April 12, 1960
v. F. DAHLGREYN
METHOD OF PREPARATION
OF THERMO
'RESI N comm PRINTED CIRC
2,932,599
STIC
Filed May 9, 1955
II
ALKALINE
BATH
COPPER
I
2
BLACK
CUPRIC OXIDE
I
0
RINSE
+
3\HCI
+'
FeCI3
I
RINSE
I,
THERMOSETTING
Fig. 2
4\‘NoCN
PHENOLIC RESIN
I
RINSE
i
5\OXIDIZ|NG
*
AGENT
I
RINSE
s\
N6
7
N
\
CURED
TH
OSETT
PHENO
LA
_,
RE
ATE
II
8\
PRESS
RE
NOLIC
BLACK
OXID
CuO
RIC
-| COPPER
‘II
I
COOL
9\
JIV-IING
_T?.
REMOVE
|o
FLUORO
CHLOR
ETHYL
-
THER
ET
PHENOLIC RE
COPPER
TRI- FLUORO
L0
TRI-FLUORO
-
YL
8w PRESS
|
COOL
I
Fig
|
CURED
THERMOSETTING
PHENOLIC RESIN
Victor F. Du
INVEN
III/Kama
Attorney
en
I
United States Patent 0
ICC
1
2,932,599
Patented Apr. 12, 1960
2
greater tendency to flow at a temperature substantially less
than the given temperature is placed in contact with the
2,932,599
imprinted surface. The plastic resins are pressed together
METHOD OF PREPARATION OF THERMOPLASTIC
at a temperature substantially less than the given tem
perature to force the second plastic resin into the follicles
to provide a bond therebetween.
In accordance with the invention there is further pro
vided a method of bonding a thermoplastic resin to a
RESIN COATED PRINTED CIRCUIT
Victor F. Dahlgren, West Windham, N.H., assignor, by
mesne assignments, to Sanders Associates, Incorpo
rated, Nashua, N.H., a corporation of Delaware
thermosetting resin. The method comprises oxidizing
10 the surfaces of a thin sheet of copper to provide the
Application May 9, 1955, Serial No. 507,032
surfaces with a homogeneous coating of black cupric
10 Claims. (Cl. 154-129)
oxide. The cupric oxide coated copper sheet is placed
The present invention relates to printed circuits. More 15
particularly, the invention relates to printed circuits
covered with insulating material.
In the prior art, printed circuits are typically formed
by a?ixing thin copper, sheets with a suitable adhesive to
an insulator base formed from a thermosetting resin such 20
as XXXP-Phenolic (a paper base, phenolic resin lami—
in contact with an uncured sheet of thermosetting resin.
A sufficient degree of heat and pressure is applied to
cure the thermosetting resin and press the copper and the
thermosetting resin sheets together, forming thereby an
imprint of the cupric oxide in the thermosetting resin.
The cupric oxide coated copper clad resin sheet is then
cooled. Cupric oxide coated copper is removed from
the resin sheet to expose the plurality of randomly dis
posed follicles in the resin provided by the imprint. The
nate). The copper is then etched away, leaving a con
thermoplastic resin is placed in contact with the imprinted
ductive pattern in the con?guration of the desired circuit.
surface.
A su?lcient degree of heat and pressure is ap
Thermosetting resins are characteristically excessively hy
plied
to
the
thermoplastic and thermosetting resins to force
groscopic. Under high humidity conditions, insulation re 25 the thermoplastic
resin into the follicles. The thermo<
sistance between conductors is substantially reduced. A
plastic and thermosetting resins are then cooled to pro
relatively new material, tri~?uoro-chloro-ethylene, termed
vide the bond therebetween.
herein “KeL-Ff’. as trade marked and manufactured by
In another embodiment of the invention there is pro
the M. W. Kellogg Company, is substantially superior to
such thermosetting resin material in that it has a very 80 vided a method of insulating a printed circuit with tri
?uoro-chloro-ethylene. The method comprises immers
low permeability to humidity as well as enhanced in
ing
a thin sheet of clean copper in an oxidizing bath
sulation resistance. It has been a great problem to ?nd
comprising a hot aqueous solution consisting essentially
a suitable means for causing Kel—F to adhere to other
of an alkali selected from the group consisting of sodium
systems.
hydroxide and potassium hydroxide and a chlorite selected
In my copending application ?led October 1, 1954, 35 from the group consisting of sodium chlorite and potas
Serial No. 459,841, a method is disclosed for causing
sium chlorite to provide the copper with a homogeneous
copper to adhere to Kel-F. In US. Patent No. 2,551,591
coating
of black cupric oxide. The oxidized copper sheet
issued to S. G. Foord, May 8, 1951, a method is dis
is placed in contact with an uncured sheet of phenolic
closed for bonding polyethylene to copper vby means of
resin laminate. A predetermined degree of heat and
a layer of cuprous oxide. Foord expressly omits the
pressure
is applied to the sheets to cure the resin and press
use of cupric oxide which is essential to the present in—
the copper and resin sheets together, forming thereby
vention.
an imprint of the cupric oxide in the resin. The cupric
It is an object of the present invention to provide an
oxide coated copper clad phenolic laminate is then cooled.
improved method of bonding thermosetting resins to
Cupric oxide coated copper is removed from the sheet
thermoplastic resins.
45 of phenolic laminate to expose the plurality of follicles
‘ It is a further object of the present invent-ion to pro
in the resin provided by the imprint. A sheet of tri
vide a method of bonding Kel-F to a phenolic resin
?uoro-chloro-ethylene is placed in contact with the im
laminate.
printed surface of the phenolic laminate. A sufficient
A still further object of the invention is to provide an
of heat and pressure is applied to the tri-?uoro
improved method for insulating a printed circuit with 50 degree
chloro-ethylene and phenolic laminate to force the tri
Kel-F.
.
a It is a further vobject of the invention to provide an im~
proved printed circuit insulated with Kel-F.
' Other and further objects of the invention will be ap
?uoro-chloro-ethylene into the follicles. The tri-?uoro
chloro~ethylene and phenolic laminate are then cooled
to provide the bond therebetween. .
parent from. the following description of preferred methods 55 Further, in accordance with the invention there is pro
vided, as an article of manufacture, an insulated printed
and embodiments thereof, taken in connection with the
circuit. The circuit comprises a sheet of phenolic resin
accompanying drawing.
lnaccordance with the invention there is presented a
paper laminate to the surface of which are af?xed thin
cupric oxide coated copper conductors. An insulating
method of bonding plastic resin having substantially
different flow characteristics relative to temperature. In 60 cover sheet of tri-?uoro-chloro-ethylene is bonded to the
laminate and the copper conductors with the conductors
accordance with the method a surface of a body of cop
held in insulated spaced relation between the sheets.
pet is oxidized to provide a coating of primarily black
In the accompanying drawing:
.
cupric oxide. The oxide coated copper surface is placed
Fig. 1 is a ?ow chart illustrating the preferred method
in contact with a surface of first plastic resin having a
?ow characteristic at a given temperature. The plastic 65 of the invention;
Fig. 2 is a sectional view of cupric oxide coated cop
‘and cooper are laminated together by means of‘a suf?cient
per clad phenolic resin;
degree of heat and pressure to form thereby an. imprint of
Fig. 3 is a plan View of a Kel-F-coated printed circuit;
the cupric oxide in the plastic. The laminated copper
Fig.
4 is a sectional view of the printed circuit taken
plastic is then cooled. The cupric oxide coated cooper is
along the lines ‘IV-IV in Fig. 3; and
removed from the plastic to expose the plurality of 70
Fig. 5 is a sectional view of a sheet of Kel-F bonded
randomly disposed follicles-in said plastic provided by. the
"imprint," A surface‘ of, a second plastic resin having a
to a thermosetting resin.
.
Referring now to the drawing and with particular refer
2,932,599
,
r
.
3
‘
XXXP from blistering, for example 5 pounds per square
ence to Fig. 1, the thin sheets of copper 1 are ?rst pre
inch pressure with the press 8;
(20) Baking at a temperature in excess of 215° C.
for 40 seconds to force the Kel-F into the exposed fol
licles in the resin provided by the imprint of the removed
pared in the following manner:
(1) Immersing the sheets of copper 1 in a mild alkaline
bath 2 such as Dy-Clene EW Metal Cleaner, as manufac
tured by MacDermid, Inc., Waterbury, Connecticut for
five seconds;
cupric oxide coated copper;
.
,
.
(21) Water cooling the printed circuit in the press;
' (2) Rinsing in cold, running water for ?ve seconds;
" (3) Dipping for 15 seconds in a 10 percent solution
and
>
(22) Removing the Kel~F insulated printed circuit
3 of hydrochloric acid (HCl) which has dissolved in it
from
the press.
'
10
8 ounces per gallon of ferric chloride (FeOl3);
in Fig. 3 a plan view of the insulated printed circuit
(4) Rinsing in cold, running water ‘for ?ve seconds;
is shown. The Kel-F ‘inside diameter is shown incom
(5) Immersing in a 10 percent solution 4 of sodium
plete to expose the copper 1 and cured thermosetting
cyanide (NaCN) for 15 seconds;
phenolic resin laminate 7. In the sectional'v'iew of Fig.
’ (6) Rinsing in cold, running water;
('7) Immersing in an oxidizing agent 5, such as a so
15
lution of 11/2 pounds per gallon of water of Ebonol “C”
Special as manufactured by Enthone Company, New
4, the cupric oxide, CuO, coating 11 is shown greatly
magni?ed.
'
'
"
It will be apparent that in place of the resin-impreg
nated paper, an epoxy resin or silicone resin may just
Haven, Connecticut, for 10 minutes at l90-205° F. to
as well be used. In the place of the impregnated paper,
provide a homogeneous coating of black cupric oxide
impregnated
glass cloth may be used to withstand higher
20
(CuO) on the copper surfaces. The Ebonol “C” Special
temperature operation.
'
‘
'
i
is substantially described in US. Patent #2,364,993 is
It has been found that polytetrailuoroethylene, fa
sued to ‘Walter R. Meyer;
miliarly termed “Te?on” as trade marked and manu
(8) Immersing in cold, running water;
(9) Rinsing in hot, running water for 10 to 20 sec
onds;
,
factured by E. I. du 'Pont de Nemours and Company,
25 may be utilized in place of the Kel~F at a press tem
p
(10) Baking in a preheated oven 6 above 212° F
until all traces of moisture are removed.
7 The sheets of cupric oxide coated copper are bonded
perature of approximately 625 ° F.
'
,
In Fig. 5 the cross-section of lamination of a thermo
plastic resin and a thermosetting resin is shown. Cured
or thermoset phenolic resin 7 is bonded to tri-?uoroe
to a thermosetting resin, for example, a laminate of
,
_
'
paper impregnated with a phenolic resin such as XXXP 30 chloro-ethylene 10 as illustrated.
The use of a thermoplastic overcoat vof the type de
Phenolic, in the following manner:
scribed to encapsulate printed circuits greatly decreases
(11) Placing a sheet of metallic foil, such as alu
'
the hygroscopic characteristics of commonly used ther
minum, on the platen of a press 8, such as manufactured
by Wabash Press Company, Wabash, Indiana. The
mosetting plastic materials by'cutting down their ex
XXXP-Phenolic and the platen;
(12) Placing, for example, six stacked sheets of un
cured XXXP-type phenolic resin impregnated paper,
each, for example, 6 inches long, 2 inches wide and
ings increase the resistivity between electrical conductors
aluminum foil is. used to prevent adherences between the 35 posed surface areas. Te?on or Kel-F insulating coat
on the surface of a printed circuit. The Te?on or Kel-F
insulating coating may be utilized as a resist mask for
plating,v and soldering purposes. Such coatings permit
sealing joints as between a baseplate and a printed circuit
.010 of an inch thick, on the aluminum foil to provide 40 cable,
whereby soldering and sealing can be accomplished
an uncured thermosetting laminate 7;
simultaneously. The present» invention naturally lends
(13) Placing the cupric oxide coated sheet of copper
itself to ?ush mounting of printed circuits.
While there has been hereinbefore describedwhat are
45 at present considered preferred embodiments of the in
sure;
vention, it will be apparent that many and various
, (l4) Baking at 300-3500 F. under pressure for 20'
changes and modi?cations may be made with respect
minutes to cure the resin and provide the cup'ric oxide
to the embodiments illustrated, without departing from
imprint in the resin of the top sheet of the laminate;
the spirit of the invention. It will be understood, ‘there
(15) Water cooling the copper and resin laminate
under maintained pressure in the press to prevent blister 60 fore, that all such changes andv modi?cations as fall
fairly within the scope of the present invention, as de
ing of the XXXP-resin;
t?ned in the appended claims, are to be considered as a
(l6) Removing the cupric oxide coated copper clad
part of the present invention.
resin laminate from the press 8 and the aluminum foil
in contact with the sheets of resin impregnated paper
and applying 300-1000 pounds per square inch of pres~
What is claimed is:
' from the laminate.
,
l. The method of bonding a thermoplastic resin to a
I The XXXP-Phenolic may, of course, be copper clad 55 thermosetting resin comprising, oxidizing the surfaces
on both sides merely by placing sheets of copper both
of a thin sheet of copper .to provide said surfaces with
above and below the sheets of impregnated paper. The
cup'ric oxide coated copper clad phenolic sheet is shown '
in crosssection in Fig. 2. The cupric oxide shown is
greatly‘ magni?ed to more clearly illustrate its imprint
upon the phenolic resin. The cupric oxide coated cop
per clad phenolic laminate is then prepared to provide‘
_ a‘ printed circuit in the desired con?guration and im
a homogeneous coating of black cupric oxide; placing
said cupric oxide coated copper sheet in contact with
an uncured sheet of thermosetting resin; applying a suf
ticient degree of heat and pressure to cure'said thermo
setting resin and press said copper and said thermosetting
resin sheets together, forming thereby an imprint. of said
mersed, for'example, in an acid etching bath 9 in ac
cupric oxide in said thermosetting resin; cooling said
cupric oxide coated copper cladresin sheet; removing
cordance with well-known, conventional photoetching
'cupric oxide coated copper from said resin .sheet to
techniques.’
’
.
~
‘
Theprinted circuit thus prepared is covered with an
insulating sheet 10 of Kel-F in the following manner:
expose the plurality of randomly disposed follicles in
said resin provided by said imprint; placing said thermo
plastic resin in contact with said imprinted surface; and
(17) Placing the printed circuit'on the platen of the 70 applying a sufficient degree of heat and pressure to said
thermoplastic and thermosetting resins to 'force said ther
moplastic resin into said follicles to provide said bond
press 8.;
_ (18) Placing a sheet of Kel-F, for example, 6, inches
long, 2 inches Wide and .002 of an inch thick, over the
therebetween.
'
g
.
t
2. The method of bonding tri-?uoro-chloro-ethylene
‘ (19) Applying su?icient pressure to prevent the 75 to aphenolic-resinilaminate comprising, immersing athin
printed circuit;
5
2,982,599
sheet of clean copper in an oxidizing bath to provide
the copper with a homogeneous coating of black cupric
oxide; placing said oxidized copper sheet in contact with
an uncured sheet of phenolic resin laminate; applying a
predetermined degree of heat and pressure to cure said
phenolic resin and press said copper and phenolic laminate
6
nate; applying a predetermined degree of heat and pressure
to cure said resin and press said copper and resin sheets
together, forming thereby an imprint of said cupric oxide
in said resin; cooling said cupric oxide coated copper clad
phenolic laminate; removing selected areas of cupric oxide
coated copper from said sheet of phenolic laminate to
sheet together, forming thereby an imprint of said cupric
oxide in said laminate; cooling said cupric oxide coated
copper clad phenolic laminate; removing cupric oxide
provide a predetermined conductive pattern and expose
phenolic laminate to force said tri-?uoro-chloro-ethylene
into said follicles; and cooling said t-ri-?uoro-ehloro
ethylene and phenolic laminate to provide said bond
chloro-ethylene and phenolic laminate to provide said
bond therebetween.
6. The method of bonding plastic resins having sub~
stantially different flow characteristics relative to tempera
the plurality of follicles in said phenolic laminate sheet
coated copper from said sheet of phenolic laminate to 10 provided by the exposed areas of said imprint; placing tri
‘ uoro-chloro-ethylene in contact with said imprinted sur
expose the plurality of follicles in said phenolic laminate
face of said phenolic laminate; applying a su?icient degree
sheet provided by said imprint; placing tri-?uoro-chloro
of heat and pressure to said tri-?uoro~chloro-ethylene
ethylene in contact with said imprinted surface of said
and said phenolic laminate to force said tri-?uoro-chloro
phenolic laminate; applying a su?icient degree of heat
and pressure to said tri-?uoro-chloro-ethylene and said 16 ethylene into said follicles; and cooling said tri-?uoro
therebetween.
3. The method of insulating a printed circuit with tri 20 ture, comprising: oxidizing a surface of a body of copper
to provide a coating of primarily black cupric oxide;
placing said cupric-oxide-coated-copper surface in contact
of clean copper in an oxidizing bath to provide the copper
?uoro-chloro-ethylene comprising, immersing a thin sheet
with a surface of a ?rst plastic resin having a ?ow charac
teristic at a given temperature; applying a su?icient
sheet of phenolic resin laminate; applying a predeter 25 degree of heat and pressure to laminate said plastic and
copper together, forming thereby an imprint of said cupric
mined degree of heat and pressure to cure said resin
oxide in said plastic; cooling said laminated copper plastic;
and press said copper and resin sheets together, forming
removing said cupric-oxide-coated-copper from said plas
thereby an imprint of said cupric oxide in said resin;
tic to expose the plurality of randomly disposed follicles
cooling said cupric oxide coated copper clad phenolic
with a homogeneous coating of black cupric oxide; placing
said oxidized copper sheet in contact with an uncured
laminate; removing selected areas of cupric oxide coated 30 in said plastic provided by said imprint; placing a surface
of a second plastic resin having a greater tendency to
copper from said sheet of phenolic laminate to provide
?ow at a temperature substantially less than said given
a predetermined conductive pattern and expose the
temperature in contact with said imprinted surface; and
plurality of follicles in said phenolic laminate sheet
pressing said plastic resins together at a temperature
formed by the exposed areas of said imprint; placing
tri-?uoro-chloro-ethylene in contact with said, imprinted 35 substantially less than said ?rst given temperature to
force said second plastic resin into said follicles to provide
surface of said phenolic laminate; applying a sufficient
said bond therebetween. ‘
degree of heat and pressure to said tri-?uoro-chloro
7. The method of bonding tri-?uoro-chloro-ethylene
ethylene and said phenolic laminate to force said tri
to a thermosetting resin laminate comprising, immersing
?uoro-chloro-ethylene into said follicles; and cooling said
tri-?uoro-chloro-ethylene and phenolic laminate to provide 40- a thin sheet of clean copper in an oxidizing bath to
thermosetting resin comprising, immersing a thin sheet
provide the copper with a homogeneous coating of black
cupric oxide; placing said oxidized copper sheet in contact
with an uncured sheet of thermosetting resin laminate;
of clean copper in an oxidizing bath comprising a hot
aqueous solution consisting essentially of an alkali selected
of heat and pressure to cure said resin and press said
said bond therebetween.
4. The method of bonding a thermoplastic resin to a
from the group consisting of sodium hydroxide and
potassium hydroxide and a chlorite selected from the
group consisting of sodium chlorite and potassium chlorite
to provide said surfaces with a homogeneous coating of
applying resin laminate; applying a predetermined degree
copper and laminate sheets together, forming thereby an
imprint of said cupric oxide in said laminate; cooling
said cupric oxide coated copper clad laminate; removing
cupric oxide coated copper from said sheet of laminate
to expose the plurality of follicles in said laminate sheet
black cupric oxide; placing said cupric oxide coated 50
provided by said imprint; placing tri-?uoro-chloro-ethylene
copper sheet in contact with an uncured sheet of thermo
in contact with said imprinted surface of said laminate;
setting resin; applying a su?icient degree of heat and
pressure to said thermosetting resin and press said copper
applying a sufficient degree of heat and pressure to said
and thermosetting resin sheets together, forming thereby
tri-?uoro-chloro-ethylene and said laminate to force said
said thermoplastic and thermosetting resins to provide
mined degree of heat and pressure to cure said phenolic
tri-?uoro-chloro-ethylene into said follicles; and cooling
an imprint of said cupric oxide in said thermosetting
said tri-?uoro-chloro-ethylene and laminate to provide
resin; cooling said cupric oxide coated copper clad resin
said bond therebetween.
sheet; removing cupric oxide coated copper from said
8. The method of bonding a thermoplastic resin to a
resin sheet to expose the plurality of randomly disposed
phenolic resin laminate comprising, immersing a thin sheet
follicles in said resin provided by said imprint; placing
said thermoplastic resin in contact with said imprinted iii) of clean copper in an oxidizing bath to provide the copper
with a homogeneous coating of black cupric oxide; placing
surface; applying a su?‘icient degree of heat and pressure
said oxidized copper sheet in contact with an uncured
to said thermoplastic and thermosetting resins to force
sheet of phenolic resin laminate; applying a predeter
said thermoplastic resin into said follicles; and cooling
said bond therebetween.
65 resin and press said copper and phenolic laminate sheets
together, forming thereby an imprint of said cupric oxide
5. The method of insulating a printed circuit with
tri-?uoro~chloro-ethylene comprising, immersing a thin
in said laminate; cooling said cupric oxide coated copper
sheet of clean copper in an oxidizing bath comprising a
hot aqueous solution consisting essentially of an alkali
clad phenolic laminate; removing cupric oxide coated
copper from said sheet of phenolic laminate to expose
selected from the group consisting of sodium hydroxide 70 the plurality of follicles in said phenolic laminate sheet
and potassium hydroxide and a chlorite selected from
provided by said imprint; placing a thermoplastic resin
the group consisting of sodium chlorite and potassium
in contact with said imprinted surface of said phenolic
chlorite to provide the copper with a homogeneous coating
laminate; applying a sufficient degree of heat and pressure
of black cupric oxide; placing said oxidized copper sheet
to
said thermoplastic resin and said phenolic laminate to
in contact with an uncured sheet of phenolic resin lami TI
force said thermoplastic resin into said follicles; and
2,982,599
circling said. thermoplastic resin and phenolic laminate
tolprovide said bond therebetween.
,
9. The method of bonding poly-tetra-?uoro-ethylene
to a phenolic resin laminate comprising, immersing a
thin sheet of clean copper in an oxidizing bath to provide
the copper with a homogeneous coating of black cupric'
8
copper with‘ a ‘homogeneous coating of black cnpri‘c
oxide; placing said: oxidized copper sheet in contact with
an uncured sheet'of epoxy resin laminate; applying a
predetermined degree of heat and pressure to cure said
epoxy resin and press‘ said copper and epoxy laminate
sheets together, forming thereby an imprint of said cupr-ic
oxide in said‘ laminate; cooling said cupric oxide coated
oxide; placing said- oxidized copper sheet'in contact with;
copper‘ clad epoxy laminate; removing cupricjoxide
an uncured sheet of phenolic resin laminate; applying a
coated copper from said sheet of epoxy laminate to ex
predetermined degree of‘heat and pressure to cure said
pose the plurality of follicles in said epoxy laminate sheet
10
phenolic resin and press said copper and‘ phenolic lami
provided by said imprint; placing tri-flnoro-chloro~ethyl
nate sheets together, forming thereby an imprint ‘of said
ene in contactwith said imprinted surface of said epoxy
cupric oxide in said laminate; cooling said: cupric oxide
laminate; applying a su?icient degree of heat and pres
coated copper clad phenolic laminate; removing cnpric
sure to said tri-?noro-ohloro-ethylene and said epoxy
oxide coated copper from said sheet of phenolic laminate
laminate to force said tri’?uoro-chloro-ethylene into said
to expose the plurality of follicles in said phenolic lami 15 follicles; and cooling, said tri—?uoro-ehloro-ethylene and
nate sheet provided by said imprint; placing poly'tetra-v
epoxy laminatepto provide said bond therebetween._
?uoro-ethylene in contact with said imprinted surface of
said phenolic laminate; applying a su?icient degree of
ReferencesCited in the ?le of this patent
heat and pressure to said poly-tetra-?uoro-ethylene ‘and
20
_
UNITED, STATES PATENTS
said phenolic laminate to force said poly-tetra-?uoro~
ethylene into said follicles; and cooling said poly-tetra
'Daly __U_____-_ ________ __ Apr. 10, 1934
fluoro-ethylene and phenolic laminate to provide said
bond
therebetween.
10. The method of bonding tri-v?uoro-chloroethylene
to an epoxy resin laminate comprising, immersing a thin 25
sheet of clean copper in an oxidizing bath to provide the
1,954,403
'
’
‘
2,551,591
.FOOl'd __‘___________ __'__ May 8, 1951
2,728,698
Rudner _____________ _._ Dec. 27, 1955
2,745,898
Hplllfd ______________ __'. May 15, 1956
2,768,923
Kepple et al. _' _________ __ Oct. 30, 1956