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AN009Application of Rheology of Polymers

revised by A. Franck, TA Instruments GermanyKeywords: Polymers, processing, Performance, MFR, rheometer

INTRODUCTION TO THE APPLICATIONOF POLYMER RHEOLOGY

Polymers are viscoelastic fluids, which behaveviscous or elastic, depending on how fast they flowor are deformed in the process. The silicone puttytest is a good demonstration of this pheno-menon(figure 1). If a ball of silicone is subjected to a rapiddeformation, for example bouncing on a table, thesilicone ball behaves like an elastic body. It is therecoverable elastic deformation, which stores theenergy and makes the ball bounce on the table. Ifthe silicone ball is at rest for a longer period of time,the gravity causes the material to flow - thus makingit behave like a viscous fluid. Whether a materialbehaves elastic or viscous depends on the time scaleof the deformation, or in other words on the ratio ofprocess time and material time, referred to asDeborah or Weissenberg number. If the material timeis short in relation to the process time, the materialshows predominantly viscous behavior. If theWeissenberg number equals unity or is larger, theelastic behavior of the fluid increases and becomesdominant.

The viscosity is the prime material parameter inmany processes. A material which did process easily

on a film blowing line twenty years ago may notrun at all on today’s lines, which operate at muchhigher draw speed. The process time has becomemuch shorter and the material under the newconditions is behaving more elastic, whichconsequently has negative effects on its pro-cessiblity. The same is true for the flow in the moldduring the injection molding process. Increasedelasticity causes more frozen-in orientation duringthe rapid cooling.

Barrel & piston

Die & pressure transducer

Fig. 2a In a capillary rheometer the material is forcedthrough a slit or round die. From the pressure drop and thevolume flow rate, a steady state viscosity as a function ofthe applied rate (piston speed) or stress (applied pressure)is measured.

torque & normal force transducer

Test geometry and air oven

Rotary actuator

Fig. 2 b In a rotational rheometer the sample is shearedbetween two plates or a cone and plate geometry. Theviscosity is calculated as the ratio of the applied stress andthe applied deformation rate (rotation speed). The rotationalrheometer, in contrast to the capil-lary rheometer,measures time dependent material behavior also.

t

Fig. 1 The silicone rubber behaves predo-minantly elasticif deformed for a short period of time and predominantlyviscous if subjected to gravity over a long time period.

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In order to guarantee a smooth operation processit is a must to control the rheology of the materialwithin the specified range. The structure–rheologyrelationship is a key to the development of polymerswith the correct rheology for processing. Theultimate goal of the design engineer is to modify

DEVELOPMENT AND PROCESSCONTROL

It is well known that materials of the same grade,but from different batches, may process quitedifferently. The trans-formation process reacts verysensitively to small variations of the material, i.e.the rheology of the polymer melt is very sensitiveto small changes of the polymer structure. Becauseof the sensitivity of rheology of the condensed phaseto structure, rheology is a most convenient methodto characterize polymers. A small amount of a highmolecular weight polymer can change the pro-cessing behavior dramatically and so does the meltrheology.

The important structure parameters defining the

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Fig. 3 The MFR is a standardized die with a barrel topreheat the sample (ASTM D1238 and DIN 53 735) Thematerial is forced through the die by applying a weight of2.14 or 21.4 kg. The measured quantity is the weight of thematerial flowing through the die within a given time period.The MFR is not only a measurement of the viscous effectsin the die, but incudes also contributions of the die inlet andexit effects.

Fig. 4 On-line rheometers are sensors inserted into theprocess line to monitor key parameters in order to allowcontrolling product uniformity and adherence to the desiredspecifications. On-line rheometers operate as return tostream or return to waste sensors. Depending on the diegeometry, on-line rheometers can provide a real timemeasurement of MFR or shear viscosity as a function ofrate. Equipped with an optical window to do NIR, UV etc.measurements the on-line rheometer can measure alsoadditive levels, degradation, color, etc.

the material structure to give better processingperformance without sacrificing the performance ofthe final product (impact resistance, wrappage,etc…). Due to its sen-sitivity to material’s structurechanges, rheology is also the desired technique tocontrol the process. Product quality can be insuredand down-time of the production line reduced aswell as the quantity of off-spec material.

STRUCTURE–RHEOLOGY RELATION-SHIP, THE KEY TO PRODUCT

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Fig. 5 Dynamic mechanical testing involves the applicationof an oscillatory strain to a material and to measure theresultant stress. The phase shift and the ratio of stressamplitude and strain amplitude allow the calculation of thedynamic moduli, the storage and loss modulus as well asthe complex viscosity as a funtion of frequency.

rheology of the polymer melts are molecular weight(MW), molecular weight distribution (MWD) andbranching /1/. Whereas an increase of the molecularweight causes the viscosity to increase, changing

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Fig. 6 The frequency sweep provides a characteristicfingerprint of the material. At higher frequency, G’ shows aplateau value referred to as plateau modulus. The plateauvalue relates to the average molecular weight betweenentanglements Me. At low frequency the loss modulus is ameasure of the materials viscosity (G”/w) and the ratio G’/G”2 of the elasticity.

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of the molecular weight distribution and branchingmainly affects the elasticity of the melt. The timedependence is affected by both. Viscosity measu-rements are performed with capillary and rotationalrheometers. The two methods are the backbone ofthe polymer rheological charac-terization (figures2a and 2b). The viscosity of a polymer melt varieswith the weight average molecular weight Mw tothe power of ~3.4. In polymer processing the MFRis used to characterize a polymer melt. Whereas theMFR (figure 3) is a very common measurement, itis not a pure measurement of viscosity, however.Off-spec material can be easily blended to make amaterial with the correct MFR, but the viscosity andthe elasticity of the blended material can be quitedifferent, which becomes evident in the Pro-cessability. Despite this fact, the MFR is thepreferred parameter to characterize and specifypolymer grades. The on-line measurement of theMFR has become an important issue in polymermanufacturing and complete QA concepts are buildaround the MFR measurement (figure 4). Most ofthe Polyolefine production lines use on-linerheometers to continuously measure the MFR,typically at the end of the extruder before thepelletizer. On-line measurements are also used tocontrol the process itself. Closed loop control is verysuccessfully applied to the peroxide degradation ofPP, in order to produce different grades of materialwith different MFR. The degradation processnarrows the MWD, which is an additionally desiredfeature /2/. The real advantages of the on-linetechnique however are:1) automatic and continuous real time display ofthe material quality, thus allowing a reduction of

the expensive, time consuming and often too latelaboratory testing and the ability to take immediatecorrective actions based on the trend analysis of thecontinuous MFR measurement ; 2) the reduction ofoff-spec material during transitions, as they can beeasily optimized with an on-line real-time controlof the material during the material change.

In order to develop materials with the correctprocessing behavior, it is not just sufficient to takeinto account the viscosity. Elasticity and timedependence of a polymer are equally important tocontrol the processability of a polymer melt. Amethod to look at the complete rheological responseof a material quickly is the dynamic mechanicalanalysis (DMA) of polymer melts. This technologyhas become a widely used tool for the productdevelopment engineer in the course of the last 20years (figure 5). The DMA measurement providesa kind of fingerprint of the material and allows tocalculate viscosity, elasticity and the material’s timedependence at melt temperatures.

Whereas the viscosity correlates with the weightaverage molecular weight Mw, the elasticityhowever, is strongly dependent on the molecularweight distribution (~Mw/Mz). In an oscillatory test,the material is mechanically probed at differentfrequencies and the polymeric chains and chainsegments relax to these external stimuli. Thedynamic mechanical analysis is therefore alsoreferred to as mechanical spectroscopy. A typicalfrequency scan is shown in figure 6. DMA is a very

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Fig. 7 The elongation viscosity is determined by stretchinga polymer filament and recording the force. The rotaryclamp rheometer developed by Meißner /5/ allowsperforming elongations of a Hencky strain of 7 (stretchingratio > 1000). The LDPE sample shows the typical strainhardening effect above eH = 3.

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Fig. 8 The DMA trace of a PSA adhesive as a function oftemperature allows determining the critical applicationparameters.1) the modulus at application temperature (G’=2 104 N/m2) /6/2) the lowest application temperature limited by the lowtemperature transition (T= -10°C) and3) the lowest processing temperature defined by the crossover point of G’and G”(T= 150 °C) /7/.

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sensitive method to fully characterize linearpolymers in terms of MW and MWD. Commercialsoftware packages are available nowadays tocalculate MW and MWD directly from theoscillatory data /3,4/. Note: Dynamic mechanicalanalysis also refers to the testing of polymer solids.However, in the solid state, the DMA providesinformation on the polymer morphology andsecondary transitions. The solid’s behavior isindependent of the polymer structure (MW andMWD, etc.).

HOW RHEOLOGY CAN MAKE THE FINALPRODUCT BETTER

Application examples are manifold for typicalprocesses like injection molding, film blowing,extrusion, etc.. In processes which involve freesurface deformations of the material (film blowingand fiber spinning), the elongation viscosity, andnot the shear viscosity is important (figure 7). Thestrain hardening behavior of polymers (elongationviscosity increase at large deformations) is essentialfor the Processability during the film blowing andfiber spinning process. In order to insure film andfiber thickness tolerances, it is important to controlthe strain hardening effect stabilizing the filamentand the bubble during the elongation process.

Pressure sensitive adhesives (PSA) are polymerblends, rheology-optimized to have a viscosity(modulus) high enough to prevent flow underapplication temperatures, but low enough to flowinto the substrate under the pressure applied by the

thumb (figure 8). At elevated temperature thematerial must show low enough viscosity for easycoating of the film during the tape manufacturingprocess.

Reactive materials evolve from a low viscosityfluid to a solid material during processing. Rheologyis an ideal technique to follow the evolution of theviscosity during cure. Today’s rheometers can easilybe programmed to simulate cure cycles and tooptimize the temperature and pressure profiles inthe mold. Changes in material (advancement as aresult of storage or moisture effects) can easily bedetected and the process parameters adjusted for thegiven batch of material /8/. Dynamic mechani-caltesting however, can provide more than just aviscosity. The gel point (figure 9) can be determinedfrom dynamic mechanical data easily.

How rheology is involved in solving material,processing and performance problems falls alwaysinto the same scheme and is illustrated in figure 10.The rheology-polymer structure relation makesrheology the ideal tool to design materials withspecific processing and end-use performance.Whereas the rheology of the melt provides directinformation on the Processability, the rheology ofthe solid and melt phase can be related to theperformance of the end product. Also, due to theviscoelastic nature of the melt which may causewanted and unwanted anisotropy during flow, thefinal product also depends on how the material isbeing processed.

CONCLUSION

Rheology is very sensitive to small changes ofthe material’s polymer structure – thus ideal for

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Fig. 9 The Gel point of a resin (here epoxy resin) can bemeasured using the DMA technique. The curing is followedin time by applying a small amplitude deformation to thesample and recording the dynamic moduli. The gel pointcan be defined as the time when G’ and G” cross over.

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Temperature T [°C]

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Fig. 8 The DMA trace of a PSA adhesive as a function oftemperature allows determining the critical applicationparameters.1) the modulus at application temperature (G’=2 104 N/m2) /6/2) the lowest application temperature limited by the lowtemperature transition (T= -10°C) and3) the lowest processing temperature defined by the crossover point of G’and G”(T= 150 °C) /7/.

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characterization of polymers. The rheology-structure relationship is the key to the developmentof new materials.

Because of the sensitivity of the process to thepolymer structure, the material needs to becontrolled within tight tolerances in order toguarantee good Processability. Rheology is a mostsuitable tool for process and quality control.

For complex materials (multi-phase system), thestructure-rheology relationship is ambiguous,because the rheological behavior is not a uniquerepresentation of the polymer structure. Additionaltest methods have to be used in conjunction withthe rheological measurement to be able to isolatethe contributions of different structure elements.Optical, thermal, dielectric, etc.. methods arepotential candidates to be used simultaneously withthe rheological measurement.

Nevertheless, today rheology is a standard in thepolymer industry to characterize materials with thegoal to develop new and better materials with thedesired processing and end-use properties.

REFERENCES

1. Ferry, J.D.; ”Viscoelastic Properties ofPolymers” 3rd edition, Wiley, New York(1980)

2. Gendron, R.; Daigneault, L.L; Dumoulin, .M.;Dufour, J.; “On-line Rheology control for the

Fig. 10 This diagram shows how rheology can be used tocorrelate end-use and processing performance to thepolymer structure. Rheology is a key characterizationtechnique for developing materials with the desired physicalproperties and for controlling the manufacturing process inorder to ensure product quality.

peroxide degradation of polypropylene”International PlasticsEngineering and Technology Vol. 2, 55-75(1996)

3. Mead, D.; J. Rheol. 38, 1797 (1994)4. Franck, A.J.; Van Dyke, T.; Mead, D.; 5th

European Conference on Rheology inPortoroz, Slovenia (1998)

5. Meißner, J.; Hostettler, J.; Rheol. Acta 33, 1-21(1994)

6. Class, J.; Chu, S.; J. Appl. Pol. Sci. 30, 683(1985)

7. Bamborough, D.; Adhesives Age, November1990

8. Macosko, C.; Adhesives Age, September 1977