UV Stabilisers: Hindered Amine, Absorber and the Outdoor Life They Buy

What UV does to a polymer

Ultraviolet light carries enough energy to break carbon-carbon bonds. When a photon is absorbed by the polymer, it creates a radical, the radical reacts with oxygen, and the result is a chain scission or a carbonyl group. Chain scission reduces molecular weight, which shows as a loss of elongation and impact strength. Carbonyl groups absorb more light, which accelerates the process.

The visible result appears late. A part that has lost half its elongation may still look perfect, because the damage starts at the surface and the surface is a small fraction of the wall. Chalking, colour shift and surface cracking are the last stages of a process that began months earlier.

That is why outdoor failures are so often a surprise. The part looked fine at the last inspection and cracked in service a month later, because the inspection could only see the surface and the surface was the last thing to fail. The measurement that would have predicted it is elongation, not appearance.

The rate depends on the dose. Dose is the product of intensity and time, so a part in southern Europe receives two to three times the annual dose of the same part in northern Europe, and a part on a horizontal surface receives more than one on a vertical wall. Any prediction of life that ignores the installation geometry is a guess.

HALS: the radical scavenger

Hindered amine light stabilisers work by trapping the radicals that the UV creates. They are not consumed in the way an antioxidant is; they cycle through a regeneration step, which is why a small loading can protect a large amount of polymer for a long time. That regeneration is also why HALS is the most cost effective class for long term protection.

The mechanism is catalytic and it operates where the radicals are, which is throughout the wall. That makes HALS effective in thick sections and in fibres, where a surface treatment would only protect the outside. It also means HALS can migrate, and migration is the main failure mode: a part in contact with water, oil or a solvent can lose its stabiliser and its protection.

There are low molecular weight and high molecular weight grades. The low molecular weight grades migrate more readily, which helps dispersion and hurts permanence. The high molecular weight grades stay where they are put, which is better for a thick part in contact with a fluid and worse for a thin film where dispersion matters more.

HALS is also sensitive to interactions. Acidic residues, some halogenated flame retardants and some pigments deactivate it, and a formulation that works in a laboratory plaque can fail in a coloured compound because the pigment was acidic. This is the most common reason a stabilised compound underperforms its data sheet.

UV absorbers: the screen

A UV absorber works by absorbing the ultraviolet before it reaches the polymer and dissipating the energy as heat. It is a sunblock rather than a repair, and it is only effective where the light is: at the surface, in the first few tens of micrometres. Behind that layer the absorber does nothing because no UV reaches it.

That makes absorbers thickness dependent in a way that HALS is not. In a 3 mm wall, an absorber protects the surface and HALS protects the wall. In a 50 micrometre film, the absorber can protect the whole thickness and the two mechanisms overlap. In a thick part, the absorber adds little to a good HALS system.

Absorbers earn their place in two situations. The first is a thin section, where the whole part is within the absorber's working depth. The second is a part where surface appearance matters and the earliest damage is surface yellowing, because the absorber delays the start of the visible change even when the bulk is protected by HALS.

The chart on the left shows the combination working. The unstabilised compound loses half its elongation within a year. HALS alone keeps 78 percent at two years and 42 at seven. HALS plus an absorber keeps 86 at two years and 60 at seven. The absorber buys a higher plateau, not a different slope.

Pigments, thickness and the dose

Carbon black is the most effective UV screen there is. A well dispersed 2 percent carbon black loading protects a polyolefin for decades, because it absorbs across the whole UV range and it does not migrate. A black part outdoors is usually the longest lived part on the site, and it is also the cheapest to stabilise.

Other pigments are less predictable. Titanium dioxide is a UV absorber in its rutile form and a photo-catalyst in its anatase form, so the same white part can be protected or destroyed depending on which grade was used and whether it was coated. A white part that chalks in a year is usually an uncoated anatase grade, and the fix is a grade change rather than more stabiliser.

Colour also changes the temperature. A black part in sun runs twenty to thirty degrees hotter than a white one, and every ten degrees roughly doubles the rate of the underlying oxidation. A black part needs more stabiliser for the dose and less for the light, and the two effects partly cancel.

Thickness is the third variable and the one most often ignored in a specification. A 1 mm wall has a much higher surface to volume ratio than a 4 mm wall, so the same loading gives less protection in the thin section. A compound specified for a 3 mm profile and used on a 1 mm profile is under-stabilised even though nothing in the specification changed.

Testing and predicting outdoor life

The standard accelerated test is xenon arc exposure to ISO 4892-2, measured in hours and reported against a dose in kilojoules per square metre. It reproduces the full solar spectrum reasonably well and it is the closest thing to a standard for outdoor durability. Fluorescent UVA tests are cheaper and faster and correlate less well.

Acceleration has a limit. A test that runs four times faster than real weather is not simply four times the dose, because the failure mechanisms change with temperature and moisture. The rule of thumb is that accelerated data ranks materials reliably and predicts absolute life poorly. Use it to choose between two compounds, not to promise a number of years.

The property to measure is elongation at break, not colour. Colour change is a late indicator and it is confounded by dirt, chalking and pigment bleaching. Elongation retention tracks molecular weight and it falls early. A specification that says the part must retain 50 percent of its elongation after a stated dose is a specification that means something.

Outdoor exposure remains the reference. A rack at the actual site, with samples at the actual orientation, gives the answer that no laboratory can give, and it costs almost nothing except time. A supplier who has been running the same compound on a rack for ten years has data that is worth more than any accelerated report.

Table 1 — Five classes and what each one does

Specifying a system

Start from the dose. The installation latitude, the orientation and the expected life give an annual dose, and the annual dose multiplied by the years gives the total. That number, not a vague requirement for outdoor use, is what a compounder needs in order to choose a system.

Then state the property that must survive. Elongation retention is the most useful, impact retention the second. A colour requirement alone will be met by a system that protects appearance and not the polymer, and the part will fail structurally while looking acceptable.

Then state the wall thickness. A specification without a thickness is not a specification, because the same loading gives different protection at 1 mm and at 4 mm. If the part has sections of different thickness, the thinnest section sets the requirement.

Finally, agree the test and the acceptance criterion. Xenon arc to a stated dose, elongation measured to a stated standard, retention above a stated percentage. That is a specification that can be checked, argued about with evidence, and met or not met. Everything else is an intention.

Table 2 — Five tests and what each one is worth