Download Chemical Resistance Testing of UV Exposed Pipe

╣JANA LABORATORIES INC.
ISO 17025
TECHNICAL REPORT
Chlorine Resistance
Testing of UV Exposed
Pipe
Confidential
CHLORINE RESISTANCE TESTING OF UV EXPOSED PIPE
J. Couch, M. Toro, K. Oliphant and P. Vibien
Jana Laboratories Inc.
Aurora, Ontario, Canada
Tel: 905-726-8550, Fax: 905-726-8609, www.janalab.com
Abstract
A test methodology to determine the effectiveness of Ultra-Violet Light (UV) stabilization on the oxidative stability
of piping materials is examined. Chlorine Resistance (CR) testing is used to determine the impact of accelerated
UV exposure on the oxidative resistance of crosslinked polyethylene (PEX) pipe. Following accelerated UV
exposure, samples are tested to failure under accelerated test conditions designed to simulate chlorinated potable
water end use environments. CR testing in conjunction with UV exposure is shown to be a sensitive method for
the evaluation of the effectiveness of UV stabilization on the oxidative stability of PEX pipe. For the particular
material examined, it is demonstrated that excellent retention of oxidative stability can be achieved when suitable
UV protection is employed.
Background
Crosslinked polyethylene (PEX) pipe is commonly
used in residential and commercial applications for
potable water delivery. The popularity of PEX in this
application is increasing in North America as the
market comes to appreciate the high quality and ease
of installation of the product.
One of the reasons for this increasing acceptance is
that PEX piping materials are highly tested in order to
ensure long term performance in hot and cold potable
water applications. Standard testing is detailed in
ASTM standards F876 and F877 for PEX piping
materials1-2. ASTM standard F2023 provides a test
methodology for examining the resistance of PEX
piping materials to chlorinated potable water3. It has
been shown previously that, while chlorinated potable
water can provide an aggressive oxidative
environment, PEX piping materials can demonstrate
excellent performance in chlorinated potable water
environments4-5.
During service, PEX piping in plumbing applications
is not exposed to direct sunlight as it is an ‘inside the
house’ application. Some exposure to sunlight can,
however, potentially occur during installation or
storage of the material at the work site. For this
reason, manufacturers provide packaging and
handling guidelines to minimize exposure to sunlight
and to ensure proper performance of the product in
the application.
The need for protection from the effects of sunlight
has been long recognized for many materials. The
spectrum of sunlight between the wavelengths 280
nm and 400 nm contains ultra-violet (UV) light that is
known to promote the degradation of many materials
There is
unless they are suitably protected6.
circumstantial evidence that some thermal stabilizers
may be affected by UV exposure. A variety of means
of providing UV protection to materials, and in
particular to plastics, have been employed to ensure
appropriate resistance to sun exposure6.
While outward manifestation of UV exposure, such as
yellowing, embrittlement or loss of physical properties
may only be apparent after prolonged exposure,
shorter exposure periods may consume some of the
stabilizers intended to provide long term oxidative
stability of PEX piping products in potable water
applications. The more traditional physical tests7-8 for
UV resistance of polymeric pipe materials (such as
percent retention of tensile elongation), therefore, do
not necessarily provide the required information to
PEX pipe manufacturers to ensure suitable UV
resistance or for evaluation of new UV stabilizer
packages for this application. A test methodology is
needed, therefore, that will evaluate the effects of UV
exposure on the long term oxidative stability of the
piping material.
In this work, the effect of UV exposure on the
oxidative stability of a PEX pipe material is evaluated
using accelerated UV exposure followed by chlorine
resistance testing. Chlorine resistance testing has
been demonstrated to be an effective method of
evaluating the long term resistance of materials to
oxidative attack4-5. Here, samples are exposed to
accelerated UV exposure and then tested to failure at
a single aggressive potable water test condition to
determine the impact of the UV exposure on the
oxidative resistance of the pipe.
A PEX pipe
formulation that is UV protected and thermally
stabilized is compared to a non-UV protected
formulation of the same material with the same
thermal stabilizer package.
A non-UV protected,
non-thermally stabilized version of the same material
was also examined. The test methodology applied is
seen to be a sensitive means of assessing the impact
of UV exposure on the oxidative stability of the pipe
material. It is also shown that, for the UV stabilized
PEX pipe, excellent resistance to UV exposure is
observed.
Experimental
The materials studied were in the form of nominal ½”
diameter SDR-9 PEX pipe with dimensions
conforming to ASTM F8761 and made from a
commercially available base resin.
Material A
contained no anti-oxidant (AO) package. Material A
was included as a control point in the chlorine
resistance testing. Material B contained a standard
commercial anti-oxidant package and material C
contained the same standard commercial anti-oxidant
package with the addition of an experimental UV
protection additive package.
UV exposure testing was performed using an ATLAS
Ci65 Xenon Arc Weather-Ometer (WOM) following
the conditions outlined in ASTM G26 standard test
method9. Three sets of specimens of materials B and
C were exposed for periods of 84, 500 and 1000
hours respectively. No specimen of material A
underwent UV exposure.
Chlorine resistance testing was subsequently
performed on the UV exposed specimens in general
accordance with ASTM F20233 at a single
temperature and pressure condition. Unexposed
specimens of materials A, B and C were also tested.
Specimens were exposed to continuous flowing hot
water of controlled quality while under constant
internal pressure. Temperature (115°C), pressure
(480kPa, 70 psig), chlorine level and pH were
continuously monitored and controlled. Testing was
conducted with reverse osmosis water with a pH of
6.8 and a chlorine concentration of 4.1 mg/L. Failure,
defined as any loss of fluid through the wall of the
pipe, was identified by computer monitored humidity
sensors. A visual examination of the specimens was
conducted after UV exposure and after chlorine
resistance testing.
Results & Discussion
Accelerated UV Exposure
Accelerated UV weathering, such as the Xenon Arc
Weather-Ometer (WOM) exposure, is widely used to
accelerate and mimic the effects of outdoor sunlight
exposure10. Correlation of the accelerated test results
with those from outdoor exposure is recommended to
confirm the acceleration effects. A WOM exposure
period of 1000 hours was chosen as approximating
six months of outdoor exposure. Correlation with
outdoor exposure samples is planned as part of an
extension of this project.
Table 1 summarizes the sample descriptions, UV
exposure periods and the appearance of the
specimens following UV exposure and prior to
chlorine resistance testing.
Table 1 – Specimen Description, UV Exposure
Period and Test Specimens Post UV Exposure
Appearance
Specimen
AO
Package
UV
Package
A0
B0
B84
B500
B1000
No
Yes
Yes
Yes
Yes
No
No
No
No
No
WOM
Exposure
Time, hrs
0
0
84
500
1000
C0
C84
C500
C1000
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
0
84
500
1000
Post
Exposure
Appearance
Not applicable
Not applicable
No change
No change
Circumferential
cracks, loss of
ductility
Not applicable
No change
No change
No change
The appearance of the non-UV stabilized PEX
specimens, material B, after 84 (B84) and 500 (B500)
hours UV exposure was comparable to the
unexposed sample (B0). After 1000 hours UV
exposure, the specimen (B1000) had closely spaced
circumferential cracks as shown in Figure 1. These
cracks were observed on the side of the pipe facing
the WOM light source only. A reduction in the
flexibility of the pipe was also noted. No color change
was, however, observed. The circumferential
cracking observed in the 1000 hours accelerated UV
specimen (B1000) is of the same nature as that
observed for samples exposed to natural sunlight.
Accelerated UV exposure testing, therefore, appears
to effectively simulate and accelerate outdoor
weathering of PEX pipe.
Figure 1 - Circumferential cracking - Specimen
B100 after 1000 hours WOM testing
exposure (A0). With further UV exposure, the failure
times decreased to 285 and 210 hours at 500 and
1000 hours UV exposure respectively, which
represent a further decrease of 29%. The failure
times of the UV stabilized specimens (C84-1000)
were essentially unaffected by UV exposure.
Table 2 - Chemical Resistance Testing Failure
Time
Specimen
A0
B0
B84
B500
B1000
C0
C84
C500
C1000
For the UV stabilized PEX specimens (material C),
no change was observed in the appearance of the
specimen after 1000 hours of accelerated UV
exposure. The specimens appeared to have retained
their flexibility and no circumferential cracking was
observed on bending.
WOM
Exposure
Time, hrs
0
0
84
500
1000
0
84
500
1000
CR
Failure
Time, hrs
399
702
343
285
210
801
825
796
753
Chlorine Resistance Testing
As has been shown previously, chlorine resistance
testing provides an aggressive means of evaluating
oxidative resistance4-5. Comparison of the chlorine
resistance failure times in Table 2 and Figure 2
shows that with no UV exposure, the UV stabilized
(C0) and the non-UV stabilized material with antioxidant (B0) have comparable failure times. Both
materials are seen to have approximately twice the
test lifetime of the no UV stabilizer / no anti-oxidant
material (A0). As shown in Table 2 and Figure 2, the
failure times of the non-UV stabilized material B
showed a steep decline with initial exposure (B84)
followed by a more gradual decline with further
exposure (B500, B1000). With 84 hours UV exposure
the chlorine resistance failure times for the non-UV
stabilized material (B84) dropped 51% from 702 to
343 hours. The failure time for this specimen is
similar to the failure time (399 hrs) of the no UV
stabilizer / no anti-oxidant material A without UV
Figure 2 - Graph of Chlorine Resistance Failure
Times.
1000
CR Failure Times (hrs)
Chlorine resistance testing was performed on the UV
exposed specimens of materials B and C. Non-UV
exposed specimens of all three materials (A0, B0,
C0) were included in order to determine the intrinsic
chlorine resistance of each material. Specimens were
exposed to the hot pressurized chlorinated water until
failure occurred. Failures times ranged from 210 to
825 hours and are summarized in Table 2. All
failures were typical of oxidation induced brittle
failures, commonly referred to as Stage III failures,
characterized by extensive oxidation of the inner wall
and radial micro-cracking extending outward through
the wall. Failure occurred when one or more cracks
penetrated through the outer wall.
800
600
Material A
Material B
Material C
400
200
0
0
200 400 600 800 1000
Accelerated UV exposure (hrs)
The comparable failure times of UV stabilized (C0)
and the non-UV stabilized (B0) specimens before UV
exposure show that these materials have essentially
the same resistance to oxidative degradation. The
difference between these failure times to that of the
no
stabilizer/anti-oxidant
specimen
(A0)
demonstrates the contribution of the stabilizers to the
oxidative resistance of the PEX pipe which is seen to
effectively double the chlorine resistance at this test
temperature.
For the non-UV stabilized material B, there would
appear to be two regimes in which the UV exposure
is reducing the chlorine resistance of the pipe: A
large decrease in the first 84 hours followed by a
more gradual decline up to the 1000 hour exposure
time. In the first regime, it would appear that the
initial UV exposure is rapidly consuming the antioxidant. With the inside pipe wall depleted of antioxidant, the oxidative resistance of the pipe to the
chlorinated water is reduced enabling more rapid
crack initiation at the wall surface and lower
resistance to oxidation of the material in advance of
the crack front as the crack propagates though the
wall. In the more gradual decline in the second
regime, the UV exposure appears to be progressively
degrading the PEX, with the observed failure times in
chlorine testing dropping below those of the unstabilized material A. The accrual of degradation is
seen to lead ultimately to circumferential cracking
even prior to chlorine resistance testing for the
longest exposed specimen (B1000).
The UV protection package added to material C was
expected to diminish the penetration depth of the UV
into the pipe wall. As a result, the bulk of the polymer
and the anti-oxidant present were expected to be
largely unaffected by the UV exposure. This would
appear to be confirmed by the results of the chlorine
testing as only a small decrease in failure time was
observed after 1000 hours UV exposure (C1000)
compared with the unexposed material (C0). The
initial large drop in the chlorine resistance failure time
that was seen with UV exposure of material B is not
observed in material C. It is believed that the
antioxidant is still present at the inside wall and
throughout most of the wall thickness and able to
provide good chlorine resistance.
The average failure time for the UV stabilized
specimens (C0-1000) was 794 hours with a standard
deviation of 4%. This variability is comparable to that
typically seen in chlorine resistance testing. The
apparent decrease in failure times for the 500 and
1000 hour UV exposed specimens (Table 2 & Figure
2) does not, therefore, appear to be statistically
significant. Further testing of specimens with longer
UV exposures would be needed to determine if a
reduction in failure time with increasing UV exposure
is in fact occurring. The UV stabilizer package added
to material C would appear to be effective at
protecting the bulk of the antioxidant and of the
polymer from degradation by the UV light resulting in
a PEX pipe formulation with very good UV resistance.
Given the effects of UV exposure observed on the
non-UV stabilized material, it appears that UV
stabilization of the pipe to protect the anti-oxidant
stabilizers is important for the particular formulation
examined in maintaining the oxidative resistance of
the material.
The range of chlorine resistance failure times has
illustrated the sensitivity of the chlorine resistance
test method in determining the relative oxidative
resistance of different pipe materials. Chlorine testing
at elevated temperature is commonly performed in
order to gain failures in an acceptable time frame.
However, results at elevated temperature may not be
indicative of performance at other conditions. Further
testing at multiple temperature and pressure
conditions testing could be used to predict
performance at service conditions5.
Conclusions
The combination of chlorine resistance testing and
accelerated UV exposure testing is shown to be
effective for evaluating PEX pipe UV stabilization.
Xenon Arc accelerated weathering appears to mimic
and effectively accelerate the effects of outdoor
sunlight exposure. Future work will examine the
correlation of accelerated weathering with outdoor
exposure. Chlorine resistance testing was shown to
be able to detect the effects of even short duration
accelerated UV light exposure.
Accelerated UV exposure was seen to have a
detrimental effect on the anti-oxidant present in the
non-UV protected pipe suggesting that UV protection
of the stabilizers is important in order to ensure long
term oxidative resistance when exposure to sunlight
is a possibility. It was shown for UV protected PEX
pipe, however, that excellent retention of oxidative
resistance can be achieved when suitable UV
protection is employed.
Testing of UV exposed pipe samples followed by
chlorine resistance testing at multiple temperature
and pressure conditions would provide for actual
performance predictions at service conditions for pipe
exposed to UV light.
1. ASTM F876 Standard Specification for Crosslinked Polyethylene (PEX)
Tubing, 2000.
2. ASTM F877 Standard Specification for Polyethylene (PEX) Plastic Hotand Cold-Water Distribution Systems, 2000
3. ASTM F2023 Standard Test Method for Evaluating the Oxidation
Resistance of Crosslinked Polyethylene (PEX) Tubing and Systems to
Hot Chlorinated Water, 2000
4. Oliphant et al., Chlorine Resistance Testing of Cross-linked Polyethylene
Piping Materials, Society of Plastics Engineers Annual Technical
Conference (ANTEC), San Francisco, USA, 2002.
5. Oliphant et al., Assessing Material Performance in Chlorinated Potable
Water Applications, Plastic Pipes XI, Munich, Germany, 2001.
6. Zweifel, H., Stabilization of Polymeric Materials, Springer Verlag,1998.
7. ASTM D1435 Standard Practice for Outdoor Weathering of Plastics.
8. ASTM D2513 Standard Specification for Thermoplastic Gas Pressure
Pipe, Tubing, and Fittings, 2000.
9. ASTM G26 Standard Practice for Operating Light-Exposure Apparatus
(Xenon-Arc Type) With and Without Water for Exposure of Nonmetallic
Materials.
10.Wypych, G., Weathering of Plastics: Testing to Mirror Real Life
Performance, Plastics Design Library, Chemtec Publishing, 1999.
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280B INDUSTRIAL PARKWAY SOUTH
AURORA, ONTARIO, L4G 3T9 CANADA
PHONE: 905-726-8550 e-mail: [email protected] FAX: 905-726-8609