Patent Application
20170081787
The present invention relates to a method of producing carbon fibers from cellulose fibers, which comprises bringing cellulose fibers having a water content of more than 20 parts by weight of water per 100 parts by weight of cellulose fiber into contact with a solution of additives and thereafter converting the additized cellulose fibers comprising not less than 1 part by weight of additives per 100 parts by weight of cellulose into carbon fibers.
Carbon fibers are obtainable by pyrolysis of polyacrylonitrile or cellulose fibers. As renewable raw materials, cellulose fibers are becoming more and more important for the growing market for carbon fibers.
Mingqiu Zhang, S. Zhu, H. Zeng, Y. Lu describe in Die Angewandte Makromolekulare Chemie 222 (1994), 147 -163 (No. 3908) the production of carbon fibers from sisal fibers. The sisal fibers are washed with water and dried. They are then treated with a solution of (NH4)2HPO4 in water, redried and converted into carbon fibers by pyrolysis.
Fanlong Zeng, Ding Pan and Ning Pan use viscose fibers for producing carbon fibers (Journal of Inorganic and Organometallic Polymers and Materials, Vol. 15, No. 2 June 2005). Again, dried cellulose fibers are treated with solutions of additives and then converted into carbon fibers.
Hui Li, Yonggang Yang, Yuefang Wen and Lang Liu (Composites Science and Technology 67 (2007) 2675-2682) impregnate the viscose fibers with an organosilicon compound before drying. After the fiber thus pretreated has been dried, the cellulose fiber is subjected to a customary treatment with additives, here an aqueous solution of ammonium sulfate and ammonium chloride, and finally carbonized.
Cellulose dissolved in ionic liquid is also used in CN 101871140 for production of carbon fibers.
Processes for producing carbon fibers should have a very high carbon yield; that is, ideally all of the carbon in the starting fiber should end up in the carbon fiber. The carbon yield is still unsatisfactory when cellulose fibers are used. Some of the cellulose carbon is lost by degradation into ultimately carbon monoxide and carbon dioxide. There is also a need to further improve the mechanical properties of the carbon fibers obtained from cellulose fibers.
The problem addressed by the present invention was therefore that of providing an improved method of producing carbon fibers from cellulose fibers.
The problem was solved by the method defined at the outset.
The Cellulose Fibers
The cellulose fibers are the starting material for the method. Cellulose fibers within the meaning of the present invention are fibers consisting of cellulose or modified cellulose to an extent of more than 60 wt %, particularly more than 80 wt %, more preferably more than 90 wt %.
In one particular embodiment, the cellulose fibers consist of cellulose or modified cellulose to an extent of more than 98 wt %, most preferably 100 wt %.
Modified cellulose is cellulose with etherified or esterified hydroxyl groups. Modified cellulose may comprise for example cellulose acetate, cellulose formate, cellulose propionate, cellulose carbamate or cellulose allophanate.
The cellulose fibers used preferably consist of cellulose in the above-specified minimum amounts.
The cellulose fibers brought into contact with a solution of an additive have a water content of more than 20 parts by weight of water, particularly more than 30 parts by weight of water, more preferably more than 50 parts by weight of water and most preferably more than 70 parts by weight of water per 100 parts by weight of cellulose fiber.
In general, however, the water content is not higher than 500, particularly not higher than 300 parts by weight of water per 100 parts by weight of cellulose fiber.
The cellulose fiber having the above water content is obtainable in a simple manner, for example by dipping a dried cellulose fiber into water. Not only natural cellulose fibers are suitable for this purpose, but also synthetic cellulose fibers.
Natural cellulose fibers are in particular cellulose fibers derived from cotton.
A preferred embodiment uses synthetic cellulose fibers.
A preferred embodiment uses synthetic cellulose fibers obtained immediately beforehand via a spinning process.
The cellulose fibers are then preferably obtained by
In the above spinning process, a spin bath is prepared by dissolving cellulose in a solvent. The cellulose fiber is derived from this spin bath by coagulating the cellulose in the form of a fiber.
There are various species of cellulose fibers depending on solvent and adjuncts used in the spin bath:
viscose fibers produced by the viscose process,
Lyocell® fibers produced from a spinning solution comprising NMMO (N-methylmorpholine N-oxide) as solvent, or
cellulose fibers obtained from spinning solutions comprising ionic liquid as solvent, as described in WO 2007/076979 for example.
In all the cases above, the cellulose fibers obtained are washed with water in order to remove adherent solvent or adherent additives from the spin bath.
The step of contacting with water is effected such that the cellulose fiber takes up water in the desired amount specified above. To this end, the cellulose fiber may be dipped into water for a sufficient length of time or, in a continuous process, be led through a sufficiently long water bath.
Preferably, the process for producing the cellulose fibers does not involve any measures for drying. The cellulose fiber obtained in the spinning process is washed with water without previous drying and thereafter, again of course without previous drying, brought into contact with the solution of the additive. The cellulose fiber concerned is therefore known as a never-dried cellulose fiber.
The Additization of Cellulose Fibers
The aqueous cellulose fibers are brought into contact with a solution of additives.
The solution of additives is preferably a solution of additives in a hydrophilic solvent, particularly in water, hydrophilic organic solvents, e.g., alcohols or ethers, or mixtures thereof. Particular preference for use as hydrophilic solvents is given to water or mixtures of water with other hydrophilic organic solvents miscible with water in any proportion, while in the last case, in a preferred embodiment, the water fraction in the solvent mixture is not less than 50 wt %.
A solution of additives in water is concerned in particular.
The solution may comprise just a single additive or a mixture of various additives.
Useful additives include particularly compounds having a water solubility of not less than 10 parts by weight, preferably of not less than 20 parts by weight, in particular of not less than 30 parts by weight, per 100 parts by weight of water under standard conditions (20° C., 1 bar).
The additives preferably comprise low molecular weight compounds having a molecular weight of not more than 1000 g/mol, more preferably not more than 500 g/mol, particularly not more than 300 g/mol.
Possible additives in one preferred embodiment include salts or acids, e.g., inorganic salts, inorganic acids, organic salts or organic acids.
Suitable organic acids are, in particular, carboxylic acids, sulfonic acids or phosphonic acids.
Suitable organic salts are, in particular, salts of the foregoing organic acids, in which case metal salts may be concerned, in particular alkali metal salts, or salts having organic cations.
Suitable organic acids include, for example, those which react with the cellulose and become attached thereto through a chemical reaction, for example a substitution reaction.
Phosphoric acid is particularly useful as inorganic acid.
Suitable inorganic salts are particularly those whose anions comprise phosphorus atoms, sulfur atoms, nitrogen atoms, for example in the form of phosphate, hydrogenphosphate, phosphite, hydrogenphosphite, sulfate or sulfite, or comprising chloride as anion.
The cations of the above inorganic salts may comprise particularly metal cations, preferably alkali metal cations such as Na+ or K+, or ammonium (NH4+).
(NH4)2HPO4, NH4SO4 or NH4Cl may be mentioned by way of example.
The above additives are frequently additives also used as flame retardants. It is believed that these additives interact with the primary hydroxyl group on the glucose ring (i.e., the CH2OH group) and work during the pyrolysis to counteract the cellulose breaking down into volatile compounds of carbon.
The total amount of all additives in the solution is, for example, from 0.05 to 5 mol/per liter of solution, preferably from 0.1 mol to 2 mol/per liter of solution.
The step of contacting with the solution of the additives is effected such that the cellulose fiber takes up additives in the desired amount. To this end, the cellulose fiber may be dipped into the solution for a sufficient length of time or, in a continuous process, be led through a sufficiently long solution bath.
In one preferred embodiment, the cellulose fiber is led through the solution of the additive in a continuous manner.
The contact time of the cellulose fiber with the solution of the additives is preferably not less than 0.5 second, more preferably not less than 2 and most preferably not less than 10 seconds. The contact time is generally not longer than 100 seconds, preferably not longer than 30 seconds.
The additized cellulose fiber obtained comprises not less than 5 parts by weight of additives per 100 parts by weight of cellulose in a preferred embodiment. The additized cellulose fiber more preferably comprises not less than 1 part by weight of additives and most preferably not less than 3 parts by weight of additives per 100 parts by weight of cellulose fiber. The cellulose fiber generally comprises not more than 30 parts by weight of additives, particularly not more than 10 and/or not more than 5 parts by weight of additives per 100 parts by weight of cellulose fiber.
The production of the cellulose fiber in a spinning process and the subsequent further processing by washing the cellulose fiber and contacting the cellulose fiber with the solution of the additives are preferably constituent parts of a continuous overall process. In it, once the cellulose fiber has been obtained, it is generally fed into the individual further processing steps via mobile rollers.
Finally, excess solvent from the solution of the additives may be removed by squeezing and the additized cellulose fiber may be rolled up.
The additized cellulose fiber may finally be dried, for example at temperatures of 50 to 300° C. Such drying is advisable when the additized cellulose fiber is first to be stored or transported before its conversion into a carbon fiber.
Lastly, the additized cellulose fiber is converted into a carbon fiber by pyrolysis.
The pyrolysis is generally carried out at temperatures of 500 to 1600° C. It may for example be carried out under air or under a protective gas, for example nitrogen or helium. It is preferably carried out under a protective gas.
The cellulose fiber may be dried before pyrolysis. In the case of previously dried and stored cellulose fibers, drying may optionally be repeated.
One possibility is a multi-step process wherein the cellulose fiber is dried at temperatures in the range from 50 to 300° C. and then pyrolyzed at temperatures in the range from 500 to 1600° C., preferably from 700 to 1500° C.
Both the drying temperature and the pyrolysis temperature may be raised stepwise or continuously.
One possibility, for example, is a drying operation in two or more steps, for example at 50 to 100° C. in a first step and at 100 to 200° C. in a second step. Contact time in the individual steps may be for example from 5 to 300 seconds in each case and range in total from 10 to 500 seconds during drying.
One possibility, for example, is a pyrolysis where the temperature is raised continuously, for example from 200° C. at the beginning until the eventual attainment of 1600 or 1400 or 1200° C. The rate of temperature increase may be from 1 to 20 kelvins/minute for example.
Exposure of the cellulose fiber to a temperature in the range from 900 to 1600° C. should preferably be for 10 to 60 minutes.
The carbon yield at pyrolysis is generally from 20 to 95 wt %; that is, the carbon fiber comprises from 20 to 95 weight percent of the carbon present in the cellulose fiber. The carbon yield is more particularly from 70 to 95, more preferably from 70 to 90 and most preferably from 70 to 85 wt %.
The process of the present invention provides an enhanced carbon yield. The carbon fiber obtained has very good mechanical properties, in particular good tenacity and elasticity.
Cellulose Fiber
The cellulose fiber employed in the example of the invention and in the comparative example is a synthetic high-tenacity cellulose fiber of the type used in the manufacture of automotive tires. Cellulose fibers of this type are known as tire cord fibers. The cellulose fiber in the example of the invention was never dried after its formation, hence its appellation as “never-dried tire cord fiber”. The cellulose fiber in the comparative example was dried. However, cellulose fibers typically comprise bound residual water, so the water content of dried cellulose fibers may be, for example, up to 20 wt %.
The never-dried tire cord fiber presented for additization has a water content of 150% and numbers 1000 filaments having a linear density of 2.2 dtex per filament.
The fiber is additized and dried in a continuous process on godets. Godets are rolls transporting the fiber in a continuous manner through the plant. Four (4) of these godets are employed. Between the first and second godets, the fiber is loaded with the additives via a dip bath. Between the third and fourth godets, a hot air duct effects a drying operation. At the end, a tension-controlled winder winds up the additized and dried fibrous material.
The godets all run at a speed of 6 m/min. The first godet serves to haul up the water-stored never-dried tire cord fibers. The fiber is wrapped around the godet 2 times, which corresponds to a contact time of 10 sec. The fiber is then directed through a dip bath containing an ammonium dihydrogenphosphate solution (concentration of ammonium hydrogenphosphate: 0.54 mol/l). The residence time therein amounts to about one second. The material is then wrapped six times around the second godet. This step is designed to allow surplus additization to drip off and to effect a homogeneous distribution of the ammonium hydrogen-phosphate in the fiber. The contact time here is 72 seconds. This is followed by drying on the heated third godet at 80° C. Here 10 wraps correspond to a contact time of 100 sec. The fiber is then led through a hot-air duct at 150° C. The residence time therein is 12 sec. The dry strand is then wrapped 4 times around the last godet (contact time 24 sec) before a tension-controlled winder winds up the material at a pre-tension of 0.1 cN/tex.
The additized cellulose fiber is subsequently carbonized in two stages under a protective gas. In the first stage, the fiber is heated at 2° K/min to 260° C. and after a residence time of 10 min heated at 10 k/min to 1400° C. and thereafter cooled down.
The carbon yield is 80 wt %, the fiber tenacity is 1.4 Gpa and the elongation at break is 3.1%.
Comparative Example with Drying Prior to Additization
The dried tire cord fiber presented for additization has a water content below 20 wt % and numbers 1000 filaments having a linear density of 2.2 dtex per filament. The experimental procedure corresponds to Example 1.
The carbon yield is 65 wt %, the fiber tenacity is 1.1 Gpa and the elongation at break is 2.2%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14168572.7 | May 2014 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/060479 | 5/12/2015 | WO | 00 |
