Theoretical modeling of ultrasonic decrosslinking of XHDPE using TSE includes four aspects.
For purpose of the simplicity, in the present study the viscoelasticity of XHDPE and decrosslinked XHDPE was described by the Lodge model :
However, the crystallization of XHDPE and decrosslinked XHDPE reduces the accessible temperature range of dynamic test leading to an insufficient experimental measurement to construct a master curve.
Therefore, only the relaxation processes of XHDPE and decrosslinked XHDPE in their rubbery plateau and transition region to the glassy state are of interest.
Swelling of XHDPE and decrosslinked XHDPE was performed by following ASTM D 2765, test method C.
A capillary rheometer (Rosand RH7, Malvern Instruments, UK) was used to measure the steady state shear viscosity of decrosslinked XHDPE at a temperature of 200[degrees]C using dies of a diameter of 1 mm and L/D ratio of 8, 16, and 24, respectively.
A stress-controlled Discover Hybrid Rheometer (DHR-2, TA Instruments, New Castle, DE) equipped with 25 mm parallel plates to perform the small amplitude oscillatory shear (SAOS) test of decrosslinked XHDPE at temperatures of 140[degrees]C, 150[degrees]C, and 160[degrees]C.
Figure 2 shows the gel fraction (a) and crosslink density (b) of the decrosslinked XHDPE obtained by ultrasonic TSE as a function of the ultrasonic amplitude at flow rates of 3.75, 6.5, and 8.25 g/min.
Ultrasonic decrosslinking of the XHDPE was carried out by using SSE.
The ultrasonic decrosslinking of XHDPE by means of SSE was performed at flow rates of 4.5, 7.5, and 15.1 g/min and ultrasonic amplitudes of 5, 7.5, and 10 /un.
The sheet of virgin HDPE and XHDPE and decrosslinked XHDPE were prepared by compression molding in a mold of dimensions of 34 X 16 X 0.185 [cm.sup.3]; 80 g of the virgin HDPE powder was molded at 150[degrees]C for 20 min; 80 g of the mixture containing peroxide was crosslinked by the same method as used for preparation of the XHDPE slabs; 100 g of the decrosslinked XHDPE pellets was molded at 200[degrees]C for 25 min.
Swelling tests of the XHDPE and decrosslinked XHDPE were performed by following ASTM D 2765, test method C.