7.5 Technische Eigenschaften von Polymerwerkstoffen
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In the processing of nanocomposites, high shear stresses at elevated tempera-tures orient two-dimensional nanoparticles like graphene. This orientationleads to anisotropic mechanical, thermal or barrier properties of the nanocom-posite. This anisotropy is addressed in this study by comparing graphene (few-layer graphene, FLG) with a nanoscaled carbon black (nCB) at a filler contentof 3 phr, by varying the vulcanization, and by comparing different FLG con-tents. Transmission electron microscopy gives insight into the qualitative ori-entation in the nanocomposite with FLG or nCB. The storage moduli paralleland normal to the orientation reveal the direction dependency of reinforce-ment through dynamic mechanical analysis (DMA). Dimensional swellingmeasurements show a restriction of the expansion parallel to the FLG orienta-tion, and an increased expansion normal to the orientation. The vulcanizationsystem and crosslinking determine the respective level of property values, andhigher crosslinking densities increase the anisotropy in DMA resulting invalues of up to 2.9 for the quantified anisotropy factor. With increasing FLGcontent, the anisotropy increases. A comparison of the results reveals swellingmeasurements as the most suitable method for the determination of anisot-ropy. Compared to recent literature, the presented processing induces higheranisotropy, leading to higher reinforcing effects in the direction of orientation
Tailored crosslinking in elastomers is crucial for their technical applications. The incorporation of nanoparticles with high surface-to-volume ratios not only leads to the formation of physical networks and influences the ultimate performance of nanocomposites, but it also affects the chemical crosslinking reactions. The influence of few-layer graphene (FLG) on the crosslinking behavior of natural rubber is investigated. Four different curing systems, two sulfur-based with different accelerator-to-sulfur ratios, and two peroxide-based with different peroxide concentrations, are combined with different FLG contents. Using differential scanning calorimetry (DSC), vulcametry (MDR) and swelling measurements, the results show an accelerating effect of FLG on the kinetics of the sulfur-based curing systems, with an exothermic reaction peak in DSC shifted to lower temperatures and lower scorch and curing times in the MDR. While a higher accelerator-to-sulfur ratio in combination with FLG leads to reduced crosslinking densities, the peroxide crosslinkers are hardly affected by the presence of FLG. The good agreement of crosslink densities obtained from the swelling behavior confirms the suitability of vulcameter measurements for monitoring the complex vulcanization process of such nanocomposite systems in a simple and efficient way. The reinforcing effect of FLG shows the highest relative improvements in weakly crosslinked nanocomposites.
Elastomers are usually reinforced by large amount of fillers like carbon black (CB) or silica in order to improve various mechanical properties, such as Young’s modulus, hardness, tear resistance, abrasion resistance, and gas barrier properties. In recent years, such improvements were also obtained by using nanoparticles at significantly lower filler loadings. Graphene is a twodimensional (2D) sheet of a thickness in the atomic scale, composed of a honeycomb structure of sp2 carbon atoms. Besides significant mechanical reinforcement, graphene harbors the potential to be used as a multifunctional filler, as it can also increase the conductivity and weathering stability of elastomer matrices. Ultraviolet (UV) irradiation and oxidative agents can lead to the degradation of elastomers due to a multistep photooxidative process, including the formation of radicals. Carbon-based fillers have an influence on these reactions, as they can absorb UV radiation and act as radical scavengers.
This chapter summarizes the results of our larger project on multilayer graphene (MLG)/elastomer nanocomposites, previously published, which present a comprehensive case study of MLG as a multifunctional nanofiller in elastomer/graphene nanocomposites. Different elastomeric matrices are compared in order to demonstrate the outstanding impact of MLG as a general benefit. The dependency of this effect on concentration is discussed in detail. Taking into account the key role of dispersion, different mixing procedures are compared, evaluating a facile implementation of graphene nanocomposites into conventional rubber processing. Finally, the most probable commercial uses of MLG nanofillers in combination with conventional CB are studied. The nanocomposites were prepared in the kg scale in order to obtain enough specimens to investigate various properties of the uncured and vulcanized rubbers at the highest quality level, including rheology, curing, morphology, several mechanical properties, abrasion, conductivity, gas permeation, burning behavior, and weathering stability. The structure property relationships are asserted and questioned, for example, by investigating the radical scavenging ability or aspect ratio of the MLG. This chapter illustrates the state of the art of graphene/rubber nanocomposites targeted for commercial mass applications.
Elastomers are usually mechanically reinforced with high loadings of carbon black (CB) to achieve the properties demanded; high amounts of mineral flame retardants are used to fulfill fire safety requirements. In this study, multilayer graphene (MLG), a nanoparticle made of only 10 graphene sheets, is applied in low loadings, 3 parts per hundred rubber (phr) to reduce the total amount of filler or boost performance in hydrogenated acrylonitrilebutadiene rubber (HNBR). In the HNBR/MLG nanocomposites, 3 phr MLG replaced 15 phr CB, 3 phr aluminum trihydroxide (ATH), or 15 phr CB + 3 phr ATH. The nanocomposites were prepared via master batch by ultrasonically assisted solution mixing and subsequent conventional two-roll milling. A comprehensive study is presented, illustrating the impact of MLG on curing and mechanical properties; e.g. replacing 2.5 phr ATH with 3 phr MLG increased the Young's modulus by over 60% and hardness by 10%, while improving flame retardancy, and reducing the total heat evolved by 10%. MLG is a multifunctional filler, as demonstrated by various enhancements in terms of the mechanical and flame retardancy properties of the rubber composites.