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GT ME 8883 - ME 8883 Physical Properties of Paper Measurement Lecture 8b Zero SPan and ZDT1

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ME 8883 Physical Properties of Paper MeasurementZD tensile testZD tensile test - considerationsZD tensile test considerationsZD tensile strength considerations1ME 8883 Physical Properties of Paper MeasurementZD tensile test and Zero Span2ZD tensile test• This is an out-of-plane tensile test• Measures the amount of stress it takes to pull apart the sheet in the ZD a measure of the internal bond strength of paper• dependent on how well the paper bonds to the double sided tape• Some confusion with other ZD tests: Scott bond and peel tests3ZD tensile test - considerations• Z directional stress-strain curve: the elastic modulus in ZD is small, elastic region is short, plastic deformation starts almost immediately, breaking strain is small less than 0.5%100 NDeformation in compression Schaffrat, 1991 International Paper Physics ConferenceForce10 N10 um5 umThis is what you get in printing or scoring comes from decreasing void volume4ZD tensile test considerationsStress MPa0.40.80.1 0.3• elastic modulus in ZD tension is 0.5 GPa about 1/10 of typical in-plane moduli• results from press drying of paper conventionally 0.05 – 0.1 GPa• in-plane modulus is 3 – 7 GPa depending on the cellulose content• Plastic deformation begins when fiber bonds begin to open• typical fiber dimension: 0.01 thick, 0.02 mm wide 1 mm long surface area to cross section are is 300 so perfect bond strength would be 0.3% of fiber strength• paper is loaded in the ZD in applications like offset printing, corrugating and folding operations5ZD tensile strength considerationsF• must ensure uniform load aplicationover face of the sample• tensile forces must be perfectly aligned through the center• lateral contraction is restrained so a transverse stress also exists paperAdhesive double sided tapeF• not a measure of intrinsic fiber to fiber bond strength: numberand size of bonds is unknown, not only bond failure: fiber wall fracture may be a part of the failure,6Scope:Equipment requirements:Note: no specific requirements on the tape properties – problem here !!7ZDT testing detailNote about the tape:Procedure:Note on applying the tape: using a hand roller with pressure to get the tape well stuck to the paper specimen no air bubbles, folds or gaps !!8ZDT test detail: TMI instrumentPress IDThen when the sample is ready and in place, press TEST9ZDT test detailPlace the specimen with double sided tape onto the bottom platen, remove tape backing Removable clevis pins for cleaning platens between tests10ZDT test detailUse the roller to firmly seal the specimen against the double sided tapeDouble sided tape dispenser11Other out-of-plane measurementspendulumAngle Al piece adhered to paper specimen with double sided tape ∆h is a measure of the delamination energy in the Scott bond testThe adhered right angle measures the delamination energy – resembles practical situations e.g. self adhesive labels, price tags, etc.12Other out-of-plane measurementsFPeel wheel, Skowronski 1991 Int. Paper Physics Conference measures the energy consumption during a stable and controlled delamination.Adhesive tape Ball bearing mountAt constant basis weight, the delamination test correlate well with each other13Comparison of ZDT tests• at very low basis weights get just the strength of the tape• above 80 gsm constant Z strength no change in bond structure• increase in Scott bond and delamination energy beyond 8- gsm is due to plastic deformation and energy consumed in bending the sampleFactors affecting ZDT• felted versus layered structure: interwoven fibers increase the ZDT since fibers are stronger than bonds • Example: at high consistency forming, you can et a felted structure:ZDT will be high but tear and tensile will be low compared to low consistency formed layer structureIncreasing the bonded area increases ZDT more so for conformable flexible fibers 1415Factors affecting ZDTThis shows that iincreasing filler (kaolin clay) increases light scattering (e.g. opacity) but decreases ZDT due to interference with bondsA higher beating level (Schopper Reigerfreeness) increases the fiber bond are and ZDT16Dennison wax pick test T459• Sealing wax sticks with different picking strength (indicated by the stick ID number) is adhered to the paper by melting the end of wax sticks by a flame• The wax number at which the paper surface does not break when removing the wax stick indicates the surface strength17Wooden paddle with hole to hold down paper upon pulling wax stickAlcohol burnerWax sticks numbered 2 through 24 on the narrow end18Zero span pre-amble• 1925 Hoffman-Jacobsen introduced this idea to distinguish the difference between the strength of the fiber network and the strength of the fibers • Assumed that fibers are being pulled apart• Research indicates that zero span is not a fundamental measure of fiber strength but isan indicator • Experimental data on individual fibers and handsheets made formthe same fibers suggestrelationships• Overall tensile load is transferred into the sheet by shear stresses –therefore need to have a lot of clamp pressure to avoid this…19Zero span test philosophyStress of sheetForce on one fiberStress on one fiber20Zero span test philosophyStress strain diagram for N fiber segments is norientatiofiberareafibertionalcrossAuluselasticfiberEsheettheofareationalcrossAffψsecmodsecFibers with orientations perpendicular to grip edges fail firstThe stress is then shifted to remaining fibers with larger orientation angles21Zero span test -preambleΤb -Breaking stress of a fiber of width wf-this is called the “Page equation”22Zero span test -preambleGauge length is zero so failure is by fiber rupture only If the fibers are all aligned in the direction of applied tension then the zero span tensile strength is :strainfailurefiberuluselasticeffectiveEEZfsfsεεmod=ffsffffEEbetospanshortforulusAssumeEAFstrengthFiber⎟⎟⎠⎞⎜⎜⎝⎛⎟⎠⎞⎜⎝⎛⎟⎠⎞⎜⎝⎛==ρρε3189:mod23Then the connection between Zero span and fiber strength is:Factors such as fiber curl may alter zero span strengthg without a change in the true fiber strength24Use of the zero span testCowan and Cowdrey (1974) developed four indices based on wet and dry curves. 1. Fiber Strength Index (FSI) – intercept of wet and dry pulp sheets as the span is extrapolated to a “true” zero span2. Fiber Length Index M (FLI) – distance from negative span R to the position on


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