Method for surface treatment of precipitated calcium carbonate

The invention relates to a process for the surface treatment of precipitated calcium carbonate, which is suitable as a functional filler and is distinguished by defined properties.

A typical area of ​​application is its use as a filler in, for example, plastisols, underbody protective compositions for automobiles, sealants, adhesives, polymers, rubber and printing inks. The precipitated, surface-treated calcium carbonate is able to control the flow behavior of the masses as well as the mechanical properties of the hardened masses. Masses with very different flow behavior can be produced by using suitable surface-treated calcium carbonates.

The yield point according to Bingham in the plasticizer mixture (SEB) is usually given as a typical characteristic for the application properties of calcium carbonates as fillers. Depending on the application, very high, medium or low flow limits may be desired. The ability of the surface-treated calcium carbonate to control the flow limits is influenced, among other things, by the surface treatment. Furthermore, the surface-treated calcium carbonates are characterized by excellent dispersibility. The very fine crystals of calcium carbonate tend to agglomerate. When incorporating these surface-treated calcium carbonates, these agglomerates have to be broken up as much as possible, which is significantly simplified by a surface treatment.

Naturally occurring calcites or aragonites are also finely ground due to their small surface area, unsatisfactory in their range of applications.

Synthetically produced calcium carbonate meets the requirements for its use as a functional filler or additive much better.

Calcium carbonate is usually produced by reacting calcium hydroxide with carbon dioxide.

It is known that the processing properties of precipitated calcium carbonate can be influenced by coating with fatty acids, waxes, inorganic salts, etc.

For example, the coating can improve the storage stability of sealing compounds or the wettability of the calcium carbonates with organic matrix, as is necessary when the fillers are incorporated in bulk, or the flow limit, or the dispersibility.

It is also known that calcium carbonate, in particular chalk, is ground in the presence of surface-active substances such as, for example, synthetic or natural fatty acids, fatty alcohols or waxes, in order to prevent the fine calcium particles from sticking (agglomerating). (DE 958 839).

EP 1 279 433 discloses a method for improving the properties of granular materials, for example sodium percarbonate, by multiple coating with coating agents known per se. Here, the coating material forms a coating matrix on the particles, the particles not being completely coated with the coating material.

The object of the invention is to provide calcium carbonate with improved properties, in particular with improved flow properties and improved dispersibility, and to provide a process for its preparation. The object is achieved by the invention specified in the claims.

The method according to the invention for the surface treatment of precipitated calcium carbonate, in which a coating agent is applied to the calcium carbonate particles, provides that the calcium carbonate particles are first brought into contact with a first coating agent and then a second coating agent is applied.

A preferred embodiment provides that the calcium carbonate particles first in a known manner with saturated or unsaturated carboxylic acids, for example fatty acids, substituted fatty acids, their salts or esters or fatty alcohols, for example monohydric alcohols with 8-22 carbon atoms or polyhydric alcohols e.g. B. glycerin, to name just a few of the suitable substances, can be coated. The calcium carbonate is contacted in a manner known per se with the first coating agent in liquid or solid form, for example during the grinding process and / or after the precipitation, the coating agent being held on the surface of the calcium carbonate.

According to the invention, the coated calcium carbonate is provided with a second coating layer in that polyhydric alcohols, preferably glycerol or ethylene glycol, or polyhydric substituted alcohols, for example substances containing alcohol groups, esters of carboxylic acids, preferably phthalic acid esters, are applied to the coated calcium carbonate particles as the second coating layer.

The coated calcium carbonate is again contacted in a manner known per se with the additional coating agent in liquid, dissolved or solid form, the second coating agent being held on the surface of the calcium carbonate. We found that the vacancies left on the CaCO after the first coating₃Surface are occupied by the second coating agent.

It is in the sense of the invention that the first and the second coating layer can be made of different or of the same material.

In a preferred embodiment, the precipitated calcium carbonate particles are first coated with stearic acid or stearic acid derivatives and then a second coating layer with glycerol as the second coating agent is applied to the already coated calcium carbonate particles.

The invention further relates to the double-coated calcium carbonate particles obtainable by the process. The double-coated calcium carbonate particles according to the invention are characterized in that the first coating layer contains saturated or unsaturated carboxylic acids, for example Fatty acids or substituted fatty acids; Contains or consists of fatty alcohols or polyhydric alcohols and the second coating layer contains or consists of polyhydric alcohols or substituted polyhydric alcohols.

Preferred double-coated calcium carbonate particles are characterized in that the second coating layer contains or consists of glycerin.

Alternative double-coated calcium carbonate particles are characterized in that the second coating layer preferably contains or consists of ethylene glycol or substances containing alcohol groups, preferably glycols, or esters of carboxylic acids, preferably phthalic acid esters.

It has been found that the calcium carbonate, which is coated twice according to the invention, has better dispersibility. It is less prone to agglomerate formation. Any agglomerates of the surface-treated calcium carbonate that are present disperse rapidly when incorporated in bulk.

With the additional coating, the yield point can be varied within a very wide range, whereby high values ​​can be set that would not be achievable without an additional second coating. For certain uses of the coated calcium carbonate, the sole coating with polyhydric alcohol may be sufficient to achieve the properties desired by the end user.

The yield point of CaCO coated with fatty acid₃ can be controlled, for example, in the range from 30 Pa to 350 Pa. Additional coating with polyhydric alcohols or substituted polyhydric alcohols can increase the yield point to over 1000 Pa.

It is also possible to correct the yield point after the first coating. If the measured yield point does not correspond to the desired value after the first coating, this desired value can be set subsequently by means of targeted additional coating.

The advantages for the user are an increase in the consistency of the yield point. The additional coating enables a very constant quality to be generated, which allows the user to work with fixed formulations. Subsequent adjustment of the flow behavior of the masses becomes superfluous. Furthermore, the dispersing process is accelerated, so that on the one hand the processing capacity of the user increases, but on the other hand better surface smoothing of the masses is achieved. Furthermore, when using surface-treated calcium carbonate with a very high yield point, the proportion of calcium carbonate in the sealing compound can be reduced, which has a positive effect on the specific weight of the compound. Sealants, underbody protection compounds and low-density adhesives are particularly preferred in the automotive sector.

Example 1:

Production of precipitated calcium carbonate (CCP).

Lime milk with a concentration of 150 g / l calcium hydroxide was heated to 18 ° C. in a container. After reaching this start temperature, CO₂ introduced into the container, the CO₂- The supply was kept constant.    CO₂-Air mixture: 30:70    CO₂-Infeed: 1m³/ h (based on 10 l milk of lime)    Fall time: 100 min

Example 2:

Preparation of the 1st coating agent

1035 g of water was heated to 75 ° C, 122 g of stearic acid were added. This mixture was heated for 15 minutes. 60 ml of 25% ammonia solution were added with stirring and the mixture was homogenized for 30 minutes. The light, homogeneous emulsion contained 10% by weight of stearic acid.

Example 3:

1. Coating

The coating agent prepared according to Example 2 was introduced into each 201 of the calcium carbonate suspension prepared according to Example 1 and the mixture was stirred at 75 ° C. for 1 hour.

The coated calcium carbonate was separated, dried and ground. The calcium carbonate thus coated contained 3% by weight of stearate coating.

Example 4:

2nd Coating

Glycerin was dissolved in water in a ratio of 1: 3 and sprayed onto the precipitated calcium carbonate particles (uncoated and coated).

The samples were dried at 50 ° C and then ground in a pin mill at 18,000 rpm.

The flow behavior of the coated CaCO₃ was determined using the Bingham method.

Example 5:

Precipitated calcium carbonate according to Example 1 was coated with different amounts of glycerin and the flow behavior was determined.<tables id="tabl0001" num="0001"><img file="EP1557442A1_D0001.tif" /></tables>

Example 6

Precipitated calcium carbonate (Example 1) was coated with 2 g of stearic acid per 100 g of calcium carbonate as in Example 3. Different amounts of glycerin were applied as the second coating layer and the flow behavior was determined.<tables id="tabl0002" num="0002"><img file="EP1557442A1_D0002.tif" /></tables>

Example 7:

Precipitated calcium carbonate (Example 1) was coated with 3 g of stearic acid per 100 g of calcium carbonate as in Example 3. Different amounts of glycerin were applied as the second coating layer and the flow behavior was determined.<tables id="tabl0003" num="0003"><img file="EP1557442A1_D0003.tif" /></tables>

Example 8:

The dispersibility of the CaCO coated according to the invention₃-Particle was determined by laser light scattering.<tables id="tabl0004" num="0004"><img file="EP1557442A1_D0004.tif" /></tables>

The results show that the flow limit, viscosity and dispersibility can be controlled by the second coating according to the invention.