4Product Shaping / Secondary Operations EXTRUSIONFinal Product (pipe, profile)Secondary operationFiber spinning (fibers)Cast film (overhead transparencies,Blown film (grocery bags)Shaping through diePreform for other molding processesBlow molding (bottles),Thermoforming (appliance liners)Compression molding (seals)
5FibersA Fiber is a long, thin thing!Aspect ratio >100At diameters > 75 , the fiber is a rodLong means:> 1 kilometerAt a density of 1.4 and a denier of 5, 1 kilometer weighs less than 5 grams> 1 kilogram1.5 kilograms at 5 dpf is 20,000 milesFew commercial fibers are produced at a scale of less than 500 tonsThe length at 5 dpf is ~ .01 lightyearTypical melt spinning speeds are in excess of 100 miles/hourTo be viable, polymer to fiber conversions must be ~ 90%Minimum property CVs are < 10%Real fibers are hard to make!!
6Griffith’s experiments with glass fibers (1921) MACROSCALE vs MICROSCALEGriffith’s experiments with glass fibers (1921)FIBER DIAMETER (micron)Strength of bulk glass: 170 MPaExtrapolates to11 GPa123TENSILE STRENGTH (GPa)20406080100120
7Griffith’s equation for the strength of materials a = length of defectg = surface energyThus, going from the macroscale to the atomic scale (via the nanoscale), defects progressively become smaller and/or are eliminated, which is why the strength increases (see equation).Note that the Griffith model predicts that defects have no effect on the modulus, only on strengthBut note: the model also predicts that defects of zero length lead to infinitely strong materials, an obvious impossibility!
8Fibers 1000 X longer than diameter Often uniaxial strength Kevlar-strongest organic fiberMelt spinning technology can be applied to polyamide (Nylon), polyesters, polyurethanes and polyolefins such as PP and HDPE.The drawing and cooling processes determine the morphology and mechanical properties of the final fiber. For example ultra high molecular weight HDPE fibers with high degrees of orientation in the axial direction have extremely high stiffness !!Of major concern during fiber spinning are the instabilities that arise during drawing, such as brittle fracture and draw resonance. Draw resonance manifests itself as periodic fluctuations that result in diameter oscillation.
9TABLE 4.2. Fiber Propertiesa Fiber TypeNaturalCottonWoolSyntheticPolyesterNylonAromatic polyamide(aramid)cPolybenzimidazolePolypropylenePolyethylene (high strength)InorganiccGlassSteelTenacityb(N/tex)0.272.65d0.31SpecificGravity1.501.301.381.141.441.430.900.952.567.7aUnless otherwise noted, data taken form L. Rebenfeld, in Encyclopedia of Polymer Science and Engineering (H. f. Mark,N. M. Bikales, C. G. Overberger, G. Menges, and J. I. Kroschwitz, Eds.), Vol. 6, Wiley-Interscience, New York, 1986,ppbTo convert newtons per tex to grams per denier, multiply by 11.3.cKevlar (see Chap. 3, structure 58.)dFrom Chem. Eng. New, 63(8), 7 (1985).eFrom V. L. Erlich, in Encyclopedia of Polymer Science and Technology (H.F. Mark, N. G. Gaylord, and N. M. Bikales,Eds.), Vol. 9, Wiley-Interscience, New Uork, 1968, p. 422.
10Polymer fibers Nylon PP, PE Normal spinning Melt spinning Super stretchingHMWPEWetspinningUHMWPEFlexiblemoleculesDyspinningCelluloseAcetateOrganicpolymersMeltspinningAromaticpolyestersStiffmoleculesWetspinningAramides
11Fibers Dry Spinning: From solution Melt Spinning: From Melt Wet Spinning: From solution into solutionKevlar, rayon, acrylics, Aramids, spandexCellulose AcetateNylon 6,6 & PETE
12Fiber Spinning: MeltBobbinExtruded Fiber Coolsand Solidifies HereMetered Extrusion(controlled flow)Melting ZonePolymer Chips/BeadsPumpFilter and SpinneretAir DiffuserHeating GridPoolLubrication by oil disk and troughPackagingBobbin driveYarn driverFeed rollsMoisture ConditioningSteamChamberFiber spinning is used to manufacture synthetic fibers. A filament is continuously extruded through an orifice and stretched to diameters of 100 mm and smaller. The molten polymer is first extruded through a filter or “screen pack”, to eliminate small contaminants. It is then extruded through a “spinneret”, a die composed of multiple orifices (it can have 1-10,000 holes). The fibers are then drawn to their final diameter, solidified (in a water bath or by forced convection) and wound-up.Nylon 6,6 & PETE
13Cellulose AcetateDry Spinning of Fibersfrom a Solution
25Aramide fibers the complete spinning line H2SO480 wt%icemachineLong washing traject(initially difficult to control)Sometimes post-strech of 1%to enhance orientationH2SO4 icePPD-T20 wt%mixerextruderair gapH2OspinneretWashingcsulf.ac. < 0.5 %neutralisingwindingdrying2000CH2SO4 + H2O
26Strong fibers from flexible chains Super-stretched polyethylene:Mw = 105 (just spinnable)conventional melt spinningadditional stretching of 30 to 50 timesbelow the melting pointWet (gel) spinning of polyethyleneMw = 106 (to high elasticity for melt spinning)decalin or parafin as solventformation of thick (weak) fibers without stretchingremoval of the solventstretching of 50 to 100 times close to melting point
27POLYETHYLENE (LDPE) Molecular Weights: 20,000-100,000; MWD = 3-20 density = g/cm3Highly branched structure—both long and short chain branchesTm ~ 105 C, X’linity ~ 40%15-30 Methyl groups/1000 C atomsApplications: Packaging Film, wire and cable coating, toys, flexible bottles, housewares, coatings
28Polyethylene (HDPE) Essentially linear structure Few long chain branches, methyl groups/ 1000 C atomsMolecular Weights: 50, ,000 for molding compounds250,000-1,500,000 for pipe compounds>1,500,000 super abrasion resistance—medical implantsMWD = 3-20density = g/cm3Tm ~ C, X’linity ~ 80%Generally opaqueApplications: Bottles, drums, pipe, conduit, sheet, film
29UHMWPE fibers: Dyneema or Spectra Gel spinning processDyneema(r), the worldﾕs strongest fiberDSM Dyneema is the inventor and manufacturer of Dyneemaｨ, the world's strongest fiber. Dyneemaｨ is a superstrong polyethylene fiber that offers maximum strength combined with minimum weight. It is up to 15 times stronger than quality steel and up to 40% stronger than aramid fibers, both on weight for weight basis. Dyneemaｨ floats on water and is extremely durable and resistant to moisture, UV light and chemicals. The applications are therefore more or less unlimited. Dyneemaｨ is an important component in ropes, cables and nets in the fishing, shipping and offshore industries. Dyneemaｨ is also used in safety gloves for the metalworking industry and in fine yarns for applications in sporting goods and the medical sector. In addition, Dyneemaｨ is also used in bullet resistant armor and clothing for police and military personnel.Structure of UHMWPE,with n = 100, ,000
30Comparison of mechanical properties Strength Modulus stretch(Gpa) (Gpa) (%)Classical fibresnylonglasssteelStrong fibressuperstretched PEwet spun PE (Dyneema)melt spun PE (Vectran)wet spun aramideidem with post-stretchRelative Flexlife: Dyneema 100, Vectran 55, ﾊAramid 8.
31Aramide fibers the spinning mechanism polymer inpure sulfuric acidat 850CSpecific points:solvent: pure H2SO4polymer concentration 20%general orientationin the capillaryextra orientation inthe air gapcoagulation in cooleddiluted sulfuric acidplatinumcapillary 65air gap 10 mm withelongational stretch (6x)coagulationbath at 100Cremoval ofsulfuric acid
32VectranVectran fiber is thermotropic, it is melt-spun, and it flows at a high temperature under pressure
41Electrospinning of Fibers 5-30 kVDriving force is charge dissipation, opposed by surface tensionForces are lowLevel of charge density is limited by breakdown voltage – Taylor cone formationFiber diameter [Voltage]-1“Inexpensive” and easy to form nanofibers from a solution of practically any polymer (Formhals 1934)Only small amount of material required
51Fig. 4. Scanning electron micrograph of a dry ribbon deposited on a glass substrate. The black arrow indicates the main axis of the ribbons, which corresponds to the direction of the initial fluid velocity. Despite the presence of a significant amount of carbon spherical impurities, SWNTs bundles are preferentially oriented along the main axis. Scale BAR=667 nm