Poly(acrylic acid) (PAA; trade name Carbomer) is a

polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

with the formula (CH₂-CHCO₂H)_n. It is a derivative of

acrylic acid Acrylic acid (IUPAC: propenoic acid) is an organic compound with the formula CH2=CHCOOH. It is the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a ...

(CH₂=CHCO₂H). In addition to the

homopolymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...

s, a variety of copolymers and crosslinked polymers, and partially deprotonated derivatives thereof are known and of commercial value. In a water solution at neutral pH, PAA is an

anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

, i.e., many of the side chains of PAA lose their protons and acquire a negative charge. Partially or wholly deprotonated PAAs are

polyelectrolyte Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are ...

s, with the ability to absorb and retain water and swell to many times their original volume. These properties – acid-base and water-attracting – are the bases of many applications. = Synthesis = PAA is produced by free radical polymerization. Initiators include potassium persulfate and AIBN. Polyacrylic acid is a

polyolefin A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More speciali ...

. It can be viewed as

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including b ...

with carboxylic acid (CO₂H) substituents on alternating carbons. Owing to these groups, alternating carbon atoms in the backbone are stereogenic (colloquially: chiral). For this reason, acrylic acid exists in atactic, syndiotactic, and isotactic forms, although this aspect is rarely discussed. The polymerization is initiated with radicals and is assumed to be stereorandom.

Crosslink In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...

ing can be introduced in many ways.

Production

About 1,600,000,000 kg were produced in 2008.

Structure and derivatives

Polyacrylic acid is a weak anionic polyelectrolyte, whose degree of ionisation is dependent on solution pH. In its non-ionised form at low pHs, PAA may associate with various non-ionic polymers (such as polyethylene oxide, poly-N-vinyl pyrrolidone, polyacrylamide, and some cellulose ethers) and form hydrogen-bonded interpolymer complexes. In aqueous solutions PAA can also form polycomplexes with oppositely charged polymers such as chitosan, surfactants, and drug molecules (for example, streptomycin).

Physical properties

Dry PAAs are sold as white, fluffy powders. In the dry powder form, the positively charged sodium ions are bound to the polyacrylate, however in aqueous solutions the sodium ions can dissociate. The presence of many metal cations allows the polymer to absorb a high amount of water.

Applications

Absorbent

PAA is widely used in dispersants and since the molecular weight has a significant impact on the rheological properties and dispersion capacity, and hence applications. The dominant application for PAA is as a superabsorbent. About 25% of PAA is used for detergents and dispersants. Polyacrylic acid and its derivatives are used in

disposable diapers A diaper /ˈdaɪpə(r)/ (American and Canadian English) or a nappy (Australian English, British English, and Hiberno-English) is a type of underwear that allows the wearer to urinate or defecate without using a toilet, by absorbing or conta ...

. Acrylic acid is also the main component of Superadsorbent Polymers (SAPs), cross-linked polyacrylates that can absorb and retain more than 100 times of their own weight in liquid. The US Food and Drug Administration authorised the use of SAPs in packaging with indirect food contact.

Cleaning

Detergents often contain copolymers of acrylic acid that assist in sequestering dirt. Cross-linked polyacrylic acid has also been used in the processing of household products, including floor cleaners. PAA may inactivate the

antiseptic An antiseptic (from Greek ἀντί ''anti'', "against" and σηπτικός ''sēptikos'', "putrefactive") is an antimicrobial substance or compound that is applied to living tissue/skin to reduce the possibility of infection, sepsis, or putre ...

chlorhexidine gluconate.

Biocompatible materials

The neutralized polyacrylic acid gels are suitable biocompatible matrices for medical applications such as gels for skin care products. PAA films can be deposited on orthopaedic implants to protect them from corrosion. Crosslinked hydrogels of PAA and gelatin have also been used as medical glue.

Paints and cosmetics

Other applications involve

paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

s and

cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protec ...

. They stabilize suspended solid in liquids, prevent emulsions from separating, and control the consistency in flow of cosmetics. Carbomer codes (910, 934, 940, 941, and 934P) are an indication of molecular weight and the specific components of the polymer. For many applications PAAs are used in form of

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

or ammonium salts, e.g. sodium polyacrylate.

Emerging applications

Hydrogels derived from PAA have attracted much study for use as bandages and aids for wound healing.

Drilling fluid and metal quenching

A few reports were made on PAA use as

deflocculant Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from pre ...

(so called

alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

ne polyacrylates) for oil drilling industry. It was also reported to be used for metal

quenching In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ...

in metalworking (see Sodium polyacrylate).

References

{{Plastics Acrylate polymers Cosmetics chemicals Polyelectrolytes Polymers