Continuous extrusion production of microcellular rigid PVC

This study examined the effect of Poly[vinyl chloride] (PVC) formulation on the cell morphology and density of rigid PVC foamed with supercritical CO₂ in a continuous extrusion process. Cell morphology and the density of foamed samples were controlled by blending two acrylic‐based processing aids (all‐acrylic foam modifier K‐400 and acrylic‐based impact modifier KM‐334), using a mixture design. The effect of blend ratios on the fusion and die swell behaviors of PVC was investigated by means of a torque rheometer and on a single‐screw extrusion capillary rheometer, respectively. Fusion was promoted as the relative amount of the all‐acrylic foam modifier increased in the blends. Similarly, the elastic constant of PVC, derived from the linear relationship between the die swell and apparent shear stress, increased upon increasing the relative amount of the all‐acrylic foam modifier in the blends, thus suggesting an increase in the melt elasticity of PVC. Microcellular rigid PVC foams with densities of approximately 0.15 g/cm³ and a tenfold volume expansion were produced. An optimum ratio of impact modifier to all‐acrylic foam modifier of 1:3 was found to maximize the foam expansion. Using impact modifier alone or all‐acrylic foam modifier alone yielded expansions considerably lower than that achieved with the 1:3 blend. The experimental results indicated that fusion is not the only criterion to control the cell morphology and density achieved in microcellular rigid PVC foams. The melt must have a viscosity low enough to allow bubble formation and growth, as well as elasticity high enough to prevent cell coalescence. J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers     

Manufacture of rigid PVC/wood‐flour composite foams using moisture contained in wood as foaming agent

Relatioships between the density of foamed rigid PVC/wood‐flour composites and the moisture content of the wood flour, the chemical foaming agent (CFA) content, the content of all‐acrylic foam modifier, and the extruder die temperature were determined by using a response surface model based on a four‐factor central composite design. The experimental results indicated that there is no synergistic effect between teh CFA content and the moisture content of the wood flour. Wood flour moisture could be used effectively as foaming agent in the production of rigid PVC/wood‐flour composite foams. Foam density as low as 0.4 g/cm³ was produced without the use of chemical foaming agents. However, successful foaming of rigid PVC/wood‐flour composite with moisture contained in wood flour strongly depends upon the presence of all‐acrylic foam modifier in the formulation and the extrusion die temperature. The lowest densities were achieved when the all‐acrylic foam modifier concentration was between 7 phr and 10 phr and extruder die temperature was as low as 170°C.     

Effect of acrylic processing aids on PVC die swell

The die swell behavior of PVC melts is a manifestation of melt elasticity and is of considerable commercial as well as fundamental importance. This behavior is a critical issue in extrusion blow molding application where die swell (i.e. parison thickness) needs to be controlled. Advantageously, the addition of high molecular weight acrylic processing aids to PVC provides better die swell control, thus, improving dramatically the processability of PVC. Hence, knowledge of molecular weight variables of such acrylic processing aids is important from both the commercial and rheological point of view. Various acrylic processing aids were prepared by polymerization designed to provide systematic variation of molecular parameters. Molecular weight distribution of the polymers was characterized by GPC, and their die swell behavior in a typical PVC blow molding formulation was determined at 200°C over various range of residence times using different L/D capillary dies. The results are presented showing effects of specific molecular variables.     

Online measurement of rheological properties of PVC/wood‐flour composites

By using a factorial design approach, this study examined the effect of the component materials on the viscoelastic properties of PVC/wood‐flour composites. Statistical analysis was performed to determine the effects of wood‐flour content, acrylic modifier content, and plasticizer content on the die swell ratio and viscosity of the composites measured online on a conical twin‐screw extrusion capillary rheometer. The viscoelastic properties of the samples also were measured using dynamic mechanical analyzer (DMA). Wood‐flour content and acrylic modifier content were the two important variables affecting the die swell ratio, whereas the addition of a low level of plasticizer did not affect this ratio. The die swell increased with the increased acrylic modifier content, but it was reduced considerably by adding wood flour into the PVC matrix. The true viscosity of neat PVC and PVC/wood‐flour composites decreased with the plasticizer content, irrespective of the acrylic modifier content. However, the addition of acrylic modifier significantly increased the viscosity of unfilled PVC, while an opposite trend was observed for the composites, owing to the differing effect of acrylic modifier on the melt elasticity and viscosity of these materials. J. Vinyl Addit. Technol. 10:121–128, 2004. © 2004 Society of Plastics Engineers.     

Die design for rigid PVC—the effect of die land length on extrudate swell

PVC profile extrusion compounds have a unique morphology. While other polymers gradually decrease in extrusion die swell with increasing length/thickness (L/D) ratio, PVC profile extrusion compounds have a low die swell, quite independent of the die's L/D ratio in the range of 5 to 20. The fact that the die land length can be changed without changing the extrudate swell is an important consideration, which makes die design and balancing dies simpler and easier for PVC profile extrusion compounds. While other polymers substantially increase extrudate swell with increased shear rate, the swell of the PVC profile compounds is not much affected by shear or extrusion rate. This unique behavior allows wider processing latitude in profile extrusion and faster extrusion rates than with other polymers. Another unique factor in the rheology of PVC profile extrusion compounds is that extrusion die swell increases with increasing melt temperature, while other polymers have decreasing die swell with increasing melt temperature. The unusual rheology of PVC profile extrusion compounds is attributed to its unique melt morphology, where the melt flow units are 1 um bundles and molecules that have low surface to surface interaction and entanglement at low processing temperatures but increased melting and increased entanglement at higher processing temperatures. Other polymers, unlike PVC, have melt flow at the molecular level.     

Foam extrusion characteristics of thermoplastic resin with fluorocarbon blowing agent. I. Low-density polyethylene foam extrusion

An experimental study was conducted to investigate the foam extrusion characteristics of low-density polyethylene resin. For the study, we used dichlorotetrafluoroethane and dichlorodifluoromethane as blowing agent and talc as nucleating agent. In the study, we investigated the effects of processing and material variables on the foam extrusion characteristics, namely extrudate swell behavior, foam density, and cell morphology. It was found that an inverse relationship exists between the extrudate swell ratio and the foam density. Also investigated was the effect of die geometry (theL/D ratio, D_R/D ratio, and entrance angle) on the foam extrusion characteristics of low-density polyethylene resin. Suggestions are made on the experimental technique that may be useful in selecting resins for foam extrusion operation. Also suggested are guidelines for selecting an optimum die geometry that would produce good quality foams of low-density polyethylene.     

Effect of molecular structure on extrudate swell behavior for different thermoplastic melts in an electro‐magnetized die

The extrudate swell ratio of five different thermoplastic melts flowing in a constant shear rate rheometer having a capillary die with and without application of magnetic field was studied. The effects of the magnetic flux direction and density, die temperature, and wall shear rate on the extrudate swell and flow properties were investigated. The experimental results suggested that an increasing wall shear rate increased the swelling ratio for the polystyrene (PS), LLDPE, and PVC melts, but the opposite effect was observed for the ABS and PC melts. The extrudate swell ratio for the PS, ABS, PC, and LLDPE melts decreased with increasing die temperature, the effect being reversed for the PVC melt. Thermoplastic melts having high benzene content in the side‐chain and exhibiting anisotropic character were apparently affected by the magnetic field, the extrudate swell ratio increasing with magnetic flux density. The effect of the magnetic field on the extrudate swell ratio decreased in the order of PS → ABS → PC. The extrudate swell ratio for the co‐parallel magnetic field system was slightly higher than that for the counter‐parallel magnetic field system at a high magnetic flux density. POLYM. ENG. SCI., 47:270–280, 2007. © 2007 Society of Plastics Engineers.     

Identification of rheological and structural characteristics of foamable poly(ethylene terephthalate) by reactive extrusion

The reactivity and efficiency of five low molecular weight multifunctional anhydride and epoxy compounds as chemical modifiers of a bottle grade poly(ethylene terephthalate) (PET) resin were evaluated by reactive extrusion under controlled conditions. The two dianhydrides and the three epoxy compounds were used at concentrations based on stoichiometry derived from the measured carboxyl and hydroxyl end group contents of the base resin. Measures of melt viscosity, melt strength, intrinsic viscosity and carboxyl group content were used as criteria of the extent of the modification. Correlations of die pressure with extrudate swell during extrusion, and melt flow index (MFI) with melt strength by off‐line testing of the extrudates permitted the ranking of the modifiers according to their chain‐extending/branching efficiency. For some systems molecular weight increases (related to die pressure/MFI/intrinsic viscosity) accompanied by broadening of the molecular weight distribution (related to die swell/melt strength) were considered excessive. Extrusion foaming experiments with one particular dianhydride modifier that increased the intrinsic viscosity of the resin from 0.71 to 0.9 dl g^?1 indicate that production of low‐density foams by a process involving one‐step reactive modification/gas injection foaming is feasible, at conditions not significantly different from those employed in the simple reactive modification of the PET resin. The rheological and structural parameters determined in this work may be used as criteria to specify PET foamable compositions in terms of types and concentrations of modifiers. Copyright © 2004 Society of Chemical Industry     

Foam extrusion characteristics of thermoplastic resin with fluorocarbon blowing agent. II. Polystyrene foam extrusion

The foam extrusion characteristics of three different grades of polystyrene resin were investigated. For the study, cylindrical dies with various values of length-to-diameter ratio, entrance angle, and reservior-to-capillary diameter ratio were used. Fluorocarbon blowing agents were used, and mixtures of citric acid and sodium bicarbonate were used as nucleating agent. It was found that the die temperature, shear rate, the type and concentration of blowing agent, and die geometry affect the quality of the extruded polystyrene foam. Foam density and open cell fraction were used in determining the quality of extruded foams. We have found that the extrudate swell ratio is correlatable to foam density, independent of the die temperature employed. However, the die temperature has been found to be a very sensitive processing variable governing the quality of extruded foams.     

Viscosity effects in rigid PVC

The viscosity of PVC is better understood if the material is treated as a fluid which contains filler. Anomalous effects such as die swell increasing with increasing melt temperature and melting history causing changes to the viscosity can be explained if the PVC primary particles are viewed as filler which disappears during melting. The fusion torque peak is well‐described by this approach. The compaction minimum is a free‐flowing powder which changes to a filler‐containing viscous liquid. If another viscous liquid is added to a PVC compound, then the fusion peak will be at a lower torque, because the effective level of filler is reduced. This reduction helps to explain the fusion curve of PVC compounds that contain chlorinated polyethylene (CPE) impact modifier.     

Coated ultrafine precipitated CaCO3 as impact modifier extender in low molecular weight PVC

Work reported earlier had shown that the addition of 0.07 micron stearate coated ultrafine precipitated CaCO₃ (CUPCC) in medium to high molecular weight PVC resulted in a significant improvement in notched Izod impact strength. The improvement was much more dramatic when the filler loading exceeded 10 phr. The addition of CUPCC to low molecular weight PVC had practically no effect on the notched Izod impact strength at equivalent loadings. More recent studies have demonstrated that when CUPCC is added to an impact modified low molecular weight PVC, it dramatically enhances the efficiency of impact modifier. This was found to be consistent with threee general classes of commercially available impact modifiers used in this study. The ability to extend the impact modifier with CUPCC in low molecular weight PVC, which has gained a wide acceptance in injection molding operations, will result in significant cost savings. The effect of CUPCC addition on other key properties and cost benefits will also be presented.     

Practical interpretation of laboratory die swell behavior of PVC extrudates—through correlating circular with non-circular die performance

Laboratory die swell measurements are routinely measured on many viscoelastic polymer extrudates to characterize thier ability to maintain a specified shape during extrusion operations. The diameters of downstream extrudates obtainable through selected round dies can then be routinely scanned by optical or laser type devices to compare the degree of swell relative to the die diameter itself. Such measurements, while yielding good relative die swell performance between compounds, frequently do not predict the actual die swell levels observed later when the same compound is extruded through production dies of different cross-sectional shapes and land lengths. This study discusses an alternate method of correlating die swell between dies of different shapes by using a technique derived from fluid mechanics. This concept, used to characterize fluid velocity through non-circular channels with that observed through circular pipes and ducts, involves the inclusion of a shape factors known as the “Hydraulic Radius” in fluid flow comparisons. When this technique was applied to die swell measurements for extrudates of both a flexible PVC wire jacket compound and a rigid PVC pipe compound, good agreement in actual die swell measurements through both round dies and dies with non-circular cross sections was obtained. This approach can lend credibility to laboratory die swell measurements and greatly expand their use in predicting production extrusion performance.     

Rheology,morphology, and mechanical and thermal properties of recycled PVC pipes

This article demonstrated the possibility of recycling PVC pipes by investigating the effect of adding PVC pipes (varying from 0 to 80 % wt) into two commercial PVC virgin grades on the rheological, morphological, mechanical, and thermal properties of the PVC blends. The results obtained showed an increase in the melt viscosity and no change in the die swell ratio as the concentration of the recycled PVC was increased. The die swell ratio was observed to increase with temperature, this being associated with the presence of gelation that occurred at high temperature. The optimum tensile and impact strengths were detected, the impact strength being explained by use of SEM micrographs of the fracture surface. The hardness result corresponded well to the density of the compounds. The glass transition, degradation, and heat-deflection temperatures were also found to shift with the recycled PVC loading. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2478–2486, 2001     

聚丙烯发泡塑料挤出胀大行为的实验研究

使用HAAKE挤出流变仪研究了聚丙烯发泡塑料挤出胀大行为，考察了毛细管数、螺杆转速和温度等参数与PP发泡塑料挤出胀大的关系，同时指出气泡的存在对发泡制品挤出胀大有较大影响，分析了气泡体积的大小等对PP发泡塑料的挤出胀大的影响。     

用毛细管流变仪研究发泡聚丙烯(PP)挤出胀大行为

利用毛细管流变仪研究发泡聚丙烯(PP)挤出胀大行为,观察了压力、温度等参数与膨胀率的关系,分析加入不同的AC发泡剂和CaCO3后其膨胀率不同的变化情况,指出这不仅仅是因为发泡体系弹性的变化,气泡的存在也对其膨胀率影响很大。     

Extrudate swell and texture of PS,LDPE, ABS,PVC melts and their blends in extrusion capillary flow using a magnetic die

The extrudate swell behavior and extrudate texture of various thermoplastic melts, namely, polystyrene (PS), low‐density polyethylene (LDPE), acrylonitrile‐butadiene styrene (ABS) copolymer, poly(vinyl chloride) (PVC), and their blends, were examined weith a magnetic die system in a constant‐shear‐rate capillary rheometer at a shear rate range 5–28 s^?1 and a temperature range 170–230 °C. The extrudate swell results obtained from the magnetic die were then compared with those produced by a nonmagnetic die. The results showed that the extrudate swell increased with shear rate, but decreased with temperature. In a pure polymer system, up to 25% increase in the extrudate swell was observed with the application of the magnetic field to the PS melt, and the effect decreased in the order ABS > LDPE > PVC. The extrudate swell changes were associated with the changes in rheological properties of the melts. The extrudate textures of the ABS and PVC melts were improved by the magnetic field. In PS/LDPE or PS/ABS blend, it was found that the magnetic die resulted in higher values of the extrudate swell than the nonmagnetic die for all blends, the magnetic effect being less as the LDPE or ABS content was increased. For PS/LDPE system, the extrudate swell of the PS melt did not change much with addition of 20% LDPE, but slightly decreased at the LDPE loading of 40%. At higher LDPE loadings, the extrudate swell increased towards the value of the pure LDPE melt. For PS/ABS system, the extrudate swell ratio progressively decreased with increasing ABS content. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 509–517, 2002     

Extrudate swell of high density polyethylene. Part II: Time dependency and effects of cooling and sagging

For two high density polyethylene resins, the isothermal time dependency of extrudate swell has been measured. Very minor differences in the large molecular weight part of the molecular weight distribution, hardly detectable with gel permeation chromatography and low angle laser light scattering techniques, dramatically influence the time dependency of extrudate swell as well as the maximum swell attainable. The presence of larger molecules in sample 802 than in 801 is reflected in a lower short time (after seconds) and a larger long time (after minutes) or maximum extrudate swell value. Extruding the polymers through a capillary die L:D = 30:2 mm into air at ambient temperature allows only the short time swelling behavior to be observed, because cooling and sagging of the strand.     

Blends of low density polyethylene/plasticized poly(vinyl chloride): The rheological,morphological, and mechanical characterization

An experimental study was conducted to investigate the rheological, morphological, and mechanical properties of a heterogeneous polymer blend system consisting of low density polyethylene (LDPE) and plasticized poly(vinyl chloride) (PVC). The components were mixed using a single-screw extruder, which was equipped with a special measuring head for the determination of rheological quantities. The morphology of blends was examined by scanning electron microscopy. Die swell was determined by photography. The velocity of ultrasound through the polymer melt was also measured. The dependencies of viscosity, die swell, and ultrasonic velocity on blend composition were qualitatively similar, exhibiting a minimum at about 70 wt % PVC. The morphology of the blend system at this blending ratio was different from morphologies of the other blends. Tensile properties of blends, except elongation at break, were not significantly inferior to those of the LDPE component.     

Quasi‐brittle to ductile transition in impact‐modified PVC

The toughness of impact‐modified poly(vinyl chloride) (PVC) compounds was examined by using a modified Charpy test. Increasing impact speed resulted in a quasi‐brittle to ductile transition in all PVC compounds. In the quasi‐brittle region, a PVC of 56,000 M_w fractured through a craze‐like damage zone that could be described by a modified Dugdale model. Furthermore, the same molecular‐weight PVC modified with either 10 pph (parts per hundred parts by weight) of chlorinated polyethylene (CPE) or 10 pph of methylmethacrylate‐butadiene‐styrene (MBS) impact modifier also conformed to the Dugdale model with the craze‐like damage zone. The CPE effectively improved the impact performance of PVC by shifting the quasi‐brittle to ductile transition to a higher loading rate. Compared to CPE, MBS was a better impact modifier, and its use resulted in a higher quasi‐brittle to ductile transition loading rate in the same PVC matrix. Fracture initiation toughness of all the materials was described by the Hayes‐Williams modification of the Dugdale model. The intrinsic brittle fracture energy obtained by extrapolation to zero craze length was determined only by the PVC matrix and was independent of the impact modifier. However, the kinetics of craze growth, and hence the response to rapid loading, depended on the impact modifier. Increasing the molecular weight of the PVC resin resulted in a more complex damage zone that was not amendable to the Dugdale analysis. J. Vinyl Addit. Technol. 10:11–16, 2004. © 2004 Society of Plastics Engineers.