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1、Chapter22Coatingcolorpreparation(D.S.35%)Total1.25MUSD22.13DeaerationofcoatingcolorsAlltheeffectsofaironthecoatingprocessarenotknown,buttheyaregenerallyconsiderednegativeorneutralatbest.Airmakescoatingcolorfoaminthemachinecirculationloopandleadstotankoverflowintheworstc
2、ase.Foamingispreventedwithcostlychemicals,whichalsoaddtotheenvironmentalload.Airissuspectedtonegativelyaffectrunnability,especiallyathighspeeds.Ithasbeennotedasapossiblereasonfordecreasedcoverageofcoatingcolor.Coatingcolorviscosityisalsoincreasedbyair.Themostevidentnega
3、tivephenomenonis,however,theuncoatedspotscausedbytheentrappedair.Togetherwithaninsufficientamountofappliedcolor,airbubblesarethemaincauseforacoatingdefectcalledskipping.Theformofanairskipisnormally"two-tailed"sothatthe"tails"pointtowardthemachine(direction).Anothersourc
4、eofairisthepaperweb.Whenairisreleasedfromthepaperpores,itleavessmall,roundcratersintheprintedpaper24.Theaimofthepapermakeristocontroltheamountofairsothatitwillnotcauseanyqualitydefectsorotherproblems.Thetraditionalwayofhandlingtheproblemhasbeentheuseofchemicals,whichcan
5、causetroubleinoffsetprinting.Themachinecirculationlayoutandpipingdesignplayacrucialpart,too.Someprocessequipmentsuchaspressurescreens,forinstance,removesairtosomeextent.Becauserequirementshavebecomestricterinthepastfewyears,wehaveseentheemergenceofspecialdeaerators,whic
6、hactivelypreventairfromaccumulatinginthecoatingcolor.Deaerationunitsreducetheaircontentincoatingbyhalf.Itisclear,though,thatallaircanneverberemovedwithoutdestroyingthecoatingcolorstructure.Thedegreeofharmoftheremainingairdependsmostlyonthebubblesizedistribution.Pilottes
7、tswithjetapplicatorcoatershaveshownthatabubblewithadiameterof1mmwillprobablycauseadefect.Bubbleswithasizebetween0.2and1.0mmdonotnormallycausedefects,buttheyhaveatendencytojointogethertoformbiggerharmfulbubbles.Thetargetindeaerationisthussettoremovebubbleslargerthan0.2mm
8、24.22.13.1ThephysicsofairincoatingcolorsAirbehaviorinaliquidsubstancecanbedescribedbythefollowing,wellknownphy
