Epoxy syntactic foams

Fig. 3. Apparent density (y) of epoxy syntactic foams versus glass microsphere concentration (C). The figures next to the curves stand for the microsphere bulk densities in kg/m3 the maximum microsphere concentration is 67% for molding compositions 11...

Chemical Principles of Syntactic Foam Formation 4.1 Epoxy Syntactic Foams... 

Epoxy syntactic foams are the best known representatives of this type of material. The brands manufactured in the USSR are EDS (with glass microspheres), EDM (with phenol microspheres). Dienic, novolac, bisphenolic, and esteric structured epoxy resins are used... 

Most epoxy syntactic foams are vacuum molded at 70-120 °C, and vibrating stirrers are used to degas the mixture40 . Recent formulations can be processed without external heating (cured for 30 days at 20 °C) and yet yield syntactic foams with properties comparable to those of hot-cured foamsthe shrinkage is less than 1 % for both. 

Kozlov et al. 90) have shown that the most influential process parameter for epoxy syntactic foams is the vacuum applied during mixing. 

The success of epoxy syntactic foams is associated with the development of the extrusion process for materials containing glass microspheres. Large articles and profiles with dimensions as large as 6 x9 m2 can be made this way (See also Sect. 7). 

Table 13. Epoxy Syntactic Foams with Carbon Microspheres...

Many papers have shown that the water absorption of a syntactic foam is proportional to that of its binder. Polyester syntactic foams, for example, absorb more water, even with dressing additives (silanes, vide infra) than do epoxy syntactic foams (Fig. 11)1. The hydrolytic stability of epoxy foams is increased when the glass microspheres are replaced by carbon ones (Table 19)40). 

Table 19. Hydrolytic stabilities of epoxy syntactic foams 40) (12% filler)...

Table 21. Comparison of methods of dressing for epoxy syntactic foams made with glass microspheres...

Decreasing the water absorption mechanically , i.e. by coating the external surface of an article, produces good results. It was shown that coating of epoxy syntactic foams with a thin layer of epoxy resin reduces the water absorption more than tenfold, even under high hydrostatic pressure 140). 

Fig. 12 a and b. Water absorption linetics of epoxy syntactic foams with glass microspheres at different concentrations. (1) without filler, (2) 10 mass %, (3) 15 mass %, (4) 20 mass%, (5) 25 mass%, a no dressing, and b filler dressed 1481... 

Fig. 13a-c. Thermomechanical curves for a unfilled epoxy binder, b epoxy syntactic foam with dressed glass microspheres, c syntactic foam with undressed filler (1) iiiitial sample and (2) sample after 2 months in contact with water us)... 

Fig. 14. Water absorption (W) versus hydrostatic pressure (P) at two different water temperatures, for an epoxy syntactic foam with glass microspheres as filler11...

The thermal expansion coefficient of an epoxy syntactic foam with carbon microspheres decreases as the filler concentration is increased. Thus the coefficients for foams with 0 (pure binder), 15, 25, and 25 vol% filler are 55 x 10-6, 45 x 1CU6, 37 x 10-6, and 13 x 10-6 deg-1, respectively. These values do not change for tempera-... 

Table 26. Superhigh-Frequency Dielectric Properties of Epoxy Syntactic Foams using Glass Microspheres...

Fig. 20. Calculated and experimental (dotted and solid curves respectively) values of (1) elastic modulus (E) and (2) ultimate compression strength (crj versus phenolic microsphere concentration (C) in an epoxy syntactic foam at 23 °C 162)...

Fig. 21. Compression diagram for (1) an epoxy syntactic foam with phenolic microspheres and (2) unfilled epoxy resin 1611...

Fig. 22a and b. Behavior of syntactic foams during thermal treatment. Key a Thermogram of the hardening process for (1) Unfilled epoxy and (2) Epoxy syntactic foam with phenolic microspheres, b Glass transition temperature of the epoxy binder versus concentration of phenolic microspheres 1621... 

Special equipment has been developed to produce 5 m long flotation sections encasing underwater pipes, using epoxy syntactic foam 46-177). Depending on the pipe size, two or four flotation sections are required, held in place by steel bands. The system is currently used for mediterranean oil exploration a similar system was developed for North Sea oil exploration, allowing the rig to operate at ocean depths of 450 m. 

Table 38 Hiyslcal and Electrical Properties of Epoxy Syntactic Foam vs Various Fillers (10)...

Polystyrene-Epoxy Syntactic Foam. Hollow polystyrene microspheres are produced by heating expandable polystyrene (in other words gas-filled spheres, e.g., propane or butane-filled polystyrene) of microscopic size. The expandable polystyrene microspheres may be added to the epoxy-resin formulation, and the exothermic heat (or the heat during oven cure) can be employed for the expansion. In this manner, foams having densities as low as 80 kg/m (5 Ib/ft ) may be developed. 

Table 42 shows properties of epoxy syntactic foam prepregs. Table 43 shows the strength of 10-ply laminates of epoxy prepregs in three orientations (8). The table indicates that the products have satisfactory isotropicity. 

Table 45 shows that epoxy syntactic foam prepreg can be used for deep-sea applications. For example, an outer hull of Synpreg 7801 can be applied to deep-submergence vehicles to be used at a depth of 20,000 ft. 

Table 45 Properties of Epoxy Syntactic-Foam Prepreg (8) Vehicle...

Song, B., Chen, W., and Frew, D.J. (2004) Dynamic compressive response and failure behavior of an epoxy syntactic foam. Journal of Composite Materials, 38, 915-936. 

Fig. 3. The Deep Quest submersible made by Lockhead for U.S. Navy made extensive use of hollow glass microspheres/epoxy syntactic foam. Courtesy of Naval Undersea Museum, Keyport, Wash.

Recently, prototype molds are being fabricated from particle-filled polyurethane and epoxy syntactic foams using computer-aided multiaxis routers. Plugs, used to mechanically prestretch polymer sheet, are also usually machined from syntactic foams. For certain pol5miers, heated aluminum plugs or solid nylon plugs are desired. 

The presence of cells marks the distinctive difference between bulk composite and composite foams. The level of porosity measures the amount of empty space within the matrix and varies with foam density. For the case of syntactic foams, different microstructures or levels of porosity can be created through varying the type and amount of microspheres. Details of the microspheres used by Wouterson et al. (2007b) to prepare the epoxy syntactic foams are listed in Table 2.1. In the fracture toughness assessment under quasistatic loading, SEND specimens were loaded in a three-point bend (3PB) geometry. Due to the difference in density between the various types of microspheres, densities of foams with equivalent amounts of microspheres vary. The difference in density makes the comparison of the properties of foams nonrelevant. In order to compare the performance of foams, the specific mechanical/fracture properties are used. 

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