Method and apparatus for producing carbon foam boards

Abstract

A method for producing carbon foam boards from starch for use as an insulation material or a filter is disclosed. The method is suitable for producing fine-pored carbon foam boards having a mainly homogeneous structure. This is achieved by sprinkling at least one very thin layer of foamable starch powder onto a temperature-resistant boardlike substrate, introducing the starch-powder-coated substrate into a furnace, introducing air, argon or nitrogen into the furnace until an internal pressure of 720 mbar to 1000 mbar has been established, heating the furnace to a foaming temperature of 180 C.-450 C. to foam the starch powder, maintaining the temperature over a prolonged duration of up to 10 hours to stabilize the foamed starch into a foam board.

Claims

1. A method for producing carbon foam boards, comprising: sprinkling a dry layer of foamable starch powder onto a temperature-resistant substrate board to form a starch-powder-coated substrate board; introducing the starch-powder-coated substrate board into a furnace; introducing argon or nitrogen into the furnace to displace air or pumping out air from the furnace until an internal pressure of 720 to 1000 mbar has been established; heating the furnace to a foaming temperature between 180 C. and 450 C. and thereby foaming the starch powder on the starch-powder-coated substrate board; and maintaining the foaming temperature over a prolonged duration between 1 and 10 hours to stabilize the foamed starch into a foam board.

2. The method as claimed in claim 1, further comprising: carbonizing the foam board at a temperature >1000 C. or graphitizing the foam board at a temperature >2000 C. under vacuum or shielding gas.

3. The method as claimed in claim 1, wherein the foamable starch powder is corn starch, tapioca starch, wheat starch, or starch powder obtained from oats or cereals.

4. The method as claimed in claim 1, wherein the heating to the foaming temperature is carried out with a heating ramp between 3 and 10 C./min.

5. The method as claimed in claim 1, wherein the substrate board is made of a graphitizable material.

6. The method as claimed in claim 1, wherein the substrate board is made of steel or graphite.

7. The method as claimed in claim 1, further comprising sieving the foamable starch powder before or during sprinkling onto a substrate board.

8. The method as claimed in claim 1, further comprising sprinkling, after foaming the starch powder, a further layer of foamable starch onto the foamed foam board, and repeating the foaming.

9. The method as claimed in claim 1, further comprising applying the starch powder to the substrate board or to a previously foamed foam board in locally varying layer thicknesses to form a structured surface of the foam board.

10. The method as claimed in claim 1, further comprising selecting the foaming temperature and the prolonged duration based on a thickness of the layer of foamable starch powder.

11. The method as claimed in claim 1, wherein the substrate board employed is polyacrylonitrile.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further explained below with reference to a working example.

(2) FIG. 1 shows a schematic representation of a conveyor furnace suitable for performing the method.

(3) FIG. 2 shows a schematic representation of a vacuum high-temperature furnace suitable for performing the method and for carbonizing and graphitizing.

DETAILED DESCRIPTION

(4) The apparatus for performing the method is composed according to FIG. 1 of an argon- or nitrogen-filled box 1 comprising a furnace 2 having a heating zone 3 for receiving boardlike substrates 4 coated with starch powder 4.

(5) The furnace 2 may for example be a conveyor furnace evacuable via the box 1 and provided with a conveyor in the form of a temperature-resistant conveyor belt 5 or the like for transporting the starch-powder-coated boardlike substrates 4 from a feeding position 6 into the heating zone 3 (foaming zone) and from said heating zone out into a discharging position 7, wherein the feeding position 6 and the discharging position 7 may also be in the same place or next to one another in space.

(6) Additionally arranged above the feeding position 6 coupled to a shaker apparatus 8 is a container 9 containing the starch powder to be sprinkled onto the boardlike substrate 4.

(7) The container 9 is further provided with a sieving apparatus 10 to ensure that only starch powder of uniform particle size is sprinkled onto the boardlike substrate 4 as a precondition for uniform foaming in the heating zone 3. It is in principle also possible instead of starch powder to employ starch in granulate form, i.e. with a somewhat coarser particle size than powder.

(8) Instead of the starch powder, it is also possible to employ other polymers/polymer mixtures if they melt at a relatively high temperature and simultaneously liberate a gas.

(9) In summary, the furnace 2 should have the following properties: A subatmospheric pressure must be producible in the box 1 and thus also in the furnace 2 via a suction means, the furnace 2 remaining open. The furnace 2 in the box 1 should be operable with different gases that are provided by the box 1. The box 1 should be filled with argon, nitrogen and be fillable/purgeable with air. The furnace 2 should be able to reach an internal temperature of 450 C. The box 1/the furnace 2 should be provided with an offgas system to prevent combustion of the gases formed in the furnace in air.

(10) To produce fine-pored carbon foam boards 11, in a first step at least one thin layer of foamable starch powder or of another foamable polymer or of a polymer mixture in powder form is sprinkled onto a temperature-resistant boardlike substrate 4. This is preferably carried out in the feeding position 7 within the box 1.

(11) The starch-powder-coated boardlike substrate 4 is then transported into the furnace 2 using the conveyor belt 5. Subsequently, air, argon or nitrogen is introduced into the furnace 2 until an internal pressure of 720 mbar has been established in the furnace by displacement of air or by pumping out air from the high-temperature furnace. It is essential here that an internal pressure of 720 mbar to 1000 mbar must be able to be established in the furnace 2.

(12) It is essential that the furnace 2 arranged in the box 1 is always open/employed without a door so that the required internal pressure in the furnace 2 may be adjusted via the box 1, wherein the required gases are also introduced into the furnace 2 via the box 1. This ensures that there is no time loss for controlling gas and pressure, thus allowing for more efficient operation.

(13) The furnace 2 is then heated to 180 C.-450 C. (foaming temperature) in the heating zone 3 to effect foaming of the starch. If air is introduced into the furnace 2 instead of argon or nitrogen, a foaming temperature of 250 C. must not be exceeded.

(14) Otherwise, the specific foaming temperature to be selected depends on the internal pressure, the type of gas, the polymer to be foamed and the gas flow into the furnace. If, for example, the gas flow increases, the foaming temperature must be increased on account of the greater cooling effect.

(15) To effect stabilization of the starch foamed on the substrate 4 to afford a foam board 11, the foaming temperature in the heating zone 3 of the furnace 2 must be maintained over a prolonged duration of up to 10 hours.

(16) To produce thicker foam boards 11, it is sufficient to return the starch-powder-coated boardlike substrate 4 to the feeding position 6 using the conveyor belt 5 to apply a further layer and then to transport it back into the furnace 2. This procedure may simply be repeated until the desired thickness of the foam board 11 has been achieved, with the temperature in the furnace remaining constant.

(17) The foam board 11 which is handleable due to the stabilization is finally carbonized at >1000 C. under vacuum or shielding gas or graphitized at >2000 C. in another furnace. Such a furnace may be a vacuum high-temperature furnace.

(18) If the boardlike substrate 4 is made of a graphitizable material, such as a polymer, e.g. polyacrylonitrile, this may remain joined to the foam board 11 during the carbonization and optionally graphitization. In the simplest case, the boardlike substrate may be made of paper. If the substrate is made of polyacrylonitrile, it may be stabilized.

(19) Alternatively, the boardlike substrate 4 may also be made of graphite or paper-covered graphite or another temperature-resistant material, so that the stabilized foam board 11 may be detached from the boardlike substrate 4 before carbonization since the boardlike substrate 4 in this case does not adhere to the stabilized foam board 11.

(20) The carbonized foam board 11 may also be graphitized in a suitable furnace, e.g. a separate vacuum high-temperature furnace, at a temperature >2000 C.

(21) If thicker foam boards 11 are to be produced, every foaming is followed by sprinkling a further layer of foamable starch powder onto the already present thinner foam board and repeating the foaming until a sandwich structure has been formed.

(22) In this case, the carbonization/graphitization is performable only once foaming is complete for all layers of the sandwich structure.

(23) A structured surface of the foam board 11 is achievable by applying the starch powder to the boardlike substrate 4 in locally varying layer thicknesses.

(24) In the case of thicker applied layers, it is advantageous and necessary to reduce the foaming temperature by up to 10% to facilitate the diffusion of the gases formed during foaming and to achieve a more uniform foaming and to prevent formation of large bubbles. However, the time for which the foaming temperature is kept constant must be correspondingly prolonged. It is alternatively also possible to carry out regular sprinkling of fresh starch onto the foamed starch.

(25) It is in principle also possible to perform the entire method foaming, carbonizing/graphitizing in the same furnace, which must then be a sealed vacuum high-temperature furnace in which the temperatures required for the particular method step can be established.

(26) FIG. 2 shows a schematic representation of a vacuum high-temperature furnace 12 suitable for performing the method for producing carbon foam boards 11 from starch powder or other foamable polymers/polymer mixtures in powder form and also for carbonizing and graphitizing.

(27) The vacuum high-temperature furnace 12 consists of a vacuum-tight housing 13 connected via a connection port 14 to a pump (not shown) for producing a subatmospheric pressure. Further arranged in the vacuum high-temperature furnace 12 is a receptacle 15 for receiving the boardlike substrate 4 for the foam board 11 to be constructed thereupon, for example in the form of a sandwich structure.

(28) Also provided in the upper region to the side in the vacuum high-temperature furnace 12 is a gas inlet 16 which projects into the vacuum high-temperature furnace 12 by a distance above the edge of the receptacle 15.

(29) This gas inlet 16 is connected to a source of inert gas 16 (not shown) and via an angled tube 17 to a conveying screw 18 and a reservoir container 19 for starch 20 arranged thereabove.

(30) The supplying of the pulverulent starch 20 into the high-temperature vacuum furnace 12 is effected using the inert gas flowing into the vacuum high-temperature furnace 12 via the gas inlet 16 which entrains the starch 20 conveyed by the conveying screw 18 from the reservoir container 19 into the angled tube 17 on account of the gas flow through the gas inlet 16 and is deposited on the boardlike substrate 4 arranged on the receptacle 15 in a manner similar to the dusting of substrates and undergoes foaming upon reaching the foaming temperature.

(31) This makes it possible to produce thick foam boards 11. Alternatively, the temperature may also be alternately increased and decreased to obtain layers of greater or lesser density.

(32) It is also possible to produce sandwich structures by initially depositing a layer of starch 20 and subsequently foaming said layer and then depositing and foaming a further layer of starch 20, etc.

(33) Upon reaching the desired layer thickness of the foam board 11 the gas supply may be terminated and the vacuum high-temperature furnace heated up to 1000 C. or 2000 C. for carbonization and graphitization, respectively.

LIST OF REFERENCE NUMERALS

(34) 1 Box 2 Furnace 3 Heating zone 4 Boardlike substrate 4 Starch powder 5 Conveyor belt/conveyor 6 Feeding position 7 Discharging position 8 Shaker apparatus 9 Container 10 Sieving apparatus 11 Foam board 12 Vacuum high-temperature furnace 13 Housing 14 Connection port 15 Receptacle 16 Gas inlet 16 Inert gas 17 Angled tube 18 Conveying screw 19 Reservoir container 20 Starch