Heading 3904.10 covers PVC that has not been mixed with any other substance — essentially the pure homopolymer resin as produced. Once the resin is compounded with plasticizers, stabilizers, fillers, or other additives, it falls under 3904.21 (nonplasticized mixtures) or 3904.22 (plasticized mixtures). The distinction turns on whether any compounding has occurred prior to import.

K-value (a measure of degree of polymerization) is a critical commercial specification: lower K-values (e.g., K-57 to K-65) are preferred for rigid applications like pipe and profiles, while higher K-values (K-67 to K-80) suit flexible applications. All grades classify identically under 3904100000, but buyers must specify K-value in purchase contracts and verify it against supplier certificates of analysis, as mismatched grades can cause processing failures.