high resolution thermogravimetric analysis · in thermal analysis, ... results with most...

Thermal Analysis & Rheology

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TGA 2950 SCHEMATIC - SAMPLE AREA

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INTRODUCTION

Resolution, the ability to separate closely occurring events,is an important criterion for most analytical instrumenttechniques since it effects the ability to obtain accurate,quantitative results. Thermal Analysis techniques are noexception to this basic rule. A good example is thermogravi-metric analysis (TGA) which measures weight changes withtemperature, providing information about material compo-sitional analysis and thermal stability. Resolution of succes-sive TGA weight losses is important for obtaining accurate,reproducible weight change values, as well as accuratecomponent identification in the evolved decomposition gasesby FTIR (Fourier transform infrared) or mass spectroscopy.

In thermal analysis, the variables affecting resolution for aspecific hardware design are typically sample size, heating/cooling rate, and purge gas composition. Generally, smallersample sizes, slower heating/cooling rates, and higher ther-mal conductivity purge gases (e.g.. helium) result in im-proved resolution. Varying (slowing) heating rate hasproven to be a particularly effective method for enhancingresolution in TGA experiments. The literature containsseveral references (1,2,3) which clearly illustrate that adjust-ing (slowing) the heating rate during weight changes im-proves resolution. J. Rouquerol in 1964 (1) used vacuumTGA and based heating rate control on sensing pressureincreases associated with the evolved decomposition gases.Paulik & Paulik (2) adjusted TGA heating rate as necessaryto achieve a constant rate of reaction (weight change).Sorenson (3) in 1978 used stepwise isothermal control toenhance TGA resolution. All three of these approachesresulted in better resolution, but with substantial increases inexperimental time.

TA Instruments has recently introduced another approachwhich delivers superior TGA resolution without the timetrade-off present in the earlier literature approaches. In fact,this approach (patent pending*) often provides better resultsin less time than conventional constant heating rate TGA.This new approach is included as part of a high resolutionTGA (Hi-ResTM TGA) accessory for the TA InstrumentsTGA 2950 Thermogravimetric Analyzer which enables theoperator to choose among several different heating rate

approaches to obtain the best compromise of resolution andproductivity for a specific situation.

PRINCIPLES OF OPERATION

Although the TA Instruments Hi-ResTM TGA accessoryinvolves software algorithms which control the TGA fur-nace, it is the unique design of the TGA 2950 Thermogravi-metric Analyzer which makes the ultimate resolution en-hancements possible. Figure 1 shows the furnace schematicfor the TGA 2950. The TGA 2950 is a vertical balancesystem which features 1.5gm capacity, 0.1µg sensitivity, andan ambient to 1000oC temperature range. It contains threefeatures critical to Hi-ResTM TGA:

HIGH RESOLUTION THERMOGRAVIMETRIC ANALYSIS -A NEW TECHNIQUE FOR OBTAINING SUPERIOR ANALYTICAL RESULTS

TA-023

� Single thermocouple design. The TGA 2950 usesa single thermocouple located close to the sample for control-ling the furnace and for monitoring sample temperature.This arrangement facilitates the rapid temperature servoresponse necessary to Hi-ResTM. The relatively low furnacemass also contributes to this rapid response.

�Horizontal purge gas flow. The open design of theTGA 2950 sample pans combined with a horizontal purgegas flow ensures good interaction between the sample andpurge gas, as well as rapid removal of decomposition prod-ucts so that undesired recombination effects do not broadenthe weight loss peaks. For high resolution experiments, theuse of an inert purge gas such as nitrogen, helium or argonis recommended. Oxygen or air as the purge gas effects thedecomposition thermodynamics such that overlap betweenclosely occurring decompositions is increased. Using a gassuch as helium has the additional benefit of being an idealcarrier gas for evolved gas analysis.

� High balance sensitivity. Since the heating ratecontrol is linked to weight change, the ability to sense smallweight changes is important to achieve optimum control.

HI-RESTM TGA ALGORITHMS

The TA Instruments Hi-ResTM TGA accessory consists offour different variable heating rate algorithms (approaches)which can be used either alone or in combination to obtainthe optimum results on a specific material. The basis foreach of the algorithms is described below:

� Dynamic Rate. This is the TA Instruments patent-pending approach. It differs from the other approaches inthat the heating rate of the sample material is dynamicallyand continuously varied in response to changes in thesample's rate of decomposition so as to maximize weightchange resolution. This approach allows very high heatingrates to be used in regions where no weight changes areoccurring while avoiding transition temperature overshoot.

Because the dynamic rate approach reduces heating ratesmoothly and only when necessary, it is the fastest and mostreliable of the various techniques. This mode gives goodresults with most temperature separable transitions. It ispreferred for fast survey scans of unknown materials overwide temperature ranges. If no other criteria exists to selecta Hi-ResTM technique, then dynamic rate is the preferredchoice.

� Constant Reaction Rate. In the constant reactionrate approach the heater control system varies the tempera-ture of the furnace as required to maintain a constantpreselected rate of weight change (%/minute). Wheneverthe rate of weight change exceeds the percent/minute thresh-old, the heating rate of the furnace is reduced, even to thepoint of cooling if necessary. When percent/minute fallsbelow the threshold the heating rate is increased up to themaximum specified for the ramp segment. Transitionresolution is improved because sample heating is reduced orreversed during transitions allowing them to complete at theselected reaction rate before moving on to the next transition.

Constant reaction rate mode is preferred for any samplewhere it is important to limit or control the rate of reaction.These may include pyrotechnics, self-heating reactions,auto-catalizing reactions and gas diffusion reactions. Con-stant reaction rate mode is also a good choice when it isimportant to accurately determine the transition temperatureat a given reaction rate.

Another area where constant reaction rate heating can behelpful is when the sample material exhibits a relativelylarge and somewhat constant background weight changeonto which is superimposed a relatively small transition. Ifthe decomposition rate threshold is chosen to be close to thebackground rate at the maximum heating rate, then theheating rate will only be changed significantly when thesmaller transition occurs.

� Stepwise Isothermal. The stepwise isothermalapproach consists of heating at a constant rate until a weightchange begins as determined by an operator chosen rate oramount of weight loss and then holding isothermally untilthe weight change is complete. This sequence of heating andisothermal holding is repeated for each weight changeencountered.

This approach has a significant advantage over the othertechniques in that the reaction onset and completion can bevery carefully controlled. If the reaction rates for thedecomposition are known, then the controller can be pro-grammed to stop heating as soon as the first indication of areaction occurs, and to continue heating once the reactionhas subsided in an insignificant level. This is particularlyhelpful for transitions which are very closely spaced intemperature. One disadvantage of stepwise heating is theneed to run several TGA scans to properly "tune" the reactionrate thresholds used to start and stop heating. Anotherdrawback is the relatively slow heating rates required toprevent transition overshoot.

INORGANICS. Inorganics are convenient materi-als for assessing TGA resolution because they are readilyavailable and often degrade with well-defined stoichiomet-ric weight losses. Cupric sulfate pentahydrate (CuSO4

.5H2O)

is a typical example. Figure 2 shows the conventional TGAresults for cupric sulfate at 20oC/minute. The solid line

indicates a series of weight losses beginning at about 70oC.

These weight losses between 70oC and 250oC represent theloss of waters of hydration. Although conventional TGAprovides a well-defined weight loss of the last hydrationwater, the initial water weight losses are difficult to resolveeven using the derivative curve (broken line).

� Constant Heating Rate (Conventional TGA). Inthis TGA approach a constant heating rate is used through-out the experiment. Using slower heating rates, 1-2oC/minute or less, is often required to obtain any resolutionenhancement from this approach.

APPLICATIONS

A series of applications which illustrate the power of Hi-ResTM TGA is included in this section. The exampleapplications shown are not meant to be all inclusive, but arechosen to illustrate the type of results possible.

COPPER SULFATE PENTAHYDRATE - HI-RESTM TGA(DYNAMIC RATE)

120COPPER SULFATE PENTAHYDRATE - CONVENTIONAL TGA

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The Hi-ResTM TGA results (Figure 3), however, shows sharpwell-resolved peaks. Figure 4 shows the results of anotherHi-ResTM TGA experiment where the area of interest iswaters of hydration. In this case, however, the weight lossesassociated with water evolution are so small and gradual thatthey cannot be detected by conventional TGA. Hi-ResTM

TGA, in effect, provides enhanced sensitivity.

HOMO POLYMERS. Figure 5 illustrates the comparativederivative curves for polyetrafluoro- ethylene (PTFE) usingconventional and dynamic rate Hi-ResTM TGA. PTFE is

representative of pure homopolymers which typically de-compose by simple, single step processes. Hence, resolutionof overlapping weight loss peaks is not an issue. However,the results illustrate that the resolution as indicated by thederivative peak sharpness (wt% per oC) is improved in Hi-

ResTM TGA even over conventional TGA at 1oC/minute.

Furthermore, the Hi-ResTM TGA results are achieved in lesstime than conventional TGA results at 20oC/minute. This isa perfect example of Hi-ResTM TGA's value as a surveytechnique.

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CEMENT - HI-RESTM TGA(CONSTANT REACTION RATE)

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POLYTETRAFLUOROETHYLENE (PTFE) DECOMPOSITIONCOMPARISON OF DERIVATIVE CURVES

SOLID HRTGA 22 MINUTES

DASHED 1C/MIN 560 MINUTES

DASHDOT 20C/MIN 30 MINUTES

homopolymer polymethylmethacrylate (PMMA). The poly-mer was prepared to be a pure homopolymer. The results,indicate, however, that a small quantity of impurity, possiblyunreacted methylmonomer or even polyethylmethacrylate,is present. Conventional TGA does not resolve this impu-rity.

The shift of the decomposition to lower temperature in theHi-ResTM TGA and 1oC/minute conventional TGA curves isas expected. In fact, these decomposition temperatures aremore representative of the theoretical isothermal values.

Figure 6 shows the Hi-ResTM TGA survey results for another

POLY METHYL METHACRYLATE HI-RESTM TGA(DYNAMIC RATE)

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COFILAMENT FISHING LINE - HI-RESTM TGA(DYNAMIC RATE)

COFILAMENT FISHING LINE - CONVENTIONAL TGA

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Hi-ResTM TGA also has value for quantifying homopolymersin finished commercial products where more than onehomopolymer is present. Figures 7 and 8 show the conven-tional and Hi-ResTM TGA results for a cofilament fishing

line. Both runs took about 20 minutes, but the Hi-ResTM TGAresults provide a much better resolution of the individualhomopolymer weight losses.

a weight ratio, which accounts for the vinyl acetate/aceticacid stoichiometry, it is possible to determine the % vinylacetate present in the copolymer (4). Figure 9 shows theweight loss curves for several different EVA formulations byconventional TGA. Although it is clearly evident that theformulations are different, there is some opportunity foroperator subjectivity when choosing the initial weight losscompletion and hence, the amount of vinyl acetate present.Figure 10, on the other hand, illustrates the results of EVAmaterials using Hi-ResTM TGA. With Hi-ResTM TGA, it iseasy to determine when the first weight loss is complete.

POLYMER BLENDS. Polymer blends represent alarge class of valuable materials which are difficult toevaluate by conventional TGA and which, therefore, repre-sent a large opportunity for Hi-ResTM TGA.

Ethylene vinyl acetate (EVA) is a common copolymer whichillustrates the ability of TGA to determine the relativepercentages of the polymer present based on the degradationprofile. In nitrogen, EVA degrades in a two step process,where the first weight loss corresponds to acetic acid. Using

IncreasingVinyl Acetate

ETHYLENE VINYL ACETATE - HI-RESTM TGA(DYNAMIC RATE)

ETHYLENE VINYL ACETATE - CONVENTIONAL TGA120

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The presence of flame retardant or other additives, whichfurther increase the complexity of the materials' weight lossprofile, provides additional indication of the benefit of Hi-ResTM TGA. Figures 11-13 are Hi-ResTM TGA curves forEVA with different additives. These survey scans obtainedin air clearly show different weight loss profiles which can

either be used quantitatively, or qualitatively as "finger-prints" to identify different formulations. The profiles forthese EVA formulations in nitrogen are not as unique as inair/oxygen indicating that atmosphere also needs to beconsidered when attempting to optimize resolution.

Figure 11

EVA FORMULATION 2 - HI-RESTM TGA(DYNAMIC RATE)

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ABS is another common polymer blend. Figure 14 shows thecomparative derivative curves for an ABS formulation eval-uated by conventional (dashed line) and Hi-ResTM TGA

(solid line). The Hi-ResTM TGA results show resolution thatis far superior to the conventional TGA curve.

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ABS - HI-RESTM TGA AND CONVENTIONAL TGA

COMPLEX MATERIALS. Foods, natural prod-ucts, and pharmaceuticals are materials that usually exhibitcomplex TGA profiles. Figures 15 and 16 show comparativeresults for a premium hand soap using conventional and Hi-ResTM TGA. The improved resolution obtained in the lattercase makes it potentially easier to identify and quantify thevarious weight loss components. Further resolution en-

hancement of specific weight loss regions can be obtainedwith additional adjustment of the Hi-ResTM TGA experimen-tal parameters. Even without further resolution improve-ment, however, these Hi-ResTM TGA results can be used asshown earlier with EVA to rapidly generate distinctive"fingerprints" that facilitate comparison of competitive prod-ucts or checking batch-to-batch variations in formulation.

PREMIUM SOAP - CONVENTIONAL TGA

PREMIUM SOAP - HI-RESTM TGA(DYNAMIC RATE)

Figure 16

Figure 15

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CONCLUSIONS

High resolution TGA is a new technique that promises torevolutionize the quality of results that can be obtained fromTGA experiments. The trade-offs that are available with Hi-ResTM TGA are illustrated in Figures 17-20. The material isa 50:50 mixture of inorganic bicarbonates which decomposeless than 50oC apart. Conventional TGA at 20oC/minuteshows two overlapping weight losses. Resolution improveswhen the conventional TGA heating rate is decreased to 1oC/minute. Using dynamic rate Hi-ResTM TGA approach yieldsresolution that is essentially comparable to the 1oC/minute

results, but the analysis time for the Hi-ResTM TGA approachis about 1/20th that for the conventional approach. Using theconstant reaction rate Hi-ResTM TGA approach providesfurther resolution enhancement but with an analysis timethat is closer but still less than the 1#C/minute conventionalapproach. (The slight temperature overshoot on the secondweight loss could be eliminated by further adjustment of thealgorithm parameters. However, since the result is notadversely affected, adjustment is not necessary.) As theseresults illustrate, the operator has the flexibility to chose thebest resolution/productivity compromise for his materials.Hi-ResTM TGA is the latest in a long line of thermal analysis

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WATER

NAHC03 KHC03

APPROX. 170 MINUTES

BICARBONATE MIXTURECONVENTIONAL TGA AT 20oC/MINUTE

BICARBONATE MIXTURECONVENTIONAL TGA AT 1oC/MINUTE

innovations from TA Instruments (former DuPont ThermalAnalysis Instruments). It joins previous advances, whichinclude dual sample DSC, differential photocalorimetry(DPC), dynamic mechanical analysis (DMA), and dielectricanalysis (DEA), and provides further indication of TAInstruments' commitment to developing innovative newproducts and services that help thermal analysts solve theirmaterial characterization problems.

REFERENCES

1. J. Rouquerol, Bull. Soc. Chim., 31 (1964).2. F.Paulik & J.Paulik, Anal. Chim. Acta., 56, 328 (1971).3. S. Sorenson, J. Therm. Anal., 13, 429 (1978).4. J. Chiu, Applied Polymers Symposia, No.2 25-43 (1966).

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9.5 MINUTES

BICARBONATE MIXTURE HI-RESTM TGA(DYNAMIC RATE)

ΤΑ−023Β

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HERMETIC PAN WITH PINHOLE133 MINUTES

BICARBONATE MIXTURE HI-RESTM TGA(CONTROLLED REACTION RATE)

*High resolution thermal analysis (includes high resolution thermogravimetric analysis) and "Hi-ResTM TGA" are conceptsfor which TA Instruments, Inc. has filed patent and trademark applications respectively.

For more information or to place an order, contact:

TA Instruments, Inc., 109 Lukens Drive, New Castle, DE 19720, Telephone: (302) 427-4000, Fax: (302) 427-4001TA Instruments S.A.R.L., Paris, France, Telephone: 33-01-30489460, Fax: 33-01-30489451TA Instruments N.V./S.A., Gent, Belgium, Telephone: 32-9-220-79-89, Fax: 32-9-220-83-21TA Instruments GmbH, Alzenau, Germany, Telephone: 49-6023-30044, Fax: 49-6023-30823TA Instruments, Ltd., Leatherhead, England, Telephone: 44-1-372-360363, Fax: 44-1-372-360135TA Instruments Japan K.K., Tokyo, Japan, Telephone: 813-5434-2771, Fax: 813-5434-2770

Internet: http://www.tainst.com

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