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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
1/2000
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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UserCom 1/2000
2
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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3UserCom 1/2000
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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UserCom 1/2000
4
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) issignificantly 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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5UserCom 1/2000
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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UserCom 1/2000
6
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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7UserCom 1/2000
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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UserCom 1/2000
8
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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9UserCom 1/2000
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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10
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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11UserCom 1/2000
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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12
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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13UserCom 1/2000
SummaryEffect on the glass transition: Special comments:
Crystallinity Increasing crystallinity → smaller For low
molecular substances, the crystallinitystep 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. 10
4 g/mol is c onstantpolymerization, 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 Tgfiller 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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14
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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15UserCom 1/2000
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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UserCom 1/2000
16
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 ⇒ C6AS3H32At 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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17UserCom 1/2000
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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19UserCom 1/2000
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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22
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:
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23UserCom 1/2000
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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24
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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25UserCom 1/2000
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 DSC821eTo 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 m