Three-Phase Carbon Microballoon Syntactic Foam Composites

S . KENIG, I. RAITER, and M. NARKIS"

Department of Chemical Engineering, Technion City, Hags 32000, Israel

This paper describes three-phase syntactic foams consisting of microballoons, resin, and air voids. The microballoons com- prise carbon and silica spheres in various proportions, always at a close-packed structure (0.6 volume fraction). Compressive, electric, and dielectric properties are demonstrated as func- tions of parameters like resin content, temperature, and various carbon/glass proportions. The good combination of mechanical, thermal, and electrical properties may qualify these foams for applications including attenuation of electromagnetic radiation.

INTRODUCTION

yntactic foam is a composite material consisting S of hollow microspheres (microballoons) embed- ded in a resin matrix. Such foams offer low density and good compressive properties. The best com- pressive strength syntactic foams are those made of glass microballoons, consequently for deep sub- mergence applications, these composites are most widely used. Hollow carbon microspheres, which are generally the carbonization product of either phenolic based hollow spheres or pitch, offer both good mechanical properties and excellent hydro- lytic stability (1).

Carbon based syntactic foam composites were the subject of a few investigations. Price and Nelson (2) studied the thermal and mechanical properties of carbon-pyrrone syntactic foams for elevated tem- perature load bearing sandwich panels. Benton and Schmitt (3), Nicholson and Thomas (4), and Sharer and Powers (5 ) investigated wholly carbon syntactic foams made of carbonized matrix and carbon micro- spheres. In such systems, high temperature insula- tion, good mechanical properties, and light weight were the properties sought. McWhirter, et a1 (6), have developed a syntactic foam comprising poly- imide resin, carbon microspheres, and carbon fibers for applications involving mechanical support and thermal protection. In another report carbon mi- croballoon filled materials were shown to be elec- trically conductive (7). Amagi, et a1 ( 8 ) have de- scribed an interesting combination of silicone rub- ber and carbon microspheres. In this composite the resistivity increased by four orders of magnitude within the temperature range of 20 to 90°C. This property-positive thermal coefficient may be useful in heating applications.

Three phase syntactic foam is a distinct composite material. It consists of hollow microspheres bonded

' To whom corr~spondence should be addressed.

together with a limited amount of polymer which does not form a continuous matrix. The void space is the third phase obtained by intentionally limiting the amount of polymer mixed with the microbal- loons. While density is an important property for all syntactic foams, the void fraction is equally im- portant for three-phase composites.

Three-phase syntactic foam composites consist- ing of a close-packed bed of microballoons, epoxy, or polyimide resin and air have been studied and reported by the authors (9). In that work both the mechanical and the dielectric properties of glass and silica microballoons based syntactic foams, were investigated. The objectives of the study (9) were to characterize these unique composites for applications such as encapsulation of electronic de- vices and radomes.

In the present publication, the study has been extended to include hollow carbon microspheres as the reinforcing material in the three-phase syntactic foam. Furthermore, the combination of silica and carbon microballoons has been investigated, with the objective of characterizing the low density com- posite mechanical, electric, and dielectric proper- ties for applications concerning shielding and ab- sorption of electromagnetic radiation.

EXPERIMENTAL

The three-phase syntactic foam composites were prepared by using a close-packed structure of the microballoons and adding predetermined amounts of the matrix material. In this way the microballoons always occupied a constant volume fraction, while the remaining constant free space was available for filling with various amounts of resin. The resins chosen were in the form of a fine solid powder. In this study Kerimid 601 (by Rhone Poulenc) a poly- imide resin, and Scotchcast 265 (by 3M), an epoxy resin, were used as the binder material.

The microballoons used were carbon (by Versar)

100 POLYMER COMPOSITES, APRIL, 1985, Vol. 6, No. 2

Three-Phase Carbon ivficroballoon Syntactic Foam Composites

:ompres- sive

strength Kg/cm2

and silica (by Emerson and Cuming). The hollow carbon microspheres have a wide size distribution, 5 to 150 pm, while the silica spheres are in the range of 40 to 175 pm. The true density of the carbon filler is 0.25 g/cm3 while that of the silica filler is 0.27 g/cm3.

The method of preparation of these three-phase composites is given elsewhere (9). Mechanical test- ing was carried out using an Instron machine. Meth- ods for measuring specific resistivities and resistiv- ity-temperature dependencies were described else- where (1 0). The dielectric properties were deter- mined by means of a waveguide type set-up at a frequency of 9 GHz.

RESULTS AND DISCUSSION Two syntactic foam composite systems were stud-

ied: a close-packed structure of carbon microbal- loons, and a close-packed structure of blends of carbon and silica microballoons.

Carbon Microballoons Figure 1 shows an optical micrograph of a dry

blend of carbon microballoons and the epoxy pow- der-resin, prior to molding. The resin adheres to the spheres’ surface and is homogeneously distrib- uted.

During the course of the experimental study the resin-filler ratio was varied, as can be seen in Table 1 , while keeping the filler volume fraction constant at the 0.6 level to yield three-phase close-packed syntactic foams. Table 1 and Fig. 2 describe the physical and mechanical properties of the resulting three-phase syntactic foams. As expected, the com- pressive strength of the foams is high compared to their respective tensile strength. No major differ- ences were observed between the epoxy and the polyimide based foams. Practically, the experimen- tal results conform to a common curve, as was the case in the previous study for the glass and silica three-phase foams (9).

Since carbon microspheres have relatively high electric conductivity, the electrical properties of the respective composites have been studied by

~~

Compres- sive

modulus (kg/cm2)

Fig. 1 . Electron micrograph of a carbon microballoonlepoxy pow- der mixture prior to molding (50 percent w/w resin).

Table 1. Mechanical Properties of Carbon Microballoon Syntactic Foams (0.6 v/v Microballoons)

Resin

SC-265 SC-265 SC-265 32-265 SC-265

Kerimid 601 Kerimid 601 Kerimid 601 Kerimid 601 Kerimid 601

folume frac- tion resin

0.05 0.10 0.20 0.30 0.40 0.35 0.10 0.20 0.30 0.40

-

-

rolume frac- tion air

0.35 0.30 0.20 0.10

0.35 0.30 0.20 0.10

-

-

- -

Den- sity

g/cm3

0.277 0.335 0.452 0.568 0.684 0.285 0.350 0.480 0.61 3 0.740 -

10760 12740 13870 3340 5360

261 11180 444 13835 660 19840

Fig. 2. Strength of carbon microballoon (0.6 v / v ) syntactic foums as function of density: A-compressive strength of carhonlpoly- imide foams, 0-compressive strength of carbonlepoxy foams, 0- tensile strength of carbonlepoxy foams.

varying both resin-filler ratio in the close-packed composites and temperature.

Figure 3 shows the resistivity change with tem- perature and resin content. A small negative tem- perature coefficient is found for the three resin concentrations in the close-packed carbon micro- balloon foams. Upon increasing the resin content (recall that the volume fraction of microballoons is constant-0.6) the resistivity increases, apparently due to the gradual reduction of contacts between the conductive microballoons. The gradual thermal expansion of the system during heating, which may increase the interparticle gaps with temperature and thus should increase the resistivity with tem- perature (positive temperature coefficient), is def- initely not the dominant mechanism for the systems in question where a negative temperature coeffi- cient is found. The latter can be explained as a net result of the other two opposing factors, namely the negative temperature coefficients characterizing the resin and the carbon microballoon themselves. The 0.6 volume fraction of the conducting particles is well above the critical percolation concentration (0.25 v/v) needed to convert continuous insulating

POLYMER COMposIlES, APRIL, 1985, V d . 6, No. 2 101

S. Kenig, 1. Raiter, and M . Narkis

XK) m 11 " 1 ' ' ' 1 ' 1 ' 1 J TEMPERATURE, OC

Fig. 3. Dependence of resistivity of close-packed (0.6 v / v carbon inicroballoons) polyimide jiiarns as function of temperature for three resin contents.

matrices into conductive systems. Thus these close- packed systems are always conductive at a conduc- tivity level similar to conductive solid polymer/ carbon black mixtures (1 1). The critical percolation concentration found for conductive carbon blacks is roughly 0.05 v/v, i.e., much less than 0.25 v/v for spherical particles. The reason for this differ- ence lies in the aggregate structure of conductive carbon blacks leading to the formation of filamen- tary conducting networks requiring significantly lower concentrations to reach the critical percola- tion concentration. The typical threshold percola- tion concentration for the insulator-conductor tran- sition in the systems under investigation can be found experimentally by replacing some of the car- bon spheres with nonconducting spheres like glass microballoons. At a certain carbon/glass microbal- loons ratio, the threshold percolation concentration (the conducting close-packed three-phase foam) will lose its conductivity.

Carbon-Glass Microballoon Blends The significant difference in electrical properties

between the carbon and glass microballoons can be utilized to tailor and control the composite electri- cal properties by proper blending. The principle of a close-packed structure was followed, as was the case in the previous section. Since the true density of the carbon and glass microballoons is similar (-0.25 g/cm') the various carbon-glass composi- tions occupied about the same volume fraction (0.6) in the composite, regardless of composition. Figure 4 shows an optical micrograph of a dry blend of carbon and glass microballoons, while Fig. 5 depicts the same blend with polyimide powder prior to molding.

Mechanical properties of three-phase syntactic foams consisting of carbon and glass microballoon blends and polyimide binder are given in Fig. 6. In this case the microballoon-resin weight ratio was always unity, corresponding to 0.12 volume frac- tion of the resin. Both the compressive strength and modulus slightly increase with a decrease in the carbon/glass ratio. Furthermore, TubEe 2 indicates that the compressive strain to failure (first visual observation of a crack) is reduced from 16 to 5 percent when increasing the carbon microsphere content from 15 to 100 percent. This may be attrib-

102

Fig 4 Optical (a ) and electron (11) micrographs of a 30170 glass/ cor1)oti microballoons dy-blend

Fig 5 Electron micrograpli of a 30170 glasslcarbon microhal- loons rly-blend mixed with 50 percent w/w polyimide powder prior to molding

uted to the fracture mechanism taking place in the three-phase composite. Photographs of fractured specimens composed of four different compositions can be seen in Fig. 7 . The carbon-glass ratio was

POLYMER COMPOSITES, APRIL, 1985, Vol. 6, No. 2

Three-Phase Carbon Microballoon Syntactic Foam Composites

varied from a pure carbon based foam to a glass rich foam. As can be noticed, when the carbon filler level is higher the fracture surface is perpendicular to the compressive stress direction. However, a Y shaped fracture is obtained in the case where the glass content is high, indicating a shear initiation mechanism.

As discussed by DeRuntz (1 2), failure by uniaxial compressive stress depends on the microsphere size and stress distribution, on the packing density, and on the strength and elastic properties of the resin. Thus, early crushing of the microspheres may be the predominant factor in one foam composition, while fracture of the resin may be important in a second composition. In the former case layers of crushed microspheres appear, as is the case in car- bon rich foams. In the latter case longitudinal cracks appear as a result of tensile microstresses acting perpendicular to the compressive load. This type

H 1 , , , , , , , , , J w L

olloo w50 low0 GLASS1 CARBON RATIO

Fig. 6. Compressive properties of close-packed (0.6 v /v carbon and glass microballoons) polyimide (0.12 v/u resin) syntactic foams as function of the carbonlglass ratio.

Table 2. Mechanical Properties of Carbon-Silica Microballoon Foams*

Filler Ratio Compressive Compressive Compressive Strength Modulus strainb

C SI kg/cm2 kglcm' 047

100 0 66 3730 5 60 40 69 4360 8 50 50 73 4920 9 40 60 67 4000 10 35 65 72 4260 1 1 30 70 73 4280 11 25 75 81 4770 13 15 85 72 4930 16

' 5 0 percenl w/w Kerimid 801 (0.12 v/v), 50 percent w/w Microballoonr, Vdume f r a f t i of micmballoons-0.6.

First visual obsenratii of a crack.

of fracture is evident, to a certain degree, in the glass rich syntactic foam. Price and Nelson (2) re- ported that in the case of a three-phase Pyrrone carbon foam of 0.347 density, some sheared in- duced failure at 17" to the cross-section phase took place.

The electric and dielectric properties of these carbon-glass blend composites have been found to possess some interesting features. Figure 8 corre- lates the specific resistivity of the foam with its carbon/glass compositional ratio. It should be em- phasized that as in the previous cases the resin content was kept at 0.12 volume fraction (50 per- cent by weight) and the microballoon bed consisted of a close-packed structure of 0.6 volume fraction. The volume resistivity shows a distinct transition at the 30/70 ratio of carbon/glass composition. This value is similar to the theoretical critical percolation concentration of spherical conductive particles.

The dielectric properties of the carbon/glass composites are given in Fig. 9. As can be seen, the

t I I I t t I

501 50 100/0 CARBON /GLASS RATIO

Fig. 8. Resistivity of close-packed (0.6 vjv carbon and glass mi- croballoons) polyimide (0.12 v/v resin) syntactic foams as function of the carbonlglass ratio.

Fig. 7. Fracture mechanisms in compression of close-packed (0.6 vIv carbon and glass microballoons) polyimide (0.12 V / O resin) syntactic foams as function of the carbon/glass ratio. ( 1 ) -15185 ( 2 ) -30170 (3) -Sol50 (4) -1OO/O

POLYMER COMPOSITES, APRIL, 7985, Vol. 6, No. 2 103

S. Kenig, 1. Raiter, and M. Narkis

I I J 0.1 0.2 0 3 04 0.5 0.6

VOLUME FRACTION OF CARBON FILLER

Fig 9 Dielectric constant and loss tangent at 9 GHz of close- packed (0 6 v/v carbon and glass microballoons) polyimide (0.12 v/v resin) syntactic foams as function of the carbon volume fraction: U-dielectric constant, 0-loss tangent. .-Dielectric con- stant of polyethylenelMT carbon black compounds and A-dielec- tric constant of polyethylenel1SAF carbon black compounds as jimction of volume fraction of the carbon black, at a frequency of I K H z .

dielectric properties are controlled by varying the foam composition. Both the dielectric constant and the loss tangent, tan 6, increase with increasing the content of carbon microballoons in the blend. For comparison, the dielectric constant of MT and ISAF type carbon-black filled polyethylene compounds are also given (13). The particle size and nitrogen surface area of MT and ISAF blacks are 5000A, 220A and 6m2/g, 120 m2/g, respectively. The crit- ical percolation concentrations of MT and ISAF blacks are approximately 20 and 8 percent v/v compared to -30 percent v/v for the carbon micro- balloons in the present syntactic foams. The initial dielectric constant values for the carbon free ma- terials are 2.3 (representing polyethylene) and 1.5 (representing a close-packed bed of glass microbal-

loons and air). Polyethylene/aluminum powder compounds (not reported here) have shown values of dielectric/constant similar to the values found for the carbon microballoon syntactic foams de- picted in Fig. 9. These polyethylene/aluminum compounds have also shown a critical percolation concentration of about 30 percent v/v. It should be noted that the density of the carbon microballoon syntactic foams is only a small fraction of the other compounds given in Fig. 9.

In conclusion, three-phase syntactic carbon com- posites having a close-packed structure possess good compressive properties. Their resistivity var- ies with resin content and decreases with tempera- ture. Three-phase syntactic carbon/glass compos- ites have good mechanical properties and their electric and dielectric properties can be controlled by varying the carbon/glass ratio. The combination of good mechanical and thermal properties, and controlled electrical and dielectric properties may be used in applications involving attenuation of electromagnetic radiation.

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6. R. J. McWhirter, G . I. Crow, R. A Daniel, and H. M. McIlory,

7 . B. G. Kimmel, NASA CR-2222 (1973). 8. Y. Amagi, Y. Nishimura, and S. Gomi, Soc. Aerospace Nat.

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1 1 . M. Narkis and A. Vaxman, J. Appl. Polym. Sci., 29, 1639

12. J. A. DeRuntz Jr., J. Appl. Mech, ASME, Sept. 1969, p. 551

13. M. Narkis, A. Ram, and F. Flashner, Polym. Eng. Sci., 18,

(19i7).

257-263 (1969).

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