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The strength of specimens made from 100% virgin material was found to have an average of 45.2 MPa with a standard deviation of 1.68 MPa. The 50/50 blend yielded an average UTS of 37.1MPa with a standard deviation of 2.56 MPa, and the 75/25 blend was slightly stronger with an average UTS of 41.2 MPa and a standard deviation of 1.45 MPa. As expected, the corresponding strength of specimens made from 100% recycled material was much weaker, with an average of 21.1 MPa and a standard deviation of 2.14 MPa. Thus, products made from the virgin material were both twice as strong and more consistent than those made from the recycled material. This dramatic weakness of recycled products, which can be attributed to the lowered material molecular weight and associated lessened degree of molecular entanglement within products, is a major reason for the severely limited usage of recycled polymers to date.
Fig. 1 Tensile strength distributions of conventionally molded
products made from blends of new and recycled material
Fig. 2 Cavity pressure profile of control sample: (a)P1 and P2 of 100/0,(b)P1 and P2 of 75/25,(c)P1 and P2 of 50/50,and (d) pressure difference between P1 and P2 of 100/0,75/25 snd 50/50
Figures 2~a!–2~c! contain typical measured cavity pressure profiles for the conventional molding of 100/0, 75/25, and 50/50 recycled PS blends. The pressure data shown was recorded at the pressure sensor locations P1 and P2 in the mold cavity. P1 is located in the gate end grip section of the specimen, whereas P2 is in the grip section opposite the gate. It is clearly seen that the pressure decreases with time during the molding cycle. This is due to the change in viscosity and the volumetric shrinkage. In conventional molding processes, the polymer shrinks and its viscosity increases toward the end of the cycle because the polymer temperature decreases. Figure 2~d! presents the temporal pressure difference between P1 and P2 for these three blends. For the 100/0,the pressure difference increases with time during the cycle. This is due to the packing pressure, which gradually packs more material into the cavity in order to compensate for the subsequent shrinkage. However, the processing at the 75/25 and 50/50 blends did not show this type of behavior. This may be due to the low molecular weight material that is contained in these blends. Because of the low molecular weight, the solidification behavior is likely to be accelerated or changed, and thus localized solidification may have been achieved prior to the application of extra packing pressure.
In comparison, Figs. 3 (a) and 3 (b) present sample pressure profiles at the pressure sensor locations P1 and P2 during VAIM processing witha2 Hz vibration frequency. The pressure profile during the VAIM process is quite different from that for conventional molding as the pressures fluctuates corresponding to the manipulation of the melt in the cavity in a compression and/or decompression manner. Furthermore, the pressure at P1 fluctuates with a larger amplitude than that at P2. Once again, P1 is closer to the gate than P2. This explains that the localized effects of the vibration may be a function of the distance between the vibration source and the particular local region of the melt. Figure 3(c) represents the pressure difference between P1 and P2. Note how the amplitude reverses sign (i.e., P2 . P1), resulting in an oscillating flow that is not seen during conventional molding processes.