Quantification of natural rubber blends by reflection/reflectance infrared and confocal Raman spectroscopy: a comparison of statistical methods.

The blend of butadiene and acrylonitrile copolymer (NBR) with natural poly-cis-isoprene (NR) shows increased resistance to swelling in solvents in comparison to the individual components. In aerospace, NBR rubber is used as thermal protection for rockets and shall not contain other polymers, even in low contents, otherwise, it can affect the protection performance and rocket safety by causing detachment of the elastomer/propellant interface; therefore, this investigation presents methodologies to determine the NR/NBR contents. This study explores different analytical techniques, such as Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy, in the mid-infrared (MIR) by reflection and in the near-infrared by reflectance (NIRA) modes, Furthermore, quantification strategies by univariate, bivariate and multivariate (chemometric) models are evaluated and compared. A proposed methodology, based on multivariate Raman microscopy with partial least squares regression (PLS), showed high linearity (R2 > 0.99) and low error (< 0.82 %). The validation of FT-MIR data for the CH3, which presented lower error (1.3%) than vinylidene band (6%), showed that both methodologies (reflection and NIRA reflectance) can be used for the quantification of NR in NR/NBR. These results constitute a contribution to the state of the art in researching industrial and aerospace elastomeric applications.

Synthesis and characterization of Poly(urea-formaldehyde) microcapsules with 5-ethylidene-2-norbornene as self-healing agent and potential use in polymeric composites.

This study describes a methodology to prepare and characterize PUF [poly(urea formaldehyde)] microcapsules containing 5-ethylidene-2-norbornene (ENB) as self-healing agent, as well as the compatibility evaluation of ENB in the presence of Grubb's catalyst with an epoxy matrix. First, the results of an adhesion assay by lap shear proved chemical compatibility between the epoxy matrix and self-healing agent in the presence of Grubb's catalyst. After evaluating the chemical compatibility, microcapsules with ENB were synthesized in an oil-in-water emulsion system. Thereafter, the microcapsules were morphologically, chemically and thermally characterized in which a granulometric dispersion between 30-140 µm and an average size of 69.8 ± 4.9 µm were observed. The SEM (scanning electron microscope) results showed that the average thickness of microcapsules shell was 3.6 ± 0.4 µm. The thermogravimetric analysis (TGA) showed that microcapsules are susceptible to rupture and consequent ENB release in temperatures greater than 230 °C, demonstrating that microcapsules are suitable for applications in materials with self-healing capacity.

Impact assessment of synthesis parameter stirring speed in final physicochemical properties of PU microcapsules incorporated into epoxy matrixes.

A considerable number of papers have been published to assess self-healing capacity of several materials, as well as several applications in different areas. However, the impact assessment of changing synthesis parameters of microcapsules, hollow fibers or microvascular systems in its final physicochemical properties are still an emerging research field. This current paper presents a synthesis process of PU microcapsules containing TDI as core agent and the characterization of microcapsules physicochemical properties. During the synthesis, the reaction parameter stirring speed was changed to assess the impact that this parameter has in the final microcapsules' physicochemical properties. Microcapsules were characterized by FT-IR, TGA and image analysis (OM and SEM). Additionally, microcapsules were incorporated to an epoxy matrix (5% weight/weight) to assess the impact in the final physicochemical and mechanical matrix properties. Epoxy-based test specimens were also obtained within aramid and silica, which are traditional reinforcing loads in rubber synthesis. Final mechanical properties of matrixes within aramid and silica were compared to the properties of matrixes within microcapsules to determine what kind of behavior the microcapsules have when incorporated to epoxy matrixes.

Qualitative and quantitative evaluation of epoxy systems by Fourier transform infrared spectroscopy and the flexibilizing effect of mercaptans.

Epoxy systems are widely applied as adhesives in the aerospace industry. They have excellent adhesion properties, however, being thermosetting, epoxy systems show fracture brittleness characteristics. Polysulfide and polymercaptans are good options to increase the flexibility of the epoxy adhesive. Thermal analysis techniques are generally used to evaluate the curing degree of epoxy systems. In most cases, when infrared (IR) analysis is used, it is employed qualitatively. This paper presents the reaction study of a DGEBA epoxy prepolymer with diethylenetriamine (DETA) and linear and branched dodecyl mercaptans as flexibilizers. Conversion data and curing time were assessed qualitatively and quantitatively by Fourier Transform Infrared Spectroscopy (FT-IR) in the medium infrared region (MIR) and in the near infrared region, using near infrared reflectance accessory (NIRA). NIRA methodology showed satisfactory results, with errors between 3 and 7%, especially in samples with lower amine contents. Mechanical tests confirmed the flexibilization of the cured epoxy system by the addition of mercaptans, indicating a lower crosslinking degree in the matrix. Young's modulus (E) significantly decreased from 2017 MPa to 578 MPa with the addition of approximately 20 wt% of normal dodecyl mercaptan to the epoxy system.

Expanded perlite/cork fillers applied to aerospace insulation materials.

Insulations for rocket motors such as Flexible Thermal Protections (FTPs) and Rigid Thermal Protection (RTPs) act as thermal barriers against the hyperthermal environment from the solid propellant combustion. FTPs present dual function: to extend RTPs performance, and to attenuate the propellant contraction. FTPs used in the Brazilian Space Program have asbestos in their composition since the 70´s; however, they are hazardous for human health. In this context, a mixture of Expanded Perlite (PExp) and Cork Powder (CP) was investigated as a replacement for asbestos. Results showed reduction about 21 % in density and an increase of 13 % in the ablative properties. The low mechanical properties not interfere in this type of FTP due to your function of attenuate the propellant contraction.