ANALYSIS OF THE ACCELERATED THERMAL AGING OF OIL INK VEHICLES USING ISOTHERMAL THERMOGRAVIMETRY

It is generally believed that the poor deinkability of aged prints fro m the coldest printing process is caused by the oxido-polymerization o f the oil ink vehicles, which solidifies the ink film and increases it s degree of bonding to paper. The accelerated thermal aging of two veg etable oils and one mineral oil used as vehicles in offset inks was st udied using isothermal thermogravimetry. The basis of the method relie s on the weight gain due to the oxygen uptake by the oil samples durin g the autoxidation process. The method was able to monitor the charact eristic stages of the autocatalytic oxidation of vegetable oils: induc tion, propagation and termination. Although the mineral oil oxidized t o a small extent and at a very low rate under harsh conditions, there was no sign of the solidification observed with soya bean and linseed oils. From the thermograms of the vegetable oils, two parameters were determined: the induction time and the oxidation time. Using a stepwis e regression algorithm, the influence of temperature and oxygen concen tration on the two parameters was determined. Temperature was found to have the largest effect on both the induction time and the oxidation time.