By Jo Ann Arceneaux, Ph.D., allnex USA, Inc.; Michel Tielemans, allnex Belgium; Kevin Poelmans, allnex Belgium; Laurence Boutreau, allnex Belgium
Energy-curable polyurethane dispersions (PUDs) are a fairly recent development. Their inherently low viscosity and combination of hardness and flexibility make them especially suitable for sprayable wood coatings. Other industrial applications, such as plastic and metal coatings, also have seen utility. Their use in graphics applications was more limited, however. With the increasing regulatory pressure on acrylate monomers, the energy-curable graphics market, and especially the inkjet market, has started to investigate the use of energy-curable waterborne resins and PUDs. Initial evaluations showed that modifications of the PUDs were necessary for use in graphics applications. The redesign of the energy-curable waterborne polyurethane dispersions for applicability in the inkjet market will be presented.
Initial Design of Energy-Curable PUDs
Energy-curable (EC) PUDs are prepared through several steps. The first step is the formation of a polyurethane prepolymer through the reaction of a diol, diisocyanate and diol acid in solvent. In a second step, the polyurethane prepolymer is capped with hydroxyalkyl acrylate. Neutralization and dispersion in water is step three. Finally, the solvent is removed in step four (Figure 1).
The structure of the EC PUD can be varied through choice of isocyanate, which builds the hard segment of the polyurethane. The isocyanates can be aliphatic or aromatic, and di- or higher in functionality. The hard segments of the polyurethane align through hydrogen bonding and provide hardness to the cured ink or coating. The diols can be polyethers, polyesters, acrylics, hydrocarbons or silicones. These are usually di-functional and build the soft segment of the EC PUD, which provides flexibility. The diol acid, which is usually dimethylol propionic acid, provides colloidal stability, increases adhesion and decreases flexibility through hydrogen bonding. The hydroxyalkyl acrylate is the source of acrylate functionality and can be mono- or higher in functionality. The combination of hard segments and soft segments in a polyurethane, and their alignment through hydrogen bonding, provide the toughness that is typical of this type of material. The typical solids content of an EC PUD is 35 to 40%, and its viscosity is less than 200 cP at room temperature.
Compared to a 100% solids urethane acrylate formulation, the EC PUD has lower crosslink density or higher molecular weight between crosslinks. It also has more urethane hard segments. The combination of these two properties provides a cured ink or coating that is both hard and flexible, a property that is difficult to achieve with a 100% solids urethane acrylate (Figure 2).
Wood coatings are typically spray applied. Because 100% solids urethane acrylates are typically high in viscosity, they require acrylate monomer dilution to obtain the necessary spray application viscosity. This can lead to a loss or decrease of the urethane acrylate properties, less adhesion due to increased shrinkage and increased odor from the lower molecular weight monomers. Also, the 100% solids urethane acrylate formulations are not easily matted because of the viscosity increase upon addition of matting agents. Water-based (WB) wood coatings allow ease of matting but lack scratch and chemical resistance and have poor gloss.
EC PUDs combine the good properties of both the 100% solids and WB formulations to provide wood coatings with no odor, no diluting monomers, good matting properties, less shrinkage, improved adhesion, and good chemical and scratch resistance. They also are easy to spray and stable in curtain coatings. Thus, the EC PUD-based wood coatings provide UV performance with WB processability, a synergistic value proposition.
EC Inkjet Requirements
Two major inkjet application techniques are continuous and drop-on-demand. Drop-on-demand is further divided into piezo and thermal (bubble jet) technologies. The 100% solids EC inkjet inks are typically applied through piezo technology. Because of viscosity constraints, the 100% solids inkjet ink is generally heated to 40°C to 60°C before jetting. EC waterborne inkjet inks could be jetted by either piezo or thermal technologies.
There are many requirements of an EC water-based inkjet ink, and some of these are discussed below. The inkjet ink must be stable, both during application and during storage. An elevated temperature stability test over a set period of time is the norm (45°C to 60°C over various time periods). Viscosity, pH, particle size and coagulum are monitored during this test. The inkjet ink must be efficiently filterable to remove particles that may block or damage the inkjet head. The inkjet ink must be jettable using whichever inkjet head is specified. Jettability is typically a function of surface tension and rheology. The inkjet ink must be resoluble before cure to avoid clogging the inkjet nozzles. Finally, the inkjet ink must be reactive or curable under the lamp and speed conditions of the specified press, and the cured ink must provide the adhesion and resistance properties required by the end application.
Figure 3 shows the results of a stability test of an EC PUD (IRR 929) held for nine to 10 days at 60°C. The IRR 929 was diluted to 10% solids for this test. The particle size of the EC PUD decreased only slightly over nine days (80 nm to 75 nm). Over a 10-day period, there was a decrease in pH from 7.9 to 7.1, and in viscosity from 1.9 to 1.5 cP at 25°C. All of these results indicate a stable system. Two EC PUDs were tested for resolubility after they were dried. Figure 4 shows that both IRR 928 and IRR 929 are resoluble, with IRR 928 having better resolubility.
Similar to wood coatings, 100% solids EC inkjet inks must contain acrylate monomers to reduce viscosity. However, in inkjet inks, higher levels of acrylate monomer are used, and more of the monomer is mono-functional due to the very low viscosity requirements. This monomer use results in odor and safety concerns, and recent REACH regulations may exclude some of the commonly used mono-functional monomers. The inkjet-printed product also has high dry film thickness and high shrinkage.
Water-based inkjet inks may dry in the inkjet nozzles, causing blockage, and have poor durability and adhesion to plastics. These water-based inks also contain emulsifiers and coalescents, leading to VOCs.
Inkjet inks based on EC PUDs separate the drying step from the curing step and are resoluble, so these inks do not block nozzles. These inkjet inks do not contain acrylate monomers, emulsifiers or coalescents; thus, they have low odor, low VOCs and ease regulatory and environmental concerns. The inkjet-printed product has lower dry film thickness, providing a better hand/feel. This product also is durable and has better adhesion to plastics due to lower shrinkage. Less crosslink density also results in a more elastic/flexible product. Thus, the value proposition for EC water-based inkjet inks is 100% solids EC performance with improved processability when compared to WB inks and improved safety as compared with 100% solids EC inks.
This value proposition also opens the door to food packaging applications. Food packaging applications require global regulatory compliance (EuPIA, Nestle List, Swiss List, German Ordinance, FDA, REACH, Prop 65…); no use of chemicals of concern; no/low residuals, extractables or migrating species; no/low odor; and no/low VOCs. The EC PUDs can be designed to meet these requirements.
Redesign of EC PUDs for Inkjet
The basic EC PUD structure for an inkjet ink is the same as that of an EC PUD for a wood coating. However, some additional design considerations exist for EC PUDs for inkjet inks. Water-based materials allow for low viscosity without the use of monomers or solvent. The molecular weight of the PUD can be quite high, resulting in low migration potential. The EC functionality provides dense crosslinking, resulting in high performance. The stability, resolubility, adhesion and other performance properties can be varied by controlling the composition of the EC PUD. There are four steps of design strategy for low migration (LM) inkjet inks: eliminating chemicals of concern (tin, BPA, emulsifier, solvent, etc.); maximizing acrylates with higher molecular weight and functionality (> 500 Daltons and >/= 6); meeting basic inkjet properties such as stability and resolubility; and validating low migration and indirect food contact compliance.
Several EC PUDs for inkjet inks have been designed. Regulatory aspects, such as tin-free, BPA-free, APEO-free and label-free, have been addressed and are shown in Figure 5. Figure 6 compares resolubility, migration, reactivity and flexibility. A spider graph in Figure 7 shows the properties of three commercial EC PUDs. Pigment wetting, hardness, adhesion, blocking, resolubility and tack before cure are compared. A variety of properties are available from the three commercial products.
Two experimental products, IRR 928 and IRR 929, were designed for low-migration inkjet inks. Both of these products contain very low levels of products with molecular weights less than 500 and functionality less than 6, which should result in very low migration potential (Figure 8). IRR 928 and IRR 929 were developed based on differentiated models, with the IRR 928 having a lower molecular weight polymer, which also is tacky before cure. Both have a very low viscosity, at 35% solids, and present a low mean particle size associated with good colloidal stability and low grit content. Importantly, the two products also are water resoluble before cure, which makes them very good candidates as inkjet binders for food contact applications (Table 1).
IRR 928 and IRR 929 have excellent UV reactivity (20 m/min) under a conventional Hg lamp at 80W/cm using 1.5% BCPK as the photoinitiator. The UV reactivity is actually increased to 25 m/min using a UV LED 365 nm lamp at 8W/cm² using 1.5% TPO-L as the photoinitiator. All of the tests were made with 15µ thick (wet) films on Leneta® charts with four-minute drying at 60°C before UV cure. The products also can be cured under electron beam, with the advantage of not requiring a photoinitiator. The cured film and adhesion properties of IRR 928 and IRR 929 are shown in Table 2 and in the spider graph in Figure 9. The adhesion was tested one hour after cure. A score of 5 equals perfect adhesion. The polyethylene and polypropylene plastics were corona-treated before coating.
A starting point formulation for an inkjet ink based on an EC PUD is shown in Table 3. An overview of the properties of the EC PUDs is shown in Table 4.
Extraction Tests of LM EC PUDs
For food packaging applications, migration tests and identification of extractables are required. These tests were performed on REFERENCE #1 (same as PRODUCT 1), REFERENCE #3 (same as PRODUCT 3), REFERENCE #4, IRR 928 and IRR 929. (REFERENCE #4 is an EC PUD but not one designed for inkjet inks.) A direct food contact protocol was used to address the most stringent extraction conditions. The polymer dispersions were formulated with a wetting agent (0 0.75%) and applied as a 30µ wet coating on an unwashed aluminum sheet. They were dried for four minutes at 60°C and then cured with an electron beam (250 keV – 5 MRad) with a conveyer speed of 10 m/min. The coated samples were immersed in 95% ethanol for 24 hours at 60°C in a closed glass flask. There was a contact area of 100 ml of solvent per dm2 of coated substrate. The extraction conditions are related to FDA § 176.170 conditions of use H, for paper and paperboard in contact with aqueous and fatty foods.
The alcoholic solution was submitted to GC-MS and LC-MS after collecting reference data from the pure polymer dispersions. Extractable components were identified and detected at suspected ppb levels by single ion monitoring (SIM). The MS response was recorded as peak areas for a comparative analysis without considering that the MS response factors are different for each type of product. The extraction data from the two analytical techniques were merged and normalized so that they could be categorized per product type. As shown in Figure 10, IRR 928 and IRR 929 show very favorable migration data, as does REFERENCE #4. Based on these data, these three products should be acceptable for indirect food packaging applications.
EC PUDs can be redesigned to meet the requirements of inkjet ink applications, including jettability, stability and resolubility. Additionally, these EC PUDs can be modified for low-migration, indirect food packaging applications. Further enhancements can be made to provide for reactivity, pigment wetting, flexibility and/or adhesion to plastics.