Emulsion polymerization surfactant-free system

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Emulsion Polymerization. Poly(vinyl acetate)-based emulsion polymers are produced by the polymerization of an emulsified monomer through free-radicals generated by an initiator system. An emulsion recipe, in general. contains monomer, water, protective colloid or surfactant, initiator, buffer, and perhaps a molecular weight regulator. 

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

In order to gain evidence for interfacial initiation, the redox initiator system was compared with a water-soluble initiator (VA-044) in terms of the emulsion polymerization behavior of butyl acrylate (BA)/[2-(methacryloyoxy)ethyl] trimethyl ammonium chloride (MAETAC). It was found that for the water-soluble initiator system, only homopoly(MAETAC) was formed and BA did not polymerize at all. In the case of VA-044, it was suggested that it may be difficult for polymeric free radicals in the aqueous phase to penetrate the viscous surfactant layer to initiate the polymerization of the BA monomer. On the other hand, it has also been found that BA could be rapidly polymerized under the same conditions if VA-044 is replaced with CHP/TEPA, indicating that radicals are formed in the interface, where they do not need to penetrate through viscous surfactant layer. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

Emulsion polymerization is an important commercial process because, in contrast to the same free-radical polymerization performed in the bulk, molecular weight and reaction rate can be increased simultaneously (1-3). Furthermore, the lower viscosity of an emulsion system compared with that of the corresponding bulk process provides better control over heat transfer. Commercial emulsion processes usually use a surfactant/water/monomer system... 

As explained before, when surfactant, water, and monomer(s) are mixed, the colloidal system obtained consists of monomer-swollen micelles (if the surfactant concentration exceeds its CMC) and monomer droplets dispersed in an aqueous phase that contains dissolved molecules of surfactant and a small amount of the sparingly water-soluble monomer(s). When free radicals are generated in the aqueous phase by action of an initiator system, then the emulsion polymerization takes place. Its evolution is such that the colloidal entities initially present tend to disappear and new colloidal entities (polymer latex particles) are bom by a process called nucleation. For convenience, we first focus on the particle nucleation mechanisms, a very important aspect of emulsion polymerization. 

When reviewing the published literature on the emulsion polymerization of vinyl acetate, one is struck with seemingly contradictory data presented by many reputable research teams. Some of these results published may not be strictly comparable because of variations in the polymerization recipes used. For example, the effect of the emulsifiers on the rate of polymerization may have a profound effect on the course of the reaction. In a persulfate-initiated system using no other surfactant, it has been postulated that the free radicals formed fixrm the decomposition of the initiator combine with the monomer in solution. As polymer forms, aggregates develop which absorb more monomer and the number of particles increases up to a constant value (at about 5% conversion). Then, while the number of particles remains constant at 1.7 X 10 per ml, the reaction rate increases. Ultimately, as a last stage of the reaction, the rate begins to drop off. The latex formed in this process is said to consist of particles of great uniformity with a diameter of 0.26 fim [137]. 

Many investigators have studied polymer surfaces for years [74,75] and have been successful in determining combinations of two or more valence states [76,77] by the mathematical process of deconvoluting the peak assignments [78]. It was only recently that latexes were examined by ESCA. Davies et al. [79] prepared a series of homopolymers of poly(methyl methacrylate) (PMMA) and poly(butyl methacrylate) (PBMA), and also poly[(methyl methacrylate)-co-(butyl methacrylate)] (PMMA-PBMA), by surfactant-free emulsion polymerization. It was found that the surface of the latex film was rich in PMMA, which may possibly be explained by the reactivity ratios for the MMA/BMA system (ri = 0.52 and rj = 2.11) [80], Recently, Arora et al. carried out angle-dependent ESCA studies on a series of films prepared from core-shell ionomeric latexes (with a polystyrene core and a styrene/n-butyl acrylate/ methacrylic acid copolymer shell) to determine the distribution of carboxyl groups in the films [81,82]. 

In batch experiments, the solids were varied from 35 to 75% [10]. The primary surfactant was Aerosol A103 (disodium ethoxylated nonyl phenol half ester of sulfosuccinic acid) with HD as the cosurfactant. These were used in concentrations of 1 and 4 wt% on monomer, respectively. Two KPS concentrations, 1 and 2 wt% on water, were tried. The miniemulsions were produced by ultra-sonification. Parallel conventional emulsion polymerizations were conducted for comparison to the miniemulsion polymerizations (75 °Q. Coagulum-free latexes resulted from miniemulsion polymerizations up to 60% solids, while only 50% solids could be achieved for the cxmventional process. These differences were attributed to the resulting particle size distributions where the miniemulsion polymerizations produced latexes with larger particles, broader distributions and lower viscosities than their conventional counterparts. As in other studies, this difference in PSDs was explained by differing nucleation mechanisms. However, as in other studies, it was not possible to determine whether the nucleation in the miniemulsion systems was predominantly by radical entry into chxjplets. 

During nucleation, monomer droplets, monomer swollen micelles and monomer swollen polymer particles coexist in the batch reactor. Polymer particles efficiently compete for radicals and as their number increases, they become the main polymerization lod. The monomer that is consumed by free-radical polymerization in the polymer particles is replaced by monomer that diffuses from the monomer droplets through the aqueous phase. Therefore, the size of the particles increases and that of the monomer droplets decreases. The number of micelles decreases because they become polymer particles upon entry of a radical, and also because they are destroyed to provide surfactant to stabilize both the polymer chains that precipitate in the aqueous phase and the increasing surface area of the growing polymer particles. After some time, all micelles disappear. This is considered to be the end of the nucleation and only limited formation of new particles may occur after this point because heterogeneous nucleation is not possible and there is no free surfactant available in the system to stabilize the particles formed by homogeneous nucleation. The stage of the batch emulsion polymerization in which particle nucleation occurs is called Interval I [24,29]. At the end of Interval I, which typically occurs at a monomer conversion... 

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