Selection of materials for carbon-carbon processing (2)-types of matrix

The selection of a suitable precursor matrix for a carbon-carbon composite is most important and the following factors should be taken into consideration:

  1. The carbon yield must be high, with consequent low weight loss during carbonization.
  2. The liquid or molten precursor must have a low viscosity to aid penetration of the matrix.
  3. The liquid or molten precursor must readily wet out the carbon fiber reinforcement.
  4. Carbonization of the matrix should produce a favorable coke microstructure.

Precursors can be divided into two general categories-thermosetting and thermoplastic resins (including pitches)

Thermosetting resin:

Kimura selected three kinds of thermosetting resins- furan, diphenylether-formaldehyde and polyimide resins- as matrix precursors to fabricate carbon fiber reinforced carbon composite (C/C composite). After heat treatment at 2000-3000C, the graphitization process of the matrix was examined by optical microscopy and X-ray diffraction. In the C/C composite derived from a polyimide, the graphite structure was not as well developed as the others. This retarded development is attributed to less internal stress between fibers and matrix as well as to less stretching of the matrix.

When using a thermosetting resin, it is usual to take the carbon fiber reinforcement and prepare a prepreg by impregnating with the chosen resin. Alternatively, a dry preform of opf can be used for conversion to carbon fiber and subsequent initial treatment by the CVI process to provide a preform with some integrity. Polymerization shrinkage will occur, which can be minimized by controlling the rate of temperature rise.

The most promising candidates for a resin matrix are the furfuryl alcohol and phenolic resin systems, which cure by a condensation polymerization releasing water, which will contribute further to porosity and shrinkage.

  1. Furan resin: Furfuraldehyde (furan), when reacted with NaOH, forms furfuryl alcohol, which can be polymerized by applying heat in the presence of an acid catalyst to initiate the reaction. Once started, the reaction becomes exothermic and can be controlled, or stopped by cooling.
  2. Phenolic resins: A phenolic resin developed by Monsanto, gives a comparatively high carbon yield when pyrolyzed. The phenolic resin is typically cured with hexamine in a condensation reaction.
  3. Polyimide resins: Although polyimides give a 60% carbon yield when pyrolyzed, they have not gained wider recognition due to processing problems. Similarly the H-resins developed by Hercules have very high carbon yields of up to 95%, but were eventually withdrawn due to processing problems when making a prepreg.

Thermoplastic matrix precursors:

  1. Pitch: Pitches can be derived from coal tar or petroleum and have been discussed as precursor materials for making pitch based carbon fibers. Pitches are oligomers and the composition will depend on the exact source and method of processing. A pitch with a high carbon yield and the ability to flow under high pressure should be selected.

An isotropic pitch gives a carbon yield of about 50%, which increases to about 85% for a mesophase pitch although mesophase pitches are more viscous. The matrix formed was mainly mesophase because the isotropic mesogenic pitch was pressed out. The composites were stabilized by treatment with air before carbonization of the matrix in order to avoid swelling. After graphitization at 2100C, flexural strengths up to 650Mpa were measured.

The composition of a pitch is very complex and, together with the processing conditions, controls the nature of the carbon matrix and it is normal to use a blend of pitches to obtain the requisite properties. A pitch based resin gives a higher yield than a phenolic resin, is cheaper and when graphitized, has greater density, but does require high pressures for processing.

  1. Other thermoplastic matrices: Thermoplastic polymers such as PEEK and PEI have been successfully used as alternative polymer matrices, but it is doubtful whether their high cost can ever be justified. Other resins have been investigated and include polybenzimidazole (FBI), which has a 73% carbon yield.

 

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