IE54017B1 - Improvements in fibre formation techniques comprising centrifugation - Google Patents

Abstract

1. Process for the formation of fibres from a thermoplastic material in which the material is conducted in a drawable state into a centrifugal device (1) provided at its periphery with orifices, the material is projected from the centrifuging device from these orifices in the form of filaments which are entrained and drawn out by a high temperature gaseous current directed along the periphery of the centrifuging device and transverse to the direction of projection of the filaments, characterised in that the peripheral velocity of the centrifuging device, at the level of the orifices from which the filaments emerge, is at least equal to 50 m/s.

Description

This invention relates to improvements to fibre formation techniques comprising a centrifugation operation.

More specifically, the invention relates to ''x techniques in which the material for processing is introduced in the drawable state into a centrifuge having multiple orifices at its periphery. By the effect of centrifugal force the attenuable material passes through these orifices and forms filaments or primary fibres. An annular stream of gas at a high temperature flows along the peripheral wall of the centrifuge. Filaments are projected into this stream of gas in which the attenuation of the filaments is completed and the resulting fibres are entrained.

In these techniques, the resulting fibres are conveyed by the gases as far as a receiving means which is usually a gas-permeable conveyor belt.

They are deposited in the form of a layer or mat.

When the final product is required to be cohesive bo some extent, the fibres are coated with a binder during their passage towards the receiving means.

The layer of coated fibres passes through a thermal processing Chamber which causes the binder to be polymerized. It is then cut and packed in the form of the final product.

These fibre formation techniques in which attenuation is due both to centrifugation and the tractive 4 017 effect exerted by the hot stream of gas are particularly advantageous. As a whole, they enable insulants to be produced with the most satisfactory qualities at an advantageous Cost under satisfactory economic conditions. The orifice system delivering me primary filaments to some extent ensures good uniformity in the formation of the primary fibres.

The formation of uniform primary fibres is an essential condition to the production of perfectly attenuated final fibres. Also, the use of hot gases to complete the attenuatioipperation enables a relatively considerable material output to pass through each orifice and so the production capacity of each centrifuge is high, and this reduces the unit costs.

These very sketchy reasons are certainly inadequate to account for the advantages of this type of process. However, they do show how the processes of the type according to the invention differ from processes which appear similar and which are very different in respect of their operation and results.

By way of information, there are techniques, for example, in which attenuation iseffected solely by centrifugation. These processes do not make use of a stream of annular gas surrounding the centrifuge and such gas does not therefore participate in the attenuation of the fibres. The alleged advantage of these techniques is to save the energy consumption for producing the high-speed high-temperature drawing gas. However, the saving obtained in this way has the effect of greatly reducing the production capacity of - 5 4017 the centrifuge and, overall, the production costs are \ not reduced.

As already indicated, the production of the fibres, and hence the production of the products made with these fibres, is subject not only to quality requirements but also certain demands in respect of production costs. As will be seen in greater detail hereinafter, these requirements affect numerous aspects. With regard to quality, this initially involves the insulating. properties of the products, although the mechanical properties also play a very important part. With regard to costs the diversity is even greater since consideration must be given to investments, labour, raw materials, energy, and so on.

The prior art discloses methods of .producing fibres and insulating products with better qualities than those conventionally obtained by the most usual processes, but such disclosure is of methods which are of little advantage in respect of the production costs, for example either because of low production or because the energy consumption is too high, or again because expensive fibre forming compositions have to be used.

The improvements previously proposed for this type of process to which the invention relates are extremely varied and this is so even if we confine ourselves to just the provisions concerning the fibre-formation stage, thus disregarding everything concerning the subsequent processing to which the fibres are subjected to obtain the final product.

Amongst the most recent publications in this connection, Applicants1 French Patent Application No. 2 443 436 contains a number o£ proposals relating more particularly to the nature of the fiberized material, a group of features intended for better control of the temperature and the passage of the material through the aentrifuge, specific structural details of the centrifuge itself, and so on.

Other prior-art publications such as French Patent No. 1 382 917, which also belongs to Applicants, relate to improving the conditions under which the primary fibres are projected into the drawing gases.

These two examples represent only a very small proportion of the literature, which is particularly plentiful on this subject. They show how diverse the proposals can be for improving this type of process.

Although the desired object, i.e. improvement of the quality-cost picture, is quite clear in every case, this is not the case with regard to the technical means to be used for this purpose.

The object of this invention is to provide means whereby mineral fibre forming processes of the type described hereinabove can be improved. It is an object of the invention in particular to increase the production capacity for a constant quality or improve the quality for a constant capacity.

The invention proposes, inter alia, to provide new operating conditions for the centrifugation means in order to achieve the results indicated. - 5 5 4 017 The invention also proposes that the insulating products obtained by the new features should have advantageous mechanical and insulating properties without the production cost being prohibitive.

The inventors have shown by their research that it is possible to achieve these objects and other results indicated hereinafter, by selecting well defined centrifuge operating conditions for the fibre formation stage.

Generally, according to the invention, centrifugation is effected, with a relatively high peripheral speed, i.e., a relatively high speed of movement of the orifices at the periphery of the centrifugation means.

The present invention provides a process for the formation of fibres from a thermoplastic material in which the material is conducted in a drawable or attenuable state into a centrifugal device provided at its periphery with orifices the material is projected fron the centrifuging device fran these orifices in the form of filaments which are entrained and drawn out or attenuated by a high tsiperature gasecus current directed along the periphery of the centrifuging device and transverse to the direction of projection of the filaments, wherein the peripheral velocity of the centrifuging device, at the level of the orifices fron which the filaments emerge, is at least equal to 50 m/s. - 6 There has been no satisfactory explanation why it is possible to improve the quality of the products by using a high peripheral speed. To some extent, in fact, this result is fairly surprising. It is well known that the attenuation of filaments of material leaving orifices at the periphery of the centrifuge is a complex phenomenon involving the movement of the centrifuge and of the stream of gas which entrains the fibres. The result of these two effects simultaneously is that it is impossible to determine with certainty the role of each of them in the attenuation mechanism and the effects that they produce.

If, nevertheless, we attempt to analyze the phenomenon, it can reasonably be assumed that the fibre is obtained by a process that can be likened to mechanical drawing, the filament being held at one end by the centrifuge and at the other end by the 54017 - 7 friction exerted by the ambient air. In this theory, the relative movement of the two ends of the fibre is the result of the combination of the rotary movement of the centrifuge and the movement of the stream of gas.

The direction followed by the filaments on leaving the centrifuge does appear to show that such a combination of movements is effectively involved in the drawing process. Projection of the path of the fila10 ment into the plane tangential to the centrifuge forms an angle with the direction of the gas flow (which is usually parallel to the centrifuge axis), and it will be found that this angle is in fact a function of the ratio of the peripheral speed of the centrifuge to the speed of the gases.

Under conventional operating conditions, the angle is relatively small, for the speed of the gases is much higher than the peripheral speed of the centrifuge.

This would tend to stress the predominating part played by the entrainment by the stream of gas in the fibre attenuation mechanism.

In the foregoing we have considered only the projection of the development of the fibre into the plane tangential to the centrifuge. Experimentally it is also found that the filaments move away from the peripheral wall very little, so that the remarks in connection with the projection also apply to the real path of the fibre. - 8 This having been said, purely geometric considerations that we shall analyze in greater detail hereinafter ought to have lead to the conclusion that in order to improve drawing of the fibres on the basis of conventional operating conditions an increase of the speed of the gases would have been the best solution.

In fact it appears that, with the other conditions unchanged, when the speed of the gases is increased substantially in order to try to obtain extremely fine fibres, products are obtained whose qualities are substantially less satisfactory, and in particular these fibres are very irregular, short and fragile while the insulating felts prepared with thermal these fibres have poor mechanical and(to some extent properties.

The inventors have surprisingly found that very substantial improvements in the quality of the insulating products are obtained if the peripheral speed is increased, all other conditions remaining unchanged.

Although it is desirable to have a high peripheral speed, it is obvious that technical considerations mean that this speed cannot he increased indefinitely.

In practice it is difficult to obtain speeds above 150 m/s and the speed according to the invention is usualfy between 50 and 90 m/s.

The peripheral speed is particularly limited by mechanical strength considerations of the centrifuges used. Although periodic replacement of the centrifuge is inevitable, the life under the operating conditions according to the invention must remain compatible with the requirements of technical reliability, and cost, 540 17 - 9 which are inseparable factors in any industrial undertaking.

Replacement of the centrifuge under conventional Operating conditions is usually necessary after some hundreds of hours of operation. It is important that the life under the conditions according to the invention should not be shorter than that with the prior-art processes and, if possible, even substantially longer.

A limitation of the speed of rotation - and hence limitation of the stresses experienced by the centrifuge without, obstructing the above-defined conditions, therefore appears to be particularly desirable for the use of the invention. Taking into account the materials conventionally used, it would appear advantageous not to exceed the limit of 8 000 rpm.

Although the speed of rotation is limited to prevent the centrifuge from being subjected to excessive stresses, the centrifugal effect must still be adequate to project the material to be formed into fibres from the centrifuge orifices under satisfactory conditions in respect of output. It is therefore preferable for the speed of rotation to be no less than 800 rpm.

In practice with regard to the choice of a speed of rotation and a peripheral speed, the problems due to the limits of the strength of the equipment usually mean that the highest peripheral speeds are associated with the lowest speeds of rotation, and vice versa. This procedure is not necessarily optimization as regards the formation of the product but is a - 10 compromise to ensure that the installation has a reasonable life.

The output of the material through each of the orifices of the centrifuge is a very important factor in this type of process. Of course this output directly determines the total production capacity of the centrifuge. Less obviously, this output per orifice is also a factor which substantially influences the characteristics of the fibres produced.

It will readily be seen that the greater the output per orifice, the more intensive the attenuation must be to obtain a constant fineness.

In the light of these practical production requirements, the output per orifice cannot be very low.

For a material comprising glass, or a similar material, an output of 0.1 kg/day may be considered as a bottom limit beyond vhich it is uneconomic to operate.

Conversely, an. excessive output does not enable very fine fibres to be obtained. For the qualities required of an insulating material the material output is preferably not more than 3 kg/day.

Advantageously, the compromises between the production capacity and the quality of the fibre result in the output per orifice being controlled to a value of between 0.7 and 1.4 kg/day.

Under the conditions of the invention, the production capacity of the centrifugation means may reach very considerable values without the quality of the fibres or of the end product being reduced. of course it is also possible to keep the production at a lower level. 4 C 1 7 - 11 more particularly in order to improve the quality still more.

The advantages provided by the invention in this respect are quite clear.

The production increase, which is an important economic factor, is of particular advantage. It has been the subject of numerous proposals in the prior art: an increasing number of centrifuge orifices, an increased orifice section, increased material fluidity, and so on. These various elements effectively modify the output of material through the centrifuge, but the proposals made in this connection have resulted in a reduction in quality or difficulties in respect of the life of the equipment.

Thus an increase in the density of the orifices at the periphery of the centrifuge, i.e. an increase in the number of orifices per unit area, rot only weakens the centrifuge but, beyond a certain threshold, appears to have an unfavourable effect on the fibre quality.

It may be assumed that the primary fibres, which are verydose to start with, collide and stick together before their attenuation Complete. This may be why the fibres collected under these conditions are less uniform, less fine and shorter.

A problem of the same kind is encountered when again to increase production capacity - the orifice section is increased while maintaining identical viscosity conditions of the material formed into fibres. In this case the primary fibres are coarser and it is difficult to obtain satisfactory drawing even if the 540 17 - 12 conditions of the annular attenuating gases are modified.

An increase in the fluidity for a given material, i.e. an increase in its temperature, gives rise to other problems. The operating temperatures are usually at the limit of what the centrifuge alloy can withstand. The nature of the composition to be formed into fibres is also often so selected as to allow for this type of limitation. It is impossible to increase the temperature on this hypothesis unless the centrifuge is made from precious metals, and this gives rise to other difficulties, particularly in respect of costs, but also mechanical characteristics.

An increase in fluidity can also be obtained by using materials of a composition such as to achieve this result, but these compositions have the disadvantage of being substantially more expensive.

Conversely, in the prior art literature, methods intended to improve the quality result in reduced productivity.

With the invention, although there is still the quality/quantity conflict, the result is at a much better level. Thus if good quality production is the objective (how this is evaluated will be explained hereinafter) comparable to that produced previously with this type of process, a production of the order of 12 tonnes of fibre material per day and per centrifuge is readily exceeded.

In practice, it appears advantageous according to the invention to maintain production at a level above 17 tonnes per day, and production of 20 to 25 4 017 - 13 tonnes or more can usefully be achieved while maintaining a product of excellent quality.

It is noteworthy that these results have been achieved without the use of the modifications indicated hereinbefore. There is no need to change the orifice density nor the orifice dimensions etc. The primary fibres formed are therefore as a whole comparable to those of the similar prior-art processes, and since drawing is improved, as we indicated Lo previously, the quality of the product is better.

Drawing of the fibres by the currents of gas is carried out by means of a generator, the annular orifice is situated in the immediate vicinity of the centrifuge. The gas flow is of a certain width so that the filaments projected from the centrifuge remain fully immersed in this flow of gas and are theitfore kept under the conditions adapted to their drawing.

The width of the flow at the start actually depends 20 on the exact geometric configuration of the fibreforming system. For a given arrangement, the useful variations are relatively limited inasmuch as to reduce the energy expenditure it is necessary to reduce the width of the gas flow as much as possible.

In the initially conventional systems the width of the flow of gas is of the order of 0.3 to 2 cm.

The pressure of the gas emitted under these conditions is between 100 and 1 000 mm water column for an emission width less than 2 cm, and is preferably from 200 to 600 mm water column for an emission width - 14 54017 Iowa: than 1.5 cm.

In analyzing hereinabove the respective effects of the peripheral speed, the material output, and the pressure of the drawing gases, we repeatedly found it necessary to specify that in practice these various parameters together.affect the quality or cost of the resulting products. Relationships without any exact physical signficance can be drawn between these various parameters to allow for the most advantageous operating conditions proposed by the invention.

Also, independently of these operating conditions, these relationships must include an index of the quality of the product obtained. In this way these relationships enable a clear distinction to be made between the operating conditions according to the invention and those of the prior art.

Thus, according to the invention, the peripheral speed v of the centrifuge orifices, expressed in metres per second, the mass of material 3 passing through each orifice in the centrifuge, expressed in kg p>er orifice and per day, and the pressure of emission 3 of the drawing gas expressed in mm water column for an emission 'width of at least 5 mm and maximum 12 mm are so selected that the ratio q.v/p is between ° °7 and 0.5 and preferably between 0.075 and 0.2.

In order that the features of the invention may be more readily apparent it is advantageous to introduce a parameter referring to the fineness of the fibres.

The fineness is not the only criterion to qualify the products, as we shall see hereinafter. We are selecting it because it is undoubtedly the most accessible 540 17 - 15 and most significant parameter for the attenuation ' quality obtained.

The fineness of the fibres is conventionally determined indirectly by the micronaire fineness (F).

Micronaire fineness is measured as follows: A sample of the product, usually 5 g, is disposed in a chamber through vfoidi passes a stream of gas emitted under defined conditions, particularly in respect of pressure.

The sample through which the gas flows forms an obstacle which tends to brake the passage of this gas. The rate, of flow of the gas is indi cated on a graduated flowmeter. These values are defined for standard conditions indicated.

The finer the fibres for a given weight of sample, the lower the rate of flow.

The most usual micronaire fineness values for insulating products are between 2 and 6 in the case of 5 g. Products of a micronaire fineness below or equal to 3 in the case of 5 g are considered as very fine. 2o They generally have a better insulating power per unit mass of product.

According to the invention, the ratio q.v/p.F is advantageously greater than 0.035, q.v, p and F (in the case of 5 g) being expressed in the units indicated above. This ratio is preferably greater than 0.040.

When specific pressure conditions are used, the invention can also be characterised by a simplified q.v/F ratio. This ratio in the case of the invention is advantageously greater than 17 and preferably greater than 18. 4 0 17 - 16 In the foregoing expressions, the ratio q/F in some way shows the advantages of the process according to the invention. Its value is all the higher, the greater the quantity and the finer the product produced. The parameters v and p for their part may be associated with the way in which the fibres are drawn. At all events, the expressions used are intended, as already stated, only to allow the invention to be properly distinguished from the prior art.

Our advantage of the kind of process to which the invention relates, is that it allows the use of a large variety of material compositions for producing fibres. Among these compositions, the following glass compositions are particularly advantageous: Si02 61 - 66 Na20 12.5 - 16.5 Ai2°5 2.5 - 5K2 0-3 CaO 6 - 9B2°3 0 - 7.5 ' MgO 0 - 5Pe2°3 less than '6 These compositions are given in percentage by weight. Other conditions are involved in performing the processes according to the invention. These relate more particularly to the material (temperature) and the stream of drawing gas (temperature). These conditions are not appreciably different from the corresponding conditions described in the prior art and it therefore appears unnecessary to repeat them here. 54G17 - 17 Other advantages and characteristics of the invention •will be apparent from the description and exemplified embodiments, which are intended only to illustrate the invention in greater detail and have no limiting force.

The following description relates more particularly to the type of apparatus used. Reference will be made to the accompanying drawings wherein: Fig. 1 is a diagrammatic perspective view of a 10 centrifuge and the path of a fibre formed at the periphery thereof. - 18 5 4 017 Fig. 2 illustrates the projection of the path of the fibre in Fig. 1 into the plane tangential to the centrifuge.

Fig. 3 is a section of the fibre-forming apparatus 5 of use according to the invention.

Fig. 4 is a larger-scale view of part of Figure'3 showing the different elements involved in the zone where the fibre drawing is effected.

Figs. 1 and 2 are intended to show the respective 10 influences of the centrifugation and the stream of gas on the drawing of the fibres.

Figure 1 diagrammatically illustrates a centrifuge, the peripheral wall of which is formed with orifices.

The path of a fibre F is also shown in this Figure, as is the plane T tangential to the wall of the centrifuge at the place where the fibre F leaves.

The direction of rotation of the centrifuge is shown by an arrow.

Although shown diagrammatically, the path of the 20 fibre in this Figure shows the essential features of the characteristics actually observed.

It will be seen that the material moves away from the wall of the centrifuge but, after a very short distance, the path is deflected as a result of the hot gas stream moving along said wall.

After this the-development of the fibre approximately follows the direction corresponding to the combination of the speeds of the centrifuge and of the stream of gas. 4 0 17 In the plane tangential to the centrifuge, the projection of this combination is of the type shown in Fig. 2, O being the orifice and the angle being the angle formed by the projection with the direction of propagation of the gas denoted by the vector OG.

The angle (X. . under the conventional centrifuge speed conditions (denoted by vector OP) and gas speed is of the order of some twenty degrees or less.

It will be seen that an increase in the speed of the centrifuge, even if relatively considerable (as shown in broken lines) ought not. a Priori· to have changed the attenuation of the fibres very substantially.

The intensity of this drawing can be represented by —·—7* —j, the sum of the vectors OG and OP. Any increase in OP results in only a small variation of the sum because the angle (%. is small. To improve drawing it would have appeared advantajeous, on the contrary, to increase the speed of the gases. We shall see in the exemplified anbodiments that it is rather the reverse that is found, and that in order to have fibres which are better drawn and of better quality it is preferable to increase the peripheral speed of the centrifuge.

We shall now see in greater detail the makeup of the fibre formation installation.

The fibre formation system shown in Fig. 3 comprises a centrifuge denoted generally by reference 1. The centrifuge is disposed horizontally and borne by the assembly 4 which forms the shaft of the system. This shaft is disposed in a bearing casing 5. The shaft 4 together with the centrifuge is rotated by a 4 0 I i - 20 motor 6 through the agency of transmission means (not shown) such as belts.

The annular hot gas stream is produced by a burner having the general reference 2.

A blower ring 3 concentric with the centrifuge emits an annular air flow at low temperature.

A high-frequency induction-heating element 12 of annular shape surrounds the centrifuge at the bottom.

To obviate stresses and strains various insulating or cooling elements (not shown) are disposed at suitable locations in the system.

The centrifuge 1 described with reference to Fig. 4 where applicable is constructed as follows: It is secured on the hollow shaft 4 by means of the hub 9. A discoid rim 16 connected to the hub 9 supports a distributor basket 11 and the part 13 of the centrifuge.

The peripheral wall 7 of the centrifuge from which the primary fibres are projected is connected to the part.13. This peripheral wall is slightly inclined with respect to the vertical so that the gases produced by the burner 2 flow uniformly along its entire surface.

The resistance of the centrifuge to deformation is reinforced by a lining 8 which follows and is connected to the peripheral wall 7.

Because of their fineness, the orifices through which the material to be formed into fibres is centrifuged are not shown in Figs. 3 and 4. They are distributed uniformly in horizontal rows. The orifices 54G 17 - 21 of two consecutive rows are offset from one another to ensure maximum spacing hetween adjacent orifices.

The centrifuge structure must be so designed as to withstand considerable mechanical stresses. It is. however, preferable to keep the bottom part open.

This arrangement, together with the facilities that it provides in respect of the centrifuge constructbn, obviates any deformation which might result from the temperature differences between the periphery and the base of the centrifuge.

The path of the material to be formed into fibres through the system illustrated is as follows: A continuous stream of molten material flows along the axis of the hdlow shaft 4. This material is received at the bottom of the distributor 11. Rottion of the latter leads the material to the peripheral part of the distributor 11, which is formed with multiple orifices. The material is projected from these offices by centrifugation to the inner surface of the 2o peripheral wall 7 of the centrifuge 1 where it forms a continuous layer. Still under the effect of centrifugation the material passes through the orifices of the wall 7 and is projected in the form of thin streams into the drawing gas where its attenuation is complied to form fibres. These gases are produced hy the nozzle 15 of the burner 2. The annular blower 3 produces a layer of gas which envelops the stream of attenuating gas aqd channels it more particularly to prevent the fibres from striking ihe ring 12. 4 G 1*7 - 22 The distributor 11 plays an important part in providing the drawing conditions. A first reserve forms there, which evens out the flow of material.

As a result, the material can particularly be distributed very uniformly over the entire inner surface of the peripheral wall 7 of the centrifuge.

A detailed study of the distributor basket shown in the aforesaid French Patent Application No. 2 443 436 will show certain characteristics of importance to the proper operation of the system. These are more particularly the size of the orifices situated at the periphery of the basket from which the material to formed into fibres is projected on to the inner surface of the peripheral wall 7, and the spatial arrangement of the basket and, particularly, of the orifices therein in relation to the centrifuge. For further detail on this matter reference should be made to the said patent.

The centrifuges for performing the invention, in 2o · addition to the features already enumerated, may differ from similar prior-art devices in respect of t very much greater dimensions. Advantageously, the diameter of the centrifuge at its peripheral wall is equal to or greater than 500 mm and it may reach and 25 even exceed 1 000 mm, whereas the centrifuges currently used to date are not greater than 400 mm.

This increase in the diameter of the centrifuge plays an important part in respect of numerous characteristics for the performance of this type of process. Thus the increased perimeter of the centrifuge, without any change in the height of the peripheral wall, increases the area available for the orifices. without changing their distribution, more particularly without changing their number per unit area, it is thus possible to provide a larger number of orifices and hence increase the production capacity. The production improvement obtained in this way is achieved without affecting the features which determine the quality of the fibres, and the increased IO effect due to the peripheral speed in relation to the produced by the stream of gas under the conditions defined by the invention even enable the said quality to be improved.

Although it is preferable to use centrifuges with diameters greater than the conventional dimensions, the use of centrifuges with diameters of the order of 200 , 300 or 400 mm under the conditions specified^ according to the invention has certain advantages. Operation at peripheral speeds above 50 m/s particularly 20 provides an improvement of the qualities of the resulting insulating products, irrespective of the centrifuge dimensions.

For conventional centrifuges, an increase in the peripheral speed necessitates an increase in the speed of rotation, which may reach up to about 8 000 rpm or more. As we have already indicated, there are practical limits to the speed of rotation, which are determined by the resistance of the centrifuge to the stresses to idiich it is subjected during operation. 4 ,j J ? For the very large diameter centrifuges according to the invention, the speed of rotation is preferably between 1 000 and 2 250 rpm.

For the proper operation of the system, the conditions should be absolutely identical at every point of the centrifuge periphery. Xt is necessary in this sense for the stream of high-temperature gas to be uniform.

The stream of gas is most usually formed from a number of combustion chambers distributed uniformly around the centrifuge. Fig. 3 shows a chamber of this type at 14. The combustion gases produced in chamber 14 are led to near the peripheral wall of the centrifuge and escape through an annular nozzle 15 common to the various combustion chambers 14 together.

When the respective dimensions of the centrifuge and of the attenuating generator are not too large, theje is no undue difficulty in rendering the stream of gas uniform. The gas emitted has substantially the same characteristics over the entire extent of the nozzle 15. This becomes increasingly more difficult with larger dimensions, as may be the case according to the invention.

To facilitate the production of uniform blowing conditions at the output of the burner, it is advantageous to use a single combustion chamber surrounding the centrifuge. This single annular combustion chamber provides better equilibrium of the pressures and gas flows over the entire width of the nozzle 15. 4 o i 7 - 25 At this point in the description and before examining the exemplified embodiments and resulting products the magnitudes shculd be specified which characterise the products in order -that the nature of the improvements provided by the invention may be more readily understood.

On these lines we shall essentially consider the production of insulants in the form of a felt or mat of fibres. These products by themselves represent a considerable proportion of all the applications of mineral fibres. Of course this does not exclude the use of the invention for the preparation of products intended for other purposes.

As already stated, the two main types of quality that are required are thermal insulating qualities and mechanical qualities. The latter relate to quite specific aspects of the insulating products. More - particularly, the product should lend itsLf to the handling and packing dictated by voluminous products 20 of low' specific gravity. For a given product, the insulating quality is defined by its thermal resistance R. This expresses the capacity of the product in question to withstand heat changes when the product is subjected to different temperatures on either side.

This value depends not only on the characteristics of the fibres and their arrangement within the product, but also on the product thickness.

The thicker a product, the greater its thermal resistance. If, therefore, the thermal resistances of different products are to be compared, the thickness - 26 of these products must be specified.' We shall see \ hereinafter that the question of thickness is closely bound up with the mechanical properties of the products. The literature also occasionally refers to thermal conductivity, a magnitude which disregards the product dimensions. It is to some extent an intrinsic measurement of the insulating quality. In practice , however, it is the thermal resistance that is most frequently used to characterise the products. It is therefore this 10 magnitude that we use in the examples. Despite numerous studies carried out on this subject, it is still impossible to establish a complete correlation between the thermal resistance, or the thermal conductivity, and the measurable data concerning the 15 structure of the product. Nevertheless, some considerations enable the production to be oriented according to the required result.

. Thus a larger quantity of fibres for a given covered area enables the insulating properties to be 2o increased, but this increase is accompanied by an increasing cost. Wherever possible, therefore, it · is advantageous to have a given thermal resistance with the minimum number of fibres. A comparison of the specific weight, i.e. the mass of fibres per unit. area for different products of the same thermal resistance, gives a measurement of their respective qualities. The quality of the product in this-case varies in inverse proportion to the specific weight.

It has also been established that the fineness The mechanical magnitudes of use for an assessment of the product are essential^ bound up with the problem of its packing.

The reason for this is that the products are very bulky and are advantageously stored in the compressed state. Nevertheless, the compressed product must resume a certain bulk once it has been released’ in order to develop its insulating qualities to the maximum.

Before compression, the fibre mat itself assumes a certain thickness which is different from the thickness before compression and from the thickness in the compressed state. Xt is desirable that the thickness resumed after unpacking should be as large as possible to obtain a product having good thermal resistance.

More specifically, for an unpacked product of standardized characteristics, and particularly of 20 guaranteed thickness, it is important that it should occupy a minimum volume in the compressed state. This guaranteed thickness resunption is also expressed by the admissible compression ratio.

It is difficult to establish a close relationship between the mechanical and the insulating qualities of a product, even if the attempt is made to analyze them in terms of structure, fibre dimensions, and so on. Experiment simply shows that the improvement of the insulating qualities is compatible with the - 28 improvement of the compression ratio.

Examples of performance of the invention are given below. 1. Comparative tests These comparative tests were carried out to show the advantage of using the conditions specified by the invention as compared with the prior-art operating conditions.

One difficulty in setting up these comparative 10 tests is, as already stated, the variety of factors likely to influence the quality of the final product. As far as possible, the attempt has been made in these tests to find operating conditions which enable the parameters sensitive to various influences to be kept constant.

The following table summarizes the various characteristics and the results obtained in respect of the prepared product. The first three tests were carried out under operating conditions not equivalent to the invention.

I IX III IV Peripheral speed (v)38. 38 38 56.5 Output per orifice (q) (kg/day) 1.1 1.30 1.16 1.1 Pressure (p) (mm water column) 758 1020 650 470 q.v/p 0.055 0.048 0.068 0.13 Micronaire fineness at 5 g (F) 3 3 4.5 3 q.v/p.F 0.0184 0.016 0.015 0.043 q.v/F 13.9 16.4 9.79 20.7 Total centrifuge output (tonnes per day) 9 18 18 18 Specific weight for R = 2 (g/m2) 945 1080 1215 945 Compression ratio 4 2 4 4 Tensile strength (g/g) 250 150 250 250 The quality of the products prepared is defined in this Table more particularly by the specific weight required for a given thermal resistance for a likewise given thickness. As a reference we chose the thermal resistance of 2 m2oK/tf measured at 297°B for a 20 thickness of 90 mm. These conditions are equivalent to a standard insulant.

The compression ratio is the ratio of the guaranteed nominal thickness after 3 months’ storage in the compressed state to the thickness of the compressed product.

The tensile strength is measured to the ASTM standard C 686 - 71 T Several conditions can be drawn from the above Table.

For example, a comparison of test IV according to the invention and test II shows that for a constant product fineness and the same total centrifuge output, the conditions specified by the invention give a better quality product (lower specific weight), a substantially improved compression ratio and better tensile strength.

For a given quantity of fibre material, the improvement found in the specific weight enables the quantity of product to be increased by about 10%. The improvement of the compression ratio gives a very considerable saving in respect of transport and storage costs.

If, as in the case of Examples I and IV,· the operating conditions are adjusted to have the same product qualities, it will be found that the centrifuge output in the method according to the invention is increased in considerable proportions, thus making the process decidedly more cost-effective.

Another way of comparing the invention with the prior-art methods is to establish conditions such that the output and mechanical qualities are identical.

In that case, as in Examples III and IV, the conditions according to the invention give an appreciable reduction in the fibre fineness, showing that the fibres produced are finer, and that is why the specific weight is lower for the same thermal resistance. 4 017 — 31No matter how the procedure according to the invention is considered, it provides an appreciable improvement over the prior-art methods, as confirmed by the results of the following tests carried out under other operating conditions. 2. Tests in respect of variations of various parameters Various other parameters were, modified according to the required product qualities while remaining in the conditions according to the invention.

The results of these tests are gitai below: V VI VII VIII IX Peripheral speed (v)rn,4s 56.5 56.5 75 56.5 71uutput per orifice (q) (kg* per day) 1 Q.9 0.7 1.25 1 Pressure (p) (mm water column) 160 325 270 440 300 q.v/p 0.35 0.15 0.19 0.16 0.24 Kiconaire fineness at 5 g (F) 4 3 3 4 3 q.v/p.F 0.038 0.052 , 0.064 0.040 0.078 q.v/F 14.12 16.95 17.5 17.65 23.6 Total centrifuge output (tonnes/day) 18 18 18 25 20 Specific weight for R = 2 (g/m2) 1080 945 915 1140 865 Compression ratio 4 4 - 3 4 Tensile strength (g/g) 300 250 - 250 250 Example V is an embodiment under the conditions of the invention to prepare a product comoarable to that of Example IV. The mechanical properties are fully retained. The micronaire fineness goes from 3 to 4, i.e. the fibres are slightly less fine. The specific weight is thus slightly greater. 4 0 1 The type of production corresponding to Exanple V is advantageous although the products have a higher specific weight than those in Exanple IV, because under identical centrifugation conditions the change of the fibre fineness is due to differences in the operation of the burner emitting the hot attenuating gases, energy consumed for operation of the burner in the case of Example V is considerably reduced by comparison with Example IV, and this can usefully make up for the increase in specific weight.

Although it is impossible to give an exact comparison because the fibre fineness is not strictly the same in both cases, extrapolation shows that for a 4.5 micronaire fineness the specific weight in the conditions of Example V would be less than that of Example III. This confirms what is apparent from a comparison of Examples II and IV, i.e., that the quality . of the insulating products produced according to the invention is better than that of the products obtained 'in the prior-art conditions, even.if this difference is greater for the finest products.

Example VI is a similar embodiment to Example IV but with the peripheral speed maintained and the speed of rotation reduced. The properties of the products are identical except for experimental approximations.

Example VII is also interesting on the same lines. The influence of the peripheral speed on the quality of the prepared insulating product is again clearly apparent from the specific weight.

Example VIII relates to a test for an output of tonnes per day, which does not represent a limit for - 33 the method according to the invention but which very clearly distinguishes it from the prior-art systems in this area when good quality fibres are required. The micronaire fineness is still relatively low and so is the specific weight, although slightly higher than Example V.

The remarkable increase in production thus obtained amply makes up for this slight increase in specific weight required to achieve the desired insulating 1° properties.

Example IX is another embodiment of the invention distinguished both by a high peripheral ^eed and a relatively low speed of rotation. These conditions enable excellent mechanical and insulating qualities 15 to be achieved (the specific weight is particularly low) for a high centrifuge output. 3.' Tests with a constant σ.ν/ρ ratio These tests were carried out with a 300 mm diameter centrifuge having 11 500 orifices.

. The output per orifice was kept constant and at the same time the tangential speed and the pressure of the burner were increased approximately in the same proportions to maintain a substantially constant q.v/p ratio.

The following table shows the results for four operating conditions: 4 ίϊ ί 7 IX X XI XII Peripheral speed (v) 68 76 85 100 Output per orifice (q) (kg per day) 1 1 1 1 Pressure (p) (mm water column) 410 460 510 600 q.v/p 0.165 0.165 0.166 0.166 Micronaire fineness at 5 g (F) 4.0 3.5 3.0 2.5 q.v/p.F 0.041 0.047 0.055 0.066 q.v/F 17 21.7 28.3 40 Total centrifuge output (tonnes/ day) 11.5 11.5 11.5 11.5 This Table shows that in the conditions according to the invention, variation of the variables v and p for a given unit output enables the fibres to be made finer in certain limits while at the same time increasing the peripheral speed and burner pressure, i.e. .the speed of the gases. The result is. a very substantial reduction of micronaire fineness and hence the useful specificweight. It is also possible to obtain a micronaire fineness as low as 2 for a material output remaining relatively high. a similar experiment but without changing the burner pressure shows that the unit cutout and hence the total output of the system must be substantially reduced to obtain a very low micronaire fineness.

The same centrifuge was used in this series of tests.

XIII XIV XV XVI XVII Peripheral speed (v) 63 72 81 90 108 Output per orifice (q) (kg per day) 1.3 1.14 1.0 0.9 0.76 Pressure (p) (mm water column) 410 410 410 410 410 q.v/p 0.20 0.20 0.20 0.20 0.20 Micronaire fineness at 5 g (F) 4.5 4.0 3.5 3.1 2.7 q.v/p.F 0.044 0.050 0.056 0.063 0.074 q.v/F 18.2 20.5 23.1 26.1 30.4 Total centrifuge output (tonnes/ day) 15 13 11.5 10.3 8.8 The above two operations, in which the peripheral ' speed and the pressure are simultaneously increased or alternatively the material output is reduced in order to increase the -insulating qualities of the product do not give completely equivalent results.

It has been found that the mechanical qualities of the resulting felts are substantially better in the case,of products obtained with a low burner pressure, as· indicated previously.

These differences are due to a better.quality of the fibre produced for a relatively moderate drawing effect of the stream of gas as compared with that resulting from the rotation of the centrifuge.

In the foregoing exanples we have shown, by cettain comparisons, differences in respect of the manufactured 25 products, and we have pointed out that theee differences - 36 5 4 017 affect the production oosts. It is· difficult exactly to evaluate the economic gain provided by the invention industrially. Approximately, taking into account • the various factors involved, such as energy, product 5 and equipment, the estimated saving may be as much as 10% of the unit cost or more depending upon the conditions selected.

During these tests analyses were carried out to try to determine the features of the fibres 10 produced according to the invention as compared with fibres produced by the prior art in order to try to give details of the improvements found, particularly by evaluating the fibre fineness for determination of the micronaire fineness. Apart from this indirect 15 approach it is difficult to determine the characteristics of the fibres, and particularly their.length.

However, a number of findings lead to the conclusion that the operating conditions according to the invention promote the formation of longer. 2o fibres. There is the improved tensile strength.

There is also the ability of the products to withstand a higher rompression ratio and the thickness of the fibre mat leaving the receiving chamber before entering the thermal processing chamber for the binder.

The conditions according to the invention give a substantial thickness increase, which may be as much as or more than 25% while the other conditions, more particularly material output, remain identical.

All these aspects to some extent may be associated with an increase in fibre length. 4 0 17 - 37 Also, after continuous operating tests for more than 200 hours, it was found that the centrifuge wear was not such as to necessitate replacement.

These are only a few examples of numerous 5 features found in performing the processes under study in the conditions of the invention.

They show the nature of the impro\enents provided by the invention and also the type of advantage arising therefrom to the user.

The production of fibre mats for insulation purposes has been studied because it is a particularly significant application of fibre-forming processes, both in respect of quantities produced and qualities required for the fibres making up these mats.

Of course the invention is not limited to this type of application and the advantages it provides for fibre formation apply irrespective of the products made with these fibres.

Claims (20)

1. Process for the formation of fibres from a thermoplastic material in which the material is conducted in a drawable state into a centrifugi/^ device provided at its periphery with orifices, the material is projected from the centrifuging device from these orifices in the form of filaments which are entrained and drawn out by a high temperature gaseous current directed along the periphery of the centrifuging device and transverse to the direction of projection of the filaments, wherein the peripheral velocity of the centrifuging device, at the level of the orifices from which the filaments emerge, is at least equal to 50 m/s.

2. Process according to Claim 1, wherein the peripheral velocity is not greater than 150 m/s.

3. Process according to Claim 1, wherein the peripheral velocity is between 50 and 90 m/s.

4. Process according to any one of Claims 1 to 3, wherein the output of material for each orifice g expressed in kg per day and the emission pressure of the high temperature gaseous current p expressed in millimetres of water column for a width greater than

5. Mm and inferior to 12 mm are so selected that the ratio q.v/p, where v is the peripheral velocity in metres per second, is maintained within the range 0.07 to 0.5. 54G17 - 39 5. Process according to Claim 4, wherein the ratio q.v/p is maintained within the range 0.075 to 0.2.

6. Process according to any one of the preceding claims wherein the output per orifice is between 0.1 5 and 3 kg/day.

7. Process according to any one of Claims 1 to 5, wherein the output per orifice is between 0.7 and 1.4 kg/day.

8. Process according to any one of the preceding 10 claims, wherein the high temperature gaseous current is expelled at a pressure of.100 to 1000 mm of water column for an emission orifice width not greater than 20 mm.

9. Process according to any one of Claims 1 to 7, wherein the high temperature gaseous current is 15 expelled at a pressure of 200 to 600 mm of water column for an emission orifice width not greater than 15 mm.

10. Process according to any one of the preceding claims, wherein the rotational velocity of the centrifuging device is between 800 and 8000 rpm. 20

11.- Process according to any of Claims 1 to 9, wherein the rotational velocity is between 1200 and 2250 rpm.

12. Process according to Claim 11, wherein the output of fiberized material from the centrifuging device is greater than 12 tons of material per day. 25

13. Process according to Claim 11, wherein the output 5 4 (ί 1 7 - 40 of fiberized material from the centrifuging device is equal to or greater than 20 tons of material per day.

14. Process for forming fibres according to Claim 1, wherein the ratio q.v/F, in which 3 represents the output of material per orifice expressed in kg/day, v is the peripheral velocity in m/s and F is the micronaire of the fibres formed, is greater than 17.

15. Process according to Claim 14 in which the output per orifice 3 is between 0.1 and 3 kg/day.

16. Process for forming fibres according to Claim 1, wherein the ratio q.v/p.F in which 3 is the output of material per orifice expressed in kg/day, v is the peripheral velocity in m/s, p is the emission pressure of the gaseous current for a diameter at emission which is not less than 5 mm and not greater than 12 mm, which pressure is expressed in mm of water column, is greater than 0.035.

17. Process according to Claim 16, in which the output per orifice is between 0.1 and 3 kg/day and the emission pressure of the gaseous current at high temperature is from 200 to 600 mm of water column.

18. Process according to Claim 16 or Claim 17 in which the ratio q.v/p.F is greater than 0.040.

19. A process for the formation of fibres substantially as herein described with reference to the accompanying drawings. 5 4 017 - 41

20. Apparatus for carrying out a process according to any of Claims 1, 14 or 16, substantially as herein described with reference to and as shown in Figures 3 and 4 of the accompanying drawings.