DE2227575B2 - Process for the production of writing and drawing leads - Google Patents

Description

The invention relates to a method for producing writing and drawing leads from one carbonizable material, in which a monofilament strand is produced from this material, this into a Carbon strand is carbonized and then cut into pieces suitable for mines.

From DE-OS 17 61133 a method for the production of mines with very small diameters is preferably less than 0.7 mm, known in which carbon fibers cut into small pieces are inserted into the in conventionally prepared with a binder lead compound are incorporated and then this is further processed according to known techniques. The production technique of the mine has developed through the Incorporation of the carbon fibers actually changed nothing.

The known method makes use of the fact that carbon fibers exceptional strength values have, so that their incorporation increases in the mine ^ their breaking strength, which is important for mines with very small diameters (so-called. Fineline) is particularly important.

The well-known mine production process, in which carbon fibers are inserted into the binder-containing lead mass are incorporated, there is a defect that the fibers are not good with the binder (matrix) Give a bond.

Mines have also been manufactured in such a way that a monofilament ^b5 strand is produced from a carbonisable material, this strand is carbonised to a carbon strand and then, if necessary, cut into lengths suitable for mines. In this process, the problem of a good bond between the carbon strand and the binder does not arise. However, the leads produced in this way have insufficient strength. This explains that the known process has so far not been able to establish itself in practice for the production of fine leads.

The invention is based on the object of improving the last-mentioned method so that it the production of very break-proof leads with a small diameter, of about 0.5 mm and less allowed

The solution to this problem is that the monofilament strand is subjected to a known oxidative thermal crosslinking at temperatures between 200 and 300 ⁰ C before carbonization. Before or during this treatment, the strand can be stretched, which contributes to a further improvement in strength

The oxidative thermal crosslinking and the subsequent carbonization of a material is in itself known (cf. z. B. Chemikerzeitung 1972, 20), so that this will not be discussed in detail However, the oxidative crosslinking of a carbonizable material has been heretofore in fields which are far removed from the manufacture of writing and drawing leads. In this respect, it is after According to the invention, the significant improvement in the breaking strength of so-called fine leads is surprising to watch.

As a starting material for the process according to the invention, those from the production can be used known carbon fibers common high molecular weight substances are used, which for a oxidative thermal crosslinking are suitable. Polyacrylonitrile and its copolymers, polyamides, Polyester and cellulose derivatives in the foreground, although the scope of the invention is neither this is still restricted by the following exemplary embodiments. The shape of the Starting material to form a monofilament strand is carried out according to the techniques known per se, e.g. B. by Extruding a thermoplastic mass or a spinning process.

The production of the leads according to the invention takes place in a multi-stage heat treatment of the strand, with oxidative crosslinking taking place in a first heating stage of about 200-300 ° C, the duration of which depends on the lead diameter. This is followed by a next heating stage (coking stage) with higher temperatures than in the first stage, in which the molecules are carbonized. Then may follow, in which the carbon is graphitized even a high temperature treatment between about 2000-3000 ⁰ C. The strand can also be kept under tension or stretched during the treatment in the work steps following the oxidative thermal crosslinking.

The extent to which graphitic components are produced in the strand depends on the desired Strength. The heat treatment in the graphitization stage is carried out, as is the previous one Carbonization (coking), carried out in an inert atmosphere, in contrast to that of the Oxidative thermal crosslinking upstream of the coking stage, in which the strand is separated from air or Oxygen containing gases is washed around.

The starting material for the production of the monofilament strand can be adjusted to the desired hardness matched proportions of soot and / or graphite are added to the mine.

The foregoing Grafitisiemng the coking of the lead string is carried out at temperatures between 300-1200 ⁰ C, wherein a Nachverkokung between 1000-1700 ° C can be advantageous for achieving high strength.

The aforementioned stretching treatment of the strand ι ο can be limited to individual stages or take place during all heating stages. In particular, an alignment of the crystallites in the Graphitization mare (so-called stretch graphitization) to one substantial improvement of the strength properties.

The carbon (soot, graphite) is incorporated into the carbonizable material used in accordance with the invention before it is extruded ^: in a manner known per se, for example by adding it to the mass plasticized for the extrusion. When producing the strand by wet spinning, for example, when cellulose is used as the starting material, the carbon can be added to the highly viscous solution of cellulose in Schweizer's reagent or into the cellulose xanthate solution and the mixture can be pressed ^{through a nozzle as a strand into an acidic r äll bath.}

When using polyamide or polyacrylonitrile, the carbon can be incorporated into the plasticized material in the highly viscous solutions of polyamide in formic acid or polyacrylonitrile in Dimethylformamide incorporated and then pressed the mixture into a strand and this then to be dried.

example 1

700 g of natural graphite are kneaded in a heated mixer with 300 g of polyamide and then mk an "n plastic extruder into a strand pressed, stretched and below the melting point in air at 200-225 ° C oxidized is then heated under exclusion of air at about 1000 ⁰ C and After cooling, the strand is cut into pieces suitable for pencils and impregnated in the usual way with oil, fat or an emulsifier

Example 2

500 g commercially available viscose (cellulose xanthate) is mixed with 50 g of carbon black and 450 g of graphite, the pre-viscosity is adjusted with 7% sodium hydroxide solution and the mixture is compressed to form a strand. The strand is immersed in an aqueous solution of 13% sulfuric acid and brought 28% sodium sulfate to decompose the xanthate. The residues of the FäJlbad are then im Washed out hot water bath, followed by drying

Be: 200-300 ⁰ C, preferably between 250-280 ⁰ C, is pre-treated in an oxidizing atmosphere and then carbonized at a temperature up to 1000 ⁰ C.

Example 3

400 g of polyacrylonitrile was dissolved in dimethylformamide and mixed with 600 g of graphite The Preßviskosität is adjusted with dimethylformamide, and the mixture z "^ is cold-pressed INEM strand. After drying, the strand this is stretched and under tension at 230-250 ⁰ C in air carbonisicrt then oxidized at a temperature up to 1000 "C under a protective gas, and finally graphitized above 2500 ⁰ C.

1 sheet of drawings

Claims (6)

1 claims: 1. A process for the production of writing and drawing leads from a carbonizable material by producing a monofiier strand therefrom, carbonizing it to a carbon strand and then optionally cutting it into pieces suitable for mines, characterized in that the strand undergoes oxidative thermal crosslinking prior to carbonization about 200-300 ⁰ C is subjected. 2. The method according to claim 1, characterized in that the carbonizable starting material Soot and / or graphite is added 3. The method according to one or both of the preceding claims, characterized in that that the strand is grefitisiert after the coking. 4. The method according to one or more of the preceding claims, characterized in, that the strand is drawn before the oxidative heat treatment. 5. The method according to one or more of the preceding claims 1 to 3, characterized characterized in that the strand during the heat treatment, preferably during the Graphitization, kept under tension. 6. The method according to one or more of the preceding claims, characterized in, that the carbonizable starting material is a high molecular weight organic compound, preferably is a polymer or a polycondensate.