Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2003-01-02
2004-02-24
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S586000, C524S588000, C351S163000, C351S164000, C351S165000, C351S166000
Reexamination Certificate
active
06696515
ABSTRACT:
This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2002-006021 filed in JAPAN on Jan. 15, 2002, which is herein incorporated by reference.
TECHNICAL FIELD
This invention relates to coating compositions for forming hard protective coats having weather resistance and optical articles having protective coats of the compositions.
BACKGROUND OF THE INVENTION
Transparent plastic articles have been used as the glass substitute owing to their advantages of light weight and ease of working. However, their surface is vulnerable, which prevents plastic articles from being directly used in the optical application. One approach for improving the surface to a glass equivalent level is surface treatment with various hard coating compositions. The coating compositions which are used in practice are generally divided into two systems, acrylic UV-curable system and silicone system. The acrylic UV-curable system capable of brief curing is advantageously used in the field where productivity is of importance, but not suitable in the field where long-term weather resistance is required, because the acrylic resin serving as the skeleton is less resistant to weathering. On the other hand, the silicone system is slow in cure and less productive, but siloxane bonds becoming the major skeleton after curing are strong. Particularly when substituent groups are methyl, the system exhibits no absorption in the UV region and good weather resistance. The silicone system is thus used in the field requiring a high degree of mar resistance and long-term weather resistance.
However, silicone base hard coating compositions have several problems. They cure by the mechanism that condensation proceeds through water- or alcohol-removing reaction in which Si—OH groups are involved (e.g., ≡Si—OH+HO—Si≡→≡Si—O—Si≡+H
2
O). Since the process entails a weight loss, the coat undergoes shrinkage during heat curing. Upon subsequent restoration to room temperature, the coat tends to incur stresses at the bonding interface due to a difference in coefficient of expansion from the substrate or a bond-improving primer thereon, increasing the likelihood of cracking. Although the silicone base coats are resistant to weathering, they lose adhesion and crack with the progress of time because the light transmitted thereby can gradually degrade the underlying substrate or a bond-improving primer thereon. Many attempts have been made to avoid the above phenomenon.
For example, JP-A 2001-79980 describes that polyethylene glycol is added to a hydrolyzed sol solution of a tetraalkoxysilane and a trialkoxysilane. Blending of the linear polymer in the system is effective for alleviating the stress generated upon curing and suppressing cracking immediately after film formation, but the cured film is less resistant to water because the polyethylene glycol is highly water absorptive. Upon exposure to light, the polyethylene glycol degrades with time and even decomposes and vaporizes off, rendering the film susceptible to cracking as before. The effect is not satisfactory.
Japanese Patent No. 3,145,298 discloses a silicone base hard coating composition obtained by partially hydrolyzing a silane compound having 1 to 4 hydrolyzable groups in colloidal silica wherein an acrylic resin having both epoxy groups and hydrolyzable silyl groups on side chains is added for the purpose of imparting toughness to coatings thereof. In this system, generation of cracks is noticeably suppressed. The acrylic resin is incorporated in the cured system as a result of the hydrolyzable silyl groups crosslinking with the silicone resin and thus imparts insufficient flexibility. Because of the lack of UV absorbing groups, the coating fails to achieve sufficient protection from UV to the surface to be coated (bonding interface). The coating tends to lose adhesion with the progress of time and eventually peel off.
JP-A 2001-220543 describes that UV absorbing groups are introduced by way of copolymerization into the acrylic resin having hydrolyzable silyl groups in the system of Japanese Patent No. 3,145,298, and a polyorganosiloxane terminated with Si—OH groups and containing phenyl groups is blended. This patent is successful in improving the degradation upon prolonged exposure to light, but is still insufficient for crack prevention because the flexibility imparting component is fixed by curing and crosslinking.
JP-A 11-58654 proposes a system in which a UV absorber, 2,2′,4,4′-tetrahydroxybenzophenone is added to a polyorganosiloxane solution containing colloidal silica. In this system, the UV absorber is not fixed within the coating and can volatilize off or be leached out due to strong hydrophilic nature. As a consequence, weather resistance is somewhat improved, but voids form within the coating with the progress of time, allowing for further shrinkage of the silicone coating. No improvement in crack prevention is made because no flexibility improver is added and cracks are likely to occur.
U.S. Pat. No. 5,679,820 discloses a system comprising a silicone hardcoat composition and an organic silicon compound having hydrolyzable silyl groups introduced into a benzophenone UV absorber of a specific structure. This system provides satisfactory light shielding property because the UV absorber is fixed. Since the coat has a reduced crosslinking density under the influence of the steric bulkiness of UV absorbing groups, the coat is made softened, failing to provide acceptable mar resistance. Crack resistance is not improved due to the lack of a flexibility imparting component.
There is a need to have a coating composition capable of forming a protective coat having high mar resistance on a substrate surface which coat is prevented from cracking with the progress of time and thus maintains its performance over a long period of time.
SUMMARY OF THE INVENTION
An object of the invention is to provide a coating composition for forming a hard protective coat having improved weather resistance, and an optical article having a protective coat of the composition.
It has been found that by blending a silicone resin with a vinyl polymer containing UV absorbing groups and hydrophilic groups, devoid of hydrolyzable silyl groups, and dissolvable or dispersible in water-containing organic solvents, there is obtained a coating composition capable of forming a hard protective coat which is flexible enough to prevent cracking and thus has improved weather resistance while maintaining a high level of mar resistance.
More particularly, for preventing initial cracks immediately after curing, it is effective to blend a linear vinyl polymer having flexibility in the system to thereby relieve the stress generated upon curing. If the vinyl polymer is fixed within the cured silicone resin, the polymer fails to exert sufficient flexibility. It is thus preferred that the polymer be devoid of hydrolyzable silyl groups so that no further crosslinking proceeds with time. However, since the vinyl polymer is not fixed within the silicone resin, the cured silicone coat becomes accordingly rather short of hardness. It is also preferred to introduce UV absorbing groups into the vinyl polymer for the purposes of protecting the polymer itself and a substrate to be coated. However, since the crosslinking density is reduced as a result of introduction of UV absorbing groups, the cured silicone coat becomes rather short of hardness.
To compensate for the shortage of hardness or mar resistance, the curable silicone resin in the silicone hard protective coat-forming coating composition must be completely cured. To this end, it is preferred that ends of the silicone resin be converted to Si—OH groups and that hydrolysis be conducted as completely as possible. To this end, hydrolysis must be conducted in the presence of excess water. As a consequence, the coating composition solution becomes a water-containing system. Then the vinyl polymer containing UV absorbing groups, but devoid of hydrolyzable silyl groups must be disso
Furuya Masahiro
Higuchi Koichi
Yamaya Masaaki
Birch & Stewart Kolasch & Birch, LLP
Sastri Satya
Shin-Etsu Chemical Co. , Ltd.
Wu David W.
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