Patent GB-816316-A (GB)
Title: Improvements relating to mica paper
IPC: H01B3/04
Filed: 1957-09-18
Abstract: Mica paper of improved tensile strength and moisture resistance is prepared by impregnating mica paper with an anhydrous alkyl orthosilicate and thereafter hydrolysing the impregnated paper under acidic conditions and removing the volatile hydrolysis products.  The initial mica paper may be formed from a 1 per cent by weight mica slurry in water by normal paper-making technique. The mica employed may be phlogophite, lepidolite or muscovite and may be fired at about 800 DEG  C. before dispersion in the aqueous medium.  The orthosilicates employed are of general formula (RO)4Si where R is alkyl, and partial hydrolysis products of such orthosilicates.  The alkyl groups preferably contain not more than 5 carbon atoms each. Methyl, ethyl, propyl, butyl, t-butyltriethyl, t-amyltriethyl, di-t-amyldiethyl, di-n-butyldiethyl, butyltriethyl, methyltributyl, methyl tripropyl, and methyltriisopropyl silicates may be employed. Such orthosilicates may be applied by immersing the mica sheet therein or by drawing the alkyl orthosilicate in solution through a supported sheet of the mica paper, or by spraying the orthosilicate on to the paper.  Impregnation may also be effected by alkyl orthosilicates dissolved in monohydric alcohols or ethers to provide solutions of at least 30 per cent by weight strength. The hydrolysis under acidic conditions may be effected by incorporating an acid forming substance in the alkyl orthosilicate which will yield the acid on contact with moisture in the air or the impregnated mica sheet may be subjected to the vapours of an acid such as hydrochloric acid.  Suitable acid forming substances are ferric, cupric, aluminium and stannic chlorides which may be employed in amounts from 0.1 to 10 per cent by weight based on the alkyl orthosilicate employed.  Thus the mica paper may be impregnated with the orthosilicate containing the metal halide by dipping and allowed to stand in air to permit hydrolysis.  When hydrolysis is completed the impregnated paper is heated to 110 DEG  to 150 DEG  C. to drive off volatile hydrolysis products.  According to Examples: (1) A mica paper prepared by firing muscovite flakes at 1800 DEG  C. for 10 minutes, slurrying the plates in water forming a paper sheet therefrom and calendering at 150 DEG C. to form a uniform sheet of 2 mils. thickness, was employed, the sheet was immersed in ethyl orthosilicate containing 6 per cent by weight of ferric chloride for 2 minutes and then allowed to stand in air for 15 seconds.  The sheet was placed in a circulating air oven at 110 DEG  C. to remove hydrolysis products and the tensile strength of the treated sheet increased from 1050 p.s.i. to 2500 p.s.i.  The thickness of the sheet increased from 1.9 mils. to 4 mils.  (2) The same mica sheet was immersed in an ethyl alcohol solution of ethyl orthosilicate and ferric chloride and allowed to stand in air and heat treated as in Example (1).  (3) A sheet of mica paper was immersed in ethyl orthosilicate and allowed to stand over an open container of concentrated aqueous hydrochloric acid for 1 minute after which the sheet was placed in a circulating air oven at 110 DEG C. and allowed to dry.  The mica paper may be employed as a dielectric medium in capacitors, as an insulator in dynamoelectric machines and an insulator in high temperature apparatus such as electron tubes.