Foam and fiber boards
Foams and fibers with a small amount of matrix (the gaps between them predominate) belong to third type composites – composites with a gas dispersion. This dispersion is usually ordinary air, it can also be (with closed pores) hydrogen, nitrogen or water vapour. These composites are generally not being prepared to improve the mechanical properties of materials, but above all for other synergy effects. It often deals with about reducing the thermal conductivity, or increasing the vibration control or sound.
Foam materials are formed as artificial microcavites in the matrix filled with a gas. These microcavities are usually approximately spherical, closed in small quantities and can include any gas; in the case of large quantities they are linked together and open micropores are formed into the outer space. This group of composite materials includes:
- Polystyrene foams (Polyfoams)
- Metal foams
- Ceramics foams
Polystyrene foams (Polyfoams)
Polyfoams represent the most known foam materials, particularly a polystyrene foam is often used. Basically any plastic material can be frothed up. Besides polystyrene, other widely used foamed materials are polyethylene, polypropylene, polyvinyl chloride and polyurethane.
Foamed (expanded) polystyrene is very often used as heat insulating material, whereits thermal conductivity depends primarily on its density. It prevents the passage of the heat by conduction and convection very well, but transfers a relatively large amount of infrared (heat) radiation. Therefore, its characteristics can be improved by adding a small amount of graphite, which absorbs the penetrating radiation. Polystyrene is very fragile and the bulk of its deformation under stress is irreversible.
Foam polyethylene and polypropylene are therefore more suitable for constructionpurposes, (foamed polypropylene-significant portion of the deformation that occurs during repeated pressure is reversible). Therefore, polyethylene and polypropylene foams are increasingly used in the automotive industry.
Polyurethane as a plastic material with a lower stiffness especially is suitable as asoundproof material.
In principle, it is possible to distinguish three types of polyfoams:
- rigid and brittle – expanded polystyrene
- rigid and tough – polyethylene foam and polypropylene foam
- flexible and tough – polyurethane foam
Metal foams are one of the promising materials of the future. Currently they are primarily being prepared from aluminium and its alloys or titanium and its alloys. Compared to dense metals the behaviour of metal foams differs in many details.
- The behaviour at the loading point – while massive metal plastic deformation in a relatively large radius of a tip occurs, in metal foam mainly compaction under the tip immediately occurs.
- The insensitivity of metal foams to notches – they are due to its structure completely insensitive to notches (unlike massive metals)
- The energy absorbed during compression – metal foams, unlike the massive metals, absorb during loading significantly more energy, and therefore are suitable as part of various body armour or shock absorbing systems, this property depends on the amount of pores in the composite.
- The electrical conductivity of metal foams decreases with increasing number of pores, especially open pores.
- There are also differences under loading pressure and it also strongly differ during the flow of massive metal at a pressure or tensile stress
An example of a ceramics foam can be a composite with a matrix of SiC, which has 90 % of the pores at a density of 0.3 g.cm-3 and is applicable to 1500 °C. It is used for heat exchangers (it has thermal conductivity about the same as steel), noise silencers, vibration absorbers, rocket nozzles, and turbine blades. It comes under the name DUOCEL.
Carbon foam is so-called net glassy carbon which is applicable up to 350 °C in the air and up to 3500 °C in a non-oxidizing atmosphere. It is especially useful for high temperature filters, vibration dampers, porous electrodes for galvanic processes, and advanced batteries.
Fiber boards with a binder
When we reduce the quantity of the matrix below about 20 % in the fiber composite, it is no longer enough to fill all the gaps between the fibers and we receive a composite with small quantities of the matrix (usually only a few percent) and a double dispersion – fiber dispersion, which is usually 10 to 30 % and pores between the fibers, which are typically 70-90 %.
Especially ceramic fiber boards are often formed for high temperatures. The binder – matrix may be either organic (starch etc.) or inorganic (heat-resistant cement). The organic matrix when first used at high temperature carbonizes and changes to a carbon and its compounds.
A good example is the Fiberfrax, a fiber with a diameter of 3 – 4 mm from mullite for manufacturing boards and simple construction components applicable up to 1250 °C. These products are highly suitable for thermal insulation.
Similar composites are made of fibers labelled as IsoFrax containing more SiO2 and MgO instead of mullite.
Fiber boards without binders
For very demanding applications, especially in space technology, it is a block of ceramic fibers without binding or with a minimum of binding identical to the material of fibers that is sintered at a high temperature so that a block of fibers bound together by diffusion bridges occurs.
The current structure (design) of space shuttles uses as a thermal insulation of the surface blocks such composites with 10 % silica fibers and 90 % of the pores in two versions. For temperatures above 700 °C, the blocks are covered with a borosilicate layer (black, for good heat radiation) and for a temperatures below 700 °C, the blocks are covered with a layer of aluminium oxide (white).
As a newly developed material, graphite fibers are sintered in a similar way.