Powdered resins

Phenol—formaldehyde (PF) was the first of the synthetic adhesives developed. By combining phenol with formaldehyde, which has exceptional cross-linking abiHties with many chemicals and materials, and a small amount of sodium hydroxide, a resin was obtained. The first resins soHdified as they cooled, and it was discovered that if it was ground to a powder with a small amount of additional formaldehyde and the appHcation of more heat, the mixture would Hquify and then convert to a permanently hard material. Upon combination of the powdered resin mixture with a filler material such as wood flour, the result then being placed in a mold and pressed under heat and pressure, a hard, durable, black plastic material was found to result. For many years these resulting products were called BakeHte, the trade name of the inventor. BakeHte products are still produced today, but this use accounts for only a small portion of the PF resins used. 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

Electrical Applications. The largest application of PTFE is for hookup and hookup-type wire used in electronic equipment in the military and aerospace industries. Coaxial cables, the second largest appHcation, use tapes made from fine powder resins and some from granular resin. Interconnecting wire appHcations include airframes. Other electrical appHcations include computer wire, electrical tape, electrical components, and spaghetti tubing. 

Overbraided hose liners are made from fine powder resins by paste extmsion, and thread-sealant tapes are produced from fine powder by calendering. Fabricated gaskets are made from granular resins and pipe liners are produced from fine powder resins. Fibers and filament forms are also available. 

The shell-molding process, introduced in the United States in 1948, is an important market for phenoHc resins. In the original process, dry sand and powdered resin (6—8%) are blended. However, because of the high binder content and the difficulty in obtaining a uniform mix, precoating methods were developed. 

Waferboard, a more recent wood constmction product, competes more with plywood than particle board. Waferboard and strand board are bonded with soHd, rather than Hquid, phenoHc resins. Both pulverized and spray-dried, rapid-curing resins have been successfully appHed. Wafers are dried, dusted with powdered resin and wax, and formed on a caul plate. A top caul plate is added and the wafers are bonded in a press at ca 180°C for 5—10 min. Physical properties such as flexural strength, modulus, and internal bond are similar to those of a plywood of equivalent thickness. 

Thin coatings consist of paints and varnishes, which are applied as liquids or powdered resin with a thickness of about 0.5 mm [e.g., epoxy resin (EP) [2]]. Typical thick coatings are bituminous materials [3] and polyolefins [e.g., polyethylene (PE) [4]], thick coating resin combinations [e.g., EP tar and polyurethane (PUR) tar [2]] as well as heat-shrinkable sleeves and tape systems [5]. 

As an alternative to the wet process described above, moulding compositions may be made by mixing a powdered resin or a methylol derivative with other ingredients on a two-roll mill or in an internal mixer. The condensation reaction proceeds during this process and when deemed sufficiently advanced, the composition is sheeted off and disintegrated to the desired particle size. This dry process is not known to be used in any current commercial operation. 

For rayon fiber based eomposites (Seetions 3 and 4) the fiber and powdered resins were mixed in a water slurry in approximately equal parts by mass. The isotropie piteh earbon fiber eomposites (Seetion 5) were manufaetured with less binder, typically a 4 1 mass ratio of fiber to binder being utilized. The slurry was transferred to a molding tank and the water drawn through a porous sereen under vacuum. In previous studies [2] it was established that a head of water must be maintained over the mold screen in order to prevent the formation of large voids, and thus to assure uniform properties. The fabrieation proeess allows the manufaeture of slab or tubular forms. In the latter case, the cylinders were molded over a perforated tubular mandrel covered with a fine mesh or screen. Moreover, it is possible to mold eontoured plates, and tubes, to near net shape via this synthesis route. 

Addition of melamine in various forms (pure melamine, MF/MUF-powder resin, melamine acetates) to an UF-resin during the application of the glue mix. In the case of the addition of pure melamine the UF-resin must have a rather high molar ratio, otherwise there is not enough formaldehyde available to react with the melamine in order to incorporate it into the resin. 

Filtration of suspended solids, primarily particulate iron oxides. Here the preferred media/process is powdered-resin precoat filters. 

Powdered resin filtration is approximately 90 to 95% efficient, whereas deep-bed polishers are perhaps 60 to 70% efficient. 

The ion-exchange capacity of a powdered resin precoat filter is perhaps only 6% that of a deep-bed polisher. 

Powdered resins systems (80-600 mesh) typically operate at lower pressure drops, generate less waste water, take up less space, and cost much less when compared to deep-bed polishers. However, they operate at only 3 to 4 gpm sq ft (although the filter element area is large) and are designed to be disposable, so that the powdered resin must be replaced whenever the bed is reconditioned. Sometimes inert resins or cellulose-based fibers are used either in place of powdered resins or as a premix, where they function as both filter aids and absorbents. 

The finely powdered resin component (Bisphenol-A) is a significant dust explosion hazard. 

Injection is by far the most used moulding process but compression and compression-transfer are used for specific cases. Rotomoulding is specifically used for polyethylene and a few other powdered resins. Slush moulding is broadly used for automotive dashboards. Generally ... 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

Figure 6.28 Photograph of a PVDC powder resin that is melting via the one-dimensional melting mechanism. Even at this early location in the melting process, the thickness of the melt film adjacent to the barrel wall was thicker than normal...

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-... 

Fine powder resins are extremely sensitive to shear and the sheared polymer cannot be processed. Because of that they have to be handled with a great care during transport and processing. 

Most commonly, fine powder resins are processed in the form of a paste. Such a paste is prepared by mixing the powder with 15 to 25% hydrocarbon lubricant, such as kerosene, white oil, or naphtha, with the resultant blend appearing much like the powder alone.7... 

Fine powder resins are shipped in specially constructed drums that typically hold 23 kg (50 lb) of resin. These shallow, cylindrical drums are designed to minimize compaction and shearing of the resin during shipment and storage. To assure further that the compaction is kept at an absolute minimum, the resin must... 

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