Syntactic Foams 101
What is syntactic foam?
Although more than 60 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 foams – and why it should be on your list of materials for your next design project.
Six things you need to know about syntactic foams
1. What is a syntactic foam?
A syntactic foam is a composite material having at least two distinct phases; this includes a cellular and binder component; see Fig. 1. The cellular phase is usually a hollow spheres that is then dispersed in the binder phase, typically an epoxy.
Features of syntactic foam that make it so appealing include:
- High strength
- Low density
- Thermally insulating
- Dissipates high-energy impacts
- Tailorable to meet a wide variety of design criteria
2. What are the most common porous phases?
Hollow microspheres typically range from 10 to 200 micrometers in diameter. An average hollow microsphere is about the diameter of a human hair. While they are small, they play a critical role in the performance of syntactic foams. There are many types of hollow microspheres, metal, polymer and ceramic but by far, the most common are hollow glass microspheres (HGMS).
Solid glass has a density of ~ 2.5 g/cm3. Because of the nature of the manufacturing process, HGMB are spherical and have a density between 0.12 and 0.60 g/cm3. Manufacturers can control the density to ±0.02 g/cm3. Hollow fly ash is becoming more popular because it is viewed as a natural and/or recycled material. Also known as, cenospheres, these hollow microspheres are a by-product of coal-fired power plants. Cenospheres are relatively inexpensive but are lower performing compared to a similar density hollow glass microsphere.
Hollow macrosphere are also very common especially in deep-sea buoyancy and energy absorption applications. The spheres range in size from 3 millimeters (mm) to >50 mm and are made from a fiber-based composite. Epoxy is the most common matrix material and the reinforcing fiber can be carbon, glass or naturally occurring fibrous minerals such as wollastonite.
3. What are the benefits of an anhydride-cured epoxy as a binder for syntactic foams?
Epoxies are the most common binder materials used in syntactic foams. These epoxies are cured by either amines or anhydrides and the selection of the curative critical to the performance of syntactic foams. Amines are convenient because they do cure at room temperatures but they have lower performance, lower strength and glass transition temperature (Tg).
Comparatively, anhydrides need heat to activate the cross-link reaction but have significantly higher strength and Tg. Anhydride curatives are used in those applications with very aggressive services conditions that demand the highest performing syntactic foams. For example, most deep-sea syntactic foam products are epoxies cured with an anhydride curative or a blend of two or more anhydride curatives. The blending of anhydrides is an attempt to optimize flexibility and high Tg in the final epoxy.
Besides a higher Tg, anhydride curatives have other advantages over amine-cured epoxies and even vinyl esters. Anhydrides do not introduce hydroxyl or other polar groups in the cured epoxy chain. For example, in amine-cured epoxies, hydroxyl groups form during the curing reaction. These become sites where water, a polar molecule, can react via hydrogen bonding and reduce performance. The absence of polar groups using an anhydride curative leads to lower moisture uptake thus better hygroscopic properties. In addition, epoxies cured with an anhydride have a lower exotherm allowing for smaller residual stresses and the ability to fabricate larger components.
4. What are the various syntactic foam design options?
Material designers can be very creative when designing the microstructures of a syntactic foam. In general, a syntactic foam can be two or three-phase, see Fig. 2. A two-phase syntactic foam can is made of hollow microsphere and a binder phase. A three-phase syntactic foam also has hollow microspheres and binder but there is not enough binder phase to totally fill the interstitial volume between the microspheres, thus leaving voids between the spheres. This void is known as interstitial or unreinforced voids. The void found within the microsphere is known as reinforced void.
5. What are the strengths of syntactic foams?
Tailorability is a major advantage in syntactic foam use. The matrix material can be selected from almost any metal, polymer, or ceramic. Hollow microspheres are available in a variety of sizes and materials, with hollow glass being the most widely used.
Controlling the diameter, size, distribution and wall thickness of the microspheres affects density. Selection of an appropriate gas contained within the hollow particles is another design feature that helps in tailoring syntactic foam to specific applications. For example, a gas can be a selection 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.
6. What are the current and future applications of syntactic foams?
Professor Nikhil Gupta from NYU’s Tandon School of Engineering published a review article on the applications of syntactic foam.
Types of Applications of Syntactic Foams include:
- Rigid pipe insulation for deep water oil and gas exploration and production
- Remotely operated underwater vehicles (ROVs)
- Autonomous underwater vehicles (AUVs)
- Boat hulls
- Aircraft radomes
- Cores for sandwich composites
- Soccer balls, golf ball cores, archery bow cores and other sporting equipment
- Automotive under hood components
- Light-weighting components in commercial aircraft
- Spacecraft ablative heat shields and thermal protection systems