PVC Paste Rheology: Study of Process Dependencies M.G. Rasteiro, 1 A. Toma ´s, 2 L. Ferreira, 1 S. Figueiredo 1 1 Chemical Engineering Department, Coimbra University, Coimbra, Portugal 2 Companhia Industrial de Resinas Sinte ´ticas, CIRES, S.A., Estarreja, Portugal Received 24 June 2008; accepted 15 October 2008 DOI 10.1002/app.29931 Published online 24 February 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: A plastisol is a dispersion (suspension) of fine particles of poly(vinyl chloride) with a plasticizer that is normally applied over a substrate, by spreading or coat- ing, before the gelification and fusion into the final prod- uct. Because of the first step, the plastisol rheology study is a very important factor which has to be taken into con- sideration to determine and fine tune the process conditions. This work is concerned with the study of the rheology of plastisols from selected PVC samples produced by the emulsion process, correlating it with the particle size dis- tribution and surface characteristics from the polymer’s powder and modifications during the plastisol aging pro- cess. The results obtained show a significant influence of the original polymer properties in the viscosity aging and viscoelastic behaviour of the plastisol. First, a significant decrease in viscosity can be achieved by changing a partic- ular surfactant during the polymerization step. Second, the type of particle aggregates present in the powder, can determine the evolution of the particle size in the plastisol which, in turn, correlates directly with the observed modi- fications in the aging and viscoelastic behavior. V V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2809–2821, 2009 Key words: PVC plastisol; rheology; particle size distribution; viscosity aging; surfactant; aggregates; oscillatory rheometry INTRODUCTION Poly(vinyl chloride) is an important world thermo- plastic with inumerous applications. Although there are four known production processes, the suspen- sion type represents around 80% of total commercial product, followed by the emulsion and micro-sus- pension type. 1 Products from these later types are normally referred as dispersion resins, because of the premixing with plasticizers and other additives in the so called Plastisol or Paste process. The plasti- sol of poly(vinyl chloride) is a suspension of fine particles in a liquid plasticizer to produce a fluid like mixture that can be spread on to a substrate, poured into a mold, sprayed on to a surface, etc. 2 The coated substrates are then heated in an oven to gel and fuse the material in to the final standard rubbery or foamed products. Because of the particu- lar application type, the rheological behavior of each paste formulation, correlating with the characteristics of the initial polymer, has been continuously studied through several publications. 3–7 The dispersion type polymers are normally made up with different particle sizes, some of them result- ing from an agglomeration process during the dry- ing of the original aqueous emulsion or micro- suspension. Nevertheless, the agglomerated particles are not sufficiently strong to resist the high shearing mixing conditions, together with the solvating plasti- cizer effect over the polymer. This effect is normally translated by an increase of the plastisol viscosity with time, in a process normally called ‘‘viscosity aging’’. 2,5 The rheology of plastisols is also sensitive to the type of surfactants used during the polymerization process, type and quantity of plasticizer, and other additives used in the formulation, such as fillers and thermal stabilizers. 2 The flow of the plastisol at room temperature nor- mally follows the general rules of a concentrated suspension of solid particles in a Newtonian liquid. That is, the pseudoplastic behavior, and also the reported thixotropy under low stress, and dilatancy and fracture at high shear rates can be observed. 6 At commonly used plasticized levels, for instance, 50–100 phr (parts of plasticizer per 100 parts of poly- mer), the particle size and size distribution (PSD) is an important control parameter for the later plastisol viscosity. Once a low viscosity level is normally required, a broader PSD is needed to achieve a max- imum packing efficiency and, thus, to have more free liquid plasticizer per volume that acts as viscos- ity depressant. 2,7 The viscosity aging phenomena can be related with several mechanisms, although the deagglomer- ation and the progressive plasticizer swelling by the polymer can justify the increase in viscosity because of the lower concentration of the liquid phase. 5,7 Journal of Applied Polymer Science, Vol. 112, 2809–2821 (2009) V V C 2009 Wiley Periodicals, Inc. Correspondence to: M.G. Rasteiro (mgr@eq.uc.pt).