Abstract:
Based on the discovery that Al 2  O 3  strongly retards the dissolving rate of insulating glass fibers in a synthetic lung solution, glass fibers having low alumina contents have been identified that dissolve significantly faster in lung fluid. Moreover, these glass fibers still have adequate durability for insulation pruposes and have physical properties which allow current processes to fabricate them into insulation.

Description:
TECHNICAL FIELD 
     The present invention relates to glass compositions and more specifically relates to soda lime aluminoborosilicate glass compositions. Even more specifically the present invention relates to glass fibers formed from these glass compositions and yet more specifically the present invention relates to glass fiber insulation made from the compositions. 
     BACKGROUND ART 
     Glass fiber insulation is well known and has been a commercial product for a long period of time. The insulation is made from intertwined soda lime aluminoborosilicate glass fiber which a binder holds together. The binder may be any suitable material but quite commonly is a phenol-formaldehyde resin or a ureaformaldehyde resin. These binders are well known and a spray nozzle generally applies them to the glass fibers as hot gases attenuate the fibers from a rotating device, commonly called a spinner. A conveyo collects the binder-coated fibers in the form of a blanket, and heat cures the blanket to produce the final insulation. The process produces various densities by varying the conveyor speed and the thickness of the cured insulation. This technology is well known. 
     It is well established that asbestos fibers when inhaled can cause significant disease in man. Though the exact mechanisms responsible for the biological activity of inhaled asbestos fibers is unknown, it is widely believed that their ability to remain in the lung for extended periods of time is an important factor. Glass fibers have not been linked to disease in man. Additionally, their durability or residence time in the lung appears much less than asbestos fibers. Further increasing the solubility of glass fibers would be expected to decrease the time glass fibers will remain in the lung if they are inhaled. 
     It been known for some time that the dissolution rate of glass fiber in saline solutions similar to those existing in the lung can be significantly increased by altering the chemical composition of the fiber, but it was not obvious that this could be done in such a way that the durability of the fiber insulation for commercial purpose was unimpaired. The present invention describes glass fiber compositions which have increased dissolution rate in physiological saline solutions thought to be similar to those found in the lung but which have adequate durability for commercial insulation applications. Moreover these compositions have physical properties which allow them to be fabricated into glass fiber insulation by the same processes currently used. 
     DISCLOSURE OF THE INVENTION 
     Conventional glass compositions, which are suited for insulations, are the soda (or alkaline metal oxide) calcia (or alkaline earth metal oxide) aluminoborosilicate compositions. The constituents in the glass which are of main interest are: SiO 2 , which is typically present in an amount of 54 to 68 percent by weight; Al 2  O 3 , typically present in amounts up to 8 percent by weight; B 2  O 3 , typically present in amounts up amounts of about 5 to about 18 percent by weight; CaO, typically present in amounts of about 3 to 14 percent by weight; MgO, typically present in amounts up to 10 percent by weight; and K 2  O, typically present in amounts up to 5 percent by weight. Other materials may also be present in the glass, either as impurities or by choice. Such components may include any one or more of: iron, in amounts up to 2 percent by weight as Fe 2  O; TiO 2  in amounts up to 4 percent by weight; F 2  in amounts up to 2 percent by weight; BaO in amounts up to 4 percent by weight; ZnO in amounts up to 4 percent by weight. Table 1 contains three typical examples of such glass compositions together with their relevant physical properties. 
     Although the exact physical/chemical conditions that a particle encounters in the lung are not known, a number of reasonable assumptions can be made, and these have been used to devise solutions thought to approximate closely those existing in the lung. These solutions have been used by a number of workers to study the dissolution rates of various materials in the lung. Correlation has been found between such laboratory studies and in vivo dissolution, and there is general agreement that such laboratory measurements have predictive power for the in vivo situation. Such a technique has been applied to a number of glass compositions spanning the compositional range typical of glass fiber insulation. Such glass compositions generally have dissolution rates in the range of 50-300 ng/cm 2  -hr. 
     The various asbestos minerals have dissolution rates less than 0.1 ng/cm 2  /hr. Existing glass fibers have solubilities many times greater than this. The glasses described in this patent are significantly more soluble in physiological salt solutions than existing glasses, while maintaining useful commercial properties. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     
                       TABLE 1______________________________________Chemical Compositions and Predicted Properties ofThree Typical Glasses for Glass Fiber Insulation.sup.1       #1a      #2b      #3a______________________________________SiO.sub.2     63.7       64.9     61.1Al.sub.2 O.sub.3         3.5        2.2      4.2CaO           7.9        8.1      9.0MgO           3.7        2.7      2.2Na.sub.2 O    15.8       15.0     14.7K.sub.2 O     0.7        0.4      0.8B.sub.2 O.sub.3         4.6        6.4      7.8Fe.sub.2 O.sub.3         0.1        0.1      0.1TiO.sub.2     0.1        0.1      0.1log3 viscosity         1074       1065     1042temperature (°C.)liquidus temperature         948        928      947(°C.)durability (%).sup.2         3/9        3/0      3.0dissolution rate.sup.3         83         211      129______________________________________ .sup.1 Property prediction is based on computer fitting of measured data on many glasses of related composition. .sup.2 % fiber weight loss after 1 week in distilled water at 96° C. .sup.3 Rate of weight loss/surface area (ng/cm.sup.2 -hr) in model physiological saline solution at 37° C. with a flow rate of 0.2 ml/min. 
    
     It is not obvious that glass fiber dissolution rate can be reduced while maintaining other necessary glass properties. In order to allow fiberization by currently used methods, glass liquidus and viscosity must both be maintained within a fairly narrow range. The glass fiber must be sufficiently durable to maintain its physical integrity over many years in the location in which it is installed. Liquidus and viscosity provide some constraint on how the glass composition can be altered, but durability provides the greatest constraint. The glass must be fairly inert to moisture attack at the installation site, but it must dissolve quickly in the lung. Since both these situations involve water attack on the glass under near neutral acid-base conditions, it is a surprising discovery that this requirement can be met by glass compositional modifications. 
     The invention consists of modifying typical glass fiber insulation compositions by a reduction of Al 2  O 3 , coupled with an increase in B 2  O 3 . Generally, but not always, Na 2  O is decreased also. This has the effect of increasing the glass dissolution rate in physiological saline solution without degrading the glass durability. Other major oxides such as SiO 2  , CaO, MgO, and K 2  O are adjusted to maintain viscosity, liquidus, and other glass properties. Trace constituents in the glass have no significant effect. The degree to which the glass dissolution rate can be increased without degrading or changing other glass properties depends on the starting glass composition. 
     INDUSTRIAL APPLICABILITY 
     Table 2 contains three examples in which this process has been applied to the three compositions in Table 1. In each case, the dissolution rate was increased to the maximum extent possible without changing other glass physical properties. Dissolution rates could be increased to over 600 ng/cm 2  -hr, a factor of 3-8 times that of the original composition in Table 1. Greater increases would be possible if some change in other physical properties were allowed. The amount of increase also depends on the original glass composition 
     Measurement of the dissolution rates of over 50 different glass fibers compositional modifications have defined the compositional region of glasses which are useful for production of glass fiber insulation and which have dissolution rate constants significantly increased over that of the typical glasses used in glass fiber insulation. This range is as follows for the major components: 
     
         ______________________________________SiO.sub.2      61.0 to 69.0Al.sub.2 O.sub.3          0.0 to  1.9CaO + MgO      7.9 to 16.0Na.sub.2 O     9.0 to 17.0B.sub.2 O.sub.3          8.7 to 15.0______________________________________ 
    
     Such glasses may typically contain up to 3% K 2  O, up to 3% BaO, and less than 1% of any or all of Fe 2  O 3 , TiO 2 , SrO or SO 2 . 
     A preferred range for the major compounds is as follows: 
     
         ______________________________________SiO.sub.2      64.0 to 68.0Al.sub.2 O.sub.3          0.6 to 1.5CaO + MgO       7.9 to 10.0Na.sub.2 O     11.0 to 16.0B.sub.2 O.sub.3           8.7 to 12.0______________________________________ 
    
     These glasses also typically contain up to 3% K 2  O, up to 3% BaO and less than 1% of any or all of Fe 2  O 3 , TiO 2 , SrO or SO 2 . 
     
                       TABLE 2______________________________________Chemical Composition and Predicted.sup.1Properties of Three Typical HighDissolution Rate Glasses with Forming andDurabilities Similar to those of Table 1.       1a       2a       3a______________________________________SiO.sub.2     66.4       66.7     64.9Al.sub.2 O.sub.3         1.2        1.0      1.5CaO           4.8        5.5      8.6MgO           3.2        2.5      0.1Na.sub.2 O    12.9       13.9     12.5K.sub.2 O     0.2        0.2      0.3B.sub.2 O.sub.3         11.0       10.0     11.9Fe.sub.2 O.sub.3         0.1        0.1      0.1TiO.sub.2     0.1        0.1      0.1log3 Viscosity         1074       1065     1042Temperature (°C.)LiquidusTemperature (°C.)         948        928      947Durability.sup.2         3.7        3.9      3.0Dissolution rate.sup.3         681        624      717______________________________________ .sup.1 Property prediction is based on computer fitting of measured data on many glasses of related composition. .sup.2 % fiber weight loss after 1 week in distilled water at 96° C. .sup.3 Rate of weight loss/surface area (ng/cm.sup.2 -hr) in model physiological saline solution at 37° C. with a flow rate of 0.2 ml/min. 
    
     The dissolution rates of the glasses of Table 2 increase to over 600 mg/cm 2  -hr, a factor of 3-8 times that of the original composition in Table 1.