CA1050717A - Process for preparing sintered tetrafluorethylene polymer resins - Google Patents

Abstract

TITLE

PROCESS FOR PREPARING SINTERED
TETRAFLUOROETHYLENE POLYMER RESINS
ABSTRACT OF THE DISCLOSURE
A procedure is described for preparing individual particles of sintered tetrafluoroethylene polymer resins by atomizing unsintered polymer particles. The unsintered par-ticles can be of polytetrafluoroethylene, non-melt-fabricable copolymers thereof, or melt-fabricable copolymers thereof;
and are atomized and sintered by passing them, with a stream of gas, through an atomizer and into a sintering chamber.

Description

~D~ n~ ~E ON
Field o~ the Invention . . . ~
ml~ inventlon is related to the preparation o~
tetra~luoroethylene pol~mer resin particles, and more par ticularly to sintered te-trafluoroethylene polymer resin particles.
Description of the Prior Art Tetrafluoroethylene polymers are used in a wide variety o~ applications in the chemical, mechanical~ electri-cal and other industries because o~ their good heat resistance,chemical resistance, and electrical properties. In sintered powder ~ormJ the polymers are use~ul as molding resins, a~
lubricants, and as coating agents, dependlng upon the type po.lymer and the ~orm o~ the powder.
To aid in understandlng ~he lnvention described herei.n, it is pointed out that traditionally tetra~luoroethyl-ene polymers have been d.Lvided in three groups: (1) granular polymers produced by aqueous suspension polymerizatlon, (2) fine powder polymers produced by dlspersion polymerizationJ
and (3) melt-~abricable polymers produced usually by aqueous dispersion polymerization. Particles o~ polymers of group (1)~ as produced, are o~ irregular shape and usually need to be ground to be adapted for specific uses; while those o~
group ~2) tend to become ~ibrous when even a low shearing ~orce is appliedO Polymers of group (3) are low ln apparent specific gravity and are difficult to handle, for this reason they are not ordinarily suitable ~or use in powder coating applications.
Polymers o~ group (1) when treated as described ;-following and sintered are used for ram extrusion because of _ 2 ~ ;

~iD7~7 their good powder flo~ and high bulk density. Polymers of groups (1) and (2)g when sintered in small particle form~ are useful in lubricating applications. In one method of producing the sintered particlesJ coarse particles obtain~d ~rom the polymerizatlon process are pulverlzed or ground, fused by sintering, and then are ground with a grinder to a size which depends on whether the particles are to be used for extrusion or lubrication. me several grindlng steps render the procedure economically unattractive, and alternative pro-cedures have been employed.
In one alternative procedure, ground powder is mixedwith a heat-resistan-t inorganlc salt so that on heating to ~intering temperatures the ground powder particles will not become fused with each other. The inorganic ~alt is then removed. This procedure is not attractive because o~ di~fi-culty in removing the inorganic salt.
Another alternative procedure is to heat the unsin-tered particles to about 450C. or higher or irradiate -them, ~ollowed by grinding the degraded particles. Thls procedure results in loss of product, generation oE noxlous gas or loss o~ p~ysical properties due to rupture of molecular chains, and is therefore undesira~le.
Still another alternative is to a-tomize the aqueous dispersion of -the polymer that has been produced by the dis-persion procedure and then sinter the atomized particles in a sintering chamber. This procedure results in the vaporiæation of other material~ present in the aqueous dispersion~ thus causing corrosion and volatiles removal problemsO
Polymers oF group (3)J being melt-~abricable~ are ordlnarily extruded after isolation from the polymerlzation 3S~7~L~
vessel into strands, which are then out to produce pellets.
Such pell~ts need to be ground into powder form, and the powder partlcles have poor flow because of thelr irregular shape.
SUMMARY OF THE INVENl'ION
The various grinding or pulverizing step3, or the removal of liquids is not necessary in the presen-t inventiong which is:
A process ~or preparing sintered particles o~ a tetrafluoroethylene polymer which comprisesJ in sequence~
mixing at a temperature below the melting point of the polyrner, particles of unsintered tetra~luoroethylene polymer powder with a stream o~ a ga~ Lnert to the powderg passing the stream of gas and powder through an atomizer capable o~ atomlzing the powder~ heating the gas and atomized powder ln a chamber at a temperature above the melting point o~ the polymer, cooling the particles, and recovering them.
By the term "melting point" as used herein is meant the temperature at wh~ch the crystalllne structure of the tetra~luoroethylene polgmers di appears.
By the term "sLntered" as used herein is meant that the tetrafluoroethylene polymers have been heated above the:Lr meLting po:Lnt and then cooled to below their meltLng poLnt.
By the term "atomizing" as used herein is meant that the tetra~luoroethylene powder is separated into minute indlvidual particles.
DESCRIPTION OF THE DRAWIMGS
Figure l is a diagram of -the equipment used ln the sequence o~ steps of this invention.
Figure 2 is a diagram o~ a variation o~ the equip-ment shown in Figure l, ~ 4 --Figures 3a, 3b and 3c are diagram o~ several atomiz-lng nozzle~ *hat can be employed in thi~ invention~
DESCRIPTION OF THE INVENTION
~ . .
The ~etraYluoroethylene polymer powder which may be employed in thi3 lnvention may b~ a powder of the homopolymer of tetrafluoroethylene or a copolymer o~ tetra~luoroethylene ana up to about 35~ by w~ight o~ the copolymer o~ at least one other copoly~erizable eth~lenically un~a~urated monomer.
The~e comonomer~ include ole~ins o~ 2-4 carbon atom such a~
ethylene or l-butene; monochlorinated per~luorin~ted olefins o~ 2_4 carbon atoms ~uch a~ chlorotri~luoroethylene, perfluori-nated olefin~ o~ 3-10 carbon atoms ~uch as hexarluoropropylene;
or perfluoro(alkyl vinyl ether~) of 3-10 carbon atom3 such a~
per~luoro(methyl v~nyl ether) or per~luoro(propyl vinyl ethor).
me powder~ o~ the polymer~ described above are produced a~ de~cribed ln the art. For example, powder o~
granular polytetrafluoroethylene i~ produ~ed by aqueou~ ~u~-pension polymerization. Fine powder polytetrafluoroethylene i~ produced by aqueou~ dispersion polymerization. Both the~e 20 procedure~ include the preparation of copolymer~ wherein the comonomer content i~ not hlgh enough to cau3e the polymer to lo~e the non-melt characterlstic o~ poly-tetra~luoroethylene, l.e., where the melt vl~cosity of the polymer lg at lea~t 1 x 109 polse~. The melt visco~ty is measured accordlng to American Society o~ Te3tine Mater~als te~t D-1238-52T~ mod~-~ied a8 de~cribed following: The cyl~nder, orif~ce and pi~ton tlp are made o~ a corro~ion-resistant alloy~ HAYNES STELT~TE*
19, made by Haynes Stelllte CompanyO The 5. 0 ~m~ ~mple ls charged to the 9.53 ~m~ ineide d~ameter cylinder~ which 1 maintained at 372G. ~ 1C. Five minute~ after the ~ple 1 charged to the cyllnder lt i3 e~truded throu2h a 2.10 n~
* denote~ trade mark ~ o~
dlameter, 8.oo mmO long square edged under a load (piston plus weight) of 5000 grams. mis corresponds to a shear s^tress of 0.457 kllograms per cm.2. me melt viscosity in poises is cal-culated as 53150 divided by the observable extrusion rate in grc~ms per minute.
Powder of melt-~abricable tetrafluoroethylene co~
polymers is produced by either aqueous dispersion or organic solution polymerization.
The resln particles produced by the above polymeri-zation procedures are isolated, and optionally dried be~oreemployment in the process of this invention~ They can be ground to smaller size after isolation, if desired. It is not nece~sary to dry the particle~ prior to atomization. mus~ ::
the s~ze of the un~intered resin particles employed hereln can range ~rom 3~ or less to 3000~ or more, wi-th the size employed depending on the use to which the products of this inventlon J
are put. Slnce tetrafluoroethylene polymer resin partlcles tend to stick together, i.e., agglomerate; it is pre~erable before emplo~ing the unsintered resin particles in -the process o~ this invention, to deagglomerate them ~ust prior to feeding them into the atomizing nozzle. This deagglomeration can be accomplished by contact with a hiC~h veloci-ty lnert gas, e.g.
as is produced in a cyclone.
The unsintered powder, a~ter deagglomeration lf needed, is ~ed into an in~ection nozzle with a stream of an inert gas, such as nitrogen or air~ maintained at a tempera-ture below the melting point o~ the polymer to prevent prema-ture sintering o~ the partlclesq Ihe powder is atomized in the nozzle and injected into a sintering chamber ~illed with an inert gas maintalned at a temperature above the melting 7~L7 polnt of the polymer powder. Preferably, in each instance9 the same inert gas is used.
In feeding the powder and the gaseous stream into the in~ectlon nozzle any type of feeding means can be employed.
Thus 5 a screw feeder, rotary ~eeder or magnetic feeder may be used -to feed the powder into the stream of gas, whereupon the gas stream carries the powder particles into the inaection nozzle (or ~irst through a deagglomerator if needed~. It is convenlent, however, to place the powder in an open tank and pass a stream of the gas through a jet located above the tank, whereupon suction created by the movement o~ gas dra~s the powder particles into the gas stream. It is convenlent also to use the gas at ambient temperatures, pre~erably between 20-30C., although the gas stream can be at any temperature below the melting point of the powder particles.
me in~ection nozzle employed is one that substan-tially evenly atomizes the powder while injecting it into the sintering chamber, such as, ~or example, an injection nozzle fitted with a "Static Mixer" therein or a nozzle which has at its throat a rotary disc or a cone~shaped dispersing plate.
Furthermore, the nozzle should be "double-tubed" so that it can be easily cooled to prevent premature melting of the particles. mese nozzles are illustrated in Figures 3a, 3b and 3c. Figure 3a illustrates a "Static Mixer" nozzle in whlch the powder enters cyllnder 70 at entrance 71, passes through a twisted plane plate 72 inside the cylinder and exits cylinder 70 in atomized form at 43. Jacket 4~ ~urrounds cylinder 70 and cooling air enters the jacket at in:Let 45 to prevent premature melting of the particlesO me cooling air exits at 46. Figure 3b illustrates an injec-tion no~zle 31~S~l7 having a rotary disc. In Figure 3b J the powder enters cylin-der 50 at entrance 51, and emerges at 52 where it strikes disc 53 and is thrown out into the sintering chamber due to the rotation of disc 53 caused by rotation of shaft 54.
Jacket 55 surrounds cylinder 50, and cooling air enters the ~acket at inlet 56 and exits at 57 to prevent premature melting of the partlcles. Figure 3c depicts an irljection nozzle having a ~ixed cone. In Flgure 3c, the powder enters cylinder 60 at entrance 61, and emerges at 62 where it strikes fixed cone 63 and is deflected lnto the slntering chamber. Jacket 64 surrounds cylinder 60, and cooling a~r enters the ~acket at inlet 65 and exits at 66 to prevent premature melting of the particles.
In the sinter~ng chamber, the atomized powder parti-cles become heated to a temperature above their melting polnt due to thelr contact with the heated gas in the chamber. ~ -Contact time of the particles with the heated gas i8 not critical provided it is long enough to melt the particles. In the pre~erred procedureJ the atomized particles will free-~all downward through the chamber and thus the melting tlme will be short, e.g., about 005 second to about 5 seconds. me tem-perature o~ the gas inside the sinterlng chamber ~hould not be so high that the polymer particles decompose through thermal cracking. A means o~ determining this temperature of decom- -position is provided by a "w~lght loss on heating test" de~
scribed ~urther below. mis test measures the degree of decomposition of the polymer powder and ~or the particles ob-tained by the proces~ o~ this invention the weight loss as measured by th:Ls test should not be more than 0.5~. In general for the polytetrafluoroethylene homopolymers ancl the~r copolymers that are non-melt-fabricable the temperature of the gas in the slntering chamber will be between about 330C. and 400C.; while for the melt-fabricable copolymers it will be between about 270C. and 400C.
The sintered particles can be cooled in the sintering chamber if the chamber is constructad to provide for "free-~all" of particles. Wlth thls construction, the particles are melted in the upper portlon of the chamber and are cooled in the lower part. Alternatively, the melted part:Lcles can be cooled ln a chamber adjacent the sintering chamber. me cooled sinteres particles are then recovered by conventional means, such as those which employ a cyclone or fil-ter bag.
me sintered powder so obta~ned comprises small sintered partlcles that are not coalesced or fu~ed together.
The particles have a slze that i~ dependent on the slze o~ the unsintered part~cles employed. I~ small size sintered part~-cles are desired, small size unsintered particles wlll be employed as the unsintered material. I~ larger size slntered particle~ are desired3 the larger si~e ~msintered particles will be employed.
The sintered powders have good flow and high bulk denslty. m e larger slze sintered partlcles, eOg~l 50-600~1 in average size are use~ul ~or ram extrusion moldlng, while the smaller size particles~ e.g., 10-50~1 in average size~ are suitable as lubricants or coatlng materials, In addltion, sintered particles of melt-~abricable tetrafluoroethylene copolymer~ are useful as coating materials, for ram extruslon purposes or mold release materials~
In the Examples descr~bed below, particle properties were determined as described ~ollowing:

~ 9 w 7~o~
Average partlcle slze is measured by placinæ 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) o~
each particle are measured, using more than 300 particles selected at random. The average particle size læ calculated as follows:
avera~e particle _ 1 ~ (a~ 2 bi) ~ 2, 3...n) Sphere ~actor is a mea~ure o~ the degree o~ round-ness of a particle and is measured by the equation:
Sphere factor = N ~ ~ (i = 1, 2~ 3...n) Apparent speciflc gravlty ls determlned by placing5.0 gm. of s~mple into a measurlng cylinder with a plug (25 - ml tube) and then adding trichlorotrifluoroethane until the total volume is 25 ml. A~ter shaking violently to make a suspended m~xture, and after settllng for 24 hours, the volume Or settled solids is recorded. men the weight of the sample, i.e., 5 gmO, is divlded by the volume o~ settled solids~ l'he unit obtained, expressed in gm./ml. is the apparent specific gravity. Measuring the apparent speclfic gravlty ln tr~chloro-tri~luoroethane is advantageous ln that the value obtalned is reproducible and can be used for ~udglng the quality o~ coating materlals made from the resin of the sample because the ~llling property in the solvent is indicative o~ usefulness as a coat-ing material. The higher the apparent specific gravity, the greater the chance of obtaining a void free film and a thicker fi~n.
Specific surface area is a measure of the sinterlng propensity of the resin and is determined by nitrogen adsorp-tion.

-- 10 -- . .

-` 10S~)7~
Angle of repose 15 a measure of the p~wder flow o~
the resin and is determined with a "Powder Tester" type PT-D
made by Hosokawa Tekko Sho, Osaka, Japan ln the following manner: The powder is passed down~ards through a glass funnel with a vibration at 23C. The funnel is 70 nm. in diameter and is provlded wlth a glass tube 40 ~n. long and 6 mm. in internal diameter. It is located 70 mm, above a table. The angle of repose of the powder plled up on the table is measured.
The powder used must be com~letely dry and free o~ static electricity.
Specific gravity ls determined with a pycnometer using trichloroethylene at 23C. as disclosed in ASTM D 153-54 (1966) method A.
Weight loss on heating as used herein is a measure o~ how much polymer has been degraded during sintering and is determined in two ways. For polytetrafluoroethylene poly-mers and ~or non-melt-~abricable copolymers, a 10 gm. sample i8 placed into a 6.45 cm.2 mold and sub~ected to a pressure of 70 kg./cm.2. The sample, removed ~rom the mold~ is heated at 300C. for one hour. It is then cooled to room temperature, and weighed to obtain value Wl. Then the sample is heated for 10 hours ln the atmosphere at 380C. ~~ 2C. and is cooled to room ~emperature and welghed to obtain value W2. The weight loss on leating measured in ~ is determined by using the equation.

~ Weight loss on heating = ~ x 100.

For melt-~abricable tetraf'luoroethylene copolymers, a 10 ~m.
sample is heated at 200C. ~or one hour and then cooled and weighed to obtaln value Wl. Then the sample is heated at 380C. ~ 2C~ for 30 m~nutes under a pressure of 10 mm. Hg9 then cooled and weighed to obtain value W2. The same equatior employed above is used to find the ~ weight loss on heating.
The degree to which the unsintered particles o~ poly-tetrafluoroethylene or of a non-melt-fabricable tetrafluoro ethylene copolymer become sintered is measured by "DTA percent sintered" valueJ 40 milligrams of sa~ple is placed in a sample tube and heated up to 350C. at a heating rate of 5C.~min~;
during this time the peak melting point of the resin is re-corded. Then the DTA percent sintered value is calculatedaccording to the equation:
DTA percen~ = 2 x dl ~ 100 wherein dl ~s the helght of the absorptlon peak o~ s~ntered polymer and d2 is the height of the absorption peak o~ unsin-tered polymer. mis Differential Thermal Analysis (DTA) is based on the ~act that sintered polytetrafluoroethylene is about 50~ cryætalline and melts at about 327C. and unsintered poly~
tetrafluoroethylene is almost 100~ crystalline and melts at about 340C. me height of the absorption peak on the DTA
graph is used in the equation above.
The following Examples are illustrative of the inven-tion. ~-~!:~
.
Re~erring to Figure 1~ air (re~erred to in the Examples herein as primary alr) pressured by air compressor (3) is forced to e~ector (2) and tetrafluoroethylene powaer in raw material tank (1) is drawn to conduit (4) by the suc tion created by the flow of air through e~ector (2)o l~le powder is carried with the stream of air into a deagglomerating equipment (5)~ which is an inverted cyclone, where any 1~]?

~2 -~sa 7~7 of the powder ls deagglomerated into initlal primary particles~
These prlmary particles are evenl~ dispersed and atomiæed into sintering chamber tlO~ by "Statlc Mixer" (7) ~itted ln in~ec-tion nozzle (6)~ Air (referred to as ~econdary a~r in the Examples herein) i~ heated by air heater (8) having a built-ln gas burner and is sent via conduit (9) into the sintering chamber (10). The conduit (9) and sintering chamber (10~ are insulated with asbestos material. The primary particle~
atomized from injection nozzle (6) mix wlth the heated air ln sintering chamber (10) and become sintered into tetrafluoro-ethylene sintered powder. The sintered powder is sent ~ia conduit (11) lnto`cooling chamber (12). A~ter being cooled to below its melting point, the powder is sent to cyclone (13) and taken into receiver (20) ~rom outlet (14). rme exhaust gas is exhau~ted by exhaust ~an (15) into the atmosphere through con-duit (16). The temperature in the system is controlled by thermometers (17, 18 and 19) placed in the secondary air con-duit, sintering chamber and at the inlet o~ cyclone (13) J
respectively.
In thi~ Example, the powder used was granular type polytetra~luoroethylene resin powder havlng an average particle size of 3.5~, an ap,parent specific gravity of 0~22 gm./ml., a speci~ic sur~ace area of 6.9 m. /gm. and an angle of repo~e o~
50 degrees. mis powder was put into raw material tank (1), ' shown in Figure 1. men a~ter actuating exhaust fan (15), the gas burner in air heater (8) was employed to heat the air in sintering chamber (10). A~ter heating tha air, compressor (3) was actuated and the powder was atomized from in~ec-tion nozzle (6~ into sintering chamber (10).
rme properties of the sintered powder obt~ined from cyclone (13) and the conditlons of manu~acture are as shown in the following Table I.

.
Granular type polytetraPluoroethylene resin powder having an average particle size of 24~, an apparent speci~ic gravity of o.46 gm./ml.3 a specific sur~ace area of 3.1l m.2/gm and an angle of repose of 49 degrees was atomized as descr~bed in Example 1 under the conditions shown in the ~ollowing Table 1. `' The properties of the sintered powder obtained are shown in the followina Table I.
Example 3 Granular type polytetra~luoroethylene resln powder compris~ng particle~ called secondary particles~ having an av~rage particle size of 330,u, which were agglomerated from primary particles having an average partlcle size of 26~, and having an apparent specific gravity of o.86 gm./ml., a speci-~ic surface area of 3.5 m.2/gm. and an angle of repose of 37 degrees was atomized as described in Example 1 under the conditions shown in the followlng Table I.
The properties of the sintered powder thus obta~ned are as shown in -the following Table I.
Exam~e 4 Granular type polytetrafluoroethylene resin powder havlng an average partlcle size o~ 125~J an apparent specific gravity of 0.62 gm./ml.~ a specific surface area of 3.2 m.2/gm and an angle of repose of 47 degrees was atomized as described in Example 1 under the conditions shown in the ~ollowing Table I.
The properties of the slntered powder thus obtained are as shown in the ~ollowing Table I.

~ 14 - :

7~7 Exam~e 5 Granular type non~melt~iabricable resin powder of a copolymer of -tetrafluoroethylene and a small ~mount o~ per-fluoro(propyl vinyl ether) and having an average par-ticle size of 240~, an apparent specific aravity of 0.62 gm./ml., a speciiic surface area oi 5.4 m.2/gm. and an angle oi repose oi 44 degrees which had been prepared accorclina to the teachirlg oi Canadian patent l,Oll,O~L~ issued May 243 1977 to Sperati.
et al, was atomized as described in Example 1 under the con~
dltions shown in the following Table I.
'rhe properties of the sintered powder thus obtained are as ~hown in the followlng Table I.
le 6 Disperslon type polyte-trafluoroethylene resin powder consisting oi primary particles having an average particle size of 0.20~ and secondary par-ticles having an average parti-cle size oi 430~, an apparen-t specific gravity of 0051 gm./ml.
a specliic sur~ace area of 10.1 m. /gm. and an angle of repose of 39 degrees was mixed with crushed dry ice powder .in order to handle the resin without deformation and atomized as described in Example 1 under the conditions shown in the following Table I.

~ 5a7~7 ~ .
Referring to Figure 2, air pressured by air compres-sor (22) is forced into ejector (23). By th~s suction of this ejector, a tetrafluoroethylene copolymer powder in tank (21) is delivered into conduit (24). The powder~ carried by the air stream through conduit (24)~ is evenly d:ispersed by "Static Mixer" (25) ~itted in injection nozzle (26) and atomized into sintering chamber (30). The noæzle (26) is prevented from being heated by cool air sent through Jacket (27). The atomlzed powder is mixed with air, which has been heated by heater t28) ha~-~ng a bu-llt-on gas burner (41) and sent via conduit (29) to slnterlng chamber (30), and meltedO The melted powder thus produced is delivered to cooling section (31) connected with the lower part of the ~intering chamber, cooled by cooling alr sent ~rom pipe (32) to a temperature below the meltlng point o~ the powder resin and then delivered via conduit (33) into cyclone (34), where it is collected and taken from outlet (35) into receiver (42), me exhaust gas ls exhausted by exhaust ~an (36) through condult (37) into the atmosphereO rme -tempera-ture ln the system is controlled by thermometers (38, 39 and40) placed in the air conduit (29), sinterlng chamber and at the inlet o~ the cyclone,respectlvely.
In thls Example, a melt-~abrlcable tetrafluoroe-thyl-ene copolymer containing 84% by wei~ht of units o~ tetra-~luoroethylene and 16~ by we~ght of units o~ hexafluoropro-pylene was employed.
Under stirring, an aqueous dispersion of the copoly-mer (12~ solids by weight) was added with nitric acid to coagulate the dlsper~ion and then trichlorotrifluoroetha.ne wa~
added thereto and stirring was continued to obtain a coagulated ~1~5~7~7 powder having an average particle size of 2.1 mm. ~This coagulated powder was deagglomeratable and when it was sub-jected, for example, to an ultrasonic wave in an organic sol-vent, it could easily be deagglomerated and dlspersed in the solvent.) The dried coagulated powder was put; into tank (21) shown in Figure 2. After starting exhaust fan (36)~ gas bur- -ner (41) fitted in air heater (28) wa~ ignited and the -temper-a-ture in slntering chamber was raised to a fixed tempera-ture.
10 Then air compressor (22) was started and the powder was atomized -~
~rom injection noæzle (26) lnto the sintering chamber.
The properties of the sintered powder obtained ~rom cyclone (34) and the conditions o~ production are shown in the ~ollowing Table II.
Example 8 An aqueous dispersion o~ a melt-~abricable copol~mer contalning 96% by weight units o~ tetra~luoroethylene and 4~ by weight units of per~luoro(propylvinyl ether) was treated as in Example 7 to obtain a dried coagulated powder having an average particle size of 2.6 mm. This powder was atomized as descrlbed in Example 7. The propert~es of the sintered powder obtalned and -the conditions of production are shown in the ~ollowing Table II.
Example 9 me coagulated powder obtained in Example 8 was atomized as described in Example 7. me properties o~ the s~ntered powder obtained and the conditions of produc-tion are shown in the following Table II.
Example 10 A pellet was prepared from the coagulated powder - 18 _ D7~L7 obtained in Example 8 and the pellet was ground by a mixer into an irregularly shaped powder having an average particle size o~ about 100 microns. This powder was then atomized as de-scribed in E~ample 7. l'he properties o~ the sintered powder obtained and the conditions of production are shown in the following Table II.

The aqueous dispersion described in Example 8 was treated as described in Example 7 to obtain a coagulated pow-der having an average particle size of 0.8 mm. me coagulated powder was heated at 290C. for 2 hours and on cooling was determined to have a specific sur~ace area of 6.9 m.2/gm. The powder was ~airly hard and not deagglomeratable on subjection to ultrasonic wave~ in an organic solvent. The powder was screened with a 420 micron sieve and the particles passing throu~h were atomized as described in Example 7. me proper-tles of the sintered powder obtained and the cond~tions of production are shown in the following Table II.
E mple 12 The aqueous disperslon described in Example 8 was treated as described in Example 7 to obtain a coagulated powder having an average particle slze o~ 0.18 mm. me powder was atomized as described in Example 7 except that the tank (21) was replaced by a hopper which fed powder to conduit 24 and a cone-shaped disc was fitted to the outlet of the injection nozzle. Air pressure of -the ~orced air ~rom compressor ~22) wa~ reduced ln this Example to prevent deagglomeration o~ the coagulated particles.
The properties of the sintered powder obtained and the conditions o~ production are shown in the ~ollowing Tab:Le II~

~ 7 xample 13 The aqueou,s dispersion described in Example 8 was treated as described in Example 7 to obtain a coagulated powder .
having an average particle size of 0.4 mm. The powder was atomized as described in Example 12. The properties of the sintered powder obtained and the conditions of production are shown in -the following Table II.

Claims (10)

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

1. A process for preparing sintered particles of a tetrafluoroethylene polymer which comprises, in sequence, mixing at a temperature below the melting point of the polymer, particles of unsintered tetrafluoroethylene polymer powder with a stream of gas inert to the powder, passing the stream of gas and powder through an atomizer capable of atomizing the powder, heating the gas and atomized powder in a chamber at a temperature above the melting point of the polymer, cooling the particles, and recovering them.

2. The process of Claim 1 wherein the particle size Or the unsintered polymer particles is between about 3µ and about 3000µ.

3. The process of claim 2 wherein the gas is air.

4. The process of Claim 1 wherein the tetrafluoro-ethylene polymer is polytetrafluoroethylene.

5. The process of Claim 1 wherein the tetrafluoro-ethylene polymer is a non-melt-fabricable copolymer of tetra-fluoroethylene and at least one other copolymerizable ethylen-ically unsaturated monomer.

6. The process of Claim 1 wherein the tetrafluoro-ethylene polymer is a melt-fabricable copolymer of tetrafluoro-ethylene and at least one other copolymerizable ethylenically unsaturated monomer.

7. The process of Claim 6 wherein the other copoly-merizable ethylenically unsaturated monomer is hexafluoropro-pylene.

8. The process of Claim 6 wherein the other copoly-merizable ethylenically unsaturated monomer is perfluoro (proply vinyl ether).

9. The process of Claim 1 wherein the gas is air.

10. The process of Claim 1 wherein the polymer powder is deagglomerated before it is atomized.