GB2185010A - Process for preparing carbon moulded articles of high anisotropy - Google Patents

Abstract

Carbon fibres are prepared from the precursor of a polymerizate of linearly condensed polycyclic compounds with a molecular weight of between 500 and 1200. It is obtained by the polymerization of linearly anellated aromatics substituted in the ??? position and, where appropriate after distilling off the monomers and polymers with less than 5 rings, it is subjected to a fractionated extraction with a supercritical gas, in order to limit the molecular weight distribution. The carbon fibres produced therefrom in known manner belong to the class of high-modulus fibres.

Description

SPECIFICATION Process for preparing carbon moulded articles of high anisotropy The invention relates to an improved process for preparing objects from carbon or graphite in the form of fibres or foils which have a high degree of anisotropy, by selecting a substance as a carbon primary product which has a specific chemical structure and specific properties. (In the following the objects of carbon and graphite are referred to generally as carbon-moulded articles).

Various methods are already known for preparing carbon-moulded articles, and in particular carbon fibres. In one method, fibres of natural or artificial high-polymer materials, such as polyacrylonitrile, polybenzimidazole, cellulose etc., are carbonized. In another method pitch is used as a starting material and converted into fibre form by melt spinning, being then subjected first to a process of rendering it infusible and the carbonization.

The second method is suitable for the preparation of products which have a uniform quality and adequate strength. However, carbon fibres, produced according to this method, characteristically have an isotropic structure, from which growth and orientation of graphite crystals are achieved only with difficulty even at high temperatures. It has consequently been difficult! until now to produce carbon fibres with a high elasticity modulus, without subjecting them to a heat treatment at graphitization temperatures (or a temperature range in the region thereof) while simultane ousiy allowing strong tensile forces to act on them.

One reason for this is that the molecular weight of the raw material, pitch, which is suitable for spinning, is extremely low compared with that of polymers such as polyacrylonitrile. Furthermore the molecules as a whole are not sufficiently uniform, so that it is scarcely possible with the aid of conventional methods to obtain pitch fibres with high molecular orientation. In addition, rendering such fibres infusible causes a three-dimensional cross-linking between the molecules at low temperatures, as a result of which poor graphitizability is imparted to the pitch fibres.

A method of preparing carbon moulded articles is known from the German Offenlegungsschrift (Laid-Open Specification) 20 15 175.

The method is characterized in that there is selected a starting material chosen from the following compounds, which have a substantially uniform condensed ring structure, a uniform melt and a melt viscosity of between 0.4 and 700 poise at the deformation temperature, is brought into the desired form:: a) condensed polycyclic compounds which are formed by condensation of at least seven rings as a basic framework and have a uniform molecular structure; b) compounds in which units of the aforesaid condensed polycyciic structure are linked to one another by an individual bridge; c) compounds in which units of condensed rings, which comprise more than two rings, are linked to form a new ring system by two bridges which contain at least one of the elements carbon, nitrogen, oxygen or sulphur, or d) a mixture of the compounds named under a) to c) and the carbonization and graphitization of the moulded article are performed without tensile stress being exerted upon the moulded article during these procedures.

The average molecular weight of the carbon fibre precursor is given at between 400 and 3000 and its viscosity at temperatures of between 320 and 480"C is given at between 0.04 and 70 Pa's. The ranges are set very widely. The average molecular weights of spinnable mesophase pitches usually lie below 1000 and at viscosities of between 1 and 20 Pa's.

The elasticity (E) modulus of the carbon fibres produced from these precursors is between 137 and 314 kN/mm2. The fibres should thus be assigned to the class of highstrength carbon fibres. High-modulus carbon fibres have an elasticity modulus of between 350 and 450 kN/mm2 and are produced almost exclusively from polyacrylnitrile (PAN). In contrast to the linear structure of PAN molecules, pitch mesophases have a plane molecular structure. This can be regarded as the reason for the lower elasticity of the carbon fibres produced from pitch precursors.

The invention sets out to develop a method of preparing carbon moulded articles, in particular carbon fibres, from polycyclic aromatics, which have an elasticity modulus which is at least as high as those produced from PAN.

The invention consists in a process for preparing a carbon-moulded article by deformation of high-polymerized polycyclic aromatic mixtures to form foils or fibres and subsequently rendering infusible, carbonizing and graphitizing the moulded article characterised in that linearly condensed polycyclic compounds, obtainable by the polymerization of linearly anellated aromatics substituted in the ss position and having not more than 6 rings, where appropriate after distilling off the monomers and polymers with less than 5 rings, are subjected to a fractionated extraction with a supercritical gas, to give a mixture of polycyclic aromatics with a close molecular weight distribution and with molecular weights in the range of between 500 and 1200, the said mixture then being processed in a manner known per se to form carbon moulded articles.

The initial raw materials are typically linearly anellated aromatics substituted only in the fi position and having the general formula:

with n between 0 and 4, which can be substituted in one or more ss positions (p, ff, ss", ss"') as desired. It has been found that these aromatics polymerize exclusively to form linearly condensed compounds.Aromatics substituted with alkyl, hydroxy or amino groups are particularly preferred, in particular aromatics substituted at least twice in the ss or ff position and the ss" and ss"' positions, such as for example 2,6-dimethyl naphthalene, 2,7-dimethyl naphthalene, 2-hydroxy-6-methyl naphthalene, 2,7-dimethyl anthracene, 2,6-dimethyl anthracene, 2,3-dimethyl anthracene, 2amino-6-methyl naphthalene. The aromatics can be polymerized individually or in a mixture, but it is advisable, in particular in the case of more highly condensed aromatics, to polymerize compounds with the same number of rings, since the fractionation is made easier by the greater molecular weight difference.

Polymerization is preferably carried out at temperatures of between 350 and 470"C. An elevated pressure is advantageous, in particular in the case of compounds whose boiling point is below or only slightly above the polymerization temperature. At the conclusion of polymerization the low-boiling monomers and polymers can be removed from the reaction product by flash distillation.

The residue is extracted with a supercritical gas from low-boiling aliphatic hydrocarbons, where appropriate using an entrainer and the extract is separated from the fluid phase by step-wise temperature increase or pressure reduction.

In this connexion, at least three fractions should be obtained, of which the middle one preferably contains not more than 4 successive polymerization stages of the aromates introduced and is used as a precursor for the preparation of the carbon moulded articles.

Example A mixture of 500 g 2,6-dimethyl anthracene and 500 g 2,7-dimethyl anthracene is polymerized for 3 h at 450"C in an autoclave under nitrogen at a pressure of 5 bar. In this connexion approximately 10% by weight-rela 'tive to the charge mixture- of a methane-rich gas is formed. The autoclave then has its pressure reduced by way of a condenser to 50 mbar. Approximately 5% by weight of an oil are obtained, which oil consists predominantly of anthracene. The residue (85% by weight of charge) is extracted with propane at 150"C at a pressure of 180 bar, and three bituminous fractions are obtained from the charged extraction medium by step-wise pressure reduction.The average fraction obtained in a yield of 65% by weight-relative to the charge mixture-has a molecular weight distribution in the range of between 550 and 1100 with an average molecular weight of 800. The viscosity at 350"C amounts to 8.2 Pa's. If the polished surface of the cooled fraction is observed under a polarization microscope the complete anisotropy of the substance is visible.

It is spun at 350"C with a draw-off speed of 5 m/s by means of an extruder and a spinning nozzle head to form fibres with a diameter of 8 ,am, without fibre breakages occurring. Next the fibres are oxidized at approximately 200"C in air for 2 h and then at 300"C for a further 1 h. The fibres which have thus been made infusible are heated in a nitrogen atmosphere at 2 K/min up to 600"C, at 10 K/min up to 1400"C and then at 20 K/min up to 300000.

The elasticity modulus of the graphite fibres obtained in this way amounts to 410 kN/mm2 and the tensile strength amounts to 2,900 N/mm2.

Claims (7)

1. A process for preparing a carbonmoulded article by deformation of high-polymerized polycyclic aromatic mixtures to form foils or fibres and subsequently rendering infusible, carbonizing and graphitizing the moulded article, characterized in that linearly condensed polycyclic compounds, obtainable by the polymerization of linearly anellated aromatics substituted in the ss position and having not more than 6 rings, where appropriate after distilling off the monomers and polymers with less than 5 rings, are subjected to a fractionated extraction with a supercritical gas, to give a mixture of polycyclic aromatics with a close molecular weight distribution and with molecular weights in the range of between 500 and 1200, the said mixture then being processed in a manner known per se to form carbon moulded articles.

2. A process according to Claim 1, characterized in that the linearly condensed polycyclic compounds are produced by polymerization of linearly anellated aromatics having between 2 and 4 rings and singly or multiply substituted in the ss position with alkyl groups, hydroxyl groups or amino groups.

3. A process according to Claim 1 or 2, characterized in that linearly anellated aromatics with the same number of rings are polymerized to form linearly condensed polycyclic compounds.

4. A process according to any of Claims 1 to 3, characterized in that the linearly condensed compounds are extracted with the supercritical gas from low boiling aliphatic hydrocarbons, where appropriate using an entrainer, and are separated by step-wise temperature increases or pressure reduction into at least three fractions, the precursor mixture for the carbon moulded articles bei ng obtained as the middle fraction.

5. A process according to Claim 4, characterized in that the precursor mixture contains not more than 4 successive polymerization stages of the aromatics introduced.

6. A process as claimed in claim 1 sub stantiaily as herein described with reference to the Example.

7. A carbon-moulded article prepred by the process claimed in any one preceding claim.