3911.90.91 - Other

This HTS category, 3911.90.91, covers "Other" products falling under the broader heading of petroleum resins, coumarone-indene resins, polyterpenes, polysulfides, polysulfones, and other specified materials, when not elsewhere classified. Specifically, it acts as a residual category for these types of resins and polymers that do not fit into more precisely defined subheadings within Chapter 39. The classification here is applied to products that are in their primary forms, such as powders, granules, or bulk liquids, and are not yet manufactured into finished articles.

Distinguishing this category from its sibling, 3911.90.70.00 (Chlorinated synthetic rubber), is crucial. While both fall under the umbrella of synthetic polymers, this "Other" category explicitly excludes synthetic rubber that has undergone chlorination. Therefore, 3911.90.91 is reserved for synthetic polymers and resins not defined by specific chemical modifications like chlorination, and which are not specifically enumerated in preceding subheadings.

The subcategories under 3911.90.91 further refine the classification based on the thermal behavior of the polymers. 3911.90.91.10 pertains to "Thermoplastic" materials, which can be repeatedly softened by heating and hardened by cooling. Conversely, 3911.90.91.50 encompasses "Thermosetting" materials, which undergo an irreversible chemical change upon heating, becoming permanently hardened and generally unable to be re-melted or reshaped. This distinction is based on the inherent physical properties and processing characteristics of these resins.