By Stephen C. Lapin and Kathryn E. Classon, PCT Engineered Systems, a division of ebeam Technologies, Comet Group
The electron beam (EB) curing properties of acrylate functional monomer/water solutions and polyurethane dispersions (PUDs) in water were characterized. Many of the materials showed good curing properties by EB alone without prior thermal drying. Gravimetric analysis showed effective removal of water from the films even at dose levels as low as 10 kGy. Many of the materials also showed excellent resistance to isopropyl alcohol after curing at low dose levels. The conversion of acrylate functional groups was characterized by FTIR. Potential advantages of water-based EB curing systems include safe handling, low viscosity, low odor, low migration and curing without added thermal drying.
The basic building blocks of UV/EB-curable formulations are acrylate functional monomers and oligomers. A common formulating strategy is to select oligomers based on the desired cured coating physical properties. Monomers serve as reactive diluents to control the viscosity of the formations. This strategy works well except when the desired application method requires very low viscosity. This may be the case for spray coating, vacuum coating, inkjet or gravure printing methods. In order to achieve very low viscosity a formulation may have high monomer content with very little allowance for oligomers. High monomer content may reduce curing speeds and may limit the physical properties of the resulting cured films. Also, from a health and safety perspective, it may not be desirable to spray formulations based on low molecular weight monomers.
An alternate method to reduce viscosity is to use water as a diluent in place of monomers. In this case specialized acrylate functional monomers or oligomers may be used which are soluble or may be dispersed or emulsified in water.1 Acrylate functional polyurethane dispersions (PUDs) are well known and have become well established in commercial sprayable, UV-curable finishes for wood.2 The main disadvantage of water-based UV curable materials is that two drying steps are needed: an initial thermal drying to remove water, followed by UV curing to further polymerize and crosslink the coating.
The UV curing properties of water-based acrylate functional materials have been widely studied and reported; however, very little is known about the EB curing properties of these materials. A US patent reference discloses EB curable compositions containing water. After EB curing, the formulations showed low migration of monomers that were used in the formulations.3 Another patent discloses a flexographic ink system, in which partial evaporation of water from the ink causes a viscosity increase that allows wet-trapping of multiple ink layers followed by EB curing after the last ink layer is applied.4 Water-based materials also were used to develop EB curable gravure ink systems.5
The purpose of this study is to characterize the EB curing properties of acrylate functional monomer/water solutions and PUDs. The results may be used as a basis for exploring new applications for this technology.
Commercial raw materials were used as received. Mixtures were prepared by weighing and hand-stirring the raw materials. Viscosities were measured at 25°C using a Brookfield DV-E viscometer equipped with small sample adapter (s34 spindle) at 50 or 100 rpm.
Coated samples were prepared by draw-down with a #8 wire-wound rod on sheets of 50 micron polyester (PET) film. Coatings were cured immediately after draw-down using a Comet EBLab 200 system operating at 125 kV with an atmosphere of less than 200 ppm O2. Samples were placed in the sample tray, which was run a speed of 18 m/min under the beam. The EBLab system automatically adjusts beam current to provide the desired dose set point.
Coating weights were determined by marking a (2.5×3-inch) area on PET film. The mass of the film and coating were determined (+/- 0.0005 g) after EB curing. The dry weight of coating was determined by removing coating and subtracting the weight of the film. It was difficult to measure weight of the coating before curing since the water would begin to evaporate from the thin film immediately after coating. For the monomer samples, a reference wet coating weight was determined using the monomer without added water. For the PUD samples a reference wet weight of 0.0545 grams (for the 2.5×3-inch area) was used. This was determined using a 100% solids nonvolatile material (Sartomer SR9020) applied with a #8 wire-wound rod. Weight loss was determined by subtracting the coating weight after curing from this reference wet coating weight.
The curing properties of the coatings were characterized by rub testing. Four layers of paper towels were wrapped on the round end of a 1 kg ball-peen hammer. The coating was examined after 10 back-and-forth strokes of the dry paper towel surface on the coating under the weight of the hammer. The same test then was repeated using paper towel layers that were saturated with isopropyl alcohol (IPA). The effect of dry and IPA rubs on the coatings were ranked on the following scale:
0 – No cure/compete removal of coating
1 – Tacky coating/severe smudging
2 – Significant effect from rub test
3 – Slight effect from rub test
4 – No visible effect from rub test
Rub testing also was conducted after thermal drying for three minutes at 80°C in a convection oven without EB curing.
Acrylate conversion was determined by FTIR by comparing spectra before and after EB curing using a Thermo Nicolet Nexus 670 FTIR spectrometer. Samples were prepared by draw-down with a #8 wire-wound rod on 90 micron low-density polyethylene film. A well type ATR cell was used for materials that were liquid before EB curing. For dry samples, the coated side of the film was clamped in a reflectance (ATR) cell on instrument. The samples were analyzed with a 4.0 cm-1 resolution and a 1.2-second scan time. The amount of unreacted acrylate was quantified using the 810 cm-1 peak from C-H out-of-plane bending from the acrylate vinyl group. Absorbance was measured from the baseline of the peak and then normalized to the carbonyl peak at 1730 cm-1. Conversion was determined by the ratio to the 810 cm-1 absorbance for the uncured material.
Web coating and curing tests were run on the EB pilot line at PCT Engineered Systems. Coatings were applied to 50 micron PET using an offset gravure coater equipped with a 130 line/inch, 15.7 BCM gravure roll. This configuration produced a wet coating weight of about 6.0 g/m2. Immediately after coating the web entered a BroadBeam EP series curing unit operating at 150 kV under a nitrogen (2) atmosphere.
Results and Discussion
Four different water-soluble monomers were investigated (Table 1). Monomers A, B, and C are water soluble by virtue of the high degree of ethoxylation. Monomer D is water soluble due to the hydrophilic nature of the two secondary hydroxyl groups. The chemical structure of Monomer D is shown in Figure 1. All four monomers were completely miscible and stable in mixtures with up to 30% water. The viscosities of the monomers as a function of added water is shown in Figure 2. A more rapid reduction of Monomer D viscosity relative to the other monomers was observed.
Figure 3 shows the weight loss for monomers A, C, and D after EB curing at 10, 20, 40 and 80 kGy. It was interesting to see that the weight loss corresponded closely to the amount of added water, indicating that water was completely eliminated from the coating even without any prior thermal drying. In addition, this weight loss occurred even at lowest (10 kGy) EB dose level.
Figure 4 shows the IPA rub resistance of the monomer series as a function of the amount of added water using a constant 40 kGy EB dose level. The IPA resistance of monomers A, B and C was poor even in the absence of water. This was not surprising due to the relatively low amount of cross-link density corresponding to the high level of ethoxylation. The most interesting monomer in this series was D, which had excellent IPA resistance with and without added water.
Figure 5 shows the IPA rub resistance of the monomer series as a function of EB dose for mixtures with 20% water. Again, monomers A B, and C show poor IPA resistance due to the high levels of ethoxylation and do not result in a significant change in IPA resistance even with dose levels up to 80 kGy. Again, monomer D is very interesting, showing excellent IPA resistance with a dose level of only 10 kGy.
Acrylate Functional PUDs
Sixteen different commercial acrylate functional PUDs were investigated (Table 2). Information available from the suppliers indicated solids levels ranging from 35 to 65% for these materials. The weight losses of these materials were determined after EB curing at 10 to 80 kGy. Representative plots of the weight losses for PUDs 9 and 16 are shown in Figure 6. Results were similar to monomers in that weight loss was constant over this dose range. The average weight losses for all dose levels for all 16 PUDs are shown in Table 2. The results closely compliment the solids levels given by the manufacturers. This indicates that all of the water is removed by EB curing even without any additional thermal drying. Furthermore, the water is removed at dose levels as low as 10 kGy.
Table 3 shows the EB curing properties of the PUDs characterized by the dry and IPA rub resistance of the resulting coatings. Some PUDs are designed to give tack-free dry coating layers upon evaporation of water even before curing. This is beneficial for some wood coating applications where it may be desirable to repair the coating layer before crosslinking. PUDs 3, 4, and 8 from this study form tack-free films that are resistant to dry rubs even before EB curing. The low IPA resistance of these materials indicates they do not crosslink without EB curing. A comparison of the IPA rub results before and after EB exposure indicates that all 16 of the PUDs are crosslinked by EB. Most of the PUDs show some increase in IPA resistance between 10 and 40 kGy. Many of the materials were completely resistant to damage by 10 IPA rubs after 40 kGy.
Mixtures of PUDs with monomer C (70/30) were prepared to check for synergistic effects on the curing properties. It was noted that many of the mixtures were not compatible (Table 4). This was indicated by a phase separation immediately upon mixing or sometime after mixing the materials. Even if phase separation did occur, the samples were remixed, drawn-down and EB cured to check for IPA rub resistance. Many of the mixtures did show an enhancement in the IPA rub resistance after 10 and 40 kGy (compare Tables 3 and 4).
Characterization of Curing by FTIR Analysis
The EB curing properties of the acrylate functional monomer/water solutions and polyurethane dispersions (PUDs) in water as characterized by weight loss and rub resistance were impressive. FTIR analysis was also used to get a direct measure of the actual conversion of acrylate groups in the EB curing process. The FTIR spectra at varying cure doses for 80/20 monomer C/water is shown in Figure 7. The spectra for PUD 5 at varying cure doses is shown in Figure 8. The characteristic 810 cm-1 peak of the acrylate group was used for quantification.
Figure 9 shows the acrylate group conversion for monomers A, C and D as a function of EB dose. It is interesting to note that all three of these monomers give more than 97 % conversion at dose levels as low as 20 kGy. This high conversion may be due to the presence of water in the early stages of the curing which may enhance the mobility of the acrylate end-groups.
Figure 10 shows the acrylate group conversion for many of the PUDs. The results show rapid conversion after 10 to 20 kGy with a gradual increase in conversion up to 80 kGy. The conversion for PUDs ranges from less than 60% to more than 90% depending on the PUDs that were used.
Note, conversion data for monomer B and some of the PUDs are missing from Figures 7 and 8. This is due to interfering bands around 810 cm-1 in the FTIR spectra that make it difficult to quantify the conversion.
Web Coating and Curing
The results of the laboratory EB curing of cut sheet samples indicate that evaporation of water and EB cross-linking can occur in a single step. Because of the time delay between coating draw-down and cure (about 30 seconds), there was some concern that evaporation of water may have occurred at room temperature during this delay period. To test the feasibility of a single EB drying step, web coating trials were conducted. In this study the coater was located immediately before the EB system to minimize any ambient drying that might occur. PUDs 2, 4 and 5 along with 70/30 monomer D/water were tested in this manner. In one sequence of tests these four materials were coated (about 6 g/m2 wet) at 50 ft/min. The dose level was started at 80 kGy and sequentially lowed to 40, 20, 10 and 0 kGy while continually checking for coating that was cloudy, wet, or easily scratched at the exit of the EB system. Surprisingly, coatings from all four materials were clear, dry, and scratch resistant even as low as 10 kGy. At 0 kGy (beam off), as expected, the coatings were wet and milky upon exit from the EB system.
In a second sequence of tests, the dose was maintained at 40 kGy and the line speed was sequentially increased in 50 ft/min increments up to 250 ft/min. The results, summarized in Figure 11, show that PUD 5 was wet at 100 ft/min while PUD 2 and 4 remained dry up to 150 ft/min. The monomer D/water (70/30) sample was the most interesting, showing good drying up to the maximum 250 ft/min test speed.
The results of this study show that water-based acrylate functional materials, including water-soluble monomers and PUDs, can be EB cured effectively. For some materials, high conversions can be achieved at relatively low dose levels. One of the most interesting aspects of the results was that good curing was achieved in a single EB curing step without prior thermal evaporation of water. This could be a unique process advantage for EB, although a direct comparison to UV curing was not part of this study.
One potential explanation for the ability to dry and cure in one step could be the nature of the EB curing process itself. EB provides uniform energy deposition throughout the thickness of the coating layer. This is in contrast to UV, where the energy is most concentrated at the surface.6 Given this EB curing process, it is possible water may be driven to the surface by the curing and crosslinking of the polymer layer. This could also be facilitated by heat generated in the exothermic curing process. Water that evaporates from the coating surface would be expected to be rapidly purged from the EB reaction chamber by the dry nitrogen that is used for inerting.
The ability to EB dry and cure water-based materials in one step could have practical implications for a number of commercial applications. These could include the following: ultra-low migration coatings for food packaging that do not contain low molecular weight monomers, low-migration inkjet inks with low viscosities for jetting enabled by using water as a diluent, or high performance industrial coatings with physical properties enabled by the use of higher molecular weight materials dispersed in water.
Another advantage of EB curing of water-based materials is curing in the absence of photoinitiators. While a large number of photoinitiators are available for UV curing, some can be challenging to use in water-based formulations. Some photoinitiators may be difficult to solubilize or disperse in water. Also, the initiator may interfere with the stability of the reactive polymer emulsion or dispersion. The absence of photoinitiator further enhances to ability to develop low-migration materials.
EB energy can penetrate opaque materials without concern for optical density. This allows curing of highly pigmented or filled coating layers. This feature may be enhanced when water is used for viscosity reduction, allowing the application of coatings with very high levels of pigments or fillers.
- C. Decker and I. Lorinczova, “UV-Radiation Curing of Waterborne Acrylate Coatings,” Journal of Coatings Technology Research, Vol. 1, No. 4, p. 247 (2004).
- R. Awad and F. Lunger, “New Developments in Waterborne UV Resins for Wood Coating,” RadTech Europe Conference Proceedings 2001, p. 415.
- S. Chatterjee, M. Laksin, D. Biro and J. D. Turgis, “Radiation Curable Aqueous Composition,” US Patent 7,037,953, May 2, 2006.
- M. Laksin, S. Chatterjee and V. Linzer, “Method and Apparatus for Wet Trapping with Energy-Curable Flexographic Liquid Inks,” US Patent 6,77,683, August 10, 2004.
- M. Laksin, S. Evans, K. Fontaine and S. Chatterjee, “EB Gravure – Novel Printing Concept for Sustainable Packaging,” RadTech North America Conference Proceedings 2010.
- S. Lapin, “Comparison of UV and EB Technology for Printing Packaging Applications,” RadTech Report, September/October 2008, p. 27.
The authors acknowledge Dr. Julie Jessop and Sage Schissel at the University of Iowa for assistance with the FTIR measurements. This collaboration is facilitated through work supported by the National Science Foundation under Grant No. 1264622. Acknowledgement also is extended to Alberdingk Boley, Allnex, Sartomer and BASF for providing materials for testing.
Stephen C. Lapin is currently at SCLapin Consulting LLC, [email protected].
Kathryn E. Classon is a University of Iowa Chemical Engineering intern at PCT Engineered Systems.