FAQs – Thermochromics

Thermochromic inks or dyes are temperature sensitive compounds, developed in the 1970s, that temporarily change colour with exposure to heat. They come in two forms, liquid crystals and leuco dyes. Leuco dyes are easier to work with and allow for a greater range of applications.

Question: I need to print a thermochromic effect onto papers and boards – which ink should I use?

Answer:  For papers and boards we would recommend the application of our water based ‘screen’ink.

Question: What colours are available?

Answer:  Standard colours are black, red, magenta and blue. Other colours can be created but with a higher minimum volume and cost per kilo, these could be orange, green, purple and turquoise for example.

Question: Does the ink start ‘invisible’ and then the colour appears?

Answer:  No, the ink will start as a solid colour, i.e black, and upon heating it will go almost completely clear. If the ink is being chilled, ie going from room temperature to below 15°C then the ink will appear white-ish and then change to its coloured state, (ie blue commonly).

Question: What temperatures are available?

Answer:  Our standard temperatures are 15°C, 31°C and 47°C, secondary temperatures are 8°C, 27°C, 29°C and 37°C (these have a higher minimum volume order). Other temperatures from -15C to +69C can also be created, again with higher minimum order levels.

Question: I’ve seen the Hypercolor t-shirts – how can I create an effect like this?

Answer:  For textiles we have a water based textile screen ink that works in the same way as our normal screen ink. We also have a plastisol ink range for screen printing fabrics

Question: What ink is used on the ‘Wow’ or Magic Mugs I’ve seen?

Answer:  For ceramics (and glass) we have a water based sprayable system that is used to spray the entire surface of the mug. And for printing only specific areas on the glass or mug we have a range of rotary screen print epoxy ink colours. The result is that the ink clears to reveal the message or image underneath it when hot water is poured into the mug.

Question: How can I get hold of some of this ink?

Answer:  We can supply a ‘sample pack’ for water based screen and textile inks that includes 3 slurries at 3 different temperatures (ie blue at 15°C, black at 31°C and red at 47°C) this also includes two binders, one for papers and one for textiles. The slurry and binder needs to be mixed together, in equal amounts, in order to create a finished ink. These can be supplied on a ‘one-off’ basis FOC in the U.K.

Question: Can the ink be used on food packaging?

Answer:  Thermochromic inks have been used on food packaging, however this will very much depend upon the substrate and if the inks will be in direct contact with the food. We would recommend thorough testing of the printed piece with the thermochromic ink.

Question: Can there be any colour message underneath the black thermochromic? Or would it need to be a solid black colour also?

Answer:  The thermochromic ink can be used to disguise messages or even images, but these may be visible through the thermochromic ink, even at room temperature. Disguise can be helped by applying two coats of thermochromic ink, if possible. Yellows seems to cause the greatest problems with thermochromic ink. Whenever possible the strength of the image or wording should be reduced to accommodate the thermochromic ink.

Question: Are there any papers or boards we cannot print on?

Answer:  Metallised papers, such as beer labels, require a particular form of thermochromic ink and these need to be discussed in great depth.

Question: Are only Screen printing inks available?

Answer:  No, inks are also available for flexographic, rotogravure and sheet-fed offset printing.

Question: How do I print onto a plastic substrate?

Answer:  Plastics may require a different method of drying, ie via UV lamps, or a by the use of a solvent based ink. These need to be discussed in greater depth.

Question: I’ve seen the colour changing spoons you sell, what ink is used to create these?

Answer:  The product that causes a colour change in the spoons is a thermochromic masterbatch. This can be mixed with various plastics, ie polypropylene, in order to create a colour change either upon heating or upon cooling. These need to be discussed in greater depth. I’ve noticed that the thermochromic ink is still soft after printing – what can I do to protect it?

Question: I’ve noticed that the thermochromic ink is still soft after printing – what can I do to protect it?

Answer:  For ‘touch and reveal’ applications we would recommend the application of a gloss laminate or varnish once the card is printed. For ‘chill and reveal’ applications a matt laminate should be used.

Question: Can thermochromic inks be used on outdoor products?

Answer:  As a general rule we would not recommend our thermochromic products for outdoor applications as U.V. light affects them rapidly affecting the thermochromic change. A U.V. protective film will prolong the lifetime of the product but not significantly enough for a permanent display.

Question: What are ‘photochromic inks’?

Answer:  Photochromic inks are inks that react to UV and sunlight. These can be applied to papers and boards or to textiles.

Question: What are ‘plastisol’ inks?

Answer:  Plastisol is used as ink for screen-printing on to textiles. Plastisols are the most commonly used inks for printing designs on to garments, and are particularly useful for printing opaque graphics on dark fabrics.

Plastisol inks are not water-soluble. The ink is composed of PVC particles suspended in a plasticizing emulsion, and will not dry if left in the screen for extended periods. Because of the convenience of not needing to wash a screen after printing, plastisol inks can be used without a source of running water. Plastisol inks are recommended for printing on coloured fabric. On lighter fabric, plastisol is extremely opaque and can retain a bright image for many years with proper care.

Plastisol inks will not dry, but must be cured. Curing can be done with a flash dryer, or more inexpensively, a home oven. Most plastisols need to reach a temperature of about 180°C (350°F) for full curing. Plastisol tends to sit atop the threads instead of soaking into them, giving the print a raised, plasticized texture. Other inks can produce a softer feel.

FAQs – Liquid Crystal Inks

Liquid Crystal inks are highly responsive to temperature change, quickly changing colour reversibly as they are warmed or cooled.

The traditional presentation of the inks is against a black background where they reveal spectral colours (from red through greens and blues) when warmed, although other coloured backgrounds will influence and alter the resulting colours viewed.

Question: What is Thermochromic Liquid Crystal?

Answer: Liquid Crystal inks and coatings are optically active mixtures of organic chemicals that react to changes in temperature by changing color.

Question: How do Liquid Crystals work?

Answer: They show colours by selectively reflecting incident white light. They should be viewed against non-reflecting backgrounds (ideally black which is totally absorbing) for best visualisation of the colours.
Liquid Crystal mixtures change colour from colourless (or black if printed onto a black background) to red at a specific formulated temperature and, as the temperature is increased, the liquid crystal changes through other colours of the visible spectrum in sequence (orange, yellow, green, blue, and violet).
Eventually it will turn colourless (black) again at a higher temperature still. The colour changes are reversible, and on cooling the colour change sequence reverses back again.

Question: What standard Liquid Crystal inks can be supplied from stock?

Answer: HAND TOUCH and STRESS formulations are available from stock. (Hand Touch formulation is: R25C5W = red start 25°C bandwidth 5°C . Blue start 30°C)
All other microencapsulated Liquid crystal inks and sprayable coatings are manufactured to order.

Question: What is bandwidth?

Answer: Bandwidth is defined as the difference between the temperature at which a red colour starts to appear, and the temperature at which a blue colour starts to appear.

Question: What range of temperatures can be supplied ?

Answer: Liquid Crystal mixtures can be produced with colour-change properties evident over a wide temperature range; red start temperatures can be made to vary from -30°C to 120°C and bandwidths from 1.0°C to 20°C.
However for most practical purposes the working temperature range is 0°C to 50°C. Below 0°C, response times increase significantly to the point where utilisation becomes slow.

Question: What are the response times of Liquid Crystals to temperature changes? How quickly do they change colour?

Answer: At ambient temperatures (around 15 to 25°C), the colour change generally only takes a few tens of milliseconds to take effect. Response times are greater at lower temperatures, and are the same in both the heating and cooling cycles.

Question: Do Liquid crystals exhibit hysteresis?

Answer: Hysteresis is not usually noticeable in most temperature-sensitive Liquid Crystal mixtures. (More details can be given).

Question: Are Liquid Crystals UV stable?

Answer: Most Liquid Crystals 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 Liquid Crystals whenever possible (e.g. in overcoats, laminations etc.).

Question: How do I print Liquid Crystals?

Answer: Liquid Crystal Inks should be printed by screen printing method. Using a screen mesh between 77T and 90T will generally give the best results.
Ensure that no solvent residues are either in the equipment, or in the air (from open containers or other print/production processes nearby).

Question: What chemicals are Liquid Crystals incompatible with?

Answer: Liquid crystal 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 Liquid Crystal which is what gives them their unique colour change properties. At all times, care must be taken to ensure that solvents and oils do not contaminate Liquid Crystal products.

Question: Do Liquid Crystal products contain any heavy metals?

Answer: No - they do not contain any heavy metals.

Question: What is the shelf life of the TLC products?

Answer: A shelf life of at least 6 months is guaranteed for all Liquid crystal products provided that they are stored correctly. All Liquid Crystal products are sensitive to UV light and elevated exposure will cause degradation. This will initially result in loss of temperature calibration and color brightness and eventually lead to complete loss of thermochromic functionality.

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