GB2365422A

GB2365422A - Bonded strontium aluminate refractory fibre materials

Info

Publication number: GB2365422A
Application number: GB0029384A
Authority: GB
Inventors: Craig John Freeman; Paul Nigel Eaton
Original assignee: Morgan Crucible Co PLC
Current assignee: Morgan Crucible Co PLC
Priority date: 2000-08-04
Filing date: 2000-12-01
Publication date: 2002-02-20
Also published as: KR20030059088A; GB0019268D0; ZA200300821B; GB0029384D0

Abstract

A refractory material comprises strontium aluminate fibres and an inorganic binder which comprises when fired greater than 35 wt% strontium oxide. The binder may comprise Al<SB>2</SB>O<SB>3</SB> and SiO<SB>2</SB> as well as SrO. The material may also comprise a filler, e.g. shot from the manufacture of the fibres, a latex binder and starch. The material is used in the manufacture of various products, including the manufacture of boards and shaped materials by vacuum forming and firing.

Description

<Desc/Clms Page number 1> BONDED FIBROUS MATERIALS This invention relates to bonded fibrous materials and is particularly applicable to materials comprising saline soluble fibres bonded with a binder.

Refractory ceramic fibres (RCF) are well known materials and typically comprise an alumino- silicate inorganic fibre formed from an oxide melt which is spun, blown, drawn, or otherwise formed into fibres. Such RCF fibres are used in the manufacture of various industrial and domestic articles. Typical uses of RCF are for applications in which resistance to temperatures in excess of 800*C is required.

Much RCF fibre is used in the form of needled blankets of fibre in which structural integrity is provided by the fibres that are tangled together in the needling process. (Such products are known as "blankef '). Sometimes a binder is used to lock the fibres together subsequent to exposure to high temperature. Blanket can be processed further to form cut shapes or folded to form insulating modules.

RCF fibre is also used in the production of so-called "Converted Products". Converted products comprise materials in which the RCF is processed further to provide materials in which the RCF is present as either a minor or major constituent. Typical converted products include the following:- "Board" - substantially rigid flat sheets containing inorganic and/or organic binders produced by a wet process (for example made by dehydrating a suspension of RCF and binders); 'Paper' a flexible fibrous insulating material with a thickness of less than or equal to 6mm, formed on paper making machinery (for example RCF in sheet form with a binder); "Shapes" substantially rigid shapes made of ceramic fibre with the addition of inorganic and/or organic binder, fired or unfired (for example, RCF formed by vacuum forming into a variety of shapes); "Fire shapes'#- RCF formed by a vacuum forming route and used for domestic and industrial fires either as radiant bodies or for decorative appearance;

"Castables"-- ceramic fibre with inorganic and/or organic binder which may be cast (for example, RCF in the form of cements, concretes and mortars); "Mastics" - A mouldable material containing RCF with binders and which may be trowelled, hand moulded, or dispensed from a pressure gun and which sets upon drying/heating; "Extrusion" - A mastic-like material that may be used in the manufacture of extruded sections and tubes; "Textiles" - ceramic fibre which has been woven with or without the addition of other filaments, wires, or yams (for example, RCF formed into rope, yam, mats and the like by textile technology).

In many of the above mentioned applications binders are used. There are two broad classes of binders:- "Organic binders" - which serve to improve the handling characteristics of the product concerned at low temperatures but which bum off at higher temperatures. Organic binders include, for example, such materials as starch.

"Inorganic binders" - which may be effective to improve the handling characteristics of the product concerned at low temperatures, but which also give integrity to the product after exposure to high temperatures. Inorganic binders include, for example, such materials as colloidal silicas, aluminas, and clays.

All of the above materials and concepts are well known in the refractory industry.

In recent years, a number of different types of fibre have been proposed which are refractory and yet soluble in body fluids. Among these fibres are the strontium aluminate fibres disclosed in W096/04214. A preferred range of compositions specified in W096/04214 was that the fibres comprise at least 90%, preferably at least 95%, by weight SrO, A1203, and a fibre forming additive, and had a composition comprising:- Sro 41.2wt% - 63.8wt% A1203 29.9wt% - 53. 1 wt%.

The applicant's currently preferred composition is:- Sro 58 0.5 wt% A1203 30 0.5 wt% Siol 12 0.5 wt% incidental impurities < 3wt%, preferably less than 2wt%, more preferably less than I wt%, which shows a good compromise between formability (the Si02 giving ease of manufacture) and high temperature performance.

As a fibre, these fibres are useable at temperatures in excess of 1260'C and some are useable at temperatures in excess of 1400T or even in excess of 1500'C. However, problems arise in trying to make converted products including inorganic binders.

Converted products including inorganic binders have to meet several criteria. These criteria include: the shrinkage of the converted product on firing (which should be low); the strength of the converted product both in the green and when fired (which should be high); and the density of the converted product (which, for a given level of thermal conductivity, should be low so as to keep the thermal mass low).

Inorganic binders conventionally used for RCF or other silicate fibres include colloidal silicas, clays, phosphates, and phosphonates. These materials seem to be incompatible with strontium aluminate fibres because:- # phosphates and phosphonates migrate in wet processing of the materials to give a converted product containing relatively high surface concentrations but relatively low concentrations in the core of the converted product (and hence low strength and machineability of the converted product) # it is difficult to get high enough concentrations of phosphates and phosphonates in the converted product for adequate strength without reducing refractoriness # colloidal silicas and clays do not migrate, but react with the fibres at temperatures of 1400"C or more.

The present invention has as its object the provision of binders that do not migrate to the same extent as phosphates or phosphonates, and which do not react adversely with the fibres to the same extent as colloidal silicas and clays.

Accordingly, the present invention provides a refractory material comprising a strontium aluminate refractory fibre and an inorganic binder comprising when fired greater than 35wt% strontium oxide.

Preferably the inorganic binder has the composition when fired (based upon the amounts of strontium, aluminium and silicon present calculated as oxide) comprising:- A1203 aluminium. oxide content of strontium aluminate fibre 65wt% Si02 silicon oxide content of strontium aluminate fibre 20wt%.

Further features of the invention will be apparent from the claims and the following description. The invention is illustrated in the following description with reference to board, but is applicable to shapes, fire shapes, and any other converted product including an inorganic binder.

The most common conventional method of forming converted products such as board is by vacuum forming, in which a dilute slurry of inorganic fibres (typically alurnino-silicate fibres) is prepared, typically containing anionic colloidal silica. On addition of cationic starch flocculation takes place due to the attraction of the opposing electrical charges and discrete agglomerates of fibre, starch, and colloidal silica are formed (known as flocs).

When a meshed (male or female) mould is placed in to the forming tank and a vacuum applied, the flocs are drawn down on to the mesh. When the mould has filled sufficiently it is removed from the slurry and a vacuum applied for a further period to remove as much water as possible. The resulting shape containing approximately 40%-50% water is carefully removed and driied and the process water is recycled.

A series of boards were made to test various binders and it was found that soluble binders such as phosphates and phosphonates are retained in the water too much, and getting a significant pick up of binder requires the use of high concentrations in the slurry. Such high concentrations reduce refractoriness leading to excessive shrinkage at high temperature. Even when a reasonable amount of binder is incorporated into the material it migrates during drying to form a surface having a relatively high binder content and a core having a relatively low binder content. This results in a product that is weak, and that on machining becomes weaker still if the surface is removed (as is often required in practice). Colloidal silica binders reacted adversely with the fibres resulting in high shrinkages. The inventors realised that by using a particulate binder with a chemistry close to that of the fibre such problems might be avoided as this will reduce concentration gradients between binder and fibre.

Accordingly, a further series of tests were made using a range of particulate binders and a spun fibre having a nominal composition SrO 58wt%, A1203 30wt% and Si02 12 wt%. Table 4 shows x-ray fluorescence analyses of three samples of thus fibre together with the mean composition. As made, fibre contains varying amount of particulate material (shot) which can result in variation in properties. Accordingly, the fibre was deshotted by hand (sieved) so as to produce a consistent material for these tests but this is not necessary to the invention.

The recipes for the boards used in these tests are given in Table I below showing amounts used by weight. The fibre, water and inorganic particulate materials were mixed together before the starch was added for flocculation. (The starch was chosen as anionic or cationic according to whether the clay was cationic or anionic respectively. Either starch may work with an amphoteric clay). This was then followed by adding latex (Acronal Latex LA420S) and finally flocculating again with Percol 230L (0.2% soln., polyacrylamide-based flocculant).

Table 2 shows x-ray analyses of the compositions of the inorganic constituents used, together with colloidal alurninas shown in other tests to be effective but not exemplified. Table 3 below shows the observed board shrinkages, the calculated inorganic binder composition (referring only to SrO, A1203 and Si02 content) and the deviation of the binder composition from the fibre composition (i.e. the absolute values of binder content less fibre content in weight percent for SrO, A1203, and SiOA

In Table 3 the first four compositions (D092, D095, D097 and D096) deviate from the S102 content of the fibre by more than 20% and have high shrinkage at a temperature of 1400'C. These compositions are ranked according to the deviation of the Si02 content of the inorganic binder from the content of the fibre and it can be seen that the more remote the Si02 content of the inorganic binder from the fibre, the worse the linear shrinkage.

The next composition (D091) has a close Si02 content to that of the fibre, but deviates ftom the A1203 content of the fibre by 70.6% and the SrO content by 57.8%. This composition has a moderately high shrinkage.

The next composition (D090) has a close Si02 content to that of the fibre but deviates from the A1203 content of the fibre by 29.4% and from the SrO content by 42.2%. This composition has an acceptably low shrinkage at 1400'C but a high shrinkage at 1500'C.

For the remaining compositions (D093, D 10 1, D 100, D094, and D098) the Si02, A1203, and SrO contents are close to that of the fibre and low linear shrinkages at both 1400'C and 1500'C are observed. It can also be seen that the lowest shrinkages at 1500'C are for those binders whose composition is closest to that of the fibre used (D098 and D099).

It should also be noted that all of the compositions for which SrO is greater than 35wt% have a low shrinkage (for example < 5%) at 1400'C.

It can be advantageous to use a particulate inorganic filler in converted products. In a fully fibrous product shrinkage of the fibres is reflected in shrinkage of the whole body containing the fibres. With a particulate filler the particles act to inhibit the shrinkage of the body so that it is not proportionate to the fibre shrinkage. Advantageously the filler will have a composition close to that of the fibre to reduce the risk of adverse reaction between filler and fibre. The shot that is formed as part of the fibre forming process can be used as this filler to advantageous effect, but will increase overall board density. For thermal mass requirements the density of the board should preferably not exceed 0.5g/cml. Table 5 shows the results of a series of test boards made using air classified (using a British Rema Mini Split air classifier) fibre of the same composition as that used in the above mentioned tests, but with some shot added back as a filler. Compositions S 113 -116 and S 121 were deshotted at 40OOrpm which removed all shot greater than 5 Olim diameter and the stated amount of shot was added back. Composition S 117 was deshotted at a lower speed resulting in approximately 50% of shot being retained so that no addition of shot was necessary.

These results are plotted in Fig. I with compositions S 113-116 and S 121 being plotted and S 117 shown as reference figures. It can be seen that addition of shot reduces shrinkage, the effect being more marked at higher temperatures. The shrinkage of boards from composition S1 17 is lower at most temperatures but this could be an artifact of damage caused by the deshotting process to the other samples, possible through separation of shot from the fibre (a proportion is usually attached to fibre) or through shorter fibre length. However, the principle of adding shot, or of using a fibre containing a lot of shot, does appear to be useful for making board.

Table 4 Oxide Run Number Mean 2 3 Na2O 0.18 0.18 0.16 0.17 A1203 29.5 29.2 29.4 Si02 12.2 12.2 12.0 12.1 CaO 0.12 0.12 0.11 0.12 Fe203 0.05 0.05 < 0.05 0.03 SrO 58.3 57.2 57.9 57.8 Y203 0.08 0.08 0.08 0.08 BaO 0.07 0.07 0.06 0.07 L.O.I. 0.22 0.31 0.16 0.23 Total 100.7 99.6 99.7 100.0

Table 5 Mix Deshot speed Binder Shot Linear Shrinkage Calculated 14000C 15000C Density S113 4000rpm. 0.5% PLV starch 0 3.45 6.64 0.25 S114 4000rpm 0.5% PLV starch 25 3.09 5.84 0.30 S115 40OOrpm 0.5% PLV starch 40 2.82 5.04 0.39 S116 40OOrpm 0.5% PLV starch 50 3.1 5.72 0.41 S121 40OOrpm 0.5% PLV starch 66 4.41 0.76 S117 2500rpm, 0.5% PLV starch -50 2.57 4.75 0.42 Following the measurements shown in Table 3, further testing was done with a range of binder compositions and using different clays. A sample using only the green binder (which had no high temperature strength) was also tested. The results are indicated in Table 6 which shows that the 35% SrO level does provide a clear difference to 1400*C shrinkages.

The clay used has little effect on shrinkage at 1400'C but may have an effect at higher temperatures. (possibly through impurities in the clays).

The closer the SrO content of the binder is to the SrO content of the fibre the more reproducibly low is the shrinkage. Preferably the SrO content of the binder is >40wt% and more preferably >50wt%. The SrO content is also preferably < 90wt%, more preferably < 80wt%, still more preferably < 70wt%. Advantageously the SrO content of the binder is within 15wt%, (more preferably 1 Owt% and still more preferably 5wtl/o. of the SrO content of the fibre.

A clay free formulation for use in vacuum forming strontium aluminium silicate boards may comvrise:-

Material Quantity Water - 10 litres Strontium Aluminate fibre (of composition as mentioned above) I 00g Strontium Carbonate powder < 5 micron 12.5g Alumina sol (20% A1203) (e.g. Nyacol A120TMeolloidal alumina from 21.85g Nyacol Products Inc.) Silica sol (25.5% SiO2- 3.8% A1203) (e.g. Bindzil CAT 220TMcolloidal 6.35g silica from Akzo Nobel) Organic charge modifier (e.g Alcofix IJOTM, a cationic polymer from 2.5g Ciba Specialty Chemicals) Starch (cold water soluble) (e.g Wisprofloc ATM, a pregelatinized 3.07g carboxymethyl ether of potato starch from Avebe) The aims of any binder system for such converted products are:

1) To be suitable for vacuum forming - all ingredients should flocculate in as stable a manner as possible 2) To bind fibres well, both when green and when fired 3) Not to have an adverse effect on the fibre In the above mix the strontium carbonate (which goes into the mix as a fine powder dispersed in water) is present as a source of strontium oxide, the alumina sol supplies aluminium oxide and a degree of strength once fired, and the colloidal silica supplies the silica and a lot of bonding, especially around 650'C. Without the colloidal silica the material may well be more refractory, but after firing at 650'C for half an hour ( i.e. when the starch has burnt out, but before any sintering has taken place), will be very weak.

The colloidal alumina is in cationic forrii to match the charge of the cationic colloidal silica so as to be compatible and not cause flocculation prematurely. Between the colloidal silica and colloidal alumina there is not enough charge to flocculate with the desired amount of anionic starch, (predetermined by the green strength desired), and so cationic polymer is added to boost the weak cationic contribution from the silica and alumina. [Of course, the charges may be chosen otherwise to provide an anionic silica and alumina and a cationic starch and anionic polymer. This may be a cheaper option.].

The elemental composition of the inorganic binder is approximately the same as the fibre; this is to promote stability and in this respect the strontium is most important element. The above binder composition has the approximate relative proportions 58.2wt% SrO, 30.9 wt% A1203, and 10.9wt% Si02.

The order of addition and charge of components is chosen so that flocculation only takes place once all the ingredients have been added.

Claims

CLAJIMMS 1. A refractory material comprising a strontium aluminate refractory fibre and an inorganic binder comprising when fired greater than 35wt% strontium oxide. 2. A refractory material as claimed in Claim I comprising a strontium aluminate refractory fibre and an inorganic binder having the composition when fired (based upon the amounts of strontium, aluminium and silicon present calculated as oxide) comprising:- Si02 silicon oxide content of strontium aluminate fibre 20Wto/o. 3. A refractory material as claimed in Claim I or Claim 2, in which the inorganic binder comprises when fired:- A1203 aluminium oxide content of strontium aluminate fibre 65wt% 4. A refractory material as claimed in Claim 3, in which the inorganic binder comprises when fired:- A1203 aluminium oxide content of strontium aluminate fibre 25wt%. 5. A refractory material as claimed in Claim 4, in which the inorganic binder comprises when fired:- A1203 aluminiurn oxide content of strontium aluminate fibre 20wt%. 6. A refractory material as claimed in Claim 5, in which the inorganic binder comprises when fired:- A1203 aluminium oxide content of strontium aluminate fibre 15wt%. 7. A refractory material as claimed in Claim 6, in which the inorganic binder comprises when fired: - A1203 aluminium oxide content of strontium aluminate fibre I Owt%. 8. A refractory material as claimed in Claim 7, in which the inorganic binder comprises when fired: - A1203 aluminium oxide content of strontium aluminate fibre 5wtO/o.

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9. A refractory material as claimed in any preceding claim, in which the inorganic binder comprises when fired:- SrO >40wt%. 10. A refractory material as claimed in Claim 9, in which the inorganic binder comprises when fired: - Sro >50wt%. 11. A refractory material as claimed in any preceding claim, in which the inorganic binder comprises when fired:- SrO < 90wt%. 12. A refractory material as claimed in Claim 11, in which the inorganic binder comprises when fired:- SrO < 80wt%. 13. A refractory material as claimed in any of Claims 9 to 12, in which the inorganic binder comprises when fired:- SrO strontium oxide content of strontium alun-linate fibre 15wt%. 14. A refractory material as claimed in Claim 13, in which the inorganic binder comprises when fired: - SrO strontium oxide content of strontium aluminate fibre 1 Owt%. 15. A refractory material as claimed in Claim 14, in which the inorganic binder comprises when fired:- SrO strontium oxide content of strontium aluminate fibre 5wt%. 16, A refractory material as claimed in any preceding claim, in which the inorganic binder comprises when fired:- S102 silicon oxide content of strontium aluminate fibre 15wt"/o. 17. A refractory material as claimed in Claim 16, in which the inorganic binder comprises when fired:-

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Si02 silicon oxide content of strontium aluminate fibre I Owt%. 18. A refractory material as claimed in Claim 17, in which the inorganic binder comprises when fired:- Si02 silicon oxide content of strontium aluminate fibre 5wt%. 19. A refractory material as claimed in any preceding claim additionally comprising an inorganic filler. 20. A refractory material as claimed in Claim 19, in which the inorganic filler has the composition (based upon the amounts of strontium, aluminium and silicon present calculated as oxide) comprising:- Si02 silicon oxide content of strontium aluminate fibre 20wt%. 21. A refractory material as claimed in Claim 20, in which the inorganic filler comprises-.- A1203 aluminiuni oxide content of strontium aluminate fibre 65wt% 22. A refractory material as claimed in Claim 21, in which the inorganic filler comprises:- A1203 aluminiurn oxide content of strontium aluminate, fibre 25wt%. 23. A refractory material as claimed in any of Claims 19 to 22, in which the inorganic filler comprises:- SrO >40wt%. 24. A refractory material as claimed in Claim 23, in which the inorganic filler comprises:- SrO >50wtO/o. 25. A refractory material as claimed in Claim 23 or Claim 24, in which the inorganic filler comprises:- Sro < 90wt%. 26. A refractory material as claimed in Claim 26, in which the inorganic filler comprises when fired: - SrO < 80wt%.

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27. A refractory material as claimed in any of Claims 19 to 25, in which the inorganic filler comprises when fired:- SrO strontium oxide content of strontium aluminate fibre 15wt%. 28. A refractory material as claimed in Claim 27, in which the inorganic filler comprises when fired:- SrO strontium oxide content of strontium aluminate fibre I Owt%. 29. A refractory material as claimed in Claim 28, in which the inorganic filler comprises when fired:- SrO strontium oxide content of strontium aluminate fibre 5wt%. 30. A refractory material as claimed in Claim 19, in which the inorganic filler comprises shot from the manufacture of the fibre. 31. A refractory material as claimed in any preceding claim comprising, before firing, both a latex binder and a starch. 32. A method of making a refractory material comprising a strontium aluminate refractory fibre and an inorganic binder containing strontium and alurninium in oxide form comprising the steps of.- a) forming a green body from a strontium aluminate refractory fibre and a particulate material; and, b) firing the green body to convert the particulate material into an inorganic binder having the composition set out in any of Claims 1 to 10. 33. A method of making a refractory material as claimed in Claim 32, in which the particulate material comprises an alurniniurn containing particulate material and a strontium containing particulate material.