Radiation Curing of Polymeric Materials - American Chemical Society

Chapter 2

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Electron-Beam Curing of Polymeric Materials James F. Kinstle James River Corporation, 1915 Marathon Avenue, Neenah, WI 54956

Electron beam induced reactions continue to grow in importance. This paper provides an introductory treatment of the equipment and materials options, including a mechanistic and kinetic view of the pertinent chemistry. Specific coverage includes several applications in polymer science, especially in curing of coatings. Electrons are very reactive, and-properly harnessed-are valuable in inducing chemical reactions. Several types of electron induced reactions are commercially important. The reactions of principal interest here are polymerization and crosslinking. This article is an introduction to the electron beam induced reactions of monomers, oligomers, and polymers, emphasizing the curing of coatings. Coverage includes the basic equipment (for process engineers), and reaction mechanisms and materials (for chemists and formulators). Overall, the intent is to provide a perspective "framework" for those interested in electron beam curing of polymers and coatings. [Please note: this is not meant to be a complete review; references are given to a few arbitrarily selected works, and to those that are presented in the following section of this book.] Electrons are very energetic species. They are from the short wave length, high energy end of the electromagnetic radiation spectrum, as can be seen from Table I. Electrons can be obtained as one component of the radiation emitted from gamma and pile sources. The most common of these for polymerization and polymer crosslinking is the cobalt-60 source. Because of the high energy of the emitted particles and waves, these sources require extensive shielding. They also cannot be turned on and off at will. Alternatively, electrons can be isolated by electrostatic methods; e.g., by using a Van de Graaff generator. 0097-6156/90/0417-0017$06.00/0 o 1990 American Chemical Society Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

18

RADIATION CURING OF POLYMERIC MATERIALS

Table I. The Electromagnetic Spectrum


Wave Length 7\, nm 1.25 1.0 6.0 4.0 1.25 2.5 6.8 1.25 3.1 1.0 1.25 2.5 1.25 1.25

X X X X X X X X X X X X X X

5

10" ΙΟ" ΙΟ" 10" ΙΟ" 10" 4

4 3

2 2

io-

2 1

ΙΟ" IO" 10 10 10 10 10

1

1

2 2

4

7

Frequency η) ,sec"

Energy eV

2.4 χ 2.9 x 5.3 x 7.5 χ 2.4 χ 1.2 χ 4.4 χ 2.4 X 9.7 χ 2.9 x 2.4 χ 1.2 χ 2.4 χ 2.4 χ

1.0 1.2 2.2 3.1 1.0 5.0 1.8 1.0 4.0 1.2

1

22

10 10 10 10 10 10 10 10 10 10 10 10 10 10

21 20

19 19 19

18 18

17 16 15

15 13 10

Source

χ 10" χ 10 χ 10 χ 10 χ 10 x 10 χ 10 χ 10 χ 10 χ 10 10 4.9 1.0 χ 10" 1.0 x 10' 7

6

5

5

4

4

4

Hard X-rays High end y range γ fl electrons Co β electrons High end soft X-ray Low end y range Η β electrons Low end soft X-ray 3

3

2

Photon UV Hg lamp

1

4

Their energies can be determined by the technique used to isolate and focus them; the units can be turned off and on. Electron beams are conventionally generated now by so-called electron accelerators. These may be linear accelerators that provide pulsed electrons emitted as a scanning beam. The development of these accelerators coincided with increased understanding of free radical polymerization mechanisms, and the availability of an increasing number of monomers. Together, these allowed rapid development of an overall science and technology base in electron induced curing and crosslinking. Further development in sources has led to the continuous "blanket" approach, whereby electrons are generated from a filament that is self-shielded, and the electrons flood the surface of the irradiated substrate rather than being scanned. Whatever the source of the electrons, there are important considerations in their utilization. Shielding is still required, both for the electrons and for the xrays that are emitted when the electrons strike a metal. Energy of the emitted electrons can be determined by the design and operation of the generation equipment. Applications requiring high penetration use high energy electrons, but most coating curing applications use electrons in the 100-350 keV range. Note that these electrons have a huge excess of energy relative to the photons provided by a UV source (more about this later). Within this energy range, the electron flux is also controllable by design and operation, as is the speed that a substrate moves past the electron source; therefore the energy, dose rate, and total dose are readily specified and controlled. When electrons strike an assembly of organic molecules, many events can be induced. They include:

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

2. KINSTLE

Electron-Beam Curing of Polymeric Materials +

+

A B"e, A"B e, etc.

dissociation

> e capture AB

19

> e ejection

AB", A B*, etc. +

A B 2e, etc.

> AB*

excitation


> That this myriad of possibilities can take place upon irradiation dramatically illustrates the energy difference between these electrons and the photons generated in a UV system. Importantly, these electrons provide sufficient energy so that a specific initiator or sensitizer is not necessary. This has several ramifications. One is economic, in that photoinitiators and sensitizers tend to be expensive. Another is that one need not worry about photoinitiator fragments that smell, turn colors, and/or are extracted, or about residual sensitizer that can assist in starting unwanted degradation reactions in the product. However, the high energy of the electron (perhaps 100X that needed for bond breakage) can also cause fragments to form within the irradiated system. Even though the electron beam is a kind of "chemical sledgehammer", the chemist/formulator still has a fair number of options. In certain ultra-pure systems, electron-induced free cation type polymerizations may be conducted. Under less rigorous conditions, addition of a cationic initiator allows cationic (or mixed cationic-free radical) polymerization [see Chapter 32]. Under more usual conditions, the coatings formulations are not sufficiently "pure" to allow ionic reactions; ions get consumed by reaction with traces of water or other additives/impurities, and free radical events predominate. Further discussion will center on these free radical systems. The polymerization that ensues after an electron initiated event is not very reflective of the process by which it was started. But some deviations do occur; e.g., at the high dose rates used in commercial practice, the rate of initiation is very high, and the overall rate of conversion can be dose rate dependent. The increasingly viscous, or even vitrifying, environment in which the polymerizations occur can also influence both rates and mechanisms of propagation and termination(1). [Also see Chapter 33 on vitrifying/crystallizing systems.] Reactions with oxygen can perturb the conversion. Still, the following rate expressions can be used to describe the "usual" coating curing process involving oligomer, multifunctional monomer, and monomer. Initiation Rj = φ l

a

where ψ is a quantum efficiency and l is the amount of absorbed energy, alternatively the amount of incident energy times an absorption efficiency times the concentration of absorber. a

Propagation R = k [~M°][M] p

p

where k is the specific rate constant for propagation, [~M°] is the p



20

concentration of growing radical, and [M] is the concentration of unreacted polymerizable group. 0

Termination R = \ [~M ]

2

t

The viscous effects mentioned above could have significant effects here. At steady state Rj = R , or 0l = ^ [~M ] . Solving for [~M ] (since this is very difficult to measure), 0


t

2

0

2

a

substituting back,

MM]

R = P

Also remember that the polymerizable species [M] can be an absorbing species, and therefore it contributes to the numerator of the square root term. In spite of the complications, the equation can be used to assess system options. One can speed up the reaction by increasing Rj. One way to do this is by increasing the intensity of the radiation, but formation of too many radicals allows them to participate in termination. One can increase the concentration of monomer, but--while increasing rate-this means there is more monomer to cure. Molecules that efficiently form useful reactive species upon irradiation can be incorporated into the formulation (organic halides often are useful here). Note that some of these modifications enter the equation in a square root term; i.e., the pertinent concentration or effect must be quadrupled to cause a factor of 2 change in R . There are also formulation variables available, like the specific functional group being polymerized. In general, initiation occurs rapidly with all readily free radical polymerizable monomers and functional groups, though aromatic monomers are sluggish due to the energy dissipation mechanisms available through the π system. Polymerizabilities/propagation of groups is like that mentioned for the UV systems. In terms of predictable, fast, and clean polymerization, the order tends to be: acrylate>methacrylate> vinyl >vinylene> vinylidene>allyl. The monomers and oligomers used in electron beam curable coatings formulations are like those discussed earlier for UV curable systems. However, the formulator has much more latitude with additives in the electron beam systems since penetration of electrons is much greater than that of photons. So highly pigmented, filled, dyed, etc. systems of many mils thick can be cured with electrons in the "regular" 150-400 keV range. This ability to penetrate can be used to advantage in other ways, too. A "buried" adhesive can be cured. Multiple chemical processes can be conducted at once, like curing a coating, grafting to a substrate, and crosslinking the substrate. The penetration also could be a negative efficiency factor, since those electrons that are transmitted through the substrate are usually wasted. One can, however, design the system so that a solid metal is p


2. KINSTLE

21

Electron-Beam Curing of Polymeric Materials

immediately behind the irradiated article/web, so that the reflected/backscattered energy can also be utilized. Electrons are useful for more than conversion of monomeric species to polymers. In fact, the following events all tend to take place upon irradiation of most curing systems:


polymerization

crosslinking

+ additive A A

Y Â I Y

crosslinking

or

Ζ

Again, opportunities are illustrated. Specifically, electron irradiation can be used to crosslink polymeric systems. This can be accomplished by irradiating the polymer alone, usually at a temperature above Tg (and above Tm if pertinent)®. Or it can be accomplished by irradiating a polymer in the presence of a polymerizable monomer, as has been done for many years in the wire and cable industry; e.g., polyvinyl chloride plus a multifunctional monomer like trimethylolpropane-trimethacrylate(3). Electron irradiation can also be used to form graft copolymers. In fact, this can be accomplished in



22


several ways(4); (a) by irradiating the polymer and then adding the monomer (the pre-irradiation grafting technique, in which case the newly grafted chain is formed from a reactive site on the preformed polymer) or (b) by irradiating the substrate polymer and the monomer together (the simultaneous irradiation grafting technique, in which the growing chain is grafted by the above mechanism plus by transfer and/or termination reactions). [Also see Chapter 36 on grafting.] Electron irradiation can also cause degradation reactions, including formation of monomer and/or other fragments. Selective crosslinking or degradation reactions can allow the use of these electron beam induced reactions in the resist field, too, where the shorter wave length (thus higher resolution) of electron, relative to UV is used to advantage®. [Also see Chapters 34 and 35 on resists and effects on organic materials.] Electron irradiation is useful in other areas, too. Foods can be irradiated, thereby allowing sterilization, and stabilization(6). For example, irradiation of potatoes kills any bugs, bacteria, etc., and eliminates germination/sprouting, which allows a great enhancement in storage-ability. Electron irradiation is increasingly used in sterilization of medical devices and equipment®, and may even have promise in treatment of various waste streams®. Summary Electrons can be generated in reliable devices and used in chemical processes. Important applications include polymer formation and crosslinking reactions. Mechanisms and kinetics of the reactions have been studied; at least a rudimentary understanding exists that can be used to guide the user. Monomeric, oligomeric, and polymeric reactants are available for exercise of the science and art. Applications of electron beam curing, especially in curing of protective and decorative coatings, is expected to continue to grow. Literature Cited 1. (a) Thompson, D.; Song, J.H.; Wilkes, G.L.; J. Appl. Polymer Sci. (34), 1063 (1987). (b) Decker, C.; Moussa, Κ.; J. Appl. Polym. Sci. (34), 1603 (1987). 2. van Aerle, N.A.J.M.; Crevecoeur, G.; Lemstra, P.J.; Polymer Commun. (29), 128 (1988). 3. Bowmer, T.N.; Vroom, W.I.; J. Appl. Polym. Sci. (28), 3527 (1983), and prior 3 papers in their series. 4. (a) Kaji, K.; Hatada, M.; Yoshizawa, I.; Kohara, C.; Komai, K.; J. Appl. Polym. Sci., 37 2153 (1989). (b) Taher, N.H.; Hegazy, E-S.A.; Dessouki, A.M.; El-Arnaouty, M.B.; Radiat. Phys. Chem., (33,#2), 129 (1989). 5. (a) Thompson, L.F.; Willson, C.G.; Bowden, M.J.; "Introduction to Microlithography," ACS, Washington, D.C., 1983. (b) Eranian, Α.; Bernard, F.; Dubois, J.C.; Makromol. Chem., Macromol. Symp. (24), 41 (1989).


2. KINSTLE

Electron-Beam Curing of Polymeric Materials

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6. (a) Josephson, E.S.; J. Food Safety (5), 161 (1983). (b) Swientek, R.J.; Food Processing. June 1985, pp. 82-90. 7. Bly, J.H.; Radiat. Phys. Chem. (33, #2), 179 (1989).


8. Trump, J.G.; Radiat. Phys. Chem. (24, #1), 55 (1984). Also see Chapiro, Α.; "Radiation Chemistry of Polymeric Systems," Interscience, NY, 1962, and Charlesby, Α.; "Atomic Radiation and Polymers," Pergamon Press, London, 1960. Dole, M. (Ed.); "Radiation Chemistry of Macromolecules, Vol. I & II," Academic Press, NY, 1972, for historical perspective. RECEIVED September 27, 1989


Radiation Curing of Polymeric Materials - American Chemical Society

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