Syntactic foams are composite materials synthesized by filling a metal, polymer, [1] cementitious or ceramic matrix with hollow spheres called microballoons [2] or cenospheres or non-hollow spheres (e.g. perlite) as aggregates. [3] [4] In this context, "syntactic" means "put together." [5] The presence of hollow particles results in lower density, higher specific strength (strength divided by density), lower coefficient of thermal expansion, and, in some cases, radar or sonar transparency.
The term was originally coined by the Bakelite Company, in 1955, for their lightweight composites made of hollow phenolic microspheres bonded to a matrix of phenolic, epoxy, or polyester. [6] [7]
These materials were developed in early 1960s as improved buoyancy materials for marine applications. [8] Other characteristics led these materials to aerospace and ground transportation vehicle applications. [9]
Research on syntactic foams has recently been advanced by Nikhil Gupta.
Tailorability is one of the biggest advantages of these materials. [10] The matrix material can be selected from almost any metal, polymer, or ceramic. Microballoons are available in a variety of sizes and materials, including glass microspheres, cenospheres, carbon, and polymers. The most widely used and studied foams are glass microspheres (in epoxy or polymers), and cenospheres or ceramics [11] (in aluminium). One can 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 manufacturing method for low density syntactic foams is based on the principle of buoyancy. [12] [13]
The compressive properties of syntactic foams, in most cases, strongly depend on the properties of the filler particle material. In general, the compressive strength of the material is proportional to its density. Cementitious syntactic foams are reported to achieve compressive strength values greater than 30 MPa (4.4 ksi) while maintaining densities lower than 1.2 g/cm3 (0.69 oz/cu in). [14]
The matrix material has more influence on the tensile properties. Tensile strength may be highly improved by a chemical surface treatment of the particles, such as silanization, which allows the formation of strong bonds between glass particles and epoxy matrix. Addition of fibrous materials can also increase the tensile strength.[ citation needed ]
Current applications for syntactic foam include buoyancy modules for marine riser tensioners, remotely operated underwater vehicles (ROVs), autonomous underwater vehicles (AUVs), deep-sea exploration, boat hulls, and helicopter and airplane components.
Cementitious syntactic foams have also been investigated as a potential lightweight structural composite material. These materials include glass microspheres dispersed in a cement paste matrix to achieve a closed cell foam structure, instead of a metallic or a polymeric matrix. Cementitious syntactic foams have also been tested for their mechanical performance under high strain rate loading conditions to evaluate their energy dissipation capacity in crash cushions, blast walls, etc. Under these loading conditions, the glass microspheres of the cementitious syntactic foams did not show progressive crushing. Ultimately, unlike the polymeric and metallic syntactic foams, they did not emerge as suitable materials for energy dissipation applications. [15] Structural applications of syntactic foams include use as the intermediate layer (that is, the core) of sandwich panels.
Though the cementitious syntactic foams demonstrate superior specific strength values in comparison to most conventional cementitious materials, it is challenging to manufacture them. Generally, the hollow inclusions tend to buoy and segregate in the low shear strength and high-density fresh cement paste. Therefore, maintaining a uniform microstructure across the material must be achieved through a strict control of the composite rheology. [16] In addition, certain glass types of microspheres may lead to an alkali silica reaction. Therefore, the adverse effects of this reaction must be considered and addressed to ensure the long-term durability of these composites. [17]
Other applications include;
A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions.
Foams are materials formed by trapping pockets of gas in a liquid or solid.
In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure or mixing with a catalyst. Heat is not necessarily applied externally, and is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.
Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, boron, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.
Wood-plastic composites (WPCs) are composite materials made of wood fiber/wood flour and thermoplastic(s) such as polythene (PE), polypropylene (PP), polyvinyl chloride (PVC), or polylactic acid (PLA).
A cenosphere or kenosphere is a lightweight, inert, hollow sphere made largely of silica and alumina and filled with air or inert gas, typically produced as a coal combustion byproduct at thermal power plants. The color of cenospheres varies from gray to almost white and their density is about 0.4–0.8 g/cm3 (0.014–0.029 lb/cu in), which gives them a great buoyancy.
Glass microspheres are microscopic spheres of glass manufactured for a wide variety of uses in research, medicine, consumer goods and various industries. Glass microspheres are usually between 1 and 1000 micrometers in diameter, although the sizes can range from 100 nanometers to 5 millimeters in diameter. Hollow glass microspheres, sometimes termed microballoons or glass bubbles, have diameters ranging from 10 to 300 micrometers.
Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes. For example, the particles of stone used to make concrete typically include both sand and gravel.
In materials science, a metal foam is a material or structure consisting of a solid metal with gas-filled pores comprising a large portion of the volume. The pores can be sealed or interconnected. The defining characteristic of metal foams is a high porosity: typically only 5–25% of the volume is the base metal. The strength of the material is due to the square–cube law.
Air entrainment in concrete is the intentional creation of tiny air bubbles in a batch by adding an air entraining agent during mixing. A form of surfactant it allows bubbles of a desired size to form. These are created during concrete mixing, with most surviving to remain part of it when hardened.
Microparticles are particles between 0.1 and 100 μm in size. Commercially available microparticles are available in a wide variety of materials, including ceramics, glass, polymers, and metals. Microparticles encountered in daily life include pollen, sand, dust, flour, and powdered sugar.
Self-healing materials are artificial or synthetically created substances that have the built-in ability to automatically repair damages to themselves without any external diagnosis of the problem or human intervention. Generally, materials will degrade over time due to fatigue, environmental conditions, or damage incurred during operation. Cracks and other types of damage on a microscopic level have been shown to change thermal, electrical, and acoustical properties of materials, and the propagation of cracks can lead to eventual failure of the material. In general, cracks are hard to detect at an early stage, and manual intervention is required for periodic inspections and repairs. In contrast, self-healing materials counter degradation through the initiation of a repair mechanism that responds to the micro-damage. Some self-healing materials are classed as smart structures, and can adapt to various environmental conditions according to their sensing and actuation properties.
Filler materials are particles added to resin or binders that can improve specific properties, make the product cheaper, or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity, etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. Particulates and fibers are the main subgroups of filler materials. Particulates are small particles of filler that are mixed in the matrix where size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.
A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.
Ultralight materials are solids with a density of less than 10 mg/cm3, including silica aerogels, carbon nanotube aerogels, aerographite, metallic foams, polymeric foams, and metallic microlattices. The density of air is about 1.275 mg/cm3, which means that the air in the pores contributes significantly to the density of these materials in atmospheric conditions. They can be classified by production method as aerogels, stochastic foams, and structured cellular materials.
Expandable microspheres are microscopic spheres comprising a thermoplastic shell encapsulating a low boiling point liquid hydrocarbon. When heated to a temperature high enough to soften the thermoplastic shell, the increasing pressure of the hydrocarbon will cause the microsphere to expand. The volume can increase by 60 to 80 times.
Robocasting is an additive manufacturing technique analogous to Direct Ink Writing and other extrusion-based 3D-printing techniques in which a filament of a paste-like material is extruded from a small nozzle while the nozzle is moved across a platform. The object is thus built by printing the required shape layer by layer. The technique was first developed in the United States in 1996 as a method to allow geometrically complex ceramic green bodies to be produced by additive manufacturing. In robocasting, a 3D CAD model is divided up into layers in a similar manner to other additive manufacturing techniques. The material is then extruded through a small nozzle as the nozzle's position is controlled, drawing out the shape of each layer of the CAD model. The material exits the nozzle in a liquid-like state but retains its shape immediately, exploiting the rheological property of shear thinning. It is distinct from fused deposition modelling as it does not rely on the solidification or drying to retain its shape after extrusion.
Nikhil Gupta is a materials scientist, researcher, and professor based in Brooklyn, New York. Gupta is a professor at New York University Tandon School of Engineering department of mechanical and aerospace engineering. He is an elected Fellow of ASM International and the American Society for Composites. He is one of the leading researchers on lightweight foams and has extensively worked on hollow particle filled composite materials called syntactic foams. Gupta developed a new functionally graded syntactic foam material and a method to create multifunctional syntactic foams. His team has also created an ultralight magnesium alloy syntactic foam that is able to float on water. In recent years, his work has focused on digital manufacturing methods for composite materials and manufacturing cybersecurity.
Titanium foams exhibit high specific strength, high energy absorption, excellent corrosion resistance and biocompatibility. These materials are ideally suited for applications within the aerospace industry. An inherent resistance to corrosion allows the foam to be a desirable candidate for various filtering applications. Further, titanium's physiological inertness makes its porous form a promising candidate for biomedical implantation devices. The largest advantage in fabricating titanium foams is that the mechanical and functional properties can be adjusted through manufacturing manipulations that vary porosity and cell morphology. The high appeal of titanium foams is directly correlated to a multi-industry demand for advancement in this technology.
Self-healing concrete is characterized as the capability of concrete to fix its cracks on its own autogenously or autonomously. It not only seals the cracks but also partially or entirely recovers the mechanical properties of the structural elements. This kind of concrete is also known as self-repairing concrete. Because concrete has a poor tensile strength compared to other building materials, it often develops cracks in the surface. These cracks reduce the durability of the concrete because they facilitate the flow of liquids and gases that may contain harmful compounds. If microcracks expand and reach the reinforcement, not only will the concrete itself be susceptible to attack, but so will the reinforcement steel bars. Therefore, it is essential to limit the crack's width and repair it as quickly as feasible. Self-healing concrete would not only make the material more sustainable, but it would also contribute to an increase in the service life of concrete structures and make the material more durable and environmentally friendly.
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