Metal coated glass fibers
TECHNICAL FIELD 
     This invention relates to a process for producing metal coated glass fibers. 
     BACKGROUND OF THE INVENTION 
     The prior art is filled with processes for metallizing the surface of glass, ceramics and fibers. Past attempts bonded a metal coating to the surfaces. More recent developments recognize the value of forming the metal coating in situ on the surface of the glass ceramics. Often, the metal coating provide heat resistance, higher strength, stress corrosion resistance or abrasion resistance. More recently, the coated surfaces find utility in the microelectronics and printed circuit arts. 
     DISCLOSURE OF THE INVENTION 
     I have developed a way of forming a metallic coating on the surface of non-crystallized, amorphous glass fiber. I have developed a two step process which prevents crystallization of the amorphous glass fibers. The first step heat treats amorphous glass fibers in an oxidizing atmosphere to form a metallic oxide coating on the surface without crystallizing the glass. The second step heats the metal oxide coating in a reducing atmosphere to form a metal coating. The resulting fibers are potential candidates for radar applications. The fibers may also have industrial uses for shielding against electromagnetic interference (EMI), radio frequency interference (RFI), and electrostatic discharge (ESD) and at the same time provide reinforcement. 
    
    
     BEST MODE OF CARRYING OUT INVENTION 
     A glass-containing copper oxide, was fiberized and then heat-treated to form a layer of copper oxide on the surface of the fiber. A subsequent reactive reduction step produces a semi-continuous film of metallic copper. This is conductive enough for radar applications. Electroplating may be used to further increase the conductivity for some applications such as EMI shielding. 
     Generally, we carried out the oxidizing heating at a temperature below 700° C. for a time ranging from 6 seconds to 6 minutes. Preferably, we carried out this heating at a temperature of 250° to 450° C. for a time of 6 seconds to 5 minutes. More specifically, the temperature ranged from 300° to 450° C. for a time of 6 seconds to 4 minutes. 
     We carried out the reducing heating at a temperature below 600° C. for a time of 10 seconds or less. Preferably, we carried out this heating at a temperature ranging from 300° to 500° C. for a time of 2 to 8 seconds. 
     Generally, the glass compositions contained 10 to 70 weight percent of the reducible metal oxide, preferably 10 to 40 weight percent. 
     The metal oxides we can employ besides CuO (Cu 2  O), include Ag 2  O, PbO, CoO, Sb 2  O 3  MnO, Cr 2  O 3 , Fe 2  O 3 , NiO or V 2  O 5 . Preferably, the metal oxide is CuO. 
     Generally, the glass fibers had the following composition: 
     
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Oxide              Weight Percent                                         
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SiO.sub.2          40.0    to 55.0                                        
CuO                10.0    to 50.0                                        
Al.sub.2 O.sub.3   0       to 30.0                                        
CaO                5       to 20.0                                        
B.sub.2 O.sub.3    0       to 10.0                                        
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     Preferably, the glass compositions are: 
     
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Oxide              Weight Percent                                         
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SiO.sub.2          45.0    to 50.0                                        
CuO                20.0    to 40.0                                        
Al.sub.2 O.sub.3   5.0     to 20.0                                        
CaO                5.0     to 15.0                                        
B.sub.2 O.sub.3    2.0     to 8.0                                         
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     A forming trial focused on developing a heat removable size, and we selected a PVA-paraffin system. Another forming trial yielded about 40 pounds each of rovable forming cakes of chemical grade and plant grade batch formulated glass. Both formulations were found to rove and heat-treat well. 
     INDUSTRIAL APPLICABILITY 
     We prepared conductive glass fibers as follows: 
     I. Glass Composition: Copper Glass 
     
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Wt. %      Oxide     Source                                               
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47.2       SiO.sub.2 Supersil                                             
24.0       Cu.sub.2 O                                                     
                     Black Copper Oxide CuO                               
12.5       Al.sub.2 O.sub.3                                               
                     Calcined Alumina                                     
9.8        CaO       Pulv. Limestone, CaCO.sub.3                          
5.4        B.sub.2 O.sub.3                                                
                     Anhydrous Boric Acid, B.sub.2 O.sub.3                
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     II. Melting 
     Mixed batch is crucible (Pt alloy) melted at 2850° F. for 21/2 hours. Cullet is crushed and mixed for remelt at 2850° F., 21/2 hours. Cullet sized for remelt forming operation. One also could melt the batch in a suitable premelt furnace to directly feed the bushing. 
     III. Forming 
     Typicially, I form the textile glass fibers using conventional bushings. While I can form a total range of fiber diameters, I prefer that the bushing deliver average fiber diameters ranging from 6 to 10 microns. 
     IV. Size 
     Belt application of an aqueous size allows for high speed roving as well as size removal via appropriate heat treatment. 
     Polyvinyl Alcohol, film former Gelvatol 20/30: 2.24% 
     Paraffin Wax Emulsion, Velvaton 77-70: 1.14% 
     Polyethylene Glycol, lubricant, Carbowax 300: 0.30% 
     Curing achieved at 270° F. for 10 hours. 
     V. Roving 
     I preferably form the roving with low tension pull and few friction points (guide eyes) to minimize damage. 
     VI. Heat Treatment--An In-Line Strand Process 
     A. Size Removal 
     Next, I thermally treat the roving at 500° C. under inert gas (helium) to vaporize size from glass surface, a necessary step in the development of metallic conducting film. The burn off also can be in air followed by slight bending of the strand to separate the individual fibers. In line residence times at as little as 6 seconds are sufficient. Burn off at 350° C. also can be used. 
     B. Oxidation 
     Thermal oxidation of the glass results in (1) the migration of Cu +   to the surface, and (2) oxidation of Cu +   to Cu +2  to form a copper oxide (CuO) film on each other fiber. Proper oxidation requires that organic size removal be affected prior to oxidation to insure that the fiber surface is directly in contact with O 2 . This oxide film is subsequently converted to a semi-continuous copper film on each fiber. Size removal also promotes &#34;filamentization&#34; of the strand, ensuring individual fibers rather than film bonded strands. 
     C. Reduction 
     Hydrogen reduction in 25 to 100% H 2  in helium at 450° C. for 5 seconds are sufficient to form a highly conductive film on each fiber. Strand resistances for a 5500 fiber tow of 7.2 micron fibers is typically less than one ohm/cm length, equivalent to similarly sized graphite fiber tows. 
     VII. Packaging 
     An effective package appears to be a moisture barrier film with a desiccant package added for insurance. 
     VIII. Electroplating 
     The conductive glass fiber tow can be electroplated using any standard electroplating bath. Copper cyanide baths work well for copper deposition on the copper film resulting from heat treatment. Iron and zinc have also been deposited on the strand. 
     With our processing oxidizing and reduction furnace temperatures, heat treatment line speeds have been increased up to a factor of 20 over the previous oxidize-electroplate system. We achieved sufficient strand conductivities on a production scale. 
     Strengths of the heat-treated products are acceptable. Single fiber specimens taken from the roving package indicate a mean unheat-treated strength of about 300,000 psi for the forming room product. Heat-treated mean strengths of about 250,000 psi have been observed. 
     Conductivity of the new material is excellent, and we had no problems with electroplating where desired.