WO1992010601A1 - Process for stretching fibers and forming composite articles - Google Patents

Abstract

This invention relates to a process for stretching fiber in which the yarn is pulled over one or more surfaces creating a frictional force between the yarn and the surfaces placing the yarn under tension after which the yarn is pulled through a heating means under tension which stretches the yarn.

Description

PROCESS FOR STRETCHING FIBERS AND FORMING COMPOSITE ARTICLES

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved process of stretching fibers. Yet another aspect of this invention relates to an integrated process for stretching fibers to improve their properties and fabricating the stretched fibers into composite articles comprising a network of fibers (preferably substantially parallel and aligned) in a polymeric matrix. Another aspect of the invention relates to apparatuses for carrying out the processes of this invention.

2. Prior Art

Stretching yarn to enhance yarn properties, such as tensile strength, tensile modulus and the like, has been practiced commercially for several decades. In these process, yarn formed from thermoplastic polymers, such as polyethylene, polyester, and nylon, are stretched between a feed roll and a stretch roll while the yarn is heated to the optimum temperature. The yarn speed is always equal to the draw roll speed and no or substantially no slippage between the yarn and the roll is allowed. One disadvantage observed with the above system is that occasionally broken filaments accumulate on the rotating roll surface. These filaments must be scrapped off the roll surface or they will cause a shut down in the stretching process. Moreover, fuzz balls and yarn loops interfer with the conduct of these conventional yarn stretching processes and often force the shut-down of conventional stretching processes to remove same. This is tolerable when stretching a single strand or up to six strands of yarn in a typical yarn 5 processing operation. However, such is not acceptable when stretching multi-ends or multi-strands of yarn, such as yarn having from about 50 to about 1000 strands, as for example those yarns commonly used to form prepregs. Therefore, the j_0 coupling of a stretching process with a prepregging process for multi-strand yarn has not been practiced commercially, nor has it been reported or published in the literature.

₁₅ SUMMARY QF INVENTION

One aspect of this invention relates to a process of stretching a fiber which comprises:

(a) pulling at least one continuous fiber over one or more surfaces of one or more friction tension _Q means, said means comprising one or more stationary bodies, one or more moving bodies, or a combination thereof provided that movement of a moving body in the direction in which said fiber is being pulled is at a velocity which is less than the linear velocity ₅ of said pulled fiber, thereby creating a frictional force between said contacted surfaces and said pulled fiber such that the tension on said fiber up-stream of said friction tension function <T₂) is greater than tension on said fiber down-stream of said 0 friction tension means (T^ and;

(b) heating all or a portion of said fiber under tension (T^ to T ) at any point from the first of said contacted surfaces and upstream thereof preferably at a temperature which is less than the 5 degradation temperature of said fiber and more preferably at a temperature which is greater than the glass transition temperature, Tg, of said fiber to stretch said fiber such that the tensile strength and tensile modulus of the stretched portion of said fiber is greater than the tensile modulus and tensile strength of unstretched portion of the said fiber. 5 Another aspect of the invention relates to a process for stretching fibers and fabricating said stretched fiber into a composite article comprising a network of fibers, preferably substantially parallel and aligned, in a polymeric matrix said proces o comprising the steps of:

(a) pulling at least one continuous fiber over one or more surfaces of one or more friction tension means, comprising one or more stationary bodies, one or more moving bodies or a combination thereof, 5 provided that the movement of a body in the direction in which said fiber is being pulled is at a velocity which is less than the linear velocity of said pulled fiber is said direction, thereby creating a frictional force between said contacted surfaces and _Q said pulled fiber such that the tension of said pulled fiber upstream of said friction tension means (T₂) is greater than said tension of said fiber down stream of pulled friction tension means (T^) ;

(b) heating all or a portion of said fiber - under tension (T_j to T₂) at any point from the first of said contacted surfaces and upstream thereof preferably at a temperature which is less than the degradation temperature of said fiber and more preferably at a temperature which is greater than the 0 glass transition temperature, Tg, of said fiber to stretch said fiber such that the tensile modulus and tensile strength of the stretched portion of said fiber are greater than the tensile modulus and tensile strength of said unstretched portion of said 5 fiber;

(c) forming said stretched fibers into a composite which comprises at least one layer comprising a network of said fibers dispersed in a matrix material employing a procedure selected from the group consisting of:

(i) aligning a plurality of said stretched 5 fibers into a network of fibers, preferably in a substantially parallel alignment, and coating said aligned and stretched fiber with a matrix material comprising one or more thermosetting polymers, one or more thermoplastic polymers, Q or a combination thereof to form at least one layer comprising a network of said fibers dispersed in said matrix material; and

(ii) coating said stretched fibers with said 15 matrix material and aligning a plurality of said stretched and coated fibers to form said layer.

Yet another aspect of this invention relates to 2_Q an apparatus for stretching fibers which comprises

(a) one or more friction tension means comprising one or more bodies having surfaces for contacting fibers pulled through said apparatus whereby a frictional force is created between said

-, -, pulled fibers and said contacted surfaces such that the tension of said fibers upstream of said contacted surfaces , T₂, is greater than the tension of said fibers downstredam of said contacted surfaces, ^;

(b) fiber heating means position at one or

30 more points from the first of said contacted surfaces and upstream thereof for heating all or a portion of that portion of said fiber under tension T₂ thereby stretching said fiber and;

(c) fiber pulling means positioned upstream of 5 said surfaces and said fiber heating means for pulling said fiber over said contaced surfaces and to said heating means. Still another aspect of this invention relates to an apparatus for stretching fibers and for fabricating said stretched fibers into a composite comprising a network of fibers in a polymeric matrix such as a prepreg, pultrusion, and filament winding (preferably a network in which the fibers are substantially parallel relative to the common fiber direction), which apparatus comprises:

(a) one or more friction tension means, comprising one or more bodies having surfaces for contacting fibers pulled through said apparatus whereby a frictional force is created between said pulled fibers and said contacted surfaces such that the tension of said fibers upstream of said contacted surfaces, T₂, is greater than the tension of said fibers down-stream of said contacted surfaces, T_1#;

(b) fiber heating means positioned at any point from the first of said contacted surfaces and upstream thereof for heating all or a portion of that portion of said fiber under tension T₂ thereby stretching said fiber;

(c) composite forming means for forming stretched fibers into a composite which comprises at least one layer comprising a network of fibers dispersed in a matrix material comprising one or more thermosetting polymers, one or more thermoplastic polymers or a combination thereof, said means selected from the group consisting of:

(i) fiber aligning means for aligning a plurality of stretched fibers into a network of fibers (preferably a network in which the fibers are substantially parallel relative to the common fiber direction) and fiber coating means for coating said aligned and stretched fibers to form said composite; and

(ii) fiber coating means for coating said stretched fibers with said matrix material and coated fiber aligning means for aligning a plurality of said coated and stretched fibers into a network (preferably a network in which the fibers are substantially parallel relative for the common fiber direction) to form said composite; and

(d) fiber pulling means for pulling said fiber through said apparatus. Several advantages flow from the processes of this invention. For example, in conventional yarn stretching processes where godet rolls are used to stretch the yarn if yarn or filament breakage occurs, broken yarns or filaments will continue to wrap around the roll surfaces resulting in an accumulation of the broken yarn on the roll surfaces. The stretching process must then be interrupted to remove the accumulation. This greatly complicates the stretching process, and increase process time and expense. If the stretching process is coupled with the composite forming process the problem is compounded, resulting in costly shut-downs. In this invention, where the friction means are kept at stationary or at lower differential speeds between the yarn and the frictional means, the accumulation of broken filaments and yarns on the frictional means is eliminated or minimized. This in an especially important feature of the invention where hundreds of yarn strands, or thousands of individual filaments, are simultaneously, stretched and fabricated into the desired composite because ocasional broken filaments or yarn do not interfere with the process operation. In general, the tension frictional means of this invention are very forgiving with respect to broken filaments, fuzz balls, yarn loops, and loose filaments, which is very important for a successful commercial yarn stretching and composite forming process. Another advantage of the present invention is that as soon as the yarns are stretched or drawn, the yarns are aligned or collimated, and ready for resin impregnation and composite forming which can be critical to the quality of the final composite product. On the other hand, if conventional godet rolls are used, the stretched yarns are not aligned, which requires additional yarn alignment or spreading means which adds additional costs to the process. Yet another advantage of the invention is cost reduction resulting from less expensive equipment. Conventional yarn stretching apparatus require godet rolls and their accessories, such as motor drive systems to synchronize the godet roll speeds to the yarn speeds. Furthermore, if conventional godet rolls are combined with conventional composite forming processes (which require the use of solvents for the resin system forming the matrix material) all motor drive systems must be explosion proof, which add additional cost to the operation. The present invention does not require the use of these expensive accessories.

Still, another advantage of the invention is that the space occupied by the tension friction means is only a fraction of the space occupied by conventional godet rolls and their accessories. Thus, the process of this invention can be carried out in smaller places.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the invention and the accompanying drawings in which:

FIG. 1 illustrates in schematic form an embodiment of this invention for stretching fiber. FIG. 2 is a detailed perspective view of bar 22_v of FIG. 3 showing the plurality of yarn 12.

FIG. 3 is a detailed sideview of the friction tension means of the embodiment of FIG. 1.

FIG 4. is a detailed sideview of an individual friction bar of the friction tension means of FIG 3.

FIG 5 is a detailed view of an alternative friction tension means. FIG. 6 is a detailed view of another alternative friction tension means.

FIG. 7 is a detailed of another alternative friction tension means.

FIG. 8 illustrates in schematic form an embodiment of this invention for stretching fiber and fabricating the stretched fiber into a prepreg.

FIG. 9 is a plot of T₂ as a function of the number of contacted surfaces of friction tension means 20 of FIG. 8.

FIG. 10 is a plot of final yarn denier as a function of the number of contacted surfaces of friction tension means of 20 of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be better understood by those of skill in the art by reference to the above figures. FIG 1 illustrates in schematic form a preferred embodiment of this invention. As shown in FIG 1, one or more creels 11 containing yarn 12 are deployed. Any type of heat stretchable yarn or fiber 12 can be used in the process of this invention. In the Figures, yarn 12 is depicted as a multi-filament yarn consisting of a bundle of from about 3 to about 2000 individual filaments of less than about 15 denier, preferably bundles of from about 100 filaments to about 1000 filaments of less than about 40 denier. For purposes of the present invention, fiber or yarn 12 is defined as an elongated body, the length dimension of which is much greater than the dimensions of width and thickness. Accordingly, the term fiber or yarn as used herein includes a onofilament elongated body, a multi-filament elongated body, ribbon, strip, film and the like having regular or irregular cross sections. The term fibers or yarns includes a plurality of any one or combination of the above.

The cross section of fibers 12 for use in this invention may vary widely. Useful fibers 12 may have a circular cross-section, oblong cross-section or irregular or regular multi-lobal cross-section having one or more regular or irregular lobes projecting from the linear or longitudinal axis of the fibers. In the particularly preferred embodiments of the invention, fibers 12 are of substantially circular or oblong cross-section, and in the most preferred embodiments are of circular or substantially circular cross-section.

Fiber 12 can be composed of any "heat stretchable polymeric composition", preferably a "thermoplastic resin". As used herein a "heat stretchable polymeric composition" is a polymeric composition which when in fiber form will stretch when heated and placed under tension, and a "thermoplastic resin" is any resin which can be heated and softened, cooled and hardened several times at least (2 or 3) without undergoing a basic alteration.

Illustrative of useful thermoplastic polymers for fabrication of fibers 12 are polyamides as for example those characterized by the presence of recurring carbonamide groups as an integral part of the polymer chain which are separated from one another by at least two carbon atoms. These polyamides can be prepared by reaction of diamines and diacids having the recurring unit represented by the general formula:

5 -NHCORCONHR¹- in which R is an alkylene group of at least about two carbon atoms or arylene of at least about 6 carbon atoms, preferably alkylene having from about 2 to about 10 carbon atoms or phenylene, and R¹ is R or 10 anyl. Exemplary of such materials are poly(hexamethylene adipamide) (nylon 6,6) poly(hexamethylene sebacamide)(nylon 6,10), poly(hexamethylene isophthalamide),poly(hexamethylene terephthalamide), poly(heptantethylene ^5 pimelamide)(nylon 7,7), poly(octamethylene suberamide)(nylon 8,8), poly(nonamethylene azelamide)(nylon 9,9), poly(decamethylene azelamide)(nylon 10,9), poly(decamethylene sebacamide)(nylon 10,10), ₂,, poly[bis(4-aminocyclohexyl)methane-l,10-decanecarboxam ide)] (Quina), ρoly(m-xylylene adipamide), poly(ρ-xylylene sebacamide),poly(2,2,2-trimethyl hexamethylene terephthalamide), ρoly(piρerazine sebacamide), poly(p-phenylene terephthalamide), ₅ poly(metaxylene adipamide), poly(metaphenylene isophthalamide) and the like.

Other useful polyamides are those formed by polymerization of amino acids and derivatives thereof, as for example lactams. Illustrative of 0 these useful polyamides are poly(3-aminopropanoic acid)(nylon 3), poly(4-aminobutyric acid)(nylon 4), poly(5-aminoρentanoc acid)(nylon 5), poly(6-aminohexanoic acid)(nylon 6), poly(7-aminoheptanoic acid)(nylon 7), 5 poly(8-aminoocatanoic acid)(nylon 8), poly(9-aminononanoic acid)(nylon 9), poly(10-amiondecanoic acid)(nylon 10), poly(ll-aminoundecanoic acid)(nylonll), poly(120aminodocecanoic acid) (nylon 12) and the like. Preferred polyamides for use in the practice of this invention are polycaprolactam and 5 poly(hexamethylene adipamide). The particularly preferred polyamide is polycaprolactam.

Illustrative of other useful themoplastic polymers for use in fabrication of fibers 12 are thermoplastic polyesters. Illustrative polyesters

10 which are suitable for use in this invention are those which are derived from the condensation of aromatic, cycloaliphatic, and aliphatic diols with aliphatic, aromatic and cycloaliphatic dicarboxylic acids. Illustrative of useful aromatic diols are

15 those having from about 6 to about 12 carbon atoms. Such aromatic diols include (bis-(p-hydroxypheny1)-methane; l,2-(bis-(p-hydroxypheny1)-ethane; 1-pheny1-(bis( -hydroxypheny1)-methane; r __.U m dipheny-(bis-(p-hydroxypheny1)-methane;

2,2-bis(4^,-hydroxy-3'-dimethylphenyl)propane; 1,1- or 2,2-(bis(p-hydroxyphenyl)-butane; 1,1-dichloro-or l,l,l-trichloro-2,2(bis(p-hydroxypheny1)-ethane; 1,l-(bis(p-hydroxypheny1)-eyelopentane; 2,2-(bis-(p-hydroxyphenyl)- propane (bisphenol A); 1,1-(bis—(p-hydroxyphenyl)- cyclohexane (bisphenol C); p-xylene glycol; 2,5-dichloro-p-xylylene glycol; p-xylene 2 , 4-diol; and the like.

Suitable cycloaliphatic diols include those

2o having from about 5 to about 8 carbon atoms.

Exemplary of such useful cycloalphatic diols are 1,4-dihydroxy cyclohexane; 1,4-dihydroxy methylcyclohexane; 1,3-dihydroxycycloheptane; 1,5-dihydroxycyclooctane; 1,4-cyclohexane dimethanol;

3 and the like. Polyesters which are derived from aliphatic diols are preferred for use in this invention. Useful and preferred aliphatic diols include those having from about 2 to about 12 carbon atoms, with those having from about 2 to about 6 carbon atoms being particularly preferred. Illustrative of such preferred diol precursors are 5 1,2-or propylene glycol; 1,3-ρropylene glycol; ethylene glycol, neopentyl glycol, pentyl glycol, 1,6-hexanediol and geometrical isomers thereof. Propylene glycol, ethylene glycol and 1,4-butanediol are particularly preferred as diol precursors of 0 polyesters for use in the conduct of this invention. Suitable dicarboxylic acids for use as precursors in the preparation of useful polyesters are linear and branched chain saturated aliphatic dicarboxylic acids, aromatic dicarboxylic acids and 5 cycloliphatic dicarboxylic acids. Illustrative of aliphatic dicarboxylic acids which can be used in this invention are those having from about 2 to about 15 carbon atoms, as for example, oxalic acid, malonic acid, dimethylmaIonic acid, succinic acid, _Q octadecylsuccinic acid, pinelic acid, adipic acid, trimethyladipic acid, sebacic acid acid, subric acid, azelaic acid, dimeric acids (dimerisation products of unsaturated aliphatic carboxylic acids such as oleic acid and alkylated malonic and succinic acids, such _c as octadecylsuccinic acid) and the like.

Illustrative of suitable cycloaliphatic dicarboxylic acids are those having from about 6 to about 15 carbon atoms. Such useful cycloaliphatic dicarboxylic acids include 0 1,3-cyclobutanedicarboxylic acid,

1,2-cyclopentanedicarboxylic acid, 1,3-cyclohexane dicarboxylic acid and 1,4- cyclohexane-dicarboxylic acid, 1,3- dicarboxymethylcyclohexane and 1,4-dicarboxymethylcyclohexane and 5 4,4'-dicyclohexylicarboxylic acid, and the like. Polyester compounds prepared from the condensation of a diol and an aromatic dicarboxylic acid are preferred for use in this invention. Illustative of such useful aromatic carboxyoic acids are terephthalic acid, isophthalic acid and a o-phthalic acid, 1,3-naρhthalenedicarboxylic acid ,1,4- naphthalenedicarboxylic acid ,2,6- naphthalenedicarboxylic acid 2,7-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenylsulphone-dicarboxylic acid, 1,1,3-trimethyl-5-carboxy-3-(p-carboxy-phenyl)-indane, diphenyl ether 4,4'-dicarboxylic acid bis-p(carboxyphenyl)methane and the like. Of the aforementioned aromatic dicarboxylic acids, those based on a benzene ring such as terephthalic acid, isophthalic acid and orthophthalic acid are preferred for use and amongst these preferred acid precursors, terephthalic acid is particularly preferred.

Preferred polyesters for use in the practice of this invention poly(p_-ethyleneoxy benzoate) , ρoly(ethylene terephthalate), poly(butylene terephthalate), and poly(l,4-cyclohexanedimethyl terephthalate) are the polyesters of choice. Among these polyesters of choice, poly(ethylene terephthalate) is most preferred. Also illustrative of useful themoplastic polymers for use in the practice of this invention are those formed by the polymerization of α,β-unsaturated monomers of the formula:

R₁R₂-C- CH

wherein:

R and R₂ are the same or different and are hydrogen, hydroxy, halogen, alkylcarbonyl, carboxy, alkoxycarbonyl, heterocycle or alkyl or aryl either unsubstituted or substituted with one or more substituents selected from the group consisting or halogen alokoxy, cyano, hydroxy, alkyl and aryl. Illustrative of such polymers of α,β-unsaturated monomers are polystyrene, polyethylene, polypropylene, poly(l-octadecene) , polyisobutylene, ( oly(l-ρentene) , poly(2-methylstyrene), poly(4-methylstyrene), poly(l-hexene), poly(l-pentene), poly(4-methoxystyrene), poly(5-methyl-l-hexene), ρoly(4-methylpentene) , poly (1-butene), polyvinyl chloride, polybutylene, polyacrylonitrile, ρoly(methyl pentene-1), poly(vinyl alcohol), poly(vinyl-acetate), ρoly(vinyl butyral), poly(vinyl chloride), poly(vinylidene chloride) , poly(vinyl fluoride), vinyl chloride-vinyl acetate chloride copolymer, poly(vinylidene fluoride), ρoly(methyl acrylate, poly(methyl methacrylate), ρoly(methacrylonitrile), poly(acrylamide), poly(vinyl fluoride),poly(vinyl formal), poly(3-methyl-1-butene) , poly(l-pentene), poly(4-methyl-1-butene) , ρoly(l-pentene), poly(vinylidene dinitrile), poly(4-methyl-l-pentene, poly(l-hexane), poly(5-methyl-l-hexene), poly(vinyl-cyclopentene), fluorinated ethylene propylene copolymer, ρoly(vinylcyclohexane), poly(a-vinyl-naphthalene), poly(vinyl methyl ether), ρoly(vinyl-ethylether), ρoly(vinyl propylether), poly(vinyl carbazole), ρoly(vinyl pyrrolidone), poly(2-chlorostyrene), ρoly(4-chlorostyrene), poly(vinyl formate), ρoly(tetrafluoroethylene) , poly(vinyl butyl ether), poly(vinyl octyl ether), poly(vinyl methyl ketone), poly(methyl-isopropenyl ketone), poly(4-phenylstyrene), poly(vinyl formate), poly(vinyl butyl ether), ρoly(vinyl octyl ether), poly(vinyl methyl ketone), poly(methyl isopropenyl ketone), poly( -phenylstyrene), poly(acrylonitrile), poly(vinyl chloride), poly(vinylidene chloride), ρoly(vinyl alcohol), ρoly(ethylene), poly(propylene) , poly(styrene) , poly(vinylidene dinitrile), co-poly(tretrafluoroethylene/ethylene) and the like.

Preferred polymers derived from α,β-unsaturated monomers are poly(ethylene) , poly(propylene) , poly(acrylonitrile) , poly(vinyl chloride), poly(vinylidene chloride), poly(tetrafluoroethylene) , poly(vinylidene dinitrile), poly(vinyl alcohol) and ρoly(styrene) . More preferred polymers are ρoly(ethylene) , poly(propylene) and ρoly(vinyl alcohol) . Poly(ethylene) if the polymer of choice. As used herein, the term "polyethylene" shall mean a predominantly linear polyethylene material that may contain minor amounts of chain branching or comonomres not exceeding 5 modifying units per 100 main chain carbon atoms, and that may also contain admixed therewith not more than about 50wt% or one or more polymeric additives such as alkene-1-polymers, in particular low density polyethylene, polypropylene or polybutylene, copolymers containing mono-olefins as primary monomers, oxidized polyolefins, graft polyolefin copolymers and polyoxymeth lens, or low molecular weight additives such as anti-oxidants, lubricants, ultra-violet screening agents, colorants and the like which are commonly incorporated by reference. Depending upon the formation technique, the draw ratio and temperatures, and othe conditions, a variety of properties can be imparted to these filaments. Many of the filaments have melting points higher than the melting point of the polymer from which they were formed. Thus, for example, high molecular weight polyethylenes of 150,000, one million and two million generally have melting points in the bulk of 138°C. The highly oriented polyethylene filaments made of these materials have melting points of from about 7 to about 13^eC higher. Thus, a slight increase in melting point reflects the crystalline perfection and higher crystalline orientation of the filaments as compared to the bulk polymer.

Still other useful fibers include polyurethane fibers, poly carbonate fibers, and various semi-synthetic fibers such as rayon.

As depicted in the figures, a plurality of fibers 12 are fed into the process from creels 10. It should be appreciated that fiber 12 can be used in the process directly from a conventional fiber forming process. Illustrative of such processes are melt spinning processes of the type described in U.S. Patent No. 4,504,432; solution spinning processes such as those described in U.S. Patent Nos. 4,376,370; 4,344,908 and 4,411,854; and gel spinning processes such as those described in U.S. Patent Nos. 4,440,711; 4,356,138; 4,457,985; 4,137,394 and 4,551,196. The fibers may be in any suitable form obtained from the final step of the process or at any suitable stage thereof, such as in the form of a cooled, dried or solvent containing fiber as for example a coagulate, a gel, an xerogel, and like.

In the embodiment of FIG 1, yarn 12 are aligned coplanarly and in a substantially parallel fashion by pulling yarn 12 through coarse comb 14 and and fine comb 16. Such an alignment minimizes yarn entanglements further down-stream in the process. However, it should be appreciated that such combs do not constitute an essential element of the process especially where other conventional means are employed to prevent entanglement or where fewer yarns 12 are employed in the process. Yarn 12 are then passed over guide bar 18 which maintains yarn alignment and are then fed through friction tension means 20. In FIG 2 and 3 is depicted a detailed view of the preferred friction tension mean 20 of the embodiment of FIG 1. As depicted in FIGs 2 and 3, friction tension means 20 of FIG 1 comprises eight substantially cylindrical bars 22^ to 2_vιn which are positioned such that the center of axis of two sets of four bars are in parallel planes and the 5 center of axis of the four bars in each set are parallel and are substantially in the same plane. As shown in FIGs. 2 and 3, yarn 12 loops about bars 22^ to 2_vnι at an angle of contact θ° to θ^β _viϋ and a contact width of w^ to w m, respectively, to create

10 a frictional force between yarn 12 and the contacted surfaces of bars 22^ and 22_vm. This frictional force results in a tension, T₂, in region 28 of yarn 12 extending from the point at which yarn 12 contact friction tension means 20 into the region upstream of

!5 friction tension mean 20, and a tension, T^, in the region 26 of yarn 12 downstream of the point of contact between yarn 12 and friction tension mean 20 when yarn 12 are pulled through tension friction means 20 by a force, F₂, exerted by some suitable

₂r means as for example by pull rolls 34 depicted FIG. 1. T₂ is greater than T . The difference between T and T₂, depends on the magnitude of the frictional forces developed between yarns 12 and the contacted surfaces of tension friction means 20. The greater

25 the difference between T-_ and T₂ the greater the frictional force being applying to yarn 12 to stretch the yarn. Conversely, the smaller the difference between T and T₂ the lower the frictional force being applied to yarn 12. The magnitude of the

30 frictional forces depends on a number of variables which can be controlled to obtain the desired degree of tension, T₂. One such variable is the area of the surface of tension friction means 20 contacted by yarn 12. In the embodiment of FIGS 1, 2 and 3, this

35 total area is equal to the summation of:

total area«∑(r)(θ°)(w)

where r is the radius r^ to r_viii of each of contacted bars 22^ to 2_vin; θ^β is the angle of contact θ°ι to θ°_viii °f each contact bar 22^ to ²²viii' ^{w s the} (width) of yarn w^ to wm contacting bars 22^ to 22_vm. It should be appreciated that the area of contact were tension friction means 20 is of another geometrical shape can be conveniently determined through use of other conventional geometric relationships. The magnitude of the frictional forces is also dependent on the coefficient of friction, which depends on the surface of materials forming yarn 12 and the contacted surfaces of tension friction means 20, contact temperature and T^ and T₂. Frictional forces are described in more detail in "Design of Machine Elements", by M.F. Spotts, published by Prentice Hall," Fundamentals of Deformation Processing", by W.A. Backofen (editor), published by Syracuse University Press and "Engineering Mechanics", by S. Timoshenko and D.H. Young, published by McGraw-Hill Book Company which are incorporated herein by reference. As a general rule, the force being applied against yarn 12 upstream of tension friction means 20 is increased when T_1# contact angles θ°ι to ^ββ _viϋ and the coefficient of friction (μ) are increased, and when the radii (r^ to r_vn) of bars 22^ to 22_vιn are decreased for a constant contacted area between yarn 12 and bars 22^ to 22_vm. Conversely, the force being exerted against yarn 12 upstream of tension friction means 20 is decreased with decreasing T_χ, contact angle θ°ι to θ°_viii and coefficient of friction(μ) , and with increasing radius

to 2_vιn for a 5 constant contacted area between yarn 12 and bars 22^ to 22 viii- In the operation of this invention, the frictional forces can be varied widely provided that the forces are less than those necessary to break all of yarn 12. Preferably forces as such that no or

10 substantially no yarn 12 are broken in the conduct of the process. In general, sufficient frictional force is applied to provide the desired "stretch ratio" when yarn 12 under tension and heat are stretched. As used herein, "stretch ratio" is the ratio of

1 cross-sectional area of feed yarn 12 to the cross-sectional area of stretched yarn 12, or the ratio of the length of stretched yarn 12 to the length of the feed yarn 12. The concept of stretch ratio or draw ratio is well known in the fiber

₂₍. processing art and will not be described herein in great detail. The frictional forces in conjunction with heating means are usually sufficient to provide a stretch ratio or draw ratio of greater than 1. In the preferred embodiments of the invention, the 2 _5_ frictional forces are sufficient to provide a stretch ratio or draw ratio of from greater than 1 to about 20, more preferably from greater than 1 to about 12 and most preferably from greater than 1 to about 8. A stretch ratio or draw ratio of from greater than 1

30 to about 6 is the stretch ratio or draw ratio of choice.

The structural configuration and materials employed in the construction of the contact surfaces of tension friction means 20 may vary widely provided

35 that means 20 provides sufficient frictional force and further provided that the material is not so abrasive as to break an undue number of yarn 12. For example, such materials may be a polymeric material such as a thermosetting resin as for example an alkyd such as those derived from esterification of polybasic acids; cross linkable acrylics; amino resins derived from reaction between formaldehyde and various amino compounds such as melamine, urea and the like; urethanes derived from reaction between polyisocyanates and polyols; unsaturated polyesters derived from reaction of dibasic acids and glycols; expoxies based on aliphatic, cycloaliphatic, aromatic and heterocyclic epoides; and phenolic resins produced by reaction of phenols and aldehydes. Useful materials for fabrication of the contact surfaces of friction tension means 20 also include thermoplastic resins such as the polymers described above for use in the fabrication of yarns 12 and silicon based polymers. Still other useful materials for fabrication of the contacting surfaces of tension friction means 20 are fiber filled compositions comprising a matrix of one or more of thermoplastic resins, one or more thermosetting resins or mixtures thereof containing dispersed organic or inorganic fibers such as carbon, S-glass, E-glass, boron, S₂-glass, ceramic, metal, polymers such as aramid, polyethylene and like fibers. The contaced sufaces of tension friction means 20 may be also formed from metals such as iron, steel, nickel, copper, aluminum and alloys thereof and metallic and non-metallic ceramic materials such as aluminum oxide, boron carbide, silicon carbide, titanium nitride, zirconia-silica, alumina-silica, aluminum carbide, silicon nitride and the like.

In the embodiment of FIGs 1 to 3, tension friction means 20 is a pluralility of cylindrical bars 22^ to 22_vm. However, such structures are not intended to be a limitation on the shape, type and/or structure of tension friction means 20 that can be used in the conduct of this invention provided that such means include one or more surfaces for contacting yarm 12 to provide a frictional force between such surface and yarn 12 to provide a tension, T₂, upstream of said means. Alternative tension friction means 20 are depicted in FIGs 5 to 7. The embodiment of FIG 5 comprises a single bar 22 around which yarn 12 is wrapped in three loops to provide sufficient contact area and frictional force so that yarn 12 has a tension, T₂, upstream of the contacted sufaces of means 20. The embodiment of FIG. 6 is a modification of the embodiment of FIG. 5 and includes an additional stationary bar 22^ over which yarn 12 is loop to provide the required surface contact area and frictional force so that yarn 12 has a tension, T₂, upstream of the contacted surfaces of means 20. The stationary bar 22^ serves the additional purpose of a yarn separator as described in more detail in U.S. Patent No. 4,916,000. In the embodiment of FIG 7, friction tension means 20 is formed form plate 30 having two sets of grooves 32 in parallel horizontal planes. Each set of grooves 32 consists of three grooves all in the same plane. Yarn 12 is looped about plate 30 in grooves 32 to provide the required contact area and frictional force so that yarn 12 has a tension, T₂, upstream of contacted surfaces of means 20. Similarly, a circular cylinder with a continuous groove (not depiected) is also applicable. As is apparent from the foregoing, the common essential element in each friction tension means 20, is that they include sufficient contact area between yarn 12 and the contacted surface of tension friction means 20 such that the frictional forces between the contacted surfaces and yarn 12 provides a tension, T₂, upstream of the contacted sufaces of means 20. As depicted in FIG. 1, from friction tension means 20 yarns 12 are pull by a force, F₂, by way of driven take-up rolls 34, over heating means 36 maintained at some predetermined temperature, or a combination of different predetermined temperatures. When yarn 12 under tension T to T₂ is heated to a certain temperature the heated yarn 12 is stretched to the desired stretch ratio. In FIG 1, yarn 12 under tension is heated upstream of the last contacted surface of tension friction means 20.

However, it should be appreciated that heating may be carried out at any point from the first contacted surface of tension friction means 20 to any point upstream thereof. For example, yarn 12 may be heated and stretched by heating one or more of the contacted surfaces of friction tension means 20 by some suitable method as for example conduction, convection, radiation or the like, and then stretched as yarn 12 slide over the heated surfaces under tension provided by friction tension means 20.

Alternatively, yarn 12 can be pulled over one or more heated surfaces of friction tension means 20 under tension where yarn 12 are partially drawn or stretched, thereafter yarn 12 are pulled through one or more additional heating zones upstream of the heated contacted surfaces of tension friction means 20 such as an electrically heated heater, plate or plates, hot air oven or radiant heaters to complete the drawing or stretching of yarn 12 to the desired extent. As shown in the Figures, heating is accomplished by heating block 38, maintained at a temperature Tι°C and heating block 40 maintained at a temperature of T₂°C. It should be appreciated that other heating means such as heating tubes, steam or hot air jets, recirculated oil, hot air ovens, radiant heat and the like may be used. The stretching may be conducted in a single stage as depicted in FIG. 7, or may be conducted in two or more stages as depicted in FIG. 1. In the preferred embodiments of this invention, stretching or drawing is conducted in two or more stages with the first stage stretching conducted preferably at room temperature or at some elevated temperature and the successive stretching stages conducted at an elevated temperature equal to or greater than that employed in the proceeding stretching stage. In general, stretching temperatures may vary widely to any temperature less than about the melting point or degradation temperature of the polymer forming yarn 12. In the preferred embodiments of the invention, stretching temperatures vary from less than about the melting point or degradation temperature of the polymer forming yarn 12 to greater than about the glass transition temperature of said polymer. Thus, the stretching temperature in any particular situation will vary widely depending on the polymer, number of stretching stages employed, stretching ratios required to achieve specific tensile strength and tensile modulus and the like. Useful and optimum stretching conditions i.e. number of stages, stretching ratios, stretching temperatures and the like for conventional yarn materials such as nylon 6, nylon 6,6, rayon, nylon 11, nylon 6,10, nylon 6,6, nylon 3, nylon 4, nylon 5, nylon 6, nylon 7, nylon 8, nylon 12, nylon 9, nylon MXD-6, nylon 6T, acetate, poly acrylonitrile, poly vinyl chloride, polyvinylidene chloride, polyvinylalcohol, polyethrene, poly(tetra fluoroethylene) , poly(vinylidene dinitrile), polyurea, poly carbonate poly(ethylene terephathalate) , poly(l,4-dimethylene cyclohexane terephathalate), poly(p-ethyleneoxy benzoate), polyethylene and polypropylene are well known in the art. For example, U.S. Patent Nos. 3,048,465; 3,376,370 and 4,504,432, describe processes to stretch high density polyethylene monofilaments in various stages at various temperatures, as for example in three stages at temperature of 100°C, 115°C and 120°C, respectively, using heated rolls. These parameters therefore will not be described herein any great detail. Stretched yarn 12 produced in the embodiment of FIG 1 is taken up on driven take-up spool 34.

In FIG 8 is depicted in schematic form an embodiment of this invention for stretching yarn and fabricating the stretched yarn into a composite article comprising at least one layer comprising a network of fibers in a polymeric matrix as for example a prepreg, pultrusion and filament winding, preferably in which the fibers are aligned parallel or substantially parallel with respect to the common fibers direction. As shown in FIG. 8, a plurality of creels 10 containing yarn 42 are deployed. Yarn 42 may vary widely and is preferably nylon 11, nylon 6,10, nylon 6,6, nylon 3, nylon 4, nylon 5, nylon 6, nylon 7, nylon 8, nylon 12, nylon 12, nylon 9, nylon MXD-6, nylon 6T, ρoly(ethylene terephthalate), poly acrylonitrile, polyvinylchloride, polyvinylidene chloride, polyvinylalcohol, polystyrene, poly (tetrafluoroethylene) poly(vinylidene dinitrile), polyurea, poly carbonate, poly(p-ethylenoxy benzoate), poly(ethylene), ρoly(proρylene) , polyvinyl alcohol, rayon, acetate, and poly(14-dimethylene cyclohexene terephthalate) yarn, more preferably polyethylene yarn and polypropylene yarn, and most preferably polyethylene yarn as for example that sold by Allied-Signal Inc. trademark under the trademark Spectra® 900 polyethylene yarn and Spectra® 1000 polyethylene yarn. Yarn 42 preferably consist of bundles of from about 30 to about 2000 individual filaments of less than about 50 denier, per filament and more preferably bundles of from about 3 to about 2000 individual filaments of less than about 40 denier per filament, and most preferably are bundles of from about 30 to about 1000 filaments of less than about 25 denier per filament. In these preferred embodiments, individual filaments are aligned coplanarly and in a substantially parallel and unidirectional fashion as in a prepreg by pulling yarns 42 through a first set of coarse combs and a second set of fine combs, indentified in FIG 8 by the numerals 44 and 46, respectively. It should be appreciated, however, that yarn 42 can be aligned in any desired fashion to form the desired fiber network. As shown in FIG. 8, the average distance between aligned yarn 42 is controlled by the distance between neighboring pins of combs 44 and 46.

In the preferred embodiments of the invention, the distance between neighboring pins of second set of combs 46 is preferably equal to or less than about twice the "equivalent diameter of the filament" of yarn 42 times the number of filaments in yarn 42. As used herein, the "equivalent diameter of the filament" is the diameter of a circle having a cross-sectional area equal to the average cross-sectional area of each of the filaments in yarn 42. In more preferred embodiments of the invention, the distance between neighboring pins of comb 44 is equal to or less than about 2.5 times the product of the equivalent diameter of the filaments and the number of filaments in yarn 42. Yarn 42 are then pulled through friction tension means 47 by driven pull rolls 48. This places yarn 42 under tension upstream of tension friction means 47 and also optionally functions as the spreader bar in the process of USP No. 4,916,000 to spread yarn 42. In the preferred embodiments of the invention, yarn 42 are spread such that the thickness of the yarn bundle is equal to or less than about 12.8 time the equivalent diameter of each of the filaments in the yarn bundle, preferably equal to or less than about 8 times the equivalent diameter of each of the filaments in the yarn bundle and more preferably from about 1.0 to about 7 times the equivalent diameter of each of the filaments in the yarn bundle. Tension friction means 47 also improves the alignment of individual filaments within the filament bundle. After passing through tension friction and optionally spreader means 47, aligned yarn 42, which have optionally been spread and are under tension, are heated over a heating means 60 and stretched to the desired ratio. Aligned and stretched yarn 42 are then coated with the matrix material using any conventional method. For example, the material can be applied as a dispersion, of the material in a suitable solvent or in the form of an emulsion or as a low molecular weight material which on consolidation cross-links to form the desired matrix material; or sprayed on as fine discrete particles of the matrix material. In the embodiment of FIG 8, yarn 42 are pulled through the apparatus at a tension T₂ supplied by driven pull roll 48 to a position directly under matrix material applying means 62 where they are coated with the matrix material. In the embodiment of FIG. 8, matrix material applying means 62 is a combination of reciprocating cylinder 64 connected to a source of resin supply (not shown) and a resin applicator 63. In the embodiment of FIG 8, resin applicator 63 is constructed of a plastic tube with orifices to meter the desired amount of resin flow. The amount of resin coated on stretched yarn 42 is determined by the gap setting between roll 66 and support plate 68, and adjustable rip rolls 72 and 70. Aligned yarn 42 is preferably coated with the matrix material. In the preferred embodiments of the invention, each filament of yarn 42 is coated with the matrix material.

The matrix material may vary widely but is usually one or more themoplastic resins or one or more thermosetting resins such as those described herein below, or a combination thereof. Illustrative of useful matrix materials are those described in U.S. Patent Nos. 4,916,000; 4,623,574; 4,748,064; 4,737,402; 4,613,534; 4,413,110; 4,650,710; 4,403,012; 4,457,985; 4,737,401; 4,543,286;

4,563,392; 4,501,856 and the like, which are hereby incorporated by reference. In one preferred embodiment of this invention, the matrix material is a combination of one or more thermosetting resins and one or more thermoplastic resins, such as a mixture of a thermosetting vinyl ester resin and a themoplastic polyurethane resin. In another preferred embodiment of the invention, the matrix material is a low modulus material such as a block copolymer of conjugated dienes, e.g., butadiene, and isoprene, and vinyl aromatic monomers, e.g., styrene, vinyl toluene and t-butyl-styrene, which is applied as a dispersion in a solvent such as water.

After application of the desired matrix material, the coated filaments are consolidated into the desired composite. Consolidation methods may vary widely depending on a number of factors, as for example, the type of matrix material and the manner in which it is applied to yarn 42, the type of yarn 42 and other factors known to those of skill in the art. In the preferred embodiments of the invention after application of the resin material, the resin coated, stretched and aligned yarn 42, are fed onto a suitable support means, as for example release paper 50, which in these preferred embodiments is a belt of silicone coated release paper. Release paper 50 is fed onto guide roll 65 and then to support plate 68 from release paper unwind 54 through release paper tension control means 56. The combination of release paper 50 and aligned, stretched and resin coated yarn 42 is pulled through adjustable resin smoother rolls 66 and support plate 68 and through a pair of adjustable nip rolls 72 and 70 to level smooth resin coated yarn 42. However, other leveling means such as doctor blades and the like can be used to level and smooth resin coated yarn 42. Under tension from pull rolls 48, coated yarn 42 are then conveyed to solvent removal means 74 to remove all or substantially all of the solvent from the matrix material coating yarn 42. In the embodiment of FIG. 8, solvent removal means 74 is a gas fired oven which heats coated yarn 42 above the vaporization temperature of the solvent and below the degradation temperature and/or melting point of yarn 42 and the matrix material. However, any solvent removal means known to those of skill in the art can be used, as for example an oven in conjunction with a vacuum means (not depicted) to allow for removal of the solvent at lower temperatures. The dried layer of coated yarn 42 preferably in which the ratio of the thickness of the layer (combination of the filaments and coating) to the equivalent diameter of the filaments is preferably equal to or less than about 12.8, together with release paper 50 are pulled from solvent removal means 74 by pull rolls 48 through guide rolls 76 and onto composite winder 78. The Figures depict the process of this invention used to form a prepreg. However, it should be appreciated that the process can be employed to form various other kinds of continuous fiber filled composites known to those of skill in the art where the fiber is in a network, such as pultrusions and filaments windings.

The composite comprising at least one layer comprising a network of resin coated fibers fabricated in accordance with this invention has many uses. For example, the composite can be used in the fabrication of composite articles, as for example the composites described in US Patent No. 4,623,574; 4,748,064; 4,737,402; 4,613,535; 4,413,110;

4,916,000; 4,403,012; 4,457,985; 4,737,401; 4,563,392 and 4,501,856. In the preferred embodiments, it is convenient to characterize the geometries of such composites by the geometries of the fiber and then to indicate that the matrix material may occupy part or all of the void space left by the network of fibers. One such suitable arrangement is where the composite fabricated by the process of the invention is a prepreg and where the prepreg is fabricated into a more complicated composite which is a plurality of layers or laminates in which the fibers coated with a suitable matrix material as for example a mixture of a polyurethane and a vinyl ester resin are arranged in a sheet-like array and aligned parallel to one another along a common filament direction.

Successive layers of such coated, uni-directional fibers can be rotated with respect to the previous layer. An example of such laminate structures are composites with the second, third, fourth and fifth layers rotated +45°, -45°, 90° and 0° , with respect to the first layer, but not necessarily in that order. Other examples include composites with 0°/90° layout of yarn or fibers.

The following examples are presented to more particularly illustrate the invention and should not be construed as limitations thereon.

Example 1 This example illustrates the effectiveness of the frictional forces created by the frictional means. The tension frictional means 20 consisted of 8 steel bars as schematically shown in FIG. 2. Each bar had a circular cross section and had a 0.5 inch (0.127cm) diameter and had a length of 6in. (15.24cm) . The coefficient of friction of the bars was 0.1. The horizontal distance between two neighboring bars 5 measured from center to center was 1.5 inch ( cm) . The vertical distance between the line drawn over the centers of the top positioned bars and the bottom positioned bars was 3 inches (7.62 cm). Spectra® 900 polyethylene fibers (1185 10 denier/118 filaments) was used for this experiment. The yarn was pulled at a speed of 6 feet per min (1.8 m/min) over guide roll 18 and wound in series around the bars (22^, 22n...22_vιn) as depicted in FIGs 1, 2 and 3. Frictional, or tensile, forces were 5 measured as a function of the 4th, 6th, and 8th bars. For instance, with the 4th bar, the frictional force was measured along the yarn between the 4th bar, 22ι_v, and the 5th bar, 22_v. Similarly, frictional forces were measured for the 6th bar, __n 22_vι, and 8th bar, 22_vιn. The position at which the frictional force was measured for the 6th bar was along the yarn between the 6th bar, 22_vι, and the 7th bar, 22_viι, and for the 8th bar along the yarn between the 8th bar, 22_v^_i, and guide box 24 (See FIG.s 1, 2 and 3). The results of these measurements 5 are set forth in the following Table 1 and plotted in Fig. 9.

X BJ_E_J_ Frictional Forces vs. Number of bar 0 Number of bars Frictional Forces

(grams)

2

4 725

6 1225

8 3300

10 B¹ 5

1. "B" indicates that yarn breakage was observed. When 8 bars were used, the frictional, or tensile, stress created was about 3300 grams/1185 denier or 2.8 gm/den.

Example 2

Example 1 was repeated except that a single strand of Spectra®-900 polyethylene fiber 1182 denier and ultimate tensile strength (UTS) of 28.3 gm/denier was heated and stretched or drawn over heater 60 after being pulled through tension frictional means 20. The apparatus depicted in Fig. 8 was used. In the experiment, the yarn was pulled over bars 47, through heater 60, by-passed resin coater 62, hot air oven 74, and wound on to take up unit 78 at a speed of 6 ft/min (1.8 m/min) . The heater was 36 in. (91.4 cm) wide long and 16 in. (40.6 cm), and was maintained at a temperature of 147^βC. The object of the experiment was to determine the optimum number of contact sufaces i.e. bars, required to achieve maximum improvement in the yarn tensile properties, such as tensile strength, at a constant heater temperature of 147°C. The yarn was pulled over four bars (22^ to 22_iv and guide bar 24), six bars (22ι to

22_vi and guide bar 24) and eight bars (22± to 22_vii and guide bar 24), respectively. The denier and tensile strength of the drawn yarn are set forth in the following Table 2 and graphically shown in FIG.

10.

IΔ__I_______ inlet outlet njim_2£j_ tension tension drawn ^yarn of bars (grams) (grams) _soj_at UTS^J

(g/denier)

2 .2

4 280 725 1146 26.2

6 280 1225 1008 30.2

8 280 3300 605 35.2

I"UTS" is the ultimate tensile strength of the yarn in gram/denier ("g/d").

2 "—" indicates that the value was not obtained. The data in Table 2 and Fig. 10 show that when less than six contacted surfaces or bars are used, yarn denier and ultimate tensile strength were only marginally affected. However, as the number of contacted surfaces or bars was increased to six, the yarn was drawn from 1182 denier (Spectra-900® polyethylene fiber as feed yarn) to 1008 denier which represented a stretch ratio of 1.17, while the ultimate tensile strength of the feed yarn was increased from 28.3 grams/denier to 30.2 grams/denier which represented an increase in tensile strength of 6.7%. As the number of bars was increased to eight, the yarn was drawn from 1182 denier (Spectra®-900 polyethylene feed yarn) to 605 denier which corresponded to a yarn stretch ratio of 1182/605 or 1.96 and a 48.9% decrease in yarn denier. The ultimate tensile strength was increased from 28.3 grams/denier to 35 grams/denier which represented an increased in tensile strength of 24%. When the number of bars was further increased fco nine, occasional yarn breakage occurred caused by excessive tension of approximately 4,500 grams.

EXAMPLE 3 Example 2 was repeated with 8 bars, except that the temperatures of the heating blocks were varied to 143^βC 145^βC, 147^βC and 150°C. In each experiment, the fed yarn was Spectra® polyethylene fiber (1182 denier) having a tensile strength of 28.3 grams/denier. The effect of block temperatures on yarn tensile strength is tabulated in the following Table 3.

The results indicated that as heating block temperatures were increased to 143°C, the tensile strength of the draw yarn was increased from 28.3 gram/denier to 32 gram/denier, which corresponded to an increase of 13% in the tensile strength over the feed yarn (Spectra®-900 polyethylene yarn). The increase in tensile strength reached the optimum of 32% when the block temperature was 148°C. As the block temperature was further increased to 151°C, the % increase in tensile strength was 34.4 g/denier which was 22% greater than the tensile strength of the feed yarn.

EXAMPLE 4 This example illustrates the process in which yarn drawing is coupled with prepregging. Experimental apparatus used is schematically shown in Fig. 8. In the experiment, 42 strands, or ends, of Spectra®-900 polyethylene yarns (1182 denier haveing a tensile strength of 28.3 grams/denier) were pulled from yarn creels 10 through coarse comb 44 having 4 pins per inch and fine comb 46 having 20 pins per inch through tension frictional means 47, over heating block 60, and to resin coating station 68, by pull roll 48, and then onto the prepreg winder 78. Heating block 60 was approximately 16 inch (40.6 cm) wide x 36 inch (91.44 cm) long and was heated by four electric heaters. Each heater had 1,250 watts X 120 volts. The temperature of the heating block was maintained at 147°C. At a yarn speed of 6 ft/min 5 (1.83 m/min), it was estimated that the yarn had reached the steady state of 147^βC. After the stretched yarn passed heating block 60, it was coated with Kraton® matrix resin, a polyestyrene -polyisoprene-polystyrene block copolymer in water 10 emulsion manufactured by Pierce and Steven

Corporation. Matrix material applying means 62 was used for coating. The coated yarn strands, or prepreg, was pulled through hot air oven 74 at a temperature of approximately 120°C where water was 1₅ evaporated resulting in a dried uniaxial prepreg tape. The prepreg tape was then wrapped around pull rolls 48 in an "S" configuration to achieve sufficient frictional pull without slippage between two nip pull rolls 48. After exiting pull rolls 48, the prepreg was wound onto prepreg winder 78 under reduced and control tension. The pregreg tape was 0.0015 in. (0.0038 cm) thick and 4.2 in (10.7 cm) wide with an areal density of 35.3 grams/m². The matrix-resin content of the prepreg tape was __ approximately 22%. The yarns were aligned and spreaded very uniformly both along and across the prepreg. The process was continued for 4 hours without interruption even though occasional fuzz balls and broken filaments were observed.

30

EXAMPLE 5

Example 4 was repeated except that artificial knots were formed in the Spectra®-900 polyethylene feed yarns before entering coarse comb 44 and fine

3 comb 46. All these knots went smoothly around the bars of tension friction means 47 with other yarn strands without interrupting the in-line stretching-prepregging process.

For relative comparison purpose, if a conventional drawing system was used, i.e., rotating godet rolls were used, broken filaments, and knots would accumulate on the godet roll surface which will interrupt the continuous drawing and prepreging process.

EXAMPLE 6 Example 2 was repeated except that a single strand of Spectra®-1000 polyethylene yarn (671 denier/118 filaments) yarn was used. The yarn was stretched using a portion of the apparatus of Fig. 8. Eight bars were used as tension friction means 40. The yarn inlet tension was approximately 200 grams while the outlet tension was 3,000 grams. The yarn was heated and drawn immediately over heating block 60 at a temperature of 149^βC. After by-passing the resin applicator 62 and oven 74, the drawn yarn was wound on a take up spool at a speed of 15 ft/min. (4.5 m/min). The tensile properties of the feed yarn, and the drawn yarn are listed in the following Table 4.

YARN

Feed Yarn 48,52 671 31.8 1401 3.07 Drawn yarn 53.9 575 35.6 1784 2.99

The data in Table 4, shows that the yarn was drawn from 671 denier to 575 denier which corresponded to a stretch ratio of 671/575 or 1.17 and resulted in a 12%, 55% and 11% increase in tensile strength, tensile modulus and energy at break, respectively. The maximum strain remained relatively unchanged.

Claims

WHAT IS CLAIMED IS :

1. A process of stretching fiber which comprises the steps of:

(a) pulling one or more fibers over one or more surfaces of one or more friction tension means, said means comprising one or more stationary bodies, or one or more moving bodies provided that the movement of a moving body in the direction in which said fibers are being pulled is at a velocity which is less than the linear velocity of said fibers in said direction, or a combination of said stationary and moving bodies thereby creating a frictional force between said contacted surfaces and said fibers such that the tension of said fibers upstream of said friction tension means, T₂, is greater than the tension of said fibers down-stream of said friction tension means, T_x; and (b) heating all or a portion of said fiber under tension (T_ to T₂) at any point from the first of said contacted surf ces and upstream thereof to stretch said fibers to any extent.

2. A process according to claim 1 wherein said fiber is heated at a temperature greater than the glass-transition temperature of the polymer forming said fiber.

3. A process according to claim 1 wherein said friction tension means comprises one or more shaped bars with a regular or irregular cross-section having a surface thereof contacted by said pulled fiber.

4. A process according to claim 3 wherein said friction tension means comprises one or more bars having a circular, oval or substantially circular or oval cross-section.

5. A process of claim 1 wherein the coefficient of friction of said contacted surfaces is at least about 0.001.

6. A process according to claim 1 wherein the ratio of T₂ to T_x is equal to or greater than about 4.

7. A process according to claim 20 wherein said ratio is from about 4 to about 500.

8. A process of stretching fibers and fabricating said fibers into a composite comprising at least one layer which comprises a network of fibers in a polymeric matrix, said process comprises the steps of:

(a) pulling one or more fibers over one or more surfaces of one or more friction tension means comprising one or more stationary bodies, or one or more moving bodies provided that the movement of at least one body in the direction in which said fibers are being pulled at a velocity which is less than the linear velocity of said pulled fibers in said direction, or a combination of said moving bodies and stationary bodies, thereby creating a frictional force between said contacted surface and said pulled fibers such that the tension of said fibers upstream of said friction tension means, T₂, is greater than the tension of said pulled fiber down-stream of said friction tension means, T_x;

(b) heating all or a portion of said pulled fiber under tension (T_x to T₂) at any point from the first of said contacted surface and upstream thereof for a time and at a temperature sufficient to stretch said fiber to any extent; and

(c) forming said stretched fibers into a composite which comprises at least one layer comprising a network of said fibers dispersed in a matrix material employing a procedure selected from the group consisting of: (i) aligning a plurality of said stretched fibers into a network of fibers and coating said aligned and stretched fibers with a matrix material comprising one or more thermosetting polymers, one or more thermoplastic polymers or a combination thereof to form a layer comprising a network of fibers dispersed in a matrix material; and

(ii) coating said stretched fibers with said matrix material and aligning a plurality of said stretched and coated fibers to form said layer.

9. An apparatus for stretching fibers which comprises: a) one or more friction tension means, said means comprising or more bodies having surfaces for contacting fibers pulled through said apparatus whereby a frictional force is created between said pulled fibers and said contacted fibers such that the tension of said fibers upstream of said contacted surfaces, T₂, is greater than the tension of said fibers down stream of said .contacted surfaces, T_x,; b) fiber heating means positioned at any point from the first of said contacted surfaces and upstream thereof for heating all or a portion of that portion of said fiber under tension T₂ thereby stretching said fiber; and c) fiber pulling means for pulling said fiber over said contacted surfaces and to said heating positioned upstream of said contacted surfaces and said fiber heating means.

10. An apparatus for stretching fiber and for fabricating the stretched fibers into a composite comprising at least one layer which comprises a network of fibers in a polymeric matrix which comprises: a) one or more friction tension means, said means comprising or more stationary bodies or one or more moving bodies provided that the movement of a moving body in the direction in which said fibers are being pulled is at a velocity which is less than the linear velocity of said pulled fibers in said direction or a combination of said moving bodies or stationary bodies, said bodies having surfaces for contacting fibers pulled therethrough such that frictional forces are created between said pulled fibers and said contacted surfaces such that the tension of said fibers upstream of said contacted surfaces, T₂, is greater than the tension of said fibers down-stream contacted surfaces, Ti,; b) fiber heating means positioned at any point from the first of said contacted surfaces and upstream thereof for heating all or a portion of that portion of said fiber under tension T₂ thereby stretching said fiber; (c) composite forming means for forming stretched fibers into a composite which comprises at least one layer comprising a network of fibers dispersed in a matrix material comprising one or more thermosetting polymer, one or more thermoplastic polymers or a combination thereof, said means selected from the group consisting of:

(i) fiber aligning means for aligning a plurality of stretched fibers into a network of fibers and fiber coating means for coating said aligned and stretched fibers to form said composite; and (ii) fiber coating means for coating said stretched fibers with said matrix material and coated fiber aligning means for aligning a plurality of said coated and stretched fibers into a network to form said composite; and

d) fiber pulling means for pulling said fiber through said apparatus.