The environmental and health problems caused by inks containing volatile organic compounds (VOC) call for new solutions in printing ink chemistry and technology.
One of the approaches may be hot melt ink, which is ink solid at ambient temperature and liquid at the moment of printing.
Such ink contains no volatile solvent to be trapped in ink film or to produce VOC.
In this work, hot melt ink for rotogravure printing process based on ethylene vinyl acetate (EVA) chemistry was formulated, analyzed for rheological properties, and printed on publication and packaging substrates. The original inks based on ethylvinylacetate and polyethylene (PE) chemistry were too viscous for rotogravure.
The polymer matrix was mixed into the carnauba wax and then the rheology was tested again. Carnauba wax significantly decreased the viscosity of hot melt formulations.
Drawdowns were made on publication (supercalendered grade A and lightweight coated) and packaging (SBS board) substrates and then the printability data was collected. The EVA/carnauba wax inks showed slightly higher optical density than PE inks.
Print gloss, delta gloss and rub resistance properties were better on PE inks. Also, mottling was lower at PE inks. SBS board exhibited highest print and delta gloss and lowest mottle index.
Rotogravure is an industrial printing process mainly used for the high-speed production of large print runs at constant and top quality, also known as “photographic quality image.” Many hundreds of millions of magazines each week are printed by the gravure process because advertisers want their products promoted in the best possible way. A large number of mail order catalogs are printed in gravure because the products must look attractive and must also demonstrate exactly what is on offer: therefore the catalogs must all look the same, which requires the constant print quality of the gravure process.
Today, 15 percent to 20 percent of printing is achieved through the rotogravure technology. In Europe, Germany is far ahead in the number of gravure printing units with approximately 100 rotogravure printing machines, Italy with 40 printing machines, U.K., France and Holland with nearly 20 for each of them.
According to Koenig & Bauer-Albert AG and European Rotogravure Association, the world demand for rotogravure publication presses, thus rotogravure printing, is increasing. Further growth of gravure printing is predicted to result from continued refinements of the process and gravure presses.
Gravure is gaining the market share with various improvements in the technology, such as ultrasonic gravure cylinder plating, which is significantly faster than traditional copper plating. Publication presses are now as wide a four meters (142 inches). Development of gravure presses for packaging is directed toward short runs, which may help the gravure process to stay competitive. High printing speeds reduce dot gain and dot distortion, improving print quality. Electrostatic assist reduces the number of skipped dots and improves the uniformity of ink distribution within the dot.
In the history of rotogravure printing, water-based inks preceded solvent-based inks, but they were abandoned because of slow drying rates. Since the beginning of the 1960s, toluene has been used as a solvent in most rotogravure publication printing plants. However, toluene was the most likely cause of the lung cancer, genotoxicity[7,8] and neurotoxicity.
For all of these health-hazard problems, water-based inks are coming into use again. The use of water-based inks for packaging and product gravure grew from 26 percent in 1993 to 50 percent in 1997.
There have been some technical problems related to the application of water-based inks to the high press speed, such as relatively low drying rates, issues such as printability on a variety of substrates and their cost/performance ratio.
However, aqueous formulations have also serious environmental disadvantages, as they cause greater water pollution and consume more energy. UV gravure inks are not yet a reality.
Research is currently being done in attempt to create a new generation of inks: hot melt inks, which do not have any sort of dangerous effluent or carcinogenic properties.
These hot melt inks operate on the premise that when heated, the components “melt” and become a homogenous liquid. These inks consist of a crystalline substance that is solid at room temperature but can be heated and melted to gravure printing viscosity. They have advantages over conventional inks because they do not contain volatile organic compounds (VOCs).
However, this is not the only advantage to hot melt inks. The fact that the ink dries by solidification prevents the ink from migrating into the pores of the substrate, which allows the ink to create better density in the solid areas with a thinner ink film.
The design of the rotogravure press must be altered in order to print using hot melt inks. The printing cylinder must be heated in some manner that the ink will not solidify on the cylinder.
This might be accomplished using a cylinder that is heated with silicone oil from the inside. Infrared heaters can also be used to heat the cylinder from the outside. The press must be fitted with a cooling roll after the nip which acts as the chilling mechanism for the solidification process necessary for hot melt inks setting. If hot melt inks are used, the need for a dryer system is totally eliminated. The overnight cleaning of the equipment is not necessary.
Hot melt inks have not yet been accepted in industry because not enough research and development work have been done to formulate optimum performance hot melt inks. Therefore, the aim of this work was to formulate hot melt inks for the rotogravure printing process.
Pigment chips: Magenta – quinacridone PE flush red 122 L280013, Yellow – AAMX yellow 13 L751349D28707, pthalo blue green shade L490714D59565, commercial pigment chips covered with polyethylene (Sun Chemical) were used for inks formulation.
Polymers: EVAC copolymer A – 85°C – melt index 400, EVAC copolymer AA – 85°C – melt index 250, EVAC copolymer B – 105°C, EVAC copolymer C – 120°C, EVAC copolymer D – 124°C, carnauba wax – from Michelman, Inc.
Additives: Slip agent – Structol.
Blending of Inks
Melt blending of polymer matrix with pigments (hot melt inks) was carried out using a Brabender Plasticorder fitted with a W50 chamber and cam blades. Mixing time was 10 minutes, mixing torque data was reported after the first minute of mixing after stabilization of the torque response. Mixing temperature was varied from 120°C to 190°C.
All composite samples were compression molded into plates of 3 mm thickness for further printing and rheology measurement.
The polymer matrix plate (EVAC A with pigment) was melted in with carnauba wax under heat and mixed with a three-blade mixer for approximately 20 minutes to ensure thorough dispersion of the polymer matrix into the wax. A 33.3 percent, a 25 percent, and 20 percent dispersion of EVAC A with pigment and the Carnauba wax were prepared.
Printing and Printability Analysis
The drawdowns were done using a Meyer rod #3. The following printing substrates were used: light weight coated –33 lb/ream (LWC), solid bleached sulfate board – (SBS), super-calendered A grade (SCA) – 34 lb/ream.
Printability analysis was done by measuring optical density (X-Rite 408 reflection densitometer), specular print gloss and delta gloss (Gardener gloss meter with 60° geometry), rub resistance (Sutherland rub tester) and mottle index (Tobias mottle tester).
A steady stress sweep test was performed on the DSR 5000 Stress Rheometer. A 25mm parallel plate in conjunction with a Peltier plate heating system was used to achieve the necessary temperatures for the inks to be liquefied. The following conditions were used: Sweep Mode: logarithmic; initial stress = 0.1 dynes/cm2; final stress = 10000 dynes/cm2; Points per decade = 10; maximum time per data point: 10 sec.; temperature: 85°C, 90°C, 95°C, 100°C, 105°C, and 110°C, respectively. Delay = 180 sec. (to normalize the sample).
Results and Discussion
The polymer blends for hot melt inks (HMI) for rotogravure printing were prepared on the basis of ethylene- vinyl acetate (EVA) or polyethylene (PE) chemistry. The starting formulations were based on 20 wt. percent of pigment (in this case Yellow 13) and EVA (PE) matrix. Properties and processing conditions of polymers are given in the Table 1.
The commercially available thermoplastic polymers are usually rheologically characterized by manufacturer using the melt flow index (MFI). It is a single-point viscosity measurement at a relatively low shear rate and temperature. Although there exists an inverse relationship between zero shear viscosity and MFI, the addition of pigment changes the whole situation. Therefore, the measurement of the ink viscosity at the shear rates close to those achieved at the moment of printing was done using DSR 5000 stress rheometer.
The hot melt ink formulations using Evatane 28-40 had viscosity oscillating in the range of 45,000-50, 000 P and Elvax 220 in the range of 7,000-9,000 P (Figure 1) and LDPE Bralen had viscosity 6,500-8,000 P (data not shown). The lowest viscosity range (3,000-3,500 P) was found at ink formulated with Elvax 210 polymer. However, all of these HMI formulations exhibited a much higher viscosity than that used for the gravure printing.
Therefore, the next goal was to reasonably decrease the viscosity of HMI. This aim was achieved by blending starting formulations with the carnauba wax. The carnauba wax was chosen because of its relative low melting range (82°C-86°C). It contains mainly fatty esters (80 percent-85 percent), free alcohols, (10 percent-15 percent), acids (3 percent-6 percent), hydrocarbons (1 percent-3 percent), esterified fatty dialcohols (up to 20 percent), hydroxylated fatty acids (up to 6 percent) and cinamic acid (up to 6 percent). The rheology of carnauba wax was measured at 95°C (Figure 2).
Steep viscosity drop was found at stresses higher than 12 dynes/cm2 to 2P at 40 dynes/cm2 and then dropping only slightly with increasing shearing rates.
The carnauba wax was mixed with the starting formulation polymer matrix at the following concentrations: 5g of yellow polymer matrix Elvax 210 (LDPE Bralen) and 10g of carnauba wax, yielding a 33 percent mixture; 5g of yellow polymer matrix Elvax 210 (LDPE Bralen) and 15g of carnauba wax yielding a 25 percent mixture; 5g of yellow polymer matrix Elvax 210 (LDPE Bralen) and 20g of carnauba wax, yielding a 20 percent mixture. All of the figures represent the percent weight.
The viscosities of the hot melt formulations measured at all three temperatures (95°C-105°C) showed a similar trend – steep drop of the viscosity to almost constant value, not changing with further stress increase from approximately 250 dynes.cm-2. For example, the 25 percent Elvax blend with carnauba had 75 P viscosity at 95°C and at the shearing stress of 35 dynes.cm-2 and this value dropped to the value of 12 P at the shearing stress of 250 Dynes. cm-2 (Figure 3).
This viscosity can be even lowered by the temperature increase (Figure 4). Temperature rise of 10 degrees reduced the viscosity of this hot melt ink formulation from 12 to 8 P (Figure 4).
These blends were used for printing trials. Three substrates were printed: lightweight coated (LWC), supercalendered (SCA) and solid bleached sulfate board (SBS). The print density (Figure 5) was excellent even after blending with the carnauba wax. EVA formulations had slightly higher reflective densities than LDPE formulations on all substrates tested and at all formulations. Print gloss (Figure 6) was higher on PE than EVA formulations (59 percent to 40 percent on SBS decreasing with increasing carnauba wax content). This is probably due to excellent ink holdout properties- due to phase change it does not have ability to penetrate into the paper pore structure. Delta gloss (Figure 7) was much higher in PE formulations (32 percent to 12 percent decreasing with increasing wax content). Negative delta gloss was found at LWC and SCA when printing with EVA inks (Figure 7).
Mottling was quite low at all inks and substrates. However, mottling was in all cases slightly higher on EVA inks than on LDPE inks and this is true for all substrates and formulations tested (Fig. 8). Mottle index was 15-95 at EVA, which is much lower than usually obtained when printing with standard toluene inks. Mottling was noticeably lowest at SBS board with all ink formulations (Figure 8).
Adhesive and rub resistance properties were slightly better on LDPE inks (Figure 9). LDPE formulations had rub resistance in the range 100 percent – 98 percent, being worst at SCA substrate (98 percent). Rub resistance of EVA inks was 99 percent – 98 percent. However, ink show-through was observed on publication grades (LWC and SCA), which may be connected with slow printing speed of drawdown technique of printing (data not shown).
Overall, very good printability results were achieved. HMI resulted in sufficient print density, very high print gloss, low mottling and excellent rub resistance. Especially, specular gloss values are very promising. Ink holdout is better than in conventional inks, due to phase change of HMI. Ink does not penetrate into pore structures of substrate.
However, further work is necessary to tune up the inks’ viscosity, especially when process colors will be printed.
EVA and LDPE polymers were used for formulations of hot melt inks for rotogravure. From all EVA and LDPE polymers tested, LDPE Bralen and the Elvax 210 were found to be most suitable ones. However, their viscosity was too high for gravure printing. To reduce the viscosity of HMI formulations, carnauba wax was used up to 80 percent of ink formulation. HMI formulations were used for printing LWC, SCA and SBS board.
Printability analysis showed very good reflective density, excellent print gloss, very low mottling of HMI and excellent rub resistance. The high print gloss is probably due to the ink’s decreased ability to penetrate to substrate pores, so that excellent ink holdout is achieved.
This research has been supported by grants from North Atlantic Treaty Organization (EST.CLG.977042) and The Eppley Foundation for Research. Donation of samples of pigments from Sun Chemical Corporation and carnauba wax from Michelman Co. is appreciated. The authors would like to extend thanks for the funding of this work.
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