THERMAL ANALYSIS By T.SHIVAKUMAR, , Kottam Institute of Pharmacy, Erravally ‘X’ Roads, MBNR-Dist.AP.
Thermal methods of Analysis This is based on the concept of heating a sample – followed by well-defined modified procedures, such as : gravimetric analysis, differential analysis and titrimetric analysis. Some property of a system is measured as a function of temperature. Thermal spectra or Thermograms
Conti…. Thermo grams characterize a single or multi component system in terms of : Thermodynamic properties, and physicochemical reaction kinetics.
Thermal Analysis is widely used … Other Polymers 13% 21% Chemicals 9% Textiles Pharma 4% 9% Aerospace 4% Petrochem 8% Metals 5% Auto Ag/Food 5% Government Academic 8% 7% 7%
Principle Sample is heated at a constant heating rate Sample’s Property Measured Wt TGA Size TMA Heat Flow DSC Temp. DTA Heat of mixing TT Tem. at which TPD gas is desorbed at the surface.
What is DTA ? Involves the technique of recording the difference in temperature between the Test and Reference material time being constant for both. Hence the Differential Thermogram consists record of difference in Temperatures.
ThermogramA differential thermogram consists of a record of thedifference in sample and reference themperature(∆T)plotted as a function of time t, sampletemperature(Ts), reference temperature(Tr) or furnacetemperature(Tf).In most of thecases, physicalchanges give rise toendothermic curves,whereas chemicalreactions give rise toexothermic peaks.
Instrumentation A differential thermal analyzer is composed of five basic components, namely : 1} Furnace 2} Sample holder 3} temperature controller and recorder 4} thermocouple 5} Cooling device
1} Furnace Tubular furnace is most commonly used because it possess the desired characteristic for good temperature regulation and programming. Dimension of the furnace is depends upon the length of the uniform temperature zone desired. The choice of resistance material is depends on the maximum temperature of the operation and gaseous environment. Grooved muffled cores preferred.
2} Sample holdero Should having low cost, ease of fabrication and inertness towards the sample.o Metallic material: nickel, stainless steel, platinumo Non-metallic material: glass, vitreous silica or sintered alumina.o Most commonly the shape of holder is cylindrical.o The nature of physical constant between the sample, thermocouple junction and the specimen holder affect the DTA signals. So to maintain it, a sample holder with dimples in which thermocouple junctions are inserted (thermocouple wells) are used.
3} Temperature controller and recorderA] Temperature Controller In order to control temperature, the three basic elements are required. These are sensor, control element and heater. The control element governs the rate of heat-input required to match the heat loss from the system. The location of sensor with respect to the heater and mode of heat transfer measure the time elapsed between sensing and variation in heat input.
Conti…B] Temperature programming It transmits a certain time-based instruction to the control unit. By this device one can achieve linearity in the rate of heating or cooling it is driven in a non-linear fashion using a special cam-drive. Heating rates of 10-20 o C / mints are employed.C] Recorder The signals obtained from the sensors can be recorded in which the signal trace is produced on paper or film, by ink, heating stylus, electric writing or optical beam.
4} Thermocouple Thermocouples are the temperature sensors. It is made up from chromel p and alumel wires are used to measure and control temperature up to 1100 0C in air. For above 1100 0C one should use thermocouple made from pure platinum & platinum-rhodium alloy wires.
5} Cooling device It is separate from the temperature programmer because it is independent from heating.
Methodologyo Insert a very thin thermocouple into a disposablesample tube 2 mm in diameter and containing 0.1- 10mg of sample,o Another identical tube is either kept empty or filledwith a reference substance, such asquartz, sand, alumina.o The two tubes are simultaneously inserted into thesample block and subsequently heated (or cooled) ata uniform predetermined programmed rate
Requirements DTA—A few of the vital aspects are : Pre-treatment of the specimen, Particle size and packing of the specimen, Dilution of the specimen, Nature of the inert diluent, Crystalline substances must be powdered, and sieved through 100-mesh sieve
Cont… Either to suppress an unwanted reaction (e.g., oxidation), or to explore the study of a reaction(e.g., gaseous reaction product)—the atmosphere should be controlled adequately.
Advantages: Instruments can be used at very high temperatures. instruments are highly sensitive. flexibility in crucible volume/form. characteristic transition or reaction temperatures can be accurately determined. Disadvantage: uncertainty of heats of fusion, transition, or reaction estimations is 20-50%.
Applications of DTA Physical Chemistry1. Heat of a Reaction2. Specific Heat of substance like Naphthalene.3. Thermal Diffusivity of samples Analytical Chemistry1. Identification of Products since no two products have identical curves.2. Determination of Melting point.
Applications of DTA1. To construct phase diagrams and study phase transitions.2. To find ∆H Peak areas depend upon sample mass, m, enthalpy change ∆H of the process, and geometric and conductivity factors such as heating rate φ and particle size (included in a constant k for a certain substance).Usually the sample peak area is compared with a standard undergoing anenthalpy change at a similar T (since the calibration constant depends onT), under the same conditions, e.g. indium MPt 156.4 C; ∆H fusion = 28.5 J g-1
3. To fingerprint substances4. To determine M.Pt., B.Pt., decomposition temperatures of organiccompounds .
5. To characterize inorganic materialsThe peak at 113 C corresponds to a solid-phase change from the rhombic to themonoclinic form, while the peak at 124 C corresponds to the melting point of theelement.Liquid sulphur is known to exist in at least three forms, and the peak at 179 Capparently involves a transition among these.The peak at 446 C corresponds to the boiling point of sulphur.
6. To quantitatively analyze polymer mixturesThis is a thermogram of a physical mixture of seven commercialpolymers. Each peak corresponds to the characteristic melting pointof one of the components. Poly tetrafluoroethylene (PTFE) has anadditional low temperature peak, which arises from a crystallinetransition. Clearly, differential thermal methods can be useful forqualitative analysis of polymer mixtures.
7. To characterize polymers Schematic DTA thermal curves for the totally amorphous polymer structure and the semi crystalline polymer structure. Both show Tg ; only the semi-crystalline polymer has a crystallization exotherm.
Cont… Quantitative analysis of Compounds Determination of Structural and Chemical changes occurring during heat treatments. Quality Control of Cement, glass, textiles, soils, explosives and resins.
Definitions • A calorimeter measures the heat into or out of a sample. • A differential calorimeter measures the heat of a sample relative to a reference. • A differential scanning calorimeter does all of the above and heats the sample with a linear temperature ramp. • Endothermic heat flows into the sample. • Exothermic heat flows out of the sample.
DSC: The Technique• Differential Scanning Calorimetry (DSC) measures the temperatures and heat flows associated with transitions in materials as a function of time and temperature in a controlled atmosphere.• These measurements provide quantitative and qualitative information about physical and chemical changes that involve endothermic or exothermic processes, or changes in heat capacity.
Conventional DSC Sample Empty Metal Metal Metal Metal 1 2 1 2 Sample Reference Temperature Temperature Temperature Difference = Heat Flow•A ―linear‖ heating profile even for isothermal methods
What can DSC measure?•Glass transitions•Melting and boiling points•Crystallisation time and temperature•Percent crystallinity•Heats of fusion and reactions•Specific heat capacity•Oxidative/thermal stability•Rate and degree of cure•Reaction kinetics•Purity
DSCMeasure Transitions: - Glass Transition Temperature (Tg) - Melting Temperature (Tm) - Crystallization Temperature (Tc)
Think First………Heat Later1. Does the sample contain volatile components? - 2 to 3% water/solvent can lower the glass transition temperature (Tg) by up to 100oC. - Evaporation creates endothermic peaks in standard (non-hermetic) DSC pans and can be suppressed with use of hermetic DSC pans.
2. At what temperature does the sample decompose?- Set the upper limit of the DSC experiment based on decomposition temperature (TGA). No meaningful DSC data can be obtained once decomposition results in a 5% weight loss.- Decomposition affect: the quality of the baseline due to both endothermic and exothermic heat flow, the quality of the baseline for future experiments and can affect the useful lifetime of the DSC cell due to corrosion.
3. How does thermal history (temperature and time) affect DSC results on my sample?4. Identical materials can look totally different based on: - Storage temperature and time. - Cooling rate from a temperature above Tg or above the melting point. - Heating rate. - Different kinds of experiments may need to be performed in order to measure the current structure vs. comparing samples to see if the materials are the same.
Amorphous Structure Glass Transition (Tg)- Detectable by DSC due to a step increase in heat capacity as the sample is heated to a temperature above the glass transition temperature (Tg).- Important transition because significant changes in physical properties, reactivity and storage stability occur at Tg.
Glass Transition (Tg) Reporting Tg as a single temp., it is necessary to state:- What point in the step change (onset, midpoint or end) is being measured.- The experimental conditions used to measure Tg: heating rate, sample weight.
Glass Transition (Tg) To increase sensitivity:- Use >10mg samples.- Quench cool sample from a temperature above the melt to maximize amorphous structure.
Glass Transition (Tg) As a little as 2-3% water can lower Tg by up to 100oC.- To measure an accurate Tg in a sample with a volatile component by running sample in a hermetic (sealed) pan.- Use a dry-box or dry-bag to prepare samples in hermetic pans. This eliminates water absorption during preparation and loss water during the measurement.
Crystalline Structure Crystalline structure in a sample is determined from the presence of an endothermic melting peak. Important complimentary techniques to DSC include:- Hot Stage Microscopy- X-Ray Diffraction (XRD)- Nuclear Magnetic Resonance (NMR)- Infrared Spectroscopy
Crystalline Structure Factors which complicate DSC analysis:- Endothermic peaks can be created by evaporation and decomposition as well as melting.- TGA should be done on all new samples prior to DSC to determine volatile content and decomposition temperature.- Dehydyration/Desolvation usually results in loss of crystalline structure.- Melting is a thermodynamic transition and therefore, the onset of melting does not change significantly with heating rate.- Decomposition is a kinetic (time-dependent) transition and therefore, the onset temperature of the peak shifts to a significantly higher temperature at higher heating rate.
Example DSC - PET Sample : PET80PC20_MM 1min Size : 23.4300 mg 1 Method: standard dsc heat -cool-heat DSC File: C:...DSCMelt Mixed 1PET80PC20_MM Operator : SAC Run Date : 05-Apr-2006 15 :34 1.001 Comment : 5/4/06 Tm Instrument : DSC Q1000 V9.4 Build 287 1.5 245.24 C 1.0 Tc TgHeat Flow (W/g) 137.58 C 20.30J/g 228.80 C 22.48J/g 79.70 C(I) 81.80 C 0.5 75.41 C Cycle 1 144.72 C 0.0 -0.5 0 50 100 150 200 250 300 Exo Down Temperature ( C) Universal V4.2E TA Instruments
Influence of Sample Mass 0 Indium at Onset not -2 10 C/minute influenced Normalized Data 15mg by mass 10mg 4.0mg -4DSC Heat Flow (W/g) 1.7mg 1.0mg 0.6mg -6 150 152 154 156 158 160 162 164 166 706 Temperature ( C)
DSC: Main Sources of Errors•Calibration•Contamination•Sample preparation – how sample is loaded into a pan•Residual solvents and moisture.•Thermal lag •Heating/Cooling rates •Sample mass•Processing errors
Sample Preparation : Shape • Keep sample as thin as possible (to minimise thermal gradients) • Cover as much of the pan bottom as possible • Samples should be cut rather than crushed to obtain a thin sample (better and more uniform thermal contact with pan)99
Other DSC TechniquesHyper-DSCBased on principle that high heating rates give large broad transitions.•Heating rates typically 400-500oC/min•Need very small sample sizes (~nanograms)Good for:•A quick overview of new sample•Picking out minute transitionPoor for:•Accuracy: transitions can be shifted by as much as 40oC•Repeatabiliy: Very sensitive to thermal lag.
Thermal AnalysisDifferential Scanning Calorimetry (DSC)DSC is a thermal method of analysis to study the thermal behaviour and thermalproperties of materials (typically polymers). The material is sealed in a sample panand subjected to a controlled temperature programme.The resulting thermograph can yield much valuable information about the propertiesof the material analysed. Main use of DSC: Material Identification (Tm and Hf) based on IS EN ISO 3146:2000; Method C2
Thermal AnalysisDifferential Scanning Calorimetry (DSC)Other uses of DSC:% Crystallinity determination by DSC (based on IS EN ISO 3146:2000; Method C2).Purity and Polymorphism analysis by DSC.Thermal Stability of materials (e.g. – oxidative induction time (OiT) of materials) byDSC.
Other DSC TechniquesModulated DSC •Composite heating profile: •Determines heat capacity and separates heat flow into that due to reversible and non-reversible events. 62 62 Modulate +/- 0.42 °C every 40 seconds Ramp 4.00 °C/min to 290.00 °C 60 60 Typicaly: Heating rates: 0 - 50C Modulated Temperature (°C) 58 58 Temperature (°C) Modulation: Period: 60 second 56 56 Amplitude: +/-10C 54 54 Note that temperature is not decreasing during Modulation i.e. no cooling 52 52 13.0 13.5 14.0 14.5 15.0 Time (min)
Modulated DSCBenefits Increased Sensitivity for Detecting Weak (Glass) Transitions Eliminates baseline curvature and drift Increased Resolution Without Loss of Sensitivity Two heating rates (average and instantaneous) Ability to Separate Complex Thermal Events and Transitions Into Their Heat Capacity and Kinetic Components Ability to Measure Heat Capacity (Structure) Changes During Reactions and Under Isothermal ConditionsDownside Slow data collection
Example MDSC 0.00 0.00 -0.02 -0.02 -0.02 -0.04 -0.04 -0.04 N onrev H eat Flow (W /g) R ev H eat Flow (W /g) H eat Flow (W /g) -0.06 -0.06 -0.06 -0.08 -0.08 -0.08 -0.10 -0.10 -0.10 -0.12 -0.12 -0.12 -0.14 -0.14 -50 0 50 100 150 200 250 Exo Up Temperature (°C) Universal V4.2E TA Instruments
Reversible Transitions•Glass Transition•MeltingNon-reversible•Crystallisation•Curing•Oxidation/degradation•Evaporation Technical Group Talk
Comparison of DSC & DTA Aspect DSC DTA1.Size of Sample 2-10 mg 50-20mg2. Sensitivity A few joule/mol 0.5kJ/mol3.Heating & Programmed ProgrammedCooling cycles Heating & Cooling heating is possible.4. Second order It can be observed It is not observedphase transition with a sample of 200mg5. Specfic heat Accurate Not accuratemeasurement
References 1. Instrumental Methods of Chemical Analysis- By, G.R. Chatwal & S.K. Anand. 2. Instrumental methods of analysis; seventh edition by Willard. Merritt, Dean Seffle. 3. www. wikipedia.com