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UV INKS: PROPERLY CURED?!Ensuring that UV inks are fully cured. By Carina Sötebier, hubergroup Deutschland.

As UV-curable inks become more widespread throughout in-dustry, there is a growing need for printers to vouchsafe the high quality of the printed material and to guarantee that the ink films are properly cured and ready for further processing. A new method enables them to do precisely that.

U V inks are becoming more and more popular due to the low temperatures needed, the nearly instantaneous curing process,

rapid processability and the elimination of traditional solvents. Ap-plications range from magazines and the commercial sector to food packaging through to the luxury segment, such as perfume and alco-hol packaging. Nowadays, printers are expected not only to be able to print and to process quickly but also to guarantee the high quality of the printed UV-ink products. Up until now, quality control has been based on subjective methods, such as wiping and scratching tests, without any guarantee on the part of the ink manufacturer that the ink films have properly cured. In this paper, we present a new method which enables printers to determine whether an ink film produced with hubergroup UV inks is properly cured, including a guarantee of whether further processing is possible.

BENEFITS OF UV INKS

The greatest advantage of a UV ink is that curing involves a UV light source, which cures the print almost instantaneously. Photoinitiators in the ink absorb a photon of UV radiation, leading to the generation

of free-radicals which initiate polymerisation of the unsaturated acrylic acid group. UV curing takes place at room temperature without the need to evaporate conventional solvents. Once the print has passed the UV lamp at high speed, it is already theoretically cured and can be further processed provided that suitable printing machine parameters have been chosen. Disadvantages include high process costs and ozone formation. However, insufficiently cured prints may also be cause by a shortage of time and a lack of process control. Reasons for this include unsuitable printing-machine parameters and defective UV lamps. The degree of curing affects not only the hardness of an ink film but also its robustness, migration behaviour and amenability to further processing. For printers, therefore, it is crucial to be able to determine whether the printing job is properly cured so that further processing may be initiated and customer complaints avoided. Up to now, the most widespread techniques for assessing the quality of printing jobs have been physical in nature, e.g. wiping, scratching and thumb tests. These are performed with specific wipes and/or by the printer’s own hand. Other mechanical and less common tests are set-off and pressure tests. Some printing companies perform chemical charac-terisation. Here, swelling tests or stability to a solvent such as acetone are common [1-3]. Nonetheless, mechanical and chemical tests yield only relative results, require expertise, and may vary from one printing company to another or even from printer to printer, because the evalu-ation is performed subjectively or against an in-house standard. Newer methods analyse the prints by spectroscopic means, for example Fou-rier transform infrared spectroscopy (FT-IR) or near-infrared spectros-

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RESULTS AT A GLANCE

ű A new method, based on UV/vis analysis and design of ex-periments, can determine if a print is properly cured and ready for further processing.

ű Principle: the concentration of a marker extracted from a printed sample correlates with the degree of curing.

ű This methodology illustrates the importance of understand-ing the ink system, the influences on the curing process, and a specific threshold value for various ink systems.

ű Printing-job-specific, objective threshold values for each ink system are defined by the ink vendor.

ű For printers, this represents a significant advantage over subjective degree-of-curing tests.

ű The test is fast, easy to use and yields comparable, reliable results.

ű The method will have a major impact on quality control directly at the printing machine.

ű Customers can expect fewer complaints, properly cured prints and a high-quality standard for end-customers.

copy (NIR). These rapid, easy-to-use instrumental techniques are able to quantify the degree of conversion of the acrylic double bonds. The advantages of NIR include the possibility of in-line measurement of both the layer thickness and the degree of acrylic double-bond conversion. To date, the NIR sensor developed by Scherzer et al. [4] is the only in-line detector which can be installed directly in the printing machine, as it has been optimised to cope with the machine’s high speed and flut-tering of the substrate. However, unlike the similar FT-IR technique, NIR cannot be performed on black inks [5-7, 4]. FT-IR is frequently used for

Figure 2: The influence of grammage on the extinction signal.Figure 1: Example of a marker embedded in t wo ink systems of different net work density.

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analysing the degree of crosslinking and yields reliable results. However, due to the low penetration depth of the FT-IR beam, it cannot be in-stalled at a printing machine. As a result, in-line analysis of the degree of curing does not lend itself to regular routine printing jobs [8-15].All the techniques and methods mentioned above do not involve an appropriate target value or a calibration, but rely instead on subjective impressions. Zarshenas et al., for example, show that the same curing conditions can yield different degrees of crosslinking for different coat-ings [16]. Given that the amount of acrylic double bonds can vary from one ink system to another, it is necessary to define a specific threshold value for each ink system. Many parameters can greatly influence the curing of an ink film, e.g. machine speed, lamp intensity, the substrate, the ink system, and lamp placement. One of the main parameters is the ink formulation. Curing of an ink system depends on the polymer backbone, such as its molecular weight and number of acrylic double bonds, as well as on the photoinitiators, pigments and other additives present. Yet most studies have been conducted on simplified ink sys-tems in a laboratory environment and so bear little resemblance to the ink formulations and industrial printing machines used in the real world [14, 17-19]. In this paper, we propose a UV/vis spectroscopic method for determining the degree of curing of commercial UV inks. Our study shows the importance of understanding the ink system, the parameters that influence ink curing, and establishes a link to industrial printing ma-chines.

EXPERIMENTAL

MaterialsCommercial UV printing inks NewV pack MGA, NewV set HS, and NewV set LED (hubergroup Deutschland GmbH, Kirchheim/Munich, Germa-ny) were chosen for the analysis. The substrate for the printing proofs was Invercote T (220 g/m2). An industrial printing machine was used to analyse various commercial coated and uncoated substrates.

InstrumentsProofs were printed on a Prüfbau printing proof machine with an in-line Hg-UV lamp (Prüfbau Dr.-Ing. H. Dürner GmbH, Peißenberg, Ger-many) with a grammage of 2 g/m2 and a speed of 0.2 m/s. An industrial printing machine KBA RA 106 – 6 +L (Koenig und Bauer AG, Radebeul, Germany) equipped with either an LED (AMS XP9, AMS Spectral UV, River Falls, WI, USA), a 200 W Fe-doped UV or two Hg-UV curing sys-tems was employed. A Lambda II photometer (Perkin Elmer, Waltham, MA, USA) with a resolution of 1 nm, a scan rate of 240 nm/min and a slit of 2 nm was used to analyse the samples over the range 190 to 750 nm. Measurements were performed with the aid of Rotilabo solvent-resistant, single-use UV cuvettes of 4.0 ml capacity (Carl Roth GmbH + Co. KG, Karlsruhe, Germany).

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Sample preparationA defined area of the printed sample was cut out and dipped into a solvent for a certain period of time. The sample area was defined by the measuring range of the UV/vis instrument. The solvent was chosen to match the analysed ink system and the marker used. The sample was removed from the solvent, which was then analysed by means of UV/vis spectroscopy.UV/vis measurementsThe samples were diluted with solvent to below a reading of approxi-mately 1.0 A.U.

METHODOLOGY

The methodology is based on an ink containing a marker substance of known concentration. Figure 1 shows an example of the marker embedded in two polymer networks of different network density. The network density can be described as a function of the functionality, the number of double bonds and the molecular weight of the net-work-forming polymer. The network on the left is fully cured, while the one on the right is mostly cured. However, the former has a lower network density than the latter.

MARKER MIGRATION

A printed, cured sample is brought into contact with a solvent. The marker can then migrate from the ink film into the solvent. The last of these is influenced by the marker and ink film characteristic, includ-ing the network density. The amount of marker available for extrac-tion depends not only on its concentration in the ink but also on the amount of ink applied. In a printing proof machine, the amount of ink applied to a substrate can be described by the grammage. The greater the availability of marker, the greater is the extinction signal (see Figure 2). As shown in Figure 2, grammage has a significant influence on the extinction signal. In contrast to the case for a printing proof machine, grammage cannot be evaluated on an industrial printing machine. However, the density can be adjusted to produce a similar correlation

Figure 3: The extinction signal as a function of extraction time for samples cured to different degrees.

Figure 4: Extinction as a function of the degree of curing.

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with the extinction. The longer the extraction time, the more marker can migrate into the solvent, until equilibrium is reached. Figure 3 shows the influence of extraction on the extinction signal for ten ink films cured to different degrees.The results in Figure 3 indicate that extraction of the marker into the solvent occurs rapidly and is accompanied by only a slight increase in extiction. For approximately half of the ink films , the extraction process is already complete after two minutes.

MARKER CONCENTRATION IN SOLVENT INDICATES DEGREE OF CURING

Analysis of the exit kinetics of the marker into the solvent enables the degree of curing to be determined. In the case of well-cured, highly crosslinked ink films, the marker migrates slowly from the ink into the solvent, but migrates faster in less extensively cured films. If the same amount of time is allowed for analysing variously cured ink films, the degree of curing is found to be proportional to the marker concentra-tion. Figure 4 shows the correlation between the extinction signal for the marker and the degree of curing.Migration of the marker from the ink film into the solvent depends on the available marker concentration, the degree of curing and the extraction time, as summarised in Equation 1:

WithE = ExtinctionM = Marker concentrationGr = GrammageD = Degree of curingtExtr = Extraction time

The available marker concentration in turn is given by the concentra-tion of the marker in the ink and the amount of ink applied to the substrate.

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Figure 5: Sample test form for evaluating the effects of different printing conditions on the degree of curing.

DESIGN OF EXPERIMENTS

An industrial printing machine differs significantly from a printing proof machine, which is incapable of fully simulating an industrial printing process. The latter machine lacks the fountain solution, runs at a much lower speed, the amount of applied ink is often higher and the UV dosage can differ. Only full, single-colour prints are possible, as overlays of different ink tones are very challenging. In industry, the machines are calibrated to ISO standard densities, without reference to the grammage. Raster overlay prints of different colours are stand-ard procedure. This makes industrial printing a much more complex process. In order to describe this printing and curing process as well

as to evaluate the influences on the extinction signal of the marker, we reverted to design of experiments. For this purpose, we created a test form, which included full-tone and raster fields, and printed it under different machine conditions, such as variable lamp intensities. A sample test form is presented in Figure 5.

CALIBRATING THE INKS

Correlation of the extinction values with the test conditions afforded a way of calibrating each ink system. The relative effects of the param-eters on the extinction value of one specific ink system are exempli-fied in Figure 6.The extinction value increases with increase in the ink amount as a function of composition, and decreases with increase in lamp inten-sity. It can be concluded that the amount and the composition of the ink exert the greatest influence on the extinction value and therefore on the degree of curing.Using our knowledge of the ink system and the printing job condi-tions, we compare the resulting extinction values against a specific curing threshold for these precise conditions with a view to establish-ing whether the system is properly cured and further processing is advisable.The extinction values for three ink systems cured under different con-ditions and printed on an industrial machine are shown in Figure 7.

PRINTING JOB-SPECIFIC THRESHOLD EXTINCTION VALUE FOR EACH INK SYSTEM

Figure 7 shows the differences in the extinction values of three differ-ent ink systems. The probability represents the quartiles of the cu-mulative frequency of occurrence relative to the normal distribution. The extinction ranges for each system vary extensively with polymer composition, density and degree of crosslinking, as well as photoiniti-ator. The ink system represented by blue crosses exhibits the smallest values up to approximately 1.3 A.U. The largest and most extensively differentiated values were found for the ink system represented by blue dots, which yields values of up to 4.5 A.U. This figure illustrates the importance of understanding the ink system concerned, the influ-ences on the curing process and a specific threshold value for the dif-ferent ink systems. We define printing job-specific, objective threshold

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Figure 6: Sample Pareto chart showing the effects of different printing conditions on the extinction value.

Figure 7: Normal probability plot for the extinction of three different ink systems as a function of curing conditions.

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“In contrast to conventional offset inks, UV offset inks offer more finishing and

substrate options.“

Dr Carina Sötebierhubergroup Deutschlandcar ina.soetebier@hubergroup.com

3 questions to Carina Sötebier

Why do UV inks gain such significance throughout the indus-try? The high speed of the curing process makes rapid processing of the prints possible. In contrast to conventional offset inks, UV offset inks offer more finishing and substrate options. In past few years, new UV lamps and low migration inks for absorbent and non-absorbent substrates have been developed, further enlarging the application portfolio. Does the deinking process after use present a challenge for UV inks? Deinking of UV inks represents a great challenge but can be solved through an optimisation of the formulation. We are in ad-vanced stage regarding this topic. An even greater challenger is the precise determination of the curing degree of UV inks with sufficient repeatability, which is why we have prioritised the method develop-ment thereof.

How elaborate is it for the printer to integrate the new meth-od into existing production processes? The method can easily be integrated in existing processes, as it is simple, fast to perform and provides an added value in terms of quality assurance and safety. It is ink system and print job specific. Therefore, no additional test fields or special machine conditions are needed. The equipment can be placed directly next to the printing machine.

values for each printing ink system. For printers, this approach represents a significant advantage over subjective degree-of-cur-ing tests commonly employed in the market. The test is fast, easy to use and yields comparable, reliable results.

MORE SATISFIED CUSTOMERS

In the future, it is expected that this method will have a major im-pact on quality control directly at the printing machine. Customers of the first printing inks manufacturer on the market to provide its customers with a tool capable of determining the degree of curing will experience a reduction in the volume of complaints, will en-sure that prints are properly cured and will provide a high-quality standard for the end-customer.

REFERENCES

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2008.[3] Schwalm, R. UV coatings; Elsevier: Amsterdam, London, 2007.[4] Scher zer, T.; Mehner t, R.; Lucht, H., Eds. Process Control of UV and

EB Cur ing of Acrylates by In-line NIR Reflection Spectroscopy, Proc. e|5 UV & EB Technology Expo & Conf erence, Char lott e / NC, USA , 02.-05.05.2004.

[5] Daikos, O.; Mir schel, G.; Genest, B.; Scher zer, T. Ind. Eng. Chem. Res. 2013, 52 (50), 17735–17743.

[6] Heymann, K .; Mir schel, G.; Scher zer, T. Applied spectroscopy 2010, 64 (4), 419–424.

[7] Mir schel, G.; Daikos, O.; Heymann, K .; Decker, U.; Scher zer, T.; Sommerer, C.; Genest, B.; St ecker t, C. Progress in Organic Coatings 2014, 77 (11), 1682–1687.

[8] Amkreutz, M.; Wilke, Y.; Hoffmann, M. Far be und Lack 2012, 118 (54), 28–33.

[9] Jančovičová, V.; Mikula, M.; Havlínová, B.; Jakubíková, Z. Pro-gress in Organic Coatings 2013, 76 (2), 432–438.

[10] Kunwong, D.; Sumanochitr apourn, N.; Kaewpirom, S. Songklana-kar in J. Sci. Technol. 2011, 33 (2), 201–207.

[11] Moht adizadeh, F.; Zohur iaan-Mehr, M. J.; Hadavand, B. S.; De-hghan, A . Progress in Organic Coatings 2015, 89, 231–239.

[12] Rauh, W.; Schnepf, J.; Epple, C. Ent wicklung eines Ver f ahrens zur Optimier ung der Abstimmung von Str ahler leistung und Emp-findlichkeit von UV-här tenden Dr uckf ar ben. Abschlussber icht über ein Ent wicklungsprojekt, geförder t unter dem Az: 24602-21/2 von der Deutschen Bundesstiftung Umwelt, 2008.

[13] Scher zer, T. Macromol. Symp. 2002, 184 (1), 79–98.[14] Stolov, A . A .; Xie, T.; Penelle, J.; Hsu, S. L . Macromolecules 2000, 33 (19), 6970–6976.[15] Studer, K .; Decker, C.; Beck , E.; Schwalm, R. European Polymer

Journal 2005, 41 (1), 157–167.[16] Zar shenas, E.; Bast ani, S.; Pishvaei, M. Ind. Eng. Chem. Res.

2013, 52 (46), 16110–16117.[17] Decker, C.; Nguyen Thi V iet, T.; Decker, D.; Weber-Koehl, E. Poly-

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