GB2124138A - Melt blowing highly oriented fibers - Google Patents

Abstract

A thermoplastic condensation polymer is melt blown at an initial velocity of 150 to 300 m/s at a temperature which is less than 50 DEG C above its crystalline melting point and at which the molten polymer has an apparent melt viscosity of less than 50 poise. The melt blown fibers are collected to form a non-woven web, e.g. while being contacted with hot air heating the fibers to 70-265 DEG C. A suitable apparatus is shown. <IMAGE>

Description

SPECIFICATION Forming non-woven webs from highly oriented melt blown fibers This invention relates to melt-blowing processes, and more particularly to a process and apparatus for forming heat shrinkable non-woven webs from highly oriented melt blown thermoplastic fibers.

Various melt-blowing processes have been described heretofore including those of Van. A. Wente (Industrial and Engineering Chemistry, Volume 48, No. 8 (1956)), Buntin et al. (U.S. Patent 3,849,241), Hartmann (U.S. Patent 3,379,811), and Wagner (U.S. Patent 3,634,573), and others, many of which are referred to in the Buntin et al. patent.

Some of such processes, e.g. Hartmann, operate at high melt viscosities, and achieve fiber velocities of less than 100 m/second. Others, particularly Buntin et al., operate at lower melt viscosities (50 to 300 poise) and require severe polymer degradations to achieve optimum spinning conditions. It has been described that the production of high quality melt blown webs requires prior degradation of the fiber forming polymer (U.S. Patent 3,849,241). At an air consumption of more than 20 kg of air/kg web substantially less than sonic fiber velocity is reached. It is known, however, that degraded polymer leads to poor web and fiber tensile strength, and is hence undesirable for many applications.

Our published application GB 2 073 098A discloses a process and apparatus for extruding through nozzles at high temperatures a molten polymer at low melt viscosity wherein the molten fibers are accelerated to near sonic velocity by gas being blown in parallel flow through small orifices surrounding each nozzle. The products produced thereby as well as in accordance with U.S. Patent No. 3,849,241 are most polyolefins with only nominal molecular orientation. Fibers produced by the prior art melt-blowing processes are weak with unoriented molecular chain structure exhibiting no heat shrinkage characteristics and low values of birefringence.

It would therefore be desirable to be able to form heat shrinkable non-woven webs comprised of highly oriented fibers from a thermoplastic condensation polymeric material, the non-woven webs possessing high tension and compression moduli, exhibiting bulk retaining properties, and being of a highly bulky web structure.

The present invention provides a process comprising melt blowing at an initial velocity of from 500 to 1000 ft/s (150 to 300 m/s) a melt blown molten at T = less than MP + 50"C thermoplastic condensation polymer at a temperature less than 50"C above the melting point thereof to form fibers of high molecular orientation, and collecting the fibers to form a non-woven web.

In one embodiment of the present invention, the fibers are collected on a rotating mandrel and heat treated during collection or subsequent to collection.

In another embodiment of the present invention, the molten polymer is passed to nozzles through a first heating zone at low incremental increases in temperature, and thence rapidly through the nozzles at high incremental increases in temperature to reach the low melt viscosity necessary for high fiber acceleration at short residence time to minimize or prevent excessive polymer degradation.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic partially cross-sectional elevational view of apparatus for forming a non-woven web of melt blown fibers; Figure 2 is a partial side view of the apparatus of Figure 1; and Figure 3 is an enlarged fragmentary cross-sectional view of the nozzle configuration of a die assembly, taken along the line 2-2 of Figure 1.

The thermoplastic polymers which are processed in accordance with the present invention are condensation polymers,such as polyethylene terephthalate and nylon 6,6. Such thermoplastic polymers, when extruded into fibers by a melt-blowing technique, exhibit high thermal shrinkage under specific process conditions of high filament extrusion velocity, low melt viscosity, low molecular weight, and at spinning temperatures less than 50"C above the melting point of the thermoplastic polymer. As described in the above-mentioned references, conventional fibers extruded in melt blowing processes are at temperatures more than about 150"C above this crystalline melting point.

The oriented fibers produced according to the present invention are generally not fuse bonded and are substantially continuous. As hereinafter more fully described, the oriented fibers are formed into a highly bulky web-like structure. The thus formed bulky web-like structures have many uses, particularly for applications considering structural resistance to compaction pressure, since the oriented fibers have higher tensile and compression moduli than unoriented non-woven webs.

The products of the present invention exhibit excellent thermal insulation properties, and are thus useful in the manufacture of sleeping bags, gloves, winter jackets, pullovers, and the like. Additionally, there is useful application based upon the shrink effect of the oriented fibers, e.g. as a filter medium. Exposure of the fibers to a temperature above the glass transition temperature of the polymer causes the web density due to the shrink effect to increase by a factor of up to twenty (20), e.g. from about 0.01 to 0.20 g/cm3. Such shrinkage characteristic produces a compact, highly entangled web of unbonded fibers possessing good mechanical strength.

In this connection, several melt-blown cartridge filters have been described in the prior art, but none with advantages provided by the present invention. Thus, Vogt et al. (U.S. 3,904,798) describes a polypropylene cartridge of self-bonded, continuous fibers. Although self-bonding increases the rigidity of the cartridge it detracts from the filtration efficiency by decreasing the open spaces through which the fluid to be filtered can flow. Pall (U.S. 4,032,688) describes a filter cartridge made of unbonded, discontinuous polypropylene fibers (made by a melt-blowing process) spirally wound on a rotating mandrel to keep the tubular web of the unbonded fibers from collapsing.

Referring now to Figure 1, a die loins comprised of a long tube 12 having a chamber 14 connected to a thick plate 16 having holes into which nozzles 18 are inserted and silver soldered to prevent slippage and leakage.

The nozzles 18 extend through an air manifold 20 and through holes in a lower plate 22 in a pattern shown in Figure 3. The air manifold 20 is provided with an air pressure gauge 24, a thermocouple 26, and an air supply tube 28 which in turn is provided with an in-line air flow meter 30 upstream of an air heater 32. Some of the hot air leaving the air heater 32 is passed through a jacket (not shown) surrounding the tube 12 to preheat a transition zone.

The die 10 is fed with hot polymer from an extruder 34. The tube 12 is provided with thermocouples 36 to measure the polymer melt temperature. A pressure transducer 38 measuring polymer melt pressure is located in a cavity 40 proximate the nozzle inlet. A resin bleed tube 42 and a valve 44 allow one to bypass resin from the extruder 34 and thus reduce resin flow rate through the nozzles 18. The bleed valve 44 permits adjustment to different temperature and heat transfer patterns in the tube 12 as well as in the nozzles 18.

Beneath the die 10, there are positioned a baffle assebly 50, a mandrel assembly 52, and an aspirating air assembly 54. The baffle assembly 50 comprises downwardly and inwardly extending side walls 56 and end walls 58, referring specifically to Figure 2, forming an elongate slot 60 for directing melt blown fibers from the nozzles 18 of the die 10 towards the mandrel assembly 52.

The mandrel assembly 52 comprises a mandrel 62 mounted for rotation on a shaft of a motor 64. The mandrel 62 is disposed in a plane parallel to and beneath the elongate slot 60 of the baffle assembly 52 for collecting the melt blown fibers, as more fully described below. The aspirating air assembly 54 comprises upwardly and inwardly extending side walls 66 and end walls 68 forming an elongate slot 70 for directing a gas, such as air, at a velocity sufficient to cause the melt blown fibers to become highly entangled as the fibers are collected on to the mandrel 62. The air stream may be heated as hereinafter discussed.

A cartridge forming assembly 72, comprising arm members 74 with rotatable gear elements 76, is provided for continuously moving on the mandrel 62 a compact mass of highly entangled melt blown fibers in cylindrically shaped cartridge form during collection of the fibers.

In operation, a condensation polymer of an intrinsic viscosity of less than 0.6 is heated to a temperature less than 50"C above the melting temperature and is extruded through the nozzles 18 towards the baffle assembly 50. As the melt blown fibers drop through the slot 60, they are contacted with a gaseous stream at ambient temperature or at a temperature sufficient to heat the fibers to a temperature of from 70 to 2650C and at an initial velocity of from 500 to 1000 ft/s (150 to 300 m/s) to cause the melt blown fibers to form a highly entangled web of unbonded fibers, which are gathered on the mandrel 62 rotating at an angular velocity of 5 to 500 rev/min, preferably 10 to 250 rev/min.

A cartridge d-shaped mass 80 is formed about the mandrel 62 to a radial thickness of from 3/4 to 5 inches (2 to 13 cm), which cartridge d-shape mass may be continuously urged from left to right, as illustrated by the arrow A, by the collection assembly 72, or alternately moved back and forth until a desired thickness is attained.

Operation of the process is described in the following Examples which are intended to be nearly illustrative, and the invention is not to be regarded as limited thereto. It will be shown that cartridges produced by a process according to the present invention comprise unbonded, continuous melt-blown filaments of condensation polymers compacted to a high density by the shrinkage effect, that high mechanical rigidity is obtained without self-bonding, and that filtration efficiency is not decreased by bonding.

The melt-blowing die assembly used in the following Examples comprises four rows of nozzles 18 with 50 nozzles per row. In such assembly, a screen, having the same spacing as the extrusion nozzles is used to form four air orifices around each extrusion nozzle. (See Figure 3.) The capillary arrangement had the following dimensions: length of capillary, 1.27 cm; inside diameter, 0.03302cm; outside diameter, 0.0635 cm; distance between capillaries, centre to centre, 0.1058cm: Apparent Melt Viscosity (AMV) is calculated from Poisseuille's equation: Q = sr pr4/8 tal, wherein Q = polymer flow through a single nozzle (cm3/s), p = polymer pressure (dyn/cm2), r = inside nozzle radius (cm), I = Iengthofcapillary(cm),and X = apparatus melt viscosity (poise).

To calculate Q (cm3/s) from the polymer flow rate measured in grams per minute, the following densities of the solid polymer have been used: 1.36 g/cm3 for polyester, and 1.15 g/cm3 for nylon 6,6.The term "intrinsic viscosity" (IV), as used herein, is defined as the limit of the ratio In(r)/C, as C approaches zero, where r is the relative viscosity, and C is the concentration in grams per 100 ml of solution. The relative viscosity (r) is the ratio of the viscosity of a solution of a polymer to the viscosity of the pure solvent per se, measured in the same units as 25"C. Intrinsic viscosity is a measure of the molecular weight of a polymer.

Apparent melt viscosity (AMV) is a measure of a combination of temperature and molecular weight.

For polyethylene terephthalate (polyester), a solvent mixture of one part trifluoroacetic acid and three parts of methylene chloride (by volume) is used; for nylon 6,6(polyhexa- methylene adipamide), ortho-cresole is used.

Fiber diameter is an average value obtained by optical or stereoscan electron microscopy.

Fiber velocity, V(cm/s), calculated from V = Q'/Ad, wherein Q' = polymer flow through single nozzle (g/s), A = fiber cross section area (cm2), d = density of solid polymer (g/cm3).

% Shrinkage = 100 (I0-lWl0, wherein lo = length of a dissected filament as initially extruded, It = length of the filament after heating for 15s at 1200C.

Birefringence is the difference of the refractive indices parallel and perpendicular to the fiber axis.

Example I Three types of dried polyethylene terephthalate resin (A,B, and C) were extruded respectively, through the above-described melt-blowing system. Type A had an initial intrinsic viscosity of 0.38; Type B, 0.50; and Type C, 0.65. The extruder (2.5 cm screw diameter, UD ratio 24/1) had three heating zones; the hopper (inlet) zone was heated to 265 C, the middle zone to 285"C and the outlet zone to 295"C. Heated air was passed to the die at a gauge pressure of 25 psi (1.7 bar), the temperature was varied and measured in the air cavity, i.e.

extrusion temperature. The die block temperature equilibrates with the air temperature after a few minutes of extrusion. The following Table I lists the results: TABLE I Melt-Blowing of Shrinkable Polyester (PET) Resin Run Extrusion Polymer Polymer AMV L.V. fiber Fiber % Birefringence Type Temper- Flow Rate Pressure Poise (Fiber) diameter Velocit Shrink ature C tbih(kglla) psi (bar) Micron mit sge A 1 320 3.17(1.41) 2 (0.14) 3 0.32 1.8 579 60 A 2 300 3.17(1.41) 5.3 (0.37) 6 0.32 2.0 469 82 0.1200 A 3 290 2.91(1.32) 21 (1.45) 25 0.33 2.6 254 39 A 4 285 2.64(1.20) 35 (2.40 45 0.35 3.2 153 21 A 5 320 27.8(12.6) 32 (2.2) 4 0.33 5.7 510 52 A 6 300 29.1(13.2) 68 (4.7) 8 0.33 6.2 440 70 0.0850 A 7 290 29.1(13.2) 256 (17.6) 30 0.34 9.3 220 34 A 8 285 29.1(1.32) 382 (20.3) 45 0.34 12.5 110 19 B 9 320 2.91(1.32) 34 (2.36) 40 0.45 2.9 200 29 B 10 330 2.91(1.32) 47 (3.2) 56 0.46 3.8 120 24 B 11 290 2.65 (1.20) 69 (4.1) 76 0.46 4.7 70 20 B 12 285 2.65(1.20) 70 (4.8) 91 0.47 5.3 55 10 0.0065 C 13 320 3.17(1.41) 69 (4.1) 64 0.60 39 126 13 C 14 300 3.17(1.41) 71 (4.9) 83 0.60 52 71 5 C 15 290 3.17(1.41) 89 (8.1) 86 0.60 73 36 0 0.008 Run 2 (low molecular weight resin, at 6 poise apparent melt viscosity, 3000C extrusion temperature) exhibited the highest shrinkage value. At higher extrusion temperature, the molecular orientation of the polymer induced by the high spinning velocity has more time to de-orient the melt phase, since cooling of the fiber takes longer. At lower extrusion temperatures, shrinkage also decreases, as melt viscosity increases and fiber velocity decreases.The same effects are seen in Runs 5 to 8, which are nearly identical to Runs 1 to 4, except that resin throughout is increased and fiber diameters are correspondingly larger. Using resins of higher molecular weight (Type B and C) shows the effect of higher apparent melt viscosities (AMV) and lower fiber velocities leading to lower shrinkage values.

Example II Two types of textile grade nylon 6,6, DuPont's "Zytel" (trade mark), (Type D = 0.45 IV, and Type E = 0.80 IV) were melt-blown under the conditions described in Example I. The results are listed in Table II, below, as Runs 1 to 7.

TABLE II Melt-Blowing of Shrinkable Nylon 6,6 Resin Run Extrusion Polymer Polymer AMV 1. V. Fiber Fiber % Type Temper- Flow Rate Pressure Poise (FiberJ diameter Velocity Shrink ature "C Iblhrkglh) psi (bar) Micron mis age D 1 320 3.25(1.47) 12(0.83) 11 0.38 2.1 511 45 D 2 300 3.25 (1.47) 20 (1.4) 18 0.38 2.4 391 72 D 3 290 3.14(1.42) 34 (2.35) 31 0.40 3.4 194 37 D 4 285 3.14 (1.42) 53 (3.65) 48 0.41 5.2 83 18 E 5 320 2.75 (1.25) 69 (4.8) 75 0.74 7.5 33 0 E 6 300 2.75(1.25) 84 (5.8) 92 0.74 12 13 0 E 7 290 2.65(1.20) 97 (6.7) 107 0.76 14 10 0 The shrinkage effects are similar to those for polyester.About 300"C, % shrinkage decreases again for low molecular weight resin, and decreases also as fiber velocities decrease at the lower temperatures. The high molecular weight resin (Type E) showed almost no shrinkage, owing to high AMV and low fiber velocities.

Example 111 Very low molecular weight polypropylene of 150 gram/10 minutes Melt Flow Rate and a crystalline melting point of 1 60"C was processed in the melt-blowing system described in Example I. The extruder zones were heated to 210"C. No fibers formed at an extrusion temperature of 210"C, owing to too high a melt viscosity.

At high extrusion temperatures of 260 to 300 C, fibers formed but exhibited no shrinkage upon heating to 125"C.

Polyester of Type A (Example I) was melt blown through the apparatus described in Figure 1, under conditions of Example I, Run 2, and collected on a rotating mandrel rod of 3/4 inch (2 cm) diameter and 12 inch (30 cm) length disposed 18 inches (45 cm) below the nozzle die. The mandrel was driven at 120 rev/min.

The baffle assembly 50 was placed between the die and the mandrel 62 to direct all fibers onto the rotating mandrel 62. The fibers having a velocity immediately below the die of about 470 m/s entangled to a fluffy, bulky web at the lower part of the baffle assembly 50. This web was then pulled down by the rotating mandrel and wrapped around it. The mandrel was moved from one end to the other to cover all 12 inches (30 cm) with a fiber sleeve. After 3 minutes of collecting, a tubular sleeve about the mandrel 62 had grown to 3 inches (7.6 cm) in diameter. The fiber sleeve was slipped off the rod. The tubular cartridge, comprising continuous, unbonded fibers, was soft, could be easily bent and collapsed by hand, and had a density of 0.055 g/cm3.

Example IV Another tube was prepared (72 grams, 3 inch (7.6 cm) diameter), as per Example Ill, and a hot stream of air at a temperature of about 200"C was directed on to the rotating fiber covered mandrel. Within about 3 seconds, the fiber sleeve had shrunk to a diameter of 1.75 inches (4.45 cm) at a density of 0.186 g/cm3. The tube, after being slipped off the rod was firm and rigid, and withstood without collapsing a pressure perpendicular to its axis of 1.2 Ib/linear inch (2.1 N/cm).

Example V In this Example a hot air stream was directed onto the mandrel 62 while the fiber web was collected on the mandrel 62, thereby simultaneously performing spinning, collecting, and shrinking. After 3 minutes, the fiber sleeve built up to a diameter of 1.6 inches (4.1 cm) at a density of 0.23 g/cm3. The tube could withstand without collapsing a pressure of 2 Ib/linear inch (3.5 N/cm).

Example Vl Example V was repeated using extrusion conditions of Table 1, Run 6 (200 g/min throughput). After 18 seconds, the tube built up to a diameter of 1.75 inch (4.45 cm) at a density of 0.19 g/cm3). The tube exhibited a porosity of 86%, where % porosity = [1 - (D/d)l x 100 wherein D = bulk density of cartridge and d = density of fiber.

The tube could withstand a pressure perpendicular to its axis of 1.8 Ib/linear inch (3.2 N/cm) and comprised unbonded, continuous, highly entangled fibers.

Example Vll Afiberweb was collected on the 12 inch (30 cm) rod (as described in Example VI). After formation to a diameter of about 1.75 inch (4.45 cm), the web sleeve was built up on the free end of the rod, the rotating tube was gripped with the clamping device pressed against the sleeve, and pulled away at a rate of about 3 ft/min (about 1 m/min). A continuous tube of a density of 0.2 g/cm3, an inside diameter of 0.75 inch (1.9 cm) and outside diameter of 1.75 inch (4.45 cm) was thus continuously formed. Example VII demonstrates continuous spinning, collecting, shrinking, and withdrawal of a continuous tube.

While the present invention has been described with reference to a melt blowing die assembly wherein the fibers are formed at sonic velocity, it is to be understood to one skilled in the art that any melt blowing die assembly may be used in the present invention.

Claims (20)

1. A process for producing a non-woven web of oriented melt blown fibers, which comprises: (a) heating a thermoplastic condensation polymer to a molten state; (b) melt blowing at an initial velocity of from 500 to 1000 ft/s (150 to 300 m/s) the molten polymer at a temperature which is less than 50"C above its crystalline melting point and at which the molten polymer has an apparent melt viscosity of less than 50 poise, to form melt blow fibers; and (c) collecting the melt blown fibers to form the non-woven web.

2. A process as claimed in claim 1,wherein the melt blow fibers of step (b) are contacted with a gaseous medium to form a highly bulky web structure.

3. A process as claimed in claim 2, wherein the gaseous medium is air.

4. A process as claimed in claim 2 or 3, wherein the gaseous medium is heated to a temperature sufficient to heat the fibers to a temperature of from 70 to 265"C.

5. A process as claimed in claim 1, wherein the non-woven web is contacted with a gaseous medium at a temperature sufficient to heat the fibers to a temperature of from 70 to 2650C.

6. A process as claimed in claim 5, wherein the gaseous medium is air.

7. A process as claimed in any preceding claim, wherein step (c) is effected on a rotating mandrel.

8. A process as claimed in claim 7 when dependent on claim 4 or 5, wherein heating is effected for a time sufficient to shrink the web to a point at which the web is of a density of at least 0.1 g/cm3.

9. A process as claimed in claim 8, wherein the fibers, after cooling, have an intrinsic viscosity (as defined herein) of less than 0.6.

10. Apparatus for producing a non-woven web of oriented melt blow fibers, which comprises: (a) means for heating to a molten state a thermoplastic condensation polymer; (b) means for melt blowing at an initial velocity of from 500 to 1000 ft/s (150 to 300 m/s) the molten polymer at a temperature which is less than 50"C above its crystalline melting point and at which the molten polymer has an apparent melt viscosity of less than 50 poise, to form melt blow fibers; and (c) means for forming a non-woven web from the melt blown fibers.

11. Apparatus as claimed in claim 10, including means for contacting the melt blow fibers with a gaseous medium to form a highly bulky web substrate.

12. Apparatus as claimed in claim 11, including means for heating the gaseous medium to a temperature sufficient to heat the fibers to a temperature of from 70 to 2650C.

13. Apparatus as claimed in any of claims 10 to 12, wherein the forming means comprises a rotating mandrel for forming the fibers into a tubular sleeve.

14. Apparatus as claimed in claim 13, including a means for moving the tubular sleeve on the rotating mandrel.

15. Apparatus as claimed in claim 14, wherein the means for moving said tubular sleeve includes wheel members for contacting the tubular sleeve.

16. A non-woven web of thermoplastic fibers, comprising oriented fibers of a thermoplastic condensa tion polymer melt blown at a temperature which is less than 50"C above its crystalline melting point and at which the polymer has an apparent melt viscosity of less than 50 poise, the fibers having an intrinsic viscosity (as defined herein) of less than 0.6.

17. A non-woven web as claimed in claim 16, in which the fibers of the web are heat shrunk to a point at which the web is of a density of at least 0.1 g/cm3.

18. A tube comprising unbonded, entangled, oriented fibers of a thermoplastic condensation polymer melt blown at an initial velocity of from 500 to 1000 ft/s (150 to 300 m/s) and at a temperature which is less than 50"C above the crystalline melting point and at which the polymer has an apparent melt viscosity of less than 50 poise, the fibers having an intrinsic viscosity (as defined herein) of less than 0.6.

19. Atube as claimed in claim 18, wherein the fibers of the tube are heat shrunk to a point at which the tube is of a density of at least 0.1 g/cm3.

20. A process as claimed in claim 1, substantially as described herein.