Effect of boron nitride on melt fracture phenomena and processability of lldpe during extrusion

Melt fractures related to processing instabilities limit processing rates in many commercially important polymer processing operations, such as fiber spinning, film blowing, extrusion, and various coating flows. Therefore, melt fracture is responsible for deteriorating the quality and the mechanical properties of final products for rates greater than a critical processing one at which melt fracture occurs. In this study, a commercial linear low-density polyethylene (LLDPE) was modified by adding a small amount of boron nitride (BN) during extrusion in order to improve processability. Capillary rheometry was used to assess processability at various temperatures, levels of applied shear rate, and the length-to-diameter (L/D) ratio for both the pure resin and resins containing boron nitride. Also, parallel-plate rheometry was used to evaluate the dynamic rheological properties of these resins. The relationship between the characteristic relaxation time and the critical shear rate for the onset of melt fracture and slip is discussed.     

Effects of polymer structure on the onset of processing defects in linear low density polyethylenes

Linear low density polyethylenes are manufactured by copolymerizing ethylene with 1-alkenes, yielding a linear polyethylene backbone with short side chains. Due to the nature of the catalyst used in the polymerizaton, multimodal branching distributions are typically obtained. In this report, we have investigated the processability of four 1-octene linear low density polyethylenes as a function of the short chain branching distribution. Analytical techniques such as ¹³C nuclear resonance spectroscopy, size exclusion chromatography, differential scanning calorimetry, and temperature rising elution fractionation, in particular were used to elucidate the molecular structure. Processability measurements were made using various extrusion techniques and dynamic mechanical analyses.It was determined that in the absence of any variations in molecular weight, the polymers with the higher proportions of linear polyethylene showed inferior processability In terms of onset of surface imperfections at lower extrusion rates. Polymers with worse processability characteristics also exhibited higher zero shear viscosities.     

超高分子量聚乙烯的特性及应用进展

超高分子量聚乙烯性能卓越、加工困难,是一种正在迅速崛起的工程性热塑性塑料。由于加工困难．国内外超高分子量聚乙烯的应用多集中在压制产品上,但是材料学家们从来没有停止过对超高分子量聚乙烯挤出制品的探讨。超高分子量聚乙烯的卓越性能源自于它具有极高的分子量,因此对超高分子量聚乙烯改性成功与否的判定在很大程度上取决于其制品的分子量保留的程度和在低温下的冲击韧性。作者利用新的挤出理念,精确的配方和精湛的工艺成功的挤出了分子量在250万以上的超高分子量聚乙烯管材制品,并对超高分子量聚乙烯的纤维、膜制品的应用进行了概要的介绍。     

Influence of poly(ethylene glycol)‐containing additives on extrusion of ultrahigh molecular weight polyethylene/polypropylene blend

The influence of poly(ethylene glycol) (PEG)‐containing additives on the extrusion behavior of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blend was studied. It was found that the addition of small amounts of PEG to UHMWPE/PP blend resulted in significant reduction of die pressure and melt viscosity, and obvious increase of the flow rate at a given die pressure, while PEG/diatomite binary additives enhanced the improvement in the processability of UHMWPE/PP blend. When pure HDPE was extruded with the die through which UHMWPE/PP/PEG blend was previously extruded, the extrusion pressure of HDPE increased with the extrusion time gradually. This meant that PEG might migrate to the die wall surface and coat it in the extrusion of UHMWPE/PP/PEG blend. FTIR spectra and SEM micrographs of the UHMWPE/PP/PEG extrudates indicated that PEG located not only at the surface but also in the interior of the extrudates. So, the external lubrication at the die wall, combined with the internal lubrication to induce interphase slippage of the blend, was proposed to be responsible for the reduction of die pressure and viscosity. In addition, an ultrahigh molecular weight polysiloxane and a fluoropolymer processing aid were used as processing aids in the extrusion of UHMWPE/PP as control, and the results showed that only minor reduction effects in die pressure and melt viscosity were achieved at their suggested loading level. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1282–1288, 2006     

后过渡金属催化剂催化乙烯制备超支化聚乙烯新进展

后过渡金属催化剂的出现改写了超支化聚烯烃的合成历史,其不仅可以催化乙烯聚合成超支化聚乙烯,还可以实现对聚乙烯产品拓扑结构的控制以及相对分子质量的裁剪。本文沿着镍系以及钯系两大类后过渡金属催化剂的路线综述了近年来合成超支化聚乙烯的研究进展,重点对控制聚乙烯拓扑结构的方法及影响因素进行了详细的综述,并对超支化聚乙烯在高分子加工助剂及润滑油改进剂等领域的应用现状进行了介绍,这为进一步研究超支化聚乙烯在材料加工助剂等方面的应用提供了新的思路。     

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.     

Molecular,rheological, and crystalline properties of low‐density polyethylene in blown film extrusion

The molecular weight and its distribution, degree of long chain branching and cooling rate strongly influence crystallinity during processing, which in turn determines the processability and the ultimate properties of the blown film. Generally a decrease in the number of branches and molecular weight of the polymer and the cooling rate results in an increase of the crystallinity. Length of the main chain and extent of branching in low‐density polyethylene (LDPE) are also factors that affect melt rheology and film crystallinity. Long chain branched polyethylene is suitable in the blown film process due to its better melt strength for bubble stability. The objective of this article is to describe the effect of molecular properties (e.g. molecular weight and its distribution, degree of long chain branching etc) of LDPE on film crystallinity at different cooling rates of blown film extrusion. Two different grades of LDPE were selected to investigate molecular characteristics, crystallinity, and rheology. The resins were processed in a blown film extrusion pilot plant using four different cooling rates. Molecular, rheological, and crystalline properties of the resins were key parameters considered in this study. POLYM. ENG. SCI., 47:1983–1991, 2007. © 2007 Society of Plastics Engineers     

On the processability of metallocene-catalysed polyethylene: effects of blending with ethylene-vinyl acetate copolymer

The rheological properties of metallocene-catalysed linear polyethylenes and those of blends prepared with ethylene-vinyl acetate copolymers were evaluated. The pure polyethylenes showed characteristic features of linear polymers in the melt state, but poor processability, as would be expected for materials of narrow molecular weight distribution. The characteristic sharkskin and slip-stick regimes appear at around 0.16 and 0.35 MPa, respectively, during extrusion. Blending polyethylene with ethylene/vinyl acetate copolymers gave rise to smooth extrusion for a characteristic blend composition. The linear viscoelastic response of the blends revealed the behaviour of heterogeneous emulsion-like polymer systems. Through the application of several rheological criteria, we were able to locate the phase inversion concentration of the system. This concentration was found to closely correspond to that at which distortion regimes disappear during extrusion.     

Improvement of processing parameters by using processing aids during the linear low‐density polyethylene film blowing process

Polymer processing aids are used to improve processing properties in the polyethylene industry. These materials improve not only the physical and mechanical properties of the final products but also their processing properties. This paper studies some of the processing variables such as die pressure, melt temperature, masterbatch activity, and die gap by examining the functions of polymer processing aids and, last but not least, the effects on the film blowing process of two‐component processing aids containing a perfluorinated additive and polyoxyethylene. J. VINYL ADDIT. TECHNOL., 2010. © 2009 Society of Plastics Engineers     

分子量200万以上UHMWPE双抗挤出制品的开发

用分子量380万的超高分子量聚乙烯(UHMWPE)树脂为基体树脂,加入高含量导电炭黑/聚乙烯蜡复合导电剂,溴-锑系与膨胀型阻燃剂组成的复配阻燃荆及常规挤出加工助剂,利用近熔点挤出理念及其加工设备,实现了分种子量200万以上UHMWPE管材、板材制品的连续挤出.制品综合性能超过了2005年国家煤炭行业的技术标准,对提高行业技术水平、解决现实煤炭生产中的安全隐患、促进UHMWPE上下游产业的发展做出了贡献.     

Compatibilized blends of polyamide-6 and polyethylene

An increasing number of polymer alloys made by reactive processing of a compatibilizer precursor (CP) to form the compatibilizer in situ have appeared in recent years. Literature reports on compatibilization studies have focused on interactions of chemistry-morphology, processing-morphology, morphology-rheology, morphology-properties, etc. In commercial alloys, the interaction of chemistry and processing results in a material that has a balance of physical properties and rheological behavior. To maximize the contribution of each phase, it is of interest to be near the phase inversion region. In this work, maleic anhydride functionalized polyethylene is used as an effective CP for polyamide/polyethylene blends. In blends containing 50% nylon 6, the melt viscosity of the alloy increases exponentially as the ratio of CP to polyethylene increases though the morphology remains dispersed polyamide 6. High ratios of CP/PE are desired for toughness, alloys containing lesser amounts of CP exhibit better processability. The balance of toughness and processability is shown to be affected by the molecular weight of the CP.     

Boron nitride and fluoropolymer combinations: Interactions and their performance as processing aids

In this work, we studied the adsorption capacity of boron nitride (BN) for fluoropolymer and polyethylene (PE) to gain a better understanding of the interactions and the performance of BN and fluoropolymer, and their combinations as processing aids in the extrusion of Ziegler‐Natta PEs. We found that BN has a relatively high adsorption capacity for both PE and fluoropolymer. As a result, simultaneous compounding of BN and fluoropolymer into the host polymer causes fluoropolymer to be trapped within the bulk of the polymer, and prevents the fluoropolymer particles from coating the die wall during flow. This limits the effectiveness of fluoropolymer and BN as processing aids. To avoid this interaction, we added fluoropolymer separately in a dry form just prior to extrusion. In this case, the synergistic effect of BN and fluoropolymer as a combination processing aid was evident. POLYM. ENG. SCI., 45:669–677, 2005. © 2005 Society of Plastics Engineers     

Optimisation of silane grafting in single screw extruder

Abstract

Reactive extrusion is an attractive means of polymer processing since the shaping and reaction takes place in a single operation. Silane grafting of low density polyethylene has been achieved in a single screw extruder. The optimum conditions for silane grafting, i.e. temperature, shear rate, and silane and dicumyl peroxide concentrations, were determined on a torque rheometer and extrusion was then performed under optimum conditions. The study shows that an optimum low level of grafting/crosslinking can be introduced into polyethylene during extrusion for better mechanical behaviour and/or thermal stability without aecting the processability.     

An investigation of acrylic processing aids for low molecular weight PVC

Acrylic processing aids have long been used in rigid PVC to increase fusion, decrease jetting and blushing down in blow molding, decrease parison draw down in blow molding, improve the rolling bank in calendering, improve thermoforming, and improve cell structure in foam extrusion. It has generally been thought that the specific viscosity (Nsp/c) of process aids were limited to a fairly limited narrow range. Recently, significantly higher molecular weight process aids have been developed and commercialized. Very little work though has been done with low molecular weight process aids as these products were assumed to be ineffective when compared to the commonly used products. This paper will investigate currently available products and some experimental products well above and below the Nsp/c of currently available products. The major area of investigation will be in very low to low molecular weight PVC resins and will deal with the applicability of these products in injection molding.     

New ultra high molecular weight acrylic processing aids for rigid PVC foaming

Foaming of rigid polyvinyl chloride (PVC) is studied as a function of high molecular weight acrylic processing aids. The industrial process to evaluate quality of foam is discussed in detail. The role of acrylic processing aids to improve melt strength and hence foaming of PVC is explained. It is demonstrated that increase in molecular weight of acrylic processing aids increases its effectiveness. It is found that ultra-high molecular weight processing aids is 25%–30% more efficient than relatively lower, but still high, molecular weight acrylic processing aids. The higher molecular weight processing aids provided comparable foaming performance at lower loading levels. Foaming reduced the density of PVC compounds to 0.32–0.34 g/cm³. More than 1000% expansion is achieved in the melt extrusion process using a chemical blowing agent. Fusion characteristics are also studied. Fusion times for initial fusion peaks are in the range of 42–44 s while the fusion times of the second fusion peaks are in the range of 74–94 s. The higher molecular weight processing aids maintained fusion characteristics of PVC compounds, warranting no significant changes in commercial process.     

Unusual rheology behaviour of ultra high molecular weight polyethylene/kaolin composites prepared via polymerization‐filling

The rheological behaviour of ultra high molecular weight polyethylene (UHMWPE)/Kaolin composites prepared by a polymerization‐filling process was investigated by conducting capillary and dynamic rheology tests. The results showed that the addition of Kaolin could significantly improve the processability of UHMWPE composites. The common viscoelastic flow was discovered for UHMWPE composites in capillary extrusion testing. The further discussion showed that the unique microstructure provided by polymerization‐filling of UHMWPE/Kaolin composites led to this unusual rheology behaviour. Copyright © 2003 Society of Chemical Industry     

超高分子量聚乙烯气辅挤出影响因素的数值模拟及分析

以超高分子量聚乙烯的圆形轴对称气辅口模挤出为研究对象,在采用Polyflow软件对气辅口模挤出时的等温流动进行数值模拟之后,就入口流率、松弛时间以及零剪切黏度等物性和工艺参数对挤出胀大、速度分布、口模压降和熔体外表面上剪切速率的影响进行了数值模拟和分析。分析表明:气辅挤出是克服超高分子量聚乙烯传统挤出时面临一系列困难的有效加工方式。     

Structure and properties of ultrahigh molecular weight polyethylene processed under a consecutive elongational flow

Herein, a novel eccentric rotor extruder (ERE) capable of generating a continuous elongational flow was used to process ultrahigh molecular weight polyethylene (UHMWPE) without any processing aids and then compare with a conventional rotational batch mixer based on a shear flow. The morphological and rheological characterization verify that the technique based on the elongational flow could effectively reduce melting defects and yield more homogeneous morphology within the extruding samples relative to the conventional bath mixing based on a shear flow. The extrusion processing under an elongational flow can largely maintain the viscosity average molecular weight (M_η) of the UHMWPE nascent powder with only a 5.0% decrease at 200 °C, implying considerably low thermal oxidative degradation in sharp contrast to the conventional processing with significantly reduced M_η by 40.3%. Furthermore, the crystallinity for the sample prepared under an elongational flow is lower than that processed under a shear flow. These differences lies in the higher normal stress, rapider heat transfer and shorter duration generated by the ERE.     

Rheology modifier additive for enhanced processability of polyethylene for blown-Film applications

Polyethylene is a versatile polymer suitable for a large variety of flexible and rigid packaging applications. Its mechanical and rheological properties can be tuned across a wide range by controlling its molecular architecture, such as the amount and distribution of olefinic comonomers (short chain branching), long chain branching, and molecular weight distribution. Linear low-density polyethylene (LLDPE) is known for its high toughness which enables downgauged film structures and low-density polyethylene (LDPE) is known for its excellent shear thinning and melt strength which enables enhanced processability and high throughput, such as on blown film lines. In order to obtain a balance of toughness and processability on films produced on blown film lines, blends of LLDPE and LDPE are commonly used. In this paper, we describe additive-based approaches, including a new product, DOWLEX™ (TM = trademark of the Dow Chemical Company (“Dow”) or an affiliated company of Dow) GM AX01, which enhances melt strength and other rheological properties of polyethylene, enabling fabrication of films with lower LDPE content while still maintaining excellent rheological properties and higher toughness versus conventional LLDPE/LDPE blends. The higher toughness enables downgauging without loss of mechanical properties, which in turn reduces consumption of polymer resulting in a more sustainable solution.     

Influence of low density polyethylene quality on extrusion coating processability

In order to obtain explicit information about the influence of different low density polyethylene (LDPE) quality parameters on extrusion coating processability, a test run was made with an autoclave reactor and the products were investigated. All the grades manufactured had melt indices (MI), densities, molecular weight distributions (MWD), and degrees of long chain branching(LCB) typical of commercial extrusion coating grades. The processability characteristics studied were maximum line speed and neck-in. The influence of MI, density, and extrusion melt temperature were systematically investigated. It was found that the maximum line speed rose with increasing MI, density, and extrusion melt temperature, and that an increasing extrusion melt temperature led to a growing difference between the maximum line speed at a constant coating thickness and the maximum line speed at a constant screw speed. Neck-in was found to increase with increasing MI, increasing density, and increasing coating thickness. These effects were more pronounced at higher extrusion melt temperatures. When using the extrusion temperature needed to achieve a certain line speed for each grade, the influence of MI on neck-in was practically non-existent.