Chemical Antecedents in EB and UV Cured Inks.
The central process in them is the insitu polymerization of the monomeric and oligomeric compounds that do not leave any residuals to the environment. The ink coating is simply treated with radiation (electrons or UV light), which performs the curing process. In spite of the safety precautions to be observed with the use of the radiation sources, these methods find special applications in inks and coatings industry.
As the names imply they utilize high energy electron beam and ultraviolet light, respectively, as the energy sources. Industrial terminology -- radiation curing -- does not differentiate the chemistry involved in these processes. However, hard core chemists who are actively involved in research and development activities with EB and UV sources have christened them as radiation chemistry and photochemistry, respectively. This is quite understandable in view of the totally different chemical processes and the initial sequences associated with the absorption of EB and UV by materials.
Radiation chemistry is a term reserved to represent the chemical consequences of absorbing high energy radiations such as [gamma] rays, electrons, neutrons and [alpha] particles.
Photochemistry represents the chemical effects of absorbing light in the visible and UV regions. With the developments in lasers, the scope of photochemistry has been extended to the infra-red region also. The interactions of electrons and photons with matter differ in many aspects, but they have many common features.
The high energy electrons are produced in a particle accelerator. In ink curing technology, typical electron energy employed ranges from 125 -- 150 keV (kilo electron volt). The radiation chemistry brought out by high energy electrons is very much similar to that induced by photon sources such as X-rays and 7-rays. Since these radiations cause ionization of the medium in which they traverse, they are also known as ionizing radiations. Conversely, UV and visible light that do not cause ionization are called nonionizing radiations.
When electrons collide with matter and interact, two types of processes take place: scattering and nuclear capture. Scattering is the interaction in which the particle retains its nature or maintains identity. It can be either elastic type or non-elastic type. In the elastic type, electrons do not lose energy irrespective of the scattering angle. In the non-elastic type, electrons lose energy. The energy loss can result in atomic displacement, excitation or ionization. Thus only the inelastic interactions are of any chemical consequence. At relatively high electron energies and large scattering angles, an energy loss mechanism referred to as Bremsstrahlung occurs, which is the light emission due to the deceleration of electrons. At sufficiently high energies, the electron will cross the barrier offered by the atomic electrons, and will be captured by the nucleus. However, such high electron energies are not used in EB curing technology.
When electrons interact with matter, they knock on the electrons in the atom and transfer some of its energy to them. These electrons can be excited to higher energy levels, and at appropriate electron beam energies, the atom can lose that electron producing a positive ion. These ejected electrons are known as secondary electrons. These low energy electrons will dissipate its energy over a small volume in the medium that results in little clusters of ionizations and excitations. These spherical clusters are called spurs and have a diameter of [sim]2 nm (nanometer).
Some of these secondary electrons may be captured by a positive ion in a process known as geminate recombination producing a highly excited state that can dissociate into other products. A fraction of these electrons might be simply trapped by the solvent molecules and solvated. In water they are referred to as hydrated electrons. These processes may be represented by the following equations:
M + [e.sup.-] [right arrow] [M.sup.+] + 2[e.sup.-]
[M.sup.+] + [e.sup.-] [right arrow] [M.sup.*]
[e.sup.-] + solvent [right arrow] [e.sup.-] (solvated)
Where M is a molecule, [M.sup.+] is a positive ion, [M.sup.*] is an excited state, and [e.sup.-] is the electron.
A wealth of information on the radiation induced chemical processes is available with [gamma] rays. Most of the secondary events in the [gamma] radiolysis is very much similar to the processes initiated by high energy electrons.
Now we will go into the details of the radiation chemistry of the most ubiquitous system, viz, water. Radiation chemistry of aqueous solutions is important as the biological systems are in such an environment. It is the most elaborately studied system in radiolysis (cleavage of molecules by radiation) literature, and the radiation source used is [gamma]-rays. Since the chemical consequences in these experiments are derived from the secondary electrons generated in the medium, radiation chemistry with high energy electrons will be operating in the same lines.
During the radiolysis of water, two types of products are formed: radical and molecular. Primarily, the excited water molecules, positive water ions and electrons are formed which decompose to give the hydrogen and hydroxyl radicals. The molecular products formed are hydrogen and hydrogen peroxide. The important steps in the radiolysis of water may be represented as follows:
[H.sub.2]O [caret]-[caret]-[caret] [right arrow] [H.sub.2][O.sup.*]
[H.sub.2]O [caret]-[caret]-[caret] [right arrow] [H.sub.2][O.sup.+] + [e.sup.-]
[H.sub.2][O.sup.*] [right arrow] [H.sup.*] + [O.sup.O]H
[e.sup.-] [right arrow] [[e.sup.-].sub.aq]
[H.sub.2][O.sup.+] [right arrow] [H.sup.+] + [O.sup.*]H
([H.sub.2][O.sup.+] + [e.sup.-]) [right arrow] [H.sub.2][O.sup.*] [right arrow] [H.sup.*] + [O.sup.*]H
[H.sup.*] + [H.sup.*] [right arrow] [H.sub.2]
[O.sup.*]H + [O.sup.*]H [right arrow] [H.sub.2][O.sub.2]
In a general way, the water radiolysis may be represented as,
[H.sub.2]O [caret]-[caret]-[caret] [right arrow] [H.sup.*], [H.sup.+], [[e.sup.-].sub.aq],
[O.sup.*]H, [H.sub.2], [H.sub.2][O.sub.2]
Here the wavy arrow [caret]-[caret]-[caret] [right arrow] represents the radiation. It may be noted that the molecular products [H.sub.2] and [H.sub.2][O.sub.2] may be formed by other mechanisms also (not shown in the scheme of equations).
The yield of radiation chemical products is expressed by G values. In the cgs system, G value is defined as the number of species formed per 100 eV of energy absorbed by the total sample,
G (X) = 100(number of species X formed)/(energy absorbed in electron volts)
In SI system, it is expressed as the number of micromoles of the species produced per joule of the energy absorbed by the sample,
G (X) = (number of micromoles of species X formed)/(energy absorbed in Joules)
The energy absorbed by the sample is to be known for calculating the G values aside from the accurate concentration of the species. In SI unit, the absorbed dose is expressed as gray (Gy). 1Gy corresponds to the absorption of 1 Joule per kilogram of the sample. The cgs unit is the rad where 1rad is defined as the dose of 100 erg [g.sup.-1]. The measurement of absorbed dose of energy in radiation chemistry is known as dosimetry. For this, both physical and chemical methods are available. Physical methods involve ion chambers and calorimeters: in the former, the ions produced by a certain amount of gas are collected to calculate the dose; in the latter, the temperature rise in a small, thermally insulated mass of material is measured to arrive at the dose.
Physical dosimeters are not convenient to handle and secondary dosimeters became popular in which the yield of a radiation-induced chemical reaction is measured. They are known as chemical dosimeters. For example, in the most popular chemical dosimeter, viz. Fricke dosimeter, the radiation induced oxidation of Fe(II) to Fe(III) ion is measured. In the ceric sulfate dosimeter, the reduction of Ce(IV) to Ce(III) ion is measured. Plastic film dosimeters are also used where the radiation-induced discoloring of a dye impregnated in the film can serve the same purpose.
The determination of G values of products of water radiolysis was one of the early interests of radiation chemists. For example, the molecular yield of hydrogen G [H.sub.2] may be manipulated by recourse to chemical elimination of hydroxyl radicals that can otherwise remove hydrogen molecule, by the addition of bromide ion. The relevant reactions are the following:
[OH.sup.*] + [H.sub.2] [right arrow] [H.sup.*]
[Br.sup.-] + [OH.sup.*] [right arrow] [Br.sup.*] + [OH.sup.-]
Similarly, the G value of [H.sub.2][O.sub.2] may be determined by measuring the oxygen evolved in the radiolysis of deaerated cerium (IV) sulfate. [H.sub.2][O.sub.2] reacts as follows:
[H.sub.2][O.sub.2] + Ce (IV)[right arrow]Ce (III) + [[HO.sup.*].sub.2] + [H.sup.+]
[[HO.sup.*].sub.2] + Ce (IV)[right arrow]Ce (III) + [H.sup.+] + [O.sub.2]
Thus the [O.sub.2] yield serves as a measure of [H.sub.2][O.sub.2]
Fundamental research in radiation chemistry was centered on identifying the initial species formed, determining their rate constants of formation and decay, and pinpointing the sequential events following the absorption of radiation. It is estimated that interaction of radiation with molecules occur in the time scale of attoseconds ([10.sup.-18]S), the time needed for the traversal of a molecular diameter by a high energy radiation. This is followed by ionization and excitation in [10.sup.-17] to [10.sup.-16] S, and dissociation of excited molecules in [10.sup.-14] S. The solvation of secondary electrons in water takes place around [10.suo.-13] S.
Identification of hydrated electrons ([[e.sup.-].sub.aq]) was one of the triumphs in radiation chemistry. This could be performed by a technique known as pulse radiolysis. Here a short pulse of radiation generated in a linear energy accelerator is made to interact with molecules and transient species formed are detected by light pulse of equally short duration as the radiation pulses. The absorption of hydrated electron was thus recorded that showed a peak at 578 nm.
The radiation chemistry of an organic molecule may be exemplified by the [gamma]-ray radiation induced oxidation of a hydrocarbon like n-heptane. Radiation produces excited states as discussed above that dissociate into radicals. In this case, 2-, 3-, and 4-heptyl radicals are formed instead of the primary 1-heptyl radical and they combine with oxygen to produce the respective hydroperoxyl radicals. Eventually, the hydroperoxyl radicals react to yield ketones such as 2-, 3-, and 4-heptanones and alcohols such as 2-, 3-, and 4-hepanols in the expected statistical ratio of 2:2:1. Figure-1 represents the dependence of products such as peroxides, carbonyls and alcohols on the absorbed radiation dose. It can be seen that heptanones are the only carbonyls formed in this reaction as the amount of total carbonyls measured by spectrophotometry and the total amount of individual heptanones determined by gas chromatography match. This indicates that the C-C bond in the hydrocarbon is not cleaved during this process. Fr om these measurements a G value of 5.4 was obtained for the heptyl radicals. Based on these observations, the following mechanism is suggested for the radiation induced oxidation of n-heptane.
[C.sub.7][H.sub.16] [[caret].sub.-][[caret].sub.-][caret][right arrow] [C.sub.7][[H.sup.*].sub.15]
[C.sub.7][[H.sup.*].sub.15] + [O.sub.2] [right arrow] [C.sub.7][H.sub.15][[O.sup.*].sub.2]
H + [O.sub.2] [right arrow] [[H.sup.*].sub.2]
[C.sub.7][H.sub.15][[O.sup.*].sub.2] + [C.sub.7][H.sub.15][[O.sup.*].sub.2] [right arrow]
[C.sub.7][H.sub.15]OH + [C.sub.7][H.sub.14]O + [O.sub.2]
[C.sub.7][H.sub.15][[O.sup.*].sub.2] + [C.sub.7][H.sub.15][[O.sup.*].sub.2] [right arrow]
[C.sub.7][H.sub.15][OOC.sub.7][H.sub.15] + [O.sub.2]
[C.sub.7][H.sub.15][[O.sup.*].sub.2] + [[HO.sup.*].sub.2] [right arrow] [C.sub.7][H.sub.15]OOH + [O.sub.2]
[[HO.sup.*].sub.2] + [[HO.sup.*].sub.2] + [[HO.sup.*].sub.2] [right arrow] [H.sub.2][O.sub.2] + [O.sub.2]
[C.sub.7][[H.sup.*].sub.16] + [O.sub.2] [right arrow] [C.sub.7][H.sub.14]O + [H.sub.2]O
Photochemical reactions spring from a state of the molecules called excited states (similar to the excited states formed with electrons). This refers to the shooting up of electrons within the molecule to higher energy positions. The light energy absorbed by the molecules is utilized in the creation of excited states. Regular chemical reactions are said to proceed through ground state mechanisms. Light absorption also helps in crossing the activation energy barriers associated with chemical reactions.
Light absorption obeys quantum mechanical rules. Light quantum has energy as given by the expression,
E = hv
Where h is the Planck's constant with a value of 6.6 x [10.sup.-27] erg.sec and v is the photon frequency in Hz. When a light quantum (photon) is absorbed by a molecule in its ground state [E.sub.1] and excited to a final state [E.sub.2], then
[delta]E = [E.sub.2] - [E.sub.1] = hv
The fundamental principles governing the photon absorption are the following: (1) Grothuss-Draper Law: Light absorbed in a system is only effective in producing a chemical change. (2) Beer-Lambert Law: The fraction of the incident light absorbed by a medium depends on the concentration c of the absorbing molecules in them and the thickness t of the layers through which light passes. Mathematically, it is expressed as
I = [I.sub.0] [10.sup.-Ect]
where [I.sub.0] is the intensity of the incident light, I the intensity of the transmitted light, a the molar extinction coefficient. [epsilon] expresses the probability of electronic transitions. (3) Stark-Einstein Law of Photochemical Equivalence: The amount of light absorbed is limited to one quantum per molecule taking part in a reaction. Such a relation is not observed in multiphotonic reactions induced by lasers. (4) Quantum Yield: The efficiency of a photochemical process is expressed in terms of quantum yield [phi], which is defined as,
[phi] = (Number of molecules reacting or formed)/(Number of quanta absorbed)
When an electron is excited, it goes to an anti-bonding orbital from a bonding orbital[4,5,6]. The electron in the excited state need not have the same set of quantum numbers as the electron with which it was paired in the bonding orbital in the ground state before absorption. The electron in the new orbital can have either parallel or opposite spin. The state in which the spins are antiparallel is called the singlet state, and the one in which the spins are parallel is called the triplet state. These states are respectively having multiplicity numbers of 1 and 3 as calculated from the 2S+1 rule, where S is the total spin (0 or 1). Among the electronic transitions, singlet-singlet and triplet-triplet transitions have a high probability and are said to be allowed. On the other hand, singlet-triplet and triplet-singlet transitions are forbidden. Forbidden transitions are recognized based on spin and symmetry considerations.
The ease and extent of photochemical transformations are controlled by the fate of the excited states. The energy acquired from light by the excited state may be drained by various mechanisms involving emission (fluorescence and phosphorescence), dissipation as heat, interchange of excited states, energy transfer to other molecules, and finally chemical reactions. The non-radiative physical processes are designated as internal conversion and intersystem crossing. The radiative processes are fluorescence and phosphorescence. The excitation process and the photophysical pathways are customarily represented pictorially in a Jablonski diagram as shown in Figure-2.
One process relevant to be discussed is sensitization. When the energy of an excited state exceeds the energy of a neighboring molecule, energy transfer may occur under designated spin conservation conditions. For example, benzophenone triplet state (donor) is known to transfer its energy to other molecules, and the newly formed triplet state of the molecule (acceptor) undergoes photophysical and photochemical processes. If D represents the donor or sensitizer and M the acceptor, then the sensitization process may be represented by the following steps:
D + hv [right arrow] [D.sup.*] (singlet)
[D.sup.*] (singlet) [right arrow] [D.sup.*] (triplet)
[D.sup.*] (triplet) + M [right arrow] D + [M.sup.*] (triplet)
M can be solvent or another solute molecule. If the object is to remove the energy from [D.sup.*], then M is said to be a quencher and the process is described as quenching. A simple case of energy transfer may be visualized in a system where the emission spectrum of the donor overlaps with the absorption spectrum of the acceptor. Some of the WV inks exploit the sensitization process as when a photoinitiator with potential polymerizing capability does not absorb in the normal UV region. Here, the sensitizer absorbs light and passes the energy on to the initiator that eventually breaks to yield intitating radicals or cations.
The actual polymerization steps are similar in EB and UV technologies. In the EB case, the excited monomer or solvent forms the initiating radicals, and in the UV case the photoinitiator absorbs light directly or indirectly producing radicals. Once the intitating radicals are formed irrespective of the process, they act on the acrylic monomer or oligomer to initiate and propagate the addition polymerization sequences. Usually acrylic compounds are the polymerizing species, which could even be mixtures of monomers and oligomers.
A general radical initiated polymerization reaction may be represented as follows.
(R-R) Initiator + hv [right arrow] 2R
Monomer + electron [right arrow]
[R.sup.*] + [H.sub.2]C=CHR' [right arrow] R-[H.sub.2]C-[CHR.sup.'*]
R-[H.sub.2]C-[CHR.sup.'*] + [H.sub.2]C=CHR' [right arrow]
n [H.sub.2]C=CHR' [right arrow]
The last step repeats itself by adding to other monomers forming the polymer. The growing radical chains terminate either by joining of two chains (combination) or by abstracting an atom from one of them (disproportionation).
This article does not attempt to cover many other aspects of EB and WV technologies, but focuses on the early events occurring immediately after the absorption of radiation by the molecules in these inks. Understandably, the curing process is nothing but the polymerization of low molecular weight compounds.
Joy T. Kunjappu received his Ph. D. in organic photochemistry in 1985 and D. Sc. in physical chemistry of surfactants in 1996. Prior to his arrival to the U.S. in 1987, he served as a senior scientific officer with the Department of Atomic Energy of India specializing on many aspects of chemistry. He worked as a postdoctoral research scientist (1987-1989) and associate research scientist (1994-1996) at the Langmuir Center for Colloids and Interfaces and the Chemistry Department of Columbia University, New York.
He has authored about 80 publications that include original research papers, review articles, general articles, book chapters, book reviews and symposium proceedings comprising the areas of surfactants, polymers, photochemistry, radiation chemistry, sterilization, spectroscopy and inks. He also served as the reviewer of technical and scientific papers of nine international publications. In 1989, he edited a special issue of Colloids and Surfaces (Aspects of Interfaces) as a guest editor.
Recently he has been appointed as deputy director general of IBC and research board member of directors of ABL Currently, Dr. Kunjappu is functioning as a consultant with "Chemicals and Consulting" and as an adjunct faculty at Yeshiva University, New York.
(1.) A portion of this article has been included in the following published material due to overlapping interests in the subject matter. Many other aspects of this topic could be obtained in that article: Joy T. Kunjappu, "Radiation Chemistry in EB- and UV Light Cured Inks," Paint and Coating Industry, October, 2000.
(2.) Radiation Chemistry, Farhataziz and A. A. J. Rodgers, (eds.), VCH publishers, 1897.
(3.) Joy T. Kunjappu and K. N. Rao, Radiation Physics and Chemistry, 13 (1979) 97.
(4.) N. J. Turro, "Modern Molecular Photochemistry," The Benjamin/Cummings Publishing Co., Inc., California, 1978.
(5.) P. Suppan, "Chemistry and Light," Royal Society of Chemistry, England, 1994.
(6.) Joy T. Kunjappu, "Fluorescence in Inks," American Ink Maker, December 2000, p. 24.
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|Author:||Kunjappu, Dr. Joy T.|
|Date:||Mar 1, 2001|
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