Method For Drying Nonwoven Fabric

Abstract

A nonwoven web, formed on a continuous screen papermaking machine and including heat shrinkable fibers, is delivered onto an endless coarse wire screen and exposed to infrared radiation to heat the fibers and dry the web. Water vapors evolved in the drying process are swept from both surfaces of the web by forced streams of dry air.

Description

This invention relates to a process and apparatus for the manufacture of fibrous material and, more particularly, to a process and apparatus for drying nonwoven fabric having particularly long fibers and produced on a papermaking type of machine where the nonwoven fabric, as it is being produced and is wet, has a low-initial tensile strength.

Paper and nonwoven fabric of synthetic fibers or in which the fibers include synthetic fibers, produced on a papermaking type of machine, are formed from a water slurry. The fibers are deposited on a wire, screen, or belt and the water is withdrawn, usually by vacuum, leaving a mat or web of felted fibers on the wire, screen, or belt. Such mat or web of felted fibers is wet and, until dried, has a very low-tensile strength. Because of the low-tensile strength, attempts to dry such mats or webs over drying rolls result in tearing or adhesion of the web. Attempts to dry the web, while suspended, by running the web through a drying duct have, likewise, proved impractical because of low-tensile strength.

In the process and apparatus of the instant invention, the tensile strength of paper and nonwoven fabric of heat-shrinkable synthetic fibers or which include heat-shrinkable synthetic fibers is increased by applying infrared radiation to the wet web or felt as it is discharged from the wet end of the papermaking type of machine. The wet web or felt is discharged from the wet end of the machine onto a belt and, while on the belt, is passed through an infrared radiation zone. Moisture is removed from the wet web or felt in the infrared radiation zone and, at the same time, the web or felt shrinks, compacting the fibers and further increasing the tensile strength of the web or felt. Because, at the time of such shrinkage, the fibrous material is not yet fully coherent, there is less resistance to such shrinking. Thus, the web or felt is more uniform and becomes a homogeneous material.

The infrared radiation zone of the instant invention heats the fibers directly and not the ambient medium as is the case in conventional dryers. The infrared radiation shrinking process of the instant invention is a dry process, the fibers heated by the radiation, heat and transfer their heat to the ambient medium. Thus, while the fibers are exposed to the radiation, and are shrinking, the ambient medium, which is water, is being driven off. This is in sharp contrast to conventional shrinking processes where hot water or steam is directed to the web and which, after shrinking, must be removed or driven off by drying.

The process and apparatus of the instant invention are particularly suited to the drying and shrinkage of webs or felts including heat-shrinkable synthetic fibers and at least some fibers which have a low melting point. The addition of low-melting fibers to the web or felt, where such web or felt is dried and shrunk in the instant process with the accompanying apparatus, results in early compacting of the fibers which contributes to increase of web or felt strength for additional processing.

Referring to the drawing, the apparatus of the instant invention includes endless wire belt 2, preferably of coarse mesh wire screen, passing around rollers 4, 6, roller 4 being driven by motor 8 through variable speed drive 10 and belt 12. Hood 14, having air inlet 16 and air outlet 18 is suspended from supports, not shown, above belt 2, the bottom of hood 14 being open over belt 2. Infrared lamps 20 are mounted in hood 14, the number of lamps depending upon the length of hood 14 and the width of belt 2, it being important, of course, that sufficient lamps be provided for the desired infrared radiation across the width of belt 2. Hood 22, having air inlet 24 and air outlet 26 is mounted on supports, not shown, below the upper run of belt 2, belt 2 running from left to right in the direction of the arrow on the drawing. Infrared lamps 28 are mounted in hood 22 and radiate upwardly through open mesh screen belt 2. Endless conveyor belt 30 passing around rollers 32, 34 is mounted at the discharge end of belt 2, roller 32 driven by motor 34 through variable speed drive 36 and belt 38 drives conveyor belt 30.

In operation of the apparatus web 40 is discharged from the wet end of a papermaking type of machine, not shown, to the left-hand end of belt 2 as shown in the drawing. The wet fiber web discharged from the wet end onto the left-hand end of belt 2 is transported on belt 2 between hoods 14, 22 and between infrared radiation lamps 20, 28. Dry and, preferably, heated air is forced into air inlets 16, 24, and is discharged through air outlets 18, 26 in hoods 14, 22, respectively. The velocity of the air fed into the hoods through air inlets 16, 24 is sufficient to remove, from the surfaces of belt 40, any steam forced out of web 40 by the shrinking and heating of the web in the infrared radiation zone by infrared bulbs 20, 28. It is important that the discharged steam be blown or swept off of the surfaces of web 40 because such steam acts as an insulator to the infrared radiation and reduces the shrinking of the fibers and drying of the web by infrared lamps 20, 28. From the infrared radiation zone on belt 2 in hoods 14, 22, web 40 is discharged onto conveyor 30 and, from conveyor 30, web 40 may be fed into conventional papermaking drying apparatus.

As has been noted hereinabove, the fibers in web 40 are shrunk in the infrared radiation zone. In order to accommodate such shrinkage, the speed of belt 2 and conveyor 30 are regulated, through variable speed drive 10, 36, respectively, to the speed at which the web is delivered from the wet end of the papermaking type of machine to the left-hand end of belt 2 and to accommodate such shrinkage. In other words, to accommodate the shrinkage in web 40, belt 2 is driven at a slower speed than the speed at which web 40 is delivered thereto. Such difference in speed is taken up in fiber shrinkage.

The method and apparatus of the instant invention may be adapted to a wide variety of nonwoven fibers particularly a wide variety of fiber contents of such fabrics. Obviously, as the content of low-melting temperature synthetic fibers and heat-shrinkable synthetic fibers in web 40 is increased, or decreased, changes in the speed of belt 2 and conveyor 30 and in the requirements for infrared radiation in hoods 14, 22 will likewise change.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible.

Claims

We claim:

1. A process for drying a nonwoven fibrous web of the type formed by a papermaking device wherein an aqueous dispersion of fibrous material or stock including heat-shrinkable synthetic fibers is discharged through a head-box across a continuous screen, said wet fibrous material being substantially evenly distributed thereacross, comprising:

a. delivering a formed wet web of said fibrous material onto a course mesh traveling wire screen, said wet web of fibrous material being delivered at a rate of speed which is faster than the rate of speed of said traveling wire screen;

b. advancing said coarse mesh traveling wire screen on which said wet web of fibrous material is being carried through an infrared radiation zone;

c. subjecting each fibrous web surface to infrared radiation until moisture is substantially removed from said fibrous web and shrinking is induced in said heat-shrinkable synthetic fibers; and

d. simultaneously sweeping each fibrous web surface with dry air to evacuate residual water vapor formed over said fibrous web surfaces during irradiation thereof.

2. A process for drying a nonwoven fibrous web of the type formed by a papermaking device wherein an aqueous dispersion of fibrous material or stock including heat-shrinkable synthetic fibers and low-melting point synthetic fibers is discharged through a head-box across a continuous screen, said wet fibrous material being substantially evenly distributed thereacross, comprising:

a. delivering a formed wet web of said fibrous material onto a coarse mesh traveling wire screen, said wet web of fibrous material being delivered at a rate of speed which is faster than the rate of speed of said traveling wire screen;

b. advancing said coarse mesh traveling wire screen on which said wet web of fibrous material is being carried through an infrared radiation zone;

c. subjecting each fibrous web surface to infrared radiation until moisture is substantially removed from said fibrous web and shrinking is induced in said heat-shrinkable synthetic fibers; and

d. simultaneously sweeping each fibrous web surface with dry air, in the machine direction of said coarse mesh traveling wire screen, to evacuate residual water vapor formed over said fibrous web surfaces during irradiation thereof.

3. The process of claim 2 including the additional step of varying the speed of said coarse mesh traveling wire screen through said infrared radiation zone to accommodate natural shrinkage of web fibers and heat-induced shrinkage attributable to said heat-shrinkable synthetic fibers.