Method for screening, separating, and removing fiber bundles, lumps, knots and foreign matter from aqueous dispersions used in forming non-woven fabrics by wet-laying

Abstract

Apparatus and method for use in making non-woven fabrics by wet-laying an aqueous dispersion. The apparatus and method screens, separates and remover fiber defects and foreign matter above a prescribed size from other dispersion fibers. The apparatus includes a housing having an inlet and several outlets, an accelerator within the housing for creating relative movement between the dispersion fluid and fibers to axially align at least some of the fibers in the direction of dispersion flow to disperse those fibers in the dispersion and a separator within the housing for separating bundles and lumps above a prescribed size from other dispersion fibers. The housing discharges the bundles and lumps above the prescribed size from the process at one outlet, and passes the other dispersion fibers through to another outlet for use in the wet-laying process. The method includes the steps of producing a stream of dispersion, directing the stream over a curvilinear path, creating relative movement between the dispersion fibers and fluid to axially align at least some of the fibers in the direction of dispersion flow, screening fiber bundles and lumps above a prescribed size from other dispersion fibers by intercepting the stream along a curvilinear path at a lateral stream boundary and washing accumulated fiber bundles and lumps above the prescribed size from the screen for discharge from the process and passing the other dispersion fibers for use in the wet-laying process.

Description

This is a continuation of application Ser. No. 267,707, filed Mar. 23, 1981, now abandoned.

This invention relates generally to the making of non-woven fabrics by the process of wet-laying an aqueous fiber dispersion and more particularly to apparatus and method for screening, separating and removing fiber defects which occur in the form of lumps, bundles and knots and foreign matter, which are above a prescribed size from other dispersion fibers.

BACKGROUND OF THE INVENTION

In recent years paper and paper product manufacturers have recognized that variants of technology commonly used in paper production can be adapted for making fabrics of inexpensive and durable form. Manufacturers recognize that acceptable substitutes for fabrics made by weaving continuous fiber filaments in the conventional manner can be found in non-woven fabrics produced by processes such as wet-laying synthetic fiber filaments with paper-making equipment.

In the wet-laying process, an aqueous dispersion containing synthetic fiber filaments of predetermined diameter, length and rigidity is distributed over a moving wire or web. The fiber filaments accumulate on the advancing web as the dispersion liquid drains away. The result is a layer of fabric composed of intertwined fibers. To enhance fabric strength, manufacturerinhancefluidcircumfectively tangentially on housing cyclindrical wall 4 at an angle φ of approximately 40° measured from the housing vertical center line as shown in FIG. 3.

Because of required process flow rates and desirable disc rotational velocity in the typical case, the substream final inlet velocity is less than the initial dispersion inlet velocity. Accordingly, the cross-section of inlet port 22 and conduit housing end 24 are greater than the cross-section of conduit source end 25. To permit smooth transition in the stream velocity and thereby minimize turbulence and fiber interaction, inlet conduit 23 is provided with the coupling section 30 of increasing cross-section in the direction of housing end 24. Therefore, as stream 51 proceeds down coupling section 30, the stream's velocity is gradually and smoothly reduced to minimize fiber entanglement.

To further minimize turbulence and the tendency for fibers to entangle, inlet 20 is mounted at the top of and tangent to housing body element 3. By mounting the inlet tangentially, the stream is admitted substantially perpendicularly to chamber 74's cross-section and in line with chamber 74's curvilinear length as best seen in FIG. 3. Introduction of dispersion substantially in line with chamber 74's curvilinear length avoids abrupt changes in stream direction and attendant turbulence at entrance. Further, by mounting inlet 22 at the top of body element 3, gravity is permitted to assist fiber movement in the curvilinear portions of chamber 74.

As best seen in FIGS. 3 and 4, and as noted previously, each annular chamber 74 has a curvilinear length defined by an initial substantially linear portion extending from inlet port 22 to approximately the center line of body element 3 and a curvilinear portion extending from the end of the linear section at element 3's center line to the chamber end at the far end 90 of outlet port 42 adjacent to outlet port 57.

As dispersion substreams are received in the linear section of each chamber 74, the respective moving walls 83, 84 and 86 engage the substream lateral boundaries to accelerate the substream in the general direction of the entering stream flow, as best seen in FIGS. 4 and 7. The width of chamber 74 as defined by the length of disc 77 is set so that the range of chamber wall velocities is capable of establishing fluid acceleration throughout the stream cross-section.

While acceleration of the dispersion fluid is generally in the direction of the stream flow, the accelerator actually has radial and angular components. The radial and angular acceleration components result from the annular form of chamber 74. Since inlet conduit 23 is mounted tangentially to the housing body element 3 and the arc subtended by this portion of the chamber is relatively small, while the radius of curvature large, the initial acceleration is in the direction of the entering dispersion substream. As noted above, inlet conduit 23 is mounted in this way to permit transition to the annular chamber with minimum turbulence.

Fluid acceleration causes the fluid to move relative to the disoriented fibers 73a bundled fibers 73b and lumped fibers 73c. This relative movement gives rise to drag forces on the filaments which tend to axially align fibers 73a in the direction of fluid flow as illustrated in FIGS. 3 and 4. As noted earlier, the shape and size of bundles 73b and lumps 73c render them either resistant or unable to align.

As fibers axially align, they tend to further disperse in stream 51, and are accelerated by the relative fluid movement. Unalignable fibers are simply accelerated by the relative fluid movement.

As the dispersion substream enters the curvilinear section of each chamber 74, the effect of the radial and angular acceleration produced by the chamber's moving walls becomes more pronounced. Since separator slot 102 is located in the chamber radial outer wall extending over its curvilinear length, chamber 74's outer wall is at least partially open and permits the dispersion substream to divert from the direction of the chamber's curvilinear length and pass out of the chamber to the dispersion outlet 40. Dispersion fluid and fibers located at the outer radial extent of chamber 74 in proximity to the outer wall, under the influence of the radial component of acceleration, are the first to attempt to pass out of the chamber. Other fibers located radially inwardly of the outer wall are moved angularly by the angular component of acceleration until the radial component and stream low flow carry them to the outer wall. Accordingly, aligned fibers and bundled and lumped fibers are accelerated radially and angularly and move outwardly toward and circumferentially of the outer chamber wall and slot 102.

Since slot 102 is oriented in chamber 74's outer wall with its width parallel to the chamber's width and its length parallel to the chamber's curvilinear length, fibers axially aligned in the substream may pass through slot 102. Fiber bundles and lumps and any foreign matter having a size larger than slot 102 however, may not pass and are rejected. In practice, manufacturers have found that fiber bundles of cross-section larger than ten times the fiber diameter in the non-woven product are objectionable. Therefore, the width of slot 102 in preferred form is set at ten times the fiber diameter.

The multiple dispersion substreams and aligned fibers that pass through slot 102 of each chamber 74 unite in outlet conduit 43 and are led away by outlet conduit 43 to further process stations.

Fiber bundles and lumps and foreign matter of size in excess of the slot width which are rejected at slot 102 are swept along the slot by the action of the dispersion flow, the chamber's moving walls and gravity to be discharged at outlet 55. Thereafter the rejected fibers and foreign matter are either discarded or processed for further use.

This invention recognizes that fiber defects and foreign matters may be screened and removed by placing a slot of restricted width only in a dispersion stream in which the dispersion fibers are axially aligned with the flow direction. Recognition of this permits the slot length to be increased as desired to increase rejection efficiency.

As noted, outlet 40 extends over the length of slot 102 and is mounted at an angle to housing 2 which permits smooth exit of the dispersion from the housing interior. In preferred form, the outlet conduit mounting angle Ψ₂ is selected to be between 30 and 50 degrees to chamber 8's vertical center line as seen in FIG. 3. Also in preferred form, the cross-section of outlet conduit 40 is selected to be sufficiently larger to reduce the dispersion stream velocity on exit from the interior housing 2 to minimize turbulence and the tendency for the fibers to intertwine.

Still further in preferred form, slot 102 is formed in chamber 74's outer radial wall with a face 131 of first width at the inner sleeve surface 79 which is larger than the width of its second face 132 at the outer sleeve surface 80. This cross-sectional profile, best seen in FIG. 4, creates a venturi effect as the dispersion substream exits housing 2 causing an acceleration of substream velocity which further aligns dispersed fibers 73 for passage through slot 102.

Finally, concerning operation of accelerator discs 75 and 76, as noted, the perimeter of discs 75 and 76 are in preferred form provided with radial extentions 103 and 104 which fit grooves 106 and 105 in sleeve 78, or in the alternate embodiment housing interior wall 206. In operation, the spacing between the faces of the projections and the grooves is set to permit the projections to move freely through the grooves without permitting fibers to become lodged therebetween to retard or obstruct disc rotation.

II

PAC Method

The method of this invention concerns the improvement of fiber dispersions used in the making of non-woven fabrics by wet-laying. As previously noted, dispersions used in a wet-laying process include fiber filaments which have been cut to length and dispersed. Typically, this is done by pouring fibers of desired length into the dispersion fluid and mechanically agitating. To improve dispersion and reduce fiber intertwining, chemical additives such as wetting agents and viscosity modifiers, well-known in the art, may also be included.

Because of fiber defects arising when the fibers are first cut to length, the fibers may fail to separate when introduced to the dispersion. Failure of the fibers to initially separate results in fiber bundles within the dispersion. Or, if the fibers do initially separate, they may subsequently entangle as a result of interaction during agitation, creating fiber lumps. This lumping problem becomes more pronounced the longer, thinner, and more flexible the fibers are as noted earlier. If left in the dispersion, these defects cause nonuniform density defects in the finished non-woven product which render the product commercially unacceptable.

The method aspect of the invention includes a series of steps which permit the aligning and dispersing of individual fibers in a fluid dispersion for the making of non-woven fabric by wet-laying and the separation of bundled and lumped fibers above a prescribed size from the dispersion. The result is a non-woven fabric having fewer and less objectionable density variations.

As a first step in the method, a dispersion stream is produced from dispersion received from a source. The dispersion includes a fluid and randomly oriented fibers. The fluid may include chemical additives such as commonly known wetting agents and viscosity modifiers. The fibers typically include disoriented individual fibers and bundled and lumped fibers. As explained with respect to the apparatus, dispersant is received at the apparatus inlet and converted to a stream having a controlled inlet velocity. In preferred form, as noted earlier, multiple substreams of controlled inlet velocity are established.

Following formation of the stream, or multiple substreams the method calls for the dispersion to be diverted over a curvilinear path. According to the invention, the path includes a first linear portion which provides a smooth transition from the dispersion direction as received, to a second curved portion. In the apparatus, the path is defined by the accelerator chamber's curvilinear path. As described previously, the chamber includes a first substantially linear portion to permit coupling of the dispersion with minimal turbulence and attendant fiber intertwining. Subsequently, the dispersion is directed over the curved portion of the path. In the apparatus, this constitutes directing the dispersion over the curved portion of the chamber's curvilinear length.

In the method, as the dispersion is directed over the curvilinear path, it is accelerated at its lateral boundaries. The method contemplates accelerating the dispersion across the entire cross-section. While it is anticipated that the acceleration may not be uniform, it is sufficient that some acceleration is provided in order to produce some relative movement between the dispersion fluid and the individual fibers. In the preferred method, the acceleration applied includes both radial and angular components with respect to the curvilinear path. These components assist movement of the dispersion over the desired path. As described with respect to the apparatus, the acceleration is applied to the dispersion boundaries by the rotating annular chamber walls. Accordingly, the acceleration is applied at the dispersion boundaries with a radial and angular component.

As a result of the acceleration of the fluid, the fluid is caused to move relative to the dispersion fibers. This relative movement causes the individual disoriented fibers to axially align in the direction of the accelerated fluid. As the individual fibers align they tend to more uniformly disperse. As explained in connection with the apparatus operation, bundled and lumped fibers are also subject to the relative movement of the fluid. However, their size and shape may prevent or limit any alignment. For example, lumped fibers arising from intertwining tend to nucleate and grow symmetrically resulting in a general spherical lump. As a result, the lump is without a preferred axis along which to align. In the case of bundles caused by fibers failing to initially separate, while an axis for alignment may exist, bundle size and flexibility may discourage alignment in the stream.

As a next step in the method, the lumped and tangled fibers are separated from the dispersion. In this step, the dispersion is presented to a screen which blocks the passage of bundled and lumped fibers and any foreign matter having a cross-section larger than a prescribed size. In the preferred method, lumps and bundles and foreign matter having a cross-sectional diameter larger than ten times the fiber diameter are rejected.

Screening is accomplished by presenting a lateral stream boundary to the screen. Preferrably, the stream boundary is along the outer lateral side of the stream as it is directed along the curvilinear path. This permits the aligned individual fibers and lumped and tangled fibers and foreign matter of cross-section below the prescribed limit to pass through the screen. In preferred form, the dispersion passing through the screen is thereafter led away in a direction which is substantially the same as the direction of the dispersion through the screen so as to minimize the turbulence and attendant fiber intertwining. In the apparatus, the screen is constituted by the separator rectangular slot located at the chamber outer radial wall.

Finally, the process includes a step of washing rejected bundled and lumped fibers and foreign matter from the screen. In this step, the dispersion itself is used to wash the rejected fibers and foreign matter from the screen. As explained in connection with the apparatus operation, the washing action arises from the direction of dispersion flow over the chamber curvilinear path caused by the acceleration chamber and its wall. It is therefore the circumferential component of dispersion flow that principally contributes to the washing action. In the preferred method, the curvilinear path is arranged so the washing action is further assisted by gravity.

While we have described the apparatus and method of the invention in the particular embodiment, it will be understood that various additions, substitutions, modifications and omissions may be made to the invention without departing from its true spirit.

Claims

1. Method for screening, separating and removing fiber defects and foreign matter above a prescribed size from a dispersion of thin, long, flexible fibers having length to diameter ratios above 400 to 1, the dispersion intended for use in the formation of non-woven fabrics by wet-laying, the method comprising:

(a) producing a fiber dispersion stream of the thin, long, flexible fibers, the dispersion including a dispersion fluid, disoriented individual fibers, and lumped and tangled fibers;

(b) directing the stream over a substantially linear path to a curvilinear path at an entrance of an annular chamber having sidewalls, and endwalls, wherein the stream is introduced substantially tangential to the curvilinear path to avoid abrupt changes in stream direction and to avoid turbulence at the entrance;

(c) rotating at least the side end walls to accelerate, throughout the cross-section in the direction of stream flow, the dispersion fluid relative to the disoriented fibers and lumped and tangled fibers to axially align and disperse the disoriented fibers in the stream;

(d) screening the lumped and tangled fibers above a prescribed size from the dispersion by presenting a lateral stream boundary to a screen comprising a plurality of tapered slots through which the disoriented fibers are aligned and accelerated to prevent passage of lumped and tangled fibers having a size above a prescribed value through the screen and permitting passage of other fibers through the screen; and

(e) washing accumulated lumped and tangled fibers prevented passage through the screen with the dispersion fluid and discharging the lumped and tangled fibers from the stream.

2. The method of claim 1 wherein the step of accelerating the dispersion fluid includes imparting radial and angular components of acceleration at the stream's lateral boundaries.

3. The method of claim 2 wherein the dispersion passing through the screen is diverted from the screen in a direction substantially the same as the direction of the stream passing through the screen so as to minimize turbulence.