Tensile Properties of Newer Fibers J. H. DILLON Textile Research Institute, Princeton, N. . I .
H E primary mission of a I n the first section of t h i s paper, stress-strain curves to larly the degree of orientation. textile fiber is to bear rupture are presented for single fibers of vismse rayon, Furthermore, some of the a load and respond to aeetate rayon, nylon, Orlon, Dacron, dynel, Acrilan, fibers chosen, especially Acrilan changes in load during procX-51, Vieara, a n d regenerated silk. T h e relative stressand X-51,are still in pilot essiog and seMce. There m e s t r a i n behavior of t h e m iihers is discussed in t e r m s of stages of development and many secondary but importheir ehemioal structures, and the limitation of this type may be expected to change tant qualities that i t should of evaluation ie stated. In the s e m n d portion of the considerably in constitution possess, such as dyeability, paper, repeated stress-strain d a t a are presented i n the and properties within the next chemical stability, and miniform of “ C y d i U g proliles,” a n d the viscoelastic properties year or two. Hence, it must mum static charge generof the various fibers thus revealed are oompared. In the be emphaaiaed that the manation. Moreover, there are t h i r d seetion of the paper the effeets of fiber morphology made fibers studied in this upon mechanical properties are discussed, a n d the effects economic factors of price and work must not he considered, availability which are often of crimp on the measured Hookean modulue are given a t this time or in the future, and interpreted. controlling. The types of a8 truly representative of loads to which textile fibers the producta of the various are subjected vary widely from suppliers. It should be menthe simple tensional to the tioned also that these fibers more com~lexbending. Rates varied widelv in amount and of loading vary from the essential!y static to the impact and high type of crimp, and this fact is probably partially responsible frequency types found in parachute shroude and tire cord. for the differences in stresestrain behavior exhibited. Response to load by the hydrophilic fibers is governed critically The natural fibers selected for the comparative stress-strain by the relative humidity of the environment, whereas the studies were: (a) a typical medium domestic wool obtained from hydrophobic fibers are more sensitive to changes in temperature. the Sheep Experiment Station of the U. 6. Department of AgriThis paper is limited to a study of the tensile properties of several culture, Duhois, Idaho (T.R.I. m t e r wool WC-5); (b) a l’/w natural and man-made fibers. I n accepting this limitation, howinch staple American rotton; and (e) a sample of Japanese silk ever, it is acknowledged that the performance in tension of any of unusually high tenerity (5.6 grams/grex). This sample of fiber is only a part of the story of its load-bearing quality-” natural silk is certainly not representative of silk used today important part, but selected mainly because i t is easier to measure but still may be considered a “target silk” in respect to the synthe properties in tension than under the more complex conditions thetic fibers. Had not international political and economic 01 processing and service. The eKects of water content, repeated factors interfered, this night have been the silk of commerce cycling, and fiber morphology are discussed in a limited manner today. A regenerated silk (natural silk dissolved and spun aince they rank in importance with the chemical structure of the into B continuous filament) was also included. The exact pmcfiber in determining its tensile behavior. em of its production in Japan is unknow. However, i t was considered to have interest as a rather unusual regenerated proSELFXXION O F FIBER SAMPLES tein fiber. For the studies of the eKects of crimp, given in Table 11, Most of the tensile data on man-made fibers in the literature special mmples of normal and crimped tow of 3-denier nylon have been obtained on fiber samples tested by scientists of the and Dacron were kindly supplied by H. F. Hume of the Du fiber supplier organizations or submitted by them to other Pont Co. The rather unusual low crimp and high crimp wool laboratories for examination. There is no question that these asmples of Table I were supplied by T. D. Watkins of the Bureau data are authoritative but, in general, they refer to fibers which of Animal Industry, U. S. Department of Agriculture, Beltaville, meet the rigid specifications of the scientists who developed them; Md. on the other hand, they often are not representative of the fibers supplied to the textile industry for proeesaing. Hence, it was EXPERIMENTAL CONDITIONS decided, for comparative stress-strain studies, to employ 3denier staple fibem of viseose rayon, acetate rayon, nylon, Orlon, Thp wight per unit length 01 each fiber was rneaeured by tbe T.R.I. vibroscope method ( 6 ) . .&uminga value for thedensity, Dacron, dynel, Acrilan, X-51, and Vicara which had been subrhe crosd-swfioiixl area was ralculated. All stress-strain snd remitted to a textile manufacturer for pilot spinning snd weaving experiments by the several fiber suppliers. Thus, it seemed re% I-inch and tested In IhQ lnstron tensile tQRtinE machine. Thr mnable to suppose that these man-made fibers could be desigconatant rate 01 crosebeud travel w88 0.5 inch per minute (50% nated a8 “specially controlled production samples.” It is well strain Der moutel fur all the stremtltrain curves to break: 0.1 known, of course, that the properties of man-made fibers can inch der minute ‘(107 strain per minute) for all nonrupture be changed over wide limits by variations in prooessing, particustress-strain curves. %treM-h-ain curve8 t o break were ob-
T
~
September 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
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 7 0 ”F.). Curves presented in Figures 1to 6 are for the averages of ten fibers in each case. Strain origin was selected as the intersection 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.
Table 1.
Cycling Test Procedure
Gage length 1 inch Relative humidity, 65% Temp., 70’ F. 3% Test 1. Extend fiber 0.03 inch (3% of initial length) a t 0.1 inch/niin. and R tract a t same rate t o zero load 2. Immediately re-extend fiber a n additional 0.03 inch a t 0.1 inch/min. m i 1 retract a t same rate t o zero loa,d 3 . Repeat step 2 4. Immediately re-extend fiber a n additional 0.03 inch a t 0.1 inch/min. 5 . Allow fiber t o relax 4 to 5 minutes a t the final elongation of step 4 6 . Retract fiber to zero load a t 0.1 inch/min. a n d immediately re-extrnrl a t 0.5 inch/min. to rupture 10% Test Same operations a s 3% test, except 0.1 inch (10%) extension employPd steps 1 t o 4
iii
20 5%Test Same Operations as 3% test, except 0 . 2 inch ( 2 0 7 0 ) extension enlployed in steps 1t o 4
’“1 2.0
k T T O N
OY
0
10
20
I
I
30
40
I
50 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 modulus, 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 vibroscope 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
2116
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 condition; 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 stressstrain 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.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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 fibersfor each wndition. Vicars (regenerated w m protein fiher) behaves ~ t ~ i k h g llike y 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 streenstrain curve to mptyre gives little information concerning the reversihility of the elasticity-i.e., resilience. In an effort to
‘“I 0
a
-
Figma 4. Silklike Fibers h 65% R.E., 700 F. e r water SOgb/PiP.
m
3 0 4 0
6.e of . e n
Finme 5.
Woollike Fibers
Y >
10
-
wet = d
J
STRUN-X
o
o
-
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 tenacity whereas the ratio of vicars is about 40%6. Undoubtedly, orotein fibers can the wet tenacitv of Vicara and other reeenerated and will be improved, but it is significant that Vicara is now being successfully blended with wool to aive Yams of unusual “loft.” Hence, tenacity is not in itself t o o k p o h n t for fibers designed for wool type processing unless an increase in abrasion resistance is desired. In that event, the use of one of the stronger but still highlyextenaible synthetic fibers would be indicated. All the synthetic fibers tested are represented in the strew strain c w e 8 (dry) of Figure 6. Again it must he emphasized that these curves are chsracteriatic only of the f i b m tested in this study. They apply today but perhaps not tomorrow. Hence, it seems necessary to w n h e comments to the fact that the nylon is unique among all these synthetic fibers in ita very low modulus and lack of a definite yield point in the low strain iegion.
September 1952
develop a simple picture of the reversible elasticity of theee vnrifibers, the cycling experiments of the next m i o n were perfomed. STRESS-STRAINCYCLING STUDIES
Repeated streae-atrain cycling bss been employed by numerow workers in experimental studies of the mechanical properties of fibers. Extended and reasonably sncceeaful theoretical h a & ments of cycling data have been given by Leaderman (4) and Eyring, Halsey, White, Burte, and others (1-3). The moat comprehensive experimental study of this sort, however, w88 that of Snsich and Backer (7). These workers carried out carefully controlled streakstrsin experiments on low twist ysrna and a few monofilaments of 26 different fibers and then devised a novel graphical means for representing the data in t e r m of immediate elastic recovery, delayed recovery, and permanent set. There is no question that the work of Suaich and Backer is a classic in this
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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 measurement 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 difficult and time-consuming, much more SO than is measurement
3l
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 relative loads (and, therefore, relative stresses) is of va.lue in indicating 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 Progression of stress with cycling, relative to breaking stress 3. Stress at 3, 70,or 20%, relative to breaking stress 2.
2
/
I n 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 probably essential For satisfactory proceMing. But, lacking knowledge 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
8
100% (BREAK)
n
n
/i
3
/7
BRK
2
3
Figure 8.
I
TEST
,-1OOX
Figure 7.
IOXTEST
Relaxation
0 S07?o/min.
(BREAK)
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 decided to employ single fibers rather t,han yarns in order to avoid m y possible ambiguities resulting from the effects of yarn geometry. The experimental technique of the cycling tebts carried out in this work has been described under “Experimental Conditions,” a,nd the test procedure is given in Table I. A typical set of cycling tests, obtained with single dyne1 fibers a t 65y0relative 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 decided simply to read directly from the recorder charts the maximum 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 sequence and fiber under investigat,ion, and plotted as shown in t.he bar charts of Figures 8 to 13 (cycling profiles). It is immediately 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,
POXTEST
CJ cling Profile for Elastically Ideal Fiber IO&%/min.
2118
n
COTTON so3265 Mg/cmZ
100% ( B R E A K )
a
-1
W L1:
ACETATE STAPLE
3XTEST
Figure 9.
IOXTEST
Cycling Tests for Cellulosic Fibers 65% R. H.,70” F.
IO%/rnin.
a Relaxation
050%/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 employed in the experiments. Herein lies the first limitation of this
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 9
S Y N T H E T I C FIBER simple form of presentation-it ignores the shorter period time effectswhich may be quite important. Each group of three pmsles may be comidered ae a fatigue pattern for a fiber Since it 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 inatability 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%) it 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 synthetic fibem which are purposely supplied in a low state of orientation. The phenomenon of thixohpy,'exhibited by animal 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, it 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 W t 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 8 8 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
h
-4PYprtge
cotton under this condition. For the 3% viscose and acetate 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, relative 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 extemibility, 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 relaxation; 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 thixotropyi.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 w a s values exhibited 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 lowfrequency crimp), these synthetics appear elaaticslly comparable ~
.
.
ACRILAN STAPLE
ln
ln
w
ACCO X-51
U
a
n
3 % TEST
IOXTEST
n
ZOXTEST
Figure 12.
Cycling Tests for I h c r o n and Dyne1 us. Wool 65% R. H.,70' F. lO%/min. Relaxation 050% min.
to wool in the 3% test, although dyne1 has considerably more relaxation. I n the 10% test, howver, wool shows its characteristic 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 considerable caution because of the probability that their constitutions 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 b y 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 remembered 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
2120
3 %TEST
IO%TEST
20VJEST
Figure 13.
Cycling Tests for Acrilan and X-51 D S . Wool a IO%/min. 65% Relaxation R. H.,70' F. 050%/min.
20% strain in Figure 14), which results when thc. rate of relaxation temporarily exceeds the rate of extension, ii often absent. Frequently, as for dry silk (Figure 1) or wet viscose (Figure a), the yield region is not sharply defined. The most striking departure is shown by nylon, which gives a curve with the yield behavior displaced to the high strain region (Figure 6). Obviously, in cases such as these, the yield stress and strain parameters have no meaning. I n other cases, there is no early linear region of the curve; thus the Hookean modulus is not defined, I n 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. I n 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 elevated 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 explanation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 9
Table 11.
FSbctr,of C r i m p on
Tenaile Propertien
(65% relative humidity, 70- F.; nte of atren. SO%/-.)
a-nsnier
3-Denier
Nylon Tow Norms1 Crimped
Daaron Tow
LOW
ai>
Normal Crimped
orimp
orimp
3.93
4.48 30 1.01
Medium Wool
3.54 40 4.80
4.27 54 3.62
2.65 24 5.05
3.42 31 4.46
5.60 4.25
4.12 2.27
6.97 8.50
8.15 5.35
18.7
10.4
115.
49.1
30
0.99
1.19
1.32 1.10
1.08 40.5
30.4
1.33
1.46
1.17
1.42
0.31
0.31
0.38
0.194
0.91
o.7a
0.179
0.183
...
13x10-
(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 e n e r g y a l m u t 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, it 81181118 quite safe to conclude that the difference in observed modulus
. . ..
21
x
10-4
5.2 X 10-1
18
x
10-4
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 a c r m the fiber in the plane of orimp. When the fiher is loaded, stress a will immediately incresse in tension along curve AD. S t r e w s 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, it is only necessary to add the forcesgenerat 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 r e w s 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~edin these experiments. 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; it is. rather, a
STRUN %
F i p m 15. Effecm of Gimp Variation on Tensile Propertics of Wool 65% R.H.. 70' F .3 RI%/mh.
September 1952
STRAIN
Figure 16. Theoretical Sueas-Strain Curve for a Crimped Fiber
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2121
MILTON HARRIS, president of Hams h e a r c h Lsboratori~: 6x paints, whereas big onen produce a earner syatsm of cross linka which may favor streas acaumulation and may not have an Dr.Mark has done his usual wonderful job elucidating the a t r u b ture of fibbers. I refer now particularly to his dincuasiou of orientation, whicb might be considered a two-djmennwnal perfedion in fiben, Venus m y n W t y , which migbt be conaidersd a tdneedimmaionel pertectiOn. I wonder if he would c u e to add mything concerning ths impixtmce of sire and dintribution of cryW A o r example, we & have wme indicationsthat in certain industrial 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 myahqt the complications of covalent or chemical cmsn linkingin 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 erydlipeamorphoua systsm of a fiber, not only eryatalline domains and their orientetiou t but 4nount 480 $he Of average siee of the erystsls. Crystalline domains really are a
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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
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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 stFessesparticularly in highly twiated structure#. With regard to the influence of chemical cross linking m celluloaic fibers: It is well known tha+aftertreatment with crosslinking agents such 88 formaldehyde, glyoxal, or diisocyanates ine m s the wet streugtb and improves the d i m e u s i o 4 stability. The problems involved in such treatments are to avoid the OFcurrence 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 t b e 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 t h e w ? 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
TND V S TR I A L A N D ' E m 0 I N B E K I N GI C HEM I S T R Y
Vol. 44, No. 0
