Preparation of acrylate IPN copolymer latexes

Radiat. Phys. Chem. Vol. 49, No. 3, pp. 371-375, 1997 © 1997 ElsevierScienceLtd Printed in Great Britain.All rights reserved PII: S0969-806X(96)00064-3 o969-8o6x/97 $17.oo+o.oo

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PREPARATION OF ACRYLATE IPN COPOLYMER LATEXES BY RADIATION EMULSION POLYMERIZATION W U M I N G H O N G , I ZHOU RUIMIN, I M A ZUE-TEH, t BAO BORONG ~ and LEI JIANQIU 2 ~Shanghai Applied Radiation Institute, Shanghai University (Jiading Campus), Shanghai 201800, People's Republic of China and 2Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai 201800, People's Republic of China (Received for publication 17 April 1996) Abstract--Radiation-induced and chemical initiation are compared in the initiation of acrylate emulsion copolymer latexes. The particle diameter, distribution and microstructure are influenced by emulsifier concentration, radiation dose and temperature. The results show that the emulsion particle diameter of radiation polymerization is smaller and better distributed in comparison to using chemical polymerization. In addition, interlude polymer net (IPN) core-shell copolymer latexes are observed by transimission electron microscope (TEM). The bounding face of core-shell acrylate copolymer latexes of radiation polymerization is clearer. The morphology of acrylate IPN copolymer latexes is further investigated. © 1997 Elsevier Science Ltd. All rights reserved.

INTRODUCTION Acrylate emulsion copolymer latexes currently used as coating are manufactured using chemical-initiated emulsion polymerization X (1983). One disadvantage of this kind of emulsion is its poor water pressure resistance. The reasons for this are as follows: (1) the emulsifier stays in the coating as a nonvolatile component, while small molecule emulsifiers acting as a hydrophilic group leads to the reduction of water pressure resistance; and (2) water pressure resistance is related to the stability of emulsion particles. The compact coating with good water pressure resistance can be made from small and well-distributed emulsion particles. Many researchers have performed investigations on stable emulsion with lesser volumes of emulsifiers, and also with small and well-distributed emulsion particles (Sakota et al., 1976; Baliter and Tomas, 1987). Compared with chemical initiation, radiation polymerization requires no initiator, and the reaction can be performed at comparably low temperatures. Thus the use of radiation initiation is of a great advantage to emulsion copolymer latexes (Hoigne et al., 1972). In order to manufacture acrylate emulsion copolymer latexes with good stability and film property, the technology has developed over the years to produce acrylate emulsion layer copolymer latexes with a particular structure, which are called core-shell copolymer latexes. It is a type of orderly interlude polymer net (IPN) (Sperling and Chiu, 1973; Pirma and Gardon, 1976), with the core being composed of a hydrophobic polymer, whilst the shell is hydrophilic. The difficulty arises in keeping the dispersive hydrophobic polymer in the core and the

hydrophibic polymer in the shell. This configuration is favorable to the dispersive stability of emulsion copolymer latexes. After film formation, the film becomes deep layer preform hydrophobic, and the water pressure resistance is enhanced. Unfortunately this kind of core-shell copolymer latexes is rather difficult to synthetize, especially in the process of making a copolymer where both the core and shell are acrylate, the bounding face of such a core-shell structure is vague and the transition layer is large (McCarty, 1984; Minghong et al., 1993). Because of this the advantage of acrylate core-shell copolymer latexes is significantly diminished. Up until now, this form of emulsion copolymer latexes with a clear bounding face of acrylate core-shell structure has not been reported. In this study we have successfully synthetized acrylate core-shell copolymer latexes using radiation initiation. EXPERIMENTAL

The radiation source used in this study was Co-60. A transimission electron microscope (TEM-800X) was employed to measure the microstructure of the particle. Tg was measured by a PE-DSC-2 variant. Preparation o f acrylate copolymer latexes Chemical method. The correct concentration of the emusifier, TX-10, R-12 and 50 g of water were placeed in a 250 ml bottle which also contained an agitator, thermometer and condensation tube. The mixture was heated to 45°C, and then a 1/5 mixture monomer (EA/BA/AA = 65/31/4) was added. After agitating the solution for about 30 rain the temperature was increased to 60°C, and 2/3 of the initiator 371

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Wu Minghong et al.

(0.45 g (NH4)2S2Os, 25 g H20) was poured into the bottle. The temperature was further increased to 80°C, and at this point the mixture monomer, N-methylol acrylamide and initiator was added to the solution. After reacting for about 2 h the temperature was increased to 85°C and kept at this level for about 1-2 h. Radiation method. After adding the reaction monomer, water, emulsifier and cross-linker to the reaction tube and blending the mixture at room temperature, air was pumped out of the tube and was replaced by nitrogen. The tube was placed in a constant temperature water pool (which had already reached the reaction temperature), and after stirring the mixture in the tube the radiation source was moved towards the container to initiate the reaction at a selected dose. Emerge core-shell structure. For the core emerge, the compound being used as the core (such as butyl acrylate (BA) or methyl methacrylate (MMA) and BA), water and emulsifier was added to the reaction bottle. This was agitated at room temperature, emulsified and, after pumping the air out, filled with nitrogen. This was now ready for radiation polymerization, and was labelled polymer I. For the shell emerge, polymer I was placed into the reaction bottle which already contained an agitator, thermometer and condensation tube. The temperature was increased to 50°C, and then to the mixture the shell compound [ethyl acrylate (EA) and acrylic acid (AA)] and the initiator was added. After being kept at this temperature for about 2 h this mixture was labelled polymer II. RESULTS AND DISCUSSION

Chemical-initiated emulsion polymerization

The particle diameter and distribution of chemicalinitiated emulsion polymerization of E A - B A - A A under different emulsifier concentrations is shown in Fig. 1. The curve indicate that with increasing emulsifier concentration, the particle diameter of the emulsion falls less and the distribution reduces, but if

/O.'--"-"

//

0.16 --

ir~ 0.12

v o

6° f

B

IA

40

20

0.5

0.6

0.7

1.4

f

a

-

0.I0

1.6

1.7

Fig. 2. The distribution of the emulsion particle: A, chemical initiation; B, radiation initiation. the emulsifier concentration attains the certain value, the change in particle diameter become less. It is considered that low emulsifier concentration is beneficial in creating a good film charateristic. The concentrations of emulsifiers TX-10 and R-12 used were 4.0 and 1.2 g/l, respectively, in the experiments. The particle diameter and distribution are depicted in Fig. 2(A). Suppose that all particle surfaces are fully covered by an emulsifier, and that the diameter of particles are equal in the same reaction system. When considering the two systems, the emulsifier concentrations are denoted as $1 and $2, and the total particle number and particle diameter are denoted as N and R, respectively. Subscripts 1 and 2 correspond to the specific systems. It is known that a emulsifier molecule covers a definite area of the emulsion surface (denoted as a,), and hence the surface area of system one or two equals the total surface area covered by emulsifier, i.e. St UNAas = 4ztr~Ni

(1)

S2UNAas = 4nr~N2

(2)

where N^ is avogadro number and U is the total volume of the system. Dividing equation (1) by equation (2), we obtain (3)

Since the monomer quantity in two systems are equal, after a complete reaction the quantity of the polymer obtained is also equal, e.g.

"

4rcr~N~po/3 = 47tr~N2Po/3

(4)

where P0 is the density of polymer, equation (4) turns out to be

/

N~/Ne = r~/r~

0.10

1.5

(x 10-1 ~m)

$1/$2 = r~Nffr~N2

0.18 --

0.14

80 -

I

t

I

I

I

0.15

0.20

0.25

0.30

0.35

I/S (g-|'l)

Fig. 1. The relationship between particle diameter and concentration.

(5)

Substituting equation (5) into equation (3), we obtain S~/$2 = r2/rl

(6)

From equation (6) it is evident that the particle diameter is inversely proportional to the emulsion

Preparation of acrylate IPN copolymer latexes concentration, which is coincident with the experimental results in Fig. 1.

0.066 --

Radiation-initiated emulsion polymerization

0.062

After the emulsifier concentration is defined, we observed the particle diameter and distribution of emulsion using radiation-initiated emulsion polymerization. The comparision with those of chemical method is presented. Influence o f radiation intensity on emulsion copolymer latexes. Fig. 3 shows the relationship between

the average particle diameter and radiation intensity under the same dose of 3.24 x 105 rad. It is clear that with the increase of radiation intensity, the particle diameter become smaller. The above results are connected with the initiation rate and polymerization rate in the process of polymerization. Under high intensity the radical-initiated rate is large and the particales formed in the forming core stage are large, because the particles have exposed to the same amount of radiation dose in a short duration. According to the Smith-Ewart theory, the rate of a polymerization reaction can be expressed as - d[M]/dt = K[M][I] 2/5

where [M] is the concentration of monomer, I is the radiation intensity and K is a constant, Integrating the above expression, we obtain the following: ln[M]o/[M] = - K I

(7)

3:5

Therefore, under the condition of equal radiation dose [equation (7)], the monomers consumed in the reaction are less when the intensity rate is higher. Due to the simultaneous influence of the two factors, the growing stage of particles under high intensity only lasts a short period. Therefore the quantity of the polymer contained in the particles is low and the dimension of the particles are small. Meanwhile, the initiating rate of radicals under a high intensity rate is large and the particle forming period is short, which results in the small difference among the lifetimes of particles and the narrow distribution of particle diameter. 0.09 --

373

0.054

0.050

I 2.0

1.6

I 2.4

•-I

Influence o f radiation dose on emulsion. The relationship between the average particle diameter and radiation dose under same intensity (30 rad/s) is now examined. The results demonstrate that the particle diameter grows in proportion to the increase in radiation dosage (Fig. 4), and there are also signs of growth in particle diameter distribution. Under the same radiation intensity, the reaction time becomes longer as the dose increases, and therefore the average duration for particle staying in the growing period becomes longer, which accounts for the size of particle diameter. In addition, with the postponement of reaction duration, the differences between the particle effective growing duration becomes larger and their distribution becomes wider. Influence o f radiation polymerization temperature on emulsion. For radiation-initiated emulsion polym-

erization, the reaction can be performed at a comparable low temperature because of its low activation energy. Figure 5 presents the average particle diameter under the same intensity (30 rad/s) and dose (3.24 x 105 rad), but at different polymerization temperatures. The results show that the particle diameter which come from emulsion radiation polymerization becomes large along with the enhance of reaction temperature.

.J"

0.07 --

0.07

0.06 -0.06 0.05

I 10

I 20

f 30

I 40

I (r/s)

Fig. 3. The relationship between the particle diameter and radiation intensity.

I 3.6

Fig. 4. The relationship between particle diameter and radiation dose.

0.08 --

\

I 3.2

Dose (105 r)

~O 0.08

I 2.8

0.05 20

/

./ I 30

./ I 40

I 50

I 60

I 70

t (°C) Fig. 5. The relationship between particle diameter and temperature (°C).

374

Wu Minghong et al. 0.14 -

"~

I~

0.12

-

0.10

-

~

0.080.06 -

o

•

J

0.04 ill/[ • 0.10 0.15

I 0.20

[ 0.25

I .. 0.30

1/S (g.1) Fig. 6. The relationship between particle diameter and emulsifier concentration.

With the rise in reaction temperature, the rate of polymerization will be speeded up. Because the temperature has little influence on the radiation ratio, it also exhibits little influence on the formation of primary particles. The rise in temperature mainly speeds up the expansion of chains in particles, which results in the end product particle diameter along with temperature. Influence o f emulsifier concentration on product for radiation initiation. Under the condition of radiation initiated, the reaction conditions were selected as follows: radiation intensity of 30 rad/s; radiation dose 3.24 × 105 rad, and a reaction temperature of 30°C. The relationship between average particle diameter and emulsifier concentration is depicted in Fig. 6. From the figure it is clear that the particle diameter shows a linear relationship with the inverse of emulsifier concentration, which is in agreement with those obtained using the chemical method. Since no initiator exists to resolve the radicals in the radiation-initiated system, the linear relationship between particle diameter and the inverse of emulsifier concentration does not occur. The distribution is shown in Fig, 2(B). The above results of radiation initiation are compared with those of chemical initiation. We discover that the emulsion particle diameter of radiation intiation is smaller, and the particle diameter distribution is narrower.

Plate 2. The particle transverse of the radiation-initiated P(BA-co-MMA)/P(EA-co-AA) latex. Preparation o f I P N acrylate emulsion layer copolyer latexes We adopt the IPN method to prepared core-shell structure acrylate emulsion copolymer latexes. The hydrophilic EA and AA are used for the shell monomer and the hydrophobic BA is used for the core copolymer monomer. These are then reacted separately under the conditions of chemical and radiation initiation. The TEM method is ultilized to observe the secondary result of the orderly IPN latexes emerging in a homogeneous phase seperation in a emmulsion particle. However, the bounding face of the core-shell is vague. T, is measured to occur at a turning point of - 32°C.The film water pressure resistance of IPN latexes is higher than that of normal latexes. All these findings indicate that homogeneous phase seperation morphology can improve film property. In order to obtain a more clear bounding face of core-shell acrylate emulsion copolymer latexes, we adjust the core monomer and add MMA. The resulting particle morphology via chemical initiation is shown in Plate 1, and the particle morphology of radiation polymerization is shown in Plate 2. It is clear that the core-shell bounding face of Plate 2 is more clear than that of Plate 1. T, of the latexes of Plate 2 is measured to be two turning points: - 27 and - 10°C, and the water pressure resistance is at its highest. In the process of chemical initiation, the shell monomer is added on the condition that the polymerization inversion ratio of the core monomer is about 80%. At a higher reaction temperature the shell monomer is apt to penetrate and so the core-shell structure is vague. However, in the process of radiation initiation, the reaction temperature is lower and the monomer is added on the condition that the core monomer is almost polymerized. Therefore radiation-initiated emulsion polymerization has the advantage in the manufacture of IPN copolymer latexes.

REFERENCES

Plate 1. The particle transverse of the chemical-initiated ax P(BA-co-MMA)/P(EA-co-AA)latex.

Baliter B. and Tomas M. A. (1987) J. Polym. Sci. A 25, 135. Hoigne H. and Neill T. (1972) J. Polyrn. Sci. A 10, 581.

Preparation of acrylate IPN copolymer latexes McCarty W. (1984) H.U.S. Us 444, 923. Minghong Wu, Shen Weiping and Zue-teh Ma. (1993) Radiat. Phys. Chem. 42, 171. Pirma I. and Gardon T. L. (1976) Emulsion polymerization. ACS Syrup., Series 24, p. 306.

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Sakota K. and Okaya T. (1976) J. Appl. Polym. Sci. 20, 1735. Sperling L. H. and Chiu Tai-wo. (1973) J. Appl. Polym. Sci. 17, 2443. Lee D. I. and Ishikawa T. (1983) J. Polym. Sci. 21, 147.