Which transparent plastics best reflect or absorb IR?

In summary: Film is good at reflecting IR. It can also be used to reflect or absorb IR. 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.
  • #1
shane2
89
3
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.
 
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  • #2
shane2 said:
reflecting or absorbing IR heat?
Near IR? Far?
 
  • #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.
 
  • #4
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?
 
  • #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?"
 
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  • #6
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.
 
  • #7
Baluncore said:
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.

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
 
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  • #8
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.

shane2 said:
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.
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.
 
  • #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..
 
  • #10
PC sheet is expensive. How many ft2 do you need?
 
  • #11
Kevin McHugh said:
PC sheet is expensive. How many ft2 do you need?

760 sq ft.

Have not shopped it around yet, at a glance saw some for greenhouses going for $1 sq ft.
 
  • #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.
 
  • #13
Kevin McHugh said:
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.
I may still, just wanted to see if anything 'new & wonderful' out there first that'd both be better and without breaking bank.

- Shane
 

1. What is the difference between reflection and absorption of IR in transparent plastics?

Reflection refers to the bouncing back of infrared (IR) radiation from the surface of a material, while absorption refers to the absorption of IR radiation by the material. In transparent plastics, reflection occurs when the IR radiation is not able to pass through the material and is instead reflected off its surface. Absorption, on the other hand, occurs when the IR radiation is absorbed by the material and converted into heat energy.

2. Which transparent plastics have the highest reflectivity for IR radiation?

Polyethylene terephthalate (PET) and polystyrene (PS) are two of the most commonly used transparent plastics that have high reflectivity for IR radiation. These materials are commonly used in food packaging and are able to reflect up to 98% of IR radiation.

3. What factors affect the ability of transparent plastics to reflect or absorb IR?

The molecular structure and composition of the plastic play a significant role in its ability to reflect or absorb IR radiation. Plastics with symmetrical and uniform molecular structures tend to have higher reflectivity, while those with more complex structures tend to have higher absorbency. The thickness and surface finish of the plastic can also affect its IR reflective or absorptive properties.

4. Are there any transparent plastics that are specifically designed for IR reflection or absorption?

Yes, there are some transparent plastics that are specifically designed for their IR reflective or absorptive properties. These include polymethyl methacrylate (PMMA) and polycarbonate (PC), which are often used in light filters and lenses for cameras and other optical devices.

5. How does the IR reflective or absorptive properties of transparent plastics affect their practical use?

The IR reflective or absorptive properties of transparent plastics can have practical implications in various industries. For example, in the food packaging industry, materials with high IR reflectivity can help to keep food products cool by reflecting heat away. On the other hand, materials with high IR absorbency can be used in solar panels to convert IR radiation into heat energy for power generation.

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