HALS (hindered amine light stabilizers) are the most widely used UV stabilizers for polyolefins. At 0.1–1% they keep PP and PE parts, films and fibers from chalking, cracking and losing strength in sunlight, often for years. As a formulation development partner, SYCHEM TECH helps compounders and film producers choose the light stabilizer grade, dosage and partner additives for their resin and climate.
This guide explains how HALS work, how they compare with UV absorbers, which type suits which part, typical dosages, what deactivates them, and how to test a formula.
Why plastics need UV stabilizers
Sunlight’s UV component breaks chemical bonds in the polymer and starts a chain reaction with oxygen, called photo-oxidation. Free radicals form, react with oxygen to peroxy radicals, and attack the next polymer chain. Polyolefins are especially exposed because catalyst residues and processing damage give the reaction an easy start. The visible result is chalking, gloss loss, yellowing and surface cracks; the invisible result is brittle parts and films that split long before their design life.
What are HALS?
Hindered amine light stabilizers are derivatives of 2,2,6,6-tetramethylpiperidine. The four methyl groups around the nitrogen are the “hindrance” that makes the chemistry work. Unlike UV absorbers, HALS do not absorb UV light at all. They stop the degradation reaction after it has started, by removing the free radicals that carry it.
How HALS work: the regenerating cycle
In use, the hindered amine is oxidised to a stable nitroxyl radical. The nitroxyl traps an alkyl radical from the polymer and becomes an alkoxyamine. The alkoxyamine then reacts with a peroxy radical, releases harmless products and gives the nitroxyl back. This loop, often called the Denisov cycle, lets one stabilizer molecule stop many degradation chains:
Because the stabilizer is regenerated rather than consumed, it loses effect mainly by physical loss, through migration, extraction or volatilisation, or by chemical deactivation with acids. Choosing the right molecular weight and avoiding acidic partners matter more than raw dosage.
HALS vs UV absorbers and other UV stabilizers
| Stabilizer type | How it works | Strengths | Limits |
|---|---|---|---|
| HALS | Traps the free radicals formed by UV, in a regenerating cycle | Works at low dosage, protects thin films, fibers and surfaces, long service life | Basic amines are deactivated by acids; little effect on colour fading of some pigments |
| UV absorbers (benzophenone, benzotriazole, triazine) | Absorb UV and release it as heat | Protect the bulk of thick or transparent parts and the contents behind them | Need path length, so weak in thin films and at the surface; slowly consumed |
| Quenchers (nickel compounds) | Deactivate excited chromophores | Historic use in agricultural film | Colour (green tint), heavy-metal concerns; largely replaced by HALS |
| Pigments and carbon black | Screen UV physically | Carbon black at 2–3% is one of the best UV screens | Only possible where the colour allows it |
The two main types work best together. A UV absorber protects the bulk of a thick part, while HALS protect the surface, where weathering starts. Automotive, ABS and transparent parts usually carry both; thin PE film and PP fiber often rely on a hindered amine alone.
Types of HALS: molecular weight matters
A light stabilizer has to stay in the part to keep working, but it also has to reach the surface. Molecular weight sets that balance:
- Monomeric, low molecular weight (for example LS 770): mobile, so it moves quickly to the surface of thick parts. In thin films and fibers it is lost too fast by volatilisation and extraction.
- Oligomeric and polymeric (LS 944, LS 622): low volatility and low migration, so they last in films, tapes and fibers and survive long service lives. They are the default for polyolefins.
- Low-basicity: N-methyl and NOR types (LS 119, LS 123): the modified nitrogen reacts less with acids, so they keep working with agrochemicals, halogenated flame retardants or acidic fillers.
- Blends (LS 783, LS 791): combine a mobile and a permanent grade, or two permanent ones, for cost-effective protection across thin and thick sections.
The HALS grades we supply, from our antioxidants and HALS range:
| Grade | CAS No. | Type | Typical use |
|---|---|---|---|
| LS 944 | 71878-19-8 | Oligomeric | PE and PP films, tapes, fibers and molded parts; the general-purpose long-term grade |
| LS 622 | 65447-77-0 | Polymeric, low basicity | PE films and PP, often blended with LS 944; low colour |
| LS 770 | 52829-07-9 | Monomeric, low molecular weight | Thick PP, ABS, PS and POM parts, where its mobility keeps the surface protected |
| LS 119 | 106990-43-6 | High-MW, N-methyl | PP fibers and tapes, TPO, agricultural film exposed to agrochemicals, talc-filled PP |
| LS 783 | Blend | Blend of two grades | Cost-effective LDPE/LLDPE agricultural film |
| LS 791 | Blend | Synergistic blend | PE, PP and agricultural film |
| LS 3853 | — | Low molecular weight | PP and PE, often supplied as a masterbatch |
| LS 123 | — | NOR type, low alkalinity | Coatings and inks, acidic environments |
Typical HALS dosage by application
| Application | Typical grades | Starting dosage |
|---|---|---|
| PP fibers, tapes and raffia | LS 944, LS 119 | 0.2–1.0% |
| LDPE/LLDPE agricultural film | LS 944, LS 783, LS 119 | 0.2–1.0%, rising with service life, sunlight and agrochemical exposure |
| HDPE and PP woven bags, FIBC | LS 944, LS 783 | 0.2–0.6% |
| Thick PP molded parts, outdoor furniture | LS 770 + LS 944 | 0.1–0.4% |
| TPO and PP automotive parts | LS 119 or LS 944 + UV absorber | 0.2–0.5% HALS + 0.1–0.3% UV absorber |
| ABS, PS, POM | LS 770 + UV absorber | 0.2–0.5% HALS + 0.2–0.5% UV absorber |
These are starting points. The final dose depends on the expected service life, the sunlight at the place of use (a film in southern Spain sees far more UV than one in northern Europe), section thickness, colour and the rest of the formula, and it should be confirmed by weathering tests.
What weakens HALS
Most hindered amines are bases, so the main threat is acid. Watch for these interactions:
| Interaction | Why | What to do |
|---|---|---|
| Acidic agrochemicals (sulfur- and chlorine-based) | Acids neutralise the basic amine, so it stops cycling | Use a low-basicity grade such as LS 119, raise the dose, and test with the actual pesticide |
| Halogenated flame retardants | HBr or HCl from the flame retardant deactivates the stabilizer | Use NOR or low-basicity grades, or move to a halogen-free flame retardant |
| Thioester antioxidants (DLTP, DSTP) | Their sulfur by-products are acidic and antagonise hindered amines | Keep thioesters low in UV-critical parts; use a phenolic + phosphite package |
| Acidic fillers and some pigments | They adsorb or neutralise the stabilizer | Raise the dose or choose a less basic grade; confirm by weathering |
| PVC | HCl released during processing reacts with the amine | PVC usually relies on UV absorbers and TiO₂ instead |
If you use halogen-free flame retardants instead, our phosphorus vs MDH comparison covers the options. In blown film, some HALS and other polar additives can also compete with the processing aid at the die wall; our PPA processing aid guide explains how to check for it.
How to choose HALS for your formulation
- Section thickness. Films, tapes and fibers need high-molecular-weight grades such as LS 944 or LS 119. Thick molded parts benefit from adding a mobile grade such as LS 770.
- Service life and climate. Agree the target, for example two seasons of greenhouse film in a sunny region, and size the dose to it.
- Chemical exposure. Agrochemicals, halogenated flame retardants, acidic fillers or acid rain call for low-basicity grades.
- Transparency and colour. Transparent and light-coloured parts usually need a UV absorber as well. Use rutile TiO₂, not photoactive anatase.
- Process stabilization. HALS do little during processing. Keep a phenolic + phosphite antioxidant package to protect the melt, and avoid high thioester levels.
- Regulations. For food contact or toys, ask for the producer’s compliance statement for the specific grade, polymer and use.
How to test UV stability
- Accelerated weathering: xenon-arc (ISO 4892-2) is closest to sunlight; fluorescent UV (ISO 4892-3, ASTM G154) is faster and harsher. Automotive parts follow OEM xenon cycles.
- What to measure: retained tensile elongation or impact strength, carbonyl index by FTIR, gloss retention and colour change (ΔE), at fixed exposure intervals.
- Always run a reference: compare against a formula with known outdoor performance, because accelerated hours do not convert directly into outdoor years.
- Outdoor exposure: for agricultural film, sites in Florida, Arizona or southern Europe, with dose measured in kLy (kilolangleys), give the most reliable lifetime data.
Frequently asked questions
Can HALS be used in food-contact applications?
Several oligomeric and polymeric grades are cleared for food-contact polyolefins under US FDA and EU rules, within concentration limits. Compliance depends on the grade, polymer and intended use, so ask us for the producer’s declaration for the grade you plan to use.
Do black parts need HALS?
Black PE pipe with 2–3% well-dispersed carbon black usually does not, because the carbon black screens the UV. Thin black parts, low carbon black loadings and parts that must keep their surface gloss still benefit from a light stabilizer.
Can HALS be used in PVC?
Rarely. PVC releases HCl, which reacts with the basic amine. PVC is normally protected with UV absorbers and rutile TiO₂.
Are HALS and antioxidants the same thing?
No. Antioxidants protect the polymer from heat and oxygen during processing and long-term heat ageing, while HALS protect it from light. Most outdoor formulas need both, and some hindered amines also add long-term thermal stability in polyolefins.
How should HALS be stored?
Keep bags sealed in a cool, dry place out of direct sunlight. The shelf life of each grade is stated on its TDS, which we send with the quotation.
Get a HALS recommendation
Tell us your polymer (for example PE or PP), part thickness, colour, target service life, place of use and any agrochemicals or flame retardants in the formula. We will suggest a light stabilizer and UV absorber package with starting dosages, and email you the TDS and arrange samples. Contact our technical team or write to info@sychemtech.com.
Related products: our antioxidants & HALS range of light stabilizers and primary and secondary antioxidants, including LS 944, LS 770 and LS 119, and the full functional additives portfolio.
