PP/EVOH 블렌드의 산소 차단 특성

여종호, 이종환, 이종훈, 김성우*,이기준

서울대학교 응용화학부

경기대학교 첨단산업공학부*

Oxygen Barrier Properties of PP/EVOH Blends

Jong-ho Yeo, Jong Hwan Lee, Chong Hoon Lee, Seong Woo Kim*, and Ki-Jun Lee

Division of Chemical Engineering, Seoul National University

Division of Advanced Industrial Engineering, Kyonggi University*

Introduction

Plastic materials with good oxygen and solvent barrier properties offer a variety of advantages in comparison to metal and glass for the applications of packaging and solvent containers. Plastic films with high barrier propeties are usually produced with multi-layer structure using lamination or co-extrusion process.  However, this process requires high capital investment and complex process control, and have a limitation of recyclability.(1)

Currently, an alternative approach to overcome these shortcomings has been proposed by incorporating a small quantity of a barrier material with a lower cost polyolefin resin. In this polymer system, the improvement in barrier properties is accomplished through the formation of a laminar morphology of the dispersed phase resulting in increase of the tortuous path.(2-4) The first successful attempt to produce blow molded high barrier containers with laminar morphology was reported by Subramanian.(5,6) The results showed that PA6, the barrier resin, could be distributed as large and thin sheets in the HDPE matrix by extruding the blends under special extrusion conditions, resulting in a decrease in solvent permeability. Kamal et al.(7) were also able to develop laminar morphology in PP/EVOH blends by incorporating a specially designed slit die into the extrusion casting film process. However, their approaches have had a limitation in enhancing the barrier property because sufficient biaxial stretching resulting in laminar structure with larger area could not be made in the molten state.

In this study, the development of polypropylene(PP)/ethylene-vinyl alcohol copolymer(EVOH) blend film with higher oxygen barrier property was attempted by utilizing the biaxial orientation process. The effects of compatibilizer content and processing variables such as stretching ratio and  temperature on the oxygen permeability and morphology of the blend films were investigated.

Experimental

For the continuous phase, a biaxially oriented polypropylene(BOPP) grade PP (HF21AGT, M.I.=2.0g/10min, Tm=160.9℃, Tc=116.3℃) supplied by Samsung General Chemicals was chosen. For the dispersed phase as a barrier material, an ethylene vinyl alcohol copolymer(EVOH; EP-E105B, ethylene content=44mol%, M.I.=5.5g/10min, Tm=166.2℃, Tc=147.1℃) was obtained from Kuraray Co. in pellet form. Since PP and EVOH are incompatible, two kinds of compatibilizer as the interfacial agents were used; a)polypropylene grafted with maleic anhydride(PP-g-MAH, PO1015; MAH content=0.3 wt%, Tm=143.9℃, Tc=102.3℃) from Exxon Co., b) ionomer (IT 7020; zinc neutralized ethylene-acrylic acid copolymer, acrylic acid content=17 mol%, Tm=83.6℃, Tc=55.1℃) from Exxon Co.

Compatibilized PP/EVOH blend sheets were prepared using a Haake single screw extruder (L/D=25) with a 10.0cm×0.14cm slit die. The temperature profiles for the three heating sections of the extruder barrel and  die were set at 190, 205, 220, and 230℃, and the screw speed was 40 rpm. The draw ratio was fixed at unity to prevent the elongation of the blends in the machine and transverse directions.

The sheets prepared from cast film process described above were variously biaxially stretched by a Toyoseiki biaxial film stretcher. The undrawn samples with a thickness of 1.05 mm were cut to dimensions, 8.7cm×8.7cm, and gripped by vacuum assisted pneumatic clips in the preheated chamber set at 160℃. The sheets were simultaneously biaxially stretched to the desired ratios at a stretch rate of 1.220m/min.

The compatibility of the blends were evaluated by FT-IR analysis, measurements of tensile properties, and morphological analysis. The oxygen permeability of the stretched blend film was measured using high vacuum time lag technique described in ASTM D3985.

Result and Discussion

Tensile properties of the compatibilized blends

The effect of compatibilizer on the tensile properties of the PP/EVOH(85/15) blends was examined. Tracing the variation of tensile property has been a method of evaluating qualitatively the compatibility by adding compatibilizer. Increasing the compatibilizer content resulted in increase of elongation of the blends regardless of the kinds of compatibilizer, which is attiribulted to the enhancement of the interfacial adhesion between PP and EVOH phase. However, the tensile strength at break of the blends was initially increased with increasing of compatibilizer content up to 4 phr, and then decreased as shown in Fig. 1. This optimum compatibilizer content for tensile strength was believed to be a result of competition between the improved interfacial adhesion and the compatibilizer's own inferior strength relative to the virgin resin.

Effect of compatibilizer content

Compatibilization reaction between alcohol group of EVOH and maleic anhydride or acrylic acid group from compatibilizers has been reported to give a finer and more homogeneous dispersion than in noncompatibilized blends.(8) As shown in Fig. 2, oxygen permeability is initially reduced with increasing of compatibilizer content because of a larger number of thinner EVOH layers. However, poor barrier properties were obtained at high level of compatibilizer content. It appears that exceeding an optimum amount of compatibilizer(4 phr in this study) tends to extremely reduce the size of laminar structure of the dispersed phase. From this result, it is confirmed that there exists an optimum level of compatibilizer content for obtaining enhanced barrier properties with a well developed laminar structure.

Effect of extension ratio

The stretching ratio in an biaxial stretching process is also significant processing parameter in controlling the morphological structure of the dispersed phase. Fig. 3 shows the permeability of the stretched film as a function of stretching ratio for the blends compatibilized with differen kinds of compatibilizers. The oxygen permeability was reduced with increaing of stretching ratio because of enlarged area of EVOH domains providing long tortuous path through the blends

Effect of stretching temperature

As the stretching temperature was lowered (160℃→155℃→150℃), it became more far from the melting temperatures of PP(161℃) and EVOH(166℃), thus each phase was not existed in easily deformable state. Fig. 4 presents that the permeability of the blend film was increased as stretching temperature decreased. This result implies that the stretching temperature close to the melting temperature of the neat resin is more favorable processing condition to obtain high barrier blend films with a pronounced laminar morphology of the dispersed phase.

References

1. J. B. Faisant, A. Ait-Kadi, M. Bousmina, and L. Deschenes, Polymer, 39,       533 (1998).

2. M. R. Kamal, I. A. Jinnah, and L. A. Utracki, Polym. Eng. Sci., 24,

1337 (1984).

3. R. M. Holsti-Miettinen, K. P. Perttila, J. V. Seppala, and M. T. Heino,         J. Appl. Polym. Sci., 58, 1551 (1995).

4. S. Y. Lee and S. C. Kim, Polym. Eng. Sci., 37, 463 (1997).

5. P. M. Subramanian, Polym. Eng. Sci., 25, 483 (1985).

6. P. M. Subramanian, and V. Mehra, Polym. Eng. Sci., 27, 663 (1987).

7. M. R. Kamal,H. Garmabi, S. Hozhabr, and L. Arghyris, Polym. Eng. Sci.,

35, 41 (1995).

8. M. G. Botros, J. Plastic Film & Sheeting, 12, 195 (1996).

Fig.1. Tensile strength of PP/EVOH    Fig.2.	Oxygen permeability of

(85/15) blends as a function	stretched blend film as a

of compatibilizer content.	function of compatibilizer

content.

Fig.3. Oxygen permeability of          Fig.4.	Oxygen permeability of

stretched blend film as a	stretched blend film as a

function of stretchig ratio.	function of stretching

temperature.