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Polymerization of di- and tri-isocyanates can be templated onto the mesoporous surface of a preformed network of sol–gel-derived silica nanoparticles, resulting in a conformal ‘crosslinked’ coating that renders the interparticle neck zone wider. Upon drying, these crosslinked networks yield aerogels which are up to ∼3× more dense than native aerogels based on the underlying silica framework, but also up to 10× less hygroscopic and they may take more than 300× the force to break. These results have been obtained with one-step based-catalyzed sol–gel silica networks, as well as with gels derived through a two-step process involving an acid-catalyzed sol and a based-catalyzed gel. Furthermore, it has been also found that crosslinking increases the dielectric constant only by ∼35% relative to values reported in the literature for native silica aerogels of about the same porosity. Chemical investigations into the polymerization reaction have shown that the process of crosslinking involves reaction of the isocyanate with: (a) OH groups at the surface of silica to form carbamate; and (b) adsorbed water, to form an amine and carbon dioxide. This amine then reacts with additional isocyanates resulting in polymer chain extension and bridging of particles with urethane-terminated polyurea.

A model was developed for the processing of recycled plastic flakes into fibers to study the effect of the various processing parameters. Verification of the fiber rolling model was done by comparing it with the standard flat rolling model, i.e., rolling into flat sheets, by looking at the sample roll-face pressure distributions for the two processes. The roll-face pressure distribution for fiber rolling and flat rolling showed the same basic curve profile, but the fiber rolling had a higher magnitude for the pressure. It seemed reasonable that the rolling and cutting of the plastic into fibers would require a higher roll pressure than the rolling flat sheets. Rolling operations have a neutral point on the rolls, which is the point of maximum or peak pressure. The neutral point for fiber rolling is slightly nearer to the entry compared with hat rolling for the same percentage reduction in cross-sectional area.After verifying the fiber rolling model with the flat rolling model, a parametric study was performed on the fiber rolling model, which showed how the processing parameters affected the roll-face pressure distribution and the torque required to turn the rolls: the coefficient of friction between the rolls and the plastic flakes, the size and shape of the fibers created, the initial thickness of the plastic flakes, and the roll radius. The model was useful in understanding the basic science of the fiber rolling process, and would be useful in optimizing the design of a rolling mill if this is developed into a commercial process. (C) 2000 Elsevier Science S.A. All rights reserved.

The use of externally bonded Steel-Reinforced Polymer (SRP) and Steel Reinforced Grout (SRG) composites is a promising new technology for increasing the flexural and shear capacities of reinforced concrete (RC) members. The reinforcement system in these composites is a sheet made up of unidirectional cords consisting of tightly wound ultra high strength steel wires. The reinforcement can be molded into resin systems (SRP) or cementitious grout (SRG). This paper reports on a study aimed at investigating the flexural strength improvement and performance of RC beams with externally bonded SRP and SRG. Three specimens were tested under four-point bending with variables considered including the number of reinforcement plies and types of bonding agents. Based on this study, it was concluded that this new technology has potential for the repair and strengthening of concrete structures. Test results indicate that up to 100% increase in the flexural capacity can be achieved using these strengthening systems.