High Refractive Index Hc Coatings Background Engineering Essay

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Plastic spectacle lenses, as those made from polycarbonates or diallyl diglycol carbonate resin CR-39 , have several advantages over lenses made from inorganic glasses. In addition to their excellent optical quality, optical plastics offer lower density and higher impact resistance. Unfortunately, the low abrasion resistance of these plastics causes a fast decrease of the optical quality of uncoated plastic ophthalmic lenses. Hard, abrasion-resistant coatings must be used in order to overcome this disadvantage. In the last years, hybrid organic-inorganic materials have been introduced in the market, with great success, as hard coatings for plastic lenses1-4. Most of these hard coatings are usually obtained by sol-gel processes5-8.

For example, a sol derived from the cohydrolysis of tetramethoxysilane (TMOS), 3-glycidoxypropyltrimethoxysilane (GPTMS), and titanium-tetraethylate (Ti(OEt)4) was applied via spin coating on top of CR 39 lenses and cured thermally at 110-C for several hours to form a highly crosslinked hybrid (inorganic-organic) network. This type of coating material has now been commercially available since more than 15 years ago. Its high scratch and abrasion resistance is presumably due to the in situ formation of titanium oxo clusters and a high degree of organic crosslinking via homopolymerisation of epoxy moieties9.

The incorporation of the titanium alkoxide resulted in a higher refractive index (nD =1.533) of the hybrid material in comparison to the simple poly(methyl)siloxanes used as scratch resistant coatings on top of polycarbonate (PC) since several decades10.In the latter formulations, solutions of colloidal silica have always been used to improve the mechanical properties of the resulting coatings and to reduce the cost of systems derived from organo(alkoxy)silanes. Meanwhile, the substitution of titanium alkoxides by zirconium and aluminium alkoxides had been described11 and the replacement of silica nanoparticles by AlOOH (boehmite) nanoparticles commercialized12 to protect ophthalmic lenses from mechanical abrasion and wear. The substitution of silicon by aluminum, zirconium or titanium in the inorganic network is effective to increase the hardness of hybrid coatings, which thereby show hardnesses close to soft glasses, like borosilicate glass.

By searching the related patents, it was found that Japanese Patent Publications No. 54331/1986 and No. 37142/1988 disclose a coating technique in which a colloidal dispersion of silicon dioxide fine particles used in the silicon based film-forming coating solution is replaced with a colloidal dispersion of inorganic oxide fine particles, such as those of Al, Ti, Zr, Sn or Sb oxide, having a high refractive index. Further, Japanese Patent Laid-Open Publication No. 301517/1989 discloses a process for preparing a compound sol of titanium dioxide and cerium dioxide; Japanese Patent Laid-Open Publication No. 264902/1990 discloses compound inorganic oxide fine particles of Ti and Ce; and Japanese Patent Laid-Open Publication No. 68901/1991 discloses a technique for treating compound oxide fine particles of Ti, Ce and Si oxides with an organosilicon compound to use for a coating solution. Moreover, Japanese Patent Laid-Open Publication No. 2102/1993 discloses a hard coat film containing compound oxide fine particles of Ti and Fe or compound oxide fine particles of Ti, Fe and Si. For example, when a colloidal dispersion of Al, Zr, Sn or Sb oxide fine particles is used in the coating compositions described in Japanese Patent Publications No. 54331/1986 and No. 37142/1988 for a high-refractive index resin lens having a refractive index of not less than 1.54, the interference fringe on the resulting coating film after curing can be reduced to a certain extent as compared with that of the silicon based coating solution. However, if fine particles of inorganic oxide of Al or Sb are used, there is a limit on the increase in refractive index of the coating film, and thus it is impossible to perfectly inhibit the interference fringe on the lens substrate having a refractive index of not less than 1.60. The reason therefor is considered that although these inorganic oxide fine particles per se have a high refractive index of not less than 1.60, they are generally mixed with an organosilicon compound or an epoxy resin as a matrix of a coating composition, so that the amount of the particles is reduced by the amount of the matrix in the composition, and thus the refractive index of the coat film is lower than that of the lens substrate.

Further, since the dispersibility of Zr or Sn inorganic oxide fine particles is unstable in the matrix, a transparent film cannot be obtained with a large amount of the particles. On the other hand, use of a colloidal dispersion of Ti inorganic oxide fine particles in a coating composition not only can provide the resulting film with a refractive index of about 1.60 or more because TiO2 itself has a higher refractive index than the aforementioned inorganic oxides, but also makes it possible to define an intended refractive index of the film within a wide range.

However, the film formed from a coating composition containing TiO2 has a disadvantage that TiO2 is extremely low in the weathering resistance, so that TiO2 induces decomposition of organic components in the composition such as the organosilicon compound or the epoxy resin and thus deterioration of the film on the surface of the resin substrate, resulting in reduced film durability. A further disadvantage of this film is poor adhesion to the substrate.

In the coating composition containing compound oxide fine particles of titanium dioxide and cerium dioxide described in Japanese Patent Laid-Open Publications No. 264902/1990 and No. 68901/1991, or in the coating composition containing compound oxide fine particles of titanium dioxide and iron oxide described in Japanese Patent Laid-Open Publication No. 2102/1993, titanium dioxide is used for improving its weathering resistance, in the form of a compound oxide with cerium dioxide or iron oxide, but the films obtained from these coating compositions are still insufficient in the weathering resistance. Additionally, cured films obtained from the coating compositions containing a compound sol of these oxides are more or less colored.

To solve the problems above, in US patent 5,789,476, the fine particulate compound oxide composed of a core comprising fine particulate titanium oxide and a cover thereon comprising silicon oxide and zirconium oxide was used in coatings. The compound oxide fine particles of this structure are preferred because stable weathering resistance can be assured. The compound oxide preferably has a mean particle diameter of 2 to 300 nm. When the mean particle diameter exceeds 800 nm, the resulting film may tend to be whitened and become opaque, whereas when the mean particle diameter is less than 1 nm, the formed film may show an insufficient hardness and thus poor scuffing resistance and abrasion resistance, and also the refractive index of the film cannot be increased sufficiently.

A film which contains a colloidal dispersion of titanium oxide alone is poor in the weathering resistance, so that deterioration of the film takes place because of lowering of adhesion between the film and the substrate or decomposition of the vehicle ingredient in the film. The reason why the unfavorable phenomenon takes place is presumably that titanium oxide absorbs ultraviolet light of 230 to 320 nm to be activated. The combined use of titanium oxide and zirconium oxide in the compound oxide can inhibit this activation of titanium oxide, and improve the weathering resistance of the titanium oxide much more than a compound oxide of titanium oxide.

By further combining these two fine particulate oxides with fine particulate, silicon oxide, and the resulting film can be enhanced in hardness and adhesion to the anti-reflection layer.

The latest developments in hard coatings by sol-gel derived coatings concentrated on UV curable systems, as long curing times are still necessary to achieve good adhesion and high abrasion resistance of hybrid systems on organic polymers of low thermal stability. Modern production technologies require high throughput and short processing times to be economic and competitive. To assure comparable mechanical properties, the hybrid network has to be designed to maximize the density and degree of condensation of the inorganic part of the final coating and reduce the space demanding size of the organic substituents. These developments have led to the commercialization of a protective coating for poly(methyl)methacrylate (PMMA) based polymeric lenses. The abrasion resistance of this UV cured hard coating proved to be fully comparable to other thermally cured systems in the relevant test procedures. The adhesion to the substrate is excellent and since 1998 the respective lenses have been produced in large quantities.

Technical Proposals

The proposals are to develop novel HI hard coatings either UV curable or thermal curable, in which the main components comprise a matrix and a fine particulate compound oxide.

1. Fine Particulate Compound Oxide

In this proposal, the fine particulate compound oxide will be composed of a core comprising fine particulate titanium oxide and a cover thereon comprising silicon oxide and zirconium oxide. The fine particulate compound oxides are preferably surface treated with an organosilicon compound. By this surface treatment, the dispersed state of the fine particulate compound oxide can be stabilized for a long period of time in the coating solution containing the compound oxide and the matrix, even when an ultraviolet curing resin is used as the matrix. Furthermore, the fine particulate compound oxide surface modified with an organosilicon compound has improved reactivity with and affinity for the matrix, so that a film formed from a coating solution containing the surface-treated fine particulate compound oxide is superior in hardness, transparency and scuffing resistance to a film formed from a coating solution containing a fine particulate compound oxide without the surface treatment.

Additionally, the surface-treated fine particulate compound oxide has much more improved affinity for a solvent used in the coating solution, as compared with compound oxide without the surface treatment.

For modifying the surface of the fine particulate compound oxide, any organosilicon compound known as a silane coupling agent is employable, and it may be properly selected depending, for example, on the type of a matrix or a solvent used in the coating solution.

The organosilicon compounds used normally include monofunctional silanes represented by the formula R3SiX (R is an alkyl group, a phenyl group, a vinyl group, or an organic group having methacryloxy group, a mercapto group, an amino group or an epoxy group, and X is a hydrolyzable group), e.g., trimethylsilane, dimethylphenylsilane and dimethylvinylsilane; difunctional silanes represented by the formula R2SiX2, e.g., dimethylsilane and diphenylsilane; trifunctinal silanes represented by the formula RSiX3, e.g., methylsilane and phenylsilane; and tetrafunctional silanes such as tetraethoxysilane.

In the surface treatment, the silane compounds may be used before or after the hydrolysable groups are hydrolyzed. After the treatment, the hydrolyzable groups are preferably in the state of being reacted with --OH groups of the fine particles, but a part of them may remain in the unreacted state. The surface modification can also be carried out by adding the hydrolyzate of the above compound and the fine particulate compound oxide to a mixture of water and alcohol, and then heating the resulting mixture. The amount of the organosilicon compound may be properly determined depending on the amount of the hydroxyl groups present on the surface of the fine particulate compound oxide.

In this proposal, 3-methacryloxypropyltrimethoxysilane (MPS) vinyltrimethoxysilane (VMS), vinyltriethoxysilane (VES) or Silane A (structure shown below) will be applied for the surface treatment of fine particulate compound oxide.

2 Matrix

As the matrix in the coating solution of the proposal, one compound selected from organosilicon compounds is Silane A which can form chemical bond with MPS, VES or VMS modified fine particulate compound oxide by thio/ene addition reaction; or the organosilicon compounds may be MPS, VMS or VES which can form chemical bond with Silane A modified fine particulate compound oxide by the same reaction.

The reaction can be done under UV irradiation. By using the Silane A, either for surface modification of fine particulate or as the matrix, may show relative high refractive index, compared to other organosilicon compounds since its sulfur content is high (three sulfur atoms in molecular). So far, the Silane A has not been found to be applied for high index hard coating in journals and patents.

Fig. 2. Thio/ene addition reaction between vinyl and thioalkyl moieties present in sols derived from the respective organosilanes

The matrix may further contain other components.

2.1 Si(OR4)

The component is used for the purpose of easily adjusting the refractive index of the resulting film with keeping the transparency of the film and for the purpose of accelerating a curing rate of the coating film. The component may include tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), or tetrapropoxysilane (TPOS)

By the use of the component, the refractive index of the cured film can be readily adjusted to match the refractive index of the lens substrate, and good adhesion between the cured film and the anti-reflection layer can be attained even in a decreased amount of the fine particulate compound oxide. Moreover, the tetrafunctional organosilicon compound can accelerate the curing rate of the coating film and inhibit discoloration of a dyed lens substrate, which tends to occur especially when a coating film is formed on a substrate made of a sulfur-containing urethane resin, resulting in a minimum change in color tone of the dyed lens after the film formation.

2.2. Curing catalyst for silanol

The catalyst should be selected from amines, amino acids, metallic acetylacetonates, organic acid metallic salts, perchloric acid, salts of perchloric acid, acids and metallic chlorides. The catalysis is used as a curing catalyst to accelerate curing of silanol and thus accelerate the film-forming reaction.

2.3. Organic Solvent

An organic solvent may be used in the coating solution of the proposal to adjust the solid concentration in the coating solution or control surface tension, viscosity and evaporation rate of the coating solution. The organic solvents which may be used in the proposal include alcohols, such as methanol, ethanol and isopropyl alcohol; cellosolves, such as methyl cellosolve and ethyl cellosolve; glycols, such as ethylene glycol; esters, such as methyl acetate and ethyl acetate; ethers, such as diethyl ether and tetrahydrofuran; ketones, such as acetone and methyl ethyl ketone; halogenated hydrocarbons, such as dichloroethane; aromatic hydrocarbons, such as toluene and xylene; carboxylic acids; and N,N-dimethylformamide. These solvents may be used singly or in combination.

2.4. Others

The coating solution may further contain, if desired, various additives such as surfactants, antistatic agents, ultraviolet light absorbers, antioxidants in small amounts, to improve coating property of the coating solution and performance of the film formed.

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