CA1130516A

CA1130516A - Open cell structure foamed cellulose acetate filters

Info

Publication number: CA1130516A
Application number: CA323,199A
Authority: CA
Inventors: David W. Trott; Carl J. Howell, Jr.; Jesse L. Riley
Original assignee: Celanese Corp
Current assignee: Celanese Corp
Priority date: 1978-03-13
Filing date: 1979-03-12
Publication date: 1982-08-31
Also published as: PH15810A; GB2091738B; NO790820L; AU4489879A; NZ189882A; YU40841B; MTP842B; AT372250B; MX150672A; GR66584B; IL56826A; IE790722L; JPS54128070A; GB2091738A; FI790848A; MW679A1; DE2909096A1; SE7902211L; GB2015921A; DE2909096C2

Abstract

ABSTRACT

This invention relates to a method of manufacturing foamed cellulose acetate and the product produced thereby. More speci-fically, the invention relates to a melt extrusion process for producing open cell structure foamed cellulose acetate having special use as a filtering element for an aerosol such as tobacco smoke.

Description

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OPEN CELL STRUCTURE FOAMED CELLULC)SE ACETATE FILTERS

This invention relates to a method of manufacturing foamed cellulose acetate and the product produced thereby. More speci-fically, the invention relates to a melt extrusion process for producing open cell structure foamed cellulose acetate having special use as a filtering element for an aerosol such as tobacco smoke.
Foamed cellulose acetate cigarette filters are known, such filters being disclosed in U.S. Patent No. 3,574,803. The cellulose acetate foams of the patent are prepared by dissolving an inert gas under pressure into ripened cellulose acetate dope at its incipient precipitation point and precipitating the cellulose acetate while reducing the pressure on the dope to produce a solid foam and then washing the foam to purge the foam of residual acetic acid and salts. In order to obtain an acceptable product, it is essential that water soluble salts and acetic acid be washed from the solid foam and therefore all areas of the foamed structure must have maximum exposure to the washing operation. This process results in a product having a water pervious area on the periphery of the foamed rod and also results in a product having a cell structure which is devoid of fiber-like elements and has an average density of about 0.13 grams per cubic centimeter.
Polyolefin open cell structures having a peripheral skin are known, such structures being disclosed in U.S. Patents Nos. 3,939,849 and 4,054.550. The polyolefin open cell structure is further characterized as a reticular structure wherein individual cells are bridged by fiber-Iike elementsO Polyolefin foams when used in cigarette filter applications function by means of physical filter-ing alone; that is to say, mechanically trapping smoke-entrained matter. A major reason for the universal acceptance of cellulose acetate cigarette filters is the consumer acceptability of the 51~

taste imparted to the tobacco smolce stream as well as the ability to chemically sequester constitutents ~rom smoke vapor as well as mechanically trapping smoke-entrained matter, e.g. tarry substances. More specifically, filters of cellulose acetate exhibit a much higher selectivity for removing phenol and other simple phenolic compounds from cigarette smoke than do filters made from polyolefins. Moreover, the capacity of cellulose acetate filters for phenol removal is increased by the presence of ether ester or simple ester type plasticizers. Commercial cellulose acetate cigarette filters, however, are high density filters (from about 0.14 to 0.15 grams per cubic centimeter) pre-pared from cellulose acetate tow. The manufacture of filter rods from tow is an expensive process involving manufacture of tow, opening of the tow bundle, precision topical application of plasticizer, forming the opened tow bundle into a rod, wrapping and gluing the rod and cutting the rod into suitable lengths.
In addition to the obvious complexity of the tow to cigarette filter rod process, the topical application of plasticizer can, on rod aging, produce internal migration of the plasticizer and consequent variations in the ability of the plasticizer to remove phenol from tobacco smoke.
It is therefore an object of this invention to provide low density open cell structure plasticized cellulose acetate foam rods wherein at least some of the individual cells are bridged by fiber-like elements.

It is still another object of this invention to provide a process for the preparation of a low density open cell structure plasticized cellulose acetate foam rod wherein the individual cells are bridged by fiber-like elements.
These and other objects of the invention will become more apparent from the following description of the invention.

~3~516 In accordance with this invention, it has now been dis-covered that an improved low density open cell foamed cellulose acetate cigarette filter may be obtalned by means of a process comprising melt extruding, through an extruder equipped with an e~ipticalor circular die, into a ternperature conditioned atmosphere, a mixture of cellulose acetate, plasticizer, blowing agent and nucleating agent. The presence of a plasticizer is found to lower the glass transition t:emperature of cellulose acetate and make melt extrusio~ of a foamed cellulose acetate possible without undesirable thermal degradation of the polymer.
Preferably, the extruder is a screw extruder.
Thus, in particular the present invention provides a process for the preparation from cellulose acetate flake of a plasticized, open cell, foamed cellulose acetate filter rod, said process comprisiny melt extruding through an elliptical or cir-cular orifice maintained at a temperature of from 180C. to 280C., of a screw extruder a mixture of cellulose acetate, a nontoxic, taste compatible thermally stable plasticizer having a boiling point of not less than 180C., a blowing agent which is at least a partial solvent for cellulose acetate and a nucleating agent, said melt being substantially free of entrained air, and the extrusion pressure profile being such as to prevent said blowing agent from backing up in said screw extruder.
The invention also provides, in another aspect, a cigarette filter rod having a density of less than 0.12 grams per cubic centimeter and a rod skin structure that is substantially water impervious, said filter rod comprising a substantially homo-genously plasticized cellulose acetate, substantially open cell nucleated foam characterized by the presence of at least some cells which are bridged by fibrils, said homogenously plasticized cellulose acetale employing a plasticizer selected from the group consisting of esters of saturated dibasic acids, esters of satur-~31~S16 ated polyhydric alcohols, fatty acid esters and sulfonamideresins.
The process must be conducted in a manner such that blow-ing agent gas cannot escape, that is to say, flow in a direction away from the direction of extrusion. I* blowing agent is allowed to escape in this manner, the process becomes difficult to control and nonuniform linear and bulk densities are produced in the end product. The escape of blowing agent is prevented by means of a pressure gradient wherein the pressure upstream of the blowing agent injection port is greater than the pressure downstream of the blowing agent injection port. Preferably, the pressure gradient is achieved by means of a dam which is either generated by molten cellulose acetate filling the barrel of the extruder or by means of a mechanical damming means which forms a portion of a screw extruder. Accordingly, the process requires that blowing agent be added downstream of the dam. The extrusion process may therefore be divided into six zones, each zone being maintained within a precise temperature range. It should be understood that the zones designated as zones two, three and four are within the screw extruder.
The first zone may be classified as a polymer feed zone wherein cellulose acetate flake is maintained at a temperature of from -20C to 100C to prevent agglomeration during feeding of the flaXe into an extruder.

- 3a -)5-16 The second zone which is within the extruder is a cellu-lose acetate polymer melt zone maintained at an inlet tempera-ture of not less than 100C and an exit temperature of not more than 240C.
The third zone which is within the extruder is a high pressure, high shear zone maintained at temperatures above the glass-rubber transition temperature of cellulose acetate. The third zone is provided with damming means to prevent gas from flowing away from the direction of extrusion. The third zone is also the zone in which blowing agent is added, downstream from the damming means.
The fourth zone which is within the extruder is a mixing zone maintained at a temperature less than zone three.
The fifth zone is the extrusion die zone which is maintained at temperatures in the range of 180C to 270Co The sixth zone is a conditioning ~one maintained at a tempera-ture of from about 45C to within 5C of the glass-rubber transi-tion temperature of the extxudate whereby the escape of blowing agent is controlled and proper rod skin formation is obtained and collapse of the foam structure is reduced.
The rod, after passage through a conditioning cabinet, is passed through a rod shaping device, the rod being maintained under sufficient tension to inhibit rod sagging while pulling the rod through the rod shaping device. The tensioning is accomplished by means of a rod puller. Rod pulling devices suitable for use in conjunction with this invention are disclosed in U.S. Patent No. 3,813,200. The rod is then cut to the desired filter rod length, the rods than being cut to the desired tip length and attached to cigarettes by gluing tipping paper around the tip and around the tobacco column of the cigarette. The tips are rigid, dimensionally stable, substantially uniformly plasticized cylinders with a substantially water impervious skin and a substantiallyuni-form cross-secti~nal density. The plasticized open cell foamed cellulose acetate filter rod may be characterized as being substantially homo-geneously plasticized, having a density of less than 0.12 gram per cubic centimeter, a pressure drop of from 20 to 200 mm of water and a hardness befor* and after smoking of at least 20 percent for a 20 mm long rod having a circumference of 24.8 mm, a reticulated cell structure wherein at least some of the indivi-dual cells contain bridging filamlent-like members and preferably a rod skin structura which is substantially water impervious. The substantially homogeneous distribution of plasticizer inhibits plasticizer migration which is known to result in a variation in phenol removing efficiency. The filtex of the instant invention, having homogeneously distributed plasticizer, is therefore substan-tially constant in phenol removal.
The term "cellulose acetate" as employed herein is meant to define an ester prepared from acetylation grade wood pulp with higher than 90 percent by weight of alphacellulos'e wherein the degree of esterification as expressed by acetyl value is about 55.0 percent, as combined acetic acid. Fully acetylated cellulose acetate contains 44.8 percent comblned acetyl or 62O5 percent as combined acetic acid. The cellulose triacetate is then hydrolyzed to give a cellulose acetate which contains approximately 39.42 percent combined acetyl or 55.O percent + or - .35 percent as combined acetic acid.
Preferab'ly, the cellulose acetate flake of this invention has a standard deviation in viscosity in 6 percent solutions of less than about 8 centipoises. The viscosity of a 6 percent solution of cellulose acetate fla~e is measured by timing the flow through a calibrated viscometer of a solution containing about 6 weight percent cellulose acetate in a solvent pre~erably 95/5 weight .
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percent acetone/water. Preferably the flake also has a standard deviation in the degree of esterification, expressed as the com-bined lower fatty acid, of less than about 0.20 percent, preferably less than about 0.15 percent.
The bound sulfate content of cellulose acetate should be con-trolled at a minimum value because of the effect of these sulfates on the thermal stability and elevated temperature viscosities. The sulfate determination is from a sample treated with hot 0.07 weight percent hydrochloric acid in a steam autoclave to split off bound sulfate groups. The~e, together with the soluble sulfate salts, are leached out and determined gravimetrically after precipitation with barium chloride. Results are reported as percent sulfate, SO4, by weight. Preferably, the bound sulfate content of the cellulose acetate flake employed herein should be less than .003 percent by weight.
A better understanding of the invention may be had from the drawings wherein FIG. 1 is a flow diagram of the process for producing the filter element of this invention;

FIG. 2 is a schematic view of one type of screw extruder suit-able for use in this invention;
FIG. 3 is a schematic view of another type of screw extruder suitable for use in this invention;
FIG~ 4 is a graph plotting pressure against screw length of a twin screw extruder suitable for use in this invention;
FIG. 5 is a projected view photomicrograph magnified 11 times of the filter element prepared according to Example 1 of this invention;
FIG. 6 is a pxojected view photomicrograph magnified 10 times of a prior art ce:llulose acetate filter element, ~3~S~i FIG. 7 is a cross-sectional photomicrograph magnified 100 times of a prior art cellulose acetate filter rod;
FIG. 8 is a cross-sectional photomicrograph magnified 100 times of the cellulose acetate filter rod prepared according to Example 1 of this invention;
FIG. 9 is a photomicrograph magnified 100 times of the skin of a prior art cellulose acetate filter rod;
FIG. 10 is a photomicrograph magnified 1000 times of the skin of a prior art cellulose acetate filter rod;
FIG. 11 is a photomicrograph magnified 100 times of the skin of the cellulose acetate filter rod prepared according to Example 1 of this invention;
FIG. 12 is a photomicrograph magnified 1000 times of the skin of the cellulose acetate filter rod prepared according to Example 1 of this invention.
Turning to Figure 1 of the drawings, cellulose acetate flake, from 10 to 26 percent by weight of a plasticizer based on the weight of the total composition and 0.5 to 15 percent by weight of a nucleating agent based on the weight of the total composition are fed into a tumbler, mixed and then dried to a moisture content of less than about 0.5 percent H2O. Alternatively, the cellulose acetate flake may be dried before mixing, drying in either case being carried out to prevent moisture from causing agglomeration.
The plasticizer is a nontoxic, taste compatable and thermally stable plasticizer having a boiling point of not less than 180C
selected from the ~roup consisting of esters of saturated dibasic acids, esters of saturated polyhydric alcohols, fatty acid esters and sulfonamide resins and more specifically a plasticizer selected from the group consisting of diethyl adipate, dicapryl adipate, dimethoxyethyl adipate, diethoxyethyl, dimethoxyethoxy adipate, triethyl citrate, tris-n-butyl citrate, acetyl triethyl citrate, ~ ~0S~6 glycerol monoacetate, glycerol diacetate, glycerol triacetate, (ethoxycarbonyl) methyl methyl phtl-alate, bis(2 methoxyethyl) phthalate, diethylene glycol diacetate, triethylene glycol di-acetate, glycerol triisopropionate, ethylene glycol dipropionate, diethylene glycol dipropionate, dimethyl sebacate, diethyl succinate, dibutyl tartrate, isophthalic propylene glycol polyester and polyalkylene glycols. The most preferred plasticizer is a plasti-cizer selected from the group consisting of glycerol triacetate, triethylene glycol diacetate and mixtures thereof. Glycerol triacetate and triethylene glycol diacetate may also be employed as plasticizers together with various amounts of polyethylene glycol.
Suitable nucleating agents preferably are inorganic materials of high surface area, preferably 0.28 square meters per gram or greater and which produce more cells in the foam than would be produced in their absence.
The nucleating agent is preferably a nucleating agent selected from the group consisting of molecular sieve materials, titanium dioxide, perlite, diatomaceous earth, limestone and talc. It should be understood, however, that other nontoxic thermally stable uniform particle size inorganic nucleating agents may be employed as long as these nucleating agents do not impart undesirable color to the extrudate. After tumbling and drying, the mixture is re-moved from the tumbler and preferably sieved so as to remove over-sized particles and obtain a sati~factory molding powder.
The molding powder is then fed at a constant rate to an ex-truder which preferably has the following characteristics: good forwarding with narrow residence time distribution, i.e., minimum back mixing; multiple zone temperature control with good heat transfer temperature control for heating and cooling; good mixing.

' ~3~6 of components, particularly the blowing agent with minimal heat buildup; minimal post screw residence time and minimal post screw laminar flow to prevent wall layer degradation. Preferably the screw revolutions per minute are controlled to provide minimum residence time with minimum shear degradation. Preferably the extruder is also divided into a plurality of temperature control zones, the first zone being the hopper wherein temperatures are controlled in the range of from 20 to 100 degrees centigrade to prevent agglomeration of cellulose acetate flake. The second temperature control zone is the melting zone wherein temperatures are maintained within the range of from 100 to 240C., and pre-ferably 150 to 200C. In this second zone, a vent port should be employed so as to allow water and entrained air to escape to the atmosphere. In the third zone, the temperature of the mater- ;
I ial in the extruder is kept above the glass-rubber transition ; temperature. The blowing agent in quantities of from 2 percent to 7 percent by weight based on the weight of the total composi~
tion is injected into the third zone while temperatures are con- ~;
trolled within the range of from 200 to 300C. and preferably 240 to 250C. The cellulose acetate melt containing a blowing agent then passes through a fourth zone which is a mixing zone, main-tained at a temperature less than zone three. The blowing agent which is injected into the third zone is at least a partial sol- ;
vent for cellulose acetate and is selected from the group con-sisting of ketones, esters and monohydric alcohols having boiling points no higher than 120C. Preferably the blowing agent is selected from the group consisting of acetone, methyl acetate, ethyl acetate, methanol, ethanol, propanol and water. ~;
A better understanding of the function of the screw extruder may be had from a discussion of Figures 2, 3 and 4 of the drawings.
FIG. 2 is a schematic illustration of a single screw extruder arrangement suitable for use in conjunction with this invention.
In any screw-type extrusion operation, a solid is rendered workable _9_ ~3~

by frictional energy and external thermal energy, transported through the apparatus and sub~ected to shear, while maintaining the correct stock temperature. In the process of the instant invention, it is also necessary to withdraw volatiles, add a blowing agent and generate enough pres~ure to force the melt through an eliptical or circular d:ie.
As can be seen in Figure 2 of the drawings, the schematically illustrated single screw extruder, which is not to scale, has varying screw depths, the increased screw depths which are apparent in the region of the feed hopper, the volatiles vent port and the blowing agent injection port reduces the total volume of the screw at those particular points and correspondingly relieves pressure at that point. It should be understood that volume re-duction and corresponding pressure reduction can also be achieved by varying screw pitch rather than screw depth. It should also be noted in Figure 2 of the drawings that any entrained moisture or volatile matter present in the mixture fed from the feed hopperinto the screw ex~uder is substantially reduced at the v~latiles vent port.
It is essential in the single screw extruder employed in this invention that the pressure profile along the length of the single screw be such as to prevent any blowing agent from backing up and exiting through the volatiles vent port. It is preferred for purposes of this invention that a twin screw extruder be employed for better heat transfer, shear mixing and pressure profile con-trol and better control of residence time distributionO
A schematic i]Llustration of a twin screw extruder arrangement suitable for use in conjunction with this invention may be seen in Figure 3 of the drawings. While for ease of illustration a single screw has been shown in Figure 3 of the drawings, it should be understood that for purposes of this invention, a co-rotating ~3~)5i6 intermeshing twin screw extruder is to be preferred.
Twin screw extrusion devices have been found to be pre-ferrable to single screw extrusion devices in that the twin screw devices minimize any tendency of the melt to surge or pulsate through the extruder due to better stabilization of the pressure profile along the length of the twin screw extruders.
Moreover, the intermeshing twin screw con$truction has a self-wiping action which prevents any portion of the polymer charge from experiencing an unduly long residence time in the extruder, prolonged residence time of course resulting in polymer degrada-tion. As can be seen in Figure 3 of the drawings, each identical screw member of the twin screw device contains sections of varying screw pitch, the scxew pitch being maximized in the area of the volatiles vent port and the blowing agent injection port so as to reduce pressures in the zones. The identical screw members of the twin screw extruder of Figure 3 also have a damming means between the volatiles exit port and the blowing agent injection port, the damming means comprising a screw section of opposite hand. The damming means formed by reversing the hand of the screw pitch pre-vents the blowing agent which is injected into the blowing agentinjection port from passing upstream and escaping through the volatiles vent port.
As previously noted, the apparatus of the instant invention may be di~ided into six distinct temperature zones, the first of which is a temperature control zone at the feed hopper and the sixth of which is a temperature control for the rod conditioning cabinet. The barrel of the extruder is divided into temperature control zones two, three and four while the extrusion die con-stitutes the fifth temperature control zone. Additionally, the barrel of the extruder may be di~ided into a plurality of pressure control zones, as can be seen in Figure 4 of drawings which is an idealized plot of presæure against barrel length. The highest pressures are experienced between the volatiles vent port and the dam member, relatively lower pressures being present between the blowing agent injection port and the extrusion orifice. While the specific diagram of Figure 4 has been plotted for the ex-trusion apparatus of Figure 3, it should be understood that a similar pressure profile is also obtained in a single screw ex-truder as illustrated by Figure 2 of the drawings, however, as previously noted, the pressure profile of a single screw extruder is much more difficult to control and therefore the twin screw extruder is preferred for purposes of this invention.
The material from the extruder is passed to an extruder die.
The die temperaturej which constitutes a fifth temperature control zone, is extremely critical and must be controlled to within plus or minus 1C of a preselected temperature of from 180 to 270C
to achieve target rod diameter and desired filter properties. The melt extrusion stage is followed by transfer of the material to a sixth temperature control zone which constitutes a conditioning cabinet which has inlet and outlet openings to permit the rod to pass through and means for controlling the temperature in the cabinet. The cabinet controls skin temperature and porosity to allow escape of the blowing agent. Temperatures of about 45 to within 5C of the glass-rubber transition temperature of the extru-date are maintained.
If the rod is extruded into a cool atmosphere, it will cool too rapidly and collapse. Once the blowing agent has evaporated, the cooled rod has lost its elasticity. A minimum residence time of about one second in the conditioning cabinet is required to prevent some rod diameter loss.

After emerg1ng from the conditioning stage, the rod passes through a heated sizing device for final trimming of the rod diameter and smoothing the rod skin. A rod puller maintains sufficient tension on the rod to prevent sagging of the rod as it passes through the sizing tube. A cutter on the discharge side of the rod puller cuts the rod to a convenient length for handling.
The following specific examples are given for purpo~es of illustration and should not be considered as limiting the spirit or scope of this invention.

Example I
Cellulose acetate flake is ground in a mill to a particle size of less than 3 mm.
A molding powder composition is then prepared by intimately mixing 78.9- weight percent undried cellulose acetate flake (5% H2O), 16.9 weight percent of glycerol triacetate plasticizer and 4.2 weight percent perlite nucleating agent. This compound is then dried for 14 hours at 65C to reduce the moisture level to less than 0.5 weight percent. The dried compound is allowed to cool to room temperature and is precision fed to a twin screw extruder through a cooled feed throat (zone 1~. The material is melted (zone 2) and vented to remove entrained air and any remaining wat~r vapor. The melt is then additionally heated and 3.75 weight percent of acetone blowing agent added based on the weight of the total composition ~zone 3). The mixture i5 then cooled to 229C
(zone 4) before exiting the screw and entering the extrusion block rom which it was extruded at a die temperature of 240C (zone 5) through a 3 mm circular orifice into a one meter long hot air cham-ber maintained at a temperature of 73C (zone 6). The solidifi~d rod is then pulled through a shaping and sizing device to ensure roundness and proper size by foam sheathed rollers at 17.7 meters/min, and cut into 120 mm long sections.

~L3~Si6 The product produced is found to be a round rod of 7.73 mm dia-meter. Both the rod skin and end appearance are found to he of uniform white color, with a small and uniform open cell struc-ture. Density is 0.109 gm/cc. The rods are then cut into 20 mm long tips and tested by methods well-known to those familiar in the practice of cigarette filtration. Encapsulated pressure drop is measured at 58 mm H20 with a smoke removal efficiency of 33.4 percent of total particulate matter. Tip weight is .102 gm/20mm tip with a firmness before and after smoking of 96 percent as measured by a "FILTRONA"* firmness tester.
Example II
The process of Example I is repeated except that the triacetin plasticizer level and extruder temperature conditions were varied as follows:

Screw Extruder Conditions -Triacetin Triacetin Zone 2 Zone 3 Zone 4 Die Zone Torque in Feed (wt%) in Final Rod 180C 313C 290C 292C 110~% 0% 0 152C 271C 237C 246~C 80-90% 14.7% 10 %
152C 260C 230C 235C 74% 18% 15 %
128C 215C 19~C 210C 80% 23% 21.5 %
The rod extrudates obtained with 14.7~ and 18% triacetin are found to be satisfactory in all respects. The rod extrudate ob-tained with 0% triacetin is found to be heavily degraded with no size control or reasonable cell structure and commercially unac-ceptable. The rod extrudate obtained with 23% triacetin is found -~
to be overplasticized, that is to say, the cell structure is large and open and commercially unacceptable.
Example III
.
A v-shell tumbler is charged with 69.84 weight percent of 30 cellulose acetate and 3.23 weight percent of limestone nucleating ~
agent and mixed for ten minutes at which time 6.79 weight percent ~ ~-*Trademark employed by Cigarette Components Limited, Friendly House 21-24 Cheiswell Street, London, EClY 4UD

of methyl alcohol is added at a rate of 8 milliliters per minute followed by the addition of 3.23 weight percent of acetone and 15.73 weight percent of triacetin. Mixing is continued for an-other thirty minutes and the mixture is screened through a 3 1/2 sieve to eliminate agglomerates. The mixture is then charged into a Model 250 extruder manufactured by ~W Brabender Instru-mentst Inc. of 48 East Wesely Street, P. O. Box 2127, South Hakensack, N.J. 07605, said extruder having a 3/4" screw and a 25 to 1 length to diameter ratio. ~he mixture is melted in the unvented extruder and extruded through a 0.18" diameter orifice.
After extrusion the rod is conditioned in a heating cabinet sized by means of a die and then cut. The operating conditions of the equipment are as follows:
sarrel lleated Extruder Zone Temperature(C) Screw Orifice Press Cabinet

2 3 4 Die (zone 5) rpm Dia. Psig Profile (&) 130 139 215 220 130 0.18" 200 1~5,110,120,100 The rod end product is found to be an open cell foam struc-ture, however, the pore size is not as uniform and consequently, the product is not as desirable as that produced by the vented screw extruder of Example 1.
Pressure drop as reported in the preceeding examples is mea-sured by the following method: Air is sucked through a 20 milli-meter length of the fully encapsulated filter at a steady rate of 1050 cubic centimeters per minute and the resulting pressure difference across the filter is measured by means of a water mano-meter. The result is expressed in a millimeters of water gauge.
Cigarette filter rod hardness as reported in the preceeding examples is measured by means of a "FILTRONA" Tester (manufactured by Cigarette Components Limited), by a test in which a rod (for example a length of say 120 millimeters of the foamed resin) having a mean diameter (D~, for example about 7.8 millimeters, is com-pressed between two plates provided in the instrument. The rod ~;

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is subjected to compression for 15 seconds by a load of 300 grams applied to opposite sides of the cylindrical surface of the rod and the average depression ~A~, that is the decrease in diameter of the rod, is measured. The hardness is the diameter of the sample measured at a load of 300 grams and expressed as a per-centage of the original diameter, that is, it is given by the following formula:
Hardness % = [(D-A)] /D x 100 Smoke removal efficiency as reported in the preceding examples is determined as follows: ~
Twenty weighed tips are attached to standard tobacco ~ . .. .
columns and smoked on a Keith-Newsome Smioklng Machine as set forth in Tobacco Science 1:51-54(1957). Traps are used to collect the particulate matter which passes through the cigarette filter.
Smoke removal efficiency is calculated by dividing the increase in weight of the tips by the sum of the weight increases of the tips and the Cambridge traps.
The product of this invention is best illustrated in the pro-jected view of Figure 5 of the drawings and in cross-section by ~20 Figure 8 of the drawings. The product, as can be seen, is cylin-drical with a well-formed skin which is found to be substantially water impervious. Within the skin is an interconnecting expanded cellular foam, at least some of the cells being further charac~
terized ~y the presence of fibrils bridging the cell walls, the entire structure having a high ratio of membranous cell walls to fibrils. Although smoke gasses in cigarette smoke moves through the foam with relative ease, a significant amount of particulate matter and condensable vapor is trapped in~the foam by impingement and absorption. The filter provides a large available surface for the capture of smoke particles due to the small cell size and the presence of fibrils within at least some of the cells. Therefore, , Si6 the smoke is relatively uniformly clistributed throughout the filter cross-section.
Figure 6 of the drawings i5 representative of a projected view of the filter tip of the precipitated cellulose acetate filter of U.S. Patent No. 3,574,803. Rather than being an expanded foam, this filter is best characterized as illustrated in cross-section by Figure 7 of the drawings, as a s1ntered structure con-taining voids. There is no evidence of membrane~ or microfi-brils normally associated with hot melt extruded foamed structures.
Distinctions between the skin structure of the precipitated cellulose acetate filter rod of U.S. Patent No. 3,574,803 as represented by Figures 9 and 10 of the drawings are readily apparent over the skin structure of the cellulose acetate filter rod of this invention as represented by Fiyures 11 and 12 of the drawings. Figures 9 and 10 which are magnified 100 and 1000 times respectively reveal large surface voids which render the rod water permeable. Figures 11 and 12 which are also magnified 100 and 1000 times respectively are free of surface voids and hence water impermeable.

Claims

1. A process for the preparation from cellulose acetate flake of a plasticized, open cell, foamed cellulose acetate filter rod, said process comprising melt extruding through an elliptical or circular orifice maintained at a temperature of from 180°C. to 280°C., of a screw extruder a mixture of cellulose acetate, a nontoxic, taste compatible thermally stable plasticizer having a boiling point of not less than 180°C., a blowing agent which is at least a partial solvent for cellulose acetate and a nucleating agent, said melt being substantially free of entrained air, and the extrusion pressure profile being such as to prevent said blowing agent from backing up in said screw extruder.

2. The process of claim 1 wherein the extrudate is passed through a sizing device.

3. The process of claim 1 wherein said blowing agent is at least a partial solvent for cellulose acetate and is selected from the group consisting of ketones, esters and monohydric alcohols having boiling points no higher than 120°C.

4. The process of claim 1 wherein said plasticizer is a plasticizer selected from the group consisting of esters of saturated dibasic acids, esters of saturated polyhydric alcohols, fatty acid esters and sulfonamide resins.

5. The process of claim 1 wherein said plasticizer is a plasticizer selected from the group consisting of glycerol tria-cetate, triethylene glycol diacetate and mixtures thereof.

6. The process of claim 1 wherein said nucleating agent is an inorganic material having a surface area of at least 0.28 square meters per gram.

7. The process of claim 1 wherein said plasticizer is present in quantities of from 10 to 26 percent by weight based on the weight of the total composition, and said nucleating agent is present in quantities of from 0.5 to 15 percent by weight based on the weight of the total composition.

8. The process of claim 1 wherein said plasticizer is glycerol triacetate.

9. The process of claim 1 conducted in six sequential temperature control zones, said zones being:
(a) a first zone classified as a polymer feed zone where cellulose acetate flake is maintained at a temperature of from -20°C. to 100°C. to prevent agglomeration of cellulose acetate flake, (b) a second zone which is a cellulose acetate polymer melt zone maintained at an inlet temperature of at least 100°C. and an outlet temperature of not more than 240°C., (c) a third zone which is a high pressure, high shear zone maintained at temperatures above the glass-rubber transition temperature melting of cellulose acetate and into which blowing agent is added, (d) a fourth zone which is a mixing zone maintained at a temperature less than zone three, (e) a fifth zone which is an extrusion die zone maintained at temperature in the range of 180°C. to 270°C., and (f) a sixth zone which is a conditioning zone maintained at a temperature of from about 45°C. to within 5°C. of the glass-rubber transition temperature of the extru-date.

10. The process of claim 9 wherein said screw extruder is a vented screw extruder.

11. The process of claim 9 wherein said screw extruder is a co-rotating, intermeshing vented twin screw extruder.

12. A cigarette filter rod comprising a substantially homo-genously plasticized cellulose acetate, substantially open cell nucleated foam characterized by the presence of at least some cells which are bridged by fibrils, said homogenously plasticized cellulose acetate employing a plasticizer selected from the group consisting of esters of saturated dibasic acids, esters of satur-ated polyhydric alcohols, fatty acid esters and sulfonamide resins.

13. The filter rod of claim 12 wherein said plasticizer is a plasticizer selected from the group consisting of glycerol tri-acetate, triethylene glycol diacetate and mixtures thereof.

14. The filter rod of claim 12 wherein said plasticizer is triacetin present in quantities of from 10 to 15 percent by weight.

15. The filter rod of claim 12 having a substantially water impervious skin structure.

16. The filter rod of claim 12 having a hardness as mea-sured by a "FILTRONA" tester of greater than 85 percent before and after smoking.

17. The filter rod of claim 12 having a pressure drop of from 20 to 200 millimeters of water for a rod having a length of 20 millimeters and circumference of 24.8 millimeters.

18. The filter rod of claim 12 having a smoke removal efficiency greather than 20 percent.