Nonwoven fabric and method of producing same

Description Submit all comments and votes

Other objects and advantages of the invention will be apparent from the following detailed description and the accompanying drawings, in which:

FIG. 1 is a partially schematic side elevation, partially in section, of a method and apparatus for producing nonwoven fabrics in accordance with the present invention;

FIG. 2 is a perspective view of a fragment of a nonwoven fabric produced by the method and apparatus of FIG. 1;

FIG. 3 is a perspective view of the fragment of nonwoven fabric shown in FIG. 2 after being subjected to an embossing operation;

FIG. 4 is a section taken along line 4--4 in FIG. 3;

FIG. 5 is a perspective view of a fragment of a nonwoven fabric produced by the method and apparatus of FIG. 1 using a different embossing pattern;

FIGS. 6-8 are scanning electron microscope photographs, at different magnification levels, of an exemplary material embodying the invention;

FIGS. 9-11 are scanning electron microscope photographs of a second exemplary material embodying the invention, FIGS. 9 and 10 showing unembossed areas of the material and FIG. 11 showing an embossed area; and

FIGS. 12-15 are graphs illustrating the data collected in certain of the examples described in the application.

While the invention will be described in connection with certain preferred embodiments, it is to be understood that the invention is not to be limited to those embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as can be included within the spirit and scope of the invention as defined in the appended claims.

Turning now to the drawings and referring first to FIG. 1, a primary gas stream 10 containing discontinuous polymeric microfibers is formed by a known melt-blowing technique, such as the one described in an article entitled "Superfine Thermoplastic Fibers," appearing in Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346, which describes work done at the Naval Research Laboratories in Washington, D.C. Also, see Naval Research Laboratory Report 111437, dated Apr. 15, 1954and U.S. Pat. No. 3,676,242, issued July 11, 1972, to Prentice. Basically, the method of formation involves extruding a molten polymeric material through a die head 11 into fine streams and attenuating the streams by converging flows of high velocity, heated gas (usually air) supplied from nozzles 12 and 13 to break the polymer streams into discontinuous microfibers of small diameter. The die head preferably includes at least one straight row of extrusion apertures. In general, the resulting microfibers have an average fiber diameter of up to only about 10 microns with very few, if any, of the microfibers exceeding 10 microns in diameter. The average diameter of the microfibers is usually greater than about 1 micron, and is preferably within the range of about 2-6 microns, averaging about 5 microns. While the microfibers are predominately discontinuous, they generally have a length exceeding that normally associated with staple fibers.

In accordance with an important aspect of one particular embodiment of the present invention, the primary gas stream 10 is merged with a secondary gas stream containing individualized wood pulp fibers so as to integrate the two different fibrous materials in a single step. The individualized wood pulp fibers typically have a length of about 0.5 to 10 millimeters and a length-to-maximum width ratio of about 10/1 to 400/1. A typical cross-section has an irregular width of 30 microns and a thickness of 5 microns. Thus, in the illustrative arrangement a secondary gas stream 14 is formed by pulp sheet divellicating apparatus of the type described and claimed in the assignee's Appel U.S. Pat. No. 3,793,678, entitled "Pulp Picking Apparatus with Improved Fiber Forming Duct." This apparatus comprises a conventional picker roll 20 having picking teeth for divellicating pulp sheets 21 into individual fibers. The pulp sheets 21 are fed radially, i.e., along a picker roll radius, to the picker roll 20 by means of rolls 22. As the teeth on the picker roll 20 divellicate the pulp sheets 21 into individual fibers, the resulting separated fibers are conveyed downwardly toward the primary air stream through a forming nozzle or duct 23. A housing 24 encloses the picker roll 20 and provides a passage 25 between the housing 24 and the picker roll surface. Process air is supplied to the picker roll in the passage 25 via duct 26 in sufficient quantity to serve as a medium for conveying the fibers through the forming duct 23 at a velocity approaching that of the picker teeth. The air may be supplied by any conventional means as, for example, a blower.

It has been found that, in order to avoid fiber floccing, the individual fibers should be conveyed through the duct 23 at substantially the same velocity at which they leave the picker teeth after separation from the pulp sheets 21, i.e., the fibers should maintain their velocity in both magnitude and direction from the point where they leave the picker teeth. More particularly, the velocity of the fibers separated from the pulp sheets 21 preferably does not change by more than about 20% in the duct 23. This is in contrast with other forming apparatus in which, due to flow separation, fibers do not travel in an ordered manner from the picker and, consequently, fiber velocities change as much as 100% or more during conveyance.

In order to maintain the desired fiber velocity, the duct 23 is positioned such that its longitudinal axis is substantially parallel to the plane which is tangent to the picker roll 20 at the point at which the fibers leave the influence of the picker teeth. With this orientation of the duct 23, fiber velocity is not changed by impingement of fibers on the duct walls. Thus, where the pulp sheets 21 are radially fed to the picker in a plane which is substantially parallel to the primary air stream, the plane which is tangent to the picker roll 20 at the point of contact with the pulp sheets is perpendicular to the primary air stream. Accordingly, since for the schematic embodiment illustrated in FIG. 1 the point of picker contact with the sheets is also the point at which the separated fibers leave the influence of the picker teeth, the longitudinal axis of the duct 23 is normal to the primary air stream 10. However, if after separation from the pulp sheets 21 the fibers are constrained to remain under the influence of the picker teeth, then the axis of the duct 23 is appropriately adjusted so as to be in the direction of fiber velocity at that point where constraint is no longer present.

As shown in FIG. 1, the width of the duct is approximately equal to the height of the picker teeth on the roll 20, the passage between the picker teeth and the picker roll housing 24 being very small. With such a duct width, the velocity of the process air supplied through the process air duct 26 remains substantially constant in its travel with the picker and thence through the duct 23. Furthermore, because the velocity of the process air approaches that of the picker teeth, which in turn is about the same as the velocity of the separated fibers, the process air causes no substantial variations in fiber velocity in the duct 23. With duct widths approximately equal to the height of the picker teeth, e.g., no more than about 1.5 times the tooth height, air velocities in the forming duct 23 of at least 70% of the picker tooth velocity are useful in the illustrated apparatus.

Duct length and transverse width, i.e., the width in a direction along the picker roll axis, are also important in order to achieve an optimum web. Preferably, the duct length should be as short as the overall equipment design will allow. For the apparatus schematically illustrated in FIG. 1, the shortest duct length is limited by the radius of the picker roll. In order to achieve a high degree of cross-width uniformity in the resultant web, the transverse duct width preferably should not exceed the width of the pulp sheets fed to the picker roll. Again referring to the apparatus illustrated in FIG. 1, it is preferred that picker teeth with relatively large heights, e.g., greater than 1/4 inch, be used. Such heights permit the use of wider ducts which, in turn, minimize the interaction of fibers with the duct walls.

As illustrated in FIG. 1, the primary and secondary gas streams 10 and 14 are preferably moving perpendicular to each other at their point of merger, although other merging angles may be employed if desired. The velocity of the secondary stream 14 is substantially lower than that of the primary stream 10 so that the integrated stream 15 resulting from the merger continues to flow in the same direction as the primary stream 10. Indeed, the merger of the two streams is somewhat like an aspirating effect whereby the fibers in the secondary stream 14 are drawn into the primary stream 10 as it passes the outlet of the duct 23. In any event, it is important that the velocity difference between the two gas streams be such that the secondary stream is integrated with the primary stream in a turbulent manner, so that the fibers in the secondary stream become thoroughly mixed with the melt-blown microfibers in the primary stream. In general, increasing velocity differences between the primary and secondary streams produce more homogeneous integration of the two materials, while lower velocities and smaller velocity differences would be expected to produce concentration gradients of components in the composite material. For maximum production rates, it is generally preferred that the primary air stream have an initial sonic velocity (within the nozzles 12 and 13) and that the secondary air stream have a subsonic velocity. Of course, as the primary air stream exits from the nozzles 12 and 13, it immediately expands with a resulting decrease in velocity.

The capacity of the air stream which attenuates the polymeric microfibers and entrains surrounding air is always larger than the volume of air used to introduce the pulp fibers. The primary air jet typically increases in volume flow more than five fold before the maximum jet velocity has decreased to 20% of its initial value. However, the pulp fibers should be introduced early in the zone of diffusion of the microfiber jet in order to expose the fiber mixture to the intense small-scale turbulence in this area of the diffusion zone, and to mix the fibers while the polymeric microfibers are in a soft nascent condition at an elevated temperature. In the later stages of diffusion of the microfiber jet, the scale of turbulence becomes large compared to the fiber entanglements, and the energy in turbulence is continuously decreasing. The combination of a high-intensity and small-scale turbulence field provides maximum mechanical containment of the small pulp fibers within the matrix of microfibers.

Deceleration of the high-velocity gas stream carrying the microfibers frees the microfibers from the drawing forces which initially form them from the polymer mass. As the microfibers relax they are better able to follow the minute eddies and to entangle and "capture" the relatively short wood pulp fibers while both fiber types are dispersed and suspended in a gaseous medium. The resulting combination is an intimate mixture of wood pulp fibers and polymeric microfibers integrated by physical entrapment and mechanical entanglement while suspended in space. It is preferred to initiate the combining action while the microfibers are still in a softened state at an elevated temperature.

Attenuation of the microfibers occurs both before and after the entanglement of these fibers with the pulp fibers. The total attenuation is from a fiber diameter of about 0.015 inch (which is a typical diameter for the die apertures) to about 5 microns (0.0002 inch) or less. Most of the attenuation occurs within about three inches of the die face, before the air velocity in the fiber stream drops below about 250 feet/second. Since the wood pulp fibers are typically introduced into the microfiber stream about one inch from the die face, attenuation of the microfibers may continue after the merger with the pulp fibers. Due to their extremely small cross-section, the polymeric microfibers are at least 50 to 100 times more flexible than conventional textile fibers made from the same polymer, and are even more flexible and conformable when freshly formed and hot.

Because the microfibers are much longer, thinner, limper and more flexible than the wood pulp fibers, the microfibers twist around and entangle the relatively short, thick and stiff pulp fibers as soon as the two fiber streams merge. This entanglement interconnects the two different types of fibers with strong, persistent inter-fiber attachments without any significant molecular, adhesive or hydrogen bonds. In the resulting matrix the microfibers retain a high degree of flexibility, with many of the microfibers being spaced apart by engagement with the comparatively stiff pulp fibers. The entangled pulp fibers are free to change their orientation when the matrix is subjected to various types of distorting forces, but the elasticity and resiliency of the microfiber network tends to return the pulp fibers to their original positions when the distorting forces are removed. A coherent integrated fibrous structure is formed solely by the mechanical entanglement and interconnection of the two different fibers.

The microfibers and the nature of their anchorage to the wood pulp fibers provide yielding "hinges" between the fibers in the final structure. The fibers are not rigidly bonded to each other, and their connection points permit fiber rotation, twisting and bending. At even moderate microfiber contents, the structure is capable of providing textile-like properties of "hand" and drape, and is conformable while retaining a degree of elasticity and resiliency. Even when wet with water, which softens the wood pulp fibers, the material exhibits flexural resiliency and a wet strength comparable to its dry strength.

Even at microfiber content levels as low as 1% by weight, the containment of the wood pulp fibers is sufficient to provide a significantly improved absorbent material; for example, such material has improved integrity and reduced linting as compared with materials prepared heretofore with similarly high contents of wood pulp fibers. Moreover, this containment of the wood pulp fibers and the other characteristics noted above are achieved in the air-formed fabric without the addition of adhesive and without any further processing or treatment. This improved material also contrasts sharply with materials in which an adhesive is used to contain the wood pulp fibers, with resulting stiffness and reduction in absorbent capacity and rate.

The spatial effect of the wood pulp fibers persists to a relatively high level of microfiber content. Because the pulp fibers maintain their shape and do not melt or undergo substantial morphological change under the temperatures and forces of the microfiber stream, they physically interfere with polymer-to-polymer interactions. This is indicated by an unexpected increase in breaking length or tensile strength at very low microfiber contents, which thereafter falls below a straight line projection of strength level vs. microfiber content, exhibiting an unexpected modification of the microfiber web strength. The wood pulp fibers are preferably distributed uniformly throughout the matrix of microfibers to provide a homogeneous material.

The wood pulp fibers also have been found to reduce the objectionable effects of the polymer aggregates or "shot" that is inevitably produced by most microfiber processes. These polymer aggregates fuse readily to themselves and to adjacent microfibers and contribute to harshness, stiffness and objectionable appearance in a 100% microfiber web. The pulp fibers apparently inhibit the bonding of "shot" particles to each other and to the microfibers and also conceal the "shot" visually and tactually.

In order to convert the fiber blend in the integrated stream 15 into an integral fibrous mat or web, the stream 15 is passed into the nip of a pair of vacuum rolls 30 and 31 having foraminous surfaces that rotate continuously over a pair of fixed vacuum nozzles 32 and 33. As the integrated stream 15 enters the nip of the rolls 30 and 31, the carrying gas is sucked into the two vacuum nozzles 32 and 33 while the fiber blend is supported and slightly compressed by the opposed surfaces of the two rolls 30 and 31. This forms an integrated, self-supporting fibrous web 34 that has sufficient integrity to permit it to be withdrawn from the vacuum roll nip and conveyed to a wind-up roll 35. The web 34 wound on the roll 35 is illustrated in FIG. 2.

The containment of the wood pulp fibers in the integrated fibrous matrix, and the other characteristics noted above, are attained without any further processing or treatment of the airlaid web. However, if it is desired to improve the strength of the composite web 34, it maybe embossed either ultrasonically or at an elevated temperature so that the thermoplastic microfibers are flattened into a film-like structure in the embossed areas. This film-like structure, which will be described in more detail below in connection with the photograph of FIG. 11, functions to hold the pulp fibers more rigidly in place in the embossed areas. Thus, in the illustrative process of FIG. 1, the composite web 34 is passed through an ultrasonic embossing station comprising an ultrasonic calendering head 40 vibrating against a patterned anvil roll 41. The embossing conditions (e.g., pressure, speed, power input) as well as the embossing pattern may be appropriately selected to provide the desired characteristics in the final product. An intermittent pattern is preferred with the area of the web occupied by the embossed areas after passage through the embossing nip being about 5-50% of the surface area of the material and the discrete embossed areas being present in a density of about 50-100/in.sup.2.

The most appropriate embossing condtions for any given material will depend on the particular components. For materials using polypropylene as the thermoplastic polymer for the microfibers, it has been found that substantial improvements in strength of the nonwoven fabric can be obtained by the use of a Branson ultrasonic system, Model 460 with continuous sonic module, operating against a patterned anvil roll 41 at a pressure of 50 psi on the ultrasonic horn, a power input of 700 watts, and a 10 inch .times. 0.5 inch horn in contact with the material being embossed. Suitable patterns for the anvil roll are those illustrated in FIGS. 3-5, and suitable web speeds through the embossing station are 25-150 feet per minute.

One of the principal advantages of this invention is that it permits utilization of all the advantages of a melt-blowing process for forming a fibrous mat, while at the same time permitting integration of the melt-blown microfibers with different amounts and types of wood pulp fibers that can be selected to provide the final product with a variety of different combinations of desired properties that cannot be realized by the use of a melt-blowing process alone. Consequently, this process can be used to produce different materials that are especially tailored for a wide variety of different applications. For example, mats of polymeric microfibers can be efficiently produced at high production rates by a melt-blowing operation, but such mats are not generally suitable for use as wipes because of their limited liquid retention and absorbency characteristics. However, by using the process of this invention to integrate wood pulp fibers with the microfibers produced by the melt-blowing operation, the liquid retention and absorbency characteristics of that mat can be improved to a level that makes the mat perfectly suitable for use as a wipe. Furthermore, the wood pulp fiber is often more readily available and less expensive than the polymeric material used to form the melt-blown microfibers so the integration of the two different types of fibers reduces the cost of the resulting composite mat. Although the nonwoven fabrics of this invention exhibit certain properties attributable to the pulp fibers, the fabric always contains a substantial amount of the thermoplastic microfibers. Consequently, the composite fabric can be modified by secondary thermal treatments such as hot calendering, embossing or spot bonding.

An additional advantage of the integration of the two different fibrous materials via turbulent mixing of the two gas streams is the attainment of a homogeneous distribution of both fibrous materials throughout the final composite web. As mentioned previously, this result is achieved by maintaining a substantial difference in the velocities of the two streams, with larger velocity differences leading to more homogeneous integration and smaller velocity differences producing concentration gradients of the secondary material throughout the primary material. If desired, a product can be made with uniform properties in any direction in the plane of the web, without any substantial variations in thickness due to embossing or the like.

A wide variety of thermoplastic polymers are useful in forming the melt-blown microfibers, so that materials can be fashioned with different physical properties by the appropriate selection of polymers or combinations thereof. Among the many useful thermoplastic polymers, polyolefins such as polypropylene and polyethylene, polyamides, polyesters such as polyethylene teraphthalate, and thermoplastic elastomers such as polyurethanes are anticipated to find the most widespread use in the preparation of the materials described herein.

The picker roll shown in the illustrative arrangement is preferred for producing the secondary air stream containing the wood pulp fibers. However, other devices may be used to generate secondary air streams containing additional fibrous and/or particulate materials, including synthetic fibers such as staple nylon fibers and natural fibers such as cotton, flax, jute and silk. If desired, the wood pulp fibers and an additional material may be carried in a single secondary air stream.

In order to achieve a particular combination of properties in the final fibrous web, there are a number of variables in both the primary and secondary air streams that can be controlled along with the composition and basis weight of the web. Process parameters susceptible to control in the primary gas stream are the gas temperature, which is preferably in the range of 600.degree. to 700.degree. F; the gas velocity, which is preferably in the sonic range within the die; the polymer extrusion rate, which is preferably in the range of 0.25 grams per hole per minute; the polymer temperature; and the ratio of air to polymer (mass flow rates) which is preferably in the range of 10/1 to 100/1. Variables that can be controlled in the secondary gas stream are the gas flow rate and velocity of the picker roll; the gas velocity wich is preferably in the sub-sonic range, e.g., 50-250 feet per second; and the fiber size which is typically on the order of 3.0 millimeters in length. The relationship between the primary and secondary gas streams can also be controlled, and it is generally preferred that the ratio of the gas velocities in the primary and secondary streams be in the range of from 5/1 to 10/1. The relative percentages of the materials introduced by the primary and secondary gas streams may vary over a wide range, but it is typical for the polymeric microfiber to comprise from about 1% to 80% by weight of the final mat. The angle between the primary and secondary gas streams at the point of their merger may also be varied, but it is generally preferred to have the two streams come together perpendicular to each other. Similarly, the particular point at which the two streams are merged, relative to the melt-blowing die in the upstream direction and foranimous forming surface in the downstream direction, may be varied.

The following examples illustrate the preparation of nonwoven materials in accordance with the present invention. The results of measurements of certain physical properties of the materials so prepared and of their individual constituents are also reported. The measurements were made substantially in accordance with the following procedures:

Uncompressed thickness

A Custom Scientific Instruments thickness tester was used with a 1 in.sup.2 foot applying pressure to the material at 0.5 oz./in.sup.2 in Examples I-X, and with a 7.07 in.sup.2 foot applying pressure to the material at 0.004 psi in the remaining examples.

Bulk density

Bulk density in g/cm.sup.3 was calculated using the measured uncompressed thickness and known sample basis weight (bulk density = basis weight/thickness).

Oil absorbency

A material sample four inches square is weighed, placed in a room temperature bath of mineral oil for 30 seconds, and then removed and drained by suspending on a glass rod for 45 seconds. The sample is then weighed again and any increase in weight is the amount of oil absorbed by the sample. This weight is then divided by the density of the oil (0.831 g/ml) to give the volumetric equivalent, which is divided by the dry weight of the sample to give "oil absorbency."

Water absorbency

Same as oil absorbency test using water in place of oil. The absorbency tests in Tables II and III were done using 0.5% aqueous solution of Aerosol OT surfactant to ensure uniform wetting of all samples.

Breaking length

A tensile strength test is conducted with an Instron tester (Model No. A70) using a material sample 1.0 inch wide and 3 inches long (a longer sample is used, but a length of 3 inches is exposed between the jaws of the tester). The sample is loaded at a rate of 10 inches/minute at 70-72.degree. F and 40-50% relative humidity. The measured tensile strength is then divided by the basis weight of the sample to give the breaking length. To measure the wet breaking length, the sample is immersed in water for 0.5 minute and then laid on a blotter to remove excess water before testing. To measure redried breaking length, the sample is wetted as just described and then air dried before testing.

Stretch

The increasing length of the sample is measured during the tensile strength test described above, and the percentage increase in length of the sample just prior to break of the sample is its stretch.

Lint count

A material sample six inches square is fastened to the peripheries of two parallel circular plates spaced four inches away from each other on a common vertical axis. The sample is then bent, twisted and crushed by moving one of the plates repetitively into engagement with the other plate while rotating the moving plate 180.degree. relative to the other plate during each advancing stroke. This repetitive plate movement is continued for 50 cycles with a Millipore filter No. HAWP-047-00, 47-mm. diameter, 0.45-micron pore size, positioned beneath the sample with the center of the filter located just slightly outside the peripheries of the two plates. The particles caught on the filter are then viewed through a microscope via a TV camera and monitor at 40X magnification, and all particles greater than 13 microns are counted in nine different fields of 1.64 .times. 2.43 mm. on the filter. Eight of these nine fields are evenly spaced around the circumference of the filter, and the ninth field is located in the center of the filter. The nine resulting particle counts are then averaged, and the resulting average count is recorded as the "lint count".

Specific volume

"Initial specific volume" is determined by dividing the uncompressed thickness (as measured by the above procedure using the 7.07 in.sup.2 foot applying pressure to the material at 0.004 psi), in centimeters, by the basis weight of the sample, in grams per square centimeter. The sample is then loaded uniformly across its surface at a pressure of 0.49 psi; after one minute the compressed thickness under this load is measured with the same thickness tester described above, and the resulting compressed thickness is divided by the basis weight to obtain the "loaded specific volume." The load is then removed from the sample; after one minute the thickness of the recovered sample is measured in the same manner described above for the uncompressed thickness (using the 7.07 in.sup.2 foot applying pressure at 0.004 psi); and the resulting recovered thickness is divided by the basis weight to obtain the "recovered specific volume."

EXAMPLE I

A composite fabric ontaining 53.5% bleached sulfite pulp fibers and 46.5% melt blown polypropylene microfibers was prepared in accordance with the general procedure described above and illustrated in FIG. 1. The polypropylene (Exxon resin, CD-523) was extruded at a rate of 22 lbs./hr. (equivalent to 0.42 g/min. per die orifice) at a final temperature of 600.degree. F., and was attenuated in the primary air streams flowing at a sonic velocity and a combined rate of 1500 lbs./hr. at a temperature of 700.degree. F. A secondary air stream containing suspended pulp fluff was generated by d