鞣制化學(xué)

9ChromeTanning

Inchrometannage,pickledpeltistreatedwithbasicchromesulphate.

Eachchromecomplexmoleculereactswithseveralcarboxylgroupsofcollagentoproducecrosslinkingbetweendifferentmoleculechainsofthecollagen.

Thiswillstrengthenthefibernetworkofthepickledpeltandconvertthepeltintochromeleather.9.1

ConventionalChromeTanningProcess

Inthisprocessthechrometanningisnormallycarriedoutinpicklingbath(orapartofpicklingbath).

WhenthehideshaveattainedanequilibriumpicklingaboutpH3,thechrometanningagent(33%basicity)isadded.

ThetanningcommenceswiththepeltsurfaceatpHabout3.0andbasicityabout33%.

AtthislowpH,thehideprotein,sinceitsionizationisrepressed,haslittlereactivitywithchromium.Thechromiumsaltwillenterintotheskinandwillbefixeduponthegrainandfleshlayers.

Asthetanningproceeds,moretanningmaterialsaredepositedonthegrainandflesh,andlesswillbedepositedinthecenter,inaccordancewiththenormaldiffusionpattern.

Afterthechromehaspenetratedtheinterioroftheskin(asshownbycuttingtheskinandobservingauniformbluegreencrosssection),thebasicityoftheliquorisincreasedbythecontrolledadditionofalkaliinordertofixthechrome.

Thisisknownasbasificationandmustbecarriedout

verycarefullytoavoidthedangersofprecipitationoftheunfixedchromestillinsolution.Uponbasification,thepHofthetannageisgraduallyraised,andthereactivityoftheproteinisgreatlyincreasedwithincreasedpH,andmorefixationofthetanningagentwilltakeplace.

Withtheincreaseinbasicity,twoormorechromiumatomsmaybeassociatedwithoneanotherthroughcommonsharingofhydroxylgroupstoformlargercomplex,whichiscalledolation.

Thehigherthebasicity,thelargerwillbethecomplexinthesolution.Thecrosslinkingreactionbetweenadjoiningchainsoftheproteinwillresult.

AfterthepHoftanningbathhasattained3.8-4.2for30-60minutes,afloatofhotwater(60-70℃)isaddedtothedrumtoraisethetemperaturetoabout40℃.

Thiswillpromotethefixationofthechromecomplexbythehideprotein,andreinforcethehydrolyzationandolationofchromecomplextoagreaterextenttherebyproducingmorecrosslinkingbetweencollagenmolecule.Thehigherthetemperature,themorequicklythereactionstakeplace.

Caremustbetakenbecauseexcessivelyhightemperaturemaycauseuneventannages,drawngrainandgreencolour.

AsthetanningproceedsatthehigherpHandtemperature,thefixationandthecrosslinkingofchromecomplexestopeltcontinue.

Thesereactionswill

resultinafulltanningandahighshrinkagetemperature.

Afterrunningforanother4-6h,itisleftovernight.

Nextmorningitisrunfor30min,andshrinkagetemperatureandpHvalue,aswellasCr2O3contentineffluent,aremeasuredtocheckthedegreeoftanning.

Thentheleathersarehorsedup.Atypicalexampleofconventionalchrometanningprocessforsheepskinisasfollows:

Pickling:50%water25℃10%-15%commonsalt5minfloatatleast60。Bé0.2%formicacid85%(1:10)10min0.6%sulphuricacid96%(1:10)60minpHoffloat2.8-3.2

Tanning:+8%chromosalB60min+%sodiumcarbonate(1:20)

addslowlyoveraperiodof90min+2hrspHoffloat3.8+100%water65℃5hrsfinaltemperature28-35℃leaveovernight,

nextmorningrun30min,horseup

Inconventionalchrometannage,sodiumbicarbonateisoftenusedasabasificationagent.Usuallyitisdosedtowardstheendoftanninginseveralportionsinatotalamountof0.8%-1.2%onpeltweightinordertoachievethefinalpH3.8-4.2inthefloat.

Magnesiaasaself-basifyingagentcontaining60%

MgOmayalsobeused.

Duetoitslowsolubility,thisagentreactsveryslowlywithacids.Itcanbeaddedatthestartoftanninginanamountof0.6%-0.8%onpeltweight,graduallyincreasingthefloatpHupto3.8-4.2.

Chrometanningagentscontainingaself-basifyingcomponentbasedonMgOordolomiteare

commerciallyavailableaswell.

Inthatcasethebasificationagentneednotbeaddedseparately.Inaconventionalprocedurethetanningandbasificationoperationshouldbefinishedatafloat

temperatureof35-40℃.

Inconventionalchrometannage,somemaskingagents,suchasformicacid,calciumformate,

sodiumacetateandsodiumphthalate,maybeaddedinordertoenhancethechromepenetrationrateandenablebasificationtohigherpH.

Theseagentscanenterintochromiumcomplex,andtherebyreducetheaffinityofchromiumcomplexfortheproteinaswellasthesensitivityofthechromiumcomplextopHvariations.

Therefore,chromewillpenetratepeltmorereadilyandtanningcanbecarriedoutataslightlyhigherpHthaninanunmaskedtannage.Thismaskingoftengivesaplumper,softerleather,whichmaybedesirable.

Italsoreducesthecationicnatureoftheleather,i.e.reducingitsabilitytofixanionicchemicals,e.g.

anionicdyestuffs,vegetableextracts,sulphatedoils.9.2

HighExhaustionChromeTanning

Methodstoimprovethechromeuptakeandtoreducethechromeconcentrationineffluentinvolveincreasingcollagenreactivityandcrosslinkingchromiumcomplex.

Thesemethodsarethebasisofamodern,environment-friendlyhighexhaustionchrometanning

process.9.2.1

IncreasingCollagenReactivity

Themainwayofincreasingthecollagenreactivityistoin-creasethenumberofcarboxylgroupontheaminoacidsidechainsinordertoproducemoresitesavailableforcross-linking.

Tothispurpose

glyoxylicacidwassuggestedtocondenseadditionalcarboxylgroupsintothecollagenusingtheMannichreaction.

Glyoxylicacidhasbeendeterminedtobeanaidtotanningwhenusedinpicklepriortothechrometanning.

Byconvertingaminogroupstocarboxylgroups,

additionalsitesarecreatedforthechrometoreactwiththecollagen.

Thechromeofferedisbetterexhaustedandmorestronglyfixedthaninaconventionaltanning.

Theresidualconcentrationofchromeoxideinthewastebathcanbereducedto0.2-0.5g/Lwhen4.4%-5.2%ofnormalchromepowderisoffered.

Tanningauxiliariesonthebasisofglyoxylicacidarecommerciallyavailabletoo.

Atypicalexampleofthechrometanningusing

glyoxylicacidisasfollows:Unsplittedpelt(cattle-hide)Pickling:50%water25℃5.0%-6.0%commonsalt10minfloatatleast7。Bé+0.2%-0.4%formicacid85%(1:10)30min

+0.4%-0.6%sulphuricacid96%(1:10)60min+1.0%FelidermCS120-180minpHoffloat3.8-4.0Tanning:+4.4%-5.2%ChromosalB120-180min+0.6%MgO60-120minpHoffloat4.0-4.4+100%water70℃5hrsfinaltemperature38-45℃horseup9.2.2CrosslinkingChromiumComplex

Crosslinkingchromiumcomplextoimprovethechromeuptakeandreducethechromeconcentrationineffluentinvolvesusingdicarboxylicacid.

Dicarboxylicacidsarethewellknownmaskingagents.

Shortchaindicarboxylicacidsaltwillproducecyclic,chelatecomplexesiftheringsizeis5-7membered;thiswillreducethereactivityofchromium(Ⅲ).

Longerchaindicarboxylicacidsaltswillcrosslinktwochromium(Ⅲ)complexions;thismakesthechromespeciesbigger,increasingreactionratebutdecreasingpenetrationrate.

Thereareseveralothertanningauxiliariesavailableinthemarket,usuallybasedonsomecrosslinkingfunction,e.g.aliphaticdicarboxylates,lowmolecularweightpolyacrylatesandsyntans.Theymaybeappliedatthestartoftanningoreven

formapartofthebasificationsystem.

Anotherexamplereferstotheuseofamixtureofaliphaticdicarboxylicacidsinthepickleandsodium

aluminiumsilicateforbasification.Amorerecentdevelopmentistocomplexunboundedchromebyapplyinglowmolecularweight

polyacrylatesintanningorinretanning.

Withthehelpoftheseauxiliaries,theexhaustionfrom80%to98%isachieved,aresidualconcentrationofchromeoxideinthewastebathvarieswithinarangefrom0.2to2.0g/L.9.3

DirectRecyclingofChromeLiquor

Anothermethodtoreducethepollutionloadofchromeinvolvesdirectrecyclingofchromeliquor.Thedirectrecyclingsystemscanbeclassifiedastheclosedandtheopen.

Closedsystemsarebasedmostlyonreusingonlyspenttanningfloatsandsammyingwaterfortanninginsuccessivecycles.

Theyareutilizedwhenworkingwithshortfloatsandpowderchrometanningagents.

Inopensystemsthefloatvolumeincreasesduringrecycling.

Thenumberofcyclesisnotlimited,however,inpracticeitdependsuponestablishinganequilibriuminthecompositionofrecycledfloats.

Severalrecyclingtechniquesofopensystemsareemployedindustrially:---Reuseofseparatedpicklingandtanningfloatsinsuccessivecycles.Reuseoftanningfloatinthenextcycleofpickling.Reuseofthemixtureoftanningfloatandsammyingwaterpartlyinpre-tanning.

partlyintanninginthenextcycle.Thepre-tanningfloatisdischargeddaily.Reuseofthespenttanningfloatinthenextcycleoftanningandthesammyingwaterforthepicklingfloatthatdailydischarged.

Ineachrecyclingoperation,spentliquorisaccumulated,filteredthroughasuitablescreenfilter,anditscompositionmaybemodifiedorreinforcedbyaddingchemicals.

Recyclingtechniquesaremostlyusedinconventionaltanning.

Accordingtotheextentoftheirintroduction,anincreaseinchromeutilizationfrom70%upto95%andadecreaseinchromedischargefrom2-5kg/t

to0.1-0.25kg/trawhidecanbeobtained.WordsandExpressionsbasicchromesulphate堿式硫酸鉻diffusionn.擴(kuò)散chromecomplex鉻絡(luò)合物

interiorn.內(nèi)部carboxylgroup羧基uniformbluegreencross-sectioncrosslinkvt.交聯(lián)

均勻的藍(lán)綠色斷面(切口)n.交聯(lián),交聯(lián)鍵basicityn.堿度

basificationn.堿化repressvt.抑制

precipitationn.沉淀hydroxylgroup羥基environment-friendly對環(huán)境友好的olationn.羧配聚

aminoacidsidechain氨基酸側(cè)鏈reinforcevt.加強(qiáng),增加…的數(shù)量

sodiumbicarbonate碳酸氫鈉hydrolyzation

n.水解

glyoxylicacid乙醛酸uneventannage不均勻的鞣制

Mannichreaction曼尼期反應(yīng)drawngrain粗面,抓面

normalchromepowder標(biāo)準(zhǔn)鉻粉shrinkagetemperature收縮溫度

unsplittedpelt未剖層的裸皮horseupvt.將…搭馬

dicarboxylicacid二羧酸

basificationagent堿化劑

maskingagent蒙囿劑dosevt.給藥

Chelatecomplex鰲合物aliphaticdicarboxylate脂肪族二羧酸鹽self-basifyingagent自堿化劑

polyaerylaten.聚丙烯酸鹽dolomiten.白云石

syntann.合成鞣劑calciumformate甲酸鈣magnesian.氧化鎂

sodiumaluminiumsilicate硅酸鋁鈉sodiumacetate醋酸鈉pollutionload污染負(fù)荷sodiumphthalate鄰苯二甲酸納

spenttanningfloat(用過的)舊鞣液affinityn.親和性

samvt.擠水unmaskedtannage不蒙囿的鞣制

powderchrometanningagent粉狀鉻鞣劑vegetableextract栲膠accumulatevt.積累,積聚sulphatedoil硫酸化油highexhaustionchrometanning高吸收鉻鞣ChapterFourProcessesforVegetableLeathersandOtherLeathers1VegetableTanningmethods

Vegetablesubstanceshavebeenusedfortanningsinceancienttimes.

Thepropertiesofvegetable-tannedleatherincludelowextendbility,goodstrength,shaperetention,storageofmoisture,embossingproperties,andpolishingpropertiesofthegrainandofthecutedgeinthecaseofsoleleathers.

Withtheintroductionofchrometannage,therewasatrendtowardssoftertypesofleather.Vegetabletannagewasusedlessandlessonaccountofitslowerproductivity.

Inaddition,importantfieldsofapplicationforvegetableleatherwerelostassyntheticsubstitutematerialswerefound(e.g.drivebelts).Recently,however,interestinvegetableleathershasbeenincreasingagainasthedesireforpropertieswhichcanonlybeenobtainedbymeansofvegetabletannagehasgrown.Oneconditionisthatthetanningrhythmshouldcorrespondtothatofachrometannage.Thedevelopmentofspecialproductsandprocesseshasmadethispossible.

Inthefollowingpages,weshalldescribehowtoproducedifferenttypesofvegetableleathers,fromfirmsoleleathertosoftupperleather,economicallyandinaccordancewithpresent-dayrequirementsbyvegetabletanningmethods.1.1

ConventionalVegetableTanningMethods

Vegetabletanninginpits,whichiscarriedoutpracticallyonlyinacountercurrentwayatthepresenttime,isabaseofallconventionalmethods.

Inthismanner,peltcomesinthemostdiluted,exhaustedsolutionoftanningmattersatfirst.Moreconcentratedsolutionsactonpretannedpeltgradually.

Thedensityofthetanningsolutionsactingonpeltincreasesfrom10Beto50Be.Tanningtimeinpitsusuallyvariesfrom2weeksto5

weeks.Inthiswaytannedpeltisthenfinishedindrumsinabathwithadensityof12-16oBe.Thetotalamountofvegetabletanningagentsusedisusually33%-40%oftanningmattersperpeltweightforsoleleather,28%-30%fortechnicalleathersand15%-20%whentanninggoatskinsandsheepskins.RapidtanninginpitsknownastheHot-pitprocessisamodificationofthisbasicmethod.Theaccelerationoftanningisachievedbyahigherconcentrationofthetanningsolutionsinpits(density10-150Be).

Diffusionofthetanningagentsintohidesisacceleratedbyagitationobtainedbyhangingthemonrockers.Thepeltispreparedforrapidtanningintheconditioning(pretanning)bathe.g.with5%ofpolyphosphateand2%ofsulphuricacid.

Thepenetrationoftanningagentsintopeltisthusacceleratedwithoutovertanningthesurfacelayer.Thetimeforrapidtanningisusuallyfrom20to25days.Thebasicmethodofdrumrapidtanningcanalsobeincludedamongconventionalmethodsatthepresenttime.Toshortenthelongproductiontimeofvegetabletannedleathersisthemainpurposeofintroducingthismethodintopractice.

Specificpropertiesofsomesyntansareutilizedforpeltpretanning.Thesesyntansdiffusefastintopeltandpreventovertanningthesurfacelayerwithvegetabletanningmatters.

Thus,theirpenetrationintothemiddlelayersisacceleratedandgoodtanning-throughachieved.Whenusingamixtureofsyntansandvegetabletanningmatters,itispossibletotanwithahighconcentrationfromthebeginningoftanningsothatthewholeprocessisshortenedsubstantially.Inallconventionalmethodsthereisarelativelyhighamountofresiduesoftanningagents(non-absorbedtanningmatters,non-tanningmattersetc.)dischargedintothesewers.Theseresiduesbelongtothebiologicallyharddegradableandresistantorganicsubstances.

1.2AlternativeMethods

Themainpurposeofthesemethodsistoincreasetheexhaustionoftanningmatterstypicalforconventionalmethods.

Itcanbeseenfromtechnicalliteratureaswellasfrompracticethatalternativelow-wastetechnologiesofvegetabletanninghavenotsofarbeendevelopedfortheactualconditionsintanneries.TheprocesscalledC-RFPcanbeconsideredasatypicallow-wastetechnology.Intherapidtanningprocessindrums,limedpeltisfirstconditionedwithacidauxiliaryagents.

Pretanningwithasuitablesyntanmakespenetrationofvegetabletanningmatters,ortheirmixturewithsyntans,intothemiddlelayersmucheasierandthusensuresagoodtanning-throughinthefollowing1to

2days.Pretanningandtanningiscarriedoutinveryshortfloatssothatrelativelyhighexhaustiontanningmattersoccurs(about75%)andonlyverysmallbathvolumesaredischarged.

Ithasbeenfoundthattheexhaustionoftanningmattersfromthetanningbathduringonetanningcycleisnotpossible.Inordertoachievegoodtanning-through,astoichiometricsurplusoftanningmattersensuringtherequiredqualityofleathersisused.

Theresultis,however,thataconsiderablepartoftanningmattersremainsinthebathtobedischargedintowastewater.Thatiswhytherecyclingmethodshavealsobeenintroducedintovegetabletanning.

OneofthemwhichhasbeenappliedinpracticeisamodernversionoftheLiritanprocess.Delimedpeltisprocessedinthebathwithpolyphosphateandsulphuricacidatfirst.Theexhaustedbath,afterbeingreinforcedwithadditionofchemicals,isusedinthenextcycle.

Insuchwaypreparedpeltisthenpretannedinpitssuccessivelyintwovegetabletanningbaths.Theprocessstartinthelessconcentratedbath(1day)atfirst.thencontinuesinthemoreconcentratedbath(1day).Apartoftheexhaustedlessconcentratedbathisusedforpreparationofthelessconcentratedpretanningbathinthenextcycle.Pretannedpeltthenproceedstopropertanningwhichtakesusuallyabout3days.

Itiscarriedoutinacountercurrentwayin6to9pitsinwhichaconcentrationisincreasedgradually.Thetemperatureofbathsiskeptat35℃.Aftermodifyingthecompositionandvolume,thebathfromthefirstpitispumpedintothelastpitfromwhichleathersaretakenout.Theleathersarepretannedthroughthecrosssectionforabouttwothirdsofthethickness.

Theleathersarethenfinishedinadrumatatemperatureof40℃bythemethodofsocalleddry-tanningfor30hours.Anextractinanamountofupt045%oftanningmattersrelatedtothepeltweightisused.

Thetannedleathersareleftforconditioningandwashedintheusualway.

AlthoughthedescribedversionoftheLiritanprocessissubstantiallymoretimeconsuming(12days)itiswidelyused.

Thereasonisthatithasasubstantiallyhighutilizationoftanningmatters(upto97%)andthecorrespondinglyreducedamountofpollutantsinthewastewater.Questions

Answerthefollowingquestionsaccordingtothetext.(1)Whatisthepropertyofvegetable-tannedleather?

Whydidpeopleusevegetabletannagelessandlessforatime?

Whyhaspeople'Sinterestinvegetableleathersbeenincreasingagainrecently?(2)Whatwaydoweemploywhenwecarryoutconventionalvegetabletanninginpits?

Howdowecarryoutitinthisway?(3)WhatistheHot-pitprocess?

Howdoweusuallyacceleratethetanningintheprocess?(4)Whatisthemainpurposeofintroducingdrumrapidtanningintopractice?Howdoweusuallyshortenthewholeprocessinthetanning?

(5)Whatresiduesdoconventionalvegetabletanningmethodsusuallyproduce?Aretheybiologicallydegradableornot?(6)WhycanweconsiderC-RFPasatypicallow-wastetechnology?

HowdowecarryoutthetanningaccordingtoC-RFPprocess?(7)Whydoweusuallyuseastochiometricsurplusoftanningagentinthevegetabletanning?

Whydoweintroducerecyclingmethodsintovegetabletanning?(8)Howdowerecyclethetanningbathintheliritan

process?

(9)WhatequipmentdoweemployintheLiritan

process?Howmuchextractshouldweuseintheprocess?(10)WhydopeoplegenerallyusetheLiritan

processalthoughitismoretimeconsuming?TopicsforDiscussion

Studentsaresupposedtodiscussthefollowingtopicsfor30minutesinpairsorgroups:(1)Vegetabletannedleatherpossessespropertieswhichcannotbeobtainedbymeansofchrometannage.Comparevegetableleatherwithchromeleather,andsumuptheiradvantagesanddisadvantages.

(2)Thecountercurrentwayisthebaseofconventionalvegetabletanningprocesses.Inthismanner,peltisputintothemostdiluted,exhaustedtanningsolutionatfirst,thentransferedtomoreconcentratedsolutiongradually,finallyistannedinthemostconcentratedtanningsolution.Whyshouldwecarryoutthevegetabletanninginthisway?Giveyourownreasonableexplanation.

2ProcessesforVegetable-tannedsoleleathersandUpperleathersTheC-RFPprocesscanbeusedtoproducealltypesofvegetableleather,e.g.soleleather,harnessleather,splithidesandliningleather.Inthefollowingpagesweshalldescribethemanufactureofsoleleatherandupperleather.

2.1ProcessesforSoleLeathers

The

charactertsticfeatureoftheC-RFPprocesscanbedemonstratedeffectivelybytakingasanexampleoffirmsoleleather,asthevegetabletannageofthickpeltsisparticularlydifficult.

Thebestrawmaterialforsoleleatherisbutts.Afterliming,thebuttsshouldbefleshedthoroughly,asanyremainingbitsoffleshimpedetanpenetration.Threetofourcutsshouldbemadealongthebackbonetopreventdrumfolds.

Asthehidesbecomestifferduringthesubsequentmaintannage,possiblygivingrisetoanabraded,drawngrain,itisimportanttoensurethatthedrumspeedis1-3r.P.m.,dependingonthedrumload.Thegreatertheloadis,theslowerthedrumspeedmustbe.Themotoroutputofthedrummustbesetat0.73KWper100kgpelt.

Procedurel:Washthepeltwithcoldwater,thenconditionwithoutfloat:

4.0%BASELC

0.1%sodiumbisulphite

2.0%formicacid85%(diluted1:2)Theadditionofsodiumbisulphitepreventshydrogen

sulphidefrombeingreleased.ThepHshouldreachapprox.3.5within2-3hours.Thenthepretannageiscarriedout:

+8.0%TANIGANRFS3-4h

(possiblyovernightonautomatic)

Aspretannedhidefibreshavelowheatresistance,thetemperaturemustnotexceed32℃.Afterpretannage,thehidesarewashedatatemperatureof25-30℃toremovethesalts.Forthemaintannage,35%-38%tanningmaterialsareusedasfollows:Maintannage:(Withoutfloat)

approx.18%vegetabletanningmaterials2h

+approx.18%vegetabletanningmaterials14-20hTanpenetrationgivesasmoothergrain.Quickertanpenetrationandalighter,moreuniformcolourcanalsobeobtainedbyreplacing4%-7%ofthevegetabletanningmaterialswithsyntans.

Theadditionof0.2%-0.4%ofPREVENTOLWBisrecommendedtoreducemouldformationafterthemaintannage.Theendtemperatureofthetannageshouldbe35-38℃andnomore.

Whilethetanningmaterialsdopenetratecompletelywithinthegiventanningtime,theyarenotyetfirmlybound.Thiscanberemediedbyfixingwithasyntantowardstheendofthetannage.Thishastheadditionaladvantageofpreventingtanningmaterialstainfromformingduringstorage.

Fixation:

+1%-2%TANIGANBL20-30min

Toallowthetanningmaterialstobindcompletely,theleathersarethencoveredandstoredfor1-2daysandthenprocessedasusual(rinsedbriefly,sammed,shavedifnecessary,retanned,setout,pre-dried,oiled,setoutagain,driedcompletely,conditionedanddrummed).

2.2

processesforUpperLeathers

Thebeamhouseworkiscarriedoutasusual.Straight

relimingimprovesthesoftnessoftheleather.Thepeltsshouldbesplitafterlimingtoobtainasmootherleatherandbetterareayield.

Afterdelimingandbating,thepeltsarewashedtoremovethesalts.Subsequentacidification

helpstoreducetheswellingofthepeltsandloosensupthefibrestructure.Theadditionaluseofformicacidhasafavourableeffectonthesoftness,andanacidequilibriumismorerapidlyobtained.

Ontheotherhand,ifinorganicacidsareused,diffusionislikelytobeslower.TheendpHshouldbeapprox.3.5.Thenextstepisthepretannage,whichisfollowedbythemaintannageinthesamebath.Thetypeoftanningextractusedisextremelyimportantforthehandleoftheleather.Chestnut,forexample,hasagraintighteningeffect,whilemimosaandquebrachogivefullerleathersandsyntanssofterleathers.

Theamountofvegetabletanningmaterialsisalsocrucialtothesoftnessofvegetableleather.Adequatetanpenetrationisobtainedwithapprox.15%puretannin.Insufficientlytannedleathershaveatinnyappearance.Iftoomuchvegetabletanningmaterialsareused,theleatherbecomestoofirm.TheadditionofsyntheticreplacementtanningmaterialssuchasTANIGANOSorBNimprovesthepenetrationofthevegetabletanningmaterials,makingitpossibletoreducetheamountused.Theleatherbecomeslighterinweightandlighterincolour.

Theuseofanelectrolyte-resistantfatliquoringagentinthepretannageormaintannageimprovesthesoftness.Increasingthetemperatureto35℃towardstheendofthemaintannageimprovestheuptakeoftheproducts.ThefollowingisanexampleofaC-RFPtannageforupperleathers:

Procedure2:Material:cattlehides,limedasusual,washed,fleshed,andsplitpercentagesonsplitweightWashing:300%water350C10min

DrainfloatDelimingandbating:100%water350C1.5%ammoniumsulphate

0.5%sodimn

bisulphite

0.2%formicacid85%(1:5)30min

+1.0%batingagent60minpH8.0-8.5Crosssectionwithphenolphthalein:colorlessdrainfloatWashing:300%water20℃10min

Conditioning:(withoutfloat)

3.0%BAYSELLC0.8formicacid85%(1:2)Temperat