Morphological and mechanical property studies of polymer extrudates obtained by the rotational extrusion process

The orientation of the reinforcing fibers in glass fiber filled polypropylene tubular extrudates has been controlled effectively by the superposition of the linear flow in a melt extruder with torsional flow generated by rotating the capillary portion of the extrusion die of the extruder. The so produced extrudates have mechanical properties which can be balanced along the hoop and extrusion direction by adjusting the extrusion rate and the rotational speed of the die for example, the breaking load of 75N along the hoop direction increases by ～40% when the capillary of the die was rotated at 80 rpm. At the same time the Young's modulus in the extrusion direction decreased from 1100 MPa under conventional extrusion conditions to 800 MPa.     

Extrusion of highly oriented polyolefin fibers

This study demonstrates that it is possible to extrude highly oriented polyethylene fibers using only a plasticating single screw extruder, specially designed and operated dies, and a simple take up mechanism. The degree of orientation achieved is as high as that achieved in solid state extrusion studies but accomplished at a fraction of the pressure. The degree of orientation is significantly greater than that of commercial grade fibers. The orientation is developed in the elongational flow of the “wine glass stem” region preceding the die entrance and is retained by crystallization in the die. Crystallization is inhibited in the pre-die region by maintaining a temperature above the effective melting point of the flowing, oriented (hence lowered entropy) melt.     

High-density polyethylene pipe with high resistance to slow crack growth prepared via rotation extrusion

Slow crack growth (SCG) is one failure principal mode in polyethylene (PE) pressure pipe applications. In the conventional extrusion process, the molecular chains in the plastic pipes are oriented along the axial direction, which are disadvantageous to their resistance to SCG. In order to change the orientation direction of molecules in the plastic pipe, a new rotation extrusion processing system was designed to extrude high-density polyethylene (HDPE) pipes, and a thorough research was done on the effect of the rotation speed on its microstructure and resistance to SCG during the rotation extrusion. The experimental results showed that when the die rotated during the extrusion process of PE pipes, the hoop stress exerted on the polymer melt could make the molecular orientation deviate from the axial direction, and therefore the consequent multi-axial orientation of molecular chains could be obtained. As a result, the PE pipe with better resistance to SCG was prepared. Compared to the PE pipe produced by the conventional extrusion, the crack initiation time of the PE pipe manufactured by the novel method increased from 27 to 57 h.     

Effect of vibration extrusion on the structure and properties of high‐density polyethylene pipes

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom‐made electromagnetic dynamic plasticating extruder was adopted to extrude high‐density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high‐circumferential‐strength pipes. RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial self‐reinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide‐angle X‐ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology. Copyright © 2008 Society of Chemical Industry     

Hydrostatically extruded glass fiber reinforced polyoxymethylene. II: Modeling the elastic properties

This paper describes an investigation into modeling the elastic properties of hydrostatically extruded short glass fiber reinforced polyoxymethylene (POM). The starting material for the extrusion was randomly arranged short glass fibers (25 wt%, average length 150 μm) in an isotropic POM matrix. Extrusion was carried out through a reducing conical die at 15°C below the melting point of the matrix phase (hence the composite was extruded in the solid state), such that after extrusion, preferential alignment along the extrusion direction was developed for both the fibers and for the crystalline fraction of the polymer matrix. The elastic properties of samples, made over a range of extrusion ratios, were measured using the ultrasonic immersion method, a technique that allows a complete set of elastic constants to be determined for a composite. Theoretical predictions for the elastic properties of the oriented extrudates were generated by combining a modification to the theory of Wilczynski to allow for the fibers' being surrounded by an oriented matrix phase, together with the aggregate model of Ward to model the effects of partial orientation of the fiber oriented matrix units.     

新型挤出机头及其挤出流动分析

介绍了可安装在现有的通用挤出机上的芯棒旋转式长玻纤维螺旋在线复配塑料管材挤出机头的结构特点和工作机理，并对口模内纤塑熔体的流动进行了理论分析，采用ANSYS软件对口模内熔体的流动进行了模拟。结果表明：熔体的速度分布符合设计要求，即聚合物颗粒塑化熔融后在线混合入一定长度的玻璃纤维后，熔体分子和玻璃纤维沿所挤出圆形管材的管壁螺旋取向。     

From polymer blends to in situ polymer/polymer composites: Morphology control and mechanical properties

In situ polymer/polymer short fiber composites were generated by a two‐step process. In the first step, a polyamide (PA) dispersed phase is blended with a polypropylene (PP) matrix in a twin‐screw extruder at a temperature at which both polymers are in molten state. The extrudate was then stretched at the die exit to generate long and thin fibers of PA in the PP matrix well oriented in the direction of flow. Adhesion between the phases was promoted by addition of PP grafted with maleic anhydride (PP‐g‐MA). During the second step, the chopped extrudates were molded by injection or compression molding at a temperature at which PA in the form of fibers is in the solid state and the PP matrix is molten. The control of the formation of such ultrafine fibers was obtained by quantitative analyses for the deformation of the minor PA‐phase during twin‐screw extrusion and stretching at the exit of the die that involve both shear and extensional flows. Morphology and mechanical properties of such polymer/polymer composites were compared to equivalent blends with dispersed spherical particles‐type morphology prepared in a batch mixer device.     

Rheological behavior and fiber orientation in slit flow of fiber reinforced thermoplastics

The flow behavior and fiber orientation in slit flow of a short fiber reinforced thermoplastic composite melt are investigated. A slit die with adjustable gap and interchangeable entrance geometries was designed and built. The slit die is fed by a single screw extruder. The bulk viscosity is calculated from the axial pressure profiles measured using three flush mounted pressure transducers. The effect of entrance geometry and gap dimensions on the fiber orientation and bulk flow behavior is specifically considered. A skin-core composite fiber orientation is observed in the thickness direction. Fibers are oriented in the flow direction and parallel to the walls in the skin region irrespective of the entrance geometry. Different fiber orientation distributions in the core region can be realized by using different entrance geometries. However, the changes in the core fiber orientation are not fully reflected by the measured viscosities, due to highly oriented skin layer. Exit pressures obtained by extrapolation of linear pressure profiles are found to be all positive, but dependent on the die geometry and entrance conditions, even for the unfilled melts.     

Flow induced fiber orientation in an expanding channel tubing dies

In the application of plastic pipes for fluid transport and for the protection of underground electrical cables, it is desirable to improve mechanical properties, particularly in the hoop direction. The use of orientable reinforcing particles such as chopped glass fibers could make possible such an improvement if the orientation of the fibers could be controlled. While conventional pipe extrusion dies tend to promote axial fiber orientation, the use of an expanding channel die has been proposed to produce a preferential hoop orientation of fibers. In this paper, a theoretical model of the flow of a fiber suspension through an expanding channel die that predicts the fiber orientation distribution at the die exit is described. The effects of Theological properties and die geometry on the final fiber orientation distribution are predicted. The results of an experimental study of fiber orientation in pipe extruded using an expanding channel die are shown to be in agreement with the theoretical predictions.     

可控制纤维取向的长玻纤增强塑料管材挤出机头

介绍笔者发明的两个关于长玻纤在线挤出增强管材机头的专利,描述了这两个机头的结构特点和工作原理,其特征为,机头可安装在现有通用的挤出机上,使用普通的聚合物粒料,在粒料塑化熔融后在线混入一定长度的玻纤,能使熔体分子和玻纤沿所挤出圆形管材的管壁螺旋取向。     

Influence of rotor on extrusion pressure

Extrusion of a paste material has a significant position in the processing of powder materials. The reduction between the barrel and die, often with a complicated cross-section, is a necessary detail of each extruder. If a deformation element of special design is situated in this zone, it has a positive influence on the process of extrusion. Its shape can be varied. In this case, it is a rotor with blades that move on the surface of the matrix, which has influence on the rate of shear strain. The effects decrease the apparent viscosity and extrusion pressure.     

The influence of fiber processing parameters on the structural properties of as‐spun polypropylene fibers: A factorial design approach

The structures of polypropylene fibers, spun according to a factorial experimental design, have been studied with the aid of wide angle X‐ray diffraction and birefringence measurements. From statistical analysis of the results, the fibers have been characterized in terms of their crystallographic order and the overall orientation of their constituent polymer chains. These properties have been quantitatively assessed as responses to seven specially selected process control parameters in the extrusion equipment used to process the fibers. For both crystallographic order and overall orientation, the metering pump speed (MPS) at which the fibers are extruded and the speed (WS) at which the extruded fibers are wound exert significant effects. Moreover, the interaction, WS × MPS, between these two control parameters also significantly influences orientation. For crystallographic order, two further significant parameters are the melt flow index (MFI) of the grade of polypropylene used and the temperature (ST) at which the polymer melt passes through the spinneret. The roles of these two factors in the development of crystallites within the fibers are discussed. No interaction effects, however, appear to be significant for crystallographic order. Models that specify the direction of change of the significant parameters for increasing or reducing both responses are given. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 568–576, 2004     

Hydrostatically extruded glass-fiber-reinforced polyoxymethylene. I: The development of fiber and matrix orientation

This paper describes the determination of fiber and matrix orientation in oriented short-glass-fiber-reinforced polyoxymethylene (POM) composites produced by hydrostatic extrusion. The starting material was random glass fibers (25 wt% and average length 150 μm) in an isotropic POM matrix, and the oriented composites were produced by extrusion through a reducing conical die at 15°C below the polymer melting point: after extrusion the average fiber length was reduced slightly to 133 μm. Fiber orientations were measured using an image analysis method developed at Leeds University, and the matrix orientation was determined using wide angle X-ray diffraction. The development of fiber orientation with extrusion ratio was found to be close to that predicted by the pseudo-affine deformation scheme although the fiber orientation was greater than that predicted by the model at low draw ratios and slightly less at the highest draw ratio. The development of the orientation of the crystalline portion of the matrix was found to be always significantly greater than that predicted by the pseudo-affine scheme.     

Solid state extrusion of vinylidene fluoride (VF2)/vinylidene trifluoride (VF3) copolymers. II: Structure development

Copolymers of vinylidene fluoride (VF2) and vinyltrifluoride (VF3) exhibit Curie transition temperatures well below their melting points. Above these endothermic transitions, they soften and this behavior helps in their solid state extrudability. In this paper, the effects of extrusion speed, temperature, and draw ratio on structure development in 60/40 and 72/28 VF2/VF3 copolymer compositions are presented. With the increase of extrusion draw ratio the Curie transition temperature of the extrudates decreased and melting temperature increased. This behavior suggested that the chains in the crystalline regions contain higher levels of conformational defects while overall crystallinity is increased. Unoriented polymers were optically opaque and extrudates were found to be transparent as a result of breakdown on the superstructural level which decreases the scattering effects in the visible wavelength range. The micro beam WAXS studies on the samples taken from the entrance of the dies revealed that the unoriented core is surrounded by alternating unoriented and oriented layers close to the core. The remainder of the skin layers are found to be oriented with local symmetry axes and main chain orientation being parallel to the die wall surface. The regions that are found to be oriented were also found to be optically translucent and unoriented regions were optically opaque. This structure turns uniformly transparent–and thus oriented?as the polymer enters the die. The existence of layered structure suggests that highly localized yielding occurs during early states of deformation at the converging entrance region of the die. Examination of the radial structural variation in extrudates with micro beam X-ray diffraction technique revealed that the local symmetry axes are tilted away from the extrusion direction and this tilt angle reduces at the sample macro-symmetry axis at the core of the samples. This indicated that the tilted structure developed at the converging entrance region is partially preserved through the die. While the orientation of local symmetry axes varies from skin to core in the extrudates, the orientation of chains with respect to these local symmetry axes remains relatively unaffected.     

Dynamic behavior of a single screw plasticating extruder part I: Experimental study

The dynamic responses of a 2–1/2 inch single screw plasticating extruder and extrusion line were investigated. Step changes in screw speed, take-up speed, back pressure, and processing materials were used to determine the transient responses of barrel pressures, die pressure, melt temperature, and extrudate thickness. Dynamic responses of the entire extrusion line can be explained by the flow mechanism of the extruder and the logical properties of the polymer used. A capillary rheometer was also used to determine if it could simulate pressure responses in the extruder for screw speed changes. Results showed that capillary rheometer was helpful in estimating the short term pressure responses in the die.     

Extrusion blow molding of long fiber reinforced polyolefins

A 58% (by weight) long glass fiber reinforced (LGF)‐HDPE master batch was blended with a typical blow molding HDPE grade. HDPE composites having between 5% and 20% (by weight) long fiber content were extruded at different processing conditions (extrusion speed, die gap, hang time). The parison swell (diameter and thickness) decreased with increasing fiber content. Although the HDPE exhibited significant shear rate dependence, the LGF/HDPE composites were shear rate insensitive. Both the diameter and weight swell results also indicated very different sagging behavior. The LGF/HDPE parisons did sag as a solid‐body (equal speed at different axial locations) governed by the orientation caused by the flow in the die. Samples taken from blown bottles showed that fiber lengths decreased to 1‐3 mm, from the original 11 mm fiber length fed to the extruder. No significant difference in fiber length distribution was found when samples for different regions of the bottle were analyzed. SEM micrographs corroborate the absence of fiber segregation and clustering or the occurrence of fiber bundles (homogeneous spatial fiber distribution) as well as a preferential fiber orientation with the direction of flow. The blowing step did not change the orientation of the fibers. Five‐percent (5%) and 10% LGF/HDPE composites could be blown with very slight variations to the neat HDPE inflation conditions. However, 20% LGF/HDPE composites could not be consistently inflated. Problems related to blowouts and incomplete weldlines were the major source of problems.     

复合应力场下制备自增强的HDPE管材

利用自行研制的先剪切后拉伸的挤管口模,对3种不同配比的HDPE6100M/HMWHDPE共混物挤出管材的周向、轴向强度进行研究,实验中发现少量的HMWHDPE有利于HDPE管材在复合引力场挤出中实现力学性能的双向自增强,预示了少量的HMWHDPE有利于诱导HDPE6100M分子沿应力场方向取向、结晶。     

A Wide-angle X-ray diffraction method of determining chopped fiber orientation in composites with application to extrusion through dies

A method of determining fiber orientation in composites using wide-angle X-ray diffraction (WAXS) is described. Oriented crystalline fibers are suspended in an amorphous polymer matrix. The WAXS reflects characteristics of the fiber are used to determine the mean orientation and orientation distribution of the crystallographic axes representing the polymer chain relative to preferred axes located in the test specimen. The chain direction crystallographic axis is taken as representing the fiber axis, and the orientation of this axis to represent the orientation of the fibers. Experimental studies were carried out using Kevlar (poly(p-phenylene terephthalamide)) fibers suspended at a 20 volume percent loading in a polymethyl methacrylate matrix. The Kevlar fibers had Hermans orientation factors of 0.92. Specific attention is given to how through circular dies. We have examined both extrudates and the material frozen-in when the composite in the reservoir and die is cooled to room temperature. Fiber orientation factors, corresponding to Hermans orientation factors, 0.3 to 0.38 were obtained for the extrudates. Orientation factors for fibers within the die is about 0.45. Specially prepared completely oriented samples had orientation factors of 0.93, which closely corresponds to the orientation of the fiber.     

Morphological studies of blends containing liquid crystalline polymers with poly(ethylene terephthalate)

The investigation involved the structure–property behavior of extruded cast films prepared from blends of thermotropic liquid crystalline copolyesters with poly(ethylene terephthalate) (PET). Data were obtained which showed not only the temperature dependence of the moduli and stress–strain behavior but also the orientation effects that must be prevalent in order to explain the differences between the moduli measured parallel and perpendicular to the extrusion direction. Only at high liquid crystal polymer (LCP) composition is the modulus particularly increased. The modulus enhancement with lower LCP content and utilization of process variables are discussed with respect to the induced morphological textures and nature of the process equipment. Specifically, the process variable extruder gear pump speed did not enhance Young's modulus at the same LCP content as extensively as did the process variable of extruder screw speed.     

聚丙烯挤出发泡中的关键技术——加工设备和成型工艺研究

从聚丙烯挤出发泡的加工设备包括挤出机的类型、发泡机头的设计、发泡剂的注入和计量控制以及聚丙烯挤出发泡的成型工艺包括螺杆转速、压力降和压力降速率、成核剂的分散、发泡机头的温度等系统介绍了聚丙烯挤出发泡中的一些关键技术。目前的研究表明：采用双螺杆挤出机进行挤出发泡时需要考虑发泡剂逃逸和压力的升高与稳定；可以通过改变机头的形状和尺寸获得不同的压力降和压力降速率，从而控制挤出发泡的成核速率。提高螺杆转速、增加压力降和压力降速率有利于优质发泡材料的获得；优选适宜的发泡机头温度可以抑制气体逃逸，提高发泡倍率，获得更低密度的聚丙烯发泡材料。