1 PHOTOINITIATOR BASIC CHEMISTRY. INTRODUCTION TO FORMULATING AND PRODUCTS FOR LED CURE AND LOW MIGRATION. Youyuan Wu IGM Resins USA Inc 1.0 INTRODUCTION What is a photoinitiator? A substance (other than reactant) which, on absorption of light, generates a reactive species (e g radical), initiates a chemical reaction or transformation, and is consumed. Photoinitiators (PI s) can be thought of as the catalysts for the polymerisation reactions that take place converting a liquid coating to a cured film. Anything that can lessen or prevent the absorption of light by the PI will reduce the amount of radicals or ionic species formed. Thus directly effecting the ability to cure. 2.0 UV CURING The polymerisation mechanisms of UV curing can be classified into 2 main types. Free Radical UV Curing, oligomers are acrylates or unsaturated polyesters, photoinitators are free radical (Type I, unimolecular cleavage and Type II, bimolecular reaction, hydrogen abstraction or electron transfer). Cationic UV Curing, oligomers are epoxies or vinyl ethers, photoinitiators are cationic (Iodonium or sulphonium salts) 2.1 TYPE I FREE RADICAL PI S When UV light energy is absorbed, they instantly cleave to generate 2 radicals. The following families of Type I PI are available: Benzil ketals, α-hydroxyalkyphenones, α- amino acetophenones, Acylphosphine oxides.
2 Figure 1: Reaction Mechanism - Type I Photoinitiator 2.2 TYPE II FREE RADICAL PI S When UV energy is absorbed they undergo a 2 stage (bimolecular) process to form their free radicals. A hydrogen atom from a donor source usually an amine synergist is needed before free radicals can be formed. Hydrogen donation (electron transfer) is required to generate radicals. This can be from a donor molecule such as an amine synergist / thiol, or by intramolecular abstraction. The following families of Type II PI are available: Benzophenones, Thioxanthones, Benzoylformate esters. Figure 2: Reaction Mechanism - Type II Photoinitiator 2.3 CATIONIC PHOTOINITIATORS Not for use with acrylate chemistry but to cure cycloaliphatic epoxides such as IGM s OC range of oligomers. Upon photolysis cationic PI produce a Strong Lewis or Brönsted acidic species which initiates the polymerisation process. Initiating species
3 The counter ion X plays a key role as it controls the strength of the acid generated and thus the cure speed. Increasing Efficiency BF 4 - < PF 6 - < AsF 6 - < SbF 6 - IGM can offer the following types of cationic initiator: Diaryliodonium, Triarylsulphonium and Thioxanthone based sulphonium salts. 3.0 PI AND FORMULATING FOR UV - LED Source : Image by Phoseon Figure 3: Spectral output comparison between Mercury Lamp and UV - LED Why is spectral output and lamp type important? Photoinitiators the catalysts for the UV curing reaction must absorb light energy to work. Without photoinitiator absorption, excitation and free radical production a coating formulation will not cure under UV irradiation. Curing lamps must emit UV energy in the spectral area that the PI s used in a formulation absorb at. Efficiency of lamp directly effects productivity and costs. Dose energy delivered to the surface per unit area is inversely proportional to speed and directly proportional to exposure (number of passes, duration).
4 3.1 OXYGEN INHIBITION Oxygen Inhibition adversely effects the curing of a formulation by quenching scavenging free radicals from the system. Quenching. PI (triplet) + O 2 PI (ground state) + O 2* O 2 + heat Scavenging Radical + O2 ROO (Inactive peroxy radical) and/or 3.2 FORMULATING FOR LED Formulating for LED lamps is difficult due to the single wavelength restricting PI choice. Oxygen inhibition can greatly effect the balance of surface and through cure. However, there are chemical and physical methods for reducing this. Chemical methods include: Multifunctional Thiols (Omnimer PE-1) N-Vinyl Amides such as N-Vinylpyrrolidone (Omnimer NVP) Tertiary Amine Additives (Omnirad EHA, EDB, Photomer 4250, 4771, 4775, 4967, 5006) Thioether Groups in Monomer Structure Physical methods include: High-Intensity Light High PI Concentrations (radicals to react with oxygen) Wax Additive (barrier to oxygen entering surface) High-Viscosity Monomers (slow oxygen diffusion) Laminate (prevents oxygen from entering coating) Inertion (eliminates oxygen) 3.3 PHOTOINITIATORS FOR UV - LED IGM has developed a number of blends suitable for LED cure. Omnirad BL 751. UV-LED Blend for pigmented systems useful with 395nm lamps. Omnirad BL 723. Excellent surface and through cure for UV and LED, particularly for whites, λmax 246, 352, 382 nm
5 Omnirad BL 724. Excellent surface and through cure for UV and LED, particularly for dark pigmented systems, λmax 275, 354, 370 nm To further aid the formulator IGM has developed a novel Type I free radical PI Omnirad 264 with enhanced long wavelength absorption and improved solubility over other PI in the same chemical class. Omnirad 264 is especially recommended for inkjet inks and curing under LED lamps. 3.4 NOVEL PI - OMNIRAD 264 PHYSICAL PROPERTIES COMPARISON Appearance: Pale yellow crystal or powder Melting Point: 74 78 C Purity: 98.0 % (HPLC) Solubility TPGDA Acetone PMA Omnirad 248 6 % 17 % 12 % Irgacure 379 24 % >50 % 47 % Omnirad 264 18 % >50 % 35 % Table 1: Solubility comparison λ max ε max Omnirad 248 318 nm 2.000 10 4 Irgacure 379 320 nm 2.341 10 4 Omnirad 264 333 nm 2.575 10 4 Table 2: Maximum UV Absorbtion and Molar Absorbtivity Figure 4: Omnirad 264 UV Absorbance Spectral Comparison Omnirad 264 is shifted further into the UVA area and has a stronger overlap at 365 nm.
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7 3.5 LED CURING PERFORMANCE STUDY - Equipment: UV-LED 101D; Slides. - Curing condition: Irradiation distance: 21mm, Irradiation time: 1s Figure 5: LED Cure in a Varnish Formulation In a Varnish Omnirad 264 performs fractionally better than Omnirad 248 or Irgacure 379. Figure 6: LED Cure in a Black Formulation In a Black ink Omnirad 264 cures faster at 365 nm than Omnirad 248 or the content of Omnirad 264 can be reduced to have same performance as Omnirad 248.
8 Figure 7: LED Cure in a Yellow Formulation In a Yellow ink Omnirad 264 cures ~30% more than Omnirad 248 or for same performance photo initiator content can be reduced by ~47.5%. 4.0 PI DESIGNED FOR LOW MIGRATION Food compliance is one of the most challenging parameters to get the right packaging for food. A food compliance crisis can ruin a brand reputation so it s very important. We have examples from both USA and EU, ITX found in baby milk and Kelloggs mineral oils in cornflakes. It s more than having raw materials complying with regulations, potential break down under light or thermal treatment is as important and migration must be carefully monitored. 4.1 WHAT IS MIGRATION? A substance transfer from the packaging into the packed goods. It is dependent on ink/ varnish formulation, substrate and design of the packaging, printing conditions (drying, speed, etc), the final use (type of food, sterilization, etc) and the storage conditions (temperature, time, pressure)
9 4.2 MIGRATION MECHANISM Figure 8: Types of Migration 4.3 MIGRATION TESTING Low Migration materials are by nature also low odor and low taint. The reverse is not always true. This does not negate need for organoleptic properties testing. Such testing was common in place prior to migration concerns and the methods /standards are well known and readily available. e.g. EN1230-2:2001 Food simulants and their use are prescribed in a number of EU Directives and Regulations. Simulants A, B, C, D1 & D2 and E are designated as suitable for testing of food contact materials. The simulant chosen is dependent on the nature of the foodstuff that is to be packaged, e.g. Simulant B; 3% acetic acid in water used for acidic foods such as fruit juices Simulant D; 95% ethanol in water or olive oil or iso-octane - used for fatty food Tenax, Poly (2,6-diphenylphenylene oxide) is a highly porous polymer - used for dry food Analytical methods such as GC, MS and HPLC are used to separate, detect and identify migrants. Results are often calculated to an EU standard model, whereby 1kg of food is assumed to be wrapped within 600 cm 2 of print.
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11 4.4 POLYMERIC PHOTOINITIATIORS IGM has developed a range of polymeric PI with molecular weights typically ranging from 700 to > 1000 Daltons. Polymeric PI typically have the structure below. Figure 9: Basic model Polymeric Photoinitiators Low migration potential is due to the relatively high MW, restricting movement in comparison to standard PI in matrices with low crosslink density. Low migration is also due to the fact that one or both PI moieties can cross link into the growing polymer chain during cure, effectively locking the PI in situ. A drawback is that products tend to have high viscosity. 4.5 IGM PI RECOMMENDATIONS IGM Product Code Product Type Swiss Ordinance TSCA Omnipol 910 Type I PI Expected 2015 YES Omnipol 9210 Type I PI diluted Expected 2015 YES Omnipol 9220 Type I PI diluted Expected 2015 YES Omnipol ASA Amine synergist YES YES Omnipol BP Type II PI YES YES Omnipol 2702 Type II PI NO NO Omnipol 2712 Type II PI NO NO Omnipol 682 Type II PI YES Expected 2015 Omnipol TX Type II PI YES YES Table 3: Low Migration Photoinitiators
12 4.6 NEW POLYMERIC BLENDS FOR EASE OF USE IGM has developed polymeric blends to aid the formulator. ECX 14-026 is suitable for dark pigmented systems. ECX 14-027 is suitable for OPV and white pigmented systems. (Swiss compliant) ECX 14-041 is suitable for OPV and white pigmented systems. (Nestle + Swiss compliant) Omnipol BL 728 is a readily usable blend at room temperature with a high concentration of Omnipol TX. These products offer the formulator the following benefits over individual polymeric PI s: Ease of use, particularly for facilities with limited or no hot room facilities. Ease of incorporation into the ink/coating formulation. Single additive PI package, no other initiators or synergists are required, although they can be used if application requires it. Improved storage stability for Omnipol TX blend vs std Omnipol TX. 5.0 OTHER ASPECTS THAT EFFECT CURE PERFORMANCE Figure 10: Pigment Effect on Particle Size and Film Thickness
13 Figure 11: Pigment Effect on Colour and Pigment Concentration 5.1 PI SELECTION DEPENDENCE ON TYPE OF LAMP A Comparison of two Type I Photoinitiators shows through its spectral absorbtion data which material may be suitable for which type of Lamp. Figure 12: Omnirad TPO
Figure 13: Omnirad 73 14
15 6.0 CONCLUSION Consider when formulating with PI s, they absorb light energy in order to work, curing lamps must emit UV energy in the spectral area that the PI absorbs. Anything that can lessen or prevent this absorption of light will reduce the amount of PI radicals or ionic species formed oxygen inhibition, pigments (some colors are more problematic than others) and coating thickness. Leads to poor curing performance to surface tack, to poor through cure and to poor physical properties. Low migration printing for food packaging is not just about the ink and the raw materials used to produce it. Multi-national companies / brand owners may require that their packaging conform to their own requirements as well as specific legislation, wherever it is manufactured on a global basis. In migration testing the simulant chosen is dependent on the nature of the foodstuff that is to be packaged. Where an efficient functional barrier cannot be applied. The converter must ensure that the Overall Migration Limit, as well as the individual substance Specific Migration Limit (SML) and other limitations when applicable are fully respected. This can be achieved by appropriate pack design, controlling the composition and migration features of the raw materials, testing directly the intermediate or finished products and controlling the process (working hygiene).