Powders and Preforms

Solid epoxy adhesives may also be fused or compacted into various shapes or preforms (see Fig. 13.2) such as sticks, rings, and beads.5 They can then be easily applied to a specifically shaped part. Similar to thermoplastic hot-melt adhesives, these thermosetting epoxy adhesives flow on the substrate by heating. However, the product will cure to an infusible thermoset stage with continued heating. Once cured, the product demonstrates properties similar to those of other structural epoxy adhesives. These solid epoxy adhesives are sometimes referred to as thermosetting hot melts. 

FIGURE 13.2 Solid epoxy preforms are designed to provide a simple method for sealing or bonding high-volume components. Courtesy Multi-Seals, Inc., Manchester, CT.) 

Preformed sticks or tubes of solid epoxy may also be applied much as a solder is.6 To apply, the area to be bonded is heated until sufficiently hot to cause the stick solder to flow when drawn across the surface. After application, the adhesive is cured by conventional thermal techniques. If two substrates are to be bonded, both must be preheated prior to 

Solid shapes or powders are formulated from solid or liquid (when used as a B stage) epoxy resins and curing agent. Fillers, additives, and other modifiers are often used as they are with liquid or paste epoxy adhesive formulations. However, consideration must be given to the flow properties of the adhesive when heated as well as the application properties. 

Guidelines for formulating solid epoxy adhesives are similar to those employed for liquid or paste adhesives. Table 13.5 shows starting formulations for a shaped solid and 

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There are several common forms of solid epoxy adhesives. These include film, tape, powder, and preformed shapes. Certain formulations are better suited for specific forms. For example, casting of tape or film adhesive from solvent solutions lends itself to working with multicomponent hybrid systems, where each resin can be solubilized and blended together in a universal solvent. B-staged systems are generally more brittle and better suited for powders or preformed adhesives. 

While, originally, hot isostatic pressing was developed and used to remove defects and/or produce parts with minimum porosity and, consequently, ultimate density from powders and preforms, the study and understanding of the mechanisms of pressure agglomeration also led to a modification of the process which is then actually used to produce parts with a controlled high porosity [B.61] (see also Section 5.3.2). 

To produce unsintered tape by paste extmsion, the fine powder is lubricated and preformed according to the procedure described above. The preform is extmded in the form of rods, which are calendered on hot roUs to the desired width and thickness (109,110). 

A PM preform could be made using recently developed prealloyed steel powders, and full densification could be achieved by hot precision forging to final configurations after the sintering operation... 

High pressure catalytic processes are developed and carried out in both preformed and powdered catalysts. Preformed catalyst are useful for fixed bed operation. Preformed catalyst pellets, are used as packing in multiphase trickling flow reactors. Trickling flow reactors have been described in detail in another part of this book (see Laurent). In this section we deal with slurry catalytic reactors, where the catalyst is used in powdered form. 

Another procedure for the preparation of modified thermosets consists of introducing preformed particles in the initial formulation. This technique is also well documented for modified thermoplastics (Paul and Bucknall, 2000). In Chapter 7 different macromolecular architectures such block copolymers, crosslinked microparticles, hyperbranched polymers, and den-drimers, were presented (Fig. 7.11). All these compact molecules can be used as thermoset modifiers. Thermoplastic powders and core-shell polymers are the more accessible preformed molecules. Some examples are given below. 

The powder is preformed to a void-free condition during the preforming stage and, as such, is moved through the die tube. The process maintains pressure on the molten PTFE in the sintering zone to coalesce the resin particles. The rate of compaction has to be slow enough to allow the air mixed with the resin to escape. 

Continuous rotary Convection Direct/indirect Direct Indirect Conduction Crystals, coarse powders, extrudes, preformed cake lumps, granular paste and fillers, cakes back-mixed with dry product Chemical ores, food products, clays, pigments, chemicals Carbon black... 

Load moulding material - as loose powder, granules or a heated preform - into the heated die cavity. Moulding temperatures vary, e.g. between 135 °C and 155 °C for urea powders and between 140 °C and... 

The samples generated for this paper were all produced in the following fashion. Firstly, a slip was developed with silicon carbide powder and a carbon based binder. The SiC powder type, 6H or 3C, and carbon content were systematically varied in the slip formulation. Secondly, a preform was cast from this slip. Finally, the preform was subjected to a reactive infiltration step in which the preform was brought into contact with a molten silicon bath under vacuum. 

One-part epoxy adhesives include solvent-free liquid resins, solutions in solvent, liquid resin pastes, fusible powders, sticks, pellets and paste, supported and unsupported films, and preformed shapes to fit a particular joint. Two-part epoxy adhesives are usually comprised of the resin and the curing agent, which are mixed just prior to use. The components may be liquids, putties, or liquid and hardener powder. They may also contain plasticizers, reactive diluents, fillers, and resinous modifiers. The processing conditions are determined by the curing agent employed. In general, two-part systems are mixed, applied within the recommended pot life (a few minutes to several hours), and cured at room temperature for up to 24 hours, or at elevated temperatures to reduce the cure time. Typical cure conditions range from 3 hours at 60°C to 20 minutes at 100 C. ... 

Due to the extremely stable C-F bond, FI FE has a very high melting temperature, 327°C, which makes the fabrication of PI PE very difficult. This difficulty is further compounded by a high viscosity above T due to restricted bond rotation and high molecular weight. FIFE is usually fabricated by sintering. PIPE powders are preformed at high pressure (2000-10 000 psi. 

Full details (see Vol. 1, p. 185) of the Japanese work on the preparation of trifluoromethylarenes from trifluoroiodomethane and iodoarenes in the presence of copper powder and a dipolar aprotic solvent have become available, and it appears that the best solvent in some cases is pyridine. This method (but with DMF as solvent) has also been used to prepare the compounds PhR [R = Me03C (CF3)3, CF3 0 (CF2)2, or perfluoro-2-tetra-hydrofurfuryl] in good yields from iodobenzene and the corresponding polyfluoroiodo-compounds. Perfluoroalkyl-copper compounds are very probably involved in such reactions, and the reactions of preformed n-perfluoroheptylcopper in dimethyl sulphoxide with the aromatic carbon-hydrogen bonds of benzene, toluene, p-xylene, nitrobenzene, and chlorobenzene also lead to (perfluoroalkyl)arenes (some replacement of chlorine occurs in the case of chlorobenzene). Homolytic substitution by perfluoro-heptyl radicals, perhaps within the co-ordination sphere of the copper atom,... 

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