Six Things You Should Know About Syntactic Foam: A Primer

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Although more than 50 years old, syntactic foam is a game changing technology for a variety of industries and applications. Lightweight, strong, low in density, a great insulator and easily adaptable, it is becoming a disruptive force. Here are six things that you should know about syntactic foam - and why it should be on your list of materials for your next design project.

What is Syntactic Foam?

What is lighter than water, self-healing in nature, and exhibits amazing properties? Dubbed “syntactic foam” by Bakelite in 1955, this material is not what we typically think of as foam in other contexts. Syntactic foams are created by combining rigid, hollow spheres of glass, ceramic, polymer or metal (called microballoons or hollow microspheres) in a polymer, metal or ceramic matrix.

Features of syntactic foam that make it so appealing include:

• High compressive strength and stiffness
• Low density
• Buoyant
• Thermally insulating

Microspheres typically range from 10 to 200 microns in diameter and macrospheres from 1/4” to 1/2”in diameter. While they are small, they play a critical role in the performance of syntactic foams.

Why Use Syntactic Foam?

Ever since its inception, syntactic foam was found to have many inherent features and benefits. Beginning with its use in marine products to provide buoyancy without crushing or absorbing large amounts of water, these foams have found use across expanding applications for a variety of reasons:

Weight

Given that syntactic foams may consist of more than 50% porosity due to the use of hollow fillers, substantial weight savings are gained. Although all foams are porous, they do not exhibit the strength, stiffness and durability of syntactic foams.

Strength

Syntactic foams exhibit high levels of compressive strength, which can be further enhanced through the optimization of raw materials, mix formulations and processing. Compressive strength is important for foams exposed to heavy loading or hydrostatic pressure. Tensile strength is comparatively low, as the hollow fillers are often unbonded, sometimes by design. This may be improved by a chemical surface treatment of the particles and/or by adding fibrous fillers. A benefit of having spherical fillers, such as microballoons, is that the components produced are isotropic, having the same strength (and all other mechanical and thermal properties) in all directions.

Density

Syntactic foams typically have lower density and higher specific strength (strength divided by density) than other foams. A balance between microsphere volume fraction and wall thickness is important to obtain the desired density, and thereby, mechanical or thermal properties required.

Insulation

Syntactic foams are inherently good insulators, exhibiting low thermal conductivity due to their discretely porous structure. They offer significant engineering and economic advantages compared to conventional insulation e.g. including structural integrity at greater ocean depths, further adding to their value.

Tailorability

Tailorability is a major advantage in syntactic foam use. The matrix material can be selected from almost any metal, polymer, or ceramic. Microspheres are available in a variety of sizes and materials, with glass being the most widely used.

Controlling the diameter, size, distribution and wall thickness of the microspheres independently of each other provides several possible variables to tailor syntactic foam properties. Selection of an appropriate gas contained within the hollow particles is another design feature, e.g. a gas can be selected for optimizing fire resistance or dielectric properties. Other factors include matrix material, particle diameter and volume fraction. Slight modifications can greatly impact porosity, compressive properties, density, water absorption, coefficient of thermal expansion and thermal conductivity.

In another example of tailorability, syntactic foams have been reinforced with a second phase such as nanoclay, carbon nanotubes, carbon nanofibers, glass or carbon fibers and other types of particles to achieve desired mechanical and thermal properties, while retaining low density.

Applications for Syntactic Foam

Early polymer matrix syntactic foams were found in marine applications because of buoyancy and low moisture absorption. Today, with enhanced performance capabilities, applications have expanded to include:

• Rigid pipe insulation for deepwater oil and gas exploration
• Remotely operated underwater vehicles (ROVs)
• Autonomous underwater vehicles (AUVs)
• Boat hulls
• Aircraft radomes
• Cores for sandwich composites
• Soccer balls
• Automotive underhood components
• Spacecraft

Figure 1: The Deepsea Challenger was able to travel to the deepest known point in the Earth’s seabed thanks to syntactic foams. (Source: Dr. Nikhil Gupta, NYU Tandon School of Engineering)Specific applications are actually quite familiar to us from today’s headlines:

A Bluefin autonomous underwater vehicle (AUV) was used in the deepwater search for the remains of the Malaysia MH370 aircraft in 2014.

The Deepsea Challenger was a manned submersible piloted by James Cameron in 2012 to reach the ocean floor in the Mariana Trench off the coast of Guam. At a depth of seven miles, this is the deepest known point in the Earth’s seabed. Syntactic foam enabled an unusual craft design with structural strength, the integration of cameras, equipment and other accessories. Unmanned and manned vehicles operate for longer periods at greater depths with the help of syntactic foams.

The USS Zumwalt Navy destroyer, built in 2008, has syntactic foam throughout its superstructure to save weight. The result is a craft that is nimble and has a reduced radar signature for elusive navigation.

Figure 2: Created by Adidas for the 2014 World Cup in Brazil, the Brazuca soccer ball uses syntactic foam technology. (Source: Dr. Nikhil Gupta, NYU Tandon School of Engineering)The Brazuca soccer ball, designed by Adidas for the 2014 World Cup in Brazil, uses syntactic foam. The lighter ball returned to original shape after being kicked, providing greater accuracy and increased goal rates. Successive iterations of the ball design still incorporate syntactic foam.

What Do They Replace?

Syntactic foams can replace a variety of traditional materials including balsa wood, natural and synthetic foams, PVC, fiber-reinforced-plastics and metals. Legacy materials simply cannot offer the same level of strength, density and weight savings as a function of temperature and moisture content. Keep in mind that a typical microsphere shell can withstand a pressure of over 25,000 pounds per square inch before rupturing, more than 5 times the compressive strength of conventional concrete pavement.

How to Design with Syntactic Foams

Syntactic foams permit unusual and unique design freedoms:Figure 3: The syntactic foam layer inside Adidas’ Brazuca soccer ball. (Source: Dr. Nikhil Gupta, NYU Tandon School of Engineering)

• Thinner parts with equivalent or greater strength, yet lesser weight than conventional materials
• Custom shapes to follow contours and tightly clad objects
• Lightweight cores for sandwich panels
• Factory or field processing and repairs

Future Needs

Syntactic foam applications are expected to grow rapidly as the science and technology of these materials continues to evolve. Theoretical models and the results of a variety of research studies involving syntactic foams are now available. There is a push to improve/optimize syntactic foam formulations, particularly at the molecular level. The focus is on resins with lower water absorption, higher strength, greater heat resistance and microspheres with specific chemistries that improve properties. Surface treatments that improve bonding or ensure debonding are also being expanded. Apart from structural applications using bulk quantities of syntactic foams, specialized parts such as electronic components, sports equipment and aerospace/automotive parts can provide value-added growth markets for syntactic foam manufacturers.