed reorganization. At low draw ratios, the height and width of the endotherm are affected by the spinline stress, and a secondary peak or shoulder is observed on the high temperature side of the main peak. The magnitude of the secondary peak increases with decreasing M_w/M_n, increasing draw ratio, decreasing draw-down ratio, and decreasing heating rate, but its position mainly depends on the heating rate. This indicates that the secondary peak may be due to the melting of structures that have been reorganized during the heating scan. As the draw ratio increases, the melting regime broadens, especially towards lower temperatures, and several maxima emerge on the DSC curve. Reorganization and shrinkage during heating may explain these observations. © 1995 John Wiley & Sons, Inc.     

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.     

A multiprocess eyring model for large strain plastic deformation

A multiprocess Eyring model is developed with a particular aim of predicting the localized instability occurring in “necking” polymers when cold‐drawn. Differences from using single and multiple Eyring processes are examined using a published data‐set for polypropylene test pieces; it is shown that a four Eyring process model can simultaneously fit both necking stretch ratio and draw force data for uniaxial stretching, whereas with a single process only one measurement could be fitted accurately. The multi process Eyring model is shown to give significantly more accurate predictions than a necking hyperelastic model. The multiprocess model is assessed against the same material undergoing a complex constant‐width elongation. It is shown that agreement is quantitatively good for both drawing force and surface deformation, with some minor differences in transverse force and surface stretch. A pronounced intermittent stretching pattern that is seen on the experimental test piece is replicated by the multiprocess Eyring simulation, but is absent using the hyperelastic model. A method is described to deform a photograph of the original specimen according to a finite element solution. The method is shown to give a clear indication of the accuracy of the model in predicting final form. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

多组份阳离子易染共聚纤维的开发

本文在分析多组份阳离子易染共聚酯切片性能的基础上,重点研究探讨了干燥温度、纺丝温度、拉伸温度和拉伸倍率以及网络压缩空气压力等因素,对多组份阳离子易染共聚酯纤维生产的影响.这一具有多种优异功能的纤维的开发成功,为国内利用改性聚酯生产涤纶仿真丝织物开辟了又一新的技术途径.     

PBT/PET conjugated fibers: Melt spinning,fiber properties,and thermal bonding

This work examines the PBT/PET sheath/core conjugated fiber, with reference to melt spinning, fiber properties and thermal bonding. Regarding the rheological behaviors in the conjugated spinning, PET and PBT show the smallest difference between their melt‐viscosity at temperatures of 290°C and 260°C respectively, which has been thought to represent optimal spinning conditions. The effect of processing parameters on the crystallinity of core material‐PET was observed and listed. In order of importance, these factors are the draw ratio, the heat‐set temperature, and the drawing temperature. The crystallinity of sheath material‐PBT, however, can be considered to be constant, independent of any processing parameters. The bulk orientation, rather than the crystallinity of PET core, dominates the tenacity of PBT/PET sheath/core fiber. Moreover, heat‐set treatment after drawing is recommended to yield a highly oriented conjugated fiber. With respect to thermal bonding, PBT/PET conjugated fibers processed via high draw ratio but low‐temperature heat setting can form optimal thermal bonds at a constant bonding temperature of 10°C above the T_m of PBT.     

333 dtex/576 f无扭矩高孔数合股丝生产工艺的探讨

在HY-7HLV S+Z型高速加弹机上,采用熔体直纺的半消光266 dtex/288 f涤纶预取向丝(POY)为原料,一步法生产333 dtex/576 f无扭矩高孔数合股丝.结果 表明:采用非"双胞胎"独立式上下两排假捻器,通过设置相关工艺参数,在较高的600～700 m/min生产速度下,拉伸比设置为1.67,变形...     

The effect of hot drawing on the properties of short fiber reinforced extruded sheets

The hot drawing of extruded composite sheets can be used to control the orientation of both matrix and reinforcing fibers. A study was made of the effect of draw ratio on the properties of an extruded polystyrene sheet containing 0 to 1 percent of short glass fibers. An increase in draw ratio resulted in an increase in fiber orientation. A model of a rigid fiber rotating in an elongational flow field was used to describe the effect of draw ratio on the final orientation distribution. An increase in draw ratio also caused an increase in the amount of fiber breakage. A shear-lag analysis was used to estimate the extent of damage as a function of draw ratio. It was also found that the mechanical properties were dependent upon both the draw ratio and fiber concentration.     

Effect of uniaxial stretching on thermal,oxygen barrier,and mechanical properties of polyamide 6 and poly(m‐xylene adipamide) nanocomposite films

In this study, the effect of uniaxial stretching on the thermal, oxygen barrier and mechanical properties of aliphatic polyamide 6 (PA6) and aromatic Poly(m‐xylene adipamide) (MXD6) nylon films as well as their in‐situ polymerized nanocomposites with 4 wt% clay were studied. Cast films were prepared by extrusion process and rapidly cooled using an air knife. The precursor films were uniaxially stretched at 110°C with draw ratios varying from 1.5 to 5. DSC results showed that the cold crystallization temperature (T_cc) of the uniaxially stretched MXD6 and MXD6/clay films drastically shifted to the lower temperatures when draw ratio increased. The aromatic nylon films had lower oxygen permeability than those of the aliphatic films, due to more rigidity and chain packing. However, the oxygen permeability of the stretched films increased with draw ratio (DR) up to a critical value for each sample, while further stretching resulted in a reduction in the oxygen permeation. This phenomenon was related to the changes in free volume upon uniaxial stretching. The ability of different geometrical models to describe the experimental relative permeability data was investigated. The Bharadwaj model that took into account clay orientation was the most successful one to predict the oxygen barrier characteristics of the stretched nanocomposites at high draw ratios. The Young's modulus and tensile strength of the aliphatic and aromatic nylons increased with uniaxial deformation, while the flexibility and elongation at break of the former decreased with increasing DR. A larger increase in the Young's modulus of the uniaxially stretched nanocomposite films compared with the neat samples was observed and could be related to the improvement in the clay orientation as well as a better alignment of the crystalline phase due to incorporating the clay platelets in the polymer matrix. In contrast, the flexibility of the stretched MXD6 improved remarkably (ca., 25 times) compared with the precursor film (DR = 1) when the draw ratio increased to 1.5. This could be related to the effect of hot stretching on the enhancement of polymer chains relaxation and mobility at low draw ratios. POLYM. ENG. SCI., 55:1113–1127, 2015. © 2014 Society of Plastics Engineers     

Ultrasonic measurements of the mechanical relaxations and complex stiffnesses in oriented linear polyethylene

The five complex stiffnesses of highly oriented linear polyethylene produced by hydrostatic extrusion and die drawing have been determined by ultrasonic measurements. The results are compared with the elastic stiffnesses for crystalline polyethylene calculated theoretically. The development of anisotropic mechanical behaviour with draw ratio is discussed in terms of present structural understanding of highly oriented polyethylene. Although the very high stiffnesses obtained at the highest draw ratios are attributed to increasing crystal continuity, it is noted that the development of anisotropy in terms of low-temperature ultrasonic behaviour can be predicted to a good approximation by the reorienting unit aggregate model. This surprising result suggests that the overall orientation may still be the key parameter at low temperatures and high frequencies where there is no molecular mobility in the structure.