PSE 322 Chapter 12 - Wet Strength Agents

relative wet strength

compares the strength of the wet paper vs. the corresponding dry strength

what tests are commonly used to measure relative wet strength?

Mullen burst strength or tensile strength

Absolute wet strength

measure of wet strength

What kind of wet strength do bank notes (money) need?

high, permanent wet-strength

what kind of wet strength do Kleenex tissue need?

temporary wet-strength

what kind of wet strength do paper towels need?

moderate wet strength

what kind of wet strength do xerographic copy paper need?

no wet strength agent

How long does it take most wet strength agents to lose effect?

ten minutes to an hour of soaking

how do the most efficient cost-effective wet-strength resins work?

form a three-dimensional cross-linked structure

what formation provides the basis for effects of temporary wet-strength additives?

hemi-acetal

glyoxylated polyacrylamide (glyPAM)

a common temporary wet-strength agent

what group reacts to form a hemi-acetal?

aldehyde

aldehyde starch

works by a mechanism similar to hemi-acetal formation

blocked-reactive-group (BRG) starch

related mechanism to hemiacetal formation, but made more storage-stable by chemical modification

How to get high retention of wet-strength agents?

adding resin to thick stock, adding resin upstream of cationic starch, adding resin where pH is alkaline

what are the benefits of longer contact time?

good mixing

what are the drawbacks of longer contact time?

the resin will decompose to a greater extent

what does foam suggest?

resin is not being retained

what are common permanent wet-strength resins?

polyamidoamine-epi (PAAE), melamine-formaldehyde (MF), urea-formaldehyde (UF)

what is the best pH for polyamidoamine-epi?

5-9

what is the best pH for melamine-formaldehyde?

4.5-6

what is the best pH for urea-formaldehyde?

4-4.5

what is the cure rate for PAAE?

Moderate

what is the cure rate for MF?

moderate

what is the cure rate for UF?

slow

what are the main reactions for PAAE?

self, and COOH’s

what are the main reactions for MF?

self cross-linking

what are the main reactions for UF?

self cross-linking

what permanent wet-strength resin is favored for alkaline paper making?

PAAE

what are the three steps of urea-formaldehyde formation?

methylolurea formation, methylolurea cross-linking, cross linking in the dryer section becoming insoluble

what are the draw backs of fast-curing wet-strength resins?

broke is not easy to repulp

what is the ratio of nitrogens to carbons in MF?

twice as many nitrogens

what results when an amine and a carboxylic acid react?

an amide linkage

what group is believed to be responsible for PAAE set-strengthening ability?

azetidinum

what is the role of the extra amine groups in PAAE?

render the polymer water-soluble and cationic, and provide a point of attachment for the epichlorohydrin

what are the key properties of an azetidinium group?

they can react with carboxyl groups at fiber surfaces, they can cross-link the resin itself, and they are highly cationic (helping with retention)

how is bleached kraft pulp repulped when wet-strength is high?

add sodium hypochlorite to the cured wet-strength broke

what are other methods for repulping with PAAE present?

high hydrodynamic shear, increased temp, hypohalous acids, increased pH

how do oxidative bleaching agents impact PAAE?

resin oxidative breakdown, usually as a result of chlorine dioxide as a biocide

how does long contact with neutral to alkaline furnish impact PAAE?

hydrolysis of the resin

how does very low pH impact PAAE?

poor retention (especially in low-yield, bleached furnish) and poor cure

how do anionic colloids in the furnish impact PAAE?

neutralization of resin charge so that it doesn’t adsorb well at the fiber surface