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49
CHAPTER 4
Conclusions and Suggested Research
Phase 1--Recycled PE Resins such as ASTM, or developed through AASHTO. Also, FTIR
spectroscopy is an alternative method to measure percent-
The purpose of this phase was to determine which types of age PP. It should be added to the method as an alternative to
recycled PE were available, where these materials could be the DSC method presented in this report.
obtained, and what their properties were. Both PIR and PCR
PE were initially examined, but the effort focused on PCR
because it is readily available, more consistent than PIR, and Phase 2--Recycled PE Blends
there is a trade association called the Association of Post- The purpose of Phase 2 was to determine how the addition
consumer Plastic Recyclers (APR). PIR should not be ruled of recycled HDPE would affect the properties of CPPI-certified
out as a source of material, but PCR will be more consistent resins and to select specific blends for trial corrugated pipe
and more widely available. A total of 25 samples of recycled manufacturing. A total of 66 blends were prepared and their
PE were obtained from nine different suppliers. There were properties evaluated. One significant finding was that stress-
three PIR samples and 22 PCR samples. Of the PCR samples, crack resistance changed in an exponential manner when two
six were natural and 16 were colored. The natural resins came different PE materials were blended together. Additionally, the
from milk bottles and the colored mostly from detergent yield strength changed in a linear manner. This means that
bottles. when one blends two materials with different strengths and
The results showed that mixed-color PCR was an excellent stress-crack resistances, the strength will change more quickly
candidate for use in corrugated HDPE pipe. The density aver- than the stress-crack resistance. This also means that one
ages around 0.949 g/cm3, which is close to virgin pipe resins, needs to find resins to blend with the recycled resins that have
so the strength and flexural modulus properties are in the similar or greater strengths and significantly better stress-crack
AASHTO specified cell class for M294 pipe. The MI averaged resistances. This project was limited by the original concept of
around 0.5 g/10 min while the M294 maximum is 0.4 g/10 min. blending recycled resins with PPI-certified corrugated pipe
It also had poor stress-crack resistance and contaminants resins. Just since this project began, data has become available
present that could be stress-crack initiation sites. about the stress-crack resistance of bimodal HDPE resins.
A test method was developed to determine the percentage There are at least six companies in the United States that produce
PP in the recycled HDPE. PP is a contaminant that comes from resins with densities in cell class 4 (required by AASHTO M294)
the tops and pour spouts of detergent bottles. It can be found and that have NCLS values in the thousands of hours. In fact,
in amounts of 10% or higher. Results showed that the PP they are tested with a more aggressive stress-crack test called
content could be as high as 5% without hurting the stress- the PENT Test (ASTM F1473) and some resin suppliers report
crack properties of the resin. Results also showed that melt values greater than 10,000 h.
filtration at a mesh size of 120 or greater can reduce the amount Besides these "super" resins, a recycled supplier submitted
of contamination to about 0.5% and raise the elongation-at- a sample for this study recently that was made from 100%
break to over 100%. These values will be specified to ensure recycled material. Its NCLS stress-crack time was 220 h, and
proper filtration occurs. it lasted for over 600 h in the BFF test at 80°C/650 psi when
The research needs from this phase involve the test method 200 h suggests 1000 years of estimated lifetime.
for percentage PE. The method needs to be written into a Research is needed to determine how the addition of these
standard test method and evaluated through a consensus group new, bimodal resins will affect the stress-crack resistance
