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Application of FTIR in structural analysis of flexographic printing plate

The aim of this paper was to characterize the chemical structure of the photopolymer material of the printing plates used in flexography. It was based on the presumption that it is possible to define the changes in surface structure of the photopolymer material on the functional properties of the printing plate by means of FTIR (Fourier transform infrared spectroscopy). Since FTIR is a characterization technique which is usually used in order to obtain an infrared spectrum of a solid, liquid or gas in this paper it was used in order to analyze the variation in chemical structure of photopolymer material which can happened during the printing plate processing phase. Flexographic printing plates are characterized by a geometrical difference between image and non-image areas and a flexible polymer material. They are mainly made from photosensitive monomers and a number of additives, such as photo initiators and plasticizers, to obtain high level of their quality. During the processing phase photopolymer material should be several times exposed to UV radiation. During the exposure the photo polymerization occurs in a polymer and exposed parts of the material form the image areas on the printing plate. Unexposed parts of the polymer material have to be chemically removed in processing solution and form non-image areas. Afterwards, in order to obtain the optimal functional properties of the printing plate, photopolymer material should be post-treated by UV radiation. This post- treatment phase, specifically UVA and UVC post- treatment, were in the focus of this paper. UV post-treatments are performed in the flexographic printing plate workflow in order to finish the photopolymerization process and determine the surface and mechanical properties of the printing plate. Those were selected for this research because it was shown in previous papers that duration of UVA and UVC post- treatments have a significant influence on the functional properties of finished flexographic plates. In order to get a detailed overview of changes and to make a correlation with FTIR results a surface free energy was calculated as well. Those results were of great importance because they have a significant influence on the adsorption of printing ink on image surfaces during the printing process and its transfer to the printing substrate. Results have proved that prolonged UVA and UVC post- treatments cause the changes in the properties of the photopolymer material. With prolonged UV post-treatments, surface free energy of the photopolymer material increased, with the emphasis on its polar component. FTIR spectroscopy of samples was then applied as a tool for determination of the cause and type of chemical changes which occurred in the samples. The results have shown that FTIR spectroscopy can be used in characterization of photopolymer printing plates. Analysis of the obtained FTIR spectra indicated that the reason for the increase of the photopolymer’s surface free energy is the increased amount of the hydroxyl and carbonyl groups in the surface of the material. Therefore, in order to achieve the printing plate of high quality, the optimal oxygen amount on the printing plate’s surface should be obtained by the correct duration of the UV post-treatment. FTIR spectroscopy, combined with the other analysis methods, proved to be a useful method for detailed research and definition of the flexographic printing plates.

flexography; photopolymerization; UV exposure; FTIR

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