Tensile Properties o f Newer Fibers

J. H. DILLON Textile Research Institute, Princeton, N. .I.

HE primary mission of a textile fiber is to bear T a load and respond to

changes in load during proc- essiog and seMce. There m e many secondary but impor- tant qualities that i t should possess, such as dyeability, chemical stability, and mini- mum static charge gener- ation. Moreover, there are economic factors of price and availability which are often controlling. The types of loads to which textile fibers are subjected vary widely from the simple tensional to the more com~lex bending. Rates

I n the first section of this paper, stress-strain curves to rupture are presented for single fibers of vismse rayon, aeetate rayon, nylon, Orlon, Dacron, dynel, Acrilan, X-51, Vieara, and regenerated silk. The relative stress- s t ra in behavior of them iihers is discussed in te rms of their ehemioal structures, and the limitation of this type of evaluation ie stated. In the semnd portion of the paper, repeated stress-strain da ta are presented in the form of “CydiUg proliles,” a n d the viscoelastic properties of the various fibers thus revealed are oompared. In the third seetion of the paper the effeets of fiber morphology upon mechanical properties are discussed, and the effects of crimp on the measured Hookean modulue are given and interpreted.

~

of loading vary from the essential!y static to the impact and high frequency types found in parachute shroude and tire cord. Response to load by the hydrophilic fibers is governed critically by the relative humidity of the environment, whereas the hydrophobic fibers are more sensitive to changes in temperature. This paper is limited to a study of the tensile properties of several natural and man-made fibers. In accepting this limitation, how- ever, it is acknowledged that the performance in tension of any fiber is only a part of the story of its load-bearing quality-” important part, but selected mainly because i t is easier to measure the properties in tension than under the more complex conditions 01 processing and service. The eKects of water content, repeated cycling, and fiber morphology are discussed in a limited manner aince they rank in importance with the chemical structure of the fiber in determining its tensile behavior.

SELFXXION OF FIBER SAMPLES

Most of the tensile data on man-made fibers in the literature have been obtained on fiber samples tested by scientists of the fiber supplier organizations or submitted by them to other laboratories for examination. There is no question that these data are authoritative but, in general, they refer to fibers which meet the rigid specifications of the scientists who developed them; on the other hand, they often are not representative of the fibers supplied to the textile industry for proeesaing. Hence, it was decided, for comparative stress-strain studies, to employ 3- denier staple fibem of viseose rayon, acetate rayon, nylon, Orlon, Dacron, dynel, Acrilan, X-51, and Vicara which had been sub- mitted to a textile manufacturer for pilot spinning snd weaving experiments by the several fiber suppliers. Thus, it seemed re% mnable to suppose that these man-made fibers could be desig- nated a8 “specially controlled production samples.” It is well known, of course, that the properties of man-made fibers can be changed over wide limits by variations in prooessing, particu-

larly the degree of orientation. Furthermore, some of the fibers chosen, especially Acrilan and X-51, are still in pilot stages of development and may be expected to change considerably in constitution and properties within the next year or two. Hence, it must be emphaaiaed that the man- made fibers studied in this work must not he considered, a t this time or in the future, a8 truly representative of the producta of the various suppliers. It should be men- tioned also that these fibers varied widelv in amount and

type of crimp, and this fact is probably partially responsible for the differences in stresestrain behavior exhibited.

The natural fibers selected for the comparative stress-strain studies were: (a) a typical medium domestic wool obtained from the Sheep Experiment Station of the U. 6. Department of Agri- culture, Duhois, Idaho (T.R.I. m t e r wool WC-5); (b) a l’/w inch staple American rotton; and (e) a sample of Japanese silk of unusually high tenerity (5.6 grams/grex). This sample of natural silk is certainly not representative of silk used today but still may be considered a “target silk” in respect to the syn- thetic fibers. Had not international political and economic factors interfered, this night have been the silk of commerce today. A regenerated silk (natural silk dissolved and spun into B continuous filament) was also included. The exact pmc- em of its production in Japan is unknow. However, i t was considered to have interest as a rather unusual regenerated pro- tein fiber.

For the studies of the eKects of crimp, given in Table 11, special mmples of normal and crimped tow of 3-denier nylon and Dacron were kindly supplied by H. F. Hume of the Du Pont Co. The rather unusual low crimp and high crimp wool asmples of Table I were supplied by T. D. Watkins of the Bureau of Animal Industry, U. S. Department of Agriculture, Beltaville, Md.

EXPERIMENTAL CONDITIONS

Thp wight per unit length 01 each fiber was rneaeured by tbe .&uminga value for thedensity,

All stress-strain snd re- T.R.I. vibroscope method ( 6 ) . rhe crosd-swfioiixl area was ralculated.

I-inch and tested In IhQ lnstron tensile tQRtinE machine. Thr conatant rate 01 crosebeud travel w88 0.5 inch per minute (50% strain Der moutel fur all the stremtltrain curves to break: 0.1 inch der minute ‘(107 strain per minute) for all nonrupture stress-strain curves. %treM-h-ain curve8 to break were ob-

September 1952 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 2115

tained on at least ten fibers a t standard “dry” conditions (65% relative humidity, 70’ F.) and on ten fibers “wet” (under water a t 70” F.). Curves presented in Figures 1 to 6 are for the averages of ten fibers in each case. Strain origin was selected as the inter- section of the tangent to the linear (Hookean) portion of the curve with the strain axis wherever crimp is present (see Figure 14). The initial length was that measured in the vibroscope test u$er a slight load which removed most of the crimp (6).

Cycling tests” were performed on five fibers of each type under dry conditions. The data presented in Figures 8 to 14, however, represent selected typical curves for each fiber type, rather than averages. The procedure used for the cycling tests is given in Table I.

’“1 2.0 k T T O N

OY I I I

20 30 40 50 0 10 STRAIN %

Figure 1. Principal Katural Fibers Dry = 65% R.H., 70’ F. Rate of strain = 50%/min.

The energy to uncrimp the fiber was measured as the area indicated in Figure 14. This procedure appears to be justified on the basis of long experience with many animal fibers. For example, human hair, mohair, and other uncrimped fibers do not show the “uncrimping region” in their stress-strain curves, but naturally crimped wools and artificially crimped fibers have this distinguishing characteristic. As will be pointed out later, the existence of crimp not only introduces this concave-upward region of the curve, but also tends to reduce the Hookean mod- ulus, the normalized slope of the stress-strain curve in the early linear portion.

COMPARATIYE STRESS-STRAIN STUDIES

811 stress-strain curves presented are plotted in terms of tensile stress on original cross section in units of megagrams per square centimeters. The ordinates may be converted to “breaking lengths” in grams per grex by dividing by the specific gravity or breaking lengths in grams per denier by dividing bv the specific gravity X 0.9. Obviously, tenacities may be calculated in the same manner from the maximum ordinates. From the purely scientific viewpoint the choice of strees units seems desirable, particularly when i t is noted that cross-sectional areas have been calculated from weights per unit length, measured by the vibro- scope technique.

The curves for the natural fibers, tested dry, are given in Figure 1. As mentioned earlier, the silk specimen is of unusual strength and extensibility. This plot is given mainly to serve as a simple reference pattern in considerations of the properties of the man-made fibers, many of which are said to be silklike or voollike. It is uell to note that the silk is uncrimped while the wool and cotton fibers have their natural crimp.

Stress-strain curves for the cellulosic fibers, dry and wet, are given in Figure 2. Also plotted for reference is a curve for highly oriented Fortisan, deduced from data published for a lowtwist yarn by Susich aud Backer ( 7 ) . The unique behavior

Table 1. Cycling Test Procedure Gage length 1 inch

Relative humidity, 65% Temp., 70’ F.

3% Test 1.

2.

3 . Repeat step 2 4. 5 . 6.

Extend fiber 0.03 inch (3% of initial length) a t 0.1 inch/niin. and R - tract a t same rate to zero load Immediately re-extend fiber a n additional 0.03 inch a t 0.1 inch/min. m i 1 retract a t same rate to zero loa,d

Immediately re-extend fiber an additional 0.03 inch a t 0.1 inch/min. Allow fiber t o relax 4 to 5 minutes a t the final elongation of step 4 Retract fiber to zero load a t 0.1 inch/min. and immediately re-extrnrl a t 0.5 inch/min. to rupture

10% Test Same operations as 3% test, except 0.1 inch (10%) extension employPd i i i steps 1 to 4

20 5% Test Same Operations as 3% test, except 0 . 2 inch (2070) extension enlployed in steps 1 to 4

of cot’ton is clearly showi, both extensibility and breaking stress increasing with moisture content. This increase of breaking stress with moisture content has been explained on the basis of a more uniform internal stress distribution in the moist con- dition; hence, it is essentially an internal morphological effect. The dry behavior of viscose and acetate is more woollike than cottonlike. Viscose in the wet state yields a curve quite similar to that of silk or nylon. Kone of the man-made fibers approaches cotton very closely in tensile behavior, although the saponified acetates, not included in these studies, reproduce it best. I t is very interesting to speculate, dipregarding economics, as t o thv possibility of achieving the behavior of the cotton fiber by making a regenerated cellulosic fiber of very high molecular weight but with a spiral subfibril structure.

Stress-strain curves for the protein fibers, dry, are given in Figure 3. As might be predicted from the experience of those who have attempted to polymerize molecules of the same state of order possessed by a corresponding natural polymer regenerated silk is more like wool than silk in its tensile behavior. The stress- strain curves of the fihers that have been called silklike (6) (nylon and Orlon) are plotted in Figure 4. A casual glance a t this plot suggests that Orlon also exhibits woollike properties in that it has the rat.her sharp yield point characteristic of wool.

The tensile behavior of two of the n-oollike synthetics, dyne1 and Dacron, is plotted in Figure 6, with wool and Vicara as reference materials. The effect of moisture on the properties of these hydrophobic fihers is, of (Bourse, very Rmall. In fact,

HIGHLY ORIENTED FORTISAN (SUSICH a BACKER)

g20

Figure 2. Cellulosic Fibers Dry = 65% R. H., 70° F. Wet = under water Rate of strain = IO%/min.

2116 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 Voi. 44, No. 9

there is an apparent revereal of the moisture effect in the case of Dacron, but thin undoubtedly resulted from the experimental error inherent in a test involving only ten fibers for each wndition. Vicars (regenerated w m protein fiher) behaves ~ t ~ i k h g l y like wool in the dry state, although it exhibite lower streen values throughout the test. However, wool increases in ultimate exten-

Examination of these initial Stre%ea-- 31rves to Npture gives only a limited general concept of relative fiber properties, and any conclusions drawn must be q d f i e d in the l i h t of known possible variations in the man-made fibers. The initial streen- strain curve to mptyre gives little information concerning the reversihility of the elasticity-i.e., resilience. In an effort to

Finme 5. Woollike Fibers

0 ‘“I a

Figma 4. Silklike Fibers h - 65% R.E., 700 F. wet = d e r water 6.e of .en - SOgb/PiP.

Y > 10 m 3 0 4 0 J o o STRUN-X

Figure 6. Synthetic Fibere h - 65% R.R., TW F. Rate of s-in = m%/min.

sibility as it becomes wet whereas Vicara shows the opposite behavior; hence, wool in the wet state retains 8.5% of its dry

the wet tenacitv of Vicara and other reeenerated orotein fibers can

develop a simple picture of the reversible elasticity of theee vnri-

fomed. tenacity whereas the ratio of vicars is about 40%6. Undoubtedly, fibers, the cycling experiments of the next m i o n were per-

- and will be improved, but it is significant that Vicara is now being successfully blended with wool to aive Yams of unusual “loft.” STRESS-STRAIN CYCLING STUDIES

Hence, tenacity is not in itself t o o k p o h n t for fibers designed Repeated streae-atrain cycling bss been employed by numerow for wool type processing unless an increase in abrasion resistance workers in experimental studies of the mechanical properties of is desired. In that event, the use of one of the stronger but still fibers. Extended and reasonably sncceeaful theoretical h a & highlyextenaible synthetic fibers would be indicated. ments of cycling data have been given by Leaderman (4) and

All the synthetic fibers tested are represented in the strew Eyring, Halsey, White, Burte, and others (1-3). The moat strain c w e 8 (dry) of Figure 6. Again it must he emphasized comprehensive experimental study of this sort, however, w88 that that these curves are chsracteriatic only of the f ibm tested in of Snsich and Backer (7). These workers carried out carefully this study. They apply today but perhaps not tomorrow. controlled streakstrsin experiments on low twist ysrna and a few Hence, i t seems necessary to w n h e comments to the fact that monofilaments of 26 different fibers and then devised a novel the nylon is unique among all these synthetic fibers in ita very graphical means for representing the data in term of immediate low modulus and lack of a definite yield point in the low strain elastic recovery, delayed recovery, and permanent set. There is iegion. no question that the work of Suaich and Backer is a classic in this

September 1952 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 2117

held and that their methods will be used effectively by many workers, Most of their work, however, was performed on yarns rather than single fibers, involved an extrapolation of the linear portion of the retraction curve, and required accurate measure- ment of the several significant component. strains. Because of the asymptotic approach of the extension and retraction curves to the strain axis, accurate measurement of strain is very dif- ficult and time-consuming, much more SO than is measurement

2 3l /

3

2

I

/7 /i BRK

Figure 7. Typical Cycling Test-Dyne1 65% R.H., 70’ F.

of maximum stress for a given cycle. Hence, it seemed debirable to develop a somewhat simpler method which would not involve an extrapolation and would employ stress values rather than stxain values as the signifimnt parameters. Further, it mas de- cided to employ single fibers rather t,han yarns in order to avoid m y possible ambiguities resulting from the effects of yarn geom- etry.

The experimental technique of the cycling tebts carried out in this work has been described under “Experimental Condi- tions,” a,nd the test procedure is given in Table I. A typical set of cycling tests, obtained with single dyne1 fibers a t 65y0 relative humidity and 70” F. is portrayed in Figure 7. The plott,ing of such a chart. is extremely laborious, of course, since it requires assignments of strain values along many feet of recorder chart paper and reading off the corresponding loads. Considerable information can be obtained from data thus presented, such as progressive changes in area of the hysteresis loops and elastic moduli corresponding to the linear portions of the extension and retraction curves. However, in order to develop a condensed visual picture of the essentials of the cycling results, it was de- cided simply to read directly from the recorder charts the maxi- mum load valuee for each of the first four strewstrain cycles and the iinal load after relaxation. These values were then expressed as per cent fractions of the breaking load for the particular se- quence and fiber under investigat,ion, and plotted as shown in t.he bar charts of Figures 8 to 13 (cycling profiles). It is immedi- ately apparent that this procedure reduces greatly the undesirable effects of fiber-to-fiber variation and makes unnecessary the accurate measurement of strain values. It is always desirable,

of course, to check the 3, 10, or 20% strain increments employed by the experimenter, but this a n be done with ease by quick examination of the recorder charts. It remains to be seen, of course, whether such a simple cycling profile involving only rela- tive loads (and, therefore, relative stresses) is of va.lue in indi- cating the reversible mechanical performance of a fiber-Le., its elasticity.

Obviously, a combination of the three profiles at 3, 10, and 20% elongations, respectively, can give only the following quantities:

1. Extent of stress relaxation 2. Progression of stress with cycling, relative to breaking

3. Stress at 3, 70, or 20%, relative to breaking stress stress

In discussing these parameters, it must be remembered that, we are thinking of an “elastically ideal” fiber, as represented in Figure 8. Such a fiber might be far from ideal in ot,her respects; in fact, a certain amount of plasticity or imperfection in elasticity is prob- ably essential For satisfactory proceMing. But, lacking knowl- edge of the true ideal fiber performance, we seem to be justified in limiting the present discussion to consideration of t.he actual performance of each fiber 1vit.h that of the elastically ideal fiber

n 8 100% (BREAK) n n

3 TEST IOXTEST POXTEST

Figure 8. CJ cling Profile for Elastically Ideal Fiber

IO&%/min. Relaxation 0 S07?o/min.

,-1OOX (BREAK)

COTTON so3265 Mg/cmZ

100% ( B R E A K )

a -1 W L1:

ACETATE STAPLE

3XTEST IOXTEST

Figure 9. Cycling Tests for Cellulosic Fibers 65% R. H., 70” F.

IO%/rnin. a Relaxation 0 50%/rnin.

as a temporary reference rriterion. This fiber, by definition, would show no progression of stress (the first four stress values would be equal) and no relaxktion of stress. Such a fiber could have no true permanent set, although i t might exhibit a delayed recovery within the time limits imposed by the rate of strain em- ployed in the experiments. Herein lies the first limitation of this

2118 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 Vol. 44, No. 9

h

simple form of presentation-it ignores the shorter period time effects which may be quite important. Each group of three pmsles may be comidered ae a fatigue

pattern for a fiber Since i t portrays the effects on the breaking load of previous repeated streakatrain cycling for the three atrain increments, follow'ed by relaktion. The progression of maximum stress for the 6rst four cycles certainly indicates in- atability of mechanical properties. This instability, however, may be related to one or more of several poseible phenomena. Most important of t h e is flow or permanent set, which may be recoverable at higher moisture codtent or higher temperature. "Delayed elastic recovery" ala0 plays a part, of course, within the time limitations of the experimen$. At the higher strains (10 or 20%) i t is also possible that a certain amount of orientation takes place which would act to increase the progression of stress, but thia is probably a minor effect except for cotton and those syn- thetic fibem which are purposely supplied in a low state of orientation. The phenomenon of thixohpy,'exhibited by ani- mal fibers in the wet state, is probably also a contributing e5ect even under the dry conditione of these tests. Thixotropic effects, however, would tend to reduce the progression of streas. Hence, i t is conceivable that a 6ber showing no prograaSon of stress for a given elongation increment might not be elastically stable in that two competing phenomena, Bow and thixotropy, might produce a Eat pattern in the Wt four bars of the prosls. Acknowledging these limitations, howevet, thia simple method of presentation has much to recommend it, and it may prove quite useful in evaluating the relative ehtic.performance of fibers.

The charta for the celluloaic fibers, given in Figure 9, me of considerable interest since they reveal several salient differences in the behavior of textile viecose rayon and acetate rayon, as compared to cotton Cotton, of course, is a low elongation fiber and could be studied only in the 3% test. AB might be expected for any fiber tested at s t r e w s in the neighborhood of the b r e w strew, the progression of streas ia high, some orientation 88 well as simple plastic Bow probably occurring. The relaxation ia mnnll, however, relative to viscose and acetate in the 3% test, indicating that orientation w a greater factor in the behavior of

-4PYprtges S Y N T H E T I C FIBER

cotton under this condition. ~ viscose and ace- tate show undesirably high relaxation although only moderate progression d stress; hence, the dominating factor is probably plastic hw *&out accompanying orientation. Even in the 10% teat, viscose shows mre relsxation than,,does cotton, rela- tive to the maximum &mea of the fou& atreee-strain cycle. Acetate baa a greater overall progreasion of stress than viscose, breaking in the fourth cycle of the 10% test. In general, then, it may be concluded that, within the limits of ita low extemi- bility, cotton orients more but flows leas than do viaoose and acetate. For ,B situation where high extensibility is demanded, however, cotton,ia not suitable; it breaks at 3.5% strain under these conditions.

Cycling data for the protein fibers are compsred in Fignre 10. Considering b t only the 3% tests, regenerated silk and Vieara show practically no progression of stress but considerable relax- ation; wool shows a moderate progression of,,Etreas but leas relaxation than occure with the other h e fibers. For the 10% tests, wool is outstanding in ita low progreseion of stress, but the fact that relaxation i% quite high suggests that thixotropy- i.e., progressive breakdown of molecular btmcture-may be a factor. Whatever the explanstion, the low progression of stress for wool in the 10% test is charwteristic and may be one of the factors contributing to "wooliness."

Nylon and Orlon, profiles for which are &n in Figure 11, would hardly be called silklike in this comparison. Nylon is unique among all the fibers studied in the low was values ex- hibited in the 3 and 10% cycles and its a1most"perfect elastic performance under those conditions (low stress prokernion and relaxation). Both nylon and Orlon are greatly superior to silk, and certainly mmpsrsble to wool, in the type of fatigue prdvided by these cycling tests. Dacron and dynel are fibers which are considered to have many

of the properties of wool, with which they are compared in Figare 12. Noting that the low stress values of the b t cycle for wool and Dacron may be related to their uniform high-frequency crimp (the dynel fibers had an entirely dieerent irregular low- frequency crimp), these synthetics appear elaaticslly comparable

For the 3%

. .

n n

3 % TEST IOXTEST ZOXTEST

Figure 12. Cycling Tests for Ihcron and Dyne1 us. Wool

65% R. H., 70' F. lO%/min. Relaxation 0 50% min.

to wool in the 3% test, although dyne1 has considerably more relaxation. In the 10% test, howver, wool shows its charac- teristic small progression of stress which must be investigated further before it is declared an unqualified virtue (note previous remarks on thixotropic breakdown).

The comparison of the experimental hcrilan and X-51 acrylic fibers with wool, given in Figure 13, must be viewed with con- siderable caution because of the probability that their constitu- tions and properties may be changed by their suppliers. Both these new fibers show almost negligible progression of stress in the 3y0 test but high relaxation; this seems to be characteristic of the acrylic or partially acrylic fibers (Figures 11 and 12).

It appears that the cycling profile method of analysis may be useful for comparing the relative elaPtic performance of the various fibers. It certainly cannot he expected to vield the more quantitative empirical parameters derived by the method of Susich and Backer (7) , nor has i t the potential for theoretical interpretation in terms of molecular structure typified by the work of Eyring, Halsey, White, Burte, et al. ( f - 3 ) . Yet, its very simplicity suggests that i t may be of practical value and perhaps might profitably be investigated beyond the fen cursory tests of its reasonableness herein presented.

EFFECTS OF CRIMP ON TENSILE PROPERTIES

h typical stress-strain curve for a crimped textile fiber is shown in Figure 14. Several of the significant mechanical parameters which may be derived from it are indicated; these include Hookean modulus; yield stress and strain; stress at 20% strain; breaking stress and strain; energies to extend 20%, to break, and uncrimp. Zero strain is arbitrarily defined by the point of intersection with the strain axis of the tangent to the linear portion of the curve. This choice is primarily a matter of convenience and may be criticized from a purely scientific viewpoint. For practical purposes, however, it must be remem- bered that the asymptotic approach of the curve to the strain axis makes it very difficult to determine the zero strain as the abscissa where the measured stress vanishes. The observed curve for a given fiber may vary considerably from that of Figure 14. For example, the dip in stress after the yield point (at about

ACRILAN STAPLE

ln ln w

ACCO X-51

U a

IO%TEST 20VJEST 3 %TEST

Figure 13. Cycling Tests for Acrilan and X-51 D S . Wool

a IO%/min. 65% Relaxation R. H., 70' F. 0 50%/min.

20% strain in Figure 14), which results when thc. rate of relaxation temporarily exceeds the rate of extension, ii often absent. Fre- quently, as for dry silk (Figure 1) or wet viscose (Figure a), the yield region is not sharply defined. The most striking de- parture is shown by nylon, which gives a curve with the yield behavior displaced to the high strain region (Figure 6). Ob- viously, in cases such as these, the yield stress and strain param- eters have no meaning. I n other cases, there is no early linear region of the curve; thus the Hookean modulus is not defined, In general, however, a careful analysis of the stress-strain curve should include measurements of most of the parameters indicated in Figure 14.

A thorough discussion of the basic factors that govern the nit'- chanical behavior of fibers is not nithin the scope of this paper. In brief, however, there are two principal factors-molecular structure and morphology. Most of the theoretical treatments of fiber mechanical behavior have been based on model? set up to represent the elastic and dissipative elements of a moleculai network; thus, it has been convenient to consider the fiber as a homogeneous cylindrical rod. Further, it has hren the custom t o assume that the effects of morphology on mechanical behavior, although admittedly appreciable, can be eliminated by simple subtraction from the total behavior. Unfortunatelv, this last assumption is far from justified, as was found in the following experiment where crimp was the independent variable.

The results of this experiment are summarized in Table 11. The parameters listed are those defined in Figure 14. Foi the nylon and Dacron samples, the imposition of crimp inci ease+ the strain a t break and reduces the tenacity and stress a t break in a manner which might be expected to result from a process involving permanent bending deformations introduced a t ele- vated temperatures. The obgerved variations in linear density are undoubtedly associated with fiber-to-fiber variations rather than with crimp. The largest and most interesting effect of crimping, however, is the reduction of modulus. It would seem doubtful that sufficient relaxation of orientation could occur in the crimping process to explain these approximately 2 to 1 reductions in modulus; nevertheless, this remains a possible ex- planation.

2120 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 Vol. 44, No. 9

Table 11. FSbctr, of Crimp on Tenaile Propertien (65% relative humidity, 70- F.; nte of atren. SO%/-.)

a-nsnier 3-Denier Medium Wool Nylon Tow Daaron Tow LOW ai>

Norms1 Crimped Normal Crimped orimp orimp

3.54 4.27

4.80 3.62 40 54

5.60 4.12 4.25 2.27

18.7 10.4

1.33 1.46

0.38 0.194

... 1 3 x 1 0 -

2.65 3.42

5.05 4.46

6.97 8.15 8.50 5.35

24 31

115. 49.1

1.17 1.42

0.91 o.7a

. . .. 21 x 10-4

3.93

0.99

1.08

30

1.19

40.5

0.31

0.179

5.2 X 10-1

4.48

1.01

1.32 1.10

30

30.4

0.31

0.183

18 x 10-4

(Figure 14. Typical Stress-Strain Curve for a Textile Fiber

The results on wool, given in the laat two columna of Table I1 and derived from the shagatrain curves of Figure 15, are free of thin ambiguity. These two samples of domestic medium wool were selected because one of them (column 6 ) had a much harsher feel in the bulk fiber form. When examined in the streagatrain teat, the harsh sample wan slightly coarser but also had a much higher uncrimping energyalmut three times that of the other sample (column 5). Table I1 and Figure 15 show that there waa an accompanying difference in modulus. All other parameters were very nearly the same for the two wools. Hence, i t 81181118 quite safe to conclude that the difference in observed modulus

STRUN %

F i p m 15. Effecm of Gimp Variation on Tensile Propertics of Wool

65% R.H.. 70' F.3 RI%/mh.

resulted from the difference in orimp and, further, that the effect of orimp is probably not entirely removed after the stress passes into the Hookean region. The resulting curve is still linear, aa may be understood from an examination of the diagram of Figure 16, which is a %hematic representation of the behavior of a crimped fiber. It is assnmrd, for simplicity, that the molecular network is perfectly elaatic up to a given strain and then yields and follows a linear viscoelastic curve. It is also assumed that the cross section of the crimped fiber illustreted in the diagram is divided into three segments of equal area, each supporting a stress, a, b, or c, depending on its location acrm the fiber in the plane of orimp. When the fiher is loaded, stress a will im- mediately incresse in tension along curve AD. Strews b and e will eventually come into play in tension, 88 the fiber is extended, along the Hwkean cnrvea, BE and CF (the initial stress b will, of course, be compressional). Yield will take place along the curve, DEFG. To obtain the theoretical resultant curve R for the crimped fiber, i t is only necessary to add the forces generat ing stresses a, b, c to give the total force on the fiber for each strain: then divide the total force hy the total crosa-sectional area Thin is accomplished numerically by adding the three s t rews for each strain and dividing by three to give the resultant strew The Hookean slope of the resultant curve, R, is clearly lesa than the slope of each of the component curves which r e p resents that ot a hypothetical uncrimped fiber having the eame molecular network propdies. Thus, the Hookean modulus should be less for a crtmped fiber, ae obae~ed in these experi- ments.

If thin interpretation is correct. it follows that the meaaured Haokean modulus ran no longer be r.onsidered aa the Young's modulus of a molecular network in simple teasion; i t is. rather, a

STRAIN

Figure 16. Theoretical Sueas-Strain Curve for a Crimped Fiber

September 1952 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 2121

MILTON HARRIS, president of Hams h e a r c h Lsboratori~: Dr. Mark has done his usual wonderful job elucidating the atrub ture of fibbers. I refer now particularly to his dincuasiou of orienta- tion, whicb might be considered a two-djmennwnal perfedion in fiben, Venus m y n W t y , which migbt be conaidersd a tdnee- dimmaionel pertectiOn. I wonder if he would cue to add my- thing concerning ths impixtmce of sire and dintribution of cry- W A o r example, we & have wme indications that in certain in- dustrial fabrIca such as tire cord, this particular qwtion hecomes impmrrnt Also, on the subject of cmee h k m g , Dr. Mark referred to the

importace of chemical cros linking in systems where you do not have many strong Latersl forces, 88 example, many intermolecular bydrogm bonds. Such in the case ~n rubber and in many of the protein fib. I wonder if he would care to nay my- ahqt the complications of covalent or chemical cmsn linking in syst8hw such an cellulose where we hare a high degree of lateral &a@ tkms4hat is, many strong fomea in the form of hydrogen bonUn. E. F. MARK: It is correct that, in d d e r i n g the erydlipe-

eryatalline domains and their orientetiou %- t but 4nount 480 $he Of amorphoua systsm of a fiber, not only

average siee of the erystsls. Crystalline domains really are a kind of mwdinking agent, only they are not strictly but extend over a Oertain area. That part of a chain which gme through a Crystsline domain is 6xed in space, whereas that pop tion which lien ip an amorphouq mea in qqucb, pop qp&. Small crystals hence rep-t a h l l y dispersed network of

6x paints, whereas big onen produce a earner syatsm of cross linka which may favor streas acaumulation and may not have an equally large resistance againat fatigue. As Dr. Dillon has pointed out, it in also nemssary to consider

the skin-core relationship of a apthetie fiber. The skin should, in @eraI, have a 6ner grain than tbe core, bemuse it ia expoaed to larger bending stFesses particularly in highly twiated structure#.

With regard to the influence of chemical cross linking m cellu- loaic fibers: It is well known tha+ aftertreatment with crosslink- ing agents such 88 formaldehyde, glyoxal, or diisocyanates in- e m s the wet streugtb and improves the dimeusio4 stability. The problems involved in such treatments are to avoid the OF-

currence ol harshness and brittlenem and to render the cmas linka sufficiently permnmnt 80 that they can mist repeated laundering. The general m n d atage cross liuking of a highly crystalline fiber is a di5cult and delicate procedure.

S. B. McFARLAIiB, Celaneae Corp.: In trying to point up tbe woollike properties of Dacron, Dr. Heck& showed us how its streakstrain is nemly the same 84 that of wool. The acetates curve &ea not vary too much from the stress-straiu curve of wool ; yet none of ua thinks acetate is woollike. How can we reconde thb with your thew? W. W. HECKBRT: Well, actually, of the cellulosics, there is

no queatmn but that acetate hes the more wmllike feel. We are not talking about redilience; we are just talking about feel. Acetate really haa tberight form of streaetltrain curve, and I have

a22 TND VS TR I A L A N D ' Em 0 I N B E KIN GI C HEM I S T R Y Vol. 44, No. 0