Ultraviolet (UV) cured coatings and heatset web offset inks are widely used in the publication segment of printing. The physical and chemical changes that occur before, during and after the curing process all contribute to the ultimate product quality.
The industry has developed a wide variety of quantitative and qualitative tools that allow the investigator to perform press audits and optimize UV coating performance. Critical factors that affect product performance, that are specific to UV coatings over heatset web offset applications, are examined using examples of system monitoring for improved process control, identification and chemical quantification of the process window and selection of quality control test methods.
Optimization of any process involves the identification of critical variables. The general chemical composition of the system is fixed in that:
1) The substrate is cellulose fiber binder and clay coating processed to a standard surface smoothness, absorbency and stock weight.1
2) Heatset inks are pigment dispersions in hydrocarbon resins, rosins and oils.2
3) The lithographic process brings water and alcohols onto the print with hydrophobic ink.
4) UV cured coatings are primarily mixtures of acrylate esters of polymeric and multifunctional alcohols.3
In the printing of heatset web offset in conjunction with ultraviolet (UV) cured coatings, interactions between the layers of the printed surface are driven by the chemistry of the components. The physical condition (coating thickness, coating volume and coating smoothness) is primary to interpreting the results of quality control tests used in standard production settings. The chemical interaction between the heatset ink and the UV coating during and after the curing process (degree of cure, chemical compatibility, adhesion) are the desired properties for measurement and observation. However, they cannot be characterized independently of the physical condition of the UV coating.
The next three sections in this paper will discuss application, cure and finally print measurement. The first section will discuss the physical application of UV coating to heatset prints with respect to standard quality control test methods. The second section will examine the changes in the curing process specific to heatset ink printing. The third section will examine what happens to values of the quality control tests that are measured on press sheets over time.
Coating Thickness vs. Coating Weight During Production
The final physical properties of a UV-cured coating depend upon the applied weight and the thickness of the UV coating over the heatset ink. This determines the quantity of performance additives on the surface and the bulk of coating separating the heatset ink from the print surface. The first section details how and why these two application variables change print quality.
Production samples of UV coatings over heatset inks were examined for both coating weight and coating thickness to determine the effect of physical changes in the coating film that occurs during the production application of UV coatings. Print quality measurements of 60 degree gloss, coefficient of friction and MEK resistance showed variability throughout normal production. All of these print quality measurements have been used to determine degree of cure of the UV coating. However, the coating thickness and the coating weight shows better correlation to the observed differences in measured physical properties.
Production samples of UV coatings over heatset inks were examined on 70#, 80# and 100# stocks. The anilox coating unit was able to apply a consistent thickness of UV coating to all of the tested stocks. However, as the speed of the press increased, the coating unit was applying a consistently lower volume of coating indicated by the decrease in #/1000 square feet of UV coating applied. (Table 1).
The reduction in coating volume for higher speed press applications manifested itself in voids in the coating surface rather than thinner UV coating film application. (Figures 1-3).
The appearance of pinholes in the production samples manifests itself in lower gloss readings and higher coefficient of friction values, both of which would indicate a lower degree of cure of the UV coating based on the standard interpretation of these results without consideration of film thickness. Chemical analysis of the UV coatings by the Ink Cure Analyzer (micro-swell coating density analysis) and potassium permanganate staining (identification of residual reducing agents in the coating film) showed that the energy output was more than sufficient to crosslink the UV film. At the highest speed, differences in acrylate conversion were negligible between the three types of examined production sheets: 70#, 80# and 100#.
Optical micrographs of the UV coating surface highlight the appearance of voids that occur in the UV coating surface under different printing conditions. The appearance of the voids explains the lack of a correlation between film thickness and coating weight as it is applied to the sheet.
Differences between performance characteristics of the UV coatings are related to the ability of the coating to completely cover the surface of the printed sheet rather than the speed of the curing of the film.
Evidence for this is seen in the optical micrographs of the prints in Figure 4. Because the overcoat is not fully covering the surface, the results of the performance testing is measuring voids in the coating in combination with the degree of cure of the coating itself.
Topographical micrographs of the bare stocks suggest that the surface roughness of the papers influence the papers’ abilities to be wet out by the coating at higher speeds. Micrographs of the bare stock show topographical differences to explain why the 100# stock has quicker wetting. The micrographs in Figures 5 – 7 show that the 100# stock is very smooth. The 80# stock is slightly more contoured, and the 70# stock is significantly more contoured.
The physical property measurements of gloss, coefficient of friction and MEK rub resistance are taken because they are indicators of the ability to process the prints and make saleable copy. This is a common example where the results of the quality control tests had been misinterpreted as a need for higher energy requirements of the UV coating.
Peak Irradiance And Cure Energy
Once a consistent application of UV coating is applied, curing conditions of the UV coating can be addressed by monitoring the condition of the UV curing system and the press speeds. The final physical properties of a UV cured coating depend upon the degree of cure of the UV coating over the heatset ink. This is controlled by lamp power (peak irradiance) and press speed (dwell time). The second section details how and why these two application variables change print quality.
During the curing process of UV coatings over heatset inks, the amount of energy that is provided to obtain the curing of the film is adjusted to meet the printing conditions. In practice, curing differences are adjusted by either slowing the speed of the press or increasing the energy output of the lamps. The press operator uses some form of coefficient of friction test, dyne test, permeation test or gloss test to determine the degree of cure of the coating. In the previous example, the results of these tests are dependent upon the physical condition of the UV coating and how it is placed on the printed sheet. The curing process is quite robust with regards to the UV coating by itself.
However, with heatset inks there are upper limits with regard to energy exposure that can be used to compensate for the natural variability in the process.
Fingernail scratch resistance is used to monitor adhesion of UV coatings to heatset inks. A weak boundary layer that is identified by failure of scratch resistance can be formed in two ways. The first mechanism of the creation of a weak boundary layer is related to the quantity of energy provided to the print during processing.
The energy output of a production curing unit was monitored to track the occurrence of adhesion failure between heatset inks and UV coatings. A radiometer having a spectral range of 250-415 nm was used to monitor the condition of the curing unit. The speed of the coating unit was adjusted to compensate for the decrease in efficiency of the UV light source as the medium pressure mercury vapor lamps and reflectors aged. (Figure 8).
By converting the press radiometer data into luminescence, the process variability could be tracked. Peak irradiance is a measure of the maximum amount of UV light energy that a print is exposed to at a single time (the higher the luminescence, the higher the peak irradiance.) The occurrence of major and minor system maintenance is identifiable from the data. System maintenance had been prompted by increased quantity of failures in the adhesion of the UV coating to the heatset ink.
As the luminescence of the curing unit decreased, the heat build-up in the coated sheet of UV coating over heatset inks increased. Whereas similar values for permeability, gloss and surface dyne could be achieved by adjusting the speed of the operation, changes in adhesion of the UV coating to the heatset ink could not be controlled as easily.
As the curing system decreases in efficiency, the heat exposure reached a threshold where solvents retained in the heatset inks from either the inks or the fountain solutions can become mobile as the chemical components of the heatset inks approach their softening point. The softening point of a UV coating and the paper stock is significantly higher than that of oil-based heatset inks. Therefore, scratch resistance problems created by heat exposure are selectively observed over large volumes of ink. These are the printing conditions where UV coatings over heatset inks are the most problematic for adhesion.
When printing conditions are far outside of the processing window, loss of scratch resistance is observed immediately. However, chemical reactions occur after the curing process that can cause loss of adhesion to appear after the print has been allowed to age.
Characterization of the Surface of a UV Coating Over Time
Once the UV coating has been applied and cured, there are chemical changes that occur in the system. The final physical properties of a UV cured coating depend upon the condition and treatment of the UV coating over the heatset ink. The third section details how changes in the UV coating can be monitored over time as the print ages.
The second mechanism for adhesion failure of UV coatings to heatset inks is related to a process that happens over time. By monitoring the susceptibility of UV coatings to oxidation by potassium permanganate over time, chemical changes in the UV coating can be observed. UV coating formulated from acrylate esters using benzophenone and amine as the photoinitiator are standard for use over heatset inks.
All of the following conditions must be met for potassium permanganate testing to be valid. Residence time of the solution changes the stain color density whereby longer residence times will result in darker stains. The age of the print changes the color density. The “post-cure” aging of a printed sheet can be monitored using potassium permanganate stain. Therefore, valid comparisons can only be made using similarly aged samples on time scale of days. Lower degree of cure will result in darker stains.The yellow density of the stain relative to the degree of cure of the sample is coating formula dependent. By measuring the change in the yellow density of the same coating formulation cured with different energy dosages, the relative degree of cure comparing different samples of the same coating can be estimated.
Dark specks or dark blotches in the stained area indicates lack of complete coverage of the coating over the substrate. Cured UV coatings typically yield a stain of 0.3-0.5 yellow density. Cellulose will yield a stain of 2.0 yellow density. Either pinholes in the coating or penetration of the coating into the substrate yields non-even staining of the sample.
By examining the change in oxidation by permanganate of a UV coating over time, the change in the chemical composition of the coating can be followed. These observations are indications of chemical changes that are occurring in the UV coating film. (Figure 10).
Tertiary amines are oxygen scavengers that generate organic peroxides and alcohols on the surface of the UV coating. Further auto-oxidation and relaxation of the cured film occurs over time that would expectedly increase the density and decrease the flexibility of the cured film.
Measured physical properties of the printed sheet follow the same trends as what is measured by observing the amount of auto-oxidation of the production sheet. As the sheet ages, the coefficient of friction decreases, the scratch resistance can get poorer, the surface tension of the print gets lower, and the solvent permeability gets lower. All of these effects make changes in the film hardness that stress the boundary layer between the UV coating and the heatset ink thus resulting in differences in scratch resistance.
Physical and chemical interactions occurring within and between the UV coating and the heatset ink are minor when compared to the fundamentally robust system that is designed for the application. However, a combination of factors determines the end properties of the resultant production print. The same quality control tests (MEK rub, coefficient of friction, dyne testing, gloss) that are used to monitor degree of cure can also examine differences in the physical application of the coating, and chemical changes in the coating after the print has been processed.
The condition of the substrate determines the effectiveness of the leveling of the UV coating and coating coverage.The maintenance schedule of the UV curing unit changes the latitude of flexibility and consistency that is obtained with a production set-up of UV over heatset. The chemistry of the cured UV film over heatset inks determines the form and degree of relaxation and oxidation of the UV coating over heatset.
The first step to optimizing any process is to develop a working model of the critical factors that affect the desired physical properties of the product. Optimization of UV coatings over heatset inks involves data collection and understanding of chemical and physical changes that occur before, during and after the curing process and how the test methods can be used to monitor the entire process.
1. Roberts, J.C., Paper Chemistry 2nd Edition, Chapman & Hall, London, 1996.
2. Hartsuch, P. J., Chemistry of Lithography, Lithographic Technical Foundation; 1961.
3. Braithwaite, M.; Davidson, S.; Holman, R.; Lowe, C.; Oldring, P.K.T.; Salim, M.S.; Wall, C.; Chemistry and Technology of UV & EB Formulation for Coatings, Inks, & Paints Volume 4, SITA Technology Ltd, Gardiner House, London, 1991.