We hear every day of tissue businesses trying to achieve better added value for their products. There are many ways of attempting this, whether it is through the construction of the tissue substrate, the ability to give improved bulk and absorbency, the style of embossing, or finally the decoration. This paper looks at the novel ways of decorating tissue utilising specialised inks.
John Hayles
Tissue printing machines are improving all the time; the quick turnaround of print units, now available by cantilever rolls and extractable sleeves, can allow many designs to be printed across the web. Quick turnarounds on the smaller machines offer bespoke printing for custom-made products. This can therefore be an opportunity for increasing added value. We are seeing more four-colour decoration being utilised by tissue printers following on the heels of our colleagues in the packaging printing business. They in turn are now progressing further with more complicated process work like Hexachrome and other variants. We, as an industry, may well benefit from their experiences as we develop. Sitting on the printing inks side of the fence, it is believed that the flexo process produces approximately 75% of all the printing of tissue. It is recognised that one of the most limiting factors for us is the substrate we have to print on. This is not a criticism but a fact of life. Printers often have difficulties with tension control and registration, together with the requirement to produce prints on tissue whilst maintaining the paper characteristics. Sharper graphics, being produced by finer and finer aniloxes, are pushing the print process ever further.
The FTA has quoted one of their member Towel, Tissue and Napkin printers as saying "every job has the same objective - to put a Mona Lisa like masterpiece onto a paper napkin". I guess that we, as ink makers, have been striving to give the industry what it asks for. An ink that will print cleanly, run fast, be stable in terms of pH, foaming and viscosity, be user friendly and achieve all the specified end use criteria. However this paper goes further by attempting to show some of the speciality and novel effects that, with the required effort and desire, could be achieved. We do not expect these to be printed in mass production at phenomenal print speeds with fine detail; in fact it is quite the opposite!
Some of the novelty products I am going to describe are based on products that have a relatively large particle size; this means that that in the case of flexo printing it is necessary to consider deeper, coarser screens to produce the best effects. The same can be said for gravure. The choice of design, print process, application method, emboss style can all play a significant part in what can be achieved. Ink makers have worked closely with cylinder makers to produce suitable rollers that allow materials with these high particle sizes to be used. Raw material suppliers to the ink industry have worked hard to provide materials, which can now be incorporated into liquid inks. Previously, they would have only been suitable for the screen ink process. It is my aim to simply bring these to your attention, for you to take away the ideas and bring them into your own arenas wherever possible.
The effects I want to highlight are as follows:
1. Fluorescent inks
2. Metal free metallic effect and optically varying inks (also known as Interference and Colour flip)
3. Invisible fluorescent inks (UV visible)
4. Phosphorescent (glow in the dark)
5. Photochromic (Light sensitive)
6. Thermochromic (heat sensitive)
7. Geometric (glitters and holographic effects)
8. Scratch and Sniff inks
Fluorescent Inks: Vibrant, Bright Colours
Pigments in general reflect certain wavelengths of visible light and absorb the rest. Depending on which wavelengths are reflected, we perceive a certain colour. The continuous range of frequencies is known as the electromagnetic spectrum. The entire range of the spectrum is often broken into specific regions. The visible spectrum is a very small part of the total electromagnetic spectrum.
VISIBLE LI6HT SPECTRUM. This visible light region consists of a spectrum of wave lengths, which range from approximately 800 to approximately 400 nm; that would be 8 x 10-7m to 4 x 10-7 m. This narrow band of visible light is known as ROYGBIV.
Normally, the absorbed wavelengths are transformed into heat energy, but in the case of daylight-fluorescent pigments, the absorbed wave-lengths are transformed into visible light that is emitted. This extra light reaching our eyes is of the same hue as the reflected light. Because of this, extra-light daylight-fluorescent pigments are perceived as being extremely bright (in some cases four times brighter than non-fluorescent pigments).
FLUORESCENT INKS SUMMARY
Will produce visible light when irradiated with UV radiation of specific wavelengths. Overall effect is one of brilliance, cleanliness of shade and brightness.
Can be water based or solvent based, made from dye or pigmented colorants.
Are susceptible to fade in sunlight, but film thickness will improve fade resistance. Generally, the higher the ink film thickness, the more brilliance is achieved.
Need to be printed on white tissue for maximum reflection.
USES
Highlighting
Promotional products
Special offers!
Contrasting with conventional colours
Pearlescent inks: Metal Free Metallics and Colour Flip ink Effects
These are also often described as Interference colours. Essentially they are metal-free metallic inks, because they are based on pearlescent (mica) pigments. Pearlescent pigments are mica platelets coated with titanium dioxide and/or iron oxide that provide both colour and visual effects. They are transparent and reflect light because of their smooth surfaces and high index of refraction. Light reflected from the platelets creates a sense of depth and a lustre that varies with particle size - smaller particles impart a satin sheen while larger particles create a more sparkly or glitter-like effect. They are characterized by having a lustre look to the ink. Lustre is accomplished initially by reflecting light rays from a surface in one direction in the case of simple Mica pigments. Rather than attempting to create lustre by overprinting multi-layers of alternating high and refractive, transparent, smooth layers of ink, the effect is obtained by using transparent, highly refractive pigments. Best effects are obtained when pigments are in parallel. Some special effect pigments have a multiple colour or "flip" in which the colour changes with the viewing angle. Although these pigments are non-metallic, they can give finishes and metallic look. Typical colour shifts are red to yellow, or green to blue. They can be easily incorporated into printing inks. Best results obtained when over printing dark colours or dark coloured tissue. The smoothest tissue surface possible provides the best base for colour Flip.
To give some idea on the size of particles, it can be shown that to get the highest lustre, the mica particle can be four times the size of a standard metallic, or one hundred times the size of a conventional coloured organic pigment. This is why cell depths of cylinders need to be considered.
PEARLESCENT AND COLOUR FLIP INK EFFECTS SUMMARY
Coarse particles make transparent films; smaller particles are more opaque but lower lustre
Safer when compared to metallic pigments - no hydrogen gas generation during manufacture and storage
Comply with waste water regulation: i.e. Germany Cu 0.5mg/l; Zn 2 mg/l
USES
Provide enhanced quality
Product differentiation
Can be used for brand protection
Invisible Fluorescent inks (UV visible): Bright Disco Colours
We saw that Ultra Violet (UV) light represents a section of the overall electromagnetic spectrum. It extends from the blue end of the visible (400nm) to the x-ray region (100nm). It has three distinct subdivided wavelength regions, described as UV-A, UV-B or UV-C in increasing order of photon energy:
UV-A 400nm-315nm: Often referred to as 'black light', this is the longest wavelength region and lowest energy; it represents the largest portion of natural UV light.
UV-B 315nm-280nm: This is the most aggressive component of natural UV light.
UV-C 280nm-100nm: Only generally encountered from artificial light sources. Invisible fluorescent materials include both organic and inorganic pigments, which are only excited by the specific wavelengths of Ultra Violet light. The chart below is typical of some of the types of products available. These are transparent and are invisible in natural daylight. They only reflect visible colour when exposed to UV light at the wavelengths indicated.
Inks that reflect UV light are also available with either a neutral or coloured body colour. For example, it is possible to have a blue body colour, visible in natural daylight, but when this is subjected to either long wave or short wave UV light it could glow fluorescent yellow. These products can also be geared to show only in either long (UV-A) wavelength or short (UV-C) wavelengths.
INVISIBLE FLUORESCENT INKS SUMMARY
Can be used as invisible ink.
Can be as a novelty or security effect.
Lightfastness is generally low, so short-term use recommended.
Must be used on optical brightener free tissue.
USES
Novelty effects
Security
Phosphorescent inks: Glow in the dark
Phosphorescent materials will "charge up" under normal visible light and emit light in darkness. These may also be charged using ultra violet light for brighter, and often longer, periods of light emission. As with fluorescent materials, the phosphorescent equivalents will glow under exposure to ultra violet light, but phosphors continue to glow when the UV light source is removed.
There are two kinds of glow in the dark pigments: "Self-luminous" and " Phosphorescent". Conventional phosphorescent pigments for inks do not contain any radioactive substance. Historically, their short afterglow time has limited their application. Conventional zinc sulphide-based phosphorescent pigments (ZnS: Cu) have been widely used for glow-in-the-dark inks and paints, but do not glow practically all night long. New products based on strontium aluminate, activated with rare earth metals, glow 10 times brighter and 10 times longer compared with the conventional phosphorescent pigments. Its energy absorption - light-emission cycles can be unlimitedly repeated. Activation can be by a wide wavelength band (200-450 nm) but best results are obtained with activation energy under 365 nm. Developments in the reduction of the particle size of these pigments by manufacturers have made products available to liquid ink makers rather than just Silk Screen ink manufacturers. Features of phosphorescent inks:
PHOSPHORESCENT INKS: SUMMARY
Long afterglow now available
Increase in luminescence and afterglow with longer activation time
Excellent weather and light fastness
Free of hazardous and radioactive substances
USES
Phosphorescent inks have been used practically for fire escape signs and as novelties for things such as wallpaper
Promotional products
Photochromic Inks: Ink that will appear/ disappear
Photochromic dyes (PC) reversibly alter their colour upon exposure to ultraviolet (UV) light. The dyes are normally clear in their unexposed state, but when exposed to UV light, the dye becomes excited and the molecular structure is changed, allowing a colour to appear. When the UV light is removed, the dye will return to its closed form, which is usually colourless. Most photochromics change colour brightly outdoors; even on fairly overcast days as most of the UV light passes through the clouds.
The exact colour of a PC dye may differ according to many factors: the temperature, the weather, the excitation source (UV lamp versus sunshine), etc. Because the PC's actually change their chemical structure when exposed to UV Light, the materials are inherently unstable. In fact, the coloured structure of the PC dye is nearly broken in half when it is excited (coloured) as shown in the drawing below. PC's are normally powdered dyes (crystals) in their pure form and must be dissolved in the inks to change colour. For waterbased products micro-encapsulation is required. A colour to colour effect is possibly by combining PC's with a UV transparent permanent colour, i.e. base pink plus blue photochromic used in sunlight will give a purple colour and then change to pink indoors. Resistances of these are generally low.
PHOTOCHROMIC INKS: SUMMARY
Reversible reaction
Limited lightfastness and resistance
Invisible in low strength light
USES
Novelties
Promotional products
Thermochromic Inks: Changing colour with Temperature
Leuco Dye (LD) thermochromic materials can be described as chemicals that change colour or disappear when a certain temperature is exceeded, then change back when the temperature decreases to below that same activation temperature. The ink changes colour by appearing or disappearing as it cools down, or heats up. By selecting the temperature change range carefully, you can make it change by breathing on it, rubbing it with your fingers, touching it with your hands, putting it in the fridge or freezer, blow-drying it or holding it in the sun.
Colours can be blended from differing temperature ranges, to create prints that change colours dependent upon the temperature they are exposed to. By incorporating transparent pigments, a colour-to-colour change can be achieved.
This leuco Dye system of materials needs to be protected using a process called microencapsulation. The micro-encapsulation process takes a small droplet of the LD system and coats a protective wall around it, enabling it to be protected in use. The LD microcapsules are typically 3-5 microns in size, which is at least 10 times larger than a typical regular ink pigment particle.
TYPICAL SHADES AVAILABLE
Generally, thermochromic inks can be made in five standard activation temperature ranges: 5°C, 15°C, 25°C, 35°C, 45°C other customised temperatures are available from -5°C to 70°C. Features of thermochromic inks:
THERMOCHROMIC INKS SUMMARY
Respond to changes in temperature
Are available in reversible and irreversible form
Are based on microencapsulated liquid crystals
Generally low lightfastness
USES
Novelty products
Used on security work - i.e. banknotes
Product verification
Geometric inks: Glitters and holographic effects
Again the particle size is one of the governing factors. As can be seen from the chart even the smallest available is still very large (50mx 50m) and requires special cylinders. An extremely small amount of aluminium at the edges of the particles is present. The diagram shows how the polyester film and epoxy coating trap the aluminium in the particle. Conventional silver inks have much higher levels of aluminium. A wide range of colours are available in the various particle sizes, some of these are shown below. Holographic particles are created in exactly the same way but by using holographic rather than vacuum metallised film. As they are polyester film particles they are resistant and can be blended with conventional pigments to give many differing effects.
GEOMETRIC INKS: SUMMARY
Embossed polyester film, Vacuum metallised
Cut up into particulate shapes and sizes
Coated with an epoxy-based sealant to provide protection
Good resistances
Adhesion to substrate can be difficult due to particle size
USES
Cosmetic effects
Novelty value
Disco appeal
Scratch and Sniff inks: Generation of fragrance and perfumes
An odour or smell can be generated in a material by many different ways. Particular essences can be impregnated into paper or printed. The following are currently available as impregnated paper and is typically produced for children at school. By utilisation of micro-encapsulation, almost any odour, either natural or synthetic, can be incorporated into ink or paper. One of the main problems with printing using ink containing microencapsulated product, is the likelihood of the capsule breaking during the printing process. This means that if an essence has been incorporated, there will be a degree of odour generated at the print stage. This can also cause contamination of the print machine unless it is cleaned thoroughly. There are ways to reduce this risk but not remove it. Encapsulation is an expensive process and it is difficult to gauge the content required. We all know very small amounts of strong perfume smell strongly. Standard everyday odours like those listed above are readily available.
Specific essences can be created to order. By incorporating into inks, the essence will be trapped to an extent within the ink.
When the print is scuffed, some of the capsules break revealing the essence. The shelf life of the printed material is good and the strength of the odour left depends on the number of unbroken capsules left.
SCRATCH AND SNIFF INKS: SUMMARY
Product recognition by smell
Inevitably some odour created around print machine- possible contamination!
Long life span of unbroken encapsulated product
Almost every essence available
USES
Potentially medicated tissue
Novelty value
Instant recognition by smell
Conclusion
As can be seen there are many special effects available, in this paper I have shown some of the possibilities. Many of the Specialty and novel effects described in this paper will have a greater impact when the film weights that are applied are significantly higher than those currently employed for tissue printing. Anilox rollers or Gravure cylinders are now available from innovative cylinder suppliers, who have specialised electro-mechanical technology that can create the necessary rollers to maximise the effects. As the recommended filmweights are relatively high, it is essential that all parties carefully consider potential problems associated with high ink coverage on tissue like cockling, creasing and web breaks. I mentioned at the start of this paper that the potential areas of use for speciality and novel effects might not be mainstream production. But by highlighting these products I feel that it is possible to incorporate the latest technology into the tissue-printing arena.
A tremendous amount of research and development takes place looking at the composition and configuration of the tissue substrate. In kitchen towels we see changes in sheet count, sheet size, paper bulk, embossing patterns, recycled content, amongst other things. We have seen process printing take a hold in our markets and we should be looking at what other attributes the printing and ink process can bring. I hope this paper has provided some food for thought in those entrepreneurs out there! Sun Chemical Corporation is the World's Leading global manufacturer of high quality printing inks and high performance organic pigments. The company has a substantial presence throughout all the continents. We draw on our international resources to bring the latest technological advances to our customers - no matter were these customers are Located.
Since its beginning more than 160 years ago Sun Chemical is committed to be the forefront in research and development in Graphic Arts Industry. Sun Chemical has over $ 3 billion in worldwide sales and employs approximately 13,000 people worldwide.