Abstract:
A stable gel is formed by reacting alumina hydrate sol and a vinyl alcohol-vinyl acetate copolymer in an aqueous solution having pH less than about 7 to form a polymeric sol and then transforming the sol to a gel. The copolymer preferably comprises polyvinyl acetate that is about 85-99% hydrolyzed. The gel is drawn into a thin layer and water is removed to form a thin, substantially crack-free film. The film makes a stable, generally transparent insulating material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to alumina-polymer gels and films and to methods for their preparation. 
     BACKGROUND OF THE INVENTION 
     Gels comprising alumina combined with various organic substances are known in the prior art. However, such gels generally take the form of a white floc rather than a stable clear gel. 
     For example, Montgomery et al U.S. Pat. No. 3,417,028 claims addition of polyvinyl alcohol, polyethylene glycols, polyethylene oxides, methyl celluloses, and polyacrylamides to solutions containing alumina hydrate. The organic polymers were reacted with the alumina in solutions made basic by addition of ammonium hydroxide. Because the reaction was carried out in basic solution, the products were white flocs rather than clear gels even though the patent refers to such products as &#34;gels&#34; or &#34;hydrous gels&#34;. 
     Andre et al U.S. Pat. No. 3,993,590 describes a process for preparing silica-alumina beads by polymerizing alumina and silica precursors with a water-soluble monomer comprising an acrylic compound. The reaction is carried out in an aqueous mixture preferably having a pH of about 3-4. One of the alumina precursors is an alumina hydrosol made by hydrolyzing an aluminum alcoholate or other water-soluble aluminum compound. A preferred monomer is acrylic acid. The polymerization reaction product comprises beads which are washed, dried, and calcined to destroy organic matter. 
     Andre et al do not suggest substituting a vinyl alcohol-vinyl acetate copolymer for the acrylic monomers preferred for their polymerization reaction. Addition of an organic plasticizer to the reaction mixture described above would be inconsistent with their ultimate objective of producing calcined alumina-silica beads which are free of any organic matter. Andre et al do not teach or suggest the toughened gel and substantially crack-free film claimed herein. 
     As used herein, the terms &#34;monolithic gel&#34; and &#34;stable gel&#34; refer to a gelled mass characterized by substantially no separation of a liquid phase. The term &#34;floc&#34; refers to a combination or aggregation of suspended particles in such a way that they form small clumps or tufts. A stable gel made in accordance with the present invention is generally transparent or slightly opalescent whereas an alumina hydrate floc is generally white. The term &#34;gel&#34; is often used in the prior art to describe what is called a floc herein. 
     The expression &#34;alumina hydrate&#34; refers to Al 2  O 3  ·x H 2  O, wherein x varies from 1 to 3. In other words, the water of the alumina hydrate varies from 15.0 to 34.6 percent by weight of the alumina hydrate, determined by calcination at 538° C. (1000° F.) for one hour. 
     It is a principal objective of the present invention to provide a method for preparing a stable clear gel comprising alumina hydrate combined with a vinyl alcohol-vinyl acetate copolymer. 
     A related objective of the invention is to provide a stable monolithic gel comprising alumina hydrate combined with a vinyl alcohol-vinyl acetate copolymer. 
     A further objective of the invention is to provide a method for transforming the clear gel of the invention into a crack-free, generally transparent film. 
     Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification and claims. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, alumina hydrate is combined with an organic polymer to form a stable clear gel. The gel is produced by a method comprising the steps of 
     (a) preparing a sol comprising a colloidal dispersion of alumina hydrate in an aqueous medium, 
     (b) mixing said sol with an organic polymer comprising a water-soluble vinyl alcohol-vinyl acetate copolymer, 
     (c) reacting said alumina hydrate with said organic polymer in an aqueous solution having a pH of less than about 7, thereby to form a polymeric sol, and 
     (d) transforming the polymeric sol to a stable clear gel. 
     A water-soluble plasticizer is preferably added to the polymeric sol before it is converted to a gel. The plasticizer may be diethylene glycol (DEG) or polyethylene glycol (PEG) or mixtures thereof. Diethylene glycol is particularly preferred. 
     The vinyl alcohol-vinyl acetate copolymer is made by partially hydrolyzing polyvinyl acetate in a basic solution. The polyvinyl acetate is generally hydrolyzed about 85-99%. About 87-97% hydrolysis is preferred. Polymers with about 88% and about 96% hydrolysis have been found quite suitable. 
     The copolymer generally has a molecular weight of greater than about 25,000, preferably greater than about 50,000 and more preferably greater than about 100,000. Two preferred copolymers have molecular weights of about 95,000 and abcut 126,000, respectively. 
     The alumina hydrate sol is preferably prepared by hydrolyzing an aluminum alkoxide in an aqueous acidic solution. A preferred aluminum alkoxide is aluminum isopropoxide. 
     The alumina hydrate sol and organic polymer are preferably reacted at an elevated temperature of about 50°-100° C. A reaction temperature of about 81° C. is particularly preferred. The reaction is carried out in an aqueous solution having a pH of less than about 7. The pH is preferably about 3-6. 
     The step of transforming the polymeric sol to a stable clear gel preferably comprises concentrating the sol by evaporation of water. For example, the sol may be concentrated in a rotary evaporator to about one-half of its initial volume. 
     A preferred gel made in accordance with the invention is nearly transparent. At room temperature, the gel retained its appearance with no syneresis for several weeks. The gel is stable upon heating to 100° C. 
     The gel may be converted into transparent film by drawing a thin layer onto an inert substrate and then drying at an elevated temperature of about 40°-100° C. A particularly preferred drying temperature is about 80° C. The layer has a thickness of less than about one millimeter, preferably less than about one-half millimeter and more preferably less than about 100 microns (0.1 mm). One suitable film has a thickness of about 25 microns. 
     The stable gel and transparent film of the invention are useful as transparent insulating materials. The gel may also be dried and calcined to form a catalyst base. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The preferred method and products of the present invention are described below with reference to some preferred examples. 
     An aqueous alumina hydrate sol was prepared by dissolving 20.4 g of aluminum isopropoxide in 195 ml distilled water and 5 ml of 50 vol% aqueous acetic acid, all in an Erlenmeyer flask. The flask contents were stirred at room temperature for various times (0-24 hours), at the end of which period an appropriate copolymer was added. The effects of different prestirring times upon thermal stability of the resulting gel product are shown by test results summarized in Table II below. The flask contents were heated with stirring at 81° C. for 21 hours to form a sol, after which the sol was allowed to cool under continued stirring. 
     Formulations were made by blending 50 ml of the sol described above, 5 ml of methyl ethyl ketone (MEK), 5 ml of acetone, and 10 ml of a diethylene glycol plasticizer in a flask at 86° C. Alternatively, 10 ml of polyethylene glycol may be substituted for the diethylene glycol. Test results summarized in Table I below show the effects of different plasticizers. The flask contents were then concentrated with a rotary evaporator to half volume, thereby forming a gel. 
     The gel was drawn into a film on a polytetrafluoroethylene substrate using a draw/down bar having a 20 micron clearance. The substrate and film were oven dried at 80° C. for one hour and allowed to cool at room temperature. The film was next peeled off the substrate and conditioned at 25° C. and 50% relative humidity. Various tests were then performed on the films. 
     Different formulations were made with vinyl alcohol-vinyl acetate copolymers designated as Copolymer B and Copolymer C. Copolymer B was 88% hydrolyzed, had a molecular weight of about 126,000, and an intrinsic viscosity of about 1.18 dL/g. Copolymer C was about 96% hydrolyzed, had a molecular weight of about 95,000, and an intrinsic viscosity of about 0.7 dL/g. 
     Fracture stress and elongation-to-failure determinations were performed with standard methods (ASTM D-882) on an Instron machine. The Instron machine was set at a crosshead speed of 50.8 cm (20 inches) per minute and a 5.08 cm (2 inches) gauge length. Stress to fracture and ultimate strain were measured on gels made with Copolymers B and C and with polyethylene glycol (PEG) and diethylene glycol (DEG) plasticizers. The polyethylene glycol had an average molecular weight of about 190-210. Results of the tests are shown in Table I. 
     
                       TABLE I______________________________________Mechanical Strength Properties of Toughened Gels        Property              Stress to Fracture                           Ultimate StrainCopolymer   Plasticizer              (10.sup.-3 psi)                           (%)______________________________________20 wt % B   PEG        0.426         73.220 wt % B   DEG        0.780        123.410 wt % B   DEG        0.380        125.020 wt % C   PEG        0.300         49.420 wt % C   DEG        1.070        120.810 wt % C   DEG        0.694         20.2______________________________________ 
    
     The test results in Table I indicate that substantially crack-free films having satisfactory toughness can be made with gels containing 20 wt. % of either copolymer. The polymeric film made with 20 wt. % Copolymer C and diethylene glycol performed best. It was not possible to obtain films at copolymer concentrations below 10 wt. %. 
     Thermal stability of the gels was tested in air using a DuPont 9900 thermal analyzer at a heating rate of 20° C. per minute. Measurements were made to determine the temperature at which 10% weight loss occurred. This temperature varied widely, depending upon length of time of prestirring for the aluminum isopropoxide solution. The gels tested were made with 20 wt. % of Copolymer B or Copolymer C and diethylene glycol as a plasticizer. Results are shown in Table II. 
     
                       TABLE II______________________________________Effect of Prestirring Time on Thermal Stability of GelPrestirring Time        Temperature at 10% weight loss (°C.)(hours)      Copolymer B  Copolymer C______________________________________0            311.1        199.01            162.8        123.02            128.5        151.93            151.9        116.124           126.6        155.6______________________________________ 
    
     Test results shown in Table II suggest that gel samples made with the 88% hydrolyzed polymer (Copolymer B) had greater thermal stability than samples made with the 96% hydrolyzed polymer (Copolymer C). These results may be explained by the higher molecular weight of Copolymer B. 
     Room temperature prestirring of the aluminum alkoxide sol prior to addition of a copolymer has a negative effect on gel thermal stability compared with substantially simultaneous introduction of the isopropoxide and copolymer. The inventors believe that when the isopropoxide and copolymer are introduced together, simultaneous exchange occurs between hydroxyl groups on the polymer and the isopropoxide. Such interaction leads to cross-linking by ionic or weakly covalent bonding and a consequent enhancement of thermal stability. On the other hand, stirring in the absence of copolymer allows the alkoxide to hydrolyze to aluminum hydroxide. The principal interaction between aluminum hydroxide and the copolymer would then be through hydrogen bonding. 
     The effect of isopropoxide solution prestirring time on optical clarity of the gels was also tested. Clarity/transparency tests were made at 500 nm according to ASTM D-1746-70 using a Beckman UV-visible spectrometer. The gels tested were made with 20 wt. % Copolymer B or C, using diethylene glycol as the plasticizer. Results of the tests are shown in Table III. 
     
                       TABLE III______________________________________Effect of Prestirring Time on Optical Clarity of GelsPrestirring Time          Light Transmission at 500 nm, %(hours)        Copolymer B                     Copolymer C______________________________________0              29.85      50.961              62.28      58.002              65.02      72.913              62.46      72.3424             72.75      78.94______________________________________ 
    
     These results show that stirring and its duration have a pronounced effect on optical clarity. The effect of prestirring and the observation that gels made with the copolymer having more hydroxyl groups (Copolymer C) demonstrate higher clarity, together indicate that alkoxide hydrolysis plays a role in producing clearer gels. The alumina hydrate sol particles produced by such hydrolysis may have enhanced compatibility with the hydroxyl group-carrying polymer, thus giving clearer gel samples. 
     While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.