WO2007099352A1

WO2007099352A1 - Extraction and/or fractionation process

Info

Publication number: WO2007099352A1
Application number: PCT/GB2007/000748
Authority: WO
Inventors: Simon William Breeden; James Hanley Clark; Stephen James Cowling; John William Goodby; Avtar Singh Matharu
Original assignee: University Of York
Priority date: 2006-03-02
Filing date: 2007-03-02
Publication date: 2007-09-07
Also published as: GB0604145D0

Abstract

A method for extracting a liquid crystal compound or group of liquid crystal compounds from a mixture containing it, said method comprising contacting the mixture with a solvent comprising carbon dioxide in liquid or supercritical form under conditions at which a liquid crystal compound or group or similar compounds is extracted into the solvent, separating the thus formed solution from the mixture and recovering the liquid crystal compound therefrom.

Description

Extraction and/or Fractionation Process

This invention relates to methods for the extraction or fractionation of liquid crystal compounds, for example, from waste liquid crystal displays (LCDs), as well as to the use of specific solvents in such processes .

The impact of LCDs in modern-day society is enormous, ranging from watches and calculators to high definition large-area displays. To combat the stockpiling of defunct electronic materials, Directive 2002/96 (WEEE directive) from the EU states that producers of electrical and electronic equipment must introduce systems for taking back waste electrical and electronic equipment that are free for private end users . LCDs containing WEEE has been identified as one of the fastest growing sources of waste in the EU, increasing by 16-28% every five years (these predictions are expected to be conservative) . These systems for recovery of WEEE should have been in place by August 2005. Targets for recovery (80%) and recycling (75%) were set for December 2005. At present there are no commercially viable, environmentally friendly solutions either for recovery and/or purification of LCD waste.

Liquid crystals are essentially low molar mass organic compounds, possessing an elongated lath-like molecular architecture, which form in an intermediate state of matter (mesophase) between the crystal and liquid states of matter. Liquid crystals possess certain properties similar to those of a crystalline solid, some similar to those of a liquid and some that are unique. The latter includes response to an electrical stimulus and hence, their ubiquitous use in flat-panel, portable electro-optic LCD devices, e.g., watches, calculators, mobile phones, laptop computer, large area TVs etc. A thin film of liquid crystal is sandwiched between two glass plates connected to drive electronics. In fact, the success and diversity of LCDs, from their inception in the early 1970s to date, now warrants an urgent strategy for their safe and effective de-commissioning of early stage displays.

Unknown to the lay-consumer, the liquid crystal material (mesophase) that is sandwiched between the glass plates is a mixture of approximately 15 - 20 compounds. The mesophase is fluid-like, immiscible with water, and has the ability to permeate skin. Stockpiling of displays or subjecting to a crusher will leave behind fluid-like sticky remains of a potentially toxic material.

Use of volatile organic solvents, such as dichloromethane, for extraction/purification of compounds from these devices may be a short-term solution but has major drawbacks including generation of high emissions of volatile organic compounds (VOCs) in the atmosphere. These organic solvents are invariably toxic and rapidly adsorbed in to skin. In addition dichloromethane belongs to the family of compounds comprising carbon tetrachloride and trichloromethane (chloroform) , both of which are now suspected carcinogens and their use is restricted. The future use of dichloromethane is under current review and is likely to be further restricted by the Environment Agency .

Carbon dioxide has emerged as an environmentally benign substitute for more conventional solvents in the food and natural products industries. For example, carbon dioxide has replaced dichloromethane for decaffeination of coffee. Similarly it has been used for extraction of hops, essential oils and more recently, high value pharmaceutical precursors. Although a greenhouse gas, it can be obtained in large quantities as a by-product of fermentation, combustion and ammonia synthesis. The applicants have found that carbon dioxide is an effective solvent for the extraction and fractionation of liquid crystal compounds.

According to the present invention there is provided a method for extracting a liquid crystal compound or a group of similar liquid crystal compounds from a mixture containing it, said method comprising contacting the mixture with a solvent comprising carbon dioxide in liquid or supercritical form under conditions at which the liquid crystal compound or group of similar liquid crystal compounds is extracted into the solvent, separating the thus formed solution from the mixture.

Liquid crystal compounds may then be recovered from the solution. The method results in the extraction of liquid crystal compounds from mixtures . Depending upon the conditions used, the method can result in the purification of individual compounds but will generally allow the fractionation of mixtures, in which groups of compounds of similar molecular weight and/or polarity are separated from a more complex mixture of liquid crystal compounds. Such fractions are included within the expression "group of similar liquid crystal compounds" used herein.

The ready availability of carbon dioxide coupled with its ease of removal and disposal/recycling makes liquid and/or supercritical carbon dioxide a potentially viable solution for recovery, purification and fractionation of substances .

Furthermore, liquid and/or supercritical carbon dioxide acts as more environmentally benign solvent for extraction and/or fractionation of liquid crystals from waste LCD devices . Carbon dioxide in liquid form is at a pressure and temperature at which it forms a liquid rather than a gas.

Supercritical fluids are of considerable interest because of their unique characteristics. These include high diffusivity, low viscosity and low surface tension compared to liquids. In addition, the large compressibility of supercritical fluids compared with ideal gas results in large changes in fluid density for slight changes in pressure, which in turn, results in highly controllable solvation power.

Strictly speaking, supercritical fluids are fluids which are at or above their critical pressure (P_c) and critical temperature (T_c) simultaneously. In practice, the pressure of the fluid is likely to be in the range (1.01 - 9.0) P_c, preferably (1.01 - 7.0) P_c, and its temperature in the range (1.01 - 4.0)T_c (measured in Kelvin).

As used herein, the term "supercritical" may also include near-critical fluids, where are fluids which are either (a) above its T_c but slightly below its P_c, (b) above its P_c but slightly below its T_c or (c) slightly below both its T_c and its P_c. The term "near-critical fluid" thus encompasses both high pressure liquids, which are fluids at or above their critical pressure but below (although preferably close to) their critical temperature, and dense vapours, which are fluids at or above their critical temperature but below (although preferably close to) their critical pressure.

By way of example, a high pressure liquid might have a pressure between about 1.01 and 9 times its P_c, and a temperature from about 0.5 to 0.99 times its T₀. A dense vapour might, correspondingly, have a pressure from about 0.5 to 0.99 times its P_c/ and a temperature from about 1.01 to 4 times its T_c. Carbon dioxide has a P_c of 74 bar and a τ_c of 31⁰C.

Suitably the mixture is a complex mixture of many liquid crystal compounds, for example, at least four liquid crystal compounds, in particular from 4 to 20 such compounds .

In a particular embodiment, a group of similar crystal compounds is extracted into the carbon dioxide and so the method provides a method for fractionation of liquid crystal compounds. As described above, where the mixture is fractionated to produce a fraction containing more than one liquid crystal compound, they will have similar molecular weights and/or polarities.

In a particular embodiment, the mixture of liquid crystal components have been mechanically separated from other components of the LCD device, such as polariser strip, epoxy resins etc prior to extraction/fractionation. The liquid crystals may be still sandwiched between the two glass plates, which may be cracked or otherwise made amenable to penetration by solvent. Preferably the temperature, pressure and time of carbon dioxide extraction is selected to enable extraction/fractionation of the desired components to take place. The extraction/fractionation pressure may be in the range, although not restricted to, 0-10,000 pounds per square inch (0-700 bar) and temperatures in the range, although not restricted to, 0-150⁰C, in particular from 0-100⁰C. For example, for certain compounds it may be found that temperatures in the range of from 10-100°C or pressures in the range of from 0 to 8,000psi may be used. The precise selection of conditions for particular compounds can be determined by the skilled person using routine methods as illustrated in the examples hereinafter. A co-solvent modifier may be added to the carbon dioxide solvent, which is suitably an organic solvent, and this may be chosen from, but not restricted to, low molecular weight alcohols, ethers, esters and halocarbon solvent such as a hydrofluorocarbon solvents.

For example, the co-solvent is selected from an Ci_₆alkyl alcohol such as methanol or ethanol, a di (Ci_₆alkyl) ether such as dimethyl ether, diethyl ether, methylethylether or methyltetrahyrofuran (methyl-THF) such as 2-methyl-THF, a Ci-εalkyl ester of a Cα-εalkanoic acid, such as ethylacetate or ethyllactate, or a halocarbon solvent such as chloromethane, dichloromethane or fluoroform, and in particular a hydrofluorocarbon solvent such as difluoromethane, 1, 1, 1, 2-tetrafluoroethane or 1,1,1,2,3,3, 3-heptafluoropropane.

In particular, the cosolvent modifier is an environmentally acceptable solvent such as methanol, ethanol, methyl-THF, ethyl acetate or ethyl lactate.

The relative amounts of carbon dioxide and co-solvent will vary depending upon factors such as the particular nature of the co-solvent, the form of the carbon dioxide (liquid or supercritical) etc.. Typically however, the ratio of carbon dioxide to co-solvent will be in the range of from 0 to 30%w/w, for example from 0.5 to 20%w/w. Generally, the co-solvent will be added to the mixture in liquid form in a pressure vessel, and the carbon dioxide at the selected temperature and pressure introduced thereafter. However, if appropriate the co-solvent can be added together with the carbon dioxide. The combination of time, temperature, pressure and cosolvent will enable the selective extraction/fractionation of material from waste LCD devices. The precise conditions which may be used in any particular circumstances will be determined by the skilled person using routine methods. Typical, the mixture is maintained in contact with the solvent at the selected pressure and temperature for from 5 to 120 minutes, for example for from 5 to 60 minutes. Thereafter, the pressure vessel can be vented. In this way, the extraction proceeds by way of an initial static extraction, followed by a dynamic extraction. The liquid crystal compounds which may be separated using the method of the invention include most of the available liquid crystal compounds.

The term "liquid crystals" is well known. It refers to compounds which, as a result of their structure, will align themselves in a similar orientation, preferably at working temperatures, for example of from -40 to 200⁰C. These materials are useful in various devices, in particular the liquid crystal display devices or LCDs.

These compounds may include a range of different types of structure, but generally all are anisotropic in nature. Either their shape is such that one molecular axis is very different from the other two, for example rod-like molecules, or in some cases, the solubility of different parts of the molecules may be different. In some cases, the compounds are chiral in nature.

Liquid crystals can exist in various phases. In essence there are three different classes of liquid crystalline material, each possessing a characteristic molecular arrangement. These classes are nematic, chiral nematic (cholesteric) and smectic.

Broadly speaking, the molecules of nematic compounds will align themselves in a particular orientation in a bulk material. Smectic materials, in addition to being orientated in a similar way, will align themselves closely in layers. A wide range of smectic phases exists, for example smectic A and smectic C. In the former, the molecules are aligned perpendicularly to a base or support, whilst in the latter, molecules may be inclined to the support. Some liquid crystal materials possess a number of liquid crystal phases on varying the temperature. Others have just one phase. For example, a liquid crystal material may show the following phases on being cooled from the isotropic phase:- isotropic - nematic - smectic A - smectic C - solid. If a material is described as being smectic A then it means that the material possesses a smectic A phase over a useful working temperature range .

Such materials are useful, in particular in display devices where their ability to align themselves and to change their alignment under the influence of voltage, is used to impact on the path of polarised light, thus giving rise to liquid crystal displays. These are widely used in devices such as watches, calculators, display boards or hoardings, computer screens, in particular laptop computer screens etc. The properties of the compounds which impact on the speed with which the compounds respond to voltage charges include molecule size, viscosity (Δn) , dipole moments (Δε) , conductivity etc.

The structure of these compounds can be variable, but generally speaking, many may be represented by a combination of rings and chains of general formula (I)

(I) where: A, B and C are independently selected from cycloalkyl, aryl, heteroaryl or heterocyclic groups/ any of which may be optionally substituted;

L¹ and L² are independently selected from direct bonds or linker groups: and

R¹ and R² are independently selected from hydrogen or terminal groups; and n is 0 or an integer of from 1 to 3.

For the avoidance of doubt, where n is other than 0 or 1, each group C and L² may be the same or different.

As used herein the term "alkyl" refers to straight or branched chain alkyl groups, suitably containing up to 20, more suitably up to 10 and preferably up to 6 carbon atoms. The term "alkylene" refers to alkyl groups which are divalent and "cycloalkyl" refers to alkyl groups which have at least 3 carbon atoms, and which are cyclic in structure. The term "alkenyl" or alkynyl refers to straight or branched unsaturated chains having from 2 to 20 and preferably from 2 to 10 carbon atoms. The term "aryl" refers to aromatic rings such as phenyl and naphthyl, but preferably phenyl.

References to "heterocyclic groups" refer to rings, which suitably contain from 4 to 20 atoms, arranged in one or more rings, up to five of which are heteroatoms selected from oxygen, nitrogen or sulphur. They may be saturated or unsaturated, but are preferably saturated. Heterocyclic groups which are aromatic in nature such as pyridyl or pyrimindinyl, are also referred to as "heteroaryl" groups. Bicyclic heterocyclic groups may comprise aromatic rings, non-aromatic rings or a combination of aromatic and non- aromatic rings.

Suitable cycloalkyl, aryl or heterocyclic groups for A, B and C would be understood by the skilled person. They may be mono or bicyclic in nature, but generally, at least one or A, B or C will be monocyclic. For example, particular aryl groups are phenyl or napthyl, such as 1,4-phenyl or 2, 6-naphthyl. Particular cycloalkyl groups for A, B or C include trans-1, 4-cyclohexyl, 1, 4-bicyclo [2.2.2] octyl, trans-1, 3-cyclobutyl, and trans-2,6- decalinyl. In particular however, where one of A, B or C is a cycloalkyl group, it is trans-1, 4-cyclohexyl .

Where the ring is a saturated heterocyclic ring, a particular example is a dioxanyl ring, in particular a 2,5- dioxanyl ring. Heteroaryl rings may comprise monocyclic rings such as pyridyl, pyrimindinyl, thiadiazolyl, thiophenyl, in particular 2-pyridyl, 3-pyridyl, or 2-4-pyrimidinyl rings. Bicyclic heteroaryl groups suitably contain combinations of 5- and/or 6- membered rings such as a system comprising two 6 membered rings fused together or a system comprising a 5 membered ring fused to a six-membered ring.

Suitably, where rings A, B and C are six-membered rings, they are attached where appropriate via a linker group L^α or L² in a para relationship to each other. In addition, terminal groups R¹ and R² are linked in a para-position on the ring. Thus for example, where rings A, B and C are 6 membered carbocyclic rings, the compounds of formula (I) may be represented as formula (II) :

(H) where R¹, R², L¹, L² and n are as defined above, the dotted lines are either present or absent depending upon whether the ring is aromatic or not, and R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are independently selected from hydrogen or optional substituent groups . Optional substituent groups for rings A, B and C are suitably small substituents which do not interfere with the alignment of the molecules. In particular these groups may be halo such as fluoro, chloro, bromo or iodo, methyl, trifluoromethyl, cyano, amino, hydroxy, methoxy or, where possible, oxo. In particular however, optional substituents are fluorine. Thus, in formula (II), R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are independently selected from hydrogen or fluorine .

In formula (II) , any of the rings may be substituted by other rings such as heterocyclic rings as described above. Suitable linker groups L¹ and L² are oxygen, -C(O)O- , - OC(O)-, azo, imine, optionally substituted silicon (for example of formula -Si(R^aR^b)-), siloxane or alkylene, alkenylene or alkynylene groups any of which may be interposed or linked by one or more oxygen, -C(O)O-, -OC(O)-, azo, imine, optionally substituted silicon (e.g. of formula -Si(R^aR^b)-), or siloxane groups where R^a and R^b are selected from hydrogen, halogen (such as fluorine or chlorine) or Cx-galkyl such as methyl or ethyl. Particular alkylene, linkers contain from 1-4 carbon atoms, in particular methylene or ethylene. Particular alkenylene or alkynylene groups in linkers L¹ and L² include from 2-4 carbon atoms such as ethylene or acetylene .

Particular examples of possible linkers L¹ and L² include oxygen, -C(O)O- , -OC(O)- , azo, imine, ethylene, methyleneoxy, acetylene or cinnamate.

In a particular embodiment however, at least one of L¹ and L² and suitably all of L¹ and L² are direct bonds.

Preferably at least one and suitably both of R¹ and R² are other than hydrogen. Suitable terminal groups would be readily apparent to a skilled person but include small polar substituents such as cyano, or chains which comprise hydrocarbon groups such as alkyl, in particular alkyl or alkoxy chains which may optionally be interposed with one or more oxygen, sulphur, optionally substituted silicon (for example -Si(R^aR^b)- where R^a and R^b are as defined above), or siloxane atoms . Chains are suitably quite long, for example from 3 to 20 carbon atoms in length.

In a particular embodiment, one of R¹ or R² is cyano, and the other is a C₃-.₂oalkyl or C₃_₂oalkoxy group.

In particular compounds of formula (I) are compounds of formula (III)

(III) where n is as defined above, but is preferably 0 or 1, and R^2a is an alkyl or alkoxy group, in particular a straight chain C₄_i₀alkyl or C₄_₁₀alkoxy group.

Other examples of liquid crystal compounds and mixtures are found in for example, EP-A-0385471, JP-64-22835, EP-A- 0824141 WO92/016500, WO92/016519, WO93/025631, WO94/006885, WO94/29405, WO95/018848, WO96/001246, WO98/013325, WO97/036947, WO2001/021606, WO2000/004111, WO2003/040812 ,

WO2003/024903, WO2005/019156 and WO2006/048620, the content of which is incorporated herein by reference. Silicon and siloxane containing liquid crystal compounds are illustrated for example in WO20060675883, USP5, 547, 604, USP4, 358, 391, GB2,145,787 and EP0478034 and the content of these also is incorporated herein by reference. However, a skilled person would be able to identify many more such compounds which may be extracted or fractionated using the method described herein. These compounds may also be extracted or fractionated from mixtures containing them using the method of the invention.

In a further aspect, the invention provides the use of carbon dioxide in liquid or supercritical form as a solvent for extraction of liquid crystal compounds. The invention further provides the use of carbon dioxide in liquid and supercritical form as a solvent for fractionation of liquid crystal compounds.

In yet a further aspect the invention provides the use of a combination of carbon dioxide in liquid and supercritical form and an organic co-solvent such as methanol for the extraction of liquid crystal compounds from mixtures containing them.

Furthermore the invention provides the use of a combination of carbon dioxide in liquid and supercritical form and an organic co-solvent such as methanol for fractionation of liquid crystal compounds from mixtures containing them.

The invention will now be particularly described by way of example with reference to the accompanying diagrammatic drawings in which Figures 1-15 are GC traces obtained from residues obtained using a method embodying the invention. Example I

Representative liquid crystal mixture E7 is a mixture of four components labelled A, B, C and D as indicated on the GC trace shown in Figure 1 and in Table 1.

Table 1

The composition contains the approximate composition values of 52% A, 25% B, 15% C and 8% D (GC area percentages relative to each other) also shown. A 10ml glass vial was charged with representative liquid, crystal mixture E7 (177mg) and placed in the extraction vessel of a Supercritical Fluid Technologies 150 SFE system. The extraction vessel was pressurised to 1,000 psi with SFC grade carbon dioxide and held at this pressure and ambient temperature for 15 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (2.2 rug) that was analysed by gas chromatography (GC) with the trace being shown as Figure 2. Chalcone was used as a standard in the GC analysis to confirm retention times. The extraction vessel was subsequently pressurised to 2,000 psi with SFC grade carbon dioxide and held at this pressure and 30⁰C for 15 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (7.4 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 3. The extraction vessel was subsequently pressurised to 3,000 psi with SFC grade carbon dioxide and held at this pressure and 30°C for 60 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give 'a residue (7.0 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 4.

The extraction vessel was subsequently pressurised to 4,000 psi with SFC grade carbon dioxide and held at this pressure and 40⁰C for 60 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (23.9 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 5.

The extraction vessel was subsequently pressurised to 5,000psi with SFC grade carbon dioxide and held at this pressure and 3O⁰C for 50 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 25 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (13.5 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 6.

The extraction vessel was subsequently pressurised to 6,000 psi with SFC grade carbon dioxide and held at this pressure and 60⁰C for 5 minutes. The extraction vessel was then vented to atmospheric pressure via the receiving flask that was washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (11.5 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 7. The residue (37.0 ing) remaining in the glass vial in the extraction vessel was analysed by GC and the trace shown as Figure 8.

The results show that the initial extractions contain a higher proportion of the lower molecular weight material

(e.g. Figure 4 is approximately 66% A, 24% B, 9% C and 1% D: GC area percentages relative to each other) , with the higher molecular weight material being extracted with higher temperatures and pressures of carbon dioxide (e.g. Figure 7 is approximately 58% A, 27% B, 12% C and 3% D: GC area percentages relative to each other) . This leaves a residue that is much enriched in the higher molecular weight components (30% A, 21% B, 22% C and 26% D: GC area percentages relative to each other) thus extraction and fractionation is demonstrated.

Example 2

A 10ml glass vial was charged with methanol (1 ml) along with representative liquid crystal mixture E7 (177mg) as described in Example 1 and placed in the extraction vessel of a Supercritical Fluid Technologies 150 SFE system. The extraction vessel was pressurised to 2,000 psi with SFC grade carbon dioxide and held at this pressure and ambient temperature for 15 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (2.0 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 9.

The extraction vessel was subsequently pressurised to 3_/000 psi with SFC grade carbon dioxide and held at this pressure and 30⁰C for 15 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from thus solution to give a residue (2.4 ing) that was analysed by gas chromatography (GC) with the trace being shown as Figure 10.

The extraction vessel was subsequently pressurised to 4,000 psi with SFC grade carbon dioxide and held at this pressure and 40⁰C for 15 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a father 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (2.1 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 11.

The extraction vessel was subsequently pressurised to 5,000 psi with SFC grade carbon dioxide and held at this pressure and 50⁰C for 15 minutes. The extraction vessel was then kept at this temperature and pressure whilst venting at a rate of approximately 0.5 litres per minute for a further 15 minutes and the extraction vessel sealed. The receiving flask was then washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (4.1 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 12.

The extraction vessel was subsequently pressurised to 6,000 psi with SFC grade carbon dioxide and held at this pressure and 60⁰C for 10 minutes. The extraction vessel was then vented to atmospheric pressure via the receiving flask that was washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (0.5 mg) that was analysed by gas chromatography (GC) with the trace being shown in Figure 13. The residue (113.6 mg) remaining in the glass vial in the extraction vessel was analysed by GC and the trace shown in Figure 14.

The exhaust gases throughout this experiment were passed through dichloromethane (100 ml) to trap any material escaping from the receiving flask. The dichloromethane was removed from this solution to give a residue (2.2 mg) that was analysed by gas chromatography (GC) with the trace being shown as Figure 15. Example 3

A cellulose soxhlet thimble was dosed with representative liquid crystal mixture E7 (312 mg) as described in Example 1 and placed in the extraction vessel of a Thar Technologies Inc. SFE-500F-2-FMC system. The extraction vessel was heated to 4O⁰C and pressurised to 250 Bar with liquid withdrawal industrial grade carbon dioxide. The extraction vessel was maintained at this temperature and pressure for the duration of the experiment (60 minutes), during which the extraction was run at a constant flow of 10 g per minute into receiving flasks 1 and 2. The first receiving flask was heated to 40 ⁰C and pressurised to 80 Bar with liquid withdrawal industrial grade carbon dioxide and the second receiving flask was heated to 40 ⁰C and maintained at atmospheric pressure. On completion of the extraction (60 minutes) the first receiving flask was isolated, vented and washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (133 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 16) . The second receiving flask was also isolated, vented and washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (169 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 17) . The relative amounts of the components A and B had increased from 51%A and 25%B (Figure 16) to 59%A and 27%B (Figure 17) , whereas the amounts of components C and D had reduced from 16%C and 8%D (Figure 16) to 12%C and 2%D (Figure 17). This demonstrates fractionation at the point of collection of the liquid crystal compounds. Example 4

Polarising filters and backing material were removed from defunct commercial liquid crystal displays with the use of a scalpel and the outer surface of the LCD panel was cleaned. The panels were then shattered to expose the liquid crystals. The shattered liquid crystal displays (200.086 g) were placed in the extraction vessel of a Thar Technologies Inc. SFE-500F-2-FMC system. The extraction vessel was heated to 40⁰C and pressurised to 250 Bar with liquid withdrawal industrial grade carbon dioxide. The extraction vessel was maintained at this temperature and pressure for the duration of the experiment (60 minutes), during which the extraction was run at a constant flow of 10 g per minute into receiving flasks 1 and 2. The first receiving flask was heated to 60 ⁰C and pressurised to 60 bar and the second receiving flask was heated to 40 ⁰C and was maintained at atmospheric pressure. On completion of the extraction (60 minutes) the first receiving flask was isolated, vented and washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (302 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 18). The second receiving flask was also isolated, vented and washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (110 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 19) . Clearly, this further demonstrates fractionation at the point of collection. Example 5

Polarising filters and backing material were removed from defunct commercial liquid crystal displays with the use of a scalpel and the outer surface of the LCD panel was cleaned. The panels were then shattered to expose the liquid crystals. The shattered liquid crystal displays (29.8833 g) were placed in the extraction vessel of a Thar Technologies Inc. SFE-500F-2-FMC system. The extraction vessel was heated to 40⁰C and pressurised to 80 bar with liquid withdrawal industrial grade carbon dioxide. The extraction vessel was maintained at this temperature and pressure for the duration of the experiment (15 minutes), during which the extraction was run at a constant flow of 40 g per minute into receiving flask 1. Receiving flask 1 was heated to 40 ⁰C and was maintained at atmospheric pressure. On completion of the extraction (15 minutes) receiving flask 1 was isolated, vented and washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (52 mg, 0.17% recovery) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 20) . Example 6 Polarising filters and backing material were removed from defunct commercial liquid crystal displays with the use of a scalpel and the outer surface of the LCD panel was cleaned. The panels were then shattered to expose the liquid crystals. The shattered liquid crystal displays (30.013 g) were placed in the extraction vessel of a Thar Technologies

Inc. SFE-500F-2-FMC system. The extraction vessel was heated to 40⁰C and pressurised to 80 bar with liquid withdrawal industrial grade carbon dioxide. The extraction vessel was maintained at this temperature and pressure for the duration of the experiment (15 minutes), during which the extraction was run at a constant flow of 40 g per minute carbon dioxide with 10 %^' w/w tetrahydrofuran (THF) co-solvent. Receiving flask 1 was heated to 40 ⁰C and was maintained at atmospheric pressure. On completion of the extraction (15 minutes) receiving flask 1 was isolated, vented and washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (116 mg, 0.38% recovery) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 21) . This example demonstrates that the presence of co-solvent can beneficially influence the percentage recovery of liquid crystals. Example 7

Polarising filters and backing material were removed from defunct commercial liquid crystal displays with the use of a scalpel and the outer surface of the LCD panel was cleaned. The panels were then shattered to expose the liquid crystals. The shattered liquid crystal displays (30.071 g) were placed in the extraction vessel of a Thar Technologies Inc. SFE-500F-2-FMC system. The extraction vessel was heated to 40⁰C and pressurised to 80 Bar with liquid withdrawal industrial grade carbon dioxide. The extraction vessel was maintained at this temperature and pressure for the duration of the experiment (15 minutes), during which the extraction was run at a constant flow of 40 g per minute carbon dioxide with 10 %, w/w ethyl acetate co-solvent. Receiving flask 1 was heated to 40 ⁰C and was maintained at atmospheric pressure. On completion of the extraction (15 minutes) receiving flask 1 was isolated, vented and washed out with dichloromethane (50ml) and the dichloromethane removed from this solution to give a residue (57 mg, 0.19% recovery) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 22) . This further demonstrates that co- solvent influences percentage recovery of liquid crystals. Example 8 Polarising filters and backing material were removed from defunct commercial liquid crystal displays with the use of a scalpel and the outer surface of the LCD panel was cleaned. The panels were then shattered to expose the liquid crystals. The shattered liquid crystal displays (30.221 g) were placed in the extraction vessel of a Thar Technologies Inc. SFE-500F-2-FMC system. The extraction vessel was heated to 40⁰C and pressurised to 80 bar with liquid withdrawal industrial grade carbon dioxide. The extraction vessel was maintained at this temperature and pressure for the duration of the experiment (15 minutes) , during which the extraction was run at a constant flow of 40 g per minute carbon dioxide with 10 %, w/w ethanol co-solvent. Receiving flask 1 was heated to 40 ⁰C and was maintained at atmospheric pressure. On completion of the extraction (15 minutes) receiving flask 1 was isolated, vented and washed out with dichloromethane

(50ml) and the dichloromethane removed from this solution to give a residue (41 mg, 0.14% recovery) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 23); thus further demonstrating that co-solvent influences percentage recovery of liquid crystals and extract composition. Example 9

Polarising filters and backing material were removed from defunct commercial liquid crystal displays with the use of a scalpel and the outer surface of the LCD panel was cleaned. The panels were then shattered to expose the liquid crystals. The shattered liquid crystal displays (200.078 g) were placed in the extraction vessel of a Thar Technologies Inc. SFE-500F-2-FMC system. The extraction vessel was heated to 60⁰C and pressurised to 60 bar with liquid withdrawal industrial grade carbon dioxide. The extraction vessel was maintained at this temperature and pressure for the duration of the experiment (15 minutes), during which the extraction was run at a constant flow of 40 g per minute into receiving flask 1. Receiving flask 1 was heated to 40 ⁰C and was maintained at atmospheric pressure. On completion of the extraction (15 minutes) receiving flask 1 was isolated, vented and washed out with dichloromethane (50ml) , the dichloromethane was removed from this solution to give a residue (5 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 24) .

The extraction vessel was subsequently cooled to 30⁰C and maintained at this temperature and 60 Bar for the duration of the extraction (15 minutes). The extraction was run at a constant flow of 40 g per minute into receiving flask 1. Receiving flask 1 was then isolated, vented and washed out with dichloromethane (50ml) , the dichloromethane was removed from this solution to give a residue (116 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 25) .

The extraction vessel was subsequently heated to 40 ⁰C and pressurised to 80 Bar with liquid withdrawal industrial grade carbon dioxide and maintained at this temperature and pressure for the duration of the extraction (15 minutes).

The extraction was run at a constant flow of 40 g per minute into receiving flask 1. Receiving flask 1 was isolated, vented and then washed out with dichloromethane (50ml) , the dichloromethane was removed from this solution to give a residue (100 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 26) .

The extraction vessel was subsequently pressurised to 250 bar with liquid withdrawal industrial grade carbon dioxide and maintained at this pressure and 40⁰C for the duration of the extraction (15 minutes) . The extraction was run at a constant flow of 40 g per minute into receiving flask 1. Receiving flask 1 was isolated, vented and then washed out with dichloromethane (50ml), the dichloromethane was removed from this solution to give a residue (108 mg) that was analysed by gas chromatography (GC) with the trace being shown below (Figure 27) ; thus demonstrating fractionation at the point of extraction.

Claims

1. A method for extracting a liquid crystal compound or a group of similar liquid crystal compounds from a mixture containing it, said method comprising contacting the mixture with a solvent comprising carbon dioxide in liquid or supercritical form under conditions at which the liquid crystal compound or group of similar liquid crystal compounds is extracted into the solvent, separating the thus formed solution from the mixture.

2. A method according to claim 1 wherein the mixture is a mixture of more than one liquid crystal compounds .

3. A method according to claim 1 or claim 2 wherein the mixture is from an LCD.

4. A method according to claim 3 wherein the mixture remains sandwiched between glass plates of an LCD, which plates are cracked or otherwise amenable to penetration by a solvent.

5. A method according to any one of the preceding claims wherein a fraction containing more than one liquid crystal compound is extracted from the mixture.

6. A method according to any one of the preceding claims wherein the solvent further comprises a co-solvent.

7. A method according to claim 4 wherein the co-solvent is an organic solvent.

8. A method according to claim 5 wherein the organic solvent is an alcohol, an ether, an ester or a hydrofluorocarbon solvents.

9. A method according to any one of claims 6 to 8 wherein the co-solvent is added to the mixture prior to or with the carbon dioxide in liquid or supercritical form.

10. A method according to any one of the preceding claims which is carried out at a pressure of from 0-10,000 pounds per square inch.

11. A method according to any one of the preceding claims which is carried out at a temperature of from 0-150⁰C.

12. A method according to any one of the preceding claims wherein the liquid crystal compound is a compound of formula general formula (I)

(0 where : A, B and C are independently selected from cycloalkyl, aryl, heteroaryl or heterocyclic groups, any of which may be optionally substituted;

L¹ and L² are independently selected from direct bonds or linker groups : and R¹ and R² are independently selected from hydrogen or terminal groups; and n is 0 or an integer of from 1 to 3.

13. The use of carbon dioxide in liquid or supercritical form as a solvent for extraction of liquid crystal compounds .

14. The use of carbon dioxide in liquid and supercritical form as a solvent for fractionation of liquid crystal compounds .

15. The use of a combination of carbon dioxide in liquid and supercritical form and an organic co-solvent for the extraction of liquid crystal compounds from mixtures containing them.

16. The use of a combination of carbon dioxide in liquid and supercritical form and an organic co-solvent such as methanol for fractionation of liquid crystal compounds from mixtures containing them.