Ultraviolet light does not just fade coloured plastics — it destroys them. A polypropylene part that survives a decade indoors can chalk, crack and lose half its impact strength within two summers outdoors, because UV photons (295–400 nm) carry enough energy to break C–H and C–C bonds and start an auto-oxidation cascade. Stabilising against that cascade is a two-part job: an ultraviolet absorber (UVA) shields the polymer from photons, and a hindered amine light stabilizer (HALS) scavenges the radicals that photons inevitably create. HALS is the more powerful half of that pair, and choosing the right grade comes down to two axes that most data sheets barely mention — molecular weight and basicity.
Why HALS outperforms every other UV protection
A UV absorber works stoichiometrically: it absorbs a photon, dissipates the energy as heat, and waits for the next one. A HALS works catalytically. In service the hindered amine is oxidised to a nitroxyl radical (>N–O•), which traps the alkyl (R•) and peroxyl (ROO•) radicals that propagate photo-oxidation and regenerates itself in the process — the Denisov cycle. One HALS molecule can therefore interrupt hundreds of oxidation events, which is why 0.2–0.5% of a good HALS routinely outperforms 1–2% of a UV absorber used alone. What HALS cannot do is stop the photons themselves, so demanding applications pair the two.
The second thing to internalise is that HALS is a surface phenomenon. Photo-oxidation begins where the light is and eats inward as the surface crazes. A HALS that stays where it was put — uniformly distributed and slow to diffuse — keeps protecting that layer for the life of the part. That is why polymeric grades dominate any application expected to last more than a few years.
The family, in four groups
| Grade | CAS | Type | Approx. MW | Form | Character |
|---|---|---|---|---|---|
| UV-770 | 52829-07-9 | Monomeric HALS | ~481 | Crystalline solid | Fast action, lowest cost, migratory |
| UV-292 | 41556-26-7 / 82919-37-7 | Liquid monomeric blend | ~509 | Liquid | Easy metering, ideal for coatings |
| UV-944 | 71878-19-8 | Polymeric (triazine backbone) | 2 000–3 100 | Solid | Non-migrating, industry benchmark |
| UV-622 | 65447-77-0 | Polymeric (polyester backbone) | 3 100–4 000 | Solid | Extraction-resistant, lower basicity |
| UV-783 | 71878-19-8 / 65447-77-0 | 944/622 blend | — | Solid | Cost/performance compromise |
| UV-119 | 106990-43-6 | Polymeric | ~2 300 | Solid | High MW, very low volatility |
| UV-2020 | 192268-64-7 | Low-basicity polymeric | ~2 000 | Solid | Survives acidic co-additives |
| UV-123 | 129757-67-1 | Low-basicity (NOR type) | ~700 | Liquid | Acid-tolerant, coatings and films |
| UV-114 | 42774-15-2 | Monomeric HALS | low | Solid | Low-MW analogue for flexible PVC and coatings |
| UV-5530 | 104810-47-1 / 104810-48-2 / 25322-68-3 | Liquid HALS + UVA blend | — | Liquid | Ready-made coating package |
Two rows deserve a note. UV-783 is not a distinct chemistry but the 944/622 blend — a supplier's answer to formulators who want most of the polymeric performance at a lower cost. UV-5530 is a liquid concentrate that combines HALS with an ultraviolet absorber, so a coatings formulator can dose both halves of the system from one drum.
Low-molecular-weight vs. polymeric — the migration problem
Molecular weight is not an academic number; it is the single best predictor of how long the stabiliser stays where you put it. Low-molecular-weight HALS such as UV-770 (CAS 52829-07-9) are excellent radical scavengers and the cheapest way to buy UV performance, but their small molecules diffuse. Three consequences follow:
- Volatility during processing. Extrusion, injection moulding and fibre spinning run at 200–280 °C. A monomeric HALS partially volatilises at the die, so the amount you weighed in is not the amount that ends up in the part.
- Surface blooming. Under service heat the small molecules migrate to the surface, forming a chalky film, fouling moulds and — in food-contact or medical applications — raising a migration question you would rather not have.
- Extraction. Agricultural film is the worst case: rain, irrigation and pesticide sprays continuously wash the surface. A monomeric HALS is leached out within one or two seasons, which is precisely why greenhouse film rated for three to five years is built on polymeric grades.
Polymeric HALS — UV-944 (CAS 71878-19-8) and UV-622 (CAS 65447-77-0) — solve all three. At molecular weights of two to four thousand their diffusion coefficient is low enough that they stay put through years of service, high-temperature processing and repeated washing. The price is a slower start (the polymer must disperse before it acts), a higher cost per kilogram and a slightly more demanding dispersion step. For any part expected to last three years or more that trade is worth taking; for a short-life injection-moulded item or a coating that will be replaced in two years, UV-770 remains the rational, economical choice.
Basicity: the criterion most data sheets omit
The active site of a HALS is the secondary amine in the piperidine ring, and that amine is basic. Basic enough, in fact, to form a salt with any acid in the formulation — and a salt cannot be oxidised to the nitroxyl radical that does the work. The HALS is not consumed; it is simply switched off. This is the most common cause of "the stabiliser was in the formula but the part still failed", and it is invisible in a standard stability test.
Acidic threats to watch for:
- Halogenated flame retardants — brominated and chlorinated systems release HBr/HCl under heat and light.
- Acidic catalyst residues — incomplete neutralisation during polymerisation leaves acidic species in the resin.
- Pesticide sprays and sulfur-containing agrochemicals — the dominant deactivation route in greenhouse film.
- Acidic fillers and pigments — certain clays, untreated carbon blacks and some organic pigments.
- Thioester and acidic antioxidant residues — covered in the section below.
The answer is a low-basicity or NOR-type HALS. In NOR chemistry the reactive N–H is replaced by an N–OR group, which is effectively non-basic and does not form salts, while still entering the same stabilisation cycle. UV-123 (CAS 129757-67-1) is the liquid NOR grade commonly used in coatings and films, and UV-2020 (CAS 192268-64-7) is the polymeric low-basicity option for plastics. If your formulation contains a halogenated flame retardant, or if it is an agricultural film that will see sulfur-based sprays, specifying a low-basicity HALS is not a refinement — it is the difference between a stabiliser that works and one that quietly does nothing.
Pairing HALS with UV absorbers
A HALS protects the polymer from the radicals that UV creates; it does not reduce the number of photons entering. A UV absorber does the opposite. Used together they cover both halves of the failure mechanism, and the effect is more than additive. The absorbers to consider, stocked alongside the HALS range:
| Product | CAS | Chemistry | Absorption | Notes |
|---|---|---|---|---|
| UV-531 | 1843-05-6 | Benzophenone | ~290–340 nm | Low cost, broad UVB coverage; can yellow at high processing temperature |
| UV-P | 2440-22-4 | Benzotriazole | ~300–385 nm | Low MW, good UVB/UVA coverage, migratory |
| UV-326 | 3896-11-5 | Benzotriazole | ~300–385 nm | Heat-stable, widely used in polyolefins |
| UV-328 | 25973-55-1 | Benzotriazole | ~300–385 nm | Excellent compatibility with polyolefins |
| UV-234 | 70321-86-7 | High-MW benzotriazole | ~300–385 nm | Low migration, for fibres and films |
Typical total loadings in a demanding outdoor system are 0.3–0.6% HALS with 0.1–0.3% UV absorber, with the ratio usually between 2:1 and 1:1 in favour of HALS. Section thickness decides who does the work: in thin sections (film, fibre, coating) the absorber is more valuable because the optical path is short and the stabilised surface layer is a larger fraction of the total, while in thick sections HALS carries the load because photons never reach the core.
Loadings by application
| Application | Typical HALS loading | Recommended type |
|---|---|---|
| Greenhouse / agricultural film | 0.3–0.6% | Polymeric (UV-944, UV-622); low-basicity where sulfur sprays are used |
| Automotive exterior (PP, TPO) | 0.2–0.5% | Polymeric plus a UV absorber |
| PP fibre and nonwoven | 0.1–0.3% | High-MW, low-volatility grades |
| Paints, coatings and inks | 0.5–2% on solids | Liquid grades (UV-292, UV-123, UV-5530) |
| Light-coloured rubber goods | 0.5–2 phr | Polymeric, non-staining |
| TPU and PU foam | 0.3–1% | Polymeric plus a UV absorber to control yellowing |
| Wood-plastic composites | 0.3–0.8% | Polymeric plus a UV absorber |
| Short-life injection mouldings | 0.1–0.3% | Monomeric (UV-770) is sufficient |
Interactions with antioxidants and other additives
HALS and antioxidants are not competitors — they defend against different mechanisms, and a hindered phenol such as AO-1010 (CAS 6683-19-8) or AO-1076 (CAS 2082-79-3) is normally used alongside a HALS without conflict. Three interactions do deserve attention:
- Thioester secondary antioxidants. Sulfur-based co-stabilisers and their oxidation products are acidic enough to deactivate conventional HALS over time. If the formulation needs a thioester, pair it with a low-basicity HALS.
- BHT. The classic phenolic antioxidant has been reported to suppress HALS efficiency in polyolefins, its transformation products interfering with the nitroxyl cycle. Where both are wanted, expect to raise the HALS loading or replace BHT with a higher-MW phenol.
- Phosphite hydrolysates. AO-168 (CAS 31570-04-4) is neutral as supplied but hydrolyses to acidic species in humid storage. Keep it dry, and avoid overdosing in systems that also rely on a conventional HALS.
In rubber goods, the amine antioxidant 6PPD (CAS 793-24-8) and the polymeric quinoline TMQ (CAS 26780-96-1) address heat and ozone ageing rather than photodegradation, and are normally specified together with — not instead of — a light stabiliser in a light-coloured compound.
What to demand on the COA
HALS grades are commodity items, but the gap between a good drum and a poor one is measurable. The data that matters:
- Molecular weight and distribution (GPC) — the defining parameter for polymeric grades. A UV-944 lot whose Mw has drifted low behaves like a monomeric HALS: it migrates. Ask for the actual curve, not just "high molecular weight".
- Volatiles — typically ≤ 0.5%. Critical for monomeric grades and for any process running above 230 °C.
- Melting range / softening point — an identity check and a processing guide; a lot that melts low may be cut with a lower grade.
- Transmittance / colour (Gardner or APHA at 425 and 500 nm) — decisive for white and pastel parts, where yellow degradation products are immediately visible.
- Purity by HPLC — ≥ 98% for monomeric grades; below that signals residual solvent or unreacted amine.
- Ash and heavy metals — a catalyst-residue indicator, and increasingly an audit item.
Procurement practice
Three habits prevent most HALS problems. First, specify the grade and the CAS number on the purchase order — "UV-770" and "HALS" are trade shorthand that different suppliers apply to different materials, and 52829-07-9 versus 71878-19-8 is the difference between a part that lasts two years and one that lasts ten. Second, always ask whether the intended end use involves a halogenated flame retardant, an acidic filler or agricultural sulfur spray; if it does, order a low-basicity grade. Third, validate by test rather than by datasheet: a QUV or xenon-arc exposure of your actual compound, with and without the HALS, tells you more in three weeks than any specification sheet will.
Shanghai Better Chemical supplies the full light stabilizer range — UV-770 (CAS 52829-07-9), UV-944 (CAS 71878-19-8), UV-622 (CAS 65447-77-0), UV-783, UV-292, UV-119, UV-123, UV-2020, UV-114 and UV-5530 liquid — together with the matching UV-531, UV-P, UV-326, UV-328 and UV-234 absorbers. Per-lot COA with molecular weight, volatiles and transmittance data is supplied as standard, along with TDS/MSDS and pre-shipment samples for your own QUV validation. Tell us your polymer, process temperature and expected service life, and we will recommend the right stabiliser package and quote current pricing.