HALS: Hindered Amine Light Stabilizers — Types & Selection

A polyolefin part that chalks and cracks after one summer outdoors. An automotive coating that loses gloss and fades on the hood first. An agricultural film that fails halfway through the season where the sulfur spray landed. These are photo-oxidation failures, and the additive that prevents them — at low loading, by interrupting the degradation chain rather than absorbing light — is a hindered amine light stabilizer, or HALS.

The short version: A HALS does not work by absorbing UV (that is a UV absorber’s job). It works by scavenging the free radicals that UV light generates in a polymer, and it does so catalytically — the active nitroxyl species is regenerated and keeps working, so a small amount protects for a long time. Selection turns on molecular weight (monomeric grades are cheap and mobile but can migrate or volatilize; polymeric grades are permanent and low-volatility), basicity (standard HALS are basic and can be deactivated by acids, so low-basicity and NOR-HALS grades exist for acidic systems, acid-catalyzed coatings, and agrochemical exposure), the polymer, and the part thickness. HALS are usually combined with a UV absorber and an antioxidant for a complete light-stabilizer package.

What is a HALS?

A hindered amine light stabilizer is a derivative of 2,2,6,6-tetramethylpiperidine. When a polymer is exposed to UV, oxygen-centered and carbon-centered radicals form and propagate a chain reaction that breaks the polymer down — you see it as yellowing, chalking, loss of gloss, embrittlement and cracking. The HALS is oxidized in service to a stable nitroxyl radical, which traps those propagating radicals; the resulting species then react back to regenerate the nitroxyl, so the stabilizer is not consumed in one pass. This regenerative (Denisov) cycle is why HALS are so efficient at low concentrations. Photostabilization chemistry and individual structures are documented at primary sources such as PubChem.

HALS vs UV absorbers vs antioxidants

These three additive classes do different jobs and are often used together.

HALS and UV absorbers are complementary: the UV absorber reduces the amount of damaging light, and the HALS mops up the radicals that still form. A typical durable formulation uses a HALS plus a UV absorber plus a processing antioxidant.

Monomeric, polymeric and NOR-HALS

The biggest selection decision is structure.

Monomeric (low-MW) HALS are mobile and effective, with good initial performance and lower cost, but can migrate to the surface, volatilize during processing, or be extracted. HALS-770 (bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate, comparable to Tinuvin 770) is the classic monomeric grade; HALS-292 (a methylated sebacate, comparable to Tinuvin 292) is the liquid monomeric standard for coatings; and a monomeric tetramethylpiperidinyl ester serves where a single low-volatility ester is wanted.

Polymeric / oligomeric (high-MW) HALS are essentially non-volatile and resist migration and extraction, so they last in thin films, hot processes and contact with water or solvents. HALS-622 (a polymeric succinate-piperidine, comparable to Tinuvin 622), HALS-944 (a polymeric triazine, comparable to Chimassorb 944), HALS-2020 and the oligomeric triazine HALS (comparable to Chimassorb-type oligomers) cover this space and are the workhorses for polyolefins and films.

NOR-HALS (low-basicity) grades, such as HALS-123 (an N-alkoxy / NOR-HALS, comparable to Tinuvin 123), carry an N–OR group instead of N–H. They are far less basic, so they survive acidic environments — acid-catalyzed coatings, flame-retarded compounds, agrochemical (pesticide) exposure and thermally demanding films — where standard HALS lose efficiency. For nylon and engineering plastics, a piperidinyl benzene-dicarboxamide HALS (an isophthalamide type) also acts as a nucleating/stabilizing additive.

How to select a HALS

Work through four questions. Polymer and process temperature: polyolefins (PP, PE) and films lean on polymeric triazine HALS (944/2020) for permanence; coatings often use liquid monomeric or NOR grades. Permanence vs cost: thin sections, long outdoor life, and water/solvent contact call for polymeric/oligomeric grades; thicker, shorter-life or cost-sensitive parts can use monomeric grades. Chemical environment: acidic systems, acid-catalyzed coatings, flame-retarded compounds and agrochemical exposure need low-basicity or NOR-HALS. The package: pair the HALS with a UV absorber (benzotriazole or triazine) and a processing antioxidant, and validate with accelerated and outdoor weathering — additive choice is confirmed by weathering data, not by theory.

Where HALS are used

Buying HALS in bulk

RawSource sources the hindered amine light stabilizer range direct from producers — monomeric grades (HALS-770, HALS-292), polymeric and oligomeric grades (HALS-622, HALS-944, HALS-2020) and NOR / low-basicity grades (HALS-123) — together with companion UV absorbers (benzotriazoles and benzophenones) for complete light-stabilizer packages. Brand references are nominative only; our grades are the generic equivalents. Tell us the polymer, the part thickness and service life, the chemical environment (acidic, flame-retarded, agrochemical), and whether you need a complete UV-absorber-plus-HALS package, and we will quote the right stabilizer or system with CoA, TDS and SDS per lot. Related amine building blocks are mapped in the amines guide.

Frequently asked questions

What is a HALS (hindered amine light stabilizer)?

A HALS is a 2,2,6,6-tetramethylpiperidine-based additive that protects plastics and coatings from UV degradation by scavenging the free radicals that light generates. It works catalytically through a regenerated nitroxyl radical, so a small amount provides long-lasting protection.

How is a HALS different from a UV absorber?

A UV absorber absorbs UV light and releases it as heat before it can damage the polymer; a HALS does not absorb UV at all — it traps the radicals that form. They are complementary and are usually used together, with the UV absorber reducing the light load and the HALS scavenging the remaining radicals.

What is the difference between monomeric and polymeric HALS?

Monomeric (low molecular weight) HALS are mobile and lower cost but can migrate, volatilize or be extracted; polymeric/oligomeric (high molecular weight) HALS are essentially non-volatile and resist migration and extraction, so they last longer in thin films, hot processes and water/solvent contact.

What is NOR-HALS and when do I need it?

NOR-HALS carry an N-alkoxy (N–OR) group instead of N–H, which makes them much less basic. They keep working in acidic environments — acid-catalyzed coatings, flame-retarded compounds and agrochemical (pesticide) exposure — where standard basic HALS are deactivated. HALS-123 is an example.

Which HALS is used for polyolefins like polypropylene?

Polyolefins and films typically use high-molecular-weight polymeric triazine HALS such as HALS-944 or HALS-2020 for permanence, combined with a UV absorber and a processing antioxidant. Confirm with accelerated and outdoor weathering at your part thickness.

Do HALS work with flame retardants?

Standard basic HALS can be deactivated by acidic flame-retardant systems; low-basicity or NOR-HALS grades are used in flame-retarded compounds. Validate the combination, because some flame retardants and stabilizers interact.

Editorial note. This article is general technical guidance for industrial and professional buyers and formulators. Mechanisms, grade characteristics and selection guidance are typical, generalized references to validate with your own accelerated and outdoor weathering; the Certificate of Analysis and Technical Data Sheet govern the grade you buy. Brand names are used only as nominative cross-references to identify the generic equivalent we supply; RawSource is not affiliated with or endorsed by those brand owners. Nothing here is a safety, health or efficacy claim. Always consult the current Safety Data Sheet (SDS) before handling, and confirm regulatory status and suitability for your application and jurisdiction. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.

RawSource Editorial

Commercial & Sourcing Desk