21 C.F.R. § 177.1640 ("Polystyrene and rubber-modified polystyrene") permits the use of rubber-modified polystyrene as a component of a food container. Under Section 177.1640(a)(2), rubber-modified polystyrene may be said to include basic polymers produced by combining styrene-butadiene copolymers with polystyrene and/or polybutadiene with polystyrene, either during or after polymerization of the polystyrene, provided that the finished basic polymers contain at least 75 weight percent of total polymer units derived from styrene monomer. We note that, in accordance with paragraph (c)(2), rubber-modified polystyrene basic polymers may not contain more than 0.5 weight percent residual styrene monomer, as determined by the method set forth in Section 177.1640(d). Depending on the actual intended use, residual styrene may need to be considerably lower than the level based on suitable purity requirements.