Thermal analysis

Interpreting DSC curvesPart 1: Dynamic measurementsThe art of interpreting curves has yet to be integrated into commercially available com-puter programs. The interpretation of a DSC measurement curve is therefore still some-thing you have to do yourself. It requires a considerable amount of experience in thermalanalysis as well as a knowledge of the possible reactions that your particular sample canundergo.This article presents tips and information that should help you with the systematic inter-pretation of DSC curves.

Recognizing artifactsThe first thing to do is to examine the curve for any obvious artifacts that could lead to apossible misinterpretation of the results. Artifacts are effects that are not caused by thesample under investigation. Figure 1 shows examples of a number of such artifacts. Theyinclude:a) An abrupt change of the heat transfer between the sample and the pan:

1) Samples of irregular form can topple over in the pan.2) Polymer films that have not been pressed against the base of the pan first changeshape (no longer lie flat) on initial warming. Afterward, on melting, they make goodcontact with the pan (Fig. 2).

b) An abrupt change of the heat transfer between the pan and the DSC sensor:1) Distortion of a hermetically sealed Al pan due to the vapor pressure of the sample.2) Slight shift of the Al pan during a dynamic temperature program due to differentcoefficients of expansion (Al: ~ 24 ppm/K, DSC sensor ~ 9 ppm/K, see also Fig. 2).This artifact does not occur with Pt pans (~ 8 ppm/K).3) The measuring cell suffers a mechanical shock: The pans jump around on thesensor and can move sideways if they do not have a central locating pin.

Information for users ofMETTLER TOLEDO thermal analysis systems

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Contents

TA TIP– Interpreting DSC curves;

Part 1: Dynamic measurements

NEW in our sales program– DSC822e

Applications– The glass transition from the point of

view of DSC measurements;Part 2: Information for the character-ization of materials

– Thermal values of fats: DSC analysisor dropping point determination?

– The use of MaxRes for the investiga-tion of partially hydrated Portlandcement systems

– Vitrification and devitrificationphenomena in the dynamic curingof an epoxy resin with ADSC

– Expansion and shrinkage of fibers

Tips– The cooling performance

of the DSC821e

Dear Customer,The year 2000 should prove to be extremely interesting for METTLER TOLEDO thermalanalysis. We plan to expand the very successful STARe product line with the introductionof an exciting new instrument for dynamic mechanical analysis.And of course the current thermal analysis instruments have been undergoing continuousdevelopment. In this edition of UserCom, we are delighted to present the new DSC822e. 11

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c) The entry of cool air into the measuringcell due to a poorly adjusted measuringcell lid leads to temperature fluctuationswhich cause a very noisy signal.

d) Electrical effects:1) Discharge of static electricity in a

metallic part of the system, or powersupply disturbances (spikes)

2) Radio emitters, mobile (cellular)phones and other sources of highfrequency interference.

e) A sudden change of room temperature,e.g. through sunshine.

f) The lid of the pan bursts as a result ofincreasing vapor pressure of the sample.This produces an endothermic peak witha height of 0.1 mW to 100 mW depend-ing on the quantity of gas or vaporevolved.

g) Intermittent (often periodic) closing ofthe hole in the lid of the pan due todroplets that condense or to samplesthat foam.

h) Contamination of the sensors caused byresidues of a sample from previousexperiments. The thermal effectscharacteristic for this substance alwaysoccur at the same temperature. Thisproblem can often be overcome byheating the system in air or oxygen.This type of artifact is very dependent onthe contaminant. Artifacts caused bypans that are not inert also look verysimilar. Figure 3 shows an example ofthis.

Artifacts can also interfere with automaticevaluations (with EvalMacro), especiallythose using automatic limits.Isolated artifacts that have been definitelyidentified as such can be eliminated fromthe measurement curve using TA/Baseline.

Measurement conditionsYou define the temperature range and theheating rate for the measurement based onyour knowledge of the physical and chemi-cal properties of the sample.• Choose a temperature range that is on

the large side. At a heating rate of 20 K/min,you do not in fact lose too much time ifthe range measured is 100 K too large.Further information on this can befound in UserCom 3.

• Use a sample weight of about 5 mg forthe first measurement. Make a note ofthe total weight of the sample and panso that you can detect a loss of weight by

Fig. 1. DSC artifacts (details are given in the text): An artifact can very often be identified by repeat-ing the measurement with a new sample of the same substance and observing whether the effect oc-curs again either at the same place or at a different place on the curve. Exceptions to this are f and h,which can be very reproducible.

reweighing after the analysis. The firstmeasurement is often performed using apan with a pierced lid and nitrogen as apurge gas.

• The first heating curve is usuallymeasured from room temperature to thedesired final temperature at a heatingrate of 20 K/min.

• Interpretation is often facilitated bymeasuring a cooling curve directlyafterward. The cooling rate that can beused depends on the cooling optioninstalled in your system.

• It is a good idea to heat the sample asecond time. Differences between thefirst and the second heating curves canbe very informative.

• Another helpful variation is to shockcool the sample after it has been heated

for the first time to the final tempera-ture. This freezes any possible meta-stable states. The sample is thenmeasured a second time. A very conve-nient way to shock cool the sample toroom temperature is to use the auto-matic sample robot. It deposits the hotsample on the cold aluminum turn-table, which cools it down to roomtemperature within a few seconds. If youdo not have a sample robot, you canwait until the sample has reached itsfinal temperature and then remove thepan with tweezers and place it on a coldaluminum surface (with a 2 mmdiameter hole for the pin) or immerse itfor about 10 seconds in liquid nitrogen.

Fig. 2. Above: Artifact due to a PE film that was not pressed down firmly in the pan (dotted line). Thesample of film that was pressed down on the base of the pan with the lid of a light Al pan gave the"correct" melting curve.Below: DSC heating curve of 1.92 mg polystyrene showing a typical artifact at about 78 °C caused bythe thermal expansion of the Al pan. This artifact, which is of the order of 10 µW, is only visible withlarge scale expansion (ordinate scale < 1mW).

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solid-solid transitions and glass transitions.The onset temperatures of the melting pro-cesses of nonpolymeric substances are, how-ever, independent of the heating rate.If several effects occur with significant lossof weight (>30 µg), you would of courselike to assign the latter to a particular peak- weight loss is usually an endothermic ef-fect due to the work of expansion resultingfrom the formation of gas. One method is toheat a new sample step by step through theindividual peaks and determine the weightof the pan and contents at each stage (atMETTLER TOLEDO we call this "off-linethermogravimetry"). The best way is tomeasure a new sample in a TGA, ands usethe same type of pan as for the DSC mea-surement.The shape of the DSC curve is usually verycharacteristic and helps to identify the na-ture of the effect.In the following sections, examples of themost important effects and their typicalcurve shapes will be discussed.

Physical transitionsPhysical transitions can in principle bemeasured as many times as desired if• on cooling, the sample reverts to the

same state as before the transition. This,however, is not always the case and

depends on the sample and the coolingrate. Many substances in fact solidifyfrom the melt at fast cooling rates to aglassy amorphous state. This is thereason why no melting peak occurs onheating the same sample a second time.Some metastable crystal modificationscrystallize only in the presence ofcertain solvents.

• the sample does not escape from the panthrough evaporation, sublimation, or(chemical) decomposition , or does notundergo transformation. Any samplelost by evaporation cannot of coursecondense in the sample pan on coolingbecause the purge gas has alreadyremoved it from the measuring cell .

Melting, crystallization andmesophase transitionsThe heat of fusion and the melting pointcan be determined from the melting curve.With pure substances, where the low tem-perature side of the melting peak is almosta straight line (Fig. 4a), the melting pointcorresponds to the onset. Impure and poly-meric samples, whose melting curves areconcave in shape, are characterized by thetemperatures of their peak maxima (Fig.4b and c). Partially crystalline polymersgive rise to very broad melting peaks be-cause of the size distribution of the crystal-lites (Fig. 4c).Many organic compounds melt with de-composition (exothermic or endothermic,Figs. 4d and 4e).An endothermic peak in a DSC heatingcurve is a melting peak if• the sample weight does not decrease

significantly over the course of the peak.A number of substances exhibit amarked degree of sublimation aroundthe melting temperature. If hermeticallysealed pans are used, the DSC curve isnot affected by sublimation and evapo-ration.

• the sample appears to have visiblymelted after the measurement. Powderyorganic substances, in particular, form amelt that on cooling either solidifies to aglass (with no exothermic crystallizationpeak) or crystallizes with an exothermicpeak.Comment: Many metals have a highmelting point oxide layer on theirsurface. After melting, the oxide layerremains behind as a rigid envelope. This

Fig. 3. Below: In an open pan, water evaporates before the boiling point is reached. Middle: In a self-generated atmosphere (50 µm hole in the lid), the boiling point can be measured as the onset.Above: In a hermetically sealed pan (at constant volume), there is no boiling point. The DSC curve isa straight line until the Al pan suddenly bursts at about 119 °C. If the ordinate scale is expanded 20times, an exothermic peak can be observed that is due to the reaction of aluminum with water (seethe expanded section of the curve).

If no thermal effects occurIn this case your sample is inert in the tem-perature range used for the measurementand you have only measured the (tempera-ture dependent) heat capacity.An inert sample does not undergo any lossof weight (except ≤30 µg surface mois-ture). After opening the pan, it looks exactlythe same as before the measurement. Thiscan be confirmed with the aid of a micro-scope for reflected light.If you are interested in cp values, you needa suitable blank curve. Check the plausibil-ity of the results you obtain: values for cpare usually in the range 0.1 to 5 Jg-1K-1.To make absolutely sure that no effects oc-cur, extend the temperature range of themeasurement and measure larger samples.

If thermal effects are visibleThermal effects are distinct deviations fromthe more or less straight line DSC curve.They are caused by the sample undergoingphysical transitions or chemical reactions.If two effects overlap, try to separate themby using faster or slower heating rates, andsmaller sample weights. Here, one shouldtake into account that faster heating ratescause a marked shift in the peak maximaof chemical reactions to higher tempera-tures. To a lesser extent, this also applies to

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is the reason why, on opening the pan,the sample looks exactly the same asbefore melting - it would in fact requiresamples weighing several grams todeform the oxide layer under the forceof gravity, so that the sample fits theshape of the pan. Precious metals haveno oxide layer and form sphericaldroplets on melting.

• its surface area is between about 10 Jg-1

and 400 Jg-1. The heat of fusion onnonpolymeric organic substances isalmost always between 120 Jg-

1and 170 Jg-1.• its width at half height (half-width) is

significantly less than 10 K (partiallycrystalline polymers can melt over awider range). The melting peak isincreasingly sharper, the purer thesubstance and the smaller the size of thesample. Very small quantities of puresubstances give peaks with half-widthsof less than 1 K.

Impure samples and mixtures often showseveral peaks. Substances with eutectic im-purities exhibit two peaks (Fig. 4b): firstthe eutectic peak, whose size is propor-tional to the amount of impurity, and thenthe main melting peak. Sometimes the eu-tectic is amorphous so the first peak ismissing. Liquid crystals remain anisotropiceven after the melting peak. The melt doesnot become isotropic until one or moresmall sharp peaks of mesophase transitionshave occurred (Fig. 4f).An exothermic peak on a cooling curve is acrystallization peak if• the peak area is about the same as the

melting peak - since the heat of fusionis temperature dependent, a difference ofup to 20% can arise depending on thedegree of supercooling.

• the degree of supercooling (the differ-ence between the onset temperatures ofmelting and crystallization) is between1 K and about 50 K. Substances thatcrystallize rapidly show an almostvertical line after nucleation until (if thesample is large enough) the meltingtemperature is reached (Figs. 5a, 5g).

If the liquid phase consists of a number ofindividual droplets, the degree of super-cooling of each droplet is different so thatseveral peaks are observed (Fig. 5b).Organic and other "poorly crystallizing"compounds form a solid glass on cooling

(Fig. 5c). Such amorphous samples canthen crystallize on heating to temperaturesabove the glass transition temperature (de-vitrification, cold crystallization). Coldcrystallization can often occur in two steps.On further heating, polymorphic transi-tions can occur before the solid phase fi-nally melts (Fig. 5e).When the melt of a sample containing eu-tectic impurities is cooled, the main com-ponent often crystallizes out (Fig. 5d). Itcan, however, solidify to a glass (Fig. 5c).Very often the eutectic remains amorphousso that the eutectic peak is missing.A polymer melt crystallizes after supercool-ing by about 30 K (Fig. 5f). Many polymerssolidify to glasses on rapid cooling(Fig. 5c).When the melt of a liquid crystal is cooled,the mesophase transitions occur first (oftenwithout any supercooling). The subsequentcrystallization exhibits the usual super-cooling (Fig. 5g).

Solid-solid transitions, polymor-phismSolid-solid transitions can be identified bythe fact that a sample in powder form isstill a powder even after the transition.The monotropic solid-solid transition ofmetastable crystals (marked α' in Fig. 6)to the stable α-form, which is frequentlyobserved in organic compounds, is exother-mic (Fig. 6a). As the name implies,monotropic transitions go in one directiononly (they are irreversible).The monotropic transition is slow and ismost rapid a few degrees K below the melt-

Fig. 4. Melting processes: a: a nonpolymeric puresubstance; b: a sample wit a eutectic impurity; c:a partially crystalline polymer; d and e: meltingwith decomposition; f: a liquid crystal.

Fig. 5. Crystallization: a: a pure substance (Tf isthe melting point); b: separate droplets solidifywith individual degrees of supercooling; c: a meltthat solidifies amorphously; d: a sample with aeutectic impurity; e: a shock-cooled melt crystal-lizes on warming above the glass transition tem-perature (cold crystallization); f: a partially crys-talline polymer; g: a liquid crystal

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ing point of the metastable phase. In spiteof this, the peak height is usually less than0.5 mW and can therefore easily be over-looked alongside the following meltingpeak of about 10 mW (gray arrow in Fig.6b). It is often best to measure the

monotropic transition isothermally.At heating rates greater than 5 K/min, it iseasy to "run over" the slow transition (Fig.6b) and so reach the melting temperatureof the metastable form. The monotropicsolid-solid transition is either not visible orit could be falsely interpreted as a slightlyexothermic "baseline shift" before themelting peak. If some stable crystals arepresent that can serve as nuclei for thecrystallization of the liquid phase formed,the melting peak merges directly into theexothermic crystallization peak. This caseis referred to as a transition via the liquidphase - on immediate cooling to room tem-perature, the sample would have visiblymelted. Finally the melting temperature ofthe stable modification is reached.If no α-nuclei are present, there is no α-crystallization peak and of course no α-melting peak (Fig. 6c). If the sample con-sists entirely of the stable form, then onlythe a-melting peak appears and the poly-morphic effect is not observed (Fig. 6d).Depending on the substance, the α-form

melts at temperatures that are 1 K to 40 Klower than the stable modification.The enantiotropic solid-solid transi-tion, which occurs less often, is revers-ible. The α→β transition, starting fromthe low temperature form a to the high

temperature form β is endothermic. Theenantiotropic transition gives rise to peaksof different shape depending on the particlesize of the sample because the nucleationrate of each crystal is different. For statisti-cal reasons, samples that are finely crystal-line give rise to bell-shaped (Gaussian)peaks (Figs. 7a and 7c). A small number oflarger crystals can give rise to peaks withvery bizarre shapes . This is especially thecase for the reverse β→α transition (Figs.7b and 7d).The peaks of enantiotropic transitions typi-cally have α half-width of 10 K.

Transitions with a distinct loss ofweightThese types of transitions can of course onlybe observed in open pans, i.e. either a panwith no lid, or a pan with a lid and a 1 mmhole to protect the measuring cell fromsubstances that creep out or that splutter.

Examples are:• the evaporation of liquid samples (Fig.

3, below and Fig. 8a),• drying (desorbtion of adsorbed moisture

or solvents, Fig. 8b),• the sublimation of solid samples (Fig.

8b) and the• decomposition of hydrates (or solvates)

with the elimination of the water ofcrystallization. In an open crucible, theshape of the curve corresponds thatshown in Fig. 8b, and in a self-gener-ated atmosphere to that in Fig. 8c.

These peaks have a half-width of ≥20 K(except in a self-generated atmosphere)and have a shape similar to that exhibitedby chemical reactions. The decompositionof solvates is known as pseudo-polymor-phism (probably because in a hermeticallysealed pan, a new melting point occurswhen the sample melts in its own water ofcrystallization) and can also be regarded asa chemical reaction.In a self-generated atmosphere (with a50 µm hole in the lid of the pan), theevaporation of liquids is severely hindered.The usual very sharp boiling peak (Fig. 3,middle and Fig. 8d) does not occur untilthe boiling point is reached.Apart from the appreciable loss of weight,these reactions have another feature incommon, namely that the baseline shifts inthe exothermic direction due to the de-creasing heat capacity of the sample.

The glass transitionAt the glass transition of amorphous sub-stances, the specific heat increases by about0.1 to 0.5 Jg-1K-1. This is the reason why theDSC curve shows a characteristic shift inthe endothermic direction (Fig. 2, belowand Fig. 9a). Typically• the radius of curvature at the onset is

significantly greater than at the endsetand

• before the transition, the slope is clearlyendothermic, and after the transitionthe curve is (almost) horizontal.

The first measurement of a sample that hasbeen stored for a long time below the glasstransition temperature, Tg, often exhibits anendothermic relaxation peak with an area of1 Jg-1 to a maximum of about 10 Jg-1 (Fig.9b). This peak can no longer be observedon cooling (Fig. 9c), or on heating a sec-ond time. The glass transition covers atemperature range of 10 K to about 30 K.

Fig. 6. Monotropic transition: a: the arrow marksthe solid-solid transition, afterward the a-modifi-cation just formed melts; b: in this case the solid-solid transition is so slow that a crystallizes; c:the pure α'-form melts low; d: the pure α-formmelts high. Fig. 7. Reversible enantiotropic transition: a: a

fine powder; b: coarse crystals; c: reverse transi-tion of the fine powder; d: reverse transition ofthe coarse crystals; at Tt, α and β are in thermo-dynamic equilibrium.

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Fig 10. Curve shapes of chemical reactions: a: anideal exothermic reaction; b: reaction with "inter-fering" physical transitions and the beginning ofdecomposition; c: chemical reaction with a sec-ondary reaction; d: partial oxidation of organicsamples with the residual oxygen in a hermeti-cally sealed pan.

You can identify an effect that resembles aglass transition by checking whether thesample is visibly soft, almost liquid or rub-bery-like above the Tg. If you do not haveaccess to a TMA or DMA instrument, youcan check this by heating a sample up to atemperature of Tg + 20 K in a pan withouta lid. After several minutes at this tempera-ture, you open the lid of the measuring celland press the sample with a spatula or aneedle. It is, however, difficult to detectsoftening in this way especially with poly-mers containing large amounts of fillers.

Lambda transitionsThese types of solid-solid transitions exhibitΛ-shaped cp temperature functions. Themost important is the ferromagnetic Curietransition, which was previously used tocalibrate the temperature scale of TGA in-struments. The DSC effect is however ex-tremely weak (Fig. 9d). To make sure, youcan check that the sample is no longermagnetic above the Curie temperature witha small magnet.

Chemical reactionsChemical reactions can in general only bemeasured in the first heating run. On cool-ing to the starting temperature, the reac-tion product remains chemically stable, sothat on heating a second time no furtherreaction takes place 1 . In some cases, how-ever, the reaction does not go to completionduring the first heating run, so that onheating a second time, a weak postreactioncan be observed (e.g. the curing of epoxyresins).The half-width of chemical reaction peaksis about 10 K to 70 K (usually about 50 K ata heating rate of 10 K /min to 20 K/min).Reactions which show no significant loss ofweight are usually exothermic (about 1 Jg-1

to 20 000 Jg-1, Figs. 10a and 10b). Theothers tend to be endothermic because thework of expansion predominates.Ideally, DSC curves of a chemical reactionshow a single smooth peak (Fig. 10a). Inpractice, however, other effects and reac-tions often overlap and distort the peakshape, e.g. the melting of additives (Fig.10b), or secondary or decomposition reac-tions (Fig. 10c).

Examples of reactions with significant lossof weight are:• thermal decomposition (pyrolysis under

an inert gas), with CO, short-chainalkanes, H

2O and N

2 as the most

frequently occurring gaseous pyrolysisproducts,

• depolymerization with more or lessquantitative formation of the monomerand

• polycondensation, for example thecuring of phenol and melamine resins.2

Reactions with a significant increase ofweight nearly always involve oxygen andare strongly exothermic. Examples are:• the corrosion of metals such as iron and• the initial uptake of oxygen at the

beginning of the oxidation of organiccompounds. During the course of thereaction, volatile oxidation productssuch as carbonic acids, CO

2 and H

2O are

formed, so that finally a weight lossoccurs (the initial increase in weightcan be seen best in a TGA curve).

Fig. 9. Step transitions: a: a glass transition; b: aglass transition with enthalpy relaxation; c: thereverse transition; d: a Curie transition

Fig. 8. Transitions with weight loss: a: evapora-tion in an open pan; b: desorbtion, sublimation; c:dehydration; d: boiling in a pan with a small holein the lid, Tb is the boiling point.

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DSC822e

Examples of reactions with no significantchange in weight are3:• addition and polyaddition reactions,

curing of epoxy resins,• polymerizations, dimerizations,• rearrangements and• the oxidation of organic samples (e.g.

polyethylene) with the residual atmo-spheric oxygen (about 10 µg) in ahermetically sealed pan (Fig. 10d).

Final commentsThis article should help you to interpretDSC curves. You will, however, often have touse additional methods for confirmation.Some important techniques are:

In the new DSC822e, both the temperatureand the DSC signal are measured with ananalog to digital converter whose resolu-tion is 16 times better than that used previ-ously. This allows the temperature to becontrolled more accurately and results in amarked reduction of the noise on the DSCsignal (Fig. 1).In the DSC821e, the DSC signal range of700 mW was defined by 1 million points,giving a resolution of 0.7 µW. In the newDSC822e, this signal range is now definedby 16 million points and is therefore muchmore accurately resolved.Operation of the DSC822e requires the latestversion of the STARe software, V6.10. Fig. 1. The above measurement of a liquid crystal demonstrates the improved signal to noise ratio.

New in our sales program

Temperature range -150 – 700 °CTemperature accuracy ± 0.2 °CTemperature reproducibility ± 0.1 °CSensor type FRS5 ceramic sensor with 56 AuAuPd

thermocouplesSignal time constant 2.3 sMeasurement range 700 mWDigital resolution 16 million pointsSampling rate Max. 10 points per second (selectable)

Specifcations

• thermogravimetric analysis, ideally incombination with DTA or SDTA. Theinterpretation of DTA and SDTA® curvesis analogous to DSC with limitationsdue to reduced sensitivity,

• thermomechanical and dynamicmechanical analysis,

• the analysis of the gaseous substancesevolved (EGA, Evolved Gas Analysis)with MS or FTIR and

• the observation of the sample on a hotstage microscope (TOA, Thermo-OpticalAnalysis in the FP82 or the FP84 withsimultaneous DSC)

In addition, various other chemical orphysical methods are available. These de-pend on the type of sample, and can be ap-plied after each thermal effect has takenplace.

1 There are very few exceptions to thisrule; one example is the polymerizationof sulfur, which begins on heating atabout 150 °C and which is then revertedon cooling at about 130 °C.

2 These slightly exothermic reactions areoften measured in high pressurecrucibles in order to suppress theendothermic vaporization peak of thevolatile side-products.

3 These reactions are often performed inhermetically sealed Al pans in order toprevent the release of small amounts ofvolatile components.

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IntroductionIn the first part of this work (UserCom 10),the basic principles of the glass transitionas well as its measurement and evaluationwere discussed. This second part describes anumber of practical aspects.A glass transition always requires the pres-ence of a certain degree of disorder in themolecular structure of the material underinvestigation (e.g. amorphous regions). It

The glass transition from the point of view of DSC measure-ments; Part 2: Information for the characterization of materials

Applications

content and consequently the intensity ofthe glass transition (step height ∆cp) de-crease.The molecular mobility in amorphous re-gions is influenced by the presence of crys-tallites. This is particularly the case withpolymers because some macromoleculesare part of both the crystalline and theamorphous components. As a result of this,the glass transition is broader and is shifted

Fig. 1. The specific heat capacity of PET is shown as a function of tem-perature in the region of the glass transition. The sample was crystallizedat 120 °C for different periods of time (tc). The crystallinity increaseswith the crystallization time, while ∆cp (DeltaCp) decreases. (Sampleweight: 14 mg, heating rate: 10 K/min).

Fig. 2. The normalized step height of the specific heat at the glass transi-tion as a function of the crystallinity. (Polymer: PET crystallized isother-mally at 120 °C), A: Behavior of a two phase system; B: Measured be-havior for a three phase system.

to higher temperature. This behavior is il-lustrated in the example in Figure 1, whichshows the glass transition of varioussamples of polyethylene terephthalate(PET) that have been crystallized underdifferent conditions. In Figure 2, the nor-malized step height at the glass transitionis shown as a function of crystallinity for anumber of different PET samples that hadbeen allowed to crystallize for different pe-riods of time at 120 °C. The line marked Arepresents a two phase behavior that canoccur with low molecular weight sub-stances in which only crystals and mobileamorphous material are present. Devia-tions from this behavior can occur withpolymers due to the molecular size be-

cause some of the amorphous regions can-not participate in the cooperative rear-rangements. This rigid amorphous phase islocated at the surface of the chain-foldedcrystals. This allows the proportion of therigid amorphous material in polymers to bedetermined by measuring the step height asa function of the degree of crystallization.

OrientationWhen thin films or fibers are manufacturedfrom polymers, a molecular orientation isintroduced that influences the glass transi-tion. Analogous to the behavior of partiallycrystalline polymers, the glass transitiontemperature is shifted to somewhat highertemperatures and the glass transition itselfbecomes broader. Orientation (e.g. stretch-ing) of partially crystalline polymers canincrease the crystallinity to a marked de-gree. This effect can also be observed at theglass transition. Stretched polymers, how-ever, very often shrink on heating. Thischanges the contact between the sampleand the DSC sensor during the measure-ment. The shrinking process begins at the

is very sensitive to changes in molecularinteractions. Measurement of the glasstransition can therefore be used to deter-mine and characterize structural differ-ences between samples or changes in mate-rials. The following article presents a num-ber of examples to illustrate the type of in-formation that can be obtained from ananalysis of the glass transition.

Partially crystalline materialsIn addition to completely amorphous orcompletely crystalline materials, there areof course materials that are partially crys-talline. In these types of material, crystal-lites and amorphous regions coexist. Withincreasing crystallinity, the amorphous

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glass transition and can result in DSCcurves that are completely unusable. Only apreheated sample (a sample that has al-ready shrunk) can be measured reproduc-ibly. However, preheating the sample elimi-nates the thermal and mechanical historyof the sample.Figure 3 shows the glass transition of ori-entated PET fibers. The beginning of theglass transition is clearly visible in the firstmeasurement. However, recrystallizationalready begins during the glass transition(exothermic peak between 80 °C and140 °C). The fiber shrinks in this tempera-ture range. If the fiber is heated to a tem-perature just below the melting tempera-ture and then cooled, the sample is par-

The glass transition temperature was deter-mined from these curves using two meth-ods: firstly as the point at which the bisec-tor of the angle between the two tangentsintersects the measurement curve, (Tg1),and secondly as the "fictive temperature"according to Richardson's method, (Tg2).While Tg1 increases with aging, Tg2 de-creases continuously. In addition, the en-thalpy relaxation was evaluated accordingto the method described in Part 1 of thisarticle. The results are shown in Figure 5.It can be clearly seen that the change of Tg2with time is analogous to that of enthalpyrelaxation. Tg2 describes the physical stateof the glass before the measurement. Thecourse of Tg1 is however, also dependent on

If an epoxy resin is cured isothermally at atemperature of Tc, the glass transition tem-perature increases with increasing curingtime. If the glass transition temperature ofthe cured material is greater than Tc, thenvitrification occurs. The sample changesfrom a liquid to a glassy state. The reactionrate thereby decreases drastically and theglass transition temperature from then onchanges only very slowly (see Fig. 8). Atthe vitrifications time, tv, the glass transi-tion temperature is equal to the curingtemperature.A similar relationship between the glasstransition temperature and the degree ofcrosslinking (degree of vulcanization) canalso be observed with many elastomers.

Fig. 3. Glass transition of stretched PET fibers (see text for details). Thearrows mark the glass transition (Sample weight: 4 mg, heating rate:10 K/min).

Fig. 4. Glass transition of samples of PET that have been stored for differ-ent periods of time at 65 °C. (Sample weight: 23 mg, heating rate:10 K/min).

tially crystalline and shows a broad glasstransition at a somewhat higher tempera-ture (2nd run in Figure 3). If the fiber ismelted and then shock cooled (3rd run),the sample is amorphous. The measurementcurve shows the glass transition and the sub-sequent exothermic recrystallization peak.

Physical agingAs has already been discussed in Part 1 ofthis article (UserCom10), both the shape ofthe curve in the region of the glass transi-tion and the glass transition itself dependon the actual storage conditions below theglass transition. Longer storage times leadto the formation of an enthalpy relaxationpeak. This process is known as physical ag-ing. To illustrate this effect , a series of heatcapacity curves are shown in Fig. 4, usingsamples of polyethylene terephthalate (PET)that had been stored for different periods at 65 °C.

the actual measurement conditions.The enthalpy relaxation peaks are depen-dent on internal stresses that, for example,originate in the processing conditions, anddepend on the thermal history during pro-cessing and storage. As can be seen in Fig.6, these peaks can occur at different placesin the glass transition region depending onthe sample and the thermal history. Thesamples were cooled rapidly before per-forming the second measurement. Thiscooling process performed under definedconditions eliminated the effects of thermalhistory.

CrosslinkingIn crosslinked systems (thermosets such asepoxy resins), the glass transition tempera-ture is dependent on the degree of crosslinking.With increasing crosslinking, the glass transitionshifts to higher temperatures (see Fig. 7).

However, the changes are relatively small(Fig. 9) because the density of crosslinkingis relatively low.

Molar massIn much the same way as a crosslinkingreaction, the glass transition temperaturein a polymerization increases with increas-ing molar mass Mw. The maximum valueof Tg is reached at a molar mass of 104 to105 g/mol. The relationship can be de-scribed to a good approximation (Fig.10)by the equation

w

ggM

JTT −= ∞

J is a polymer-specific constant.

PlasticizersFigure 11 shows the effect of the plasticizercontent on the glass transition of a polyvi-

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crease depending on which componentswere mixed together. In such cases, at leasttwo glass transitions are observed afterseparation.

CopolymersWith copolymers, the glass transition is de-pendent on the type of polymerized mono-mers and their configuration in the macro-molecule. If the monomers are miscible orstatistically distributed, then one singleglass transition is observed. With block andgraft polymers, a phase separation oftenoccurs. Two glass transitions are then mea-sured. If the blocks are too short, then forchemical reasons no phase separation can

nyl acetate (PVAc). Increasing concentra-tions of plasticizer cause the glass transi-tion temperature to shift to lower values(Fig. 12). With some materials, it is pos-sible for water (moisture) absorbed fromthe air to act as a plasticizer. Solvent resi-dues, originating from the manufacture orprocessing of the material, can also behaveas (unwelcome) plasticizers.

Polymer mixturesBecause of the large variety of polymermixtures (polymer blends), only a few as-pects of the glass transition can be men-tioned here.

Fig. 5. Glass transition temperature Tg1 (intercept of the bisector; opencircles) and Tg2 (according to Richardson; black dots) as well as the en-thalpy relaxation -∆Hrelax of PET (aged at 65 °C) as a function of the ag-ing time.

Fig. 6. First and second measurements of the glass transition of anacrylic copolymer and PMMA. The arrows mark the relaxation peaks.

In principle, polymers are either miscible(compatible) or immiscible (incompat-ible). With immiscible polymers, the indi-vidual components occur as separatephases. Regions of different phases exist atthe same time alongside one another. Eachof these phases can individually undergo aglass transition which means that severaldifferent glass transitions are measured. Acomparison of the step heights and theglass transition temperatures with those ofthe pure components can provide informa-tion on the relative content of the phasesand possible interactions between thephases, as well as on the quality of the mix-ing process. If the various glass transitionslie very close to each other, it is very diffi-cult to separate them in a "normal" analy-sis. Annealing at a temperature just belowTg produces relaxation peaks that often al-low a separation to be made.

An example of an incompatible mixture isshown in Figure 13. A polycarbonate (PC)was mixed with ABS. The two glass transi-tions can be clearly seen in the measure-ment curve of the mixture. The PC glasstransition temperature is lowered by about3 K due to interaction with the ABS. Fromthe ratio of the step heights of the PC glasstransition (∆cppure/∆cpmixture), it can beestimated that the mixture consists of 67%PC and 33% ABS.

With miscible substances, a homogeneousphase is formed and one single glass tran-sition is measured. The glass transitiontemperature Tg depends on the concentra-

tion of the individual components. The re-lationship between the glass transitiontemperature and the composition can bedescribed by the semi empirical Gordon-Taylor equation:

21

2211

kww

TkwTwT

gg

g ++

=

Tg1 and Tg2 are the glass transition tem-peratures of the pure components and w1and w2 are the proportions by weight. k canbe looked upon as being a fit parameter.The change of the glass temperature as afunction of concentration of the concentra-tion of PS-PPE blends is shown in Figure14. (PPE is polyphenylene ether).

A homogeneous mixture need not necessar-ily be stable. A phase separation can occuras a result of a temperature increase or de-

take place, and only one transition is ob-served. Figure 15 shows the glass transi-tions of a gel consisting of two block co-polymers. The substances differ only in thelength of the blocks. In sample 2, theblocks are relatively long and a phase sepa-ration occurs. In sample 1, a phase separa-tion is not possible because the blocks areshort.

Chemical modificationChemical modification can also influencemolecular mobility. Phase separation is inthis case also possible. Chemical modifica-tion can be deliberate or can occur throughchemical aging. In chemical aging, degra-dation or oxidation takes place. An ex-ample of a deliberate modification is thechlorination of polyvinylchloride (PVC).Figure 16 shows the effect of the chlorine

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concentration on the glass transition.Higher concentrations of chlorine decreasethe molecular mobility. As a result of this,the glass transition shifts to higher tem-peratures.The broadening of the glass transition withincreasing chlorine content is particularlynoticeable. The reason for this is the rela-tively large degree of inhomogeneity of thechlorine distribution.

In chlorination, a hydrogen atom is re-placed by a chlorine atom. This does notchange the number of degrees of freedomof a monomer unit. The step height (∆cp)with respect to the mole therefore remainsunaffected by chlorination. The reductionof the step height with increasing chlorina-

Fig. 9. Glass transition temperature as a function of the degree of vul-canization of an NBR rubber (Nitrile-Butadiene-Rubber). The sampleswere vulcanized isothermally at 70 °C, 130 °C and 150 °C.

Fig. 7. Glass transition temperature as a function of the degree of cross-linking of an epoxy resin system.

tion, which is apparent in Figure 16, istherefore due to the increase in size of themolar mass. This allows the change of∆cp to be used to estimate the chlorinecontent. The molar mass of a PVC mono-mer unit, MPVC, is 65.5 g/mol. Because themolar mass of chlorine is 35.5 g/mol, thisgives a value of 56.8% for the chlorine con-tent of PVC. The ∆cp step height, ∆cPVC is0.28 J/gK. This corresponds to18.34 J/molK. The height of the ∆cp step ofthe chlorinated PVC sample with the lowercontent of chlorine can determined rela-tively accurately (∆cPVCC= 0.24 J/gK). Themolar mass of the chlorinated PVC, MPVCC,can be estimated from the equation

In the case considered, this gives a value ofMPVCC=76.41 g/mol. This corresponds to1.31 chlorine atoms per monomer unit andhence a chlorine content of 60.8%. Thisagrees very well with the data given for thissample.

FillersInert substances such as glass fibers, chalkor carbon black are often added to poly-mers as fillers. They lower the polymer con-tent of the materials and thereby reduce thestep height of the glass transition. The stepheight ∆cp is proportional to the polymercontent. In general, the glass transitiontemperature is independent of the fillercontent. Only with active fillers can rela-tively small changes in Tg be observed.

Fig. 8. Change of the glass transition temperature during the isothermalcross-linking of an epoxy resin system at Tc = 100 °C. New samples werecured for different periods of time at Tc and then cooled rapidly. The glasstransition temperature was determined from the heating measurement at10 K/min.

Fig. 10. Glass temperature of polystyrene (PS) as a function of the reci-procal mole mass (Tg∞ = 101 °C, J = 2.2 kgK/mol).

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Fig. 11. Heat capacity as a function of temperature in the glass transitionregion of PVAc containing different concentrations of plasticizers.

Fig. 12. Glass transition temperature of PVAc as a function of the plasti-cizer content (data from the measurements in Fig. 11).

Fig. 13. Glass transition of samples of pure PC and a PC-ABS blend(sample weight about 10 mg, heating rate: 10 K/min).

ConclusionsThe glass transition is a phenomenon thatcan be observed in (partially) disorderedsystems as a step in the heat capacity curve.

Fig. 15. Glass transition region of gels of block copolymers made of thesame components but with different block lengths. The arrows mark theglass transitions (sample 1: short blocks; sample 2: long blocks).

Fig. 14: Glass transition temperature as a function of the composition ofPS-PPE mixtures. The continuous curve corresponds to the Gordon-Taylorequation with k = 0.63.

Fig. 16. Glass transition of samples of PVC and PVC that have been chlo-rinated to different extents. In the sample with 66.5% Cl, the glass tran-sition is so broad that it has still not been completed at 150 °C.

It is normally characterized by the glasstransition temperature, Tg, the step height,∆cp, and the width of the transition. Vari-ous methods can be used to determine the

glass transition. The glass transition is pri-marily a result of molecular interactionsand can therefore be used to detect smallchanges in the structure of samples.

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SummaryEffect on the glass transition: Special comments:

Crystallinity Increasing crystallinity → smaller For low molecular substances, the crystallinity

step height; can be determined from ∆cp ; for polymers theThe glass transition is larger and broader. proportion of the Tg rigid amorphous phase

Crosslinking, curing, Tg shifts to higher temperature with Tg

bei Mw ab ca. 104 g/mol is c onstant

polymerization, molar mass increasing molar mass or crosslinking.

Orientation and storage Internal stresses and storage shift Tg Possible crystallization in the glassbelow T

g and increase the size of the enthalpy transition region;relaxation peak. Often, the first measurement cannot be used;

Possibly use the evaluation, according toRichardson.The relaxation peaks contain informationabout the sample history.

Plasticizers Plasticizers shift Tg to Solvent residues and moisture often behavelower temperatures. as plasticizers (Tg is higher in the 2nd

measurement if weight loss occurs)

Mixtures Incompatible mixtures give two The content can be determined from Tg as atransitions, compatible mixtures only one. function of the composition or the step height;

Copolymers Block and graft copolymers of Tg and the width of the transitions depend oncompatible monomers and the interactions of the phases.statistical copolymers showone transition; otherwise two transitions.

Chemical modification Tg, step height and the width of the transition By specific chemical modification orcan change; several transitions can occur. chemical aging such as oxidation or

degradation of polymers

Fillers The step height decreases with increasing Hardly any effect on Tg

filler content.

One problem that affects the measurementand evaluation of the glass transition is thefact that the change in heat capacity can bevery small (particularly with filled or par-tially crystalline materials). To improve theresolution, it is best to measure relativelylarge samples (e.g. with polymers typically10 mg to 20 mg). In addition, thermal con-tact should be optimized, for example bycompacting powders or by premelting inthe pan. Usually a combination of mea-surements involving heating, cooling andthen heating a second time yields the infor-mation required. The investigation can besupplemented by measuring samples thathave been annealed just below the glasstransition temperature. With these types ofsample, both temperature-dependent and

time-dependent peaks occur. Broad and flattransitions are particularly difficult to de-tect. In this case, subtraction of a blankcurve often makes the evaluation easier.A major problem when determining theglass transition temperature is where todraw the tangents. A lot of care should betaken in the evaluation of the curve. It isessential to use adequate scale expansionfor the relevant part of the curve. If severalglass transition are to be compared withone another, it is best to normalize thecurves with respect to sample weight or toevaluate the heat capacity. Furthermore ithelps to display the curves in a coordinatesystem and to choose the tangents so thatin all the curves the high and the low tem-perature tangents run parallel to each

other. This allows even small changes inthe glass transition temperature to be sys-tematically detected and evaluated.The glass transition temperature is not athermodynamic fixed point . It depends onthe heating and cooling rates, the thermaland mechanical history and the methodused to determine it. Especially when largeoverheating peaks occur, Richardson'smethod (glass transition temperature asthe fictive temperature) gives results for theglass transition temperature that are moresignificant and more reproducible thanthose from other methods. In any case, thestep height should also be included in theevaluation, because this value contains im-portant information about the material un-der investigation.

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Many of the pure starting materials used inthe pharmaceutical industry and in foodtechnology can be routinely analyzed andcharacterized with the help of meltingpoint determination. The situation is quitedifferent, however, for edible oils, fats, andwaxes.

Thermal valuesThe variable composition and differentcrystal modifications of such productsmean that they cannot effectively be char-acterized by one single thermal value, e.g.the melting point.Nevertheless, at least for comparison pur-poses, a number of different procedureshave been developed to obtain thermal val-ues that can be easily measured in routineanalysis, e.g. softening points, droppingpoints, slip melting points , melting pointaccording to Wiley and Ubbelohde, etc.

DSCIn contrast, DSC analysis, which measuresthe heat absorbed when the temperature ofa sample is raised at a linear rate, offersmany more possibilities. The result is nowno longer a single temperature value, but acomplete measurement curve that recordsall the thermal effects occurring in thetemperature range investigated. This tech-nique allows a much more detailed com-parison and characterization of oils fatsand waxes to be made. But can we convertthe data from such complex measurementcurves into the numerical values that inthe end are required for comparative as-sessments and as characteristic values?One method often used is to measure thearea between the measurement curve andthe instrument baseline at discrete tem-perature intervals. These areas are thencalculated as percentages of the total areaunder the melting curve and the resultspresented in tabular form. In the literature,the values obtained by this method are re-ferred to as the liquid fraction, LF, or thecomplementary term solid fat index.

Comparison DSC - thermal valuesCan the results from different methods becorrelated in order to obtain a uniform setof results from various different sources? Inprinciple, no, because in fact very differentproperties are measured. In the slip melt-ing point and dropping point methods, thetemperature-dependent viscosity of thesample plays an important role in additionto the actual physical melting. In compari-son, DSC measures only the heat requiredto melt the crystallites. The following tablecompares the results obtained from theanalysis of five different samples with bothtechniques. The dropping point tempera-

tures were measured with a METTLER TO-LEDO FP900 system and FP83HT measur-ing cell. The DSC results were obtained us-ing a METTLER TOLEDO DSC821e

equipped with an IntraCooler accessory andshows the temperatures at which 95% ofeach sample (as measured by the surfacearea under the curve) melted.

Sample preparation and measure-mentReproducible sample preparation is essen-tial for these measurements. With droppingpoint measurements, the fat was first com-pletely melted at 65 °C and then trans-

ferred to the standardnipple using a pipette(about 0.5 ml). It wasthen allowed to cool atroom temperature for 1hour and then stored for12 hours in the deep-freezer compartment of arefrigerator.For the DSC measure-ments, about 10 µl of each

Thermal values of fats: DSC analysis or dropping point deter-mination?Dr. B. Benzler, Applikationslabor METTLER TOLEDO, Giessen

FatFatFatFatFat Dropping point in Dropping point in Dropping point in Dropping point in Dropping point in °CCCCC T at 95% LF in T at 95% LF in T at 95% LF in T at 95% LF in T at 95% LF in °CCCCC

# 1 29.2 29.3# 2 38.1 39.8# 3 43.7 43.9# 4 49.6 52.1# 5 54.7 53.5

Table: Comparison of the dropping point temperature with the tem-perature at which 95% has melted (DSC).

Fig. 1. The DSC curve in the upper part of the diagram shows the complex melting behavior of asample of fat with a heat of fusion of 67.7 J/g. In the lower part of the diagram, the percentageamount of the sample that has melted at any particular temperature is shown as a curve and in tabu-lar form between 50% and 95%.

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The use of MaxRes for the investigation of partially hydratedPortland cement systemsDr. Jordi Payá , Dr. María Victoria Borrachero and Dr. José Monzó, Grupo de Investigación en Química de los Materiales (GIQUIMA), Departamentode Ingeniería de la Construcción, Universidad Politécnica de Valencia, Camino de Vera s/n, E- 46071 Valencia (España)# Direktor der Forschungsgruppe GIQUIMA. E-mail: [email protected]

Fig. 1. TG and DTG curves of Portland cement in an open pan after 4 hours hydration.

of the liquid fat samples were pipetted intostandard aluminum pans, and the samplepretreatment integrated into the DSC mea-surement program. This consisted of a pe-riod at 60 °C, then programmed coolingdown to –30 °C at a cooling rate of 5 K/min,storage for 5 minutes at –30 °C and thenthe heating measurement at 5 K/min. Theresults of a typical measurement are shownin Figure 1. The DSC heating curve isshown in the upper part of the diagram; thearea under the broad, complex meltingcurve was integrated in order to obtain thetotal heat of fusion. In the lower part of thediagram, the percentage amount of thesample that has melted at any particular tem-perature is shown both as a continuous curveand at discrete intervals in tabular form.

The rate at which a sample is cooled to itscrystallization temperature influences thepolymorphic composition of the crystal-lites: the more rapid the cooling, thesmaller is the proportion of the stable(high melting) part. The cooling rate of5 K/min is a good compromise between ashort measurement time and degree of su-percooling that is not too large.

ConclusionsThe characterization of fats and oils bytheir dropping points has the advantage ofbeing simple with respect to both the actualmeasurement and the determination of theresult. The FP83HT measuring cell deter-mines the latter automatically so that theuser does not have to make any decisions at

all. The only disadvantage is that this onesingle value can only to a limited extentdescribe the complex melting behavior ofoils and waxes.DSC analysis,DSC analysis,DSC analysis,DSC analysis,DSC analysis, however, yields much more in-formation regarding the composition and therelative proportions of the fractions with re-spect to temperature. Although stored evalua-tion methods (EvalMacro) can often auto-matically calculate the desired numerical val-ues from the measurement curves, a criticalcheck and possible correction by the user is,however, often appropriate.In both cases, the sample preparation mustbe clearly defined in order to obtain repro-ducible results. This applies in particular tothe crystallization conditions for the mol-ten fats (temperature and time).

• 3CaO.Al2O3.6H2O (C3AH6),• 2CaO.Al2O3.8H2O (C2AH8) and• 4CaO.Al2O3.19H2O (C4AH19)

IntroductionIn cement chemistry the following symbolsare used for simplicity:AAAAA for Al2O3 , CCCCC for CaO, HHHHH for H2O, SSSSS forSiO2 and SSSSS for SO3. For example,tricalcium aluminate, 3CaO.Al2O3 becomesCCCCC33333AAAAA and gypsum, CaSO4.2H2O, becomesC S HC S HC S HC S HC S H22222.The addition of water to Portland cementinitiates the setting or hardening reaction,which binds the whole mass together. Thehydration of Portland cement leads to theformation of different hydrates and is a verycomplicated process:• Portland cement contains various

components that take up water ofcrystallization at different rates.

• Many different hydrates, some of whichare not stoichiometric, are formed.

• The degree of crystallinity of thehydrates is low.

In the first few hours after mixing waterwith Portland cement, CCCCC33333AAAAA reacts rapidlywith the formation of a number of differentcalcium aluminum hydrates:

The presence of calcium and sulfate in theaqueous phase (dissolved gypsum) causesC3A to hydrate to ettringite (C6AS3H32):

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TG measurements in an opencrucibleCrucible: 70 µl alumina, heating rate:20 K/min, temperature range: 35 °C to250 °C, purge gas: 75 ml/min nitrogen.

In an open crucible, any volatile compo-nents evolved from the sample are free toleave the crucible. Two weight loss steps canbe observed (Fig. 1). The first, in the range80 °C to 140 °C, is assigned to the dehydra-tion of ettringite and CSH. The second, be-tween 140 °C and 200 °C is due to the lossof water of crystallization from gypsum,which should in fact show two steps:

It was clearly not possible to separate thetwo steps in an open crucible [2].

Measurement in a self-generatedatmosphere to improve the resolutionCrucible: 100 µl aluminum, with a lid witha 50 µm hole, heating rate: 20 K/min, tem-perature range: 35 °C to 250 °C, purge gas:stationary air atmosphere, no flow.In a self-generated atmosphere a large pro-portion of the evolved products remainwithin the volume of the crucible. Thesample is almost in equilibrium with itsgas phase. The result of this is that thermaleffects are shifted to higher temperatureand the weight loss steps are often betterseparated (Fig. 2).

Under these conditions, three steps areclearly visible. The first (from 80 °C and150 °C) is again assigned to the dehydra-tion of CSHCSHCSHCSHCSH and ettringite, the second(150 °C to 180 °C) to the partial dehydra-tion of calcium sulfate dihydrate to thehemihydrate, and the final step (from180 °C to 210 °C) from the hemihydrate tothe anhydrous form of calcium sulfate. TheDTG peak of ettringite has shifted from123 °C (in the open crucible) to 143 °C.And instead of the single peak originallyobserved in the open crucible at 158 °C,there are now two peaks at 169 °C and201 °C.From equations 4 and 5 it is clear that theratio of the step heights for gypsum shouldbe 3:1. In fact a ratio of 2.33:1 was ob-tained, which means that part of the dehy-dration occurred during the ettringite step.

Fig. 2. TG and DTG curves of Portland cement in a self-generated atmosphere after 4 hours hydration.

Fig. 3. MaxRes TG and DTG curves of Portland cement in a self-generated atmosphere after 4 hourshydration. Weight loss as a function of time and temperature.

3CaO.A12O3+3CaSO4.2H2O+26H2O⇒ 6CO.A12O3.3SO3.32H2O

C3A+3CSH2+26H ⇒ C6AS3H32

At the same time, a small amount of colloidal calcium silicate gel (CSH) is formed fromthe CCCCC33333S.S.S.S.S.

C3S+nH2O ⇒ C3S.nH2O (gel)The interpretation of the thermogravimetric curves in the early stages of this hydration ismade more difficult because the decomposition temperatures of CSHCSHCSHCSHCSH, ettringite and cal-cium sufate dihydrate lie close together. The thermogravimetric measurements were performed with a METTLER TOLEDO TGA/SDTA850. The adaptive event-controlled heating rate option (MaxRes [3 - 5]) was used toimprove the separation of the dehydration processes.

Sample preparationA standard mixture of Portland cement and water was allowed to set for 4 hours at 20 °C.At this stage, further uptake of waterof crystallization was stopped by the addition of acetone. The solvent was then removed atroom temperature under vacuum. The resulting powder was stored under nitrogen to pre-vent contact with moisture and carbon dioxide.

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Vitrification and devitrification phenomena in the dynamic curingof an epoxy resin with ADSCS. Montserrat, Y. Calventus und P. Colomer, Departament de Màquines i Motors Tèrmics, Universitat Politècnica de Catalunya, Carrer de Colom 11,E-08222-Terrassa, España

Fig. 4. Effect of the various TGA measurement techniques on the TGA curve form of Portland cementafter 4 hours hydration.

The overlapping of the first two steps is evi-dent from the fact that the DTG curve doesnot return to zero.

Measurement with the adaptiveevent-controlled heating rate option(MaxRes) to improve resolutionA further improvement in resolution is to beexpected through the use of the MaxRessoftware option. The DTG signal is used tocontrol the heating rate [3, 5] .Crucible: 100 µl aluminium, lid with50 µm hole, heating rate: MaxRes (stan-dard conditions [4]), temperature range:35 °C to 250 °C, purge gas: stationary airatmosphere, no flow.

The first step (60 °C to 115 °C) in Figure 3is assigned to the loss of weakly-bondedwater from the CSH gel. The weight lossbetween 120 °C and 150 °C is attributed tothe overlapping of the dehydration ofettringite and the partial dehydration ofcalcium sulfate dihydrate (two peaks in theDTG curve). Finally between 150 °C and200 °C the hemihydrate dehydrates to theanhydrous form of calcium sulfate. The ra-tio of the overlapped second step to thethird step is now 3.47:1 and slightly greaterthan the 3:1 ratio expected. The differenceis ascribed to the simultaneous dehydrationof a certain amount of ettringite.

Literature[1] P.C. Hewlett (Ed). Lea´s Chemistry of

Cement and Concrete, 4th

edition, Arnold,London, pp. 241-298 (1998)

[2] F. Gomá . El Cemento Portland y otrosAglomerantes. Editores Técnicos AsociadosSA, Barcelona, pp. 27-31 (1979).

[3] USER COM 4. Information for user ofMETTLER TOLEDO thermal analysissystems. December 1996, page 4.

[4] B. Schenker and R. Riesen. MaxRes: event-controlled adaption of the heating rate.USER COM 6, December 1997, pp. 10-12.

[5] R. Riesen, Adjustment of heating rate for maxi-mum resolution in TG and TMA (MaxRes),J. Thermal Anal. 53 (1998) 365 – 374.

IntroductionAlternating differential scanning calorim-etry (ADSC) is a DSC technique in which aperiodically varying temperature is super-imposed on a linear heating rate. In thecase of a sinusoidal modulation of ampli-tude AT and frequency ω, the heating rate,β, is described by the equation:

β = βo + AT cos (ωt) (1)

In conventional DSC, the temperature pro-gram is defined by the initial and finaltemperatures and the heating rate. InADCS, however, in addition to the underly-

ing heating rate βo, there are two addi-tional parameters, namely the modulationamplitude AT and the modulation fre-quency ω. These parameters must be care-fully chosen in order to obtain meaningfulinformation from the experiment (see alsothe article in USER COM 6).The modulation of the heating rate resultsin a modulated heat flow signal, Φ. Thismodulated signal is subjected to Fourieranalysis and separated into different com-ponents. One of these components is thetotal heat flow, which corresponds closelyto the signal obtained from a conventional

DSC measurement at a heating rate of βo.In addition, the curve of the complex heatcapacity |Cp∗ | is calculated according tothe equation:

(2)

where AΦ and Aβ are the amplitudes of theheat flow and the heating rate respectively.The phase angle between the modulatedheating rate and the modulated heat flow isalso calculated. This allows certain asser-tions to be made about relaxation processesin the sample.

Figure 4 summarizes the improvement inthe resolution of the TGA curves in one dia-gram. Thanks to the use of MaxRes, the for-mation of ettringite in cement/water mix-tures can be quantitatively measured bysubtracting the height of the hemihydratedehydration step multiplied by three fromthe weight loss in the range 120 °C to150 °C (the second step).

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with an amine hardener based on 3,3'-dim-ethyl-4,4'-diaminodicyclohexylmethane(HY 2954). The fully cured resin exhibiteda maximum glass transition temperature ,Tg∞, of 159 °C measured by ADSC.

The measurements were performed using aMETTLER TOLEDO DSC821e equipped withan IntraCooler cooling accessory. Theresults were evaluated with the STARe soft-ware.An amplitude of 0.2 K and a period of1 minute were used for all the measure-ments described in this article. The averageheating rate was varied between 1 and0.1 Kmin-1. All necessary blank and cali-bration measurements were performed be-fore the actual measurements in order toensure optimum results.The experiments were performed withsample weights of about 10 mg in standardAl pans.

Results and discussionFigure 1 shows the total heat flow, the com-plex heat capacity and the phase angle ofan epoxy amine hardener system duringdynamic curing (average heating rate0.4 K/min, amplitude 0.2 K, period 1 min).The glass transition of the uncured resin isvisible in all three signals (endothermicshift of the DSC curve, the increase in the cpcurve and the relaxation peak in the phaseangle signal). Evaluation of the DSC curvegave a value of –42 °C (midpoint) for theglass transition temperature, Tgo.

At an average heating rate of 0.4 K/min,the exothermic curing reaction begins atabout 20 °C. The maximum reaction rateoccurs at about 70 °C and curing is com-pleted between 180 °C and 200 °C. The in-tegration of the peak using a linearbaseline yields a value of 460 J/g for theheat of cure. As with conventional DSC, theconversion of the reaction can be deter-mined by dividing the partial areas by theheat of fusion (Fig. 1). During the courseof the reaction, the heat capacity increasesdue to the crosslinking. The constant phasesignal shows that no relaxation processesoccur.

The heat capacity decreases at about 90 °Cand then increases again at about 110 °C.These changes of cp correspond to the vitri-fication (at 80% to 90% conversion) and

Fig.2. The same as in Figure 1 but measured with an average heating rate of 0.25 K/min.

Fig.1. Total heat flow, complex heat capacity and phase angle of an amine-hardened epoxy system(average heating rate 0.4 K/min, amplitude 0.2 K, period 1 min). The degree of curing is shownabove the DSC curve.

The use of ADSC allows the isothermal cur-ing of epoxy resins to be investigated. Of par-ticular interest in this respect are vitrificationand the determination of the temperature-time-transformation diagram [2, 3]).This article describes how the ADSC tech-nique can be used to investigate dynamiccuring. Vitrification (liquid→solid transi-tion) followed by devitrification(solid→liquid transition) can be observedon the heat capacity and the phase anglecurves if the heating rate is sufficientlyslow. The corresponding temperatures aredetermined from the |Cp*| signal and en-

tered in the continuous heating cure dia-gram (CHT diagram). The CHT diagramshows the temperatures and times that arerequired to reach these transitions at vari-ous different constant heating rates (4).Analogous to the isothermal TTT diagram,the CHT diagram is used to investigate theproperties and the influence of curing con-ditions on such resins.

Experimental detailsThe epoxy system investigated was an epoxyresin based on a diglycidyl ether of bisphe-nol A (DGEBA) (Araldite LY564) and cured

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then the subsequent devitrification (at 95%conversion) of the epoxy resin. The epoxyresin used shows the vitrification moreclearly than the devitrification. Values of97 °C and 121 °C were determined for themidpoints of the two effects.At lower heating rates, vitrification occursat a lower temperatures, while devitrifica-tion is shifted to slightly higher tempera-tures (Fig. 2). This means that the separa-tion of the two effects increases with de-creasing heating rate. This has also been

observed with other amine-hardened andanhydride-hardened systems using tor-sional braid analysis [4]) and temperaturemodulated DSC [5].A second ADSC measurement of the fullycured resin gave a value for the maximumglass transition temperature of the system,Tg∞, of 159 °C (midpoint of the |Cp*| sig-nal) and a cp change of about 0.20 Jg-1K-1

(Fig. 3). This value for the cp change issmaller than that at Tgo (0.6 Jg-1K-1) and isin agreement with conventional DSC mea-

surements made on other epoxy systems[6]. As expected, the glass transition can beobserved in the DSC curve and as a relax-ation peak in the phase angle.The different vitrification and devitrifica-tion temperatures measured with variousheating rates are shown in the CHT dia-gram (Fig. 4). They define the regionwithin which the glass transition occurs.The values of Tgo (-40 °C) and Tg∞(159 ° C) are also shown. In other epoxyresin systems, devitrification does not occuruntil Tg∞ [4, 5]. According to Verchère etal [7], the reason why devitrification oc-curs at a lower temperature in our system isthe effect of steric hindrance of the methylgroup, which inhibits the reaction with theamine hydrogen atom. Consequently, thefully cured epoxy is only obtained on fur-ther heating up to 250 °C.

ConclusionsThe non-isothermal ADSC technique allowsthe measurement of vitrification and devit-rification temperatures during the curingof epoxy resin systems. This is not possiblewith conventional DSC. The data obtainedcan be used to construct a CHT diagram.Compared with torsional braid analysis,ADSC has the advantage of determining thedegree of cure at the same time.

Fig. 3. Total heat flow, complex heat capacity and phase angle of a fully cured epoxy amine hardenersystem (average heating rate 0.4 K/min, amplitude 0.2 K, period 1 min). This is the second measure-ment of the same sample from Figure 1.

Fig. 4. Continuous heating transformation cure diagram (CHT diagram) of the measured epoxy resinamine hardener system. The dashed lines show the average heating rates used. Filled black squaresmark the vitrification temperatures, and black triangles the devitrification temperatures. White tri-angles show the glass transition temperatures of the fully cured resin, and white squares the glasstransition temperatures of the uncured resin-hardener mixture.

Literature[1] C. T. Imrie, Z. Jiang, J. M. Hutchinson,

Phase correction in ADSC measurementsin glass transition, USER COM No.6,December 97, p.20-21

[2] S. Montserrat, Vitrification in the isother-mal curing of epoxy resins by ADSC, USERCOM No.8, December 98, p.11-12

[3] S. Montserrat, I. Cima, Thermochim.Acta, 330 (1999) 189

[4] G. Wisanrakkit, J. K. Gillham, J. Appl.Polym. Sci., 42 (1991) 2453

[5] G. Van Assche, A. Van Hemelrijck, H.Rahier, B. Van Mele, Thermochim. @σa,286 (1996) 209

[6] S. Montserrat, Polymer Commun., 36(1995) 435

[7] D. Verchère, H. Sautereau, J. P. Pascault,C. C. Riccardi, S. M. Moschiar, R. J. J.Williams, Macromolecules, 23 (1990) 725

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Expansion and shrinkage of fibers

IntroductionFibers are produced worldwide in enormousquantities. More than 20 million tons ofsynthetic fibers and 20 million tons ofnatural fibers are manufactured each year.The total length of these fibers correspondsto about 10 000 times the distance from theearth to the sun.A characteristic feature of a fiber is that itslength is much greater than its diameter.The great anisotropy of the microstructureand the physical properties originatingfrom spinning and stretching processes aretwo of the main reasons for the specialproperties and peculiarities of fibers [1, 2].Spinning, stretching and annealing are infact the most important steps in themanufacture of fibers. These processesdetermine properties such as the modulusof elasticity (Young’s modulus, E) andtoughness that are required for theapplication envisaged. Coloring properties,shrinkage (contraction of fibers) andthermal stability are determined by thesize, number and orientation of thecrystallites, as well as the molecularstructure in the amorphous regions.Thermomechanical analysis (TMA) inparticular, as well as DMA, DSC, TGA andTOA are all excellent techniques for theinvestigation of the effects of temperatureand mechanical loading on fibers andyarns. They allow the relationship betweenstructure, properties and themanufacturing process [3] to beinvestigated. Very often comparativemeasurements under identical conditionsare sufficient to characterize transitiontemperatures, expansion and shrinkingbehavior. TMA measurements also yieldnumerical values such as the coefficient oflinear expansion, Young’s modulus, E, andthe force of contraction as a function oftemperature.

TerminologyFiber strength is normally characterized byits linear density. The SI unit is the tex. Theunit decitex (dtex) is often used, which isthe weight in grams of a length of10 000 m of fiber (or in other words: 1 dtex= 1 µg/cm). In order to compare fibers of

different linear density with respect to theirexpansion behavior, the samples are usu-ally heated under the same tensile force,e.g. 0.1 mN/dtex.Example: a piece of silk thread has alength of 22 cm and a weight of 0.363 mg.The linear density is therefore 16.5 dtex.The thread was subjected to a load of0.002 N in the TMA.

The average linear coefficient of expansion,αl, in the temperature range T1 to T2 canbe calculated from the change in length inthis temperature range, ∆L, and the origi-nal length L0 according to the equation:

The module of elasticity, E, is determinedby the ratio of the tensile force to the ex-pansion:

Here ∆F is the change in the tensile force, Ais the cross-sectional area of the fiber and

DL is the change in length as a result of thechange in the tensile force. This assumesthat the change in length, DL, is smallcompared with the total length, L0.In the TMA, the change in the tensile forceis caused by a stepwise change in the load.During the heating measurement, the ten-sile force exerted on the sample is, for ex-ample, modulated with a constant value of0.06 N with a period of 12 s and an ampli-tude of 0.01 N. This mode of operation isknown as Dynamic Load TMA (DLTMA).

Experimental detailsThe measurements described in this articlewere performed with a METTLER TOLEDOSTARe System and the TMA/SDTA840 mod-ule. The samples were prepared for mea-surement by mounting them in the fiberattachment accessory. The fibers wereplaced in copper clips and fixed in place bymechanically squeezing the clips together.The effective length of fiber between the twoclips was always 13 mm. Samples preparedin this way were mounted between thehooks of the sample holder (see Fig. 1).During the heating measurement, the soft-

Sample Description Linear density Tensile force[dtex] in the TMA [N]

Wool Woll yarn 1157 0.116Cotton Cotton yarn, merceried 298 0.030Silk Silk thread 17 0.002Hemp Hemp fibers from a piece of string 57 0.006Hair (horse tail) Horse hair, black from a horse tail 324 0.033Hair (human) Human hair 47 0.005PAN Polyacrylnitril, yarn 219 0.022PA 66 bulky Nylon, crimped (Helanca) 252 0.025PA 66 Nylon yarn 1400 0.144PA 66 Nylon, 6 fibers (from yarn) 44 0.004PA 66 1 fiber, 0.1 mm (Viscosuisse type 162) 90 various forcesPET 1 fiber 0.048 mm (Viscosuisse, type 200) 25 0.003PET 1 fiber, 0.1 mm (Viscosuisse, type 260) 108 0.011PE 1 fiber (Dyneema®) 13 0.002Kevlar Several fibers 85 0.009Carbon Several fibers 101 0.050Aluminum Aluminum wire, 0.3 mm - 0.050Copper Copper wire, 0.2 mm - 0.050Fused Silica Quartz fiber glass 0.1 mm - 0.050

Table 1. List of the various fibers measured with details of their origin, linear density and the tensileforce used in the experiment.

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Fig. 1. Quartz glass sample holder with fibersample mounted. A piece of indium is attached tothe fiber.

Fig. 2. TMA and SDTA curves showing the temperature check with indium on a PET fiber (see Fig.1).Heating rate: 10 K/min, stationary air atmosphere. SDTA curve: exothermic in the upward direction;TMA: expansion in the upward direction.

ware compensates for both the expansion ofthe clips (the effective length is 1 mm) andthe expansion of the quartz sample holder.

The sample temperature was checked andadjusted using an indium melting pointreference sample. To do this, two smallpieces of indium with a total weight about10 mg were squeezed together around asample of fiber (see Fig. 1). This allowedthe melting point of indium to be mea-sured several times at different heatingrates - the melting point of the fiber mustof course be appreciably higher. The ther-mocouple for the measurement of thesample temperature was positioned about3 mm away from the center of the fiber. Ascan be seen in Figure 2, the SDTA signalrecords the melting of the indium sample.The SDTA signal is the temperature differ-ence between the measured temperature ofthe sample and the program temperature[4]. The SDTA curve in Figure 2 shows asmall peak due to the melting of the in-

dium standard. The onset temperature wasevaluated in the same way as for DSCcurves. The TMA curve also shows a smallstep in the same temperature range. Thereason for this is that the temperature of

the short section of fiber that is enclosed bythe indium sample remains constant. Thissection of the fiber does not therefore ex-pand while the indium melts.

The fiber samples were measured in therange 30 °C to 270 °C at a heating rate of10 K/min in a stationary air atmosphere witha tensile force 0.1 mN/dtex. Table 1 shows alist of the fibers used for the measurements.Any deviations from the experimental con-ditions given above are noted together withthe results of that particular sample.

Fig. 3. Natural fibers (see Table 1). For clarity, dry hair is shown as a dotted curve and horsehair as adashed curve.

ResultsShrinking behaviorExamples of TMA curves of natural fibers,synthetic fibers, and special fibers and wiresare shown in three separate diagrams.A detailed discussion of the thermoanalyticalmeasurement of fibers is given in reference [2].

Natural fibers (Fig. 3)Human hair and silk both shrink (i.e. con-tract) initially due to drying. Decomposi-tion begins above 220 °C and the fibersrapidly tear. Horsehair and hemp show

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relatively little change in length below200 °C (< 0.1 %) under the tensile forceused. Wool, however, expands in the samerange by more that 2 %. Dry human hairshows a similar behavior. Cellulose fibers(e.g. cotton and hemp) show far greaterthermal stability compared with fibers ofhuman or animal origin and expand untilthey decompose and break at about 400 °C.

Synthetic fibers (Fig. 4)Synthetic fibers, in contrast to fibers ofnatural origin, nearly always show amarked shrinkage that is very dependent onthe manufacturing process, and also be-have thermoplastically. With special ex-

identical in form to those of an individualfiber taken from the same yarn. This com-parison shows the excellent reproducibilityof such measurements (see PA66 with 44and 1400 dtex). The PET fibers used havedifferent type designations and their curveforms also show somewhat larger differ-ences. A comparison of the curve of PA66(252 dtex) to the other PA66 curves showshow great the influence of processing onthermal expansion can be. Polyacryloni-trile, (PAN), is dimensionally very stable upto about 130 °C and shows only smallchanges in length of less than 0.5%. Athigher temperatures, however, PAN expandsmore rapidly than wool for example.

αl for aluminum and copper are entered inthe diagram (calculated from the averageslope over a range of 40 K). The literaturevalues for the relevant temperature rangesare also given (upper left).

Effect of conditioningTMA is not just a technique that can beused to measure a new sample of a fiber. Itcan also be used to condition samplesthermally. Both the temperature and theapplied tensile force have a large effect onthe subsequent thermal behavior, whichagain can then be measured with TMA.This conditioning procedure allows processconditions to be simulated or understood,and their effect on the thermal behavior ofthe fibers to be investigated. To illustratethis, a polyamide fiber was cooled withdifferent tensile forces and then heatedagain using a weak tensile force of 0.1 N(see Fig. 6a). Figure 6b shows the heatingcurves for different values of the tensileforce, whereby the cooling beforehand wasperformed with a tensile force of 0.1 N. Thelarger the tensile force used on cooling, thegreater was the shrinkage afterward onheating. If the tensile force used for coolingwas lower that used for the subsequentheating, then the fiber expands until theforce of contraction is sufficiently large tocounteract the expansion.

Determination of the force ofcontractionOne would sometimes like to determine theforce of contraction that develops when afiber is heated but held at constant length.This type of measurement is only possible ifthe TMA is equipped with a suitable acces-sory (e.g. a converter). If, however, theheating curves of individual samples of thesame fiber are measured with different ten-sile forces in the TMA, then the force of con-traction can be determined directly as afunction of temperature from the measure-ment curves (Fig. 7). The temperatures atwhich the length of the fiber after thermalexpansion is the same as its initial lengthare read off from the array of curves. InFigure 8, the temperatures corresponding tothe points of intersection of each TMA curvewith the horizontal straight line throughthe starting point (at 30 °C) are plotted asa function of the force applied. The datapoints show a pronounced increase of theforce of contraction above the glass transi-

Fig.4: Synthetic fibers made from different polymers (see Table 1)

Special fibers and metal wires(Fig. 5)Carbon fibers and quartz glass fibers showonly a very low degree of expansion over awide range of temperature. Quartz glassfibers are brittle and are therefore difficultto mount. They are, however, useful as”inert” material for the determination ofthe baseline (blank curve).The fiber attachment can also be used tomount thin wires. The example shows thedetermination of the linear coefficient ofexpansion (αl) of aluminum and copperwires. In contrast to polymer fibers, αl formetals is only slightly temperaturedependent and the values are much smaller(e.g. 25 ppm/K for aluminum compared to125 ppm/K for wool). The mean values of

tremely orientated fibers (e.g. Kevlar, Fig.5), the degree of shrinking is low (< 0.5%)up to high temperatures (450 °C) and isalso reversible from the second heatingmeasurement onward. Normal, irreversibleshrinkage begins above the glass transitiontemperature (e.g. PET: 80 °C; PA66: <50 °Cdepending on the moisture content; PAN:90 °C) and increases shortly beforemelting. Melting is indicated by a veryrapid increase in length of the fibers. Theextremely rapid shrinkage of PE beforemelting is a result of the special manufac-turing process, in which the fibers arestretched after the spinning process. Sincethe measurement force is normalized to alinear density (0.1 mN/dtex), the TMAcurves of a yarn (with many fibers) are

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Fig. 6a. Thermal conditioning and measurement of the expansion/shrinking behavior of a Nylon fiber (PA66, 90 dtex) using different ten-sile forces. The fibers were conditioned by cooling from 190 °C to35 °C under a tensile force of 0.1 N. The subsequent measurementswere performed with the tensile forces noted next to the curves.

Fig. 6b. Measurement of the expansion and shrinking behavior of aNylon fiber (PA66, 90 dtex) after conditioning the fiber by coolingfrom 190 °C to 35 °C under the tensile forces noted next to the curves.The subsequent measurements were performed with a tensile force of0.1 N.

Fig. 5. Special fibers and metal wires

Determination of Young’s modulusIn addition to the investigation of shrink-age, one of the main applications ofthermomechanical analysis for the charac-terization of fibers is the determination ofYoung’s modulus, E, and its dependence ontemperature. With the TMA/SDTA840, a pe-riodically changing force is used instead ofthe constant force (DLTMA operatingmode). The resulting expansion is used inthe evaluation to calculate the value ofYoung’s modulus. During heating, thesample is modulated with a periodic, step-wise change of force (period usually 12 s,amplitude typically 0.01 N). This also al-lows the temperature dependence ofYoung’s modulus to be measured duringshrinking. Figure 9 shows the DLTMAcurves of a PET fiber. Young’s modulus iscalculated from the amplitude of the peri-

Fig. 7. TMA curves of PET fibers (108 dtex). A different constant tensile force was used for eachsample for each heating run (30 °C to 220 °C at 10 K/min). This yields an array of shrinkage/expan-sion data curves.

tion temperature of 80 °C. Recrystallization and relaxation pro-cesses [5] that take place above 100 °C are the cause of the slowdecrease of the force of contraction at higher temperatures.

The great advantage of TMA measurements with different loadsis that with relatively few measurements, the force of contrac-tion and the shrinking behavior can be simultaneously mea-sured without having to change the configuration of the instru-ment. A second heating measurement performed using the samemeasurement parameters does not show any force of contrac-tion.

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Fig. 8. The force of contraction of PET (108 dtex): the data points were determined from the curves inFigure 7 as described in the text.

Fig. 9: DLTMA curves of a PET fiber (108 dtex) showing the first and second heating runs: heating to220 °C at 10 K/min with a tensile force which changes every

odic change of length (storage modulus)using Fourier analysis (see lower diagramin Figure 9). The value of Young’s modulusstarts to decrease as soon as the glass tran-sition begins (onset 68 °C). It in fact de-creases by a factor of ten due to the glasstransition. A comparison of the first andsecond heating curves shows that at lowtemperatures the value of the Young’smodulus for the stretched fiber is somewhatlarger than that of fiber after it hasundergone shrinkage. Above 120 °C, i.e.above the glass transition, the values ofYoung’s modulus are the same because thephysical conditions are similar.

fibers and even thicker yarns and wires tobe reproducibly mounted - this is of courseabsolutely essential for accurate results.The measuring system can also be used tocondition fibers at different temperatures,or under different tensile forces or gasatmospheres. DMA, DSC, TGA and thermo-optical analysis are additional techniquesthat can be used to determine theproperties of fibers.

ConclusionsThe TMA measurement technique and theevaluation the resulting curves is an excel-lent way to characterize the expansion andshrinking behavior of fibers. Effects origi-nating in the manufacturing process andsubsequent processing steps can be detectedand described. The TMA curves allow prop-erties such as the glass transition tempera-ture, the degree of shrinking and the melt-ing temperature to be determined. Values ofthe expansion coefficients, Young’s modu-lus and the force of contraction can be cal-culated and displayed as a function of tem-perature. The copper clips allow very fine

Literature[1] L.H. Sperling, Introduction to physical

polymer science, 2nd ed., Wiley-Interscience, New York (1992), p. 263.

[2] M. Jaffe, J. D. Menczel, W. E. Bessey,Chapter 7 in Thermal Characterization ofPolymeric Materials, 2

nd ed. (E. A. Turi,

Ed.), Academic Press, New York (1997)1767 - 1954.

[3] ibid., Seite 1785.[4] J.A. Foreman, R. Riesen, G. Widmann,

Thermal Trends, Vol. 5, No. 3 (Summer1998), 18.

[5] R. Riesen, J.E.K. Schawe, J ThermalAnalysis, Vol. 59 (2000) 337-358.

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IntroductionIn many DSC experiments the sample hasto be cooled under full control at a con-stant cooling rate, i.e. program cooled. To-ward the end of such a measurement, red

brackets may appear on the measurementcurve, indicating that the cooling capacityis no longer able to maintain the givencooling program. This of course depends

on the type of cooling option used and thecooling rate chosen. In order to complete acooling program without these warningsigns appearing, one needs to know thelowest temperature which can be reached at

a given cooling rate. This article presentsmeasured cooling curves which can be usedto estimate the maximum cooling rate as afunction of the end temperature.

Free cooling of the DSC821e

To measure the maximum cooling rate, atemperature program consisting of two iso-thermal segments (start temperatur andend temperature) is used. When the seg-ment changes, the measuring cell tries toreach the temperature of the second seg-ment as rapidly as possible. The rate oftemperature change then corresponds tothe maximum possible cooling rate at thatparicular temperature. Figure 1 shows thecooling curves measured in this way forvarious cooling options.

On the assumption that cooling is above allthe result of thermal conduction, the cool-ing behavior can be described by a simpleexponential equation. In this case, thecooling rate τ at a particular temperature,T, can be estimated from to the equation

(1)

where τ is the time constant characteristicfor the DSC furnace and T0 is the tempera-ture of the cooling flange. The value of T0is about -70 °C for the IntraCooler andabout 22 °C for air cooling. This model as-sumes that the temperature of the coolingflange is constant and that the time con-stant of the instrument can be described bya single value. To a good approximation,this is in fact the case for normal air cool-ing, the IntraCooler or normal cryostats.The cooling time constant is about 4 min-utes. If the system is cooled with liquid ni-trogen, the temperature of the coolingflange no longer remains constant and thecooling behavior can no longer be de-scribed by the above equation.Figure 2 shows the cooling rates as a func-tion of temperature for the various coolingoptions available. To a good approxima-tion, the results for air cooling and coolingwith an IntraCooler are given by thestraight lines described by equation 1,where the slope corresponds to the recipro-

Tips

Fig. 1. Cooling curves for the DSC821e with air cooling, IntraCooler and liquid nitrogen cooling.

Fig. 2. Cooling rates for different DSC821e cooling options (air cooling, IntraCooler, liquid nitrogen).

The cooling performance of the DSC821e

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cal of the cooling time constant. Withliquid nitrogen cooling, three different"cooling ranges" can be distinguished,which are determined by the mode ofoperation of the feedback control cir-cuit that regulates the amount of liquidnitrogen supplied. Above about 100 °C,only a small amount of liquid nitrogenis used for cooling and the coolingflange remains at an almost constanttemperature. Between 100 °C and about-100 °C more and more liquid nitrogenis supplied, the cooling power increasesand the cooling rate has a value that ismore or less independent of the actual

furnace temperature. Below -100 °C, the cool-ing flange reaches its lowest value (typicallyabout -170 °C), and the cooling rate decreasesrapidly. The diagram shows that liquid nitro-gen cooling is superior to IntraCooler coolingexcept in the range between about 100 °C and150 °C. Table 2 summarizes the maximumcooling rates for the 3 cooling options.One would often like to know how long themeasuring cell takes to cool down from thestarting temperature, T1, to the final tempera-ture, T2. For cooling options whose cooling be-havior can be described by the time constant,τ, the time can be estimated with the help ofequation (2).

Cooling option Minimum Temperature Advantages Disadvantages

Air cooling Room temperature No additional cooling unit Low cooling rates,required, no costs involved limited temperature range

Cryostat variable, depending on the Variable end temperature Coolant must be checkedcoolant down to –50 °C ) from time to time from time to time

IntraCooler > -60 °C Easy to use, good value Cools continuously(can be switched off withthe power switch

Liquid nitrogen > -150 °C Highest cooling rates Requires liquid nitrogen,

complexity

Table 2. Summary of the different DSC821e.cooling options

Temperature [°C] Cooling range with different cooling options [K/min] Air cooled IntraCooler Liquid nitrogen

-40 - 4 27-20 - 8 30

0 - 12 3120 - 16 3040 2 20 3260 6 25 3580 11 19 34

100 15 34 34

Table 1. Maximum cooling rates for different DSC821e cooling options at various temperatures.

(2)

T0 is the temperature of the cooling flange.The value of T0 is about -70 °C for theIntraCooler and about 22 °C for aircooling. With liquid nitrogen cooling, thecooling behavior can no longer be de-scribed with a time constant, so thatequation (2) can no longer be used. Thecooling time constant τ is about 4 minutes.This is valid for the IntraCooler, air coolingand cryostat cooling).

ConclusionsThe most rapid cooling rates can beachieved using liquid nitrogen as a cool-ant. Between 100 °C and 150 °C, the maxi-mum cooling rate with the IntraCooler isslightly higher than with liquid nitrogencooling. Table 2 summarizes the advan-tages and disadvantages of the variouscooling options. The measuring cell shouldbe purged with about 200 ml/min of drygas when using the IntraCooler, liquid ni-trogen or cryostat cooling options in orderto avoid icing of the furnace.

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TA Customer Courses and Seminars in USA and CanadaBasic Thermal Analysis Training based upon the STARe System version 6 is being offered April 21-22 and October 12-13 at our Columbus, OhioHeadquarters. Training will include lectures and hands-on workshops.For information contact Jon Foreman at 1-800-638-8537 extension 4687 or by e-mail [email protected] course June 21 – 22, 2000 Columbus (OH)TA course October 10 – 11, 2000 Columbus (OH)

TA Customer Courses and Seminars in UKFor details of training courses and seminars please contact:Rod Bottom at METTLER TOLEDO Ltd., Leicester, Tel.: ++44-116 234 50 25, Fax: ++44-116 234 50 25.TA Information Day:7 June 2000 Warrington14 June 2000 Bristol

TA Customer Training Courses in the South East Asia regional office, Kuala LumpurFor information on dates please contact:Malaysia: Jackie Tan/Ann Owe at ++ 603-7032773, fax: 603-7038773Singapore: Lim Li/Clive Choo at ++ 65-7786779, fax: 65-7786639Thailand: Warangkana/Ajjima Sartra at ++ 662-7196480, fax: 662-7196479Or SEA regional office: Soosay P. at ++ 603-7041773, fax: 603-7031772

For further information regarding meetings, products or applications please contact your local METTLER TOLEDO representative.Bei Fragen zu weiteren Tagungen, den Produkten oder Applikationen wenden Sie sich bitte an Ihre lokale METTLER TOLEDO Vertretung.Internet: http:/www.mt.com

RedaktionMETTLER TOLEDO GmbH, AnalyticalSonnenbergstrasse 74CH-8603 Schwerzenbach, Schweiz

Dr. J. Schawe, Dr. R. Riesen, J. Widmann, Dr. M. Schubnell, U. Jörimanne-mail: [email protected].: ++41 1 806 73 87, Fax: ++41 1 806 72 60

Layout und ProductionPromotion & Dokumentation Schwerzenbach, G. UnterwegnerME-51710020

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