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Патент USA US2407526

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Sept. 10, 1946.y
G, sMoLAK
2,407,520
MANUFACTURE oF DIELECTRIC sHAPEs
Filed Dec. 16, 1943
40
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60
NVE-*JCR
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2,407,520
Patented Sept. 10, 19.46
f UNITED lsTA'rEsi PATENToFFlcE
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ALlwrllxuIJFACTUREiÍ),Í‘«“)17;Íî:`(i).E*o'rmc sHAPEs ¿
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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.
,
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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
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'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
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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
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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
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,.
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.
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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.
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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.
~
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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.
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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:
.
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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.
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