Methods of manufacturing fibrous granulates

Abstract

This invention relates to methods of manufacturing fibrous granulates. Various fibers of animal, vegetable, mineral or synthetic origin can be granulated by a method in accordance with the invention in which the fibers are first intertangled or felted in a dry state and then tightened relative to each other by treatment with a liquid followed by drying. The liquid is used in an amount which is sufficient to form menisci between the fibres. Additional bonding between the fibers can also be effected using bonding agents etc. The present invention enables fibrous granulates to be produced which are coherent, substantially spherical, light, flexible and resilient.

Description

This invention relates to methods of manufacturing fibrous granulates.

It has hitherto been proposed to manufacture fibrous granulates in two stages, firstly by subjecting the fibres to a dry treatment in which individual fibers assembled loosely in a tunnel having a horizontal axis are stirred by rotation of the tunnel so as to start imbrication of the fibers into groups and to form fibrous aggregates having little cohesion, and then to treat the aggregates with a liquid carrier during which the aggregates are suspended in a liquid and agitated with this liquid either by simple rotation or rotation combined with pulsation to consolidate the fibrous aggregates and transform them into granulates having desired properties.

The products of such a method are desirably coherent, substantially spherical, light, flexible and resilient fibrous granulates which are sufficiently solid not to come apart easily, and are relatively inexpensive to produce from the base fibers.

According to the present invention there is provided a method of manufacturing a fibrous granulate which method comprises agitating natural or synthetic fibers which are dry on their surface and have a moisture content of not more than 55% of the total weight of the fibers under dry conditions in order to initiate their imbrication in groups and form fibrous aggregates having little cohesion; depositing a liquid on the surface of the aggregates so as to form liquid menisci between the surfaces of the fibers of each aggregate and cause tightening of the fibers; and evaporating the liquid to intensify the tightening.

The fibers may be of natural origin, for example vegetable fibers such as wood fibers, annual plant fibers e.g. straw, cotton linters or the like; animal fibers such as wool or various hairs; or mineral fibers such as asbestos or various fibrous crystals. The fibers may also be of artificial origin, for example organic fibers such as viscoses or rayons; or mineral fibers such as spun glass or stone or the like. The fibers in addition may be of synthetic origin such as polyvinyl chloride, a polyolefin, a polycarbonate, or one of various copolymers.

As used herein, the word "aggregate" refers to organised groups of fibers obtained by dry treatment and the word "granulate" refers to spherules obtained by consolidation of such aggregates.

Using a method in accordance with the present invention it is possible to reduce the duration of the wet treatment of the aggregates compared with hitherto, the energy consumed in effecting this treatment and the energy in extracting water from the granulates. The cost of the resulting granulates can thereby be reduced considerably.

Apparatus for effecting a method in accordance with the invention can be simpler than that used for hitherto proposed methods and this makes it possible to reduce the amount of capital investment in the apparatus and this influences the cost of the granulates. Furthermore, it is possible to establish profitable small capacity installations near to places where the fibrous granulates are produced.

Where the product is to consist only of fibrous granulates, the fibrous mass resulting from the dry treatment will generally be graded in order to separate the fibrous aggregates of desired size from other components. These other components can be pulverulent products, fibers that have remained free, and fibrous groups that are loose or are of unsuitable size. The fibrous groups tend to disintegrate and completely free the fibers that consitute them. The free fibers removed during separation and the free fibers produced by disintegration, can be mixed with fresh fibers and subjected to a dry treatment. The liquid is then only deposited on fibrous aggregates which have been graded.

Fibrous aggregates extracted from the fibrous mass resulting from the dry treatment are preferably spread in a layer of substantially uniform thickness and the liquid atomized on to this layer while the aggregate is being vibrated.

The liquid may be water which may, if desired, contain a surfactant and/or a binder.

The fibrous granulates are advantageously dried after moistening.

In a practical embodiment of the present invention, the fibers used were distinct from one another, although in the case of vegetable fibers, the mass of fibers treated contained a few slivers, that is to say thick bundles of fibers in which numerous parallel fibers are connected to one another and constitute a group that does not have the form of a fiber.

Irrespective of the origin of the fibers, which may be animal, vegetable, mineral or synthetic, the fibers were dry. The fibers were not moist on their surfaces at this stage but they could have internal moisture. Thus, in the case of wood fibers, the moisture content could be from 3 to 55% of the total weight of the fibers but it should not make them appear damp to the touch.

For the dry treatment, the individual dry fibers were assembled in a tunnel in the form of a body with a horizontal axis of revolution. The tunnel may have partitions. The tunnel was arranged to be driven in rotation about its horizontal axis. The tunnel was filled to more than half its internal volume. Moreover, no liquid was distributed in this tunnel, so that the treatment of the fibers was carried out under really dry conditions.

The fibers were subjected to mechanical felting interactions during rotation of the tunnel. The fibers were slightly deformed resiliently and their interlacing was consolidated by the stresses developed by these resilient forces. Fibrous aggregates were thus formed in the moving mass of fibers undergoing treatment.

Most of the resulting cohesive aggregates, which were in the form of spherules, had dimensions which corresponded substantially to the mean length of the fibers and these dimensions varied little even when the treatment was prolonged. Moreover, the fibrous structures that were formed were cohesive only in so far as their dimensions did not greatly exceed the mean length of the fibers. Larger structures had a tendency to break during sliding of the layers of fibers relative to one another, and when falling.

The effectiveness of the dry treatment generally depends on the extent to which the tunnel is filled, since the active driving surface must be small with respect to the volume treated, and on the speed of rotation of the tunnel.

Particularly good results were obtained when the extent of filling of the tunnel was between two thirds and four fifths of the volume of the tunnel.

For example, when 850 kg. of wood fibers are subjected to the dry treatment in a tunnel 2.5 m in diameter and 8m long, the extent of filling is 65%. After 2 hours rotation at a peripheral speed of about 80 m/min, 80% of the fibrous mass had been transformed into aggregates.

When the grain size of the aggregates had been stabilized, the dry treatment was complete and the wet treatment could begin.

Although not always necessary, it may be found desirable to grade the treated fibrous mass before it is subjected to the wet treatment in order to separate fibrous aggregates of appropriate sizes, fibers that have remained free, fibrous groups that are loose or are of unsuitable size, and pulverulent products and the like.

This grading may be effected by screening, which also makes it possible to eliminate fibrous aggregates of appropriate size which have insufficient cohesion. These latter aggregates tend to fail to withstand the various stresses imposed by the screens, and they break up.

The groups that are loose, or are of unsuitable size, and the remains of aggregates destroyed during screening, are disintegrated, for instance by carding, so that the fibers of which they are made are freed from one another. These fibers, and those that remained free, can be mixed with fibers being used for the first time and subjected to a fresh dry treatment so that fibers that are not aggregated or are badly aggregated are recycled.

The graded fibrous aggregates cannot be used in such a state because their internal cohesion is insufficient.

It is therefore essential to consolidate the aggregates and the wet treatment hereinafter described enables this to be effected.

The wet treatment is preferably applied to fibrous aggregates which have been separated from the rest of the mass that has been subjected to the dry treatment. However, the wet treatment can be applied to the whole of the mass, provided that the product containing the granulate can contain free fibers or loose groups of fibers, and also provided that the proportion of fibrous aggregates in the mass is sufficiently high.

The wet treatment consists of depositing a quantity of a liquid on the surface of the fibrous aggregates which is just sufficient to form liquid menisci between the surfaces of the fibers. These menisci cause tightening of the fibers and this tightening is intensified by evaporation of the liquid.

Advantageously the fibrous aggregates are spread in a layer of substantially uniform thickness on a moving surface and the liquid distributed by atomization. In order to provide uniform moistening, the moving surface is preferably vibrated. The moving surface, which may be horizontal, rising or falling, and may be flat, cylindrical, helical or of some other shape, guides the suitably moistened fibrous aggregates towards a dryer so that the liquid forming the menisci is completely and rapidly removed.

In most cases, and more particularly when the base fibers are of natural origin, the liquid can be water. The mean moisture content of the whole mass is then preferably between 50 and 85% based on the total weight of the moist aggregates.

In certain cases, however, other liquids can be sprayed on. For example, if the base fibers are polyethylene the liquid may be a hydrocarbon or a solvent which moistens the fibers.

To show clearly the transformation which results from the wet treatment, and by which the fibrous aggregates having little cohesion become fibrous granulates having better cohesion, two types of tests have been effected.

A first series of tests was designed to measure the diametrical contraction of the fibrous aggregates subjected to wet treatment. Aggregates based on wood fibers were disposed on a motionless plate, and the variation in the diameter of a large number of aggregates was measured by means of a cathetometer.

Atomization of water was found to be accompanied by a contraction varying from 5 to 10%, according to the aggregates, and drying was accompanied by a further contraction of about 1%.

A second series of tests was designed to measure the cohesion of the fibrous granulates. Two needles were inserted diametrally into each granulate. Then, by means of a dynamometer, a force tending to move the needles apart was exerted until the granulate subjected to the test disintegrated. The value of the breaking force, arbitrarily called the cohesion, is significant and was given as a criterion of estimation. The example described hereinafter refers to the results of several tests.

Fibers of conifers such as those which make up "wood pulp" paste were subjected to a dry treatment in accordance with the invention. The fibrous aggregates obtained had weak cohesion because the cohesion was between 10 and 12 g for a diameter of 8mm.

The aggregates were moistened to 80% with water, and then dried. The cohesion of the fibrous granulates obtained was then between 25 and 30g.

It may be advantageous to reinforce the effect of moistening, which results in producing a physical and mechanical effect in which the fibers of the aggregates are tightened, by combining the effect of moistening with that of another kind of connection between the fibers.

Reinforcing connections can be produced by moistening with the liquid. In this case the liquid serves to swell the surface fibers, and if these fibers have been brought sufficiently close together under the action of the surface tensional forces, connections between fibers appear and these further consolidate the granulates.

For example, if polymer fibers are to be treated, a swelling agent which permits bonding by evaporation may be used in the moistening liquid.

With certain thermoplastic fibers, the granulates can be subjected to a thermal treatment which makes it possible to accelerate evaporation of the moistening liquid and the drying of the granulates. Furthermore, it is thereby possible to soften the surface of the fibers and produce on their surfaces a pasty state similar to that obtained using a swelling agent. The interfacial tension forces than produce local welding by bringing the fibers close together.

For example, 5 to 8mm diameter polyvinyl chloride granulates having a cohesion of a few grammes between needles may attain a cohesion of more than 800g when subjected to thermal treatment for a few minutes at 100.degree. to 110.degree.C.

Fibers which are "thermocontractile", and react to an increase in temperature by changing their dimensions, can be subjected to a thermal treatment in the form of a granulate which also makes it possible to accelerate evaporation of the moistening liquid and the drying of the granulate, to produce intensified imbrication and tightening of the fibers.

In general, the moistening liquid used with any type of fiber can include a substance capable of consolidating the approach of the fibers towards one another by glued connections. It is possible for this purpose to use various amylaceous materials, various polymers and more generally any product capable of producing a glued connection between the fibers by drying and/or thermal treatment. For example, fibrous aggregates obtained by dry treatment of conifer fibers can be moistened with a 20g/l suspension of pre-gelled starch in water so that their weight is doubled.

After drying, the cohesion of the fibrous granulates thus obtained can reach a value between 80 and 100g.

A method in accordance with the invention may be used in most cases in which the fibrous granulates are required to be economical and are not subjected to tensile stresses or are only subjected to weak tensile stresses.

Particularly important applications are the manufacture of light, porous insulating fillers and the manufacture of base products for the production of moulded or extruded parts forming sub-layers of various coatings.

Claims

We claim:

1. A method of manufacturing a fibrous granulate, which method comprises the steps of agitating resilient fibers which are dry on their surfaces and which have a moisture content of not mre than 55% of the total weight of the fibers under dry conditions, to initiate imbrication of the fibers in groups and form fibrous aggregates having relatively low cohesion; thereafter depositing on the surface of the fibrous aggregates a suitable liquid selected, in accordance with the fibers being treated, to form liquid menisci on the fibers; said depositing step comprising the step of spraying only a limited amount of liquid onto the fibrous aggregates which limited amount of liquid is just sufficient to form liquid menisci between the surfaces of the fibers of each aggregate at the crossing points of the fibers, thereby to deform the fibers and cause mechanical tightening and increased mechanical interengagement and cohesion of the fibers; and thence evaporating the liquid to intensify the tightening and increase the mechanical interengagement and cohesion of the fibers.

2. A method according to claim 1, wherein said agitating step comprises agitating the fibers in a rotary tunnel having a substantially horizontal axis, the tunnel being filled with fibers to between two thirds and four fifths of its volume.

3. A method according to claim 1, including the steps of grading the fibrous mass resulting from the dry agitating step before said depositing step to separate fibrous aggregates of a predetermined size from pulverulent products, fibers that have remained free, loose fiber groups and fiber groups of less than a predetermined size, thereafter disintegrating the fiber groups to free the fibers that constitute them, collecting the free fibers removed during the grading step and the free fibers obtained by said disintegrating step and mixing the collected free fibers with fresh fibers and subjecting them to the dry agitating step, and depositing said liquid solely on the fibrous aggregates of predetermined size separated during the grading step.

4. A method according to claim 1, wherein said depositing step comprises atomizing the liquid on the fibrous aggregates.

5. A method according to claim 3 including the steps of spreading the separated fibrous aggregates in a layer of substantially uniform thickness and vibrating the aggregates during distribution of the liquid.

6. A method according to claim 1, including the step of drying the fibrous aggregates after distribution of the liquid.

7. A method according to claim 1, wherein the liquid deposited on the fibrous aggregates is water and the mean moisture content thereof after deposition is from 50 to 85%, based on the total weight of the moist aggregates.

8. a method according to claim 1, including the step of forming additional connections between the fibers to reinforce the physical and mechanical action of tightening of the fibers due to dampening with the liquid.

9. A method according to claim 8, wherein the fibers are thermoplastic and additional connections are obtained by subjecting fibers tightened by the liquid to a thermal treatment which produces local welding between the fibers.

10. A method according to claim 8, wherein the fibers are thermocontractile and the additional connections are obtained by subjecting the granulates tightened by the liquid to a thermal treatment which intensifies this tightening.

11. A method according to claim 8, wherein the damping liquid contains a binding agent which causes localised gluing between the fibers when the granulates tightened by this liquid are dried and additional connections are thereby produced.