SYNOPSISDrawing on the difference in melting points of UHMPE fiber (150°C) and HDPE matrix ( 13OoC), single-polymer composites were fabricated under various processing conditions. Because of the chemical similarity of the composite components, good bonding a t the fibermatrix interface could be expected. The matrix, the fiber, and unidirectional composite laminae were studied using TMA and DSC analyses, a hot-stage crystallization unit attached to a polarizing microscope, and an universal tensile testing machine. The TMA showed negative thermal expansion of the fiber over the complete temperature range of the experiment. Three regimes of contraction according to the values of the thermal expansion coefficient were detected. DSC analyses of either the fiber or the composite specimens did not show any appreciable changes after various thermal treatments. They also showed no evidence of fiber relaxation during manufacture, probably because of the pressure-related transverse constraint. The tensile strength and modulus values of the composite appeared to be fairly high and close to those reported for other composites reinforced with polyethylene ( P E ) fibers. An apparent maximum on the temperature dependencies of tensile properties was observed. A study of the matrix microstructure did not give any proof of transcrystalline growth at the fiber-matrix interface even for chemical or plasma surface-treated fibers.
SYNOPSISThe microstructure of polyethylene (PE)/PE composites, consisting of the high-density P E (HDPE) matrix and ultrahigh molecular-weight P E (UHMWPE) fibers, was investigated. Single-fiber composites were prepared and analyzed in a hot-stage crystallization unit attached to a polarizing microscope, aiming to find out how the conditions of crystallization affected the transcrystallinity ( t c ) growth at the fiber-matrix interface. Thermal treatments leading to two extreme microstructures, of either maximum or minimum thickness of the transcrystalline zone, were sought. It was found that a uniform transcrystalline layer was developed on the UHMWPE fiber from the HDPE melt under isothermal conditions, whereas rapid cooling from the melt prevented the generation of tc. The mechanical properties of unidirectional composite laminae either with or without the transcrystalline zone were measured.A comparison of the transverse strength predicted by theoretical models with the experimental values revealed good interfacial adhesion in the PE/PE system. It was shown that the t c growth had a negligible effect on the composite mechanical properties in the longitudinal direction, whereas it resulted in a 50% decrease of the transverse tensile strength and strain to failure. Scanning electron microscopy attributed that observation to premature brittle failure at tc/tc contact regions. 0 1995 John Wiley & Sons, Inc. I NTRODU CTl ONUltrahigh molecular weight polyethylene (UHMWPE) fibers are currently a prime subject of extensive ixwestigations pertaining to composite materials. A combination of highly oriented crystalline structure and of low density results in high specific mechanical properties of UHMWPE fibers and make them an attractive candidate for composite applications. However, the low surface energy and chemical inertness of polyethylene (PE) fibers cause their poor interfacial bonding with polymeric matrices's2 and restrict their use in structural applications, because the stress-transfer ability of their interface and, in turn, the mechanical properties of the composites are greatly affected by the level of fiber-matrix * To whom correspondence should be addressed. The concept of the single-polymer composite, consisting of a matrix and fibers of the same chemical nature (e.g., PE/PE or polyamide-polyamide) presents an alternative way of improving the adhesion in the case of UHMWPE fibers. An inherent chemical compatibility of the composite components is supposed to promote good bonding at the interface and to eliminate the need for a coupling agent. Indeed, based on the difference in melting points of UHMWPE fiber (147°C) and PE matrix (110-13OoC), single-polymer composites were manufactured, whose specific mechanical properties in the 959
ElectroInk, developed and manufactured by HP Indigo Division for its digital printing presses, is a complex fluid of unique rheological and electrical properties. Depending on the shear and the electric field the ink may be solid-or fluidlike. As ElectroInk propagates in the press from the ink reservoir to the substrate, the concentration increases while the structure of the ink changes from a dispersion of non-interacting particles to an elastic solid. The authors' model treats ElectroInk as an interwoven structure of two continuous phases, one being a network of pigmented resin particles and the other the incorporated liquid. In an external electric field the network shrinks like a sponge and partial phase separation occurs. Application of the model is shown for the design of 100% transfer of ink from roller to roller in the press, which is necessary to meet the principal requirement of digital printing, that each printed page can be different.
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