Poly(Ethylene-co-Acrylic Acid)을 이용한 Ethylene-Modified Latexes의 제조

신진섭, 김중현

연세대학교 화학공학과 기능성 초미립자 공정연구실

Preparation of Ethylene-Modified Latexes by Emulsion Polymerization Using Poly(Ethylene-co-Acrylic Acid) As a Polymeric Emulsifier

Jin-Sup Shin, Jung-Hyun Kim

Department of Chemical Engineering, Yonsei University, 134 Shinchon-Dong, Sudaemoon-Ku, Seoul 120-749, Korea

Introduction

Recently, alkali-soluble resin (ASR) containing carboxylated ramdom copolymers was used as a polymeric surfactant in the emulsion polymerization of polystyrene (PS) and polymethyl methacylate (PMMA) latex particles.[1]

Ethylene-acrylic acid resin (EAA) containing the ethylene group are flexible thermoplastics having water resistance and barrier property similar to low-density polyethylene (LDPE). Introduction of ethylene-acrylic acid resin containing ethylene groups on emulsion polymer also reduces the permeability of water. The role of adsorbed surfactants in the process of film formation has received some interest and has been summarized by Bindschaedler et al.,[2] and the barrier properties caused by them have been discussed.[3]

The importance of the interface in multiphase polymer systems has been long recognized. Physical and chemical interactions across the phase boundaries are known to control the overall performance of immiscible polymer blends. Searches have shown that the blending of PET and PA in the melt produces polyester-polyamide block copolymers, which could improve the compatibility of the blends.[4]

Experimental

Batch polymerizations were carried out in a double-wall round-bottom reactor equipped with a temperature controller, nitrogen inlet, and stirrer. In the reactor, EAA and NaOH were dissolved in distilled deionized water, and monomer was added. NaCl was added in the low NaOH concentration systems in order to adjust the ionic strength. The reactor was left for about 30 min for the contents to attain the reaction temperature 70℃. During this time, the contents of the reactor were stirred and flushed with nitrogen. The initiator was dissolved in the remaining water and added to the reactor. The addition of the initiator marked the start of the reaction. The polymerization was carried out under nitrogen atmosphere until the reaction was substantially completed.

The average particle size and particle size distribution of the latex particles were determined using capillary hydrodynamic fractionation (CHDF 2000, Matec Applied Science, USA). The polydispersity index (PDI) was given as Dw/Dn, where Dw is the weight average diameter of latex particles and Dn is the number average diameter.

For DMA, we worked with the simple extension mode. The dynamic mechanical behaviour of the films was investigated using a dynamic mechanical thermal analysis (DMTA) (Polymer Labotatories, Model MK-III, UK) equipped with an attachment for temperature control. This apparatus enabled us to measure the extension storage modulus (E'), the loss modulus (E'') and the loss tangent (tan□) over a wide range of temperature (-50℃~200℃). The isochronal temperature dependence of moduli and tan  was obtained for a frequency of 10 Hz at a constant heating rate of 2℃ min-1 under a nitrogen atmosphere. The accuracy of the data values was checked by twice repeating measurements in the same temperature range and the reproducibility was found to be satisfactory.

The barrier properties of ethylene-modified poly(n-butyl methacrylate) (PBMA) films were evaluated by measuring the weight loss of water in the bottles. The weight loss of water was determined after 14 days. To gain the accuracy of the data, the data was obtain by three test bottles and the data collected for each bottles at same condition were averaged.

Results and discussion

It is suggested that both the solubility of the monomer in water and its polarity affect the mechanism of particle formation.[5] For the conventional emulsion polymerization of styrene, examined by Harkins,[5] the most important sites of particle generation are monomer-swollen emulsifier micelle; with the increase of monomer solubility in water, the locus of particle nucleation may move to the aqueous phase. The particle formation in emulsion polymerization using EAA would be affected by the properties of EAA aggregates which depend on the neutralization of EAA, as well as the content of electrolytes, and the water solubility of monomer.

The polystyrene (PS) prepared at 100% degree of neutralization of EAA showed the relatively broad particle size distribution (PSD) because the size of EAA aggregates was large and random, contributed to high electrostatic repulsion between charges along the EAA chain. As the EAA concentration is higher, the PSD is narrower(Figure 1). This result was coincident with conventional surfactant system. The particle size and polydispersity of latexes prepared using EAA as a polymeric surfactant are listed in Table 1.

However, the PS latexes prepared using excess neutralizing agent, NaOH, shows the small particle size (ca. 70 nm) compared to that observed for conventional emulsion polymers. In this system, emulsion latexes which are very stable, and their particles in submicron sizes are approaching monodisperse. The particle sizes of latexes increased from 69.1 to 82.4 nm in diameter with decreasing weight ratio of EAA/styrene and the particle sizes distribution became monodisperse with increasing weight ratio of EAA/styrene(Figure 2). This result was similar to that obtained in the emulsion polymerization using conventional surfactants. These results are listed in Table 1.

In case of ethylene-modified PS, however, the damping curve shows the single, sharp glass transition temperature(Figure 3), that is, the miscibility between PS and SAA resin in ethylene-modified PS improved by ethylene-modified emulsion polymerization. It can be also interpreted from the improved miscibility between two phases without adding any compatibilizer. The phase separation sometimes occurs during film formation resulting in a drop of film characteristics. The EAA used as a polymeric emulsifier would be grafted to main chains or keeps sticking to them during polymerization or the chain transfer to EAA molecules would be result in the improvement of miscibility and then there is no phase separation during film formation.[1]

It is clear that the ethylene-modified PBMA latex films give lower the permeability than pure PBMA film.(Figure 4) The permeability decreases with increasing EAA concentration in the PBMA latex films. The improvement of water resistance is owing to the ethylene group in the EAA.

Table 1. Particle Size Analyses of Ethylene-Modified PS Latexes Prepared Using EAA at 100% degree of neutralization of EAA

EAA*	Degree of Neutralization	Dn (nm)	Dw (nm)	PDI (Dw/Dn)
40	100	114.9	235.2	2.04
60	100	82.7	114.9	1.37

40	120	104.7	181.0	1.73
40	140	82.4	101.1	1.23
50	140	75.6	81.2	1.07
60	140	69.1	74.7	1.08
40	160	71.7	78.6	1.10
40	180	70.0	76.0	1.09

PDI : polydispersity index

Dn : number average diameter, Dw : weight average diameter

* wt% based on monomer

Figure 1. Particle distribution index of ethylene-modified PS using 40wt% EAA based on PS with different degree of neutralization of EAA.

Figure 2. Particle size and size distribution of ethylene-modified PS with different EAA concentration at 140% different degree of neutralization of EAA.

Figure 3. Dynamic mechanical properties of ethylene-modified PS film containing EAA 40wt% as a function of temperature : damping curve (tan□).

Figure 4. Water permeability of PBMA and ethylene-modified PBMA films with EAA concentration.

References and notes

1. Lee, D. Y., and Kim, J. H., J. Appl. Polym. Sci. 69, 543(1998).

2. Bindschaedler, C., Gurny, R., and Doelker, E., J. Appl. Polym. Sci. 34, 2631(1987).

3. Issacs, F. K., J. Macromol. Chem., 1, 163(1966).

4. Inoshita, K., Terakawa, M., and Yasuda, T., Chem. Abstr. 78, 3075(1973).

5. Harkins, W. D., J. Am. Chem. Soc. 69, 522(1947).