By Clinton E. Clyburn III, Stewart Superabsorbents LLC
Dry water-blocking materials for cables come in many forms, including powder, coatings, binders and tapes. In power cables, water-blocking materials have replaced mastics around the conductors and concentric wires. The benefits of using dry water-blocking materials are numerous: cost savings in manufacturing and installation, effectiveness and reduction in cable weight.
Previous investigations of dry water-blocking materials in cable designs have found that the materials meet current cable performance standards. These standards measure the performance of the superabsorbent in a dry, virgin state, prior to activation by water from cable damage. This certainly has relevance in the ability of the materials to immediately stop water. What has not been examined is the ability of the superabsorbent to hold the water over time once activated.
Current Requirements and Issues
Water penetration requirements vary depending on the cable type and application. Previous studies have shown some superabsorbent polymers to be quite stable in a dry state under aging at 80 C. They are still capable of meeting water penetration requirements after prolonged heat exposure. Other superabsorbents show degradation in performance after aging, typically in reductions of swell height and capacity to absorb water. These are usually superabsorbents characterized by initial high swell capacities and expansion ratios. Conventional thinking in the industry is that those superabsorbents that absorb more and at a faster rate are better candidates for use in cables. However, testing has shown that the actual penetration distance in a cable does not depend solely on the swelling speed or expansion of the superabsorbent. Other characteristics of the polymer (and its carrier) are important factors. So, the proper selection of material can ensure long-term dry performance if a cable jacket is cut many years after installation.
The long-term stability of the superabsorbent blockage, a hydrated gel, is an important factor in preventing the migration of water down the cable over time.
Superabsorbent Polymers and Testing
Current superabsorbent polymers used in cable applications are typically crosslinked acrylates, neutralized with sodium or potassium salts. Differences in manufacturing methods determine performance characteristics such as absorption capacity, absorption speed and ability to swell against pressure. For a cable application, a superabsorbent must be able to swell enough to block the interstitial spaces of a cable core with sufficient speed to stop the water ingress within one meter. Additionally, the resulting superabsorbent hydrated gel must be strong enough to hold the head of water and prevent migration over time.
Superabsorbents for cable water-blocking require a combination of these performance metrics to block water within requirements. However, the best performing materials in a dry virgin state may not be the best in an aged hydrated state.
To measure the gel’s integrity and stability to replicate real-life conditions of the blocking mechanism in a hydrated state, tests were conducted. Measurements included viscosity, extrudate and flow.
Eight different commercially available superabsorbents currently used in cable water-blocking were selected. These grades are incorporated in cable designs through direct powder application, water-blocking tapes and binders. Not all samples were used in every test.
Aged Gel Viscosity
The first test examined the viscosity change of a hydrated gel as a function of aging. For each sample, the maximum water absorption capacity in deionized water was determined using an industry standard test. Each superabsorbent was then hydrated in deionized water to 50 percent of maximum for a total test volume of 250 cc. A beginning viscosity was obtained using a Brookfield viscometer (Spindle 5, 20 rpm).
Afterwards, the samples were put into a closed container and placed in an oven at 80 C to simulate aging. The viscosity of each sample was tested after one, five, six, 10, 14 and 76 days. The samples were removed from the oven and allowed to cool to room temperature. Afterward, the viscosity of each sample was measured and the samples were returned to the oven.
The change in viscosity was used as the indicator of gel stability. If the superabsorbent exhibited a large drop in viscosity it was considered to have low gel stability and vice versa. The results of the aging experiment showed dramatic differences in gel stability over time (see Figure 1).
All samples tested exhibit high viscosity when first made in the laboratory. However, after only one day in the simulated aging conditions, six of the superabsorbents being tested showed a significant drop in their viscosity. Samples SAP-4 and 6 retained only about 20 percent of their viscosity from the day before and samples SAP-1, 2, 3 and 5 dropped to near-water levels. By day five, sample SAP-4 also showed no gel structure. After 14 days, sample SAP-6 viscosity decreased to near-water value. The dramatic decrease of viscosity with aging at 80 C is interpreted as a sign of low gel stability and increase of the water-front inside the cable.
Only the sample SAP-7 displayed resistance to the simulated aging conditions. In fact, its viscosity was stable for at least two weeks. Even after 76 days of 80 C aging, sample SAP-7 displayed a viscosity of 2500 mPas.
Gel Flow Time
In similar fashion to the viscosity test, samples were hydrated to 50 percent of maximum absorption in deionized water. The hydrated gel was carefully poured into a polyethylene funnel with a 120 mm top inner diameter, 12 mm bottom exit diameter and 45 degree walls. After settling three minutes, the gel was allowed to flow out of the funnel and the total time to empty was recorded. The gel was then placed into a closed container and aged at 80 C. Samples were removed every 24 hours for four days and allowed to cool to room temperature before repeating the test.
Similar to the viscosity test, dramatic differences in flow times were observed after the first day of aging (see Figure 2).
All initial samples exhibited low propensity to flow and were quite slow to exit the funnel. After only one day, samples SAP-1, 4 and 5 flowed immediately out of the funnel. Sample SAP-8 displayed more resistance after 24 hours but was also water-like after 48 hours. Only sample SAP-7 maintained similar exit times across the test period with a high resistance to gravity flow. One would expect similar resistance to the pressure head experienced in a water penetration test.
Four samples of each test superabsorbent were prepared as per the previous tests, 50 percent of maximum absorption in deionized water. These were placed in closed containers inside an 80 C oven. At 24 hour intervals, a sample of each was removed from the oven and allowed to cool to room temperature.
The hydrated gel mass was poured into a conical wire mesh filter with 100 micron openings. Any liquid extrudate from the gel was allowed to drain for three hours and collected. The liquid extrudate was weighed and compared to the original gel mass to determine the percent lost. The gel with higher stability retains more of its absorbed liquid under aging and subsequently prevents further water propagation (see lead art for this story as an example of the test).
After only 24 hours of aging, sample SAP-5 released 96 percent of the water it had absorbed. Other samples released up to 40 percent in the first day. Lost extrudate continued to climb each subsequent day such that samples SAP-1 and SAP-4 had also drained most of their absorbed liquid by day four. Sample SAP-8 was relatively stable at first. It retained about 80 percent of its water after 96 hours but showed an increased rate of water release at day four. Only sample SAP-7 showed consistent extrudate weight with only 3 to 4 percent loss during the test period (see Figure 3).
Discussion and Conclusions
The samples contained various types and levels of crosslinking, indirectly characterized by their absorption capacity. Our initial hypothesis suggested the superabsorbent with the highest degree of crosslinking would display more stability in the tests. Heat is one factor which degrades polymers and a higher number of beginning links in the polymer network would take more time to revert to linear form. Results basically showed this outcome to be true as sample SAP-7, the most reticulated polymer, showed the least deviation from initial testing.
In addition to crosslink density, it is thought that the type of crosslinker used in the superior performing superabsorbent offers more resistance to hydrated aging, thus holding the polymer network together. As most superabsorbents are produced for the diaper/hygiene market, hydrolytic stability of the crosslinker is not a major concern. These materials see a short service life of less than one day upon which they are disposed and replaced. Only a superabsorbent chemically tailored for hydrated stability would use an appropriate crosslinker.
Shape of the expanded superabsorbent can also have an impact. The jagged edges of the crystalline particles lodge to each other more than spheres or cauliflower morphologies, resulting in better viscosity and flow numbers. Even as the gel ages, this increased resistance to movement can help in prolonging superior test results and water-blockage.
Results of the tests were very consistent in ranking the integrity of the aged superabsorbent gel. By far, the worst performer in viscosity drop, flow and extrudate drain was a fast swelling suspension polymer commonly found in water-blocking tapes. With a low crosslinking density and high surface area morphology, this superabsorbent was very sensitive to hydrated aging conditions and rapidly lost its gel integrity.
The best candidate was a crystalline-shaped solution polymer with the highest degree of crosslinking density and a hydrolytically stable crosslinker. This reticulated polymer displayed very stable performance through the aging study and showed little change in gel formation for the first two weeks, while the other samples suffered major degradation.
For a full consideration of water-blocking performance employing superabsorbents, the proper candidate must encompass the dry swelling criteria outlined by the relevant cable requirements. However, this must be balanced by hydrated gel stability to ensure that the superabsorbent maintains a secure block in the case of an undetected jacket breach.
Clinton E. Clyburn III is division manager for Stewart Superabsorbents. He previously worked in the R&D departments of Raychem, Alcatel and Siecor before joining Stockhausen (now Evonik Stockhausen) for superabsorbent development.
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