What is the Difference Between Carbon Fiber Graphite Soft Felt and Hard Felt?

 

 

Carbon Fiber Graphite soft felt and hard felt are both commonly used thermal insulation materials for high temperature thermal fields. They both have the characteristics of high temperature resistance, low density, and low thermal conductivity, but there are also differences between the two.

 

1. Key Points of the Process and Product Characteristics of Carbon Fiber Graphite Soft Felt

 

Carbon Fiber Graphite Soft Felt is generally made of viscose fiber, PAN fiber and other raw fibers through non-woven process to make felt, and then through impregnation (mainly viscose base needs to be impregnated), pre-oxidation, carbonization, graphitization, cutting, and finally get the finished product.

 

The forming of graphite soft felt is mainly in the non-woven process at the front end. Non-woven is a collective name for a series of complex processes. There are many factors that affect the quality of the finished product. For example, when the fibers are combed into a web, PAN fibers or viscose fibers are thin, light, and relatively dry. They are prone to static electricity under the mechanical action of the equipment, which affects the normal operation of the equipment. Therefore, static electricity needs to be reduced by adding antistatic agents and other methods; when laying the web, there are many types of laying methods such as parallel, cross, combination, and vertical, and the number of laying layers can reach dozens of layers; when acupuncture reinforcement, different types of needles, different needle arrangements, and different acupuncture times can be selected.

 

These are only a small part of the dry web and acupuncture reinforcement used in the non-woven process. There are many different technical routes for non-woven.

 

 

Carbon Fiber Graphite Soft Felt is Formed by Non-woven Process

 

After the raw fiber is formed, the length, width and thickness of the graphite soft felt will be reduced compared with the felt before pre-oxidation due to the gradual reduction of non-carbon elements in the felt from the beginning of pre-oxidation. However, the uncut graphite soft felt is still a huge piece of material, with a length of up to 40m and a width of up to 1.6m.

 

According to the application requirements, the uneven parts at both ends and both sides of the graphite soft felt can be cut, and it can also be cut into rectangular soft felts, round soft felts with suitable areas, and various shapes of blanks for making hard felts.

 

From the main performance indicators, the density of graphite soft felt is only about 0.10g/cm³, which is about one-tenth of the density of water. The graphitized fibers are light and thin, and the small space formed between a large number of fibers makes the density of the soft felt so low.

 

The thermal conductivity of high-purity/high-efficiency viscose-based graphite soft felt is about 0.10-0.15W/m·k at 1150℃. The lower the thermal conductivity, the better the thermal insulation performance.

 

2. Key Points of the Process and Product Characteristics of Carbon Fiber Graphite Hard Felt

 

Hard felt is mainly formed by soft felt lamination curing molding and wet molding. The common lamination method for making carbon fiber graphite hard felt uses soft felt as raw material. The production process begins with cutting: cutting multiple pieces of blanks slightly larger than the final hard felt size required from the soft felt. After cutting, impregnate with resin and other bonding materials, and stack the impregnated blanks in multiple layers according to the required hard felt thickness.

 

The impregnation in soft felt production is mainly the impregnation of catalysts to make viscose fibers more stable in subsequent heat treatment; while the impregnation of hard felt is mainly the impregnation of thermosetting resins. In the subsequent heating and curing process, the liquid resin solidifies, and the multiple layers of soft felt are tightly combined with the resin to form.

 

Resin + carbon fiber graphite soft felt allows the material to achieve the effect of “1+1>2” in some properties. After the multi-layered soft felt impregnated with resin is heated and cured, it can no longer be rolled up like soft felt, and the density and compressive strength of the material are also increased.

 

 

Carbon Fiber Graphite Hard Felt is Formed by Curing Multiple Layers of Soft Felt and Resin

 

But this is not Carbon Fiber Graphite Hard Felt. Because the resin contains a lot of elements other than carbon, it needs to be removed through processes such as carbonization and graphitization.

 

The carbon in the resin is the matrix, and the carbon fiber graphite soft felt is the reinforcement. They are combined together to eventually become a composite material such as hard felt. Therefore, carbon fiber graphite hard felt is also called graphite hard composite felt.

 

After the small space between the soft felt fibers is filled with resin and cured, the density of the semi-finished product must be higher than that of the raw soft felt; after carbonization and graphitization, the resin loses hydrogen and oxygen atoms, and the density is reduced, but the carbon left in the resin makes the density of the graphite hard felt still higher than that of the raw soft felt. The density of hard felt is about 0.13-0.25g/cm³.

 

The area of ​​the hard felt after graphitization is almost the same as the area of ​​the semi-finished product after curing, and is generally smaller than the raw soft felt before cutting.

 

According to the shape and appearance requirements, the hard felt also needs to be machined, coated or carbon cloth, graphite paper, etc.

 

Compared with carbon fiber graphite soft felt, hard felt has a slightly higher thermal conductivity and is not as good at keeping warm as soft felt, but has a longer service life. For example, in a single crystal furnace, the service life of soft felt is about 6 months, while hard felt needs to be replaced about every 12 months.