A new coupling agent for aramid fibers

Various compounds were used for the surface treatment of aramid fibers in order to promote adhesion to polymeric matrices. The improvement of adhesive bond could be based either on the roughness of fiber surface or on chemical modification introduced by grafting. The compounds used are more or less reactive to the secondary amide groups, present on the aramid chain. Thus, the fibers were impregnated with acetic acid anhydride, sulfuric acid–acrylamide, and methacryloyl chloride. The effect of such treatment was first evaluated by optical microscopy and infrared analysis. Tensile measurements were also carried out in order to define any changes of strength and modulus. Finally, tensile specimens were prepared using unsaturated polyester reinforced with aramid fibers and the effect of chemical treatment on the tensile strength was determined. From the above study, methacryloyl chloride was proved an effective coupling agent with possible grafting to aramid fibers.     

Controlled release systems based on poly(lactic acid). An in vitro and in vivo study

A new biodegradable delivery system based on poly(lactic acid) has been formulated, with potential applications in sustained antibiotic release against bone infection. The in vitro release of a new quinolone (pefloxacin) from low molecular weight poly(D,L-lactic acid) Mw = 2×10³ lasted for 56 d whereas the in vivo delivery lasted 33 d. In both cases, the release rate is controlled by the drug diffusion and the polymer degradation, which seems to be the predominant factor. For the release experiments, discs were prepared from poly (D,L-lactide) Mw = 2×10⁴ with drug loadings of 2% and 10% w/w. It was concluded that pefloxacin concentration remains higher than the Minimum Inhibitory Concentration (MIC) against the major causative bacteria of bone infection. The results indicate that the two different types of poly(lactic acid) can be used effectively in an implantable antibiotic release system. ©2000 Kluwer Academic Publishers     

Synthesis and properties

Poly(D,L lactic acid) was prepared by bulk polymerization of D,L lactide, both under atmospheric pressure and in vacuum. The obtained polymeric products were characterized in terms of molecular weight, Mw, melting point, calorimetric response and swelling behaviour. All products were amorphous. Their molecular weights were determined by viscosimetry and ranged from 2×103 to 9×104. Similarly, the melting points ranged from 90 to 210°C. Swelling experiments, with specimens immersed in buffer solutions, showed that hydrolytic degradation started in a few days for the low Mw material, whereas for the higher molecular weight products it took much longer and probably followed a two-stage mechanism. This study suggests that the high molecular weight material could be an interesting carrier for the preparation of controlled release products, in cases where prolonged delivery is necessary. © 1999 Kluwer Academic Publishers     

Permeability of soft porous media under one-dimensional compaction

A novel experimental approach to measure permeability of porous material samples under variable longitudinal compaction has been developed. The material has a non-linear structural behavior and exhibits a small hysteresis during mechanical loading and unloading experiments. The new permeameter includes a piston moving inside a Plexiglas cylinder with controllable speed and a test section where the porous material sample is placed under compaction by two grids with adjustable positions. Time-dependent pressure was recorded at four different locations along the sample together with the velocity of the piston. Experiments with two different sample lengths have been performed at three different Reynolds numbers based on the apparatus diameter. The results show that pressure gradient and permeability data do not depend on initial uncompacted sample length. All experiments included measurements at various compaction ratios of the material followed by measurements during relaxation/expansion of the material. No hysteresis was observed in the pressure gradient and permeability data during compaction and expansion of the material for a wide range of compaction ratio. The effects of small velocity fluctuations due to variable friction of the moving piston with cylinder’s wall were also considered. These velocity fluctuations cause pressure fluctuations within the sample which are high close to the inlet part of the material sample and are reduced almost completely towards its outlet. However these pressure fluctuations when scaled with the corresponding mean pressure retained their time-dependent amplitude and phase unchanged along the material. These relative pressure fluctuations cancelled out the flow velocity fluctuations resulting insignificant fluctuations in permeability. It was found that permeability, which is a material property, is drastically reduced with increased compaction ratio of the material while its solid fraction changes substantially but its porosity remains practically unchanged. A comparison with the Cármán–Kozeny expression for random porous media was also examined. Cármán–Kozeny expression predicts qualitatively the reduction of permeability with compaction. However, the predicted values of permeability are very sensitive to the initial value of porosity.     

Treated polyethylene fibres as reinforcement for epoxy resins

Ultra-high-modulus polyethylene (UHMPE) fibres were treated in order to develop favourable surface and, possibly, microstructure characteristics. The main aim was to eliminate the microfibrillar morphology of the fibre and improve interfacial bonding between fibre/matrix so that better compressive properties can be achieved in reinforced resins. Calendering at 130 °C was performed, and the surface treatment used oxidative solutions. Adhesive bonding to epoxy matrices was highly improved in chromosulphate-treated material exceeding that of a commercial, corona-treated product, but the mechanical properties of these fibres deteriorated. Calendering did not significantly affect fibre strength and only improved adhesive bonding slightly. The use of these treated reinforcements is expected to improve the performance of composite materials, especially at low fibre volume fractions, because of their improved interfacial characteristics.     

Stability of DL‐poly(lactic acid) in aqueous solutions

DL ‐poly(lactic acid) of molecular weight about 2500 was prepared by polycondensation of lactic acid and characterized by viscosimetry, infrared spectroscopy, light scattering, GPC, and NMR. Tablets made of the above polymer were immersed in buffer solutions at 37°C, and their swelling behavior was recorded as a function of time, in terms of weight gain. In the same experiments, the hydrolytic stability of PLA was assessed by measuring the weight loss after drying the tablets. To inhibit any degradation due to bacteria, formaldehyde was added to the solution as a biostatic factor. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 795–804, 2003     

Factors Which Influence the High Temperature Adhesive Characteristics of Liquid Crystalline Copolyesters

Liquid crystalline copolyesters of p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid, with compositions 20/80 and 24/76, were studied in order to evaluate the effect of higher melting HBA/HNA copolyesters on the high temperature adhesive properties compared with data for the recently reported 73/27 system. A tetramer of HBA/HNA with a composition of 24/76 was synthesized to examine the effect on adhesion of wetting of the substrate at much lower temperatures. From the results of this study it is now possible to define approaches for achieving very good adhesion characteristics at temperatures well in excess of 250°C.