back to index

Thermochromic silicone


Why
How
      First attempt, fail
      Second attempt, silicone
            Timelapse videos
Todo

Why

Seeing the approximate temperature of an object is pretty handy, whether it is a chip, its heatsink, stepper heatsink, coffee mug, or a machine gearbox.


How

A set of three microencapsulated leuco-dye thermochromic pigments with different threshold temperatures and compatible colors, was obtained:

They were mixed together, yielding the color range of black-purple-red-white as the temperature increases over the thresholds.

(Sourcing pigments with different thresholds than 28-30°C was a logistical exercise of pain and patience. Multiple vendors had to be used, and a proxy employed to avoid high transportation-related charges.)

To be possible to apply, the pigments are mixed into a suitable vehicle/binder. This binder has to be compatible with the materials to be applied to (well, duh). It also has to not be chemically aggressive, nor contain certain solvents and other chemicals that could diffuse into the microcapsules and alter their internal composition, or damage the microcapsule itself. Both cases would result in loss of color change functionality, usually leaving the pigment stuck in colored or partially colored state. The pigments are also sensitive to ultraviolet radiation and degrade upon exposition, and also degrade at prolonged high temperature exposition.

To protect against UV radiation, a UV filter can be added into the mixture. Here, a nanometer-sized titanium dioxide powder is employed, selected for being on hand.

The color change is slightly gradual and with a few degrees C of hysteresis. The pigments change from colored to colorless. The black one appears gray on the images, due to high ambient temperature nearing the threshold at the time of the test.

First attempt, fail

The first mixing attempt was done using a clear UV-curable nail gel. The gel failed to set under the UV LED (probably due to UV filter compound leaching from the microcapsules, the additional filter was not added here), and the test coupon stayed sticky. The thermochromic action worked well; however the color change was slightly muddy, due to the pigments not reverting completely to white; probably due to capsule content degradation by diffusion of the acrylate monomer from the uncured gel resin. The same mechanism led to the loss of thermochromic functionality in few days.


First test, heating

First test, heating

First test, heating

First test, heating

First test, cooling

First test, cooling

First test, cooling

First test, cooling

Second attempt, silicone

The second attempt used additional UV filter, appearing as a suspicious white powder. Neutral single-component silicone was employed as the base. The UV filter was mixed in. The pigments were mixed in one by one, adjusting the ratio to achieve sufficiently significant hue differences. Slight excess of black was added to accord for higher ambient temperature that made it less black.

The material was deposited on a sheet of paper in a thin layer. Some was also used to coat parts of stepper motors of a 3D printer, the Raspberry Pi main and USB/LAN chip, and heatsinks of the stepper drivers. After curing, strips of the coated paper were attached to double-sided tape and applied to other objects, e.g. a water faucet. A thin strip was also attached across the raspberry pi main chip. One test strip was attached to the outside of a window, to be exposed to direct sunlight as weather permits, to test the UV degradation.


Pigments and filter

Pigments, filters, silicone

After mixing

After mixing, layer deposited

The silicone cured in nominal time without the slightest problem. The color change persisted, and tests shown the resulting color is clear white.


Printer head indication, warm ambient

Printer head indication, warm ambient

Chip temperature indication, hotter in the center

Chip temperature indication, hotter in the center

Heatsink temperature indication, cold

Stepper temperature indication, cold

Stepper temperature indication, cold

Timelapse videos

The 3D printer microscope was used to obtain annotated timelapses of color changes during heating and cooling. The printer heated bed was used as a convenient regulated hot plate. The temperatures in the video are somewhat off, especially at the black-purple threshold; due to high ambient temperature the bed glass had to be cooled down with a pack of frozen spinach (cue Popeye theme), and there is a time difference between what the thermistor senses and what temperature gets to the top side of the glass. The cooling timelapse is ten times faster, as natural cooling of an insulated printer takes its sweet time.



Timelapse of active heating, high magnification
(part of field of vision covered with white paper to bypass the camera's auto-adjustment and highlight the color change)


Realtime show of sticker applied to a water faucet, showing changes with hot and cold water
(water is not hot enough to achieve white, sticker is slightly not adhering at two places, showing nonhomogeneity at thresholds)


Timelapse of active heating, 12x speed up
(autoadjustment of white balance makes color changes less evident)


Timelapse of spontaneous cooling, 120x speed up
(autoadjustment of white balance makes color changes less evident)



Heating video test, black

Heating video test, black-purple

Heating video test, purple

Heating video test, purple-red

Heating video test, red

Heating video test, red-white

Todo


If you have any comments or questions about the topic, please let me know here:
Your name:
Your email:
Spambait
Leave this empty!
Only spambots enter stuff here.
Feedback: