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Polyacrylamide (IUPAC poly(2-propenamide) or poly(1-carbamoylethylene), abbreviated as PAM) is a polymer (-CH2CHCONH2-) formed from acrylamide subunits. It can be synthesized as a simple linear-chain structure or cross-linked, typically using N,N'-methylenebisacrylamide. In the cross-linked form, the possibility of the monomer being present is reduced even further. It is highly water-absorbent, forming a soft gel when hydrated, used in such applications as polyacrylamide gel electrophoresis, and can also be called ghost crystals when cross-linked, and in manufacturing soft contact lenses. In the straight-chain form, it is also used as a thickener and suspending agent. More recently, it has been used as a subdermal filler for aesthetic facial surgery (see Aquamid).
Linear polyacrylamide is a water-soluble polymer. It is typically non-ionic polymer but due to hydrolysis of some amide groups they could convert into carboxylic groups giving polyacrylamide some weak anionic properties.
Uses of polyacrylamide
One of the largest uses for polyacrylamide is to flocculate solids in a liquid. This process applies to water treatment, and processes like paper making and screen printing. Polyacrylamide can be supplied in a powder or liquid form, with the liquid form being subcategorized as solution and emulsion polymer. Even though these products are often called 'polyacrylamide', many are actually copolymers of acrylamide and one or more other chemical species, such as an acrylic acid or a salt thereof. The main consequence of this is to give the 'modified' polymer a particular ionic character.
Another common use of polyacrylamide and its derivatives is in subsurface applications such as Enhanced Oil Recovery. High viscosity aqueous solutions can be generated with low concentrations of polyacrylamide polymers, and these can be injected to improve the economics of conventional waterflooding.
The linear soil conditioning form was developed in the 1950s by Monsanto Company and was marketed under the trade name Krilium. The soil conditioning technology was presented at a symposium on "Improvement of Soil Structure" held in Philadelphia, Pennsylvania on December 29, 1951. The technology was strongly documented and was published in the June 1952 issue of the journal Soil Science, volume 73, June 1952 that was dedicated to polymeric soil conditioners.
The original formulation of Krilium was difficult to use because it contained calcium which cross-linked the linear polymer under field conditions. Even with a strong marketing campaign, Krilium was abandoned by Monsanto.
After 34 years, the journal Soil Science wanted to update the soil conditioning technology and published another dedicated issue on polymeric soil conditioner and especially linear, water-soluble, anionic polyacrylamide in the May 1986 issue, volume 141, issue number 5.
The new water-soluble soil conditioners may, if used according to established procedures
- increase pore space in soils containing clay
- increase water infiltration into soils containing clay
- prevent soil crusting
- stop erosion and water runoff
- make friable soil that is easy to cultivate
- make soil dry quicker after rain or irrigation, so that the soil can be worked sooner
Consequently, these translate into
- stronger, larger plants with more extensive root system
- earlier seed emergence and crop maturity
- more efficient water utilization
- easier weed removal
- more response to fertilizers and to new crop varieties
- less plant diseases related to poor soil aeration
- decreased energy requirement for tillage
The cross-linked form which retains water is often used for horticultural and agricultural under trade names such as Broadleaf P4, Swell-Gel, and so on.
The anionic form of linear, water-soluble polyacrylamide is frequently used as a soil conditioner on farm land and construction sites for erosion control, in order to protect the water quality of nearby rivers and streams.
The polymer is also used to make Gro-Beast toys, which expand when placed in water, such as the Test Tube Aliens. Similarly, the absorbent properties of one of its copolymers can be utilized as an additive in body-powder.
The ionic form of polyacrylamide has found an important role in the potable water treatment industry. Trivalent metal salts, like ferric chloride and aluminum chloride, are bridged by the long polymer chains of polyacrylamide. This results in significant enhancement of the flocculation rate. This allows water treatment plants to greatly improve the removal of total organic content (TOC) from raw water.
Polyacrylamide is also often used in molecular biology applications as a medium for electrophoresis of proteins and nucleic acids in a technique known as PAGE.
It was also used in the synthesis of the first Boger fluid.
Molecular biology laboratories
Polyacrylamide was first used in a laboratory setting in the early 1950s. In 1959, the groups of Davis and Ornstein and of Raymond and Weintraub independently published on the use of polyacrylamide gel electrophoresis to separate charged molecules. The technique is widely accepted today, and remains a common protocol in molecular biology labs.
Acrylamide has many other uses in molecular biology laboratories, including the use of linear polyacrylamide (LPA) as a carrier, which aids in the precipitation of small amounts of DNA. Many laboratory supply companies sell LPA for this use.
The majority of acrylamide is used to manufacture various polymers. In the 1970s and 1980s, the proportionately largest use of these polymers was in water treatment. Additional uses include as binding, thickening or flocculating agents in grout, cement, sewage/wastewater treatment, pesticide formulations, cosmetics, sugar manufacturing, soil erosion prevention, ore processing, food packaging, plastic products, and paper production. Polyacrylamide is also used in some potting soil. Another use of polyacrylamide is as a chemical intermediate in the production of N-methylol acrylamide and N-butoxyacrylamide. In oil and gas industry Polyacrylamide derivatives especially co-polymers of that have a substantial effect on unconventional production and hydraulic fracturing. As an nonionic monomer it can be co-polymerize with anionic for example Acrylic acid and cationic monomer such as diallyldimethyl ammonium chloride (DADMAC) and resulted co-polymer that can have different compatibility in different applications.
The primary functions of polyacrylamide soil conditioners are to increase soil tilth, aeration, and porosity and reduce compaction, dustiness and water run-off. Secondary functions are to increase plant vigor, color, appearance, rooting depth and emergence of seeds while decreasing water requirements, diseases, erosion and maintenance expenses. FC 2712 is used for this purpose.
In dilute aqueous solution, such as is commonly used for Enhanced Oil Recovery applications, polyacrylamide polymers are susceptible to chemical, thermal, and mechanical degradation. Chemical degradation occurs when the labile amide moiety hydrolyzes at elevated temperature or pH, resulting in the evolution of ammonia and a remaining carboxyl group. Thus, the degree of anionicity of the molecule increases. Thermal degradation of the vinyl backbone can occur through several possible radical mechanisms, including the autooxidation of small amounts of iron and reactions between oxygen and residual impurities from polymerization at elevated temperature. Mechanical degradation can also be an issue at the high shear rates experienced in the near-wellbore region.
Concerns have been raised that polyacrylamide used in agriculture may contaminate food with acrylamide, a known neurotoxin and carcinogen. While polyacrylamide itself is relatively non-toxic, it is known that commercially available polyacrylamide contains minute residual amounts of acrylamide remaining from its production, usually less than 0.05% w/w.
Additionally, there are concerns that polyacrylamide may de-polymerise to form acrylamide. In a study conducted in 2003 at the Central Science Laboratory in Sand Hutton, England, polyacrylamide was treated similarly as food during cooking. It was shown that these conditions do not cause polyacrylamide to de-polymerise significantly.
In a study conducted in 1997 at Kansas State University, the effect of environmental conditions on polyacrylamide were tested, and it was shown that degradation of polyacrylamide under certain conditions can cause the release of acrylamide. The experimental design of this study as well as its results and their interpretation have been questioned, and a 1999 study by the Nalco Chemical Company did not replicate the results.
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