Growth of Polymer Particles in Vinyl Chloride Emulsion Polymerization

9 Growth of Polymer Particles in Vinyl Chloride Emulsion Polymerization

Downloaded by RUTGERS UNIV on February 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch009

G. GATTA, G. BENETTA, G. P. TALAMINI, and G. VIANELLO Montecatini Edison S.p.A., Research Center of Petrochemical Division, Porto Marghera Venezia, Italy

In the seeded

emulsion

—e.g., styrene—it form,

large

particles

the quantity particles

polymerization

is possible

by adjusting,

of added

is prevented

For vinyl chloride,

emulsifier; emulsifier

latex, the surface

a given

volume

that the growth of formation

amount

these are taken by the surface

of new

of

emulsifier.

is not sufficient

is related of sized

to pre-

in fact, to obtain

of the particles

useful radicals

uni-

polymerization,

must be controlled.

of new nuclei

of primary

monomers

the formation

of new particles;

monodispersed water

during

by the limited

limited

vent the formation

of some

to obtain final latexes with

a

seeded in

It is assumed

either to the rate or to the rate that particles.

"Cor some polymers it has been shown that monodispersed latexes with ·*• large particle sizes can be obtained by a seeding technique (10, 11). According to this technique, some particles of a preformed latex are grown by polymerizing monomer on them which is added continually during the process together with emulsifier. To obtain monodispersed final latexes, the addition of the emulsifier must be adjusted accurately (limited emulsifier); in fact, the surface of the seeded particles must never be saturated in order to prevent the formation of new particles. Few works have appeared on the seeded emulsion polymerization of vinyl chloride ( V C ) . Giskehaug (5) recently used this technique in a kinetic study of the emulsion polymerization of V C , but he has not determined the number and distribution of particles in the final latexes. Kotlyar et al. (6) do not give sufficient experimental data for an exhaustive analysis of the results; moreover, most of the growth experiments seem to have been carried out in the presence of free emulsifier. The data reported in some industrial patents (1,9) point out only the impor158 In Addition and Condensation Polymerization Processes; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

9.

GATTA ET AL.

Growth

of Polymer

Particles

159

tance of the way i n which emulsifier is added to produce the desired characteristics of the final product. A multiple reseeding technique (7) has been suggested to obtain uniform, large particles, but it is very elaborate and time consuming.


In the present work the conditions used to obtain uniform growth of the seeded particles, i n a single step, i n the emulsion polymerization of V C , are reported (4), i n particular, we show that limited emulsifier is not sufficient to prevent new particle formation. A qualitative interpreta tion of results is suggested. Experimental

A l l polymerizations were carried out at a temperature of 55 ° C . either i n a 50-gallon kettle (Experiments A , B, C ) or i n a 100-gallon kettle (Experiments D , E ) ; i n both cases, reactors were glass lined and fitted with a device for feeding soap solution and V C continuously. The potassium persulfate-sodium bisulfite redox system was used as the initiator for Experiments A , B, C , D , whereas potassium persulfate only was used i n Experiment E . Quantities of initiator were chosen accurately so that the reaction rate was always about the same. The polymerization procedure was as follows: water and the other additives were introduced, deaeration followed, some of the V C was added, and the reaction was started. The remaining V C and soap solution were introduced continuously, following a procedure that took into account particle total surface, so that at each moment added quantities never reached complete saturation. As soon as the kettle pressure reached 75 p.s.i.g., the reactor was vented, and the following determinations were carried out on the resulting latexes: solids percent, surface tension, par ticle size, and diameter distribution [measured by electron microscope (Philips model E-M-200 type P W 6000) calibrated with polystyrene latex ( D o w ) : φ = 0.365 and φ = 0.557/Λ]. Particles were counted, and diameters were measured with a particle size analyzer (Zeiss, model T G - Z 3 ) . H i g h purity V C and distilled water were used. Potassium per sulfate and sodium bisulfite were C . E r b a reagent grade. The soap used was Empicol Ser (a product of Marchon Italiana), as received. The active ingredient is sodium lauryl sulfate = 88%. A l l the quantities i n weight, mentioned i n the present work, refer to the product as such. Experiment A—Initial Seed Latexes for Growth Experiments. Four different P V C latexes were prepared for use as seeds i n growth experi ments according the well-known batch procedure. The properties of these seeds are given i n Table I. Experiment Β—Growing Particle Size of Seed Latex No. 2. Water (90 kg.), vinyl chloride (13.27 kg.), and seed latex No. 2 (7.85 kg.) (2.73 kg. solids) were placed i n the kettle. About one hour after the start of the reaction, 64 kg. of the remaining V C was fed continuously, while simultaneously a solution of 0.336 kg. Empicol Ser i n 2.0 kg. water was introduced, continuously, dropwise. V i n y l chloride (64 kg.) and soap (0.336 kg.) were introduced by the following procedure. In the

In Addition and Condensation Polymerization Processes; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

160

ADDITION

AND

CONDENSATION

POLYMERIZATION

PROCESSES

first hour of reaction 23.5 kg. of V C and 62.5 grams of soap were intro duced; during the second hour 23.5 kg. V C and 87.5 grams soap were added; during another hour, 17 kg. V C and 186 grams soap were added. About four and a half hours from the beginning, the reaction was complete. Runs 63 and 67 were carried out according to the described procedure; the conversions were 89.6 and 90.0% respectively, and the latex surface tensions were 66.6 and 66.1 dynes/cm.


Table I.

Properties of Seed Latexes

Seed No.

Conversion,

Solids,

%

Particle Diameter, A .

σ, Α.*

σν, % »

1 2 3 4

90.6 88.6 75.0 27.3

35.3 34.8 31.0 14.1

4200 4000 3100 2290

120 115 66 100

8.8 8.7 6.6 13.2

%

Surface Tension, dynes/cm.

—

62

—

66.3

Standard deviation of particle diameter. * Coefficient of volume variation.

α

Experiment C—Growing Particle Size of Seed Latex No. 1. The procedure used for Experiment Β was followed, except that 8.95 kg. of seed latex No. 1 (3.16 kg. solids) and 12.84 kg. of V C were initially placed i n the kettle. R u n 72 was done by this procedure, and a 94.0% conversion resulted; the surface tension of the latex was 65.0 dynes/cm. Experiment D—Growing Particle Size of Seed Latex No. 3. Quanti ties of H 2 0 , V C , and seed (see Table I) are given i n Table II. A total of 720 grams of soap was introduced dropwise during the reaction. E m p i c o l Ser was dissolved in water. Conversion was about 90% for all runs. Table II.

a

H20,

H 2 0 , V C , and Seed Used in Experiment D kg.

Latex Run

Initial

Final

Dry Seed No. 3, kg.

913 945 951 952 953 954 962 1277 1278 1279 941

222.8 223.0 223.3 223.0 223.0 222.5 222.5 223.0 223.0 223.0 223.1

269 269 269 269 269 269 269 269 269 269 269

2.6 2.9 5.76 7.2 8.65 11.5 11.5 15.7 23.5 31.4 48.4

VC Feed, kg. 1st

2nd

36.0 35.7 32.8 31.4 30.0 27.1 27.1 22.9 15.1 161.2 a 144.2°

154 154 154 154 154 154 154 154 154

Reaction Time, hours: min.

Solids in Final Latex, %

5:30 5:30 5:30 5:15 5 5:30 4:26 4:42 4:20 4:45

39.2 39.0 39.4 39.8 39.1 405 40.2 38.8 40.2 40.0 40.9

A l l the VC was introduced, continuously, at the beginning of the reaction.


9.

GATTA E T A L .

Growth

of Polymer

161

Particles

Experiment Ε — G r o w i n g Particle Size of Seed Latex No. 4. The quantities of water, V C , seed (Table I ) and soap used i n these polymeri zations are given i n Table III. In Runs 935,1313, and 1314 all the V C was


Table III.

H 2 0 , V C , Seed, and Soap Used in Experiment Ε

Η Ο Ice

Latex Run

Initial

Final

Dry Seed No. 4, kg.

936 937 938 939 935 940 1313 1314

139.6 143.4 144.0 145.0 147.0 147.0 147.0 147.0

147 147 147 147 147 147 147 147

21.2 10.6 8.5 6.35 4.24 4.24 2.12 1.06

VC

Monomer, kg.

108.5 60.3 50.6 41.0 31.5 70.0 35.0 17.5

Empicol Ser, kg.

0.325 0.162 0.130 0.0975 0 0 0 0

Reaction Time, hours: min.

Solids in Final Latex, %

3:55 3:10 2:45 2:05 1:30 3

44.2 30.6 26.7 22.8 17.5 30.6

— —

— —

introduced at the beginning of the run; i n the other runs, part of the V C was placed initially i n the kettle, and the remaining V C was fed during the reaction. In Runs 936, 937, 938, and 939 only the proper soap solution was introduced continuously drop b y drop. The amount of E m p i c o l Ser is different i n each run, and it was calculated, on the grounds of particle surface increase, that the surface saturation would never be reached. In Runs 935, 940, 1313, and 1314 no soap was added because the low solids percent of final latex assures latex stability. A l l runs were made at constant reaction rate; since quantities of V C subjected to polymerization are quite different, reaction times are different. Conversions were always about 90%. Results

Seed Latexes. In preparing the four different P V C latexes to be used as seeds i n growth experiments, uniform-sized particles were obtained, as shown i n Figure 1. [The main properties of the seed latexes have been given i n Table I.] The amount of soap required to coat the particles completely was determined for each latex b y the method of surface tension. A n example of these determinations is given in Figure 2 for seed latex N o . 2, where an aqueous dispersion of the seed latex at 3.55% solids was used. Figure 2 also shows the surface tension variation of aqueous soap solutions. In the absence of particles the surface tension decreases sharply up to a soap concentration of 0.0045%; at higher con centrations, the values are practically constant. In the presence of 3.55% of seed latex N o . 2 surface tension becomes practically constant at a much higher soap concentration because the soap molecules are adsorbed on the polymer particles up to complete saturation of their surface.



162

ADDITION

Figure

1.

Electron

AND CONDENSATION

micrographs

POLYMERIZATION

of seeds used in growth

PROCESSES

experiments

(A) Seed No. 1 (B) Seed No. 2 (C) Seed No. 3 (D) Seed No. 4

The amount of soap required for complete coverage of the particle surface lies i n the range 1.62-1.94 mg./10 2 0 sq. A . Taking into account the value of covering power of sodium lauryl sulfate as reported i n literature, the value of 1.62 is preferred (2,3). G r o w i n g Particle Size. According to Vandegaer's (JO) procedure, the total surface (TS) of all the particles during the growth can be calculated, so that the emulsifier addition can be adjusted not to exceed the 100% surface saturation, thus preventing formation of new particles. The following equations were used: (1) (2)


9.

GATTA ET AL.

Growth

of Polymer

163

Particles

Χ

=

grams of V C added

time t

a

=

grams of seed solids per 100 grams latex

ρ

=

P V C density =

TS

=

total surface ( i n square centimeters) of particles i n 100 grams of latex.

1.4 grams/ml.


Two experiments i n growing seed No. 2 were carried out—Runs 63 and 67 (Experiment B ) ; 7.85 kg. of seed latex (34.8% solids) were intro duced (a = 2730 grams). Calculated values of TS and of φ for different values of X are reported i n Table I V . Ί

ι ι ι ι

111

I Γ

ΓΠΤΠ—I I

1 I I I MM

Γ

60 h 5a \

Ε ο

40

Q> C

20 10

I I I I 1 III 0.001

Figure

2.

Surface

tension

LJ

I I I I I III 0.01 SOAP

vs. soap concentration latex present

I I

I I I I I I II 0.1

with (b) and without

(a)

During the first stage of growth, no soap was introduced to prevent formation of new particles; only the small amount of soap introduced with the seed is present (this is neglected i n the calculations). During this stage, since the solids content is very low, no coagulation is observed. For the same reason, when conversion is high, soap is added i n an amount to cause less than 100% saturation. The final latex has a surface tension greater than 60 dynes/cm., which shows how low the concentration of free emulsifier is i n water. During these stages small samples of the latex were taken, with great care to avoid coagulation; the particle sizes were measured by


164

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

electron microscopy. Unexpectedly, we observed the formation of new particles, which grow i n competition with the seed particles (Table V ) .


Table I V .

X , grams

(X + 2730)

0 7270 13270 36770 60270 77270

2730 10000 16000 39500 63000 80000

β

Calculations for Growth Experiment Β (Runs 63 and 67)

(X +

2730f'3

195.33 464.15 635.01 1214.50 1583.29 1856.59

j ^ — sq. A. X 10~22

292 694 950 1815 2367 2772

Weight of Empicol Ser, grams Required0

47 112 154 294 384 449

Used

φ, A .

0 0 0 62.5 150.0 336.0

4000 6166 7211 9746 10875 12332

Amount needed for 100% saturation. Table V .

Run

Results of Growth Experiment Β

Polymerization Time (from beginning of Solids, reaction), min. %

Numerical Distribution of Particles in Latex

63

0 70 170 295

— 9.30 29.75 40.46

Monodispersed around 4000 A. Two groups around 1300 and 6100 A. Three groups around 500, 3200, 9500 A. Three groups around 750, 4200, 11500 A .

67

0 155 270 300

— 26.0 39.0 40.46

Monodispersed around 4000 A. Two groups around 2500 and 9200 A . Three groups around 800, 3800, 11500 A . Three groups around 1500, 4200, 12500 A .

The percent numerical distribution of particles, at different poly merization times, is plotted i n Figures 3 and 4 and is shown in Figure 5. Each group of particles exhibits a fairly uniform distribution around a main diameter (the one indicated i n Table V ) . Diameter dispersion is wide; for example, we have calculated the σ value for the sample taken at 70 minutes i n R u n 63 and for the sample taken at 300 minutes i n R u n 67. They are reported in Table V I . In a few cases, diameter distribution is not strictly gaussian i n nature. B y observing diameter distribution i n the final latex, one can recog nize the following: some large particles, resulting from the growth of the seed; a second group of particles, which clearly results from the growth of particles formed i n a nucleation phase at the beginning of polymeriza tion (see sample taken at 70 minutes i n R u n 63 and the one taken at 155 minutes i n R u n 67); finally a third group with very small diameters whose origin is obscure.


9.

GATTA ET AL.

Growth

of Polymer

165

Particles

20


10

20

10

0.2 Figure

3.

V

S

il

0.4 DIAMETERS

Run 63:

Top: after 70 min.

η 0.6 0.8 OF PARTICLES,

1.0 MICRONS

A

numerical distribution of particles polymerization times Middle:

after 170 min.

Bottom:

at

5Ώ

1.2

different

after 295 min.


166

ADDITION A N D CONDENSATION

POLYMERIZATION

PROCESSES

20


10

0

0.2

0.2

0.4

0.6

0.4

0.8

0.6

0.8

1.0

1.2

1.0

1.2

20

i\

10

COG.

i

0.2

V

Q4

DIAMETERS

Figure

4.

0.6

OF

0.8 PARTICLES.

Middle:

after 270 min.

1.2

MICRONS

Run 67: numerical distribution of particles polymerization times

Top: after 155 min.

I

1.0

at

different

Bottom: after 300 min.



9.

GATTA ET AL.

Figure 5.

Growth

of Polymer

Run 63: electron micrographs

167

Particles

of the particles obtained at different

polymerization

times

(A) after 70 min. (B) after 170 min. (C) after 295

Table V I .

min.

σ-Values for Experiment Β

Run

Sampling Times, min.

Mean Diameter, Α.

σ, A.

63

70

1300 6100

380 220

67

300

1500 4200 12500

670 530 510

To confirm the results of Runs 63 and 67, another growth experiment (Experiment C , of R u n 72) was carried out, using seed latex No. 1. A total of 8.95 kg. of seed latex was used, at 35.3% solids—i.e., a = 3160. According to Equations 1 and 2, values of TS and φ for different values of X were obtained and are given in Table V I I . As i n the previous cases small samples of latexes were taken during polymerization, and the particles were photographed. The results are i n Table V I I I .


168

ADDITION

A N D CONDENSATION

POLYMERIZATION

PROCESSES

Table VII. Calculations for Growth Experiment C (Run 72)

X 0 6840 12840 36340 59840 76840 Downloaded by RUTGERS UNIV on February 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch009

TS, Sq. A.

X + 3160

(X + 3160)*"

3160 10000 16000 39500 63000 80000

215.34 464.15 635.01 1214.50 1583.29 1856.59

Table VIII. Sampling Times (from beginning of reaction), min.

0 45 137 273

Weight of Empicol Ser, grams

x io-*2

Required

Used

ψ, A .

322.36 694.8 950.6 1818.1 2370.2 2779.3

52 112 154 294 384 450

0 0 0 62.5 150.0 336.0

4200 6176 7212 9746 11392 12332

Results of Growth Experiment C, Run 72

Solids, %

No. of Counted F article s

Numerical Distribution of Farticles in Latex

— 6.4 21.0 41.0

580 372 442 538

Monodispersed around 4200 A. Two groups around 1100, 6000 A. Three groups around 800, 2300, 8200 A . Three groups around 600, 3700, 11000 A .

Experiment C confirms the previous results. Other growth experi ments were carried out with seed latexes No. 3 and 4. In a first series of runs (Experiment D ) seed No. 3 (φ = 3100 A . ) was grown b y intro ducing different amounts of seed i n each polymerization run, so that the ratio of the total introduced monomer to the weight of seed ( M / F ) , ranges from 3 to 73 as a maximum. Polymerizations were carried out, as before, using the redox system potassium persulfate—sodium bisulfite as activator and using an amount of sodium lauryl sulfate always less than the amount required for complete coverage of the particle surface ( details are given i n the Experimental Section). The results are reported i n Table I X and i n Figure 6. Runs 954 and 962 are similar and were con ducted to test the reproducibility of results. In another series of runs (Experiment E ) , various amounts of seed No. 4 were introduced ( φ = 2290 A . ). The following modifications were made i n the polymerization procedure: (a) Potassium persulfate alone was used as activator, i n such a con centration that the polymerization rate was almost the same as with the redox system. (b) The M/F ratio was kept within a narrow range. F o r the first five runs (see Table X ) the M / F ratio ranges from 5.12 to 7.38. For the other experiments, since the amount of seed was very small (and there fore the amount of V C to be introduced would have been too small), the M / F ratio had to be increased to 16.51. However, two identical runs


9.

GATTA

E T AL.

Growth

of Polymer

169

Particles

were made, w i t h two different M / ' P ratios—i.e., 7.38 ( R u n 935) and 16.51 (Run 940), and similar results were obtained. The main results are reported i n Table X . In the last two series of experiments, poly- or monodispersed latexes were obtained according to the adopted formulation.


Discussion

and

Conclusions

In the experiments which were carried out i n absence of free soap, new groups of particles arising from new nucleation phases appeared i n the final latexes; i n other experiments monodispersed final latexes were obtained. If the experimental data are rearranged according to the sur face value S (per ml. of initial water) of seed or according to the number of seeded particles (per ml. of initial water) (Table X I ) , it is possible to notice that, for the two series of experiments using seeds with φ = 3100 and 2290 Α., uniform growth of seed particles begins only above a certain value of S or correspondingly of N. S and Ν are calculated according to the following equations:

Φ •Ρ · 10 2 4

N=

b =

grams seed/ml. initial water at 55 ° C .

φ =

seed diameter, A .

ρ =

density of P V C ( 1.4 grams/ml. ) Table IX.

Results of Growth Experiment D

Run

M/P

No. of Counted Particles

913 945 951 952 953 954 962 1277 1278 1279 941

73.0 72.96 32.43 25.75 21.27 15.75 15.75 11.26 7.19 5.13 2.98

2338 355 229 426 362 637 233 160 194 184 428

Numerical

Distribution of Particles in Final Latex

Three groups around: 1100, 4250, 1100, 4250, 800, 3300, 900, 2900, 900, 2500, 900, 1800, 700, 2000, Monodispersed around: 6360 A. 5900 A. 5200 A . 4300 A.


10000 A. 8600 A. 8100 A. 7500 A. 6900 A. 6600 A. 6800 A.

ADDITION

AND

CONDENSATION

POLYMERIZATION

PROCESSES


170


GATTA

ET AL.

Growth

of Polymer

Particles


9.


171

172

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES

RUN 9 5 4

ι


1 A/

0

0

i

0.2

0.2

0.4

I

(X6

0.8

0.4

1.0

oo^

6

0.6

0.8

1.0

RUN 1277

}

f 0

0.2 DIAMETERS

0.4

!

0J6

OF PARTICLES,

0.8

1.0

MICRONS


9.

GATTA E T AL.

Growth

of Polymer

υ (Τ

1278

RUN

40

is ιs

30

&

Ιι

20

2 D 10


z

173

Particles

0.2

0.4

Ko

αβ

1279

RUN

40

0.6

30

20

ft

2

D Ζ

10

Q2 DIAMETERS

Table X .

0.4 OF PARTICLES,

Q6

0.8

MICRONS

Results of G r o w t h Experiment Ε

Run

M/P

No. of Counted Particles

Numerical Distribution of Particles in Final Latex

936 937 938 939 935 940 1314 1313

5.12 5.69 5.95 6.45 7.38 16.51 16.51 16.51

421 245 325 480 304 301 305 394

Monodispersed around: 3900 A . 3700 A . 3900 A . 3900 A . 3700 A . 5400 A . Two groups around: 2300, 5700 A . 2600, 5500 A .

From the available experimental data, i n the experiments with φ = 3100 Α., uniform growth begins i n the range of S = 0.7 Χ 10 2 0 to 0.96 X 10 2 0 A . and i n the range of IV = 2.34 Χ 10 1 2 to 3.16 Χ 10 1 2 . Similarly, i n


174

ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N PROCESSES


the experiments with seed φ = 2290 Α., uniform growth begins i n the range of S from 0.26 Χ 10 2 0 to 0.53 Χ 10 2 0 sq. A . and i n the range of Ν from 1.6 χ 10 1 2 to 3.24 Χ 10 1 2 . F o r small seed sizes, such as those used here, the limiting value of S seems to increase with increasing diameter; the limiting value of Ν seems to be more constant. To confirm this behavior, we are carrying out many growth experiments on seeds with diameters i n the range 1000-6000 Α., using the conditions as i n Experi ment D—i.e., varying the initial amount of seed so that the number Ν of seeded particles per m l . water and the respective surface S are varied Table X I .

Results of G r o w t h Experiments Ν X JO"' 2

S X 1Q~ , Sq. A. (per ml. H 0 initial)

(per ml. H 0 initial)

φι, A.

M/P

Distribution of Particles in Final Latex

72

0.312

0.307

4200

24.31

Polydispersed

63 67

0.285 0.285

0.569 0.559

4000 4000

28.30 28.30

Polydispersed

913 945 951 952 953 954 962 1277 1278 1279 941

0.155 0.177 0.352 0.440 0.529 0.705 0.705 0.961 1.438 1.922 2.960

0.514 0.588 1.168 1.462 1.756 2.340 2.340 3.168 4.771 6.376 9.825

3100 3100 3100 3100 3100 3100 3100 3100 3100 3100 3100

73.07 72.96 32.43 25.75 21.27 15.75 15.75 11.26 7.19 5.13 2.98

Polydispersed

1313 1314 940 935 939 938 937 936

0.133 0.266 0.533 0.533 0.809 1.091 1.366 2.885

0.8105 1.621 3.243 3.243 4.923 6.636 8.311 17.07

2290 2290 2290 2290 2290 2290 2290 2290

16.51 16.51 16.51 7.38 6.45 5.95 5.69 5.12

Polydispersed

20

2

Run

2

Monodispersed

Monodispersed

widely. Thus, for each seed diameter it is possible to determine the range of Ν and S values over which the seed growth behavior is varied. Pre liminary results of two sets of experiments with φ = 4570 and 5700 Α., respectively, are reported i n Table X I I , together with previously reported results of experiments with φ = 2290 and 3100 A . S and Ν values cannot be plotted as functions of φι since sufficient experimental results are not available, although the results adequately describe the phenomenon qualitatively. The smaller the seed particle


9.

GATTA ET

AL.

Growth

of Polymer

175

Particles

size, the higher seems to be the'limiting number of particles (per m l . of aqueous phase) to which the system approaches. If the number of seeded particles is less than that limiting value, polymer molecules formed i n the aqueous phase give rise to new nuclei, in which polymeri zation occurs in competition with aqueous-phase polymerization. The limiting value of Ν increases, whereas the S value obviously decreases ( Table X I I ) with decreasing seed diameter.


Table XII. Range of S and Ν Values of Seed Particles Over Which Growth Behavior Varies (uniform at higher values) φ,,Α.

S X 10~20, Sq. Α.

Ν X 10~12

2290 3100 4570 5700

0.26-0.53 0.70-0.96 0.53-1.07 0.62-1.29

1.62-3.24 2.34-3.16 0.94-1.88 0.79-1.58

This behavior can be explained as follows. V i n y l chloride is mod erately soluble in water (8); thus, the initiator radicals react with V C molecules dissolved i n water, and polymerization begins. If there are no polymer particles present, a particle is supposed to be formed for each primary radical. If there are a few particles present, some of the primary or growing radicals may be captured by polymer particles. If the capture rate is equal to that of formation of primary radicals, no new particles form. The capture rate is proportional to the particle's total surface. The formation rate of new particles is given by Equation 3. ^^Ri-KiTS)

(3)

Ri is the formation rate of primary useful radicals, Κ is the capture rate of particles per unit surface, and TS is the total surface. In order for no new particles to form (dn/dt = 0), it is necessary that: (TS),

= ^

(4)

Therefore, at a constant value of Ri and K, there is a surface value above which no new particles form. In growth experiments where the surface was increased gradually, no formation of new particles could be observed above a certain surface value, and latexes appeared monodispersed. For spherical particles, capture rate per unit surface ( K ) is inversely propor tional to particle radius; in fact we have: (5)


176

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

where D is the diffusion constant, c * is the radical stationary concentra tion i n the aqueous phase, and r is the particle radius. F r o m Equations 4 and 5 it appears that the limiting surface above which no new particles form, increases with increasing particle size. This agrees with our experi mental results (see Table X I I ) . If Ν is the number of particles with radius r, giving total surface TS, TS = 4τττ2 Ν


From Equations 4 and 5 we can obtain Equation 6 for the limiting number N, above which new particles do not form. N l

= --^—

(6)

It follows that the limiting number of particles decreases with increasing dimension of particles, also i n agreement with our experimental results. The growth ratio M/P of introduced monomer to the initially present seed polymer seems to have no effect. In fact, although most cases of uniform growing have been obtained with small M / P ratios, some runs carried out with almost equal M/P ratios (Runs 940 and 962) have given opposite results. Behavior of V C i n emulsion-seeded polymerization is quite different from that of other vinyl monomer such as styrene and vinyl toluene. For instance, i n styrene-seeded polymerization, Vanderhoff ( I I ) d i d not observe any anomalous seed growing. H e reports uniform growing for a mixture of two seeds with a φ = 2640 and 5570 Α., respectively, by seeding 0.193 Χ 10 1 2 particles/ml. H 2 0 , whose surface per ml. of water is, according to our calculations, equal to 0.121 X 10 2 0 sq. A . Therefore, Vanderhoff ( I I ) obtained uniform growing with styrene with values of Ν and S with which V C would have exhibited a new nucleation phase. In the above mentioned work by Vandegaer (10), i n his growth Experiments Β and D with poly (vinyl toluene) seed (initial diameters 1420 and 1470 A . ) he obtained uniform growing. Unfortunately we cannot compare our results because we do not know the S and Ν limiting values for seeds of P V C having such small diameters. The differences i n behavior between V C and styrene would be related to different water solubilities of monomers, as well as to different values of the constant D of radicals into the polymer particles. Acknowledgment

The authors thank F. Gheda, A . F i n z i , and G . Valori for their help in the experimental work.


9.

GATTA ET AL.

Literature

(1) (2) (3) (4)

Growth of Polymer Particles

177

Cited

Β. F. Goodrich Co., British Patent 1,036,468 (1966). Brodnyan, J. G., Brown, G. L., J. Colloid Sci. 15, 76 (1960). Corso, C., Bedeschi, M., Materie Plastiche 12, 1165 (1961). Ferri, Α., Gatta, G., Benetta, G., Talamini, G. P., Italian Patent 794,693 (1967).


(5) Giskehaug, K., Symp. Chem. Polymerization Proc., London, 1965, pre

print. Kotlyar, I. B., Mukhina, I. Α., Soviet Plastics (March 8, 1966). Peggion, E., Testa, F., Gatta, G., Chim. Ind. 46, 9 (1964). Peggion, E., Testa, F., Talamini, G. P., Makromol. Chem. 71, 173 (1964). U. S. Rubber Co., British Patent 753,832 (1956). Vandegaer, J. E., J. Appl. Polymer Sci. 9, 2929 (1965). Vanderhoff, J. W., Vitkuske, J. F., Bradford, E. B., Alfrey, T. Jr.,J.Poly mer Sci. 20, 225 (1956). R E C E I V E D March 18, 1968. (6) (7) (8) (9) (10) (11)


Growth of Polymer Particles in Vinyl Chloride Emulsion Polymerization

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