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Preparation of Poly(Vinyl Chloride) Latexes Using a Dual Surfactant System: the Effect in the Particle Size Distribution

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Preparation of Poly(Vinyl Chloride) Latexes Using a Dual Surfactant System: the Effect in the Particle Size Distribution Preparation of Poly(vinyl chloride) Latexes Using a Dual Surfactant System: The Effect in the Particle Size Distribution A. Toma´s,1 M. H. Gil,2 J. C. Bordado,3 P. Gonc¸alves,1 P. Rodrigues1 1Companhia Industrial de Resinas Sinte´ticas, CIRES, S.A., Development Department, Avanca-Estarreja, Portugal 2Chemical Engineering Department, Coimbra University, Coimbra, Portugal 3Chemical and Biological Engineering Department, Instituto Superior Te´cnico, Lisboa, Portugal Received 31 May 2008; accepted 10 October 2008 DOI 10.1002/app.29509 Published online 5 February 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The control of the average particle size and the derivative diffusional swelling techniques, no kind of size distribution in the emulsion polymerization of vinyl high shear is applied to the monomer and/or surfactant chloride monomer is an important parameter to determine, mixtures. In fact, the concentration and selection of the ani- not only the latex characteristics, but also the properties of onic surfactant, together with the fatty-alcohol, can play the final dispersion powder in several industrial applica- an interesting role on the final PSD of the polymer’s particles. tions. It is possible to manipulate the particle size distribu- VC 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1416–1424, 2009 tion (PSD) by applying a mixture of an anionic surfactant- fatty alcohol before the start of the free radical polymeriza- Key words: miniemulsion; diffusional swelling; VCM; tion. Contrary to the procedures of the miniemulsion and particle size distribution; fatty alcohol INTRODUCTION cizer dosage or to improve the application process of the plastisol.5 The control of the average particle size and size dis- It has long been known, from the pioneer work of tribution during the emulsion polymerization of Ugelstad and coworkers,6–8 that the addition of vinyl chloride monomer (VCM) has been studied for long-chain fatty alcohols increases the dispersion decades, since they represent an important parame- capacity of anionic surfactants. With a mixed system ter for the definition of the rheological behavior of of sodium lauryl sulfate (SLS) and cetyl alcohol the polymer dispersion into the liquid plasticizer, in (CA), Ugelstad et al.6 reported that the polystyrene the so called paste or plastisol’s process.1 polymer particles at the end of the polymerization In the conventional emulsion polymerization, an had a similar size of the starting monomer droplets. ionic surfactant, an inorganic initiator and the vinyl This discovery gave rise to a new technique where monomer are dispersed in a water phase to produce the micellar and homogeneous particle nucleation a monomodal particle size distribution (PSD) in the could be replaced by the nucleation in the monomer submicron range.2 Adding this, the utilization of a droplets. Following these early publications, soon seeding technique from the start of polymerization the term Miniemulsion was applied for the case, allows the production of higher particle sizes and where the liquid monomer droplets are stabilized even multimodal distributions.3,4 The distribution of against molecular diffusion (mechanism 1) and coa- the different class sizes at the end of polymerization lescence (mechanism 2) from the beginning of the will define mostly the flow type of the final plastisol. polymerization.3 Thus, for an optimum packing, the broadening of From the work of Higuchi and Misra,9 it was pos- the distribution will contribute for a low viscosity sible to conclude that if the small droplets are not level, since more liquid plasticizer will be available. stabilized against diffusional degradation (Ostwald This effect is normally applied to decrease the plasti- ripening), they will disappear with the observed increase on the average droplet size. Therefore, mechanism 1 is normally prevented by adding a Correspondence to: A. Toma´s ([email protected]). small quantity of a highly monomer-soluble and Contract grant sponsor: FCT; contract grant number: water-insoluble agent (hydrophobe), referred as SFRH/BDE/15534/2005. cosurfactant or coemulsifier. Journal of Applied Polymer Science, Vol. 112, 1416–1424 (2009) The effects of the second mechanism are sup- VC 2009 Wiley Periodicals, Inc. pressed as much as possible, by adding an PREPARATION OF POLY(VINYL CHLORIDE) LATEXES 1417 Figure 1 Procedures for: (a) miniemulsion followed by polymerization; (b) diffusional swelling followed by polymerization. appropriate surfactant (emulsifier), assuring that the merization, the Z compound normally matches to droplets do not aggregate by collision and the vinyl monomer.11 coalescence.3 Figure 1(a,b) illustrates the differences between The process of miniemulsion follows general rules the standard miniemulsion and the diffusional swel- where the selected monomer, surfactant and cosur- ling methods. factant are normally subjected to a previous homog- Although both methods have a wide range of enization step with a variable shear applied by applications, the homogenization conditions, as well mechanical or ultrasound devices. However, for the the type and concentration of the selected compounds case of vinyl chloride, this procedure is very compli- were found to be fundamental on the dispersion stabil- cated by the fact that the monomer is gaseous at ity and final properties of the emulsion polymers.2,10 normal pressures, requiring special precautions due Because of the high water solubility of the mono- to his high flammable and toxic nature. mer and the strongly exothermic free radical reac- To overcome that problem, Ugelstad et al.10,11 tion, a semicontinuous reactor operation is often reported a new procedure where the monomer was applied to have a tight control over all the process added to a previous mixed emulsifier system, con- parameters,12 namely, the surfactant concentration, sisting of a long fatty alcohol and an ionic emulsi- over the particle size evolution, followed by the dos- fier. This so called the Diffusional Swelling method, age of monomer and initiator over the conversion involved, in a first step, an intense homogenization rate. For example, the application of several reaction of an aqueous mixture of a high insoluble com- steps, with a starting mixture of water, emulsifier pound (Y compound) and an emulsifier. After this, a and initiator to which some portion of monomer is new compound (Z compound), which has higher added. After this so called nucleation step, others water solubility, was added to the said pre-emulsion steps can be made, charging more monomer, water to diffuse through the water into the Y droplets. or other ingredients over the pre-establish feed strat- When applying this procedure to the emulsion poly- egy of the polymerization recipe.13 Journal of Applied Polymer Science DOI 10.1002/app 1418 TOMA´ S ET AL. Figure 2 Scheme of the pilot plant. Following the coemulsifier technique, the aim of The critical micellar concentration of all anionic this article is to examine the effect of applying a pre- emulsifiers was determined according to the method vious mixture of a surfactant and a fatty alcohol Wilhelmy-Plate at 298.15 K (25C). before the first nucleation step correlating with the final polymer’s particle size and size distribution. Procedure Nevertheless, the homogenization step, normally applied to achieve a higher number of small mono- The polymerizations reactions were carried in a pilot mer droplets, was substituted by a mild stirring of plant equipped with a 5 L reactor (designed by the surfactant and cosurfactant at a convenient CIRES, S.A.). Figure 2 shows a scheme of the pilot temperature. plant installation, with several tanks and dosing Adding that, by changing the concentration and pumps for the dosage of surfactants, water and type of the surfactant, the micellar nucleation can be monomer into the reactor. The reacting mixture was strongly affected with the correspondent differences stirred with an anchor agitator, which was con- at the end of the emulsion polymerization. nected to a variable speed motor. The emulsion tem- perature was measured by a thermocouple installed inside the reactor, whose outlet signal was directly EXPERIMENTAL connected to a PID temperature controller of an external water bath that, through a closed circuit into Materials the reactor jacket, controls the reaction temperature. For the production of the emulsion polymer sam- After the initial charge with deionized water, the ples, VCM, from Shin Etsu B.V. [purity 99.99% (w/ (mixed) emulsifier system and the initiator (solid) w) from chromatographic method] was used with were charged, the reactor was closed and the mix- À deionized water (conductivity < 10 lsm 1) (indus- ture heated to 318.15 K (45C). At this temperature, trial utility). As main surfactant, ammonium laurate vacuum was applied to remove oxygen that, other- (AL), produced from the neutralization of lauric acid wise, would affect the reaction kinetics.14 After the (min. 99% C12 purity) and ammonia solution (20% vacuum operation, the reaction temperature (Tpoly) w/w) (commercial grades) and as a free radical ini- set-point was set to 324.65 K (51.5C) and the desig- tiator, ammonium persulfate (purity 99% w/w) nated quantity of VCM for the first step was charged (commercial grade) from Brenntag S.A. As others with a reciprocating pump. When it was detected a studied surfactants, commercial grades of SLS, so- 0.5 Â 105 Pa G difference between the reactor’s pres- dium octyl sulfate (SOS) and CA (min. 95% C16 pu- sure and the correspondent saturation pressure of rity) from Cognis Iberia S.A. VCM (at 324.65 K), a new step was initiated with a Journal of Applied Polymer Science DOI 10.1002/app PREPARATION OF POLY(VINYL CHLORIDE) LATEXES 1419 TABLE I were previously diluted with water and added a Polymerization Recipe (Three Steps) small portion of a selected surfactant and ammonia. Tpoly: 51.5 C 200 rpm All LDS analyses were repeated twice for the same Initial charge Mass, g sample.
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