nonwovens containing immobilized superabsorbent polymer
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AbstractIn the early 1980s Johnson & Johnson developed a revolu-
tionary process for the production of webs containing superab-sorbent polymers (SAP) produced by in-situ polymerization ofpartially neutralized acrylic monomers directly on a syntheticnonwoven substrate [1, 2, 3, 4]. A fresh look at this forgottentechnology will be presented from both a manufacturing andapplication perspective. In particular, In-situ, SAP-containingnonwovens offer many unique properties for application in thepersonal hygiene industry, such as, improved fluid acquisition,permeability, compressibility and pH control. These materialsalso provide a more homogenous SAP distribution, eliminate theneed for SAP powder handling and have superior wet integrityas compared to conventional fluff pulp/SAP air laid structures.This technology also offers some unique opportunities fordesigning and manufacturing profiled absorbent articles withspecific zones tailored to perform specific functions.
IntroductionTypically, conventional superabsorbent powder polymer pro-
duction begins with solution polymerization of partially neutral-ized acrylic acid along with a small amount of a crosslinkingagent in water. The polymerization results in a water insoluble,water swellable gel containing approximately 25 to 40 % solids,which must then be cut, dried, milled and sifted to produce apowdered SAP product with a typical particle size rangingbetween 100 to 800 m. The sifting operation typically generatesa fines stream that must be recycled back into the productionprocess creating a production bottleneck. The finished SAPproduct is then shipped to a hygiene-industry converter where itis blended with fibrillated wood fluff to form the absorbent corestructure of a personal hygiene article such as a diaper.
In an in-situ SAP process, the partially neutralized acrylic acidmonomer solution is applied directly to a nonwoven substrateand polymerized. The web may be fed to the process either as apre-manufactured roll good or, preferably, made in-line from
bulk staple fiber using a carding operation. The monomer solu-tion may be applied to the web using a variety of applicationtechniques such as brush coating, pressurized liquid spray, air-assisted spray or airless spray. Initiation of the polymerizationmay be carried out by using a redox package, thermal initiation,UV, electron beam radiation or a combination of methods.Partial drying of the web may be achieved using the heat of poly-merization if the acrylic acid concentration of the monomer solu-tion is above 30%. The moisture content of the final product maybe adjusted to the desired level using either a through-air, airflotation, or forced-air infrared dryer. The dried product is thenmechanically softened, slit to the desired use width and wound.A schematic comparison of a conventional SAP productionprocess and an in-situ process is shown in Figure 1.
The in-situ SAP process offers a number of potential advan-tages for hygiene converters over conventional technology. Sincethe process produces stable, immobilized SAP structures, it elim-inates the need for SAP powder handling and the associateddust exposure issues. It also offers the potential to provide moreuniform SAP distribution in absorbent core structures and willremain stable during transport. The immobilized SAP particlesalso remain stable even in the hydrated state. Other possibilitiesinclude zoned SAP gradients and strategic placement of SAPswith different performance properties within the core structure.The technology further offers the possibility of reducing produc-tion costs through the minimization of SAP processing steps.
Nonwoven SubstrateOne of the major objectives of the in-situ process is the forma-
tion and bonding of discrete SAP micro-droplets to individualweb fibers as shown in Figure 2. An optimized nonwoven struc-ture is critical to the achievement of this state. Typically, a high-loft nonwoven structure is required, depending on the desiredSAP loading and application. Key substrate properties includethe basis weight, bulk density, loft and porosity. A delicate andintricate balance of these properties must be maintained in order
Nonwovens ContainingImmobilized SuperabsorbentPolymer ParticlesBy Darryl L. Whitmore, BASF Corp., Portsmouth, Virginia
35 INJ Fall 2003
to avoid saturation of the web structure during the monomercoating process. Saturation, where droplets of monomer solutioncoalesce on the web, leads to agglomeration and film formation.Figure 3 shows the functional relationship between SAP basisweight and weight percent loading for 55, 80 and 100 gsm sub-strates at a constant substrate density of 0.018 g/cc. Once the sat-uration zone is reached for a given substrate, film and agglomer-ate formation begins to occur and negatively impacts the flexibil-ity of the structure. In addition, this phenomenon alsoadversely affects the absorbency rate of the applied SAP.
The monomer coating process is also critical to SAPdroplet formation. For spray application, the degree ofatomization is controlled by: (1) the viscosity of thecoating (the higher the solution viscosity with the highshear rate encountered going through the nozzle ori-fice, the larger the particle size), (2) atomization gaspressure (higher gas pressure giving smaller particlesize), (3) diameter of the nozzle orifice (smaller orificegiving smaller particles), (4) pressure forcing the coat-ing through the orifice (higher pressure leading tosmaller particle size), and (5) surface tension (lowersurface tension yielding smaller particle size) . Bymaintaining a critical balance among all of these para-meters, the SAP particle size on the web can be con-trolled. Both fiber type and finish also play a crucialroll in droplet formation. As shown in Figure 4,hydrophobic fibers and finishes promote more spheri-cal droplet formation, while hydrophilic types tend topromote film formation of the applied hydrophilicmonomer mixture.
Other factors to consider when choosing a nonwoven for in-situ SAP polymerization include flexibility, softness, compress-ibility, resilience and elasticity. For hygiene applications, the finalproduct must, of course, remain soft and flexible even though itis generally compressed to produce a thin structure. Resiliencyand elasticity are important aspects of the nonwoven in that thesubstrate must be able to expand as the SAP particles begin toswell. Any impedance to the swelling of the SAP particles in the
36 INJ Fall 2003
Figure 1PAD FORMATION CONVENTIONAL VS. IN SITU
Figure 2IN-SITU SAP POLYMERIZATION ON
A FIBROUS SUBSTRATE
SAP Particle (
xyz directions may prohibit the SAP from achieving its fullabsorptive capacity.
ApplicationsOne unique property of in-situ SAP structures is their ability to
rapidly generate a large, mechanically stabilized pore volume,which makes the structure become highly permeable . In-situmaterials provide such a mechanism through the swelling of theimmobilized SAP particles following exposure to fluid. Both thedegree and rate of expansion may be optimized by adjustingsuch parameters as: the degree of loading of SAP particles on theweb, the SAP particle size, the degree of neutralization of the SAPparticles and their respective crosslink density. The degree ofexpansion is an indication of the generated pore volume avail-able for fluid uptake (i.e. larger volume correlates with better per-formance) and by increasing the speed at which the pore volumeis generated, the likelihood of leakage during rapid fluid appli-cation is diminished.
The degree and rate of expansion of in-situ materials may bedetermined by measuring the free swell expansion volume(FSEV) . The FSEV is determined by measuring the height(thickness) change, in millimeters, of a compressed, 6.0 cm diam-eter circle of the test material during hydration using a single 20ml dose of 0.9% saline under an applied load of 0.6 KPa. The
thickness of the sample as a func-tion of time is measured with theaid of a linear variable differentialtransducer (LVDT) interfaced to acomputer (additional details aboutthis method may be found in ref-erence 6). The results obtained foran in-situ material, containing 100gsm of SAP applied to a 55 gsm,thermally bonded high-loft non-woven, is shown in Figure 5a. Asimilar test, expansion volumeunder load (EVUL), was also con-ducted on the same sample usingan applied load of 3.0 KPa and isshown in Figure 5b. As shown inFigures 5a and 5b, the in-situ mate-rial expands quite rapidly oncehydrated, even under an appliedload of 3.0 KPa.
Following both the FSEV and EVUL tests, the hydrated in-situsample was removed from the sample cell and the free fluid con-tained within the void spaces of the material was removed byblotting the sample between two stacks of filter paper under anapplied load of 3.0 KPa for two minutes. The resulting weightloss allowed an estimation of the void volume of the sample. Thecorresponding gel volume, measured by the weight change ofthe dewatered sample, gives the amount of the fluid containedwithin the in-situ SAP particles on the web. The results of thisdetermination are shown in Table I. As can be seen from the data,the in-situ material generates a substantial pore volume com-pared to the uncoated high loft nonwoven substrate and the SAPparticles provide fluid storage.
Since in-situ materials swell rapidly and generate a large openpore volume, they lack sufficient capillary pressure to effectivelywick and distribute fluids. This limitation is effectively overcomethrough the formation of laminate structures with cellulosicmaterials. Suitable laminates may be prepared by either air lay-ing fluff pulp directly on the in-situ web or by simply laminatingthe webs with an