USER COM December 95 1

With the selection of the heating rate you can facilitate or complicate theinterpretation of the measurement.

1. Temperature difference between sample and temperature sensorAs the temperature sensor does not measure the temperature directly inthe sample, but only in the vicinity of the sample, a temperature differencebetween sample and temperature sensor appears with every thermalanalysis measuring cell. This depends on the heating rate and thetemperature. If it can not be calibrated, the temperature results must becorrected by hand when the heating rate differs from the calibration.With Mettler you can calibrate this heating rate dependence (tau lagcalibration). In accord with the physics, the onset values become practi-cally identical even with different heating rates (Fig. 1: Indium meltingpeaks).

USER COM

Contents

TA-TIP:– Selection of the heating rate

NEW in sales program

– New DSC821e module

– New cooling option (intra cooler)

– Automatic crucible opening before

the measurement

Applications

– New possibilities with ADSC cp

– Measurement with TGA/SDTA

– Curing behavior of adhesives

Dear customerMany thanks for the confidenceyou have shown in us this year. Itis a pleasure for us to be in regularcontact with you as this continual-ly leads to new improvements inour products. Once again, we caninform you of new products inaddition to interesting applicationexamples.May we also urge you to use UserCom as a platform for the ex-change of experiences with otherpersons interested in thermal analysis.To ensure we can give even greater consideration to your wishes, please let us know youropinion of User Com.

Selection ofthe heating rate

Information for users ofMETTLER TOLEDO thermal analysis systems

December 1995

TA-TIP

2

Fig. 1: Indium melting peaks at different heating rates

METTLER TOLEDO TA 8000

Indium with different heating rates 18.12.1995 07:30:53

USER COM December 952

Smectic transformations of a ferroelectricliquid crystal measured at 2 and 5 K/min.The first 3 phases have a narrow stabilityrange of only approx. 1°C. As the trans-formations are not really clearly defined, the baseline is not reached completely evenwith 2°C/min. The resolution of the lowercurve with 5°C/min is clearly poorer. Alwaysremember that the peak areas of DSC curvesplotted against the temperature do not correspond to the heat effect (the enthalpychange is the integral of the DSC curve withrespect to time).

3. Improvement in the resolutionwith decreasing heating rateFor the peak separation, a low heat-ing rate and a low signal time con-stant is necessary. With the ceramicsensor you have a rapidly reactingsensor available (signal time con-stant approx. 3 s with the standardaluminum crucible). To separateeffects cleanly, the time betweenthem should be approx. 5 time con-stants. The deflection drops back to1% of the original baseline. If, for example, your effects areseparated by around 5°C, you cancalculate the maximum heating rateat a signal time constant of 3 s as follows:5°C/5 * 3 s) * 60 s/min= 20°C/min(Fig. 3: Peak separation)

4. Blank value correctionThe shape of the blank curve alsodepends on the heating rate. With theTSW870, the database automaticallysearches for the latest blank curvemeasured with an identical heatingrate and performs an online correc-tion during the measurement.

You thus save yourself the manual result corrections with their associatederrors. If you wish to use your module over a wide temperature range, weadvise you to perform the tau lag calibration with several different sub-stances so that the system can also take the temperature dependence of thiscorrection function into account.In the case of chemical reactions, the onset temperature is naturally not con-stant at different heating rates (however, the difference between the onsettemperatures would be even greater if the tau lag calibration were wrong orif it were not carried out).

2. Increase in the sensitivity with increasing heating rateAs you have certainly already discovered, melting peaks, for example,become larger with increasing heating rate. You can thus amplify minoreffects very simply by selecting a higher heating rate (Fig. 2: Glasstransition).

However, if you have a substance which exhibits two effects which lie closetogether, these peaks will merge to a single peak at a high heating rate.

Fig. 3: Peak separation of a liquid crystal

Fig. 2: Glass transition

USER COM December 95 3

Intra CoolerYou now have an intra cooler (LabPlant or Haake) available as a newcooling option for the DSC820/821e.In addition to the air cooling, thecryostat or water cooling and theliquid nitrogen cooling, you can nowequip your existing system with the intra cooler. The cold finger iscooled directly with the coolant,the intermediate circulation systemfound with the cryostat is no longerused. You can thus perform low temperatu-re measurements down to the region of -65°C very quickly and at a very favorable price. You will find the cooling times in Figure 4.

The DSC821e was introduced inJanuary as a successor to theDSC820 module and is based on leading edge technology. The module is available in two tempera-ture versions. The modular conceptof the DSC820 has been extendedfurther so that we can now give evenbetter consideration to your specificwishes. Countless system combi-nations are possible with the old andthe new options.We are convinced that we can nowoffer you exactly the right solution.You pay only for what you actuallyneed.But the future is still open to you.You can expand your instrument atany time with additional options tomeet your needs.

The new DSC module DSC821e

The properties of the DSC821e module at a glance:

• Large measurement range ±350 mW at RT• High resolution 0.7 µW at RT• Wide temperature range -150°C – max. 500/700°C• High temperature accuracy ±0.2°C• Excellent peak separation low signal time constant (3 s)• Modular construction open for the future• Automatable with sample changer 34 samples• Further options Automatic sample chamber opening

Gas controllerControl of peripheralsPower output switched by softwareLocal module operationLiquid nitrogen (-150°C)

• Cooling systems Intra cooler (-65°C),cryostat (-50°C) and air cooling (standard)

Fig. 4: Cooling curve with intra cooler

METTLER TOLEDO TA 8000

With the DSC820/821e and theTGA850, the sample changers cannow be equipped with an accessorythat automatically opens the crucibleshortly before the measurement. Amechanism attached to the gripperpunctures the crucible on the sampleturntable.

During the wait time on the sampleturntable, the sample thus exchangesno moisture with the surroundings.This is a great advantage especiallywith the readily volatile sample frac-tions found in the pharmaceutical,paint and food industries.

TGA850 specific heat fromthe SDTA curve?

G. Widmann

Several users have asked us whetherat least a semiquantitative measure-ment of the specific heat from theSDTA signal is possible.As the temperature function of thecalorimetric sensitivity is not (yet)known, only the sapphire cruciblemethod can be used. As the cptemperature function of sapphire isknown, use is made of a standard-ization employing a sapphiremeasurement corrected by theblank curve. The software used therule of three to calculate thespecific heat from the samplemeasurement corrected by the blank curve:

cp = cpSap • mSap • SDTAProbe /(mProbe • SDTASap)

The following Figure (Fig. 5) showshow this is achieved.

In the superimposed coordinatesystem you can see the sapphire andthe sample SDTA curves (both cor-rected by the blank curve). The

sample used was a quartz with itssolid-solid transition at around570°C. The calculated cp curve ofquartz is shown in the large coordi-nate system.

Enthalpy changesAs you know, enthalpy changes withDSC curves are calculated by inte-gration of the heat flow with time.By analogy, heat effects can bedetermined by integration of the spe-cific heat with respect to tempera-ture. However, version 3.01 of oursoftware always integrates with re-spect to time. The result obtained in

At present two types of crucibles suited to this type of measurementare available:The present standard aluminum cru-cible 40 µl (DSC) and the newaluminum 100 µl crucible (DSC andTGA). The following new cruciblesets are available:• Standard aluminum crucible 40 µl,

set of 160 without lid• Standard lid, set of 160• Membrane lid, set of 160 (for the

automatic opening)

Existing customers of a DSC820 orTGA850 module have the followingupgrade possibilities:

Conversion kit for the (DSC) samplechanger• Calibration crucible• Needles (3 types)• Drive shaft• Crucible holding-down ring

Conversion kit for the (TGA) samplechanger• Calibration crucible• Needles (3 types)• Drive shaft• Crucible holding-down ring– Sample turntable type 1 (7.8 mm)– Gripper type 1 (7.8 mm)– Crucible holder(weighing pan plus deflector)

the unit Js/gK must therefore be divided by the heating rate in K/s toobtain the result in J/g (the next software version will also include integration with respect to the abscissa).The circuitous route via the cp deter-mination with the sapphire methodthus allows at least semiquantitativecalorimetric measurements. Youshould use the Pt crucible for suchmeasurements, when possible withlid.

Abb. 5 cp Messung mit TGA 850

Automatic crucible opening before the measurement

USER COM December 954

USER COM December 95 5

Fig. 7: ADSC curve of 20.425 mg PET in the temperature range 40 to 140°C. Each heating segmentincreases the temperature by 4°C with an instantaneous rate of 3°C/min. The subsequent cooling segment lowers it at the same rate by 3°C. A period thus lasts 2 min. and 20 s. The mean heating rate is1°C/2.33 min = 0.43°C/min. The two envelope curves needed to calculate the curves in Fig. 8 are alsoshown.

Dr. B. Benzler, G. Widmann

In DSC, the heat flow of the sampleunder investigation is usuallymeasured at a constant heating orcooling rate. High heating and cool-ing rates – 10 K/min and higher –have the advantage of high measure-ment sensitivity (large peak), how-ever, this is associated with lowresolution relative to the tempera-ture axis. At low rates, on theother hand, the conditions are theopposite: virtually invisibly smallpeaks which are often very wellseparated. ADSC with its periodic temperatureprogram combines the advantages ofboth measurement modes:– High sensitivity thanks to high

instantaneous heating rate

– High temperature resolution thanksto low average heating rate

The temperature program of ADSCcomprises a periodic succession ofshort, linear heating and cooling pha-ses. The heating or cooling rates liebetween 2 and 5 K/min. For a hea-ting measurement the final tempera-ture of the cooling phase is higherthat the start temperature of the

previous heating phase by a small amount thus leading to the low, aver-age rate:The measured heat flow comprisesfractions which arise from theheat capacity Cp and those due tophysical transformations or chemicalreactions. The Cp part is thus alsocalled sensible heat (specific heatis synonymous with sensible heat).

The other part is called the latentheat flow.

After the stabilization time, theADSC measurement signal of eachsegment is proportional to theheat capacity of the sample. Thesoftware connects the points of themeasured curve at the end of eachheating segment to the lower en-velope curve and those at the end ofthe cooling segments to the upperenvelope curve. Half of the dif-ference between the two envelopecurves corresponds to the sensibleheat flow.

Cp changes appear with the follow-ing effects:• Glass transition of amorphous sub-

stances• Cold crystallization• Chemical reactions (the reactants

do not have the same heat capacityas the products formed).

The additional latent heat flowappears with heat effects in both theheating and cooling phase. The meanvalue of the two envelope curvesthus corresponds to the latent heatflow.

Alternating differential scanning calorimetry, ADSC, opens up new possibilities

Fig. 6: Section from an ADSC measurement. The top part shows the alternating temperature program.The corresponding startup deflections are shown on the associated ADSC curve. After approx. 20 min., a heat effect starts to shift both envelope curves upward.

Fig. 8: Two endothermic peaks (enthalpy relaxation) during the glass transition at 70°C as well as theexothermic crystallisation peak at 110°C appear on the curve of the latent heat (mean value of the en-velope curves). The curve of the sensible heat (half the difference between the envelope curves) showsthe corresponding Cp changes.

Fig. 9: Comparison of the usual, dynamic DSC curve at a constant heating rate of 0.43 °C/min with thetwo separated DSC curves from the ADSC measurement. If the curve of the latent heat and the curve ofthe sensible heat divided by the heating rate factor are added, the classical DSC curve is obtained. Theheating rate factor is the ratio of the instantaneous rate to the mean rate: 3/0.43 = 7.

USER COM December 956

This allows separation of certainlatent effects from cp changes:• Enthalpy relaxation peak during

glass transition• Cold crystallisation in the heating

of supercooled melts• Crystallization of melts during cooling• Chemical reactions• Vaporization of volatile substances

(drying)

Such effects are also called nonre-versing. More problematic are rever-sible transformations such as meltingand crystallization; the crystalswhich have just started to melt onattainment of the melting pointwould immediately crystallize in thefollowing cooling segment. To prevent this, in such cases iso-thermal holding times are pro-

grammed instead of cooling phases.The separation of the ADSC heatingcurve into the sensible and latentheat flow is shown in Figure 8 usingthe example of polyethylene tereph-thalate, PET. The usual DSC heatingcurve would falsify the glass transi-tion owing to the simultaneous re-laxation peak. Moreover, the cpchange of the cold crystallizationwould scarcely be visible (Fig. 9).The separated curves can be evalu-ated with the software in exactly thesame manner as measured curves.The separation can also be achievedby the optional Fourier analysis. (In this case, the phase shift appearsas a curve.)

Summary

Compared with classical DSC, theADSC measurement technique incombination with the TSW870 TAsoftware offers the following advan-tages:• High resolution of the processes

associated with heat effects thanksto low average heating rate. Themeasured temperatures are near theequilibrium values.

• High sensitivity of the measure-ment signal through relative highinstantaneous heating and coolingrate.

• Separate representation of sensibleand latent heat flow.

However, there are also a few dis-advantages:• Additional parameters in the

method development widen therange of selection possibilities.

• Longer measurement times owingto the low average heating rate.

• Standard methods require constantheating rates.

USER COM December 95 7

The launch of the new softwareversion also saw the introductionof the model-free kinetics in the lastissue of User Com. As numerousmeasurements have shown, inmost cases these new model-freekinetics allow a considerably moreaccurate prediction than the traditio-nal model concepts, even in thecase of complex or multi-stage reac-tions.

Only three or more dynamic measu-red curves (recorded using DSC orTGA) are needed to analyse thedynamic or isothermal behaviorof a reaction. The activation energyE(α) is calculated as a functionof the conversion α.

With simple reactions, this E(α) iscomparable with the classicalactivation energy Ea, but its profileallows additional information onthe complexity of a reaction to begained.

The curing of a 2-component resinwas recorded at heating rates of2, 5, 10 and 20 K/min (1. Dynamiccuring measured). The associatedactivation energy E(aα) is calculatedautomatically (3. Activation energycalculated). For comparison, thekinetic model of the nth order wasused, with the following parametersresulting for a heating rate of 5 K/min:ln (ko) = 88, Ea = 267 kJ/mol and n= 1.76.These two kinetic data records wereused to simulate the isothermalcuring at 60°C and compare it withthe curve actually measured. (4. Isothermal conversion at 60°C). Itis clear that the model-free kineticscan describe the behavior verymuch better than the nth orderkinetics.

The DSC analysis of a special adhesive from the automobile branch showsthe rapid setting-in of curing, which is manifested in a very steep reactionpeak (1. DSC curves measured). The complexity of this reaction also be-comes apparent in the analysis of the activation energy E(α), which assumesvery high values at the start of the reaction (2. Activation energy calculated).Even with this complex reaction, the model-free kinetics allow simulation ofthe dynamic curing at different heating rates as the comparison between thesimulated and measured curves shows (3. Simulated curves). The relationbetween conversion, time and temperature can also be shown in tabular formand/or graphically (4. Iso-Conversion).

Fig. 10: Curing of a 2-component resin

Fig. 11: Curing of a special adhesive

Model-free kinetics

Dr. J. de Buhr

Exhibitions/conferencesPittsburgh Conference March 4 – 7, ’96 Chicago (USA)Forum Laboratoire April 1 – 5, ’96 Paris (F)Analytica (hall 18/stand B14) April 23 – 26, ’96 München (D)Instrurama June 2 – 7, ’96 Brussels (B)ICTAC August 12 – 16, ’96 Philadelphia (USA)Het Instrument October 7 – 10, ’96 Utrecht (NL)GEFTA/AFCAT/STK Sept. 23 – 26, ’96 Freiburg im Brsg. (D)Ilmac 96 Nov. 19 – 22, ’96 Basle (CH)

TA customer courses and seminars (CH)For further information, please contact Mettler-Toledo AG, AnalyticalPhone ++41 1 806 72 74Fax ++41 1 806 72 40TA customer course (French) May 7 – 9, ’96 Greifensee (CH)TA customer course (German) May 21 – 23, ’96 Greifensee (CH)TA customer course (English) May 29 – 31, ’96 Greifensee (CH)TA customer course (German) Nov. 12 – 14, ’96 Greifensee (CH)TA customer course (English) Nov. 19 – 21, ’96 Greifensee (CH)

Lab Talk advance notices ’96 (CH)Interested parties requiring further information should contactMettler-Toledo (Switzerland) AG (phone 01/944 45 45, fax 01/944 46 60)TA8000 user seminar April Greifensee (CH)TA8000 user seminar May Greifensee (CH)2nd specialized seminar oncontrol of inspection, measuringand test equipment in Q systems May Basle (CH)

TA information days and training courses (USA)Please contact your local instrument specialists or R. Truttman Phone 1 800 METTLER (63 88 537) 88 21 or fax 1 609 448 47 77

If you have any questions concerning other meetings, the products or appli-cations, please contact your local METTLER TOLEDO dealer.

Internet: http://www.mt.com

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U. Jörimann, Dr. B. Benzler, Dr. J. de Buhr, Dr. R. Riesen, J. WidmannLayout and production:MCG – MarCom GreifenseeME – 51 709 521