Compositions and processes for paper
Compositions and processes that enhance the brightness and improve color of pulp or paper when applied during different stages of the papermaking process involving optical brighteners are identified. Used in combination with optical brighteners and/or chelants the described agents produce a synergistic effect not previously identified in the paper process.
1. A method of activating an optical brightening agent, the method comprising the steps of:
contacting an optical brightening agent with a reducing agent before the optical brightening agent is added to pulp in the wet end of a papermaking process or tissue making process wherein the reducing agent is in an amount of 0.05 to 2% on oven-dried pulp.
2. The method of claim 1 in which the reducing agent is one selected from the list consisting of: sulfites, bisulfites, metabisulfites (pyrosulfites), sulfoxylates, thiosulfates, dithionites (hydrosulfites), polythionates, formamidinesulfinic acid and salts and derivatives thereof, formaldehyde bisulfite adduct and other aldehyde bisulfite adducts, sulfinamides and ethers of sulfinic acid, sulfenamides and ethers of sulfenic acid, sulfamides, phosphites, and thiophosphites and wherein said bisulfites are selected from monobasic metal salts of sulfurous acid and said sulfites are selected from dibasic metal salts of sulfurous acid provided said reducing agents are not used in combination with an acid of formula:
where R 1 =—CHOHCH 2 OH or —CHOHCOOH, or salts thereof.
3. The method of claim 2 in which the optical brightening agent is added to lignin free bleached kraft pulp material.
4. The method of claim 3 in which the optical brightening agent is added to bleached kraft pulp material in a papermaking or tissue making process (a) before or after the fan pump to the paper machine headbox; (b) to the paper machine white water.
5. The method of claim 1 in which additional reducing agent is added after a wet-end of the papermaking process is complete.
6. The method of claim 1 in which the reducing agent chemically alters at least one quenching group in the paper such that the quenching group absorbs less of the light that is emitted by the optical brightening agent.
7. The method of claim 6 in which the quenching group is a carbonyl group in the hemicellulose component of the pulp.
8. The method of claim 1 in which the paper is made from bleached pulp.
9. The method of claim 1 in which the paper is made from non-bleached pulp.
10. The method of claim 2 in which the reducing agent is one selected from the list consisting of: metabisulfites (pyrosulfites), sulfoxyiates, thiosulfates, dithionites (hydrosulfites), polythionates, formamidinesulfinic acid and salts and derivatives thereof, formaldehyde bisulfite adduct and other aldehyde bisulfite adducts, sulfinamides and ethers of sulfinic acid, sulfenamides and ethers of sulfenic acid, sulfamides, phosphites, and thiophosphites.