Feldspar and Syenites

Abstract

Feldspars are a group of tectosilicate minerals being the most important ones for rock formation of the Earth’s crust having a share of 51 % on it.

Syenite is a group of plutonic (or intrusive) rocks being rich on feldspars – nepheline syenite is the most important member of that group. In ceramics and glass, it is used like feldspar as a source of alumina and as a flux.

In polymer-bond systems, the feldspars and nepheline syenite are common as fillers in paints and lacquers as well as in pigment pastes. Refractive indices are in the region of 1.50–1.55 with little to no birefringence, so they match refractive indices of several resins very well to form highly transparent films with high abrasion and scratch resistance.

The use in thermoplastics is – due to their high Mohs hardness – limited to few applications like antiblocking in polyolefin films and lately also as a filler for dental composites. Apart from their use in ceramics and glass making, the most common use in polymer or resin bond systems is in paints and coatings where they provide excellent chalking and weathering resistance in exterior paints, high UV transparency in radiation-curable films (parquet coatings), and scratch resistance as well as high transparency for visible light in resins matching their refractive index due to their low birefringence.

In thermoplastics feldspars are used for antiblocking. Polyolefin films (mostly blown or cast polyethylene (LLDPE) and polypropylene (PP) – but also PVC and to some extent PET) tend to adhere to each other due to strong van der Waals interaction or electrostatic charges when being in close contact. To avoid the adherence of layers due to a close contact, particulate matter is introduced into the film in a highly diluted concentration. By that measure the contact area of film layers is minimized and adherence suppressed. Due to their optical properties, feldspars are preferably used in this respect.

Another application is light and thermal management in agricultural films. Greenhouses made from polyolefin films (LDPE) offer cost-effective solutions for intensified agriculture. Transmission and reflection of the solar spectrum are important key parameters for the growth of plants and the heat management in the greenhouse. The biologically active UV/VIS radiation for plant growth should pass the film as completely as possible, but heat should be reflected during the day to avoid overheating during the day but should be kept inside during cold nights to avoid cooling during the night. Feldspars are used to make those LDPE films suitable for thermal management. Advantages are early and high-quality crops, protection against cold/frost, and higher yields.

Apart from their use in veneers, highly pure feldspars become more and more interesting to dental applications like dental fillings, inlays, or artificial teeth in polymer compounds. Besides that these applications do not represent mass markets, they use similar principles in optical appearance and transparency and color like the applications mentioned before.