Wet end softeners / debonders
Tissue and toweling manufacturers add debonders to the furnish in the wet end to impart bulk softness through disruption of interfiber hydrogen bonds (see Figure 1 below) and to develop surface softness through the presence of the fatty alkyl groups contained within them. Previously, softening was primarily accomplished by mechanical methods, but the current trend for premium tissue includes a combination of mechanical and chemical processes.
Softener/debonders, primarily quaternary ammonium compounds with multiple fatty alkyl groups, are cationic surfactants which are highly substantive to fibers. Their low cost allows producers to economically modify and improve the properties of tissue and towel products.
In the wet end, cationic softener/debonder specialty surfactants are added to increase bulk, improve softness, and impart antistatic properties. As such, they must rapidly and completely exhaust onto the fibers without interfering with the effectiveness of other additives such as wet and dry strength resins, retention aides, flocculents, etc. Although fines and anionic trash may interfere with performance, the softener/debonders must be effective at low concentrations in order to be cost efficient.
Softener/debonder formulations must be tightly controlled to balance increasing environmental restrictions with customer requirements for ease of handling and dispersibility. Solvents and specialty surfactants are often added to these formulations to ensure their liquidity and cold water dispersibility, but they can contribute to Volatile Organic Compound (VOC) levels and can accumulate in the white water circuit. Evonik addresses these problems by offering highly concentrated, liquid formulations based on non-flammable solvents exhibiting reduced or zero VOC contents (as determined by EPA Method 24).
Since softener/debonder formulations can be introduced at several locations in the wet end such as the head box, or preferably, the fan pump, they must not contribute to foam creation. Therefore, the specialty surfactants in the debonder formulation must be carefully selected to control foam.For wet end applications, Goldschmidt formulates debonders with low corrosivity.
Incorporation of wet end debonders into the sheet often increases release on the Yankee dryer. When this occurs, the Yankee coating package must be adjusted to maintain machine runnability.
Frequently, debonded pulps are used in towels and non-woven wipes for food service applications. As a consequence, all of the additives used to treat these pulps must be safe for indirect food contact. Recently, Evonik was the first company to successfully petition the FDA to amend food additive regulations for the safe use of imidazolinum debonders in the manufacture of paper and paperboard for use in contact with aqueous and fatty foods (21 CFR 176.170) and with dry food (21 CFR 176.180).
Requirements for Wet End Tissue and Towel Softener/Debonders:
Easy to handle liquid formulations
Compatible with other paper additives
Rapid pickup by fibers
Minimal impact on Yankee release
Concentrated to minimize solvent and surfactants
Reduced or zero VOC-content
FDA sanctioned for food service applications
Cost effective performance
Our softener/debonders can be introduced at several points in the wet end: headbox, fan pump, stuff box or machine chest. They should be incorporated into furnish at the point in the wet end which maximizes contact time with the fibers, thus allowing complete and uniform exhaustion.
Typical application dosages are 1 - 4 kg/ton of fiber, but this is highly dependent on the quality and compostion of the furnish and the desired product attributes. Fines and anionic residue are known to interfere with the adsorption and performance of the debonder, so scavangers should be first added to deal with the cationic demand of the fines and trash. The debonders should also be introduced after the wet-strength resin to enhance the performance of both additives.
Since debonders can also affect release properties on the dryer, they must be matched to the Yankee coating package to ensure that maximum benefits are achieved by both additives.
To prevent overdosing with a concentrate, formulated debonders can be easily predispersed prior to contact with the furnish. Dispersions of 1 - 6 % solids levels are prepared by adding the debonder concentrate to water that is 20 °C or warmer with adequate mixing. Care should be taken when water is added to the softener formulation, as undesired gellation may occur. For continuous, in-line mixing the softener/debonder concentrate can be pumped into the water circuit. An in-line mixer is recommended to facilitate complete dispersion.
Tests, which are necessary to assess the performance of wet end debonders, are conducted in our Technical Service Centers in Hopewell, Virginia and Steinau, Germany. In order to match the best product with the customer's furnish and performance criteria, screening evaluations are willingly conducted in the above-mentioned Tech Service Centers. Of course, plant mill trials are necessary to confirm these preliminary results.
The following tests are practiced at our technical locations:
TAPPI test method T 205 (test sheet preparation)
TAPPI test method T 402 (conditioning)
TAPPI test method T 220 (tensile strength, density)
Absorption rate and capacity
Sensory hand panels (surface softening)
Although bulk softness is related to the reduced sheet density and loss of tensile as measured by TAPPI method T 220, surface softness is more subjective and difficult to assess. This is best left to the manufacturer as it is a composite of the chemical and mechanical softening methods employed to produce the finished tissue.
Our wet end debonders are highly efficient with regard to substantivity, tensile strength modifications and absorbency maintenance. The following test results are on handsheets treated with AROSURF® PA-842 (FDA conformance) and Z-QUAT™ TO10ZH from our patented Z-Quat™ technology.Test results on wet end softeners/debonders