CA1050719A

CA1050719A - Sintered hollow tetrafluoroethylene polymer resins

Info

Publication number: CA1050719A
Application number: CA220,935A
Authority: CA
Inventors: Takumi Saito
Original assignee: Mitsui Fluorochemicals Co Ltd
Current assignee: Chemours Mitsui Fluoroproducts Co Ltd
Priority date: 1974-02-26
Filing date: 1975-02-24
Publication date: 1979-03-20
Also published as: DE2508361C2; GB1476272A; FR2262069B1; FR2262069A1; IT1033208B; NL7502295A; NL180395C; JPS5313229B2; JPS50114447A; DE2508361A1; NL180395B

Abstract

TITLE

SINTERED HOLLOW SINTERED
POLYMER RESINS

ABSTRACT OF THE DISCLOSURE

Void containing particles of sintered non-melt-fabricable tetrafluoroethylene polymer resins are prepared by atomizing an aqueous dispersion-containing unsintered tetra-fluoroethylene polymer particles, a surface active agent and a foaming agent, and passing the atomized dispersion into a sintering chamber where the unsintered polymer particles coalesce and sinter.

Description

~5~7~L9 Fleld o~ the In~ention This invention relates to te~rafluoroethylene polymer resins and their preparation; and more particularly to sintered tetra~luoroethylene polymer resins.
BACKGROUND
Tetrafluoroethylene polymer resins have good heat resistance and chemical resistance, as well as low ~riction and non stick properties. Thus they are useful as coating materials and lubricants. However, the resins have a high specific gravi~y and a low affinity for organic solvents which make them susceptible to precipi~ating from an organic liquid dispersion. It thus is difficult to apply the resins to sub-strates from an organic liquld vehicle such as trichloro-ethylene.
SUMMARY OF THE IN~ENTION
-A sintered tetrafluoroethylene polymer resin has now ~een obtained which remains suspended and dispersed in an organic solvent such as tri~hloroethylene to a much greater extent than any heretofore obtainable.
This resin comprises sintered particles of a non-melt-fabricable tetrafluoroethylene polymer wherein the particles have an average particle size o~ between about 5 and 500 microns, and a sphere factor of between 1.0 and about 1.2, said resin particles containing at least one enclosed void which causes the resin to have a precipitation rate in trichloroethylene o~ no greater than 20%.
The particles are prepared b~ (a~ atomlzing an aqueous dispersion obtained from the dispersion polymerization of tetrafluoroethylene, containing (i) 5-80% by weight of' unsintered non-melt f`abricable tetrafluoroethylene polymer

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1~507~
resin particles, based on the weight of the water and the particles, wherein the particles have an a~erage particle size o~ about 0.05 to about 0.5 micron, (li) about Q.2 to about 20% by weight based on the weight o~ the resin parti-cles of a nonionic or anionic surface active agent having a boiling point less than the sintering temperature of the polymer, and (iii) about 0.2 to about 20% by weight based on the weight of the dispersion of a liquid or solid foaming ~
agent, said liquid foaming agent having a boiling point ;
below the sintering temperature of the polymer and having a solubility in water o~ 15 gm./100 ml. or less at 25C., said soli.d foaming agent being decomposible into volatile compon-ents at a temperature below the sintering temperature of the polymer; (b) heating the atomi~ed aqueous dispersion at a temperature above the sintering temperature of the polymer; (c) cooling the sintered polymer particles to below ~ -the slntering temperature of the polymer; and (d) recovering the sintered polymer particles.
DESCRIPTION OF T~E DRAWINGS
_ Figure 1 is a schematic cliagram of the equipment u~ed in this invention.
Figure 2 is an enlargement of a photograph of resin particles of this invention.
DESCRIPTION OF THE I~ENTION
The tetrafluoroethylene polymer powder-employed in this invention may be the homopolymer of tetrafluoroethylene or a non-melt-fabricable copolymer thereo~ with at least one other copolymerizable ethylenically unsaturated monomer. The comonomers include monochlorlnated perfluorinated olefins of 2-4 carbon atoms, such as chlorotrifluoroethylene, per~luori-. . . ... .. _ ~ ~ 5~
nated (alkyl vinyl) monomers o~ 3~10 carbon atoms such as hexa~luoropropylene; or per~luoro(aIkyl vinyl ethers~ o~ 3;10 carbon ato~s such as per~luoro(methyl vinyl ether) or perfluoro (propyl v~nyl ether). me copolymers mu~t be non-melt-~abricable, i.e., copolymers ln which the melt vi~co~ity o:E
the copolymer i8 at lea~t 1 x 109 poise~ at 380C. ~Che amount of each comonomer pre3ent must therefore not be great enough to render the copolymer melt-~abricable. The amount will vary dependin~ on the comonomer used, but can easily be determined by ~inding ~he melt visco~ity. The melt viscosity 1~ mea~ured according to A~Lerican Society of Testing Materlals test D-1~38-52T, modi~ied as descr~bed ~ollowing: me cylinder~ ;
ori~ice and piston tip are made o~ a corro~ion-re~l~tant allo~, H~YNES STELLITE~ 19, made by Hayne~ Stellite Co~Lpany. 'rhe 5.0 æm- 5ample iS charged to the 9.53 mm. inslde diameter cgllnder, whtch 1~ maintalned at 372C, * l~C. Five minutes after the sample i~ charged to the cylinder it ig extruded through a 2~10 mm. diameter, 8.oo mm. lon~ square edged under a load (piston plus weight) o~ 5000 grams. This corresponds to a shear stress o~ 0.457 kilograms per cm.2. m e melt visco~ity in poises is calculated as 53150 divided by the ob~ervable extru~ion rate in grams per minute.
me un~intered tetra~luoroethylene resln powder emplo,yed herein æhould be resin powder prepared b~ the aqueous di3per~ion polymerization method. Resin particles produced by thls method generally have an a~erage particle ~ize of between about 0.05 and 005 micron~ and hence, it is convenlent to use this size particle herein~ However, it is under~tood th~t smaller or larger ~ize particle~ can be employed.
me amount o~ the tetrafluoroethylene resin powder * denotes trade mark ~a151~7~
present in the aqueous dispersion used in the process of this invention should be between about 5 and about 80% by weight based on the weight of the dispersion, and preferably between ~,, about 20 and 60%. If more than 80%, the dispersion tends to become unstable and coagulates; whlle if less than 5%, the heat load becomes lar~e which makes the process economically ,, un~avorable.
The aqueous dispersion used in the process of this invention also conkains about 0.2 to about 20% by weight based on the weight of the polymer present 7 preferably 0.5 to 10%, of ,~
a nonionic or anionic surface acti~e agent, which acts to ~
stabilize the dispersion and assist the foamlng agent. If the ~ ' sur~ace active agent is not present, the particles obtained from the process are generally not hollow. The surface active agents should have a boiling point less than the sintering temperature of the polymer.
Suitable nonionic surface active agents include the reaction products of ethylene oxide with other compounds which impart hydrophobic moieties to the resultant surfactant, such as propylene oxide, amines, saturated and unsaturated alcohols and acids, and alkyl phenols. For purposes of illustration, some of the foregoing mentioned nonionic surfactants are fur-ther illustrated hereinafter by the formulae:
A. R ~ (A)n H ~x wherein (A)n is the group ~C2H40~n or a mixture of the groups ~C2H40~a and ~C3H60~b, wherein n in each instance is an integer of from 2 to 50 and preferably 2 to 18, b is an integer of 0 to 30, and a is an integer of at least 2, a ~ b being equal to n; x is an integer of 1, 2, or 3; and

3 R is an aliphatic hydrocarbon group which can be saturate,d 1~5~7~
or unsaturated, straight-chain, branched, or cyclic, or combinations thereof and will generall~ contain from 8 to 24 carbon atoms, preferably from 8 to 18 carbon atomsi examples o~ R groups include oleyl, stearylg tridecyl, lauryl, decyl and the groups derived from aliphatic glycols and triols;
B. R'-C6H40 (B)mH, wherein B is the group ~C2H40~C or a mixture of the groups ~C2H40~C and ~C3H60~d, wherein m in each instance is an integer o~ from 2 to 50 and preferably 8 to 20, d is an integer of O
to 30, c ig an integer of at least 2, c ~ d being equal to m;
R' is a monovalent aliphatic and usually saturated and con-taining 4 to 20 carbon atoms and pre~erably 8 to 12 carbon atoms;
~. R~
R3 - N ~(CH2CH20)pH ~z and R3 - CON C(CH2CH20)pH ] , wherein p is an integer of 2 to 50, z is an integ.er of 1 or 2, R3 is an alkyl group containing 1 to 8 carbon atoms, R2 is a chemical bond to a group ~CH2CH20)pH , when z is 2 and an alkyl group of 1 to 8 carbon atoms when z is 1, with the proviso ~hat at least 5 carbon atoms are provided by R2 + R3 ;
and D. The polyalkylene oxlde block copolymers o~ the ~ormula HO (c2H4o)e(c3H6o)f(c2H4 )g ~
wherein f is an integer of from 15 to 65 and e and g are 3 lntegers sufficiently large that e ~ g total 20 to 90% of the 11~5~
total weight of the polymer. Preferred compounds of this class `~
are those in which g is zero. For each o~ the surface active agents A-D of the foregoing described formulae, the hydrophobic and hydrophilic moieties are proportioned such and the total molecular weight is such tha~ the copolymer is a surface active agent, the aforementioned requirement of water solu~ility is met, and preferably the aforementioned degree o~ vola~ility is also met. Additional specific surface active agents include CH3(cH2)4cH2(oc~I2cH2)30Hi CH3 ( CH2) 6CH2 ( oCH2C~I2) 30H;

3(cH2)locH2(ocH2cH2)l2(ocH~cH3)cH2)5oH;
CH3 ( CH2) 8CH2 ( OCH2CH2) 10 CH3 ( CH2) 8CH2 (0CH2CH2) 50H; and CH3c(cH3)2c}l2c(cH3)2~3-(ocH2cH2)loo~I. , Suitable anionic surface active agents include those commonly used in fluorocarbon polymerization such as those descr~bed in U.S. Patent Nos. 2,559,6297 2,559,752, 2,816,082, and ln German Patent No. 1,194,580 and French Patent No~

13406,778. Surf`ace active agenks con~alning fluoroalkyl groups of at least 6 carbon atoms are commonly used, wherein the term fluoroalkyl is understood to include also the completely fluorinated alkyl groups containing one or more of hydrogen or chlorine atoms in addition to fluorine.
Another class of the fluoroalkyl surface active agents, as disclosed in U.S. Patent No. 2,559,752, have the structure BtCF2)nCOOY, wherein B is hydrogen or fluorlne; Y
is hydrogen, the ammonium radical, a lo~Jer alkyl amine radical or a ~etra(lower alkyl ammonium) radical, and n is an integer of from 6 to 20 and preferably ~rom 6 to 12.
3 Suitable hydrocarbon-based surface active agents whlch can be used under the conditions described herein include :
both ionic types such as isodium lauryl sulfate and nonionlc t~pes such as TRITON* X 100.
The ioamina agent e~ployed in the aqueou~ di~per~ion should be volatile or decomposable into volatlle components.
It ~hould be one that i~ liquid or ~olid until it reaches the noz~le opening and ~hould have a bolling point or decompo~i-tion temperature, as the case ma~ be, bel~w the sinterlng temperature o~ the tetra~l~oroethylene polymer. Pre~erably liquid ~oaming agents should have a solub~lity in water of 0-15 ~m./100 ml. or less at 25C. and a boiling point of between about 25 and about 120C. If solid, they should have a decomposition point of between about 25 and about 120C.
The ~oamlng agent ~hould be pre6ent ~n an amount o.~ betw~en about 0.2 to about 20~ by welght ba~ed on the weight o~ the a~ueou~ dlæpersion, and preferably between about 0.5 and about 10~. Ammonlum bicarbonate and carbonate are especially ef~ective ~ince thelr decompos~tion product~ are gases.
Suitable llquid or ga~eou~ foam~ng agents include halocarbon3 of 1-6 carbon atom~ such as trichlorotrifluoroethane, tr~
chloromonofluoromethane, chloroform, methylene chlorlde9 1,1-di~luoro~2,2,2-trichloroethane, 1,1-dichloro-2J2,2-tri.~luoroethane, or l,l-di~luoroethane; hydrocarbons o~ 5-~8 carbon atoms such a~ pentane or hexane; organic hydrocarbon or halohydrocarbon ether~ or esters such as ethyl acetate, dlethyl ether, petroleum ether or diprop~l ether. Halocarbon foaming agent~ are preferred among liquid ~oaming agents becau~e o~ lower ~lammabilit~
The disper~ion used in the proce~s i~ prepared by polymerl3~ng the monomers~ tetra~luoroethyl0ne alone or a * denote~ trade mark '7~
mixture thereo~ with comonomers3 provided the amount o:f comonomer 1~ not suf~ic~ent to re~ul~ in a melt-fabricable copolymer, by the aqueous di~persion polymeriza~ion procedure.
To the re~ulting aqueous dispersion can be adde~ the surEace active agent and the foa~ing agent.
To aid in obt~ining a ætable3 unifo~ dispersion, especially when the foaming agent is a volatile one~ it is preferable to emulslfy or suspend the lngredlents by sub~ect-ing the aqueous dispersion to an ultrasonic wave. A suitable ultrasor~c wave generator i~ SONIFIER* B--12 Model B-12A manu-~actured by Branson Sonlcpower Compar~. Alternativel~, or additionally, a stable, ~ni~orm dlsper~on may be~ obtained by increa~ing the solubility of the foaming agent eikher through lowerlng the temperature o~ the aqueous dispersion or through malntalning the di~persion under pressure.
In addiyion to the ingredients di~cussed above, the aqueous dispersion may contain flller materials in powder ~orm such as pigments, carbon black, colloidal silica powder, molybdenum disulfide power or graphite powder. me size o~
the riller material should be ~mallg e.g., about the ~ame ~ize as the tetra~luoroethylene polymer particles or less~
The amount o~ flller that can be preæent is at most l~bouk 3%
based on the ~eight of polymer.
The dispersion is ~ed to an ~to~izer at a tempera-ture below the slntering temperature of the polymer, and pre~erably at room temperature, e.g., 20-30Co ~ where it passes throucgh an in~ec~io~ nozzle lnto the a~mo~phere of a ~ntering chamber malntained æt a temperature above the s1nterinca temperature of ~he polymer~ By the term "atom~ze"
i~ meant that the a~ueou~ disperæion is separated into minute * denotes trade mark 1~5~7~
droplets. The e~uipment employed is similar to the spray drying equipment employed in, e.g., the production of dried foods such as dried milk.
The atomizer employed can be any atom~zer that can distribute fine drops of the aqueous disperslon substantially uniformly into the sintering chamber. A binary fluid nozzle, high pressure nozzle, or the like, are among the atomizers that can be employed. Preferably a binary fluid nozzle is used because less shear force detrimental to the polymer particles is appl~ed as compared with some other types of atomizers.
In the sintering chamber, the temperature o~ the atmo~phere therein must be above the sinterlng temperature of the polymer particles and high enough to cause the particles to sinter within about 0.5 ~o about 5 seconds, i.e., during free fall. The temperature of the atmosphere should not be so ~ -high, however, as to cause substantial decomposition of the polymer. The atmosphere can be any gas inert to the polymer particles, e.g., nitrogen or helium, but pre~erably it will be air, which o~ course, will contain fuel exhaust gas -lf a gas burner is used to heat the atmosphere. Pre~erably also it wlll be composed of the same gas used in the atomizer. Dur-ing slntering, moisture, surface active agent and foaming agent are vaporized and the resin particles are coalesced and sintered into hollow particles.
The sintered particles are then cooled to below the slntering temperature and collected by any usual means~ such as by means of a filter bag or cyclone.
The sintered particles have an average particle size between about 5 and 500 microns and thus are produced by the coalescin~ o~ the smaller unsintered polymer partic]es of ~L0507~9 the atomized dispersion. The sintered particles are round having a sphere factor of between 1.0 and about 1.2. The ~intered par~icles also contain enclosed voids which result rrom the coalescing action of the unsintered part;icles. Each sintered particle appears to contain one large void, but this is not known with certainty. The void content is measured indirectly by measuring the precipitation rate of the resin particles in trichloroethylene, as more fully described below.
As mentioned above, the precipitation rate in trichloroethylene should be no more than 20%, preferably no more than 10%, and most preferably no more than 2%.
The size of the sintered particles can be altered as desired by varying the pressure of the inert gas flowing through the atomizer (referred to in the Examples as "primary"
gas, as opposed to the heated gas wlthin the sintering chamber), by varying the construction of the atomizer nozzle, by varying the velocity of the aqueous dispersion traveling into the atomlzer, or by varying the amount of foaming agent in the a~ueous dispersion.
The sintered particles are useful as additives for coating materials, grease and inks, and Gan be applled ~rom a dispersion in an organic liquid.
In the Examples described below, particle properties were determined as described following:
Average particle size is measured by placing a small amount of sample on a glass slide and dispersing it into a thin layer by shaking. The plate is then microphotographed.
On a print, the largest and shortest diameters (a and b) of - each particle are measured~ using more than 300 particles 3 selected at random. The average particle size is calculated ~.

, - - ,,~

50'7~
as rOllOws: ~ ;
average particle = _ ~ 2 Sphere factor is a measure of the degree o~ round-ness o~ a particle and is measured by the equation:

Sphere ~actor = n ~ (b-i) (i ~ l, 2, 3...n) The degree to which the unsintered partlcles of polytetrarluoroethylene or of a non-melt-~abricable betra-fluoroethylene copolymer become sintered is measured by "DTA
percent sintered" value. 40 milligrams o~ sample is placed in a sample tube and heated up to 350C. at a heating rate Ol' 5C./min.; during this time the peak meltlng point of the resin is recorded. Then the DTA percent sintered value is calculated according to the equation:

DTA percent _ 2 x dl 10 sintered ~2 x dl) ~ d2 x 0 wherein dl is the height o~ khe absorption peak of sintered polymer and d2 ls the height o~ the absorption peak of unsintered polymer. This Differential Thermal Analysis (DTA) is based on the ~act that sintered polytetrafluoroethylene is about 50% crystalline and melts at about 327C. and unsintered polytetra~luoroethylene is almost 100% crystalline and melts at about 340C. For copolymers, d1 an~ d2 will be lower. The height Or the absorption peak on the DTA graph is used in the equation above.
The void content of the sintered particles is determined by measuring the precipitatlon rate in trichloro-ethylene. This determination is made by placing 1 gm. of the sintered particles into a 25 ml. glass stoppered bottle, adding 25 ml. o~ trichloroethylene, shaking ~or 20 seconds to disperse the contents and then allowing the contents to stand in the bo~tle for two hours~ The trichloroethylene ~s~9 ~:
and sintered particles suspended in it are then removed by syphoning them out wi~h an aspirator. The sintered particles that have precipitated to the bottom of the bottle are then dried and weighed to obtain weight value W2. The precipl-tation rate, measured in %, is calculated as follows: ~
W2 ' :
% Precipitation Rate = 1~ gm x 100.
Apparent specific gravity is determined according to the procedure described in A5TM D 1457-62To The following Examples are illustratlve of the invention.
Exam~e 1 Referring to Figure 1, air (referred to in the Examples herein as primary air) pressured by air compressor (3) is sent through pipe (4) to binary fluid nozzle (5). By suction and gravity, the aqueous dispersion in tank (1) is delivered through pipe (2) to binary fluid nozzle (5). The primary air atomizes the dispersion emerging from the outlet (6) of nozzle t5) into fine drops. These ~ine drops then emerge inko sintering chamber t9). Air (referred to as secon-dary air in the Examples herein) is heated by air heater (7) having a built-in gas burner (19) and is sent via pipe (8) into the sintering chamber (9). The pipe (8) and sintering chamber (9) are ordinarily insulated with asbestos material.
The fine drops atomized from nozzle t5) are mixed with the heated air in sintering chamber (9) where they foam. The moisture and additives volatize and the tetrafluoroethylene particles sinter. The sintered powder is cooled in cooling section (10) by cool air from cooling pipe (11). After being cooled to below its melting point, the sintered powder is sent to cyclone tl2) and recovered from outlet (13) into receiver (20). The exhaust gas is exhausted by exhaust fan _ ~3 - , ~ "

~507~L~
(14) into the atmo~phere through conduit (15). The tempera-ture in the system is controlled b,y thermo~eter~ ~16, 17 and 18~ placed in the secondary air pipeJ slntering chamber and at the inlet of the cyclone, respect~vely.
In thls Example,3 gm. of TRITON X-100 (nonion~c ~ur~ace actlve agent ma~ufactured by Rohm and Haas Co.) was dis~olved ln 50 gm. o~ water and then 50 gm. of 1,1,2-tri chloro-1,2,2-trifluoroethane wa~ added thereto. me mixture wa~ made lnto an emulsion at 8C~ with the use of an ultra-30nic wave. ml~ emulsion was added to, mlxed with and di~-persed in 1000 gm. of aqu~ous dlsper~ion containing 35~ by weight o~ polytetrafluoroethylene flne particles havln~ an average ~i~e of 0.2~.

The dl~perslon wa~ put ln tank (1) shown ln Flgure 1. After starting exhaust fan (14~, gas burner (19) in ~ir heater (7) was ign~ted to heat 3intering chamber (9~0 men, after ~tarting primary air compre~sor (3), the disperslon wa~
atomi~ed ~rom blnary fluid nozzle (5) havlng a diameter of o.8 mm. into sintering chamber (9~.
me propertie~ of the ~intered powder obtained from cyclone (12) and the condltion~ of manu~acture are a~ ~hown ln Table 1 under column 1.

1000 Gm~ of aqueou~ disper310n contain$n~ 35% by weight of a copolymer conslsting of WQi1i`8 of tetrafluoroethyl-ene and about 0.15~ by weight of unit~ of hexafluoropropylene was added with 106 gm. of water emulslon containing 6 Km. o~
TRITON X-100 and 50 gmO of 1,lg2-trichloro-1,2,2-tri~luoro thane.
me disper~ion thu~ prepared was atomized in the same manner as ~n Example 1.

The properties and the condit~ons of manufacture o~
the slntered powder thus obtained are ~hown in Table 1 under c o Lu~ 2 0 Ex~m~e 3 1000 Gm. of aqueous disper~ion containing 20~ by welght o~ polytetrafluoroethylene was addecl ~lth 112 gm. of water emul~ion containing 12 gm. o~ TRITO~ X-100 and 50 gm.
o~ 1~1,2-trichloro-1,2,2~tri~1uoroethane. The di~persion thus prepared was atomized in the ~ame manner a5 in Example 1.
me propertles and the condition~ of manuf~cture o~
the ~intered powder thus obta~ned are ~hown in Table 1 ~n column 3. ` ' Example 4 1000 ~m. o~ aqueous di~per~ion conta~nin~ 60~ by weight o~ pol~tetrafluoroethylene was added with 136 gmO o~
water emulslon containing 36 gm. of TRITON X-100 and 50 gmO
o~ 1,1,2-trichloro-1,2,2-tri~luoroethane. me dispersion thus prepared was atomized ln the same manner as ln Exa~ple lo The properties and the conditions o~ manu~acture o~
powder thus obtained are shown in Table 1 in colum~ 4.
~ '' ' 1000 ~m. of aqueous di~per~ion containlng 35% by weight of polytetra~luoroe~hylene was added with 81 gmO of wa~er emul~on containing 6 gm. o~ TRITON X-100 and ~5 gm. o~
propyl ether. The dispersion thus prepared was atomlzed in the same manner a~ in Exa~ple 1.
The propertles and the conditions o~ manu~acture o~
powder thu~ obtained are shown in Table 1 in column 5.
Ex~m~le 6 1000 ~m. o~ aqueou~ dl~perslon containing 35% by , ~)507~9 weight of polytetra~luoroethylene was added wlth 6 gm. of TRITON X-100 and 50 gm. of ammonium carbonate. The di6per-sion thu~ prepared was atomized in the same manner a in Example 17 me properties and the conditions of manufacture o~
powder thu~ obta~ned are shown ln Table 1 ln column 6.
Example 7 1000 Gm. of aqueou~ di3persion containing 35% by weight of tetrafluoroethylene was added with 106 gm. o~ water emul~ion containing 6 gm. of dodecyl benzene ~ulfo~ate and 25 gm. o~ petroleum ether~ The d~per~ion thus prepared was atomized in the ~ame manner as ln Example 1.
The propertie~ and the conditions of manu~acture of powder thu~ obtained are shown in Table 1 in column 7.
Ex~mple 8 me Bame dispersion as in execution Exa~ple 1 above was atomized ~rom hi~h pressure nozzle having a diameter o~
1.5 mm. in the 3ame manner as in Example 1.
me properties and the conditions o~ manufacture o~
~0 powder thus obtained are sho~n in Table 1 in column 8.

.~

7~ ~
(No use of foaming agent) 1000 GmP o~ aqueous dl~per~ion containing 35~ by welght o~ polytetra~luoroethylene was add d with 3 gm. o~
TRITON ~-100. me di~persion thu~ prepared wa~ atomi~ed in the ~ame manner a~ in Example 1.
The properties and the condition~ o~ manufacture of p~wder thu~ obtalned are sho~n in Table 2 under column 1.
(No u3e o~ ~ur~ace active a~ent) 1000 Gm. o~ aqueou~ di~persion containing 35~ by welght o~ polytetrafluoroethylene wa~ added with 25 gm. of propyl ether, whlch was atomized in the 8am2 manner a~ ln Example 1.
me propertie~ and the condition~ o~ manufacture of powder thus obtained are shown ln Table 2 under column 2~ :
Com~ari~on Example C (Example o~ a copolymer that i8 melt-~abricable) 1000 ~m. of aqueous di~per~ion containlng 20~ by wei~ht of tetra~luoroethylene - hex~fluoropropylen~ copolymer (84/16 by weight) was added with 112 gm. of water emulsion con--tainin~ 12 ~m. o~ TRITON X-100 and 50 gm. of 1,1,2-trich:Loro-1,2,2-trifluoroethane. me dlsperslon thu~ prepared was atomized ~n the ~ame manner a3 in Bx~mple lo The properties and -the condltions of manu~acture o~
powder thus obtained are shown in Table 2 under column 3.

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Claims

The embodiments of the invention in which an exclusive prop-erty or privilege is claimed are defined as follows:

1. A resin comprising sintered particles of a non-melt-fabricable tetrafluoroethylene polymer wherein the particles have an average particle size of between about 5 and 500 microns, a sphere factor of between 1.0 and about 1.2, said resin particles containing at least one enclosed void which causes the resin to have a precipitation rate in tri-chloroethylene of no greater than 20%.

2. The resin of Claim 1 wherein the polymer is polytetrafluoroethylene.

3. The resin of Claim 1 wherein the polymer is a copolymer of tetrafluoroethylene and at least one other co-polymerizable ethylenically unsaturated monomer.

4. The resin of Claim 3 wherein the other copoly-merizable ethylenically unsaturated monomer is hexafluoropro-pylene.

5. The resin of Claim 3 wherein the other copoly-merizable ethylenically unsaturated monomer is perfluoro (propyl vinyl ether).

6. The resin of Claim 1 wherein the tetrafluoro-ethylene polymer was prepared by aqueous dispersion polymeri-zation.

7. The resin of Claim 6 wherein the polymer is polytetrafluoroethylene.

8. The resin of Claim 5 wherein the polymer is a copolymer of tetrafluoroethylene and at least one other co-polymerizable ethylenically unsaturated monomer.

9. Process for preparing sintered particles of non-melt-fabricable tetrafluoroethylene polymers having at least one enclosed void therein, which comprises:
(a) atomizing an aqueous dispersion obtained from the dispersion polymerization of tetrafluoro-ethylene, containing (i) 5-80% by weight of unsin-tered non-melt-fabricable tetrafluoroethylene polymer resin particles based on the weight of the water and the particles, wherein the particles have an average particle size of about 0.05 to about 0.5 micron, (ii) about 0.2 to about 20% by weight based on the weight of the resin particles of a nonionic or anionic surface active agent having a boiling point less than the sintering temperature of the polymer, and (iii) about 0.2 to about 20% by weight based on the weight of the dispersion of a liquid or solid foaming agent, said liquid foaming agent having a boiling point below the sintering temperature of the polymer and having a solubility in water of 15 gm./100 ml. or less at 25°C., said solid foaming agent being decomposible into volatile components at a temperature below the sintering temperature of the polymer;
(b) heating the atomized aqueous dispersion at a temperature above the sintering temperature of the polymer;

(c) cooling the sintered polymer particles to below the sintering temperature of the polymer;
and (d) recovering the sintered polymer particles.