Long-Term Stability of Superabsorbent Gel for Cable Water-blocking ...

Long-Term Stability of Superabsorbent Gel for Cable Water-blockingPerformanceClinton E. Clyburn IIIStewart Superabsorbents LLCTaylorsville, NC, USA+1-828-632-5664 · kclyburn@stewartgroup.comAbstractCurrent cable water penetration requirements only examine theperformance of water-blocking superabsorbents in a dry state,either virgin or aged. With today’s robust designs, a leak may goundetected allowing the interior of a cable to be exposed to awater head for years. In this scenario, the long-term stability ofthe hydrated superabsorbent gel is critical to prevent watermigration down a cable to sensitive closure electronics.This paper subjects standard superabsorbents currently used forwater-blocking applications to accelerated aging tests in thehydrated state. These tests measure viscosity, flow and extrudatedrainage of the hydrated gel as a function of heat aging. Theresults are used to rank gel stability and highlight whichsuperabsorbent should perform best for long-term blockage.The superabsorbents degraded at various rates, from total loss ofgel integrity within 24 hours to stable gel structure for at least 14days. Given these differences in aging, it is essential to balanceinitial superabsorbent swelling performance with the ability of thesuperabsorbent to maintain water-blocking ability over time.Keywords: Superabsorbent; Water-Blocking; Dry; Aging; HeatStability; Water Penetration.IntroductionDry water-blocking materials for cables come in many forms,including powder, coatings, binders and tapes. They all use acrosslinked acrylate polymer as their active ingredient. Thissuperabsorbent polymer has a high affinity for aqueous-basedliquids and can absorb over 300 times its weight in distilled water.Since the 1990’s, dry water-blocking materials have beenincorporated into cable designs, including fiber optic and powerconduction constructions. These materials are tasked to stop theaxial penetration of liquid from a damaged outer jacket or splice. Inpower cables, water-blocking materials have replaced masticsaround the conductors and concentric wires. Initially used outside ofbuffer and core tubes in fiber optic cables to replace floodingcompounds, dry materials have migrated inside the tubes around theoptical fibers. They are found in virtually all outdoor optical cablestoday.The benefits of using dry water-blocking materials to replacemastics, flooding and filling compounds are numerous. Foremostare cost savings in cable manufacturing and installation.Superabsorbents are so effective, the volume required in a cable isa fraction of the compound it replaces. Cable weights aresubstantially reduced and diameters often decreased, resulting ineasier pulls into conduits. Installation, handling and splices arevastly simplified, as compounds do not need to be cleaned.Investigations of dry water-blocking materials in cable designshave been presented multiple times in the past. The consensus ofthese various studies is that dry cable designs typically improvethe prior art for the aforementioned reasons and meet currentcable performance standards. These standards measure theperformance of the superabsorbent in a dry, virgin state, prior toactivation by water from cable damage. This certainly hasrelevance in the ability of the materials to immediately stop water.What has not been examined is the ability of the superabsorbent tohold the water over time once activated.Current RequirementsWater penetration requirements vary depending on the cable typeand application. Outdoor fiber optic cables are typically governedby Telcordia GR-20 and FOTP-82 which allow a maximum axialpenetration of one meter when exposing the cable cross section toa one meter pressure head of water. Energy cables have a similarrequirement, one meter of cable exposed to 3-5 psi of water.GR-20 outlines water penetration testing of fiber optic cables inan unaged state and after an accelerated aging program of -40°Cto 70°C for two cycles, 168 hours at 85°C, followed by two morecycles. The accelerated aging is designed to mimic the cablelifespan of 20 years. The water-blocking materials are aged in adry state inside the cable before being exposed to water.IssuePrevious studies have shown some superabsorbent polymers to bequite stable in a dry state under aging at 80°C. They are stillcapable of meeting water penetration requirements afterprolonged heat exposure. Other superabsorbents showdegradation in performance after aging, typically in reductions ofswell height and capacity to absorb water. These are usuallysuperabsorbents characterized by initial high swell capacities andexpansion ratios. Conventional thinking in the industry is thatthose superabsorbents that absorb more and at a faster rate arebetter candidates for use in cables. However, testing has shownthat the actual penetration distance in a cable does not dependsolely on the swelling speed or expansion of the superabsorbent –other characteristics of the polymer (and its carrier) are importantfactors. This said, the proper selection of material can ensurelong-term dry performance if a cable jacket is cut many yearsafter installation.Cable constructions today are quite robust. The sheath of a cablecan be cut without disturbing the optical fibers or conductors inside.As long as the transmission is satisfactory, the cut can go undetectedand the outside water head will be present at this cut for the life ofthe cable.Of course, the larger fear would be the travel of water to a junctionwhere sensitive electronics are present. The long-term stability ofInternational Wire & Cable Symposium 147 Proceedings of the 59th IWCS/IICIT

diaper/hygiene market, hydrolytic stability of the crosslinker isnot a major concern. These materials see a short service life ofless than one day upon which they are disposed and replaced.Only a superabsorbent chemically tailored for hydrated stabilitywould use an appropriate crosslinker.Shape of the expanded superabsorbent can also have an impact.The jagged edges of the crystalline particles lodge to each othermore than spheres or cauliflower morphologies, resulting in betterviscosity and flow numbers. Even as the gel ages, this increasedresistance to movement can help in prolonging superior testresults and water-blockage.Chart 3. Gel mass lost from agingDiscussionPopular superabsorbents currently available for cable waterblockingapplications were sampled. These were superabsorbentsfound in water-blocking tapes, yarns/binders or applied directlysuch as with electrostatic applicators. The samples originatedfrom either solution or suspension polymerization, displayingvarious morphologies including spheres, porous structures andjagged shapes. The samples contained various types and levels ofcrosslinking, indirectly characterized by their absorption capacity.To maintain blockage over time in an undetected jacket cut, thepremise is that the superabsorbent hydrated gel must continue tobehave similar to its initial form and maintain a gel-type state.Degradation over time will allow liquid to penetrate past theblockage and progress further down the cable. This eventuallywill reach junctions with sensitive components such as repeatersand amplifiers reside, causing loss of service.To measure this aging integrity characteristic, three tests weredesigned to determine gel viscosity, flow and liquid/extrudateloss. Samples were hydrated to 50% of their maximumabsorption capacity in deionized water. Note that at only 50%hydration, a superabsorbent still has a significant hold onabsorbed water such that it requires a strong input of energy tofree this moisture. Also, note that the testing medium wasdeionized water, presenting the most favorable conditions for gelformation and retention. As it is known that ionic contentsdegrade superabsorbent swell performance, their presence in ahydrated aging study would likely magnify the results ofdeterioration [3].Our initial hypothesis suggested the superabsorbent with thehighest degree of crosslinking would display more stability in thetests. Heat is one factor which degrades polymers and a highernumber of beginning links in the polymer network would takemore time to revert to linear form. Results basically showed thisoutcome to be true as sample SAP-7, the most reticulatedpolymer, showed the least deviation from initial testing.In addition to crosslink density, it is thought that the type ofcrosslinker used in the superior performing superabsorbent offersmore resistance to hydrated aging, thus holding the polymernetwork together. As most superabsorbents are produced for theConclusionsHydrated superabsorbent gel stability was examined in a number oftests. These tests were designed to characterize the integrity of thegel in accelerated aging conditions of 80°C in a closedenvironment. The goal was to extrapolate aging conditions of atypical cable service life of 20 years.Results of the tests were very consistent in ranking the integrity ofthe aged superabsorbent gel. By far, the worst performer inviscosity drop, flow and extrudate drain was a fast swellingsuspension polymer commonly found in water-blocking tapes.With a low crosslinking density and high surface areamorphology, this superabsorbent was very sensitive to hydratedaging conditions and rapidly lost its gel integrity.The best candidate was a crystalline-shaped solution polymer withthe highest degree of crosslinking density, as determined byabsorption capacity. This reticulated polymer displayed verystable performance through the aging study and showed littlechange in gel formation for the first two weeks, while the othersamples suffered major degradation.For a full consideration of water-blocking performance employingsuperabsorbents, the proper candidate must encompass the dryswelling criteria outlined by GR-20 requirements. However, thismust be balanced by hydrated gel stability to ensure that thesuperabsorbent maintains a secure block in the case of anundetected jacket breach.AcknowledgmentsSpecial thanks to Alton Deaton and Martin Tennie of Evonik fortheir experimental contributions to this paper.References[1] N. Patel, “Functional Performance of Ocean Water BlockingTapes,” International Wire and Cable SymposiumProceedings, pp.136-146, (1998).[2] F. Buchholz and A. Graham, Modern Superabsorbent PolymerTechnology, Ed. Wiley-VCH, (1997).[3] A. Bringuier and C. Clyburn III, “Reliability of DryWaterblocking Materials,” International Wire and CableSymposium Proceedings, pp.779-787, (1996).[4] M. Gourmand and Y. Taupin, “Superabsorbent Polymers(SAP) for Cable Application, ” Index Congress 99, (1999)[5] J. Gruhn, “Characterizing and Selecting Superabsorbing CableComponents,” International Wire and Cable SymposiumProceedings, pp.126-135, (1998).International Wire & Cable Symposium 150 Proceedings of the 59th IWCS/IICIT

Clinton E. Clyburn IIIStewart Superabsorbents LLC200 Commerce DriveTaylorsville, NC 28681 USAClinton Clyburn (Kip) received a B.S. degree in MechanicalEngineering from North Carolina State University. He previouslyworked in the R&D departments of Raychem, Alcatel and Siecorbefore joining Stockhausen for superabsorbent development. Heis currently the Division Manager for Stewart Superabsorbents.International Wire & Cable Symposium 151 Proceedings of the 59th IWCS/IICIT