of C 5 -C 18 perfluorocarbon alkanes, perfluorocarbon amines, and/or perfluorocarbon ethers (CAS No. 86508–42–1) (provided for in subheading 3824.99.92) Free No change No change On or before 12/31/2020 . 946. Certain specialty monomers Subchapter II of chapter 99 is amended by inserting in numerical sequence the following new heading: 9902.10.58 Mixtures of 2-methyl-N-[2-(2-oxoimidazolidin-1-yl)ethyl]prop-2-enamide (CAS No. 3089–19–8), 2-methylprop-2-enoic acid (CAS No. 79–41–4), 1-(2-aminoethyl)imidazolidin-2-one (CAS No. 6281–42–1) and benzene-1,4-diol (CAS No. 123–31–9) (provided for in subheading 3824.99.92) Free No change No change On or before 12/31/2020 . 947. Silicon dioxide Subchapter II of chapter 99 is amended by inserting in numerical sequence the following new heading: 9902.10.59 Amorphous synthetic silica gel, silanized (CAS No. 112926–00–8) (provided for in subheading 3824.99.92 Free No change No change On or before 12/31/2020 . 948. Sodium ethylate Subchapter II of chapter 99 is amended by inserting in numerical sequence the following new heading: 9902.10.60 Sodium ethylate (sodium ethanolate) (CAS No. 141–52–6), solution in ethanol (CAS No. 64–17–5) (provided for in subheading 3824.99.92) Free No change No change On or before 12/31/2020 . 949. Organo-modified siloxanes combined with silica Subchapter II of chapter 99 is amended by inserting in numerical sequence the following new heading: 9902.10.61 Siloxanes and silicones, di-Me, polymers with Me silsesquioxanes and polypropylene glycol mono-Bu ether (CAS No. 68554–65–4), greater than 70 percent by weight, with less than 8 percent percent by weight siloxanes and silicones, di-Me, polymers with Me silsesquioxanes and polyethylene-polypropylene glycol mono-Bu ether, less than 8 percent by weight poly(oxy(methyl-1,2-ethanediyl)), α-butyl-ω hydroxyl (CAS No. 9003–13–8), less than 2 percent by weight oxirane, methyl-, polymer with oxirane, monobutylether (CAS No. 9038–95–3), less than 8 percent by weight silanamine, 1,1,1, trimethyl (CAS No.