An agricultural film is the most hostile environment a polyolefin stabiliser package will ever face. A greenhouse cover is 100-200 microns thick, sits in full sun for three to five years, and is simultaneously attacked by sulphur-based fungicides, chlorine disinfectants, acidic pesticide actives and condensation. Nowhere else does the difference between a cheap stabiliser and the right one become visible as quickly – usually in the second season, as a cover that tears along the fold.
This guide explains how Light Stabilizer 944 (HALS 944, CAS 70624-18-9) is used in greenhouse and mulch film, why its triazine-based oligomeric structure tolerates agrochemical exposure better than monomeric HALS, how much to add, and how to test so that a five-year claim is actually supported.
Why greenhouse film fails
Photo-oxidation in polyethylene proceeds by a radical chain reaction: UV photons abstract hydrogen from the chain, oxygen adds to form peroxy radicals, and those radicals abstract further hydrogen, branching the reaction. The visible outcome is embrittlement, then cracking along the folds, then total loss of mechanical integrity – a cover that shatters instead of tearing.
Three factors accelerate it in the field beyond what a laboratory UV test reproduces:
- Chemical attack. Sulphur-based fungicides and chlorine-based disinfectants react with the stabiliser itself, especially with basic (secondary amine) HALS.
- Thinness. A 150 micron film has a huge surface-to-volume ratio, so any additive that can volatilise or migrate does so quickly.
- Thermal cycling and condensation. Daily wet-dry cycles promote additive extraction at the surface, exactly where UV damage starts.
What HALS 944 brings to film
| Property of HALS 944 | Consequence in agricultural film |
|---|---|
| Mn 2000-3100 (oligomeric) | Low volatility and slow migration – the stabiliser stays in a thin film for years |
| Triazine-based, lower basicity than monomeric HALS | Less deactivation by acidic pesticide actives and by halogenated or sulphur-containing chemicals |
| Transmittance 425 nm min 90.0%, 450 nm min 93.0% | Keeps the film clear and colour-neutral, preserving light transmission for the crop |
| Volatile matter max 1.0% | Less plate-out and die deposit on high-output blown film lines |
| Ash max 0.1% | No inorganic haze in a film whose whole purpose is transmitting light |
| Granular form | Dust-free handling; suited to masterbatch preparation |
The transmittance specification matters more here than in any other application. Greenhouse film is a optical product: yellowing reduces photosynthetically active radiation at precisely the wavelengths the crop needs, and it does so before mechanical failure becomes visible. Buyers therefore specify transmittance on the additive, not only on the finished film.
Formulating a multi-season greenhouse film
| Component | Typical level (% w/w) | Function |
|---|---|---|
| HALS 944 | 0.3 – 0.6 | Bulk radical scavenging, long-term persistence |
| Additional HALS (lower MW or different basicity) | 0 – 0.3 | Surface-near protection; tune response to agrochemicals |
| UV absorber (benzotriazole type) | 0.1 – 0.4 | Surface screening of the UV fraction |
| Primary antioxidant (hindered phenol) | 0.05 – 0.15 | Processing and long-term thermal stability |
| Phosphite secondary antioxidant | 0.03 – 0.1 | Melt stability, colour control |
| Acid scavenger (stearate / hydrotalcite) | 0.05 – 0.15 | Neutralises catalyst residues and acidic contaminants |
| Anti-fog / anti-dust additives | As required | Light transmission and dust shedding |
The numbers are a starting point for a three-season cover, not a recipe. Two structural choices decide the outcome more than the levels: whether the HALS package is pre-dispersed in a masterbatch (it must be, in a 150 micron blown film), and whether the acid scavenger is adequate for the resin’s catalyst residues.
Mulch film: a different balance
Mulch film is thin (8-30 microns), used for a single season, and often black or silver-black. Here the economics change: the film’s life is limited by the crop cycle, so the target is not maximum persistence but enough persistence to survive the season without tearing during mechanical laying and weeding. Lower HALS loadings are common, carbon black contributes substantial UV screening, and the dominant risk shifts to processing stability and to cost.
| Film type | Thickness | Service target | HALS strategy |
|---|---|---|---|
| Greenhouse cover | 100 – 200 micron | 3 – 5 seasons | HALS 944 at 0.3-0.6% plus UV absorber; acid resistant package |
| Small tunnel / low tunnel | 60 – 120 micron | 1 – 2 seasons | HALS 944 at 0.2-0.4% |
| Mulch film | 8 – 30 micron | 1 season | Lower HALS with carbon black; focus on laying strength |
| Silage / bale wrap | 25 – 50 micron | 1 season, high mechanical demand | Balanced HALS plus UV absorber; toughness over longevity |
Agrochemical interaction: the hidden failure mode
Field failures in agricultural film rarely match laboratory predictions, and the reason is almost always chemical. Sulphur-based powdery mildew treatments, chlorine-releasing disinfectants and acidic formulations deactivate basic HALS at the film surface, which is where the UV attack begins. A film that passes 3000 hours of clean xenon arc exposure can fail in eighteen months under a sulphur programme.
- Choose lower-basicity HALS. The triazine-linked structure of HALS 944 reduces protonation compared with monomeric secondary-amine HALS.
- Do not rely on one molecule. A blend covering both bulk and surface-near regions tolerates partial deactivation better than a single high-loading grade.
- Test with the chemical in the loop. Apply the actual agrochemical formulation to the film, then expose. Suppliers who have done this can tell you which combination to avoid.
- Increase loading where the risk is known. In high-sulphur programmes a 20-30% increase in HALS is cheaper than an early cover replacement.
Processing: blown film specifics
| Issue | Cause | Response |
|---|---|---|
| Die deposit / plate-out | Volatiles and low-MW fractions condensing on the die lip | Specify volatile matter max 1.0%; use a masterbatch; check melt temperature |
| Gels and specks | Undispersed HALS agglomerates | Pre-disperse in carrier resin; verify masterbatch quality |
| Haze increase | Poor dispersion or incompatibility | Check transmittance of incoming lots; adjust carrier |
| Bubble instability | Melt strength reduced by over-loading | Optimise HALS/UV absorber split rather than raising total loading |
How to validate a multi-season claim
Three tests, run in this order, will tell you whether a film formulation deserves a three-season label:
- Accelerated weathering at final thickness – xenon arc per ISO 4892-2, with retained elongation at break as the criterion. Test the actual film, not a pressed plaque.
- Chemical exposure plus weathering – apply the agrochemical programme used by the grower, then weather. This is the step that separates film that is good on paper from film that survives.
- Field trial in the target climate – one season of real exposure, with mechanical testing at the fold line. Accelerated tests rank; field data validates.
Frequently asked questions
How much HALS 944 does a greenhouse cover need? Starting point 0.3-0.6% for a three-season cover at 150 micron, together with a UV absorber and a balanced antioxidant system. Validate by xenon arc testing at final thickness.
Can HALS 944 be used alone? It can, but bulk-only protection leaves the surface under-defended. Most durable film formulations pair an oligomeric HALS with a UV absorber.
Is HALS 944 food-contact approved for agricultural use? Eligibility depends on jurisdiction and on the final article; documentation for the destination market is confirmed at enquiry stage.
Why does my film yellow even with a HALS? Yellowing usually comes from the stabiliser’s own colour, from phenolic antioxidant oxidation, or from agrochemical deposits. Transmittance specifications on the HALS and a phosphite in the package address the first two.
Contact PolyBlueChem for HALS 944 samples, COA and formulation support.