report and analysis:mechanical properties of polymers & ceramics
TRANSCRIPT
Lab # V:
Mechanical Properties of Polymers and Ceramics
Submitted By
DeAndria L. Hardy
Laboratory Instructor: A Samant
Date of Experiment: April 5, 2007
I. IntroductionThe purpose of this lab was to better understand microstructures of polymers and
to correlate the microstructures present in the polymer to its mechanical properties. For this lab four different polymers were used: High Density Polyethylene (HDPE), Low Density Polyethylene (LDPE), Polymethyl Methacrylate (PMMA), and Polystyrene (TPS). To observe these polymer’s mechanical properties a tensile test and Izod test were performed.
II. ProcedureThe tensile test was performed on each of the four polymers. Each polymer to be
tested had a width of 0.5”, a thickness of 1/8” at its center and a 4” gage length. Each polymer was tested at a speed of 2in/min. HDPE was tested two additional times at speeds of 10in/min and 20in/min. Load versus extension data was recorded for each polymer.
An Izod test was performed on the HDPE polymer. The HDPE was impacted at five different temperatures: 100oC, 25oC, 0oC, -60oC, and -196oC. The energy absorbed by the impact registered on the tester’s gauge and was recorded.
III. Results and Discussion
All data, results, tables, and graphs can be found in supplement sheets.
In polymers there are many aspects that affect its ductility, strength and elastic modulus. When considering these mechanical properties the primary factor is the polymers chemical make-up. Other factors such as the degree of crystallinity also affect a polymer’s mechanical properties. Crystallinity is a function of the known densities for a polymer in its crystalline and amorphous forms.
For this lab the degree of crystallinity was calculated for both HDPE and LDPE. As intuition would suggest the low density polymer has a lower degree of crystallinity than the high density polymer. In the parameters of elastic modulus and yield strength the HDPE, which has a higher degree of crystallinity, has a higher elastic modulus and higher yield strength than LDPE. But the higher degree of crystallinity in HDPE lowers its ductility. So, according to the comparison between HDPE and LDPE, the lower the degree of crystallinity in a polymer the more ductile the polymer will be.
Also it appears that the degree of crystallinity affects physical characteristics of a polymer, particularly its color appearance. The higher degree of crystallinity within HDPE causes it to appear slightly more of an opaque white than the LDPE. Based on the comparison between HDPE and LDPE it is assume that TPS has the highest degree of crystallinity because it appears completely white. Likewise the lowest degree of crystallinity is seen in the PMMA polymer and it appears transparent.
Along with calculating a polymer’s degree of crystallinity, common methods like tensile tests can also be used to determine mechanical properties. When comparing two polymer’s crystallinity the data used is constant and specific to that polymer alone. When comparing two polymers using a tensile test there must also be a constant present for adequate comparison. For this lab the constant was a strain rate of 2in/min. By using the same strain rate during the tensile test of all four polymers it ensures their parameters like yield strength and elastic modulus will be comparable. If the strain rates were not the
same for all four polymers there would be no standard for data and the polymers wouldn’t be able to be compared to one another. Case in point, the HDPE was tested at 2in/min like all the other polymers but it was also tested at 10in/min and 20in/min. This increased strain rate increased HDPE’s elastic modulus and decreased its yield strength.
The final test preformed during this lab was an IZOD impact test. It was observed that as the temperature increased, and the polymer moved through the brittle temperature range into the ductile temperature range, so did the impact energy. The impact energy steadily increased until the final testing temperature where there was decline in impact energy. This decline at the final temperature of 100oC was due to the polymer entering into “alpha relaxation”, the point in which the crystals of the polymer became more mobile.
The data obtained from the IZOD test of HDPE is also useful in design formation. HDPE is used as a material for items such as milk jugs, toys and ice trays. By performing the IZOD test the glass transition temperature of approximately -90oC was identified. This is helpful knowledge because it allows an engineer to know around what temperature the material becomes increasingly brittle and unfit to be used in certain applications. For instance, a polymer used to make ice trays needs to be able to withstand low to freezing temperature and not become so brittle that when a consumer is retrieving ice the ice trays don’t shatter causing injury. The IZOD test provides information about the Tg and lets a design engineer know that using HDPE for an ice tray is safe because the ice tray can withstand as low as -90oC and still be ductile enough to handle a consumer twisting to retrieve the ice.
IV. ConclusionIn this lab it was observed that mechanical properties of a polymer are dependant
on the strain rate at which it is tested and its degree of crystallinity. The higher the degree of crystallinity of a polymer the higher its elastic modulus and yield strength were. Also as the strain rate increased during the testing of HDPE so did its elastic modulus. In the IZOD test, the higher the temperature was the higher the impact energy was.
HDPE
2in/minHDPE
10in/minHDPE
20in/min LDPE TPS PMMAElastic
Modulus(psi) 75599.83 87336.07 161409.8 12938.63 137928.83 221828.2
Yield Strength(psi)Approx. 4080 2288 2400 1456 3136 640
% ElongationLo-4” 361.7563 15.4485 14.8345 399.6163 3.46325 17.80675
Toughnessin-lbf/in3 -- -- -- -- 35.93225
Degree of Crystallinity 0.925589 0.925589 0.925589 0.607158 -- --
Load vs. Extension: TPS
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
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Load vs. Extension: PMMA
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0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
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Load vs. Extenion: LDPE
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0 2 4 6 8 10 12 14 16 18
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Load vs. Extension: HDPE 2in/min
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Load vs. Extension: HDPE 10in/min
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Load vs. Extension: HDPE 20in/min
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Load vs. Extension: Comparison PMMA/HDPE
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PMMA
Izod Test Results
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-196 -60 0 25 100
Temperature
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Izod Test Results
Tg
Load vs. Extension: HDPE 20in/min
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Load vs. Extension: Comparison PMMA/HDPE
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PMMA
Izod Test Results
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-196 -60 0 25 100
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Izod Test Results
Tg