Method and device for manufacturing ultrafine fibres from thermoplastic polymers

A process and device for manufacturing ultrafine fibers and ultrafine-fibre mats from thermoplastic polymers with mean fibre diameters of 0,2-15 .mu.m, preferably 0,5-10 .mu.m, by a melt blowing technique. The polymer melt (12) flows through at least one bore (15) in a melt blowing nozzle (18). Immediately on emerging from the bore, gas is blown against the extrusion from both sides of the bore exit (15), thus breaking up the melt to form fibers. To this end, the gas is accelerated to supersonic speed in Laval nozzles (25, 26; 31, 32), disposed in mirror symmetry round the bore exits (15), and decelerated to just below the speed of sound in channels (27) with constant cross-section, or a cross-section which decreases in the direction of flow, fitted downstream of the Laval nozzles, and the melt (12) fed into the gas stream emerging from the channels (27).

BACKGROUND OF THE INVENTION 
The invention is based on a process for producing microfibres and non-woven 
microfibre webs from thermoplastic polymers by the melt-blowing technique 
in which a polymer melt flows through at least one orifice in a melt die 
and is separated into fibers by a gas which impinges on the melt from both 
sides immediately after its exit from the orifices. The invention also 
relates to a device for carrying out the process. The melt-blowing process 
has been disclosed in numerous publications (see e.g. U.S. Pat. Nos. 
3,755,527, 3,978,185, 4,622,259 and 3,341,590), and German Patent No. 
2,948,821. According to the melt-blowing technique a stream of polymer 
melt extrusion issuing from a melt orifice is separated into individual 
fibers and drawn out while attenuated by means of an inert gas, in most 
cases air, which has a temperature higher than or equal to the temperature 
of the melt and is blown against the melt in the direction of flow. One 
main object is to increase the economic efficiency of the process by 
appropriately regulating the melt viscosity. Thus the prior art discloses 
the use of polymers with an extremely low viscosity and correspondingly 
high extrusion flow rates, since this enables relatively fine fibers to be 
produced with a lower degree of energy consumption by reducing the 
temperature of the melt and the gas stream. The following parameters are 
known to have a crucial effect on the economic efficiency of the process: 
a) The number of melt orifices (per unit of length) and the throughput of 
the melt per orifice, 
b) the melt temperature and viscosity of the melt, 
c) the gas inlet pressure for obtaining a uniform gas stream with high flow 
rate over the whole length of the die, 
d) the temperature of the gas stream, and 
e) the mass flow rate of the gas. 
According to the prior art the gas temperature is adjusted to a value 
higher than or equal to the temperature of the melt. In all known 
processes the gas stream issues from the die in direct proximity to the 
melt orifices and on either side thereof via exit slots arranged in the 
longitudinal direction of the die. Complicated hydrodynamic brake means 
and air distributing systems have to be provided in the gas inlets to 
ensure a uniform rate of flow over the entire slot length. PCT application 
WO 87/04195 describes appropriate technical means for achieving optimum 
results. 
The use of relatively large gas exit slots (1 mm to 3 mm) has also been 
disclosed. One disadvantage of this method is the high quantity of gas 
required since high rates of flow are necessary in particular for the 
production of very fine fibers of an average diameter of &lt;3 .mu.m. The 
rate of flow at the slot exit is usually 0.5 to 0.7 times the sonic speed 
of the gas (0.5 V.sub.s to 0.7 V.sub.s ; V.sub.s =sonic speed). 
SUMMARY OF THE INVENTION 
A principle object of the invention is to provide an additional increase in 
the economic efficiency of the melt-blowing process. In particular the 
object is to provide higher economic efficiency in the production of 
fibers of mean fibre diameters of &lt;10 .mu.m, preferably &lt;5 .mu.m. An 
additional object is to considerably increase the melt throughput per 
orifice and thus the total spinning capacity of the installation when 
producing fibers of mean diameters of between 0.5 .mu.m and 3 .mu.m. 
According to the invention the above objects are achieved by accelerating 
the rate of flow of the gas to supersonic speed in Laval nozzles arranged 
mirror-symmetrically in relation to the melt orifices and reducing the 
rate of flow of the gas in flow channels arranged downstream of the Laval 
nozzles, and having a constant cross-section or a cross-section tapering 
in the direction of flow to a rate just below sonic speed and by directing 
the polymer melt into the gas stream issuing from the flow channels. "Just 
below sonic speed" is understood to be a range between 0.8 V.sub.s, 
preferably 0.9 V.sub.s and 0.99 V.sub.s (0.8 V.sub.s &lt;V&lt;0.99 V.sub.s, 
preferably 0.9 V.sub.s &lt;V&lt;0.99 V.sub.s). Whereas in the known melt-blowing 
process the rate of flow of the gas stream at the exit to the slot nozzles 
is considerably lower than sonic speed, the solution provided by the 
invention is based on a gas exit speed just below sonic speed ("transonic 
speed") which is obtained in a particular manner. This solution is 
effected technically with the aid of Laval nozzles which are oriented in 
the direction of the gas stream adjacent to the tip of the melt die and 
are arranged at a small distance upstream of the melt orifices. Thus the 
device for carrying out the process is characterised according to the 
invention in that the gas nozzles are designed in the form of Laval 
nozzles with flow channels arranged downstream thereof and having a 
convergent or constant cross-section, which are arranged in direct 
proximity to the wedge-shaped die tip and terminate with a sharp edge 
maximally 3 mm above or below the level of the melt orifices. 
The Laval nozzles can either have a square or a circular cross-section with 
an orifice diameter of 0.3 to 2 mm. 
Preferably widened sections leading into the flow channel are arranged 
downstream of the Laval nozzles. The inlet cross-section of the flow 
channel should be 1 to 2.5 times the sum of the widened cross-sections of 
the Laval nozzles and the length of the flow channels should be 1 to 30 
times the widened cross-section. 
According to a further embodiment a gas smoothing chamber (tranquilizing 
chamber) is arranged upstream of the Laval nozzles and several linearly 
arranged Laval nozzles are assembled together with the corresponding gas 
smoothing chambers in the form of individual units to form a modular gas 
supply element. 
The gas supply elements, whose width is 25 mm to 500 mm and preferably 50 
mm to 200 mm are advantageously connected in a gas-tight manner both to 
the melt die and to each other. In a further advantageously designed 
modification the gas supply elements can be displaced parallel to the 
wedge-shaped contour of the melt die tip in order to allow the adjustment 
of the distance between the Laval nozzles and the melt orifices. 
Compared to the previously known melt-blowing processes a considerably 
higher space time yield (production rate) is achieved under stable and 
uniform operating conditions. Of considerable importance is the reduction 
in the rate of flow of the gas streams which issue from the Laval nozzles 
at supersonic speed, in the flow channels arranged downstream of the Laval 
nozzles. The flow channels are constructed in such a manner that one side 
of each flow channel is formed by the outer wall of the melt die tip. The 
Laval nozzles and the flow channels can be displaced parallel to the outer 
walls of the wedge-shaped melt die so as to allow the use of either 
position typically employed in the melt-blowing technique, i.e. either the 
stick out or the set back position. The following advantages are obtained 
by the invention: 
1. Due to the presence of the particularly uniform transonic region of flow 
in the vicinity of the melt orifices the draw rate of the melt from the 
orifices and thus also the yield, is greatly increased without trading off 
between product quality and yield. 
2. It has been found that it is possible to considerably increase the melt 
throughput per hole for fibre thicknesses of less than 5 .mu.m and in 
particular less than 3 .mu.m. 
3. It has also been found that, compared with the conventional process for 
the production of fibers of the same fineness, considerably lower gas 
throughput quantities are required for identical melt throughput 
quantities. 
4. Static pressures in the gas smoothing chamber of less than 4 bar (abs.), 
and preferably less than 2.5 bar (abs.) are sufficient for obtaining a 
transonic region of flow. 
5. In the Laval nozzles the gas is distributed over the length of the die 
in an absolutely uniform manner, so that additional means for ensuring 
uniformity which are necessarily associated with a loss in pressure can be 
dispensed with. 
6. Compared to the conventional melt-blowing process the specific energy 
consumption can be reduced by a factor of 2 for an identical fibre 
fineness in the range d&lt;=5 .mu.m, preferably &lt;=3 .mu.m. 
7. Due to the reduced quantity of gas required the fibers can be deposited 
more uniformly and without any secondary entangling on to the 
fibre-collecting belt, especially in the case of very fine fibers. Also 
flying fibers are avoided in the production of very fine fibers (&lt;2 .mu.m) 
and low web densities. 
8. Due to the increased rate of attenuation in the transonic region of flow 
the gas temperature can be considerably decreased in comparison with the 
conventional process for producing identical fibre thicknesses. As a 
result of the reduced quantity of gas there is also less compression of 
the web material as it is deposited on to the fibre-collecting belt; i.e. 
a web with reduced density is produced without adhesion of the fibers. 
9. Due to the absolutely uniform distribution of gas over the width of the 
die disadvantageous edge zone effects can be avoided. 
10. The process has proven particularly effective for the production of 
fibre webs with fibre finenesses of less than 3 .mu.m, and in particular 
less than 2 .mu.m. 
11. The fibre webs produced by the process have excellent filtration 
properties as a result of their reduced density and homogeneous structure. 
Compared to prior art the following superior results of such filter webs 
are predominant: 
a. higher particle filtration efficiency at a reduced flow resistance 
b. a higher dust collecting capacity, 
c. higher electrostatic charge accumulating capacity, for example if an 
electric corona discharge technique is used for charging the web. 
In the following, working examples of the invention are explained in more 
detail with reference to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Sheet fibre products, in particular fleece materials or fibre webs, 
manufactured by the melt-blowing process are of great economic importance 
in present-day technology. They are used for many applications and in 
particular in cases where very fine fibres are required in conjunction 
with high surface coverage. Virtually all melt-extrudable thermoplastic 
polymers can be used as starting materials. Possible applications are for 
example: Filtration media, hygienic filters, medical applications, 
protective clothing, absorbent media, battery separating media, insulating 
clothing etc. Materials combined with other textiles or non-woven webs are 
also known. It is thus highly important to improve the economic efficiency 
of the melt-blowing process. An increase in the melt throughput rate 
and/or a reduction in the specific air consumption is a necessary 
requirement for achieving an improvement in economic efficiency. It goes 
without saying that product quality must not in any way suffer as a result 
of such improvements; i.e. product quality must at least remain constant. 
For the production of filtration media with a high degree of filtration 
efficiency and low flow resistance microfibre media are required having a 
lower density at identical or higher fibre fineness than that disclosed in 
the prior art. It is also advantageous to be able to produce the sheet 
fibre products at lower gas and melt temperatures than those employed in 
the prior art. This permits a reduction in the tendency of the fibres to 
adhere to each other on being deposited on the fibre collecting belt and 
simultaneously decreases the tendency of temperature-sensitive polymers to 
undergo thermal decomposition during the extrusion and spinning process 
and at the same time increases the lifetime of the spinning nozzles. In 
order to obtain uniform and homogeneous product quality over the entire 
width of the web an absolutely uniform and constant distribution of air 
with regard to space and time is required. 
The production of a fibre web by the melt-blowing process is first 
described generally (i.e. according to the prior art) with reference to 
FIG. 1. The extruder 1 driven by a motor 2 is fed with a polymer via 
funnel 3. The polymer melt is delivered to the melt-blowing die 6 via melt 
filter 5 by means of a spinning pump 4. The extruder, the spinning pump, 
the melt filter, the die and the transition zones are heated in order to 
obtain the required temperature and viscosity of the melt. The 
melt-blowing nozzle 6 has inlets for the fibre-forming gas 7 which is 
supplied by means of a compressor and is heated to the required 
temperature by means of a heat exchanger (not depicted) before it enters 
the melt-blowing die 6. The melt-blowing die 6 has at least one linear row 
of fine orifices from which the melt issues by means of an inlet pressure 
produced by the spinning pump 4 and is attenuated by means of gas 7 to 
form microfibres which are deposited on a mechanically driven 
fibre-collecting belt 9 to form the finished web 10. A portion of the gas 
stream is removed by means of a suction box 11 arranged beneath the 
fibre-collecting belt 9. FIG. 2 shows a cross-section through the 
embodiment of the melt-blowing die on which the invention is based. The 
polymer melt 12 flows into slot 14 via melt distributor 13 and then into 
the orifices 15 from which it issues while being attenuated into 
microfibres by means of a gas 16 (air) supplied from both sides at a high 
rate of flow. The melt distributor 13 is arranged inside a die block 17, 
below which the melt-blowing die 18 is arranged in a melt-tight manner. 
The die block 17 and the melt-blowing die 18 are heated by means of 
electric resistance wires 19 arranged within the surrounding heating 
jacket 20. The wedge-shaped die tip 21 of the melt-blowing die 18 has an 
angle of 20.degree. to 100.degree., preferably 40.degree. to 80.degree.. 
The melt exit holes 15 are arranged linearly (perpendicularly to the 
drawing plane) and have a diameter of 0.1 to 0.6 mm, preferably 0.1 to 0.4 
mm, and a channel length 2 to 10 times their diameter. 
The fibre-forming gas 16 is fed from both sides via openings 22 into gas 
smoothing chambers 23 arranged inside the gas supply elements 24. The 
smoothing chambers 23 lead into very small, linearly arranged gas openings 
25, which are located in direct proximity to the die tip 21 and oriented 
in a direction parallel to the wedge-shaped contour of the die tip 21. The 
gas openings 25 are provided with widened sections 26 and represent 
fluidically (with regard to the flow configuration) widened Laval nozzles 
(25, 26). A flow channel 27 is arranged downstream of each of the widened 
sections 26 which is defined on the one side by the contour of the melt 
die tip 21 and on the other side by the bottom plates 28, the bottom 
plates 28 terminating with a sharp edge in the region of the apex of the 
die tip 21. The gas supply elements 24 together with the smoothing 
chambers 23 and the Laval nozzles 25, 26 are arranged on either side of 
the melt orifices 15 or die axis 29 and mirror-symmetrically in relation 
thereto. 
The gas supply elements 24 are arranged adjacently to the contour of the 
wedge-shaped die tip 21 in a gas-tight manner and can be displaced in a 
parallel direction to the wedge-shaped surfaces. It is thus possible to 
adjust the distance between the Laval nozzles 25, 26 and the melt orifices 
15. Depending on the polymer specifications and the required physical web 
properties it is therefore possible to displace the outlet of the melt 
orifices 15 in relation to the sharp-edged outlet of the issuing gas jets 
to the required extent upwards or downwards in the direction of flow. The 
bottom plates 28 can also be displaced transversely to the die axis 29, 
thus allowing the flow slot 30 or the flow channels 27 to be accurately 
adjusted. The gas openings 25 of the Laval nozzles have a diameter of 0.3 
to 2.0 mm, preferably 0.4 mm to 1 mm, and a length 0.3 to 5 times the 
diameter. The widened section 26 beneath the gas openings 25 has a total 
angle of 5.degree. to 30.degree., preferably 10.degree. to 20.degree.. The 
widened section 26 is conically shaped, and is either axially symmetrical 
to the axis of the gas opening 25 or is inclined at an angle in relation 
thereto (as shown in FIG. 3). The latter embodiment has the advantage that 
the Laval nozzles 25, 26 can be arranged in direct proximity to the die 
tip 21. The cross-section of the flow channels 27 downstream of the Laval 
nozzles converges or remains constant in the direction of flow. The length 
of the flow channels 27 is 1 to 30, preferably 3 to 20 times the largest 
diameter of the widened sections 26 of the Laval nozzles. The main purpose 
of the flow channels is to form a homogeneous region of transonic flow in 
the longitudinal direction of the flow channels 27. 
With the aid of the Laval nozzles 25, 26 and by establishing a pressure 
ratio between the flow channel 27 and the gas smoothing chamber 23 which 
corresponds at least to the critical Laval pressure ratio of 0.53, a rate 
of flow is formed in the Laval orifice 25, as a result of the known flow 
parameters, which corresponds to sonic speed at the given temperature. 
This parameter applies to all Laval nozzles 25 so that throughout the 
length of the melt-blowing die 18 (perpendicularly to the plane of 
projection) an absolutely uniform stream of gas issues from the flow slot 
30. Inlet pressures in the gas smoothing chambers 23 of 1.9 to 5 bar 
(abs.), preferably 1.9 to 2.5 bar (abs.) are sufficient for obtaining such 
flow conditions. The widened section 26 of the Laval nozzles serves to 
accelerate the flow to supersonic speed and to improve the cross-sectional 
homogeneity of the gas stream as it enters flow channel 27. Due to the 
parallel or convergent shape in flow channel 27 the ultrasonic diffusion 
effect causes the rate of flow to be reduced to transonic speed with 
optimum cross-sectional homogeneity in the proximity of the flow slot 30. 
"Transonic speed" is understood to refer to a flow rate of at most 20% (at 
maximum), and preferably at most 10% below sonic speed. The inlet 
cross-section of the flow channels 27 is 1.0 to 2.5 times the sum of the 
cross-sections of the widened sections 26 of the Laval nozzles and the 
outlet cross-section is 0.8 to 2.5 times this sum. Such conditions provide 
a high degree of flow stability and homogeneity in the critical region of 
the orifices 15. 
FIG. 3 shows the assembly of the gas smoothing chamber 23, the Laval nozzle 
25, 26 and the flow channel 27 once again in magnified form. The wall 
thickness of the portion of the gas supply element 24 adjacent to the 
outer wall of the die tip 21 at the level of the gas openings 25 (Laval 
nozzles) is as small as technically possible. The sharp-edged outlet to 
the flow channel 27 (referred to as the flow slot 30) is flush with the 
melt orifice in this embodiment. The gas smoothing chamber 23 begins with 
a relatively large cross-section and tapers continuously towards the Laval 
orifices 25, thus helping to minimise flow resistance in the subsonic 
region. The distance a, i.e. the length of the flow channel 27, is in the 
range from 1 mm to 50 mm, preferably 2.5 mm to 30 mm. 
FIG. 4 shows an alternative slot-shaped embodiment of the Laval nozzles. 
Both the Laval orifice and the widened section downstream thereof are 
slot-shaped in this embodiment. Thus the Laval nozzle consists of Laval 
slot 31 and the slot-shaped widened shaft 32 downstream thereof. The 
slot-shaped cross-section of the Laval nozzles 31, 32 extends over the 
whole width of the die tip (perpendicularly to the drawing plane). The 
widened shaft has a total angle of 5.degree. to 30.degree., preferably 
10.degree. to 15.degree.. As in the embodiment according to FIG. 2 a flow 
channel 27 with a convergent or constant cross-section which terminates 
with slot 30, is arranged downstream of the widened shaft 32. In all of 
the embodiments shown in FIGS. 2 to 5 the fibre-forming gas which produces 
the fibres and attenuates the melt streams issuing from the melt orifices 
15, is formed by gas streams directed on to the melt strands from both 
sides by means of flow channels 27. 
FIG. 5 shows a particularly advantageous modular construction in which a 
number of air supply elements 33a, 33b, 33c, 33d . . . are arranged next 
to or behind one another at the side of the melt-blowing die 18 in the 
form of an assembly of individual units. Each unit is connected via pipe 
34a, 34b . . . to a distributor pipe 35 which is supplied with the 
fibre-forming gas 16. Each gas supply element comprises a gas smoothing 
chamber 23 which supplies several Laval nozzles 25, 26 with a circular 
cross-section or one slot-shaped Laval nozzle 31, 32. 
The gas supply elements 33a, 33b . . . are sealed at their front ends, so 
that they represent individually effective units which are juxtaposed to 
each other in a gas-tight manner. As shown in FIG. 5 and in accordance 
with the basic embodiment according to FIG. 2 the gas supply elements are 
arranged mirror-symmetrically (to the central plane of the melt-blowing 
die 18) on either side of the die tip 21. 
The embodiment according to FIG. 5 has the following advantages especially 
for the production of fibre webs of large widths: 
1. The gas stream in slots 30 is absolutely uniform over the whole width, 
even where dies of large dimensions are employed. 
2. Provided the width of the individual units is not too large, 
misalignment of the Laval openings 25 or the Laval slot 31 during the 
manufacture of the Laval nozzles can be avoided. Appropriate unit widths 
are in the range from 25 to 500 mm, preferably 50 to 200 mm. 
3. The unit assembly allows the air supply elements to be connected to the 
melt-blowing die 18 in the best possible manner. 
4. Webs of different widths can be obtained in a simple manner. 
EXAMPLE 
Polypropylene produced by Exxon, Type PD 3495 having a melt flow index of 
800 g/10 min was melted according to FIG. 1 and delivered to a 
melt-blowing die according to FIGS. 2 and 3 having the following 
characteristic dimensions: 
Diameter of the melt orifices 15: 0.3 mm 
Channel length: 3.8 mm 
Lateral distance of melt orifices 15: 1.25 mm 
Apex angle of the melt die tip 21: 60.degree. 
Diameter of the Laval orifice 25: 0.6 mm 
Length of the Laval orifice 25: 0.3 mm 
Widened section 26 of the Laval nozzle: Total angle 15.degree.; 
End diameter: 0.7 mm 
Lateral distance of the gas openings 25: 0.8 mm 
Flow channel 27: initial width 0.8 mm; width at exit (at the level of the 
sharp-edged outlet): 0.7 mm; 
Length: 2.3 mm 
Unit width: 50 mm 
Number of units: 2 on each side. 
The sharp-edged outlet of the air exit slot (flow slot) 30 was flush with 
the die tip 21. Air was used as the fibre-forming gas; it was compressed 
in a screw compressor and heated to the required temperature in an 
electric heater arranged downstream of the compressor. 
During web formation a portion of the volumetric stream of the 
fibre-forming gas was removed with the aid of the suction removal means 
11. 
Table 1 shows the results obtained in relation to fibre load, fibre 
diameter and specific energy consumption, the apparatus was operated with 
the following process parameters: 
Static pressure of the air in the gas smoothing chamber 23: 3 bar (abs.), 
Temperature of the air: 285.degree. C., 
Melt temperature: 230.degree. C., 
Inlet pressure of the melt upstream of the filter 5 (see FIG. 1): 35 bar. 
With the above process parameters a sonic speed of about 440 m/s results in 
the Laval nozzles and a rate of flow of about 5% below sonic speed is 
obtained at the flow slot 30. The distance between the melt-blowing nozzle 
21 and the fibre-collecting belt 9 was 0.3 m. Table 2 shows the results of 
a further series of tests, in which the static pressure of the air in the 
smoothing chambers 23 was decreased to 2.2 bar (abs.) and the gas 
temperature was increased to 294.degree. C. No changes were made in the 
remaining operating parameters. 
In the tables: 
m.sub.F,B is the melt mass throughput per hole, 
.lambda. is the air stream load (ratio of the mass flow rate of the fibres 
to the mass flow rate of the blowing air) 
1/.lambda. is the consumption of blowing air in relation to the quantity of 
fibres produced d.sub.F is the mean fibre diameter 
E.sub.L /m.sub.F is the specific net energy consumption necessary for 
compressing and heating the blowing air, based on the quantity of fibres 
and at an inlet temperature of 40.degree. C. of the air into the electric 
air heater. 
TABLE 1 
______________________________________ 
.lambda. 
. m.sub.F,B 
*10.sup.-3 
1/.lambda. d.sub.F 
E.sub.L /. m.sub.F 
(g/min) (kg.sub.F /kg.sub.L) 
(kg.sub.L /kg.sub.F) 
(.mu.m) 
(kWh/kg.sub.F) 
______________________________________ 
0,089 3,37 297 1,25 36,6 
0,16 6 165,6 1,68 20,4 
0,23 8,7 114,8 2 14,2 
0,3 11,3 88,8 2,3 11 
0,43 16,2 61,7 2,6 7,6 
0,56 21,1 47,3 2,95 5,8 
0,87 32,9 30,4 3,25 3,7 
______________________________________ 
TABLE 2 
______________________________________ 
.lambda. 
. m.sub.F,B 
*10.sup.-3 
1/.lambda. d.sub.F 
E.sub.L /. m.sub.F 
(g/min) (kg.sub.F /kg.sub.L) 
(kg.sub.L /kg.sub.F) 
(.mu.m) 
(kWh/kg.sub.F) 
______________________________________ 
0,086 4,47 223,8 1,44 25,1 
0,15 7,9 126,6 1,61 14,2 
0,23 12 83 1,77 9,3 
0,3 15,8 63,3 2,11 7,1 
0,43 22,2 45,1 2,65 5,1 
0,57 29,6 33,8 3,1 3,8 
______________________________________ 
In both test series it was found that very fine fibres can be obtained with 
very efficient energy consumption values. In the second test series 
considerably lower energy consumption values were obtained especially for 
fibre finenesses of less than 2.5 .mu.m. 
The graph (FIG. 6) shows a comparison between the two test series operated 
with a transonic rate of gas flow and the conventional melt-blowing 
process operated with the same number of melt orifices 15 per cm of die 
width. All of the average fibre diameters were measured by the same 
aerodynamic measuring method. The advantages of a transonic rate of gas 
flow, in particular for average fibre diameters of less than 3 .mu.m are 
clearly evident. 
As far as the physical properties of the fibre web products are concerned 
those produced by the process according to the invention are distinguished 
by a very low density and very soft handle. No adhesion of the fibres 
could be detected. Irrespective of the distance between the die and the 
fibre-collecting belt 9 virtually no flying fibres were detected even with 
very fine fibre diameters of &lt;2 .mu.m. 
There has thus been shown and described a novel method and device for 
manufacturing ultrafine fibres from thermoplastic polymers which fulfills 
all the objects and advantages sought therefor. Many changes, 
modifications, variations and other uses and applications of the subject 
invention will, however, become apparent to those skilled in the art after 
considering this specification and the accompanying drawings which 
disclose the preferred embodiments thereof. All such changes, 
modifications, variations and other uses and applications which do not 
depart from the spirit and scope of the invention are deemed to be covered 
by the invention, which is to be limited only by the claims which follow.