WO2008113793A1

WO2008113793A1 - Process for preparing a layered double hydroxide

Info

Publication number: WO2008113793A1
Application number: PCT/EP2008/053187
Authority: WO
Inventors: Jan Pieter Coen De Ruiter; Cornelis Johannes Govardus Van Strien; Marianne Frederika Reedijk; Jozef Johannes Maria Baltussen
Original assignee: Akzo Nobel N.V.
Priority date: 2007-03-21
Filing date: 2008-03-18
Publication date: 2008-09-25

Abstract

The invention relates to a process for preparing a layered double hydroxide containing a charge-balancing anion comprising the steps of: a) preparing a mixture comprising a trivalent metal ion source, a divalent metal ion source, a suspending medium, and optionally an anion precursor of the charge-balancing anion, the total amount of divalent, trivalent metal ion sources and optional anion precursor being 30 wt% or higher, based on the total weight of the mixture; b) treating the mixture in a mixing device to form the layered double hydroxide.

Description

PROCESS FOR PREPARING A LAYERED DOUBLE HYDROXIDE

The invention relates to a process for preparing layered double hydroxides.

Various processes for preparing layered double hydroxides (LDHs) are known in the art. In most processes the LDH is prepared by heating a slurry or solution of divalent and trivalent metal ion sources where the amount of divalent and trivalent sources is relatively low, typically less than 20 percent by weight (wt%). These processes require the solvent or suspending medium to be removed to obtain the solids of the LDH. This removal is expensive and also provides a considerable waste stream.

Alternatively, mechanochemical processes have been described, for example in JP 2004-224590 and in an article by V. Isupov et al. (J. Materials Synthesis and Processing, Vol. 8, Nos. 3/4, 2000, pp. 251 -253). In these references the formation of layered double hydroxides from magnesium and aluminium in the presence of a small amount of water, while grinding the mixture in a mill, is disclosed. Such grinding operation in a mill is expensive as a result of the high energy costs and intensive maintenance due to wear of the mill. Moreover, the capacity of such milling operations is low due to the high residence times required to convert a sufficient amount of the layered double hydroxides. Scale- up of such a process is complex and expensive.

It is an object of the present invention to provide an improved process for preparing a layered double hydroxide.

This objective is achieved with a process for preparing a layered double hydroxide containing a charge-balancing anion comprising the steps of: a) preparing a mixture comprising a trivalent metal ion source, a divalent metal ion source, a suspending medium, and optionally an anion precursor of a charge-balancing anion, the total amount of divalent, trivalent metal ion sources and optional anion precursor being 30 wt% or higher, based on the total weight of the mixture; b) treating the mixture in a mixing device to form the layered double hydroxide.

The process of the invention is conducted using a relatively low amount of solvent or suspending medium, which enables easy removal of the solvent or suspending medium if necessary, thus giving dry product. This renders the process simpler and more attractive economically, as less energy is required to remove the solvent or suspending medium. Compared to the grinding operations disclosed in the art, the present process using a mixing device has a much lower energy consumption and allows for a higher production rate. Additionally, the equipment required for the process of the invention is relatively small, more robust, less complex, and requires less maintenance. It is furthermore noted that conducting conventional processes in conventional reactors with amounts of divalent and trivalent metal ion sources exceeding 20 wt% generally results in clump formation, which is highly undesirable.

The process is generally performed in a mixing device. A mixing device generally comprises a chamber in which the reaction mixture can be stirred/- mixed and a mixer suitable for stirring or mixing the reaction mixture. This mixing device allows highly viscous reaction mixtures to be mixed sufficiently so as to enable the process to proceed faster. A suitable mixing device is a mixing device capable of handling (highly) viscous suspensions, pastes and crumbly (solid) particles. The process according to the invention may be conducted batchwise or continuously. Suitable mixing devices for batchwise processing are stirred vessels and tanks, batch mixers and kneaders, blenders, batch extruders, and other agitated vessels. Suitable mixing devices for conducting the process in a continuous mode include tube reactors, horizontal kneaders, twin- or single-screw extruders, plow mixers, compounding machines, and other suitable high-intensity mixing devices. Various mixers are suitably used in the mixing devices of the present invention. Examples of such mixers are vertical and horizontal mixers, which are described by E. Paul et al. in Handbook of Industrial Mixing: Science and Practice, Chapters 15 and 16. Examples of suitable vertical mixers are helical ribbons, screws, anchors, coaxial mixers, vertical orbiting screw mixers, and planetary mixers. Examples of suitable horizontal mixers are double-arm kneaders with sigma, dispersion, multiwing overlap, or double Naben blades, double cam mixers, ribbon blenders, and mullers. Preferred mixers are horizontal mixers, in particular horizontal single- or double-axis mixers.

The total amount of divalent, trivalent metal ion sources and optional anion precursor generally is at least 30 wt%. In one embodiment, the total amount of metal ion sources is at least 50 wt%, preferably at least 55 wt%, more preferably at least 60 wt%, even more preferably at least 65 wt%, and most preferably at least 70 wt%, and at most 99 wt%, preferably at most 95 wt%, and most preferably at most 90 wt%.

In another embodiment, at least one of the divalent and/or trivalent metal ion sources is present as solids in the amounts given above. Preferably, both the divalent and the trivalent metal ion source are present as solids.

In the process of the invention the suspending medium or solvent can be any suspending medium known in the art. The suspending medium is generally liquid but can also be gaseous, although gaseous media are less preferred. The suspending medium can be a liquid in which the divalent and trivalent metal ion sources and the optional anion precursor can be suspended or dissolved. In the context of the present application, the wording "suspending medium" or "solvents" when associated with a liquid medium refers to liquids in which the divalent and/or trivalent metal ion sources dissolve either completely or partially and/or disperse. Examples of such suspending media include water and organic solvents including alcohols such as methanol, ethanol, n-propanol, isopropanol, n- butanol, i-butanol, and tert-butanol; alkane polyols such as ethylene glycol, propylene glycol, and glycerol; ethers such as dimethyl ether, diethyl ether or dibutyl ether; diethers of alkane polyols such as dimethyl ethylene glycol, diethyl ethylene glycol, dimethyl propylene glycol, and diethyl propylene glycol; and alkoxylated alcohols according to the formula

wherein Ri is a Ci-Cs alkyl or phenyl, R₂ is hydrogen or methyl, and n is an integer from 1 to 5; amines such as thethyl amine; non-ionic polymeric solvents such as polyethylene glycols, polypropylene glycols, lauryl polyethylene glycol; ionic liquids; pyridines; dimethyl sulfoxide; pyrrolidones such as n-methyl pyrrolidone, and more hydrophobic suspending media which are less miscible with water such as alkanes such as pentane, hexane, and heptane; ketones such as methyl amyl ketone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; unsaturated acrylic esters such as butyl acrylate, methyl methacrylate, hexamethylene diacrylate, and trimethylol propane thacrylate; aromatic hydrocarbons such as benzene, toluene, and xylene; cumene; cymene, and ortho-dichlorobenzene. In one embodiment, the suspending medium comprises water and an organic solvent. In a preferred embodiment the suspending medium comprises water and a solvent that can form an azeotrope with water.

Preferably, the solvent is an alcohol having one hydroxyl group. Examples of such alcohols are monoalcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, i-butanol, and tert-butanol, and alkoxylated alcohols as defined above.

In another embodiment of the present invention, the solvent is an alkoxylated alcohol. Examples of such alkoxylated alcohols are ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol mono-t-butyl ether, ethylene glycol monohexyl ether, ethylene glycol monophenyl ether, ethylene glycol, 2-ethylhexyl ether, diethylene glycol mono- methyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n- propyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monoisopropyl ether, propylene glycol monobutyl ether, propylene glycol mono-t-butyl ether, propylene glycol monohexyl ether, propylene glycol monophenyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol monoisopropyl ether, and dipropylene glycol monobutyl ether. Of these alcohols ethylene glycol monomethyl ether and ethylene glycol monoethyl ether are less preferred, because they are teratogenic and may cause health problems. The most preferred alkoxylated alcohols are propylene glycol monomethyl ether and propylene glycol monoethyl ether. Solvents are available, e.g., from Shell (Oxitol/Proxitol) and Dow (Dowanol) and Union Carbide (Carbitol/Cellosolve). It is also envisioned to use two or more solvents in the process of the invention.

In the process of the invention the total amount of hydroxide in the mixture should be present in such an amount, i.e. at or near stoichiometric level, as to allow the layered double hydroxide to be formed. The hydroxide source can be hydroxide originating from the divalent or trivalent metal ion sources, a hydroxide originating from the anion precursor, and/or hydroxide originating from the suspending medium. The hydroxide source originating from the metal ion sources may be hydroxide which as such is part of the metal ion source, but it may also be the crystal water present in the solids of the metal ion source, or a combination of both. If the amount of hydroxide originating from the metal ion sources is sufficient to provide the required amount of hydroxide, addition of an anion precursor and/or a suspending medium (such as water) may not be required. If the amount of hydroxide is not sufficient, an anion precursor and/or suspending medium should be added so as to allow the LDH to be formed.

If a suspending medium comprising a mixture of water and an organic solvent is used in the process, the amounts of water and organic solvent used can vary over a wide range. In one embodiment of the invention the amount of organic solvent is less than 50 wt%, preferably less than 40 wt%, and most preferably less than 30 wt%, based on the total weight of suspending medium.

The divalent metal ion source and the trivalent metal ion source used in the processes of the present invention can be any source known to the man skilled in the art. These sources include soluble salts of the divalent and/or trivalent metal ions as well as insoluble or partially insoluble divalent and trivalent metal ion sources, or mixtures thereof. Soluble salts of metal ion sources include nitrates, chlorides, perchlorates, and also aluminates. The insoluble or partially insoluble divalent and trivalent metal ion sources generally include oxides or hydroxides, carbonates of the divalent or trivalent metal ions. Preferably, the sources are insoluble or partially soluble. Most preferably, the divalent and trivalent metal ion sources are oxides or hydroxides.

In the context of the present application, "soluble salts" refers to divalent and trivalent metal ion sources that dissolve completely and form a clear solution at the temperature and in the amount used in the process of the invention. In the context of the present application the term "insoluble or partially insoluble" refers to sources that do not dissolve completely and form a suspension at the temperature and in the amount used in the process of the invention. Examples of divalent metal ions are Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺, Fe²⁺, Cu²⁺, Sn²⁺, Ba²⁺, Ca²⁺, and Mg²⁺. Examples of trivalent metal ions are Al³⁺, Cr³⁺, Fe³⁺, Co³⁺, Mn³⁺, Ni³⁺, Ce³⁺, and Ga³⁺. It is also contemplated to use three or more different metal ions in the layered double hydroxide prepared with the process of the invention. Among the above metal ions the combination of Mg²⁺ and Al³⁺ is preferred.

Examples of suitable magnesium sources which are insoluble or partially insoluble include magnesium oxide, magnesium hydroxide, magnesium hydroxycarbonate, magnesium bicarbonate, dolomite, and sepiolite. A combination of two or more magnesium sources is also contemplated. The aluminium source which is insoluble or partially insoluble typically is a hydroxide or an oxide of aluminium. Examples of such aluminium sources are aluminium trihydroxides such as gibbsite and bayerite, aluminium oxohydroxides such as boehmite, diaspore or goethite, and transition aluminas, which are known to the man skilled in the art.

The use of the above insoluble or partially soluble divalent metal ion and trivalent metal ion sources in the process of the invention provides a process that is more environment-friendly, as considerably less salt - if any - remains in the waste stream resulting from the process. Moreover, the divalent and trivalent metal ion sources, and in particular the magnesium and aluminium sources, generally are less expensive than the corresponding salts commonly used in the production of layered double hydroxides. In addition, the process of the invention generally is simpler, as it requires fewer steps and/or does not require an after-treatment of the waste stream. Furthermore, these processes can be performed in a much shorter time, which in turn may lead to a higher production rate of the organically modified layered double hydroxide compared to conventional processes.

In a preferred embodiment of the present invention, the insoluble or partially soluble divalent and/or trivalent metal ion sources, and in particular the magnesium and/or aluminium sources, are milled prior to step (b). In the processes of the invention the divalent and/or trivalent metal ion sources generally have a d50 value of less than 1 mm and a d90 value of less than 2.5 mm. Preferably, they have a d50 value of less than 20 μm and a d90 value of less than 50 μm; more preferably, the d50 value is less than 15 μm and the d90 value is less than 40 μm; even more preferably, the d50 value is less than 10 μm and the d90 value is less than 30 μm; more preferably still, the d50 value is less than 8 μm and the d90 value is less than 20 μm; and most preferably, the d50 value is less than 6 μm and the d90 value is less than 10 μm. The particle size distribution can be determined using methods known to the man skilled in the art, e.g. laser diffraction in accordance with DIN 13320. This milling step allows the formation of the layered double hydroxide to proceed faster. It further may reduce the amount of impurities such as gibbsite or brucite if the divalent and trivalent metal ion sources are magnesium and aluminium sources.

In the context of the present application, the terms "treatment" and "treated", such as the treatment of step (b), refer to a treatment of the suspension at elevated temperatures. Such a treatment can be a thermal treatment or a solvothermal treatment. In the context of the present application, the terms "thermal treatment" and "thermally" refer to the treatment of the precursor suspension or solution at a temperature between 30⁰C and the boiling point of the precursor suspension or solution at or below atmospheric pressure. The temperature generally is from 40 to 120⁰C, preferably from 50 to 100°C, and most preferably from 60 to 90⁰C. Additionally, the terms "solvothermal treatment" and "solvothermally" refer to the treatment of the precursor suspension or solution at a pressure above atmospheric pressure and a temperature which generally is above the boiling point of the precursor suspension or solution at atmospheric pressure. The pressure generally is from 1 bar to 200 bar, preferably from 2 bar to 150 bar, and most preferably from 3 bar to 100 bar. Generally, the temperature is 100°C or higher, preferably from 100°C to 300⁰C, more preferably from 110°C to 250°C, and most preferably from 120⁰C to 200°C.

The process of the invention may be conducted in the absence of CO2 or any carbonate in the precursor suspension, so as to ascertain that no carbonate is incorporated into the layered double hydroxide as charge-balancing anion, when this is desired.

The process of the invention is suitable to prepare LDHs comprising carbonate as charge-balancing anion. The carbonate can be part of the divalent and/or trivalent metal ion source, and/or the carbonate can be added to the mixture (also referred to as "carbonate source") as a separate anion precursor. In one embodiment, the process is conducted by preparing a slurry or solution comprising a trivalent metal ion source, a carbonate-containing divalent metal ion source, and a carbonate-free divalent metal ion source. If in such case the amount of carbonate in the slurry or solution is sufficient to form an LDH with the desired amount of carbonate in the interlayer, the addition of a carbonate source is not necessary. However, if the amount of carbonate is not sufficient, a carbonate source is generally added before, during or after step b) of the process. Preferably, the carbonate source is added during or after step b).

Alternatively, if a carbonate-containing divalent metal ion source is absent from the slurry or solution of step a), the addition of the carbonate source before, during or after step b) is necessary in order to obtain an LDH comprising carbonate as charge-balancing anion. In a preferred embodiment, the carbonate source is added during or after step b).

The carbonate source can be any suitable carbonate source known in the art. Examples of such a carbonate source are carbon dioxide (CO2), an alkali metal carbonate such as sodium or potassium carbonate, and an alkali metal bicarbonate such as sodium or potassium bicarbonate. It is also contemplated to use one or more carbonate sources. These sources may be added simultaneously or at different stages in the process. Of these carbonate sources carbon dioxide is preferred, as no salt which ends up in the waste stream and needs to be removed is added to the slurry or solution.

In the context of the present application, the term "charge-balancing anion" refers to anions that compensate for the electrostatic charge deficiencies of the crystalline clay sheets of the LDH. As the clay typically has a layered structure, the charge-balancing anions may be situated in the interlayer, on the edge or on the outer surface of the stacked clay layers. Such anions situated in the interlayer of stacked clay layers are referred to as intercalating ions.

The LDH comprising charge-balancing organic anions have a layered structure corresponding to the general formula:

[M^⁺ (OH)_2m+2n Jx-- bH₂O (I)

wherein M²⁺ is a divalent metal ion such as Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺, Fe²⁺, Cu²⁺, Sn²⁺, Ba²⁺, Ca²⁺, and Mg²⁺, M³⁺ is a trivalent metal ion such as Al³⁺, Cr³⁺, Fe³⁺, Co³⁺, Mn³⁺, Ni³⁺, Ce³⁺, and Ga³⁺, m and n have a value such that m/n = 1 to 10, and b has a value in the range of from 0 to 10. It is also contemplated to use three or more different metal ions in the layered double hydroxide prepared with the process of the invention. Of the above metal ions the combination of Mg²⁺ and/or Zn²⁺ as divalent metal ions and Al³⁺ as trivalent metal ion is preferred. X is a charge-balancing anion known to the man skilled in the art. The charge- balancing anion can be an organic anion or an inorganic anion. It is envisaged to use one or more organic anions and/or one or more inorganic anions.

Examples of inorganic anions known in the art include hydroxide, carbonate, bicarbonate, nitrate, chloride, bromide, sulfonate, sulfate, bisulfate, vanadates, tungstates, borates, phosphates, pillaring anions such as HVO₄ ^", V₂O₇ ^4", HV₂Oi₂ ^4", V₃O₉ ^3", V₁₀O₂₈ ^6", Mo₇O₂₄ ^6", PW₁₂O₄₀ ^3", B(OH)₄ ^", B₄O₅(OH)₄ ^2", [B₃O₃(OH)₄]^", [B₃O₃(OH)₅]^2" HBO₄ ^2", HGaO₃ ^2"' CrO₄ ^2", and Keggin-ions. Preferably, the inorganic anion is selected from the group consisting of hydroxide, carbonate, bicarbonate, nitrate, chloride, bromide, sulfonate, sulfate, bisulfate, or mixtures thereof. For the purpose of this specification, carbonate and bicarbonate anions are defined as being of inorganic nature. The process of the invention also pertains to the preparation of a layered double hydroxide comprising an organic anion as charge-balancing anion. Such layered double hydroxides are referred to as "organically modified layered double hydroxides" or "organoclays".

The charge-balancing anion precursor can be a salt of an alkali metal or alkali earth metal, or a salt of the divalent and/or trivalent metal ion, or an acid, or mixtures thereof. In the process of the invention less than 50 wt% of the charge- balancing anions, based on the total weight of the charge-balancing anion precursors, typically is introduced into the suspension in the form of a salt. In order to reduce the amount of salt in the waste stream, it is preferred to use a mixture of the acid of the organic anion and the salt of the divalent and/or trivalent metal ion and the organic anion, or the acid of the organic anion. As indicated above, less than 50 wt% of the charge-balancing anions, based on the total weight of the charge-balancing anion precursors, is used in the form of a salt; preferably, less than 30 wt% of the charge-balancing anions is a salt, and more preferably, less than 10 wt% of the charge-balancing anions is a salt. It is also envisaged that salts of the charge-balancing anions are absent from the process of the invention, in order to reduce the amount of salts in the waste stream and/or the final product even further.

In one embodiment of the invention, the charge-balancing anion is an organic anion. The precursor of the organic anion can be a salt of an alkali metal or alkali earth metal, or a salt of the divalent and/or trivalent metal ion, or an acid, or mixtures thereof. The organic anion can be used in the above-indicated amounts.

Such a stacked LDH comprising an organic anion or organoclay may also be delaminated or exfoliated, e.g. in a polymeric matrix. Within the context of the present specification the term "delamination" is defined as a reduction of the mean stacking degree of the LDH particles by at least partial de-layering of the LDH structure, thereby yielding a material containing significantly more individual LDH sheets per volume. The term "exfoliation" is defined as complete delamination, i.e. disappearance of periodicity in the direction perpendicular to the LDH sheets, leading to a random dispersion of individual layers in a medium, thereby leaving no stacking order at all. Swelling or expansion of the LDHs, also called intercalation of the LDHs, can be observed with X-ray diffraction (XRD), because the position of the basal reflections - i.e. the U(OOI) reflections - is indicative of the distance between the layers, which distance increases upon intercalation.

Reduction of the mean stacking degree can be observed as a broadening, up to disappearance, of the XRD reflections or by an increasing asymmetry of the basal reflections (00/).

Characterization of complete delamination, i.e. exfoliation, remains an analytical challenge, but may in general be concluded from the complete disappearance of non-(hkθ) reflections from the original LDH. The ordering of the layers and, hence, the extent of delamination, can further be visualized with transmission electron microscopy (TEM).

The LDH of the invention may be any LDH known to the man skilled in the art, except that the morphology and the physical and chemical properties may be different. Typically, these LDHs are mineral LDHs which are able to expand or swell. Such LDHs have a layered structure comprising charged crystalline sheets (also referred to as individual LDH layers) with charge-balancing anions sandwiched in between. The terms "expand" and "swell" within the context of the present application refer to an increase in the distance between the charged crystalline sheets. Expandable LDHs can swell in suitable solvents, e.g. water, and can be further expanded and modified by exchanging the charge-balancing ions with other (organic) charge-balancing ions, which modification is also known in the art as intercalation.

The organic anion used in the process of the invention can be any organic anion known in the art. The organic anion which can be suitably used in the process can be derived from a salt or an acid of the organic anion. Use of a salt-derived organic anion such as an alkali metal salt of stearate may be advantageous due to its higher solubility in the solvent compared to the corresponding acid-derived organic anion. Alternatively, use of an acid-derived organic anion may be advantageous, as salt ions will not be introduced into the waste stream, so that the waste stream does not need additional treatments to remove the salt ions, rendering the process cheaper and simpler. Such organic anions include mono-, di- or polycarboxylic acids, sulfonic acids, phosphonic acids, and sulfate acids. Preferably, the organic anion comprises at least 2 carbon atoms, more preferably at least 8 carbon atoms, even more preferably at least 10 carbon atoms, and most preferably at least 12 carbon atoms; and the organic anion comprises at most 1 ,000 carbon atoms, preferably at most 500 carbon atoms, more preferably at most 100 carbon atoms, and most preferably at most 50 carbon atoms. The organically modified layered double hydroxides prepared with the process of the invention preferably have a distance between the individual layers of above 1.5 nm. This has advantages in the use of these organically modified layered double hydroxides, e.g. if they are used in polymeric matrices. In polymeric matrices (e.g. in nanocomposite materials or coating compositions) the larger interlayer distance renders the layered double hydroxides of the invention easily processable in the polymeric matrix, and it further enables easy delamination and/or exfoliation of the layered double hydroxide, resulting in a mixture of the modified layered double hydroxide and the polymer matrix with improved physical properties. Preferably, the distance between the layers in an LDH according to the invention is at least 1.5 nm, more preferably at least 1.6 nm, even more preferably at least 1.8 nm, and most preferably at least 2 nm. The distance between the individual layers can be determined using X-ray diffraction and transmission electron microscopy (TEM), as outlined above.

It is further contemplated that the charge-balancing organic anion comprises one or more functional groups such as hydroxyl, amine, carboxylic acid, vinyl, ether, thiol, thiol ether, amide imide, ester, thioester, urethane, ketone, thioketone, imine, imidazole, etc. If such organically modified LDHs are used in polymeric matrices, these functional groups may interact or react with a polymeric matrix. Suitable examples of organic anions of the invention are monocarboxylic acids such as fatty acids and rosin-based ions.

In one embodiment, the organic anion is a fatty acid or a salt thereof having from 8 to 22 carbon atoms. Such a fatty acid or salt thereof may be a saturated or unsaturated fatty acid. Suitable examples of such fatty acids or salts thereof are derived from caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, decenoic acid, palmitoleic acid, oleic acid, linoleic acid, linolenic acid, and mixtures thereof.

In another embodiment of the present invention, the organic anion is rosin or a salt thereof. Rosin is derived from natural sources, is readily available, and is relatively inexpensive compared to synthetic organic anions. Typical examples of natural sources of rosin are gum rosin, wood rosin, and tall oil rosins. Rosin commonly is a suspension of a wide variety of different isomers of monocarboxylic tricyclic rosin acids usually containing about 20 carbon atoms. The tricyclic structures of the various rosin acids differ mainly in the position of the double bonds. Typically, rosin is a suspension of substances comprising levopimaric acid, neoabietic acid, palusthc acid, abietic acid, dehydroabietic acid, seco-dehydroabietic acid, tetrahydroabietic acid, dihydroabietic acid, pimaric acid, and isopimahc acid. Rosin derived from natural sources also includes rosins, i.e. rosin suspensions, modified notably by polymerization, isomehzation, disproportionation, hydrogenation, and Diels-Alder reactions with acrylic acid, anhydrides, and acrylic acid esters. The products obtained by these processes are referred to as modified rosins. Natural rosin may also be chemically altered by any process known in the art, such as for example reaction of the carboxyl group on the rosin with metal oxides, metal hydroxides or salts to form rosin soaps or salts (so-called resinates). Such chemically altered rosins are referred to as rosin derivatives. Such rosin can be modified or chemically altered by introducing an organic group, an anionic group or a cationic group. The organic group may be a substituted or unsubstituted aliphatic or aromatic hydrocarbon having 1 to 40 carbon atoms. The anionic group may be any anionic group known to the man skilled in the art, such as a carboxylate or a sulfonate.

Further details of these rosin-based materials can be gleaned from D. F. Zinkel and J. Russell (in Naval Stores, production-chemistry-utilization, 1989, New York, Section II, Chapter 9) and J. B. Class ("Resins, Natural," Chapter 1 : "Rosin and Modified Rosins," Kirk-Othmer Encyclopedia of Chemical Technology, online posting date: December 4, 2000).

In one embodiment, the intercalating anions are a mixture of fatty acid and rosin.

Generally, at least 10% of the total amount of intercalating ions in the LDH types according to the invention is organic anions, preferably at least 30%, more preferably at least 60%, and most preferably at least 90% of the total amount of intercalating ions is organic anions. In a preferred embodiment, at least 10% of the total amount of intercalating anions is fatty acid-derived or rosin-based anions or a suspension of both anions, preferably at least 30%, more preferably at least 60%, and most preferably at least 90% of the total amount of intercalating ions is fatty acid-derived or rosin-based anions or a mixture of both anions.

The molar ratio between charge-balancing anions and aluminium contained in the LDH as used in the preparation of the organically modified LDH may have any value desired. This ratio can range from 10:1 to 1 :10, preferably from 5:1 to 1 :5, and most preferably from 2:1 to 1 :2. It is contemplated to use a ratio of 1. It is also envisaged to add part of the organic anion prior to or during step (b) and to add the remaining part after the layered double hydroxide is formed. The LDHs of the invention include hydrotalcite and hydrotalcite-like anionic LDHs. Examples of such LDHs are hydrotalcite and hydrotalcite-like materials, meixnehte, manasseite, pyroaurite, sjόgrenite, stichtite, barberonite, takovite, reevesite, and desautelsite. A preferred LDH is hydrotalcite, which is an LDH having a layered structure corresponding to the general formula:

[Mg² _m ⁺Air(OH)_2m+2nJX_n7_z- bH₂O (II)

wherein m and n have a value such that m/n = 1 to 10, preferably 1 to 6, and b has a value in the range of from 0 to 10, generally a value of 2 to 6, and often a value of about 4. X is a charge-balancing ion as defined above. It is preferred that m/n should have a value of 1 to 4, more particularly a value of 1.5 to 3.

The LDH may be in any crystal form known in the art, such as described by Cavani et al. (Catalysis Today, 11 (1991 ), pp. 173-301 ) or by Bookin et al. (LDHs and LDH Minerals, (1993), Vol. 41 (5), pp. 558-564). If the LDH is a hydrotalcite, the hydrotalcite may be a polytype having 3Hi, 3H₂, 3Ri or 3R₂ stacking, for example.

In one embodiment of the invention, the LDH is treated with a coating agent in order to render it more hydrophobic. This is particularly advantageous for LDHs which comprise charge-balancing anions other than organic anions, such as LDHs comprising one or more charge-balancing anions selected from the group consisting of hydroxide, carbonate, bicarbonate, nitrate, chloride, bromide, sulfonate, sulfate, bisulfate, vanadates, tungstates, borates, phosphates, pillaring anions such as HVO₄ ^", V₂O₇ ^4", HV₂Oi₂ ^4", V₃O₉ ^3", Vi₀O₂₈ ^6", Mo₇O₂₄ ^6", PWi₂O₄₀ ^3", B(OH)₄ ^", B₄O₅(OH)₄ ^2", [B₃O₃(OH)₄]^", [B₃O₃(OH)₅]^2" HBO₄ ^2", HGaO₃ ^2"' CrO₄ ^2", and Keggin-ions. Preferably, the inorganic anion is selected from the group consisting of hydroxide, carbonate, bicarbonate, nitrate, chloride, bromide, sulfonate, sulfate, bisulfate, or mixtures thereof. Such a coating agent can be any coating agent known in the art. Examples of such coating agents include mono-, di- or polycarboxylic acids, sulfonic acids, phosphonic acids, and sulfate acids, thiols, benzothiols, phenols, and salts thereof. Suitable examples are fatty acids having from 8 to 22 carbon atoms, or salts thereof. Such a fatty acid may be a saturated or unsaturated fatty acid. Suitable examples of such fatty acids are caprylic acid, capric acid, lauric acid, myhstic acid, palmitic acid, stearic acid, arachidic acid, decenoic acid, palmitoleic acid, oleic acid, linoleic acid, linolenic acid, and mixtures thereof. A preferred fatty acid is stearic acid. The coating agent is used in order to increase the hydrophobic nature of the LDH and improve the compatibility with polymeric matrices such as polyvinyl chloride (PVC). This fatty acid treatment can be conducted in any way known in the art. The fatty acid can be added before, during or after step b) of the process of the invention. After the LDH is formed, the fatty acid can be added to the slurry in a molten state or in solid form. The fatty acid may also be added to the slurry or solution before or during the (solvo)thermal treatment of step b). This latter route is preferred over the former, because the resulting product is more hydrophobic and its compatibility with a polymeric matrix such as PVC is improved. Moreover, the treated LDH may form less agglomerates and may be more finely and uniformly distributed throughout a polymeric matrix. It is believed - without being bound by any theory - that compared to a treated LDH obtained via the after-treatment route, the treated LDH is coated more efficiently and more extensively. Alternatively, the fatty acid can be added as a magnesium or zinc salt to the slurry or solution in step a) or during step b) of the process of the invention. This has the advantage that the magnesium and/or zinc ions of the fatty acid salt can be used in the formation of the LDH, so that no salts remain in the waste stream. Suitable examples of such fatty acid salts are magnesium stearate and zinc stearate. The amount of coating agent used in the process of the invention generally is from 0.01 to 10 percent by weight (wt%), preferably from 0.1 to 8 wt%, and most preferably from 0.2 to 5 wt%, based on the weight of the divalent and trivalent metal ion sources.

The present invention is further illustrated in the Examples below.

EXAMPLES

Example 1

MgO (Zolitho 40 ex MAF) was milled to a d50 value of 2.6 μm and a d90 value of 7.2 μm. Separately, AI(OH)₃ (Alumill F505 ex Nyalco) was milled to a d50 value of 3.2 μm and a d90 value of 9.0 μm. Consequently, 77 g of MgO and 73 g of AI(OH)₃ were fed to an IKA laboratory kneader HKDT06D, where the mixture was mixed and heated up without suspending medium being present. After reaching a certain temperature of 65°C, 135 g of hot water having a temperature of 90⁰C were added. Subsequently, the kneader was closed and the temperature increased to 145°C. After mixing the mixture at this temperature for a period of 2 hours, the mixture was cooled rapidly to 60⁰C. The solids content of the powder was determined gravimethcally by drying the solids at 120⁰C until the weight of the solids was constant. The resulting powder had a solids content of about 80 wt%.

The powder was analysed using XRD and found to predominantly contain a Mg/AI layered double hydroxide having hydroxyl as charge-balancing anion.

Example 2 MgO (Zolitho 40 ex MAF) was milled to a d50 value of 2.6 μm and a d90 value of 7.2 μm. Separately, AI(OH)₃ (Alumill F505 ex Nyalco) was milled to a d50 value of 3.2 μm and a d90 value of 9.0 μm. Consequently, 77 g of MgO, 73 g of AI(OH)₃, and 135 g of hot water having a temperature of 90 °C were fed to an IKA laboratory kneader HKDT06D, where the mixture was mixed and heated up to a temperature of 120°C. Subsequently, 14 I of carbon dioxide were injected into the mixing chamber of the kneader within 50 minutes. After addition of the

CO2, the mixture was cooled rapidly to room temperature.

The solids content of the powder was determined gravimethcally by drying the solids at 120⁰C until the weight of the solids was constant. The resulting powder had a solids content of about 71.5 wt%.

The powder was analysed using XRD and infrared spectroscopy and found to predominantly contain a Mg/AI layered double hydroxide having carbonate as charge-balancing anion.

Claims

1. A process for preparing a layered double hydroxide containing a charge- balancing anion comprising the steps of: a) preparing a mixture comprising a trivalent metal ion source, a divalent metal ion source, a suspending medium, and optionally an anion precursor of the charge-balancing anion, the total amount of divalent, trivalent metal ion sources and optional anion precursor being 30 wt% or higher, based on the total weight of the mixture; b) treating the mixture in a mixing device to form the layered double hydroxide.

2. Process according to claim 1 wherein the total amount of divalent, trivalent metal ion sources and optional anion precursor is between 70 wt% and 95 wt%.

3. Process according to either of claims 1 and 2 wherein the suspending medium is water and/or an organic solvent.

4. Process according to any one of the preceding claims wherein the divalent metal ion is magnesium and/or zinc and the trivalent metal ion is aluminium.

5. Process according to any one of the preceding claims wherein the layered double hydroxide comprises an inorganic anion as charge-balancing anion.

6. Process according to any one of the preceding claims wherein the layered double hydroxide comprises an organic anion as charge-balancing anion.

7. Process according to any one of the preceding claims wherein a fatty acid, preferably stearic acid, is added to the slurry or solution before, during or after step b) to form a treated layered double hydroxide.