thermal shrinkage and mechanical crimping during false-twist texturing

7/24/2019 Thermal Shrinkage and Mechanical Crimping During False-Twist Texturing

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283

It ~ is difficult to establish formulas, even empiricalones, to predict contraction, which is due to circum-stances explained in the preceding conclusions. How-

ever, in certain concrete cases a modification of for-

mulas of either Obukh or Besset-Barella, adaptingthe coefficient that affects the term (/100)2, seemsto be

applicable.In some machines (BD 200 S) it seems that in theformulas recommended for the change wheel, cor-

responding to the linear density of the spun yarn,there is already a correction that takes account ofthe probable contraction of the yarn which, then, isminimized in relation with that corresponding to aconventional yarn of the same twist.

In the studied cases, it seems that contraction of

rotor yarns tends to be less than that of conventional

yarns for the same twist, which is explained by thedifference of structure of the two types of yarn.

Literature Cited

1. Barella, A., Law of Critical Yarn Diameterand

Twist, Textile Res. J. 20, 249 (1950).2. Barella, A. and Vigo, J. P.,An Application of

Mini-Computers to the Optimization of Open-End Spinning. Part IV: The Influence of

Winding Tension on Yarn Properties, J. TextileInst. 68, 143 (1977).

3. Braschler, E., "Die Festigkeit von Baumwoll-

gespinste," Doctoral Thesis, Zurich PolytechnicUniversity, 1935.

4. Carminati, C., "Il Filatore de Cotone," Hoepli,Milan, 1960.

5. Goswami, B. C., Martindale, J. G., and Scardino,F.

L.,"Textile Yarns

Technology,Structure and

Applications," Wiley, New York, 1977.6. Hearle, J. W. S., Grosberg, P., and Backer, S.,

"The Structure Mechanism of Fibers, Yarns and

Fabrics," Wiley, New York, 1969.7. Manich,A. M., "StructuralAspects of Open-End

Yarns," Doctoral Thesis, Polytechnic Universityof Barcelona, 1980.

8. Obukh, I. G., "O krapkovike i protnosti pochatkai usadke pryazhi na vaterakh," 1936, In: W.Zurek, "The Structure of Yams," see Ref. [10].

9. Ormerod,A.," Management of Textile Production,"

Newnes-Butterworths, London, 1979.10. Zurek, W., "The Structure of Yams" (Strukturaprzedzy) (from a Polish book published byW.N.T., Warszawa, 1971).

11. Zurek, W. and Piwowarska G., Blended Irregu-larity in Blended Yarns, Textile Res. J. 48, 528

(1978).

14.&dquo;$&dquo;&dquo; recdwdAugust 15, /Wt.

Thermal Shrinkage and Mechanical CrimpingDuring False-Twist Texturing

RICHARD G. QUYNN1AND WILLIAM H. POISSON

FRL,AnAlbany International Company, Dedham, Massachusetts 02026, U. S.A.

ABSTRACT

A mass flow analysis of the conventional double-heater false-twist process shows that the yarn

thermal shrinkage over the first heater equals exactly the imposed first overfeed.A

relationship isderived between the yarn " mechanical shrinkage," thermal shrinkage over the second heater, and the

imposed second overfeed. Single-heater results are shown to result from a special case of themore

general analysis. Some supporting experimental results are given.

Introduction

The false-twist texturing of continuous-filament yarnsconstitutes a truly revolutionary development in thetextile industry, primarily because the textured product

resembles in several important respects a spun yam.The transformation from the linear continuous-filament

bundle to the bulked yarn can be effected at high speedand with a good measure of control over the final

product. The mechanics of the process has been de-

veloped by Backer and his colleagues [ 1 ~ and by

Thwaites [9~.As fiber and textile

processes go,the

1 Present address: Jet Propulsion Laboratory, California Insti-

tute of Technology, Pasadena, California 91103, U. S.A.


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false-twist process is rather complex and not at alldevoid of scientific interest. One of us has had

occasion to comment on one of the morphologicalquestions involved [51.The present paper deals with what might be called

the macroscopic aspects of the so-called &dquo;constant-

extension&dquo; process, in which the yarn is held in thefalse-twist zone by positively-driven rolls. The anal-

ysis of mass flow is, of course, independent of the

particular fiber type and of the particular type ofmachine used. It is confined to conventional false-

twist texturing, but the same principles apply to draw-

texturing, and this was done by Brookstein and Backer[2~. The &dquo;black box&dquo; approach can yield certaingeneral principles, at the same time bypassing indi-vidual technical questions. Emphasis is placed on thedouble-heater process; it will be seen that single-heaterresults are obtained as a simpler, special case. The

analysis applieseither to

everyindividual filament or

to the yarn bundle as a whole.

Mass Flow

It is useful to visualize the two-heater false-twist

process as two &dquo; black boxes&dquo; in sequence, and to con-

sider in Figure 1 the mass flow of material through themachine. Consider the first two rolls,. having linear

speeds vi and v2, and everything in between as com-prising the first black box. In a given short intervalof time a small mass di X h, consisting of a straightlength of yarn tie and denier di, is introduced into the

box, and over the same period of time a mass d2 X t2is expelled. The yarn is straight on entering v2 andbecomes bulked immediately on leaving v2. Since inthe steady-state condition there cannot be any buildupor decay of mass within the box, and assuming no

slippage of the rolls,2

FIG. 1. Sketch of conventional two-heaterfalse-twist texturing process.

By definition, the fractional thermal shrinkage occur-ring over the first heater is

so that

or the first important result is that (TS), equals the

first overfeed, basedon

the speed of the firsl roll. If theyarn did not thermally shrink as much as the first over-feed allowed, then there would be a mass buildup in the

zone, and the machine would not operate.Similarly, vz, the second heater, and v3 can be con-

sidered a black box. In a given short interval of timeAt (e.g., 0.001 s) the roll V2 (the roll that has a linear or

peripheral speed V2) delivers a certain mass of yarnequal to t2 X d2. During the same time intervalAt,the roll v3 expels the same mass of yarn, which is con-fined to an end-to-end distance which we define as L$in Figure 2. That /2 is&dquo;straight&dquo; is an explicit assump-

tion whichhas

alreadybeen made. The mass of

yarnrepresented by L~ consists of a (sinuous or nonlinear)length of yarn t3 having a denier d3, i.e., if L3 were

straightened out but not stretched, it would equal ~G$.

FIG. 2. Sketch illustrating definition of mechanical shrinkage.

Thus, because of the equal mass flow, d212 = d3l3, or

The fractional &dquo;mechanical shrinkage&dquo; (M Sh, or more

properly the decrease in end-to-end yarn length byvirtue of the individual filaments passing from a straightto a curled or bulked configuration

We note that (MS)2 is basedon

the twice-shrunklength ts, and is essentially a geometrical quantity.

It must now be clearly understood that V2 = 12/At and

vs = Lsllt (and not t3/Al). This last expression is

subtle, and bears close pondering.3 Then

2Throughout the analysis we use the symbol d to denotedenier, as used in the traditional sense of linear density.

3

The yarn speed equals &upsi;s

onlyat

the nip ofthe

rolls;outside

the nip each filament goes off in some other direction, and itsactual speed multiplied by the cosine of the angle of inclinationis equal to &upsi;s.


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285

Since, by definition,

and

Equation 3 expresses the relationship between thefractional &dquo;mechanical shrinkage,&dquo; the fractional ther-mal shrinkage over the second heater, and the fractional

imposed second overfeed. The second overfeed is thatbased on the speed of the second roll. In order for

(MS)2to be a

positivenumber (i.e., in order to get any

bulking), (ON)2 must be greater than (TS)2. Thus, ifthere is a positive second overfeed, the yarn musteither bulk or shrink, and in general it does both.

Equation 3 can be rewritten in a form thatshows that (MS)2 is a linear function of (OF)2- IfK = 1/[1 - (TS)2], then

which shows that (MS)2 is linear with (OF).2, of slopeK and intercept (1 - K). Figure 3 shows this relation-

ship, for (TS)2 values rangingfrom 0 to

15%.

Fm. 3. Graphical relationship between mechanical shrinkage(MS)&dquo; thermal shrinkage over the second heater (TS):, and

imposed second overfeed (OF)t.

Equation 3 can be approximated in two ways. If

(TS)2 is a number small compared to 1, then one canuse the approximation that 1/ 1-- x = 1 ~- x, and

Equation 3 becomes

This expression corresponds to the intuitive feeling thatthe excess length represented by the overfeed is &dquo;usedup&dquo; by the mechanical and thermal decrease in length.The reason that Equation 5 is an approximation ratherthan an exact expression is that the basis length for thecalculation of (MS)2 is different than that for the othertwo terms. If one assumes that the thermal

shrinkageover the second heater (TS)2 is not only small butzero, then

Equation 3 (or 5) says that as far as (MS): is con-cerned, the first heater temperature and dwell time

play no role whatever, as long as the machine runs,and that (OF) influences (MS)2 only by its influenceon (TS)2. These purely formal consequences are validto the extent that the assumptions underlying Equation3 are valid.

ExperimentalIn most modem double-heater machines, the yam is

not taken up at vs, but rather at a later roll V4- We

now show how to calculate (MS)2 by measurementsmade on yarn collected (at some arbitrary packageoverfeed) at V4. Referring to Figure 1, the &dquo; mechanical

shrinkage&dquo; of yarn issuing from roll v, is

where L,=

94AI and Ls=

>g4l.For mass

continuity,tsd3 = l,d,. But since there is no denier change (there

being no heater between vs and V4), d= = d,, whereuponl3 = l,, regardkss of 1M ,soBut

so that

and therefore it iseasy

to show that

i.e., for a double-heater machine (MS)3 - (1~IS)! if

there is 0% overfeed to the package. For a single-heater machine, (MS)s is always equal to (MShbecause there is no M, and v, is the takeup roll. The

last expression can be rearranged to give

where by definition the package overfeed


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286

which enables us to calculate (MS)2 from experimentallymeasured values of (MS)a prepared under known

package overfeeds. Some results are given in Table I,

which refer to double-heater polyester runs. Thepolyester runs were conducted with du Pont Dacron

type 56 150/34/R10 yarns on anA.R.C.T. type FTF-440B machine equipped with a cooling or &dquo;extendedyarn path&dquo; device; single-heater runs were conductedon the same machine by bypassing the second heater.

TABLE I. Calculation of (MS)2 from (M S),.

Celanese 200/3Z/32 secondary acetate was run on anA.R.C.T. type FT-411 (single-heater) machine, whichhas no cooling attachment. Each experimental numberis the average of that obtained on two runs.

EXPERIMENTAL MEASUREMENT OF (MS)3 OR (MS)2

By definition,

which is simply the fractional amount of extension(based on the straightened length) required to straightenout but not stretch the bulked

yarn.The measure-

ment is most conveniently made on an Instron tester,although it can be done on a table top. A 5-inch(12.7-cm) initial gauge of nonwavy, nonsnarled, butunstretched yarn is established, by allowing the yarnto hang under its own weight.4 A pip marker marksthe chart position at which the bottom jaw beginsto move (in our measurements, at a rate of 5 in.per minute). The Instron curve is allowed to extendsomewhat beyond the yield point of the yarn. Theinitial Hookes law tangent is drawn down to intersectthe X- or extension axis. If x is the distance between

this intersection and the pip mark .(in units of inchesof jaw movement), then (MS)3 [or (MS)2 as the casemay be] is x/ (5 + x). This method of measurementdiffers somewhat from that utilized by the TextileResearch Institute [8] ; in the latter, the extensionis not carried back entirely to the point of first jawmovement, and the extension is divided by the initialunextended or crimped length. It is clear from itsdefinition that (MS)2 takes no account of the detailed

geometry of the bulk, and that many different geome-tries could lead to the same (MS)2 value.The entire notion of (MS)2 represents an oversimpli-

fication of the actual situation. Once the yarn has

been twisted, heated, untwisted, and bulked, it is

quite impossible to restore it to a state in which atthe same instant every individual filament is &dquo;straight-ened but not stretched.&dquo; Microscopic inspectionshows that even at the breaking point of the textured

yarn some filaments retain a significant curvature.The problem has been discussed by Skelton [7].

EXPERIMENTAL VERIFICATION OF THEANALYSIS

For a single-heater machine (MS)2 = (OF)3 exactly,because there is no second heater and (TS)2 = 0; inthis case (MS)2 is based on the once-shrunk length.Table II shows that the agreement here is fairly good.The overfeed figures are presumably quite accurateand derive ultimately from gear tables supplied by themachine manufacturer. The mechanical shrinkagemeasurement, however, is subject to some experimental

error and inherent yarn variability.TABLE II. Single-heater runs.

In order to demonstrate the validity of the analysisfor a double-heater machine, one must have some inde-

pendent measurement of thermal shrinkage, which is

provided by measurement of the deniers before and after

passing through the machine.

4

One must be very careful here. At this point the yarnpossesses a great deal of latent crimp, as testified to ir. [4].Familant [3] has also commented on this matter.


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287

The reason that (TS),,,,,,i is not simply the sum of thetwo individual shrinkages is that the basis length forthe shrinkage calculation is not the same for (TS)1and (TS)2.

For a double-heater run, we can take the corre-

sponding (MS)2 value, calculated from Equation 6,and with the help of Equation 3 calculate (TS)2; we

know also that (TS)1 equals (OF)1. (TS)~o~8, can thenbe determined by Equation 7 and compared with the,value obtained by the denier measurement. Such a

comparison is given in Table III. Comparison of thelast two columns shows that the agreement is quitegood, considering that Equation 3 has some substantial

assumptions built into it. For the comparison to be

legitimate, however, the (MS)s and denier measure-

TABLE III. Double-heater polyester runs.

ments must be made under the same loading. In these

(MS)a measurements with polyester yarns, the stress

immediately above the point x on the extension axiswas

approximately 0.1 gpd, whichis

thesame as the

loading under which the deniers used for the lastcolumn of Table III were obtained. The agreementis probably better than could be expected, because ofthe well-known difficulty of measuring the (inherent)denier of a textured or bulked yarn. The textured

yarn deniers were determined by loading to 0.1 gpd,and cutting out and weighing 90-cm lengths. The

weights in each case were corrected for the amount offinish on the yarn (approximately 1.0%) as determined

by perchloroethylene extraction. ,

Factors

AffectingMechanical

Shrinkagein

Double-Heater Operation

According to Equation 3, the mechanical shrinkage(MS)a during double-heater operation is determined

by the imposed second overfeed and thermal shrinkageover the second heater. The latter, of course, refersto the actual thermal shrinkage occurring in the par-ticular situation and, as such, depends on both machinefactors and the propensities of the particular feed yarninvolved. For common operating variables of manyfalse-twist machines, we would estimate that the total

potential thermal shrinkage over both heaters, for the

customary types of polyester feed yarn, would notexceed about 10%. Of this 10%, an amount equal to

the imposed first overfeed is used up over the firstheater, according to Equation 1. The remainder isthen available for shrinkage over the second heater(to the approximation indicated by Equation 7).Whether the full amount of this remainder is actuallyrealized obviously depends on such things as the tem-perature and dwell time on the second heater, the

ef6ciency of yarn-heater contact, etc. Table III indi-cates that in these particular runs about 5% (TS)2 wasrealized. Even if one knows all the machine conditions,it is difficult to predict (MS)2, because the requireddata on rate of thermal shrinkage as a function of

temperature and tension, especially at very short times,are not ordinarily available. A further complicationarises with polyester because of its &dquo; thermal memory&dquo;:the exact amount of thermal shrinkage over the secondheater will depend on the amount and circumstancesof the first heater shrinkage. Additionally, in its

passage over the second heater, the yarn is in a bulked,

albeit nearly untwisted,state. One would

expect that,other things being equal, increased second-heater tem-

peratures5 and increased second-heater dwell timeswould lead to higher (TS)2 values, and hence lower

(MS)2 values.(MS)! or (MS)a is a (rather crude) measure of the

degree of crimp development obtained from anyparticular set of machine conditions. According to

Equations 3 or 6 it is determined only by the imposedoverfeeds, and not at all by the number of turns/inchof twist inserted, except insofar as the latter mightinfluence (TS)2- The space-filling propensites of abulked yarn will depend on the crimp amplitude and thenumber of crimps/inch. For any given (MS)2 thelatter would be expected to be determined largely bythe inserted turns/inch, and the crimp amplitude wouldbe influenced by the total denier.

Excess Length and Start-Up of the Machine

In the operating steady-state, if vl > V2 the machineis said to be operating at a positive or (+) overfeed.At zero first overfeed (VI = 11t) no thermal shrinkagecan occur, and (TS) equals precisely the first over-

feed, as long as this is positive. The latter cannot beincreased indefinitely, however, because the yamwould not thermally shrink enough to take up the slack.Similarly, increasing the negative first overfeed moreand more would increasingly stretch the yarn, raise the

running tension, and eventually break the yam.The yarn that is required to accommodate twist con-

traction in the operating steady-state has already beenstored in the machine. When the machine is first

strung up and started, the situation is different from the

operating steady-state. Although the practice varies

among machine operators, an operable sequence of

5 It is probably true [6] that polyester thermal shrinkage passes

througha maximum near 200C, but the shrinkage is cumulative

and theyarn must pass through 200C on its way to highertemperatures.


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288

operations is to engage V2 (see Fig. 1). then engage thespindle (with the heater turned on), and finally-actually only a fraction of a second later-engage vi.As long as vi is not engaged, the twist can run all the

way back to the supply package, and the machine is

pulling in whatever length is required to accommodatethe twist contraction (determined by the spindle speed,among other things) and thermal shrinkage over thefirst heater. Once vi is engaged, however, the machine

quickly comes to a new steady-state, and the twist doesnot extend beyond vi. As long as vi is not engaged, themachine is operating in a positive overfeed conditionin the sense that the yarn speed at roll Il is greaterthan that at V2-

II is perfectly true that when the machine is operating(whether vi is engaged,or not), at every instant thereexists between vi and v~ a length of yarn greater thanthe straight-line distance between vi and V2, because oftwist contraction. Just beyond the spindle, this excess

length is &dquo;given back,&dquo; so to speak, until at v2 the yarnis untwisted (for Rotoset feed yarn) or back to the lowtwist characteristic of the feed yarn. The amount

of this excess length could be calculated if one knew(a) the twist contour--,i.e., the actual twist at everyposition between vi and V2, and (b) the analytical rela-tionship between the twist and the twist contraction,which involves the total denier. However, this

&dquo;storage&dquo; function of the machine has nothing what-ever to do with the black-box argument, which de-

pends solely on what enters y during a given time in-terval, and what exits from v2 during the same interval.

Yarn Stretching During Steady-State Operation

The question is often asked: Is there any stretching ofthe yarn within the first zone during steady-stateoperation with zero or positive first overfeed? Theanalysis of mass flow provides no answer, of course, be-cause it deals only with the itet effect. We believe thatthe answer is in general no for several reasons:

1. No stretching is required in order to provide theextra length demanded by twist contraction, becausethat excess length is already stored in the machine.

2. I)enier measurements on yarn made before and

after single-heater runs show no sign of a significantreduction of denier, although, to be sure, the accuracyof the measurement is not suflicient to detect a small

amount of stretching. One would have to measure thedenier of individual filaments. The tensile propertiesof yarns after single-heater processing are consistentwith their having been slightly shrunk rather than sub-stantially extended-i.c., they show a somewhat re-duced tenacity, and modulus and somewhat increasedelongation.*

3. In order for Equation 1 to be satisfied, stretchingwithin the first zone would have to be followed by re-

covery from that stretching (&dquo; elastic recovery&dquo;).Photography of yarn under simulated running tensionreductions shows absolutely no length recovery. The

implication is, then, that there was no significant priorstretching.

ACKNOWLEDGMENTS

We are indebted to J. W. Whitworth and E. Nortonof Milliken Research Corporation, Spartanburg, S. C.for experimental work, to Dr. J. Skelton of FRL for the

photographic work, and to Drs. K. R. Fox and M. M.Platt of FRL for encouraging discussions.

Literature Cited

1. Backer, S. and colleagues, Mechanics of TexturingThermoplastic Yarns. Part I :, Textile Research J.46, 599-610 (1976); Part II :, 46, 724-733 (1976);Part III :, 46, 802-809 (1976); Part VI:, 48, 198-218 (1978); Part VII:, 48, 722-732 (1978);Transient Threadline Behavior in False-Twist

Texturing, J. Textile Inst. 67, 224-226 (1976).2. Brookstein, D. and Backer, S., Mechanics of Tex-

turing Thermoplastic Yarns. Part V: Steady-State Mechanics of Draw Texturing, Textile Res. J.

47,256-266 (1977).3. Familant, H. M., Dependence of Latent Crimp on

Wind-Up Tension in the False-Twist TexturingProcess, Textile Research J. 47, 448-449 (1977).

4. Gupta, V. B. and Natarajan, M., Latent Crimp in

False-Twist-Textured Polyethylene TerephthalateYarn, Textile Research J. 46, 417-419 (1976).

5. Quynn, R. G., The False-Twist Texturing of Various

Fibres, J. Textile Inst. 62, 510-511 (1971).6. Ribnick,A., The Thermal Shrinkage of an Oriented

Polyester Yarn as a Function of Time, Tempera-ture, and Stress, Textile Research J. 39, 742-748

(1969).7. Skelton, J., The Effects of Planar Crimp in the

Measurement of the Mechanical Properties of

Fibres, Filaments, and Yarns, J. Textile Inst. 58,T533-556 (1967).

8. Textile ResearchInstitute, "Stability

of

Crimpand

Mechanical Properties of Textured PolyesterYarns to Solvent Treatments," Notes on Re-search No. 226, Princeton, N. J., November 1972.

9. Thwaites, J. J., Mechanics of Texturing Thermo-

plastic Yarns. Part IV: The Origin and Signifi-cance of the Torsional Behavior of the False-Twist

Threadline, Textile Research J. 46, 886-892 (1976) ;Thwaites, J. J., Brookstein, D. S., and Backer, S.,Deductions about the False Twist Process from

Observations of the Variation of Torque on De-

twisting a Twisted Heat Set Yarn, J. Textile Inst.

67, 183-186 (1976).

Manuscript received July 18, 1979.

thermal shrinkage and mechanical crimping during false-twist texturing

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