Dispersion containing cerium oxide particles and the use thereof for polishing glasses

The invention relates to a dispersion which contains cerium oxide particles and polyacrylates, and the use of this dispersion for polishing glasses.

Cerium oxide is considered to be the most efficient agent for polishing glass surfaces. As a rule, the cerium oxide is used in the form of a dispersion. With the increase in requirements regarding the quality of glass surfaces, improved polishing compositions are required.

Thus, for example, US6221119 discloses a dispersion which contains sodium polyacrylate in addition to cerium oxide particles. This additive is intended to increase the stability of the dispersion and the polishing rate.

A disadvantage of this dispersion is that the quality of the surfaces treated therewith still has uneven areas which cannot be tolerated, particularly in precision optics .

It was therefore an object of the present invention to provide a dispersion which overcomes this disadvantage.

The invention relates to a dispersion which contains cerium oxide particles and one or more polymeric, anionic dispersing additives which are soluble in the liquid phase of the dispersion, wherein

- the dispersion has a pH in the range from 6 to 8, preferably from 6.5 to 7.5, and

- the cerium oxide particles are positively charged in this range and are present in a proportion of 0.01 to 10% by weight, preferably 0.1 to 2% by weight, in the dispersion .

The positive charge of the cerium oxide particles can be determined via the zeta potential. The zeta potential is a measure of the surface charge of the particles, which can be shifted by polymeric, anionic dispersing additives which accumulate on the surface. Zeta potential is to be understood as meaning the potential at the shear plane within the electrochemical double layer cerium oxide particle/electrolyte in the dispersion. An important parameter in relation to the zeta potential is the isoelectric point (IEP) for a particle. The IEP indicates the pH at which the zeta potential is zero.

The zeta potential of the cerium oxide particles is determined in the pH range 6-8 by means of the electrokinetic sound amplitude. For this purpose, a dispersion which contains 1% by weight of cerium oxide with water as the liquid phase is prepared. The dispersing is effected using an ultrasound rod (400 W) . The dispersion is stirred with a magnetic stirrer and pumped via a peristaltic pump through the PPL-80 sensor of the ESA-8000 apparatus from Matec. The potentiometric titration with 5M NaOH to pH 8 starts from the initial pH. The back- titration to pH 6 is carried out with 5M HNO3. The evaluation is effected according to

ESA -η φ-Ap-c-\ G(a)\-ε-ε _r where ESA = Electrokinetic sound amplitude,

ζ = Zeta potential, φ = Volume fraction,

δp = Density difference between particle and liquid, c = Sound velocity in the suspension, η = Viscosity of the liquid, ε = Dielectric constant of the suspension, G (α) I = Inertial correction.

The zeta potential of the cerium oxide particles present in the dispersion according to the invention is preferably +20 to +60 mV, particularly preferably +30 to +40 mV.

It is true that the polishing rates which are achieved with the dispersion according to the invention are as a rule lower than polishing rates achieved with the dispersion disclosed in US6221119. However, the quality of glass surfaces can be substantially improved with the dispersion according to the invention.

A polishing process with the dispersion disclosed in US6221119 is based on the theory that the interaction between polyacrylate and cerium oxide particles during the polishing process is only slight at neutral or basic pH .

In the case of the dispersion according to the invention, on the other hand, the interaction between cerium oxide particles and anionic dispersing additive during the polishing process is a strong one, i.e. the cerium oxide particles are surrounded by a layer of the anionic dispersing additive. Such a strong interaction between cerium oxide particles and anionic dispersing additive is explicitly ruled out in US 6221119 since the interaction between cerium oxide particles and glass surface would be

adversely affected thereby.

However, it is now surprisingly being found that it is precisely the strong steric interaction that produces the quality of the polished glass surface.

Furthermore, it may be advantageous if the cerium oxide particles present in the dispersion according to the invention have a BET surface area of 25 to 150 m ² /g. In particular, cerium oxide particles having a BET surface area of 30 to 100 m ² /g can be used. The BET surface area can be determined by means of DIN 66131.

The cerium oxide particles may be present in the form of isolated individual particles as well as in the form of aggregated primary particles.

The mean particle diameter, the mean aggregate diameter in the case of aggregated primary particles, is preferably less than 200 nm. A range from 50 to 150 nm may be particularly preferred. The value can be determined, for example, by means of dynamic light scattering .

Depending on the type of use of the surface to be polished, the dispersion according to the invention may comprise cerium oxide particles with a proportion of sodium of not more than 5 ppm and of chlorine of not more than 20 ppm.

A particularly preferred dispersion may be one in which the cerium oxide particles

- are present in the form of aggregates of primary particles,

- the primary particles

• have a mean diameter of 5 to 50 nm and

^• contain carbonate groups on the surface and in a layer close to the surface.

A very particularly preferred dispersion may be one in which the cerium oxide particles

- are present in the form of aggregates of primary particles, the proportion of cerium oxide, calculated as Ceθ2, being at least 99.5% by weight,

- the proportion of carbon, comprising organic and inorganic carbon, is 0.01 to 0.3% by weight,

- the primary particles have a mean diameter of 5 to 50 nm,

- carbonate groups are present on the surface and in a layer close to the surface, where in a layer close to the surface, i.e. a layer about 5 nm thick, the carbonate concentration decreases inwards, starting from the surface, on which the carbonate concentration is highest, and • the carbon content originating from the carbonate groups is 5 to 50 area percent on the surface and is 0 to 30 area percent in the layer close to the surface, at a depth of about 5 nm.

Such cerium oxide particles are disclosed, for example, in DE-A-102005038136.

In addition to cerium oxide particles, the dispersion according to the invention also comprises polymeric, anionic dispersing additives. The proportion thereof is preferably 0.1 to 100 parts by weight, particularly preferably 0.5 to 5 parts by weight, based in each case on cerium oxide.

The polymeric, anionic dispersing additive can preferably

be selected from the group consisting of acrylic acid polymers and salts thereof, methacrylic acid polymers and salts thereof, ammonium lauryl sulphate and polyoxyethylene lauryl ether ammonium sulphate.

Particularly preferred are salts of polyacrylic acids, in particular ammonium polyacrylates . The average molecular weight (number-averaged) can preferably be 500 to 50000, a range from 1000 to 30000 being particularly preferred.

The liquid phase of the dispersion according to the invention is as a rule one whose main constituent is water. The proportion of water is at least 80% by weight in a preferred embodiment and at least 95% by weight, based in each case on the dispersion, in a particularly preferred embodiment.

The preparation of the dispersion according to the invention can be effected, for example, by dispersing a predispersion containing cerium oxide particles with an energy input of at least 200 KJ/m ³ and subsequently adding one or more polymeric, anionic dispersing additives with lower energy input, for example by stirring.

Suitable dispersing units having an energy input of at least 200 KJ/m ³ are in particular systems according to the rotor-stator principle, for example Ultra-Turrax machines, or stirred ball mills. Higher energy inputs are possible with a planetary kneader/mixer .

With high-pressure homogenisers, two predispersed suspension streams under high pressure are depressurised via a nozzle. The two dispersion jets strike one another exactly and the particles mill themselves. In another embodiment, the predispersion is likewise subjected to a

high pressure but the collision of the particles is effected against armoured wall regions. The operation can be repeated as often as desired in order to obtain smaller particle sizes.

Furthermore, the energy input can also be effected by means of ultrasound. The dispersing and milling apparatuses can also be used in combination.

The invention furthermore relates to the use of the dispersion according to the invention for polishing glasses, in particular in high-precision optics.

Examples

Starting materials

Cerium oxide particles: as described in DE-A-102005038136, Example 2. Analytical data: BET 60 m ² /g, particle diameter 65 nm, CeO ₂ content 99.79% by weight, C content 0.14% by weight, zeta potential 48 mV at pH=5, IEP at pH=9.8.

Polishing dispersions: - Cerium oxide dispersion (according to the invention) : 2% by weight of cerium oxide particles (DE-A-102005038136, Example 2), 3% by weight of ammonium polyacrylate (based on a polyacrylic acid having an average molecular weight M _w of 2000, the pH of which is adjusted with ammonia), based on cerium oxide; remainder water;

- Cerium oxide dispersion CERI 3000 (comparison) : proportion of cerium oxide: 60 - 65% by weight, total

proportion of rare earth metals 86 - 91% by weight, average individual particle size: 0.7 μm (Sedigraph) , maximum particle size (95% less than:) 2.0 μm, pH: 8 - 10, specific gravity: 1.8 - 2.0.

Glass materials to be polished (plane discs ø25.4mm x 5mm) :

- Quartz glass Ql, Schott AG

- Boron crown glass N-BK7, Schott AG

- Heavy flint glass SF6, Schott AG

- Zerodur® "Zero-thermal expansion glass ceramic" - Crystalline quartz ølβ mm x 4 mm

Polishing machine used:

Lapping and polishing machine LOH HLP 150-2

2-spindle polishing machine with lever control, adjustable weights and additional weights can be added, speed of the spindle and lever arms continuously electronically controllable independently of one another.

Evaluation :

- Talystep, from Taylor-Hobson . It is based on the principle of mechanical linear sampling of the surface with a very fine diamond needle and defined tip radius. With this apparatus, micro roughnesses down to the sub nanometre range can be determined. The lower limit of measurement is about 0.7 - 0.5 nm rms .

- White light interferometer microscope MICROMAP, ATOS GmbH

Procedure corresponding to optical pitch polishing technology: pitch polishing technology utilises the flow of the pitch in order to achieve "independent" levelling or adaptation to the ideal form to be achieved (auto correction principle) . The pitch applied to the metal dish

serves here as a polish carrier. Accuracies of shape down to a few nanometres and roughnesses to 0.2 nm rms are obtained.

Carrying out the experiments a) 3 - 4 test discs per material, cemented onto substrate support plates b) Lapping or fine lapping (particle size-dependent and stepwise from grade 240 to 800) of the substrates with loose SiC abrasive. c) Optical polishing with maintenance of the planarity with CERI 3000 on a dressed pitch dish to a customary standard quality. d) Determination of the micro roughness on the surfaces obtained after optical polishing by means of a roughness-measuring apparatus (Taly-Step) . e) A Ql sample very finely lapped with SiC 800 was processed directly with the cerium oxide dispersion under standard conditions for about 20 minutes. This surface showed slightly polished zones. f) Thereafter, polishing is effected with cerium oxide dispersion according to the invention for about 30 min,

Figure 1 shows the roughness of the surface (quartz glass Ql) after polishing with CERI 3000. Figure 2 shows the roughness of the surface after polishing with the dispersion according to the invention. On polishing, the cerium oxide dispersion according to the invention shows uniform material removal with a finely structured surface. The planarities are noticeably improved. Similar results are also achieved with the other glass qualities tested.

Compared with the Syton used in the prior art, an alkaline suspension with colloidal Siθ2 particles, the dispersion

according to the invention has the advantage that a high accuracy of shape is achieved even with a long processing time .