THERMAL ANALYSIS OF POLYMERIC MATERIALS

Transcription

1 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 18 TH JUNE 2005

2 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 Figure 1 A linear polymer chain 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 (1) (2). atoms, but the backbone chain usually has hundreds of thousands of atoms 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.

3 Table 1 Important Properties of Polymers Type of Property Thermo-physical Other physical Transport Thermo-chemical Processing Product Quantities *Volume: density, molar volume, thermal expansion *Calorimetric: heat capacity, enthalpy, entropy *Transition: glass transition temperature, melting temperature *Interfacial: surface energy, interfacial tension *Electrical: conductivity, dielectric constant *Magnetic: magnetic resonance *Acoustic: sound absorption *Rheological: shear, extension, elasticity *Mass transfer: diffusion *Polymerization rate coefficients *Thermal degradation *Extrusion *Molding *Spinning *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

4 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 T g and T m. 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.

5 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 Al 2 O 3, 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.

6 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.

7 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

8 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) Notched Impact Strength (Kj/m 2 ) Thermal Coefficient of Expansion (10-6 ) Maximum Service Temperature ( C) Density (g/cm 3 )

9 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) Notched Impact Strength (Kj/m 2 ) 3-15 Thermal Coefficient of Expansion (10-6 ) Maximum Service Temperature ( C) Density (g/cm 3 )

10 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/m 2 ) 3-9 Thermal Coefficient of Expansion (10-6 ) Maximum Service Temperature ( C) Density (g/cm 3 ) 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 They were a big hit, but they became hard to get, because the next year the United States entered World War II, and nylon

11 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) Melting Temperature ( C) Thermal Coefficient of Expansion (10-6 ) Maximum Service Temperature ( C) Density (g/cm 3 ) 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

12 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

13 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

14 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

15 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 (N 2 ). 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

16 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 C P values and other important points on the curves. DSC( T2 ) DSC( T1 ) C P = x60 Eqn.1 HeatingRate DSC( t2) DSC( t1) C P = x60 Eqn.2 HeatingRate

17 Equation 1 is for the calculation in temperature scaling; Equation 2 is for the calculation in time scaling. T 1 and T 2 are temperatures; t 1 and t 2 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 1 ST 2 ND 3 RD Onset Point ( C) Middle Point ( C) Inflection Point ( C) End Point ( C) C P (mv.s/g. K) 8.49x x x10-2

18 Table 8 Glass Transition Evaluation results of PVA specimen RUN 1 ST 2 ND 3 RD Onset Point ( C) Middle Point ( C) Inflection Point ( C) End Point ( C) C P (mv.s/g. K) 43.98x x x10-2 Table 9 Glass Transition Evaluation results of PS specimen RUN 1 ST 2 ND 3 RD Onset Point ( C) Middle Point ( C) Inflection Point ( C) End Point ( C) C P (mv.s/g. K) 5.43x x x10-2 Table 10 Glass Transition Evaluation results of PA-66_DSC specimen RUN 1 ST 2 ND 3 RD 4 TH Onset Point ( C) Middle Point ( C) Inflection Point ( C) End Point ( C) C P (mv.s/g. K) 99.71x x x x10-2

19 Table 11 Glass Transition Evaluation results of PA-66_DTA specimen RUN 1 ST 2 ND 3 RD Onset Point ( C) Middle Point ( C) Inflection Point ( C) End Point ( C) C P (mv.s/g. K) 0.99x x x10-2 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 (T g ) 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 T g 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.

20 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.

21 VII. BIBLIOGRAPHY 1. CALLISTER, William D. Jr., Materials Science and Engineering an Introduction, Fifth Edition, John Wiley&Sons, Inc., GRULKE, Eric A., Polymer Process Engineering, Prentice-Hall, Inc., ROSEN, Stephen L., Fundamental Principles of Polymeric Materials, Second Edition, John Wiley&Sons, Inc., NETZSCH, STA 409 CD, Operation Manual, 2000