ionb

n Wtitute

HAAKE MiniLabSteady-state torqueDynamic speed test methodRepeatability

n appheolo

to thound

in polymer processing laboratory for torquemeasurements.It can simulate a mixer and the extrusion process in nearly

ch as mixing timeost impossible toer at a given tem-rrelation of steady-temperature, it isotation rates andrque with time atming process with

For the above reasons, the application of torque-rheometers for characterizing the properties of polymermelts has been greatly restricted.

MiniLab Micro Compounder and Rheometer, devised byHAAKE Company, Germany, is specially designed for thecompounding of small volume (7 mL only) samples.

* Corresponding author. Tel.: 86 028 85460817; fax: 86 02885402465.

E-mail address: wgli2007@126.com (W. Li).

Contents lists available at ScienceDirect

Polymer T

.e lse

Polymer Testing 33 (2014) 138144mer ow behavior [14].The torque-rheometer is the most commonly used tool

no certainty of reliable data. Only a few researchers havetried such experiments with no satisfactory results [1517].melting process for determining suitable processing con-ditions, as well as to monitor industrial extrusion produc-tion for process quality control. Steady-state torque is themeasurement of the torque at equilibrium as the plasti-cized material is in a stable and homogeneous melt state.For a pure polymer in a given process, the steady-statetorque is a characteristic of the polymer, which can beused as a rheological parameter to characterize the poly-

material and experimental conditions suand rotation rate. As a result, it is almobtain a consistent torque for a polymperature [13,14]. As for measuring the costate torque with rotation rate at a givengenerally required to select several rmeasure separately the variation of toeach rotation rate. This is a time-consuand mixed with rotation of screws or rotors. It has beenused as an engineering indicator to learn the polymer

heat from viscous dissipation, the pre-set temperature al-ways changes during the mixing process, dependent uponReliability

1. Introduction

In the polymer eld, torque refersmet when polymer resins or comp0142-9418/$ see front matter 2013 Elsevier Ltdhttp://dx.doi.org/10.1016/j.polymertesting.2013.12.0reliability of torque data were also evaluated. Results showed that the torque could becalibrated by subtracting the torque without samples. Also, a feed quantity of ca. 6 g with adynamic speed test range of 10105 r/min was suitable for the determination of steady-state torque of polyolen samples. The new method was quick, effective and reliable tocorrelate the steady-state torque with rotation speed. Therefore, MiniLab would be a veryuseful tool in exploring and characterizing polymer ow behavior through its torquemeasurements.

2013 Elsevier Ltd. All rights reserved.

e hindering forces are plasticized

real processing conditions. Therefore, many researchershave used it to study the processing properties and mixingquality of polymers [512]. However, as the mixing ofpolymer materials with the rotation of rotors generatesKeywords:

melts. The choices of sample feed quantity and screw rotation speed, as well as calibrationfor real torque, were carefully studied before torque measurements. The repeatability andAccepted 2 December 2013 namic speed test method for the rapid determination of steady-state torque of polymerTest method

A novel method for the determinatof polymer melts by HAAKE MiniLa

Cong Wang, Jing Wang, Chenyang Yu, BingtiaState Key Laboratory of Polymer Materials Engineering, Polymer Research InsPeoples Republic of China

a r t i c l e i n f o

Article history:Received 25 October 2013

a b s t r a c t

HAAKE MiniLab is aon-line testing of r

journal homepage: www. All rights reserved.01of steady-state torque

u, Ya Wang, Wenguang Li*

of Sichuan University, Chengdu, Sichuan 610065,

aratus specially designed for compounding polymer material andgical properties. For the rst time, it was used to establish a dy-

esting

vier .com/locate/polytest

unit (2), extruder housing (3) and feeding device (5). Thecontrol system includes themanual operating panel (4) andthe application software (1). Both the dynamic speedmeasurement and the experimental data documentation

Table 1Characteristics of materials used.

Material Grade Melt ow index(g/10 min)

Supplier

HDPE 6070EA 7.2 (190 C/2.16 kg) Dushanzi Petrochemical,China

HDPE 60550 7.0 (190 C/2.16 kg) Lanzhou Petrochemical,China

LDPE 18D 1.5 (190 C/2.16 kg) Daqing Petrochemical,China

iPP T30S 2.0 (230 C/2.16 kg) Lanzhou Petrochemical,China

Fig. 1. The constitution of HAAKE MiniLab: (1) and (4) control system, (2)drive unit, (3) extruder housing, (5) feeding device.

C. Wang et al. / Polymer Testing 33 (2014) 138144 139Simultaneously, the rheological properties can be recordedto document structural changes. Its main features includeintegrated torque and viscosity measurements, co- andcounter rotation twin screws and an automatic bypassoperation for circulation/extrusion with pneumaticfeeding. Therefore, MiniLab is considered as a combinationof a batch mixer, a twin-screw extruder and a rheometer.

Based on the units specications, we found that, inaddition to small usage of samples, there are two distinctadvantages in determining steady-state torque of polymermelts by MiniLab over other torque-rheometers. First,MiniLab has a special backow channel integrated with theextrusion housing for circulation, which ensures a preciseand stable operating temperature, thus signifying that thevariation of polymer melt state with temperature due toviscous dissipation can be negligible. Second, MiniLab canperform dynamic speed measurements in a selected speedrange by a pre-set test program. We will show that in thisway the correlation of steady-state torquewith rotation ratecan be quickly obtained through only a single test, making itpossible and convenient to explore the rheological proper-ties of polymermelts with signicant time and cost savings.

Despite the prominent characteristics ofMiniLab as statedabove, published research still relates to using it only as aconventional mixer to prepare experimental samples and tocompound expensive materials such as nano-composites,bio-polymers and pharmaceuticals [1827]. No attentionhas been paid to its potential powerful function in torquemeasurements. The reason may be that the reliability ofrheological measurements by MiniLab has not been evalu-ated. Moreover, no appropriate method has been establishedfor thedeterminationof steady-state torqueofpolymermelts.

This situation was motivation for the current research.This paper presents a novel method, termed dynamic speedtest method, for the determination of steady-state torque ofpolymer melts by means of the MiniLabs functions. We willintroduce how to obtain repeatable, reliable torque data aswell as the correlation of steady-state torque with rotationrate in theMiniLab test. Thecalibrationof torquedata, sampleloads and screw rotation speeds were carefully investigatedas regards the repeatability of data. The comparison of thenew method with the common xed speed method forsteady-state torque measurements was also done todemonstrate the reliability of data. Our results show that thedynamic speed test is a quick, effective and reliable methodfor determining steady-state torque of polymer melts.

2. Experimental

2.1. Materials

All the samples in this study were carefully selectedfrom commercial resins. These were high-density poly-ethylene (HDPE), low-density polyethylene (LDPE) andisotactic polypropylene (iPP). The characteristics of thematerials were shown in Table 1.

2.2. Instrumental details

The HAAKEMiniLab, as illustrated in Fig. 1, is made up ofve functional elements: control system (1 and 4), drivecan only be performed through the software. The drive unitoffers motor motion and precisely controls experimentalconditions, such as testing temperature and rotation speed.It also measures the motor torque from a mixer sensor. Thestructure of the extruder housing with co-rotation twinscrews is displayed in Fig. 2 (the co-rotationmodewas usedin this study). The lled-in sample can be extruded in acirculation via an integrated backow channel. The back-ow channel is constructed as a rheological slit capillarywith two pressure sensors. While the sample is extrudingthrough the backow channel, the rheological informationabout the viscosity of the sample can be obtained from thetwo pressure sensors. Lastly, the feeding device enablesperiodic sample feeding by the piston of a pneumaticcylinder.

2.3. Dynamic speed test method

There are two stages in the dynamic speed test method.In the rst stage, samples were dynamically plasticized andmelted under a rotation speed range predetermined ac-cording to the requirement of the experiment and the na-ture of the material. The number of speed points was set to520 with equal distance logarithmic intervals (rpm)within the speed range. At each speed, the software pro-gram measured the torque of the material, and then

ran again in the same way as in the rst stage. Then, the

rotation speed for calibration purposes. In the second step,

Fig. 2. The structure of extruder housing: (1) backow channel, (2) pressure

C. Wang et al. / Polymer Testing 33 (2014) 138144140the torque measurements were done after the sample wasfed into the extruder and plasticized. The correlation ofsteady-state torque with rotation speed was achieved viacalculating the data exported from the above two-stepmeasurements.

2.5. Choice of feed quantity

To choose a suitable feed quantity for torque measure-ments, iPP (T30S) and HDPE (60550) with a dosage of 47 gmeasured data were recorded to correlate steady-statetorque with rotation speed for polymer melts at the testtemperature. Generally speaking, it only took about 15 minto nish the whole measurement with a speed range of 10105 r/min.

2.4. Torque measurements by MiniLab

There were two main steps for the determination ofsteady-state torque of the material. In the rst step, devicesetup was carried out and the measuring procedure wasdened using the software. When the temperature reachedthe set point, torque measurements without samples wereperformed to obtain a relationship between torque andselected 20 data points that met the programs requiredstability criteria. Afterwards, the instrument would auto-matically switch to the next speed point until the operatingprogram was over. In the second stage, the same program

sensor, (3) conical twin-screw, (4) bypass valve, (5) temperature sensor, (6)extrusion channel.were used. The test was carried out under a dynamic speedrange of 10105 r/min and the temperature was xed at190 C.

2.6. Choice of rotation speed

HDPE (6070 EA) with 6 g was selected as the sample andthree experiments were conducted as follows. Two sampleswere separately plasticized for 5 min under a xed speed of50 r/min and 100 r/min, respectively, and another under adynamic speed range of 10105 r/min. All the samples weremeasured in the dynamic speed range of 10105 r/min. Thetemperature was set at 150 C for both plasticizing andmeasurement.

approach to calibrate the torque of polymer melts. That is,the real torque value was obtained by subtracting the tor-

que value without samples from the torque values withsamples. For any dynamic torque measurement, it wasnecessary to preferentially establish the correlation of tor-que with rotation speed without samples before beginningthe measurement. Thus, the real torque of polymer meltscould be obtained throughout the dynamic speed testmethod.2.7. Study on repeatability of torque data

Two polyethylenes, HDPE (6070 EA) and LDPE (18D)with a feed quantity of 6 g, were chosen as test samples.Plasticizing and measuring were done with a dynamicspeed range of 10105 r/min. The temperature was set at190 C for 6070 EA and at 150 C for 18D.

2.8. Study on reliability of torque data

HDPE (6070 EA) was chosen as test material with ausage of 6 g. The comparison of steady-state torquedetermined by the common xed speed method was madewith that measured by the new dynamic speed testmethod. In the xed speed method, the test samples weremixed for 8 min at speeds of 30 r/min, 50 r/min, 80 r/minand 100 r/min. The torque values from 5 min to 8 minwereaveraged to obtain the steady-state torques under differentrotation speeds. For the dynamic speed test method, afterbeing plasticized under a dynamic speed range of 10105 r/min, the sample was measured in the same speed range toacquire the steady-state torques. The experimental tem-perature in both methods was kept at 150 C.

3. Results and discussion

3.1. Torque calibration without samples

According to the instruction manual for the HAAKEMiniLab, before beginning the measurement, motor torquewithout material but with rotating screws must be cali-brated to its torque zero point. It was found that the torquecalibration in the manual worked well for a xed speed butnot for a dynamic speed test. Some torque values were notalways accurate over a wide rotation speed range. Hence inthis paper, we calibrated torque through subtraction on thebasis of the following.

Fig. 3 depicts the torque plotted against rotation speedwithout samples in the MiniLab. It can be seen that thereis a linear relationship between torque and rotation speedwithin a rather wide range of 5205 r/min, which can beexpressed as y 0.0018*x 0.019, where x is the rotationspeed, y is the corresponding torque value. It should benoted that the slope or intercept of the linear line mightvary slightly with the time passed. However, our MiniLabinstrument, after many years of usage, has consistentlydemonstrated this linear relationship between torqueand rotation speed. The instruments performancewas not subject to atrophy that may apply to otherinstruments.

Based on the above nding, we proposed a subtraction

repeatable and valuable data from torque measurements inthe MiniLab be obtained.

3.3. Effects of rotation speed on torque measurements

Fig. 4. The relationship between steady-state torque and feed quantity toT30S (a) and 60550 (b).

C. Wang et al. / Polymer Testing 33 (2014) 138144 1413.2. Effects of feed quantity on torque measurements

The torque, by denition, is obviously dependent on theamount of material used in a measurement. For the Mini-Lab, the feed quantity needs to be suitable for obtaining aconsistent measuring torque value.

Fig. 4 outlines the effects of feed quantity on the steady-state torque of T30S (a) and 60550 (b). It can be seen fromFig. 4(a) that the torque of T30S signicantly increased withfeed quantity varying from 4 g to 6 g at given rotationspeeds. When continuing to add up to 7 g, the steady-statetorque leveled off. It should be noted that the torque startedto fall with increased feed quantity from 6 g to 7 g at arotation speed of 105 r/min. Themost probable explanationof this trend was that the PP chains degraded with rotationat such a high speed.

As shown in Fig. 4(b) for 60550, it was also found thatthe torque increased with the increased feed quantity from4 g to 6 g. However, unlike T30S, the steady-state torque of60550 was constant when the feed quantity was between6 g7 g, even at a high rotation speed of 105 r/min. Thismay be due to the better chain stability of HDPE than that ofPP.

Fig. 3. Relationship of torque-rotation speed without samples.The above results indicated that loads of ca. 6 g weresuitable for obtaining consistent torque values in theMiniLab. This is reasonable if the following calculation isconsidered. As bulk densities of PP and HDPE are about0.91 g/cm3 and 0.96 g/cm3, respectively, the volume of 6 gsample could correspond to 6.6 mL for PP and 6.3 mL forHDPE at room temperature. When PP or HDPE is molten,the polymer volume will expand somewhat. Since MiniLabhas a cavity of 7 mL volume, the feeding quantity of ca. 6 gpolyolen samples should completely ll the cavity, whichleads to the maximum torque as expected. Therefore, theexperimental results are in agreement with the theoreticalcalculations.

It should be mentioned that lling with 45.5 g of ma-terial, as suggested in the operatingmanual of MiniLab, wasnot suitable for torque measurements. Since the steady-state torque of polymer melts is strongly related to feedquantity, it is important to determine the optimum quan-tity for any unknown material. Only by doing that, canSteady-state torque depends, to an extent, on bothplasticizing and measuring conditions, especially on thescrew rotation speed. If samples are plasticized ormeasured at a very high rotation speed, polymer chains canbe easily broken down, leading to a low torque value. Ifsamples are measured at a very low rotation speed, thetorque may be too small to be accurately determined.Hence, there is an optimum rotation speed range for torquemeasurements, in which the measured torque can accu-rately represent the rheological character of the material.

Fig. 5 gives the steady-state torques of 6070 EA plasti-cized under three different plasticizing conditions. It wasobserved that the torque-rotation speed curve of 6070 EAplasticized at a dynamic speed range of 10105 r/min wasin accordance with that plasticized at a xed speed of 50 r/min, revealing that two plasticizing conditions couldgenerate almost identical polymer melts with homoge-neous and stable states. However, for the sample

Fig. 5. The steady-state torque of 6070 EA plasticized under different rota-tion speed.

Fig. 7. Torque-speed curves of 18D measured in three consecutives.

C. Wang et al. / Polymer Testing 33 (2014) 138144142plasticized at a xed speed of 100 r/min, the torque-rotation speed curve dramatically declined withincreasing speed as compared to that of other two samples.This trend indicates that polymer chains had been greatlydegraded under that plasticizing condition.

Thus, from the above results, a dynamic speed range of10105 r/min was chosen for torque measurements. Thisspeed range was appropriate enough for the preparation ofsamples with identical melt states. Therefore, the truetorque values can be obtained within a relatively widespeed range.

3.4. Evaluating the repeatability of torque data

Repeatability of measured data must be a primary cri-terion of how well an instrument works. Fig. 6 shows thetorque-rotation speed curves of 6070 EA measured atdifferent times (on March 23, May 17 and June 6 of 2012,respectively). It can be seen that, although there was asmall deviation of data at the speeds of above 80 r/min, the

highest standard deviation was only 2.15% (at a speed of

Fig. 6. Torque-speed curves of 6070 EA measured at different times.105 r/min). Therefore, the data repeatability was very goodfor the same samples tested at different times. The resultsalso indicated that the working state of MiniLab was verystable.

Fig. 7 shows the relationship between torque and rota-tion speed of 18Dmeasured on three consecutive tests afterbeing plasticized at 150 C. The torque curve in the rst testwas almost completely overlaid with that in the secondtest. In the third test, it started to decline slightly, probablydue to degradation of LDPE chains with time at high tem-perature. The results demonstrated that the data repeat-ability was also very good for the sample tested in the rsttwo replicates after the sample was plasticized.

In general, if test temperature and dynamic speed rangeare well set up to minimize the degradation of polymerchains, we can always get repeatable torque measurementsfor any sample by MiniLab.

3.5. Evaluating the reliability of torque data

Data reliability is critical for a measuring method.Although the measurements were obtained quickly, theFig. 8. Torque measurements of 6070 EA under xed speed.

Fig. 9. Torque measurements of 6070 EA under dynamic speed.

C. Wang et al. / Polymer Testing 33 (2014) 138144 143reliability of the torque data was not compromised. Fig. 8exhibits the variation of torque with time for 6070 EAunder the xed speed test method. It can be seen fromFig. 8 that, after feeding the sample, the torque valuesunder each xed speed from 30 r/min to 100 r/min allchanged with time in the rst 5 min. Thereafter, theyreached equilibrium values up to 8 min. As expected, thesteady-state torques increased with increase of rotationspeed. It is important to note that, if the sample is addedinto the extruder cavity all at once, the feed path becomesobstructed. Thus, the samplewas divided into two portions,resulting in two peaks on the graph.

The dynamic speed test method includes plasticizingand measuring stages. After the plasticizing stage, thesample becomes a homogeneous and stable melt. Thus, thetorque of a sample quickly reaches an equilibrium valuewhen the rotation speed changes from one speed toanother. Fig. 9 describes the variation of torquewith time inthe measuring stage, by which the steady-state torqueswere directly measured with 10 separate rotation speeds.Please note that only one sample was used in the methodand the steady-state torques could be obtained for 10different rotation speeds within no more than 15 min.Fig. 10. Comparison of two methods.measuring the same samples separately at different timesor with sample replicates. The data reliability wasconrmed by comparing the new method with the com-mon xed speed method for steady-state torque mea-surements. The results showed that the dynamic speed testwas a direct, effective and rapid method for the determi-nation of steady-state torque of polymer melts by MiniLab.The torque data obtained were repeatable and reliable.Moreover, the newmethod had a remarkable time and costsavings as compared to the conventional method. Thecorrelation of steady-state torque with rotation speed wasgenerally considered as a rheological curve for polymermelts. This promises great potential in academic researchand industrial applications.

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4. Conclusions

We proposed a novel method for determination ofsteady-state torque of polymer melts using the features ofthe MiniLab instrument. The torque was calibrated bysubtracting that without material. The optimum feedquantity of ca. 6 g and a dynamic speed range of 10105 r/min were determined for torque measurements of poly-olen samples. The data repeatability was evaluated by

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C. Wang et al. / Polymer Testing 33 (2014) 138144144

A novel method for the determination of steady-state torque of polymer melts by HAAKE MiniLab1 Introduction2 Experimental2.1 Materials2.2 Instrumental details2.3 Dynamic speed test method2.4 Torque measurements by MiniLab2.5 Choice of feed quantity2.6 Choice of rotation speed2.7 Study on repeatability of torque data2.8 Study on reliability of torque data

3 Results and discussion3.1 Torque calibration without samples3.2 Effects of feed quantity on torque measurements3.3 Effects of rotation speed on torque measurements3.4 Evaluating the repeatability of torque data3.5 Evaluating the reliability of torque data

4 ConclusionsReferences