Ultraviolet (UV) inks are a major growth area for ink companies. While they share many similarities to conventional inks, UV inks are cured by radiation, and as a result, they have ingredients and properties that differ from conventional inks.
For ink companies, understanding these differences and having strict controls on quality and testing ensures that their own customers will receive the best possible products.
Importance of Testing
To test UV inks, it is important to study how the ink cures in the lab. That requires ink manufacturers to create similar operations on a laboratory scale to what their customers are using.
“The most important factor in testing UV inks is developing a test procedure that simulates curing of the actual production equipment (UV lamp and set-up),” said Dr. Scot Pedersen, project manager for energy cure inks (UV/EB) at Color Converting Industries. “Once the comparable cure procedure is established, it is expected that all the other physical properties will fall in line. This requires adjusting lab-curing parameters to closely mimic the press cure conditions.”
As for conventional inks, end-use applications govern the specifications UV inks need to meet.
“The most important aspect is to consistently measure performance parameters of the ink that relate to the needs of customers,” said Glenn Webster, Sun Chemical’s marketing manager, UV inks. “The tolerance of the manufacturing process has to meet with the customer’s process window. This includes not only the printer, but also his customers’ process and end-use needs as well. Measurements must be consistent from one location to another.”
For Zeller+Gmelin, quality control involves a virtual gauntlet of steps to make sure that the ink is being properly manufactured.
“We consider it imperative that quality control testing be integrated into every segment of the manufacturing process,” said Stephen Lazure, vice president of operations at Zeller+Gmelin. “We conduct an average of nine in-process quality control tests and a final quality control approval before the packaging of our ink products. It is critical that the product passes each in-process quality control test before proceeding to the next phase of manufacturing. This technique allows for minor adjustments to the batch if necessary as opposed to major adjustments during the final phase of manufacturing.”
Testing UV vs. Conventional Inks
As is the case with conventional inks, fundamental ink tests such as particle size and color strength are often carried out in a similar way for both ink types. Mr. Webster pointed out that differences arise in testing methods for drying speed or curing rate.
“Generally speaking, quality control testing for printing inks is basically the same regardless of the chemistry,” Mr. Lazure said. “The test methods may differ though, depending upon the type of ink system. For example, in order to determine the tack of a conventional versus UV ink, the same inkometer may be used, but the durometer and composition of the rollers is different. Tests such as adhesion and solvent resistance of a printed UV ink film can be accomplished immediately after subjecting the ink to UV energy, while some conventional inks may not be tested immediately as their oxidation process requires more time to complete.”
Drying of conventional inks occurs over a period of time, whereas curing of UV inks is immediate.
“The conventional inks simply require evaporation of solvents to dry or set,” Dr. Pedersen said. “Therefore, they are tested for proper drying speed in the lab. Sometimes, additional tests are performed for ink offsetting (ink blocking). On the other hand, the UV flexo inks are made with monomers, which react to become dry plastic while exposed to the UV light.
All the other physical properties of both liquid inks and UV inks are tested using the same equipment such as gloss meter for gloss; Sutherland rub tester for rub resistance; Scotch tape for adhesion and coefficient of friction tester for slip characteristics.”
“In its essence, UV is just another way to polymerize (dry) the ink on the substrate,” said John Valla, UV technical service for INX International Ink Company. “Therefore, the most divergent aspect of UV testing would be to evaluate cure with a UV curing unit rather than some kind of dry rate testing. Secondly, the viscosity of a conventional ink is usually measured with some sort of viscosity cup. Since UV generally has higher viscosities, it is desirable to measure UV viscosity with a rotational viscometer. The rest of a finished ink testing parameters – color, strength, adhesion, etc. – are basically the same.”
“Testing for curing is the primary difference (light energy vs. oxidation or heating),” said Rodney Balmer, Flint Ink’s technical director. “It is important to make sure that the testing equipment is compatible with the UV/EB materials (i.e. inkometer must have EPDM or Buna-N rollers). When testing for color strength, the bleaching white must be compatible with the UV/EB ink system.”
Key Properties for Testing UV Inks
When testing for any type of ink, there are a number of critical properties to analyze. For UV inks, the most critical aspect is whether the ink has cured properly. Pigment wetting is another area that affects ink color and press performance.
“The two most important aspects in testing UV inks are cure rate and pigment wetting, as UV tends to be somewhat inferior to many conventional materials for this,” said JohnBraddock, Akzo Nobel Resins’ technical marketing manager, UV/EB curing chemicals. “Key properties to test when manufacturing UV inks are rheology, cure rate, adhesion aspects and flexibility.”
Why proper cure is paramount to UV ink performance: all other properties stem from it. Poor cure results in detriments in many other areas.
“The amount of cure and the degree of cross-linking to ensure that the finished product meets the necessary requirements for scratch resistance, mar resistance, product resistance, etc. are important, as is the viscosity and the rheology of the ink to ensure proper flow of the ink in the ink fountain and down the rollers,” Mr. Balmer said. “The color and color strength need to be measured to ensure that the ink will be the proper color when the correct amount of ink is being carried on the rollers. Too little or too much ink can influence the printability of the ink.”
UV ink rheology has been problematic due to questionable affinity of pigment and monomeric/oligomeric components.However, rheology is crucial to proper ink transfer.
“Tack is very important to measure to make sure the ink will transfer from roller to roller,” Mr. Balmer added. “It will also determine how the ink interacts with the substrate and the other inks, if trapped wet on wet. Grind is measured to determine how well the pigment is dispersed in the system. If the pigment is not dispersed well enough, that can impact color strength, gloss and printability.”
If the ink is not cured properly, there could be problems with adhesion. Mr. Webster said UV cure rate at a consistent film weight or color density is a key performance parameter that affects press speed and adhesion to substrates. Related to that is color strength and shade. Rheological parameters such as viscosity, flow and tack are also key parameters for all types of UV ink, and are especially important for high-speed printing processes such as offset and flexo.
“The most important aspects are curing and adhesion,” said Lisa Hahn, president of Flexo Tech. “In addition, UV flexo inks tend to be more viscous and pseudoplastic so the leveling of these inks tends to be poorer than for conventional inks, although you do get better dot structure. UV inks tend to shrink when they are cured, which can be detrimental to proper adhesion.Hence, tape tests are important.”
“For a UV flexo ink, the ink must cure adequately, adhere to the prescribed substrate, be on color, have good color strength and have good viscosity with low thixotropy,” said Mr. Valla. “If any of those aspects are not satisfied, then the ink may be considered unusable for a particular application.”
The list of tests that ink companies conduct is lengthy.
“For proper press performance, the color strength, viscosity and cure need to be tested in the lab,” said Dr. Pedersen. “Also, in many applications, the gloss, rub resistance, adhesion and other physical resistance are also being tested in the lab. Some customers require wet rub resistance of cured UV inks with MEK or toluene, which in principle, is used to confirm a complete cure of UV inks.”
“Viscosity and stability are also factors,” Ms. Hahn said. “You have to be careful not to get the temperature too high when dispersing pigment, and you often have to use three-roll mills, where there is much better cooling than in horizontal media mills. The wetting of pigment is usually poorer, so dispersability issues are magnified.”
“Depending upon the specific product and the end use application, we test grind, cure speed, tack, viscosity, gloss, density, water pick-up, opacity, color value, rub, solvent resistance, scratch, tape, dye strength, MEK, xylene and crinkle,” Mr. Lazure said. “We utilize a statistical process control method to identify trends during the manufacturing process. Our manufacturing equipment utilizes PLC (program logic controller), which allows for each individual formulation to be manufactured consistently every time.”
Any well-stocked laboratory has an abundance of equipment, and leaders at ink companies have their own recommendations for testing UV.
“The most critical testing tools for UV inks are a UV lamp, bladed hand-proofer and an advanced rheometer,” Mr. Webster said. “A UV lamp curing system with variable speed bed is critical. It is important to calibrate and test the performance of the lamp regularly, using both speed calibration for the moving bed and UV wavelength output intensity measurement for the lamp itself. Since UV flexo is slightly more viscous than conventional inks, when test proofing in the lab, a good quality doctor-bladed hand-proofer is necessary to apply consistent film weight and color density of ink to substrates. With UV flexo inks, advanced rheometry techniques, such as controlled-stress steady-state viscosity measurement, gives far more information about ink performance than simple flow cup measurements.”
“A radiometer is critical to adjust the UV lamp speed to simulate the press conditions,” Dr. Pedersen said. “Also, using a lamp of the same type as those used at customer facilities is very important. These are, of course, in addition to any physical property testers that are needed.”
Mr. Valla recommended a UV curing unit and a viscometer and proofing device, each designed to handle the higher viscosities of UV.
Mr. Balmer recommended a wide variety of equipment, including an inkometer, ink cure analyzer (ICA), Kershaw/ Lithotronic, tack-o-scope, gloss meter, spectrophotometer, UV/VIS spectrophotometer, Sutherland rub tester, UV lab curing unit, EB lab curing unit, rheometer and Prufbau.
Mr. Lazure suggested a cone and plate or rotational viscometer, inkometer, spectrophotometer, densitometer, grind gauge, lab curing unit, proofing equipment and water balance equipment.
“The equipment is the same for UV testing vs. conventional, so use of proofers, inkometers, rheometers, other viscosity testing, crosshatch adhesion, Sutherland rub, etc are the same,” Mr. Braddock said. “Another aspect is that inkometer rollers need to be EPDM rubber to avoid swell. Another equipment need is a lab UV cure unit.”
There are areas specific to UV that also need to be addressed by manufacturers and end-users alike. For example, Mr. Balmer said that shelf life of UV ink is a concern that companies need to keep in mind.
“UV ink typically has a shelf life stability that is less than that of its conventional counterpart, and as such should be monitored more closely,” Mr. Balmer said. “Determining factors could be things such as ambient temperature conditions in storage and the type of packaging that has been utilized. Shelf life may also be decreased by incorrect selection of raw materials and their interaction with other materials contained within the formulation. Another factor which can have a detrimental effect is the manufacturing process used to manufacture the ink.”
Dr. Pedersen said that standardized testing procedures would help the ink industry.
“Developing proper test procedures for UV flexo ink is an industry-wide challenge,” Dr. Pedersen said. “This is due to the complexity in curing devices, variety of lamp set-up, age of the bulb and equipment maintenance schedule. Variation in any of these parameters can impact on the amount of energy being applied to the wet UV inks to cure, and therefore, performance of the final product.”
Ultimately, meeting the specifications of customers is the goal for all ink manufacturers.
“Testing and quality control of UV inks is relatively straightforward,” Mr. Webster said. “As with other inks, it is key to use appropriate testing equipment that is properly calibrated. The challenge is in setting customer specifications and tolerances for inks that actually relate to performance on press and customer end-use needs so that productivity and consistency is maximized for all.”