Charakterisierung von Elastomeren mittels dynamischer Indentation und vergleichender Methoden
This work deals with a new developed method of dynamic-machanical characterization of elastomeric surfaces. Hereby a conical indenter penetrates the surface of elastomers. The indentation depths are in the order of about 20 – 100 mm. Quasistatic indentations and hysteresis-curves as well as sinusoidal indentations in the micro-range are possible to be performed because of the use of a piezo-actuator. Therefore the stiffness can be devided in an elastic and a viscose term. Defined scanning of the surface in x- and y-direction enables one to estimate the homogeneity of the material by the help of the average value and the standard deviation of the stiffness values and mechanical imaging of the same as well. The obtained mechanical images have the length of a few millimeters with local resolution of a few micrometers. It could be shown, that the theories according to the literature are not able to describe the indentation curves. A reason might be, that adhesion and friction between indentation probe and sample as well as the special material properties of elastomers were not regarded (by derivating the theory). By the help of different compound formulations (variation of the microstructure of the polymers, crosslink-density, filler content and type of filler) vulcanizates with different mechanical properties were investigated. Linear correlations between stiffness values and conventional values obtained on macroscopic samples were obtained (stress strain curves, dynamic-mechanical analysis). There are some hints indicating that an interaction between tip and surface has to be regarded. For elastomers typical properties, e.g. the “Payne effect”, the “frequency-temperature equivalence principle” according to Williams, Landel and Ferry and the “structure-concentration equivalence principle” according to Kraus was obtained. Investigations in the region of the glass transition temperature of an elastomer revealed, that Tg indicates the limit for sinusoidal indentation, because of the loss of contact between tip and sample below Tg. Evaluation of the relaxation spectra revealed, that for successful dynamic indentation the condition (f = frequency, = main relaxation time) has to be fulfilled. Indentation experiments below the glass transition temperature show, that the shape of the indentation curves is varied. Within this work, it could not be shown, if these oscillations contain specific information of the investigated samples. When this method was used locally on vulcanizates, different outlets were demonstrated: on an NR / SBR composite sample a stiffness profile across the borderline was detected, which apparently results from diffusion of crosslinker during vulcanization. In the case of filler loaded rubbers a deviation of the dynamic indentation values in the millimeter range of 7-9 % could be shown. In this case the distribution curves of the stiffness values, which could be described as Gauß-distributions in a first step approximately, could be shifted on a common curve. Investigations on carbon black loaded rubbers with different incorporation times showed tendencially, that the average value and standard deviation of the indentation values are characteristic quantities for vulcanizates. On different samples mechanical imaging was demonstrated and for an unfilled natural rubber vulcanizate such an image was compared with an NMR-image. In the case of crosslink-density inhomogeneities an accomodation of the images could be confirmed. Indentation experiments on a carbon black loaded rubber were accompanied with images of atomic force microscopy. Therefore the range of investigations was enriched up to the aggregate structure of the filler. By the evaluation of so called “mechanical units” (= harder regions) with fractal dimension and size distribution the conclusion can be drawn, that there is a common principle in the formation of the filler-super-structures. Furthermore the selectivity of the introduced indentation method on the density distribution of carbon black aggregates, resp. -agglomerates is outlined. Furthermore NMR-investigations with the surface sensitive NMR-MOUSE® and DMTA-investigations on elastomers with different microstructure and crosslink density were performed. These showed, that there is a correlation between dynamic-mechanical analysis and NMR-parameters, which are suitable for imaging as well (here T2). By the help of dynamic mechanical thermal analysis (DMTA) and the WLF-shift principle, molecular parameters especially the molecular jump frequency could be obtained. This presents a quantity indepentend on the microstructure of the polymer and the crosslink-density which represents the molecular mobility of the elastomer and correlates with T2. Therefore in principle a relation between the dynamic-mechanical analysis and the NMR-method exists, whereas both methods are suitable for the characterization of elastomers by imaging.
info:eu-repo/classification/ddc/540, Chemie
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