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Natural mineral fillers from Neuburg  #8 (102), 2011

Depending on their origin fillers are divided into natural and synthetic. The most important synthetic fillers are the carbon black and precipitated silicic acid. Natural mineral fillers are related mainly to a group of silicates, carbonates and oxides and are mined in natural deposits where are bedded mainly as dense rock or in granular form. The silica from Neuburg deposit supplied by the German company Hoffmann Mineral GmbH, gives a number of advantages as a functional filler

Neuburg silica
Deposits with granular structure are divided into different classes by grain size via methods of physical sorting, e.g. separation in hydrocyclones. On the basis of the determined by BET data on the specific surface area in the range of 1 to 50 g/m2, the natural mineral fillers are divided into semi-active and inactive. According to the grains’ spatial structure they are mainly being spherical, lamellar and also rod- or needle-shaped.

Among the natural mineral fillers Neuburg silica takes a special place. It belongs to the group of silicates and presents itself an agglomerate of cryptocrystalline silicic acid and lamellar kaolinite. Both these components form a friable amorphous structure that being used as a filler provides specific advantages in terms of application technology. Material uniqueness is due to the typical size of grains of silicic acid within the range of 1 to 5 microns, caused by natural weathering.

The origin history of material that became later the raw stock for Neuburg silica, has 95 million years. In the former times a large part of southern Germany was covered by sea. On limestone depositions formed in the early Jurassic period, in the bay to the west of Neuburg the silica strata were settling. With this silica itself sedimentated onto the coarse sand layer, and then was settled upon with a fine sand layer. When the sea receded, under the influence of warm moist climate there began the karstic process of the limestone strata under silica. Hence there appeared karstic cavities and cavings. The latter resulted in so-called karstic craters where silica slipped down. Eventually after the strata of Tertiary period the silica deposits got their present appearance.

Properties and dosage
Unlike the too coarse quartz that after grinding contains particles with sharp edges, the particles of silicic acid in Neuburg silica are rounded due to the natural aging processes lasting millions years. This gives much lower abrasiveness as compared with grinded quartz powder. The increased specific surface area (determined by BET) is a result of combination of cryptocrystalline silicic acid and lamellar kaolinite. In the process of deposition in prehistoric era both minerals have formed a porous structure. Today this binding has become so strong that their separation by physical methods is almost impossible. Characteristic properties of Neuburg silica cannot be corrected via physical mixing of cryptocrystalline silicic acid and lamellar kaolinite as in adulteration process the demixing occurs due to a very different dispersive capacity of individual unbound minerals, and at that the most essential problem here is the kaolinite component. Depending on the size of grains the Neuburg silica is related to semi-active or inactive fillers.

Along with such types of silica as Sillitin N, Sillitin Z and Sillikolloid P, differing in grain size, the species of Aktisil (Aktisil VM 56, Aktisil PF 216 and Aktisil MM) are used. The latter have a specially processed surface and therefore some properties not available for unprocessed grades:
 - increased tensile strength;
 - reduced abrasion;
 - increased tension values;
 - improved swelling characteristics;
 - reduction of residual strain in compression and tension;
 - maximal tensile strength at a higher filling degree;
 - increased resistance stability when stored in water.

Neuburg silica is mainly used in elastomers and thermoplastic polymers as a functional filler. As a rule, its content can be up to 400 parts per 100 parts of rubber that improves physical and technological properties, leads to increase of hardness, to cost reduction.

The most important advantages of Neuburg silica are:
 - the ability for rapid dispersion;
 - no tendency to form agglomerates;
 - very low residual strain in compression;
 - excellent extrusion properties;
 - relatively high surface activity of the source material, that can be even boosted by surface processing

Calcined silica
Besides the classic products of Hoffmann Mineral that are derived from silica, namely Sillitin and Aktisil, the company offers calcined or heat treated Silfit and Aktifit series. Having a modified surface Aktifit AM is used where a reduced ability to warping in combination with high quality surface treatment are as important as the good melt fluidity, high unit elongation at break and high impact strength in ready for casting or dry state.

The potential application areas of this material - transparent parts and enclosures with smooth surfaces in engine compartment, wheel caps, handles, housings, i.e. products with high demands on dimensional stability and surface finish. Aktifit AM is preferably used for filling polyamids, particularly PA-6,6. It is also possible to use it in other engineering thermoplastics and polar polymers, e.g. in PE/EVAC, PVAC.

In comparison with polymer without fillers Aktifit AM demonstrates the following
 - reduction of shrinkage during processing;
 - increase in hardness;
 - increase in rigidity;
 - increase in strength at tension and bending;
 - increase in heat resistance;
 - increase in dimensional stability at varying humidity.

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