H(2)O Outgassing Properties of Fumed and Precipitated Silica Particles by Temperature-Programmed Desorption

Area of Science:

• Materials Science
• Surface Chemistry
• Physical Chemistry

Background:

• Silica particles are widely used as fillers in polymers, influencing material properties.
• Water adsorption on silica surfaces can affect polymer performance and processing.
• Understanding water desorption kinetics is crucial for controlling moisture content in silica-filled materials.

Purpose of the Study:

• To investigate the desorption kinetics of water from fumed and precipitated silica particles.
• To determine the activation energies for physisorbed and chemisorbed water desorption.
• To correlate desorption behavior with water outgassing in silica-reinforced polysiloxane formulations.

Main Methods:

• Temperature-programmed desorption (TPD) up to 850 K was employed.
• Analysis of water desorption from fumed silica (Cab-O-Sil-M-7D) and precipitated silica (Hi-Sil-233).
• Calculation of activation energies for water desorption based on TPD profiles.

Main Results:

• Physisorbed water on both silica types had low activation energies (38-61 kJ/mol) and desorbed readily at moderate temperatures.
• Chemisorbed water exhibited higher activation energies: 80 to >247 kJ/mol for fumed silica and 96 to 155 kJ/mol for precipitated silica.
• Water desorption rates were highly dependent on temperature and activation energy, with significant implications for long-term moisture stability.

Conclusions:

• Physisorbed water can be removed efficiently at room temperature or 320 K.
• Chemisorbed water with high activation energies (>126 kJ/mol) is strongly bound and persists for extended periods.
• Adsorbed H2O and Si-OH bonds on silica surfaces are primary sources of water outgassing in silica-filled polymers.