U.S. patent number RE32,319 [Application Number 06/799,479] was granted by the patent office on 1986-12-30 for vitreous carbon and process for preparation thereof.
This patent grant is currently assigned to Plastics Engineering Company. Invention is credited to Louis L. Korb, Phillip A. Waitkus.
United States Patent |
RE32,319 |
Korb , et al. |
December 30, 1986 |
Vitreous carbon and process for preparation thereof
Abstract
The vitreous carbon disclosed herein is prepared from intimate
uniform mixtures of powder or otherwise blendable form of 20-80% by
weight of a solid phenolic-aldehyde Novolak resin and of 20-80% by
weight of a solid phenolic-aldehyde resol resin, the percentages
being based on the combined weight of the Novolak and resol resins,
and the aldehyde in said Novolak resin comprising at least 50 molar
percent, preferably substantially 100 percent, furfuraldehyde,
together with a carbonaceous filler, preferably graphite, in a
proportion as high as 76% by weight based on the total composition.
Generally the graphite may comprise 30-70%, advantageously 35-65%
and preferably 40-60% of the molding condition. The vitreous carbon
is improved in electrical properties and in the capability of being
shaped into large thin plates which are much more stress-free than
otherwise produced. The intimate mixture of the resins used for
this preparation is advantageously made by blending resin
components having a particle size of less than 40 mesh, preferably
less than 100 mesh.
Inventors: |
Korb; Louis L. (Sheboygan,
WI), Waitkus; Phillip A. (Sheboygan, WI) |
Assignee: |
Plastics Engineering Company
(Sheboygan, WI)
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Family
ID: |
27413410 |
Appl.
No.: |
06/799,479 |
Filed: |
November 19, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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477235 |
Mar 21, 1983 |
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397497 |
Jul 12, 1982 |
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220361 |
Dec 19, 1980 |
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50531 |
Jun 21, 1979 |
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Reissue of: |
599737 |
Apr 12, 1984 |
04550015 |
Oct 29, 1985 |
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Current U.S.
Class: |
423/445R;
264/29.1; 264/29.7; 501/99; 525/501 |
Current CPC
Class: |
C04B
35/524 (20130101) |
Current International
Class: |
C04B
35/524 (20060101); C01B 031/00 () |
Field of
Search: |
;423/445,447.1,447.2,449
;264/29.1,29.2,29.3,29.4,29.5,29.7 ;501/99 ;525/501 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1281514 |
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Dec 1960 |
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FR |
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623271 |
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May 1949 |
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GB |
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1098029 |
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Jan 1968 |
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GB |
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Primary Examiner: Dixon, Jr.; William R.
Assistant Examiner: Capella; Steven
Attorney, Agent or Firm: Monacelli; Walter J.
Parent Case Text
This is an application for reissue of U.S. Pat. No. 4,550,015
issued on Oct. 29, 1985 based on application Ser. No. 599,737,
filed Apr. 12, 1984.
This application is a continuation-in-part of copending application
Ser. No. 477,235, filed Mar. 21, 1983, now abandoned which in turn
is a continuation-in-part of application Ser. No. 397,497 filed
July 12, 1982, now abandoned, which is a continuation of
application Ser. No. 220,361 filed Dec. 19, 1980, now abandoned,
which in turn is a continuation-in-part of application Ser. No.
50,531 filed June 21, 1979, now abandoned.
Claims
The invention claimed is:
1. A process for preparing a vitreous carbon from a solid
thermosettable intimate blend comprising 30-76 percent by weight of
a finely divided carbonaceous particulate filler and 24-70 percent
by weight of a finely divided phenolic resin mixture comprising
20-80 parts by weight of a phenolic-furfuraldehyde Novolak resin,
which is fusible upon heating, and 20-80 parts by weight of a
phenolic-aldehyde resol resin, which is thermosettable upon
heating, the sum of said parts of Novolak and resol resins totaling
100 parts by weight, the aldehyde in said Novolak comprising at
least 50 molar percent of furfuraldehyde, comprising the steps
of:
(1) grinding said phenolic furfuraldehyde Novolak resin to a
particle size of less than 40 mesh on a U.S. Standard sieve;
(2) grinding said phenolic-aldehyde resol resin to a particle size
of less than 40 mesh on a U.S. Standard sieve;
(3) mixing said ground resins together with said finely divided
carbonaceous powder to a homogeneous mixture;
(4) molding the resultant mixture at a temperature of
100.degree.-180.degree. C. and a pressure between 500 pounds per
square inch and 8 tons per square inch; and
(5) heating the resultant molded product gradually up to a
temperature of 600.degree.-700.degree. C. with the temperature
increased at a rate of 1.degree.-5.degree. C. per hour, then above
the range of 600.degree.-700.degree. C. at an increasing rate of
10.degree.-50.degree. C. per hour up to 800.degree.-850.degree. C.
and thereafter at 20.degree.-25.degree. C. per hour up to a maximum
temperature of 1800.degree.-3000.degree. C., which maximum
temperature is held for at least 24 hours.
2. The process of claim 1, in which after being held at maximum
temperature for at least 24 hours the temperature is gradually
dropped at a rate of 10.degree.-20.degree. C. per hour.
3. The process of claim 2 in which both said resins are ground to a
particle size not exceeding 32 microns in diameter.
4. The process of claim 1 in which said mixture of resin and
carbonaceous powder is further pulverized to a finer size, then
densified and reground to a particle size suitable for molding.
5. The process of claim 4, in which said step of mixing said ground
resins is performed in an extruder or on heated rolls.
6. The process of claim 5 in which said step (2) is effected in an
air mill, impact grinder or ball mill.
7. The process of claim 6 in which said step (3) is effected on a
differential 2-roll heated mill.
8. The process of claim 7, in which the front roll of said mill is
maintained at 220.degree.-230.degree. F. and the back roll at
80.degree.-100.degree. F.
9. The process of claim 4, in which said steps (1) and (2) are
effected to produce a particle size of less than 100 mesh for both
said Novolak and said resol resins.
10. The process of claim 4, in which said step (3) is effected to
produce a homogeneous blend of particles having a size no greater
than 32 microns.
11. The process of claim 4 in which said molding is effected at a
temperature of 149.degree.-166.degree. C.
12. A process for preparing a shaped molded object from an intimate
blend of a finely divided carbonaceous powder and a mixture of a
phenolic-furfuraldehyde Novolak resin, the aldehyde in said Novolak
comprising at least 50 molar percent of furfuraldehyde, and a
phenolic-aldehyde resol resin comprising the steps of:
(1) grinding the solid phenolic furfuraldehyde Novolak resin to a
particle size of less than 40 mesh on a U.S. Standard sieve;
(2) grinding the solid phenolic-aldehyde resol resin to a particle
size of less than 40 mesh on a U.S. Standard sieve;
(3) mixing said ground resins together with said finely divided
carbonaceous powder to a homogeneous mixture suitable for molding,
said carbonaceous powder being used in a proportion of 30-76
percent based on total weight of carbonaceous powder and resin
powders, and said Novolak and resol powders being used in a
proportion of 20-80 percent by weight of said Novolak and 20-80
parts by weight of said resol, the combined weight of said Novolak
and said resol totalling 100 parts by weight; and
(4) molding the resultant mixture at a temperature of
100.degree.-180.degree. C. and a pressure between 500 pounds per
square inch and 8 tons per square inch.
13. A thermoset resin shaped object produced according to the
process of claim 12.
14. The process of claim 12 in which both said resins are ground to
a particle size not exceeding 32 microns in diameter.
15. The process of claim 12 in which said mixture of resin and
carbonaceous powder is further pulverized to a finer size, then
densified and reground to a particle size suitable for molding.
16. The process of claim 15, in which said step of mixing said
ground resins is performed in an extruder or on heated rolls.
17. The process of claim 16 in which said resol resin grinding is
effected in an air mill, impact grinder or ball mill.
18. The process of claim 17 in which said step mixing is effected
on a differential 2-roll heated mill.
19. The process of claim 18 in which the front roll of said mill is
maintained at 220.degree.-230.degree. F. and the back roll at
80.degree.-100.degree. F.
20. The process of claim 15 in which said resin grinding steps are
effected to produce a particle size of less than 100 mesh for both
said Novolak and said resol reins.
21. The process of claim 15 in which said mixing step is effected
to produce a homogeneous blend of particles having a size no
greater than 32 microns.
22. The process of claim 15 in which said molding is effected at a
temperature of 149.degree.-166.degree. C.
23. A thermoset resin shaped object prepared according to the
process of claim .[.12.]. .Iadd.15.Iaddend..
24. A shaped object of claim .[.23.]. .Iadd.13 .Iaddend.comprising
a thin plate having a thickness of 0.04-0.05 inch.
25. A shaped object of claim 24 having other dimensions up to 50
inches by 50 inches. .Iadd.26. The process of any one of claims 1
through 12 and 14 through 22 in which the phenolic component of
said Novolak resin is phenol. .Iaddend. .Iadd.27. The process of
any one of claims 1 through 12 and 14 through 22 in which the
phenolic component of said resol is phenol. .Iaddend. .Iadd.28. The
process of any one of claims 1 through 12 and 14 through 22 in
which the aldehyde component of said resol resin is
formaldehyde. .Iaddend. .Iadd.29. The process of any one of claims
1 through 12 and 14 through 22 in which the aldehyde in said
Novolak is at least 75 molar percent furfuraldehyde. .Iaddend.
.Iadd.30. The process of any one of claims 1 through 12 and 14
through 22 in which the phenolic component of said Novolak is
phenol, the phenolic component of said resol resin is phenol, the
aldehyde component of said resol resin is formaldehyde, and the
aldehyde in said Novolak is at least 75 molar percent
furfuraldehyde. .Iaddend. .Iadd.31. The shaped object of claim 13
in which the phenolic component of said Novolak is phenol, the
phenolic component of said resol resin is phenol, the aldehyde
component of said resol resin is formaldehyde, and the aldehyde in
said Novolak is at least 75 molar percent furfuraldehyde. .Iaddend.
.Iadd.32. The shaped object of claim 23 in which the phenolic
component of said Novolak is phenol, the phenolic component of said
resol resin is phenol, the aldehyde component of said resol resin
is formaldehyde, and the aldehyde in said Novolak is
substantially 100 percent furfuraldehyde. .Iaddend. .Iadd.33. The
shaped object of claim 24 in which the phenolic component of said
Novalak is phenol, the phenolic component of said resol resin is
phenol, the aldehyde component of said resol resin is formaldehyde,
and the aldehyde in said Novalak is substantially 100 percent
furfuraldehyde. .Iaddend. .Iadd.34. The shaped object of claim 25
in which the phenolic component of said Novolak is phenol, the
phenolic component of said resol resin is phenol, the aldehyde
component of said resol resin is formaldehyde, and the aldehyde in
said Novolak is substantially 100 percent furfuraldehyde. .Iaddend.
.Iadd.35. The shaped object of claim 31 in which the graphite
proportion is 40-60 percent by weight based on the total resin
composition. .Iaddend. .Iadd.36. The shaped object of claim 35 in
which there is the aldehyde in said Novalak resin is substantially
100 percent furfuraldehyde. .Iaddend. .Iadd.37. The shaped object
of claim 36 in which the shaped object comprises a thin plate
having a thickness of 0.04-0.05 inch. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to new compositions comprising a mixture of
a Novolak resin and a "resol" resin. More specifically, it relates
to such mixtures in intimate, uniformly blended form which are
capable, upon heating, of thermosetting and eventually, under
appropriate carburizing conditions, of forming vitreous carbon.
2. State of the Prior Art
Vitreous carbon, as normally prepared, is at least 99.9% pure,
having an ash content of 200 PPM or less. Thus, with its low
permeability, negligible porosity and low specific surface,
vitreous carbon carbon is very inert. Its resistance to
hydrochloride, hydrofluoric, nitric, sulfuric and chromic acids and
to mixtures of nitric acid with oxidizing agents is much better
than with non-vitreous carbon. The rate of attack on vitreous
carbon by molten zinc, lead, tin, phosphorus, silver, arsenic,
etc., is very low.
The extreme inertness, impermeability and non-porosity qualifies
vitreous carbon as an important and useful material of construction
for use in a number of applications and in various industries. For
research and development work this material has been used in
fabricating beakers, basins, boats, reaction tubes, etc. and for
extensive use in the processing of semiconductors, fluoride laser
materials, zone refining of metals, zone refining of chemicals,
biomedical applications, fuel cell electrodes, etc.
However, industrial applications of vitreous carbon have been made
only in recent years. Since vitreous carbon is not wet by a wide
range of metals including zinc, silver, copper, tin, lead,
aluminum, gold, platinum and others, it has found application in
the processing of some of these metals and their alloys, for
example, in the dehydrogenation of molten aluminum with chlorine
gas. Dip pipes of this material for corrosive liquids have also
been successfully used.
The growth of industrial applications for this material has been
restricted in large measure by the inability to produce large and
properly cursed small moldings or extrusions in the required shapes
and by use of conventional molds or dies at conventional rates and
reasonable cost. There has been difficulty in making large thin
plates suitable for use in fuel cells.
In the parent application, Ser. No. 397,497, the Patent and
Trademark Office has relied on the following references: Redfern
U.S. Pat. No. 3,109,712; Appleby et al U.S. Pat. No. 3,626,042;
British Pat. Nos. 623,271, 1,020,441, 1,098,029 and 1,330,296;
Japanese Pat. No. 54-20991; Grazen et al U.S. Pat. No. 3,879,338;
Rice et al U.S. Pat. No. 3,927,140; and French Pat. No. 1,281,514.
However, none of these references show the superiority of a
phenol-furfuraldehyde Novolak in admixture with a
phenol-formaldehyde resol for the preparation of vitreous carbon,
particularly when there is a filler such as graphite in the mixture
which makes it more urgent that the resin mixture has superior flow
properties so as to avoid the strains and stresses that otherwise
occur in the molded and vitreous carbon products. As pointed out
hereinafter, the plasticity and flow properties contributed by the
phenol-formaldehyde Novolak-phenol-aldehyde resol combination,
particularly with a substantial amount of graphite filler, is not
attainable by a phenol-formaldehyde Novolak combination with a
phenol-aldehyde resol, nor by any such individual resin.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been found that
vitreous carbon can be made in large, thin plates as well as
various intricately shaped articles by starting with an initial
composition of 20-80 percent by weight of a solid phenolic-aldehyde
Novolak resin and 20-80 percent by weight of a solid
phenolic-aldehyde resol resin, the percentages being based on the
combined weight of the two resins, and the aldehyde in said Novolak
resin comprising at least 50 molar percent, preferably
substantially 100 percent furfuraldehyde, together with a finely
divided carbonaceous filler, preferably graphite, in a proportion
as high as 76 percent by weight based on the total composition.
Generally, the graphite may comprise 30-70 percent, advantageously
35-65 percent and preferably 40-60 percent of the molding
composition. Although the two resins may be cured together with no
added curing agent, it is very often advantageous to have a small
amount of a curing agent such as hexamethylenetetramine (hexa)
present. Thus the composition may contain 0-12 parts, preferably
0.12-8 parts of the hexa per 100 parts of Novolak resin. Another
important factor is that the solid resins have an initial fine
particle size to permit intimate and uniform mixture of blending of
the two resins. The particle size is advantageously less than 40
mesh, preferably no greater than 100 mesh and most preferably no
greater than 32 microns in size. (Mesh sizes are measured on U.S.
Standard sieves and screen analyses are performed according to ASTM
Method D1921-63.)
A "resol" resin is the resinous reaction product of a phenol and an
aldehyde which has been condensed (reacted) only to a stage where
it still melts when heated and is still soluble in acetone, and the
resin still has sufficient residual reactivity that it may be cured
by heat without the addition of a curing agent to an insoluble and
infusible condition. A resol resin is also known as an "A" stage
phenolic resin, or also as a "single stage" resin, because it is
curable without the addition of any crosslinking agent. Upon
progressing from the "resol" or "A stage" resin by heating, an
intermediate stage is reached before the final insoluble, infusible
cured condition is reached. This intermediate stage, or "B stage"
resin is termed a "resitol".
A "resitol" is a resin of the same type as the "resol" except that
the aldehyde-phenolic condensation has been carried out to the
stage where it has become infusible but upon heating, will
decidedly soften, but not melt. The "resitol" swells in acetone but
is insoluble in it. When heated, a resitol distorts substantially
and when a formed or shaped resitol piece is ejected from a mold,
the lack of dimensional stability makes the product practically
useless.
The "resol" and "resitol" resins are prepared by using the aldehyde
in a molar proportion greater than 1--1 with the phenol. Since
sufficient aldehyde is already present to give a cure to the
insoluble infusible state, there is no need to add a curing agent
such as hexa for final curing. However, in preparing the resol
resin it may be desirable to add a small amount of hexa to obtain a
harder and more easily grindable resol. For example, 0.005 to 0.03
mole, preferably 0.01 mole of hexa per mole of phenol is
advantageous for this purpose. In any case the amount of hexa is
small enough that the resol retains its properties of fusibility
and acetone solubility and is fused only by continued heating.
However the amount of hexa used in preparing the resol is not
calculated in the amount which may be subsequently added to aid in
the curing of the Novolak-resol mixture.
When the resol is difficult to grind it is possible to use larger
particles of the resol with finely divided Novolak and to extend
the milling or subsequent blending step so that the Novolak is
worked into intimate contact with the resol and the resol is
comminuted by the milling or blending operation into an intimate
blend of the two resins.
The final, or "C stage" of a phenolic resin is characterized by
insolubility, lack of swelling in acetone, infusibility and freedom
from softening upon heating. A "C stage" phenolic-aldehyde resin is
also known as a "resite".
In contrast, a "Novolak" resin is one prepared with a deficiency in
aldehyde so that it may not be cured unless a curing agent such as
hexa is added. Therefore, a "Novolak" resin may be defined as the
resinous reaction product of a phenol and an aldehyde that, for all
practical purposes, does not harden or convert to an insoluble,
infusible condition upon heating but remains soluble and fusible.
(See "The Chemistry of Synthetic Resins" by Carleton Ellis, Vol. 1,
page 315, Reinhold Publishing Co., New York, N.Y. 1935.)
In curing a Novolak resin, a substantial amount of a curing agent
is used such as hexa to overcome the deficiency of
aldehyde-bridging groups. This added curing agent may be an
aldehyde such as formaldehyde or an alkylene-providing compound,
such as hexamethylenetetramine, which provides methylene groups for
curing. However when a substantial amount of such a curing agent is
used in preparing a cured Novolak resin for ultimate vitreous
carbon formation, there is generally sufficient by-product gas
formed or retained so that in the later stages of the resin
processing, molded-in stresses may be formed. In any case, the
products do not have the properties desired.
By the composition and process of this invention it has been found
possible to prepare resins for ultimate formation of vitreous
carbon of very good properties by virtue of the fact that at least
most or substantially all of the final bridging between phenolic
groups is effected through methylol groups present in the resol
resin. Thus the necessity for adding a curing agent to provide
bridging groups for the Novolak molecules is eliminated or reduced.
In any case, the amount of curing agent required to produce the
desired curing is reduced substantially to the extent that the
problem of stresses and strains and accompanying weaknesses in the
vitreous carbon have been avoided or reduced to a tolerable amount.
Thus the carbonizable phenolic resins produced from the composition
of this invention is the coreaction product of a Novolak resin and
a resol resin.
For most purposes for which the vitreous carbons of this invention
are to be applied, those prepared from phenol-furfur-aldehyde
Novolaks are preferred. It is believed that the Novolak resin has a
plasticizing effect on the resol resin, thereby allowing the
viscous mass to flow more uniformly and readily at lower pressure
to completely fill the mold before a high degree of gelation and
crosslinking occurs. This contrasts with the initially faster
curing rates of the single stage resins (resols) which create large
initial portions of gelled or crosslinked polymer molecules,
thereby also forming local strained areas in the molded part. This
plasticizing effect is believed to allow the production of molded
shapes and forms which are substantially free of molded-in stress.
This allows the formed product to be ejected from a hot mold
without distortion or deformation. Moreover, when the molded
product is carburized to vitreous carbon, this strain-free
condition carries over into the final product.
The results obtained by molding the compositions of this invention
also contrast with the results obtained in molding a mixture of
Novolak and hexa. To provide a cure in a reasonable time schedule,
it is generally necessary to use between eight and twenty or more
parts of hexa per one hundred parts of Novolak. This amount of hexa
tends to soften and plasticize the curing and cured resin. This,
together with the by-product nitrogenous gases introduce molded-in
stresses and strains in the formed product which cause distortions
in the product after ejection from the mold or when subjected to
carbonization temperatures.
The reduction in strains in products molded and carbonized from the
compositions of this invention can be demonstrated by transmitted
polarized light through products molded with unfilled, clear
specimens as described more fully hereinafter.
Novolak and resol resins may be prepared by the condensation of a
large variety of phenols and aldehydes as described in the
above-mentioned "The Chemistry of Synthetic Resins". The author
describes numerous phenolic-aldehyde resins in Chapters 13-18,
modified phenol-aldehyde resins in Chapter 19, and modified
phenol-formaldehyde resins in Chapter 20.
Typical examples of the phenols which may be used are: phenol
itself and its various homologs such as meta-cresol; the various
xylenols, hydroquinone; pyrogallol; resorcinol; the halogenated
derivatives of phenol which leave two or more positions available
for condensation with the aldehyde, such as p-chlorophenol,
p-bromophenol, p-fluorophenol, etc.; the various naphthols, the
various hydroxy-benzoic acid esters; p,p'-dihydroxydiphenylmethane;
p,p'-dihydroxydiphenyl-2,2'-diphenylpropane, etc. Ortho- and
para-cresol may also be used when mixed with another phenol, such
as metal-phenol, which has three available reactive positions such
as ortho and para to permit crosslinking. For economic and
availability reasons, phenol itself is preferred.
Similarly, a wide variety of aldehydes may be used in preparing the
resol resins. Typical examples are formaldehyde, paraformaldehyde,
acetaldehyde, butyraldehyde, glyoxal, acrolein, benzaldehyde,
terephthaldehyde, etc. Again, for reasons of cost and availability
and for ease in processing, formaldehyde is preferred. The term
"aldehyde" is intended to include not only aldehydes per se, that
is, compounds containing the --CHO group, but also compounds which,
under reaction conditions, can engender an aldehyde or provide the
same type of alkylene group for bridging as provided by the
aldehyde. For example, hexamethylenetetramine provides methylene
bridging groups and acetylene, under appropriate conditions with
phenol, produces resins similar to those produced from phenol and
acetaldehyde. When less than 100% furfuraldehyde is used to make
the Novolak, the other aldehyde used may be selected from those
listed above for the resol resins.
The appropriate molar ratio of aldehyde to phenol for preparing the
Novolak and for preparing the resol respectively depends upon the
nature of the aldehyde and the nature of the phenol and the
conditions under which they are reacted. A generalization can be
made, however, when the phenolic compound is phenol per se and the
aldehyde is formaldehyde or furfuraldehyde. For example,
phenol-formaldehyde Novolaks are usually prepared using aqueous
formaldehyde under acidic conditions due to the formic acids
present in the formaldehyde solution or by the addition of acids
and/or salts to establish acid conditions. Under these conditions,
a formaldehyde-phenol molar ratio of 0.55/1 to 0.95/1, preferably
0.7-0.88/1, in the original reaction mixture, together with a pH of
0.8 to 5.5, will produce a Novolak resin, that is a resin which,
when isolated and heated, will not cure. The formaldehyde may be
added all at once or in two or more stages.
When furfuraldehyde (furfural) is used as the condensing aldehyde
the proportion is advantageously in the range of about 0.6-0.9,
preferably about 0.70-0.75 mole per mole of phenol. Here again the
furfural may be added all at once or in two or more stages.
However, if acrolein is used as the aldehyde, gelation and
crosslinking can occur when 0.5 mole of acrolein is used per mole
of phenol. This is because the vinyl group in the acrolein also
reacts to cause crosslinking. Moreover, larger amounts of
formaldehyde than indicated above can be used when mixtures of
ortho- and meta-cresol or ortho- and para-cresol are used.
The primary requirement is that the Novolak conforms to the
accepted definition of a Novolak, namely that the phenolic-aldehyde
resin will not cure merely upon heating and therefore for all
practical purposes, is not heat-convertible to an insoluble,
infusible product.
A further requirement of the Novolak is that it must be capable of
conversion to an insoluble, infusible product by heat reaction with
added amounts of an aldehyde, such as formaldehyde or an aldehyde
reactive type of compound such as hexa. (See Ellis as cited above,
p. 327.) Thus the Novolak may be tested by the addition of 10% by
weight of hexa and heated. A true Novolak will be cured to an
insoluble, infusible resin. If the material is not so cured upon
testing, an additional amount of hexa may be added and the test
repeated. If still no cure, the material is definitely not a
Novolak. For use in the practice of this invention, the Novolak
must meet this curing test to insure that it will be capable of
curing with the resol to an insoluble, infusible state.
Thus in the practice of this invention part or all of the
crosslinking bridges of the Novolak are formed by the resol. Some
hexamethylenetetramine may be used to supplement the crosslinking
bridges formed by the resol. Advantageously a small amount of hexa
is used to assist in a faster cure. However the amount of hexa may
be an amount that will produce the desired effect but still be low
enough to avoid the disadvantages described above. As indicated
above, this amount is 0 to 12 parts, preferably 0.12 to 8 parts by
weight per hundred parts of the weight of Novolak resin.
Resols are prepared from phenols and aldehydes over a wide molar
ratio of reagents depending on the particular phenol and particular
aldehyde. In this case also, a generalization can be made when the
phenolic compound is phenol and the aldehyde is formaldehyde.
Phenol-formaldehyde resols are usually prepared under alkaline or
basic conditions, or in the presence of metal salts such as zinc
acetate, to give resinous condensation products having a number of
unreacted methylol group as well as methylene bridges, both derived
from the aldehyde. The ratio of formaldehyde to phenol varies for
resols, preferably from a ratio of 1.05/1 to 1.5/1. In some
formulations some of the bridging groups may be supplied by
hexa.
The available methylol groups are the active functional groups that
allow the Stage A resol to progress to the Stage B resitol and then
to the Stage C resite. Thus it is a primary requirement that the
resols used in the practice of this invention conform to the
standard definition of resol, namely that heat alone will effect
the progress of the resol to the resitol and then to the
resite.
These available methylol groups in the resol are likewise the
active functional groups which coreact with the Novolak to effect
bridging and thereby produce the thermoset resin used in subsequent
carbonization to produce vitreous carbon.
When furfural is used as the aldehyde in the reaction with the
phenol to produce either the Novolak or the resol, an alkaline
catalyst is preferred to obtain a controlled condensation. An acid
catalyst is avoided since the acid is likely to initiate additional
polymerization through the ethylenic unsaturation in the furan
ring, which has a cyclic diene-ether structure.
In contrast, when producing a Novolak with furfural under alkaline
conditions, such as with sodium carbonate, a furfural-to-phenol
molar ratio of 0.60/1 to 0.90/1, preferably 0.70 to 0.75, produces
a Novolak which upon normal heating will not cure but will do so
upon the addition of formaldehyde or hexa. However if an acid is
present during the initial condensation or is added thereafter the
resin can undergo additional reaction which may be misinterpreted
and the resin considered to be resol in character. For example, if
a furfural Novolak is treated with a strong acid, a vinyl type
addition reaction will be promoted and the resultant additional
polymerization will effect bridging between polymer molecules.
As discussed above, the methylol groups in the resol resin react
with aromatic rings in the Novolak resin to form bridging or
crosslinking and since it is desirable for uniform coreaction
between the Novolak and the resol, it has been found that very
intimate and uniform distribution of one resin in the other is
conducive to improved strength and avoidance of stresses in the
molded and carburized products. Therefore, in order to have
intimate and uniform blending of the two resins, it is important
that they are of a small particle size that forms such intimate
mixture. It has been found advantageous, therefore, that the resins
should be of a particle size no larger than 40 mesh, preferably no
larger than 100 mesh, or on the basis of micron size, the particles
advantageously are no larger than 50 to 60 microns and obviously
even smaller particles are desirable, for example 5-32 microns or
even down to 0.01 micron.
These fine particle sizes permit homogeneity in the resultant
mixtures and aid in the uniform and more complete interaction that
produces greater strength, freedom from stress, and other desirable
properties in the ultimate products.
Uniform dispersion or blends of the two resins may be prepared by a
variety of techniques. A practical method is to grind the resins to
about 60 mesh size or smaller and then pass the mixture of the
powdered resins through a single to two-worm extruder. This method
requires close control of the barrel and nozzle temperature of the
extruder as well as of the residence time in the extruder to avoid
premature gelation which is detrimental since this causes
deformation in a molded piece on cooling and causes stresses in the
carbonized product. This close control is also necessary if the
resin mixture is processed further in a Banbury or on heated
rolls.
Another method comprises pregrinding the Novolak and the resol to
less than 40 mesh, preferably to less than 100 mesh, followed by
blending together with the various appropriate additives, if such
are to be used, such as hexa, furfural, graphite, mold lubrican,
etc. The resultant blend is then fine ground by air-milling or
impace grinding to form a homogeneous blend. This homoegeneous
material is further compounded and densified by any of the standard
procedures used in processing phenolic resins, such as in a banbury
extruder and particularly on a two-roll heated mill. Typical
processing conditions on the mill are in the range of
210.degree.-245.degree. F. on the front end, 75.degree.-110.degree.
F. on the back roll and processing time of 1-5 minutes, depending
on the flow properties in the densified product.
The densified material is processed for molding by grinding in a
unit grinder to specific particle sizes, usually determined
experimentally for the particular shape to be molded. A typical
distribution range (determined after 15 minutes on a Ro-Tap unit)
is:
______________________________________ Percent
______________________________________ On 10 mesh 25-35 20 mesh
30-27 40 mesh 10-18 60 mesh 2-10 100 mesh 2-10 140 mesh 2-2 Through
140 mesh 5-75 ______________________________________
For reasons of efficiency, the fine grinding of the resins to low
micron dimensions is preferred to achieve a uniform blend.
For general commercial application, the method of processing on
heated rolls may be preferred for blending the resol and the
Novolak together or with additives.
As used herein, the expression "densify" is intended to mean the
step of intimately mixing or blending particles of two or more
types by rolling, milling, extruding, etc.
The thermoset resins produced from the compositions of this
invention are suitable for molding and carburization to prepare
filled vitreous carbon products adapted to various industrial
applications. The cured resite resins of this invention are
adaptable for use as electrodes in electrochemical systems, such as
in chlorine cells, in molten aluminum systems, in electroplating
systems, in direct electrical generating systems using strong
electrolytic acids such as sulfuric and phosphoric acids together
with methane and air, hydrogen and oxygen or chlorine, etc.; as
fuels; as Barnes capture devices or control rods in nuclear
reactors; in aerospace systems; as supports or walls in catalytic
systems, as large metallurgical crucibles; diffusion sheets or
plates in diffusion devices; as large zone refining units, etc.
The production of formed shapes from the Novolak-resol compositions
of this invention is achieved by well known techniques, such as by
compression, injection, transfer and impulse molding. In each case
the resin mixture is introduced into a hot mold under pressure at
least sufficient to force the mixture to fill all parts of the
mold, and the resin mixture is cured to an insoluble, infusible
state.
The temperature used will vary over a wide range depending on the
composition of the Novolak, the composition of the resol, the
presence or absence of hexa, the presence or absence of added
furfural as either an external cross-linking agent or a reactive
plasticizer, the amount and type of carbonaceous fillers and other
non-gassing fillers, etc. However, most of these compositions can
be molded in the range of 100.degree. to 166.degree. C.
(212.degree. to 330.degree. F.) but in some cases temperatures as
high as 180.degree. C. (356.degree. F.) may be used. The preferred
range is 149.degree.-166.degree. C. (300.degree.-330.degree.
F.).
The same factors recited above as affecting the molding
temperatures used, as well as the temperature itself, in many cases
also affect the choice of pressure used in molding to a shaped
form. For example, a higher molding pressure is required for a
blend of a phenol-formaldehyde Novolak (PFN) and a
phenol-formaldehyde resol (PFR) than the corresponding blend
containing a phenolfurfural Novolak (PFUN). Similarly, a higher
pressure is required to mold a blend containing 60% graphite filler
than for one containing 40% or 20% or 0% graphite. Thus, a PFUN-PFR
blend containing 60% graphite may require 6-8 tons per square inch
compared to 1-2 tons per square inch for a corresponding blend
containing no graphite, and 500-1000 psi for the corresponding
composition containing 1-5% of furfural as a reactive carbonizable
plasticizer.
Moreover, the shape of the molded part will also influence the
selection of a suitable molding pressure which may require one
pressure for compression molding and a higher pressure for transfer
molding, and a still higher pressure for injection molding which
can be 20 tons per square inch for injection in contrast to 10 tons
per square inch for transfer and 5 tons per square inch for
compression molding. The general range of pressures lies between
0.5 to 25 tons per square inch, and the preferred ranges are 2-20
tons per square inch and 1-5 tons per square inch for compression
molding.
Extractability tests on the molded products of this invention show
that there is less than 2 percent and generally less than one
percent of material extracted by acetone based on the resin content
of the product provided there is no non-reactive additive present,
such as a plasticizer. In fact, in most cases the molded product
has very little extractible material even when strong solvents,
such as dimethylformamide, are used. The extractability tests are
performed according to ASTM Procedure D494-46.
The weight ratio of the respective Novolak and resol resins, as
well as the presence or absence of various additives or modifiers,
such as external curing agents, e.g., hexa, reactive plasticizers,
such as furfural, processing aids, such as molding plasticizers,
e.g. zinc stearate or stearyl alcohol, and various type of fillers,
such as graphite, etc., will depend on the application for which
the vitreous carbon is to be utilized. As discussed herein, the
applications for the products of this invention are extremely
diversified, including chemically resistant piping and equipment,
and walls and electrodes for fuel cells, etc.
In these compositions, the PFUN:PFR ratio can be from 80:20 to
20:80. Hexa may advantageously be added in a proportion of zero up
to 12 parts by weight, preferably 0.12 to 8 parts by weight of hexa
per 100 parts weight of PFUN. Also 0-5 parts by weight of furfural
per 100 parts by weight of PFUN plus PFR may be added.
Mold lubricants may be incorporated in the resin blends. Suitable
lubricants include fatty acids of 14-22 carbon atoms, their esters
of alcohols containing 1-22 carbon atoms and their metal salts,
such as Ca, Zn and Mg salts. Typical of these which may be used are
oleic acid, stearic acid, montan wax, stearyl stearate, glyceryl
mono-oleate, glyceryl monostearate, the commercial wax sold under
the brand name "Acrawax", zinc, calcium and magnesium stearates,
etc. For biomedical applications the lubricants should be free of
metals and metallic compounds. The lubricants may be used in
proportions of 0.05-3 percent by weight based on total resin
composition.
A very useful class of fillers comprises carbonaceous fillers, for
which vitreous carbon itself exhibits high adhesion. Such materials
include pyrolytic graphite; the normal graphites such as that
formed of flat, parallel lamellae of carbon held together by van de
Waal's forces (distance approximately 3.35.ANG.); carbonized
celluloses; etc. For economic reasons, the regular graphites
generally find greater use in vitreous carbon than the other
fillers.
In general, with regular graphite as a reference, the proportion of
such fillers in ready-to-use molding powders of this invention may
be as little as 5% to produce a noticeable effect but is
advantageously between 35 and 65%, preferably about 40-60% by
weight of the molding composition. By adjusting the amount of
Novolak in the mixture together with the use of hexa, lubricants,
furfural, etc., the proportions of graphite may be adjusted to 30
to 70% of the molding composition. Moreover the compatibility of
the graphite may be improved by the use of finer grades of graphite
as compared to more coarse grades. In some cases it is desirable to
use a combination of varying fineness or coarseness in the
graphite. Similar considerations apply to the other carbonaceous
fillers.
The amount of carbonaceous filler can also be expressed as parts
per 100 parts of the combined weight of Novolak and resol resins.
Thus the moldable compositions of this invention comprise a heat
curable, pressure moldable blend of 80-20 parts by weight of a
phenolic-aldehyde Novolak and 20-80 parts by weight of a
phenolic-aldehyde resol, with the combined weight of Novolak and
resol totaling 100 parts by weight, plus 0-230, preferably 0-150
parts by weight of a carbonaceous filler, 0-5 parts by weight of
furfuraldehyde, 0-12 parts by weight, preferably 0.15-8 parts by
weight of hexa, and 0-3 parts by weight of mold lubricant, with the
proportion of each of the additives being based on 100 parts by
weight of the combined Novolak and resol. With extreme dispersion
methods, such as ball-milling carefully to extremely fine particle
size, the amount of graphite may exceed somewhat the amount defined
above.
A typical "high" graphite composition may be comprised as follows:
60% graphite, 22.1% phenol-formaldehyde resol, 13.0%
phenol-furfural Novolak, 1.48% hexa, 0.6% stearyl stearate, 1.25%
zinc stearate and 1.25% furfural. Another typical composition may
comprise: 40% graphite, 35.75% phenol-formaldehyde resol, 20.17
phenol-furfural Novolak, 0.1% stearyl stearate, 1% zinc stearate
and 0.6% furfural. The percentages are percent by weight based on
the total composition.
Another typical "high" graphite composition containing 49.9 percent
by weight of graphite, 29.2% PFR, 16.6% PFUN, 1.9% hexa, 0.4%
stearyl stearate, 1.0% zinc stearate and 1.0 furfural, after being
transfer molded as described herein, is ground as tested for
acetone soluble content. An average of these tests shows 0.48%
soluble material which analyzes to show a mixture of stearate
plasticizer and unreacted furfural.
In special applications, finely dispersed pyrolytic graphite may be
preferred as a filler over normal graphite because of its
anistropic properties which appear to reinforce the vitreous carbon
with its unique thermal and mechanical properties. Pyrolytic
graphite is pure crystalline graphite deposited from carbon-bearing
vapor at temperatures in excess of 2000.degree. C. (See "Ablative
Plastics", G. F. D'Alelio and John A. Parker, Marcel Depper, Inc.,
N.Y., N.Y. 1971, pp. 119-120). This material is metallic in
appearance, impervious to gases and contains no binders such as are
found in the regular commercial graphite used as electrodes. X-ray
diffraction patterns of pyrolytic graphite show significant
deviations from those of normal graphite as evidenced by an
unusually high degree of peripheral orientation. The crystals have
their basal planes aligned parallel to the surface of deposition.
This orientation is a function of deposition temperature and
density and is responsible for the marked anistropic properties of
pyrolytic graphite. When pyrolytic graphite is used as filler in
the composition of this invention it appears that vitreous carbon
has increased regions of graphite (or diamond) crystallinity which
are dispersed among the small stacks of graphite-like layers. The
reason for this is not clear but it is probably due to a nucleating
effect. To some measure this allows some variability in the ratio
of the small stacks of graphite-like layers interpersed with
regions of graphite crystallinity.
The co-reactive blends of this invention are converted to vitreous
carbon by the intermediate steps of molding and curing the shaped
form. Adequate cure and therefore curing temperature and time, are
of paramount importance since undercured articles usually crack
during the carbonization process. The extent of cure may be checked
by determining the amount of acetone extractable material.
Satisfactory products are obtained when the acetone extractable
value is no greater than 2 percent, preferably less than 1 percent,
as determined by ASTM Method D494-46 based on the weight of resin
content.
Many methods of molding may be used. As previously indicated, the
formed precursor part may be compression, transfer, extrusion or
injection molded. High production volumes are readily obtained with
the blends of this invention by transfer and injection molding in a
multicavity mold. In some cases, shaped precursor parts can be
machined from other molded or extruded shapes.
In a typical operation the ground molding composition is first
preformed to eliminate trapped air and is electrically preheated to
230.degree. F. (110.degree. C.). This preform is then molded in a
preform press using a 5.5" diameter ram operated at 500 psi line
pressure. The parts may be cured at 300.degree. F. (149.degree. C.)
for 4-5 minutes with a cavity pressure of approximately 2000 psi.
Obviously various other techniques may be used.
The precursor shaped article is converted to a shaped vitreous
carbon article by closely controlled thermal degradation of the
article in an inert atmosphere in a furnace until the maximum
heat-treating temperature is reached. Then the vitreous carbon
article is cooled in the closed furnace and in the inert
atmosphere. Usually large numbers of precursor moldings are treated
in a single heating operation while retained in open graphite
containers in the furnace.
In most cases the optimum firing cycle of time versus temperature
is determined experimentally for each different shape since the
geometry, particularly the wall thicknesses, of the molded part has
a direct bearing on the rate. Also the time-temperature
relationship is dependent on the degradation characteristics of the
final cured resin. For example, it is logical and obvious that
cured products derived from (1) a PFN-PFR blend; (2) a PFUN-PFR
blend; (3) a PFN-PFR blend containing 5% furfural; (4) a PFUN-PFR
blend containing 10% hexa; and (5) the admixture of (4) containing
an equal amount of graphite would require different
temperature-time firing cycles for the same shaped article. The
variations in firing cycles will be even greater if the articles
have different wall dimensions.
A typical firing cycle generally has a continually increasing
temperature and usually has variable rates of increase in different
parts of the cycle. During the firing a substantial volume
shrinkage of the article occurs, which is usually in the range of
15 to 25% in those moldings which contain no carbonaceous or other
filler.
During the firing of such unfilled molded parts, about 30-35% by
weight of the part is lost as volatile gas. When carbonaceous
fillers are used with the resin mixture, the shrinkage and weight
loss is reduced in proportion to the filler content. To flush out
the large amount of gas generated during the vitreous carbon
formation, a stream of inert purge gas, such as nitrogen, helium or
argon is used, or alternatively, a reduced pressure of 10.sup.-2
torr or less may be applied. The outgassing is predominant in
certain temperature ranges. For example, from room temperature up
to 600.degree.-700.degree. C. the temperature increase is typically
at a rate of 1.degree.-5.degree. C. per hour. Above about
700.degree. C., the temperature can be increased much more rapidly,
as from 10.degree. to 50.degree. C. per hour. Generally for most
parts the temperature above 700.degree. C. is increased at a rate
of 10.degree. C. per hour up to 800.degree.-850.degree. C., and
thereafter at 20.degree.- 50.degree. C. per hour to the maximum
temperature which generally need not be above 1800.degree. C. In
very special cases where high thermal stability is required in the
shaped vitreous carbon product, the heating may be continued up to
2000.degree. C. and in some very special cases up to 3000.degree.
C. and held at that temperature for a least 24 hours. Then the
temperature is dropped gradually at a rate of 10.degree.-20.degree.
C. per hour.
Where desired, the critical temperature regimes can be determined
by thermogravimetric (TGA) and differential thermal analysis (DTA).
The thermal regimes thus determined are characteristic of the
chemistry and thermal history of the molded phenolic resin
part.
It has been found that the blends of Novolak and resol resins
prepared as described above may be used for preparing molding
compositions for purposes other than the production of vitreous
compositions and that such molded products have various improved
properties by virtue of the stress-free interaction of the Novolak
and resol resins. In such cases the proportions of fillers and
additives correspond substantially to those reported above for use
in the comositions to be converted to vitreous carbon.
SPECIFIC EMBODIMENTS OF THE INVENTION
The invention is illustrated by the following examples which are
intended merely for purpose of illustration and are not to be
regarded as limiting the scope of the invention or the manner in
which it may be practiced. Unless specifically indicated otherwise,
parts and percentages are given by weight.
EXAMPLE I
Preparation of Phenol-Formaldehyde Novolak Resin (PFN)
Into a 4 liter resin flask equipped with a mechanical stirrer,
reflux condenser and thermometer is placed 2000 gms (21.28 moles)
USP phenol; 882 gms (15.28 moles) aqueous formaldehyde (52%); 200
gms water and 12 gms (0.10 mole) phosphoric acid (85%). The pH of
the resulting mixture is 1.05. This mixture is then heated to
reflux and refluxed a total of 5 hours. The free formaldehyde
content of the mixture at this point is found to be 0.84%. At this
point the reflux condenser is replaced with a distillation
condenser and batch distilled under atmospheric pressure for one
hour until the batch temperature reaches 160.degree. C. At this
point a mixture of 173 gms (3.0 moles) of 52% aqueous formaldehyde,
mixed with 70 gms of water is slowly added to the mixture over a
period of 36 minutes. During the addition, the batch temperature
drops to 142.degree. C. When all the aqueous formaldehyde has been
added, the batch is held at 142.degree.-150.degree. C. for 15
minutes. Then the receiver on the distillation condenser is
replaced with a vacuum receiver to allow completion of the batch
under vacuum. The resin is then dehydrated to a batch temperature
of 165.degree. C. under vacuum of 28 inches of mercury. The vacuum
is released and the resin discharged from the vessel to yield 2027
gms of product, which exhibits a gradient bar melting point of
223.degree. F. and a glass transition temperature as measured by
differential scanning calorimetry of 70.degree. C. (158.degree.
F.). This resin is a non-curing Novolak as shown when tested on a
hot plate at 330.degree. F. However, when thoroughly blended with
10 parts per hundred (pph) of hexa, it has a set time of 24-25 sec.
when heated at 330.degree. F. (166.degree. C.).
EXAMPLE II
Preparation of Phenol-Furfuraldehyde Novolak Resin (PFUN)
Into a 4 liter resin vessel equipped with a mechanical stirrer,
distillation condenser, heating mantel and thermometer is placed
2000 gms (21.28 moles) of USP phenol and 1480 gms (15.25 moles) of
furfural. This mixture is heated to 66.degree. C. and 30.0 gms
(0.36 mole) sodium carbonate added. The charge is then slowly
heated to 121.degree. C., at which temperature the heating mantel
is removed. The reaction then becomes exothermic and the
temperature continues to rise until boiling begins at 135.degree.
C. The distillate is collected in a separating device and the
furfural layer is periodically drawn off and returned to the batch
during the course of the reaction. Distillation is continued for 3
hours, 40 minutes with the batch temperature maintained between
133.degree. and 139.degree. C. The resin is then discharged from
the vessel and allowed to cool to a solid which has a melting point
of 89.degree. C. (192.degree. F.), a yield of 3230 gms, a glass
transistion temperature of 330.degree. K. (134.6.degree. F.). This
is a non-curing Novolak resin, as shown when tested on a hot plate
at 330.degree. F., but when thoroughly mixed with 10 pph of hexa,
has a set time at 330.degree. F. of 65-69 sec.
EXAMPLE III
Preparation of PFUN with Mold Lubricant Added
The procedure of Example II is repeated except that midway in the
distillation 30 gms of glycerlymonooleate is added, thoroughly
mixed and the distillation continued. The resin has a melting point
of 85.5.degree. C. (186.degree. F.), a glass transition temperature
of 327.degree. K. This is also a non-curing Novolak resin and with
10 pph of hexa, has a set time of 67-68 sec.
EXAMPLE IV
Preparation of Phenol-Formaldehyde Resol Resin (PFR)
(a) Into equipment as used in Example I there is placed 1500 gms
(15.96 moles) of USP phenol, 1197 gms (20.75 moles) of aqueous
formaldehyde (52%), 23 gms (0.16 moles) of hexa, and 7 gms (0.170
mole) of sodium hydroxide (97%). This mixture is warmed to
90.degree. C. and maintained at this temperature for one hour. At
the end of this period the reflux condenser is replaced with a
vacuum distillation condenser and receiver. The batch is then
maintained at a vacuum of 26" of Hg and heat supplied until the
batch temperature reaches 90.degree. C. (194.degree. F.). Then the
vacuum is adjusted to 28" Hg and distillation continued for one
hour. The reaction is then terminated and the product discharged
from the vessel. The amount of distillate water collected is 940
gms. The product weighs 1855 gms and has a gradient bar melting
point of 79.5.degree. C. (175.degree. F.), a glass transition
temperature as measured by differential scanning calorimetry of
321.degree. K. (118.4.degree. F.) and a hot plate set time of 17-18
seconds at 330.degree. F. (165.5.degree. C.).
(b) The above procedure is repeated using 1252.7 gms (21.71 moles)
of aqueous formaldehyde but omitting the hexa, and the distillation
is continued until about 980 gms of distillate water is collected.
The CH.sub.2 O/phenol ratio of the resol is 1.36/1. However the
product is softer than that obtained in the above procedure (a) so
the product is placed in a pan and heated in an oven at 60.degree.
C. (140.degree. F.) until a specimen indicates that the melting
point is about 175.degree. F. (79.5.degree. C.) and is similarly
grindable as the resol produced in (a).
(c) The procedure of Example IV(a) is repeated except that 1058 gms
(18.34 moles) of the aqueous formaldehyde solution is used. The
product is a grindable resol.
(d) The procedure of Example IV(a) is repeated using 1335 gms
(23.15 moles) instead of the 1197 gms of the aqueous formaldehyde.
The product is a grindable resol.
EXAMPLE V
Densifying and Molding of Mixtures of Phenol-Formaldehyde Novolak
(PFN) and Phenol-Formaldehyde Resol (PFR)
The Novolak resin of Example I and the resol resin of Example IV(a)
are ground separately in a laboratory Wiley mill and passed through
a 0.05 inch screen. A blend is made on a small ribbon blender using
600 gms of PFN and 1400 gms of PFR. This blended mixture is next
fine ground through an impact grinder to yield an intimate mixture
having a maximum particle diameter of approximately 10 microns.
This mixture is compounded on a differential 2-roll heated mill.
The front roll of this mill is maintained at
220.degree.-230.degree. F. (104.degree.-110.degree. C.) and the
back roll at 80.degree.-100.degree. F. (26.7.degree.-37.8.degree.
C.). The milling is continued for 1.5 minutes after sheet
formation, and then the sheet is removed, cooled and ground. The
particle size distribution of this material suitable for molding
is:
______________________________________ Mesh Size %
______________________________________ On 10 28-34 On 20 32-36 On
40 12-16 On 60 5-9 On 100 4-8 On 140 1-3 through 140 4-5
______________________________________
The molding flow properties of this product is measured in a C. W.
Brabender Torque Rheometer equipped with a one-half size mixing
bowl equipped with roller blades. The instrument is operated at
125.degree. C. with 60 RPM with a connector setting of 1.5, a
sensitivity of 45 (.times.5) with no suppression of torque. This
test gives a minimum torque reading after melting of 1700-1800
meter-grams and a charge-to-setting time of 80 sec. This material
is also moldable in a standard multicavity transfer mold using 600
psi of line pressure on the transfer ram. The parts so molded are
clear and amber in color.
EXAMPLE VI
Demonstration of PFUN-PFR Blends with Wide Range of Graphite and
Other Additives
Large batches of the PFUN resin of Example II and the PFR resin of
Example IV(c) are prepared so that identical resins may be used in
a large number of comparative mixtures. Four mixtures of varying
ratios of resins with varying amounts of hexa, graphite and
additives are prepared. The amounts of hexa, graphite, zinc
stearate, stearyl stearate and furfural are shown in the table
below. The resins are ground and passed through a 0.05" screen,
blended with graphite and the respective additives, and compounded
on a 2-roll differential mill with the front roll maintained at
200.degree. F. (93.degree. C.) and the back roll at 300.degree. F.
(149.degree. C.). The milling is continued for 15 seconds after
complete melting of the resin. The resulting sheets are cooled,
ground as in the preceding examples to a particle size of 6-80 mesh
as measured on U.S. Standard sieves. This is molded according to
the transfer molding techniques of the earlier examples into plates
22.5 inches.times.27.5 inches.times.0.045 inch thickness using
compression molding techniques with 868 grams of 2" preforms
weighing 62 grams each, on a large 600 ton compression press. The
cavity pressure in this molding is 1500 psi and the plates molded
at 300.degree. F. (149.degree. C.) for 3-4 minutes. These plates
are successfully converted to vitreous carbon of excellent
properties by the method described by Thornbury and Morgan (Soc.
Plast. Eng. Pactec. 1975, Sept. 16-18, 1975, p. 47). These are
placed in an open graphite container in a furnace having nitrogen
atmosphere. The temperature in the furnace is raised gradually at a
rate of about 2.degree.-3.degree. C. per hour to a temperature of
700.degree. C., at which point the temperature increase rate is
adjusted to about 10.degree. C. per hour up to
800.degree.-850.degree. C. and thereafter at a rate of about
30.degree. C. per hour to the maximum temperature of 1800.degree.
C., which temperature is maintained for about 25 hours. Then while
the nitrogen atmosphere is maintained in the closed furnace, the
temperature is decreased gradually at a rate of about
10.degree.-20.degree. C. per hour to room temperature.
TABLE ______________________________________ Run No. 1 2 3 4
______________________________________ Graphite/resin 50/50 40/60
50/50 60/40 (Wt. ratio) PFUN/PFR 50/50 40/60 30/70 40/60 (Wt.
Ratio) PFR-Ratio 1.15 1.15 1.15 1.15 of Form./Phenol PFUN (Gms)
1180 1146 801 764 PFR (Gms) 1180 1719 1604 1146 Hexa (Gms) 138.8
134.8 94.3 98.9 Graphite (Gms) 2498 2000 1498 3000 Zn Stearate
(Gms) 43.83 53.20 44.66 8.87 Furfural (Gms) 50 50 50 50 Brabender
300 305 257 237 (Duration of Mix) Flex. Strength, psi 8640 9344
8384 9792 Tens. Strength, psi 5867 6293 6080 5547 Flex. Mod.
.times. 10.sup.6, 1.07 0.842 1.08 1.36 psi Spec. Grav. of 1.578
1.48 1.579 1.685 molding On Carbonization: Wt. Loss (%) 15.56 19.43
16.47 11.54 Shrinkage in 6.96 8.15 6.84 4.59 Length (%)
______________________________________
EXAMPLE VII
The procedure of Example VI is repeated five times using the PFUN
resin prepared in the master batch of that example and also using
PFR prepared in another master batch according to Example IV(a).
The respective amounts of materials and the results are shown in
the following Table:
______________________________________ Run No. 5 6 7 8 9
______________________________________ Graphite/ 40/60 40/60 60/40
60/40 50/50 Resin (wt. Ratio) PFUN/ 50/50 30/70 30/70 50/50 40/60
PFR (Wt. Ratio) PFR (Ratio 1.30 1.30 1.30 1.30 1.30 of Form/ Phenol
PFUN 1417 962 641 944 955 (Gms.) PFR 1417 1925 1283 944 1433 (Gms)
Hexa 166 113 75.4 111 112.3 (Gms) Graphite 2000 2000 3000 3000 2500
(Gms) Zn Stearate 52.63 53.61 8.93 8.77 44.34 (Gms) Stearyl 94.74
96.50 64.32 63.11 79.83 Stearate (Gms) Furfural 50 50 50 50 50
(Gms) Brabender 345 305 182 230 263 (Dur. of Mix) Flex. 7936 8320
9216 7872 8072 Strength, psi Tens. 6293 5813 4747 5333 5067
Strength psi Flex. 0.826 0.866 1.28 1.31 1.05 Mod. .times.
10.sup.6, psi Sp. Grav. 1.482 1.487 1.678 1.678 1.576 of Molding
Firing by the method described in Example VI gives vitreous carbon
of excellent properties with the following weight loss and
shrinkage values: Wt. Loss 20.14 18.59 11.17 11.17 15.73 (%)
Shrinkage 8.5 8.15 3.76 4.0 6.13 in length (%)
______________________________________
EXAMPLE VIII
The procedure of Example VI is repeated four times using portions
of the same master batch of PFUN and using PFR from a master batch
prepared according to the procedure of Example IV(d). The
respective amounts of materials and the results are shown in the
following Table:
______________________________________ Run No. 10 11 12 13
______________________________________ Graphite/Resin 50/50 40/60
50/50 60/40 (Wt. ratio) PFUN/PFR 50/50 40/60 30/70 40/60 (Wt.
ratio) PFR (Ratio of 1.45 1.45 1.45 1.45 form./phenol) PFRN (Gms)
1181 1146 801 746 PFR (Gms) 1181 1719 1604 1146 Hexa (Gms) 138.8
135 94 89.8 Graphite (Gms) 2500 2000 2500 3000 Zn Stearate (Gms)
43.86 53.21 44.66 8.79 Stearyl Stearate 78.96 95.77 80.40 63.24
Furfural (Gms) 50 50 50 50 Brabender 275 293 250 204 (Dur. of Mix)
Flex. Strength, 8512 8516 7680 8704 psi Tens. Strength 5787 5413
5680 4293 psi Flex. Mod. .times. 1.05 .841 1.18 1.38 10.sup.6, psi
Sp. Grav. of 1.578 1.493 1.576 1.680 Molding Firing by the method
described in Example VI gives vitreous carbon of excellent
properties with the following weight loss and shrinkage values: Wt.
Loss (%) 16.99 20.73 16.88 11.27 Shrinkage in 6.25 7.79 6.13 3.88
length (%) ______________________________________
EXAMPLE IX
Dramatic improvement in flexural and tensile strength is shown in
unfilled PFUN-PFR mixtures, particularly those having a proportion
of at least 50% PFUN in the mixture. A master batch of PFUN resin
is prepared according to Example II, the PFR master batch is
prepared by the procedure of Example IV(a) and the mixtures are
prepared by the procedure of Example VI. No hexa or other additive
is added. The roll conditions are 240.degree. F. (116.degree. C.)
front roll, with back roll unheated. The roll time is measured
after melt of the resin has occurred and is continued until the
material on the rolls has achieved proper molding plasticity.
______________________________________ Run No. A B C D
______________________________________ PFUN/PFR Ratio 5/95 20/80
30/70 50/50 Roll Time (min.) 0.75 1.75 4.75 8.5 Piston line 450 800
800 800 pressure (psig) Cure Time (min. 4 4 4 4 at 335.degree. F.)
Flexural strength* 5,030 8,688 7,808 17,473 (psi) Flexural Mod.*
830,000 700,000 700,000 640,000 (psi) Tensile strength* 4,064 4,896
5,888 7,824 (psi) Brabender Minimum 1,590 1,090 1,350 1,550 Torque
(meter-gms) ______________________________________ *These values
are the average values for five specimens each prepared according
to ASTM D1896.
EXAMPLE X
The procedure of Example VI is repeated a number of times except
that no filler, hexa or additive is added. As shown in the
following table, increasing proportions of the phenol-furfural
Novolak (PFUN) are used, with the corresponding conditions and
results reported in the table. It will be noted that with 40% PFUN
and higher proportions of PFUN, the strength properties show a
tremendous increase in values, more than double in some cases. The
roll conditions are 240.degree. F. (156.degree. C.) for the front
roll and the back roll is unheated. The roll time is measured after
melting of the resin has occurred. Roll time is prolonged until
proper molding plasticity is achieved. Even without plasticizer,
hexa and other additives, the compositions containing 20% or more
PFUN are found to mold easily without sticking by transfer,
compression and injection molding to give shaped molded objects of
excellent properties.
TABLE ______________________________________ A B C D E F G
______________________________________ Ratio of 5/95 20/80 30/70
40/60 50/50 60/40 70/30 PFUN/ PFR Molding 450 800 800 900 675 550
550 Transfer Piston Line Pressure, psig Cure Time 4 4 4 -- 4 -- --
(Min.) at 335.degree. F. Flexural 5,030 8,688 7,808 15,590 16,839
18,240 14,899 Strength* psi Flexural 8,300 7,000 7,000 14,000
15,000 14,000 15,000 Modulus* psi .times. 10.sup.-2 Tensile 4,064
4,986 5,888 5,808 7,776 8,912 6,592 Strength* Brabender 1,590 1,090
1,350 17,750 1,525 1,850 1,600 Min. Torque (meter- gms)
______________________________________ *These values are the
average for five specimens prepared in accordance with ASTM
D1896.
EXAMPLE XI
The procedures of Examples I, IV and VI are repeated a number of
times with satisfactory results using:
(a) An equivalent amount of meta-cresol in place of the phenol in
preparing the Novolak and the resol and these are used in the
procedure of Example VI;
(b) An equivalent amount of p,p'-diphenylolmethane is used in place
of phenol in preparing the Novolak and this is used with the PFR in
the procedure of Example VI;
(c) An equivalent amount of beta-naphthol is used in place of
phenol in preparing the resol and this is used with the PFUN in the
procedure of Example VI.
The vitreous carbon products made from the intimate blend of resins
described above, densified and molded as also described above, are
very much improved in freedom from pits and holes in the products
and also the percentage of unsatisfactory items as a result of
cracking or failures of any type are very much reduced in
comparison with vitreous carbons made from individual resins or
even with mixtures of resins other than those described and claimed
herein. Where appropriate mixtures are not used, cracking and
failures may occur during the carburization to vitreous products,
particularly of plates of low thickness and in some cases, during
the molding of the same.
This improvement with respect to freedom from pits and holes and
cracks in the vitreous carbon products is evidenced by improved
resistance to permeability and also in the reduction in the number
of rejects when the vitreous carbon products, such as large thin
plates, are subjected to various tests to determine whether they
will meet the conditions to which they will be exposed for use in
fuel cells.
As shown in Examples VI-VIII, the preferred ratios of PFUN/PFR run
from 30/70 to 50/50. As also pointed out above, the
phenol-furfuraldehyde Novolak has a plasticizing effect on the
resol resin allowing the viscous mass to flow more uniformly and
readily at lower pressure to completely fill the mold. This is
particularly evident when the resin is filled with a substantial
amount of filler such as graphite. This improved plasticity has
made it possible to mold and convert to vitreous carbon
PFUN-PFR-graphite filled plates as large as 50".times.50" and
having a thickness of 0.04-0.05 inch, and having excellent
resistance to permeability and freedom from cracks and pits.
PFN-PFR-graphite filled mixtures do not have satisfactory
plasticity and flow for this purpose.
Where there is poor plasticity during molding, it is generally
necessary to use pressures over 2000 psi. As indicated in above
Example VI and several subsequent Examples, pressures of 1500 psi
are used satisfactorily with PFUN-PFR-graphite mixtures and, in
most cases, the molding plasticity may be sufficiently improved to
permit lower pressures, preferably 800-1500 psi. Pressures above
1500 psi and more particularly above 2000 psi cause more stresses
and strains in the molded products and in the vitreous carbon
produced therefrom. The stresses and strains very often cause
cracks and particularly microcracks in the products. These stresses
and the accompanying cracks and microcracks are avoided or at least
reduced by the lower molding pressures allowed by the improved
molding plasticity of the PFUN-PFR combinations.
While the above description of the present invention has stressed a
preference for starting with finely divided Novolak and resol
resins, it is nevertheless contemplated that other methods of
producing equivalent intimate blending of these resins by starting
with larger particles than specified above will be suitable for the
practice of this invention. For example, where appropriate
processing by prolonged rolling, etc. of larger particles will
produced comminuted and equivalent intimately and uniformly mixed
compositions of the Novolak and resol resins, such procedures may
be used. However, such prolonged processing may be much less
practical then the preferred methods described above.
Moreover, where reference is made to phenol-furfural Novolaks, it
is intended that this include Novolaks in which other aldehydes may
be used to replace a minor amount of furfuraldehyde in the
formation of the resin. Thus in referring to a phenol-furfural
Novolak, it is intended to include Novolaks in which the major
molar amount of the aldehyde used to condense with the phenol is
furfural. In other words, while 100 molar percent of furfural is
preferred for this purpose, it is found that so long as at least
50.Iadd., advantageously at least 75, molar .Iaddend.percent of the
condensing aldehyde is furfural, the resulting Novolak gives
preferred results for the purpose of this invention.
Normally, a phenol-aldehyde resol is liquid. When this is blended
with a phenol-aldehyde Novolak, the resol functions first as a
plasticizer. The resultant mixture is extremely difficult to body
sufficiently to permit molding pressures. During molding of such
mixtures, gas is given off which produces pits and holes in the
molded product.
To avoid the problems described above, applicants have found that
hard, solid resols may be obtained which are pulverizable to finely
divided particles by the addition of hexa, or ammonia (preferably
in aqueous solution), during or after the preparation of the resol
and described in above Examples IVa, b, c and d.
The amount of hexa to be added should be in the range of 1 to 10
percent by weight or even higher, preferably 1.3-10 percent, based
on the total weight of resin solids. The amount of ammonia is the
molar equivalent of the specified amount of hexa. Another method of
calculating the desired amount is to calculate that the methylene
content derived or derivable from the hexa and the formaldehyde
should be in the range of 1.05 to 1.5 per mole of phenol.
In one of the parent applications, the Patent Office cited U.S.
Pat. Nos. 3,927,140 and 3,879,338 which involve mixtures of
phenolaldehyde Novolaks and resols. However, in addition to other
distinctions, the resols are the normal type and therefore such
mixtures would present the problems discussed above.
In another parent application Bentolila et al U.S. Pat. No.
3,280,231 and Huschka et al U.S. Pat. No. 4,013,760 were cited. The
lowest permeability shown by either of these references is in col.
5 of the Bentolila et al patent where permeabilities of about
10.sup.-4 cm.sup.2 /sec for graphitized products are reported. This
patent teaches against the use of more than 19% binder. The Huschka
et al patent teaches the use of 5 to 30% binder.
Emmanuelson et al U.S. Pat. No. 4,360,485 shows that separator
plates used in fuel cells need to have a hydrogen permeability of
less than 0.03 cc H.sub.2 /ft.sup.2 /sec, preferably less than 0.02
cc H.sub.2 /ft.sup.2 /sec. Applicants' plates made according to the
process of this application have been found to be very satisfactory
for such use in fuel cells which means that they meet this
permeability test.
In accordance with the discussion starting at line 26 of page 22
running through line 7 of page 23 of this application there is a
substantial shrinkage of 15-25% in volume during a carbonization
cycle of a molded resin of this invention if there is no filler
present and about 30-35% by weight is lost as volatile gas during
the firing of such unfilled molded parts. When carbonaceous
fillers, such as graphite are used, the shrinkage and weight loss
is reduced in proportion of the filler content. This accounts for
the limits of 19% and 30% maximum binder content fixed by the cited
Bentolila et al and the Huschka et al patents. It is surprising
therefore that with larger amounts of resin binder that a
carbonized product could have the low permeability shown by
applicants' products.
While it is easier to obtain low permeability in thicker objects,
low permeability or impermeability is particularly difficult to
achieve in the molding and firing of very thin plates such as the
plates of 0.045 inch thickness produced in Examples VI-VIII.
The description of improved freedom from pits and holes is
particularly important with respect to such thin plates since it
will affect the permeability. It is obvious therefore that the
permeability of the vitreous products of this invention are much
less than the 10.sup.-4 cm.sup.2 /sec of the products of the
Bentolila et al reference.
In the tables following Examples VI-VIII of this application it is
shown that fired molded products made from 40-60% graphite and
60-40% of the resin composition in which the PFUN/PFR ratio runs
from 30/70 to 50/50 give very good flexural and tensile strengths.
As also indicated above, preferred graphite contents are in the
range of 35-65%.
While certain features of this invention have been described in
detail with respect to various embodiments thereof, it will of
course be apparent that other modifications can be made within the
spirit and scope of this invention, and it is not intended to limit
the invention to the exact details shown above except insofar as
they are defined in the following claims.
* * * * *