WO1992016469A1

WO1992016469A1 - Insulation material

Info

Publication number: WO1992016469A1
Application number: PCT/GB1992/000433
Authority: WO
Inventors: Stephen David Ellacott
Original assignee: Calcarb Limited
Priority date: 1991-03-14
Filing date: 1992-03-11
Publication date: 1992-10-01
Also published as: GB9318048D0; GB9105415D0; GB2269394B; GB2269394A

Abstract

The invention relates to a method for producing refractory material comprising a shared, rigid carbon-fibre based cladding material which is coated as required by at least one layer of finely-divided graphite in a carbonisable vehicle which may be suitable resin solution. The coated surface is then heat treated in a non-oxidising atmosphere to carbonise the vehicle and the coated surface is then fired under reduced pressure in the presence of a carbon-rich gas such as methane to form a deposit of pyrolytic carbonaceous material in an outer layer of the area. The heat treatment stage may be carried out at a temperature gradually to, say, 400 C and then brought up to about 900 C, the subsequent firing temperature range being between about 970 C and 1200 C.

Description

INSULATION MATERIAL

The present invention is concerned with improvements in or relating to insulation material, particularly but not exclusively, material for use as lining in a furnace for example in the heat treatment of metals including brazing, hardening, sintering and refining or in the ceramics or crystal-growing industries The furnace may be one in which a non-oxidising atmosphere is required and may be designed to operated under conditions of reduced atmospheric pressure.

In order to protect the structure of the furnace from the conditions of high temperature, chemical attack and the like, it is customary to provide a lining which may be replaced from time to time as required. Advantageously, a lining should exhibit thermal stability, abrasion resistance and mechanical strength, and should be readily conformable to the interior contours of the furnace giving good reflectance of radiated heat and uniformity of temperature characteristics to maximise the utilisation of the operative zone of the furnace. Moreover, the lining should be highly abrasion-resistant and relatively dust-free. It is common practice for such high-temperature resistant linings to be formed from a carbon material, for example, cladding panels or blocks formed from carbon fibres which may be bonded by a carbonised resin to produce a low bulk density material having the desired thermal properties. However, the requirements of abrasion resistance for example are not met unless the panels are provided with a protective layer comprising a sealant coating or a layer of suitable sheet material.

The types of protective layers which may be used include the provision of a carbon foil bonded to the cladding panels or blocks by a cement bonding technique. Although this has many advantages, including erosion resistance and impermeability, over for example, a paint coating, there are problems of de-lamination under adverse furnace conditions and the extreme difficulty of cladding complex shapes and surfaces exhibiting more than slight curvature. In such cases, it is often necessary to use a carbonisable paint in very difficult areas.

It is also known to use a composite coating of non-woven and/or laminated carbon fibre in a matrix which forms an abrasion-resistant composite, but the resultant cladding contains impurities and requires a degree of purification before successful use in, for example, crystal growing furnaces. The cladding is only satisfactorily bonded or mechanically fixed to flat surfaces. Where the cladding is bonded, subsequent failure of the bonding because of chemical attack is, however^', frequently encountered.

It is an object of the present invention to provide insulation material the use of which minimises the effects of the above mentioned disadvantages. The present invention therefore provides a method of producing a refractory cladding material comprising the steps of (a) procuring an at least substantially rigid, shaped portion of low-density carbon-fibre based material, (b) applying to a surface area of the material at least one coating of finely-divided graphite in a carbonisable vehicle, (c) heat-treating the coated surface area in a non-oxidising atmosphere to carbonise the vehicle, and (d) firing the coated surface area under non-oxidising conditions of reduced pressure and in the presence of a carbon-rich gas to form a deposit of pyrolytic, carbonaceous material in an outer layer of said surface area.

Advantageously, the carbon-rich gas may be selected from the group including methane, propane, ethane, acetylene.with or without the addition of hydrogen or other suitable gases.

Conveniently, the carbon fibre material may be carbon bonded carbon fibre material or may be any other suitable material in which carbon fibres are felted or otherwise secured into an at least substantially rigid condition.

Preferably, the finely-divided graphite of the applied coating may comprise particles which are generally below 1 micron in size. Conveniently, the particles are bonded using a resin, which when carbonised acts to stabilise the coating(s) applied to the carbon-fibre based material, and any necessary bond-enhancing additives. Preferably, a plurality of coatings may be applied to the material in step (b) , each coating being dry before the next is applied.

Advantageously, at least the final coating is allowed to dry at room temperature.

If desired, each coating may be lightly abraded prior to the application of the next coat.

When the coatings are complete the top coating may be buffed or rubbed and a reflective surface may be produced.

Conveniently, the heat treatment of step (c) may be carried out in a substantially nitrogen atmosphere, the temperature being raised gradually over predetermined periods of time.

Advantageously, the firing step (d) includes the introduction of a diluent gas, for example, nitrogen, argon, helium or possibly hydrogen at a desired rate suitable to control the rate of deposition of carbon from the carbon-rich gas which may be methane.

Conveniently, to exemplify the method of the invention to be described below, the firing step (d) may be carried out in a chamber initially evacuated to approximately 1 bar or less, and heated to a temperature in the approximate range of 970° - 1200^βC, typically about 1010"-1090 " C. Methane and an optional diluent gas, for example, nitrogen, are supplied to the chamber. Proportions by volume may vary from 100% methane to 10% methane and 90% nitrogen, typically 25% methane, 75% nitrogen, and the pumping rate adjusted to maintain a total system pressure of between 10 and 150 mbar, preferably approximately 20-70 mbar, for at least four or five hours, preferably at least twenty five hours, and, if required, up to one hundred hours or more.

If preferred, steps (c) and (d) may be carried out sequentially in a furnace or, if more convenient, as independent steps in accordance with requirements.

The invention further provides a refractory cladding material comprising a body portion consisting of a carbon fibre-based material having a relatively low density, said body portion having a surface area comprising a pyrolytic deposit of carbon having a high density relative to that of the body portion.

Advantageously, the pyrolytic deposit of carbon is a botryoidal type deposit, for example in the form of fine clusters of spheroidal aggregations. The formation of such a deposit may be enhanced by the presence of carbon converted from carbon/graphite and carbonisable binder from a coating of the same applied to the body portion according to steps (a) to (d) of the method described above.

It will be apparent that the carbon/graphite/ carbonisable binder coating, in which the graphite may be in a colloidal form, may act as a barrier to the penetration of carbon from the carbon-rich gas into the body portion as a result of which the carbon deposit formed may be largely confined to a surface zone of the body portion. The deposit has a dense appearance and is very hard and wear-resistant.

Under certain circumstances, it may be found convenient to provide an interrupted deposit to facilitate de-gassing. It may therefore be found advantageous to provide for example, chamfers on edges of blocks or panels to provide outlets for the gases.

There will now be described several examples of the method according to the invention. It will be understood that the description is given by way of example only and not by way of limitation.

In the Examples, the general conditions of operation involve the use of a so-called graphite paint having between 10% and 20% by weight of solids in a solvent or solvent blend. A typical blend of solvents may comprise isopropanol (80-85% by weight) with other solvents such as 2-ethoxyethanol, n-butyl alcohol and hexyleneglycol.

In the following examples, the solvent blend comprises: Isopropanol 83% 2-ethoxyethanol 5% N-butyl alcohol 6% Hexy1eneglyco1 6%.

The solids content in the example is a resin-bonded micrographite present as 12% by weight in a colloidal suspension. The resin is a thermo-setting or chemically-enhanced thermoplastic resin which forms a stable carbon residue on thermal decomposition in a non-oxidising atmosphere. In the examples, the resin is a phenolformaldehyde resin, with a hexamine bonding additive.

The paint is conveniently applied to blocks, panels or machined shapes of low density carbon-bonded carbon fibre material using a spray, although a brushing technique may be used if preferred, but the results are generally less smooth and less evenly distributed.

Several coats of paint may be applied and dried between the application of each layer, at a temperate of, say, 60^βC. Each drying period is approximately 20 minutes. In the examples a total of five coats were applied, but between five and seven are typical. However, the ideal number of coats differs according to the solids content of the paint; the lower the solids content, the more coats are required up to as many as fifteen.

The resin content of the paints may be between 22% and 28% by weight solids. Lower levels of resin content were found to produce deposits which tended to flake under some conditions and higher levels tended to produce a degree of shrinkage which could cause cracking of the surface. A convenient figure may be found to be about 25% to 26% by weight.

The coatings on the panels, blocks or machined shapes are then to be heat-treated to achieve stabilisation, reducing the resin to carbon prior to the firing step. The initial temperature range for the treatment may be between 650" - 1200^βC, but a convenient temperature is approximately 900"C, in an inert atmosphere, e.g. nitrogen, helium or argon. The temperature should preferably be raised gradually, a rise no greater than 30^βC per hour until 150^βC is reached, and no greater than 55"C per hour from 150^βC to 400^βC. Above 400"C, the heating rate can without adverse effect be accelerated until the temperature of the coated panels, blocks or machined shapes has been brought to about 900"C. Where the firing step is to be carried out without interruption in the process the treatment chamber is then evacuated to a low level of pressure, the firing step being carried out between 970^βC and 1200°C preferably between 1000^βC and 1100^βC. For panels, blocks and machined shapes used in Examples I and II, the firing temperature was 1090^βC, and in Example III was 1010°c.

The pressure in the chamber is firstly reduced to 1 mbar and the next stage comprising introducing nitrogen at a rate sufficient to produce and maintain pressure of 38 mbar. Methane is added at a rate sufficient to produce and maintain a partial pressure of 9 mbar. During the firing step the pumping rates of the gases were adjusted to maintain a total system pressure of about 67-68 mbar. The duration of the firing step depends on conditions of temperature, gas ratios, chamber pressures and gas flow rates, but, the Examples below, in Example I, the firing time was eight hours, Example II, 24 hours, and for Example III 100 hours. However, this time could be as short as four or five hours or up to 100 hours or more if required.

All fired blocks and panels which had been coated exhibited a more or less lustrous appearance in the coated areas, these areas appearing to be of increased wear-resistance and having a higher density than the remainder of its associated block or panel. Example I

A number of cubic blocks of material were coated with various grades of graphite paint as listed in Table I. One block was left un-coated as a control. The firing time was 8 hours.

All coated surfaces exhibited a lustrous appearance which under microscopic examination showed a fine botryoidal type deposit of spheroidal carbon aggregations.

Table I

Initial Initial Final Total C Densit wt. (g) density density deposited factor (g/cc) (g/cc) (g)

Sample A 100.71 0.199 0.229 8.83 1.15

(coating with large graphite particles c.30 micron)

Sample B 103.27 0.199 0.235 6.32 1.18 (coating with c. 1 micron particle size)

Sample C 97.89 0.191 0.226 5.72 1.18 (coating with < 1 micron particle size)

Uncoated sample 101.30 0.200 0.385 93.43 1.93

It is apparent from the above table that the uncoated block allowed a considerable penetration of carbon vapour into the block to take place to an extent that there was a barely discernible difference between the surface zone of the block and the remainder thereof.

By contrast, all the coated blocks exhibited the above mentioned hard lustrous surface which may be polished by rubbing to a more or less reflective sheen. The coating has therefore acted as a barrier to the carbon vapour, which leads to a carbon deposit being formed in a surface zone or layer of the block. lt will also be apparent that where the coating includes relatively coarse graphite particles, a satisfactory density increase figure obtained but is accompanied by additional total carbon deposit, representing some carbon penetration into the block beyond the surface zone. Example II

A further run was conducted using coating C on cubic blocks of standard material to ascertain the effect of longer residence time in the chamber (24 hours) at appropriately lower gas flow rates. Some sooty deposits were observed and the surface layer on the coated blocks had a rougher appearance. However, there was little sign of penetration of the carbon vapour deposit beyond the surface layer which exhibited a satisfactory degree of abrasion resistance.

The results are listed in Table II which includes data for an uncoated control block.

Table II

Sample D 100.28 0.196 0.245 14.65 1.25

Sample E 97.79 0.196 0.247 15.08 1.25

Sample F 100.85 0.202 0.253 14.47 1.25

Uncoated Sample 100.12 0.196 0.331 69.39 1.69 The coated samples D, E and F were subjected to abrading and cutting treatment and the surface layer was found very difficult to break.

Sample E was examined using a projection microscope with polariser and analyser attachments. The results indicated a varied orientation of the graphite particles, the deposited carbon seeming to have coated the paint particles in an apparently spheroidal formation with inclusions within the coating. Where the sample had been polished, there was preferential erosion at interfaces of the composite and sample-mounting resin, evidence of a significant degree of wear-resistance of the deposit.

Sample E was further examined with a scanning electron microscope. The coated layer was found to be 40 micron thick, lying parallel to the true composite surface. The coated layer was consistent in thickness and followed accurately the slight surface deviations. The coated layer appeared as dense carbon with only slight evidence of any porosity.

High magnification showed very slight porosity, with mean pore size less than 0.1 micron. The carbon vapour showed penetration into the carbonized paint layer to approximately two thirds of the total depth.

Alternative process conditions may be used in the coating process producing some variations in the thickness of coating. EXAMPLE III

At a system temperature of 1010*C and pressure of 25 mbar with a gas feed of mix of 2 parts methane to 5 parts nitrogen by volume and a process time of 100 hours, an increased thickness of surface coating was achieved.

The variation of the carbon-rich gas concentrations, the process duration times and firing temperature produces satisfactory coatings with variation in thickness of deposit. The coating and substrate exhibit compatible thermal expansion.

Runs using blocks of more intricate shapes, including re-entrant and curved surfaces, gave results which were satisfactory in wear characteristics and hardness.

Various modifications may be made within the scope of the invention as defined by the following claims.

Claims

1. A method of producing a refractory cladding material comprising the steps of (a) procuring an at least substantially rigid, shaped portion of low-density carbon-fibre based material, (b) applying to a surface area of the material at least one coating of finely-divided graphite in a carbonisable vehicle, (c) heat-treating the coated surface area in a non-oxidising atmosphere to carbonise the vehicle, and (d) firing the coated surface area under non-oxidising conditions of reduced pressure and in the presence of a carbon-rich gas to form a deposit of pyrolytic, carbonaceous material in an outer layer of said surface area.

2. A method as claimed in claim 1, wherein the carbon-rich gas is selected from the group including methane, propane, ethane and acetylene.

3. A method as claimed in either one of claims 1 and 2, wherein the carbon-rich gas is diluted with a gas selected from the group including nitrogen, argon, helium and hydrogen.

4. A method claim as claimed in any one of the preceding claims wherein the carbon fibre material is carbon bonded material in which the carbon fibres are secured together into an at least substantially rigid "condition.

5. A method as claimed in any one of the preceding claims, wherein the finely-divided graphite of the coating to be applied in step (b) comprises particles having a size less than 1 micron.

6. A method as claimed in either one of claims 4 and 5, wherein the carbonisable vehicle is a resin in solution in an organic solvent.

7. A method as claimed in claim 6, wherein the resin is a phenolformaldehyde resin.

8. A method as claimed in either one of claims 6 and 7, wherein a bonding agent is added to the carbonisable vehicle.

9. A method as claimed in claim 8, wherein the bonding agent is hexamine.

10. A method as claimed in claim 1, wherein the heat-treatment of the coated surface area comprising subjecting the area to a temperature between 900^βC and 1200^βC, raising the temperate to a level within that range by gradual increase in temperature over an extended period of time.

11. A method as claimed in claim 10, wherein the temperature is first raised at a rate no greater than 30°C per hour until a temperature of approximately 150^βC and thereafter no greater than 55°C per hour until a temperature of about 400^βC is reached, thereafter continuing to raise the temperature to the required level of between 650^βC and 1200^βC.

12. A method as claimed in claim 11, wherein the temperature is raised to about 900"C.

13. A method as claimed in claim 1, wherein the firing of the coated surface area takes place at a temperature between about 970"C and 1200^βC.

14. A method as claimed in claim 13, wherein the firing temperature lies between 1000^βC and 1100^βC.

15. A method as claimed in either one of claims 1 and 13, wherein the firing step is carried out in a chamber in which the pressure is first reduced to about lmbar prior to the introduction of the carbon-rich gas at a rate appropriate to maintain a pressure of between approximately 10 and 150 mbar.

16. A method as claimed in claim 15 wherein the pressure is maintained at a level between 20 and 70 mbar.

17. A refractory cladding material comprising a rigid, shaped portion of low-density carbon-fibre based material, heat-treated and fired according to steps (c) and (d) thereof respectively to produce thereon an abrasion-and wear-resistant outer layer.

18. A material as claimed in claim 17, wherein said outer layers comprise a fine deposit of carbon aggregates.