Polymerization fine powder resins

The first step in the manufacture of fine powder resins is to prepare an aqueous colloidal dispersion by polymerization with initiator and emulsifier present.21 Although the polymerization mechanism is not a typical emulsion type, some of the principles of emulsion polymerization apply here. Both the process and the ingredients have significant effects on the product.22 The solids contents of such disper-... 

Commercially PTFE is available in granular and fine powder resin forms and aqueous dispersions. The granular form of PTFE resin is prepared by suspension polymerization process in an aqueous mediiun with little or no dispersing agent. These forms of PTFE resins are mainly used for compression molding and ram extrusion. The fine PTFE powder is prepared by controlled emulsion polymerization and it is useful for paste extrusion into tapes, tubes, pipe liners, insulation layer of wires, gas-liquid separation membranes and fibers. 

The discovery of PTFE (1) in 1938 opened the commercial field of perfluoropolymers. Initial production of PTFE was directed toward the World War II effort, and commercial production was delayed by Du Pont until 1947. Commercial PTFE is manufactured by two different polymerization techniques that result in two different types of chemically identical polymer. Suspension polymerization produces a granular resin, and emulsion polymerization produces the coagulated dispersion that is often referred to as a fine powder or PTFE dispersion. 

Emulsion polymerization a sufficient amount of dispersing agent and mild agitation is employed. This produces small colloidal particles dispersed in the aqueous reaction medium. In this procedure, called aqueous dispersion polymerization, precipitation of the resin particles is avoided. The coagulated dispersion produced by emulsion polymerization is often called a fine-powder or PTFE dispersion. 

It is made by dimerizing cyanamide in basic aqueous solution, and is a colorless solid melting at 208°C. Dicyandiamide is soluble in polar solvents, but at room temperature is insoluble in bisphenol A epoxy resins. It can be made into a very fine powder and milled into epoxy resins to form stable dispersions. Because the dicy is insoluble in the epoxy, the only possible reaction sites are at the particle surfaces. Although some reaction certainly occurs over a short time, the adhesives easily can have a useful shelf life of six months. On heating to about 150°C, the dicyandiamide becomes soluble in the epoxy resin, and the adhesive polymerizes rapidly. Cure can be accelerated by incorporation of tertiary aromatic amines or substituted ureas. 

Pure tetrafluoroethylene monomer under ambient conditions is an odorless, colorless, tasteless gas with low toxicity. It may be polymerized by either suspension or emulsion techniques. Both procedures require use of high pressures in an autoclave in order to maintain the monomer in liquid form. These techniques produce chemically identical product, the first a granular resin, and the second a fine powder (Eq. 23.7). 

PTFE is produced by free-radical polymerization mechanism in an aqueous media via addition polymerization of tetrafluoroethylene in a batch process. The initiator for the polymerization is usually a water-soluble peroxide, such as ammonium persulfate or disuccinic peroxide. A redox catalyst is used for low temperature polymerization. PTFE is produced by suspension (or slurry) polymerization without a surfactant to obtain granular resins or with a perfluori-nated surfactant emulsion polymerization) to produce fine powder and dispersion products. Polymerization temperature and pressure usually range from 0 to 100°C and 0.7 to 3.5 MPa. 

Daylight fluorescent pigments, however, are comprised of a solid state solution of fluorescent dyes in a friable polymeric resin. Once the dyes are incorporated into the resin, they are ground into a fine powder for use as a pigment or colorant. As these pigments are resinous solutions of dyes, they tend to be transparent in nature. 

Commercially, PTFE is produced from the monomer tetrafluoroethylene by two different polymerization techniques, namely, suspension and emulsion polymerization. These processes give two vastly different physical forms of chemically identical PTFE. While suspension polymerization produces granular PTFE resin, emulsion polymerization produces an aqueous PTFE dispersion and PTFE fine powders (after coagulating the dispersion). 

Fluid or Pour-Tjpe Resins. Fluid or pour-type resins are modified acryHc systems that can be cured chemically. A fine-particle-size polymer powder consisting mostly of high molecular weight material is preferred to prevent a rapid increase in viscosity during mixing and pouring. Polymerization occurs in flexible... 

---