J. Cent. South Univ. Technol. (2008) 15: 324-328
DOI: 10.1007/s11771-008-0061-4
Preparation of self-crosslinked acrylate emulsion with high elasticity and its rheological properties
CHEN Li-jun(陈立军)1, 2, WU Feng-qin(武凤琴)3, LI Dong-shuang(李冬霜)1, 2,
YANG Jian(杨 建)2, LI Rong-xian(李荣先)1, 2
(1. Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China;
2. Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China;
3. Novel Company of Shenzhen, Shenzhen 518054, China)
Abstract: Using butyl acrylate(BA), methyl methacrylate(MMA), methacrylic acid(MAA) and mixed emulsifier as raw materials, the self-crosslinked emulsion was prepared via pre-emulsified and semi-continuous seeded emulsion polymerization technology in the presence of N-hydroxymethyl acrylamide and poly solidum maleate. The influence of mass ratio of BA to MMA, amount of N-hydroxymethyl acrylamide and poly solidum maleate on the rheological properties of the self-crosslinked emulsion was studied. Possible cross-linked mechanism of self-crosslinked monomer was investigated. And the relationship between emulsion viscosity and shear rate was investigated. The results show that the self-crosslinked acrylate emulsion with high elasticity can be synthesized when the mass fractions of BA is 60%, MMA is 40%, and added amount of N-hydroxymethyl acrylamide is 2.5%-3.0% and added amount of poly solidum maleate is 0.3%-0.4%. The self-crosslinkage process of N-hydroxymethyl acrylamide involves two steps. One is copolymerization of N-hydroxymethyl acrylamide and acrylate, the other is cross-linkage among polymer molecules via condensation reaction of methylol. The emulsion is of rheological properties of pseudo-plastic fluid and belongs to non-Newtonian fluid.
Key words: self-crosslinkage; acrylate emulsion; rheological properties; condensation reaction
1 Introduction
Emulsion paints have widespread application in finishing architectural wall[1-3]. However, conventional emulsion paints often lead to the crack of basal wall surface, which has an influence on the practicability and functionality of emulsion paints. Drawbacks of conventional film can be overcome via selection of elastic paints, but the main performance of elastic paints is dependent on the binder, i.e. polymer emulsions. Polymer emulsions with high performance are vital to improve the properties of elastic coatings[4-5].
Acrylate emulsions are widely used in preparing emulsion paints owing to their good water resistance, weather resistance, ageing resistance and flexibility at low temperature[6-7]. Self-crosslinked acrylate emulsions can be prepared via molecular design when some functional groups are introduced into molecular chain of polymer[8-9]. The properties of polymer emulsion, such as heat resistance, water resistance and weather resistance, are further improved for polymer emulsions crosslink and cure via reaction among functional groups of polymer molecules. Compared with cross-linked and cured methods of external cross-linked agent, self- linkage has many merits such as high degree of cross-linkage and convenience of use[10-12]. In this work, self-crosslinked acrylate emulsion with high elasticity was prepared via pre-emulsified and semi-continuous seeded emulsion polymerization technology by using N- hydroxymethyl acrylamide as self-crosslinked monomer and poly solidum maleate as protective colloid. Possible cross-linked mechanism of self-crosslinked monomer was put forward. In addition, the rheological properties of the prepared acrylate emulsion were analyzed.
2 Experimental
2.1 Raw materials
Methyl methacrylate(MMA) and butyl acrylate(BA) were from Guangdong Carpoly Coatings Corporation, China. Monomers were distilled under reduced pressure to remove the polymerization inhibitor before use. Potassium persulfate was recrystallized from water. Polyethylene glocol mono-pnonyl phenyl ether(OP-10), sodium dodecyl benzene sulfonate(DSBS), NaHCO3, ammonia water, methacrylic acid(MAA), N-hydroxymethyl acrylamide, poly solidum maleate and other reagents were all used as received. Distilled water was manufactured in our laboratory.
2.2 Pre-emulsified and semi-continuous seeded emulsion polymerization
2.2.1 Preparation of pre-emulsified liquid
The pre-emulsification of parts of the monomers was carried out in a four-neck glass flask equipped with a stirrer, a thermometer and a dripping funnel. The procedures are as follows[13].
1) Charge the distilled water, part of emulsifiers and self-crosslinked monomer into the four-neck flask under moderate agitation.
2) Set the water-bath to keep the temperature of mixture at 45 ℃.
3) Evenly drip part of mixed monomers of MMA, BA and MAA into the four-neck flask within 45 min under the agitation. Then the stable pre-emulsified liquid, whose appearance was milk white, was obtained for further use.
2.2.2 Preparation of self-crosslinked acrylate emulsion
Semi-continuous seeded emulsion polymerization was carried out in a four-neck glass flask equipped with a stirrer, a reflux condenser and two dripping funnels. The procedures are as follows.
1) Add the distilled water, part of emulsifiers, NaHCO3 and protective colloid into the four-neck flask under moderate stirring.
2) Set a water-bath to keep the temperature of mixture at 80 ℃.
3) Drip the rest of mixed monomers of MMA, BA and MAA and part of initiator solution into the four-neck flask within 15 min.
4) After reacting for 15 min the seeded emulsion, whose appearance was blue and fluorescent, was obtained.
5) Drip the pre-emulsified liquid into the reactor within 4 h. After the reaction lasted for 1 h, the rest of initiator solution was added into the reactor simultaneously. The reaction lasted for 30 min after all the raw materials were completely dripped into the reactor.
6) Raise the temperature of water-bath to 90 ℃ and maintain the reaction for 45 min.
7) Cool the resulted emulsion to 60 ℃ and adjust pH value of the reaction mixture to 6.0-7.0 with ammonia water.
Self-crosslinked acrylate emulsion was obtained after the reaction mixture was filtered.
2.3 Characterization
MDSC 2910 was applied to analyzing glass transition temperature (tg) of the emulsion. The raised temperature was -50-100 ℃. The velocity of the raised temperature was 20 ℃/min. The viscosity of the emulsion was tested by Brookfield viscometer with No.2 rotor at 20 r/min and room temperature. The viscosity of emulsion at different shear rates was tested by NXS-11A rotary viscometer with type A rotor at room temperature.
2.4 Gel rate
All the condensation products were collected after reaction and dried to constant mass. Gel rate was calculated by
(1)
where w is the gel rate; m1 is the mass of all the collected dry condensation products; m2 is the mass of all the monomers.
2.5 Cross-linked degree of emulsion film
The emulsion film was formed at room temperature and then dried for 7 d. 1 g dried film was weighed and extracted in a Soxhlet extractor with THF in 24 h. The cross-linked degree of the emulsion film was calculated by
(2)
where Dc is the cross-linked degree of the emulsion film; 
is the sample mass after extraction; 
is the sample mass before extraction.
3 Results and discussion
3.1 Determination of mass ratio of BA to MMA
Fig.1 shows that the mass ratio of BA to MMA has an effect on tg of the polymer emulsion. tg of the polymer emulsion can be preliminarily estimated from FOX equation[14]:
(3)
Fig.1 Effect of mass ratio of soft monomer to hard monomer on tg of polymer emulsion
where mi is the mass fraction of copolymerized monomer; tgi is the value of tg of homopolymer. The determination of tg is related to other components of polymer, the tested method and instrument, and the velocity of raised temperature. However, FOX equation neglects the influence of relative molecular mass of polymer on tg. Therefore, there is some difference between the theoretical value and the measured value of tg. For two kinds of copolymerized monomer, e.g. MMA and BA, FOX equation can be turned into
(4)
where m1 is the mass fraction of BA; m2 is the mass fraction of MMA; tg1 is value of tg of BA homopolymer; tg2 is the value of tg of homopolymer MMA. The following equation can be obtained via differentiating Eqn.(4):
(5)
In Eqn.(5), tg2 is higher than tg1, so 1/tg2 is smaller than 1/tg1, and dtg/dm1 is negative, i.e. tg of polymer decreases with the increase of the amount of BA. And the lower the tg of polymer emulsion is, the higher the elasticity of film is[15]. But tg of polymer emulsion cannot be too low, otherwise the film of the polymer emulsion is easy to be tacky. In view of elasticity of film and service performance of the polymer emulsion, the mass fraction of BA was selected to be 60% of total mass of copolymerized monomers, and that of MMA is 40% in this work.
3.2 Determination of added amount of self-cross- linked monomer
Fig.2 shows that the cross-linked degree of the emulsion film is increased with the increase of the added amount of self-crosslinked monomer. N-hydroxymethyl acrylamide is copolymerized with vinyl monomer to form thermoplastic polymer since there exist two reactive functional groups in the molecular structure of self-crosslinked monomer, i.e. vinyl group and methylol group, which can run addition reaction and condensation reaction, respectively. There is the active methylol group in the molecular chain of the formed thermoplastic polymer. The polymer with cross-linked structure is formed after drying, dehydration and condensation of the film. Water resistance, weather resistance and ageing resistance of the film are further improved as the emulsion is turned from linear structure to network molecule. The cross-linked points are increased with the increase of added amount of self-crosslinked monomer, leading to the increase of the cross-linked degree of emulsion film. The cross-linked degree of the film should be controlled because the hardness and elasticity of the film vary with it. The cross-linked degree of the emulsion film must be suitable for elastic paints, otherwise the film will lose softness, elasticity and extensibility. Fig.2 also shows that gel rate is increased with the increase of the added amount of self-crosslinked monomer. This can be explained via water solubility and cross-linkage of self-crosslinked monomer. Homo- polymeric degree in the water phase is increased, at the same time, solvation of the emulsion particles is increased, and the emulsion becomes thicker with the increase of the added amount of self-crosslinked monomer, thus causing diffusion of monomers to be more difficult. Therefore, the rate of polymerization is decreased, the viscosity of the emulsion is increased, and elimination of heat is more difficult and degree of self-raised temperature is also increased, which lead to the decrease of polymerization stability and the increase of gel rate.
Fig.2 Influence of added amount of self-crosslinked monomer on cross-linked degree of emulsion film and gel rate
3.3 Possible cross-linked mechanism of self- crosslinked monomer
Possible cross-linked mechanism of self-crosslinked monomer is put forward as follows.
1) Copolymerization of N-hydroxymethyl acryl- amide and acrylate:
2) Cross-linkage among polymer molecules via condensation reaction of methylol:
3.4 Selection of protective colloid and determination of its added amount
Fig.3 shows the effect of added amount of protective colloid (poly solidum maleate) on the viscosity of emulation and gel rate. From Fig.3, it can be seen that the gel rate during the polymerization is decreased with the increase of the added amount of poly solidum maleate. This is mainly caused by good consistency between the protective colloid and acrylate. The compatibility of acrylate is increased due to methyl in the main chain of protective colloid, which can be easily absorbed onto the surface of emulsion particles and helps emulsifiers to stabilize the emulsion. Therefore, gel can be prevented effectively by poly solidum maleate. However, the viscosity of the emulsion is increased with the increase of the added amount of poly solidum maleate.
Fig.3 Effect of added amount of protective colloid on viscosity of emulsion and gel rate
So, the mass fraction of poly solidum maleate should be less than 0.4%, otherwise the viscosity of the emulsion is too larger to polymerize smoothly.
3.5 Rheological properties of emulsion
In Fig.4, the viscosity of the emulsion(η) is decreased quickly with an increase of shear rate(Ds) in a certain range. Furthermore, the viscosity of the latex does not change obviously with an increase of the shear rate when the viscosity is decreased to a certain value. These results show that the emulsion is of rheological property of the pseudo-plastic fluid[16]. This phenomenon is called shear-thinning and can be explained with Mooney equation[17]. Mooney equation can be expressed as
(6)
where ηe is the external phase viscosity of the emulsion particles; ke is the deformation constant; Vi is the internal phase volume of emulsion particles; φ is the accumulation coefficient. Under the action of shear force, the emulsion particles are distorted and deformed, ke is decreased, but φ is increased, which leads to the decrease of η. The shape of the emulsion particle does not change any longer under the action of the shear force, and η changes inconspicuously.
Fig.4 Relationship between viscosity of emulsion and shear rate
Ostwald De Waele equation[18]:
(7)
where η is the apparent viscosity; Ds is shear rate; n is the flow index (the emulsion belongs to Newtonian fluid when n is equal to 1 and emulsion belongs to non-Newtonian fluid when n is less than 1); K is the consistency coefficient. Taking the logarithm to the two sides of Eqn.(7) yields
ln η=ln K+(n-1)ln Ds (8)
The values of K and n can be obtained from linear relationship of ln η-ln Ds. At room temperature, the relationship between ln η and ln Ds of acrylate emulsion is shown in Fig.5.
Fig.5 ln η—ln Ds curve of acrylate emulsion
Fig.5 is analyzed by Origin6.0 professional software. And the results are listed in Table 1.
Table 1 Analytical results of Fig.5 by Origin6.0 professional software
Table 1 shows that ln K is equal to 5.515 23 and n-1 is equal to -0.303. Therefore, K equals 248.45 and n equals 0.697. Acrylate emulsion belongs to non- Newtonian fluid since n is less than 1.
4 Conclusions
1) Self-crosslinked emulsion with high elasticity is prepared by using N-hydroxymethyl acrylamide as self-crosslinked monomer and poly solidum maleate as protective colloid. And the pre-emulsified and semi-continuous seeded emulsion polymerization is adopted.
2) Possible cross-linked mechanism of self- crosslinked monomer involves two steps. One is copolymerization of N-hydroxymethyl acrylamide and acrylate, the other is the cross-linkage among polymer molecule via condensation reaction of methylol.
3) The relationship between viscosity of the emulsion and shear rate shows that the emulsion is of the rheological properties of pseudo-plastic fluid. Data fitting shows that n is equal to 0.697, and the emulsion belongs to non-Newtonian fluid.
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(Edited by CHEN Wei-ping)
Foundation item: Project(2003B10506) supported by Guangdong Provincial Department of Science and Technology, China
Received date: 2007-08-29; Accepted date: 2007-09-30
Corresponding author: CHEN Li-jun, PhD; Tel: +86-755-26551409; E-mail: chenlijun1975@sohu.com
