Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2002-12-23
2004-08-17
Berman, Susan (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S026000, C522S029000, C522S168000, C522S170000, C522S181000, C522S182000
Reexamination Certificate
active
06777460
ABSTRACT:
FIELD OF THE INVENTION
Novel catalyst systems provide cationically photocured compositions with reduced color, improved degree of cure, and increased thermal stability. Such compositions are useful as optically colorless or low color coatings, adhesives, encapsulants, sealants and abrasives for optical and electronic applications.
BACKGROUND OF THE INVENTION
Salts that have an organic, inorganic or organometallic cation and a nonnucleophilic counteranion are widely used in industry as initiators, particularly photoinitiators, for cationic addition polymerization reactions. Common initiator salts include onium salts such as diaryliodonium, triarylsulfonium, and (cyclopentadienyl)(arene)iron
+
salts of the anions PF
6
−
, AsF
6
−
, or SbF
6
−
. It has been well documented that the identity of the anion associated with the onium cation can significantly affect the rate of polymerization as well as the color and thermal stability of the resulting cured polymer. For example when a sulfonium SbF
6
−
salt is used to catalyze photocationic addition polymerization reactions, highly colored cured materials are generally produced whereas when a sulfonium PF
6
−
salt is used as the catalyst essentially colorless polymers result. Measurement of the degree of cure of these two systems, however reveals that while the sulfonium PF
6
−
catalyst produces a colorless product there is a high level of unreacted epoxy monomer present in it, producing thermal instability in the cured product. And the sulfonium SbF
6
−
salt, while producing a highly colored material, has essentially no unreacted epoxy as determined by infrared spectroscopy.
Recently it has been found that polymers catalyzed by anions having three highly fluorinated alkylsulfonyl, fluorinated arylsulfonyl or perfluoroalkylsulfonyl groups, and combinations thereof, exhibit very strong catalytic activity. For example, epoxy formulations that have a tris-trifluorosulfonylmethide (“methide”) anion provide very rapid cures in cationic polymerization reactions. Although methide-containing initiators seem to be a direct replacement for SbF
6
−
anion-containing initiators in cationic addition polymerization reaction systems as far as cure rate is concerned, these systems impart significantly reduced thermal stability to the cured material.
SUMMARY OF THE INVENTION
Briefly, the present invention provides a curable composition comprising
a. at least one cationically curable monomer, and
b. a curing agent comprising Photocurative A or Photocurative B, wherein
Photocurative A comprises
1) an effective amount of an accelerating photochemically active salt comprising a photochemically active cation and an accelerating anion, and
2) an effective amount of an inhibiting non-photochemically active salt comprising a non-photochemically active cation and an inhibiting anion; and
Photocurative B comprises
1) an effective amount of an inhibiting photochemically active salt comprising a photochemically active cation and an inhibiting anion, and
2) an effective amount of an accelerating non-photochemically active salt comprising a non-photochemically active cation and an accelerating anion;
wherein the inhibiting and accelerating anions are defined by photo differential scanning calorimetry (pDSC) and thermal differential scanning calorimetry (tDSC) such that anions of triarylsulfonium salts that have a total DSC exotherm energy value in the range of 0 to 300 Joules per gram (J/g), preferably 0 to 250 J/g, and more preferably 0 to 200 J/g, are classified as inhibiting anions, and anions of triarylsulfonium salts that have a total DSC exotherm energy value in the range of greater than 300 to 750 Joules per gram (J/g), preferably 325 to 600 J/g, and more preferably 350 to 500 J/g, are classified as accelerating anions.
In another aspect, the present invention provides curatives for producing high performance, cationically cured compositions, the curatives being designated Photocurative A and Photocurative B, as defined above.
In yet another aspect, the present invention provides a cured composition comprising the curable composition described above that has been subjected to curing conditions. These compositions preferably have one or more of the following characteristics: increased degree of cure, increased T
g
, increased thermal stability, and reduced color or colorless cured compositions compared to conventional cationically cured compositions.
In a further aspect, the present invention provides a method for photopolymerizing a cationically polymerizable composition comprising Photocurative A or Photocurative B, as defined above, and subjecting the composition to photocuring conditions to produce high performance compositions.
Cationically polymerizable materials that can be cured or polymerized by the curatives of the present invention, using the latter in a catalytically effective amount, are those known to undergo cationic polymerization and include 1,2-, 1,3-, and 1,4-cyclic ethers (also designated as 1,2-, 1,3-, and 1,4-epoxides), vinyl ethers, N-vinyl compounds, ethylenically unsaturated hydrocarbons, cyclic formals, and cyclic organosiloxanes. An extensive list of cationically polymerizable monomers which can be used in this invention are given in U.S. Pat. Nos. 3,347,676 and 3,842,019, which patents are incorporated herein by reference.
We have found that the addition of additional anions to a conventional photochemically active initiator salt can improve the properties of a cationically cured composition depending upon the nature of the added anion and the anion of the photochemically active salt. It is the added anion that produces the effect. The anion can have any cation associated with it that does not interfere with the photochemical activity of the initiator salt.
This invention provides an improved catalyst system for cationically photocurable compositions. The invention utilizes conventional cationic initiators with the addition of effective amounts of additional anions. Choice of optimal initiator/anion combinations provide compositions with high performance characterics not previously achievable using conventional curatives. Colorless or low color compositions of the present invention can represent an improvement in the art compared to compositions made with conventional cationic initiator systems that can impart a deep yellow/brown color to the cured composition or have poor curing characteristics.
In this application:
“colorless”, “low color”, and “reduced color” means colorless to the naked eye, or reduced or lower in color compared to a corresponding polymer cured by a conventional photocurative. Conventional photocuratives are those known in the art to cure cationic monomers. Examples of conventional curatives include the photoinitiators mentioned in the background of this application and are disclosed, for example, in U.S. Pat. Nos. 4,250,311; 3,708,296; 4,069,055; 4,216,288; 5,084,586; 5,124,417; 4,985,340; 5,089,536; and 6,025,406;
“high performance” means having at least one of the following characteristics: increased degree of cure, increased T
g
, increased thermal stability, reduced color or colorless cured compositions compared to conventional compositions;
“initiator” means a photochemically active salt;
“normal cure conditions” is defmed as comprising the steps of exposing the sample to light, generally on the order of 1 to 2 Joules of UVA and if desired heating the composition to between 100 to 150° C. to complete cure;
“photochemically active compositions” are those that are cationically curable, such as epoxy and epoxy containing materials, and comprise a photochemically active salt;
“photochemically active salt” is one that, upon photolysis, produces an acid (can be a Bronsted (H
+
) or Lewis acid (M
+n
)) capable of initiating polymerization of cationically curable material;
“resin mixture” means a mixture of curable materials without a photochemically active salt;
“substituent” or “substituted” group means halogen or any other group that doesn&apo
Gryska Stefan H.
Palazzotto Michael C.
3M Innovative Properties Company
Berman Susan
Sherman Lorraine R.
Weiss Lucy C.
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