WO2004076350A1

WO2004076350A1 - Method for the production of highly exfoliating organophilic clays

Info

Publication number: WO2004076350A1
Application number: PCT/IT2004/000088
Authority: WO
Inventors: Valerio Cittadini; Cinzia Della Porta
Original assignee: Laviosa Chimica Mineraria S.P.A.
Priority date: 2003-02-28
Filing date: 2004-02-26
Publication date: 2004-09-10
Also published as: ITFI20030052A1

Abstract

A method for the production of an organoclay in which a clay having a cationic exchange capacity is reacted with a quaternary ammonium salt in aqueous suspension and the reaction product is separated by filtration, dried and ground, the clay being preliminarily reacted with tannin. The organoclay obtained in this way has the property of completely exfoliate in a nanocomposite production process.

Description

METHOD FOR THE PRODUCTION OF HIGHLY EXFOLIATING ORGANOPHILIC CLAYS.

Field of the Invention

The present invention relates to a method for the production of highly exfoliating organophilic clays. The invention further relates to the organophilic clays produced according to this method. State of the Art

It is known that clays having cation exchange capacity, in particular those being part of the group of the smectites, especially the montmorillonite, can be reacted with organic molecules, such as for example the nitrogen quaternary derivatives, e.g. the quaternary ammonium salts, or the phosphorous quaternary derivatives. As a result of the reaction these molecules are allowed to occupy the spaces among the layered structures forming the clay mineral. See US 2531427, US2859234 and US 3974125 as exemplary references .

The synthesis strategy, through which these materials are produced, consists of a simple cation exchange reaction between the exchangeable cations naturally present in the starting mineral and the ammonic cations selected for the clay material modification. The products obtained in this way are generally known as organophilic clays, or "organoclays" . The most commonly used organic modifiers are the alkyl ammonium cations such as for example the dimethyl-ditallow ammonium ion and the dimethyl-tallow benzilammonium ion.

For the production of the organoclays the starting material is suspended in water and purified and then is subjected to the cation exchange reaction and finally the obtained product is isolated by filtration and subsequently dried and ground.

A typical field of application of the organophylic clays is that of paints and varnishes: in these products they are used as rheological additives to control the viscosity of the organic solvent varnishes and to prevent pigments and fillers sedimentation.

In recent years organoclays have been increasedly used even in the field of plastic materials for the production of the so-called polymeric nanocomposite, in particular in the field of thermoplastic polymers. The organoclays can be dispersed in the polymeric matrix during the extrusion process or added to the precursor monomer before the polymerization takes place, thereby improving the mechanical properties (elastic modulus and impact resistance increase) and the thermal properties (HDT increase) of the polymer, the gas barrier effect and the flame retarding properties. See US 4889885, WO 01/148080 and US 554670 as exemplary references.

It is known that the mechanical and physical properties of a nanocomposite depend on the interaction surface between the polymer and the organophylic clay, that is to say of the higher or lower capacity of the clay to disperse in the polymeric matrix. Making reference, for example to a clay with montmorillonite structure, the structure of the montmorillonite is a layered structure, wherein each layer or platelet has a thickness of about 1 nanometer and a diameter up to a micron. The diameter/thickness ratio is defined as "aspect ratio" and is strictly related to the specific surface of the material. The higher is the aspect ratio, the higher is the contact surface between the polymer and the clay and hence better are the physical and mechanical characteristics of the final nanocomposite. However, in the starting mineral the platelets are packed to form a structure similar to that of a pack of cards. Therefore, unless the packing is broken during the synthesis of the nanocomposite, the aspect ratio of the material dispersed in the polymer is lower than the attainable value, this resulting in a decrease of the polymer-clay interaction and a strong reduction of the possibility to achieve the wanted improvements of the nanocomposite features.

The synthesis of the organophylic clays according to the conventional technique allows for a significant spacing among the platelets to be established, thus increasing the affinity of the clay surface to the polymer, due to the fact that the ammonium radicals come between the platelets and extend almost perpendicularly to them. In this way, the inlet of the polymeric chains within two adjacent platelets is made easier, as schematically shown in figure 1. However the polymer is unable to completely open the packing of the organoclay layered structure and hence an homogeneous dispersion of the clay cannot be achieved and the surface available for the polymer interaction is not exploited in full. On the other hand, a more in-depth analysis of the structure of the layers shows that a considerable amount of hydroxyl groups can be found at they vertices tending to hinder the inlet of the polymeric chain between the platelets.

The technical problem of the exfoliation of the inorganic part within the polymeric matrix is, therefore, the main problem in the nanocomposite synthesis . According to US patents no. 5698624 and 5844032, in order to promote the clay delamination, clay is treated, before reacting with the polymer, with intercalating/compatibilizing polymers and resins without subjecting to the reaction with quaternary ammonium salts. The polymers and the resins proposed in the examples are in particular dimethylteraphthalate, polyethylenterephthalate, hydroxy- ethyltherephthalate, hydroxy-buthylterephthalate, polycarbonate, polyvinylpyrrolidone, polyvinylalcohol .

US patent no. 6271298 discloses a method for increasing the exfoliation of a smectite clay by treating the edges of the platelets with negatively charged organic molecules. In particular, a high charge density amionic polymer such as polyacrylate is used. The treatment is carried out in liquid phase after the clay purification when it is a very diluted (1-6%) aqueous suspension. Summary of the Invention

The object of the present invention is to provide a method for the production of organophylic clays having a high exfoliating efficiency, higher than that of the known organoclays, whereby they can be completely dispersed in the polymeric matrix during the production process of the polymeric nanocomposite.

Another object of the present invention is to provide an organoclay, to be used for the production of polymeric nanocomposite, capable of being completely exfoliated and thus exposing all its available interaction surface to the polymer to grant higher phisico-mechanical properties to the polymer than what can be obtained with the conventional organoclays. The above objects are attained with the method according to the present invention in which the clay, before being reacted with the quaternary ammonium salt, is submitted to a preliminary treatment with tannin according to what set forth in claim 1.

Brief description of the drawings

The features and the advantages of the invention will appear clearly from the following description of some embodiments thereof given as non-limiting examples with reference to the attached drawings, wherein:

Figure 1 schematically shows the polymer/clay interaction according to the prior art; - Figure 2 schematically shows the polymer/clay interaction according to the invention;

- Figure 3 shows a X-ray diffraction pattern of two samples of nanocomposite according to example 1 obtained by using an organoclay not treated with tannin and, respectively, treated with tannin;

- Figures 4a and 4b are TEM images of two samples of nanocomposite according to example 2 obtained by using an organoclay not treated with tannin and, respectively, treated with tannin; - Figure 5 shows a X-ray diffraction pattern of two samples of nanocomposite according to example 3 obtained by using an organoclay not treated with tannin and, respectively, treated with tannin. Detailed description of the invention In the present description the term "tannin" is generally used to identify a category of both natural and synthetic compounds, all being suitable to achieve the objects of the invention. In particular, natural hydrolysable tannins can be used, constituted by glucose chains in which the hydroxy groups are esterified by gallic acid (gallitannins) or by ellagic acid (ellagitannins) , and natural condensed tannins, characterized by the presence of condensed flavonoid units. These tannins are of vegetable origin and in particular are extracted from the bark of some trees such as chestnut-tree, walnut-tree, oak, mimosa and other trees. Among the synthetic tannins that can be used to the ends of the present invention, hydroxy-phenylsulfonic tannins, naphthalenic tannins and phenolureic tannins can be mentioned. Synthetic tannins are oligomers with a very eterogeneous composition both as the molecular weight and the functional groups distribution over the chains are concerned.

As is known, many of the above mentioned tannins are used since longtime in the field of tanning industry. Therefore, in the present specification the term "tannin" will be used to indicate any of the natural or synthetic tannins mentioned above.

Due to the presence of acid and hydroxyl groups tannin is characterized by a very high water solubility, whereby, by bringing tannin into contact with the clayey material, for example a bentonite, which, as known, is characterized by a naturally high water content (20 to 40% wt . ) , tannin dissolves in the bentonite. A particularly preferred and advantageous way to bring tannin into contact with the bentonite is to subject a tannin- bentonite mixture to an extrusion process.

According to an innovative aspect of the invention, the starting clayey material, for example a bentonite, is subjected to the treatment with tannin in the solid phase before its purification, by using the water naturally present in the starting material. This pre-treatment, beside reaching the aim of increasing the clay exfoliation ability, allows some advantages to be attained in the successive treatment steps when the clayey material is dispersed in water in view of the next purification step.

The presence of tannin allows an easier dispersion of the clayey material in water without giving rise to viscosity in the dispersion, as instead occurs with processes according to the known art such as for example the process disclosed in US 6271298.

During the extrusion process water inside platelet structure of the bentonite acts as a carrier for the tannin, which, once subjected to the mechanical stress of the extrusion, can penetrate inside the clay mineral and bind to the single platelets of which it is formed.

Due to the chemical structure of the platelets, the first reaction site between tannin and mineral is at the outer vertices of the platelets. As a matter of fact, the hydroxylic groups of the mineral, which are at the ^• platelet vertices, can become bound to the functional groups of the tannin by way of interactions that can be of electrostatic nature, hydrogen bridge or covalent nature in the case of formation of ester groups between the hydroxyl group of the mineral and the acid group of tannin. In this way the hydroxyl groups which tend to hinder the polymeric chain entrance in the nanocomposite formation process, are the first to interact with the tannin molecules. The final organoclay, containing both tannin and ammonic salt, has a structure that makes easier the polymer entrance (as shown schematically in figure 2) , which finds a preferential pathway thanks to the presence of the tannin organic chains that make easier their entrance between the platelets. Tannin can react with the groups at the platelet vertices only or can enter inside the platelets depending on the amount of tannin which has been added; the amount of tanning has also influence on the polarity of the final product. In any case, the first interaction site is at the platelet vertices.

As a starting mineral a clay with a cationic exchange capacity can be used, in particular selected from the group of smectites (montmorillonite, hectorite, actapulgite, illite, beidellite, nontronite, saponite and others) . In particular montmorillonite-based bentonites can be used, already used in the nanocomposite synthesis processes, with a cationic exchange capacity comprised between 40meq/100g and 130 meq/100 g. Tannin can be any of the above mentioned tannin types, in particular a natural chestnut-tree or mimosa tannin or a hydroxyphenylsulfonic- based synthetic tannin. The reaction between bentonite and tannin is carried out in solid phase inside an extruder, for example a single-screw type extruder, to which the two reactants are fed. The reaction product, characterized by the presence of tannin bound to the mineral substrate, is dispersed in deionized water through mechanical stirring, for such a time as to allow the complete dispersion of the material (typically about 30 in) . The dispersion is then left at rest by stopping the stirring, in such a way to settle and dispose of the heavier impurities which are present in the mineral. The supernatant fraction is centrifuged to remove other impurities such as quartz and cristobalite which are typically present in the starting bentonite. The purified suspension obtained in this way at this point consists of the only purified bentonite to which tannin is bound. The purified suspension obtained in this way is reacted with a quaternary ammonium salt in a known way to form a solid which is reduced to poweder by way of filtration, drying and grinding. The product obtained in this way constitutes the organoclay as an object of the present invention to be used for the production of polymeric nanocomposite according to known processes . The minimum weight percentage of tannin necessary to obtain a completely exfoliable product is equal to 0,5% dry basis with respect to the weight of the used clay. The percentage varies with the type and the features of the used clay. The preferred values have been found between 1 and 3% wt dry basis. In this range of compositions neither the organoclay production process, nor its way of use in the nanocomposite production process, undergo significant variations with respect to the known methods.

The amount of quaternary ammonium salt to be employed for the production of the organoclay from a bentonite treated with tannin corresponds to that currently used for the production of the conventional organoclays and is on average, equal to 100% of the cationic exchange capacity (CEC) of the starting bentonite. Anyway, it is possible to vary the amount of ammonium salt of ± 40% with respect to bentonite CEC, still obtaining a filtrable and workable product according to the proposed method.

In the following examples it is shown how the method according to the invention can be put into practice. Example 1

A raw bentonite (750 g dry basis) with a cationic exchange capacity equal to 80 meq/100 g is added, in a single-screw extruder, with 7,5 g of natural chestnut-tree tannin commercially available, for example from Riverchimica S.p.A. under the commercial name "Chestnut- tree Extract" . The product obtained in this way is dispersed in 10 1 of deionized water, by keeping it under mechanical stirring for 90 minutes. Once the stirring is over, the sospension is left to settle for 30 minutes, after which the settled fraction is removed, while the supernatant portion is subjected to centrifugation in a vertical centrifuge operating at 10.200 rpm.

400 g of hydrogenated dimethyl-ditallow ammonium chloride is added to the purified suspension obtained in this way under stirring at 55 °C. The formed solid is isolated by filtration, dried at 80 °C overnight and ground to obtain a powder with a 100% grain size below 45μ

(sample A) . The same treatment is made to the same bentonite, with the only difference that it is not treated with tannin (sample B) . The two samples obtained in this way are separately mixed with polypropylene in a double-screw extruder using a formulation containing 75% by weight of polypropylene omopolymer, 22% by weight of polypropylene grafted with maleic anydride and 3% of the sample containing the organoclay. Two polymeric nanocomposite are obtained in this way, one of which is prepared from the tannin-treated organoclay and the other one prepared from the untreated organoclay. The two products are subjected to X-ray diffractometric analysis (XRD) and the relevant diffraction patterns are shown in figure 3. In the same figure there is also shown, as a reference, the diffraction pattern of the organoclay.

As can be seen, in the case of the nanocomposite prepared from the tannin-treated organoclay, there is no presence of any signal that can be referred to the starting organoclay: this means that the platelets of the montmorillonite forming the clay are completely exfoliated. In contrast, in the case of the nanocomposite produced from bentonite not treated with tannin a peak due to the presence of organoclay not completely dispersed is still appreciated: the displacement towards lower angles with respect to the starting organoclay indicates that some polymeric chains are inserted within the organoclay, slightly increasing the distance between the montmorillonite platelets, but without giving rise to a complete dispersion thereof. The diffraction patter shows that, the raw material being the same, the treatment with tannin is essential to obtain a nanocomposite in which the organoclay is completely dispersed. Example 2 A raw bentonite (100 g dry basis) , with a cationic exchange capacity equal to 65 meq/100 g, is added, in a single-screw extruder with 20 g of hydroxyphenilsulfonic based synthetic tannin, for example the commercial product named "Setasun" from Riverchimica S.p.A.. The product obtained in this way is dispersed in 10 1 of deionized water, by keeping it under mechanical stirring for 90 minutes. Once the stirring is over, the suspension is left to settle for 30 minutes, after which the settled fraction is removed and the supernatant portion is subjected to centrifugation in a vertical centrifuge operating at 10.200 rpm.

380 g of hydrogenated dimethyl -ditallow ammonium chloride is added to the purified suspension obtained in this way under stirring at 55°C. The formed solid is isolated by filtration, dried at 80°C overnight and ground to obtain a powder with a 100% grain sizes below 45μ (sample Al) . The same treatment is made to the same bentonite with the only difference that it is not treated with tannin (sample Bl) .

The two samples are separately mixed with EVA

(ethylene-vinyl acetate copolymer) in a Brabender type mixer, at a temperature of 110-120°C using a formulation formed by 95% EVA and 5% organoclay.

Figures 4a and 4b show the images obtained by TEM of the two materials obtained in this way. It can be seen how, in the case of the product obtained from tannin- treated organoclay, the dispersion of the montmorillonite platelets is much more homogeneous, contrary to the other case in which the presence of a certain number of undispersed aggregates can be seen. Example 3 A raw bentonite (700 g dry basis) with a cationic exchange capacity equal to 128 meq/100 g is added, in a single-screw extruder, with 10 g of natural mimosa tannin, for example the commercial product named "Mimosa Extract" from Riverchimica S.p.A.. The product obtained in this way is dispersed in 10 1 of deionized water, by keeping it under mechanical stirring for 90 minutes. Once the stirring is over, the suspension is left to settle for 30 minutes, after which the settled fraction is removed and the supernatant portion is subjected to centrifugation in a vertical centrifuge operating at 10.200 rpm.

512 g of bis-2-hydroxyethyl-tallow ammonium chloride is added to the purified suspension obtained in this way under stirring at 55°C. The formed solid is isolated by filtration, dried at 80°C overnight and ground to obtain a powder with a 100% grain size below 45μ (sample A2) .

The same treatment is made to the same bentonite with the only difference that it is not treated with tannin (sample B2) .

Both products obtained in this way are mixed with polyamide-6 in a double-screw extruder using a formulation containing 95% by weight of polyamide-6 and 5% of modified organoclay. The XRD analysis of the products, as well as of the organoclay, is shown in figure 5.

In this case too, it can be seen how in the case of the nanocomposite prepared from a tannin-treated organoclay there is no absorption peak, this meaning that the product is completely exfoliated. In contrast, in the case of the nanocomposite without tannin, a peak is observed which is representative of the presence of a polymer intercalation between the organoclay platelets, responsible of the displacement of the peak towards lower angles.

Although in the present description reference is made to the use of quaternary ammonium salts as more commonly used cationic exchange reactants, it is clear that other cationic exchange reactants known and used in the organoclay production processes can be used as an alternative, as fully equivalent.

The presence of the tannin groups bound to the clay platelet does not influence in any way the cationic exchange reaction. Moreover, even if in the present description the use of organoclays according to the invention has been exemplified in nanocomposite preparation processes by extrusion, they can be used however even in different nanocomposite preparation processes, for example processes in which the organoclay is added in the polimerization step.

Claims

1. A method for the production of an organoclay in which a clay having a cationic exchange capacity is reacted with a quaternary ammonium salt in aqueous suspension and the reaction product is separated by filtration, dried and ground, characterized in that said clay is preliminarly reacted with tannin.

2. Method according to claim 1, wherein the reaction between clay and tannin is a solid phase reaction carried out in an extruder.

3. Method according to claims 1 or 2 , wherein the reaction product between clay and tannin is dispersed in water under stirring, then is settled and the supernatant is centrifuged to obtain a suspension of said reaction product in a substantially pure form.

4. Method according to any of the previous claims, wherein the weight percentage of tannin with respect to said clay used in said reaction is at least 0,5% dry basis.

5. Method according to claim 4, wherein the weight percentage of tannin with respect to said clay used in said reaction is comprised between 1 and 3% dry basis.

6. Method according to any of the previous claims, wherein said tannin is selected from hydrolysable and condensed natural tannins and hydroxyphenylsulfonic, naphtalenic, phenolureic synthetic tannins.

7. Method according to any of the previous claims, wherein said clay having cationic exchange capacity is bentonite.

8. Method according to any of the previous claims, wherein the percentage of ammonium salts reacted with said tannin- treated clay is comprised between 60 and 140% of the cationic exchange capacity of said clay.

9. Organoclay comprising a clay having cationic exchange capacity wherein a substantial portion of the exchange cations has been exchanged with cations of a quaternary ammonium salt characterized in that it further comprises tannin groups bond to the clay structure through a chemical or electrostatic bond.

10. Organoclay according to claim 9, wherein tannin groups are at least 0,5% by weight with respect to the weight of said clay on a dry basis.

11. Organoclay according to claim 10, wherein the tannin groups are 1-3% by weight with respect to the weight of said clay on a dry basis.

12. Organoclay according to any of the claims 9 to 11, wherein said tannin groups result from hydrolysable or condensed natural tannins, or hydroxyphenil sulphonic, naftalenic, phenolureic synthetic tannins.