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Which transparent plastics best reflect or absorb IR?

  1. Sep 29, 2016 #1
    Among thin plastics, sheets and films, that are highly transparent to visible light, which are also best at reflecting or absorbing IR heat?

    Are there any spray coatings to enhance a clear plastic surface reflecting even more IR without significantly degrading transparency to visible light?

    Thank you for any comments.
     
  2. jcsd
  3. Sep 29, 2016 #2

    Bystander

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    Near IR? Far?
     
  4. Sep 29, 2016 #3
    All plastics absorb in the IR. To determine which ones absorb the most, you would have to look up or determine the molar absorptivities of the functional groups in each polymer. Don't forget path length. Sheet will absorb much more IR than film. I suggest polyethylene would absorb the least because of the lack of strongly absorbing functional groups. Styrenics absorb across a broader range of IR than other plastics. Anything with carbonyl functionality will strongly absorb between 1600 and 1750 cm-1. Polyesters, polycarbonates and acrylics absorb around 1745 cm-1. You need not concern yourself with nylons as they have poor transparency. Polyvinyl alcohol absorbs strongly around 3500 cm=1.
     
  5. Sep 29, 2016 #4

    jim mcnamara

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    Instead of a general question consider giving us the specifc requirements or your intended use. You got two good responses. One like this one, and another more like a shotgun attempt - cover a lot and hope you hit the target.

    Your question is somewhat like asking: I drive to work, what is the best car for me?
     
  6. Sep 29, 2016 #5
    Jim,
    Was purposely trying not to muddy the waters with too much detail and to minimize discussion expanding into other than the original inquiry.

    OK, the application is for a greenhouse in hot sunny desert, where maximum 400-700nm PAR light, mostly the visible range, is desired for plants, but IR gain in or outside that range is unwanted heat gain inside greenhouse requiring shading which cuts down on beneficial visible light intensity.

    I'd like to forestall shading as much as possible, to maximize light intensity, by first minimizing IR heat gain by smarter selection of greenhouse covering that had overall best IR rejection via reflection and/or absorbance.

    Thus, my inquiry, for best "thin plastics, sheets and films, that are highly transparent to visible light, which are also best at reflecting or absorbing IR heat?"
     
    Last edited: Sep 29, 2016
  7. Sep 29, 2016 #6

    Baluncore

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    A greenhouse is used to keep plants warm, not cool. In a desert during the day you want heat to escape, not stay in there.
    You are neglecting the very significant UV component.

    You need shade. Maybe you should get some of the anti-UV film used on house or car windows.
    Open vents low down and have temperature controlled vents along the ridge, so air flows up through the greenhouse, to exchange the hot air and cool the plants, without too much loss of water.
     
  8. Sep 29, 2016 #7
    I'm well aware of how greenhouses are used & cooled, but sometimes greenhouses are used in warmer weather, too, and even with aggressive ventilation it can be a struggle requiring ever more shading which reduces overall light being received by the plants.

    That's why I've inquired here if any plastic sheets or films are better than others in absorbing or reflecting the IR before it gets inside the greenhouse.

    You wrote that UV is a "very significant component", are you saying here that it is a significant portion of the total sunlight spectrum of heat passing through the sheet or film and entering the greenhouse, or something else? My understanding was that the UV was only about 5% and the IR 50% and the rest was in the visible light range between them.

    - Shane
     
    Last edited: Sep 29, 2016
  9. Sep 29, 2016 #8

    Baluncore

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    UV that enters the greenhouse heats the contents because after UV is absorbed it is converted to IR. In dry air and at altitude there are significantly higher levels of UV in the sunlight.

    There is more heat in a greenhouse than just the incident IR component. In a hot environment you want to dispose of that heat, not trap heat inside. Avoid film absorption that heats the envelope that contains the plants.

    One solution might be to build the shelter / (green)house from film or shade-cloth, then raise or lower the walls to regulate the temperature. I have worked on systems that attach one edge of shade-cloth to the top of the wall with the bottom edge rolled around a long steel water pipe. When the water pipe is rotated the bottom of the wall rolls up or down. A chord wound around a pulley at the end of the pipe is pulled in or out by the temperature controller.
     
  10. Sep 29, 2016 #9
    One of the materials I'm looking at is polycarbonate, like Lexan, high transmission of visible light, lower than glass or PE transmission of IR.

    One set of numbers I'd seen for standard LEXAN solid sheet 3 mm...
    Light transmittance of 88%
    And, solar heat of only 68%

    ...but I've got to get into the weeds of their numbers to see where they came up with that.

    Spectral charts also show good cut off of UV.

    I hear you, Baluncore, about heat absorption conducting through the film, that's a prominent issue..
     
  11. Sep 29, 2016 #10
    PC sheet is expensive. How many ft2 do you need?
     
  12. Sep 29, 2016 #11
    760 sq ft.

    Have not shopped it around yet, at a glance saw some for greenhouses going for $1 sq ft.
     
  13. Sep 30, 2016 #12
    I'd still opt for PE ag film with a shade cloth deployed over the film to reduce heat . Much cheaper and effective all the way around.
     
  14. Sep 30, 2016 #13
    I may still, just wanted to see if anything 'new & wonderful' out there first that'd both be better and without breaking bank.

    - Shane
     
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