® Produces Brilliant Colors
Wide-format advertising banners and vehicle wraps are genuine eye-catchers. Their colored surfaces are created with professional inkjet printers that mostly employ solvent-borne digital printing inks. WACKER has developed the polymeric VINNOL® E 18/38 binder specifically for such inks.
Everywhere we look in our cities, wide-format advertisements compete for our attention: giant banners hanging from building fronts, scaffolding and construction site hoardings push their way into our field of vision while standing signs entice us to enter hotels, museums and shops. Meanwhile, buses, trams and delivery trucks are adorned with visually impactful vehicle wraps – which cling like a second skin.
However different these advertisements may appear at first glance, they all have one thing in common: they are made of plastic films which are custom-printed and processed in relatively small numbers. These films mostly consist of plasticized PVC and are printed using wide-format inkjet printers.
“Use by Dates” are printed on the packaging of different consumer goods. Thanks to VINNOL® E 18/38, the printing ink adheres to a wide variety of substrates.
Advertising banners, standing signs and vehicle wraps are exposed to the elements and are also cleaned from time to time. Water-borne printing inks like those found in many domestic inkjet printers are generally not durable enough for this application. Nor do they adhere sufficiently to plasticized PVC. So instead, makers of advertising resort to either UV-curable or solvent-borne digital printing inks, whose binder systems provide not only the high quality print image, but also the necessary durability.
The printing industry expects inkjet printing to deliver even better results in the future and that the inks will no longer be a noticeable odor nuisance. VINNOL® E 18/38 is a polymeric binder from WACKER that will help ink makers to meet these requirements.
Samples have already been provided to all major customers. “The feedback is extremely encouraging. VINNOL® E 18/38 not only is a binder of the highest quality, but it also enables us to respond to customers’ needs and provide them with tailored technical service support,” says Feike van der Heide, who heads WACKER POLYMERS’ global Resins business. “Our goal is to continually develop new business fields and promote continued growth in existing markets through our innovative products.”
One of the first customers to formulate its products with VINNOL® E 18/38 was Chinese ink maker Beijing KinColor Digital Technology. “We have noticed a vast improvement in printing performance since then. One of the reasons for this is that the pigments are more evenly distributed in the printing ink thanks to the new VINNOL® E product,” explains Rong Jin, head of Beijing KinColor Digital Technology. This company has been using WACKER binders for many years because they offer a consistently high level of quality and performance. It collaborated with WACKER application technologists on the development of a suitable formulation for a printing ink containing VINNOL® E 18/38 that would be very well received on the market. “Thanks to WACKER, our new ink has strengthened our market position,” says Rong Jin.
“These requirements – low viscosity and no settling – pose a dilemma for ink makers.”Dr. Ulrich Lauter, Head of the applications laboratory for VINNOL® resins, WACKER POLYMERS
The low-viscosity VINNOL® E 18/38 binder can also be used to print high-resolution images on wide-format advertising banners.
Inkjet printing is a non-impact process. The ink is deposited as very fine droplets on the substrate, without the need for an image carrier or “printing plate” as it is called in the industry. Instead, the digital print data determine which areas receive the ink droplets and which do not. This eliminates the costs of producing a printing plate and reduces the turnaround times for the printing press, enabling smaller print-runs of very few or just one single article to be economically viable.
There are two types of inkjet printing - drop on demand (DOD) and continuous inkjet (CIJ). DOD is used for printing paper and films, even in large formats. CIJ, in contrast, is mainly used in printers that are integrated into industrial production lines because it is capable of printing on uneven surfaces. This affords a way of providing the produced articles with a serial number, a brand name, a QR code or a shelf-life date.
Long-Lasting Print Head
VINNOL® – BINDERS FOR INDUSTRIAL APPLICATIONS
The surface coating resins that WACKER sells under the VINNOL® brand are polymers that can be used as coatings in many industrial applications, including industrial coatings and adhesives, paints and printing inks, and heat-sealable coatings used in food packaging. VINNOL® surface coating resins are vinyl-chloride-based copolymers and terpolymers. Each grade differs from the others in terms of its composition and degree of polymerization, resulting in differences in their physical and chemical properties. The line-up also includes grades containing carboxyl groups for strong adhesion to metal surfaces, and grades containing hydroxyl groups that can be used, for example, in coatings that cure via chemical reaction.
Overall, the VINNOL® range encompasses a diverse array of product grades, all of which can be used together in any combination and are compatible with many other common binders. For the production of VINNOL®, WACKER uses two different polymerization processes: suspension and emulsion. Grades intended for use in digital printing inks are predominantly made by the emulsion process.
“At the heart of the inkjet printer is the print head – it determines whether printing is a success or a failure. Keeping it working for as long as possible is the topmost priority,” explains Dr. Ulrich Lauter, co-developer of the new polymeric binder VINNOL® E 18/38 and a technical service manager at WACKER. The print head, together with the printing ink, determines the quality of the print.
For the highest resolution, the print head must be designed in such a way that the smallest-possible ink droplets form and strike the printing surface. However, the smaller the droplets, the lower the viscosity of the ink has to be. The ink also determines the degree of color saturation. Particularly brilliant color prints are obtained with pigments that are dispersed very finely and homogeneously in the liquid printing ink – the tiny pigment particles must not form large agglomerates and should remain in suspension for long periods of time.
“These requirements – low viscosity and no settling – pose a dilemma for ink makers,” says Dr Lauter. “The thinner the fluid ink, the more readily the pigment particles will settle.” Settling not only impairs color brilliance, but also shortens the service life of the print head. If individual nozzles in the print head become clogged with pigment particles, the inkjet printer will stop working. A key role regarding viscosity and dispersibility is played by the polymeric binder employed. “As inkjet printing technology advances, it is essential that the binders in particular be optimized,” stresses Dr. Lauter. The binder is the film-forming component of a printing ink – it envelops the pigment particles, binding them together and fixing them on the substrate.
WACKER has been offering binders under the brand name VINNOL® E for many years. These enable ink makers to keep settling to a minimum, explains the WACKER expert: “The binders are vinyl chloride–vinyl acetate copolymers, which we produce by emulsion polymerization. These polymers are recognized for their ability to interact with the pigment surface, stabilizing the finely ground particles and keeping them in suspension.” That is why VINNOL® E represents a good compromise between the two conflicting goals of low viscosity and little or no settling.
Suitability for Mild Solvents
An example of an application for CIJ inks based on VINNOL® E 18/38: cable encoding.
In the last few years, a further requirement has come to the fore: to reduce employees’ exposure to smelly solvent fumes, the printing sector is looking for inks that are formulated with the mildest-possible solvents. The fumes of mild or “eco” solvents are neither harmful to health nor noticeably malodorous. Examples are certain glycol ethers and glycol esters. Such “eco-solvent printing inks” are intended for print shops or advertising agencies that use DOD printers. In the case of inks for CIJ printers, the odor is a minor problem as these printers are used exclusively in an industrial production environment where solvent vapors are extracted as necessary.
Mild solvents have their downsides too: a great many binders are not very soluble in them. In addition, inkjet inks formulated with mild solvents are less likely to adhere to plasticized PVC and other polymeric substrates, with the result that the print image is less resistant to abrasion. This is also true of the VINNOL® E grade, which has been available for use in digital printing inks for some considerable time. Dr. Lauter and his team therefore set out to fashion a new vinyl chloride-vinyl acetate copolymer from this product which would offer the best compromise between the somewhat contradictory requirements. Their efforts resulted in the new VINNOL® E 18/38.
This new grade not only improves solubility in mild solvents, but also yields a polymer solution that has particularly low viscosity. In addition, VINNOL® E 18/38 is resistant to water, alcohol and weakly basic cleaning agents.
There are two types of inkjet printing process: discontinuous (drop on demand or DOD) and continuous inkjet (CIJ)
DoD – Drop on Demand: In this technology, the ink droplets are generated and then ejected from the print-head nozzles as required, i.e. only if they are needed to produce a dot. The print heads found in professional DOD printers typically have 256 nozzles of 20 to 50 micrometers in diameter per color. The droplet volume varies from 2 to 40 picoliters in accordance with the print-head technology. The pigment particles of the ink must not be larger than 5 to 8 micrometers. A filter is installed in front of each nozzle, blocking the passage of large particles. 2-Butoxyethyl acetate is currently the principal solvent used for DOD printers.
CIJ – Continuous Inkjet: This process ejects a continuous jet of electrically conducting ink from a nozzle located in the print head. The flow of inkjet disintegrates into individual microscopic droplets with a volume of between 10 picoliters and 1 nanoliter. From 60,000 to 100,000 such droplets are formed every second. The droplets move through two electric fields. The first provides them with an electrostatic charge. The second determines the trajectory of the droplets and is itself controlled by the digital print data: depending on whether the field is switched on or off, the droplets are either deflected toward the substrate with varying degrees of force to produce a color dot or they are allowed to pass by the substrate without deflection. The non-deflected droplets are collected and returned to the printer’s ink reservoir. Methyl ethyl ketone is a common solvent for CIJ printer inks.