The present invention concerns a process for producing granulated polyacrylic acid. The polyacrylic polymers are highly swollen in aqueous media. The polyacrylic acid granules produced by the process of the present invention are useful in controlled release formulations (such as pharmaceutical tablets). The powdered form of polyacrylic acid, which was previously used in controlled release applications, created material handling problems due to its poor powder flow characteristics, dust, and static charges associated with the dust.
Many compounded solids originate or are manufactured as fine, light, and/or loose powders. Such powders often have poor flow characteristics and are resistant to blending and dispersion in liquids due to clumping and poor wetting. The dust associated with the powders can exhibit static charge effects. Additional problems include difficulty in handling and, difficulty in feeding through volumetric metering equipment. Many such powders have historically been granulated to vary their particle size distribution in order to improve their characteristics. In these applications the larger granules are a temporary state with the granules being easily broken back down into the smaller powder particles by shear or solvents in the final product.
Polyacrylic acid resins, which are to be used in applications involving swelling with aqueous electrolyte solutions, are commonly polymerized in nonaqueous polymerizations where the insoluble polymer can be isolated as powders. These powders, comprised of aggregated or agglomerated polymer chains, are significantly easier to disperse and dissolve in water than the bulk polymer. However these polyacrylic acid powders have been noted for their static electricity charge, poor powder flow and some difficult in making dispersions in water since their introduction in 1958.
While some of the difficulties in using and dispersing polyacrylic acids have been addressed by various improved polyacrylic acids e.g. U.S. Pat. No. 5,349,030 and by adding components to minimize the effects of ionic charges (e.g. counterions), the problems of product dusting and poor flowability continue to be significant issues, especially with the use of very pure polyacrylic acid resins used in the pharmaceutical industry.
There are a variety of methods, which have been employed by powdered material suppliers and users in an attempt to reduce handling difficulties of powders. Slugging, hot roll milling, and fluidized bed or wet agglomeration processes are well known processes for converting powders to granules. Slugging compresses the powder into large tablets. Hot roll milling uses heat along with pressure to squeeze the powder into flakes or sheets. In either case, the compacted material is then reground into particles larger than the original powder grains. Both slugging and roll milling are relatively slow, low-capacity, energy-intensive processes. Roll milling has the additional disadvantages of requiring constant attention by a skilled operator.
Wet agglomeration techniques involve adding liquid to the original powder to increase the particle sizes and then drying the larger particles in trays or a fluidized bed. The resulting agglomerates can be used as is or ground to smaller sizes for specific uses.
Dry granulation eliminates several problems inherent in conventional processes. Dry granulation of powder material is a two-step process, requiring no heating or wetting (depending on the starting material), in which the powder is first densified (compacted) into solid form, then broken into smaller particles and separated into predetermined sizes.
To perform these steps, a granulation system combines several different kinds of specialized machines (usually in a vertical, gravity-assisted arrangement) to achieve a closed-loop operation. System components typically include: a feeding hopper, horizontal and vertical screws, compaction rollers, a prebreaker, a granulator, sizing/sorting screens, and a recycling elevator.
Granulated particles formed by the above processes are more easily handled than the powder from which they are formed. However, the granular particles may be too hard, too soft, too friable or not particularly suitable, due to the particle size distribution, for their end use (e.g. tablet forming processes).
Thus, there is a need to develop a process for preparing granulated polyacrylic acid suitable for controlled release applications e.g. pharmaceutical applications, from the polyacrylic acid powder. Desirably the process would produce granules, which retain similar properties (attributes) during formulation, forming tablets, and releasing actives from tablets to the powder without the handling problems associated with the powder.
The present invention pertains to a method for forming polyacrylic acid granules and granules formed therefrom wherein the granules are flowable, have comparable swelling characteristics and provide comparable tablet properties to powdered polyacrylic acid, have increased bulk density, and contain minimal amounts of very small particles that can cause dusting and/or static adherence. The granules from this process vary from other granules of similar materials in that they retain more of their dissolution and swelling characteristics in both aqueous solutions in slow release tablets than do prior art polyacrylic acid granules. The granules formed by the method of the present invention can be used to prepare controlled release tablets, especially controlled release pharmaceutical tablets where the granules have surprisingly similar characteristics during tablet formation with powders and form tablets with similar controlled release rates to tablets from the harder to handle powders. They can also be used as thickeners; emulsifiers and suspending agents in water based formulations based on other polar solvents.
Thus, a first advantage of the present invention is that polyacrylic acid powder is formulated into a granular product with better dry flow characteristics facilitating metering and mixing operations.
An additional advantage is the production of a granular polyacrylic acid having better control over particle size, higher bulk density to minimize packaging, and lower static adherence compared to unprocessed powdered polyacrylic acid.
A further advantage is that the granular polyacrylic acid produced in accordance with the present invention has relatively low dusting compared to the powder form of polyacrylic acid.
A still further advantage of the granular polyacrylic acid of the present invention is that it results in the unexpectedly better controlled release of various active material from tablets formed from the granules than from tablets formed from granules produced by other granulation processes.
The term polyacrylic acid is used to include various homopolymers and copolymers wherein at least 50 or 75 mole percent of the repeating units have pendant carboxylic acid groups or anhydrides of dicarboxylic acid groups. While acrylic acid is the most common primary monomer used to form polyacrylic acid the term is not limited thereto but includes generally all xcex1-xcex2 unsaturated monomers with carboxylic pendant groups or anhydrides of dicarboxylic acids as described in U.S. Pat. No. 5,349,030.
Other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description of the preferred embodiments.