Ultraviolet paints and luminophores

Over the recent five years, the demand for UV-paints has risen from 4,59 billion $ in 2014 up to 7,59 billion $ in 2020. The average growth rate is 10,6% per year. The largest consumer is still the Asian-Pacific region. The figures say that in 2013 the region was already responsible for a half of total consumption of UV paints. Now it’s clear that for the region, the percentage is way after 50% of world total consumption. It is expected that the demand will give a jump of 9,9% per year. 

The biggest consumers among countries are China and Japan, then comes South Korea. The young and developing Chinese market is in a bad need of high-quality UV paints because the country’s manufacturing industry, as well as electronics and many other industries, is showing good growth rate. Consequently, this leads to a higher demand for paints and varnishes in the domestic market. As the government often prefers green technologies better, the manufacturers start bringing green innovations. UV paints is one of the most promising. Being an ecologically friendly technology to use, UV paints are reserving their place in the international market.

    UV paints and UV-paint-based materials are most often used for wood coatings (1). The experts attribute this to high quality of the paints themselves and of UV-painted coatings, low VOC (Volatile Organic Compound) and quick drying. The demand for UV wood coatings is the highest in Germany, China and the USA. Wood coatings accounted for 75% share of total UV-coatings demand in 2019. 

UV paints are becoming more popular in other end-use industries, too. For example, industrial coatings (2) segment stands out. The electronic industry (3) is catching up with it. The Electric Ultraviolet UV Shoes Dryer is presented in the picture below.

UV-based paints give the customer a number of advantages:

1. printing on non-absorbent surfaces;
2. paint layer is resistant to chemical and mechanical impacts;
3. glossy print surface;
4. paint can be applied using much thicker layers (compared to water-based paints);
5. not volatile;
6. imprints are the same throughout printing workflow;
7. printing process is simple;
8. paint contains no harmful substances ⇒ it can be used for producing packages for food and beverage industries;
9. ready-to-use surfaces can be left unattended for a few days;
10. post printing is acceptable; 
11. no wasted time for washing out.  

And here are some disadvantages:

1. be cautious when handling UV rays generators: enclosures or something like this will defend you against toxic ozone emissions;
2. possible film shrinkage lowers adhesion;
3. if the layers are too thick or high-pigmented, it is possible that UV rays will giver in;
4. high price for consumables;
5. high hygroscopicity level – UV paint absorbs water too fast.

UV paints are divided into two big groups: luminescent paints and fluorescent paints. Luminescent paint can gradually accumulate light energy and then release it, shining brightly even when the lights are off. Fluorescent paint can only be seen under UV rays.

Luminophores

Luminophore means ‘carrying light’ in translation from Greek. Luminophore is a pigment which can absorb light or heat energy. Later, when the lights are off, luminophores begin to glow. They give away the energy that was previously accumulated. In the picture, you can see the actual colour of luminophores (above) and their glowing at night (below). 

How to charge luminophores? There are a few ways of doing that. For example, you can charge them with the sun energy (this one is the most effective way). You can use a lamp instead, too. Another solution is to use a special UV-lamp. Whichever you choose, the whole process will take approximately 20-30 minutes. By the way, it is not recommended to charge them more than an hour. An hour is enough. Anyway, it won’t glow longer than it can. 

How to buy luminophores?

Before buying luminophore pigments, it is advisable to turn flashlight on your telephone and attach the telephone tightly to the package. Wait 10-15 seconds. If the pigment you’re holding is good, there should be a permanent glowing spot on the lumiphore surface. During the day, it can be visible for as long as five minutes. Do not buy luminophore if the glowing spot varnishes earlier.    

Basically, there are two types of luminophores: aluminate-based and zinc-based. It is thought that the aluminate-based luminophores win the race: they provide much more glowing time. 

There are three stages of glowing:

1. The glowing stage – luminophore glows and illuminates space around it. It is both possible in darkness and in the daylight. The stage lasts for 20-30 minutes.
2. The afterglow stage – afterglow can also be observed in darkness (after eyes get used to it, of course). This stage can be as long as 10-12 hours.  
3. The fading glow stage – glowing gradually fades away. To watch it, special devices should be used.

    ‘Pure luminophores’ (not containing UV pigments) can be coloured three different colours – white, green and grey (daylight colour). When in darkness, white-coloured luminophores can glow purple, turquoise and red, green-coloured luminophores can glow green-yellow or green, and grey-coloured luminophores can only glow blue.

If you want to see another colour, luminophores should by dyed with UV pigment, then. In this case, the colour of glowing luminophore will be the same as the UV pigment. When dyed with UV pigments and under UV-rays influence, luminophore is being in the so-called charging state. The glow is in fact provided by the fluorescent component. And when UV-rays flow is ceased, luminophore starts giving away accumulated energy. 

The most popular UV-pigmented luminophores are coloured:

1. Lemon (bright lemon – daylight, greenish lemon – nightlight);
2. Blue (blue – daylight, bright blue – nightlight);
3. Magenta (crimson – daylight, pink – nightlight);
4. Red (bright red – daylight, pink red – nightlight);
5. Orange-yellow (yellow – daylight, bright yellow – nightlight);
6. Orange-red (bright orange with red tint – daylight, hot pink – nightlight);
7. Purple (bright ultramarine – daylight, light blue – nightlight);
8. Pink (pink – daylight, pale pink – nightlight).

    Luminophores are environmentally friendly pigments. Though, they contaminate surfaces plus it is not recommended to put them into the mouth.

    Luminophores are most widely used in luminescent paints production (can be used to make luminous toys and T-shirts). Also, photoluminescent films are the basis for plans of evacuation in schools, banks, offices, etc. The reason is such a film is self-adhesive and has a luminophore layer.

This pigment can’t glow as powerfully and as long as a usual light bulb. If you want to keep the glowing for as long as possible, then the best advice would be to charge luminophore often, if possible – use UV-lamp.  

Returning to UV paints, there is an obvious division in what is the basic substance of UV paints is. There are water-based UV paints and alkyd-based UV paints. Water-based paints contain no harmful substances at all, so they can be used to decorate indoor design, for example, residential premises and entertainment centers. Alkyd-based paints cannot be washed off using detergents. Some toxic fumes can be emitted when the paint is drying. So, it is advised to use alkyd-based paints for outdoor design.

UV paints is an environmentally friendly, safe and efficient tool. The most important properties of UV paints include effectiveness, safety, easy of use, etc. No evaporating or self-absorbing solvents are used. So, when being dried, the paints do not change their colour. This feature is an obvious advantage over water-based paints. 

Another advantage of using UV paints is that monomers which are often used for conventional water-based paints, are fully or partially excluded. Although UV paints do contain some monomers, the manufacturers are still doing their best to minimise their bad effect. The bad effect is that monomers emit an unpleasant smell. When contacting mucous membranes of the human eye or nose or mouth, these substances can also cause skin irritation.

The film-forming component of the paint is something of a huge importance. Resin is the most popular film-forming component for conventional, non-UV paints. Vice versa, chemically active oligomers is the basis for most UV paints. 

The vast majority of all the modern UV paints is based on the following oligomers:

1. Epoxy acrylates (one of the cheapest type of oligomers and that is why one of the most often used);
2. Polyester acrylates (although a bit more expensive, but is also less tough and more elastic);
3. Oligourethane acrylates (perhaps the most expensive of all; noticeable for high adhesion and abrasion level; suitable for label packages);

    The key properties of UV paint will be different depending on which type of oligomers you choose. Reactivity, fluidity, elasticity and hardness – all of these are directly related to the type of oligomer used. The right combination allows you to create paint structure that fully meets production requirements. 

Also, UV paints contain a completely new component – photoinitiator. The UV rays that affect the paint are usually not strong enough to break the monomer-oligomer bond. To make sure UV rays are effective, special compounds – photoinitiators – are used. Photoinitiators are essential for photopolymerization. Photoinitiator is a chemical substance that absorbs UV-light and begins to generate free radicals (unpaired electrons). The function of the free radicals is to react with the future film-former substance. As a result, the UV ink layer changes from liquid state to solid state. A three-dimensional cross-linked structure is being formed at a time. It is important that the photoinitiator only absorbs UV-rays of certain wavelength. It is the chemical structure of the photoinitiator that defines whether the initiator can absorb UV-rays or not.

So, there are 3 stages of photopolymerization: initiation, chain growth, chain breakage. The first one includes the generation of free radicals. As for the second one, radicals react with paint molecules. This way, the number of radicals is increasing. The last stage is responsible for turning the radicals into molecules. The result of it is generating high-molecular polymer.

The speed of polymerization is difficult to measure. It is influenced by the following factors:

1. exposure-to-UV duration;
2. chemical nature of monomer-oligomeric components;
3. photoinitiator concentration;
4. type of photoinitiator;
5. UV radiation: intensity and wavelength;
6. paint film thickness;
7. oxygen.

So, the composition of UV paints is as follows:

1. Epoxy acrylate resin (30%)
2. Epoxy acrylate with fatty acids added (25%)
3. Blue рhthalocyanine (16%)
4. Initiator (11%)
5. Monomer (viscosity modifier) (8%)
6. Pigment (4%)
7. Photo-sensitive aromatic amine (4%)
8. Stabiliser, wax, etc. (2%).

Simple instructions on how to apply UV paint

Before applying paint on a surface, pay attention to the surface itself: it should be smooth, not rough. Make sure that the surface is clean and there is no dirt and dust on it. Then mix the paint with some water (in case of dispersion emulsion). After that, paint the surface, gently stirring the paint. Paint brushes, rollers and sprayers can be useful. UV paint allows to apply 2, 3 or even 4 layers. Bear in mind that after applying, for instance, the first layer, you should leave it alone for some time before paint becomes dry, and only then it’s time for the second layer. The best way to apply a picture, a scheme, etc., is to use a special stencil. 

Interestingly enough, UV paints cure much faster on a white substrate than on a dark substrate. The reason is white substrate reflects UV rays, whereas dark substrate absorb UV rays.

Even if we are sure about UV paint quality, there could be some other paint-non-related problems when printing. Here we’ll let you know:

When it comes to print business industry, UV paints are primarily used in label packaging industry. UV paints provide most accurate colour reproduction. They also guarantee high level of mechanical resistance and chemical defense. When using UV paints for printing labels, it is desirable that you add a special UV-module. After that, the printer will be ready for using UV paints. It is important that UV paints are compatible with water- and flexo-based inks.

The UV paints usage field is wide, but it is best recommended to apply UV paints on paper or cardboard. Here, UV-paint printing and offset printing can both do a good job. But when it comes to much more complicated surfaces – such as polymer films, metallised paper and foil – the printing process will bring much more difficulties. When printing on such materials, UV-based flexo-printing can be a good solution. In this case though, much more attention should be paid to which material to choose. Potential shrinkage may lead to low adhesion level. So,  manufacturers usually do it this way: non-UV paints are applied, and after drying, the surface is additionally covered with UV varnish. This method ensures high resistance to wear and other negative impacts.

To solve the above-mentioned problems with low adhesion, unique cationic UV paints were also developed. The cationic UV paints have a lot of advantages. First, they are not sensitive to the absence of oxygen. Second, they become dry very fast. This can even be processed in complete darkness. Third, increased adhesion is a defining characteristic of cationic UV paints (and the solution to the problem, too). Fourth, they can be combined with a wide range of materials. Finally, this kind of UV paints emits no smell at all, and the skin is never irritated after printing job. Thus, the paints are 100% safe for human health. 

The only drawback of such unique paints is that their price is quite high. As a result, cationic UV paints are used only when particularly high adhesion level is required.

One of the world’s leading UV-paint producing companies is Flint Group. Last year Flint Group managers managed to earn 2,6 billion $ by selling high-quality solutions for offset, flexo, UV, etc., printing. For example, their Flint Group LITO FLORA FTX. According to the website, this kind of paint is unique because of the following:

1. unique rheological and technical properties;
2. suits coated glossy surfaces;
3. fast curing;
4. high resistance to paint abrasion;
5. maintaining water-paint balance;
6. suits UV varnishing perfectly well;
7. ‘dry pigment technology’ ensures low hygroscopicity level;
8. two-sided printing is acceptable;

Summarising all that have been said on UV paints, it is a great time-saving effect that makes UV paints so liked. Using UV paints for printing saves much space, too: no large warehouse and workshop premises are required. The only weak point is that additional heavy equipment should be installed (such as UV lamps, UV reflector, cooling system and drying machine). Also, UV paints allow you save a lot of energy. Solvent regeneration is not required, as this role is taken by monomers. When polymerizing, they do not evaporate into the air.

Now, the UV paints are suitable for any type of printing, including screen printing, flexo, offset, etc. Definitely, UV paints will become even a more powerful tool for printing in the future.

Image source: fxsupply.com, lankwitzer.com, flintgrp.com

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