Photoinitiators and UV absorbers/blockers: spectra, applications and approvals

Ultraviolet radiation (roughly 200–400 nm) is a double-edged tool in modern materials and consumer goods chemistry: it is used deliberately to cure liquid coating, printing ink, adhesive and dental filling systems within fractions of a second – and at the same time it has to be kept away from plastics, coatings, packaging and human skin in order to prevent photo-oxidation, yellowing, embrittlement or skin damage. The first task is performed by photoinitiators, which absorb UV light and generate reactive radicals (or cations) from it that start a polymerisation. The second is performed by UV absorbers (chromophoric molecules that convert UV radiation into heat), inorganic UV blockers (semiconductor particles such as titanium dioxide and zinc oxide, which reflect and scatter UV light and absorb it via their band gap) and HALS (hindered amine light stabilisers), which absorb no UV light themselves but scavenge the radicals formed during photo-oxidation.

Both groups of substances are present in practically every area of life: in furniture coatings, automotive clear coats, 3D printing resins, dental fillings, label and packaging inks, sunscreens, plastic films and housings and in cosmetic formulations.

This article summarises the most important industrially used photoinitiators (type I and type II) as well as UV absorbers and blockers (organic, inorganic, HALS) – with λ_max values and absorption ranges, typical applications, approval status (cosmetics, food contact, medical devices) and trade names. All values are taken from manufacturers’ data sheets, PubChem, ECHA/REACH, CosIng and EU Annex VI databases, FDA documents and the specialist literature; the corresponding sources are linked directly in the text and listed again at the end.

Photoinitiators are divided into two classes according to their reaction mechanism:

Type I (Norrish type I, α-cleavage): the molecule absorbs a photon and breaks down directly into two radical fragments. Typical representatives are hydroxyacetophenones, benzil dimethyl ketal, benzoin ethers and (bis)acylphosphine oxides.

Type II (hydrogen abstraction): the excited photoinitiator molecule abstracts a hydrogen atom from a hydrogen donor (usually a tertiary amine acting as co-initiator or synergist); this produces an initiating radical on the donor and an inactive ketyl radical. Typical representatives are benzophenone, thioxanthones (for example ITX) and quinones (for example camphorquinone).

Regulation (EU) No 10/2011. This regulation establishes the “Union list” of authorised monomers and additives for plastics intended to come into contact with food (generic specific migration limit, SML, of 60 mg/kg; overall migration limit 10 mg/dm²) – but it does not cover printing inks directly, leaving them to national provisions (for example Annex 10 Part A of the Swiss Consumer Goods Ordinance 817.023.217) and to the NIAS principle (“non-intentionally added substances”) (legislation.gov.uk; Printcolor).

The EuPIA suitability list. The European printing ink association EuPIA publishes an annual list of photoinitiators and photosynergists regarded as suitable for indirect food contact; TPO was added in April 2023 (EuPIA 2025).

TPO – an example of shifting classifications. TPO was classified as CMR category 2 in 2012; in 2021 ECHA’s Committee for Risk Assessment recommended reclassification to CMR 1B (implemented by Delegated Regulation (EU) 2024/197); SVHC listing followed on 14 June 2023, and since 1 September 2025 TPO has been listed as a prohibited substance in Annex II of the EU Cosmetics Regulation – a ban that primarily affects UV gel and hybrid nail products, not printing inks or industrial coatings (HPRA).

Dental composites and ISO 10993. Dental filling materials containing camphorquinone are classified in the United States as class II medical devices (21 CFR 872.3690); their biocompatibility is tested to the ISO 10993 series of standards, which the FDA’s guidance on device biocompatibility also refers to (FDA guidance; ACS Omega).

US rules for food contact. In the United States, UV and EB cured food contact coatings received a broader FDA clearance in 2008 through Food Contact Notification (FCN) 772; individual photoinitiators in adhesives and coatings additionally fall under 21 CFR Part 175 (RadTech).

The tables above give one or more absorption maxima for every initiator. Those figures are not a footnote but the actual selection criterion – because excitation follows the overlap of two curves, not the power of the lamp: the spectral irradiance of the source and the absorption of the initiator.

The rate at which polymerisation starts is proportional to the number of photons absorbed by the initiator, not to the number arriving. Where the initiator does not absorb, radiation passes through the formulation or ends up as heat, and neither contributes to cure. From that follows a sentence that is regularly missing from procurement discussions:

A 395 nm LED at twenty times the irradiance does not cure an HCPK formulation twenty times faster – it does not cure it at all. HCPK absorbs at 243 and 331 nm; by 395 nm the band has long since died away. Conversely, a few milliwatts per square centimetre at 385 nm are enough to cleave TPO reliably, with its maximum at 379 nm.

This becomes practically important when moving from the mercury medium-pressure lamp to UV LEDs. The medium-pressure lamp radiates broadly and hits every common initiator somewhere; it forgives an unsuitable formulation. A UV LED radiates in a band ten to twenty nanometres wide and forgives nothing. Formulations that ran reliably for thirty years stop curing after the change – not because the new source is weaker, but because it radiates past the initiator.

The second consequence concerns depth. Short-wavelength radiation is consumed by a strongly absorbing initiator within the top few micrometres – the surface cures and it stays soft underneath. Long-wavelength bands (TPO 379 nm, BAPO 380 nm, ITX 383 nm) penetrate further. The acylphosphine oxides bring a property with them that makes them indispensable for thick sections and white-pigmented systems: they photobleach as they react – the reaction product no longer absorbs, and the layer already cured lets light through to the molecules below.

With white-pigmented systems the titanium dioxide adds to this. It scatters and absorbs so strongly below about 400 nm that nothing is left of a 365 nm source after a few micrometres. Here only what works on the long-wavelength flank still cures.

The assessment follows the position of the absorption band relative to the lines or peak wavelengths of the source; the λmax values are taken from the tables above. It replaces neither a data sheet nor a cure trial – the molar absorption coefficient on the flank, the concentration, the layer thickness and the pigmentation all shift the result considerably. But it answers the question that comes first: whether source and initiator match at all.

The bottom row is the exception that proves the rule. Camphorquinone absorbs in the visible – which is why dental units work with blue light and not with UV. A UV lamp, whatever its power, is the wrong source for a CQ system.

The overlap can be looked at rather than guessed. The spectral database explorer shows measured spectra of mercury low- and medium-pressure lamps, doped lamps and UV LEDs; put the λmax values from the tables above next to them and it is immediately visible whether the initiator's band lies inside the emitted range or beside it. The underlying spectral database of UV lamps lists the measurements individually.

For design this implies an order worth reversing: first fix the initiator that the formulation and the regulatory position allow, then choose the source to match – not the other way round. The source is interchangeable, the formulation rarely is.

• The source. Opsytec supplies UV LED light sources at 365, 385, 395, 405 and 450 nm – one peak wavelength for each column of the table above. Where the spectrum has to be broad because several initiators work alongside each other, the mercury lamp remains the simpler answer.
• The controlled trial. The UV LED chambers of the BSL series irradiate a sample area at a known irradiance with dose-controlled exposure. That turns “cures” or “does not cure” into a dose in mJ/cm² that can be compared between formulations.
• The cross-check. Whether the source really radiates where the data sheet promises is shown by a spectral measurement in the sample plane – with the UVpad E, or with the SR900 where the full range is needed. Peak wavelengths of LEDs scatter from production and shift with junction temperature; on a steep absorption flank, five nanometres are not a trifle.

UV absorbers and blockers prevent photodamage by three different mechanisms:

Organic UV absorbers (benzotriazoles, triazines, benzophenones, cinnamate esters, salicylates and others) have chromophoric ring systems that absorb UV photons and release the energy non-radiatively as heat.

Inorganic UV blockers (titanium dioxide, zinc oxide) are semiconductor particles that absorb UV light via their band gap and additionally reflect and scatter it (a “physical” filter).

HALS (hindered amine light stabilisers) absorb no appreciable amount of UV light themselves; instead they scavenge the radicals formed during photo-oxidation through a nitroxide radical cycle – which is why they are practically always used together with a genuine UV absorber.

HALS compounds (hindered amine light stabilisers) have no appreciable UV absorption – they protect polymers by scavenging the radicals formed during photo-oxidation through a catalytic nitroxide radical cycle. They are therefore practically always formulated together with a genuine UV absorber (a benzotriazole or a triazine).

EU Cosmetics Regulation (EC) No 1223/2009, Annex VI. Annex VI is a closed positive list: only the 33 substances named in it (plus separate nanoform entries) may be used as UV filters in cosmetics in the EU, each with its own maximum concentration (CosIng; ECHA). Oxybenzone and octocrylene received tighter concentration limits through Regulation (EU) 2022/1176 after the Scientific Committee on Consumer Safety (SCCS) had raised endocrine concerns (Regulation (EU) 2022/1176). Industrial benzotriazole additives such as Tinuvin 328, 234, 326, 327 and P are not part of this list, because they act as pure polymer and coating additives rather than as cosmetic UV filters.

The FDA sunscreen monograph (GRASE status). Of the 16 sunscreen active ingredients covered by the US monograph, only zinc oxide and titanium dioxide are considered GRASE (“generally recognized as safe and effective”) as the FDA proposal stands (FDA fact sheet). PABA and trolamine salicylate are considered not GRASE; the remaining twelve active ingredients (including avobenzone, oxybenzone, octocrylene, homosalate, octinoxate, octisalate and ensulizole) remain in the status “insufficient data”. Newer filters such as bemotrizinol (Tinosorb S) and octyl triazone (Uvinul T150) are not yet part of the final FDA monograph, as they run through a separate TEA authorisation procedure that has been pending for years.

EU Food Contact Materials Regulation (EU) No 10/2011. This regulation governs additives for plastic packaging with specific migration limits; several benzotriazole UV stabilisers and HALS (for example Tinuvin 326 and Tinuvin 770 DF) are marketed by manufacturers as suitable for food contact; titanium dioxide is authorised separately as a pigment and colourant for plastics (EU 10/2011).

Titanium dioxide: carcinogenicity classification and its annulment. In 2020 the EU classified titanium dioxide in powder form as a category 2 carcinogen by inhalation (a CLP adaptation); on 23 November 2022 the General Court of the European Union annulled this classification for want of a reliable scientific basis (Ecomundo; Court press release). Its use as a sunscreen UV filter and as a food packaging pigment is regulated independently of this.

The debate about endocrine effects. Since 2019 the SCCS has been reassessing several UV filters (oxybenzone, octocrylene, homosalate, ethylhexyl salicylate) with regard to possible endocrine effects; in each case continued use was assessed as safe, but at reduced maximum concentrations or for restricted product categories (SCCS on octocrylene; SCCS on homosalate).

“Reef-safe” bans. Since 1 January 2021 Hawaii has prohibited the sale of sunscreens containing oxybenzone and octinoxate (Act 104) because of their effect on coral reefs (Safe to Swim Hawaii). Palau goes further with its Sunscreen Regulations, in force since 1 January 2020, prohibiting practically all synthetic organic UV filters as “reef-toxic”, while mineral filters (zinc oxide and non-nano titanium dioxide) remain permitted as alternatives (Palau regulations).

Photoinitiators and UV absorbers/blockers address apparently opposite goals – deliberate UV activation versus UV protection – yet they follow the same photophysical principles: absorption of a UV photon and its conversion into a chemical (radical formation) or physical (heat, scattering) consequence. Choosing the right substance depends decisively on the spectral range required – from short-wave α-cleavage at 240–280 nm (HCPK, HMPP), through long-wave LED curing at 380–405 nm (TPO, BAPO), to visible blue-light curing in dental composites (camphorquinone, 468 nm) and the UV-B/UV-A protection range of 280–400 nm in sunscreen. Regulation shows a clear trend: substances with demonstrated or suspected CMR or endocrine effects (TPO, Michler’s ketone, oxybenzone, octocrylene, homosalate) are increasingly restricted in their permitted concentration or banned outright, while low-migration and mineral alternatives (zinc oxide, titanium dioxide) and low-migration photoinitiators such as BAPO gain in importance.

What distinguishes type I from type II photoinitiators?

A type I photoinitiator breaks down directly into two radicals after absorbing a photon (Norrish type I cleavage) and therefore works without any further additive. A type II photoinitiator, in its excited state, abstracts a hydrogen atom from a hydrogen donor – usually a tertiary amine acting as co-initiator – and only then is the initiating radical formed. Type II systems therefore always need a synergist, but they can often be excited at longer wavelengths.

Which wavelength does a photoinitiator need?

The wavelength of its own absorption maximum. Hydroxyacetophenones such as HCPK and HMPP absorb at around 240 and 330 nm and can still be excited at 365 to 385 nm; acylphosphine oxides such as TPO and BAPO have a long-wave band at 379 to 380 nm and therefore match UV LEDs at 385, 395 and 405 nm; camphorquinone absorbs in the blue visible range with a maximum at about 468 nm. Anyone replacing a mercury lamp with an LED must therefore check whether the narrow LED band still reaches the absorption band of the initiator at all.

Why is UV dose a regulatorily relevant parameter for photoinitiators?

Because incomplete polymerisation increases the proportion of unreacted photoinitiators and residual monomers, and it is precisely these substances that can migrate out of the cured layer. For food contact materials, reproducible and complete cure is therefore the lever for meeting migration limits – demonstrable through the spectrum, the irradiance and the dose formed from them at the substrate.

What happened to TPO?

Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide was classified as CMR category 2 in 2012; in 2021 ECHA’s Committee for Risk Assessment recommended reclassification to CMR 1B (implemented by Delegated Regulation (EU) 2024/197); SVHC listing followed on 14 June 2023, and since 1 September 2025 TPO has been listed in Annex II of the EU Cosmetics Regulation. The ban primarily affects UV gel and hybrid nail products, not printing inks or industrial coatings; for indirect food contact, TPO has been on the EuPIA suitability list since April 2023.

How do UV absorbers, UV blockers and HALS differ?

Organic UV absorbers have chromophoric ring systems, absorb UV photons and release the energy non-radiatively as heat. Inorganic UV blockers – titanium dioxide and zinc oxide – are semiconductor particles that absorb UV radiation via their band gap and additionally reflect and scatter it. HALS absorb practically no UV light themselves but scavenge the radicals formed during photo-oxidation through a catalytic nitroxide radical cycle; they are therefore always formulated together with a genuine absorber.

May any UV absorber be used in cosmetics?

No. Annex VI of the EU Cosmetics Regulation (EC) No 1223/2009 is a closed positive list: only the substances named in it – currently 33 entries plus separate nanoform entries – may be used as cosmetic UV filters in the EU, each with its own maximum concentration. Industrial benzotriazole stabilisers such as Tinuvin 328, 234, 326, 327 or P are not on this list; they are polymer and coating additives, not cosmetic UV filters.

Why do different sunscreen filters apply in the United States than in the EU?

Because the FDA regulates sunscreens as drugs through a monograph. As the FDA proposal stands, only zinc oxide and titanium dioxide are considered GRASE out of the 16 active ingredients covered; PABA and trolamine salicylate are considered not GRASE, and the remaining twelve – including avobenzone, oxybenzone, octocrylene, homosalate and octinoxate – remain in the status “insufficient data”. Newer broadband filters such as bemotrizinol and octyl triazone are not yet part of the final monograph, as they run through a separate TEA procedure that has been pending for years.

How can curing be monitored by measurement?

Through the quantities the photoinitiator actually sees: the spectrum of the source, the irradiance at the location of the layer and the dose formed from it over the exposure time, each within the wavelength range of the absorption band. Initial characterisation and every change of source require a spectrally resolved measurement; for ongoing monitoring a band-limited sensor is sufficient, provided it has been matched against a traceable spectral reference at that source.

Photoinitiators – primary data

• IGM Resins product data sheets: Omnirad 184, Omnirad 1173, Omnirad 2959, Omnirad 369, Omnirad 907, Omnirad TPO, Omnirad 819, Omnirad BP Flakes
• Sartomer/Arkema photoinitiator catalogue: PDF, SpeedCure 2-ITX
• PubChem entries, including: HCPK, Irgacure 369, Irgacure 907, BDK, TPO, TPO-L, benzophenone, Michler’s ketone, ITX, 2-chlorothioxanthone, anthraquinone, 2-ethylanthraquinone
• ChemicalBook: HCPK, BAPO
• Sinocure: camphorquinone
• Fisher/Thermo Scientific: camphorquinone
• Royal-Dent technical data sheet: PDF
• ACS Omega: leachables study on dental composites
• FDA biocompatibility guidance: PDF
• ScienceDirect: cytotoxicity study Irgacure 2959
• REACH Baden-Württemberg: Irgacure 907
• ECHA: SVHC dossier TPO / HPRA notice, substance evaluation of benzophenone, SVHC dossier Michler’s ketone
• Printcolor customer information on TPO: PDF
• EuPIA Suitability List 2025: PDF
• BBC News: ITX/Tetra Pak incident
• DairyReporter: Nestlé recall
• European Parliament: parliamentary question E-6-2005-4616
• legislation.gov.uk: consolidated text (EU) 10/2011
• RadTech Food Packaging Alliance: FCN 772
• OSTI: historical absorption data for benzoin ethers

UV absorbers and UV blockers – primary data

• PubChem entries, including: UV-328, Tinuvin 326, Tinuvin 327, Tinuvin P, oxybenzone, Bemotrizinol, octyl triazone, avobenzone, homosalate, ethylhexyl salicylate, Ensulizol, octocrylene, titanium dioxide
• BASF data sheets and product pages: Tinuvin 328, Tinuvin 326, Tinuvin 400, Tinuvin 479, Tinosorb S, Tinosorb M, Uvinul T150, Tinuvin 770 DF, Tinuvin 292, Tinuvin 622, BASF press release Tinosorb M, BASF press release Tinosorb S
• CosIng / Annex VI tracker: cosingchecker.com
• ECHA: overview of cosmetic UV filters, guidance on the classification of titanium dioxide
• Official Journal of the EU: Regulation (EU) 2022/1176, Regulation (EU) No 10/2011, consolidated
• SCCS opinions: homosalate, ethylhexyl salicylate, octocrylene, benzophenone-4
• FDA: sunscreen fact sheet (GRASE status), FDA food contact substance database
• Ecomundo: annulment of the TiO₂ carcinogen classification
• Court of Justice of the European Union: press release
• Safe to Swim Hawaii: reef ban in Hawaii
• Government of Palau: Sunscreen Regulations 2020
• Wikipedia: avobenzone, zinc oxide
• Specialist literature: MDPI review of zinc oxide, InstaNano UV-Vis database, PMC review of photocatalysis, BenchChem Tinosorb comparison, ScienceDirect octocrylene, madebygenie oxybenzone, Plastemart UV stabilisers, Patsnap zinc oxide

Note: the λ_max values and absorption ranges given in this article are taken from manufacturers’ data sheets and published sources; they do not replace spectrophotometrically measured raw data for the particular formulation. Concentration limits under Annex VI of the EU Cosmetics Regulation and regulatory classifications are amended regularly – before any regulatory use, the current status on EUR-Lex or CosIng should be checked.

• UV curing and photopolymerisation – the chain reaction the photoinitiator starts, with oxygen inhibition and dose control.
• UV bonding, potting and encapsulation – photoinitiator choice and through-cure in adhesives, potting compounds and encapsulants.
• Additive manufacturing and 3D printing – resins for stereolithography and DLP, in which TPO, BAPO and HMPP determine the cure.
• Packaging and filling technology – low-migration printing inks, set-off and food contact compliance.
• Pharmaceuticals and photostability – photodegradation as a risk and the role of UV absorbers in product protection.
• Medicine and phototherapy – camphorquinone systems in dental composites and blue-light curing.
• Photochemistry – the photophysical fundamentals of absorption, excitation and quantum yield.

The full picture is in the UV applications overview.

Are you moving from a mercury lamp to UV LED and need to know whether the narrow LED band still reaches the absorption band of your photoinitiator – or do you have to demonstrate the complete cure of a low-migration formulation? We measure the spectrum of your source in our calibration laboratory, determine the irradiance at the location of the layer with a spectroradiometer and set up dose monitoring with the appropriate UV sensor. Get in touch.

Dr Mark Paravia
Managing Director and Head of the Calibration Laboratory
Opsytec Dr. Gröbel GmbH, Ettlingen, Germany
Tel. +49 (0)7243 / 94 783-50 · mark.paravia@opsytec.de