Merkon H-640 is described as an acrylonitrile-butyl acrylate-methyl methacrylate-styrene copolymer (CAS # 25852-38-4). It is said to consist of 61.9 percent styrene, 17.7 percent polymethyl methacrylate, 13.7 percent acrylonitrile, 4.9 percent butyl acrylate and less than 2 percent additives. The product will be imported as a white powder. You state that the product will be used as a process aid 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/N319986