EFFECT OF THE PRESSURE DRIP RATE ON CELL NUCLEATION IN CONTINUOUS PROCESSING OF MICROCELLULAR POLYMERS

Microcellular plastics are cellular polymers characterized by cell den sities greater than 10(9) cells/cm(3) and cells smaller than 10 mu m. One of the critical steps in the continuous production of microcellula r plastics Is the promotion of high cell nucleation rates In a flowing polymer matrix. These high nucleation rates can be achieved by first forming a polymer/gas solution followed by rapidly decreasing the solu bility of gas in the polymer. Since, in the processing range of intere st, the gas solubility in the polymer decreases as the pressure decrea ses, a rapid pressure drop element, consisting of a nozzle, has been e mployed as a continuous microcellular nucleation device. In this paper , the effects of the pressure drop rate on the nucleation of cells and the cell density are discussed. The experimental results indicate tha t both the magnitude and the rate of pressure drop play a strong role in microcellular processing. The pressure phenomenon affects the therm odynamic instability induced in the polymer/gas solution and the compe tition between cell nucleation and growth.