Nonacrylate Curing Mechanisms - American Chemical Society

Chapter 12

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Nonacrylate Curing Mechanisms Photoinitiated Cross-Linking Using the Amine-Ene Reaction G. Κ. Noren and E. J. Murphy DeSoto Inc., P.O. Box 5030, Des Plaines, IL 60017

The use of polymer bound amine synergists for UV-cure of non-acrylate unsaturated resins initiated by intermolecular hydrogen abstraction photoinitiators has been investigated and has been utilized to effect crosslinking of organic coatings. Benzophenone was found to be the most efficient photoinitiator for this system. Tertiary amine concentrations of about 10 equivalents per gram of resin were found to give the best cure at the lowest dosages. Good cure was obtained with methyl or ethyl substituted amines and high functionality unsaturated resins. Thermal mechanical analysis of one film showed a distance of 8.3 x 10 cm between crosslinks. -4

-8

The a r e a o f u l t r a - v i o l e t c u r a b l e r e s i n s has undergone considerable growth during t h e l a s t 15 t o 20 y e a r s . U s e f u l r e s i n systems which can be c u r e d by u l t r a - v i o l e t l i g h t i n i t i a t e d f r e e r a d i c a l mechanisms can be c l a s s i f i e d as f o l l o w s : 1. U n s a t u r a t e d p o l y e s t e r r e s i n / s t y r e n e 2. T h i o l - e n e 3. M u l t i f u n c t i o n a l ( m e t h ) a c r y l a t e s The p o l y e s t e r / s t y r e n e systems a r e low i n p r i c e b u t have slow cure speed and p r e s e n t e n v i r o n m e n t a l and h e a l t h problems due t o t h e h i g h v o l a t i l i t y o f t h e s t y r e n e monomer. They a r e used p r i m a r i l y i n wood finishing. T h i o l - e n e systems a r e h i g h i n p r i c e and c u r e much f a s t e r b u t a l s o p r e s e n t h e a l t h problems due t o t h e t h i o l component i n the system. They a r e used i n PVC f l o o r i n g and i n g a s k e t s . Acrylate systems a r e m o d e r a t e l y p r i c e d and a r e a l s o f a s t c u r i n g b u t c a n cause skin i r r i t a t i o n and have r e c e n t l y come under s c r u t i n y f o r l o n g term t o x i c i t y . M e t h a c r y l a t e s a r e n o t c o n s i d e r e d t o c r e a t e h e a l t h problems b u t a r e v e r y slow c u r i n g . ( M e t h ) a c r y l a t e m a t e r i a l s have been u s e d i n many a p p l i c a t i o n a r e a s . The photoinitiation of polymerization by the use o f intermolecular hydrogen abstraction i s well known. (JL) Diaryl k e t o n e s , such as benzophenone, a r e u s e d as t h e p h o t o i n i t i a t o r and a

0097-6156/90/0417-0151$06.00A) ο 1990 American Chemical Society Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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RADIATION CURING OF POLYMERIC MATERIALS

coinitiator consisting of a low molecular weight tertiary amine having at least one abstractable hydrogen atom on a carbon atom that is alpha to the nitrogen is required. These amine synergists or photoactivators form an exiplex with the excited state of the diaryl ketone and subsequently transfer a hydrogen atom to the diaryl ketone producing a ketyl radical and an alpha-amino radical. (Scheme I) While the ketyl radical has proved to be quite inefficient as an initiator, the alpha-amino radical has been shown to be an effective initiator for acrylate polymerization. (2_) We wish to report the application of this initiation mechanism to a system consisting of polymeric or oligomeric tertiary amines and unsaturated oligomers which do not contain (meth)acrylate functionality. Results and Discussion Photoinitiator Studies. A tertiary amine pendent acrylic copolymer (ACRYLIC 1) was synthesized by free radical polymerization and used for the evaluation of various initiators. Thus, 414 g of a solution containing 43% methyl methacrylate, 32% 2-ethylhexyl acrylate, 5% hydroxypropyl acrylate and 20% dimethylaminoethyl methacrylate and 1.9% (based on monomers) of AIBN was added over a 2.5 hour period to 172 g of η-butyl acetate at reflux. The resulting copolymer solution was diluted to 55% NVM with η-butyl acetate which resulted in a final viscosity of about 4000 cps. The copolymer had a theoretical equivalent weight per nitrogen of 785 corresponding to a functionality of 110 based on a theoretical molecular weight of about 87,000. Triallylcyanurate (equivalent weight per double bond = 83) dissolved in MEK (80% NVM) was used as the unsaturation source for these experiments. The ratio of unsaturation equivalents to amine equivalents (UE/AE) was 4 to 1 and a photoinitiator level of 4% based on resin solids was used. The degree of cure was determined by the solvent resistance as measured by the number of methyl ethyl ketone double rubs (MEKDR) of 1 mil films cured on an 1ST unit (pulsed xenon lamp UV source) at 3 J/sq-cm dosage. These results are shown in Table I. Table I. Effect of Photoinitiator on The Cure of ACRYLIC 1/Triallylcyanurate System Photoinitiator(4% by wt) Benzophenone Xanthone Isopropylthioxanthone Methylthioxanthone 4,4'-dichlorobenzophenone 10-thioxanthone Benzopinacol

MEKDR (à 3 J/sa-cm 30 23 17 16 15 9 8

Based on these results benzophenone was chosen as the photoinitiator to be used in further studies. Using the same oligomer system as above the effects of benzophenone level and dosage were studied. The results are shown below in Table II.

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Nonacrylate Curing Mechanisms

Table II. The Effect of Benzophenone Level and UV Dosage on ACRYLIC 1/Triallylcyanurate System


Dosage, J/sq-cm 1 2 3

MEKDR @ Benzophenone Level (PHR) 2 i> 1 5 6 8 12 16 22 24 32 52

From this data i t can be seen that both increased amounts of benzophenone and higher dosages result in increased degrees of cure. A photoinitiator level of 4% was considered practical and representative and used in further experiments. The Effect of the Structure of the Unsaturation Source. The effect of the structure of the unsaturated component was studied using two types of amine sources. The first was the copolymer ACRYLIC 1 described above. The second was a linear polyurethane (LPUR) prepared by reacting bisphenol A (1.0 mol) with trimethylhexane diisocyanate (1.5 mol) in urethane grade MEK at 70 C for 4 hours in the presence of dibutyl t i n dilaurate. Dimethylethanol amine (1.1 mol) was then added at 50 C and reacted at 70 C for 18 hours. The resultant material was 55% NVM in urethane grade MEK, had an equivalent weight per nitrogen of about 63 0 and had a theoretical functionality of two. The studies were done using a UE/AE ratio of 4/1 and 4% of benzophenone photoinitiator based on solid resin. The results are shown in Table III. Table III. Effect of Unsaturation and Amine Sources on the Degree of Cure at 3 J/sq-cm

Unsaturation Source Eq. Wt. Diallyl phthalate(f = 2) 123 T r i a l l y l trimellitate(f = 3) 110 Poly(diallyl phthalate) 453 (f = 10) Poly(diallyl isophthalate) 317 (f = 17) Unsaturated polyester 213 (f = 4) A l l y l terminated unsaturated 594 polyester (f = High)

MEKDR Amine Type LPUR(f = 2) ACRYLIC l ( f = 4 80(a,b) 6 25 81 — 150 8 >200

— 12 >150(c)

a. Dosage was 3.5 J/sq-cm. b. 6% Benzophenone based on solids. c. 8% Benzophenone based on solids. Based on this data i t was indicated that the higher the functionality the better the degree of cure for a l l y l compounds. The use of an unsaturated polyester in place of an a l l y l compound appears to be detrimental to the cure but a synergistic effect occurs when both terminal a l l y l unsaturation and internal maleic



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type unsaturation exist together. In fact, the system containing the a l l y l terminated unsaturated polyester (ATUPE) and the linear polyurethane had >200 MEKDR at 2 J/sq-cm and 30 at 1 J/sq-cm. When the ACRYLIC 1 copolymer was substituted for the LPUR, the films had >150 and 31 MEKDR at 2 and 1 J/sq-cm respectively. The a l l y l terminated unsaturated polyester was chosen for further study. No advantage was observed for either amine. The Effect of Amine Structure. In order to evaluate the effects of amine functionality and substitution on the crosslinking reaction, several amine terminated urethane oligomers were synthesized by the same procedure as used in the synthesis of the LPUR. The oligomers were prepared in Urethane Grade MEK. These amines were evaluated with the a l l y l terminated unsaturated polyester as the unsaturation source. The composition and the properties of the oligomers are shown in Table IV. Table IV. Oligomers Used for the Study of Amine Structure Mole Ratio Components 1/2.8/2.8 TMP/IPDI/DMEA TMP/IPDI/DEEA 1/2.8/2.8 TMP/IPDI/DIPEA 1/2.8/2.8 Hexane Diol/ TMDI/ 2/3/2 DEEA 1/3/3 5 TMP/IPDI/DEEA

No. 1 2 3 4

Eg. Wt. 335 361 388 551

%NVM 73 73 80 80

384

74

Viscosity(cps) 10,200 4,080 5, 680 6, 500 12,000

TMDI = Trimethylhexamethylene diisocyanate TMP = Trimethylol propane IPDI = Isophorone diisocyanate DMEA = Dimethylethanolamine DEEA = Diethylethanolamine DIPEA = Diisopropylethanolamine Formulated coatings were made at a UE/AE ratio of 4/1 using 6% benzophenone as the photoinitiator. The degree of cure of the films after exposure to dosages of 1 and 2 J/sq-cm was again measured as the number of MEK double rubs. The results of the testing of these systems are shown in Table V. Table V. Effect of Amine Structure on The Cure of Oligomers

Resin No. 1 2 3 4 5 ACRYLIC 1

Amine Type DMEA DEEA DIPEA DEEA DEEA Dimethylamino

Variable Methyl Ethyl Isopropyl f = 2 f = 3 f = 110

MEKDR @ Dosaqe, J/sq-cm 2_ 2 >150 100 92 >150 45 >150 55 >150 40 >150 31 >150

Very l i t t l e difference was observed in the degree of cure between methyl and ethyl substitution on the nitrogen. However, in the case of the isopropyl group somewhat slower curing was observed. This was probably due to both statistical and steric effects. There also


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seems to be a slight advantages to lower degrees of functionality and having the functionality located on a terminal position rather than pendant to a chain. The Effect of Unsaturation Equivalents to Amine Equivalents Ratio. The ratio of unsaturation equivalents to amine equivalents should be an important factor in controlling the cure speed of this system and would be an important factor in the determination of the contribution that the addition of the hydrogen on the alpha-carbon atom of the amine makes to the crosslinking reaction. This effect was studied by keeping the initiator concentration constant while varying the UE/AE ratio. This set of results was again obtained with the ACRYLIC 1 copolymer as the amine source and triallyl cyanurate as the unsaturation source. The benzophenone photoinitiator concentration was held constant at 4% over the whole range of UE/AE ratios studied. Curing was accomplished at 3 J/sq-cm and 6 J/sq-cm. The results are shown in Figure 1. A maximum was observed at an UE/AE ratio of 10/1 which is equivalent to 8 hydrogen atoms per double bond. However, i f only 1 hydrogen is active, which might be expected due to steric hindrance, a degree of polymerization (DP) per a l l y l of 10 would result. This is a reasonable DP for a l l y l polymerization.Q) This UE/AE ratio also corresponds to an amine concentration of 6.19 χ 10~ equivalents of nitrogen per gram. This concentration of amine is almost equal to that used in the benzophenone initiated photopolymerization of methyl acrylate (2.3 χ 10~ ) (2.) ^ also corresponds well to the level of tertiary amine synergist (4.2 χ 10" to 4.2 χ 10~ ) used with benzophenone in epoxy acrylate crosslinking systems. (_4) At this point we decided to separate the terminal a l l y l unsaturation from the internal unsaturation being contributed by the maleic type double bonds in the unsaturated polyester. A simplex mixture design was used to study a system consisting of the ACRYLIC 1 copolymer, an a l l y l terminated polyester (ATPE) and an unsaturated polyester (UPE). The a l l y l terminated polyester was prepared by reacting diethylene glycol (1 mol), azelaic acid (1 mol) and trimethylolpropane diallyl ether (0.3 mol) at 160 to 200°C while removing water as the xylene azeotrope. The ATPE had an equivalent weight per double bond of 560 and a functionality of about four. The unsaturated polyester was prepared by reacting diethylene glycol (1.5 mol), fumaric acid (1.5 mol) and benzoic acid (0.44 mol) at 160 to 200°C while removing water as the xylene azeotrope. The UPE had an equivalent weight per double bond of 332 and a functionality of about 6-7. The three component mixtures were prepared containing 4% benzophenone and were cured at 3 J/sq-cm. The results are shown in Figure 2 which indicates a maximum at a mixture consisting of 66.6% UPE, 16.7% ATPE and 16.7% of ACRYLIC 1. This mixture is at an UE/AE ratio of about 10/1 which corresponds to 2.1 χ 10~ equivalents of nitrogen per gram. This also compares favorably with the concentration used for low molecular weight amine synergists. 4

4

ana

5

4

4

Thermal Behavior of Cured Films. A film from triallylcyanurate and ACRYLIC 1 copolymer was analyzed by DSC and TGA. The sample was cured at 3 J/sq-cm. The glass transition temperature was -4 2° C (under Nitrogen) and the isothermal weight loss after 1 hr at 200°C in air was 31%. Finally, Thermal Mechanical Analysis was performed on the




OH

2

NCH

3

0

: to +

R NCH 2

3

R^NCH.

EXIPLEX

200

Dose

= 3 Joule/sq-cm

Dose

= 6 Joule/sq-cm

100

U n s a t u r a t i o n Equivalents / Amine Equivalents

Figure 1.

Ratio

Effect of UE/AE ratio on the degree of cure of ACRYLIC 1/triallylcyanurate system.



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Nonocrylate Curing Mechanisms

100% ACRYLIC 1

ACRYLIC 1 U/A - Unsaturation to amine equivalents ratio. Dosage » 3 J/sq-cm. Figure 2.

Simplex experimental design study of the degree of cure of ACRYLIC 1/ ATPE/UPE blends as measured by MEK double rubs.


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sample that had the best performance in the simplex design. The sample was cured at 3 J/sq-cm. The results are shown in Figure 3. A calculation of the crosslink density using the modulus data at 120°C gives a value of 1.75 χ 10 per cc or 8.3 χ 10~ cm between crosslinks. Theoretical calculations using statistical methods yield crosslink densities of 1 0 to 10 for trimethylol propane triacrylate, pentaerythritol triacrylate and urethane oligomers at 60% conversion.(5) Thus, amine-ene systems have an equivalent crosslink density to acrylates. 21

8

20

22


Conclusions Formulations containing tertiary amine substituted oligomers and non-acrylate unsaturated resins cured well at moderate dosages (1-2 J/sq-cm). Based on the effective concentration of amine (10~ equivalents of nitrogen per gram) i t appears that the amine substituted oligomers function as polymer bound amine synergists. Benzophenone was found to be the best photoinitiator for these systems. The amine nitrogen can contain either methyl or ethyl groups and s t i l l function equally well as a coinitiator. Increasing the functionality of the unsaturated resin also increases the degree of cure. 4

Figure 3.

Thermal Mechanical Analysis of a film containing 16.7% ACRYLIC 1, 16.7% ATPE and 66.6% UPE cured at 3 J/sq-cm.


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Acknowledgments The authors would like to thank DeSoto, Inc. for the opportunity to present the findings of this research. We would also like to extend our appreciation to R. S. Conti and J. J. Krajewski for helpful discussions and advice and to R. W. Johnson for conducting the ΤMA work.


Literature

Cited

1. Pappas, S. P. J. Radiation Curing, July 1987, page 6. 2. Sander, M. R.; Osborn, C. L.; Trecker, T. J. J. Polym. Sci., Polym. Chem. Ed., 1972, 10, 3173. 3. Lenz, R. L. Organic Chemistry of Synthetic High Polymers, Interscience, New York, 1967, p 294. 4. Christensen, J. E.; Wooten, W. L.; Whitman, P. J. J. Radiation Curing, July 1987, page 35. 5. Ishikawa, H.; Motomura, M. paper presented at the 66th Annual Meeting of the Federation of Societies for Coatings Technology, Oct. 19-21, 1988, Chicago, IL. RECEIVED September 13, 1989


Nonacrylate Curing Mechanisms - American Chemical Society

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