Feb 9, 2017 at 2:58 pm #3449760
Any suggestions for a lightweight material for a sun umbrella or sunshade? Ideally reflective on top, dark underneath, like on the chromedome umbrellas. Is this material available anywhere?
Another alternative might be mylar over a thin, dark silpoly, though I imagine I’d lose a lot of flexibility bonding 2 fabrics together…
Any other ideas?Feb 9, 2017 at 7:43 pm #3449803
Seattle Fabrics lists several variants of reflective nylon fabrics on their website, including a laminated 70d nylon. Google lists several European distributors of metallic ripstop nylon. AliExpress lists many vendors of metallized nylon fabrics for EM shielding (I don’t know if those would be appropriate for your application). Have you considered reinforced metallized films? Aluminized Cuben or scrim-reinforced aluminized mylar might be options. Maybe you have ruled these out already.
Do you want the material to provide darkness, protection from solar heat, or both? I ask because an optimal solar heat barrier would be metallized film (metal layer out) on both sides, not black inside. If all you want is darkness, you have more options.Feb 9, 2017 at 10:31 pm #3449829
No, I haven’t ruled these out already :-). Just started thinking about it, really…
I’m looking for a material to act as a sunshade, like a parasol, so was thinking highly reflective on top to bounce the sun off, and dark underneath to absorb (hence avoid reflecting) the sun back onto me. You’ve listed a few materials/sources I was unaware of, and will look into – thanks!Feb 10, 2017 at 1:46 am #3449844
It sounds like you might have some misconceptions about how heat is reflected and absorbed, but I’m sure your project will turn out fine. Good luck.Feb 10, 2017 at 9:59 pm #3450073
I admit my ignorance. What would you recommend for a material for a MYOG umbrella? My priorities are 1) reduce sunburn; 2) reduce heat; and 3) keep my head dry, in that order.Feb 11, 2017 at 12:30 am #3450092
Others could explain it better than I can, and maybe someone with more expertise will chime in. This topic gets pretty regularly discussed, and just like the evaporation of mixed liquid stove fuels in pressurized canisters, it sometimes attracts some good technical explanations.
The only theoretical foundation you need is this: every material absorbs and emits radiant heat to some unique extent, and, in general, materials that are good at reflecting heat are bad at emitting it. Aluminum foil is very good at reflecting heat, so it is bad at emitting it. If a flat object (a brim of a hat, say) has shiny aluminum on both sides, and is turned to face the sun, the sunlit side will reflect most of the heat, and the shaded side will hardly emit any of the small amount of heat absorbed by the sunlit side. So the head under the hat gets almost no radiant heat. If the brim was aluminum above and dark fabric below, the top would reflect and the bottom would emit onto the wearer much more of the small amount of heat absorbed by the top side, because dark organic materials (like darkly colored plastics) are very good at emitting radiant heat.
If you are considering materials for your project, the property I described above is related to a quantity called emissivity. If the emissivity of a material is low (like aluminum), then it doesn’t absorb or emit very much radiant heat. If the emissivity is high (like plastics, asphalt, and water), then it absorbs a lot of radiant heat and emits a lot. For your project, you want the upper and lower surfaces to have low emissivity, like the aluminum surface of a space blanket, not high emissivity, like dark fabric.Feb 11, 2017 at 12:45 am #3450093
Emissivity varies between zero and one. Materials that absorb and emit a lot, like plastics, have high emissivity values (0.94, say). Materials that reflect a lot and don’t emit very much, like metals, have low emissivity values (0.07, say). Some examples:
Aluminum foil: 0.04
Polished brass: 0.03
Polished magnesium: 0.03
Silicone rubber (surface of silnylon fabric): 0.93
Epoxy (surface of carbon fiber and fiberglass articles): 0.89
Cotton cloth: 0.77
Polypropylene plastic/fabric: 0.97
Teflon PTFE plastic: 0.92
PVC plastic: 0.92Feb 11, 2017 at 1:06 am #3450096
Makes sense from a heat perspective – thanks for the explanation. But what about UV? Would not a highly reflective, low emissivity material on the underside be likely to reflect reflected UV back onto my skin?
I don’t know how great this risk is. Presumably high if walking across snow or paddling a canoe, but maybe not significant walking across the dessert? I’m going to be spending 2 weeks in the Grand Canyon, and have very fair skin; while comfort is a priority, sun protection is a greater one.Feb 11, 2017 at 7:31 am #3450105
Jerry AdamsBPL Member
@retiredjerryLocale: Oregon and Washington
white reflects sunlight, as does aluminized material
like Colin said, white does not reflect IR but aluminized material does. This isn’t that big a deal for sunlight. A big factor for something hot like a steel mill or a fire fighter.
Lightweight silnylon is somewhat transparent so would let some through. Depending on the weight. So it could let some heat from the sun through.
You could have two layers of nylon – white on top and dark on the bottom – sew them – hold them up to the sun first to make sure it’s blocking most of the sunlight – maybe use heavier fabric like 1.9 oz/yd2. Maybe go to local fabric store and look at whatever nylon they have and see how white it is and how transparent it is. That should give you good protection from UV for sunburn. I think that since UV is more energetic, it tends to get absorbed more easily so any fabric should work.
You could have aluminized mylar on top and dark nylon that isn’t siliconized on the bottom. Use the same tape as is used for cuben fabric.Feb 11, 2017 at 9:21 pm #3450203
Gumbo, your understanding of the UV problem sounds right to me. If you are actually standing on a surface that is highly reflective in the UV, then I would expect a wide brim hat with a reflective underside to increase your UV exposure a little. I don’t know the solar reflection spectra for common outdoor surfaces like soils, ice, and water.Feb 12, 2017 at 7:01 am #3450227
Ken MozdenBPL Member
@kenmozLocale: Louisville, Oh
I paddle often so wide brimmed hats that are light colored or white are my choice. The underside of the brims are almost always light color also … not good. I always mask off the brim and spray with a flat black spray paint. Several light coats so it doesn’t soak through. Makes a big difference as to how much glare /sun gets reflected back into my face.
Possibly if you had a light colored or aluminized umbrella and it wasn’t dark underneath you could try my flat black spray method. Use a spray made for plastic. It wouldn’t add any weight. A silicone coated fabric might be a problem with adhesion. -KenMFeb 12, 2017 at 9:23 am #3450246
Ken T.BPL Member
I’d buy an umbrella and see if I like hiking with it first. My black umbrella performed well in the sun too. Still shade.
I have the aluminized Euroschirm umbrella and love it. Bought a golf sized umbrella at the local home store for $5. It too gets the job done.
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