372 TURNEft-"TENDI.:RIKG OF VAT DYED TEXTILES ON h:X POSURE TO LIGHT'" Dee. 1917

Tendering of Vat Dyed Textile Materials on Exposure to Light H. A. TLXNRI~

A review is given of some of the tlevclopmcrrts thut Iiave ttilrrn place 111 this ficlrl durmg the past An attempt is made to connect these duvelopments n r i d to show where g a p still exist

'l'htwc iq , in paitirulcw, a p u r i t y of relidde References are given to work in othcr ficlds whir11 may bc cupahle of frnitfiil

fourteen years. 111 the experimental investigntion of vat dyes on tho fibre. spertrographic tltita. extension to vat dye systems.

Introduction In 1933, at the Annuiil Conference of the Tcxtile

Institute, an attempt' was made to review the situation as it then stood on the subject of light-tendering actions conditioned by dyes, especially dyes of t.he vat range. At tlrut time, this light-tendrring action and certain associutcd phenomena had been defined in some detail, and the general p ih i re RO present,ed has not changed very greatly. Thcre was a sufficiency of connected data to warrant some spec~iletion on tho causal mcchanisn~s of these phenomena and to suggest certain means for their further investigation.

Later work hits tended to pursuc intercsting side-issues rather thttn to contrihute steadily to elucidation of the fundamental problem. The number of people directly engaged on the problem has never been large, or at least the volume of published matcrial has been comparatively small. Meanwhile, the general body of physics and physical chemist.ry has been increasing in detail, colierence and certainty in those fields which appear most likely to afford help in the dcvelopnie,rit of satisfactory tlieories. In thiH review complcte liberty hus been reserved to spernliit,e upon possibilities that are not yet capable of bririg rrferred dircct,ly to experimental evidenctc, for its ohjwt is to shnw whrit opport,nnitirs exist for further work ctntl what giips tire to 1)c found in the data on which satis- furtory theories mrty be constructed. By this means i t is hoped that a greater interest may he moused and that more workcrs will bc mcouragctl to givc tittention to the prohlcm.

The gtmtwtl tendering behnvionr of va.t dyes exposcd as tlyeings to light hits heon doscribed a number of times in prrvioiis papers' 3zJ. The observed results appear qnitlitittively to be closely connected with two other sets of plienomona, viz. ( I ) t,he fading of ttssociated dyes and (2) thc tendc!ring produced by vibt dyes during chemicnl rediiction and oxidation of the tlyc as in the normal dyeing process. All these manifestations of special activity serve to group the dycs into two classes, viz. active arid inactive. Among the former, individual members differ in their ability to promote tendering. Work by Henk' and lSgertons indicates that the non-act'ive residue should be further subdividcd into trnly neutral dyes and pro- tective dyes. Dyeings of protrct,ivr dycs, when exposed iinder compar~hle conditions, suffer less modification than the corresponding undyed materials.

For the simple phenomena of tendering by exposure to light we need as ossential partners (1) light, ( 2 ) tho 811h- strnt.e, i.e. a textile fibre, and ( 3 ) the active dye. These are essential almost by definition. Other factors, r iz . (4) the presence of gaseous oxygen and (5) the presence of moisture, nlso play very important parts, but, owing to the grewt experimental difficwlty of oxcluding them absohitely, i t is difficult to show that either is truly essential.

Fe.ding of the non-active component of a dyeing in which an active and a non-active dye are simultaneously present' is a well-established phenomenon, and in thin case the dye which is faded must also be considered as a substrate. When tendering occurs in dyeinqs in which an fictive vat dye is successively reduced and oxidised, a reducing agent must be included as an initiating factor, and t,he presence of air or other oxidising agent appears as R further essential factor.

With this introduction as a guide i t is now convenient to consider each factor separately, showing the possible contribution i t may reasonably be assumed to make to the total effect., and then addncing the experimental evidence for or against the nssumption.

Light The light which promotes chemical modification of the

exposed dyed fibre may be absorbed primarily by the

fibro substunce (most of the evidence is confined to the bohavioiir of celhilosc) and the dye rnuy purticipato non- photochcmically in assisting the modificution, c.g. as an electron acceptor. Altcrriatively, thorpt,ion of light enorgy by tho dye may constitute the true initittting mechunisrn. The weiglit of available evidence does not soem to support tho first alterntttive, for this shows thut cellulosc is scarcely affected by light with LL wavelength greatcr than about ~ , O O O A ~ . ~ . Vcry complicatcti changes occur in undyed cellulose on illuminrttiug with iiltra-violet light, as shown by Stillings and vun Nostrand', chenges which are not dependent on the presence of oxygon. In exporimentnl invcstigci,tions it is tliereforo simpler in the first instance to c!onfine attention to the twtion of longer wavelengths if the specific effect of the dye is to he shown. The specific effect ohltiinecl by exposure of tlyeings of active dyes, additional to that produccd by comparublc oxposures of the cellulose alone or d y ~ d with non-active dyes, appears, on the ruthor me'ngre nnd mostly qualitative evidence at present available, t,o come from wavelengths in the visible or in thc longest ultra-violet region of the spcotrnm. BlinovO stat,Ps thttt, tlic coeffii.irmt of weakening, expressed hy-

loss in strength of dyeing loss in atrenKth of undyed matiwiril

increasrs when exposure is perfornwd behind glttss. Egerton5, in it detitilrtl inr~extigution of thc c+Kv:ts of exposure, mounted his dyoings beliind gliiss :: in. thick. Whittaker has stated tliut when an iiicrrr~sing nnmber of t~hickness~?s of glitss tire intrrposcd between the liglit source and tho sprcimen, and ulso whctn spocimons are protected by Crlloplianc sheets, thc rut0 of tentlering is Rcurcely affected.

The tendency for uctivit,y to occur m o ~ t orange and rod vat. dyes would nt first siglit plain indirution thut srlective absorption by the activc dye is esse11ti~1. A further quaIit,utivc intiicution is that common tthsorpt.ion in the short,-wuve visible rungu is conncc.ted with the c:orninon unt,ivit,y in tendoring. The umonnt of satisfaot,ory piihlishcd work on the liglit H l ~ 0 1 y - tion of t,hr various dyes dyed on tt fihrolis sul)strate is very snitill indccd. There is not genrrully ttvailablc ut present any systeninbic expcrimentitl cwitlcnce which connects vut dye-promotcd tcnrlevirig witli i ~ n y part,icular frequency found in the ebnorption spcctrum of the dyeing. The fact thiit i t is possible to find ii r i t in ihar of y~l low. ormge and rod dye^ which do not C U I I X ~ npprecitthlf~ tendering therrforn rrqui res t,ho following altornntives--. ( u ) The light energy cansing t,endoiing is containrtl in a comparatively naimw frcqnency htind in the short.-wtive region of the visible spectrum corninon to the tendering dyes, biit not necessiirily coincident with n rnuin absorption band of any indivitlnal dye. ( b ) Light of the sume I'requenry is not necessarily effective in prorliicinp tendering as between one dye and another, hut i p ubsorbed a t freqnencies chnracteristic of the dye nnd re-emitted at a second (and longer) wavelength able to effect decom- position of the cellnlose. ( D ) The distinction betwcen tondering and non-tentlcring dyes of similar absorption characteristics consists in the presence within the latter of chemical groups which may undergo change themselves and utilise t-he absorbed energy rather than allow it to he transferred to the cellulose. This suggestion has heen made from time to time and will be examined latcr. Part of the consequcnc!e could be that non-active dyes wonld he fadcd more rendily than active dycs on exposure, and there is little ronsistcnt evidcncc for this nssumpt.ion. ( d ) ' One hypothesis of the tentloring action requires the photo-reduction of the dye .prior to an nutoxiclntion of the cellulose conditioned by reoxitiution of tho retluoeti form'. It has also been shown that tho illuniina.tion of

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