CNRS                  Saint-Gobain
 

Silica surfaces lubrication by model sizings

Un article de Surface du verre et interfaces.

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Laurence Serreau (PhD 2003-2006), Caroline Heitz, Muriel Beauvais, Etienne Barthel


Within milliseconds of fibers forming and cooling, glass filaments are coated with an aqueous dispersion (sizing). The sized filaments are then brought together in a strand (fiber) which is mainly used to reinforce composite materials. Sizing is a key component in the mechanical performances of glass fiber-reinforced composites, as it controls the glass fiber toughness as well as the fiber – matrix interactions level. This is explained by the complex nature of the sizing formulation which includes a cationic or non ionic surfactant (softener), an organofunctional silane (coupling agent) and additives (anti-static, wetting and sticking agents, emulsifiers). While the silane promotes the strong interfacial adhesion between the fibers and the matrix in the composite, the surfactant reduces the high friction characteristic of silicate surfaces in water during sliding contacts between filaments within the fiber. Such a filament - filament contact lubrication protects the filaments from surface damage which can weaken the fiber tensile strength.


However, a question relates to the possible interaction between the silane and the surfactant, since both are able to adsorb on the glass surface. Model sizing formulations are considered to investigate the contact properties (repulsive barrier, adhesion, and friction) of silicate surfaces as well as the adsorption kinetics of the surfactant in the initial stage of the sizing adsorption. These investigations are of primary importance for a better understanding on the mechanisms involved in the protecting effect of the sizing.


An aminosilane and a C18 double-chained cationic surfactant were chosen. The silane on its own has no lubricant property unlike the surfactant. However, silica surface lubrication by the surfactant is not directly connected to the surfactant surface excess but originates from the transition to a defect-free bilayer that generates limited dissipation. Lubrication is obtained faster at higher ionic strength and under shear because both facilitate the bilayer organization. When the silane and the surfactant are mixed together, we have showed that the silane reduces or even suppresses the cationic surfactant adsorption on silica. However, the silica-silica contact is lubricated even if the surfactant is not adsorbed. The repeated contacts and shear due to the friction experiment itself induce accumulation, organization and compactness of surfactant bilayers


 

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