Viscoelastic investigation on the cure of unsaturated polyester coatings

Abstract

The curing process of unsaturated polyesters has been investigated chemically or electrically, whilst the changes of physical properties with time during the cure have not been dealt with because of experimental difficulty. Previously, authors reported that it is possible to obtain the complex moduli of attached paint films using a vibrometric technique, and this technique may be considered to be applicable for the aim described above. In this study, the differences in the rate of polymerization and physical properties were determined viscoelastically with polyester films which had been formed under different dnying conditions.
The experimental method employed was the same reported in the previous paper, but unsaturated polyesters were coated on to the one side of substrate steel panels. The polyester resin used was prepared from phthalic anhydride, maleic anhydride, diethylene glycol and propylene glycol, the molar ratio of which was 5 : 2 : 2 : 5, and 70 parts of the resin prepared were dissolved in 30 parts of styrene monomer. Methylethylketon peroxide solution (60% dimethylphthalate soln.) and cobalt naphthenate solution (1.2% metal cntent) were used as a radical initiator and an accelerator respectively. The drying schedule employed in this experiment is shown in Table 1.
Table 1 Drying schedule of unsaturated polyester coatings
Component ratio
Sample Drying temp.°C
polyester soln.initiator soln.accelerator soln.
A 100 1 1 20, 30, 40 B 100 2 2 20, 30, 40 C 100 5 5 20, 30, 40
Imediately after coating, the panels to be tested were mounted on the vibrometric apparatus as illustrated previously, and the progress of polymerization of the coated polyesters was followed by the measurements of natural resonance frequency and band width under forced oscillation. From the result of experiments, it is obtained that the dynamic modulus (E) increases with time, while the loss modulus (É) decrease after reaching its maximum value. The temperature (TÉ) for which loss modulus takes a maximum value has been considered to relate with the glass transition temperature, which rises with the development of crosslinkage in polymer. Since the time (tÉ) required for the arrival of the maximum É may be assumed to be inversely proportional to polymerization rate, we can evaluate the rate of dry by means of the determination of tÉ. In this experiment, the rate of polymerization indicated by the reciprocal of tÉ seemed to be nearly proportional to the amount of the initiator incorporated, though a little discrepancy had been recognized. For the heat treated films at 120°C for 4 hours, in spite of the decrease of E with increasing initiator, the rise of TÉ has been observed. This, the reason for which has not been explained, is assumed to indicate the change of chemical compositions caused by the variation of drying schedule.