Sodium α-hydroxyalkyl phosphinates as inhibitors of photoyellowing of high-yield pulps

Previous work on sodium hydroxymethylphosphinate as an inhibitor of photoyellowing of mechanical pulps was extended to other sodium salts of α-hydroxyalkyl phosphinic acids. Sodium α-hydroxyethyl phosphinate, sodium α-hydroxybutyl phosphinate, sodium bis(α-hydroxybutyl) phosphinate, sodium (α-hydroxyl, α-methyl)ethyl phosphinate, sodium bis((α-hydroxyl, α-methyl)ethyl) phosphinate, and sodium (α-hydroxyl, α-methyl)butyl phosphinate were prepared from hypophosphorous acid and the appropriate aldehydes or ketones. The products were characterized by ³¹P NMR (nuclear magnetic resonance) spectroscopy. All the compounds were stable, colourless, odourless and showed some inhibition of photoyellowing on bleached chemithermomechanical pulp sheets. The primary α-hydroxyethyl phosphinate was the most effective, and the secondary bis(α-hydroxybutyl) phosphinate was the least effective in inhibiting photoyellowing. The existence of hydrogen on the carbon attached directly to the phosphorus of α-hydroxyalkyl phosphinates has a small positive effect on brightness stabilization, compared to those compounds with substitution of the hydrogen by a methyl group. The inhibition of yellowing correlated qualitatively with a decrease in the formation of chromophores around 360 nm during irradiation, as measured by diffuse reflectance UV spectrometry.