ME 8883 lecture 2, caliper and porosityME 8883 Physical Properties of Paper Lecture 2• Paper caliper (thickness) – is the perpendicular separation between the two surfaces of a sheet• Paper is sold by caliper as a quality spec – uniformity in the MD and CD is a concern• Used to make “apparent” density calculations where Density = Basis weight/caliper – g/cm3if basis weight is in g/cm2and caliper is in microns• Density of pure cellulose is 1.55 g/cm3paper and paperboard products are in the range of 0.3 to 1.1 g/cm3ME 8883 lecture 2, caliper and porosity• the Tappi standard measurement specifies the use of parallel platens of a fixed size, applied at a prescribed rate, under a fixed pressure, the reading is taken after a fixed time after the platens contact the paper surfacePlaten diameter: 16 mmPressure: 50 kPA Time duration: 2 seconds• the parameters work to predict thicknesses of paper stacks, books, the method measures the “hilltops” of the paper surfaceME 8883 lecture 2, caliper and porosityME 8883 lecture 2, caliper and porosityL&W caliper gaugeLCD readout can be adjusted to produce a resettable running average of successive readingsPushbutton on the side raises the platenME 8883 lecture 2, caliper and porosityEmveco caliper gaugePut switch on “Run”when readyNumber of readings shown hereME 8883 lecture 2, caliper and porosityCalibration of electronic caliper gaugesUse the calibration shim standards in the black binderME 8883 lecture 2, caliper and porosity• Electronic gauges have a transducer whose electrical output is processed by a differential amplifier which has a gain and offset.• The linear output of the transducer, e.g. strain gauge is converted to engineering units on a readout using a gain and offset1. select a high and low caliper shim covering the range of interest to be measured. 2. Place the high shim in the caliper platen, adjust the gain or “Span” on the Emveco panel to get the readout value to coincide with the known high caliper shim thickness value3. Place the low reading shim into the caliper platen and adjust the offset or “zero” of the Emveco front panel to coincide with the low caliper shim value4. Reiterate steps 2 and 3, until no further adjustments are necessary.ME 8883 lecture 2, caliper and porosityTurns out that with the right neoprene platens “exercised” for 15 + minutes that the soft platen caliper coincides with the “effective”thickness concept, the platens must be continually cycling due to the viscoelastic nature of neopreneME 8883 lecture 2, caliper and porosityEffective thickness concept123EtBS =Bending stiffness of sheet can be measured by one instrument and tensile stiffness by another, (we will soon see how this is done).Bending stiffness BS per unit width of a sheet is related to the appropriate modulus EMDand caliper t by the relation :Tensile stiffness per unit width of a sheet TS measured from a tensile test (load displacement curve) TS = E x tCombining the 2 measurements and solving for t gives an effective t of TSBSt12=ME 8883 lecture 2, caliper and porosityEmveco soft caliper gaugeTwo sets of gain/slope adjustments Sample advancing wheelHard PlatenSoft platenME 8883 lecture 2, caliper and porosityUltrasonic longitudinal transducersNeoprene rubber coated platens Weight to provide Tappi caliper pressure LVDT transducer in the back measure platen separation Longitudinal C33 ultrasonic stiffness testC33 = V2/ρV is the velocity of soundwaves through the sheet. So the density is calculated from the basis weight divided by the mechanical equivalent of caliper or “soft”caliper.Velocity is “soft”caliper divided by timeME 8883 lecture 2, caliper and porosityCaliper relation to porosity• In papermaking processes, caliper is influenced by:1. Beating or refining of fibers2. Pressing and calendering3. Furnish additives4. Fiber type5. Sheet moistureIf caliper too low:Furnish changes can be made to increase bulk,Decrease refining, wet pressing or calender pressureIncreasing density increases smoothness and gloss as desired for printing papers but decreases opacity and paper porosityME 8883 lecture 2, caliper and porosityPorosityThe low apparent density of paper means that paper is about 50% air by volume, most of the air is within pores of the sheet structureRatio of pore volume to total volume is the porosity.We measure this indirectly as air resistance or air permeability.Air permeability can be measures by Sheffield, Bendtsen, Print-Surf instruments using different pressures, areas and clamping configurations.The inverse to air permeability, air resistance is measured by a “densitometer”Air resistance is generally related to strength properties, tensile burst, fold, inversely related to opacity. Most importantly porosity indicates how it will react to fluid penetration in coating or printing operations.ME 8883 lecture 2, caliper and porosityDescription of test•Paper specimen is clamped exposing 1 square inch of surface to an air pressure drop = 4.88 inches of water through the sheet. •Pressure is applied by a weighted cylinder freely floating in an outer cylinder f which is filled with oil for an air – seal ( Do not press down on the cylinder – oil will leak out !!)•Test consists of measuring the time in seconds for the flow of 100cc of air through the top orifice, through one inch of sample area through the bottom orifice., the reported value is Gurley seconds (t) or may be converted to micrometers per Pascal second by P = 135.5/tME 8883 lecture 2, caliper and porosityAnother check: run a test using a certified calibration flow restrictor plate compare read value to specified valueNote: some samples may display a “two sidedness” in air flow, number of repeats (on different non-overlapping areas of the sample should be 5 or greater , Tappi method says 10ME 8883 lecture 2, caliper and porosityL&W “densitometer”Cylinder flows under gravity providing air flow Timer – starts automatically once the cylinder has reached the start mark stop when 100 cc of air have flowedMove handle up to clamp specimen in placeME 8883 lecture 2, caliper and porosityThe upper part of the clamping mechanism contains an elastomeric gasket whose integrity should be inspected and checked. Always run a check test using the standardized orifice sample, should read 26 seconds +/- 5%ME 8883 lecture 2, caliper and porosityTop cylinder falls under gravity, do