GB2600974A

GB2600974A - Thermal insulation

Info

Publication number: GB2600974A
Application number: GB2017916.4A
Authority: GB
Inventors: Modarresifar Farid
Original assignee: Thermal Ceramics UK Ltd
Current assignee: Thermal Ceramics UK Ltd
Priority date: 2020-11-13
Filing date: 2020-11-13
Publication date: 2022-05-18
Also published as: GB202017916D0

Abstract

Thermal insulation is disclosed for use in applications requiring continuous resistance to temperatures of 1200°C or more. The thermal insulation comprises bio-soluble inorganic fibres with a diameter of less than 6.0μm. The fibres have a composition comprising: 61.0 to 71.0 wt% SiO2; 27.0 to 39.0 wt% CaO; and >0 to 2.0 wt% incidental impurities. The incidental impurities comprise less than 1.0 wt% MgO; and the sum of SiO2 and CaO is greater or equal to 98.0 wt%.

Description

THERMAL INSULATION

FIELD OF THE INVENTION

This invention relates to thermal insulation comprising bio-soluble inorganic fibre compositions and more particularly insulation materials comprising said fibre. The invention also relates to the use of said fibre at temperatures in excess of 1200CC.

BACKGROUND

The insulation material industry has determined that it is desirable to utilize fibres in thermal, electrical and acoustical insulating applications, which do not persist in physiological fluids. That is, fibre compositions which are considered have low biopersistence (i.e. bio-soluble) in physiological fluids.

While candidate silicate materials have been proposed, the use temperature limit of these materials have not been high enough to accommodate many of the applications to which high temperature resistant fibres are applied. For example, such bio-soluble fibres exhibit high shrinkage at use temperatures and/or reduced mechanical strength when exposed to use temperatures ranging from 1000° C. to 1500° C. as compared to the performance of refractory ceramic fibres.

The high temperature resistant, low soluble fibres should exhibit minimal shrinkage at expected exposure temperatures, and after prolonged or continuous exposure to the expected use temperatures, in order to provide effective thermal protection to the article being insulated. In addition to bio-solublity and high temperature resistance the fibres should possess a low diameter and low shot content for the resultant insulation materials to have low density and thermal conductivity. The multitude of requirements do not end there, with fibres also needing to be non-reactive to other materials in the insulation system they may form part of.

In 1987 Manville Corporation developed bio-soluble high temperature resistant fibres based on a calcium magnesium silicate chemistry (US5,714,421). Such material not only had a higher temperature capability than traditional glass wools, but also had a higher solubility in body fluids than the aluminosilicate fibres mostly used for high temperature insulation. US5,714,421 taught the necessity to combine silica, calcia and magnesia with a variety of other metal oxide additives to obtain the desired combination of fibre properties and form.

While there are many commercial examples of the biosoluble high temperature resistant fibres which have stemmed from magnesia, calcia, silica systems, there is still a need for improved biosoluble high temperature resistant fibres and insulation material thereof.

International Application WO 87/05007 discloses inorganic fibres consisting essentially of Si02, CaO with specified ranges of MgO and A1203, which were obtained from metal oxides rather than raw byproduct materials with variable composition. It was observed that lower A1203 levels resulted in a surprisingly high bio-solubility level.

International Application WO 94/15883 discloses CaO/MgO/SiO2fibres with additional constituents A1203, Zr02, and Ti02, for which saline solubility and refractoriness were investigated. The document states that saline solubility appeared to increase with increasing amounts of MgO, whereas ZrO2 and A1203 were detrimental to solubility. The presence of Ti02(0.71-0.74 mol %) and A1203(0.51-0.55 mol %) led to the fibres failing the shrinkage criterion of 3.5% or less at 1260° C. The document further states that fibres that are too high in 5i02 are difficult or impossible to form, and cites fibres having 70.04, 73.09, 73.28 and 78.07 wt%SiO2as examples of compositions which could not be fiberized.

D56,953,757 discloses an inorganic high silica fibre composition comprising predominately silica, calcia, magnesia and zirconia and optionally viscosity modifiers, such as alumina and boria, to enable product fiberisation.

Despite advances in the field, there is still a need for a simplified fibre composition which is not reliant upon a range of additives to obtain the required combination of fibre properties and form.

SUMMARY OF THE INVENTION

The applicant has found that, contrary to received wisdom in the field of refractory alkaline earth silicate fibres, that refractory fibres with high utility are able to be produced without the deliberate additional of additives, such as viscosity modifiers or solubility or refractory enhancers, to a Si02-CaO system, within a specified compositional range.

According to a first aspect of the prevent invention there is provided bio-soluble inorganic fibres with a diameter of less than 6.0 pm usable as refractory insulation requiring continuous resistance to a temperature of 1200°C having a composition comprising or consisting: 61.0. to 71.0 wt%5i02; 27.0 to 39.0 wt% CaO; and 0 (or >0) to 2.0 wt% incidental impurities wherein the incidental impurities comprises less than 1.0 wt% MgO; and the sum of 5i02 and CaO is greater or equal to 98.0 wt%.

According to another aspect of the present invention, there is provided thermal insulation for use in applications requiring continuous resistance to temperatures of 1200°C or more (e.g. 1260°C or 1300°C), the thermal insulation comprising bio-soluble inorganic fibres with a diameter of less than 6.0p.m, said fibres having a composition comprising: 61.0 to 71.0 wt% Si02; 27.0 to 39.0 wt% CaO; and 0 (or >0) to 2.0 wt% incidental impurities wherein the incidental impurities comprises less than 1.0 wt% MgO; and the sum of Si02 and Ca0 is greater or equal to 98.0 wt%.

The amount of incidental impurities is typically at least 0.2 wt% or at least 0.3 wt% or at least 0.4 wt% or at least 0.5 wt%. While the use of more pure raw materials is possible, this is often accompanied with an increased carbon footprint and cost due to the need for additional purification processes to be used.

In some embodiments, a small amount of additives may be included to fine-tune the properties of the fibres. Additive addition may be greater than 0.0 wt% or greater than 0.1 wt% or greater than 0.2 wt% or greater than 0.3wt%. Additive addition may be less than 1.5 wt% or less than 1.2 wt% or less than 1.0 wt% or less than 0.8 wt% or less than 0.6 wt% or less than 0.4 wt% or less than 0.3 wt% or less than 0.2 wt% of the fibre composition.

In some embodiments, additives are added to assist in fiberisation (melt viscosity modifiers) to facilitate the formation of finer fibre diameters whilst maintaining the required bio-solubility and high temperature usage characteristics.

The viscosity modifiers may include oxides or fluorides of one or more of the lanthanides series of elements (e.g. La, Ce), Li, Na, K, Sr, Ba, Cr, Fe, Zn, Y, Zr, CaF2, 13203, P205 or combinations thereof.

The viscosity modifiers are preferably sourced from a naturally occurring mineral deposit. The addition of viscosity modifiers is particularly advantageous when added to fibre compositions with a Si02 content of greater than 66.0 wt% or 67.0 wt% or 68.0 wt% or 69.0 wt%.

It has been found that within this compositional window, bio-soluble high temperature resistant fibres are melt formable. This is particularly surprising given the reported use of a variety of additives to modify the inorganic fibre composition characteristics, such that the fibres have utility at high temperatures. Additionally, when the fibre composition has greater than 65.8 wt% 5i02 the fibre is also non-reactive in the presence of mullite at high temperature.

In some embodiment, the inorganic fibres are non-reactive with an alumina rich composition (such as mullite) when exposed to 1200°C for 24 hours. Alumina rich composition preferably include composition with at least 50 wt% A1203 Incidental impurities stem from impurities in the raw materials used to make the inorganic fibres, including coal ash, when coal is used as an energy source to melt in the inorganic fibre precursor material, such as silica sand and lime.

The main impurity in lime is magnesia Other impurities include alumina, iron oxide and alkali metal oxides, such as K20 and Na20 In some embodiments, the sum of Si02 and CaO and MgO in the incidental impurities is greater or equal to 99.0 wt% or greater than 99.2 wt% or greater than 99.4 wt% of the fibre composition. Preferably the inorganic fibre composition comprises less than 1.0 wt% MgO or less than 0.90 wt% or less than 0.80 wt% or less than 0.70 wt% or less than 0.6 wt% MgO or less than 0.50 wt% or less than 0.45 wt% MgO derived from the incidental impurities. Higher contents of MgO has been found to detrimentally affect the thermal stability of the fibres at 1300°C.

Preferably, inorganic fibre composition comprises less than 0.8 wt% A1203 or less than 0.70 wt% or less than 0.6 wt% or less than 0.5 wt% or less than 0.4 wt% or less than 0.3 wt% or less than 0.25 wt% A1203 derived from the incidental impurities. Within the current Si02-CaO composition, higher levels of A1203 have been found to adversely affect the bio-solubility and thermal stability of the inorganic fibres.

In other embodiment, the inorganic fibre from which a vacuum cast preform of the fibre has a shrinkage of 4.0% or less (or 3.0% or less or 2.5% or less) when exposed to 1200°C for 24 hours. In another embodiment, the inorganic fibres from which a vacuum cast preform of the fibre has a shrinkage of 4.0% or less (or 3.0% or less or 2.5% or less) when exposed to 1300°C for 24 hours.

The melting temperature of the inorganic fibres is preferably at least 1350°C or at least 1380°C or at least 1400°C or at least 1420°C.

Preferably, the impurities of the raw materials is such that the sum of SiO2 and CaO is greater or equal to 99.0 wt% or greater or equal to 99.3 wt% greater or equal to 99.4 wt% greater or equal to

S

99.5 wt% of the inorganic fibre composition. The upper limit of the purity is likely be constrained by the cost and availability of raw materials To aid fiberisation, in the absence of additives, the Si02 content of the inorganic fibre composition is preferably less than 70.5 wt% or less than 70.0 wt% or less than 69.5 wt% or less than 69.0 wt% or less than 68.5 wt% or less than 68.0 wt%. To aid resiliency at high temperature and minimise reactivity with alumina containing substrates, the the Si02 content of the inorganic fibre composition is preferably at at least 61.5 wt% or at least 62.0 wt% or at least 62.5 wt% or at least 63.0 wt% or at least 63.5 wt% or at least 64.0 wt% or at least 64.5 wt% or at least 65.0 wt% or at least 65.5 wt% or least 66.0 wt% or at least 66.2 wt% or at least 66.4 wt% or at least 66.6 wt% or at least 66.8 wt% or at least 67.0 wt% or at least 67.2 wt% or at least 67.4 wt%.

The CaO content of the inorganic fibre composition preferably varies accordingly, with the lower limit of CaO preferably at least 27.5 wt% or at least 28.0 wt% or at least 28.5 wt% or at least 29.0 wt% of at least 29.5 wt% or at least 30.0 wt%. The upper limit of the CaO content of the inorganic fibre composition is preferably no more than 38.5 wt% or no more than 38.0 wt% or no more than 37.5 wt% or no more than 37.0 wt% or no more than 36.5 wt% or no more than 36.0 wt% or no more than 35.5 wt% or no more than 35.0 wt% or no more than 34.5 wt% or no more than 34.0 wt% or no more than 33.5 wt% or no more than 33.0 wt% or no more than 32.5 wt% or no more than 32.0 wt%.

In one embodiment, the incidental impurities comprise: * 0 to 1.0 wt% MgO or 0.1 to 0.8 wt% MgO or 0.2 to 0.6 wt% MgO or 0.3 to 0.5 wt% MgO * 0 to 0.6 wt% A1203 or 0.1 to 0.4 wt% A1203or 0.2 to 0.3 wt% A1203 or 0 to 0.2 wt% A1203 * 0 to 0.3 wt% alkali metal oxides or 0.1 to 0.2 wt% alkali metal oxides or 0.05 to 0.1 wt% alkali metal oxides.

In some embodiments, at least 80 wt% of the alkali metal oxides comprise Na20 or K20.

The preferred range of other incidental impurities in the inorganic fibres is provided below: Ba0: 0 to 0.05 wt% or 0 to 0.01 wt% B203: 0 to 0.1 wt% or 0 to 0.05 wt% Cr203: 0 to 0.08 wt% or 0 to 0.03 wt% Fe203: 0 to 0.25 wt% or 0 to 0.15 wt% Hf02: 0 to 0.05 wt% or 0 to 0.01 wt% La203: 0 to 0.1 wt% or 0 to 0.03 wt% Mn304: 0 to 0.05 wt% or 0 to 0.01 wt% Li20: 0 to 0.15 wt% or 0 to 0.08 wt% Na20: 0 to 0.15 wt% or 0 to 0.08 wt% K20: 0 to 0.5 wt% or 0 to 0.20 wt% P205: 0 to 0.05 wt% or 0 to 0.01 wt% Sr0: 0 to 0.08 wt% or 0 to 0.03 wt% Ti02: 0 to 0.08 wt% or 0 to 0.03 wt% V205: 0 to 0.05 wt% or 0 to 0.01 wt% Sn02: 0 to 0.05 wt% or 0 to 0.01 wt% ZnO: 0 to 0.05 wt% or 0 to 0.01 wt% Zr02: 0 to 0.1 wt% or 0 to 0.02 wt% The sum of BaO + Cr,01+ Fe201+ HfC), + La,C) + Molar + Na,0 + K,0 +13205+ Sr0 + TiO, + V705+ Zr02 + ZnO is preferably less than 1.2 wt% or less than 1.0 wt% of less than 0.8 wt% or less than 0.6 wt% or less than 0.5 wt% or less than 0.4 wt% or less than 0.3wt% or less than 0.25 wt% or less than 0.2 wt% of the total weight of the inorganic fibres. The sum of BaO + Cr203+ Fe203+ Hf02+ La203+ Mn304+ Na20 + K20 + P205+ Sr0 + Ti02+ V205+ Zr02 + ZnO is typically at least 0.1 wt% or at least 0.2 wt% or at least 0.3 wt% of the total weight of the inorganic fibres.

In a preferred embodiment, the inorganic fibres comprise: 66.0 to 69.0 wt% Si02; 30.0 to 34.0 wt% CaO; 0 to 1.0 wt% (or 0.3wt% and 1.0 wt%) incidental impurities; the sum of Si02 and CaO is greater or equal to 99.0 wt%; and the incidental impurities comprise: 0 to 0.45 wt% (or 0.1 to 0.45 wt%) MgO 0 to 0.35 wt% (or 0.1 to 0.35 wt%) A1203 0 to 0.25 wt% (or 0.05 to 0.25 wt%) alkali metal oxides.

Within this embodiment: * the fibre diameter is preferably less than 3.5pm or less than 3.0 p.m; and * the fibre has a dissolution rate, in the flow solubility test (pH 7.4), is preferably at least 150 ng/cm2hr or at least 170 ng/cm2hr or at least 200 ng/cm2hr; and * the shrinkage of the fibre at 1300°C (24hrs) is preferably less than 3.5% or less than 3.0% of less than 2.5%.

By maintaining the incidental impurities within the above limits, the inorganic fibres of the present disclosure are able to maintain excellent high temperature utility. While it may be possible to individual impurities levels to vary from their preferred range, through maintaining an overall low level of incidental impurities, the need for adding additives (e.g. viscosity modifier, solubility enhancer, refractory temperature stabiliser, etc.) to the calcia and silica mixture is not required.

In some embodiments, the fibre diameter is less than 5.0 pm or less than 4.5 pm or less than 4.0 pm or less than 3.5 pm or less than 3.3 pm or less than 3.0 pm or less than 2.8 pm or less than 2.5 pm. Minimum fibre diameter is typically at least 1.5 pm or at least 2.0 pm to enable the fibres to have sufficient mechanical strength in use.

Fiberisation techniques as taught in U54,238,213 or U52012/247156 may be used to form the disclosed fibres of the present invention. The apparatus and techniques disclosed in W02017/121770 (which is incorporated herein in its entirety by reference) may be preferably used, particularly for compositions comprising higher silica contents (e.g. >68 wt% or > 69 wt%).

In a second aspect of the present invention, there is provided an insulation system comprising: a. the inorganic fibres according to any one of the preceding claims; and b. an A1203 rich refractory component, wherein the inorganic fibres and refractory component are in contacting engagement.

The A1203 rich refractory component preferably comprises at least 50 wt% A1203 or at least 60 wt% or at least 70 wt% A1203. Examples of A1203 rich refractory components include mullite are clay-based components.

In a third aspect of the present invention, there is provided a process for the formation of the inorganic fibres of the first aspect the present invention comprising: * combining a source of silica and calcia together; * melting the silica and calcia to form a molten mass; and * forming said inorganic fibres from the melt The source of silica is preferably silica sand. The source of calcia is preferably lime. The silica and calcia are each preferably derived from a single source. The quality of the silica and calcia raw materials are preferably monitored to ensure that incidental impurities are kept below 2.0 wt% or below 1.0 wt% or below 0.5 wt%.

For the avoidance of doubt it should be noted that in the present specification the term "comprise" in relation to a composition is taken to have the meaning of include, contain, or embrace, and to permit other ingredients to be present. The terms "comprises" and "comprising" are to be understood in like manner. It should also be noted that no claim is made to any composition in which the sum of the components exceeds 100%.

Where a patent or other document is referred to herein, its content is incorporated herein by reference to the extent permissible under national law.

Further it should be understood that usage in compositions of the names of oxides [e.g. alumina, silica, potassia] does not imply that these materials are supplied as such, but refers to the composition of the final fibre expressing the relevant elements as oxides. The materials concerned may be provided in whole or in part as mixed oxides, compounded with fugitive components [e.g. supplied as carbonates] or indeed as non-oxide components [e.g. as halides].

Incidental impurities are defined as impurities which are derived from the raw material, fuel source or other sources during the formation of the inorganic fibres.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Fibres according to the invention and comparative fibres described herein have been produced at either the French production facilities in Saint Marcellin, France by spinning [made from the melt by forming a molten stream and converting the stream into fibre by permitting the stream to contact one or more spinning wheels]; or at the applicant's research facilities in Bromborough, England by spinning or alternatively by blowing [fibres made from the melt by forming a molten stream and converting the stream into fibre by using an air blast directed at the stream]. The invention is not limited to any particular method of forming the fibres from a melt, and other methods [e.g. rotary or centrifugal formation of fibres; drawing; air jet attenuation] may be used. The resultant fibres were then fed onto a conveyor belt and entangled by needling methods, as known in the art.

The raw materials used to produce the inorganic fibres of a preferred embodiment of the present invention are lime and silica sand. The chemical analysis (normalised) of the lime used is provided in Table 1 below. The incidental impurities (100-CaO -Si02) in the lime is typically less than 2.0 wt%. The silica sand purity may be 98.5 wt% of 99.0 wt% or higher. Typically, the silica sand had a purity of greater than 99.5 wt% silica and less than 200 ppm Fe203; less than 1000 ppm A1203; less than 200 pmm Ti02, less than 100 ppm CaO and less than 100 ppm K20.

Table 1

Lime bag CaO A1203 Fe203 K20 MgO Si02 Zr02 Un-normalised XRF total B1 97.97 0.28 0.21 0.04 0.41 1.09 0.01 98.39 B2 98.12 0.30 0.21 0.04 0.38 0.93 0.00 99.17 B3 97.79 0.30 0.21 0.04 0.37 1.26 0.02 99.39 B4 97.56 0.35 0.21 0.04 0.38 1.43 0.01 99.00 B5 97.64 0.54 0.21 0.04 0.38 1.15 0.01 99.94 B6 97.61 0.49 0.22 0.04 0.41 1.15 0.04 99.92 B7 97.97 0.33 0.20 0.04 0.40 1.01 0.01 98.93 B8 95.15 0.34 0.20 0.04 0.40 3.85 0.00 99.94 The fibres/blankets made therefrom were then evaluated using the test methodology as described: Test methodology Thermal Stability (Shrinkage) The method for determination of dimensional stability of refractory materials, including the refractory glass fibre insulation materials, is based on the EN ISO 10635. This method is a shrinkage test that measures the change of a flat specimen's linear dimensions after a heat treatment.

The shrinkage test requires a relatively rigid specimen's so that the linear dimensions could be accurately determined before and after the heat treatment. In cases where a needled fibre blanket specimen were not available, starch bonded vacuum formed boards were prepared from the glass fibre samples.

To prepare the vacuum formed boards, the as made fibre material were chopped using a small-scale industrial granulator through a #6 mesh (-3mm opening). Chopped fibre samples were lightly cleaned using a sieve to remove any debris and large glass residues. 40g of chopped clean fibre was mixed in 500 ml of 5 wt% concentration potato starch in water solution to create a slurry. Subsequently a vacuum former was used to produce 75x75 mm boards with a thickness of 10-15 mm. The vacuum former consists of a sealed acrylic mould with a 100 p.m mesh bottom, a vacuum pump was used to remove the water from the slurry while manually compressing the shape using a flat plate. Vacuum formed boards were dried at 120°C.

To measure permanent linear shrinkage, the linear dimensions of specimen were measured to an accuracy of ±5 p.m using a travelling microscope. The specimens were subsequently placed in a furnace and ramped to a temperature 50°C below the test temperature (e.g. 1300°C) at a rate of 300°C/hour and then ramped at 120°C/hour for the last 50°C to test temperature and held for 24 hours. Specimens were allowed to cool down naturally to room temperature at the end of this heat treatment. After heat treatment, the specimen's linear dimensions were measured again using the same apparatus to calculate the change in dimensions. Shrinkage values are given as an average of 4 measurements.

Reactivity with Mullite Needled fibre blanket specimens with approximate dimensions of SOmm x 100mm were used for this test. Blanket specimens were placed on a fresh mullite Insulating Fire Brick (JM 28 IFB). The specimen, along with the IFB substrate, was heated treated at 1200° C for 24 hours to confirm the reactivity after the heat treatment. The specimen and IFB were inspected for any sign of melting or reaction. The sample which did not react with the IFB at all were evaluated as good (0). The sample which reacted with the IFB (the sample was adhered to IFB or sign of melting was observed) were evaluated as poor (X).

Bio-solubility The biological solubility of fibrous materials can be estimated in a system in which the material is exposed to a simulated body fluid in a flow-through apparatus (i.e., in vitro). This measurement of solubility is defined as the rate of decrease of mass per unit surface area (Kdis). Although several attempts have been made to standardize this measurement, there is currently no international standard. Major protocol differences among laboratories include different simulated body fluid chemistries (and, most significantly, different buffering and organic components), flow rates, mass and/or surface area of samples, determination methods for specific surface area, and determination of mass loss. Consequently, Kdis values should be regarded as relative estimates of chemical reactivity with the simulated body fluid under the specific parameters of the test, not as measures of absolute solubility of fibrous particles in the human lung. The flow through solubility test method used in this study is a 3-week long solubility test in pH 7.4 saline. Two channels of each unique specimen are simultaneously tested. Samples of saline solution flowing over the fibre specimens are taken after 1, 4, 7, 11, 14, 19 and 21 days. The saline samples are analysed using the ICP method to measure the oxide dissolution levels in ppm level. To validate the flow test results and calculate the final dissolution rates for each specimen, the square root of remaining fibre mass against sampling times are plotted. Deviation from a linear trend could suggest an issue with the results. A good linear regression fit was observed in the flow test results conducted in this study. Based on the historical data collected by authors, a minimum of 150 ng/cm2hr dissolution rate is required for a fibre to have exoneration potential. In the static solubility test method, fibre specimens are agitated in saline solution at 37°C to replicate conditions within the lungs. The test monitors fibre dissolution after 5 or 24 hours using the ICE' method.

Fibre diameter Fibre diameter measurements were carried out using the Scanning Electron Microscope (SEM). SEM is a micro-analytical technique used to conduct high magnification observation of materials' microscopic details. SEM uses a tungsten filament to generate an electron beam, the electron beam is then rastered over a selected area of the specimen and the signal produced by the specimen is recorded by a detector and processed into an image display on a computer. A variety of detectors can be used to record the signal produced by the sample including secondary electrons and backscattered electrons detectors.

The particular SEM equipment used operates under vacuum and on electrically conductive specimens. Therefore, all glass/ceramic fibre specimens need to be coated with gold or carbon prior to SEM analysis. Coating was applied using a automated sputter coater at approximately 20 nm. In order to prepare the fibrous specimens for diameter measurements, fibre specimens were crushed using a pneumatic press at 400 psi. The aim of crushing is to ensure the sample is crushed enough to be dispersed without compromising the fibre length, crushing results in fibres with aspect ratios >3:1. The crushed fibre specimens is then cone and quartered to ensure representative sampling. Crushed and quartered fibres are dispersed in IPA. Typically 50 Ftg of fibres are placed in a 50 mL centrifuge tube and 25 m L IPA is added. A SEM stub is then placed at centre of a petri dish, then the centrifuge tube is vigorously shaken and emptied into the petri dish containing the SEM stub. The petri dish is left in fume cupboard for 1 hour for the fibres to settle on the SEM stub. The SEM stub is then carefully coated with gold in preparation for SEM imaging.

Following this sample preparation step, an automated software on the SEM equipment is utilised to collect 350 unique secondary electron images at 1500x magnification from the SEM stub. Following the image collection step, the images are processed by a computer software to measure the diameter of fibres. The process involves manual inspection of measured fibres in every image to ensure only the fibres particles with aspect ratios greater than 3:1 are measured. The final fibre diameter distribution is reposted in a graph as well as numerical average diameter.

Fibre composition Fibre composition was determined using standard XRF methodology. Results were normalised, with results un-normalised results discarded if the total weight of the composition fell outside the range 98.0 wt% to 102.0 wt%. Preferably, the un-normalised results are between 98.5 wt% to 101.5 wt% and more preferably between 99.0 wt% and 101.0 wt%.

Results Referring to Table 2 & 3, there is shown the composition of inorganic fibres as % weight of the total composition according to Examples 1 to 11 and Comparative Examples C1 to C6. As illustrated in Table 3, inorganic fibre compositions with silica levels less than 65.8 wt% were found to be not compatible with mullite based bricks, adhering to the bricks after being in contact at 1200°C for 24hrs. Inorganic fibre compositions with higher silica levels had generally higher shot content and higher fibre diameter.

Table 2

Example 5102 CaO A1203 K20 MgO Ca0+5102 Un-normalised XRF total C-1 72.8 24.9 1.1 0.6 0.6 97.7 100.72 C-2 71.3 28.2 0.3 0.1 0.2 99.5 101.09 1 70.7 28.9 0.3 0.0 0.1 99.6 100.43 2 70.6 28.9 0.3 0.0 0.2 99.5 100.55 3 70.6 28.5 0.5 0.1 0.2 99.1 101.11 4 70.5 28.4 0.7 0.2 0.2 98.9 100.89 70.3 29.1 0.4 0.0 0.2 99.4 100.80 6 69.5 30.0 0.3 0.0 0.1 99.5 100.64 7 69.4 30.1 0.3 0.0 0.1 99.5 101.01 8 67.7 31.9 0.2 0.0 0.1 99.6 100.83 9 67.1 32.4 0.3 0.0 0.1 99.5 101.1 66.1 33.1 0.6 0.0 0.2 99.2 99.39 11 65.8 33.8 0.2 0.0 0.2 99.6 101.49 12 65.7 33.9 0.2 0.0 0.1 99.6 100.92 13 65.3 34.2 0.2 0.0 0.2 99.5 99.92 14 65.0 34.5 0.3 0.0 0.2 99.5 100.88 64.5 35.1 0.2 0.1 0.2 99.6 100.79 16 63.3 36.1 0.2 0.1 0.3 99.4 100.83 17 62.8 36.7 0.2 0.1 0.2 99.5 100.60 18 61.6 38.0 0.2 0.1 0.2 99.6 100.84 C-3 60.7 38.9 0.3 0.1 0.2 99.6 100.53 C-4 64.9 29.8 0.1 0.0 5.2 94.7 100.92

Table 3

Example Mu!lite Shrinkage at Shot content Fibre diameter Reactivity @ 1300°C (24 % wt (11111) 1200°C hrs) C-1 0 2.0 6.9 C-2 0 1.4 59.3 1 0.9 51.9 5.7 2 0 1.4 52.0 3 0 2.2 54.5 4 0 2.7 53.4 0 1.1 50.6 6 0 49.5 7 0 1.2 47.8 8 0 2.0 34.6 9 0 1.4 47.3 0 1.2 36.6 3.02 11 0 0.8 37.7 12 X 1.3 37.4 3.33 13 X 2.0 39.7 14 X 38.2 2.87 2.2 16 1.7 17 2.6 18 3.3 C-3 8.6 C-4 X 14.5 Effects of impurities To assess the effect of impurities, additional amounts of A1203, MgO and Zr02 were added to the existing incidental impurities. With reference to Table 4, increasing amounts of MgO and A1203 results in reduced thermal stability at 1300°C (24 hrs), as measured by the % shrinkage. Example E is a repetition of sample E-174 from US5,714, 421.

The lowest shrinkage (best high temperature performance) was observed in samples F & G. Sample G was a control sample with no additives, whereas Sample G has a slightly elevated MgO level, although in both samples, the sum of Si02 and Ca0 is greater than 99.0 wt%. Sample F appears to be an anomaly in the correlation between shrinkage and MgO content of Samples D to G. Likewise, Example K is also considered a suspect result, with the shrinkage result expected to be below 4%.

Table 4

Example 5102 CaO A1203 K20 MgO Zr02 Ca0+5102 Shrinkage at 1300°C A 60.0 35.2 0.3 0.1 4.3 0.0 95.2 24.1 B 62.5 35.5 0.2 0.1 1.7 0.0 98.0 6.1 C 62.6 35.7 0.2 0.1 1.4 0.0 98.3 11.3 D 65.7 33.1 0.2 0.1 1.0 0.0 98.8 7.0 E 65.5 33.4 0.2 0.1 0.8 0.0 98.9 3.4 F 66.1 33.1 0.2 0.1 0.6 0.0 99.2 1.7 G 66.1 33.4 0.2 0.0 0.2 0.0 99.5 2.6 H 63.4 34.9 0.8 0.1 0.5 0.3 98.3 5.7 1 65.6 32.6 1.5 0.1 0.2 0.0 98.2 6.6 J 65.5 33.1 1.0 0.2 0.2 0.0 98.6 4.1 K 65.5 33.6 0.6 0.1 0.3 0.0 99.1 5.0 Apart from the main incidental impurities of A1203, MgO and K20, the XRF analysis measured the metal oxides listed in Table 5. The maximum incidental impurity level of each of the metal oxides is provided. Typically, these minor incidental impurities are typically less than 0.3 wt% or less than 0.25 wt% or less than 0.20 wt%.

Table 5

Incidental Max level impurities % wt Ba0 0.01 Cr203 0.02 Fe203 0.13 Hf02 0.00 La203 0.07 Mn3O4 0.00 Na20 0.03 P205 0.00 Sr° 0.03 TiO2 0.03 V205 0.01 SnO2 0.01 ZnO 0.00 Zr02 0.02 Thermal conductivity of bodies of inorganic fibres Thermal conductivity of a body of melt formed fibres (e.g. a blanket or other product form) is determined by a number of factors including in particular:- * Diameter of the fibres; and * "Shot" (unfiberised material) content Fine diameter fibres provide low thermal conductivity to a body of fibres by reducing the scope for conduction through the solid and permitting finer inter-fibre porosity increasing the number of radiate-absorb steps for heat to pass by radiation from one side of the body to the other.

The presence of shot in a blanket increases thermal conductivity of the blanket by increasing the scope for conduction through the solid. Shot also increases the density of a blanket. The lower the shot content, the lower the thermal conductivity and density. For two bodies of identical fibre content and chemistry, the body with the lower shot content will have both the lower density and lower thermal conductivity.

In reference to Table 6, inorganic fibres were produced with a fibre diameter between approximately 2.6 to 3.0 pm and a shot content between 33 and 41 wt%. From the small data provided in Table 5, there is no clear correlation between fibre characteristics and thermal conductivity, although a larger data set should provide this expected relationship.

Table 6

Normalised XRD results (%wt) Shot SEM Fibre (>45pm) diameter SAMPLE S102 CaO A1203 K20 MgO % wt (Pm) 12 65.8 33.8 0.2 0.0 0.1 32.5 2.65 13 66.9 32.5 0.4 0.0 0.2 40.6 14 66.8 32.7 0.4 0.0 0.1 38.3 2.70 66.7 32.7 0.4 0.0 0.1 38.7 2.76 66.1 33.1 0.6 0.0 0.2 36.6 3.02

Table 7

Conductivity (W/m.K) SAMPLE 400° 600° 800° 1000° 1100° 1200° Density Kg/m3 Strength kPa Density Kg/m3 12 0.07 0.12 0.21 0.32 0.39 0.46 96 SO 95 13 0.08 0.13 0.20 0.28 0.33 0.39 111 50 121 14 0.07 0.11 0.18 0.27 0.33 0.39 105 48 115 0.07 0.12 0.19 0.29 0.35 0.41 105 56 123 0.08 0.13 0.22 0.33 0.40 0.47 88 35 91 Bio-solubility Referring now to Table 8, there is shown data for bio-solubility testing.

A 21 day static and long flow through solubility test in saline pH 7.4 was conducted on the compositions shown in Table 8. Two samples of each fibre composition were simultaneously tested, with the average results reported. The saline samples were analysed using the ICP method to measure the oxide dissolution levels in ppm level. The results confirm that the fibres have low biopersistence. A low biopersistence fibre composition is taken to be a fibre composition which has a dissolution rate, in the flow solubility test, of at least 150 necm2hr or at least 170 ngfcm2hr or at least 200 ng/cm2hr.

The inorganic fibres under the present invention have comparable or improved bio-solubility in comparison with prior art fibre compositions Cl and C2. As indicated by the specific surface area measurements, fine fibre dimensions promote increased bio-solubility.

Table 8

Sample Normalised XRF Results mit%) Static Solubility Now through Specific Surface Area

Description 4 H 7 Dissolution Rate

(p. saline) (pH 7.4 saline) A1203 CaO K20 MgO 5i02 (total PPm) (ng/cm2hr) (m 2/g) C-1 1.1 24.9 0.6 0.6 72.8 230 125 0.1652 C-2 0.1 29.8 0.0 5.2 64.9 313 379 0.2526 11 0.2 33.8 0.0 0.1 65.8 378 348 0.2887 16 0.4 32.7 0.0 0.1 66.7 295 326 0.3375 17 1.5 32.6 0.1 0.2 65.5 167 18 1.0 33.1 0.2 0.2 65.5 208

Summary of results

The above results highlight that the fibre composition of the present disclosure is able to produce a refractory fibre with great utility without the need for the deliberate additional to additives to enhance one or more fibre properties. This unexpected result also enables refractory fibres to be produced with a lower carbon footprint due to the reduced number of raw materials required for its production.

Potential uses The fibres of the present invention can be used, subject to meeting relevant performance criteria, for any purpose for which fibrous inorganic materials, and particularly alkaline earth silicate and aluminosilicate materials, have been used heretofore; and may be used in future applications where the fibre properties are appropriate. In the following reference is made to a number of patent documents relating to applications in which the fibres may be used, subject to meeting relevant performance criteria for the application. The fibres of the present invention can be used in place of the fibres specified in any of these applications subject to meeting relevant performance criteria.

For example, the fibres may be used as:- * bulk materials; * deshotted materials [W02013/094113]; * in a mastic or mouldable composition [W02013/080455, W02013/080456] or as part of a wet article [W02012/132271]; * as a constituent in needled or otherwise entangled [W02010/077360, W02011/0844871 assemblies of materials, for example in the form of blanket, folded blanket modules, or high density fibre blocks [W02013/046052]; * as a constituent of non-needled assemblies of materials, for example felts, vacuum formed shapes [W02012/132469], or papers [W02008/136875, W02011/040968, W02012/132329, W02012/132327]; * as a constituent (with fillers and/or binders) of boards, blocks, and more complex shapes [W02007/143067, W02012/049858, W02011/083695, W02011/083696]; * as strengthening constituents in composite materials such as, for example, fibre reinforced cements, fibre reinforced plastics, and as a component of metal matrix composites; * in support structures for catalyst bodies in pollution control devices such as automotive exhaust system catalytic converters and diesel particulate filters [W02013/015083], including support structures comprising: o edge protectants [W02010/024920, W02012/021270]; o microporous materials [W02009/032147, W02011019394, W02011/019396]; o organic binders and antioxidants [W02009/032191]; o intumescent material [W02009/032191]; o nanofibrillated fibres [W02012/021817]; o microspheres [W02011/084558]; o colloidal materials [W02006/004974, W02011/037617] o oriented fibre layers [W02011/084475]; o portions having different basis weight [W02011/019377]; o layers comprising different fibres [W02012065052]; o coated fibres [W02010122337]; o mats cut at specified angles [W02011067598]; [NB all of the above features may be used in applications other than support structures for catalytic bodies] * as a constituent of catalyst bodies [W02010/074711]; * as a constituent of friction materials [e.g. for automotive brakes [JP56-16578]]; * for fire protection [W02011/060421, W02011/060259, wo2o12/068427, W02012/148468, W02012/148469, W02013074968]; * as insulation, for example; o as insulation for ethylene crackers [vv02009/126593], hydrogen reforming apparatus [U54690690]; o as insulation in furnaces for the heat treatment of metals including iron and steel [US4504957]; o as insulation in apparatus for ceramics manufacturing.

The fibres may also be used in combination with other materials. For example the fibres may be used in combination with polycrystalline (sol-gel) fibres [W02012/065052] or with other biosoluble fibres [W02011/037634].

Bodies comprising the fibres may also be used in combination with bodies formed of other materials. For example, in insulation applications, a layer of material according to the present invention [for example a blanket or board] may be secured to a layer of insulation having a lower maximum continuous use temperature [for example a blanket or board of alkaline earth silicate fibres] [W02010/120380, W02011133778]. Securing of the layers together may be by any known mechanism, for example blanket anchors secured within the blankets [U54578918], or ceramic screws passing through the blankets [see for example DE3427918-A1].

Treatment of the fibres In formation of the fibres or afterwards they may be treated by applying materials to the fibres. For example:- * lubricants may be applied to the fibres to assist needling or other processing of the fibres; * coatings may be applied to the fibres to act as binders; * coatings may be applied to the fibres to provide a strengthening or other effect, for example phosphates [W02007/005836] metal oxides [W02011159914] and colloidal materials such as alumina, silica and zirconia [W02006/004974]; * binders may be applied to the fibres to bind the fibres subsequent to incorporation in a body comprising such fibres.

Many variants, product forms, uses, and applications of the fibres of the present invention will be apparent to the person skilled in the art and are intended to be encompassed by this invention.

By providing biosoluble fibres having maximum continuous use temperature higher than alkaline earth silicate fibres, the present invention extends the range of applications for which biosoluble fibres may be used. This reduces the present need, for many applications, to use fibres that are not biosoluble.

Claims

Claims 1. Thermal insulation for use in applications requiring continuous resistance to temperatures of 1200°C or more, the thermal insulation comprising bio-soluble inorganic fibres with a diameter of less than 6.0pm, said fibres having a composition comprising: 61.0 to 71.0 wt% Si02; 27.0 to 39.0 wt% CaO; and >0 to 2.0 wt% incidental impurities wherein the incidental impurities comprises less than 1.0 wt% MgO; and the sum of Si02 and CaO is greater or equal to 98.0 wt%.
2. The inorganic fibres of claims 1, wherein the inorganic fibres have a melting temperature of greater than 1350°C.
3. The inorganic fibres of claim 1 or 2, wherein incidental impurities account for at least 0.3 wt% of the composition of the inorganic fibres.
4. The inorganic fibres of any one of the preceding claims, wherein the sum of Si02 and CaO and MgO in the incidental impurities is greater or equal to 99.0 wt% of the fibre composition.
S. The inorganic fibres of of any one of the preceding claims, wherein the inorganic fibres consist of Si02, CaO and incidental impurities.
6. The inorganic fibres of of any one of the preceding claims, wherein the fibre composition comprises less than 0.6 wt% MgO derived from the incidental impurities.
7. The inorganic fibres according to any one of the preceding claims, wherein the amount of A1203 from the incidental impurities is less than 0.6 wt%.
8. The inorganic fibres according to any one of the preceding claims, wherein the amount of alkali metal oxides from the incidental impurities is no more than 0.3 wt%.
9. The inorganic fibres of any one of the preceding claims, wherein the sum of Si02 and CaO is greater or equal to 99.0 wt%.
10. The inorganic fibres of any one of the preceding claims, wherein the sum of Si02 and CaO is greater or equal to 99.5 wt%.
11. The inorganic fibres according to any one of the preceding claims, wherein the composition comprises less than 70.0 wt% 5i02.
12. The inorganic fibres according to any one of the preceding claims, wherein then composition comprises greater than 64.0 wt% Si02.
13. The inorganic fibres according to any one of the preceding claims, wherein the composition comprises greater than 65.8 wt% Si02.
14. The inorganic fibres according to any one of the preceding claims, wherein the fibre composition further comprises 0.1 to 1.0 wt% of one or more of oxides or fluorides of lanthanides, Li, Na, K, Sr, Ba, Cr, Fe, Zn, Y, Zr, CaF2, B203, P205 or combinations thereof.
15. The inorganic fibres according to any one of the preceding claims wherein the incidental impurities comprise: 0 to less than 0.6 wt% MgO 0 to less than 0. 6 wt% A1203 0 to 0.3 wt% alkali metal oxides.
16. The inorganic fibres according to claim 1, wherein the composition comprises: 66.0 to 69.0 wt% 5i02; 30.0 to 34.0 wt% CaO; 0.3 to 1.0 wt% incidental impurities; the sum of Si02 and CaO is greater or equal to 99.0 wt%; and the incidental impurities comprise: 0.1 to 0.45 wt% MgO 0.1 to 0.35 wt% A1203 0.05 to 0.25 wt% alkali metal oxides.
17. The inorganic fibres according to any one of the preceding claims, wherein the shot content (>45 pm) is no more than 52.0 wt%.
18. The inorganic fibres according to any one of the preceding claims, wherein the shot content (>45 p.m) is no more than 50.0 wt%.
19. The inorganic fibres according to any one of the preceding claims, wherein the numerical fibre diameter is less than 5.0 pm.
20. The inorganic fibres according to any one of the preceding claims, wherein the numerical fibre diameter is less than 4.0 pm.
21. The inorganic fibres according to any one of the preceding claims, wherein the inorganic fibres are non-reactive with mullite when exposed to 1200°C for 24 hours.
22. The inorganic fibres according to any one of the preceding claims, wherein the inorganic fibres have a shrinkage of 3.0% or less when exposed to 1300°C for 24 hours.
23. The inorganic fibres according to any one of the preceding claims wherein the composition of the incidental impurities comprises: 0 to 0.05 wt% Ba0; 0 to 0.08 wt% Cr203; 0 to 0.25 wt% 1e203; 0 to 0.05 wt% Hf02; 0 to 0.1 wt% La203; 0 to 0.05 wt% Mn304; 0 to 0.5 wt% K20; 0 to 0.15 wt% Na20; 0 to 0.05 wt% P205; 0 to 0.08 wt% Sr0; 0 to 0.08 wt%Ti02; 0 to 0.05 wt% V205; 0 to 0.05 wt% 5n02; 0 to 0.05 wt% Zn0; and 0 to 0.1 wt% Zr02.
24. The inorganic fibres according to any one of the preceding claims, wherein the sum of BaO + Cr203+ Fe203÷ R102 La203+ Mn304+ K20 + Na20 ÷ P205 +Sr0 + Ti02+ V205+ Zr02 + ZnO in the incidental impurities is less than 1.2 wt%.
25. Use of the inorganic fibres of claims 1 to 24 at temperatures at or above 1200°C or above 1260°C or above 1280°C.