MEASURING GRINDING PERFORMANCE IN UV RESIN SYSTEMS FOR PIGMENT PASTES AND CONCENTRATES Paulo Roberto Vieira Junior, Marc Heylen, Paul Gevaert, Francis Bergiers, Luc DeWaele, Rosalyn Waldo, and Peter Yuan. Allnex ABSTRACT Cost of ownership along with performance, quality and compatibility are all key universal issues that formulators must consider when evaluating and adopting new components for energy curable inks, coatings and overprint varnishes. In emerging markets, inconsistency of raw material quality is an equally important issue that formulators must confront and have limited tolerance for in UV cured ink formulations for consumer product packaging. Herein reviewed are pigment grinding resin options that provide good printability and process saving performance attributes including grinding efficiency, load capacity, color development and wetting properties, for UV cured printing inks that provide good printability and process savings. INTRODUCTION Given the current market environment and the global reach of products and technologies, versatility, performance and competitiveness are the three most important attributes of printing inks. There is currently an urgent need in the marketplace for improvement in the efficiency of printing inks, with demand increasing measurably for systems that are capable of doing more with less. Possible routes to increasing efficiency include having versatile products, product lines or in-plant systems that can be used in a number of different applications. Development of more productive and/or faster processes is an alternative approach to increasing efficiency that can provide cost savings for the overall manufacturing process. Products recently launched by Allnex, EBECRYL 452 and EBECRYL 3203, enable the improvement of the efficiency via both strategies (versatility and enhanced productivity/competitiveness) in the grinding step, thus allowing our partners to have cuttingedge formulations aligned with the most up-to-date global market demands. The importance of grinding media and the improvements obtained using these new products are discussed below. INFLUENCE OF THE GRINDING STEP ON THE FINAL INK A fully prepared ink that is ready for application is produced using the following two basic steps: 1. Pigment paste (concentrate) production: Grinding Step. 2. Addition of additives and final ink adjustment to achieve the desired physical/chemical properties (reactivity, leveling/slip, etc.): Let-down Step.
Of these two steps, the grinding step is the most important, not only because it is the step that introduces the color to the system, but also because the quality of the pigment paste directly impacts the quality of the final ink (Figure 1). Figure 1. Visual difference between poor (left) and high quality (right) pigment concentrates The key parameters of a pigment concentrate include the pigment wetting and dispersion capability and the viscosity. Pigment wetting & dispersion capabilities: o This means how efficiently (how good and fast) the grinding media (resins and additives) may wet and disperse the pigments in order to achieve the optimal color strength, density and stability of the dispersed pigments. Wetting and dispersion occur during the grinding step, which occurs in three stages (Figure 2). First, the larger pigment particles, or aggregates, that are present in a pigment when it is first combined with the grinding medium, are wetted by the medium, which stabilizes the surfaces of the particles. Second, once the surfaces of the aggregates are stabilized by the grinding medium, they begin to break apart into smaller particles (de-aggregates), which enables better color development because the chromophores are then fully exposed throughout the dispersion. Finally, the grinding medium helps ensure that the de-aggregates remain stable and dispersed and do not settle or re-aggregate, which would result in a poor quality pigment concentrate.
Figure 2. Stages of the grinding step for pigment concentrate production. Viscosity and processing characteristics The viscosity range of the raw materials used in the pigment concentrate determines the type of equipment and processing that is required for pigment paste production. Typically, a 3- roll mill is used for higher viscosity pigment formulations, while a bead (pearl) mill, which may be horizontal or vertical, is used for lower viscosity pigment pastes (Figure 3). (a) (b) Figure 3. (a) Triple-roll mill (image courtesy of Exakt Mills) and (b) bead/pearl mill (image courtesy of Buhler Mills) CONVENTIONAL UV GRINDING MEDIA In energy curing systems, polyester based acrylates are specially used for organic pigments on grinding and pigment paste production due to their good pigment wetting characteristics. A typical pigment past formulation for energy-cured inks includes oligomers and monomers, a dispersing agent, a stabilizer and the pigment. A very basic starting-point formulation is presented in Table 1.
Table 1. Basic starting-point formulation for pigment paste production intended for use in energy-cured systems Component % Oligomer 45 60% Monomer 10 15% Dispersing agent 2-5 % Stabilizer 1% Pigment 30 40% When choosing the oligomers used in a pigment paste formulation, it is critical to consider not only their grinding performance, but also the impact they will have on the final ink formulation. Because the oligomers in a pigment paste are also reactive, when an ink is cured, the oligomers present in the pigment paste will also be polymerized into the film and can influence the film properties. In addition, the oligomers in pigment concentrates can also have an impact on the viscosity of a final ink formulation. As mentioned above, polyesters are most often used as the oligomers, not only because of their grinding performance, but also because they have a positive impact on the properties of the final ink formulation. The range of polyester options is quite varied in order to meet the performance criteria for different types of energy cured inks. More flexible polyesters, such as EBECRYL 810 or EBECRYL 411, which are more fluid and have good flexibility but are slightly less reactive are typically used for flexography systems, while chlorinated polyesters, such as EBECRYL 436 and EBECRYL 438, often used for lithography applications, brings also additional features as a good adhesion profile, are. In some cases, highly reactive polyesters are needed, such as EBECRYL 450, which has hexa-functionality. The role of the monomer is as a diluent. Thus the choice of monomer is largely based on its dilution capability. However, because pigment wetting is crucial in a pigment concentrate, the monomer should, in combination with a good dilution capability, also have good pigment wetting properties. As a result, the most widely used monomer is OTA-480. A typical pigment paste for energy cured inks as formulated following the above guidelines, with pigment concentration up to 40% (usually lower than this) is typically highly viscous, and must be ground in a 3-roll mill. While these mills are very effective, the grinding process is often very time consuming and thus has a negative impact on the cost of the overall manufacturing process. NEW HIGH-PERFORMANCE OLIGOMER ACRYLATES FOR IMPROVED GRINDING PERFORMANCE In order to meet the market needs for improvements in both processability and versatility, Allnex has developed and recently launched two new high-performance oligomer acrylates as grinding media for the formulation of pigment pastes intended for use in energy cured inks and coatings. EBECRYL 452 and EBECRYL 3203 are low viscosity oligomers with increased pigment load capability for high-performing systems.
These two new resins, which combine low viscosity with excellent pigment wetting properties, provide both process and formulation advantages: Because the new oligomers are low viscosity UV grinding resins, pigment concentrates based on this technology can be produced using bead mills. Because of their excellent pigment wetting properties and low viscosity, pigment concentrates with pigment loadings of 40% or higher can be produced using three roll mills. In addition to the ability to formulate low viscosity pigment concentrates, it is also possible to prepare and process pigment pastes with higher pigment loadings using a three roll mill. Furthermore, the new oligomers provide formulators with increased versatility: Including TSCA and REACH approval, both oligomers have broad regulatory status. The resins can be used for a wide range of organic and inorganic pigments and have been successfully tested with different pigment quality grades. The resins can be used to formulate a very wide range of final paste viscosities. As a result, it is possible to use them as the sole in plant grinding oligomers for the preparation of all different types of final ink systems (flexo, litho, screen), generating savings by reducing inventory and logistics costs and increasing the efficiency of manufacturing processes. Due to its low viscosity, EBECRYL 452 is suitable for use on inkjet dispersions applications Formulation with the new resins make it possible to avoid the use of lower molecular weight and low functionality products. EBECRYL 452 is a polyester triacrylate designed for organic pigments. It is a dark amber resin with a viscosity of approximately 600 mpa.s at 25 C. EBECRYL 3203 is a polyol acrylate designed specifically for black pigment concentrates and inorganic pigments and fillers. It is an amber resin with a viscosity of approximately 1000 mpa.s at 25 C. Representative pigment paste formulations designed for use in bead mills and three-roll mills are shown below. Table 2 presents the low viscosity pigment paste concentrate formulations designed for grinding in a bead mill. Note that these formulations include typical pigment loading levels (on the order of 40%). The formulations designed for grinding in a 3-roll mill are presented in Table 3. In these cases, higher pigment loads of up to 50% were used.
Table 2. Bead mill formulations using EBECRYL 452 and EBECRYL 3203 Yellow Magenta Cyan Black Stabilizer 1 1 1 1 Dispersing agent 5 6% 5 5.5% 5 6% 5 5.5% Pigment 35-40% 40-45% 35-40% 40-45% OLIGOMER Up to 100% Up to 100% Up to 100% Up to 100% Table 3. Three-roll mill formulations using EBECRYL 452 and EBECRYL 3203 Yellow Magenta Cyan Black Stabilizer 1 1 1 1 Dispersing agent 6 6.5% 4.5 5% 6.5 7% 5 5.5% Pigment 42-45% 45-50% 40-45% 40-45% OLIGOMER Up to 100% Up to 100% Up to 100% Up to 100% Pigment pastes produced with the new low viscous systems enable the production of more versatile and efficient pigment pastes. As a result, it is possible to use them as the sole grinding oligomers for a full printing ink range. Below can be seen comparisons of the viscosities and rheological profiles of systems produced using the standard oligomer EBECRYL 450 and the new grinding medium EBECRYL 452 (Figure 4).
Figure 4. Comparison of the viscosity and rheological behavior of EBECRYL 452 with the standard oligomer EBECRYL 450 on typical pigments. Conclusion Formulators of energy-cured inks are looking to improve the efficiency of their manufacturing processes and increase product performance. Two new acrylates developed by Allnex address these important needs. EBECRYL 452 and EBECRYL 3203 are high-performing grinding oligomers that allow more efficient and versatile pigment pastes production. With these new grinding media, it is possible to: Achieve pigment wetting and dispersion properties that are better than those obtained with standard resins Achieve the same color strength, density and stability in ink formulations with less pigment concentrate; Produce inks with higher optical densities using existing formulations. Grind typical pigment concentrate formulations using a bead mill; Grind high concentration pigment pastes using a 3-roll mill; Increase ink formulating options with the capability to use pigment concentrates with higher pigment loadings. These new products are just the most recent examples of new technologies developed by Allnex to address the needs of our customers for improved performance and processing efficiency. The exploration of new chemistries is ongoing, and further developments in resin technologies for energy cured inks and coatings will be forthcoming. Notice: trademarks indicated with the, or * are registered, unregistered or pending trademarks of Allnex Belgium SA and its direct and indirect affiliates. Disclaimer: Allnex Belgium SA in its own name and on behalf of its directly or indirectly affiliated companies (collectively, "Allnex") decline any liability with respect to the use made by anyone of the information contained herein. The information contained herein represents Allnex's best knowledge thereon without constituting any express or implied guarantee or warranty of any kind (including, but not limited to, regarding the accuracy, the completeness or relevance of the data set out herein). Nothing contained herein shall be construed as conferring any license or right under any patent or other intellectual property rights of Allnex or of any third party. The information relating to the products is given for information purposes only. No guarantee or warranty is provided that the product and/or information is adapted for any specific use, performance or result and that product and/or information do not infringe any Allnex and/or third party intellectual property rights. The user should perform its own tests to determine the suitability for a particular purpose. The final choice of use of a product and/or information as well as the investigation of any possible violation of intellectual property rights of Allnex and/or third parties remains the sole responsibility of the user. 2013 Allnex. All Rights Reserved.