EPS

EPS beads are produced in a polymerisation reaction in which the liquid monomer styrene is heated in water in a vessel with initiators and additives while constantly stirring for several hours. A metastable emulsion of styrene droplets thus forms in water which should be as finely distributed as possible. Under the effect of heat, long chains of or branched polymers gradually build up from the styrene molecules. This way, growing and increasingly solid polystyrene beads are formed from the liquid styrene droplets throughout the polymerisation reaction. Once the target size for the particles is reached, the reaction is stopped, further polymer growth is inhibited and bonding of the beads by adding a stabiliser such as dicalcium phosphate is prevented. Pentane is added to the reactor as the propellant which makes the expandable granules from the polystyrene beads. A final heating of the system ensures complete hardening of the polymer into solid beads which are separated, washed and dried after draining the reactor of the water and then divided into the desired size fractions using screen cuts.

The on-line ultrasonic measuring technology with OPUS is available both for monitoring the droplet size of the finely distributed liquid styrene in the emulsion mixed with water and for monitoring and controlling the polymer growth of the continuously forming polystyrene beads. The ultrasonic measuring finger is incorporated in the reactor for real-time analysis via an adapter flange (AF) through the floor or the wall, meaning that the measurement zone is located directly in the stirred mixture (in-situ). The droplet and particle size, as well as the concentration is measured under process conditions by means of ultrasonic extinction. Around every 2 minutes, the measurement probe delivers a particle size distribution divided into 31 size categories. The characteristic values x10, x50 and x90 can be shown in a Q(t) diagram applied over time thus allowing close monitoring and control in the polymerisation reaction.