STUDIES ON LASER-INDUCED IRREVERSIBLE SURFACE SOFTENING IN A THERMOSET POLYMER OF ALLYL DIGLYCOL CARBONATE (CR-39)

It is shown that controlled irreversible surface softening can be obtained in thermoset polymer of allyl diglycol carbonate (CR‐39) without degrading its bulk properties on treating it with a cw‐CO2 laser. An average value of the threshold fluence for the onset of softening is found to be about 9 J/cm2, which changes slightly with the interaction time and power density of the laser beam. Beyond this threshold the hardness of the treated surface decreases on increasing the power density and/or the interaction time till the onset of volatile decomposition in this polymer, which takes place at the laser fluence of 25 J/cm2. Thereafter, the hardness tends to saturate at nearly 60% of its original value for the untreated surface. Formation of a new heterogenous interlinked porous microstructure has been observed in the laser softened polymer surface. Solution of the 1‐dimensional heat flow equation incorporating the temperature‐dependent decomposition energy of CR‐39 has shown that at 9 J/cm2 the surface attains the maximum temperature of about 280°C and then cools at a rate of about 103°C/s. The starting value of the surface cooling rate increases with the fluence. A part of the absorbed energy goes in for the depolymerization, which is found to increase from about 0.004 to 4.5 J/cm2 when the laser fluence is increased from 9 to 25 J/cm2. The laser‐induced depolymerization and subsequent rapid cooling of the surface explain the observed effects. Copyright © 1991 John Wiley & Sons, Inc.