Study On The Process Of Preparing Rigid Thermal Insulation Felt by Wet Method

 

The method of preparing carbon fiber rigid insulation felt by wet method is to use carbon fiber with a length of less than 10mm, dispersant, binder, resin powder, anti-ablation filler, etc. to fully disperse in water in a certain proportion, then pour it into a mold, filter, mold, heat and pre-cure it to form a preformed carbon felt, immerse the preformed carbon felt in a solution containing a foaming agent and resin, fully infiltrate it, filter out excess resin, and heat it for secondary foaming and curing. Finally, other treatments such as carbonization, graphitization, and surface treatment are carried out. The carbon fiber rigid insulation felt obtained by this method has a designable fiber arrangement, and the process is simple, easy to operate, and has little impact on the environment.

 

Carbon fiber rigid insulation felt is a carbon/carbon (C/C) composite material made by a special process. It is a new type of high-performance structural and functional composite material with excellent characteristics such as high strength, high modulus, high fracture toughness, high thermal conductivity, excellent thermal insulation and low density.

 

In this paper, a low-density, high-insulation carbon fiber rigid insulation felt is prepared by wet molding process. The fiber arrangement and density can be designed, the raw materials have a wide range of applications, so the cost is low, and the product performance can be highly designed. It is widely used in alloy sintering furnaces, single crystal silicon furnaces, vacuum smelting furnaces, vacuum heat treatment furnaces, vapor deposition furnaces and other furnaces. In addition, the key points of controlling the performance factors and density of carbon fiber rigid insulation felt during the preparation process are also discussed.

 

1 Experimental Part

1.1 Raw Materials

Carbon Fiber: pitch carbon fiber with carbonization temperatures of 750℃, 950℃, and 1200℃ (Sichuan Chuangyue Carbon Material Co., Ltd.), polyacrylonitrile carbon fiber (Shanxi Coal Chemistry Research Institute, Chinese Academy of Sciences).

Dispersant: methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose (commercially available).

Binder: polyvinyl amide, soluble starch, sucrose, polyvinyl alcohol are configured into an appropriate proportion of aqueous solution (self-made).

Anti-ablation Filler: coke powder (Lingshou County Chengheng Mineral Powder Factory).

Impregnating Agent: phenolic resin solution with phenolic resin content of 5%, 8%, and 10% respectively (commercially available).

Curing Resin: thermosetting phenolic resin below 200 mesh (commercially available).

 

1.2 Sample Preparation

① Short-cut carbon fiber, dispersant, felting binder, resin powder, anti-ablation filler are fully stirred and mixed with appropriate amount of water in a mass fraction ratio of 10:0.05:0.1:1:1;

② Pour the evenly dispersed mixture into a special mold, filter, mold, and heat to 100-150℃ for pre-curing and molding, demold, and obtain preformed felt;

③ Immerse the preformed felt in water-soluble phenolic resin solution with solid content of 5%, 8%, and 10% respectively for 30 minutes, then vacuum filter the excess resin, dry it, and heat it again at 250℃ for pressurization and curing.

④ Carbonize the impregnated and cured carbon felt in vacuum or inert atmosphere at a carbonization temperature of 950℃, and further perform mechanical processing or surface treatment to obtain carbon fiber hard insulation felt.

 

2 Results and Analysis

2.1 Dispersion of Carbon Fiber

In the process of wet preparation of carbon fiber hard insulation felt, the fiber dispersion uniformity has a great influence on the performance of the insulation felt. Since carbon fiber is easy to flocculate in water and has no binding ability, it affects the uniformity and strength of the felt.

 

2.1.1 Effect of Fiber Type on Dispersion

The structural morphology of carbon fiber directly affects the physical and mechanical properties of carbon fiber itself, and the surface structure and performance of carbon fiber are closely related to dispersion.

 

At present, carbon fiber can be divided into polyacrylonitrile carbon fiber, viscose-based carbon fiber, and asphalt-based carbon fiber according to its type. Although the structure of these carbon fibers is transitional carbon converted from the original organic matter, and its carbon content is generally more than 90%, its morphological structure varies with the original silk and heat treatment conditions, which results in different dispersion effects under the same conditions.

 

Some literature points out that the carbon on the surface of carbon fiber can easily react with oxygen and moisture in the air to form polar groups such as carboxyl and carbonyl, and the number of these polar groups is also closely related to the dispersion performance.

 

2.1.2 Effect of Fiber Length on Dispersion

During the wet felting process, carbon fiber is difficult to disperse, which directly affects the density and strength of carbon fiber hard insulation felt.

 

Therefore, we need to cut carbon fiber into 1-10mm in the wet felting process to avoid re-flocculation of carbon fiber and improve the bonding force between carbon fibers. The longer the fiber length, the greater the energy dispersed between fibers. Moreover, since slender fibers are very easy to bend and entangle with each other, it is difficult to disperse.

 

2.1.3 Effect of Dispersant On Dispersion

In the experiment, polyacrylamide, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose all have good dispersion effects on carbon fibers under certain temperature and concentration. After amplification experimental comparison, it is found that when a single dispersant faces a large number of relatively complex fibers (different diameters, different lengths, and fibers themselves are clustered), a single dispersant has its own limitations. The composite dispersant used in this paper can effectively avoid uneven dispersion, stratification and bubble problems.

 

2.3 Effect Of Impregnation Liquid Density and Curing Pressure On The Density Of Finished Felt

 

2.3.1 Effect Of Impregnation Liquid On Density

This experiment configured phenolic resin solutions with a dilution mass fraction of 5%, 8%, and 10% as impregnation liquids, and impregnated preformed felts of the same size A (density 0.21g/cm3) and B (0.23g/cm3) respectively to study the effect of impregnation liquid concentration on the density of sample felt types. The results are as follows:

 

 

It can be seen from the chart that for the same type of preformed felt, the higher the concentration of the impregnation liquid, the higher the density of the sample after curing and carbonization. This is because the preformed felt has a large open porosity. As the concentration of the impregnation liquid increases, the resin absorbed by the felt increases.

When the same concentration of impregnation liquid is used for impregnation, the density of preformed felt A after curing and carbonization is lower than that of preformed felt B. This is mainly because the initial density of preformed felt A is lower than that of preformed felt B. When the concentration of the impregnation liquid is 5%, the density of the prepared carbon fiber rigid insulation felt is the lowest.

 

2.3.2 Effect of Curing Pressure On Density

In order to make the binder play a better effect, a certain pressure needs to be applied during curing. Using preformed felt A as the preform, polyvinyl alcohol as the binder, and a phenolic resin solution with a concentration of 5% of the impregnation liquid, the changes in sample density under different curing pressures are studied. As shown in the following figure:

 

 

As can be seen from the figure, as the applied pressure increases during curing, the density of the sample increases. This is mainly because the sample itself is soft in texture. As the applied pressure increases during curing, the sample will be compressed to a certain extent, resulting in a decrease in thickness, and the volume of the sample will be correspondingly reduced, making it more dense. Therefore, when preparing the secondary curing of carbon fiber rigid insulation felt, we appropriately added a certain amount of foaming agent, which also correspondingly reduced the density of carbon fiber insulation felt, increased the internal voids, greatly reduced the number of macropores in the product and increased the closed porosity, and reduced the thermal conductivity of the product.

 

3 Conclusion

① The short-cut carbon fiber, dispersant, binder, etc. are fully dispersed in the aqueous solution, and a carbon fiber rigid insulation felt with a designable density and low thermal conductivity can be prepared by a wet molding process.

② The uniformity of carbon fiber dispersion in the aqueous solution directly affects the performance and density of the carbon fiber rigid insulation felt.

③ During the preparation process of carbon fiber rigid insulation felt, the greater the density of the phenolic resin solution used as the impregnation liquid, the greater the density of the rigid insulation felt after carbonization, because in the fixed gap, the absorbed resin increases, and after carbonization, the residual carbon increases, which increases the quality of the rigid insulation felt.

④ After carbonization, the carbon fiber rigid insulation felt has a certain volume shrinkage. If low-temperature carbonized raw silk is used when forming the felt, the shrinkage rate is the largest. If high-temperature carbonized raw silk is used, the shrinkage rate is relatively small. This is because the non-carbon atoms in the carbonized raw silk are continuously removed, and the carbon atoms are more closely connected to form a chaotic layer structure.