EP0381662B1 - Process for the manufacture of abrasive bodies - Google Patents

Abstract

In a process for the production of abrasive bodies having particulate abrasive material, such as, for example, SiC, corundum, Al2O3 or B4C, bound in a binder, plastic and MgO and MgCl2 being used as binders, the plastic employed is a polycarboxylic acid. The moulded abrasive bodies are bound or hardened without heating, by reacting the polycarboxylic acids with magnesium ions.

Description

The invention relates to a method for producing abrasive articles with granular abrasives embedded in a binder, such as e.g. SiC, corundum, Al₂O₃ or B₄C, plastic and MgO and MgCl₂ being used as binders.

For the manufacture of abrasive products, it is known to use granular abrasives with Sorel cement, i.e. a magnesite binder, which is formed by stirring burned magnesite (MgO) with a concentrated magnesium chloride solution, and allowing the mixture obtained to dry in molds. In the case of such grinding bodies, which are bonded exclusively with Sorel cement, a soap-like surface forms during operation, in particular when wet grinding, and constituents of the binder are lost due to their water solubility during grinding. For this reason, the service life of such grinding wheels, which are bonded exclusively with Sorel cement, is limited, and the effort for the production time is considerable in view of drying at ambient temperature.

To improve the service life, it has already been proposed to add phenolic resins to the mixtures of granular abrasives and Sorel cement, it being known to use phenolic resins in an amount of about 4 to 10% by weight, based on MgO, cf. eg DE-A-24 01 099. Such starting mixtures mixed with phenolic resins subsequently have to be thermally hardened, and exact thermal control and a corresponding effort for the hardening furnaces are required for the thermal hardening. For the hardening, it must be ensured that predetermined maximum temperatures are not exceeded, and the temperature rise in the hardening phase or the temperature curve over the hardening phase must also be adjusted to the limit values precisely adapted to the respective mixing ratios being held. With such a thermal hardening, shorter manufacturing times can be achieved, so that in addition to the advantage of an improved service life, a shortening of the manufacturing time is also possible. However, due to the requirements for an exact setting and maintenance of the temperatures during the hardening process, disturbances in the hardening process can occur, and such grinding wheels can become brittle and thereby reduce the output.

The invention now aims to provide a method for producing grinding wheels of the type mentioned, which can be carried out without expensive hardening ovens and in which no critical conditions have to be observed during setting, so that the proportion of rejects in production is reduced becomes. At the same time, the invention aims to create a product with a significantly longer service life than a pure Sorel cement bond, which is also characterized by high breaking strength. To achieve this object, the invention consists essentially in that polycarboxylic acids are used as the plastic and that the shaped abrasive bodies are bonded or hardened without additional external heating by reaction of the polycarboxylic acids with Mg ions. Because polycarboxylic acids are used as the plastic, the mixture thus obtained can set or harden at ambient temperature, and the expensive furnace control or the expensive furnace can be dispensed with. The fact that polycarboxylic acids are generally water-soluble and are present as an aqueous solution before hardening results in good miscibility with the other components of the grinding wheel, so that better homogenization and thus uniform strength over the entire cross section of the grinding wheel is ensured. In the case of the use of polycarboxylic acids, the hardening or setting takes place by salt formation with magnesium ions, the addition being the Hardening or setting occurs through the Sorel cement bond. The combination of magnesium ions and the carboxylate ions of the polycarboxylic acid leads to a particularly strong bond, since the withdrawal of magnesium ions shifts the equilibrium of the solution and increases the degree of supersaturation of chloride ions.

The process according to the invention is advantageously carried out in such a way that polycarboxylic acids are used in amounts of 0.5-10% by weight, preferably about 5% by weight, based on MgO, which results in particularly high final strengths and good breaking strength.

As already mentioned above, the temperature control is not critical during setting or hardening and it is possible to work at ambient temperature without regulating or controlling the temperature. In any case, however, the temperature should be controlled in such a way that the temperature is kept below 100 ° C., preferably below 60 ° C., during setting or hardening in order not to impair the final strength properties in this way. If the proportion of polycarboxylic acid is correspondingly high, it may be necessary to provide cooling in order to maintain these temperatures in order to reliably avoid local overheating during setting or hardening.

In a particularly advantageous manner, the process according to the invention is carried out in such a way that MgO and MgCl are used in a manner known per se in amounts of 25-35% by weight, MgCl₂ preferably being used in an aqueous solution at 30 ° Be and the polycarboxylic acids in an aqueous solution with a solids content of 25-35% by weight, preferably 30% by weight. Such an adjustment of the proportions of the respective components of the mixture leads to particularly homogeneous and long-lasting grinding wheels, the Risk of cracking during setting or hardening is avoided.

The invention is explained in more detail below on the basis of exemplary embodiments.

Example 1:

1.2 l of a 30% aqueous solution of a polycarboxylic acid, which has a pH of about 8 and an acid number of 220 mg KOH / g, was mixed with 2000 cm 3 of a magnesium chloride solution at 30 ° Be. 2.4 kg of corundum were added to this mixture as an abrasive. In order to achieve grinding stones with a fine grain size, finely ground corundum with a grinding number of 1200 was used. 2.4 kg of homogeneous magnesium oxide with a purity of 75% were added to this mixture. The mixture was poured into suitable molds immediately after the mixing, and a heat shade could be determined already during the pouring, which resulted from the setting reaction of the polycarboxylic acid with the magnesium ions and the Sorel cement bond. The moldings were stored at room temperature and their properties were examined after 3 days. After 3 days, a test molded body had a compressive strength of 398 kg / cm², a bending tensile strength of 93.8 kg / cm² and a shrinkage of 0.17 to 0.19%. The further batch of the shaped bodies prepared in this way was allowed to harden further and another shaped body was examined after 7 days. After 7 days, the molded body had a compressive strength of 664 kp / cm², a bending tensile strength of around 121.9 kp / cm² and a shrinkage of 0.19%. Since these values were still too low compared to a conventional grinding wheel mixture, which was only produced on the basis of Sorel cement, the grinding wheel mixture was left unchanged for a further 3 weeks. After a total period For 4 weeks, the grinding wheels consistently had compressive strengths in the range from 775 to 785 kp / cm², a bending tensile strength from 146 to 149 kp / cm² and an average shrinkage of 0.16%.

A comparative grinding body, which was only produced by mixing 2.4 kg of magnesium oxide, 2.4 kg of corundum and 2000 cm³ of a magnesium chloride solution of 30 ° Bé, had a compressive strength of 1000 kp / cm² and a bending tensile strength of 140 kp / cm² at the same time and the shrinkage was 0.14%.

Example 2:

In a second experiment, grinding media with a medium grain size were produced, 1300 cm 3 of a 30% aqueous solution of a polycarboxylic acid being used. The polycarboxylic acid had a pH of about 8 and the viscosity of the aqueous solution was 3 Pa.s at 20 ° C. The solution of the polycarboxylic acid was mixed with 1935 cm³ of a magnesium chloride solution at 30 ° Bé, and 3.6 kg of ground corundum were added. The corundum had a meal of 220. The mixture was mixed with 2.4 kg of 80% magnesium oxide and poured into molds. Within the first 5 hours of curing, a heat tone of 38 ° C was observed. The abrasive bodies cast in this way were again examined after 3 days, 7 days and 28 days. After 3 days, the abrasives had a compressive strength of 405 kg / cm², a bending tensile strength of 95.8 kg / cm² and the shrinkage was 0.16%. After 7 days the compressive strength was 671 kp / cm², the bending tensile strength was 124.6 kp / cm² and the shrinkage was 0.17%. After 28 days (4 weeks) the abrasives had a compressive strength of 780 to 791 kg / cm², a bending tensile strength of 147 to 155 kg / cm² and the shrinkage was 0.18%.

Example 3:

In an analogous procedure, an attempt was made to produce grinding tools with a coarse-grained structure. For this purpose, 1300 cm 3 of the polycarboxylic acid solution described in Example 1 were mixed with 1770 cm 3 of magnesium chloride solution (30 ° Be) and 4.8 kg of corundum (number of grinding = 20) were added to this mixture. After adding 2.4 kg of magnesium oxide with 85% purity, the mixture was poured into shaped bodies. After 3 days, these moldings had a compressive strength of 391 kp / cm² and a bending tensile strength of 91.2 kp / cm². The shrinkage was 0.13%. After 7 days, the abrasives had a compressive strength of 659 kp / cm² and a flexural tensile strength of 119.8 kp / cm². The shrinkage was 0.16%. After 28 days, the coarse-grained abrasives showed a 0.16% shrinkage. The compressive strength of these grinding wheels was 770 kp / cm² and the bending tensile strength was 145.9 kp / cm².

From all the examples it can be seen that with the addition of polycarboxylic acids to the conventional grinding wheel mixture it is possible to produce grinding wheels of various grits which consistently have bending tensile strengths which are higher than those of conventional grinding wheels which were only produced using the Sorel cement bond. The compressive strengths of such abrasive products produced using polycarboxylic acids are somewhat lower than those which were obtained only by using Sorel cement and corundum. However, in spite of the somewhat lower compressive strength, it is expected that the service life of such abrasive products made using polycarboxylic acids will be longer than that of abrasive products made only by using the Sorel cement bond, since their water solubility is based on polycarboxylic acids produced grinding wheel is significantly reduced. The shrinkage of the grinding wheels, which were produced using polycarboxylic acids, was consistently in the same range as that of the grinding wheels according to the prior art.

Claims (6)

1. Process for the manufacture of abrasive bodies with a granular abrasive material such as SiC, emery, Al₂O₃ or B₄C, embedded in a binding material, wherein synthetic material and MgO and MgCl₂ are used as binding material, characterised in that polycarboxylic acids are used as synthetic material and that the premolded abrasive bodies are set or hardened by reaction of the polycarboxylic acids with the Mg-ions without additional heating from outward.
2. Process according to claim 1, characterised in that the polycarboxylic acids are used in amounts of 0,5 to 10 weight%, especially approximately 5 weight%, relative to MgO.
3. Process according to one of claims 1 or 2, characterised in that the temperature is held during the setting or hardening under 100°C, especially under 60°C.
4. Process according to any one of the claims 1, 2 or 3, characterised in that MgO and MgCl is used, in a manner known per se in amounts of 25 to 35 weight%.
5. Process according to any one of the claims 1 to 4, characterised in that MgCl₂ is used in aqueous solution with 30° Bé.
6. Process according to any one of the claims 1 to 5, characterised in that the polycarboxylic acids are used in aqueous solution with an amount of solid state bodies of 25 to 35 weight%, especially 30 weight%.