Chemical derivatives of natural rubber in primary forms are explicitly included in heading 3913, while unmodified natural rubber falls in the rubber chapter. The test is whether the chemical modification has given the material the character of a plastic rather than a rubber — consult the chapter notes to Section VII and Section VI, and consider a binding ruling if the degree of modification is borderline.

For pharmaceutical-grade alginates, importers should retain the manufacturer's certificate of analysis confirming chemical identity (alginic acid or a specific salt/ester), the product's CAS number, and any pharmacopoeia grade designation. FDA may review pharmaceutical excipient imports, so ensuring the supplier is registered and the product meets USP or equivalent standards is advisable alongside the customs entry.