Chemical Resistence Technology
Introduction
Standard Hydrostatic Testing focuses primarily on testing with water as the internal pressurizing medium and either air or water as the external test environment. Under end-use conditions, however, plastic pipes may be exposed to a wide variety of chemicals and their behavior under these conditions can be dramatically different from the traditional hydrostatic water/air test environment. As a consequence, a number of testing approaches have been developed to determine the performance of plastic piping materials under specific chemical environments. In developing an appropriate validation test protocol it is important to understand how the chemical environment may impact pipe performance in each of the potential failure regions and the testing methods that may be applied to assess this performance.
Stage I
The primary potential influence of chemicals on the Stage I performance of piping materials is through a softening of the pipe material which leads to a reduction in its stiffness. This can occur with materials that are solvents or partial solvents for the piping material. The net result is a lowering of the stress required to cause ductile failure and hence a reduction in the pressure carrying capability of the pipe. For very strong solvents, significant swelling of the pipe material can occur to the point of making the piping material unsuitable for the handling of that particular chemical.
Simple immersion testing, where piping material samples are immersed at different temperatures in the chemical of interest, is often used for screening purposes to determine the compatibility of a particular piping material with a particular chemical. This testing can provide an indication as to the effect the chemical may have on all three failure modes. In this testing the swelling, weight gain or loss and change in physical properties are often measured as a function of temperature and exposure time. The data are typically expressed in terms of chemical resistance tables, which list the influence a particular chemical has on a general class of piping materials.
Pressure testing with the chemical of interest as the internal environment can also be employed to determine the influence of that chemical on the stage I performance of a piping material. This testing would typically be performed to confirm performance after demonstrating potential chemical compatibility through immersion testing. The influence of testing under pressure may reveal a lowering of Stage I performance that was not indicated by simple immersion testing.
Stage II
The Brittle Mechanical failure mode can be accelerated by a number of different chemical environments. This effect is termed Environmental Stress Cracking. A material's resistance to this phenomenon is termed Environmental Stress Crack Resistance (ESCR). Agents that promote stress cracking do so by promoting the slipping or untangling of the polymer chains across the advancing crack tip. The result can be a dramatic reduction in pipe performance through a rapid acceleration of the slow crack growth mechanism. Standard ESCR tests for materials (ASTM D1693) and pipe (ASTM F1248: "Standard Test Method for Determination of Environmental Stress Crack Resistance (ESCR) of Polyethylene Pipe") are in use. Testing of actual pipe samples, either notched or un-notched, in the chemical environment of interest can also be used to assess the ESCR of a piping material. Testing is typically performed at elevated temperatures to accelerate any potential effects.
Stage III
As Stage III failures are of a Brittle-Chemical nature, the influence of the chemical environment was discussed in the Stage III section of the Testing Technology Overview on Hydrostatic Pressure Testing . That discussion is continued in more detail in this section.
In general, any chemical or chemical environment that can act as an oxidant, a catalyst in the oxidation process or as an inhibitor of the effectiveness of the stabilizers in the pipe formulation can impact the Stage III performance of a plastic piping material. Proper performance testing and a thorough understanding of the end use environment are essential to validate a pipe material's performance in an application over the desired lifetime. Below is a brief discussion of some of the potential chemical environments to which plastic pipe materials are typically exposed and the testing methodologies that can be applied to ensure performance in these environments.
Oxidizers
Essentially all piping applications involve exposure to air and, therefore, to the potential for oxidation. For chemical transport piping, the chemical or fluid being transported can also cause oxidation if the chemical is an oxidizer (for example strong acids). A common piping application is for potable water transport. At the levels found in potable water in North America, the chlorine used to disinfect the water can promote oxidation of piping materials. Chlorine Resistance Testing of piping products is used to validate a material's performance in this application. This methodology illustrates the sophisticated testing and analysis approaches that have been developed to validate long term piping material performance in a specific chemical environment.
Chlorine Resistance Testing
All piping products that are potentially exposed to potable water have the potential to be exposed to chlorinated, chlorine dioxide treated or chloramines treated water, and hence to an oxidative environment. Fortunately, if materials are properly selected and formulated to be resistant to oxidation, excellent performance of piping materials and appurtenances in potable water applications can be achieved.
1. Water Quality
Chlorine is the most commonly used disinfectant for potable water in North America. It is added to water supplies for initial disinfection and to ensure a residual chlorine level that prevents bacteria growth in the water as it travels through the distribution network. The most common method of addition of chlorine to potable water is the addition of chlorine gas. When chlorine gas is dissolved in water, an equilibrium is established between chlorine, hypochlorous acid and the hypochlorite ion Cl2, HOCl and OCI- respectively:
At a pH level above 6, essentially complete conversion of chlorine to hypochlorous acid occurs (Eqn. 5). It is the hypochlorous acid that is the primary disinfecting species. The equilibrium for Eqn 6 is also pH dependent, with the formation of hypochlorite ion favored by a higher pH. As the hypochlorous acid is 80 to 300 times more effective than the hypochlorite ion as an oxidizer, pH has a strong influence on the oxidizing ability of the chlorinated water. At a pH of 7, approximately 80% of the free chlorine will be present as hypochlorous acid. At a pH of 8, approximately 80% of the free chlorine will be present as the much less aggressive hypochlorite ion. Obviously, this difference in water quality can have a significant impact on pipe lifetime.
In North America, actual water quality varies considerably in terms of chlorine levels and pH. Chlorine levels range from 0 - 4 ppm (or higher), with the majority of systems in the 1-2 ppm range. pH levels range from 6.5 - 8.5, with the majority of systems in the 7 - 8 range. Testing for Chlorine Resistance is commonly conducted at a chlorine level of 4 ppm and a pH of 7 to provide test data under an aggressive environment.
In addition to monitoring the chlorine level and pH, Oxidation Reduction Potential (ORP) is another commonly used measure of the oxidizing ability of chlorinated water solutions. ORP electrodes measure a voltage in the water (typically expressed in millivolts (mV)) that is dependant on the type and amount of oxidizing species present. The more oxidizing species present (for example the higher the chlorine level) the higher the ORP. "Pure water" typically has an ORP of 200 to 300 mV, primarily due to the dissolved oxygen (which is an oxidizer). Water with a chlorine level of 4 ppm and a pH of 7 has a nominal ORP of approximately 875 mV. (The term nominal ORP is used here to signify "pure water" to which chlorine has been added. In a typical water supply there could be other species present that can influence the ORP in addition to chlorine). Chlorinated water can, therefore, be a much more aggressive oxidation environment than "pure water". ORP is also useful in measuring the change in oxidizing ability of chlorinated water with pH. For example, chlorinated water with 0.5 ppm chlorine has a nominal ORP of 775 mV at a pH of 7 and a nominal ORP of 560 mV at a pH of 8.5; the lower ORP being less aggressive.
The above discussion illustrates the importance of understanding the end-use chemical environment for piping applications and taking that environment into consideration in the development of a testing methodology. Simply testing products intended for use in potable water applications with water of unspecified quality could result in testing under conditions that are much less aggressive than end-use conditions. The issues become even more complex for other chemical environments.
2. Mechanism of Chlorine Degradation of Piping Materials
While materials that are susceptible to chlorine induced degradation can fail through a number of mechanisms, the following mechanism has been found to apply to many materials:
- Step 1: Chlorine consumes the anti-oxidants on the inner layer of the component that is exposed to the potable water.
- Step 2: The inner exposed layer begins to oxidize and degrade.
- Step 3: Micro-cracks are initiated on the inner surface.
- Step 4: The cracks propagate through the wall of the component through a slow crack growth mechanism. Depending on the slow crack growth resistance of the material, this may or may not be accompanied by the chlorine induced degradation of the material in advance of the crack tip.
- Step 5: Ultimately, brittle failure occurs.
For materials that are not properly selected and formulated or for materials that are improperly used, failure can occur from chlorine degradation much more quickly than would be indicated by standard material tests.
Figure II shows the fracture surface of a fitting that failed due to chlorine induced degradation. The fitting was installed in an application for which it was not intended for use, the drain valve of a hot tub. It was exposed continuously to hot chlorinated water, a very aggressive oxidizing environment, and failed after only a few years service. This resulted in significant water damage to the owners' home. It should be mentioned that the plastic pipe and fittings designed for use in this environment were still functioning well after more that 15 years service. This example highlights the importance of validating the performance of materials intended for use with chlorinated water.
Chlorine Resistance Testing Approach
To validate the performance of piping materials in chlorinated potable water applications, a sophisticated testing methodology has been developed. Testing is conducted on actual product samples at elevated temperatures, in pressurized continuous flow systems where the pH, chlorine level, pressure and temperature are continuously monitored and controlled.
The general approach is to simulate end-use conditions as closely as possible. Actual product samples are tested due to the many variables that can influence chlorine resistance such as metal components, sample geometry and manufacturing or surface defects. Tests are conducted under pressure because potable water systems are pressurized and the failure is driven by pressure dependant slow crack growth. A continuous flow of freshly chlorinated water is used to maintain the chlorine levels in the samples. Due to the reaction of chlorine with the samples, chlorine levels in static systems would quickly drop to zero and little oxidative influence would be observed. Elevated temperatures are used to accelerate the failures so that testing can be conducted in a reasonable time frame. Various extrapolation methods can then be employed to extrapolate results to end-use temperatures.
Typically, testing is performed at multiple temperatures and pressures and a multiple linear regression is performed based on the three coefficient rate process extrapolation method:
where A, B and C are constants. Validation of log [stress] versus log [failure time] and of Arrhenious behaviour is also carried out. The model derived from Equation 7 can then be used to predict the extrapolated test lifetime under end-use temperatures and pressures.
When testing is performed under proper water conditions (Chlorine level = 4 ppm, pH = 7) this testing approach provides an aggressive and useful validation tool. In practice it has been found to identify material, formulation and manufacturing limitations and provide a scientifically based means of validating material and product performance. Chlorine Resistance Testing is also useful for simulating field failures and, thereby, resolving product liability claims or aiding re-engineering to address product deficiencies. The general approach can also be extended to other chemical environments.
Oxidation Catalysts
In addition to chemicals that are potentially direct oxidizers of piping materials, certain substances can accelerate the oxidation process through catalyzing oxidation. An often overlooked example is water. Oxidation of many polymeric materials occurs faster in water/air environments than in simply air alone. This has been shown to be a chemical phenomenon and not simply an issue of the extraction of additives. The pH of the water influences the rate of acceleration of the oxidation process, with more basic (higher pH) water providing a greater catalytic effect. (Note: this is for water without any other impurities or additives. As discussed above, the opposite influence of pH is seen for potable water containing chlorine, where lower pH solutions are more aggressive. This highlights the importance of thoroughly understanding the end use environment for the development of appropriate performance validation test methodologies.) The mechanism appears to be the catalytic degradation of the hydroperoxides that are formed in the auto-oxidation cycle. In other words, the water is accelerating one step in the oxidation process which results in the overall oxidation process occurring more quickly. This example demonstrates the importance of testing in the actual end use chemical environment to validate oxidative stability.
Metal ions are also known to be potential oxidation catalysts for piping materials. They are present in many applications and can also be introduced through metal fittings and components in the piping system. Again the mechanism is through the catalytic degradation of the hydroperoxides that are formed in the auto-oxidation cycle. The influence of metal ions on the oxidation of piping materials at elevated temperatures is a further illustration of the importance of understanding the fundamental mechanisms of degradation in the design of appropriate test methodologies. In accelerated testing, little influence of metal ions on oxidation behavior is observed at temperatures greater than 110 degrees C. As the test temperature is lowered, however, a greater and greater catalytic effect of metal ions is observed. This is a result of the decomposition of hydroperoxides, the step in the oxidation cycle catalyzed by metal ions, occurring through thermal cleavage of the oxygen-oxygen bond in the hydroperoxides at significant rates for temperatures >110 degrees C. The thermal process overshadows the metal ion catalysis at high temperatures and no difference in oxidative lifetime is observed. At lower temperatures, when the thermal decomposition of the hydroperoxides is not significant, metal ion catalysis of the oxidation process is observed and the presence of metal ions is seen to reduce the oxidative lifetime. Unless properly accounted for in piping system design, a significant reduction in pipe material lifetime can be observed. For improperly stabilized materials, a 2 to 4 fold reduction in lifetime will result. Testing at multiple elevated temperatures is required in order to properly assess the influence of metal ions on the oxidative lifetime of a piping material. This also demonstrates the importance of considering the total end-use system of pipe, fittings and appurtenances in designing a meaningful end-use validation test.
Stabilizer Inhibition
Some formulation additives can interfere with the functioning of the stabilizers added to piping formulations to protect against oxidation either through chemical transformation or physical binding of the stabilizers. As the stabilizers are added to provide oxidative stability and extend the oxidative lifetime of the material this can lead to a reduction in Stage III performance. For example, calcium carbonate has been found to have a marked influence on the efficiency of some stabilizers. This appears to be a result of the adsorption of stabilizer molecules on the surface of the calcium carbonate, rendering them ineffective. Care, therefore, needs to exercised in making any formulation changes to ensure that the Stage III performance is not compromised.
The fluid to be transported through the piping system can also have a profound effect on the functioning of the additive system. For example, certain chemical environments can accelerate the extraction of additives from piping system components. This can result in a lower effective level of stabilizers and reduce the Stage III life expectancy. Properly designed testing programs, conducted with the fluid to be used in the end-use application, can ensure proper product functioning.
Chlorine Dioxide and Chloramines Resistance Testing
Similar to chlorinated water, the other two common potable water disinfectants (Chlorine Dioxide and Chloramines) can also induce oxidative degradation of piping systems exposed to potable water.
Chlorine Dioxide
A third disinfectant found in potable water is chlorine dioxide.
Chlorine dioxide can be produced many different ways. One method using sodium chlorite and sodium hypochlorite is shown below:
There appears to be an indication that on a parts per million (ppm) concentration basis, chlorine dioxide may be more oxidatively aggressive than chlorine and chloramines.
Chloramines
Chloramines are also used as disinfectants in potable water applications. Chloramines impact specific materials differently from chlorine. For certain materials, such as elastomers, chloramines have been observed to be more aggressive than chlorine.
There are three types of chloramines:
HOCl + NH2Cl NHCl2 (dichloramine) + H2O
HOCl + NHCl2 NCl3 (trichloramine or nitrogen trichloride) + H2O
Summary
The end-use chemical environment can have a profound influence on the performance of piping materials. While the above discussion is by no means complete, it does illustrate the importance of validating product performance under conditions that accurately reflect the end-use environment. In developing testing methodologies, careful consideration needs to be given to: the specific chemical environment, other components of the system (such as metal fittings) that could alter the chemical environment, the nature of the failure mechanism(s) and the physical environment (stresses, strains, temperatures, etc...). Fortunately, test methods are available, or can be developed, to validate proper product functioning in the intended end-use environment. The amazing variety of chemical environments in which piping products have been, and continue to be, very successfully employed is testament to the success of product developers and specifiers in applying the proper testing methodologies to validate product performance.