Composition for polishing optical surfaces

 

(57) Abstract:

Composition for polishing optical surfaces of parts made of glass or plastic, is an aqueous suspension containing alpha-aluminum oxide and cerium oxide. A specific ratio of components and the size of the particles of alpha-alumina and cerium oxide in suspension to provide a high quality polishing. 3 C.p. f-crystals, 4 Il., table 1.

The present invention relates to the creation of a composition for polishing optical surfaces of parts made of glass or plastic.

It is well known that optical surfaces must not have scratches and should have less possible roughness coefficient Ra. This ratio Rameasured as the average distance between the highest and lowest points on the surface, in the direction perpendicular to the plane of the polished glass sheet. Thus, this factor serves as a measure of variation between the highest and lowest points, for a surface that is not perfectly flat on the submicron level. It is clear that the smaller the value of this ratio, the better optical transparency and less remote is STIC in the aquatic environment (usually on the basis of deionized water) is introduced into contact with the surface, which need to be polished, and moving on the surface in certain areas of the polishing pad (medium abrasive), which leads to the polishing surface. In accordance with the second approach, used tool in the form of a polymeric matrix with inclusions of abrasive particles used for polishing optical surfaces. The present invention relates to the first approach, in which the use of a suspension (slurry).

Have already been proposed various structures of these suspensions. In U.S. patent 4576612 suspension get to the place of use in controlled quantities by using the polishing pad having a surface layer which contains abrasive particles in the polymer and which in use will gradually dissolve, releasing the polishing particles. In this patent indicate that is useful particles are particles of cerium oxide, zirconium oxide and iron oxide.

In the patent EP 608730-A1 describes an abrasive slurry for polishing a surface of an optical element, which contains an abrasive selected from the group comprising aluminium oxide, glass, diamond powder, silicon carbide, tungsten carbide, silicon carbide or boron nitride, with dimensions of cha and hard contact lenses made of polymethyl methacrylate, which contains particles of alpha-alumina, magnesium hydroxide and ammonium chloride.

A significant number of trains suspensions also proposed in the related fields of chemical-mechanical polishing of metal and semiconductor substrates, and are also usually use the same abrasives with variations of the components in the dispersion medium.

Needless to say that achieving success in polishing glass to some extent depends on the hardness of the glass itself. Polishing very hard glass can be time-consuming, and can cause problems in the end of processing, if you use a harder abrasives.

Already known compositions of the suspensions, which can often very effective to achieve the desired result, but it requires quite a long time. In accordance with the present invention have developed a new structure, in which two work together oxide, namely, aluminum oxide and cerium, in such a way that their interaction gives better results than the sum of the effects of any of the individual components. This composition allows to reach a very high level of optical quality in a much smaller Brent to increase chemical activity. In addition, such a composition allows the polishing very hard glass is very efficient and virtually no surface damage. Such a suspension can be used with polishing device type polisher or a pitch.

According to the present invention, the composition for polishing optical surfaces is an aqueous dispersion containing from 5 to 20% of solid abrasive particles, and from 85 to 95% of the content of solids is alpha-alumina and from 15 to 5% cerium oxide, and the particle size of the alpha alumina of less than 0.5 μm, and mainly from 0.15 to 0.25 μm, and the cerium oxide is a powder with a particle size of from 0.2 to 4 μm, mainly from 3 to 4 μm. In the context of the present invention, the term "particle size" corresponds to the value "D50"that is measured by using a particle size analyzer, Horiba L-910. For carrying out the invention can be used alpha-alumina, obtained, for example, using the process described in U.S. patent 4657754.

A commercially available cerium oxide is a mainly a mixture of oxides of rare earth metals, among which the main component is otstavat smaller amounts of oxides and other rare earth metals. In practice it was found that the purity of the cerium oxide does not greatly affect the quality of the abrasive particles for polishing, so the recommendations in accordance with the present invention can be more or less attributed to all other oxides of rare earth metals which accompany the cerium oxide in the commercial mixtures of cerium oxide. Therefore, in this specification, a mixture of oxides of rare earth metals, in which the weight percentage content of cerium oxide is the dominant, referred to as "cerium oxide". Among the commercially available "oxides of cerium" specify "50D1 and Superox 50" (manufactured by Cercoa PenYan N. Y.), which contain, respectively, approximately 75 and 34% of cerium oxide; and Rhodox 76" (manufactured by Rhone Poulenc), which contains approximately 50% cerium oxide.

Purchased on the market cerium oxide typically has the form of particles with a bicomponent (bimodal) distribution of particle size (grain size), with peaks near the particle sizes of 0.4 and 4 μm, and particles of larger size form the bulk of the particles, so that a total score of D50powder is less than 4, and usually 3-3,5 μm. It was found that if the producing grinding cerium oxide relative to the major, unless you want to produce a very polished solid glass with achievement of low-level visual defects. So often it is more efficient to use the specified structure and without conducting particle size reduction.

Wednesday dispersion of abrasive particles is water, although they can be added and a small amount of the water-soluble liquids such as alcohols. Usually use deionized water with surface-active substances that promote good dispersion of the abrasive particles. The content of solids in the suspension is usually from 5 to 15 or even up to 20% by weight, and less maintenance for use of tool type of pitch. Usually the suspension with a lower solids content slows down the process of polishing, and the suspension with a high content of solid substances may have the problem of stratification of the suspension and deposition of the abrasive material in the sediment. Therefore, for practical reasons, use the content of solids in suspension from 5 to 15, mainly from 8 to 12% by weight.

These and other features of the invention will be more apparent from the subsequent detailed description, given with reference to examples, without exhaust gas is modify the purity of the component of cerium oxide and particle size.

Example 1

In this example, comparison of characteristics of abrasive mixture in accordance with the present invention with compositions containing mixtures of the individual components.

Tests polishing were carried out using a bilateral machine type AS Peter Wolters, equipped polishers "Suba 500" manufactured by Rodel, Inc. Glass samples for polishing were made of fused silica (Corning), which is considered a very hard material (560-640 Knoop).

Polishing of the samples was conducted using three abrasives with solids in suspension, equal to 10%. As a first abrasive used 100% of aluminum oxide, and the second is 100% cerium oxide, and the third used a mixture of 90:10 components of the same aluminum oxide and cerium oxide. Aluminum oxide, which was purchased by the company Saint-Gobain Industrial Ceramics, Inc., contained particles of alpha-alumina with a size of approximately 20 to 50 nm in the form of agglomerates with a diameter of approximately 0.15 to 0.25 μm, and the agglomerates with sizes larger than 1 μm was largely absent. As cerium oxide used material Rhodox 76, which is a mixture of oxides of rare earth metals, the content is AI bred in deionized water, to which was added 0.07 per cent by weight of surfactant (sodium polyacrylate, which can be purchased at the firm of R. T. Vanderbilt under the trade name Darvan 811).

Conducted measurement of surface finish in time and the data obtained have constructed a graph shown in Fig. 1. In Fig.2 shows the same data with the expanded scale of surface roughness, in order to more clearly show the improvement obtained.

From a consideration of Fig.1 and 2, we can conclude that although the sample polished 100% of cerium oxide and has the best initial surface finish (i.e. before polishing it had a smaller roughness) than the other two samples, polishing is not possible to obtain a good surface finish. From a consideration of Fig.2 we can conclude that using a single aluminum oxide can never reach the surface (Ra) 200. On the other hand, this level of cleanliness of the surface was achieved by using cerium oxide approximately 19 minutes of polishing, and using the mixture in accordance with the present invention a level of cleanliness of the surface was achieved after 10 minutes. If you use a different view, we can say that after polishing for orientalistentag aluminum oxide sample had a surface finish of about 600 units, and after polishing suspension in accordance with the present invention, the sample had a surface finish of less than 200 units.

Example 2

In this example, discusses the impact of changes of the particle size of cerium oxide on the quality of the polishing specimen of quartz glass (fused quartz).

This example used the same composition in accordance with the present invention, as in example 1, and the cerium oxide used material Rhodox 76 manufactured by Rhone Poulenc. However, Rhodox 76 used four different particle sizes (measured value of D50using particle size analyzer, Horiba LA910), and conducted 4 separate evaluation of the quality of the polishing. Used particles with sizes 3,17 μm, and 2.14 ám, 0,992 μm and 0,435 μm. The results shown in the graph of Fig.3, which shows that when the polishing of the specified glass changing the particle size of the cerium oxide is of little use. Similar results were obtained when using materials "Superox 50 and 50D-1" containing particles of cerium oxide.

Example 3

In this example, we studied the source of cerium oxide, in particular the influence of the purity of the material on the efficiency of polishing. Were prepared with the compositions of estwenno about 90% aluminum oxide (this composition was used in Example 1). These compositions were tested polishing sample of quartz glass using equipment and procedures similar to those described in Example 1. Were obtained the results shown in Fig.4. Were used in the first composition with the material "Superox 50", which contained approximately 34% of cerium oxide, the second composition with the material "Rhodox 76", which contained approximately 50% cerium oxide, and the third part with the material "50D1", which contained approximately 75% of cerium oxide. You can see a small difference in the quality of polishing for the three compositions. We can conclude that in the compositions in accordance with the present invention all of the other oxides of rare earth materials behave similar to cerium oxide.

Example 4

In this example, we studied the efficiency of polishing glass type V (solid glass 530 Knoop) and the influence of the particle size of cerium oxide. If in the previous examples were made only to the assessment of surface cleanliness using the values of Rain this example, used an additional visual assessment of the surface quality by an experienced operator. This assessment allows you to evaluate the "grayness" that arise due to defects on top of the used bilateral polishing installation type 4800 P. R. , equipped polishers "Suba 10" by Rodel. When carrying out polishing to detail has been applied pressure of about 1.5 psi (1,034104PA). Polishing was continued to achieve the desired surface finish (transparency).

For polishing used 3 composition in accordance with the present invention. All 3 composition contained components aluminium oxide and surfactants, in amounts and distributions described in Example 1, and the component cerium oxide, in the same relative proportions in deionized water. The difference between the components is the different sizes of particles of cerium oxide. In the first part And component cerium oxide was crushed to a value of D50is equal to 0.4 μm. In the second and third trains and' was used cerium oxide in the form in which it comes directly from the manufacturer (Superox 50), and the only difference was the size of the sample of the treated glass. In the example in' the sample size was smaller and therefore exerted on him during polishing on the same machine as the pressure was more that made it possible to achieve the end result faster. In the fourth composition was also applied to the cerium oxide in the form in which the bimodal distribution of particle sizes, with a peak near the particle size 4 μm, forming the main mass of particles (the measurements were carried out using particle size analyzer type Horiba 910). The results are summarized in table.

When using the composition (which was applied powdered component cerium oxide), received a uniform gray color after 90 minutes of polishing, and removal of "greyness" it took another 30 minutes of treatment, then received a flatness of less than 0.1 wavelength. The trains and In the' enable very active (aggressive) polishing, obtaining good uniformity over the entire surface of the workpiece. The composition also gives a very good and rapid results polishing. After polishing glass products V receives excellent surface flatness. In contrast to these compounds, other polishing materials instead of homogeneous polishing give rather "spotty" the polished surface of the workpiece.

From the above we can conclude that in the case when the critical parameter is the transparency, carrying out polishing using formulations with non-cut component cerium oxide provides significant advantages. In tostnet, but when you use them to achieve visual perfection takes more time.

1. Composition for polishing optical surfaces, which is an aqueous suspension containing alpha-alumina, wherein the aqueous suspension contains from 5 to 20% by weight solids, and from 85 to 95% of the content of solids is alpha-alumina with a particle size of D50measured by using a particle size analyzer, Horiba L-910 lower than 0.5 micron, and from 15% to 5% by weight content of solid substances is a cerium oxide powder with a particle size of D50from 0.2 to 4 μm.

2. Composition for polishing optical surfaces on p. 1, characterized in that the solids content in the suspension is from 8 to 12% by weight.

3. Composition for polishing optical surfaces on p. 1, characterized in that the aluminum oxide has a particle size of D50from 0.15 to 0.25 μm.

4. Composition for polishing optical surfaces on p. 1, characterized in that the cerium oxide has a bimodal distribution of particle sizes with two components, with a particle size of D50ranges from 3 to 4 μm.

 

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1 cl, 3 ex, 1 dwg

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