Syntactic adhesives

Core-Bond, Polyester-based syntactic adhesives, ATC Chemical Corp. 

Although film adhesives account for the overwhelming bulk of stmctural adhesive used by the aerospace industry, expandable and syntactic adhesive foams, paste adhesives, and adhesive primers are all necessary parts of a complete bonding system. 

The second range of products contains the foaming and syntactic adhesives. These are the materials which can be used to sphce sheets of honeycomb together, to join honeycomb to metallic channelling. 

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. 

The influence of stable adhesion between binder and filler on the water absorption of a syntactic foam is corroborated by the thermomechanical behavior of the foam. 

The data in Fig. 13 show that the glass transition temperatures of all materials is reduced by the absorption of water. This seems to be due to the plasticizing effect of the water on the binder. There is a marked difference between the elastic states of the dressed and undressed foams, the latter becoming much more plastic after immersion in water. Increased plasticity is due to the loss of adhesion between the binder and the filler, indicating that water absorption by syntactic foams is multistaged. 

Experience has shown that, even under hydrostatic pressure up to 75% of the collapsing pressure, syntactic foams do not absorb large amounts of water 74). Epoxy and glass syntactic foams resist water the best, but after prolonged exposure they weaken considerably (Table 22), as the binder—filler adhesion fails. 

Syntactic foams are less combustible than their chemically foamed counterparts for the same reason. A syntactic foam s fire resistance can be increased using modifiers and additives in much the same way as for ordinary plastics, the only additional precaution being that the filler—binder adhesion should not be impaired in the process. Specially compounded polyester resins have been used in the USSR to obtain syntactic foams, whose combustion times and mass losses are, respectively, 4-60 and 24 to 180 times lower than those of the unmodified plastic 155). 

The adhesion of syntactic materials depends primarily on the adhesive and the substrate in question. Epoxy materials, for example, (in the absence of an extra glue) have the following adhesion strengths2) ... 

An examination of the experimental findings and the calculation model shows that the deformability of a syntactic foam depends mainly on the elastic properties of the polymer matrix, whereas the filler concentration mainly affects its compressibility. In fact, monolithic (unfilled) samples do deform elastically at the start of the compression curve, but when the material is deformed further, the forced elasticity limit is reached (Fig. 21). Thus, the nominal ultimate strength for non-brittle failure is determined by the fact that the forced elastic limit is reached, and not because the adhesive ties have lost their stability (as it is the case with light plastic foams) 8 10). 

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. 

Syntactic silicone foams have also been formed. They cure at room temperature or at slightly elevated temperatures and become flexible insulators and ablators. They can be cut, trowelled, molded, or air-injected into place, and oued pieces can be bonded to substrates by using RTV silicone adhesives (13). 

Syntactic Foams Adhesives should be selected based on the resin matrix, which is usually epoxy or phenolic (18). 

From syntactic foams and other lightweight structural composites to the new generation of adhesives and batteries, Ross mixers are helping to create the materials necessary to build the New Economy. 

Accompanying the proliferation of non-metallic products used in building aircraft has been a growing base of information about how different materials can affect passenger and crew survivability in the event of an on-board fire or crash. For example, FAA 14 CFR 25.853-compliant adhesives and syntactics do not propagate flame when exposed to the 60- or 12-second vertical Bunsen burner test (see sidebar on p. 28). 

Table 1. Comparison of Flame-Retardant Adhesives and Syntactics...

Several of the recommendations under consideration hy the ESTG to help standardize flammability testing on adhesive and syntactic-containing composite components include ... 

Testing an adhesive or syntactic used for insert reinforcement alone to a 12-second vertical test... 

A syntactic core material made of a 120C curing epoxy film adhesive filled with glass micro spheres and supplied with a lightweight carrier scrim is also available. In the uncured state, the material is in... 

SynCore, Syntactic film core, Dexter Adhesive Coating Systems (CA)... 

SynSkin, Composite surfacing film, Dexter Adhesive Coating Systems (CA) SynSpand, Expandable syntactic core, Dexter Adhesive Coating Systems (CA) Synthemul, Nitrile, Reichhold Syntron, Parts feeders, FMC Corp., Material Handling Equipment... 

Figure 37 Jet engine nacelles are insulated sandwiches which are bonded with heat resistant adhesives including paste or fdm adhesives and syntactic fdm...

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