PP/PA 블렌드의 산소투과 특성

이종환, 이기준, 남재도*, 김현준**, 김성우**

서울대학교 응용화학부, 성균관대학교 고분자공학과*, 경기대학교 화학공학과**

Oxygen Permeability of

Polypropylene/Polyamide Blends

Jong Hwan Lee, Ki-Jun Lee, Jae-Do Nam*, Hyun-Joon Kim**, and Seong Woo Kim**

School of Chemical Engineering, Seoul National University

Department of Polymer Science & Engineering, Sung Kyun Kwan University*

Department of Chemical Engineering, Kyonggi University**

Introduction

Plastic films with high barrier properties for the application of packaging are usually produced with a multilayered structures using coextrusion or lamination process. However, these processing techniques require high capital cost and complex process control because of requirements for multiple extruders and dies, appropriate adhesives, and multi-step operations. In addition, multi-layer coextruded films present a limitation of recyclability because dissimilar resins are adhered together in composite structure. Currently, an alternative approach to overcome these shortcomings has been proposed by blending a small quantity of impermeable polymers such as polyamide or EVOH resin with a lower cost polyolefin resin.(1-3) In this polymer blend 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.

Blending of polypropylene and polyamide has been made to improve the thermal and mechanical properties of polypropylenes. However, addition of polyamide to polypropylene results in improvement in barrier properties as well as mechanical properties. Polypropylene exhibits excellent barrier proper- ties to moisture, but it has a poor barrier property to O2 or CO2. On the other hand, polyamide has high barrier properties to gases such as O2 and CO2, and high resistance to hydrocarbons.(4) However, compatibilizer must be used to enhance the interphase adhesion and phase stability in immiscible blends of PP and PA.(5) The ultimate behavior of the polymer blend, to a large extent, depends on the morphology of the blend such as the size and the shape of the dispersed phase, and on the nature of the interface between the two phases.(6-7)

In this study, in other to obtain high barrier PP/PA blend films using casting film process, the effects of material parameters and processing parameters such as draw ratio, die temperature, and screw rpm on the oxygen permeability and morphology of the blends were investigated.

Experimental

For the continuous phase(matrix phase), a film grade PP(HF21AGT, M.I.= 2.0g/10min, Tm=160.9℃, Tc=116.3℃)was chosen which had been manufactured by Samsung General Chemicals. For the dispersed phase as a barrier material, PA(2835TF, a copolymer of Nylon 6 and Nylon 66, Tm=191.5℃, Tc=143.9℃) was obtained from Hyosung Co. The compatibilizer used in this study was polypropylene grafted with 0.3 wt% maleic anhydride(PP-g-MAH, PO1015;  Tm=143.9℃, Tc=102.3℃), which was obtained from Exxon Co.

Before processing, the resins were pre-dried over night at 60℃ in a vacuum oven and dry blended before extrusion. In all blends, the weight ratio of PP/PA was fixed at 80/20, and the content of the compatibilizer was 1, 2, 4, 7, and 10phr. Cast films of compatibilized PP/PA were prepared by first mixing the dried resins with a pellet shape, followed by melting blending in a Hakke Rheomix 252 single screw extruder with a 10cm X 0.14cm slit die and a take-up system. The temperature profiles for the three heating sections of the extruder barrel and die were set at 190, 205, 220, and 230℃. The various cast film processing conditions such as screw speed, draw ratio, and die temperature were used to examine the influence of processing parameters.

Oxygen permeability of the sample was measured according to the method based on ASTM D3985. The permeation cell consisted of stainless steel short cylinder divided into an upper and lower part. The sample with a diameter of 4.4cm was inserted into this permeation cell separated by O-ring for sealing. The permeation system was evacuated at 10-3 torr for about one hour, and then oxygen(760 torr) was introduced to the upstream side. The permeated pressure as a function time on the downstream side was monitored by an MKS-Baratron transducer(MKS-627B). The temperature was controlled by water bath and maintained at 25℃ during each measurement. Two cylinders were connected to both upstream and downstream side to minimize the fluctuation of pressure with time by increasing the volume. The oxygen peremability could be determined from the slope in the plot of permeated pressure variation with time.

Results and Discussion

Modeling of permeability

Fricke extended Maxwell's model to describe the conductivity of a two-phase system in which low permeability phase with ellipsoid shape is dispersed in continuous phase with high permeability. The permeability of the blend system can be obtained using the permeabilities of the two phases, the volume fraction, and the aspect ratio of the ellipsoids. The measured and calculated permeabilities using three different models are compared in Figure 1. As shown in the figure, the measured values are located between Maxwell and series model, more over they are in good agreement with values predicted by Fricke model. The predicted permeabilities of the blends were obtained using L/W ratio value of 19.6, which was measured from SEM micrographs.

Effect of material parameters

Compatibilization reaction is expected to occur between maleic anhydride group of functionalizaed polypropylene and the amine end group of the polyamide. Figure 2 shows the permeability of PP/PA(80/20) blend films as a function of compatibilizer content. The permeability decreased as the amount of PP-g-MAH increased, but there was not a considerable change above the 7phr. It appears that the compatibilizer has a little influence on the size or shape of the ellipsoids dispersed in continuous phase at content exceeding some level of compatibilizer content(7 phr in this study). In the case of uncompatibilized blend films, poor barrier property(permeability is 28.5 cm3-mm/m2-24hr-atm) were observed because oxygen molecules can diffuse through the microvoids formed at the interface during film process. This may be due to poor interfacial adhesion between PP and PA.

In the binary blend system, the viscosity of each resin and the viscosity ratio of the dispersed phase to the continuous phase(□=□d/□m) are important factors in controlling the blend morphology. The influence of the viscosity of PP on the oxygen permeability is depicted in Figure 3. The permeability of the blends decreased as the viscosity ratio decreases. It can be seen from these results that lower viscosity ratio is a more favorable material condition for obtaining high barrier PP/PA blend films with a well developed ellipsoidal structure.

Effect of process parameters

Draw ratio is a important parameter affecting the blend morphology. The extent of deformation of a dispersed phase was enlarged with draw ratio, consequently L/W ratio became larger. Figure 4 shows that higher screw rpm is less effective in the reduction of the oxygen permeability. Higher shear rate corresponding to high screw rpm gives higher viscosity ratio, which can be confirmed by results obtained from rheological analysis. It can be seen that these results are consistent with the results exhibited in Figure 3. In addition, higher screw rpm gives insufficient compatibilization reaction due to short residence time, resulting in poor barrier properties.

References

1. M. B. Kamal, I. A. Jinnah, and L. A. Utracki, Polymer Engineering and         Science, 24, 17, 1337 (1984)

2. G. W. Lohfink and M. R. Kamal, Polymer Engineering and Science, 33, 21,       1404 (1993)

3. S. A. Jabarin and W. J. Kollen, Polymer Engineering and Science, 28, 1156       (1987)

4. R. M. Holsti-Miettinen, K. P. Perttila, J. V. Seppala, and M. T. Heino,         Journal of Applied Polymer Science, 58, 1551 (1995)

5. M. Xanthos and S. S. Dagli, Polymer Engineering and Science, 31, 13, 929       (1991)

6. P. M. Subramanian, Polymer Engineering and Science, 25, 8, 483 (1985)

7. P. M. Subramanian and V. Mehra, Polymer Engineering and Science, 27, 9,       663 (1987)

Fig.1. Comparison of oxygen permeability  Fig.2. Oxygen permeability of PP/

data for predictions based on	PA blend films as a function

the permeability model.                of PP-g-MAH content.

Fig.3. Oxygen permeability of PP/	Fig.4. Oxygen permeability of PP/            PA/PP-g-MAH blend films as        PA/PP-g-MAH blend films            a function of the viscosity of         as a function of screw rpm.

PP.