An improved class of alumoxanes and a method of synthesizing them have been invented. Alumoxanes are aluminum oxy hydroxide particles that have been modified with compounds containing carboxylic acid groups. For typical applications in which alumoxanes are required to be compatible with polymers, it is desirable that the modifying compounds be carboxylic acids that have high molecular weights (>500 Daltons) and/or are somewhat hydrophobic (characterized by solubility <5% in boiling water). Heretofore, the hydrophobicity of such compounds has made it difficult or impossible to synthesize alumoxanes in sufficiently high yields in acceptably short reaction times, and the alumoxane products have exhibited nonuniformities, both within and between batches. The present invention overcomes these shortcomings of prior approaches to synthesis of alumoxanes.
All of the known prior approaches have involved the use of boehmite or other unmodified aluminum oxy hydroxides as the particulate starting materials. In contrast, the starting particulate material in a synthesis according to the present method is an aluminum oxy hydroxide (possibly boehmite) that has already been modified with a suitable organic compound, usually a low-molecular-weight carboxylic acid (or possibly a sulfonic acid) that is sufficiently soluble in water to react with boehmite within an acceptably short processing time (<24 hours).
More specifically, the particulate starting material should be one in which the loading of the organic modifier on the aluminum oxy hydroxide particles is low enough to leave enough unreacted surface for subsequent reactions to take place. Typical starting materials consistent with this requirement are alumoxanes with acid-to-boehmite ratios between 5 and 20. The advantage of using these alumoxanes as the starting materials is that in comparison with unmodified boehmite, they are substantially more soluble in water and/or organic solvents. The greater solubility makes the reactions with high-molecular-weight and/or hydrophobic acids faster, and the resulting products are more nearly uniform.
In one example of a synthesis according to this method, the particulate starting material is propionato alumoxane having an acid-to-boehmite ratio between 8 and 20. The other starting material is a carboxylic acid-functionalized polyester having a molecular weight between 500 and 50,000 Daltons. The carboxylic acid-functionalized polyester is dissolved in a solvent (for example, ethyl acetate or butyl acetate), making a solution. Separately, the propionato alumoxane is dispersed in water. The solution and the dispersion are mixed together. A third solvent (for example ethanol) can be added to make the mixture homogenous. The mixture is then refluxed for 1 to 72 h. The product of the reflux process is a homogenous powder or gel.
This work was done by Silvia Luebben, Ron Cook, Andy Myers, Cory Kreutzer, Carolina Wilson, and Manfred Meiser of TDA Research, Inc., for Johnson Space Center. MSC-23727-1