SynopsisThermoplastic polyurethane is a relatively new development in the elastomers field, combining elastomeric behavior with thermoplastic processing characteristics. Thus, high production rates and reduced scrap can be achieved but, increased versatility for the incorporation of additives is also possible. In this work, thermoplastic polyurethane matrices were prepared and Kevlar pulp w a s dispersed as a reinforcing agent. Since good dispersion and appropriate filler volume fraction are critical factors for a reinforcing effect, special attention was paid to the optimization of these parameters. Thermoplastic polyurethanes were prepared by mixing a low melting polyol with a low melting diisocyanate and the subsequent addition of small amounts of a chain extender, namely, a low molecular weight diol such as 1,4 butanediol. The Kevlar pulp was added under stimng either in the polyol or the diisocyanate melt. This incorporation technique allows the dispersion of small fiber percentages. Thus, calendering of the materials prepared was also performed and additional fibers were incorporated. Changes in the properties of the specimens prepared as a function of the filler volume fraction and the dispersion technique were determined, taking into consideration the possibilities of chemical interaction between fibers and matrix.

SynopsisCrosslinked polyethylene finds extensive use in applications where improved thermal resistance is required. Furthermore, reinforced and crosslinked polyethylene presents high strength at elevated temperatures, so that the field of applications of such materials is considerably enlarged. Aramid fibers seem to be a promising reinforcement for the above materials since they combine high modulus with thermal resistance and light weight. The development of an adequate adhesive bond between aramid fiber and polyethylene matrix is of course the main problem, taking into consideration that both polyethylene and aramid present reduced chemical affinity to other substances. Thus, fiber pretreatment was performed in order to insert unsaturated groups in the aramid chain. These groups could be, in turn, grafted in the polyethylene backbone during the crosslinking reaction. The aramid fibers were treated with methacryloyl chloride, and then dispersed under stirring in 2% aqueous sodium hydroxide containing 0.5% of a nonionic surfactant. After washing and drying, the fibers were dry blended with polyethylene powder containing 2 phr dicumyl peroxide. The mixture was melt-blended in a Brabender plasticorder and tensile specimens were prepared. Tensile measurements and microscopy on the fracture surface were carried out in order to evaluate changes introduced in the fiber by surface treatment.

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The incorporation of BaS04 containing cotton threads into acrylic matrices was studied, as a means of reinforcing the polymer and producing radiopacity at the same time. Coating the threads with a thin poly(vinyl acetate) film containing BaS04 was defined as the most suitable method, providing strength and wettability to the fibres. A percentage of about 10 p.h.r, fibres can be characterized as satisfactory for both objectives, i.e. radiopacity and reinforcement. |ntroduetionPoly(methyl methacrylate) (PMMA) is a well established material in dental prosthetics because of the combination of properties such as biocompatibility, versatility in preparation and repair as well as excellent appearance. A recognized disadvantage of the acrylics used for the construction of dentures, is their brittleness leading to breaks during service as a result of fatigue failure in the mouth or impact failure out of the mouth.Many attempts have been made to overcome this problem by the introduction of new materials or the modification of existing acrylics. Stafford and Smith [1] investigated polycarbonates and pointed out that this polymer is superior to acrylic resins in deflection, impact strength and thermal expansion. Grant and Greener [2] attempted to reinforce acrylic resins using sapphire whiskers and reported an improvement of physical properties such as flexural strength.The development of new, high-strength and highmodulus reinforcing fibres has, of course, attracted the interest of many scientists. Schreiber I-3, 4] investigated the reinforcing effect of carbon fibres and reported considerable improvement. The same reinforcing medium was also used by Manley et al. [5,6], whereas other researchers studied the effect of carbon fibres on the fatigue resistance and bending properties of denture resins, the modification of ftexural strength and modulus of carbon fibre-acrylic resin composites [8]. Apart from carbon fibres, many other reinforcements have been tested, such as carbon black [9], aramid fibres [10], or combinations of carbon and glass fibres leading to hybrid composites [-11]. On the other hand, the strengthening of acrylic resins has also been attempted by the incorporation of shock absorbers such as small rubber particles [12,13].* Author to whom all correspondence should be addressed.An additional problem of the acrylics as well as other polymers used as prosthetic materials is that they are radiolucent, i.e. transparent to X-rays. There is, therefore, a danger in their use in dentistry, medicine and surgery. Dentures or parts of dentures may be aspirated or swallowed and it is also possible that in case of traumatic injuries to the face (as in road traffic accidents) dentures or fractured portions of dentures may become impacted into the tissues. The radiolucency of the material causes problems in the detection of such foreign bodies, Many attempts were made therefore to modify acrylics and other prosthetic polymers to radiopaque materials, including the dispersion of finely divided inorganic salts, such as bismu...