Can't explain an observation with material properties

In summary, the clear PETE plastic tub stored next to a PP tub did not deform, while the same tub stored in direct sunlight did. The PP tub had a lower heat-deflection point than the PETE, which is why it did not deform. The two experiments measured the temperature vs. time for a clear plastic box in calm wind and shade with the temperature reaching 138 degrees Fahrenheit in 16 minutes. The PP tub had a higher reading on the thermocouple than the PETE tub, which failed the lab 101 lab test for not replacing the thermocouple.
  • #1
Steve_
7
0
I beg forgiveness if this should be in chemistry; to me it seems more of a physics problem.
This summer I had two Polyethylene terephthalate (PETE) spice bottles deform due to the sun shining on the clear plastic tub, Polypropylene(PP) in which they were stored.
The first time the big tub was on a picnic table in the desert and it was about 105F and in direct sun.
The second was inside my camper but with the sun shining through a window onto the clear plastic tub.
OK, that should be expected during the summer on a California desert. I unintentionally made a solar oven.
What is not understandable is that these two bottles were not alone and had PP bottles next to them in the same environment but they did not deform.
I looked up the heat-deflection for both and found: PP 100C, PETE 115C

Why would the plastic with the lower heat-deflection point not deform?

One difference it that the PETE bottles are clear and the PP are milky.
These are the McKormick spice bottles.

Happy Trails
 
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  • #2
The objects exposed to sunlight can warn up much more than their environment, as they absorb radiation and the absorbed solar energy transforms into heat. Just recall how hot the sand can be on a sunny day. If PETE absorbed more the solar energy as PP the PETE bottles warmed up more. It also could happen that PP absorbed sunlight at some wavelengths and the bottles of the same material got less of those wavelengths they could absorb. The PETE bottles absorb at different wavelengths, where the PP tube was transparent so they were able to absorb more energy from the sunlight and got warmer.

ehild
 
  • #3
I think that this phenomenon is more complex.
Almost all materials(in this case, plastics) have shape-memory,less or more.
They "remember" the steps of the technological process of fabrication
(injection molding,sintering etc.) and may accumulate some frozen strains.
On heating, this strains are released and deforms the shape of the plastic.
Also, the absorbed water(or organic solvents) may contribute to this.
In this case, the heating rate makes the difference.
Take a look at:
http://en.wikipedia.org/wiki/Shape-memory_polymer
 
  • #4
vortextor said:
I think that this phenomenon is more complex.
Almost all materials(in this case, plastics) have shape-memory,less or more.
They "remember" the steps of the technological process of fabrication
(injection molding,sintering etc.) and may accumulate some frozen strains.
On heating, this strains are released and deforms the shape of the plastic.
Also, the absorbed water(or organic solvents) may contribute to this.
In this case, the heating rate makes the difference.
Take a look at:
http://en.wikipedia.org/wiki/Shape-memory_polymer

And even without shape-memory, a different shape of the bottles or wall thickness can make the difference. You would have to compare equally shaped bottles.
 
  • #5
Thanks to all for the thoughts. I will study the shape-memory wiki entry. It's still over 100F here and I can set up an experiment using my Fluke and a thermocouple to get some temperature data. The wall thicknesses will be interesting to know also. Then I will immerse the bottles in hot water and slowly raise the temperature. These tests may allow me to see if they have very different points at which they release the frozen strains. This is more interesting now. I'll post again later this week. Being able to do some kitchen physics is one of the few good things about being old enough to retire. I hope I can remember to turn off the stove...
Happy Trails.
 
  • #6
Greetings
It might also be worthwhile to try to discover something of the composition and manufacturing processes of each. since plastics, like glass, are made from "recipes" and with processes that affect such properties. I'm not familiar with plastics specifically, but I do know that a lot of commercial glass, for example, has a high Iron (Fe) content that interacts with sunlight in a manner that reflects some frequencies and absorbs others, making it unsuitable for most solar collectors. These require reduced or Iron-Free compositions of glass. I would not be at all surprised that plastics would include (or not) elements with similar effects. In fact. I'd wager they do.
 
  • #7
Sorry this took so long; life got in the way.
I made some basic measurements and have a picture to share.
There were two experiments:

First was the solar oven or temperature vs. time for the clear plastic box.
The instrumentation was an old Fluke 77AN DVM with the 80TTK thermocouple module.
I did a quick cal check on the thermocouple and found it to read 1.5 deg higher than my Hg lab thermometer across the range tested.
The setup was to place a type K thermocouple on the surface of a PETE bottle with a small amount of thermal paste.
Ambient conditions were calm wind, shade temperature 104 degF.
The temp reached 138 degF in 16 min. with ambient conditions and then the battery in the Fluke failed.
I failed Lab 101 for not replacing the battery before testing.
The temp was still changing at 0.5 degF/min when the battery failed.
Conclusion is the box will get hotter than 138 degF.

Second was to submerge the bottles, one at a time in a 2 Qt. pot of tap water on very low flame heat.
The bottles labeled PETE both deformed noticeably at 160-170 degF.
The PETE bottles were so soft at 185 degF it was difficult to lift them from the water.
The PP bottle was not even pliable under finger force at 208 degF.
The PP bottle never showed any deformation and remained sturdy to finger forces.

It is curious that the PETE with material properties that would indicate better thermal performance failed significantly before the PP plastic. What I wonder now is are the other chemicals like release agents getting released from the plastic and contaminating the spices?
Here is a link to an image of the bottles after heating.
http://s1376.photobucket.com/user/Steve_Imager/media/_D8E1857tiny_zps4262d01b.jpg.html

Happy Trails
Steve
 

1. What do you mean by "material properties"?

"Material properties" refers to the characteristics and behaviors of a substance that make it unique. This can include physical properties such as color, shape, and density, as well as chemical properties such as reactivity and stability.

2. How can an observation not be explained by material properties?

Sometimes, an observation may not fit within the expected behaviors or properties of a material. This could be due to a number of factors, such as the presence of impurities, external forces acting on the material, or unknown interactions with other substances.

3. What other factors could be affecting the material's behavior?

There are many external factors that could impact a material's behavior, such as temperature, pressure, and electromagnetic fields. Additionally, the structure and arrangement of molecules within a material can also play a significant role in its properties and behavior.

4. How can scientists study and understand materials with unexplained behavior?

Scientists use a variety of methods and techniques to study materials with unexplained behavior. This can include conducting experiments, analyzing data, and developing models and theories to explain the observations. Collaboration and interdisciplinary approaches are often necessary to fully understand complex material behaviors.

5. Why is it important to understand material properties?

Understanding material properties is crucial for many fields of science and technology. It allows us to design and develop new materials for specific purposes, improve existing materials, and predict and prevent potential issues or failures. Additionally, studying material properties can also deepen our understanding of the natural world and help us make advancements in various industries, such as medicine, energy, and electronics.

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