Carbon Foam: A Next-Generation Structural Material
Researchers at Touchstone Research Laboratory Ltd. struck a chord with the U.S. Dept. of Defense when they demonstrated the uniqueness of their CFOAM, product. The material, made from coal under controlled conditions, is now being investigated for use in a wide range of military applications from ship bulkheads, smokestacks and blast deflectors to armor, thruster nozzles and stealth materials.
The material is an enabling technology for a host of next-generation material systems and components according to Touchstone's president Brian Joseph. It can be used as the core material in a structural composite that makes ships more fireproof, as a firewall in airplanes and as energy absorbing structural components in automobiles. The foam material performs well in a marine environment, unlike many metals, which are susceptible to corrosion in such an environment.
The carbon foam is an attractive alternative material to traditional material in many applications due to its unique properties. For example, you could make an oven out of the foam, and it could serve both as the insulation and as the actual oven structure. The black foam feels like foam insulation, but is a thousand times stronger; one square inch of the material could support the weight of a full size automobile without crushing. It also is possible to have a flame (from an acetylene torch, for example) impinge directly onto the foam without initiating combustion of the material. Such properties are what make the material so different to work with.
The list of possible applications is long and varied. The carbon foam conceivably can replace balsa wood, intumescent mats, polymer matrices, metallic honeycombs, ceramic fibrous insulation, ceramic tile polystyrene, plastics, fiberglass, rubber and various metals that currently are in use as basic, conventional materials of construction.
Every conventional material has some limitations that can make it less attractive than carbon foam. For example, high material cost, structural limitations, fire hazards, foreign sourcing, corrosion susceptibility, and weight are problems associated with many traditional materials that can be overcome using carbon foam. As a result, the material could become the future material of choice when shipbuilders are designing vessels and aeronautical engineers conceptualize the next fighter for U.S. defense. Major aerospace companies, ship builders, automotive companies and the home construction industry have inquired about using the foam material in different applications because it will not burn and because it presents significant opportunities for cost savings and extended life cycle.
The coal-based material can be made having a wide range of densities from 5 to 40 lb/ft3 (0.08 to 0.64 g/cm3) and having tensile strengths approaching 3,000 psi (20 MPa). The foam can be made either as an electrical conductor or an electrical insulator. While the foam currently is produced as a thermal insulator, it also is possible to manufacture it as a thermal conductor.
The foam also can be graded in density and pore structure through its thickness to provide localized stiffness and thermal expansion control while maintaining an overall weight-efficient structure. As a result, the foam is being evaluated for use commercially as well as for defense. Possible commercial uses include structural panels and firewalls for automobiles, fireproof attic doors on new houses, and even in recreational equipment like canoes.
The material's versatility is based on properties/characteristics including:
- Low cost; the foam is produced from coal, which is inexpensive and readily available
- Product forms; CFOAM is manufactured both as panels of various thicknesses and as foamed-to-shape parts
- Fire resistance; after heat treatment, the foam generally does not contain a sufficient amount of volatile material to support combustion
- Thermal conductivity; low-temperature heat treatment versions of the material have thermal conductivities of less than 1.0 W/mK, roughly equivalent to rock wool. When graphitized, conductivity is in the range of 70 W/mk approaching that of metals.
- Thermal expansion; the foam has an extremely low coefficient of thermal expansion (typically from 0.5 and 6.5 x 10-6/C), which can be tailored
- Thermal stability; the material can be used in air to a temperature of about 540 C (1000 F), and in an inert environment to 3000 C (5430 F).
- Design flexibility; properties can be readily engineered to meet different requirements, such as density, cell size and cell connectivity, and its mechanical properties can be varied over a broad range.
- Finishing; the material can be integrated with other materials, such as lamination with fiber reinforced face sheets, impregnation with resins or metals and thermal spraying with metals or ceramics.
- Machinability: the foam is easily worked with most woodworking tools or machining equipment.
- Formability; it can be foamed to virtually any shape.
- Joining; the foam can be joined by means of pitch bonding and heat treatment or, more simply, using low-temperature cure graphite-phenoic adhesives, allowing the use of carbon-foam building blocks to create larger or more intricate structures, as well as repair to damaged structures
- Impact absorption; the foam outperforms conventional polymer foams in mechanical properties and impact resistance.
These properties/characteristics combined with the fact that production of the material is based on a readily available low cost precursor have attracted attention for using the material for defense purposes. Combustion-resistance is one of the key attractions of the material for use in military and commercial applications; the material will not support ignition. Its strength and weight properties and its possibilities as a fire-protection material could lead to incorporation of the carbon foam in ships where on-board fires always are a concern, in aircraft and in space applications where weight problems always exist.
There also is a great deal of interest in the material for crash protection. For example, race cars have crashed at speeds of 200 miles per hour and the driver walked away. This same protection could be used in small aircraft, which often hit the ground at much slower speeds. Until now, materials have not been available for designers to create airplanes for impact absorption. Tests results show that CFOAM performs better than conventional polyurethane foams that are currently used for impact absorption.
The U.S. Army, Air Force and Navy are aware of the possibilities of using this carbon-foam material and are supporting additional research, which has lead to the creation of pilot plant facilities to manufacture the foam. Touchstone's pilot plant at Triadelphia was commissioned this fall and has the capacity to make 40 tons of foam per year. A full-scale plant is on the drawing board.
CFOAM is a next-generation structural material offering a wide range of user benefits. It is inexpensive, lightweight, fire resistant, impact absorbing, can be thermally insulating or conducting, and has an electrical conductivity that can be varied over seven orders of magnitude.
Touchstone's innovative technology transforms high-sulfur bituminous coal, using a proprietary process, into a lightweight, strong, fire-resistant and thermally insulating material. The material potentially could be suitable for use in a number of applications including U.S. Navy ships and other military vehicles, thermal protection for future spacecraft and aircraft, ballistic impact protection for law-enforcement vehicles, energy-absorbing car bumpers, and fire-proof walls for the construction industry.
Two principal microstructure types possible are open, reticulated foam and cellular foam. Reticulated foam (Fig 1) can be produced in densities between 0.08 and 0.80 g/cm3 (5 to 50 lb/ft3). They graphitize readily to exhibit high thermal and electrical conductivities for use in thermal management and electrical applications. In addition, they offer carbon fiber-like performance in the ligaments that define each foam cell and, thus, are an excellent reinforcement choice for resin systems.
The cellular foams were developed for use as a structural core, where it is not desirable for resin or adhesive systems to penetrate the foam significantly; for example, where the foam is used as a fire-resistant core, separating polymer-based structures. Adhesion to the polymer-based structures is critical, but penetration of a flammable resin through the foam thickness would eliminate the foam's fire protection benefit. Such foams are composed of sintered, hollow spheres (Fig 2), with only small pinholes in the cell walls and are available in densities between 0.35 and 0.80 g/cm3 (22 to 50 lb/ft3). They have very low fluid permeability and thicker, more complete cell walls. Less graphitizable than their reticulated counterparts, they are more suitable for use in structural and thermal insulator applications.
Typical mechanical properties for CFOAM material having a density of around 0.4 to 0.5 g/cm3 (25 to 31 lb/ft3) include:
- Compressive strength of 2,200 to 3,000 psi (15 to 20 MPa)
- Compressive modulus of around 80,000 psi (550 MPa)
- Tensile strength of 300 to 1,000 psi (2 to 7 MPa)
- Tensile modulus of 100,000 to 200,000 psi (689 to 1,379 MPa)
- Shear strength of 300 psi (2 MPa)
- Impact resistance of 0.3 to 0.4 ft lbf/in.2
Mechanical properties, especially compressive strength, are strongly related to foam density. In compression, the foam fails by a sequence of shear and compressive buckling. Tensile strength varies with material density and the heat treatment used to produce the material as shown in Table 1.
CFOAM typically is supplied in the calcined, or carbonized, form. That is, it has been heat treated at a temperature sufficient to remove the majority of the volatile matter and make the material resist ignition, but not sufficient to develop graphitic order.
When heat treated to a very high temperature to develop graphitic order within the foam, thermal conductivity increases greatly as shown in the table for low and high density products.