Wet strength

1. Check addition rate.
— as more wet strength resin is added, wet strength increases but at diminishing rate of return.
2. Both pulp consistency and contact time affect resin retention
— add as far back in the system as practical.
— increased residence time improves retention and wet strength response of alkaline curing resins.
— absorption is more rapid and complete in thick stock; efficiency falls off as stock consistencies are lowered.
3. More highly refined stock develops more wet strength as higher surface area of fines permits better resin retention.
— fines retention is maximized as system charge (zeta potential) approaches zero.
— good first-pass retention is important to resin efficiency.
— fines typically hold 4 to 8 times the cationic resin as long fibers so maximizing fines retention is imperative.
— at low levels of resin addition, effect of refining is less pronounced as there is sufficient surface area for sorption.
4. Ensure good, rapid mixing of diluted resin with stock.
5. Use fresh water for dilution with good agitation.
— modest hardness levels (up to »100 ppm) can improve resin performance.
— soft water (< 100 ppm) may reduce efficiency of cationic additives.
— high hardness interferes with resin retention as calcium ions can complex free carboxylate groups and also reduce swelling.
— salts (high conductivity) compete with fiber for resin and compete with resin for reactive sites on the fibers.
— carbonate and bicarbonate ions can give both higher wet and dry strength from a given dosage for PAE and epoxide resins.
6. PAE and other alkaline curing wet strength agents are generally used in a PH range of 6 to 8.
— efficiency is adequate in pH range of 5 – 9.
7. Low pH impedes self cross-linking of resins and carboxylate groups on fiber are converted to electrically neutral carboxyls.
— effectively reduces the number of sites to retain and react with the wet strength resin.
— high acidity or alumina content reduces alkaline curing wet strength efficiency or may retard cure rate.
— the retention of cationic additives is proportional to the charge on the fibers, i.e., carboxyl content.
— cationic additives are more effective at neutral or slightly alkaline conditions.
8. In hot stock with long exposure times, both polyamide and urea formaldehyde resins lose efficiency.
9. Anionic trash anywhere in the system is detrimental to wet strength efficiency.
— especially interferes with sorption of polyamide resin on pulp.
— lignosulfonates may form precipitates or complexes of variable composition with resins; these have limited attraction to pulp.
— poorly washed pulps interfere with wet strength efficiency.
10. Wet strength agents can alter dye intensity by acting as retention aids.
— excess optical brightener (a direct dye) interferes with resin retention by tying up potential sites.
— cationic resins act as scavengers for any color bodies in water supply and adversely affect brightness.
11. Anionic dyes can interfere with cationic resins, especially in deep colors, as dye forms a complex with the resin which is then poorly retained by the pulp.
— may be partially offset by using CMC with polyamide resin in ratio of 1:3 and add dye to the system last.
— increased alkalinity, i.e., with bicarbonate, reduces dye interaction with wet strength agents.
12. Presence of other cationic agents (starch, optical brighteners, etc.) competes for sites and can reduce cationic wet strength efficiency.
13. Chlorine interactions:
— bleach plant (or color stripping) residuals should be avoided as they can break down the freshly added wet strength resin.
— system should be treated with antichlor such as sodium sulfite; excess sulfite will attack functional groups of PAE and other amine-epichlorohydrin wet strength resins.
— free chlorine and sulfite ions should be avoided.
14. Drying conditions are important as most wet strength resins are thermo-setting.
— too rapid drying (hot early cylinders) impairs efficiency; gradient drying is best.
15. Zeta potential:
— a slightly negatively charged system favors retention of cationic additives.
16. The relative effect of different pulp types influences wet strength performance in the following decreasing order:
— CTMP, unbleached kraft, bleached HWK, bleached SWK, recycled fiber, bleached sulfite, NSSC.
17. Most wet strength problems can be related to general dry strength problems. Also may be due to:
— poor resin quality.
— improper resin addition practices.
— low resin retention.
— poor curing resin.
— improper resin dose rate.