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28 700 700 600 600 VR1 + MCR y = -3.425x + 551.5 500 Theoretical 500 R2 = 0.6076 Break Strain (%) Break Strain (%) y = -4.155x + 478 400 400 300 300 200 200 y = -3.14x + 478 100 y = -1.585x + 246.5 100 VR1 + MCRG R2 = 0.7001 Actual 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 % Recycled % Recycled 700 700 600 y = -5.845x + 647 600 VR2 + MCR VR3 + MCR 500 500 Break Strain (%) Break Strain (%) y = -5.765x + 639 400 400 300 300 200 200 y = -3.655x + 456.5 y = -4.305x + 490 R2 = 0.9852 100 R2 = 0.6047 100 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 % Recycled % Recycled Figure 29. The effect of recycled content on the break strain of PCR-MCR blends. The results of the blending and testing with mixed-color 6. The stress-crack resistance can be dramatically improved PCR HDPE have produced the following findings: by the addition of MDPE to the mixed-color PCR. A 50:50 blend would produce a resin with about 200 hours in the 1. The maximum amount of mixed-color PCR that can be 15% NCTL test. The yield stress would be reduced to about blended with one of the pipe resins and meet 24 hours 3,250 psi, so this must be kept in balance. of stress-crack resistance is about 20%. And, since the 15% 7. The PIR-MD evaluated also improved the resistance to NCTL is less aggressive that the NCLS test, a conservative cracking, but not as much as the virgin MD. number is closer to 15%. 2. At 15% added MCR, all the AASHTO requirements of Blends Made with Natural PCR pipe would be met. 3. The two different batches of mixed-color PCR (MCR1, There were 27 blends made with natural PCR HDPE. They MCRG) behaved dramatically different. The latter produced included much better correlation to theory and had a much higher break strain, showing that there were fewer contaminants VR2 + NAT @ 20, 40, 60, and 80%, in the sample. NAT + LLDPE @ 20, 40, 60, and 80%, 4. The difference between the predicted and actual values of NAT + MDPE @ 20, 40, 60, and 80%, percentage strain-at-break might be used to evaluate the VR1 + N10LL @ 20, 40, 60, and 80%, level of contamination in the recycled material. VR2 + N10LL @ 20, 40, 60, and 80%, 5. Much greater stress-crack resistance may be required to VR1 + N35LL @ 20, 40, 60, and 80%, offset the deleterious effects of contamination. 50% NAT + 50% MDPE,

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29 60 60 VR1 + MCRG 50 50 VR1 + MCR NCTL Failure Time (hrs) NCTL Failure Time (hrs) 40 Theoretical 40 30 30 y = 45.8e-0.018x 20 20 y = 45.8e-0.018x y = 46.94e-0.0202x 10 y = 37.231e-0.0219x 10 R2 = 0.9794 R2 = 0.9941 Actual 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 % Recycled % Recycled 60 60 50 VR2 + MCR 50 NCTL Failure Time (hrs) NCTL Failure Time (hrs) VR3 + MCR 40 40 y = 55.392e-0.0211x y = 36.2e-0.0156x 30 R2 = 0.9647 30 20 y = 38.5e-0.0162x 20 10 10 y = 28.342e-0.0188x R2 = 0.9541 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 % Recycled % Recycled Figure 30. The effect of recycled content on the 15% NCTL on PCR-MCR blends. 65% NAT + 35% PIR-LD, and ingly, the series of NAT with MDPE also has high scatter, but 50% VR3 + 25% MDPE + 25% NAT. the averages happen to fall in line so it doesn't present as dra- matically as the NAT + LL results. The COVs for 20% and The effect of recycled content on the yield strength for four 40% MDPE were 36% and 28%. blends of virgin pipe resins with natural PCR is shown in These results show a good correlation between the actual Figure 31. and theoretical. This may be due to the fact that the applied The N10LL and N35LL are natural PCR with added LLDPE loads are based on the yield strengths of the samples. Differ- at 10% and 35% by weight. The correlation coefficients for the ences from plaque to plaque, which may contribute to the scat- lines and the agreement between the actual and theoretical ter in the yield stress, are minimized by the applied load being are poor. 15% of the yield stress. Regardless, these results show a definite These results are similar to those seen for the mixed-color exponential relationship between the stress-crack resistance resin. Once again the correlation between the actual results and and the recycled content. the theoretical ones are relatively poor. Appendix C, Section C.10 contains summary tables for the Similar plots for the breaking strain and the 15% NCTL blend series, plots of properties versus percentage recycled, stress-crack resistance are shown in Figures 32 and 33. and individual property reports for the 27 blends. The poor correlation in the break strain for the NAT + The results of the blending and testing with natural PCR- LLDPE series is easily explained by contaminants. The NAT HDPE have produced the following findings: itself has a COV of 38%, which is carried over into the blends with low amounts of LLDPE. The COVs for 5%, 10%, and 1. Only about 10% of natural PCR-HDPE can be added to 20% LLDPE were 33%, 38%, and 25% respectively. Interest- virgin pipe resins and meet a 15% NCTL time of 24 h.