Liquid Crystals

liquid_crystalsThermochromic Liquid Crystals (LCs) can be highly temperature sensitive, change to many colors, and are more expensive than leuco dyes.  LC's start black below their temperature range; go through the colors of a rainbow, and then black again above the temperature range.  LC's are reversible in that they can be used over and over again. The picture shows an example of a liquid crystal sheet in response to warming.

Popular liquid crystal applications include medical devices, forehead, aquarium and room thermometers, promotional pieces and advertising applications. Additionally functional devices such as a propane tank gas level indicator are achieving much notoriety.  Liquid crystal thermometers are being used for thermal mapping and other industrial applications where custom inexpensive temperature monitoring is warranted.   We offer a wide range of LC thermometers as stock products, but also offer literally thousands of products for your label.

LC's are very similar to the liquid crystal displays used in watches and laptop computers, but our thermochromic LC's change color with temperature instead of voltage.  LC's can be formulated to change temperature from -22 to +248ºF   (-30 to 120ºC), and can be sensitive enough to detect changes as small as 0.2ºF.  If kept out of UV light and away from high temperatures and strong solvents, LC products will last for years.

The most prevalent use of liquid crystal is as a self adhesive reversible temperature indicator label that continually monitors temperature offering a visual readout that ranges from traditional numeric displays to custom graphics that can serve as an alert or warning.

What They Are:

These self adhesive labels consist of a series of temperature-sensitive elements containing microencapsulated Thermochromic Liquid Crystal TLC coated on a black backing.  Each element changes color distinctly as its rated temperature is reached, passing through the colors of the spectrum in sequence (Tan, Green, and Blue before turning black at a higher temperature. The TLC strips are calibrated so that the indicator that shows green indicates the actual temperature. The color changes are reversible and the reflected colors will be observed in the reverse order upon cooling.

How the labels work:

The temperature-sensitive elements contain TLC molecules that are very sensitive to temperature and change position / twist in relation to changes in temperature. This change in molecular structure affects the wavelengths of light that are absorbed and reflected by the liquid crystals, resulting in an apparent change in the color of each temperature event.

When the rated temperature of an indicator is reached the TLC molecules twist slightly causing the TLC substance to absorb the red and blue portions of the visible light and reflect the green part. This causes the temperature event to appear green. When the temperature decreases, the molecules begin to twist in the opposite direction, and the TLC reflect a different portion of the spectrum.

Label Construction:thermochromic_liquid_crystal_label_construction_illustration

Polyester film

>75, 125, 175µ
(Depending on label type)

Graphic Print
Liquid Crystal Ink
Black Backing
Adhesive & carrier
Release Liner

Above values are general guides using the standard products as a representative example.

Adhesive Tape Double-coated Adhesive Tape
Covering Film Polyester 5 mil stock
Color Change Material Non toxic mixture of liquid crystal esters
Accuracy - 30 to 59°C ±1°C
60 to 89°C ±2°C
90 to 120°C ±3°C
Shelf Life 12 months from invoice date when stored at room temperature (i.e. 21ºC (70ºF) & 50% relative humidity).

Reading TLC Thermometers:

The correct temperature is indicated by the square on the TLC thermometer that turns green.  If green is not visible the temperature will be mid way between the indicators that are illuminated tan and blue as follows:


Please note the above colors are a guide for demonstration purposes and the actual colors observed could be slightly different depending on label type.

Custom Products:

In addition to the standard Digi-Temp  labels available, which cover the temperature range of -30 - 120°C, a full custom manufacturing service is available and products can be manufactured to meet customers' precise requirements of, for example, size, shape, color, temperature range, number of temperature elements, graphic printing (logo's etc).  Please ask for details. 

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