PVAc particles

We have found that in the system of presulfate initiator, the PVAc latexes are not dissolved transparently in the methanol-water mixture [8], and in the system of HPO initiator, the extraction of the polymer from the PVAc latex films with acetone greatly depends on the polymerization condition [9]. These results suggest that if a polymerization method can be found in which the grafting polymerization of VAc onto PVA is controlled to the minimum, a large portion of PVAc in the latex film will have a chance of extraction with solvents. In this Chapter, the preparations of the unique porous films from the PVAc latexes containing PVA as a protective colloid by an extraction of the PVAc particles with acetone and the characteristic properties of the porous films are summarized. 

As shown in Fig. 2a, the size of PVAc particles was found to have a range of approximately 0.1-1.0 /u,m from electron micrograph of the crack surface of the latex... 

From the electron micrographs, assuming that PVAc particles in the latex are the same size, the formation model of the porous film from the latex film can be illustrated as in Fig. 3 [19]. When the latex forms a dried film over minimum film-forming temperature, it is concluded that PVA coexisted in the latex and is not excluded to the outside of the film during filming, but is kept in spaces produced by the close-packed structure of PVAc particles. 

This prediction is drawn according to the following model. Figure 5 illustrates that in the latex state the grafting PVA protects hydrophobic PVAc particles in water by concentrating on the surfaces of PVAc particles, but in the porous film after acetone extraction, the insoluble grafting PVAc conversely exists as an important component on the inner surface of spherical cells of PVA. 

There has been a conventional sense that the PVAc latexes prepared in the presence of PVA as a protective colloid contain the graft copolymer of PVA and PVAc, so that PVAc particles in the dried latex film are not extracted at a high ratio with solvents. In this Chapter, it has been defined without an influence by the usual sense that the porous PVA-PVAc composite can be prepared from the PVAc latex film with acetone extraction. The porous film consists of the spherical cells of PVA... 

Oxygen Quenching in Poly(vinyl acetate) Particles. In order to study a system in which fluorescent groups could be Introduced into both the stabilizer and core polymers, we turned our attention to poly(vinyl acetate) [PVAc] particles sterically stabilized by poly(2-ethylhexyl methacrylate) [PEHMA] (14,15). Phenanthrene [Phe] was chosen as the fluorescence sensor. It was Introduced into the stabilizer by mixing a small amount (ca. 1%) of 9-phenanthrylmethyl methacrylate 1. with EHMA in the synthesis of PEHMA. It was introduced into the core polymer by mixing a trace (ca. 0.01%) of J with VAc in the presence of unlabelled PEHMA in the particle synthesis step. 

These principles are demonstrated in experiments on PVAc particles sterically stabilized with PEHMA. Phenanthrene [Phe] is a particularly useful label for these studies. It does not form excimers singlet selfquenching is inefficient. Its emission profile virtually always decays... 

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

Plasticizers soften the film and increase the adhesion and the setting speed. The most common are phthalates, adipates and benzoates. The amount added can be in a broad range of 10-50%. They affect the swelling and softening of the PVAc emulsion particles, ensure film formation at room temperature, and the tack of the still wet adhesive. They also provide improved moisture resistance of the bond. Disadvantages are the lower resistance of the bond line against heat, possible migration of the plasticizers and enhanced cold flow. 

Expt no. HPO amount (%) Conversion (%) Average particle size (/Lirn) Residual PVAc in porous film i%r Extraction ratio vs. total PVAc (%)... 

The determination of adsorption isotherms at liquid-solid interfaces involves a mass balance on the amount of polymer added to the dispersion, which requires the separation of the liquid phase from the particle phase. Centrifugation is often used for this separation, under the assumption that the adsorption-desorption equilibrium does not change during this process. Serum replacement (6) allows the separation of the liquid phase without assumptions as to the configuration of the adsorbed polymer molecules. This method has been used to determine the adsorption isotherms of anionic and nonionic emulsifiers on various types of latex particles (7,8). This paper describes the adsorption of fully and partially hydrolyzed PVA on different-size PS latex particles. PS latex was chosen over polyvinyl acetate (PVAc) latex because of its well-characterized surface PVAc latexes will be studied later. 

Fig. 41. Molecular weight dependence of Dz of linear PVAc (O), and of branched PVAc synthesized in latex particles ( )188,189)...

Fig. 48. The o-parameter for PVAc obtained by emulsion polymerization. The transition indicates gel-formation in the latex particle. (Compare also Figs. 41 and 42188 16,>)...

Presence of non-ionic surfactants such as Igepal CO-630 seems to prevent the thickening of PVAC latex by NaLS, as shown in Table IV. This can be interpreted as to show that the presence of non-ionic surfactant at the PVAC latex surface prevents the penetration of NaLS into the particle. 

Polyfvinyl acetate) (PVAc) latexes produced by batch and continuous emulsion polymerization were used in this study. Details for the apparatus and the polymerization procedure can be found in Penlidis et al. (6,12,K3). Samples taken during the reaction were subsequently analyzed to follow conversion- and particle growth-time histories. The batch experimental runs were designed to yield similar conversion-time histories but different particle sizes. Conversion was measured both off-line, by gravimetric analysis, and on-line using an on-line densitometer (a U-tube DPR-YWE model with a Y-mode oscillator with a PTE-98 excitation cell and a DPR-2000 electronic board by Anton Paar, Austria). A number of runs were repeated to check for reproducibility of the results. Four batch runs are described in Table I below and their conversion histories are plotted in Figure 1. 

A dry packed column with porous material was used for the characterization according to size of the PVAc latex samples. The packing employed was CPG (Controlled Pore Glass), 2000 A, 200-400 mesh size. Deionized water with 0.8 gr/lit Aerosol O.T. (dioctyl sodium sulphosuccinate), 0.8 gr/lit sodium nitrate and 0.4 gr/lit sodium azide served as the carrier fluid under a constant flowrate. The sample loop volume was 10 pC A Beckman UV detector operating at 254 nm was connected at the column outlet to monitor particle size. A particle size-mean retention volume calibration curve was constructed from commercially available polystyrene standards. For reasons of comparison, the samples previously characterized by turbidity spectra were also characterized by SEC. A number of injections were repeated to check for the reproducibility of the method. 

Polyvinylacetate (PVAc) has not been used in the pharmaceutical held until recently. During the polymerization, especially at high conversion, free radicals are transferred to dead polymers, resulting in the formation of branched polymers. These branched polymers are susceptible to deterioration. Because the PVAc latex particles are produced by an emulsion polymerization technique, this provides a good process for the water-based dispersion in him coatings. The main purpose of this polymer is the him coating of sustained release dosage forms. The polymer is used as a precursor in the production of polyvinylalcohol (PVA), which cannot be prepared directly by polymerization due to the unstable, isomeric monomer of acetaldehyde. 

Fig. 6 Influence of the granule particle size on the theophylline release properties of the tablets containing hot-melt extruded granules (n = 3). Tablets 20% extruded granules (Formula 3 25% theophylline, 2% PEG, and 73% PVAc), 79.5% Avicel PH200, and 0.5% magnesium stearate ( ) Less than 125 pm (i) 180-212 pm (a) 300 25 pm (O) 500-600 pm. (From Ref. l)...

Particle size (nanoparticles, carbon black, and fumed silica are examples of small particles which typically contribute to an increase in tensile strength compare the effect of particle size on PVAc adhesive properties where different sizes of calcium carbonate were used)... 

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