GB2489491A - Cellulose acetate and plasticizer blends - Google Patents

Abstract

A composition comprises a blend of cellulose acetate having a degree of substitution (DS) of greater than 2.1 and an environmentally friendly water soluble plasticizer, preferably selected from the group consisting of citrate based plasticizers, such as triethyl citrate, tributyl citrate or acetyl triethyl citrate; triglycerides such as triacetin or tripropionin; or low molecular weight polyethylene glycols. Preferably the cellulose acetate and plasticizer are both soluble in a solvent such as acetone. Preferably, the plasticizer is included in the material in an amount of up to 25% by weight. Preferably the cellulose acetate has a degree of substitution from 2.1 to 2.8, in particular 2.5. The composition may be prepared by dissolving the cellulose acetate and plasticizer in a solvent to form a single phase after which the solution is cast, moulded or spun. The composition may also be prepared by melting cellulose acetate and blending with the plasticizer to form a homogeneous blend after which the blend is cast, moulded or spun. Preferably, the composition is formed into melt spun or solvent spun fibres which may be formed into a filter for a smoking article, such as a cigarette. Such compositions can, for instance, exhibit an increased rate of degradation compared to conventional, unmodified cellulose acetate.

Description

Cellulose Acetate and Plasticizer Blends

Field of the Invention

The present invention relates to cellulose acetate compositions, including but not limited to compositions comprising a homogeneous blend of cellulose acetate with a degree of substitution of greater than 2.1 and a plasticizer, preferably an environmentally-friendly, water soluble plasticizer. Such compositions can, for instance, exhibit an increased rate of degradation compared to conventional, unmodified cellulose acetate.

Background to the Invention

Cellulose acetate (CA) is an important organic ester \vhich is widely used for many industrial applications in the form of films and fibres. CA is derived from cellulose using the acetylation process and its main properties are its hardness, good resistance to impact, high shine, transparency, pleasing texture, lack of static electricity and resistance to hydrocarbons. Furthermore, CA has been reported to be potentially biodegradable. It has been suggested that the biodegradation rate of CA depends on its degree of acetyl-group substitution (DS). As the DS of CA decreases, the biodegradation rate increases. CA with a DS of less than 2.1 is considered to be biodegradable, whilst CA with a DS above 2.1 is only poorly or slowly biodegradable.

The biodegradation of CA is believed to involve a first step in which the acetyl group of cellulose acetate is cleaved (for example, by an enzyme released from microorganisms). As the acetyl groups are removed, the DS of the CA is reduced.

Then, the CA with decreased DS is subjected to enzyme decomposition by, for example, cellulase, which is widely present in the environment. The rate-limiting step of the biodegradation rate is thought to be the removal of the acetyl group at the start of the process. It is also considered that the biodegradation rate is further dependent on the surface area (and therefore the direct exposure of the CA to the environment).

Biodegradation rates for CA with a degree of substitution ranging from 1.85 to 2.57 were reported by Komarek, R. J. et al in "Biodegradation of radiolabeled cellulose acetate and cellulose propionate" J. Appl. Polym. Sci. 1993, 50, (10), 17394746.

Their studies showed that microorganisms were able to extensively degrade CA with a DS ranging from 1.85 to 2.57 over periods of 14 to 31 days.

Various approaches to improving the degradation of CA have been proposed. In JP 6-199901 an acid compound having an acid dissociation constant greater than that of acetic acid is added to cellulose acetate. This has the effect of removing the acetyl groups from the CA, thereby reducing its DS, but has the disadvantage of generating acetic acid.

WO 93/07771 discloses the addition to cellulose acetate of fine particles of a compound which is either water-soluble or degraded by bacteria. When the additive particles are removed, the surface area of the remaining CA structure is increased.

However, this approach does not enhance the degradability of the CA itself.

US Patent No. 6,739,344 discloses a biodegradable cellulose acetate composition comprising a biodegradation promoting agent contained in cellulose acetate and comprising at least one compound selected from the group consisting of a salt of an oxygen acid of phosphorus, an ester of an oxygen acid of phosphorus or a salt thereof, carbonic acid and a salt thereof.

WO 2007/01119 discloses biodegradable resin compositions comprising cellulose diacetate with a molecular weight of 10,000 to 500,000, and a plasticizer, the resin composition being kneaded and extruded to form a moulded product. The plasticizer is said to make processing, such as moulding and injection, easy and to improve the glass transition temperature (Tg), tensile strength and Young's modulus.

The disclosure suggests that the inventors have discovered that the cellulose diacetate used in the invention is inherently biodegradable, and the plasticizer appears to be included to improve processability of the composition, and to afford other beneficial properties mentioned to above, thereby assisting in the melting of the compositions and moulding them into products such as containers and cutlery.

One important use of CA is in the filters of smoking articles such as cigarettes, where it contributes to the selective removal of semi-volatile compounds. One disadvantage associated with the conventional CA filter material is, however, that it is slow to degrade. Whilst most of the components of a spent smoking article dissociate into their individual constituent parts and degrade within a relatively short period of time when exposed to moisture and/or mechanical abrasion, CA filter material is slow to degrade because the CA fibres themselves are not water soluble and are poorly degradable.

The CA generally used in filter materials for inclusion in smoking articles has a DS of around 2.5. The downside is that this relatively high DS means that CA is poorly degradable. However, this DS is selected as it renders the CA soluble in solvents such as acetone. Solvent solubility of the CA is important and allows the material to be processed in useful ways, such as solvent film casting and solvent fibre spinning, which is the process used to form the fibres of the cellulose acetate tow conventionally used in the filters of smoking articles.

It is common to treat CA for use in smoking article filters with plasticizers. This involves applying the plasticizer (usually in liquid form) to the surface of the CA fibres, for example by spraying the liquid plasticizer on to the CA tow. The plasticizer acts by binding adjacent fibres to one another at their contact points, thereby affording the filter rods sufficient hardness for cigarette manufacture and use. Thus, although the materials added to CA in this way are generally referred to as plasticizers, they are really acting as binders or hardeners rather than as plasticizers. Suitable plasticizers for this use include triacetin (glycerin triacetate), TEC (triethyl citrate) and PEG 400 (low molecular weight polyethylene glycol).

Plasticized cellulose acetate to\v is also known to improve the selective removal of semi-volatile compounds found in smoke (e.g. phenol, o-cresol, p-cresol and m-cresol). For this effect, it is considered to be necessary for the plasticizer to be present on the surface of the CA fibres. Unfortunately, the addition of a plasticizer which binds fibres actually can result in a reduction in the degradability of the filter material. The binding of the fibres certainly slows the separation of the individual fibres making up the tow in a spent smoking article, thus maintaining the bundle of fibres and reducing their exposure to the elements that will catty out any degradation process.

Because of the fibre-binding effect of plasticizers, CA filters generally include less than lO% plasticizer and frequently less than 7%. It has been found that including plasticizer in greater amounts than this has a detrimental effect on the cellulose acetate tow, causing holes to be formed.

It is also known to form CA compositions from a blend of CA and plasticizer. CA has previously been melt blended with two different citric acid esters: triethyl citrate and acetyl triethyl citrate (see Ghiya et al. "Biodegradability of Cellulose Acetate Plasticized with Citrate Esters" Journal of Macromolecular Science, Part A (1996) 33(5):627-638). The authors observed that both plasticizers are miscible with CA and the addition of plasticizer reduces the tensile modulus and increases the elongation of CA.

In Rosa DS, et al, "The effect of the Mw of PEG in PCL/CA blends" Polymer Testing (2005) 24:542-548, it was reported that blending CA with PEG(400) or PEG(1500) decreases the Tg of CA and enhances its tensile strength. The authors pointed out the effect of the interactions between the free hydroxyl groups of PEG and the chains of CA as an explanation of the enhanced the resistance of CA.

In light of the foregoing, it would be desirable to produce a composition comprising cellulose acetate having a degree of substitution of greater than 2.1 which exhibits an improved rate of degradation compared to conventional compositions comprising cellulose acetate with the same degree of substitution. It would also be desirable to provide a method of manufacturing a film or fibre from a CA composition, with the resultant CA film or fibre exhibiting an improved rate of degradation.

Summary of the Invention

According to a first aspect of the present invention, there is provided a composition comprising a blend or mixture of cellulose acetate having a degree of substitution of greater than 2.1 and plasticizer. The plasticizer is preferably environmentally-friendly and water soluble. Preferably, the blend or mixture is homogenous. The composition may be in the form of a film or fibres.

According to a second aspect of the invention, a method is provided for manufacturing a composition comprising a blend of cellulose acetate having a degree of substitution of greater than 2.1 and a plasticizer, the method comprising forming a mixture or blend of the cellulose acetate and plasticizer, and spinning, moulding or casting the mixture or blend. Preferably, the method includes the step of solvent spinning or melt spinning to form fibres.

According to a third aspect of the present invention, a cellulose acetate tow material is provided, comprising a composition according to the first aspect of the present invention, and/or a composition manufactured by the method according to the second aspect of the invention.

According to a fourth aspect of the present invention, a filter element for a smoking article is provided, comprising a composition according to the first aspect of the present invention, and/or a composition manufactured by the method according to the second aspect of the invention.

According to a fifth aspect of the present invention, a smoking article is provided, comprising a composition according to the first aspect of the present invention, and/or a composition manufactured by the method according to the second aspect of the invention.

Detailed Description of the Invention

The present invention relates to cellulose acetate compositions with an improved rate of degradation. In one embodiment, the compositions according to the invention are biodegradable.

Preferably, the cellulose acetate starting material is a CA which is not inherently degradable and/or biodegradable or is inherently only poorly or slowly degradable and/or biodegradable. Compositions of the present invention comprising a blend of CA and a plasticizer have an improved rate of degradation compared to that of a composition of the same CA without the plasticizer.

In one embodiment of the invention, the CA starting material is CA having a DS of greater than 2.1. This DS means that the CA starting material is soluble in solvents such as acetone and this is generally a requirement for solvent-based processing of CA. A CA solution is formed and this may be processed in a number of ways, including film casting and fibre spinning. The products manufactured using these solvent-based processes exhibit a variety of benefits.

According to the invention, a blend of CA and a plasticizer is formed. In one embodiment, the blend is soluble in acetone or other suitable solvent and the composition formed from the solution comprises an intimate blend of CA and plasticizer. In another embodiment, the CA and plasticizer may be melt blended.

Following either mixing or blending process, the plasticizer is integrated within the structure of the CA and is retained.

The CA starting material used in the present invention may be the CA used to prepare conventional cellulose acetate tow for smoking article filters. This CA generally has aDS of between 2.4 and 2.6, more commonly of around 2.5.

The cellulose acetate used in the present invention preferably has a DS of greater than 2.1 and more preferably of between 2.1 and 2.6. In one embodiment, the cellulose acetate has a DS of 2.5 to 2.6. This DS means that the CA starting material is particularly suited to the manufacture of cellulose acetate fibres by a solvent spinning process, or to the manufacture of cellulose acetate films by a solvent film casting process.

A variety of known plasticizers may be used in the present invention. There are, however, a number of properties that they preferably have. Firstly, in one embodiment of the invention, the plasticizer used is environmentally-friendly and water soluble. This is beneficial because it is desirable for the compositions of this invention to be environmentally-friendly, as well as ideally water dispersible, with water soluble elements.

In addition, the plasticizer is compatible with cellulose acetate. Ideally, the plasticizer should be soluble in the solvents that may be used to form a cellulose acetate solution, such as acetone. CA is soluble in other solvents like chloroform, dimethyl sulfoxide and tetrahydrofuran, but the industrial use of these solvents could be limited, Furthermore, it is important that the CA and plasticizer are miscible, so that the mixture of solution of CA, plasticizer and optionally solvent has a single-phase.

In addition, where the CA and plasticizer are mixed by melt processing, the plasticizer xvill preferably have a boiling point which is higher than the melting point of the CA. This will minimise the loss of the plasticizer when the CA is heated and melted during melt processing.

Polyols are a class of compounds \vhich have been studied as plasticizers for degradable polymers. Glycerol (also referred to as glycerine or glycerin), which is often used with degradable polymers, has been found to reduce thermal degradation of thermoplastic starch (]TPS) reinforced with cellulosic fibres. However, the poor miscibility of glycerol in acetone reduces its application by solvent blending \vith CA and therefore makes this plasticizer less attractive for use in the present invention where a solution of the plasticizer and CA is to be formed for solvent processing. Therefore, in one embodiment of the present invention, the plasticizer is not glycerol. In a preferred embodiment of the invention, the plasticizer exhibits high miscibility in acetone and good solvent blending with CA.

The chemical structures of some of the preferred plasticizers for use in the present invention are shown below: jOrO Dy Glycerol triacetate, Glyceryl tripropionate, Triacetin (TA) Tripropionin (TP) oOzt

OH OH

Triethyl citrate (TEC) Tributyl citrate (TB C) o0/ 1 in

OO

Tributyl 2-acetyl citrate T'oly(ethylene glycol) (PEG) (TB2C) Average M = 200 Two families of low molecular weight plasticizers have been considered: citrate esters and simple triglycerides, together with poly(ethylene glycol) of low molecular weight.

PEGs with an average molecular weight of up to 1000 daltons may be used in the present invention. These plasticizers are environmentally-friendly and water soluble, they are soluble in acetone and they form a single-phase solution with CA having a DS of greater than 2.1. In a preferred embodiment, PEGs with an average molecular weight of greater than 1000 are not considered to be "low molecular weight" in the context of the present invention and are preferably not used as plasticizers.

Citrate-based plasticizers can be derived from naturally occurring citric acid. They are non-toxic and are used as plasticizers with some biodegradable polymers.

Simple triglycerides such as triacetin can be used as a food additive and triacetin is already applied to cigarette filters as a binder, in the manner discussed above. The high boiling point of triacetin compared to citrate-based plasticizers (Table 1) has the advantage of reducing the amount of plasticizer loss which occurs during melt processing, especially compared with the lower molecular weight citrates (see Labrecque LV, et al "Citrate esters as plasticizers for poly(lactic acid)" Journal of Applied Polymer Science (1997) 66(8):1507-1513), Table 1. Boiling points and densities of triacetin and citrate-based plasticizers.

Plasticizer Boiling point (°C) (g/cm3) Triacetin 259 1.15 Tributyl citrate 169 1.10 Triethyl citrate 126 1.14 Acetyl tributyl citrate 173 1.05 Acetyl triethyl citrate 131 1.14 In a preferred embodiment of the present invention, the plasticizer is selected from the group consisting of: triacetin (glycerol triacetate) (TA), tripropionin (glyceryl to tripropionate) (TP), triethyl citrate (TEC), tributyl citrate (TBC), and low molecular weight poly(ethylene glycol) (PEG), preferably PEG 200 or PEG 400.

The right balance between hydrophilicity and plasticization efficiency (reduction in T5) must be struck in order to increase the rate of degradation of CA. It has been found that degradation rate is, at least in part, a function of the hydrophilicity and plasticization efficiency and these two parameters have to be balanced in order to increase the degradation rate. The plasticizer is therefore preferably included in the compositions of the present invention in an amount xvhich increases the rate of degradation of the resultant CA composition, but which also provides the composition with an acceptable Tg In a preferred embodiment of the invention, the Tg of the composition comprising a blend of CA and plasticizer is at least 35°C lo\ver compared to that of a composition comprising the CA without plasticizer, and preferably wherein the of the composition is at least 65°C lower compared to that of a composition comprising the CA without plasticizer.

-10 -The amount of plasticizer included in the compositions according to the present invention is preferably up to 30% by weight of the CA composition. More preferably, up to 25 or up to 20% by weight plasticizer is included. Preferably at least S%, or at least IO%, by weight of plasticizer is included, based on the weight of the CA composition. In one embodiment between 10 and 2O% by weight, or about 2O% by weight of plasticizer is included. Including these amounts of plasticizer does not have the effect of forming holes in a CA tow, as it has been noted in the prior art when plasticizers are sprayed onto the surface of the CA tow.

It should be noted that blending CA with the plasticizers according to the present invention is completely different to spraying the plasticizer onto the surface of CA fibres, as is done when preparing fibrous tow for constructing filter material for smoking articles in order to bind the adjacent fibres at their points of contact and to give the fibrous material structural strength and rigidity. As mentioned above, adding too much plasticizer to the surface of CA fibres can lead to the formation of holes. According to the present invention, the plasticizer is blended with CA to form a composite material (homogenous blend). When the plasticizer is incorporated into the CA fibre in this way, as proposed in the present invention, greater amounts can be included as the formation of holes is not an issue.

In an embodiment of the invention, the composition comprising a blend of CA and plasticizer exhibits an increased rate of degradation compared to a composition comprising the CA without plasticizer. This increased rate of degradation may be quantified by an increase in the average molecular weight loss percentage over a defined period of time. In a preferred embodiment, the compositions of the present invention exhibit an increase in the average molecular weight loss percentage of at least SO% over a period of 425 hours under accelerated weathering conditions, compared to a composition comprising the CA without plasticizer.

In one method according to the present invention, a solution comprising CA and a plasticizer is formed. The raw material CA (for example, in the form of flakes) and the plasticizer are dissolved in a solvent, preferably acetone. The CA and plasticizer may be mixed before being added to the solvent, or they may be added to the solvent separately and/or sequentially. The result is a solution comprising CA and ii plasticizer. This enables the plasticizer to become integrated into the cellulose polymer structure and, upon drying the solution, for example by a film casting or solvent spinning step, the resultant composition is a resin in which the plasticizer is retained in the structure of the cellulose acetate.

In an alternative method according to the present invention, the CA and plasticizer are mixed by a melt blending step. In one embodiment, the CA is heated to a temperature above its melting point and the plasticizer is blended with the molten CA to form a homogenous blend using any suitable mixing apparatus, such as a Brabender internal mixer (model 50 EHT). Once again, this enables the plasticizer to become integrated into the cellulose polymer structure and, following processing of the blend by moulding, casting or spinning the molten composition, the resultant composition is a resin in which the plasticizer is retained in the structure of the cellulose acetate.

It should be noted it is not essential for the processing to be conducted at a temperature that is above the melting point of the CA. CA can also be processed successfully at lower temperatures, as the key is that the processing of the blend is carried out at a temperature above the melting point of the blend, which will be affected by the inclusion of the plasticizer. Indeed, the only reason the study described herein used a temperature above the CA melting point was to be able to also process CA without plasticizer, in order to use the unplasticized CA as a reference.

Thus, the compositions of the present invention may be prepared by a solvent mixing approach or by melt blending and both methods result in similar compositions in terms of their rate of degradation.

In one embodiment, the compositions of the present invention are preferably provided in the form of fibres. The fibres may be formed by a solvent spinning process or by a melt spinning process. The fibres may be used to form a CA tow suitable for use in the filter element of a smoking article. Once the fibres have been prepared according to the present invention, these may be processed in the 12 -conventional way to form a tow. The CA tow may or may not be treated with further plasticizer to bind adjacent fibres to one another to ensure that the filter element has the required hardness and structural stability. The fibres of the tow formed according to the present invention may have the same dimensions as those used in the tow of conventional smoking article filters. In this context, the fibre used in the filter production is generally characterised as having a particular denier per filament (DPF). The tow denier is also used to describe the filter fibres. In the case of conventional CA filters, the lowest DY!? is 1.5 and the highest is approximately 8 or 9. In terms of diameter, conventional filter fibres tend to fall within the range of 20 to 100 p.m. In a preferred embodiment of the invention, the fibres made according to the present invention have a DFP of between I and 10, preferably 1.5 to 9 or 1.5 to 9, and a diameter of from 10 to 150p.m, preferably 20 to 100p.m.

In a preferred embodiment, a CA tow is made from CA compositions of the present invention, and the tow is preferably treated with a plasticizer such as triacetin, in order to bind adjacent fibres of the tow and to achieve the hardness needed for use in the filter element of a smoking article.

A filter element comprising the CA according to the present invention may be incorporated into a smoking article in the conventional manner and using conventional processes and apparatus. The filter element will have the benefit that it will degrade at a much faster rate compared to a conventional filter element once the smoldng article has been used.

The effects of the plasticizers on thermo-mechanical properties and weathering stability of the related polymer materials made with CA having a DS of 2.5 has been investigated. It has not previously been demonstrated that the modification of CA with a plasticizer has an effect on the weathering stability of the polymer material and that the blending of CA and plasticizers according to the present invention has the effect of increasing the rate of degradation of CA.

Brief Description of the Figures -13-

Examples of the invention will now be described, with reference to the accompanying drawings in which: Figure 1 shows the effect on the Young modulus, stress and strain at break of CA/PEG blends with varying PEG content.

Figure 2 is a photograph showing the visual appearance of a series of film samples of: (a) Pure CA; (b) CA + PEG 10 wt%; (c) CA + PEG 20 wt%; (d) CA + TP 10 wt%; and (e) CA + TP 20 wt%. The samples were photographed after being subjected to 0, 225 and 425 hours of accelerated weathering.

Figure 3 shows high resolution thermogravimetric analysis (HiRes TGA) curves of CA + PEG (20 wit %) blend before and after 425 hours of accelerated weathering.

Figure 4a shows the average molecular weight loss percentages, for CA and plasticizer Mends subj ected to accelera ted weathering tests.

Figure 4b shows the average molecular number loss percentages, for CA and plasticizer blends subjected to accelerated weathering tests.

Examples

CA with a DS of 2.5 was plasticized using the environmentally-friendly plasticizers triacetin, tripropionin, triethyl citrate, tributyl citrate, tributyl 2acetyl citrate and poly(ethylene glycol) of low molecular weight. The thermo-mechanical properties and hydrophilicity of the modified CA were measured and correlated with the content and nature of the plasticizer used and compared with these properties of unplasticized CA.

The increase in toughening and the change in the hydrophilicity by the plasticization were evaluated in terms of aging and weathering stability under accelerated conditions. Samples were exposed to TJV-degradation with water spray periods.

The treated samples \vere removed periodically and characterized by several analytical techniques. The results show the effects of plasticization on enhancement of the degradation rate of CA. The plasticization of CA triggered an increase in weight loss of between 50 and 90%, and the low molecular weight plasticizers were shown to be more effective. The results indicate that the right balance between hydrophilicity and plasticization efficiency (reduction of T5) is needed in order to increase the degradation rate of CA.

Experimental 1.1. Materials Cellulose Acetate (flakes, M65.400 and polydispersity index of 3.4) with a degree Triacetin (TA) (purchased from Acros), tripropionin (TP), triethyl citrate (TEC), tributyl citrate (TBC) and tributyl 2-acetyl citrate (TB2C) (all purchased from Merck) and poly(ethylene glycol) 200 (PEG) (purchased from Fluka).

These chemicals and the solvents used were all guaranteed reagent grade and used without further purification.

1.2. Procedures 1.2.1. Film Preparation by solvent casting and melt processing Films of blends of CA and plasticizers were prepared by both solvent casting and melt processing methods. The films were cast using 1O% solutions (w/w) of the materials in acetone which were drawn on Petri dishes. The thickness of the films was approximately 0.5 mm after the removal of the solvent.

Melt blending of CA (dried overnight at 105°C in a ventilated oven) with plasticizers was performed in a Brabender internal mixer (model 50 EHT, 80 cm3 free volume) equipped with cam blades at 235°C and 60 rpm during a total time of minutes. 500 pm films were prepared by hot-pressing moulding at 235°C using an Agila PE2O hydraulic press (low pressure for 120 seconds without degassing cycle, followed by a high-pressure cycle at 150 bars for 180 seconds and cooling by tap water at 50 bars for 120 seconds).

1.2.2. Aging The plasticized CA films were boxed and aged at room conditions for 3 months.

The films were then characterized with Modulated Differential Scanning Calorimetry (MDSC) in order to investigate their stability.

1.2.3. Accelerated weathering Accelerated weathering testing was performed in a Q-Sun Xenon arc test chamber.

Square (3x3 cm) specimens were exposed to 0.65 W/m2 at a chamber temperature of 50°C and a relative humidity of SO% for up to 475 hours. The samples were water sprayed during 30 minutes of each 5 hour cycle. Samples were placed perpendicular to the irradiation into sample cups of 92 mm of diameter with three films placed in each cup. After each time period, a film was removed for further characterization.

1.3. Measurements The number average molecular weights and molecular weight distributions of the polymers were determined in tetrahydrofuran (THF) at 23°C using a Agilent size exclusion chromatograph equipped with a Knauer 2320 refractometer index detector and two PLGe1 columns (MIXED-D and 103A). Samples were dissolved in THF (5 mg/i ml), 20 iL of the solutions were injected into the columns using a flow rate of 1 mL/min. Monodisperse polystyrene standards (Polymer Laboratories Ltd.) were used for the primary calibration.

Differential scanning calorimetry (DSC) measurements were performed by using a DSC 2920 from TA Instruments calibrated with indium under nitrogen flow (50 mL/min). The following procedure was used: first heating at 20°C/mm from room temperature to 150°C, keeping at this temperature for 60 seconds to eliminate the dehydration, which occurs at around 120°C. Then the temperature was reduced to 30°C at 10°C/mm and finally a heating scan at 10°C/mm to 270°C.

Modulated DSC spectra were obtained using a MDSC 2920CE from TA Instruments calibrated with indium under nitrogen flow (50 rnL/min). The samples were heated at 20°C/mm from room temperature to 150°C and held there for 60 seconds. Then the temperature was reduced to 30°C at 10°C/mm and finally the -16 -samples were heated to 270°C at 5°C/mm while applying a temperature oscillation of 1°C/mm.

Thermogravimetric analyses (TGA) were performed by using a TGA Q500 (TA instruments) with a heating rate of 20°C/mm in air, from room temperature to 800°C (platinum pans, 60 cm3/min air flow rate). High resolution TGA analyses were performed on a Hi-Res TGA 2950 from TA Instruments, using nitrogen as purge gas and a resolution parameter of 5, which means that a continuously variable heating rate is applied in response to changes in the sample decomposition rate.

This resolution parameter can be tuned within an eight-step scale to maximize weight loss resolution.

Tensile testing measurements were performed by using a Lloyd LR 10K tensile bench in accordance with ASTM D 882 "Standard Test Method for Tensile Properties of Thin Plastic Sheeting", at room temperature using a crosshead speed of 20 mm/mm and a distance of 25.4 mm between grips. Rectangular test specimens (64 x 10 mm) were cut from 500.tm films and were previously conditioned for at least 48 hours at 20 ± 1°C under relative humidity of 45 ± 5% and values were averaged over five measurements.

The static contact angle of water drop deposited onto film surface was measured using a drop shape analysis system (DSA 10 MK2, Kruss) at 25°C. A drop of deionized water (20 kL) was placed onto the sample surface and the images of the water menisci on the sample surface were recorded through a digital camera. These images were analyzed by DSA software to yield the contact angle values. A total of 4 measurements on different areas of the surface were averaged.

2. Results and discussion 2.1. Plasticization of CA Good clear and transparent films of neat CA and CA blends with 10 and 20 wt% of the selected plasticizers \vere prepared by either solvent casting in acetone or by melt processing methods. Only when tributyl 2-acetyl citrate was used were white and opaque films obtained by casting. Further DSC analysis demonstrated the -17-presence of a crystalline melting peak. The macroscopic bending presented on these films was then attributed to contraction at the film surface produced by crystalline phase formation during solvent evaporation. However, those problems were avoided when films were prepared by melt processing. Indeed, clear transparent films were easily prepared.

2.2. Tensile properties Plasticization is assumed to increase the flexibility of chains and lead to a decrease in the stiffness. The strength usually also decreases, whereas deformability becomes stronger at the same time. The effect of PEG concentration on the mechanical properties of CA/PEG films was investigated by tensile tests. Figure 1 presents the Young's modulus, stress at break and elongation at break results for the complete series of CA/PEG blends (0, 10, 15 and 20 wt%).

The effect of plasticizing is clearly observed with an increase of the ductility of the films containing PEG. However, no significant differences in tensile properties were observed between the three compositions. This result is discussed later on in terms of miscibility threshold. For this reason, only 10 and 2O% compositions were studied for the other plasticizers.

Then, the tensile properties of CA2.5/plasticizer films with 10 and 20 wt% of plasticizer content and prepared by either solvent casting or melt processing method are compared in Table 2. As observed, the main differences between results obtained by film casting and melt processing concern the tensile strength. However, observed tendencies in function of the nature of the plasticizer remained unaffected. Comparison \vith neat CA2.5 results reveals that blends are slightly more ductile and have similar ultimate properties.

Table 2. Comparison of the tensile properties of CA2.5/plasticizer films prepared by solvent casting and melt processing with 10 and 20 \vt% of plasticizer content.

-18 -Young modulus Stress at break Strain at break (MPa) (MPa) (%) wt Melt Melt. Melt Plasticizer % Cast. Proc. Cast. Proc. Cast. Proc. None 0 2073±81 2158±77 75.4±3.5 76.8±21.1 10±2 6±3 1711±50 1644±81 49.0±1.8 50.6±1.3 11±4 31±15 PEG(200) 1754±25 1600±77 49.0±1.7 38.3±3.3 19±13 35±15 1817±69 2153±163 43.0±2.6 59.3±1.3 4±1 11±5 Triacetin -____________ ___________ ____________ _________ _________ 1550±105 1712±72 51.3±4.0 49.9±2.9 9±2 29±17 1995±66 1776±21 50.0±6.9 52.7±8.8 5±2 27±13

TP

1540±44 1959±193 45.0±3.4 52.5±4.8 11±3 28±16 2027±101 1861±196 57.0±1.4 50.8±64 6±1 9±6

TEC

1534±86 1435±59 52.7±3.2 21.8±11.2 11±3 3±0.4 Note: Mean values along with the standard deviation (±).

Taking into account the effect of plasticizer concentration, blends prepared with PEG 200 and TP are more effective at low concentration and blends prepared with TA and TEC are more sensitive to plasticizer concentration. At the 20 wt% content the final properties of the films are almost independent of the plasticizer.

2.3. Thermal properties and hydrophilicity.

Thermal properties (via DSC and TGA) and hydrophilicity (via contact angle measurements) of the plasticized cellulose acetates have also been studied. Table 3 summarizes the transition temperature and contact angle values for the blends examined in this study, together with the corresponding data for the neat CA as a reference sample.

Table 3. Effect of plasticizer nature and content on thermal properties and contact angle of CA2.5. -19-

Plasticizer ___________ TGA Tg Contact Type (wt%) (°C) angIe (°) None 0 190 337 372 58.5 154 327 370 55.4 PEG(200) 156 296 372 38.0 151 313 369 82.7 Triacetin _______________ 118 279 371 76.2 157 327 371 76.6 Tripropionin 122 288 372 66.9 153 315 367 83.5 Triethyl citrate _______________ 109 300 372 81.6 a Decomposition temperatures measured by TGA for a lO% of weight loss (°Td) and for the maximum decomposition rate (Td).

The results of the DSC measurements confirm the miscibility of the plasticizers with CA. Tg shifted as the plasticizer content increased, with a single Tg, which indicated miscibility with CA. The low molecular weight plasticizers are able to decrease Tg of CA in a range of 60 and 80°C, However, at 10 wt% all plasticizer showed the same effect on Tg, a decrease of 30 and 40°C. Furthermore, for these plasticizers a proportional relationship of approximately 4°C/wt% between concentration and Tg drop can be observed. It is well known that PEG with a MW400 is miscible with CA at this concentration range. However, our results indicate that a reduction is reached, about 35°C, but shows no dependence on the PEG content. For this reason, we interpret our data in terms of miscibility threshold: at a concentration lower than 20 wt% the miscibility threshold was reached, implying that these films could be over-plasticized. This interpretation agrees with data reported by Yuan et al. (Effects of Polyethylene Glycol on Morphology, Thermomechanical Properties, and Water Vapor Permeability of Cellulose Acetate-free Films. Pharmaceutical Technology. 2001;25:62-73) regarding the preparation of CA/PEG blends using PEGs of different molecular weights (400, 1000 and 3350). Their results indicate that increasing the molecular weight of -20 - PEG permits one to increase the content of PEG in the blend without film over-plasticization.

The thermal stability of plasticized GAs is not strongly dependent upon the nature of the plasticizer as revealed by oxidative TGA results. In this study, the contact angles are used as a measure to follow the changes in surface hydrophilicity due to the type and concentration of the plasticizer. A significant difference in hydrophilicity was observed for the blends prepared; plasticization with PEG increased the hydrophilicity and the low weight plasticizers have the opposite effect.

Stability of the CA blend films at room temperature has been examined by MDSC analysis at I and 3 months after the preparation. Table 4 summarizes the MDSC data.

Table 4. Glass transition temperature of CA blends after 1 and 3 months.

Plasticizer Tg (°C) Content Initial After 1 After 3 Nature ___________ (wt %) __________ -month months PEG(200) 10 154 156 151 156 162 160 Triacetin 10 151 152 146 118 117 116 Tripropionin 10 157 -158 151 128 127 128 Triethyl 10 153 152 151 citrate 20 110 109 110 As Table 4 shows, only very small differences could be measured for the CA/PEG blend, as Tg increase up to 2-6°C. This loss of plasticization could be explained as an effect due to migration of the plasticizer to the surface of the film which has previously been reported for PEG of higher molecular weight (i.e. 600) (Yamashita Y. et al "Deacetylation behavior of binary blend films of cellulose acetate and various polymers" (2006) Journal of Applied Polymer Science;1 00(3): 1816-1823).

Results could also be correlated with the over-plasticized interpretation of CA/PEG films suggested before. In contraposition, the stability of films containing the low molecular weight plasticizers was not significantly altered by time.

-21 - 2.4. Accelerated weathering Film degradability was studied by using an in-house weatherometer method, which uses UV light and water to simulate an accelerated degradation. Selected film samples were subjected to weathering conditions up to 475 hours. Figure 2 shows the visual appearance of series of degraded films after 225 and 425 hours, together with the original ones for comparison. The change in visual appearance of the films could be clearly correlated with the content and the nature of the plasticizer. Table reports the remaining weight of the films as a function of the weathering time, for up to 425 hours; after this time, it was not possible to correctly determine the weight of the samples due to the high level of degradation reached.

Table 5. Remaining weight of samples subjected to accelerated weathering testing.

Accelerated weathering time (hours) 0 125 175 325 425 Sample % remaining weight a CA2.5 100 100 98 98 100 2Owt%PEG 100 92 89 81 80 2Owt%TA 100 100 100 95 94 2Owt%TP 100 100 100 94 87 2Owt%TBC 100 98 95 96 94 Measurement error ± 4 % It appears that the samples showed a gradual weight loss over time (Table 5). As the accelerated weathering method used in this study has water spray periods and the plasticizers used arc water-soluble, a reasonable explanation for the weight changes observed is the fact that a certain amount of plasticizer was dissolved from the films by water. High Resolution TGA (Hi-Res TGA) measurements have been conducted to quantify the amount of the plasticizers remained in the samples after 0 and 425 hours of the accelerated weathering. Plasticizer content is determined as the weight loss observed on the Hi-Res TGA curves (as it is shown on Figure 3) due to volatilization of the plasticizer. Table 6 summarizes these data in relation to the remaining weight percentage.

-22 -Table 6. Plasticizer content, determined by Hi-Res TGA, and Remaining Weight of samples subjected to accelerated weathering testing.

Hi-Res TGA Remaining weight (RW) Plasticizer content (wt%) after 425 b Sample Initial After 425 h Difference 100 -RW (%) CA2.5 0 -0 0 wt% PEG 20.3 2.7 17.6 20 2Owt% TA 17.4 12.3 5.1 6 2Owt%TP 18.5 11.0 7.5 13 2Owt% TBC 20.9 18.6 2.3 6 Table 6 shows that the weight loss by the samples subjected to the accelerated weathering could be correlated \vith the plasticizer loss, at least to some extent. For example, the highest loss of weight (20% at 425 hours) of the CA/PEG blend could be correlated with the higher hydrophilicity of this blend.

Molecular weights of the samples subjected to accelerated weathering were estimated by gel permeation chromatography (GFC). Figure 4 shows the evolution of the average molecular weight loss percentage for neat and plasticized CA with 20 wt°/o plasticizer. The blended CA showed a clear reduction of the molecular weight, significantly higher than that of the neat CA. In general, plasticization of CA triggered an increase of the molecular weight loss between 50 and 90%, for \Vhich the low molecular weight plasticizers are more effective than PEG.

Table 7. Evolution of Mn (Da) of plasticized CA with accelerated weathering time.

CA2.5 PEG TP Time lOwt% 2Owt% lOwt% 2Owt% 0 35400 51200 53800 56500 62700 200/225' 22500 41900 26900 36300 12300 425 19000 24200 8200 11100 4300 a 200 or 225 hours for CA plasticized with 10 or 20 vt%, respectively -23 -In order to correlate the visual appearance with the molecular weight of the representative samples presented in Figure 2, in Table 7 are summarized their number average molecular weights after 0, 200 or 225 and 425 hours of accelerated weathering. Film breakdown is evident when M is under the value of 10,000 Da (approximately 40 anhydroglucose units) and indicates that the critical molecular weight for entanglements has been reached. It is remarkable that the breakdown rate is tripled when the plasticizer concentration is doubled.

Differences observed in Table 7 on molecular weight of the initial samples were attributed to thermo-mechanical degradation of CA occurring during melt processing. As the plasticizer content increases the degradation effect on molecular weight of CA decreases. It is well known that plasticization reduces the melt viscosity and thus the shearing stress during processing, which promotes chain scission.

3. Conclusions

Plasticized cellulose acetates have been prepared by blending with environmentally-friendly plasticizers such as triacetin, tripropionin, triethyl citrate, tributyl citrate, tributyl 2-acetyl citrate and poly(ethylene glycol). Transparent films of CA and CA blends with 10 and 20 wt% of the selected plasticizers could be prepared by either solvent casting in acetone or by melt processing methods. Thermo-mechanical properties of these films were assessed by means of DSC, TGA and tensile testing.

The low molecular weight plasticizers were able to decrease Tg of CA (190°C) by 60 to 80°C and by about 35°C in case of PEG, while thermal stability of the CA2,5 and plasticizer blends proved not to be highly dependent upon the plasticizer nature. In comparison with neat CA, the blends comprising plasticizer \vere slightly more ductile and their ultimate properties were also almost independent of the plasticizer nature. Hydrophilicity of the films was estimated via contact angle measurements.

Only the CA/PEG blend showed higher hydrophilicity than CA. In general, CA/plasticizer blends present good stability. Uniquely, CA/PEG blends displayed some plasticizer migration with time.

-24 -Accelerated weathering tests results showed that plasticization of CA triggered an increase of the molecular \veight loss, up to 90%. A good balance between hydrophilicity and plasticization efficiency (reduction of T5) seems needed to increase the degradation rate of CA.

Claims (13)

1. Claims 1. A composition comprising a blend of cellulose acetate having a degree of substitution of greater than 2.1 and an environmentally-friendly water soluble plasticizer.
2. 2. A composition as claimed in claim 1, wherein the cellulose acetate and plasticizer are both soluble in a solvent, such as acetone.
3. 3. A composition as claimed in either of the preceding claims, wherein the plasticizer is selected from the group consisting of: citrate-based plasticizer, such as triethyl citrate, tributyl citrate or acetyl triethyl citrate; triglycerides such as triacetin or tripropionin; or low molecular weight polyethylene glycols.
4. 4. A composition as claimed in any one of the preceding claims, wherein the plasticizer is included in the material in an amount of up to 25% by weight, preferably of between 10 and 20% by weight.
5. 5. A composition as claimed in any one of the preceding claims, wherein the cellulose acetate has a degree of substitution from 2.1 to 2.8, preferably 2,3 to 2.7, more preferably from 2.4 to 2.6 and most preferably of about 2.5.
6. 6. A composition as claimed in any one of the preceding claims, wherein the composition exhibits an increase in the average molecular weight loss percentage of at least SO% over a period of 425 hours under accelerated weathering conditions, compared to a composition comprising the CA without plasticizer.
7. 7. A composition as claimed in any one of the preceding claims, wherein the Tg of the composition is at least 35°C lower compared to that of a composition comprising the CA without plasticizer, and preferably wherein the T5 of the composition is at least 65°C lower compared to that of a composition comprising the CA without plasticizer.

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8. 8. A composition as claimed in any one of the preceding claims, in the form of a fibre.
9. 9. A method of preparing a composition as claimed in any one of the preceding claims, wherein the cellulose acetate and plasticizer are dissolved in a solvent to form a single phase, and the solution is cast, moulded or spun.
10. 10. A method of preparing a composition as claimed in any one of claims I to 8, wherein the cellulose acetate is melted and blended with the plasticizer to form a honiogenous blend, and the blend is cast, moulded or spun.
11. 11. A fibrous material comprising melt spun or solvent spun fibres comprising the composition as claimed in any one of claims 1 to 8.
12. 12. A filter for a smoking article comprising a fibrous material as claimed in claim 11.
13. 13. A smoking article comprising a filter as claimed in claim 12.