Method of producing high dry strength paper and cardboard and paper and cardboard thus obtained
In a method of producing a sheet of paper and/or cardboard and similar products, prior to the formation of the sheet from a fibrous suspension, at least two dry strength agents are added to the suspension either separately in any order or mixed together and at one or more injection points, namely: a first agent corresponding to a product for Hofmann degradation in solution on a base (co)polymer comprising at least one non-ionic monomer which is selected from the group containing acrylamide and/or methacrylamide, N,N dimethylacrlamide and/or acrylonitrile; and a second agent corresponding to a (co)polymer having an anionic charge density greater than 0.1 meq/g. The Hofmann degradation product comprises an organic polymer produced at a concentration greater than 3.5% by weight, and preferably greater than 4.5% and the first and second agents are each introduced into the pulp suspension at a concentration of between 0.01 and 2% by weight of active polymer material in relation to the dry weight of the pulp suspension.
1. Production method for a sheet of paper and/or cardboard, according to which, before forming said sheet from a fibrous suspension, are added to the fibrous suspension, separately or in a mixture, in any order of addition, in one or more points of injection, at least two dry strength agents, respectively:
a first agent corresponding to a Hofmann degradation product in a solution on a base (co)polymer containing at least one non-ionic monomer selected from the group including acrylamide and/or methacrylamide, N,N dimethylacrylamide and/or acrylonitrile,
and a second agent corresponding to a (co)polymer with an anionic charge density greater than 0.1 meq/g,
wherein the Hofmann degradation product comprises an organic polymer produced at a temperature of 0 to 45° C. and at a concentration greater than 3.5% by weight, and wherein the first and second agents are added to the fibrous suspension at 0.01 to 2% each by weight of active polymer material in relation to a dry weight of the fibrous suspension.
2. Method as claimed in claim 1 , wherein the Hofmann degradation product has anionicity less than 10 moles percent.
3. Method as claimed in claim 2 , wherein the Hofmann degradation product has anionicity less than 5 moles percent.
4. Method as claimed in claim 1 , wherein the base (co)polymer has a molecular weight greater than 5,000.
5. Method as claimed in claim 1 , wherein the Hofmann degradation product is obtained by reaction of an alkaline earth hydroxide and an alkaline earth hypohalide with a hydroxide/hypohalide mole ratio between 2 and 6.
6. Method as claimed in claim 5 , wherein the hydroxide/hypohalide mole ratio is between 2 and 5.
7. Method as claimed in claim 1 , wherein the first agent corresponds to a Hofmann degradation product on a base (co)polymer containing:
at least one non-ionic monomer selected from the group including acrylamide and/or methacrylamide, N,N dimethylacrylamide and/or acrylonitrile, and
at least one unsaturated cationic ethylene monomer.
8. Method as claimed in claim 7 , wherein the at least one unsaturated cationic ethylene monomer is selected from the group including dialkylaminoalkyl (meth)acrylamide, diallylamine, methyl diallylamine and their quaternary ammonium or acid salts, and/or at least one non-ionic monomer selected from the group including N-vinyl acetamide, N-vinyl formamide, N-vinyl pyrrolidone and/or vinyl acetate.
9. Method as claimed in claim 1 , wherein, when the base (co)polymer is a copolymer, the copolymer contains monomers with anionic charges at a level less than 5 moles percent.
10. Method as claimed in claim 1 , wherein the second agent is obtained from:
1 to 100 moles percent of at least one monomer with an anionic charge,
and 0 to 99 moles percent of at least one monomer with a neutral and/or cationic charge.
11. Method as claimed in claim 10 , wherein the second agent is obtained from monomers comprising anionic monomers with a carboxylic function selected from the group including acrylic acid, methacrylic acid and their salts, etc., and/or having a sulfonic acid function selected from the group including 2-acrylamido-2-methylpropane sulfonic acid (AMPS), vinyl sulfonic acid, methallyl sulfonic acid and their salts, etc., and/or non-ionic monomers selected from the group including acrylamide, methacrylamide, N,N dimethylacrylamide, N-vinyl pyrrolidone, N-vinyl acetamide, N-vinyl formamide, vinyl acetate, acrylate esters, allyl alcohol, etc, and/or cationic monomers selected from the group including dimethylaminoethyl acrylate (DMAEA) and/or dimethylaminoethyle methacrylate (DMAEMA), quaternised or salified, diallyldimethyl ammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC) and/or methacrylamidopropyltrimethylammonium chloride (MAPTAC).
12. Method as claimed in claim 1 , wherein the second agent is branched.
13. Method as claimed in claim 12 , wherein the second agent is branched during polymerisation.
14. Method as claimed in claim 1 , wherein the Hofmann degradation product is added before the (co)polymer with an anionic charge density greater than 0.1 meq/g.
15. Method as claimed in claim 1 , wherein the Hofmann degradation product is obtained by Hofmann degradation reaction on a base (co)polymer at a concentration greater than 20% by weight.
16. Method as claimed in claim 1 , wherein the Hofmann degradation product comprises an organic polymer produced at a concentration greater than 4.5% by weight.