CA1107022A - Method of drawing nylon monofilaments - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process is provided for improving the longitudinal and loop tenacity properties of polyamide monofilaments. The process contemplates the treating of an undrawn, spun polyamide monofilament with a vapor medium such as steam and, while the monofilament is exposed to such vapor, drawing the monofilament to a draw ratio of from 2.0:1.0 to 4.0:1.0, and further drawing the monofilament in a subsequent drawing operation to an aggregate draw ratio of from 5.0:1 to 8.0:1. The drawn products are particularly useful as reinforcing elements in rubber bodies such as tires.

Description

B~CICGROUND OF THE INVENTION AND PRIOR ~RT
.. . .

The presPnt invantion ralates to a procass of treatin~

polyamid~ monofilam~nts with a vapor medium while drawing th~
monofilaments to improve longitudinal and loop tenacity proper-ties. The process is aspecially advanta~eous for the devalopment of such proparties in relatively high deni2r monofilaments having deniers in the ran~ o ~rom about 500 to about 50,000.
In accordanca with the procass ! improved ~enacity properti~s and combination~ of tenacity properties are obtained as well as improvements in resistanc2 to fibrillation and ~atigue propPr-ties, all of these proparties ~eing o~ particular signiicance in tire cord materiaI where ~etter flex li~e and tir~ lifa are results of these improvements~ The process does not s~bstantially alter the surface smoothness of th2 monofilament which is desir-able in tire applications since it has ~een ound that ~ur~ace irregularities or etching of the surface can reduce the adhesion between the monofilament and the elastomPric ti~a material~

The improvements in lon~itudinal and loop t~nacity properties as well as tha improved combination~ of these tenacity propertiPs in polyamide monofilaments obtained in accordance wi~h tha process of the prasent invention~ re~lect ~h2 di~cov2ry that an initial vapor treatment of a substantially noncrystallinP
polyamide monofilament which has not been previously drawn for purposes of orienta~ion and that the partial drawing of th~

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monofilament durin~ such vapor tr~atment lead to a morphological condition of the partially drawn monofilament which provides increased tenacity values and the selective modification of both lon~itudinal and loop tenacity properties.

The ahility to selectively modify ~he tenacity properties through the use o an initial orientation drawing step in a vapor medium in accordance with the subject process has not heretofore been recognized in the art. In conventional oven drawing, loop strength increases with increasing draw ratio at low drat~ ratios, then decreases with increasln~ draw ratio.
Provided the material is not overdrawn, longitudinal tenacity continues to increase with increasing draw ratio~ Tha selsctive modification of tenacities is not possibl~ in conventional oven drawing techniques, and the vapor medium treatment in accordance with the subject pxocass is not a mere substitution of heating media as demonstrated in ~rea~er detail below.

The prior ar~ discloses numerou~. processes for heat treating polyamide monofilaments to increasa their crystallinity or otherwise modi~y particular properties thereof. A process for improving loop strength and knot strength of polyamide monofilaments is disclosed in U. S. Patent No. 3,59S,952. In accordance with this patent~ polyamide monofilamen~ which has t ` ~ G _ 7~

been ini-tially, conventionally drawn to a draw ratio of at least 3.0 and preferably at least 3.5 is then drawn in a steam ~apor environment at a speed ratio in the range of 0.85 to 1.15 As demonstrated hereinafter, the initial or first vapor drawing step o~ the subject process is essential and the advantages of the present invention cannot be otherwise obtained. The use of steam vapor is also disclosed in U.S. patent Nos. 3,671,381 and 3,650,884 to provide monofilaments having highly etched surfaces and porous surfaces contrary to the present process.
The invention, as herein claimed, is a process for treating a polyamide monofilament to improve its flex life and tenacity properties, essentially comprising the sequential steps of: partially drawing, prior to any previous orientation drawing thereof, an extensible spun polyamide monofilament having a relative viscosity of at least 2.5 and a denier of from 500 to 50,000 in a draw ratio of at least 2.0:1.0 and less than 4.0:1.0 while contacting said monofilament with a vapor medium at an elevated temperature and pressure; and then further drawing said monofilament in a subsequent drawing operation to an aggregate draw ratio of from about 5.0:1.0 to about 8.0:1.0, and wherein the subsequent drawing operation is done while the monofilament is heated to an effective temperature of at least 550F in dry heat.
The preferred technique involves a partial first draw in which the vapor medium contains entrained liquid in droplet form.

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Tha polyamides contempla~ed in this invention are of fiber fo~min~ molecular weight, crystallizabl~ and have a xela~ive viscosity of at least 2 . 5 as determined on a 1~8 solu tion in concentrat~d sulfuric acid at 25C., and include, for example, polycaprolactam, polyhexamethylene adip~mide, poly-hexamethylene sabac~lide, copolymers of the for~oin~ polyamides, and the poly~mides based on the condensation product of 1,4-cyclohexanebis(methylamine) and a linear aliphatic dicàrboxylic acid. The crystallizable polyamide materials can be formed into filam~nts using conventional technqiues such as melt spinning.

The undrawn danier of the spun monofilament is pref~rably in the range of from about 5qO to about 50,000. The monofilamants may have circular or acircular cross sections such as r~ctan~ular or ribbon-like forms.

Th~Q term "substantially noncrystalline" as used herain is intended to in~lude "noncrystallin~" as wQll as unstabl~ crystal forms which may, during spinning and handling imm2diately thereaftex and prior to any drawin~o~ the monofil~men~ occur spontaneously or as a result of handling or tempQrature chanyes.
Accosdin~ly~ the phrase substantially noncrystallin~ necessarily excludes those polyamide monofilaments which have been d~aw~
for purposes of orientation and, for example, hav~ been drawn to , a draw xatio o~ 1. 5 :1. O . ~side from positive drawin~ s~eps, there is a comparatively minimum ~lount of preorientation and stable crystal structure in monofilamsnts of the size ox d~nier contemplated herPin for polycaprolactam. The unstable he~a~onal crystal form can form on spinning. Durin~ drawing, th~ crystal structùre i~ transformed to the thermodynamically favored mono-clinic form. Typical characteri2ing properties o~ substantially non~rystalline polycaprolactam monofilaments include a bire fringence valus of 0.01 and a sonic modulus of 20 g/dO Tha birefringence and sonic modulus are determin~d in the manner described in Morgan, Vol. 42, ~ ,8~6, (1966). The wide angle x-ray pattern is essentially an amoxphous halo with some hexa~onal crystallinity present. Tha density is 1.12 g~cm3 as m2asured by the density gradiant method using a toluene~carbontetrachlorid~

Tha vapor medium may comprise any suitàble liquid vapor ox mixtures of liquid vapors which plasticize without physically de~rading polyamides, and which can be maintained as a vapor medium at a pressure and ~empexature relationship within the ran~es contemplated hexein. Examplas o suitable materials include water, aliphatic alcohols such as methanol and mixtures of these materials. In addition, monofilament modifying agents may be incorporated in the ~apor medium in rela~ively lessPr amounts to achieve particular modifications of tha monofilament.

~ a practical and commercial consideration, liva st_am is typically employea as the major or sole component of tha vapor medium. Accordin~ly, the process of th~ pxas~nt invention is herainaEtar d2scrib2d and illustratPd with raference to the use of live staam as a vapor medium, it being understood that the use of the aforemantioned liquid vapors or mixtures . .,, ~ .
thereof~ar~--subject to paramat2rs corraspondin~ to those discussed with resp~ct to stsa~. Furthar, the vapor medium or st~am may initially be in a dry condition i~ the s~onse that entrained liquid in droplat form is not presant since the vapor will condense upon contactin~ th~ relatively cool sur~ace of the monofilament. Typically~ live steam containing about 3~ by waight an~rainad water in droplet form has provided suitable processing, and water levels on the order of 1% by weight have also bePn found satisfactory.

The ef~eative prassure xang~ for tha st~am treatman~
~s from 1 psig. to 150 p5ig. Howev2r, it should be apprecia~ad th~ the minimum possible operatin~ pressure merely must assure a monofilament temperature which is ~reater tha~ the apparent second order transition temperature of the particular polyamide.
Similarly, the maximum operating pressure is such that tha corresponding temperature is less than the effectiva melting tempPrature of the polyamida. Howev~r, the pr~erred maximum opPrating pressure is ganerally limited by the occurr~nce G~
surfacP irregularities ar surface etchin~ o~ ~he monofilamen~.
The specific pressure at wh~ch etchin~ first occurs is dependent on tha dwoll or contact timo. For a dwall time of 4 to 6 secands, a maximum operatin~ pressur2 of about 70 psig. is typical~

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The residence or contact time of -the mono~il.ament in the steam treatment is generally less than 10 seconds. Although a longer residence time wlll also provide the desired results, a ~u~ther advantage of the steam medium lies in the fact that such results are typically obtained in less than 10 seconds which is commercially quite practical. The preferable contact time resul-ting in the improved physical properties obtainea herein is related to variables including the particular vapor or steam treatmen-t, draw ratio, and the characteristics of the particular polyamide monofilament being treated. As a practical matter, the linear p~ocess.ing speed is usually greater than 10 feet per minute and the s~l~ctcd steam pressure has a corresponding temperature in excess of 100 C.
The term draw ratio as employed herein is generally in accor-dance with the prior art usage of this term, and it is employed to denote the weight ratio of a unit length of the monofilamentbefore and after drawing. In this instance, the weight ratio is appro-ximated by the reported speed ratio of the rolls be-tween which the monofilament is drawn.

It has been observed that the use of live steam in the dra-wing operation tends to facilitate the draw and permi-t eEfective drawing at somewhat lower temperatures, These observations are believed to be associated with the superior heat transfer proper-ties o~ a vapor medium such as steam and a plasticiæing effect by the absorbed water. The plasticizing eEfect of the absorbed water or similar plasticizing agents such as low molecular weightalcohols on the monofilament is considered to effec-tively lower -the appar~lt second orcler transition temperature and perlllit drawiny to occur at a lower temperature. An initial draw of this -type results in a somewhat lesser degree of molecular arrangement but enables an increased amount of further molecular modification as compared with prior art technlques, and it is believed -to result in an in-terme-. 7-~7`~

diate monoEil~ment mor~hology which uni~u~ly ~nables improved tenacity properties and combinations oE ten~city propcrties.
The process of the present in~ention may be used to advantage in either a ''coupled" or a " split " process for producing polyamide monofilaments. The apparatus is composed of known units arranged and utilized, however, in a different manner from that commonly used in treating pol~amide monofilaments.
Basicall~, the apparatus includes an extruder and liquid quencll asscmbly, godet rolls, ~ st~(~m tube or tubes s~lch as tllat disclosed in sritish Patent No. 1,167,696, driers, and possi~ly hot air ovens or infrared ovens, and takeup reels or spools.
In accordance with known technology, a suitable polyamide is initially extruded and spun wi-th conventional apparatus. The spun monofilament is immediately quenched in a conventional quench tank wnerein it is trained about rollers, and the monofilamen-t is substantially noncrystalline at this poin-t. In a discontinuous split process, the monofilament is drawn from the quench tank and wound on storage reels for subsequent processing. The orientation and/or crystalliæation (to stable crystal structures) which may occur in this processing and possibly storage phase of the split process have been found to be of a minor degree which does not detract from the unique benefits ob-tained by a subsequen-t vapor or steam treatment of the monofilamen-t as disclosed herein.
In a coupled or continuous process, the monofilaments is guided from the quenching tank directly to a first set of godet rolls.
In a split or discontinuous.process, the monofilament is passedfrom the storage reel to the first set of godet rolls a-t the time of its subsequen-t processing, and the further monofilament trea-tment is thereafter the same in the coupled or split process as described below. The monofilament passes from the first set of godet rolls through a steam tube and then about a second set of godet rolls which cooperate wi-th the first set of godet rolls to draw the ~7~

monofilament.
As previously i~dicated, the s-team.tube is of a conventional structure. The steam tube includes entrance and exit ports at its axial ends, and it has an outer jacket and a perforated core which cooperate to define an annular chamber for receiving the pressu-rized steam. The perforated core defines a passageway through which the monofilament passes as it is being drawn. The perforated core has a plurality of perforations or steam outlets along its length and about its periphery so as to permit radially inward impingement of the steam upon the monofilament and abou-t the entire periphery thereof regardless of the particularcon~ a-tion ot the monofilament. Thus, the vapor or steam impingement may be aptly described as impingement a-t substantially right angles to the longitudianl axis of the monofilament about the entire periphe-ry thereof. The steam impingement velocity is related with other process parameters, and it may approach and approximate a zero value with appropriate adjustment of other parameters such as the resi-dence time of the monofilament within the steam tube.
The drawin~ of the monofilament and, more particularly, the actual physical deformation of the monofilament between the first and second set of godet rolls occurs primarily within the steam tube. The draw or physical deformation of the _g _ monofilam_nt within th~ Steam tub~ is charact~rized by a ~radual taperin~ of the filament from its undrawn dimensions to its dxa~n dimensions, The ~mpPraturP of the monofilament during tha drat~in~ oper~tion is bliVed to approximat2 th~ cond~n-sation t~mperature of ~h~ steam a~ thP selacted oparating pr~ssure.

As previously indicated, the monofilam2nt is drawn in ~he vapor or st2am enviroNmen~ to a draw ra~io ~rom abouk 2.0:1 to about 4.0:1. The steam pressure within the steam tube Is in the ran~a of from about 1 psi~. to 150 psigO ard, typicaLly, betwe~n 20 psig. and 90 psig.

Upon exiting from the steam ~ub~, ~he monofilam~nt passas through the second s~t of godet rolls and than throuqh an air stripper or drier which r~moves moisture carried on the surface of the monofilamant without elevatin~ the t~mperature thereof~ Thereafter, the monofilamsnt is further drat~ to provida an agyr~ate draw ratio o from about 5.0:1 to about 8.0:1 at an ele~ated tempexatura by maans of the se~ond set o ~odet rolls and a third set of ~od~t rolls. This second sta~e draw may be a dry, hot air draw under conv~ntional operatin~ conditions. k~tcr~a~ V ~t~C~second sta~e draw may also be performed in a vapor or steam environment. The /~) ~ .

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drawin~ of monofilaments in first and second s~a~Q vapor environments has also been ~ound to be 2~fective in accordance with the prssent invention.

Followin~ the second draw stage, the monofilament ma~
optionally bs expos~d ~o a rel~xation treatment wh~rein a dra~ ratio of about 1.0:1.0 is maintained while exposin~ the monofilaman~ to elevated temp2raturQs. Th~ ~unction o~ this treatment is to provide impro~ed linear stability without signi-ficant changa in the lon~itudinal tenacity~ loo~ tenacity and oth2r physical properties of the monofilam2nt.

As previously indicated, the presen~ invention enables the pr~vision of improv~d lon~itudinal and loop tenacity proper-ties as well as pref~rr~d combinations of these properti~s as r~sult of ths ability to s lectively modify th2 sam~ in ~h~
xelatively high denier polyamida monofilaments contemplat~d harein. In illustration of the improved combinations o~.
properties, the subject mQthod may be amployed to simul~an~ously increasa both longitudinal and loop tenacity properties to absolute values, depanding upon the par~icular polyamide selected, not heretoore possible in accordance with prior art ~echniques.
For example, as characterized by th~ product of the longitudinal and loop tenacity values, monofilaments processed in accordance with the subject method can be typically provided with //

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tsnacity product values greater than 20 ~r~ms2/denier . Further, eith~r tenacity p~operty can b~ prefer~ntially incre~ased .while maintainin~ an ac~ptable lev~l for th2 oth2r~

The improv2m~nts obtained in acCor~anc2 with the present invPntion and the selactive modiication of tenaoities is not possible in conventional ov~n drawing techniques as illustrated below by th2 ~xamples of Table I and Table II.
In each o~ th2se tablas, tha polya~ide was polycaprolactam ana tw~ stag~, conventional oven drawin~ techniques w~re employ2d.
Examples A to F in Table I and A to G in Ta~le II demonstrate thP effscts of increasin~ the first stage draw ratio at a constant oven temperature, and th~ remainin~ examples demonstrate the effects o~ increasing the first stage oven tamp~rature at a constant draw ratio. In all o the examples~ th~ se~ond staga draw ratio is maximizad or selected at a level just b~low that at which the monofilament break6.

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~1 In reviewing Tables I and II, it is apparent ~.hat the loop ~enaCity is unaffectad b~ chan~es in the first stage draw ratio and temperature at the high overall draw ratios necessary to obtain tire ~rade monofil. Further, ~he loop tenacity values ara in all instances l~ss than 2.0 gxam~s per denier at the moderate longi~udinal ~enacity values. It is also appara~t from these examples ~hat the primary prior art drawing parameters, draw ratio and oven temperature, cannot be effectively used to salectively modiy lon~itudinal and loop ten~city properties or provide improved combinations thereof.

` In contrast with the ~a~iciencies of tha prior a~t techniques, the.advantages of th~ present inve~tion are readily illustrated by means of designad experim~nts rep~rted in Table III as Examples 1 to 5 and the resulting contoux plots which are reproduced in Fi~ures 1 to S and 1~ to 5~ The particular designed experime~t used wa~ a two actor, pentagonal design ra~uirin~ six data points pPr dapendent variable examined.
In each of the plots, the vertical axis is tha indepande~
variable steam pressure and the hori20ntal axis is the ind~pen-dent variable draw ratio~ In Figures 1 to 50 the con~our plots of the longitudinal tenacity a~d the loop tenacity have been imposea upon one another ~or conveniance of illustxa~ion~ In ~7~

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Fiqures 1~ to 5~t the contour plots of th~ product of the lon~itudinal an~ loop t~nacity valuas are depicted ~or the correspondin~ d~siqn experiments shown in Fi~ures 1 to 5.
'rhus, ~hs t~nacity and tenacity product valua~ available for each of tha monofilam~nts of E~amplas 1 to 5 of Tabl~ III, proc~ss~d as report2d in the table, ara graphically d~picted in thn respective Figures 1 to 5 and lA to 5A. In all o~ th~
figures, only a portion of the con~our plot betwe~n pPrtinent draw ratio values and steam pressure values is shown.
In the t~t specimens of the desi~nPd ~xperim~nts, th~ polyamid2 was poly`caprolactam and the variables in the extrusion and drawing pxocess, except for the independznt variables bein~ examined, were maintained constant. For example, tlle extruder, spin head asssmbly and the die in th~
extrusion sta~e of ths monofilaments were the same, and the same conventional water quench was also used in all casas. The axtruded mono~ilamen~ size or denier and the linear velocity or spPed o~ the drawin~ apparatus was varied as indicated for purposes of comparison. In all cases, th~ longitudinal and loop t~nacitias comprise th~ d~pendent variable$.
In the first stage steam draw, t`he st~am pxessure and draw ratio were varied in tha indicated ran~s as ths indepen~nt variables. The steam tube employed in the Pirst sta~e ~rawin~ operation ~ras o convantional desi~n a5 de5cribed above, and live steam containin~ about 3% by wei~ht entrainsd - _~
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water in droplet-type ~orm was employe~ as the vapor medium.
The second sta~e draw was perfQrmed in accordance wi~h conv~ntional oven drawin~ techniques and maximization of th~
draw ratio was used in all cas~s. Tha 3.2 relative viscosity polymers were drawn at an ov~n ~emperature of about 800F.
and the 3.5 rslative viscosity polymers wer~ drawn at a t~mpara-ture o~ about 870F.

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In Table III, pertinent monofilament characteristics and proca~sing parameters togeth~r ~ith the r~sultinq tenacity prop~rties are summari~d. IJpon comparison with Tables I and II, it is apparent that th~ maximum t2nacity valuss obtained in accordance with the process of the present invPntion are improvad and, mor~over, improved comhinations o~ longitudinal and loop tenacity propPrties are o~tain d. For example, the indicated Steam Draw Condi~ions and T~naci~y Values illus~rat~q operatin~ ranges which pxovide minimum longit~dinal tenacity values of about 7.0 grams par deni~r and minimum loop t~acity value~ of about 2.0 ~ams p~r deni2r. Thes~q minimum tenacity propqrti~s ara desirable in tir~ applications. The indicated operating ranges are not exhaustive in that oth2r steam dra~
conditions may be employed to obtain tha desirea tenacity pxop~rties~ For ~xample, in Figure 2, it is also po~sible to obtain the ore~oin~ minimum tenacity valuas by employing a draw ratio in the range of from 3.0 to 3.2 and a steam pressure in the range of from 40 to 50 psig.

In addition to the foregoing improvad tenacity values and combinations thereof, it i5 also readily apparant from Fig~res 1 to 5 that tha tanacity values can be selactivaly modified~

For ~xample, if a particular application requirzd a maximum loop strenqth ~alue with only moderate t~nacity properties, the -~r-7~f~

monofilament o~ ~igura 4 would be drawn at a draw ratio of ~2.7 and a steam pressure of ~51 psig. in thP first stage steam draw process in order to provide a loop tenacity valua of ~5.5 ~r~ns per deni~r and a lon~itudinal tenacity value of ~8,3 grams pPr danier. Contrarily, an applicati~n dictating maximization of longitudinal tenacity could be prapaxed by drawing the mono~ilament o~ Figure ~ at a draw ratio of ~3.2 and a steam pressur.e o~ ~50 psig. in the first stag~ steam dxaw ~o provide a longitudinal tenacity of about ~9.5 grams par denisr and a loop tenacity of about ~2.2 gxams per d~niar.
Cons~quently, it is readily apparent upon r~vie~ing Figures 1 to 5 that the process of the presant invention enablas selectiva modification of the t~nacity propertias.

The improved t~nacity values and combinations theraof are urther illustrated by comparison of the product of th~
longitudinal and loop tenacities in TablP III or Fisur~s lA to 5A with the similar product in Tables I and II. As shown in Table III, the tenacity product value is above 20 in all cases and it ~as bean found pre~erable to maintain this value at a level o~ at lea~t 20 for tire applications. In contrast, ths prior ar~ tenacity product values as shown in Tables I
and II are generally ~ignificantly lawer.

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;97~;22 The impro~ed physical prop~rties which are obtainahlP
in accordance with the subject process are fur~har demonstrated by the examples report~d in Tabls IV. ~h~ monofilam2nts in each of ~hese examples wPre spun from the same polyamide pol~mer (polycaprolactam) havin~ a relativ~ viscosity of 3.2 and a rectan~ular cross sectional configuration. The mono-ilaments ware procsssed as indicated, and in all cases the s~cond stage draw was optimized. Examplas A to D wPre processed in a steam/oven drawin~ technique, and thay illustxake ~he ran~e of values obtainable in accordance with the present invention. Examples F a~d G ware processed in a staam/steam drawin~ techniqua in accordance with the subject process, and they illustrate the improved ph-ysical propsrties obtained in a multipl~ staam drawing technique. Ex~mple E is a control, and it wa5 processed in accordance with conventional ovan drawing techniques.

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In addition to illustxating improved tenacity proparties, th~ examples of Tabl~ IV also illustrate tha improvsd fatigu8 life obtainable in accordance with the subject process as xeflacted hy the report~d SC~F properties. The SCEF fati~ua testex is desi~ned to apply compression and tension to cords or monofilaments ~hrough a sh~arin~ action as descri~ed by Forster et al, ~ Msw Improved Tir~ Cqrd Fatigue Testar, Vo~ 42, No, 4, Rubbar Ch~mistry and Technology, Septemb~r, 1?69~ In tlliS test, the monofilaments are cured in rubbar and they ara disposed in a iengthwise direction in a resulting test strap.
The strap is secured adjacent a central portion thereof in a floating clamp which is arrangad for oscillation. Tha strap is positioned at an angl~ of 45 with raspect to th2 direc~ion of oscillation. The exposed ends of the test strap arP
securèd in stationarv blocks, and an oscillating load is applied to tha ~loating clamp through a pair of springs. The floating clamp is oscillated until the monofilament fails, and th~
reported tast result is minutes to failure.

In Tabla IV, ths impravemant in f 12x life resulting from an initial steam drawing treatment rangsa from about 29 to about 83 percent bas~d upon the control Exampla E. The greatest improvemants wers displayed in Examplss B and D which correlates with the relatively hi~h loop tenacity proper~ies ~;,, ._ .

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, provided as a result of particular processing conditions.
However, it should be appreciated that improved flex life was found in all cases when an initial steam draw was employed.
The examples of Table IV were also examined with respect to their surface smoothness after processing. The surfaces of the monofilaments were examined with a scanning electron microscope at magni~ications of 65 and 260 in order to evaluate the occurrence of etching or su`race irregularities.
Each of the monofilaments was subjectively evaluated for smoothness as reported in Table IV. The designation ~ (not etched) was assigned to those monofilament surfaces being found substantially ree of surface etching or other irregularity.
The monofilament surfaces found to be subs-tantially continuously etched or irregular with surface contour variations in height on the order of 0.01 millimeters were assigned a designation HE (highly etched). Monofilament surfaces found to have infrequent occurrences of etching or other irregularity and which were predominantly characterized by a continuously smooth characteristic were assigned the designation SE (slightly etched). A monoilament surface displaying a significant but not continuous surface etching or other irregularity condition was assigned a designation ME (moderately etched).

The monofilam~nts of th2 examples oE Table IV, except for Excampl2 ~, ware all found to hava smooth surfac_ characteristics ~hich ~ould not be d~trimantal to adhesion wi~h ~lastomsric or rubb~r materials as indicat2d belowO The monofilament in Example G was found to ba moderately e~ched reflQctin~ th~ relativaly hi~her steam pressure ~rawing conditions.

Referrin~ to Table V, the monofilament employed in Examples A to J is identical with that employed in Table IV.
In this table, a number of processin~ variations are examined as discusssd in datail below.

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~7fD~J2 ExamplPs A to C in Table V illustrate the eff2ct o~ a staam drawinq trsatm~n~ followin~ conventional prior art ov2n drawin~ techniques. Th2 monofilam2nts of Examples A to C
~ere initially drawn in multiple ov~n drawin~ st2ps Wit]l optimi-zation in the sacond draw, and th2n subjected to a third stage draw wherein the draw ratio was substantially maintained at a 1.0 value. The steam pressura in th~ third stage drawing of Examples A to C was prograssively incraased as reported.

As shown by Exampl ~ A to C in Table V, the use of a subsaquent st~am drawin~ sta~a was not found to provid~ selective modification of the tenacity properties although incrsases in loop tenacity wexe found to occur with increasing ~team pxessur~.
However, the increases in the loop tenacity values we~e accom-panied by correspondingly increasing 12vels of etching or ixragularities in the monofilament surfaces.

It has been found that etchin~ of the mono~ilament surface decr~ases the adhesion of the monofilamant to rubber stock. A T-pullout adhesion t~st was used to compare the adhesion of the undipp~d mono~ilament of Examples ~ to C to adhPsiva a~tivated rubber stock with that of control Exampl~ D.
In the T-pullout test, the monofilament is cur d into a rubher block ~ith an axial p2nstration into the block of one-~ourth ~,~

inch. The rub~er block is heated to 230F., and the monofilament is then pulled along its longitudinal axis until it is separatea from thè rubber ~locl;. Control Example v was assi~ned an adhesion l~vel of 100, and ~amples ~ to C resulted in the re~orte~
relative rankiny ba~ed upon their p2rformance in the T-pullout adhesion t~st. The rela~.ive d~cxease in adhesion with increasincJ
surface etching or irrecJularity is apparent, and it is most significant with regard to Example C wherein the flex life has also beenrelatively severely decreased.

Examples E to I of Table V further illustrate the advanta~es obtained by the process of the pr~sent inventlon.
Examples H and I are directly comparable with the control Example J, and they demonstrate the maintenance of adhesion and flex lie properties with improving loop tenacity strength~

: The reasons for the decrease in adhesion with an increase in ~he degree of monofilament surface etching or irregularity are not clearIy understoodO In view oE the physical characteriskics of a highly etched monofilament surface, it i5 theorized that it may be more diffiçult for the rubber material ~ith which adhesion i5 sought to be in contact with the entire surface of the monofilament. In regions where there is a void at the interface between the rubber and mon~filament surfaces, ;'' ` ~P
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propa~ation of separation d~uring st~ess would be readily ini~ia-ted so as to result in a relatively low cle~ree of adhesion. In the absence of such voids, it would be necessary to generate }ligher stress forces in order to initiat~ separation and the ul-timate adhesion failure. Also, the higher steam pressures employed during the effective steam treating of drawn nylon could result in too high a level of crystallinity on the filament surface.
Referring to Table VI, the processing conditions and physical properties for monofilaments prepared from a polycapxolactam mate-rial having a relative viscosity of 3.5 are set forth. The achie-vement of high tenacity properties and good flex life without sur-ace etching or irregularities are attained. In addition, compari-son t~ith control Example E illustrates the inabili~y to attain improved tenacity properties employing prior art drawing techniques.
Accordingly, the process of the subject invention is advantageousiy employed to develop improved tenacity properties and combinations thereof, flex properties, and adhesion properties in polyamide monofilaments of increased relative viscosi-ty.

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Re~rrin~ to Table VII, th2 advantages of the subject proces.s are furtller illustrated with respect to equivalent monofilam~nt matexials and variations in processing conditions, and compared with prior art techniques.

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In Examplas A and B of Table VII, the polyamide is poly~trans-1,4-cyclohexyl2nedimethylen2 a~elamide) having r21ative viscosity of 3.2. Control Example B is reprasen-tative of prior art processing techniqu2s.

Examples C and D of Table VII illustxate split processing techniques and the use of an oil ~uanch. The poly-amide used in these examplas was polycaprolactam having a relative viscosity of 3.2. The monofilaments ware spun into a conventional oil ~uench and then wound on a spool and storad or ona day prior to fur~har processin~O The monofilamen~s were also wound on spools and stored for one day periads betwasn ~ach of the fur~her drawin~ steps. Control Example D illustrate prior art proc~ssing techniques.

3g~

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for treating a polyamide monofilament to improve its flex life and tenacity properties comprising the sequential steps of:
a) partially drawing, prior to any previous orientation drawing thereof, an extensible spun polyamide monofilament having a relative viscosity of at least 2.5 and a denier of from 500 to 50,000 in a draw ratio of at least 2.0:1.0 and less than 4.0:1.0 while contacting said monofilament with a vapor medium at an elevated temperature and pressure; and then b) further drawing said monofilament in a subsequent drawing operation to an aggregate draw ratio of from about 5.0:1.0 to about 8.0:1.0, and c) wherein the subsequent drawing operation is done while the monofilament is heated to an effective temperature of at least 550°F in dry heat.

2. The method of claim 1 wherein the subsequent drawing operation is done while exposing the monofilament to a vapor medium containing entrained droplet-type liquid at an elevated temperature.

3. The method of claim 1 wherein said vapor medium contains at least 1% by weight entrained droplet-type liquid.

4. The method of claim 1 wherein said vapor medium is live steam containing at least 1% by weight entrained water in droplet form.

5. The method of claim 4 wherein said monofilament is exposed to said live steam at a pressure of from 1 psig. to 150 psig. in said first drawing operation.

6. The method of claim 5 wherein said monofilament is exposed to said live steam for a period of less than 10 seconds in said first drawing operation.