Low molecular weight phosphorus-containing polyacrylic acids and use thereof as scale inhibitors in water-carrying systems
The invention relates to an aqueous solution of acrylic acid polymers, obtainable by polymerization of acrylic acid in feed mode with peroxodisulfate as initiator in the presence of hypophosphite in water as solvent, wherein (i) water and optionally one or more ethylenically unsaturated comonomers are initially charged, and (ii) acrylic acid in acidic, unneutralized form, optionally one or more ethylenically unsaturated comonomers, aqueous peroxodisulfate solution and aqueous hypophosphite solution are added continuously, and (iii) a base is added on completion of the acrylic acid feed to the aqueous solution, wherein the comonomer content does not exceed 30% by weight, based on total monomer content.
1. An aqueous solution of an acrylic acid polymer, obtained by polymerization of acrylic acid in feed mode with peroxodisulfate as initiator in the presence of hypophosphite in water as solvent,
wherein:
(i) water and optionally one or more ethylenically unsaturated comonomers are initially charged; and
(ii) acrylic acid in acidic, unneutralized form, optionally one or more ethylenically unsaturated comonomers, aqueous peroxodisulfate solution and aqueous hypophosphite solution are added continuously; and
(iii) a base is added on completion of the acrylic acid feed to the aqueous solution, and wherein:
the comonomer content does not exceed 30% by weight, based on total monomer content;
said acrylic acid polymer has a weight average molecular weight ranging from 1000 to 2500 g/mol; and
at most 8% of phosphorous contained in said acrylic acid polymer is present in the form of phosphinate and/or phosphonate groups bound at a polymer chain end.
2. The aqueous solution according to claim 1 , wherein the weight average molecular weight of the acrylic acid polymer ranges from 1200 to 2500 g/mol.
3. The aqueous solution according to claim 1 , wherein the M w /M n polydispersity index of the acrylic polymer is <2.5.
4. The aqueous solution according to claim 1 having a total phosphorus content of organically and possibly inorganically bound phosphorus, wherein
(a) a first portion of the phosphorus is present in the form of phosphinate groups bound within the polymer chain,
(b) a second portion of the phosphorus is present in the form of phosphinate and/or phosphonate groups bound at the polymer chain end,
(c) possibly a third portion of the phosphorus is present in the form of dissolved inorganic salts of phosphorus,
wherein at least 70% of the total phosphorus content is present in the form of phosphinate groups bound within the polymer chain of the acrylic acid polymer.
5. The aqueous solution according to claim 1 , wherein the acrylic acid polymer is an acrylic acid homopolymer.
6. The aqueous solution according to claim 1 , wherein the acrylic acid polymer is an acrylic acid copolymer comprising up to 30% by weight, based on all ethylenically unsaturated monomers, of one or more ethylenically unsaturated comonomers selected from the group consisting of methacrylic acid, maleic acid, maleic anhydride, vinylsulfonic acid, allylsulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid, as polymerized units.
7. An acrylic acid polymer obtained from the aqueous solution according to claim 1 .
8. The aqueous solution according to claim 1 , wherein said process consists of:
(i) water and optionally one or more ethylenically unsaturated comonomers are initially charged; and
(ii) acrylic acid in acidic, unneutralized form, optionally one or more ethylenically unsaturated comonomers, aqueous peroxodisulfate solution and aqueous hypophosphite solution are added continuously; and
(iii) a base is added on completion of the acrylic acid feed to the aqueous solution.
9. The aqueous solution according to claim 1 , wherein 6-8% of phosphorous contained in said acrylic acid polymer is present in the form of phosphinate and/or phosphonate groups bound at a polymer chain end.