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UV absorbers vs HALS: choosing light stabilizers for plastics.
Two families, two completely different jobs. One soaks
up the ultraviolet light before it reaches the polymer; the other never
touches the light at all and instead mops up the radicals it leaves behind.
Here is how each works, which plastics suit which, how to combine them, and
how to verify the package by weathering.
Chelora Chemicals Technical TeamPublished 12 October 202611 min readPolymer Additives
Two families, one jobUV absorbers
vs HALS
UV absorbers
Absorb UV and release it as heat — benzotriazoles, benzophenones, triazines, cyanoacrylates, oxanilides
HALS
Scavenge radicals and regenerate — derivatives of 2,2,6,6-tetramethylpiperidine
Protects best
Absorbers in thick sections; HALS at the surface and in thin films and fibres
Polymer lead
HALS in polyolefins — absorbers in polycarbonate, PVC, polyesters
Typical loading
About 0.1 to 1% of polymer weight, for each type
Named examples
CAS 1843-05-6 benzophenone absorber — CAS 52829-07-9 low molecular weight HALS
The two families are not strong and weak versions
of one another — one stops light, the other stops radicals.
One stops light, one stops radicalsThey act at different points in the degradation chain, which is why they combine well.
Thickness decides the absorberBeer–Lambert: in films, fibres and tapes the light path is too short to absorb in.
HALS regenerate, absorbers depleteA small amount of HALS neutralises many radicals over the life of the part.
Polymer first, then the packageHALS lead in polyolefins; absorbers lead in polycarbonate and PVC, where HALS struggle.
UV absorbers and HALS (hindered amine light
stabilizers) are the two main families of light stabilizer for plastics, and they
protect polymers in fundamentally different ways. A UV absorber soaks up harmful
ultraviolet light and converts it to harmless heat before it can damage the
plastic. A HALS does not absorb light at all; instead it mops up the free radicals
that sunlight creates and is regenerated to keep working.
Because of this difference, UV absorbers suit thick sections, HALS suit thin films,
fibres and surfaces, and the choice also depends on the polymer: HALS are the
backbone of polyolefin stabilization but are not recommended in polycarbonate and
perform poorly in PVC, where UV absorbers dominate. Choosing wrongly shows up as
yellowing, chalking, cracking and loss of strength long before the product’s
expected life is over.
This guide from Chelora
Chemicals compares UV absorbers vs HALS, explains how each works, shows which
plastics suit which, and sets out how to select and combine them. The sections run
in the order a formulation project actually moves: why the problem exists, how each
family answers it, which polymer takes which, whether to combine them, how to
choose, and how the result is tested.
Plastics need light stabilizers because ultraviolet radiation from sunlight starts
photo-oxidation, a radical chain reaction that breaks and cross-links polymer
chains and ruins appearance and strength. Without stabilization, many common
plastics degrade within months outdoors.
When a polymer absorbs UV, bonds break and free radicals form. These radicals react
with oxygen to create peroxy radicals and hydroperoxides, which in turn generate
more radicals, so the damage feeds itself. The visible results are yellowing or
fading, surface chalking, loss of gloss, micro-cracking and embrittlement, while
the invisible result is a steady loss of tensile and impact strength.
Polypropylene is especially sensitive because of its tertiary carbon–hydrogen
bonds, and even plastics that look durable, such as polycarbonate, yellow and craze
under prolonged exposure. Light stabilizers interrupt this process, either by
stopping UV reaching the polymer or by breaking the radical chain, which is why
every outdoor or high-exposure plastic part relies on one or both of the two
families.
The two families intervene at different points in the same chain. A UV
absorber works upstream, removing the light before bonds break, and needs a
reasonable path length of plastic to do it. A HALS works downstream, capturing
the radicals once they have formed, and is regenerated rather than consumed,
which is why it protects thin sections and surfaces so effectively at low
loadings.
What Are UV Absorbers and How Do They Work?
UV absorbers are additives that absorb ultraviolet light, mainly between roughly 290
and 400 nanometres, and release the energy as harmless heat before it can break
polymer bonds.
The most common families are benzotriazoles, benzophenones, hydroxyphenyl triazines,
cyanoacrylates and oxanilides. In benzotriazoles, triazines and
hydroxybenzophenones, an internal hydrogen bond lets the excited molecule shift a
proton and relax back to its starting state within a very short time, turning light
into heat without being destroyed, which allows the absorber to do its job many
times over.
Their performance follows the Beer–Lambert law: the more absorber and the
greater the thickness of plastic, the more light is blocked, so UV absorbers protect
thick parts well but give little help in thin films, fibres and tapes, where the
light path through the absorber is too short. They are also slowly consumed over a
long period by photochemical and physical loss, so loading and thickness set the
service life. Benzotriazoles and triazines are widely used where low colour and high
absorption are needed, as in polycarbonate and polyester, while benzophenones such
as 2-hydroxy-4-octyloxybenzophenone (CAS 1843-05-6) are a common choice in clear
flexible PVC and polyolefins.
Two different “benzotriazoles”
The benzotriazole UV absorbers are 2-(2-hydroxyphenyl)benzotriazole
derivatives, carrying the phenol group that makes the light-to-heat conversion
work. They are not the same material as 1H-benzotriazole (CAS 95-14-7), the
azole used as a
yellow-metal
corrosion inhibitor in metalworking fluids, although that compound is an
intermediate on the route to some of them. Quote the full chemical name and CAS
number on every enquiry so the two are never confused on a purchase order.
What Are HALS and How Do They Work?
HALS are additives built on a 2,2,6,6-tetramethylpiperidine ring that stabilise
plastics by trapping the free radicals formed during photo-oxidation, rather than
by absorbing UV light.
During weathering, the amine is converted to a nitroxyl radical, which captures alkyl
radicals from the degrading polymer and is then regenerated through a series of
reactions known as the Denisov cycle, so a small amount of HALS can neutralise many
radicals over time. This regeneration is why HALS give such long protection at low
loadings, and why they are the most effective light stabilizers for polyolefins,
helping polypropylene to be used in demanding automotive applications. Because they
work at the point where radicals form, they protect the surface layer and thin
sections well. Low molecular weight HALS such as
bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate (CAS 52829-07-9) are mobile and give
good surface protection but can volatilise or migrate, while polymeric and high
molecular weight types resist extraction and loss and suit thin films and fibres in
demanding conditions. Mechanisms of HALS in polypropylene are described in
this
open-access study.
The main weakness is basicity. Conventional HALS react with acids, so they lose
activity in acidic environments such as degrading PVC, or in the presence of some
acidic flame retardants or acidic agrochemicals on greenhouse films. Low-basicity
and N-alkoxy HALS were developed for these cases.
Note
A UV absorber and a HALS are not interchangeable and are not simply
“strong” and “weak” versions of one another. One stops
light, the other stops radicals. If a part fails by surface chalking and
cracking while the bulk looks fine, the surface likely needs more radical
protection (HALS); if it fails by deep yellowing or loss of strength through
the thickness, it likely needs more light absorption (UV absorber).
Which Stabilizer Suits Which Plastic?
The best stabilizer depends mainly on the polymer. HALS usually lead in polyolefins,
UV absorbers lead in polycarbonate and PVC, and combinations are common in
styrenics, polyurethanes and engineering plastics.
Starting points by polymer — orientation for a formulator, not a specification.
Polymer
Typical approach
Points to watch
Polypropylene, polyethylene
HALS as primary stabilizer; UV absorber added for thick, pigmented or heavily exposed parts
Thin films and fibres rely on HALS because UV absorbers cannot absorb enough light
PVC (clear flexible and rigid)
UV absorbers, often benzophenones or benzotriazoles; titanium dioxide in opaque rigid PVC
Conventional HALS are deactivated by HCl; use low-basicity HALS only
Polycarbonate
UV absorbers such as benzotriazoles or triazines; coatings or co-extruded UV layers for long life
Conventional HALS can accelerate hydrolysis and are generally avoided
ABS, polystyrene
HALS plus UV absorber combinations, with antioxidants
Yellowing is the main failure; test the HALS to UV absorber ratio
Polyesters, polyamides
UV absorbers (triazines, benzotriazoles), sometimes with suitable HALS
Processing temperature and volatility of the additive matter
Polyurethane
HALS plus UV absorber
Choose grades compatible with the urethane chemistry
This table is a starting point, not a specification: stabilizer selection also
depends on part thickness, pigments and the exposure climate. A good overview of
selection rules by polymer is given in this
light
stabilizer selection guide.
Do UV Absorbers and HALS Work Better Together?
Often yes. Because UV absorbers and HALS act through different mechanisms, combining
them frequently gives better protection than either alone, but the effect depends on
the pairing and the ratio and is not guaranteed.
What the UV absorber contributes
It reduces the amount of light reaching the bulk of the polymer, working
through the thickness of the part. The more absorber and the greater the path
length, the less light gets through — which is why it earns its place in
mouldings and sheet rather than in film.
What the HALS contributes
It migrates toward the surface layer, where photo-oxidation is most intense,
and neutralises radicals there. Because it is regenerated rather than consumed,
it keeps working for a long time at a low loading, covering exactly the zone the
absorber protects least.
One study of four common polymers found maximum efficiency
at HALS to UV absorber ratios of about 75:25 in polypropylene and high-density
polyethylene, 90:10 in ABS and 80:20 in crystal polystyrene, as reported in
this
synergism paper. However, other work has found weaker effects or antagonism with
some benzotriazole absorbers in polypropylene, while pairing with benzophenones
showed clear synergy. The practical conclusion is to treat every combination as
something to test, not assume.
How Do You Choose a Light Stabilizer for a Plastic Part?
Choose a light stabilizer by working through the polymer, the part thickness, the
exposure and the other additives in sequence, then confirm the shortlist by
weathering tests.
Define the polymer and processing route. Note the resin, the
melt temperature and whether the part is film, fibre, moulding or sheet, since
processing temperature affects additive volatility and thickness decides UV
absorber effectiveness.
Define the exposure and service life. Set the target life and
the climate, including UV intensity, temperature and humidity, because tropical
outdoor use needs much more protection than indoor or shaded service.
Check chemical interactions. Look at acids, halogenated flame
retardants, sulfur or chlorine-containing agrochemicals and metal residues that
can deactivate a basic HALS, and at pigments that compete for the light.
Choose chemistry and molecular weight. Pick the UV absorber
family and HALS type, using low molecular weight grades for surface mobility and
polymeric grades where volatility, migration or extraction are concerns.
Set the loading and ratio. Start from supplier guidance,
commonly a few tenths of a percent of each, and adjust the ratio for thickness
and polymer.
Verify by weathering and check regulations. Run accelerated QUV
or xenon-arc tests, confirm with outdoor exposure where possible, and check
food-contact, toy or other regulatory requirements for the finished
article.
Important
Light stabilizers and the processing aids used with them are industrial
chemicals and should be handled according to the Safety Data Sheet of each
grade, with dust control and protective equipment during compounding. Never
assume that an additive suitable for one polymer is safe in another: an
unsuitable HALS in polycarbonate or an acidic environment can reduce the
performance of the part rather than improve it. Where articles contact food,
drinking water, toys or medical products, additives must meet the relevant
regulatory approvals for that use.
How Are Light Stabilizers Tested?
Light stabilizers are tested by accelerated weathering in a fluorescent UV or
xenon-arc chamber, with results confirmed by outdoor exposure, and the exposed
samples are measured for colour, gloss, mechanical strength and chemical
change.
Fluorescent UV (QUV) chambers use UV lamps and condensation cycles to speed up
degradation, while xenon-arc chambers reproduce the full solar spectrum more
closely, and outdoor exposure in high-UV sites gives real-world confirmation.
Samples are checked at set intervals for gloss retention, colour difference or
yellowness index, tensile and impact retention, and the carbonyl index measured by
infrared spectroscopy, which tracks the chemical progress of oxidation. Because
acceleration never matches outdoor ageing exactly, results are used to rank
formulations and check that a package will reach its target life rather than to
predict exact years of service.
How Did Light Stabilizers for Plastics Develop?
Light stabilizers developed in stages as plastics moved outdoors, from early UV
absorbers in the mid-twentieth century to HALS in the 1970s and more advanced
absorbers since.
Early UV absorbers, including salicylates and hydroxybenzophenones, appeared in the
1940s and 1950s to protect early plastics and coatings, and benzotriazoles followed
in the 1960s with better light stability and lower colour. The introduction of
hindered amine light stabilizers in the 1970s was a major step: because they are
regenerated rather than consumed quickly, they transformed the durability of
polypropylene and polyethylene and opened up outdoor uses such as automotive parts,
garden furniture and agricultural film. Later decades brought hydroxyphenyl triazine
absorbers for demanding engineering plastics, polymeric and high molecular weight
HALS with lower volatility, and low-basicity HALS for acidic systems such as PVC and
flame-retardant formulations. Today stabilizer packages are usually designed as
combinations of absorber, HALS and antioxidant matched to the polymer, part
thickness and climate.
Glossary of Terms
UV absorber (UVA)
An additive that absorbs ultraviolet light, mainly 290–400 nm, and releases the energy as heat.
HALS
Hindered amine light stabilizer — a 2,2,6,6-tetramethylpiperidine derivative that traps radicals rather than light.
Photo-oxidation
The radical chain reaction, started by UV and fed by oxygen, that degrades a polymer.
Denisov cycle
The sequence through which a HALS is regenerated after trapping a radical, so it keeps working.
Nitroxyl radical
The active species formed from the hindered amine; it captures alkyl radicals from the polymer.
Beer–Lambert law
Absorption rises with concentration and path length — why UV absorbers need thickness to work.
Low-basicity HALS
A HALS modified to resist deactivation by acids, for PVC and flame-retardant or agrochemical exposure.
Polymeric HALS
A high molecular weight grade that resists volatility, migration and extraction in thin sections.
QUV / xenon-arc
Accelerated weathering chambers — fluorescent UV with condensation, and full-spectrum solar simulation.
Carbonyl index
An infrared measure of oxidation products, used to track chemical degradation over exposure.
Yellowness index
A measured colour change used to quantify yellowing during weathering.
Chalking
A powdery surface left when the polymer at the surface degrades and releases pigment.
Loading
The concentration at which an additive is dosed, as a percentage of polymer weight.
COA / SDS / TDS
Certificate of Analysis (batch results), Safety Data Sheet (hazard) and Technical Data Sheet (properties and handling).
Before You Specify the Package
A short routine that catches most of the avoidable problems
Start with the polymer, not the additive. Write down the resin, the processing
route and the part geometry, because thickness is what decides whether a UV
absorber can do anything at all, and melt temperature is what decides whether a
low molecular weight grade will survive compounding. Then set the exposure: the
climate, the target service life and whether the part is shaded, vertical or
facing the sun, since tropical outdoor service is a different problem from
indoor use behind glass.
Next, check what else is in the formulation. Acids, halogenated flame retardants,
sulfur or chlorine-containing agrochemical residues and some metal catalysts
will deactivate a conventional basic HALS, and pigments compete for the same
light the absorber is trying to capture. Only then choose chemistry, molecular
weight and loading, and fix the HALS to UV absorber ratio as something to be
tested rather than inherited from a different polymer. Finally, confirm the
package by accelerated weathering against your current baseline, back it with
outdoor exposure where the timeline allows, and check the regulatory position
for the finished article if it will contact food, drinking water, toys or
medical products. Ask your supplier for the grade-specific Safety Data Sheet and
Technical Data Sheet at the quotation stage, not after the order.
CT
Chelora Chemicals Technical Team
The Chelora Chemicals Technical Team supports plastics processors and
compounders with specialty chemicals and polymer additives, with documentation
matched to each application. Every batch is tested and reported against measured
results, with a Certificate of Analysis, Safety Data Sheet and Technical Data
Sheet accompanying each shipment.
The right stabilizer package depends on your polymer, part thickness and
climate. Chelora
Chemicals supplies specialty chemicals and polymer additives for
plastics processors, with documentation matched to each application. Send us
your polymer, application and exposure conditions and our technical team
will advise on grades and send a quotation.
The questions formulators and buyers ask most about this
chemistry.
What is the difference between UV absorbers and HALS?
UV absorbers work by absorbing harmful ultraviolet
light and releasing it as harmless heat before it can damage the polymer, so
they protect best in thick sections. HALS (hindered amine light stabilizers)
do not absorb UV; they trap the free radicals that form when degradation
starts and are regenerated in the process, so they protect best at the
surface and in thin sections. The two act at different points in the
degradation chain, which is why they are often combined.
Which is better for polypropylene, a UV absorber or HALS?
HALS are usually the primary light stabilizer for
polypropylene and polyethylene, because they work well in thin parts such as
films, fibres and tapes where a UV absorber cannot absorb enough light. A UV
absorber is often added on top for thick, pigmented or highly exposed parts.
The best ratio and combination depend on the part thickness, the exposure
and the other additives, so weathering tests are recommended.
Can HALS be used in polycarbonate?
Conventional HALS are generally not recommended in
polycarbonate, because their basic amine groups can accelerate hydrolysis of
the polymer during processing and use. Polycarbonate is normally protected
with UV absorbers such as benzotriazoles or triazines, sometimes with
specially designed low-basicity HALS in blends. Always confirm compatibility
with trials before committing to a formulation.
Why do HALS perform poorly in PVC?
Conventional HALS are basic, and PVC releases
hydrochloric acid as it degrades, which reacts with the amine and
deactivates it. For that reason clear flexible PVC is commonly stabilised
with UV absorbers such as benzophenones, and where HALS are used they are
typically low-basicity types designed for acidic environments, together with
a UV absorber.
Do UV absorbers and HALS always work better together?
Often, but not always. Because they act by different
mechanisms, combinations frequently outperform either alone, with studies in
polypropylene and polyethylene finding best results at roughly three parts
HALS to one part UV absorber. However, some pairings show weaker or even
antagonistic effects, for example with certain benzotriazoles, so each
combination should be verified by weathering tests rather than
assumed.
How much light stabilizer is added to plastic?
Loadings are usually a few tenths of a percent by
weight of the polymer, commonly in the range of about 0.1 to 1 percent for
each type, and higher in thin or severely exposed applications. The right
level depends on the polymer, the part thickness, the colour, the expected
service life and the climate, so formulators set it by testing.
How are light stabilizers tested?
Accelerated weathering in a fluorescent UV (QUV) or
xenon-arc chamber is the standard laboratory approach, backed by outdoor
exposure in sunny locations for long-term confirmation. Samples are measured
over time for gloss retention, colour change or yellowness, tensile and
impact retention, and carbonyl build-up by infrared spectroscopy. Results
show whether the chosen stabilizer package will last the target service
life.
Does Chelora Chemicals supply UV absorbers and HALS?
Chelora Chemicals supplies polymer additives and
specialty chemicals for plastics processors. Contact our team with your
polymer, application and exposure conditions, and we will advise on grades,
supply documentation and quotations.
Start the Conversation
Send us the polymer and the exposure, and we’ll send the grades.
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