HYDROPHOBIC OR HYDROPHILIC FUMED SILICA AS FILLER OF POLYURETHANE ADHESIVES

Two hydrophilic and two hydrophobic fumed silicas of different charact eristics were added to solvent-based polyurethane adhesives (PU). IR s pectroscopy, contact angle measurements and rheology (viscosity measur ements, determination of viscoelastic properties) were used to monitor the variation of properties of PU adhesives produced by addition of s ilica. Immediate (green) adhesion was determined by T-peel testing of halogenated synthetic rubber/PU adhesive/halogenated synthetic rubber joints. Silica addition produced a noticeable increase in the PU adhes ive viscosity which Fan be related to the variation of viscoelastic pr operties. Viscosity of PU adhesives containing hydrophilic silica slig htly increased with time alter preparation; the increase was less sign ificant in PU adhesives with hydrophilic silica. In the rheological st udies, silica imparted shear thinning and negative thixotropy to PU ad hesives due to a better dispersion of the silica in the polyurethane d uring shearing. The addition of silica produces an increase in the sto rage modulus (G') of PU adhesives, the values obtained being independe nt of the hydrophilic or hydrophobic nature of the fumed silica. The i ncrease of G' and the changes in tan delta of PU adhesives containing silica corresponded to an improvement in the green adhesion properties of chlorinated rubber/PU adhesive/chlorinated rubber joints. In gener al, in disagreement with previous results,(1) the presence of silica d id affect the properties of solvent-based PU adhesives, but these prop erties were not dependent on the type of silica (hydrophobic or hydrop hilic) used in this study.