Process for the manufacture of intermediates for the production of molded electric insulating elements
United States Patent [191 Nitzsche et al.  
 [ Apr. 15, 1975 PROCESS FOR THE MANUFACTURE OF INTERMEDIATES FOR THE PRODUCTION OF MOLDED ELECTRIC INSULATING ELEMENTS [75] Inventors: Siegfried Nitzsche; Ignaz Bauer;  
 Bernward Deubzer; Wolfgang Seidel, all of Burghausen, Germany [73] Assignee: Wacker-Chemie GmbH, Munich,  
 Germany 22 Filed: Apr. 12, 1973 21 Appl. No.: 350,452  
 [30] Foreign Application Priority Data Apr. 12, 1972 Germany 2217610 [52] US. Cl. 264/86; 264/91; 264/110; 264/126; 264/219 [51] Int. Cl B28d 1/32 [58] Field of Search 264/91, 110, 126, 311,  
 [56] References Cited UNITED STATES PATENTS 3,133,217 5/1964 Wohlferth et a1 264/110 3,523,061 8/1970 Purvis 264/110 FOREIGN PATENTS OR APPLICATIONS 1,239,607 1967 Germany 264/110 Primary Examiner-*Robert F. White Assistant Examinei&#39; -Gene Auville [57] ABSTRACT 5 Claims, No Drawings PROCESS FOR THE MANUFACTURE OF INTERMEDIATES FOR THE PRODUCTION OF MOLDED ELECTRIC INSULATING ELEMENTS The invention relatesto a method for manufacturing intermediates which are used in the production of molded electric insulating elements. More particularly, the invention relates to an improved method for manufacturing intermediates used in the production of molded electric insulating elements which are obtained from mica and thermosetting resins. These intermediates are prepared by applying an aqueous suspension consisting of mica and thermosetting resins to a molding device; removing the water by mechanical means and the mold thus obtained is heated for from 0.1 to 60 seconds at a temperature between the softening point of the organopolysiloxane resin and 99C. and thereafter the resultant mold is dried.  
  Methods for the production of electric insulating molds by the pressure and heat-hardening of intermediates consisting of mica and binding agents are well known, especially, the so-called prepregs&#34; and heated micanite prepregs. It is essential that these intermediates be self-supporting and that they can be handled easily without damage. Also, it is essential that they be free of tackiness at room temperature and that they be capable of being stored at room temperature without adhering together.  
  In comparison with the prior process for the production of intermediates, whereas mica molds were subsequently impregnated with organopolysiloxane resin solutions, the process of the instant invention has several advantages. One advantage of the present process is that at least one manufacturing step is eliminated. Also the process of this invention eliminates the necessity of working with highly flammable and/or toxic solvents. thereby eliminating the necessity of making provisions for recovering the same.  
  In comparison with the well-known processes in which an aqueous organopolysiloxane resin emulsion or solution was employed for the preparation of the aqueous mica suspension from which the intermediates were produced, or where the mica moldings were subsequently impregnated with aqueous organopolysiloxane resin emulsions or solutions, the process of the present invention also offers the advantage of eliminating the expense for the production, storage and transportation of the emulsions or solutions. Of even greater importance is the fact that the cleaning of the equipment which produced the intermediates, from mica particles is eliminated while at the same time avoiding the loss of resin from the solution or emulsion.  
  Compared to the heretofore known process (DT-AS 1,239,607) for the production of molds from an aqueous mica suspension and a binding agent, in which the silicone resin particles have diameters which are considerably larger than those of the mica flakes, the process of the present invention utilizes silicone resins (i.e. organopolysiloxane resins) having a greater proportion of fines, i.e. particles whose diameters are smaller than those of the mica particles. Furthermore the process of this invention will provide for the handling of the inter mediates for the manufacture of electric insulating tioned German Patent under the conditions indicated. when organopolysiloxane resins are used as binding agents.  
  It is therefore an object of this invention to provide a process for manufacturing intermediates for the production of molded electric insulating elements. Another object of this invention is to provide a process for manufacturing intermediates which avoids the use of highly flammable and/or toxic solvents. A further object of this invention is to provide a process which molds prior to drying, an embodiment which greatly fa- I avoids the formation of bubbles or holes in the molds.  
  The foregoing objects and others which will become apparent from the following description are accomplished in accordance with this invention, generally speaking, by providing a processfor manufacturing intermediates for the production of electric insulating molds. In this process, an aqueous suspension prepared from mica having laminating properties and thermosetting organopolysiloxane resins which are solid at ambient temperature, is placed in a molding device, the water is removed by mechanical means and the form thus obtained is dried. The method is further characterized by the fact that between the water removal step and the drying step, the form thus obtained is heated for from 0.1 to 60 seconds at a temperature between the softening point of the organopolysiloxane resin and 99C. The heating step is preformed prior to the removal of the mold from the support on which it was formed during water removal. 7 v  
  This invention can be carried out either continuously or intermittently. It is particularly well suited tothe preparation of prepregs, which are intermediates in the production of heating micanite [see VDE 0332/1 1.68, Specifications for Mica Products,&#34; of the Verband Deutscher Elektrotechniker&#34; (Association of German Electrical Engineers)], effective Nov. 1, 1968, (Section 3a, 1.2). Mica having laminating characteristics is fine mica, i.e., chemically or physically decomposed mica (see the above mentioned VDE regulation Section 3b, 5).  
  Examples of thermosetting organopolysiloxane resins which are solid at room temperature are hydrolysates or mixed hydrolysates of &#39;methyltrichlorosilane which contain at least 0.1 percent by weight of Si-bonded hydroxyl groups and if desired. up to 3 percent by weight of SiOC-bonded alkyl groups having up to 10 carbon atoms, such as methoxy, ethoxy and/or isopropoxy groups. These hydrolysates may be mixed with up to 5 mole percent of dimethyldichlorosilane or methyltrimethoxysilane and if desired up to 5 mole percent of dimethyldiethoxysilane. Additional examples of thermosetting organopolysiloxane resins which are solid at ambient temperature are organopolysiloxanes which contain at least 0.1 weight percent of Si-bonded hydroxyl groups with an average of from 0.3 to 1.2, preferably about 1.1, phenyl groups per Si atom and an overall average of from 0.9 to 1.7, preferably about 1.1 SiC-bonded organic groups per Si atom, as well as up to 3 percent by weight of SiOC-bonded alkyl groups such as methoxy, ethoxy and/0r isopropoxy groups. The remaining SiC-bonded organic groups which are generally present in addition to the phenyl groups are preferably methyl groups.  
  In order that an aqueous suspension can be prepared from these resins, they must be insoluble in water as well as existing in a solid and finely divided form at ambient temperature. Although the diameter of the resin a collodial suspension. Thermosetting &#39;organopolysiloxane resins which are solid at ambient temperature are described in British Pat. Nos. 1,001,743 and 1,054,352.  
  In order to accelerate the hardening, thermosetting organopolysiloxane resins which are solid at ambient temperature are often used in combination with a hard- 4 surfac e afnd between the mica particles are resin particles and possibly particles of the hardening catalyst.  
 - a Duringthe heating&#39;step, at least a portion of the resin particles&#39;should become plastic, soften, or melt and ening catalyst. The hardening catalyst must be inactive at ambient temperature and must not cause any undesired hardening of the resin, not only during the steps of the process of forming the intermediate according to .lats, which have a boiling point of at least 140C. (at 7.60 Torrabsolute) and a melting point below the tem- 1 perat&#39;ure used in the formation of the precursors. Examples of suitable ketones or metal enolates are acetylacetone and aluminum acetylacetonate (German Pat. Application. No. 1,920,691, of Nov. 5, 1970). When inorganic lead compounds are used as hardening catalysts, they areusually used in quantities of from 0.01 to 5 weight&#39; p teem. based on the weight of the organopolysiloxanesyFurther examples of suitable hardening catalyst s aregmixtures of aluminum and zinc acetylacetonates &#39;or mixtures of aluminum and titanium acetylacetonates As is generally known, the lead compoundsdescribed above as well as mixtures of metal enolates can be used in combination with higher alcohols or carboxylic acids.  
 . The thermosetting organopolysiloxane resins which are solid at ambient temperature are used preferably in an amount of from 2 to weight percent, preferably from 4 to 12 weight percent based on the total weight of the mica and the organopolysiloxane resin.  
  The amount of water used for the preparation of the aqueous suspension of mica having laminating characteristics and thermosetting organopolysiloxane resins which are solid at ambient temperature is not critical. A sufficient amount must be used to obtain a suspen sion, il e., enough water should be used in order that each particle is surrounded by water. The upper limit on the amount of water is determined only by the desired thickness of the intermediate, or by economic considerations. That is, the thicker the initial products desired, e.g. in the shaping by the screen ofa paper machine,- the smaller the amount of water whichmust be used for the preparation of the initial products. The amount of water can, vary from about 50,,to 1200 weight percent based on the weight of the mica.  
  The forming support can, e.g., be a suction filter (Buchner funnel), a shaping screen, a filter cloth or the screen network of a paper machine.  
 ],l e&#34;ad carbonate (PbCO3). lead mon- The mechanical means by which the waterisi sremoved from the suspension of mica having laminating characteristics and thermosetting organosiloxaneresms which are solid at ambient temperature can. consist ijn should thereby cement the mica particles together. The heating step ranges from about 0.1 to 60 seconds and is carried out at a temperature between the softening pointof the organopolysiloxane resin and 99C.&#39;The brief, shock-like heat treatment is insufficient to harden the resin. The heating step is carried out after the water has been removed by mechanical means, but prior to the drying step. In addition the heating step is carried out prior to the removal of the mold from the support on which it was formed, or before the mold resting on a flexible support such as a filter paper, a filter cloth, or a shaping screen, is lifted from the flexible support site during the water removal. Heat carriers with temperatures of from to C. are preferred.  
  The brief, shock-like heat treatment, which is insufficient to harden the resin, can be conducted; for example, byspraying or pouring hot water over the form obtained by the removal of water by mechanical means. or with hot steam which, if applicable, can be superheated, or by means of heating baths, hot air, heated rollers as heat transfer agents and/or thermal radiation. The use of hot water is preferred. Generally 2 to 3 liters of water at 90C. are required for the preparation of intermediates which, after hardening under heat and pressure, result in a sheet having dimensions of about 1500 X 300 X 0.5 mm. Hot steam is especially suitable for the preparation of intermediate products which. after hardening under heat and pressure, result in a sheet with dimensions of about 1500 X 300 X 0.5 mm. The brief, shock-like heat treatment which is insufficient to harden the resin is carried out by subjecting the form obtained by the removal of the water to steam at a temperature of about C. for 20 to 30 seconds.  
  In order to prevent the mold consisting of mica and a binding agent from adhering to the support or other processing parts during the brief, shock-like heat treatment, especially when a medium other than water or steam is used to conduct the shock-like treatment, it may be expeditious to coat the supports or other equipment which will come in contact with the mold, such as filter cloths or rollers, with an organopolysiloxane resin-repelling coating such as polytetrafluoroethylene and/or to cool the form consisting of mica and binding agent to room temperature immediately after the shock-like heat treatment. This cooling may also be required when the form consisting of mica and binding agent is removed, before drying from the support on which it was formed, or when the form is on a flexible support and is lifted from the site occupied by the flexible support during the removal of the water by mechanical means, if hot water or steam is not used at a temperature below about C. as the heat transfer agent. Cooling can be carried out by spraying or pouring cold water or by means of cooling baths, cold air and/or cooled rollers. Each of the cooling methods mentioned can be combined with eachof the heating methods previously described. Thefollowing steps can be carried out with a suspension &#39;of Emicaand thermosetting organopolysiloxane screening, suction, filtration and[orcentrifugation ln using these techniques, a sheet-like for&#39;mi s obtainedin;  
 which the mica particles are oriented parallel to the resin on aFourdrinier&#39; machine; such as-is used in paper production. The water is removed from the suspension by. screening and the formremaining on the screen is passed through a heating tunnel at C. for 30 secsource of heat. The temperature of the form consisting of mica and binding agent does not exceedabout 95C.  
 and immediately thereafter. cooling is accomplished with a cooling roller. I The drying of the form consisting of binding agent and mica. which contains residual water. can occur by means of hot air. heated rollers. thermal radiation or heating tables. in this step. it is advantageous not to exceed a temperature of about 90&#39;C. because it was found that with the relatively long action of high temperatures. premature hardening of the organopolysiloxane resin and/or the formation of voids in the prepreg can occur. During drying. all of the water in excess of 1 percent by weight is removed. because otherwise the finished product. which was produced from the prepregslby hardening with pressure and heat. will often contain voids. 1  
  vFurtherprocessing of the intermediates prepared ac cording to the process of this invention may occur in any manner known in the art for processing intermedi ates consisting of a binding agent and mica into electric insulating molds by hardening under pressure and heat.  
  Various embodiments of this invention are illustrated in the following examples in which all parts are by weight unless otherwise specified.  
  The values given .in the following examples for tensile strength and water absorption of electric insulating molds. which were produced from intermediates prepared according to the invention. are determined in accord with DIN (German Industrial Standard) 53.455 or DIN 53.475.  
 . EXAMPLEI Approximately I parts of finely ground mica is combined with about l5 parts of methylpolysiloxane.  
 tion. The hydrolysate was washed with water and evaporated to dryness and the residue was ground to a particle size of not more than about 0.2 mm. About 0.0.l5 parts of basic lead carbonate having a lead content of 8! weight percent. consisting of particles below about 0.l mm. is added to the mica-resin mixture and the entire mixture is suspended in 50 parts of water by means of a high-speed laboratory stirrer. The water is removed froin the suspension by suction through a Buchnerfunnel with a filter surface area of 125 cm.  
  About 300 parts of water at a temperature of from 90&#39; to 99C. is then drawn through the filter cake. The latter is then lifted off the filter paper and removed from the funnel. This is easily done without any damage to the form consisting of the binding agent and the mica. which contains residual water. This form is stored in a drying oven at 60&#39;C. for 48 hours. after which the water content is less than 1 weight percent. The filter cake is then heated to l75C. for one hour in a hydraulic press at a pressure of 8 kg/cm&#39;. the temperature is reduced to l00&#39;C. and the pressure is relieved. The resuit is a mieanite sheet of 4 mm. thickness and a speeific gravity of 2.2 g/cm.  
  The plate is removed from the press and cut into standard small bars..usi n g a high-speed diamond saw. forthe mechanical strengthmeasurement. The tensile value is less than i weigjlitpercent;-  
 &#39;EXAMPLECZ Finely ground mica having average particle diameter of about 0.00! mm. is mixed in a Hollander with l0 weight percent (based on the weight of the mica) of a powdered phenylmethylp&#39;olysiloxane resin having a melting point of 65&#39;C.. an average of 0.75 phenyl groups per silicon atom and an overall average of L0 SiC-bonded methyl and phenyl groups per silicon atom. 3.5 weight percent of Si-bonded hydroxylgroups and 0.5 weight percent of ethoxy groups. About O. l weight percent (based on the weight of the organopolysiloxane resin) of finely divided lead monoxide is added&#39;to :the mica-resin mixture and the entire mixture .issuspe&#39;nded in- [00 weight percent of water (based .on ti e weight of the mica). The suspension is placed on a Fourdrinier machine. such as used in the production of paper. where it is sprayed for about 5 seconds with water at about 85&#39;C. to increase the residual water content to about weight percent before removing it from the machine. The sheet-like form thus obtained is cut into smaller pieces. during which no undesired damage occurs. The pieces are then stored in a drying oven at 60&#39;C. for 48 hours. in order to dry them to a water content of at least i weight percent. The prepregs are then ready for processing and can be stored for a relatively long time.  
  In each case. ten pieces of these initial products are stacked on top of each other. placed in a preheated hydraulic press for 15 minutes of heating at ZSO&#39;C. under a pressure of i0 ltg/cm and cooled to room temperature in a second press which is likewise under a pressure of i0 lrglcm to convert them to a heating micanite. The tensile strength value is 1000 ltglcm and the water absorption value is less than about 0.5 weight percent.  
 EXAMPLE 3 Finely ground mica is combined with 10 weight percent (based on the weight of the mica) of a methylpolysiloxane resin consisting of 95 mole percent methyisiloxane units and 5 mole percent dimethylsiloxane units and having a sofiening point of &#39;C. The resin contains about 0.6 weight percent Si-bonded hydroxyl groups and 2 weight percent ethoxy groups and consists only of particles below about 0.5 mm. Approximately 0. l weight percent (based on the weight of the organopoiysiloxane resin) of lead monoxide. consisting only of particles less than about 0.01 mm. is added to the micaresin mixture and the entire mixture is suspended in l000 weight percent of water. The water is removed from the suspension in a screening centrifuge. through a filter cloth which is impregnated with polytetrafluoroethylene and which lies on the inner side of the centrifuge screen. While the centrifuge is still operating. the filter cake is sprayed with a volume of water. at C. equal to its own volume. After this water is centrif uged off. the filter cloth. together with the filter cake. is removed from the centrifuge. The filter cake is heated at 75&#39;C. for hours on a level timber frame. in order to dry it to a water content of at most l weight percent. The product thus obtained whose width correobtained is very quickly-transferred, while still hot, to-  
 an adjacent cold press andcooled to room temperature under a pressure of &#34;kg&#39;/c&#39;m The heating micanite thus obtained hasa tensile strength of 750 ltg/cm and a water absorption of. Zweight percent.  
 Although specificfexamples of the invention have been described herein. ltzlS not intended to limit the invention solely thereto. but: to include all the variations and modifications falling within the spirit and scope of the appended claims. i  
 What is claimed is: 1  
  l. Animproved method for&#39;preparing&#39; intermediate products-forwmanufacturing electric insulating molded formsin&#39; which an aqueous suspension containing mica having laminating characteristics and thermosetting organopolysiloxane resin particles havinga diameter up toaboutOj cm. and which are solid at ambient temperature is applied to a shaping support, said resin particles are presentin an amount of from Z&#39;to&#39; 20 percent by weight based on the total &#39;weightof the organopolysiloxan&#39;e resin and&#39;mica, water is removed by mechanical means and the form thus formed is removed from thesupport and dried. the improvement which comprises heating the form after removing the water by mechanical means and prior to removing the form from the support on which it was formed to a temperature between the softening point of the organopolysiloxane resin and 99C. for from 0.] to 60 seconds. 2. The method of claim 1 wherein the form is heated from 0.1 to 30 seconds to a temperature of from to C. 3. The method of claim 1 wherein the form isf heated by means of heat transfer agents; i i  
  4. The method of claim 1 wherein the form isyhe&#39;a&#39;t ed by means of hot water. 1 1 a S. The-method of claim 1 wherein thea&#39;queous suspension contains a hardening catalyst.