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26 Table 6. Component resins for blend preparation. Resin Abbreviation Description Virgin Resin 1 VR1 PPI-certified AASHTO HDPE pipe resin. Virgin Resin 2 VR2 PPI-certified AASHTO HDPE pipe resin. Virgin Resin 3 VR3 PPI-certified AASHTO HDPE pipe resin. Virgin LLDPE LLDPE Commercial linear low-density polyethylene resin from a supplier that makes AASHTO pipe resin. Virgin LMDPE LMDPE Commercial linear medium-density polyethylene resin from a supplier that makes AASHTO pipe resin. Mixed-Color PCR 1 MCR1 Mixed-color post-consumer reprocessed HDPE pellets composed of colored and natural bottles. Mixed-Color PCR 2 MCRG Mixed-color post-consumer regrind HDPE chips composed of colored and natural bottle. Natural PCR NAT Post-consumer reprocessed HDPE pellets made from milk, juice, and water bottles. Natural PCR + 10% LLDPE N10LL Blend of NAT with 10% LLDPE to enhance the properties of the NAT. Natural PCR + 35% LLDPE N35LL Blend of NAT with 35% LLDPE to enhance the properties of the NAT. PIR Low Density PIR-LD Post-industrial low-density polyethylene reprocessed pellets believed to contain mostly film and bags. PIR Medium Density PIR-MD Post-industrial linear medium-density polyethylene regrind chips from the sheet market. PIR High Density PIR-HD Blend of PCR high-density bottles with PIR polyethylene. what those limits were and to also enhance the properties of Blends Made with Mixed-Color PCR recycled HDPE by blending it with nonpipe virgin resins such as LLDPE and LMDPE. A secondary but important objective A total of 29 blends were prepared with the use of Mixed- was to determine the relationships between the percentage Color PCR bottle resin. They included component in a blend and the resulting blend's properties. Resins used during the blending study are given in Table 6. VR1 + MCR1 @ 20, 40, 60, and 80%, A few selected properties of these resins are shown in VR1 + MCRG @ 20, 40, 60, and 80%, Table 7. Notice the wide variability in the yield stress and VR2 + MCR1 @ 20, 40, 60, and 80%, NCTL values. VR3 + MCR1 @ 20, 40, 60, and 80%, Table 7. Properties of component resins for blending. Resin Density Yield Stress Break Strain 15% NCTL (g/cm3) (psi) (%) (h) VR1 0.950 3,688 58 478 45 45.8 2.5 VR2 0.953 3,924 58 639 98 38.5 3 VR3 0.949 3,764 52 647 83 36.2 2.5 LLDPE 0.919 1,616 19 771 94 >1,000 LMDPE 0.934 2,732 24 645 49 >1,000 PCR-MCR1 0.960 3,620 86 62.5 27 7.6 1 PCR-MCRG 0.960 3,527 58 164 30 7.6 1 PCR-NAT 0.960 4,525 42 302 115 2.3 0 PCR-N10LL 0.957 4,037 60 411 191 3.0 0 PCR-N35LL -- 3,203 54 655 32 19.5 3 PIR-LD 0.952 1,686 42 727 14 >300 PIR-MD 0.942 2,662 27 692 43 >300 PIR-HD 0.968 3,157 32 684 40 97.5 9 Note: "--" = data not available.

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27 MCRG + MDPE @ 25, 50, and 75%, line, and the scatter in the results is quite high. This is a reflec- MCR1 + MDPE @ 25, 50, and 75%, tion of the contaminants found in PCR resins and demon- MCRG + PIR-MD @ 25, 50, and 75%, strates the need to control contamination. 75% MCR1 + 25% PIR-HD, and Plots of the NCTL stress-crack resistance determined at 50% VR3 + 25% MDPE + 25% MCR1. 15% of the yield strength are shown in Figure 30. In this case, the curves are obviously exponential in nature and the match The effects of recycled content on the yield strength for between theoretical and actual is much better. four blends of virgin pipe resins with mixed-colored PCR are Appendix C, Section C.9 contains summary tables for all shown in Figure 28. the blends made with PCR-MCR, plots of properties versus It is fairly clear from these graphs that the yield strength is a percentage recycled content, and individual property reports linear function with respect to recycled content. That means that for the 29 blends. Examination of the results reveals that all a simple mixing equation can be used to approximate the yield the properties change in either a linear or an exponential man- strength of a blend. This information will allow one to blend dif- ner. More specifically, all the property changes are linear except ferent resins to make sure that the resulting blend always stays for the melt flow (both loads) and the stress-crack resistance. within the specified yield strength requirements. The correlation This is powerful information because the properties of blends is not particularly good, but this is likely caused by the combi- can be predicted based on these relationships. However, some nation of the two blend components not being too far apart in of the inherent scatter found in certain properties makes such strength and the higher scatter found with recycled materials. predictions unreliable. It is believed, though, that the rela- Similar plots are shown for the breaking strain in Figure 29. tionships can be used as a guide for preparing blends with the Notice that these are even farther away from the theoretical understanding that actual blend testing will still be required. 4000 4000 Actual y = -3.62x + 3934.5 y = -0.65x + 3779 R2 = 0.8439 R2 = 0.412 3800 3800 Yield Strength (psi) Yield Strength (psi) 3600 Theoretical 3600 y = -1.44x + 3764 y = -0.68x + 3688 3400 3400 3200 3200 VR1 + MCR VR3 + MCR 3000 3000 0 20 40 60 80 100 120 0 20 40 60 80 100 120 % Recycled % Recycled 4000 4000 3800 3800 y = -3.04x + 3924 Yield Strength (psi) Yield Strength (psi) y = -1.61x + 3688 3600 3600 y = -3.945x + 3886 3400 3400 R2 = 0.9563 y = -0.665x + 3564 R2 = 0.2812 3200 3200 VR1 + MCRG VR2 + MCR 3000 3000 0 20 40 60 80 100 120 0 20 40 60 80 100 120 % Recycled % Recycled Figure 28. The effect of recycled content on the yield strength of PCR-MCR blends.