STRUCTURAL MODIFICATION OF SEPIOLITE (NATURAL MAGNESIUM-SILICATE) BY THERMAL-TREATMENT - EFFECT ON THE PROPERTIES OF POLYURETHANE ADHESIVES

A sepiolite silicate was heat-treated at 550 and 1000 degrees C to mod ify its structure, and was used as a filler in a solvent-based polyure thane (PU) adhesive. The treated sepiolites were characterized by X-ra y diffraction and infra-red spectroscopy, and it was observed that the water was irreversibly removed from the structure and pores of the se piolite, changing the structure. The increase of temperature produced a collapse of the sepiolite structure. The rheological, mechanical, th ermal and adhesion properties of the filled PU adhesives were measured . In general, the addition of treated sepiolite to PU adhesives result ed in a loss of adhesive properties with respect to the blank (PU adhe sive with untreated sepiolite). The loss in properties was more notice able as the treatment temperature increased. Thus the PU adhesives con taining treated sepiolite had reduced rheological properties (lower vi scosity, lower storage and loss moduli, and they did not provide thixo tropy and pseudoplasticity to the solutions) with respect to the PU ad hesive filled with untreated sepiolite. On the other hand, the additio n of treated sepiolite decreased the mechanical and thermal mechanical properties of PU films. The T-peel strength of roughened and roughene d + chlorinated (with 1 wt% trichloroisocyanuric acid in 2-butanone) s tyrene-butadiene rubber/PU adhesive joints was improved if the PU adhe sive contained untreated sepiolite, but it decreased if the sepiolite was heat-treated. Interactions between the untreated sepiolite, the so lvent and the polyurethane were responsible fbr the improved propertie s of PU adhesives. These interactions disappeared when the sepiolite w as heat-treated, because of the destruction of the structure of the se piolite and the removal of surface silanol groups. (C) 1997 Elsevier S cience Ltd.