paper and paperboard converting（下）

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1、Chapter6Extrusioncoatingandproductapplicationscoating-betterheat-sealability:extrudingthesurfacelayeratalowertemperaturereducestheoxidationofthepolymer.Selectionoftheheat-sealableskinpolymercanprovidegoodheatsealingpropertiesbyusing,forexample,anionomer-layerscontainingadditive

2、sandpigments:onlythesurfacelayerneedstocontainanyadditivestoreducecostandprotectthediemetal.Twodissimilarcolorsarealsopossible.-lessmanufacturingprocesses:machinetimesavings-polymersthataredifficulttoprocesscanberunsupportedbyotherpolymers-increaseincapacity-combinepolymershavi

3、ngspecialbarrierpropertiesasthinlayersasarenecessary-nonslipsurfacesorultralowheatsealtemperaturesbytheselectionoftheskin-polymer.6.3.4LaminatingsystemFigure24showsthelaminatingsystemconsistingofpressure,stripping,andchillrolls.Figure24.Structureofatypicalchill-rollinextrusionc

4、oating.Thepurposeofthelaminatingoperationistocreateadhesionbetweentheresinandthemovingwebandtosolidifytheresin33.Thelaminatingsystemnormallyhasapositionsuchthatonecanvarytheverticalandhorizontaldistancesfromthedieliptothelaminatingnip.Figure25showsthisairgap.Thetypicalairgapadj

5、ustmentprovidesanoptimumtimeforoxidationofthepolymermeltsurfacetooccur.Atypicalresidencetimeis100ms.Ahighresidencetimeleadstoexcessiveoxidation.Thisimprovesadhesionbutalsocausescoolingofthemelt.Suchcoolingcanweakenadhesion.Highresidencetimesathighlinespeedsrequireairgapsofappro

6、ximately30cmleadingtoalargeneck-in.Thiscauseswastefromthickandwideedgebeads.Figure25.Laminator33.Thesolidificationofthemoltenresininthecontinuousextrusioncoatingprocessrequiresefficientcoolingperformancefromthechillroll.Double-walled,spiralchillrollscanprovideadequatecooling.Th

7、etemperaturerangeofthecoolingwaterisoften15−25癈.Thesurfacefinishofthechillrolliscritical.Itdeterminesthereleasecharacteristics,theopticalproperties,andthecoefficientoffriction(COF)ofthecoating.Aglossyorpolishedfinishproduceshighopticalpropertiesofexcellentgloss,goodtransparency

8、,andlowhaze.PolishedfinishesalsoprovidehighCOFandpoors