Kinetics of Photopolymerization of Acrylate Coatings - ACS Publications

Chapter 10

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Kinetics of Photopolymerization of Acrylate Coatings 1

1

Igor V. Khudyakov , Michael B. Purvis , and Nicholas

J.

2,*

Turro

1

Alcatel Telecommunications Cable, 2512 Penny Road, Claremont, NC 28610 Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027

2

Kinetics of free radical polymerization (FRP) of acrylate coatings was studied by photoDSC and real-time IR. Kinetic treatment of polymerization of acrylates in bulk and of acrylate coatings is critically analyzed. Experiments on postpolymerization of two commercial coatings ran by photoDSC allow an estimation of dimensionless ratio of parameters 2k/k at different conversions. It was determined that the ratio 2k /[k (1-ξ)], where ξ is acrylate conversion, weakly depends upon ξ within an experimental error of its determination. The latter conclusion is in accordance with a popular concept of "reaction diffusion" (Schultz, 1956) as a main mechanism of FRP of multifunctional acrylates. A different performance of the studied coatings under short UV -irradiationwas discussed. An effective mode of cure of the coatings with a short exposure to light was suggested. t

p

t

p

© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

113

114


Different mono- and multifunctional acrylate monomers and acrylate oligomers (or macromonomers) are widely used in UV-curable coatings (7-5). UV-cure of acrylates is afreeradical photopolymerization (FRP), and kinetics of this process is of academic and of practical interest. A natural intention of manufacturers of articles with polyaerylate coatings to increase productivity results in a demand to run polymerization faster and faster. That stimulates an additional interest to kinetics of photopolymerization. Kinetic treatment of photopolymerization is usually started with the expression given in eq 1 for a chainfreeradical reaction with bimolecular chain termination in a quasi-stationary regime (7-5):

where v, is a time- dependent rate of polymerization (disappearance of acrylate groups), k (2k ) is a rate constant (M .$~ ) of chain propagation (bimolecular chain termination), [M], is a concentration of a monomer (M), here a concentration of acrylate groups, w (Mis) is the rate of chain initiation. There are two other useful kinetic expressions for postpolymerization, i.e., polymerization after termination of initiation (4): l

p

l

t

in

[M], / f ~[M] / vo = (2V V V

0

2

*

()

o+2*,?[R- ] ?tr n

0

Here a subscript "o" stands for the initial moment when postpolymerization is observed; [R* ] is the initial concentration of macroradicals. Eq 1 is successfully applied for such liquid-phase chain reactions as oxidation of hydrocarbons, kinetics of FRP of vinyl monomers in diluted solutions, and some others. It is known, that polymerization of acrylates in a bulk (or neat acrylates) is accompanied by dramatic changes of the reagents and the media (2,5,5,6). In the case of multifunctional acrylates a network is formed with pending non-reacted acrylates. Viscous liquid of monomers (oligomers) is quickly converted into polymer, into elastomer or even into a hard polyaerylate in the course of photopolymerization (3,5,6). Evidently a formal kinetic treatment (eqs 1-3) is applicable only for an initial stage of cure under such circumstances if at all. Very limited application of formal kinetics to photopolymerization compels researchers to more or less convincing qualitative explanation of changes in a system under irradiation. The following concepts are widely used in the current literature under discussion of photopolymerization: time/conversion/radical length - dependent k and 2k \ entanglements of macroradicals; cure is a photocrosslinking of a multifunctional monomer; cross0

p

t


115 link density M ; free volume; intramolecular chain transfer, backbiting mechanism; branching; cyclization; gel or Norrish-Trommsdorff-Smith effect (1939); autoacceleration, autodeceleration; reaction diffusion (Schultz, 1956); postpolymerization, dark polymerization; trapped macroradicals, linear termination; microgelation; vitrification; final conversion ξ < 1.0; polymerization below or above T of a forming polymer; 3D-network formation, network structure; volume shrinkage, volume relaxation; 3D-network formation, network structure; polychromatic or dispersive kinetics at high degrees of polymerization. Photopolymerization was successfully described in a number of cases as an autocatalytic reaction (3,6). Certainly, the list above does not aim to be complete, and an order of concepts or notions is arbitrary. A detailed discussion of the concepts one can find in a current literature; we refer the reader to recent review articles (3,5) and to publications (2, 7-7/). Eqs 1-3 do not account for a chain transfer to monomer (solvent, polymer), reaction with molecular oxygen and impurities. (The latter result in an appearance of induction period.) These phenomena can play a role in FRP of acrylates in a bulk and acrylate coatings as well. There are a number of successful simulations of kinetics of polymerization of monofunctional vinyl monomers in a bulk; these works are mainly based on a dependence of diffusivity and/or reactivity at a high conversion ξ on the free volume of a system, cf., e.g. refs (7,8). It is known that kinetics of reactions in solid polymers and in a solid state is controlled by other principles rather than kinetics in a liquid state, cf., e.g. refs (12,13). These principles are related to a polychromatic kinetics, where the same chemical species have a certain distribution of their reactivates, and these principles may be applied to polymerization at high ξ. c


g

Conversion Dependent Parameters k and 2k p

t

The present status of polymer science and kinetics does not allow practically useful description kinetics of polymerization of multifunctional acrylates and acrylate coatings. At the same time, as it was mentioned above, there is a demand for analysis of rates and efficiency (final conversion) of cure. A coating is cured when ξ =1.0. A possible approach to kinetics of polymerization of multifunctional acrylates and acrylate coatings is still to use eqs 1,2 with conversion-dependent parameters k and 2£,(2,3,9,74 75) . It is a

p

a

A constant, which is in fact a variable, is an oxymoron. In such a cases a term "reaction coefficient" is sometimes used instead of "rate constant". However, "reaction coefficient" is often used in the sense of "rate constant" in particular in a gas phase kinetics (16). Certainly, one might use a term "specific reaction rate", but that term had been suggested by S. Arrhenius in the sense of "rate constant" already (77). In this paper we use term "parameter" k or 2k . p

t


116 hard to justify this approach. The possible logic can be that afreeradical polymerization in a complex acrylate system still consists of three main steps: initiation, propagation and (bimolecular) termination, and one ascribes parameters w , k , and 2k to characterize these steps, respectively. During certain conversion k and 2k are (or are considered) more or less ξ independent. Both k and 2k decrease with ξ increase, usually a ratio 2k /k also decreases with ξ increase (2, 14,15). One can expect that at ξ _ 1.0,2k md k become close to each other (2,14,15). Close values of parameters k and 2k mean that both propagation and termination are controlled by the same type of diffusion in a supposedly "highly crosslinked" media. Schultz (18) advanced a concept of "reaction diffusion" or "residual diffusion." Reaction diffusion means that at a high degree of polymerization termination occurs not as a result of a mutual diffusion of macroradicals with a subsequent reaction, but radicals move as a result of propagation, and 2k ~ k [M] or 2k ~ R k [M] (3,7,8,10,18). The proportionality coefficient R has dimension of Af\ and RD is constant during the rest of polymerization or at least weakly depend upon ξ. In other words, one can expect at high ξ that a dimensionless value in

p

t

p

p

t

t

t

p

t


p

t

p

t

D

p

t

p

D

2k l[kp(\-Q\ « const

(4)

t

for a given system at a given temperature. For monofunctional vinyl monomers (so called "linear systems") an onset of reaction diffusion control may be expected at ξ > 0.8, cf., e.g., réf. (10). For multifunctional monomers and acrylate coatings one can expect an onset of reaction diffusion at low ξ due to a formation of a polymer network (3). Efforts were devoted to an experimental verification of reaction diffusion (eq 3) as a main mechanism of polymerization of multifunctional acrylates (3). It is tempting in photoDSC experiments to get individual values of k and 2k using eqs 1,2. However, eq 1 has an additional unknown value w . In order to get w one needs to estimate an absorbed light W(einstein/Ms), to know quantum yield of formation of free radicals 0d«ss and a cage escape value e. Eq 5 holds true for a case of photodissociation of an initiator into two reactive free radicals: p

t

in

in

Win = 2Iabs

0diss β = 2 I b s / , a

(5)

b

We consider a rather common case of a photoinitiator, which is a benzoyl derivative, and it dissociates into a pair of reactive free radicals in a triplet state. Such photoinitiators are known under trade name of Irgacure® or Darocur®.


117 where/is often called a photoinitiator efficiency, / i s a cage escape e in the case of thermoinitiation. A value of 0 1 ms. Start up accessory triggered simultaneously IR and the shutter. Kinetic IR traces obtained and presented in this work demonstrate a good reproducibility despite a moderate S/N ratio. 1

2


119

PhotoDSC Results Irradiation of coatings during several min result in a total heat Q t = 150 ± 10 J/g for the primary and 200± 10 J/g for the secondary coating. These values were used in the calculation of Q , J for these two coatings, cf. the Experimental section. Figures 1,2 present experimental data obtained during pulsed irradiation of coatings. Data of Figures 1,2 allowed to get estimations of2k/k (cf. the Experimental section). These values 2k/k decrease with conversion as expected (3,4,14,15). Figure 3 presents a dependence of the calculated ratio to

0

p


p

2Λ//£ρ(1-ξ)] vs. ξ. It followsfromthe data of Figure 3 that the measured ratio does not vary more than several times during polymerization. We may conclude, that eq 4 is more or less valid, and it processes a certain predictive power.

Experiments with R T I R

Polymerization and Postpolymerization Irradiation of coatings with pulses (10-200 ms) of light of high intensity (50-100 mW/cm ) allowed us to monitor postpolymerization by RT IR till a partial or complete cure. Short irradiation can result in a negligible or in a very low conversion during the time of irradiation v and to a large final conversion up till ξ = 1.0 Postpolymerization plays an important role in cure of coatings. Evidently, the higher intensity I, the shorter v in order to cure coatings ( ξ = 1.0)We have noticed that in order to cure the primary coating it is necessary to have Ε = I. %r = 5-10 mJ/cm , whereas in order to cure the secondary coating the same value is 20-25 mJ/cm . An increase of light intensity resulted in acceleration of cure. 2

m

irt

2

2

Initiation of Polymerization: One Long or Two Short Pulses? In practice the cure of coatings, especially coatings for opticalfiber,takes place not by one short irradiation but by two or more short irradiation. An interesting and an important for practice question is: Which is a better way to cure coatings: by two short pulses with one or another delay Δ between them or by one long pulse with a duration equal to sum of duration of two short pulses?


120

100-


80-

3025c

E * σ

2015105o-

τ 4

Figure 1. PhotoDSC trace obtained during polymerization of the primary coating with a light of intensity 10 mW/cm (a); calculated ratio Q/v vs. time polymerization φ). 2



121

Figure 2. PhotoDSC trace obtained during polymerization of the secondary coating with a light of intensity 10 mW/cm (a); calculated ratio Q/v vs. time polymerization (b). 2


122

Ο _|


Ο

,

,

,

,

,

0.2

0.4

0.6

0.8

1 ξ

Figure 3. Dependencies of 2k/[k (l-§] vs. ξ for the primary coating (circles) and for the secondary coating (squares). p

This way the total exposure of coating to light is the same. We will try to get a answer below. We use a symbol ξ ι to define a conversion during the first pulse, ξ stands for an additional conversion during postpolymerization between the two irradiation, ξβ stands for an additional conversion during the second irradiation, ξ ^ stands for an additional conversion during postpolymerization till the termination of a reaction, ξ stands for a conversion during irradiation with one long pulse, ξ stands for an additional conversion during postpolymerization after a long pulse till the termination of a reaction. Figure 4 shows a typical relative position of kinetic traces pertinent to polymerization by one pulse and by two pulses. Experiments with primary coating led to an expected result: two pulses lead to higher conversion than one due to an additional conversion ( ξ ^ ) by postpolymerization between two pulses: Δ

ΐ 2

ΐ 2 ρ

ξΐ + ξ

Δ

+ ξ2 + ξ 2 > Ρ

ξΐ2+ξΐ2ρ

(6)

We have found in experiments with primary coatings that ξ ι ~ ξ and ξ ι + ξ ~ ξ ι . Further, ξ ^ ~ ξ « ξ ι , and eq 6 can be simplified as: 2

2

ρ

2

2

2 ρ

ξ

Δ

+ ξ 2 Ρ

>

ξΐ2

Ρ

(6a)

Thus, a higher ξ can be achieved in the course of two irradiation pulses rather than by one. Two postpolymerization is better than one, and the larger is A, larger is a beneficial effect of two irradiation. We have noticed a difference in two ways of irradiation delay between pulses Δ > 150 ms.


th

123


c

0.4

H

0

6

4

2

8

t,s

Figure 4. Kinetics of cure of the primary coating under irradiation during: a) two times by 50 ms with a delay between pulses of Is; b) a pulse of 100 ms mW/cm ). Initial absorption in these experiments is ~0.45. 2

On the contrary, for the secondary coating we have found that:

ξ ΐ + ξ Λ + ξ2 + ξ 2
0.1- 0.2. It is possible to ascribe a certain energy of light (E, mJ/cm ) directed to a surface of a thin coating film required for its cure. In the case of short irradiation (10-200 ms in our experimental conditions) a conversion during irradiation can be very low, and the rest of cure up till ξ =1.0 occurs as postpolymerization. We have studied kinetics of cure of coatings by two short pulses and by one long pulse. In our experiments the primary coating was cured at a temperature higher than T , and cure by two pulses is preferable, and it results in a larger ξ than a cure by one pulse. In experiments with secondary coating ran at temperature below T we did not observe any benefits of cure in two pulses. t

p

p


2

g

g

Acknowledgments The author at Columbia University is grateful to NSF for financial support of this work (grant NSF-CHE 98-12676). This work was supported in part by the MRSEC Program of the National Science Foundation under award No. DMR-9809687.

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5. 6.

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8.

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Kinetics of Photopolymerization of Acrylate Coatings - ACS Publications

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