Merkon P-390 is described as an acrylic copolymer. It is said to consist of approximately 43 percent polymethyl methacrylate, 26 percent chlorinated polyethylene, 16 percent styrene, 8 percent butyl acrylate and 6 percent additives. The product will be imported as a white powder. You state that the product will be used as an additive in the production of polyvinyl compounds.
[1] Please importantly note that only the relevant portion of the related is displayed above. You may refer to the said customs ruling for the entire ruling explanation.
[1]. https://rulings.cbp.gov/ruling/N320540