An optical brightener does not make a plastic whiter. It makes it look whiter, by adding blue light that was never there.

Understanding that distinction is the difference between using brighteners well and creating an expensive mess.

How a brightener actually works

Most polymers carry a slight yellow cast. It comes from the resin itself, from trace degradation during processing, from stabiliser residues and from any recycled content.

Yellow is what you see when blue light is missing. So there are two ways to correct it.

The old way – bluing. Add a trace of blue or violet pigment. This absorbs the remaining yellow wavelengths, which neutralises the cast. It works, and it has an unavoidable cost: you have removed light rather than added it, so the article becomes darker and greyer as it becomes less yellow. Whiter in hue, duller in brightness.

The brightener way – fluorescence. An optical brightening agent absorbs ultraviolet light in roughly the 340-370 nm band – energy that was invisible to the eye anyway – and re-emits it as visible blue light at around 420-470 nm.

The article now returns more visible light than it received. It reads as brighter and whiter at the same time. In the strict sense it can reflect more than 100% of the visible light falling on it, because it is converting invisible UV into visible output.

That is the whole trick, and it is why brighteners replaced bluing agents almost everywhere.

OB-1 and the common grades

OB-1 is the workhorse of the plastics industry – a benzoxazole-type brightener whose defining property is very high thermal stability. It survives the processing temperatures of engineering polymers, which most brighteners do not, and it is why OB-1 turns up in polyester, polyamide, ABS, PVC and polyolefins alike.

Its trade-off is a slightly greenish emission tone compared with some alternatives, and it is not the cheapest.

OB (a thiophene-based type) is widely used in polyolefins and PVC where the processing temperature is more moderate. KSN and KCB appear in higher-temperature engineering applications.

The choice is driven by processing temperature first, and by the emission tone second. A brightener that decomposes in your extruder does nothing except cost money and potentially discolour.

The overdose effect – the thing everybody gets wrong

This is the single most important practical fact about optical brighteners, and it surprises almost every first-time user.

Whiteness does not increase with dosage. It peaks, and then it falls.

Below the optimum, more brightener gives more whiteness. Above it, two things happen. The molecules begin to quench each other’s fluorescence, so extra brightener contributes less and less. And the brightener’s own faint body colour starts to show through – a greenish cast that gets stronger with every additional gram.

The result is a curve with a clear maximum. Overdose and you get a dull, greenish white that looks worse than a lower dosage, while paying more for it.

This is why brightener dosages are so low. Typical addition rates are in the range of 50 to 300 ppm in the finished article – that is 0.005% to 0.03%. At those levels, weighing errors matter enormously, which is precisely why brighteners are supplied and dosed as a masterbatch rather than as neat powder.

Find the optimum experimentally for your polymer, your process and your gauge. A ladder trial across the range takes a morning and settles it permanently.

What a brightener will not do

Four limits, each of which causes real problems when ignored.

It is not a UV stabiliser. Brighteners absorb UV, which sounds protective, but they immediately re-emit that energy as visible light rather than dissipating it harmlessly, and they do not scavenge radicals. A brightener provides essentially no weathering protection. If your part goes outdoors it needs a HALS package regardless.

It adds no opacity. Brighteners are dissolved in the polymer, not dispersed as particles, so they scatter no light. Hiding power comes from titanium dioxide and nothing else. A brightener makes a thin white film look brighter; it does not make it hide better.

It does not work without UV. No UV in, no blue out. Under a light source with little UV content – many LED fixtures, some warehouse lighting – a brightened article loses the effect entirely and looks as yellow as it truly is. An article that looked brilliant in daylight can look disappointing under a customer’s inspection lamp.

It breaks colour measurement. A spectrophotometer measuring with a UV-included source will read the fluorescence; measuring with UV excluded, it will not. The same sample gives two different readings. If you and your customer measure differently, you will disagree about a shade that is in fact identical.

If you use brighteners, agree the measurement condition – UV included or UV excluded – with your customer in writing, before the first delivery. This is a common and entirely avoidable dispute.

Where brighteners genuinely earn their keep

Recycled and PCR content. Recyclate carries a yellow-grey cast from thermal history and contamination. A brightener is one of the most cost-effective ways to lift the appearance of a recycled product, and it works well alongside pigment. More on this in masterbatch for recycled and PCR plastic.

Thin films and fibres. Where there is not enough material in the light path for TiO₂ to deliver brightness on its own.

Cost reduction in whites. Because a brightener lifts perceived whiteness without adding pigment, it can allow a modest reduction in TiO₂ at the same visual result. TiO₂ is expensive; brightener at 150 ppm is not. This is a legitimate optimisation, provided opacity is separately verified – the two properties are not interchangeable.

Masking the yellowing of hot-processed material. Engineering polymers processed near their limits pick up a cast. OB-1’s heat stability is why it is the standard answer there.

What to ask your supplier

  • Which brightener chemistry, and its thermal stability limit against your process temperature
  • The recommended dosage window in ppm of active in the finished article, and the whiteness peak
  • Whether the recommendation was established at your gauge
  • Interaction with your TiO₂ grade and your stabiliser package
  • Food contact compliance, if relevant – brighteners are regulated substances in food-contact plastics
  • The measurement condition their whiteness figures were taken under

FAQs

What does an optical brightener do in plastic? It absorbs ultraviolet light and re-emits it as visible blue light, which cancels the yellow cast and makes the article look brighter and whiter. It adds light rather than removing it, which is why it does not darken the part the way a blue pigment does.

What is OB-1 used for? OB-1 is a benzoxazole-type optical brightener with very high thermal stability, used where processing temperatures are too high for other brighteners – polyester, polyamide, ABS, PVC and polyolefins.

How much optical brightener should I add? Typically 50 to 300 ppm of active in the finished article. Whiteness peaks and then declines with further addition, so an overdose produces a dull, greenish white. Establish the optimum with a ladder trial for your polymer and gauge.

Why did more optical brightener make my part look worse? Above the optimum, brightener molecules quench each other’s fluorescence and the brightener’s own faint colour begins to show, producing a green cast. Whiteness genuinely falls with overdose.

Is an optical brightener a UV stabiliser? No. It absorbs UV but re-emits it as visible light rather than dissipating it, and it does not scavenge radicals. Outdoor parts need a HALS package regardless of whether a brightener is present.

Does an optical brightener improve opacity? No. Brighteners dissolve in the polymer and scatter no light. Opacity comes from titanium dioxide. A brightened film looks brighter but hides no better.

Why does my brightened part look different under different lights? Because the effect depends on UV in the light source. Under a low-UV source such as some LED lighting there is nothing to fluoresce, and the article’s true yellowness shows. Agree lighting and measurement conditions with your customer.