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aliens
Patented Dec. 31, 1946 ‘
Uldl’i‘hi? sra'rss PAra
‘
I
2,413,498
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_
MOLDING PROCESS
Julian W. Hill, Wilmington, Del., asslgnor to E. I.
du Pont de Nemours & ‘Company, Wilmington,
Del, a corporation of Delaware
‘
No Drawing. Application March a, 1944,’
Serial No. 525,307
r 4 Claims.- (Cl.18—55)
,
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.
E
This invention relates to the manufacture of
shaped products from polymeric materials and
more particularly to ‘a process for obtaining
shaped articles from polytetrafluoroethylene._ '
Polytetra?uoroethylene is described in U. S.
Patent 2,230,654 as a white or brown powder or
jelly which becomes incandescent in the presence
of a flame but which does not burn when the
and
lulose ?ashes off almost immediately leaving a
grayish film of polytetra?uoroethylene. Heat
treatment is continued for about 10 minutes or
until the residue is rendered, clear and trans
parent. Film and support are then passed quickly
into cold water. A translucent, coherent, tough,
?exible ?lm of polytetrafluoroethylene approxié
mately 0.005" thick results.
The polytetra?uoroethylene in the above ex
name is removed and melts only at red heat.
This patent also states that by the use of sum 10 ample can be replaced by the co-polymerization
ciently high pressure and temperature the poly
product of tetrafluoroethylene in major amount,
mer can be molded into articles which are rela
‘ preferably above 75%, with other polymerizable
impraetlcai or uneconomical on a manufactur
chloride, etc. Film-forming materials other than
that mentioned in the example comprise cellulose
acetate, ethyl cellulose, and other cellulose ethers
materials examples of which are ethylene, ~iso
tively clear and colorless. However, methods
butylene, chlorotri?uoroethylene, vinylidene
generally used for shaping thermoplastics, such
as injection molding and extrusion, have been 15 chloride, vlnylidene ?uoride, vinyl ?uoride, vinyl
ing scale because of the peculiar characteristics
of polytetrafiuoroethylene,
Polytetra?uoroethylene undergoes a reversible
and esters, polyvinyl acetate, polyesters obtained
polymer is transparent in thin sections, is
markedly less crystalline as 'shownby X-ray di
polystyrene, and protein fiber-forming materials
I transition at 327° C. Above this temperature the’ 20 from polyhydric a‘cohols and polybasic acids,
such as gelatin and zein. The most valuable
products, however, are those obtained from nitro
agrams, and has a very low'tensile strength. On
cellulose and polytetra?uoroethylene.
» cooling below 327° C. the polymer becomes trans
The ratio of the tetrafluoroethylene polymer
lucent or opaque, becomes more crystalline, and 26
and
the volatile or combustible ?lm-forming ma
regains its tensile strength. However, above 327°
terial will vary depwding upon the properties
O. and even at temperatures as high as 450° C.,
desired. Thus, when high proportions of the
the polymer assumesnone of the ordinary prop
film-forming material are used in the prepara
erties of a liquid or ?uid.
'
30 tion of ?lms, thin ?lms are generally obtained.
This invention has as an object a new and
Since the ?lm-forming material is destroyed in
‘ practical method for forming articles from tetra
?uoroethylene polymer. Other objects will \ap- 7 the processing steps, it is usually desirable to use
pear hereinafter.
.
' as small quantities as possible and still maintain
.
In the best method of practicing the invention -
the desired article is formed by shaping a mix
ture of comminuted polytetra?uoroethylene. and
a heat-decomposable ?lm-forming material, and
then removing the ?lm-forming material at a
su?lcient rigidity to facilitate handling of the
initial shaped object. In most instances the com
bined weight of the tetra?uoroethylene polymer
and ?lm-forming material will consist of from
10% to ‘75% of the latter.
The time and temperature of heating of the
temperature sumclently high to ?ash off said
?lm-forming material and continuing the heat '40 shaped object will be dependent upon the type
of ?lm-forming material used and the nature of
ing until the polytetra?uoroethylene coalesces as
the tetrafluoroethylene polymer. In general the
more fullydescribed hereinafter. By “?ash o?”
temperature of heating will be above the melting
is meant the burning, volatilizing or degrading
or transition point of the tetra?uoroethylene
to volatile products.
'
45 polymer. Thus, in compositions containing poly
The invention is illustrated by the
tetrafluoroethylene, the temperature of heating
example in which parts are by weight.
will usually be between 350 and 500° C.
One part of comminuted polytetrafluoroethyl
The heating step is preferably carried out in
ene is stirred into four parts of commercial col
‘following
the presence of oxygen supplied by the atmos
volatile solvent mixture). The resulting viscous 50 phere, from a tank, or by oxygen-liberating ma
terials such as inorganic nitrates, e. g., ammo
slurry or suspension is cast ‘on a glass plate and
nium nitrate.
the solvent permitted to evaporate. The plate
This invention provides an improved, and
containing the cured, polytetra?uoroethylene
economical method for obtaining shaped poly
?lled nitrocellulose ?lm is next passed into an
"oven maintained at- 430—450° C. The nitrocel 55 tetra?uoroethylene articles such as rods, tubes, lcdion (a solution of nitrocellulose in a highly
2,418,498
4
and unsupported ?lms. Films, ?bers, and mas
sive objects can be advantageously used as elec
trical insulation. v".i'he ?bers and ?lms can be
' an amount or from 10% to 75% ‘or the combined
wrapped onto the article to be insulated. These
wrappings can be set by heat treatment to elimi
nate their tendency to unwrap. This heat-set
mixture thus obtained into‘ the shape of the
desired article, heating the shaped mixture to
a temperature of from 350° C. to 500° C. and
continuing said heating until said polymer
weight of said tetra?uoroethylene polymer and
said organic ?lm-forming material, forming the
ting treatment, particularly of oriented ?bers
and ?lms, results in an increase in the ?exible
coalesces.
life, i. e., the resistance to heat emhrittlement,~
21. A process for obtaining shaped articles
of the ?bers and ?lms. The process disclosed 10 from polytetra?uoroethylene which comprises
herein can also be used to coat porous materials,
mixing the polymer with nitrocellulose inv the
such as asbestos sheets and ?bers, carbon, mica,
proportion of from 10% to 75% of the nitro
bentonite, glass, and metal wires with the poly~
cellulose on the basis of the combined weight of
mer.
.
said ‘polymer and nitrocellulose, forming the
As many widely di?erent embodiments of this 15 mixture thus obtained into the shape of the de
invention may be made without departing from
sired article, heating the shaped mixture to a.
the spirit and scope thereof, it is to be under
temperature of from 350° C. to 500° C.. and
stood that thisiinvention is not to be limited to
continuing said heating until said polymer
the speci?c embodiments thereof except as de
coalesces.
?ned in the appended claims.
'
,
I claim:
20
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v
3. The process set forth in claim 1 in which
,
said organic ?lm-forming material is a cellulose
1. A process for obtaining shaped articles
from tetra?uoroethylene polymer which com
4; The process set forth in claim 1 in which
prises mixing the polymer with another organic
said organic ?lm-forming material is a cellulose
?lm-forming material which decomposes at a 25 ester.
temperature of from 350° C. to 500° C'., said
- JULIAN W.
.
ether.
organic ?lm-forming material being present in
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