When talking about temperature, high is a relative term. When things get really hot (above 1830°F), however, cost versus performance analysis reveals the value of carbon-based insulation materials. Because of their unique carbon content and structure, they lend themselves well to providing the required thermal protection at extremely high temperatures.

Fig. 4. Examples of different shapes that are available or can be achieved

Fig. 1. Insulation wall thickness using graphitized soft felt

Industrial furnaces are designed to work at a variety of maximum operating temperatures. A wide range of insulation material is commercially available to provide the necessary thermal protection for many of these furnaces. When temperatures get “high” – above 1830°F (1000°C) – insulation material choices start to decline. It is at this point when the cost versus performance criteria point to carbon-based insulation materials.

Insulation materials based on carbonized and graphitized carbon fiber from rayon, petroleum-based pitch or polyacrylonitrile (PAN) sources can provide the solution to difficult heat insulation problems. Starting with raw fibers, the material is processed through controlled oxidation and carbonization processes and eventually graphitized at over 3630°F (2000°C). These high-carbon-content fibers can be turned into flexible soft-felt insulations or rigidized insulation structures, which can be used to protect many high-temperature environments.

Combining the soft or rigid insulation with layers of carbon-fiber board for armoring and rigidity and flexible graphite sheets for heat reflection and gas impermeability creates a series of materials that can be customized to provide solutions to even the toughest heat-barrier problems. The wall thickness of the insulation is based on the internal operating temperature of the furnace and the desired outside temperature, referred to as hot wall (inside furnace) or cold wall (outside surface of the insulation). Figure 1 provides a guide to insulation wall thickness using graphitized soft felt.

Fig. 2. Soft-felt insulation

Carbon-Based Insulation - Soft

Flexible soft felt is supplied in either a carbonized or graphitized form. Certain furnaces do not require the graphitized soft felt and can utilize the more economical carbonized felt. Since different manufacturers will process the materials to different end carbonization and graphitization temperatures, it is recommended to review the application with the material manufacturer. Flexible soft-felt insulation is typically available in four main thicknesses – 1/8 inch, ¼ inch, ½ inch and 1 inch. Supplied in rolls with a typical width of 48 inches, this insulation is easy to handle for wrapping suceptors, crucibles or other furnace shapes (Fig. 2).

Creating the needed wall thickness is a simple matter of layering the soft felt to the desired thickness. Its soft texture allows for easy bending around corners or a radius and also for filling in voids or other spaces. In addition, it is easily sewn using carbon cord if additional reinforcement is needed. Sewing can also be incorporated if a metal structure is used to provide the rigid shape of the insulation package. The flexible felt can be easily sewn through, just like sewing cloth, and then intertwined with a metal superstructure. Soft insulation is also available as a flock or wool.

Fig. 3. Rigidized insulation combined with other carbon-based products such as carbon fiber or flexible graphite

Carbon-Based Insulation - Rigid

Rigidized insulation can be made from soft felt, resin-impregnated soft felt or chopped carbon fibers held together in a resin matrix. This material is manufactured in board form or as a custom cylindrical shape with custom-made top and bottom features, which allow for maximum use of the furnace. It can be layered with carbon-fiber board and flexible graphite sheets to provide additional armoring and heat reflection (Fig. 3). The flexible graphite will also provide the advantage of resisting gas permeation and will have a longer life. Rigid insulation in board form can be machined into complex shapes to create the exact insulation package needed. Machining key slots and staves to form large cylinder shapes or rectangular boxes is a common practice (Fig. 4).

Fig. 5. Thermal conductivity vs. temperature in a nitrogen atmosphere for SGL Group’s SIGRATHERM GFA10 graphite soft flexible insulation; typical properties for SIGRATHERM GFA10

Applications of Carbon-Based Insulation

Typical applications for these materials are resistance- or induction-heated vacuum furnaces and inert-gas furnaces. As with other related carbon products such as extruded or iso-molded graphite, carbon-fiber products or flexible graphite foils, exposure to temperature above 932°F (500°C) in the presence of oxygen results in a chemical reaction of the carbon content of the material into carbon dioxide. The carbon dioxide is vented away, and the remaining material starts to take on a pitted or porous appearance. This reaction, or oxidation, results in deteriorating material properties, hence the emphasis on vacuum or inert-gas applications.

Carbon-based insulations have low thermal conductivity, making them superior to metal radiation shields and cleaner than loose-particle filling like carbon black. Figure 5 illustrates thermal conductivity versus temperature values in a nitrogen atmosphere for Sigratherm®. Its low specific heat permits rapid furnace cycling from cold to hot and hot to cold, resulting in the ability to gain efficiencies in volume production runs. The carbon-based materials are very stable in oxidizing temperatures up to 500°C and protective atmospheres (vacuum or inert) up to 5430°F (3000°C).

One commonly asked question regarding soft or rigid carbon-based insulation is, will it couple with an induction field? The short answer is some will, some won’t. It may depend on the underlying fiber source and the production processes used to make the insulation. Please ask the manufacturer.

Fig. 6. Versatility of the materials is shown. Cylindrical shapes with mating lids and bottoms can be provided.

Material Versatility

The material is easy to work with. It’s lightweight, easy to cut by hand and it can be machined into intricate shapes (Fig. 6). Because it is carbon-based, it does not wet to most molten metals, and because it has a small specific surface, it has low adsorption capacity.

The final graphitization step results in a very clean and high carbon-content material. Additional thermal or halogen-gas purification can be done to bring the total ash content to a very low level (<5 ppm). Usually very smooth and consistent in thickness, it makes for a predictable product to work with, and it does not create electrostatic charging when used together with plastics or composite materials.

All in all, when the temperature gets too hot to handle with more conventional insulations, turn to carbon-based materials for the cool long-lasting solution. IH

For more information: Contact Joseph Labant, product manager, high-temperature applications, SGL Group, The Carbon Company, 900 Theresia St., St. Marys, PA 15857; tel: 814-781-2729; fax: 814-781-2697; e-mail: joe.labant@sglcarbon.com; web: www.sglcarbon.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: carbon-based insulation, carbonized, graphitized carbon fiber, thermal conductivity, specific heat, composite