Stabilisation of diazo compounds used in lithographic systems.
The research undertaken has been an investigation of the deterioration that occurs during lithographic plate manufacture and storage. The aim of the research was to identify and eliminate the causes of the deterioration. Deterioration was found to be due to the presence in the formulation of diazonium compounds that were thermally unstable. These compounds were studied in coating solutions and printing plates. Alternative analytical techniques for diazonium analysis were examined. It was found that HPLC and UV/vis spectroscopy were most useful. These techniques gave information on loss of specific diazonium compounds or loss of diazonium character respectively. Identification of the decomposition products and an assessment of the components of the matrix that influenced the rate of decomposition provided information on the decomposition mechanism. This was found to be largely free radical based and hence accelerated by the presence of electron donating solvents. Several diazonium salts were examined notably to determine what substituents influenced the rate of decomposition. Ortho and para electron donating substituents improved stability. The counterion used also had an influence on diazonium stability. Tetrafluoroborate salts were found to be more stable than hexafluorophosphates. Diazonium salts with other counterions were also prepared and the short chain alkyl sulphonates were particularly stable. Additives and stabilisers were assessed but only the use of crown ethers and high molecular weight polyethers gave a significant diazonium stability improvement. A comparison was made between the effects of photolytic and thermal decomposition. Fortunately, compounds that are thermally stable react readily to light, resulting in good lithographic properties. This was explained as being due to a heterolytic mechanism predominating for photolytic reaction compared to a homolytic mechanism for thermolysis.
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Sheffield Hallam University (United Kingdom).