Synthesis of Large Platelets of Egyptian Blue via Pseudomorphosis after NaRUB‐18

Patrick Loch,Günther Kaupp,Wilhelm Schwieger,Thomas Martin,Michael Grüner,Josef Breu

doi:10.1002/zaac.202000203

Abstract

A colored platy substrate with appreciable aspect ratio can improve the hiding power while extending the color range of pearlescent pigments. One of the oldest synthetic pigments, Egyptian blue, possesses a layered structure and a platy morphology, making it a promising colored substrate for pearlescent pigments, which derives its blue color from CuII. Unfortunately, existing synthesis routes are either not technically benign or lack the natural platy morphology. Here, we introduce a new synthesis route starting with large, square‐shaped platelets of a synthetic layered sodium silicate (NaRUB‐18). As evidenced by Rietveld refinement and scanning electron microscopy, NaRUB‐18 can be converted with conservation of this attractive morphology (pseudomorphosis) into Egyptian blue.

Highlights

Pearlescent pigments draw their attractiveness from a combination of lustrous appearance and color, leading to their widespread application ranging from automotive paints to cosmetics
Crystalline layered sodium silicates like Kanemite, Magadiite, and NaRUB-18 are known for some time.[11]
A conversion via topotactic reaction has been suggested.[12e,12f,12h,12i] Of the above mentioned phyllosilicates, NaRUB-18[13] (Na8[Si32O64(OH)8]·32H2O) possess a O:Si-ratio of 2.25, which among the above mentioned compounds is most similar to the ratio of 2.5 found in Egyptian blue

Summary

Introduction

Pearlescent pigments draw their attractiveness from a combination of lustrous appearance and color, leading to their widespread application ranging from automotive paints to cosmetics. The silicate layers carry a negative charge, which is compensated by CaII in the interlayer space.[3,4] The layered structure is reflected in a platy morphology with atomically flat surfaces, rendering Egyptian blue a potentially interesting colored substrate for pearlescent pigments.[1a]

Results

Conclusion