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USER COMDear customer

Thank you for the interest you

have shown in our customer

magazine.In compliance with your wishes,

we are including more application

examples.

Yet again, we were able to present

new products at important exhibi-

tions last spring. Even if you are

on a tight budget, we can offer

you an extremely attractive and

powerful solution. The new

STARe software and a new appli-

cation collection covering thetopic of thermoplasts have been

available since the beginning of

May 96.

TA TIP

Inform ation for users of

METTLER-TOLEDO thermal analysis systems

3July 1996

Investigating unknown

samples

Contents

TATiP: Investiga tin g un kn ow n sa mpl es

NEW in the sales program:

STARe software V3.10 Applica tio n co llection

Thermoplastics

i.e.: PE, melting curve andthermal prehistory

Engineerin g po lym ers

Applications Elasto m er an alys is in the TGA8 5 0

Selection of exp erim enta lparam eters for the cpdetermination with ADSC

Sample Preparation

With the right sample preparation you can avoid unreasonable results. The

main goals of sample preparation are:

Minimum Temperature Gradients Within the Sample Pan to achieve sharp

thermal effects. Sharply defined, well pronounced effects increase the

precision of numeric results as well as the resolution of overlapping peaks.Small temperature gradients are achieved by a good thermal contact

between sample and pan and by good thermal conductivity of the sample.

In this connection, ideal sample forms are plane disks, dense powders as

well as liquids. Improve the sample shape of irregular objects, e.g. plastic

parts by sawing or face grinding of at least the bottom surface. Pulverize

brittle objects in a mortar. Fill the powder in a pan and compress it with

the teflon bar supplied. Also press down pasty samples. Care: the pan must

not be deformed, place it on a flat surface (with a hole for pans with center

pin). Remove the punch brow of punched samples or simply place them in

the pan with the flat side down. Liquids: immerse a spatula in liquid and

touch the inner surface of the pan. The suspended droplet will flow down.

You may use a small syringe, but some plastic parts of the syringe could

be attacked by certain organic liquids. Fibers can be difficult, too. Cut

sufficiently thick fibers into short pieces that fit flat in a pan. Wrap thin

fibers in the smallest piece of degreased aluminum foil possible and

compress the package. Place it in the pan with the flat side down.If you

expect very highly exothermic reactions or wish to facilitate the diffusion

of gases you can mix a powdered or liquid sample with an inert solid

diluent such as relatively coarse alumina or glass powder.

Defined Atmosphere in the Area of the Sample: An open pan allows free

access of the furnace atmosphere. Such a measurement takes place at the

practically constant atmospheric pressure (isobar). However, there exists adanger that the measuring cell can be spoiled by material creeping or

splashing out of the pan. Use a pierced lid to avoid such problems. A

hindered gas exchange (self-generated atmosphere) is necessary, e.g. for

determination of the boiling point (in an open pan the liquid would

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vaporize before reaching the boiling point). For this purpose put the alu-

minum lid upside down on a solid support, e.g. the crucible box and pierce

the lid once with a sharp pin. Check the hole diameter under the

microscope or by holding against a light source. The diameter should be

20 through 50 m. Fill the sample into pan and close it with the pierced

lid in the sealing press.

If you seal your sample hermetically, you suppress any volume work, e.g.

the vaporization endotherm. Since the sample is exposed to the increasingvapor pressure and the pressure of its decomposition products, you shift

the onset of the decomposition to higher temperature. Such a

measurement takes place at practically constant volume (isochor) up to the

pressure strength of the crucible (Al standard pan approx. 200 kPa above

ambient, gold plated steel pan approx. 15 MPa).

Experimental Conditions

Advice for first measurements:

Organic substances: Amount of sample 1 10 mg in Al pan with piercedlid. Temperature range: ambient 350 C, rate: 10 or 20 C/min,

atmosphere: N2

(approx. 50 ml/min).

With inorganic substances use 10 30 mg and a higher maximum

temperature of, e.g. 600 C.

In DSC compare the total pan weight before and after the measurement to

detect a possible weight loss ("offline thermogravimetry").

Examine the measured sample:

Does it look molten? Can you identify a fusion peak on the DSC trace? If

you are interested in the crystallisation behaviour, run a new sample

with a cooling segment (-10 K/min) after the fusion peak. Avoid a super

cooling of 150 C! Many substances are difficult to crystallise from

the melt, they form a glass on cooling.

Did it become discolored? Organic compounds turn brownish when

decomposing.

Are there gas bubbles? Together with a weight decrease of >30 g this

indicates decomposition.

Has there been a reaction with the crucible material? A pan that is not

inert begins to dissolve or can be destroyed completely. Try to find a

really inert pan material.

Sometimes chemical analyses of the investigated sample can provide

important information.

Very often a second run under identical conditions is helpful. Compare the

curves of first and second run. To separate overlapping effects try a lower or a higher (!) heating rate or

apply self- generated atmosphere. In most cases a smaller sample mass

gives better separation.

With organic substances: measure a new sample in air or oxygen. The

exothermic oxidation reaction occurs at a heating rate of 10 C/min in the

range approx. 150 to 300 C.

When only small effects appear (DSC < 2 mW, TGA < 1 mg), run a blank

curve under identical conditions and subtract it from the sample curve.

Sample preparation

Selection of the sampleatmosphere

open crucible, self-generated atmosphere orclosed crucible

good thermal contactbetween sample and crucible

Selection of the measure-ment conditions

organic substances inorganic substances

Offline Thermo-gravimetrie

comparison of the weightbefore and after measurement

Visual check

open crucible and studysample changes

Possible 2nd measurement

identical or new

parameter offline blank curve correction

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USER COM July 96 3

STARe software V3.10

General expansions

TA basis software

With the new STARe SW the data

from AE, AG, AT and MT balances

can be transferred directly to the

computer at a keystroke.

The DOS data export is now

included in the basic software and

contains the following data formats:

- HPGL

- PostScript

- TIFF (PACK and LZW)

- BMP (mono and color)

- PCX (mono, gray and color)

A new feature is support for

numerous printers.

The data backup possibility via the

network to a different PC instead of

on DAT or tape will also be found

extremely valuable.

To ensure greater clarity, only the

menu points for which you have

purchased a licence appear.

The deletion of database entries hasalso been considerably simplified.

Several objects can now be selected

at the same time and deleted.

In data import, complete diskettes

can be loaded simultaneously. In

addition, you also have many data

formats of other manufacturers

available.

Method window

In this window together with the re-

lative loops option you can generate

sinusoidal temperature programs.

You have the following parameters

available:

Start temperature

Mean heating rate

Amplitude Period (= 1/frequency)

Experiment window

The possibility to screen a sample

without the need to develop a

method is included as standard in the

Experiment window (earlier Rapid

Experiment option).

By popular request, you can now

define the position on the sample

turntable when developing theexperiment.

With the new STARe SW (a further

development of the TSW870), we

are now in a position to offer a spe-

cific solution to customers in all

branches. If need be, the SW can beexpanded at a later date to meet the

latest requirements.

We are convinced that this will pro-

vide you with a tool which can be

used to analyze both simple and very

complex problems.

If you already possess the TSW870

SW, you can update your SW at a

favorable price and make use of the

numerous new possibilities. If a SW

option which you have already pur-chased has been improved, all new

possibilities of this SW option can be

used (e.g. if the kinetics SW option

has been enhanced with an addi-

tional ASTM evaluation).

NEWS

1st option: Routine window

With this option a numeric experi-

ment and method editor can be

started directly in the Module control

window. Without the need to switch

windows, you need only develop

simple methods and supplement

them by data specific to an experi-

ment such as sample weight and you

are ready to start the measurement.

In the same window you can observe

the measurement or store it for the

analysis. At the same time, however,

additional new methods or experi-

ments can be developed. These arethen processed in their order in the

experiment buffer.

Illustration "Module control window

with Routine window"

2nd Option: Application database

As you have continually requested

more applications, we are pleased to

introduce you to the first application

collection dealing with

thermoplastics. As all curves andevaluations in the handbook are also

integrated in the database, you can

call these up at any time for

comparison or training purposes.

Module control window with Routine window

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Evaluation window

The following new improvements

have been implemented in the basic

evaluation window:

Various coordinate systems can be

automatically arranged one above

the other.

Curve and curve name are linkedwith each other (if the curve is de-

leted, the name also disappears)

Huge expansion of the Eval Macro

capabilities (all points under Info

can now be integrated in a macro,

moreover this list has been ex-

panded, e.g. numeric output of the

temperature program).

At the request of our customers, it

is now possible to configure the

numeric printout (what should beprinted out with what title)

The calibration printout can be gen-

erated directly at a keystroke in

conformance with GLP.

In curve searching, the search can

now also be made on the basis of

the order number of the experi-

ment

The temperature calibration has

been expanded so that with more

than 3 different substances a 2ndorder correction is now made.

In addition to these numerous, minor

improvements, existing evaluation

options have been expanded:

1st SW option ADSC (formerly

FFT)

This option has been expanded by 2

additional evaluations and today of-

fers you 3 different evaluations for

periodically induced (force or tem-

perature) signals:

- Fourier analysis (FFT)

(force or temperature excitation)

- Steady state ADSC

(temperature excitation)

- ADSC (temperature excitation)

Fourier analysis is used to split the

measured signal into its harmonic

signal parts. Depending on the set-

ting, separation into the amplitudes

of the sine and cosine or the cosinewith phase is possible.

Steady state ADSC is a special

evaluation for temperature-modu-lated signals. In contrast to the sinu-

soidal excitation, work is performed

here with a saw-tooth temperature

excitation. If the individual segments

are sufficiently long, the sample

changes to a quasi-stationary state.

Errors due to frequency and ampli-

tude response of the system (furnace

and sample) are thus minimized.

This evaluation allows a very accu-

rate quantitative determination of thereversing (cp) signal component.

Further, the non reversing signal

component is also calculated.

With the ADSC evaluation you have

a new evaluation available which

provides a huge wealth of informa-

tion. All temperature modulated TA

curves can be evaluated.

In addition to the non reversing

curve, the reversing signal (cp) is

calculated and is also separated into

the inphase and outphase compo-

nents thanks to the phase informa-

tion. As an additional curve, the

phase signal is also calculated for

each harmonic excitation; this signal

is very sensitive.

2nd SW option Kinetics nth order

The ASTM E1641 TGA kinetic

evaluation has been newly imple-

mented. Following the calculationusing the new procedure, you can

use the applied kinetics as before.

3rd SW option DSC Purity (for-merly Purity)

To date, the purity has been calcu-

lated based on the simplified Vant

Hoff law.

Impurities up to approx. 5 mol%

could be detected by this method.

This evaluation is eminently suitable

for unknown samples or those which

already start to decompose during

melting.

The purity can now also be calcu-lated using the complete Vant Hoff

law. This allows the evaluation of

samples with up to around 10 mol%

impurities.

4th SW option Mathematics

This option has been extended by a

mathematical integration evaluation.

The curve is simply integrated over

the x axis.

A new feature is the possibility to

enter a mass unit in the multiplica-

tion or division. This allows, e.g. cp

curves to be reconverted into heat

flow curves.

It goes without saying that we are in-

terested in your future criticism,

however we believe that an optimum

product can be developed only

through cooperation between pro-

ducer and user.

ADSC evaluation

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Application collection ThermoplasticsWe are now in a position to intro-

duce the collection of thermoplastic

applications as the first part of a

planned series of such brochure. As

all evaluations in the brochures canalso be integrated in the STARe SW,

you have innumerable comparison

curves available.

If you have purchased the SW new,

you can call up these applications di-

rectly with the SW option Applica-

tion Database.

All measurements are described us-

ing the same scheme enabling you to

find your way around very quickly.

Thanks to the detailed description,

you will find it childs play to com-

prehend all steps up to and including

the evaluation.

PE, melting curve and thermal history

Sample PE-HD film

Measurement Measuring cell: DSC821e with air cooling

conditions

Crucible: Aluminum standard 40 l, lid hermetically sealed

Sample preparation: disk of 2.33 mg punched from film

DSC measurement Pretreatment: 60 min isothermally at 129 C and

cooling to 40C at program: 5 K/min.

Heating from 30 to 160 C at 5 K/min gives the

measured curve Tempered at 129 C, cooling

from 160 to 40 C with 5 K/min.

Second heating from 30 to 160 C at 5 K/min gives

the measured curve Deleted Thermal.

Atmosphere: Quiet air

Interpretation In the isothermal pretreatment crystal segregation appears: some amorphous

regions can form crystallites with a melting point sufficiently high to lead to

a melting gap at this temperature ("memory effect"). After complete melt-

ing, the thermal history is again cleared.

Evaluation No numeric evaluation is needed here. Naturally, the melting gaps can be

evaluated as, e.g. onset or the melting behavior assessed with the conversion

curve and table.

Conclusion The DSC melting curve depends on the thermal history of polyethylene. Themelting gap is often used to check the conditioning temperature of e.g. PE

high voltage cables. Complete melting of PE deletes the thermal history and

is a prerequisit for the comparison of different PE qualities.

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Engineering PolymersSamples Polyetheretherketone, PEEK Victrex 450G, injection moulded,

Polyethersulfone, PES Victrex 200P, injection moulded,

Polytetrafluoroethylene, PTFE Film supplied in the sample set of the Ar-

beitsgemeinschaft Deutsche Kunststoff-Industrie

Conditions Measuring cell: DSC821e with air cooling

Pan: Aluminum standard 40 l, lid pierced

Sample preparation: Disk cut from injection moulded parts or punched

of film

DSC measurement: Heating from 30 to 400C at 20 K/min

Atmosphere: Nitrogen, 50 ml/min

Interpretation PES is entirely amorphous and just shows the glass transition (one of the

highest of all organic matter!). At room temperature PEEK and PTFE are

semicrystalline. They cannot be frozen in the glassy state by quenching

from melt in the sample robot (the mean cooling rate amounts to approx.

3000 K/min when the robot places the hot pan on the cold turntable).

The chosen representation of the DSC curves in W/g and with automatic

blank subtraction is directly proportional to the specific heat capacity of the

samples. The proportionality constant is the heating rate of 0.333 K/s. Thus,

the heat capacity of PTFE surprisingly amounts to approx. 0.5 J/gK at 60C

whereas PES and PEEK are in the range of 1 J/gK that is typical for organic

matter.

Conclusion The high sensitivity (low signal noise level) of the DSC821e allows problem

free determination of small effects that occur especially with highly filled

polymers.

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Elastomer analysis in the TGA850

aluminum oxide crucibles. Without a

sample changer, rubber samples canalso be placed directly on the sample

support. There are two reasons for

the relatively low amount of sample:

Fast reaction and hence in general

good separation of neighboring

elastomer steps.

Low amount of depolymerization

and the sample changer removes

the ashed sample. At the sametime, the automatic evaluation

starts using the evaluation proce-

dures defined in the EvalMacro of

the method.

For quantitative TG measurements,

a blank curve is recorded before-

hand and is automatically sub-

Horst Wyden, Georg Widmann

Introduction

Thermogravimetric analysis deter-

mines the mass change of a sample

subjected to a temperature program

and a defined atmosphere. The first

derivative of the TG curve, called

DTG, is used for the interpretation of

the reactions of the sample. In the

analysis of the main components of

elastomers, the classical extraction

processes and also the qualitative

and quantitative analysis of theelastomer components with IR

spectroscopy or gas chromatography

have been virtually completely sup-

planted by the more elegant

thermogravimetric rubber analysis.

The main components usually deter-

mined are:

1. Volatile components, which are

driven off between room temperature

and approx. 300C. They chiefly

comprise added oils and other plasti-

cizers, as well as moisture, solvent

residues, monomers and, e.g. stearic

acid.

2. Content of elastomers, such as

natural rubber and EPDM. Under a

nitrogen atmosphere and the usual

heating rate of 30 K/min, pyrolysis

follows between 300 and 550C, de-

pending on the chemical structure of

the elastomer molecule.

3. Content of carbon blackby burn-

ing in air or oxygen (automatic gasswitching!).

4. Residue: Inorganic fillers (plus

ash). Any CaCO3

loses CO2

at

approx. 800C. The stoichiometric

CaCO3content follows from the

weight loss.

Measurements

An NR/EPDM rubber and an NR/

SBR rubber with known composition

(see table) were investigated. Sam-ples with a mass of 4 6 mg were

cut out with a sharp knife.

In work with a sample changer, they

are placed in reusable 70 l

products. Elastomers frequently

produce oily and tacky decomposi-

tion products which are deposited

on cool parts and necessitate clean-

ing from time to time.

After insertion of the sample in themeasuring cell (isothermally at

25C), atmospheric oxygen is dis-

played by nitrogen as early as the

temperature equilibration phase.

This is possible within a short

space of time as the furnace vol-

ume is very low (and as the bal-

ance chamber is continuously

purged with nitrogen). The opti-

mum heating rate is 30 K/min.

Gas switching to air occurs at600C without interruption of the

dynamic temperature program. At

the final temperature of 800C the

measuring cell starts to cool down

tracted in the subsequent measure-

ments. This compensates influ-

ences such as a buoyancy change

and flow effects.

The automatic step analysis is so

designed that components whichare sufficiently well separated

(clear DTG minimum) are auto-

matically recorded ("multi limits").

The usually extensively overlap-

ping volatile components are

evaluated with fixed temperature

limits.

The DTG peak temperatures (in-

flection point of the TG curve)

characteristic for the identification

at 30 K/min are:

NR: 390C, BR: 445C,

SBR: 460C, EPDM: 480C

Fig.1: Example of an automatically evaluated rubber analysis with 4

steps. Here, the usual representation of a TG curve in percent of the

sample weight as a function of temperature is selected.

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n is the number of measurements, M

are the mean values, the standard

deviations.

The first weight step up to approx.

300C corresponds to the vaporization

or sublimation of volatile substances

such as oil, sulfur and stearic acid, as

well as readily volatile components ofthe elastomers. As a result, the natural

rubber content found is somewhat too

low. The clearly lowest standard

deviation is attained with carbon black

as the step in question is completely

isolated from neighboring effects. The

residue comprises ZnO and the sum of

the ash contents of the components.

Conclusions

The TGA850 assures high

analytical accuracy with minimal

labor and a high level of operating

convenience. Thanks to the sample

changer, the measurements are

fully automatic. Even the

evaluation of the experimentalcurves obtained can be performed

automatically with the new STARe

software. An analysis takes less

than 30 minutes (increasing the

start temperature to, e.g. 100C

would save time).

References

[1] G. Widmann und R. Riesen,

Thermoanalyse, Hthig, Heidelberg

[2] H. Wyden, Kunststoffe-Plastics, 5,

1982

Table, formulas and results in percent.

Formulation NR/EPDM-rubber, n=5 NR/SBR-rubber, n=4

Formula M Formula M

Volatiles: 4,42 0,21 3,96 0,15

Stearic acid,

sulfur and 3,78 3,56

accelerators

Natural rubber, NR 49,66 47,55 0,43 29,22 30,74 0,21

EPDM 12,41 13,49 0,12 Styrene-butadiene 29,22 27,0 0,22

Carbon black 31,06 30,81 0,02 35,07 34,88 0,08

Residue: 3,52 0,23 3,29 0,13

zinc oxide 3,10 2,92

Fig.2: Overview of the TG and DTG curves of the investigated rubber samples. The

ordinate unit is percent per degree Celsius and corresponds to the reaction rate.

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Selection of experimental parameters forthe c

pdetermination with ADSC

the relatively large mass and

expansion of the furnace are

considered. However, it is surprising

that for substances with a thermalconductivity (0.1 to 1 W/(m K)) in

the range of typical polymers, the

minimal time is determined by the

sample and not by the DSC furnace.

In a forthcoming publication [1], the

autor describes the simulation of a

greatly simplified DSC. In this

simulation, the partial differential

equation which describes the heat

transport in the sample was solved

by a finite element analysis. Figure 2

shows a scheme of the simulation.

This work has shown that weights of

typically not more than 5 mg and

cycle times of 2 to 4 minutes are

needed for accurate determinations

of the heat capacities of polymers.

Measurements

The heat capacity of polystyrene and

sapphire was measured with the

ADSC method for different cycle

times. The polystyrene samples were

cut from a sheet. The sample cuboidhad a thickness of 1.34 mm, side

lengths of approx. 3 x 3.5 mm and a

mass of 12.755 mg. The sapphire

sample was a circular disk of

thickness 0.3 mm, diameter 4.5 mm

and a mass of 27.245 mg. The

measurements were performed qua-

si-isothermally at a temperature of75C.

The ADSC evaluation of the STARe

software was used for the evaluation.

4 DSC runs were performed for each

frequency with the same crucibles:

a no-load curve, without crucible at

the sample and reference positions

a blank curve with a crucible with lid

on the sample side and a crucible

without lid on the reference side and

a measured curve for both the

polystyrene and the sapphire sample

in a crucible with lid and a crucible

without lid on the reference side.

The following heat capacities were

calculated from these experimental

data:

cp(corr): The no-load curve and the

blank curve are used for calibration

of the DSC cell and for

compensation of the cell asymmetry.

cp(conv): Only the cell asymmetry is

compensated.

Results

Table 1 and Figure 3 show the

results of the individual

Benedikt Schenker, Technical chem-istry laboratory, Swiss Federal

Institute of Technology, Zurich

Tel: + 41 1 / 632 30 59

Fax: + 41 1 / 632 12 22

E-mail: schenker@tech.chem.ethz.ch

Introduction

ADSC allows simultaneous

determination of the heat capacity

and thermal events in the sample.Here, a periodic signal, the so-called

modulation, is superposed on the

conventional, generally linear

temperature program:

Tp

= T0

+ bt + Asin(t2/p) where

Tp

is the program temperature, T0

the

start temperature, b the heating rate,

A the modulation amplitude, t the

time and p the modulation period.

In the evaluation, the differential

heat signal is split into a cpcomponent and a heat of reaction

component (thermal event). The

question now arises regarding the

"correct" choice of the experimental

parameters: sample size, mean

heating rate, modulation amplitude

and modulation period.

As the period of the modulation

determines the time resolution and

through this together with the mean

heating rate of the basic temperature

program also the resolution in thetemperature range, the aim is to have

periods as short as possible.

However, the period can not be

shortened at will as both the DSC

furnace and the sample have a finite

thermal conductivity and hence

periods which are too short are

misrepresented. Figure 1 outlines the

falsification, which is manifested as

damping (too small an amplitude)

and as a phase shift (time shiftcompared with the excitation signal).

The limited ability of the furnace to

handle short cycle times is

intuitively easy to understand when

Heater

Sample Reference

TC

TS

TR

TC

TS

TR

...Tf1 Tf2 Tf3

Figure 2

Figure 1

DSC system

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measurements for the different cycle

times. It can clearly be seen that the

relatively thick 1.3 mm polystyrene

sample even with relatively longperiods shows a cpvalue that is

obviously too low. The agreement

between the simulation and the

measurements is satisfactory. The

differences can be explained by the

fact that the simulation is based on

the assumption that the heat is

transferred only via the base area of

the sample and not by the other

surfaces. As in the actualmeasurement the heat is also

transferred by the other surfaces,

however, this leads to a lower

influence of the thermal

modulation in the sample finally

leads in the evaluation to a value of

the thermal conductivity which is too

low.

Recommendations

The simulations and measurements

lead to derivation of the following

recommendations for the procedure

in the determination of the experi-mental parameters:

With an unknown sample, a

traditional measurement must first be

performed. This measurement shows

the extent of the effects important for

the determination of the mean

heating rate and supplies values for

cp

for verification of the measured

values of the ADSC method.

Sample

Use samples as flat as possible

having a large contact surface with

the crucible.

Sample weight

For substances with a moderate ther-

mal conductivity of 0.1 to 1 W/(m

K) of typical polymers, even for flat

samples no more than approximately

5 mg should be weighed in. Accurate

weighings require an accurate

(micro) balance. A weighing error of

0.1 mg means a relative error of at

least 2%.

Period

Sufficiently long periods should be

used. 60 seconds appear to be the

lower limit for polymers even with

favorable sample geometry and 5 mg

sample weight. For good heat

conducting samples, modern DSCfurnaces allow markedly shorter

measurement times. The

measurements for sapphire show that

even for cycle times of 30 seconds

reasonable results can be obtained. If

possible, the results should be

verified by the conventional cp

determination.

Figure 5 shows the maximum

sample thickness for different cycletimes which allows the

determination of cp

with a relative

error of 2%. The characteristic

parameter which describes the

conductivity. The results for sapphire

in Table 1 show that the DSC

furnace can readily propagate the

modulation down to a period of 30seconds.

Figure 3 also clearly shows the

effect of the correction by the no-

load and the blank curve when

cp(corr) is compared with c

p(conv).

An additional sapphire run is not

needed.

Figure 4 shows the calculated

temperature from the simulation for

the polystyrene sample at a cycletime of 45 seconds. The

inhomogeneous temperature

distribution is clearly visible. The

pronounced damping of the

Period Polystyrene, = 1.09 W /(mK) Sapphire, = 34.7 W /(mK)

cp (corr) cp error cp (corr) cp error

[s] [J/(gK)] [%] [J/(gK)] [%]

20 0.75 -48.9 0.69 -20.2

30 1.00 -31.4 0.78 -10.4

45 1.17 -19.7 0.85 -1.660 1.31 -10.0 0.88 1.3

90 1.37 -6.2 0.89 2.5

120 1.38 -5.5 0.88 1.2

180 1.42 -2.3 0.89 2.2

240 1.43 -1.6 0.86 -0.9

Table 1: Measurement results

50 100 150 200 250 300

Period time [s]

0.5

1.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

*

*

* *

*** *

x

x

x

x x

xx

*

Heatcapacity[J/(gK)]

Figure 3

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USER COM July 96 11

effects of heat conduction is the ther-

mal diffusivity a=l/(rcp). a is 110-4

m2/s for aluminum, 910-6 m2/s for

sapphire, 810-7 m2/s for quartz glass

and 110-7 m2/s for polystyrene. The

importance of flat samples and large

cycle time is clearly apparent.

Modulation amplitude

The size of the modulation

amplitude must be selected so that

the periodic temperature changes do

not have any great influence on the

processes occurring in the sample.

Amplitudes from 0.5 to 1 K have

proved their worth in practice.

However, it should be ensured that

the maximum cooling rate of the

furnace is not reached.

bmin

= b - A2/p >> bcooling system

In the present measurements, the

amplitudes for the measurements

with cycle times of less the 90

seconds were so reduced that a

maximum cooling rate of approx.

5 K/min was not exceeded. The

maximum possible cooling rate of

the DSC furnace used was approx.

15 K/min under the conditions

employed.

Heating rate

The selection of a suitable mean

heating rate is determined by many

factors. The evaluation for the cp

determination always takes into

account all values within a period.

The effects are thus smeared over

a period. For a high resolution in the

temperature range, preferably low

mean heating rates should thus be

selected. On the other hand, low

mean heating rates lead to

correspondingly long experiment

times and to low heat generation

rates of the thermal events in thesample. The results obtained at a

particular mean heating rate should

therefore be verified by

measurements with other mean

heating rates. The mean heating rate

should be chosen so that during an

event (e.g. a glass transition) around

6 cycles occur. Here, also ensure that

the amplitude selected is not so large

that the temperature range of the

event is already totally exceeded inthe initial cycles.

References

[1] B. Schenker, F. Stger, Influence

of the Heat Conductivity on the cp

Determination by Dynamic

Methods, Thermochimica Acta,

1996, submitted for publication.

Figure 5

10-8 10-7 10-60

0.5

1

1.5

2

Sam

plethickness[mm]

Thermal diffusivity [m^2/s]

Period duration: 240 180 120 90 60 45 30 20

Figure 4

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USERCOM J l

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