Fibrillar Structure of Rayon Fibers L a XI. WELCH
ASD
W. E. ROSEVEARE
E . 1. d u Pont d e IYeernours & C o m p u r ~ yI,n c . , Richmond, Vu.
H . MARK Polytechnic Institute of Brooklyn, A , Y .
A
method has been deleloped for breaking down ralon filaments into fibrils which may be seen with the optical microscope. Photomicrographs and x-ray diffraction patterns illustrate the differences in the structure of rayon filaments h b i n g +arious degrees of crystallite orientation. The fibrillar structures of highly oriented ralonq are compared t o those of cotton linters and ramie.
swollen in 700, nitric acid. This filament shows a lomgitudinal split with fibrils crossing from one side to the other before receiving :my riieclianical treatment. The application of pressure disintegrates the filament further and reveals clearly the fibrillar hewn in Figure 1B. I n this fiber the fibrils are imntely parallel to the filament asis. Filaments of high-tenacity saponified cellulose acetate yarns, :is n-ell as those of highly oriented viscose rayons, are characterized by spontaneous splitting when swollen in 70% nitric acid. 111the initial stages of disintegration under the application of low pressures, the fibrils appear to be oriented parallel to tile fiber axis. The principal differencc between these two types is the Fcivcr crossover> ot the fibrils in the saponified cellulose acetate filament$, indicating a higher orientation of fibrils in the latter c:tse. Figure 1C shows a single filament, of a high-tenacity saponified cellulow acetate yarn broken d o ~ into ~ n fibrils. Figure I D shows the fibrillar structure of a viscose yarn for tire cord. ( I ) hnviiig a degree of orientation somen-hat lower than that foi the tin) 3arnples nieritioned Intviously. Fila~neritsof the tirc y n n do not split spontaneously on s i d l i n g , but rathei tlic fibrils tend to stick togi:t,her. With the applicatiou of pressure the i\~-ollenfilament first qeparates into compiiratively large filanicntlike rtrwtures, wliicli can be divided further into fibrils. 011 the other liarid, fi1:iineiit.: oi the more highly oriented viscose :,rid thc. s:tpr)nified cellulose :xetnte yarns hrenli do\w into fibril- of fairly uniform . i i i tlie fir-t stage oi niecli:itiic:ildisinregration. llite orient:itioiis of the above-mentiolied consiiierd>ly :ind tire much higlier than til typicnl visroae textile yarn. Comp on of the x-ray patternr of Figures lE, F, : i d I sliows this for highly oriented y a r ~ i ,tire y x n , and testile yarn, rcyiectivcly. Filaments of normal visc o w tcstile yarn do not sepxrate into distinct fibrils urider the s\velliiig and traii~vrrse-pressliretreatments described previously, :is sliown by the disintegrated textile filament in Figure 1H. Lonaitudinal disintegration and the beginning of fibril formatiol I a r c :ipparent. Different samples of textile yarns sliow more or ICJS distinct fibril formation. ding upan the degree of orientation. Filtlnients Iiaving no lite orientation (Figure 1G) do not break c1on.n into fibrils. In t h e first stage of disiutegration the unoriented filament cleaves: approximately to the same estent in different directions (Figure 111). Furt,her mechanical treatment gives only irregularly shaped pieces of cellulose. When this regenerated yarn is highly stretched and t,hen treated with nitric< acid, the filament tends to break longitudinally, and the beginning of fibril formation is apparent (Figure 1J). The same viscose and spinning bath conditions were used iri spinning the filaments shown in Figures 1B and K. Therefore, the differences in breakdown structures of these filaments are attributed to the different stretching treatments of the extruded filaments. A comparison of the s-ray diffr:ict,ion patterns and
F
IBERS of natural cellulose are knorrn to break down into fibrils under the action of chemical agents or by mechanical disintegration (3, 6, 7 ) . In studying the action of si\-elling agents
on rayon with the optical microscope, it \vas found that certain types of rayoii filaments break down into fibrils similar in appearance to those of the natural fibers, except for the spiriil arrangement found in the Iat,ter. Similar fibrillar st,ructures in Lilienfeld and other high-tenacity cellulose yarn have been described (3, 10). Formerly, fibrillar structures \\-ere bcli in natural fibers (8). h method of fibrillation and its application to various cellulosic fibers are described in this paper. Several svelling agents-cuprammollirlnl, sodium hydroxide, and sulfuric acid-which have been used (2, 6 ) in htudying the fibrillar structures of natural cellulose fibers, mny be used in breaking down rayon filaments; however, the tendency for complete solution with these reagents led to the trial of other snclling agents. Seventy per cent nitric acid was found to he very sntisfactory. This solution does not nitrate at room t,emperatnre but forms an addition compound ( 5 ) rritli cellrilose and has an advantage in that it dissolves the cellulose less retldih.. The technique involved in breaking dciu II ?ingle filtunerits under the optical microscope is simple. The specimen is mounted on a glass slide and covered with :Ltliin cover gla-s. A small amount of nitric acid is placed at the edge of the COVFI’ glass in contact with the specimen, n.hicli swell penetrates tlie structure. The application of :I slight prci.surn u t i the cover glass causes the sxollen filnnieiit or fiber to disiutc~grntv into fibrils. The arrangement of these fihrils wit1 original fiber or filament axis is ea3y to SIN‘ ii esi act*ion is avoided. If it is desired to preserve tlic fibrils for a period of time, the nitric acid may be iwshed uway by applying drops of water to one side of the cover glass and blotting up the liquid at the other side. FIBRILLAR
STRUCTURES
The use of this technique on rayon fibers has shown difference,? in the structure of the following rayons having varying orientations: very highly oriented viscose and saponified acetate yarns, highly oriented viscose tire yarn, viscose textile yarn, and unoriented viscose yarn, having tenacities about 5 , 5 , 3.7, 1.8, and 1.0 gram per denier, respectively. Figure 1,4 is a single filament oi a highly oriented viscose rayon
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June, 1946
581
INDUSTRIAL AND ENGINEERING CHEMISTRY
C
A
E
F
I
A.
Filament of rcry highly nriented viscose rayon i n 70% nitric acid
I f . Disintegrated filament of v e r y highly oriented viacose rayon
C. Diaintegraied filament of
%cry
highly oriented saponified cellulose acetate rayon
I ) . Disintegrated filament of viacosc rayon tire barn
E.
X-ray diagram of \ p r y highly oriented viscose ra5on
F. X-ray rayon tire diagram Tarn
of
tisc0.e
G. X-ray diagram of unoripnted \isrose rayon I f . Disintegrated filament of riscom textile rayon
I.
X-ray dingram of rayon tectile yarn
J.
Filament of unoriented viscose rayon after atretching and partial disintegration
K . Partially
viscose
disintegrated filam e n t of unoriented viscose
rayon
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IND'JSTRIAL AND ENGINEERIKG CEENISTRY
Vol. 38, No. 6 fibers is l a r p ~ l y :I measure of the crystallite vrientation within the fibril because the spread in orientation as determined by x-rays is much greater than the spread in oiientation of the fibril3. I n mitural fiber. fibrils exist beforc any merit, hut there is no direct evidence th:it fibrilb exist in the rayon fibers beforc sn.elling. Howver, the new ruriaces produced by fibrillation of the rayon fiiiers must come from regions of the filaments having less than average cohesion. These regions may have aboveaverage amounts of holes, strains, and amorphous cellulose which is more readily dissolved and weaker than t,he more crystalline portions. The fibrils break up into smaller and smaller ones as the mechanical treatment is increased, but their HPpearanee with a givrn amount of mechmiea1 a c t i o n i s c h a r a c t e r i s t i c ot t h e t y p e of rayon.
Figure 2 1. Partiall, disintegrated fiher of c o t t o n lintprs B . Partiall) disintegrated fiber of r a m i e C. Intermediately disintegrated filler of rnmie I). Disintegrated fiber of ramie
the corresponding photomicrographs shon-. that fibrillar formation parallels crystallite orientntion, which is k~ioivnto be dependent upon spinning cmditioiis (4). Figures 2-1 and B are photoniicrogr3l)lis of cotton linters and ramie broken d o \ m into fibril. by the use of 7OGc iiitric acid. They are given to compare the fibrillation ot' rayon fibers with that of natural fibers untlcr ttic nits conditions. The natural fibers show the w l l knovn spirnl arr,ingcn(~iitof the, fibrils around the fiber axis; rmiio ha$ :I s n i : i I l ~:ciiglc ~ ~ ~ bet\t-wn the fibrils arid the filament asii. Figure- 'LC m c l Z i -lioi\ completely separated by f u r larger fibrils obtained in thi. fii broken don-n into smaller Ebri I' mechanical treatment. There seems to be little diffcrcnce in appearance bet\+-een the fibrils of natural fibers n n d thosr of highly oriented rayon, with the exception of orientation (E'igurer 1X :ind 2 0 ) . The orientation ohserved by meail- of s-rays in the highly oriented viscose fibers is ciomparable with tlint obPerved in ramie fibers. The crystallite oricntntion within a fibril of natural cellulose is t,hought to be almost completeiy parallel to the axis of the fibril, so that the over-all crystallite orientation of natural fibers is determined by t,he fibrillnr orient;Ltion ( 9 ) . On the other hand 111 oriantatioii of thc ( illites in these synthPtiC
CONCLCSIOY
The nirLiinerin ivhich rayon filaments break doxn by mecliaiiical or chemical treatments depends principally upori the crystallite orientntion. Rayons having high crystallite orientation separate int,o fibrils, wlierexs unoriented rayons shov no cvidencc of ii fibrillar structure. The fibrils from viscose and snponificci cellulose acetate p r n s a p p w r iniilar to the fibrils ir(,m cottori :uid ramie except for the s p i r ~ larrungemcnt in thii h t t e r . In contrast 10 imturai fiber.;, t!ie ovcr-dl orientntioii 01 tlii tiown by x-rays, is dc.tcrmined largc-ly by tl orientation withiti thta fibril
