3913 - Natural polymers (for example, alginic acid) and modified natural polymers (for example, hardened proteins, chemical derivatives of natural rubber), not elsewhere specified or included, in primary forms:

This HTS category, 3913, encompasses natural polymers and their modified forms that are not classified elsewhere in Chapter 39. This includes materials like alginic acid derived from seaweed, and modified natural polymers such as hardened proteins (e.g., casein plastics) and chemical derivatives of natural rubber (e.g., vulcanized latex, chlorinated rubber). These polymers are generally supplied in their primary forms, such as powders, granules, or bulk liquid states, prior to further processing into finished articles.

Unlike other categories within Section VII, which often deal with synthetic plastics (Chapter 39) or rubber articles (Chapter 40), this category specifically focuses on materials originating from biological sources that undergo chemical modification or are presented in a form not yet considered a manufactured article. It serves as a residual category for natural polymers that don't fit into more specific classifications for their raw or semi-processed states.

Within this category, the primary subdivisions distinguish between specific types of natural polymers and a general "other" category. Subheading 3913.10.00.00 specifically addresses alginic acid, its salts, and esters, reflecting their common commercial significance. The remaining products, such as modified proteins or rubber derivatives, fall under the broader "other" category, 3913.90, indicating a need for careful examination of the material's origin and modification process to ensure accurate classification.