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Cellulose Coalescence

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. [Pg.386]

The lignin layer may coalesce when molecular weight is increased, thus removing it from contact with much of the cellulose. It is possible that irradiation, thermal treatment, or even chemical cross-linking would be approaches to using this strategy. [Pg.23]

The life of an Avicel suspension can be extended by coprecipitating the rodlike structures with a protective colloid after trituration. Avicel-RC19 is limit cellulose that has been physically modified by coprecipitation with CMC to facilite dispersibility. Avicel-RC water suspensions simulate the properties of a hydrosol. At low aqueous concentrations, the apparendy hydrated crystallites assemble into a thixotropic, heat- and acid-stable structure whose viscosity depends direcdy on pH to about pH 10, whereupon it declines precipitously. The suspension coalesces at low pH. The addition of salt after mixing increases viscosity above what it would be if the salt were added at the time of mixing or shearing. [Pg.170]

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly (vinyl acetate)—polytyinyl 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 well as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a colloid protection system. The protective colloids are similar to those used paint (qv) to stabilize latex. For poly (vinyl acetate), the protective colloids are isolated from natural gums and cellulosic resins (carboxymethylcellulose or hydroxyethylcellulose). The hydrolized 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 application. 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. Applications are found mostly in the area of adhesion to paper and wood (see VlNYL POLYMERS). [Pg.235]

Lesions created in both bovine and human enamel, in an acidified methyl cellulose gel system, displayed many of the same qualitative trends [Lynch, unpubl. data]. After an initial period of approximately 3 days when dissolution was negligible, mineral loss was typically found at a series of discrete locations, with no apparent mineral loss between these pockets of demineralisation. Surface zones were typically poorly defined or absent. After 5 or more days, the isolated pockets had coalesced and lesions were uniform in terms of both depth and mineral loss across the bulk of the lesion body, with well-defined surface zones. When observed under polarised light, these initial pockets of demineralisation were very often coincident with Hunter-Schreger banding. This was particularly noticeable in bovine enamel. Shellis [64] reported variations in solubility related to enamel microstructure and suggested that structure/solubility relationships are likely to influence lesion formation. [Pg.79]

Suitable protective colloids for the preparation of acrylic suspension polymers include cellulose derivatives, polyacrylate salts, starch, poly(vinyl alcohol), gelatin, talc, clay, and clay derivatives (95). These materials are added to prevent the monomer droplets from coalescing during polymerization (110). Thickeners such as glycerol, glycols, polyglycols, and inorganic salts are also often added to improve the quality of acrylic suspension polymers (95). [Pg.169]

Figure 2 Schematic representation of cellulose synthesis from Acetobacter xylinus (not to scale). Microfibrils of cellulose are secreted into the fermentation medium via terminal complex transmembrane synthetic sites. In the extracellular medium, a number of elementary microfibrils coalesce to form a flat, twisting and highly persistent ribbon of cellulose. The presence of polysaccharides in the fermentation medium allows interactions to occur both before and after the assembly of microfibrils into ribbons. The right angle bend at the point of ribbon assembly is purely schematic... Figure 2 Schematic representation of cellulose synthesis from Acetobacter xylinus (not to scale). Microfibrils of cellulose are secreted into the fermentation medium via terminal complex transmembrane synthetic sites. In the extracellular medium, a number of elementary microfibrils coalesce to form a flat, twisting and highly persistent ribbon of cellulose. The presence of polysaccharides in the fermentation medium allows interactions to occur both before and after the assembly of microfibrils into ribbons. The right angle bend at the point of ribbon assembly is purely schematic...
Viscosity Maxima. The low-shear-rate viscosities of both commercial and model associative thickeners below their c /, values will increase with the addition of conventional low molecular weight surfactants or coalescing aid (22). With HEUR polymers, solution viscosities are observed to increase, achieve a maximum value, and then decrease with continued increase in surfactant concentration (23). This type of behavior is illustrated (Figure 5) for four commercial HEURs with a nonionic surfactant (typical of that used in coating formulations). A similar behavior has been observed (24) with a classical anionic surfactant and hydrophobically modified (hydroxy-ethyl)cellulose (HMHEC) and is reviewed in Chapter 18. Intermicellar networks, formed by the participation of one or more hydrophobes from different polymers in the micelles of conventional surfactants, were again recently suggested (25) to affect viscous solutions. [Pg.507]

Emulsification is the most important act of the washing process. To prevent secondary soil deposition, formation of a coalescence-stable low-concentration emulsion is needed. As it is shown above (see section 6.4), the formation of such an emulsion is possible under real conditions considering the surfactant concentration in the washing solution and hydrodynamic conditions of the soil deposition process. As far as solid soils are concerned, the process of dispersion of particles is important here. To prevent their re-deposition on the surface washed, water-soluble polymers are used, e.g. carboxymethyl cellulose. Effective dispersion agents are also inorganic salts, e.g. alkali metal silicates. [Pg.546]


See other pages where Cellulose Coalescence is mentioned: [Pg.268]    [Pg.239]    [Pg.313]    [Pg.395]    [Pg.112]    [Pg.298]    [Pg.92]    [Pg.362]    [Pg.395]    [Pg.26]    [Pg.163]    [Pg.266]    [Pg.310]    [Pg.469]    [Pg.392]    [Pg.82]    [Pg.169]    [Pg.175]    [Pg.436]    [Pg.228]    [Pg.597]    [Pg.304]    [Pg.1743]    [Pg.252]    [Pg.405]    [Pg.124]    [Pg.551]    [Pg.164]    [Pg.341]    [Pg.1085]    [Pg.1094]    [Pg.41]    [Pg.42]    [Pg.1149]    [Pg.395]    [Pg.27]    [Pg.137]    [Pg.298]    [Pg.19]    [Pg.269]   
See also in sourсe #XX -- [ Pg.45 ]




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Coalesce

Coalescence

Coalescent

Coalescents

Coalescer

Coalescers

Coalescing

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