Every polyolefin stabiliser quotation eventually comes down to a comparison between four names: HALS 944 (CAS 70624-18-9), HALS 622, UV-770 and UV-783. They are all hindered amine light stabilizers, they all work by the same radical-trapping cycle, and they are priced differently. Choosing between them on price per kilogram is how formulations end up failing in year two.
This guide gives the comparison that matters in practice: what separates the four grades physically, which application each fits, how to blend them deliberately, and how to run the substitution test that settles the question in your own plant.
The one mechanism, four physical profiles
All HALS are converted in service to a nitroxyl radical that traps alkyl radicals and is regenerated, so the chemistry is catalytic and the same for every grade. What differs is what happens to the molecule as a physical object inside the polymer:
- Molecular weight sets volatility and diffusion rate.
- Basicity sets compatibility with acidic co-additives.
- Melting point and physical form set how it is fed and dispersed.
- Colour and transmittance set whether it can be used in white, clear or light-coloured articles.
Side-by-side comparison
| Grade | Type | Mn | Volatility | Basicity | Form | Best fit |
|---|---|---|---|---|---|---|
| HALS 944 | Oligomeric, triazine-based | 2000 – 3100 | Low (max 1.0% spec) | Low-moderate | Slightly yellow granules | PP fibres, agricultural film, pipe, TPO, long-life thin sections |
| HALS 622 | Oligomeric, ester-based | approx. 2000 – 4000 | Low | Moderate | White to off-white granules | Thick sections, pipe, general polyolefins where colour is critical |
| UV-770 | Monomeric, secondary amine | approx. 480 | High | High | White crystalline powder | Thick moulded parts, cost-driven indoor applications |
| UV-783 | Blend (770/622 type) | Mixed | Medium | Moderate | Powder or granules | Balanced cost and persistence; retrofit into existing formulations |
Volatility: the variable that decides thin sections
A monomeric HALS with Mn around 480 can leave a 25 micron film almost entirely within a normal extrusion run, and continues to leave it during service. The stabiliser was paid for and is no longer in the product. An oligomeric HALS with Mn above 2000 cannot diffuse far, so it stays where it was dispersed.
| Section thickness | Dominant loss mechanism | Practical consequence |
|---|---|---|
| Under 50 micron (film, fibre) | Volatilisation and surface migration | Oligomeric HALS are effectively mandatory |
| 0.5 – 2 mm (injection moulded) | Moderate; both work | Grade choice can be cost-driven |
| Over 2 mm (pipe, thick profile) | Diffusion-limited; surface protection matters | Oligomeric HALS plus a UV absorber in the outer layer |
Basicity: the variable that decides chemical compatibility
HALS are amines, and amines react with acids. A strongly basic HALS is protonated by acidic pigments, halogenated flame retardants, acid-catalysed paint residues and agrochemical actives, and a protonated HALS is out of the catalytic cycle. The triazine-linked structure of HALS 944 lowers its basicity relative to UV-770, which is the technical reason it is preferred in agricultural film and in flame-retarded compounds.
| Environment | Risk to high-basicity HALS | Preferred choice |
|---|---|---|
| Halogenated flame-retardant compound | High – acid antagonism | HALS 944 with increased loading, or halogen-free FR system |
| Agricultural film with sulphur/chlorine programmes | High | HALS 944 plus UV absorber |
| Acidic pigments, some carbon blacks | Moderate | HALS 944 or 622, with dispersion check |
| Neutral, dry indoor mouldings | Low | UV-770 is often adequate and cheapest |
Colour and transmittance
HALS 622 is generally closer to white; HALS 944 is slightly yellow as granule and carries a transmittance specification (425 nm min 90.0%, 450 nm min 93.0%) precisely because that colour must be controlled for white and clear articles. For a light-grey automotive interior or a clear greenhouse cover, transmittance at 425 nm is a purchasing specification, not a cosmetic detail.
Blending two HALS on purpose
Many commercial “783 type” products are blends, and blending is a legitimate strategy when done deliberately. The logic is to cover two zones: a low-MW grade concentrates near the surface where UV is most intense, an oligomeric grade holds the bulk. The risk is a blend whose ratio is chosen by the supplier rather than by your weathering data.
- Set the bulk level first with an oligomeric HALS (944 or 622) to meet the long-term target.
- Add surface-near loading only if the application’s failure mode is surface cracking rather than bulk embrittlement.
- Keep acid-base compatibility in mind: pairing a high-basicity grade with an acidic system simply wastes the high-basicity portion.
- Re-test after any blend change, because two HALS do not necessarily add up linearly.
Cost-in-use calculation
The comparison that survives a purchasing review is cost per year of service, not cost per kilogram:
| Item | UV-770 route | HALS 944 route |
|---|---|---|
| Additive price per kg | Lower | Higher |
| Loading needed for 3-season film | Not achievable – leaves the film | 0.3 – 0.6% |
| Replacement frequency | 1 – 2 seasons | 3 – 5 seasons |
| In-use cost | Low per kg, high per season | Higher per kg, low per season |
The substitution test that settles it
When moving from one HALS to another, run the same four-step protocol every time:
- Fix the rest of the package. Keep antioxidant, acid scavenger and UV absorber constant, otherwise the comparison is meaningless.
- Test at final thickness with the process the product actually uses.
- Include the chemical environment where relevant – pesticide contact, flame retardant, or pigment loading.
- Report the failure criterion: retained elongation, carbonyl index or colour shift, at a defined percentage. Vague “better” conclusions do not survive a customer audit.
Frequently asked questions
Which is better, 944 or 622? Both are oligomeric and both persist. 944 is triazine-based with lower basicity and is preferred where acidic chemicals are present; 622 is often preferred where colour is critical. Neither is universally “better”.
Can I replace UV-770 with HALS 944 at the same loading? No – they differ in molecular weight by a factor of four to six. Recalculate on a molar and persistence basis, then validate.
Is UV-783 a distinct chemical? Usually it is a blend of 770-type and 622-type HALS. Ask the supplier for the ratio and treat it as a formulation, not a molecule.
Do HALS work in filled compounds? Yes, but fillers adsorb stabiliser and acidic fillers deactivate basic HALS; increase loading and validate with the full filler package.
Contact PolyBlueChem for HALS 944 samples, COA and selection support.