Lithia-containing frit additives for MgO coatings
United States Patent 1 1 1111 3,879,234  
 Lee et a1. Apr. 22, 1975 [54] LlTHlA-CONT-AINING FRIT ADDITIVES 2.533.351 12/1950 Carpenter 148/113 3.184.320 5/1965 Michael 106/48 FOR MGO COATINGS 3.227.587 1/1966 Martin 148/113 inventors: Leonard a y y; os 3.375.144 3/1968 Taylor 148/113 Uyeda, San Mateo; Samuel W. Sopp, 3.390.021 6/1968 Michael 148/20 Foster City, an of c lm 3.583.887 6/1971 Steger et a1 148/113 3.594.240 7/1971 Foster et a1 148/113 1 1 Asslgneei Merck &amp; 4 1119-, y. 3.615.903 10/1971 Perry et a1. 148/122 3.653.984 4/1972 Urushiyama et al..... 148/113 [22] 1973 3.697.322 10/1972 Lee et al. 148/113 [21] Appl. No.: 415,477  
  l&#39; D 6 Rel ated U S App canon ata Primary E.\&#39;aminer--Wa1ter R. Satterfield l 3] fSx S E E of 2] L126 Attorney, Agent, or FirmHesna J. Pfeiffer; Julian S.  
  an one Levitt; .1. Jerome Behan [52] us. Cl. 148/113; 117/127; 117/222; 148/31.5; 148/122; 148/27 [51] Int. Cl. H0&#34; 1/04 [58] Field 61 Search 148/113, 122. 27, 120, [571 ABSTRACT 129, 1 18, 135.1, 222 Novel frits of the composition alkali metal oxide-MgO useful for coating silicon steel and as an additive for [56] References Cited MgO coatings.  
  UNITED STATES PATENTS 2.354.123 7/1944 Horstman et a1. 148/113 1 Claim, No Drawings LlTHlA-CONTAINING FRIT ADDITIVES FOR MGO COATINGS This application is a continuation-in-part application of U.S. Ser. No. 21 1,126. filed Dec. 22. l97l and now abandoned.  
  This invention relates to novel alkali metal hydroxide-MgO materials. The invention further relates to the use of said materials as coatings for grain oriented silicon steel. The invention also concerns employing the alkali metal hydroxide-MgO material as an additive for magnesium oxide/magnesium hydroxide coatings for ferrous substrates.  
  In many fields of use and, in particular, in the electrical industry, it is necessary to provide a coating on ferrous material. This coating desirably performs the function of separating and purifying the ferrous material and reacting with surface silica in the steel to form an electrical insulating layer. For example. in the transformer art. the cores of the transformers are usually formed of a ferrous material, such as silicon steel, which may be provided with a preferred grain growth orientation to provide optimum electrical and magnetic properties. It has been found necessary to provide a coating on the ferrous material prior to the final high temperature grain growth anneal. This coating will perform three separate functions. The first function of the coating is to provide separation of the various turns or layers of the coiled material to prevent their sticking or welding together during high temperature anneals. A second function is that of aiding in the chemical purification of the ferrous material to develop the desired optimum magnetic characteristics of such material. The third function of the coating is to form on the surface of the ferrous material a refractory type coating which will provide electrical insulation of one layer of ferrous material from the next during its use as a core in a transformer.  
  In the present state of the electrical apparatus art. the most widely used coating for the ferrous material which is used as the magnetic core of the electrical apparatus is a coating of magnesium oxide and/or magnesium hydroxide. These coatings are. in general. applied to the ferrous material in the form of a suspension of magnesium oxide and/or magnesium hydroxide in water. The suspension comprises a quantity of magnesium oxide in water and is mixed sufficiently for the desired application; the magnesium oxide being hydrated to an extent dependent on the character of the oxide used, the duration of mixing and the temperature of the suspension. Therefore, the term magnesium oxide coating is with reference to a coating of magnesium hydroxide which may include magnesium oxide which has not been bydrated.  
  As set forth in U.S. Pat. No. 2,385,332. in the names of Victor W. Carpenter et al., portions of an annealing separator of magnesium oxide can, during a heat treatment at suitable temperatures, be caused to react with silica particles on or near the surfaces of previously oxidized silicon-iron sheet stock to form a glass-like coating, which coating is useful as an interlaminary insulator in the use of silicon iron in electrical apparatus, e.g. in the cores of transformers.  
  In the production of silicon steel for the magnetic cores of transformers, the steel is generally annealed to provide optimum grain growth and grain orientation which develops the magnetic properties of the silicon steel. This anneal is usually carried out in a hydrogen atmosphere at temperatures ranging from approximately 950 to l.500C. from about 2 to about 5 hours. This anneal also aids in purifying the steel, aided by the coating placed on the steel. During this anneal a portion of the magnesium oxide coating reacts with the silica on the surface of the silicon steel to form a glass-like coating of magnesium silicate. This glass-like coating provides electrical insulation during the use of the silicon steel in electrical apparatus. e.g., in the cores of transformers.  
  A number of additives have been proposed in the past to be added to the magnesium hydroxide and/or magnesium oxide in order to improve the MgOSiO- reaction. For example, U.S. Pat. No. 2.8091 37 (Robinson) involves the use of silica to be combined with the MgO and/or Mg(OH). for the purpose of improving the insulating properties of the glass-like film obtained after high temperature annealing. U.S. Pat. No. 2,394,047 (Elsey, et al.) relates to the use of additives to produce oxidized surface metal and to enhance glass film formation.  
  This invention relates to alkali metal hydroxide-MgO materials which form a superior insulating glass film when applied to silicon steel surfaces. The novel materials of the invention may be characterized as an X- ,OMgO frit wherein X is an alkali metal. Representative alkali metals that may be employed in the formation of the novel frits of the invention include lithium. sodium. potassium. rubidium and cesium. The term frit as used herein refers to a complex that is formed by heating materials near their melting point. The alkali metal hydroxide-MgO materials or alkali metal oxide MgO frits include the following:  
 LiOH-MgO (Li O--MgO frit); NaOH-Mg0 (Na O--MgO frit); KOH-Mg0 (K ()MgO frit); RbOH-Mg0 (Rb OMgO frit) and CsOHMgO (Cs- O--MgO frit).  
 A preferred member of this class which produces outstanding insulation properties upon application to silicon steel surfaces by itself or in combination with MgO coatings is LiOH-MgO which may be characterized as Li OMgO frit.  
  In addition to utilizing the frits per se as coatings for silicon steel, these novel materials may be employed as an additive for conventional MgO coatings. Accordingly, this invention further relates to coatings containing magnesium oxidelmagnesium hydroxide and alkali metal-MgO frits which when applied to silicon sheet steel impart unexpected and improved insulation qualities to the silicon steel after the final high temperature anneal.  
  Procedures for the preparation of grain-oriented silicon steels are well known in the art. Representative patents which teach the preparation of grain-oriented silicon steels and indicate typical analysis of silicon steels which may be employed in the practice of the invention and are incorporated herein by reference, include the following: U.S. Pat. Nos. 3.582.409; 3,544,396; 3.333.991; and 3,676,227 which relates to silicon steels of high permeability. V  
  The alkali metal oxide MgO frits of the invention may be prepared by solid reaction of an alkali metal hydroxide such as LiOl-l and MgO at elevated temperatures. A convenient and effective process for the preparation of the frits of the invention is by calcining a mixture of the alkali hydroxide powder and MgO at temperatures ranging from 100C. to l.500C. It may be noted that any conventional device. technique or apparatus may be employed to effect the solid state reaction at elevated temperatures in the production of the frits of the invention. The novel frits of the invention include those materials wherein the alkali metal hydroxide is present by weight from about 0.] to 50 percent. Accordingly. the MgO component may vary from about 50 to 99.9 percent of the total frit by weight. Preferred frits include those wherein the alkali metal hydroxide component is present within the range of 3-8 parts per 100 parts of the MgO.  
  The alkali metal hydroxide-MgO materials may be applied as a coating to grain-oriented silicon steel using techniques well known to the art. Among the well known procedures that may be employed in applying the frit coatings include passing a continuous strip of the ferrous material through a bath containing a suspension of the frit followed by subjecting the coated steel to a drying furnace. The concentration of the frit in the coating bath is not critical and may vary from about 1 to 50 percent.  
  When the frit is used as an additive for the MgO/M- g(OH coating. the concentration of frit additive with respect to the amount of the MgO employed in the coating is not critical and may vary from about 0.1 to about 30 weight percent of the magnesium oxide. A satisfactory concentration for most practical purposes has been found to be from about 0.2 to 12.5 weight percent of the MgO. It should be noted that the particular grade of MgO to be utilized is not critical and any commercially available MgO may be employed in the practice of the invention such as Maglite S 333l manufactured by Merck &amp; Co.. Inc.  
  The alkali metal hydroxide-MgO additive may be employed in conjunction with the MgO/Mg(OH) coatings in accordance with procedures well known in the coating of grain-oriented silicon steel.  
  The amount of MgO/Mg(Ol-l) (exclusive of additive) that is applied to the silicon steel in the practice of this invention is similar to those amounts that heretofore had been conventionally employed in MgO/M-- g(OH). coatings and in general will vary from about 0.020 to 0.070 ounces of MgO per square foot of steel surface.  
  The manner and time at which the additive is combined with the magnesium oxide is not critical. For example, procedures which may be utilized include adding the frit to a magnesium material, such as magnesium basic carbonate or Mg(OH) prior to its conversion to the magnesium oxide; blending the additive with the MgO or Mg(OH adding the frit separately during coating slurry make-up; or mixing the frit in the water used for coating slurry make-up prior to the addition of the MgO powder.  
  The annealing of the silicon steel that has previously been coated with the coating composition of the invention may be carried out in a neutral or reducing atmosphere at temperatures ranging from approximately 950 to l.500C. for from about 2 to 50 hours using techniques well known to the art.  
  The unobvious and unexpected properties of the instant invention are clearly revealed by the following examples:  
 EXAMPLE 1 A mixture of 100 g. of MgO and 5 g. of lithium hydroxide powder is calcined at l.000C. for three minutes to yield a Li OMgO frit.  
  Following the procedure of Example I. the Li- ,OMgO frits of the invention may be prepared by varying the amount of LiOH powder utilized. As indicated above. the amount of alkali metal hydroxide may vary from about 01-50 percent of the total frit by weight.  
  Replacing the LiOH powder of Example 1 with representative .alkali metal hydroxide powders will result in the preparation of the alkali metal oxide-MgO frits of the invention. For example, when in place of the LiOH powder, there is utilized NaOl-l. KOl-l. RbOH or CsOH the following frits result. respectively:  
 EXAMPLE 2 a. A coating slurry is made by mixing in a Waring Blender 60 g. of a commercial steel grade. MgO. 60 g. of the Li OMgO frit prepared in Example I and 1000 ml. of deionized water. The mixture is allowed to stand to stabilize the viscosity. The resulting slurry is coated onto silicon steel strips (size 3 cm. X 30.5 &#39;cm.) at a coating weight of 0.061 oz./ft. based upon MgO and dried at 250270C. The coated strips are then boxannealed in hydrogen atmosphere for 30 hours at l,200C.  
  b. For comparative purposes. identical steel strips are coated in the above manner with an identical MgO slurry of the same concentration as employed in (a) but which does not contain the Li OMgO frit additive.  
 After annealing and cooling. the excess coating was scrubbed off all samples with a nylon brush and a cloth.  
 These strips were tested for resistance on both surfaces with a Franklin tester (ASTM-A344-6OT). The results a. A coating slurry is made by mixing in a Waring Blender 60 g. of a commercial steel grade, MgO. 30 g. of the Li O-MgO frit prepared in Example 1 and 750 ml. of deionized water. The mixture is allowed to stand to stabilize the viscosity. The resulting slurry is coated onto silicon steel strips (size 3 cm. X 30.5 cm.) at a coating weight of 0.06l oz./ft. based upon MgO and dried at 250270C. The coated strips are then boxannealed in hydrogen atmosphere for 30 hours at 1.200C.  
  b. For comparative purposes. identical steel strips ar coated in the above manner with an identical MgO slurry of the same concentration as employed in (a) but which does not contain the Li OMgO frit additive.  
  After annealing and cooling. the excess coating was scrubbed off all samples with a nylon brush and a cloth. These strips were tested for resistance on both surfaces with 21 Franklin tester (ASTM-A344-60T). The results are:  
 No additive Li O-MgO frit additive (33 A391 (a) Resistivity 4,9 ohms/cm 85.5 ohms-cm Surface particle size 0.2 l.5 p. 0.5 l.5 a Ductility. No. of 90 bends to break steel l0 l0 Coating appearance smooth smooth Adhesion pass pass EXAMPLE 4 a. A coating slurry is made by mixing in a Waring Blender 60 g. of a commercial steel grade, MgO, 60 g. of the Li- O--MgO frit prepared in Example 1 and L000 ml. of deionized water. The mixture is allowed to stand to stabilize the viscosity. The resulting slurry is coated onto silicon steel strips (size 3 cm. X 30.5 cm.) at a coating weight of 0.061 oz./ft. based upon MgO and dried at 250-270C. The coated strips are then box-annealed in hydrogen atmosphere for 30 hours at l.200C.  
  b. For comparative purposes, identical steel strips are coated in the above manner with an identical MgO slurry of the same concentration as employed in (a) but which does not contain the Li- O-MgO frit additive.  
  After annealing and cooling, the excess coating was scrubbed off all samples with a nylon brush and a cloth. These strips were tested for resistance on both surfaces with a Franklin tester (ASTM-A344-60T). The results are:  
  The above experiments demonstrate that magnesium oxide currently employed to coat grain-oriented silicon steel give relatively low resistance whereas the same MgO coating containing a frit additive results in the production of a film having a considerably higher resis- Lil LII  
 tance. Comparable results to that indicated above are achieved employing other representative alkali metal oxide-MgO frits encompassed within the scope of the invention.  
  Illustrative of the procedures that may be employed in the coating of silicon steel utilizing solely a representative frit of the invention follows:  
 EXAMPLE 5 a. A coating slurry is made by mixing in a Waring Blender 60 g. of the LLO-MgO frit prepared in Example l and 500 ml. of deionized water. The mixture is allowed to stand to stabilize the viscosity. The resulting slurry is coated onto silicon steel strips (size 3 cm. X 30.5 cm.) at a coating weight of 0.061 oz./ft.&#39;- based upon MgO and dried at 250-270C. The coated strips are then box-annealed in hydrogen atmosphere for 30 hours at l.200C.  
  b. For comparative purposes, identical steel strips are coated in the above manner with an identical MgO slurry of the same concentration as employed in (a) but which does not contain the Li- ,O-MgO frit.  
  After box-annealing and cooling, the excess coating was scrubbed off all samples with a nylon brush and a cloth. These strips were tested for resistance on both surfaces with a Franklin tester (ASTM-A344-60T). The results are:  
 What is claimed is:  
  l. A method of producing a permanent electrical insulating film on magnetic silicon steel which comprises applying a coating composition consisting essentially of MgO. Mg(OH) or mixtures thereof and a Li OMgO frit to silicon steel and annealing said silicon steel at an elevated temperature wherein said frit is a complex that is formed by heating lithium hydroxide and MgO at about their melting points said lithium hydroxide beingpresent from about 3-8 parts by weight per parts by weight of MgO, wherein said frit being present at from about 0.l to about 30 weight percent of the MgO. Mg(OH) or mixtures thereof, and wherein the annealing occurs at about 950 1,500C. for from about 2 to 50 hours.