Radiative Cooling Fabric

Background:

    Last year scientists in China demonstrated it may be possible to use a wet-spinning method to weave chitosan polymer with suspended SiO2 microparticles that appears to offer enough radiative cooling performance to stay ~10-20* F cooler than ambient in sunlight. If we can replicate their experiment and produce enough fabric, we may be able to use the loom and coordinate with the sewing committee to make shirts that offer excellent cooling performance. If successful, this could be expanded on in the future; similar methods could be tested with different polymers, chitosan extraction from shellfish waste could be explored, we could test the anti-microbial effects of chitosan, different spinning techniques, etc.

Why?: 

    Radiative cooling textiles are not a new concept, but they aren’t really a consumer product you can buy either. Chitosan, being derived from chitin (the second most abundant biopolymer in the world behind cellulose), has potential as a relatively low-cost polymer with much better thermal properties than cotton. And silica microspheres are literally just fancy sand, how much could they cost right? If we can make a radiative cooling fabric that is both inexpensive and effective, it could potentially be used to reduce the risk of heat exposure and heat stroke, and improve the quality of life of homeless people, construction workers, and anyone else stuck in the Texas sun all day.

    Completion of this project would involve the creation of a wet-spinning machine which could be used for all sorts of polymer projects in the future. Additionally, telling a kid that you made a shirt that gets colder in the sun instead of hotter and it’s made out of crabs and tiny crystals might be one of the most effective ways to convince them that science is this world’s magic system.

Materials: 

    Chemicals
        -ammonia
        -acetone
        -DI water
        -tetraethoxysilane or tetraethylorthosilicate (TEOS)
        -methanol
        -chitosan 
        -sodium sulfate
        -sodium hyrdoxide
        -acetic acid
        -glycerol

    Tools
        -light microscope
        -thermal camera
        -spectrometer
        -glassware, hotplates, etc.
        -fume hood and PPE
        -loom
        -centrifuge (vacuum chamber may work as alternative)

The plan: This experiment can be broken into a few phases, some easier than others.

    -Preparation of silica microspheres

        -add 62.7 mL of NH3*H2O, 14.6 mL acetone and 22.7 mL ultrapure (probably just mistranslated DI water?) to a round bottom glass reactor.
        -chill both the reactor and TEOS to 0* C
        -Add 2.0 mL of TEOS under vigorous stirring for ~ 5min
        -continue feeding ethanol ammonia solution at 0.5 mL/min and TEOS at 0.2 mL/min into the reactor for 4 hours
        -allow stirring to continue for 16 hours at 0* C
        -Let solution sit for an additional 4 hours without stirring
        -Filter, wash, and characterize microspheres
        
    -Preparation of chitosan solution

        -Make a 7.5% solid solution of acetic acid and SiO2 microspheres
        -Add .8 mL of the microsphere/acetic solution to a round-bottom beaker
        -Top off to 40 mL with DI water
        -Add 2g of chitosan
        -Add 25.5 mL of methanol
        -Add 1.7 mL of glycerol
        -Stir vigorously for at least 24 hours
        -centrifuge at 3000 rpm for 5 minutes

    -Extrusion, spinning, and weaving of the polymer

        -Extrude polymer through a syringe into a coagulation bath containing 30.0 wt% sodium sulfate, 10.0 wt% sodium hydroxide and 60.0 vol% DI water
        -Allow extruded fiber to soak in coagulation bath for 24 hours and wash with DI water several times
        -Soak fibers in 30.0 vol% methanol for 5 hours
        -Soak fibers in 50.0 vol% methanol overnight 12 hours
        -Wash in DI water several times
        -Dry the fibers
        -Twist the fibers
        -Stretch to final dimension and spool
        -Use the loom to weave textiles for testing
        
    -Testing
        
        In order to verify that we have actually made the thing that we wanted to make, we will need to test it at a couple of stages. A light microscope should be enough to verify both that the sizing of the silica microspheres is correct, and that they have actually been extruded and embedded in the fiber as intended. A spectrometer should be used to verify that the microspheres and final product are both reflective and emissive in the correct wavelengths. A thermal camera would be nice so we can see the effect once the fabric has been woven.

    -Demo

        What’s the point of making something neat if you aren’t going to show it off? Once we have an acceptably cool fabric, we can make a small bolt and coordinate with the sewing committee to turn it into something wearable.

Likely challenges:

    One of the keys to success for this experiment is going to be the wet-spinner, which will need to be made in-house assuming we don’t have tens of thousands to drop on a commercial machine. Conceptually it doesn’t seem that difficult, but things like what diameter syringe should we use to extrude the fiber, what rate should the fiber be extruded, how fast should the spinners and twisters be going, etc. were not discussed in the original paper, so this process will likely involve a lot of trial and error. This means that most components on the machine should be easily and individually adjustable so that settings can be tweaked iteratively until we have a satisfactory result. This will likely be the most tedious and time consuming step.

    The original paper goes into a good bit of detail on how they prepared the CS solution, so hopefully we don’t run into issues if we just copy what they did. That being said, chitosan can be difficult to work with and I wouldn’t be surprised if we have to troubleshoot a bit to get the viscosity and solubility right. Fortunately, there is a pretty big body of literature on chemically modifying chitin and chitosan so if it does become an issue we should be able to find a solution.

Where to start if you’re interested:

    -https://doi.org/10.1016/j.cej.2023.146307 - original paper
    -https://www.youtube.com/watch?v=JIGPV_-MI9w - youtube video of guy attempting the same thing.
    -https://www.youtube.com/watch?v=O4pVny7NV8E - video of the same guy but with more info on wet-spinning specifically.
    -https://youtu.be/N3bJnKmeNJY?si=UwBSfYu4UqcfqODs - video of different guy making radiative cooling fabric using a different method, he has a whole series with some good general info on radiative cooling and atmospheric transparency window.