Dispersing Divided powders

Suspensions can be prepared by either dispersing finely divided powders in an appropriate vehicle or... 

Pigments. These arc finely divided powders (particles between 0.1 and 50 micrometers in diameter) which are dispersed throughout llie binder. In addition to reinforcing the final film, much as they do in composite plastics, they influence a coating s resistance to abrasion and corrosion, and they also are the major factor in the gloss, color, and opacity of a coating. 

An ideal sample consistency for ASE is similar to that desired for liquid extraction, that being a dry, finely divided powder. To achieve this, ASE samples are often ground prior to placing in the bomb. To avoid particle aggregation, a dispersing agent such as Ottawa sand or diatomaceous earth is... 

For the first live atomization sources listed in Table 8-1, samples arc usually iiilroduccd in the form of aqueous solulions (occa.sionally, nonaqueous soUilion.s are used) or less often as slurries (a slurry is a suspension of a ftnely divided powder in a liquid). For samples lhai are difficull lo dissolve, however, several methods have been used lo introduce samples into the atomizer in ihe form of solids or finely dispersed powders. Gen-crallv, solid sample-introduction techniques arc less reproducible and more subject lo various errors and as a result are not nearly as widely applied as aqueous solution techniques. Table 8-2 lists the common sample-introduction methods for atomic spcciro.scopy and the ispe of samples lo which each method is applicable. 

In the analysis of particulates in air, the samples are collected by passing measured volumes of air through filter paper (or some other suitable filter media). One method of standardization is to pipette small volumes of solution containing known amounts of the elements to be determined onto the same filter medium. Depending on the energy of the x-ray line and the size of the particulates, there may be systematic errors in the analysis using standards prepared from solutions. Another approach to standardization is to disperse known quantities of elements in the form of finely divided powders onto paper or glass-fiber filters. Problems with this approach include variation in particle size between standards and unknowns, and quantitative collection of the standard powders by the filter. 

The majority of samples for mid-infrared investigations are presented as finely divided powders dispersed in an excess of a dry powdered nonabsorbing matrix, commonly KCl. This ensures that superimposed interferences from specular (front-surface) reflections are minimized in the recorded diffuse reflection spectrum. A useful approach for sampling intractable, composite, or gross objects is to abrade a fine powder from the article s surface with some SiC... 

Disperse dyes are materials that have very low water solubility (typically 10 mole fraction). Disperse dyes are supplied as finely divided powders containing dispersing agents (e.g., naphthalene sulfonic acid and lignin sulfonate), which keep the particles suspended in the dyebath. They are applied to polyester and other synthetic materials from neutral to slightly acid baths by procedures typically similar to the following ... 

The copolymer is colloidally dispersed in the aqueous phase. The polymer is recovered by coagulation. Normally, coagulation is carried out by mechanically shearing the dispersion. The product is a finely divided powder with poor flow characteristics. Buckmaster and Morgan have described a process in which combinations of agitation and chemicals are applied to the dispersion to obtain the desired larger particles. In... 

Having achieved the required degree of fineness in the powder it is then necessary to ensure that the powder is satisfactorily dispersed in the ointment Finely divided powders have a marked... 

Powders or fluff of polymers can be studied with the transmission technique by forming a KBr pellet. This approach disperses the sample in an infrared transparent matrix like KBr or NaCl. These inorganic salts have the property of cold flow so they revert to a glass-like consistency when sufficient pressure (10-15,000 Ib/in ) is applied to a finely divided powder. In this method, KBr is usually mixed with 1% of a finely ground sample in a special die. Pressure is applied with a press. The sample pellet formed is placed in the spectrometer and measured. The use of an alkali halide matrix to support and surround a solid sample for IR analysis was first presented by Stimson in 1952 [11]. This technique has been used widely because of its general applicability. The limitation of the KBr method (in addition... 

---