- Q: What are Thermochromic Liquid Crystals?
- Q: How do they work?
- Q: What is bandwidth?
- Q: What is color play?
- Q: Are there any standard color plays available?
- Q: Over what temperature range can TLC's be used?
- Q: What are the response times of TLCs to temperature changes?
- Q: Do TLCs exhibit hysteresis?
- Q: Are TLCs UV stable?
- Q: What chemicals are TLCs incompatible with?
- Q: Do the TLC products contain any heavy metals?
- Q: What is the shelf life of the TLC products?
- Q: Can two different TLC products be used together?
What are Thermochromic Liquid Crystal?
They are optically active mixtures of organic chemicals that react to changes in temperature.
How do they work?
TLCs show colors by selectively reflecting incident white light. They should be viewed against non-reflecting backgrounds (ideally black which is totally absorbing) for best visualization of the colors.
Temperature-sensitive TLC mixtures turn from colorless (black against a black background) to red at a given temperature and, as the temperature is increased, pass through the other colors of the visible spectrum in sequence (orange, yellow, green, blue, and violet) before turning colorless (black) again at a higher temperature still. The color changes are reversible and, on cooling, the color change sequence is reversed.
What is bandwidth?
Bandwidth is defined as the difference between the red start and blue start temperatures
BS - RS = BW
36°C - 37°C = 1°C
Band Width (BW) = 1°C
BS - RS = BW
99°F - 97°F = 2°F
Band Width (BW) = 2°F
What is color play?
The color change properties of TLC mixtures, and products made from them, are identified by a code called the COLOR PLAY. This specifies the temperatures at which the colors are shown by the TLC change.
The color play describes 3 features; EITHER the red start temperature (R) OR start of green temperature, the temperature scale (C or F) and the bandwidth (W). The bandwidth is defined as the difference between the red start and blue start temperatures. For example, R35C1W describes a TLC mixture with a red start at 35°C and a bandwidth of 1°C, (i.e.) a blue start 1°C higher at 36°C; G60C5W describes a mixture with a green start at 60°C and a bandwidth (red start to blue start) of 5°C
Red start temperatures can be made to vary from -30°C to 120°C and bandwidths from 1.0°C to 20°C. Green starts, blue starts and clearing points vary accordingly. Tolerances depend on the color play.
Details are given in the tables below
Red Start Temperature (R)
Green Start Temperature (G)
Blue Start Temperature (B)
Clearing Point (CP)
For Start Temps from -30 to 60°C:
≥8W: 20% of Bandwidth
For Start Temps from 60 to 90°C:
≥8W: 30% of Bandwidth
For Start Temps from 90 to 120°C:
≥8W: 40% of Bandwidth
Are there any standard color plays available?
All microencapsulated TLC slurries and sprayable coatings are manufactured to order. However, HAND TOUCH formulations (R25C5W - red start 25°C bandwidth 5°C [blue start 30°C]) are routinely used to make promotional products and this formulation is available as the microencapsulated TLC slurry and sprayable TLC coating, in small packs, from the www.thermometersite.com website.
Over what temperature range can TLCs be used?
Even though TLC mixtures are available with color-change properties over a wide temperature range, for most practical purposes, the working temperature range for TLC mixtures is 0°C to 50°C. Below 0°C, response times increase significantly to the point where utilization becomes impractical.
What are the response times of TLCs to temperature changes?
At ambient temperatures (15 to 25°C), generally accepted values vary from a few tens of milliseconds to a few hundreds of milliseconds. Response times are greater at lower temperatures and are the same in both the heating and cooling cycles.
Do TLCs exhibit hysteresis?
Hysteresis is not usually noticeable in most temperature-sensitive TLC mixtures. More details are given in the Handbook of TLC Technology.
Are TLCs UV stable?
Most TLCs are very sensitive to UV light and exposure should be avoided. However, for applications requiring some degree of UV stability, protection can be achieved by using materials with UV absorbing properties in combination with the TLCs whenever possible (e.g. in overcoats, etc.).
What chemicals are TLCs incompatible with?
TLC mixtures are essentially oils and are very sensitive to contact with common organic solvents, oils and waxes, particularly low molecular weight materials. These have small molecules which can penetrate the microcapsule walls and destroy the highly ordered molecular structure of the TLC which is what gives them their unique color change properties. At all times, care must be taken to ensure that solvents and oils do not contaminate TLC products.
Do the TLC products contain any heavy metals?
No - they do not contain any heavy metals.
What is the shelf life of the TLC products?
12 months from invoice date when stored at room temperature (i.e. 21ºC (70ºF) & 50% relative humidity).
Can two different TLC products be used together?
Unsealed TLC mixtures cannot be used together without changing their properties. However, microencapsulated products - slurries and Sprayable coatings - can. The properties of the individual TLC mixtures are protected by the microcapsule walls. For sprayable coatings, best results are achieved if the coatings are applied individually (rather than mixed). Coatings with the lowest temperature color plays should be closest to the observer.