Introduction to Thermal Analysis Methods

Introduction to Thermal Analysis Methods HKUST Introduction to Thermal Analysis Methods HKUST • Definition: Thermal analysis refers to a variety of techniques in which physical property of a sample is continuously measured as a function of temperature, whist the sample is subjected to a pre-determined temperature profile. • Where is Thermal Analysis used? • Thermal Analysis techniques are used in virtually every area of modern science and technology. The basic information that these techniques provide, such as crystallinity, specific heat and expansion, are relied on heavily for the research and development of new products. Thermal Analysis techniques also find increasing use in the area of quality control and assurance, where demanding requirements must be met in an increasingly competitive world. And of course thermal analysis instruments are used in universities for applications ranging from basic undergraduate studies to the most sophisticated postgraduate research. Introduction to Thermal Analysis Methods HKUST • Major Techniques: – Thermal Gravimetric Analysis (TGA) – Differential Thermal Analysis (DTA) – Differential Scanning Calorimetry (DSC) – Dynamic Mechanical Analysis (DMA) – Thermo-mechanical Analysis (TMA) • Other techniques: – Melt Elasticity Index – Dielectric Analysis – Thermally Stimulated Current/Relaxation Map Analysis – Free Volume Microprobe – Differential Photo Calorimetry – Thermal Conductivity Analysis Introduction to Thermal Analysis Methods HKUST TG or TGA----Thermal Gravimetric Analysis TGA examines the process of weight changes as a function of time, temperature, and other environment conditions that may be created within the apparatus. Heating Temperature Rate Sample Weight Change Atmosphere Introduction to Thermal Analysis Methods HKUST Typical TGA curve W e i g h t L o s s ( w t . % ) 1 s t D e r i v a t i v e ( m g / m i n x 1 0 - 2 ) 100 200 300 400 500 0.0 -2.0 -4.0 -6.0 -8.0 -10.0 -12.0 -14.0 -16.0 0.0 Temperature (ºC) Ti Te Td ∆W 2.0 -2.0 -4.0 -6.0 -8.0 -10.0 Introduction to Thermal Analysis Methods HKUST ∗ Effect of Atmosphere on Mass Buoyancy: the change in density of the gas phase with temperature. ρair ↓ ⇒Wspecimen↓ example: ρdry air =1.3 mg/cm3 , 25 oC ρdry air =0.3 mg/cm3 , 1000 oC For : 20 mg Sample ( ρ =1.0g/ cm3 ) 25 oC -----→ 1000 oC A 0.1 wt% loss will be introduced due to Buoyancy Introduction to Thermal Analysis Methods HKUST TABLE MAJOR FACTORS AFFECTING THERMOGRAVIMETRY Mass Temperature Buoyancy Heating rate Atmospheric turbulence Thermal conductivity Condensation and reaction Enthalpy of the process Electrostatic and magnetic forces Sample, furnace, and sensor arrangement Electronic drift Electronic drift Introduction to Thermal Analysis Methods HKUST • The range of materials can be studied by thermal analysis – Biological materials – Building materials – Catalysis – Ceramics and Glasses Applications of TGA: Composition Moisture content Solvent content Additives Polymer content Filler content Dehydration Decarboxylation Oxidation Decomposition Dm = mass change dm/dt = rate of mass change/decomposition DTG = derivative thermogravimetry DTG Peak = characteristic decomposition temperatures ® identification Tonset = thermal stability Introduction to Thermal Analysis Methods HKUST Example of using TGA to identify the composition of a PP/PE blend Introduction to Thermal Analysis Methods HKUST (1) Additives Oxidation → Weight gain → Temperature ~ time → Anti Oxidation additive concentration (2) Extent of Cure Residual Weight loss → Degree of cure (3) Thermal Stability (4) Reactivity & Phase Equilibration in Ceramics Introduction to Thermal Analysis Methods HKUST TGA Curve --- I Introduction to Thermal Analysis Methods HKUST Introduction to Thermal Analysis Methods HKUST Effect of Sulfonation Treatment ⇓ Degree of Stability Introduction to Thermal Analysis Methods HKUST Introduction to Thermal Analysis Methods HKUST Introduction to Thermal Analysis Methods HKUST DSC Introduction to Thermal Analysis Methods HKUST DSC and DTA are techniques by which the difference in heat flow to or from a sample and to or from a reference is monitored as a function of temperature or time, while the sample is subjected to a controlled temperature program. Applications: •characteristic temperatures identification •glass transitions •melting and crystallization behavior •heat of melting and crystallization •purity •compatibility •polymorphism •solid-liquid ratio •specific heat capacity •reaction behavior •heat of reaction •reaction kinetics •oxidative stability •thermal stability Differential Thermal Analysis (DTA) Differential Scanning Calorimetry (DSC) Introduction to Thermal Analysis Methods HKUST Typical DSC curve H e a t F l o w ( m W ) 50 100 150 200 250 300 20 15 10 5 0 -5 -10 -15 Tonset TC Tp Exo Temperature (ºC) 25 Introduction to Thermal Analysis Methods HKUST Measurement of Glass Transition Temperature (Tg) by DSC and Rate Effects Tg measurement by DSC Introduction to Thermal Analysis Methods HKUST TABLE VIII EVENTS DETECTED BY DTA AND DSC AND THEIR EFFECTS DURING EATINGH Transformation Observation Reaction Observation First-order Endothermic Liquid-solid Either Higher-order Step in base Homogeneous Either Lambda Endothermic Polymerization Exothermic Metastable to stable Exothermic Gas-condensed phase Either Introduction to Thermal Analysis Methods HKUST Introduction to Thermal Analysis Methods HKUST Typical DTA and DSC curves for the melting of ice, illustrating several methods of defining the baseline. Relative error of the enthalpy due to each baseline construction is given relative to an exponential function taken as the “true” baseline. Introduction to Thermal Analysis Methods HKUST Typical DTA and DSC curves illustrating several methods of defining the baseline Introduction to Thermal Analysis Methods HKUST Introduction to Thermal Analysis Methods HKUST Representations of two heat-flux calorimeters showing (a) the Boersma therr Couple placement (b) the Tian-Calvet design (c) The schematic diagram is appropriate. Introduction to Thermal Analysis Methods HKUST General principles and schematic for a DTA apparatus Introduction to Thermal Analysis Methods HKUST (1) Solid State Transitions (2) Solid State Reactions (3) Solid State Decompositions Applications of DTA & DSC (A) Crystallinity DSC can determine the presence & concentration of a crystalline phase in a solid, as well as the melting point of the crystals. (B) The Glass Transition, Tg DSC can detect the glass transition of an amorphous materials, such as polymer. (C) Characterization of Alloys & Composites (D) Aging and Degradation (E) Phase Diagram of alloys Introduction to Thermal Analysis Methods HKUST Differential Scanning calorimetry (DSC) Samuel S. KIM Differential scanning calorimetry (DSC) is one of the most useful techniques applicable to the assessment of enthalpy changes that accompany various material transformations. DSC measures, for example, the endothermic step-like change in heat flow rate that occurs at the glass transition temperature (Tg). It can identify and characterize polymerization, cross linking, crystallization, and fusion ( or melting) of crystallites in terms of heat evolved or absorbed and the associated transition temperatures. Furthermore, these thermal data enable one to deduce the reaction rate constant, kinetic reaction order, activation energy, etc. DSC analyses can be made either isothermally as a function of time of dynamically as function of temperature. DSC is particularly suited to the characterization of mold compounds. For example, DSC enables one to assess how fast the mold compound is curing at the transfer molding temperature and to measure the effect of a particular catalyst on the cure rate, the initial as-molded Tg, and the final post-cured Tg. Experimental procedures for DSC are rather simple. For analysis by DSC, a 5-20 mg sample of a solid is placed in a small Al pan, which is subsequently covered with an Al lid and tightly crimped around the edge. It is then transferred into the DSC cell and cured either isothermally or dynamically, depending on the desred cure mode. In the case of a volatile or liquid sample, a high-pressure steel sample cup is available for hermetic sealing. Introduction to Thermal Analysis Methods HKUST Figure 2.20 is a typical dynamic DSC thermo gram. In the beginning of the thermal scanning, an increase in the observed heat flow is due to the thermal stabilization of the instrument and the specimen. The first sharp peak is an endothermic characteristic representing the T g of uncured starting material. The second peak, denoted by A, is due to exothermic hear liberated during the cure reaction. From a series of dynamic scans of samples cured isothermally at a given temperature for different times one can monitor the progressive change in T g as well as the degree of cure. In parallel with this, multiple isothermal analyses at different temperatures enable one to evaluate various reaction kinetic parameters. The reaction rate is given by the following equation: Rate =dα/dt =kf(α) (1) Where α is the degree of conversion or cure, k is the rate constant, and f(α) is some function of the reactive group conversion. The reaction rate constant, k, in turn, is expressed in the form of Arrhenius equation. k= A-E/RT (2) Where A is the frequency factor, E is the activation energy, R is the gas constant, and T is the absolute temperature. Combining Equations 1 and 2 and taking the logarithm of both sides one obtains ln(dα/dt )=ln[Af(α )] – E/RT (3) Introduction to Thermal Analysis Methods HKUST In DSC measurements, the degree of cure of conversion can be estimated from the ratio of the amount of heat evolved for the partial conversion after time t at a given temperature, Ht , to the total heat evolved for the complete conversion, H 0 : α = Ht / H0 (4) The rate Equations 1 and 3 can be rewritten as dα/dt = (1/ H0)(d Ht/dt) (5) ln(dα/dt ) = ln [(1/ H0)(d Ht/dt)]= ln[Af(a)]-E/RT (6) If f(α) is a function of conversion but not of temperature, the activation energy, E, can be obtained by plotting ln [(1/ H0)(d Ht/dt)] versus 1/T at a fixed conversion. From Equation 6 one can see that E can be calculated from the slope of the line. It is also generally assumed that f (α) can be expressed in terms of both α and the reaction kinetic order, n, as follows: f(α) = (1- α)n (7) The rate Equation 6 then can be rewritten in combination with Equation 7 as ln(dα/dt ) = lnA-E/RT + nln(1-a) (8) Introduction to Thermal Analysis Methods HKUST One can see from Equation 8 that a plot of ln(rate) versus ln(1-α) should provide a straight line for isothermal runs at a given temperature at various conversions, The slope of the line should yield the kinetic reaction order, n. With the use of this formalism and a series of isothermal experiments, as shown in Figure 2.20, the kinetic order and the activation energy were calculated to be 1.41 and 107.1kJ/mole, respectively, for the mold compounds, which consist of a diglycidyl ether of bisphenol A type and phenolic resin. References: 1. C.C.Riccardi, H.E.Adabbo, and J.J.Williams. J.Appl.Polym.Sci. 29. 2481, 1984 2. V.M.Gonzalez and N.Casillas. Polym.Eng. and Sci. 29, 295, 1989 3. H.E.Kissinger. Anal. Chem.29, 1702, 1957 4. Internal technical report. Rohm and Haas Co., May 1989. Introduction to Thermal Analysis Methods HKUST