Beads, particle size distribution

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. 

Among new mill developments, annular-gap bead mills and stirred bead mills are being used. These have a high cost, but result in a steep particle-size distribution when used in multipass mode [Kolb, Ceramic Forum International, 70(5), 212-216 (1993)]. Costs for fine grinding typically exceed the cost of raw materials. Produces are used for high-performance ceramics. 

Two-phase suspension systems produce beaded products with broader particle-size distribution (e.g., 1-50 /rm). The microspherical particles usually need to be classified repeatedly to reduce the particle-size distribution in order to improve the resolution and efficiency in the separation for use in chromatography. The actual classification process depends on the size range involved, the nature of the beaded product, and its intended applications. Relatively large (>50 /rm) and mechanically stable particles can be sieved easily in the dry state, whereas small particles are processed more conveniently in the wet state. For very fine particles (<20 /rm), classification is accomplished by wet sedimentation, countflow setting, countflow centrifugation, or air classification. 

Each of the PLgel individual pore sizes is produced hy suspension polymerization, which yields a fairly diverse range of particle sizes. For optimum performance in a chromatographic column the particle size distribution of the beads should be narrow this is achieved by air classification after the cross-linked beads have been washed and dried thoroughly. Similarly, for consistent column performance, the particle size distribution is critical and is another quality control aspect where both the median particle size and the width of the distribution are specified. The efficiency of the packed column is extremely sensitive to the median particle size, as predicted by the van Deemter equation (4), whereas the width of the particle size distribution can affect column operating pressure and packed bed stability. 

The particle size distribution of AP was determined and Compared using the Ro-Tap, the Alpine Air Jet Sieve and the Micromerograph. For this purpose six different samples of ground AP were chosen and separated using sieves calibrated with National Bureau of Standards spherical glass beads. The results were plotted... 

The particle-size distribution of two test powders, spherical glass beads and crushed quartz was determined with different types of app (Ref 32), and indicate that the micro-mesh sieve data is in good agreement with those of other methods (Table 6) ... 

Fig 25 Comparison of microscopic particle size distribution with the average distribution of four identical runs made with the Coulter Counter (Sharpies standard glass bead sample, type XC-3)... 

Glicksman and Farrell (1995) constructed a scale model of the Tidd 70 MWe pressurized fluidized bed combustor. The scale model was fluidized with air at atmospheric pressure and temperature. They used the simplified set of scaling relationships to construct a one-quarter length scale model of a section of the Tidd combustor shown in Fig. 34. Based on the results of Glicksman and McAndrews (1985), the bubble characteristics within a bank of horizontal tubes should be independent of wall effects at locations at least three to five bubble diameters away from the wall. Low density polyurethane beads were used to obtain a close fit with the solid-to-gas density ratio for the combustor as well as the particle sphericity and particle size distribution (Table 6). 

In general, the efficiency of the columns used in ion chromatography is limited by the large-sized particles and broad particle size distributions of the resin packings. Resin beads are currently available in the ranges of 20-30, 37-74, and 44—57 pm. 

Figure 7 shows the effect of filler particle shape on the viscosity of filled polypropylene melts, containing glass beads and talc particles, of similar density, loading and particle size distribution. The greater viscosity of the talc-filled composition was attributed to increased contact and surface interaction between these irregularly shaped particles. 

A wide variety of polymers have been analyzed by gel-permeation, or size-exclusion, chromatography (sec) to determine molecular weight distribution of the polymer and additives (86—92). Some work has been completed on expanding this technique to determine branching in certain polymers (93). Combinations of sec with pyrolysis—gc systems have been used to show that the relative composition of polystyrene or acrylonitrile—polystyrene copolymer is independent of molecule size (94). Improvements in gpc include smaller cross-linked polystyrene beads having narrow particle size distributions, which allow higher column efficiency and new families of porous hydrophilic gels to be used for aqueous gpc (95). 

Suspension Polymerization. The suspension or pearl polymerization process has been used to prepare polymers for adhesive and coaling applications and for conversion to poly(vinyl alcohol). Suspension polymerization are carried out with monomer-soiubie initiators predominantly, with low levels of stabilizers Continuous tubular polymerization of vinyl acetate in suspension yields stable dispersions of beads with narrow particle size distributions at high yields. 

The eventual dispersion process consists of grinding the moist dye, generally in a ball, bead, or sand mill, in the presence of dispersing agents. Since the dyeing rate depends on particle size, the aim is to reduce the particle size to <1 qm. A narrow particle-size distribution is important to minimize recrystallization during storage and application. 

Figure 8.6. Separation of polystyrene latex beads of four different diameters (indicated in the figure) by a disc centrifuge operated at 3586 rpm. (From ref. 44. Reprinted with permission from R. M. Holsworth, T. Provder, and J. J. Stansbrey, in T. Provder, Ed., Particle Size Distribution, ACS Symposium Series No. 332, American Chemical Society, Washington, DC, 1987, Chapter 13. Copyright 1987 American Chemical Society.)...

Polystyrene Latex (PSL) Bead Solution Filtration Experiments were conducted to obtain filter retention, flow rate, and Ap data for a DI water based PSL bead mix solution prepared using particles ranging from bead diameters of 0.772 to 20 pm. It is a common practice to use PSL bead challenge solutions (created by mixing different size PSL bead standards in specific volumetric ratio to simulate slurry-like particle size distribution for the bead mix solution) to obtain relative quantitative retention data for various filters. These solutions are expected to retain stable PSD and provide more consistent information compared to real CMP slurries, which may change particle characteristics over time. 

Attempts to produce large particles with narrow bead distribution ranges have been suggested. In a patent [72], the application of hydrodynamic conditions is described, using tall reactors with volumes > 60 m3 to achieve a standard deviation of particle size distributions of < 25 %. An independent claim is included for a hydrodynamic system, where the ratios of stirrer diameter to... 

The use of insoluble polymeric carriers has greatly simplified the synthesis of peptides purification of the growing peptide chain in the repetitive steps is achieved by filtration procedures that simply remove all soluble reagents and byproducts from the reaction medium, whilst the covalently resin-hnked macromolecule is retained on the insoluble polymeric support. In all sohd-phase reactions of this type, the polymeric support represents the medium on or in which the chemical reaction takes place. Correspondingly, this medium is represented by the total amount of insoluble polymer present, which in the case of polymeric beads is divided into small, individual reaction compartments. Resin parameters such as the degree of crosshnking, the polarity of the resin, its sweUing properties, mass-transport, phase transitions, bead size, and the particle size distribution therefore have to be taken into serious consideration. 

The miscible monomers, ethenylbenzene (styrene) and diethenylben-zene (divinylbenzene, DVB), undergo a free radical induced copolymerization reaction initiated by a benzoyl peroxide catalyst. The exothermic reaction is carried out in an aqueous suspension whereby the mixed monomers are immiscibly dispersed as spherical droplets throughout the reacting medium resulting in discreet beads of copolymer being formed. Correct reaction conditions and the use of suspension stabilizers enable the particle size distribution of the... 

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