Polyolefins fail in sunlight for one reason: ultraviolet radiation initiates a radical chain reaction that cannot stop itself. Hindered amine light stabilizers interrupt that chain, and among them Light Stabilizer 944 (CAS 70624-18-9) has held a leading position for four decades in the applications where permanence matters most – polypropylene fibres, polyethylene agricultural film, pipe, geomembrane and exterior automotive parts.
This guide explains what HALS 944 is, why its oligomeric structure (Mn 2000-3100) changes the economics of stabilisation, how to interpret the transmittance specification at 425 and 450 nm, where it sits relative to UV-770, UV-622, UV-783 and UV-292, and how to choose loading and co-additives for a target service life.
What Is Light Stabilizer 944?
Light Stabilizer 944 is an oligomeric hindered amine light stabilizer built from 2,2,6,6-tetramethylpiperidinyl groups attached through a triazine ring and a hexamethylene linker. It is sold as slightly yellow granules with a number-average molecular weight between 2000 and 3100 – a range that places it firmly in the “polymeric HALS” class, alongside HALS 622, and well above the monomeric HALS such as UV-770 and UV-292.
The class matters more than the individual grade. All HALS protect by the same catalytic cycle: the piperidinyl amine is oxidised to a nitroxyl radical, which traps alkyl radicals and is regenerated, so a single molecule can repeat the cycle many times. What separates the grades is physical behaviour – how fast the molecule volatilises, migrates to the surface, is extracted by water or chemicals, or plates out on processing equipment. That is why molecular weight and basicity, not “activity”, are the practical selection criteria.
Chemical Identity: CAS 70624-18-9
| Item | Detail |
|---|---|
| Common names | Light Stabilizer 944, HALS 944, UV-944 |
| CAS Registry Number | 70624-18-9 |
| Chemical class | Hindered amine light stabilizer (HALS), polymeric / oligomeric, triazine-based |
| Number-average molecular weight (Mn) | 2000 – 3100 |
| Appearance | Slightly yellow granular |
| Transmittance | 425 nm min 90.0%; 450 nm min 93.0% |
| Volatile matter | Max 1.0% |
| Ash | Max 0.1% |
| Typical loading | 0.15 – 0.6% in polyolefins |
Because the product is an oligomer rather than a single molecule, a molecular formula is not a meaningful specification; molecular weight distribution and the transmittance/volatility/ash set are. Buyers comparing quotations should insist on all five numbers, since a low-Mn lot will behave like a different product in a fibre line.
How HALS Work
The mechanism runs in four repeating steps:
- Activation. The hindered amine is oxidised – by processing peroxides or by photochemically generated species – to the corresponding nitroxyl radical.
- Radical trapping. The nitroxyl radical combines with alkyl radicals formed when UV light abstracts hydrogen from the polymer chain, forming a hydroxylamine.
- Regeneration. The hydroxylamine reacts with peroxy radicals and returns to the nitroxyl form, closing the catalytic loop.
- Hydroperoxide decomposition. HALS also decompose hydroperoxides, removing the branching points that would otherwise accelerate photo-oxidation.
Two practical consequences follow. First, HALS are catalytic, so a few tenths of a percent can protect a thick section for years – performance is not proportional to loading in the way an additive that is consumed would be. Second, HALS only work where they are: an additive that has volatilised out of a 30 micron film cannot protect it, no matter how active it is in a beaker.
The Oligomeric Advantage
| Failure mode | Monomeric HALS (e.g. UV-770) | Oligomeric HALS 944 (Mn 2000-3100) |
|---|---|---|
| Volatilisation during processing and service | Significant, especially in fibres and thin films | Low – volatility max 1.0% by specification |
| Surface migration and bloom | Fast; can plate out on rollers | Slow; the oligomer cannot diffuse far in the polymer matrix |
| Extraction by water, detergents, oils | Appreciable | Low |
| Extraction by agricultural chemicals | High for basic HALS exposed to sulphur and halogenated actives | Lower, and the triazine structure reduces acid-base antagonism |
| Fogging in automotive interiors | Contributes | Minimal |
| Cost per unit | Lower | Higher – justified by persistence, not by activity |
The trade-off is dispersion: an oligomer with Mn above 2000 does not dissolve into the polymer melt as easily as a monomer, so thin-film and fibre producers typically pre-disperse HALS 944 in a masterbatch rather than feeding the powder or granule directly.
Specifications We Supply
| Property | Specification | Why it matters |
|---|---|---|
| Appearance | Slightly yellow granular | Granular form feeds cleanly and is low-dust |
| Transmittance at 425 nm | Min 90.0% | Colour and haze contribution in natural and light-tinted articles |
| Transmittance at 450 nm | Min 93.0% | Protects optical quality of clear and white film |
| Molecular weight (Mn) | 2000 – 3100 | Determines volatility and migration resistance |
| Volatile matter | Max 1.0% | Volatiles cause mould deposit and plate-out at high throughput |
| Ash | Max 0.1% | Inorganic residue affects film clarity and electrical properties |
| Packaging | 25 kg bag or 500 kg jumbo bag | Sealed liner protects colour quality |
Why Transmittance Is a Real Specification
Two HALS with identical molecular weight can differ in colour. Transmittance at 425 nm sits at the edge of the visible spectrum – the region where slight yellowing of a stabilizer becomes visible yellowing of a white or clear article. A specification of min 90.0% at 425 nm and min 93.0% at 450 nm is what separates a grade suitable for white film and light-coloured fibre from one that will force a tint correction.
This is not cosmetic. Agricultural film users specify high transmittance because a yellowed film reduces photosynthetically active radiation transmission at the exact wavelengths that matter; automotive interior suppliers specify it because colour drift in a light-grey trim is a warranty claim.
Applications
Polypropylene fibres, tapes and nonwovens
Fibres have the highest surface-to-volume ratio of any polyolefin article, which makes volatilisation the dominant loss mechanism. HALS 944 is standard in BCF carpet yarn, spunbond nonwovens, woven bags and strapping. Loadings typically start at 0.3-0.6% and are validated against a target retained strength after accelerated weathering.
Polyethylene film and agricultural film
Greenhouse covers, silage film and mulch film combine thin sections, intense UV and chemical exposure – sulphur-based fungicides, chlorine disinfectants and acidic pesticide actives. The triazine-linked, less basic structure of HALS 944 tolerates those chemicals better than monomeric secondary-amine HALS. See the dedicated guide on HALS 944 in agricultural film and greenhouse covers.
Pipe, geomembrane and cable jacketing
Buried and exposed PE pipe, landfill geomembrane and cable jacketing are designed for 25-50 year service. Extraction resistance and low volatility dominate the choice; HALS 944 is commonly combined with a high-MW antioxidant package and a UV absorber for the outer layer.
TPO and PP automotive parts
Exterior trim, bumpers and interior components combine thermal load, UV and a low-fogging requirement. Oligomeric HALS meet the fogging limit that monomeric grades struggle with, and their low plate-out keeps tooling clean over long runs.
EVA, TPU and elastomers
In softer matrices the risk is migration to the surface and adhesion loss in multilayer structures. The oligomer’s low diffusion rate helps maintain interlayer bond strength.
Formulation Guidance
| Application | HALS 944 loading (% w/w) | Typical co-additives |
|---|---|---|
| PP fibre / tape | 0.3 – 0.6 | Primary antioxidant, phosphite, acid scavenger |
| PE film, thin | 0.2 – 0.5 | UV absorber (benzotriazole), antioxidant |
| Agricultural film | 0.3 – 0.6 | UV absorber plus a lower-basicity HALS partner for surface protection |
| PE pipe / geomembrane | 0.15 – 0.4 | High-MW phenolic antioxidant, carbon black in outer layer |
| TPO automotive | 0.2 – 0.5 | UV absorber, low-fogging antioxidant package |
| Masterbatch concentrate | 5 – 20 (in carrier) | PE or PP carrier matched to the let-down resin |
Three formulation rules matter more than the exact numbers. First, pair bulk and surface protection: HALS work in the bulk, UV absorbers protect the surface layer where UV is strongest. Second, balance the antioxidant system: HALS need a low level of peroxides to activate, but a poorly stabilised melt produces so many that the HALS is consumed early. Third, validate at final thickness: a 200 micron plaque test says little about a 25 micron film.
HALS 944 Versus Other Grades
| Grade | Type | Mn | Volatility | Basicity | Best fit |
|---|---|---|---|---|---|
| HALS 944 | Oligomeric, triazine-based | 2000 – 3100 | Low | Low-moderate | Fibres, agricultural film, pipe, automotive |
| HALS 622 | Oligomeric, ester-based | ~2000-4000 | Low | Moderate | Thick sections, pipe, general polyolefins |
| UV-770 | Monomeric, secondary amine | ~480 | High | High | Thick moulded parts, cost-driven indoor use |
| UV-783 | Blend (770/622 type) | Mixed | Medium | Moderate | Balanced cost and persistence |
| UV-292 | Monomeric, liquid | ~509 | High | Moderate | Coatings, low-melting systems |
A fuller comparison – including how to blend two HALS deliberately rather than by accident – is in Choosing a HALS: 944 vs 622 vs 770 vs 783.
Interaction with Flame Retardants, Fillers and Chemicals
HALS chemistry is acid-sensitive. Strongly acidic species protonate the amine and take it out of the catalytic cycle, so a halogenated flame retardant or an acidic pigment can consume a high-basicity HALS quickly. The lower basicity of HALS 944 reduces that loss, but it does not eliminate it: in halogenated systems the stabiliser package must be re-validated, and a higher HALS loading is often needed to compensate.
| Co-component | Interaction | Practical response |
|---|---|---|
| Halogenated flame retardants | Acid antagonism deactivates basic HALS | Use a lower-basicity HALS, increase loading, or switch to a halogen-free system |
| Acidic pigments and carbon black | Adsorption and acid-base reaction | Increase HALS loading; check dispersion quality |
| Metal hydroxides (ATH/MDH) | High loading can adsorb the stabiliser | Validate weathering with the full filler package |
| Sulphur and chlorine-based agrochemicals | Extraction and deactivation at the surface | Combine with a UV absorber; increase HALS level for long-life film |
| Phenolic antioxidants | Generally synergistic with HALS | Keep the ratio balanced; excess phenolic can retard HALS activation |
Processing and Dispersion
| Stage | Practice |
|---|---|
| Feeding | Granular product feeds well; for fibre and thin film use a masterbatch to avoid undispersed particles that become stress points |
| Compounding | Normal polyolefin profiles; avoid prolonged residence at peak temperature to protect transmittance |
| Film extrusion | Check gel count and haze after adding HALS; poor dispersion shows up as optical defects before it shows up as a weathering failure |
| Fibre spinning | Filter pressure rise indicates agglomerates – pre-dispersion is the fix, not a finer filter |
| Storage of compound | Keep dry; moisture does not degrade HALS 944 directly but degrades the antioxidant system around it |
Testing and Service-Life Prediction
Weathering tests rank stabiliser packages; they do not predict years. What they do give you is a defensible comparison, provided three things are held constant:
- Thickness – always test at the thickness the product will be made in.
- Exposure condition – xenon arc, fluorescent UV or natural exposure produce different rankings; pick the one closest to the application and keep it fixed.
- Failure criterion – retained tensile or elongation at a defined percentage, carbonyl index by FTIR, or colour change (delta E). State it up front.
For agricultural film, an additional chemical-exposure step (pesticide or disinfectant contact followed by UV exposure) is far more informative than UV alone, because that combination is what destroys covers in service.
Safety, Handling and Regulatory Status
HALS 944 is an industrial additive supplied as granules. The controls are standard: local exhaust ventilation where the material is weighed, dust masks and eye protection when handling open bags, and normal good housekeeping to avoid dust accumulation. Store away from strong acids and oxidising agents.
Regulatory status – including food-contact eligibility, which varies by jurisdiction and by final article – is confirmed for the destination market at enquiry stage. We supply SDS in English and a per-batch COA with every shipment rather than relying on a generic datasheet.
Packaging, Storage and Supply
| Item | Detail |
|---|---|
| Standard pack | 25 kg net multi-layer bag with inner PE liner |
| Bulk pack | 500 kg jumbo bag on request |
| Storage | Dry, ventilated, out of direct sunlight; keep bags sealed to preserve colour |
| Documents | COA per batch (appearance, transmittance, Mn, volatiles, ash), SDS, shipping documents |
| Samples | Available for compounding and weathering trials |
Frequently Asked Questions
Is HALS 944 a UV absorber? No. It is a radical scavenger that works in the bulk of the polymer; UV absorbers protect the surface. Most durable formulations use both.
Why is Mn 2000-3100 important? It is the physical basis of persistence: above roughly 2000 the oligomer volatilises and migrates slowly, which is what makes it suitable for thin sections and long service life.
Can HALS 944 replace UV-770 one-for-one? Not on weight. 770 is more active per gram in thick sections but volatile; substitutions need a loading check and a weathering re-test.
Does it work in halogenated flame-retardant compounds? It works better than more basic HALS, but the acid antagonism still costs performance – validate and expect to raise the loading.
What is the difference between HALS 944 and 622? Both are oligomeric; 944 is triazine-based with lower basicity, 622 is ester-based. Selection usually turns on acid exposure and on colour, not on activity.
Contact PolyBlueChem for specifications, samples, COA and quotations.