The scope of this paper is to review peroxide curing of diene elastomers. At high dicumyl peroxide loading and high curing temperature, changes in the crosslinking mechanisms were observed. Both Abstraction of allylic hydrogen and addition to the double bond can act as mechanisms of crosslinking. IR spectroscopy reveals a significant decrease in double bonds, and a new relaxation at temperatures above the Tg of the elastomers is present.

SYNOPSISThe effects of different types of elastomeric matrix (NR, SBR, CR, NBR) and several levels (10,15, and 20 parts phr) of short fibers on mechanical properties of uncured and cured composites and on swelling behavior of composites in hydrocarbon solvent are studied.The variation of the dynamic properties, E', E", and tan 6 is determined as a function of deformation amplitude, temperature, and vibration frequency in composite materials subjected to dynamic deformation. The increase of fiber level does not limit the orientation ability of the fibers, which in all materials seemed to be above 70%. The addition of fiber markedly reduces maximum swelling and entails a n increase in material stiffness. In addition, the amount of dissipated energy is increased and hence transformable into heat upon fiber incorporation, which can reach up to 16 times the value corresponding to the matrix alone, in addition to an increase with strain amplitude. The effect is most pronounced in the presumed direction of fiber orientation.The marked reduction of elongation at break ( u p to values of 7-9% of those of unfilled samples) and the shape of stress-strain curves point to a good fiber-matrix adhesion. Dynamic glass transition temperature is displaced toward higher values as a consequence of matrix-fiber interaction, which increases proportionally to fiber level, thus proving a linear relationship between thermal displacement and the number of interactions between the two phases. By the same token, the apparent activation energy of the relaxation process is enhanced for fiber-containing materials as compared to the fiber-free matrix. The fiber composites present a less prominent yet broader transition zone.

A step forward in the understanding of rubber vulcanization with organic peroxides is provided by combining a proper arrangement between polymer, vulcanizing agent and cure conditions. For this purpose, an ethylene-vinyl acetate copolymer with a high content of vinyl acetate (70 mol%) was used since a fully saturated polymer backbone allows its vulcanization via peroxide. For the range of conditions analysed here, it is shown that the predominant process taking place is crosslinking via radical recombination, minimizing or even avoiding undesirable secondary reactions such us polymer degradation. Once conditions had been optimized, peroxide vulcanization was analysed in more depth in the presence of 2,2,6,6tetramethylpiperidinyloxyl, which is a mediating stable free radical commonly used in controlled radical polymerization. Consequently, it was possible to differentiate the termination reaction from the initiation and propagation steps, allowing the determination of the enthalpy of formed C-C crosslinks as measured using calorimetry. It was possible to isolate and determine the contribution of the crosslinking pathway from the global vulcanization reaction by means of calorimetric methods at optimum conditions. In fact, this simple methodology could be an important tool for understanding in detail the complex peroxide vulcanization of elastomers since reactions involved in this process determine the final network structure, and thus the final elastic properties of these compounds.

SynopsisThis paper reports on the study of the mechanical and dynamic properties of composites with an EPDM matrix and short fiber, either polyester or carbon, at three different fiber concentrations. In general, the properties prove to be dependent on fiber concentrations and type, in particular on the final ratio fiber length/diameter. This ratio ranges for carbon fiber over 35-45 and for polyester over 135-175, thus placing the former near the lower limit of acceptability. Evidence is supplied of the existence of adhesion between the matrix and the fiber, in the form of measurements of the swelling and dynamic properties, especially through variation of relative damping and the displacement of the dynamic glass transition temperature towards higher ranges. Composites present a marked property anisotropy.

This paper analyzes the effects of the addition of a three‐component dry type adhesive system on the properties of composite SBR (styrene‐butadiene rubber) material containing 20 percent short polyamide (5 mm length and 60 μm diameter). The curing times are extended and green strength is doubled. The replacement of the silica component in the system by a natural silicate is not only feasible, but proves to be favorable for certain properties, such as the storage modulus E′, which becomes less sensitive towards temperature changes. Dynamic property measurements provide evidence of an additional dispersion corresponding to the matrix‐fiber interface located around 40 to 50°C, which does not occur in the composites without adhesive. The activation energies of the main relaxation process of these materials are determined, amounting to 175 KJ/mol for the matrix and 250 KJ/mol for adhesive‐containing composites, as a consequence of the formation of strong bonds between both phases.