The current economic climate has many executives, managers and engineers pursuing cost-reduction measures. This is a natural and just response, but one that is not without significant risk, particularly when it comes to carbon-fiber-based, high-temperature insulation materials.

Fig. 1. Felt generally has better insulation properties than rigid board.


Carbon-fiber-based insulation materials are relied upon to ensure consistent, high-quality product, equipment integrity and personal safety in furnaces that can often exceed 2000°C (3630°F). An error in insulation-material selection can lead to failure in one or all of these critical areas. A well-qualified and technically capable supplier can be an invaluable resource in identifying the optimum insulation material for achieving each of these requirements at the lowest possible cost.

When considering cost, an important concept to understand is the total cost of ownership (TCO) for a given insulation material. Beyond simply the cost of purchasing the material, TCO represents the net cost associated with using the material in the process. Factors such as material price, service life, product yield, product quality, equipment maintenance expenses and safety are taken into account when their value can be shown to correlate with the insulation material. TCO is an important concept because each insulation type, grade and source possesses their own unique set of properties and performance potential. Two seemingly similar insulation materials may have very different performance in your application.

Technical Information

To fully appreciate this, a bit of material science is helpful. Carbon is a relatively unique element in that it is allotropic, meaning that it exists as a solid in nature in many different forms. The two most common allotropes are diamond and graphite, the only difference being the carbon atoms’ crystallographic arrangement. Graphite is the allotrope that is common in insulation products. Further investigation into the graphite structure reveals that there is technically no such thing as the theoretical graphite structure but rather a range between amorphous carbon (non-crystalline) and graphite (crystalline). Between these two limits lies a region termed turbostratic graphite where the atomic structure is more or less crystalline and layers are aligned. Where a given material falls along this continuum determines its unique properties, and this is highly influenced by the precursor raw materials and the process used to produce it.

The progression from amorphous carbon to graphite is a two-step process – chemical and physical. At temperatures below approximately 1500°C (2730°F), non-carbon volatiles are chemically volatilized, a process generally known as pyrolysis. Volatiles are evolved during this stage resulting in significant mass loss of the material. It is important to closely control this process in order to preserve the integrity of the product. Above 1500°C, the carbon atoms become more mobile and begin to arrange themselves into the well-known layered hexagonal (graphite) structure.

Generally, the higher the process temperature, the more graphitic it becomes. It is important to note, however, that some precursor raw materials are more readily graphitized than others. Therefore, the final properties of an insulation material are determined by both the precursor raw materials as well as the details of the process parameters and the specific process path. If the same raw materials are processed differently, products of differing properties result. Similarly, if two different raw materials are processed identically, products of differing properties also result. In summary, carbon-fiber-based insulation products are highly unique materials between various grades and manufacturers.

Fig. 2. Rigid-board insulation is used for its structural and insulative properties.

Material Makeup

Carbon fiber is the core of all carbon-fiber-based insulation products. The most common precursors to produce carbon fibers are rayon, petroleum-based pitch and polyacrylonitrile (PAN). Rayon is the more traditional fiber precursor for insulation materials due to its relatively lower thermal conductivity. While PAN is generally used for high tensile-strength applications and pitch for high modulus/high thermal-conductivity applications, each is also employed as insulation material by some manufacturers. Differences in the base fiber properties result in differences in the insulation products produced from them. Therefore, an understanding of the impact on the application is critical to ensuring a performance matched to the requirements.

Basic Types
The two basic carbon-fiber insulation materials are felt insulation and rigid-board insulation. Each of these can be produced from any of the fiber precursor raw materials described above. Felt insulation (Fig. 1) is produced by carbonizing/graphitizing a needled blanket of the precursor fiber and does not contain any resin binders. The resulting insulation material is soft and pliable and has little structural form. Felt insulation is generally a better insulator than a rigid-board insulation material made from the same precursor raw-material fiber type. Rigid-board insulation materials (Fig. 2) are most commonly produced by chopping carbonized fiber into short staples and then mixing into a slurry with a phenolic or similar resin. The slurry is formed into blocks, thermally processed and further processed into boards or other machined forms. Rigid-board insulation materials trade off insulative properties for structural strength and stiffness.

A third form of carbon-fiber-based insulation materials is generally referred to as rigidized felt. It is produced by impregnating felt insulation layers with resin and bonding them together at high temperature to form a rigid insulation material. These materials can be manufactured in such a way that they are more like felt insulation or rigid-board insulation in performance. Carbon/carbon-composite or graphite-foil layers can be incorporated to further modify their performance (Fig. 3).

Fig. 3. Rigidized felt can be produced with a range of properties.

Leave it to the Experts

Developing a more thorough understanding of the breadth of carbon-fiber-based insulation materials that are available may serve only to add confusion and indecision as to which material provides the lowest TCO for a given application. The adage “you don’t know what you don’t know” applies here. It is common to see companies using inefficient legacy insulation materials blindly because “it’s what we’ve always used.” Many small companies lack the dedicated technical staff to become insulation-material experts, while many larger companies have reduced workforce levels to the point that they are rarely able to pull focus away from their primary process-control efforts. The culmination of these and other factors is that opportunities to improve the TCO of insulation materials are often overlooked.

Identifying and cultivating a relationship with a world-class supplier of insulation products is key to capitalizing on these opportunities. More than just providing insulation products, you should expect your supplier to become a valued technical resource. The ideal insulation supplier should possess the following capabilities:

  • A range of insulation products. If only one type of insulation is offered, can you trust that your supplier is presenting an unbiased opinion on which insulation type offers the best TCO for your application?
  • Applications engineering support. Your supplier should be an insulation-materials expert and have the technical staff to back this claim. While they may not have the solution for your specific application up front, they should demonstrate the high-temperature problem-solving aptitude required to provide engineered solutions. As an established customer, this service should be provided with no consulting fee.
  • Directly manufacture the products or be able to demonstrate a strong partnership with those that do. Having a direct link to the insulation-materials manufacturer will help ensure a stable supply chain, responsiveness to technical or quality concerns, and provide access to the insulation-material experts that can be leveraged to identify the material with the lowest TCO for your application.


Conclusion

One can be assured that the leaders in your industry understand the TCO concept and have ensured that their furnace insulation components provide the maximum possible benefit. That is in part why they are the industry leaders in the first place. More than simply weathering the economic storm, aligning yourself with an insulation supplier that has the capability to partner with you on TCO reduction will position your company well to capitalize when the economy improves. The high-temperature equipment and processes that you employ are critical to the success of your business and the safety of your workforce. Are you using the insulation product that provides the lowest possible TCO? IH

For more information: Contact Jamie Snipes, business development manager for Morgan AM&T, 251 Forrester Dr., Greenville, SC 29607; tel: 864-458-7700 x210; fax: 864-281-0180; e-mail: Jamie.Snipes@morganplc.com; web: www.morganamt.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 fiber, allotropic, pyrolysis, rigid-board insulation, carbon composite