Melt blends of polyethylene with a phenoxy

Pergamon

001~3057(95)ooo38-0

MELT BLENDS OF POLYETHYLENE

Eur. Polym.J. Vol. 31, No. 8, pp. 705-708.1995 Copyright 0 1995ElsevierScienceLtd Printed in Great Britain.All rigbta reserved 00143057/95$9.50+ 0.00

WITH A PHENOXY

CHI HOON CHOI* and BYUNG KYU KIM? Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea (Received 22 August 1994; accepted in final form 5 September 1994)

Abstract-Melt blends of high density polyethylene (PE) with a phenoxy have been prepared over a full composition range using a Brabender Plasticorder. Scanning electron microscopy @EM) micrographs showed a clear phase separation, viz. particle-in-matrix or co-continuous morphology depending on the composition. T,,,(crystalline melting temperature) of PE decreased by 3-6”C, whereas TF(glass transition temperature) of phenoxy increased up to ca 11°C in the blends. Mechanical propertIes of the blends showed a negative deviation from linear additivity due to large phase separation (-several pm) and poor interfacial adhesion.

Blending and injection moulding

INTRODUCTION

Polymer blends are commercially important when certain specific performance requirements cannot be satisfied with a single type of polymer [l-3]. There have been a great number of studies to investigate miscible polymer blends. However, most polymers are thermodynamically immiscible. In immiscible polymer blends the dispersed phase can deform during the melt processing, which results in a wide variety of sizes and shapes of the minor phase [4, 51. The change in morphology has a pronounced effect on melt rheology as well as mechanical properties. Therefore, morphology control has received broad attentions [6-91. We consider the melt blends of high density polyethylene (PE) with a phenoxy, which is a poly(hydroxy ether of bisphenol A). Phenoxy is a relatively tough and ductile amorphous polymer with excellent oxygen barrier properties. Thermal, mechanical, morphological, and rheological properties of the blends, prepared by melt extrusion followed by injection moulding, are considered. In addition, the effect of phenoxy on the crystalline structure of PE was also studied.

EXPERIMENTAL Base resins

A medium viscosity grade of phenoxy (PKHH, Union Carbide), and in&son grade of high density PE (M. = 1.86 x 104.M,, = 9.66 x 104a/mol MFI = 4.8ai 10 iin;’ Korean PetrochLmicals) were used for blending. Figure 1 shows the complex viscosity of the base resins at 220°C. Shear viscosity of PE is slightly higher than that of phenoxy. It is generally accepted that the finest breakup of a dispersed phase is obtained when the melt viscosities of the component polymers are alike [4, lo]. Therefore, a rheologically optimum condition for fine breakup is expected. *Present address: Material Engineering & Test Department, Hyundai Motor Co., Ulsan 681-791, Korea. tTo whom all correspondence should be addressed.

Phenoxy was dried for 3 days under vacuum at 80°C to remove moisture thoroughly, and PE for 5 hr at the same temperature using a convection oven. Samples were dry mixed at the desired composition, followed by melt-mixing at 230°C in a single crew extruder (Brabender Plasticorder, L/D = 30, D = 2.5 cm). Test specimens were injection moulded using a BOY injection moulding machine. The injection moulding conditions were as follows: barrel temperature 23O”C, mould temperature 4o”C, injection pressure 500 kg/cm*, and total cycle time 45 sec. Thermal properties

Thermal properties of the samples were determined using a differential scanning calorimetry (Perkin-Elmer DSC-7) at a heating and cooling rate of lO”C/min between 30 and 160°C. X-ray diffraction profiles The crystalline structure of the samples was determined by a wide angle X-ray diffractometer (Rigaku 2013) using CuK, at 30 kV, 15 mA with a scan speed of 2 20/m. Injection moulded specimens were mounted for through direction.

The morphology of the blends was observed using an SEM tJSM820). SEM microgranhs were taken from cryogenicaily (in l&id nitrogen) &a&red surfaces of injectionmoulded tensile specimens in the middle of a cross-section. The fractured surfaces were sputtered with gold before viewing. A transverse view with respect to the flow direction in mould cavity was examined. Mechanical properties

Flexural properties were determined following the standard procedure described in ASTM D 790 (3-point bending method). A testing machine (Tinius Olsen Series 1000) was operated at 5 mm/min. Tests were made at room temperature, and at least five runs were made to report the average. Notched impact strength was measured using an Izod impact tester (Zwick 5102) (ASTM D256). RESULTSAND DISCUSSION Thermal properties

Shear and thermal history of the samples was first erased by heating the sample at 190°C for 5 min. DSC 705

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o(rad/sec) Fig. I, Complex viscosity of the base resins at 120 C‘: (C ) PE and (0) phenoxy. thermograms were taken during the second cycle between 50 and 190°C at lO”C/min. Figure 2 shows the DSC thermograms of the blends. PE homopolymer shows a typical endothermic peak (T,) at 134 C. and phenoxy shows a glass transition (T,) at 86°C. T,, of PE decreased by 3-6°C and T, of phenoxy increased up to about 11 C in the blends. In a crystalline/amorphous system T,,, decreases with the addition of amorphous polymer due to the dilution effect [l l] and T, of each component moves toward each other when there are specific interactions between them [12, 131. Since no specific interactior! is expected between PE and phenoxy, the increased Tg

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28(degree) I tg. 3. X-ray diffraction profiles of PE/phenoxy blends: (a) 1OO:O: (b) 901’10;(c) 70/30; (d) SO/SO;(e) 30/70; (f) 10/90; and (g) Oi100. of phenoxy is due to the migration of low molecular weight phenoxy molecules into the PE phase [14, 151, which also leads to the decreased T,,, of PE. .k’-rq~ d@raction prqfiles Figure 3 shows X-ray diffraction profiles of the blends. The crystalline structure of PE is orthorhombit. with lattice constants (I = 7.42 A, 6 = 4.95 A, c = 2.55 A corresponding to 20 = 20.7 (100 plane), 23.1 (200), and 35.4 (020), respectively [16, 171. Our X-ray diffraction profile for PE slightly deviates from the cited data. Addition of phenoxy does not exert an effect on the crystalline structure of PE throughout the composition range, indicative of no molecular level interaction. Only the peak intensity decreased with increasing phenoxy due to dilution.

Figure 4 shows SEM micrographs of the blends I‘ractured transversely to the flow direction in the mould. The blends show a two-phase morphology at all compositions, i.e. particles-in-matrix or cocontinuous structure. The dispersed domain is _ several pm (diameter), and it is larger when PE forms a dispersed phase than when phenoxy does. In addition, a number of holes (traces of pull-out of dispersed phase upon fracture) are observed especially when phenoxy forms a dispersed phase. Debonding is caused by the poor interfacial adhesion because PE is a typical non-polar material, whereas phenoxy is a polar material containing hydroxy and ether groups. Phenoxy forms a dispersed phase up to 50% phenoxy, probably due to its slightly higher viscosity as compared to PE. 20

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Temperature(V) Fig, 2. DSC thermograms of PE/phenoxy blends: (a) 100/O: (b) 9O/rO; (c) 70/30; (d) SO/SO;(e) 30170; (f) 10/90; and (g) O/100.

Mechanical properties Flexural modulus and strength (Fig. 5) show a negative deviation from the additivity. The negative deviation is generally seen in immiscible bIends of polyolefins [18, 191, and it is mainly caused by the

Melt blends of polyethylene with a phenoxy

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Fig. 4. SEM micrographs of PE/phenoxy blends.

poor interfacial adhesion. The modulus of blends (E) has been predicted by using simple models, a parallel model for miscible and a series model for immiscible blends. These models, however, are not able to explain the synergism or pronounced negative deviation. Thus the modified rule of mixtures is proposed by Nielsen [20] E=w,E,+w,E,+~,,w,wj

(1)

where E is the modulus, w is weight fraction, and j3,z is an empirical parameter defined by & = 4E,, - 2E, - 2E,

(2)

where E, and ‘pirepresent tensile modulus and volume fraction of each component, respectively. The inter-

action term, &, expresses t’ie magnitude of deviation from linearity, and can be used as a relative measure of compatibility. When the interaction term was calculated from Fig. 5, it was -200 MN/m2 for the blends. The negative sign indicates the immiscibility of the blends. In crystalline/amorphous polymer blends, a considerable decrease in impact strength is obtained when the component polymers have limited miscibility. The impact strength (Fig. 6) of the PE/phenoxy blends shows a large negative deviation from the linearity with the minimum in a PE-rich blend. Large phase separation up to several pm (Fig. 3) with poor interfacial adhesion should be the reason for embrittlement of the blends. Debonding in PE-

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