L. Irusta

Photooxidative behaviour of segmented aliphatic polyurethanes

Segmented polyurethanes based on aliphatic diisocyanate (hexamethylene diisocyanate) were synthesised varying the chemical nature (polyester or polyether) and molecular weight of the soft segment. These polymers were exposed to UV radiation and changes in the chemical structure were analysed by FTIR spectroscopy. Chemical nature and molecular weight of the soft segment are shown to be important factors which control the photodegradative behaviour of the urethane linkages.

The effect of a miscible and an immiscible polymeric modifier on the mechanical and rheological properties of PVC

Abstract PVC/copoly(ester-urethane) (Baymod PU®) and PVC/EVA copolymer blends have been analysed from the point of view of the effect of miscibility on the mechanical and rheological properties. The FTIR and NMR analysis of Baymod PU modifier leads to the conclusion that its miscibility with PVC (determined by dynamic mechanical analysis) is due to the presence of 31.9% poly(e-caprolactone) and 38.4% adipates in the main chain of the copoly(ester-urethane) polymer. However the PVC/EVA system considered in this work results in an immiscible blend, due to the low content of vinyl acetate, 33 wt%. The comparison of the mechanical and rheological properties of both blends with those of a PVC plasticised with a low molecular weight common plasticiser (PVC/DOP system), reveals that compatibility is not a determinant point in what some ultimate properties are concerned. The molecular weight (related to the viscosity of pure polymers, which is similar for both polymeric modifiers) seems to play a more important role than the miscibility. The stress–strain curves of both PVC/copoly(ester-urethane) (PU) miscible blends and PVC/EVA immiscible blends resemble each other and are very different with respect to PVC/DOP curves. The PVCs plasticised with DOP show a considerably higher elongation at break and a lower Young’s modulus than PVC mixed with polymeric modifiers. Miscible systems, PVC/PU and PVC/DOP, show similar values of the stress at break, slightly higher than those of immiscible PVC/EVA blends. The results of the latter two-phase system are well fitted to a two-parameter equivalent box model developed by Kolarik, considering a value very close to zero for the parameter, which accounts for the adhesion in between the phases. The reduction of the viscosity, induced by PU and EVA polymeric modifiers, with respect to that of pure PVC, is remarkable at shear rates such as those involved in calendering and milling, but is less significant at shear rates above 1000 s −1 which correspond to injection moulding processes. A better performance is obtained with DOP plasticiser at such high shear rates. For immiscible PVC/EVA blends the variation of the viscosity with composition has been adjusted using an adaptation of the Kolarik model, being this a novel approach to relate the viscous and mechanical behaviour of immiscible but in some way compatible blends [1] . As expected miscible PVC/PU blends viscosity data follow a free volume additivity model.

Thermal and mechanical behaviour of self-curable waterborne hybrid polyurethanes functionalized with (3-aminopropyl)triethoxysilane (APTES)

Two series of self curable polyurethanes were synthesized using an aliphatic diisocyanate (isophorone diisocyanate), an anionic diol (2-bis(hydroxymethyl) propionic acid) and two different soft segments poly(1,4-butylene adipate) end capped diol (semicrystalline polyester) and poly (propylene glycol) end capped diol (amorphous polyether). In both cases the polyurethanes were end-capped with (3-aminopropyl)triethoxysilane to impart the films the ability to crosslink at room temperature. The thermal and mechanical properties of the cured films can be tailored with the alkoxysilane concentration. Thus, as the alkoxysilane content increases, the resulting systems present a higher degree of phase separation, according to both DSC and DMTA results. In addition, TGA results confirm that their thermal stability also increases and finally, the modulus increases and the strain decreases as a function of the crosslinking degree.

Aromatic poly(ester–urethanes): effect of the polyol molecular weight on the photochemical behaviour

Abstract Poly(ether-urethanes) based on 4,4′-methylene bis (4-phenylisocyanate) or toluene diisocyanate and poly(tetramethylene oxide) have been synthesised varying the molecular weight of the soft segment. The hard/soft segment mixing has been studied by differential scanning calorimetry and Fourier transform infrared spectroscopy. The polymers have been exposed to UV radiation, and the changes in the chemical structure have been analysed by FTir spectroscopy. The extent of the photodegradation in the urethane linkage increases with soft segment molecular weight. The phase separated structure is a critical factor which must be taken into account in order to study the photodegradation of the hard segments in poly(ether-urethanes).

Infrared spectroscopic studies of the urethane/ether inter-association

Abstract Infrared spectroscopy has been used to determine the inter-association equilibrium constant for mixtures containing urethane/ether functionalities. This constant has been obtained both in the presence and absence of solvent and the results have been compared. The effect of the solvent nature, urethane structure and calculation method on the obtained equilibrium constant has also been studied.

Aromatic diselenide crosslinkers to enhance the reprocessability and self-healing of polyurethane thermosets

Dynamic structures containing polymers are considered a new class of polymeric materials with very attractive properties, since they can behave as thermosets at room temperature but at the same time they can be reprocessed to any shape. Recently, aromatic disulfides have been explored as dynamic bonds because of their good exchange reaction rate at low temperatures. In this work, we explore the utilization of aromatic diselenides on the basis of their lower bond energies in comparison with their disulfide counterparts. By comparing a model exchange reaction between aromatic diselenides and aromatic disulfides, the ability of aromatic diselenides to exchange faster than aromatic disulfides has been demonstrated. These results are supported by molecular quantum chemical calculations, confirming the faster exchange of aromatic diselenides in comparison with disulfides. In addition, a [2 + 1] radical-mediated reaction mechanism is proposed for aromatic diselenides to understand their faster kinetic exchange. An aromatic diselenide has been incorporated into polyurethane networks using a para-substituted amine diphenyl-diselenide. The resulting materials not only exhibit faster self-healing properties than the corresponding disulfide based materials, but also show the ability to be processed at temperatures as low as 100 °C. The fact that aromatic amines are already used as crosslinkers in a wide range of commercial products makes this system very attractive.

Preparation of superhydrophobic silica nanoparticles by microwave assisted sol–gel process

Hydrophobic silica nanoparticles were obtained by microwave assisted sol–gel method using a two-step procedure. In the first step different size silica particles were generated from tetraethyl orthosilicate and in the second one the silica particles were hydrophobized using hexadecyl trimethoxysilane (HDTMOS). Under microwave irradiation, high conversion degrees were obtained at relatively short reaction times. The HDTMOS added in the second step instead of coating the silica nanoparticles generated new ones and therefore the final product showed a bimodal size distribution. All the synthesized nanoparticles gave rise to high water contact angles (≈150°) and low hysteresis values.

In situ monitoring of isophorone diisocyanate-based flexible polyurethane foams formation:

Novel isophorone diisocyanate-based flexible polyurethane foams were prepared by the one-step method in a computerized foam qualification system (FOAMAT). The experimental conditions to obtain this type of foams, in relation to the nature and concentration of catalysts as well as the reaction temperature, were established as no data were available in scientific literature. The chemical reactions occurring during the foam generation process were monitored in situ by attenuated total reflectance-FTIR spectroscopy. The kinetics of the foam generation was fitted to an nth order model and the data showed that the foaming process adjusted to a first-order kinetics. The physical changes as pressure, foam height, and dielectric polarization were monitored by the FOAM software (FOAMAT). According to these parameters, the foaming process was divided into four steps: bubble growth, bubble packing, cell opening, and final curing.

The role of cellulose nanocrystals incorporation route in waterborne polyurethane for preparation of electrospun nanocomposites mats

Electrospinning offers the possibility of obtaining fibers mats from polymer solutions. The use of environmentally-friendly waterborne polyurethane (WBPU) allows obtaining electrospun polyurethane mats in water medium. Furthermore, the incorporation of water dispersible nanoentities, like renewable cellulose nanocrystals (CNC), is facilitated. Therefore, in this work, a WBPU was synthesized and CNC were isolated for preparing WBPU-CNC dispersions nanocomposites with 1 and 3wt% of CNC following both the classical mixing by sonication, and the innovative in-situ route. The dispersions were used for obtaining electrospun mats assisted by poly(ethylene oxide) (PEO) as polymer template. Moreover, the extraction of PEO with water resulted in continuous WBPU-CNC mats, showing different properties respect to WBPU-CNC mats containing PEO. The effective addition of CNC led to more defined cylindrical morphologies and the two alternative incorporation routes induced to different CNC dispositions in the matrix, which modified fibers diameters, and thus, mats final properties.

Oxygen Barrier Properties of Waterborne Polyurethane/Silica Hybrids

The oxygen barrier properties of films obtained from waterborne polyurethane/silica hybrid dispersions were analyzed. Two different types of polyurethanes were used, based on poly(propylene glycol) and poly(1,4-butylene adipate). Three different strategies were followed in the preparation of the hybrid dispersions. In the first type of materials (series 1), the inorganic part came exclusively from the covalent incorporation of trifunctional silane groups into the polymeric chains. The other two series contained, in addition to the trifunctional silane groups, tetrafunctional silane groups either physically blended (series 2) or “in situ” generated (series 3). Materials of series 1 showed an increase of the oxygen permeability coefficient with the silane content. However, the other two types of materials presented just the opposite dependence. In this latter case, the systems containing “in situ” generated silica (series 3) presented higher permeability coefficient values, probably because of the steric hindrance imposed by the polyurethane that gave rise to silica networks containing silanol groups and free volume holes. Moreover, lower permeability coefficient values were obtained when larger size particles were added. This fact could mean that the polyurethane/silica interface effects were not totally hindered even when the organic/inorganic phases were covalently bonded.