NONDESTRUCTIVE EVALUATION OF THE QUALITY (CURE) OF POLYMERIC COATINGS ON STEEL FOOD CANS BY MEANS OF HIGH-FREQUENCY LAMB WAVE-PROPAGATION - A PRELIMINARY-STUDY

Lamb wave propagation in the frequency range from 13 to 30 MHz has been used to monitor structural variations in thin polymer coatings on steel plate used in the food packaging industry. The amplitude of compressional waves reflected from water-plate interfaces was measured as a function of a continuously variable angle of incidence, to a precision of ±0.05 dB. The minima in these reflectivity curves, which signal the onset of Lamb wave propagation, were measured to a precision of ±0.1°. This precision was adequate to allow shifts in Lamb wave phase velocities due to changes in polymer structure and polymer-metal delamination, to be distinguished from changes due to overall thickness variations in the plate and structural inhomogeneities in the steel. This conclusion was reached from a statistical analysis of a large amount of data taken with polymer coatings of variable cure obtained by varying annealing times from 1 to 20 min and annealing temperatures from 170° to 237° C. The lack of systematic relationships between phase velocities and the heat treatment variables suggested that besides the degree of cure, the object of interest, the structure of the batch of polymer coatings studied also varied in other ill-defined ways. However the data remains well worth reporting to introduce a novel technique. © 1990 Chapman and Hall Ltd.