EVOH의 비닐알코올 함량이 PA/EVOH 블렌드의 물성에 미치는 영향

성원영, 김성우

경기대학교 화학공학과

wookim@kyonggi.ac.kr

The Effects of Vinyl Alcohol Content in EVOH

on the Properties of PA/EVOH Blends

Won Young Sung, Seong Woo Kim

Department of Chemical Engineering, Kyonggi University

wookim@kyonggi.ac.kr

Introduction

Blending of polymers has been the most useful industrial technique to produce new polymeric systems with enhanced properties. This methodology has been also attempted to produce barrier packaging materials by incorporating small quantity of gas barrier resins of polyamide(PA) or ethylene-vinyl alcohol copolymer(EVOH) with polyolefin resins having moisture barrier property.(1,2) In this blend system, various properties including barrier property of ultimate products are largely dependent on the physical properties of each constituent and on the final blend morphology. EVOH has been widely used as packaging materials for food, drugs, and cosmetics because of its superior gas barrier property, excellent transparency, and high oil resistance.(3-5) The commerically available EVOH grades contain 56 to 73 mole % vinyl alcohol. Its excellent barrier properties are attributed to the high cohesive energy density of the resin arising from the dense inter and intra molecular hydrogen bonding. The physical properties including barrier property of EVOH resin are largely dependent on the content of vinyl alcohol in EVOH.

However, EVOH has some shortcomings such as relative high brittleness, high cost, and difficulty of processing. On the other hand, nylon that is more advantageous in cost relative to EVOH exhibits high resistance to the permeation of organic solvents, and also has good toughness. In this work, hence, incorporation of EVOH with PA was attempted to produce a new blend system with attractive properties of each resin. It can be expected that this blend system should be utilized to produce polyolefin based blend materials with enhanced propeties of gas/solvent barrier property and  mechanical property. It has been well known that blends of EVOH with PA have a good miscibility due to occurrence of hydrogen bonding interactions between hydroxyl group of EVOH and amide group of PA.(6) This indicates that the degree of compatibility between these two resins would be varied with the content of vinyl alcohol in EVOH copolymer.

In this study, the effects of content of vinyl alcohol in the EVOH and composition in PA/EVOH blend system on the rheological, morphological, thermal, mechanical, and barrier property were investigated. The compatibility of this blend system was also evaluated by performing FT-IR and XRD analysis.

Experimental

The polymers used for the study were PA(2835TF, a copolymer of Nylon 6 and Nylon 66, Tm=191.5℃, Tc=143.9℃), which was produced from Hyosung Co. As EVOH resin containing various vinyl alcohol content, EVOH1(EP-E105B; vinyl alcohol content=56mol%), EVOH2(EP-F101A; 68mol%), and EVOH3(EP-L105B; 73mol%) were chosen, which were supplied from Kuraray Co. Before mixing and processing, the resins were pre-dried over night at 60℃ in a vacuum oven and dry blended before extrusion. The weight ratio of PA/EVOH was 100/0, 80/20, 60/40, 40/60, 20/80, 0/100, respectively. PA/EVOH blends with various compositions were obtained using a twin screw co-rotating extruder(Brabender, L/D=20) operating at a screw speed of 60 rpm. A processing temperature profile of 190-205-220-230oC in the barrel and die was fixed for all experiments. Rheological properties of both virgin polymers and blends were measured by a Rheometrics Mechanical Spectrometer(RMS 800) with rotational parallel plate fixture, the parallel plates had a radius of 25mm, and the gap between two plates was set to 2mm. The complex viscosities were determined by varying the frequency from 0.1 to 100 rad/s. All measurements were performed under a nitrogen atmosphere to prevent oxidation.

The oxygen permeability of the samples was measured by the method based on ASTM D3985. Tensile strength and elongation at break were measured using an Instron testing machine(Model : IX series 6.05) with a cross  head speed of 200 mm/min. A minimum of five samples were tested in each case, and the standard deviation was less than 20%. The specimen dimensions were in accordance with ASTM-D638 at room temperature.

Result and Discussion

Rheological and Morphological property

The effect of the content of EVOH on the complex viscosity of the PA/EVOH blends is shown in Figure 1. As shown in the figure, the viscosities of EVOH2 and EVOH3 are higher than that of PA, but EVOH1 has a low viscosity than PA because it contains larger amount of ethylene. The measured viscosities at a fixed shear rate of 1s-1 as a function of EVOH content are compared with the viscosity data obtained from additive rule, which is represented by the straight line. It shows positive deviation in complex viscosities over entire shear rates with a fixed PA/EVOH  compostion. The degree of positive deviation becomes greater as the shear rate decreases because the interfacial hydrogen bonding interactions may dominate over at low shear rates. It can be seen that the maximum positive deviation points shift to lower value of EVOH content as vinyl alcohol content contained in EVOH increases, implying that the weight ratio of PA/EVOH blends corresponds approximately to the molar ratio of the hydroxyl group of EVOH and amide group of PA by simple stoichimetric calculation. This result also indicates that vinyl alcohol content in EVOH as well as blend composition are significant parameters to improve the compatibilization of PA/EVOH blend system. It was confirmed from morphological observations exhibited in Figure 2 that the domain size of dispersed EVOH phase decreased at fixed composition of PA/EVOH blend system as the vinyl alcohol content of EVOH increases, indicating that highger amount of vinyl alcohol induces compatibilization of the blend due to increased intermolecular interactions between two polymeric materials. This result was consistent with that obtained from FT-IR analysis.

Tensile property

The tensile properties of the PA/EVOH blends with EVOH contents are shown in Figure 3-4. Highr amount of vinyl alcohol in EVOH gives rise to increased tensile strength and reduced elongation of the blend. Meanwhile, both of strength and elongation decrease with increasing of EVOH content. From these results, vinyl alcohol content was found to be importnat variable to control the mechanical properties of the PA/EVOH blends. The measured tensile strength and elongation of the blend also exhibit the positive deviation from simple additive rule, which is similar to the rheological results in all range of composition, but the maximum deviation values obtained by the mechanical tests seem to be somewhat different from those of rheological measurements.

Oxygen permeability

The measured oxygen permeabilities of PA/EVOH blend with various compositions are shown in Figure 5. As shown in the figure, the barrier property of the blend is enhanced as the amounts of vinyl alcohol as well as EVOH resin itself increase, indicating that vinyl alcohol content of EVOH is also significant parameter to obtain higher barrier blend system. It is noted  that the plateau region is observed in range of 60-80 wt% of EVOH for the  EVOH2 and EVOH3, which may be attributed to mositure absorption of larger amount of vinyl alcohol containing EVOH.

References

1. S. W. Kim and Y. H. Chun, Korean J. Chem. Eng., 16(4), 511 (1999).

2. J. H Lee, K. Lee, J. Nam, S. W. Kim, Theories and Application of Chem. Eng.,    7, 2322 (2001).

3. C. A. Finch, Polyvinyl Alcohol, John Wiley and Sons, New York (1992).

4. W. J. Karos, Barrier Polymers and Structures, ACS Symposium Series 423,

American Chemical Society, Washington, DC, 1989.

5. K. M. Finlayson, Plastic Film Technology: High Barrier Plastic Film for

Packaging, Technomic Publishing Co., Lancaster, Pa. (1989)

6. Meijer, H. E. M.; Lemstra, P. J.; Elemans, P. H. M. Makromol Chem,

Macromol Symp, 16, 113 (1988).

Fig.1 Shear viscosity of PA/EVOH blends

as a function of EVOH content(wt%)

at a fixed shear rate of 1s-1

Fig.3 Tensile strength of PA/EVOH as

a function of EVOH content.

PA/EVOH1                 PA/EVOH2                 PA/EVOH3

Fig.2 SEM micrographs of fractured cross-section of PA/EVOH(80/20) blends

with varying vinyl-alcohol content.(×3,000;         10□)

Fig.4 Elongation at break of PA/EVOH

as a function of EVOH content.

Fig.5 Oxygen permeability of PA/EVOH

blends with varying EVOH content.