Durability and integrity studies of environmentally conditioned interfaces in fibrous polymeric composites: critical concepts and comments

Fibre reinforced polymer (FRP) composites are the most promising and elegant material of the present century. Their durability and integrity in various service environments can be altered by the response of its constituent i.e. fibre, polymer matrix, and the existing interface/interphase between the fibre and polymer matrix, in that particular environment. The interphase is generally manifested by chemical bonding, molecular segregation and also by van der Waals bonding. The sizing of fibres generally influences the chemistry and character of the interface/interphase and might generate structural gradient in the polymer matrix. Their susceptibilities to degradation are dependent of the nature of environments and each of the constituents' responds differently and uniquely. Amongst the three constituents, the interface/interphase has a very critical role to play on the performance and reliability of FRP composites. The reduced glass transition temperature of the interphase may induce low modulus area, which subsequently affects fracture toughness and local stresses of the composite. These result in high fracture toughness at ambient temperatures, but significantly reduced performance at high temperatures.

Fluorescent labels to study thermal transitions in epoxy/silica composites

The analysis of the fluorescent response from the dansyl moiety as a function of temperature has been used to estimate the thermal transitions in silica particle/epoxy-based composite materials. Silica particles were surface-coated with 3-aminopropyltriethoxysilane (APTES) and 3-aminopropylmethyldiethoxysilane (APDES). 5-Dimethylamino-1-naphtalenesulfonyl chloride (DNS) and 5-dimethylaminonaphthalene-1-(2-aminoehyl)) sulfonamide (DNS-EDA) were selectively attached to the silanized silica particles and to the epoxy matrix respectively. The fluorescence results, interpreted in terms of the model of interpenetrating polymer networks, suggest that (i) independent of the silane coating, the interfacial region is slightly more rigid and heterogeneous than the epoxy bulk and (ii) the interface generated with APTES seems to be more flexible than that obtained with APDES.

Kinetic study of epoxy curing in the glass fiber/epoxy interface using dansyl fluorescence

The fluorescence response of the dansyl chromophore has been used to study the kinetic of epoxy curing processes. With this new method, comparison between the curing at the interface of a glass fiber/epoxy and in the epoxy bulk of a composite material was studied. The effect of two glass fiber surface treatments was investigated. Commercial E-glass fibers were surface coated with 3-aminopropyltriethoxysilane (APTES) and 3-aminopropylmethyldiethoxysilane (APDES). Fluorimetry (using fluorescent labels) and FT-NIR (Fourier transformed infrared spectroscopy in the near range) techniques were used to monitor the curing process in these composite materials. From the analysis of the data obtained, different simple kinetic models were discussed and apparent activation energies were obtained. Furthermore, from those techniques the respective results were compared to obtain complementary information. Independently of the sample and the technique used for the kinetic analysis, no variation of the activation energy of the epoxy curing reaction was found, which suggests that there are no changes in the mechanism of the reaction along the process. Fluorescence from dansyl located at the glass fiber/epoxy interface reflected that the kind of reinforcement treatment clearly affects the epoxy curing process exactly in that region. However, when analytical response comes from the whole system the mechanism of the reaction does not seem to change with the silane coating used although is quite different in comparison with the process at the interface.

The nature of the structural gradient in epoxy curing at a glass fiber/epoxy matrix interface using FTIR imaging

The curing process of an epoxide system was studied at the interface formed between a silane-coated glass fiber and an epoxy matrix. The gradient in the structure of the epoxy resin as a result of the cure process at the fiber/matrix interfacial region was monitored by FTIR imaging. For comparison, the epoxy curing at the interface formed between the epoxy resin and (a) an uncoated glass fiber and (b) a polyorganosiloxane (obtained from the silane used for the glass-fiber coating) were also monitored. Chemically specific images of the OH and the H-N-H groups near the interface region were obtained. These images suggest that there is a chemical gradient in the structure of the matrix from the fiber surface to the polymer bulk due to different conversions. The basis of the different kinetics of the curing reactions is a result of amino group inactivation at the interface. This deactivation translates into an off-stoichiometry of the reaction mixture, which is a function of the distance from the surface of the glass fiber.