This invention generally relates to the pelletization of polymers. More particularly, the invention relates to the pelletization of polyolefin copolymers and elastomeric materials. It further relates to the improvement in the ease of fabrication of extruded or molded articles, or otherwise shaped objects, formed from the melt processing of said polymeric materials.
Pelletization of polyolefin copolymer, for example, polybutylene copolymer, especially high melt flow, high ethylene (ethylene content higher than 0.75 wt %), has been very difficult when using either the underwater face cutter or the strand cutting pelletizer. The problem appears to relate to the slow crystallization rate of these polymers which exhibit extreme tackiness in pelletizing. It is known that as the pellets leave the cutting blades, they are very clear and tacky, and collide with other pellets to form agglomerates. With a longer residence time, the pellets change to an opaque color, as they complete their crystallization, become hard and lose their tackiness. These agglomerates frequently plug the pelletizer bowl and the spin dryer. The extrusion line has to be shut down in order to clean the plugged section resulting in undesirable production interruptions. Also, the high number of pellet marriages shown in the final product is not acceptable in the customers' fabrication process.
Using polyethylenes (such as HDPE powder) as an external pelletizing aid to eliminate the surface tackiness is known in the art. The success of this method depends on good dispersion and suspension of HDPE in the slurry water. Therefore, it generally requires the addition of extra portable mixers to the slurry water tank to keep the HDPE from foaming. This method of reducing surface tackiness is very messy and labor intensive. Even under the best of circumstances, a significant fraction (about 20%) of the production has to be discarded due to poor pellet quality and pellet agglomerations. Thus, there continues to exist the need for new and better methods and/or agents for preventing or minimizing stickiness of polymers and copolymers.
The use of a nucleating agent(s) to accelerate crystallization is also known in the art. However, the art clearly teaches that there is no evidence of a universally strong nucleating agent for all polymers, and that an effective nucleating agent for one polymer may be ineffective for even a closely related polymer. The art further teaches that even a compound closely related to an effective nucleating agent for one polymer may be ineffective for that same polymer. Thus, it can be correctly said that no cross-utility exists or is expected for nucleating agent(s), and that the discovery of new and effective nucleants for polymers, especially if such nucleants exhibit synergism continues to be unexpected and desirable.
USPTO Backgrounds
USPTO Backgrounds