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Hindered amine light stabilizers (HALS) are chemical compounds containing an
The structure of the HALS makes them resistant to side reactions. The use of a hindered amine possessing no alpha-hydrogens prevents the HALS being converted into a
amine
In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
functional group
In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
that are used as stabilizers in plastics and polymers. These compounds are typically derivatives of tetramethylpiperidine
2,2,6,6-Tetramethylpiperidine, abbreviated TMP, HTMP, or TMPH, is an organic compound of the amine class. In appearance, it is a colorless liquid and has a "fishy", amine-like odor. This amine is used in chemistry as a steric effects, hindered bas ...
and are primarily used to protect the polymers from the effects of photo-oxidation; as opposed to other forms of polymer degradation
Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycl ...
such as ozonolysis
In organic chemistry, ozonolysis is an organic reaction where the Saturated and unsaturated compounds, unsaturated bonds are Bond cleavage, cleaved with ozone (). Multiple carbon–carbon bond are replaced by carbonyl () groups, such as aldehydes ...
.
They are also increasingly being used as thermal stabilizers, particularly for low and moderate level of heat, however during the high temperature processing of polymers (e.g. injection moulding
Injection moulding (U.S. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals (for ...
) they remain less effective than traditional phenolic antioxidants.
Mechanism of action
HALS do not absorb UV radiation, but act to inhibit degradation of the polymer by continuously and cyclically removingfree radical
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s that are produced by photo-oxidation of the polymer. The overall process is sometimes referred to as the Denisov cycle, after Evguenii T. Denisov and is exceedingly complex. Broadly, HALS react with the initial polymer peroxy radical (ROO•) and alkyl polymer radicals (R•) formed by the reaction of the polymer and oxygen, preventing further radical oxidation. By these reactions HALS are oxidised to their corresponding aminoxyl radicals (R2NO• c.f. TEMPO
In musical terminology, tempo (Italian for 'time'; plural 'tempos', or from the Italian plural), measured in beats per minute, is the speed or pace of a given musical composition, composition, and is often also an indication of the composition ...
), however they are able to return to their initial amine form via a series of additional radical reactions. HALS's high efficiency and longevity are due to this cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process.
The structure of the HALS makes them resistant to side reactions. The use of a hindered amine possessing no alpha-hydrogens prevents the HALS being converted into a nitrone
In organic chemistry, a nitrone is a functional group consisting of an Amine oxide, ''N''-oxide of an imine. The general structure is , where R3 is not a hydrogen. Their primary application is Chemical Intermediate, intermediates in chemical syn ...
species and piperidines are resistant to intramolecular Cope reaction
The Cope reaction or Cope elimination, developed by Arthur C. Cope, is the elimination reaction of an N-oxide to an alkene and a hydroxylamine.Typically, the amine oxide is prepared from the corresponding amine with a peroxy acid or comparable ...
s. In commercial HALS the reactive piperidine group is usually bonded to bulky chemical scaffold, in order to reduce its volatility during the melt processing of plastic.
Application
Even though HALS are extremely effective inpolyolefin
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 ...
s, 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 bott ...
and polyurethane
Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...
, they are ineffective in polyvinyl chloride
Polyvinyl chloride (alternatively: poly(vinyl chloride), colloquial: vinyl or polyvinyl; abbreviated: PVC) is the world's third-most widely produced synthetic polymer of plastic (after polyethylene and polypropylene). About 40 million tons of ...
(PVC). It is thought that their ability to form nitroxyl radicals is disrupted due them being readily protonated by HCl HCL may refer to:
Science and medicine
* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia
* Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development
* Hollow-cathode lamp, a s ...
released by dehydrohalogenation
In chemistry, dehydrohalogenation is an elimination reaction which removes a hydrogen halide from a substrate (chemistry), substrate. The reaction is usually associated with the synthesis of alkenes, but it has wider applications.
Dehydrohalogen ...
of PVC.
See also
*BTMPS
Bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate (abbreviated BTMPS) is a hindered amine light stabilizer used to protect plastics and coatings such as paint from oxidation caused by sunlight weathering. Like most compounds of this class, the activ ...
*Steric hindrance
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivi ...
References
{{Reflist Amines Plastics additives