Polymerization kinetics of photocurable acrylic resins

Polymerization Kinetics of Photocurable Acrylic Resins

EWA ANDRZEJEWSKA, MACIEJ ANDRZEJEWSKI

Poznan University of Technology, Institute of Chemical Technology and Engineering, Pl. Sklodowskiej-Curie 2,60-965 Poznan, Poland

Received 25 March 1997; accepted 25 August 1997

ABSTRACT: The polymerization kinetics of photocurable compositions based on an ep-oxyacrylate oligomer and three analogous diacrylate monomers were investigated. Theeffects of the oligomer-to-monomer ratio, curing conditions, and monomer structurewere considered. The polymerization is characterized by a synergistic effect observedin a wide temperature range and occurring for the polymerization rate both in air andAr and for final conversions in air. The final conversion in Ar is determined by viscosityof a formulation. The presence of a heteroatom (S or O) in the ester group of thereactive diluent is beneficial for the polymerization course, especially in air atmosphere.The best results were obtained for the sulfur-containing monomer. q 1998 John Wiley &Sons, Inc. J Polym Sci A: Polym Chem 36: 665–673, 1998Keywords: epoxyacrylate; diacrylates; photopolymerization; kinetics; synergistic ef-fect; temperature effect; heteroatom effect

INTRODUCTION While final film properties result from theblend of ingredients, each compound lends its spe-cial character to the coating. The oligomers, gen-Photosensitive resins that polymerize readily un-

der intense UV radiation are being increasingly erally acrylate functional, impart properties asso-ciated with their basic structure. Typical commer-used in various sectors of applications, mainly in

the coating industry, the graphic arts, and micro- cial oligomers are acrylated urethane, polyesters,silicones, and epoxides. Acrylated epoxides, for in-electronics. UV curing technology allows the pro-

duction of high resolution relief images that are stance, impart chemical resistance, hardness, andadhesive strength. The monomers, also typicallyrequired for the manufacture of integrated cir-

cuits or printing plates, as well as to achieve fast acrylate functional, provide viscosity control, aswell as impart properties based on their structure.hardening of clear or pigmented coatings, adhe-

sives, and composites.1–3 Photocurable composi- Highly functional monomers (containing at leasttwo double bonds) increase crosslink density oftions are used for the surface protection of all kind

of materials (metals, plastics, glass, paper, wood, the polymer, increasing in this way hardness ofthe coating. Alkoxylated monomers can reducefabrics, etc.) by fast-drying varnishes, paints, or

printing inks as well as in dentistry.1,2,4 Most of- surface tension, enhance the wetting and adhe-sive character, but can also reduce water resis-ten these photosensitive resins consist of multi-

functional oligomers and monomers that polymer- tance and toughness.2,5 Recently studied sulfur-containing monomers improve thermo-oxidativeize to form highly crosslinked polymer network,

additives of various types, and a photoinitiator stability6 reduce solvent swellability and mois-ture absorption7 as well as increase refractive in-that yields reactive initiating species upon UV ex-

posure. dex8 of the coating.However, the nature of the resultant polymer

may depend not only on the properties of the reac-Correspondence to: E. Andrzejewska

tive components but also on cure kinetics. Gener-Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 36, 665–673 (1998)q 1998 John Wiley & Sons, Inc. CCC 0887-624X/98/040665-09 ally, the rate of polymerization depends first of

665

97050T/ 8G66$$050T 01-05-98 14:31:52 polca W: Poly Chem

666 ANDRZEJEWSKA AND ANDRZEJEWSKI

all on the reactivity of the functional groups, on (b) curing conditions (temperature and atmo-sphere), andtheir concentration and on the viscosity of the

resin, whereas the chemical structure and func- (c) monomer structure (the presence and typeof the heteroatom in the ester group).tionality of both the monomer and the oligomer

determine the final degree of polymerization.9 Fora given formulation, the polymerization kineticsdepend on reaction conditions, i.e., atmosphere, EXPERIMENTALtemperature, light intensity, photoinitiator con-centration, etc.2,10,11 Thus, to obtain the best phys-

Synthesis and characteristics of the monomersical and mechanical properties of the polymeriza-are described elsewhere.7 Chemical formulae oftion product one must not only consider the mate-the acrylates used are given in the following:rials used, but also the conditions under which the

polymer was formed. These reaction conditions,along with the type of reactant system chosen,will completely control the polymerization rateand conversion of double bond in the system. Theconversion, in turn, will determine the mechani-

where: n É 0.1514cal, physical, and wear properties of the materialformed.

The polymerization of multifunctional mono-mers shows a complex behavior.2,4,10,11 One of themost characteristic features is autoacceleration,i.e., the increase in rate of polymerization despitethe consumption of monomer, which occurs dueto extremely restricted diffusion of radicals in thehighly crosslinked polymer. Another process, au-todeceleration occurs because the propagationeventually becomes diffusion controlled alongwith the termination. Both these processes leadto the appearance of a maximum polymerization

X|S: TEDArate. Polymerization is continued until it stopsfrom vitrification. Depending on the monomer sys- X|O: OEDAtem and the reaction conditions by which the sys-

X|CH2: PDAtem was polymerized, the polymerization typi-cally ceases before complete conversion of thefunctional groups.4 The polymerization kinetics were monitored by

a differential scanning calorimeter (DSC 605 M,The aim of this work was to investigate thepolymerization kinetics of photocurable composi- UNIPAN-TERMAL, Warsaw, Poland) equipped

with a lid especially designed for photochemicaltions based on an epoxyacrylate oligomer (EPA)and three analogous diacrylate monomers: 2.2 *- measurements and computer-aided data analysis

software. The initiator, 2,2-dimethoxy-2-phenyl-thiobisethanol diacrylate (TEDA), 2,2 *-oxybis-ethanol diacrylate (OEDA), and pentane-1,5-diol acetophenone (Irgacure 651t from Ciba–Geigy,

Basel) was applied in concentration 0.01 mol/kg.diacrylate (PDA). Due to a variety of technologi-cal curing conditions (air atmosphere or an inert Accurately weighed (Ç 10 mg) samples of the pho-

tocurable composition were polymerized in 6.8gas blanketing, often elevated temperatures dueto large amounts of heat emitted by UV lamps), mm diameter open aluminum DSC pans. When

polymerization was carried out in an inert atmo-the polymerization kinetics were followed in thepresence and in the absence of atmospheric oxy- sphere, the sample was allowed to equilibrate in

the apparatus under argon (0.0005% O2) for 10gen and at a wide temperature range.The results obtained in this work are discussed min at the chosen polymerization temperature. A

medium-pressure mercury lamp equipped with ain terms of the effects of the following factors onthe polymerization kinetics: glass filter was used for the irradiation. The trans-

mitted light was in the range of 310–400 nm with(a) oligomer to monomer ratio; lmax at 366 nm. The incident light intensity at the


PHOTOCURABLE ACRYLIC RESINS 667

Table I. The Concentration of Double Bonds in the of these parameters obtained for the polymeriza-Compositions and in the Components tion of individual formulations.

The exemplary comparison of the polymeriza-Concentration of Double Bonds tion courses of OEDA, EPA, and their mixtures

(mol/kg) at 507C in Ar is given in Figure 1(a) and (b). Ascan be seen, the formulations containing mixturesInof the oligomer and the monomer polymerizeCompositionsmuch faster (higher Rmax

p and shorter time neededto reach Rmax

p ) than individual components. TheEPA tolowest reactivity of the oligomer results from itsMonomer Molar

Ratio high viscosity (as high as Ç 105 of that of mono-mers), which restricts diffusional mobility of EPA

Components In Components 1 : 2.5 1 : 1 molecules. Such dependence is typical for lowerand medium polymerization temperatures and for

TEDA 8.70 6.35 5.28 all the monomers, both in Ar and air. At higherOEDA 9.35 6.59 5.40 temperatures, about 807C, this dependence is of-PDA 9.43 6.62 5.41 ten disturbed by an increase in rates of processesEPA 3.80

parallel to propagation (e.g., degradation, chaintransfer, oxidation, etc.) . The plots of p f andRmax

p vs. polymerization temperature obtained forsample pan position was measured to be 1.8 mW/ Ar and air atmospheres for the monomers, oligo-cm2 (by the carbon black method). mers, and their compositions are presented in Fig-

For calculations, a value for the heat of poly- ures 2–5. The values of p f and Rmaxp for polymer-

merization DH0 Å 86 kJ/mol7 (per one double ization carried out in air are lower than thosebond) was taken. Conversion at the time at which for polymerization in Ar due to the well-knownthe polymerization rate decreased to 0 (ca. 20 inhibitory effect of atmospheric oxygen. However,min.) was taken as the final conversion. The DSC it is worthy to note that detrimental effect of oxy-data obtained were analyzed for the corrected gen is much less pronounced in the polymeriza-base line. tion of the oligomer, which is associated with re-

duced oxygen diffusion to the highly viscous me-dium.

RESULTS Registered by DSC, overall heat effect resultsfrom several various processes, besides the mostimportant—propagation, also from those men-The most often used in practical applications

oligomer-to-monomer ratio varies between 50 : 50 tioned above, like degradation, depolymerizationor oxidation (H-abstraction is a zero-enthalpy pro-to 70 : 30 by weight. Both these ratios were chosen

to formulate the oligomer and monomer mixtures cess2) . The contribution of these processes to theoverall heat flow (and heat evolved) depends to aused in this work for the investigation of the cure

kinetics. Thus, the compositions contained: (a) high degree on the state of the polymerizing sys-tem, mainly on the conversion degree of doubleoligomer and monomer in weight ratio 50 : 50,

which corresponds to molar ratio 1 : 2.5; and (b) bonds. For this reason the discussion of the re-sults also includes the temperature dependence ofoligomer and monomer in weight ratio 70 : 30,

which corresponds to molar ratio 1 : 1. the polymerization rate during an early reactionstage—at 3% of double bond conversion (R3

p , Figs.The concentration of double bonds in the com-positions and in the components are given in Ta- 6 and 7). The lack of some points related to bound-

ary temperatures on these plots result from theble I. The polymerization was carried out isother-mally in the temperature range from 30 to 1007C fact that corresponding Rmax

p values were reachedat conversions lower than 3%.in the presence or absence of atmospheric oxygen.

The most important parameters characterizing Another factor that influences the results ob-tained at the highest temperatures is possible evap-the cure kinetics of multifunctional monomers

are: the rate at peak maximum (maximum poly- oration of the monomers during the polymerization.For instance, whereas volatility of PDA at 807C ismerization rate Rmax

p ) and final degree of doublebond conversion (p f ) . The discussion of the re- negligible, at 1007C the weight loss after about 20

min can reach several percent. This may somewhatsults in this work is based mainly on the values



Figure 1. Polymerization of EPA, OEDM, and their mixtures in 1 : 1 and 1 : 2.5oligomer to monomer molar ratios in Ar at 507C: (a) reaction rate profiles; (b) doublebond conversion profiles.

lower the parameters obtained, especially pf values. increased reactivity, which is manifested by a sub-stantial increase in the polymerization rates, bothOn the other hand, EPA is completely nonvolatile

in the temperature range studied. in air and Ar, in comparing to Rp values of theindividual components. The enhanced reactivity

DISCUSSION of such compositions is reflected also by the in-creased final conversions achieved in air atmo-The Influence of the Oligomer-to-Monomer Ratiosphere (not in Ar). The best results were obtained

As was mentioned above, the formulations based for equimolar mixtures of EPA and reactive dilu-on mixtures of EPA and the monomers show an ents. When the registered effect of a mixture of

two or more components is not additive butgreater than would be expected from the additiv-ity law, synergism between the components of themixture is said to occur.12 This synergistic effectis illustrated by Figure 8, which shows the de-pendencies of Rmax

p and p f at 707C on the composi-

Figure 2. Temperature dependence of final conver-sions for polymerization in Ar. (h ) TEDA, (s ) OEDA,(n ) PDA, (L ) EPA. Open symbols and dashed linesdenote monomer and oligomer polymerizations; filledsymbols denote polymerization of compositions con-taining corresponding monomer; solid lines denote po-lymerization of compositions with 1 : 2.5 oligomer-to-monomer molar ratio, and dotted lines denote polymer-ization of compositions with 1 : 1 oligomer-to-monomer Figure 3. Temperature dependence of final conve-

sions for polymerization in air. Symbols as in Figure 2.molar ratio.



Figure 6. Temperature dependence of the polymer-Figure 4. Temperature dependence of maximum po-ization rates at 3% of double bond conversion in Ar.lymerization rates in Ar. Symbols as in Figure 2.Symbols as in Figure 2.

tion of EPA/monomer mixtures. The appearancepends on the crosslink density (the lower initialof a synergistic effect was also shortly mentioneddouble bond concentration, the higher networkby other authors for the polymerization rate ofmobility), nevertheless, the diffusional limita-mixtures of an epoxy–acrylate oligomer and mo-tions caused by high initial viscosity of the formu-noacrylates containing heterocyclic structures inlation are the crucial factors in this case.their ester groups.13

Synergistic effect is observed also for the poly-Synergistic effect does not occur in the case ofmerization rate at low degrees of conversion (R3

pfinal conversion for the polymerization carried outFigs. 6 and 7). In Ar it exists only to about 607Cin Ar. That means that in the inert atmosphere,due to fast increase in R3

p of EPA with tempera-the oligomer-to-monomer ratio determines the po-ture that results from very strong temperaturelymerization rate, whereas the final conversion isdependence of the oligomer viscosity (high activa-determined by viscosity of a formulation: thetion energy of viscosity equal to 129 kJ/mol14) . Atlower viscosity, the higher conversion. The mobil-907C, R3

p value of EPA reaches the R3p value ob-ity of the polymer network plays a dominant role

in determining the value of p f . Although it de-

Figure 7. Temperature dependence of the polymer-ization rates at 3% of double bond conversion in air.Figure 5. Temperature dependence of maximum po-

lymerization rates in air. Symbols as in Figure 2. Symbols as in Figure 2.



Figure 8. Synergistic effect in the polymerization of EPA and monomer mixtures at707C in Ar and air: (a) maximum polymerization rate; (b) final conversion. (j ) TEDA,(l ) OEDA, (m ) PDA.

tained for equimolar mixture of the reactants, in- sional limitations, but simultaneously reducespossible interactions between EPA and monomerdicating, that under conditions where diffusion of

reactants is not strongly restricted (low conver- molecules.The existence of an association may explain thesion, reduced viscosity) the polymerization rate

tends to increase with the oligomer content in the occurrence of the synergistic effect in the case offinal conversions in air. The close neighborhoodcomposition.

As the reaction proceeds, the network density of the interacting comonomers causes substantialreduction of the inhibitory effect of atmosphericand viscosity of the polymerizing system increase

and near Rmaxp the propagation and termination oxygen due to lowering of oxygen diffusion, which,

in turn, enables the polymerization to proceed tobecome diffusion and reaction diffusion con-trolled,15,16 respectively. Because the strongest higher conversions. This effect, observed to about

907C, may find practical importance in productiondiffusional limitations occur for EPA homopoly-merization, its Rmax

p values are the lowest in the of protective coatings cured in air atmosphere.Thus, the 1 : 1 composition gives the optimumwhole temperature range (Fig. 4). Very high vis-

cosity of EPA results from hydrogen bonding due effect of increasing the polymerization rate at lowand medium polymerization temperatures and re-to the presence of hydroxy group.14 The enhanced

reactivity of the oligomer / monomer mixture duction of oxygen diffusion in air.needs a good compatibility between the reactants,for example, solubility, which is usually facili-

The Influence of Temperaturetated by intermolecular interactions. One may ex-pect that a type of intermolecular interactions As can be seen in Figures 2 and 4, the values of

Rmaxp and p f for the polymerization in Ar increasemay exist also between oligomer and monomer

molecules (C|O and OH groups), especially at with temperature up to about 60–807C, de-pending on formulation, but further grow withequimolar oligomer-to-monomer ratio. This, to-

gether with the reduced viscosity of the formula- temperature is hampered, and even a decliningtendency is observed in some cases.tion, would favor an increase in the polymeriza-

tion rate. It was found, that biunsaturated acrylic Although evaporative loss of the monomersmay complicate the interpretation of the resultsmonomers are able to form associates and that

their polymerization rate is sensitive to intermo- obtained at the highest temperatures (from about907C), the main reason of such polymerizationlecular interactions.17 Further increase in the

monomer content continues to decrease diffu- behaviors must be chemical in nature, most prob-



ably chain–transfer reaction and thermal degra- The difference between the strong growingtemperature tendency of Rmax

p of monomers anddation or depolymerization,7,14–16,18 which ratesincrease with temperature. The existence of a pla- EPA in air and its tendency to reach a plateau

in Ar in the polymerization above 807C may beteau region on the final conversion–temperatureplots suggests a topologically controlled conver- explained, assuming that the increase in the poly-

merization rate with temperature rise in the pres-sion limit.15 At temperatures higher than Ç 807Cthe conversion appears to decrease, suggesting in ence of oxygen is faster than the increase in rates

of decelerating processes like degradation/depoly-turn, the occurrence of degradation/depolymer-ization processes. On the other hand, the chain– merization or chain transfer at low conversion de-

grees reached in air. The exothermic effects of oxi-transfer reaction, i.e., the abstraction of mobilehydrogens from initial components or polymer by dation processes may contribute to a degree to the

observed heat flow. The fast increase in Rmaxp ofthe growing macroradical can increase the rate of

OEDA and PDA with temperature in air is proba-bimolecular termination and decrease in this waybly associated with acceleration of initiation bythe polymerization rate. Activation energy ofperoxides formed.7,20 At low initiation rates (as inchain–transfer reaction is higher by about 20–40the present work), this effect may significantlykJ/mol from activation energy of propagation,19

affect the temperature dependence of the poly-so, its contribution increases with polymerizationmerization rate. However, at higher initiationtemperature. This reaction is probably responsi-rates (higher light intensity and/or initiator con-ble for the drop in the polymerization rate withcentration), the effect of oxygen will be less pro-temperature rise above 907C for EPA polymeriza-nounced because the contribution of peroxides totion in Ar at the early polymerization stagesthe initiation process will be reduced. Thus, under(R3

p , Fig. 6) due to the presence of readily ab-conditions of fast initiation the temperature de-

stractable hydrogensu

H{C(OH)v

. For formula- pendencies of polymerization parameters in airshould resemble those in Ar.

tions based only on monomers or containing EPA The lack of the maximum on the plots Rmaxp

and monomers in the molar ratio 1 : 2.5, R3p in- and R3

p vs. temperature for EPA polymerizationcreases monotonically with temperature, indicat- in air may result in part from earlier increase ining that the possible degradation/depolymeriza- chain–transfer effects due to oxygen assistancetion processes are slow at very low polymer con- in this reaction.2 The reason of the rapid drop ofcentrations (and at low concentration of trapped Rmax

p , R3p , and p f values for monomer / oligomer

and mobile radicals) and that the chain–transfer compositions above Ç 807C in air is still uncer-reaction for the oligomeric component of the tain.formulation is of little importance. The increaseof EPA content in the formulation (1 : 1 ratio )

The Influence of the Heteroatomstops further increase of R 3p with temperature

above 807C. From the plots presented in Figures 2–7 it isAs the polymerization proceeds, the degrada- clearly seen that the presence of the heteroatom

tion/depolymerization and chain–transfer pro- in the ester group of the reactive diluent exerts acesses intensify, due to an increase in polymer deep influence on the polymerization kinetics. The(and radical) concentration and resulting in- beneficial influence of the heteroatom is muchcrease in concentration of tertiary hydrogen more pronounced in air atmosphere and is consid-atoms in the polymer backbone. This is mani- erably stronger for the sulfide than for the etherfested by the appearance of a plateau on the plots group.7,21 Obviously, the effect of the heteroatomof Rmax

p vs. temperature and a declining tendency is the strongest in the polymerization of the mono-of Rmax

p and p f at the highest temperatures (Figs. mers, but it is also kept in composition of the2 and 4). monomers with EPA. The exemplary comparison

Somewhat different temperature dependencies of the reaction rate profiles for the monomers,are observed for polymerization in air (Figs. 3, 5, EPA, and their mixtures in the 1 : 1 molar ratioand 7). The values of p f , Rmax

p , and R3p for the at 807C in air is given in Figure 9. The results of

monomers and oligomer increase monotonically TEDA, OEDA, and PDA polymerization are con-with temperature, whereas p f , Rmax

p and R3p val- sisted with those obtained previously.7

ues of the mixtures of monomers and oligomer The differences in the polymerization kineticsof the monomers studied result from the abilitypass through a plateau at about 50–807C.



attached to sulfur atom are considerably higherthan those for hydrogens in a CH2 group attachedto oxygen atom and higher than for tertiary hy-drogens19) . The importance of this reaction in thepolymerization of diacrylates was shown for theformulations containing sulfide additives of vari-ous structures.23 Thus, the TEDA-based formula-tions are least sensitive to oxygen inhibition aswell as give the best results when polymerizedin Ar. These properties, together with improvedthermooxidative stability and reduced solventand moisture absorption6,7 of the resulting poly-mer, makes TEDA useful for potential applica-tions as the reactive diluent in production of pro-

Figure 9. Reaction rate profiles of the monomers, tective coatings.EPA, and their mixtures in the 1 : 1 molar ratio at 807Cin air.

CONCLUSIONSof the CH2 group attached to heteroatom to donate

The photoinitiated polymerization of acrylic res-hydrogen.7 In air, this reaction leads to significantins based on the epoxyacrylate oligomer and dia-reduction of oxygen inhibition due to accelerationcrylate monomers is characterized by the syner-of oxygen consumption in the peroxidation pro-gistic effect occurring in a wide temperaturecess:range. Synergism is observed for the maximumpolymerization rate both in air and in Ar and forfinal conversions in air. The strongest effect oc-curs for the equimolar oligomer-to-monomer ratio.Such composition gives the optimum effect of in-creasing the polymerization rate at early and me-dium polymerization stages and low and medium

ROOı 1©CH¤©X© ROOH 1©CH©X© (1)

©CH©X© 1 O¤ı

ı

©CH©X©

OOı

where X 5 S or O.

(2)

reaction temperatures as well as reduction of oxy-gen diffusion in air. In Ar, the final conversions

This is the main reason of faster polymerization increase with the decrease in viscosity of a formu-rates and higher conversions of heteroatom-con- lation. The presence of a heteroatom (S or O) intaining monomers and their compositions in air. the ester group of the reactive diluent is beneficial

Another possible reaction, hydrogen abstrac- for the polymerization course, especially in air at-tion by growing macroradical both from the poly- mosphere. The best results were obtained for themer and monomer [reaction (3)] , leads to en- sulfur-containing monomer.hanced network crosslink density due to graftingand introduces some mobility to radical sites The authors thank Dr. W. Charmas for synthesis of theattached to the network. oligomer. This work was supported by grant of the Poznan

University of Technology number DS-32/282/97.

REFERENCES AND NOTES

Mı 1 ©CH¤©X ı MH 1 ©CH©X©

where ; Mı is macroradical.

(3)

1. S. P. Pappas, Ed., Radiation Curing Science andThis reaction affects the kinetics especially inTechnology, Plenum Press, New York, 1992.Ar and may be responsible in part for larger con- 2. J. G. Kloosterboer, Adv. Polym. Sci., 84, 1 (1988).

versions of TEDA- and OEDA-based formula- 3. Ullmann’s Encyclopedia of Industrial Chemistry,tions. A14, VCH Verlagsgesellschaft mbH, D-6940 Wein-

The hydrogen abstraction occurs easier in the heim, 1989, p. 171.case of {CH2{S{ group19,22 (the chain–trans- 4. K. S. Anseth, S. M. Newman, and C. N. Bowman,

Adv. Polym. Sci., 122, 176 (1995).fer constants for hydrogens in a CH2 group



5. H. R. Ragin, in ref. 1, ch. 7. 15. W. D. Cook, Polymer, 33, 2152 (1992).16. W. D. Cook, J. Polym. Sci., Part A: Polym. Chem.,6. A. Voelkel, E. Andrzejewska, R. Maga, and M. An-

drzejewski, Polymer, 37, 455 (1996). 31, 1053 (1993).17. L. A. Sukhareva, Poliefirnye pokrytya, struktura i7. E. Andrzejewska, Polymer, 37, 1039 (1996).

8. E. Andrzejewska and M. Andrzejewski, J. Polym. svoistva, Khimiya, Moskva, 1987.18. E. Andrzejewska, Polymer, 37, 1047 (1996).Sci., Part A: Polym. Chem., 31, 2365 (1993).

9. C. Decker, Acta Polym., 45, 333 (1994). 19. G. Odian, Principles of Polymerization, 2nd ed., Wi-ley, New York, 1981.10. C. E. Hoyle, in ref. 1, ch. 3.

11. E. Selli and I. R. Bellobono, in Radiation Curing 20. C. Decker and K. Moussa, J. Appl. Polym. Sci., 34,1603 (1987).in Polymer Science and Technology, Vol. III, J. P.

Fouassier and J. F. Rabek, Eds., Elsevier Applied 21. E. Andrzejewska, Polym. Bull., 37, 199 (1996).22. As showed semiempirical calculations (on the HF/Science, London, 1993.

12. J. F. Rabek, Mechanism of Photophysical Processes 6-31G* level) the energy needed to break the C{Hbond in the {CH2{S{ group is by about 8.5 kJ/and Photochemical Reactions in Polymers. Theory

and Applications, Wiley, Chichester, 1987, p. 574. mol lower than that needed to break the C{Hbond in the {CH2{O{ group. E. Andrzejewska13. C. Decker, in ref. 11, ch. 2.

14. D. J. Broer, G. N. Mol, and G. Challa, Polymer, 32, and M. Andrzejewski, unpublished work.23. E. Andrzejewska, Polymer, 34, 3899 (1993).690 (1991).


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Polymerization kinetics of photocurable acrylic resins