Man-made vitreous fibres

Abstract: Man-made vitreous fibres have a solubility at pH 4.5 of at least 20nm per day and a melt viscosity of 10-70 poise at 1,400°C. A composition which will provide these properties is selected for their manufacture and contains at least 105 by weight Al 2 O 3 . Novel fibres contain at least 6% by weight Na 2 O + K 2 O.

Description: This invention relates to man-made vitreous fibres (MMVF) which are durable in use but which can be shown to be biologically advantageous. MMV fibres are made from vitreous melt, such as of rock, slag, glass or other mineral melts. The melt is formed by melting in a furnace a mineral composition having the desired analysis. This composition is generally formed by blending rocks or minerals to give the desired analysis. The mineral composition often has an analysis, as oxides, which includes at least 32% SiO 2 , below 30% Al 2 O 3 and at least 10% CaO. The elemental analyses in the specification are by weight and calculated as oxides. The iron oxide may be a mixture of FeO and Fe 2 O 3 but is quoted herein as FeO. Efficient and cost-effective formation of the melt in the furnace and of the fibres from the melt requires that the composition should have a suitable liquidus temperature and should have a suitable viscosity during the fibre-forming process. These requirements impose constraints on the selection of the composition that is to be melted. Although there is no scientific evidence establishing that there is a health risk associated with the manufacture and use of MMV fibres, commercial interests have led manufacturers to provide MMV fibres that retain the required physical properties of MMV fibres (e.g., durability at elevated temperatures and under humid conditions) but which can also be alleged to be of improved biological safety. This allegation of improved safety is usually made on the basis of an in vitro test that examines the dissolution rate or degradability of the fibres in a liquid which is intended to simulate lung liquid, such as Gamble's solution with pH 7.4 to 7.8. A consequence of enhanced dissolution rate at pH 7.5 is that the fibres will normally have reduced resistance to humidity. Numerous patent applications have been published describing fibres that give enhanced dissolution rate in such an in vitro test, such as WO87/05007, WO89/12032, EP 412878, EP459897, WO92/09536, WO93/22251 and WO94/14717. A characteristic of many of these patent applications, and of fibres which are alleged to have enhanced dissolution rate in such in vitro tests, is that the fibre should have reduced aluminium content. For instance it is stated in W087/05007 that the Al 2 O 3 amount must be below 10%. The aluminium content of rock wool and slag wool is generally in the range 5 to 15% (measured as Al 2 O 3 by weight) and many of these allegedly biologically suitable fibres have an aluminium content of below 4%, and often below 2%. It is known to include phosphorous in these low-Al 2 O 3 compositions in order to increase the dissolution rate in this pH 7.5 dissolution rate test. A problem with many of these low-Al 2 O 3 fibres (in addition to uncertainty as to whether they do have enhanced biological suitability) is that the melt properties are not entirely satisfactory for manufacture in conventional or easily adapted melting and fibre-forming apparatus. For instance the melt viscosity at convenient fibre-forming temperatures may be rather low. Another problem is that a high dissolution rate at pH 7.5 may tend to result in reduced durability under humid conditions which may be experienced after installation. Fibres which are described as being susceptible to decomposition in a physiological medium and which have an analysis containing 7.5% to 15% Al 2 O 3 are described in FR-A-2,662,687. The examples show the greatest susceptibility to decomposition for the fibres having analyses containing 7.5 and 7.6% Al 2 O 3 . In addition to the in vitro tests, in vivo research testing has been conducted. For instance Oberdörster in VDI Berichte 853, 1991, pages 17 to 37 showed that two basic mechanisms are involved in the clearance of fibres from the lungs, namely dissolution in the near-neutral lung fluid and dissolution in the acidic environment (maintained at pH 4.5 to 5) created around fibres surrounded by macrophages in the lung. It is believed macrophages promote removal of the fibres from the lung by promoting local dissolution of the surrounded fibre area leading to weakening and breaking of the fibres so as to reduce the average fibre length, thereby enabling macrophages to engulf and transport the shorter fibres out of the lung. This mechanism is illustrated in the article by Morimoto et al in Occup. Environ. Med 1994, 51, 62-67 and especially Figures 3 and 7 and articles by Luoto et al in Environmental Research 66 (1994) 198-207 and StaubReinhaltung der Luft 52 (1992) 419-423. Traditional glass fibres and many of the MMV fibres claimed to have increased solubility in lung fluid (at pH 7.5) have a worse solubility at pH 4.5 than at pH 7.5 and so presumably attack by macrophages would not contribute significantly to the shortening and final removal of the fibres from the lung. Existing MMV fibres formed from rock, slag and other relatively high alkaline earth mixtures can have a higher dissolution rate at pH 4.5 than pH 7.5 but tend to have a low melt viscosity. Existing fibres which are promoted as being biologically acceptable do not have a satisfactory combination of dissolution rate at pH 4.5 with melt properties. Fibres which are currently alleged to be preferred on the basis of in vitro tests tend to have low melt viscosity when they have the required low aluminium content. The low melt viscosity inevitably reduces production efficiency compared to normal production. It would be desirable to provide MMV fibres which can be shown to be biodegradable in the lung, have melt properties which allow for normal, high, production efficiency and which can be made from inexpensive raw materials. Preferably they have good weathering resistance when exposed to ambient humid conditions in use. In the invention we use a particular melt composition to make fibres which can be shown to be biodegradable in the lung. They can have a dissolution rate measured at pH 4 to 5 of at least 20nm per day and are formed of a composition having a melt viscosity at 1,400°C of 10-70 poise. For instance the dissolution rate at pH 4.5 can be at least 30 or even at least 50 nm per day or more. In particular, in the invention we use a melt composition to make man-made vitreous fibres which are shown to be biodegradable in the lung, wherein the fibres have an analysis, measured as weight of oxides, which includes SiO 2 32 to below 45% Al 2 O 3 above 16 to 28% CaO 10 to 30% MgO 2 to 20% FeO 2 to 15% Na 2 O + K 2 O 0 to 12% TiO 2 0 to 4% Other Elements 0 to 8% and the composition has a melt viscosity at 1400°C of 10 to 70 poise. It is surprisingly possible, in accordance with the invention, to use fibres which can be shown to have a good dissolution rate at pH 4.5 thereby facilitating clearance from the lungs by macrophages (thus promoting genuine biodegradability), even though the fibres can have low or moderate dissolution rate at pH 7.5. This allows maintenance of good stability under humid conditions (without loss of biodegradability). The fibres can have reasonably conventional melt characteristics such as liquidus temperature, crystallisation rate and melt viscosity. The fibres can be formed using inexpensive raw materials. Another advantage of the fibres is that when they are exposed to humidity and condensed water, the resultant solution that is formed containing dissolution products has increased pH but the fibres may have reduced solubility at increased pH and so they may dissolve less and have increased durability. Accordingly the invention includes the use of the defined composition to make fibres which are exposed to humidity or condensed water during use of the fibres. The selected fibres which are shown to be biodegradable may be used in any of the forms conventional for conventional MMV fibres. Thus they may be used in the form of a product consisting of loose, unbonded fibres, for instance as free fibres for reinforcement of cement, plastics or other products or as a filler. More usually the invention includes the use of the melt composition to make fibres which are shown to be biodegradable in the lung and which are the fibres of a bonded MMVF product, for instance as a result of forming the fibres and bonding them in conventional manner. Generally the product is consolidated as a slab, sheet, tube or other shaped article. The products formed of the fibres may be used wherever it is desirable to be able to show biodegradability in the lung, for instance as thermal insulation, fire insulation and protection, or noise reduction and regulation, or as horticultural growing media. The manufacture, promotion or sale or use of the fibres used in the invention generally involves showing the biodegradability of the fibres in the lung and this generally involves measurement of, or reference to measurement of, solubility at about pH 4.5 (e.g., 4 to 5) and/or in the environment of macrophages in the lung irrespective of whether the dissolution rate is measured during the actual production of such products. The MMV fibres are generally made from a composition which has been selected in order to provide the stated solubility. For instance the selection of the composition may include the measurement of pH 4-5 solubility and the melt viscosity of one or more compositions and selecting a composition partly or wholly on the basis of observing melt viscosity and solubility value at pH 4-5, and utilising compositions having the same or substantially the same analysis for making MMVF products. Any deviations in the analysis must be sufficiently small that they do not significantly alter the pH 4-5 solubility. When conducting the measurements to allow a selection of fibres to be made, the solubility can be determined at any pH (usually in the range 4-5) which correlates to the pH at 4.5. The melt viscosity can be determined either by deduction from data or by measurement and/or calculation, for any temperature (usually in the range 1,370-1,450°C) which gives a value which correlates with the value at 1,400°C. The selection of the composition does not have to be conducted at the same location or at about the same time as carrying out the commercial production using the selected composition. Thus a manufacturer may conduct tests, or sponsor others to conducts tests, to determine solubility and use the information from these tests as part of the basis for selecting the composition which is used for making the fibres commercially. It is possible to select elemental analyses within the general ranges given above so as to obtain the defined combination of melt viscosity and dissolution rate at pH 4.5. Also, it is easily possible to select the composition such that the composition and the fibres comply with other desirable properties, such as liquidus temperature and sintering temperature. For instance, if it is found that the viscosity at 1400°C of any particular melt is too high, it may be possible to reduce it by reducing the total amount of SiO 2 + Al 2 O 3. Similarly, if the melt viscosity is too low, it may be possible to increase it by raising the total amount of SiO 3 + Al 2 O 3 generally within the range of 55 to 75%, often 60 to 75%, or by increasing the amount of alkali oxide. Similarly, it may be possible to decrease the viscosity by increasing the total amount of alkaline earth metal oxide components and FeO. If the rate of dissolution at pH 4.5 is too low it may be possible to increase it by decreasing the amount of SiO 2 , but it may then be necessary to increase the amount of Al 2 O 3 (and or to add a component such as P 2 O 5 ) in order to maintain melt properties. The amount of SiO 2 is normally at least 32%, often at least 34% and preferably at least 35%. It is often 38-42%. The amount of Al 2 O 3 is above 16 or 17%, especially at least 18%, but preferably at least 20% and often at least 24%. It is preferably below 26%. Amounts of 20-23% are often preferred. The combined amount of SiO 2 + Al 2 O 3 is normally 55 to 70%, usually at least 56% and preferably at least 57%. In preferred products it is often above 60%, most preferably at least 61 or 62%. It is normally below 68% and preferably below 65%. The amount of CaO is normally at least 14% and preferably at least 18%. It is normally below 28% and preferably below 25%. Amounts of 14-20% are often preferred. The amount of MgO is normally at least 5%, preferably at least 6% and most preferably at least 8%. It is normally below 15%, preferably below 11%. The amount of FeO is normally at least 3% and preferably at least 5%. It is normally below 12%, preferably below 10% and most preferably below 8%. Amounts of 5-7% are often preferred. Preferably CaO + MgO + FeO is 25 to 40%. The composition often includes TiO 2 in an amount of up to 3% or 4%, usually up to 2%. The amount of TiO 2 is usually at least 0.2%, often at least 0.5 or 1%. A variety of other elements can be present in the composition in any amount that does not detract from the desired properties Examples of other elements that can be included are P 2 O 5 , B 2 O 3 , BaO, ZrO 2 , MnO, ZnO and V 2 O 5 . It is often desirable to include P 2 O 5 and/or B 2 O 3 for instance to adjust melt properties or to adjust solubility. The total amount of P 2 O 5 and B 2 O 3 is generally not more than 10%. The amount of P 2 O 5 is usually more than the amount of B 2 O 3 and is usually at least 1% or 2%. Often B 2 O 3 is absent. Preferably there is 1 to 8%, usually 1 to 5%, P 2 O 5 and 0 to 5% B 2 O 3 (often 1 to 4% B 2 O 3 ). The total amount of these various other elements is below 8%. Each of the other elements which is present is normally present in an amount of not more than 2%, except thaT P 2 O 5 and/or B 2 O 3 may be present in larger amounts as mentioned above. The melt can have normal crystallisation characteristics, but when it is desired to minimise crystallisation this can be achieved by including magnesium in a rather low amount for instance 2 to 6% MgO. When it is desired to provide fibres having improved fire resistance, it is generally desired to increase the amount of FeO, which preferably is then at least 6%, for instance up to 8% or higher, for instance 10%, and the MgO should then be at least 8%. The analysis of the composition is preferably such that the fibres have a dissolution rate at pH 4.5 of at least 25, and preferably at least 40, nm per day. It is desirable for the dissolution rate to be as high as possible (consistent with retention of adequate humidity and heat resistance properties) but it is generally unnecessary for it to be above 150 or 100nm per day and it is usually below 80nm per day. Although a high dissolution rate at pH 7.5 has been proposed as a desirable property (as an indication of alleged biodegradability), in fact it is often an undesirable property since it is an indication of poor weathering resistance when exposed to humidity. Dissolution in the lungs at pH 7.5 is not exclusively necessary for the fibres to be biodegradable. Preferably the fibres have a dissolution rate in Gambles solution at pH 7.5 of below 25, and most preferably below 15, nm per day. The viscosity of the composition at 1400°C is usually at least 12 or 15 poise and is preferably at least 18 poise. Although it can be as high as, for instance, 60 poise it is generally below 40 poise and preferably it is not more than 30 poise. When it is desired that the fibres should have good fire resistance, the analysis is preferably such that the sintering temperature is at least 800°C and preferably at least 1,000°C. The liquidus temperature is usually at least 1200°C but often at least 1240°C. It can be as high as, for instance, 1400°C but preferably it is not more than 1340°C. An advantage of the use of the moderate aluminium melts defined for use in the invention is that it permits the inclusion in the composition of readily available materials having a moderate aluminium content such as rock, sand and waste. This therefore minimises the need to use expensive, high alumina materials such as bauxite or kaolin, and minimises at the same time the need to use expensive very low alumina materials such as silica sand or olivine sand, iron ore, etc. These more expensive materials may however be useful if desired. Typical, readily available, medium alumina materials that may be used as part or all of the composition include anorthosite and phonolite and gabbros. The composition is typically formed by blending appropriate amounts of naturally occurring rock and sand materials such as anorthosite, gabbros, limestone, dolomite, diabase, apatite, boron-containing materials, and waste materials such as mineral wool waste, alumina silicates, slag, in particular high alumina (20-30%) slags such as ladle slag, foundry sand, filter dust, fly ash, bottom ash and high alumina waste from the production of refractory materials. The composition can be converted to a melt in conventional manner, for instance in a gas heated furnace or in an electric furnace or in a cupola furnace. An advantage of the invention is that the composition can easily have a reasonably low liquidus temperature (while maintaining adequate viscosity at 1400°C) and this minimises the amount of energy that is required for forming the melt. The melt can be converted to fibres in conventional manner, for instance by a spinning cup process or by a cascade rotor process, for instance as described in W092/06047. The fibres used in the invention can have any convenient fibre diameter and length. In this invention, dissolution rate is determined using the following test protocol. 300mg of fibres are placed in polyethylene bottles containing 500ml of a modified Gamble's solution (i.e., with complexing agents), adjusted to pH 7.5 or 4.5, respectively. Once a day the pH is checked and if necessary adjusted by means of HCl. The tests are carried out during a one week period. The bottles are kept in water bath at 37°C and shaken vigorously twice a day. Aliquots of the solution are taken out after one and four days and analysed for Si on a Perkin-Elmer Atomic Absorption Spectrophotometer. The modified Gamble's solution has the following composition: g/l MgCl 2 .6H 2 O 0.212 NaCl 7.120 CaCl 2 .2H 2 O 0.029 Na 2 SO 4 0.079 Na 2 HPO 4 0.148 NaHCO 3 1.950 (Na 2 -tartrate).2H 2 .O 0.180 (Na 3 -citrate).2H 2 O 0.152 90% lactic acid 0.156 Glycine 0.118 Na-pyruvate 0.172 Formalin 1 ml The fibre diameter distribution is determined for each sample by measuring the diameter of at least 200 individual fibres by means of the intercept method and a scanning electron microscope or optical microscope (1000 x magnification). The readings are used for calculating the specific surface of the fibre samples, taking into account the density of the fibres. Based on the dissolution of SiO 2 (network dissolution), the specific thickness dissolved was calculated and the rate of dissolution established (nm/day). The calculations are based on the SiO 2 content in the fibres, the specific surface and the dissolved amount of Si. In this specification, the sintering temperature is determined by the following test protocol. A sample (5 x 5 x 7.5 cm) of mineral wool made of the fibre composition to be tested is placed in a furnace preheated to 700°C. After 1.5 hours exposure the shrinkage and the sintering of the sample were evaluated. The method is repeated each time with a fresh sample and a furnace temperature 50°C above the previous furnace temperature until the maximum furnace temperature, at which no sintering or no excessive shrinkage of the sample is observed, was determined. In this specification, the viscosity in poise at 1400°C is calculated according to Bottinga and Weill, American Journal of Science Volume 272, May 1972, page 455-475. The following are examples of the invention. Compositions were formed by blending appropriate proportions of raw materials as shown in the table and each was melted in a crucible furnace and was fiberised by the cascade spinner technique. The melt viscosity and solubility of each was determined. The analyses of the compositions and their properties are quoted in the following tables. In the invention, any of compositions A to X are judged to be suitable and are selected for the subsequent manufacture of MMVF products which are labelled as having good biological solubility. Those having viscosity above 20 and pH 4.5 solubility above 30 are preferred. Product 1 is similar to, commercial slag wool and gives a poor (low) viscosity. Product 2 is a high aluminium product but the proportions of all the components are such that the melt viscosity is too high for convenient spinning. Product 3 is similar to a conventional rock wool product with normal good product properties but has a very low dissolution rate at pH 4.5. Accordingly the use of the melts for products 1, 2 and 3 are not within the invention. The use of the melts for products G, I, J, R, S and T is also not within the invention. The selected fibres may be provided in any of the forms conventional for MMV fibres. Thus they may be provided as a product consisting of loose, unbonded fibres. More usually they are provided with a bonding agent, for instance as a result of forming the fibres and connecting them in conventional manner. Generally the product is consolidated as a slab, sheet or other shaped article. Products made according to the invention may be formulated for any of the conventional purposes of MMV fibres, for instance as slabs, sheets, tubes or other shaped products that are to serve as thermal insulation, fire insulation and protection or noise reduction and regulation, or in appropriate shapes for use as horticultural growing media, or as free fibres for reinforcement of cement, plastics or other products or as a filler.

Claims: Use of a melt composition to make man-made vitreous fibres which are shown to be biodegradable in the lung wherein the composition has an analysis, measured as weight of oxides, which includes SiO 2 32 to below 45% Al 2 O 3 above 16 to 28% CaO 10 to 30% MgO 2 to 20% FeO 2 to 15% Na 2 O + K 2 O 0 to 12% TiO 2 0 to 4% Other Elements 0 to 8% and the composition has a melt viscosity at 1400°C of 10 to 70 poise. Use according to claim 1 in which the fibres are the fibres of a bonded product. Use according to claim 1 or claim 2 in which the fibres are the fibres of a bonded product which is used as thermal insulation, fire insulation or protection, or noise regulation or protection. Use according to claim 1 or claim 2 in which the fibres are the fibres of a bonded product which is used as horticultural growth medium. Use according to claim 1 in which the fibres are used in free form as reinforcement or as a filler. Use according to any preceding claim in which the fibres are shown to be biodegradable in the lung by determination of solubility in the environment of macrophages in the lung. Use according to any preceding claim in which the fibres have a dissolution rate as defined herein of at least 20nm per day calculated from the amount of Si in solution in modified Gambles solution at pH 4.5 after one day's immersion and after four day's immersion. Use according to any preceding claim wherein the sintering temperature of the fibres in mineral wool formed from the fibres is at least 800°C. Use according to any preceding claim in which the amount of SiO 2 + Al 2 O 3 is below 68%. Use according to any preceding claim in which the amount of magnesium expressed as MgO is at least 5% up to 20%. Use according to any preceding claim in which the amount of iron, expressed as FeO, is below 10%. Use according to any preceding claim in which the amount of iron, expressed as FeO, is above 5%. Use according to any preceding claim in which the amount of aluminium, expressed as Al 2 O 3 , is at least 18%. A use according to claim 13 in which the amount of Al 2 O 3 is at least 20%. A use according to claim 13 in which the amount of Al 2 O 3 is 20 to 26%. A use according to any preceding claim in which the amount of SiO 2 is below 42%. A use according to any preceding claim in which the amount of titanium, expressed as TiO 2 , is at least 0.5%. A use according to any preceding claim in which the fibres have a dissolution rate at pH 7.5 below 15nm per day. A use according to any preceding claim in which the composition provides a liquidus temperature of 1240 to 1340°C.

Inventors: CHRISTENSEN VERMUND RUST, GULDBERG MARIENNE, JENSEN SOREN LUND, CHRISTENSEN, VERMUND RUST, GULDBERG, MARIENNE, JENSEN, SOREN LUND
Applicants: ROCKWOOL INT, ROCKWOOL INTERNATIONAL A/S
Application year: 1995
Main classification: C03C13
IPC codes: C03C13