US3118161A - Foamed polypropylene filaments - Google Patents

Description

Jan. 21, 1964 F. R. CRAMTON FOAMED POLYPROPYLENE FILAMENTS 2 Sheets-Sheet 1 Filed March 13, 1963 m m m FRANK R. CRAMTON ATTORNEY Jan. 21, 1964 F. R. CRAMTON FOAMED POLYPROPYLENE FILAMENTS 2 Sheets-Sheet 2 Filed March 13, 1963 m mlh INVENTOR I FRANK R. CRAMTON BY 6M 9M ATTORNEY United States Patent sci or. 15 -159) stretching the filaments to increase the molecular orientation along the fiber axis. The resultant stretch oriented rlaments are character zed by increased tensile strength, ow elongation, and increased stifiness and resiliency.

in the melt extrusion of polypropylene filaments, it is necessary to solidify the fdaments by cooling prior to softening for stretch orientation such as by quenchlng in a non-sol ent liquid. Subsequently, the filaments are heat softened and longitudinally stretched to orient them. In stretch orienting heat softened filaments of polypropylene, the filament is often supported by some solid surface after it has been softened, but before it is stretched. For examole, the filament may be passed through a heating zone wherein the filaments are heated to a temperature within their softening range by the action of a hot liquid or a hot gas in the heating zone. in order to expose the polypropylene filaments to the fluid source of heat for a period of time sufiicient to raise the temperature thereof to a point within its softening range, the filaments are frequently conducted through a sinuous path within the heating zone by means of supporting rolls. Such rolls may themselves be independently heated so that they will serve as the source of heat for heating the filaments. In such a case, the fluid source of heat in the zone may be dispensed with. When polypropylene filaments are passed directly through a heating zone unsupported by rolls or other surfaces while in the zone, there may be provided a device such as a roll assembly on the outside of the zone for snubbing the filaments after they have been heat softened but before they are stretched or drawn.

When a polypropylene filament having a circular cross section is cooled to solidify it subsequent to extrusion, the filament undergoes severe cross sectional distortion. Thus, when an extruded polypropylene filament having a circular cross section is cooled to solidify it prior to softening it for stretch ori station, the filament does not retain its circular cross sectional shape but acquires instead an elliptical or nearly elliptical cross sectional shape. This phenomenon is known as out of roundness. The degree of out of roundness (i.e., amount of cross sectional distortion) increases as the rate of cooling of the extruded filament increase Moreover, the out of roundness is accentuated when the filament, after being heat softened, is supported by a solid surface prior to and/ or during the drawing thereof; The tension applied to the filament during the stretching operation while the heat softened filament is supported on a solid surface is responsible for this source of out of roundness.

The out of roundness, or cross sectional distortion, may be so pronounced that the length of the major axis of the elliptical cross sectional shape of the distorted filament may be two or more times greater than the length of the minor axis thereof. This phenomenon may best be illus- Patented Jan. 21', l

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trated by reference to the relative dimensions of the cross sectional shape of the filament. The relative dimension of the cross sectional shape of a filament or dimensional ratio, is the ratio of the length of the major axis to the length of the minor axis. Thus, if the cross sectional shape of the filament is substantially circular, both axes are substantially the same length and the relative dimension of the cross section or dimensional ratio is substantially unity. However, when the length of the major axis is twice as great as the length of the minor axis, the relative dimension or dimensional ratio is 2.0.

When a non-foamed polypropylene filament is extruded and rapidly cooled, notable diameter variations, as well as crystalline irregularities in the form of large vacuoles within the filament, are found to occur along the length of the filament; The large low strength vacuoles ap pear to be a result of rapid thermal and crystallization contraction. Although the thermal and crystallization contraction may be controlled to some extent by the use of controlled slow cooling of the filament af er extrusion, filaments possessing the best abrasionresistance are obtained by rapid quenching or cooling. When stretch oriented, the low strength vacuole points yield more than normal to the orientation stress and create extreme diameter variations resulting in points of low tensile strength along the filament. The uniform small cells of the foamed filaments of the present invention absorb the thermal and crystallization contraction along the axis to the point where the large vacuoles no longer exist. The foamed filaments maintain a substantially round, uniform diameter throughout the length of the filament thus substantially increasing the tensile strength.

The foregoing described disadvantages are avoided by the practice of this invention which, briefly, comprises produciug'a dimensionally stable filament consisting essentially of oriented isotactic foamed polypropylene having an efiective blow-up of from 1% to about 15%, and having a cross section which is substantially round. Such a filament is produced by preparing a mixture of isotactic polypropylene and a quantity of foaming agent sulfiient to produce from about 1% to 15% blow, introducing the mixture into a heating zone, melt extruding a filament from the mixture, quenching or otherwise solidifying'the filament, subsequently softening the filamerit and longitudinally stretching said filament. Prior to and/or during the stretching of the filament, it may be supported on a' solid surface.

The preferred polypropylene which may be used in the practice of this invention is iso-tactic polypropylene which is a high molecular weight (i.e., above about 45,- C6 0) solid polymer exhibiting a crystalline X-ray diffraction pattern. Such a polymer has a density between 0.9-3 and 0.94 and a melting point above about 320 F. These polymers may be prepared by methods now well known in the art such as the procedures described by G. Natta in the Journal of Polymer Science, Vol. XVl, pp. 143 to 154 (1955) and in US. Patents 2,882,263; 2,874,153 and 2,913,442.

The preferred foaming agents which may be used in the practice of this invention are those which will decompose at temperatures approximating the extrusion processing temperatureof'polypropylene. For instance, foaming agents-such as 1,1-azobisforrnamide (Kempore R425), 4,4 oxybis (benzenesulfonyl sem carbozide), trihydrazinosym.-triazine (THT), bis-benzenesulfonyl hydraeide EBSH) and barium' azodicarboxy late ("Expandex 177) are typical of those which may be used in the practice of this invention.

The foaming agentshor'rl'd be present in an amount sufiicient to produce up to 15% blow in the extruded Percent blow= It has been found that filaments containing up to about blow will give the desired properties of improved dimensional stability and reduced diameter variation with no significant loss of stiffness or abrasion resistance.

The polypropylene, which is preferably in particulate form, is thoroughly admixed with the blowing agent an the admixture is introduced into a heating zone. In the heating zone, the polypropylene is heated to a suitable extrusion temperature above its melting temperature. After the admixture has been heated to said temperature, it may be maintained in gastight environs until it has been extruded to prevent premature blowing. The melt of polypropylene is then extruded into a filament. After the filament has been extruded, it may be solified as by quenching in a non-solvent bath or by air cooling. Subsequently, it is heat softened by some suitable means. This may be accomplished by conducting it through a heating chamber wherein it is supported, preferably, by a plurality of rolls or equivalent supporting members, the peripheral surface of each providing a supporting surface. The filament may be heated while in the heating zone by means of a fluid source of heat, either gas or liquid, or by means of independently heated rolls. Alternatively, the supporting rolls may be eliminated from the heating zone and the filament passed directly through the zone which is heated by some fluid source of heat, or by radiant heat, the filament being unsupported by any member. The softened filament may be snubbed by means of a suitable device such as a conventional snubbing roll assembly after it leaves the heating chamber but prior to the drawing operation. Thus, the heat softened filament may be supported by the surface of the snubbing device after it leaves the heating zone and while in a softened condition.

After the filament has been softened, it is stretched longitudinally to increase the molecular orientation along the fiber axis. Any amount of stretching will increase the molecular orientation. However, maximum benefits are attained by stretching the filaments from about 6 to about 11 or more times of their length.

The drawn filaments of foamed polypropylene have diameters which range from 10 to 500 mils and preferably from 75 to 259 mils.

The cross sectional shape of the drawn filament prepared according to this invention is substantially the same as that of the undrawn filament after it is extruded but before it is solidified and subsequently heat softened prior to the drawing thereof. That is the relative distance between the midpoint of the cross section and the periphery, or the relative dimensions, remain approximately or nearly the same so that, although the size or area of the cross section are considerably less after drawing, the round configuration remains substantially the same.

The filament or bristle produced according to this invention has a structure such that a major part of the foamed portion is located at the center of the bristle and extends lengthwise therethrough. Since the foamed portion is located at approximately the center area of the filament, the thermal and crystallization contraction, which occurs primarily at this center area is apparently absorbed by the foam, resulting in a rounder filament. Substantially none of the foam is in the outer part. Thus the outer portion of the bristle is continuous and nonporous after extrusion. ther polymers, such as polyethylene, must be drawn in order to obtain a continuous surface.

Uniformly shaped filaments produced by the process of this invention are extremely useful in the production of uniform tufts of bristles in a brush. This results in a tmore uniform distribution of wear of the bristles. Moreover, such tufts are more easily mounted in brush heads. Flex action in any direction is much more nearly uniform than in oval filaments-ie, filaments which have gone out of round during processing. Also, the foamed filements of this invention possess better flaggability than do non-foamed polypropylene filaments.

The aspects of this invention which are capable of illustration, are shown in the accompanying drawings,

FIG. 1 is an end view of a single bristle made according to the present invention.

FIG. 2 is a schematic view of a suitable overall arrangement of apparatus for carrying out the method of this invention.

FIG. 3 is a schematic View of a section of a modification of the apparatus shown in FIG. 2.

FIG. 4 is a schematic view of another apparatus suitable for carrying out this invention in which the filament is softened in a heating chamber which does not contain any supporting surface in the heating chamber itself.

FIG. 5 is a view of a section of a brush in which the bristles of this invention are adapted for use.

FIG. 1 is an end view of a 10% foamed mil bristle under magnification of x. The bristle 14 has a structure such that the major part A of the foamed portion is located at the center area of the bristle while the outer part B is continuous and non-porous. Approximately 70% of the foamed portion is limited to not more than 30% of the center area.

In FIG. 2, a hopper it contains pellets 11 of the polypropylene and foaming agent starting material such as crystalline isotactic polypropylene and Kempore R125. The pellets 11 may be preheated in the hopper 10 if desired. From the hopper 1% the pellets are conveyed by means of a heated extrusion chamber 12 to a heated nonvented extrusion head 13 which contains an extrusion die 13a. In the extrusion chamber 12, the temperature of the mixture is raised to above the melting point of the polypropylene and above the decomposition point of the foaming agent. It is extruded through suitably shaped orifices in the extrusion die 13a into one or more filaments 14. A preferred extrusion temperature for the isotactic polypropylene is about 480 F. when the filaments 14 are extruded at a linear rate of from about 18 to about 50 feet per minute from the orifices, the size of which may range from about 10 mils to about 500 mils in diameter.

In making filaments of isotactic foamed polypropylene, it is advantageous to quench the extruded filaments in order to solidify them. Quenching at a temperature below about 60 F. imparts valuable properties to the drawn polypropylene as described in U.S. Patent 3,059,- 991 to P. C. Munt (the disclosure of which is incorporated herein by reference). This may be accomplished, as shown in FIG. 2, by placing a quench bath 15 between the extrusion head 13 and the heating chamber 22. The extruded filaments 14 are guided into the quench bath 15, containing a liquid non-solvent for isotactic polypylene, e.g., water, by a guide roll 16. The bath 15 is maintained in a suitable tank 17 at a temperature of 60 F. or below. Temperatures of about 40 F. are preferred. A much lower temperature, such as 10 F., may be used for the quench bath if precautions are taken to keep the nonsolvent from freezing. Thus, a salt water brine at temperatures below 32 F. may be used. An immersion time of from about 8 seconds to about 20 seconds of the filaments 14 in the bath 15 is generally sufficient. The filaments 14 are transported around a stationary pin 18 in the quench bath 15 and then over the roll 19 and into the hot air conditioning oven 22. In the oven, the extruded filaments are transported over a number of rolls 20, which may be heated, in a sinuous or zig-zag path, as heated air is circulated from overhead as indicated by the arrows. As the filaments pass through the heating zone,

each succeeding driven roll 20 over which the filaments 14 pass is driven at a slightly increased peripheral speed from that or the proceeding roll so as to prevent the filaments from sagging appreciably. The primary purpose of the series of driven rolls 21 is to provide a heat exchange relaand the amounts of foaming agent (Kempore R-125) indicated in the following table are fed into a screw extruder of the type shown in FIG. 2 having a 2.5 inch screw diameter. The die plate contains 3 extrusions orifices, each having a circular shape. The non-vented tionship between the filaments 14 and the heated air in 5 jacket is heated to a temperature of about 480 and the the oven whereby the filaments are uniformly softened by filaments are extruded at a linear rate of '35 feet per min heat. Since isotactic foamed polypropylene has a softenute from the orifices. The cross sectionsof each of the ing temperature in the range of from about 260 F. to extruded filaments are substantially round. The filaments about 305 F., it is preferred to maintain the temperature are then passed through a quench bath maintained at a i th oven t abgut 300 F temperature of about 40 F. to solidify them. They are After leaving the last and uppermost driven roll 2%, the then fed into a heating chamber and are transported over filaments are snubbed-With a three roll assembly 21, each a series of rolls. Hot air is circulated through the heatroll of which is driven atabout the same as or a higher ing chamber, thereby heating the filaments to a temper- ,en'pheral speed than that of the last driven roll 20. A attire of about 300 F. before they leave the heating fast snub roll assembly 23 is provided just outside the chamber. After leaving the heating chamber, the filaoven ZZZ-which is drivenat a peripheral speed of about 6 ments are stretched to about eight times their original to l1 times-that-of the assembly rolls 21. Thereby, the length to give 80 mil diameter filaments. The cross secfilaments 14 are stretched from about 6 to 11 times their tions of the drawn foamed filaments are substwtially of length. This increases the rnolecular orientation along the same shape as the cross sections of the filaments after the fiber axis. The drawn, oriented filaments 14 are they are extruded but before they are quenched and taken thereafter collected on a reel 24 which is supported on a over the series of rolls in the heating chamber. frame 25. The following table lists the percent blow and the FIG. 3 illustrates a modification of the apparatus shown specific gravity as well as the measured volumetric abrain FIG. 2 in which extruded filaments 14 are solidified sion rate expressed in (in. /min.) 10 the ratio of the by air cooling rather than by quenching. The filaments 29 major axis to the minor axis (D /D and the diameter 14 are passed over the rolls 31 to allow them to solidify variation for the filaments produced by each example. prior to being transported into the hot air conditioning As can be seen from this table, the non foamed filament oven 22. (Example 8) is much more out of round and shows much In PEG. 4, filaments 4%) having a substantially round greater diameter variation than the foamed filaments (Exsectional shape prepared by means, previously described, amples 1 to 7). Moreover, the abrasion resistance of the are passed over a snub roll assembly 39 and thence into foamed filaments compares favorably with the abrasion the heating chamber 41. A fluid heating means, such as resistance of the non foamed filament.

Table Amount Standard Percent Foaming Volumetric Deviation Variation Extremes Example Agent. Percent Dz/D Abrasion, Specific of Diaof Diaoi Varia- Percent Blow infi/min. Gravity meter meter tion By from From Vr'eight Av. Av.

0. 01 4. 4 1. s9 0. 00055 0. s0 0. 0030 3. 75 i0. 015 0. 03 0. 7 1. s3 0. 00003 0. 8t 0. 0020 2. 3 i0. 010 0. 05 '1. s 1. 265 0. 00075 0. s2 0. 0026 3 i0. 012 0. 0s 0. s 1. 24 0. 00070 9. s1 0. 0010 1.10 :b(). 014 0. 07 11 1. 20 0. 0005s 0. s0 0. 0022 2. 5 i0. Q10 0. 00 13. 3 1.11 0. 00005 0. 7s 0.0010 1.16 it). 010 0.10 15. 5 1.15 0. 00070 0. 7s 0. 0012 1. 5 i0. 008 0 0 1. 450 0. 00060 0. e0 0. 0055 0 i0. 010

uperheated steam, is circulated through the heating EXAMPLE 9 Chambgr' The .fluid is introduced .into the heating cham' A series of bristles varying in diameters of from about bar P l i The 30 mils to about 500 mils, specifically 30 mils, 250 mils temperature lfeatmg Chamber 1s mamtamed at a and 560 mils, in thickness as measured after drawing telfiper'ature sufiiflen to soften the filaments heat are extruded and quenched as described in the above filamems .are snubbed by i 3 assemoly examples using snfficient blowing agent to achieve a 15% lmmedlatili. afteimeyfezfve l heating cnamber' These blow. The quench bath is maintained at a temperature mus am drivel} a shghdy higher penpher.a1 Speed than of about 40 F. The bristles are then fed into a heating l rate Whlch The filamnts are passed Into heatchamber where they are heat softened and stretch oriented. mg q F 5 keep me filaments p Saggmg On each foamed bristle, the ratio of the major axis to preclably Whfiefn shamber' After being snubbed by the minor axis before quenching, as in the 80 mil bristle the roll assembly 4-4 th e softened filaments are drawn to of Example 1, is about unity. After they are quenched, sgYeral tunes f then lgngth by a fast mu 45 Whlch 13 heat softened and stretched, there is little variation in the f at Penpheral of about 6 11 or more dimensional ratio of the cross sections and little diameter tunes that of the Snub mu ass'ambly 44' Th5 filaments variation. The abrasion resistance of these bristles is good. may then be collected on a conventional reel assembly 1 claim: Such as desc'nbed m f 1. An abrasion resistant filament consisting essentially In FIG. 5, a plurality of bristles 14 are grouped toof oriented isotacfic foamed polypropylfim; gether at one end 5% and inserted into retaining means the mic of tbs di-fiersnce between the Specific gravity a hole) 5 on a street brush base of said foamed filament and the specific gravity of The fonOWmg flluslrate best mode 7 the original polypropylene from which it is formed, to templaied for carrymg out thls mvemlon: 0 the specific gravity of the original polypropylene from EXAMPLES 1 To 8 said filament is formed having a value of up to In these examples, mixtures of isotactic polypropylene the ratio of the length of the major axis to the length having an aver-age molecular Weight of about 100,000, a of the minor axis of the cross section of said filadensity of 0.90 and a crystalline melting point of 333 F.,

merit being not greater than about 1.39;

*3 the diameter of said filament being substantially constant along the length thereof; said filament having a major part of the foamed portion located at the center area and extending length- Wise through said filament. 2. A filament as defined in claim 1 having a diameter of from 10 to 500 mils.

3. A filament as defined in claim 1 having a diameter of from 75 to 250 mils.

4. An abrasion resistant brush bristle consisting essen- V tially of oriented isotactic foamed polypropylene;

the ratio of the ditierence between the specific gravity of said foamed bristle and the specific gravity of the original polypropylene from which it is formed, to the specific gravity of the original polypropylene from Which said bristle is formed having a value of up to 0.15;

the ratio of the length of the major axis to the length of the minor axis of the cross section of said bristle being no greater than about 1.39;

the diameter of said bristle being substantially constant along the length thereof; said bristle having at least approximately 70% of said foamed portion limited to not more than approximately 30% of the center area of said bristle. 5. A brush containing a plurality of bristles as defined in claim 4.

References Cited in the file of this patent UNITED STATES PATENTS 2,341,823 Smith Feb. 15, 1944 2,659,921 Osborn Nov. 24, 1953 2,812,530 Vfhitesel Nov. 12, 1957 2,888,415 Jankens May 26, 1959 2,905,648 Haas Sept. 22, 1959 2,907,096 Halbig Oct. 6, 1959 2,913,769 Kastli Nov. 24, 1959 2,948,048 Jankens Aug. 9, 1960 2,950,495 Stingley Aug. 30, 1960 3,050,070 Sidelman Aug. 21, 1962 3,059,991 Munt Oct. 23, 1962

Claims (1)

1. AN ABRASION RESISTANT FILAMENT CONSISTING ESSENTIALLY OF ORIENTED ISOTACTIC FOAMED POLYPROPYLENE; THE RATIO OF THE DIFFERENCE BETWEEN THE SPECIFIC GRAVITY OF SAID FOAMED FILAMENT AND THE SPECIFIC GRAVITY OF THE ORIGINAL POLYPROPYLENE FROM WHICH IT IS FORMED, TO THE SPECIFIC GRAVITY OF THE ORIGINAL POLYPROPYLENE FROM WHICH SAID FILAMENT IS FORMED HAVING A VALUE OF UP TO 0.15; THE RATIO OF THE LENGTH OF THE MAJOR AXIS TO THE LENGTH OF THE MINOR AXIS OF THE CROSS SECTION OF SAID FILAMENT BEING NOT GREATER THAN ABOUT 1.39: THE DIAMETER OF SAID FILAMENT BEING SUBSTNATIALLY CONSTANT ALONG THE LENGTH THEREOF; SAID FILAMENT HAVING A MAJOR PART OF THE FOAMED PORTION LOCATED AT THE CENTER AREA AND EXTENDING LENGTHWISE THROUGH SAID FILAMENT.

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