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Applied Rheology

SPE Library content related to rheology
New Boron Nitride Processing Aids for the Extrusion of Molten Polymer
Franky Yip, Savvas G. Hatzikiriakos, Thomas M. Clere, May 2000

It is already known that the use of Boron Nitride (BN) in the extrusion of molten polymers may eliminate surface melt fracture and postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type and additive content. In this work several new Boron Nitride powders (Carborandum Co.) are tested that exhibit superior behavior from the previous studied ones. Critical parameters for this unique behavior (elimination of gross melt fracture) are good dispersion, small average particle size and free of agglomeration. The equipment used for the testing of the new powders include both an Instron capillary rheometer with special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and a parallel-plate rheometer. A metallocene polyethylene with all types of boron nitride is tested at various additive concentrations. The additive having the finest particle size and that is free of agglomeration was found to have the greatest influence on the gross melt fracture performance of the polymer tested in crosshead dies and tips. Moreover, one of the additives was found to enhance melt slippage and as a result relieved internal shear stresses. This action is believed to eliminate surface melt fracture and postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on the additive concentration. The influence of the boron nitride type and its concentration on the polymer rheology is also discussed.

Properties of an Oil Resistant TPV
S. Cook, J. Patel, A.J. Tinker, May 2000

A TPV based on a dynamically vulcanized blend of epoxidized natural rubber and polypropylene is described. Morphological and rheological properties are briefly reviewed. Basic physical properties compare well with those of other TPVs whilst oil resistance is comparable to that of a well-compounded NBR vulcanizate (34% acrylonitrile). Excellent heat resistance with good retention of properties on ageing for extended periods at 100° and 150°C is also demonstrated as is good weathering and ozone resistance.

Rheology of TPV’S
Paul Steeman, Wim Zoetelief, May 2000

The rheological properties of various TPV's have been studied in shear flow using dynamical mechanical spectrometry, capillary rheometry, transient stress buildup and shear creep. The TPV's which are commercial dynamically vulcanised PP/EPDM blends show a typical rheological behaviour with an apparent yield stress value at low shear rates, a shear-thinning viscosity at high(er) shear rates and, moreover, they do not obey the Cox-Merz law. This study revealed that the observed phenomena could be explained by the role of the three main components: PP, EPDM and oil.

On-Line Rheological Measurements
John M. Dealy, May 2000

Rheometers are being used increasingly as sensors for process control. This is because of the sensitivity of rheological properties to polymer characteristics such as molecular weight. The process rheometers now being used are mostly of the on-line, pressure flow type, in which a gear pump feeds a capillary or slit, and the pressure drop is measured. For process control applications, the signal delay associated with the flow of melt from the main flow to the capillary or slit slows the system response, and this problem has been addressed by the use of a larger sampling line together with a bypass around the rheometer. Other recent advances are aimed at providing a more extensive characterization of the melt.

Polymer Rheology-Challenges for the 21st Century
John M. Dealy, May 2000

Rheological data are used in the plastics industry for polymer characterization, quality control, process control, and process modeling. In each of these fields, there remain significant barriers to progress. The challenges in polymer characterization are to develop experimental techniques for the rapid and precise measurement of linear viscoelastic properties and for evaluating the response of a melt to extensional deformations. A powerful aid in keeping a manufacturing process under statistical control is the use of an on-line rheometer as a process sensor for automatic control. A remaining challenge is the development of an in-line melt rheometer that would eliminate the need for gear pumps and reduce the time delay resulting from the transit of melt from the sampling point to the rheometer. While computational fluid dynamics has been applied with success to the simulation of several melt forming processes, major problems remain when melt elasticity plays an important role. In addition, present simulation techniques do not incorporate models for gross melt fracture, sharkskin and wall slip.

Influence of Chemical Composition on the Rheological Behaviour of Condensation Reaction Resins
Marc Doyle, Jan-Anders E. Månson, May 2000

A study of Melamine Formaldehyde (MF) resins of different molar ratios was undertaken to asses their chemorheological and thermorheological properties. MF resins polymerise via a polycondensation reaction involving condesation of up to 10 wt. % of water on cure. This results in rapid and extensive foaming of the resin when cured under atmospheric pressure. To accomplish the study, both foaming dynamics and bulk properties of MF were analysed; the former yielded results on the influence of initial moisture content on reaction rate and on curing kinetics including gel and vitrification times, while the latter yielded true G' and G' values and consistent glass transition temperature readings. The different molar ratios were found to affect curing kinetics in terms of large differences in gelation and vitrification times, and to yield high glass transition temperatures.

The Effects of Comonomer Type on the Blown Film Performance of LLDPE Resins Made Using a Metallocene Single-Site Catalyst
Ashish M. Sukhadia, M. Bruce Welch, Raj K. Krishnaswamy, Syriac J. Palackal, May 2000

In this paper, we report on the effects of comonomer type viz. 1-butene, 1-hexene and 1-octene, on the blown film performance of linear low density polyethylene (LLDPE) resins made using a metallocene single-site catalyst. The effect of film thickness on blown film properties was also examined. The resins were characterized in detail with respect to their rheological, thermal and molecular characteristics. It was established that these three copolymers, despite the great similarities in their molecular, rheological and thermal properties, exhibited blown film performance that clearly increased with increasing length of the ?-olefin employed.

The Effect of Molecular Structure on the Extensional Melt Rheology of Conventional and Metallocene Polyethylenes
Choon K. Chai, May 2000

Extensional melt rheology and processing characteristics of conventional high pressure low density polyethylene (LDPE) and Ziegler-Natta linear low density polyethylenes (LLDPE) are compared with both narrow and broad molecular weight distribution (MWD), long chain branched (LCB) metallocene polyethylenes. The effects of MWD and LCB on the melt behaviour of these different types of polymers will be presented in terms of their dynamic linear viscosities and their strain-hardening behaviour from transient tensile stress growth experiments. Film processability properties will also be discussed.

Rheological and Blown Film Characterization of Binary Blends of Metallocene-Catalyzed LLDPEs
Rajendra K. Krishnaswamy, David C. Rohlfing, Bryan E. Hauger, May 2000

The advent of metallocene catalyst technology has created several opportunities in the worldwide packaging film marketplace. In this work, we explore the effects of blending two metallocene-catalyzed LLDPEs of reasonably differing molecular weights. Specifically, the shear rheology, quiescent and shear-induced crystallization characteristics, and blown film performance were investigated as a function of blend composition. The presence of small amounts of longer molecules was found to have significant effects on the shear-induced crystallization kinetics and blown film orientation of the blends; this was explained in terms of an increasing number of extended chain nuclei for crystallization with increasing molecular weight. Lastly, the biaxial orientation features in the crystalline and non-crystalline phases of the blend blown films were also characterized and used in explaining the Elmendorf tear and dart impact performance of the blown films.

Viscoplastic Analysis of Hot Embossing in Microfabrication
Yi-Je Juang, L. James Lee, Kurt W. Koelling, May 2000

In MEMS (micro electromechanical system), the hot embossing process has been mentioned as one of the major fabrication techniques, which is capable of making polymer microstructures for both low volume prototyping and high volume production. This is due to its simple process, relatively low tooling cost, high replication accuracy, and relatively high throughput. In order to minimize the process cycle time, the embossing temperature is set slightly above the glass transition temperature (Tg), while the de-embossing temperature is slightly below Tg. Since the polymer is deformed near Tg, its flow behavior during molding is substantially different from that at high temperature processes such as injection molding and extrusion. Also, the residual stresses resulting from the thermal cycle and compression force will profoundly influence the replication accuracy and optical properties of the molded parts. In order to establish the relationship between the residual stresses, replication accuracy, rheological behavior, and processing variables, we carried out a viscoplastic analysis and a series of hot embossing experiments. Two optical polymers, polyvinyl butyral (PVB) and polycarbonate (PC) were used in this study. The rheological property was characterized through both dynamic and transient shear viscosity measurements using the RMS-800 and a tensile test. Optical and SEM photos of replicates were taken at different processing conditions to determine the replication accuracy and birefringence pattern.

Rheology and Degradation Kinetics of Poly(ethylene terephthalate)/Poly(ethylene naphthalate) Blends
S.R. Tharmapuram, S.A. Jabarin, May 2000

Blends and copolyesters of poly(ethylene terephthalate)/poly(ethylene naphthalate), PET/PEN, have shown promise in high performance container applications. Both rheology and degradation kinetics of these blends have been studied as a function of material composition. Melt viscosity loss was measured as a function of time and temperature. Activation energies for degradation were calculated from experimental data. Results show that blends containing a minimum of 10% PEN by weight are as stable as PEN. Addition of low amounts of PEN to PET causes a depression in melt viscosity. A critical composition of 10% PEN by weight is required before we observe an increase in blend viscosity.

Dependence of Draw Resonance on Extensional Rheological Properties of LLDPE
Michael J. Bortner, Donald G. Baird, May 2000

The effect of extensional rheology on draw resonance during fiber melt spinning is studied. Two linear low-density polyethylenes(PE's) synthesized using metallocene catalysts are compared. The LLDPE's appear to have different amounts of long chain branching, resulting in differences in extensional rheology. However, both materials behave almost identically in shear. During melt spinning, the PE's exhibit different critical draw ratios at which the onset of draw resonance occurs. Furthermore, the period of resonance changes with increasing draw down ratio for both PE's.

Apparent Viscosity Measurements for Determining Injection Molding Dimensional Variations
Gary L. Freiberg, May 2000

Injection molders presently use the Apparent Viscosity Curve" to determine the optimum fill time for a particular mold. Once the fill time has been determined the Viscosity Curve is set aside and the goal of the molder is to maintain the fill time. The motivation for my work is not only to determine fill time but also to quantify the melt viscosity using an injection molding machine/mold combination. The ability to determine the melt viscosity would assist the molder with "root cause" analysis when evaluating small dimensional shifts. The study will focus on determining if the derived viscosity using traditional rheological equations is of value when compared to dimensional or cosmetic changes."

Processing and Properties of Polymer Nano-Composites
Hua Wang, Mark Elkovitch, L. James Lee, Kurt W. Koelling, May 2000

Polymer nano-composites are prepared by melt intercalation in this study. Nano-clay is mixed with either a polymer or polymer blends by twin screw extrusion. The clay-spacing in the composites is measured by X-ray Diffraction (XRD). The morphology of the composites and its development during the extrusion process are observed by SEM. Rheological behavior of the composites are measured. It is found that the clay spacing and composite morphology are influenced by the concentration of the nano-clay and the type of polymer used. The addition of the nano-clay can greatly increase the viscosity of the polymer when there is strong interaction. If such polymer/nano-clay mixture is used as the matrix phase, it would lead to improved dispersion and distribution of the minor phase in the polymer blends. The effect of nano-clay on polymers and polymer blends is also compared with Kaolin clay under the same experimental conditions.

The Effect of Tie-Layers on Optical Properties of Coextruded Multilayer Films
Dean A. Zimmerman, May 2000

The tie-layer is a critical component of multilayer films and can effect the optical properties of the overall film structure. This paper evaluates various tie-layers for their effect on optical properties of the film. The see-through and contact clarity of the film is explained based on a combination of the physical properties of the tie-layer resin and the effect of the interfacial region on flow instability. Rheology of the interfacial region is discussed based on controlled experiments simulating the reaction that occurs during multilayer extrusion and shows that the shear stress and complex viscosity can be used to estimate the see-through clarity in multilayer coextruded films.

Flow Behavior of Sparsely Branched Metallocene-Catalyzed Polyethylenes
Phillip J. Doerpinghaus, Sujan E. Bin Wadud, Donald G. Baird, May 2000

The dramatic effect of chain architecture on the melt flow behavior of polyethylenes is well established. The advent of metallocene and other constrained-geometry catalysts now allows the precise control of molecular weight, branch content, and branch distribution. In the present study, the rheological responses of three different metallocene-catalyzed polyethylenes are analyzed. Despite similarities in their shear properties, the differences in transient extensional viscosities can be attributed to the presence of sparse long-chain branches and/or high molecular weight fractions. Melt fracture observations as well as full-field flow birefringence measurements are collected to further corroborate this idea.

Evidence for a Master Curve Behavior for Polypropylene (PP) Degradation and Durability
Samuel Ding, Craig L. Sandford, Michael T.K. Ling, Atul Khare, Lecon Woo, May 2000

We have studied material degradation and durability in a broad based program in selecting medical materials. In addition, the specific requirement that medical devices and packaging must have specified shelf-lives supported by real time data also prompted for quantitative evaluation. A large number of PP types and samples were studied under diverse conditions. These included high temperature oxidative stability by oxidation induction time (OIT), high oxygen pressure OIT, oven aging, aging and oxidative stability after ionizing radiation and real time ambient storage. The activation energies from the surface embrittlement processes were also found to have a striking similarity to the thermal processes. To overcome the difficulties in generating long term ( greater than 10 years) data, well characterized historic samples of up to 23 years in age were included in this study. When all data were combined, a striking feature was apparen very few data points at higher temperatures could allow a reasonable determination of the shift factor and quite accurate estimates for lower temperature durability. This methodologyt: that for PP, a self-similarity existed among all systems examined. This allows a simple vertical shift of data to construct a master curve" similar to rheological master curves. The implication being if the master curve can be constructed when supported by further studies could lead to broader applications and deeper understandings on polymer degradation"

Investigation of Rotational Moulding Characteristics and Mechanical Properties of Metallocene Polyethylene
X. Wang, R.J. Crawford, A.M. Fatnes, E. Harkin-Jones, May 2000

Metallocene grades of polyethylene offer exciting opportunities for the rotational moulding industry. However, as they have only become commercially available in the last few years, their full potential has yet to be exploited. This paper reports on the results of an experimental investigation in which processing conditions are related to the microstructure and mechanical properties of three grades of metallocene polyethylene. The materials have been characterised by dynamic parallel plate rheological tests, NMR spectra and DMTA. The results obtained from equivalent standard grades of polyethylene are used as a basis for comparison. It is shown that the metallocene materials possess many rheological and physical properties that are desirable in rotational moulding. A detailed understanding of the unique nature of these materials is necessary in order to take full advantage of their properties in rotational moulding.

Structure-Property Relationships for Propylene/?-Olefin Copolymers: Effect of Short Branch Length
Catherine Jung Hyun Cho, João B.P. Soares, Costas Tzoganakis, May 2000

A single active-site-type catalyst was supported onto silica and used to produce propylene/ a-olefin copolymers with high isotacticity and high molecular weight. The effects of different reaction conditions such as temperature, pressure and hydrogen content on structural properties were investigated. Several comonomers with varying length and bulkiness such as hexene, decene, octene, dodecene, hexadecene, eicosene, and styrene were copolymerized with propylene. With the knowledge of the reactivity ratios and the effect of different reaction parameters on polymer microstructure, copolymers with the same level of comonomer content and similar molecular weight were made with different comonomer types. The produced copolymers were analyzed for their structural and rheological properties using GPC, DSC, CRYSTAF, NMR and RMS. Thus, the isolated effects of comonomer type (length and bulkiness) on structural and rheological properties were studied and correlated.

Sharksin Melt Fracture in High-Performance Hexene-LLDPEs
Harry Mavridis, Ramesh Shroff, May 2000

Several types of High-Performance Hexene (HPH)-LLDPE have been introduced in recent years for high-strength blown film applications, with substantial enhancement of film properties over conventional Ziegler-Natta LLDPEs. The present work is a study of the comparative behavior of the various types of LLDPEs under processing conditions designed to induce sharkskin melt fracture (SSMF). Both capillary rheology and blown film studies were conducted. The ability of capillary rheology to capture the difference between resins in terms of their relative tendency to sharkskin melt fracture was investigated in parallel with blown film studies at different shear rates and die gaps. Film blowing is more effective in discriminating between resins and identifying melt fracture tendency. The influence of sharkskin melt fracture on the film properties was also quantified, showing the film impact strength to be affected most sensitively, and negatively, by the presence and severity of sharkskin melt fracture (SSMF).

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