emulsion polymerization of tetrafluoroethylene: effects of reaction conditions on particle formation

Emulsion polymerization of tetra¯uoroethylene:effects of reaction conditions on particle formation

C.U. Kima, J.M. Leea,*, S. K. Ihmb

aChemical Process and Engineering Research Center, Korea Research Institute of Chemical Technology, PO Box 107,

Yusong-gu, Taejon 305-343, South KoreabDepartment of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong,

Yusong-gu, Taejon 305-701, South Korea

Received 23 September 1997; accepted 25 March 1998

Abstract

The emulsion polymerization of TFE was carried out with a surfactant (FC-143) and an initiator (ammonium persulfate) under various

reaction conditions. Two different shapes of PTFE dispersion particles were produced by the emulsion polymerization, rods and spherical

particles. Variables, except for surfactant concentration, do not affect the initial and ®nal particle morphology when no coagulation occurs.

Rod-like particles are formed when the surfactant concentration is near or above the critical micelle concentration (CMC) of the surfactant.

At below CMC, however, spherical particles are dominant. No hexagon-shaped particles or abrupt changes in the particle morphology are

observed during polymerization.

When no coagulation occurred, the size and number of polymer particles suddenly increased in the early stages of emulsion

polymerization and then steadily increased as time progressed. Most particles are formed in the early stages (within 5 min), and the size of

particles formed was in the range 60±80 nm in diameter. The particle sizes ranged from 100 to 230 nm at 250 g/l of a speci®c TFE uptake

concentration (the PTFE dispersion had a solid content of about 20%).

It was found that the size and number of polymer particles formed are almost independent of the temperature, pressure, concentration of

initiator and stabilizer (anti-coagulant), whereas the surfactant concentration, kinds of stabilizer, and agitation speed are important. The

particle size decreased gradually as the dispersion concentration increased, whereas the particle number increased exponentially.

When the PTFE dispersion coagulated during polymerization under certain conditions (high temperature and agitation speed, and low

concentration of surfactant), the particle size suddenly increased, the particle number suddenly decreased, and the rate of the polymerization

decreased, indicating that the polymerization occurred on the surface of the polymer particles. # 1999 Elsevier Science S.A. All rights

reserved.

Keywords: Emulsion polymerization; Tetra¯uoroethylene; PTFE dispersion

1. Introduction

A polytetra¯uoroethylene (PTFE) dispersion, which is

normally prepared by the emulsion polymerization of tetra-

¯uoroethylene (TFE), has many applications because of its

excellent properties ± thermal and chemical resistance, in

particular. The dispersion of colloidal particles is very useful

in ®elds where colloidal suspensions are applied such as dip-

coating, electrodeposition, and ®lm casting, etc. [1±4].

Many patents have been registered for the emulsion

polymerization of TFE with chemical initiators and surfac-

tants [5,6]. However, only a few scienti®c papers have been

published regarding the quantitative analysis of the kinetics

and the mechanism of the emulsion polymerization, includ-

ing the size and numbers of the PTFE particles formed.

Generally, emulsion polymerization has been discussed

on the basis of Harkins' hypothesis and the Smith±Ewart

theory [7,8]. In their theories, it is assumed that the particle

generation occurs with each entry of a dissolved free radical

into an emulsi®er micelle and continues until the micelle

population vanishes. But, the accepted concepts of the

mechanism of emulsion polymerization have been ques-

tioned and new ideas have been proposed [9,10]. Roe [11]

re-evaluated the Smith±Ewart theory, particularly for parti-

cle generation, on the basis of many experimental results. He

proposed that the particle generation is independent of the

presence of micelles; it occurs at each interaction of a

dissolved free radical with a dissolved monomer molecule

and continues until the emulsi®er is depleted to a level below

Journal of Fluorine Chemistry 96 (1999) 11±21

*Corresponding author.

0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.

PII: S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 3 2 8 - 5

which new particles cannot achieve stability by adequate

adsorption of emulsi®er.

In the formation of particles of the PTFE dispersion,

Punderson [4] observed that the polymerization of the PTFE

dispersion passes through two distinct periods or phases.

The initial period of the reaction is a nucleation phase in

which a given number of polymerization sites or nuclei are

established. Subsequently, there occurs a growth phase in

which the predominant action is the polymerization of TFE

on established particles with little or no formation of new

particles. The transition from nucleation to the growth stage

occurs smoothly when the solid particle is between 4% and

10% in solution. Suwa et al. [12] found that most particles

are formed within 5 min after the initiation of the reaction in

an emulsi®er-free emulsion polymerization of TFE by

radiation, and the polymerization loci change is governed

by TFE pressure/dose rate ratio.

The shape of the PTFE particles during the polymeriza-

tion of TFE would be expected to be different from amor-

phous spherical polystyrene particles due to the high degree

of crystallinity [13]. The possibility of controlling the PTFE

particle size and geometric shape by changing the surfactant

type and/or its concentration was ®rst suggested by Berry

[14]. He showed that, besides creating approximately sphe-

rical PTFE dispersion particles, it is possible to synthesize

dispersions of rod-like PTFE particles by polymerizing TFE

in the presence of high concentrations of selected ¯uori-

nated surfactants. Rahl et al. [15] observed that the rod-like

PTFE particles are more likely in polymerizations of a low

conversion, while polymerizations of a high conversion

gave mostly spherical particles.

Suwa et al. [16] related the PTFE particle morphology to

the molecular weight (MW) by the radiation emulsion

polymerization of TFE. The PTFE polymer of low MW

forms rod-like aggregates, while the high-MW PTFE

emerges in the form of approximately spherical dispersion

particles. Luhmann and Feiring [17] reviewed the PTFE

particle morphology using electron microscopy and

observed the thermal behavior by DSC. Essentially three

types of PTFE dispersion particles were distinguished:

dispersion particles resembling hexagons, approximately

spherical dispersion particles resembling cobblestones,

and rod-shaped dispersion particles. Thermal behavior by

DSC in virgin PTFE samples monitored in a temperature

range between ÿ108C and �408C was shown to be related

to the particle morphology.

Particle number directly affects the reaction rate in

emulsion polymerization and depends on the mechanism

of particle formation (micellar initiation, precipitation from

the aqueous phase, or polymerization in monomer droplets),

as well as on coalescence processes. Although the particle

formation of the PTFE dispersion was known to be domi-

nated generally by the action of the surfactant, the exact

mechanism of particle formation remained unclear.

This study is mainly concerned with the morphology of

the PTFE dispersion in polymerization and the effects of the

reaction conditions on the polymer particle size and number

in the PTFE dispersion polymerization; FC-143 was used as

surfactant and ammonium persulfate was used as an initiator.

The reaction conditions of this study involve the effect of

agitation speed, the concentration of surfactant and initiator,

temperature, pressure, concentration and type of stabilizer.

2. Experimental

2.1. Material

TFE monomer was supplied by 3F (China). Before using

TFE, impurities such as the inhibitor d-limonene, octa¯uor-

ocyclobutane (RC-318), di¯uoromethane (R-32), etc.) were

removed by passing the sample through silica gel and a

granular activated carbon bed at ÿ258C. The TFE purity,

checked by gas chromatography was about 99.9995%.

Water (about 18.2 M), puri®ed by Millipore Milli-Q,

was used as the polymerization medium for the PTFE

dispersion. n-Hexadecane from Aldrich and paraf®n waxes

described in Table 1 were used as stabilizers without further

puri®cation. Ammonium persulfate from Sigma was pur-

i®ed by recrystallizing it twice from pure water. The purity

of the ammonium persulfate was above 99.5% and was

analyzed by the iodine titration method. Ammonium per-

¯uorooctanoate (FluoradTM Fluorochemical surfactant FC-

143) from 3M was used as a surfactant without any pur-

i®cation. The pH value of the aqueous solution was about 5.

2.2. Polymerization procedure

Fig. 1 is a diagram of the experimental apparatus. The

experimental setup consists of the TFE puri®cation system

and glass vessel (model Buchiglauster). The reactor is a

cylindrical jacketed glass vessel with a dished bottom

Table 1

Type of stabilizer

Type of stabilizer Melting point (8C) Iodine valuea Manufacturing company

p-Wax(1) 56±58 0.06 Wako (Japan)

p-Wax(2) 56±58 0.06 Avondale laboratory (England)

p-Wax(3) 56±58 0.12 Sinyo (Japan)

p-Wax(4) 44±46 0.23 Matsunoen chemical (Japan)

n-Hexadecane 18 0.06 Aldrich (USA)

a Analysis method; ASTM no. D1959.

12 C.U. Kim et al. / Journal of Fluorine Chemistry 96 (1999) 11±21

equipped with an anchor-type impeller, and an injection/

sampling port that allows for the injection of initiator/water

and sampling during polymerization.

A detailed diagram of the vessel in the equipment is

shown in Fig. 2. TFE uptake is controlled at a constant

pressure by the Buchi press¯ow gas controller (model bpc

6010) capable of detecting a ¯ow change of 1-l.

The desired amounts of surfactant, stabilizer and water

were charged in the reactor and the reactor was degassed

with slow stirring. The reactor was then evacuated for about

10 min and ¯ushed out ®ve times with nitrogen at 10 bar and

three times with TFE at 3 bar. The monomer was transferred

to the buffer tank after it was passed through a silica gel

column at ÿ258C to remove the inhibitor in the TFE. The

inhibitor free TFE was transferred to another buffer tank,

operated by a gas booster which kept the pressure at about

10 bar. This gas was passed through the activated carbon

column at ÿ158C and transferred into the reactor.

After the stirring, speed and temperature reached the

desired values, the pressure inside the reactor was adjusted

to a reaction pressure. A desired amount of the initiator

dissolved in water was injected into the reactor. The TFE

supply line was opened, allowing the TFE to ¯ow into the

reactor while TFE uptake (liter) and reaction time at the

constant pressure were recorded and controlled by the Buchi

pressure gas controller.

The reaction temperature was controlled with an accuracy

of �0.18C by a circulating heating medium (silicone oil),

which included a thermostatic bath and circulator (model

Hakker 320). The standard experimental conditions are

given in Table 2.

2.3. Measurement of average particle size

The particle size of the PTFE dispersion was determined

by an electron microscope and a sub-micron particle ana-

lyzer. Samples were withdrawn from the reactor through a

sampling valve.

To measure the particle size using electron microscopy

(JEOL JEM-7), PTFE dispersions were diluted 100 times

with pure water. A drop of the diluted dispersion was

deposited on a glass plate and evaporated at room tempera-

ture and then the particles were observed under the micro-

scope. (The detailed calculation procedure of the average

particle number and size from the microscopic data can be

found elsewhere [12].) About 100 particles from the SEM

were used to estimate the average particle size, and the data

were calculated by an image scanner.

The average particle size was also measured by a sub-

micron particle analyzer (COULTER1 model N4SD). The

PTFE dispersion was diluted within the measurement range

10ÿ4±10ÿ6 counts per s; the average particle diameter, d,

was estimated by the Stokes±Einstein equation at a constant

temperature and a given viscosity of the solution.

D � kT=3�d;

where D is the diffusion coef®cient, k the Boltzmann's

constant, T the temperature, � the viscosity of the diluents,

and d is the equivalent spherical hydrodynamic diameter.

The number of particles in 1 ml water, np, was calculated

by the relation:

np � Vp � 10ÿ3=�Vx;

where Vx is the volume of a particle by an equivalent particle

radius, � the PTFE density (2.2 g/cm3), and Vp is the

Fig. 1. Schematic diagram of the experimental apparatus: (1) high purity nitrogen cylinder; (2) TFE cylinder; (3) silica gel column; (4) (6) buffer tank; (5)

gas booster; (7) activated carbon column; (8) pressure flow controller; (9) glass reactor; (10) and (11) thermostatic bath and circulator.

Table 2

Standard recipe for the emulsion polymerization of tetrafluoroethylene

Condition and variable Standard recipe

Pure water 500 ml

Surfactant: FC-143 3.48�10ÿ3 mol/l (0.75 g)

Initiator: ammonium persulfate 2.19�10ÿ4 mol/l (0.05 g)

Paraffin wax: p-wax(2) 40 g/l (20 g)

Agitation speed 500 rpm

Reaction pressure 4 bar

Reaction temperature 758C

C.U. Kim et al. / Journal of Fluorine Chemistry 96 (1999) 11±21 13

concentration of PTFE dispersion (g/l of H2O) calculated

from the sum of the dispersion and coagulated (or deposited)

polymers.

2.4. DSC Measurement

DSC (differential scanning calorimeter) measurements

were carried out using a 2910 DSC (TA Instruments). The

temperature of the sample was raised from 08C to 408C at a

constant rate of 108C/min while an exothermic curve was

recorded. The sample amount was 10�0.5 mg for all runs.

3. Results and discussion

3.1. Particle morphology

Fig. 3 shows the electron micrographs of the polymerized

PTFE dispersion for various periods at the standard experi-

mental condition shown in Table 2. The particle size

increases continuously as the polymerization progressed,

as shown in Fig. 3(A)±(D). Observed particle shapes were a

mixture of rod and spherical particles. The hexagonal-

shaped particles reported by Luhmann and Feiring [17]

were not observed in our experiments. Also, the change

of particle morphology from rod in initial stage to spherical

particles in the ®nal stage in the polymerization reported by

Rahl [15] was not observed in this PTFE formation.

These results were also con®rmed by the DSC analysis in

the temperature range between 08C and�408C, as shown in

Fig. 4. Luhmann and Feiring mentioned that the rod-like

and cobblestone dispersion particles are assigned to the

presence of exothermic phase transformation, I, around

�208C, the appearance of a third exothermic peak, I0, as

a shoulder from transformation I on its lower-temperature

side is attributed to the hexagonal dispersion particles. Our

results showed in peak I, suggesting that the polymer sample

consisted of cobblestone-like dispersion particles; these

results agreed qualitatively with the electron micrograph

observations.

The ®nal form of PTFE dispersion particles is primarily

affected by the concentration of the surfactant used. Chanzy

and Smith [18] assumed that the spherical particles are

developed from rod-like dispersion particles, and then

initially formed rod-like particles are stabilized by the

surfactant. As long as the surfactant encapsulates the grow-

ing rods, the particle growth continues in this mode, and the

transformation into spherical entities occurs when the sur-

factant/polymer concentration ratio is no longer suf®cient.

In our results, at various reaction conditions, only surfactant

concentration affects the initial and ®nal particle morphol-

ogy when no coagulation occurs.

Coagulation induced the rapid change of particle mor-

phology. Fig. 5 shows the effect of surfactant concentration

on the shape of PTFE dispersion particles. As shown in

Fig. 5(A) and (B), granular particles are the main population

when the polymerization is initiated at surfactant concen-

trations below the CMC, whereas the rods seem to form

frequently with increasing surfactant concentration, having

a broad size distribution [17]. As shown in Fig. 5(C) and

(D), rod-like particles establish the main population when

polymerization is initiated near or above the CMC. Typical

particle widths are in the range 50±100 nm at a solid content

of 20 wt%. Rods having widths of 5 nm have been detected

at the initial stage of polymerization.

3.2. Size and number of PTFE dispersion particles

Fig. 6 shows the typical results of particle size from PTFE

dispersion, as measured by a sub-micron particle analyzer at

50 min by the standard conditions shown in Table 2. The

standard deviation of the unimodal distribution obtained is

48 nm. A similar result was obtained by electron micro-

scopy, but the standard deviation is always larger. This

discrepancy in the particle size distribution may be due

to the non-spherical shape of the particles. Our experiment

was carried out as a semi-batch type with a continuous

Fig. 2. Detailed diagram of a reaction vessel (1000 ml): (1) anchor-type

agitator; (2) thermocouple; (3) baffle plate; (4) jacketed vessel.


uptake of TFE. Therefore, polymer concentration increases

continuously with reaction time; coagulations occur in any

concentration. We reviewed the effects of the reaction

variables in the experimental range until 300 g/mol of

TFE uptake concentration (about 23% of PTFE in solution)

was attained. Here, the TFE uptake concentration is de®ned

Fig. 3. Electron micrographs of PTFE dispersion polymerized under standard condition of Table 2 for various reaction times: (A) 8; (B) 32; (C) 52; (D)

103 min.

Fig. 4. DSC traces of virgin PTFE arranged as a function of surfactant concentration : (A) no addition; (B) 3.48�10ÿ3; (C) 9.28�10ÿ3; (D) 20.0�10ÿ3; (E)

46.4�10ÿ3 mol/l. Baseline at 408C adjusted to the concentration axis.


as 1 of TFE fed per 1 of water. It was used instead of the

PTFE concentration due to the occurrence of coagulation

under the reaction conditions.

3.2.1. Effects of agitation speed

Fig. 7 shows the effects of agitation speed (range from

250 to 750 rpm) on the size and number of PTFE dispersion

particles as a function of TFE uptake concentration. TFE

uptake concentration was used instead of reaction time for

the comparison in the same concentration (PTFE concen-

tration in solution). As shown in Fig. 7, the particle size and

number increased gradually with TFE uptake concentration,

as the agitation speed increased. At an agitation speed under

500 rpm, the particle diameter increased slowly and steadily

until it reached the concentration of about 300 g/l of the TFE

uptake concentration.

Fig. 5. Electron micrographs of PTFE dispersion polymerized for various surfactant concentrations: (A) 3.48�10ÿ3; (B) 9.28�10ÿ3; (C) 20.0�10ÿ3; (D)

46.4�10ÿ3 mol/l. Other reaction conditions are given in Table 2.

Fig. 6. Typical results of particle size of PTFE dispersion measured by

sub-micron particle analyzer at 50 min. Other reaction conditions are

given in Table 2.

Fig. 7. Particle diameter and particle number vs TFE uptake concentra-

tion, for polymerization at different agitation speeds: (*) 250; (&) 350;

(�) 430; (r) 500; (}) 750 rpm. Other reaction conditions are given in

Table 2.


However, when the agitation speed was extremely high

(above 750 rpm), the particle diameter suddenly increased

at 250 g/l of TFE uptake concentration, while the polymer-

ization rate reduced (Fig. 17) and coagulation increased.

When the agitation speed was beyond 850 rpm, the particle

size and the amount of coagulation of PTFE dispersion

increased sharply even at a low PTFE concentration. Thus,

high speed agitation induced a strong shearing force which

broke the PTFE dispersion and formed the coagulated

polymer. The decrease in the number of polymer particles

accompanied by an increase in particle size was caused by

the coagulation of the dispersion particles and by the

decrease of the polymerization rate. This may be a random

coagulation or ¯occulation of the particles which was more

pronounced at a high agitation speed.

3.2.2. Effects of the surfactant and initiator concentrations

Figs. 8 and 9 show the variation of size (Fig. 8) and

number (Fig. 9) of the particles formed as a function

of the TFE uptake concentration at various concentrations

of the surfactant (FC-143), ranging from 0 to 46.4�10ÿ3 mol/l. Here, CMC of FC-143 or C7F15COONH4 is

in the range 0.5±1.422% (33.0�10ÿ3 mol/l) depending

on the experimental techniques and temperature [19±22].

When the polymerization of TFE was carried out without

the surfactant, or the surfactant concentration was very

high (over CMC), the particles aggregated easily as the

reaction proceeded. Particularly, most of the particles

of the PTFE dispersion near or above CMC were the

rod shaped (Fig. 5(C) and (D)). When the surfactant

concentration was below CMC, as shown in Figs. 8 and

9, the particle size and number of polymer particles

increased abruptly in the initial stage and increased slowly

in the later stage. At 250 g/l of a speci®c TFE uptake

concentration, a particle size was about 100±180 nm,

due to the difference of surfactant concentration, whereas

the particle number (Fig. 10) was about 2�1012±20�1012

number per ml of water.

In Fig. 10, the particle size and number obtained by

Figs. 8 and 9 are plotted against surfactant (FC-143) con-

centration at a speci®c 250 g/l of TFE uptake concentration.

As noted previously, the particle size decreased gradually as

the dispersion concentration increased, whereas the particle

number increased exponentially.

Fig. 11 shows the effect of the initiator concentration on

the particle size and number with TFE uptake concentration

ranging from 0.87 to 8.77�10ÿ4 mol/l. As shown in Fig. 11,

the size and number of particles gradually increased with the

TFE uptake concentration, irrespective of the initiator con-

Fig. 8. Particle diameter vs TFE uptake concentration, for polymerization

at different surfactant concentrations: (*) 1.40�10ÿ3; (&) 3.48�10ÿ3;

(r) 9.28�10ÿ3; (�) 20.0�10ÿ3; (}) 34.4�10ÿ3 mol/l. Other reaction

conditions are given in Table 2.

Fig. 9. Particle number vs TFE uptake concentration, for polymerization

at different surfactant concentrations: (*) 1.40�10ÿ3; (&) 3.48�10ÿ3;

(r) 9.28�10ÿ3; (�) 20.0�10ÿ3; (}) 34.4�10ÿ3 mol/l. Other reaction


Fig. 10. Particle diameter and particle number vs surfactant concentration

at 250 g/l of TFE uptake concentration. Other reaction conditions are

given in Table 2.


centration. The concentration does not appreciably affect

the size and number of polymer particles produced. Similar

behavior had been observed in the emulsion polymerization

of a water-soluble monomer such as in methyl methacrylate

and vinyl acetate [16,23].

3.2.3. Effects of temperature and pressure

Figs. 12 and 13 show the effects of temperature on the

size and number of particles when the TFE uptake con-

centration was examined at 500 and 750 rpm, respectively.

In Fig. 12 (500 rpm), particle size and number increased

gradually with TFE uptake concentration as the temperature

increased, except at 1058C. At a high temperature (1058C),

however, the particle diameter suddenly increased and the

particle number suddenly decreased at 140 g/l of the TFE

uptake concentration. The decrease in the number of poly-

mer particles accompanied by a sudden increase in the

particle size was caused by a coagulation of the PTFE

dispersion particles. The amount of coagulated polymer

measured at 250 g/l of a speci®c TFE uptake concentration

was about 100 g per one liter of water.

In Fig. 13 (a high agitation speed of 750 rpm), a rapid

increase and decrease in the size and number of polymer

particles were observed irrespective of the reaction tem-

perature. This behavior may be due to the fact that high

speed agitation induces a strong shearing force which breaks

the PTFE dispersion. The amount of coagulated polymer at

250 g/l of a speci®c TFE uptake concentration and at 1058Cwas about 160 g per one liter of water.

Fig. 14 shows the effect of the reaction pressure on the

size and number of particles when the TFE uptake con-

centration was examined at three different pressures (3, 4,

6 bar). The particle size and number increased gradually

with the TFE uptake concentration irrespective of the

reaction pressure. No coagulation occurred in the range

of this experiment.

3.2.4. Effects of the stabilizer

The stabilizer was used to avoid coagulation during

polymerization or bulk polymerization in the gas phase

[13,14]. Fig. 15 shows the effect of the stabilizer type on

the size and number of particles with the TFE uptake

concentration. The stabilizers used are shown in Table 1.

Here, the iodine value represents the degree of double

bonding in the stabilizer. As shown in Fig. 15, the increase

of the particle size and number with TFE uptake concen-

tration can be divided into two groups due to the difference

of the iodine value. Polymerization using a stabilizer and


tion, for polymerization at different initiator concentrations: (*)

0.87�10ÿ4; (&) 2.19�10ÿ4; (�) 4.39�10ÿ4; (r) 7.72�10ÿ4; (})

8.77�10ÿ4 mol/l. Other reaction conditions are given in Table 2.


tion, for polymerization at different reaction temperatures and 500 rpm:

(�) 658C; (*) 758C; (r) 858C; (&) 958C; (}) 1058C. Other reaction



tion, for polymerization at different reaction temperatures and 750 rpm:

(*) 758C; (&) 958C; (}) 1058C. Other reaction conditions are given in

Table 2.


having a high iodine value resulted in larger particles than

those found in the lower iodine-valued polymerizations. It is

supposed that the impurities involved in p-wax can be

affected by the particle size and number. No coagulation

occurred in these experiments.

Fig. 16 shows the effect of the p-wax(2) concentration

(ranging from 0 to 60 g/l) on the particle size and number

with TFE uptake concentration. As shown in Fig. 16, the

size and number of particles tend to increase as polymer-

ization proceeds. When the concentration of p-wax(2) was

above 20 g/l, a linear relationship between the particle size

versus the TFE uptake concentration was observed. When p-

wax(2) was below 10 g/l, the amount of the coagulated

polymer increased as the amount of p-wax(2) decreased. In

polymerization without p-wax(2), most of the coagulated

polymer was deposited on the agitation blade at 250 g/l of

the TFE uptake concentration; at this condition, the particle

size suddenly increased and the particle number suddenly

decreased (at 170 g/l of TFE uptake concentration.)

3.2.5. Effects of TFE uptake rate and coagulation

Fig. 17 shows the relation between the TFE uptake rate

(polymerization rate) and reaction time, in order to review

the coagulation effects on polymerization rate. Coagulation

occurred in an extreme reaction condition. The typical

polymerization rate under standard conditions (Table 2)

sharply increases reaction time and levels off at any time.


tion, for polymerization with different reaction pressures: (*) 3; (&) 4;

(�) 6 bar. Other reaction conditions are given in Table 2.


tion, for polymerization with different stabilizers: (*) n-hexadecane; (})

p-wax(1); (&) p-wax(2); (�) p-wax(3); (r) p-wax(4). Other reaction



tion, for polymerization with different p-wax(2) concentrations: (})

without p-wax(2); (r) 10; (*) 20; (&) 40; (�) 60 g/l. Other reaction


Fig. 17. TFE uptake rate vs reaction time, for polymerization with

different conditions: (~) 758C; (&) 850 rpm; (&) 1058C; (5)

3.48�10ÿ3 mol/l of surfactant; (}) without surfactant. Other reaction



The coagulation of PTFE particles is restrained up to 300 g/l

of the TFE uptake concentration.

The polymerization rate was always reduced when the

polymer particles coagulated during polymerization. Co-

agulation leads to a decrease in the surface area of the

particle, which results in a decrease in the rate of supply

of the monomer and initiating radicals from the aqueous

phase. In addition, the growing polymer radicals on the

particle surface plug the particle inside, which leads to a

termination of the reaction. In the initial stage, the monomer

dissolved in water reacted with the initiating radicals and

formed polymer chains, which, in turn, are the particle

nuclei. The nucleation or particle generation is completed

after a few minutes and the particle growth continues as

polymerization proceeds. Crystallinity of the as-polymer-

ized PTFE was as high as 90%; therefore, the polymer

particles are rigid and the monomer does not diffuse to the

inside. Accordingly, the polymerization after the particle

generation proceeds largely on the surface of the particle on

which monomer is mainly located. This results in the

reduction of the polymerization rate when the polymer

particles coagulate.

3.3. Mechanism of particle formation

According to Harkins' theory [8], polymer particles are

generated principally in micelles swollen with a solubilized

monomer. The reaction theoretically starts when a free

radical from the aqueous phase invades a swollen micelle

and activates a solubilized monomer molecule. Chain

growth follows at the expense of the solubilized monomer

until it is terminated with another similarly produced radical

chain in the same micelle. The Harkins' hypothesis does not

seem to be applicable to TFE polymerization system studied

in this paper, because the coagulation suddenly occurred at a

low PTFE concentration in the polymerization when the

surfactant concentration was over CMC. The morphology of

the PTFE dispersion changed in a complicated manner with

varying surfactant concentrations.

As shown in Fig. 3(a), the polymer particles formed

rapidly in the initial stages of polymerization (within

5 min) and have granular particles of an average size of

about 60 nm. Thus, the nucleation or particle generation was

completed in a few minutes and particle growth continued as

polymerization proceeded.

In polymerization, the polymer formed in the aqueous

phase as particles having a diameter of 100±230 nm at

250 g/l of a speci®c TFE uptake concentration. The size

and number of particles were closely related to the reaction

conditions. In the initial stage, the monomer dissolved in the

aqueous phase reacted with the initiating radicals to form

polymer chains, which formed the nucleus of the particle.

Crystallinity of as-polymerized PTFE in this polymerization

was as high as 90%; therefore, the polymer particles were

rigid and the monomer did not diffuse to the inside. Accord-

ingly, polymerization after particle generation proceeded

largely on the surface of the particle, on which the monomer

is mainly located. This accounted for the reduction in the

polymerization rate when the polymer coagulated. When no

coagulation occurred, the particle generation during the

TFE emulsion polymerization occurred via a precipitation

of aqueous oligomeric radicals, the particle size and number

of polymer particles suddenly increases in the initial stage

(in a few minutes), and then steadily increases at a relatively

slow rate, as presented in Figs. 7±16.

4. Conclusion

The effects of reaction conditions on the particle forma-

tion in the emulsion polymerization of TFE were studied. In

our results, at various reaction conditions, several variables

(except surfactant concentration) do not affect the initial and

®nal particle morphology when no coagulation occurs.

Essentially, two types of PTFE dispersion particles were

formed, rod-shaped and spherical-shaped particles, depend-

ing on the reaction conditions (such as surfactant concen-

tration). Rod-like particles were formed when the surfactant

concentration was near or above the CMC. Spherical par-

ticles were dominant when the surfactant concentration was

below CMC.

When no coagulation occurred, the particle size and

number of polymer particles suddenly increases in the initial

stage and then steadily increases at a relatively slow

rate. Particle sizes ranged from 100 to 230 nm at 250 g/l

at a speci®c TFE uptake concentration. The size and number

of polymer particles were almost independent of the

temperature, pressure, and the concentration of initiator

and stabilizer, whereas the surfactant concentration

and type of stabilizer closely affected them. The particle

size decreased gradually as the dispersion concentration

increased, whereas the particle number increased exponen-

tially.

When the PTFE dispersion coagulated during polymer-

ization under certain conditions (high temperature and

agitation speed, low surfactant concentration), the particle

size suddenly increased, the particle number suddenly

decreased, and the polymerization rate decreased, probably

because polymerization proceeds mainly on the polymer

particle surface.

5. Nomenclature

D rate of diffusion or diffusion coefficient

d equivalent spherical hydrodynamic diameter

k Boltzmann's constant (1.38�10ÿ16 erg/K)

T temperature in degree Kelvin

Vp concentration of PTFE dispersion (g/l of H2O)

Vx volume of a particle by equivalent particle radius

� PTFE density (2.2 g/cm3)

� viscosity of the diluents


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