US2821771A - Method of making a papermaker's felt - Google Patents

Description

No Drawing. Application-April 5,1957 Serial No. 650,832

4 Claims: (Cl. 28-72) The present inventionrelates to papermakers felts and a method forthe manufacture thereof and moreparticularly to papermakers felts madefrom synthetic fibers and a method by which such felts may be manufactured. The-papermakers felts to whichthis invention is directed are those which are used at any stage in the wet end or aheadof thedriersiofa papermaking machine: The-term papermaking machine includes machines which are used for the manufacture of cementand asbestos-cement-products-,- wallzboard, roofing felts, pulp laps and similar and relatedproducts. Such felts customarily have: been made from; natural -wool l or occasionally from ablend of natural wool and synthetic fibers. These" felts' are hereinafter referredto' as papermakers. feltsor as papermakersv wet felts.

Papermakers wet felts are usually woven fabrics which have been fulled to cause the wool fiberstherein to felt. In the manufacture thereof from wool or a blend of wool and synthetic material which contains sufiicient wool to render the fabric fullable, it has been customary to weave a fab'ric of a given width and to full it to a width ap-- proximately one-half the woven width while maintaining the woven length approximately the same throughout the operation. In this manner the woven material assumes the desired firmness in texture" and the felting of the. wool fibers with one another gives the felt a high degree of dimensional stability under the conditions of use. The fulling operation also cl'oses up the fabric givirig it'the ability to retain" the fibers of the paper while being sufiiciently porous to permit rapid removal of the water in the freshly laid paper. Beyond this, the natural wool papermakers felt will remain resilient while wet and thu's has theability to pick up a freshly laid'web'of paperfrom the wire and conduct it through the presses. This highly exacting combination of requirements has heretofore been-met only we natural wool fabric or one in which natural wool has been blended with a limited amount of other fibers and in each instance ithas been es's'entialthat the fabrics be capable of fulling;

According to the present invention, I providea' method for manufacturing entirely suitable papermakers felts having great dimensional stability and having a' 'life far in-excess' of 'that achieved by the best of woolen papermakers felts, my new felts being made substantially wholly from synthetic materials or blends thereof which are not capable of felting and which heretofore have been used only in limited amounts in combination with large-pen centages of wool irlthe manufacture of papermakers felts. According to my process, papermakers felts may be made entirely or substantially entirely from fibers or blends of. fibers which have-dry tensile strengths substantially above 20,000 pounds per square inch, which is-the average dry tensilezstrength of natural wool, and which. do notlose tensilestrength appreciably when wet and which qualities are: nylon and fibers derived from polyvinylidene r 2,821,771 Patented Feb. 4;, 1958 chloride. and acrylonitrile, copolymers of .polyvinylacetate, polyvinylchloride andpolyacrylonitrile and condensation products of terephthahc acid and ethylene glycol. The trade names. "saran, Orion, "dynel" and Dacron. are used to identity particular fibers within this group.

Many syntheticfibers suchlasthose enumerated above are supplied by the manufacturers thereof in the form ofa staple fiber which has been manipulated or treated by the manufacturer to resemble,. as closely as possible, anatural woolfiber. buchfibers are generally identified as "crimped" although the particular conformations formed inthe fibers thus identified may comprise crimps,,crinltles, curls, spirals, indentations or serrations in one or more lhus, tor. example, nylon is supplied as a staple fiber having, a. diameter and staple length t'alling within the rangeot' diameters and staple lengths encountered in natural wool. While these nylon fibers do not have the charaeteristicot' felting, they aresupplied in a delustered tormtan'd with a large'num'ber of sharp crimps throughoutrtheir individual lengths which are permanent individual fibers will remainso fiber willtexhibitsubstantially the bulk and springiness of wool 'and although it has a moderate initial shrinkage, it does not: exhibit the tendency to felt. This-makes such materialzhig-hly desirable as a substitute for wool in many fields-wherein progressive hardening and t61'lSIiC10f wool is undesirable. 'ihe lack of this characteristic, however,- has rendered such fiberwholly unsuitable for the manufacture of papermakers felts in accordance with known processes. following particular made wholly from such synthetic fibers.

The process and product of the presentinvention will be first described in general terms to enable those skilled in the art to manufacture papermakers felts from a relaselected for the manufacture of papermakersfelts having certain desired characteristics. Thus, synthetic resin staple fi ers are available in variousstaple lengths and in various engage one another in a relatively permanent relationship, thus contributing to dimensional stability of the final product.

.carding and spinning,

makers felts may be made wholly from synthetic resin as of the date of filing of fibers exhibit no definite 'equal to the corresponding ratio in natural wool which averages about 77%, that the fiber retain a large degree of its original resilience when wet and that the selected synthetic fibers have an abrasion resistance at least equivalent to wool when wet. The selected fiber must also have the property of taking a permanent set when subjected to heat treatment within the ranges of from 150 F. to 350 F. Furthermore, any crimping or curling imparted to the fibers should be permanent in the sense that they will not straighten out under the tensions found in weaving or finishing or heat treatment or under continued operation while wet at the temperatures encountered by papermakers felts upon a papermaking machine or during such cleaning or reconditioning operations as are normally practiced. Otherwise the fabric will lose its bulk and resilience.

In the manufacture of paper the advantage to be gained by the decrease in viscosity of water through the application of heat is recognized and in many cases, temperature of the paper stock is raised for this purpose. Temperatures rarely exceed 140 F. for economic reasons although it is conceivable that temperatures ranging from 33 F. to 212 F. might be encountered in the normal manufacture of paper. In any event the felt is not at all likely to be subjected to temperature about 212 F. and for practical purposes a felt set at 212 F. or slightly higher, as for example from 212 F. to 240 P. will be quite adequate for use on machines used in the manufacture of paper, asbestos cement products, wall board, lap pulp and similar products.

Crimped nylon, Dacron or Orlon" fibers will retain substantially their original crimped form throughout manufacture and use of a papermakers felt made from such materials and they have a ratio of wet strength to dry strength considerably higher than natural wool. the case of saran fibers there are two forms presently available which fall within the general classification of crimped fibers. One of these is identified by the manufacturer as crimped and the other is identified as curled. Apparently or processing, the saran fibers identified as crimped do not have sufficient stability under the conditions met in the manufacture and use of papermakers wet felts while the saran fibers identified as curled have satisfactory stability. In the appended claims the word crirnped is used in the general sense so as to include the curled saran fibers having the other characteristics set forth in the claims. Cellulosic fibers such as rayon or cellulose acetate are Wholly unsuitable because of lack of strength, wet or dry, as well as their lack of characteristics leading to dimensional stability under the conditions herein involved.

It is desirable that the synthetic resin fibers exhibit at least a moderate amount of initial shrinkage when a woven fabric made therefrom is subjected to heat in the presence of moisture as by immersion in hot water or exposure to wet steam. When hot Water is used any suitable additional swelling agents may be added if so desired as is customary in the shrinking of fabrics. This characteristic is exhibited by the nylon, Dacron fibers ident'fied hereinabove as otherwise suitable for use in the present invention.

The selected synthetic fiber or blend of such fibers may be carded and spun into yarn in accordance with the technique ordinarily employed in the carding and spinning of natural wool fibers. The yarns may be twisted and plied in any of the manners desired for particular due to differences in manufacture final results. The warp and weft yarns may be identical or may difier from one another in any of the respects already well known in connection with the weaving of papermakers felts from natural wool.

The type of weave may be selected at will and excellent results have been had with plain weaving as well as with twills, etc., commonly used for producing papermakers felts of various grades and for various intended uses. In view of the fact that only a moderate initial shrinkage can be expected and the fabric cannot be fulled, the ordinary technique of weaving the fabric relatively loosely and shrinking and fulling it will not produce a fabric having the necessary final qualities. I have found that desired final construction may be achieved by supplying a proportionately greater number of warp ends on the one hand and so operating the loom on the other hand as to bring about a proportionately higher pick count to compensate for the lack of fulling quality. Thus the take up motion and the motion of the lay should be so adjusted that successive picks are beat up with relatively greater force than would be applied in the manufacture of a substantially identical final product from fiber including sufficient natural wool to render the same fullable.

Preferably the length of the synthetic papermakers felt of the present invention is maintained and the effect of such shrinkage as the synthetic fibers may produce is chiefly confined to a reduction in width of the finished felt. Thus, in the case of a felt which is to be spliced, the woven length is the warp length and it is this dimension which is maintained so that shrinkage occurs only along the filling. When the felt is woven endless the filling length becomes the finished length and in that case shrinkage will be chiefly confined to the warp yarns while the filling threads are maintained substantially at their original length. With these considerations in mind, it will be apparent to those skilled in the art that in accordance with the present invention appropriate variations in the weight of yarn, count of warp ends and picks and in the degree of beating up may be resorted to to produce finished felts of widely varying specifications. Specific examples of endless and spliced felts will be set forth hereinbelow.

After the fabric has been woven from the selected synthetic resin fiberor blends thereof, it is shrunk by heating in the presence of moisture as by placing it in relaxed condition in water which is preferably heated to a temperature sufnciently to induce relatively rapid initial shrinkage. To the same end it may be exposed to an atmosphere of steam at or near normal atmospheric pressure. If it is desired to maintain the length of the felt substantially constant and to realize the major shrinkage in width, the felt is maintained under tension only in the direction of its length while permitting shrinkage in the cross-wise direction. A considerable amount of shrinkage can be achieved with any of the synthetic resin fibers set forth above as suitable for use in the present invention. While in some instances the fibers themselves 3 will exhibit a very small amount of shrinkage, the woven Orlon and I result of the twisting and fabric will shrink considerably more as a tensioning of the yarns during spinning, weaving.

After the initial shrinking the fabric is dried and during the drying operation the fabric is subjected to an elevated temperature for a period of time sufficient to not only remove any moisture remaining from the immersion or steaming thereof but also to induce a permanent set of the synthetic resin fibers in the positions and configurations in which they find themselves when the felt is held at the desired final dimensions. The drying apparatus may be essentially the same as that heretofore employed in the drying of natural woolen felts and usually comprises a heated drum and an idler roll spaced therefrom full length of the felt may be shrink'age thusefieeted many natural 'themanu-factureof a nylon wiI lbe maintained at about 220 be left in contact therewith long enough for evaporaf d 'While inthemanufaeture of e i I A felts it is customary -to rotate the drying drum continuously, and thus to pass'the feltcontinuously thrjoughan endlesspath, it hasbeen found preferable in the drying] and setting of the synthetic felts of the present invention torot'ate the drying drum in steps. Each increment of advance imparted to the felt in successive steps-is approximately equal to the length of felt which is ex-posed in direct contact witht e' heated drying drum. In this manner, each successive area of the felt may be maintained in contact-with the surface jof the heated drum for aperi'od of time sufficient teary the felt and s o'ftejn the-fibers somewhat so that they "will relax into the shapes'and positions assumed at that/time. The felt is then moved by rotating the drying drum to bring a new increment thereof "into contact with the drum. The inerementleaving the drum is thus given an opportunity to"'cpol"a nd "the fibers therein will assume a'set which "willbe retained solong as the felt is not subjected to a temperature a roaching the temperature at which the set" was established.

The drying oper'atio'hjust described-causes further shrinkingh'r the fabric in the filling direction. The plus the initial shrihkagedescribed abov wil he by thepartictilar fibers from which the fabric is woven. rot-example, =an-y'lon fiber which will shrink only about 2 whenfrelaxed'inhot water maybe used for the manuf'a'c 'ture of a papermakers felt as described herein and the fi'n'ished *fabr'ic will have-shrunk by asm-uch as from to=40% iritht-z direction of t efiller yarns.

The temperatures at which drying andsettin'g occur 'may be varied at will in accordance with the particular fiber 'ojrblend of fiber used and in accordance with any ticul'ah'con'tirtiorfs to which a felt maybe subjected in further" processing or in ultimate use. For example,

1 W001 felts are impregnated with a therino setting resin andheated to set such resin during the manufacturing operatidns. In some instances the thermosetting resin requires heat treatment'at "about 300 F. to When "such materials are' incorporated into a *feltm'ad'e in accordance with the present invention and jsu'eh higher temperatures are appliedthe synthetic resin fibers will'tak'e the permanent set, herein described, at the highertemperature;

-A lso, whether or not the :feltis impregnated, it may be "desired to have the felt withstand some particular tern- 'p'erature to which it will be subjected in use. In such ieven'tthe felt is set "at a temperature somewhat above that particular temperature. Ordinarily a paperrnakers wet felt may be set at or slightly above the boiling temjrerature'of wat'er and itflwill maintain its-desirable characjterrst cs throughout it's life in the usual papermaking "b2 fio S h St p" at div e t s lto the p e ent n enayfbeav'ailed of to set the fibersas described. Th drying drum described above is normally heated by is am, While'the fibers may be dried and set at a temperature as'low as about 150 F., for practical purposes felt which will be subjected as ordinary papermaking conditions, the drum preferably -F. and each increment tion of all of the water and for the dried felt to assume approximately the temperature of the 'drum. The fibers will thus be set at about 220 F. and the finished felt will be entirely suitable for normal papermaking uses wherein it will not be subjected to temperatures in excess of about 2-12" F.

If the feltis to be impregnated with a thermosetting resin or if it is desired that itwithstand higher temperaturesit may; be passed through further heating devices {maintained at suitable. temperatures. In the case of was. he .maaia am emp ature sho ld not s zy x greatly in excess of the shrinkage exhibited *w'o'ol felts has been exceedingly short.

ceed about 350" F. if all of theoriginal characteristics of the fiber are'to be preserved.

The selection of the drying and setting temperature "for synthetic resin fibers other than nylon may be governed by the same considerations. The temperature must be at least highenough to set the particular fiber or blendbf fiber and it should be sufiiciently high to condition the :felt'f'or the use which it will encounter. The maximum temperature should not exceed that at which'the original desired characteristics of the fiber or blend might belost.

Felts made wholly from synthetic resin fibers in 'ac- 'cord'ance with *the present invention have successfully de'mbnstrated their utility on paper machines ofyarious types in connection with *the production of widely differing grades-o'f-paper-as Well as in'the production-of other related products such as those-made from asbestos cement aha 'wallboard's-and thelike made "from ditferent types of furnish. In some 0f these instances theffurnish contains materials-ofhigl-ily abrasive natureand the life of natural The felts of the pr'esentinvention not only have adequately demonstrated the greatly increased tensile strength and life whieh might be expected as a "result of the substitution ofsyntheti'cresin 'fibers having greate'r tensile strength and abrasion resistance than natural wool, but also exhibit desirable 'grea'ter porosity and have been completely satisfactory in dimensional stability and ease of cleaning and maintenance.

Throughout the specification, emphasis has been laid upon thefact that in accordance with the teachings'h'ereof, it is possible Eto make papermakers wet felts by 'weav- *in'g afabric from yarns consisting wholly of a particular synthetic resin fiber or a blend of two or more such fibers. Obviously minor amounts o-f-otherfibe'rs, including natural wool, -m'ight be blended therewith without departing from the essentially novel features of :this inve'nti'on. The felts heretofore made from blends ref natural and synthetic fibers have always contained sufiicient natural wool fibe'rsto render the fabric capable of being ful'led whereas the felts of the present invention c'onta'in no in'atural wool fibers or, 'at most, insufiicient quantity thereof to ma'kethe fabric fullable. Thelprocess :of the present invention provides for the-production 0f pap'e'rmakers" felts from fibers whichare essentially incapable of 'felting and the product of the .present invea tion is a pap'ermakers felt made in the :form ofa woven fabric which is essentially incapable of fulling.

Specific examples of papermakers felts and of the :process for producing the same will now be given and, for purposes of illustration,- there also will be ig-iven-a description of theip'rocess and product resulting from the manufacture "of a natural wool felt having substantially the same final specifications.

Example L A papermakers felt was :made from fiber comprising crimped nylon staple fiber havinga staple length of 3 :inches and a .fiber'diameter lying within the range of fiber diameters encountereduin natural wool. Such fibers have a dry tensile strength of about 58,000 pounds :per square inchand a wet tensile strength of about 56,000 pounds per square inch. Each fiberhatl been sharply and permanently crimped throughout its length to the extent that when straightened out under tension each fiberis approximately four inches in length. The fiber was car ed, spun and twisted intoy'arn in accordance with ordinary. technique applicable to natural wool fiber. Yarn thus made weighing 700 *grains per 100 yards, in 'the grease, was used for both warp and fill. -A reed having 7 dents per inch was used with two Warp ends per dent. The takeup'mechanism and lay were adjusted to supply "1'6 picks per :inch in a conventional 4-harness satin weave. Under these conditions wherein there are 14 warp ends per inch and 1 6-picl s per inch of relatively heavy yarn, it was essential that each pick be 'beaten up with suificient force topabk thje .1'6.picl sinto alineal inch. t it a t i i This felt was woven as a piece and after the requisite length was woven it was spliced to form an endless loop.

.The fabric was then placed under longitudinal tension but without crosswise tension in a bath of water maintained at a temperature of 180 F. for about 20 minutes in order to permit the fabric to take an initial shrinkage in the filler direction. The felt was then dried by passing it around a rotary drier drum and an appropriately spaced idler roll. The distance between the idler roll and the drier drum was established so as to place the felt under tension and bring it back to its original woven length. The drier drum was heated to a temperature of 220 F. and was held against rotation for approximately minutes during which the water was evaporated from that portion of the felt in direct contact with the surface of the drier drum. After the water was evaporated the felt achieved a maximum temperature of about 220 F., the temperature of the drum. At the latter temperature the crimped nylon fibers were relaxed into the shape and position which they assumed in the woven fabric and they become set in such relationship upon cooling of the fabric to room temperature. During the drying and setting operation the fabric shrunk in width and the ratio of woven width to the finished width was 140%.

The drier drum was rotated stepwise to move successive lengths of felt into and out of direct contact with the surface thereof for drying and setting as aforesaid until the entire length of felt has been dried and set.

The felt may be then napped and singed or otherwise processed in accordance with known procedures which will vary with regard to the particular use for which the felt is intended.

In its finished form the felt of this example has a weft count of 16 per inch and a warp count of 19.6 per inch. The increase in warp count is the result of the shrinkage in the filler direction, the ratio of woven width to finished width being, as stated above, 140%.

For purposes of comparison, a papermakers felt was woven from 100% natural wool fiber wherein the warp yarns weigh 700 grains per 100 yards in the grease and the filling yarns weigh 500 grains per 100 yards. The reed was provided with 5 dents per inch with two warp ends per dent. The take-up mechanism and the lay were so adjusted as to supply 16 picks per inch but it should be noted that since the filling yarns have less weight than those used in the manufacture of the nylon felt of the present example, the successive picks need not be so firmly beaten up. The weave was the same conventional 4-harness satin. The fabric was then spliced after which it was fulled in accordance with known procedure and placed upon the drier with the tension roll so positioned as to tension the felt at its original woven length. The drier drum was heated to 220 F. and rotated continu- 011313 to progressively dry the felt as is usual with the manufacture of felts from natural wool. The ratio of the woven width to the finished width was 200% and the finished felt bad a warp count of 20 and a pick count of 16. The warp yarns, of course, retained their original 700 grain weight and thus were completely comparable in weight with the warp yarns in the finished nylon felt of the present example. The filling yarns of the wool .felt weighed 500 grains when woven but after the reduction in width due to shrinkage and fulling, the tiller yarns are equivalent to 1000 grain yarn and thus comparable with the final 960 grain weight of the nylon yarn used as fillers for the nylon felt. It is apparent, therefore, that the two finished felts are very closely comparable in finished construction.

The two felts of the present example were then subiected to identical abrasive tests on a special testing machine having d 'ven press, tension and elongation rolls, showers, dragxbars, speed indicators and other instruments to simulate a press section of the common paper machine. This testing machine is particularly constructed to provide a means of duplicating, with reasonable accuracy, the widely varying pressures, tensions, temperatures, chemical conditions and abrasive conditions which exist in paper machines of various types. The tests applied to the wool and nylon felts of the present example are known to be identical within an accuracy of plus or minus 2% as the result of many years of experience with this particular testing machine.

The tensile strength of the warp and filling yarns was tested at the beginning of the test and at frequent intervals during the progress thereof and the decrease in tensile strength was plotted against time. As might be expected, the nylon yarns exhibited tensile strength much higher than that of wool at the beginning of the test. As the test proceeded, the nylon warp yarns showed a rate of wear about the same as that for the wool warp yarns but in view of their greater original strength the nylon warp yarns exhibited after 48 hours more than twice the tensile strength of the wool warp yarns after about 36 hours. Warp yarns are relatively straight and protected by the filler yarns so that a felt seldom fails as a result of wear on the warp yarns. The closely packed filler yarns present knuckles on the surfaces of the felt and are most subject to wear from abrasion. The nylon filling yarns had considerably greater tensile strength than the wool filling yarns at the beginning of the test. as might be expected, but the rate of wear upon the nylon was somewhat less than that upon the wool so that at the end of 48 hours the nylon filling yarns exhibited greater tensile strength than that exhibited by the wool filling yarns after only 14 hours. This test indicated that the nylon felt could be expected to last almost four times as long as the wool felt insofar as abrasive wear is concerned. However, the life of a felt is not solely dependent upon its resistance to abrasion and the very much greater tensile strength of the nylon felt makes it less vulnerable to damage from mishaps of various sorts. Therefore. the superiority of the nylon felt over the woolen one under actual conditions of use has been found to be even greater than that indicated in the abrasion tests.

Example 2.A papermakers felt was woven in endless form from yarns having a weight of 700 grains per yards, in the grease, for both warp and fill, the yarns being made from 100% crimped nylon fibers identical with those described above in Example 1. The reed was provided with 8 dents per inch with 4 warp ends per dent and one warp end per eye on 8 harnesses. Since the felt was being woven endless the warp count in top and bottom was therefore 16 per inch. The take-up mechanism and lay were adjusted to supply 11 picks per inch top and bottom and thhe weave was a plain endless weave.

In this felt the woven length, i. e., the length warp-wise, becomes the finished width and the woven width becomes the finished length. The woven fabric was shrunk in water at F. for about 20 minutes, with the filler yarns maintained under tension so as to retain the original woven width which was destined to become the finished length. After shrinking the felt was dried and set in stepwise manner as described in Example 1. The ratio of the woven length to the finished width was 116%. The amount of shrinkage was considerably less than that encountered in Example 1 since, in the present example, the relatively straight warp yarns were permitted to shrink while in Example 1 the filler yarns were permitted to shrink. The finished warp count remains at 16 per inch while the finished filler count is increased as a result of shrinkage to 12.75 per inch.

For purposes of comparison, a similar endless felt was woven from yarns comprising 100% natural wool fiber, the yarn weight being 640 grains per 100 yards, in the grease, for the filler yarns supplied to the loom and 350 grains per 100 yards, in the grease, for the warp yarns supplied to the loom. The reed was provided with 7 dents per inch with 4 warp ends per dent and one warp end per eye in 8 harnesses. The warp count, as woven,

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was therefore 14 per inch. The take-up motion and the lay were so adjusted as to supply 6 picksjper inch top'and bottom. After weaving, this woolen. felt was shrunk and fulled and thereafter dried in accordance with known technique and the ratio of woven width to finished length was 110% and the ratio of woven length to finishedwidth was 200%. The finished warp count was 15.5 per inch and the finished filler count was 12 per inch. Since the warp yarns were reduced to half their woven length by "shrinkage and fulling the finished warp yarn weight was increased to 700 grains; thus being comparable to the finished nylon warp yarn Weight of the present example. The filler yarns remained at 640 grains and thus are comparable to the 700 grain nylon filler yarns of the present example.

Tests similar to those described in Example 1 indicated that the nylon felt of the present example should last at least four times as long as the wool felt of this example.

Example 3.-Papermakers felts identical in construction with those of Examples 1 and 2 were made with blends of crimped nylon fibers and fibers sold under the tradename saran," the latter having a fiber diameter lying within the range of diameters encountered in natural wool. The proportion of saran fibers to nylon fibers may be varied considerably without noticeable elfect upon the durability of the finished felt. Specifically, a blend containing 90% nylon staple fibers and saran staple fibers produces papermakers felts having all of the desirable attributes of those of Examples 1 and 2 which in addition exhibit a desirable great tendency to pick up a freshly laid web of paper.

Example 4.Papermakers felts having the same desirable attributes of those in the preceding examples may be made in accordance with said examples except that the initial shrinkage is effected by exposure of the fabric to an atmosphere of wet steam instead of by immersion in hot water. Thus any of said felts may be steamed in any of the several ways which are customarily employed for shrinkage of fabrics. A suitable procedure is to conduct the fabric through a path wherein jets of steam are released into the atmosphere and directed onto the fabric whereby the fabric is moistened and heated to induce shrinkage comparable to that achieved by immersion in hot water.

Felts manufactured from synthetic resin staple fibers in accordance with the present invention exhibit considerably greater porosity than felts made from natural wool and of identical construction. For this reason and be cause the synthetic resin fibers herein availed of are usually much stronger than natural wool, it is not necessary that a particular natural wool felt be replaced by a synthetic fiber felt which is identical, pick for pick, end for end and weight for weight. Thus in some instances it may be possible to replace a particular natural wool felt with a synthetic fiber felt of heavier construction because of the relatively greater porosity of the synthetic fiber felt. Furthermore, the greater tensile strength of felts made from synthetic materials will make it possible to substitute a considerably lighter construction where maximum porosity is particularly desired. The synthetic felts of the present invention greatly expand the area within which felts may be designed and the value of this will be apparent when it is considered that in many instances the design of a natural wool felt must constitute a compromise between desired porosity and requisite tensile strength.

The present application is a continuation-in-part of my application Serial No. 313,367, filed October 6, 1952, now abandoned.

What is claimed is:

1. The method of making papermakers wet felts from yarn composed substantially entirely of thermoplastic synthetic resin fibers having crimps which are stable below a predetermined temperature and are capable of taking and retaining a set in addition to said crimps at temperature base are predetermined temperature a s ,ab'oye "the temperatures ordinarily encountered in using above the temperatures ordinarily encountered in using papermakers wet felt torset the fibers in said shrunken fabric in the positions and conformations existing during said drying and heating step while retaining substantially all of the original crimps therein.

2. The method of making papermakers wet felts from yarn composed substantially entirely of thermoplastic synthetic resin fibers having crimps which are stable below a predetermined temperature and are capable of taking and retaining a set in addition to said crimps at a temperature below said predetermined temperature and above the temperatures ordinarily encountered in using papermakers wet felt, comprising the steps of weaving a fabric tightly from said yarn, immersing said tightly woven fabric in hot water to shrink the same transversely of the felt, applying tension to said tightly woven fabric during said shrinking step to prevent shrinkage of the same longitudinally of the felt, drying and heating a relatively small portion of the shrunken fabric under longitudinal tension and at a temperature below said predetermined temperature and above the temperatures ordinarily encountered in using papermakers wet felt to set the fibers in said portion of the shrunken fabric in the positions and conformations existing during said drying and heating step while retaining substantially all of the original crimps therein, and periodically advancing said shrunken fabric under longitudinal tension to dry and heat the entire length thereof by increments.

3. The method of making papermakers wet felts from yarn composed substantially entirely of thermoplastic synthetic resin fibers having crimps which are stable below a predetermined temperature and are capable of taking and retaining a set in addition to said crimps at a temperature below said predetermined temperature and above the temperatures ordinarily encountered in using papermakers wet felt, comprising the steps of weaving a fabric tightly from said yarn, heating said tightly woven fabric in the presence of moisture to shrink the same transversely of the felt, applying tension to said tightly woven fabric during said shrinking step to prevent shrinkage of the same longitudinally of the felt, and drying and heating the shrunken fabric under longitudinal tension and at a temperature below said predetermined temperature and above the temperatures ordinarily encountered in using papermakers wet felt to set the fibers in said shrunken fabric in the positions and conformations existing during said drying and heating step while retaining substantially all of the original crimps therein.

4. The method of making papermakers wet felts from yarn composed substantially entirely of thermoplastic synthetic resin fibers having crimps which are stable below a predetermined temperature and are capable of taking and retaining a set in addition to said crimps at a temperature below said predetermined temperature and above the temperatures ordinarily encountered in using papermakers wet felt, comprising the steps of weaving a fabric tightly from said yarn, heating said tightly woven fabric in the presence of moisture to shrink the same transversely of the felt, applying tension to said tightly woven fabric during said shrinking step to prevent shrinkage of the same longitudinally of the felt, drying and heating a relatively small portion of the shrunken fabric under longitudinal tension and at a temperature below said predetermined temperature and above the temperatures ordinarily encountered in using papermakers wet felt to set the fibers in said portion of the shrunken fabric in the positions and conformations existing during said drying and heating step while retaining substantially all of the original crimps therein, and periodically advancing said shrunken fabric under longitudinal tension to dry and heat the entire length thereof by increments.

References Cited in the file of this patent UNITED STATES PATENTS 1,889,902 Moore Dec. 6, 1932 2,161,766 Rugeley June 6, 1939 2,197,896 Miles Apr. 23, 1940