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Studies on high density polyethylene/polycarbonate blend system compatibilized with low density polyethylene grafted diallyl bisphenol A ether.

INTRODUCTION

The study of polyolefin-based engineering plastics is currently an important research field of polymeric materials and science (1). Polyolefin resins mainly polyethylene and polypropylene, constitute more than 35% of the global resin and plastic production of [10.sup.9] tons per year. Because of the poor heat resistance and low mechanical properties, polyolefins are always used as general plastics. Therefore it is useful research to modify polyolefin and raise its heat resistance and mechanical properties to the range of engineering plastics. Blending polyolefin with general engineering plastics is a main route for achieving this goal (2). Polycarbonate exhibits excellent mechanical properties and dimensional stability, which makes it suitable to blend with polyolefin (3). In several recent papers (4-7), it was found that for injection molded polypropylene (PP) or polyethylene matrix/polycarbonate blends, the stiffness increased with PC concentration despite the absence of interfacial adhesion. This effect was attributed to the higher thermal expansion coefficient of PP and PE that shrank around the dispersed PC phase. However, the fact that the viscoelastic properties, structures and morphology were very different between PE (or PP), and PC rendered PE (or PP)/PC blends incompatible. Because of this, much work should focus on the developing compatibilization of the blends to enhance the interface interaction and improve the heat resistance and mechanical properties of the blends.

In this paper, a new compatibilizer, LDPE grafted diallyl bisphenol A ether composed in our laboratory, was employed for HDPE/PC blending system to improve phase dispersion and strengthen the interfacial adhesion. The effects of the addition of LDPE-g-DBAE on mechanical properties, heat resistance, crystallization, and rheological behavior are discussed.

EXPERIMENTAL

Materials

The polymers used in this work were the commercial products of Teijin Chemical Ltd., Japan. Polycarbonate (PC K1300, number average molecular weight approximating 2.8 [approximately] 3.5 x [10.sup.4]). HDPE (5000S, melting flow index was about 0.98 g/10 min) was made by Yangzi Petroleum Chemical Co., China. LDPE grafted diallyl bisphenol A ether (LDPE-g-DBAE) used in this work was prepared in our laboratory, having been reported elsewhere (8).

Specimens Preparation and Testing

The materials containing 0 [approximately] 35% of PC in weight were prepared on a co-rotating twin screw extruder (L/D = 30); the processing conditions were: temperature ([degrees] C): 190, 250, 260, 260, 230 from hopper to die respectively; screw rotation (r/m): 30. The blends were extruded through rod die and subsequently pelletized and injection molded into test specimens on a reciprocating screw injection molding machine. The injection molding conditions were: temperature for each zone: 220/250/260/240; injection pressure 75 Mpa, holding pressure: 55 Mpa; injection time 3 s, holding time: 12 s, cooling time 40 s. The mold temperature was kept at 60 [degrees] C constantly throughout injection processing, and the hopper was heated to 100 [degrees] C by convection heating air to prevent the material from absorbing moisture during injection. The materials were dried at 120 [degrees] C for 12 hours and at 80 [degrees] C for 10 hours in a vacuum oven prior to extrusion and injection, respectively. Dogbone-shaped specimens were tensile tested on an Instron Universal Testing Instrument, model AG-10T; the crosshead speed was 50 mm/min. The testing temperature was [approximately] 23 [degrees] C. Rectangular plaque specimens were used for impact testing according to the Izod Impact test (GB1843-80) standard. For every test, five specimens of each composition were tested and the average value reported. The heat distortion temperature was conducted according to the regulation mentioned in GB1634-79.

For morphological characterization, scanning electron microscopy (SEM) photographs were taken on the fracture surface from the impact test, and the fracture surface was made conductive by the deposition of a layer of gold and palladium in a vacuum chamber. The investigation was done on a Hitachi SEM.

Thermal properties and crystalline behavior were determined using a differential scanning calorimeter (DSC, Perkin Elmer-7) in [N.sub.2]. Samples, 5 [approximately] 10 mg, were heated to 200 [degrees] C at 10 [degrees] C/min scanning rate and held at that temperature for 5 min, followed by cooling to 30 [degrees] C at the same scanning rate. The melting point and the melting enthalpy of HDPE in the blends were recorded. The crystallinity of HDPE in the blends could be calculated by means of the following equation:

[X.sub.c] = ([Delta]H/[Delta][H.sub.o]) x 100% (1)

where [Delta][H.sub.o] = 68.4 x 4.18 J/g (9) was the enthalpy of HDPE of 100% crystallization. [Delta]H was the enthalpy of HDPE in the blends.

Rheological measurements were made on an auxiliary capillary viscometer of the AG-10T (Instron Tensile Testing instrument). The diameter of the capillary was 1 mm and the aspect ratio (L/D) was 20.

RESULTS AND DISCUSSION

For the PC matrix/PE blend system, previous authors (3, 10) have reported developing a compatible effect between PC and PE phases using a grafted or block copolymer. Endo et al. used a triblock copolymer containing styrene, ehtylene, and butadiene units (SEEKS) to compatibilize the PC/PE blending system. Mekhilef et al. studied the effect of a styrene butadiene copolymer on the miscibility of the PC/PE blends. They found that these kinds of copolymers could control the morphology of the blends and prevent phase growth in PC/PE systems by reducing the interfacial tension. However, there is little literature dealing with the compatibilizing of the HDPE matrix/PC blending system.

In our research work, a new compatibilizer, LDPE grafted diallyl bisphenol A ether, was made to improve the interfacial behavior of the blend where the HDPE as a matrix blended with the dispersed phase, PC. It is possible that the dially bisphenol A ether unit might entangle with the molecular segment of polycarbonate and the LDPE unit of the gafted copolymer might have a strong interaction with HDPE because of the similarity of the molecular structures of the different components.

Morphology

Figure 1a and b are micrographs of the HDPE/PC (63/35) blends mixed with 0% and 10% compatibilizer, respectively. The blend containing 35% polycarbonate without compatibilizer [ILLUSTRATION FOR FIGURE 1A OMITTED] showed a sperhical dispersion of the PC phase in the HDPE matrix. No interaction between HDPE and PC was seen. A smooth interrace in the blends was observed, which resulted from the fact that there was almost no interfacial adhesive between PC spheres and HDPE matrix, and the majority of PC spheres were pulled out and the voids remained. However, the addition of 10 wt% LDPE-g-DBAE made the morphology undergo a considerable change in the dimension of the dispersion phase and the interfacial behavior. More fine PC particles and a dim phase are observed in Fig. 1b. All the changes should be attributed to the compatibilization imparted by LDPE-g-DABE in the HDPE/PC binary blend.

Heat Resistance and Maim Properties

Figure 2 shows the heat distortion temperature (HDT) of the blends as a function of PC concentration. It was found that the HDT of the blends increased with PC content in the blends, and the HDT of the blends containing 10 wt% of the compatibilizer LDPE-g-DBAE was always higher than that of the noncompatibilized blends.

It can be further concluded from Fig. 2 that whether the blends contained the compatibilizer or not, the HDTs of the blends were between those of the neat HDPE and neat PC and higher than the HDT predicted by the additivity rule over the entire composition range. This was a very interesting result. Favis et al. (5) explained a similar phenomenon from tensile modulus testing of the PP/PC blends. The effect might result from the thermal history of the blends. The matrix would shrink more than the dispersed phase did since the coefficient of thermal expansion of the HDPE was higher than that of the PC. As it cooled from processing temperature to room temperature, the matrix tightly embedded the dispersed phase PC and caused macromolecular chains to orient partially around the PC particles. This effect was of much benefit in increasing the stiffness and heat resistance of the blends.

Figures 3 and 4 show the results obtained from tensile and impact tests, respectively. It was found that the tensile strength of the blends with or without the compatibilizer increased substantially with an increase of PC content over the entire composition range. Furthermore, the tensile strength of the compatibilized blends was always higher than that of the noncompatibilized ones for the same blend [ILLUSTRATION FOR FIGURE 3 OMITTED]. As shown in Fig. 4, when the blends contained 10 wt% of PC, there was an abrupt change in notched impact strength, and when PC content exceeded 10 wt%, the impact strength decreased slightly. This result was amazing; it could be explained by the toughening mechanism of rigid organic filler (ROF) (11). When the PC phase, as rigid particles was dispersed in the ductile matrix HDPE, owing to their different thermal expansion coefficients, PC droplets were embedded tightly in HDPE. When external force was applied to the specimens, a large amount of energy absorption occurred, which resulted from the large elongation (cold drawing) caused by tensile stress under the influence of the compressive pressure acting on the dispersed PC particles and HDPE matrix. When the PC content was above 10 wt%, it was difficult for the PC to disperse in the HDPE matrix. Therefore, the decrease in impact strength resulting from too-large PC particles in the blends balanced or even exceeded the increase produced by cold drawing, which resulted in reducing the impact strength of the blends at high PC concentrations.

Crystallizing Behavior

The data listed in Table 1 show a decrease in both melting point and crystallinity. of HDPE in the blends and a further decrease in the presence of the compatibilizer, which indicates that HDPE/PC blends with LDPE-g-DBAE as a compatibilizer are at least partially compatible.

[TABULAR DATA FOR TABLE 1 OMITTED]

Apparent viscosity-shear rate curves are presented in Figs. 5, 6 and 7. The apparent viscosity of the blends [ILLUSTRATION FOR FIGURE 5 OMITTED] was constantly lower than that of the neat HDPE and PC over the range of shear rates, indicating that the PC and HDPE phases were immiscible with each other in the blend system. Since the viscosity of HDPE with flexible chains was more sensitive to the shear rate (or shear stress) than PC with rigid chains, it was reasonable that blends containing 90 wt% HDPE decreased significantly in apparent viscosity with shear rate increasing, as compared with the blends containing 65 wt% HDPE.

It was found from Figs. 6 and 7 that the viscosity of the blends with LDPE-g-DBAE was always higher than that of the blends without LDPE-g-DBAE at the same PC concentration. This result should obviously be attributed to the efficient compatible effect of LDPE-g-DBAE, which developed the interaction between HDPE and PC interfaces.

CONCLUSION

A blend of HDPE and PC exhibited a phase growth and no adhesion between the phases. The efficient compatible effect of LDPE-g-DBAE on the HDPE/PC blends was confirmed through the investigation of the morphology, heat resistance, mechanical properties, crystallizing behavior, and rheological measurement. The effect of the compatibilizer resulted from the interaction between the diallyl bisphenol A ether unit of LDPE-g-DBAE and PC, and the miscibility of the LDPE unit and HDPE.

REFERENCES

1. Xu Xi, Advances in Polymer Science and Industry, Chengdu, China (1994).

2. Xu Xi, Preprints of '93 National Symposium on Special Engineering Plastics Applications and Technology, 4, Chengdu, China (1993).

3. N. Mekhilef, A. Ait-Kadi, and A. Ajji, Polym. Eng. Sci., 32, 894 (1992).

4. B. Fisa. B. D. Favis, and S. Bourgeois, Polym. Eng. Sci., 80, 1051 (1990).

5. B. D. Favis and D. Therrien, Polymer, 32, 1474 (1991).

6. J. M. Wills and B. D. Favis, Polym. Eng. Sci., 28, 1417 (1998).

7. L. Leclair and B. D. Favis, Ninth Annual Meeting of the Polym. Proc. Soc., p. 301, England (April 1993).

8. Z. Li, M. Yang and J. Feng, Polymeric Materials Science and Engineering, 3, 45 (1996).

9. B. Wunderlich, Macromolecular Physics, Vol. 3, Academic Press, New York (1980).

10. S. Endo, K. Min, J. L. White, and T. Kyu, Polym. Eng. Sci., 26, 45 (1986).

11. T. Kurauchi and T. Ohta, J. Mater. Sci., 19, 1699 (1984).
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Author:Yang, Mingbo; Li, Zhongming; Feng, Jianming
Publication:Polymer Engineering and Science
Date:Jun 1, 1998
Words:2047
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