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Collected Applications Thermal Analysis EVOLVED GAS ANALYSIS Preface Over the past decades, the development and characterization of materials has become increasingly specialized due to the ever-increasing demands placed on quality. In addition, the widespread use of products such as blends, composites, new stabilizers and additives has resulted in relatively complex chemical systems. The great interest in material sciences has created a need for specific analytical tools to characterize the constituent inorganic and organic molecules. One particularly important development has been to combine well-established separation and identification techniques into one instrument system. This drastically increases the amount of specific information that can be obtained and shortens analysis times. This booklet provides an insight into two such so-called hyphenated techniques, TGA-FTIR and TGAMS. The first part of the booklet focuses on basic principles and describes the techniques. It also includes a practical section and an introduction to the interpretation of spectra. The second part discusses some 15 different applications performed in our applications laboratory using TGA-MS and/or TGA-FTIR. Two additional applications illustrate the use of the rather unusual combination of TMA with MS. I would like to thank the Materials Characterization Section of the Market Support Group in Schwerzenbach, Switzerland for their helpful discussions, Mrs. Helga Judex for preparing the layout and Dudley May for checking the text. Schwerzenbach, October 2001 Cyril Darribère This application booklet presents selected application examples. The experiments were conducted with the utmost care using the instruments specified in the description of each application. The results were evaluated according to the current state of our knowledge. This does not however absolve you from personally testing the suitability of the examples for your own methods, instruments and purposes. Since the transfer and use of an application is beyond our control, we cannot of course accept any responsibility. When chemicals, solvents and gases are used, general safety rules and the instructions given by the manufacturer or supplier must be observed. ® TM All names of commercial products can be registered trademarks, even if they are not denoted as such. METTLER-TOLEDO Collected Applications EVOLVED GAS ANALYSIS Page 1 Contents Preface ......................................................................................................................................................1 Content......................................................................................................................................................2 Abbreviations and Acronyms ...................................................................................................................3 Introduction to TGA-EGA........................................................................................................................4 TGA-MS ...................................................................................................................................................5 TGA-FTIR ..............................................................................................................................................11 Introduction to the Applications .............................................................................................................20 Applications List.....................................................................................................................................21 1 Decomposition of Acetylsalicylic Acid....................................................................................22 2 Thermal Degradation of BHET ................................................................................................24 3 Influence of the Heating Rate on MS Response.......................................................................27 4 Decomposition of Technical Lauryl Alcohol ...........................................................................30 5 Detection of Residual Solvents in a Pharmaceutical Substance...............................................33 6 Decomposition of Calcium Oxalate Monohydrate (Tutorial) ..................................................35 7 Influence of Sample Weight on MS Sensitivity .......................................................................37 8 Decomposition of Copper Sulfate Pentahydrate (Tutorial)......................................................39 9 Desorption of a Zeolite Filled with Organic Matter .................................................................41 10 Pyrolysis of PVC Powder .........................................................................................................43 11 Investigation of Fluorinated Cable Wires.................................................................................46 12 Detection of Methyl Salicylate in a Sample of Rubber. ...........................................................49 13 Degradation of a Silicone Polymer...........................................................................................51 14 Curing and Decomposition of an Amino Resin........................................................................54 15 Identification of BR and NR Rubbers. Study of Thermal Transitions and Decomposition.....57 16 Delamination of Printed Circuit Boards ...................................................................................61 17 Expansion and Decomposition of the Polymer Used to Encapsulate a Blowing Agent ..........63 Index .......................................................................................................................................................66 Notes .......................................................................................................................................................68 Page 2 EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications 2 Thermal Degradation of BHET Purpose The aim of the experiment was to demonstrate how the evolved gas profile and evolved group profile techniques can be used to identify degradation products. Sample BHET (bis-hydroxylethyl terephthalate). A common industrial synthetic route involves the esterification of terephthalic acid (TPA) with ethylene glycol (EG). BHET is usually polymerized to polyethylene terephthalate (PET). O O OH + 2 HO O OH HO O HO + 2HO 2 O O TPA Conditions OH BHET EG Measuring cells: TGA/SDTA851e coupled to a Nicolet Nexus FTIR Pan: Alumina 70 µl, no lid Sample preparation: As received, no preparation. 18.122 mg TGA measurement: Heating from 50 °C to 900 °C at 10 K/min, blank curve corrected Atmosphere: Nitrogen, 80 ml/min Degradati on of BHET (A) % TGA curve Step -36.6665 % -6.6447 mg Step -54.8433 % -9.9387 mg 50 0 %min^-1 10.09.2001 13:47:28 DTG curve -5 -10 -15 0.10 0.05 Gram-Schmidt curve 0.00 -0.05 100 Interpretation Page 24 200 300 400 500 600 700 °C The two-step degradation of BHET results in maxima in the DTG curve at about 300 °C and 450 °C. Theses peaks correspond to the maxima in the GramSchmidt curve. EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications Evolved gas profile Interpretation Functional group profile The evolved gas profile method (see chapter on interpretation procedures of TGA-FTIR measurements) was used to identify the cause of the first weight loss step (DTG maximum at 300 °C). Direct comparison of the FTIR spectrum at 300 °C with database spectra characterizes the first component evolved as pure ethylene glycol. The doublet between 2340 and 2360 cm-1 (A) is due to residual carbon dioxide in system. Absorption bands in the range 4000-3500 cm-1 and 2000-1200 cm-1 (B) are attributed to water (background moisture). Degradati on of BHET (B) 10.09.2001 13:47:37 %min^-1 DTG curve -10 -20 3000-2850 cm-1 Alkane -30 1280-970 cm-1 Alcohol 1800-1600 cm-1 Carboxylic acid 20 1790-1650 cm-1 Ester 200 METTLER-TOLEDO Collected Applications 400 600 EVOLVED GAS ANALYSIS °C Page 25 Interpretation Evaluation Conclusion Page 26 Direct comparison of the spectrum at 450 °C with the database did not identify any one particular decomposition product. The functional group profiles of possible decomposition products are displayed above. As expected, the maxima correspond to the maxima of the DTG curve. The identification of the degradation products is based on the measurement of the absorption bands corresponding to different chemical groups, for example the characteristic absorption bands of C-O for alcohols, carboxylic acids and esters. The profiles for aromatic ring and ketones were measured but no significant signals were detected. In fact the side chains of BHET are cleaved at temperatures above 400 °C with the formation of hydroxy formic acid ester (CH-CO-O-(CH2)2-OH). Step, % Peak temperature DTG, °C Possible products 36.7 298.5 ethylene glycol 54.8 446.1 hydroxy formic acid ester The two closely lying weight loss steps cannot be interpreted with TGA alone. The use of a TGA-FTIR combination allows a qualitative characterization of the decomposition process. Based on an analysis of the spectrum at 300 °C, we concluded that the first decomposition step is due to the loss of ethylene glycol. The second step was assumed to be due to the cleavage of the side chains of the aromatic group, i.e. the elimination of hydroxy formic acid ester. This study illustrates the application of the evolved gas profile and the functional group profile methods described in the introduction. The two techniques provide complementary information. EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications 3 Influence of the Heating Rate on MS Response Purpose The aim of the experiment was to investigate the influence of different heating rates on the response of the mass spectrometer. Sample Malonic acid (COOH-CH2-COOH). Conditions Measuring cells: TGA/SDTA851e coupled to a Balzers Thermostar© mass spectrometer Pan: Aluminum 40 µl, no lid Sample preparation: The crystals were ground in a mortar. Sample weights of approx. 10 mg were used. TGA measurement: Heating from 50 °C to 640 °C at 5, 10, 20, 30, 40 and 50 K/min, blank curve corrected Atmosphere: Nitrogen, 50 ml/min ^exo Influence Heati ng Rate on MS (A) % 10.09.2001 13:49:49 5 10 20 30 40 50 TGA curves 50 K/min K/min K/min K/min K/min K/min 0 °C SDTA curves -5 -10 -15 80 Interpretation 100 120 140 160 180 200 220 240 260 280 °C Samples of malonic acid were measured with TGA at heating rates between 5 and 50 K/min and the corresponding TGA signals recorded. The thermal decomposition of malonic acid exhibits a single weight loss step that shifts toward higher temperatures with increasing heating rates. The SDTA curves indicate that the sample begins to melt at approximately 134 °C (onset temperature), and that this is independent of the heating rate. The decomposition process in the liquid phase depends on the heating rate; higher heating rates shift the onset of decomposition and the decomposition range to higher temperatures. METTLER-TOLEDO Collected Applications EVOLVED GAS ANALYSIS Page 27 Influence Heati ng Rate on MS (B) 10.09.2001 13:49:59 m/z 60 Heating rate Integral 50 2326.99 nsA Integral 40 50 nA 2287.02 nsA Integral 30 2273.32 nsA Integral 2213.86 nsA 20 10 5 0 Interpretation Evaluation Page 28 10 20 Integral 2185.62 nsA Integral 2154.70 nsA 30 min The decomposition of malonic acid results in the elimination of acetic acid, which was confirmed by the m/z 60 ion curve (see MS spectrum of acetic acid in Application 1). The m/z 60 ion curve was measured at each different TGA heating rate and the corresponding peak areas calculated. The results are summarized in the above diagram. The TGA curve showed that the weight losses were the same for each sample, i.e. the amount of acetic acid evolved was independent of the heating rate. Constant peaks areas for the m/z 60 curves were therefore also expected. In fact the calculated peak area shows a slight increase at higher heating rates. Heating rate, K/min Onset / peak temperatures SDTA, °C MS peak area (m/z 60), nsA Melting Decomposition 5 134.2 173.6 2154 10 134.1 182.9 2185 20 134.1 188.7 2213 30 134.0 194.3 2273 40 134.0 191.5 2287 50 134.0 203.7 2326 EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications Peak area m/z 60 [nsA] 3000 2000 1000 0 10 20 30 40 50 60 Heating rate [K/min] Conclusion The weight loss step due to the decomposition of malonic acid to acetic acid is independent of the heating rate. The m/z 60 peak area depends to a slight extent on the heating rate, although similar quantities of material are transported to the mass spectrometer. This effect might be due to the fact that because the heating rates are different, the concentration of acetic acid in the evolved gases is in each case very different. METTLER-TOLEDO Collected Applications EVOLVED GAS ANALYSIS Page 29 4 Decomposition of Technical Lauryl Alcohol Purpose The aim of the experiment was to investigate the possible decomposition products of lauryl alcohol. Sample Technical 1-dodecanol; CH3(CH2)11OH, lauryl alcohol. Conditions Measuring cells: TGA/SDTA851e coupled to a Nicolet Nexus FTIR Pan: Aluminum 40 µl, pierced lid (diameter: 50 µm) Sample preparation: The waxy material was melted in a water bath at 50 °C. The homogeneous liquid was then filled directly into the crucible and cooled to room temperature before measurement TGA measurement: Heating from 30 °C to 600 °C at 5 K/min, blank curve corrected Atmosphere: Nitrogen, 50 ml/min Decomposition of Lauryl Al cohol % TGA curve 50 0 %°C^-1 DTG curve 10.09.2001 13:47:14 Step -57.2342 % -18.6028 mg Step -9.0136 % -2.9297 mg Step -32.2443 % -10.4804 mg -1 -2 Gram-Schmidt curve 0.05 0.00 100 Interpretation 200 300 400 500 °C The measurements were performed in crucibles sealed with pierced lids to improve the separation of the two weight loss steps in the TGA curve compared with measurements in an open crucible. The Gram-Schmidt curve shows two peaks that correspond to the weight loss steps at 207 °C and 395 °C (maxima of the DTG curve). Page 30 EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications Interpretation The FTIR spectrum measured at about 207 °C was very similar to that of 1-decanol in the spectrum database (e.g. stretching and bending vibrations of the alkane part at 2970, 2930, 2860, 1460 and 1380 cm-1 and the primary alcohol at 3670, 1350 and 1050 cm-1). A detailed comparison of the measured spectrum and the reference spectrum of dodecanol in the range 1500-900 cm-1 confirms that no dodecanol evaporates. A distinct shoulder can be observed on the high-temperature side of the peaks in both the DTG and GS curves of the first weight loss step. Spectra recorded in this shoulder however showed no new absorption bands. Possible explanations for this could be presence of other dodecanol isomers in the original sample, i.e. isomers with a higher boiling point, or polymerization. This would have to be checked with TGA-MS measurements. METTLER-TOLEDO Collected Applications EVOLVED GAS ANALYSIS Page 31 Interpretation Evaluation Conclusion Page 32 Some additional peaks were observed in the spectrum at 395 °C. Besides the characteristic bands for alkanes and alcohols, the absorption bands at 3085 cm-1 (C-H stretching), 1640 cm-1 (C=C stretching) and 990/910 cm-1 (out-of-plane C-H bending) indicate the presence of a vinyl group. The second weight loss step could result from the decomposition of a polymerized compound. Step, % Peak temperature DTG, °C Possible products 57.2 206.9 1-decanol 9.0 237.7 isomerization / polymerization product 32.2 395.1 alkyl chain with a vinyl and alcohol group The main functional groups such as alkane, alcohol and alkene were identified by FTIR measurements. The cleavage of the dodecanol chain results mainly in a shorter aliphatic alcohol (first step) and an alkene (third step). An isomerization or polymerization reaction is proposed to explain the degradation products of the second weight loss step. EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications 5 Detection of Residual Solvents in a Pharmaceutical Substance Purpose The aim of the experiment was to identify residues of solvents left in a pharmaceutical substance after recrystallization in a solvent mixture. Sample Pharmaceutically active substance. Conditions Measuring cells: TGA/SDTA851e coupled to a Balzers Thermostar© mass spectrometer Pan: Alumina 70 µl, without lid Sample preparation: As received, no preparation. 1.3640 mg TGA measurement: Heating from 30 °C to 350 °C at 10 K/min, blank curve corrected Atmosphere: Nitrogen, 20 ml/min Detecti on of Resi dual Sol vents % TGA curve 10.09.2001 13:48:21 Step -2.2889 % -31.2205e-03 mg 95 Step -4.6543 % -63.4850e-03 mg 90 85 lg(nA) m/z 43 -2 MS curves m/z 31 -3 -4 50 Interpretation 100 150 200 250 300 °C The TGA curve shows a number of weight loss steps in temperature range measured. The initial weight loss below 60 °C is probably due to the gradual evaporation of adsorbed moisture or solvent(s) while the final step (above 250 °C) no doubt arises from the decomposition of the product. The change in the intensities of the m/z 31 (methanol) and m/z 43 (acetone) fragment ions measured by TG-MS in the MID mode correspond to the weight loss steps in the range 70-190 °C and 190-225 °C respectively. The MS spectra of methanol and acetone are shown in the following diagrams. While methanol is evolved over a wide temperature range, the acetone (m/z 43) peak is appreciably sharper. The higher temperature indicates that acetone is more firmly bound in the substance in the form of a solvate. METTLER-TOLEDO Collected Applications EVOLVED GAS ANALYSIS Page 33 MS spectrum of methanol 100 Rel. Abundance 80 60 40 20 0 10 20 30 40 50 60 40 50 60 m/z MS spectrum of acetone 100 Rel. Abundance 80 60 40 20 0 10 20 30 m/z Evaluation Conclusion Page 34 Evaluation range, °C Step, % Possible products 70-190 2.3 methanol 190-225 4.7 methanol, acetone From the TG-MS results, it was possible to identify the solvents used to recrystallize the active ingredient. In this case (and in general), it is possible to distinguish between residual solvent that is merely adsorbed and solvent molecules that are more strongly bound in the form of solvates. EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications 6 Purpose Sample Conditions Decomposition of Calcium Oxalate Monohydrate (Tutorial) Calcium oxalate monohydrate is used as a tutorial sample to demonstrate the decomposition reaction and stoichiometry. Calcium oxalate monohydrate decomposes in three reaction steps given by the following equations: (1) CaC2O4.H2O Ö CaC2O4 + H2O (2) CaC2O4 Ö CaCO3 + CO (3) CaCO3 Ö CaO + CO2 Calcium oxalate monohydrate (CaC2O4.H2O). Measuring cells: TGA/SDTA851e coupled to a Balzers Thermostar© mass spectrometer Pan: Alumina 70 µl, no lid Sample preparation: As received, no preparation. 26.465 mg TGA measurement: Heating from 70 °C to 950 °C at 30 K/min, blank curve corrected Argon (50 ml/min) was used instead of nitrogen in order to be able to detect CO since CO and N2 ions appear at the same m/z ratio Atmosphere: Decomposition of CaC2O4 (A) % 10.09.2001 13:46:48 TGA curve 100 Step -12.4872 % -3.3049 mg Step -18.8643 % -4.9926 mg 80 60 Step -30.2613 % -8.0089 mg Residue 38.3819 % 10.1581 mg 40 100 Interpretation 200 300 400 500 600 700 800 °C The TGA curve shows three weight loss steps in agreement with the threestep reaction scheme given above. The loss of water of crystallization and the two decomposition steps yielded weight losses of 12.5%, 18.9% and 30.3% respectively. This is in good agreement with the theoretical stoichiometric values based on the fact that one mole of CaC2O4⋅H2O gives rise to one mole each of H2O, CO and CO2. METTLER-TOLEDO Collected Applications EVOLVED GAS ANALYSIS Page 35 ^exo Decomposition of CaC2O4 (B) mgmin^-1 10.09.2001 13:46:56 DTG curve -1 -2 MS: H2O MS: CO MS: CO2 5 nA 100 Interpretation Evaluation Conclusion Page 36 200 300 400 500 600 700 800 900 °C From the above reaction scheme, one would expect H2O, CO and CO2 to be evolved in turn at each step. The TG-MS ion curves showed this is in fact the case. However, in contrast to the reaction scheme, a small CO2 peak was observed during the decomposition of the anhydrous oxalate in the second weight loss step. This is due to the disproportionation of CO to CO2 (2CO → CO2 + C). Stoichiometric step % Measured step % Peak temperatures DTG, °C Possible products 12.3 12.5 221.3 water of crystallization 19.2 18.9 528.5 CO, CO2 30.1 30.3 847.6 CO2 According to the above reaction scheme, calcium oxalate monohydrate is expected to decompose in well-defined steps with the release of H2O, CO and CO2. It seems that this process is more complicated as slight differences were observed between the measured and theoretical weight loss steps. The TG-MS results help clarify the matter by showing that CO2 is also formed during the second reaction step. This was concluded as arising from the disproportionation reaction 2CO → CO2 + C. EVOLVED GAS ANALYSIS METTLER-TOLEDO Collected Applications