3907.91 - Unsaturated:

This HTS category, 3907.91, encompasses unsaturated polyesters, which are a class of polymers characterized by the presence of double bonds within their molecular structure. These unsaturation points are crucial as they allow for cross-linking reactions, typically through copolymerization with vinyl monomers, leading to the formation of rigid, thermosetting plastics. Common applications for these materials include fiberglass reinforced plastics used in boat hulls, automotive parts, and construction materials, as well as casting resins and coatings.

In contrast to its sibling category, 3907.99, which covers "Other" polyesters not otherwise specified, 3907.91 is specifically reserved for those polyesters that possess an unsaturated nature, indicating their potential for further chemical reaction and curing. This distinction is fundamental for accurate classification, as the presence or absence of unsaturation dictates the material's processing characteristics and ultimate properties.

Within this category, a key subdivision is the classification of allyl resins. These are a specific type of unsaturated polyester derived from allyl alcohol. The subcategory 3907.91.50.00 further refines this by covering "Other" allyl resins, implying that there may be more specific sub-classifications for allyl resins based on particular compositions or forms that fall under the broader umbrella of unsaturated polyesters.