专业外语课后阅读第六课page-64--71-Adhesives

6Adhesivespage64--71-1-6Adhesivespage64--71Adhesivescanbeclassifiedaseithernaturalorsynthetic.1.NaturalNaturaladhesivesincludesubstancesofplantorigin(starchandsoyaglue)andanimalorigin(gluefromhidesorbones,casein,andbloodalbumin).Uptoabout1930,theuseofnaturalglueswasextensive(theyweretheonlyonesavailable),butafterwardtheywerereplaced,almostentirely,bysyntheticadhesives.Thisreplacementhastakenplacebecausenaturalglueslackdurabilityunderadverseconditions;whereassyntheticresinsareeasytouse,aresuperiorinresistancetomoisture,microorganisms,hightemperatures,andchemicals,andcomparativelycost-effective.Amongnaturalglues,onlycaseinhasimportantapplicationsinthewoodusingindustry.Therefore,thefollowingdiscussionofnaturalgluesisprimarilyofhistoricalinterest.(1)PlantOrigin①StarchGlue:TherawmaterialisstarchfromvarioussourcesmainlycornandCassawarootsrarelywheat,potatoes,orbananas.Theglueispreparedbymixingstarchwithwaterandthenheating.Insteadodheating,sodiumhydroxidemaybeadded,althoughahighproportionmaycausediscolorationoflightcoloredwoods.Starchglueisviscousandhardensbyevaporationordiffusionofmoistureinthewood;therefore,redryingofthegluedproductmaybeneeded.Thedisadvantagesofthisglueisthatthejoinsarenotresistanttomoisture,fungi,andinsects,althoughimprovementispossiblebytheadditionofasyntheticresin.②SoysGlue:Thisglueispreparedfromproteincontainedintheseedsofsoya,ansasiaticplant,theseedsofwhichareutilizedtoproduceoil,flour,andotherproducts.Inthetrade,itisofferedintheformofapowdercontainingotherchemicalsubstances,suchascalciumhydroxide,orsodiumhydroxide,orthesechemicalsareaddedduringpreparationbysimplemixingwithwater.Soyaglueischemicallysimilartocaseinglue.Ithardenspartlybylossofwaterandpartlybychemicalreaction.Itisalkaline,likecasein,butthejointsarecomparativelylessresistanttomoisture,andaresusceptabletoattackbyfungiandinsects.Itspropertiesmaybeimprovedbytheadditionofasyntheticresin.(2)AnimalOrigin①HideorBoneGlue:Thilsglueisknownasthemainanimalglue.Theadhesivecapacityisduetocollagen-aproteinfoundinhides,bones,andmusclesofanimals,especiallycattle.Animalglueisusedintheformofawarmaqueoussolutionatatemperatureofabout40~60℃.Toensureuniformheating,thecontainingvesselisheatedinawarmwaterhath.Hardeningtakesplacebycoolingandlossofmoisture.Thejointsarenotresistanttowaterorhighrelativehumidity,butmaybeimprovedbyadditionofchemicals,suchasoxalicacidandparaformaldehyde.Anotherdisadvantageisthatthegluemaybeattackedbyfungiandinsects.②CaseinGlue:Caseinisaproteincontainedinmilk.Itispreparedbyoxidation,whichseperatesasedimentfromwhey.Wheymaybeusedasanimalfood,whereasthesediment(rawcasein)iswashed,pressed,driedandreducedtopowder.Theglueispreparedbymixingsuchpowderwithwater,althoughliquidcaseinisalsoavailable.Theadditionofcalciumhydroxideincreasestheresistanceofjointstomoisture6Adhesivespage64--71-2-(insolublecalciumcaseinisformedintheglueline),whereassodiumhydroxideprolongsthe“workinglife”(i.e.theperiodduringwhichtheglueissuitableforuse).Otherchemicalsarealsousedtoimprovethejoints,orasantifoamadditives.Caseingluemaybeusedatlowtemperatures,evenout-of-doorsinthewinter(aslowasto0℃),andkeepsitsadhesivecapacityaspowder(manymonths),afterpreparation(severalhours),andafterspreading(upto1.5h).Theglueprovidesstrongjoins,butisnotsuitableforproductsexposedtotheweatherortoenvironmentsofhighrelativehumidity.Althoughjointsmaybeattackedbyfungiandinsects,improvementispossiblebytheadditionoftoxicchemicals.Machingofgluedwoodmaywearcuttingtools.Caseinglueisalkaline,itfacilitatesgluingsomewoodsandsurfacesthathaveagreasyappearance,butmaycausediscolorationoflightcoloredwoodsandskinirritationtoworkers.③BloodAlbuminGlue:Albumincontainedinthebloodofanimals,mainlycattle,possessadhesiveproperties.GluepreparedfromthissourcewasectensivelyusedduringthefirstWorldWarforbuildingwoodenairplanes,asthejointswereresistanttomoisture.Gluingrequiredhightemperatures(above70℃)and,therefore,hotpressing.2.SyntheticSyntheticadhesives,calledsyntheticresinsbecausetheirappearanceissimilartonaturalresins,areproductsofthechemicalindustry.Therawmaterialsnecessaryfortheirproductionarepetroleum,coal,ornaturalgases.Experimentalpreparationstartedin1872,andtheseadhesivesappearedonthemarketinthe1930s.Inthefollowingyears,theuseofsyntheticresinsincreasedcontinuouslyattheexpenseofnaturalglues.Thefirstsyntheticresinwasphenol-formaldehyde,whichoriginally(1929)appearedintheformofthinsheets,andlater(1935)asaliquid.Urea-formaldehydebecameavailablealmostsimultaneously(1931),melamine-formaldehydebytheendofthesamedecade,andresocinol-formaldehydealittlelater(1943).Othersyntheticresinsareproductsofthesameperiod(e.g.polyvinylemulsions,1929)butneweronesalsoexist.Syntheticresinsaredividedintothermosettingandthermoplastic.Undertheinfluenceofheat,thermosettingresinsatfirstsoftenandlaterhardenpermanently.Incontrast,thermoplasticresinsstaysoftaslongastheirheatingiscontinuedandhardenwhencooled.Thermosettingresinshardenbychemicalreaction,whichisacceleratedbyheatorcatalyst;whereasinthermoplasticresins,hardeningisaphysicalproce