Cellulose esters having mixed-phase titanium dioxide particles for improved degradation   

Abstract: Degradable cellulose esters are disclosed that have incorporated therein or thereon mixed-phase titanium dioxide particles. The esters may be in the form of filters prepared by applying a plasticizer, having the mixed-phase titanium dioxide particles dispersed therein, to cellulose ester fibers to obtain plasticized cellulose ester fibers; and thereafter forming the plasticized cellulose ester fibers into a filter. Alternatively, the particles may be added to the dope from which the fibers are spun, or blended with a cellulose ester intended for molded articles.

FIELD OF THE INVENTION

The present invention relates to cellulose esters, and specifically, to cellulose esters that exhibit improved degradation.

BACKGROUND OF THE INVENTION

Typical cigarette filters are made from a continuous-filament tow band of cellulose acetate-based fibers, called cellulose acetate tow, or simply acetate tow. The use of acetate tow to make filters is described in various patents, and the tow may be plasticized. See, for example, U.S. Pat. No. 2,794,239.

Instead of continuous fibers, staple fibers may be used which are shorter, and which may assist in the ultimate degradation of the filters. See, for example, U.S. Pat. No. 3,658,626 which discloses the production of staple fiber smoke filter elements and the like directly from a continuous filamentary tow. These staple fibers also may be plasticized.

Acetate tow for cigarette fibers is typically made up of Y-shaped, small-filament-denier fibers which are intentionally highly crimped and entangled, as described in U.S. Pat. No. 2,953,838. The Y-shape allows optimum cigarette filters with the lowest weight for a given pressure drop compared to other fiber shapes. See U.S. Pat. No. 2,829,027. The small-filament-denier fibers, typically in the range of 1.6-8 denier per filament (dpf), are used to make efficient filters. In constructing a filter, the crimp of the fibers allows improved filter firmness and reduced tow weight for a given pressure drop.

The conversion of acetate tow into cigarette filters may be accomplished by means of a tow conditioning system and a plugmaker, as described, for example, in U.S. Pat. No. 3,017,309. The tow conditioning system withdraws the tow from the bale, spreads and de-registers (“blooms”) the fibers, and delivers the tow to the plugmaker. The plugmaker compresses the tow, wraps it with plugwrap paper, and cuts it into rods of suitable length. To further increase filter firmness, a nonvolatile solvent may be added to solvent-bond the fibers together. These solvent-bonding agents are called plasticizers in the trade, and historically have included triacetin (glycerol triacetate), diethylene glycol diacetate, triethylene glycol diacetate, tripropionin, acetyl triethyl citrate, and triethyl citrate. Waxes have also been used to increase filter firmness. See, for example, U.S. Pat. No. 2,904,050.

Conventional plasticizer fiber-to-fiber bonding agents work well for bonding and selective filtration. However, plasticizers typically are not water-soluble, and the fibers will remain bonded over extended periods of time. In fact, conventional cigarette filters can require years to degrade and disintegrate when discarded, due to the highly entangled nature of the filter fibers, the solvent bonding between the fibers, and the inherent slow degradability of the cellulose acetate polymer. Attempts have therefore been made to develop cigarette filters having improved degradability.

U.S. Pat. No. 5,947,126 discloses a bundle of cellulose acetate fibers bonded with a water-soluble fiber-to-fiber bonding agent. The bonded fibers are wrapped in a paper having opposing ends secured together with a water-soluble plug wrap adhesive, and a plurality of cuts are made to extend more than one half way through the bundle wrapped fibers. A tobacco smoke filter is thus provided that disintegrates and degrades in a relatively short period of time.

U.S. Pat. No. 5,947,127 discloses a filter rod produced by adding a water-soluble polymer in the form of an aqueous solution or dispersion, or in a particulate form, to a tow of cellulose ester fiber. The tobacco filter is said to be highly wet-disintegratable and, hence, contributes to mitigation of environmental pollution. The environmental degradability of the fiber can be increased by incorporating a biodegradation accelerator such as citric acid, tartaric acid, malic acid, etc. and/or a photodegradation accelerator such as anatase-form titanium dioxide, or titanium dioxide, preferably anatase-form titanium dioxide, may be provided as a whitening agent.

Research Disclosure, June 1996, pp. 375-77 discloses that the use of plasticizers used to form filters from acetate tow decrease the degradation of cigarette filters by holding the fibers together, but that simply leaving off the plasticizer will not allow the rapid disintegration of the filters in the environment due to fiber entanglement. The authors therefore propose environmentally disintegratable filters made using uncommon types of tow, that is, fibers which have properties that will significantly reduce entanglement when wet.

U.S. Pat. No. 7,435,208 discloses cigarette filters that comprise an elongate filter component having a longitudinal axis. A plurality of spaced-apart slits, generally perpendicular to the longitudinal axis of the filter component, partially extend into the component. The slits enable the filter to disintegrate and more readily degrade after being used and discarded.

U.S. Pat. Nos. 5,491,024 and 5,647,383 disclose a man-made fiber comprising a cellulose ester and 0.05 to 5.0% by weight of a titanium dioxide having an average particle size of less than 100 nanometers. The titanium dioxide is added to the “dope” (i.e., the solvated cellulose ester) prior to extrusion into the tow. Addition of the titanium dioxide may be at any convenient point prior to extrusion. The titanium dioxide is preferably an uncoated anatase material having an average particle size less than 10 nanometers and a specific surface area of about 250 m2/g.

U.S. Pat. No. 5,512,230 discloses a method for spinning a cellulose acetate fiber having a low degree of substitution per anhydroglucose unit (DS/AGU) of the cellulose acetate. The addition of 5 to 40 weight percent water to cellulose acetate (CA)/acetone spinning solutions (dopes) is said to produce dopes that will allow fibers to be solvent spun using CA with a DS/AGU from 1.9 to 2.2.

U.S. Pat. No. 5,970,988 discloses cellulose ester fibers having an intermediate degree of substitution per anhydroglucose unit (DS/AGU) that contain pigments which act as photooxidation catalysts. The fibers are useful as filter materials for tobacco products. The filter materials thus provided are easily dispersible and biodegradable and do not persist in the environment. The pigment may be titanium dioxide and is provided within the fiber, but in amounts greater than are typical for use as a whitening agent.

U.S. Patent Publication. No. 2009/0151738 discloses a degradable cigarette filter that includes a filter element of a bloomed cellulose acetate tow, a plug wrap surrounding the filter element, and either a coating or a pill in contact with the tow. The coating and/or pill may be composed of a material adapted to catalyze hydrolysis of the cellulose acetate tow and a water-soluble matrix material such that when water contacts the water-soluble matrix material, the material adapted to catalyze hydrolysis is released and catalyzes the hydrolysis, and subsequent degradation, of the cellulose acetate tow.

WO 2010/017989 discloses a photodegradable plastic comprising cellulose esters and also, if appropriate, additives. The photodegradable plastic comprises a dispersed photocatalytic carbon-modified titanium dioxide. The photodegradable plastic is said to exhibit a surprisingly high increase in photocatalytic degradability when compared with products in which a conventional or other modified titanium dioxide is used. The photodegradable plastic can, for example, first be further processed to give a filter tow.

WO 2009/093051 and U.S. Patent Publication. No. 2011/0023900 disclose a tobacco smoke filter or filter element comprising a cylindrical plug of a substantially homogeneous filtering material of circumference between 14.0 and 23.2 mm, wherein the substantially homogeneous filtering material comprises a plurality of randomly oriented staple fibers.

The photocatalytic activity of mixed-phase titanium dioxide has been investigated. See “Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR”, J. Phys. Chem. B 107 (2003) 4545-4549. See also “Probing reaction mechanisms in mixed phase TiO2 by EPR”, Journal of Electron Spectroscopy and Related Phenomena, 150 (2006) 155-163.

Titanium Dioxide P25, Manufacture—Properties—Applications, Technical Bulletin Fine Particles, Number 80, Degussa Aerosil & Silanes Product Literature (Undated) discusses commercial uses of mixed-phase titanium dioxide, including use as a photocatalyst and as a photo-semiconductor.

U.S. Pat. No. 5,720,803 discloses a composition comprising a cellulose ester including at least 10 weight % of a low-substituted cellulose ester having an average degree of substitution not exceeding 2.15 and giving a 4-week decomposition rate of at least 60 weight % as determined using the amount of evolution of carbon dioxide as an indicator in accordance with ASTM 125209-91. The composition may contain a plasticizer, an aliphatic polyester, a photolysis accelerator such as anatase type titanium dioxide or a biodegradation accelerator such as organic acids and their esters. The low-substituted cellulose ester may be a cellulose ester having an average degree of polymerization from 50 to 250, an average degree of substitution from 1.0 to 2.15 and a residual alkali metal/alkaline earth metal-to-residual sulfuric acid equivalent ratio of 0.1 to 1.1. The biodegradable cellulose ester composition is said to be suitable for the manufacture of various articles including fibrous articles such as tobacco filters.

U.S. Pat. No. 5,478,386 discloses a composition that includes a cellulose ester including at least 10 weight % of a low-substituted cellulose ester having an average degree of substitution not exceeding 2.15. The composition may contain a plasticizer, an aliphatic polyester, a photolysis accelerator such as anatase-type titanium dioxide, or a biodegradation accelerator such as organic acids and their esters.

U.S. Pat. No. 5,242,880 discloses novel titania comprising anatase titanium dioxide and sodium, potassium, calcium, magnesium, barium, zinc, or magnesium salts of sulfuric or phosphoric acid. The titania are said to be useful in the pigmentation of oxidizable polymers, while at the same time providing a catalyst system for the photooxidation of the oxidizable polymers.

U.S. Pat. No. 5,804,296 discloses a composition comprising a cellulose acetate or other cellulose ester, and an anatase-type titanium oxide having a specific surface area of not less than 30 m2/g, a primary particle size of 0.001 to 0.07 μm, or a specific surface area of not less than 30 m2/g and a primary particle size of 0.001 to 0.07 μm. For improving the photodegradability and the dispersibility, the surface of the titanium oxide may be treated with a phosphoric acid salt or other phosphorus compound, a polyhydric alcohol, an amino acid or others. The composition may further contain a plasticizer and/or an aliphatic polyester, a biodegradation accelerator (e.g. organic acids or esters thereof).

WO 1995/29209 discloses pigmented cellulose acetate filaments produced by mixing a dispersion of titanium dioxide in a carboxylate ester of a polyhydric alcohol with cellulose acetate and a solvent for cellulose acetate. The resulting dispersion is dry spun to produce pigmented cellulose acetate filaments.

Balázs, Nándor et al.; “The effect of particle shape on the activity of nanocrystalline TiO2 photocatalysts in phenol decomposition”; Applied Catalysis B: Environmental, 84 (2008), pp. 356-362, investigated the effect of the morphology, that is spherical versus polyhedral, on the photocatalytic activity of nanocrystalline titanium dioxide photocatalysts.

Byrne, H. E. et al.; “Characterization of HF-catalyzed silica gels doped with Degussa P25 titanium dioxide”; Journal of Non-Crystalline Solids, 355 (2009), pp. 525-530, synthesized SiO2/TiO2 composites by adding Degussa P25 TiO2 to a liquid sol that was catalyzed by HNO3 and HF acids. The composites were then characterized by several different analytical techniques.

Hurum, D. C. et al., in “Probing reaction mechanisms in mixed phase TiO2 by EPR”; Journal of Electron Spectroscopy and Related Phenomena, 150 (2006), pp. 155-163, investigated charge separation processes in mixed phase TiO2 photocatalysts by electron paramagnetic resonance spectroscopy.

Janus, M. et al., in “Carbon-modified TiO2 photocatalyst by ethanol carbonisation”; Applied Catalysis B: Environmental; 63 (2006), pp. 272-276, investigated the effect on photocatalytic activity of modifying titanium dioxide powder by carbon via ethanol carbonization.

Janus, M. et al., in “Carbon Modified TiO2 Photocatalyst with Enhanced Adsorptivity for Dyes from Water”; Catal. Lett.; 131 (2009), pp. 506-511, obtained a new photocatalyst by modifying a commercial anatase titanium dioxide in a pressure reactor in an ethanol atmosphere. The photocatalytic activity of the material was tested during three azo dyes decompositions.

Lu, Xujie et al., in “Intelligent Hydrated-Sulfate Template Assisted Preparation of Nanoporous TiO2 Spheres and Their Visible-Light Application”; ACS Applied Materials & Interfaces; December 2010, investigated nanoporous titanium dioxide spheres and their applications, including their photocatalytic activities.

Juergen Puls et al., in “Degradation of Cellulose Acetate-Based Materials: A Review”; Journal of Polymers and the Environment: Volume 19, Issue 1; 2011; pp. 152-165, reviewed studies conducted on the biogradability of cellulose acetate, including photo-degradation.

There remains a need, however, for degradable cellulose esters such as those used to make cigarette filters, especially those that exhibit an increased ability to degrade in the environment using materials that may be applied to the filters in a variety of manners.

SUMMARY

In one aspect, the invention relates to articles comprising a cellulose ester having incorporated therein or thereon mixed-phase titanium dioxide particles. The article may be in the form of a molded article, or in the form of cellulose ester fibers, for example in the form of a yarn or a fabric.

Alternatively, the cellulose ester fibers may be in the form of a filter prepared by a process that includes the steps of applying a plasticizer, having the mixed-phase titanium dioxide particles dispersed therein, to the cellulose ester fibers to obtain plasticized cellulose ester fibers; and thereafter forming the plasticized cellulose ester fibers into a filter. In this aspect, the plasticizer may include one or more of: triacetin (glycerol triacetate), diethylene glycol diacetate, triethylene glycol diacetate, methyl phthalyl ethyl glycollate, dibutyl phthalate, tripropionin, acetyl triethyl citrate, triethyl citrate, and mixtures of triacetin and one or more polyethylene glycols. When the articles of the invention are in the form of filters, the filters may be provided with one or more slits, and may be useful, for example, for use in cigarettes.

The mixed-phase titanium dioxide particles useful according to the invention may comprise, for example, an anatase phase present in an amount from 50% to 98%, and a rutile phase present in an amount from 2% to 50%, in each case as measured by x-ray crystallography. The particles may have a surface area, for example, from about 10 to about 300 sq. m/g, and may independently have an average particle size from about 1 nm to about 100 nm, or from 5 nm to 50 nm.

The amount of mixed-phase titanium dioxide particles provided to the articles according to the invention may vary widely, for example from about 0.01 to about 20 wt. %, based on the weight of the article.

When the articles of the invention are in the form of fibers, for example, they may be dispersed in the cellulose ester fibers, for example by being dispersed in the solvent from which the fibers are spun.

The cellulose esters useful according to the invention may comprise one or more of a cellulose acetate, a cellulose propionate, a cellulose butyrate, a cellulose acetate propionate, or a cellulose acetate butyrate, for example a cellulose acetate having a DS/AGU from about 1.8 to about 2.7, or from about 1.9 to about 2.5.

Further aspects of the invention are as disclosed and claimed herein.

DETAILED DESCRIPTION

Although rutile-based titanium dioxide particles roughly 200 nm in size may be provided to cellulose esters to provide good light scattering and minimal photo activity, and anatase-type titanium dioxide particles used to enhance the degradation of cigarette filters, the present invention relates rather to titanium dioxide particles of mixed phase, hereinafter described as mixed-phase titanium dioxide particles, which provide enhanced degradation under UV radiation compared with those comprised either of anatase or rutile form alone. The mixed-phase titanium dioxide particles of the invention are especially those which are uncoated and have ultrafine particle sizes (<100 nm) to achieve optimum photo activity.

We have determined that titanium dioxide particles having a mixture of anatase and rutile crystal structure, that is, mixed-phase particles, provide the greatest amount of degradation in the presence of UV radiation, when compared with particles either entirely rutile or entirely anatase in crystalline structure. The invention thus relates to cellulose esters having incorporated therein titanium dioxide particles with mixed phases of anatase and rutile crystals, especially those having uncoated surfaces, and an ultrafine (<100 nm) particle size. The mixed-phase titanium dioxide particles may be provided to the cellulose esters in a variety of manners.

When we say “rutile phase” or “rutile crystalline structure,” we refer to the structure which is the most common natural form of titanium dioxide as identified by its unique x-ray diffraction pattern. This phase or crystalline structure may be identified, for example, by x-ray crystallography. When we say “anatase phase” or “anatase crystalline structure,” we refer to the known structure as identified by its x-ray diffraction pattern. This phase or crystalline structure may likewise be identified, for example, by x-ray crystallography. These phases or crystalline structures are well known in the art of titanium dioxides, and are thus readily identifiable to those skilled in the art in the manner described herein.

Mixed-phase titanium dioxide particles are thus used according to the invention, and are characterized as having both rutile and anatase crystalline structures present in the same particle. By the term “mixed-phase,” we do not intend to include particles having only trace amounts of one or the other of the two structures. Thus, the amount of anatase phase present in the mixed phase particles may vary, for example, from about 50% to about 99%, or from 60% to 98%, or from 75% to 95%, in each case as measured by x-ray crystallography. The rutile phase present in the particles may likewise vary in a similar manner, for example from about 1% to about 50%, or from 2% to 40%, or from 5% to 25%, in each case as measured by x-ray crystallography. We have found these particles to be especially suitable at enhancing degradation of the filters in which they are used. Not wishing to be bound by any theory, we believe that the suitability of such mixed phase particles may be because the rutile phase provides better UV absorption while the anatase phase provides better photocatalytic behavior. The present invention thus relates to the use of mixed phase titanium dioxide particles in cellulose esters such as those used for cigarette filters, without regard to the method of incorporation, that is, whether provided to the plasticizer, within the fiber, or otherwise provided.

The mixed-phase titanium dioxide particles useful according to the invention may be prepared, for example, by methods such as the flame hydrolysis of titanium tetrachloride. Relatively high purity mixed-phase titanium dioxide particles may be obtained by this process. See, for example, the Titanium Compounds (Inorganic) entry of Kirk-Othmer, Encyclopedia of Chemical Technology, pp. 225-274, Vol. 24, Fourth Edition, John Wiley and Sons, 1997. The gas-phase oxidation of titanium tetrachloride can also be used. Id. Other processes may be used in which the particles obtained have a mixture of anatase and rutile phases in the same particle, as already described.

The surface area of the mixed-phase titanium dioxide particles of the invention may vary, for example, from about 10 to about 300 m2/g, or from 20 to 200 m2/g, or from 40 to 100 m2/g, as measured by gas absorption in a BET surface area method.

The amount of the mixed-phase titanium dioxide particles provided to the cellulose ester may vary within a wide range, for example, from about 0.01 to about 20 wt. %, or from 0.2 to 10 wt. %, or from 0.2 to 5 wt. %. In some aspects, the amount of titanium dioxide particles provided may depend upon the solution viscosity during fiber manufacturing. A variety of particle sizes of titanium dioxide are useful according to the invention, for example from about 1 nm to about 10 microns, or from 1 nm to 1 micron, or from 1 nm to 500 nm, or from 1 nm to 250 nm, or from 3 nm to 100 nm, or from 5 nm to 50 nm. We have found that nanoscale particles are particularly suited for use according to the invention. Not wishing to be bound by any theory, it may be that the use of a smaller particle size provides higher surface area. A second possible reason is that small particle size allows the UV radiation to penetrate further into the cellulose ester, so that the degradation is further from the surface, thus causing degradation deeper within the cellulose ester.

Although the particle sizes given refer to the primary particle size, the photoactive agent may be present not just in discrete particles, but also in agglomerates. We have found that particles present as agglomerates suitably enhance degradation of the resulting filters, but the particles may be milled, for example, if desired, in order to obtain a more uniform and primary particle size.

Both coated and uncoated titanium particles are suitable for use according to the invention. Coating agents that may be applied to the titanium oxide particles include, for example, carbon coatings. Coating agents that may be incorporated on the surface or with the titanium dioxide include, for example, carbon coatings and hydrated metal sulfates (MSO4*xH20, M=Zn, Fe, Co, Mg, etc.) Not wishing to be bound by any theory, certain coatings, for example carbon coatings, may assist in the desired photodegradation of the filters, for example by allowing visual light absorption.

Both modified and unmodified titanium particles are suitable for use according to the invention, for example carbon-modified particles. Although the mixed phase titanium oxide particles according to the invention are predominately rutile and anatase phases, the particles may have carbon modifications into or onto the particles. Carbon modifications or doping may be accomplished, for example, by ethanol carbonization or by the adsorption of dyes. Not wishing to be bound by any theory, certain modifications, for example carbon coatings, may assist in the desired photodegradation of the filters, for example by allowing visual light absorption. A second theory is that the carbon modifications, such as carbon doping, may suppress the phase transformation from anatase to rutile during high temperature treatments and reduce the crystal sizes.

In one aspect, the mixed-phase titanium dioxide particles of the invention may be provided to the cellulose ester “dope,” that is, to the cellulose ester dissolved in acetone from which cellulose ester fibers are spun. The mixed-phase titanium dioxide particles may be provided to the dope in a variety of manners, for example by the inclusion of a mixture of cellulose acetate, triacetin and TiO2. Alternatively, the particles may be provided by mixing the TiO2 in a hydrophilic solvent which would allow optimum particle dispersion. Further examples of dispersing titanium dioxide particles in cellulose ester fibers include those set out in U.S. Pat. No. 5,970,988, incorporated herein by reference, although the titanium dioxide particles of the present invention are characterized as being mixed-phase titanium dioxide particles.

In molded articles, the mixed-phase titanium dioxide particles of the invention may be provided, for example, by high shear blending with a polymer concentrate.

We have determined also that, in the manufacture of cigarette filters, the use of a photoactive agent in the plasticizer also causes an increased rate of breakdown of the resulting filter structure, as measured on filters exposed to UV radiation in an outdoor environment. This is distinguished from adding the photoactive agent to the fiber at the time the fiber is formed, for example by adding the photoactive agent to the cellulose ester dope, as just described. The present invention, however, relates to the use of mixed-phase titanium dioxide particles, whether provided in the cellulose ester fibers, or in the plasticizer, or both. The use of titanium dioxide particles in the plasticizer, whether or not the particles are mixed-phase in nature, is being separately pursued in a copending application filed on the same date herewith.

As used herein, the term “plasticizer” is intended to describe a solvent that, when applied to cellulose ester fibers, solvent-bonds the fibers together. Plasticizers useful according to the invention include one or more of: triacetin (glycerol triacetate), diethylene glycol diacetate, triethylene glycol diacetate, tripropionin, acetyl triethyl citrate, triethyl citrate, and mixtures of triacetin and one or more polyethylene glycols. The blends or mixtures may optionally contain polymers, for example water-soluble polymers such as cellulose mono-acetate with a DS/ADU of 0.6 to 0.9, polyesters, such as, methyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose, polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH).

As used herein, the mixed-phase titanium dioxide particles of the invention are also termed “photoactive agent” and serve as such agents, meaning that, when added to a plasticizer that is applied to a cellulose ester fiber, or to the cellulose ester itself, increases the rate at which the ester degrades upon exposure to UV radiation.

When applied with the plasticizer, the mixed-phase particles may be dispersed in the plasticizer in any of a number of ways, for example by use of high shear mixing devices, such as media mills or ultra-sonic probes. The stability of the mixed-phase particles in the plasticizer, that is, the tendency of the particles to remain suspended in the plasticizer during filter manufacture, may be enhanced by adding an amount of cellulose ester to the plasticizer, for example in an amount from about 0.1% to about 20%, or from 0.2 to 10%, based on the viscosity in the resultant plasticizer solution. Stability may be further enhanced by providing to the plasticizer an amount of a polyethylene glycol, one having a molecular weight, for example, from about 100 to about 400, or from 200 to 1000, in an amount from about 0.1% to about 20%, or from 0.2% to 10%.

Those skilled in the art will readily appreciate that providing a photoactive agent in the plasticizer rather than the fiber allows conventional acetate tows to be used, without any change in the ester or tow formulation. However, mixed-phase particles in the plasticizer may affect, for example, the viscosity of the plasticizer, especially if stabilizers such as a cellulose ester and/or a polyethylene glycol are incorporated. Thus, the stabilizer may best be chosen so as not to affect the viscosity, for example by providing a relatively low molecular weight cellulose ester, or polyethylene glycol, or both.

As used herein, the term “cellulose ester” means one or more cellulose esters, such as cellulose acetate, that may be, for example, melt-spun or solvent-spun into fibers. The cellulose esters useful according to the invention thus include, without limitation, cellulose acetates, cellulose propionates, and cellulose butyrates with varying degrees of substitution, as well as mixed esters of these, that is, cellulose acetate propionate, cellulose acetate butyrate, and cellulose acetate propionate butyrate. The cellulose ester of the present invention may be a secondary cellulose ester. Examples of suitable esters thus include cellulose acetates, cellulose acetate propionates, and cellulose acetate butyrates, as described in U.S. Pat. Nos. 1,698,049; 1,683,347; 1,880,808; 1,880,560; 1,984,147; 2,129,052; and 3,617,201, incorporated herein by reference.

Thus, although cigarette filters are traditionally made with cellulose acetate fibers, the invention is not strictly limited to traditional esters. Further, while the typical degree of substitution per anhydroglucose unit (DS/AGU) of acetate for cigarette filters is about 2.45, esters may be readily used and fibers constructed with a range of acetyl levels, such as from 1.5 to 2.8, or from 1.8 to 2.7, or from 1.9 to 2.5, or for example, an average DS/AGU of about 2.0. We note that lower DS/AGU values may provide a faster biodegradation.

The cellulose esters of the present invention can be spun into a fiber, for example by melt-spinning or by spinning from an appropriate solvent (e.g., acetone, acetone/water, tetrahydrofuran, methylene chloride/methanol, chloroform, dioxane, N,N-dimethylformamide, dimethylsulfoxide, methyl acetate, ethyl acetate, or pyridine). When spinning from a solvent, the choice of solvent depends upon the type of ester substituent and upon the DS/AGU. A suitable solvent for spinning fiber is acetone containing from 0 to 30% water. For cellulose acetate having a DS/AGu of 2.4-2.6, a preferred spinning solvent is acetone containing less than 3% water. For cellulose acetate having a DS/AGU of 2.0-2.4, a preferred spinning solvent is 5-15% aqueous acetone. For cellulose acetate having a DS/AGU of 1.7 to 2.0, a preferred solvent is 15-30% aqueous acetone.

When melt-spinning fibers, the cellulose ester or plasticized cellulose ester may have a melt temperature, for example, from 120° C. to 250° C., or from 180° C. to 220° C. Examples of suitable plasticizers for use in melt spinning of cellulose esters include, but are not limited to, diethyl phthalate, dipropyl phthalate, dibutyl phthalate, triacetin, dioctyl adipate, polyethylene glycol-200, or polyethylene glycol-200, or polyethylene glycol-400. Preferred plasticizers for melt-spinning include dibutyl phthalate, dioctyl adipate, or polyethylene glycol-400.

The cellulose ester fibers used may be continuous fibers, or may be staple fibers having a shorter length, rendering the fibers more susceptible to degradation. Thus, the staple fibers may have a length from about 4 to 20 mm, or from 5 to 18 mm, or from 7 to 16 mm.

The cellulose ester fibers useful according to the invention are typically crimped, having, for example, from 4-20 crimps per inch, or from 10 to 15 crimps per inch. The fibers typically have a denier/filament (DPF) of 20-0.1, or from 5-1.5 DPF. For processing, the fibers may optionally contain lubricants or processing aids such as mineral oil, used in amount from 0.1 to 3%, or from 0.3 to 0.8%.

While particulate additives are commonly added into fibers to enhance filter whiteness, these additives are typically titanium dioxide particles roughly 200 nm in size, a size which provides good light scattering but with minimal photo activity. Such titanium particles commonly have an inorganic coating on the surface to enhance the particles\' dispersion in spinning solutions. Titanium dioxides are not traditionally added to the plasticizer, perhaps because it might limit the filter\'s hardness without enhancing the whiteness.

Filters produced according to the invention may further incorporate other features to enhance their degradation, for example by being slit perpendicular to the longitudinal axis, by incorporating staple fibers, or otherwise shorter fibers which tend to increase the rate of degradation in the environment. Further measures to increase the rate of degradation may include incorporating in the plasticizer one or more polymers, for example water-soluble polymers, although this may, in fact, reduce the rate of degradation if this affects the ability of the plasticizer to solubilize the ester such that the photoactive agent does not penetrate the fiber during the plasticizing step. Water-soluble polymers that may nonetheless be useful include cellulose mono-acetate with DS/ADU of 0.6 to 0.9, polyesters, such as, methyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose, polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH). The novel processes and filters provided by the present invention are further illustrated by the following examples.

As noted, the invention is not limited to cellulose esters in the form of fibers for cigarette filters, but may relate also to other cellulose ester fibers, such as those known in the industry as acetate yarn, and used in various textile uses including clothing. The cellulose esters according to the invention may also be in the form of molded articles, or any other known form of cellulose esters in which an increased rate of breakdown in the environment is desired.

In the following examples, filter samples were assembled with a conventional filter maker with 10% triacetin applied to the fibers during filter making.

For degradation testing, the filter samples were placed on the roof of a building in individual wire mesh cages to allow sufficient UV radiation to reach the filters. The cages were positioned approximately four inches from the ground so as to minimize the samples sitting in water puddles present on the roof top. Each roof top study consisted of ten 21-mm filter tips per example, placed in the mesh cage with the paper removed leaving only the fibers that formed the filter. The paper was removed so the fibers in the filters could be directly exposed to UV radiation and thereby determines the effects of the photoactive agent role in degradability. The filters were collected for weighing periodically to assess the degradation of the fibers in the filter. The results are a comparison of the weight of the ten filter samples at each test point.

Two separate outdoor weathering experiments were performed at different times. Examples 1-4 represent the first set of samples tested and Examples 5-8B represent the second set of samples tested by roof top outdoor weathering. Each set was tested by the degradation testing described above.

Ten cigarette filters with cellulose acetate fibers containing no TiO2 were constructed. The outdoor weathering results are shown in Table 1 for comparison with Examples 1 through 4. The results show the limited photo degradation rate for the cellulose acetate fibers with no TiO2 present.

Ten conventional cigarette filters were constructed with cellulose acetate fibers containing 0.5% pigment-size (210 nm) uncoated anatase TiO2. The properties of the TiO2 (Tioxide A-HR) are listed in Table 2 for comparison to the other experimental samples.

To illustrate the importance of the amount of mixed phases and crystal size, ten filters were made with cellulose acetate fibers containing 0.5% of Nanotek, an ultrafine-size (32 nm), uncoated TiO2 available from Nanophase. The percent crystal phase structure for the ultrafine-size TiO2 was 94% anatase and 6% rutile. The crystal structure was greater than 500 Å, which indicates large crystals. The remaining properties of the ultrafine-size TiO2 (Nanotek from Nanophase) are listed in Table 2.

Ten filters were prepared with cellulose acetate fibers containing 0.5% ultrafine-size (21 nm) uncoated mixed phase TiO2. (P25, available from Degussa). Note the mixed phases in the TiO2 particles, 89% anatase and 11% rutile, and the small crystal sizes (301 Å). The properties of the TiO2 (P25 from Degussa) are listed in Table 2.

As shown in Table 1, the fastest degradation was provided by Example 4, in which the ultrafine uncoated TiO2 particles consisted of a mixed phase of anatase (89%) and rutile (11%). A filter with no TiO2 had a remaining weight of 81% after 10 months; while a filter with the pigment size (210 nm) uncoated anatase (94%) particles had a remaining weight of 38%. The mixed phase ultrafine-size (21 nm) TiO2 in Example 4 had a remaining weight of 33%. Note the other mixed phase TiO2 in Example 3 had a remaining weight of 72% and shows the importance of the higher blend of rutile to anatase along with the smaller crystal sizes.

TABLE 1 Outdoor Weathering Results Example No. 1 2 3 4 Tioxide A-HR TiO2, % in Fiber — 0.5 — — Nanophase TiO2, % in Fiber — — 0.5 — Degussa P 25 TiO2, % in Fiber — — — 0.5 Time, Months Wt % Wt % Wt % Wt % 0 100 100 100 100 2 93 89 91 84 4 93 85 87 76 6 91 73 88 62 7 89 64 86 53 8 85 52 81 42 9 82 44 76 34 10  81 38 72 33

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