Polymeric powders

Melting temperatures of as-polymerized powders are high, ie, 198—205°C as measured by differential thermal analysis (dta) or hot-stage microscopy (76). Two peaks are usually observed in dta curves a small lower temperature peak and the main melting peak. The small peak seems to be related to polymer crystallized by precipitation rather than during polymerization. 

Free-radical multicomponent copolymerization of dialkylstannyl maleates or dialkylstannyl dimethacrylates with methallyl alcohol (or (J-hydroxyalkyl acrylates) and vinyl monomers (sryrene, methacrylic acid or methacrylamide) yields polymeric powders. Due to their storage and thermal stability and impact strength they are used as protective coatings 79). 

Activity in the field was, however, expanding. For example, in 1927 Drew and Haworth (65) obtaind a crystalline polymeric powder by the action of hydrogen chloride on the lactone of 2,3,4-trimethyl-l-arabonic acid. Citing the increase in melting point and molecular weight, and loss of specific optical rotation, they ascribed a cyclic, high polymer structure to this polyester. 

Crawford, R.l. and Paul, D.W., Radial and Axial Die Pressures During Solid Phase Compaction of Polymeric Powders, Ear. Polym. /., 17, 1023 (1981)... 

During high temperature compression molding, the polymeric powder is poured into a positive pressure mold that is heated and then cooled under pressure. The cooled mold is then opened to yield a fully sintered UHMWPE article. 

The very inexpensive material sold undo- the name dibutyltin oxide is an amorphous, insoluble, polymeric powder with the composition BujSnO. A suspension of this powder in an appropriate organic solvent in the presence of a 1,2- or 1,3-diol becomes clear more or less quickly because of the formation of a stannylene. These derivatives are soluble in most solvents, polar or nonpolar. Equation (1) indicates the stoichiometry of the process ... 

Melting temperatures of as-polymerized powders are high, i.e., 198— 205°C. As-polymerized PVDC does not have a well-defined glass-transition temperature because of its high crystallinity. The amorphous polymer has a glass-transition temperature of — 17°C. Once melted, PVDC does not regain its as-polymerized morphology when subsequently crystallized. 

D. M. Bigg, High Pressure Molding of Polymeric Powders, Proc. of the Society of Plastics Engineers 33rd Annu. Tech. Conf., Atlanta, May 1975, pp. 472 176. 

Rotational Molding Throne et al.4 investigated heat-transfer problems in rotational molding of polymeric powders. One of the simulation models for heat transfer they have considered is depicted in the accompanying figure. The lower... 

In 1972, Liepins and Sakaoku [7] reported that polymeric powders were formed nearly exclusively in the radio frequency reactors shown in Figures 8.12 and 8.13, in which an organic vapor was introduced into the glow discharge of a carrier gas. The monomers that formed powders nearly exclusively and the yield of powder formation are summarized in Table 8.1. Monomers that did not form powders exclusively (i.e., formed plasma polymer in the form of a film or a film with powders) are shown in Table 8.2. The significant points about these experiments are as follows ... 

Figure 8.19 Infrared spectra of (a) low-density polyethylene film, (b) plasma-polymerized film, (c) plasma-polymerized powder. Adapted from Ref. 11.

The Coulter principle is also standard for dry toners [8,9] and an accepted method for aluminum oxide powder [10], chromatography media [11], polymeric powders [12], plutonium [13], filter evaluation [14], catalytic material [15] and comparing particle size distribution using alternative types of particle counters [16]. In ASTM method C-21 it states that the experience of several laboratories indicates that the method is capable of a repeatability of 1% and a reproducibility of 3% at the 95% confidence level. Operating procedures for this technique are also covered in BS3405 [17]. The method is also the subject of an international standard [18]. 

ASTM3451 -92 (1992), Testing polymeric powders and powder coating, 450... 

Hot mounting uses hot-press equipment as shown in Figure 1.18. A specimen is placed in the cylinder of a press and embedded in polymeric powder. The surface to be examined faces the bottom of the cylinder. Then, the specimen and powder are heated at about 150°C under constant pressure for tens of minutes. Heat and pressure enable the powder to bond with the specimen to form a cylinder. Phenolic (bakelite) is the most widely used polymeric powder for hot mounting. Hot mounting is suitable for most metal specimens. However, if the microstructure of the material changes at the mounting temperature, cold mounting should be used. 

Composite Particles, Inc. developed two methods of surface modification of polymeric materials which are used for materials of different shapes and compositions. Here, only the spherical, non-rubber particles are discussed. Further information is included in the section on rubber particles below. One method of surface modification is based on exposing the polymeric powder to a chemically reactive gas atmosphere which oxidizes surface groups to form OH and COOH functionalities. These functionalities are then available for reaction with the components of the matrix into which modified particles are introduced. Vistamer HD and UH are manufactured by this method from polyethylenes of different molecular weights. Two factors can be regulated here the properties of the core particle and the type and density of functional groups on the surface of these particles. Polyethylene is a material, which without this modification, will not be compatible with most systems. The surface modification allows the incorporation of the material into resins. This improves abrasion resistance, tear strength, and moisture barrier properties and reduces the fiiction coefficient. 

Nitrile Barrier Resins Polymerization in Air Emulsion Polymerization Powders... 

For clinical applications in dentistry, it is convenient to work with mixtures made from a polymeric powder and a liquid monomer. The mixture attains a doughy consistency, and it can then be formed into intricate shapes. Subsequently, as a result of the combination of reactants in the powder and liquid, the monomer polymerizes to provide a rigid load-bearing material. This is usually a two-phase material in which the dispersed phase is attributable to particles originally present in the powder. Many proprietary materials comprise two phases of poly (methyl methacrylate), although other ingredients may be included to modify appearance and properties. 

Ikezaki, K. Murata. Y. "Derivation of Intrinsic Thermally Stimulated Current Spectra of Polymeric Powder Samples"). Inst. Electrostatics Jpn., Vol.30, No.l, pp.14-19, (2006) [in Japanese]... 

To make the demonstration even more spectacular, it is advised to use three cups, where the polymeric powder is only present in one of them. The water is poured from one empty cup to another empty cup, and the places of all three cups on the table switch. The students are then asked to follow the water and tell, at each pouring and switching step, which cup has the water in it. Then, the demonstrator pours the water from one of the two cups into the third one, which contains the polymeric powder, switches the cups, and asks the students about the whereabouts of the water. When the students point to the third cup, the demonstrator turns over all three cups, and the water suddenly seems (to the students) to vanish The experiment is repeated. 

Core-shell rubbers Rather than just use polymeric powders, it is now possible to formulate with pre-formed rubber particles, the so-called core-shell rubbers. These carefully designed materials enable both the particle size distribution and the volume fraction of particles in the cured resin to be closely conttolled. 

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