A comparative kinetic study of commercial photoinitiators for UV/visible curable acrylate clear coatings

A comparative kinetic study of commercial photoinitiators for UV/visible curable

acrylate clear coatings N SAllen ,J Segurola, M Edge, E Santamaf i a n d A M c M a h o n

T h e M a n c h e s t e r M e t r o p o l i t a n Universi ty, D e p a r t m e n t o f C h e m i s t r y and Materials, J o h n Da l ton Building, C h e s t e r Street, M a n c h e s t e r M1 5GD, Uni t ed K i n g d o m

S Wilson

Techn ica l Centre , Swale Process ,Tay lo r Road,Traf ford Park, Manches te r , Un i t ed K i n g d o m

Introduction The curing of coatings and inks by ultraviolet or visible light has become an established tectmologT for many industrial applications, >5 including offset lithographic inks, flexo- graphic inks, screen printing, metal decoration, basecoats for beverages caius, 6 wood coatings, pigmented coatings for textile applications7 coatings exhibiting a stereoscopic pattern, 8 pigmented primary9 and secondary optical fibre coatings. 10

Over the years, several types of photoinitiators have been developed to induce the photopol}~nerisation or pho- tocrosslinking of acrylate systems, Free radical photoinitia- tots induce a free radical chain process in which low molecular weight monomers and prepolymers are converted by the absorption of UV/visible light into highly crosslinked films. Measurements of the rates of curing or crosslinking vary from the simple, such as the thumb test, pencil and pendulum hardness methods to the more scientific methods involving real tmle infra-red (RTIR) spectroscopy. The basic mechanism for any photocurable free radical system involves the formation of free radical species through the absorption of light by the photoinitiator. The active radical will then add to the monomer, inducing a chain growth polymerisation and eventually termination. The photophysi- cal and photochemical properties of the photoinitiators are extremely important in controlling the reactivity and they should possess the follo~4ng properties:

e High absorptivity in the region of activation E High quantum yield for free radical formation

Solubility in the resins I Non yellowing, odourless, low volatility m Non toxic, low migration and cost effective

There are two basic categories of photoinitiators which meet these requirements. The first group involves type I photoinitiators which undergo a direct photoffagmentation process (ct or less common ~ cleavage) upon absorption of light and the formation of initiating radicals capable of inducing polymerisation. The second group, kmown as type II photoinitiators, undergo a primary process of hydrogen atom abstraction from the resin, itself, or a solvent, but usually here a tertiary amine co-synergist is used for improved efficiency. Usually, the reaction occurs from the lowest excited triplet state of the ketone and depends on the intersystem crossing (ISC) rate, the co,Ng~ration of the triplet state (n, rc* or re,g9

Surface Coatings International 1999 (2)

and its corresponding energy. Here, the excited triplet state of the ketone forms an intermediate excited electron transfer complex (exciplex) with the tertiary amine. Electron transfer occurs with the subsequent formation of radicals, the amino radical is then believed to be the main initiating radical. The UV curing industry commonly uses combinations of type I and type II photoinitiators in order to overcome oxygen inhibition. Oxygen can quench the lowest excited state of many aromatic ketones. Additionally, the initiating and propagating radicals are quenched especially on the surface of thin films. The selection of a photoinitiator is of prime importance in the design of LW curing systems, since the polF~erisation and/or crosslinking rate depends on the photoinitiator package, and the physical properties of inks and coatings such as flexibility-, hardness, scratch, rub and chemical resistance properties are dependent on the degree of cure of the system. One of the main directions in this field is the development of non-migratory photoinitiators, specially where food contact is required, since a percentage of photoinitiator remains unreacted at the high levels often used in radiation curing.

Type I Photoinitiators c -Clem, age photoinitiators ct-Cleavage photoinitiators are the most important type I photoinitiators due to their high reactivity and thermal stability. Some of the applications in which g-cleavage photoinitiators are used are clear coatings, printing inks, printing plates, white lacquers, The most important families are:

Benzil ketds Benzil ketals represent a versatile family of photoinitiators. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) is one of the most important commercial photoinitiators of the (x-cleavage family. The major drawback of this compound is its consid- erable yellowing. Acrylate based inks and coatings for metal, paper, plastic and wood commonly make use of these types of photoinitiators. Methyl benzoate was found to be tile main photoproduct besides benzaldehyde and benzil in the photocuring. The primary benzoyl radical can dimerise to give benzfl, which can also react with a methyl radical to fom~ acetophenone or benzaldehyde 11 via hydrogen atom abstraction from a molecule in the close proximity. In the

67

presence of molecular oxygen it can form benzoic acid and methyl benzoate. Benzil DimethylKetal (BDK) is difficult to deactivate by quenchers due to its very short life, precluding bimolecular reactions and making it suitable for curing styrene systems.

- -Didko• g-g-Dialkoxyacetophenones are structurally similar t o ben- zilketal, g-gDiethoxy-acetophenone (DEAP) is the most fre- quently used photoinitiator of its class. The excited state undergoes two competitive reactions.~2 Norrish Type I cleavage gives a benzoyl radicaJ and a ketyl radical. The latter fragments to give an alkyl ester of formic acid and a initiating alkyl radical. Botl~ the benzoyl and the alkyl radicals initiate the polymerisation process. Norrish type II intramdec- ular hydrogen atom abstraction gives a 1,4 radical intermediate which decomposes to an oxetane by cyclisation, an acetophenone derivative and an aldehyde by elimination. The Norrish type II reactions do not directly initiate polymerisation reactions.

-Hydro• dkylphenones g-Hydroxyalkylphenones are highly reactive and possess high thermal stability as benzil ketals. Additionally, the?/" show better colour characteristics imparting very low yellowing in comparison with other commercial photoinitiators, This low yellowing is a necessity when clear coatings are designed.

1-Hydroxy-cyclohexTlphenylketone (HCPK or Irgacure 184) is the most common representative photoinitiator of this family. Triplet lifetimes for g-hydroxy alkylphenones~ are considerably longer than benzoin ethers, DEAP and acylphospt~ae oxides, Due to this fact, the former can be easily quenched with a subsequent decrease of reactivity observed in s~rene formulations. Upon irradiation benzoyl radicals are formed. As with other photoinitiators, the benzoyl radical is mainly responsible for the initiation of the pol?~nerisation. The g-hydroxyalkyl radical can also partici- pate in the initiation process.

-Aminodky{phenones 0t-~m• are a relatively new type of type I photoinitiators. They are of particular use in pigmented systems due to their absorption in the near ultraviolet region, in applicatior~s such as printing inks. Different attempts have been made to elucidate their curing behaviour in pigmented systems. ~4 Two types of c~-aminoacetophenone photoinitiators are currently available: 2-methyl-l-4-(methylthiophenyl)- 2-morpholino-propan-2-one (MMMP/Irgacure 907) and 2- benzyl- 2-dimethylamino-l-(4-morpholinophenyl)-butan-1 - one (BDMB/Irgacure 369). BDMB is more extemively used due to its lower odour in comparison to MMMP. MMMP has been replaced in some applications such as printing inks due to its odour. It has been shown by chemically induced dynamic nuclear polarisation, 1H-NMR-ClDNP ~5 experiments that BDIVlB in solution undergoes a predominantly (x-cleavage from its triplet state and minor t~-cleavage. ~6

Acy{phosphine Oxides Acylphosphine oxides were introduced some years ago as a new class of s-cleavage photoinitiators, derived from DEAP by replacing C-H by P=O and alkoxy by awl groups. A rela-

tively high oxygen inhibition may decrease their reactivity in the curing of thin films,17

Mono-acylphosphine (MAPO e.g. Lucirin TPO) and bis- acylphosphine oxides such as BAPO1 (Irgacure 1700 and 1800) and BAPO 2 (Irgacure 819) have absorption bands in the near UV/visible region, and so are specially indicated for use in pigmented systems,~S Additionally acylphosphine oxides bleach on irradiation, hence there is a decrease of absorptivity in the near UV-visible range and radiation can penetrate into deeper layers, Acylphosphine oxides produce little yellowing immediately after curing and on long term exposure, therefore, they have been used in applications where low yellowing is required as in white and pale inks, Acylphosphine oxides possess short-lived excited states and present low- quenching characteristics being suitable in stF.wene-based coatings for the furniture manufacturing industry.

Type II photoinitiators A variety of bimolecular photoinitiators have been used in the curing of pigmented and clear systems. Tertiary amines are mostly used as co-synergism, since they increase the reactivity of t37pe II photoinitiators and can react also as ox T- gen scavengers and, thereby, help to overcome oxygen inhibition.

Benzophenones Due to their low cost despite their relatively low reactivity and hardness of cured films, benzophenone (BP) and its derivatives are the most widely used bimolecular photoinitiators. Benzophenone combined v,~ith tertiary amines is fre- quently used in pi~t ing inks in blends with other type I and type II photoinitiators. A eutectic liquid 1:1 blend of benzophenone and 1-bydroxycyclohexyl-phenyl ketone (Irgacure 500) has high reactivity which is only slightly infe- rior to 1-hydroxycyclohexyl-phenyl ketone. ~

In this study some coir~l.ercial oligomeric photoinitiators have been compared kinetically with non-polymeric photoinitiators in order to assess the effect of the oligomeric structure on the reactivity and the inter-comparison of type I and ~ e II initiators in clear coatings,

Experimental Materials The solvents methylcyclohexane, acetonitrile and 2- propanol (HPLC grade) were obtained from the Aldrich Chemical Company, UK, The reactive a u e n t Actilane 440 PPTTA (alkoxylated tetrafunctional acrylate monomer) was supplied by Akcros, Manchester, UK and the resin Crodamer U2/297 (acrylated urethane alkyd resin in 20% glyceryl propoxylated triacrylate) was supplied by Croda, Liverpool, UK, A list of the photoinitiators and reactive diluents studied are show~a in Tables 1, 2.

Spectroscopic properties of photoinitiators U{travidet spectroscopy Absorption spectra were obtained using a Perkin-Ehner Lambda 7 absorption spectrometer. In order to study the effect of solvent polari~ on the absorption properties of photoinitiators, ultraviolet spectra have been carried out on

68 Surface Ceetings International 1999 (2)

Table 1: List o[ pholoiniliators

Commerdd Name Chemical Name Supplier

1 IRGACURE 184 HCPKl-Ilydroxycydohexyl phenylketone ClBA-GEIGY 2 IRGACURE 369 DBMP 2-1lenzyl-2-N,N-dimethylamino-l-(4-morpholinophenyl)-l-butanone CIBA-GEIGY 3 IRGACURE 500 HCPK/Benzophenone (BP) 1:1 CIIIA-GEIGY 4 IRGACURE 651 BDK Benzil DimelhylKetal or DMPA 2,2-dimelfioxy-2-phenyl acetophenone CIBA-GEIGY 5 IRGACURE 907 MMMP 2-Methyl-l-(4-(Meitlyllhio)phenyl)-2-morpholino propan-l-one CIBA-GEIGY 6 DAROCUR 1173 HMPP 2-Hydroxy-2-methyl-l-phenyl-propan-l-one ClllA-GEtGY 7 IRGACURE 1700 25% DMBAPO Oor IIAP01 Bis(2,6-dimelhoxybenzoyl) CIBA-GEtGY

-2,4-,4-trimethylpenlyl phosphine oxide 75% IIMPP 4-(2-hydroxyelfioxy)phenyl-(2-hydroxy-2-methylpropyl)ketone 50% TPO/50% HMPP 25% DMBAPO or BAP01 Bis(2,6-dimethoxybenzoyl) -2,4-,44rimethylpentyl phosphine oxide 75% ttCPK (1184) Ilis(2,4,6-1Timelhytbenzoyl)-phenylphosphineoxide BAP02 CIBA-GEtGY Benzophenone Lambson BP/melhyl-B P mixture Lambson Melhyt o- benxoylbenzoate Lambson Melhylbenzoytformate I~ttN D[AP 2,2-diethoxyacetophenone I~HN DBAP 2,2-Di-sec-butoxyacetophenone First Chemicals (4-(4-melhylphenylthio) phenyl)phenylmethanone. G. Lakes 4-benxoyl-4'-m elhyldiphenyl sulphide Eutectic mixture of 2,4,6-trimelhylbenzophenone and Lambedi 4-melhylbenzophenone 70% Oligo (2-hydroxy-2-hydroxy-2-melt@l-(4-(1-methytvinyl) phenyl) Lambedi propanone 30% HAg~P Oligo (2-hydroxy-2-hydroxy-2-melhyM-(4 (1-methylvinyl) phenyl) Lambedi propanone (llMPPoligomer) p-phenyl benzophenone AIgO 70% TIT + 30% KiP 150 Lambedi MAPO 2,4,6-trimelhyl benzoyldiphenyl phosphine oxide BASF

8 IRGACURE 2959 CIIIA-GEIGY 9 DAROCUR 4265 CIBA-GEIGY 10 IRGACURE 1800 CIBA-GEIGY

11 IRGACURE 819 12 SP[EDCUR[ BP 13 SP[EDCUR[ BEM 14 SP[EDCUR[ MllB 15 GENOCUR[MBF 16 GENOCUR[ D[AP 17 DBAP 18 QUANTACUR[ BMS

19 ESACURE TZT

20 [SACUR[ KIP 100 F

21 [SACUR[ KIP 150

22 TRIGANOL 12 23 [SACUR[ 1(137 24 LUCIPdN TPO

Table 2: List of reactive diluents

Commercial Supplier Chemical Type Functionality Name

Actilane 441 AKCROS Actilane 440 AKCROS Lucirin LR 8889 BASF Actilane 421 AKCROS Actilane 422 AKCROS Actilane 432 AKCROS Laromer TPGDA BASF Ebecryl 53 UCB

DiTMPTA 4 PPTTA AIIkoxylated telJafunctional acrylate monomer 4 Amine modified potyelher acrylate 2 Aliphafic difunclional acrylate ester 2 DPGDA dipropylene glycol diacrylate 2 GPTA glyceryl propoxylated triacrylate 3 Tripropylene glycol diacrylate 2 Purified OTA 480, similar to GPTA 3

solutions of photoinit iators in three solvents of varying polarity. Methylcyclohexane, acetonitri le and 2-propanol were the solvents chosen. The solutions were analysed by UV spec t roscopy at different concentrat ions to obtain the best correlat ion by regression analysis. From the s lope of the l inear regression lines the photoinit iators extinction coefficients were calculated hi the solvents ment ioned previously.

Reactivity of photoinitiators and reactive diluents Different techniques have been used in this study for mea- suring the reactivity and curing of UV coatings systems, Both direct and indirect methods of analysis have been used to

evaluate and compare the reactivi W of photoinit iators and reactive diluents,

Real time infrared spectroscopy 2~ 2s (RTIR) is based on the decrease in absorbance at 812 cm -i, which corre- sponds to the deformation of die carbon-carbon double bond, during the polymerisat ion reaction, The sample is po lymer ised with UV radiation whilst the infrared b e a m measures tile decrease in absorbance of the vinyl group. Pendulum hardness measures die number of oscillations to decrease from 6 to 3 degrees from the vertical, the harder the film the higher number of oscillations.

Real time infrared spedroscopy RTIR The irradiation source used was a Xenon lamp ILC 302UV (Laser Lines Ltd.) connected to a dispersive infrared spectrophotometer . A sample of a solution of a photoinit iator in a resin or reactive diluent was pos i t ioned be tween two sodium chloride plates covered ~adth two pieces of low density polyethylene film using a Teflon sepa- rator to give a film thickness of 50 microns. Two polyethylene films were p l aced in the reference b e a m in order to subtract the polyethylene absorpt ion from the sample. Different samples of photoinit iators were dissolved at 2% in PPTTA and cured following the method described.

Another method has been set up in order to evaluate tl~e effect of oxygen

inhibition on the curing efficiency of a range of photoinitiators. The liquid samples were appl ied using calibrated wire bars 22 (No 0) on sodium chloride plates to give a film thickness of 4 microns and they were posi t ioned uncovered in die sample holder as before. Consequently, the reactivity of photoinitiators, resins and reactive diluents can be assessed in the presence of atmospheric oxygen as is common practice in industrial applications. 2s The reactivity of the photoinitia- tots at 8% weight at 4 microns has been d e t e n ~ n e d on a b road range of commercial photoinitiators using an acrylated pol>qozethane alkyd at 20% GPTA Crodamer U2/297. Also, another comparative study of reactivi W at 8% Irgacure 500 has been performed on a range of reactive diluents.

Plots of conversion percentage and polymerisat ion rate versus irradiation tJ~qe were obtained. The derivative of %conversion against irradiation time plots were calculated and the maximum polymerisat ion rates (Rp maximum) were obta ined from the maxima of the derivative curves (Fig-ure 6). For those photoinit iators extdbiting slow cure rates, the pol?~nerisation rates were calculated by regression analysis from the slope of % conversion against time plots at short irradiation times. For each sample replication has been done in order m have an estimate of the pure errors and to prevent gross errors. The results were expressed as the mean values and the s tandard deviations gave an indication of the spread of the results. The reactivi W plots sho~am in the figures were averaged.

Surface Coatings International 1999 (2) 69

Pendulum hardness measurements A comparative stud?" of photoinitiator reactivity at 2~ weight has also been per formed on a b road range of commercial photoinftfators using PPTTA as the reactive diluent. The measurements were carried out on fih-ns of 75 microns thickness on glass. A K wired bar (No /-'3. was used to app ly the films. The films were cured ~fith a medium pressure mer- cury lamp source of about 80 VcTcm (Prinmrc) at 50 if/minl ( lpass ) using a tunnel conveyon The spectral output, was measured with a Solascope radiometer (Solatell) connected to a Sola-Probe. The hardness measurements were made after 24 hours. The average number of oscillatfons was obta ined fl'om at least 5 repeat measurements.

Also, a comparative stud>" of pendu lum hardness at 8% Irgacure 500 has been per formed on a range of reactive dilu~ ents. The measurements were made on f lms of 75 microns thickness on glass. The films were cured at 100 feet/'n~n with the same tunnel conveyon In this series of exper iments the films har@~ess was measured immediately after curing.

Results and Discussion Spectros opk properties of photoinitiators Ultraviolet spectroscopy The UV spectra of several photoinit iators are shown in Figures 3-.5. Absorption max in~ and molar extinction coefficients (logarithmic) of some photoinitiators are compared in Table 3 in three solvents of different polarity.

LW curing is associated with the n,s ~ or ~,~* transitions (or combinat ion of these.) that occur be tween 250 to 400 nn~ Transitions of ~,r configuration are shifted to longer wavelengths by an increase of the e~zension of the conjugation of the ch romophore by substitution and by an increase in solvent polarity< These transitions are also more intense than n,~* transitions due to conjugation of the ~* electrons. The latter which are shifted to shinier wavelengths by an increase in solvent polarity due to greater stabilisation of the ground-state e n e r ~ n,~qevel.

The majority of photoinitiators exhibit ~,~* transitions be low 300nm (Figure 5). The ct aminoalkylphenones Irgacure 369 and Irgacure 907 exhibit a weak bathochromic effect, a bathochromic shift is a red shift in the absorption maximum of the spectrum of the initiator, showing absorptions ( r * transitions) over 300 nm due to substitution at the

Figure l: Structure of Type II photoinitiators

% The instrument has bee~ calibrated in feet per minute which is commonly used m indusW,

Figure 2: Structures of Type I photoiniliators

Figure 3: Uu spectra of Type II pho|oinitia|ors in isopropenol (l.O • 10 -~ M)

2~

ff

para posit ion by electron donating groups such as morpholino and alkylthio respectively within the aron~t ic ring of the benzoyl group. Acylphosphine oxides (Lucfrin TPO and Irgacure 819) also have red shifted absorptions over 300 nm tailing into the visible mainly due to extension of aromatic conjugation. In this case the absorption maxim~ are blue shifted with increasing solvent polari ty and are theretbre associated with n,~r �9 transitions. The absorption maxima of

70 Surt~ce Coatings International ! 999 (2)

Figure 4: Uu speclra d n~ar UV p~loinilialors in isopropa,ol (1.0 x 10 4 M)

8a

8~S

S~S ~5

Figure 5: UV speClla of Type I pholoinilialors ill isopropa,ol (C=l.O x 10 4 M)

11"iganol 12 and Quantacure BMS (re,n* transitions) exhibit bathochromic effects by charge transfer due to extension of conjugation by phenyl and methylthiophenyl groups respectively in the pare position of the basic initiator chro- mophore. These red shifted transitions are particularly use- ful in pigmented wstems since some pigments give rise to absorption windows in the near UV.

Readivity of photoniliators and reactive dibe.|s The reactivity of a range of" commercial phot.c, initiators and reactive dfluents is compared by RTIR. RTIR kinetic curves provMe informa- tion on the inhibition time, polymerfsa~ tion rate, cure extent (% conversion) and residual unsaturation content.

Readivily of pholoiniliators at 2% in PPTIA in the absence of air The c.uring performance of 20 cons~mer- cial photoinitiators has been compared in formulations containing 2% photoinf- tiator in PPTTA by RTs and pendulum hardness measurements. The data are sumzs~arised in Table 4 where the Rp values provMe a good comparison of pol3axnerisation rates. Presentation of the actual curves in this instance would be too congestive. TWo important fea- tures were evident from the curing data. The first is that. photGnitiat.ors fi'om the phosphine oxide and cr

Figure 6: Plols d % conversion and poly~risalion tale (Rp- dConversion/dl) verst~ irradialion time

~ ~ ~ # ~ ~ .........

g

aminoalk-ylphenone families exhibit the highest reactivity of all the photoinitiators studied. The second is that Wpe II photoinitiators from the benzophenone family showed a lower reactiviW than the Wpe I photoinitiators in the absence of a tertiary amine.

Reactivity of near UV photoinitiators Near UV photoinitiators of the ct-alk34aminoketone and acylphosphfne oxide families (Irgacure 819, Lucirin TPO and Irgacure 569) have very high initiating efficiency, which induces higher polymerfsation rates after short exposure times than for the other photoinitiators studied. The eutectic blends of acylphosphfne oxides vt4th Irgacure 184 and Darocure 1173 (Darocure 4265, Irgacure 1800 and Irgacure 17000 exhibited lower polymerisation rates than the pure acylphosphine oxides. On the other hand they gave similar % conversions at long exposure times and similar hardness with the exception of Irgacure 1800 which exhibited lower polymerisation rates, % conversion and hardness than its counterparts (Table 4). Commercial eutectic blends of acylphosphine oxide photoinftiators and far LW ct-hydrox-

Table 3: Wavelength m~ima and logarithm of extindio, coeffidenls s (Lmol -I cm -1) of photoiniliatars in isopropanol, acetonitrile and meltr~,kTdohexane

Isopmpenol Acelonitrik Melb/kydolmxarm

me{ Log ~'~x s max Log S~x s max Log s~x (impropanol) (ace|onilrile) (mBll~'kyclol~xa~)

Irgacure 651 251.8 (~,~*) 4.12 252.0 4.11 24&3 4.13 DEAP 246.5 (~,~*) 4.33 246.7 4.11 245.5 4.10 Irgacure 184 244.0 (~,~*) 4.05 243.0 4.00 243.5 4.05 lrga~:ure 2959 275.3 (:~,~*) 4.21 272.4 4.21 Dar0cul ~ ] 173 244.0 (~,~*) 4.00 243.0 4.04 242.7 4.05 lrgacure 819 367.9 (nS) 2.98 368.7 2.97 371.4 2.93 Ludlin TPO 379.9 (nS) 2.76 380.4 2.72 381.8 2.61 Irgacure 907 307.0 (~c,~C*) 4.25 302.8 4.27 300.4 4.31 Irgecure 369 317.0 (s,s*) 4.31 315.0 4.24 311.7 4.36 Benzophenone 251.5 (~:,~*) 4.27 250.3 4.19 248.2 4.30 Trigano112 289.7 ('~,~*) 4.36 288.5 4.35 283.4 4.40 Speedcure A,~.BB 245.2 (~,~") 4.21 243.9 4.19 239.5 4.18 hacure T~ 249.6 (~,~") 4.15 247.6 4.15 244.3 4.16

$udete Co~ings M e r n ~ n d 1999 (2) 71

Table 4: leadivty of photaiaili~lors at 2% ~/win PPTTA by rTIr ~150 microns (covered method) in the absenco d oxygen and pendulum hardness measurements of coatings cured at 50 ft/min with a film thickness of 75 microns (measured ~ter 24 hours)

Phatoinliators Rpmaximurn % Conversional Noofoscil~iom Nod at 2% in PPTTA (real L% l) • SD 15 s • �92 • SD (5 rum) rum Ludrin TPO 2.401 • 0.301 12.0 • 3.1 156 _ 15 2 hgemre 819 2.435 • 0.291 71.1 • 2.7 155 • 16 2 Irg<ure 369 2.416 _ 0.057 77.7 _ 0.7 142 • 18 2 Dar0<ur 4265 1.950 • 0.250 75.6 • 2.5 152 • 15 2 Irtecure 1700 1.853 _ 0.031 74.6 • 1,6 145 • 4 2 Irgleure %7 1.835 • 0.035 78.6 _+ 3.5 154 +_ 17 2 Irgtcure 184 1,240 • 0.020 77,6 • 0.2 126 • 8 2 Ot, AP 1.356 • 0.149 78.5 • 2.6 157 • 8 3 Irg~cure 1800 1.309 • 0.291 65.5 • 7.4 147 • 5 2 Esacure KIPIOOF 1.335 • 0.60 74.0 • 7.7 147 • 12 2 hl0cure 651 1.265 • 0.587 68.8 • 10.6 145 • 9 2 DIAP 0.875 • 0.147 75.0 • 3.1 152 • 6 4 It%cure 2959 1.147 • 0.262 78.0 • 5.2 112 • 9 2 0en0cure MBF 0.965 • 0.007 70.2 • 1.5 1 ] 9 • 7 2 hgacure 500 0.708 • 0.124 65.8 • 7.1 138 • 13 2 Esacure KT37 0.470 • 0.085 57.6 • 2.2 108 • 17 2 Quai~acure B/~S 0.146 • 0.053 64.9 • 8.5 75 • 5 3 Spee@ure BIM 0.132 • 0.042 61.8 +_ 4.8 47 +_ 12 2 TZT 0.121 • 52.8_+9.4 26• 2 T@nol TI 2 0.051 • 0,021 57.1 +_ 5.8 rio (ul~d 2

yalkylphenone photoinitiators (Darocur 1173 and Irgacure 184), such as Darocur 4265 (5C% Lucirin TPO and 50% Darocur 1173), Irgacure 1700 (25%DMBAPO and 75% Darocur 1173) and Irgacure 1800 (25% DMBAPO and 75% Irgacure 184), give cost effective formulations with relatively lower pol>~erisat.ion rates than the pure acylphosphine oxide photoinftiators.

Kinetic plots of several acylphosphine oxide and c~- aminoalleflphenone photoinitiators are shown in more detail in Figure 7. Irgacure 819, Ludrin TPO and Irgacure 369 possess ve W high initiating efficiency as is reflected by their low inhibition time, high pol)a:nerisation rates and high extent of cure. Irgacure 819 exhibited a better surface cure than Ludrin TPO due to the formation of three initiating radfcals instead of the two formed with Lucirin TPO. These pho-

Figure 7: Rec~ctivily d r~ar IV photoiniliators at 2% w/w in PPTTA by RTIR at 51 microns in tl~ absence d oxygen

t /

/ / 1.4 I I I .I

< i ii . . . . . . . . I

toinitiators are specially formulated in printing inks due to their high reactivity and extended absorbance in the near UV/visible region. Irgacure 907 exhfbted a lower reactix,~ty than the previously mentioned photoinitiators. The films cured with g-aminoaltcyl phenone type photoinitiators such as Irgacure 369 and 907, and with acylphosphine oxide type such as Lucirin TPO> Irgacure 819, Irgacure 1700 and Irgacure 1800 gave ye w hard films as shown by the data in Table 4 fi'om pendulum hardness.

Reactivity of Jar UV photoinitiators The reactivity and hardness of several Lgv" photoinitiators absorbing below 300 nm co~27~-nonly used in UV coatings are shown in Figure 8 using RTIR plots. Several ~- -dialkoxwacetophenones ~rgacure 651, DEAP and DB$,P) and 0~-hydroxwalk3dphenones (Irgacure 184, Irgacure 2959 and Esacure KIP 100) have been compared with some benzophenone derivatives. DEAP exhibited a lower po>eTneri- sation rate than the other t~@e I photoinitiators studied possibly due to a Norrish %,'pe 1I intramolecular hydrogen atom abstraction process which yields non initiating prod- ucts. The other photoinitia~ors presented, similar polymerisa.tion rates, % conversion and hardness properties as shown by the data in Table 4. The n2aximum poi~er i sa~ tion rates, % conversion and pendulum hardness of PPTI~A films cured with different type I photoinitiators showed approximately the following order:

Rp~x: DBAP - Esacure KIP100 ~ Irgacure 184 - Irgacure 2959 -= Irgacure 651 =- DEAP > Irgacure 500 > Esacure KT37. % Conversion2s .~: DBh~P m Irgacure 184 = Irgacure 2959 - DEAP = Esacure KIP100 > Irgacure 651 > Irgacure 500 > Esacure KT37.

Figure 1: Re~Ivly of photoinliators at 7% ~v/w in PPTTA by ITII~ at $0 miroas in the a~nce o| oxygen

I I

I I

51

Ii

71

r / I

, i ' S . ~ - ~

J

E

r 11 I,I 1,I

I ii

/2 Suttee C~tings Inter~tio.d ! 999 (2)

Pendulum Hardness: DBAP > DEAP > Esacure KIP100 > Irgacure 651 > Irgacure 184 > Irgacure 500 > Irgacure 2959 - Esacure KT37.

Esacure KT37 (7@4 TZT and 30% KIP150) and Irgacure 500 (50% BP and 50% Irgacure 184) exhibited a lower reac- tivf W and gave softer films (lower oscillations) than their counterpmts because of its content in Wpe If photoinftfators (Table 4). Type II photofnitfators fi'om the benzophenone f a ~ I y in the absence of a tertiary amine gave very poor results. Quantacure BMS (para methylphenylthfo derivative of benzophenone) eydqibfted higher polymerfsatfon rates and gave harder films than the other benzophenone deriva~ tires such as Speedcure BEM, Esacure TZT (eutectic ~ u r e of methylated benzophenones) and ~ iganol 12 (paraphenyl derivativ@ in the absence of a tertiaW amine co-synergist (Figure 8).

Repeatability analysis The covered method gave love" reproducibility due to the varfabflfW of the polyethylene absorption in the region 790-850 cm 4, Different experiments were performed fn order to elucidate the origin of this phenomenon. It was found that. when polyethylene films were placed in both sample and reference beams respect.ively the spectra did not subtract to give a constant zero absorption due to an inte> ference pattern that is critically dependent on the film thickness. Consequently, the measurement of the baseline is affected by the variable position of the shoulders of the peak at 811cm 4 which reflects on the accuracy and reproducibility of the results.

Reactivity d photoinitiators at 8% in Crodamer U2/297 in the presence of air The curing performance of a range commercial photoinitia- tots has been compared at a high level of photofnitfator (8% w/ve9 at love" film thickness around 4 microns in the presence of air (uncovered methodZ0 to simulate industrial applications of overprint varnishes used in the printing ink industry. All the initial Rp values and % conversions are compared in Table 5.

Acylphosphfne oxide and 0:.-aminoalkylphenone Wpe I photoinftfators induced higher polymedsation rates than photoinitiators of the cq~-dialko>wacetophenone and c~- hydrox%,alkylphenone type (Figure 9).

Reactivity of type I and type I! photoinitiators Kinetic plots of some type I and type II photoinitiators have been compared in Figure 9 to gain a clearer picture of the initial rate conversions. Lucirin TPO gave a higher polymerb sation rate than the other photoinitiators studied. Blends of TPO and DMBAPO with ~-hydro~,~allcylphenones (Irgacure 1800) were also ve W effTcient. Irgacure 907 showed lower reactiviW than Lucirin TPO at short exposure times. Darocure 1173 exhibited a higher polymerfsation rate and final % conversion (34elding a h~gh conversion reaction of 95%) than the other ~hydroxyalkylphenone Wpe photoinF tiators compared in this study thus:

Rp and % conversion: Darocure 1173 > Irgacure 184 > Irgacure 2959

Type 111 photoinitiators, such as Benzophenone, Esacure TZT, Triganol T12, Speedcure BEM, Speedcure MBB,

Toble 5: Readivily af photoini|i~ers at 8% w/w in (rodamer 02/297 by RTIR in the presence of air (uncovered method)

Phatoiniliators at Rp maximum (reel % Convasion No 8% in Credamer L4S 4) _+ SD at 25s _+ SD runs UV297 [br0cur 1173 0.3959 _+ 0.0360 95.1 _+ 2.0 3 Dar0cur 4265 0.4198 _+ 0.1305 90.6 • 2.2 2 Irgacure 184 0.3037 • 0.0252 87.9 • 2.9 4 Luddn TPO 0.6272 • 0.0400 85.2 • 0.1 2 Esncure KIPIOOF 0.2596 _+ 0.0771 85.3 • 1 2 Irgacure 907 0.5653 • 0.0385 84.9 • 0.8 2 Irgqcure 1800 0.5625 • 0.0545 83.7 • 5.0 3 Irgacure 1700 0.4446 • 0.0357 81.2 • 14.0 2 Irgacure 2959 0.1704 • 0.0248 78.2 • 2.3 3 Esacure KiP ] 50 0.06275 • 0.002 58.8 • 1.20 2 Esaure KTS7 0.06960 • 0.0071 45.5 • 0.3 2 genocure MBF 0.0215 • 0.0025 32.3 _+ 2.0 3 Spesdcure BEI~. 0.00991 • 0.00021 20.8 • 1.1 3 Benzophenone 0.0]03 • 0.00017 19.6 • 0.6 3 ,!'~,~.BB 0.0061 • 0.0009 11.3 • 0.6 3 T/r 0.0057 • 0.0003 11.7 • 2.2 4 T@nd T]2 0.0050 • 0.0001 9.1 • 0.4 3 Ebecru P36 0.0030 • 0,00004 8.1 • 0.9 3

Figure 9: Reactivity of type I p~toiniliators (compared 1o benzophenone) at 8% w/w in Croctamer U2/297 by I{TIR inll~ presence of air

|

@

|

N

9 a

F

showed the lowest reactivity of all the photofnftiators covered in the study (Table 7).

ReactMty of HMMP digomer (Esacure KIP150) versus the single HMMP (Darocur 1173) The curing performance of an olfgomeric photofnftiator Esacure FXP 150 (HMMP oligomer) has been compared in Figure 10 with its single counterpmt Darocur 1173 (HMMP) and with eutectic blends such as Esacure KIP 100 (70% HMMP oligomer and 3@/~ HMMP) and Esacure KIP KT37 (70% Esacure TZT and 30%HMMP olfgomer). s RIP 150 exhibited lower reactivity than Darocure 1173 and Esacure KIP 100 mainly due to the lower mobility of the initiating radical. Esacure KT 37 gave vei T low" reactivity due to its high contep/t in Esacure TZT (type 1I photoinitiator%

Sud~:e r I ~ e m ~ [ ! 999 (2) 73

Figure l O: Re~|ivily o| phatoinfiators at 8% w/w in Crr~amer U2/297 by RTIR in the p~ence of air

which imparted low reactivity in the absence of a tertiary amine (Table 9).

Reactivity of type II photoinitiators The reactivity of some type II photoinitfators is compared f ' om the data shown in Figure 11 and Table 5. Benzophenone and Speedcure BEM (eutectic mixture of benzophenone and methylated benzophenones) had similar polymerisation rates and final % conversion. Esacure TZT exhibited lower reactivity than Esacure BEM despite having a similar composition. This lower photoacti~ty is due possibly to the presence of a methyl group in the ortho position of the benzophenone, which can induce Norrish t?pe II intramolecular hydrogen atom abstraction by the ketone group and thus decrease the initiating efficiency. ~4 Genocure MBB (Methyl ortho-benzoylbenzoate) gave similar results to that of Esacure TZT. Substitution of benzophenone by an ester group in the o, tho position in the absence of an amine cc~synergist decreased its reactivity by a factor of two. Triganol 12 (paraphenylbenzophenone) also e~hibited low efficiency in the absence of an amine possibly due to the low energy of its lowea lying triplet state by charge transfer, which decreases the proton abaractfon ability. Ebecryl P36

Figure 1 h Reaclivily of Type II phatoinil~ors at 8% w/w in Croda U2/207 by RTIR at 4 microns (K bar No. O) in lhe presence of air

~5

~5

5.0

# Y

(copol3amerfsable unsaturated benzophenone derivative) had ve W love* reactivity in the absence of ain

Repeatability' analysis The 'uncovered' me thod pro~ 'ded more reproduc ib le results with systems of high vfscos%~ and high yield value where the film thickness and consequently the optical path length can be maintained constant. Systems presenting low yield values have been shown to be less reproducible than systems with high 3field values, because of the variation in film thickness due to gravity. The advantage of this method is the absence of any interference pattern and organic residual absorption background beionging to the polyethylene film, consequently, the relative peak shoulder position is not affected by the polyethylene absorption.

Reactivity and hardness of different reactive di luents Functionality and reactivity are impo~ant factors to consider when choosing reactive dfluents, for example hardness, scratch and thumb twist resistance increases with fhnctfonal- ity. Also flexibility can be obtained with low functionality reactive diluents due to their slow cure ra~e and low crosslfnking density.

Pendulum hardness measurements have been carried out with a number of commercial reactive dfluents of diff%rent functionality a~ 89s Irgacure 500. The coatings were printed at 75 microns film thickness with a K bar No 7 and cured under a conveyor at one pass at 100 feet per minute. The hardness was measured immediately after curing. The results obtained are shown fn Table 6. The reactiviW of a range of low" viscosity acwlates is compared by R TfR at 8% Irgacure 500 at 4 microns (Figure 12 and Table 6).

Laromer 8889 (amine modified ether acrylate) has the highest polymerfsation rate and fnal % conversion among

Fig~m 12: Re.a(lJv#y d dJ[f~renl re~l~ diluents at 8% Irgacure 500 by RTiR in tt~ ~resence of air

|

g

A I _ - -

.... 2~ 2Si ~ %

. . . . [ ~ !~PC~I~ ~ ~ ; ] t L ~ ~

....... [~[~g S ~}> ....... ~8~ ~ IA~I~[I~

74 Surf~e Coatings Inter~tiond | 999 (2)

Table 6: Reactivity of different reactive diluents at 8% Irgacure 500 by RTIR in the presence of air. Pendulum Hardness of films (75 microns) tared under a conveyor of 80 W/cm at l O0 fi/min measured immediately after curing

Acrylate Rp maximum % Conversion He Number of (mol L% -~) _+ $D at 25 s + SD runs oscillations + SD

Di-TMPTA 1.16545 • 0.0678 70.7 • 3.1 2 104 • 3 PPTTA 0.9798 _+ 0.2265 71.6 _+ 6.3 2 94 • 2 LR8889 1.1520• 82.9• 3 55• ACT 421 0.1584 • 0.0206 28.7 • 2.3 3 <30 DPGDA 0.01465 • 0.0047 15.6 • 3.1 2 no cured GPTA 0.91435• 63.4• 2 66• TPGDA 0.02435 _+ 0.0028 29.3 • 6.9 2 no cured OTA480 0.71145• 57.6• 2 74•

all the acrylates investigated, possibly due to its co-synergistic amine function which can act as an electror~/proton donor with the excited triplet state of the benzophenone, On the other hand, it gives low hardness due to its low functionality and crosslinking density.

Di-TMPTA and PPTTA impart at the same time high reactivity and hardiness due to their high functionality and high concentration of growing radicals capable of overcoming oxygen inhibition. GPTA and OTA 80 exhibit lower reactivity and hardness due to their lower functionality which results in higher oxygen inhibition affecting the overall polymerisation rate and in a lower crossli,lking density. Difunctional reactive diluents such as DPGDA, TPGDA and Actilane 421 exhibit even lower reactivity due to low" crosslinking density and low surface cure, The latter is associated witla the high h~hibition effect of oxygen acting as a terminating agent of the pol}wnerisation process.

Viscosity is another factor to be born in mind, since the diffusion rate constant of molecular oxygen depends on the viscosity of the system. Hence, since high functional systems have higher viscosity due to their higher branching and molecular weights, they exhibit lower oxygen inhibition than low functional acrylates. On the other hand, the viscosity of the system affects the mobility of the propagating macro- radical, consequently high viscosity systems may show reduced reactivity due to mobility restrictions of the propagating macro-radicals.

Actilane 440 (PPTTA) is a fomi of ATFA (alkoxytated tetracrylate) that has little odour and gives hard films wqth good toughness and abrasion resistance. The Draize rating is very low at 0.1 (non toxic), therefore it is ideally suited for food contact inks. GPTA has a sin~ilar viscosity and a slightly lower reactivity and hardness than PPTFA. GPTA has a good hydrophilic-lipophilic balance making it particularly important for lithographic inks. GPTA has good wet offset lithographic performance and a good balance of tensile strength and elongation. The Draize value of GPTA is lower than 0,1.

Conclusions From this study, type I photoinitiators of the acylphosphine oxide and the c~-aminoalkylphenone families are the most efficient free radical photoinitiators for UV curing acrylic clear coats. Due to their extended high absorptivity in the near UX(/visible range they are specially suited to fast curing pigmented systems such as printing inks. Mso Acylphosphine oxide photoinitiators can be used in clear systems for overprint varnishes to improve the adhesion of the coating due m their extended absorptivity in the near

UV/visible range. Irgacure 819 (BAPO2) has a higher reactivity than Lucirin TPO (MAPO) possibly due to its higher absorptivity and to the formation of three initiating radicals in comparison to the two initiating radicals formed by Lucirin TPO upon absorption of l i~t , ~,7 Irgacure 819 presented better tack free surface cure properties than Lucirin TPO.

Far UV photoinitiators of the 0~-dialco~-acetophenone and the c~-hydroxTalkylphenones families are specially effective in clear systems such as overprint varnishes and in pigmented systems for obtaining cost effective coatings, Oligomeric photoinitiators such as the oligomeric HMMP (Esacure KIP 150) with low m.igration properties and hence low- toxicological hazards have shown comparable reactivity to single photoinitiators. The use of oligomeric photoinitiators is acceptable in food contact inks and varnishes in order to prevent photoinitiator migration and contamination of foodstuffs.

Type II photoinitiators exhibit lower reactivity titan type I photoinitiators in the absence of a co-synergist. The use of type II photoinitiators is recommended in combined systems in con.junction with tertia W amines and type I photoinitiators in order to design cost effective synergistic photoinitiamr blends.

The selection of high performance low viscosity diluents influences the final properties of the cured coatings and inks, High. functional acrylates such as PPTFA and DiTMPTA are indispensable for obtaining high tack free crosslinked systems with good hardness characteristics especially in the printing ink industry due to the high oxygen inhibition effect found in thin films, Low functionality acrylates have low crosslinking density and inadequate surface cure due to oxygen inhibition, This inadequate cure and the low" crosstitlking density has a plasticising effect which can be used to improve the flexibility of the cured film and to improve the adhesion on difficult substrates.

Future work will report on the photoinitiator activity and photochemical properties of the initiators.

Acknowledgements The authors thank Swale Process for financially supporting one of them (JS) and for materials used in this study.

References 1. Pappas SP, Ed, UV Curing: Science and Technology,

TecbndogyMarbeting Co,potation, Stanford, CT, USA, 1978.

2. Roffey CG, Photopolymerisation of surface Coatings, John Wiley, New-York, USA, 1982.

3. Ol&ing Pt,2T, Ed, Chemist W and Technology of LW and EB Formulation of Coatings, I,lks and Paints, SITA TechndoR}; 1-6, 1991.

4. Holman R and Oldiing P, Eds, UV and EB Curing Formulation for Printing I,~ks, Coatings and Paints, SITA Technology, 1988.

5. Allen NS, Ed, Photopolymerisation and Photoimaging Science and Tecimology, Elxezder, 1986.

6. Limure T, In Proceedings Conference Radiation Curing Asia, 461, 1988.

7. Marsman M, Luiken A and Holweg RBM, In Proceedings Conference Radtech Europe, 440, 1991.

8. Yoshiyuki K, 1,2ayuzuki M, TosI~o S, Mitsuhiro M and Hiromi M, US Patent 5013768, 1989.

Surface Coatings lnternat|onal |999 (2) 75

9. Borzel P and Hating E, German Patent Application 3304524, 1983.

10. Murray KP and Bishop T E, Int. Patent Application WO 9W13579, 1989.

11. Phan XTPJ, Radiation Curing, 13, (1), 11, 1986.

12. Borer A, Kh'chmayr R and Rfst G, Helv. Chfm. Acta, 61, 305, 1978.

13. EichlerJ, Herz CR NMto I and Schnabel VcJ, Photochem., (12),225, 1980.

14. Putsch W, Angerer H, Desobry V, Dfetliker K and Huesler R, Proceedings of the 16th International Conference in Organic Coatings Science and Technology, Athens, Greece, 423, 1990.

15. Desobw V, Dietliker K, Miser L, Reinhold M, Rist G and RutschW, In Radiation Curing of Polymeric Materials, Eds Hoyle CE and Kfnstle JF, Am#r~ca~ Chemica~ ,Society, Washington, DC, USA, 92, 1990.

17. Desobry V, Dfetl/ker K, Miser L, Reinhold M, Rist G and RutschW, In Radiation Curing of Polymeric Materials, Eds Hoyle CE and K/nstle JF, Ame,~can Cbemicd Society, Washington, DC, USA, 92, 1990.

18. Jacob/M, Henne AJ, Radiation Curing, 19, (4) 16, 1983.

19. Valet A, Jong T and Kohler M, Fourth Nuremberg Congress, 4, 1997.

20. Desobw V, Dietliker K, Huster R, Rutsch W and Loliger H, Pdym. Paint ColoarJ., 178, 913, 1988.

21. Phillips R, J. Oil Colour Chem. Association, 6,233, 1978.

22. Collins GL, ~ u n g DA and Cosmnza JR, J. Goat. 'Technology, 48, 1976.

23. Kerchiss TR, Polymers Paint Colour Journal, 182,566, 1992.

24. Alien NS, Salleh NG, Edge M, Garrales T, Shah M, Weddelt I, Carolina F and Green A, Journal of PhotochemistI"y and Photobfolo~%~, 99, 192, 1996.

Key to acronyms used in this paper DMPA 2.2-Dfmethoxy-3- ph enylace tophen one

BDK Benzil Dimethylketat

DEAP cz-ct-Die th oxy-ace top he no ne

HCPK 1-Hydroy-cyclohexylphenylketone

MMMP 2-Methyl-l-4 <meth ylthiophen yl~2-morpholi no- propan-2-one

BDMB 2-Be nzyl-2-dfmeth ylamf no- 1 -<4-morpholinophenyl>- butan-l-one

MAPO Mono-ac wlphosphine

BAPO Bis-acylphos phfne Oxides

BP Benzophenone

PPTA Alkox~qated Tetrafunctinal Acwlate Monomer

GPTA Glyceryl Propoxytated Triacrylate

DMBAPO Dfmethoxy Bis-Acylphosphine Oxide

HMMP Hydroxymethyl Methyl Phenyl Propanone

DiTMPYA Ditrfmethylol Propanetriacrylate

DPGDA Dipropylene Glycol Diacwlate

ATTA Alkoxylated Tetra Acwlate

DB~4_P Dibutoxy Acetophenone

[]

76 Surface Coa|ings Inter.ationd 1999 (2)

Documents

A comparative kinetic study of commercial photoinitiators for UV/visible curable acrylate clear coatings