A UV absorber stops the light. A HALS stops the damage the light has already started. They are not competing options and they are not interchangeable, which is why almost every serious outdoor formulation contains both.
If you make anything that lives in Indian sunlight – agricultural film, woven sacks, tarpaulin, water tanks, garden furniture, cable, pipe – this is the chemistry you are buying when you buy UV masterbatch.
What sunlight actually does to a polymer
Ultraviolet photons carry enough energy to break covalent bonds in a polymer chain. When a bond breaks, the fragments are free radicals – highly reactive species that immediately attack neighbouring chains.
The sequence, once started, sustains itself:
A radical reacts with atmospheric oxygen to form a peroxy radical. The peroxy radical abstracts hydrogen from a nearby chain, creating a hydroperoxide and a new radical. The hydroperoxide decomposes under heat or further UV into two more radicals. One initiating photon has produced a chain reaction that continues in the dark.
This is photo-oxidative degradation, and its visible symptoms are familiar: chalking, fading, surface crazing, loss of gloss, and – the one that matters – catastrophic loss of impact strength and elongation. A tarpaulin that still looks acceptable can tear at a fraction of its original load.
Polypropylene is the most vulnerable of the commodity polymers, because its tertiary carbon is easy to attack. Polyethylene is more tolerant but not immune. This is why PP woven sacks and PP raffia need considerably more UV protection than a comparable PE product.
Mechanism one: UV absorbers
A UV absorber is a molecule that absorbs ultraviolet photons and dissipates the energy as heat instead of allowing it to break bonds. Benzotriazoles and benzophenones are the common families.
The strengths. Direct, immediate protection. They intercept the photon before the polymer does.
The limitations, and they are significant.
They need thickness to work. Absorption follows the Beer-Lambert law – the amount of light stopped depends on the path length through the material. In a 3 mm moulded wall, a UV absorber has plenty of depth in which to intercept photons. In a 25 micron mulch film it has almost none, and the majority of incident UV passes straight through. This single fact is why UV absorbers are close to useless in thin film and effective in thick sections.
They are consumed. A UV absorber is used up over time, and once depleted the polymer is unprotected.
They do not protect the surface. Even in thick sections the outermost layer receives full-intensity radiation before any absorption has occurred. Surface chalking happens anyway.
Mechanism two: HALS
Hindered amine light stabilisers do not absorb ultraviolet light at all. They work downstream, interrupting the radical chain reaction after it has already begun.
The important property is that HALS are regenerative. The active nitroxyl radical scavenges a polymer radical, forms an intermediate, and is then reformed – returning to scavenge again. This cycle, known as the Denisov cycle, means a HALS molecule is not consumed in the way a UV absorber is. Small quantities deliver protection out of all proportion to their loading.
Why HALS dominate film applications. Because the mechanism has no dependence on path length. A HALS works exactly as well in a 25 micron film as in a 3 mm section – it is scavenging radicals wherever they form. For mulch film and agricultural film, HALS is the primary protection and a UV absorber is an optional supplement.
Molecular weight matters. Low molecular weight HALS migrate readily, giving excellent surface protection but risking loss through volatilisation, extraction by rain, or migration into contact media. High molecular weight and polymeric HALS stay put – essential for thin film with a large surface-to-volume ratio, for anything in contact with food or soil, and for long service lives.
Comparing the two
| UV absorber | HALS | |
| Mechanism | Absorbs UV, releases it as heat | Scavenges free radicals |
| Consumed in use | Yes | Largely regenerative |
| Depends on part thickness | Strongly | No |
| Effective in thin film | Poor | Excellent |
| Protects the surface layer | Weak | Good |
| Typical loading | Higher | Lower |
| Cost per unit protection | Higher | Lower |
Used together they are more than additive. The absorber reduces the total radiation reaching the polymer, so fewer radicals form; the HALS deals with the radicals that form anyway and keeps working after the absorber is spent. Most quality outdoor packages combine them.
The interactions that catch people out
Three antagonisms are worth knowing, because they cause failures that look inexplicable.
Sulphur-containing antioxidants deactivate HALS. Thioester secondary antioxidants and basic HALS interact, and the HALS loses effectiveness. If your formulation already contains a thio-synergist, the UV package has to be designed around it.
Acidic residues neutralise HALS. HALS are basic. Halogenated flame retardants, acidic pigments and residual acidic catalyst can protonate the amine and shut down the Denisov cycle. Flame-retardant plus UV-stabilised formulations need specific HALS chemistries selected for acid tolerance.
Some pigments help, some hurt. Carbon black is an outstanding UV screen in its own right – at pipe and geomembrane loadings it does most of the protective work. Certain titanium dioxide grades are the opposite: anatase TiO2 is photocatalytic and actively accelerates degradation. Only surface-treated rutile grades belong in an outdoor product. This is covered in titanium dioxide in plastics and, for black, in carbon black and UV protection.
Agrochemical attack is a special case. Sulphur and halogen-based pesticides used under greenhouse and mulch film attack conventional HALS aggressively. Films for Indian agriculture should use HALS chemistries specifically qualified for pesticide resistance, or service life will fall far short of the laboratory prediction.
How much protection does an Indian product need
The honest answer is that dosage is set by the required service life, not by a rule of thumb – and the inputs are the site’s UV exposure, part thickness, polymer type, pigmentation and the chemical environment.
What is specific to India is the exposure. Much of the country receives high annual total solar radiation, and large parts of Rajasthan, Gujarat and the Deccan sit at the top of that range. A film formulated for a European service life will not deliver that life in Kutch.
The industry convention for outdoor agricultural films is to specify in kilolangleys (kLy) – cumulative solar radiation the film must survive rather than a period in months. A film rated to a stated kLy value delivers a predictable calendar life once you know the site’s annual radiation. It is a far more honest basis for a guarantee than “one season”, and it is what a competent supplier will quote against.
For PP woven sacks and FIBC, protection is usually specified as retained tensile strength after a defined exposure – commonly 50% retention as the end of useful life. FIBC in particular carries a safety obligation, since a bag that has lost strength invisibly is a load-handling hazard.
Specifying UV masterbatch properly
Put these on the enquiry and you will get a formulation rather than a price:
– Polymer and grade – PP needs materially more protection than PE – Part thickness – it decides whether an absorber contributes anything at all – Required service life, ideally in kLy, or the region and expected exposure – Pigmentation – carbon black changes the requirement completely – Chemical environment – pesticides, fertiliser, chlorine, cleaning agents – Other additives present – flame retardants, thio-antioxidants, acidic fillers – Regulatory constraints – food contact, soil contact, potable water – Whether the part is also in contact with hot water or steam
Our additive masterbatches are formulated against these inputs. We manufacture under ISO 9001:2015 and can supply RoHS and REACH declarations where the application requires them.
Testing: what a claim is worth
Two accelerated methods dominate.
Fluorescent UV (QUV-type) cycles UV lamps with condensation to simulate day and dew. It is inexpensive and correlates reasonably for pure UV degradation.
Xenon arc reproduces the full solar spectrum including visible and infrared, with controlled humidity and optional water spray. More representative, more expensive.
Both accelerate; neither converts cleanly to calendar years. Treat accelerated results as a ranking tool between formulations, not as a prediction of life. Real outdoor exposure at a site with the same radiation profile as the end use is the only honest confirmation, and it takes as long as it takes.
Be sceptical of any supplier quoting years of service life from an accelerated test alone with no exposure basis stated.
Talk to us
Send us your polymer, thickness, region and required service life and we will formulate the UV package around them. If the product is for agriculture we will also want to know what is being sprayed under it, because that decides which HALS chemistry survives.
Related: agriculture applications, packaging, building and construction.
FAQs
What is the difference between HALS and UV absorbers? A UV absorber absorbs ultraviolet photons and releases the energy as heat, preventing bond breakage. A HALS does not absorb UV at all – it scavenges the free radicals produced after degradation has started. Absorbers are consumed over time; HALS are largely regenerative.
Why are UV absorbers ineffective in thin film? Because absorption depends on path length. In a 25 micron film there is almost no depth in which to intercept photons, so most UV passes straight through. HALS have no thickness dependence, which is why they are the primary protection in film.
Do HALS get used up? Not in the way UV absorbers do. The active nitroxyl radical is regenerated through the Denisov cycle, so a HALS molecule can scavenge repeatedly. It is still lost slowly through migration, extraction and chemical attack, which is why molecular weight and chemistry selection matter.
Can HALS and flame retardants be used together? Only with care. Halogenated flame retardants generate acidic species that protonate and deactivate basic HALS. Formulations needing both require HALS chemistries specifically selected for acid tolerance.
Does carbon black replace the need for UV stabiliser? In many applications it does most of the work. Carbon black is an excellent UV screen at the loadings used in pipe, geomembrane and cable. Coloured and natural products have no such screen and depend entirely on the stabiliser package.
What is a kilolangley and why do film suppliers quote it? A kilolangley is a unit of cumulative solar radiation. Quoting UV life in kLy rather than months is more honest, because a film exposed in Rajasthan receives far more radiation per year than the same film in a low-exposure region. Once you know the site’s annual radiation you can convert kLy to a calendar life.
Do pesticides affect UV stabilisers? Yes, substantially. Sulphur and halogen-based agrochemicals attack conventional HALS and can cut a greenhouse or mulch film’s life dramatically. Films for Indian agriculture should use pesticide-resistant HALS chemistries.
How reliable are accelerated weathering tests? They are reliable for ranking formulations against each other and unreliable as a prediction of service life in years. There is no universal conversion factor from QUV or xenon arc hours to outdoor months. Real exposure at a site matching the end use is the only firm confirmation.