CA1224608A

CA1224608A - Process and apparatus for forming non-woven webs from highly oriented melt blown fibers and products produced thereby

Info

Publication number: CA1224608A
Application number: CA000428921A
Authority: CA
Inventors: Eckhard C.A. Schwarz
Original assignee: Biax Fiberfilm Corp
Current assignee: Biax Fiberfilm Corp
Priority date: 1982-06-07
Filing date: 1983-05-26
Publication date: 1987-07-28
Also published as: DE3320058A1; GB8315616D0; GB2124138A; GB2124138B; JPS591758A

Abstract

Abstract of the Disclosure There is disclosed a process and apparatus for melt blowing at an initial velocity of from 500 to 1000 feet per second a molten 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 aspect of the present invention, the fibers are collected on a rotating mandrel one heat treated during collection or subsequent to collection.

-- i --

Description

6~
Field of the I_vention This invention relates to melt-blowing processes, and more particularly -to a proce~ss and appara-tus for forming novel heat shrinkable non-woven webs from highly oriented mel-t blown thermoplastic fibers.
Background of the Invent1on Various melt-blowiny processes have been described heretofore including those of Van A. Wente (Industrial and Engineering Chemistry, Volume 48, No. 8 (1956), Buntin et al (United States Paten-t 3,849,241), Hartmann (United States Patent 3,379,811), and Wagner (United States 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 30G 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 (United States Patent 3,849,241). At an air consumption of more than 20 lb. of air/lb. 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.
In Canadian Patent No. 1,157,610 there is disclosed a process and apparatus for extruding through nozzles at high temperatures a molten polymer at l.ow me].-t viscosity wherein the molten Eibers are accc?lerated to near soni.c veloci.ty by gas bei.ng bl.own -i.n parall.el ~I.ow through small orifices surround:incl each noxxle. 'rhe products prodwced thereby as well as in accorcl-ance with Unitecl States Palen-t No. 3,8~9,2~l are mostly poly-

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olefins with only nominal molecular orientation. Fibers pro-duced by the prior art melt-blowing processes are weak with unoriented molecular chain structure exhib:iting no heat shrink-age characteristics and low values of birefringence.
Ob1ects of the Invention It is an object oE the present invention to provide a novel apparatus and process for forming novel non-woven webs.
A further object oE the present invention is to pro-vide a novel apparatus and process for forming novel heat shrinkable non-woven webs comprised of highly oriented fibers from a thermoplastic condensation polymeric material.
Another object of the present invention is to provide a novel apparatus and process for forming novel heat shrinkable non-woven webs possessing high tension and compression moduli.
Still another object of the present invention is to provide a novel apparatus and process for forming novel heat shrinkable non-woven webs exhibiting bulk retaining properties.
A still further object of the present invention is to provide a novel apparatus and process for forming novel heat shrinkable non-woven webs of a highly bulky web structure.
Yet another object of the present invention is to pro-vide a novel heat shrinkable non-woven web formed of highly oriented fibers and in a highly bulky web structure.
Summary of the Invention These and other objects of the present invention are achieved by a process for producing a non-woven web of oriented melt blown fibers, which comprises:
(a) heating a thermoplastic condensation polymer to a rnolten state, (b) rnelt blowing at an ini.tial velocity of.:~rQm 500 to lO00 ~eet p~r second saicl molten thermop:Lastic~ cc~ndensat.ion pol~mer at a temperature of less than about 50~ abovc3 the mel.t-`- ~22~6C18 ing point thereof and whereat said molten polymer has an apparent melt viscosity of less than 50 poise to form melt blown .Eibers, and (c) collecting said melt blown fibers to form said non-woven web. In one aspect oE 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 the nozzles through a first heating zone at low incremental increases in temperature, and thence rapidly through said nozzles at high incremental increases in - 2~ -V j ~iLZ24~

temperature to reach the low melt viscosity necessary for high fiber accelera~ion at short residence time to minimize or prevent excessive polymer degradation.
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A better understanding of the present invention as well ~s other objects and S advantages thereof will become apparent upon consideration OI the det~ileddisclosure thereof, especially when taken with the accompanying drawings, wherein lilce num~rals designate lil~e parts throughout; and wherein Figure 1 is a partially schematic cross-sectional elevational view of the apparatus OI the present mvention;
Figure 2 is a partial side view of the apparatus of Figure 1; and Figure 3 is an enlarged partial cross-sectional view of the nozzle configurationfo~ such die assernbly, taken along the line 2-2 of Figure 1.

The tnermoplastic polymers which are processed in accordance with the present invesltion are condensation polymePs, such as polyethylene terephthalate, nylon 6,~, etc. i.e. thermoplastic polymers when extruded into ~ibers by a melt-blowing technique exhibit high thermal shrinkage under specific set of process conditions of high filament e~ctrusion velocity, low melt viscosity, low molecular weight and at spinning temperatures of less than 50C. above the melting point of the thermoplastic polymer. As described in the hereinabove references, conventional fioers e:~truded in melt blowing processes are at temperatures above about 150C.
above the crystalline melting point thereof.
The oriented fiber o~ the present invention are generally not fuse bonded and are essentially continuous. As hereinafter more fully described, the oriented fibers of the present invention are formed into a highly bulky web~ e structure. 'rhe thus ¦ formed bulky web-like structures have many uses~ particularly for applications ¦ consi~erin~ structulal resistcmce to cornpaction pressure, since the oriented fibers have hig~her ten~sile and compressiorl moduli than unoriented non-woven web,.

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l ~Z;~D~6C~3 The products of the present invention exhibit excellent thermAI 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 media. Exposure of the .ibers ¦ to a temperature above the glass transition temperature of the polymer causes the web density due to the shrink effect to increase by a Iactor of up to twenty (20), i.e., from about 0.01 to 0.20 grams/cc. Such shrinkage characteristic produces a compact, highly ent_nged web of unbonded fibers possessing good mechanical strength.
In this connection, sever~l melt-blown cartridge ~ilters have been described in the prior art, but none with advantages of the present invention. Thus, Yogt et al.
(U.S. 3,904,79d,~ describes a polyprop~lene cartridge of self-~onded, continuous fibers.
Although, self-bonding increases the rigidity of the cartridge, it detracts from the filtration efficiellcy by decreasing the open spaces through which the fluid to be l filtered can flow. Pall (U S 4/032~688) describes a filter cartridge made of unbonded, 15 ¦ discontinuous polypropylene Iibers (made by a melt-blowing process) spirally wound on ~ rotating mandrel to keep the tubular web of the unbonded ~ibers from collapsing.
¦ Referring now to Figure l, a die, ~enerally indicated as lO, is comprisad of a long tube 12 having a chamber 1~ connected to a thick plate 1~ into which nozzles 18 are inserted through holes in plate 16, and silver solder (not shown) disposed to preve~t slippage and leakage. The nozzles 18 e~ctend 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 '~8 which in turn is provided with an in line air flow meter 30 upstream of an air heate~
32. Some o~ the hot air e:citing air heater 32 is passed through a jacket (not shown) surrounding tube 12 to preheat a transition zone.
The tubular die 10 is fed with hot polyrner from an e~ctrucler 34. ~he tube l2 i~
provided with thermocouples 36 to measure the polymer Inelt tampe ~turs. -~
pressure tr~n~ducer 38 measuring ?olymer melt pressure is loc/lted in ~ ClVity 10 proximllte the noæ~le inlet. There is ~rovidecl ~ resin bleed tube ~2 and fl valve -~ tc .~ . n ~

.

bypass resin from the e~truder 34 and thus reduce resin flow rate through the nozzles 18. The bleed valve 44 permits adjustment to different ~.emperature and heat transfer patterns in the tube 12 as well as in the nozzles 18.
Eleneath the die 10, there is positioned a baffle assembly, a mandrel assembly S and an aspirating air assembly~ generally indicated as 50, 52 and 54, respectfully.
The baffle assembly S0 is comprised of downwardly and inwardly extending side walls 36 and end waLts v~? referring specifically to Figure 2, forming an elongated slot 60 for directing ~elt blown fibers from the nozzles 18 of the die assembly 10 towards the mandrel assembly 52.
The mandrel assembly 52 is comprised of mandrel 62 mounted for rotation to a shaft of a motor 64. The mandrel 62 of the mandrel assembly 52 is disposed in a plane parallel to and beneath the elongated slot 62 of the baffle assembly 52 Ior collectillg the mèlt blown flbers, ~ more fuUy hereinafter described. The aspirating air assembly Sd~ is comprised of upwardly and inwardly extending side walls 66 and end walls ~8 forming an elongated slo~ 7û for directing a gas, such as air, at avelocity sufficient to cause the melt blown fibers to become highly entangled as the Iibers are collected on to mandrel 62. The air stream may be heated as hereinafter ¦ discussed.
'. cartridge forming assembly, generally indicated as 72, comprised of arm ¦ members 7~ includin~ rotatable gear elements 76? is provided for continuously moving ¦ on the manarel 62 a compact mass OI highly entangled melt blown fiber in ¦ cylindrically sllaped cartridge form during collection of the fibers.
In operation, a condensation polymer of ~n intrinsic viscosity of less than 0.6 is heated to a temperature of less than 50C. above the melt temperature and is extruded through nozzle 18 towards the baffle assembly 52. As the melt blown fibers drop through thc ilot f)ll, the melt blown fibers are contacted with oaseous stream at ambient temperature or at a temperature sufficient to heat the fibers to a temperatul g OL` ~. om, O to 26~ C. and at an initial velocity of from ~00 to lO00 t`eet per secolld to c~use the melt blown fibers to torm a highly entan~led web o~` unbondes~

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fibers which are gathered on the rotating m~drel 62 rotating ~t an angular velocity of 5 to 500 revolution per minute, preferably 10 to 250 per minute.
A cartridge ~shaped mass 80 is formed about the mandrel 62 to a radial thickness of from about 3/d~ to 5 inches, which cartridge ~shape mass maybe continuously urged from lef t to right, as ill~trated by the arrow l'A", by the S collection assembly 72, or alternately moved back and forth until a desired thickness is attained.
ExamDles of the Invention O~ f ~ process is described in the following Examples which are intended to be mer~ly illustrative, and the invention is not to be regarded as limited thereto. It will be shown that the cartridges o~ the present invention are comprised of unbonded, continuous melt-blown filaments of condensation polymers compacted to a high density by the shrinkage effect, that high mechanical rigidity is obtained without sel~-bonding, and that ~iltration e~ficiency is not de~reased by bonding.
The melt-blowing die assembly used in the following Examples is comprised of four rows of nozzles 18 with 50 nozzles per row. In such assembly, a screen, having the same spacirlg as the extrusion nozzles is used to form four air orifices around each e2~trusion nozzle. (See Figure 3.~ The capillary arrangement had the following dimensions: length OI capillary-1.27 cm; inside diamete~O.û3302 cm; outside diamter, 0.0635 cm; distance between capi~laries, center to center: 0.1058 cm:
Apparent MeltViscosity is calculated from Poisseuille's equation:
Q ~4 whe~ein:

Q = polymer flow through a single nozzle (CC/sec) p = polymer pressures (dynes/cm.
r = inside nozzle radius (~
1 = length o~ capillary (cm.), and )7 ~ apparent melt viscosity (poise) 'I I
?vv~v . ~22~

To calculate Q (cc/sec) from the polymer fLow rate measured in gram/minutes, the following densities of the solid polymer have been used: 1.36 gram/cc for polyester, and 1.15 gram/cc for nylon 6,8. The term "intrinsic viscosity" (I~/), as used herein, is defined as the limit of the fraction ln(r~/C, as C approaches zero, where (r) is the relRtive viscosity, ~nd 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 o~ the pure solvent per se, measured in the same units at 25C. Intrinsic viscosity is a measure of the molecular weight of a polymer. Apparent melt viscosity (AMV) is a measure of For polyethylene terephthalate (polyester), a solvent mixture of one part trifluoroacetic acid and three parts of methylene chloride (by volume~ is used, forI
nylon 6,6 (polyhexamethylene adipamide), orth~cresole is used.
Fiber diameter is an average value obtained by optic I or stereoscan electron micros~opy.
Fiber velocity is calculated by:
~= ~
fiber cross section area x d For these calculations, the density d (glcc) of the solid polymer has been used.

% Shrinkage =~ x 100, wherein lo lo = length of a dissected filament as initially extruded.
t -length of the ~ilament after heating for 15 seconds at 120 C.
Birefringence is the difference of the refractive indices parallel and vertical to the fiber axis.
EXAMPLE I

Three types ot dried polyechylene terephthalate resin, (~ , and C) are e~;trudedre~pectively, through the herein~bove described melt-blowin~ system. Type A had -m InItiaS IntrIngIc vIscosIty of 0.38; Type B of 0.50; and Type C of ~.39. The ~.YtrucIer ~1"

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3L2~0~3 screw diameter, L/D ratio 24/1) is provided with threeheating zones; the hopper (inlet) zone was heated to 2~5C., the middle zone to 285C. and the outlet zone to 295C. Heated air is passed to the die at 25 psg pressure, the tefnperature was varied and measured in the air cavity i.e., e~trusion temperature. The die ~lock S temperature equilibrates with the air temperature after a fe~N minutes of e~trusion.
The following Table I lists the results:

~i i .j i ~j i ~L~Z4~

~ ! ~ Q ~ ~ ~ t~ ~ D ~ ~
I ~, ~ 1~ 1-- 1~ i~ I--Cl~ CO -~ cn crl ~ c~ ta I-- :~t 1~
cn ~ W ~ 1~ 0 ej ~ c~ w r~ ~ ~ ~ ~ ~ ~ W C~ -C~ O 1~:1 GO ~ O C~ 00 ~0 0 N CO CO O C;l ~ ~ tS~
C~ ~=1 O cn Cl O O C~l 2 0 0 C~l O O O C 3' x Q~ o.
i~ ~ O
~ 5 3'C
P~

~ W ~ c~ 0 c~ ~ t ~~
c~ 1- eD O CO ~ ~ X cn ~ , ul ~D

C~ ~ _~
~w~ l-0cnoc~o cJ~ ~ ~ l . ~D 3~ 1 ooo oo~o~c:: ooo~o~ sij~
o c~ c ~ o . ~5 ~c~
n C~ ~ ~ ~ ~ ~ ~ ~
_. _. M
. _~ ~
W ~ l~ I ~ ~ ~
z~ ~ O O O o 'P C C~ CO ~n 'O ~
. ~

C~ O ~ O ~ O ~ ~

O O

_9_ ?v. 3 i .

~un #2 (low molecular weight resin, at 6 poise app~rent melt viscosity 300C
extrusion temperature) e~hibited the highest shrinkage value. At higher extrusion temperature, the moleculsr orientation o~ the polymer induced by the high spinning velocity, has more time to de-orient in the melt phase since cooling of the fiber takes longer. At lower extrusion temperatures, shrinkage also decreases, as melt YiScosity increases and fiber velocity decreases. The same effects are seen in Buns ~ through 8, which is nearly identical to Runs i through 4, except that resin throughput is increased and fiber diameters are correspondingly larger. Using resins of highermolecular weight (Type B and C) shows the effect of higher apparent melt viscosities ~= A l'? V) and lower fiber velocities leading to lower shrinkage values.
E~AMPLE II
Two types of textile grade nylon 6,~, DuP~nt's "Zytel" TE, (Type D - O.~S IY, and Type E = 0.8û ~V) were mel~-blown under conditions described in Example I. The 15 ¦ results are listed in Table Ll, below as Runs 1 through 7.

~ ,~ .k lZ~:4608 , ~ cn ~ c ~ ~ I
1~;1 W C`~ ~ N C'~ C.
C~ C~ ~:) CO C~ C~l c: o o cn o o o ~ 5 ~j;

~:1 ~ N C~ CW C.
~ l ~ 5~ l ~ o cn cn c;l ~P ~ cn cn a~ x 5~ c~ c~ ~

~ ~ ~ ~ o ~ U~ ~ ~ o O~ 1 ô~,2' ~, ~ 'o I O o o C ~ o C~ o ~ ~ X' ~
~ OW~ C~ ~C

I ~ ~cn c~ ~ ~ ~ CL ~ _ i O~_~

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C~c~ C~C51- 3= _~, o C o 1- C,' ~ ~ P
W.

=

- :~ v v v l i l ~Z;~6~8 The shrinkage effects are similar as for polyester. About 300C ~6 shrinkage decreases again for low molecular weight resin, and decreases also as ~iber velocities decrease at the lower temperature~. The high molecular ~veight resin (Type E) showed almost no shrinkage due to high AMV and low fiber velocities.
EXA~IPLE II][
Yery low molecular weight polypropylene of 150 gram/10 minutes .~lelt Flow R~te and a cr~stalline melting point of 160~. is processed in the melt-blowing system described ln Example I. The extruder zones are heated to 210 C. No fibers formed at an extrusion temperature of 210C. due to too high a melt viscosity. At high extrusion temperatures of 260degree C. to 300~., fibers formed but exhibited no shrinkage upon heating to 125C.
Polyester of Type A (Example I~ is melt blown through the app~ratus described in ~igure 1, under conditions of Example Ig Run ~2, and collected on a rotating mandrel rod of 3/4 inch diameter and 12 inch length dispersed 18 inches below the nozzle die. The mandrel was driven at 120 RPM. The baffle assembly 52 is 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 meter/second entangled to a fluffy, bulky web at the lower part of the baffle assembly 52. This web is then pulled down by the rotating mandrel and wrapped around it. The mandrel is 20 moved from one end to the other to cover all 12 inches with a fiber sleeve. After 3 minutes of collecting, a tubular sleeve about the mandrel 62 is grown to 3 inches in diameter. The fiber sleeve is slipped off the rod. The tubular cartridge, comprised of continuous, unbonded ~ibers, is soft, could be easily bent and collapsed by hand, and had a density of 0.055 gram/cc.

? .,~3 ~ L6C~3 EX~MPLE IY
Another tube was prepared (72 grams, 3 inch diameter), as per Example III, and a hot stream of air at a temperature of about 200C. is directed on to the rotatina fiber covered mandrel. Within about 3 seconds, the fiber sleeve had shrunk to a S diameter of 1.75 inches at a density of 0.186 gram/cc. The tube, after being slipped off the rod is firm and rigid, and withstood without collapsing a vertical pressure o its axis of 1.2 lb/linear inch.
EXAMPL~ V
In this Example a hot air stream is directed onto the mandrel 62 while the fiberweb is collected on the m~ndrel 62 thereby simultaneously performing spinning, collecting and shrinking. After 3 minutes, the fiber sleeve is built up to a diameter o~ 1.6 inches at a density OI 0.23 grarn/cc. The tube could withstand without collapsing a pressure of 2 lb/linear ~nch.
EXAMPL13 Vi Example V was repeated using extrusion conditions of Table 1, Run #6 ~200 gram/minute throughput). After 1~ seconds, the tube is ~uilt up to a diameter of 1.75 inch at a density of û.19 gram/cc. The tube exhibited a porosity of 86%, vvhere bulk density of cartride ~6 porosity =1- x 100 density of fiber 2 0 The tube could withstand a pressure vertical to its a2~is of 1.8 lb/linear inch, and is comprised of unbonded, continous, highly entangled fibers.
EXAMPLE VII
. _ _ A fiber web is collected on the 12 inch rod ~as described in Example VI). After formation to a diameter of about 1.75 inch, the web saeeve is built up on the free end of the rod, the rotating tube is ~ripped with the clamping device pressed against the sleeve~ and pulled away at a rate of about 3 feet per minute. A continuous tube of a density of û.2 grarn/cc, an inside diameter of 0.75 inch and ou-tside diameter o~
diumeter is thus continuously formed. Example VII demonstrates continuous spinnin~, collectirn~t ~hrinking and withdrawal of a continuous tube.

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~Z~6~8 While the present invention has been described with reference to a melt blowing die assembly wherein the fibers are formed at sonic velocity, i-t is to be understood to one skilled in the art that any melt blowin~ die assembly may be used in the present invention.
While the present invention has been déscribed in connection with an exemplary embodiment thereo~, it will be understo.od that rnany modifications ~ill be apparent to those of ordinary skill in the art and that this application is intended to cover any adaptation or variation thereof. Therefore, it is manifestly intended that this invention be only lirnited by the claim and the equivalents thereof.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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 feet per second said molten thermoplastic condensation polymer at a temperature of less than about 50°C above the melting point thereof and whereat said molten polymer has an apparent melt viscosity of less than 50 poise to form melt blown fibers, and (c) collecting said melt blown fibers to form said non-woven web.

2. The process as defined in claim 1 wherein said melt blown fibers of step (b) are contacted with a gaseous medium to form a highly bulky web structure.

3. The process as defined in claim 2 wherein said gaseous medium is air.

4. The process as defined in claim 2 wherein said gaseous medium is heated to a temperature sufficient to heat said fibers to a temperature of from 70 to 265°C.

5. The process as defined in claim 1 and wherein said non-woven web is contacted with a gaseous medium at a temperature sufficient to heat said fibers to a temperature of from 70 to 265°C.

6. The process as defined in claim 5 wherein said gaseous medium is air.

7. The process as defined in claim 1 wherein step (c) is effected on a rotating mandrel.

8. The process as defined in claims 4, 5 and 7 wherein heating is effected for a time sufficient to shrink said web to a point at which said web is of a density of at least 0.1 gms/cc.

9. The process as defined in claims 4, 5 and 7 wherein heating is effected for a time sufficient to shrink said web to a point at which said web is of a density of at least 0.1 gms/cc, and where said melt blown fibers, after cooling, have an intrin-sic viscosity of less than 0.6.

10. A non-woven web of thermoplastic fibers, comprised of oriented fibers of a thermoplastic condensation polymer melt blown at a temperature of less than bout 50°C. below the cry-stalline melting point and whereat said polymer has an apparent melt viscosity of less than 50 poise, said fibers after cooling, having an intrinsic viscosity of less than 0.6.

11. The non-woven web as defined in Claim 10 wherein said fibers of said web are heat shrunk to a point at which said web is of a density of at least 0.1 gram/cc.

12. A tube comprised of unbonded, entangled, oriented fi-bers of a thermoplastic condensation polymer melt blown at an initial velocity of from 500 to 1000 feet per second and at a temperature of less than about 50°C below the crystalline melt-ing point and whereat said polymer has an apparent melt viscosity of less than 50 poise, said fibers after cooling, having an in-trinsic viscosity of less than 0.6.

13. The tube as defined in Claim 12 wherein said fibers of said tube are heat shrunk to a point at which said tube is of a density of at least 0.1 gram/cc.