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Abstract

Cationic urea/formaldehyde resins are obtainable by condensing urea and formaldehyde in a molar ratio of from 1:1.5 to 1:3 in the presence of polymers which contain not less than 1 mol % of polymerized vinylamine units and have Fikentscher K values of from 5 to 300, in an amount of from 5 to 50 g per mole of urea in the end product, where the mixture is first a) precondensed at a pH of from 8 to 14, then acidified and b) condensed at a pH of from 1 to 5 until gel formation begins, c) then from 0.3 to 1.5 moles of formaldehyde are added per mole of urea used, d) post-condensation is carried out and e) the resin solution is subsequently neutralized. The resins thus obtained are used in papermaking as assistants for increasing the dry and wet strength of paper and result in less dulling of the whiteness of the paper.

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