Flocculation of a synthetic rubber latex with homopolymers and copolymers of N-vinylcaprolactam and N-vinylimidazoles

Article abstract of DOI:10.1023/B:RJAC.0000018696.17325.de

Flocculation of a latex of SKS-30 ARK commercial rubber with homopolymers and copolymers of N-vinylcaprolactam, 1-vinyl-2-methylimidazole, and its quaternization product was studied. The flocculating power of the nonionic polymers in acid solution was evaluated. The performance of the copolymers grows with increasing content of cationic units in the macromolecules and on adding an acid.

Full text of DOI:10.1023/B:RJAC.0000018696.17325.de

Russian Journal of Applied Chemistry, Vol. 76, No. 11, 2003, pp. 1839 1843. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 11,
003, pp. 1890 1894.
Original Russian Text Copyright
2
2003 by Shatalov, Verezhnikov, Churilina, Kuznetsov, Poyarkova.
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
Flocculation of a Synthetic Rubber Latex
with Homopolymers and Copolymers
of N-Vinylcaprolactam and N-Vinylimidazoles
G. V. Shatalov, V. N. Verezhnikov, E. V. Churilina,
V. A. Kuznetsov, and T. N. Poyarkova
Voronezh State University, Voronezh, Russia
Received March 19, 2003
Abstract Flocculation of a latex of SKS-30 ARK commercial rubber with homopolymers and copolymers of
N-vinylcaprolactam, 1-vinyl-2-methylimidazole, and its quaternization product was studied. The flocculating
power of the nonionic polymers in acid solution was evaluated. The performance of the copolymers grows
with increasing content of cationic units in the macromolecules and on adding an acid.
An urgent problem in rubber production is devel-
opment of methods of salt-free coagulation of latexes
with synthetic and natural polymers, including prote-
ins [1, 2]. An important advantage of coagulants based
on natural products is that they are nontoxic and en-
vironmentally friendly. Protein hydrolyzates used as
coagulating additives in industry [3, 4] are unstable
in storage and nonuniform in composition; they also
have other drawbacks. The undesirable properties of
protein products are partially eliminated by perform-
ing coagulation with addition of salts. Promising
coagulants for latexes are synthetic polymers with an
amide (peptide) bond, containing ionic groups in the
macrochain. These groups enhance the coagulation
power of macromolecular compounds.
of preparing these monomers from readily available
2-methylimidazole (MI).
CH CH2
CH CH2
N
CH CH2
N
O
N
CH3
CH3
_
I
N
N
C3H7
VII
+
VC
VMI
EXPERIMENTAL
VC (mp 33 34 C, bp 92 93 C/1 mm Hg) was
prepared from caprolactam, acetaldehyde, and alcohol
by the procedure described in [8]. VMI was prepared
by thermal dehydration of 1-(2 -hydroxyethyl)-2-meth-
ylimidazole with a mixture of KOH and NaOH (1 : 1
by weight) at 150 160 C in the presence of hydroqui-
none. Fractionation yielded VMI with bp 64 66 C
It is known [5] that polymers based on N-vinyl-
caprolactam (VC), containing an amide bond in the
ring, are low-toxic and capable of complexation; their
aqueous solutions have the lower critical solution
point in the temperature interval of functioning of
living bodies. Lozinskii et al. prepared a protein-like
copolymer from VC and 1-vinylimidazole [6] with
the structure resembling that of a globular protein.
It was shown previously that 2-methylimidazole (MI)
used in production of drugs, polymeric materials, and
other products is suitable for latex coagulation [7].
at 4 mm Hg and n 1.5250, which agrees with refer-
D
ence data [9].
Synthesis of 1-(2 -hydroxyethyl)-2-methylimidazole
involved the reaction of MI with oxirane in DMF.
Vacuum fractionation of the reaction mixture gives a
product with mp 64 65 C (from ethyl acetate).
VII was prepared by quaternization of VMI with
propyl iodide. A solution of VMI in absolute ethyl
In this work, we studied coagulation of a latex
of SKS-30 ARK commercial rubber with a VC homo-
polymer and its copolymers with N-vinylimidazoles.
As comonomers we used 1-vinyl-2-methylimidazole
acetate was mixed with C H I at 5 C, stirred for 1 h,
3
7
and allowed to stand for 12 h at the same temperature.
The product was recrystallized from isobutyl alcohol
and dried in a vacuum; mp 129 130 C, in agreement
with published data [10].
(
VMI) and 1-vinyl-2-methyl-3-propylimidazolium
iodide (VII). This choice was governed by the ease
1
070-4272/03/7611-1839$25.00 2003 MAIK Nauka/Interperiodica

1840
SHATALOV et al.
Poly-N-vinylcaprolactam (PVC) and poly-1-vinyl-
-methylimidazole (PVMI) were prepared by radical
ing, we studied their radical copolymerization in
2
ethanol under conditions of thermal initiation. From
the data on composition of the copolymers obtained
at low (up to 5 7%) conversions, we calculated ana-
lytically [13] the apparent copolymerization constants:
r = 0.54 0.07 and r = 0.03 0.02. Thus, r and r
polymerization in isopropanol at 70 C. The monomer
concentration was 3.0 M, and the initiator [azobis(iso-
2
butyronitrile), AIBN] concentration, 3.0 10 M.
Under such conditions, we prepared PVC with M =
1
2
1
2
4
4
2
.65 10 , and at [VC] = 4.5 M, with M = 8.2 10 .
differ considerably, with VII showing relatively low
0) activity. This is due to the fact that VII is
4
(
r2
PVC with M = 3.5 10 was prepared by polymeri-
zation in benzene solution at 50 C for 5 h at [AIBN] =
charged, which prevents its addition to the propagat-
ing macroradical bearing like-charged units and favors
addition of VC units.
2
1
.5 10 M. The polymers were precipitated with
hexane and vacuum-dried at 55 60 C. The molecular
weights M were calculated from the intrinsic viscosi-
We found that VC VII copolymers with the VC
mole fraction exceeding 0.55 are water-soluble like
PVC of widely varied M . As the content of VC units
4
0.68
ties [ ] by the formula [ ] = 1.5 10 M
[5].
PVMI had [ ] = 0.17 dl g 1 in C H OH at 25 C.
2
5
The radical copolymerization of VC with VII was
performed in ethanol, and that with VMI, in isoprop-
anol (total concentration of comonomers 1 M) under
decreases, the products become water-insoluble be-
cause of increased content of hydrophobic VII units.
The coagulation performance of the polymers was
evaluated from the rubber flocculation curves, i.e.,
from the dependences of the weight fraction of the
recovered polymer on the flocculant concentration c.
conditions of thermal initiation (70 C, [AIBN] = 1
2
1
0
M). The mole fractions of the comonomers in the
mixture were varied from 0.1 to 0.9. The products
were precipitated with hexane and dried similarly to
the homopolymers. Over the entire composition range,
VC homopolymers differing in M and the VC
[
] of the homopolymers was 0.10 0.12 dl g 1 (for
VMI copolymer without acidification showed no floc-
culating power. Nonionic PVMI tested for comparison
behaved similarly. The curves of rubber recovery
with PVC, PVMI (Fig. 1a), and VC VMI copolymer
VC VII in CH OH and VC VMI in H O) at 25 C.
3
2
The copolymer compositions were determined by
IR spectroscopy, from the intensity ratio of the ab-
(
Fig. 1b, curve 8) pass through a maximum (floccula-
sorption bands in the regions of 1635 (C=O vibrations
of the VC units) and 1440 cm 1 (vibrations of the
tion optimum), i.e., their shape is typical of floccula-
tion of negatively charged sols (including latexes)
with nonionic polymers and cationic polyelectrolytes
imidazole ring [11]).
Coagulation experiments were performed with a
latex of commercial SKS-30 ARK rubber prepared
with an emulsifying system based on disproportion-
ated tall oil [12].
[
14, 15]. A decrease in the efficiency of rubber recov-
ery at flocculant concentrations above the optimum is
accounted for, according to the existing concepts [16],
by restabilization of the system due to recharging of
particles (addition of polyelectrolytes) and/or forma-
tion on their surface of a dense layer of adsorbed
hydrophilic macromolecules, exerting a stabilizing
effect (addition of nonionic polymeric flocculants).
The coagulation power of the polymeric products
was evaluated gravimetrically (by the weight of the
coagulum formed) and visually (by the serum trans-
parency). Coagulation was performed in a stirred flask
containing 20 ml of a latex solution containing 1%
dispersed phase (dry residue 18.65%). After ther-
mostating at room temperature, the prescribed amount
of an aqueous solution of a polymer was added with
stirring. The consumption of the acidifying agent
The appearance of the flocculating power in an
acidic solution may be due to two factors. First, the
aggregative stability of the latex decreases owing to
hydrolysis of carboxyl-containing surfactants (salts of
fatty and other acids) to form insoluble and virtually
nondissociating carboxylic acids showing no protec-
tive effect. In the process, the charge and potential of
the particles and the thickness of the electrical double
layer should decrease. As a result, the particles be-
come able to approach each other to a distance at
which the thickness of the adsorption layer of the
macromolecules is sufficient for formation of inter-
particle bridges [16].
2.0% sulfuric acid) was 15 kg t 1 rubber. The result-
(
ing coagulum was separated from the serum, washed
with water, and dried to constant weight at 75 80 C.
Experiments on latex coagulation were performed
3
6
with PVC (M 26.5 10 and 3.5 10 ) and with
VC VMI (VC mole fraction 0.57) and VC VII (VC
mole fraction 0.57 and 0.66) copolymers.
Since data on VC (M ) VII copolymers are lack-
1
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 11 2003

FLOCCULATION OF A SYNTHETIC RUBBER LATEX
1841
Second, the flocculating effect of PVC and VC
VMI copolymer, and also of PVMI in an acid solution
may be associated with appearance of positively
charged sites in the macromolecules due to protona-
tion. In polymers containing VMI units, the protona-
tion can occur at nucleophilic pyridine-like nitrogen
atoms of the imidazole ring [17]. The protonation of
azoles at the nitrogen atoms containing lone electron
pairs in interaction with mineral acids is indicated in
,
wt %
(a)
2
1
c
10 , g dl
[18]. This is also suggested by the previously revealed
7] dramatic enhancement of the coagulating power of
,
wt %
[
(b)
2-methylimidazole in acid solution upon addition of
increasing amounts of sulfuric acid.
In PVC and its copolymers, the possible protona-
tion site in acid solutions is the C=O group of lactam
units. This process was detected experimentally with
a related polymer, polyvinylpyrrolidone [19], by
NMR spectroscopy. According to [19], in acid solu-
tions, at pH < 4, the 13C NMR signal of the carbonyl
carbon atom is regularly shifted downfield with in-
creasing acidity. This suggests an increase in its posi-
tive charge and may be due to protonation of the car-
bonyl oxygen atom. Protonation of PVC is confirmed
by certain experimental data. For example, with hy-
2
1
c
10 , g dl
, wt %
(c)
+
drogen ions in the latex replaced by Na ions in an
equimolar amount, the flocculation efficiency is ex-
tremely low, = 1 3% (Fig. 1, curve 4). Hence, spe-
cifically appearance of cationic groups in PVC macro-
molecules, rather than an increase in the ionic strength
of the medium, is the decisive factor responsible for
coagulation. The efficiency of rubber recovery is also
very low when an acid is added without a flocculant,
whereas an increase in the acid concentration in the
presence of PVC causes a sharp growth of (Fig. 1,
curves 9 and 10), presumably due to increasing proto-
nation of the polymer macromolecules.
3
ca 10 , M
Fig. 1. Weight percentage of the recovered rubber as a
function of the concentration of (a) homopolymers (c),
b) copolymers (c), and (c) acid (c ). (1, 4) PVC (M =
(
a
6
4
3
(
0
.5 10 ), (2) PVMI, (3) PVC (M = 2.65 10 ), (5, 7) VC
mole fraction 0.66) VII copolymer, (6) VC (mole fraction
.57) VII copolymer, (8) VC (mole fraction 0.57) VMI co-
polymer, and (9) PVC (M = 8.2 10 ). (4) NaCl concen-
tration 3 10 M. H SO concentration, M: (1 3, 5, 8) 1.5
4
3
2
4
3
4
1
(
0
and (4, 6, 7) 0. PVC concentration (M = 8.2 10 ):
Thus, PVC (like PVMI) in acid solutions acquires
the properties of a cationic polyelectrolyte. Its addi-
tion to a latex under these conditions should result in
chemical binding of surfactant (stabilizer) anions with
the cationic centers of flocculant macromolecules to
form insoluble ionic complexes exhibiting no stabiliz-
ing effect. Such an interaction was detected nephelo-
metrically in the presence of an acid in aqueous solu-
3
9) 2.2 10 and (10) 0.
of the insoluble oligomer polymer complex [20]. In
the absence of an acid, no interaction is observed in
a wide range of Leukanol concentrations (Fig. 2,
curve 2).
It should be noted that, at the acid concentrations
used in flocculation experiments (Fig. 2, point in
curve 2 denoted with a circle), there is no noticeable
increase in the turbidity. This fact apparently suggests
that, under these conditions, the degree of protonation
is low and the concentration of the forming complex
is below the solubility limit at which the new phase
starts to precipitate.
1
tions of a mixture of PVC with Leukanol [a nonhy-
drolyzable (in contrast to soaps) stabilizer present in
latexes] (Fig. 2). Figure 2 shows that, as the acid con-
centration is increased, the turbidity first grows slight-
ly and then sharply, suggesting formation of particles
1
A mixture of sodium salts of an oligomeric product formed by
condensation of -naphthalenesulfonic acid with formaldehyde.
Thus, appearance of positive charges in polymer
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 11 2003

1842
SHATALOV et al.
Copolymers of VC with VII exhibit a higher floc-
culating power compared to PVC. Figure 1 (curves 5
) show that, on adding VC VII copolymers to the
latex, the yield of the recovered rubber sharply grows,
reaching a virtually constant limiting value, , start-
1
03, cm 1
7
ing from a certain critical concentration c of each co-
polymer. The flocculating power grows (c decreases,
increases) on acidification (Fig. 1, curves 5 and 7)
and with increasing content of VII units in the macro-
molecules (curves 6, 7). This is due to an increase in
the number of cationic groups in macrochains, which
enhances the contribution of the neutralization factor
to the latex destabilization. Furthermore, there is no
restabilization branch in the flocculation curves ob-
tained with the copolymers. This suggests a decrease
in the adsorbability of copolymer macromolecules
with hydrophilic charged groups. In the ascending
portions of the flocculation curves, the adsorption of
macromolecules is provided by electrostatic forces.
After neutralization of the surface charge, such phe-
nomena as further adsorption, densification of the
adsorption layer of macromolecules, and enhancement
of its protective function cease to be manifested, and
the system remains strongly destabilized with excess
copolymer also.
2
ca 10 , M
3
cL 10 , M
Fig. 2. Turbidity (1) of solution of PVC (M = 8.2
4
3
1
0 ) Leukanol mixture (3.6 10 M each) as a function
of H SO concentration c and (2) of PVC solution (3.6
2
4
a
3
1
0
M) as a function of the Leukanol concentration c in
L
the absence of the acid.
macrochains in acid solutions results in manifestation
of the neutralization mechanism of flocculation, along
with the bridging mechanism. The neutralization
mechanism involves binding of surfactant (stabilizer)
anions with cationic groups of the flocculant macro-
molecules.
Thus, coagulation of a latex of a synthetic rubber
can be effected with nonionic PVC and VC VMI
copolymer in acid solutions; copolymers of VC with
a cationic monomer, VII, exhibit a flocculating effect
in the absence of acid also, and this effect is enhanced
upon acidification.
Comparison of the curves for PVC and PVMI
Fig. 1a) shows that the polymer with an imidazole
ring exhibits a stronger flocculating effect. This is
indicated by the fact that the optimal flocculating
(
3
1
concentration of PVMI (9.4 10 g dl ) is consid-
2
erably lower than that of PVC (3.9 10 and 5.3
CONCLUSIONS
2
1
4
1
3
0
g dl for samples with M = 2.65 10 and
.5 10 , respectively). The higher flocculating activ-
ity of PVMI and VC VMI copolymers is due to the
presence in the macromolecules of more basic imida-
zole rings which are protonated in acid solutions
more efficiently than caprolactam units.
6
(1) Products of homopolymerization of N-vinyl-
caprolactam with a wide range of molecular weights
and water-soluble copolymers with 1-vinyl-2-methyl-
imidazole and 1-vinyl-2-methylpropylimidazolium io-
dide exhibit flocculating power and can be used for
recovery of a synthetic rubber from a latex.
The influence of the molecular weight of the poly-
mers on their flocculating effect was considered with
PVC samples differing in M by a factor of more than
000. We found that the floculation efficiency notice-
ably grew with increasing average molecular weight
(2) Copolymers of N-vinylcaprolactam with N-vi-
nylimidazole derivatives exhibit stronger flocculating
effect than poly-N-vinylcaprolactam.
1
6
of PVC (with the sample having M = 3.5 10 , the
coagulum yield in the flocculation optimum is 100%,
whereas with the relatively low-molecular-weight
sample it is about 70%). Such a trend is characteristic
of nonionic and weakly charged polymers (with which
PVC in a weakly acidic solution can be classed) and
is not observed with highly charged polyelectrolytes
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