FR2529878A1 - Improvements to the techniques for forming fibres comprising centrifuging. - Google Patents

Abstract

Techniques for forming inorganic fibres by centrifuging and gas drawing. According to the invention the peripheral velocity of the centrifuging member is maintained at a value above 50 m/s. The invention enables the results of this type of technique to be improved.

Description

IMPROVEMENTS IN FIBER FORMATION TECHNIQUES
INCLUDING CENTRIFUGATION
The invention relates to improvements in fiber-forming techniques including a centrifugation operation.

 More specifically, the invention relates to techniques in which the material to be treated is introduced in the stretchable state into a centrifuge having at its periphery multiple orifices. Under the effect of centrifugal force the stretchable material passes through these orifices and forms primary filaments or fibers. An annular stream of high temperature gas runs along the peripheral wall of the centrifuge. The filaments projected in this gas stream have their draw completed.

 In these techniques, the fibers formed are transported by the gases to a receiving member usually consisting of a gas permeable conveyor belt. They are deposited in the form of a tablecloth or mattress. This sheet is possibly subjected to other treatments before being cut and packaged according to the shape of the final product.

 When the final product must have a certain cohesion, the fibers are coated with a binding product during their journey to the receiving member. The ply of coated fibers passes through a heat treatment enclosure which fixes the binder.

 Of particular interest are these fiber-forming techniques in which the drawing results from both centrifugation and the pulling effect exerted by the hot gas stream.

On the whole, they make it possible to reach products of the best quality under satisfactory economic conditions.

The system of orifices delivering the primary filaments to a certain extent guarantees good regularity of the fibers. Furthermore, the completion of the stretching by the hot gases makes it possible to pass a relatively large flow of material through each orifice. Consequently, the production capacity of each centrifuge is high, which allows a reduction in unit costs.

 These very schematic reasons are certainly insufficient to account for the advantages of this type of process. However, they make it possible to underline in what the methods of the type of those of the invention are distinguished from apparently neighboring methods and which however, in their operation and their results, are very different.

 By way of example, techniques are known in which the drawing is only centrifugal. In these techniques the annular gas stream surrounding the centrifuge is either absent or does not participate in the drawing of the fibers. The supposed advantage of these techniques is to save the energy expenditure corresponding to the creation of the drawing gas at high temperature and high speed. The economy achieved in this way, however, results in a significantly lower production capacity of the centrifuge, and overall the production costs are not lowered quite the contrary.

 As has just been recalled, the manufacture of fibers, and consequently that of products prepared with these fibers, is subject not only to quality requirements but also to those of production costs. These imperatives cover many aspects as we will see in more detail later in the description.

As far as quality is concerned, these are first of all the insulating properties, but the mechanical properties also play a very important role. For the costs the diversity is even greater it is necessary to take into consideration the investments, the manpower, the raw materials, the energy ...

 The prior art teaches ways of preparing fibers and insulating products of good quality, but this usually corresponds to an implementation which is disadvantageous from the point of view of production costs, either for example that production is low or that consumption energy is too high, or that it is necessary to use costly fiber compositions ...

 The improvements proposed previously for the type of process to which the invention relates are extremely varied and this even if we limit ourselves to the provisions which relate only to the fiber formation step and that consequently we discard everything that is relating to the subsequent treatments to which they are subjected to reach the final product.

 Among the most recent publications on this subject, French Application No. 2,443,436 belonging to the Applicant Company contains a number of proposals. They relate in particular to the nature of the fiber material, a set of provisions intended to better control the temperature and the path of the material in the centrifuge, specific structures of the centrifuge itself, etc.

 Other previous publications, such as French Patent No. 1,382,917, also belonging to the applicant, endeavor to improve the conditions for spraying the primary fibers into the drawing gases.

 These two examples represent only a small treks of the particularly abundant literature on this subject. They show what can be the diversity of the proposals made to perfect this type of process.

 If in all cases the goal to be reached, namely the improvement of the quality-cost package is very clear, it is not the same with regard to the technical means to be implemented to achieve it. .

 The object of the invention is to provide means making it possible to improve the processes for forming mineral fibers of the type described above. The object of the invention is in particular to increase the production capacity at constant quality or the quality at constant capacity.

 The invention proposes in particular to provide new operating conditions for centrifugation devices to achieve the results indicated.

 The invention also provides modified centrifuges allowing the implementation of the conditions in question.

 The invention also provides insulating products prepared according to the new provisions and having improved insulating and mechanical properties.

 The inventors have shown by the research undertaken that it was possible to achieve these aims and other results which will be present in the following description by choosing to proceed to the formation of the fibers under well-defined operating conditions. of the centrifuge.

 In general, according to the invention, centrifugation should be carried out in such a way that the peripheral speed, that is to say the speed of movement of the orifices located at the periphery of the centrifuge member, is relatively high.

 Although the reason for the improvements observed when operating under the conditions defined by the invention is not exactly known, hypotheses can be put.

 Thus it can be assumed that the speed of movement of the orifices forming the primary fibers is closely linked to the intensity of the stretching of these fibers. In a pictorial fashion, it can in fact be considered that the pulling is carried out by traction on the fiber retained at one "end on the centrifuge member and at the other end" by the part of the fiber already fixed and which is retained. by the friction of the air surrounding the centrifuge. In this model of representation of the phenomenon, the stretching before freezing is all the more intense as the speed of movement of the periphery of the centrifuge is greater. For the global interpretation of the phenomenon, one can indeed make the approximation according to which, in the direction of rotation, the surrounding air is practically immobile.

 This is just a simplified interpretation and other important mechanisms are involved in the stretching. It will be discussed from time to time in the following description. Be that as it may, the inventors do not intend to link the invention to any interpretation whatsoever.

 More precisely, it appeared to achieve the objectives set according to the invention that it is necessary to operate at a peripheral speed - greater than or equal to 50 m / s.

 When the centrifugation is carried out at a perphenolic speed of less than 50 m / s, the advantages with respect to the previous operating modes become apparent and tend to disappear; the quality of the fibers is less good, possibly the specific production decreases ...

 If it is desirable to have a high peripheral speed, it goes without saying that for technological reasons this speed cannot be increased indefinitely. In practice it is very difficult to exceed speeds of the order of 150 m / s and most often the speed according to the invention is between 50 and 90 m / s.

 Furthermore, the increase in peripheral speed must be obtained by keeping the speed of rotation within limits compatible with the mechanical strength of the centrifuge.

 In the devices traditionally used, the centrifuge is the element subjected to the harshest conditions. The weakest point is formed by the peripheral wall pierced with the orifices forming the die. The presence of these very numerous orifices reduces its resistance capacity while it is subjected to centrifugal forces which are exerted first on itself but also on the reserve of molten material which it retains. In addition, these efforts are supported with the temperature removed, which has the consequence of reducing the mechanical resistance.

 In prolonged use, the centrifuge undergoes two kinds of modifications which by developing end up disturbing the operation. These are deformations such as the bulging of the peripheral wall towards the text. This deformation, among other consequences, modifies the position of the orifices with respect to the gas stream, thereby causing a defective draft which may require replacement of the centrifuge.

 Another kind of modification is constituted by the wear of the orifices. The circulation of the material tends to gradually increase the diameter of the orifices. If the other conditions are not changed, this results in an increase in the material flow rate. When this increase remains low it is possible to partially remedy it by modifying for example the temperature of the material or the regime of the drawing gases. Beyond a certain wear, these modifications are no longer sufficient to maintain the quality of the products. In this case also, the centrifuge must be replaced.

 Replacement of the centrifuge under traditional operating conditions is usually necessary after a few hundred hours of operation. For this reason, the cost of the centrifuge is a significant part of the total production cost. It is therefore particularly important to ensure that the lifetime, in the arrangements according to the invention, is not shorter than that corresponding to the previous implementations and even, if necessary, is significantly longer.

 A limitation of the speed of rotation - and consequently an imitation of the forces supported by the centrifuge - which does not obstruct a high peripheral speed therefore seems particularly desirable for the implementation of the invention.

 It is desirable to limit the speed of rotation so that the centrifuge is not subjected to excessive forces, but it is necessary that the centrifugal effect remains sufficient to ensure the projection of the material to be fiberized out of orifices of the centrifuge under satisfactory conditions , especially debit. It is thus preferable that the speed of rotation is not less than 800 revolutions / minute.

 In practice in the choice of a rotational speed and a peripheral speed, the questions raised by the limits of resistance of the apparatus make that usually the highest peripheral speeds are associated with the most rotational speeds. small and vice versa. This procedure does not necessarily represent an optimization in terms of product formation but a compromise to ensure a reasonable longevity of the installation.

 Under the conditions which have just been stated, it can be seen experimentally that the production capacity of the centrifuge member can reach very high values without the quality of the fibers or that of the final product; be reduced. it goes without saying that it is also possible to keep production at a lower level, in particular to further improve quality.

 The advantages provided by the invention on this point are very clear.

 The increase in production which is an important economic factor is of particular interest. I1 has been the subject of numerous proposals in the prior art: increase in the number of orifices of the centrifuge, increase in the section of these orifices, increase in the fluidity of the material, etc. These various elements effectively make it possible to modify the flow rate. of material passing through the centrifuge, but the proposals made on this subject usually have as a counterpart a reduction in the quality or difficulties as regards the longevity of the apparatus.

 Thus an increase in the density of the orifices at the periphery of the centrifuge, that is to say the number of orifices per unit of surface area, not only weakens the centrifuge but, passes a certain threshold, appears to be detrimental to the quality. fibers. We can assume that the primary fibers, too tight at the origin, collide and stick together before their stretching is complete. This may explain why the fibers collected under these conditions are less regular, less fine and shorter.

 A similar problem is encountered when, still with the aim of increasing the production capacity, the section of the orifices is increased while maintaining the same viscosity conditions of the fiber material. In this case the primary fibers are larger and it is difficult to achieve satisfactory drawing even by modifying the speed of the annular drawing gases.

 The increase in fluidity of the fiber material, that is to say the increase in its temperature, poses other problems. The operating temperatures are usually at the limit of what the alloy of the centrifuge can withstand. Often even, the nature of the composition to be fiberized is chosen to take account of this type of limitation. The increase in temperature in this assumption is impossible except to have recourse to precious metals to constitute the centrifuge, which raises other difficulties in particular of cost but also of mechanical characteristics.

 Conversely, in the previous literature, the processes whose object is to achieve an improvement in quality lead to a reduction in production. One mode consists, for example, in using orifices of small dimensions to form finer primary fibers which are themselves drawn into finer fibers.

 Thanks to the invention, although the quality / quantity opposition remains, the result is at a much better level. Also when we set a goal of producing good quality (we will see in the following description how it is evaluated) comparable to that which was previously obtained - with this type of process, we easily exceed a production of around 13 tonnes of fiber material per day per centrifuge.

 In practice, it appears advantageous according to the invention to maintain production at a level higher than 17 tonnes per day, and productions of 20 to 25 tonnes or more can usefully be achieved while retaining a product of excellent quality.

 It is remarkable that these results are achieved without resorting to the modifications indicated above. It is not necessary to change the density of the orifices or their dimensions, etc.

The primary fibers formed are therefore on the whole comparable to those of analogous prior techniques, and the drawing being improved as a result of the increase in peripheral speed, the quality of the products is better.

 A particularly advantageous prior arrangement is that according to which the diameter of the orifices of the centrifuge varies according to the level at which these orifices are situated on the peripheral strip. This arrangement tends on the one hand to prevent the primary filaments emitted at different levels, and folded down by the drawing gas, from mixing and on the other hand to adapt the amount of glass passing through each orifice to the evolution of the conditions of speed and temperature of the drawing gases along the peripheral surface. For this the diameter of the orifices, concurrently with other arrangements, is chosen to obtain what has been called an umbrella fiber drawing ". In this embodiment the primary fibers the first emitted on the path of the drawing gases are also those which are projected farthest from the centrifuge. They "wrap the fibers emitted at the level immediately below and so on.

 In practice, the diameter of the orifices is not the only factor which determines this "umbrella" effect. The length of the orifice, the temperature conditions, the thickness of the reserve of material on the internal face of the peripheral strip also play a role. Ordinarily when the thickness of the peripheral wall of the centrifuge is uniform, and for a strip height not too large, so that the temperature conditions are substantially the same at all points, the umbrella fiber drawing is obtained by arranging the orifices by diameter decreasing in the direction of propagation of the drawing gases.

 When the thickness of the wall is not uniform or when the temperature conditions are different depending on the position of the row, the diameter of the orifices may vary according to another method of distribution. To this end, reference can be made to the teaching of the aforementioned patents of the applicant.

 Other conditions are involved in the implementation of the procedures according to the invention as we have seen previously. They relate in particular to the material (nature, temperature) and the gaseous stretching current (temperature, speed). These conditions are not significantly different from the corresponding conditions previously described. It therefore does not seem necessary to repeat the study here.

 Other advantages and characteristics of the invention appear in the following description and examples of implementation. These are only intended to illustrate the invention in more detail and are in no way limiting.

The rest of the description describes in particular the type of device used. Reference is made to the drawing boards in which
- Figure 1 is a sectional view of the assembly of a fiberizing device usable according to the invention;
- Figure 2 is, on a larger scale, part of Figure 1 showing the different elements involved in the area where the fibers are drawn.

 - Figure 3 is a schematic perspective view showing the path of the fibers from their formation to the receiving member in a traditional embodiment (right) and according to the invention (left).

 The fiber-forming device shown in FIG. 1 comprises a centrifuge proper, generally designated by the reference 1. This centrifuge has - horizontally is carried by the assembly 4 forming the shaft of the device. This shaft is placed in a bearing box 5. The assembly of the shaft 4 and of the centrifuge is rotated by a motor 6 by the intermediary of transmission members not shown such as belts.

 The annular hot gas flow is sown by a burner generally designated by the reference 2.

 A blowing ring 3 concentric with the centrifuge also emits an annular current of air at low temperature.

 A high-frequency induction heating element 12, in the form of a ring, surrounds the centrifuge at its lower part.

 To avoid stresses and deformations, different insulating or cooling elements, not shown, are arranged in the appropriate locations of the device.

The composition of the centrifuge 1 given with reference where appropriate to Figure 2 is as follows
The fixing on the hollow shaft 4 is ensured by the hub 9.

Solid with the hub 9, the disc-shaped rim 16 supports, on the one hand, a distributor basket ll by means of a fixing crown 17 and the part 13 of the centrifuge called 'tulip because of its shape.

 The peripheral strip 7 of the centrifuge from which the primary fibers are projected is integral with the tulip 13. This peripheral strip is slightly inclined with respect to the vertical so that the gases withdrawn by the burner 2 uniformly run along its entire surface.

 The resistance to deformation of the centrifuge is reinforced by a reverse 8 which follows the peripheral band 7 and is integral with the latter.

 In FIGS. 1 and 2, the orifices through which the material to be fiberized is centrifuged are not shown because of their fineness. These holes are evenly distributed in horizontal rows. The orifices of two consecutive rows are offset from one another to ensure maximum distance between the neighboring orifices.

 The structure of the centrifuge must be designed to withstand significant mechanical forces. However, it seems preferable to keep the lower part open. This arrangement, in addition to the facilities which it provides for the production of the centrifuge, possibly avoids the deformations which could result from temperature differences between the periphery and the bottom of the centrifuge.

 The path of the material to be fiberized in the mode shown is as follows. A continuous stream of molten material rolls in the axis of the hollow shaft 4. This molten material is received at the bottom of the basket 11. The rotation of the latter leads the material to the peripheral part which is pierced with multiple orifices. From these orifices, under the effect of centrifugation, the material is projected onto the internal face of the peripheral strip 7 of the centrifuge. There, the material forms a continuous layer and is evacuated through the holes drilled in the wall of the centrifuge. As the centrifugal force continues to be exerted, the material passes through the orifices and is projected in the form of thin threads into the drawing gases where it finishes being drawn in the form of fibers.

These gases are emitted from the nozzle 15 of the burner 2. The annular blower 3 generates a gas layer which envelops the gaseous drawing current and channels it in particular to prevent fibers from striking the inducing ring 12.

 The distributor basket 11 plays an important role in the formation of the stretching conditions. A first reserve is created there which equalizes the material flow. This makes it possible in particular to distribute the material very regularly over the entire circumference of the internal face of the peripheral strip 7 of the centrifuge.

 The detailed study of the distributor basket which appears in the aforementioned French patent application No. 2443 436 highlights certain characteristics which are important for the proper functioning of the assembly. These are in particular the dimension of the orifices located at the periphery of the basket from which the material to be fiberized is projected onto the internal face of the peripheral band 7, and the spatial arrangement of the basket and in particular the orifices of the latter. shown opposite the centrifuge. For more information on this subject, it is advantageous to refer to the aforementioned prior patent.

 Centrifuges for implementing the invention, in addition to the characteristics already stated, can be distinguished from similar devices known previously by very substantially increased dimensions. Advantageously, the diameter of the centrifuge at its peripheral wall is equal to or greater than 500 mm and it can reach and even exceed 1000 mm, while the centrifuges commonly used to date do not exceed 400 mm.

 This increase in the diameter of the centrifuge plays an important role for many of the characteristics of implementing this type of process apart from the effects directly dependent on the conditions of these speeds which are the basis of the invention. Thus increasing the perimeter of the centrifuge, without changing the height of the peripheral strip, increases the surface available for the orifices.

Without modifying their distribution, in particular without changing their number per unit of area, it is thus possible to have a larger number of orifices and consequently increase the production capacity.

The improvement in production which is obtained in this way is done without affecting the provisions which determine the quality of the fibers, and the increase in peripheral speed under the conditions defined by the invention even allows an improvement in this quality. .

 Although the use of centrifuges with diameters greater than traditional dimensions is preferred, the use of centrifuges with diameters of the order of 200, 300 or 400 mm under the conditions of peripheral speeds provided for according to the present invention some advantages. Operation at peripheral speeds greater than 50 m / s allows, in particular, independently of the dimensions of the centrifuge, an improvement in the quality of the insulating products formed.

 For traditional centrifuges, increasing the peripheral speed requires an increase in the rotation speed which can reach up to about 6000 rpm or more. your speed limits, as mentioned above, are practical. They are determined by the resistance of the centrifuge to the stresses exerted during its operation.

 The use of very large diameter centrifuges used according to the invention has other less direct effects on the products obtained. For example, there are modifications in the way the formed fibers behave beyond the stretching zone.

 The fibers which are entrained by the drawing gases towards the receiving member form a kind of continuous circular web.

In traditional techniques the shape of this veil, at the level of the centrifuge and immediately downstream, is very characteristic.

 FIG. 3 represents the general evolution of the fiber web from the centrifuge to the receiving member shown diagrammatically in 19 on which the fiber mat is formed.

 For the right half of Figure 3 representing the traditional mode. Initially the sail 21 opens out and deviates from the axis of the device then, in a second step, it retracts into an area 22 situate a little below the centrifuge before expanding again .

 The presence of this tightening of the veil is not desirable. In this movement the homogeneity of the distribution of the fibers in the gas stream which carries them is broken. We are indeed witnessing the formation of irregular folds which are as many incipient fractures of the veil and the development of wicks or bundles of fibers which we know to harm the homogeneity of the final product.

 By operating under the conditions of the invention and with the corresponding centrifuges, it can be seen, as shown on the left side of FIG. 3, that this phenomenon of tightening of the web is considerably reduced. This change is probably explained by the fact that the "pumping" effect exerted by the drawing gases by induction of the air located under the centrifuge is distributed over a larger volume and is better compensated because of this. that this zone located under the centrifuge opens more widely on the surrounding atmosphere. In other words, the depression which characterizes this zone in the prior techniques is little or not sensitive in the embodiments of the invention. This results in a better deployment of the web and a reduction in the tendency to form bundles of fibers.

 Limiting the speed of rotation makes it possible to have centrifuges which in their shape are comparable to those used previously. The mechanical forces to which they are subjected vary as the square of the speed of rotation. Even if these efforts also increase in proportion to the dimensions and the mass, the combination of these factors makes it possible to keep them at the level to which those prior devices were subjected or even possibly to reduce them appreciably. For this reason, as a whole, it is not necessary to confer a structure different from that described in the previous documents.

 The use of large centrifuges requires, if necessary, certain adjustments.

 It is essential for the proper functioning of the assembly that the stretching conditions in the same horizontal plane are strictly identical at all points. This is true with regard to the centrifuge as we have said; this is also true for the other elements involved in drawing and first of all for the high temperature gas generator.

 In the most usual modes, the gas stream is formed from several combustion chambers distributed regularly around the centrifuge. A chamber of this type is shown at 14 in FIG. 1. The combustion gases produced in chamber 14 are conducted near the peripheral wall of the centrifuge and escape through an annular nozzle 15 common to all of the various chambers combustion 14.

 When the respective dimensions of the centrifuge and the drawing gas generator are not too large, the uniformity of the gas stream is obtained without much difficulty. The gas emitted has the same characteristics over the entire extent of the nozzle 15.

This becomes all the more difficult to obtain as the dimensions are larger as is the case according to the invention.

 To facilitate obtaining uniform blowing conditions at the outlet of the burner, it seems advantageous to use a single combustion chamber surrounding the centrifuge. This unique annular combustion chamber allows a better balance of pressures and gas flows over the entire width of the nozzle 15.

 At this point in the description and before examining the examples of methods of implementation and of products obtained, it is necessary to specify the quantities which characterize the products in order to better understand the nature of the improvements brought about by the invention.

 In this approach we mainly consider the production of insulators consisting of a felt or fiber mat. These products alone represent a considerable part of all mineral fiber applications. Of course this does not exclude the implementation of the invention for the preparation of products intended for other uses.

 As we have indicated, the two main types of quality that we strive to obtain are thermal insulating qualities but also mechanical qualities. These relate to very specific aspects of insulation products. In particular, the product must perform well in the handling and packaging required by bulky products with a low specific mass.

 For a given product, the insulating quality is defined by its thermal resistance R. It expresses the ability of the product in question to oppose thermal replacements when two different temperatures prevail on either side of the product. This value depends not only on the characteristics of the fibers and their arrangement within the product, but also on the thickness of the product.

 The thicker a product, the greater its thermal resistance.

 Although the products are generally presented according to standardized dimensions, it is often more convenient to draw up a comparison to refer to thermal conductivity X, a quantity which does not take into account the dimensions of the product. In a way, this is the intrinsic measure of the insulating quality.

 Despite numerous studies on this subject, it is not yet possible to establish a complete correlation between thermal resistance or thermal conductivity, and measurable data concerning the structure of the product. Nevertheless, a certain number of facts are well established which, if necessary, make it possible to orient production according to the desired result.

 Thus, for constant characteristics of the fibers and within certain limits, the denser the product the more it is insulating. But of course a denser product requires more fiber and is therefore more expensive. For products with insulating properties of normalized value, account for the differences by referring to what is called the grammage ", that is to say the mass of fibers required per unit area to obtain the resistance The quality of the product in this case varies in the opposite direction to the grammage.

 it is also established that the fineness of the fibers is an important factor in the formation of the insulating qualities. For the same grammage, the product is all the more insulating as the fibers are thinner.

 It is also certain that the distribution of the fibers in the product plays a very significant role and that it is desirable to have a product as homogeneous as possible.

 One way to assess the homogeneity of a mattress of insulating fibers consists in cutting small elements of constant dimensions from a sample of mattresses and determining the variations in mass of these elements. When the dimensions of the elements are sufficiently small, the variations satisfactorily reflect the irregularities in the distribution of the fibers.

 Mass variation measurements made in this way show a relationship between the coefficient of variation (determined as the standard deviation divided by the average value) and the insulating qualities of the products. Overall, it can be seen that the smaller the coefficient of variation, the better the insulating qualities of the product.

 The products prepared according to the invention exhibit a substantial improvement in homogeneity. The coefficient of variation for the measurements carried out, under the conditions specified in the examples remains less than 5% and even less than 3 t.

 The mechanical quantities useful for the appreciation of the product are essentially related to the problem of its packaging.

 In. all of these are very voluminous products which it is advantageous to store in a compressed state. However, the compressed product must resume a certain volute once released in order to best develop its insulating qualities.

 After compression, the fiber mat takes on a certain thickness, different from the thickness before compression, and that in the compressed state. It is desirable that the thickness taken up after unpacking is as large as possible.

 More precisely for a product (unpacked) whose characteristics are well standardized, and in particular whose thickness is guaranteed, it is important that in the compressed state it occupies a minimum volume. This guaranteed thickness recovery is also expressed by the allowable compression ratio.

 It is difficult to establish a close relationship between the mechanical qualities and the insulating qualities of a product, even when we try to analyze both in terms of structure, fiber dimensions, etc. .. Experience only shows that the improvement in insulating qualities is compatible with that of the compression ratio.

 Comparative tests have been carried out in order to highlight the advantages which one notes to operate under the conditions prescribed by the invention compared to the previous operating conditions.

 A difficulty in establishing these comparative tests lies in the multiplicity of factors likely to influence the quality of the final product.

 As far as possible, in these tests, research is carried out on the operating conditions which make it possible to keep the parameters sensitive to multiple influences constant. Thus to reproduce the temperature conditions of the centrifuge from one test to another, it is essential in these comparative tests to ensure that the average flow rate per orifice remains constant. Indeed, the thermal equilibrium of the centrifuge is the result of several combined actions, that of the burner; of the inductor, the ambient air, but especially that of the material which passes through the centrifuge. A variation of the must per orifice modifies this equilibrium and requires, on the contrary, to return to the temperature conditions (and especially viscosity which result), a modification for example of the burner operation.

in this case this results in modifications in the drawing itself.

 These indications show the difficulties raised by these tests as we have emphasized above in connection with the search for the improved conditions which constituted the object of the invention.

10 / Comparative tests.

 In this series of examples, the flow rate per orifice, the temperature conditions of the centrifuge as well as the speed and the temperature of the drawing gases and the nature of the fiberglass are kept identical.

In the present case, the tests were carried out with a glass whose weight composition is as follows
- SiO2 64.1-% - Na20 15.75%
- A1203 3.4 Z - K20 1.15 7.

- CaO 7.2 X - B203 4.5%
- MgO 3% - Fe2O3 0.45%
- SO3 0.25% - Impurities 0.20%
The following table summarizes the variable characteristics and the results obtained with regard to the prepared product. The first three tests are carried out under operating conditions which do not correspond to the invention.

I II III IV
- rotation speed
(rpm) 2400 2400 2400 1800
- peripheral speed
(m / s) 38 38 38 56.5
- total flow of the centri
runaway (tonnes / day) 9 18 18 18
- micronaire under 5 g 3 3 4.5 3
- specific mass
insulating quality Cte (g / m2) 945 1080 1215 945
- compression ratio 4 2 4 4
- resistance to stage fright
tion (g / g) 250 150 250 250
We have previously indicated what the properties listed in this table consist of. It remains to clarify the meaning of the values presented.

 The micronaire is a quantity linked to the fineness of the fiber.

It is measured as follows. A sample of the product of 5 g is placed in a compartment crossed by a gaseous current emitted under conditions, in particular of pressure, well fixed. The sample traversed by the gas forms an obstaele which tends to slow down the passage of this gas. The gas flow measurement is taken on a graded flow meter.

It is these values defined for standard conditions that are noted.

 The finer the fibers for the same sample weight, the lower the flow rate.

 The most common micronaire values for insulating products are between 2 and 6 under 5 g. Products with a micronaire less than or equal to 3 are considered to be very fine. They generally have better insulating power per unit mass of the product.

 The insulating value of the fibers prepared is defined in this table by the specific mass necessary to obtain a determined thermal resistance for a thickness also determined. We have chosen for reference the thermal resistance value of 2 m2 K / w measured at 2970K for a thickness of 90 mm. This value corresponds to a good quality insulator.

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

Tensile strength is measured according to ASTM standard
C 686 - 71 T.

 The preceding table makes it possible to draw several conclusions.

 By comparing test IV according to the invention with test II it is found that for a constant fineness of product and for the same overall flow rate of the centrifuge, by operating at the speeds prescribed by the invention, one obtains on the one hand better insulating property (or what is equivalent for the same insulating property, the mass of product is less), a significantly improved compression ratio and better tensile strength.

 The improvement in the specific mass allows for the same quantity of fiber materials to increase the quantity of product of constant quality by 10 Z. The improvement in the compression ratio allows a very significant saving in transport and storage costs.

 If, as in Examples I and IV, the operating conditions are adjusted in order to have the same qualities of product, it can be seen that the flow rate of the centrifuge in the mode according to the invention is increased in considerable proportions, which makes the process significantly more economical. .

 Another way of comparing the invention with the previous modes of implementation consists in establishing conditions such that the flow rate and the mechanical qualities are identical. In this case, which is that of Examples III and IV, it is observed by operating according to the invention on the one hand a significant reduction in the micronaire showing that the fibers produced are finer and a decrease in the thermal conductivity (or an increase in thermal resistance) which reduces the specific mass.

 Other comparative tests have been carried out to determine the influence of the procedure according to the invention on the homogeneity of the mattresses of fibers formed.

Two series of tests were carried out under the following conditions:
X XI
- rotation speed (rev / min) 2100 1800
- peripheral speed (m / s) 44 56.5
- centrifuge flow 20 20
- micronaire under 5 g 3 3
- specific mass (g / m2) for R = 2, e = 90 mm 1050 960
- coefficient of variation (%) 6.1 2.6
To determine the coefficient of variation, the mattress studied was cut into 25 x 25 x 45 mm elements. The masses of these elements are compared with the average value as indicated above. The coefficient of variation corresponds to the standard deviation divided by the average value of the mass of the elements.

 It can be seen that the production of fiber mats carried out under the conditions of the invention (XI) results in a significant improvement in the homogeneity of the distribution compared to that corresponding to a traditional mode (X).

 This improvement in the homogeneity of the products translates in another way the improvement in their qualities as thermal insulation insofar as, as we have indicated above, the experience shows that these two aspects are linked .

 Whatever the way of considering the way of proceeding according to the invention, it constitutes a significant improvement over previous methods, which is confirmed by the results of the following tests carried out under other oppressive conditions.

20 / -Testing variations of various parameters.

 Various other operating parameters have been modified in relation to the desired product qualities.

The results of these tests are shown in the following table
V VI VII VIII IX
- rotation speed
(rev / min) 1800 1400 2250 1800 1350
- peripheral speed
(m / s) 56.5 56.5 70 56; 5 71
- centrifuge flow
(tonnes / day) 18 18 18 25 20
- micronaire under 5 g 4 3 3 4 3
- specific mass (g / m2)
for R 2 2, e = 90 mm 1080 945 915 1140 865
- compression ratio 4 4 - 3 4
- resistance to stage fright
tion (g / g) 300 250 - 250 250
Example V is an implementation under conditions of the invention to prepare a product which can be compared to that of Example IV. The mechanical properties are fully preserved.

The micronaire goes from 3 to 4, that is to say that the fibers are a little less fine. I1 results in a slightly stronger grammage.

 The type of production corresponding to example V, even if the products are a little less efficient than those of example IV, is interesting. Indeed, under identical centrifugation conditions, the change in the fineness of the fibers results from differences in the operation of the burner emitting the hot drawing gases. The energy consumed for the operation of the burner in the case of Example V is reduced by about 15 z compared to Example IV, which can usefully compensate for the increase in grammage.

 Although an exact comparison is not possible since the micronaire is not strictly the same in the two cases, it can be deduced by extrapolation that for a micronaire of 4.5 the specific mass for example V would remain lower to that of Example III. This confirms what appeared on comparison of Examples II and IV; namely that for the same quality of fibers the products obtained according to the invention are better insulators, or, if desired, that the production can be increased.

 Example VI is an implementation analogous to that of Example IV but in which the peripheral speed is maintained and the speed of rotation reduced. The properties of the products, apart from the experimental approximations, are identical.

 In the same order dtiddes, example VII is also interesting. The influence of the peripheral speed on the quality of the insulating products prepared again appears clearly on the value of the grammage. Likewise, in a certain way, independence from the speed of rotation seems to confirm. The speed of rotation in this example is at the limit of those which are considered acceptable according to the invention for reasons of longevity of the centrifuge, particularly when the must is high as is the case.

 In Example VIII, a test is available for a flow rate of 25 tonnes / day which does not constitute a limit for the process according to the invention, but which distinguishes it very clearly from previous achievements in this field when one wants fibers with good qualities. The micronaire remains relatively low and the grammage also; although slightly higher than in Example V.

 The remarkable increase in production thus achieved largely compensates for this slight increase in grammage necessary to achieve the required insulating qualities.

 Example IX is yet another embodiment of the invention which is distinguished both by a high peripheral speed and a relatively low speed of rotation. These conditions of implementation make it possible to achieve excellent mechanical and insulating qualities (the grammage is particularly low) and this for a high flow rate of the centrifuge.

 In the previous examples we have highlighted by comparison some of the differences in terms of products produced and we have said on occasion that these differences are reflected in terms of production costs. It is difficult to assess precisely the economic advantage represented by the implementation of the invention on an industrial level. Approximately, taking into account the various factors involved, synergy, product, material, the estimated gain can reach 10 Z of the unit cost or more depending on the conditions chosen.

 During these tests, analyzes were undertaken to try to determine what particularities present the fibers produced according to the invention vis-à-vis the fibers obtained previously and which would make it possible to account for the improvements observed.

 Apart from that relating to the micronaire there is no very precise method for directly assessing the characteristics of the fibers. It is particularly difficult to assess the length of the fibers. Several observations however suggest that the operating conditions according to the invention favor the formation of longer fibers. It is improved tensile strength. It is also the increased compression ratio. It is also the thickness of the fiber mat leaving the receiving chamber before entering the binder heat treatment enclosure. There is indeed a significant increase in thickness by operating according to the invention, which can reach or exceed 25 Z while the conditions in particular of material flow remain identical. All of these aspects can in some way be traced to an increase in fiber length.

 Furthermore, after continuous operating tests of more than 200 hours, it is found that the wear of the centrifuges was not such that it requires replacement.

 This constitutes only a few examples of the numerous particularities noted in the implementation of the procedures studied under the conditions of the invention.

 They allow to appreciate the nature of the improvements brought by the invention and also the type of advantages which can flow for the user.

 The production of fiber blankets for insulation has been studied because it constitutes a particularly significant application of fiberizing processes, both in terms of the quantities produced and the qualities required of the fibers that make up these blankets.

 The invention is of course not limited to this type of application, the advantages which it provides at the stage of fiber formation are found whatever the products prepared with these fibers.

Claims (10)

 1. A method of forming fibers in which the material in the stretchable state passes through a centrifuge member provided at its periphery with orifices through which the material is projected in the form of filaments which are entrained and drawn by a gas stream at high temperature directed along the periphery of the centrifuge member transversely to the direction of projection of the filaments, characterized in that the peripheral speed at the level of the orifices from which the filaments escape is at least 50 m / s.  2. Method according to claim 1, characterized in that the peripheral speed is not greater than 150-m / s.  3. Method according to claim 1, characterized in that the peripheral speed is between 50 and 90 m / s.  4. Method according to one of the preceding claims, characterized in that the speed of rotation is not less than 800 rpm.  5. Procedure according to claim 4, characterized in that the rotation speed is at most equal to 6000 rpm.  6. Method according to any one of claims 1 to 3, characterized in that the speed of rotation is between 1200 and 2250 rpm.  7. Method according to any one of the preceding claims, characterized in that the flow rate of fiber material from a centrifuge member is greater than 13 tonnes per day.  8. Method according to any one of claims 1 to 6, characterized in that the flow rate of fiber material from a centrifuge member is greater than 17 tonnes per day.  9. Insulating product obtained by the process according to any one of claims 1 to 8.  10. centrifuge member for implementing the method according to any one of claims 1 to 7, the dimensions of which are such that the peripheral speed is at least 50 m / s for a rotation speed at most equal to 2250 revolutions per minute.