engleski

Evaluation of the Rub Resistance of Offset UV Ink Layers on Papers with Wheat Pulp

Paper composition, from the aspect of the printing substrate, and especially its surface characteristics, are of immense importance for its printability and the achieved quality of the final graphic product. When we talk about print quality, the stability of printed papers under various influences such as time, chemical agents, transport, pressure, and others are also needed to consider them as quality prints. The ink rub resistance, as one of the most important properties of printed material during the shipping or handling processes, refers to the degree of removal of an ink layer under the action of rubbing. The quality of the printed product is determined not only by the printing substrate and the selected ink (which is defined by the printing technique) but also by their interaction. Therefore, it is particularly important to choose an adequate printing substrate to achieve a quality printing. Increased concern for the environment has led to the idea that wood fibres in the pulp and paper industry must be replaced, as much as possible, with non-wood fibres. Nowadays, more and more studies indicate that fibres of various non-wood raw material can be a good substitute for wood raw fibres in the paper industry. Wheat pulp is a by-product of crop production and it has been selected as an alternative source of fibre because it is available from the annually renewable crops and is produced abundantly in numerous regions all over the world. In laboratory paper production, the collected and purified wheat straw was first cut manually and processed by the soda pulping method. The obtained unbleached semi chemical wheat pulp was added into the pulp of recycled wood fibres in a ratio of 30% to wood fibres, and laboratory papers of approximately 42.5 g/m2 were formed by Rapid-Köthen sheet former (FRANK-PTI) according to standard EN ISO 5269- 2:2004. Laboratory paper made only from pulp of recycled wood fibres was used as a reference sample. For a better insight into the utilisation of wheat pulp in pulp and paper industry, a control sample of commercially produced newsprint paper made from the pulp of recycled wood fibres was used for comparison with laboratory-made samples. In this research, the emphasis was placed on laboratory papers made with the addition of wheat pulp and on stability of the prints after they were printed with cyan, magenta, yellow and black inks by offset printing technique. Offset is an indirect lithographic printing technology and is the main printing technique in various publications. The offset printing process depends on many chemical and physical characteristics of materials and components which are involved in the process. The full tone areas with cyan, magenta, yellow and black inks have been achieved with Suncure Starluxe UV curable ink (manufacturer Sun Chemical) by Prüfbau (Multi-purpose printability tester) which simulates offset printing. After printing and drying, the samples were subjected to a mechanical resistance test (i.e., rub resistance test) on a Hanatek T4 Rub and Abrasion Tester according to the standard BS 3110 with 20, 40 and 60 circular motions at a constant speed of one rotation per second. Evaluation of the rub resistance of the UV ink layer on papers with and without wheat pulp is based on the CIE L*a*b* colorimetric values. Colorimetric values were measured before and after performed rub resistance test. The Euclidean colour difference (ΔE00) was calculated to define the tolerance between changes in coloration, i.e., fading of the colour. As mentioned, the ink rub resistance strongly depends on the surface characteristics of printing substrate. It is important to emphasize that laboratory-made papers have not gone through the final stages of surface treatment as commercially produced papers. One of the most important surface characteristics that directly affects the rub stability is roughness of the paper. Laboratory papers have a far higher surface roughness than commercially produced ones due to the lack of surface treatment. Roughness profiles of the samples' surfaces was measured with a Mahr MarSurf PS 10 profilometer to study the correlation between surface characteristics of papers with wheat pulp and rub resistance after printing them with offset UV inks. As expected, commercial papers showed better rub resistance of all printed cyan, magenta, yellow and black offset inks compared to laboratory papers. Of all analysed offset inks on commercial paper, the black ink layer showed the lowest rub resistance (ΔE00 < 1.5), while the yellow ink layer showed the highest rub resistance (ΔE00 < 0.6). Compared to commercial papers, laboratory-made reference samples after printing with cyan, magenta and yellow ink showed lower rub resistance i.e., greater ΔE00 value, while black ink layer showed approximately the same rub resistance on both papers. Adding 30% wheat pulp into the pulp of recycled wood fibres, resulted with laboratory papers that provide lower rub resistance after printing. Observing all process inks, the cyan ink layer on papers with wheat pulp had the lowest rub stability. However, the calculated ΔE00 values of ink layers of all prints were within the tolerated range, according to the tolerance definition where ΔE00 ≤ 2 is classified as a very small noticeable difference for the standard observer. In this study, it is observed that the highest colour degradation occurs after only 20 rotations of the rubbing test, while further rubbing up to 60 rotations has a negligible impact on the colour degradation of the offset UV ink layers. This research contributes to the necessary knowledge about the possibility of using non-wood sources of cellulose fibres in the paper and graphic industry, given that wood sources are increasingly limited.

offset printing, paper, roughness, rub resistance, UV ink, wheat pulp

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