Some companies have developed carbon bonded carbon fiber insulation board made by mixing chopped viscose rayon fibers about 2mm long together with ground reworked CBCF, into an aqueous slurry, adding a phenolic resin and binder. It is then filtered and vacuum-formed into slabs, dried and carbonized at 1000C. The process gives a 50% yield and is followed by a further heat treatment to a temperature about 200C above the maximum intended use temperature to minimize outgassing. The final product has a carbon content>99.9% and the insulative properties are governed by the porosity.
The material must not be used in the presence of air above 200C. These products are particularly useful for vacuum furnace insulation and can be used in vacuum or in the presence of an inert gas up to 2750C.
Carbon felts can be used as an insulation material, made from scrap material which is relatively cheap, where further carbonization continues in use. Under these conditions, the possibility of shrinkage and ourgassing products has to be accommodated. Superior, but expensive, carbon and graphite felts can be made by carbonizing long small diameter organic filaments, which are very stable and have low shrinkage.
In aircraft, fire protection can be offered to the cabin by using a layer of insulating batting between the aircraft skin and the cabin interior panels. The fuselage insulation is placed in bags to resist moisture penetration and the bas are fitted between the outer skin and interior trim panels.
The fiber offers fire resistance and high thermal insulation together with low smoke emission, low electrical conductivity and a weight saving. Current applications include aircraft fuselage thermal insulation, aircraft fire blockers, fire protective clothing, personal insulation and fire retardant insulation boards for special lightweight applications.
Work undertaken at Auburn University describes the basic technology as a heat treatment of a crimped opf fiber in an inert atmosphere at a temperature of 600-700C. The work involved preparation of battings for an improved insulating layer in, for example, military clothing and started with a cloth woven made from 12k opf, heat treated at 600C, deknitted, staple cut to about 75 mm, opening up the staple and blending with polyester, rumbling in a prefeeder followed by carding. The opened fibers were then made into battings using a Rando Webber and bonding was achieved by passing through an oven at 165C. Batt densities of 1.6-2.4 kg/m3 were achieved, controlling the density by the compression applied during the thermal bonding stage. Due to the spring-like crimp, the battings were very resilient, exhibiting hardly any permanent set after repeated loadings.
The performance is due to the non-conducting nature of the fiber, whilst the high temperature emissivity as a black body radiator provides a cooling mechanism in a high temperature flame. Since Curlon is about 9 um in diameter, it can be classed as a microfiber and, therefore, is a good insulator. The loss on ignition of the basic fiber is about 55%. About 25% of a polyester fiber binder can be incorporated to thermobond to thermobond the lightweight battings and a water repellent coatings is added to enhance the water repellency.
Comparing this with the best aircraft glass insulation, which has a K value of 0.039Wm/K at 6.72 kg/m3, shows that the Curlon has the potential for a weight saving of 33-50% in aircraft insulation and related applications.
It is believed that the fire resistance is enhanced by the ability to act like a flame arrester.
When the flame hits the batting, the flammable fiber pulls away from the flame leaving a Curlon gauze in front of the batting. The Curlon gauze does not shrink or pull back, only slowly oxidizing in the flame. It is non-melting and has an insulating property and through its good emissivity, throws some the heat back as light.