MAKING IMPROVEMENTS IN ENERGY CURABLE PLASTIC SUBSTRATE ADHESION PERFORMANCE

Transcription

1 MAKING IMPROVEMENTS IN ENERGY CURABLE PLASTIC SUBSTRATE ADHESION PERFORMANCE Paulo Roberto Vieira Junior, Marc Heylen, Paul Gevaert, Francis Bergiers, Luc DeWaele, Rosalyn Waldo, and Peter Yuan. Allnex ABSTRACT Over the past decade, significant improvements have been made in the overall print quality, curing speed and cost of ownership of UV Flexo inks for consumer product printing applications. While Flexo technology continues to gain acceptance and take market share away from Lithography, both printing techniques still have ample room for improvement with respect to their adhesion to plastic substrates. Herein is presented a review of the energy-curable chemistries currently available to the market for rigid and flexible non-porous plastic printing applications. Also discussed in this paper are approaches developed by Allnex for each printing technique. Results provided demonstrate step change improvements in adhesion performance for both flexible and rigid substrates commonly available to printers and converters in the Americas. INTRODUCITION Plastic substrates (filmic, flexible and rigid) were introduced several decades ago, but recently have become increasingly popular as cost-effective alternatives to metal and wood, particularly in the packaging and consumer products segments. Because packaging is an important aspect of consumer product brand identity, printing inks must be durable and high-performing. In the packaging sector, the most widely used printing techniques for plastic substrates are flexography, lithography, gravure and screen printing, and a large amount of inks are formulated as energy-curable systems. Given the great variety of possible polymer chemistries, structures, and compositions (Table 1), as well as the wide range in quality, found in the plastics that are currently on the market, there are significant variations in substrate characteristics that present challenges to ink formulators and printers. The development of inks that exhibit high adhesion to the many different plastic substrates is the biggest challenge. There is, as a result, strong demand in the packaging sector for inks that are robust, highperforming and have very broad applicability in terms of adhesion properties to different plastic compositions.

2 Table 1. Commonly used plastic packaging substrates Polyethylene Terephthalate (PET) Polyethylene Terephthalate, glycol modified (PET-G) Polylactic Acid (PLA) Oriented Polystyrene Bioriented Polypropylene (BOPP) Polystyrene (PS) Polycarbonate (PC) Polyethylene (PE) Poly Vinyl Chlorine (PVC) Poly Methyl Methacrylate (PMMA) MAIN PARAMETERS AFFECTING ADHESION Specific properties of the resins found in ink formulations play an important role on adhesion performance of the applied ink. The most important properties include the affinity of the polymeric backbone to the substrate, the polarity, the shrinkage of the cured film and the glass transition temperature (Tg). Pre-treatment of the substrate can also have a significant influence on the ability of an ink to adhere to it. One of the primary parameters that determine adhesion performance, however, is surface tension. The concept of surface energy can be more easily understood when considering a liquid, where molecules in the center of a liquid may freely move in order to reach positions that have the lowest potential energy (law of thermodynamics). At these sites, the forces (attractive and repulsive) acting in all directions on the molecules are in equilibrium. The molecules on the surface of the liquid, on the other hand, experience only the forces that are directed toward the center. As a result, surfaces, including the surfaces of solid substrates and liquid coatings, are always high energy regions. The difference between the energies on the surface and those in the interior is called SURFACE TENSION (Figure 1). Figure 1. Surface tension at the liquid-solid interface. A challenge in radiation curing formulations when dealing with surface tension comes from an often polar profile of the components that are commonly used in such systems. The polarity of the ingredients leads to higher surface tensions, thus impacting the leveling and adhesion performance on certain substrates.

3 The polarity of a given ink formulation is essentially determined by the choice of oligomers and monomers, and more specifically by the type and number of functional groups present in its ingredients. Polar groups, such as hydroxyl or carboxyl groups, increase the surface tension, while nonpolar groups, such as long alkyl chains, siloxanes, and (fluoro)alkyl groups, reduce it. Plastics present a particular challenge, because they are naturally low surface tension substrates. Some plastics, in fact, have a surface tension of 32 dynes/cm or less. Most coatings also have surface tensions in this range. The general literature suggests the plastic parts must have a surface energy that is 8-10 dynes/cm greater than that of the ink in order to obtain optimal adhesion. Complicating the issue is the fact that, for plastics compounded using blends of polymers, the surface energy can vary across the surface of a single plastic substrate. Incorporation of recycled materials and polymers of varying quality in the production of the substrate can also lead to small variations in dyne from one region to another. Values for the surface tension of selected acrylate monomers and oligomers and various plastics are presented in Table 2.. Table 2. Surface tension values for typical acrylate products and widely used plastics.

4 CONVENTIONAL ROUTES TO IMPROVING THE ADHESION OF ENERGY-CURED FORMULATIONS AND THEIR LIMITATIONS There are several common strategies that are employed to ensure the proper adhesion of inks to plastic substrates. Some are related to formulation choices, while others involve modification of the substrate surface in some way. One approach involves lowering the Tg of the ink formulation. Typically, to achieve this goal, low Tg ingredients, such as acrylic acrylates, are selected. The tack of formulations with lower Tg values increases, and as a result, the films are stickier and adhere with greater strength to substrates. EBECRYL 745 from Allnex is a resin system for energy-cured inks that has a reduced Tg value. EBECRYL 767, meanwhile, is an acid modified acrylic acrylate with a low Tg. Lowering the Tg is also important when considering that less reactive compounds, meaning less double bounds in the system will lead to a lower shrinkage of the film, helping on the adhesion. A second strategy is to work around the interaction between the film phase and the substrate by using modified components, such as: EBECRYL 436, EBECRYL 438 where chlorination plays an important adhesion promoting role in polyester acrylates or EBECRYL 524 which is acid modified. A third approach involves developing a balance between the surface tensions of the ingredients used in the overall ink formulation by using carefully selected blends of monomer and oligomers, such as has been achieved with EBECRYL 303. A fourth strategy involves the addition of acidic modified methacrylates adhesion promoters to the ink formulation to enhance adhesion performance. These adhesion promoters work based on an acidic characteristics provided by such products. Examples of adhesion promoters from Allnex include EBECRYL 168, EBECRYL 170 and EBECRYL 171; With respect to modification of the substrate, pre-treatments, such as corona or flame treatment, can help polarize the surface and increase the surface tension, leading to better substrate wetting conditions. Primers, typically chlorinated systems, have also been used to promote adhesion establishing a more compatible interlayer. Although these strategies have proven to be very beneficial for the improvement of adhesion, they are not 100% effective solutions for the ever increasing number of plastic substrates used in packaging applications that are produced from blends of polymer resins with differing physical properties and suffering from variations in surface energy across their surfaces. NOVEL CHEMISTRIES FOR IMPROVEMENT OF THE ADHESION OF ENERGY-CURABLE FORMULATIONS Specially designed modifications of the typical backbone chemistries of resins used in energycured ink formulations are creating possibilities in terms of adhesion improvement and robustness for different types of substrates. Because of the good balance that epoxy acrylates offer between performance and cost, they are widely used in energy-cured coatings for wood, metal and plastics, overprint varnishes (OPVs), and lithographic and silk screen inks. Allnex has developed and introduced to the market EBECRYL 3415, a modified epoxy acrylate suitable for screen printing inks that exhibits excellent reactivity and good adhesion to various nonporous plastic substrates, including polycarbonate, polystyrene, glycol-modified polyethylene terephthalate (PET-G), rigid, matte and glossy vinyls and polyvinyl chloride (PVC). As can be seen in Figure 2, even matte finishes with good adhesion over plastic substrates may be achieved when using EBECRYL 3415.

5 Figure 2. EBECRYL 3415 matte finish formulation applied over semi-glossy polystyrene. Polyester acrylates are attractive because of their lower viscosities and reasonable cost. For this category of oligomers, Allnex is developing chlorine-free solutions that address current environmental concerns about the ingredients in ink formulations whilst ensuring adhesion to a broad range to substrates. Recently launched products in Allnex s portfolio include EBECRYL 571 for lithography inks (Figure 3) and EBECRYL 411 for flexography inks (Figure 4). In Figure 3, EBECRYL 571 was used for a shrink sleeve application over a PET-G substrate, with no wrinkling and good transverse directional flexibility, color maintenance and adhesion possible even at shrink rates greater than 60%. Figure 3. EBECRYL 571 shrink sleeve application on PET-G. From the images shown in Figure 4, the good printability, and adhesion can be seen, with a good reactivity and low color. EBECRYL 411 is a versatile, low viscosity, competitively priced oligomer for non-food contact UV/EB Flexo and Gravure inks, with excellent adhesion to a variety of corona treated, un-primed filmic substrates.

6 Figure 4. EBECRYL 411 narrow web application on BOPP With these new products added to Allnex s existing portfolio of resin and additive systems designed for improving the adhesion of energy-cured inks on plastic substrates, formulators have a wider range of options and increased flexibility for developing effective solutions for all of their printing ink applications. A summary of the available products from Allnex and they different plastics substrates for which they are suitable for adhesion improvement is presented in Table 3. A summary of Allnex enhanced adhesion performance energy curable resins and their substrate compatibilities is presented below in Table 3. Table 3. Adhesion performance products

7 CONCLUSION Currently available energy curable chemistries combined with newly developed technologies designed by Allnex for different printing techniques offer a step change improvement in adhesion performance for inks used in plastic packaging applications. Improvement in adhesion has been demonstrated on the flexible and rigid substrates commonly available to printers and converters that are widely used in rigid and flexible non-porous plastic printing applications. Existing and newly developed energy curable resin chemistries designed by Allnex offer a step change improvement in adhesion performance for inks used in plastic packaging applications. Improvement in adhesion has been demonstrated on the flexible and rigid substrates commonly available to printers and converters that are widely used in non-porous plastic printing applications. Further work is also underway to develop additional options for improving the adhesion robustness of energy-cured ink formulations and address the need for high performance on a wide range of plastic materials with varying surface characteristics both between and within different substrates. Allnex is committed to bringing value to ink formulators by developing innovative resin and additive solutions that address the challenges that they face. 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 Allnex. All Rights Reserved.