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.