Modacrylic is a modified acrylic fiber (typically 35–85% acrylonitrile content) with inherent flame-retardant properties, classified together with acrylic because of their chemical and structural similarity. At the SITC level they share a heading, but at the HTS 10-digit level, duty rates and any applicable trade remedy orders may differ between acrylic and modacrylic constructions. Importers should confirm the specific fiber type with the mill and verify the applicable HTS subheading accordingly.

Fiber content is typically determined by quantitative chemical analysis per AATCC Test Method 20A or equivalent ISO methods, which dissolve or separate fiber components to measure their proportional weight. For acrylic blends that visually resemble wool or cotton, laboratory testing is especially important because visual inspection alone cannot reliably confirm the fiber ratio required for this classification.