Can't explain an observation with material properties

  1. 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
     
  2. jcsd
  3. ehild

    ehild 12,387
    Homework Helper
    Gold Member
    2014 Award

    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
     
  4. 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
     
  5. A.T.

    A.T. 6,205
    Gold Member

    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.
     
  6. 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.
     
  7. 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.
     
  8. 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
     
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