Polymerization microsuspension

Microstructures. See also Microstructure fatigue properties and, 73 484-486 in lotus effect surfaces, 22 117-120 Microsuspension polymerization, of PVC, 25 670... 

An inverse suspension polymerization involves an organic solvent as the continuous phase with droplets of a water-soluble monomer (e.g., acrylamide), either neat or dissolved in water. Microsuspension polymerizations are suspension polymerizations in which the size of monomer droplets is about 1 pm. 

Microsuspension Polymerization. Microsnspension polymerization uses a monomer-soluble initiator. The monomer is homogenized in water... 

Figure 8.2. SEM analysis of PS-DVB (polystyrene-co-divinylbenzene) microspheres prepared by two-step microsuspension polymerization method, particle diameter 2-20 pm (39).

If surfactant is added to a suspension polymerization system, a number of phenomena may occur. If the surfactant is added in small amounts (below the critical micelle concentration or CMC), the reduction in interfacial tension between the organic and aqueous phases will result in smaller monomer droplets, but it has hardly any other effect. If surfactant is added above the CMC, and an oil-soluble initiator is used, the process is commonly termed a microsuspension polymerization. Due to the reduced interfacial tension, the droplet diameter (and hence bead diameter) is reduced to approximately 10-40 pm. Little polymerization takes place in the aqueous phase or in particles generated from surfactant micelles because of the hydrophobic nature of the initiator. However, some smaller particles initiated from surfactant micelles may be found. The kinetics are still essentially those of a bulk free radical polymerization. Microsuspension polymerization is used to produce pressure-sensitive adhesives for repositionable notes. 

To compensate for the low toughness in ASA when using small rubber particles, large particles prepared using the microsuspension polymerization process can be added to the products (700 nm to 100 xm) [22]. Further work showed that the use of particles with a diameter of 0.15-0.8 p,m brings better toughness than a particle size of less than 0.1 xm [33]. 

The use of core-shell impact modifiers for sPS is also patented in EP 318793 [15] (see Table 19.1). These impact modifiers are usually prepared using the emulsion polymerization process, although other methods such as the microsuspension polymerization process are possible. The core usually consists of polymers prepared from an acrylate, especially butyl or 2-ethylhexyl acrylate or butadiene. These rubber particles are then grafted with vinyl monomers, where... 

Emulsion polymerizations normally produce polymer particles with diameters ofO.I-l pm(l pm= I micron= 10 cm), although much larger particles can be made by special techniques mentioned in Chapter 8. Tlie polymer particles made by suspension reactions have diameters in the range of 50-500 pm. Recall that free-radical initiation in suspension reactions is in the monomer phase, whereas the aqueous phase is the initiation site in emulsion polymerizations. The two processes often dilTer also in the types of stabilizers that are used. Microsuspension polymerization is an alternative technique which can yield particles in the same size range as emulsion processes. This method uses a monomer-soluble initiator and anionic emulsifiers similar in nature and concentration to those used in emulsion polymerizations. A microdispersion of the mixture of the reaction ingredients is first produced mechanically and is then polymerized to provide polymer with essentially the initial fine particle size distribution. 

Usually, monomer droplets are bdieved not to play any role in emulsion polymerization other than as a source of monomer. Ugelstad and associates have shown, however, that in cases with very small monomer droplets, these may become an important, or even the sole, loci for particle nucleation. The system may then be regarded as a microsuspension polymerization with water-soluble initiators. It has therefore been pointed out (Hansen and Ugelstad, 1979c) that particle nucleation mndels should include all three initiation mechanisms— micellar, homogenous, and droplet—since all these mechanisms may compete and coexist in the same system, even if one of them usually dominates. 

Microsuspension and Inverse-microsuspension. In suspension polymerizations, particle formation occurs through a droplet breakup-coalescence mechanism, with the diameter controlled by the temperature, interfacial tension, agitation intensity and conversion. Suspension polymerizations have typically been characterized by an initiator soluble in the monomer phase and particle diameters in the 50-1000 pm range [40]. Smaller particles (0.2-20 pm) have been produced at higher agitation speeds (lower interfadal tensions) [41] and in such cases a prefix micro has been added to the nomenclature (microsuspension) to reflect both the dominant synthesis conditions (suspension) and the nominal particle size (1 micron). Therefore, microsuspension polymerization has historically referred to a subdomain of suspension polymerization occurring at smaller particle sizes. Based on an analogy to this nomenclature, inverse-microsuspension polymerization has been proposed for similar water-in-oil... 

Figures 11 and 12 show experimental data for copolymerizations of acrylamide with DMAEM at 60 and 50 C. The solid line is the kinetic model for inverse microsuspension polymerization in isoparaffinic solvents stabilized with fatty acid esters of sorbitan. This model was derived for acrylamide homopolymers (66) and has been extended to include copolymerizations with cationic monomers. Good agreement with the data can be observed. The details of the mechanism will be discussed in a subsequent publication (67).

Figure 12. Conversion vs. time data of an inverse microsuspension polymerization of acrylamide and DMAEM at 50 C and 50 wt % total monomer concentration (fjo = 0.875). The phase ratio of water to oil was 0.74 1 y with 10 wt % sorbitan monooleate (based on the organic phase). Polymerization was initiated with 7.373 X 10 mol AIBN. The solid line is the model...

Table 2.4 gives data on commercial grades of microsuspension polymerized... 

Bulk or mass polymerization" Gas-phase pol3mierization Precipitation polymerization Suspension polymerization Microsuspension polymerization Dispersion polymerization Emulsion polymerization Miniemulsion polymerization Microemulsion polymerization... 

PlaStiSOlS. Plastisol-forming pol5rmers are produced by microsuspension polymerization or dispersion polymerization (9). Microsuspension produces very fine particles of monomer to ensure that small particle sizes of polymer are produced. In emulsion or dispersion polymerization the vinyl chloride is dispersed in a water-soluble initiator the vinyl chloride particles are small and stabilized using surfactants. There are also several variations of these two basic techniques. 

Microsuspension polymerization, in which a monomer-soluble initiator is used. Polymerization takes place within the fine droplets in which the monomer is dispersed by passing a coarse emulsion of monomer, initiator, and surfactant through a mechanical homogenizer (24-26). The resulting particle size distribution is Gaussian between 0.2 and 1.2 /xm. 

Microsuspension polymerization is a process used in the PVC industry to produce resins for plastisols [125], In this process, which resembles miniemulsion polymerization, a mixture of monomer and an oil-soluble initiator are dispersed in an aqueous solution of surfactants using intensive shear. The monomer droplets are polymerized yielding particles usually <2 pm, which are normally isolated by spray drying as they cannot be separated by centrifuging or filtering. These particles are solid and nonporous. The polymer particles are larger than the monomer droplets (0.1-2 pm) because the combined effect of the Ostwald ripening (as no costabilizer is used in the formulation) and droplet/particle coalescence. 

Issacs H, Schwartz R, Garti N, Lemer F. Microsuspension polymerization of vinyl chloride. Tenside Det 1987 24 220-226. 

Small-particle PVC (< 1 pm diameter) can be produced either by true emulsion or by microsuspension polymerization. VCM, surfactant (usually a soap, such as sodium lauryl sulfate), and a peroxide or azo initiator are charged to a reactor along with water. Polymerization yields latex at about 40% solids, which is steam stripped and then spray dried. 

Recently, Cozens [34] has reported the suspension polymerization of vinyl chloride using a LPO-Cu metal chelate redox pair system. The suspension polymerization of vinyl chloride [35] was also studied using a diacyl peroxide such as BzaOj-Cu as the redox initiator. The microsuspension polymerization of vinyl chloride was carried out at 40-60°C. The conversion of 85% was obtained after 10 hr of polymerization according to the typical recipe presented in Table 5. 

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