Poly(acrylic acid) (PAA; trade name Carbomer) is a
polymer with the formula (CH
2-CHCO
2H)
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 ...
(CH
2=CHCO
2H). In addition to the
homopolymers, 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 polymer, i.e., many of the side chains of PAA lose their protons and acquire a negative charge. Partially or wholly deprotonated PAAs are
polyelectrolytes, 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 with carboxylic acid (CO
2H) 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.
Crosslinking 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. 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 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. 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 p ...
(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 metalworking (see Sodium polyacrylate).
References
{{Plastics
Acrylate polymers
Cosmetics chemicals
Polyelectrolytes
Polymers