Thermal Protective Properties of ALD Coated Fibers

Thermal Properties of ALD Coated Fibers

Nik Sigmon

Abstract

One method of producing flame resistant textiles is with intumescent surface coatings.  In this study, Atomic Layer Deposition (ALD) coatings were studied for their effects on thermal properties of nylon, cotton, and a nylon-cotton blend (Nyco).  This procedure uses gas phase precursors to form nano-scale, uniform, ceramic oxide coatings on the substrates at specific temperatures and pressures. An example of a reaction to form aluminum oxide (Al₂O₃) coating involves trimethyl aluminum (TMA) reacting with the surface hydroxide groups (OH), followed by a reaction with H₂O, creating new OH groups, with nitrogen purges between each gas dose.  The reactions are self-limiting and form a layer of coating of desired thickness based on the number of cycles deposited.    The process was used to form common Al₂O₃, zinc oxide (ZnO), and titanium oxide (TiO₂) coatings, as well as phosphorus based phosphate (PO_x), aluminum phosphate (AlP_xO_y), titanium phosphate (TiP_xO_y), and zinc phosphate (ZnP_xO_y) coatings. 

The traditional Al₂O₃, ZnO, and TiO₂ coatings show small improvements in the thermal properties of the fibers based on thermogravimetric analysis (TGA) testing.  Al₂O₃ and ZnO coatings also show decreased flame times on Nyco and cotton fibers based on standard vertical flame tests.  Extensive characterization techniques were used to confirm phosphorus deposition.  FTIR, EDS, XPS, and ellipsometry tests all confirmed phosphorus presence in the coatings.  EDS tests in particular show increased phosphorus atomic percent from 2.3 to 6.2 with increased deposition temperature from 110 to 180 °C.  This process offers one method of creating thermally protective coatings on textiles.