AU546017B2

AU546017B2 - Ultraviolet light, cationic curable hard coating compositionsof polysiloxanes

Info

Publication number: AU546017B2
Application number: AU70375/81A
Authority: AU
Inventors: Rack Hun Chung
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1980-03-11
Filing date: 1981-03-02
Publication date: 1985-08-08
Anticipated expiration: 2001-03-02
Also published as: JPS57500247A; AU7037581A; KR840000820B1; EP0047299A4; IT1194138B; EP0047299A1; IT8120293A1; WO1981002579A1; KR830005289A; JPS6365115B2; IT8120293A0

Description

ULTRA VIOLET LIGHT, CATIONIC CURABLE HARD COATING COMPOSITIONS OF POLYSILOXANES.

Background of the Invention This invention relates to an ultraviolet radiation curable protective coating composition. More particularly, it relates to a silicone coating composition which, when applied to a substrate, forms a protective, abrasion-resistant coating thereon.

Recently, the substitution of .glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example,transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eyeglasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.

While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and/or ordinary weathering. Continuous scratching and marring results in impared visibility and poor aesthetics, and often times requires replacement of the glazing or lens or the like. One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan^® sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethyimethacrylate, are also widely used transparent plastics for glazing.

Attempts have been made to improve the abrasion-resistance of transparent plastics. For.example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Patents 3,708,225, 3,986,997 and 3,976,497, for example, describe such compositions. In copending U.S. Application Serial No. 964,910 coating compositions having improved resistance to moisture and humidity and ultraviolet light are disclosed. It. was discovered therein that, in direct contrast to the teachings of U.S. Patent 3,986,997, compositions having a basic pH, i.e., 7.1 - 7.8, do not immediately gel but in fact provide excellent abrasion-resistant coatings on solid substrates.

The present invention offers a significant advantage over many of the heretofore known coating compositions in that it does not require heat in order to initiate the cure reaction. The radiation cure system of the present invention expends considerably less thermal energy than conventional heat cure systems.

Ultraviolet light is one of the most widely used types of radiation because of its relatively low cost, ease of maintenance, and low potential hazard to industrial users. Rapid photo-induced polymerization utilizing UV light rather than thermal energy for the curing of hard coating offer several other significant advantages. First, faster curing coatings offer substantial economic benefits. Furthermore, heat sensitive materials can be safely coated and cured with UV light without the use of thermal energy which could damage the substrate. Additionally, the essentially solvent free media reduces the necessity for expensive and time consuming pollution abatement procedures.

Summary of the Invention The invention is accomplished herein by a coating composition comprising a mixture of ingredient (A) which is the acid hydrolysis product of an alkoxy-functional si lane and ingredient (B) which is the acid hydrolysis product of an acryloxy-functional si lane or the acid hydrolysis product of a glycidoxy-functional silane, or mixtures thereof. To the mixture of ingredients A and B is added a catalytic amount of a cationic photoinitiator which is effective for facilitating a cure reaction of the hydrolysis products. Such a cationic photoinitiator can be a radiation sensitive halonium, phosphonium or sulfonium salt. An additional amount of a radical type photoinitiator as opposed to the above- described cationic type can be combined with the cationic photoinitiator. This radical photoinitiator facilitates a cure reaction on the part of the acryloxy-functional portions of the hydrolyzed silanes, thereby providing a hard coating having an even tighter cure and exhibiting improved abrasion-resistance. Detailed Description of the Invention One of the major constituents of the coating composition of the present invention is ingredient (A) which is the hydrolysis product of an alkoxy-functional si lane. Such a si lane will ordi- rily have the following general formula:

(I) - si - (OR²)_4-a wherein R ¹ and R² are the same or different monovalent hydrocarbon radicals, including halogenated species of such radicals. Prefer- . ably, R ¹ and R² will be lower alky! radicals such as methyl, ethyl, propyl, etc., but may include other saturated and unsaturated species including vinyl, aryl, etc. The letter a is aninteger from

0 to 3 such that there are 4-a alkoxy groups in the si lane molecule.

Since tetra-alkoxy silanes are particularly effective, a will often equal zero.

The hydrolysis product of such silanes is obtained by contacting the silanes with an excess of water in the presence of a catalytic amount of acid. When less than a stoichiometric amount of water is utilized, a partial-hydrolyzate is obtained. Such partial-hydrolyzates can also be used to obtain the hard coatings of present invention. Among the particularly useful alkoxy- functional silanes are the following: Tetraethoxysilane, ethyl triethoxysi 1 ane , diethyl di ethoxsi 1 ane , tri ethy1 ethoxysi 1 ane, tetramethoxysilane, methyl trimethoxysi lane, dimethyl dimethoxysi lane, and trimethylmethoxysilane. The second major ingredient is ingredient (B) which is the acid hydrolysis product of a acryloxy-functional si lane or the acid hydrolysis product of a glycidoxy-functional si lane or mixtures thereof. The acryloxy-functional si lane has a general formula given by

(II) wherein R ³ and R ⁴ are the same or different monovalent hydrocarbon radicals as described above for R ¹ and R². R⁵ is a divalent hydrocarbon radical having from 2 to 8 carbon atoms. R⁶ is a hydrogen or a monovalent hydrocarbon radical. The letter b is an integer from 1 to 3, c is an integer from 0 to 2 and d is an integer equaling 4-b-c. In many of the embodiments of the present invention, b will ordinarily be 3, c will be 0 and d will equal 1.

Specific examples of acryloxy-functional silanes include:

3-methacrylQxypropyltrimethoxysilaιιe

3- aeryloxypr opy 1 triaietfioxysilane 2-methacryloxyethylurimethoxysilane 2-acryloxyethyltrimethoxysilane 3-methacryloxypropyltrimethoxysilane 3-acryloxypropyltriethoxysilane

2-methacryloxyethyltriethoxysilane 2-acryloxyethyltrie-thoxysIlane

Such acryloxy-functional silanes are corπmerically available. For example, 3-methacryloxypropyl tri methoxysi lane can be obtained from Union Carbide. The second major constituent (Ingredient B) of the coating composition may also be a glycidoxy-functional silane instead of the acryloxy-functional silane just described, or it may be a combination or mixture of both types of silanes. A glycidoxy-functional silane has the general formula given by (III)

(III)

wherein R⁷ and R⁸ are the same or different monovalent hydro carbon radicals, as described above for R ¹ and R² R⁹ is a divalent hydrocarbon radical having from 2 to 8 carbon atoms The letter e is an integer from 1 to 3, f is an integer from 0 to 2 and g is an integer equaling 4-e-f. Specific examples of useful glycidoxy-functional silanes are the following:

3-glycidoxypropyltrimethoxysilane 3-giycidoxyathyltrimethoxysilane

3-glycidoxypropyItriethoxysilane 3-glycidoxyethyltriethoxysilane

These glycidoxy-functional silanes are also commercially available. One source, for example, is Petrarch Systems, Inc. The ultraviolet radiation curable coating composition of the present invention will be comprised of 100 parts by weight of the acid hydrolysis product of the alkoxy-functional silane given by formula I which is combined with from approximately 10 to 1000 parts by weight. of either the acid hydrolysis product of the acryloxy-functional silane given by formula II or the glycidoxy-functional silane given by formula III, or combinations thereof. To this mixture must be added a catalytic amount of a cationic photoinitiator. Effective photpinitiators are radiation sensitive aromatic halonium, sulfonium or phosphonium salts which have been described in the literature. Cationic photoinitiators have been described by Crivello in numerous U.S. Patents and applications, such as the following, for example, which are hereby incorporated by reference: U.S. 4,136,102 issued.January 23, 1979 and U.S. 3,981,897 issued September 21, 1976. Such cationic photoinitiators can have the general formula given by (IV In this formula, X is a radical selected from I, P or S. M is a metal or metalloid and Q is a halogen radical selected from Cl, F, Br, or I. R¹⁰ is hydrogen or a monovalent hydrocarbon radical having from 1 to 12 carbon atoms. The letter h is an integer having the value of 4 to 6 inclusive, an n is an integer having the value of 2 or 3. The expression[MO_h] applies to any number of ionic species but preferably will be selected from SbF₆-, AsF₆, BF₄- and PF₆-.

It is ordinarily preferable to utilize approximately 0.20. parts by weight of the cationic photoinitiator for every 100 parts by weight of the mixture of ingredients A and B as described above.

However, depending upon individual desired process parameters such as rate of cure and ultimate abrasion-resistance, the amount of the photoinitiator can range from approximately .01 to 5 parts by weight per 100 parts of the mixture of ingredient A and B. The cationic photoinitiators are particularly effective for initiating a cross-linking reaction between the hydrolyzed alkoxy groups of the compositions given by formulas I, II, and III upon exposure to ultraviolet radiation. Good hard coatings having excellent adhesion can thus be obtained when the coating composition is applied to a substrate and exposed to radiation such as that provided by UV lamps.

Improved abrasion-resistance can be obtained with the same hard coating compositions when in addition to the cationic photo- initiators described above, there is also utilized a radical-type initiator which is effective for cross-linking or self-condensing the acryloxy-functional portions of the silanes contained in the composition. Such radical photoinitiators include among others, benzoin ethers, alpha-acyloxime esters, acetophenone derivatives, benzil ketals and ketone-amine derivatives. Specific examples of these photoinitiators include ethyl benzoin ether, isopropyl beπsoin ether, and dimethoxyphenyl acetophenone.

The acid hydrolysis products of ingredients A and B can be effectively catalyzed to form satisfactory radiation curable hard coatings by combining 100 parts by weight of such hydrolysis products and mixtures with from approximately, 0.5 to 5.0 parts by weight of a combination of photoinitiators. The photoinitiator combination will be comprised of, approximately, 10 to 90% by weight of a cationic-type initiator such as diphenyliodonium- hexafluoroarsenate and the remaining portion is a radical -type initiator such as ethyl benxoin ether. Alternative embodiments of the present invention are achieved when the coating composition as discussed above is optionally further combined with from 5 to 50 parts by weight of additional acryloxy-functional. materials such as trimethylolpropanetriaerylate. The UV-curable coating composition of the present invention is ordinarily coated on at least one surface of some solid substrate. The solid substrate may be comprised of. a synthetic organic polymer or a metal or even glass, also included are synthetic organic polymer substrates which themselves have a metallized surface. Prior to the composition being coated upon a substrate there may optionally be included .a priming step wherein a primer such as a thermosetting acrylic emulsion could first be applied to the substrate. After the coating composition is applied to the substrate or the primed substrate, the coating may be cured thereon by an effective amount of UV-radiation, which may be obtained from for example, a. Hanovia 550 watt lamp or a PPG Processor, Model QC1202.

The coating compositions of the present invention can be applied to a variety of solid substrates, by conventional methods, such a flowing, spraying or dipping, to form a continuous surface film. Optimum coating thicknesses are obtained by slow dip coating procedures. Substrates which are especially contemplated herein are transparent and non-transparent plastics and metals. More particularly, these plastics are synthetic organic polymeric substrates such as acrylic polymers like poly-(methylmethacrylate), polyesters, such as poly(ethylene terephthalate), poly(butyl ene terephthalate), etc, polyamices, polyimides, acrylonitrile- styrene copolymers, styrene-acrylonitrile-butadiene copolymers,. polyvinyl chloride, butyrates, polyethylene and the like. The coating compositions of this invention are especially useful as coatings for polycarbonates, such as poly(bisphenol-A carbonate) and those polycarbonates known as lexan^®, sold by General Electric

Company, and as coating for injection molded or extruded acrylics, such as polymethylmethacrylatεs. Metal substrates on which the present protective coatings are also effective include bright and dull metals like aluminum and bright metal ized surfaces like sputtered chromium alloy. Other solid substrates contemplated herein include wood, painted surfaces, leather, glass, ceramics and textiles. By choice of the proper formulation, application conditions and pretreatment of the substrate including the use of primers, the coatings can be .adhered to substantially all solid substrates. A hard coating having all of the aforementioned characteristics and advantages is obtained by the removal of any residual solvent and volatile materials such as methanol or ethanol byproducts of the hydrolysis reactions. Note that except for such residual moieties the present invention provides essentially sol ventless coating compositions.

Coating thicknesses may vary but for improved abrasion- resistance coating thicknesses of 3-10 microns and preferably 5 microns, are utilized.

In order that those skilled in the art may better understand how to practice the present invention, the following examples are given by way of illustration and not by way of limitation.

Example 1. Into a 200 ml three-necked flask equipped with a thermometer, a magnetic stirring bar and ice bath, was placed a mixture of 52 grams (0.25 mole) of tetraethoxysilane (TES) and 9 grams (0.5 mole of water. This heterogeneous solution was cooled to approximately 0 to 3 C whereupon 0.4 grams of perchloric acid was added. The reaction mixture was stirred as the ice melted away. After removal of some insoluble particles by filtration, a clear solution of TES-hydrolyzate having a viscosity of 5.6 centisstokes was obtained. Next, a second emulsified solution was obtained by mixing 24.8 grams (0.1 mole) of 3-methacryloxypropyltrimethoxysilane (MPTMS) and 2.7 grams (0.15 mole) of water to which 2 drops of perchloric acid were added at room temperature. After stirring overnight at room temperature, a pale, greenish solution of MPTMS-hydrolyzate was obtained having a viscosity of 8.6 centi- stokes. Into a mixture of 5 grams of TES-hydrolyzate and 4 grams of MPTMS-hydrolyzate was added 60 mg of a cationic catalyst, diphenyliodoπiumhexafluoroarsenate. A clear solution was obtained which was flow coated upon a sheet of lexan^®and the,coated panel was dried at room temperature for 30 minutes, whereupon it was irradiated under a single Hanovia 550 watt lamp. A hard coating with good adhesion was obtained within 12 to 60 seconds.

When the same process was repeated. on a PPG QC1202 UV processor, similar hard coatings with good adhesion were obtained in as little as 3 seconds.

Example 2 To a mixture of eight parts of tetraethoxysilane, four parts by weight of 3-methacryloxypropyltrimethoxysilane (MPTMS), and a catalytic amount of perchloric acid, was added three molar equivalents of water at ice-bath temperature. The reaction mixture was sitrred overnight at room temperature. One hundred parts of the resultant hydrolyzate was combined With 16 parts by weight trimethyl opropanetriaerylate and 2.0 parts of a mixture of photoinitiators comprising 50% by weight ethylbenzoin ether. This catalyzed mixture was flow coated on lexan^®and subsequently air dried for 30 minutes whereupon it was cured on a UV processor. The cured composition showed good adhesion and it was cured on a UV processor. The cured composition showed good adhesion and abrasion-resistance, and required only 3 to 6 seconds to cure.

Example 3 To a mixture of 152.2 g (1 mole) of tetramethoxysilane (TMS) and 36 g (2 mole) of water was added 1 g of perchloric acid at ice-bath temperature. The reaction mixture was then stirred overnight (approximately 16 hours) as the ice melted. To 50 parts by weight of this TMS-hydrolyzate was added 40 parts MPTMS-hydrolyzate as obtained in Example 1. To the resulting hydrolyzate mixture was added 1.5 parts by weight of a mixture of photoinitiators comprising 40% by weight diphenyliodonium hexafluoroarsenate and 60% by weight ethyl benzoin ether. This mixture was flow coated on lexan^®and drained for 30 min. whereupon it was cured under UV-light to vie a clear hard coating with good adhesion. Example 4 A mixture of 104 g (0.5 mole) of tetraethoxysilane and 18 g (1 mole) of water was cooled in an ice-bath. 0.5 g trifluoroacetic acid was then added. A clear TES-hydrolyzate was obtained after stirring overnight at room temperature. To 100 grams of this hydro lyzate was added 80 grams MPTMS hydrolyzate as obtained in Example 1. To 100 parts of this resulting hydrolyzate-was added 1.5 parts by weight of the 50-50 photoinitiator combination of Example 1. After the mixture was coated and air-dried upon lexan^®it cured to an abrasion resistant hard coating showing good adhesion upon exposure to ultra-violet radiation.

Example 5 Into a 500 ml, three-necked flask equipped with a thermometer, mechanical stirrεr, and ice-bath was placed 236 g (1 mole) of glycidoxypropyltrimethoxysilane (GPTMS) and 54 g (3 mole) of water. The suspension mixture was. cooled at the ice-bath temperature while 0.5 g of perchloric acid was added. This mixture was filtered and a.GPTMS-hydrolyzate was obtained. A mixture of 208 g (1 mole) of tetraethoxysilane, 27 g (1.5 mole) of water and 1 g of perchloric acid was heated to 80 C for 4 hrs. This resulted in a partial TES-hydrolyzate. To 10 parts by weight of the GPTMS- hydrolyzate was added 7 parts by weight of parital TES-hydrolyzate. To 100 parts by weight of this resulting hydrolyzate-partial hydrolyzate mixture was added 1 part by weight of the 50-50 photoinitiator combination of Example 1. The catalyzed mixture was flow coated upon lexan^®and cured to a hard coating showing good adhesion upon exposure to UV-radiation.

Obviously, other modifications and variations of the present invention are possible in the light of the above teachings. For example, additives and other modifying agents, such as pigments, dyes and the like,, may be added to the compositions of this invention. It is to be understood, however, that changes may be made in the particular embodiments described above which are within the full intended scope of the invention as defined in the appended claims.

Claims

1. A radi ati on curabl e coati ng composi ti on , compri si ng ;

A) 100 parts by wei ght of an aci d hydrolys i s product of an al koxy-functi onal s i l ane havi ng a formul a : - S i - (OR² ) _{4- a} wherein R ¹ and R² are the same or different monovalent hydrocarbon radicals, and a is an integer from 0 to 3;

B) 10 to 1000 parts by weight of an acid hydrolysis product of a compound selected from acryloxy-functional silanes having a general f wherein R ³ and R⁴ are the same or different monovalent hydro- carbon radicals, R⁵ is a divalent hydrocarbon radical having from

2 to 8 carbon atoms, R⁶ is hydrogen or a monovalent hydrocarbon radical, b is an integer from 1 to 3, c is an integer from 0 to 2, and d is an integer equaling 4-b-c; or glycidoxy-functional silanes having a general formula;

where R and R are the same or different monovalent hydrocarbon radicals, R⁹ is a divalent hydrocarbon radical having from 2 to 8 carbon atoms, e is an integer from 1 to 3, f is an integer from

0 to 2, and g is an integer equlaing 4-e-f, and mixtures thereof; and

C) a catalytic amount of cationic photoinitiator.

2. A composition as in Claim 1 wherein said alkoxy-functional silane is selected from the group consisting of tetraethoxysilane, ethyl triethoxysilane, diethyl diethoxysilane, triethylethoxysilane, tetra methoxysilane, methyl trimethoxysilane, dimethyl dimethoxysilane, and trimethyl methoxy silane.

3. A composi ti on as i n Cl aim 1 wherei n sai d acryl oxy-functi onal si l ane i s sel ected from the group consisti ng of

3-methaeryloxypropy1trimethoxysilane 3-acryloxγpropyltriraethoxysilane 2-methacryloxytrimethoxysilane

2-acryloxyethyltriπιethoxysilane 3-methacryloxyρroρyltrimethoxysilane 3-acrylc-xyproρyltriethoxysilane 2-methacryloxyethyltriethoxysilane 2-acryloxyethyltriethoxysilane

4. A composition as. in Claim 1 wherein said glycidoxy-functional silane is selected from the groups consisting of

3-glycidoxypropyltrimethoxysilane 3-glycidoxyethyltrimethoxysilane 3-glycidoxypropyltriethoxysilane 3-glycidoxyethyltxiethoxysilane

5. A composition as in Claim 1 wherein said photoinitiator is a radiation sensitive aromatic halonium or sulfonium .salt having a formula: (R¹⁰- C₆H₄) X⁺ [MO_h]-

wherein X is a radical selected from I, P or S; and M is a metal or metalloid and Q is a halogen nadical selected from Cl , F, Br, or I; R is hydrogen or a monovalent hydrocarbon radical having 1 to 12 carbon atoms, h is an integer having a value of 4 to 6 inclusive, and n is an integer having a value of 2 or 3.

6. A composition as in Claim 5 wherein [MO_h]- is selected from SbF₆-, AsF6-, BF₄-, and Pf₆-.

7. A composition as in Claim 1 further comprising approximately 5 to 50 parts by weight trimethylolpropanetriaerylate.

8. A composition as in Claim 1 wherein said cationic photoinitiator is present in an amount from approximately .01 to 5 parts by weight per 100 parts of said alkoxy-functional silane.

9. A composition as in Claim 5 wherein said cationic photoinitiator is further combined with a radical -type photointiator.

10. A composition as in Claim 9 wherein said radical-type photoinitiator is selected from ethyl benzoin ether, isopropyl bensoin ether, and dimethoxyphenyl acetophenone.

11. The cured product of Claim 1.

12. A solid substrate having at least one surface coated with the aqueous coating composition of Claim 1.

13. An article as defined in Claim 12 wherein the solid substrate is comprised of a synthetic organic polymer.

14. An article as defined in Claim 12 wherein said solid substrate is a metal.

15. An article as defined in Claim 12 wherein said solid substrate is a synthetic organic polymer having a metallized surface.

16. An article as defined in Claim 13 wherein said polymer is a transparent polymer.

17. An article as defined in- Claim 16 wherein said polymer is a polycarbonate.

18. An article as defined in Claim 17 wherein said polycarbonate is transparent.

19. An article as defined in Claim 17 wherein said polycarbonate is a poly(bisphenol-A carbonate).

20. An article as defined in Claim 12 wherein the coating composition has been cured on said surface of said solid substrate by an effective amount of ultraviolet radiation.

21. An article as defined in Claim 12 wherein said surface of said solid substrate has been primed with a primer composition prior to having been coated with the aqueous coating composition of Claim 1.

22. An article as defined in Claim 21 wherein said primer composition is comprised of a thermosetting acrylic emulsion.

23. An article as defined in Claim 13 wherein said polymer is polymethylmethacrylate.