Principles and Applications of Thermal Analysis

Edited by

Paul Gabbott

•A Blackwell ·~ Publishing

Contents

Abbreviations

List of Contributors

A Practical Introduction to Differential Scanning Calorimetry 1.1 Introduction 1.2 Principles of DSC and types of measurements made

1.2.1 A definition of DSC 1.2.2 Heat flow measurements 1.2.3 Specific heat ( Cp) 1.2.4 Enthalpy 1.2.5 Derivative curves

1.3 Practical issues 1.3.1 Encapsulation 1.3.2 Temperature range 1.3.3 Scan rate 1.3.4 Sample size 1.3.5 Purge gas 1.3.6 Sub-ambient operation 1.3.7 General practical points 1.3.8 Preparing power compensation systems for use

1.4 Calibration 1.4.1 Why calibrate 1.4.2 When to calibrate 1.4.3 Checking performance 1.4.4 Parameters to be calibrated 1.4.5 Heat flow calibration 1.4.6 Temperature calibration 1.4.7 Temperature control (furnace) calibration 1.4.8 Choice ofstandards

XV

XVI

Paul Gabbott 1 2 2 2 3 3 5 5 6 6 8 8

10 10 11 11 11 12 12 12 13 13 13 15 16 16

vi Contents

1.4.9 Factors affecting calibration 16 1.4.10 Final comments 17

1.5 Interpretation of data 17 1.5.l The instrumental transient 17 1.5.2 Melting 18 1.5.3 The glass transition 22 1.5.4 Factors affecting 'fg 24 1.5.5 Calculating and assigning 'fg 25 1.5.6 Enthalpie relaxation 26 1.5.7 T g on cooling 30 1.5.8 Methods of obtaining amorphous material 31 1.5.9 Reactions 34 1.5.10 Guidelines for interpreting data 40

1.6 Oscillatory temperature profiles 42 1.6.l Modulated temperature methods 42 1.6.2 Stepwise methods 44

1.7 DSC design 46 1.7.1 Power compensation DSC 46 1.7.2 Heat flux DSC 47 1.7.3 Differential thermal analysis DTA 48 1.7.4 Differential photocalorimetry DPC 48 1.7.5 High-pressure cells 49

Appendix: standard DCS methods 49 References 50

2 Fast Scanning DSC Paul Gabbott 51 2.1 Introduction 52 2.2 Proof of performance 52

2.2.1 Effect ofhigh scan rates on standards 52 2.2.2 Definition of HyperDSC™ 54 2.2.3 The initial transient 54 2.2.4 Fast cooling rates 54

2.3 Benefits of fast scanning rates 57 2.3.1 Sensitivity 57 2.3.2 Measurement of sample properties without unwanted

annealing effects 57 2.3.3 Separate overlapping events based on different kinetics 59 2.3.4 Speed of analysis 59

2.4 Application to polymers 61 2.4.1 Melting and crystallisation processes 61 2.4.2 Comparative studies 64 2.4.3 Forensic studies 65 2.4.4 Effect ofheating rate on the sensitivity of the glass transition 67 2.4.5 Effect ofheating rate on the temperature ofthe glass transition 68 2.4.6 Effect ofheating rate on Tg of annealed materials

(and enthalpic relaxation phenomena) 72

Contents vii

2.5 Application to pharmaceuticals 76 2.5.1 Purity of polymorphic form 76 2.5.2 Identifying polymorphs 78 2.5.3 Determination of amorphous content of materials 79 2.5.4 Measurements of solubility 81

2.6 Application to water-based solutions and the effect of moisture 82 2.6.1 Measurement ofTg in frozen solutions and suspensions 82 2.6.2 Material affected by moisture 83

2.7 Practical aspects of scanning at fast rates 83 2.7.1 Purge gas 83 2.7.2 Sample pans 84 2.7.3 Sample size 85 2.7.4 Scan rate 85 2.7.5 Instrumental settings 85 2.7.6 Cleanliness 85 2.7.7 Getting started 86

References 86

3 Thermogravimetric Analysis RodBottom 87 3.1 Introduction 88 3.2 Design and measuring principle 89

3.2.1 Buoyancy correction 90 3.3 Sample preparation 92 3.4 Performing measurements 93

3.4.1 Influence ofheating rate 93 3.4.2 Influence of crucible 94 3.4.3 Influence of furnace atmosphere 95 3.4.4 Influence of residual oxygen in inert atmosphere 95 3.4.5 Influence of reduced pressure 96 3.4.6 lnfluence ofhumidity control 97 3.4.7 Special points in connection with automatic sample changers 97 3.4.8 Inhomogeneous samples and samples with very

small changes in mass 98 3.5 Interpreting TGA curves 98

3.5.1 Chemical reactions 99 3.5.2 Gravimetrie effects on melting 101 3.5.3 Other gravimetric effects 101 3.5.4 Identifying artefacts 103 3.5.5 Final comments on the interpretation of TGA curves 104

3.6 Quantitative evaluation of TGA data 104 3.6.1 Horiwntal or tangential step evaluation 104 3.6.2 Determination of content 106 3.6.3 The empirical content 107 3.6.4 Reaction conversion, a 110

3.7 Stoichiometric considerations 111 3.8 Typical application: rubber analysis 111

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3.9 Analysis overview 112 3.10 Calibration and adjustment 112

3.10.1 Standard TGA methods 113 3.11 Evolved gas analysis 114

3.11.1 Brief introduction to mass spectrometry 115 3.11.2 Brief introduction to Fourier transform infrared spectrometry 115 3.11.3 Examples 117

Reference 118

4 Principles and Applications of Mechanical Thermal Analysis John Duncan 119 4.1 Thermal analysis using mechanical property measurement 120

4.1.1 Introduction 120 4.1.2 Viscoelastic behaviour 121 4.1.3 The glass transition, Tg 123 4.1.4 Sub-T g relaxations 124

4.2 Theoretical considerations 125 4.2.1 Principles ofDMA 125 4.2.2 Moduli and damping factor 127 4.2.3 Dynamic mechanical parameters 127

4.3 Practical considerations 128 4.3.1 Usage ofDMA instruments 128 4.3.2 Choosing the best geometry 129 4.3.3 Considerations for each mode of geometry 133 4.3.4 Static force control 134 4.3.5 Consideration of applied strain and strain field 134 4.3.6 Other important factors 135 4.3.7 The first experiment -what to do? 136 4.3.8 Thermal scanning experiments 137 4.3.9 Isothermal experiments 137 4.3.10 Strain scanning experiments 138 4.3.11 Frequency scanning experiments 138 4.3.12 Step-isotherm experiments 138 4.3.13 Creep - recovery tests 138 4.3.14 Determination ofthe glass transition temperature, Tg 139

4.4 Instrument details and calibration 140 4.4.1 Instrument drives and transducers 140 4.4.2 Force and displacement calibration 141 4.4.3 Temperature calibration 141 4.4.4 Effect of heating rate 142 4.4.5 Modulus determination 142

4.5 Example data 143 4.5.l Amorphous polymers 143 4.5.2 Semi-crystalline polymers 145 4.5.3 Example of a and ß activation energy calculations using PMMA 14 7 4.5.4 Glass transition, T8, measurements 150

Contents

4.5.S Measurements on powder samples 4.5.6 Effect of moisture on samples

4.6 Thermomechanical analysis 4.6.1 Introduction 4.6.2 Calibration procedures 4.6.3 TMA usage

Appendix: sample geometry constants References

5 Applications ofThermal Analysis in Electrical Cable Manufacture 5.1 Introduction 5.2 Differential scanning calorimetry

5.2.1 Oxidation studies (OIT test) 5.2.2 Thermal history studies 5.2.3 Cross-linking processes 5.2.4 lnvestigation of unknowns 5.2.S Rapid scanning with DSC

5.3 Thermomechanical analysis 5.3.1 Investigation of extrusion defects 5.3.2 Cross-linking 5.3.3 Material identification 5.3.4 Extrusion studies 5.3.S Fire-retardant mineral insulations

5.4 Thermogravimetric analysis 5.4.1 Practical comments 5.4.2 Investigation of composition 5.4.3 Carbon content 5.4.4 Rapid scanning with TGA

5.5 Combined studies 5.6 Concluding remarks References

6 Application to Thermoplastics and Rubbers Martin/. Forrest 6.1 Introduction

John A. Bevis

ix

152 155 156 156 157 157 162 163

164 165 165 165 166 168 171 172 175 176 177 177 178 179 180 180 181 185 186 188 189 189

190 191

6.2 Thermogravimetric analysis 192 6.2.1 Background 192 6.2.2 Determination of additives 194 6.2.3 Compositional analysis 199 6.2.4 Thermal stability determinations 206 6.2.S High resolution TGA and modulated TGA 208 6.2.6 Hyphenated TGA techniques and evolved gas analysis 210

6.3 Dynamic mechanical analysis 212 6.3.1 Background 212 6.3.2 Determination of polymer transitions and investigations into

molecular structure 215

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6.3.3 Characterisation of curing and eure state studies 218 6.3.4 Characterisation of polymer blends and the effect of additives

on physical properties 220 6.3.5 Ageing, degradation and creep studies 224 6.3.6 Thermal mechanical analysis 227

6.4 Differential scanning calorimetry 228 6.4.1 Background 228 6.4.2 Crystallinity studies and the characterisation of polymer blends 231 6.4.3 Glass transition and the factors that influence it 236 6.4.4 Ageing and degradation 238 6.4.5 Curing and cross-linking 241 6.4.6 Blowing agents 243 6.4.7 Modulated DSC 244 6.4.8 HyperDSC™ 245 6.4.9 Microthermal analysis 246

6.5 Other thermal analysis techniques used to characterise thermoplastics and rubbers 247 6.5.1 Dielectric analysis 247 6.5.2 Differential photocalorimetry (DPC) 248 6.5.3 Thermally stimulated current (TSC) 248 6.5.4 Thermal conductivity analysis (TCA) 249

6.6 Conclusion 249 References 250

7 Thermal Analysis ofBiomaterials Showan N. Nazhat 256 Abbreviations 257 7.1 Biomaterials 257

7.1.1 Introduction 257 7.2 Material classes ofbiomaterials 260

7.2.1 Metals and alloys 260 7.2.2 Ceramics and glasses 260 7.2.3 Polymers 261 7.2.4 Composites 262

7.3 The significance of thermal analysis in biomaterials 262 7.4 Examples of applications using dynamic mechanical analysis (DMA)

in the development and characterisation ofbiomaterials 263 7.4.1 Particulate and/or fibre-filled polymer composites as hone

substitutes 264 7.4.2 Absorption and hydrolysis of polymers and composites 270 7.4.3 Porous foams for tissue engineering scaffolds 271

7.5 Examples of applications using DSC in the development and characterisation ofbiomaterials 275 7.5.1 Thermal history in particulate-filled degradable

composites and foams 275 7.5.2 Plasticisation effect of solvents 276 7.5.3 Thermal stability and degradation 278 7.5.4 Setting behaviour of inorganic cements 278

Contents xi

7.6 Examples of applications using differential thermal analysis/thermogravimetric analysis (DTNTGA) in the development and characterisation ofbiomaterials 280 7 .6.1 Bioactive glasses 280 7.6.2 Thermal stability ofbioactive composites 282

7.7 Summary 283 References 283

8 Thermal Analysis of Pharmaceuticals Mark Saunders 286 8.1 lntroduction 287 8.2 Determining the melting behaviour of crystalline solids 288

8.2.1 Evaluating the melting point transition 288 8.2.2 Melting point determination for identification of

samples 289 8.3 Polymorphism 290

8.3. l Significance of pharmaceutical polymorphism 291 8.3.2 Thermodynamic and kinetic aspects of polymorphism:

enantiotropy and monotropy 292 8.3.3 Characterisation of polymorphs by DSC 293 8.3.4 Determining polymorphic purity by DSC 297 8.3.5 Interpretation of DSC thermograms of samples exhibiting

polymorphism 302 8.4 Solvates and hydrates (pseudo-polymorphism) 303

8.4.1 Factors influencing DSC curves ofhydrates and solvates 304 8.4.2 Types of desolvation/dehydration 305

8.5 Evolved gas analysis 308 8.6 Amorphous content 310

8.6.1 Introduction 310 8.6.2 Characterisation of amorphous solids: the glass

transition temperature 8.6.3 Quantification of amorphous content using DSC

8.7 Purity determination using DSC 8. 7 .1 Types of impurities 8.7.2 DSC purity method 8.7.3 Practical issues and potential interferences

8.8 Excipient compatibility 8.8. l Excipient compatibility screening using DSC 8.8.2 Excipient compatibility analysis using isothermal

calorimetry 8.9 Microcalorimetry

8.9.1 Introduction 8.9.2 Principles of isothermal microcalorimetry 8.9.3 High-sensitivity DSC 8.9.4 Pharmaceutical applications of isothermal

microcalorimetry References

311 313 315 315 316 317 320 321

321 323 323 324 324

325 327

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9 Thermal Methods in the Study ofFoods and Food Ingredients Bill MacNaughtan, Imad A. Farhat 330 9.1 Introduction 331 9.2 Starch 332

9.2.1 Starch structure 332 9.2.2 Order in the granule 335 9.2.3 The glass transition 336 9.2.4 Extrusion and expansion 337 9.2.5 Mechanical properties 338 9.2.6 Starch retrogradation 338 9.2.7 Effect of sugars 339 9.2.8 Lipid-amylose complexes 339 9.2.9 Multiple amorphous phases: polyamorphism 339 9.2.10 Foods 341

9.3 Sugars 343 9.3.l Physical properties 343 9.3.2 Sugar glasses 345 9.3.3 Sugar crystallisation 345 9.3.4 Effect of ions on crystallisation 347 9.3.5 Effect of ions on the glass transition temperature 348 9.3.6 Ageing 349 9.3.7 The Maillard reaction 350 9.3.8 Mechanical properties 350 9.3.9 Foods 350

9.4 Fats 353 9.4.1 Solid/liquid ratio 353 9.4.2 Phase diagrams 355 9.4.3 Polymorphie forms and structure 356 9.4.4 Kinetic information 356 9.4.5 Non-isothermal methods 359 9.4.6 DSC at high scanning rates 361 9.4.7 Mechanical measurements 362 9.4.8 Lipid oxidation and the oxidation induction

time test 362 9.4.9 Foods 363

9.5 Proteins 365 9.5.1 Protein denaturation and gelation 365 9.5.2 Differential scanning microcalorimetry 366 9.5.3 Aggregation 369 9.5.4 Glass transition in proteins 371 9.5.5 Ageing in proteins 371 9.5.6 Gelatin in a high-sugar environment 373 9.5.7 Mechanical properties of proteins 373 9.5.8 Foods 373 9.5.9 TA applied to other areas 377 9.5.10 lnteractions with polysaccharides and other materials 377

Contents xiii

9.6 Hydrocolloids 378 9.6.1 Definitions 378 9.6.2 Structures in solution 379 9.6.3 Solvent effects 381 9.6.4 The rheological Tg 381 9.6.5 Glassy behaviour in hydrocolloid/high-sugar systems 382 9.6.6 Foods 386

9.7 Frozen systems 387 9.7.1 Bound water? 387 9.7.2 State diagram 388 9.7.3 Mechanical properties of frozen sugar solutions 390 9.7.4 Separation of nucleation and growth components of

crystallisation 391 9.7.5 Foods 392 9.7.6 Cryopreservation 394

9.8 Thermodynamics and reaction rates 395 9.8.1 Studies on mixing 395 9.8.2 Isothermal titration calorimetry 396 9.8.3 Reaction rates 398 9.8.4 Thermogravimetric analysis 399 9.8.5 Sample controlled TA 401

References 402

10 Thermal Analysis of Inorganic Compound Glasses and Glass-Ceramics David Furniss, Angela B. Seddon 410 10.1 Introduction 411 10.2 Background glass science 411

10.2.1 Nature of glasses 411 10.2.2 Crystallisation of glasses 413 10.2.3 Liquid-liquid phase separation 416 10.2.4 Viscosity of the supercooled, glass-forming liquid 416

10.3 Differential thermal analysis 418 10.3.1 General comments 418 10.3.2 Experimental issues 420 10.3.3 DTA case studies 421

10.4 Differential scanning calorimetry 426 10.4.1 General comments 426 10.4.2 Experimental issues 427 10.4.3 DSC case studies 427 10.4.4 Modulated Differential Scanning Calorimetry case studies 432

10.5 Thermomechanical Analysis 432 10.5.1 General comments 432 10.5.2 Experimental issues 433 10.5.3 Linear thermal expansion coefficient (ex) and dilatometric

softening point (Mg) 433

xiv Contents

10.5.4 Temperature coefficient of viscosity: introduction 10.5.5 TMA indentation viscometry 10.5.6 TMA parallel-plate viscometry

10.6 Final comments References

Appendix

Glossary

Further Reading

Web Resources

Index

436 438 443 447 448

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