Medium evaporation rate Solvents such as n-propanol or n-propyl acetate offer greater benefits in Flexo

Flexographic printing is one of the most common types of printing process used for long-run printed materials such as flexible packaging, magazines, catalogues or art prints. In comparison to other printing processes such as offset printing, gravure inks require a lower viscosity, i.e. they contain more solvent. Conventional solvents such as ethyl acetate evaporate fast and require retardants to reduce their evaporation rate. Pure n-propyl acetate or mixtures with n-propanol – i.e. Propyls – evaporate moderately fast only. Accordingly, Flexo can avoid or reduce the use of retardants such as Ethoxy propanol both when preparing the base ink and when refilling ink in the printing process.

While slower than Ethyls, the Propyls evaporation rate is still fast enough to ensure the printing ink dries properly during the process. As a result, levels of solvent loss are generally lower in the press. This means less refilling is needed during the printing process to maintain the correct ink viscosity.

Compared with conventional solvents, the slightly longer period required for drying in a propyl solvent system gives the pigments in the printing ink more time to distribute uniformly at the moment they are applied to the polymer film. When compared with conventional solvent systems, this results in a higher color density for the same quantity of pigment. Accordingly, the same printing quality can be achieved with less pigment, which in turn leads to a lower consumption of concentrated ink.

The point of using Propyls is, that you can produce the same color density with a formula that has more solvent but less pigments than with Ethyls (less color base). This means lower costs. This effect can be attributed to the different way in which Propyls solubilize the binder components. As a result, less of the valuable concentrated ink is needed to make the same volume of ready-to-use printing ink. This, in turn, leads to further savings per square meter of printed film due to the fact that the ink is generally more expensive than the solvents.

Advantages for flexography in packaging printing

In film printing with flexography, Propyls offer a 1:1 replacement for the Ethyls conventionally used. A simple swap is possible, with no further modifications needed. Print shops can continue to use the same printing ink, the same equipment and the same photopolymers as before. Especially in packaging printing, customers are demanding more sustainable printing from providers: with Propyls, print shops use less ink and less solvent while also producing a lower percentage of scrap. As a result, resources are conserved and emissions of volatile organic compounds (VOCs) are reduced due to reduction of solvent. Overall, productivity also increases while using the same machinery as before.

The advantages of Propyls over conventional solvents have long been recognized all over the world. In North America in particular, Propyls have traditionally been favored in flexography printing. In recent years, this trend has also gathered pace in Central and South America. With new production capacity coming upstream in 2018 and the years to come, European printers and converters can finally enjoy the advantages of the use of n-propyl acetate in flexography printing as well.

Studies show potential savings of Propyls versus Ethyls

Several industrial studies published by the chemical company Oxea have examined the substitution of Ethyls for Propyls in flexography at ink- and print shops. The studies looked at print runs of over 2,000 km and production periods of up to one month. In all of the studies, considerable savings in printing ink and solvents were achieved, simply by substituting Propyls for conventional ethyl-based solvent systems.

By using Propyls, study participants achieved major improvements in terms of increased cost-effectiveness while maintaining or even improving print quality, and were able to reduce or avoid entirely the use of retardants such as Ethoxy propanol while preparing the base ink. Propyls feature the same chemical and physical compatibility as Ethyls with the typical binders used in printing ink such as nitrocellulose (NC) and polyurethane (PU). In the studies, advantages were demonstrated in terms of improved process stability, higher printing speeds and less scrap. Printed materials also contained a significantly lower volume of solvent.

In gravure, the delayed evaporation of Propyls lessens the incidence of cylinder cell ‘plugging’ by lowering the tendency of ink to dry onto the cells in the first place while simultaneously improving the reconstitution of dried-on ink. This effect is the main reason for the improvements to quality observed. The same also applies to the anilox roller cells in the flexography process. Thanks to the slower evaporation rate of Propyls, it was possible to increase the speed of the press in many cases without compromising print quality. In several printing trials, Propyls demonstrated less foam formation tendency than Ethyls, especially with the color black, which again led to improvements in quality. In a Brazilian study, one other advantage observed was improved color stability due to lower water absorption.

Where solvents were reused, it was possible to utilize the exact same procedure for concentration (activated charcoal bed), condensation or distillation. N-Propyl acetate also has another advantage over ethyl acetate: since the enthalpy of vaporization for n-propyl acetate is lower than for n-ethyl acetate, less energy is required for both vaporization and condensation. The higher boiling point of n-propyl acetate exerts a larger effect across the larger temperature differential of the cooling medium, which further promotes condensation. When considered in conjunction with the lower overall volume of solvent in the propyl-based recycling process, these effects lead to energy savings when reclaiming used solvent.

On the basis of the experience gained at industry customers, detailed models for replacing conventional solvents with moderately evaporating Propyls were developed at the HdM University of Media Studies (Stuttgart) in late 2017/early 2018. In the University’s Print and Media faculty, a research group from partner association DFTA and a group led by Professor Armin Weichmann conducted test runs on standard press under lab conditions with runs of 17,000 m in flexography and 10,000 m in gravure. The results, which definitively confirmed the previous insights gained by Oxea in the industry studies, documented comparable savings in solvents and printing inks while meeting the same high standards required for the output quality of the printed matter.

Authors:

Dr. Jens Klabunde

An industrial chemist and biotechnologist by profession, his career has encompassed a variety of roles in biotech firms, where he developed and marketed processes for manufacturing chemical products using procedures from both biotechnology and conventional chemistry. He joined Oxea in Germany in 2011, where his work as a specialist in product/application development involves expanding the existing portfolio while identifying and developing new application fields.

Sebastian Anton

After graduating in business informatics, he then worked for over 17 years in various departments and roles both in Germany and internationally for both Oxea and its predecessors. Since early 2018, the focus of his work has turned to developing Oxea’s solvent business in the printing and printing ink industries.