Abstract:
A glass fiber capable of withstanding temperatures in excess of 1900° F. is produced by treating a glass, preferably E-glass, fiber. The glass fiber is first leached with selected acids, and then the leached fiber is treated with organo-metallic materials of low viscosity, such as a dispersion of low molecular weight water-in-oil emulsion of dimethyl polysiloxane. The fiber is used in such applications as embedding it in a fire-resistant active coating material or embedding it into one surface of a polyolefin or composite plastic, such as a polypropylene sheet. The treated fiberglass can be used as a sole component or in concert with a fire-resistant or fire retardant material to further enhance its fire-resistant properties.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a glass form, preferably a fiber, which resists high temperatures, at least 1900° F. and higher, while retaining at least some of its tensile strength and other physical properties. 
     In numerous applications, fabrics are utilized in systems which resist high temperatures. An example of the use of such fabrics is in reinforced coating systems. In these systems, the fabric is embedded in a char-forming, fire-resistive coating such as those described in Deogon, U.S. Pat. No. 5,591,791. Briefly, such coatings include ablative coatings, which swell to less than twice their original thickness when exposed to fire or other thermal extremes, intumescent coatings such as those disclosed in Nielsen et al., U.S. Pat. No. 2,680,077, Kaplan, U.S. Pat. No. 3,284,216, or Ward et al., U.S. Pat. No. 4,529,467, which swell to produce a char more than five times the original thickness of the coating, and subliming char-forming coatings of the type disclosed in Feldman, U.S. Pat. No. 3,849,178, which undergo an endothermic phase change and expand two to five times their original thickness to form a continuous porosity matrix. The intumescent and subliming coatings are denoted &#34;active&#34; thermal protective coatings. 
     The time required for a given temperature rise across a predetermined thickness of the composition, under specified heat flux, environmental, and temperature conditions, is a measure of the composition&#39;s effectiveness in providing thermal protection to an underlying substrate. 
     Eventually, the char is consumed by physical erosion and by chemical processes, such as oxidation by oxygen in the air and by free radicals produced by the coating or otherwise in a fire environment, and protection is substantially reduced. Before the char is totally consumed, degradation of the char layer leaves it crumbled and without the necessary strength to sustain itself, causing it to fail by being blown off or simply falling off (spalling). 
     Some of these chars degrade rapidly during exposure to high temperature, high heat flux environments. In the case of coatings which swell when exposed to thermal extremes, the degradations are usually in the form of fissures which are formed in the char as a result of differential thermal stresses produced by the high thermal gradients with them, and differential thermal expansion between the virgin material and the char. 
     To increase the strength of char layers during exposure to thermal extremes, and to limit spalling and fissures, fabrics have long been incorporated in the coating materials. As set out in Feldman et al., U.S. Pat. No. 5,622,774, fiberglass fabric provides an inexpensive, easy to install, reinforcement in many high temperature applications. In certain applications, however, such as coatings which may be exposed to high velocity petroleum fires or to high-temperature, high heat flux fires which will raise the fabric to temperatures above the softening point of the glass (around 1600° F.), the fiberglass fabric has disintegrated. Other fabrics have therefore been required. Graphite cloth, as taught in the foregoing Feldman et al. U.S. Pat. No. 5,622,774 and in Kobayashi et al., U.S. Pat. No. 5,401,793, is very expensive. Refractory materials, such as quartz (Refrasil) fabric is also expensive. Metal mesh is inexpensive but it is heavy and difficult to install, particularly because it generally requires welding metal studs to the substrate to be protected. 
     Other examples of fabric-reinforced systems are laminates in which the fabric is embedded directly in a structural resin material itself, for example in the structure of a furnace or a rocket nozzle. Generally, these materials also produce a char when exposed to sufficiently high temperatures, although in many applications they are routinely exposed to high temperatures below their char-forming temperature for extended periods. In other applications they are exposed for short periods to temperature, heat flux, and environmental conditions which do not cause a char to form, but which are sufficiently high to cause serious loss of structural properties. Examples of these latter systems are automobile gasoline tanks and trunks, which can be made of plastic material if they can pass a test involving preventing structural failure (such as drop through or explosion in the case of a gasoline tank) when the tank or trunk is placed over a fire of a specified temperature and intensity for a predetermined period such as two minutes. In all of these conditions, a fabric which resists complete degradation under the foregoing conditions can provide sufficient structural integrity to impede failure of the system. 
     Attempts have been made for many years to produce a glass fiber which retains a substantial portion of its mechanical properties even when subjected to very high temperatures, greater than 1600° F. (871° C.), and preferably on the order of 1900° to 2000° F. (1038° to 1093° C.). Examples are Nordberg, U.S. Pat. No. 2,461,841, Parker et al., U.S. Pat. No. 2,491,761, and Leeg et al., U.S. Pat. No. 2,992,960. These patents all involve leaching of the glass fiber with mineral acid, followed by treatment with a sizing material. Heretofore, such attempts have failed to provide a reliable, reproducible, and efficient process of converting commercial grade fiberglass (such as Type E and Type F glass fibers) into a material capable of withstanding elevated temperatures and aerodynamic shear which may be coupled with elevated temperatures. 
     BRIEF SUMMARY OF THE INVENTION 
     One of the objects of the present invention is to provide a reliable, reproducible, and efficient method of converting commercial grade fiberglass into a material capable of withstanding elevated temperatures. 
     Another object is to provide such a method which produces a fiberglass material capable of withstanding aerodynamic shear which may be coupled with elevated temperatures. 
     Another object is to provide such a method which produces a fiberglass material which retains at least a substantial part of its &#34;hand&#34; after prolonged exposure to elevated temperatures. 
     Another object is to provide such a method which is adaptable to a wide variety of applications. 
     Another object is to provide such a method which is easy to apply and extremely effective. 
     Another object is to provide an improved fiberglass cloth material. 
     Another object is to provide improved structural elements incorporating the improved fiberglass cloth material. 
     Another object is to provide improved structural elements incorporating a structural resin, a fabric, and an active thermal protective material. 
     Other objects will become apparent to those skilled in the art in light of the following disclosure. 
     In accordance with one aspect of the present invention, generally stated, a method of making a heat-resistant fiberglass fabric is provided comprising soaking a fiberglass fabric in a carefully selected acid bath for a selected time at a selected temperature. We have found that soaking the fiberglass in a mixture of acids containing at least 15% sulfuric acid at room temperature for a period of more than twenty-four hours, preferably at least about forty-eight hours, produces a material which maintains a substantial part of its physical characteristics even when heated to a temperature of at least 1600° F. for a period of at least one hour. The molality of the mixed acids is below 8, preferably around 5. It is anticipated that shorter soak times may be achieved with higher temperatures. 
     We have also found that the acid-treated fiberglass can be given superior qualities, such as improved &#34;hand&#34; (texture) both before and during exposure to high temperatures, by soaking the fiberglass in a low viscosity organo-metallic material, such as a low molecular weight silicone to fill the pores of the glass. The silicone is preferably in the form of a water-in-oil emulsion of low molecular weight silicone, such as dimethyl polysiloxane. The use of a water-in-oil emulsion, rather than an oil-in-water emulsion, inhibits the formation of silicone micelles and enhances absorption of the silicone into the pores of the glass fibers. We have found that soaking a fiberglass fabric for a period of more than forty-eight hours, preferably at least seventy-two hours, at room temperature in a low molecular weight, low viscosity, silicone bath produces superior results. 
     Certain combinations of acid treatment and treatment with low molecular weight silicone have been found to produce materials which consistently will withstand being heated to a temperature of 1900° F. for at least one hour in a muffle furnace. 
     The improved fabric has numerous uses, as will be apparent to those skilled in the art. It provides a superior reinforcement in thermal protective coating systems of the types previously described. For example, it may be embedded in fire-protective coatings of subliming, intumescing and ablative types, and has been found to provide excellent results as a reinforcement in sprayed-on subliming coating systems. It may also be used in active or passive cast or molded self-supporting thermal protective systems such as the system described in Feldman, U.S. Pat. No. 4,493,945. 
     In accordance with another aspect of the invention, structural members formed of thermoplastic resins are strengthened and protected from fire for extended periods by incorporating in them a high-temperature fiberglass fabric made in accordance with the present invention. The fabric is preferably pressed into a resin sheet as the sheet is formed into a container or other functional entity. In accordance with yet another aspect of the invention, such structural members are protected with a fabric or mesh which is pretreated with an active (intumescing or subliming) thermal protective coating material and then embedded in the thermoplastic material. The fabric is preferably a high-temperature fiberglass made by the method of the invention. Preferably, the fabric comprises an open weave mesh having from 0.5 to 20 openings per square centimeter. The active fire retardant material can be impregnated into the fiberglass or surface coated. It can be completely cured in a rigid or elastic form; it can also be supplied in a precured condition or in a semi-cured condition to be cured during processing. The cells of the treated or impregnated fiberglass can be open or filled. For some materials such as polypropylene, an open cell is preferred as it enables the softened polypropylene to pass through the opening and form a mechanical lock. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To determine the effect of different acid compositions and exposure times at a constant temperature and acid molality, and to determine the effect of further treatment in a low molecular weight polysiloxane at a constant temperature for different time periods, a series of tests was made. The results of these tests are shown in Table 1. 
     
                                           TABLE 1__________________________________________________________________________    Acid   Bath Time Wt Time in  Wt ChangeSample    Mole Ratio       in Bath           Change                Silicone                    Wt Change In   FromNo. Cl NO3    SO4       Hr. %    Hr. From orig                         From acid                              muffle                                   orig.                                        Comments__________________________________________________________________________Blank                              -12.44%                                   -12.44%                                        shrunk, melted  A-1 1 1 1  3 -0.08%  0 -0.08% 0.00% -11.76% -11.83% fragile, crisp                                         A-2 1 1 1  3 -0.77%  6                                        5.82% 6.64% -16.89% -12.05%                                        slightly fragile  A-3 1 1 1  3 -0.34% 24 6.28% 6.63% -17.40% -12.22% slightly fragile                                         A-4 1 1 1  3 0.00% 72 7.68%                                        7.68% -17.38% -11.04%                                        slightly fragile  A-5 1 1 1  8 -0.30%  0 -0.22% 0.08% -13.80% -13.98% less fragile                                         A-6 1 1 1  8 -0.69%  6                                        5.66% 6.39% -17.24% -12.55%                                        less fragile  A-7 1 1 1  8 -0.35% 24 7.51% 7.89% -18.56% -12.44% less fragile                                         A-8 1 1 1  8 -0.60% 72                                        7.01% 7.64% -17.37% -11.58%                                        very nice  A-9 1 1 1 24 -2.92%  0 -2.93% -0.10% -17.81% -20.21% less fragile                                         A-10 1 1 1 24 -3.88%  6                                        2.84% 6.83% -21.10% -18.86%                                        less fragile  A-11 1 1 1 24 -1.92% 24 6.92% 8.97% -20.33% -14.81% less fragile                                         A-12 1 1 1 24 -2.12% 72                                        0.18% 2.30% -20.07% -19.92%                                        nice  A-13 1 1 1 48 -6.59%  0 -6.15% 0.04% -23.14% -27.86% very nice - fuzzy                                         A-14 1 1 1 48 -7.04%  6                                        3.91% 11.22% -26.89% -24.03%                                        soft - fuzzy  A-15 1 1 1 48 -6.38% 24 3.98% 10.62% -26.33% -23.39% very nice - fuzzy                                         A-16 1 1 1 48 -4.54% 72                                        -3.38% 1.00% -26.16% -28.66%                                        very nice - fuzzy  B-1 2 3 1  3 -0.25%  0 -0.27% -0.02% -10.58% -10.82% fragile  B-2 2 3 1  3 -0.39%  6 6.86% 7.28% -14.91% -9.07% fragile  B-3 2 3 1  3 0.02% 24 6.01% 5.98% -18.29% -13.38% fragile  B-4 2 3 1  3 -0.32% 72 7.62% 7.97% -16.37% -10.00% less fragile                                         B-5 2 3 1  8 -0.85%  0                                        -0.79% 0.05% -13.56% -14.24%                                        fragile  B-6 2 3 1  8 -0.95%  6 4.26% 5.25% -17.62% -14.11% less fragile                                         B-7 2 3 1  8 -1.10% 24                                        5.56% 6.71% -15.79% -11.11%                                        nice  B-8 2 3 1  8 -0.85% 72 5.48% 6.38% -18.26% -13.78% slightly fragile                                         B-9 2 3 1 24 -5.47%  0                                        -5.27% -0.10% -16.44%                                        -20.85% less fragile                                         B-10 2 3 1 24 -3.24%  6                                        1.86% 5.16% -20.06% -18.56%                                        nice  B-11 2 3 1 24 -4.40% 24 4.70% 9.31% -20.89% -17.18% less fragile                                         B-12 2 3 1 24 -4.31% 72                                        0.79% 5.14% -20.59% -19.96%                                        nice  B-13 2 3 1 48 -10.86%  0 -9.58% 0.23% -19.86% -27.54% very nice - fuzzy  B-14 2 3 1 48 -10.29%  6 1.02% 11.42% -24.77% -24.00% nice - fuzzy                                         B-15 2 3 1 48 -9.63% 24                                        3.29% 13.24% -24.99% -22.52%                                        nice - fuzzy  B-16 2 3 1 48 -10.89% 72 -0.31% 10.54% -24.76% -24.99% nice - fuzzy                                         C-1 4 2 1  3 -0.09%  0                                        -0.10% -0.01% -13.10%                                        -13.19% fragile  C-2 4 2 1  3 0.00%  6 5.52% 5.52% -16.48% -11.87% slightly fragile                                         C-3 4 2 1  3 -0.15% 24                                        7.62% 7.78% -16.79% -10.45%                                        less fragile  C-4 4 2 1  3 -0.08% 72 7.30% 7.39% -18.17% -12.20% slightly fragile                                         C-5 4 2 1  8 -0.94%  0                                        -0.85% 0.07% -13.07% -13.81%                                        slightly fragile  C-6 4 2 1  8 -1.04%  6 4.57% 5.66% -18.14% -14.40% less fragile                                         C-7 4 2 1  8 -0.67% 24                                        4.30% 4.99% -18.83% -15.34%                                        slightly fragile  C-8 4 2 1  8 -0.53% 72 4.78% 5.34% -19.15% -15.28% slightly fragile                                         C-9 4 2 1 24 -4.83%  0                                        -4.63% -0.02% -16.68%                                        -20.54% less fragile                                         C-10 4 2 1 24 -5.38%  6                                        1.54% 7.00% -18.95% -17.70%                                        slightly fragile  C-11 4 2 1 24 -5.18% 24 3.98% 9.36% -19.70% -16.50% less fragile                                         C-12 4 2 1 24 -4.62% 72                                        1.52% 6.21% -19.55% -18.32%                                        less fragile  C-13 4 2 1 48 -11.42%  0 -10.10% 0.17% -18.76% -26.96% nice  C-14 4 2 1 48 -11.08%  6 1.59% 12.85% -23.59% -22.38% less fragile                                         C-15 4 2 1 48 -10.30% 24                                        4.90% 15.71% -25.06% -21.38%                                        less fragile  C-16 4 2 1 48 -10.51% 72 0.06% 10.57% -22.90% -22.85% less fragile                                         D-1 1 1 4  3 8.87%  0 1.78%                                        -7.24% -15.64% -14.14%                                        slightly fragile  D-2 1 1 4  3 1.56%  6 10.81% 9.08% -17.73% -8.84% slightly fragile                                         D-3 1 1 4  3 0.88% 24                                        12.22% 11.23% -16.50% -6.30%                                        fragile  D-4 1 1 4  3 1.29% 72 9.14% 7.73% -20.98% -13.76% fragile  D-5 1 1 4  8 1.32%  0 1.36% 0.02% -15.18% -14.03% nice  D-6 1 1 4  8 0.77%  6 9.52% 8.67% -21.82% -14.38% slightly fragile                                         D-7 1 1 4  8 0.27% 24 7.21%                                        6.93% -19.01% -13.17%                                        slightly fragile  D-8 1 1 4  8 0.58% 72 4.50% 3.90% -19.29% -15.66% less fragile  D-9 1 1 4 24 1.45%  0 1.71% 0.24% -18.33% -16.93% nice  D-10 1 1 4 24 0.63%  6 12.25% 11.54% -22.71% -13.25% nice fuzzy                                         D-11 1 1 4 24 2.10% 24                                        12.89% 10.52% -22.24%                                        -12.21% nice fuzzy  D-12 1 1 4 24 2.09% 72 0.08% -2.01% -21.21% -21.15% nice  D-13 1 1 4 48 4.73%  0 8.04% 2.93% -23.89% -17.77% nice  D-14 1 1 4 48 10.98%  6 18.84% 5.78% -24.70% -10.52% soft fuzzy                                         D-15 1 1 4 48 10.92% 24                                        22.15% 8.80% -24.48% -7.75%                                        nice fuzzy  D-16 1 1 4 48 11.01% 72 -1.13% -12.01% -23.98% -24.84% nice fuzzy                                         E-1 1 0 4  3 0.29%  0 0.31%                                        0.02% -11.51% -11.23%                                        slightly fragile  E-2 1 0 4  3 0.54%  6 10.84% 10.25% -18.64% -9.82% slightly fragile                                         E-3 1 0 4  3 0.46% 24                                        12.85% 12.33% -18.33% -7.84%                                        slightly fragile  E-4 1 0 4  3 0.56% 72 9.88% 9.27% -21.75% -14.02% less fragile  E-5 1 0 4  8 2.41%  0 2.55% 0.08% -17.69% -15.60% slightly fragile                                         E-6 1 0 4  8 2.64%  6                                        11.64% 8.70% -18.82% -9.37%                                        slightly fragile  E-7 1 0 4  8 3.02% 24 8.06% 4.80% -18.54% -11.97% less fragile  E-8 1 0 4  8 3.49% 72 3.58% -0.03% -18.44% -15.52% slightly fragile                                         E-9 1 0 4 24 7.70%  0 8.08%                                        -0.24% -22.04% -15.74% very                                        nice  E-10 1 0 4 24 5.88%  6 14.80% 8.05% -24.62% -13.47% soft fuzzy  E-11 1 0 4 24 7.78% 24 17.76% 9.78% -24.16% -10.69% very nice  E-12 1 0 4 24 7.04% 72 -0.18% -7.21% -21.80% -21.94% nice fuzzy                                         E-13 1 0 4 48 10.58%  0                                        11.47% -0.32% -22.83%                                        -13.98% soft fuzzy  E-14 1 0 4 48 8.31%  6 18.63% 8.78% -25.45% -11.56% very nice fuzzy                                         E-15 1 0 4 48 11.11% 24                                        15.43% 2.62% -25.56% -14.07%                                        nice fuzzy  E-16 1 0 4 48 3.15% 72 0.37% -2.79% -24.33% -24.05% nice fuzzy__________________________________________________________________________ Examined E13, A15, B15, C15, D15 &amp; E15 E13 &amp; E15 showed no visible differences under 30× microscope Weave threads of samples E13, E15 &amp; D15 looked swollen These samples also had a softer &#34;hand&#34; and drooped more. 
    
     In making these tests, the following procedure was followed. 
     An acid bath was prepared by weighing the relative quantities of acid, mixing the acid thoroughly, and adding sufficient water to obtain a 5.0 molal solution. The selection of the 5.0 concentration was arbitrary, but it has been found that molalities of 8 or above do not function as well. 
     Samples of the unraveled fiberglass material (Type E, J. P. Stevens Type 1353) were weighed and then totally immersed in the acid bath for the indicated number of hours. 
     At the completion of each individual immersion period, the fiberglass sample was removed from the acid bath, carefully washed with clean water, dried, and weighed. This weight was compared to the weight of the sample in its virgin state. 
     Some of the acid-treated fiberglass samples were then totally immersed in a bath of low molecular weight dimethyl polysiloxane in the form of a dispersion of water in the polysiloxane oil. The dispersion was formed from a concentrate of low molecular weight dimethyl polysiloxane in a water-in-oil emulsion, as sold by Blackhawk Specialty Products, Inc., Rock Island, Illinois, as its BSP-89W. The time spent in the silicone bath is recorded in Table 1 for each sample. 
     The sample was then placed in a muffle furnace which was maintained at a constant temperature of about 1600° F. This temperature was selected to represent a typical average temperature within a char layer which results from the exposure of a subliming or intumescing material which is applied to a steel substrate in thicknesses that are capable of meeting relevant ASTM E-119 type fires for one, two, or more hours in duration. 
     The sample was kept in this environment for a period of about sixty minutes, then removed from the muffle furnace and cooled. Upon completion of cooling, it was examined for embrittlement, &#34;hand&#34; retention, and weight loss. If the sample retained its &#34;hand&#34; (indicated by the word &#34;nice&#34; in the comments section of Table 1) then it was subjected to further tests. It will be seen from Table 1 that samples A-8, A-12 through A-16, B-7, B-10, B-11 through B-16, C-13, D-5, D-9 through D-16, and E-9 through E-16 met the foregoing criterion and were tested further. All of these samples were soaked in the room temperature acid for a period in excess of three hours. All but C-13 had been soaked in a mixture including in excess of 15% sulfuric acid. 
     Fresh samples taken from corresponding lots as the foregoing samples were further exposed to 1600° F. in the muffle furnace for a period of sixty minutes. After cooldown, they were again checked for &#34;hand retention,&#34; and results recorded. The samples which were embrittled were eliminated from further testing, leaving samples A-13 through A-16, D-13 through D-16, and E-13 through E-16 to be tested further. 
     Fresh samples taken from corresponding item lots as the foregoing samples were exposed to a muffle furnace fire at a temperature of about 1900° F. to about 2000° F. for a period of one hour. This temperature was selected because it represents a thermal environment most likely encountered within the char layer of a subliming or intumescing material which is exposed to a hydrocarbon fire environment as defined by Underwriters Laboratory procedure 1709 or a jet fire environment as defined by British Offshore Technology Report OTO 93 028 (Dec. 21, 1993). After cooldown, the samples were checked for &#34;hand&#34; retention, and results were recorded. The three item samples which were not embrittled were A-16, D-13, and E-16. All the most successful samples had been soaked at room temperature for over twenty-four hours in an acid solution containing in excess of 15% molal sulfuric acid, and two of the three samples had been soaked at room temperature for over twenty-four hours in the low-viscosity silicone emulsion. 
     Preliminary fire tests of the treated fiberglass fabric in a subliming fire-protective coating system indicate that the system provides superior results. 
     To further test the treated fabrics, and to demonstrate their usefulness in composite materials, the material was embedded in small (approximately 10 cm square) test polypropylene sheets as follows. In separate tests, two active fire protective materials were applied to the fabric: a subliming material (Thermal Science, Inc. THERMO-LAG 440-1) and a thin-film intumescent material (Thermal Science, Inc. THERMO-LAG 2000). In each case, the active thermal protective material was thinned in a solvent or low viscosity resin diluent. The treated fiberglass fabric was placed in a container of the thinned material and squeezed in a wringer to remove excess coating. This produces a coating of material on the fabric which covers the fibers of the fabric but does not close the individual cells in the fabric mesh. The coated fabric was heated to semicure the subliming material or to cure the intumescent material. The fiberglass remained somewhat elastic. 
     Polypropylene is a hard material with a surface that is difficult to adhere to. The test square was heated to 200° to 210° C. The female portion of a mold was heated to 60-70° C. The mold includes a ram with a platen which is heated to 60-70° C. The thermoplastic square was quickly placed into the mold with the fabric on top of it, and the ram was be moved quickly to mold the plastic before a skin is formed on the plastic. The sample was removed from the mold. This process caused the polypropylene to exude through the openings in the woven fabric and physically lock the fabric to the plastic sheet. Comparative flame tests without reinforcement, with untreated fiberglass, with treated fiberglass, and with treated fiberglass coated with each of the active thermal protective materials showed that the treated fiberglass systems provided substantial increases in time to failure, and the coated fiberglass provided dramatic increases in time to failure. 
     Based on the foregoing tests, it is believed that the treated fiberglass fiber of the present invention can be embedded into thermoplastic structures by the same process of incorporation during the molding process. Other fibers, coated with active thermal protective materials, can also be embedded into thermoplastic structures. Composite sheets including the treated and/or coated fabric may also be formed either by passing a single heated thermoplastic sheet and a structural automotive elements, firewalls and other high temperature barriers such as used on jet engines. These variations are merely illustrative.