Investigation of the Effect of Delayed Reflux on PVC Grain Properties Produced by Suspension Polymerization N. Etesami, M. Nasr Esfahany, R. Bagheri Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Received 16 May 2009; accepted 22 January 2010 DOI 10.1002/app.32130 Published online 13 April 2010 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The effects of the condenser operation on properties of polyvinyl chloride (PVC) particles produced by suspension polymerization process were investigated in a pilot scale reactor. It was observed that delaying reflux operation increased the cold plasticizer absorption of the final resin. Both bulk density and K-value of the PVC powder decreased by increasing time delay in the reflux operation. It was also found that commencement of reflux- ing before 20% conversion resulted in bimodal particle size distribution (PSD), while monomodal PSD was obtained for longer delays in refluxing. SEM micrographs showed that surface of produced particles were rough and smooth when reflux started before and after 20% conver- sion, respectively. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2506–2514, 2010 Key words: polymerization; suspension; morphology; reflux; polyvinyl chloride; particle size INTRODUCTION The great number of applications of polyvinyl chlo- ride (PVC) is largely due to its particle morphological characteristics together with its low price. Particle morphology is an important factor influencing the removal of unreacted vinyl chloride monomer (VCM), plasticizer uptake, processability and bulk density (BD) of the powder, which are important for downstream processing. In general, the morphology of suspension polymerization (S-PVC) particles is determined by the size distribution, shape, and the average porosity of the grains. About 80% of the global PVC production is by suspension polymerization process. 1 In suspension polymerization process, VCM is dispersed as drop- lets into the continuous aqueous phase containing suspending agents by vigorous agitation. Each dis- persed droplet acts as a minibulk polymerization re- actor. The polymerization reaction is strongly exo- thermic (1540 kJ/kg). Batch reactors which are used to carry out the polymerization reaction at constant temperature are equipped with a cooling jacket for removal of the reaction heat. Large scale reactors are also equipped with overhead condensers to remove part of the reaction heat through the evaporation and subsequent condensation of the monomer. The operation of the reflux condenser affects the particle morphological properties of the final PVC grains. Particle formation can be described considering the fact that PVC is not soluble in its own monomer. PVC molecular chains start to precipitate from monomer phase inside droplets at very low conversions (X < 0.1%). By increasing the conversion, formation of pri- mary particle nuclei takes place. Because of limited stability, primary particle nuclei rapidly undergo coagulation leading to the formation of primary par- ticles (X < 1%). 2–4 Primary particles grow by coagula- tion to form a continuous ‘‘cloud’’ inside the dis- persed droplets (4.5% conversion 4 ). Continuous three-dimensional primary particle network within the droplets has formed at critical conversion (X C ¼ 15–20%). 5 Primary particles grow further and fuse until the limiting conversion in which monomer rich phase disappears is reached (X f ). Although the effects of polymerization temperature, quality of agitation, type, and concentration of the suspending agents on properties of the final PVC particles have been stud- ied, 3,6–16 very few reports are available in the litera- ture addressing the effect of the reflux condenser operation on the final product properties. Koyanagi et al. 17 used a 1-m 3 polymerization reac- tor equipped with a multitubular condenser and showed that the commencement of refluxing before 5% conversion resulted in coarser final product. Kobayashi et al. 18 reported that monomer refluxing increased porosity and decreased BD of the final product. Cheng and Langsam 19 investigated the interaction between some process variables such as: rate of heat removal by the condenser, the reflux start time and suspending agent concentration on Correspondence to: M. N. Esfahany (mnasr@cc.iut.ac.ir). Contract grant sponsor: Petrochemical Research and Technology Co.; contract grant number: 84269. Journal of Applied Polymer Science, Vol. 117, 2506–2514 (2010) V C 2010 Wiley Periodicals, Inc.