novel chemistry for uv coatings

8/16/2019 Novel Chemistry for UV Coatings

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European Coatings Journal

10/2005

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Novel chemistry for UV coatings

Photolatent base system optimises performance forrefinish clearcoats.Kurt Dietliker, Katharina Misteli, Tunja Jung, Patrik Contich,Johannes Benkhoff, Eugene Sitzmann.Radiation curing offers clear economic and environmentaladvantages. It combines low VOC formulations and lowprocessing temperatures with fast cure speed.Nevertheless, current systems have some well-understoodlimitations. The development of a new photolatent base UVcuring system is described, which is particularly well suitedfor use in the difficult application area of refinish clearcoats.Most radiation curable coatings today are based onphotoinitiated free-radical polymerisation. A wide variety ofresin components and photoinitiators allows the formulationof photocurable coatings for a broad spectrum ofapplications. The technology does, however, havesignificant limitations, notably 'oxygen inhibition', whichretards surface cure, and high shrinkage upon curing, whichlimits adhesion in some applications.Photoinduced cationic polymerisation has also beenexploited. It eliminates oxygen inhibition and offers improvedadhesion. Some inherent limitations and higher costs have,however, restricted the use of this technology.

Amine catalysts can also be used in UV curingVery few photoinduced curing processes usingbase-catalysed crosslinking reactions are known [1]. Thephotoactive compounds used rely on photolabile blockinggroups for primary or secondary amines, developed asprotective groups in organic synthesis [2]. A few coatingapplications have been reported using photogeneratedamines as crosslinkers in reactions with epoxides [3-5] orisocyanates [4, 6]. A stoichiometric quantity of photons isrequired in relation to the number of crosslinks to be formed,making the energy efficiency of this curing process ratherlow.Due to their low basicity and nucleophilicity, primary andsecondary amines are less efficient as catalysts than tertiaryamines. However, the photogeneration of tertiary amines isconsiderably more difficult. The introduction of additionalphotolatent substituents on the amine results in theformation of ammonium salts, while transformation of onesubstituent into a photolabile group gives a precursor of asecondary amine (Figure 1).Most examples of photolatent tertiary amines reported in theliterature are thus salts [7-12], with a correspondingly limited

solubility and low chemical stability. One of the fewexamples of a neutral photolatent tertiary amine is atetramethyl guanidine precursor [13].The lack of suitable photolatent bases has prompted theintroduction of a research programme to develop atechnology platform based on novel photobase generators[14]. The first photolatent catalyst successfully developed foruse in UV-A curable car refinish coatings is describedbelow.

Vehicle finishing poses many problems for UV curingRadiation curing offers considerable advantages for theautomotive industry. However, this technology has not yetfound widespread use due to some major challenges:- Automotive topcoats are applied to complex 3-D carbodies, resulting in the formation of shadowed areas which

are not fully cured.- The distance between the lamp and the coating is largerand more variable than in conventional UV applications. The

resulting low light intensity gives rise to strong oxygeninhibition in free-radical coatings.- The long-term exterior durability of an automotive topcoatrequires an optimised stabiliser package, but UV absorberscan compete with the photoinitiator for the incident lightmaking through cure more difficult.- Residual photoinitiator and unreacted unsaturated groupsin the cured coating were believed to contribute to fasterdeterioration of UV-cured lacquers under outdoorweathering conditions.Over recent years, much work has been devoted toovercoming these difficulties. The radiation curing of OEMtopcoats has been proved to be feasible, if an optimisedcombination of resins, photoinitiators, stabiliser package andcuring equipment is used.

Features such as low VOC values, fast drying at ambienttemperature which increases productivity and avoidsproblems with heat sensitive parts, are also very attractivefor car refinish applications. Consequently, a UV curablerefinish primer has recently been introduced [15].

Refinish clearcoats are especially hard to cureThe development of a radiation-curable clear topcoat for carrefinish applications is a greater challenge. Such a materialmust offer performance similar to an OEM topcoat but alsobe processable under the working conditions of a bodyshop.The technology must rely on simple, low cost, mobile andflexible equipment instead of the sophisticated equipmentdeveloped for OEM use.Radiation sources are limited to lamps emitting in theUV-A/visible range which do not require extensive protective

equipment for the worker. This low light intensity increasesoxygen inhibition in free radical curing formulations.Consequently, a different curing mechanism is preferable.'Dark cure' must also occur in areas that are not sufficientlyirradiated, preferably at room temperature since theapplication of heat to a fully equipped car would offset manyof the advantages of UV.

Thiol-isocyanate chemistry avoids oxygen inhibitionAn efficient solution to these problems requires a new resinand initiator chemistry. A suitable resin chemistry hasrecently been developed [16], based on a two-componentsystem crosslinked by the addition of thiol groups toisocyanate functionalities. After mixing, the formulationundergoes slow crosslinking in the dark even in the absenceof a catalyst. In the presence of an amine catalyst, full cure

is achieved within a few minutes at room temperature. Thisis very attractive in terms of high productivity, but makeshandling and application very difficult.Only a photolatent amine, allowing the catalyst to beactivated after application, can provide the required controlover the curing process. This paper reports on thedevelopment of the catalyst, while a later paper will focus onthe use and performance of the formulation in car refinishapplications [16].

Photolatent amine must meet several requirementsThe processing and application conditions alreadydiscussed define the requirements for the photolatentamine:- Activation must occur under UV-A light;- UV absorbers should not affect the efficiency of the

initiator;- Highly efficient photoreaction is needed to give fast curingat low light intensities;

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- The amine produced must be an efficient catalyst for thecrosslinking reaction;- The catalyst must not cause yellowing after cure or uponlong-term exposure.In contrast to conventional photoinitiators, the latent form ofthe catalyst need not be completely inactive, since moderatecatalytic activity improves dark cure. Hence a compoundpossessing an optimum balance of reactivity in the latentand non-latent forms had to be developed.

Two main factors affect amine efficiencyThe efficiency of an amine catalyst depends primarily on itsbasicity and steric hindrance [17]. Two novel concepts forthe blocking of tertiary amines by photolabile groups haverecently been developed: photoinduced oxidativeintroduction of an amidine double bond and photoinducedsteric release [14].The first concept has been proved to be useful forcompounds liberating strong bases such as DBN or DBU

[18]. Photoinduced steric release has been used for tertiaryamines to catalyse the ring-opening addition of carboxylatesto epoxides [19].An initial evaluation revealed that tertiary alkyl amines arethe most suitable catalysts for thiol-isocyanate crosslinking.Therefore, research was focused on developing photolatenttertiary amines which utilise the steric release concept.

Steric uncoupling changes molecular configurationNew compounds were developed, based on a-aminoacetophenone structures [20, 21] possessing a highlycrowded quaternary carbon atom in the a-position to thenitrogen atom. Calculations performed on one compound,1-(4-methylthio-phenyl)-2-methyl-2-(morpholin-4-yl)-1-propanone (PLA-1), suggest a folded configuration in which theactivity of the amine is hindered by the shielding effect of the

benzoyl moiety [19].The benzoyl group can be removed by an efficient NorrishType I cleavage that produces an a-aminoalkyl radical [22].Since the formulation does not contain radicallypolymerisable groups, stabilisation of the radical occurs viahydrogen abstraction, which is greatly facilitated by the thiolfunctionalities in the resin.This photoreaction efficiently generates a new tertiary aminethat is sterically much less crowded. Calculations for theamine thus formed from PLA-1 (designated A-1) indicate astretched conformation in which the nitrogen atom isaccessible for reactions (Figure 2). Thus, this transformationmeets all the requirements of the steric release concept.Oxygen inhibition is not an issue since no radical chainreaction is involved in the curing process.

Efficiency and yellowing effects studiedSeveral classes of photolatent amines were evaluated. Theircuring efficiency strongly depends on the structure andbasicity of the amine produced. For example, amine A-1formed from PLA-1 efficiently catalyses thecarboxylate-epoxide reaction [19], but is a poor catalyst forthe crosslinking investigated here.Compounds possessing other types of amino groups and ahigher steric hindrance were found to be more efficient(Figure 3). The absorption of the substituted benzoyl groupallows for efficient activation with UV-A light, and fast curingof the formulation upon irradiation was observed.

Most α-amino ketone photoinitiators impart yellowing to the

cured coating. Hence another challenge was thedevelopment of a photolatent moiety that gives no yellowing.Finally, suitable modifications to the chromophore allowedthe combination of high initiation efficiency under near-UVirradiation with very low yellowing after cure in the structure

designated PLA-2 (Figure 3).

Factors controlling cure speed are identifiedBoth the latent amine (PLA-2) and its active form (A-2) aretertiary amines and thus catalysts for the crosslinkingreaction. The catalytic efficiency of the two compounds is,however, different and modulated by the steric andelectronic environment of the nitrogen atom. This effect isillustrated by potlife studies on ready-to-usepolythiol/polyisocyanate formulations containing either thelatent or the active catalyst (Table 1). Potlife is defined asthe time to doubling of the initial viscosity.The stability in the absence of a catalyst of 36 hours at roomtemperature is reduced by a factor of two upon addition of0.33% PLA-2. Increasing its concentration to 1% furtherdecreases potlife to four hours. This shows that the latentamine has a weak but measurable catalytic activity.To confirm the nature of the photoactivation process, theactive catalyst A-2 was synthesised independently. If only

0.15% of this compound is added to the formulation, shelflife is drastically reduced to less than 2 minutes. Thischange closely resembles the light-triggered activation ofthe catalyst.Under irradiation conditions, conversion of the latent into theactive form is not complete. When 0.33% catalyst is usedand undergoes photoreaction with 50% efficiency, theconcentration of the active catalyst after irradiation would bein the range of 0.15%. The accompanying change from 18hours to less than 2 minutes potlife corresponds to anincrease of catalytic efficiency by a factor of more than 500.This is in good agreement with experimental data, where acure time of three minutes was determined for a formulationcontaining 0.8% latent catalyst under a Philips "TL 40W/03"fluorescent lamp emitting UV-A light. An optimum balance ofshelf life, dark cure and cure speed can thus be achieved by

optimising the initiator concentration.

Amine properties must be matched to curing reactionTo provide a better understanding of the catalytic efficiency,basicity (pKa) values of the amine in its latent and activeforms were calculated [23] (Table 2). The difference in

basicity of δpKa = 2.8 is attributed to electronic changes in

the vicinity of the amino group. In addition, the sterichindrance in the highly crowded latent form PLA-2 furtherreduces the catalytic efficiency of the latter.PLA-1 and its corresponding free amine A-1 exhibit an even

larger δpKa difference. The basicity of A-1 is, however,

lower than that of A-2 by 2.35 pKa units, explaining the lowefficiency of PLA-1 in the thiol/isocyanate addition reaction.DBN, though it is a considerably stronger base than either ofthese free amines, is less efficient and not useful for thisapplication (Table 1). Clearly, base strength is not the onlyfactor controlling the catalytic activity of the amine, and acareful selection of the amine to match the crosslinkingreaction is crucial for efficiency.

New chemistry has clear advantages in refinishingA new photolatent catalyst has been developed, whoseefficiency can be switched from a low to a highly reactivestate under the influence of light. A two-pack formulation canbe produced with extended pot life, very fast cure uponirradiation, and ultimately full cure even in shadowed areas.The sensitivity of the photolatent catalyst to UV-A light andthe absence of oxygen inhibition allows curing to be carriedout by low intensity lamps that do not emit harmful radiationand can be easily used in a bodyshop environment.

REFERENCES[1] K. Dietliker, J. V. Crivello, Photoinitiators for Free



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Radical, Cationic and Anionic Photopolymerization, 2ndedition, Vol. 3, in the series Chemistry and Technology ofUV and EB Formulation for Coatings, Inks and Paints (G.Bradley, ed.), John Wiley and Sons/SITA Technology,London, 1998, chapter IV.[2] R. W. Blinkley, T. W. Fletchner, Synthetic OrganicPhotochemistry (W. M. Horspool, ed.), Plenum Press, NewYork 1984, p. 407.[3] G. Kutal, G. Willson, J. Electrochem. Soc. 1987, 134,2280.[4] T. Nishikubo, E. Takehara, A. Kameyama, Polym. Sci.Part A: J. Polym. Chem. 1993, 31, 3013.[5] K. Ito, M. Nishimura, M. Sashio, M. J. Tsunooka, Polym.Sci. Part A: Polym. Chem. 1994, 32, 1793.[6] S. Bull, Photogenerated Amines as Novel CrosslinkingAgents, e¦5 2004 (RadTech USA), Technical Conference,Proceedings May 2-5, 2004.[7] W. Mayer, H. Rudolph, E. de Cleur, Angew. Makromol.Chem. 1981, 93, 83.

[8] A. Noomen, H. Klinkenberg, Eur. Pat. Appl. 448154 A1(1990).[9] J. E. Hanson, K. H. Jensen, D. Gargiulo, D. A. Motta, A.E. Pingor, D. Novembre, A. Mixon, J. M. Kometani, C.Knurek, Polym. Mater. Sci. Eng., 1995, 72, 201.[10] A. M. Sarker, A. Lungu, D. C. Neckers, Macromolecules1996, 29, 8047.[11] G. Baudin, S. C. Turner, A. F. Cunningham, PCT Pat.Appl. WO 00/10964 (2000).[12] H. Tachi, T. Yamamoto, M. Shirai, M. Tsunooka, Polym.Sci. Part A: J. Polym. Chem. 2001, 39, 1329.[13] W. A. D. Stanssens, J. F. G. A. Jansen, US Pat.6124371.[14] K. Dietliker, T. Jung, J. Benkhoff, Photolatent Amines:New Opportunities in Radiation Curing, e¦5 2004 (RadTechUSA), Technical Conference Proceedings, May 2-5, 2004.

[15] P. Leeseman, RadTech Report 2001, Nov/Dec, 15.[16] N. Dogan, H. Klinkenberg, L. Reinerie, D. Ruigrok andP. Wijnands, A New Generation of UV-A CurableClearcoats, EC J. 2005,in press.[17] S.-W. Wong, K. C. Frisch, J. Polym. Sci. Part A: Polym.Chem. 1986, 24, 2877.[18] G. Baudin, K. Dietliker, T. Jung, PCT Pat. Appl. WO03/033500.[19] H. Kura, H. Oka, J.-L. Birbaum, T. Kikuchi, J.Photopolym. Sci. Technol. 2000, 13, 145.[20] K. Dietliker, J. V. Crivello, Volume: as Ref [1], chapterIII.[21] K. Dietliker, A Compilation of PhotoinitiatorsCommercially Available for UV Today, SITA TechnologyLimited, Edinburgh/London 2002.[22] G. Rist, A. Borer, K. Dietliker, V. Desobry, J.-P.

Fouassier, D. Ruhlmann, Macromol. 1992, 25, 4182.[23] Advanced Chemistry Development (ACD/Labs)Software Solaris V4.67.[24] W. Erdle, K.-H. Vogel, R. Tinsley, Farbe UND Lack2004, 2, 22.

Results at a glance- A new type of photolatent amine catalyst has beendeveloped, whose efficiency can be switched from a low to ahighly reactive state under the influence of UV-A light.- This catalyst can be used in a two-pack thiol/isocyanateradiation curable formulation, which does not suffer from the'oxygen inhibition' found in free-radical type coatings.- A formulation can be produced with extended pot life, lowyellowing, very fast cure upon irradiation, and ultimately fullcure even in shadowed areas.

- These features allow refinish clearcoats to be formulatedwhich cure under low intensity lamps that do not emit

harmful radiation and can be easily used in a bodyshopenvironment.

The authors:> Kurt Dietliker, Senior Research Fellow, is responsible forresearch on photolatent amines in Coating EffectsResearch, Basel.> Katharina Misteli is Lab Technician in Coating EffectsResearch, Basel.> Johannes Benkhoff is Head Technology Center BusinessLine Coatings in Basel.> Tunja Jung is Technical Manager New Technologies atTechnology Center Business Line Coatings in Basel.> Patrik Contich is Lab Technician at Technology CenterBusiness Line Coatings in Basel.> Eugene Sitzmann is Technical Manager UV Curing for theBusiness Line Coatings in Newport (DE), USA.This paper is one of two on a novel UV-A curable carrefinish clearcoat. Part II will be published in the November

2005 issue of European Coatings Journal.



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Figure 1: Modification of a tertiary amine by a photoremovable group.

Figure 2: Photoinduced steric release provides latency in PLA-1.

Figure 3: Development of the new photolatent amine PLA-2.



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novel chemistry for uv coatings

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