Editor’s Note: “Novel Lamination Ink Resin” received the 2012 Lawter Best Paper Award during the 2012 National Printing Ink Research Institute’s (NPIRI) Annual Technical Conference.
The flexible packaging market is very complex, composed of packaging materials made from composites of film, foil and paper.Printing inks for film and foil flexible packaging are a wide and diverse market dominated by solvent-based ink systems.
The film printing market from an ink perspective can be further divided into surface and lamination applications.Inks for single web, surface or reverse printed webs are used to package baked goods, confectionery or other low-barrier applications.Inks for multi-laminated structures are typically reverse printed on the outer web then laminated to the secondary webs with an adhesive and used for more demanding barrier applications such as snack food, meat and poultry, coffee and dairy applications.This paper focuses on the lamination segment.
Typical laminates are produced by coating a secondary or inner film(s) with an adhesive, drying the adhesive (if solvent or water based) and then combined together with the primary or outer film, which is typically printed, as depicted in Figure 1.The flexible packaging industry is becoming more complex as a variety of substrates, as well as a broad range of adhesives with different performance properties, are introduced into the market.Various types of adhesives are used to bond the various films.The adhesive could either be molten polyethylene, also referred to as extrusion lamination, solvent- or water-based adhesives, or solvent-less lamination utilizing 2K urethane chemistry.Under heat and pressure, an industrial laminator combines the structure.
Printing Inks for Laminations
Printing ink formulations must be suitable for complex packaging and meet all fitness-for-use requirements.The inks for laminations have to withstand the lamination process, while yielding bond strengths consistent with the product application.In addition, these inks have to print consistently at high speeds, i.e. minimal down time and maintain a high level of graphic quality over long periods.
To meet these increased demands, ink manufacturers must formulate inks that meet all of the performance requirements.Adhesion to a variety of substrates, good bond strengths, compatibility with various adhesives as well as good opacity for white inks and good print quality for colors are the primary requirements of successful ink formulations.Various printing applications require different line screens and anilox cell volumes.The trend is to utilize higher line screen, lower volume anilox rolls to enhance the quality of the graphic image. These lower volumes, higher line screen anilox rolls require the use of high strength inks to maintain the color intensity at lower volumes.The combination of high strength inks with higher line screen anilox rolls give printers the opportunity to reach for higher graphics with finer vignettes, crisp lines and type, and higher quality process work.
As Nitrocellulose (NC) is widely used as the grinding resin for many different types of pigments as well as many ink systems, there is a need for ink lamination resins that have good NC compatibility.This paper discusses the performance of lamination inks based on different resin chemistries when combined with NC-based color dispersions.The first ink was formulated with a polyurethane (labeled as Polymer-A).The second ink was formulated with a newly developed polyester resin (labeled as Polymer-B).Both inks were prepared using NC-based dispersions.Standard lamination ink test protocols were used.
Novel Polymer-B is a multi-functional polyester with a relatively low molecular weight as compared to Polymer-A.While most low molecular weight polymers used as co-binders are incorporated at low levels, typically less than 20 wt% of total resin solids, it was found that Polymer-B can be blended with NC in a broad range.In addition, it is noted that ink formulations prepared with 65~80 wt% of Polymer-B to total resin solids yielded the highest bond strength values in this study.
Characterization of Polymers (Polymer-A, Polyurethane vs. Polymer-B, Polyester)
Properties of Ink Resins
Polymer-A was a non-reactive, high molecular weight polyurethane synthesized by a di-isocyanate with a polypropylene glycol.Polymer-A was synthesized at two molecular weights (Polymer A-1 and Polymer A-2).Polymer-B, a saturated polyester, was synthesized by reacting a di-acid monomer with a monomer with hydroxyl functional groups.See Table 1.
Rheology of Resin Solutions
Viscosities of typical polymer solutions depend on concentration and size (i.e., molecular weight) of the dissolved polymer.It was expected that the measured solution viscosity would be higher for higher molecular weight polymers.As depicted in Figure 2, the viscosity of Polymer-B solution is much lower than either Polymer A-1 or A-2 in a solvent blend of 80% N-propanol with 20% N-propyl acetate.
The resin solution with a lower viscosity is potentially capable of making a coating or ink with higher solids, or lower VOC.In fact, total solids of the white ink made from Polymer-B was over 6% higher than Polymer A-2; Polymer-A-1 was about 4% higher than the one made from Polymer-A-2 as shown in Table 2.
Ink Compositions and Performance
Both white and blue ink compositions and properties are outlined in Tables 2 - 5.White inks were prepared with Polymer-B and two polyurethanes; Polymer-A-1 with a molecular weight (Mw) ~20,000 and Polymer-A-2 with Mw ~35,000.The viscosity / solids curves for the white ink formulations correlate with the molecular weight of polymers, as shown in Table 2.However, the lamination performance does not correlate across the tested samples.Within the same polymer chemistry, i.e. polyurethane, a higher molecular weight yielded higher bond strength, as shown in Ink-A-1 vs. Ink-A-2.The ink based on Polymer-A-1 was higher in solids vs. the A-2 ink, but lamination bond strength of the Polymer-A-1 ink was much lower than the Polymer A-2 ink in both PET//PE and OPP//OPP experiments.However, the molecular weight of Polymer-B is much lower than either A-1 or A-2; it formed a much higher lamination bond strength than Polymer-A-1.The white ink formulated with Polymer-B has the unique combination of higher solids plus high bond strength as compared to the inks based on the two variations of Polymer-A.
Color ink formulations utilizing NC-based phthalocyanine blue dispersions were also evaluated using Polymer-A-2 and Polymer-B.Note: Polymer-A-1 was not further evaluated due to its poor lamination results in the white ink formulation test.Table 4 and Table 5 illustrate ink compositions and properties.Lamination bond strength was measured for PET//PE and OPP//OPP structures with a solvent-less adhesive.At the print viscosity, total ink solids in the blue ink with Polymer-B were approximately 6% higher than Polymer-A-2 ink.
Ink-A-2 and Ink-B, made with Polymer-A-2 and Polymer-B, respectively, have equal lamination bond strength performance in PET//PE and OPP//OPP structures.The ink formulations demonstrated that Polymer-B allows higher solids, stronger inks (i.e. less solvent or lower VOC) with equivalent lamination bonds.
Characteristics of Polymer-B, the Novel Polyester Ink Resin
• High solids inks made with Polymer-B
As discussed previously, Polymer-B not only has very good lamination bond strength but also enables high solids inks.Higher solids, higher pigmented inks allow converters to use a finer line, lower volume anilox rolls for higher quality printing and may help to enable increased press speeds.
Ink rheological behavior at press is complex.The solids volume fraction (SVF) influences the rheological characteristics of a solid/ liquid dispersion as an ink.In a pigment suspension, the shape and packing of solid pigment particles during flow affects the viscosity.Resin is
typically dissolved in the ink solvents, but will gradually precipitate out when solvent evaporates during the drying process. For this reason, researchers will often focus on determining the maximum packing fraction (MPF, φm) of an ink system.MPF is defined as the highest volume fraction of solids, also called the immobilization point, which is the given solids point at which the ink has lost any plasticity or deformation ability, and its viscosity increases abruptly to a very high value.
Figure 3 shows viscosities of white inks made with Polymer-A-2 and Polymer-B, respectively, as a function of increasing solids volume fraction (SVF).Ink viscosities are exponentially correlated with SVF.Noticeably, Polymer-A-2 ink viscosity is much higher than Polymer-B ink viscosity at a given solids level.In other words, the ink formulated with Polymer-B, at print viscosity, contains approximately 30% more solids (by volume) than the Polymer-A-2 ink.Therefore, Polymer-B can increase formulation latitude and enable higher pigment loadings.Both attributes have the potential to improve the graphic capabilities of the ink.
•Lamination bond strength of inks made with Polymer-B
Lamination bond strength is assessed by delaminating the printed specimen with the use of an Instron tension tester or equivalent.In a peel experiment where the lamination sample is being separated, the behavior of the peel forces for cohesive and adhesive failure can be observed by checking the peeled interface.Cohesive failure occurs when fracture appears within either the ink or adhesive if the system experiences an external force of sufficient intensity.However, if the adhesion at the interface is lower than the cohesion of the ink or adhesive layer, adhesive failure is observed at either interface between ink and substrate, between ink and adhesive, or between adhesive and substrate.In lamination applications, the printing ink can be designed to incorporate functional polymers that will result in optimal behavior, i.e. high peel energy and adhesive failure, for a given film construction.
The effect of Polymer-B on the peel strength of polyester-
Figure 4. Peel energy of lamination as a function Polymer B in the ink; an optimum concentration is about 73% of Polymer B.
polyethylene film lamination has been examined with a solvent-less adhesive by formulating Polymer-B with various amounts of NC, as shown in Figure 4.Peel energy for delaminating the laminate samples increased as the percentage of Polymer-B increased, then drastically dropped in cohesive fracture mode.The ink formulated with ~73% Polymer-B in total polymer solids presented the optimum peel strength.
It is well known that nitrocellulose (NC) resin, by itself, is rigid, inflexible, with poor adhesion to polymeric substrates.It is usually modified with plasticizers or blended with co-resins to achieve acceptable performance.Figure 4 illustrated the synergistic effect of combining Polymer-B with NC in an ink formulation.The ink system incorporates NC, which provided cohesive strength due to its relatively high molecular weight (Mw ~36,000 and Mn ~10,000 Daltons), and Polymer-B, which augmented adhesive strength through its functional moieties.An ink system with optimal bond strength can be generated by carefully balancing the amount of NC and Polymer-B.
Flexible film lamination bond strengths are highly correlated with ink resin compositions.Novel ink resins can be developed that will improve upon existing technologies and satisfy the performance improvement requirements of high quality flexographic printing of laminated packaging.This new resin technology enables ink formulators to design inks that exhibit the desired properties of high color strength, high opacity, high solids, lower VOC and as well as suitable bond strength and good print characteristics.
• Harper web site information.
• PAPEL vol. 72, num. 11, pp. 55 - 59, Nov. 2011
Chien Lu (Charlie) Hsu is senior research scientist, Resins & Performance Additives, Printing & Packaging, North America for BASF Corporation. Born in Taiwan, Mr. Hsu earned a master’s of science degree in chemical engineering from Lehigh University, Bethlehem, PA, and a bachelor of science in chemical engineering from the National Taiwan University in Taiwan.Mr. Hsu has more than 20 years’ experience in the printing ink and polymer industry, and has been teaching the rheology section for the NPIRI summer course for more than seven years.Mr. Hsu joined BASF in 2008, and is the senior research scientist for BASF’s packaging printing inks. He is responsible for BASF resins used in ink and overprint varnish applications for the printing and packaging industry in North America, headquartered in Wyandotte, MI.
Rick Grandke is industry manager, Resins & Performance Additives, Printing & Packaging, North America for BASF Corporation. A native of Michigan, Mr. Grandke earned a master’s degree in business administration, from Clark University in Massachusetts, and a bachelor of science in management from Wayne State University in Michigan.Mr. Grandke has more than 35 years’ experience in the ink and polymer industry, and has held several technical management positions with Inmont BASF and Johnson Polymer.Mr. Grandke re-joined BASF in 2006, and is currently industry manager for BASF’s printing and packaging resin business. He is responsible for BASF resins used in ink and overprint varnish applications for the printing & packaging industry in North America, headquartered in Wyandotte, MI.