Technical Field This invention relates to a process for isolating copolymers comprising repeating units from 2-vinylpyridine and styrene ^' by drowning the reaction emulsion into an acetic acid solution or by adding an acetic acid/water solution to the reaction emulsion. The copolymer isolated by this technique filters and washes much more rapidly than the copolymer isolated using sodium chloride, and the wet cake has a high solids content.

Background of the Invention Copolymers of 2-vinylpyridine and styrene are known for use as a pH-sensitive coating for orally administrable drugs, nutrients, and medicaments, particularly for the ruminants (U.S. Patents 4,177,255, 4,181,708, 4,181,709, 4,181,710). These polymers when prepared by emulsion polymerization are generally precipitated by drowning into sodium chloride solution. Our experience has shown that the filtration and washing of the cake is slow and that the wet cake has % solids of 30-35%. The present invention overcomes these problems in the prior art by providing a technique for isolating the copolymer so that it filters and washes more rapidly, and the wet cake has a higher solids content. The low solids and the fact that such cake is thixotropic makes the drying process difficult. Furthermore, solubility problems have been experienced during the coating process which were traced to excessive NaCl left from the wet cake.

A typical procedure for isolating copolymer of 2-vinylpyridine and styrene calls for drowning the reaction emulsion into a sodium chloride/water solution. This affords a product that is small in particle size

and filters and washes slowly. The wet cake produced from such work-up generally has 30 to 35% solids. A method that improves the filtration, washing, and % solids of the wet cakes also saves time in the subsequent drying.

A procedure is disclosed in U.S. Patent No. 2,830,975 whereby a polymer is coagulated with an acidic material, such as sulfuric acid, hydrochloric acid, acetic acid, or the like. A highly ionizing acid is preferred. One method of effecting, coagulation is to pass separate streams of the latex and coagulant to a coagulating vessel. Another method comprises blending separate streams of the latex and coagulant just prior to the entrance of the combined streams into the coagulating vessel. The materials are agitated during coagulation and the pH of the serum is preferably maintained below a pH of 4.

Description of the Invention According to the present invention, there is provided a method of isolating copolymers comprising repeating units from vinylpyridine and styrene from an aqueous emulsion of the copolymer which comprises mixing the aqueous emulsion with an acidic material such that the mixture has a pH of 4-8 to aggregate the copolymer, and heating the resulting slurry containing the aggregated copolymer to a temperature of 55-100 ^* C for 10-120 minutes to increase the size of the aggregated particles. The method uses an acid in place of sodium chloride to break the reaction emulsion after the emulsion polymerization of the said polymer. The product isolation can be done by adding acid/water solution to the emulsion or by drowning the emulsion in an agitated solution of acid/water such that the end pH is from 4 to 8. The

aggregated product slurry becomes finely divided particles after heating from 55 'C to 85'C for 10 minutes to 2 hours. A temperature of lower than 55'C does not provide good particle size for easy filtration and washing. A temperature of greater than 85'C tends to ball up the product to an unmanageable gum. The time of heating does not appear to be very critical but longer heating provides no advantages. As an example, for _. 65/35 poly(2-vinylpyridine-co-styrene) , the preferred pH after breaking the emulsion with acetic acid is from 6 to 8. The preferred temperature is from 70 'C to 80 ^* C, and the preferred time is from 10 minutes to 30 minutes. - Such product can be filtered and washed in a much shorter time than that worked up in a sodium chloride solution. The wet cake prepared from such a process generally, has 70-95% solids and can be dried much more readily than the conventional sodium chloride workup. An additional advantage is that no material is introduced which could cause solubility problems during the coating process.

The invention is useful in reducing the manufacturing cost of the poly(2-vinylpyridine-costyrene) and the polymer is useful as a coating for drugs, nutrients, and medicaments orally administrable to ruminants.

The copolymer isolated according to the method of the present invention preferably is pol (2-vinyl¬ pyridine-co-styrene) , which is produced in an aqueous emulsion thereof by techniques, e.g., according to the teachings of U.S. 4,593,082. The copolymer may contain repeat units from other monomers in addition to vinylpyridine and styrene, and the term copolymer as used herein is intended to include such polymers.

Preferably the acidic material used in the isolation process is a water-soluble inorganic acid, or a C -C ₄

carboxylic acid. The preferred water-soluble organic acid is acetic acid. Examples of other suitable carboxylic acids include formic acid, pro ionic acid, butyric acid and isobutyric acid. The copolymer of vinylpyridine and styrene is produced by the well-known latex system (U.S. 4,593,082) which results in the copolymer being in an aqueous emulsion. The first step of the present process is to mix the emulsion with one of the acids described above until the mixture has a pH of 4-8, preferably 6-8.

Such lowering of the pH causes the copolymer to aggregate. The resulting slurry containing the aggregated or precipitated copolymer is then heated by conventional means to a temperature of 55-100"C (preferably 65-85 ^* C) for 10-120 minutes (preferably 10-20 minutes) during which time the size of the aggregated particles is increased.

The following examples are submitted for a better understanding of the invention.

Examples la, lb and lc illustrate the high solid content obtained from the wet cake isolated by acid work-up according to this invention.

Example la - Preparation of 65/35 2-Vinylpyridine/

Styrene Monomers

A mixture of 2-vinylpyridine (650 g) and styrene

(350 g) is washed twice with 5% NaOH in water (each

320 mL) and twice with water (each 640 L) to remove the inhibitors. The monomers which contain some residual water have a volume of 1039 mL and are used in b below.

Example lb - Preparation of 65/35 Poly(2-Vinyl- pyridine-co-styrene) Emulsion

The pre-washed 65/35 2-vinylpyridine/styrene (1000 g from a), pre-mixed 5% sodium oleate (33 g sodium oleate in 660 mL of distilled water) , 50% NaOH

(30.0 g) and distilled water (5340 mL) are charged into a 10-litre 3-neck flask equipped with a mechanical stirrer, thermometer and a condenser. The reaction mixture is purged with nitrogen and sodium persulfate (5.0 g) is added. The reaction is heated to 55 'G. under nitrogen and the reaction is maintained at 55 'C for 8 hours. The reaction emulsion at this point has a pH of 12.9. The batch is divided into four equal parts, of which one is worked up as c.

Example c - Work-up by Acetic Acid .

An emulsion equivalent to 250 g polymer from b is added to an agitated solution of acetic acid (10 g) and distilled water (600 mL) in a 5000 mL 3-neck flask equipped with a mechanical stirrer and a condenser.

The emulsion breaks into a thick slurry of cottage-cheese like product with a pH of 8.1. The reaction mixture is heated to and maintained at 80 'C for 15 minutes. The batch is cooled to room temperature over 2-4 hours. The product is filtered on a 25 cm Bϋchner funnel with house vacuum and washed with distilled water (6000 mL) . The filtration is nearly instantaneous and the washing takes 1 minute and 14 seconds. The wet cake is pulled further on the Bϋchner funnel for 3 hours to give solids of 76.1%. The cake is dried in a 55"C oven to give.241.0 g white powder (96.4% yield) with an inherent viscosity of 1.165.

Examples 2a, 2b and 2c are submitted for reference. They illustrate the low solid content of wet cake isolated by the conventional sodium chloride work-up.

5 Example 2a - Preparation of 65/35 2-Vinylpyridine/ styrene Monomers

Same procedure as in example la is followed.

Example 2b - Preparation of 65/35 Poly(2-vinyl- 10 pyridine-co-styrene) Emulsion

Same procedure as in example lb is followed.

Example 2c - Work-up by Sodium Chloride/Water

An emulsion equivalent to 250 g polymer from b is

15 added to an agitated solution of sodium chloride (48 g) and distilled water (600 mL) in a 5000 mL 3-neck flask equipped with a mechanical stirrer and a condenser. The emulsion breaks into a thick slurry of cottage-cheese like product. The reaction mixture is heated to and ^' 20 maintained at 80"C for 15 minutes. The batch is cooled to room temperature over 2-4 hours. The product is filtered on a 25 cm Bϋchner funnel with house vacuum and washed with distilled water (6000 mL) . The filtration takes 3 minutes 39 seconds and the washing takes 46

25 minutes 29 seconds. The wet cake is pulled on the

Bϋchner funnel for 3 hours to give solids of 39.8%. The cake is further dried in a 55"C oven to give 250.0 g white powder (100% yield) with an inherent viscosity of 1.132.

30

Inherent viscosity (I.V.) is determined in a 60/40 mixture (wt/wt) phenol/tetrachloroethane at 25"C at a concentration of 0.5 gram polymer per 100 mL.

Unless otherwise specified, all parts, percentages,

35 ratios, etc. are by weight.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.