L. Ibarra

Dynamic properties of thermoplastic butadiene-styrene (SBS) and oxidized short carbon fiber composite materials

In composites consisting of a thermoplastic butadiene-styrene (SBS) elastomer matrix reinforced with oxidized short carbon fiber, scanning electron microscopy ( SEM ) reveals the existence of matrix-fiber interactions, which are not detected when employing commercial carbon fiber. Interpretation of the dynamic properties and other parameters, such as equivalent interfacial thickness, and glass transition temperature, measured in terms of maximum damping temperature, as well as the apparent activation energy of the relaxation process, helps to explain the existence of such interactions.

Overcoming the disadvantages of fumed silica as filler in elastomer composites

Elastomeric composites have been prepared using pre-treated fumed silica as a reinforcing system. The processing disadvantages of fumed silica were overcome by a novel one-pot modification method based on silanization treatment of silica particles after an ultra-high energy sonication process that breaks down the micro-aggregates/agglomerates of pristine silica into nanoparticles. Grafted organosilane molecules on the nanoparticle surfaces avoid the natural tendency for silica particles to re-agglomerate obtaining a stable dispersion of organo nanosilica particles that are compatible with the rubber matrix. The modified nanosilica can be easily incorporated into the elastomer matrix by conventional methods employed in rubber technology, contrary to pristine fumed silica which shows significant compounding disadvantages leading to high viscosity compounds. The improved dispersion and enhanced interface significantly enhance the final properties of the composites. Finally, the filler–rubber interface was characterized combining 1H low field Double Quantum (DQ) NMR spectroscopy and equilibrium swelling experiments. This novel experimental methodology demonstrates the existence of improved interactions in the interface between silica particles and rubber macromolecules.

Vulcanization of carboxylated nitrile rubber (XNBR) by a mixed zinc peroxide–sulphur system

Vulcanization of carboxylated nitrile rubber (XNBR) with a mixed system based on zinc peroxide and sulphur accelerators produces materials with favourable mechanical properties because of the formation of ionic aggregates which give the vulcanized compounds a certain rigidity. These properties are drastically reduced by the effect of saturated ammonia vapour which plasticizes the ionic aggregates. This plasticization, however, is reversible and the aggregates can be regenerated by addition of a solvent, which results in recovery and even improvement of the original properties such as stress at constant deformation, tensile strength, crosslinking density and storage modulus. The temperatures of the two transitions observed in the relaxation spectra, the glass transition of the polymer and the ionic transition corresponding to ionic aggregates, are displaced to higher values when ionic structures are regenerated. © 2000 Society of Chemical Industry

Elastomer composites based on improved fumed silica and carbon black. Advantages of mixed reinforcing systems

The combination of reinforcing fillers is one of the most promising tendencies in elastomer reinforcement, where a synergic effect between them is desired. Up to now, the use of mixed fillers has been focused on the preparation of dual fillers based on a complex synthesis process where silica particles are in situ generated in a carbon black matrix. In this paper, mixed filler systems are prepared following a straightforward methodology, where carbon black is gradually and partially substituted by modified fumed silica. The presence of reinforcing fillers with different surface energies significantly decreases filler networking, reducing the non-linear viscoelastic response of the composites. The hybrid reinforcing systems prepared were easily incorporated into the elastomer matrix by conventional methods in rubber technology. Physical and dynamic responses of the elastomeric composites were characterized in order to find the optimum mixed system combination. Filler–rubber interface was characterized combining 1H low field DQ (Double Quantum) NMR spectroscopy and equilibrium swelling experiments.

Mechanical properties of thermoplastic butadiene-styrene (SBS) and oxidized short carbon fibre composites

This work reports on some results of research conducted on composite materials consisting of a butadiene-styrene (SBS) thermoplastic elastomer matrix filled with short carbon fibres previously subjected to oxidative treatment to increase the surface functionality. Scanning electron microscopy confirms the existence of interactions between the matrix and the fibre, which are not observed for commercial fibre fillers and which translate into mechanical strength increments, in terms of the Youngs modulus, tensile and tear strengths for the oxidized fibre composites. The stress-strain curves of the composites show yield point phenomena as strain is applied longitudinally to the main fibre orientation. In oxidized fibre composites the stress and strain coordinates are a function of the degree of oxidation (greater strain for more strongly oxidized fibre) and fibre strength (lower stress for longer treatment times).

Effect of covalent cross-links on the network structure of thermo-reversible ionic elastomers

The effect of forming additional covalent cross-links on the thermo-reversible network structure of ionic elastomers was evaluated. Ionic elastomers are characterized by a strong physically cross-linked network resulting from a phase separation of ionic-rich nano-domains. The ionic domains are formed by the association of ionic groups that act as cross-links, promoting the elastic behaviour of these polymers. In addition, the trapped glassy rubber around the ionic associations improves the physical properties of these materials acting as reinforcing points of the soft rubbery matrix. These materials are considered as thermoplastic elastomers because of the thermo-labile nature of the ionic associations; however, this property limits their potential applications at elevated temperatures. In order to overcome this disadvantage some permanent covalent cross-links were formed in the ionic structure, improving their properties at high temperature but without altering their reversible thermoplastic nature. The formation of covalent crosslinks increases the number of ionic nano-domains that present a smaller and more homogeneous size distribution. This variation in the network structure is closely related to the enhanced properties shown by these materials. Therefore the proper combination of covalent and ionic cross-links allows the control of the network structure of ionic elastomers in order to obtain tunable properties for these materials.

Vulcanization of carboxylated nitrile rubber (XNBR) by zinc peroxide

The vulcanization of carboxylated nitrile rubber (XNBR) with zinc peroxide, which produces ionic crosslinks, has been studied in relation to vulcanization time. Vulcanized compounds present two transitions, corresponding to the glass transition of the polymer at low temperatures and the ionic transition resulting from the formation of ionic aggregates. Both transitions are displaced to higher temperatures with increasing crosslink density. The ionic associations which give rise to high values of mechanical properties disappear on exposure of the vulcanized compounds to saturated ammonia vapour. This treatment produces a decreased crosslink density resulting in the disappearance of the ionic transition. When the action of ammonia is terminated by immersion in solvent followed by drying, the original crosslink density is recovered and the ionic transition reappears, although at higher temperatures. However, with increasing crosslink density, the difference between the temperatures at which both transitions take place diminishes. All these factors can be interpreted as reflecting the generation of a new and more compatible arrangement of the newly-appearing ionic clusters. © 1999 Society of Chemical Industry

The effect of crosslinking type on the physical properties of carboxylated acrylonitrile butadiene elastomers

Vulcanization of carboxylated nitrile rubber (XNBR) with different vulcanizing agents (i.e., zinc peroxide, sulphur, and a zinc peroxide-sulphur mixed system) was studied. Properties of the vulcanized compounds depend on the type of crosslinking produced (i.e., ionic or covalent) rather than the crosslinking density. Ionic crosslinks gave rise to greater stress relaxation, relaxation rates, and a greater generation of heat. In the relaxation spectra, tan δ versus temperature, two transitions appeared. Those occurring at the lower temperature corresponded to the polymer T g , while the transition occurring at the higher temperature was associated with ionic structures. The properties of the vulcanized compounds with ionic crosslinks decreased drastically after treatment with ammonia, which acts as a plasticizer of the ionic aggregates formed. The effect of ammonia disappeared on expansion in solvent, which resulted in the recovery of the original crosslinks, producing a value of υ r -volume fraction of swollen rubber in equilibrium-close to the original value and the reappearance of the ionic transition.

Mechanical and viscoelastic properties of butadiene–styrene thermoplastic and oxidized carbon fibre composites

The mechanical and dynamic properties of oxidized carbon fibre and butadiene-styrene thermoplastic elastomer (SBS) composites were studied as a function of the level of fibre oxidation and in comparison with the properties of composites reinforced with untreated commercial carbon fibre. As a general rule, fibre oxidation gives rise to materials with improved mechanical properties-greater tensile and tear strengths. The improvements accomplished depend on the degree of fibre oxidation. The effects of long exposure times to oxidizing agents were tested on the experimental samples, i.e. increase in the number of functional surface groups and loss in mechanical strength due to a decrease in the L/d ratio, properties which act in opposite directions in the composite. Storage modulus retention with increasing strain amplitude is directly proportional to the number of functional groups incorporated into the fibre surface, whereas at low strain amplitude it is proportional to fibre strength, measured in terms of the L/d ratio after processing. It is suggested that improved adhesion at the matrix-fibre interface is obtained through the functional groups of the oxidized fibre. As a consequence of fibre-matrix interface and at any frequency, the damping peak temperature is shifted towards higher ranges and at the same time the apparent activation energy of the relaxation process is observed to increase.

Reactivity of short oxidized carbon fibres and functionalized elastomer

Using the curves obtained with a Monsanto oscillating disk rheometer, a clear reaction can be seen between short oxidized carbon fibres and the epoxidized natural rubber matrix, which does not occur, or occurs to only a very small extent, when non-treated carbon fibres are used. This reaction causes a continuous, although less accentuated, increase in the elastic component of the S force couples. The extent of the reaction depends on the fibre content, on the degree of oxidation of the fibres and on the temperature. The reaction among the matrix and fibre components is translated into an increase in the percentage of non-solvent extractable rubber.