[advances in polymer science] chromatography/foams/copolymers volume 73/74 || syntactic polymer...

61
Syntactic Polymer Foams F. A. Shutov Mendeleev Institute of Chemistry and Technology, Dept. of Polymer Processing, Moscow-125820, USSR The review deals with a special kind of gas-filled polymeric materials: the syntactic polymer foams, or spheroplastics, consisting of a polymer matrix and a hollow spherical filler. The survey covers the following aspects: preparation and properties of polymeric hollow micro- and macrospheres ; physical principles of syntactic foam formation: rheology, regulation of apparent density, spacefactor and packing of syntactic compositions; chemical and technological principles of formation; preparation of syntactic foams based on epoxy, polyester, phenol, polyimide, and other resins, as well as the newest types of foams -- prepregs, poly-phase structures, elastomeric and reinforced foams. The specificity" of physical properties andmethods of calculation of strength parameters are discussed. Attempts are made to evaluate the effect and contribution of size and packing of spheres on the principal physical properties. The main applications and trends of syntactic foams are outlined. Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Physieaehemicai Symbols . . . . . . . . . . . . . . . . . . . . . . . 65 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 2 Formation and Properties of Hollow Sphere Fillers . . . . . . . . . . . . 67 2.1 General Principles . . . . . . . . . . . . . . . . . . . . . . . 67 2.2 Glass Microspheres . . . . . . . . . . . . . . . . . . . . . . . 68 2.3 Phenolic Microspheres . . . . . . . . . . . . . . . . . . . . . 69 2.40ligo(ester acrylate) Microspheres . . . . . . . . . . . . . . . . 72 2.5 Carbonized Microspheres . . . . . . . . . . . . . . . . . . . . 73 2.6 Miscellaneous Other Microspheres . . . . . . . . . . . . . . . . 74 2.7 Macrospheres . . . . . . . . . . . . . . . . . . . . . . . . . 75 3 Physical Principles of Syntactic Foam Formation . . . . . . . . . . . . 76 3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 3.2 Rheology of Syntactic Mixtures . . . . . . . . . . . . . . . . . 71~ 3.3 Casting and Molding Compositions . . . . . . . . . . . . . . . . 77 3.4 Regulation of the Apparent Density . . . . . . . . . . . . . . . 78 3.5 Microsphere Space Factor . . . . . . . . . . . . . . . . . . . . 79 3.6 Two-Phase and Three-Phase Packings . . . . . . . . . . . . . . . 81 4 Chemical Principles of Syntactic Foam Formation . . . . . . . . . . . . 83 4.1 Epoxy Syntactic Foams . . . . . . . . . . . . . . . . . . . . . 83 4.2 Polyester Syntactic Foams . . . . . . . . . . . . . . . . . . . . 85

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Page 1: [Advances in Polymer Science] Chromatography/Foams/Copolymers Volume 73/74 || Syntactic polymer foams

Syntactic Polymer Foams

F. A. Shutov Mendeleev Inst i tute o f Chemistry and Technology, Dept. o f Polymer Processing, Moscow-125820, USSR

The review deals with a special kind of gas-filled polymeric materials: the syntactic polymer foams, or spheroplastics, consisting of a polymer matrix and a hollow spherical filler. The survey covers the following aspects: preparation and properties of polymeric hollow micro- and macrospheres ; physical principles of syntactic foam formation: rheology, regulation of apparent density, space factor and packing of syntactic compositions; chemical and technological principles of formation; preparation of syntactic foams based on epoxy, polyester, phenol, polyimide, and other resins, as well as the newest types of foams -- prepregs, poly-phase structures, elastomeric and reinforced foams. The specificity" of physical properties andmethods of calculation of strength parameters are discussed. Attempts are made to evaluate the effect and contribution of size and packing of spheres on the principal physical properties. The main applications and trends of syntactic foams are outlined.

Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Physieaehemicai Symbols . . . . . . . . . . . . . . . . . . . . . . . 65

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

2 Formation and Properties of Hollow Sphere Fillers . . . . . . . . . . . . 67 2.1 Genera l Principles . . . . . . . . . . . . . . . . . . . . . . . 67 2.2 Glass Microspheres . . . . . . . . . . . . . . . . . . . . . . . 68 2.3 Phenolic Microspheres . . . . . . . . . . . . . . . . . . . . . 69 2 . 4 0 l i g o ( e s t e r acrylate) Microspheres . . . . . . . . . . . . . . . . 72 2.5 Carbonized Microspheres . . . . . . . . . . . . . . . . . . . . 73 2.6 Miscellaneous Other Microspheres . . . . . . . . . . . . . . . . 74 2.7 Macrospheres . . . . . . . . . . . . . . . . . . . . . . . . . 75

3 Physical Principles of Syntactic Foam Format ion . . . . . . . . . . . . 76 3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 3.2 Rheology o f Syntactic Mixtures . . . . . . . . . . . . . . . . . 71~ 3.3 Casting and Mold ing Composi t ions . . . . . . . . . . . . . . . . 77 3.4 Regulat ion o f the Appa ren t Densi ty . . . . . . . . . . . . . . . 78 3.5 Microsphere Space Fac to r . . . . . . . . . . . . . . . . . . . . 79 3.6 Two-Phase and Three-Phase Packings . . . . . . . . . . . . . . . 81

4 Chemical Principles of Syntactic Foam Formation . . . . . . . . . . . . 83 4.1 Epoxy Syntactic Foams . . . . . . . . . . . . . . . . . . . . . 83 4.2 Polyester Syntactic F o a m s . . . . . . . . . . . . . . . . . . . . 85

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64 F.A. Shutov

4.3 Phenolic Syntactic Foams . . . . . . . . . . . . . . . . . . . . 85 4.4 Organosilicone Syntactic Foams . . . . . . . . . . . . . . . . . 86 4.5 Polyimide Syntactic Foams . . . . . . . . . . . . . . . . . . . 86 4.6 Carbonized Syntactic Foams . . . . . . . . . . . . . . . . . . . 86 4.7 Syntactic Foams based on Unconvent ional Binders . . . . . . . . . 88 4.8 Syntactic Prepregs . . . . . . . . . . . . . . . . . . . . . . . 89 4.9 Elastomeric Syntactic Foams . . . . . . . . . . . . . . . . . . 89 4.10 Three-Phase and Four-Phase Syntactic Foams . . . . . . . . . . . 89 4.11 Reinforced Syntactic Foams . . . . . . . . . . . . . . . . . . . 92

5 Physical Properties of Syntactic Foams . . . . . . . . . . . . . . . . . 92 5.1 Strength Properties . . . . . . . . . . . . . . . . . . . . . . . 92

5.1.1 Gener~il . . . . . . . . . . . . . . . . . . . . . . . . . 92 5.1.2 Effect of Microspheres . . . . . . . . . . . . . . . . . . . 94 5.1.3 Effect of Binder . . . . . . . . . . . . . . . . . . . . . . 95 5.1.4 Strengthening Effects . . . . . . . . . . . . . . . . . . . 95

5.2 Water Absorpt ion and Resistance to Hydrostatic Pressure . . . . . . 97 5.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.2.2 The Binder-Filler Interface Problem . . . . . . . . . . . . . 99 5.2.3 Water Penetration Mechanisms . . . . . . . . . . . . . . . 100 5.2.4 Effect of Hydrostatic Pressure on Water Absorption . . . . . . 103

5.3 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . 104 5.4 Dielectric Properties . . . . . . . . . . . . . . . . . . . . . . 106 5.5 Other Properties . . . . . . . . . . . . . . . . . . . . . . . . 108

6 Calculation of Strength Parameters . . . . . . . . . . . . . . . . . . 109 6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.2 Macrostructural Models . . . . . . . . . . . . . . . . . . . . 110 6.3 Microstructural Models . . . . . . . . . . . . . . . . . . . . . 112 6.4 Strength Calculation . . . . . . . . . . . . . . . . . . . . . . 113 6.5 Elastic Modulus Calculation . . . . . . . . . . . . . . . . . . 115

7 Main Applications . . . . . . . . . . . . . . . . . . . . . . . . . 117

8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

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Syntactic Polymer Foams 65

Abbreviations

ABS BJO BMC BV DMC DNPMTA EDM EDS EK EMS ENS FREP FTD HTF OEA PhFO PMMA PPU RIM RRIM SMC SPAB SPB SPM SPS TrPSF

Acrylonitrile-butadiene-styrene copolymer USA grade phenolic microspheres Bulk molding compound USSR grade phenolic microspheres Dough molding compound Dinitrosopentamethylenetetramine USSR grade syntactic foam from epoxy resin and phenol microspheres USSR grade syntactic foam from epoxy resin and glass microspheres USSR grade epoxy resin USSR grade syntactic foam based on epoxy resin USSR grade syntactic foam based on epoxy resin Fabricated reinforced epoxy tooling USA grade glass microspheres USA grade syntactic foam from polyimide resin Oligo(ester acrylate) Phenol-formaldehyde oligomer Poly(methyl methacrylate) USSR grade polyurethane foam Reaction injection molding process Reinforced reaction injection molding process Sheet molding compound USSR grade syntactic foam based on OEA USSR grade syntactic foam based on polyester resin USSR grade syntactic foam based on polyester resin USSR grade syntactic foam based on polyester resin Thermosetting three-phase syntactic foam

Physicochemical Symbols

a inner radius of a hollow sphere A coefficient b outer radius of a hollow sphere B coefficient C concentration, content D packing factor, diffusion coefficient, diameter E elastic modulus Ef elastic modulus of filler Es elastic modulus of syntactic foam Eo elastic modulus of matrix G shear modulus Grt shear modulus of a hollow sphere Gs shear modulus of a solid sphere Go shear modulus of matrix

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66 F . A . Shutov

h H J K Kr Ks Ko L P P~ol pr

eo P~ r

S T V WT W~

%

Y % Yt Yo tan E E;'

Ob

80 k v

vo O'cy I

~ f Cs

~ t (So

Zi

half-thickness of a specimen long-term elastic modulus long-term shear modulus bulk elastic modulus bulk elastic modulus of filler space factor bulk elastic modulus of matrix length of specimen molding pressure, external pressure collapsing hydrostatic pressure relative hydrostatic pressure pressure needed to get Ks hydrostatic long-term pressure radius surface area temperature volume water absorption in time eqhilibrium water absorption coefficient of thermal expansion for filler coefficient of thermal expansion for matrix parameter of cellular structure apparent density density of binder theoretical apparent density apparent density of matrix dielectric losses compressive deformation dielectric constant binder volume fraction filler volume fraction maximum filler volume fraction true volume fraction of microspheres packed volume fraction of spheres thermal conductivity Poisson's ratio Poisson's ratio for matrix cylindrical strength of filler flexural strength compressive strength shear strength tensile strength compressive strength of matrix current time relaxation time dimensionless numbers

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Syntactic Polymer Foams 67

1 Introduction

Syntactic foamed plastics (from the Greek aovz~t~, to put together) or spheroplastics are a special kind of gas filled polymeric material. They consist of a polymer matrix, called the binder, and a filler of hollow spherical particles, called microspheres, micro- capsules, or microballoons, distributed within the binder. Expoxy and phenolic resins, polyesters, silicones, polyurethanes, and several other polymers and oligomers are used as binders, while the fillers have been made of glass, carbon, metal, ceramics, polymers, and resins. The foamed plastic is formed by the microcapsular method, i.e. the gas-filled particles are inserted into the polymer binder ~' 2).

Syntactic foamed materials are classified as foamed plastics because they are for- mally 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~

On the other hand, syntactic materials may also be thought of as reinforced or filled plastics, with the gas-containing particles being the reinforcing component. This classification is also justifiable in view of the manufacturing technology. The matrix is not foamed chemically, but is filled mechanically with the hollow spheres. Syntactic foamed plastics are thus called "physical" foams 6.7).

The cellular structure of the foam depends on the size, quantity, and distributive uniformity of the filler. Since the microspheres have continuous shells, the final material will, as a rule, have completely enclosed cells, and thus they can be called absolute foamed plastics or "absolute" closed-cell foams (by analogy with "absolute" open-cell foamed plastics, i.e. reticulated foams) s~. This, together with the absence of microstructural anisotropy (because the microspheres have practically all the same size and are uniformly distributed in the matrix), gives a syntactic plastic its valuable properties. They have better strength-to-weight ratios than conventional "chemically" foamed plastics, absorb water poorly and can withstand considerable hydrostatic pressures. Using a hollow filler means that the final material is lighter than one containing a compact filler, such as glass powder, talc, kaolin, quartz meal, or asbestos. Moreover hollow fillers increase the yield, and the final material has lower residual stress 9~.

The properties of a syntactic material can be varied quite significantly by changing the filler type, the binder-filler ratio, and the manufacturing and curing techniques. Syntactic plastics are heavier than conventional foams, with apparent densities of 200-800 kg/m 3.

2 Formation and Properties of Hollow Sphere Fillers

2.1 General Principles

The filler microspheres may be glass, polymeric, carbon, ceramic, or metallic. However, the main requirements are that the microspheres are spherical, non-cohesive, strong, intact, moisture and chemically resistant, and hydrolytically stable. They should be

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68 F.A. Shutov

available in a variety of granulometric compositions and with a variety of space (packaging) factors (see Sect. 3.6).

The main technological advantage of microspheres is that the viscosity of systems with spherical fillers is always less than that of a system with fillers of any other shape, because a sphere has the smallest surface. Moreover the isotropic materials with the best strength properties are those with spherical gas inclusions lo).

Microspheres are 1-500 lam in diameter, have a wall thickness of 1~4 ~tm, bulk densities of 70-500 kg/m a, and apparent densities of 50-250 kg/m 3. Hollow macro- spheres (1-100 mm in diameter) are also used as fillers for syntactic plastics.

It is interesting to note that microspheres were originally developed as a means of creating a floating layer to decrease the evaporation (by 90 %) of oil and petroleum in tankers and large containers, and to increase the buoyancy of ships and subma- rines t~. They are still used for this purpose, but most microspheres are now used in the manufacture of syntactic foamed materials.

2.2 Glass Microspheres

Glass microspheres are widely used because of the strength of the glass and the substantial difference between the elasticities of glass and polymer ~2-14~. Several types of glass microspheres are produced in the Soviet Union in the form of non- cohesive white powders with particle sizes of 10-150 I~m and densities of 200 to 400 kg/m 3.

The process technology and raw material sources for making glass microspheres are well established in many countries ~s), and hence glass microspheres are cheaper than polymeric ones.

Glass microspheres are manufactured in vertical tube furnaces heated by gas, e.g. a propane-butane mixture. A finely dispersed powder containing glass and a porofore is sprayed into the bottom of the tower. The porofore (i.e. chemical blowing agent) evolves gas at the melting point of the glass and inflates the partially fused monolithic particles. The microspheres thus formed are carried by the hot gas to the top of the tower where they are cooled. They are then treated with acid to improve their chemical resistance and raise their softening temperature, and finally washed with water to remove defective microspheres ~' 1~.

Sodium and borosilicate glasses are generally used, but some low-cost "Sirasu" brand microspheres have started to be manufactured in Japan using volcanic glass and ashes 16, ~7~. The properties of some glass microspheres manufactured in USA are given in Table 1 18~

Glass macrospheres, 0.5-10 mm in diameter, are manufactured in a rotating ho- rizontal apparatus with mixing arms. A binder together with a powder filler (ground glass fibre) is applied to prefoamed polystyrene granules, then the binder is hardened into the macrosphere's shell and the granules are melted at high temperature ~)

The most important factor determining the mechanical properties of a syntactic foam is the distribution of its microspheres with respect to size, shape, and strength, the best properties being attained if these distributions are homogeneous 19). Micro- spheres with the same shape and size can be obtained by sieving or flotation. Their mechanical strengths still differ considerably, even after sorting, due to wall thickness

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Syntactic Polymer Foams

Table 1. Properties of US-manufactured glass microspheres ls~

69

Parameter Eccospheres R Eccospheres SI Eccospheres VT Spheres IG-101

Bulk density, kg/m a 220 180 180 190-240

Apparent density kg/m 3 420 280 280 340

Diameter ~trn 30-300 30-125 30-125 10-250

Wall thickness I~m 2 2 2 2

Type of glass borosilicate SiO2 (95 ~) SiO2 SiO2

differences and surface defects. This problem can be overcome by subjecting the microspheres to high hydrostatic pressures thereby destroying the weaker spheres. Wakayama and Hayashida 16) have shown that this treatment also raises the apparent density of the microspheres and changes their size distribution (increasing the relative number of smaller spheres), and this may increase the strength of syntactic foams made from epoxy resins and these glass microspheres by 40 %.

The Philadelphia Quartz Company in USA produces "Q-CeI" quartz micro- spheres 2o,2~). These have an apparent density of 300 kg/m 3, a bulk density of 100 kg/m 3, and an average diameter of 75 ~tm. They are mechanically very strong and are very cheap (half the price of glass and one third of the price of phenolic micro- spheres).

2.3 Phenolic Microspheres

Microspheres made from polymers or reactive oligomers (RO's) are manufactured by thermally treating sprayed solutions or emulsions. A solution of any film-forming polymer can be used. When an RO is used the solvent has to be evaporated, the sprayed monolithic particles are then heated to expand the gas or vapor within the particle, and the final microspheres are hardened 22, 23),

Foams with polymeric microspheres have poor mechanical properties because the filler and binder have similar elasticities. Consequently the binder's strength is not reflected as deafly as it is when glass spheres are used.

Microspheres based on phenol formaldehyde oligomers (PhFO's) are brown non- cohesive powders with particle sizes of 10--300 ~tm, apparent densities of 100 to 150 kg/m a, and sinkages of less than 5--10 mass ~o ~1,24-26)

Phenolic microspheres are obtained by feeding the phenolic resin and the other components (e.g. surfactants, see below) 27) into a mixer, heating them to the required temperature, and then pumping them via a displacement pump into a disk sprayer. At the top of the spray chamber the condensation products are dispersed and heated

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70 F.A. Shutov

by the heat carrier, the combustion products of burning propane in the furnace. As the solvent is evaporated the particles expand and solidify. They are then separated from the heat carrier in a cyclone and discharged into a collector. The heat carrier flow is promoted by a ventilator.

Masumy and Tasomy 28~ found that for regularly shaped resol PhFO microspheres it was best to use dilute aqueous solutions with viscosities of 15-50 Pa - see, water contents of up to 50 %, and free phenol contents of 6--9 %. The resultant spheres included unexpanded monolithic particles, but these could be eliminated by using a chemical blowing agent such as dinitrosopentamethylenetetramine (DNPMTA) or azodiisobutyronitrile, or a surfactant (0.25 mass %).

Porofore concentration does not determine the yield of microspheres as much as the spraying temperature. In particular, the best choice of entry and chamber tempera- tures are 480 and 220 °C, respectively (Table 2) 28~

At present, phenolic microspheres based on grade B bakelite are produced commer- cially in the USSR 29) They are similar to those produced in USA ffable 3) 1~)

Phenolic microspheres are weaker than glass and they cannot withstand a hydro- static pressure greater than 2.5 MPa, whereas glass microspheres can stand pressures up to 12 MPa 30j.

Macrospheres made from resol phenolic resins, with diameters of 0.2-5.0 mm, are also used for syntactic foams 1, ~8)

Table 2, Process conditions and properties of phenolic microspheres

DNPMTA Temperature of heat Fraction of spherical Density of Concentration, carrier, °C particles mierospheres, % % kg/m 3

at entry inside point chamber

2.0 570 250 61.5 140 2.0 480 220 80.5 170 0.2 570 250 64.2 150 0.2 480 290 82.4 190

Note: DNPMTA is dinitrosopentamethylenetetramine, a chemical blowing agent

Table 3. Properties of phenolic microspheres

Parameter BV- 10 BJO-930 USSR USA

Bulk density, kg/m a 9%147 120-140 Apparent density, kg/m 3 207-264 250-270 Degree of hardening, % 96-99 95-97 Sinkage, % 1-4 1-3 Weighted average diameter, ~m 140-150 87

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Syntactic Polymer Foams 71

Little is known, at present, about the formation mechanism of hollow microspheres, and of the influence of the composition or process parameters on their quality and properties. It is extremely difficult to study the formation mechanism in a spray drier, as for this purpose individual droplets should be followed as they are treated.

In one of the few papers on the subject, Zhigalov et al. 31} looked at the behavior of dispersed resol phenol droplets. These were studied under the microscope as they were heated with air at 20(b400 °C for periods between 1-20 sec under conditions close to those in a spray drier. Gas bubbles were found to be present (Table 4~. Even before the thermal treatment, bubbles were observed, their number depending on the size of the drop. Droplets smaller than 20 l~m had no gas bubbles while those 80 ~tm or larger contained more than one (Table 5). Since there is no gas in the initial liquid resin, the gas phase must have formed during the spraying. Note that the solid particles obtained from industrial sprayers are less than 40 pm in size. Note too that droplets containing no gas bubbles are not swollen and remain monolithic even after the heat treatment 31). This remains the same even if a porofore (azodiisobutyronitrile) is added to the initial mix (10 ~o). Moreover, some of the droplets originally containing bubbles loose them during the treatment, and for droplets with only one initial bubble this results in a monolithic final particle.

Table 4. Percentages of inflated and polycellular microspheres obtained at different temperatures 3t}

Number of gas bubbles in resin droplets

Heat treatment temperature, °C

200 250 300 350

0 0 0 0 0 1 46.6; 0 58.7; 0 53.5; 0 70.9; 0 2 81.4; 4.5 100.0; 28.6 93.4; 35.8 100.0; 22.2 3 97.5; 35.0 100.0; 87.0 100.0; 60.4 100.0; 30.4

Note: The first number is the percentage of expanded particles and the second number is the percentage of expanded particles that are polycellular (i.e. contain gas bubbles)

Table 5. Percentage and nature of initial droplets of dispersed resol PhFO 3t~

Droplet Percentage of Droplets size, ~tm without with l gas bubbles with 2 gas bubbles with 3 gas bubbles

gas bubbles

0-20 35.2; 94.0 2.4; 6.0 0 0 20-40 17.2; 60.0 10.5; 37.0 0.9; 3.0 0 40-80 2.6; 9.9 18.2; 70.5 3.7; 14.3 1.3; 5.3 60-160 0 1.1 ; 14.3 2.2; 28.6 4.3; 57.1 over 160 0 0 0 0.4; 100.0

Note: The first number is the percentage of the total number of droplets in the class and the second number is the percentage of droplets in this class having the given number of bubbles

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72 F.A. Shutov

Thus the initial oligomer droplets should contain bubbles if hollow microspheres are to be obtained (although this is not a sufficient condition). The proportion of droplets with bubbles is increased by choosing the correct spraying conditions and resin viscosity and by introducing active centres to the original mix to promote bubble formation.

The main reason why the solid particles are mostly smaller than 40 ~tm is apparently that not enough small bubbles can penetrate into them and be retained during the heat treatment. It is thus important to investigate the ability of liquid resol phenol oligomers to incorporate and retain air bubbles as they are mixed and sprayed. It is clear from a consideration of the physical chemistry of foaming that this ability is closely related to the aggregative stability of the polymer foam 10, 32). The emulsifi- cation of a component and the formation, growth, and stability of gas bubbles are aided by surfactants 3a~. Until recently, almost nothing was known about the effect a surfactant would have on hollow microsphere formation.

Zhigalov et al. carried out an investigation of the topic for microspheres based on PhFO resol 34). The surface tension of the system was reduced as the concentration of surfactant (various kinds) was increased up to one ~o (mass), after which the surface tension remained constant. At room temperature the volume of air in a mixture, saturated using a paddle mixer, depended on the surfactant a4~. Very small bubbles are formed when the mixture is dispersed by a pneumatic sprayer, decreasing the probability of small solid particles. Surfactants increase the number of polycellular microspheres because they stabilize foams. The data in 31) show that organo-silicon surfactants can stabilize a system up to 24 hrs. However, other surfactants (Sintonom DS-3, Pronsanol-305) are preferable because they promote the incorporation of more gas bubbles into the particles and increase their coalescence rate (after 15-60 min most of the bubbles have coalesced).

Under the real conditions of high temperature spraying, the surface tension and the solubility of the surfactant (e.g. Proxanol 305) are changed and the droplets are not as stable. The number of microspheres is the same as it is for organo-silicon sur- factants, while there are fewer polycellular microspheres than were produced in the absence of a surfactant. Those surfactants that increase the gas content in the resin also increase the yield of hollow microspheres ~). The addition of a surfactant in- creases the yield of polycetlular particles in most cases, though their number decreases as the temperature increases.

The addition of a surfactant to liquid phenol resol oligomers affects the dispersity of the gas bubbles in the resin droplets and thus can increase the yield of hollow microspheres 33.34)

2.40ligo(ester acrylate) Microspheres

Spray methods for obtaining microspheres have a number of disadvantages. High temperatures and bulky and complicated equipment are needed; moreover they pose fire and explosion hazards. Low temperature methods, such as using emulsified thermoplast solutions, saturated polyester resins in liquid heat carriers, or suspension polymerization, are preferable 1, 3s)

The low temperature method of producing hollow microspheres from oligo(ester

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Syntactic Polymer Foams 73

acrylates) (OEA), first developed in the USSR by Balyerbin et al. 35, 36) is noteworthy. By using microspheres based on OEA's, which are obtained by suspension polymeriza- tion in the presence of a blowing agent, the assortment and properties of the final syntatic plastics can be enhanced. The microspheres are made by emulsifying OEA in aqueous solutions of cellulose ethers. The dispersity of the emulsion, and thus the size of the microspheres, can be altered by varying the ratio of the phases in the OEA/ether solution system (from 1 : 4 to 4:1), and by changing the rotational speed of the mixer, the mixing time, and the solution concentration. A partial or complete phase reversion occurs in the system when the emulsion concentration becomes >40 vol. ~ . By changing the initiator to blowing agent ratio, the OEA gelation can be varied from 30 sec to 12 min.

OEA microspheres are manufactured in the following stages: 1) the OEA, blowing agent and polymerization initiator are mixed, 2) the emulsion is prepared, 3) the emulsion is dispersed, 4) the emulsion is thermally treated, 5) droplets volume is in- creased by the action of the blowing agent, accompanied by the coalescence of the droplets into one bubble, 6) bubble growth and simultaneous shell thickness decrease, and 7) microsphere hardening.

Short heat treatments were found to result in a loss of blowing gas. It is probable that in this case gas evolution precedes polymerization and the formation of a film to keep the gas within the particle. By contrast, for longer treatment times polymeriza- tion precedes gas evolution and microspheres are formed. Data on the reaction kinetics during microspheres formation are given in 36).

The sizes of the droplets in the initial emulsion significantly affect the size and prop- erties of the microspheres (Table 6)36). As the mixing rate used to produce the emulsions is increased, the average size of the droplets decreases whereas the micro- sphere size increases. Very dispersed emulsions seem to be more likely to coalesce, thus yielding larger droplets and hence larger microspheres after the heat treatment. This method can produce spherical unicellular particles with diameters of 200-400 ~m, densities of 260-700 kg/m 3, and space factors of up to 59 ~ 35, 36).

Table 6. Size and density of OEA microspheres. Obtained from emulsions of different dispersities 36~

Mixing Size of emulsion droplets, Microsphere size, Microsphere density I~m lam (by air displacement)

kg/cm 3 Size Range Main Fraction Size Range Main Fraction

1 10-100 10-50 00) 210-460 290-380 (310) 483 2 10-90 10-50 (15) 310-560 350-460 (370) 500 3 10-60 10-20 (10) 420-530 450-500 (450) 360

Note: The main particle size is given in brackets

2.5 Carbonized Microspheres

Carbonized microspheres are a new type of hollow filler for syntactic plastics. They are very strong and bind well to the polymer matrix 1, 37).

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74

Table 7. Properties of kresosphere carbonized microspheres 39)

F. A. Shutov

Parameter Mierosphere Type

A-50 A-100 A - 2 0 0 A-300

Diameter, gm 45-75 75-150 150-250 250-420 Average diameter, gm 60 110 200 330 Wall thickness, lam 1-2 2-3 3-8 6-12 Bulk density, kg/m a 100-250 100-250 70-200 50-200 Apparent density, kg/m 3 150-400 150-400 150.350 100-300 Percentage of microspheres damaged by a hydrostatic pressure of 2.8 MPa 5-10 20.25 25-35 35-45

The "Carbosphere" brand of filler is manufactured in USA by carbonizing BJO phenolic microspheres at 900 °C in an inert atmosphere. They are 5-150 ~rn (average 40 gm) in diameter, have wall thicknesses of 1-4 gm and bulk densities of 130 to 140 kg/m 3 a8~ Four types of "Kresosphere" microspheres are produced in Japan. They are more than 95 ~ carbon and made by carbonizing microspheres made from wood resins (pitches) at 800-1100 °C in an inert medium (Table 7)39~

Shaver et al. 4°"'m have shown that a considerable number of the initial microspheres are defective due to pores being eroded in the cell walls by oxidation during carboniza- tion. The defective spheres are removed by sieving or by flotation fractionation, e.g. in acetone.

Carbonized microspheres, even with defects in their shells, are stronger than glass microspheres. At a gas (nitrogen) pressure of 7 MPa, 43 ~o of glass spheres, but only 5 ~ of carbon microspheres are damaged 39~. Production of carbonized microspheres based on glass 42~ and polymers 43~ has been started in the USSR.

2.6 Miscellaneous Other Microspheres

The expanding number of applications of syntactic foams and the increasingly stringent economic and technical requirements for their production have fueled the search for new types of rigid and elastic microspheres made from commercial reactive oligomers and polymers. As a rule, the microspheres are produced by spraying low viscosity solutions and melts. The method is used for spheres made from poly- urethane 1.44~, polyimide,5~, unsaturated polyester 35.46~ and epoxy 47~ oligomers, carbamide #s~ and melamine formaldehyde 1) oligomers, polyethylene, poly- propylene 49), PVC and copolymers of vinyl chloride 50~, poly(divinyl chloride) 51, s2j, acrylonitrile poly(divinyt chloride)1~, polyamide 1~, poly(methyl acrylate)14.53-55), polystyrene 53-56~, polyacrylate 53, ss), and polyaldehyde sT)

A recent achievement worthy of note is the manufacture of microspheres containing an inert gas, e.g. nitrogen, or a volatile liquid, such as the freons ~). The patent litera- ture contains methods for producing microspheres based on poly(vinyl chloride) and poly(divinyl chloride), containing isobutane or carbon tetrachloride 52~, and based on poly(methyl methacrylate), containing neopentane t~ Microspheres containing liquid dyes and oils are also used to make syntactic foams ss)

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Syntactic Polymer Foams 75

Quartz microspheres are now manufactured in many countries, e.g. the Q-cel microspheres. Metal, metal oxide, metal salts, and ceramics microspheres have also been used to make syntactic foams 1, 57-6o)

2.7 Macrospheres

Polymeric ,7,61), inorganic 1), and carbon 37) hollow macrospheres are used too. They have densities of 200-500 kg/m 3 and are larger than 1 ram. By using macrospheres and microspheres together the apparent density of a syntactic foam can be reduced, however its specific strength is also less than that of a foam made only with micro- spheres is, 62~

Soviet scientists have developed a method of producing macrospheres made from foamed polystyrene granules 63> A thin layer of binder is applied to the surface of the granules followed by the saturation of the layer with a powdered filler (to prevent the granules' coalescing), and the hardening of the layer accompanied by the fusion of the granules to produce the syntactic material. The EK-1, EK-2 and EK-3 epoxides, whose viscosities are, respectively, 0.43, 3.2 and 21.8 kg/s • m, were used as binders, while ground fiber glass (7 lam diameter and 70-500 ~tm length) or hollow glass micro- spheres (10-250 lam diameter and 320 kg/m a density) were used as the powdered filler. The polystyrene granules were 5-12 mm in size with apparent densities of 20 kg/m 3. The quality of foams made from macrospheres depends on the thickness of the binder layer, the binder viscosity, the mixing time for binder, granules, and filler, the binder: filler ratio, and the granulation of the macrospheres 64, 65~

It was shown 63) that the concentration of filler in the epoxy binder layer depends on the mixing time, the layer thickness, and the nature of the filler, given the same bin- der/filler ratio (2:1). The layer is filled most rapidly at the start of the mixing and is then gradually saturated with filler; the thinner the binder layer, the higher the con- centration of filler in it. As the layer thickness increases the macrosphere density increases at first and then decreases, as sections of the filled binder layer peel of the surfaces of the polystyrene granules. The volumetric concentration of filler in the binder depends on the filler type, being higher for microspheres than for ground fiber. The binder with the lowest viscosity (EK-I) was filled faster than those with higher viscosities 0~K-2 and EK-3). Layers peel off the granule surfaces for the same reason, being the fastest for EK-1. There is more peeling when glass fiber is the filler.

The process in which a thin layer is filled can be visualized in the light of these data. After the polystyrene granules have been mixed with the binder, an agglomerate of particles is formed, with a bonding strength dependending on the viscosity of the bind- er. The filler is added to the system which is then fed into a condenser. The agglomerates are broken down into separate particles by a pin-type mixer. They are separated because the particles cohere less well as a result of the filler sticking to their surfaces. About the same amount of filler sticks to the surface of the binder layer, irrespective of the thickness of the latter. Hence, as the thickness of the initial binder layer decreases, the binder/filler ratio, and thus the fraction of filler in the final layer, increases.

The density of the macrospheres, and thus the apparent density of the final syntactic material, can be controlled by varying the thickness of the binder layer and by altering the concentration of filler. The quality of the filled layer and the density of the macro-

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76 F.A. Shutov

spheres depend, however, mostly on the process conditions, i.e. the mixing time and the volume of charge in the mixer 1). Increasing the charge volume with a constant mixing time increases unit throughput, but also results in macrospheres with peeling coatings. It was found, however, that by decreasing the thickness of the binder layer and increasing the glass-fiber/binder ratio, spheres with intact coatings were obtained.

The most important factor affecting the apparent density and properties of the final materials is the unifomity of the macrospheres with respect to density and granula- tion 65)

Given the same bulk density and binder/filler ratio, an increase in the space factor of the polystyrene granules from 52 to 62 vol % decreased the macrosphere density from 270 to 220 kg/m a 6a). In order to obtain macrospheres with the same density, their volumes should be the same, even though they may have different surface areas.

The materials studied in 63.6s) can be regarded as "binary" or three-phase syntactic foams because they contain two types of filler, namely macrospheres and microspheres in 63), and macrospheres and glass fiber in 65~. The latter may be called reinforced syntactic foams and have also been referred to as foamed 9lass-reinforcedspheroplasts 66, 67) (see Sect. 4.11).

3 Physical Principles of Syntactic Foam Formation

3.1 General

The binders used to make syntactic foams must satisfy the following specifications. They must have low viscosities, easily controlled gelation times, small exothermal effects during curing, small curing shrinkages, and good adhesions and wettabilities to the filler, and they must be compatible with modi,qers and fillers such as diluents, plasticizers, dyes, and antipyrogenes.

Syntactic plastics are processed using free pouring, molding, casting, or extrusion, depending on the binder and filler fraction. A typical manufacturing process consists of the mixing of the microspheres and the binder, filling of the mold or structure, with the mixture, and curing.

The main difficulty in manufacturing a syntactic loam lies in the choice of the process parameters for mixing the components (temperature, duration, and addition sequence). When the mixture leaves the mixer, its viscosity must be low enough for the mold to be filled rapidly, although once there, the viscosity should rise rapidly to prevent the mixture from becoming laminated. If possible, it is best to mix and pour the materials under vacuum so as to prevent the formation of pores and cavities in the finished material.

3.2 Rheology of Syntactic Mixtures

Irrespective of the method used, the quality of a syntactic foam depends substantially on the rheological proper'ties of the initial mixture with the microspheres 68-7°). The investigation of the rheology of syntactic compositions by Petrilaenkova et al. 71)

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Syntactic Polymer Foams 77

considerably simplifies the choice of the best process conditions. If the microsphere concentration exceeds 5 mass ~o, the mixture becomes thixotropic O.e. the shear stress increases as the viscosity decreases); and the larger the microsphere concentration, the greater the viscosity decrease. The mixers must thus provide rates of shear that will maximize mixture liquefaction. For unsaturated oligomer binders containing 45 to 50 volvo microspheres, the rate of shear must be 300--350 Pa. To obtain good quality materials, the gelation time must be shorter than the lamination time.

The binder must be able to withstand the heat applied during the mixing to produce a homogeneous mixture.

Telegina et al. 72) showed that the activation energy for the viscous flow of a poly- ester oligomer filled with glass microspheres is 46.9 kJ/mol, while that of an epoxy oligomer is 78.3 kJ/mol. They also established the important fact that the addition of microspheres to an oligomer composition does not change the temperature viscosity coefficient. This means that the viscosity of a mixture with microspheres can be controlled, if the temperature dependence of the viscosity of the binder is known.

3.3 Casting and Molding Compositions

The consistency (fluidity) of an initial mixture depends on the binder:filler weight ratio, all other parameters (binder viscosity, microsphere type, shape, size, density, and mixing conditions) being equal. The mixtures are casting compositions (viscous fluids) at small microsphere concentrations, while they become molding compositions (pastes) at higher concentrations. Thus, the fluidity of a syntactic composition depends primarily on the filler concentration and not on the binder viscosity (Fig. 1) 73. 74)

Semi-finished articles, cast or molded, can be obtained in either of two ways. The first method is to mix the components under vacuum to prevent air inclusion. The mixture can be produced either batchwise or continuously 1). The mixture is then poured into a mold (preferably under vacuum); as it solidifies external heating may or may not be needed. This method gives a high throughput and is used for large castings. The second method consists in charging the filler together with the binder

I I A I _ I I~

j t I I I I

#0 67 ?OO c(% ~oU

Fig. 1. Qualitative relationship between the ap- parent density (~/) of a syntactic foam and the filler concentration (C) A: casting compositions; B : molding compositions 39~

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78 F.A. Shutov

into an airtight mold, the vacuum forcing the binder in between the particles. The composition is then cured.

The most important thing is to get the microspheres evenly distributed in the poly- mer matrix. It is therfore recommended to vibrate fluid compositions in the mold, especially in the case of large articles 2). Since molding compositions do not easily fill the molds, tamping and low pressures (0.5-2 MPa) are used for small, simple objects, while the molds for large or complicated objects are filled layer by layer 1). The molds are made from plastic, reinforced with metal, wood or fibre; their surfaces are degreased or covered with a separation layer.

3.4 Regulat ion o f the Apparent Dens i ty

As mentioned above, the binder/filler ratio determines the fluidity and hence the process technology for a given syntactic composition. The solid line in Fig. 1 shows how the apparent density depends on the relative microsphere concentration in syn- tactic compounds, provided there are no air inclusions. The lower limit of apparent density is at the concentration at which the filler is most densily packed; at this point the material has its highest specific strength (strength per unit apparent density). Experience has shown that the closest packing of the spheres within the binder is obtained when the material is mixed and cast or molded under vacuum 8-10).

Lower apparent densities can be obtained by allowing air intrusions into the binder (dotted line in Fig. 1). If the concentration of filler exceeds a critical value (67 vol ~o), the quantity of binder falls below that of the free volume between the microspheres. The binder then no longer covers all microspheres, the homogeneity of the system is disturbed, and defects (cavities) occur. This can formally be regarded as the appearance of open pores in the structure, the consequence of which is the deterioration of all macroscopic properties of the material.

Powdered binders based on phenolic and epoxy oligomers, and also one based on poly(vinyl chloride), have attracted interest as they extend the range of products and sometimes simplify the process technology 75).

.,.., 60

20

2

I I I

~0 O0 120

700

ooo %

$00

~0

Fig. 2. Influence of concentration (C) and diam- eter (D) of carbon microspheres on the fluidity (1, relative units) and the apparent density (2) of epoxy syntactic foams 4o) The A and B regions are the cast and molding compositions, respectively

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The properties and densities of the mixtures and their resultant syntactic foams not only depend on the binder/filler ratio but also on the microspheres themselves, their size, sphericity, polydispersity, apparent and bulk density, the thickness and uni- formity of their shells. Thus, at a given binder/filler ratio, the fluidity of a mixture depends on the size of the microspheres (Fig. 2) and the apparent density depends on their bulk density (Fig. 3) i). As the bulk density of the microspheres increases (the filler particles become larger), the final strength of the material decreases 3.76).

The lowest apparent density, without voids, should be obtained when monodisperse microspheres are packed in the closest possible way. Materials made from large mone disperse microspheres have lower apparent densities than those made from small ones, even though theory L~ indicates that the void depends only on the packing arrangement and not on the particle dimensions. This enigma is resolved by the fact that larger spheres have lower apparent densities (Fig. 4) because they have thinner walls 7s, 79). If they could be made with thicker walls, the resultant syntactic material would be stronger 7s).

8o

67 _\V~C-j250 60

20

0 I i 320 6¢0 g60

~" (kg m "s) 1280

Fig. 3. Apparent density 0') of epoxy syntactic foams versus glass microsphere concentration (C). The figures next to the curves stand for the microsphere bulk densities in kg/m3; the maximum microsphere concentration is 67% for molding compositions 1)

00, )

Fig. 4. Apparent density (7) versus fractionated carbon microsphere diameter (D) ,o)

3.5 Microsphere Space Factor

If the microsphere concentration exceeds a certain threshold value, called the space factor (Ks), the mixture loses its fluidity and turns from a casting composition to a press-molding composition. Each type of microspheres has its own binder/rifler ratio

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80 F.A. Shutov

at which this transition occurs, marking the boundary between castable and moldable compositions 9.77}

The space factor is given by the volume the microspheres occupy when packed the closest s, vs~, i.e.,

Ks = ~)s~/~o (1)

where lls~ is the true volume fraction of the spheres, 9o is the closest packed volume fraction of the spheres.

The theoretical apparent density of a syntactic foam 7t is calculated, according to Bredenbach 79), as:

Ks 8b 7, = "[so, ~oo + 7b ~oo (2)

where 7sr~ is the apparent density of the microspheres in kg/m 3, 7b is the binder density in kg/m a, and 8b is the binder volume fraction.

The apparent density Vt, can also be calculated according to 79):

~,, = ~,s¢.(1 - ~ ) + ~ , ~ , (3)

In order to obtain in practice the theoretical highest apparent density of the syntactic foam, the mixture has to be molded at the pressure at which the microspheres pack the closest. Several methods (mold vibration, high molding pressures, etc.) have been recommended as ways of obtaining a syntactic foam density close or equal to 7t, by helping the microspheres pack more closely and thus approach the foam's maximum strength. It should be remembered that ideal packing is never actually achieved and there is always some disorder 8o~

Packing microspheres more closely by using external force results in some sections in which the spheres touch each other and other sections in which there is an interven- ing layer of binder polymer. The first type is markedly weaker than the second, and syntactic foam failure starts there. The practical conclusion is obviously that, in order to obtain strong syntactic foams, the microspheres should be packed somewhat less densely, so that thin binder layers are present between all filler particles 7, 8)

According to Krenzke and Kiernan 61) an effective method of decreasing the appar- ent density of a syntactic material is to use microspheres with a large value of Ks. Another method is to use macrospheres and microspheres together as filler (see Sect. 2.7). Orlova et al. 81) studied how the molding pressure depends on the concen- tration and viscosity of the binder (unsaturated polyester resin) and on concentration and type of the microspheres (BV-01 grade). The microspheres had the following specifications:

Lot Number 1 2 3 Apparent Density, kg/m 3 235 262 274 Ks x 100 64.8 57.7 64.2 Specific Surface Area, m2/cm 3 0.0486 0.0484 0.0592

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Syntactic Polymer Foams 81

... 0 . 5 ~

0.¢1 i o 20

c(% rot) /j#

Fig. 5. Molding pressure (P) versus binder con- centration (C) for BV-01 phenol microspheres and binder viscosities of 125 (1) and 10.8 (2) Pa • s 21

Fig. 5 shows a linear relationship P = F(C); in the form of clusters of straight lines for varying binder viscosity, emanating from a single point on the ordinate axis. This indicates that with no binder present, the pressure needed to reach Ks for a particular microsphere does not depend on the microsphere's properties but on the mold geome- try. The binder reduces the pressure needed to obtain the best packing, i.e. the binder helps the microspheres to move with respect to each other. The different slopes indicate that, for the same binder, the molding pressure depends on the binder's viscosity and the microsphere's specific surface. Thus, for the same microspheres, an increase of the binder viscosity from 1.250 to 10.805 kg/m • s increases the pressure. By constrast, for the same Ks and binder concentration and viscosity, the molding pressure falls as the microsphere specific surface rises. The following empirical Equation relates the molding pressure, P, to the mixture parameters sl).

P = P0 -- B (~[ _ "¢bKs) (4) 7b

where Po is the pressure needed to reach Ks in the absence of binder, and B is a co- efficient that depends on the viscosity of the binder and the specific surface of the microspheres.

It follows that, when formulating a manufacturing composition for a syntactic foam, all of the binder and filler properties, as well as the process parameters have to be taken into account. Special computer methods have been proposed for selecting the best formulations whilst accounting for economic efficiency s 2 - ~

3.6 Two-Phase and Three-Phase Packings

The density of a syntactic foam is determined by the relative proportions (volume fractions) of microspheres, resin matrix, and dispersed air. The theoretical lower limit of the density in a two-phase system (without dispersed air) is determined by the close packing rules for spheres. Various packing possibilities in systems containing uniform-size spheres are summarized below s, ss):

Packing type Volume fraction occupied

Cubic 0.52 Body-centered cubic 0.60 Face-centered cubic 0.74 Hexagonal 0.63 Random 0.90

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82 F.A. Shutov

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

Figure 6 is a graphic representation of foam structures in which the microspheres are dispersed randomly (a) and uniformly in close packing (b). In both structures, the two phases fill completely the whole volume (no dispersed air voids) and the density of the product is thus calculated from the relative proportions of the two. Measured density values often differ from the calculated ones, due to the existence of some isolat- ed or interconnected, irregularly shaped voids as shown in Fig. 6c. The voids are usually an incidental part of the composite, as it is not easy to avoid their formation. Nevertheless, voids are often introduced intentionally to reduce the density below the minimum possible in a close-packed two-phase structure. In such three-phase systems the resin matrix is mainly a binding material, holding the structure of the microspheres together.

It has been shown by Price and Nielsen s6) that intentional introduction of voids and a change of the relative proportions of resin to voids can significantly vary the compression strength of three-phase syntactic foams (Table 8).

The principle of composition of three-phase syntactic foams can be represented by a ternary phase diagram (Fig. 7). Point A on the diagram denotes a composite of the following volume fractions: resin: 0.15, microspheres: 0.60, voids: 0.25 84). The "pure" void free syntactic foam has a two-phase composition which falls along the

Table 8. Composition and Properties of Carbon/Carbon Three-phase Syntactic Foam 8~)

Volume fraction a Density, Compressive kg/m 3 strength,

Resin ~ Air void b MPa

0.05 0.35 210 0.92 0.08 0.32 250 3.10 0.15 0.25 354 5.92 0.18 0.22 388 5.10 0.20 0.20 426 12.00

a) Microsphere volume fraction is 0.60; b) Values calculated using microsphere particle density of

237 kg/m 3 and resin density of 1400 kg/m a

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Syntactic Polymer Foams 83

o~/ / \ \ ~ ,w/ / \ \'%

/ . . . . . . . . . . \ \&

/ / .~(.,m--- Hexagonal

l.O Void 1.0 Microsphere Cub,"c ~cecking limit

Fig. 7. Phase diagram of three-component syntactic foam 86)

polymer-microsphere border of the diagram. It is subject to the packing considera- tions of spheres (see above). The hexagonal packing is the upper limit for uniform sized spheres. Thus, the volume fraction of the microspheres cannot exceed 0.74 s, 10)

The two-phase "'composite" of microspheres and voids only (i.e. packed spheres without any binding resin) has also a minimum limit. Cubic packing (volume fraction 0.53) is the lower limit, below which the structure is not selfsupporting any more, and some resin is required to fill in between the spheres, s~.

Another two-phase composite is chemically or physically blown foam, composed of polymer and voids only (i.e. conventional foamed or cellular polymer). Its compo- sitions tie along the polymer-void border of Fig. 7, and it, too, is limited by the maxi- mum volume fraction of voids allowed, while still maintaining the definition of a foam. The limits mentioned define the allowed compositions for syntactic foams and determine the hrea within the diagram where they are located. One limiting case is point B which represents the composition of microspheres (0.74), polymer (0.11), and voids (0.15). The microspheres, in this case, are arranged in a hexagonal close packing as).

4 Chemical Principles of Syntactic Foam Formation

4.1 Epoxy Syntactic Foams

Epoxy oligomers satisfy the specifications on syntactic foam binders the best. Their main disadvantage is that they have high viscosities at room temperature, but this can be circumvented by using diluents.

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84 F.A. Shutov

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 1) 16, 5s, 81, s7).

A formulation used in USA is: 54.7 parts epoxy resin, 10.3 parts aromatic amine, and 30.0 parts BJO phenolic microsphere. It has an apparent density of 336 kg/m 3 and is viable for two hours. The mixture is hardened at 71 °C for 2 hours or at 82 °C for 1 hour as). Recently, Prigozhin and Krasnikova 89) successfully applied "simplex planning" to the formulation of EDS materials with good properties. In addition to glass and phenolic microspheres, polystrene 18~, carbon 3s-4o), and mineral micro- spheres 18, 59) have also been used.

Most epoxy syntactic foams are vacuum molded at 70-120 °C, and vibrating stirrers are used to degas the mixture 4o~ Recent formulations can be processed without external heating (cured for 30 days at 20 °C) and yet yield syntactic foams with prop- erties comparable to those of hot-cured foams 1); the shrinkage is less than 1% for both.

Kozlov et al. 90) have shown that the most influential process parameter for epoxy syntactic foams is the vacuum applied during mixing.

Soviet scientists have developed a cold-cure EDS material 87~. Note that the epoxy binders used are in fact generally hot-cure oligomers (from the point of view of the functional groups used and the completion of cure). It turned out that the forced elastic limit and the compression elastic modulus of the matrix are the same for both hot and cold cures. The good mechanical properties of a cold cured syntactic foam seem to be due to strong intermolecular bonds.

The success of epoxy syntactic foams is associated with the development of the extrusion process for materials containing glass microspheres. Large articles and profiles with dimensions as large as 6 x9 m 2 1) can be made this way (See also Sect. 7).

Resnik has shown 92) that an effective way of reducing the apparent density of a syntactic foam is to decrease the apparent and bulk densities of the filler. Reducing the apparent density of glass microspheres from 450 to 350 kg/m 3 reduces the apparent density of the syntactic foam from 707 to 630 kg/m a, the binder concentration remain- ing the same.

As unfilled epoxy oligomers harden, large quantities of heat are evolved 32). When it is difficult to remove this heat from large articles, the material may be thermally destroyed, which shows up as crack formation, shrinkage, and sometimes material carbonization. These problems do not occur in epoxy syntactic foams because of the low content of binder (10 to 25 mass%), reducing the maximum temperature of the exothermic hardening reaction by 40 to 60 °C. This is because the hardening reaction takes place in thin films covering the microspheres and not in a large mass, and the microspheres absorb some of the heat of reaction.

When manufacturing large articles, the thickness of the binder layer must be chosen such that the temperature in the centre of the semi-finished product remains low. It has been shown, for instance 2), that an EDS layer should not exceed 160 mm.

The aftercure and cooling conditions of epoxy materials have a major influence on the quality andproperties of the final material. Orlova et al. 92~ studied the adiabatic curing of EDS materials by varying the hardening temperature and the degree of conversion of the reactive groups. They developed a mathematical model of the

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Syntactic Polymer Foams 85

system which takes into account the kinetics, diffusion, and heat exchange. The model was used to select the optimum curing and cooling conditions for articles with dimen- sions of 1.2 x 1.2 x 1.2 m. It was found that the best aftercure treatment was to put the block into a tempering chamber immediately after being poured. The chamber temperature is kept at the temperature inside the block (100 °C) and the block is kept there for 24 hours. Cracking is prevented by cooling either slowly and evenly at 2 to 3 °C/hr, or in 10 °C steps with the temperature maintained at each step for 7-9 hours to relieve internal stresses 1)

The brittleness of syntactic foams is effectively decreased by adding modifiers to the binder. Polysulfide rubbers, oligo(ester acrylate), and dioctyl phthalate, either together or individually, have been used as plasticizers for syntactic foams of glass and epoxy. It has also been recommended that curing should be conducted with dimethylaminomethylpheno193). The following types of compounds can be used as modifiers: 1) compounds without epoxy groups but capable of reacting with the hardener, 2) elastomers with functional groups that react with the epoxy resin, 3) com- pounds that react with neither the resin nor the binder.

Modifiers also increase the thermal resistance and enhance the stability towards wet or corrosive media. New epoxy resins can be used to obtain syntactic foams (y = 700 kg/m 3) with high unit elongations and good thermal resistance (up to 200 °C), without the need of modifiers 1,46)

4.2 Polyester Syntactic Foams

These materials are produced in the USSR as the SPS (with glass microspheres) and SPM (with phenol microspheres) brands 1,2~. The process technology for press molding is similar to that used for the epoxy materials. Formulations that can be cast at room temperature have recently been developed s3, 94~. The materials have limited applications because they shrink strongly during hardening 46,9s), and this results in decarbonation and cracking. They are, however, cheaper than epoxy foams 1)

Examples of new, very strong and light syntactic foams using quartz microspheres are given in 20, 96).

Recently Sands et al. 97) and Methven et al. 9s~ developed sheet molding compounds (SMC) and sandwich structures based on polyester syntactic foams.

4.3 Phenolic Syntactic Foams

Novolac and resol cold hardening oligomers habe been used 19, 75, 99)° In the case of resol foams the process technology is not different from that used for epoxy foams. Glass, phenolic resins, carbon, polystyrene, polyacrylonitrile and poly(vinylidene chloride) microspheres have been used as fillers 1~

The novolac foams are made by mixing the solid (powdered) oligomer (70 parts), carbon filler (30 parts), and hardener (hexamethylenetetramine) in a vibromixer and then press-molding the mixture for one hour at 150 °C, at a compression of 30-40~o of the initial volume 39~. Casting compositions curable at room temperature are made from resol cold hardening oligomers a3~.

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86 F.A. Shutov

In the USSR syntactic materials are manufactured from phenolic microspheres and phenolic binders (18~15 vol~/o) using press-molding at 170-190 °C and at low pressure. They apparent densities of 400-700 kg/m 3 1~.

4.40rganosilicone Syntactic Foams

Syntactic foams have been made from organositicone polymers and glass, ceramic, or polymer microspheres 23,100). They are used mainly for heat insulation and abla- tion coating lot~. For the latter, the two components are sprayed together onto the exterior of rockets, and cold setting silicone glues are used to improve ablation 102~. Syntactic materials with carbon microspheres and silicones have also been proposed 39~.

4.5 Polyimide Syntactic Foams

The HFT-60 brand is a syntactic foam made from a polyimide and glass microspheres and is manufactured in USA ~o3~. The binder is the aromatic polyimide formed by the cyclization of the polyimide acid obtained from pyromellitic anhydride and an aromatic diamine. High strength glass microspheres with an apparent density of 325 kg/m 3 are used as filler. The principal manufacturing technique is molding and heating. For an article 15.2 x 15.2 x2.5 cm, the composition (40 g solid polyamide, 60 g glass microsphere, 52 g acetic anhydride, 5.2 g 4-benzoylpyridine) is cured in three stages: 2 hours at 66 °C, 15 hours at 66-316 °C, and 2 hours at 316 °C. It is then aftercured at 150-316 °C for 15 hours and at 316 °C for 4 hours. The apparent density of the material is controlled either by changing the binder/filler ratio or by altering the apparent density of the glass microsphere filler lo3-ios~ Mclllroy's vacuum molding process ~03~ makes it posible to make complicated configurations and to reduce the hazard of toxic or explosive gas evolution during the hardening.

Syntactic materials based on polybenzimidazole and glass or phenolic microspheres (7 = 40-500 kg/m 3) have been described in the literature ~o9-1m. They have been used as ablation materials that consist of two layers, one of which is a monolithic carbon plastic. Carbon fiber was added to the polyimide binder to improve the me- chanical properties of the material.

4.6 Carbonized Syntactic Foams

These are produced by carbonizing foams made from various binders and micro- spheres. The binders include polyurethane ~, resol ~2~ and novolac 39~ phenolic oligomers, and epoxy oligomcrs 39~, while glass ss~, carbon 39~, and ceramic ~8~ micro- spheres fillers have been used for carbonized foams.

Benton and Schmitt 3s~ described several carbonized syntactic materials using phenolic and carbon microspheres. The binders included wood resin, partially polymerized furfuryl alcohol~ isotruxene, decacyclenc, starch, and epoxy oligomers. Furfuryl alcohol and maleic anhydride were used as hardeners (Table 9). In addition solvents (methyl ethyl ketone, tetrahydrofuran, and acetone) were added to improve

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Syntactic Polymer Foams 87

Table 9. Formulation and Properties of Carbonized Syntactic Foams Using BJO ~ Phenolic Micro- spheres

Microsphere wall Binder composition Apparent Compression thickness, density, strength, (~tm) Resin Furfuryl acetone b kg/m 3 10 -5, N/m 2

(g) (g) (ml)

3 wood resin 10 10 50 270 60.5 2 wood resin 10 10 50 200 38.8 1 woodresin 10 10 50 170 31.5 2 isotruxene 10 10 50 20 37.5 3 decacylene 10 10 50 280 52.7 3 starch 9 30 50 300 61.5 3 Varcum ~ 10 10 50 20 32.3 3 Varcum 40 30 50 440 136.0 3 Varcum 25 30 50 570 107.0 2 epoxy resin 20 -- 50 260 62.0 2 - - 30 50 210 39.0

a microsphere mass 60 g, molding conditions: 150 °C hardening temperature, load 550 N, carboniza- tion temperature 900 °C;

b contains 5 g catalyst (maleic anhydride); Varcum is the trademark of a partially polymerized furfurol alcohyl (USA)

the adhesion of the binder to the microspheres. For example, a typical formulation is 10 g wood resin, 10 g furfuryl alcohol, 5 g maleic acid, 50 ml acetone, and 60 g microsphere. The mixture is press molded at 150 °C for 12 hours, demolded and placed in the carbonization chamber. The temperature is raised slowly during 60 hours up to 900 °C, and then the carbonized moldling is cooled. The whole process is carried out in an argon atmosphere throughout, and a slow cooling rate prevents crack formation in the material as it is cooled. Furfuryl alcohol materials lose 41-46~ of their weight and 17-20 ~o of their volume.

The properties of the final material are greatly influenced by the mold pressure, and the strength of a carbonized syntactic material can be doubled in this way (Fig. 8).

Nicholson and Thomas 113) reported carbonized plastic foams containing 51 carbon, made from novolac or epoxy oligomers and phenolic microspheres. A dry charge of resin and filler is mixed in a vibrating mill, then placed in a mold, and heated at 150 °C for 3 hours under 2 xl04 Pa.

v /

ol 2 3 # 5

(MPa)

Fig. 8. Effect of molding pressure P on the stress in compression (cr~) for a carbonized foamed plastic made from a compound binder (wood resin, furfuryl alcohol, maleic anhydride) and phenolic microspheres as)

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88 F, A. Shutov

1oo

80

"~ oo

20

I

z k 10 . . . . 0bz00

Fig. 9. Size distribution of the volume fraction of open cells in a carbonized syntactic foam. For a phenolic syntactic foam with "Kerasphere" carbon microspheres, specimen apparent densities are (1) 130, (2) 220, and (3) 390 kg/m 3 ag)

During the carbonization (800-900 °C in an inert atmosphere) the material shrinks 18-22 ~, but its apparent density remains unchanged because the shrinkage is com- pensated for by material loss 38~

The carbonization of a syntactic foam opens up the previously closed celt 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)

4.7 Syntactic Foams based on Unconventional Binders

The search for ways of improving the properties of syntactic foams has led to a number of new materials based on some unconventional binders.

Light weight (~ = 400-700 kg/m 3) insulating compounds using glass or phenolic microspheres (18-45 vol ~ ) with an oligoester acrylate (tentatively called SPA B-1) or epoxy (tentatively ENS-6T) binder have recently been developed in the USSR 1~ These compounds are made up just before use by mixing the microspheres and the liquid binder. The SPAB compound is available as a two component system that is stable in storage and can be used on-site. The ENS-6T compound hardens at 80 to 160 °C, while SPAB-1 hardens at 60-100 °C or even at room temperature.

Foams made from poly(methyl acrylate) including a plasticized form) with glass microspheres are now manufactured in the USSR 114). The apparent density of this material is 700 kg/m 3 for a filler concentration of 24 mass~.

Systems in which a polyolefin is the binder have attracted world-wide attention. These include the polyethylene--phenolic microsphere v4'lls~, polyethylene or polypropylene--glass microsphere H4-~16~, polyethylene or polybutylene--PVC microsphere (containing isobutane) 52~, and polyethylene/vinyl acetate copoly- mer--glass microsphere 1~ systems. Syntactic foams have been made from polystyrene (and its copolymers with chlorostyrene or polychlorostyrene) and microspheres made from polyethylene or polypropylene 46, ~15~ and' foams from styrene/acrylonitrile 1~7~

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Syntactic Polymer Foams 89

and styrene/butadiene 118~ copolymers with glass microspheres. The binders that have been used include also polyurethanes 55-57,119-123), polyamides 12,124), poly- (vinyl chloride) 57, 93~, ABS copolymers 116,1~), polysulfides zo, 106~, polysulfones 106), synthetic rubbers zo), polymethacrylates 9a) polyacrylates 125), carbamide oligomers 1), pyrones 126), and asphalts 17)

Glass, carbon, ceramic, quartz, and polymer microspheres together with inorganic materials such as cement, gypsum, lime, and metals have all been used as fillers aT, 88).

4.8 Syntactic Prepregs

Submarines must be made from materials that are light, have good strength to weight ratios, and a good buckling resistance. Kausen and Corse report that these specifica- tions are met by the "syntactic prepregs" such as Synpreg, made in USA. These are syntactic foams applied over a textile substrate 127). Laminates and sandwich struc- tures are made, creating a surface crust. In terms properties, prepregs are between syntactic foams and monolithic (unfoamed) laminated and reinforced materials. They have half the apparent densities of fibre glass plastics, with the compression strengths only 25 % less. They are isotropic because a woven fabric forms the substrate. The surface crust does not change the bearing capacity of a prepreg, but it does affect its rigidity. A crust 0.1--0.25 mm thick gives a prepreg a reduced modulus of elasticity three times that of a fiber-reinforced plastic. Five Synpreg materials are produced for different applications (Table 10).

Prepreg structures can withstand considerable impact loads of up to 830 J. The main danger in the underwater use of prepregs, viz. the hydrostatic failure of the glass microspheres, has for the most part been eliminated. No more than 50% of the microspheres are destroyed at 21 MPa. The application ofprepregs to the manufacture of deep sea equipment is 30 % more efficient than fiber glass. These materials are also used for internal sheathing in rockets and aircraft where high impact loads and large local compression stresses are encountered 127).

4.9 Elastomeric Syntactic Foams

Elastomeric syntactic foams have recently attracted attention. They use an elastic matrix and either elastic or rigid microspheres 114,125,128)

A family of elastomeric foams has been developed by Rand 129) for use as stress relief coatings on electronic components in encapsulated electronic assemblies. Polysulfide, silicone and polyurethane elastomers blended with glass and phenolic microspheres have been used to formulate syntactic foams (Fig. 10) These foams are used to minimize the stress caused by differential thermal expansion between the component and the encapsulant.

4.10 Three-Phase and Four-Phase Syntactic Foams

Three-phase syntactic foams with unique properties have been developed during recent years (see also Chapter 3.6 and Table 8).

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Tab

le 1

0.

Pro

pert

ies

of s

ynta

ctic

pre

preg

s ba

sed

on e

poxi

de r

esin

and

gla

ss m

icro

sphe

res

127)

Par

amet

er

Syn

preg

-780

1 S

ynpr

eg-7

802

Syn

preg

-780

3 S

ynpr

eg-7

804

(for

dep

th

(for

dep

th

(for

dep

th

(cov

ered

wit

h <

6000

m)

< 30

00 m

) <

300

m

glas

s fa

bric

)

Syn

preg

-720

1 (f

irep

roof

)

App

aren

t de

nsit

y, k

g/m

3 70

0-90

0 70

0-90

0

Thi

ckne

ss,

mm

9.

3 10

.1

Ten

sile

str

engt

h •

10- 5

, H/m

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3.9

913.

9 m

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us

70.3

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70.3

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Fle

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us

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4218

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0

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900

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ter

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ter

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• 10

6 P

a hy

dros

tati

c pr

essu

re

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Syntactic Polymer Foams 91

J

15

V 0 I I I

0 lo 90 30 40

(%)

Fig. 10. Compression stress-strain properties of various elastomeric syntactic foams 129): (1) urethane elastomer binder and glass microspheres: V = 640 kg/m 3, void fraction 0.321 ; (2) polysulfide elastomer binder and phenolic mierospheres: 3' = 1500 kg/m 3, void fraction 0.133; (3) silicone elastomer binder and glass microspheres: V = 610 kg/m 3, void fraction 0.407

A common method to produce three-phase foams is by blending the dry components (resin and microspheres), covering the microspheres with finely divided powdery resin as). The spheres are then mixed with a solution of the polymer to form a low viscosity mixture. After removal of the spheres from the solution they are charged wet into a mold, and the final solvent removal is done in the mold before or during curing. Air voids in the binder are generated by air dispersed in the resin during blending and mixing. According to another method, developed by Woodhams 99) a cellular matrix of these foams is produced by a conventional foaming process, with chemical or physical blowing agents.

Three-phase syntactic foams based on epoxy and polyimide resins were developed by Puterman and Narkis ss). Narkis et al. 104~ worked out thermosetting three-phase syntactic foams (TTPSF) by a rotational molding technique. These materials consist of glass or silica microspheres, polyimide or epoxy resin, and air voids (having open- cell structure) dispersed in the resin. The TTPSFs possess an interesting combina- tion of properties including light weight (densities from 120 to 240 kg/m3), com- pressive strength from 0.3 to 2.0 MPa, low dielectric properties and high temperature resistance.

Recently, so-called four-phase syntactic foams were produced by using two micro- spheres types together, e.g. glass and polystyrene with a polyurethane binder 1).

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92 F.A. Shutov

The two fillers with their widely differing strength parameters, produced a material that has good compression, tensile, and impact strengths.

4.11 Reinforced Syntactic Foams

Light weight reinforced syntactic plastics are a new application of microspheres, an example being made from an epoxy resin, glass fiber, and carbon or glass microspheres. These materials have also good heat insulation and ablation properties 4~)

A light weight polyester glass-reinforced syntactic plastic has been developed in the USSR. It has been applied to ship building and contains phenolic microspheres, glass fiber filler, and polyester resin 66). The structure consists of layers of glass rein- forcement impregnated with binder, alternating with foamed binder containing microspheres. The strength properties of the material can be varied between those of the glass-reinforced plastic and those of the foamed plastic, by altering the microsphere concentration. At an apparent density of 940 kg/m 3, the reinforced foam is 30-50% more rigid than the unfoamed reinforced plastic, while at equal rigidities a structure made from the reinforced foam is 20-30 % lighter than one made from the unfoamed reinforced plastic. The reinforced foam is processed in practically the same way as the ordinary reinforced plastic, with the exception of a vacuum mixer needed to mix microspheres and binder.

LeWark 13o~ developed a family of Fabricated Reinforced Epoxy Toolings (FREP) for thermoset molding processes, based on epoxy syntactic foam. Reinforcements used are typically continuous fibers of E-glass in woven-fabric form or, for some applications, fabric of carbon-graphite. FRET tools exibit enormous advantages over conventional metal tooling, such as: faster fabrication, light-weight for easy process- cycle handling, much lower cost than metal, and faster part cycle times. Hi-modulus composite parts based on syntactic foams were developed in other work of the same author 131) Epoxy, urethane, polyester, and vinyl ester resins were reinforced by E-glass, aramid, or graphite/boron fibers. These materials were applied for the produc- tion of missile fins, skis, windpropeller blades, etc.

Other new materials are frothed or lightened syntactic plastics, which can be either isotropic 107,11o, 122,132-134) or anisotropic 116). They have also been called integral (structural) syntacticfoarns and the smallest density obtained is 67 kg/m 3 67).

5 The Physical Properties of Syntactic Foams

5.1 Strength Properties

5.1.1 General

The properties of syntactic materials are influenced by several factors including the binder/filler ratio, the process and hardening conditions, and the physicochemical processes at the binder/filler interface 12, 76, 99~. The best syntactic foams, at given apparent densities of 680-700 kg/m 3, have a compression strength of I0 MPa, shear and tension elastic moduli of 2500--3000 MPa; ultimate bending strengths of 40 to

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Syntactic Polymer Foams 93

Table 11. Syntactic Foams (USSR) Using Phenolic (EDM, SPB) and Glass (EDS, SPS) Microspheres

Parameter Epoxy Binder Polyester Binder

EDM EDS SPB SPS

Cast Materials Apparent Density, kg/m 3 600-750 6 0 0 - 7 5 0 600-750 Ultimate Strength, MPa

in compression 29-55 55-100 18-25 in flexure 15-25 25-42 10-12 in tension 14-42 19-25 5-8

Elastic Modulus, MPa in compression 800-1500 1500-3000 300-500

Impact Strength, kJ/m 2 1-3 3-7 1-2 Molded Materials Apparent Density, kg/m 3 280-500 2 8 0 - 5 0 0 400-480

Ultimate Strength, MPa in compression 6-23 7-27 2-3 in flexure 3.5--10 4-15 3-5 in tension -- -- 2-3

Elastic Modulus, MPa in compression 15-43 10-40 22-43

Impact Strength, kJ/m 2 0.5-1.0 1.5-2.5 0.3~).4

600-750

40-55 20-25 10-13

900-1200

1-2

m

m

E

50 MPa, impact strength of 3 ~ kJ/m 2, and ultimate tensile strengths of 25-30 MPa 1) Table 11 indicates that syntactic plastics are similar in strength properties to mono-

lithic filled systems (glass-reinforced plastics), although their apparent densities are 2-3 times smaller. Hence, syntactic plastics appear to have the highest specific strengths of all known plastic materials.

The mechanical characteristics of the epoxy and polyester cold-cured syntactic molded foams do not differ much from the hot-cured materials (Table 12) 1).

Table 12. Properties of Cold-Cured Syntactic Foams

Type of Material Apparent Bulk Compressive Density, Modulus, Modulus, kg/m 3 MPa MPa

Oligoester resin - -

P h e n o l microspheres (USA) 670 1 t30 1050

Epoxy Oligomer -- Phenol microspheres (USA) 530 80 915 (7 = 610)

Epoxy Oligomer -- Glass microspheres (USA) 610 2500 2700

Epoxy Oligomer - -

G l a s s microspheres (USSR) 700-730 -- 1900-1720

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94 F.A. Shutov

5.1.2 Effect o f Microspheres

Unti l recently, the materials made from epoxy binders and glass microspheres were believed to be the strongest syntactic foams. However, several papers 26, 39) have shown that, when carbon microspheres replace those of glass, the material becomes stronger, more water resistant, and more capable to withstand hydrostat ic pressure (for the same filler concentrat ion) (Table 13). The smaller the carbon microspheres, the stronger are the resulting foams 19.135~. Carbon microspheres also improve the mechanical propert ies of phenolic and resol syntactic materials (Table 14) 38~.

Table 13. Epoxy Syntactic Foams with Carbon Microspheres

Parameter Krecasphere FTD-202 Carbon Microspheres Glass

Microspheres A-50 A-100 A-200

Microsphere Average Diameter, ~m 50 ! 00 200 200

Apparent Density, kg/m 3 660 680 680 650

Ultimate Compression Strength, MPa 3.9 8.45 7.1 6.6

Compression Modulus, MPa 24.0 23.0 19.7 18.6

Bulk Modulus, MPa 33.5 32.0 31.2 23.5

Collapsing Hydrostatic Pressure, MPa 14.0 13.2 11.0 9.8

Table 14. Novolac and Resol Syntactic Foams with Carbon Microspheres

Parameter Apparent Density y, kg/m 3

200 250 300

Ultimate Strength, MPa in compression 2.0 4.8 5.8 in flexure 2~9 4.3 6.0 in shear 1.1 2.8 5.2

Moduli, MPa: compression 84 125 231 flexure 580 700 1260 shear 28 70 105

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Syntactic Polymer Foams 95

5.1.3 Effect of Binder

Unsaturated polyester syntactic foams are cheaper than epoxy foams, although the latter are stronger, more water resistant, and shrink less if cured at room or high temperatures 83,136~. One merit of polyester syntactic foams is the low apparent density that can be obtained. But the mechanical characteristics depend upon the apparent density e.g. for the Soviet polyester syntactic foams (SPB), using phenolic microspheres (BV-01) 1):

Apparent Density, kg/m 3 580 445 360 310

Ultimate Strength, MPa in compression 121.5 11.5 9.0 6.0 in flexure 10.0 6.0 3.4 2.2 in tension 6.0 4.8 2.6 2.4

However, the syntactic materials with the lowest apparent densities are the carboniz- ed foamed plastics. During the carbonization of these materials open pores are formed (see Fig. 9) which lead to a whole series of valuable properties such as strength, low heat conductivity, and controllable gas and water permeability (Table 15)39)

The substitution of carbon microspheres for phenol ones reduces the carbonization shrinkage and material losses 1, 39~. The strength properties of the final material are, however, lower (Table 16). This seems to be caused by a weakening of the adhesion of the binder to the carbonized filler (compared to the uncarbonized one) 75~.

The properties of the syntactic plastics in which polystyrene or organosilicone polymers are the binder and glass microspheres the filler, are shown in Table 17 1~

Table 15. Properties of carbonized syntactic foams (Novolac phenolic oligomer and carbon mi- crospheres) 39)

Parameter Apparent Density, kg/m 3

130 220 390

Average Pore Size, lam 180 54 20 Volume Fraction of Open Pores, ~ 32 43 55 Volume Fraction of Microspheres, ~ 60 44 21 Thermal Conductivity, W/mK 0.035 0.087 0.122 Ultimate Strength, MPa

in compression 0.7 2.8 10.5 in flexure 0.6 i .2 3.8 in shear 0.6 1.2 3.8

5.1.4 Strengthening Effects

Syntactic foamed plastics are generally three component systems. For example, a syntactic system with 60 ~ glass microspheres (bulk density of 300 kg/m 3) consists

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96 F.A. Shutov

Table 16. Properties of carbonized syntactic foams (phenolicbinder and phenolic microspheres)

Parameter Filler: Binder ratio ( ~ mass)

90/10 60/40 40/60 30/70

Apparent density, kg/m 3 prior to carbonization 130 210 300 410 after carbonization 150 220 300 390

Ultimate compression strength at 900 °C, MPa 0.7 3.9 8.1 16.8

Compression modulus at 900 °C, MPa 42.0 120.0 294.0 715.0

Table 17. Properties of syntactic foamed plastics

Parameter Binder type

Crosslinked Organosilicone polystyrene resin

Apparent density, kg/m 3

Ultimate compression strength, MPa

Operating temperatures, °C

510 400

35.0 52.5

--30 to +175 --50 to +430

of 40 vol ~opolymer, 53 ~air, and 7 ~glass. The inference from this example is impor- tant for an understanding of the strength of syntactic plastics. A reduction of the poly- mer content by 60 ~ reduces; the strength only by 55 ~ because of the strengthening effect of the microspheres 9,137 )

This strengthening effect is even more prominent if the binder is weak, i.e. its modulus is low. Thus, it has been shown 13s) that the absolute values of the strength and elastic parameters of an epoxy syntactic foam with glass microspheres were lower than those of the monolithic plastics, while the absolute values of a paraffin syntactic foam with the same number of glass microspheres are higher than those of the monolithic plastic (Table 18). The paraffin and epoxy polymers were chosen as binders for their very different elastic and strength parameters. The specific strength values of the syntactic materials are higher than those of the monolithic plastics in all cases, but the change is more substantial in the case of the paraffin binder. Exactly why this is so has not been understood. We can assume, however, that these effects occur in the binder layers close to the filler shells, and are probably associated with variations of the density and order of the polymer supermolecular structure 7, i37).

An increase in the microsphere concentration that changes the composition from a casting to a molding one is accompanied by qualitative changes in the final material's strength and elastic properties. Zavalina et al. 139~, for example, demonstrated that

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Syntactic Polymer Foams 97

Table 18. Variation of strength and elastic parameters of epoxide resin and paraffin on filling with glass microspheres ~3s~

Parameter Epoxide resin Paraffin

Unfilled filled loss of strength (%)

Unfilled filled increase of strength (%)

Apparent density, kg/m 3 1200 640 -- 936 540

Ultimate strength in uniform compression, MPa t47.0 84.0 --43.4 86.0 86.5 +0.3

Strength in uniform com- pression at 0.2 % deforma- tion, MPa

Bulk modulus, MPa

116.0 72.0 -38.0 51.5 75.5 +46.0

4000.0 3200.0 -20.0 300.0 590.0 +95.3

the proportional elastic limit of the SPM-1 syntactic foam does not depend linearly on its apparent density in the case of the casting material, while it is linearly dependent for the molding material. By contrast, other elastic properties (e.g. speed of sound) depend in the same nonlinear way on the apparent density, for molded as well as cast materials. A comparison of syntactic foam (Soviet grade SPM-1) and two foamed polyurethanes (Soviet grades PPU-3 and PPU-10) show, that for a given strength, the product of dynamic modulus and apparent density is the greatest in SPM-1 1~ This means that syntactic foams based on polyester appear to be promising dampening materials.

Two major factors governing the strength and elasticity of a syntactic foam are the residual internal stresses in its bulk and the adhesion strength between the binder and the filler x4o~ Accordingly, an obvious way of improving these parameters is to use strong binders and to strengthen the microspheres. Simply rejecting defective spheres can increase a syntactic foam's strength by 15-20 % 5s}. Kenyon 141) applied an elastic adhesive to the filler which not only reduced the stresses at the binder--filler interface but also altered the way the material strength depended on the microsphere concen- tration. Elastic binders increase the unit elongation of a syntactic foam from 1 to 10 ~o, and its ultimate tensile strength to 30 MPa.

5.2 Water Absorption and Resistance to Hydrostatic Pressure

5.2.1 General

The small degree to which syntactic foams absorb water (because their cells are com- pletely closed) puts them above all other foamed plastics. The differences in the ab- sorptivities of different syntactic foams are due to the binder chemistry, filler type, and filler concentration.

Hobaica and Cook 142) demonstrated that for microsphere concentrations below 67 vol %, syntactic-foam water absorptivities are virtually independent of the apparent density, but above that concentration the absorptivities rise rapidly due to the loss of binder integrity and the appearance of cavities and ducts.

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98 F.A. Shutov

Water absorption is also reduced for large articles, i.e. for low surface to volume (S/V) ratios 2) For example, for two samples with S/V ratios 6.0 and 1.5 cm -1 the absorption figures were 0.55 and 0.12 vol ~ , respectively (after 24 hours at 7 MPa). In practice the S/V ratio should be less than 0.5 cm -1 for syntactic foams used for boats 142)

Many papers have shown that the water absorption of a syntactic foam is propor- tional to that of its binder. Polyester syntactic foams, for example, absorb more water, even with "dressing" additives (silanes, vide infra) than do epoxy syntactic foams (Fig. 11) 1). The hydrolytic stability of epoxy foams is increased when the glass micro- spheres are replaced by carbon ones (Table 19)40).

The methods used to increase the water resistance of a glass microsphere foam are basically those applied to glass-reinforced plastics, filled thermoplasts, and elastomers, viz. hydrophobic adhesion compounds are added to binder and microsphere dress- ing 147). The compounds added are alkyl alkoxysilane derivatives, amino or epoxy alkoxysilanes for epoxy and phenolic resins, vinyl or methacryloxy alkoxysilanes for polyester resins. The dressing agents used are aminoethoxysilanes (~,-aminopropyl-

Table 19. Hydrolytic stabilities of epoxy syntactic foams 4oj (12 ~/o filler)

Filler type Compression strength, Strength Water MPa Loss, absorption,

% % Before After testing a testing a

Glass microspheres without dressing 77 62.3 -- 19. t 1.0 dressed with silane 80 73.5 -- 8. I 0.6

Carbon Microspheres 96 97.0 + 1.5 0.4

a Test was 6 hours in boiling water

f I ~ I i I

2 q ~:(month)

Fig. 11, Water Absorption (W) of different syntactic foams as a function of time at atmospheric pressure: (1) SP-1, (2) SP-1A, (3) EDM-7, (4) EDS-6, (5) EDS-7, and (6) EDS-7A 2)

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Syntactic Polymer Foams 99

triethoxysilane) for the epoxy and phenolic resins, and methyl or vinyl silicones for polyester resins 91). Note that the hydrophobic adhesion agents migrate from the binder bulk to the filler surface, whereas the dressing agents diffuse into the bulk 144)

5.2.2 The Binder--Filler Interface Problem

The properties of filled systems, including syntactic plastics, are not only influenced by the properties of the system's components, but also by actions at the binder--filler interface. This is where dressing additives are most likely to have their effect. A number of investigators have examined the role of dressing agents from the point of view of the "squeezing" of the filler into the binder, and of the friction between the two. The coefficient of thermal expansion of a polymer is an order of magnitude higher than that of glass (80-150 × 10 -6 versus 5 x 10-6), and as a result the glass filler will be "squeezed" into the polymer matrix during cooling of the syntactic foam. Any inter- action between binder and filler will depend on the pressure and friction between them. Landsman and Klumer 145) contend that the effect of water is due to its "lu- bricating" effect at the polymer--glass interface, reducing the friction. One function of a dressing agent is therefore to prevent water from reaching the interface. On the other hand a dressing agent may also decrease the internal stresses that arise at the interface when the filled system is forfiaed. Trostianskaya et al. 146) examined one mechanism by which the internal stresses in a glass filled phenol formaldehyde plastic is reduced: a hydration sheath on the glass inhibits the oligomer hardening, thus changing the network compaction close to the filler; dressing the filler eliminates the hydration sheath and thus the binder hardens more evenly.

The formation of strong adhesion between the polymer and filler is, however, more important than mechanical squeezing. Many papers have shown that dressing the glass surfaces increases the adhesion.

Dressing EDS plastics (Table 20) reduces their water absorptivities considerably 1) However, dressing agents added to the binder (active additives) are less effective than dressing of the glass surfaces (Table 21) 2).

In order to obtain water resistant materials the curing temperature should be raised to 130 °C or more, because this increases the strength of the adhesion layer. But the presence of dressing formulations reduces the temperature of the hardening reaction, while retaining the degree of conversion, and this simplifies the technology

Table 20. Influence of dressing on the water absorptivities of soviet syntactic foams with glass fillers

Hydrostaticpressure, MPa Water absorption (mass %) after 30 days in water

EDS EDS~

0,9 1-1,2 0,4-0.6 20 1.5-2.5 0.8-1.2 40 2.8-3.2 1.2-1.4 50 4.0-4.5 1.5 60 8-10,0 less than 3,0

Note: Specimen dimensions 20 x 20 x 20 mm

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100 F.A. Shutov

Table 21. Comparison of methods of dressing for epoxy syntactic foams made with glass micro- spheres

Brand Dressing Properties

Apparent After 10 hr boiling Water absorbed density after 20 days, kg/m 3 Water Loss in % mass

absorbed, Compression % mass Strength,

%

EDS Without Dressing Agent

Added to Binder

675 4.1 18 1.60

678 2.3 10 0.85

Microspheres Dressed 690 1.2 8 0.70

EDS-6 Without Dressing Agent

Added to Binder

680 6.2 37 2.9

690 3.4 10 2.1

Microspheres Dressed 700 2.3 11 1.0

needed to produce large articles 1~. Note that the cold-set epoxy materials based on hot-set resins (see Sect. 4.1) are very water resistant if the microspheres are dressed sT). The EDS and EDM foams are stable in prolonged thermohydrolytic tests at atmo- spheric pressure, up to 10,000 hours at 30--70 °C and a relative humidity of 98 ~o. In cyclical loading (to test cold resistance) water saturation impairs the mechanical properties 147)

Decreasing the water absorption "mecl~anically", i.e. by coating the external surface of an article, produces good results. It was shown that coating of epoxy syntac- tic foams with a thin layer of epoxy resin reduces the water absorption more than tenfold, even under high hydrostatic pressure 1~)

5.2.3 Water Penetration Mechanisms

Almost none of the cited papers dealing with water absorption by syntactic foams deals with the mechanism by which water or other small molecules penetrate into these materials. Filyanov et al. 14s, 149) undertook one of the first attempts using the ED-20 epoxy oligomer--glass microsphere system. Water absorption by a filled polymer is known to depend on the sorptive properties of the binder, the stability of

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Syntactic Polymer Foams 101

20

16

*.3.. ~fe

a 5 0 b

o;

# 8 12 o # 8

Fig. 12a and b. Water absorption linetics of epoxy syntactic foams with glass microspheres at different concentrations. (1) without filler, (2) 10 mass %, (3) 15 mass %, (4) 20 mass %, (5) 25 mass 9/0, a no dressing, and b filler dressed 1,8~

the binder--filler adhesion, and the material 's manufacturing conditions. Sorption and diffusion studies are, in principle, easy and informative ways for estimating the influence o f a filler on the polymer structure, but the data in the literature cover only solvents in epoxy binders, and few deal with water diffusion.

The addition o f a filler changes the kinetics o f the water absorption by an epoxy binder, water absorption becoming a multistage process (Fig. 12). Crank and Park 15o) have given the equation for the kinetics of water sorption by a thin plate, as well as a solution of the Fickian diffusion differential Equation as:

W = h \ r c / (5)

W---~ - rt-- 5 exp - 4h 2 f (6)

where W, is the water absorbed at time x, W~ is the water absorbed at equilibrium, h is one half o f the specimen's thickness, and D = diffusion coefficient.

As the filler concentration increases so does the initial slope of W, = f ( ~ ) , see Fig. 12. According to Eq. (5) this may be due either to an increase of the diffusion coefficient or to the increased effective thickness of the specimen due to the addition o f the filler that does not absorb water. Initially, there are small increments o f the relative content of absorbed water, but the absolute quantity (referred to the binder) is almost independent o f the filler concentration; however in the next stages the abso- lute quantity of absorbed water becomes proportionM to the microsphere concentra- tion. Thus there are different absorption mechanisms effective in the course of time.

Since the filler does not affect the sorptive properties o f the binder in the initial stages, D and W~ appear to remain constant, irrespective of the degree of filling. Thus we have:

WjW~ = h, ff/h = const (7)

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102 F.A. Shutov

where W~ and W; are the water absorptions of filled and unfilled specimens at tirhe z, calculated in terms of binder, h is the half-thickness of the unfilled specimen, and h~ff is the effective half-thickness of the filled specimen 148)

Equation (7) is satisfied quite well for various degrees of filling. Hence, the diffusion coefficient at the first stage does in fact not depend on the filler and is equal to the diffusion coefficient in the unfilled epoxy resin (D = 1.25 x 10 -5 m2/s).

The addition of microspheres lowers the glass transition temperature of the epoxy binder (Fig. 13). This seems to be because the filler causes defects in the matrix network. Equal diffusion coefficients of filled and unfilled epoxy binder indicates, therefore, that the diffusion processes are insensitive to binder changes. The sorption of water by epoxy resins is in fact known to depend mainly on their polarity and only slightly on the three-dimensional compactness of the network.

During the second stage of water absorption (Fig. 12), a considerable quantity of water is absorbed, proportional to the filler concentration. It seems that, after prolong- ed contact with water, the epoxy resin--glass adhesion begins to deteriorate and water is absorbed at the resin--glass interface. Water absorption due to true diffusion slows down and the diffusion coefficient in the second stage is only one third of that in the first stage. It is assumed that, as soon as the binder ceases to adhere to the micro- spheres, water is mainly sorbed onto the hydrophilic surfaces of the latter.

The second stage of water absorption can be used to trace the effect of a dressing agent (-/-aminopropyttriethoxysilane) on the mechanism of water absorption by a syntactic foam. Dressing the glass microspheres beforehand with the agent eliminates the second stages, and the kinetics are like the first stage kinetics (Fig. 12b). The decrease of the amount of water absorbed and the altered absorption pattern indicate the formation of water resistant chemical bonds between the binder and the filler.

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.

O.3

0 0.6

.~ 0.0

O.2

0 0.6

8.#

#,g

Y C

g

gO ¢#0 2go T(°g)

Fig. 13a-c. Thermomechanical curves for a unfilled epoxy binder, b epoxy syntactic foam with dressed glass microspheres, c syntactic foam with undressed filler; (1) iriitial sample and (2) sample after 2 months in contact with water 1482

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Syntactic Polymer Foams 103

The data in Fig. 13 show that the glass transition temperatures of all materials is reduc- ed 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 toss of adhesion between the binder and the filler, indicating that water absorption by syntactic foams is multistaged.

Manson and Chin 151) reported that the addition of filler to an epoxy binder reduces the epoxy's permeability coefficient (P), as well as the solubility of water in the resin (S) and that the reduction is stronger than expected from theory t52). Diffusion coef- ficients calculated from P and S for the unfilled resin were found to be somewhat higher than those for filled resin. The difference seems to be due to the formation of ordered layers, up to 4 lam thick, around every filler particle. The layers form because of residual stresses caused by the difference between the binder and filler coefficients of thermal expansion. The effective activation energy for water to penetrate into these materials, calculated in the 0--100 °C temperature range, is 54.3 k J/tool 1517.

5.2.4 Effect of Hydrostatic Pressure on Water Absorption

Syntactic foams absorb considerable amounts of water only at hydrostatic pressures above 100 MPa (Fig. 14), but absorb very little between 20 and 100 MPa 1~. A good feature of syntactic materials is their stability in cyclic hydrostatic tests. They can withstand up to 1000 cycles of alternating 60 MPa and atmospheric pressure 16)

Experience has shown that, even under hydrostatic pressure up to 75 ~o 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.

Filler dressing improves the water resistance. Thus EDS and EDS-A (a dressed foam) samples had residual compression strengths of 80 % and 95 %, respectively, after they had been immersed in water at 60 MPa for a month 2).

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 gaso- line, 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

3

12

~/ J , ,

l*g

tt# 8# 190 16# P(MPa)

Fig. 14. Water absorption (W) versus hydrostatic pressure (P) at two different water temperatures, for an epoxy syntactic foam with glass micro- spheres as filler 1~

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104 F.A. Shutov

Table 22. Changes in the strength of EDS materials a

Pressure, MPa Strength loss in compression (%) after staying in water for

30 days 60 days

0.1 1.6 18.5 30 1.5 15.7 40 13.8 37.0 50 21.1 51.0

a Apparent density: 650 kg/m ~

bulk moduli, while only lithium and the syntactic foams have bulk modufi greater than that of water, but lithium reacts with water. Conventional plastic foams, wood, glass foam, and plastics cannot withstand pressures of the order of 60 MPa, and metal foams corrode. Thus only syntactic foams meet all the specifications for this work, and those of epoxy and glass, or epoxy and carbon, are the best 16, lsa)

5.3 Thermal Properties

The thermal properties of a syntactic foam are primarily determined by the binder. Epoxy foams are more thermally stable than polyester foams; the former can be used at temperatures up to 20 °C, while polyesters can only be used up to 100 °C a2). Foams made from modified epoxy resins have comparatively low thermal stabilities, although those of epoxy rubber 52) and epoxy novolac foams are quite high, with Mart~ns thermal stabilities in excess of 170 °C 131) The EMS materials retain 50% of their initial compression strength when the temperature is raised from 20 to 100 °C, while their bending strengths falls from 65-70 MPa to 30-35 MPa 2). Accelerated tests have shown that the foams with glass and polymer microspheres (EDS and EDM brands, respectively) are very stable to prolonged heating. They can withstand tem- peratures of 75-100 °C for up to 10,000 hours, as well as prolonged exposure to below zero temperatures 1,7~

The strength of cured oligoester acrylate foams with phenol microspheres is propor- tional to the binder concentration, i.e. inversely proportional to the apparent density 15,). The decrease of the material's strength with temperature is also linear, whereby the strength of the material with the highest binder concentration decreases most rapidly. Apparently the binder strength and thermal stability are decisive, the filler strength being quite low. The SPAB-2 materials have an operating temperature of 150 °C, and the stability of their strength parameters at elevated temperatures depends more on their binder:filler ratio, than on the degree of binder hardening 1~

Syntactic materials with very good thermal resistance are now manufactured in the USSR. They are made from phenolic binders and glass-carbon or carbon micro- spheres 1) and are thermally stable at 300-350 °C.

It should be mentioned that they have also good corrosion resistance under condi- tions of severe thermal and gas-dynamic flows 82).

Polyimide syntactic foams are also exceptionally stable mechanically at elevated

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Syntactic Polymer Foams | 05

Io I

I 0 gO ~0

c(°lo)

Fig. 15. Compression diagram for a polyimide syntactic foam with NT-60 (USA) glass micro- spheres (apparent density: 370 kg/m 3) at various temperatures (1) 20 °C, (2) 204 °C, (3) 260 °C, (4) 316 °C, and (5) 370 °C ~0s)

temperatures (Fig. 15). For example HTF-60 (apparent density = 370 kg/m 3) loses less than 20 ~ of its initial strength as the temperature rises fron~ 200 to 370 °C. The material can remain intact under compressive strains up to e = 4 0 ~ at 370 °C, while under tension at the same temperature fracture starts at e = 1.8~o (Or = 3.2 MPa) lo5). The foam loses 10 ~o of its mass when heated to 528 °C in air or to 557 °C in an inert atmosphere. The material's shortcomings are that it shrinks on curing by up to 20 ~ , and curing takes a long time with the evolution of toxic and inflammable gases (acetic acid, acetic anhydride, N-methyl pyrrolidone etc.)~03~

Polybenzimidazole foams with phenolic or glass microspheres also have good ther- mal properties. Since they are not combustible they retain good mechanical properties up to 350 °C, starting to lose mass at 600 °C lo9).

The filler type has some effect on a syntactic foam's thermal resistance. The replace- ment of glass microspheres by phenolic ones improves the thermal oxidation stability of epoxy foams, especially at 100-150 °C 2).

Interestingly, a temperature increase lowers the strength of a novolac syntactic foam with carbon filler less than it reduces the strength of the unfilled foam (Fig. 16) 39) This is undoubtably because there is destructive thermal oxidation in the plastic foamed by gas, due to the oxygen in the gas. Thermal oxidation in syntactic foams is much lower, because the microsphere shell forms a protective barrier between the gas within the sphere and the polymer matrix.

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 ~' 1~s~

The filler type has some effect on the fire resistance of the syntactic material. Using carbon microspheres instead of organic fillers makes the foam less combustible 39). The carbonized materials are completely incombustible ~13)

The thermal expansion coefficient of an epoxy syntactic foam with carbon micro- spheres decreases as the filler concentration is increased. Thus the coefficients for foams with 0 (pure binder), 15, 25, and 25 vol~/~ filler are 55 x 10 -6, 45 x 10 -6, 37 x 10-6, and 13 x 10-6 deg- 1, respectively. These values do not change for tempera-

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106 F.A. Shutov

45

~ 3

¢.5

I

-700 0

2

100 200 390

r(°g)

Fig. 16. Variation of ultimate compression stress (co) with the test temperature for a novolac syntactic foam with carbon microspheres, ap- parent densities (1) 250 and (2) 200 kg/m 3 ls~

tures up to 370 °C 41); this is important for materials used as thermal insulation where the heat loads can vary abruptly. If a phenolic binder (novolac) is used instead of the epoxy binder, the coefficient of thermal expansion is even lower, i.e. 7 × 10 - 6 deg-1 for an apparent density of 200-300 kg/m 3 39)

Syntactic materials made with carbon microspheres of low apparent density have substantially lower thermal conductivities (0.049~).064 W/InK for 7 = 200 to 300 kg/m 3) than other syntactic foams 39).

Table 23 shows the therrnophysical properties o f some syntactic foams 2). Polyimide syntactic foams have a thermal expansivity o f 20 × 10 - 6 deg-1 and thermal conduc- tivities of 0.085 W / m K at 22 °C and 0.111 W / m K at 370 °C lo5, lo6). The low thermal conductivity of the carbonized foams (Fig. 17) is due to their open pore structure 113)

In general, the thermal conductivity o f syntactic foams is the same as that of conven- tional "chemical" foamed polymers of the same apparent density 8,11s).

5.4 Dielectric Properties

Syntactic foams have good dielectric properties (Table 24) 2); they can be varied over a broad range by changing the binder and filler, as well as the filler concentration s, 32)

Table 23. Thermophysical Properties of Syntactic Foams

Parameter Type of material

EDS EDM

Conductivity, 105 m2/s 0.139-0.167 0.111

Thermal conductivity, W/mK 0.116 0.091

Specific heat, 10 -3 J/kg °K 1.2-1.42 1.21

Expansion coefficient, 106 °C-1 at 50 °C 50 73

100 °C 76 82 150 °C 87 94 200 °C 80 103

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Syntactic Polymer Foams 107

~-0.I0 1

0 200 #0O 000 r(°g)

Fig. 17. Heat Conductivity (~.) versus temperature and apparent density for a carbonized phenolic foam with phenolic microspheres, (1) 200, (2) 250, (3) 300 and (4) 350 kg/m 3 113)

The dielectric properties of the syntactic foams used for electric insulation (SPAB-I and ENS-6T) are very good. Even an increase in temperature in humid environments does not raise their e' or tan 6 values by more than 20 ~ . The electric resistance of a SPAB-1 sample did not exceed 400-500 M O h m even after 24 hours in an environment of 95 ~ ( + 3 ~o) relative humidity 1)

The most noticeable difference between syntactic foams with the same filler but different binders is seen in the tangent of the dielectric loss angle (Table 25) 11). I f glass microspheres replace organosilicon ones for the same binder, not only tan 6, but also e' decrease 1). But also the dielectric properties and the concentration of the binder affect the final foam's e' (Fig. 18) 11)

Table 24. Dielectric properties of EDS-6 syntactic foam at different temperatures z)

Parameter Test Temperature, °C

20 100 150 200

Dissipation Losses at 5 ×106 Hz, xl0 -2 0.86 2.3 4.6 2.4 Dielectric Constant 2.04 2.30 2.40 2.13 Resistivity, Ohm • cm 1.2 × 1013 0.2 × 10 TM 0.3 x l0 s 0.5 × 107 Electric Strength, kV/mm 8-13.8 -- -- --

Table 25. Dielectric properties of syntactic foams with glass microspheres 11)

Parameters Type of binder

Epoxy Crosslinked Organosilicone oligomer polystyrene polymer

Apparent density, kg/m 3 370 e' at 106-10 TM Hz i.55 tan 5 at 106-10 t° Hz 0.0t0

510 400 1.67 1.60 0.001 0.002

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108 F.A. Shutov

2.2

l.a

L~

LO 100

3

t

0 c(O/ovot )

Fig. 18. Variation of the dielectric constant s' with binder concentration C for polyester syntactic foams with PVC microspheres; Original polyester dielectric constants (1) 2.0, (2) 2.2, (3) 2.4, and (4) 2.6 52~

Carbon microspheres yield syntactic foams with resistivities that are astonishingly low for these materials. Novolac syntactic foams with carbon microspheres have resistivities o f 0 . 0 2 - 0 . 5 0 h m . m (depending on the filler concentration)77); this is ten orders of magnitude lower than for glass microspheres in the same binder! For materials made from carbon microspheres and silicone rubbers, the resistivity depends exponentially on the temperature, viz. 0.08 Ohm • m at 20 °C, 0.2 O h m . m at 60 °C, and 200 O h m - m at 95 °C 1~. Consequently, carbon microspheres make it possible to produce syntactic foams with electric properties appropriate for semiconductors.

Their good superhigh-frequency dielectric properties (Table 26) explain why syntactic foams have been so widely used in radar blast furnaces and lenses, and for absorbing electromagnetic waves 156).

Table 26. Superhigh-Frequency Dielectric Properties of Epoxy Syntactic Foams using Glass Microspheres

Apparent Density, kg/m 3 Frequency, MHz s' tan 8 × 103

320 2279.6 t.49 6.06 457 2284.8 1.73 6.96 641 2277,6 2.05 7.35 801 2215 2.39 8.20 961 2071.8 2.73 8.15

1280 1782.1 3.50 8.76

5.5 Other Properties

Syntactic foams are chemically very stable. The EDS, SPS, and E D M materials gain less than 1.5 mass ~o if exposed to environments such as propyl alcohol, t ransformer oil, gasoline, or petroleum for up to 12 months, whilst their compression strength is reduced by less than 12-20 ~ (Fig. 19). However, epoxy materials have poor resistance to acids or alkali, and are completely decomposed within 24 hours by benzene or acetone 1)

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Syntactic Polymer Foams 109

80

"~60

20

[] m [] []

A B C B E

Fig. 19. Effect of exposure in corrosive environments on the ultimate compression stress for various syntactic foams. Key: (A) initial samples, (B) gasoline, transformer oil, and isopropyl alcohol, (C) carbon tetrachloride, (D) 10 ~ sulphuric acid, (E) 10 % alkali. Exposure duration: EDS for 12 months in each medium, SPS for 6 months in each medium, EDM for 12 months in C, D, and E, 6 months in B 2~

Dressing additives increase chemical stability. For example, the undressed foam EDS-7 gains more than 3 mass ~ after 4 months in 30 ~ sulphuric acid, while the dressed foam EDS-7A gains only about 1 m a s s ~ after 10 months 2).

Syntactic materials weather well and are stable under warehouse storage condi- tions 1~.

The adhesion of syntactic materials depends primarily on the adhesive and the substrat~ in question. Epoxy materials, for example, (in the absence of an extra glue) have the following adhesion strengths 2):

Strength, MPa EDM EDS

with metals in tension 1.0 - - in shear 4.0 8.7 with glass reinforced plastics in tension 2.0 - - in shear 7.5 25.0

The strength of adhesion also depends on the filler, being better with glass than with phenolic microspheres. This is due to better adhesion at the binder-filler interface (See Sect. 5.2.2) 6, i24)

6 C~_lculation of Strength Parameters

6.1 General

In order to improve the properties of gas filled, particularly of syntactic, foams, a general theory covering their deterioration and deformation should be developed, along with continued technological research. The absence of reliable techniques for

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110 F.A. Shutov

calculating and predicting the strength of the foams has hitherto restricted consid- erably their application 6.67).

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 spheric- al particles have not proved successful 1, s, 76) Models which rely on close microsphere packing 157.15s~ have not been very accurate either. For example, Krzhechkovsky et al. 159) showed that syntactic foams can be treated as homogeneous uniform ma- terials with small Ks (up to 20 %) only if there is no stress field in the polymer matrix around the microspheres (see Sect. 3.5).

6.2 Macrostructural Models

Dementyev and Tarakanov 8,160) used another approach by adopting a macrostructur- al model of syntactic foam morphology x61) to calculate the strength properties of an epoxy foam with phenolic microspheres. They made two restrictive assumptions, i.e. that the mechanical properties of the microsphere walls and the binder are the same, and that the volume fraction of filler is substantially smaller than that of the matrix. The macrostructural parameters of the syntactic foam are then defined in terms of the dimensions of the microspheres, and their displacements have the same nature as the deformations of the nodes and edges of an imaginary latice. We then get:

132 E = Eo 1 +[3 ~ c r~ = oOl +13 ~ (8)

where E and Eo are the elastic moduli of the syntactic foam and the polymer matrix, respectively, o¢ is the nominal ultimate compression strength of the foam, ~o is the compression strength of the polymer matrix, and 13 is a parameter of the cellular structure defined by the equation

v 3t3 2 +13 3

Y0-- (1 +[3) 3 (9)

I

N 0

, , , 2 > .

i t~

30 60 vol )

2O

I

12 1.,4 Fig. 20. Calculated and experimental (dotted and

solid curves respectively) values of (1) elastic modulus (E) and (2) ultimate compression strength (¢rc) versus phenolic microsphere concentration (C) in an epoxy syntactic foam at 23 °C 162)

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Syntactic Polymer Foams 111

100

2O f I I 8

Fig. 21. Compression diagram for (1) an epoxy syn- tactic foam with phenolic microspheres and (2) unfilled epoxy resin 161)

where q, is the apparent density of the foam and 7o is the density of the polymer matrix. The calculated data agree reasonably well with experimental results (Fig. 20). 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 compres- sion 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 de- termined 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) s - lo~

It should, however, be borne in mind that, whilst ordinary and syntactic foams are similar, there is an important difference: a syntactic foam has an additional phase boundary between the binder and the filler. The agreement between the theoretical and experimental results indicates that there is strong adhesion at the binder--filler interface, and only small internal stress. The form of the hardening thermogram (Fig. 20a) confirms in fact that there is a strong chemical interaction between the binder and filler. In addition, the filler causes the polymer near to it to be structured differently from that of the bulk polymer, leading to the "interphase" polymer layer. This is confirmed by the variation of the glass transition temperature of the epoxy resin as the degree of filling is increased (Fig. 22b). It should however be pointed out that, though the data of Dementyev et al. 162) clearly indicate that the glass transition temperature decreases markedly as the degree of filling rises, this is at variance with most other data which indicate that the transition temperature increases, or remains constant, as the filler concentration rises 146~. If the glass transition tempera- ture decreases, this means that the filler changes the structure of the base polymer, in particular the compaction of the three-dimensional polymer network. However, this has very little effect on the strength parameters of the syntactic material at room temperature. It seems that the macrostructural model can only be used to calculate the mechanical characteristics of syntactic materials if the elastic modulus of the matrix polymer is greater than or equal to that of the filler. Since glass microspheres ha.ve an elastic modulus much greater than that of epoxy resins, the modulus of the syntactic foam increases with increasing filler concentration and thus, the macrostructural model cannot be used for this material.

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112 F.A. Shutov

. . . . . I I I

I00 200 TC°C)

,--~#6

78

b

#.2 ~6 c(°/oVOO

Fig. 22a and b. Behavior of syntactic foams during thermal treatment. Key: a Thermogram of the hardening process for (1) Unfilled epoxy and (2) Epoxy syntactic foam with phenolic micro- spheres, b Glass transition temperature of the epoxy binder versus concentration of phenolic micro- spheres 162~

6.3 Microstructural Models

The mechanical parameter of a highly filled syntactic foam must in general be analyzed taking into account the interactions at the binder--filler interface and the system's stresses since both of these factors are important for highly filled systems 8,140)

Telegin et al. 163) have proposed a model for syntactic foams which assumes that a) the microsphers fill a large fraction of the volume, and b) the average thickness of the filler film is of the same order of magnitude as that of the microsphere walls. We can then, as a first approximation, regard a syntactic foam as consisting of two-layer shells, with the external layer being the binder. It may also be assumed that deformation "compatibility" at the interface is assured by the existence of adhesion between the two layers and that if the microspheres are made of polymers, they will tend to creep under normal conditions. The elastic moduli and hydrostatic strengths of syntactic foams are predicted very well using this model.

Bobrov also used this model of a syntactic foam to calculate hydrostatic strengths 164). At the same time, he showed that this parameter cannot be obtained theoretically for a syntactic foam using traditional micromechanical, macromechanic- al, or statistical approaches, as they are unsuitable for these foams. The first approach requires a three-dimensional solution of the viscoelasticity boundary value problem of a multiphase medium, and this is very laborious. The second and third methods assume the material is homogeneous overall, and so produce poor estimates for syntactic materials.

De Runtz 165) has made an interesting and promising attempt of estimating the strength parameters of syntactic foams. He applied the concepts and mathematical apparatus of the mechanics of discontinuous media and the theory of plasticity.

It is easy to see that these models are all based on the same (microstructural) prin- ciple, 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 element 8-10). 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 prop- erties 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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Syntactic Polymer Foams 113

estimates differ from experimental results by a factor two to three (see Sect. 6.5) 16~) One way out of this situation has been suggested by Telegin et al. 168). They consider

deformation and strength from a "more general" level, i.e. that of a "working cell". This is made large enough for the actual structure of the syntactic foam to be replaced by a quasi-homogeneous element. Since 1 mm 3 contains 1000-1500 microspheres, Telegin et al. consider a working cell with dimensions of 2-3 mm to contain enough structural elements for homogeneity, and generally to be small compared to the di- mensions of the final article. The mathematical discription of the behavior of this celt is made by compiling the equilibrium equation for the forces acting in damaged and intact sections, and the deformation compatibility equation ("preservation of flat ends" condition)169) This approach can be used not only to predict the strength parameters of a syntactic foam, but also to calculate some of its other properties, from the known properties of the foam's main constituents, i.e. the binder and filler 17o, 171)

6.4 Strength Calculation

Let us examine some concrete examples of calculating the strength properties of syntactic foams.

The following expression was suggested by Lavrenyuk 7s) for the strength of a syntactic foam in axial compression. It assumes that the stresses are distributed within the foam in proportion to the volumetric concentration of its components:

Es(Ef -- E0) (10) cr~ = t~cy I Ef(E s _ E0 )

where ere is the ultimate strength in compression, Cr~y~ is the cylindrical strength of the filler as given by USSR state standard 97-58-61, and E~, El, Eo are the elastic moduli of the syntactic foam, the filler, and the matrix, respectively.

Krzhechkovsky et al. 132,172) have calculated the deformability and long term strength of syntactic foams under external hydrostatic pressures. Their deformability e(x) as a function of time is given by:

_ p ~K2z 2 Z1 toxp (ll)

where z is the duration of pressure hardening, r is mean radius of sphere, P is the external pressure, G and J are the instantaneous and long term shear moduli, K is the bulk elastic modulus, r* is the relaxation time, Zz and %2 are dimensionless numbers that depend on the filler concentration ~s~ in the foam, as shown in Eqs. (12).

1 8s~ (12) - ' x 2 -

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114 F.A. Shutov

Elastic deformations are found if z* = 0. The ultimate long term strength can be obtained from criteria given in 173). The hydrostatic pressure that the foam can withstand for an infinitely long time, P~ is given as

4(1 + v ) Z 4 Poo = rlPco~ ; q = ~q 2(E/H - 1) •3 (13)

where Pool is the collapsing pressure, v is Poisson's ratio, and E and H are the instan- taneous and long term elastic moduli of the matrix.

The dimensionless numbers )~3 and Z~ are determined as

j 9 X3=~(l-a)x~+ (1 + a 2 ) - 9b 2 Z~ + ~ b2)c22 ;

1 - 2v 2 - + Vsp h

3 ] + v I~ , l . b - Io, I 7~4 = -~ ( 1 - a~Q~sph) 2 ; a - [Cycl, I~h[

(14)

where crt, ~c, and Cr~h are those stresses that in axial tension, compression, and shear, respectively, cause the syntactic foam to fail in the same lifetime; here q and 9sph may vary only from 0.5 to 0.65.

The influence the volumetric concentration of the filler ltsph has on the hydrostatic strength has been studied by Krzhechkovsky et al. 132). They assumed that the filler particles were spheres, had all the same dimension, and that O~ph did not exceed 30 %. The relative concentration in a working cell is assumed to be C = C%h/D, where D is the packing factor. Values for the relative collapsing pressure (pr), as it depends on C, were found by solving numerically and simultaneously the equilibrium equation for a spherical element, given a filler concentration C, and the Equation that gives the onset of the limiting strength of the syntactic foam 172). Figure 23 shows the results

/.2

Fig. 23. Calculated relative collapsing pressure of syntactic foams (Pr/O~) versus volume fraction of filler ,gsp ~ t32)

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Syntactic Polymer Foams 115

for a syntactic foam with ~ / ~ = 0.657, where o~ and ~ are the ultimate stresses for axial and hydrostatic compressions. The collapsing pressure is given by

I ( 1 ) 1 1 1 pr @+0 ,5 D 1 - + -~ + D(1 -q) )+q~ Pool . . . . (15) 0¢ 1 + *

where

q0 = 1 -- C)/(1 + qC); ~ = 4Go/3Ko

where K 0 and G o are the bulk and shear moduli of the matrix, respectively. Calculations have shown that at 0sph = 15 ~ the hydrostatic strengths of a syntactic

foam decreases by 35-40~/o, and for 0sp~ = 28 9/o by 60 ~ as compared to the matrix 132~ 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~

The thermal and shrinkage stresses in syntactic foams and the deformations due to polymerization and matrix solidification are still not clearly understood. Fillers hinder free shrinkage, and also hinder the thermal expansion of the matrix, resulting in the appearance of stresses. These stresses radically reduce the strength of the foam because the expansion (contraction) coefficients of matrix and filler are so large. Krzhechovsky et al. 174) calculated the thermal and shrinkage stresses by considering the overall deformation due to the polymer shrinkage and the contraction as it cools, the elastic shrinking depending on the relative length changes (L). The radial stresses at the binder--filler interface for a two-phase system are given as

3(cq T - L) o = (16) 1 ~+Of +

Kf (1 - Of) Ko

where T is temperature, Ke and Ko are the bulk moduli of the filler and matrix respec- tively, Of is the volume fraction of filler, and ar is the coefficient of thermal expansion of the filler.

The relative linear deformation of a syntactic loam is given as

L* = (arT -- L) (I + @) 9f + L (17) 1<2o(1 -- 0f)/Kr + (,ll + Of)

The expansion coefficient can be obtained from this equation by taking L = %, where % is the thermal expansion coefficient of the matrix. This approach gives results in good agreement with experiment.

6.5 Elastic Modulus Calculation

Until recently no theory has been published for the shear and Young's moduli of syntactic foams. Recently a simple method of estimating these moduli was presented

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116 F.A. Shutov

by Nielsen 167). According to this method, the modulus of syntactic foams can be calculated by well-known Equations if one can estimate the apparent modulus of the hollow spheres in terms of their inner and outer radii. From the theory of Kodama 175~, it follows that the apparent density of a hollow sphere is related to the cubes of the outer and inner radii. Thus

Gn 1 - - (a/b) 3 (18) Gs 1 + (a/b) ~

where Gn is the apparent shear modulus of a hollow sphere, Gs is the shear modulus of a solid sphere, a and b are the inner and outer radii of the hollow" sphere, respectively. Gn determined according to Eq. (18) may be substituted for the modulus of the filler phase in known Equations for the modulus of filled systems, such as 152):

G/Go = (1 + AB8~)/(1 - - B~)~r~ ) (19)

where

A = ( 7 - - 5 V o ) / ( 8 - - 1 0 v 0 ) (20)

for spheres

B - G n / G o - 1 (21) GH/Go + A

----- 1 + [(1 - - 9~')/~] ~)sph (22)

In these Equations, G is the modulus of the syntactic foam, Go is the modulus of the polymer matrix, v o is Poisson's ratio of the polymer matrix, and 9~' is the maximum packing fraction of the filler phase. For uniform spheres, ~)~ ~ 0.64 (see Sect. 3.6). The volume fraction of spheres in the syntactic foam is 0sr~. The slope of the G/G0 vs. 0sin curve depends strongly upon whether or not G/Go is greater or less than 1.0. The slope is negative if the apparent modulus of the hollow spheres is tess than the modulus of the polymer matrix.

Recently, Kinra and Ker 137~ published data of the shear modulus of syntactic foams consisting of hollow glass spheres in a poly(methyl methacrylate) matrix. The glass spheres had a mean radius of 45 ~tm and a wall thickness of 1.2 gm. Reliable values are known for the shear modulus of the polymer Go, the shear modulus of glass Gs, and Poisson's ratio of the polymer: Go = 1120 MPa, Gs = 2800 MPa, and v o = 0.35. Using these values, the upper curve 1 of Fig. 24 was calculated by Nielsen for the modulus of the foam as a function of the volume fraction of hollow spheres. These calculated values are, however, too high compared with the experimental values reported by Kinra and Ker.

This is not surprising since the calculations are based on a perfectly uniform wall thickness of 1.2 gm for all spheres. In actual practice, as it was mentioned above, there are frequently thin spots in the glass shells, and some spheres have shells thinner than the mean value. Since the apparent modulus of the hollow spheres depends

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Syntactic Polymer Foams 117

"1.0

a61 i I I t t 0 0.2 ~4

Fig. 24. Relative shear modulus of a PMMA/hollow- glass microspheres syntactic foam t67): 1) values calculated with a uniform shell thickness of 1.2 ~tm and Gu/Go = 1.01 ; 2) values calculated with a shell thickness of 0.6 txm and Gn/Go = 0.5, Filled circles are the experimental data of Kinra and Ker at 1 MHz x3~)

strongly on the wall thickness, spheres with thin shells and spheres with thin spots greatly reduce the apparent modulus of hollow spheres, With an effective shell thick- ness of 0.6 Ixm the calculated values for the syntactic foam are given by the lower curve 2 of Fig. 24. Thus the calculated results can be adapted to the experimental data in a reasonable manner. However, a much more detailed characterization of the structure of the glass spheres would be required to prove the validity of the made assumptions.

According to Nielsen's method, Young's modulus E can also be calculated using the well-known equation

E = 2G(1 + v) (23)

where v is Poisson's ratio for the filled system (in most cases v ~ Vo). However, it should be realized that the theory of the mechanics of syntactic foams is so complex that these equations are only approximations.

An excellent survey on the numerical analysis of the strength properties of syntactic foams was recently presented by Luxmoore and Owen 5)

7 Main Applications

Syntactic foams are used extensively for the construction of boats and deep-water submarines 46. 55, 58, 79,127,142,169) They are also used to make floats, buoys, under- water rescue apparatus, and equipment for raising sunken ships ,23.176). Other applica- tions include cements and putties for repairing hydraulic structures, submarine bodies and bulkheads 127)

One of the most promising recent applications for syntactic foams is in an infant industry: the deep-sea mining of minerals. A 5 m long unmanned shuttle with a 250-ton payload is exploring the Pacific Sea bed; the project is sponsored by the French government 46). The hull of the shuttle has been constructed by bonding 20 syntactic foam sections to ametal frame, joining the sections with adhesive. Each section has a wall thickness of 800 mm. The shuttle has made several hundreds dives to depths of 6000 m, withstanding pressures of up to 600 bars. Using the same basic design, it is planned to construct a 1000-ton payload sea-floor shuttle that will be 25 m long, 10 m high, and I0 m wide.

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118 F.A. Shutov

Special equipment has been developed to produce 5 m long flotation sections encasing underwater pipes, using epoxy syntactic foam 46.177). Depending on the pipe size, two or four flotation sections are required, held in place by steel bands. The system is currently used for mediterranean oil exploration; a similar system was developed for North Sea oil exploration, allowing the rig to operate at ocean depths of 450 m.

One of the more ambitious underseas projects involving syntactic foam is a 23-ton flotation collar that provides 32 tons of buoyancy 46). The part, 5.5 m in diameter and 3 m long, encases cold-water pipes, in an oceanic thermal-energy conversion system being developed by the US Department of Energy; it has been tested off the coast of Hawaii, at depths of 915 m.

Syntactic foams are broadly used in civil and industrial engineering 78.123,144, 155) The foams have been used in large-diameter municipal water installations, where iron pipe previosly had made few inroads, and for insulation of metal pipes i78). Recently 179) two innovations were reported: microspheres preblended in the polyester resin bath of a filament winding operation; and integral winging of the bell with the rest of the pipe, the bell retaining a rubber gasket. The microspheres increase the pipe's wall thickness and stiffness, imparting greater earth-load-bearing capabilities without incurring as much extra expense or weight as in the case of walls built with more glass. In comparison with concrete and ductile iron pipe, the weight of the new pipes is only one-ninth and one-third, respectively.

For many years syntactic foams have been used in construction, in sandwich structures, or as imitation wood or marble 1.22.57,180). Many types of these materials are applied in the boat-building industry 17.66). For example, recently PVDC micro- spheres with polyester binder were used to produce the decks of a 10 m sail-boat, with a weight reduction of I00 kg (from 250 to 150 kg) 46).

Weight reduction is not the only property that makes syntactic foam attractive for underwater and boat-building applications. The material also offers substantial improvement in core bonding, reportedly increasing productivity, decreasing cycle time in molds, and reducing material waste. Syntactic foam is applied with specially designed spray equipment either to PVC core, or directly into the laminate in the mold, either before or after the laminate has gelled 46)

While most of the development in syntactic foam processing has involved thermosets and compression molding, a reaction injection molding (RIM) process 181) was also successfully developed. The latest achievements in syntactic foam technology consist of the production of sheet molding compounds (SMC) 97.98,125,130, 181,182), bulk molding compounds (BMC) 125,181), and dough molding compounds (DmC) 98)

SMC based on hollow microspheres and polyester resin are being used in the manufacture of food containers for replacing aluminum containers 46).

Growing interest was reported in syntactic foam for car applications 46,182). Syntactic foams have been used successfully in the electrical and electronics indus-

tries lO4,129, xsa, 184). Recent developments indicate a widening range o f applications in microwave and automotive applications, in the manufacture of precision-molded radomes 46). Light weight syntactic foam is used for potting marine starter units and subminiature connectors, while other foam is used to raise the working capacity of transformers in a broad range of operating temperatures from --60 ° to + 135 °C 92)

These materials have been used for many years in the aerospace industry for

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Syntactic Polymer Foams 119

insulation and ablation materials 1,43, 82,13o, 131,156, 185), in the petrochemical in- dustry 2,131,282), in nuclear physics 186), and in analytical chemistry for gas-solid and gas-liquid chromatography . By using polymer compounds containg microspheres filled with die, "syntactic" carbon paper has been produced 46,186-188)

Applications of polymeric microspheres themselves have been increasing recently. In the USSR an additive based on phenolic microspheres ("Plamilon") has been used in the oil extracting industry to obtain light-weight drilling suspensions which decrease hydrostatic resistance and rise drilling rates 32,189); it is also added to flushing fluids and plugging solutions 1). The high specific surface area and good absorptive capacity of microspheres makes it possible to use them to collect oil from water surfaces.

Polymeric microspheres have recently been applied for medical uses: for drug delivery 190,1917, and for serological tests ~4,83). A new class of immunochemical reagents consists of antibodies covalently bonded to polymeric microspheres. In- terestingly, the addition of fluorochromes or magnetic particles during the synthesis of polyaldehyde microspheres resulted in the formation of fluorescent or magnetic microspheres, respectively 192) These microspheres may be used as a simple toot for mapping cell surface receptors, for specific labeling of human red blood cells, and for the separation of the cells.

It is quite obvious that the potentials of syntactic foams have not been exhausted yet, nor can their useful characteristics by overestimated 193,194)

Acknowledgments: The author would like to express his deep grat i tude to the colleagues in different countries who kindly took par t in the discussion o f the manuscript , or sent to the author copies of their papers or reports. The author wishes to thank for contr ibut ions from the following scientists: Drs. A Heuchon (Belgium); R. S. Molday , R. T. W o o d h a m s (Canada) ; J. M. Methven (England); T. Braun (Hungary) ; K. Ogura (Japan); J. A. DeRuntz , B. LeWark, P. Rand , W. Sands, W. Watkins (USA). The author is very grateful to his Soviet colleagues Drs. T. Krasn ikova and E. Petr i lenkova- Telegina for the very useful and numerous correct ions and discussions o f the Russian version o f the manuscript .

8 References

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Moscow: Sov. Encyclop. 1974, vol. 2, pp. 617 (Russ.) 3, Shutov, F. A. : IUPAC 29th Intern. Symp. on Macromolecules, Bucharest Romania, 1983 4. Hylyard, N. C., Young, J.: Introduction. In: Mechanics of Cellular Plas~:ics. Hylyard, N. C.

(ed.). London: Applied Science, 1982, pp. 1-26 5. Luxmoore, A. R., Owen, R. J. : Syntactic Foams. In : Mechanics of Cellular Plastics. Hylyard,

N. C. (ed.). London: Applied Science, 1982, pp. 359-391 6. Shutov, F. A.: Adv. Polym. Sci. 39, 1 (1981) 7. Shutov, F. A. : 17th Europhys. Conf. Morphology of Polymers, Praga Czechoslovakia, 1985 8. Shutov, F. A. : Adv. Polym. Sci. 51, 155 (1983) 9. Shutov, F. A.: 30th Inter. Symp. on Maeromolecutes, Hague Netherland, 1985

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10. Berlin, A. A., Shutov, F. A. : Chemistry and Technology of Gas-Filled High Polymers. Moscow: Nauka, 1980 (Russ.)

11. Volk, M. C. : Syntactic Foams. In: Encylopedia of Polymer Science and Technology. New York: Reinhold 1969, vol. 8, pp. 752-757

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65. N. N.: Offic. Plast. Caoutch. 18, No 27, 266 (1971) 66. Petrov, B. P., et al. : See [26], p. 196 67. Shutov, F. A.: Polymer Foams. Heidelberg: Springer 1985 68. Krasnikova, T. V., et al.: Plast Massy No 4, 58 (1974) 69. Park, H., et al.: Pol. Eng. Sci. 15, 761 (1975) 70. Collins, W. T., et al. : Proc. 30th Ann. Conf. Reinforced Plastics, Wilestone USA, 1975 71. Petrilenkova, E. B., et al.: See [28], p. 116 72. Telegina, E. B., et al.: See [26], p. 159 73. Ohji, K., Ogura, K.: Proc. 16th Jap. Congr. Mater. Res., Kyoto Japan, 1973; J. Soc. Mater.

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118. Evrard, G., Nottin, J. : Intern, Semp. Macromolecutes, Helsinki Finland, 1972 119. Barber, E., et aL: 32nd ANTEC Soc. Plast. Eng., Boston USA, 1977 120. N. N.: Plast. Technology 21, No 5, 30 (1975); No 1i, 21 (1975) 121. Einborn, I. N.: J. MacromoL Sci. l-D, No 2, 113 (1971) 122. Barber, E., et al.: J. Cell. Plast. 13, 383 (1977) 123. Shutov, F. A. : Intern. Symp. Polyurethanes, PRI, London England, 1983 124. Ozerov, G. M., et al.: Plast Massy No 2, 43 (1981) 125. Howes, W. C.: Soc. Plast Ind., Thermoset Press Molding Committee, 1981 126. Kimmel, B. G. : Proc. 19th SAMPE Symp., Buena Park USA, 1972 127. Kausen, R. G., Corce, F. E.: Mod. Plast. 46, No 12, 146 (1969) 128. Brokenbrow, B. E., et al. : US Nat. Techn. Inform. Serv., ADR No 750563, 1972 129. Rand, P. B.: J. Cell. Plast. 14, 277 (1978) 130. LeWark, B., Kanemoto, E. : SPE ANTEC, San Francisco USA, 1982 131. LeWark, B.: SPE ANTEC, San Francisco USA, 1982 132. Krzhechkovsky, P. G., et al.: See [26], p. 163 133. Kanovich, M. Z., et al.: Mekh. Polim. No 2, 225 (1977) 134. Artem'ev, V. I., et al.: See [26], p. 169 135. Shutov, F. A.: 2nd Intern. Conf. Roofing Technology, Gaithersburg USA, 1985 136. DeRuntz, J. A.: J. Appl. Mechanics No 3, 23 (1971) t37. Kinra, V. K., Ker, E. : J. Compos. Mater. 16, 117 (1982) 138. Lee, K. J., Westmann, R. A.: J. Comp. Mater. 4, 242 (1970) 139. Zavalina, I. N., et al.: See [29], p. 124 140. Shutov, F. A. : 28th Microsymp Polym. Composites, Praga Czekhoslovakia, 1985 141. Kenyon, A. S.: J. Coll. Interface Sci. 27, 761 (1968) 142. Hobaica, E. C., Cook, F. D. : J. Cell. Plast. 4, 143 (1968) 143. Lones, H. L.: Proc. 29th ANTEC Reinforced Plastics, Washington USA, 1974 144. Shutov, F. A. : Inter. Symp. Plastics in Building, Liege Belgium, 1984 145. Landsman, P., Klumder, J. : Kunststoffe 54, 791 (1964) 146. Trostyanskaya, E. B., et al.: Mekh. Polim. No 1, 26 (1967) 147. Belova, E. V., et al.: See [26], p. 135 148. Filyanov, E. M., et at. : Mekh. Polim. No 2, 290 (1972) 149. Filyanov, E. M., et al. : Komp. Polim. Mater. No 4, 16 (1979) 150. Crank, J., Park, G.: Diffusion in Polymers. New York: Reinhold 1968, p. 16 151. Manson, J. A., et al. : ACS Polym. Preprints 14, 469 (1973) 152. Nielson, L. E. : Mechanical Properties of Polymers and Composites. New York: Dekker 1974 153. Whitaker, T. E., et al. : AIAA Papers No 1308 (1980) 154. Anderson, T. F., et al. : J. Cell. Plast. 6, 171 (1970) 155. Reusova, L. A., et al. : Plast Massy No 10, 31 (t982) 156. Lane, F. L. : Tech. Reprt No 32-1433, Pasadena USA, 1973 157. Karnaukhov, V. G.: Mekh. Polim. No 5, 875 (1976) 158. Chapelt, M. J., et al.: J. Mater. Sci. 11, No 1, 57 (1976) 159. Krzhechkovsky, P. G., et al. : Tr. Nikol. Karablestr. Inst. No 98, 87 (1975) 160. Dement'ev, A. G., et al.: Kolt. Zh. 36, 233 (1974) 161. Dement'ev, A. G., Tarakanov, O. G. : Mekh. Polim. No 4, 594 (1970) 162. Dement'ev, A. G., et at.: Plast Massy No 11, 59 (1972) 163. Telegin, V. A., et al.: See [29], p. 114 164. Bobrov, B. S., et al.: See [29], p. 115 165. DeRuntz, J. A., Hoffman, O. : J. Appl. Mechanics No 9, 551 (1969) 166. Nico|ais, L., et al. : Polym. Eng. Sci. 11, 194 (1971); 13, 469 (1973) 167. Nielsen, L. E.: J. Polym. Sci., Polym. Phys. 21, 1567 (1983) 168. Telegin, V. A., et al.: See [26], p. 165 169. Sadrakyan, L. C. : Statistical Theory of Deformation. Erevan: Apastan 1968 (Russ.) 170. Allen, J. D., et al.: Proc. 19th SAMPE Symp., Buena Park USA, 1974 171. Kanovich, M. Z., et al.: Mekh. Kompos. Mater. No 1, 73 (1979) 172. Krzhechkovsky, P. G., et al. : Probl. Prochn. No 2, 53 (1978) 173. Bezukhov, N. I. : The Basics of Theory of Flexibility and Bouyancy. Moscow: Vyssh. Shkola

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175. Kodama, M.: Kobunski Kogaku (Eng, Ed) 2, 535 (1973) 176. N. N.: Mod. Plast. Intern. t0, No 2, 18 (1980) 177. Nishimura, E., Woodhams, R.: Proc. Fracture Conf., Alton Canada, 1980 178. Shutov, F. A., Ivanov, V. V.: Intern. Symp. Plastics in Building, Liege Belgium, 1984 179. N. N.: Mod. Plast. Intern. 12, No 2, 42 (1982) 180. Klapproth, D. K., et al.: 26 th SAMPE Symp., Boston USA, 1981 181. Jones, G. T., et a l. : Europ. J. Cell. Plast. 2, 163 (1979) 182. N. N.: Brit. Plast. Rub. 14, Sept, 20 (1981) 183. N. N. : Europ. Plast. News 10, No 5, 4 (1983) 184. Shutov, F. A. : 1st JSPS Intern. Conf. on Polymers, Kyoto Japan, 1984 185. N. N. : Chem. Eng. News December 5, 55 (1983) 186. Solodovnik, V. D. : Mierocapsutation. Moscow: Khimia 1980 (Russ.) 187. Alyshuller, M. A., Deryagin, B. V. : Koll. Zh. 42, 819 (1980) 188. Thies, C. : Polym. Plast. Teclmol. Eng. 5, t (1975) 189. Minkhairov, K. L., et al.: See [26], p. 201 190. Davis, S. S., Ilium, L.: Brit. Polym. J. 15, No 4, 160 (1983) 191. Bala, K., Vasudevan, P. : J. Macromol. Sci.-Chem. ,416(4), 819 (1981) 192. Margel, S. : Ind. Eng. Chem., Res. Dev. 21, 313 (1982) 193. Shutov, F. A. : Conf. Technology of Composites, Varna Bulgaria, 1985 194. Shutov, F. A.: Intern. Conf. Polym. Science and Technology, Mar-deI-Plata Argentina, 1985

G. Henrici-Oliv~, S. Oliv6 (Editors) Received March 22, 1985