C. C. CONRAD
A N DA. G . SCROGGIE
R a y o n D e p a r tm e n t, E . I. d u P o n t d e N e m o u r s & C o m p a n y , R ic h m o n d , Va.
T he hyd rolysis and ca ta ly tic o x id ation o f c ellu lo se (4, 5, 6, 7, 9) by h yd rochloric acid in th e p resen ce o f ferric c h lo ride is app lied to th e ch aracterization o f a n u m b er o f rayon yarns an d cellu lo sic raw m a teria ls u sed in rayon m a n u factu re. T h e a m o u n t o f carbon d ioxide evolved from th e cellu lo sic m a teria ls is com pared w ith th e a m o u n t evolved from g lu cose to o b ta in an e stim a te o f th e m o s t reactive m aterial. T h is e stim a te is called th e “ a cce ssib ility ” o f th e m ateria l. A ccessib ility is revealed a s sh o w in g a h ig h
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H E development of th e rayon industry has resulted in the production of a series of fiber types having a wide variety of properties. Uniform ity of properties is a characteristic which can be relied upon, because the fibers are m anufactured, under close chemical and physical control, from a uniform supply of cellulose.Fam iliar properties characterizing these cellulosic fibers as cur
rently produced are: controlled filam ent length, uniform strength and elongation, resistance to certain types of m echanical flexing, controlled luster, brilliant and uniform dyeing, and high dry strength. In contrast to these desirable attributes, th e high wet elongation and loss of appreciable strength when w et are less de
sirable qualities which further characterize these fibers.
For such regenerated cellulose fibers—in fact for all fibers whether natural or man-made— attem pts have been m ade to characterize their chemical and physical behavior by hypotheses concerning their internal molecular structure. Studies by Nicker
son (4-, S, 6, 7, 9) on the hydrolysis and catalytic oxidation of cellulose by hydrochloric acid in th e presence of ferric chloride show differences in the degree of reactivity of cellulose, depending on the history of the sample. O ur interest in this field has been to apply this chemical m ethod to the characterization of a num
ber of cellulosic materials.
The work reported presents several modifications of the m ethod which have greatly improved its reproducibility, and some information is given regarding the extent of the oxidation which takes place. Results when the improved technique is applied to a variety of viscose rayons and cellulosic raw m aterials are given. Accessibility as determ ined by chemical tests is com
pared w ith the “crystallinity” , calculated from x-ray diffraction intensity measurem ents, and a high degree of correlation is ob
served in the rayons tested.
PREPARATION O F SAM PLES AND EX PERIM EN TA L M ETHOD All tests on cellulosic m aterials were carried out on samples conditioned to an equilibrium m oisture content a t 60% relative hum idity and 75° F. (24° C.). T he oven-dry cellulose in the sam ple was calculated from th e m oisture regain of a duplicate sample dried to constant weight a t 105° C. in a current of dry air.
E lim ination of an actual oven-drying operation on the sample it
self was a safeguard provided to prevent any change in the reac
tivity of the m aterial as a result of strong heating or dehydration.
d egree o f co rr ela tio n w ith c e r ta in x-ray p a r a m eters an d s u b s ta n tia te s to a c e r ta in d eg ree t h e c o n te n tio n t h a t a c c e s sib ility is ch iefly a m e a su r e o f a m o rp h o u s c e llu lo s e . A c c essib ility is a lso sh o w n to vary in v ersely w it h t h e a lp h a c e llu lo se c o n te n t o f n a tiv e c e llu lo s e s . M o d ifica tio n s o f th e c h a r a cteriza tio n te c h n iq u e are d escrib ed w h ic h lea d to a n im p roved r ep ro d u c ib ility fo r t h e m e th o d . A d d itio n a l d a ta are g iv en o n t h e r e a c tio n b e tw e e n g lu c o se a n d th e sy s te m h y d ro ch lo ric a c id -fer ric ch lo rid e .
T he glucose used in th is w ork w as M erck reagent-grade an h y drous dextrose. I t was dried to c o n stan t w eight a t 105° C. be
fore use.
T he ap p aratu s is essentially th a t described b y N ickerson (7) w ith th e exception of c ertain m odifications designed to im prove th e reproducibility of carbon dioxide evolution an d assist in con
venience of operation. T his ch aracterizatio n te s t is based on th e fact th a t carbon dioxide is evolved from glucose by hydrochloric acid-ferric chloride reagent a t a ra te pro p o rtio n al to th e glucose concentration. U nder sim ilar conditions carbon dioxide is also produced from cotton and o th er cellulosic m aterials although a t som ew hat lower rates. T he m echanism proposed for cellulose is th a t glucose is first produced b y hydrolysis an d is subsequently partially oxidized to carbon dioxide. T hus, th e ra tio of th e ra te of evolution of carbon dioxide for glucose to t h a t for th e test sam ple gives a m easure of th e am o u n t of m aterial hydrolyzed.
In conducting th e hydrolysis, sam ples of glucose (1.4 to 1.6 grams) or cellulosic m aterials (1.2 to 1.6 gram s) a re digested w ith 150 ml. of reagent (2.4 =*= 0.01 N hydrochloric acid an d 0.6 =*=
0.01 M ferric chloride) a t its boiling p o in t. T h e arran g em en t of th e ap p aratu s is shown in Figure 1. T h e essential featu res of th e apparatus are: a purification train , A , for rem oving carbon di
oxide from th e carrier air stream (th e tra in includes a m anom eter and a flowmeter); a reaction flask, B , w ith stirrer; condenser, sulfuric acid trap , and purification tubes, C; and N e sb itt bulbs, D, filled w ith A scarite-D rierite m ixture for carbon dioxide ad
sorption.
A gravim etric m ethod for determ ining carbon dioxide is sub
stitu te d for th e volum etric m ethod described b y N ickerson (7).
The gravim etric m ethod appears to be superior to th e volum etric, and this behavior is supported by independent observations (8).
Actual practice has been to set up th e equipm ent in duplicate, utilizing th e same oil b a th so th a t check ru n s on a sam ple can be made a t th e same tim e. T he hydrolysis an d cataly tic oxidation are carried o u t under th e following se t of controlled conditions:
1. COi-free carrier air stream, flo* 1 0 ± 0.5 liters per hour >
2. Constant pressure in reaction vessel (745 mm. Hg)
3. Oil heating bath regulated so th a t reagent boils at a constant temperature; approximately 134° ± 0.3° C. oil bath temperature (dependent on radiation, size of flask, etc.)
4. Carrier air stream entry tube, 5-10 mm. above level of reauo in reaction flask
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6. C arborundum crystal boiling stones
7. S tirrin g by T-shaped mercury-seal glass stirrer (blade
8 X 15 mm.) at 350-400 r.p.m.
This carefully controlled set of conditions is necessary to en
sure reproducibility of th e car
bon dioxide evolution rates.
T hrough parallel arrange
m ent of th e N esbitt bulbs, m easurem ent of th e am ounts of carbon dioxide evolved can be m ade over any desired period. The oil b ath is heated electrically a t such a rate th a t about 35 m inutes are re
quired to raise it from room tem perature to a point high enough to prom ote boiling of the reagent. T his period is used for sweeping th e ap
paratu s free of carbon dioxide (no significant am ount of car
bon dioxide is evolved from the sam ple during this heat- ing-up interval). An alter
nate procedure m ay be em
ployed (8) whereby the warm
ing-up period is conducted in th e absence of th e sample.
After th e reaction m ixture sta rts to boil, th e sample can be introduced into th e reac
tion vessel through a p o rt in the side which is norm ally c lo s e d b y a g r o u n d g la s s stopper. T he usual practice is to m easure the am ount of carbon dioxide evolved for each half-hour interval after initial boiling of th e reagent and to continue th e half-hour m easurem ents for a period of 2 hours; th en m easurem ents are m ade a t intervals of one hour. Zero tim e for the reac
tion is considered to be the onset of boiling, and m easure
m ents are m ade over the fol
lowing 7-hour period. In the case of glucose th e carbon dioxide evolved is expressed as moles carbon dioxide per mole glucose; in th e case of cellulosic m aterials, as moles carbon dioxide per mole of glucose which occurs as glu
cose anhydride in th e sample.
T ypical carbon dioxide evolu
tion curves for glucose, a representative high-tenacity rayon yam , and commercial cotton linters are shown in Figure 2. A ctual carbon di
oxide evolution d a ta for these m aterials are given in T able I.
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T! T „ v ...
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wRE A C T I O N T I M E - H O U R S
F igu re £ . T ypical Carbon D ioxide E v o lu tio n -T im e Curves for R ep resen tative M aterials
value as th e accessibility of th e sam ple or th e p er cent a c c e s s i
bility because of th e som ew hat vague con n o tatio n of th e wor
"am orphous” as applied to cellulose. T his p o in t of view is !D agreem ent w ith th e term inology followed in recent papers y
o th er authors ( /, #). .
Tw o ad ditional m ethods of calculating accessibility frorQ^ , raw carbon dioxide evolution-tim e d a ta have been developed.
These are em pirical m ethods b u t have certain advantages over th e m ethod of slopes described above. These alte rn ate m et o are as follows: th e m ethod of areas, based on th e ratio o e area under th e curve [from U, — 0 to U = x, where t = (** ~~ W is th e tim e of digestion] of th e test sam ple, to th a t for glucose, and th e m ethod of heights, based on th e ratio of th e moles o C O j/m ole of anhydroglucose from th e te s t sam ple, to th e moles of C O j/m ole from glucose, for th e sam e tim e. A com parison of these th ree m ethods for calculating p er cent accessibility is given for a sam ple of viscose ray o n in Figure 4. T he tw o a lte rn ate m ethods avoid th e p roduction of an S-shaped curve. T h e m ethod of heights offers some adv an tag e in being m ore easily com puted th an th e o th er two.
TREATMENT O F DATA
The raw carbon dioxide evolution tim e d a ta for cellulosic m a
terials are converted to “percentage hydrolyzed-tim e curves”
by obtaining the ratios of the rate of carbon dioxide evolution for th e test sample to th a t for glucose a t selected tim e intervals.
The rates of carbon dioxide evolution are conveniently measured graphically from a plot of the carbon dioxide evolution tim e data.
A plot of these ratios against tim e gives typical curves of a pro
nounced S-shape. This m ethod of calculation will be referred to as th e m ethod of slopes. Figure 3 shows such curves for cotton linters and a commercial viscose rayon. T he pronounced S- shape of th e curves is a result of a decrease in the ra te of carbon dioxide evolution for glucose (the reference m aterial) which sets in after 5-6 hours of digestion. A sim ilar decrease for cellulosic m aterial does no t take place in m ost cases until the reaction has proceeded for 7 hours or more. Actually, Nickerson’s percentage hydrolyzed-tim e curves do not show this S-shape, since he ex
trapolated th e linear 3-5 hour rate through the 5-7 hour period and disregarded the decreasing rate period. The curves are char
acterized by an initial region of relatively high slope which gradu
ally falls off to one of lower slope as th e m ost reactive portion of th e m aterial is consumed. E xtrapolation of the latter region (2-5 hour period) to zero tim e gives a value corresponding to the am ount of th e m ost reactive p a rt of th e sample. This treatm ent is in accordance w ith th e theory for consecutive reactions where tw o m aterials are present which react a t different rates. Nicker
son (6, 9) calls this extrapolated value th e “per cent amorphous cellulose” in th e sample. W e have designated this extrapolated
Ta b l e I. Ty p i c a l Ca r b o n Di o x i d e Ev o l u t i o n Da t a f o r Re p r e s e n t a t i v e Ma t e r ia l s
.--- Moles COi per Mole of Sample11--- , AnhvrfrmiQ TTio h . t û nonitir Vmnnnn I _
Time, glucose viscose rayon of cott
Hr. a b a b a
0.5 0.048 0.049 0.005 0.006 0.002
1.0 0.130 0.131 0.027 0.027 0.007
1.5 0.223 0.226 0.060 0.060 0.015
2.0 0.317 0.320 0.102 0.099 0.025
3.0 0.501 0.501 0.195 0.186 0.049
4.0 0.670 0.678 0.298 0.284 0.074
5.0 0.824 0.835 0.409 0.394 0.103
6.0 0.961 0.972 0.523 0.504 0.132
7.0 1.076 1.085 0.637 0.617 0.163
1 Columns p and b represent check analyses on duplicate samples.
0.003 0.007 0.01S 0.027 0.051 0.076 0.106 0.138 0.168
A calculation of per cent accessibility for several sam ples of yam and pulps b y th e th ree m ethods shows a reasonably con
sistent relation betw een them . F o r th e m ost p a r t an y v a ria tio n is believed to be due to th e difficulty in deciding from w h a t p o in t on th e curve th e extrapolation is to be m ade. T h is em phasizes one of th e chief w eak points of th e hydrochloric acid -ferric chlo
ride hydrolysis technique in th a t th ere is no sh arp b re ak in th e curve, and values can th u s be in error by several p e r cen t because of th e u n certainty entering in to th e ex trapolation. I n order to reduce th is variability, we have arb itrarily adopted th e practice of extrapolating th e straig h t line best representing th e d a ta for the 2-5 hour period w ith th e m ethod of slopes, an d for th e 4 -7 hour period w ith th e m ethod of areas and m ethod of heights.
ese tim e intervals were selected after exam ination of a num ber of curves based on a v ariety of n atu ra l an d regenerated cellu- oses. Accessibilities m entioned in th is p ap er are based on th e average of the values obtained from th e m eth o d of slopes and th e method of heights. A lthough th e tw o calculations are b a se d on the same experim ental d ata, averaging th e results in th is m an n er tends to balance o u t errors involved in th e extrapolations.
The alternate procedures yield hydrolysis-tim e curves which are decidedly different from those o b tained b y Nickerson’a
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R E A C T I O N T I M E - H O U R S F ig u re 3. P er C en t H y d r o ly s is -T im e C u rves
(M eth o d o f S lo p es)
t s s t t s w ^ I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y m ethod of slopes. T h e p lo t (Figure 4) m erely serves to illu strate
th e graphical m ethods followed in obtaining th e in tercep t (acces
sibility) , and as such is n o t necessarily concerned w ith w hich of th e three m ethods of trea tin g th e raw d a ta represents th e actual degree of hydrolysis. T h e criticism of th e m ethod of slopes is solely from th e p o in t of view of th e inconvenience of using t his m ethod in determ ining th e accessibility o r intercept. T h e curve for th e m ethod of slopes appears to give a close approxim ation of th e tru e degree of hydrolysis of th e cellulose (6). T his m ethod accordingly should b e em ployed w hen it is desired to know th e tru e degree of hydrolysis a t any specified tim e; for example, samples could be com pared on th e basis of th e am ount of cellulose hydrolyzed in one hour.
All m aterials described in th is rep o rt have a ty p ical carbon dioxide evolution c j . tim e curve s i m i l a r to th a t shown in Figure 2.
T o save space, th e original experim ental d a ta are n o t given here b u t m erely th e derived accessibility values.
M OD IFIC A TIO N S O F ANALYTICAL TE C H N IQ U E W hen applying th e m ethod described by Nickerson (7) to replicate sam ples of a given m aterial, large variations in th e rate of carbon dioxide evolution were observed. These resulted in erratic accessibility values, p articularly on samples of wood pulp or sheeted linters.
A detailed stu d y of th e procedure was m ade to find th e chief factors causing th e variability. T he results obtained indicate th a t one or m ore of th e reactions occurring during digestion has a high tem perature coefficient. Accordingly, possible causes of variab ility in th e tem perature a t w hich th e reagent boils and vari
ations due to local superheating were studied.
T he use of glass beads, d a y chips, or C arborundum crystals did n o t reduce superheating sufficiently to produce uniform re
sults. Stirring has been found to reduce superheating m aterially and m ust be employed in order to obtain a satisfactory degree of reproducibility. A lthough C arborundum crystals were used in all experim ents employing stirring, we have n o t established w hether th ey are essential, and it m ay be th a t stirring alone is sufficient. Superheating in th e absence of stirring is lowest for glucose (a homogeneous system ), som ew hat greater for regener
ated cellulose fibers, and m uch higher for cotton cellulose and wood pulp. T his difference betw een th e pulps on th e one hand and regenerated cellulose fibers on th e other arises from th e m an
ner in which th ey react w ith th e reagent. A fter a few m inutes' contact w ith boiling hydrochloric acid-ferric chloride reagent, fibers of regenerated cellulose are dispersed in to very fine par
ticles which are circulated b y th e boiling liquid. Fibers from wood pulp or sheeted linters are n o t dispersed to th e sam e extent and ten d to accum ulate a t th e bottom of th e flask. A gitation by boiling alone does n o t keep these fibers away from prolonged contact w ith th e h o t walls of th e flask; as a consequence, pro
nounced superheating, occasionally even charring, occurs.
In th e absence of stirring, variations as high as 20-30% be
tw een cum ulative carbon dioxide evolution values have been ob
served for m aterials such as pulps. T he use of stirring has re duced th e maximum v ariation to th e order of 8-10% and norm al v ariations to th e order of 3 -5 % . Experim ental d a ta on three samples are included in T able I.
T h e minim um ra te of stirring capable of reducing superheating to th e level m entioned has n o t been determ ined. N o change in th e ra te of carbon dioxide evolution has been found b y varying th e stirring ra te over th e range 375-575 r .p jn . F o r our work, therefore, th e lower ra te of stirring was adopted since less wear and te a r on th e equipm ent results a t slow speeds.
F o r m ost of th e tests, 500-mL reaction flaaVs containing 150 ml of reagent an d 1.2-1.6 grams of sam ple were used. Increasing th e size of th e reaction vessel to 1000 mL and doubling reagent volume and sam ple size showed no change in carbon dioxide evolu
tion ra te or accessibility level.
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R E A C T I O N T I M E -H O U R S F ig u re 4. C o m p a ris o n o f M e th o d s o f D e te r m in in g P e r
C e n t A c c essib ility
In view of th e high tem perature coefficient and th e fact th a t th e reaction is carried o u t a t th e boiling point which is dependent on barom etric pressure, experim ents were perform ed to establish th e effect on carbon dioxide evolution of norm al atm ospheric variations. F o r glucose th e change in th e am ount of carbon di
oxide evolved in th e 7-hour reaction period am ounts to about 0.04 m ole carbon dioxide per mole glucose (about 4 % variation) for a change in pressure of 1 cm. of m ercury. B arom etric pressure changes from day to day are of th is m agnitude; as a consequence, regulation of th e pressure a t some stan d ard level is desirable in order to reduce variations from th is source as m uch as possible.
A pressure of 745 t t it t i. of m ercury has been selected as stan d ard for th e reaction as carried o u t in this laboratory. This pressure is som ew hat below th e prevailing barom etric pressure and facili
tate s sweeping th e carrier air stream through th e ap p aratu s by suction. T he pressure coefficient under th e conditions described has been found to be 0.375° C. per 1 cm. of m ercury.
A n altern ate procedure which could be adopted would be to operate a t a stan d ard tem perature of, say, 100° C. This would elim inate th e dependence of th e reaction tem perature on barom et
ric pressure and would, therefore, be m ore generally applicable in laboratories a t different elevations.
T he carbon dioxide evolution, expressed as th e ratio of to ta l num ber of moles of carbon dioxide produced during th e 7-hour reaction period to th e initial glucose concentration in moles, is n o t a constant b u t is dependent on th e num ber of moles of glucose initially present in th e system . Thus, strictly speaking, th e re
action is n o t to be considered a pseudo unim olecular or first- order bimolecular reaction since th e ratio for such a process
action is n o t to be considered a pseudo unim olecular or first- order bimolecular reaction since th e ratio for such a process