GB2243834A - Adding thermoplastic fibres to cellulose fluff pulps - Google Patents

Abstract

Thermoplastic fibres, such as polylethylene fibres, are added to cellulose fluff pulp in a mill in which the fibres are disintegrated and the mixture homogenized. In accordance with the invention, the thermoplastic fibre material is first introduced into a controlled humid environment and thereafter fed to the mill in a wet state. Such pro-treatment of the thermoplastic fibre eliminates variations in the moisture content of the fibre material and allows the material to be charged to the mill in accurately weight-determined quantities.

Description

A method and apparatus for adding thermoplastic fibres to cellulose fluff pulps.

The present invention relates to a method of adding thermoplastic fibres, such as polyethylene fibres, to cellulose fluff pulp, in which method thermoplastic fibre material and cellulose fluff pulp are charged to a mill and there pulped and the mixture homogenized. The invention also relates to apparatus for carrying out the method.

In one known method of the aforesaid kind, the polyethylene is used in sheet form. The polyethylene sheets are coarsely pulped mechanically and conveyed on a conveyer belt to a mill, into which air-transported cellulose fluff pulp is also introduced, and in which the fibres are disintegrated and the mixture homogenized.

The mixture of cellulose fluff pulp and thermoplastic fibres is used as an absorbent in absorption bodies intended for disposable absorbent articles, such as disposable diapers, sanitary napkins and incontinence guards. The function of the thermoplastic fibres is to improve the mechanical strength of the absorption bodies and the proportion of admixed fibres is normally within the range of 15-20%, depending upon the desired properties of the product concerned. Because the thermoplastic fibres are from four to five times more expensive than cellulose fibres, manufacturers are anxious not to use a greater proportion of thermoplastic fibres than is absolutely necessary. To this end, it is important that the thermoplastic fibres are metered to the mill in precise quantities.It has been found difficult, however, to meter thermoplastic fibres in precise quantities when practising the known method, because the moisture content of the polyethylene material used varies and because the irregular surface weight of the polyethylene sheets can influence the end result. It is therefore difficult to determine the weight percentages of water and thermoplastic fibres introduced to the mill when charging polyethylene material of a given weight.

The object of the present invention is to solve these problems by means of a method and apparatus which, when adding thermoplastic fibres to cellulose fibres, will enable the thermoplastic fibres to be metered in precise to the mill in which pulping homogenization of the mixture takes place.

This object of the invention is achieved by means of a method of the aforesaid kind which is characterized by introducing a thermoplastic fibre material into a controlled humid or moist environment and thereafter introducing said fibre material into the mill in a wet state.

Because the thermoplastic fibre material endeavours to take a state of equilibrium with its surroundings, the material will absorb moisture through capillary effect until this state is reached. This enables the variations in moisture content of the fibre material introduced into the controlled humid atmosphere to be equalized such that the fibre material leaving said humid environment will have a constant moisture content. The fibreweight of the fibre material discharged from the humid environment can readily be determined, therewith enabling the fibre material to be charged to the mill in accurately weight-determined quantities.

Apparatus for carrying out the inventive method has the characteristic features set forth in Claim 5 A preferred embodiment of the invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 illustrates schematically plant-apparatus for carrying out the inventive method, and Figure 2 illustrates a preferred embodiment of a device incorporated in the plant-apparatus of Figure 1 and operative to provide a controlled humid environment, and a device for feeding thermoplastic fibre material in a wet state from the humid environment to the mill incorporated in the plant.

Figure 1 illustrates highly schematically the components incorporated in the device for adding polyethylene fibres to cellulose fluff pulp. The cellulose fluff is charged to a mixing mill through a conduit line, in which the cellulose fluff pulp is air-transported and which discharges into the mill. The polyethylene is introduced into the station A in the form of sheets which at the time of manufacture have a moisture content of about 50% but which subsequent to storage and transportation have a varying moisture content, due to the different storage and transport environments to which said sheets have been subjected. The polyethylene sheets are coarsely pulped mechanically in station A, by means of appropriate apparatus, such as a hammer mill.The coarsely defibred polyethylene material is then allowed to fall onto an appropriate conveyer means, for instance a belt conveyer, and is carried by the conveyer means into station B. The polyethylene material is exposed to a humid environment in station B. The polyethylene material will absorb water from the surrounding atmosphere until a state of equilibrium with said atmosphere is reached, the drier parts of the material absorbing more water than the wetter parts thereof. Consequently, all parts of the polyethylene material exiting from station B will have mutually the same moisture content, and consequently the fibre-weight of said material can be readily determined.

Figure 2 illustrates schematically a preferred embodiment of the station B to which the coarsely-pulped polyethylene material is supplied from station A with the aid of a belt conveyer 1. The discharge end of this conveyer is located above a feed funnel 2, which in turn is located centrally in the opening of a container 3, which has a sloping bottom 4. The container 3 is also provided with a laterally extending outlet tube 5, which extends obliquely upwards parallel with the bottom 4.

The outlet tube has the form of an obliquely truncated cone and thus narrows in the upward direction. A feed screw 6 extends along the container-bottom and somewhat into the outlet tube 5, said screw being journalled for rotation in an appropriate manner. Spray nozzles 7 are disposed around the container opening.

The container 3 is mounted within a tank 8, to which water can be supplied through a pipe 9. The pressure side of a pump P is connected to the nozzles, whereas the suction side of the pump is connected to the tank 8.

A row of holes or slots 10 in the side wall of the container 3 form a spillway to the tank 8.

The apparatus illustrated in Figure 2 operates in the following manner.

In the starting state of the apparatus, the container 3 shall be empty, so as to avoid the presence of a water surface in which down-falling polyethylene material can collect, said material being lighter than water.

The container 3 is first emptied, when necessary, where after polylethylene material is introduced into the container 3, from the conveyer 1 and through the funnel 2, until the whole of the bottom 4 is covered. The pump P, which may be a variable displacement pump, is then activated. The pump is driven so that a relatively small quantity of water per unit of time is delivered from the nozzles 7. The polyethylene material falling from the funnel 2 will therefore pass through a water shower prior to falling into the container and on top of the polyethylene material previously supplied. When polyethylene material has been supplied in a quantity such as to fill the container 3 to the level of the rows of holes 10 in the container wall, the supply of polyethylene material is interrupted, although spraying with water is continued.After a given period of time, the polyethylene material present in the container will have absorbed so much water as to reach saturation point, and water subsequently sprayed onto the material will drainoff through the row of holes 10.

This terminates the starting-up phase of the process and the feed screw 6 and the conveyer belt 1 are then set into motion. A continuous length of polyethylene material, or polethylene string, is then fed-out through the outlet tube 5 and falls gravitationally onto a conveyer, or, as is preferred, directly into the mill.

As will be understood, the feed of material into the container 3 must be controlled in a suitable manner, so as to correspond to the outfeed of material through the tube 5. This can be achieved, for instance, by controlling the conveyer speed with the aid of a level sensor mounted in the funnel 2 and operative to indicate the level of material in the funnel.

The reason why water is supplied over a relatively long period during the starting-up phase is to allow the absorption mechanism of the polyethylene material to work to the full, this absorption, or suction, being effected primarily as a result of capillary forces.

Subsequent to the absorption capacity having been utilized to the full, which is evidenced by the fact that water will then flow through the holes 10, the material is no longer bouyant but has essentially the same density as water, while at the same time having a porridgelike consistency. The bonding forces in this porridgelike material are sufficiently strong to hold the material together, even if a water surface should form ontop of the material. When the starting-up phase has been terminated, the flow of water from the nozzle 7 is increased so as to obtain a water-excess. This excess of water, however, shall not be greater than the amount of water that can drain-off through the holes 10 in the container wall.

As beforementioned, the outlet tube 5 narrows towards its outlet end. The outer part of the tube 5 has perforations 11 disposed along the undersurface thereof. The polyethylene material will therefore be compressed and dewatered in the outlet tube and the water extracted by compression will drain-off through the perforations 11.

Consequently, the moisture content of the polyethylene material exiting from the tube 5 will be lower than it was in the container 3.

When mixing cellulose fluff-pulp and polyethylene in the mixing mill, it is necesary for the mixture to have a given moisture content in order to avoid problems created by static electricity. Consequently, an advantage is afforded when the polyethylene material, as in the case of the inventive method, introduced into the mill has a higher moisture content than in the case of the earlier known method, in which it is necessary to add water to the mill in order to counteract occurrent static electricity. It can be mentioned in this respect that the homogenous mixture of cellulose fibres and binder fibres leaving the mill will generally have a moisture content of between 7 and 15%.

Since the moisture content of the cellulose fluff fed to the mill can also vary, it is preferred to configure the tube 5 in a manner which will enable the moisture content of the polyethylene string leaving the tube to be varied. To this end, that part of the tube 5 located externally of the screw 6 can be formed in two mutually coacting parts, a lower rigid part of semi-circular cross-section and an upper part of U-shaped cross-section, said upper part being pivotal in a vertical direction, e.g. with the aid of a hinge, so as to enable the common cross-sectional shape of the two parts to be varied, by locating a larger or smaller part of the legs of the U above the upper defining line of the lower cross-section. It will be understood that other constructions are also possible for controlling the moisture content of the material fed from the outlet tube.

For instance, conical tubular extensions can be arranged in the free end of the outlet tube.

In one variant (not shown) of the apparatus illustrated in Figure 2, the container 3 has a horizontal bottom and the feed screw 6 and the outlet tube 5 also extend horizontally. In order to prevent the occurrence of a free water-surface in the container 3, the bottom part of the container is perforated. In order to shorten the time taken to complete the starting-up phase, the flow of water delivered by the nozzle 7 is controlled so that excess water will constantly be supplied to the polyethylene material. Subsequent to filling the container 3 to a given level, it will be seen that the starting-up phase is terminated when the amount of water draining from the container 3 per unit of time is equal to the amount of water delivered per unit of time from the nozzles 7.This comparison between the aforesaid quantities, however, requires the provision of relatively complicated and expensive measuring equipment, and consequently it is preferred in the case of this variant to determine the duration of the starting-up phase emperically.

The polyethylene material is introduced into the described apparatus in an environment such that its intrinsic absorption capacity is utilized to the full.

The advantage afforded by the use of such an environment is that it is possible to determine when the full absorption capacity of the material has been utilized, for instance by comparing the amounts of water supplied to the apparatus and drained therefrom per unit of time, or by allowing the water-flow over the spillway to indicate that the material is incapable of absorbing more water.

It is not necessary, however, for the environment to be such as to achieve maximum absorption in order to obtain a polyethylene material of homogenous moisture content.

In order to achieve this state, it suffices for the humid environment to have a higher moisture content than the moisture content of the incoming polyethylene material and to hold this material in said environment for a length of time sufficient for said material to achieve by absorption a state of equilibrium with the environment, in all parts of said material. However, due to initially varying moisture contents of the incoming material, the material will absorb varying quantities of water from the surroundings per unit of time, which means that it is necessary to vary the quantities in which water is supplied to the humid or moist environ ment per unit of time, in order to maintain continuity in the moisture content of said environment.Consequently, it is preferred to use an environment in which it is possible to work with a water excess, therewith enabling continuity of the moist or humid environment into which the polyethylene material is introduced to be ensured in a simple fashion.

It will be understood that the described and illustrated embodiments can be modified within the scope of the invention, particularly with respect to the arrangement for creating said humid environment. For instance, the polyethylene material may be sprayed with water whilst on the conveyer path, in which case the conveyer path will be perforated. Furthermore, outfeed devices other than feed screws can also be used. In accordance with one variant of the arrangement, the polyethylene material can be fed from the coarse-pulping station through a water bath, with the aid of a feed screw, a wormpiston pump or the like, and then compressed in a conical, drainage channel. Since such compression contributes to homogenization of the moisture content of the outfeed material, when correctly dimensioned, an ar rangement of this kind can be operated in the absence of a starting-up phase.

It will also be understood that thermoplastic bindingfibres other than polyethylene fibres can be used.

The invention is therefore solely restricted by the subject matter of the following claims.

Claims (8)

CLAIMS:

1. A method for adding thermoplastic fibres, such as polyethylene fibres, to cellulose fluff pulp, in which method thermoplastic fibre material and cellulose fluff pulp are introduced into a mill in which the fibres are disintegrated and the mixture is homogenized, which method comprises introducing a thermoplastic fibre material into a controlled humid environment and thereafter introducing the material into the mill in a wet state.

2. A method as claimed in claim 1, wherein the humid environment is created by spraying the thermoplastic fibre material with water during its passage to the mill.

3. A method as claimed in claim 1, wherein the humid environment is created by passing the thermoplastic fibre material through a water bath.

4. A method as claimed in any one of the preceing claims, wherein the thermoplastic fibre material exiting from the humid environment is compressed prior to introducing the material into the mill, and carrying away the water pressed from the material.

5. A plant for carrying out the method as claimed in claim 1, which plant comprises an air-transport device for the transportation of cellulose fluff pulp, into a mixing mill, an arrangement operative to create a humid environment, means for transporting fibre material to the arrangement operative to create a humid environment, means for feeding fibre material into the humid environment, and means for feeding wet fibre-material from the humid environment and introducing the fibre material into the mixing mill.

6. A plant as claimed in claim 5, wherein the menas for feeding wet fibre-material from the humid environment includes means for compressing the material fed from the environment.

7. A method for adding thermoplastic fibres to cellulose fluff pulp substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

8. A plant for carrying out the method for adding thermoplastic fibres to cellulose fluff pulp substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.