Colloid-type protection

Surface Phenomena Stabilizer type (protective colloid or inorganic powder) Stabilizer concentration ... 

The other this type protective colloids include aluminum and magnesium hydroxides. They can be precipitated directly in the aquatic enviromnent through an action on the water-soluble salts of these metals with sodium hydroxide. In this way the cumbersome process of filtration, fragmentation... 

Such agents are of importance in the plating baths used for the deposition of decorative as well as corrosion-protective coatings, e.g. bright nickel plate which confers protection to nickel-chromium coatings. The addition agents in this instance are more specific, more stable and more controllable in their effects than the colloidal type of agent they are characterised by the presence of unsaturated bonds, e.g. dimethyl fumarate. 

Lead azide is not readily dead-pressed, ie, pressed to a point where it can no longer be initiated. However, this condition is somewhat dependent on the output of the mixture used to ignite the lead azide and the degree of confinement of the system. Because lead azide is a nonconductor, it may be mixed with flaked graphite to form a conductive mix for use in low energy electric detonators. A number of different types of lead azide have been prepared to improve its handling characteristics and performance and to decrease sensitivity. In addition to the dextrinated lead azide commonly used in the United States, service lead azide, which contains a minimum of 97% lead azide and no protective colloid, is used in the United Kingdom. Other varieties include colloidal lead azide (3—4 pm), poly(vinyl alcohol)-coated lead azide, and British RE) 1333 and RE) 1343 lead azide which is precipitated in the presence of carboxymethyl cellulose (88—92). 

A typical recipe for batch emulsion polymerization is shown in Table 13. A reaction time of 7—8 h at 30°C is requited for 95—98% conversion. A latex is produced with an average particle diameter of 100—150 nm. Other modifying ingredients may be present, eg, other colloidal protective agents such as gelatin or carboxymethylcellulose, initiator activators such as redox types, chelates, plasticizers, stabilizers, and chain-transfer agents. 

In the pharmaceutical field, agar is commonly the base colloid for stabilizing mineral oil in water emulsions, used for laxative purposes. The concentration of agar is kept below the gel point, so that the emulsion will pour. Other gums, like tragacanth, Irish moss extract, or carboxymethylcellulose, may replace the agar, where desired. Usually, from 0.5 to 0.8% of the gum, based upon the weight of the aqueous phase, suffices to protect this type of emulsion, which is somewhat of a neutral variety. 

The protective colloid/wetting agent may be a single anionic agent Turkey Red Oil, for example, combines both functions but is prone to form a precipitate in hard water. Only anionic types are suitable, since nonionic and cationic types generally cause precipitation [31]. Most protective colloids are of the following types ... 

The chemistry of these product types has been described previously (section 10.6.1). The anionic polyelectrolyte helps to stabilise the colloidal solution of the naphtholate, through a mechanism similar to that already described. Where the protective colloid itself does not give adequate wetting of the fabric a suitable wetting agent, which in batchwise dyeing must function well in the cold, should be added the alkylnaphthalenesulphonate types are suitable. 

Products of this type seem to protect the humus from rapid incorporation into new biological processes. Additional factors that appear to be associated with the accumulation of organic matter in Mollisols are high exchange capacities, saturation with calcium, an abundance of mineral colloids and a high content of minerals of the smectite group (Fenton, 1983). 

Because of the low level of emulsifiers and protective colloids, the suspension polymer types are most suitable for electrical applications and end uses requiring clarity, This form is also employed in the bulk of extrusion and molding applications. Cost is lower than for emulsion and solution forms. The emulsion or dispersion resins are employed mainly for organosol and plastisol applications where fast fusion with plasticizer at elevated temperature will occur as a result of the fine particle size of the resin. 

Fig. 33 Various types of colloid DNA-mediated interactions, either conventional or self-protected - switched off - through intra-particle hybridization, (a, b) Normal, hairpin-free pair of complementary sticky ends, either grafted to separate beads (a) or mixed on the same bead (b). (c, d) Self-complementary sticky ends. Besides self-protective loops as in (b), this sequence can form two hairpin structures the sticky end sequence can fold on itself (1), or it can bind to the backbone (2). (e, f) As in (a), but each of the sticky ends can fold into a protective hairpin. Adapted with permission from [140]...

Dispersion polymerization differs from emulsion polymerization in that the reaction mixture, consisting of monomer, initiator, and solvent (aqueous or nonaque-ous), is usually homogeneous. As polymerization proceeds, polymer separates out and the reaction continues in a heterogeneous manner. A polymeric surfactant of the block or graft type (referred to as protective colloid ) is added to stabilize the particles once formed. 

Products comprising hydrophilic polymers dissolved in water are well-known and used widely as adhesives but are of little general significance for bonding plastics. The present chapter is concerned only with products based on polymer dispersions, which consist of small discrete particles of diameter about one micron (1 pm, or 10-3 mm) suspended in a continuous water phase. In most instances a protective colloid is present at the interface between the particles of polymer and the water and this helps to stabilize the dispersion and prevent premature coalescence of particles. Dispersions such as these are known as oil-in-water types. With them, the molar mass of the polymer species comprising the dispersed particles does not affect the viscosity and so polymers of high molecular weight can be applied in this way. 

Poly(vinyl acetate) emulsions are excellent bases for water-resistant paper adhesives destined for use in manufacturing bags, tubes, and cartons. Glue-lap adhesives, which require moderate-to-high resistance to water, exemplify this type. When routine water resistance is required, a homopolymer vinyl acetate emulsion containing a cellulosic protective colloid is effective for most purposes. Next effective are emulsions containing fully hydrolyzed poly(vinyl alcohol) as a protective colloid, followed by those containing partially hydrolyzed poly(vinyl acetate). 

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