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15
CHAPTER 3
Findings
Phase 1--Recycled There are certainly resins that would be appropriate for
Polyethylene Resins incorporation into pipe available from the PIR market. How-
ever, there is a downside. PIR material is sold mainly through
There are many different types of recycled PE available. brokerage firms and is commonly sold on a lot-to-lot basis.
One way to classify PE resins based on density is found in This means that reliable and continuous waste streams are
Table 2. Notice that at the extremes of density one finds LDPE not commonly found. Additionally, it is often in a bulk form
and HDPE homopolymers. These were the first two types
and/or co-mingled with different grades of PE. The result is
of PE commercially produced. LDPE has the most branch-
that the PIR material is used mostly in noncritical applications
ing and HDPE has the least. The others are all made by co-
by plastic processors who have the capabilities to accommo-
polymerizing ethylene gas with other -olefins, which produces
date different grades of PE.
small branches in a controlled manner. The -olefins are
Despite the limitations mentioned above, three PIR PE sam-
butene, hexene, and octene, which produce 2, 4, and 6 carbon
ples were received for evaluation in this study. Two [HDPE
branches, respectively. The density is varied by the amount
and low-density PE (LDPE)] were provided by a re-processor
of -olefin added.
that also produces post-consumer polyethylene. The third
All of these types of PE are available on the recycled-resin
sample, medium-density PE (MDPE), was regrind scrap from
market. However, some are certainly more available than
others. The main subclasses for recycled PE are post-industrial a manufacturer of geomembranes for landfills. Properties of
recycled (PIR) and PCR. There are also several different phys- each of these recycled resins are found in Table 3.
ical forms one can buy. PIR resins are often sold in their Notice that each of these has stress-crack resistance (15%
bulk forms or the plastic parts may be ground into chips NCTL) higher than most, if not all of, the current AASHTO
and sold as regrind. PCR resins are sold as regrind chips, or resins for pipe. They are also much higher in cracking resis-
as a reprocessed resin (repro). Reprocessed resins are chips tance than post-consumer HDPE. Therefore, these resins could
that have been melted, filtered and pelletized. They are more be useful for enhancing the properties of post-consumer
blended than chips, cleaner from the melt filtration step, and HDPE. They are also much lower in density, which would
in a better form to feed into an extruder. limit how much could be used in a blend. Notice that the den-
There were about 50 different recycled polyethylene sup- sity of the low density material is actually higher than the den-
pliers identified and many were contacted for samples. A list sity of the MDPE. This is due to the high level of inorganic
can be found in Appendix B, Section B.8. fillers displayed by the percentage ash. In fact, each of the three
density values are misrepresented because of fillers in the cases
of the HDPE and LDPE and carbon black in the case of the
Post-Industrial Recycled Polyethylene
MDPE sample. The density can be calculated from the yield
This is a large category that includes scrap from processes stress through the following relationship:
such as pipe, sheet, thermoforming, injection molding, blown
film, tubing, and more. This includes low density, linear low Yield Stress = 81, 250 ( Density ) - 73, 500
density, linear medium density, and high density (homo- and
co-polymer). The molecular weight also varies from injection The calculated values for the HDPE, MDPE, and LDPE are
molding grade (low molecular weight [LMW], high MI) to 0.943, 0.937, and 0.925 g/cm3, respectively. One can also correct
thermoforming or blown film (high MW [HMW], low MI). for the carbon black from the MDPE sample because each 1
