FAQs – Photochromics

Photochromic dyes reversibly change colour upon exposure to ultraviolet light in the range of 300 to 360 nanometers.

What are ‘photochromic inks’?

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

How do they work?

Photochromic means ‘ability to change colour when exposed to sunlight’.

Photochromic inks, dyes and masterbatches change their molecular structure and darken on exposure to specific types of light of sufficient intensity, most commonly ultraviolet (UV) radiation,  (UV light in the range of 300 to 360 nanometers).

These Photochromic dyes are reversible.  When placed into sunlight or ultra violet rays they become activated, in an “excited” state, allowing the photochromic compound to turn into a darker colour. In the absence of activating light when the UV light source fades, the effect is reversed and they return to their clear state.

What colours are available?

Standard colours in water-based screen inks are  red, yellow, blue, green, orange, purple. Standard colours in plastisol-based screen inks are  red, yellow, blue, green, orange, magenta.

Other colours can be created but with a higher minimum volume and cost per kilo.

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

Yes, the ink will start virtually clear, and upon activating 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).   Please note that the colour may appear stronger on days with more intense sunlight.

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

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

I want to print onto t-shirts – how can I create an effect like this?

Photochromic inks and dyes can be applied to fabrics through conventional printing and dyeing methods.

For textiles printing 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 on fabrics.

Plastisol tends to sit atop the threads instead of soaking into them, giving the print a raised, plasticised texture. Water-based inks usually produce a softer feel.

When used in textile ink printed onto clothing garments such as t shirts, the change to colour when exposed to sunlight occurs within 15 seconds or less, and returns to a near invisible clear shade after 5 minutes or less indoors when removed from the uv/sunlight

What are ‘plastisol’ inks?

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. Water-based inks can produce a softer feel.

How do I print onto a black or dark T shirt ?

First print a white base or ‘discharge white ink’ in the normal way, BUT the white ink MUST be formaldehyde-free. Otherwise the formaldehyde will permeate into the photochromic ink and degrade the ability to change colour.

This can still happen after the discharge white ink has been cured/dried and then the photochromic ink printed on top is being cured.

Suitable formaldehyde-free water-based discharge white inks are available and should be used, such as ‘Magnaprint Discharge Super White ULF Ultra GA’

Note- The dyes on some printed fabrics can attack the photochromic ink, even after treatment with a discharge white ink. A small test print is advised before moving on to full production.

Note- The colour strength of the photochromic ink will depend on the amount of ink printed on the surface; a white cotton fabric will absorb and hold more photochromic ink than a cotton that is first printed with white ink layer. Therefore printing photochromic ink through the same screen mesh onto a discharge white or direct onto white cotton will probably show different strengths of photochromic colour; the photochromic colour achieved on the white will probably be reduced due to the reduced ink deposit, therefore a heavier screen deposit is needed and you should change to a wider mesh in order to increase the colour strength..

Tell me more about “discharge white” ink

For printing onto coloured fabrics, in order to get bright water-based print colours on dark fabrics, a bleaching ‘discharge’ white ink is usually applied to the surface first to give better colour to the photochromic ink. Discharge inks normally include a zinc-formaldehyde formulation  -ZFS (Zinc Formaldehyde Sulfoxylate) – in order to bleach the coloured cotton back to a natural pale cotton colour.

However, this formaldehyde will have a bleaching effect on the photochromic ink printed on top, even after curing or during subsequent t shirt washing.

To be compatible with photochromic inks a non-formaldehyde based discharge ink must therefore be used.

Is Photochromic ink formaldehyde free?

We can offer two versions of photochromic inks low-formaldehyde and formaldehyde-free.  The formaldehyde-free system is plastisol only and may not be suited to use with some discharge white inks.

What type of Photochromic products do you sell?

Photochromics are available in the following formats-

•             Dyes – For direct use in materials able to solubilise them

•             Slurries – Microencapsulated for use in aqueous systems

•             Powders – Microencapsulated for use in non-aqueous systems where protection is required

•             Inks – For printing onto papers, plastics and textiles

•             Labels – Used on finished products

•             Masterbatch-   For addition to plastic moulding and extrusion

How can I try some of this ink?

We supply a ‘sample pack’ for water based screen and for plastisol inks, that contain smaller

Are only Screen printing inks available?

No, inks are also available for water-based flexographic printing.

I’ve seen plastic cups that change colour in the sun, what ink is used to create these?

If the plastic is changing colour, rather than a printed image on the cup, the product that causes a colour change is a photochromic masterbatch. This can be mixed with various plastics, i.e. polypropylene, at the time the cups are being moulded. (See Masterbatch section).

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