WO2000064969A1

WO2000064969A1 - Nanocomposite of polymeric materials with surface modified clay and silica xerogel

Info

Publication number: WO2000064969A1
Application number: PCT/US2000/010711
Authority: WO
Inventors: Arthur Jing-Min Yang; Yuehua Zhang
Original assignee: Industrial Science & Technology Network, Inc.
Priority date: 1999-04-22
Filing date: 2000-04-21
Publication date: 2000-11-02
Also published as: WO2000064969A9; AU4365500A

Abstract

A grafted silica or other metal oxide xerogel is interpenetrated or impregnated by a polymer and sandwiched between layered clay plate molecules. The grafted segments on the backbone of the xerogel provide phase compatibilization between the polymer and layered clay plate molecules. The grafted xerogel is produced by a gel precursor with a coupling reactant, under specified conditions, to cause the coupling reactant to condense and react with functional, e.g., hydroxyl (from silanol) groups on the gel precursor to form grafted sol. The grafted sol is mixed under stirring with clay to exfoliate the clay plate molecules. The exfoliated clay molecules may be mixed with oligomers or blended with a polymer to form the grafted xerogel as shown in Figure 1.

Description

NANOCOMPOSITE OF POLYMERIC MATERIALS WITH SURFACE MODIFIED CLAY AND SILICA XEROGEL

Technology Background and Comparison with Existing Art:

During the past five years, a new generation of thermal insulation material was developed through the effort of an ATP project¹. This new generation of material consists of a group of nanopore low-density materials. The thermal conductivity of the new Aerogel material, approx. 23 W/mK, is only half that of conventional insulation. In the same project, clay molecules (layered silicates) were used with aerogel to create anisotropic pore structure. The strategy was to reduce thermal conductivity in one direction at the expense of properties in the other two directions. Such a maneuver is beneficial because in a field application, heat flows only in the direction of a temperature gradient, not the other two. In our experiments, adding layered silicates into aerogel reduced its thermal conductivity from 23 to 16 mW/mK. Our experimental results confirmed the prediction based on the modeling work² of property enhancement through morphology control.

Controlling morphology at the nanometer level results in impressive performance in other properties as well. A nanocomposite of nylon and layered silicates made by researchers at the Toyota research center³ shows that with just 2 volume % of silicates, the tensile modulus and strength are doubled. For properties such as thermal, diffusive, and fire retardation, the same amazing degrees of improvements were observed for polymer-clay nanocomposites.⁴'⁵

¹ "Thermal Insulation Materials-Morphology Control and Process for the Next Generation of Performance", ATP award to Armstrong World Industries, Inc., 1992

² Yang A. J., Tech. Report, Armstrong World Industries, June 21, 1989

³ Usuki A., Kawasurni M., Kojima Y., Okada. A., Kurauchi T., Karnigaito 0., /. Mater. Res. 1993, 8, 1174; Usuki A., KojimaY., Kawasumi M., Okada A,, Fukushima Y., Kurauchi T., Karnigaito 0., /. Mater. Res. 1993, 8, 1179; Yano K., Usuki A., Karauchi T., Karnigaito 0., /. Polym. Sc , Part A: Polym Chem. 1993, 31, 2493; Kojima Y., Usuki A., Kawasurni M., Okada, A., Kurauchi, T., Kamigaitc 0., / Polym. ScL: Part A: Polym. Chem 1993, 31, 983; Kojima Y., Usuki A., Kawasumi M., Okada. A., Kurauchi T., Karnigaito 0., Kaji K., J. Polym ScL Part B: Polym Phys.1994, 32, 625; Kojima Y., Usuki A., Kawasumi M., Okada, A., Kurauchi T., Karnigaito 0., Kaji K., J Polym. ScL: Part B:Polym.Phys. 1995, 33, 1039 Giannelis, E. P., Polymer Layered Silicate Nanocomposites. Advanced Materials. 1996, 8, 29. ⁵ Lan T., Pinnavaia T. J., Chemical Materials, 1994, 6, 468 Extensive characterization work demonstrated that the superior enhancement in property comes directly from the unique morphology of the composite. A silicate is composed of plate molecules with high aspect ratios (100 to 1000) comparable to conventional fibers. Because of the electrostatic forces between them, plates are stacked together to form a particle. Without layer separation, silicate additives perform no plate surface, organic polymers can penetrate into the space between host layers and form an organic-inorganic nanocomposite. Two types of structures⁶ are distinguished: intercalated. in which only a single layer of polymer chains is confined between the hosting plates, and delaminated. in which layers are exfoliated and dispersed in a continuous matrix of polymer.

When fully delaminated, the plate molecules of silicates exhibit exceptional enhancement of properties not attainable by conventional inorganic fillers for polymers. Compared with fiber reinforcements, the delaminated plates strengthen mechanical properties in two dimensions rather than one. Moreover, the planar barrier geometry, which considerably increases the torturosity⁷, effectively reduces both the mass and heat transport rates within the composite. Data reported in literature⁹'¹⁰ regarding the remarkable effects of delaminated plates in a nanocomposite support this concept. Without using any harmful chemical additive, a Polymer-Clay nanocomposite is one of the best available approaches to reducing the flammability of a polymer⁸.

The progress on the front of nanocomposite processing is equally exciting. In certain organically modified layered silicates, polymer molecules can enter into the gallery between host layers by direct melt intercalation. Consequently, processing techniques such as casting, extrusion, and injection molding, commonly used for thermoplastic materials, may be applied to polymer-clay nanocomposites. This advancement in processing opens the entire plastic industry to applications of clay nanocomposites. In a project funded by DOE⁹, microcellular foaming technology for the polymer-clay nanocomposite is being developed. The foaming technology has the potential to immediately realize a widespread application of nanocomposite in the foam market, one of the biggest market sectors of plastic industry.

⁶ S D Burnside, E P Giannelis, Chemistry Of Materials, Vol. 7, No. 9 (September 1995), pp. 1597-1600

⁷ Torturosity may be defined as the average distance a diffusant travels between a unit change in concentration or temperature.

⁸ J. W. Gilman, T Kashiwagi, and J. D. Lichtenhan, SAMPE J., Vol. 33, No.4, 41(August 1997)

⁹ SBIR Phase I project performed by Industrial Science and Technology Network, Inc., September 1998 to March 1999.

L The clay contains many layers of plate-like silicate molecules. The aspect ratio of the plate is extremely high. The lateral dimensions of a plate are several microns while its thickness is about one nanometer. The plate contains stable Si-O bonds and is negatively charged. Small cations such as Na⁺, K⁺, Mg⁺², and even Al⁺³, stay in the gallery between the anionic silicate layers. The strong electrostatic attraction between the oppositely charged species holds the plates together to form a micron-sized particle. The size of the cations determines the spacing between the stacked layers.

By ion-exchange reaction, an organocationic surfactant may replace smaller cations in the gallery; increasing the layer spacing and setting up galleries for intercalation of additional organic oligomers. Polymerization of the oligomers after the penetration led to a Polymer-Clay nanocomposite. At present this is the standard treatment procedure for the intercalation of organic species into the gallery of the layered structure of clays. The cationic surfactants including primary, secondary, tertiary, and quaternary ammonium ions have been widely used for this purpose.

However, there are at least three disadvantages to using this standard system for compatiblizing the polymer clay composite. First, the reactive amine functional groups could interfere with the subsequent polymerization reaction among the oligomers. Secondly, the long alkyl chain of the surfactant, with a high hydrocarbon content, is thermally unstable and highly flammable, diminishing the value of using exfoliated clay to improve the fire resistance of the composite. Most importantly for the processing, a clay solution loaded with surfactants is difficult to process because of the large numbers of bubbles generated during stirring. Eliminating the use of the surfactant considerably would reduce the material cost and the processing time of the clay modification.

In this invention, we demonstrated an alternative approach that did not require the pretreatment of clay with an organo-cationic surfactant. We first washed the clay with an acid to exchange the cations between the layers with the hydronium ions in the acid. After several consecutive acid washings, the inner surface of the gallery, with an abundance of -OH groups, is capable of forming strong hydrogen bonding with the incoming oligomers such as polyol or polyether. It was demonstrated with full characterization in this method that such treatment was effective in separating the clay layers with the intercalation of hydrophilic (i.e. hydrogen-bond forming) oligomers. Admittance of macromolecules into the gallery leads to complete exfoliation of the plates. Treating the clay by acid washing before the intercalation of organic oligomers was effective, but also required additional processing efforts. The greater the number of acid washings, the more difficult the processing becomes. However, also demonstrated in this invention is the extreme ease and efficiency of exfoliating the clay plates with the silica sol (silicic acid). Silicic acid (H₂SiO₃) can be obtained by hydrolyzing TEOS or replacing the sodium ions (by ion-exchange) of sodium silicates with protons. Due to its similar composition (Si-OH) and structure to the clay, silicic acid can completely exfoliate the clay just by mixing and stirring. The processing time and cost can be considerably reduced with the use of silicic acid in the separation of clay plate molecules.

The significant technical advancement of using the silicic acid system, besides the ease and efficiency in the separation of clay molecules, is that the active silanol groups (Si-OH) introduced into the gallery between the clay plate molecules by the silicic acid offer ample opportunities for chemical variations in the resulting composite. Reaction of these silanol groups with other coupling agents can be used for the development of additional schemes in the modification of composition and morphology of a composite at the nanometer scale.

Recent successes of incorporating functional groups onto silica by reacting mercaptoalkyltrialkoxy silane or similar coupling agents with silica precursors in a one- phase reaction (see, e.g., WO 99/39816, published 12 August 1999, the entire disclosure of which is incorporated herein by reference thereto) provide a new scheme to compatibilize the clay silica with oligomers or polymers which are hydrophobic (i.e. incapable of forming hydrogen bond).

Summary of the Invention

According to one aspect, the present invention provides a grafted xerogel (preferably a silica xerogel) which is interpenetrated or impregnated by a polymer and is sandwiched between layered clay plate molecules. The grafted segments on the backbone of the xerogel provides phase compatibilization between the polymer and layered clay plate molecules.

The present invention also provides a method for preparing the grafted xerogel. Thus, silicic acid or other gel precursor is first reacted with a modifying reagent to chemically incorporate hydrophobic functional groups (such as alkyl) with the silica backbone. Then, the modified silica sol (grafted silica oligomers) can be used to separate the clay plate molecules. The nanocomposite of clay and organo-grafted silica can be gelled and then either mixed with intercalating oligomers or blended with a polymer to form a Clay-Xerogel-Polymer (CXP) nanocomposite. The morphology for this Clay- Xerogel-Polymer composite contains a layered structure of exfoliated clay separated by the porous silica xerogel of which the pores are penetrated by the organic polymer.

The functional groups to be incorporated with the xerogel silica for making a CXP composite can be engineered to further improve the performances of the composite. These property enhancements may include, but are not limited to, fire and smoke retardation, phases compatibilization, crosslinking for mechanical enhancement (e.g., strength properties, rolling resistance for the polymer, e.g., rubber, substrate used in tire manufacture, anti-skid resistance, and the like), foaming gas generation, gas nucleation, radiation (UN) resistance, anti-oxidation, and the like.

Representative of coupling agents readable with the silica include, but are not limited to, silanes such as methacryloxymethyltrimethylsilane, CH₂=C(CH₃)COOCH₂- Si(OCH₃)₃; methacryloxypropyltriethoxysilane, CH₂=C(CH₃)COO(CH₂)₃-Si(OCH₃)₃; 7-octenyltrimethoxysilane, CH₂= CH-(CH₂)₆-Si(OCH₃)₃; styrylethyltrimethoxysilane, CH₂= CH-C₆H₄-(CH₂)₂-Si(OCH₃)₃, vinyltriethoxysilane, CH₂= CH-Si(OCH₂CH₃)₃, bis(2-hydroxyethyl)-3-aminoproρyltriethoxysilane, Ν-[3-(triethoxysilyl)propyl]-4,5- dihydroimidazole, and the like. See also, for example, the functionalizing groups disclosed in U.S. 4,650,784.

In addition, while the xerogel is conveniently produced from silica gel, the xerogel may be produced using metal oxide or metal alkoxide sol-gel precursors, such as, for example, aluminum alkoxide, zirconium alkoxide, vanadium alkoxide and titanium alkoxide or their corresponding metal hydroxides. The alkoxides may have, for example, from about 2 to about 30 carbon atoms, preferably, from about 3 to about 22 carbon atoms. Brief Description of the Drawings

Fig. 1 is a schematic diagram illustrating foaming of a polyurethane-clay nanocomposite according to the invention;

Fig. 2 are X-ray diffraction patterns of Volclay and ICP-0 showing the d(100) peaks;

Fig. 3 are X-ray diffraction patterns of ICP-1, ICP-2, ICP-3, TTMSPI-Clay, and SS-Clay, according to Example 5;

Fig. 4 are X-ray diffraction patterns of nanocomposits according to the invention containing 1%, 3% and 5%, by weight, of exfoliated clay;

Fig. 5 is a TEM micrograph of a thin section of 1% polyurethane-clay nanocomposite according to the invention;

Fig. 6 are FTIR spectra of Nolclay, polyol and polyol-clay, showing intercalation of polyol in the Nolclay;

Fig. 7 are FTIR spectra of polyurethane-clay nanocomposites according to the invention, as a function of time;

Fig. 8 is a scanning electron micrograph (SEM) of a polyurethane-clay composite according to the invention; and,

Fig. 9 is a graph plotting weight loss ( ) of polyurethane-clay nanocomposites as a function of temperature (°C). Detailed Description of the Invention and Working Principles:

Nolclay is an inexpensive raw material (approximately $0.05/lb). It is a specific bentonite composed of 90% montmorillonite. The other 10% consists of minute fragments of other minerals. Montmorillonite is composed of two sheets of silicate tetrahedra linked through shared oxide ions and sandwiched above and below a central sheet of aluminate octahedra that, likewise, share oxide bonds with adjacent silicate tetrahedra. On occasion a magnesium (+2) (or ferrous) ion substitutes for one of the trivalent aluminum cations in the octahedral layer.

The present invention provides a new technology for the modification of clay has now been developed, that not only makes the surface of the silicate layer compatible with organic compounds, it can also overcome the drawbacks with the use of conventional alkyl ammonium surfactants. Using alkyl ammonium surfactants not only prolongs the processing time of clay; the presence of the amine functional groups also complicate and may even terminate a subsequent polymerization reaction. We treated the clay with a HC1 solution to exchange the metal ions existing in the gallery of clay with H₃O⁺ before the intercalation procedure. The H₃O⁺ exchanged surface of the silicate layer abounds with OH groups, and exhibits compatibility to organic compounds, especially to polyol due to the driving force of hydrogen bonding between hydroxyl groups on the surface of the silicate layer and hydroxyl and ether groups of polyol.

Polyol-intercalated volclay can be mixed with an equivalent of tolyl 2,4- diisocyanate (TDI) for the polymerization reaction to make a cured polymer-clay nanocomposite. The degree of reaction was characterized dynamically by FTIR spectra. In addition, polyurethane-clay foams can be made from the controlled entrapment of gases that are generated during the polymerization reaction between polyfunctional alcohol and polyisocyanates which form urethane linkages. The foaming formulation used for the nanocomposite consists of intensively mixed polyol, diisocyanate, intercalated / exfoliated clay, water, surfactant, and a combination of amine and tin catalysts. Silicone surfactants are the class of surface-active materials used in making almost all polyurethane for controlling foam stability, cell size, and cell size distribution. Tertiary amine and tin catalysts are used in combination, and are balanced to give the desired reactivity as reflected in the foam development and cure.¹⁰ The chemical reactions occurring in polyurethane foam formation mainly include:

(1) reaction of isocyanate of TDI with hydroxyl of polyol to form urethane,

ROH + R'-N=C=O R-O-CO-NH-R'

(2) reaction of isocyanate with water,

R'-N=C=O + H₂O (R-NH-COOH) R-NH₂ + CO₂ carbamic acid

(3) reaction of amine with isocyanate to form an urea linkage. R'-N=C=O + R-NH₂ R-NH-CO-NH-R'

¹⁰ Wong, S.- W., Frisch, K. C, /. Polym. ScL, Polym Chem. Ed. 1986, 24, 2867 & 2877; Malwitz, N. Manis, P. A., Wong, S.- W., Frisch, K. C, Proceedings of the SPI 30^th Annual Polyurethanes Technical/Marketing Conference, 1986, Technomic: Lancaster, 1986, p 338. Water and TDI react first, to produce a transient carbamic acid, which on loss of carbon dioxide, yields an aniline derivative, which can then react further with isocyanate to produce a urea linkage. The reaction of isocyanate with water is the most exothermic reaction that takes place during the foaming process. The carbon dioxide which is generated, diffuses into the nucleation sites. At such sites, carbon dioxide begins to grow by the accretion of additional liberated carbon dioxide and by thermal expansion causing the cell to expand.

Foaming of the Polyurethane-Clay nanocomposite was achieved by mixing the desired amount of polyol-intercalated volclay, polyol, and water along with an amine catalyst and surfactant in a volumetric reaction container. After degassing, the mixture was rapidly mixed with a tin catalyst and an equivalent amount of TDI for 30 seconds, and then put in an oven at 80 °C over night for curing. The attached diagram displays the route of the working procedures (Figure 1).

The most effective way of exfoliating the clay molecules discovered by this invention is by using silicic acid. The X-ray data demonstrated a full exfoliation of clay by simply mixing and stirring with silicic acid. Moreover, the reactive silanol groups can be utilized for grafting various additional functional groups. For example, silicic acid obtained from sodium silicate solution can be reacted with a 3- mercaptopropyltriethoxysilane or other functional groups of choice in a one-phase reaction to produce a stock solution of the modified (organo-grafted) silica sol. This modified silica sol solution may be used to separate layers of the volclay. Additional organic oligomers can be added into this mixture in order to achieve the intercalation of organic specie.

Alternatively, the organically modified silica sol solution can be mixed with the volclay and then, after plate separation, gelled to create a solid composite of layered clay separated by the porous modified xerogel. Those skilled in the arts of drying xerogel can dry the composite without shrinking the porous structure. The dried composite of the clay and the modified xerogel can then be impregnated with hydrophobic oligomers followed by polymerization, or directly blended with a hydrophobic polymer to form a nanocomposite of Clay-Xerogel-Polymer with a specific morphology of a layered structure of clay filled by a xerogel composite of interpenetrating silica and polymer. Examples and Characterization

Materials used in Examples

Nolclay SPN-200 was purchased from American Colloid Company. The cation exchange capacity (CEC) of the clay was 120 meq/lOOg. Glycerol propoxylates, Toluene 2,4-diisocyanate (TDI), stannous 2-ethylhexanoate, and other general chemicals were purchased from Aldrich Chemical Company, Inc. without further purification. Νiax amine catalysts (A-l, A-230, A-107, C-5) and Νiax silicone surfactants (L-560, L-6906, Y- 10366, L-620) were obtained from OSI Specialties, Inc.

Experimental Characterization of polyol-intercalated clay and PU-Clay nanocomposites.

X-ray diffraction (XRD) experiments were performed on the polyol oligomer- intercalated volclay powder and nanocomposite samples using a Rigaku Rotaflex diffractometer with Cu Kα (λ= 1.5405 A) irradiation at a scanning rate of 2° 2Θ/min. The voltage and the current of the x-ray tubes were 40 kN and 100 mA, respectively. The basal spacing of the volclay was estimated from the position of the (001) peak in the

XRD pattern. FTIR spectra of the products in the form of KBr pellets were recorded on a Νicolet 5-DX infrared spectrometer. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer thermal analyzer with a 7 Series Thermal Analysis System at a heating rate 20°C/min under an atmosphere of flowing air or nitrogen ( 40 cm³/min). The elemental analysis was performed using Perkin-Elmer 2400 CHΝ elemental analyzer. Scanning electron micrographs were obtained with a XL 30 Teaching SEM using an acceleration voltage of 1 kN.

Example 1: Separation of clay plates by washing with acid and intercalating oligomers

H₃O⁺ exchanged volclay was made by treatment of volclay with a hydrochloric acid solution (2N). Nolclay was immersed in the hydrochloric acid solution and shaken at room temperature for 2 hours. The H₃O⁺ exchanged volclay was collected by centrifugation. The highly swelled volclay was placed in distilled water and mixed with glycerol propoxylate dissolved in alcohol. The mixture was heated to 60°C with vigorous stirring for 24 hours. The polyol oligomer-intercalated volclay was collected by centrifugation and washed with 50% alcohol several times. The amount of polyol oligomer intercalated in volclay was determined by elemental analysis (EA) and thermogravimetric analysis (TGA). The polyol oligomer content was between 45.6-51.3%. The degree of intercalation was characterized by X-ray diffraction.

To compare the effectiveness of clay separation, hydrolyzed tri[3- (trimethyloxysilyl)-propyl] isocyanurate (TTMSPI) and silicate sol (SS) were applied as an intercalator respectively by the same procedure given above except without the pretreatment by acid. Silica sol was obtained by flowing sodium silicate solution through an cation-exchange column at a rapid flow rate.

Example 2: PU-Clay Nanocomposite

A mixture of the desired amount of polyol oligomer and Polyol intercalated Nolclay was stirred at 30 -40 °C for about 48 hours, and vacuum dried at room temperature with phosphorous pentaoxide over night. Then, an equivalent of tolyl 2,4- diisocyanate was added into the mixture with vigorous stirring and degassing. When mixing was stopped, the mixture was rapidly poured into a container and put into an oven at 70-80°C for 48 hours. The clay content of the nanocomposites was 1%, 3%, and 5% respectively.

An alternative method uses a mixture of polyol and the desired percentage of volclay in distilled water-alcohol (1:1) co-solvent. The mixture was stirred at room temperature for 2-4 hours. The co-solvent was removed by vacuum, and the mixture was dried with vacuum under phosphorous pentaoxide over night. Then, an equivalent of tolyl 2,4-diisocyanate was added into the mixture with vigorous stirring. After degassing, the mixture was rapidly poured into a container and heated to 70-80 ^QC for 48 hours. Example 3: foaming of PU-Clay Nanocompsoite

A mixture of the desired amount of polyol-intercalated volclay, polyol, and water was sufficiently mixed with an amine catalyst and surfactant in a volumetric reaction container. After degassing, the mixture was rapidly mixed with a tin catalyst and an equivalent amount of TDI for 30 seconds, and then put in an oven at 80 °C over night for curing. An electric timer was used to record the mixing time and to signal the end of mixing. The following table is an example of a formulation for water-blown polyurethane-clay nanocomposite foam. Parts are by weight unless otherwise indicated.

Table 1 Formulation of ingredients for a water-blown polyurethane-clay nanocomposite foam

Ingredient Parts per 100 parts polyol

Polyether polyol¹ 100.0

Volclay 1 - 5

Water 2.5

Surfactant² 1.0

Amine catalyst³ 0.08

Stannous octoate 0.2

TDI (105 index)⁴ 50.3

¹ A polyether triol with a hydroxyl number of 168. ² Silicone surfactant, L-1000

³ Bis(N,N'-dimethylamlinoethyl)ether Isocyanate index, by convention, is 100 times the ratio of the free isocyanate groups to isocyanate reactive groups, i.e., hydroxyl, amine, and water, before reaction has taken place. Thus, an isocyanate index of 100 is the exact number of equivalents of isocyanate as the number of equivalents of hydroxyl, amine,hydrogens of water. Water has two equivalents per mole, a primary amine, two, and a secondary amine, one. Example 4: Grafting Functional Groups onto the backbone of silica oligomers

Silica sol was prepared from TEOS, H₂O, ethanol and HC1, in the total molar ratio 1: 2 : 4 : 0.0007. The mixture of 50ml of silica sol and a variable amount (depending on the desired % of functional group loading) of 3- mercaptopropyltrimethoxysilane were added into a reaction vessel equipped with agitator, heating mantel, thermometer and nitrogen purge system. Additional amount of water or ethanol were used to adjust the water/ethanol ratio in the solvent mixture so that their proportions are suitable for the amount of functional groups desired. The reaction mixture was heated to 50-60°C for 1 to 2 hours.

Example 5: Nanocomposite of Clay and Surface Modified Xerogel

The mixture of 50ml stock solution of modified silica sol obtained from the above example is mixed with a desired amount of volclay in a reaction vessel equipped with agitator, heating mantel, thermometer and nitrogen purge system. Additional amounts of water or ethanol were used to adjust the water/ethanol ratio in the solvent mixture so that their proportions are appropriate for the amount of clay desired. The reaction mixture was heated to 50-60°C and stirred vigorously for 1 to 2 hours. A NH₄OH solution was then added to the mixture to induce gelation. After cooling, the composite of clay and modifed xerogel was filtered and washed thoroughly with ethanol and water successively.

Characterization and Analysis

Separation Of Clay By Washing With Acid And Intercalating Oligomers

Experimental results demonstrated that the HC1 treatment of volclay greatly affected the extent of clay expansion upon polyol intercalation. It was found that basal spacings for a series of polyol-clay were proportional to the HC1 treatment times. XRD analysis was used to measure the degree of intercalation of polyol-clay. The XRD patterns in figure 2 and 3 show the relationship between HC1 treatment times and the basal spacing of polyol-clay. For convenience, we term the intercalated polyol- clays as IPC-n (n is the number of HC1 batch treatment before intercalation, n = 0, 1, 2, and 3). The (001) peaks of volclay and IPC-0 are shown in Figure 2. The main silicate reflection in volclay centered at 2Θ = 7.14 corresponds to a d spacing of 12.4 A, van der Waals gap of Mⁿ⁺montmorillonite. After mixing untreated volclay and polyol at 60°C in a water and alcohol co-solvent, a new reflection at 2Θ = 4.9 (d _m) = 18.0 A) was found (See pattern IPC-0) which represents an increase in spacing of approximately 5.6 A.

The XRD patterns in Figure 3 show a further expansion of basal spacing of silicate layers in polyol-clay when H₃O⁺ exchanged clay was employed. The XRD pattern of ICP-1 shows that volclay, when treated one time with HC1 solution, undergoes a 1.6 A increase in basal spacing from 18.0 A (ICP-0) to 19.6 A. Both patterns for ICP-2 and ICP-3 contain two reflection peaks at 2Θ = 4.4°, 0.90° and 2Θ = 4.15°, 0.65° corresponding to d spacings of 20.1 A, 98.0 A, 21.3 A, and 135.4 A respectively. The d spacings of 98.0 A for ICP-2, and 135.4 A for ICP-3 indicate that some of clay layers in the polyol-clay in both cases have been exfoliated (> 80 A¹¹). The d spacing increase for ICP-2 and ICP-3 compared to that for ICP-1 indicates that more cations in the silicate gallery are exchanged by H₃O⁺ so that the surface of the silicate layer is more compatible to polyol. The pattern for ICP-3 also shows a further increase of d spacing of the gallery compared to that of ICP-2 due to an extra HC1 batch treatment. In addition, the relative intensity of the reflection peak at 2θ = 0.65° to that at 2Θ = 4.15° for ICP-3 has increased significantly. This result suggests that more silicate layers were exfoliated by polyol due to more H₃O⁺ exchange occurring on these surfaces after three HC1 treatments to volclay. All of these findings can be accounted for in terms of the proposed mechanism that the H₃O⁺ exchanged surface of silicate layers have better compatibility to polyol than those silicate layers without HC1 treatment. The trend illustrated in Figure 3 summarizes the overall mechanism for the formation of the polyurethane-clay nanocomposite.

The amount of polyol oligomer that can be intercalated into the gallery depends on the clay layer charge density. For clay with a high layer charge density, a strong electronic attraction must be overcome to achieve an exfoliation. The charge density decreases with an increase in the degree of H₃O⁺ exchange in the silicate layers. The hydrogen bonding between the hydroxyl groups on the surface of the silicate layers and the hydroxyl and ether groups of polyol is the main driving force for the intercalation and exfoliation.

¹¹ Lan, T.; Kaviratna, P. D.; Pinnavaia T. J., Chem. Mater. 7, 2144 (1995) To further verify our hypothesis, we investigated the effect of the molecular structure of the intercalator on intercalation and exfoliation. We chose hydrolyzed tri[3-(trimethyloxysilyl)-propyl] isocyanurate (TTMSPI) and silicate sol (SS) as an intercalator respectively. They are expected to have better separation ability with silicate clay than polyol because they contain many Si-OH groups. They should be more compatible and more easily able to form hydrogen bonds with the surface of the silicate layers. The XRD pattern of TTMSPI-Clay in Figure 3 shows that TTMSPI can fully exfoliate silicate layers up to 147.0 A, and the pattern of SS-Clay (Figure 3) exhibits no Bragg scattering, which suggests that the silicate sol has stronger separation ability than TTMSPI due to its similar structure to clay, and can easily exfoliate the clay into single layers with no regular repeat distance between the layers. The ability to exfoliate volclay and their compatibility to silicate layers of clay decreases in the order: silicate sol > TTMSPI > polyol.

Polvurethane-clay nanocomposites Under the appropriate polymerization condition, more polyol and TDI can penetrate the gallery space of the polyol-intercalated clay layers. The galleries continue to expand beyond a bilayer spacing, and the parallel orientation of the nanolayers is lost as the polymerization of polyol in the gallery with TDI proceeds with increasing reaction time. Therefore, the stacking registry of the former nanolayers was completely exfoliated and became disordered. XRD patterns of polyurethane-clay nanocomposites formed with 1, 3, and 5 wt % of clay are shown in Figure 4. For all cases, no d(00ϊ) Bragg reflection peaks are observed, indicating that the clay nanolayers are completely exfoliated in the polyurethane matrix.

On the basis of the above XRD results, we conclude that H₃O⁺ exchanged clay presents a very low charge density in the gallery of silicate layers, and exhibits an excellent swelling property toward polyol owing to a strong driving force resulting from the hydrogen bonding between hydroxyl groups on the silicate layer surface, and hydroxyl and ether groups of polyol. During the polymerization of polyol and TDI, more polyol and TDI will diffuse into the gallery of the silicate layers. The mtragallery polymerization further expands the space between the layers and results in a complete exfoliation. The XRD results are confirmed by transmission electron microscopy (TEM) (Figure 5) on carbon-coated microtoned sections of the 1% PU-nanocomposite. The dispersed silicate layers are viewed edge-on as dark lines. The individual silicate nanolayers are separated by 20-50nm of the PU component. TEM characterization works were performed by Dr. Jun Liu of the DOE Pacific Northwest National Laboratory (PNNL). FTIR Spectra

FTIR spectra, although more qualitative in nature, provide additional clear evidence for intercalation of polyol into the volclay. The FTIR spectra of volclay, polyol, and polyol-intercalated clay are shown in Figure 6. The volclay spectrum shows its characteristic bands of free SiO-H stretching at 3634 cm^"1, hydrogen bonding SiO- H stretching at 3458 cm^'1, and Si-O-Si stretching at 1053 cm^'1. After intercalation of polyol into the clay, there are several new adsorption bands at 2882-2980 cm^"1 and 1377 cm^"1, which are attributed to CH and CH₂, and OCH₂ groups of polyol. This observation clearly indicates the formation of polyol and clay intercalated material. In order to investigate the reactions during the polyurethane-clay nanocomposites formation, infrared measurements have been used. For the infrared studies, the nanocomposite reaction mixture was filmed on a KBr pellet after mixing in a reaction container. The KBr pellet was then put into an oven at 80°C. IR spectra were taken at different reaction times during the polymerization of toluene diisocyanate with polyol in the presence of polyol-intercalated-clay. The change in the infrared absorbance at specific wavenumbers was recorded as a function of time. IR spectra of the polymerization mixture are shown in Figure 7. The adsorbances of interest are the isocyanate group and urethane group. At the beginning of the reaction, a strong characteristic N=C=O stretching band at 2270 cm^'1 was observed due to the high concentration of isocyanate groups of TDI.

Meanwhile, two weak bands appeared at 3290 cm^"1 (υ_N._H) and 1728 cm^"1 (υ_c=α) due to the new formation of the urethane group (-O-CO-NH-). With an increase in reaction time, the N= C= O stretching band of isocyanate was diminished, whereas the C=O and N-H stretching bands of formed urethane bonds was increased. It was noticed that the absorbances follow a rapid early decrease of N=C=O bands and a rapid early increase of C=O and N-H bands. The change became very slow after 2 hours. This is because the concentration of the isocyanate group and the hydroxy group decreased dramatically, and the molecular weight of polyurethane increased rapidly with the reaction time. After a 12 hour reaction, the N=C=O band completely disappeared, and the C=O and N-H bands became strongest. This indicated that the isocyanate groups of TDI had completely reacted with the hydroxyl groups of polyol.

Water-blown polyurethane-clay nanocomposite foaming

The nucleation sites are small air bubbles entrained in the liquid mixture during intensive mixing of the formulation, and are stabilized by the surfactant. The number of these nucleation sites, determined by the mode of mixing and by the surfactant, is generally taken to be essentially constant during a foaming process, leading to a stable foam.¹² It was found that there was a pronounced whitening of the reaction mixture after an interval of a few seconds. At the end of mixing, there was a given percentage of volume expansion achieved with time: 30 percent in 30 seconds; 50-60 percent in 60 seconds; 90 percent in 90 seconds and full rise after about two minutes. The final volume is about 20 times the original volume. The density of the nanocomposite foam is about 0.05 g/cc. The scanning electron micrograph of the polyurethane-clay nanocomposite foam in Figure 8 shows that the foam process produces many closed cells with different cell sizes. The diameter of the cell is about 100 - 200 μm. The foam cells have polyhedral windows with not much visible accumulation of material in the struts. In comparison with open cell foam, the closed-cell nanocomposite could have lower permeability to gas and vapor, and more effective insulation capabilities for either heat or electricity. Thermal stability

The thermal behavior of the polyurethane-clay nanocomposites was investigated by thermogravimetric analysis (TGA) in both air and nitrogen. The results presented in Figure 9 show that the weight loss of polyurethane clay nanocomposites began above 310°C, followed by rapid thermal decomposition (major decomposition) at 340-350°C on further heating. The 50% weight loss temperature was observed within the 370-390°C range. As can be seen by comparison of the figures of nanocomposites with that of plain polyurethane, the introduction of exfoliated clay into the polyurethane leads to a significant increase to the point at

¹² Kanner, B.; Prokai, B. Advances in Urethane Science and Technology; Technomic: Westport, 1973; Vol. 2, 221. which weight loss begins and major decomposition and 50% weight loss occur. Onset temperatures are increased by about 10°C, 25-25°C, 20-40°C, respectively. The improved thermooxidative stability of nanocomposites can be attributed to the oxidation retardant and barrier properties of separated silicate clay. A further encouraging feature is that all polyurethane clay nanocomposites gave substantially higher char than the plain polyurethane at the same temperature under airflow, which indicates an improved performance in fire resistance.

Claims

WHAT IS CLAIMED IS: Claim 1. A composition comprising a grafted xerogel which is interpenetrated or impregnated by a polymer and is sandwiched between layered clay plate molecules; the grafted segments on the backbone of the xerogel providing phase compatiblization between the polymer and layered clay plate molecules.

Claim 2. A composition according to claim 1 wherein the xerogel is a silica xerogel.

Claim 3. A method for producing the composition and morphology described in claim 1 comprising the steps of: (a) reacting a gel precursor comprising reactive groups on the surface thereof, with a coupling reagent, in an aqueous alcoholic medium under an inert atmosphere and at an elevated temperature within the range of from about 40°C to about 80°C to cause the coupling reactant to condense and react with said reactive groups on said gel precursor to form a grafted sol; (b) mixing and stirring the grafted sol with clay to exfoliate the clay plate molecules; and (^c)(l) mixing the exfoliated clay plate molecules with oligomers or (2) blending the exfoliated clay plate molecules with a polymer to form the composition as described in claim 1.

Claim 4. The method of claim 3, further comprising (d) adding oligomers to the exfoliated clay plate molecules, followed by polymerization.

Claim 5. The method of claim 3, further comprising (e) gelling and drying the mixture of clay and grafted xerogel to form a clay- xerogel-polymer nancomposite.

Claim 6. A method according to any one of claims 3, 4 or 5, wherein the gel precursor is a silica gel precursor and wherein said reactive groups comprise hydroxyl groups from silanol groups.

Claim 7. A method according to any one of claims 3, 4 or 5 wherein the blending is achieved by a process of batch mixing, injection molding, or extrusion.

Claim 8. A method according to claim 3, wherein the gel precursor is an alkoxide or hydroxide of a metal selected from the group consisting of aluminum, zirconium, vanadium, and titanium.

Claim 9. A method of producing exfoliated clay structure by (a) repeated washing volclay with strong acid, and (b) after the acid washes, intercalating and exfoliating the clay with the addition of oligomers or polymer which are capable of forming hydrogen bonding with silanol groups.

Claim 10. A method according to claim 3, wherein the gel precursor comprises methacryloxymethyltrimethylsilane, methacryloxypropyltriethoxysilane, 7- octenyltrimethoxysilane, styrylethyltrimethoxysilane, or vinyltriethoxysilane.

Claim 11. A method according to claim 3, wherein the coupling agent includes a functional group selected from a foaming agent, a nucleating agent, a radiation resistance agent, an anti-oxidation agent, a fire retardation agent, and a cross-linking agent.

Claim 12. A composition according to claim 1, wherein the xerogel is grafted with an agent including a functional group selected from the group consisting of a foaming agent, a nucleating agent, a radiation resistance agent, an anti-oxidation agent, a fire retardation agent, and a cross-linking agent.