Ultradrawing of thermoplastics by solid state coextrusion illustrated with high density polyethylene

Ultra-thin films of high density polyethylene of high orientation have been produced by the recently developed technique of solid state coextrusion. The films were prepared under moderate conditions, without lubricant in continuous lengths by extruding through conical dies of extrusion draw ratio up to 36. This is a draw ratio higher than achievable by conventional solid state extrusion at comparable processing conditions through slit dies. The ultra-thin films of high orientation were transparent and exhibited dead bend. The physical and mechanical properties were evaluated and compared with the properties of the same high density polyethylene extruded through a slit die. The increase in the melt point, crystallinity, tensile modulus, and birefringence indicates that the method is very efficient for the production of ultra-thin and highly oriented films. An experimental technique is also presented for preparing billets of controlled and uniform initial morphology and free of voids.     

Effect of gamma radiation on low density polyethylene

Low density monofilaments of polyethylene with varying draw ratios were subjected to high energy radiation using Co⁶⁰ gamma radiation source. It was found that the tensile strength is sharply improved with the increasing dose up to 20 Mrd beyond which a sharp decrease in tenacity and initial modulus was observed. The swelling behaviour also decreased with the increased in dose of irradiation. However, density showed an increasing trend. The shrinkage behaviour of all the filaments was found to increase with increasing dose and draw ratios. The results have been explained in terms of continuous increase in crosslink density with the increasing dose of irradiation along with loss of crystallinity, which appears to be more severe in the latter stages of exposure. The samples exhibited memory effect after redrawing at 100°C and cooling under tension followed by further heating at 110°C under relaxed condition. The shrinkage was still much higher but independent of draw ratio.     

The preparation of ultra-high modulus polypropylene films and fibres

Following the discovery that linear polyethylene can be drawn to very high draw ratios to produce oriented fibres and films with ultra-high initial moduli, a similar study has been undertaken for polypropylene. In particular, the modulus/draw ratio relationship has been obtained for a range of polymers of different molecular weight and molecular weight distribution. The effects of thermal history and draw temperature were studied, and it was shown that under optimum conditions material with an initial modulus at room temperature of 1.9 × 10¹⁰ Nm^?2 (205 gdtex, 3 × 10⁶ psi) can be obtained. This value is at least 50 percent greater than those previously recorded for drawn fibres and about one half of the theoretical modulus.     

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.     

The influence of the draw ratio of melt-spun polyethylene fibres on γ-irradiation induced effects as observed by ESR

The influence of draw ratio on free radical behaviour in melt-spun polyethylene fibres has been examined using ESR spectroscopy. The stability of free radicals produced by γ-irradiation is greatest some where between draw ratio 1 and 10. The general trend of radical stability has been found to correlate with the trend of gel content with draw ratio. The possible link between the extent of crosslinking and the production of stable radicals has been discussed in terms of morphological aspects.     

PA6/PE-LD共混物流延薄膜成型过程的理论分析与实验研究

研究了聚酰胺6/低密度聚乙烯（PA6/PE-LD）共混物流延薄膜成型工艺,从模拟和实验两方面分析了空气间隙和拉伸比对薄膜厚度和颈缩的影响。结果表明,实验和模拟有比较好的吻合性,拉伸比对薄膜厚度的影响大于空气间隙,而空气间隙对薄膜颈缩的影响大于拉伸比。     

Anisotropy of optical and mechanical properties of stretched low density polyethylene

Summary Parallel measurements of IR dichroism, birefringence and microhardness anisotropy of drawn low density polyethylene films have been carried out. The angle α between the dipole moment vector of the IR band at 1367 cm⁻¹ and the chain axis has a value of 50°. The longitudinal and transverse moduli of the drawn samples, measured by microhardness indentations, increase as the draw ratio increases after passing through a minimum for a draw ratio of around 1.5. Received: 27 April 1998/Revised version: 22 July 1998/Accepted: 22 July 1998     

Hot rolling and quench rolling of ultrahigh molecular weight polyethylene

The mechanical properties, the crystal orientation, and the microstructure of hot rolled and quench rolled ultrahigh molecular weight polyethylene (UHMW-PE) were investigated. The tensile strength of hot rolled and quench rolled UHMW-PE sheets increased with increasing draw ratio. The crystallographic axes a, b, and c of the rolled sheets tended to be oriented to the normal direction (ND), the traverse direction (TD), and the rolling direction (RD), respectively. The small-angle X-ray scattering patterns with incident X-ray beam parallel to TD suggested the presence of inclined lamellar structure in the RD–ND plane. At the initial stage of rolling, partial breakup of crystallites along the (100) plane was observed. The lamellar structure is deformed by the slippage mechanism along the (100) plane in the chain direction.     

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.     

Orientation processes in the drawing of dry gel films of polyethylene and polypropylene

Changes in the orientation with draw ratio of dried gel films of high density polyethylene and isotactic polypropylene were monitored by polarizing microscopy, birefringence measurements and X-ray diffraction. The results suggest that the gels, formed from 0.5 or 1% w/v solutions, contain a continuous gel network within which the bulk of the crystallized polymer is held. Tie molecules link the initial gel network with the crystalline morphology formed as the gel cools. The distribution of tie molecules is suggested as the reason why the undrawn gel breaks up into interconnected fibrils as drawing proceeds. The relatively low number of tie molecules allows draw ratios of greater than 100 to be achieved without sample failure.     

Relaxation Behavior of Polyethylene Oriented by Various Techniques

Abstract

Stress relaxation of high-density polyethylene extrudates and those crystallized from highly deformed melt (PE-1) have been investigated in a wide range of temperatures (?50 to +120°C) and draw ratios from 5.5 up to 12.2 at the different constant tensile strains from 1 up to 20%. The experimental data obtained have been summarized by the time-temperature superposition principle. Relative intensity of stress relaxation (the stress drop in 10³ s divided by the initial stress) has been observed to increase together with the growth of draw ratio despite the enhancement of the short-term properties. The radiation cross-linking of the PE-1 samples may only decrease the stress relaxation intensity by 30%. The relaxation properties of a number of oriented polyethylene samples produced by various techniques were compared. It has been established that all the investigated materials are characterized by similar values and high relative drops in stress, whereas the short-term properties are essentially different. It points out to the relaxation processes being intensive both in the oriented and unoriented PE.     

Preparation of ultra-high modulus linear polyethylenes; effect of molecular weight and molecular weight distribution on drawing behaviour and mechanical properties

A systematic investigation of the effect of molecular weight and molecular weight distribution on the cold drawing behaviour of linear polyethylene has been undertaken. In the molecular weight range studied, the natural draw ratio was very sensitive to the morphology of the initial material; spectacular effects on the natural draw ratio were observed provided that an optimum initial morphology was achieved. These effects can be related to both molecular weight and molecular weight distribution.The extensional modulus and melting behaviour of the drawn material was also examined. To a first approximation the extensional modulus related to the natural draw ratio, and at very high draw ratios (～30) extremely high extensional moduli (～700kbar) were obtained. The structure and properties of the drawn material did, however, also depend on the molecular weight and molecular weight distribution. In particular, when certain molecular weight requirements were satisfied, the oriented samples showed the presence of extended chain material. It does, however, appear that although differences in molecular weight and molecular weight distribution give rise to differences in extensional moduli, the presence of extended chain crystallization per se is not a necessary requirement for the production of high modulus material.     

Experimental study on cold press workability of aluminum-polyethylene sandwich laminates

The cold press workability of aluminum-polyethylene sandwich laminates is experimentally investigated by a deep drawing process with a conical die. “Planium” of 2 mm thickness, in which the core layer, high density polyethylene, is sandwich-laminated between two aluminum sheets, is used as a test specimen. Soybean oil is employed as a lubricant. The dimensions of the drawing die set are determined for a circular, blank so that the tensile fracture of the drawn.cup occurs at the top corner of the punch. The limiting draw ratio (LDR) is experimentally explored by using many blanks of various initial diameters. In general the maximum punch load in successful drawing increases linearly with increasing draw ratio. Because of the sandwich structure, the fracture of the drawn cup occurs twice. The initial fracture (LDR_s) corresponds to a fine bending fracture at the aluminum surface. The second fracture (LDR_B) is the complete fracture of the whole laminate. The effect of punch corner radius, working velocity and thickness fraction of aluminum and polyethylene on LDR_S and LDR_B are studied. The strain distributions of deep drawn cup in three orthogonal directions are analyzed experimentally.     

Preparation of ultradrawn polyethylene by a novel radial-compression method

High density polyethylene has been ultradrawn at 80°C by a novel radial–compression method which involves the gradual buildup of highly-stretched elastic (spandex) filaments around the circumference of a solid polymer cylinder. The sheath of tightly-wound filaments generates a high-radial pressure which forces the polyethylene cylinder to neck down, uniaxially extend, and ultradraw. The physical and mechanical properties of radially-compressed polyethylene of maximum draw ratio, 40-45X, were evaluated and compared with the same polymer ultradrawn in other stress fields. In most physical property categories, the radially-compressed samples are virtually indistinguishable from samples prepared by solid-state extrusion at higher temperature and pressure. The transparency and the maximum tensile modulus, 62 GPa, further confirm the effectiveness of radial compression in achieving ultradraw. An experimental technique for measuring the radial pressure generated by the winding of the highly-stretched elastic filament is presented. The upper-bound pressure in the radial-compression experiments was less than 100 MPa.     

Effect of postdrawing on the permeability of gases in blown polyethylene film

The effect of orientation on the structure and transport properties of high-density polyethylene film has been studied. Microstructure was characterized using small-angle light scattering, birefringence, and wide-angle x-ray scattering. Water vapor and oxygen transmission rates were determined as a function of film draw ratio. The object of the present work is to correlate the effects of postprocessing conditions on the transport properties and morphology of linear polyethylene. High-density spherulitic polyethylene films were produced by blown film extrusion and subsequently oriented by longitudinal stretching in a postoperation. Various degrees of orientation were imparted to the films, with percent crystallinity, sample orientation and transport properties measured as a function of draw ratio. For the postoriented films, results indicate there was no significant change in percent crystallinity with increasing draw ratio although water vapor and oxygen permeability decreased substantially. This is attributed to the increased orientation of the crystalline and amorphous regions and rod-like and microfibril structure formation brought about by the drawing process. Lower processing temperatures result in increased orientation which improves the vapor barrier properties.     

Uniaxial-planar deformation of poly(ethylene terephthalate) films: 1. Characterization of the crystalline phase

Amorphous isotropic poly(ethylene terephthalate) films were deformed under uniaxial-planar symmetry conditions at constant engineering stress close to and above the glass transition temperature. The structure of the crystalline phase is characterized in terms of orientation, size and organization of the crystalline blocks by X-ray scattering techniques. The orientation of the crystallographic axes and the size of the crystalline blocks are well rescaled by the macroscopic draw ratio, which is a function of the drawing temperature and the applied stress. The large-scale organization of the crystalline phase is not determined by the same parameter but changes mainly with temperature.     

Structure-property relationships on uniaxially oriented carbon nanotube/polyethylene composites

Multi walled carbon nanotubes have been incorporated into a linear low density polyethylene matrix through high energy ball milling technique at room temperature, without any chemical modification or physical treatment of the nanotubes. Highly oriented samples, with different draw ratios, were obtained by drawing at 80 °C the composite films. SEM and FTIR results on the drawn PE films demonstrate that the molecular chains in both crystalline and amorphous phases are well oriented along the drawing direction. The effect of different weight percent loadings of nanotubes and draw ratio on the morphology, thermal, mechanical and electrical properties of the composite fibers have been investigated.     

In-situ SAXS study of the mesoscale deformation of polyethylene in the pre-yield strain domain: Influence of microstructure and temperature

The deformation of polyethylene spherulites at the scale of the lamellar stackings is investigated by in-situ SAXS as a function of draw temperature in comparison with the macroscopic strain. Local elongation strain in equatorial region and local compressive transverse strain in polar region both increase in absolute value and follow linear relations versus macroscopic strain in the visco-elastic range. However, local strains turn out heterogeneous over the whole spherulite volume. This is attributed to the different type of mechanical coupling between the crystalline lamellae and the amorphous layers in the polar and equatorial regions. Increasing the temperature in the range 50–100 °C gradually promotes homogeneous distribution of strain at the local scale. The origin of this phenomenon is discussed in terms of activation of the chain mobility in the crystalline phase that modifies the mechanical coupling between the two phases. This ratio between local and macroscopic deformation increases with temperature up to the macroscopic value.     

Properties of oriented polyethylene bars of large cross section

The solid-state extrusion technique has been used to prepare uniaxially oriented polyethylene bars with rectangular end cross sections of 6 × 50 mm². They were extruded at 110°C from billets of high density polyethylene. The tensile modulus and strength for the extrudate with a draw ratio (DR) of 14 were 17.9 and 0.32 GPa, respectively. The mechanicals were also measured in the transverse direction by means of the proportional elastic limit (PEL) bending test. The PEL results do not change after DR 14 due to the fibrillate structure formation. Crystallinity and shrinkage tests were made on samples taken over the bar cross section. They show that uniform properties were achieved across the width of the bar with proper die design. © 1994 John Wiley & Sons, Inc.