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CROSS-DIRECTIONAL DISTRIBUTION OF
ADDITIVES IN SYNTHETIC PAPERS
BACKGROUND OF THE INVENTION
The present invention relates to synthetic, wet-laid, nonwoven sheets and more particularly relates to synthetic, wet-laid, nonwoven sheets having an additive distributed in a cross-directional pattern across the sheets.
Various types of synthetic, wet-laid, nonwoven sheets such as papers and processes for making such sheets are known and are described in, for example, U.S. Pat. Nos. 2,999,788 and 3,756,908. As disclosed in U.S. Pat. No. 3,756,908, the papers produced from fibrids and floe of non-fusible, aromatic polyamides are particularly useful due to excellent thermal and electrical insulation properties.
For papers of the type disclosed in U.S. Pat. Nos. 2,999,788 and 3,756,908 and in other papers produced from synthetic fibrous stocks, it is desirable for some specialized end uses for an additive to be distributed in higher and lower additive quantities across the width of the sheet while being consistent along the sheet length, i.e., distributed in a "cross-direction" in the sheet. For example, in the manufacture of "honeycomb" structures from such papers, it is desirable for an additive or colorant to be distributed in the paper so that one face of the honeycomb contains the additive or colorant whereas the other face has a lower amount of such additive or colorant. In papers for making honeycomb and for other uses, it may be desirable for the additive to be entirely absent in some cross-directional areas of the sheet or for a particular cross-directional pattern of 35 additive to be provided. A process is needed for producing such synthetic nonwoven sheets having additives distributed in a cross-direction in the sheet.
In accordance with the invention, a process is provided for making an elongate, nonwoven sheet from synthetic fibrous stocks. The sheets have an additive distributed in a predetermined cross-directional pattern of areas of higher and lower additive quantities across 45 the width of the sheet. In the process, at least two synthetic fibrous stocks, each containing solids capable of forming an elongate, nonwoven flexible sheet, are provided, one of the stocks containing an additive in a concentration higher than in the other stock. Each of 50 the stocks is supplied to a paper machine having a headbox for depositing the stocks on a wire to form a wet sheet of the solids. The stocks are introduced into the headbox from a plurality of cross-directional positions equally spaced-apart along the headbox with a generally equal amount of solids being introduced at each position to produce a wet sheet generally uniform in weight per unit area across its width. The stock with the higher concentration of additive is introduced in higher quantity than the other stock at least at one of the crossdirectional positions corresponding to a higher additive quantity area of the sheet so that the additive is distributed in the predetermined cross-directional pattern. The wet sheet is dewatered and dried to form the nonwoven sheet.
In accordance with a preferred form of the present invention, the stocks are blended before introduction into the headbox at least at one of the positions to adjust
the amount of additive in the stock introduced at that position.
In accordance with another preferred form of the invention, the stocks are introduced into the headbox from a plurality of equally spaced apart, discrete discharge openings corresponding to the cross-directional positions along the headbox. It is also advantageous in some applications to segregate the stocks from each of the discharge openings and prevent cross-mixing until the stocks are in the headbox adjacent the wire.
In accordance with another preferred process in accordance with the invention, the solids in the stocks comprise between about 15 and about 90 percent fibrids by weight. Preferably, the fibrids employed are of a non-fusible aromatic polyamide and the stocks further comprise short fibers of a non-fusible aromatic polyamide. Preferred aromatic polyamides are wholly aromatic such as poly-(meta-phenylene isophthalamide).
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may best be understood by reference to the following detailed description of preferred embodiments when read in conjunction with the accompanying drawings in which:
FIG. 1 is a general schematic illustration of apparatus for the practice of the preferred form of the present invention;
FIG. 2 illustrates apparatus for employing a preferred process of the invention capable of producing a linear cross-distribution profile of the additive;
FIG. 3 illustrates apparatus for producing an "S" curve additive distribution profile;
FIG. 4 illustrates apparatus for producing an exponential distribution profile of an additive;
FIG. 5 illustrates various additive distribution profiles produced in accordance with Examples 1-2; and
FIG. 6 illustrates relative slurry flows using the apparatus of FIG. 2 to produce an alternate "S" curve additive distribution profile.
The present invention is useful for the cross-directional distribution of additives in a wide variety of sheets such as papers produced from synthetic fibrous stocks. "Synthetic fibrous stock" is intended to refer to aqueous stocks containing at least a major portion of solids of man-made origin and being capable, with or without a resinous binder, of forming a wet-laid nonwoven sheet. Preferred for the practice of the present invention, are stocks as disclosed in U.S. Pat. No. 2,999,788 which contain fibrids as a binder for the sheets. U.S. Pat. No. 2,999,788 is hereby incorporated by reference. In the preferred embodiment of the invention, the stocks are the type disclosed in U.S. Pat. No. 3,756,908, hereby incorporated by reference, which contain solids with between about 15 percent and about 90 percent by weight of fibrids of a non-fusible aromatic polyamide and about 10 and about 85 percent by weight short fibers (floe) also of a non-fusible aromatic polyamide. It is desirable for both polyamides to be wholly aromatic, most preferably poly-(meta-phenylene isophthalamide). For the practice of the preferred form of the invention, fibrids and floe are prepared as disclosed in U.S. Pat. No. 3,756,908.
In the process of the invention, two or more stocks are prepared with the stocks containing differing additive concentrations, i.e., the concentration of the additive in one stock is higher than in the other stock or is
entirely absent from the one stock. It will be understood
that for the purposes of the present application, additive
is intended to refer to any of a number of materials to be
distributed in synthetic nonwoven sheets in a desired
cross-directional pattern and which are retained on the 5
wire during sheet formation. Additives include solid
materials with a variety of morphologies such as fibrous
materials, powders, and platelets. Fibrous additives
include materials such as staple, floe, pulp or fibrids
which have characteristics which differ from the other 10
fibrous materials in the stocks. Many other types of
materials are intended to fall within the meaning of the
term additives provided that they are incorporated into
the sheet. For example, dyes, colorants and other mate-
rials such as dispersions of liquids which become incor- 15
porated into or associated with materials which form
the sheet can be distributed in a cross-directional pat-
tern in the sheet.
It will also be understood that the present invention is intended to encompass the distribution of more than one 20 additive in cross-directional patterns which may be the same or different for the different additives. As will become more apparent hereinafter, additional stocks are needed to provide separate distribution patterns for each additive desired to be distributed differently from 25 other additives.
Referring now to the drawings and with particularity to FIG. 1 illustrating preferred apparatus generally for distributing one additive in a process in accordance with a process of the invention, it is shown schemati- 30 cally that stocks "A" and "B" with "A" having a higher concentration of an additive than stock "B" are provided in tanks 10A and 10B, respectfully. As will be explained in more detail hereinafter, stocks "A" and "B" are supplied to a continuous paper-forming ma- 35 chine 12 such as a Fourdrinier paper machine having a headbox 14 for depositing the stocks on a wire (not shown) to form a wet sheet from the solids in the stocks.
FIG. 1 illustrates generally that a stock supply system 16 including pumps (not shown) is employed which 40 introduces the two stocks into the headbox 14 at inlet 15 from a plurality of discrete discharge openings 18 into the inlet 15 of the headbox 14. The discharge openings 18 are equally spaced-apart along the width of the headbox with eight such positions being employed in the 45 apparatus of FIG. 1. It will be understood that the number of discharge openings 18 to be employed will vary with the width of the paper being made and the type of pattern to be provided for the additive in the paper consistent with good flow distributor design practices. 50
In accordance with the process of the invention, the amount of solids introduced into the inlet by each of the discharge openings 18 (identified individually as I-VIII) into the headbox is generally equal so that the wet sheet formed on the wire is generally uniform in 55 weight along its width although the amount of additive in the streams varies to produce the cross-directional distribution pattern. After discharge into the inlet 15 of the headbox 14, the stock flows to the headbox 14 and onto the wire at a position along the width of sheet 60 corresponding to the position of discharge into the head box inlet and the concentration of the additive in the stock at that discharge opening 18 determines the amount of additive in the sheet. Depending on the type of distribution pattern desired, it may be necessary to 65 minimize the level of the pond in the headbox so that the retention time in the headbox is decreased to minimize cross-mixing. Nevertheless, for some patterns, at
least limited cross-mixing may be desired. In some paper machines, the wire is normally oscillated to promote cross-mixing to decrease sheet directionality. In general, this is undesirable in the practice of the present invention and the oscillation should not be used. In addition, it is sometimes desirable to provide baffles or partitions in the paper machine headbox inlet 15 which provide confined flow areas of approximately equal size corresponding to each of the discharge openings 18 so that the stock introduced into the headbox from that opening is deposited on the wire in a localized fashion. If desired, the partitions extend substantially though the headbox inlet to segregate streams of the stock from each of the discharge openings 18 until the streams are very close to the wire and cross-mixing is prevented until that point.
The stock supply system 16 illustrated includes two flow distribution devices 20A and 20B suitably provided by headers which are capable of dividing aqueous slurries into a number of different flows. Additional flow distribution devices and additional separate stock tanks will be needed for additional additives to be distributed in different patterns. Flow distribution device 20A is used to divide a flow from stock tank 10A containing the maximum additive concentration into a number of streams flowing to the discharge openings 18 in the headbox inlet 15. Similarly, flow distribution device 20B provides one or more flows of the stock flowing from the tank 10B containing minimum additive as is necessary to achieve the desired pattern of additive distribution. Valves 22A and 22B and flow meters 24A and 24B are preferably used to control the flow from the tanks 10A and 10B to the flow distribution devices 20A and 20B.
As will become more apparent hereinafter, one or more flows from the flow distribution devices 20A and 20B are introduced to the discharge openings 18 in the paper machine headbox. For some applications, where a maximum additive concentration is desired at a discharge opening 18,-a direct connection such as connection 26A between a flow from the flow distribution device to discharge opening I is provided with valve 28A and flow meter 30A for control of the flow. A similar connection 32B is illustrated for the minimum additive concentration at discharge opening VIII. When the amount of additive at a position is to be intermediate the maximum and minimum, two flows from the flow distribution devices 20A and 20B are tied together with a Y-connector 34 connected to discharge opening III so that the stocks are blended together to result in an intermediate amount of additive at that position. Again, it is desirable to use a maximum additive control valve 36A, a minimum additive control valve 36B, flow meters 38A and 38B on those streams and a combined flow meter 40. It is thereby possible to monitor and control the flows to provide the appropriate amount of additive while maintaining the solids in the paper generally uniform across the sheet width. Other means of calibrating the flows can be used such as by visually comparing flows of water flowing from the distribution devices when disconnected. It is also desirable to monitor the basis weight of the paper as formed to insure uniform deposition of solids on the wire and to monitor the additive distribution pattern when possible.
Referring now to FIG. 2, there is shown a piping scheme from the flow distribution devices to the headbox which can be used to produce a linear additive distribution profile in the paper, i.e., the concentration