Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1998-07-15
2001-04-03
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S331000, C428S404000, C428S407000
Reexamination Certificate
active
06210790
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to modified silica-polymer composites and methods for their preparation.
BACKGROUND OF THE INVENTION
In many applications, the formability, flexibility, lower processing temperatures, resistance to breakage and lighter weight of transparent plastics are features which make these materials attractive as substitutes for glass. However, plastics also have inherent drawbacks in comparison to glass. For example, poor abrasion resistance causes transparent plastics to scratch and haze, making them unsuited for glazing.
One means of making a plastic with glass-like properties has been to utilize a matrix of a highly crosslinked polymer with a finely divided silica. These materials are intrinsically hard and have been applied as coatings to the surface of plastic articles to improve abrasion resistance. Such abrasion resistant hardcoat compositions based on the photoinduced free radical polymerizations of multifunctional acrylates have been described recently in U.S. Pat. No. 5,708,048 and are also commercially available.
UV-cured plastics can be processed at higher rates and consume far less energy than thermally-cured systems. In addition, these materials can be applied to even thin plastics with low glass transition temperatures without causing distortion and warping usually encountered in processing at higher temperatures. In these compositions, silyl acrylate coupling agents undergo hydrolysis by water present in the aqueous dispersion of colloidal silica. By this reaction, the alkoxy groups are replaced by hydroxy groups which hydrogen bond to, or form covalent bonds with, the hydroxy groups present on the surface of the silica particle.
However, these materials possess drawbacks that diminish their attractiveness. The sensitivity of free radical polymerizations to inhibition by oxygen often causes a soft layer to be formed at the exposed surface. On the other hand, it is difficult to stabilize these materials against adventitious free radical polymerization in the absence of oxygen. As a result, the materials have relatively short shelf lives and tend to gel on standing.
Cationic photopolymerizations have the advantage that they display no appreciable oxygen inhibition and in addition, using this approach, a wide variety of different monomer systems, including epoxies, vinyl ethers, and oxetanes may be used. However, until now, it has not been possible to prepare silica-polymer composites having glass-like properties by cationic photopolymerization. There are several reasons for this. First, cationic photoinitiators are not useful with acrylate monomers. Second, a process for preparing composites based on acrylate polymers entails surface modification of colloidal silica with acrylate groups by hydrolysis and condensation of acrylate-functional alkoxysilane coupling agents in order to incorporate the silica into the polymer matrix. However, conditions required for the hydrolysis have prevented use of cationically polymerized epoxies: acid-catalyzed hydrolysis results in opening of the epoxy ring while use of basic catalysts causes inhibition of the cationic polymerization.
It is therefore an objective of this invention to provide a polymer composite which possesses glass-like properties. It is a further objective to retain the advantages of UV curing: a system which is solvent-free, low energy, and cures rapidly at low temperatures. It is still a further objective to overcome the difficulties inherent in using free radical initiators for polymerization.
SUMMARY OF THE INVENTION
The present invention achieves these objectives and provides for a surface-modified colloidal silica containing epoxy- or 1-alkenylether functionality covalently bonded to a surface of a finely divided silica particle, and a process for its preparation which incorporates the steps of mixing a colloidal silica, an epoxy- or 1-alkenyl ether-functional-trialkoxysilane and an ion-exchange resin and removing the ion-exchange resin. The functionality covalently bonded to the surface of the silica particle is a residue derived from an alkoxysilane chosen from the group consisting of:
wherein x=1 to 8. The colloidal silica may be in the form of an acidic aqueous dispersion.
In another embodiment, the invention provides for a photocurable composition comprising an epoxy- or 1-alkenyl ether-functional silica and a multifunctional epoxy- or 1-alkenyl ether monomer. Preferred monomers are 1,2-epoxy octane, 1,2-epoxydecane, 1,2,13,14-tetradecane diepoxide, 1,2,7,8-octane diepoxide, epichlorohydrin, limonene dioxide, &agr;-pinene oxide, dicyclo-pentadiene dioxide, 4-vinylcyclohexene dioxide, 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexenecarboxylate, cyclopentene oxide, cyclohexene oxide, cycloheptene oxide, cyclooctene oxide, 1,2,5,6-cyclooctadiene dioxide; butanediol diglycidyl ether, bisphenol-A-diglycidyl ether, bisphenol-A-extended glycidyl ethers, phenol-formaldehyde glycidyl ethers, cresol-formaldehyde glycidyl ethers; diglycidyl phthalate; epoxidized linseed oil, epoxidized soybean oil, epoxidized safflower oil, epoxidized sunflower oil, epoxidized rapeseed oil, epoxidized canola oil; silicone epoxy resins; epoxidized polybutadiene, epoxidized polyisoprene and epoxidized polystyrene-co-butadiene.
In yet another embodiment, a polymer composite is provided which is composed of a highly cross-linked silica-reinforced epoxy or 1-alkenyl resin wherein silica particles are covalently bonded to the resin. In still another embodiment, the present invention provides a method for producing composites by preparing an epoxy- or 1-alkenyl ether-functional colloidal silica; adding a multifunctional epoxy- or 1-alkenyl ether monomer; adding a polymerization initiator; applying the mixture to a substrate and curing, by exposure to heat or radiation.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of the invention contain a modified colloidal silica having epoxy or 1-alkenyl groups attached to the surface of the particle. In one aspect, the invention relates to these surface-modified silicas. In another aspect, the invention relates to a combination of the modified silica with a multifunctional epoxy or 1-alkenyl ether monomer. In yet another aspect, the invention relates to having the modified silica incorporated as part of a crosslinked polymer network based on epoxy or 1-alkenyl ether monomers.
The term colloidal silica includes silica in the form of a colloidal dispersion and is intended to represent a wide variety of finely divided SiO
2
forms which can be utilized to form the compositions of the invention. Further description can be found in U.S. Pat. No. 4,027,073, the disclosure of which is incorporated herein by reference.
Colloidal silica is a dispersion of submicron-sized silica (SiO
2
) particles in an aqueous or other solvent medium. Dispersions of colloidal silica are available from chemical manufacturers such as DuPont and Nalco Chemical Company. Colloidal silica is available in either acidic or basic form. For purposes of the present invention, it is preferable that the acid form be utilized. Alkaline colloidal silica may be converted to acidic colloidal silica by the addition of acids such as HCl or H
2
SO
4
with high agitation.
Nalco 1034, available from Nalco Chemical Company, is an example of a colloidal silica used in the compositions of the invention. NALCO 1034 is a high-purity, acidic aqueous colloidal silica dispersion in water, having a mean particle size of 20 nanometers (nm), and a low Na
2
O content, a pH of approximately 3.1 and an SiO
2
content of approximately 34% by weight. In the examples below, parts by weight of the colloidal silica includes an aqueous component of the dispersion. Thus, for example, in 520 grams of NALCO-1034, colloidal silica represents, approximately, 117 grams of SiO
2
by weight. Aqueous dispersion is a convenient way of handling the colloidal silica and does not form a necessary part of the compositions of the present invention. However, because water is required for hydrolysis of the alkoxy si
Heslin & Rothenberg, P.C.
Le H. Thi
Rensselaer Polytechnic Institute
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