Introduction to Polymer Science and Technology

Thermal properties

7.5.2 Vicat softening temperature

This is a similar method to that of HDT, but instead of a test bar, the specimen consists of a small flat piece and the Vicat softening temperature (VST) is the temperature at which a flat-ended pin of 1 mm² circular cross section, under a specified load and heating rate, penetrates the specimen to a certain depth. The standard methods for conducting VST include:

ISO 306:2004 Plastics - Thermoplastic materials - Determination of Vicat softening temperature (VST) ASTM D1525 - 09 Standard test method for Vicat softening temperature of plastics.

The test piece is either a disk of 10 mm diameter or a square piece of 10 mm² surface area and 3 mm to 6.5 mm thickness. The alternatives in the test methods recommend employing loads of either 10 N or 50 N, and the heating rates of either 50 °C/h or 120 °C/h. The test determines the temperature at which the needle penetrates the specimen by 1 mm. The penetration is mainly an outcome of the decrease in elastic modulus, or viscous flow above T in lower molecular weight portions of amorphous thermoplastics. The material must become quite soft for the Vicat pin/needle to penetrate to a depth of 1 mm, therefore, for most polymers the VST values are higher than the ones obtained with other methods for measuring softening points, such as HDT, as can be seen in Figure 7.31.

Figure 7.31VST, HDT and Tm for various thermoplastics, showing the influence of structural variations and fibre reinforcement (source: DuPont Engineering Polymers)

The test maybe open to misinterpretation when testing specimens that contain high molecular orientation. The specimens with high molecular orientation will relax and increase in thickness when heated during the test and may push the pin up or at least slow down its penetration, producing a distorted result. Similar concerns apply to the HDT test. Therefore, in such cases the material should be annealed prior to the test run.

Introduction to Polymer Science and Technology Thermal properties

7.5.3 Thermal conductivity

Thermal conductivity is a measure of how easily heat moves through/along a material. It is a material property that is primarily dependent on its state, temperature, density, and molecular bonding. Covalent bonding in polymer molecules are not conducive to conduction: in covalent bonding the electrons are localised and not free to move and the absence of free/ mobile electrons make most ceramics and polymers poor conductors of electricity and heat. Furthermore, when heated, polymer molecules undergo random internal molecular motion absorbing/dissipating energy rather than transferring it along. Polymers, therefore, make good insulating materials, provided the application temperature is not too high as to cause thermal degradation. Crystallinity, therefore density, influences the magnitude of thermal conductivity: a material with highly crystalline/ordered structure facilitates higher conductivity than the equivalent amorphous material. In the crystalline state, the molecular vibrations to facilitate conduction are coordinated more effectively.

Thermal conductivity, k, is defined as the 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. If the uniform temperature on one face of a flat slab of a material is T and (Т-ЛТ) on the other face, then the rate of heat flow or heat transfer, q, through the thickness of the slab is given by Fourier's law of heat conduction:

q= к А (ЛТ / L)

where, 'A' is the heat transfer area (the surface area of the rectangular slab) and 'L' is the material thickness.

The equation is often expressed with a minus sign to indicate that the heat flows in the direction of decreasing temperature gradient. The equation can be rearranged to express for conductivity

k= (q/A)x(L/AT) (inW/(m°C)).

where, 'q / A' is known as the heat flux.

The methods for measuring thermal conductivity include the un-guarded hot plate and the guarded hot plate methods. The unguarded hot platemethod is based on the well known Lees' disc method, see Figure 7.32. The standard test procedures for the unguarded hot plate method include:

BS 874-2.2:1988 Methods for determining thermal insulating properties. Tests for thermal conductivity and related properties. Unguarded hot-plate method

ASTM C518 - 10 Standard test method for steady-state thermal transmission properties by means of the heat flow meter apparatus.

The test is open to heat losses from the exposed edges. Ways of reducing heat loss from the exposed edges are described by Hands (1999, p602). Using thin specimens and thin heater plates with large surface areas, the side losses maybe reduced. Surrounding the apparatus with a low conductivity material such as vermiculite also reduces heat loss. An improvement

Introduction to Polymer Science and Technology Thermal properties

on the unguarded hot plate method is the guarded hot plateone, which is the most accurate method available for solid materials (including foams). The associated standard test methods include:

ISO 8302:1991Thermal insulation - Determination of steady-state thermal resistance and related properties - Guarded hot plate apparatus

ISO 8990:1994 Thermal insulation - Determination of steady-state thermal transmission properties - Calibrated and guarded hot box

BS EN 12667:2001 Thermal performance of building materials and products. Determination of thermal resistance by means of guarded hot plate and heat flow meter methods. Products of high and medium thermal resistance

BS 874-2.1:1986 Methods for determining thermal insulating properties. Tests for thermal conductivity and related properties. Guarded hot-plate method

ASTM C177 - 10 Standard test method for steady-state heat flux measurements and thermal transmission properties by means of the guarded-hot-plate apparatus

ASTM C1044 - 07 Standard practice for using a guarded-hot-plate apparatus or thin-heater apparatus in the single-sided mode

Introduction to Polymer Science and Technology

Thermal properties

DIN 52612-2 Testing of thermal insulating materials; determination of thermal conductivity by means of the guarded hot plate apparatus; conversion of the measured values for building applications.

The guarded hot plate test, see Figure 7.33, is described in the IDES web site (http://www.ides.com/property_descriptions/ ISO8302.asp): two identical samples are placed on opposite sides of the main heater. The main heater and guard heaters/ rings are kept at the same temperature. Heat sinks are maintained at a lower temperature. The guard heaters minimize the amount of lateral heat transfer from the main heater. Temperatures are monitored at each surface. The heat transferred through the specimens is equal to the power supplied to the main heater. Thermal equilibrium is established when temperature and voltage readings are steady. Thermal conductivity is calculated by inserting the equilibrium values obtained into the conductivity equation:

^к={-2

f-)x[

(T_h-T_s)^J

(inW/(m°C))

where, P is the power supplied to the main heater (W = VI), A is the surface area of the specimen (m²), L is the specimen thickness (m), T_h is the temperature of the heater and T is the temperature of the heat sink. Note that the power supplied is divided by 2 since there are two specimens.

Figure 7.32 Lees'disk apparatus

Figure 7.33 Guarded hot-plate set up

There is commercial equipment for conducting guarded hot-plate tests, e.g., GUNT WL376, designed to meet the DIN 52612-2 standard requirements is used for thermal conductivity of building materials such as polystyrene , PMMA and cork sheets, and plaster board.

Introduction to Polymer Science and Technology

Thermal properties

Hands (1999, p597) presents a good account of testing for thermal diffusivity and specific heat as well as thermal conductivity.

Values for thermal properties of various polymeric materials (obtained under standard laboratory conditions of approximately 23 °C and 50 % relative humidity on dry-as-moulded samples) are presented in Table 7.1.

Table 7.1Thermal properties of various polymers (sources:; Netzsch, CAMPUS, Plastics International, Joel Fried (1995, p473),Ehrenstein (2001, p264))

Introduction to Polymer Science and Technology

Thermal properties

Introduction to Polymer Science and Technology

Thermal properties

Factors that affect data: crystallinity, orientation, types of copolymer, ratios of comonomers, grades of TSs, e.g., epoxy is such a generic name covering all sorts of resins with varying properties, type of curing (cold cured, stove cured at different temperatures for different lengths of times, etc.) and degree of curing. Thermosets such as MF, PF and UF mainly contain filler.

7.6 Self-assessment questions

1. On the same graph paper show how the heat flow during a DSC analysis and the elastic modulus of a polymeric
material changes with temperature over a temperature range that passes through the glass-transition temperature
of the polymer.

2. When a squash ball is immersed in liquid nitrogen and then brought out and hit immediately with a racquet
against the wall, what would happen to it and why?

3. Sketch, on the same graph, the plots of specific volume vs. temperature for PP and PS, both with syndiotactic
stereoregularity

4. The effect that increasing the temperature of a polymer has on its elastic modulus is similar to that of

a) increasing the time under load

b) decreasing the cross-link density

c) decreasing the time under load

5. What is meant by the 'free volume' of a polymer?

Introduction to Polymer Science and Technology

Thermal properties

If you hang a weight from a strip of rubber so that it stretches about 300%, then heat the rubber, which of the following would happen?

a) it stretches some more

b) it contracts

c) it maintains the same length

Consider these three labelled specific volume vs. temperature plots that may be displayed by various types of materials. Indicate the one, which best describes the behaviour expected from PP with a spherulitic structure.

8. Why is the value of T dependent on the method of measurement?

9. Based on the way polymer chains behave above and below T , why do you think glasses tend to be brittle whereas
rubbers are not?

10. Explain why bungee jumping cord should be made from butyl rubber (especially if it is to be used in a sunny
climate).

Hint: consider influence ofu.v. light on degree of cross-linking.

11. Which of the following correctly represents the sequential change in mechanical state with increasing temperature
for an amorphous polymer?

a) viscous liquid; rubbery region, glass

b) glass; viscous liquid; rubbery solid

c) glass; rubbery solid; viscous liquid

d) rubbery solid; glass; viscous liquid

12. It is intended to manufacture coiled cords for telecommunication equipment using PVC flex, and for efficiency
in production the relaxation time should not be greater than 3 h at the production temperature. Determine what
should be the minimum temperature of the processing oven in order to achieve this relaxation time. The PVC
used has a relaxation time of 28 h at 27 °C, take the activation energy for this process to be 20 kj / mol and the
molar gas constant is 8.314 J / (mol.K).

Answer: T= 142 °C

13. A plot of DMTA damping term against temperature can be used to determine a temperature at which

a) tensile strength becomes maximum

b) the specific heat shows a minimum

c) Youngs modulus undergoes a significant drop

d) the crystalline phase melts.

Introduction to Polymer Science and Technology

Thermal properties

14.

15.

16. 17.

18. 19.

20. 21.

22.

23.

The presence of aromatic groups in a polymer chain results in

a) intermolecular attraction

b) potential for crosslinking

c) increase in T and T

G m

d) tensile strength becomes a maximum.

The outcome of a DSC test is usually presented in the form of a plot of heat flow against temperature and/or

time, why is it useful to also present the derivative of this plot?

Indicate the thermal properties that can be determined using DSC. Make a definition of these properties.

From the definition of the specific heat capacity (C ), derive an equation that expresses Cp in terms of DSC

parameters.

„ , heat flow .

Answer: L_v = (------------------- ).

^F heating rate

Distinguish between "dynamic" and "static" OIT methods. Briefly describe an application of OIT.

Hint: consider heating programme/conditions.

DSC analysis for a PA 66 sample produces a heat of fusion value of 40 J/g, with reference to Table 7.1 determine the % crystallinity for the sample.

Answer: approximately 22 %.

Indicate which other analytical techniques TGA can be coupled with and what would be the advantages? Indicate the uses for TGA. A plastic containing a fire retardant (magnesite) is examined by TGA in order to determine the % of magnesite used. The analysis shows that, after the pyrolysis of the polymer, at approximately 600 °C , 12% CO₂ is emitted. Determine the magnesite (MgCO₃) content.

Answer: 22.8 %.

Describe in terms of molecular-relaxation transition, how some rigid plastics exhibit good impact strength. Give examples of such plastics.

Describe how the Arrhenius relationship can be used to determine the energy needed for a relaxation transition to occur. Using the Arrhenius relationship and the data for the P-transition temperature at various frequencies for a thermoplastic in the table below, determine the activation energy for the (3-relaxation in kj/mol.

24.

25. 26.

27.

Answer: £_a = 61 kj/mol.

Describe, giving an example, how annealing affects the dynamic-mechanical thermal (DMT) properties of crystalline thermoplastics.

Why is there a variation between the DMT properties of various nylons?

Compatible Polymers A and В are to be blended to achieve a glass-transition temperature of 60 °C. T s of Polymers A and В are 30 and 85 °C, respectively, using an appropriate equation determine the composition of the blend.

Answer: Polymer A should be approximately 23 % by weight.

Describe, briefly, with examples how to achieve polymers with damping properties that are not affected by changing temperature over a wide range of temperatures, and also how to improve the impact toughness of a resin widely used in aerospace applications.

Introduction to Polymer Science and Technology Thermal properties

28. With reference to Figure 7.27, highlight the changes that occur to the damping characteristics of PVC with
plasticisation.

29. HDT is popularly used in industry to determine softening temperature for polymers, what precautions should be
exercised in its use for product design to establish the upper limit of safe operating temperatures for products?

30. In comparison, which one of the methods for measuring softening points, HDT and VST, usually indicates a
higher value than the other and why?

31. A fibre reinforced thermosetting polymer composite is autoclave cured at 160 °C, determine the differential
strain that is generated between the component materials as the composite is cooled down to 20 °C following
the curing process, and indicate if the level of strain is significant and why? The linear coefficients of expansions
are 5 x 10~⁶/°C for fibre and as high as 10 x 10~⁵/°C for the resin. The resin exhibits 1.5 % elongation at failure.

Answer: approximately 1.3 %.

32. A PMMA rod, wedged in between two solid walls, parallel to the ground, suffers negligible stress at room
temperature (20 °C). The rod in that position is subjected to heat, by using the data from Tables 6.1 and 7.1,
determine the magnitude of the compressive stress that would be generated in the rod if the temperature reaches
70 °C?

Answer: 1.5 MPa.

33. A plane wall of 1 m² surface area is constructed from a 50 mm thick solid polymeric material of 2 W/(m °C)
thermal conductivity. When the temperature is 80 °C on one side and 20 °C on the other, calculate the conductive
heat transfer through the wall.

Answer: 2.4 kW.

Introduction to Polymer Science and Technology References

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