By Tom Kerchiss of RK Print Coat Instruments Ltd
Environmental concerns and legislation to reduce the use of solvent-based materials in the late ‘90s and early years of the new century resulted in water-based inks and UV curable inks dominating trade press headlines in terms of product development and usage. A state of affairs that continues up to and including the present, very rarely is there any mention of solvent-based materials; indeed, for those who for very good reason need to employ solvents – oft find that in some circles they are considered to be in the same category as smokers, pariahs. Solvent-based ink usage does however continue, albeit at a much lower rate; bad publicity, the need for costly solvent-reclaim devices and better UV ink formulations and improved associated equipment have impacted on solvent usage. In some situations though, solvent materials remain the only option, notably for some difficult substrates.
Solvent-based inks contain organic pigments, a binder or varnish, and additives as well as the solvent component. The varnish is one of the most important elements of the ink.
With a solvent-based ink the binder or varnish is usually a polymer, having long chain of molecules, with a high molecular weight. Full synthetic and half synthetic polymers are used, the former made of oil, the latter of modified natural products including cellulose. Varnishes made from these products are modified by nitration, ethering or estering. Cellulose derivatives like cellulose acetopropoinate and nitrocellulose also have a reputation as good varnishes. Although different varnishes are available including pure types solvable with alcohol and the co-solvent types, which can only be solved with petrol/ethanol, the most often used are nitrocellulose inks based on pure ethanol. There are many types of formula such as polyvinylbuitiral (PVB) resins made from polyvinylalcohols, which can be solved in alcohols and are used for applications involving polyethylene’s, OPP’s and co-extrusions.
UV inks and coatings on the other hand have increased marked share considerably in recent years and while it is true that they are not right for every job, it is true to say that they are no longer regarded as the preserve of the few. Most are 100% solids with no volatile or flammable solvents; new generation UV curable flexo inks for example: offer a wide operating window for printability and performance, delivering ‘more bangs for the buck.’ When using uncoated stock for instance, and when text and graphics need to look crisp, UV inks are often the answer in that the ink does not have time to dive into the stock, curing in milliseconds.
Wet film thickness does not decrease significantly during UV curing from evaporation of ingredients and the high solids allow very high gloss to be achieved under proper conditions.
Since UV inks flash dry they remain pristine and vibrant, more resistant to damage that may arise as a result of handling, stacking and shipment.
As mentioned early the water-based inks also received attention and have expanded market share, though arguably they do not have the same cachet as UV inks. These inks are based on combinations of acidic resins held in solution with amine and water-based resin dispersions – collectively termed emulsions. Solution resins aid print quality while the emulsion provides resistance properties in the dried ink.
On absorbent surfaces such as paper and board the ink will start to penetrate into the substrate. Evaporation of water and amine occurs together, generally aided by on-press dryers. The need to achieve ‘balance’ sums up the production/formulation of water-based inks for there is a degree of contradictory requirements. While a highly fugitive amine assists drying speed, print quality can suffer. Loss of amine in low-coverage print decks can lead to print defect, monitoring and maintaining pH will keep the ink in balance. In sectors such as corrugated box printing, presses are not equipped with dryers; due to the penetration of the binder, the inks have a mechanical anchor for ink to substrate, providing strong adhesion. For this reason water-based flexo inks largely dominate the printing of paper/board.
Water-based inks for obvious reasons do not dry or adhere well to non-absorbent surfaces. For this reason these substrates are printed more frequently though not exclusively with solvent-based inks.
Irrespective of the ink technology involved ink producers and others in the production process supply chain have to work closely and keep lines of communication open when formulating ink systems with the proper strength, viscosity and end use structure. Ink, resin, additives, substrate suppliers and printer/converters always require equipment that will enable them to develop new products and ensure that batch consistency remains within set tolerances.
Consistency throughout the supply chain is critical; with colour variation perhaps being one of the most common complaints. Colour matching is a major issue not only facing the printer but also the ink maker and substrate provider. Sometimes the substrate and ink will interact to produce the unexpected. Colour variation can be caused by variations in the amount of pigment transferred as well as in variations in the colour and texture of the substrate. Ink formulators and printers need to be sure that the ink will run smoothly on press and that variables such as gloss, flexibility, durability and wear resistance are under control.
Devices such as the new Paste Ink Proofer with UV curing (PIPuv) enable users to obtain fast, repeatable colour matching of UV litho, UV web offset and other paste inks. Employing automatic ink metering, the PIPuv eliminates the need to weigh or measure inks samples. It is also possible to proof two inks simultaneously.
For further information email sales@rkprint.com, www.rkprint.com
Source: RK Print Press Release
|
