How it works – Liquid Crystal

Thermochromic Liquid Crystals (TLC’s, or sometimes simply LC’s) are optically active mixtures of organic chemicals that can be formulated to be highly temperature sensitive and when used in certain conditions show many different colours as they pass through their liquid crystalline state, or can be tuned to only change through a single colour.

Liquid Crystals are highly anisotropic fluids that exist between the boundaries of the solid phase and the conventional isotropic phase. The phase is a result of long range orientational ordering among constituent molecules that occurs within a certain range of temperature in melts and solutions of many organic compounds. The ordering is sufficient to impart some solid-like properties to the fluid but the forces of attraction usually are not strong enough to prevent flow. This dualism of physical properties is expressed in the term ‘liquid crystal’. A more proper name is ‘mesomorphic phase’ (mesomorphic: of intermediate form), often abbreviated to ‘mesophase’.

Liquid crystal exhibits optical properties typical of the crystalline state (anisotropy to light, birefringement) and mechanical properties typical of the liquid state (fluidity and surface tension).

TLCs react to changes in temperature by changing colour. Starting as black (when viewed against a black background) below their temperature range, they go through the colours of a rainbow as temperature is increased finally changing back to black again when above their temperature range. They are reversible and can be used over and over again.

The technology is similar to that used in TV and Watch displays however instead of reacting to electrical impulses they respond to fluctuations in temperature. They can be formulated to react to temperatures between -30°C (-22°F) and 120°C (248°F) and react to changes in temperature as small as 0.1°C (0.2°F). Accurate to up to +/- 1°C if stored and used correctly they will remain functional for many years.

TLC’s have chiral (twisted) molecular structures and are optically active mixtures of organic chemicals. The correct scientific name for the materials is CHOLESTERIC or CHIRAL NEMATIC liquid crystals. The term cholesteric is an historical one, and derives from the fact that the first materials to show the characteristic properties and structure of this particular type of liquid crystal were esters of cholesterol. This can be misleading, as many non-sterol derived optically active chemicals (and mixtures containing them) also show the cholesteric liquid crystal structure. It is important to differentiate these sterol and non-sterol derived materials because, although they change colour in the same way, they have different properties and can be used in different ways to achieve different effects. TLC mixtures can therefore be divided into 3 types based on their compositions:

(a) CHOLESTERIC – comprised entirely of sterol-derived chemicals;
(b) CHIRAL NEMATIC – comprised entirely of non-sterol based chemicals.
(c) COMBINATION – containing both cholesteric and chiral nematic components. Combination mixtures extend the application possibilities and working ranges of TLC formulations by combining the respective advantages of both groups of component chemicals.

TLC’s in the MICROENCAPSULATED form (SLURRIES AND COATINGS)

To date, the microencapsulation process has been the most versatile and successful way of packaging and protecting TLC mixtures. The LC is isolated from the atmosphere by a protective barrier and, at the same time, converted into a comparatively easy-to-use form. In simple terms, a microcapsule is a small sphere with a uniform wall around it, and in the microencapsulation process tiny droplets of liquid crystal are surrounded with a continuous polymer coating to give discrete microcapsules. Microcapsule diameters are generally between a few microns and a few millimetres. The product of the microencapsulation process is an AQUEOUS SLURRY. This can be used directly (e.g. in hydrophilic liquids as tracer particles in flow field studies) or can be incorporated as a pigment, into a COATING FORMULATION optimised for a particular method of application (e.g. spraying, etc.). The dry coating should ideally support the liquid crystal in a uniform film with the minimum degradative effect on the intensity and purity of the reflected light. Microencapsulated TLC mixtures offer improved stability and versatility of use over their unsealed precursors. Further protection can be achieved by using materials with UV absorbing properties in combination with the TLCs whenever possible. Water resistant coatings can also be made.

TLCs can be used as colour change pigments in the form of microencapsulated slurries in water. The products are available as 40% (weight) solids content with microcapsule diameters centred in the range 10-15 microns. They are custom formulated to the required colour change properties. These slurries can be used to make sprayable TLC coatings by addition to aqueous binders.

Two series of TLC coatings are available. Both are aqueous, acrylic-based and designed for application by spraying through an airbrush or similar compressed gas sprayer. They have good adhesion to most surfaces and matt and gloss finishes are achievable by varying the coating thickness.

SPN100 Series – For optimum colour brightness and ease-of-use. They can be removed easily by washing with water.

SPN300 Series – For some degree of water resistance. They can be used underwater for limited periods of time.

The use of microencapsulated sprayable TLC coatings overcomes many of the problems associated with using unsealed TLC mixtures, although the reflected colours are less bright. Applied as a thin film, the coatings will dry to give a finish, which will resist light abrasion. They can be sprayed directly onto the test surface and are particularly useful when the surface is not flat.

COATED (PRINTED) SHEETS

Most TLC temperature indicating devices are comprised of a thin film of liquid crystal sandwiched between a transparent substrate (sheet), and a black background. They are usually made by printing an ink containing microencapsulated TLC onto the reverse side of the substrate. A black ink is then applied on top of the dry TLC coating and colour change effects are viewed from the uncoated side. The different forms of the materials each have relative advantages and disadvantages and are suited to different applications.

Their most common use is in self-adhesive Thermometer Labels or Strips where they have been used in a large number of common applications including:-

  • Forehead Thermometers
  • Fridge/Freezer Thermometers
  • Monitoring storage of cold chain produce
  • Nursery Thermometers
  • Bath Thermometers
  • Aquarium Thermometers
  • Gas Level Indicators
  • Overheat/Too Hot Indicators
  • Wine Thermometers
  • Temperature Mapping of surfaces
  • Preventative Maintenance Indicators

Each temperature on the thermometer is individually formulated and laid down to form an independently functioning temperature indicator. When placed alongside other formulations a graduated scale can be created allowing the user to identify current temperature. As each segment will pass through the rainbow of colours the formulations are generally set so that the green window indicates the temperature. In the case of more than one window showing green, it is the highest one that should be read.

LC Thermometer

SPECIFICATIONS

Layer – Material – Thickness
Protective Film – Polyester or Polycarbonate – 75-175µ
Graphics – Printing Ink – 10-20µ
Temperature Window – Liquid Crystal Ink – 10-50µ
Black Backing – Printing Ink – 10-20µ
Adhesive & Carrier – Modified Acrylic with polyester carrier – 150-180µ
Release Liner – N/A – 75-120µ

LC Label Construction

CUSTOM PRODUCTS

Due to the nature of construction any design or shape can be produced and a full custom manufacturing service is available to meet customers’ precise requirements. Please contact us for details.

Normally printed onto a black background they initially show in a black state below their temperature rating and then pass through the colours of a rainbow as light is reflected at different wavelengths as they pass through their “liquid crystalline” state and then show as black again once the temperature is outside the range and they are fully liquid in their isotropic state.

Different formulations can be formulated to show just a single colour either below or above their rating, providing simple clear and easy to read indicators. All these materials are reversible and as such can be used time and time again.

Request more information

If you would like to read our Privacy Policy please click the link below.

PRIVACY POLICY