Syntactic foams applications

Syntactic foams are composites made of micro-balloons or hollow macrospheres bound into a plastic matrix. The polyimide syntactic foams are used in specific electronic equipment for microwave and RF applications. They act as structural, dielectric and heat-barrier materials. Micro-balloons can be made of polyimides for special applications. 

The low water absorptivity and good resistance to hydrostatic pressure make syntactic foams very useful for marine and submarine construction. Materials to be used for deep-sea application must have 1) low compressibilities at high hydrostatic pressure, 2) low thermal expansion coefficients, 3) low water absorption, and 4) good fire resistance. The fluids used for buoyancy in deep water submersibles include gasoline, ammonia, and silicone oil, while the solids include plastic, glass and aluminium foams, lithium, wood, and monolithic polyolefins. The liquids are dense but have low... 

Calculations have shown that at 9sph = 15 % the hydrostatic strengths of a syntactic foam decreases by 35-40 %, and for 9sph = 28 % by 60 % as compared to the matrix132>. Note that for practical applications the hydrostatic strength of a syntactic foam does not depend on scale factors (between 1 and 100) nor on the shape of the final article 75). 

Syntactic foams are used extensively for the construction of boats and deep-water submarines46155,58,79,127 142-169). They are also used to make floats, buoys, underwater rescue apparatus, and equipment for raising sunken ships 153,176). Other applications include cements and putties for repairing hydraulic structures, submarine bodies and bulkheads127). 

Growing interest was reported in syntactic foam for car applications 46,182). 

From an experimental point of view, typically ayt/hydrostatic pressure is not negligible and must be taken into account for submarine applications (pipelines, submarines, syntactic foams, etc.). 

Fillers with a density lower than that of epoxy can be used to provide reduced specific gravity in cured products. These are usually gas-filled microballoons. Although they generally bring about a significant increase in viscosity, the microballoon filled epoxies (sometimes called syntactic foam adhesives) are often used in marine applications where low density and buoyancy are important criteria. 

Due to the extremely low density of the microspheres, the resultant syntactic foam is low in density. It also has high physical strength, especially compressive strength. Other features of the syntactic foam include isotropic physical properties and low water absorption. The latter has led to application in deep sea submersible vehicles. Recently applications have expanded to other areas, including aircraft. 

Matrix resins to be used for syntactic foams include thermosetting resins and thermoplastic resins, as shown below. Epoxy resin, unsaturated polyester and phenolic resin have been the resins of choice for industrial applications because the resulting foams have remarkably high compressive strengths. Examples of the resins used are given below. 

Uniaxial compressive properties are important to the design engineer who can utilize the foams inherent high compressive strength in reinforcing other structural members. Sandwich construction is a typical example of such a use, as in submarine-hull construction. Syntactic-foam prepregs have been developed for this application (7). 

The major advantage of syntactic foams is the high strength-to-weight ratios. This advantage has ied to applications in deep-submergence vehicles for hydrospace use (7), aerospace applications such as interior floor panels of aircraft (8), nose cones, fins, and bodies of rockets, sonar windows (some acoustic properties of the foam are similar to those of sea water, radomes, etc. (11). 

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. 

In 1977 the National Materials Advisory Board (NMAB) of the National Academy of Sciences held a workshop on Acoustic Attenuation Materials Systems. The sponsoring agency was the Office of Naval Research, and the proceedings were published by the NMAB Conunittee on Structural Applications of Syntactic Foam (15). 

This excellent book, a revision of the 1978 Handbook of Fillers and Reinforcements, covers only fillers and has a chapter on hollow microballons for use in syntactic foams. A companion volume by the same authors covering reinforcements does not consider foam applications. 

Proceedings of the Workshop on Acoustic Attenuation of Materials Systems, prepared by the National Materials Advisory Board (NMAB) Committee on Structural Application of Syntactic Foam, NMAB-339, 1978, 147 pp. 

Applications are for use as lightweight filler materials for syntactic foam shapes or for parts for dielectric applications. 

Covers material and process requirements for fabricating sandwich radomes with polyimide-resin-impregnated quartz cloth shells and polyimide-resin syntactic foam cores. Application is primarUy as a... 

Syntactic foam is made by dispersing hollow microballoons into a liquid polymer and then solidifying it. Microballoons are typically hollow glass or hollow phenolic microspheres, and the most common liquid polymer is an epoxy prepolymer, which is then cured. Although some products are notably woodlike in their properties and machinability, primary applications are high-performance products such as deep-sea instrumentation. 

Next, we will present applications of the CTH scheme in several real-world composite stractures. They arc fiber reinforced syntactic foam cored sandwich structures, sandwich stractures with a continuous fiber reinforced grid stiffened syntactic foam core, and 3-D woven fabric reinforced polymer composite structures. Of course, the core or the polymer composite is based on the same thermosetting PSMP as discussed above. 

Wood is the most appropriate material for a solid sandwich core and has been used extensively for many applications, such as doors and partitions. For lightweight constructions flat or end grain balsa is still used, although this is being superseded by synthetic materials. Bonded microspheres of inorganic materials, glass, ceramics, etc. to form syntactic foams are useful core materials. 

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