Ethylene-Modified Latex 필름의 동적기계적, 내화학성 및 기계적 물성에 관한 연구

신진섭, 김중현

연세대학교 화학공학과 기능성 초미립자 공정연구실

Dynamic Mechanical, Barrier, and Mechanical Properties of Ethylene-Modified Latex Films

Jin-Sup Shin, Jung-Hyun Kim

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

Introduction

It is well known that the amphiphilic polymers which consist of both hydrophobic and hydrophilic groups can stabilize polymer particles and can form aggregates, like micelles. Poly(ethylene-co-acrylic acid) (EAA) containing the ethylene group are flexible thermoplastics having water resistance and barrier property similar to low-density polyethylene (LDPE). Due to a high degree of inertness to chemicals, polyethylene is an appropriate material for containers and automobile tanks. Introduction of EAA 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.[1], and the barrier properties caused by them have been discussed. However, few publications can be found concerned with EAA as a polymeric emulsifier in emulsion polymerization.[2] The properties of EAA are expected to be similar to those of other polymeric surfactants, but the behavior and contribution in emulsion polymerization would be different from those of conventional polymeric surfactants. 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.[3] Unfortunately, very few investigations on the improvement compatibility of the blends with compatibilizer or functional group seems to be available in the literature.

The present study was undertaken to investigate the contributions of EAA in the emulsion polymerization of styrene with a water-soluble initiator, potassium persulfate. The improvement of miscibility between PS and EAA during emulsion polymerization and the barrier properties and mechanical property of ethylene-modified latex films were investigated.

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. 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.

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. 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 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. The tensile strength measurements were done in a Universal Testing Machine (Lloyd LR-10K, USA) according to ASTM D638M at a strain rate of 10 mm/min).

Results and Discussion

Before the DMTA analysis of simple blending film of PS latex and EAA solution, let us set the stage by defining the relevant features in the tan  and storage modulus of individual PS latex film and EAA film. Both PS latex film and EAA film show the single, sharp glass transition temperatures.(Figure 1) Figure 2 shows various dynamic mechanical properties, such as damping (tan□), of simple blending film of PS latex and EAA solution with respect to variation in the temperature, range from 50℃ to 200℃. The simple blending film of PS and EAA shows the two glass transition peaks. The peak at 130~135℃ corresponds to the glass transition peak of PS and the other peak at 60~70℃ corresponds to the glass transition peak of EAA.

In case of ethylene-modified PS, however, the damping curve shows the single and sharp glass transition temperature (Figure 2), that is, the miscibility between PS and EAA in ethylene-modified PS was in-situ improved during ethylene-modified emulsion polymerization. It can be also interpreted as the improved miscibility between two phases without adding any compatibilizer. The EAA used as a polymeric emulsifier would be grafted to main chains or keep sticking to them during polymerization. This EAA-grafted-PS due to the chain transfer would result in the improvement of miscibility and then there is no phase separation during film formation.

Figure 3 showed the chemical resistance of ethylene-modified PS and the simple blends PS and EAA. Because of the good compatibility between PS and EAA originated in PS-grafted-EAA, the chemical resistance of ethylene-modified PS films is dramatically better than that of the simple blends PS and EAA. In case of EMPS-E40, the chemical resistance is about 20 times higher than that of SBPS-E40, as shown in Figure 3. Figure 3 shows the percentage weight losses of ethylene-modified latex films and the simple blending films of PS and EAA after their immersion to methyl ethyl ketone for 5 hours. These results are in general agreement with recent work by Ju et el..[4] However, they used the special functional monomers for compatibilizing those immiscible blends. In the ethylene-modified system, during emulsion polymerization the improvement of miscibility between PS and EAA in-situ gained without any compatibilizer. Also, with an increase of EAA concentration, the chemical resistance of ethylene-modified PS films increased.(Figure 3) It is attributed to the content of ethylene group in these films. The tensile strength of ethylene-modified latex films was better than that of simple blends due to the enhancement of miscibility between EAA and PS.(Figure 4)

References

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

2) J. S. Shin, D.Y. Lee, and J.H. Kim, Macromolecular Symposia, in press.

3) K. Inoshita, M. Terakawa, and T. Yasuda, Jpn. Pat. 7, 728, (1972) 916.

4) M. Y. Ju and F.C. Chang, J. of Appl. Polym. Sci. 73 (1999) 2029.

Figure 2. Dynamic mechanical properties of ethylene-modified PS latex film and simple blending film of PS and 40wt% EAA based on PS as a function of temperature : damping curve (tan ).

Figure 1. Dynamic mechanical properties of PS homepolymer film and EAA homopolymer film as a function of temperature : damping curves (tan ).

Figure 4. Stress-extension behavior for ethylene-modified PS latex film and simple blend PS and 40wt% EAA based on PS.

Figure 3. Weight loss of ethylene-modified PS and the simple blends of PS/EAA as a function of EAA concentration after their immersion to methyl ethyl ketone for 5 hours.