Post 744 –by Gautam Shah
Foams are air-entrained objects, where nominally a gas is trapped in solids or liquids. Foams, are called ‘examples of dispersed media’. The gas, the main dispersing agent is divided into bubbles of different sizes, called ‘polydisperse’. Foams are usually disordered with many different sizes of bubbles. In liquid-foams the bubbles continuously resize through collapse or assimilation. When the dispersed medium is very thin, it is called a type of colloid. Though some claim, ‘A Foam is a colloidal solution of liquid and gas, with liquid acting as dispersing medium, and the gas acting as the dispersed phase. So it cannot definitely be said that it is a solid, liquid, or gas. It is a colloidal solution’.
Foams are nominally, air entrained compounds, but, the air (or a vacuum) pockets could be continuous- interconnected, or could be isolated. In the first case, the structure is fairly stiff (non-compressible), so, called solid foams (just a ‘light-weight hollowed mass’). In the second case, for the soft foams, the structure is not stiff, it can collapse, and which may or may not recover to the original shape.
Aerogel is a synthetic ultra-light ‘porous’ material (98.2% air) that is derived, when a liquid is replaced by air. The result is a solid with extremely low density and low thermal conductivity. Aerogel is a synthetic porous ultra-light material derived from a gel, in which the liquid component of the gel has been replaced with a gas without significant collapse of the gel structure. It results into a solid form of extremely low density. Such materials show extremely low thermal conductivity. These have been called (mainly due to the translucent mass) frozen smoke, solid smoke, solid air, solid cloud, and blue smoke. Silica Aerogel feels like fragile expanded polystyrene to the touch, while some polymer-based Aerogels feel like rigid foams.
Froth: The fizz on beer or the soap suds (bubbles) in bath water, are foams, a mixture of gas and liquid. Whisking the milk and egg-whites produces foam, air bubbles as an emulsion. Blowing a gas through a molten plastic fills it with bubbles, and when the plastic cools and solidifies, the gas bubbles are trapped inside, making a foamed-plastic, used for filling cushions, and packing of goods.
Materials have three states or phases of matter, namely Gas, Liquid, and Solid. These states also denote the structural rigidity and resistance to change of shape or volume. The states or phases are due to the temperature and pressure.
One of the earliest realizations of a foamed structure was the pumice stone (very porous, froth like volcanic glass, cooled fast without the crystallization). It is used as an abrasive for cleaning (rubbing the body-hair and fabrics), polishing, and as scouring compound. Another was the dried froth, spongy material (locally, W. India, called Ush) that gets deposited against the slightly alkaline shores of rivers and rivulets. It was used for washing the clothes. Ceramic materials, such as bricks are produced by addition of organic fibers (typically rice husks, chopped hay or mustard leaves and stalks) into the mud. On firing the organic material burns out leaving a lighter and hollowed mass.
Right from early days, the rubber-based foams had issues of density, heat dissipation from the mass, deterioration of the material due to defective vulcanization and release of VOCs. Some of the issues were partly solved by providing hollow cores on the back-face, which achieved additional compressibility (even with high density structure) and better aeration. The foams were excellent materials for thermal insulation, floating devices, packing, padding and stuffing of toys.
Foamed or Cellular materials are called expanded plastics or foams. These are made in various types, from soft and flexible to hard and rigid. There are three types of cellular plastics.
• Blown, an expanded matrix such as in a natural sponge.
• Syntactic, encapsulation of hollow organic or inorganic micro-spheres (or nodules) in a matrix.
• Structural are composed or formed materials with dense outer skin surrounding a foam core.
✓ Gas in Liquids or Liquid foams These are like, froth, aerated soda, washing soap suds. These become little more stable or last longer, when a stabilizer or surfactant is present or added. In foods Proteins (eggs, oils, gums) act as foaming agents. Nearly all fermented foods are like foams. In aerated drinks and fire extinguishing foams, Carbon dioxide is dissolved. Foaming makes many foods lighter. Gas-liquid foams have high surface area and is exploited for flotation and foam fractionation. Many foaming conditions are not always desired, such as lubricating oils, chemical processes. To break the foams air releasing agents or skimmers (blades moving over the surface).
✓ Gas in Solids or Solid foams. These are like breads, cakes, rubbers, polystyrene (Thermocole), polyurethane, paper pulp, slag, ceramic foam, sponge iron and tantalum or titanium sponge (for prosthetics). Solid foams are mostly used as lightweight cellular engineering materials.
✓ Solid foams are of two classes, based on their pore structures. Foams with open-cell structures, called reticulated foams, where the gas pockets are connect to each other. Foams with closed-cell structures, called cellular solids, where the gas is trapped into discrete pockets, each completely surrounded by the solid material. A bathing sponge is an open-cell foam, the water can be sucked in as well as squeezed out. A shoe base or sole is a closed-cell foam, though ‘porous’, cannot soak water as the gas pockets are sealed and separated from each other.
✓ There are other classes of closed-cell foams, known as syntactic foams. These have embedded hollow particles or nodules placed in a matrix material. The nodules are made of glass, ceramic, polymers, etc. Such syntactic foams offer very high strength-to-weight ratio and so are useful for deep-sea and space applications. Memory Foams also use a syntactic shape-memory polymer, as a matrix.
✓ Foams can be forms of Composites. It is a product that is made by mixing fibers like straw, hair, coir, hemp, jute, papyruses, rice-husk etc. into fillers, like latex, mud, gum, etc. The filler fibres are arranged into patterns such as, random, unidirectional (aligned in a single direction), multi-directional (oriented in two or three directions) or spaced continuous strands. There is no compaction of the mass. On curing or hardening the mass has cavities that gives an identity of air-filled material. The materials are not very compressible, but have resilience. Such composites can absorb shocks, vibrations, sound and in few instances water or moisture. Rubberized-Coir fibre sheets are used with polyurethane foams and other foams for mattress making.
✓ Aluminium foam sandwich (AFS) is a product made of two metallic dense face sheets and a metal foam core made of an aluminium alloy. It is produced, once the raw MMC (Metal Matrix Composite) is melted, then transferred to the foaming apparatus, where gas is injected into the melt and dispersed using either rotating impellers or vibrating nozzles. Such panels are used as insulation face in very hot environments, as sound damping layer, reduced weight, increased energy absorption in case of crashes, and in military operations to combat the concussive force of IEDs.
✓ Integral skin foams, also known as self-skin foams, have a high-density skin and a low-density core of the same material. Examples of integrated skin foams include, furnishing fabrics like rexines, insulated rails and handles, arm rests, baby seats, shoe soles, and mattresses. Exterior sandwich or composite panels (Aluminium composite panels -ACP), are used for external and internal architectural cladding, partitions, false ceilings, signage, machine coverings, container construction, etc. Materials like ACP may not be a sandwich construction, but through application like a coating on one face (that foams and insulates). ACPs often have highly combustible Polyethylene (PE) core, which was the principal cause of the rapid spread of flame in the 2017 Grenfell Tower fire in London. Similarly the coating on the the aluminium sheets with polyvinylidene fluoride (PVDF), fluoropolymer resins (FEVE), or polyester paint may contribute to combustibility and spread. In packaging, fluted polypropylene boards and polypropylene honeycomb boards are used for impact resistence. Ship cabins use lightweight door shutters, table tops, cupboards and shutters formed of wood veneers with core of foam.
✓ Under appropriate conditions almost any Polymer resin, thermoplastic or thermosetting can be foamed. Plastics, commonly foamed include vinyl, polystyrene, polyethylene, phenolic, silicone, cellulose acetate, and urethane, polystyrene or polypropylene. Hydroxyl terminated polyethers are often used to prepare flexible foams, typically for furniture cushioning. Hydroxyl-terminated polyesters, are popular for making rigid foams such as those used in custom packaging of appliances.
✓ Unlike many polymer foams, metal foams remain deformed after impact and can therefore only be deformed once. Reticulated foam, compressible and porous material. For orthopedic applications, tantalum or titanium foams are used for their tensile strength, corrosion resistance and bio-compatibility.
✓ Many natural substances such as rocks and soil (e.g. aquifers, petroleum reservoirs), zeolites, biological tissues (e.g. bones, wood, cork), and man made materials such as cements and ceramics can be considered as porous media and not as Foams.
Foams are used as shock absorbers and vibration retarders. There are many other shape configurations that absorb the impact. Air-filled constructions or compositions, such as the air-filled tubes and bubbles, tubes and tyres, fibers like carded cotton, rayon, coconut, wool, hay, straw and bird-feathers are pressed into mats or sheets. Mineral wool is a fibrous mass formed by spinning or drawing the molten mineral or rock materials such as slag, glass and ceramics. These have resilience as well compressibility. Multi layered corrugated papers, paper pulp formed items (egg crates), layered beds of hair (brushes), coiled springs, pre-stressed and shaped shock absorbers of spring steels, etc.