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16 Table 2. Types of PE. Common Name Type (ASTM D1248) Density (g/cm3) LDPE (homopolymer) 0 <0.910 LLDPE (copolymer) 1 0.9100.925 LMDPE (copolymer) 2 >0.9250.940 LHDPE (copolymer) 3 >0.9400.960 HDPE (homopolymer) 4 >0.960 Note: LDDPE = linear low-density polyethylene, LMDPE = linear medium-density polyethylene, LHDPE = linear high-density polyethylene. percentage of carbon black increases the density 0.0044 g/cm3. recycling code. The resin used in colored bottles is an HDPE This also produced a calculated value of 0.937 g/cm3 for MDPE. co-polymer which is not as strong as the homopolymer but has better cracking resistance. Both the natural and colored HDPE resins are available as Post-Consumer Recycled High-Density regrind (chips) or reprocessed (pellets). The reprocessed pel- Polyethylene lets have the advantage of being melt-filtered during the pel- This is a smaller category, in terms of different resins, that letizing process. This is essentially one additional purification includes primarily recycled bottles and high-strength shop- step. The colored-bottle recycled resin contains a significant ping or trash bags. The volume of good recycled bags is low amount of natural bottle in it. There is an increasing number because the bulk density (volume/weight ratio) is low and the of recyclers who are separating the natural bottles from the high quality bags are usually contaminated with lower quality mixed-color bottles because the natural resin has significantly LDPE bags. Additionally, the volume of plastic bags is shrink- higher value on the recycled-resin market. The difference can ing because their use is being legislated against in a growing be $0.10 per pound, or more. number of communities throughout the United States. Most of the samples found during this study came through The bottles can either be natural or colored. The natural contact with the Association of Post-Consumer Plastic Re- bottles are most often the 1/2 to 1 gallon water, juice, or milk cyclers (APR). jugs. They are made from HDPE homopolymer, which is a Twenty-two samples of PCR HDPE were received from six resin with high strength and poor stress-crack resistance. member companies of the APR and two nonmembers. The The colored bottles include liquid detergent, cleaners, sham- samples included natural regrind (2), natural reprocessed (4), poos, fabric softeners, and others marked with a number "2" mixed-color regrind (MCRG) (5), MCR (8), and three special Table 3. PIR resin properties. Resin Type HDPE MDPE LDPE Property Reprocessed Regrind Reprocessed Density (g/cm3) 0.970 0.942 0.952 Calculated Density1 0.943 0.937 0.925 (g/cm3) Melt Index (g/10 min) 0.32 0.66 0.80 MFR (21.6/2.16kg) 101 46 34 % Volatiles 0.24 -- 0.18 % Color 0.28 1.05 0.15 % Ash 3.69 0.05 3.75 Yield Stress (psi) 3143 2631 1686 Break Strain (%) 628 662 727 15% NCTL (h) 104 >800 >300 OIT (min) 18.2 61.4 6.3 1 Calculated from the yield stress. Note: MFR = melt flow rate; "--" = no data available.