THERMAL PROPERTIES

1. THERMAL PROPERTIES

2. THERMAL PROPERTIES • Plays a vital role in evaluating the product performance & processibilty characteristics in polymers. • Thermal analytical methods monitor differences in some sample property as the temperature increases, or differences in temperature between a sample and a standard as a function of added heat. These methods are usually applied to solids to characterize the materials.

3. Heat Deflection Temperature (HDT) Vicat Softening Temperature (VSP) Thermal Endurance Thermal Conductivity Thermal Expansion Low Temperature Brittleness Flammability Melting Point, Tm, and Glass Transition, Tg (DSC) Thermomechanical Analysis THERMAL PROPERTIES

4. Heat Deflection Temperature Defined as the temperature at which a standard test bar (5 x ½ x ¼ in ) deflects 0.010 inch under a stated load of either 66 or 264 psi. • Significance: • HDT values are used to compare the elevated temperature performance of the materials under load at the stated conditions. • Used for screening and ranking materials for short-term heat resistance. • HDT values do not represent the upper temperature limit for a specific material or application. • The data are not intended for use in design or predicting endurance at elevated temperatures.

5. Test Methods, Specimen & Conditioning • Test Method: • ASTMD 648, ISO 75 -1 and 75-2 • Test Specimen: • 127mm (5 in.) in length, 13mm (½ in.) in depth by any width from 3mm (⅛ in.) to 13mm ((½ in.) • Conditioning: • 23 ± 2oC and 50 ± 5% RH for not less than 40 hrs prior to test. • Two replicate specimens are used for each test

6. Apparatus • Specimen Supports: Metal supports for the specimen of 100 ± 2mm • Immersion Bath • Deflection Measurement Device • Weights: 0.455 MPa (66 psi) ± 2.5% or 1.82 MPa (264 psi) ± 2.5%. • Temperature Measurement System Apparatus for Determination of HDT

7. Procedure • Measure the width and depth of each specimen • Position the test specimens edgewise in the apparatus • Position the thermometer bulb sensitive part of the temperature • Stir the liquid-heat transfer medium thoroughly • Apply the loaded rod to the specimen and lower the assembly into the bath. • Adjust the load to obtain desired stress of 0.455 MPa (66 psi) or 1.82 MPa (264 psi) • Five minutes after applying the load, adjust the deflection measurement device to zero or record its starting position • Heat the liquid heat-transfer medium at a rate of 2.0 ± 0.2oC/min. • Record the temperature of the liquid heat-transfer medium at which the specimen has deflected the specified amount at the specified fibre stress.

8. Calculation The weight of the rod used to transfer the force on the test specimen is included as part of the total load. The load (P) is calculated as: P = 2Sbd2 / 3L Where, S = Max. Fibre stress in the specimen of 66 Psi / 264 Psi b = Width of specimen d = Depth of specimen L = Width of span between support (4 in)

9. Results & Conclusion • A bar of rectangular cross section is tested in the edgewise position as a simple beam. • Load applied at the center to give maximum fibre stresses of 66 /264 psi. • The specimen is immersed under load in a heat-transfer medium provided with a means of raising the temperature at 2 ± 0.2oC/min. • The temperature of the medium is measured when the test bar has deflected 0.25mm (0.010 in). • This temperature is recorded as the deflection temperature under flexural load of the test specimen.

10. Factors influencing • HDT of unannealed (heat treatment) specimen is usually lower than that of annealed specimen. • Specimen thickness is directly proportional to HDT because of the inherently low thermal conductivity of plastic materials. • Higher the fibre stress or loading lower the HDT. • Injection moulded specimen tend to have a lower HDT than compression – moulded specimen. • Compression moulded specimen are relatively stress free.

11. Vicat Softening Point (VSP) Defined as the temperature at which a flat ended probe with 1 mm2 cross section penetrates a plastic specimen to 0.04 inch (1 mm) depth. • Significance • Data obtained by this test method may be used to compare the heat-softening qualities of thermoplastic materials. • This test method is useful in the areas of quality control, development and characterization of plastic materials.

12. Test Methods, Specimen & Conditioning • Test Method: • ASTMD 1525 or ISO 306 • Test Specimens : • The specimen shall be flat, between 3 and 6.5mm thick and at least 10 by 10mm in area or 10mm in diameter. • Conditioning: • 23 ± 2oC and at 50 ± 5% relative humidity of not less than 40 hrs A minimum of two specimens shall be used to test each sample.

13. Apparatus • Immersion Bath • Heat-Transfer Medium • Specimen Support • Penetration-Measuring DeviceMasses: 10 ± 0.2N or 50 ±1.0N • Temperature-Measuring Device • Needle Fig. 2 Apparatus for Softening Temperature Determination

14. Procedure • Prepare the immersion bath so that the temperature of the heat-transfer medium is between 20 and 23oC at the start of the test • Place the specimen, which is at room temperature, on the specimen support. • The needle should not be nearer than 3mm to the edge of the specimen. • Gently lower the needle rod, without the extra mass, so that the needle rests on the surface of the specimen and holds it in position. • Position the temperature-measuring device so that the sensing end is located within 10mm from where the load is applied to the surface of the specimen. • Lower the assembly into the bath and apply the extra mass required to increase the load on the specimen to 10 ± 0.2N (Loading 1) or 50 ± 1.0N (Loading 2). • After a 5-min waiting period, set the penetration indicator to zero. • Start the temperature rise. • Record the temperature of the bath when the needle has penetrated 1 ± 0.01mm into the test specimen.

15. Results & Conclusion • Vicat softening temperature is expressed as the arithmetic mean of the temperature of penetration of all specimens tested. • If the range of penetration temperatures for the individual test specimens exceeds 2oC, record the individual results and repeat the test, using at least two new specimens.

16. Thermal Conductivity • Rate at which heat is transferred by conduction through a unit cross sectional area of a material when a temperature gradient exists perpendicular to the area. • ` • The coefficient of thermal conductivity (K factor), is defined as the quantity of heat that passes through a unit cube of the substance in a given unit time when the difference in temperature of the two faces is 10C. • Mathematically, thermal conductivity is expressed as • K = Qt/A(T1-T2) • Q = amount of heat passing through a cross section, A causing a temperature difference, ∆T (T1-T2), t = thickness of the specimen. • K is the thermal conductivity, typically measured as BTU.in / (hr.ft2.0F) indicates the materials ability to conduct heat energy.

17. Significance • Thermal conductivity is particularly important in applications such as headlight housings, pot handles & hair curlers that require thermal insulation or heat dissipation properties. • Computerized mold-filling analysis programs requires special thermal conductivity data derived at higher temperatures than specified by most tests.

18. Test Methods & Specimen • Test method: Guarded hot plate test • ASTM D177, ISO 2582 • Test Specimen: two identical specimens having plane surface of such size as to completely cover the heating unit surface • The thickness should be greater than that for which the apparent thermal resistivity does not change by more than 2% with further increase in thickness

19. Apparatus • The apparatus is broadly of two different categories of the following: • Type I (low temperature) Temperature of cold plate : 21 K, Temperature of heating unit:<500 K • Type II (High temperature) Temperature of heating unit range:>550 K -<1350K • Heating units • Gap & Metering Area • Unbalance Detectors • Cooling units • Sensors for measuring Temperature difference • Clamping force • Measuring system for Temperature detector outputs

20. Guarded Hot plate Apparatus Guarded Hot plate Apparatus Courtesy: Bayer Material Data Sheet

21. Two test specimens are sandwiched between the heat source (main heater) & heat sink; one on either side of the heat source. The clamping force is so adjusted that the specimens remain in perfect contact with the heater & sink Guard heaters are provided to prevent heat flow in all except in the axial direction towards the specimen The time of stabilization of input & out put temperature is noted. Temperature difference between the hot & cold surfaces of the specimen should not be less that 5 K or suitable differences as required. Procedure

22. Calculation The relationship between the quantity of heat flow and thermal conductivity is defined as Q ~ K/ x Q = Quantity of heat flow K = Thermal Conductivity X = The distance the heat must flow Thermal conductivity is calculated as : K = Qt / A (T1 – T2) Q = Rate of heat flow (w) T = Thickness of specimen (m) A = Area under test (m2) T1 = Temperature of hot surface of specimen (k) T2 = Temperature of cold surface of specimen (k)

23. Results & Conclusion • Thermal conductivity is calculated by using the value of rate of flow at a fixed temperature gradient. • Data are obtained in the steady state

24. Factors influencing • Crystallites have higher conductivity. • As the density of the cellular plastic decreases, the conductivity also decreases up to a minimum value and rises again due to increased convection effects caused by a higher proportion of open cells.

25. Thermal Expansion (Coefficient of Linear Thermal Expansion, CLTE) • Measures the change in length per unit length of a material, per unit change in temperature. • Expressed as in/in/0F or cm/cm/0C • Mathematically, CLTE (α), between temperatures T1 and T2 for a specimen of length L0 at the reference temperature, is given by : • α = (L2 – L1)/[L0(T2 – T1­)] = L/L0ΔT

26. Significance • Determines the rate at which a material expands as a function of temperature. • The higher the value for this coefficient the more a material expands and contracts with temperature changes. • Plastics tend to expand and contract anywhere from six to nine times more than materials that are metallic. • The thermal expansion difference develops internal stresses and stress concentrations in the polymer, which allows premature failure to occur.

27. Test Method: ASTMD 696 Test Specimen: 12.5 by 6.3mm (½ in. by ¼ in.) 12.5 by 3mm (½ by ⅛ in.), 12.5mm (½ in.) in diameter or 6.3mm (¼ in.) in diameter. Conditioning: 23 ± 2oC and 50 ± 5% RH for not less than 40h prior to test.

28. Apparatus • A vitreous silica dilometer • Dial gage • The weight of the inner silica tube + the measuring device reaction shall not exert a stress > 70 kPa on the specimen so that the specimen is not distorted or appreciably indented. • Scale or Caliper • Controlled Temperature Environment • Means shall be provided for stirring the bath • Thermometer or thermocouple

29. Procedure • Measure the length of two conditioned specimen at room temperature • Mount each specimen in a dilatometer, install the dilatometer in the –30oC control environment. • Maintain the temperature of the bath in the range –32oC to –28oC ± 0.2oC until temperature of the specimen along the length is constant • Record the actual temperature and the measuring device reading. • Change to the + 30oC bath, so that the top of the specimen is at least 50mm below the liquid level of the bath. • Maintain the temperature of the bath in the range from + 28 to 32oC ± 0.2oC • Record the actual temperature and the measuring device reading. • Change to –30oC and repeat the above procedure & measure the final length of the specimen at room temperature. • If the change in length per degree of temperature difference due to heating does not agree with the change length per degree due to cooling within 10% of their average investigate the cause of the discrepancy and if possible eliminate. • Repeat the test until agreement is reached.

30. Calculate the CLTE over the temperature range as: α = ΔL/LoΔT α = Average coefficient of linear thermal expansion degree Celsius. ΔL = Change in length of test specimen due to heating or to cooling, Lo = Length of test specimen at room temperature (ΔL and Lo being measured in the same units), and ΔT = Temperature differences, oC, over which the change in the length of the specimen is measured. The values of α for heating and for cooling shall be averaged to give the value to be reported. Calculation

31. Result & Conclusion • Provide a means of determining the CLTE of plastics, which are not distorted or indented by the thrust of the dilatometer on the specimen. • The specimen is placed at the bottom of the outer dilatometer tube with the inner one resting on it. • The measuring device, which is firmly, attached to the outer tube is in contact with top of the inner tube and indicates variations in the length of the specimen with changes in temperature. • Temperature changes are brought about by immersing the outer tube in a liquid bath or other controlled temperature environment maintained at the desired temperature. • The nature of most plastics and the construction of the dilatometer make –30 to +30oC a convenient temperature ranges for linear thermal expansion measurements of plastics. • This range covers the temperatures in which plastics are most commonly used.

32. Thermal expansion is substantially affected by the use of additives especially fillers Wt% Of loading Lowers the coefficient of thermal expansion. Factors influencing

33. Differential Scanning Calorimetry (DSC) • DSC measures the heat flow into or from a sample as it is heated, cooled or held under isothermal conditions • Applications of DSC includes characterization of • Polymers • fibres • Elastomers • Composites • films • pharamaceuticals • foods • cosmetics

34. DSC provides the following important properties of materials • Glass Transition Temp. (Tg) • Melting point (Tm) • Crystallization times & Temp. • Heats of melting & crystallization • Percent Crystallinities • Heat set temp. • OIT • Compositional Analysis • Heat capacities • Heats of cure • Thermal Stabilities

35. Apparatus • DSC apparatus consists of • Furnace • Temperature Sensor • Differential Sensor • Test Chamber Environment • Temperature Controller • Recording Device • Sealed pans • Balance

36. Samples: Powder, Liquids, crystal

37. Procedure • DSC apparatus consists of two sealed pans sample and reference aluminum pans • The pans are heated, or cooled, uniformly while the heat flow difference between the two is monitored. • This can be done at a constant temperature (isothermally), but is more commonly done by changing the temperature at a constant rate, called temperature scanning. • The instrument detects differences in the heat flow between the sample and reference & plots the differential heat flow between the reference and sample cell as a function of temperature.

38. First Order Transitions (Tc, Tm) • Specimen mass appropriate of 5-mg is taken in the pan • Intimate thermal contact between the pan and specimen is established for reproducible results. • Heat the sample at a rate of 10oC/min under inert gas atmosphere from 50oC below to 30oC above the melting point to erase the thermal history . • The selection of temperature and time are critical when effect of annealing is studied. • Hold temperature for 10min. • Cool to 50oC below the peak crystallization temperature at a rate of 10oC/min and record the cooling curve. • Repeat heating as soon as possible under inert purge gas at a rate of 10oC/min, and record the heating curve.

39. For Second order Transition (Tg) • Use a specimen mass of 5-mg. • Perform and record a preliminary thermal cycle as up to a temperatures 30oC above the extrapolated end temperature, Te, to erase previous thermal history, heating at a rate of 20oC/min. • Hold temperature for 10min. • Quench cool to 50oC below the transition temperature of interest. • Hold temperature for 10min. • Repeat heating at a rate of 20oC/min, and record the heating curve until all desired transition have been completed.

40. Typical DSC curves

41. Measurement of various Properties/Explanations • Heat Capacity • Heating the sample & Reference pans, the the difference in heat output of the two heaters is plotted against temperature. i.e the heat absorbed by the polymer against temperature.

42. Heat Capacity • The heat flow at a given temperature is represented units of heat, q supplied per unit time, t. • The heating rate is temperature increase T per unit time, t. Dividing,

43. Glass Transition • Property of the amorphous region • Below Tg: Disordered amorphous solid with immobile molecules • Above Tg: Disordered amorphous solid in which portions of molecules can wiggle around • A second order transition ( Increase in heat capacity but there is no transfer of heat

44. Crystallization • Above Tg, the polymers are in mobile conditions. • When they reach the right temperature, they gain enough energy to move into very ordered arrangements, which we call crystals, • When polymers fall into these crystalline arrangements, they give off heat. • When this heat is dumped out, there is drop in the heat flow as a big dip in the plot of heat flow versus temperature: We call crystallization an exothermic transition.

45. Melting Above Tc, we reach the polymer's melting temperature, or Tm, those polymer crystals begin to fall apart, that is they melt. The chains come out of their ordered arrangements, and begin to move around freely. Melting is a first order transition (Tm).

46. Putting it all together

47. Polymer crystallinity Measure the area of under the melting of the polymer. Plot of heat flow per gram of material, versus temperature. multiply this by the mass of the sample

48. Degree of crystallinity is given by X 100% = Xc Where H’= Heat of Fusion determined from DSC thermogram H*m= Heat of fusion of a 100% crystalline sample

49. Results & Conclusion • DSC thermograms provides an elaborate picture of various transitions in a polymer. • The degree of crystallinity in a polymer sample, specific heat etc. can be determined. • Any side reaction (for example, crosslinking, thermal degradation or oxidation) shall also be reported and the reaction identified if possible.

50. Factors affecting • Addition of fillers affects the transitions in DSC • Previous thermal history of the samples also affects the DSC transitions. • There should be proper contact between the samples & pans