In the Laboratory
Thermal Analysis of Plastics Teresa D’Amico, Craig J. Donahue,* and Elizabeth A. Rais Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI 48128; *
[email protected] This lab experiment illustrates the use of differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA or TG) to explore polymer properties: percent crystallinity, glass transition temperature (Tg ), melting temperature (Tm), and filler content, among others. This dry lab, consisting of a series of exercises, has been performed by second-semester students in a new two-semester general chemistry sequence designed for the undergraduate engineering major. The overarching theme of “chemistry and the automobile” has been adopted for these courses. Half of the second semester involves surveys of organic chemistry and synthetic polymer chemistry, topics beneficial to engineering students because they will likely encounter at least some of the following substances and materials in their professional career: fuels, lubricants, solvents, additives, elastomers, thermoplastics, thermosets, fibers, composites, adhesives, and coatings. This lab experiment was designed to bridge the gap between molecular structure and bulk properties of plastics. In their second-year engineering materials course, the mechanical engineering (ME) students examine the linear interrelationships between processing, structure, properties, and performance (1). Without a rudimentary understanding of organic chemistry the ME students struggle to grasp the nuances in structure from one polymer to the next and therefore struggle with the relationship between polymer structure and properties. Several other lab experiments developed to support this approach to teaching engineering students general chemistry have appeared in this Journal (2–5). Laboratory Procedure In addition to several books (6–8) that survey thermal analysis techniques and experiments, a number of thermal analysis experiments have appeared in this Journal including several that examine polymer samples. References to these JCE thermal analysis experiments are detailed in the online supplement.
Heat Flow / (mJ/s)
23
melting endotherm glass transition
22
21
cold crystallization exotherm
20
19 50
100
150
200
250
300
Temperature / pC Figure 1. DSC trace of PET preform body. Heating rate is 10 oC/min.
404
A PerkinElmer Pyris 6 DSC and a PerkinElmer TGA 7 were used to compile the data for the exercises. Of necessity this lab is run as a dry lab. The size of the class, the time involved in preparing a sample and running a scan, the students’ lack of familiarity with the instrumentation, and the cost of materials dictated how the lab was structured. The students were given a demonstration of how each of the instruments works. Students work in four-member groups and completed all the exercises described below in one three-hour lab period. Although all seven exercises are identified here, only four are described in detail. The other three are described in detail in the online supplement as are the protocols used to obtain the DSC and TGA traces for the samples. Exercise 1: Determination of the Percent Crystallinity in a Polypropylene Dog Bone before and after Pulling See the online supplement for details. Exercise 2: Determination of the Percent Crystallinity in PET Preforms and Pop Bottles Background Strong has described the two-step process of making a PET pop bottle (9). First, the parison or preform is made by injection molding at 250–280 °C and the resulting preform (complete with threads for attaching a cap) is quenched. In the second step, the preform is reheated to 95–100 °C and stretch-blow molded to yield the finished bottle. During the blow-molding process a telescoping mandrel stretches the preform in the longitudinal direction while the injected air stretches the preform radially against the walls of the mold. Collectively the process is labeled stress-induced crystallization and the PET polymer chains are biaxially oriented in the finished product. Procedure One, two, and three liter preforms and bottles were used in this study. The preform and bottle did not come from the same manufacturer,1 that is, a generic two liter preform is compared to a generic two liter pop bottle in this study. For each preform and bottle, a sample from the neck and the body was examined. Results The DSC traces obtained for these samples were of two general types. The DSC traces for the neck of both the preform and bottle and the body of the preform exhibited the same features: Tg was observed at 75 °C, followed by a cold crystallization at 135 °C, and finally Tm at 246–248 °C. A representative DSC trace for these PET samples is shown in Figure 1. The body of the bottle gave a different DSC curve. It still contained Tg and Tm, but the cold crystallization exotherm was absent (Figure 2). The fractional crystallinity where cold crystallization occurs was calculated from
}% m Hsamp} }%c Hsamp} fractional 100% crystallinity }% m H100%}
Journal of Chemical Education • Vol. 85 No. 3 March 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory 24
Glass Transition Temperature / pC
110
Heat Flow / (mJ/s)
melting endotherm 23
22
glass transition
21
20
50
100
150
200
250
300
100
90
80
70
60 0
1
2
3
4
[1/(Mn)] / (10ź4 mol/g)
Temperature / pC Figure 2. DSC trace of PET bottle body. Heating rate is 10 oC/min.
Figure 3. Glass transition temperature (Tg) versus 1/M n for polystyrene samples. Heating rate is 10 oC/min.
where ΔmHsamp is the enthalpy change associated with Tm, ΔcHsamp is the enthalpy change associated with the cold crystallization exotherm, and ΔmH100% is the reference value2 of 140.1 J∙g for 100% crystalline PET.3 This equation subtracts the contribution due to cold crystallization from the melting endotherm so that the percent crystallinity calculated reflects only the crystalline domains that were present before the sample was heated. The fractional crystallinity for the body of the bottle was calculated from
units of °C g mol‒1. This initial study involved the examination of a series of low molecular weight atactic polystyrene samples and the following expression was obtained (11)
%m Hsamp
fractional crystallinity %m H100% 100%
The preform neck and body exhibited low fractional crystallinity values ranging from 1–5%, while the pop bottle neck samples exhibited a fractional crystallinity around 7%. In contrast, the pop bottle body samples had a fractional crystallinity in the range of 23–27%. These results are in good agreement with those reported by Iler, Rutt, and Althoff (10). They found for a PET bottle neck (thread) and body (sidewall) fractional crystallinity values of 5% and 23%, respectively. Exercise 3: Examination of Molecular Weight Change on the Glass Transition Temperature of Polystyrene Background Fox and Flory were the first to provide a theoretical analysis of the relationship between glass transition temperature and molecular weight (11). This relationship takes the following form,
Tg Tg,e
K Mn
where Tg is the glass transition temperature observed with a number average polymer molecular weight of Mn, Tg,∞ is the glass transition temperature at infinite molecular weight, and K is a constant specific to the polymer under investigation and has
Tg 100 . 0 pC
1. 8 t 10 5 Mn
pC g mol
When atactic polystyrene samples having molecular weights ranging from 4000 to 1,800,000 g mol‒1 were examined by DSC (12), the following equation was obtained Tgc 106 . 0 pC
2. 1 t 10 5 Mn
pC g mol
where Tge is the glass transition temperature extrapolated to a heating rate of 1 °C∙min. Procedure The polystyrene samples4 used in this study were obtained from Aldrich Chemical Company. Ten samples were examined ranging in molecular weight from 2,500 to 4.5 × 106 g mol‒1. DSC scans were performed at a heating rate of 10 °C∙min. Results A table containing the experimentally determined Tg values obtained for the ten polystyrene samples and a plot of Tg versus Mn are provided in the online supplement. The plot of Tg versus Mn demonstrates that the glass transition temperature rises rapidly as molecule weight increases and then begins to level off beginning at a molecular weight around 100,000 g mol‒1. A plot of Tg versus 1∙Mn for the data collected is shown in Figure 3. The best-fit straight line for this plot is
Tg 104 . 0 pC
1. 1 t 10 5 Mn
pC g mol
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In the Laboratory Table 1. Composition of Tire Tread Compound
Amount (PHR)
Natural rubber
50
29.5
Styrene–butadiene rubber
50
29.5
Carbon black
50
29.5
Processing oil
7.5
4.4
Antioxidant
1.0
0.6
Stearic acid
2.0
1.2
Zinc oxidant
5.0
3.0
Accelerator
1.25
0.7
Sulfur
2.5
1.5
100
Wt %
Note: PHR = parts per hundred parts of rubber, by mass.
80
Weight (%)
loss of moisture at 110 pC under N2 switch from N2 to air
60
volatilization of elastomers at 110–900 pC under N2
40
combustion of carbon black at 900 pC in air
20
0
0
2
4
6
8
10
12
14
16
18
20
22
24
Time / min Figure 4. TGA trace of automobile tire tread. Heating rate from 110 oC to 900 oC is 80 oC/min.
Since the numerical value of Tg,∞ (the y intercept) is a function of the heating rate and decreases as the heating rate increases, our result is reasonable when compared to the literature results. At an extrapolated heating rate of 1 °C∙min, Tg,∞ was found to be 106.0 °C (12) and at higher heating rates (presumably 20 °C∙min), Tg,∞ was found to be 100.0 °C (11). Our Tg,∞ of 104.0 °C, obtained at an intermediate heating rate, falls between the two literature values. Our K value, 1.1 × 105 °C g mol‒1 falls slightly below the literature values of 2.1 × 105 and 1.8 × 105 °C g mol‒1. Exercise 4: Identification of the Plastic in Automobile Headlights and Taillights Background Two amorphous plastics, polycarbonate (PC) and poly(methyl methacrylate) (PMMA), have replaced glass as the lens material in automotive lenses yielding a significant weight reduction (13–16). Acrylic (as PMMA is referred to) has a density of 1.2 g∙cm3 as compared to glass, which has a density of 2.6 g∙cm3 (15). PMMA, the plastic of choice for taillights, also possesses superior weathering performance and UV and scratch resistance (14). PC is preferred for headlights because of its superior impact and heat distortion performance. Engineeringgrade glasses only have an impact–strength range of 0.5–1.5 kJ∙m2 as compared to PC, which has an impact–strength range of 28–35 kJ∙m2 (15). Finally, replacing glass and metal with plastics allows for parts reduction and design freedom because plastics can be molded into complex shapes. Procedure Automobile headlight and taillight fragments were collected from parking lots and along roadsides. No attempt was made to match the sample to the make, model, and year of the vehicle. Students were provided both a DSC thermogram and an IR spectrum of the lens material. The IR spectra were recorded using a Pike Technologies HATR attachment. The protocol used
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here to identify the lens materials parallels that used in industry to perform failure analysis on a taillight where scanning electron microscopy (SEM), IR, DSC, and thermomechanical analysis (TMA) were used (17). SEM and TMA were not available to us. Results The DSC traces of the taillight samples only exhibited a Tg in the range of 101–106 °C and no Tm indicating the plastic was amorphous. The observed Tg range for the taillight samples falls within the range of 85–105 °C reported by PerkinElmer for PMMA.4 The DSC traces of the headlight samples only exhibited a Tg in the range of 146 ± 1 °C and no Tm. The much higher Tg value for the headlight samples compared to the taillight samples clearly demonstrate the two lenses are made of different plastics. The observed Tg value for the headlight samples falls within the range of 140–150 °C reported by PerkinElmer for PC.5 Comparison of the IR spectra of the lens samples with reference IR spectra also confirmed these assignments. Further details are supplied in the online supplement. Exercise 5: Identification of the Type of Nylon (Polyamide) Present in Commercial Samples See the online supplement for details. Exercise 6: Investigation of Filled and Reinforced Plastics by TGA See the online supplement for details. Exercise 7: Examination of the Rubber Tread from an Automobile Tire by TGA Background (18) A tubeless automobile tire consists of a carcass, sidewall, and tread. The tread is that portion of the tire in contact with the road. The carcass is made up of layers of cord (e.g., nylon or polyester) and wire strands. The cords are laid in layers and
Journal of Chemical Education • Vol. 85 No. 3 March 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
impregnated with rubber to form plies. Both the sidewall and tread are applied to the carcass and vulcanized in place. The key ingredients found in a tire tread (18) are listed in Table 1. This is a representative recipe. Carbon black is the chief reinforcing agent used in tires. It improves the rubber tire’s resistance to tearing and abrasion and increases traction and durability. Carbon black increases life expectancy of tires by approximately a factor of ten. Procedure Over the first 22 minutes the rubber sample was heated under N2(g). The sample was first held at 25 °C for five min, then heated to 110 °C at 85 °C∙min, and then held at 110 °C for 6 min. At 12 min, the sample was heated to 900 °C at 80 °C∙min. At 22 minutes while the sample was at 900 °C, the atmosphere was switched to air. The run was concluded at 25 min. Results A TGA trace of an automobile tire tread is shown in Figure 4. It shows three distinct weight loss steps. The first weight loss of 0.6 wt % occurring at 110 °C under N2(g) is attributed to the loss of moisture. A second weight loss of 63.3 wt % occurring under N2(g) while the temperature was ramped from 110 °C to 900 °C is attributed to the volatilization of elastomers, processing or extender oil, and other additives. The last weight loss of 31.1 wt % occurs at 900 °C after the purging gas is switched from N2(g) to air and this is attributed to the combustion of the carbon black. At the conclusion of the experiment, a residue of 4.7 wt % remained. This is attributed to inert filler (e.g., clays). These results are in good agreement with the data presented in Table 1. The wt % contribution of carbon black, 31.1 wt %, found in our analysis is close to the value of 29.5 wt % listed in Table 1. Acknowledgment One of the authors (CJD) gratefully acknowledges the financial support of this work from a NSF Course, Curriculum, and Laboratory Improvement grant, DUE-0088729. Notes 1. Plastipak Packaging Inc., Westland, MI, donated the preforms. 2. Value was obtained from PerkinElmer Technical Bulletin. 3. Percent crystallinity values are the average of three measurements. 4. The polystyrene samples have a monodispersity index of one or close to one. 5. Data were obtained from the PerkinElmer “MP, Tg and Structure of Common Polymers” wall chart.
Literature Cited 1. Callister, W. D., Jr. Materials Science and Engineering: An Introduction, 5th ed.; John Wiley & Sons: New York, 2000; Chapter 1. 2. Donahue, C. J. J. Chem. Educ. 2002, 79, 721. 3. Donahue, C. J.; D’Amico, T.; Exline, J. A. J. Chem. Educ. 2002, 79, 724. 4. Donahue, C. J.; Exline, J. A.; Warner, C. J. Chem. Educ. 2003, 80, 79. 5. Mayotte, D.; Donahue C. J.; Snyder, C. A. J. Chem. Educ. 2006, 83, 902. 6. Brown, M. E. Introduction to Thermal Analysis: Techniques and Applications; Chapman and Hall: London, 1988. 7. Haines, P. J. Thermal Methods of Analysis: Principles, Applications and Problems; Blackie Academic & Professional: London, 1995. 8. Hoehne, G.; Hemminger, W.; Flammersheim, H. J. Differential Scanning Calorimetry: An Introduction for Practitioners; Springer: Berlin, 1996. 9. Strong, B. A. Plastics: Materials and Processing, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 2000; Chapter 13. 10. Iler, H. D.; Rutt, E.; Althoff, S. J. Chem. Educ. 2006, 83, 439. 11. Fox, T. G., Jr.; Flory, P. J. J. Appl. Phys. 1950, 21, 581. 12. Blanchard, L.; Hesse, J.; Malhotra, S. L. Can. J. Chem. 1974, 52, 3170. 13. Lange, W. Polymers in Automobile Applications. In Plastics and the Environment; Andrady, A. L., Ed.; John Wiley & Sons, Inc.: New York, 2003; Chapter 17. 14. Maxwell, J. Plastics in the Automobile Industry; Woodland Publishing Limited: Cambridge, U.K., 1994. 15. Culp, E. Modern Plastics 1993, 70, 48. 16. Chipalkatti, M. H.; Laski, J. J.; Trickett, E. A. Engineering Plastics 1996, 9, 293. 17. Jansen, J. A. Automotive Plastics 2001, 34. 18. Mark, J. E.; Erman, B.; Eirich, F. R. Science and Technology of Rubber; Academic Press Inc.: New York, 1994.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Mar/abs404.html Abstract and keywords Full text (PDF) Links to cited JCE articles Supplement Student handouts
All exercises are described in detail
Protocol to obtain the DCS and TGA traces
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