Thermal Analysis of Polymers - Mettler Toledo

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ElastomersKineticsKinetics describes how fast a chemicalreaction proceeds. It provides us withvaluable information about the influenceof temperature, time, concentration, catalysts,and inhibitors.Different kinetics programs are availablefor kinetics evaluation. These includen th order kinetics, Model Free Kinetics(MFK) and Advanced Model Free Kinetics(AMFK).The activation energy can be calculatedfrom the conversion curves. This canthen be used to predict isothermal reactionbehavior under conditions wheremeasurements are difficult to perform orwhere the reaction times are very shortor very long.Figure 37 shows the evaluation of the vulcanizationreaction of an unvulcanizedNBR system using Model Free Kinetics.MFK requires at least three measurementsto be performed at three differentheating rates. In this case, the measurementswere made at 1, 2 and 5 K/min.The conversion curves were then determinedand are displayed on the left of thefigure.The apparent activation energy is calculatedfrom the three curves as a functionof conversion. The shape of the activationenergy curve reveals changes in thereaction mechanism of the reaction. Thecurve for the vulcanization of NBR showstwo reaction steps. The first step hasan apparent activation energy of about90 kJ/mol. At 60% conversion, the activationenergy of the second step increasesto about 110 kJ/mol.The activation energy curve can be usedto predict reaction conversion as a functionof time for isothermal reactions atdifferent temperatures. The accuracy ofsuch predictions should however be verifiedby performing suitable isothermalmeasurements.6.3.2 Thermogravimetric analysis(TGA)Figure 37.Model Free Kineticsof the vulcanizationof NBR.Compositional analysisTGA measures sample mass as a functionof temperature or time. The technique isoften used in quality control or productdevelopment to determine the compositionof elastomers. Different componentssuch as moisture or solvents, plasticizers,polymers, carbon black or inorganic fillerscan be determined. Figure 38 shows atypical TGA analysis of an SBR elastomer.The TGA curve exhibits three steps. TheDTG curve (the first derivative of the TGAcurve) is used to set the correct temperaturelimits for the evaluation of the steps.Figure 38.TGA analysis of anSBR elastomer.The first step below about 300 °Camounts to 3.1% and corresponds to theloss of small quantities of relatively volatilecomponents. Pyrolysis of the elastomertakes place between 300 and 550 °C.The step corresponds to a polymer contentof about 62.9%.The atmosphere is then switched fromnitrogen to air (oxidative) at 600 °C.The carbon black filler in the elastomerburns. With many elastomers, theamount of carbon black formed duringpyrolysis can be neglected. The carbonblack filler content can therefore be determinedfrom the third step between 600and 700 °C and yields a value of 31.5%.The residue of 2.3% corresponds to theash content, which in this case containsinorganic fillers.30Thermal Analysis of Polymers METTLER TOLEDO Selected Applications
TGA at reduced pressureElastomers often contain an appreciableamount of oil as a plasticizer. In manycases, accurate determination of the oiland the polymer content is difficult becausethe oil vaporizes in the same temperaturerange in which pyrolysis of thepolymer begins.Figure 39.Determination ofthe oil content of anSBR/NR elastomerusing TGA at ambientpressure andreduced pressure.The separation of the vaporization ofthe oil and the pyrolysis of the polymercan be improved by using lower heatingrates for the measurement or by performingthe measurement at reduced pressure(under vacuum).Figure 39 demonstrates the influence ofpressure on the determination of the oilcontent of an SBR/NR blend. Measurementswere performed under nitrogen at1 kPa (10 mbar) and 100 kPa (1 bar) usinga heating rate of 10 K/min. The figuredisplays the resulting TGA and DTGcurves.Figure 40.TGA analysis of EVAcontaining differentflame retardants.The DTG curve shows that at a pressureof 1 KPa (10 mbar) the oil vaporizes ata lower temperature while the pyrolysisof the polymer takes place at the sametemperature as before without vacuum.This means that the separation of thetwo effects is better at a pressure of 1 kPa(10 mbar). This in turn allows the oiland polymer contents to be more accuratelydetermined.The results obtained for the oil and polymercontents of the sample were approximately9.9% oil and 35.7% polymer.Flame retardantsFlame or fire retardants are often addedto elastomers for fire prevention dependingon the field of application. The effectof flame retardants on the decompositionbehavior of materials and the energy involvedin the process can be investigatedby thermal analysis.Figure 40 shows the TGA analysis ofthree ethylene-vinyl acetate copolymers(EVA) containing different flame retardants:Al(OH) 3 ; (ATH: aluminum trihydrate);and Mg(OH) 2 (MDH: magnesiumdihydrate). The diagram displays the TGAand DTG curves of the samples as a functionof temperature. Water is first eliminatedfrom the hydroxides of the flameretardants between 300 and 400 °C.This is followed by the pyrolysis of the EVAat about 460 °C. Quantitative analysis ishowever difficult because EVA eliminatesacetic acid between 360 and 400 °C.The presence of flame retardants in polymerscan therefore be investigated usingsimple TGA measurements. Informationabout the energy involved in the dehydrationprocess of the flame retardant canbe obtained by DSC. This is importantbecause it corresponds to the energy extractedfrom the fire.6.4 References[1] A. Hammer, Thermal analysis ofpolymers. Part 1: DSC of thermoplastics,UserCom 31, 1–6.[2] A. Hammer, Thermal analysis ofpolymers. Part 2: TGA, TMA and DMAof thermoplastics, UserCom 32, 1–5.[3] A. Hammer, Thermal analysis ofpolymers. Part 3: DSC of thermosets,UserCom 33, 1–5.[4] A. Hammer, Thermal analysis ofpolymers. Part 4: TGA, TMA and DMAof thermosets, UserCom 34, 1–5.[5] METTLER TOLEDO Collected ApplicationsHandbook: ELASTOMERS,Volume 1.[6] METTLER TOLEDO Collected ApplicationsHandbook: ELASTOMERS,Volume 2.METTLER TOLEDO Selected Applications Thermal Analysis of Polymers 31
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