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MARMARA UNIVERSITY FACULTY OF ENGINEERING
METALLURGICAL AND MATERIALS ENGINEERING DEPARTMENT
SENIOR PROJECT
THERMAL ANALYSIS OF
POLYMERIC MATERIALS
PREPARED BY Serkan ÇAKIROĞLU
ADVISOR Prof. Dr. Ersan KALAFATOĞLU
DATE 18TH JUNE 2005
I. INTRODUCTION
The purpose of this senior project is to make thermal analysis of chosen polymer
specimens and determine their thermal and other properties. Determination of thermal
properties of polymeric materials needs specific experiments which will be done in this senior
project period according to the prepared management plan. Necessary equipments for the
experiment are available in the university laboratories and are ready to use. In addition to
these, another goal of this senior project is to compare the experimental results with the
results given in literature and discuss the results with respect to their structure. Commercial
importance of thermal analysis of polymeric materials is another important point that will be
mentioned.
II. BACKGROUND
Most polymers are organic in origin. Many organic materials are hydrocarbons; that is,
they are composed of hydrogen and carbon. Small molecules which are made up of
hydrocarbon groups, polymers -
which are very large molecules - are
made up of hundreds of thousands or
even millions of atoms all strung together,
usually in long chains. Most of the polymers are linear polymers. A linear polymer is a
polymer molecule in which the atoms make up is a long chain such as Figure 1. This chain is
called the backbone. Normally, some of these atoms in the chain will have small chains of
atoms attached to them. These small chains are called pendant groups. The chains of pendant
groups are much smaller than the backbone chain. Pendant chains normally have just a few
atoms, but the backbone chain usually has hundreds of thousands of atoms (1) (2).
There are several areas of polymer analysis such as; qualitative evaluations, chain
micro structure, polymer macrostructure, thermo-physical properties, and thermal analysis. A
number of important properties of polymers are shown in Table 1.
Figure 1 A linear polymer chain
Table 1 Important Properties of Polymers
Type of Property
Quantities
Thermo-physical
*Volume: density, molar volume, thermal expansion
*Calorimetric: heat capacity, enthalpy, entropy
*Transition: glass transition temperature, melting temperature
*Interfacial: surface energy, interfacial tension
Other physical *Electrical: conductivity, dielectric constant *Magnetic: magnetic resonance *Acoustic: sound absorption
Transport *Rheological: shear, extension, elasticity *Mass transfer: diffusion
Thermo-chemical *Polymerization rate coefficients *Thermal degradation
Processing *Extrusion *Molding *Spinning
Product *Mechanical: deformation, toughness, hardness, wear
Thermal properties probably are the most important characteristic of a polymer
material. They determine whether the material will perform as a solid, an elastomer, or a fluid
in the end-use application. They affect the processing methods used to convert the reactor
product into finished parts. There are three thermal performance properties. These are:
Melting or flowing characteristics
Flammability
Thermal degradation
THERMAL ANALYSIS
Thermal analysis of polymers is done by measuring physical properties of the polymer
as it is subjected to controlled temperature changes. Thermal analysis is performed on
condensed matter, specifically solids, glasses, liquids and solutions. Table 2 contains a list of
the more popular methods for thermal analysis. Four methods which are thermogravimetric
analysis, differential scanning calorimetry, thermo mechanical analysis and dynamic
mechanical analysis are widely practiced in the polymer and composites industries. Thermal
properties are very important to end-use applications and to the processing methods used to
make polymer products. There are a number of important thermal transitions that relate to
processing, including Tg and Tm. Many polymers degrade or depolymerize when they are
heated 100 ˚C or more above their processing temperature. Except in a few cases, polymers
do not degrade to reform monomer units, but react with oxygen or with themselves to form a
wide variety of volatile and nonvolatile products (3).
Table 2 Thermal Analysis Methods
METHOD
ABBREVIATION
USE
Thermogravimetric Analysis TGA Change in gain or loss of weight Differential Scanning Calorimetry DSC Change in specific heat Differential Thermal Analysis DTA Heat capacity (rate of enthalpy change) Thermo Mechanical Analysis TMA Change in dimensions Dynamic Mechanical Analysis DMA Loss moduli Thermodilatometry Change in volume Dielectric Thermal Analysis DETA Change in dielectric constant Evolved Gas Detection EGD Pyrolysis or degradation products Evolved Gas Analysis EGA Solvent loss
RESEARCH FACILITY
• Thermogravimetric Analysis (TGA)
Thermogravimetric analysis (TGA or TG) is used to measure a variety of polymeric
phenomena involving weight change. Typical phenomena include rate of sorption of gases;
desorption of volatile contaminants (monomers, solvents, plasticizers and other additives);
diffusion and permeation of gases; and polymer degradations in oxidative, inert and vacuum
environments.
When gaseous materials evolve from the sample, TGA is often used in tandem with
gas chromatography (GC) or mass spectroscopy (MS) to identify the lost materials. For
example, it can be used to determine the carbon content of rubbers and the composition of
polymer composites. TGA, GC and MS pyrolysis of samples is used for identification and
characterization of homopolymers, copolymers, blends and mixtures. In some cases branching
and tacticity can be detected.
Thermal testing has become easier with appearance of automated equipment for
testing small samples. These systems speed the characterization of new and modified
polymers, allowing efficient research and development as well as accurate production
monitoring. The chemical composition tests are also important, although the chemical
compositions of most materials sold commercially are well-known. These tests are useful in
determining the composition of unknown samples or verifying that the expected composition
was achieved. They also can be very helpful in identifying impurities, side reactions and
additives that can have a big effect on performance properties.
• Differential Thermal Analysis (DTA)
Differential thermal analysis (DTA) is a technique involves heating or cooling a test
sample and an inert reference such as Al2O3, under identical conditions, while recording any
temperature difference between the sample and reference. This differential temperature is then
plotted against time, or against temperature. Changes in the sample which lead to the
absorption or evolution of heat can be detected relative to the inert reference.
Differential temperatures can also arise between two inert samples when their response
to the applied heat treatment is not identical. DTA can therefore be used to study thermal
properties and phase changes which do not lead to a change in enthalpy. The baseline of the
DTA curve should then exhibit discontinuities at the transition temperatures and the slope of
the curve at any point will depend on the micro structural constitution at that temperature.
Schematic illustration of a DTA device is shown in Figure 2.
Figure 2 DTA device
• Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) is a technique for measuring the energy
necessary to establish a nearly zero temperature difference between a substance and an inert
reference material, as the two specimens are subjected to identical temperature regimes in an
environment heated or cooled at a controlled rate.
There are two types of DSC systems in common use which are shown in Figure 3. In
power-compensation DSC the temperatures of the sample and reference are controlled
independently using separate, identical furnaces. The temperatures of the sample and
reference are made identical by varying the power input to the two furnaces; the energy
required to do this is a measure of the enthalpy or heat capacity changes in the sample relative
to the reference. In heat flux DSC, the sample and reference are connected by a low-resistance
heat flow path (a metal disc). The assembly is enclosed in a single furnace. Enthalpy or heat
capacity changes in the sample cause a difference in its temperature relative to the reference;
the resulting heat flow is small compared with that in differential thermal analysis (DTA)
because the sample and reference are in good thermal contact. The temperature difference is
recorded and related to enthalpy change in the sample using calibration experiments.
Figure 3 DSC device
The system is a subtle modification of DTA, differing only by the fact that the sample
and reference crucibles are linked by good heat flow path. The sample and reference are
enclosed in the same furnace. The difference in energy required to maintain them at a nearly
identical temperature is provided by the heat changes in the sample. Any excess energy is
conducted between the sample and reference through the connecting metallic disc, a feature
absent in DTA. As in modern DTA equipment, the thermocouples are not embedded in either
of the specimens; the small temperature difference that may develop between the sample and
the inert reference (usually an empty sample pan and lid) is proportional to the heat flow
between the two. The fact that the temperature difference is small is important to ensure that
both containers are exposed to essentially the same temperature program.
The main assembly of the DSC cell is enclosed in a cylindrical, silver heating black,
which dissipates heat to the specimens via a constantan disc which is attached to the silver
block. The disc has two raised platforms on which the sample and reference pans are placed.
A chromel disc and connecting wire are attached to the underside of each platform, and the
resulting chromel-constantan thermocouples are used to determine the differential
temperatures of interest. Alumel wires attached to the chromel discs provide the
chromel-alumel junctions for independently measuring the sample and reference temperature.
A separate thermocouple embedded in the silver block serves a temperature controller for the
programmed heating cycle. An inert gas is passed through the cell at a constant flow rate. The
thermal resistances of the system vary with temperature, but the instruments can be used in
the `calibrated' mode, where the amplification is automatically varied with temperature to
give a nearly constant calorimetric sensitivity.
EXPERIMENTAL MATERIALS
• Acrylonitrile-Butadiene-Styrene (ABS)
ABS is an ideal material wherever superlative surface quality, colorfastness and luster
are required. ABS is a two phase polymer blend. A continuous phase of styrene-acrylonitrile
copolymer (SAN) gives the materials rigidity, hardness and heat resistance. The toughness of
ABS is the result of sub-microscopically fine polybutadiene rubber particles uniformly
distributed in the SAN matrix.
ABS standard grades have been developed specifically to meet the requirements of
major customers. ABS is readily modified both by the addition of additives and by variation
of the ratio of the three monomers Acrylonitrile, Butadiene and Styrene: hence grades
available include high and medium impact, high heat resistance, and electroplatable. Fiber
reinforcement can be incorporated to increase stiffness and dimensional stability. ABS is
readily blended or alloyed with other polymers further increasing the range of properties
available. Fire retardancy may be obtained either by the inclusion of fire retardant additives
or by blending with PVC. The natural material is an opaque ivory color and is readily colored
with pigments or dyes. Transparent grades are also available. Physical properties of ABS are
shown in Table 3.
Table 3 Properties of ABS
Tensile Strength (MPa) 40-50
Notched Impact Strength (Kj/m2) 10-20
Thermal Coefficient of Expansion (10-6) 70-90
Maximum Service Temperature (°C) 80-95
Density (g/cm3) 1.0-1.05
As a result of its good balance of properties, toughness/strength/temperature resistance
coupled with its ease of moulding and high quality surface finish, ABS has a very wide range
of applications. These include domestic appliances, telephone handsets computer and other
office equipment housings, lawn mower covers, safety helmets, luggage shells, pipes and
fittings. Because of the ability to tailor grades to the property requirements of the application
and the availability of electroplatable grades ABS is often found as automotive interior and
exterior trim components.
• Polystyrene (PS)
Polystyrene (PS) is one of the styrenic family (two of the others are
ABS - acrylonitrile butadiene styrene and SAN - styrene acrylonitrile) and all of the family
tend to be relatively brittle with poor outdoor performance. Basic PS is brittle, rigid,
transparent, easy to process, is low cost and free from odor and taste. High Impact grades
(PS-HI) are a rubber modified grade of PS where elastomers are introduced into the base
polymer to improve the impact resistance and deformation before fracture. Sometimes PS is
referred to as crystal PS, this refers to the clarity of the finished product and does not imply
that there the molecular structure is crystalline. In fact the lack of a crystalline structure is
responsible for many of the good points of PS such as the clarity of the product, the low
energy input required for processing -no crystal to melt- and the ease of processing with low
shrinkage. Some basic properties are shown in Table 4.
Table 4 Properties of PS
Tensile Strength (MPa) 55-80
Notched Impact Strength (Kj/m2) 3-15
Thermal Coefficient of Expansion (10-6) 50-100
Maximum Service Temperature (°C) 70-85
Density (g/cm3) 1.0-1.2
• Polyvinyl Alcohol (PVA)
Polyvinyl alcohol is a water-soluble polymer. It is prepared by hydrolysis of a
polyvinyl ester (polyvinyl acetate). It is used as a starting material for the preparation of other
resins. It can be used as a component of elastomers used in the manufacture of sponges. This
polymer is used in sizing agents that confer resistance to oils and greases upon paper and
textiles, to make films resistant to attack by solvents or oxygen. It is used as a component of
adhesives, emulsifiers, suspending and thickening agents. In pharmaceutical industry,
polyvinyl alcohol is used as an ophthalmic lubricant and viscosity increasing agent. It
thickens the natural film of tears in eyes. General properties are shown in Table 5.
Table 5 Properties of PVA
Melting Temperature (°C) ≈230
Notched Impact Strength (Kj/m2) 3-9
Thermal Coefficient of Expansion (10-6) 70-100
Maximum Service Temperature (°C) 75-85
Density (g/cm3) 1.27-1.31
• Nylon 66 (PA 66)
In the years following the World War I, a number of chemists recognized the need for
developing a basic knowledge of polymer chemistry. In the early 1930’s, Wallace M.
Carothers and his associates at E. I. DuPont de Nemours & Company began fundamental
research of dicarboxylic acids and diamines. This research led to the synthesis of the first
purely synthetic fiber, a polyamide-- Nylon 66. Nylon 66 is so named because it is
synthesized from two different organic compounds, each containing six carbon atoms.
Nylons are one of the most common polymers used as a fiber. Nylon is found in
clothing all the time, but also in other places, in the form of a thermoplastic. Nylon's first real
success came with its use in women's stockings, in about 1940. They were a big hit, but they
became hard to get, because the next year the United States entered World War II, and nylon
was needed to make war materials, like parachutes and ropes. It may be surprising to learn
that before stockings or parachutes, the very first nylon product was a toothbrush with nylon
bristles. The main properties of PA-66 are shown in Table 6.
Table 6 Properties of PA-66
Tensile Strength (MPa) 33-52
Melting Temperature (°C) 190-240
Thermal Coefficient of Expansion (10-6) 45-60
Maximum Service Temperature (°C) 57-150
Density (g/cm3) 1.14
In addition to all these general information and properties, Figure 4 shows the
appearance of all specimens.
Figure 4 PS_PVA_PA-66_ABS specimens
EXPERIMENTAL EQUIPMENTS
• MEASURING PARTS
The measuring parts are the most important parts of the experiment. All reactions and
data records occur in these parts. The whole picture of these parts can be seen in Figure 5.
Measuring parts consist of following equipments;
FURNACE
HOISTING DEVICE
CROSS HEAD
Figure 5 Measuring parts
There were two types of furnaces in the laboratory. First one is SiC with S-type
thermocouple which has maximum temperature of 1600 °C. The other furnace is kanthal with
B-type thermocouple which has maximum temperature of 1700 °C.
The DTA-DSC experiment facility is supplied with a single or a double hoisting
device. The furnace can be moved vertically up the hoisting column and can be swung out
from the top in parking position. The double hoisting device allows the furnace to be swung
out 180°.
A vacuum tight connection between the sample and balance chamber is achieved with
a snap closure by the cross head. The cross head and radiation shield protect the balance
chamber thermally from the furnace. Purge gas can be lead in or out through the cross head
valves.
• SAMPLE(SPECIMEN) CARRIER SYSTEMS
The sample carrier system is plugged to the balance system into the furnace. The
following sample carrier systems which are shown in Figure 6 in two parts can be used. They
can be supplied with different thermocouples depending on the measuring temperature
required.
Figure 6 Sample (Specimen) carrier system
The pans or crucibles (sample carriers) are placed on top of the sample carrier head.
The crucibles are placed on the thermocouple measuring sensor. The examples of measuring
heads and crucibles are illustrated in Figure 7.
Figure 7 Measuring heads and crucibles
• POWER UNIT
The power unit gives power to the fan of
the furnace for a faster cooling. This unit also
indicates the problems which related to fuses,
furnace heating or exceeding of a preadjusted
limit values.
• GAS CONTROL UNIT
This unit helps to control and adjust the
gas flow during the experiment. The gas control
equipment shown in Figure 8.
Figure 8 Gas control unit
• TA SYSTEM CONTROLLER (TASC)
The TA System Controller is a microprocessor system with the following functions;
Temperature programming and control
Temperature linearization
Data acquisition
Measurement range switching
The system is working with a computer software which the data and records can be
followed easily during the experiment. Additionally, the setting of the experiment can be
adjusted with the software program. The TASC is equipped with a sample temperature
controller (STC). With the aid of the STC, the sample temperature is included in the furnace
control. The difference between the sample temperature and desired temperature is
minimized. STC can be switched on or off via software.
III. PROCEDURE
First of all, after entering the laboratory, all of the plugs and switches must be checked
in order to prevent a hazardous accident due to electricity. Turn on the computer and run the
measurement program. Adjust the desired instrument settings, the temperature program and
measurement type according to the subject or project. Set the gas control device to the desired
value by using the gas meter on the gas control device. The inert gas for all experiments is
Nitrogen (N2).
Specimen for the experiment is weighed by electronic balance. Open the snap closure
by turning counter-clockwise. Move the furnace upwards. Swing the furnace to the left in
park position. Pick up the sample carrier in the middle of the capillary between your fingers
and thumb. Insert the connection plug of the sample carrier centrally into the opening of the
cross head. Move the sample carrier downwards until the connection plug reaches the
bushing. Find the arresting position of the connection plug by carefully turning the capillary.
Push the sample carrier slightly into the bushing. Place the selected crucibles related to the
measuring type on to the sample carrier head such as in Figure 9. After positioning the
crucibles, specimen is put into the crucible and furnace is replaced downwards.
Figure 9 Positioning of crucibles
Using the computer program and according to the temperature program, 3 runs are
done for all specimens. A run consists of heating the specimen to a temperature which is
below its melting temperature. The heating rate is 20 °K/min for all runs. After all runs the
furnace is waited until its temperature come to room temperature. It is important to make a
correction run which means that run the same program without specimen. This correction
helps to make more correct determination during analysis.
In curve analysis program, glass transition evaluation is done for every curve. The
program automatically applies the following equations in the selected region to find ∆CP
values and other important points on the curves.
60)()( 12 x
eHeatingRatTDSCTDSC
CP−
=∆ Eqn.1
60)()( 12 xeHeatingRat
tDSCtDSCCP−
=∆ Eqn.2
Equation 1 is for the calculation in temperature scaling; Equation 2 is for the
calculation in time scaling. T1 and T2 are temperatures; t1 and t2 are time; DSC values are in
calorimetric units and finally heating rate’s unit is °K/min.
IV. RESULTS
During a run, computer takes data continuously and periodically up to the end of the
experiment. These data form a curve related to the measuring type -DTA or DSC curve- and
saved to a specific directory for curve analysis. As a result of a run, a single curve is obtained.
By using the curve analysis program, the glass transition temperatures are found. The used
curves are mostly DSC curves. Only PA-66 specimen has both DTA and DSC results but it is
seen that two analyses give nearly the same results.
The three curves which are obtained from three runs for each specimen are put into a
single graph to see the changes and shifts of the temperatures. The resultant graphs and
related important temperature values for ABS, PVA, PS and PA-66 are illustrated and listed
in Figures 10, 11, 12, 13, 14 in Appendix and Tables 7, 8, 9, 10 and 11 respectively. The data
in the below tables also can be seen in figures in the Appendix.
Table 7 Glass Transition Evaluation results of ABS specimen
RUN 1ST 2ND 3RD
Onset Point (°C) 81.6 91.9 92.3
Middle Point (°C) 94.9 102.2 101.4
Inflection Point (°C) 86.6 99.7 100.2
End Point (°C) 108.2 112.2 110.6
∆CP (mV.s/g.°K) 8.49x10-2 12.11x10-2 10.74x10-2
Table 8 Glass Transition Evaluation results of PVA specimen
RUN 1ST 2ND 3RD
Onset Point (°C) 58.3 60.1 60.7
Middle Point (°C) 72.1 73.6 74.0
Inflection Point (°C) 66.2 68.3 69.0
End Point (°C) 85.9 87.1 87.2
∆CP (mV.s/g.°K) 43.98x10-2 46.49x10-2 49.34x10-2
Table 9 Glass Transition Evaluation results of PS specimen
RUN 1ST 2ND 3RD
Onset Point (°C) 87.7 96.1 97.4
Middle Point (°C) 100.0 105.0 106.1
Inflection Point (°C) 93.3 101.5 103.3
End Point (°C) 112.2 113.9 114.8
∆CP (mV.s/g.°K) 5.43x10-2 5.47x10-2 4.37x10-2
Table 10 Glass Transition Evaluation results of PA-66_DSC specimen
RUN 1ST 2ND 3RD 4TH
Onset Point (°C) 27.8 38.4 28.5 40.6
Middle Point (°C) 48.3 55.4 50.7 58.3
Inflection Point (°C) 37.6 39.2 39.2 42.4
End Point (°C) 68.7 72.4 72.8 76.1
∆CP (mV.s/g.°K) 99.71x10-2 61.95x10-2 75.35x10-2 61.32x10-2
Table 11 Glass Transition Evaluation results of PA-66_DTA specimen
RUN 1ST 2ND 3RD
Onset Point (°C) 59.0 59.5 65.1
Middle Point (°C) 89.1 90.2 90.6
Inflection Point (°C) 78.8 79.8 81.7
End Point (°C) 119.3 120.9 116.1
∆CP (mV.s/g.°K) 0.99x10-2 0.17x10-2 2.13x10-2
V. DISCUSSION OF RESULTS
The most important point in tables and on figures is the inflection points. The
inflection point is the point which the aim of the tangents on the curves changes positive value
to negative value or vice versa. This indicates the glass transition (Tg) zone and the
temperature of transition reaction. Inflection points are indicated different then other points in
the table to have a better look to the results.
As it can be seen in the tables and on the figures, the inflection point increases with
respect to the previous runs. In other words the Tg values shift to right and transition reaction
occurs later. Temperature changes and the rate of temperature changes, affects the chain
orientation, stress relaxation and free volume in the polymer. The heating of the specimens
under their melting points acts as an annealing process. During cooling after a run, polymer
chains find necessary time for reorientation and crystal portion of the specimen increases.
This reorientation occurs in every cooling step after each run. As a result of the increase in
crystal structure in the polymer leads to a rise in inflection points. In other words, the glass
transition reaction occurs harder than the previous run.
VI. CONCLUSION
In conclusion, general knowledge about polymers and thermal analysis are gained.
Additionally thermal analysis methods are studied and experienced by using the related
laboratory equipments. Thermal analysis of chosen polymeric materials is done and the results
of the related experiments are discussed and compared with each other according to the
previous lectures knowledge.
VII. BIBLIOGRAPHY
1. CALLISTER, William D. Jr., Materials Science and Engineering an Introduction,
Fifth Edition, John Wiley&Sons, Inc., 2000
2. www.psrc.usm.edu/macrog/kidsmac/basics.htm
3. GRULKE, Eric A., Polymer Process Engineering, Prentice-Hall, Inc., 1994
4. ROSEN, Stephen L., Fundamental Principles of Polymeric Materials,
Second Edition, John Wiley&Sons, Inc., 1993
5. NETZSCH, STA 409 CD, Operation Manual, 2000