Let us understand how this Conventional metallic inks vs UV varnish chemistry works,
To test metallic inks with UV varnish compatibility , it is important to study this compatibility only in the lab. That requires ink manufacturers to create similar operations on a laboratory scale to what their customers are using or customer can do this in their laboratory.
We did some laboratory test on metallic inks with UV varnish with specified time gap to study the behavior. This article covers the results of the study. This Lab test can be done by the client or ink manufacturer for different drying time to see if there are any improvements in the scratch test. We thank SIEGWERK for providing their lab for testing purpose.
Let us understand about metallic inks:
Metallic ink is a varnish or vehicle containing metallic particles. Common metals used to manufacture metallic ink include copper, aluminum, bronze or zinc. Metallic inks consist of aluminum powder in silver inks (bronze powder in golds), in an offset vehicle system. The primary reason for using a metallic ink is to achieve a metallic sheen in a printed piece. These inks are formulated using either aluminum or bronze (silver and gold respectively) colorant. That colorant, by design will align at the surface of the printed ink film to achieve maximum metallic sheen.
When metallic ink is printed and left to dry, the metallic particles rise to the surface, reflecting light and creating a metallic sheen. It is important to note that metallic inks are opaque. The brilliance is dependent on the leafing properties of the metal powder. These leafing properties can also lead to rub issues. In general, the more brilliant the color, the poorer the rub resistance.
The vehicle or carrier, by design, allows this “float” to occur; in fact it is designed to encourage this surface alignment. There is also a certain amount of penetration of vehicle into the surface of the substrate. Substrates such as SBS board have less “hold – out” so there is more absorption of vehicle into the substrate. Bear in mind that the vehicle is also going to function as the binder of pigment to substrate. The more of the binder that absorbs into the substrate, the less that will be available to bind the metallic colorant to the substrate. What is left is a loosely bound layer of metallic colorant at the printed surface which makes for a “brilliant” metallic look. (Anyone who has run their finger over an unprotected metallic print can attest to the loose bind. See picture .) This also results in rubbing & marking issues, particularly on packaging and covers. Thus the need or desire to protect metallic printed material with a varnish or coating.
Overprinting with varnish will improve the rub resistance, however, the brilliance will suffer since the metallic ink will not be able to leaf, the metallic particles are forced to lay flat. Due to the nature of metallic ink, it is important to be aware that rub resistance issues exist, and therefore, advisable to apply a protective coating on top of the surface. There are trade-offs however, because the protective coating may not react well with the metallic surface causing adhesion problems (such as with a film laminate) or the protective coating can lessen the metallic effect. In most cases an aqueous coating applied in-line will provide adequate protection against rubbing and marking as well as tape “adhesion”. An aqueous coating is low in viscosity and easily flows into the peaks & valleys and interstices in the print surface. The drying mechanism for aqueous coatings is not instantaneous and provides a flexible “plastic” film that flexes and moves as both ink and coating go through their final drying phases. The result is a well bound pair of films that provide adequate rub resistance, but not a high gloss finish. The desire for extremely high gloss on many packages leads to experimentation with UV cured coatings which do provide extremely high gloss, but bring new problems.
One of the most effective methods to achieve intercoat adhesion between the UV coating and a “true” metallic ink is to coat the metallic ink with an aqueous primer coating, prior to UV coating the ink. If an aqueous primer coating is used, it should be applied only after the metallic ink dries/cures thoroughly, or the wet metallic ink will be sealed under the aqueous primer coating and the metallic ink and aqueous primer coating will not dry sufficiently enough to allow the UV coating to achieve acceptable adhesion. An aqueous primer coating (or other UV coating compatible chemistry primer) is most effective in achieving intercoat adhesion due to a couple of reasons. The first is that, in general, UV coating chemistry has very limited compatibility with the lubricant(s) added in the manufacturing process of a metallic ink. During the ink drying process, the lubricant(s) can migrate to the surface and interfere with the UV coating wetting and/or adhesion. Since the level of the lubricant(s) present at the ink/coating interface varies with the degree of dryness of an ink, including the chemistry of
the ink itself and even the porosity of the substrate (which affects solvent retention and drying), the ink/coating interface may always potentially be contaminated with the lubricant(s).
The other most prevalent reason is the generally poor cohesive binding of the metal particles in the ink – the inks are, in essence, tinted varnishes with metallic particles; and, as the varnish dries, the metallic particles migrate to the surface and can rub off easily, creating An unstable surface. Consequently, when the ink is UV coated or laminated, adhesion only occurs on the surface or top layer of the metal particles in the ink.
First of all, there is a radical difference in
1) the general chemistry and 2) the drying/curing process between “conventional” materials and UV cured materials. The different chemistry drives difficulties in “wetting” and “binding” a UV coating to conventional ink and coating. Most coating films shrink slightly as they cure. A UV cured coating shrinks essentially instantaneously which can cause a break in the overall layer at the weakest link. If there is poor surface adhesion of coating to print then the UV cured coating film will easily break away from the surface it’s applied to. (Tape adhesion, scratch tests)
2) Because of the behavior of metallic ink colorant (orienting at the print surface) as the metallic ink sets and dries (either with or without a primer) this leaves this poorly bound layer of aluminum or bronze powder at the surface of the metallic ink film. When a subsequent UV cured coating is applied this loosely bound layer can fail in terms of adhesion of the UV cured coating to the printed film. A primer coating will likely not substantially prevent this as the loosely bound layer forms in the hours after printing and in-line primer coating. If the primer is functioning well, the UV cured coating adheres well and during the UV curing as the UV coating film shrinks, the break-away takes place between the primer and the ink film (the weakest link). A tape adhesion test will demonstrate this. Usually, in these cases the tape will pull a layer of the metallic ink film along with the UV and primer coatings. This is a result of the metallic ink film splitting due to the loosely bound layer of metallic colorant. This failure is not due to a materials failure – it is because all the materials are doing what they are designed to do.
1)There are options to evaluate that will minimize (not eliminate) issues with UV coating adhesion over metallic inks. The key to this is having sufficient binder remaining in the ink film to hold on to the metallic colorants.
The single most influential component in this combination is the absorptive characteristic of the substrate. The more absorbent the substrate is, the more problematic it will be. A lower absorbency substrate keeps more ink binder on the surface of the substrate thereby providing improved binding of metallic colorant in the ink film which improves the overall binding of materials applied over the metallic ink. In order to increase and maximize rub resistance, it is recommended that printers apply metallic ink on coated stock only. The harder and less porous the stock (such as a gloss coated stock) the greater the metallic effect produced. If a customer is interested in printing metallic ink on uncoated or textured stock, the metallic sheen will not be as pronounced because the ink will sink into the pores of the paper. If they are looking to apply metallic ink to an uncoated or textured sheet, the printer may apply two hits of the metallic ink or lay down a thicker film of ink.
20 ) The secondary option is to use a metallic ink that is higher in binder level to begin with. Manufacturers of metallic inks have alternative formulations available which are essentially lower metallic content and higher in oxidative components and binders. This allows a more positive binding of colorant into the ink film. The trade-off here is that there is a lower metallic “luster” proportional to the reduction of metal colorant content. The fast drying time reduces the metallic effect. Longer drying metallic inks tend to have a greater metallic lustre because the particles that reflect light have had more time to rise to the surface of the ink.
A third option is to in-line apply an oil based OPV designed to accept off line UV curable coatings. We tested with OPV and then UV varnish. The result was very good. Tape test passed. The opv we used was duct based with wax content . Twice we did tape test to make sure of report . Both the time the tape test passed. The below pucture shows both the tapes pasted in the same sheet on the edges.
We will do ine kore test with transparent ink which is again a opv without wax content. If that passes then that would be a good process to evaluate in press.
We also tested with water based varnish and uv on that. The drying time between metallic and opv/primer/wb varnish was 6hrs -12hrs and again 6-12hrs for uv.
Metallic ink + water based varnish + uv ; the tape is pasted on the corner of the sheet with metallic ink traces.
A final consideration is that it is usually desirable to minimize the time between printing of the metallic and UV coating the printed film. It is highly recommended that various combinations of these options be tested and monitored under production conditions, as specific printing conditions (ambient environment, fountain solution type and mix ratios, impression, etc.) can and do vary greatly from one pressroom to another.
Additionally we tried with
Tests for UV lamps, inks and coatings
UV Lamp efficiency
Detex test strips can be used to record the efficiency of UV lamps. The strips are attached across a sheet of paper which is then passed through the press with the UV lamps on and the colour changes of the strips will indicate the profile of lamp efficiency.
Adhesion tape test
The adhesion of an ink or coating to a substrate can be assessed using adhesive tape. The principal is that if the tape adheres more strongly to the ink or coating surface — than the ink or coating to the substrate — then the tape will remove ink or coating. Tapes with various levels of adhesion are used and care is needed to select the most appropriate tape for the purpose. Commonly used standard tapes include:
- Moderate adhesive strength: Commonly used by printers and also to test the foil adhesion in foil blocking. 3M ScotchTM range (http://solutions.3m.com) product 3M 616 is made in the USA.
- Moderate to strong adhesion: Used by ink makers to test inks and coatings on paper and board. Scapa red tape H101 (www.scapa.com/products).
- Strong adhesion: Testing adhesion of UV inks and coatings to plastics. TESA blue tape code 4104 (www.tesa.co.uk)
- Lay the print on a hard flat surface (a thick glass slab is ideal) and stick a 5 cm length of the selected tape to the surface with firm thumb pressure. Since the tape is transparent, we can visually check if the tape is sticking evening without air bubble. The pressure is to ensure the tape is evenly sticked to the printed sheet.
- Peel the tape away immediately, quickly, cleanly and smoothly at 90° to the print.
- Study both the print and the adhesive side of the tape.
- Make an estimate in % terms regarding the amount of ink/coating that is adhered to the tape. Always ensure reels of tape are properly stored at ambient temperature (out of direct sunlight and away from sources of heat) and respect use-by dates: adhesive tape deteriorates with age.
Health and Safety first for testing:Throughout the tests wear appropriate clothing (overalls or a laboratory coat), protective glasses and protective gloves (made of vinyl or nitryl rubber) during cleaning. Environment: Ensure all waste products, solvents and cleaning materials are disposed of correctly in proper waste bins. Do not mix materials between bins and keep solvents, paper and plastic waste separate.
Ethanol has a dissolving action, especially on the non-reacted components of the ink. This test assesses the degree cure of the ink film, and consists of rubbing over the print sample with an ethanol soaked cotton pad.
- Soak cotton (or similar) with ethanol. Then place the cotton on the face of the print sample and rub backwards and forwards at constant pressure and at regular speed and amplitude.
- Count the necessary numbers of cycles needed to register
- The beginning of the ink film deterioration,
- The complete destruction of the ink film.
The results of the test will depend on:
- The size of the cotton pad, • quantity of solvent used, • exerted pressure,
- Deplacement speed on the print sample, • ambient temperature (solvent evaporation).
- A specification for the number of cycles to produce this deterioration (initial or complete) can be established after having performed the test on validated prints.
MEK resistance test:
Like Ethanol, MEK has a dissolving power action on varnishes, especially on its non- reacted components. MEK is used to test the curing of UV varnishes only, since it would be too aggressive on UV inks (a test using MEK would not give conclusive results on UV inks).
Refer to the Ethanol resistance test for the application mode which we have mentioned above.
It must be noted that a negative test result cannot only be due to a lack of cure, but also due to an insufficient varnish coating weight.
Scratch resistance (Nail test):
This simple test is not standardised and depends on the printer’s experience. It consists of passing the fingernail over the printed area with a light pressure. A scratch will indicate an insufficient through cure or too soft a film. In this case over varnishing of the print is recommended. Finer nail test is not a standard testing procedure recommended any Certification body but this practice is done globally. Remember, the UV material is not friendly for human skin. There is a possibility the varnish or ink residue get stuck in nail can affect the person or he may accidentally swallow the material .Any testing with UV substances should be done with gloves only. So there is no way of doing nail test.The MSDS if UV inks and varnishes recommend to use glove while operating so with glove nail test is rules out.
Potassium Permanganate Test: Control of the curing of UV lacquers:
Potassium permanganate in aqueous solution oxidises the non-reacted acrylate sites present in the lacquers (monomers or oligo- mers). The optical density of the stains created by the solution on the surface is proportional to the quantity of double bounds that have not reacted in the dried product. This test involves a comparison to a standard value, which can be obtained by carrying the test on a previously validated job.
- With a pipette deposit a drop of solution on the varnished but not inked zones. • Let the reaction continue for 5 minutes. • Sponge up the excess with an absorbent paper, with a dabbing method (do not wipe).
- Calibrate the densitometer to zero on a varnished but not inked part of the print. • After having selected the yellow filter measure the optical density of the stain.
- Record the value.
- If the reading is inferior or equal to the standard value, the tested product has passed. If the reading is superior to the standard value, the tested product does not pass (insufficient curing).
- Precision: The comparison must always be made against a standard varnished with the same UV lacquer. Any comparison between two different varnishes would be meaningless.
Rub resistance test:
- After printing, the product can be subjected to a multitude of strains during the post printing operations, such as cutting, folding, and embossing to name a few. Also during the transformation chain of the product or during its transport to a customer. It can therefore be very useful to simulate these strains using a rub resistance test.
- Specifications can be defined in order to detect any possible defaults due to:
- Excessive deposit of ink,
- Insufficient protection from the UV lacquer (e.g. Low film deposit),
- Lack of cure.
PIRA rub resistance test:
- This consists of rubbing the printed surface to test with another piece of substrate, printed or unprinted and need not be identical. The dry rub resistant limit of the printed sample is characterised by a degradation that can be accompanied by ink transfer due to the friction with a blank substrate.
- The number of cycles necessary for degradation or transfer to occur measures the limit of resistance. Aspect modification or alteration (powdering, scratches, transfer) can be assessed with a qualitative scale (from 0 to 5) or by comparing with a standard (inferior, equal or superior to standard).
- The PIRA rub resistance test is often used by SIEGWERK INK Packaging to assess resistance of print outs designed for packaging. There also exist other tests such as the Sutherland test, which is generally used for paper substrates.
A Final Word
For best results, contact your UV equipment and chemistry supplier and collect the MSDS , TDS , some theory training , laboratory tests , some management systems, good QC/QA Process etc are necessary . The above are just a reference information and do not treat it as bible. The information is more or less same or similar between different manufacturers.
Thank you siegwerk for lab support and we have made our best efforts to compile the required datas from different resources combined with our experience. The above are just the theory behind the working of metallic ink or uv varnishes.
We hope the answer was upto your requirement. Your Feedbacks / suggessions / corrections are required for updating our knowledge or improving this FAQ page.