Abstract:
A method of making refractory alkaline earth silicate fibres from a melt, including the use as an intended component of alkali metal to improve the mechanical properties of the fibre in comparison with a fibre free of alkali metal.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of priority from applicants&#39; provisional application 60/717,516 filed Sep. 15, 2005 and British patent applications GB 0424190.7 filed Nov. 1, 2004 and GB 0502701.6 filed Feb. 9, 2005, all of which are relied on and incorporated herein by reference. 
     
    
     INTRODUCTION AND BACKGROUND  
       [0002]     This invention relates to alkaline earth silicate fibres.  
         [0003]     Inorganic fibrous materials are well known and widely used for many purposes (e.g. as thermal or acoustic insulation in bulk, mat, or blanket form, as vacuum formed shapes, as vacuum formed boards and papers, and as ropes, yarns or textiles; as a reinforcing fibre for building materials; as a constituent of brake blocks for vehicles). In most of these applications the properties for which inorganic fibrous materials are used require resistance to heat, and often resistance to aggressive chemical environments.  
         [0004]     Inorganic fibrous materials can be either glassy or crystalline. Asbestos is an inorganic fibrous material one form of which has been strongly implicated in respiratory disease.  
         [0005]     It is still not clear what the causative mechanism is that relates some asbestos with disease but some researchers believe that the mechanism is mechanical and size related. Asbestos of a critical size can pierce cells in the body and so, through long and repeated cell injury, have a bad effect on health. Whether this mechanism is true or not regulatory agencies have indicated a desire to categorise any inorganic fibre product that has a respiratory fraction as hazardous, regardless of whether there is any evidence to support such categorisation. Unfortunately for many of the applications for which inorganic fibres are used, there are no realistic substitutes.  
         [0006]     Accordingly there is a demand for inorganic fibres that will pose as little risk as possible (if any) and for which there are objective grounds to believe them safe.  
         [0007]     A line of study has proposed that if inorganic fibres were made that were sufficiently soluble in physiological fluids that their residence time in the human body was short; then damage would not occur or at least be minimised. As the risk of asbestos linked disease appears to depend very much on the length of exposure this idea appears reasonable. Asbestos is extremely insoluble.  
         [0008]     As intercellular fluid is saline in nature the importance of fibre solubility in saline solution has long been recognised. If fibres are soluble in physiological saline solution then, provided the dissolved components are not toxic, the fibres should be safer than fibres which are not so soluble. Alkaline earth silicate fibres have been proposed for use as saline soluble, non-metallic, amorphous, inorganic oxide, refractory fibrous materials. The invention particularly relates to glassy alkaline earth silicate fibres having silica as their principal constituent.  
         [0009]     International Patent Application No. WO87/05007 disclosed that fibres comprising magnesia, silica, calcia and less than 10 wt % alumina are soluble in saline solution. The solubilities of the fibres disclosed were in terms of parts per million of silicon (extracted from the silica containing material of the fibre) present in a saline solution after 5 hours of exposure. WO87/05007 stated that pure materials should be used and gave an upper limit of 2 wt % in aggregate to the impurities that could be present. No mention of alkali metals was made in this patent.  
         [0010]     International Patent Application No. WO89/12032 disclosed additional fibres soluble in saline solution and discusses some of the constituents that may be present in such fibres. This disclosed the addition of Na 2 O in amounts ranging from 0.28 to 6.84 wt % but gave no indication that the presence of Na 2 O had any effect.  
         [0011]     European Patent Application No. 0399320 disclosed glass fibres having a high physiological solubility and having 10-20 mol % Na 2 O and 0-5 mol % K 2 O. Although these fibres were shown to be physiologically soluble their maximum use temperature was not indicated.  
         [0012]     Further patent specifications disclosing selection of fibres for their saline solubility include for example European 0412878 and 0459897, French 2662687 and 2662688, PCT WO86/04807, WO90/02713, WO92/09536, WO93/22251, WO94/15883, WO97/16386 and U.S. Pat. No. 5,250,488.  
         [0013]     The refractoriness of the fibres disclosed in these various prior art documents varies considerably and for these alkaline earth silicate materials the properties are critically dependent upon composition.  
         [0014]     As a generality, it is relatively easy to produce alkaline earth silicate fibres that perform well at low temperatures, since for low temperature use one can provide additives such as boron oxide to ensure good fiberisation and vary the amounts of the components to suit desired material properties. However, as one seeks to raise the refractoriness of alkaline earth silicate fibres, one is forced to reduce the use of additives since in general (albeit with exceptions) the more components are present, the lower the refractoriness.  
         [0015]     WO93/15028 disclosed fibres comprising CaO, MgO, SiO 2 , and optionally ZrO 2  as principal constituents. Such fibres are frequently known as CMS (calcium magnesium silicate) or CMZS ((calcium magnesium zirconium silicate) fibres. WO93/15028 required that the compositions used should be essentially free of alkali metal oxides. Amounts of up to 0.65 wt % were shown to be acceptable for materials suitable for use as insulation at 1000° C. WO93/15028 also required low levels of Al 2 O 3  (&lt;3.97%).  
         [0016]     WO94/15883 disclosed a number of such fibres usable as refractory insulation at temperatures of up to 1260° C. or more. As with WO93/15028, this patent required that the alkali metal oxide content should be kept low, but indicated that some alkaline earth silicate fibres could tolerate higher levels of alkali metal oxide than others. However, levels of 0.3% and 0.4% by weight Na 2 O were suspected of causing increased shrinkage in materials for use as insulation at 1260° C. The importance of keeping the level of alumina low was stressed is stressed in this document.  
         [0017]     WO97/16386 disclosed fibres usable as refractory insulation at temperatures of up to 1260° C. or more. These fibres comprised MgO, SiO 2 , and optionally ZrO 2  as principal constituents. These fibres are stated to require substantially no alkali metal oxides other than as trace impurites (present at levels of hundredths of a percent at most calculated as alkali metal oxide). The fibres have a general composition  
                                                       SiO 2     65-86%           MgO   14-35%                      
 
 with the components MgO and SiO 2  comprising at least 82.5% by weight of the fibre, the balance being named constituents and viscosity modifiers. Such magnesium silicate fibres may comprise low quantities of other alkaline earths. The importance of keeping the level of alumina low was stressed is stressed in this document. 
 
         [0018]     WO2003/059835 discloses certain calcium silicate fibres certain calcium silicate compositions for which fibres show a low reactivity with aluminosilicate bricks, namely:— 
         [0019]     65%&lt;SiO 2 &lt;86%  
         [0020]     MgO&lt;10%  
         [0021]     14%&lt;CaO&lt;28%  
         [0022]     Al 2 O 3 &lt;2%  
         [0023]     ZrO 2 &lt;3%  
         [0024]     B 2 O 3 &lt;5%  
         [0025]     P 2 O 5 &lt;5%  
         [0026]     72%&lt;SiO 2 +ZrO 2 +B 2 O 3 +5*P 2 O 5    
         [0000]     95%&lt;SiO 2 +CaO+MgO+Al 2 O 3 +ZrO 2 +B 2 O 3 +P 2 O 5    
         [0027]     This patent also discloses the use of La 2 O 3  or other lanthanide additives to improve the strength of the fibres and blanket made from the fibres. This patent application does not mention alkali metal oxide levels, but amounts in the region of ˜0.5 wt % were disclosed in fibres intended for use as insulation at up to 126° C. or more.  
         [0028]     WO2003/060016 claims a low shrinkage, high temperature resistant inorganic fiber having a use temperature up to at least 1330 C, which maintains mechanical integrity after exposure to the use temperature and which is non-durable in physiological fluids, comprising the fiberization product of greater than 71.25 to about 85 weight percent silica, 0 to about 20 weight percent magnesia, about 5 to about 28.75 weight percent calcia, and 0 to about 5 weight percent zirconia, and optionally a viscosity modifier in an amount effective to render the product fiberizable.  
         [0029]     EP 1323687 claims a biosoluble ceramic fiber composition for a high temperature insulation material comprising 75-80 wt % of SiO 2 , 13-25 wt % of CaO, 1-8 wt % of MgO, 0.5-3 wt % of ZrO 2  and 0-0.5 wt % of Al 2 O 3 , wherein (ZrO 2 +Al 2 O 3 ) is contained 0.5-3 wt % and (CaO+MgO) is contained 15-26 wt %.  
         [0030]     Alkaline earth silicate fibres have received a definition in the Chemical Abstract Service Registry [Registry Number: 436083-99-7] of:—
        “Chemical substances manufactured in the form of fibers. This category encompasses substances produced by blowing or spinning a molten mixture of alkaline earth oxides, silica and other minor/trace oxides. It melts around 1500° C. (2732° F.). It consists predominantly of silica (50-82 wt %), calcia and magnesia (18-43 wt %), alumina, titania and zirconia (&lt;6 wt %), and trace oxides.”.        
 
         [0032]     This definition reflects European Health and Safety regulations which impose special labelling requirements on silicate fibres containing less than 18% alkaline earth oxides.  
         [0033]     However as is clearly indicated in relation to WO2003/059835, WO2003/060016 and EP 1323687, the silica content of alkaline earth silicate fibres is increasing with the demand for higher use temperatures and this is leading to lower alkaline earth contents.  
         [0034]     The present invention is applicable not only to alkaline earth silicate fibres in this narrow definition reflected in the Chemical Abstracts definition, but also to alkaline earth silicate fibres having lower levels of alkaline earth oxides.  
         [0035]     Accordingly, in the present specification alkaline earth silicate fibres should be considered to be materials comprising predominantly of silica and alkaline earth oxides and comprising less than 10 wt % alumina [as indicated in WO87/05007—which first introduced such fibres], preferably in which alumina, zirconia and titania amount to less that 6 wt % [as indicated in the Chemical Abstracts definition]. For regulatory reasons, preferred materials contain more than 18% alkaline earth metal oxides.  
         [0036]     The prior art shows that for refractory alkaline earth silicate fibres, alkali metals have been considered as impurities that can be tolerated at low levels but which have detrimental affects on refractoriness at higher levels.  
       SUMMARY OF THE INVENTION  
       [0037]     The applicant has found that, contrary to received wisdom in the field of refractory alkaline earth silicate fibres, the addition of minor quantities of alkali metals within a certain narrow range improves the mechanical quality of fibres produced (in particular fibre strength) without appreciably damaging the refractoriness of the fibres.  
         [0038]     Accordingly, the present invention provides a method of making refractory alkaline earth silicate fibres from a melt, comprising the inclusion as an intended melt component of alkali metal to improve the mechanical and/or thermal properties of the fibre in comparison with a fibre free of alkali metal.  
         [0039]     Preferably, the amount of alkali metal (M) expressed as the oxide M 2 O is greater than 0.2 mol % and preferably in the range 0.2 mol % to 2.5 mol %, more preferably 0.25 mol % to 2 mol %.  
         [0040]     By “a fibre free of alkali metal” is meant a fibre in which all other components are present in the same proportions but which lacks alkali metal.  
         [0041]     The alkali metal is preferably present in an amount sufficient to increase the tensile strength of a blanket made using the fibre by &gt;50% over the tensile strength of a blanket free of alkali metal, and less than an amount that will result in a shrinkage as measured by the method described below of greater than 3.5% in a vacuum cast preform of the fibre when exposed to 1250° C. for 24 hours.  
         [0042]     It will be apparent that the alkali metal may be provided either as an additive to the melt (preferably in the form of an oxide), or by using as ingredients of the melt appropriate amounts of materials containing alkali metal as a component or impurity, or both as an additive and as a component or impurity. The invention lies in ensuring that the melt has the desired quantity of alkali metal to achieve the beneficial effects of the invention.  
         [0043]     The invention may be applied to all of the prior art alkaline earth silicate compositions mentioned above. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0044]     The scope and further features of the invention will become apparent from the claims in the light of the following illustrative description and with reference to the drawings in which:— 
         [0045]      FIG. 1  is a graph showing tensile strength/density plotted against melt stream temperatures as determined in a production trial for a number of fibres of differing Na 2 O content;  
         [0046]      FIG. 2  is a graph plotting maximum, average, and minimum values of tensile strength/density against Na 2 O content for the same fibres;  
         [0047]      FIG. 3  is a graph of experimentally determined temperature/viscosity curves for a range of compositions;  
         [0048]      FIG. 4  is a graph showing shot content plotted against Na 2 O content for the fibres of  FIG. 1   
         [0049]      FIG. 5  is a graph of shot content against Na 2 O content for a different range of alkaline earth silicate fibres  
         [0050]      FIG. 6  is a graph of linear shrinkages for alkaline earth silicate fibres of varying composition, compared with known refractory ceramic fibre (RCF) fibres  
         [0051]      FIG. 7  is a graph of the effect on blanket strength of sodium addition to a range of alkaline earth silicate fibres  
         [0052]      FIG. 8  contrasts micrographs showing various fibres after exposure to a range of temperatures  
         [0053]      FIG. 9  is a graph comparing measured thermal conductivities for a range of fibres. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0054]     The inventors produced fibre blanket using a production trial line at their factory in Bromborough, England. Fibre was produced by forming a melt and allowing the melt to fall onto a pair of spinners (as is conventionally known).  
         [0055]     The base melt had a nominal composition in weight percent:— 
                                                       SiO 2     73.5           CaO   25           La 2 O 3     1.5                      
 
 with other components forming minor impurities and sodium oxide being added in specified amounts. 
 
         [0056]     The melt stream temperature was monitored using a two colour pyrometer.  
         [0057]     Fibres produced from the spinners were passed onto a conveyer and then needled to form blanket in a conventional manner.  
         [0058]     The blanket thickness, density, and tensile strength were measured for fibres produced using a range of conditions.  
         [0059]     The blanket was produced with a view to determining the effect on fibre quality of melt stream temperature, since it was believed that this had an effect on fibre quality.  
         [0060]     The inventors also decided to add alkali metal oxides with the view of flattening the viscosity-temperature curve of the melt as this was thought a relevant factor in fibre production as explained further below.  
         [0061]     The results of these tests are set out in Table 1 and illustrated graphically in  FIGS. 1 and 2 . In Table 1, the melt stream temperature, blanket thickness, blanket density, tensile strength and tensile strength divided by density is shown for all compositions. [The tensile strength divided by density is calculated to counteract the variation attributable to different amounts of material being in the blanket]. Also for selected compositions the shrinkage of a preform at 1150° C. and 1250° C. was measured in the same manner as in WO2003/059835.  
         [0062]     The first thing that is noteworthy is that the blanket strengths show a high variability. This is because the manufacture of a blanket involves many variables, including:—
        Composition of the melt     Temperature of the melt     Melt stream temperature     Shot content (melt that has solidified in the form of droplets rather than fibres)     Fibre diameter     Fibre length     Needling conditions     Post-solidification thermal history        
 
         [0071]     By producing a range of fibres on a single line and significantly varying only melt stream temperature and composition (each of which will have an affect on shot content, fibre diameter and fibre length) it was hoped to reduce such variability. However because a blanket is an aggregated body of individual fibres, there is inevitably a statistical variation in such aggregate properties as tensile strength.  
         [0072]     As can be seen from  FIG. 1  there appears to be relatively little variation in strength with melt stream temperature, but since the range of melt stream temperatures chosen was selected to encompass ranges previously found to be effective, this is not surprising.  
         [0073]     However, it can be seen that with progressive increases in Na 2 O content, the strength tends to increase.  FIG. 2  shows the maximum, minimum, and average strengths found for a range of compositions and it can be seen that blanket strength shows a strong positive correlation with Na 2 O content. In contrast, the shrinkage of the fibres seemed barely affected.  
         [0074]     The fibres with nominal zero Na 2 O content of course had minor trace amounts (average measured content 0.038%-maximum 0.11%). Extrapolating back to zero Na 2 O gives an average tensile strength/density of 0.0675 kPa/[kg/m 3 ]. The average tensile strength/density for the addition of 0.3% Na 2 O is 0.1426. The increase in blanket strength is over 100% and smaller additions (e.g. 0.25 mol %) would be expected to exceed a 50% improvement.  
                                                                                                                                                                                       TABLE 1                                   % linear   % linear   Tensile Strength           Melt Stream   Blanket   Blanket   shrinkage   shrinkage   kPa (average of   Tensile       Temperature   thickness   density   1150° C./24   1250° C./24   three   strength/       ° C.   (mm)   (kg/m 3 )   hours   hours   mesurements)   density                                Zero nominal Na 2 O content            1750   7.20   118           5.67   0.048023       1750   8.18   109           8.33   0.076453       1750   16.87   161           15.33   0.095238       1750   15.12   169           15.67   0.092702       1750   15.71   134           16.00   0.119403       1750   20.51   141           13.67   0.096927       1750   19.14   138           11.33   0.082126       1750   18.58   125           9.67   0.077333       1750   18.87   141           12.00   0.085106       1750   25.92   130           14.00   0.107692       1750   24.49   140           17.00   0.121429       1750   15.88   166           13.47   0.081124       1750   17.34   144           7.33   0.050926       1750   11.00   174           16.20   0.093103       1750   22.01   124   0.52   0.88   7.91   0.06379       1760   16.60   133           18.47   0.138847       1800   8.06   129           8.67   0.067183       1800   22.04   132           12.92   0.097904       1800   21.97   139           13.62   0.09801       1850   7.75   120           9.33   0.077778       1850   18.49   133           9.31   0.069962       1850   18.12   128           8.56   0.066901       1850   17.19   123           5.33   0.043333       1850   24.49   125           5.26   0.042107       1900   21.83   114           10.57   0.092708       1910   8.50   127           12.33   0.097113       1950   8.14   115           9.33   0.081159       1950   8.92   115           10.00   0.086957       1990   19.39   123           10.67   0.086764            0.3 wt % nominal Na 2 O content            1800   22.82   107           19.83   0.185327       1850   17.10   149           16.91   0.113512       1900   24.40   137           17.66   0.128881            0.5 wt % nominal Na 2 O content            1795   20.32   169   0.43   1.70   48.64   0.287811       1800   19.98   147           24.81   0.168913       1800   25.25   136           16.17   0.118922       1800   18.64   153           34.24   0.223769       1800   18.02   190           42.65   0.224456       1800   24.22   175           37.26   0.212895       1800   22.47   165           36.83   0.223212       1835   14.54   150           42.01   0.280067       1850   23.50   164   0.31   1.04   27.29   0.166789       1850   25.15   162           27.85   0.171681            0.7 wt % nominal Na 2 O content            1800   21.91   166           47.12   0.283835       1800   21.25   166           38.32   0.230863       1800   18.44   161           53.64   0.333188       1800   19.22   163           38.74   0.237669       1800   19.95   144   0.48   1.11   33.35   0.231597       1850   26.04   175   0.48   0.90   38.41   0.219467       1850   23.48   166           54.11   0.325984       1850   27.73   165           37.03   0.224404       1900   29.30   166           41.69   0.251165       1900   21.16   135           44.09   0.326617       1900   19.49   135           40.93   0.30316       1950   25.88   151           39.12   0.259073                  
 
         [0075]     Encouraged by this, and with a view to determining the upper limit of alkali metal oxide that was appropriate, the inventors produced a range of further alkaline earth silicate fibres using an experimental rig in which a melt was formed of appropriate composition, tapped through a 8-16 mm orifice, and blown to produce fibre in a known manner. (The size of the tap hole was varied to cater for the viscosity of the melt—this is an adjustment that must be determined experimentally according to the apparatus and composition used). Shrinkage of performs of the fibre at 1150° C. and 1250° C. were measured in the same manner as in WO2003/059835. Total solubility in ppm of the major glass components after a 24 hour static test in a physiological saline solution were also measured for some of the examples.  
         [0076]     The results of these studies are shown in Table 2. The fibres in the left of the table were aimed at assessing the effect of adding approximately equimolar amounts of alkali metal addition to calcium silicate fibre containing La 2 O 3  (as in WO2003/059835), whereas those to the right were aimed at assessing the effect of varying the quantity of Na 2 O in such a fibre. While not conclusive, the results indicate that for these fibres Na 2 O and K 2 O show shrinkages no worse or even better than fibre free of Na 2 O, whereas Li 2 O appears detrimental to shrinkage.  
         [0077]     However, this latter conclusion is thought unsafe since it was determined that the lithium had been added in the form of lithium tetraborate, and the boron addition may have had a significant effect. Until proven otherwise, the applicants are assuming that all alkali metals can be used in the invention, but that the absolute amount of alkali metal may vary from metal to metal and fibre to fibre. The solubility figures show that total solubility is slightly increased by the addition of alkali metal oxide.  
                                                                                                                                                                                                   TABLE 2                           PAT   PAT   PAT Li 2 O   PAT K 2 O   BG-X-04-   BG-X-04-   BG-X-04-       Sample   STD 01   Na 2 O 02   03   04   0305   0277   0279                                Component            Na 2 O   0.26   0.95   0.12   0.24   0.6   0.72   1.14       MgO   0.38   0.39   0.36   0.36   0.35   0.38   0.36       Al 2 O 3     0.6   0.64   0.56   0.62   0.38   0.02   0       SiO 2     72.58   72.47   72.43   72.40   73.26   73.58   73.76       K 2 O   0.08   0.08   0.07   1.05   0.07   0.08   0.08       CaO   24.05   23.27   23.62   22.67   22.82   23.52   23.22       TiO 2     0.1   0.10   0.11   0.15   0.1   0.1   0.1       Fe 2 O 3     0.16   0.19   0.21   0.23   0.16   0.18   0.18       La 2 O 3  (estimated)   1.3   1.3   1.3   1.3   1.3   1.3   1.3       Li 2 O           0.34*            % Linear Shrinkage             850° C./24 hours                   0.38   0.21   0.22       1150° C./24 hours   1.05   0.88   1.58   0.63   0.47   0.36   0.59       1250° C./24 hours   1.08   1.08   1.71   0.79   0.48   0.69   0.84            % Thickness shrinkage             850° C./24 hours                   0.42   0.71   1.31       1150° C./24 hours                   0.93   0.71   1.44       1250° C./24 hours                   0.91   0.72   6.43            Static Solubility 24 hrs (ppm)                191   202   200   N/A                      
 
         [0078]     The right side of Table 2 shows firstly that only a 1% higher silica content has a big effect on shrinkage, giving a much lower shrinkage. For these fibres, linear shrinkage at 850° C./24 hrs seemed unaffected by all soda additions tested, however the same is not true for thickness shrinkage, although it is still low. At 1150° C./24 hrs there is a slight increase in both linear and through thickness shrinkage, but at 1250° C./24 hrs through thickness whilst still acceptable grows more significantly for the highest soda addition. All of these figures are acceptable for some applications whereas other applications could not tolerate the highest Na 2 O level tested.  
         [0079]     The improvement in shrinkage with higher silica levels led the inventors to look to materials containing still higher silica levels and the results are set out in Table 3 below.  
                                                                                                                                                   TABLE 3                           PAT Na 2 O   PAT Na 2 O   PAT Na 2 O   PAT Na 2 O   PAT Na 2 O   PAT Na 2 O       Sample   05   06   07   08   09   10                                Component            Na 2 O   0.5   0.5   0.5   0.5   0.5   1.1       MgO   0.4   0.3   0.3   0.4   0.3   0.4       Al 2 O 3     0.6   0.5   0.6   0.8   0.6   0.8       SiO 2     73.9   74.3   74.5   75.2   76.3   77.7       K 2 O   0.1   0.1   0.1   0.1   0.1   0.1       CaO   23.6   22.9   22.6   22.0   21.4   19.3       TiO 2     0.1   0.1   0.1   0.1   0.1   0.1       Fe 2 O 3     0.2   0.2   0.2   0.2   0.2   0.2       La 2 O 3     1.3   1.3   1.3   1.3   1.3   1.3            % Linear Shrinkage            1150° C./24 hrs   0.54   0.8   0.61   0.56   0.65   0.58       1250° C./24 hrs   1.1   1.07   N/A   0.84   0.86   N/A            Static Solubility 24 hrs (ppm)                199   208   165   194   245   107                      
 
         [0080]     These results show low shrinkage and a reasonably high solubility across the range. It appears that addition of alkali metal oxide may increase the amount of silica that can be added to produce a workable alkaline earth silicate fibre, and perhaps with an acceptable solubility. This is of great significance since, in general, increasing silica content permits higher use temperatures for alkaline earth silicate fibres.  
         [0081]      FIG. 6  shows the shrinkage at various temperatures of preforms of a range of alkaline earth silicate fibres. The reference SW613 refers to lanthanum containing materials of composition similar to those set out in Table 3 with varying silica contents as indicated but absent any alkali metal addition. [Silica and calcia comprising most of the material with lanthanum oxide being present in about 1.3%]. One of these fibres also has an addition of 2 wt % MgO. Also shown are shrinkages for a conventional aluminosilicate fibre (RCF) and a magnesium silicate fibre (MgO Silicate).  
         [0082]     It can be seen that all of the SW613 fibres have a shrinkage lower than that of RCF and the MgO silicate fibres up to 1350° C. but rise thereafter. However, there is a progressive increase in refractoriness with increasing silica content. For the SW613 fibre containing 77 and 79% SiO 2 , the shrinkage remains below that of RCF and the MgO silicate fibres up to 1400° C. and better could be expected for higher silica contents. In contrast, it can be seen also that addition of 2% MgO to the SW613 compositions is detrimental to shrinkage. High silica alkaline earth silicate fibres are difficult to make and addition of alkali metals to such compositions should improve the quality of such fibres and ease manufacture.  
         [0083]     Having shown such effects the applicants conducted a trial to make blanket on a production line, to see whether the initial results on shrinkage were confirmed. A base composition comprising:— 
                                                       SiO 2     72.5-74 wt %           CaO   24-26.5 wt %           MgO    0.4-0.8 wt %            Al 2 O 2       &lt;0.3 wt %           La 2 O 3      1.2-1.5 wt %                       
 
 was used and varying amounts of Na 2 O were added. Blanket having a density 128 kg/m 3  was produced having a thickness of 25 mm. The results, summarised in  FIG. 7 , show a dramatic increase in blanket strength with Na 2 O addition. 
 
         [0084]     These findings relate to compositions containing La 2 O 3  as a component, but similar effects of alkali metal additions are found with alkaline earth silicate fibres not containing La 2 O 3  as a component.  
         [0085]     The inventors also tested other alkaline earth silicate fibres comprising predominantly magnesium as the alkaline earth component (magnesium silicate fibres) and the results are set out in Table 4.  
         [0086]     This table shows that whereas Na 2 O and K 2 O have a small or large respectively detrimental effect on shrinkage, Li 2 O has hardly any effect on shrinkage. This does not imply no effect at all, the inventors observed that whereas the fibres with Na 2 O and K 2 O were similar to fibres without such additives (coarse) the fibre with Li 2 O addition was significantly finer and of better quality. At lower quantities, Na 2 O and K 2 O may still give shrinkages that are tolerable in most applications.  
                                                                                                                                 TABLE 4                                       Sample                04 MgO 01   04 MgO 02   04 MgO 03   04 MgO 04                        Component            Na 2 O   0.0   0.5   0.0   0.0       MgO   20.0   19.1   19.6   18.3       Al 2 O 3     1.7   2.0   1.8   1.7       SiO 2     77.6   77.5   77.8   78.2       K 2 O   0.0   0.0   0.0   1.0       CaO   0.5   0.5   0.6   0.5       TiO 2     0.1   0.1   0.1   0.1       Fe 2 O 3     0.5   0.5   0.5   0.5       Li 2 O           0.3            % Linear Shrinkage            1150° C./   2.53   3.53   2.34   5.59       24 hrs       1250° C./   2.16   3.57   2.3   9.94       24 hrs            Static Solubility 24 hrs (ppm)                297   N/A   331   N/A                      
 
         [0087]     The purpose of adding alkali metal is to try to alter the viscosity temperature curve for alkaline earth silicates so as to provide a more useful working range for the silicates.  FIG. 3  shows a graph experimental viscosity/temperature curves for:— 
         [0088]     a high soda glass having the approximate composition in wt %:— 
                                                       SiO 2     68           Na 2 O   13.4           CaO   7.94           B 2 O 3     4.74           MgO   2.8           Al 2 O 3     2.66           Fe 2 O 3     1.17           TiO 2     0.09           ZrO 2     0.08           Cr 2 O 3     0.06                      
 
         [0089]     an alkaline earth silicate melt comprising the approximate composition:— 
         [0090]      CaO 29  
         [0091]      MgO 6%  
         [0092]      SiO 2  64.5  
         [0093]      +others to 100%  
         [0094]     and the same alkaline earth silicate melt comprising respectively 1 wt % Na 2 O and 2 wt % Na 2 O as an additive.  
         [0095]     The viscosity/temperature graph of the high soda glass is a smooth line rising as temperature falls.  
         [0096]     For the known alkaline earth silicate melt (SW) the viscosity is lower and then rises steeply at a critical temperature value (this is shown as a slope in the graph but that is an artefact of the graphing process—it actually represents a much steeper change).  
         [0097]     Addition of Na 2 O to the melt moves this rise in viscosity to lower temperatures.  
         [0098]     This extends the working range of the melt so that it becomes less dependent upon temperature so increasing the tolerance of the melt to fibre forming conditions. Although the melt stream temperature is important, the melt cools rapidly during the fibre forming process and so a longer range of workability for the composition improves fibre formation. The addition of the alkali metal oxides may also serve to stabilise the melt stream so that for a given set of conditions there is an amount that reduces shot.  
         [0099]     Additionally, it is surmised that in small quantities the alkali metal oxides serve to suppress phase separation in alkaline earth silicate fibres.  
         [0100]     Since the alkaline earth silicate systems have a two liquid region in their phase diagrams, the applicants suspect that addition of alkali metal oxides may move the melts out of a two-liquid region into a single phase region.  
         [0101]     The addition also has the effect of lowering melt stream temperature which may assist in stability.  
         [0102]     The effectiveness of these measures is also shown by the amount of shot present in the finished material. In the fibre forming process, droplets of melt are rapidly accelerated (by being flung off a spinning wheel or being blasted by a jet of gas) and form long tails which become the fibres.  
         [0103]     However that part of the droplets that does not form fibre remains in the finished material in the form of particles known in the industry as “shot”. Shot is generally detrimental to the thermal properties of insulation formed from the fibres, and so it is a general aim in the industry to reduce the quantity of shot.  
         [0104]     The applicants have found that addition of minor amounts of alkali metal to the melt has the effect of reducing the amount of shot, and this is shown in  FIG. 4  for the lanthanum containing materials of Table 1, where it can be seen that the shot content was reduced from 51% to 48%.  
         [0105]     Similar effects apply to lanthanum free materials. Table 5 shows the analysed compositions of a range of alkaline earth silicate fibres (having a lower maximum use temperature) made in accordance with the compositions of WO93/15028, which were made by spinning using a melt stream temperature of 1380-1420° C., and with a pair of rotating spinners.  
         [0106]      FIG. 5  shows experimentally determined shot contents with error bars indicating one standard deviation about mean. It can be seen that in the range 0.35 to 1.5 wt % Na 2 O, there is a statistical improvement in the shot content as a result of the addition. In particular, a 3% reduction in shot for a 0.35 wt % soda content is significant.  
         [0107]     Since there seems no detrimental effect on shrinkage at such levels (and indeed a slight improvement) it can be seen that addition of alkali metal oxides is beneficial for the production of such materials.  
                                                                                                                           TABLE 5                       Sample   04-C43-1   04C56-7   04C46-5   04C47-2   04C51-6   04C50-8   04C49-6                                Component            Na 2 O   0.11   0.35   0.66   1.01   1.47   2.03   2.46       MgO   4.78   5.90   5.18   5.47   5.71   5.76   6.20       Al 2 O 3     1.07   0.40   0.35   0.27   0.30   0.36   0.30       SiO 2     65.1   65.16   65.07   64.96   65.91   66.15   65.24       P 2 O 5     0   0.00   0.00   0.00   0.00   0.00   0.00       K 2 O   0.08   0.08   0.08   0.07   0.07   0.07   0.07       CaO   28.92   27.84   28.47   28.12   26.25   25.36   24.79       TiO 2     0.02   0.02   0.03   0.01   0.02   0.03   0.02       Cr 2 O 3     0.02   0.02   0.02   0.02   0.02   0.02   0.02       Mn 3 O 4     0.03   0.03   0.03   0.03   0.03   0.03   0.03       Fe 2 O 3     0.2   0.19   0.19   0.18   0.18   0.18   0.18       ZnO   0   0.00   0.00   0.00   0.00   0.01   0.00       SrO   0.01   0.01   0.01   0.01   0.01   0.01   0.01       ZrO 2     0   0.00   0.00   0.00   0.00   0.00   0.00       BaO   0   0.00   0.00   0.00   0.00   0.00   0.00       HfO 2     0   0.00   0.00   0.00   0.00   0.00   0.00       PbO   n/a   n/a   n/a   n/a   n/a   n/a   n/a       SnO 2     n/a   n/a   n/a   n/a   n/a   n/a   n/a       CuO   n/a   n/a   n/a   n/a   n/a   n/a   n/a            % Linear Shrinkage            1000° C./24 hours   1.42       1.33   1.54       4.18           1100° C./24 hours   1.39       1.20   1.77       4.85                  
 
         [0108]     Addition of the alkali metal should be at levels that do not excessively detrimentally affect other properties of the fibre (e.g. shrinkage), but for different applications what is “excessive” will vary.  
         [0109]     The fibres can be used in thermal insulation and may form either a constituent of the insulation (e.g. with other fibres and/or fillers and/or binders) or may form the whole of the insulation. The fibres may be formed into blanket form insulation.  
         [0110]     Although initial work was primarily related to the addition of Na 2 O to alkaline earth silicate fibres, the applicants discovered that when Na 2 O was used as the additive to high calcium—low magnesium fibres it had a tendency to promote crystallisation (and hence powderiness of the fibres) after exposure to temperatures of ˜1000° C. This can be seen in  FIG. 8  in which fibre a)-e) had base compositions falling in the region:— 
                                                           SiO 2     72-75   wt %           CaO   22-26.5   wt %           MgO   0.4-1   wt %           Al 2 O 2     &lt;0.3   wt %           La 2 O 3     1.2-1.5   wt %                      
 
         [0111]     Fibres a), b) and c) show the effect on surface appearance of fibres after exposure to 1050° C. for 24 hours on fibres containing increasing amounts of Na 2 O (from ˜0 through 0.5 wt % to 1.06 wt % respectively). As can be seen, the fibre absent Na 2 O has a smooth appearance indicating little crystallisation, whereas increasing Na 2 O leads to an increase in surface roughness indicative of crystallisation.  
         [0112]     In contrast, fibres d) and e) show that at 1100° C. a fibre containing ˜0.5 wt % K2O is little different from a fibre free of K 2 O, and only starts to show slight surface roughness at 1150° C.  
         [0113]     Table 6 shows relative thermal conductivities of blankets having approximate density of 96 kg·m −3  formed from fibres having the principal ingredients shown. It also shows thermal conductivities of these blankets and these figures are shown in  FIG. 9 . It can be seen that addition of Na 2 O and K 2 O seems to result in lower thermal conductivity from the blankets so showing improved insulating ability.  
                                                               TABLE 6                                       Ca Silicate   Ca Silicate           Ca Silicate   Mg Silicate   with   with Na 2 O           blanket   Blanket   K 2 O addition   addition               Na 2 O   0.22   0   0   1.06       MgO   0.4   19.13   0.74   0.96       Al 2 O 3     0.79   1.58   0.15   0.13       SiO 2     73.94   79.08   74.7   72.1       K 2 O   0.06   0   0.75   0       CaO   22.69   0.25   22.3   24.5       TiO 2     0   0.06   0   0       Fe 2 O 3     0.16   0.38   0.04   0       La 2 O 3     2.07   NA   1.36   1.26                    Temperature ° C.   Thermal Conductivity (w · m −1  · K −1 )                     200           0.06   0.06        400           0.12   0.11        600   0.35   0.35   0.21   0.2        800   0.59   0.57   0.33   0.34       1000   0.9   0.85   0.49   0.52       1200   1.3   1.2   0.67   0.75                  
 
         [0114]     The applicants have therefore identified further advantages of the use of alkali metal oxides as additives to alkaline earth silicate blanket materials, and particular advantage to the use of potassium. In particular, to avoid promotion of crystallisation by sodium, preferably at least 75 mol % of the alkali metal is potassium. More preferably at least 90%, still more preferably at least 95% and yet still more preferably at least 99% of the alkali metal is potassium.  
         [0115]     To test the mutual interaction of La 2 O 3  and K 2 O on the fibre properties a range of fibres were made into blankets and tested for shrinkage at various temperatures [24 hours at temperature].  
         [0116]     It was found that La 2 O 3  could be reduced and replaced by K 2 O without significant harm to the shrinkage properties of the materials, but this led to onset of crystallisation at lower temperatures than for the La 2 O 3  containing materials. However, replacement of La 2 O 3  in part by alumina cured this problem. Table 7 indicates a range of materials tested, the temperature at which crystallisation commenced, and temperature at which the crystals reached ˜1 μm in size. The materials all had a base composition of approximately 73.1-74.4 wt % SiO 2  and 24.6-25.3 wt % CaO with all other ingredients amounting to less than 3% in total.  
                                           Crystallisa-               tion   Crystals           Starts @   Coarsen ˜1       Composition   ° C.   mm @ ° C.                   CaO—SiO2—La2O3 (1.3%)   1100   1200       CaO—SiO2—K 2 O (0.75%)   1000   1100       CaO—SiO2—K 2 O (0.75%)—La 2 O 3  (1.3%)   1050   1150       CaO—SiO2—K 2 O (0.75%)—La 2 O 3  (1.3%)   1050   1150       CaO—SiO2—K 2 O (0.8%)—La 2 O 3  (0.4%)   1050   1200       CaO—SiO2—K 2 O (0.6%)—La 2 O 3     1100   1200       (0.15%)—Al 2 O 3  (0.94%)                  
 
         [0117]     Accordingly, a preferred range of compositions comprises:— 
         [0118]     72%&lt;SiO 2 &lt;79%  
         [0119]     MgO&lt;10%  
         [0120]     13.8%&lt;CaO&lt;27.8%  
         [0121]     Al 2 O 3 &lt;2%  
         [0122]     ZrO 2 &lt;3%  
         [0123]     B 2 O 3 &lt;5%  
         [0124]     P 2 O 5 &lt;5%  
         [0125]     95%&lt;SiO 2 +CaO+MgO+Al 2 O 3 +ZrO 2 +B 2 O 3 +P 2 O 5    
         [0126]     M 2 O&gt;0.2% and &lt;1.5%  
         [0000]     in which M is alkali metal of which at least 90 mol % is potassium.  
         [0127]     More preferably SiO 2  plus CaO&gt;95%, and usefully a preferred range of compositions comprises:— 
         [0128]     72%&lt;SiO 2 &lt;75%  
         [0129]     MgO&lt;2.5%  
         [0130]     24%&lt;CaO&lt;26%  
         [0131]     0.5%&lt;Al 2 O 3 &lt;1.5%  
         [0132]     ZrO 2 &lt;1%  
         [0133]     B 2 O 3 &lt;1%  
         [0134]     P 2 O 5 &lt;1%  
         [0135]     M 2 O&gt;0.2% and &lt;1.5%  
         [0000]     in which M is alkali metal of which at least 90 mol % is potassium.  
         [0136]     A particularly preferred range is  
         [0137]     SiO 2  74±2%  
         [0138]     MgO&lt;1%  
         [0139]     CaO 25±2%  
         [0140]     K 2 O 1±0.5%  
         [0141]     Al 2 O 3 &lt;1.5%  
         [0142]     98%&lt;SiO 2 +CaO+MgO+Al 2 O 3 +K 2 O  
         [0143]     And these preferred ranges may comprise additionally R 2 O 3 &lt;0.5 wt % where R is selected from the group Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, Y or mixtures thereof.  
         [0144]     During further trials a second range of fibres was found that gave good results. These fibres had the composition:— 
         [0145]     SiO 2 =67.8-70%  
         [0146]     CaO=27.2-29%  
         [0147]     MgO=1-1.8%  
         [0148]     Al 2 O 3 =&lt;0.25%  
         [0149]     La 2 O 3 =0.81-1.08%  
         [0150]     K 2 O=0.47-0.63%  
         [0151]     These fibres had a high strength (80-105 kPa for a blanket of thickness ˜25 mm and density ˜128 kg·m 3 ) and and low shot content (˜41% total shot).  
         [0152]     The fibres may also be used in other applications where alkaline earth silicate fibres are currently employed (e.g. as constituents of friction materials).