miniemulsion polymerization of butadiene 2015

RESEARCHARTICLE
Copyright © 2015 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Colloid Science and Biotechnology
Vol. 4, 1–6, 2015
Miniemulsion Polymerization of Butadiene:
Kinetics Study
Ahmed F. Moustafa1 2
1
Sekisui Specialty Chemicals America, LLC. 900 Gemini Avenue, Houston, 77058, Texas, USA
2
Polymers and Pigments Department, National Research Center, Dokki, Giza 12622, Egypt
Kinetics of miniemulsion polymerization (MEP) and conventional emulsion polymerization (CEP) of
Butadiene (BD) were studied using Vazo 50; oil soluble initiator, Sodium lauryl sulfate surfactant and
Hexadecane (HD) co-stabilizer, at 50 C using tumbler water baths in a batch process mode. The
MEP was carried out by first, miniemulsification of HD using ultrasound and micro-fluidizer, and then
allows BD swelling onto the HD particles before the initiation step for 45 and/or 120 minutes. The
maximum swelling capacity of HD miniemulsion particles by BD liquid monomer was determined.
The effect of HD and BD concentrations on the initial conversion and propagation rate (Rp) is
discussed. Polymerization kinetics was determined gravimetrically. Particle size analysis was done
using capillary hydrodynamic fractionation (CHDF) technology.
Keywords: Butadiene, Hexadecane, Miniemulsion, Batch Polymerization, Kinetics, Capillary
Hydrodynamic Fractionation.
1. INTRODUCTION
Miniemulsion polymerization (MEP) provides many
distinct advantages over conventional emulsion polymer-
ization (CEP) because monomer droplets are directly poly-
merized into polymer particles, whereas in case of the
CEP, the monomer is polymerized in micelles after migra-
tion through the aqueous phase from relatively large
monomer droplet reservoirs.1
The MEP does not suffer
from the limitation from the consideration of degree of
hydrophilicity or hydrophobicity of monomers as the CEP
does. The technique of MEP is based on creating relatively
small monomer droplets by high mechanical shear together
with the aid of co-stabilizer. The Co-stabilizer is a very
hydrophobic organic and water immiscible compound such
as Hexadecane (HD) or Cetyl alcohol. The high hydropho-
bicity of HD helps in minimizing the Ostwald ripening;
which is the agglomeration of small monomer droplets into
larger ones. Breaking down the small monomer droplets
into smaller droplets eliminates the micellar polymeriza-
tion by the probability theory. It is also important to
prevent micellar nucleation by lowering the surfactant con-
centration below its critical micelle concentration (CMC).
MEP can be used to have both droplet and micellar nucle-
ation, if multimodal particle size distribution is required
for high solids latex applications.
The role of the co-stabilizer (HD) in the miniemulsion
system is critical. It allows the system of submicron size
monomer droplets to be sufficiently stable, such that, these
can serve as the main locus of particle nucleation. Com-
pared with another commonly used co-stabilizer, Cetyl
alcohol, HD acts to stabilize the miniemulsion system by
its low molecular weight and low water solubility (the
solubility of Cetyl alcohol and HD in water are 10−5
and 10−6
g/dm3
, respectively), and its presence inside the
monomer droplets reduces Ostwald ripening effect, which
results in a relatively long stability of the miniemulsion
system. When using Cetyl alcohol as co-stabilizer, the sur-
face of the monomer droplets is covered by a “condensed
phase” of the surfactant and co-stabilizer in addition to the
Cetyl alcohol molecules present inside the droplets. These
act to reduce the droplet degradation, consequently, mak-
ing the miniemulsion system stable, although less so than
HD systems. Schork et al.2
used Lauroyl peroxide (LPO)
as a co-surfactant as well as an initiator, in MEP of methyl
methacrylate.
The propagation step in free radical polymerization of
Butadiene involves 3 pathways;3–8
1, 4 cis-addition, 1, 4
trans-addition or 1,2 vinyl addition. The last mode of addi-
tion is responsible for a pendant reactive double bond that
can polymerize with active radicals causing cross linking.
Chain transfer agent, limited conversions and low polymer-
ization temperatures are the techniques used to minimize
crosslinking.
El-Aasser et al. discussed previously the kinetics
of MEP of styrene-butadiene copolymer9
using reac-
tion calorimetry. Kinetic models were formulated for
J. Colloid Sci. Biotechnol. 2015, Vol. 4, No. 1 2164-9634/2015/4/001/006 doi:10.1166/jcsb.2015.1112 1

RESEARCHARTICLE
Miniemulsion Polymerization of Butadiene: Kinetics Study Moustafa
synthesis of styrene butadiene block copolymer using
reversible addition-fragmentation chain-transfer (RAFT)
seeded mini emulsion polymerization.10–12
Preparation
techniques parameters influencing product property and
reaction kinetics of large carboxylic acid-functionalized
SBR latex particles synthesized through MEP were
studied.13
MEP of high BD level in SBR rubber latex
up to 50% solid content was synthesized. MEP approach
offered an efficient hetero phase route for synthesizing
SBR copolymer latex with narrow size distribution. Sec-
ondary nucleation was successfully prevented by using
a hydrophobic initiator. Even at high conversions, a low
cross linking degree and, therefore, low gel contents were
obtained.14
A direct miniemulsification approach followed
by a subsequent MEP process has been utilized to synthe-
size a series of hybrid composite latexes.15
MEP of BD
and methyl methacrylate in the presence of urethane pre-
polymer at 30 C was discussed.16
The synthesis of monodisperse poly BD latex will find
new applications in building complex polymer structures
as core shell, soft latex systems for construction and adhe-
sives applications, emulsion blends, acrylonitrile butadiene
styrene plastics, etc. The scope of this article is to study the
miniemulsion polymerization kinetics of BD homopoly-
mer. Polymerization of BD monomer reservoirs instead
of micellar nucleation and polymerization will have an
impact onto latex film formation, molecular weight distri-
bution, rheological properties, film mechanical properties.
2. MATERIALS AND METHODS
2.1. Materials
The chemicals used in this work are reagent grade (Sigma–
Aldrich, St. Louis, MO, USA), Butadiene (Air Products
And Chemicals, Allentown, PA, USA), Hexadecane (HD;
Fisher Scientific, Springfield, NJ, USA), Sodium lauryl
sulfate (SLS; Fisher), and Vazo 50 initiator; 2,2 -Azobis(2-
amidinopropane) dihydrochloride from Wako Chemicals,
VA, USA. BD was first cleaned by passing through
two successive columns to remove the moisture (Drierite,
Fisher) (Ascarite II; Thomas Scientific, Swedesboro, NJ).
All other chemicals were used as received. Deionized
water was used in all polymerizations.
Table I. Miniemulsion polymerization recipe versus, conventional one to polymerize butadiene.
Ingredient Weight (g) Concentration Weight (g) Concentration
Polymerization type MEP CEP
Water 80 4.4 mol 80 4.4 mol
Sodium Lauryl sulphate 0 023 10 mmolL−1 a
0 023 10 mmolL−1 a
Butadiene 20 36.98 mol 20 36.98 mol
NaHCO3 0 1 1.2 mmolL−1 a
0 1 1.2 mmolL−1 a
Hexadecane 0 8 3.5 mmolL−1 b
0 0
Vazo 65 0 0064 1.3 mmolL−1 b
0 0064 1.3 mmolL−1 b
2,4,6-trimethyl benzyl mercaptan 0 07 8 4×10−3
mmolL−1 b
0 07 8 4×10−3
mmolL−1 b
Notes: a
(aqueous phase), b
(butadiene monomer phase).
2.2. Methods
2.2.1. Miniemulsification of Hexadecane (HD)
Miniemulsification of BD is difficult, because it boils at
−4.4 C. HD instead was emulsified in SLS solution by
sonification for 90 seconds at 50% duty, power 7 (Branson
sonifier model 450, Ultrasonic, Danbury, CT, USA). The
crude HD miniemulsion then was subsequently passed
through the Microfluidizer (Model 110T, Microfluidics
Corp., Newton, MA) 10 times with a pump inlet pressure
set point of 80 psig. The BD miniemulsions were then cre-
ated by tumbling the liquid BD with HD miniemulsions
and other polymerization ingredients in capped pressure
glass bottles.
2.2.2. Miniemulsion Polymerization of BD
BD liquid was added volumetrically to the HD miniemul-
sion in an ice bath pre-cooled pressure glass bottles.
NaHCO3 buffer and mercaptan chain transfer agent to con-
trol molecular weight and cross linking were added. The
glass bottles were then capped and were allowed to tum-
ble for 2 hrs at room temperature to allow swelling of BD
liquid phase into the HD miniemulsion droplets, then tem-
perature was raised to 50 C to activate the initiator for the
initiation step. The polymerization reaction was left to pro-
ceed overnight for 18 hrs. A typical MEP and CEP recipe
are shown in Table I. During the polymerization reaction,
2 ml of the emulsion phase were withdrawn after 4 hrs of
initiation then every 2 hrs over 18 hrs polymerization time.
For monomer conversion measurements, 1 ml of emulsion
phase was precipitated out in methanol/hydroquinone solu-
tion. On the other hand, the second ml was dispersed in
hydroquinone aqueous solution for particle size analysis.
2.2.3. Instrumentation
BD gas was condensed using a dry ice/isopropanol
bath, and then charged into a pre-cooled 300 mL stain-
less steel cylinder. BD was transferred gravimetrically
using prechilled flasks to the polymerization bottles. Par-
ticles sizes were measured by capillary hydrodynamic
fractionation (CHDF-1100, Matec Applied Sciences, and
Northborough, MA) by injecting deionized 0.01% Poly
BD dispersion into the capillary with retention time of
2 J. Colloid Sci. Biotechnol. 4, 1–6, 2015

RESEARCHARTICLE
Moustafa Miniemulsion Polymerization of Butadiene: Kinetics Study
Fig. 1. Swelling of BD by HD miniemulsion droplets in absence of
Vazo 50 initiator at room temperature.
5 minutes. The capillary was washed twice with deionized
water before next measurement.
3. RESULTS AND DISCUSSION
3.1. Effect of BD Swelling Time onto MEP Kinetics
The first step in MEP of BD is the partial swelling
of BD liquid phase into the HD pre-emulsion droplets.
This swelling process was followed up during the first
12 hours in order to measure the time required for maxi-
mum swelling of but adiene in HD mini emulsion droplets
in absence of Vazo 50 initiator and before starting poly-
merization. The swelling was quantified by measuring
the equilibrium height of BD liquid monomer over HD
miniemulsion using graduated volumetric glass bottles.
Figure 1 shows that the maximum partial swelling ratio of
15 g of BD onto 1 g of HD droplets is achieved in less
than 3 hours.
After reaching the maximum swelling of BD into HD
mini emulsion droplets at room temperature, the polymer-
ization reaction was started by increasing the temperature
of the reaction mixture to 50 C, and then adding Vazo
50 initiator. The rate of BD conversion (for both mini
emulsion and conventional emulsion polymerizations) as a
Fig. 2. Effect of Vazo 50 initiator concentration onto the conversion
rate of CEP and MEP of BD at 50 C.
0
10
20
30
40
50
60
70
80
0 5 10 15 20 25 30
Conversion%
Time (H)
1.3 mmol (CEP) 1.3 mmol (MEP) 2.6 mmol (MEP)
3.9 mmol (MEP) 5.2 mmol (MEP)
Fig. 3. Effect of Vazo 50 initiator concentration onto the conversion
rate of BD in CEP and MEP after 45 minutes of swelling of BD onto
HD droplets.
function of Vazo 50 initiator concentration was studied at
50 C, using the recipe in Table I.
The rate of propagation of CEP17 18
had a quite differ-
ent profile from that of MEP; as shown in Figure 2. The
first 14 hours of the polymerization time; CEP has low
Rp value of 0.68%/hr however Rp of MEP is 4.3%/hr.
The previous result is in agreement of the proposed
MEP mechanism of polymerizing the monomer reservoirs
instead of monomer micelles. The high conversion rate of
MEP also confirms the success of miniemulsification and
swelling of HD miniemulsion droplets by experimental
design and open the gate foe polymerizing other gaseous
monomers in faster and with higher precision of parti-
cle size disribution and morphology control. CEP slow
Rp before the 14th hour of polymerization is attributed
to the poor diffusion of liquid BD into SLS aqueous
phase.19 20
The slow BD diffusion into the SLS phase
makes the nucleation the slow rate determining step. The
visual observation of CEP bottles confirmed that the sys-
tem remains phase separated for 14 hours, after which, one
bluish white emulsion phase appears. The sudden jump in
19.5
20
20.5
0
1
2
3
4
5
0 1 2 3 4 5 6
RpofCEPafter14thhr
(Conversion%//hr)
Rp(Conversion%/hr)
Vazo 50 concentartion (mmol)
MEP (2H swelling)
MEP (45 min swelling) CEP (Before 14H) 2H swelling
CEP (45 min swelling) CEP (After 14 h) 2H swelling
Fig. 4. Effect of Vazo 50 initiator concentration onto the CEP and MEP
propagation rates.
J. Colloid Sci. Biotechnol. 4, 1–6, 2015 3

RESEARCHARTICLE
0
1
2
3
4
5
6
7
0 2 4 6 8 10 12 14 16 18 20
Np×1016
/cm3
Time (H)
1.3 mmol (CEP) 1.3 mmol (MEP) 2.6 mmol (MEP)
3.9 mmol (MEP) 5.2 mmol (MEP)
Fig. 5. Number of particles (Np) per unit volume evolution of CEP
versus MEP.
the Rp of CEP is responsible for the relatively high gel
content in the Poly BD latex obtained.7
The importance of
MEP of BD is that it shows steady propagation rate with
fast swelling and nucleation with expected much less gel
content. Figure 2 also reveals the Rp independency onto
vazo 50 initiator concentrations which is also another big
advantage of this system; which is the molecular weight
control without mass transfer problems.
The swelling time of BD onto HD miniemulsion was
reduced to 45 instead of 120 minutes at room temperature
before elevation to 50 C. The conversion-time curves, in
Figure 3, show a signiﬁcant reduction in Rp for CEP from
19% to 0.68%/H and remain almost constant through the
polymerization course.
BD has water solubility of 0.735 g/100 g of water; in
addition, it has a density of 0.615 gcm−3
, which hinder
the diffusion into the SLS aqueous phase. No change in
diffusion or dispersion of BD was observed according to
our test method; which is measuring the equilibrium height
of BD separated liquid phase, in different tumbling times
of 45 to 120 minutes. The previous facts do not explain
the jump increase of conversion after the 14th hour CEP
time after 2 hours of tumbling. The slight reduction in
Rp of MEP, when reducing the swelling time to 45 min-
utes; explains the role of the HD miniemulsion droplets
Fig. 6. Particles diameter variations of CEP versus MEP of polybutadi-
ene latex.
Fig. 7. Variation of particle size polydispersity index during CEP and
MEP of BD.
in increasing BD concentration in the HD mini emul-
sion droplets phase. The conversion-time curves shown
in Figure 3 reveal that MEP propagation rate of BD is
5 times faster than that of CEP, where the ﬁrst propagates
at 3% and the later propagates at 0.68% conversion/H.
The faster propagation rate is attributed to enhancement of
BD diffusion between aqueous and organic phases by the
HD hydrophobic layer. Therefore, the higher BD concen-
tration in the miniemulsion droplets than that in conven-
tional SLS micellar solution is a major contribution to Rp
enhancement.
More interestingly, Figure 4 shows that Rp of MEP of
BD is independent of Vazo 50 organic soluble initiator
irrespective of the BD/HD swelling time. MEP with 2 hrs
swelling time shows higher Rp than that of 45 minutes at
all Vazo 50 concentrations.
3.2. Particle Size and Polydispersity of
CEP versus MEP of BD
Figure 5 shows the number of CEP particles per unit vol-
ume is increasing as conversion increases due to the slow
diffusion rate of BD into SLS solution. The increase in Np
Fig. 8. Monodisperse (20% solids) polybutadiene latexes prepared by
CEP and MEP.

RESEARCHARTICLE
Moustafa Miniemulsion Polymerization of Butadiene: Kinetics Study
Fig. 9. BD conversion% as a function of HD/BD weight ratio percent-
age, using 2.6 mmolL−1
Vazo 50 initiator at 50 C.
after 10 hours is in agreement with the sudden increase of
Rp after the 14th hour shown in Figure 1.
On the other hand, the Np of MEP is independent of
either the initiator concentration or the conversion rate,
because of the preferential BD swelling onto the con-
stant number of HD droplets over time. The weight aver-
age diameter (Dw) of CEP starts very big at 400 nm as
shown in Figure 6, due to tumbling mixing mode then
decreases to below 100 nm at maximum conversion. Dw
of MEP grows over time from 100 nm, which are the orig-
inal Dw of HD miniemulsion particles, to 180 nm after
swelling and maximum conversion to polybutadiene latex.
Figure 7 shows the particle size polydispersity polymer
conversion profile of CEP and MEP. The MEP course
shows monodisperse particles through the initiation, prop-
agation and termination stages; however the polydispersity
of CEP declines as the polymerization propagates then
overlap with the MEP one after the 14th hour of reaction
time.
Figure 8 shows MEP pinkish and CEP bluish latexes
pictures prepared using Table I recipes. The particles
monodispersity and nano scale diameters allow compact
hexagonal close packing and light diffraction.
Fig. 10. Effect of HD/BD weight ratio percentage onto the initial and
propagation conversion rates of MEP of BD using 2.6 mmolL−1
Vazo
50 initiator at 50 C.
Fig. 11. Polybutadiene latexes prepared at different HD/BD weight
ratio percentage.
3.3. Effect of HD Co-Surfactant Concentration on the
Kinetics of the MEP of BD
Figure 9 shows the conversion-time curves of MEP of BD
at various HD/BD ratios (w/w%) (1, 2, 3, 4, 10 and 15%).
The increase of HD/BD weight ratio from 1 to 15% led to
a significant increase in BD conversion%, and in parallel,
a decrease in the overall MEP conversion rate from 5 to
1%/hr. However, by increasing HD content in the polymer-
ization recipe, a significant increase in the initiation rate
from 2 to 20%/hr was observed, as shown in Figure 10.
The higher HD concentration increases the initial rate of
BD diffusion and solubilization into the HD miniemulsion
particles and this explains the high initial MEP conver-
sion rate and increase of monodispersity level as shown in
Figure 11.
3.4. Effect of BD/Water Ratio% onto the CEP and
MEP of BD
The CEP and MEP of BD were studied at different
BD/water weight ratio percentages of 9, 30 and 40%
in presence of fixed Vazo 50 initiator concentration
at 2.6 mmol and fixed emulsifier (SLS) concentration
at 10 mmol. In case of MEP, a constant costabilizer
10% HD/BD concentration ratio percentage was used. At
BD/water ratio of 9%, a very high initial and polymeriza-
tion propagation rates were observed for MEP as shown in
Fig. 12. Effect of BD/water ratio percentage on conversion rate of CEP
and MEP using 2.6 mmolL−1
Vazo 50 at 50 C.
J. Colloid Sci. Biotechnol. 4, 1–6, 2015 5

RESEARCHARTICLE
Fig. 13. Effect of BD/water ratio percentage on initial and overall rate
of CEP and MEP of BD using 2.6 mmolL−1
Vazo 50 at 50 C.
Figures 12 and 13; however CEP at the same BD/water%
showed lower initial and Rp rates of 9.5 and 3.35%/hr,
respectively. No back pressure in the sampling syringes
after 4 hr for MEP and after 10 hr for CEP at BD/water of
9% which means depletion or consumption of BD from the
reaction medium. The enhanced initial MEP conversion is
attributed to higher swelling of BD onto HD miniemul-
sion particles. At Higher BD/water ratio percentages of 30
and 40; the initial and Rp rates of CEP and MEP become
similar due to the maximum swelling capacity of HD par-
ticles is 15 g BD/1 g HD could not be exceeded. In addi-
tion, the MEP shows again a higher rate of reaction than
the CEP when increasing the BD/water% to 30 and 40, in
the ﬁrst 14th hr of reaction time, after which Rp of CEP
becomes higher.
4. CONCLUSIONS
The miniemulsion polymerization of BD is advantageous
to the conventional one with respect to the gradual particle
size and propagation rate increase through the course of
polymerization. The MEP does not show sharp exponential
propagation rate which will result in much less internal
crosslinking or gel content inside the rubber particles. The
HD concentration can be used to enhance or suppress the
propagation rate of polymerization. The propagation rate
is shown to be independent of initiator concentration.
References and Notes
1. V. Mittal, Miniemulsion Polymerization Technology, Technology
and Engineering, edited by John Wiley & Sons (2011), p. 1.
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3. M. Morton, Science and Technology of Rubber, edited by F. R.
Erich, Academic Press, New York (1978), p. 26.
4. M. Morton and P. P. Salatiello, J. Polym. Sci. 6, 225 (1951).
5. W. Hofmann, Rubber Technology Handbook, Hanser, Munich
(1989).
6. M. S. El-Aasser and E. D. Sudol, J. Coat Technol. Res. 1, 21 (2004).
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Symposium Series 801 (Polymer Colloids) (2002), pp. 357–373.
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Workshop on Polymer Reaction Engineering (2001), pp. 1–13.
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Received: 10 October 2014. Accepted: 9 January 2015.

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