Analytical Identification of Non Intentionally Added Photoinitiator Degradation Products in Recycled Film Fractions

Ink

Polyethylene and polypropylene flexible packaging webs run through high-speed printing lines carrying radiation-cured surface coatings. Photoinitiators generate the free radicals needed to polymerize acrylate monomers in ultraviolet-cured inks and varnishes. Absorbing light causes Type I photoinitiators to undergo unimolecular cleavage, breaking into two radical fragments that start rapid polymerization.

Type II photoinitiators work through a bimolecular reaction, pulling a hydrogen atom from a donor molecule to create the reactive radical. If these ink systems are applied to polyolefin films without electron-beam curing or functional barrier layers, unreacted photoinitiators stay trapped inside the cross-linked ink matrix.

Mechanical recycling processes printed post-consumer flexible films straight into recovered polymer pellets. Collection streams bring together surface-printed shopping bags, frozen food pouches, and shrink wraps. Standard washing strips off water-soluble dirt and superficial adhesives, but it cannot remove cross-linked UV-cured inks from polyolefin surfaces.

The inks travel through the shredder, wash bath, friction washer, and extruding re-granulator right alongside the base polymer.

Storage reels and tightly stacked sheets squeeze the unprinted contact layer against freshly cured ink under mechanical pressure. During storage, photoinitiators migrate out of the ink and into the inner food-contact layer long before post-consumer collection. Re-granulation then melts film, cured ink, and migrated compounds together into a single molten mass at temperatures between one hundred eighty and two hundred forty degrees Celsius.

Set-off occurs under pressure.

Thermal extrusion turns intact photoinitiators into non-intentionally added substances. Heat, residual shear, and trace oxygen partially break down parent photoinitiator molecules through pyrolytic and oxidative pathways. The resulting degradation products end up distributed throughout the recycled resin pellets.

Blending those pellets into new film structures carries these breakdown fragments straight into the finished package wall.

Photoinitiator Selection and Printing Set off Mechanisms

Radiative curing relies on photoactive compounds that generate reactive radicals when exposed to specific ultraviolet wavelengths. Common industrial photoinitiators include benzophenone, isopropylthioxanthone, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methyl-1-phenylpropan-1-one. Printers choose these molecules because they offer high quantum yields and fast reaction kinetics on non-porous films.

Type II systems also need tertiary amine co-initiators, such as ethyl 4-dimethylaminobenzoate, to donate hydrogen atoms during radical generation.

Unreacted photoinitiators linger in cured ink films whenever UV exposure is incomplete, ink deposits are thick, or oxygen inhibits curing at the web surface. Standard industrial curing reaches monomer conversion rates above ninety-eight percent, but residual photoinitiators still remain trapped in the solid matrix. Within low-density polyethylene, these unreacted compounds stay physically mobile.

When printed rolls sit under mechanical tension in warehouses, set-off migration rapidly drives lipophilic photoinitiators into the unprinted reverse surface.

Thermal and Mechanical Stress in Polyolefin Recycling

Mechanical reprocessing exposes recovered polymers to temperatures over two hundred degrees Celsius during melt filtration. Extrusion equipment exerts shear forces that break apart cross-linked ink particles. This combination of heat and shear degrades parent photoinitiators via carbon-carbon bond cleavage, oxidation, and secondary radical reactions, yielding breakdown products with lower molecular weights and higher volatility than the original structures.

Contamination pathways can be traced by sampling post-consumer film bales prior to washing and extrusion. Analytical screening of raw washed flake reveals intact photoinitiator compounds on film surfaces, while pelletized recyclate contains complex mixtures dominated by photolytic and pyrolytic breakdown fragments.

Cleavage

Homolytic bond rupture drives the transformation of photoactive molecules exposed to high heat during melt re-granulation. Type I photoinitiators undergo alpha-cleavage at the carbonyl carbon atom when they absorb light or thermal energy. Hydroxyacetophenones break down into benzoyl radicals and substituted hydroxyalkyl radicals.

Benzoyl radicals then pull hydrogen from surrounding polyolefin chains, forming benzaldehyde, benzoic acid, and substituted alkyl benzenes, while the hydroxyalkyl fragment rearranges into volatile ketones and aliphatic aldehydes.

Acylphosphine oxides decompose through phosphorus-carbon bond scission. Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide splits into 2,4,6-trimethylbenzoyl radicals and diphenylphosphinyl radicals. Secondary oxidation converts the trimethylbenzoyl fragments into 2,4,6-trimethylbenzaldehyde and 2,4,6-trimethylbenzoic acid, while the diphenylphosphinyl radical reacts with oxygen to form diphenylphosphinic acid and oxidized phosphine oxides.

These ionic and polar breakdown products alter the chemical profile of recycled polyethylene resins.

Alpha-aminoacetophenones undergo thermal cleavage at elevated processing temperatures. Photoinitiator 907 generates 4-methylthiobenzoyl radicals and morpholinoisobutyl radicals. Secondary reactions convert the 4-methylthiobenzoyl moiety into 4-methylthiobenzaldehyde and 4-methylthiobenzoic acid.

The morpholine fragment undergoes elimination, yielding free morpholine, 4-morpholinyl derivatives, and volatile nitrogenous species. Morpholine poses a specific migration hazard because it can undergo nitrosation reactions in downstream processing environments.

Photolytic and Thermal Degradation Pathways

Free radical formation triggers chemical cascades as soon as UV-curable molecules absorb light or encounter melt temperatures. Type II photoinitiators undergo hydrogen abstraction rather than direct homolytic fragmentation. Benzophenone extracts hydrogen from polyolefin backbones to form the hydroxydiphenylmethyl radical, and two of these radicals dimerize into benzpinacol.

Heat and shear in the extruder cleave benzpinacol back into benzophenone and benzhydrol, while oxidation yields 4-hydroxybenzophenone and hydroxybenzhydrol derivatives.

Isopropylthioxanthone breaks down along pathways involving its thioxanthone core. Photo-oxidation converts thioxanthone into thioxanthone-5-dioxide and isopropylthioxanthone-5-oxide, whereas thermal degradation produces thioxanthone, methylthioxanthone isomers, and sulfurous volatile organic compounds. Amine co-initiators accelerate these oxidative cascades while simultaneously breaking down under heat.

Ethyl 4-dimethylaminobenzoate breaks down via oxidative dealkylation. Processing heat oxidizes the N-methyl groups, yielding ethyl 4-methylaminobenzoate and ethyl 4-aminobenzoate, with formaldehyde released as a volatile byproduct. Inside the melt, these primary and secondary amine products react with residual free fatty acids or oxidized polymer fragments to form complex amides and imines.

Benzophenone migration into ten percent ethanol reaches eight milligrams per square decimeter after ten days at forty degrees Celsius in unwashed recycled polyethylene fractions.

Recycling amplifies these degradation pathways. Repeated thermal processing cycles accumulate secondary breakdown products in the resin, and continuous re-granulation converts primary photolysis fragments into tertiary oxidation products. Identifying these compounds requires systematically mapping specific fragment families.

Secondary Reaction Products and Structural Elucidation

Reactive radical intermediates undergo recombination, hydrogen abstraction, and atmospheric oxidation to form stable secondary non-intentionally added substances. Benzoyl radicals attack unsaturated sites on oxidized polyethylene chains to produce alkyl-aryl ketones, while substituted benzaldehydes undergo aldol condensation with aliphatic ketones present in post-consumer waste. These reactions yield high-molecular-weight aromatic condensation products that alter the UV absorption spectra of recycled film fractions.

Breakdown routes produce identifiable compound families based on parent chemical structures:

• Benzoyl-derived volatiles form through alpha-cleavage of hydroxyacetophenones and acylphosphine oxides, yielding benzaldehyde, acetophenone, and benzoic acid that contribute to the odor of recycled pellets.
• Substituted alkyl benzenes originate from secondary radical abstraction where benzoyl radicals pull hydrogen from polyolefin branches, generating toluene, ethylbenzene, and propylbenzene inside the melt.
• Oxidized thioxanthones produce polar sulfoxide and sulfone derivatives during extrusion, shifting the chemical profile of isopropylthioxanthone toward more strongly retained compounds in chromatography.
• Morpholine and amine derivatives emerge from amino-photoinitiator cleavage, creating nucleophilic nitrogenous species capable of reacting with trace epoxides or residual acrylate monomers.
• Phosphine oxide fragments turn into acidic organophosphorus compounds, raising the acidity index of recycled polyolefin fractions and interacting with metallic extrusion equipment.

As thermal stress breaks molecular bonds during processing, unreacted photoinitiators break down into predictable fragment patterns. Mapping these cleavage pathways provides the framework required to interpret complex mass spectrometry spectra of unknown film extracts. Whether tertiary peroxide structures formed during long-term storage of re-granulated film pellets decompose into novel migrating aldehydes remains undetermined by current analytical literature.

Extraction

Isolating unbound organic compounds from post-consumer polyolefin matrices requires solvents capable of penetrating the crystalline structure. Matrix swelling opens amorphous regions, allowing trapped photoinitiator degradation products to diffuse out into the liquid phase. Total dissolution techniques destroy the polymer matrix entirely by dissolving the sample in boiling toluene, xylene, or tetrahydrofuran.

Precipitating the base polymer with methanol or ethanol then leaves low-molecular-weight additives and non-intentionally added substances in solution.

While total dissolution recovers embedded non-intentionally added substances completely, solvent precipitation co-extracts low-molecular-weight polyolefin oligomers. Synthetic oligomers between C12 and C36 precipitate incompletely when cold alcohol is added. These residual oligomers stay in the filtered solution, causing chromatographic interference during mass spectrometry screening.

Solid-liquid extraction through controlled swelling offers an alternative to total polymer dissolution. Swelling agents such as dichloromethane, tetrahydrofuran, or ethyl acetate expand the polyolefin matrix without dissolving high-molecular-weight polymer chains. Microwave-assisted extraction speeds up this process by heating solvents above their atmospheric boiling points in sealed pressure vessels.

At temperatures between sixty and eighty degrees Celsius, equilibrium extraction of semi-volatile photoinitiator fragments takes thirty to sixty minutes.

Solvent Selection and Polymer Matrix Swelling

Targeting low-molecular-weight degradation fragments in polyethylene requires balancing solvent polarity against polymer dissolution parameters. Dichloromethane swells low-density polyethylene efficiently at forty degrees Celsius without extracting high concentrations of long-chain wax oligomers. Isopropanol and acetonitrile act as non-swelling solvents, targeting surface-bound set-off contaminants while leaving internal polymer matrices intact.

Ultrasonication improves solvent penetration into micro-cracks and surface features of shredded film flakes.

Extraction efficiency and recovery ratios vary with solvent choice, contact temperature, matrix density, and mechanical agitation. Polarity matching ensures high solubility for target fragments like benzhydrol, benzoic acid, and 4-methylthiobenzaldehyde.

Food Simulant Contact Protocols and Migration Setup

Standardized testing procedures require immersing film samples in liquid chemical media or solid adsorbents under controlled temperatures. EN 1186 and EN 13130 specify conditions simulating various food contact types and storage durations. Standard media for compliance verification include Food Simulant A (10% ethanol), Simulant B (3% acetic acid), Simulant C (20% ethanol), Simulant D1 (50% ethanol), Simulant D2 (vegetable oil or 95% ethanol), and Simulant E (poly(2,6-diphenyl-p-phenylene oxide), known commercially as Tenax).

Polyolefin films containing recycled content undergo specific migration testing based on their intended end use. High-temperature short-term exposures call for testing at one hundred twenty-one degrees Celsius for two hours in closed pressure cells using Simulant D2 or Tenax. Long-term ambient storage requires contact for ten days at forty degrees Celsius, while accelerated testing for extended storage runs for ten days at sixty degrees Celsius in 95% ethanol or Tenax.

Evaluating extraction yields involves running parallel dissolution tests across three solvent mixtures. Swelling in dichloromethane for sixteen hours at forty degrees Celsius gives comprehensive recovery of non-volatile photolysis fragments. Tenax contact at sixty degrees Celsius for ten days selectively collects migrating volatile and semi-volatile fractions.

Comparing total extract concentrations against migration amounts determines the migration partitioning coefficient for each identified degradation compound.

Higher extrusion temperatures during film re-granulation consistently increase the concentration of low-molecular-weight aromatic breakdown products.

Sample preparation procedures convert raw film extracts into clean analytical solutions through structured sequential steps:

1. Cut recycled polyolefin film samples into uniform pieces measuring one square decimeter per specimen.
2. Immerse the film specimen in fifty milliliters of dichloromethane inside an amber glass extraction vessel.
3. Incubate the sealed vessel at forty degrees Celsius for sixteen hours under gentle orbital agitation.
4. Decant the liquid extract through a zero point two micron PTFE syringe filter into a clean glass flask.
5. Concentrate the filtered solvent to a final volume of one milliliter using a gentle stream of high-purity nitrogen gas at thirty degrees Celsius.
6. Add internal standards, including deuterated benzophenone-d10 and atropine, at known concentrations.
7. Transfer the final extract into an amber high-performance liquid chromatography vial equipped with a PTFE-lined septum for instrumental analysis.

Because solvents that completely dissolve the recycled polyolefin matrix invariably co-extract high-molecular-weight oligomers that contaminate mass spectrometer ion sources, clean-up steps are necessary to remove matrix interference.

Detection

Instrumental workflows for identifying unknown degradation products combine gas-phase separation with high-resolution ion measurement. Gas chromatography coupled to electron ionization mass spectrometry isolates volatile and semi-volatile fragments like benzaldehyde, acetophenone, and cyclohexanone. Coupling gas chromatography to quadrupole time-of-flight mass spectrometry provides accurate mass data for molecular ions and fragment peaks, establishing empirical formulas within a mass error tolerance of less than three parts per million.

Non-volatile, polar, or thermally labile fragments require liquid chromatography instead. Reverse-phase liquid chromatography using C18 or phenyl-hexyl stationary phases resolves polar degradation products, including hydroxybenzophenone isomers, phosphinic acids, and amine oxidation products. Coupling liquid chromatography to electrospray ionization Orbitrap mass spectrometry delivers resolving power above one hundred thousand at mass-to-charge ratio two hundred, separating isobaric matrix interferences from target degradation products.

Spectral interpretation relies on accurate mass measurements, isotopic abundance patterns, and collision-induced dissociation spectra. MS/MS fragmentation trees reveal characteristic structural losses. Benzoyl derivatives display a prominent fragment ion at mass-to-charge ratio 105.0335, corresponding to the C7H5O cation, while acylphosphine oxides yield characteristic product ions at mass-to-charge ratio 217.0413 for diphenylphosphinyl moieties.

Comparing experimental spectra against reference libraries confirms structural identities.

High Resolution Mass Spectrometry Elucidation Protocols

Accurate mass measurements from quad-time-of-flight analyzers yield elemental formulas for unknown chromatographic peaks. Calculating nitrogen rule compliance, double bond equivalents, and ring-plus-double-bond values narrows candidate structures. Retention index matching on gas chromatography columns provides added confidence by comparing experimental Kovats indices against theoretical values calculated from structure-property relationships.

Quantifying non-intentionally added substances presents technical hurdles because commercially available authentic reference standards for complex degradation products are scarce. Analytical laboratories use surrogate internal standards with similar chemical structures and ionization efficiencies. Because relative response factors introduce quantitative uncertainty, exposure modeling requires conservative estimation parameters.

What Triggers False Positives in Spectrometry Screening?

Polyolefin oligomers bearing identical isobaric nominal masses frequently mimic photoinitiator degradation fragments during non-target screening. Breakdown products of synthetic antioxidants, such as oxidized Irgafos 168 fragments, yield mass spectra that overlap with substituted phosphine oxide signals. High-resolution mass spectrometry resolves this overlap by distinguishing the exact mass of organophosphites from organophosphines.

Phthalate plasticizers and slip agent breakdown products can contaminate laboratory reagents, producing background chromatographic peaks. Blank extraction runs identify ambient laboratory contributions, and gas chromatography isolates volatile fragments. Any peaks appearing in both sample extracts and procedural blanks must be subtracted prior to structural assignment.

European Commission Regulation 10/2011 Annex I dictates that unlisted non-intentionally added substances require risk assessment according to established scientific principles.

Assigning identification confidence to non-target chromatographic peaks follows established criteria based on structural reporting scales:

• Level one confirmed structures require matching exact mass, retention time, and MS/MS spectrum against an authentic reference standard analyzed on the same instrument platform under identical operational conditions.
• Level two probable structures rely on matching high-resolution MS/MS spectra and retention index data against public spectral libraries or detailed literature fragmentation spectra.
• Level three candidate structures represent tentative assignments where exact mass and fragmentation patterns suggest a specific chemical class or isomer family without definitive position confirmation.
• Level four unequivocal formulas provide confirmed empirical elemental formulas derived from exact mass and isotopic abundance ratios without sufficient structural data to propose a molecular skeleton.
• Level five exact mass features consist of reproducible chromatographic peaks defined solely by accurate mass-to-charge ratios and retention times, lacking structural or elemental formula resolution.

High resolution mass spectrometers resolve isobaric peaks, but injecting unpurified film extracts can quickly foul a high-resolution ion source and result in severe instrument downtime and maintenance.

Toxicity

Evaluating the biological hazard posed by migrating degradation products requires systematic application of structural exposure thresholds. Unknown or non-listed substances identified in food-contact film fractions undergo toxicological evaluation through the Threshold of Toxicological Concern framework. European Food Safety Authority guidelines organize chemical structures into Cramer Classes based on toxicological potential, setting acceptable human exposure thresholds for compounds lacking empirical toxicity data.

Cramer Class I compounds possess simple chemical structures with efficient metabolic detoxification pathways, corresponding to a human exposure threshold of one thousand eight hundred micrograms per person per day. Cramer Class II covers moderately toxic structures with a threshold of five hundred forty micrograms per person per day. Cramer Class III comprises complex aromatic, heterocyclic, or reactive functional groups suggesting significant toxicity, setting a protective threshold of ninety micrograms per person per day.

Threshold of Toxicological Concern Application

EFSA scientific opinions govern exposure evaluations for chemical species lacking dedicated toxicological feeding studies. Converting human daily intake thresholds into maximum permissible migration limits assumes a default food consumption model of one kilogram of food consumed daily by a sixty-kilogram individual in contact with six square decimeters of packaging film. Under these standardized conversion parameters, a Cramer Class III threshold translates to a specific migration limit of zero point fifteen milligrams per kilogram of food.

Structures carrying specific chemical alerts for genotoxicity fall outside standard Cramer Class thresholds. Genotoxic carcinogens demand protective human exposure limits based on a toxicological threshold of zero point one5 micrograms per person per day. Expressed as a migration concentration in food, this genotoxicity threshold establishes a maximum legal migration limit of zero point zero ten milligrams per kilogram of food (10 parts per billion).

In Silico Hazard Profiling and Genotoxicity Screening

Computational software models predict mutagenic potential by cross-referencing molecular connectivity maps against established structural alerts. Quantitative Structure-Activity Relationship systems including Derek Nexus, VEGA, and the OECD QSAR Toolbox evaluate bacterial reverse mutation performance (Ames test) in silico. Compounds containing aromatic amines, unsubstituted heteroatoms, or alpha,beta-unsaturated carbonyl groups trigger automated alerts for DNA reactivity.

Structural evaluation using in silico tools determines whether a photoinitiator degradation product requires full empirical mutagenicity testing or qualifies for clearance under standard threshold levels.

Regulatory dossiers for recycled film fractions must include detailed toxicological data packages:

• Chemical identity documentation specifying empirical formulas, CAS numbers, IUPAC names, and confirmed structural drawings for every migration feature exceeding ten parts per billion.
• Chromatographic quantitation reports detailing calculated specific migration values in food simulants alongside stated method detection limits and measurement uncertainties.
• Cramer classification rationale detailing the logical decision tree path used to assign the substance to Cramer Class I, II, or III.
• In silico mutagenicity predictions containing consensus output reports from at least two independent QSAR software platforms targeting Ames mutagenicity.
• Risk characterization statements demonstrating that estimated daily consumer intake remains below the applicable Threshold of Toxicological Concern value.

Because unbound photoinitiators migrate rapidly, standard procurement contracts for food-grade recycled polyolefin resins now mandate that every unassigned chromatographic peak exceeding 0.01 milligrams per kilogram migration equivalent carries a formal in silico genotoxicity evaluation.

Obligation

Legal responsibility for food contact safety rests firmly upon the entity placing the finished packaging material onto the single market. Article 3 of Regulation (EC) 1935/2004 dictates that materials and articles must not transfer their constituents to food in quantities that endanger human health, bring about an unacceptable change in food composition, or cause deterioration in organoleptic characteristics. Article 19 of Regulation (EU) 10/2011 extends this mandate explicitly to non-intentionally added substances, requiring business operators to perform toxicological risk assessments for chemical species generated during manufacturing or recycling processes.

Regulation (EU) 2022/1616 establishes legal controls over recycled plastic materials intended for food contact applications. Decontamination processes must demonstrate suitable technology clearance through European Food Safety Authority evaluations. Recyclers must operate continuous quality assurance schemes under Regulation (EC) 2023/2006 to control input scrap quality, decontamination efficiency, and chemical output uniformity.

Testing isolated resin samples does not fulfill legal duties if converting processes alter the chemical baseline through secondary thermal stress.

Regulatory Frameworks for Recycled Polyolefin Films

European packaging directives demand strict verification that recycled plastics introduce no chemical agents capable of altering food composition. Importers and converters assume civil liability for non-compliant migration values detected during market surveillance audits. National enforcement authorities utilize rapid screening protocols to identify unauthorized photoinitiators and non-assessed breakdown fragments in flexible food packages taken from retail shelves.

Rapid Alert System for Food and Feed notifications trigger product withdrawals, inventory seizures, and financial fines. Importers face customs holds when shipping documentation lacks adequate supporting analytical files. Declarations of Compliance that rely on generic supplier statements without batch-specific migration data fail regulatory audits.

Solvent extraction of thin flexible films reaches equilibrium significantly faster than thick rigid container walls.

Declaration of Compliance and Supply Chain Dossiers

Traceability requirements link raw post-consumer resin batches to final converter conformity declarations through audited analytical dossiers. Declarations of Compliance must explicitly state the presence of restricted additives, dual-use additives, and evaluated non-intentionally added substances. Supporting documentation dossiers must remain accessible to national authorities upon demand, containing raw chromatograms, mass spectra, identification confidence logic, and toxicological risk characterizations.

Auditing declaration packages against raw mass spectra verifies that non-listed substances match underlying chromatograms, ensuring that declarations meet analytical standards while the importer retains full financial liability.

Establishing verified decontamination protocols and continuous chemical monitoring guarantees that recycled film fractions maintain legal compliance without exposing brand owners to unquantified migration liabilities.