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24 from, but it is known that the rubber bladder found in the caps cult in the presence of colorants, particularly carbon black. of some wide-mouth bottles is made from silicone rubber. These absorb IR radiation and can affect the peak heights and Rubber has been seen in ketchup, mustard, jelly, and dessert shift the linear portion of a calibration curve. topping bottles. Unfortunately, the rubber particles are so The other measure is with the use of DSC. A DSC measures flexible that they pass through the filter screens. However, it thermodynamic transitions, like melting or decomposition. is believed that the particles can be broken up by the screens, This is the method of choice during this study and details of the so better filtration should produce smaller particles. If this method can be found in Appendix, Section A.2.6. is combined with improvements in stress-crack resistance, Samples of VR1 with 2%, 5%, and 10% PP were pre- then it is believed that the effect of this contaminant can be pared and tested. The properties evaluated were density, melt minimized. index (MI), break strain, and two different stress-crack tests (15% NCTL and NCLS). The results are shown in Figures 24 through 27. The Effect of Polypropylene The density and MI change in a predictable way because this PP is a contaminant in post-consumer MCRG and re- PP has a lower density and a higher MI than VR1. processed resins that comes from the colored-bottle closures. The break strain values reflect the lack of miscibility between The recyclers report PP at levels up to 20% by weight. There- HDPE and PP during extrusion. fore, it is important to know its effect on the properties of The most interesting results were from the stress-crack tests HDPE. (Figure 27). Notice that the stress crack resistance actually There are two obvious ways to measure percentage PP. The increased between 2% and 5%, all three times the test was run. first is with the use of FTIR spectroscopy. FTIR is an analyti- The effect is not so great in the NCLS tests as in the 15% NCTL cal technique that takes advantage of the fact that different test. The NCLS results are effectively normalized by all the combinations of atoms absorb IR radiation at different fre- samples being placed under the same applied load. It is clear quencies. The technique produces a chemical fingerprint of though, that the stress-crack resistance begins to be compro- absorption bands of different intensities and at different fre- mised around 5% PP. quencies. This can be used as a quantitative tool because the height of a particular band is directly related to its concentra- Phase 2--Recycled-Resin Blends tion, assuming the specimen thickness (path length) is a con- stant. With a blend of PE and PP, one can ratio two peaks, The results from Phase I of the project showed that recycled each specific to one of the polymers. The ratio of these peaks HDPE had properties that were below the established limits will be linearly related to the relative concentrations, up to a of AASHTO-approved pipe. Therefore, the percentage of re- certain limit that can be determined experimentally. This tech- cycled material that can be blended with pipe resin will be lim- nique works well for natural resins but becomes more diffi- ited by these properties. Efforts were undertaken to determine 0.95 0.949 0.948 y = -0.0005x + 0.9489 Density (g/cc) R2 = 0.9875 0.947 0.946 0.945 0.944 0.943 0 2 4 6 8 10 12 % Polypropylene Figure 24. The effect of percentage PP on density.

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0.4 0.35 0.3 Melt Index (g/10 min) 0.25 0.2 y = 0.0089x + 0.2695 0.15 2 R = 0.9804 0.1 0.05 0 0 2 4 6 8 10 12 % Polypropylene Figure 25. The effect of percentage PP on MI. 800 700 600 Break Strain (%) 500 400 300 y = -23.388x + 628.65 200 2 R = 0.8882 100 0 0 2 4 6 8 10 12 % Polypropylene Figure 26. The effect of percentage PP on break strain. 80 15% NCTL 70 Failure Time (Hrs) 60 50 NCLS - 1 40 NCLS - 2 30 0 2 4 6 8 10 12 % Polypropylene Figure 27. The effect of percentage PP on the stress-crack resistance.