HK1150013A - Method and apparatus for the plasma processing of filter material - Google Patents

Description

Method and apparatus for plasma treatment process of filter material

Technical Field

The present invention relates to filters, including filters for smoking articles such as cigarettes.

Background

Most cigarettes include filters for absorbing smoke and accumulating particles of smoke constituents. The main component of most filters is a bundle of cellulose acetate wrapped in filter paper. This material is usually made of cotton or tree pulp as a synthetic fiber. Plasticizers such as triacetin (glycerol triacetate) may be used to facilitate bonding of the fibers together.

Cigarette filters must achieve a balance of removing undesirable components from cigarette smoke while maintaining a product that is satisfactory to consumers. Accordingly, there is an interest in improving the performance of cigarette filters.

Various mechanisms are well known to include charcoal or charcoal in cigarette filters to enhance their filtering properties. For example, the carbon may be included in the filter as a separate component, or the carbon particles may be dispersed in the filter tow and/or filter wrapper. US2006/0151382 discloses non-porous carbon (excluding carbon nanotubes) and nanostructured materials suitable for use in cigarette filters. The metal may be deposited onto the carbon from the vapour phase, for example using chemical or physical vapour deposition. One of the disadvantages of using chemical vapor deposition is that it generally involves the use of reagents, which can lead to contamination or residue problems.

Disclosure of Invention

The present invention provides a method of manufacturing a filter material suitable for use in a smoking article comprising carbon. The method includes improving selective filtration characteristics of the filter material by altering the surface of the filter material. The surface modification is carried out by a plasma treatment process.

Non-equilibrium or low temperature Plasma treatment processes (see, e.g., Wertheimer et al; LowTemperature Plasma Processing of Materials: Past, Present and Future; Plasma processes and Polymers, 2, 7-15, 2005) are capable of improving the chemical deposition and morphology of the surface of Materials, including filter Materials, while retaining most of their properties. The use of plasma allows the deposition of coatings of tailored chemical composition and properties onto the filter material, or grafting of predetermined chemical functional groups. By appropriate adjustment of plasma parameters, a wide range of chemical compositions can be used for these surfaces, including compositions that are not available in solution. In addition, the plasma treatment process does not need to use a solvent, thereby avoiding pollution and residual risks and reducing the environmental impact of the modification process. Generally, such plasma-induced changes in surface composition and morphology will affect filtration characteristics. For example, an increase in surface area (e.g., due to increased roughness) may result in improved absorption of the volatile component, while enriching for new surface chemical components having suitable chemical groups.

Generally the improved filtration characteristics are not uniform for all smoke constituents, but they can become more selective. This allows a targeted improvement of the filtration properties of the filter material to some extent, with some smoke components being reduced much more than others. The result can be an enhanced smoking article that is still appealing to consumers.

Possible modifications to the filter include enhancing the acidic nature of its surface, intended to potentially alter the absorption of basic chemicals present in the smoke, and vice versa, enhancing its basic nature to potentially alter the absorption of acidic substances.

Thus, selectively improving the filtration characteristics of the filter material may include enhancing the absorption of acidic components.

In addition, selectively improving the filtration characteristics of the filter material may include enhancing the absorption of alkaline components.

In addition, selectively improving the filtration characteristics of the filter material may include enhancing the hydrophilic nature of the surface of the material.

Enhancing the hydrophobic character of the filter may provide better moisture resistance.

It will be appreciated that a particular filter material may undergo one or more modification processes to impart one or more characteristics to the final filter. Thus, the filter material may, for example, be subjected to two different plasma processing treatments. It will be appreciated that a particular filter may comprise different aliquots of material, each plasma treatment in a different manner being aimed at imparting a range of desired filtration characteristics to the final filter. Thus, a filter material may comprise a first material subjected to a first plasma processing treatment and a second material subjected to a second plasma processing treatment.

Examples of gases that can be used in the plasma process include NH₃(for grafting basic groups containing nitrogen) with a substituent such as N₂Or H₂A surrogate; o is₂(for grafting acid groups) with, for example, a water vapor substitute. Plasma grafting, as well as plasma etching processes, are highly substrate-dependent methods, the final chemical composition of the grafted surface being strongly dependent on the properties of the substrate material. However, the plasma enhanced chemical vapor deposition (PE-DVD) process is less dependent on the properties of the substrate. Thus, for example, the plasma treatment process may comprise plasma enhanced chemical vapor deposition of acrylic acid. The plasma treatment process may also, or alternatively, include the use of O₂And/or NH₃Etching of (4).

Examples of gas/vapor mixtures that can be used in PE-CVD processes to produce coatings on surfaces may be mixed with argon or other inert buffer gases, including Acrylic Acid (AA) or other organic acids (for coatings with surface acidic groups and properties), and allylamine (AAm) and other organic amines (for coatings with surface basic groups and properties). Under appropriate diagnostic regulation, suitable adjustment of plasma parameters such as power, voltage, feed properties and flow rate therein generally enables adjustment of fragmentation of the feedstock in the discharge, thereby adjusting the density of active species (radicals, atoms, ions, etc.) capable of interacting with the substrate, and then the composition and properties of the modified substrate. Changing the properties of the feed gas/vapour compound, commonly referred to as "monomer" in PE-DVD, results in a variety of possible coatings (e.g. silica based, teflon based and others) of different nature and character, some of which have wide industrial application.

The filter material may comprise carbon in particulate form. This carbon in particulate form is plasma processed in a properly constructed plasma reactor before being added to the filter (e.g., by impregnation into cellulose acetate tow). Another possibility is to add the carbon to the support before the plasma treatment process. For example, the carbon may be incorporated into a thread or sheet material such as paper. The manufacturing apparatus may employ a continuous reel arrangement for the thread or sheet material to allow the filter material to be fed through the plasma processing chamber. One possibility is to pass the filter material through a plurality of treatment chambers, each for a different form of plasma treatment process.

The invention also provides filter materials including those suitable for use in smoking articles. The filter material includes carbon that is subjected to surface modification by a plasma treatment process to improve selective filtration characteristics of the filter.

Smoking articles (e.g., cigarettes) incorporating such filters are also provided according to the invention.

Thus, the methods described herein generally involve altering the surface chemical composition and other properties of the filter material using non-equilibrium plasma treatment processes such that more efficient uptake of smoke by cigarettes and other such products occurs at the surface of the filter material. In one embodiment, activated carbon in the form of carbon particles is used as a filter material (e.g., for cigarettes) and is plasma treated to improve its surface characteristics. Simulated smoking experiments have been conducted with the above-described plasma-treated carbon particles and have been found to have enhanced filtration properties for removing certain components of smoke as compared to untreated carbon particles.

Brief description of the drawings

For a better understanding of the present invention, reference is made to the following drawings by way of example.

FIG. 1 is a schematic view of a plasma reaction chamber suitable for use in a uniform plasma treatment process for particulate materials, in accordance with one embodiment of the present invention;

FIG. 2 is a schematic view of a continuous roll-to-roll plasma reaction chamber suitable for processing substrates in roll form, in accordance with one embodiment of the present invention; and is

Figure 3 shows Water Contact Angle (WCA) data for aqueous solutions of different pH for graphite treated in the same plasma treatment process used to impart a predetermined acidic/basic character to the carbon particle surface, in accordance with one embodiment of the present invention.

Detailed Description

Plasma treatment process

Low pressure non-equilibrium cold plasmas (e.g., room temperature plasmas rather than thermal plasmas at thousands of degrees) provide an effective tool for modifying the surface composition and profile of materials without altering their bulk properties. Plasma processing processes are well known in a variety of different industries, including microelectronics, semiconductors, food and pharmaceutical packaging, automotive, preservation, and biomaterials. Three main categories of plasma treatment processes that can be defined are: plasma etching, wherein the material is ablated by the interaction of the material and active substances generated in plasma to generate volatile products; plasma enhanced chemical vapor deposition (PE-DVD) to deposit thin (5-1000nm) organic or inorganic coatings; and plasma treatment, grafting functional groups on the material using glow discharge. The grafted functional groups may be partially associated with some degree of crosslinking of the treated surface.

Plasma etching, deposition and processing can be carried out in a suitably constructed low pressure reactor-e.g., at 10^-210 torr (. about.1.3-1300 Pa). The electromagnetic field is transferred to the gas inlet by electrodes or other means (e.g. coils located outside the dielectric reaction vessel) to generate a glow discharge. Typically, alternating (e.g., radio frequency at 13.56MHz) rather than continuous electric fields are utilized. Materials exposed to glow discharge are modified by the interaction of species generated in the gaseous plasma phase (atoms, radicals, ions) with the surface of the material. After the plasma treatment process, low molecular weight molecules are formed in the plasma by recombination reactions and unreacted monomer molecules are pumped out.

The plasma treatment process modifies the surface of the material by forming a stable interface. Covalent bonds are formed between the active species and the matrix material in the plasma phase. As is known to those skilled in the art, plasma treatment processes increase the thickness of the coating produced by PE-CVD, the amount (depth) of material etched during etching, and the extent of grafting in plasma treatment. More generally, the resulting surface modification can be controlled by appropriately adjusting and controlling experimental parameters such as input power, frequency and amplitude of the applied electric field; the nature, flow rate and pressure of the gas inlet; temperature, bias voltage and substrate position, among others. These external control parameters in turn affect various internal factors, such as the degree of ionization of the gas inlet; density of reactive (atomic, ionic, radical, etc.) species in the plasma phase; uniformity of the treatment process; deposition, etch, and process rates. Internal parameters can be controlled using diagnostic tools such as Optical Emission Spectroscopy (OES), Laser Induced Fluorescence (LIF), and absorption spectroscopy (ultraviolet, visible, and infrared).

As described herein, low pressure plasma treatment processes are used to process surface chemistry and char properties, which in turn affect the filtration properties of the char. Figure 1 shows a reactor suitable for plasma treatment of particulate material. This particulate material may be in the 18-40 mesh range, which corresponds to approximately 420-1000 microns. The reactor shown in FIG. 1 is a rotating device capable of uniformly treating up to 500g of carbon particles at a glow discharge frequency (13.56MHz) while stirring. The reaction chamber comprises a rotating glass cavity 1 with glass wings 2, a fixed radio frequency external electrode 3, a ground electrode 4, a fixed edge 5 and a rotating vacuum edge 6. Another form of carbon matrix, such as graphite, may also or alternatively be provided.

FIG. 2 shows another plasma chamber that uses a movable roll input arranged in a continuous roll-to-roll fashion. The reaction chamber comprises a front chamber 7 housing a first reel 8, a reaction chamber 9 having a radio-frequency electrode 10 and a rear chamber 11 housing a second reel 12. The reaction chamber further comprises a series of pumps 13. This configuration is suitable for use with wire or sheet materials, rather than powder or granular forms, and enables continuous processing. For example, the present machine can be used for the treatment of cellulose tow containing carbon particles. In this case, the tension and curvature of the material are strictly controlled in view of the characteristics of the tow. In particular, the path of the tow 14 as shown in fig. 2 does not include corners or sharp curvatures to avoid damage to the tow. The continuous reel apparatus of fig. 2 can also be used to treat carbon paper (i.e., paper impregnated or coated with carbon particles).

Fig. 3 shows the data relating to the adjustment of the acidic/basic surface properties of carbon materials by means of a plasma treatment process. In this case, a radio frequency glow discharge (13.56MHz) plus O₂/NH₃(grafting) or AA/AAm gaseous mixtures (PE-CVD) are used to modify the surface of graphitic substrates bearing acidic (oxygen-containing) and/or basic (nitrogen-containing) surface groups.

O is carried out at a pressure of 0.250mbar and a radio frequency output power of 100Watt₂/NH₃Graft discharge was carried out for 2 minutes. The total flow rate is 10sccm (standard milliliters per minute) with O₂/NH₃The flow ratios were 10/0, 5/5 and 0/10 sccm/sccm. An AA/AAmPE-CVD discharge was carried out at a pressure of 0.120mbar and a radio frequency output power of 100Watt for 10 minutes. The total flow rate was 10sccm with AA/AAm flow ratios of 4/0, 2/2 and 0/4sccm/sccm and 6sccm argon as the buffer gas. Untreated and grafted/coated graphite was added to 2 μ l drops of acidic (HCl) and basic (NaOH) aqueous solutions.

Because there are no acidic/basic groups present on the surface, the untreated graphite surface exhibits a WCA value of about 90 degrees, independent of the pH of the probe solution. All discharges observed reduced the WCA value of graphite because all types of added groups containing O-and N-grafted or contained in the coating were polar and hydrophilic with respect to bare carbon.

100％O₂And 100% AA discharge increases acid oxygen-containing groups (-COOH, OH and others) on the graphite surface; in fact, the WCA value is higher at low pH values due to the interaction between the solution and the acidic groups on the substrate surface, and therefore becomes lower when alkaline (high pH) solutions are used. When the surface of the graphite is added with a nitrogen-containing group (-NH)₂And others), 100% NH was found₃And 100% AAm discharge is exactly the opposite behavior; in this case, these trends are found to be higher WCA values when using high pH solutions, which decrease at acidic (low) pH values, due to the interaction between the solution and the basic groups on the substrate surface. Using 1/1O₂/NH₃And AA/AAm discharge, both acidic and basic types of groups are added to the graphite surface simultaneously, and amphoteric behaviour is observed, with a decrease in WCA values at low and high pH values compared to neutral pH (strong surface/solution interaction). These examples illustrate that a degree of control can be exercised over a substrate during plasma processing by using reactants having different characteristics.

Plasma treatment process for carbon particles

Carbon particles are treated in a plasma reactor as shown in fig. 1 using various surface treatment processes aimed at imparting acidic/basic surface characteristics as shown in fig. 3.

PE-CVD in acrylic acid/argon radio frequency glow discharge

The PE-CVD process was run in a discharge fed with AA gas and argon. The argon/AA flow ratio, the radio frequency power, the pressure, the rotation of the reactor and the process duration are controlled in such a way that: i.e. containing CH_xO_yThe crosslinked coating of the ingredient is tightly bound to the surface of the carbon particle, and has an average thickness capable of being adjusted to a range of 5 to 50 nm.

For samples produced according to the method, the characteristic data obtained with X-ray photoelectron spectroscopy (XPS), infrared spectroscopy and WCA diagnostic techniques showed a very hydrophilic coating, as expected (100% AA compared to the WCA data in fig. 3), with discontinuity causing the WCA to be undetectable (water absorbed) on the particle layer. As shown in fig. 3, the acidic nature of the coating is attributed to the presence of oxygen-containing groups, including carboxyl, hydroxyl, and carbonyl groups therein. The surface density of the groups in the coating depends on the degree of fragmentation of the AA monomer in the plasma phase, which can be controlled by appropriate adjustment of the plasma parameters; for example, by reducing the input power and/or increasing the pressure.

Plasma deposited layers have a very different composition and structure compared to conventional polyacrylic acids, with only carboxylic acid groups present as oxygen-containing groups. There is a degree of cross-linking (C-C and C-O bonds) in the plasma deposited coating, making the coating itself stable in air and water. In fact, samples aged in air and water were analyzed at some time after deposition and no changes in the relevant components were detected.

Plasma treatment in O glow discharge

The process is using O₂The fed discharge was run with argon gas mixed in some cases. The process parameters can be controlled so that an oxide layer containing oxygen chemical groups (carboxyl, hydroxyl and carbonyl) is formed on the carbon surface, thereby enhancing its polar (hydrophilic, acidic) character. Oxygen atoms in the plasma from fragmented O₂Molecules are formed which have a high reactivity with the carbonaceous material. Due to the fact thatRaw CO and CO₂The etching (ashing) reaction of the molecules, the carbon is consumed, leaving an oxide layer on the carbon. The average thickness of the modified layer is very thin; the etch rate can be adjusted using plasma conditions. Generally, the higher the density of oxygen atoms in the plasma, the faster the etch rate, with a concomitant increase in the roughness and surface area of the oxidized carbon.

For the correspondingly prepared samples, the combined XPS and WCA data showed a significant hydrophilic surface on the carbon with undetectable WCA on the particle layer (water absorbed). Due to the presence of oxygen containing functionality, the grafted surface exhibits certain acidic properties, such as the planar graphite shown in fig. 3. According to the aging combination data, the stability of the treated surface in the air is very good.

NH₃Glow discharge plasma treatment

The treatment (grafting) being with NH₃The fed discharge was run with argon gas in some cases. The process parameters are controlled such that the nitrogen-containing chemical group (e.g., amino, imino, etc.) layer passes through and is dissociated from NH₃The formed nitrogenous free radicals interact and are grafted on the carbon surface. And use of O₂Plasma treatment of (3) NH₃The discharge initiates a milder surface modification process and the etch rate is slow. The average thickness of the modified layer is very thin, NH₃The roughness and surface area of the plasma treated carbon changed only slightly.

The combined XPS and WCA data showed a significantly hydrophilic carbon surface with undetectable WCA on the particle layer (water absorbed). As shown in fig. 3, the nitrogen-grafted surface exhibits certain basic characteristics due to the presence of nitrogen-containing functionality. According to the aging combination data, the stability of the treated surface in the air is very good.

Discussion of results

Carbon: chemistry of tobacco

In a rotary reactor of the type systemized in fig. 1, 6 standard samples of coconut coal (about 10g each) were plasma treated with feed gas and operating parameters shown in table 1:

sample (about 10g)	Feed gas and flow rate	Pressure of	Power of	Rotational speed	Time of day
						1	O210sccm	0.250mbar	20W	20rpm	15min
2	O210sccm	0.250mbar	100W	20rpm	15min
						3	NH310sccm	0.250mbar	20W	20rpm	15min
4	NH310sccm	0.250mbar	100W	20rpm	15min
						5	AA5sccm，Ar20sccm	0.300mbar	20W	20rpm	60min
6	AA5sccm，Ar20sccm	0.300mbar	100W	20rpm	60min

Table 1: feed gas and operating parameters

After plasma treatment, every 60mg of the treated carbon additive was added to the filter cavity (12mm cellulose acetate tip/5 mm filter additive/10 mm cellulose acetate rod tip) attached to a tobacco rod containing a density of 229mg/cm³The Virginia tobacco of (1) has a length of 56mm and a circumference of 24.6mm throughout the cigarette. No filter end ventilation was used to avoid other variables.

Two controls were set up. First, 60mg of untreated charcoal was added to the cigarette in the same design as above. Second, a 5mm long cavity was used in the filter. The cigarettes were treated at 22 ℃ and 60% relative humidity for 3 weeks prior to smoking. Smoking was performed under ISO conditions-i.e. a 35ml volume was taken over a 2 second per minute period. The product was normalized to unit tar and the percent reduction relative to the cigarette with untreated char was calculated and shown in table 2 below (21% and more percent reductions are shaded.

Table 2: percent reduction obtained from the treated carbon (BDL ═ below the limit of detection)

There was essentially no difference in smoke chemistry between the treated and untreated chars, e.g., about 10mg/cig tar, similar CO levels were seen, etc. Samples 2 and 5 have a great improvement in some gas phase components compared to untreated carbon, while samples 3 and 4 provide no improvement. This may be based on the fact that the surface density of carboxyl groups strongly depends on the degree of fragmentation of the monomers in the plasma phase, for example by lowering it by increasing the power.

While numerous variations of the specific embodiments will be apparent to those skilled in the art, the invention is not limited to any one particular embodiment described herein, but is defined by the appended claims and their equivalents.

Claims (21)

1. A filter material suitable for use in a smoking article, wherein the filter material comprises carbon which has been subjected to surface modification by plasma treatment to alter the selective filtration characteristics of the filter.

2. The filter material of claim 1, wherein the carbon is in particulate form.

3. The filter material of claim 2, wherein the granular carbon is added to a carrier.

4. The filter material of claim 3, wherein the carrier is cellulose acetate tow.

5. The filter material of claim 3, wherein the carrier is paper.

6. The filter material of any preceding claim, wherein a first portion of the carbon is subjected to surface modification according to a first plasma process and a second portion of the carbon is subjected to surface modification according to a second plasma process.

7. A smoking article comprising the filter material of any one of the preceding claims.

8. A method of manufacturing a filter material suitable for use in a smoking article comprising carbon, the method comprising modifying the selective filtration properties of the filter material by modifying the surface of the filter material, wherein the surface modification is performed by a plasma treatment process.

9. The method of claim 8, wherein the selective modification of the filtration characteristics of the filter material comprises increasing adsorption of acidic components.

10. The method of claim 8, wherein the selective modification of the filtration characteristics of the filter material comprises increasing adsorption of alkaline components.

11. The method of claim 8, wherein the selective modification of the filtration characteristics of the filter material comprises increasing the hydrophilic nature of the material surface.

12. The method of any one of claims 8-11, wherein the plasma treatment process comprises acrylic plasma enhanced chemical vapor deposition.

13. The method of any of claims 8-11, wherein the plasma treatment process comprises O₂And/or NH₃And (6) etching.

14. The method of any one of claims 8-13, wherein the filter material is subjected to two different plasma processing treatments.

15. The method of any of claims 8-13, wherein the filter material comprises a first material subjected to a first plasma processing treatment and a second material subjected to a second plasma processing treatment.

16. The method of any one of claims 8-15, wherein the filter material comprises carbon in particulate form.

17. The method of any one of claims 8-15, wherein the filter material comprises a thread or sheet material.

18. The method of claim 17, wherein the thread or sheet material incorporates carbon in particulate form.

19. The method of claim 17 or 18, wherein the plasma treatment process comprises using a continuous reel arrangement of the thread or sheet material to allow the filter material to be fed through a plasma treatment chamber.

20. A filter material substantially as herein described with reference to the accompanying drawings.

21. A method of manufacturing a filter material substantially as described herein with reference to the accompanying drawings.