A coextrusion process for the manufacture of short-fiber-reinforced thermoplastic pipe

The advantageous use of short-fiber-reinforced thermoplastic (SFRTP) resins in the manufacture of pipe requires that the fiber orientation be controlled and that the surface finish not be adversely affected by the presence of the fibers. It is proposed here that a coextrusion process be combined with the use of a Specially designed, expanding channel die to achieve these objectives. Conventional pipe dies tend to promote the axial orientation of fibers, which has an undesirable effect on the hoop properties of pipe. The use of an expanding channel die promotes fiber orientation in the hoop direction. However, even in this case, there is still a strong component of the shear field near the wall of the die, and in this region, axial orientation still predominates, The use of a three-layer, coextrusion process in which unreinforced resin is used for both the inner and outer layers makes it possible to minimize the shear effects on the fiber orientation in the middle SFRTP layer. At the same time, it provides a smooth, fiber-free pipe surface. A theoretical analysis of this process is presented. The analysis shows that the presence of unreinforced surface layers increases the level and uniformity of fiber orientation. The results of an experimental study of fiber orientation in the middle SFRTP layer are found to be in qualitative agreement with the predictions of the theory.     

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.     

The influence of hoop shear field on the structure and performances of glass fiber reinforced three-layer polypropylene random copolymer pipe

Three-layer pipe has many advantages over single layer one, especially for the pipe of glass fiber (GF) reinforced materials. But the hoop strength of the pipe produced via convention extrusion is poor because GFs orient along axial direction. In this work, a self-designed rotation extrusion system was adopted to extrude GF reinforced three-layer polypropylene random copolymer (PPR) pipe, in which a hoop shear field was applied to the polymer matrix and fibers in the middle layer. The structure and performance of pipes were investigated via scanning electronic microscope (SEM) and synchrotron two-dimensional wide-angel X-ray diffraction (2D-WAXD). Due to the hoop shear field, the orientation of GFs in middle layer deviated from axial direction. As a result, PPR pipes with enhanced hoop tensile strength were obtained. Because of the three-layer structure and the production process, the molecular chains of middle layer did not emerge distinct orientation after rotation shear, as shown in 2D-WAXD and SEM experimental results. This three-layer pipe rotation extrusion system offers a novel method for the modification of pipes in manufacture industry. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46985.     

短纤维在扩芯式口型中的复合挤出

为了解决短纤维增强热塑性橡胶管或塑料管的表面粗糙度问题，并得到所期望的纤维取向和厚度方向取向的一致性。本文阐述了有关机头口型的种类及扩芯式口型的复合挤出法。     

挤出机头内流场对玻纤取向和分布的影响

本文分析了平直和发散两种挤出机头内的流场,推得发散流道内熔体周向拉伸应变速率的表达式;研究了两种挤出流率下由这两种机头挤出的制品壁内玻纤的取向和分布,并通过流道内的剪切和周向拉伸应变速率,对玻纤取向和分布的形成机理进行解释。结果表明：经平直机头挤出的制品内,玻纤在剪切作用下基本沿流动方向排列。发散机头内熔体受剪切和周向拉伸的共同作用,使制品壁厚方向形成了“表层-次表层-芯层-次表层-表层”的五层结构,并首次发现芯层呈“W”形排列。玻纤的排列不仅受流动过程中的应变影响,更取决于应变速率的大小。     

The influence of processing on fiber orientation and creep in short carbon-fiber reinforced low density polyethylene and polycarbonate

This paper is concerned with the creep behavior of short fiber-reinforced thermoplastics, especially with regard to the role of fiber orientation. Rectangular samples of low density polyethylene (LDPE) and polycarbonate (PC) containing varying amounts of carbon fibers were prepared by compression and injection molding. The materials were compounded using a technique producing a concentration independent fiber length distribution. The orientation distribution, on the other hand, was found to be strongly influenced by fiber concentration. The creep parameters were measured for both LDPE and PC. The contraction ratio was determined for the PC samples. In the case of LDPE reliable data could not be obtained due to the low modulus of the matrix. The creep properties of the PC and LDPE samples varied significantly with the orientation of the fibers. The creep strain was measured as a function of time for both polymers for different fiber concentrations and orientations. The predictions of the Halpin-Tsai equation underestimated the experimental strain figures somewhat when the stress direction coincided with that of the fiber orientation. When the stress acted across the fibers the theoretical and experimental results showed satisfactory agreement. A plausible explanation is that incorporation of carbon fibers changes the morphology of the matrix material. We conclude that the contraction ratio is an important measure of the volume and its changes during deformation, especially with regard to its relation to the free volume and similar quantities. We have shown that the necessary data can be obtained, in spite of experimental difficulties even for anisotropic samples.     

Effects of abrupt expansion geometries on flow‐induced fiber orientation and concentration distributions in slit channel flows of fiber suspensions

Flow‐induced fiber orientation and concentration distributions were measured in channel flows of fiber suspension. The test fluids used are a concentrated fiber suspension (CFS), a semidilute one (SDFS), and a dilute one (DFS). The channel has a thin slit geometry with a 1:16 expansion. In the present work, fiber orientation and concentration distributions are quantitatively evaluated by direct observation of fibers even in the CFS flow. It is found that the weak fiber–fiber interaction of the SDFS largely affects the fiber orientation in the flow with a sudden change such as in the expansion flow, while it is ineffective upon the fiber orientation in the flow without a sudden change such as in the far downstream region. Fiber concentration in the CFS has a flat distribution over a channel width in both the entrance region of the expansion and the downstream region. However, fiber concentration distributions in the SDFS and the DFS have a small and a large peak near the sidewall in the entrance region, respectively, due to the fiber‐wall interaction at the channel wall. These peaks, however, disappeared in the far downstream region after the fibers passed through the expansion. POLYM. COMPOS., 26:660–670, 2005. © 2005 Society of Plastics Engineers.     

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.     

Evolution of the three-dimensional orientation distribution of glass fibers in injected isotactic polypropylene

The mechanical properties of fiber reinforced thermoplastics depend on the characteristics of the fibers, such as their three-dimensional orientation distribution, which is the result of the processing conditions and of the initial products before mixing. Different composites were obtained by injection of reinforced isotactic polypropylene containing 30 wt% fibers. Different size distributions of fibers were investigated. Their average lengths range from 60 to 470 μm. Specific new tools were developed by combining microscopy and image analysis to determine the microtextural characteristics of the fibers, such as their size distribution or their orientation in the matrix. Our technique allows the following of the size distribution evolution of the fibers as the composite is processed. The three-dimensional orientation of the fiber is established using the in-plane angle and the angle of inclination. The evolution of the three-dimensional orientation distribution is studied in the sample, by differentiating central and border behavior and also as a function of fiber size. It is concluded that the coupling of scanning electronic microscopy with image analysis is an interesting technique that allows the 3-D orientation of the fibers to be determined. This technique is validated by showing that the results obtained agree with those that other authors obtained on comparable materials.     

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.     

Dynamics of semi-flexible and breakable fibers under Poiseuille flow

In the processing of fiber-reinforced polymer composites, especially in injection molding, the fiber's orientation, length, and distribution vary depending on the location of the channel flow and its properties, which affects the performance of final products. To investigate the intricate behavior of fiber suspensions under Poiseuille flow, we used the hybrid simulation approach, multiparticle collision dynamics–molecular dynamics (MPC-MD), which takes hydrodynamic interactions and fiber properties (strength, flexibility) into account. For non-breakable and rodlike fibers, fibers align well along the flow direction while showing more alignment near the wall. As fiber becomes breakable and/or flexible, the length and orientation of fibers strongly depend on their properties. The interesting phenomenon is specifically seen for breakable and semiflexible fibers, where the orientation of the fiber exhibits non-monotonic behavior depending on the flow rate. This complex behavior highlights the importance of comprehending the dynamics of many types of fibers and necessitates research into the best conditions for injection molding.     

A simple illustration of structure-properties relationships for short fiber-reinforced thermoplastics

In this paper, a simple theoretical model for elastic properties of short fiber-reinforced thermoplastic (SFRTP) composite systems is described, which considers the effect of two important structural parameters, viz., fiber aspect ratio distribution and fiber orientation distribution. An experimental technique for producing SFRTP samples featuring a fairly uniform fiber orientation is described, and a broad set of experimental results on the mechanical properties of SFRTP systems based on polystyrene (PS) and polyethylene (PE) resins with several fiber loadings is presented. The use of the simple theoretical model to analyze the experimentally-determined elastic properties is discussed.     

The prediction of flow and orientation behavior of short fiber reinforced melts in simple flow systems

A model for the prediction of changes in fiber orientation in simple flows of fiber suspensions is proposed. Fiber interactions are modeled as randomizing forces over the rotation of fibers in closed orbits in simple shear. The resulting Fokker-Planck type convection-diffusion equation in orientation space is solved using a finite difference technique. The solution technique permits the use of periodic boundary condition for the convection-diffusion equation and different initial conditions for the orientation distribution. The model predictions for simple shear flow demonstrate the interaction between the structural changes and the bulk rheological properties. The effect of non-Newtonian fluid properties on the orientation distribution was also incorporated at the slow flow limit. Structural changes are assumed to be irreversible. The irreversibility is incorporated through an orientation distribution dependent diffusion coefficient.     

直接纤维数值模拟法与柔性复合纤维悬浮液流变性能

纤维复合材料的性能取决于其加工过程中纤维的变形、取向以及悬浮液的流变性能等因素,因此认识纤维的上述变化及流动诱导取向的机理就非常重要。与刚性纤维的研究相比,柔性纤维的变形及取向更复杂,是目前复合材料领域中开始关注的焦点。介绍了柔性纤维悬浮液的直接数值模拟法——颗粒模拟法(PSM)及其主要结果。通过研究方法可以分析柔性纤维的弹性变形、运动轨迹、取向分布以及悬浮液的流变性能变化。     

Poly(lactide) nanofibers produced by a melt‐electrospinning system with a laser melting device

A new melt‐electrospinning system equipped with a CO₂‐laser melting device was developed. Rod‐like samples were prepared from poly(lactide) pellets, and then fibers were produced from the samples using the new system. The effects of producing conditions on the fiber diameter were investigated. Furthermore, the physical properties of the fibers were investigated. The following conclusions were obtained: (i) in a special case, fibers having an average fiber diameter smaller than 1 μm could be obtained using the system developed; (ii) the fiber diameter could be decreased with increased laser output power, but the physical properties of the fibers such as the melting point and the molecular weight were decreased; and (iii) the electrospun fibers exhibited an amorphous state, and the annealed fibers exhibited an isotropic crystal orientation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1640–1645, 2007     

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.     

TCS工艺对PET FDY纤维成形的影响

采用热管纺丝 ( TCS)技术 ,生产 5 5 dtex/2 4f,78dtex/36 f PET-F DY长丝。探讨了纺丝工艺对纤维成形及产品力学性能的影响。结果表明 :张力对纤维结晶取向影响不大 ,热管温度对纤维取向、结晶及力学性能均有影响 ,结晶随热管温度的升高进一步完善 ,冷却吹风温度、湿度 ,集束位置均影响纤维成形 ,一般选择冷却风温度 2 2± 1℃ ,相对湿度 6 0 %～ 80 %。集束位置根据热管长度、丝条长度等进行选择     

The Effect of Rotational Extrusion on the Structure and Properties of HDPE Pipes

High density polyethylene (HDPE) pipes were prepared using a novel rotational extrusion processing system. The experimental results showed that the hoop stress exerted by either mandrel rotation or die rotation could have the macromolecular chains oriented in hoop direction and alter the crystallization behavior of HDPE during rotational extrusion, resulting in forming transcrystals with larger crystalline size, thicker lamellae, higher Tm. Therefore, the mechanical properties of HDPE pipes were greatly improved in hoop direction, which was attributed to the changes of the crystalline morphology and the molecular orientation under the action of the hoop stress field.     

Morphology and property of polyethylene pipe extruded at the low mandrel rotation

During the rotation extrusion of polyethylene (PE) pipes, with the rotating mandrel, compressed air as a cooling medium was introduced through their interior to achieve the quick cooling of the inner wall. The experimental results showed that the hoop stress exerted by mandrel rotation could promote the molecular orientation in the hoop direction; moreover, the introduction of compressed air could quicken its inner wall's cooling rate so as to slow down the relaxation of the oriented molecule and to reserve the orientation structure. Therefore, the hoop orientation degree increased with the increasing inner wall's cooling rate. As a result, the performance of the PE pipe was greatly enhanced. The hoop tensile strength of the PE pipe produced by the novel extrusion method increased from original 24.1 MPa up to 35 MPa; the pipe's crack initiation time increased from 27 to 60 h and the crack growth rate slowed down. POLYM. ENG. SCI., 50:1743–1750, 2010. © 2010 Society of Plastics Engineers