Granular polymer additives and their preparation

#This invention relates to the provision of improved polymer additives—i.e., additives which are for use in polymers—where the additives are in granular form such as pellets.

#A wide variety of particulate additives are used in thermoplastic polymers in order to improve the properties of the polymer and/or the utility of products formed from such polymers. Included among the types of particulate polymer additives used are, for example, such substances as antioxidants, flame retardants, flame retardant synergists, thermal stabilizers, UV stabilizers, nucleating agents, acid neutralizers, polymer clarifiers, and the like. In order to facilitate blending operations it is desirable to provide additives in a granular form because many additives can cake up or form rat-holes in feed hoppers, and/or feed unevenly through metering equipment. Moreover, certain additives, especially finely-divided additives, often used in the manufacture of polymers, such as high melting nucleating agents and inorganic acid neutralizers can create hazardous airborne dusts during handling and blending operations.

#Various methods for converting polymer additives into granular forms have been described heretofore. Such methods include melting at least one component of a dry blend of additives to thereby bond the particles of the blend together, or including in a blend of the additives a special component such as a wax, fatty acid, a compound containing a fatty acid chain or fatty alcohol chain, or metal salt of a fatty acid, and then converting the dry blend into granules or pellets by compacting or milling the blend. All such methods require use of extraneous components to serve as binders, which components are not necessarily desired as components in the finished polymer composition, and which may actually interfere with product specifications of the polymer producer. Moreover, use of some previously used binders can detract from performance properties of the host polymer. And, in some cases the scope of the additives that can be converted into granules or pellets is quite specific and thus not of widespread utility.

#4000 Grams of 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl4-hydroxybenzyl)benzene (ETHANOX® 330 antioxidant obtained from Albemarle Corporation) and 444 grams of anhydrous acetone were combined in a Kahl Model KDHJ-20 batch kneader and then blended for about 3 minutes. The resulting pasty powder blend was then transferred to a polyethylene bag, the bag was sealed and the material was stored for about 15 minutes. The blend was then manually fed to a Kahl Model 14-175 pellet mill operating at 100 rpm and equipped with a die plate having 2 mm diameter holes and a 6 mm pressway length. The pelletized product was then dried in a 200° F. (ca. 93° C.) oven for about 2 hours to yield a product consisting of 94% pellets and 6% fines (−20 mesh particles). The resulting pellets were relatively very hard, in the sense that considerable force was required to break them down into powder when subjected to manual compression and therefore could be handled with typical conveying equipment with minimal attrition. For comparative purposes neat ETHANOX® 330 antioxidant powder was processed on a pellet mill operating under the same conditions as above described but in which no acetone was introduced. The products from the pellet mill consisted of nearly all powder with a few very soft pellets. This comparative example illustrates that the processing solvent such as acetone of the instant invention is necessary to impart the desired hardness to the pellets.

#Fourteen batches of feed material were prepared as follows: 1.33 kg of ETHANOX® 330 antioxidant powder and 2.67 kg of tris(2,4-di-tert-butylphoshite (Irgafos® 168 powder) (obtained from Ciba Specialty Chemicals Corporation) were combined in a Kahl model KDIIJ-20 batch kneader then dry blended for 5 minutes. 571 Grams of acetone was added to the powder in the kneader and blending was then continued for an additional 3 minutes. The resulting pasty solid blend was discharged from the kneader into a polyethylene bag which was then sealed. The fourteen batches of feed material were then manually fed to a Kahl Model 33-390 pellet mill operating at 80 rpm speed and equipped with a die plate having 3 mm diameter holes and a pressway length of 9 mm. The product obtained from the pellet mill was dried in a 200° F. (ca. 93° C.) oven for about two hours. Dry sieving of the dried product with a US Standard mesh screen indicated that the product consisted of 95.8% pellets (+12 mesh) and 4.2% fines (−12 mesh). The dried pellets exhibited adequate hardness and thus were judged suitable for handling with conventional conveying equipment without significant attrition.

#A pelletized blend composition consisting of ETHANOX® 330 antioxidant blended with Irgafos® 168 secondary phosphite antioxidant, dihydrotalcite from Kyowa Chemical Company and glycerol was prepared as follows: 2.63 kg of ETHANOX® 330 antioxidant, 1.215 kg of Irgafos® 168 and 0.790 kg of dihydrotalcite were combined in a Kahl Model KDHJ-20 batch kneader and dry blended for 5 minutes. 0.263 kg of glycerol and 0.556 kg of anhydrous acetone were added to the kneader and blending was continued for an additional 3 minutes. The pasty mass from the kneader was transferred to a polyethylene bag for about 15 minutes storage. The mass was then manually fed to a Kahl Model 14-175 pellet mill operating at 100 rpm and equipped with a die plate having 3 mm diameter holes and a pressway length of 9 mm. The product from the pellet mill was collected and dried at 200° F. (ca. 93° C.) for about 2 hours. The dried product consisted of 93.9% pellets and only 6.1% fines (i.e., −12 mesh particles). The dried pellets were subjected to manual characterization and judged to have very good hardness and therefore good resistance to particle attrition during pellet conveying operations.