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18
Table 5. Post-consumer mixed-color recycled priate for different applications and there have also been mis-
PE properties. interpretations about the meaning of the results.
In the OIT test (ASTM D3895), a small sample (5.0 mg)
Property TRI Repro 1 Average Range is maintained at 200°C in a pure oxygen environment until
Density the protective stabilizers are consumed by the oxygen and the
(g/cm3) 0.960 0.955 ± 0.005 0.9460.960
Melt Index
sample decomposes exothermally (it basically catches on fire).
(g/10 min) 0.54 0.48 ± 0.10 0.370.64 The time it takes from the addition of oxygen to the onset of
oxidation is the OIT. It is known that in well-behaved addi-
MFR (21.6/2.16kg) 83 97 ± 14 75122
tive packages, there is a relationship between the OIT value
% Color 0.4 0.31 ± 0.32 01.3 and the amount of additives present in the sample (1, 2). It is
also known that the OIT value does not relate to the long-
% Ash 1.2 1.1 ± 0.3 0.41.5
term performance of the material. The test merely shows the
% PP 5.7 3.6 ± 1.5 0.86.3 oxidation behavior at 200°C, which has little to do with real-
Yield Strength
(psi) 3,685 3,728 ± 218 3,3274,037
time aging at ambient temperatures. However, if one exposes
Break a sample to a condition that consumes additives (oven, chlori-
Strain (%) 46 90 ± 81 9302 nated water, UV light) then the residual lifetime can be deter-
15% NCTL
(h) 8.8 7.8 ± 2.9 4.514.8 mined by the amount of OIT left after the exposure. This seems
to be a reasonable approach. It is known from assessments of
OIT (min) 12 13.6 ± 7.2 939
old geomembrane liners (3 - TRI, Confidential files) that as the
OIT value approaches zero, other changes can be seen such as
a reduction in stress-crack resistance or change in melt index,
· The percentage PP reported was typical in 2007. The aver- both signs of polymer oxidation.
age is closer to 10% in 2010, due to the popularity of 2X The OITemp test has historically been used on scrapings
and 3X detergent bottles. The percentage PP by weight can from the inside walls of thick-walled pipe after manufacturing.
be as high as 30% in the 3X bottles. The caps and spouts are The inside wall is exposed to more heat longer and is the first
made from PP. place that will become oxidized during the process if adequate
· The stress-crack values are typical for these products. Note stabilizers are not present. Therefore, when a sample shows
that the NCLS stress-crack times (F2136) are even lower an OITemp value less than 220°C, that means it has been
because the yield stresses are less than 4,000 psi. The NCLS degraded during processing. So, it really does not say how good
test assumes a 4,000 psi yield stress and loads the samples something is, but instead that it has not been made worse. That
at 600 psi, which is 15% of 4000. The load as a percentage said, there is a relationship between OIT and OITemp.
of the yield stress will increase for resins with yield stresses Figure 14 shows the relationship between OITemp and OIT
less than 4,000 psi. For example, a sample with a yield stress for 76 data points. The data includes eight PPI-certified virgin
of 3,327 will be loaded at 18% of its yield stress during the corrugated pipe resins, two geomembranes, 21 PCR samples,
NCLS test. and 45 points from the Pennsylvania Deep Burial Project (13).
· The OIT values show that some stabilizers are present, These results should settle the controversy. Since the rela-
although it is not possible to know what they are from the tionship appears to be logarithmic, there will always be a larger
results. That some stabilization is present, even after repro- change in the OIT value for a given change in the OITemp
cessing, shows that the material is probably not affected by value. That means the OIT test is more sensitive and can dis-
the additional stress of reprocessing. The rather large vari- tinguish materials better. Additionally, the OIT test is likely to
ability suggests that either some reprocessors are adding be more sensitive to long-term antioxidants, while the OITemp
stabilizers, or additives are being consumed at different is better suited to evaluate process stabilizers, since these are
rates by different reprocessors. made to be active at higher temperatures. For these reasons,
the OIT test will be used in the recommended specifications
for the use of recycled resins.
OIT vs. OITemp
This has been a controversial issue for about 5 years now.
The Effects of Contamination
ASTM D3350 requires the OITemp test to be performed on
resins and that a minimum temperature of 220°C be obtained. The types of foreign matter that may be found in PCR
Other industries, most notably geosynthetics, have used the HDPE include labels, paper, cardboard, dirt, aluminum foil,
OIT test for over 15 years to specify HDPE geomembranes. adhesives, and other polymers like PP (from caps), ethylene
There have been discussions about which one was most appro- vinyl alcohol (EVOH), Nylon, PET or polyvinyl chloride
