Evaluation of chemical degradation on prints with solvent based ink and UV ink.

Bates, Irena ; Zjakic, Igor

1. INTRODUCTION

Flexographic printing technique has the greatest application in the field of flexible packaging because of the ability of the quick ink drying on the non-absorbent substrate (Leach & Pierce, 1993). The conventional solvent based inks are still broadly used, while their replacement with the energy curable inks (UV or EB inks) has slowly begun. According to the Decision of the European Council of 1999/13, the printing houses are obliged to change the conventional inks or to dispose VOC. The technology with UV radiation enables quick drying of a very thin UV ink layer by radical polymerization, without emission of VOC. (White, 1998). The prints with UV ink have higher gloss and they provide a better reproduction quality (Baro et al., 2007). The aim of this paper is to compare the chemical resistances of samples printed with conventional ink and ecologically acceptable ink--UV ink on same substrate. Measurements were performed on the full-tone process colors (CMYK). The chemical resistance of samples was determined with agents: [H.sub.2]O+C[O.sub.2], soap, [C.sub.3][H.sub.6][O.sub.3], oil, C[H.sub.3]COOH, [C.sub.6][H.sub.8][O.sub.7], [C.sub.2][H.sub.5]OH, detergent, [H.sub.2]S[O.sub.4], HCl, NaOH (Flexographic Technical Association, Inc., 2003). The samples' resistances were ranked according to subjective and objective evaluations.

The subjective evaluation was performed with ten observers under the illumination D50.

2. EXPERIMENTAL PART

The printing with conventional inks was performed on a Soloflex printing machine of Windmuller und Holscher Corporation and the printing with UV inks was performed on a Nilpeter FA machine. During the printing the same printing form and same printing material were used. The conditions under which the printing process has been conducted rendered possible the production of good quality reproductions. The resistance to different chemicals was tested according to the standard ISO 2836-2004. For testing the resistance to liquid agents, two sheets of filter paper with the sample between them and two glass plates were used. The sheets of filter paper were previously soaked in the agent. The resistance to alcohol was tested by immersing the sample into a half filled test tube. Subjective evaluation of the color changes of filter paper and alcohol was performed after the treatments. The objective evaluation of the color changes on the print was determined with spectrophotometer. Spectrophotometric measurements were obtained by the instrument Eye One and the software Key Wizard.

3. RESULTS

The subjective evaluation was performed with five male and five female observers of the same age who all had normal vision. The subjective evaluation of samples is presented in the following table. The criteria of ranking the colour changes on the filter paper have been done as follows: 4--great changes on the sample, 3--medium to great changes on the sample, 2 medium changes on the sample, 1--small changes on the sample, 0--no changes on the sample.

Spectrophotometric values of samples were measured before and after the treatment. [L.sup.*][a.sup.*][b.sup.*] values represent the three dimensional color space CIE [L.sup.*][a.sup.*][b.sup.*], where a*and b* are the coordinates and they represent the hue, and the coordinate [L.sup.*] the lightness of the color. Value [a.sup.*] describes red-green hue and value [b.sup.*] yellow-blue hue. CIE [L.sup.*][a.sup.*][b.sup.*] color space is defined for describing the visual perception of a color by a human eye.

The numerical value for discribing difference between two colors is determined by Euclidean diference (AE) (Equation 1.) (Kipphan, 2001; Field, 1999).

[DELTA][E.sup.*] = [square root of ([[DELTA][a.sup.*]).sup.2] + [([DELTA][b.sup.*]).sup.2] + [([DELTA][L.sup.*]).sup.2]] (1)

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

4. DISCUSSION

When comparing the results obtained by visual inspection and objective examination, the correlation of results can be detected. Minor differences are visible in visual inspection of changes in the filter paper during the treatment of yellow ink, due to a weak contrast between yellow ink and the paper. By performing visual inspection, (Table 1) a very low resistance of magenta samples printed with solvent ink in the treatment with hydrochloric acid, sulphuric acid and detergent has been determined. Samples of magenta printed with UV ink also have a low resistance in the treatment with hydrochloric and sulphuric acid. In case of treatment with alkali (sodium hydroxide), a great resistance has been determined with all process inks in relation to samples printed with solvent ink and those with UV ink, except in case of magenta where minor changes are detectable in the filter paper. When testing the resistance to acids (hydrochloric acid, sulphuric acid, citric acid, acetic acid and lactic acid) major changes are observed in the filter paper treated with hydrochloric acid and sulphuric acid. Changes have also been detected with magenta and slightly less with black ink in the testing of samples printed with solvent based ink, while in the testing of samples printed with UV ink changes have been detected with magenta and to a much lesser degree with cyan ink. According to the visual inspection, magenta and yellow inks have a low resistance to carbonated water, which is more evident in samples with UV ink than the samples with solvent based ink. All process inks of samples printed with solvent based and UV ink have a high resistance when treated with oil.

It is not possible to determine precisely through the visual inspection how big the change in the colour of samples actually is, that is, the chemical resistance; therefore, it is necessary to conduct spectrophotometric measurements.

The colour differences (AE) are determined by comparing spectrophotometric values ([L.sup.*][a.sup.*][b.sup.*]) of samples before and after the treatment with agents. The colour differences on the samples printed with solvent ink are presented on Figure 1. A major difference in the colour can be spotted in the figure ([DELTA]E = 30.51) in case of magenta when testing its resistance to hydrochloride acid. Minor differences in the colour of magenta were also obtained in the testing with sulphuric acid. When subjected to ethanol black and yellow ink have a low resistance, while other inks are within the permissible limits. Yellow ink, black ink and cyan have a lower resistance when subjected to carbonated water. When treated with soap colour changes occur with regard to yellow ink. Figure 2 presents colour differences on samples printed with UV ink. It is evident from the figure that the colour difference of a sample of magenta, when mixed with hydrochloric and sulphuric acid, is significant. When treated with carbonated water major differences occur in the colour of a yellow sample. According to objective examination of other inks and samples minor colour differences occur which are not visible to an average observer.

5. CONCLUSIONS

Magenta on the sample printed with solvent based ink has a distinctly low chemical resistance to acid hydrochloride. The same sample with magenta has a slightly higher resistance to sulphuric acid. Magenta sample printed with UV ink has a low resistance but a slightly higher one when compared to the sample printed with solvent ink when both are treated with same acids. The reason for that is a low chemical resistance of magenta pigment. Samples printed with the solvent based ink in comparison with the samples printed with UV ink contain lower resistance to chemicals in all process inks. Among the samples printed with UV ink the lowest resistance is detected in yellow ink when treated with carbonated water.

It can be concluded from the obtained results that the ecologically acceptable inks--the UV inks, can replace conventional inks in case of the researched substrates.

6. REFERENCES

Baro at al. (2007). UV technology: A practical Guide for all Printing Process, Committee for UV printing, ISBN 9783000220272, Wiesbaden

Field, G. G. (1999). Color and its reproduction, 2th edition GAFTPress, ISBN 0-88362-201-7, Sewickley

Flexographic Technical Association, Inc. (2003). Flexographic Image Reproduction Specifications & Tolerances, Foundation of Flexographic Technical Association, Inc., ISBN 0-9720474-4-1, Ronkonkoma

Kipphan, H. (2001). Handbook of Print Media: Technologies and Production Methods, Springer, ISBN 3-540-67326-1, Berlin

Leach, R. H. & Pierce, R. J. (1993). The Printing Ink Manual, 5th edition, Society of British Printing Ink, ISBN 9870948905810, London

White, A. (1998). High Quality Flexography, 2nd edition, Pira International, ISBN 9781858020082, Leatherhead

Tab. 1. Presents the results of the visual evaluation

 chemical resistance

 SOLVENT based ink

 c M Y K

NaOH 0 1 0 0
HCl 0 4 0 2
[H.sub.2]S[0.sub.4] 0 a 0 1
Detergent 0 a 0 0
[C.sub.2][H.sub.5]0H 0 1 2 2
[C.sub.6]{h.sub.8][O.sub.7] 0 1 0 0
C[H.sub.3],COOH 0 1 1 1
Oil 0 0 0 0
[C.sub.3][H.sub.6][O.sub.3] 0 0 0 0
Soap 0 1 0 0
[H.sub.2]0 + C[0.sub.2] 1 2 2 0

 chemical resistance

 UV ink

 c M Y K

NaOH 0 1 0 0
HCl 0 4 0 0
[H.sub.2]S[0.sub.4] 1 3 0 0
Detergent 0 1 0 0
[C.sub.2][H.sub.5]0H 0 0 0 0
[C.sub.6]{h.sub.8][O.sub.7] 0 1 0 1
C[H.sub.3],COOH 0 1 0 0
Oil 0 0 0 0
[C.sub.3][H.sub.6][O.sub.3] 1 0 0 0
Soap 0 0 0 0
[H.sub.2]0 + C[0.sub.2] 0 1 2 0