For structural composite applications, often sandwich composite constructions are being used. Sandwich composites are made up of thin but stiff fibrous skin layers and a lightweight core material. The core material is normally made of a low-strength material, but due to its larger thickness, it provides the sandwich composite with high bending stiffness with overall low density. Typical core materials include materials like polyvinylchloride, polyethylene, polystyrene, and balsa wood. However, for the past few years, there is an increasing trend in the use of a special type of foam material, called syntactic foam. The concept of a syntactic foam is shown in Fig. 18.

The term syntactic,which originates from the Greekword “syntaktikos” (= orderly disposed system), indicates a constructed foam, in the sense that the material is manufactured by a specific mixing procedure of fillers and binders with appropriate volume fractions. The hollow filler is randomly dispersed in the matrix, creating a homogeneous material exhibiting isotropic macroscopic behavior. Since the spheres have continuous shells, the final material can be considered as a closed-cell foam. Typical photograph and SEM image of syntactic foam are shown in Fig. 19.

The advantages of syntactic foam over the conventional foams are as follows:

• The mechanical properties of syntactic foam, in particular the compression properties, are several orders of magnitude higher than the properties of the lighter (traditional) foams.
• No blowing agents are required to create the porosity.
• The density of the syntactic foam before curing is fairly the same as the density after curing which makes the synthesis of these materials fairly simple (see Fig. 20).
• Due to its closed-cell structure, syntactic foams absorb only minimal amounts of liquid which makes syntactic foam an interesting material for the marine and offshore applications.

Although the use of syntactic foam has only increased sharply in the twenty-first century, it has been around for nearly half a century. Syntactic foams originate from the early 1960s (Shutov 1991). It was initially used as a buoyancy material for marine applications. Current applications for syntactic foam include buoyancy modules for marine riser tensioners, boat hulls, deep-sea exploration, autonomous underwater vehicles (AUV), parts of helicopters and airplanes, soccer balls, etc.

Processing of Syntactic Foam

This paragraph describes how, in general, a syntactic foam can be processed in a laboratory-based environment. The first step is to mix the base resin and the hardener according to a predefined stoichiometric ratio. Mixing can be done in a glass beaker or disposable cup and stirring can be performed using a glass rod. The obtained mixture has to be stirred by hand for a couple of minutes until the filaments visible with the naked eye, due to the contact of the hardener and base resin component, are no longer visible. Stirring should be performed cautiously to avoid the formation of air bubbles.
Once the base resin and hardener have been stirred thoroughly, the microspheres can be added to the mixture. The amount of microspheres to be added depends on the preferred microstructure of the syntactic foam. It is important to understand that the microspheres should be added to the mixture in small portions to avoid resin necking. Resin necking occurs when a group of microspheres is trapped inside the resin, affecting the mechanical and fracture properties of syntactic foam.
The viscosity of the resulting uncured syntactic foam depends on the amount of microspheres added. For microsphere volume fractions of 30 % and below, the viscosity of the uncured syntactic foam is low enough to be poured into a pretreated mold. For mixtures having more than 30–40 volume percentage of microspheres, the viscosity of syntactic foam gets too high and the mixture is not able to flow into the mold. The mixture has to be scooped into the mold and requires some pressure to fill up the mold.
The main challenge in processing syntactic foam lies in the choice of the process parameters, i.e., mixing temperature and mixing time and the addition sequence of the components (base resin, hardener, hollow microspheres). It is important that the viscosity of the mixture should rise rapidly once placed into the mold to avoid lamination of the mixture.

Mechanical Behavior of Syntactic Foam Versus Filler Volume Fraction

It is well known that the mechanical behavior of syntactic foam is far superior compared to conventional foams (see Table 4). It is obvious that the density and the properties of these foams will vary with the quantity and type of microspheres. Wouterson et al have done extensive research on how the content of microspheres affects the mechanical and fracture properties of syntactic foams (Wouterson et al. 2004, 2005). Besides looking at various content of microspheres, they also studied the existence of various toughening mechanisms in syntactic foams as well as the kind of toughening strategies that can be used to improve the toughness of this kind of foams (Wouterson et al. 2007a, b).

The work presented here highlights the typical mechanical behavior of a syntactic foam as well as highlights the change in mechanical behavior observed when the content of microspheres is changed. The work as presented focuses on three types of microspheres, namely, K15 and K46 hollow glass microspheres from 3M and BJO-093 phenolic hollow microspheres from Asia Pacific Microspheres. The details of these different microspheres are given in Table 5.

Specific Compression Properties

As was already highlighted in Table 4, syntactic foams exhibit phenomenal compression strength that have been reported extensively by various researchers (Gupta et al. 2001; Bunn and Mottram 1993; Kim and Plubrai 2004; Rizzi et al. 2000). To understand the performance, it is important to understand what happens inside the syntactic foam during compression loading. Examples of typical compression stress–strain curves of syntactic foam are shown in Fig. 21.

It is clear that all curves are made up of three different sections. The first region (I) is characterized by an almost linear-elastic behavior of the syntactic foam. The region ends when the material starts to yield and reaches its compressive yield strength. Upon yielding, the microspheres become crushed under the compression load and severe damage occurs. Yielding and inelastic damage occur during the test as the sides of the specimen barrel outward under compression loading.
The second region (II) of the flatwise compression curves in Fig. 21 is characterized by relatively horizontal plateaus. The horizontal plateau is attributed to the implosion of the hollow microspheres under the increasing compression load (see Fig. 22). The elliptical voids indicate that the microspheres have been crushed under compression loading and the remaining pore has been deformed to an elliptical shape. Intact microspheres and sphere-shaped voids can also be observed indicating that the syntactic foam has not reached it maximum compression strain but is still in the horizontal plateau of the load vs. displacement curve. It is also clear from Fig. 18 that syntactic foams with higher fractions of hollow microspheres show a larger horizontal plateau and thus a larger strain which is in-line with the above observation of microsphere crushing leading to compression strain.
The third region (III) is characterized by a steep increase in the load–displacement curve. The steep increase is caused by a large number of microspheres being crushed and compacted, and the maximum density is being reached. The point, where region III starts, is considered to be the point of failure for the syntactic foam as this is the point where most of the load-bearing microspheres have been crushed.
Although the results presented in Fig. 21 are given as absolute values, it should be understood that in order to compare the different types of foam, the values should be represented as specific values to compensate for the change in density. The overall results of the specific compression strength and modulus for various contents of microspheres are given in Fig. 23a, b. From Fig. 23a, it can be observed that the specific compressive yield strength, σyc/ρ, decreases with increasing filler content for phenolic and K15 glass microspheres. The microspheres act as voids and weaken the structure. Nevertheless, an upward trend in σyc/ρ is observed for K46 microspheres. The upward trend is attributed to a relatively minor decrease in the compressive yield strength compared to the relatively larger decrease in density. From the difference in trends between the K15 and K46 glass microspheres, it is induced that σyc/ρ is influenced by the wall thickness of microspheres. The results for the specific compressive modulus, Ec/ρ, are shown in Fig. 23b. Again K46 microspheres perform better as shown. The performance of K46 microspheres is also attributed to their higher wall thickness-to-radius ratio. For all microspheres investigated, it appears a densification process takes place at low filler content (~10 vol%). The densification is caused by a compaction process of the matrix polymer in the presence of voids in curing.
As the content of microspheres increases and, consequently, the matrix volume decreases, the microsphere takes up more load under compression and, as a result, the specific compressive moduli increase. K15 glass and phenolic microspheres show a rather similar behavior in Ec/ρ. Both show a minimum around 20 vol%, after which the Ec/ρ slightly increases. This increase is higher for K15 microspheres. The specific compressive stiffness levels off as the increase in stiffness is counterbalanced by the increases in density of the composite.

Specific Tensile Properties

Besides exhibiting high compression strength, Table 4 also shows clearly that the tensile properties of syntactic foams are significant. Syntactic foam behaves like a linear-elastic material up to failure when loaded in tension. The material experiences catastrophic failure across a plane perpendicular to the tensile axis. Luxmoore and Owen (1980) concluded that a crack will initiate from an oversized void, which basically acts as a stress concentration, when a composite is subjected to tensile loading. Similar findings were observed by Wouterson et al (2004, 2005). On the fracture surfaces of the tensile specimens, voids could be identified. The presence of voids in syntactic foam, prepared in a lab-based environment, is difficult to prevent. The number of voids can be minimized by avoiding vigorous stirring as well as by applying a pressure onto the mixture during curing.
The outcome of the tensile testing of various compositions of syntactic foam is highlighted in Fig. 24. The trends in the tensile strength, σt, with increasing microsphere content are rather similar for the three types of microspheres, except for 50 vol% phenolic microspheres. The results indicate that the failure under tension is matrix-dominated or dominated by an external factor (i.e., voids). Figure 24a shows that σt/ρ increases upon inclusion of a small amount of microspheres when compared to neat epoxy resin. However, beyond 10 vol%, a decreasing trend in σt/ρ is observed for all types of microspheres. The decreasing trend in σt/ρ with increasing filler content indicates that the relative reduction in strength is larger than the relative reduction in density. Indeed, a decrease in σt with increasing filler content is observed. The comparable trends and values suggest that σt/ρ is independent of the microsphere type and size but only varies with the microsphere volume fraction.

Luxmoore and Owen (1980) also suggested that the failure of the foam is attributed to the failure of the resin matrix. The nonlinearity in σt/ρ is caused by the reduction in the area of the epoxy matrix in a cross-sectional area. Introduction of hollow microspheres reduces the epoxy volume fraction and increases the inhomogeneity content, consequently reducing the tensile strength of the composite as a result of poor interfacial strength between the matrix and the filler. The authors assume that the matrix serves as the load-bearing phase in the composite, whereas the hollow microspheres only provide lightweight and minimal strengthening effect. The reduction in load-bearing volume outweighs the increase in stiff microsphere shells. Around 40 vol% of filler content, a minimum is observed for all the microspheres. The authors suggest that 40 vol% of microspheres is to be the maximum amount of filler which can be fully wetted by the epoxy matrix in this system. The change in properties and behavior of syntactic foam around 40 vol% was also observed by Bunn and Mottram (1993). Nevertheless, clear advantages regarding other mechanical properties arise upon introduction of hollow microsphere as shown in the discussion that follows.
It has been reported that the Young’s modulus, Et, generally decreases with increasing content of hollow microspheres (El-Hadek and Tippur 2002). Bardella and Genna (2001) are some of the few who have reported an increasing trend in Et for syntactic foams containing K37 (ρ= 0.37 g/cm3) microspheres. The results presented in Fig. 22b to Fig. 24b shows similarity to those in Bardella and Genna (2001) in the sense that the trend for the specific Young’s modulus, Et/ρ, with increasing filler content depends on the type of microspheres used. K46 microspheres show a rather linear increase in Et/ρ, whereas K15 shows a constant trend. Based on the data presented, it is known that the constant trend in Et/ρ is caused by a proportionate decrease in the Young’s modulus and the density. For hollow phenolic microspheres, a decrease in Et with increasing filler content is observed. The value obtained for neat epoxy resin is too low considering the trends in Et for syntactic foams containing the three different types of microspheres. The lower value for neat epoxy resin could be caused by the presence of voids. The differences between the results for K46 and K15 microspheres can be attributed to their size and wall thickness. It has been shown by several researchers that, in general, for microspheres of the same material composition, improved mechanical properties are obtained for the microspheres with a higher density. Higher density for microspheres of the same material is often associated with a larger thickness-to-radius ratio, t/R (see Table 5). Syntactic foam containing microspheres of the same material having a larger t/R often exhibits improved mechanical properties (Fine et al. 2003).
The difference between the phenolic and K15 glass microspheres explains that apart from microsphere size, the material composition of the microsphere is of equal importance for the mechanical properties. Phenol formaldehyde has an Et of about 6.8 GPa whereas soda lime glass has an Et of about 77 GPa. The difference in Et is believed to be an important factor to the difference in the trends of hollow glass and phenolic microspheres. This relationship between the material of the matrix and the inclusion was studied by Pawlak and Galeski (2002). It was found that the higher the Young’s modulus of the inclusion compared to the epoxy used, the higher the stresses developed in the material. Further they observed that the position of the maximum stress at the spherical inclusion changed from the pole for hard inclusion to the equator for soft inclusion.

Figure 25 shows a typical SEM fractograph of a tensile specimen; the micrograph confirms the role of the binder in the failure of syntactic foam under tensile loading. The micrograph shows a rough surface of the epoxy resin after brittle fracture. Numerous step structures can be identified in Fig. 25 which indicates the plastic yielding of the epoxy resin. Besides plastic yielding, intact microspheres are observed on the fracture surface. The fact that the crack has grown over the interface between the matrix and the microsphere indicates the presence of debonding. Debonding of hard inclusions was also reported by Pawlak and Galeski (2002). The debonding is caused by the complex stress state around the particle–matrix interface which will not be further discussed here.

Specific Flexure Properties

Figure 26a, b show typical flexure stress–strain curves for syntactic foams containing K46 glass and phenolic microspheres, respectively. The flexure behavior of K15 microspheres is not shown as it is similar to the flexure behavior of BJO-093 microspheres. For all specimens, it is observed that the strain is reduced with increasing filler content. Especially, the syntactic foams containing glass K46 microspheres show a larger reduction in failure strain. The reduction in strain suggests that the failure of the syntactic foam under flexure loading occurs at the side loaded in tension. The larger reduction in strain for glass K46 microspheres is attributed to the higher strength of the K46 microspheres (see Table 5). Fewer microspheres will fracture under the applied load which prevents stress relief in the material. On the contrary BJO-093 and K15 microspheres fracture more easily, relieving the stress in the material and thus showing a larger strain.

Upon fracture, plastic deformation is observed for most compositions of syntactic foam with up to 30 vol% of microspheres. The amount of plastic deformation decreases with increasing filler content since the plastic deformation is attributed to the behavior of the epoxy binder. Syntactic foams containing hollow phenolic microspheres show larger plastic deformation compared to K46 and K15 microspheres. The latter might be attributed to ductile deformation of hollow phenolic microspheres as shown in Fig. 27, an observation that has not been seen for hollow glass microspheres. The plastic deformation of syntactic foam containing hollow phenolic microspheres can be clearly seen on the fracture surface of specimens due to stress whitening. The stress whitening cannot be distinguished on syntactic foam containing hollow glass microspheres since the resulting syntactic foam is white in color.
During three-point flexure loading, the specimen is subjected to compressive stresses on the top part of the specimen and to tensile stresses at the lower part of the specimen. From the tension and compression data presented in the previous paragraphs, it is expected that syntactic foam subjected to flexure loading will fail at the side that is loaded in tension. Indeed, failure at the lower specimen side under tensile stresses is observed during experimental work. All specimens fail at the center of the support span. No indentation of the indenter is observed. The comparable failure mode under tensile loading and three-point bending explains why there is some resemblance between Fig. 24 and Fig. 28. The only difference observed is the behavior of syntactic foams with 0–10 vol% of microspheres.

For the specific flexural strength, σf/ρ (see Fig. 28a), some differences between the different compositions are observed. The differences are mainly caused by the difference in density between the different compositions as the values and trends are rather similar for the maximum flexural strengths. All types of microspheres show a decreasing trend in the maximum specific flexural strength with increasing filler content. Similar to the tensile test results, the specific strength approaches a minimum around 40–50 vol% of filler content.
The trends and values in the specific flexural modulus, Ef/ρ (see Fig. 28b), are consistent with the results presented in Fig. 24b. The similarity is again attributed to the tensile failure mode of syntactic foam under three-point bending. The K46 microspheres show an increase in Ef/ρ with increasing filler content. K15 microspheres show a constant value for Ef/ρ whereas phenolic microspheres show a decrease with increasing filler content. The constant value in Ef/ρ from K15 microspheres is attributed to a similar relative decrease in the flexure modulus and density.
SEM fractographs of the specimens tested under three-point bending (see Fig. 29) reveal the different loading modes at the upper and lower parts of the cross-sectional area of the specimen. Figure 29a shows the fracture surface of the lower part of the cross section, which is loaded in tension. The fractograph is consistent with that in Fig. 25. The surface is characterized by a rough surface. In between the tension and compression sides of the specimen, an area with relative smooth features is observed. The transition between the tensile surface and the compressive surface is shown in the right side of Fig. 29b. The compressive surface shows step structures behind the microspheres. The step structures diminish with increasing filler content and are less pronounced for phenolic microspheres. Besides the step structures, debonded microspheres can be identified. Debonded microspheres indicate a poor interface between the matrix and the filler.

Specific Fracture Properties

Besides the mechanical properties, fracture properties were also investigated for various contents of microspheres. Figure 30 shows the specific fracture toughness, KIc/ρ, for various compositions of syntactic foam. The most distinctive features are the significant increase in KIc/ρ for filler content up to 30 vol% for all types of microspheres and the decrease in KIc/ρ beyond 30 vol% of filler content. The maximum in the trend for the fracture toughness has been reported for other solid particulate composites (Lee and Yee 2000; Rothon 1995; Kawaguchi and Pearson 2003). The increase in KIc/ρ for 0–30 vol% filler content suggests the presence of a toughening mechanism which increases the fracture energy compared to neat epoxy resin. For KIc, the hollow glass microspheres outperform the hollow phenolic microspheres with K46 microspheres resulting in the highest value for KIc/ρ, suggesting that the wall thickness and density of the microspheres affect the fracture toughness of the syntactic foam. However, the hollow phenolic microspheres outperform the hollow glass microspheres for the specific energy release rate. This is mainly caused by the lower value of the Young’s modulus of the BJO-093 samples.
Figure 31 shows the fracture surface of pristine epoxy resin after being subjected to three-point bending. The featureless fracture surface of epoxy resin is indicative of the well-known brittle deformation of epoxy resin. Uniformly oriented white lines on the fracture surface suggest minor shear deformation of the epoxy resin prior to fracture.
From Fig. 32, it is clear that the morphology of the fracture surface changes dramatically if hollow spheres are added to the epoxy resin. The lines observed behind the hollow spheres are referred to as step structures.

The characteristic tail structure which is created by the mismatch between two planes of crack propagation is often regarded to be the evidence for the action of the crack front bowing mechanism. Lange (1970) first proposed the crack front bowing mechanism. The mechanism proclaims the resistance of rigid particles to crack propagation. Because of the impenetrability of the particles, the primary crack has to bend between particles. There will be a point when the crack front breaks away from the rigid particle. At this point, the arms of the secondary crack fronts will come together and form a characteristic step structure as both secondary crack fronts propagate at a different crack plane. These characteristic step structures are also called “tails” or “lances.” Many studies on the fracture toughness of particulate composites consider the crack bowing as the main toughening mechanism. The bowed secondary crack front has more elastic energy stored than the straight unbowed crack front. Therefore, more energy is needed for a crack to propagate.
Besides the presence of step structures, debonding of microspheres is observed on the fracture surface of syntactic foam. Debonding can be identified by the absence of the microsphere or by a gap in between the microsphere and the matrix. According to Lee and Yee (2001), the debonding is typical for microspheres with a size larger than 10 μm. The presence of debonding of microspheres from the polymer matrix suggests the weak interfacial adhesion at the microsphere–matrix interface.
Several researchers have suggested that the debonding of glass beads is one of the major toughening mechanisms or is a mechanism, which triggers matrix plastic deformation in glass bead-reinforced polymers. According to Evans (1972), debonding of glass beads was the most prominent mode of micro-cracking in glass bead filled epoxies as it exhibited the ability to reduce the crack driving force resulting in toughening behavior. Debonding involves energy dissipation and will thus impede the crack growth process.
Figure 33 shows the fracture surface of syntactic foam containing low and high volume fractions of microspheres, respectively. Clearly, Fig. 33a shows that step structures prevail for the microstructures containing low volume fractions of microspheres, whereas debonding of microspheres is observed to be the dominant mechanism for syntactic foams containing large quantities of microspheres (see Fig. 33b). The maximum value in KIc/ρ suggests a transition in dominating toughening mechanisms in syntactic foam.

According to Lee and Yee (2000), the increase in microsphere content will decrease the interparticle separation between microspheres. The increase of microsphere content beyond the complete wetting ability of epoxy also introduces intersphere sliding. Higher volume fractions of microspheres allow more microspheres to debond from the matrix. Debonding is accompanied by premature cracks. If the direction of these cracks is parallel to the crack growth direction, the subcritical cracks act as precursors and facilitate crack propagation.
From the results presented, it can be concluded that the specific properties of syntactic foam depend on the types and volume fractions of microspheres utilized in the syntactic foam. Increase in microsphere density (K46 vs. K15) and the thickness-to-radius ratio led to an increase in specific tensile stiffness. The results for the tensile and flexural tests were comparable due to the fact that both types of tests exhibited the same failure mode. Both tests elicited a decreasing trend in specific strength with increasing filler content. A distinct trend in compressive behavior was noted in contrast to the tensile failure. The compression tests revealed the excellent compressive properties of syntactic foam and in particular the superior performance of K46 microspheres, giving rise to higher compressive yield strengths and moduli compared to K15 and phenolic microspheres.
From the fracture toughness tests, it can be concluded that all types of studied microspheres show a similar trend in the specific fracture toughness with increasing filler content. For lower filler content, an increase in the specific fracture toughness was observed. The increase reached a maximum after which a decrease in the specific fracture toughness was seen. The change in behavior was attributed to a change in the dominant toughening mechanisms from filler stiffening, crack front bowing, to excessive debonding. The usefulness of a combination of desired properties for syntactic foam such as lightweight high stiffness, high compression, and high toughness was clearly demonstrated.