30| PLASTICS ENGINEERING | OCTOBER 2011| www.4spe.orgment of tougher plastics and poly-mers."We had a paper published abouttwo years ago where we added carbonnanotubes that were functionalizedinto a polydicyclopentadiene matrixthat showed a 900% increase in thetensile toughness by adding less thana few percent of modified carbonnanotubes," Kessler notes.Polymer synthesis has facilitated thecreation of heretofore hard-to-producecomposites, thanks to the recentlydeveloped ruthenium catalyst thatpolymerizes that polymer, he adds.Polydicyclopentadiene "is a materialpolymerized by a ring-openingmetathesis polymerization that onlyreally was done in the late '90s whenthe Grubbs catalyst was developed,"Kessler explains. "That polymer isreally tough. It's a crosslinked ther-moset polymer. You can shoot a bul-let, and it may just get lodged into athick block of that thermoset. It's alsogot good toughness at lower tempera-tures, so it's used in snowmobile cowl-ings and hoods or parts of machinerythat used to be made out of maybe afiberglass composite, but now it's justmade out of a pure polymer becausethe polymer's tough enough and theycan make it without needing the fiberreinforcement." Meantime, the U.S. Army is doinga lot of work in composite armor,Kessler notes. "These are very com-plex composite systems that includeceramic tiles and polymer compositesand adhesives between the tiles tohave function where they can absorbhuge amounts of energy from a ballis-tic impact but be light enough to be adeployable force, (for example) a tankthat can be flown in an airplane."With self-healing polymers, "interms of applications there's beenmore in the area of coatings that healfrom a scratch," he says.One of Kessler's priorities is civilinfrastructure, primarily repairingsome of the thousands of miles ofaging pipeline crisscrossing the U.S."The traditional way to repair apipeline is to take that pipe offline,"Kessler notes. "The damaged sectionis cut out, and a new section is weld-ed into the spot. What we've devel-oped are external composite wraps.You can restore the structural integrityand strength of the pipeline."Some of the same technology isapplied to bridges and docks, hepoints out, with crews retrofittingdeteriorating infrastructure. Supersonic StrengthThe U.S. Department of Defense(DoD) has assembled so-called ero-sion working groups and indicatedthe need for a test facility for super-sonic rain erosion testing. Before thisyear, the only means of such testinghad been the "rocket sled" atHolloman Air Force Base in NewMexico, says Mike Spicer, programmanager for the AFRL's coatings lab."That's a very expensive test,"Spicer says. "They needed a less-expensive test facility that can weedout some of the poor polymer per-formers prior to going to the rocketsled."Using a Small Business InnovativeResearch program, the AFRL devel-oped a one-of-a-kind prototype fortesting materials against raindrops attwice the speed of sound. Such testingis primarily intended to evaluate thedurability of the leading edges of air-craft, "whether it's fixed-wing aircraft,missiles, or rockets," Spicer notes. "It'sjust about anything that could seerain droplets at supersonic speeds."The AFRL soon will develop a testprocedure and evaluate a number ofmaterials. "The main thing we test iscoatings: organic coatings or inorganiccoatings," he says. "Some of the sub-strates are polymers such as canopies,windshields, and things of thatnature."Prior to venturing into supersonictesting, the AFRL had engaged inplenty of subsonic testing-up toabout 650 mph-"that simulates aone-inch rainfall, and there are somemilitary specifications written aroundthat method." Heal ThyselfIn the early to mid-2000s, Arkemabegan exploring the idea of elastomersMichael R. Kessler
that featured strong butreversible chemical bonds.The final result, Reverlinkself-healing elastomers, is atriumph of supramolecularchemistry."Reverlink technologybegan as a joint researchprogram [with chemistryprofessor Ludwik Leiblerof the Paris-based Écolesupérieure de physique etde chimie industrielles dela Ville de Paris]," says Dr.Michael E. Smith, techni-cal development managerfor Arkema. "It was part of our corpo-rate initiative for novel materials, ide-ally biobased materials. Most of whatis in a Reverlink self-healing rubber isbiosourced: 70% to 80%, dependingon the grade." While many researchers have pur-sued the concept of assembling largemolecular structures from smallerbuilding blocks-without necessarilyhaving them be permanent -[Leibler] "came up with the idea oftrying to simulate plastic and rubberstructures where you have a controllednumber of reversible connections,"Smith says.Based on hydrogen bonds that aresimilar to the link between the basepairs of amino acids in DNA strands,Arkema's Reverlink technology wascreated with the idea that a linkerstructure could allow the attachmentof complementary hydrogen bonds toplastics and rubbery materials. The results, Smith says, are promis-ing: "You can imagine you have seals,where you've got some sort of gasketor connection, and if some eventoccurs (and) you can design it proper-ly so the pieces stay in place-even ifthere's crack-ing-as long asthey are in con-tact with eachother, they willreform theirbonds. [Othertechnologies] arevaluable for things like coatings,where you have wear and tear andchipping and such; you bring it in tothe base and you can very quicklyreseal or recure the coating."In comparing self-healing technolo-gies, Smith notes that unlike those thatrequire an activator, Reverlink technol-ogy "does not require heating (or) anysort of (UV) light or anything else. Youjust need to bring the cut surfaces backinto contact within about an hour, andthe actual chemistry will allow the con-nections to form."Prospects for the adoption ofReverlink materials are in the earlystages. "At the moment, the quantitythat can be generated is large enoughto do hundreds of pounds, thousandsof pounds," Smith says. "You canmake real trials in real-world applica-tions; it's not a lab material where youcan only get a one-pound jar."Arkema and its partners are workingon potential applications in the mili-tary/defense and commercial sectors.The FutureThe trend toward exploring non-petroleum, biobased polymers(derived from vegetable oils or chemi-cal feedstock that comesfrom biomass) like polylac-tic acid will present contin-uing challenges for materialtoughness, Iowa State'sKessler predicts. "Some of the vegetableoil-based thermosets thathave been developed in the past sever-al years could have application (in therepair of significant infrastructure)"-competing with unsaturated polyestersand epoxy resins, he says.Meantime, the AFRL's Spicer says,commercial polymer development isthe primary focus of research stem-ming from the supersonic rain erosionstudies as well as the AFRL's other testcapabilities. "We have developed aCRADA (Cooperative Research andDevelopment Agreement), and thatallows third-party polymer developersto use this facility to evaluate theirpolymers such that they can thenclaim the performance and then comeback and sell it to DoD or the com-mercial world," he says. "With oursubsonic rain erosion-and we dohave a dust and particle erosion testcapability-we do a lot of work forcommercial companies that developmaterials for both DoD and (commer-cial uses), and they come through theCRADA and get the testing done."Noting that not even NASA has thetest capability of the AFRL, Spicersays: "We have some customers wait-ing in the queue. They are probablygoing to come in here [soon] to testmaterials to develop parameters fortest methods that we'll put into amaterial specification. We will eventu-ally be developing better polymersbecause we've got some applications(in which) polymers aren't performingas we would have hoped."www.4spe.org | OCTOBER 2011| PLASTICS ENGINEERING | 31Photos courtesy of PBI Performance Products.