Shaojie Lua,
Kangde Yaob,
Songbai Linb,
Deyong Caob,
Yiping Chenb
a
School of Science, Tianjin University, Tianjin, Peoples Republic of China
b
School of Materials Science and Engineering, Tianjin University, Tianjin, People's Republic of
China
A Study on Preparation and Thickening Efficiency of an Inverse Emulsion of Anionic Starchgraft-Polyacrylamide
A series of graft copoiymers of starch with polyacrylamide was made by using a K2S2O8 initiation
system in inverse emulsion. The corresponding anionic graft copoiymers, which have a different
hydrolysis degree, are prepared by alkaline hydrolysis of the uncharged graft copoiymers. Adequate
hydrolytic conditions including type and amount of hydrolyzing agent, temperature, and time were
studied. These graft copoiymers have been tested for their temperature sensitivity, salt tolerance and
shear stability, and a comparison has been made with ungrafted polyacrylamide (PAM). Experiments showed that a high hydrolysis degree can be obtained in a shorter time when NaOH and
Na2CO3 were used together as hydrolyzing agents, and the highest solution viscosity was observed
when the hydrolysis degree of the graft copoiymers was approximately 30%. In addition, the shear
stability, temperature sensitivity and salt tolerance in the graft copoiymers were also investigated.
Keywords: Acrylamide; Graft copolymerization; Inverse emulsion; Alkaline hydrolysis
1 Introduction
Polyacrylamide (PAM) has found various applications, for instance as efficient flocculant, thickener
and friction reduction agent [1-3]. Because of its excellent characteristics, the production rate of
PAM is increasing more strongly than that of other polymer materials. Drawbacks of this product
are a high temperature sensitivity, low salinity tolerance, poor shear stability and poor dissolution
properties. To improver these properties, in recent years, many working groups worldwide have
studied preparation and application of modified products of polyacrylamide (e.g. [2-7]).
In this paper, a series of starch-graft-polyacrylamide coplymers with different anionic degree were
prepared by partial hydrolysis [4, 8]. These copoiymers were synthesized in inverse emulsion [911], to overcome such disadvantages of polymerization in aqueous solution as low solid contents
and poor solubility [12]. Hydrolytic conditions and properties of inverse emulsion of anionic starchgraft-polyacrylamide were investigated.
2 Experimental
2.1 Materials
Solubilized starch and oxidized starch were supplied by Tianjin Tongyuan Company (Tianjin,
People's Republic of
Correspondence: Shaojie Lu, School of Science, Tianjin University, Tianjin 300072, People's
Republic of China, e-mail: jiesl@public.tpt.tj.cn.
China). Hydrolyzed starch was prepared by treating com starch with 2% hydrochloric acid solution
at 50 °C for 2.5 h. Other materials used in this study were purchased from the following suppliers:
acrylamide from Jiangxi Agriculture and Science Chemical Industry, Ltd. (Nanchang, China); corn
starch from Langfang Plant of Starch (Lang-fang, China); white oil (liquid paraffin) from Tianjin
Zhenda Chemical Industry, Ltd. (Tianjin, China); and all other chemicals from Tianjin Plant of
Agents (Tianjin, China).
2.2 Synthesis
The reaction was carried out in a four-necked round-bottom flask with stirrer, thermometer and
nitrogen gas inlet. About 160 mL white oil and the emulsifier (mixture of Span 80, Span 40 and op4) were added. The emulsifier concentration was 7% (w/w) for the continuous phase and the HLB
value was 6.13. Nitrogen was allowed to bubble through the solution for 30 min. The mixture was
heated in a water bath and was stirred until the emulsifiers dissolved in the oil phase.
Starch (12 g) was slurried in 25 mL water, 0.15 g of urea were added and the mixture treated at 45
°C for 15 min. Then potassium persulfate (0.1g) and 48 g acrylamide in aqueous solution were
added. This solution was added to the oil phase under stirring. Dispersion of the aqueous solution in
the oil phase took 20 min, and a water-in-oil system was formed. The pH value of the system was
adjusted to 9.0 by adding aqueous ammonia. After the reaction was initiated, stirring rate was raised
to 420 rpm and temperature ramped with increase in conversion.
When the temperature rise came to an end, the temperature of the water bath was adjusted to 50 °C.
Polymerization was conducted in a nitrogen atmosphere. The reaction lasted for 2 h, and then the
hydrolyzing agent added into the emulsion of starch-graft-polyacrylamide (S-g-PAM). The reaction
lasted 8 h. During the reaction, samples were withdrawn periodically and were analyzed.
2.3 Analysis
2.3.1 IR spectroscopy
The emulsion with prepared starch-graft-polyacrylamide (S-g-PAM) was added dropwise to
ethanol, and the precipitate (S-g-PAM) was washed with acetone to remove the unreacted monomer
and oil phase. The product was dried at 40 °C.
The homopolymer in uncharged graft copolymer was removed by extraction with a 60:40 (v/v)
mixture of ethan-1,2-diol and acetic acid, then the copolymer was washed with ethanol, and vacuum
dried at 40 °C to constant weight [13]. The product was analyzed by IR spectroscopy.
2.3.2 Graft level
The purified starch-g-PAM copolymer (1.0 g) was placed in a 10OmL flask with 50 mL 1M HCI.
The flask was fitted with a condenser, and the mixture was refluxed for 3 h to split off the grafted
side chains. The solution was then neutralized with 1 M NaOH and the grafted side chains were
precipitated with cold ethanol. The product was then dried at 40 °C to constant weight. The
percentage of grafting (G%), grafting efficiency ( E%) and grafting frequency (F) were calculated
as follows [14].
G (%) = weight of grafted polyacrylamide/weight of starch x100
E (%) = weight of grafted polyacrylamide/weight of reacted acrylamide x100
F (AGU/ per graft) = (w1 /162) / (w2 /Mw)
Where w1 and w2 are weight of starch and weight of grafted polyacrylamide in copolymer,
respectively and Mw is the molecular weight.
2.3.3 Hydrolysis degree and molecular weight
Hydrolysis degree (HD%) and molecular weight (Mw) were measured according to GB 12005.6-89
(National Standard of China). The hydrolysis degree was determined by conductometric titration
using a electric conductivity meter DDS-11A (Shanghai, China) and molecular weight was determined by a viscometric
method.
Measurements of the molecular weight: First, the solution viscosity of the grafted chains was
measured with a Ubbelohde viscometer at 30 °C using 1 M aqueous NaNO3 as solvent, and then the
molecular weight was calculated by the Mark-Houwink equation:
where K= 3.75 x 10-4, and α = 0.66.
Measurements of hydrolysis degree: a dry sample (0.03-0.05 g) was dissolved in distilled water and
titrated with a standardized solution of hydrochloric acid (0.02 mol/L). The endpoint of titration was
determined conductometri-cally. The following formula was used to calculate the hydrolysis degree
of the product:
Where C is molar concentration of hydrochloric acid, and V is the volume of the consumed
hydrochloric acid for the sample (mL); W is the weight of the sample (g). The difference between
the molecular weights of sodium acry-late and acrylamide is 23. One milliliter of consumed standard hydrochloric acid (1 mol/L) is equivalent to 71 g of acrylamide.
2.3.4 Apparent viscosity of the graft copolymers
A surfactant (0.1 mL) was added to the emulsion (5 g) under stirring, and the emulsion dissolved in
100 mL of distilled water. The apparent viscosity of the graft copolymers was measured by a
rotational viscometer.
3 Results and Discussion
Graft copolymerization of starch with acrylamide in inverse emulsion and alkaline hydrolysis of
uncharged graft copolymers was discussed in [15].
3.1 Preparing route
Copolymers of acrylamide and acrylate with anionic groups in the chains may be obtained by
alkaline hydrolysis of polyacrylamide. There are two methods for adding alkali: before
copolymerization and after copolymerization. The reaction mechanism of the first method of adding
materials is as follows:
There scheme (1) is dominant, but scheme (2) cannot be excluded. Even if scheme (2) is neglected,
a complicated process of copolymerization may occur due to the presence of Na2CO3 or NaOH.
Because of this reason, a new method was used. That is, first starch was added to the system and
reacts with acrylamide for a period of time, then the alkaline solution was dropped into the inverse
emulsion of starch-graft-polyacrylamide. Using this way of adding materials, we found that
molecular weight of grafted PAM is higher and obtained hydrolysis degree is ideal. In addition,
grafting efficiency and grafting frequency were also measured. In the system, the maximum
grafting efficiency is 62.50%, and grafting frequency is 8749 AGU / per graft.
3.2 Choice of hydrolyzing agents and hydrolytic conditions
The process of hydrolysis includes two steps. First the hydrolyzing agent enters the copolymer latex
particles through the water-in-oil inferfacial film, then the amide groups in the polyacrylamide are
converted to carboxy-late at high temperature and high pH. According to this mechanism, the
process of hydrolysis is closely related to the agents' ability to dissolve in water, the pH value, time,
and temperature.
Using NaOH and Na2CO3 as hydrolyzing agents, respectively, and comparing their characteristics,
we first found that addition of strong alkali destroyed the emulsion because NaOH can lower
pasting temperature of starch and lead to swelling of the liquid beads. In addition, NaOH can cause
cleavage of the starch chains. Therefore, it is not suitable to use strong alkali as hydrolyzing agent.
For this reason, we used Na2CO3 instead. The system caused less coagulation and was emulsified
very easily. Hydrolytic degree was close to 30% at 4 h when using Na2CO3 as hydrolyzing agent,
with equimolar amounts of Na2CO3 and AM were added (see Fig. 1).
A further study indicated some disadvantages to use Na2CO3 as hydrolyzing agent. First, it requires
a large volume of agent. Second, a slow hydrolyzing process causes incomplete hydrolysis and
more Na2CO3 remains. However NaOH has a catalytic action, and it can increase hydrolytic rate.
Therefore, it is better to use NaOH and Na2CO3 together, both in low concentration, for hydrolysis.
Experiments showed that this method can reach a high hydrolysis degree in a shorter time when
adequate concentration of hydrolyzing agent was used (see Tab. 1).
Temperature also had effect on hydrolysis (see Tab. 2). At a low temperature, it took a long time to
hydrolyze and the hydrolysis degree was not sufficient. If the temperature was too high, latex
particles would cohere. Experiments
Fig. 1. Effect of hydrolysis time and temperature on percent hydrolysis.
(●-●) Reaction curve at 30 °C;
(▲-▲) Reaction curve at 40 °C;
(o-o) Reaction curve at 50 °C;
The acrylamide/alkali molar ratio was 1/1.
Tab. 1. Effect of different hydrolyzing agent on hydrolysis
Tab. 2. Effect of reaction temperature on hydrolysis.
showed that the appropriate temperature was 50-60 °C. At the temperature, copolymer latex
particles swelled easily and alkali easily entered into its inside. Therefore hydrolysis was complete
and hydrolysis degree was higher.
3.3 Stabilities of polymer solutions
Here stabilities imply temperature sensitivity, salinity resistance and shear stability. The effect of
temperature on the stability of the grafted copolymer can be explained by the change of its viscosity
with temperature. The relationship between the viscosity of the polymer solutions and temperature
conforms to the Arrhenius formula [16]:
To compare the dependence of the viscosity of grafted copolymers and ungrafted copolymers on
temperature, In r\ was plotted versus 1/T. From the plot, Ea, the activation energy of viscous flow
was calculated (see Tab. 3). It shows that the activation energy of the grafted copolymer is lower
than that of ungrafted copolymer. The viscosity change was less with temperature increase, which
shows stability of the copolymer. In addition, it can be seen that temperature stability of anionic
starch-g-PAM is better than uncharged grafted copolymer.
Tab. 3. The activation energy of viscous flow (Ea) for the polymer solution.
Fig. 2. Viscograms for graft copolymers at different shear
rate.
Polymer concentration, 0.5%; temperature 30 °C; Mw of
PAM graft, 4.22 x 106; Mw of PAM, 4.23 x 106.
1 S-g-PAM, HD 31%;
2 S-g-PAM, HD 29%;
3 S-g-PAM, HD 27%;
4 S-g-PAM, HD 21%;
5 PAM, HD 14%;
6 PAM;
7 S-g-PAM.
The shear stability of a series of starch-graft-polyacryl-amide with different hydrolysis degree are
illustrated in Fig. 2. It shows that rheological properties of the products are related to the hydrolysis
degree. Viscosity increased with increase in hydrolysis degree. When hydrolysis degree is about
30%, the viscosity of anionic starch-graft-polyacrylamide is six times as much as that of polyacrylamide and uncharged grafted copolymer. Moreover, the viscosity of copolymers with different
hydrolysis degree decreased with increasing shear rate. The higher the hydrolysis degree, the more
viscosity decreased with the increasing shear rate. But viscosity does not change any longer when
the shear rate reached 90 rpm, and uncharged grafted copolymer is the most stable.
Salt tolerance of starch grafted copolymers is expressed as:
The results are shown in Tab. 4. As can been, the grafted copolymers have a better salt tolerance
and the uncharged polymers more stable.
In addition, the stability of anionic grafted copolymers of different kinds of starch are also
compared. The results are shown in Tab. 5. As illustrated in Tab. 5, the shear stability of oxidized
starch-g-PAM is the best, and the salt tolerance of hydrolyzed starch and solubilized starch is
slightly lower. Salt tolerance of the different starch graft
Tab. 4. Salt tolerance of different polymers.
Tab. 5a. Stability of anionic graft copolymers of different starches.
copolymers is as follows: hydrolyzed starch-g-PAM >sol-ubilized-g-PAM >oxidized starch-gPAM.
3.4 Proof of formation of starch graft copolymer
Fig. 3 shows IR spectra of starch and its grafted copolymer, respectively. Pattern A indicates the
characteristic absorption bands of starch at 1020 cm-1, 1080 cm-1 and 1150 cm-1 due to C-0
stretching vibration. Comparing pattern A and B can be seen that in pattern B, the characteristic
absorption bands of the sample at 1620cm"1 and 1670 cm-1 due to -NH2 and C-0 stretching
vibration in -CONH2 group have been observed besides the characteristic absorption peak of starch.
The result proved that starch has been grafted with acrylamide.
4 Conclusions
Anionic starch-graft-polyacrylamide was obtained using alkaline hydrolysis in inverse emulsion.
This method avoid the use of acrylates; temperature control of the
process is easier; polymerization in concentrated solution of acrylamide becomes possible and the
molecular weight of grafted PAM is higher.
Fig. 3. IR spectra of starch and the grafted copolymer. (A) starch; (B) grafted copolymer.
Both NaOH and Na2CO3 are common hydrolyzing agents. A combined use of both not only
increases the hydrolyzing speed but also improves the effect of hydrolyza-tion. A synergistic effect
of NaOH and Na2CO3 on the hydrolysis is observed.
Temperature stability of anionic starch-graft-polyacryl-amide depends on graft level and hydrolysis
degree. The higher the grafting and hydrolysis, the better is its stability. When hydrolysis degree is
about 30%, anionic starch-graft-polyacrylamide has high viscosity. The high viscosity at low shear
rate will be beneficial in oil recovery.
Anionic graft polyacryiamides of different starches show a variation in stability. The salt tolerance
of anionic graft polyacryiamide of hydrolyzed corn starch is better than that of solubilized starch
and oxidized starch, and the shear stability of anionic graft polyacryiamide of oxidized starch is the
best.
References
[1] J. Goin: "Water Soluble Polymers", Marketing Research Report 582.0000 D-E. August 1991.
[2] M. Camail, A. Margaillan, I. Martin: Synthesis of N-alkyl and N-arylalkylacrylamides and
micellar copolymehzation with acrylamide. Eur. Polym. J. 2000, 36, 1853-1863.
[3] E. E. Kathmann, L. A. White, C. L. Mccormick: Water-soluble copolymers. 67. Polyelectrolytes
of N-vinylformamide with sodium 3-acrylamido-3-methylbutanoate, sodium 2-acrylamido-2methylpropanesulfonate, and sodium acry-late: synthesis and characterization. Macromolecules
1996, 29, 5268-5272.
[4] L. Y. Liu, S. Z. Han, G. T. Jin: Synthesis of APAM by inverse suspension and adding alkali for
hydrolysis polymerization. Journal of Beijing University of Chemical Technology 2000, 2, 36-39.
[5] S. R. Deshmukh, K. Sudhakar, R. P. Singh: Drag-reduction efficiency, shear stability, and
biodegradation resistance of
carboxymethyl cellulose-based and starch-based graft copolymers. J.Appl. Polym. Sci. 1991,
43, 1091-1101.
[6] J. H. Luo, R.Y. Pu, P. M. Wang: Performance properties of salt tolerant polymer KYPAM for
EOR. Oilfield Chemistry 2002, 1, 64-67.
[7] Zhang Liansheng, EP Pat. 356, 241, (1990): Manufacture of Graft Copolymers for Oil Recovery
Applications.
[8] V. F. Kurenkoy, A. S. Verihznikova, V. A. Myagchenkov: Peculiarities of the preparation of
copolymers of acrylamide with sodium acrylate in inverse emulsion. Eur. Polym. J. 1984,20,357360.
[9] Huang He, Li Jianzong: Inverse emulsion polymerization. China Synthetic Rubber Industry
1993, 4, 238-242.
[10] Li Jianzong, Cheng Shiyuan, Huang He: The research advances in inverse emulsion
polymerization. Polymer Bulletin 1993, 2, 71 -76.
[11] A. Renken, D. Hunkeler: Effect of the surfactant blend composition on the properties of
polymerizing acrylamide-based inverse-emulsions: Characterization by small-angle neutron
scattering and quasi-elastic light scattering. Polymer 1999, 40, 3545-3554.
[12] YenjerShih EP Pat. 216091 (1987): Inverse Emulsions.
[13] K. Varma, O. P. Singh, N. K. Sandle: Graft-copolymerization of starch with acrylamide, 1.
Angew. Makromol. Chem. 1983, 119, 183-192.
[14] J. Gao, J. Yu, W. Wang: Graft copolymerization of starch-AN initiated by potassium
permanganate. J. Appl. Polym. Sci. 1998, 65,1965-1972.
[15] S. Lu, T. Lin, D. Cao: Inverse emulsion of starch-graft-poly-acrylamide. Starch/Starke 2003,
55, 222-227.
[16] Xu Xi, He Qing-gong, Hu Jun-ming: A study on stabilities of aqueous polymer solutions used
for exploration of oil field. Journal of Chengdu University of Science and Technology 1981, 1, 2737.
(Received: September 4, 2002)
(Revised: December 18, 2002/March 1, 2003)
(Accepted: May 20, 2003)