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Color Change Basics:Liquid Crystal Thermometers

Liquid Crystal Thermometers

Thermochromic Liquid Crystals (LCs) can be highly temperature sensitive, change to many colors, and are more expensive than leuco dyes. LCs start black below their temperature range, go through the colors of a rainbow, and back to black again above the temperature range. LCs 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 propane tank gas level indicator are achieving much notoriety. Liquid crystal thermometer strips are being used for thermal mapping and other industrial applications where custom inexpensive temperature monitoring is warranted. We offer a wide range of liquid crystal thermometers as stock products, but also offer literally thousands of custom products for your label.

LCs are very similar to the liquid crystal displays used in watches and laptop computers, but our thermochromic LCs change color with temperature instead of voltage. LCs 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 from tan to green to 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.

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.

Component:
Polyester Film
Graphic Print
Liquid Crystal Ink
Black Backing
Adhesive & Carrier
Release Liner

Thickness:
>75µ, 125µ, 175µ (Depending on label type)
10-20µ
10-50µ
10-20µ
150-180µ
75µ

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

Adhesive Tape
Covering Film
Color Change Material
Accuracy



Shelf Life

Double-Coated Adhesive Tape
Polyester 5 mil stock
Non toxic mixture of liquid crystal esters
- 30 to 59°C ±1°C
60 to 89°C ±2°C
90 to 120°C ±3°C
75µ
12 months from invoice date when stored at room temperature (i.e. 21ºC (70ºF) & 50% relative humidity).

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, size, shape, color, temperature range, number of temperature elements, graphic printing (logo's etc). Please ask for details.

LCR Hallcrest LLC - US

1911 Pickwick Lane - Glenview, IL 60026
847.998.8580
sales@hallcrest.com
customerservice@hallcrest.com
info@hallcrest.com

LCR Hallcrest Ltd – UK

Riverside Buildings
Dock Road
Connah's Quay
Flintshire
CH5 4DS
+44 (0) 1244 817107
salesuk@hallcrest.com

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