Graphite Insulation Takes the Heat
The BasicsInsulation materials based on carbonized and graphitized carbon fiber are most often formed from rayon, petroleum-based pitch or polyacrylonitrile (PAN) raw material sources. Rayon is the more expensive fiber, while PAN is a lower cost alternative.
Starting with raw fibers, the material is processed through controlled oxidation and carbonization processes; it is eventually graphitized at up to 2000°C (3630°F). These high-carbon-content fibers can be turned into flexible soft-felt insulations or rigidized insulation structures, which can then be used to protect high-temperature environments and specialized equipment.
The progression from amorphous carbon to graphite is a two-step process that includes both chemical and physical shifts. At temperatures below approximately 1500°C (2730°F), non-carbon volatiles are chemically volatilized in a process generally known as pyrolysis. Because volatiles are evolved during this stage, a significant loss of the material occurs. Above 1500°C, the carbon atoms become more mobile and begin to arrange themselves into the layered hexagonal structure-graphite.
In general terms, the higher the process temperature, the more graphitic the carbon becomes and the more expensive the resulting product. The final graphitization step results in a very clean material with high carbon content. Additional thermal processing or halogen gas purification can be done to reduce the total ash content to less than 5 ppm.
Soft Carbon-Based InsulationFlexible soft felt is supplied in either a carbonized or graphitized form, depending on whether it was heat treated at a temperature higher or lower than 1500°C. The felt-needled blanket is woven from raw material fibers and heat treated as a formed cloth. Certain end use furnaces and processes, such has the heat treating of metals, do not require the graphitized soft felt and can use the more economical carbonized felt. Since different manufacturers process the materials to different end carbonization and graphitization temperatures, it is a good idea to review the application and end use with the material manufacturer or supplier.
Flexible soft felt insulation is typically available in four thicknesses: 1/8, 1/4, 1/2 and 1 in. The felt is supplied in rolls in widths of 40 to 48 in., is easy to handle and can be cut with common scissors or a knife.
The soft, flexible felt is also versatile and allows easy bending around corners or a radius. Soft felt insulation can fill in voids or other spaces and be layered to a desired thickness. In addition, it is easily sewn using carbon yarn, graphite tow or even moly wire if extra reinforcement is needed. Sewing can be incorporated if a metal or carbon composite structure is used to provide the rigid shape of the insulation package. Soft insulation is available as a flock or wool.
Rigid Carbon-Based InsulationRigidized insulation is often made from resin-impregnated carbon felt layers or chopped carbon fibers held together in a resin matrix. This material is manufactured in board form or as customized shapes, such as cylinders, that allow for maximum use of the furnace space. Rigid insulation can also be formed and machined into complex shapes to incorporate specific features, such as lap joints or barrel stave cuts. Rigidized felt is formed to shape and size using layers of resin-impregnated soft felt, which provides additional resistance to fracture.
Carbon fiber in resin is also shaped into large rigid blocks. Boards are formed by slicing the carbon fiber resin blocks on large band saws. Covering the boards with graphite foil seals the surface for lower gas permeability and provides some durability against abrasion. In addition, layers of carbon fiber cloth and graphite foil sheets provide structural support, surface protection and heat reflection.
ApplicationsTypical applications for these materials are resistance- or induction-heated vacuum furnaces and inert gas furnaces. Because carbon-based insulations feature low thermal conductivity, they are cleaner than loose-particle filling (like carbon black) and superior to metal radiation shields. A low specific heat permits rapid furnace cycling, resulting in the ability to gain efficiencies in volume production runs. Their high surface area makes these materials excellent oxygen "getters" in vacuum and inert atmospheres.
The carbon-based materials are very stable in oxidizing temperatures up to 300°C and protective atmospheres (vacuum or inert) up to 3000°C. Due to the high surface area of the fibers, carbon and graphite insulation will start to oxidize as low as 200°C, so care must be taken to allow the furnaces to cool sufficiently before opening to air. Quenching in nitrogen gas is often used to assist cooling, especially when heat treating metals.
BenefitsCarbon-based insulation materials are easy to work with, inert and safe to handle. They are lightweight, easy to cut by hand, and can be machined into intricate shapes. Because this type of insulation is carbon based, it is not wet by most molten metals, and with its high specific surface, it offers high adsorption capacity. Carbon and graphite insulation offer durable cost performance for high-temperature processing in oxygen-free and vacuum environments.
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