Syntactic structural foam

As an alternative to blowing agents, it is also possible to introduce lightweight void-forming pellets or prill , manufactured from ultrafine ceramic foam. These are strong, insulating, and flame resistant, with a density of 100-200 kg m . Grades (such as Tecpril, from Filtec) are stable to 1050°C and exhibit no 

A syntactic foam produced by mixing a polyurethane compound with millions of tiny gas-filled microspheres has been used for high-performance footballs. Developed by the sports company Adidas and Bayer, the foam makes up the top one-third of the outer skin of the footballs. When a football is kicked, the microspheres restore its shape very quickly, producing a very high elastic recovery compared with other football constructions, with the effect that the ball deviates very little from its initial flight path. 

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In order to obtain a reliable mathematical apparatus there has to be a reasonably simple, yet accurate, model of the structure of a syntactic foam. Attempts to extend the known models of monolithic plastics to syntactic structures filled with solid spherical particles have not proved successfull 8,76). Models which rely on close microsphere packing 157,158> have not been very accurate either. For example, Krzhechkovsky et al.,59) showed that syntactic foams can be treated as homogeneous uniform materials with small Ks (up to 20%) only if there is no stress field in the polymer matrix around the microspheres (see Sect. 3.5). 

The problem of ozone depletion considered in the Montreal Protocol is discussed in detail. Qiapter 8 considers methods of manufacture, including molding methods, spraying, frothing, laminating, structural foam molding, syntactic foam preparations, and foam-in-place techniques. Chapter 9 on sources of information covers journals, manufacturers bulletins, technical conferences and their published proceeding, seminars and workshops, standardization activities, trade associations, consultants, and information centers, and books. 

Special Processes. Several more recent developments have opened the possibility of special foam processes that may grow to tremendous importance and perhaps even change the entire concept of foamed plastics. These are structural foam, reaction injection molding (RIM), and syntactic foam. 

Another important distinction is based on density. For flotation, sandwich construction, thermal insulation, and ecomsmy, very low-density foams are preferred, often as low as 1 Ib/ft or less. Vinyl, polyolefin, and syntactic foams are generally of medium density. Structural foams are of medium to fairly high density and have a graded structure from solid skin to fairly low-density core. RIM foams are similar, often exhibiting a fairly high density. 

FIGURE 2.58 Schematic representations of section through different types of cellular polymer, (a) Low-density open-cell foam, (b) High-density closed-cell foam, (c) Single-component structural foam with cellular core and integral solid skin, (d) Multicomponent structural foam, (e) Fiber-reinforced closed-cell foam, (f) Syntactic foam. 

Some special types of foams are (1) structural foams (2) syntactic foams and multifoams and (3) reinforced foams. Structural foams (Figure 2.58c and d), which possess full-density skins and cellular cores, are similar to structural sandwich constructions or to human bones, which have solid surfaces but cellular cores. Structural foams may be manufactured by high pressure processes or by low-pressure processes (Figure 2.61). The first one may provide denser, smoother skins with greater fidelity to fine detail in the mold than maybe true of low-pressure processes. Fine wood detail, for example, is used for simulated wood furniture and simulated wood beams. Surfaces made by low-pressure processes may, however, show swirl or other textures, not necessarily detracting from their usefulness. Almost any thermoplastic or thermosetting polymer can be formulated into a structural foam. 

El-Hadek, M.A. and Tippur, H. V. (2003) Dynamic fracture parameters and constraint effects in functionally graded syntactic epoxy foams. International Journal of Solids arul Structures, 40, 1885-1906. 

The glass transition temperature lies considerably above the temperature of use with rigid foams and considerably below the temperature of use with flexible foams. The cellular structure may be open, closed, or mixed. All cells are open to each other with open cellular structures. In contrast, each cell is sealed off or encapsulated from the other cells by a plastic wall in closed-cell structures. Structural foams are foams with a dense outer skin and an interior of lower density they are also called integral foams or self-skinning foams. In contrast to normal foams syntactic foams do not directly enclose the gas but contain small hollow bodies of glass, ceramics, or plastics which are under vacuum or are filled with a gas. 

For the structural paste and film adhesives (Table 6) some indication as to the maximum service temperatures obtainable, under these eure eyeles, are given. In the case of the syntactics and foams (Table 7), the eured Sg or expansion ratio is given. 

In addition to developing solid RP structures, work has been conducted on sandwich structures such as filament-wound plastic skins with low-density foamed core or a plastic honeycomb core to develop more efficient strength-to-weight structures. Sandwich structures using a syntactic core have been successfully tested so that failures occurred at prescribed high-hydrostatic pressures of 28 MPa (4,000 psi). 

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. 

Abstract— The use of organosilanes as adhesion promoters for surface coatings, adhesives and syntactic foams is described and reviewed in the light of published work. Data are presented on the beneficial effect of silanes, when used as pretreatment primers and additives, on the bond strength of two pack epoxide and polyurethane paints applied to aluminium and mild steel. It is shown that silanes when used as additives to structural epoxide and polyurethane adhesives are less effective than when used as pretreatment primers on metals but are highly effective on glass substrates. The compressive properties of glass microballoon/epoxide syntactic foams are shown to be markedly improved by the addition of silanes. 

Syntactic foams manufactured from hollow glass or silica microspheres and an epoxide, phenolic or other matrix resin represent a class of lightweight structural materials used for buoyancy purposes, insulation and packaging. The effect of silanes on the mechanical properties of syntactic foams at a nominal density of 0.35 g/cm3 is shown in Tables 14-16. The Proportional Limit is defined as the greatest stress which the foam is capable of sustaining without any deviation from proportionality of stress to strain (Hooke s Law). 

Syntactic foamed materials are classified as foamed plastics because they are formally similar in structure to cellular gas-expanded plastics in that they are heterophase, gas-solid systems. In general, however, they differ from ordinary foamed plastics in that they are not binary but tertiary systems because the filler and binder are made usually from different materials 3 5). 

Fig. 6a-c. Graphic representation of syntactic foam structures 85) a Random dispersion of spheres, two-phase composite b Hexagonal closed-packed structure of uniform-sized spheres, two-phase composite c Three-phase composite containing packed microspheres, dispersed voids, and binding resin... 

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... 

Recently Sands et al.97) and Methven et al. 98) developed sheet molding compounds (SMC) and sandwich structures based on polyester syntactic foams. 

The carbonization of a syntactic foam opens up the previously closed cell structure. The size and proportion of micropores formed depend mainly on the size and shape homogeneity of the filler. Thus, there is quite a narrow size distribution of open pores in the carbonized material made from a novolac oligomer and Krecasphere carbon microspheres (Fig. 9)39). 

Other new materials are frothed or lightened syntactic plastics, which can be either isotropic 101 n0,111 132 —134.) or anisotropic 116). They have also been called integral (structural) syntactic foams and the smallest density obtained is 67 kg/m3 67). 

It is easy to see that these models are all based on the same (microstructural) principle, viz. that there is an elementary structural unit that can be described and then used for calculation. Remember that the corresponding unit cell for foamed polymers is the gas-structure element8 10). Microstructural models are a first approximation to a general theory describing the deformation and failure of gas-filled materials. However, this approximation cannot be extended to allow for all macroscopic properties of a syntactic foam to be calculated 166). In fact, the approximation works well only for the elastic moduli, it is satisfactory for strength properties, but deformation... 

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