专业外语课后阅读第四课page-42--44-Preservation-of-Wood

4PreservationofWoodpage42--444PreservationofWoodpage42--44Woodhasmanyuniquecharacteristicswhichmakeitavaluablematerialforconstruction.However,inapplicationswhereitisexposedtowetconditionnsitissubjecttodegradationbymicroorganisms.Inaddition,itisvulnerabletoinsectattackinareaswherethewood-destroyingspeciesexists,anditisalsosusceptibletofire.Fortunately,woodpreservativesystemshavebeendevelopedwhichincludeseveryprocessofchemicalorphysicaltreatmentwhichundertakentoextendthefireofwoodinservicebyincreasingitsresistancetobiologicalattack,fire,shrinkageandswellingduetochangesinmoisturecontentand,infact,toanyoftheagenciesmentionedabove.1.WoodPreservativesWoodpreservativesaregenerallyclassifiedintobroadgroups,namely,oilborneandwaterborne.Themajorpreservativesintheoilbornegrouparecreosote,whichisanoilyliquidthathasasatisfactoryviscosityforwoodtreatment,andpentachlorophenol(penta)whichisoil-soluble.Coppernaphthenate,tributyltinoxide(TBTO)andcopper-8-quinolinolate(Cu-8)arealsooil-solublecompoundswhichhaveminorusesforspecialapplications.Anewbiocide,propynylbutylcarbamate,hasbeenevaluatedasawoodpreservativeandappearstohavepotentialforabove-groundapplications.Thewaterbornepreservativesaremainlyinorganicsaltsandincludethefireretardants.(1)OilbornePreservativesThehydrocabonsolventsusedtodissolvetheoilbornepreservativesfortreatmentofwoodrangefromheavyoilstoverylightsolvents,andhavesignificanteffectsontheendproduct.ThesolventscurrentlybeingusedaredescribedbyAmericanWoodPreservingAssociation(AWPA)StandardP9.Abriefdescriptionofthevarioustypesofsolventsisasfollows:TypeA:heavy-oil-typehydrocarbonTypeB:liquidpetroleumgas(volatilesolvent)TypeC:lightpetroleumsolvent(volatilesolvent)TypeD:methylenechloride(volatilesolvent)(2)WaterbornePreservationThemajorwaterbornepreservativesareinorganicsalts.Theseincludechromatedcopperarsenate(CCA),ammoniacalcopperarsenate(ACA),acidcopperchromate(ACC),chromatedzincchloride(CZC)andfireretardants.CCAandACAarebyfarthemostwidelyusedpreservativesinthisgroupbecauseoftheirsuperioreffectivenessagainstwood-destroyingorganismsandthefactthattheyundergochemicalreactionsinthewoodwhichmakesthemnonleachable.nonleachable:不渗析的，不沥滤的InsomecasesTBTOandCu-8areformulatedasaqueousolutions,dispersionsoremulsionsandsotheycouldalsobeclassifiedaswaterbornepreservatives.Furthermore,becauseoftherecentsurgeinthecostofpetroleumsolvents,severalwaterbornepentaformulationsarebeingdevelopedandthesesystemsmaywellbeusedextensivelyinthefuture.Althoughwoodpreservativesarenormallythoughtofasprovidingprotectionagainstdecaybymicroorganisms,fireretardantscanalsobeclassifiedaswoodpreservativesforprotection4PreservationofWoodpage42--44againstthermaldecomposition.Therearetwobasictypesoffireretardantsandthesearegenerallyclassifiedasinteriortypes.Theinteriorfireretardantscontainmixturesofinorganicsaltssuchasammoniumphosphates,ammoniumsulfate,boraxandzincchloride.Theactualformulationsusedbythevariousmunufacturesareproprietarybutareknowntocontainsomeorallofthechemicalslistedabove.Thecommerciallyavailableexteriorfireretardantsutilizereactivecomponentsthatforminsolublecompoundsinthewoodwhentheyarecuredathightemperatures.Thistreatmentresultsinanonhygroscopic,noncorrosive,paintableproductthatcanbeusedforsuchitemsascedarshakesandshinglesandexterior-gradelumberandplywood.Acceleratedlaboratorytestshaveknownthatthecurrentlyavailableexteriorfire-retardanttreatedwoodprovidesareasonabledegreeofresistancetobothdecayandtermites.Hence,thismaterialcanbeusedforabovegroundapplicationswithoutadditionalpreservativetreatment.2.TreatingMethodsEssentiallytherearetwomethodsofimpregatingwoodwithapreservativesubstance-pressuretreatingandnonpressuretreating.Ingeneralthenonpressureprocessesareoflowercost,however,thetreatmentstendtobesuperficialandthedegreeofprotectionisthereforelower.Inorderforpreservativestobeeffectivetheymustbeappliedtothewoodinamannerthatprovidesuniformdistributionandsufficientpenetrationtocontrolthewooddestroyingagents.Thisiscariedoutbydissolvinganddilutingthepreservativewithasuitablesolventwhichisthenimpregnatedintothewoodbypressuremethods.Pressuretreatmentofwoodisbyfarthemosteffectivewaytotreatwoodwithpreservativesbecausedeeperpenetrationandclosercontroloverpreservativeretentionsandpenetrationcanbeobtained.Thisprocessiscarriedoutinlargepressurevesselsbycompletelyfloodingthewoodwithpreservativesolutionandapplyingpressure.Followingthisthesolutionisreturnedtostoragetanksforuseinsubsequentcharges.4PreservationofWoodpage42--44