Патент USA US2407526код для вставки
Sept. 10, 1946.y G, sMoLAK 2,407,520 MANUFACTURE oF DIELECTRIC sHAPEs Filed Dec. 16, 1943 40 50 60 NVE-*JCR fia/ass _iva/wr. 2,407,520 Patented Sept. 10, 19.46 f UNITED lsTA'rEsi PATENToFFlcE ,y l ALlwrllxuIJFACTUREiÍ),Í‘«“)17;Íî:`(i).E*o'rmc sHAPEs ¿ Y V`George Smolak, Somerville, N. J., assigner tok Johns-Manville Corporation, New York, N. Y., a'corporation ofNew York ‘ `Application December 16, 1943,Y Serial No. 514,446 ' 2 Claims. ,(Cl, 26o-38) 1 , Thisginvention relates to the manufacture of ‘moistureabsorption of a panel with changes in tough and strong structural dielectric sheets or proportions of über and resin; and shapes having properties particularly adapted for Y use as switchboard panels and the like. .Serious diliicultie's have been encountered lby those attempting to solve the problem of pro viding dielectric panels Yof suitable large size and dielectric properties and having the impact and flexural strength to withstand the severe mechan ical strains> which are imposed particularly >by naval and marine service. - Such service demands panels having highY mechanical and dielectric strength and which exhibitV dielectric stability , f Fig. 6 is a perspective View (partly in section) showing a fragment of a dielectric panel manu factured in accordance with the invention. , The first step in preparing structural dielectric sheets in accordance with the present invention is to subject a supply of crocidolite asbestos fibers to a thorough willowing treatment, to open the v fibers, to flberize coarse ñber pencils, and to pro duce Well-'opened fine fibers of average lengthy approximating 1A inch; - Afterthe crocidolite lîbers have been thoroughly,wil1owed',l they are subjected to a further opening and flull‘lng'opera-V 15 tion, asfbybeing passed through an lair b1ower,'to' An object of the present invention is to pro break up any clots of‘matted fibers. Thoroughly vide a flbrO-cementitious sheet orshape having opened ‘andflulîed fibers are then introduced into high dielectric, impact and'llexural strengths and a batch mixervand mixed therein with a measured heat and moisture resistance adapting it forse proportion of finely pulverized and >’screened heat over a widetemperature range and in moist at mospheres.V vere navalservice. , " c ' ' Another object is to provide a simple and an economical method forrmanufacturing dielectric sheets and panels having the requisite dielectric and physicalproperties and size for use as naval 20 hardenabl'e" “B” 'stage ‘ phenolic 'or` Bakelite resins ' (for example, type'BRf7095);v ' It'should be under n stood'that the 'composition of such resins maybe varied considerably, and‘that "they may incorpo; ’ rate' plasticizing and'm'odifying agents and may ~ ' ' 'be preparedA by condensing 'and polymerizing re-` --The present invention provides a tough and,y 25 actions involvingv r hydroxy-aromatic homologues hard surfaced moldedV shape which is highly re of phenolY and "compounds including formaldehyde sistant to moisture penetration, has'goodj dielec having a >reactive"'methylene' radical; The pre tric-strength and dielectric stability in moist at mixture incorporates approximately 62.5% mospheres, vpossesses dimensional stability >and 30 ferred by weight‘of crocidolite‘ñbers andv 37.5% of ' phe physical strength throughout a wide temperature noi-formaldehyde ltype* 'resinr The ' fibers ' and range and which resistsY charring and burning on resin are thoroughlymixedin adry state to ef-switchboard panels. exposureto arcìng or short circuiting. t ~ The invention consists in the improved Istruc- Y, tural dielectric- sheets'. ory shapesl and' method of manufacture which arek hereinafter described and more vparticularly defined in the accompanying claims. ï f K _ A ¿ fectïsubstantially uniform dispersion of the resin particles over the exposed surfacesof theffluiied fiberz: This mixing -operationgrnay be carried out in a. rotary paddle type mixer-„or in a hammer mill type of mixer. From the mixer the thoroughly dispersed, dry mixture of ñbers and heat harden `-In the following'description of a preferredform ableÍ resin may be conveyed to a preforming or of the invention, reference will be made to the ac 40 cold molding unit or directly to a hot press mold companying drawing inLwhich: _ ',Fig. il 'presents acurve in which the impact strengths of dielectric panels made up as herein described, are plotted against the'proportions of íîlg and Curing. “.nîtf ,A „ f , Y ’ ,. Wet mixing is avoided because it has been found that whenV the resin binder is incorporated in the resin-fiber mixture as` a liquid, it is diiiicult to 45 produce aïmixtu're of uniform composition, sincev Fig.`2 presents a curve showing how thetrans-V , the liquid resins arek comparatively viscous, and vfers'e strength or modulus of rupture is alfected since thev volume of resin is small in comparison. by 'changes inthe proportions of liber and resin; ' with thevvolume of highly lluffed asbestos liners. Fig. 3 presents a curve plotting changes in Moreover, whena solvent is employedlto increase densitylwith changes in proportions of liber and 50 theavolume and .to decrease the viscosity of the :liber and resin inv their composition;Y I » resin; resin binder, an additional process step is re , ïFig'. 4 presents a curve plotting changes in mod-- ` ' quired to remove this solvent, and this solvent re ulus of elasticity with changesv in proportions of moval operation, and the problem of replacing or flberand resin; recovering the solvent, adds substantially to the _Fimv 5 presents a curve v plotting changes'Y 55 expense of the process. 3 lay and uniformly coated with and bonded by the phenolic resin binder l-2. The curing cycle preferably includes a short The process is particularly adapted for manu facturing structural dielectric shapes and panels of monolithic (as distinguished'from laminate) breathing period under reduced pressure. For ex structure and of comparatively large size. For eX ample the full cycle may include initial curing ample, the' process is applied particularly to the between hot platens at a temperature of approxi manufacture of naval switchboard panels having mately 300° F, andunder> 2,000 pounds per square inch pressure fora period of say 5 minutes. This face areas of 15 to 30 sq. ft. and varying in thick- v ness from 1/4.-2 inches. 'The dry molding mii; ture of opened and ilufi'ed crocidclite übers and period of initial cure is followed by a period of during which the sheet ¿reduced pressure heating resin particles which is required for producing a -10 er panel is held at curing temperature under a panel of the indicated size is comparatively bulky.' reduced pressure not exceeding about 500 pounds The preferred method of operation therefore con per square inch for a period of say 3 minutes. ‘ _ . n templates a step of compressing andpreforming rl‘his low pressure curing period is then followed the dry mixture oi' asbestos i'fibers‘and resin binder ¿ . by >prolonged final curing treatment at the indi under high pressure in a cold mold, as a pirelli/nie ‘lâÍ cated molding temperature and under full mold nary to completing the consolidation of the sheet> y pressure of about 2,000 pounds per square inch. and cure hardening of the resin'A under 'neat The final curing period may last as long as 45 to pressure. ln a preforming operation for produc»` ¿ 1 50 minutes, depending on the thickness of the ing a panel of l inch thickness, for example, the panel. dry resin-fiber mixture is preferably subjected in Appended hereto is a tabley illustrating the ef feet on the properties of cured sheets or panels of comparable size when employing crocidolite as bestos fibers, in place of chrysotile asbestos fibers, as the reinforcing element. By replacing chryso a batch mold at normal temperature to pressures of at least i500 lbs. per sq. inch and preferably to pressures Vof 3000 vor more pounds per square inch. In this preforming operation, the bulky mixture of ñbers and resin is compressed and shaped to a tile fiber in the same formula with crocidolite rigid handleable preform mat or sheet which may have substantially the ultimate face dimensions of the panel and which may have a thickness of ysay 21/4 to 3 inches. Since the preforming opera fiber, the impact strength of the resulting panel is increased from 0.4 ft. lbs/cu. inch to 0.9 ft. lbs/cu. inch. The dielectric strength of a` sheet reinforced with crocidolite fiber is 55 volts per mil, tionY is applied directly to the ?lurfed’nber-resin 30 as compared to only 37 -volts per mil for a sheet mixture, there is developed a certain heterogene reinforced with chrysotile fibers. The sheets in ous or random lay of the fibers in the c-o-ld pre form shape. By reason of this random lay ofthe fibers' and the preforming operation on the full charge of fiber-resin mixture, the final product has a monolithic structure which is more rmiform in Strength than a laminated product of the same generalcomposition. " ' ` i ‘ ' ' corporating chrysotile fib er as the reinforcing ma terial exhibit high electrical leakage near the breakdown value when subjected to electrical stress. Whereas the sheets made with crocidolite fiber display relatively small electrical leakage under the same conditions. ' Preform snape's of the indicated range of thick ness may be reduced to approximately 1 inch thickness during a final curing operation in a hot press mold under a pressure of at least approxi mately 2000 lbs. per square inch pressure, and under a curing temperature of approximately 30G-350° F. ' The final shaping and curing op? eration may be carried out/„or at least initiated, in a hot press mold having' confining side walls which are dímensioned to the final dimensions of the panel which it is desired to produce. Be cause of the large size and thickness of thepan els Which’are manufactured‘in accordance with the present invention, a prolonged curing period of at least 30 minutes to 1 hour is required tade velop complete cure-of the resin throughout a sheet of say 1 inch thickness. After forming and’ initiating cure of the sheet. by molding under pressure and elevated temperature, the cure of the `resin can be completed by oven heat treat ment. ~ . Composition of sheet Flexural strength , 45 70% Crocidolite über and ‘30% phenol-formaldehyde resin. Dielectric strength Ft. lbs./ yLbs/sq. in. 70% Chrysotile fiber and 30% 12,000 phenol-formaldehyde resin. Impact strength 22, 000 cu. in.V 0. 4 0.9 V./mil 37 55 - Fibro-cementitious sheets and> panels of suita ble dielectric and structural strength for severe naval surface 'can best be manufactured from compositions of the type 'herein described in which crocidolite> asbestos fiber is the principal constituent. For maximumrstrength, it is essen tial that the crocidolitel asbestos be thoroughly willowed and opened, andthat'the average length of theiibers shall be not substantially less than about 1/4 inch. A suitable grade of crocidolite as bestos liber4 has been found‘to classify as to length: 23.8% of 0.1jínch length or less, 25.1% of The curing temperature and pressure must be 60 0.1-0.2 inch, 24.6% _of 0.2-0.3 inch, 16.7% vof 0.3-0.4 inch, and 9.8% of 0.4-1.0 inch. To pro such as to soften the >resin Yand to insure com duce a structural dielectric panel 'which exhibits plete filling o-f the mold by the resin-liber miX optimum impact and flexuralV strength, dielectric ture. The curing temperature and pressure must stability, and optimum low moisture absorption also be such as to consolidate and densifythe it has been found that the proportions of fiber mold charge and to effect polymerization reac and resin in the Vcomposition are critical. Refer tion hardening of the resin binder.A Some varia ring to the charts presented in Figs. 1_5 of. the tions in both temperature and pressure may be drawing, curve A shows that panels or sheets in necessary in employing other types of bonding which the fiber proportions lie within the range resin and in molding sheets or shapes of difier 70 (S0-70% ñber and :iO-40% resin exhibit maximum impact strength. Curve B shows that the maxi ent dimensions. ' mum modulus of rupture or flexural strength is The cured shape (Fig. 6) has a smooth hard also exhibited by panels having a composition of surface and a dense monolithic structure in about 65% fibers and 35% resin. Curves C and. which the crocidolite asbestos fibers l0 are dis posed in heterogeneous or random> intermeshed 75 D Vshow that the maximum density and the max 2,401,520 i'mum modulus of elasticity are exhibited yby a Brinell hardness number of approximately 100, panels having a composition of about 70% ñber and aibout 30% resin. For satisfactory ' an impact strength of approximately 0.9-1 ft. lb. per cu. inch, a flexural strength of approximately dielectric stability the moisture absorption of 24,000 lbs. per sq. inch, a density of approximately the panel (curve E) should not exceed about 130 lbs. per cu. ft. and a maximum water absorp 0.4% by Weight after immersion for 24-48 . tion after 24-48 hours immersion of about 0.2 hours. Panels having> a composition of 'S0-40% 0.3%. Such sheets or panels have ‘a modulus of resin and 6'0-70% fiber exhibit satisfactorily elasticity in the neighborhood Aoi’ 3 million lbs. low Water absorption. The optimum com-position for crocidolite fiber -B stage phenolicA .» per sq. inch, and have a dielectric strength of at resin panels is approximately S21/2% fibers and 10 least 55 vl per mil thickness. Since many variations may be made from the approximately 371/2% resin. ' l illustrative details give , Without departing from the scope of the invention, it is intended that the invention should be limited only by the terms by standard test procedures. Information for plotting the curves of Figs. 2 and 4 was ob 15 of the claims interpreted as broadly as consistent The physical properties of `>cured panels, as plotted in the curves of Figs. 1-5, were obtained tained by applying a load to the center of a test panel strip having a 9 inch span between sup ports, at the rate of V2,000 lbs. per minute, until failure occurs. The center deiìections. were de with novelty over the prior art. What I claim is: . n 1. - A hard and tough dielectric panel having an impact strength of not less than 0.9-1.0 ft. lb. per 20 cubic inch, a dielectric strength of at least 55 v. termined at uniform Vincrements of load. The per mil thickness, and a ñexural strength exceed impact resistance, as plotted in the curve of Fig. ing 22,500 lbs. per sq. inch, and comprising 60 1, was determined for test panels by measuring '70% by weight of long crocidolite asbestos fibers, _the force of a bloW which will just causefailure said ñbers being coated and bonded together with in one blow, such blow. being delivered to the cen 30-40% of heat hardened phenol-aldehyde resin. ter of a 9 inch span by a Weight rounded at its 2. A hard and dense fibro-cementitious panel striking end to a 1 inch radius. The impact re of monolithic structure, comprising approxi sistance is reported as unit energy (single blow) mately 65% by weight of crocidolite asbestos per unit volume of sample. Water absorption was determinedV by immersing test specimens in ñbers averaging 1/4 inch in length and arranged water for a period of 48 hours at room tempera 30 in heterogeneous lay, and approximately 35% heat hardened phenol-aldehyde resin binder, said ture. Brinell hardness tests were made on the shape having a maximum water absorption of hard surface of a cured panel by measuring in dentations eiîected by application of a 1500 kilo gram load through a 10 mm. Ysteel ballV over a period of 10 seconds. not to exceed 0.4%, an impact strength of at least 0.9 ft. lbs. per cubic inch, and a flexural strength ' exceeding 22,500 lbs. sq. inch. After curing, the panel sheets normally exhibit GEORGE SMOLAK.