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19 275 270 265 260 255 OITemp (C) 250 245 y = 11.836Ln(x) + 207.94 2 R = 0.871 240 235 230 225 0 20 40 60 80 100 120 140 160 180 OIT (minutes) Figure 14. Comparison between the OITemp and the OIT. (PVC). There is also a new plastic called poly(lactic acid) (PLA), The Effects of Particulates which is derived from corn and is biodegradable. The recycled Angular "playground" sand was obtained and sifted with HDPE may be contaminated with other grades of PE, like LDPE, which could affect the cracking resistance. Finally, there both a 100 mesh and a 120 mesh screen. The sand that passed may be milk and detergent residue, which produce smoke and the 100 mesh, and retained by the 120 mesh was collected. The odors during processing. sand particles ranged in size from 0.0052 to 0.0061 in. From The foreign particulate matter can be removed during the this material, a master batch containing 2.5% sand in Virgin washing step at the recycler and filtered out during extrusion Resin 1 (VR1) was prepared. The 2.5% master batch was diluted by either the recycler or pipe manufacturer. However, the re- with more VR1 to produce samples of about 1.6%, 0.8%, 0.4%, cyclerswhoproducereprocessed HDPE are more capable of this and 0.2% sand. The actual values measured by ash analysis because some of them already melt filter to 100120 mesh on duplicate samples were 1.61%, 0.79%, 0.41%, and 0.25% while the pipe manufacturers generally filter at a mesh size of respectively. 80 or below. Compression-molded plaques were prepared and the break- PP comes from bottle closures (caps) and is found in mixed- ing strain and the BAM stress-crack test were performed on color reprocessed HDPE at levels as high as 20%. When pro- the samples. The BAM test is a stress-crack test without a notch cessed, the PP will blend in with sufficient mixing, or be melted and is described in Appendix A, Section A.3.2. The results are and spread out in the final part. This could affect the behavior shown in Figures 15 and 16. of the pipe, especially the stress-crack resistance. The HDPE Notice that this size of sand particle has an extremely strong milk bottle resin contains no PP because the caps are made effect on the break strain, even at a level of 0.25%. The percent- from LDPE. In fact, the recycled natural homopolymer (milk age break strain went from 468% to 148%, or to 32% of the bottle) is much cleaner than the mixed color. original value. This demonstrates that it's important to keep the The effects of contamination have been investigated three particle size small and the amounts as low as possible. ways. First, samples containing a known size and percentage The effect is similar for the BAM stress-crack test results. of angular sand were prepared to see how the sand affected Sand in the amount of 0.25% reduced the stress-crack resis- properties such as tensile elongation and stress-crack resis- tance to 62% of the original value. tance. These results will offer some guidance concerning how much filtration to specify. The Effects of Melt Filtration Secondly, two samples containing MCRG were prepared at three levels of filtration. Two types of samples were prepared for this study. The And, finally, samples containing different amounts of PP first was 100% MCRG and the second was 50% MCRG + 50% were prepared and evaluated. Virgin MDPE. Each of these was melt filtered at three different

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20 500 400 Break Strain (%) 300 200 100 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 % 100-120 Sand Figure 15. The effect of sand on the break strain. levels. It should also be mentioned that the two sample sets Both sets of results indicate that melt filtration dramati- were made with two different batches of MCRG HDPE. cally improves the properties of recycled HDPE and blends The 100% MCRG was blended without filtration and also containing HDPE. Notice in Figure 18 that the scatter in results melt filtered at 100 and 150 mesh. gets smaller with better melt filtration. This is important data The 50/50 MCRG/MDPE samples were melt filtered at 40, because tensile specimens always fail at a defect. The plotted 100, and 150 mesh. The filtration at 40 mesh was to help the results clearly show that melt filtration removes defects. The material process better by removing larger particles that would coefficients of variation (COV) (standard deviation/mean cause the melt strand to break. 100) for the three results were 69%, 30%, and 10% with increas- The tensile properties were measured on the two sample ing mesh size. A 10% COV is acceptable, even for virgin resins. types. The percentage strain-at-break was chosen as the best This is a direct result of fewer larger particles in the samples. result to follow the effects of blending. Plots of the effects of The BAM stress-crack test is usually performed at 80C in melt filtration on both types of samples are shown in Fig- a surfactant (Igepal CA720) on a specimen without a notch. ures 17 and 18. Five replicates were tested for the 100% MCRG It is basically a flaw detector. The results were similar to the and 10 were tested for the 50/50 MCRG/MDPE. tensile results. Longer failure times were associated with higher 50 40 BAM Failure Time (hrs) 30 20 10 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 % 100-120 Mesh Sand Figure 16. The effect of sand on the BAM stress-crack resistance.