Photoinitiators and applications therefor

Details Photoinitiators and applications therefor Photoinitiators and applications therefor

1999-09-28

2001-07-24

Seidleck, James J. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C522S001000, C522S049000, C522S050000, C522S051000, C522S053000, C522S063000, C522S066000

Reexamination Certificate

active

06265458

ABSTRACT:

TECHNICAL FIELD
The present invention relates to novel photoinitiators and methods for generating a reactive species using the photoinitiators. The present invention further relates to methods of polymerizing or photocuring polymerizable material using the above-mentioned photoinitiators. The photoinitiators of the present invention find particular utility in photocurable inks as used in ink jet printers or on a printing press with and without nitrogen blanketing.
BACKGROUND OF THE INVENTION
Polymers have served essential needs in society. For many years, these needs were filled by natural polymers. More recently, synthetic polymers have played an increasingly greater role, particularly since the beginning of the 20th century. Especially useful polymers are those prepared by an addition polymerization mechanism, i.e., free radical chain polymerization of unsaturated monomers, and include, by way of example only, coatings and adhesives. In fact, the majority of commercially significant processes are based on free-radical chemistry. That is, chain polymerization is initiated by a reactive species, which often is a free radical. The source of the free radicals is termed an initiator or photoinitiator.
Improvements in free radical chain polymerization have focused both on (1) more reactive monomer and pre-polymer materials and (2) the photoinitiator. Whether a particular unsaturated monomer can be converted to a polymer requires structural, thermodynamic, and kinetic feasibility. Even when all three exist, kinetic feasibility is achieved in many cases only with a specific type of photoinitiator. Moreover, the photoinitiator can have a significant effect on reaction rate which, in turn, may determine the commercial success or failure of a particular polymerization process or product.
A free radical-generating photoinitiator may generate free radicals in several different ways. For example, the thermal, homolytic dissociation of an initiator typically directly yields two free radicals per initiator molecule. A photoinitiator, i.e., an initiator which absorbs light energy, may produce free radicals by one of three pathways:
(1) the photoinitiator undergoes excitation by energy absorption with subsequent decomposition into one or more radicals;
(2) the photoinitiator undergoes excitation and the excited species interacts with a second compound (by either energy transfer or a redox reaction) to form free radicals from the latter and/or former compound(s); or
(3) the photoinitiator undergoes an electron transfer to produce a radical cation and a radical anion.
While any free radical chain polymerization process should avoid the presence of species which may prematurely terminate the polymerization reaction, prior photoinitiators present special problems. For example, absorption of the light by the reaction medium may limit the amount of energy available for absorption by the photoinitiator. Also, the often competitive and complex kinetics involved may have an adverse effect on the reaction rate. Moreover, some commercially available radiation sources, such as medium and high pressure mercury and xenon lamps, may emit over a wide wavelength range, thus producing individual emission bands of relatively low intensity. Many photoinitiators only absorb over a small portion of the emission spectra and, as a consequence, much of the lamps' radiation remains unused. In addition, most known photoinitiators have only moderate “quantum yields” (generally less than 0.4) at these wavelengths, indicating that the conversion of light radiation to radical formation can be more efficient.
Many commercially available photoinitiators, including IRGACURE® 369, are presently used in ink compositions to accelerate ink drying in “radiation-drying printing.” As used herein, the term “radiation-drying printing” refers to any printing method which utilizes radiation as a drying means. Radiation-drying printing includes, for example, off-set printing operations, such as on a Heidelberg press, flexographic printing, and flat-bed printing. Commercially available photoinitiator systems have a number of shortcomings. First, most of the commercially available photoinitiator systems require a relatively large amount of photoinitiator in the ink composition to fully cure/dry the ink composition. This leads to undesirable extractables within the ink composition. Second, most of the commercially available photoinitiator systems require a hi energy radiation source to induce photocuring. Moreover, even with the high energy radiation source, often the cure results are unsatisfactory. Third, many commercially available photoinitiator systems are highly reactive to oxygen and must be used under a nitrogen blanket. Fourth, even with a large amount of photoinitiator and a high energy light source, the commercially available photoinitiator systems require a dry/cure time only accomplished by multiple passes, as many as 15 passes, under a light source, which significantly limits the output of a radiation-drying printing press.
What is needed in the art is a new class of energy-efficient photoinitiators having unsurpassed photoreactivity even when exposed to a low energy light source, such as a 50 W excimer cold lamp. What is also needed in the art is a new class of energy-efficient photoinitiators that may be cured in air, as well as, a nitrogen atmosphere. Further, what is needed in the art is a class of photoinitiators having unsurpassed photoreactivity, for use in the radiation-drying printing industry, which will significantly increase the output of a radiation-drying printing press due to reduction in ink drying/curing time.
SUMMARY OF THE INVENTION
The present invention addresses some of the difficulties and problems discussed above by the discovery of energy-efficient photoinitiators having the following general formula:
wherein X
1
is a conjugated system such as one or more aryl groups or substituted aryl groups; Z
1
is —O, —S, an alkyl group having from one to six carbon atoms, an ester moiety, a ketone moiety, an amine moiety, an imine moiety, an ether moiety, an aryl or substituted aryl group, a metal or non-metal, or a metal or non-metal containing group, such as a zinc-containing group or a boron-containing group, respectively; and M
1
is an alkyl group, a substituted alkyl group, or forms a five-member ring with Z
1
. By selecting particular “X
1
”, “Z
1
”, and “M
1
” groups, photoinitiators are produced having a desired absorption maximum, which substantially corresponds to an emission band of a radiation source and selectively varies from less than about 222 nm to greater than about 445 nm.
The present invention is directed to the above-described photoinitiators, compositions containing the same, and methods for generating a reactive species which includes providing one or more of the photoinitiators and irradiating the one or more photoinitiators. One of the main advantages of the photoinitiators of the present invention is that they efficiently generate one or more reactive species under extremely low energy lamps, such as excimer lamps and mercury lamps, as compared to prior art photoinitiators. The photoinitiators of the present invention also efficiently generate one or more reactive species in air or in a nitrogen atmosphere. Unlike many prior photoinitiators, the photoinitiators of the present invention are not sensitive to oxygen. Further, the photoinitiators of the present invention are as much as ten times faster than the best prior art photoinitiators.
The present invention is further directed to a method of efficiently generating a reactive species by matching a photoinitiator having an absorption maximum to an emission band of a radiation source, which corresponds to the absorption maximum. By adjusting the substituents of the photoinitiator, one can shift the absorption maximum of the photoinitiator from less than about 222 nm to greater than 445 nm.
The present invention is also directed to methods of using the above-described photoinitiators to polymerize and/or photocure a polymerizable

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