J. C. Fernández-García’s research while affiliated with INESCOP Footwear Technological Institute and other places

Four silicas, two fumed silicas (one hydrophilic and one hydrophobic) and two precipitated silicas (one hydrophilic and one hydrophobic), were added as filler to solvent-based polyurethane (PU) adhesive formulations. In general, the addition of silica increased the viscosity, the storage and loss moduli of the PU adhesives but only the hydrophilic fumed silica exhibited pseudoplasticity and thixotropy. The rheological properties imparted by adding filmed silicas to PU adhesive solutions were more noticeable than that of precipitated silicas. Interactions between the hydrophilic fumed silica, the polyurethane and/or the solvent seemed to be responsible for the improved rheological properties of filled PU adhesives.

Fumed silicas of different specific surface area (90-380 m/g) were added to a thermoplastic polyurethane (PU) solution. After solvent removal, solid fumed silica-PU composites were obtained. The viscoelastic properties of PU were improved by adding fumed silica and only a solid-like behavior in PU-fumed silica composites was obtained. The increase in the specific surface area of the fumed silica up to 200 m / g increased the moduli of the composites. Fumed silica-PU interactions were responsible for the improved rheological properties of the composites. The activation energies for viscous flow of the composites were 14-16 kcal/mol and increased as the specific surface area of fumed silica increased. The glass transition temperature (obtained from DMTA and DSC experiments) and the crystallization rate of fumed silica-PU composites decreased compared with PU and also decreased with increasing surface area of the fumed silica. The contact angle values were similar in all the composites and the strength of PVC/fumed silica-PU composite joints was not affected by the specific surface area of the fumed silica.

—Different amounts (5-20 wt% with respect to the solids content in the adhesive) of a hydrated magnesium/aluminum silicate (attapulgite) were added to solvent-based polyurethane (PU) adhesive formulations. The rheological, thermal, mechanical, surface, and adhesion properties of the filled PU adhesives obtained were measured. The addition of attapulgite provided an increase in viscosity, imparted thixotropy and pseudoplasticity to the PU adhesive solution, and produced an increase in storage and loss moduli. Increase in the amount of attapulgite enhanced these effects. Some interactions (not well defined in this paper) between the attapulgite, the polyurethane, and/or the solvent seemed to be responsible for the improved properties of the filled PU adhesives. Furthermore, the addition of attapulgite increased the mechanical properties, decreased the glass transition temperature, affected the crystallization rate and melting process, and modified the surface properties of the PU adhesive films (without solvent). On the other hand, the immediate (green) T-peel strength of roughened or (roughened + chlorinated with 1 wt% trichloroisocyanuric acid solutions in 2-butanone) styrene-butadiene rubber (SBR)/PU adhesive joints was greatly improved if the adhesive contained attapulgite. However, similar T-peel strength values in fully cured adhesive joints (measured 72 h after bond formation) were found irrespective of whether the PU adhesives contained attapulgite or not.

A sepiolite silicate was heat-treated at 550 and 1000°C to modify its structure, and was used as a filler in a solvent-based polyurethane (PU) adhesive. The treated sepiolites were characterized by X-ray diffraction and infra-red spectroscopy, and it was observed that the water was irreversibly removed from the structure and pores of the sepiolite, changing the structure. The increase of temperature produced a collapse of the sepiolite structure. The rheological, mechanical, thermal and adhesion properties of the filled PU adhesives were measured. In general, the addition of treated sepiolite to PU adhesives resulted in a loss of adhesive properties with respect to the blank (PU adhesive with untreated sepiolite). The loss in properties was more noticeable as the treatment temperature increased. Thus the PU adhesives containing treated sepiolite had reduced rheological properties (lower viscosity, lower storage and loss moduli, and they did not provide thixotropy and pseudoplasticity to the solutions) with respect to the PU adhesive filled with untreated sepiolite. On the other hand, the addition of treated sepiolite decreased the mechanical and thermal mechanical properties of PU films. The T-peel strength of roughened and roughened + chlorinated (with 1 wt% trichloroisocyanuric acid in 2-butanone) styrene-butadiene rubber/PU adhesive joints was improved if the PU adhesive contained untreated sepiolite, but it decreased if the sepiolite was heat-treated. Interactions between the untreated sepiolite, the solvent and the polyurethane were responsible for the improved properties of PU adhesives. These interactions disappeared when the sepiolite was heat-treated, because of the destruction of the structure of the sepiolite and the removal of surface silanol groups.

Fumed silica is a well-known mineral filler of epoxy and polyurethane adhesives. Although effective, the small particle size and the relative high cost of fumed silicas suggest the need for an alternative filler. In this study, the usefulness of adding a natural hydrated magnesium silicate (sepiolite) as a new filler in solvent-based polyurethane (PU) adhesive formulations has been demonstrated. The rheological and adhesion performance of the sepiolite-filled PU adhesive was compared with that in PU adhesives containing fumed silicas. The addition of a filler to PU adhesives provided an increase in viscosity, imparted thixotropy and pseudoplasticity to the adhesive solution, produced an increase in storage and loss moduli, and improved the rheology of the PU. The mechanical properties of adhesive films were increased by adding filler, mainly with fumed silica. On the other hand, the immediate T-peel strength of roughened or (roughened + chlorinated) styrene-butadiene rubber/PU adhesive joints was greatly improved in filled PU adhesives. The effects produced by adding sepiolite or fumed silica to the adhesives were very similar, although in general more noticeable in fumed silica filled PU due to the formation of hydrogen bonds between the filler and the solvent and/or the polyurethane (in sepiolite-filled adhesives, van der Waals forces seemed to be responsible for the interactions between the filler and the solvent and/or polyurethane).

Heat-treated sepiolite silicate at 550 and 1000°C was used as a filler in solvent-based polyurethane (PU) adhesives. The increase of the treatment temperature produced a collapse of the structure, due to an irreversible removal of water from the pores of the sepiolite. The Theological, mechanical, thermal and adhesion properties of the filled PU adhesives were measured. The addition of the treated sepiolite to PU adhesives provided a loss in properties in respect to the PU adhesive containing untreated sepiolite. The loss in properties was more noticeable as the treatment temperature increased.

Different amounts (5-30 wt% with respect to the polyurethane content in the adhesive) of a new filler, a partially defibrillated natural hydrated magnesium silicate (sepiolite), were added to solvent-based polyurethane (PU) adhesive formulations. The rheological, mechanical, surface, and adhesion properties of the PU adhesives obtained were measured. Increase in the amount of sepiolite added to PU adhesives led to an increase in viscosity and imparted thixotropy and pseudoplasticity to the adhesive solution. However, the addition of sepiolite produced an increase in storage and loss moduli and a decrease in the mechanical properties but did not affect the surface properties of the PU adhesive films. On the other hand, the immediate (measured 30 s after joint formation) T-peel strength of roughened or (roughened + chlorinated with 1 wt% trichloroisocyanuric acid solutions in 2-butanone) styrene-butadiene rubber/PU adhesive joints was greatly improved if the adhesive was filled with up to 10 wt% sepiolite. The T-peel strength measured 72 h after bond formation was similar for the joints prepared with PU adhesives without and with up to 10 wt% sepiolite. The joint strength decreased if the amount of sepiolite in the PU adhesive was 20-30 wt%, due to the poor mechanical properties and too high moduli of the adhesives. Some interactions between the sepiolite, the polyurethane, and/or the solvent seemed to be responsible for the improved properties of filled PU adhesives. These interactions were responsible for the increased storage and loss moduli and the displacement of Tg of the PU adhesive films to higher temperature when it contained sepiolite. Furthermore, the addition of a high amount of sepiolite changed the rheological behaviour of the PU to a solid, giving rigidity to the structure, which is responsible for the reduced adhesion in roughened rubber joints produced with polyurethanes containing 20-30 wt% sepiolite.

Aromatic hydrocarbon resins with different molecular weights (Mw = 1300-50400 daltons) were added to a solvent-based polychloroprene adhesive. The hydrocarbon resins were characterized using infra-red (IR) and differential scanning calorimetry (DSC) measurements. The properties and compatibility of the polychloroprene/resin blends were studied using mechanical tests, DSC measurements, scanning electron microscopy (SEM), and stress-controlled rheology. Tack measurements were also carried out and the adhesion strength was obtained from T-peel tests on roughened styrene-butadiene rubber/polychloroprene adhesive joints. The addition of low-molecular-weight tackifiers produced a compatible polychloroprene/tackifier system (only one Tg was found in DSC measurements), while the addition of a high-molecular-weight (and broad molecular weight distribution) tackifier produced a partially incompatible system (two Tg's were found in DSC measurements). The compatibility of polychloroprene/tackifier blends was also assessed with stress-controlled rheology and SEM. An increase in the T-peel strength and tack were produced when the molecular weight of the tackifier increased, although the addition of a hydrocarbon resin with a Mw higher than about 50 000 reduced the tack. A broad molecular weight distribution in the tackifier favoured incompatibility with the polychloroprene, resulting in a reduction in the tack and rheological properties.

Different amounts (0.5–3 wt%) of fumaric acid (FA) were added to a solvent-based polyurethane (PU) adhesive. Addition of FA produced a decrease in viscosity and changed the rheological and viscoelastic properties of the liquid PU adhesive, which was more marked upon increasing the time after the adhesive was prepared and more noticeable for high amounts of FA. According to molecular weight distribution, differential scanning calorimetry, thermal mechanical analysis and thermogravimetry measurements, the addition of FA seemed to produce a reaction with the polyurethane (possibly an acid-catalyzed transesterification reaction) which resulted in chain cleavage, disruption of polyurethane crystallinity and a loss of physical properties. These modification in PU structure did not affect the surface energy value of the PU adhesive although the T-peel strength of roughened rubber/PU adhesive/roughened rubber joints increased when the PU adhesive contained FA. This improvement was due to the removal of antiadherent substances (zinc stearate, paraffin wax) from the rubber surface produced by migration of FA to the PU adhesive surface once the adhesive joint was formed. Therefore, the carboxylic acid seemed to act as a compatibilizer between the rubber surface and the PU/adhesive interface.