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Syntactic foams are particulate composite materials synthesized by filling a metal, polymer or ceramic matrix with hollow particles called microballoons, "syntactic" meaning "put together". The presence of hollow particles results in lower density, higher specific strength (that is the strength over the composite density), a lower coefficient of thermal expansion, and, in some cases, radar or sonar transparency.
Tailorability is one of the biggest advantages of these materials. The matrix material can be selected from almost any metal, polymer or ceramic. A wide variety of microballoons are available, including cenospheres, glass microspheres, and carbon and polymer microballoons. The most widely used and studied foams are glass microballoon-epoxy, glass microballoon-aluminium, and cenosphere-aluminium.
The compressive properties of syntactic foams, in most cases, strongly depend on the properties of microballoons, whereas the matrix material has more influence on the tensile properties. The tensile strength may be highly improved by a chemical surface treatment of the particles, like the silanization which allows the formation of strong bonds between glass particles and epoxy matrix. There are ways of adjusting the properties of these materials. One can either change the volume fraction of microballoons or use microballoons of different effective density, the latter depending on the average ratio between the inner and outer radii of the microballoons. A more recent product adds fibrous materials to increase the tensile strength. In general, the compressive strength of the material is proportional to its density.
These materials were developed in early 1960s as buoyancy aid materials for marine applications; the other characteristics led these materials to aerospace and ground transportation vehicle applications. Current applications for syntactic foam include buoyancy modules for marine riser tensioners, boat hulls, deep-sea exploration, autonomous underwater vehicles (AUV), parts of helicopters and airplanes, and sporting goods such as soccer balls. Structural applications include the use of syntactic foams as intermediate layer (that is, the core) of sandwich panels.
Other applications include;
- Deep sea buoyancy foams
- Thermoforming plug assist
- Radar transparent materials
- Acoustically attenuating materials
- Blast mitigating materials
See also 
- Shutov, F.A. (1986). "Syntactic polymer foams". Advances in Polymer Science. 73-74: 63–123.
- "What is Syntactic Foam?". Cornerstone Research Group. Retrieved 2009-08-07.
- Bardella, L.; Genna F. (2001). "On the elastic behavior of syntactic foams". International Journal of Solids and Structures 38 (2): 7235–7260. doi:10.1016/S0020-7683(00)00228-6.
- Kudo, Kimiaki (January 2008). "Overseas Trends in the Development of Human Occupied Deep Submersibles and a Proposal for Japan’s Way to Take" (PDF). Science and Technology Trends Quarterly Review 26: 104–123. Retrieved 2009-08-10.
- Karst, G (2002). "Novel Processing of High-Performance Structural Syntactic Foams". Society for the Advancement of Material and Process Engineering. Retrieved 2009-08-07.
- Thim, Johann (3 February 2005). "Performing Plastics - How plastics set out to conquer the world of sports". European Chemical Industry Council. Retrieved 2009-08-10.