This invention relates to a method for the preparation of porous, crosslinked polyester-containing particles.
Porous, polyester-based particles, also referred to as beads, have projected uses as adsorbents, ion exchange resins, opacifying additives for coatings, matting and texturizing agents, and fillers. Several methods have been disclosed for the production of such particles or beads. All of these methods, however, suffer from specific limitations as to the morphologies of the beads which can be produced, the composition of the polyesters which can be successfully used, and the feasibility of the process for large-scale production.
Perhaps the most widely reported method involves the rapid crosslinking of a water-in-oil-in-water emulsion in which the oil phase contains an unsaturated polyester and one or more monomers which serve to copolymerize with the double bonds in the backbone of the polyester thereby crosslinking the particle. The presence of individual water droplets within the organic phase serves to create voids within the resultant polyester particles, hence introducing the porosity. This process is described in U.S. Pat. Nos. 3,979,342, 4,089,819, 4,363,888, 3,822,224, 3,923,704, 4,137,380, 3,933,579, and in GB 1,395,065. This process suffers from several limitations. First, this process is useful for only a narrow class of polyesters. The process cannot be used for polymers with a high degree of water-solubility. Thus, polymers with a high density of water-soluble functions, such as sulfonates, PEO segments, or quaternary ammonium units, cannot be used. This method is primarily useful for low molecular weight polyesters with acid numbers within a specific range and with carboxylic acid end groups which are neutralized using specific agents. Second, process control is difficult as the particle size is very highly dependent on pH and degree of agitation. Third, the colloidal stability of beads prepared by this process is often poor, and settling of the product dispersions can be a problem. Most importantly, the pore morphology of the beads prepared by this process is described in the patents as xe2x80x9cnoncontinuousxe2x80x9d or xe2x80x9cvesiculated.xe2x80x9d In other words, the hollows within the bead are not interconnected, but exist as discrete voids. This severely limits the utility of the beads in such applications as chromatography supports, ion exchange resins, and adsorbents. Also, it is difficult to produce very small particles, such as particles having a mean diameter of less than 1 xcexcm, using this method.
A second method called xe2x80x9cgentle cohesionxe2x80x9d is disclosed for the preparation of porous polyester beads in Japanese Kokais 07-179533, 07-137432, 07-137433, and 07-070331. In this method, a dispersion of very small polyester precursor particles is formed by dissolving a polyester which contains charged groups, either pendant charged units or neutralized carboxylic acid end groups, in a water-miscible organic solvent, adding water, and removing the organic solvent by evaporation. These precursor particles are coalesced into larger xe2x80x9cclusteredxe2x80x9d particles by the addition or generation of carefully controlled levels of electrolyte. These larger particles are fused by heating the clustered particles above the glass transition temperature. They are then swelled with a crosslinking monomer and an initiator and heated to yield the final, crosslinked beads. Like the water-in-oil-in-water emulsion method, the utility of this method is limited to only specific types of polyesters. If the process is applied to polymers with a high degree of water-solubility, the precursor particles cannot be formed. Secondly the procedure will not work for polymers with a glass transition temperature below room temperature or above the boiling temperature of the solvent, such as water, in this case. This procedure is difficult to apply to large scale due to the preponderance of process variables, and it is difficult to use this method to produce particles with a mean diameter of less than 1 xcexcm. Finally, this procedure yields only one type of bead morphology.
It is an object of this invention to provide a method for making small porous polyester particles, which demonstrate excellent stability, using a broad range of polyesters in a process that is easily controlled.
The present invention relates to a method of forming porous particles comprising preparing a mixture of an organic phase and an aqueous phase comprising an unsaturated polyester, a water immiscible organic solvent, at least one organic-soluble ethylenically unsaturated monomer, and initiator, subjecting said mixture to high energy emulsification, initiating crosslinking of said polyester after emulsification, removing said water immiscible organic solvent to recover porous polyester particles.
The method of the invention provides advantages with respect to the ability to use polyesters of an extremely broad compositional latitude with no restrictions due to glass transition temperature. In addition, this method offers, as an advantage, relatively simple process control and is ideally suited for high volume production. The colloidal stability of particles prepared by this method is excellent and a wide variety of pore morphologies can be attained. The porous, polyester-based particles or beads made by this method are useful as adsorbents, opacifying additives for coatings, matting and texturizing agents, and fillers.