The Physical Properties of Microcellular Composite Foams
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THE PHYSICAL PROPERTIES OF MICROCELLULAR COMPOSITE FOAMS Alice M. Nyitray and Joel M. Williams, Los Alamos National Laboratory, Los Alamos, NM David Onn and Adam Witek, Applied Thermal Physics Laboratory, University of Delaware, Newark, DE ABSTRACT Recently we reported on a method of preparing microcellular composite foams. In this procedure an open-celled polystyrene foam is prepared by the polymerization of a high-internal-phase water-in-oil emulsion containing styrene, divinylbenzene, surfactant, free-radical initiator and water. After drying, the cells of the polystyrene foam are then filled with other materials such as aerogel or resoles. The physical properties of these materials e.g., surface area, density, thermal conductivity, and compressive strength will be presented. INTRODUCTION Physics experiments require very special materials. For inertial confinement fusion (ICF) experiments, foams have been suggested as ideal material to hold uniformly a mixture of liquid deuterium and tritium in a fusion target. To do this, the foam must be a material of low ptomic number with very small cells (< 0.1 pm), low density (< 40 mg/cml) but handleable. Other experiments require open-celled foams with specific densities that can range from 7 mg/cm (5 X the density of air) to nearly full density, and range in length from a millimeter to meter§. One type of foam that we use extensively is a polystyrene emulsion foam prepared by a technique introduced by Unilever. The foam is prepared by mixing styrene, divinylbenzene, sorbitan monooleate and free radical initiator with water to form a water-in-oil emulsion that can be polymerized by heating at 60°C. The solid mass is then oven-dsied to remove the water. In this way, foams with densities from 0.012g/cm to 0.20 g/cm3 and cell diameter of 3-100 pm can be prepared depending on the concentration of monomers, surfactant, initiator and salt used to prepared them. (Fig. 1)
10/.m
I 0.025 g/cm 3 Figure 1. density.
0.133 g/cm 3
The size of the cell "windows" decreases with increasing foam
Mat. Res. Soc. Symp. Proc. Vol. 171. ©1990 Materials Research Society
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Composite foams were developed to reduce the cell size of the foam matrix by backfilling tfie cells of the polystyrene foam with materials such as aerogel, or resole.' The preparation of the composite foams have been reported on elsewhere.5 The polystyrene foam can be filled completely (Fig. 2,3), or the cell walls can be coated depending upon the concentration of filler. This method has been extended to other filler materials such as backflling with styrene and divinylbenzene to prepare foams of higher density. In the remainder of this paper we will examine some of the physical properties of the composite materials.
Fig.2
Fig. 3
Figure 2. Composite foams prepared by backfilling polystyrene emulsion foam with (10%) Si aerogel. Figure 3. Composite foams prepared by backfilling polystyrene emulsion foam with (6%) Resorcinol-Formaldehyde (RF).
RESULTS AND DISCUSSION Specific surface area measurements were made on a M
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