Superabsorbent particles

The elastic modulus of the swollen superabsorbent polymer is important for several reasons. The powdered polymer is typically used in a physical blend with fibers such as cellulose and thermoplastic binder fibers. The resulting structure is called the absorbent core. The absorbent core relies on pore spaces between the fibers and polymer particles to provide liquid transport volume for the overall structure. When the swollen superabsorbent particles have low modulus, they are easily deformed by body pressures and can fill in the interfiber and interparticle pores. This prevents liquid flow and wicking of liquid into the drier portions of the core. Higher modulus limits the deformation of the particles and helps maintain the pore space. 

The superabsorbent can be designed to absorb higher amounts of liquids (with less retention) or very high retentions (but lower capacity). In addition, a surface crosslinker can be added to the superabsorbent particle to help it move liquid while it is saturated. This helps avoid formation of gel blocks , the phenomenon that describes the impossibility of moving liquids once a SAP particle gets samrated. 

Superabsorbent material was also reported to play a role as a matrix material or binder. Superabsorbent particles can occlude odor control additives, such as zeolites (15). 

Agglomerated particles have been made from metal oxides and fine superabsorbent particles (20) and also from clays and fine superabsorbent particles (21,22). 

The previous swelling equations are valid for molecularly homogeneous systems. However, some polymer gels contain pores or voids which contribute to the volume of the system. Such is the case with many porous ion-exchange resins or porous superabsorbent particles. Therefore, the measured polymer volume fraction in the swollen gel, )2,app> s larger than the actual polymer volume fraction in the swollen polymer, U2 s. The apparent polymer volume fraction in a porous system may be... 

The water-absorption rate of the polymers can be increased by mixing inorganic particles such as kaolin, talc, etc. with superabsorbent polymers. This mixing procedure results in the decrease in the formation of fish-eyes in polymer gels. In addition, the increase in the water-absorption rate is because of the fact that inorganic particles adsorbed on superabsorbent copolymers give space between polymer particles. 

The water permeable polymer matrix allows the water or moisture contained in the adhesive to migrate to the superabsorbent water retentive particles at a rate that allows bonding to occur within a desirable time. The rate of bonding is dependent upon the type of adhesive used, the amount of water found in the adhesive, and the rate of moisture migration into the water retentive particles. The rate of drying of the adhesive is controlled, such that it does not dry too quickly, thereby resulting in a loss of adhesion, and allowing appropriate time for installation. 

In 2003, global demand for superabsorbents was 1.05 million tones and was found to increase at an average of 3.6% per annum. Out of the total demand, baby diapers account for about 81%, feminine hygiene products about 5%, adult incontinence about 8%, and other 6%. Other applications of superabsorbents are in removal of toxic heavy metal ions from industrial effluents, as drying agents, and removal of colloidal particles from potable water [106]. 

The elastic modulus also influences the swelling kinetics of the superabsorbent polymer in the blend (11). For a cross-linked polymer, different shaped particles have different swelling kinetics. This results from the coupling between the diffusion of water into the polymer network through the surfaces of the particles, and the relaxation of the cross-linked polymer chains in the presence of the swelling agent. Of the simple shapes, spherical particles have the fastest kinetics whereas flat disks have the slowest kinetics. Spherical particles that are deformed into a disk-like shape swell more slowly, and therefore the more deformable swollen particles (lower elastic modulus) will be slower than similar sized particles with higher modulus that are less deformable. 

Elastic Modulus. The elastic shear modulus of swollen superabsorbent polymer is commonly measured by means of oscillatory rheometry, using any of a variety of commercially available instruments (31). The sample to be analyzed is screened to the desired particle size cut such that the swollen particle size is smaller than the gap between the measuring plates. The sample is swollen in the desired liquid, usually 0.9 wt% NaCl solution or synthetic urine. Excess liquid is removed from the swollen hydrogel by blotting to minimize interparticle slippage. The hydrogel typically is placed onto the lower circular plate. An upper conical plate is positioned at the proper sample gap, which is packed with the gel. An oscillating torque is applied at a known frequency to the upper plate and the... 

Swelling Kinetics Methods. Swelling kinetics for superabsorbent polymers may be measured by a viscometric method (37), based on the dependence of suspension viscosity on the volume fraction of the suspended particles. However, the swelling rate is most often measured by determining the speciflc absorbency as a function of time, for example, by means of the centrifuge capacity analysis. An absorption rate may also be obtained from data of swelling vs time using the demand absorbency method (38). 

Determination of polyacrylate superabsorbent powders and particle size distribution — Sieve Fractionation NWSP 220.0.R2 (15) ISO 17190-3 2001. 

Polyacrylate superabsorbent powders — Determining the content of respirable particles NWSP 405.0.R0 (15). 

The absorption rate of the diaper must not be slower than the urination rate of the baby otherwise, leakage will occur. The absorption rate of the composite is influenced by the absorption rate of the SAP. On the other hand, fast swelling of the polymer may or may not be desirable. In some diaper designs, fast swelling may cause the diaper to leak if the porosity and permeability of the composite are reduced. The absorption rate of superabsorbent polymers is affected by the maximum absorption capacity of the polymer and its particle size and shape (Bartkowiak Frydrych, 2011). 

Plasmonic metamaterials can be utilized to match the impedance of environment (air) to the substrate (semiconductor material) in the optical range (or acmally any other range) directly by optimizing their particles design. Thus one can obtain perfect black surfaces denoted as superabsorbers [229, 230]. Such nanostructured surfaces are able to absorb light in a wide bandwidth regardless of its polarization and for literally any incident angle. 

The texture composition consists essentially of calcium carbonate in an amount of 30-95% of the dry composition, starch, and a superabsorbent pol uner that can absorb at least about 50 times its mass and having a particle size of less than about 250 pm. The superabsorbent polymer is added in an amount of 0.02-5% of the dry-composition. Superabsorbent polymers that have been disclosed are shown in Table 5.3. 

Water-agglomerated superabsorbent fine particles dissociate upon contact or swelling with an aqueous solution. This results in a concentration of swollen free fine particles that will contribute to an increased gel blocking. 

D.C. Roe, Absorbent structures containing specific particle size distributions of superabsorbent hydrogel-forming materials mixed with inorganic powders, US Patent 5419956, assigned to The Procter Gamble Company (Cincinnati, OH), May 30,1995. 

Based on an inquiry made on behalf of the superabsorbent polymer manufacturers, performance testing items include appearance, smell, water uptake, water retention, rate of water absorption, particle size distribution, bulk density, pH, gel strength, durability, dry weights, residual monomer, and water soluble components. The testing items that should be standardized are water uptake and rate of water absorption [151]. Testing method preferences include ... 

Four common superabsorbent polymers were examined for their water uptake at seven organizations under many different conditions. These included using two types of testing solution, two materials for tea bags, immersion time, temperature, measurement temperature, type of solution, conditioning of the sample, the amount of the sample, type, and particle size. The obtained results have been analyzed and the reproducibility from each organization and each measurement have been evaluated. 

In a manner similar to the method described in the water uptake information four types of superabsorbent polymers were used as common samples and eight organizations determined the rate of water absorption by varying conditions that included particle size, the amount of sample, the method of sample addition to the solution, stirring condition, method to identify the end point, two types of solution, the amount of solution, measurement temperature, and the number of measurements. Based on the results and their analysis, the method for measuring the rate of water absorption for superabsorbent polymers is proposed as follows. 

Remarks A similar expression as for the water uptake measurement method is used for the water uptake, particle size, and reference specifications of superabsorbent polymers. 

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