GB2054681A

GB2054681A - Flexible sheet materials

Info

Publication number: GB2054681A
Application number: GB8019982A
Authority: GB
Original assignee: Courtaulds PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1979-06-19
Filing date: 1980-06-18
Publication date: 1981-02-18
Also published as: GB2054681B

Abstract

A flexible sheet material useful, for example, as a heat and sound insulating material, a carpet underlay or as an absorbent tissue for wiping oil and water, comprises 50 to 98 per cent by weight dry pulp fibres and from 20 to 50 per cent by weight thermoplastic synthetic polymer fibrils. The flexible sheet material has a bulk density of from 10 to 100 kg/m<3>. The thermoplastic synthetic polymer fibrils may be of polyethylene or polypropylene, are randomly distributed throughout the sheet material, are entangled with wood pulp fibres and are fused to each other at their points of contact to form a network retaining wood pulp fibres in the sheet. The flexible sheet material is formed from a layer comprising 50 to 98 per cent by weight dry wood pulp fibres and from 2 to 50 per cent thermoplastic synthetic polymer fibrils entangled with the wood pulp fibres by heating the layer while at a bulk density of from 10 to 100 kg/m<3> to fuse the thermoplastic synthetic polymer fibrils at their points of contact.

Description

SPECIFICATION Flexible sheet materials This invention relates to flexible sheet materials formed from dry wood pulp fibres. Dry wood pulp fibres do not have the bonding capabilities of wet wood pulp fibres. Coherent flexible sheet materials made from them are generally made by application of adhesive to a layer of dry fibres followed by compression.

The present invention seeks to provide a sheet material which is both bulky and flexible and which is formed mainly of wood pulp fibres and could be prepared without any application of adhesive to the layer of pulp fibres.

Thermoplastic synthetic polymer fibrils have become available to the pulp and paper industry as additives for pulp in paper making to give increased strength. The synthetic polymer fibrils are sold under the trade marks "Hostapulp", "Pulpex" and "Carifil" in a hydrophilic chemically treated form for addition to wet wood pulp furnish.

Recommended applications are in wallpapers, filter papers, tea bags, electrical insulation paper and thermoformable paperboard. British Patent Specification 1 453,053 describes a process for the production of thermoformed sheets by first forming a sheet comprising 50-90 per cent by weight wood pulp and 50--10 per cent by weight thermoplastic synthetic polymer fibrils, heating the sheet to a temperature above the softening point of the polymer of which the fibres are composed, and shaping the hot sheet under pressure. The thermoformed sheet produced is described as being rigid, as having a dense structure and as retaining the shape imparted to it.

According to one aspect of the present invention, a flexible sheet material comprises from 50 to 98 per cent by weight dry wood pulp fibres and from 2 to 50 per cent by weight thermoplastic synthetic polymer fibrils and has a bulk density of from 10 to 100 kg/m3, the thermoplastic synthetic polymer fibrils being randomly distributed throughout the sheet material, being entangled with the wood pulp fibres and being fused to each other at their points of contact to form a network retaining wood pulp fibres in the sheet.

According to another aspect of the invention, a method of forming a flexible sheet material comprises forming a layer comprising from 50 to 98 per cent by weight dry wood pulp fibres and from 2 to 50 per cent by weight thermoplastic synthetic polymer fibrils entangled with the wood pulp fibres and heating the said layer while at a bulk density of from 10 to 1 00 kg/m3 to fuse the thermoplastic synthetic polymer fibrils at their points of contact.

We have found that when the dry wood pulp sheet material is heated to fuse the synthetic polymer fibrils at a low bulk density according to the invention, good bonding can be achieved which holds the wood pulp in a coherent sheet without the sheet being set to a rigid thermoformed product. The synthetic polymer fibrils give much better bonding of the dry wood pulp than polymer adhesives in powder, split film or continuous filament form because of their ability to become mechanically entangled with pulp fibres.

The thermoplastic synthetic polymer fibrils are separate, discontinuous very fine filaments having a thickness of the order of 1 micron and an average length of from 0.2 to 10 mm. They preferably have a branched structure. They are generally prepared by suddenly reducing the pressure on a two-phase mixture comprising a molten thermoplastic polymer and a solvent. The pressure reduction is produced by allowing the mixture to pass through an orifice, for example as described in British Patent Specification 1,355,912. The fibrils are formed of a thermoplastic synthetic polymer and preferably of an a-olefin homopolymer or copolymer containing more than 50 per cent a-olefin units, for example polyethylene or polypropylene.

The wood pulp fibres are preferably formed by mechanical or thermomechanical pulping, for example by thermomechanically pulping wood chips in a double disc refiner. The wood pulp fibres can alternatively be formed from pulped waste paper. After pulping the fibres are dried. A preferred process of drying uses the apparatus described and claimed in U.S. Patent 3,069,784 but àlternatives include flash drying or drying in sheet form followed by separation of the pulp fibres in a hammer mill.

The dry wood pulp fibres and the thermoplastic synthetic polymer fibrils are blended and formed into a dry layer, for example by depositing them on a permeable conveyor through which air is drawn by suction. The wood pulp fibres and the synthetic polymer fibrils are preferably fed together to the permeable conveyor to aid in the distribution of the synthetic polymer fibrils throughout the dry fibre layer. A preferred apparatus for forming the dry fibre layer uses a pulp spreader as described and claimed in our pending British Patent Application No. 78.48840 and described in more detail hereinafter with reference to the drawing.

The pulp spreader makes the layer more uniform both in the distribution of the synthetic polymer fibrils in the layer and in evening out variations in the basis weight of the layer. Using such a spreader, satisfactory distribution of the synthetic polymer fibrils in the layer can be achieved by separateiy feeding the dry wood pulp and the synthetic polymer fibrils in a substantially continuous manner to the hopper used to lay the fibres on the permeable conveyor. Without the use of the spreader it may be preferred for best results to initially blend the dry wood pulp and the synthetic polymer fibrils in a separate process.

We have found that a dry formed layer of wood pulp and synthetic polymer fibrils produced as described above using a suction of up to 60 cm water (gauge) below the permeable conveyor, generally has a bulk density of from 10 to 30 kg/m3. Satisfactory bonding of the sheet can be achieved by this low density and may be preferred for some products; alternatively compression to a bulk density of from 30 to 60 kg/m3 which is the bulk density usually preferred, can be achieved by a consolidating roll positioned to control the thickness of the layer.

The dry fibre layer is then heated to fuse the thermoplastic synthetic polymer fibrils at their points of contact. The heat applied must be sufficient to raise the temperature of the synthetic polymer fibres above their softening point (1300C for high density polyethylene or 1 600C for polypropylene). A preferred method of heating is to draw a hot gas, preferably air, through the dry fibre layer while it is supported on a permeable conveyor.

One preferred product according to the invention is a heat and sound insulating material, useful for example in insulating roof spaces. Such a product can, for example, have a bulk density of from 30 to 60 kg/m3, a basis weight of from 1000 to 3000 g/m2 can contain from 5 to 10 per cent by weight of synthetic polymer fibrils. The insulating material is sufficiently coherent to be rolled up and transported and then laid down in position, for example in a house loft. The insulating material can be formed from a cheap source of wood pulp fibres, such as waste paper. The pulp or the waste paper from which it is formed is preferably impregnated with a flame retardant, for example borax, boric acid or a phosphorus compound before being dried and mixed with the synthetic polymer fibrils.

Another preferred product according to the invention is carpet underlay material. This is generally prepared having a similar bulk density to the insulating material and can also be formed from a cheap source of wood pulp fibres but preferably has a higher content of synthetic polymer fibrils, for example from 10 to 20 per cent by weight to ensure that the underlay remains coherent during the life of a carpet.

Another preferred product is a material useful for absorbing oil and water. Such a material is preferably formed at a basis weight of 250 to 750 grams per square metre and a bulk density of 60 to 100 kilograms per cubic metre and preferably contains from 20 to 30 per cent by weight of synthetic polymerfibrils to give oil absorbency.

The absorbent material can be a tissue useful for wiping up oil and water. The use of 20 to 30 per cent by weight of synthetic polymer fibrils gives the tissue sufficient strength to be crumpled up into a pad and reopened to a sheet. The absorbent material can be used as a disposable floor covering in places where oil and water stains are likely to occur, for example in garages to protect the floor of a customer's car.

Products according to the invention can generally be used, for example, rolled up, unrolled and laid in position without substantial loss of pulp fibres from the sheet. Loss of pulp fibres from the surface of the sheet can be further reduced by the application of a lightweight covering web or scrim to the surface of the sheet and/or the application of a thin film of adhesive binder to the surface of the sheet. For example, a water-based adhesive can be applied to both surfaces of the sheet while it is stiil hot from the heat treatment, to fuse the synthetic polymer fibrils so that the water vehicle is evaporated and a layer of the non-woven fibrous network described in British Patent Specification No. 1,525,001 can be applied to these surfaces while the adhesive is still tacky.Alternatively an open scrim or net can be bonded to one or both surface of the sheet. Conveniently a polyethylene or polypropylene net can be applied to the surface of the sheet and bonded to the sheet by the heating step used to fuse the synthetic polymer fibrils.

The invention will now be described by way of example, with reference to the accompanying drawing, the single Figure of which is a diagrammatic, partially sectioned, side elevation of an apparatus for producing a coherent sheet of thermal insulating material.

The apparatus comprises generally a conveyor 1, a hopper 2, a spreader 3, a housing 4, consolidation rolls 5 and 6 and a heater 7.

The conveyor 1 is a gas-pervious mesh conveyor, preferably of metallic wire. It moves in the direction shown around rolls 11, 12, 13 and 14. Below the hopper 2, the spreader 3 and housing 4, the conveyor 1 moves over a stationary gas-pervious plate 1 5.

The hopper 2 is of generally rectangular crosssection having a front wall 16 and a back wall 1 7.

The conveyor 1 acts as the base of the hopper.

The hopper 2 is positioned to receive dry wood pulp via a duct 1 8 which may, for example, lead from a cyclone separator (not shown) and to receive thermoplastic synthetic polymer fibrils via a duct 19. The dry wood pulp can for example be produced by the process of our British Patent No.

1 564202. At the bottom of the front wall 16, a roller 23 is free to rotate. The exit 20 of the hopper 2 is defined by the conveyor 1 and the roller 23.

The length of the hopper from the front wall 1 6 to the back wall 17 is small, for example about three times the height of the exit 20. The pulp and polymer fibrils flow down the hopper 2 as a block and are then held on the conveyor 1 by suction and move with the conveyor. At the exit 20 the movement of the pulp causes rotation of the roller 23 and this rotation aids in the smooth passage of pulp through the exit 20. Rotation of the roller 23 carries pulp upwards but the pulp soon falls back under its own weight so that the layer of pulp on the feed conveyor 1 often has ridges extending across the conveyor The action of the pulp spreader 3 removes these irregularities.

A suction box 22 is positioned below the gaspervious plate 1 5 and the conveyor 1 in the region of the hopper 2. The suction box 22 extends across the width of the hopper and lengthwise from the back wall 17 of the hopper to beyond the exit 20 of the hopper.

A duct 25 opening into the suction box 22 communicates with a fan (not shown) arranged to draw gas from the hopper 2 through the pulp and polymer fibrils in the hopper and through the gaspervious conveyor 1 into the suction box 22.

The degree of suction applied can be varied, for example, between 1 cm water gauge and 60 cm water gauge. Increased suction holds the mixture of pulp fibres and polymer fibrils more tightly against the conveyor 1 forming a more dense mass on the conveyor. Increased suction thus increases the bulk density of the layer of pulp and polymer fibrils carried through the exit 20 and hence the weight per unit area (basis weight) of the layer on the conveyor 1.

The conveyor 1 carries the layer of pulp from the exit 20 of the hopper 2 to the spreader 3.

The spreader 3 consists of a first member in the form of a fan 26 and a second member in the form of a deflector 37. The fan 26 has a hub 28 and radially-extending blades 27 which just clear the conveyor 1 at their lowest point-24. The deflector 37 comprises two curved plates 46 and 47 connected at 45. For example, the plates 46 and 47 can be hinged at 45 to allow adjustment of the deflector. As the pulp fibres and polymer fibrils are blown off the conveyor by the fan 26 they encounter the low curved plate 46 of the deflector 37. The suspension df pulp fibres and polymer fibrils in turbulent air is constrained to a convergent path between the fan 26 and the curved plate 46 up to the point at which the deflector 37 most nearly approaches the fan 26 which is at or near 45.The path of the pulp and air is divergent between the fan 26 and the upper curved plate 47 of the deflector 37, allowing the pulp fibres and polymer fibrils to be thrown forward as a loose fluffy mass.

The preferred speed of rotation of the fan 26 of spreader 3 is from 100 to 3000 r.p.m. At these high speeds of rotation the pulp fibres and polymer fibrils are blow off the feed conveyor 1 against the deflector 37. The stream of air created by the fan 26 separates the pulp fibres, distributes them substantially uniformly in air and conveys them between the first and second members 26 and 37 of the spreader 3.

The fan 26 needs to draw in large amounts of air to suspend the wood pulp fibres and polymer fibrils in air. The spreader 3 is provided with an air injection nozzle 81 which injects air at superatmospheric pressure below the deflector 37 in a downwards and forwards direction. This aids in the break-up of fibre bundles in the pulp and increases the distance over which the spreader 3 discharges the pulp fibres and polymer fibrils, thus allowing the evening out of greater irregularities in the feed of pulp and polymer fibrils.

The housing 4 is divided into two compartments 40 and 41 by a wall 42 on which the upper plate 47 of the deflector 37 can be mounted. The spreader 3 throws the pulp fibres and polymer fibrils forwards into the compartment 41 as a loose fluffy mass which lies on the conveyor 1. A suction box 48 is positioned below the conveyor 1 and the gas-pervious plate 1 5 in the area where the fibres and fibrils fall. They are thus gathered above the suction box 48 as a layer of mechanically entangled dry wood pulp fibres and thermoplastic synthetic polymer fibrils. The layer so formed is carried by the conveyor 1 under a trimming roll 51 which is a rotating assembly of blades, for example having a speed of rotation of 50 to 1000 r.p.m.

A suction box 55 is mounted below the gaspervious plate 1 5 and conveyor 1 in the region approaching the trimming roll 51 to hold the layer of pulp fibres and polymer fibrils on the conveyor 1 against the action of the trimming roll 51. The suction boxes 48 and 55 are both connected to a duct 49 through which a suction of, for example, 5 cm water gauge can be applied. The air thus removed can be recycled through a pipe 83 to the air injection nozzle 81.

The layer of pulp fibres and polymer fibrils passes to the consolidation rolls 5 and 6 which compress the pulp, for example from a bulk density of 20 kg/m3 to a bulk density of 40 kg/m3.

Roll 5 is positioned to control the thickness of the layer and is sealed against the housing 4 and 60.

The consolidation achieved by rolls 5 and 6 is generally sufficient to prevent pulp fibres being blown off the layer but the layer is not coherent without the support from the conveyor 1.

The layer of pulp fibres and polymer fibrils is carried to heater 7 which comprises a duct 63 for supplying hot air and a suction box 64 positioned under the outlet of duct 63. A duct 65 opening into the suction box 64 communicates with a fan (not shown) which draw the hot air through the layer of pulp fibres and polymer fibrils and through the gas-pervious conveyor 1 into the suction box 64. The hot air from duct 65 can be recycled to heating means, for example a burner and thence to duct 63. The temeprature and flow rate of the hot air is such that it raises the temperature of the thermoplastic synthetic polymer fibrils above their softening point The fibrils fuse at their points of contact and the wood pulp fibres entangled with the polymer fibrils are securely held in the network of polymer fibrils so formed.Some of the synthetic polymer may melt and coat adjacent wood pulp fibres giving additional bonding.

The coherent sheet of thermal insulating material so formed is separated from the conveyor 1 at roll 12.

The invention is illustrated by the following Examples in which percentages are by weight.

EXAMPLE 1 Waste paper was impregnated with 10.5 per cent of borax and 4.5 per cent of boric acid flame retardants (on a dry weight basis) and fed to a hydropulper. The pulp formed was pressed to 50 per cent solids then fed to an "Atritor" (Trade Mark) high turbulence mixer used as a dryer as described in U.S. Patent 3,069,784. The pulp contacted hot air at a temperature of 4500C at the entry of the dryer and was discharged as a suspension of dry wood pulp fibres in air at 1200 C. The pulp fibres were separated from the majority of the air in a cyclone separator and fed to the hopper 2 of the apparatus shown in the drawing at a rate of 230 kg/hour. Simultaneously 20 kg per hour of thermoplastic high density polyethylene fibrils were fed to the hopper 2.

The dry wood pulp fibres and polyethylene fibrils were laid on the permeable conveyor 1 as a dry layer 7 cm thick having a bulk density of about 20 kg/m3. The pulp spreader 3 spread the pulp fibres and fibrils more evenly within the layer and the rolls 5 and 6 compressed it to a bulk density of 40 kg/m3.

The layer-so formed was treated with air at 2000C drawn through the fibrous layer at 1.4 m3/min. The polyethylene fibrils were heated sufficiently to bond them at their points of contact.

They remained substantially as distinct fibrils mechanically entangled with the wood pulp fibres.

The product was a coherent sheet of thermal insulating material of density 40 kg/m3 which could be rolled up for transport and then unrolled and laid in position in a loft without substantial loss of pulp fibres from the sheet.

EXAMPLE 2 Waste paper was pulped in a hydropulper, pressed to 50 per cent solids and dried in an "Atritor" high turbulence mixer as described in Example 1. The dried wood pulp fibres formed were fed to an apparatus similar to that shown in the drawing together with 25 per cent of thermoplastic high density polyethylene fibrils.

The dried wood pulp fibres and polyethylene fibrils were laid on a permeable conveyor as a dry layer having a basis weight of 450 grams per square metre and a bulk density of about 25 kilograms per cubic metre. A pulp spreader spread the pulp fibres and fibrils more evenly within the layer and compression rollers compressed the layer to a bulk density of 80 kilograms per cubic metre.

Polyethylene nets were fed into contact with both surfaces of the compressed layer and the assembly was treated with air at 200cm, drawn through the fibrous layer at about 1 cubic metre per minute. The polyethylene fibrils were heated sufficiently to bond them at their points of contact.

They remained substantially as distinct fibrils mechanically entangled with wood pulp fibres.

The polyethylene nets were fused to the surface of the sheet. The product was a flexible coherent sheet material which could be used as a disposable floor covering without substantial loss of pulp fibres from the sheet.

Claims

1. A flexible sheet material comprising from 50 to 98 per cent by weight dry wood pulp fibres and from 2 to 50 per cent by weight thermoplastic synthetic polymer fibrils and having a bulk density of from 10 to 100 kg/m3, the thermoplastic synthetic polymer fibrils being randomly distributed throughout the sheet material, being entangled with the wood pulp fibres and being fused to each other at their points of contact to - form a network retaining wood pulp fibres in the sheet.

2. A flexible sheet material according to claim 1 in which the wood pulp fibres are formed by thermo-mechanical pulping of wood chips.

3. A flexible sheet material according to claim 1 in which the wood pulp fibres are derived from pulped waste paper.

4. A flexible heat and sound insulating sheet material according to any of claims 1 to 3 having a bulk density of from 30 to 60 kg/m3, a basis weight of from 1,000 to 3,000 g/m2 and containing from 5 to 10 per cent by weight of synthetic polymer fibrils.

5. A flexible sheet material according to claim 1 useful for absorbing oil and water and having a basis weight of from 250 to 750 g/m2 and containing from 20 to 30 percent by weight of synthetic polymer fibrils.

6. A method of forming a flexible sheet material comprising forming a layer comprising from 50 to 98 per cent by weight dry wood pulp fibres and from 2 to 50 percent by weight thermoplastic synthetic polymer fibrils entangled with the wood pulp fibres and heating the said layer while at a bulk density of from 10 to 100 kg/m3 to fuse the thermoplastic synthetic polymer fibrils at their points of contact.

7. A method according to claim 6 in which the layer of dried wood pulp fibres and thermoplastic synthetic polymer fibrils is formed by depositing dried wood pulp fibres and thermoplastic synthetic polymer fibrils together on a permeable conveyor through which air is drawn by suction.

8. A process according to claim 6 or claim 7 in which the layer of dry pulp fibres and thermoplastic synthetic polymerfibrils is heated by drawing a heated gas through the dry fibre layer while it is supported on a permeable conveyor.

9. A method of forming a flexible sheet material substantially as hereinbefore described with reference to the accompanying drawing.

10. A method of forming a flexible sheet material substantially as described in Example 1.

11. A method of forming a flexible sheet material substantially as described in Example 2.

12. A flexible sheet material produced by the process of any of claims 6 to 11.