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Polymer Matrix Nanocomposites

Involved People: Alfonso Maffezzoli, Mariaenrica Frigione, Antonio Greco, Carola Esposito Corcione
Polymer matrix nanocomposites reinforced with inorganic clay minerals of dimensions in the nanometer range are the most studied, due to their unexpected synergistically properties derived from the two components. This new class of materials is characterized by improved thermal, mechanical and barrier properties compared either to the matrix either to the conventional composites (called also microcomposites) because of their unique phase morphology deriving by layer intercalation or exfoliation, that maximizes interfacial contact between the organic and inorganic phases and enhances bulk properties. The most popular nanocomposites are composed of thermoplastic or thermosetting matrix and an organically modified montmorillonite (OM). On the other hand, a few examples of incorporation of boehmite (AlOOH) as nanofiller are found in literature. The research activity is focused on the incorporation of both type of nanofillers in polymeric matrices as well as on the use of graphene nanoplatelets. Different nanocomposites were prepared and characterized:
  1. Polyurethane matrix nanocomposites. Nanocomposite adhesives obtained using an organically modified montmorillonite (OM) at different concentrations in a polyurethane matrix PU adhesives are used to produce multilayer laminates for food packaging. However their function in laminated films is limited to bond the films and their contribution to the overall barrier performance of the laminate is usually neglected. If the adhesive could contribute to the barrier performance of the laminate, besides adding a new value to the adhesive component, this would lead to a reduction in laminate thickness. A polyurethane gas-barrier coating can also be used. Increased permeation-barrier properties of PU adhesives can be achieved by incorporating nano-clay. In contrast to microcomposites, impressive improvements in performance were achieved with a small amount of filler. A typical adhesive formulation used in laminated films for food packaging is studied. The effect of different preparation procedures was compared in order to correlate exfoliation, rheological and mechanical properties. Nanocomposite adhesives obtained using a montmorillonite, modified with two different organic cations, in a polyurethane matrix were synthesized and characterized. A mixed of exfoliated and intercalated layers was observed in all composites. A good increase of tensile properties and Tg of nanocomposites with nanoclay content is also observed, confirming the good dispersion of the nanofiller. Rheological analysis on OMM-filled polyol indicated that as the clay content increases, the clay lamellar crystals acts as weakand labile physical cross linking points inducing either an increase of viscosity of more than two order of magnitudes either a strong non-Newtonian behavior (Figure 1).The analysis of micromechanical behaviour of the nanocomposite showed that a partially exfoliated structure is obtained, confirming experimental results (Figure 2) [1-4].
  2. Optically transparent nanocomposites. An organically modified boehmite at different concentrations in a conventional diglycidyl ether of bisphenol A (DGEBA) epoxy matrix was used to prepare transparent nanocomposites. A solvent based procedure, aimed to improve the dispersion of an organic modified boehmite, was successfully adopted. Incorporation of the nanofiller in the epoxy matrix slightly reduces the optical transparency of the neat epoxy (Figure 3a and 3b). On the other hand an appreciable increase of the glass transition and of all mechanical properties was observed. The dependence of the flexural modulus on the filler volume fraction was analysed using the Halpin-Tsai equation obtaining an average aspect ratio of bohemite particles between 2 and 3. Improved properties were obtained incorporating boehmite in the epoxy network up to 10% by wt. A higher amount of nanofiller is associated to a very limited increase of the properties and a relevant reduction of the transmittance. In summary the nanosize dispersion of boehmite, confirmed by X-ray , SEM and UV-VIS-NIR spectroscopy, results in a overall property increase even associated with a toughness increase, typically observed in composites with fillers of nanometric size [5-6].
  3. UV curable nanocomposite coatings. Nanocomposites were also prepared using an organically modified boehmite at different concentrations in a cycloaliphatic epoxy resin (CE), specific for UV-curable coatings, photopolymerized in the presence of triarylsulfonium hexafluoroantimonate as photoinitiator. Organic modified boehmite (modified with p-toluenesulfonic acid, PTA) was used in order to prepare nanocomposite coatings obtained through a photopolymerization process. Different amounts of the nanofiller, ranging from 5 up to 10 wt.% were dispersed into an epoxy cycloaliphatic resin, specific for UV-curable coatings, namely 3,4-epoxycyclohexylmethyl-3’,4’-epoxycyclohexane carboxylate (CE) and exposed to the UV light in the presence of triarylsulfonium hexafluoroantimonate as photoinitiator. The kinetics of photopolymerization was deeply investigated by means of two different techniques, photocalorimetry and real time FT-IR spectroscopy. The effect of the presence of nanofiller on the thermo-mechanical properties of the obtained nanocomposites was investigated and correlated to their composition and morphology. This work exploits the possibility of preparing nanocomposite coatings by using photochemically initiated cationic polymerization. The recent growing interest in cationic photopolymerization is related to the development of very efficient photoinitiators and to the distinct advantages and applications of this radiation curing method. The experimental conditions in order to have the possibility of comparing the results obtained through p-DSC and RT-IR analyses were settled and optimized. These techniques evidenced a substantial correspondence as far as the final conversion of the epoxy groups is concerned; on the contrary, the shape of the kinetic curves was quite different: in particular RT-IR spectroscopy was found to be quicker with respect to photocalorimetry, thus evidencing, at constant times, higher kinetic profiles than p-DSC. The optical transparency found for the nanocomposites indicated that Boehmite nanoparticles were homogeneously dispersed in the polymer matrix. Some physico-thermo-mechanical properties of the final UV-cured products were evaluated and correlated to the type and the amount of the nanofiller employed [7,8].
  4. Development of polyamide and polyester nanocomposites through incorporation of nanodispersed clay. Organic modified montmorillonite can have a significant impact on Poly(ethylene terephthalate) (PET) properties, including flame resistance, oxygen permeability, elastic modulus. For this reason different amounts of nanofiller were incorporated in the polymer matrix. In this work, an alternative route for the production of PET based nanocomposite is proposed. An intermediate masterbatch is produced by melt mixing of organically-modified montmorillonite clay (OM) with amorphous PET (aPET) at low temperatures (150C) for long times (1h). In order to avoid polymer hydrolysis during melt compounding, the static mixer was modified to work under nitrogen atmosphere. Different amounts of nanofiller were incorporated in the polymer matrix. X-ray analysis was performed on the aPET masterbatches, showing that intercalation and probably exfoliation take place during static mixing (Figure 4). Water permeability measurements on PET150 samples are shown in Figure 5, where the evolution of Q*L is reported as a function of time. After a non steady state time interval a linear weight increase is observed, indicating that the steady state conditions are very soon achieved. The results obtained show a significant decrease of the permeability of PETg with addition of the nanofiller [9-11]. Another apoproach was developed for the production of thermoplastic poilyester matrix nanocomposites throught the ring opening polymerization of cyclic buthylene-terephthalte, for the production of poly-buthyleneterephthalate nancompsosites [12].
  5. Development of polyurethane based nanocomposites for the production of flame retardant foams. In this activity, the nanofiller was properly modified by the addition of a compatibilizer which allows to improve the intercalation properties in polyether poylol dispersions. Further addition of the isocyanate allows to obtain a polyurethane foam in which the nanoparticles are completely exfoliated. Such foams show an improved fire resistance than conventional foams, and the fire retardancy effects of few amounts of nanofiller is equivalent to that of much higher quantities (>20%) of other melamine-based flame retardants [13].
  6. Nanofilled suspensions. The rheology of fluids suspension containing nanodispersed particles was studied. In particular, a rheological characterization of bentonite dispersions in isododecane at different clay content and shear history was performed. For each inorganic content, both mixed samples and samples subjected to several calendering runs were studied. The effect of shear and clay content on the viscoelastic properties was investigated by a combination of oscillatory shear experiments under small-deformation conditions and by X-Ray diffraction. The tested samples showed a gel-like behaviour with a final structure depending on the applied shear stress. By increasing the inorganic content in the dispersion, a reduction in the gel stability to a further shear application was observed. Two models, developed for colloidal gels, were used to fit the rheological results enabling to evaluate the microstructure and the degree of dispersion of the tested samples and to relate the colloidal structure to the elastic properties [14].
  7. Grafene based nanocomposites. The last research efforts were devoted to the synthesis and characterization of thermoplastic and thermosetting resin filled with graphene. Preliminary activities are ongoing using either amorphous PET either an epoxy resin as matrices.

FIG. 1 - Comparison between the experimental results of the viscosity as function of shear rate of unfilled polyol and that of polyol filled with OM clay, with the Ellis model predictions.

FIG. 2 - Comparison between experimental Yong's modulus data and Halpin-Tsai predictions for PU nanocomposites
  Figure 3a and 3b: a) Light transmittance of the nanocomposites compared with that of the unfilled epoxy resin in the wavelength range 200-1500 nm. b) transparent sample of an epoxy matrix nanocomposite sample

Figure 4: X-ray analysis performed on the aPET masterbatches

Figure 5: Permeability tests results for PETg and nanofilled samples.


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University of Salento   Facoltà di Ingengeria    Department of Engineering for Innovation