Continuous processing of waste EPDM and PP blends. (Tech Services).

Commercial polyblends consist of at least two polymeric components. They allow the combination of favorable properties of known polymers as well as the development of new quality characteristics (ref. 1). It is, in general, far more cost-effective to develop a new polyblend rather than a new polymer (ref. 2). During the processing of polyblends, the mostly single-phase starting materials form different microstructures inside.

Thermoplastic olefin elastomers (TPOs) are single blends of non-crosslinked elastomers with polyolefin polymers (refs. 3-6). These combine the processing characteristics of thermoplastics with the functional performance of conventional thermoset rubber (ref. 5). Thus, they offer a number of practical advantages over conventional rubbery materials. However, the future of these system is based on dynamic vulcanization which is one of the most interesting processes recently applied for the preparation of new thermoplastic elastomers (refs. 7 and 8). For example, several studies were performed in the last few years (refs. 9-11) on the dynamic vulcanization process with semicrystalline polymer (isotactic polypropylene-iPP) and amorphous rubber as ethylene-propylene-diene terpolymer rubber (EPDM). The term dynamic vulcanization is commonly used to describe the process of vulcanizing the elastomer during its intimate melt mixing with a thermoplastic polymer. The process needs to be carried out under high shear stresses and above the melting point of the thermoplastic.

The barrel and screw configuration as well as the processing conditions such as screw speed and temperature strongly affect the phase morphology and mechanical properties of the blends. These dependencies are experimentally shown by using various screw configurations of the extruder adapted in this work. The particle size of the waste EPDM in a polypropylene (PP) matrix can be substantially reduced in the melting zone of the extruder. If the rubber particles are small enough (a diameter of 1-2 [micro]m or less) and fully cured, the properties of the material improve in relation to the corresponding mechanical blends (TPOs), and the TPV systems exhibit more elastomer-like properties. The final morphology and mechanical properties of the blends, are determined by the design of the geometry loads to the flow and mixing conditions, as well as the screw speeds.

Experimental

Materials

The characteristics of the polymers used in this study are summarized in table 1. The PP was a commercial extrusion grade. The waste EPDM powder was obtained by using a cooling crush method from the vulcanized EPDM compound.

Blending

The waste EPDM/PP ratios were 25/75 and 30/70. The screw speeds were selected as 50, 70, 100 and 150 rpm in a co-rotating twin screw extruder. We decided to optimize mechanical properties of waste EPDM/PP blends manufactured under a screw speed of 100 rpm. We used a co-rotating intermeshing twin screw extruder (D = 19 mm, L/D = 40) and five different screw configurations, which are shown in figure 1 and table 2. Such configurations involve different combinations of right handed and left handed screw and neutral kneading disc elements. The cylinder temperature was maintained at 60, 200, 220, 230, and 235 [degrees] C through the way from the hopper to the die.

[FIGURE 1 OMITTED]

Morphology

The surface analysis was studied by using cryogenically fractured etched samples in p-xylene. The morphology of the sample was investigated by using a scanning electron microscope (SEM) after coating the samples with gold on a Fine Coat in sputter (JEOL JFC-1100E).

Mechanical and rheological properties

The extrudates were palletized and then injection molded. The test specimens pertained to ASTM standards (dumbbell-shaped samples) for tensile test (ASTM D412) and elongation test (ASTM D412). The temperature of the injection molding cylinder was kept at 235 [degrees] C. The mold temperature was 35~40 [degrees] C, and the injection pressure was 2,000~2,400 psi. The mechanical properties, (i.e., tensile strength and elongation at break) were determined by using a tensile testing machine. The crosshead speed was chosen as 500 mm/min, for tensile tests, and a load cell of 10KN capacity was used.

The rheological properties of the samples were studied by using a capillary rheometer with the length and diameter of 0.591 and 0.0276 in., respectively. The L/D ratio of the capillary was 20 and measurements were carried out at 250 [degrees] C.

Results and discussion

The results obtained by compiling the comparison of different screw geometries show different morphologies of the blends. Figure 2 shows the SEM micrographs produced using different screw configurations of the waste EPDM/PP blend. The results indicate that the better morphology of blends is shown in screw configurations B and E because of the kneading disc block and left handed screw element in their screw configurations. The kneading disc or left handed screw has induced the complicated flow, leading ultimately to good dispersion.

[FIGURE 2 OMITTED]

Mechanical properties

The tensile strength, the elongation at break and the 50% modulus with different screw configurations are shown in figure 3. The better mechanical properties are shown in configurations B and E than in the other screw configurations. These results indicate that the mechanical properties are affected by shear stress and residence time during the process.

[FIGURE 3 OMITTED]

Figure 4 shows the mechanical properties obtained by using different screw speeds in the case of screw configuration E. The blends which contain 30 wt % PP at 100 rpm show higher tensile strength, elongation at break and 50% modulus compared to the other samples, i.e., compounded at 50, 70 and 150 rpms. The reason is that there are higher shear rates at 100 rpm than the shear rate at the 50 and 70 rpm, as well as longer residence time compared to the screw speed of 150 rpm. Figure 5 shows the mechanical properties obtained by using different blend ratios in the case of screw configuration E. The blends that contain 30 wt % PP at 100 rpm showed better tensile strength and 50% modulus compared to 25 wt % PP but the elongation at break was decreasing.

[FIGURES 4-5 OMITTED]

Rheology

The effects of the screw configurations and blend ratio upon the rheological behavior of the waste EPDM/PP blends are shown in figure 6. It shows that the shear viscosity strongly depends on the waste EPDM/PP blend ratios rather than on the screw configuration. It is obvious that the thermoplastic elastomer vulcanizate shows shear thinning behavior that follows the power law model over the entire range of the shear rates. These are attributed to the formation of crosslinks between the rubber chains within the rubber particles, which increases their stability toward shear breakdown during mixing, and therefore, imparts the less reduction in the shear viscosity of the blends.

Conclusions

The purpose of this study is to recycle the waste EPDM powder by blending PP. Therefore, the attempt has been made to determine the optimum conditions of extrusion that involve are screw configurations and screw speed. We have investigated the morphology and mechanical properties (tensile strength, elongation at break and 50% modulus) of the waste EPDM/PP blends to find out the optimum conditions. From experimental results, the best conditions are shown in screw configuration E containing two left handed screws and three kneading disc blocks because of the best condition of dynamic vulcanization. In the screw configuration E, the best screw speed was 100 rpm. These results indicate that the properties strongly depend on the dynamic vulcanization during blending. The result produced in this study proves the high potential to produce TPEs by using the waste rubber.

[FIGURE 6 OMITTED]

Table 1 - polymer characteristics

Material       Properties

PP             MFI (melt flow index) = 4.5g/10 min.
               Mw = 200,000
               Density = 0.9
               Melting point = 152 [degrees] C
Waste EPDM     Size = 5~[20.sup.[micro]m]
               Density = 1.057g/[cm.sup.3]
Table 2 - modular screw assemblies used

Designation    Right handed       Left handed       Right handed
               screw element      screw element     kneading disc
               number(length)     number(length)    number(length)

Screw A           40(760mm)              0                 0
Screw B         32.5(617.5mm)         2(38mm)         22(104.5mm)
Screw C          34(655.5mm)             0            22(104.5mm)
Screw D           32(608mm)              0             32(152mm)
Screw E           30(570mm)           2(38mm)          32(152mm)

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