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OCR for page 42
42
Table 18. Recent BFF test results.
Sample/Formulation BFF Failure Time (h) (COV)
80ºC/650 psi 80ºC/465 psi 70ºC/650 psi
B1 411 ± 135 (33%) 1,601 ± 537 (34%) 1,627 ± 680 (42%)
100VR1
A2 257 ± 85 (33%)
-- --
85VR1 + 15MCR
L5 431 ± 86 (20%)
-- --
50VR1 + 20MD + 30MCR
A5 323 ± 60 (19%) 1,364 ± 571 (42%) 1,615 ± 644 (40%)
40VR1+ 30MD + 30NAT
B3 330 ± 66 (20%)
-- --
20VR1+40MD+24MCR+16NAT
B5 280 ± 95 (34%)
-- --
40MD + 36MCR + 24NAT
L1 201 ± 39 (19%)
Proprietary Blend 50% Recycled -- --
content
RPM Proprietary Blend 620 ± 376 (61%) 1,480, 4>3,0001
--
100% Recycled content
1
Four specimens terminated after 3,000 h.
Note: "--" = data not available.
cited in the FDOT protocol. However, the results generated are Since there are failure times under three sets of condi-
believed to be unreliable due to the presence of residual sur- tions for samples B1 and A5, one can make a service-lifetime
factant in the baths. Therefore, there are no reliable service- estimate through bidirectional shifting. The master curves for
lifetime estimates for all of the six final candidates. stress-crack resistance for Samples B1 (100% Virgin) and A5
Once the issue with the baths was resolved, the BFF tests were (30% Recycled) are shown in Figures 42 and 43.
repeated under one set of conditions for eight samples, under These results suggest that the service lifetime under 500 psi
two sets of conditions for three samples, and under three sets of of stress will be well above 1,000 years and that even at stresses
conditions for two samples. The results are shown in Table 18. of 900 psi, the estimated service lifetime is over 100 years.
Notice that a new sample was evaluated. This material is a
fully formulated pipe blend containing 100% PIR recycled
Combined SIM (Stage I) and BFF (Stage II)
and PCR PE, submitted by a recycled-resin supplier. It repre-
Service-Lifetime Estimates
sents the possibilities for recycled resins when they are not
based on typical virgin corrugated pipe resins. Some of the The SIM test produced information about the long-term
important properties of this resin are shown in Table 19. tensile strength and the BFF test provided information about
The stress-crack resistance for this material was outstand- the long-term stress-crack resistance. Now, the two results
ing, but the yield stress was a little low, the break strain was can be combined into one global service-lifetime estimate.
poor, and it contained about 0.7% of ash. Melt filtration at This is shown in Figures 44 for Sample B1.
150 mesh reduced the percentage ash to 0.4% and raised the This curve shows both the long-term tensile strength that
break strain to 225%. This is the type of material that can be might relate to a buckling failure and the long-term stress-
developed for use in corrugated pipe. cracking strength. The first thing to keep in mind is these data
Table 19. Properties of pipe resin containing 100% recycled PE.
Property Result
Corrected Density 0.948 g/cm3
Melt Index 0.11 g/10 min
% Color 3.9
% Ash 0.7
% PP 4.8
Flexural Modulus 128,606 psi
Yield Stress 3,260 psi
Break Strain 21%
NCLS Stress-crack Resistance 220 ± 54 hrs
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3.15
BFF Results Shifted to 20C
3.1
Sample B1
y = -0.236x + 4.3928
Log Stress (psi)
R² = 0.9835
3.05
3
500 psi = 1717 years
100 years = 979 psi
2.95
5.4 5.5 5.6 5.7 5.8 5.9 6 6.1
Log Failure Time (hrs)
Figure 42. BFF stress-cracking master curve at 20 ºC for sample B1.
were generated on tensile dumbbell specimens with very sim- psi, the pipe is more likely to crack than to buckle. In fact, as
ple geometry. A pipe is likely to behave differently. Secondly, most involved in this project thought, slow crack growth is
if these data relate to pipe, they relate to an unconfined pipe. the key to service lifetime in corrugated pipe, according to
A properly installed pipe would also be a different situation. these results.
That said, there is still valuable information contained in the The curve also shows that even at the very high service
master curves. stress of 900 psi, the estimated lifetime is over 100 years. Once
The curve shows a ductile-to-brittle transition stress of more, at a typical operating stress of 500 psi, the service life-
1,153 psi. This is just above 30% of the material's initial yield time estimate is over 1,500 years.
stress. This also means that at applied stresses less than 1,153 A similar master curve is shown for Sample A5 in Figure 45.
3.150
BFF Results Shifted to 20C
3.100
Sample A5
y = -0.2331x + 4.3625
Log Stress (psi)
R² = 0.9988
3.050
3.000 500 psi = 1563 Years
100 years = 949 psi
2.950
5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 6.1
Log Failure Time (hrs)
Figure 43. BFF stress-cracking master curve at 20ºC for sample A5.
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3.4
3.3
3.2
SIM
3.1 y = -0.0459x + 3.3237
Log Stress (psi) R² = 0.9527
1153 psi
3
979 psi
2.9
BFF-POP
y = -0.236x + 4.3928
2.8 R² = 0.9835
2.7
Sample B1
Shifted to 20C
50 yrs 100 yrs
2.6
0 1 2 3 4 5 6 7 8
Log Time (hrs)
Figure 44. Combined master curve for Sample B1 (100% virgin).
Sample A5 contained 30% PCR-NAT, which is basically the LRFD design manual is a long-term yield strength. The
ground up milk and water jugs. The service-lifetime estimates results generated during this project suggest that the long-
are similar to 100% virgin. Because of limitations of time and term stress-crack resistance is more important and will limit
funding, more of the recycled-content-containing pipes were the lifetime of the pipe. So, it might be more appropriate to
not evaluated. determine a long-term stress-crack strength, which is the stress
One important aspect here is to define the "long-term a material can be subjected to without cracking for its ser-
strength." The long-term strength referred to in Section 12 of vice lifetime. In the two examples above, the 100-year tensile
3.4
3.3
3.2
SIM
Log Stress (psi)
3.1 y = -0.0471x + 3.3272
R² = 0.9658 1116 psi
3
950 psi
2.9
BFF-POP
2.8 y = -0.2331x + 4.3625
R² = 0.9988
2.7 Sample A5
Shifted to 20C
50 yrs 100 yrs
2.6
0 1 2 3 4 5 6 7 8
Log Time (hrs)
Figure 45. Combined master curve for Sample A5 (30% recycled).
