Superabsorbent polymer
Superabsorbent ( super-absorbent polymer, SAP) are called plastics, which are able to absorb many times their own weight in polar liquids. These are mainly water or aqueous solutions. When taking the liquid of the superabsorbent swells and forms a hydrogel. The sum of the volume of the liquid and the volume of the dry superabsorbent remains the same.
Use
The product comes as a white, coarse-grained powders with particle sizes of 100-1000 microns ( = 0.1-1.0 mm ) are used. It takes place mostly in diapers, but also in products for feminine hygiene, incontinence care, in bandages and in small amounts in cable sheathing for deep lines use. However, establishing themselves also gradually other purposes, such as to sleep on in the so-called gel beds, as gelling extinguishing agent in fire fighting, as a mechanical stabilizer for cut flowers in a vase or as an additive to potting soil to permanently store water. However, this is because of better environmental kalilaugeneutralisierte acrylic acid used. Large manufacturers for superabsorbent polymers include BASF SE, Evonik, or Nippon Shokubai. Large superabsorbent customers are, for example, Procter & Gamble, Kimberly- Clark, Attends Healthcare Group or SCA, Paul Hartmann AG in Germany and ONTEX GmbH.
In the testing and implementation phase are super absorbent to increase the water storage capacity of soils. Besides Kompositgranulaten, consisting of a highly porous mineral matrix and attached thereto hydrogel, the alternative production of superabsorbent polymers of starch as a renewable resource for worldwide use in the agricultural sector is explored.
Chemical nature
Chemically, it is at the superabsorbent is a copolymer of acrylic acid (propenoic acid, H2C = CH- COOH) and sodium (sodium salt of acrylic acid, H2C = CH- COONa ), the ratio of the two monomers can vary from one another. In addition, a so-called Kernvernetzer (Core -Cross - Linker, CXL ) is added to the monomer solution, which connects the chain polymer molecules in places with each other by chemical bridges ( "cross-links "). Through these bridges, the polymer is insoluble in water.
According to the current state of development is called the product in this form " base polymer ", as the requirements are grown on the superabsorbent over the years and further processing steps are applied. As most important is mentioned here only the so-called surface post-crosslinking (Surface -Cross - Linking, SXL). In this case, a further chemical is applied to the surface of each particle, and a reaction takes place under heat, a second network, subject only to the outer layer of the grain. This network helps the particle to retain its form in the swollen state and under pressure. This in turn favors the holding capacity of the superabsorbent even in situations when a baby with all his weight sitting on the diaper.
Operation
The surface of the superabsorbent polymer particles are observed microscopically, fissured and porous. Channels that run towards particle core, acting through the capillary liquid Attractive. Upon contact between superabsorbent and a large liquid surface of the particle is wetted so already. Due to the existing sodium ions within the polymer structure, the molecule forms a strong salt character, and it comes because of osmotic pressure for further fluid absorption. This osmotic pressure ultimately determine the capacity of the superabsorbent.
The more sodium ions are incorporated in the polymer, the greater amounts of liquid can be added: This has the following relationships arise. The higher the concentration of salts in the liquid, the lower the absorption will be. This explains that pure water is absorbed in magnitude to the several hundred times the superabsorbent own weight, whereas salt solutions - for example urine - limit the capacity of the superabsorbent significantly. Furthermore, derived from the operation also, which is why non-polar liquids - eg oils - can not be absorbed.
Within the particle, the liquid is now seeking the molecule, which presents itself as a salt to dissolve - to separate so in this case the sodium from the anion radical. In this case, the sodium dissolves in the liquid and in fact on forms by its electric charge ( Na ) and the dipole character of water a hydration shell around them. However, the organic radical is insoluble in water due to its polymer structure, and particularly by the cross-linking. At the points at which still have been in the dry state, sodium ions, but now vacant positions remain open with a negative charge. There, too, hydrate, which, however, does not compensate for the charge imbalance form. Since these vacancies can occur in the polymer chain next to each other and like charges repel each other, the molecule becomes more and more stretched in order to obtain the greatest possible distance between. This is clearly visible on swelling during absorption.