Nuevos materiales híbridos de poliuretano termoplástico y nanocargas inorgánicas. Propiedades reológicas, térmicas y de adhesión

Resumen

Polyurethane adhesives are commonly used to join different kind of materials in the footwear, textile, building, construction, automotive, aeronautic and electronic industries. Polyurethanes should be synthesized and formulated to improve the adhesion of materials, including composites in which durable joints must be assured. The properties of thermoplastic polyurethane adhesives and polyurethane composites (TPU) depend on their segmented structure (i.e. degree of phase separation), the amount and nature of the reactants and the reaction conditions, among other factors. Inorganic fillers can be added for controlling the rheological, mechanical and viscoelastic properties of the TPUs. Fumed silica is the most common filler for TPUs because of its nanometric particle size and the presence of silanol groups on the particle' surfaces. It has been demonstrated that the silanol surface groups of nanosilica interact by hydrogen bond with polymer chains and disrupted the phase separation in the polyurethanes which changed the rheological, mechanical and adhesion properties. However, although effective, nanosilica is expensive and difficult to handle because of its extremely low density and small particle size. Therefore, alternative inorganic fillers to fumed silica for polyurethane adhesives are currently found, and in this study micronized calcium carbonate, precipitated calcium carbonate and carbon black were tested. The effectiveness of filler addition in polyurethanes is controlled by the extent of the filler-polymer interactions. Therefore, inert filler should be surface modified to enhance the filler-polymer interactions. In fact, since the last 10 years the use of nanofillers to prepare organic-inorganic hybrid materials had increased to impart improved properties to nanocomposites. As a result, it is crucial analyse the interactions produced between inorganic fillers and polyurethane chains to determine the influence of them in the final properties of the polyurethane adhesives and composites, mainly the adhesion and improved ageing resistance in construction and regenerative medicine. The addition of calcium carbonate, precipitated calcium carbonate, silica and carbon black fillers in polyurethane adhesives has been scarcely studied and there are several issues that have not been addressed yet. In particular, the extent of the polyurethane-filler interactions has been only indirectly studied. Furthermore, the incidence of the addition of precipitated calcium carbonate or carbon black fillers on the polyurethane adhesion property has not been considered in detail yet. The synthesis of hybrid polyurethane-nanofillers adhesives and composites was the main objective of this study. It was necessary determine the structure-properties relationship, principally the adhesion properties. In particular, a good interfacial interaction between the polyurethane chains and filler surface particles, and an appropriate filler distribution in the polyurethane matrix should be achieved. Under a scientific and technologic point of view, getting a control of the properties imparted by adding fillers is a current challenge in construction and regenerative medicine (by using biomaterials). In this study, interfacial interactions between nanofillers of different chemical nature and polyurethane model were accessed by flow microcalorimetry (FMC) and diffuse reflectance infrared spectroscopy (DRIFTS). The influence of the addition of different amounts of precipitated calcium carbonate nanofiller in thermoplastic polyurethane adhesive on the rheological, thermal, surface, mechanical and adhesion properties were analysed. Furthermore, the effect of adding two kinds of calcium carbonate (micro- and nanometric particle size) filler on the polyurethane properties was also considered. In addition, the properties of the polyurethanes containing nanofillers with different size and nature were compared. Moreover, the synergic effect of the nanofiller mixtures (fumed silica and carbon black) in the polyurethane properties was studied. Finally, the incidence of the dispersion of the filler in the thermal and rheological properties was also evaluated. Dimethyl adipate (DMA) was selected as model compound of polyester diol in the thermoplastic polyurethane (TPU). Carbonyl groups of DMA interacted strongly with silanol groups on the fumed silica surface via hydrogen bond formation. The interaction of dimethyl adipate and precipitated calcium carbonate was weak and produced via Van der Waals forces. Thus, the stearate treatment of the precipitated calcium carbonate greatly reduced the strength of interaction with the DMA due to blockage of the surface adsorption sites. Carbon black had micropores and thus the surface groups were less accessible to DMA. The addition of precipitated calcium carbonate (PCC) caused a moderate increase in the rheological and viscoelastic properties of the polyurethane adhesive solutions likely due to the weak interactions of the PCC filler with the polyurethane chains because of the stearate coated PCC. However, the addition of filler improved the rheological and viscoelastic properties in the solid polyurethane films. The crystallinity and the melting enthalpy decreased by adding filler because of the decrease in the degree of phase separation in the polyurethane. Immediate peel strength value was improved in joints produced with higher content in PCC filled adhesive but the addition of an inferior amount of PCC caused significant increase in the final adhesion of the polyurethanes. Furthermore, the addition of precipitated calcium carbonate (PCC) filler increased moderately the rheological and mechanical properties of the polyurethane with respect to the addition of natural calcium carbonate filler. The chemical nature of the nanofillers affected the rheological properties of the polyurethane adhesives and composites. Only fumed silica modified drastically the rheological behavior because of the silanol groups-polyurethane interactions. Due to the different nanofiller-polyurethane interactions, both fumed silica and carbon black improved the thermal stability of the TPUs. However, the crystallinity increased in carbon black filled TPU and the mechanical properties improved as well. The highest immediate adhesion was achieved in the fumed silica filled TPU adhesives, but there were not differences in final adhesion in the joints produced with the other nanofilled polyurethane adhesives. The rheological and thermal properties of the hybrid polyurethane-nanofiller materials were enhanced when fumed silica was added meanwhile the crystallinity and the mechanical properties increased by increasing the carbon black content. In general, viscoelastic properties improved in the fumed silica containing polyurethane due to strong SiO2-polyurethane interactions by hydrogen bonding. The fumed silica and carbon black mixtures filled TPU adhesives showed an adequate immediate adhesion property. However, fumed silica and carbon black illustrated a synergic effect in the final adhesion properties in both joints made with flexible and rigid substrates. The dispersion of the different nanofillers (PCC, fumed silica and carbon black) into the TPU matrix via polymer solution and melt intercalation by extrusion was investigated and the final properties of the hybrid polyurethane-nanofillers composites were compared. By using both dispersion procedures, the formation of filler aggregates was observed. The dispersion procedure of the nanofillers influenced significantly the extent of interactions with the polyurethane. In general, better filler dispersion was obtained by melt mixing by extrusion although clusters formation into the polyurethane matrices was observed. The dispersion procedure neither the filler nature did not affect the melting of the soft segments in the TPU. Furthermore, the viscoelastic behaviour of the fumed silica filled polyurethanes changed strongly for nanocomposite prepared by melt mixing by extrusion because of stronger filler-TPU interactions. In most cases, melt mixing by extrusion procedure caused more rigid TPUs than solvent polymer mixing procedure because of a decreasing of storage modulus and consequently a decreasing of the viscoelastic properties was found.