Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H5/00—Special paper or cardboard not otherwise provided for
  • D21H5/12—Special paper or cardboard not otherwise provided for characterised by the use of special fibrous materials
  • D21H5/20—Special paper or cardboard not otherwise provided for characterised by the use of special fibrous materials of organic non-cellulosic fibres too short for spinning, with or without cellulose fibres
  • D21H5/205—Special paper or cardboard not otherwise provided for characterised by the use of special fibrous materials of organic non-cellulosic fibres too short for spinning, with or without cellulose fibres acrylic fibres
• • D—TEXTILES; PAPER
  • D04—BRAIDING; LACE-MAKING; KNITTING; TRIMMINGS; NON-WOVEN FABRICS
  • D04H—MAKING TEXTILE FABRICS, e.g. FROM FIBRES OR FILAMENTARY MATERIAL; FABRICS MADE BY SUCH PROCESSES OR APPARATUS, e.g. FELTS, NON-WOVEN FABRICS; COTTON-WOOL; WADDING ; NON-WOVEN FABRICS FROM STAPLE FIBRES, FILAMENTS OR YARNS, BONDED WITH AT LEAST ONE WEB-LIKE MATERIAL DURING THEIR CONSOLIDATION
  • D04H3/00—Non-woven fabrics formed wholly or mainly of yarns or like filamentary material of substantial length
  • D04H3/08—Non-woven fabrics formed wholly or mainly of yarns or like filamentary material of substantial length characterised by the method of strengthening or consolidating
  • D04H3/16—Non-woven fabrics formed wholly or mainly of yarns or like filamentary material of substantial length characterised by the method of strengthening or consolidating with bonds between thermoplastic filaments produced in association with filament formation, e.g. immediately following extrusion
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H13/00—Pulp or paper, comprising synthetic cellulose or non-cellulose fibres or web-forming material
  • D21H13/10—Organic non-cellulose fibres
  • D21H13/12—Organic non-cellulose fibres from macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds
  • D21H13/18—Polymers of unsaturated acids or derivatives thereof, e.g. polyacrylonitriles

Definitions

• the product is useful as a textile fabric in the prep aration of a variety of materials ranging from hard, boardy sheets to soft, fluffy fabrics to stretchable, tough fabrics.
• the invention relates to nonwoven fibrous products made from gel fibers, and to improved methods for manufacturing such products.
• a wet leaf is de- Patented Jan. 26, 1971 "ice posited on a screen or cylinder and must be stripped from the substrate, passed between dewatering squeeze rolls, and dried to produce the final paper.
• it In order to strip the wet leaf from the substrate, it must have enough selfsupporting strength to withstand the jump between the moving substrate and the squeeze rolls employed in the next processing step.
• Cellulosic fibers produce a self-supporting wet leaf in the manufacture of paper because the fibers, while in the original dispersion and before being deposited on the moving porous substrate, are subjected to a violent beating action which provides the fibers with a greater surface area, partly by reason of breaking and splitting the fibers into smaller pieces and branched struc tures.
• the beating action also softens the fiber and permits hydrogen bonding to occur between fibers.
• the beaten fibers produce a self-supporting wet leaf by the combination of various forces, such as surface tension between fibers, interlocking of fibrilous branches, and interfiber hydrogen bonding.
• Synthetic fibers are not as responsive to a beating action as cellulosic fibers and normally must be beaten much more strenuously in order to produce even a small portion of the breaking and splitting which is found in cellulosic fibers.
• any of the compacted and dried synthetic fibers such as nylon, polyesters, acrylics, etc.
• the wet leaf is so fragile that it cannot be stripped from the screen without causing it to fall apart.
• the prior art workers who fabricated non-woven materials from acrylonitrile fibers recognized the necessity of beating the fibers to cause fibrillation or of providing some other means of making a self-supporting wet leaf (see US. Pats. 3,047,455 and 3,047,456).
• thermosetting resins as binders
• these resins after being thermoset, generally leave the fabric stiff and boardy.
• thermoplastic resins or polymers have been suggested for binders, for example, in the form of a latex but, again these resins often leave the fabric stiffer than desirable, and the bonding between the fibers is frequently observed to be short-lived, subject to dis coloring with time, and not durable when subjected to many uses.
• a method is proposed in US.
• thermoplastic polymer fibers having highly acceptable Wet strength and excellent web properties without having to resort to a fibrillation or beating treatment and without having to apply binding agents to the web.
• gel fiber in an uncollapsed, never dried condition has sufficient amount of self-bonding properties to cause interfiber bonding between adjacent fibers in the routine preparation of the wet leaf to the extent that it can be stripped from a moving substrate and conducted without support to squeeze rolls or other processing equipment.
• the selfbonding property of the gel fiber is less pronounced and it may be desirable or necessary to subject the wet leaf to a combination of pressure and temperature to make the wet leaf self-supporting.
• This invention provides a process for preparing a nonwoven fibrous product by forming a web of self-bonding, nonfibrillated, gel fibers of a thermoplastic fiber-forming polymer and subjecting said web to a combination of pressure, temperature and time which is insufiicient to cause dry fibers of said polymer to fuse but is sufiicient to produce a web having a rewet tensile strength of at least about 0.034 lb./in./oz./sq. yd.
• This web of gel fibers may, if desired, be then subjected to a heated aqueous medium to relax the fibers in the fabric.
• the web may be treated further in accordance with this invention by employing any of several drying methods, or by impregnating the web with resinous solids before or after drying. The dried web may then be annealed by a final heat treatment.
• the products provided by this invention include a nonwoven fibrous sheet materials consisting essentially of gel fibers, whether in the form of continuous filaments or staple fiber, bonded to each other at points of interfiber contact.
• the gel fibers are composed of a polymer of acrylonitrile.
• the sheet may contain other types of fibers blended with the self-bonding gel fibers, and the sheet may be impregnated with resinous solids, e.g., from a latex.
• the appearance of the product may vary from a hard, boardy sheet to a soft flufiy fabric to a stretchable, tough fabric.
• the present invention thus provides an efficient, expedient, and economical means for preparing non-woven fibrous sheets from gel fibers. It make unnecessary the normal practice of first fibrillating the fibers prior to formation of the web. Thus, the considerable energy requirements of the fibrillation treatment are obviated. Furthermore, elimination of the fibrillation step minimizes the loss of material which occurs due to fibers breaking into smaller particles or fines that may be lost in the operation or might have to be removed as undesirables from the final product. No binders are required to achieve suitable and highly acceptable web strengths, however, these can be added to the product by subsequent treatments if desired. Inducement of fusion by employing solvents for the polymer is unnecessary.
• the present invention provides for the manufacture of non-woven fibrous sheets and other products from gel fibers having highly desirable properties which heretofore were believed unobtainable except by the employment of fibrillation or by the use of bond-forming or bond-inducing agents.
• the critically important component of the products and processes of this invention is the gel fiber, which is the fiber form of a thermoplastic polymeric material which is capable of forming a gel network structure.
• the fiber may be in the form of continuous filament or cut staple.
• the gel network structure is a combination of fibrillar polymeric material and liquid from the bath employed to form the fiber.
• This bath may be a coagulating bath which coagulates streams of polymer solution into fibers in the spinning process, or, alternatively, it may be a wash bath employed to replace an organic solvent with an aqueous liquid.
• the gel network structure is formed consisting of interconnecting capillary spaces filled with the liquid intimately intertwined with a fibrillar structure of polymeric material.
• Such structures are described from various technical viewpoints in an article by J. P. Craig, J. P. Knudsen, and V. F. Holland, entitled, Characterization of Acrylic Fiber Structure, Textile Research Journal, vol. 32, No. 6, pages 435-448 (June 1962). These authors state that in the case of acrylonitrile polymers the structure is of a fibril-void type with fibrils and voids of the order of 200 angstroms in diameter.
• the gel fiber If the gel fiber is subjected to drying conditions, it collapses from its original swollen condition to form a dry fiber having the same characteristics and properties as fibers spun by other techniques which never involve passing through a gel network structure. So long as the gel fiber is not completely collapsed, it retains its gel network structure and can be reswollen to its original condition. Once the gel fiber has been dried to a completely collapsed condition, however, it cannot be reswollen. Only the gel fiber which has never been dried to a completely collapsed condition has the self-bonding properties which are necessary to this invention.
• Gel fibers, as prepared, do not have fibrils branching out from the main portion of the fiber, but such fibrils can be produced by a beating action such as is used on wood pulp in the production of paper.
• the characteristic self-bonding property is present in both the fibrillated and nonfibrillated form of the gel fiber permitting either to be used in the process of this invention although it is preferred to use the nonfibrillated form and thereby elim inate the expense of beating the fiber.
• One of the most revealing tests for showing the improvement achieved by making non-woven fabrics by the process of this invention is the measurement of the tensile strength of the fabric at various stages of its preparation.
• the product is a wet leaf.
• this product is subjected to a combination of elevated pressure and temperature to remove more water and to produce interfiber bonding, the product is a pressed wet leaf. If the pressed wet leaf is then submerged in water or otherwise saturated with water it is a rewet wet leaf.
• wood pulp made into ordinary paper through these stages might typically have an initial wet leaf tensile strength of 0.10-0.12 lb./in./oz./sq. yd. (at a solids content of 30%), and a rewet wet leaf tensile strength which is unmeasurable because the paper is too weak to test.
• the initial wet leaf is too weak to test, the pressed wet leaf tensile strength is about 0.02 0.04, and the rewet wet leaf disintegrates into individual fibers when placed in water again.
• the initial wet leaf tensile strength is about 0.2-0.4
• the pressed wet leaf tensile strength about 1.2l.8
• the rewet wet leaf tensile strength about 0.8 1.2.
• a web is prepared from the gel fibers as the first step in the process of this invention.
• Any convenient method of distributing the gel fibers in the form of a web or sheet is permissible, e.g., random deposition by a scattering mechanism, distribution by a stream of air or water, deposition by filtering from a liquid, and the like.
• a preferred method for use with staple fiber is the papermaking technique of forming a wet leaf from a suspension of staple fibers in water, followed by couching and pressing.
• a dispersion is prepared of the gel fibers in a large quantity of water, e.g., 1 part by weight of gel fiber per 1,0005,000 parts water, in the presence or absence of an added dispersing agent.
• the dispersion is then drained through a screen or other porous substrate to produce a sheet-like mass of fibers. This sheet-like mass is the wet lea which, after further treatment, is the product of this invention.
• the preferred method of forming a web is alsofrom a suspension in water whereby the filaments may be laid substantially parallel, or alternatively, may be laid down in random direction in a tangled fashion. It is not critical to employ any single procedure since many are equally satisfactory.
• a stream of water into which continuous filaments are introduced is an excellent carrier for the filaments. When this stream of water is directed onto a porous substrate, the water drains away leaving the filaments on the substrate.
• the filaments may be laid down substantially parallel or in a random, tangled fashion. Other methods may also be employed.
• a tow of filaments may be deposited on a moving substrate in the form of a web of parallel filaments.
• a tow of filaments may be dropped into a pool of Water causing the filaments to separate more or less randomly depending on the angle of incidence at which the tow contacts the surface of the pool. Still other procedures will be apparent to those skilled in this art.
• the preparation of a web from substantially parallel continuous filament is employed when it is desired to prepare a web with a maximum of tensile strength in the machine direction, i.e., the length of the web, and tensile strength in the transverse direction is relatively unimportant. Binding tapes for packaging are typical of this type of use.
• the web prepared from continuous filament laid down in a random fashion results in one having an initial strength essentially the same as that of a web prepared from staple fiber.
• the web of continuous filament has a secondary strength which is the strength of the individual filaments which have been straightened out into parallel strands without any substantial interfiber bonding.
• the aqueous medium in the gel structure of the filament (i.e., that portion other than the polymer), have a surface tension greater than about dynes per centimeter. Lower surface tensions tend to interfere with lateral fiber bonding and an integral -web may not result. It is, therefore, desirable in order to obtain excellently laterally bonded filaments that the aquagel be free of solutions containing surfac- 6 tants, lubricants, wetting agents and the like textile finishes.
• the aqueous medium of the gel structure (including any extrinsic liquid associated therewith) consists essentially completely of water, which has a surface tension of about 74 dynes per centimeter at 25 C.
• the synthetic polymeric materials which are intended to be employed in the process of this invention are the thermoplastic fiber-forming materials such as polyacrylonitrile, polyamides, and polyesters.
• the preferred material is that which contains at least '80 weight percent of polymerized acrylonitrile. This includes, therefore, the homopolymer of acrylonitrile as well as copolymers, terpolymers, and interpolymers of acrylonitrile.
• Suitable comonomers which may be employed in amounts up to 20% by weight of the final product include allyl alcohol, vinyl acetate, acrylamide, methacrylamide, methylacrylate, vinyl pyridine, ethylene sulfonic acid, and its alkali metal salt; vinyl benzene sulfonic acid, and its salts; 2- sulfoethylmethacrylate, and its salts; vinyl lactams, such as vinyl caprolactam; and vinyl pyrrolidone, and mixtures thereof.
• the preferred gel fibers employed in this invention are acrylonitrile polymer gel fibers which are preferably produced by wet spinning a solution of the polymer into an aqueous coagulating bath, washing the solvent therefrom, and recovering the fiber in the form of a swollen gel containing from about 50 to about 500 weight percent Water based upon the dry weight of the polymeric fiber.
• solvents such as ethylene carbonate, dimethylformamide, dimethylsulfoxide, butyrolactone, and various salt solutions, such as aqueous solutions of calcium thiocyanate, sodium thiocyanate, lithium bromide, zinc chloride, and others well-known in the art. Many of such systems are disclosed in United States Letters Patent Nos. 2,140,921; 2,425,192; 2,648,593; 2,648,646; 2,648,648; 2,648,649; 2,790,700; and 2,949,435.
• the polymeric fibers After the polymeric fibers have been wet spun into an aqueous coagulating bath they are usually washed -with water or with an inert, aqueous solution to remove any residual polymer solvent from the freshly formed filaments.
• the product thus formed is a gel fiber (sometimes called an aquagel fiber).
• Acrylonitrile polymer gel fibers prepared in this way may contain up to about 6 parts by weight of water for each part by weight of dry polymer therein, although more frequently, the proportion is about 1 to 3 parts by weight of water per part of polymer.
• the fiber in this condition, or after it has been oriented by stretching, may be employed in the process of this invention.
• Another technique which may be employed for preparing acrylonitrile polymer gel fibers for use in the process of this invention is to dry spin the fiber, evaporate a portion of the solvent, and wash out the remainder of the solvent with water so as to form the aqueous gel network structure.
• the gel fibers which are employed in the process of this invention may be staple fiber of short or long lengths or continuous filaments to produce end products of different qualities and characteristics.
• the staple length is not more than about two inches, although this is not intended to be a limitation on the process and product of this invention.
• the remaining portion may or may not be fibrillated and may be any type of fiber or any substance, such as synthetic polymer fibers or cellulosic fibers used in the manu facture of paper.
• the degree of self-bonding for any given type of gel fiber is proportional to the combination of pressure, temperature, and time applied to the area of such fiber. These factors cooperate in the sense that the same tensile strength for a web of bonded fibers can be obtained by employing a combination of low pressure and high temperature or by employing a different combination of high pressure and low temperature. Time is a factor within certain limitations since longer times under a given set of conditions of pressure and temperature normally produce higher tensile strengths in the webs being treated. Experimental data indicate that a larger effect is produced by varying the temperature and a smaller effect by varying the pressure.
• Minimum temperature and pressure conditions are about 25 C. and p.s.i.
• a common means is to press the entire web uniformly with heated platens or rolls. This procedure produces interfiber bonding more or less uniformly over the entire web.
• Another means is to subject the web to a pattern of heated pressure points to produce interfiber bonding only at the points in the pattern, leaving the remaining fibers substantially unbonded.
• the bonded web is subsequently subjected to drying conditions which collapse and dry the gel fibers, the fibers tend to shrink and various useful and ornamental effects can be produced by using different types of bonding means and different types of drying means.
• the final dried fabric product may be fiuffy, boardy, textured, paper-like, leatherlike, or film-like.
• the individual fibers in the fabric may be straight, folded, kinked, or bulked.
• the patterned pressure means may be a pair of rolls, a pair of flat plates or a combination of rolls or plates with screens.
• the pattern is made by cooperating pressure points which are relatively small in diameter and are spaced from each other a distance which is not greater than about the length of the staple fiber in the wet leaf.
• the size and shape of the points is not critical, although a preferred size is in the order of A to -inch in maxi- 8 mum dimension. These points may be arranged in a square pattern or a diamond pattern or any other design which covers the desired area of the wet leaf.
• One suitable pressure means is a coarse, woven screen which. when used in combination with a continuous surface, produces intermittent pressure points at the location where the screen wires overlap each other.
• a pair of plates or rolls may be etched or machined to produce a knurled effect or other patterns of pressure points which can be applied to both sides of the fabric being treated.
• a preferred combination is to employ cooperating rolls which are machined to produce parallel ridges on each roll. If the ridges on one roll are at an angle to the ridges on the other roll, the effect produced on a fabric passing between the two rolls is a regular design of spaced points corresponding to the intersections of ridges.
• the wet leaf in the pressed, partially bonded condition is the preferred form of the web to be treated by impregnation with resin solids.
• the impregnation is preferably accomplished by dipping the web in a solution or dispersion off the resin solids, squeezing the material to remove excess liquid, and thereby producing a sheet having resin solids, desirably 550% by weight, incorporated therein.
• the resin may require fixation by evaporation of the liquid carrier, curing by the application of heat or a change in pH, or it may require coagulation, cross-linking,
• sheets prepared by either of these general procedures have been dried, it will be found that they have a combination of a high tensile strength, a high tear strength, along with the general characteristics of a good handle and drape.
• binders which may be employed to impregnate the non-woven fibrous product of this invention.
• These binders may be organic or inorganic, thermoplastic or thermosetting, or elastomeric or brittle.
• the binder is, however, preferably embodied in an aqueous carrier such as being dissolved in an aqueous solvent, being dispersed in an aqueous medium, or suspended in an aqueous emulsion or latex. Any chemical compound having the characteristics of a binder may be used if it is compatible with the acrylonitrile fibers employed in this invention.
• binder compositions which may be mentioned specifically are:
• butadiene/styrene (2) butadiene/acrylonitrile (3) carboxylated butadiene/acrylonitrile (4) blends of phenolic resins or other thermosetting resins with latices of butadiene/styrene or butadiene/acrylonitrile (5) polyvinyl esters (6) polyvinyl halides (7) copolymers of vinylidene chloride (8) chlorinated rubber (9) polyacrylates and polyalkylacrylates (10) polymers of acrylic acid, methacrylic acid, or
• water-dispersible resins such as:
• metal hydroxides e.g., aluminum, zinc, copper, etc. which are adhesive in the hydrated form and may be formed in situ by raising the pH of aqueous solutions of soluble salts of the metals.
• the amount of such solids employed to impregnate the gel fiber webs of this invention is normally from about 5- 50% by weight of the final dry sheet depending on what type of properties are desired.
• the impregnating composition one of the resinous latices listed under A above, particularly desirable ones because of the excellent results achived by employing them are the butadiene/acrylonitrile latices and the latices of polyacrylates, polymethacrylates, acrylic acid polymers, and others of the family of acrylic polymers.
• the aqueous composition containing the resin normally is one having 10-30% or higher by weight of resin solids and it is usually convenient to apply these resin solids to the non-woven fabric by dipping the fabric in the aqueous, resin-containing composition, whether this is a latex, dispersion, solution, or the like.
• Other methods of application which are operable although not preferable, in-
• the web of gel fibers with particles from a resin latex and then to coagulate the latex particles in situ before drying the impregnated sheet.
• This treatment frequently produces improved properties as compared to a product made without the coagulation step.
• the coagulation may be accomplished by any of many known methods, usually relating to a change in the pH of the system, a change in the ionic environment of the particles, or by applying heat.
• acrylonitrile polymer gel fibers are incompletely washed when produced from a polymer solution in which the solvent is an aqueous solution of zinc chloride, there will be enough zinc ions and chloride ions associated with the gel structure of the fibers to cause coagulation of the latex applied to the web.
• Another coagulation procedure is to alter the pH with hydrochloric acid or some other fugitive material which would disappear upon later drying of the impregnated fabric.
• the fibrous material must be in the form of a web of gel fibers, e.g., a wet loaf, which have never been completely irreversibly dried and collapsed. In this never-dried condition, the gel fiber is remarkably receptive to the treatment with an aqueous impregnating composition. It is not known why this is so, but completely different results are obtained when one treats a fabric made from these gel fibers, which are irreversibly dried and then rewet before being impregnated with an aqueous binder composition.
• the drying of the web is accomplished by driving off the free water and solvent in the fabric, and all water and other volatile materials which may be in the gel structure of the individual fibers making up the fabric.
• the drying must cause the gel to collapse irreversibly to a dry fiber which cannot thereafter be reformed into a gel by the addition of water.
• the actual drying may be accomplished by any suitable means, e.g., hot air ovens, infra red lamps, heated metal surfaces, etc., which will volatilize the water and other materials in a reasonably short period of time.
• temperatures of C. to 200 C. are sufiicient to dry a sheet of a few mils in thickness in about one minute of time. If subatmospheric pressure is employed, the temperature may be correspondingly lower and yet accomplish the same result.
• the final drying step may cause the fabric to shrink 20-40% in area and the shrunken fabric may have a tensile strength of less than 0.1 lb./in./oz./sq. yd. This reduction in tensile strength is principally due to the breaking of interfiber bonds by the shrinkage forces of the fibers.
• the final product When this pressed wet leaf was dried at 40 C. in an air oven, the final product had a tensile strength of about 3.5, and when it was dried at 150 C. in an air oven, the final product had a tensile strength of about 7.6. Thus, it is important in certain embodiments of this invention to press the web at elevated temperatures and pressures in order to obtain the highest tensile strengths in the final product.
• One of the simplest methods of drying the fabric is to subject the fabric to drying conditions while the fabric is in a completely relaxed condition. This means that the fabric is not restrained from movement in any direction nor are the fibers restrained except insofar as the interfiber self-bonding has restrained them. In other words, there is no external stress applied to the fabric nor to the fibers to prevent their movement during the drying operation.
• This drying is conveniently accomplished in a hot air oven which may be a continuous oven in the sense of having a moving screen upon which the fabric travels without restraint. It is to be understood that the weight of the fabric itself causes some minute stresses in the fabric but this is not regarded as an external force.
• Another method of drying is accomplished by passing the web of gel fibers, whether it has been prepressed to provide some interfiber bonding or not prepressed and therefore has substantially no interfiber self-bonding, between two parallel barriers which restrict the movement of the web in its thickness direction but not substantially in either of the other two directions.
• the barrier may be a tangible solid material such as a screen or a platen, or the equivalent.
• the barrier may be an energy barrier such as heat or heat-producing energy. The requirement of the barrier is that the normal curling, kinking movement of individual fibers or networks of fibers be so restrained that the fibers and the sheet do not expand or curl beyond the thickness barrier.
• a heat barrier produces this effect by causing the fiber or sheet to be heated much more rapidly near the barrier than at some greater distance from the barrier, causing the fibers and the sheet to move away from the heat to the zone midway between the two barriers.
• the tangible, solid barrier accomplishes the same effect by physical contact.
• the tangible solid is the heat source for the drying operation, thus providing both a heat barrier and a solid barrier.
• a preferred procedure which is convenient because of the ease of handling the material which is to be dried and because it accomplishes a certain smoothing effect, is to perform the drying between two heated surfaces which are parallel and spaced apart sufficiently for the web to pass therebetween without being placed under any substantial stresses.
• the individual fibers tend to shrink and to crimp, thus causing wrinkles to form in the fabric.
• the presence of the two spaced, heated surfaces prevents the wrinkle from growing into waves any 12 larger than the distance between the heated surfaces.
• a certain smoothing is produced, which causes the finished sheet to be essentially flat and capable of being conveniently stored in a roll.
• the space between the heated surfaces is made smaller, even to the extent of being less than the thickness of the web before drying. While these spaced heating plates may produce small localized stresses in the wet leaf as it is being dried, these stresses are essentially negligible with respect to the overall fabric.
• the spacing between the barriers may be from about 0.8 to 30 times the thickness of the web entering the drying zone. Preferably this range will be from about 0.9 to about 15 times the predried thickness of the web.
• the heating of the wet leaf first raises the temperature sufficiently to cause the free water to evaporate, and then drives off the water in the gel fiber. During the first portion of the heating, rapid shrinking of the fabric takes place and it is important to keep the fabric between the barriers during that time. This situation permits various combinations of equipment and process conditions. In a batch process the fabric might be taken from its form as a wet leaf to complete irreversible dryness of the fiber in a fixed barrier.
• the web In a continuous process, however, the web might pass through continuous or semicontinuous barriers for the rapid shrinkage period, and finally into a heated zone without barriers for the final bit of shrinkage. In most instances, the Web will experience its most rapid shrinkage within a few seconds of being subjected to the drying temperature.
• the fabric When the fabric dries, it causes the individual fibers to shrink, and, depending upon the degree to which the fibers are in a random, or oriented position, the shrinkage of all of the fibers produces varied results in the appearance and handle of the fabric. In some instances, the fabric will be fiuffy with the appearance and feel of a lamb fleece. In other instances, the fabric will have a laminar structure with the outside relatively stiff and the center relatively fluffy. Other effects can be produced by varying the process steps. In any event the individual fibers normally acquire a crimp by reason of the shrinkage forces. If compressive forces or tension forces were applied to the fabric during the drying step, the shrinkage would be substantially lessened. When such forces are employed in the process of this invention they are preferably in an amount necessary to restrain at least about 50% of the area shrinkage.
• Various textured effects can be produced in the fabric by a combination of pressing the web with certain combinations of pressure and temperature, followed by subjecting that web to the drying step of this invention. If the combination of pressure and temperature which is applied to the web leaf is very mild so as to produce very little increase in the wet tensile strength of the web, the interfiber bonding will be sparse, and the subsequent drying and the accompanying shrinkage will produce a material which is fluffy and has a low tensile strength. The texture of such a material will be quite similar to that of a light wool felt.
• the shrinkage forces will not be able to overcome as much of the interfiber bonding and the result will be a randomly mottled, leathery appearance and a relatively high tensile strength.
• the product has a laminar appearance with kinked, wavy fibers forming a flufiiness in the center and somewhat fiat, smoothed appearance on the outside.
• the drying temperature and the procedure for accomplishing the drying of the web are important in producing products of any desired tensile strength. In general, if the drying temperature is high, the tensile strength will be higher, and if the drying temperature is low, the tensile strength will be lower.
• a material is dried at-a low temperature, e.g., 40 C., it may later be heated to a high temperature, e.g., 150 C., to increase the tensile strength of the dried sheet. This occurs, presumably, because more interfiber bonding occurs during this period of heating the sheet to a high temperature.
• Still another method of drying is one in which the web is dried while under sufiicient restraint to prevent a substantial portion of the normal shrinkage from taking place.
• the web may be restrained by a compressive force applied to the face of the web or by a tensile force applied along two or more edges or the web.
• the restraining force may be applied so as to prevent any amount up to 100% of the shrinkage, it is usually desirable to apply sufiicient force to resist at least one-half of the gel fiber axial shrinkage forces.
• restraining half the axial shrinkage force shall mean restraining half the area shrinkage of a web of the gel fibers that would occur if the web were allowed to dry in a relaxed, free-toshrink condition.
• shrinkage forces will depend in large measure on the history of the filament, for instance, to an extent on the polymer composition of the gel, the amount of swelling or water in the gel structure, the degree of orientation given the fibers, and somewhat on the fiber denier.
• the drying temperature will also have an influence on these forces. Since the axial shrinkage forces depend on many variables, it will be appreciated that the exact range of restraining forces required to overcome the axial shrinkage forces under all conditions, cannot adequately be set forth. However, these forces can readily be determined for any given set of conditions so that an appropriate restraining force for resisting the desired portion of the shrinkage can be applied.
• the amount of restraining force applied to the drying web in the practice of the invention will be in the range of from about 5 to 75 or 100 p.s.i., although very useful products can be obtained at pressures in the range of 0.5-5 p.s.i. under regulated drying conditions.
• Restraining or resisting forces greater than the minimum amount to overcome the axial shrinkage forces of the fiber can, of course, be utilized in the drying cycle. Care should be exercised that the applied restraining force is not so great that the gel fibers become substantially crushed to an extent such that the porous network of the web is destroyed. Undue crushing and general lateral fiber to fiber fusion frequently reduce the tensile strength and porosity or permeability of the web.
• the restraining force is preferably applied throughout the drying cycle, i.e., maintained constantly once the drying of the gel fiber web is begun and imposed until the gel fibers are essentially completely irreversibly dried to a characteristically hydrophobic nature and the gel structure is collapsed or destroyed.
• the applied restraining force is compressive force it can be made great enough to prevent any shrinkage. If this type of force is not applied the axial shrinkage forces cannot be completely counteracted or restrained since the web is formed of the fibers lying in random directions. For instance, if the necessary restraining forces are applied as tension forces to the web in the direction of web travel only, i.e., as might be accomplished by tensioning the web between tension rolls or tensioning over a large roll,
• the most convenient and effective means to achieve a compressive restraining force over the entire web surface in an omnidirectional, planar manner is to confine the web between two solid sheets of a suitable material, such as by confining it between two belts that are at least coextensive with, and can be adapted to move with the web during the irreversible drying thereof.
• a suitable material such as by confining it between two belts that are at least coextensive with, and can be adapted to move with the web during the irreversible drying thereof.
• at least one of these belts, or similar restraining surface is foraminous in nature to allow for the escape of water vapor or other volatile material during the drying of the web and collapse of the gel fibers.
• drying web can, of course, be utilized to apply this coextensive compressive force to the drying web, such as, for erample, by passing the web confined on one side with a screen or similar foraminous structure, while the bottom side is a stationary solid surface, or when the bottom side is a rotating roll, which can also be heated.
• the web is dried while being confined by one of the above indicated means or equivalents thereof while passing through a forced hot air oven.
• the gel fibers are first collected in a generally parallel and side-by-side manner, such as a tow of fibers might be handled.
• a ribbon or tow several filaments thick is utilized to prepare a flat ribbon-like overall structure so as to facilitate uniform temperature and restraint throughout the plurality of filaments during the irreversible drying cycle. It is important that the gel filaments be combined before drying in a way to assure predominate lateral fiber-to-fiber contact with a certain amount of random array so that adequate fusion points develop between the filaments effecting an integral Web.
• the web of gel filaments After the web of gel filaments is collected as indicated, it may be dried using any conventional means suited to the purpose, such as by passing the web through a forced hot air oven or by passing it under infrared lamps or over heated metal drums, as long as the drying means utilized is adapted to impose on the drying web a resultant compressive force that acts upon the planar surfaces of the web and adequately restrains or resists the axial shrinking forces of the gel fibers released during the irreversible drying thereof. As indicated, the web during its drying cycle must be restrained to the extent of resisting at least up to one-half and preferably to the extent of essentially completely counteracting or resisting the fiber axial shrinkage forces released during the irreversible drying cycle.
• suflicient restraining force is apparently necessary in order to prevent differential motion between filaments and particularly at cross-over areas between filaments. Such differential motion apparently leads to debonding of the drying filaments or at least prevention of forming a bonded juncture.
• the minimum amount of restraining force applied to the drying ribbon in the practice of the invention in order to obtain an adequately bonded ribbon will be in the range of from about 2 to 6 p.s.i. when a direct exterior compressive restraining force is applied without accompanying axial restraint (tension) on the drying ribbon.
• the exterior compressive force minimum is generally in the range of about 0.1 to 0.6 p.s.i.
• the restraining force should be controlled throughout the drying cycle so as to govern the differential shrinkage during the critical bonding period. That is, the restraining force should be controlled constantly once the drying of the web of gel fiber is begun and until the gel fibers are essentially completely irreversibly dried to a characteristically hydrophobic nature and the gel network structures of essentially all fibers in the ribbon are collapsed.
• the control of the restraining force can be maintained by constantly applying the external compressive force or tension force throughout the drying; or, control can be accomplished, particularly when the web is dried around heated rolls, by intermittent application of the restraining force suitable to control the shrinkage or differential motion of the individual filaments.
• intermittent application of the restraining force may be present when the ribbon or web is dried by being passed continuously over a series of dryer cans or rolls and wrapped alternately clockwise and counterclockwise, while being restrained to about constant length.
• the most convenient and effective means to achieve a restraining force over the entire ribbon surface when an external compressive force is desired is to confine the ribbon between two solid sheets of a suitable material as has been previously described.
• the ribbon can, for example, be held between tensioning rolls while being passed through a hot air or radiant heated oven, or, as has been indicated, and preferably, the ribbon can be held under the requisite tension forces while being passed over and/ or around a rotating or stationary heated drum so that a resultant compressive force is effected.
• T glass transition temperature
• T glass transition temperature
• the time for holding the fiber at the annealing temperature may be from a few seconds to a few minutes. Thus, it may require only a few seconds if the annealing temperature is 50-100 C. above T while the time may be a few minutes for a lower annealing temperature. Since the sole purpose of this treatment is to relieve stresses, it is emphasized that annealing is not a necessary step in the process of this invention.
• the Wet gel fiber was found to contain 2.13 parts water per part polymer in the wet gel structure. Axial shrinkage on relaxed drying at 140 C. was 28%, which represents 48% area shrink in a wet leaf. The dried fiber had a tenacity of 3.8 grams per denier.
• Tensile strengths were measured on the wet leaf after pressing as described above, and also on each wet leaf which, after the pressing operation, was rewet by saturating it with water and then testing the tensile strength of the rewet sample. Test conditions and results are shown in Table I and are averages of many individual tests.
• EXAMPLE 2 Properties of dry webs prepared from gel fibers Webs were prepared from acrylonitrile homopolymer gel fibers as described in Example 1. In each instance the web was formed as a wet leaf which was then subjected to certain conditions of pressure, temperature, and time, followed by drying to cause the gel fibers to irreversibly collapse. The dried sheet was subjected to various tests to determine its properties, with the following results:
• the wet leaf was placed in one of two air ovens at 40 C. or at C. and completely dried at that temperature and the dried sample was then tested to determine its tensile strength.
• the values varied from as low as about 0.7 to as high as about 8.8, with the, higher values occurring at the higher temperatures and pressures and the lower values occurring at the lower temperatures and pressures.
• the shrinkage varied from about 49% to about 30%, as the Wet leaf pressing temperature varied from about 40 to about 85 0., although in some instances the shrinkage went through a maximum at a temperature of about 60 C.
• Non-woven sheets prepared as described above in this example exhibited densities from about 19.5 pounds per cubic foot to about 40 pounds per cubic foot, depending upon the conditions of the wet leaf pressing and the final drying.
• Non-woven sheets prepared as described above in this example exhibited porosities which varied from about 140 to 20 cu. ft./min./sq. ft. (measured on Gu'rley air permeometer) 18 EXAMPLE 3 Blends of gel fibers with non-gel fibers In this series of experiments, wet gel fibers of polyacrylonitrile were blended with other fibers which did not exhibit a gel network structure. The ability of the self-bonding gel fiber to act as a binder for other fibers which do not have self-bonding properties provides the possibility of producing non-woven materials of many different properties.
• Polyacrylonitrile wet gel fiber was prepared as described in Example 1 and cut to /z-inch staple.
• the non-gel fibers of the blends were a polyamide (66 nylon a polyester (polyethylene terephthalate), a polyolefin (polypropylene), and an irreversibly collapsed gel fiber of polyacrylonitrile.
• the properties of the various fibers is shown in the following tabulation.
• Blends were prepared of various mixtures of the gel fiber and one of the other fibers (also cut to /2-inch staple) and formed into webs on a laboratory handsheet mold.
• the aqueous dispersions from which the webs were formed contained 0.3% (based on the dry fiber weight) of an acrylamide polymer as a dispersing agent and drainage assistant.
• the webs were made to have basis weights of 2.5 dry oz./Wt. sq. yd.
• sample was pattern bonded, it was accomplished by passing the sample through patterned pressure rolls at 5 feet/ minute to produce a square pattern of -inch spots on /e-inch x Arinch centers.
• the pressed webs were suspended in a hot air oven and dried at 150 C. for 2 /2 minutes without restraining the webs in any way. The results are shown in the following tabulation.
• aqueous suspension of these wet gel fibers blended in a 1:1 ratio was prepared by dispersing the filaments at about 0.04% by weight of dry fiber in water containing 1% polyacrylamide (based on dry fiber weight) as a dispersing agent and drainage assistant.
• the wet leaf was then formed on a continuously moving screen and transferred at about by weight of polymer (2 /2 oz. wet gel fibers/sq. ft.).
• Wet leaf thus formed was pattern bonded with /Q, -inch spots on a rectangular pattern of At-inch x %-inch by passing the sheet through the nip of heated pattern bond rolls.
• Nip pressure was 10 pounds/ linear inch of nip and temperature was 85 l00 C.
• COPOLYMER SERIES 30 32. 1 10. O 1. 20 2. 8 4. 0 28 56 22 0. 3 0. 6 1. G 30 40. 0 12. 5 1. 09 2. 7 3. 2 20 51 33 0.6 3. 8 0. 5 15 42. 0 12. 5 0. 55 2. 7 5.0 23 63 14 0.3 1. 9 4. 0 9 42. 0 12. 5 0. 59 2. 3 5. 8 23 65 14 0. 2 0. 6 1. 8 31 32. 5 10. 0 1. 14 2. 9 4. 1 26 (i0 17. 5
• Blends were made of gel fibers of the last homopolymer and the last copolymer shown in Table V.
• the blends were made into webs by the papermaking procedure and each wet leaf was subjected to combinations of pressure, temperature, and time, and the pressed wet leaf tensile strength was measured. The results are shown in Table VI.
• wet gel fibers of acrylonitrile homopolymer were prepared, spun washed, and stretched as described in US. Pat. 2,790,700. These fibers were so sized to give 3 denier pattern bonded wet leaf was then dipped in an aqueous binder composition of Hycar 157la latex emulsion of butadiene/acrylonitrile.
• the solids in the aqueous latex system were varied from 10-40% to provide impregnated wet leaf samples with latex solid contents of 10-50%. To regulate the latex solids pickup in the sheets still further, they were sandwiched between blotter papers and passed through the nip of rubber squeeze rolls.
• high tenacity means tire cord quality of 11 denier/filament and approximately 89 grams/denier
• normal tenacity means textile quality of 6 denier/filament and approximately 5 grams/denier.
• V-inc staple Dry, 86% polyacrylom'trile, 3 denier/filament, 54-inch staple; 20% polyethylene terephthalate, high 8 13 1.7
• EXAMPLE 6 EXAMPLE 8 impregnation of webs made-of blends of fibers This example demonstrates the utility of other varieties of non-woven fibrous sheets in the preparation of impregnated fabrics.
• the same general procedure as described in Example 5 was employed on fibrous sheets made from polyacrylonitrile wet gel fibers of different lengths and properties, and on other sheets made from blends of different fibers.
• the results are shown in Table VIII in which the fibers employed in the non-Woven sheet are characterized by chemical composition, denier, and staple length.
• the polyacrylonitrile fiber was a wet gel fiber, except for the predried, high tenacity fibers of Samples 13 and 14 which were wet gel fibers that had been irreversibly collapsed and dried before being used in the preparation of a wet leaf.
• the impregnating material was the same as that employed in Example 5.
• Impregnation of webs with various types of resin solids Wet leafs were formed, couched, and pattern bonded employing wet gel fibers of acrylonitrile homopolymer as described in Example 5. The webs were then impregnated by placing them between ZO-mesh nylon screens and dipping the combination in one of several types of aqueous latices as described below. The web was then subjected to squeeze rolls to produce a web containing approximately 23% by weight resin solids and 77% by weight gel fibers on a dry basis. The impregnated webs were then dried by being placed for 5 minutes between two plates spaced 0.1- inch apart and maintained at 150 F.
• adipamide 11 denier/filament, l-inch staple. 11 50% polyacrylonitrile, denier/filament, Mg-inch staple; 50% polyhexamethylene 24 4.2 30 6,3
• edipamide 11 denier/filament, l-inch staple. 12 10% polyacrylonitrile, V denier/filament, %.)-inch staple; polyhexamethylene 13 3.2 17 4.8
• sheets were prepared as described above, with the modification that immediately after impregnation the impregnated web was dipped in a homopolymer) were cut to /2-inch staple length and blended with various amounts of non-gel fibers also cut to /2-inch staple length.
• the non-gel fibers were:
• gel fibers of acrylonitrile polymer were prepared by Wet spinning polyacrylonitrile from an aqueous zinc chloride solution containing about 60% by Weight zinc chloride into a coagulation bath (an aqueous zinc chloride solution containing about 43% zinc chloride) at about 12.5 C., washing the fibers substantially free of zinc chloride, and orienting them by stretching the gel filaments about 12 times their original Tensile Impreg- Porosity, Tear Tensile Elongaenergy Sample nating it. /min./ strength, strength, tion, absorbed, 0. latex sq. ft. lb. 1b./in. percent; ft. lb./lb.
• the wet gel fiber was cut to 4-inch staple, made into a wet leaf, felt-pressed to 28.5% solids, and then subjected to a drying operation to cause irreversible collapsing of the fibers.
• sample 1 there was no restraint whatsoever on the drying fabric, while in samples 2 and 3, the spaced barriers of this invention. were employed in the form of heated flat bed press platens, the temperatures of which were about 180 C.
• samples 4, 5, and 6 the fiber was Ai-inch staple, was felt-pressed to 28.5 solids, and then calendered at 220 lbs/linear inch at 25 C.
• the gel fiber was formed into a wet leaf and treated as described above with respect to samples the roll was determined to be about 0.8 p.s.i., and about 0.4 p.s.i. average shrinkage generated resultant force between fiber layers, the ribbon being about 5 filament layers thick.
• the irreversibly dried ribbon was observed to be a strong, coherent, self-bonded structure.
• the individual filaments could not be separated from one another when the ribbon was flexed and pulled.
• the filaments remained in a coherent, continuous ribbon.
• a microscopic examination of a cross-section of the ribbon showed that the filaments were excellently self-bonded to one or more adjacent filaments.
• Ribbons 4%. inches wide and 1 inch wide were made according to the foregoing, with similar excellent results.
• EXAMPLE 13 Sample 5Pattern bonded wet leaf was dried without The procedure of Example 12 was repeated, excepting Festramt on Sheet, m a clrclll'fmng a1 to utilize gel filaments of varying degrees of swelling, lmPregnated by dlppmg resm and to vary the surface tension of the gel fluid, the re- Solids Hycar squeezed and redned same straining force applied to the ribbon during drying, and 5 manner as beforethe drying temperature. Drying was accomplished on a Sample samPle 5 except that Pa bonded heated drum or hot air oven. These results are set forth Wet leaf was bolled In Water for 10 minutes before in Table XV. drying, impregnating, and redrying.
• 102 74 (C) do l Copolymer of about 96% acryloriitrile.
• EXAMPLE 14 Hot Wet treatment of wet leaf Staple gel fiber (%-inch length) prepared substantially as described in Example 1 was formed into a wet leaf The results are shown in Table XVI.
• the hot wet treatment provided by boiling the Wet leaf causes a shrinkage to take place before the drying step and, because of the relaxation of the fibers, produces a stronger sheet per unit of fabric weight (tensile strength x fabric weight).
• said gel fibers are staple gel fibers and said combination of pressure, temperature of at least 25 C., and time, is applied to said Web in the form of a pattern of spaced, heated pressure points spaced apart not greater than the staple length of said fibers, and wherein said drying conditions are imposed while maintaining said web substantially free of externally applied stresses other than those inherent in said pattern of spaced, heated pressure points.

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• 1967
  • 1967-02-10 NL NL6702029A patent/NL6702029A/xx unknown
  • 1967-02-22 DE DE19671696185 patent/DE1696185A1/de active Pending
  • 1967-02-23 FR FR96135A patent/FR1514481A/fr not_active Expired
  • 1967-02-24 GB GB47025/69A patent/GB1188325A/en not_active Expired
• 1968
  • 1968-07-25 US US747455A patent/US3558429A/en not_active Expired - Lifetime

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