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- Mechanical properties (4)
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Organisationseinheit der BAM
Polymer-based electrolytes based on commercially available epoxy resins were prepared through the addition of a liquid electrolyte, a solution of a lithium salt in an ionic liquid. The polymer monoliths were characterized using impedance spectroscopy, 3-point bending test, scanning electron microscopy (SEM) and nitrogen adsorption (BET). The balance of ionic conductivity and flexural modulus is crucially dependent on the relative proportions of epoxy resin to electrolyte. Also the effect of the liquid electrolyte on curing kinetics and processing was assessed by complex viscosity measurements and differential scanning calorimetry (DSC). Only one out of the three resins investigated exhibited a significant acceleration effect.
For the concept of using structural materials such as carbon fibre reinforced plastics as
energy storage devices, new matrix polymers are required. These polymers must provide ionic
conductivity as well as adequate mechanical strength. In the EU-Project StorAGE this
requirements are fulfilled by adding ionic liquid to commercial polymers. The mechanical
properties of these mixtures materials were characterized by using a 3-point-bending device.
In addition, single fibre pull test were performed in order to get information on the interfacial
shear strength. Adding of ionic liquid has an impact on the mechanical performance of the
materials. A decrease of the flexural strength and modulus of less than 10% of the value of the
reference materials took part. The interfacial shear strength decreased to a value of around
one third compare to the reference material.
Atmospheric-plasma fluorination was used to introduce fluorine functionalities onto the surface of carbon fibers without affecting their bulk properties. The interfacial adhesion between atmospheric-plasma-fluorinated carbon fibers and poly(vinylidene fluoride) (PVDF) was studied by means of direct wetting measurements and single fiber pullout tests. Measured contact angles of PVDF melt droplets on modified carbon fibers show that short exposure times of carbon fibers to atmospheric-plasma fluorination (corresponding to a degree of surface fluorination of F/C = 0.01 (1.1%)) leads to improved wettability of the fibers by PVDF melts. The apparent interfacial shear strength as a measure of practical adhesion, determined by the single-fiber pullout test, increases by 65% under optimal treatment conditions. The improved practical adhesion is not due to the formation of transcrystalline regions around the fibers or a change of the bulk matrix crystallinity or to an increased surface roughness; it seems to be due to the compatibilization of the interface caused of the atmospheric-plasma fluorination of the carbon fibers.
The interaction between direct fluorinated carbon fibres and various fluoropolymers (ethylene-chlorotrifluoroethylene, poly vinylidene fluoride, fluorinated ethylene propylene copolymer and tetrafluoroethylene-perfluoro alkoxy vinyl ether copolymer) was studied by means of direct wetting measurements between fibres and the polymer melts and single fibre pull-out tests. The results of both techniques allow the adhesion behaviour between the fibres and the matrices to be predicted. The results obtained show that a low degree of surface fluorination of carbon fibres leads to an improved wettability between the fibres and fluoropolymer melts and this is an indicator for an improved thermodynamic work of adhesion. The apparent interfacial shear strength as measure of practical adhesion, determined by the single fibre pull-out test, increases with increasing degree of surface fluorine content up to a maximum, which depends on the degree of fluorination of the matrix used. The improved interaction between the fibre and the matrix is due to an enhanced compatibility at the fibre/matrix interface.
The quality of interfacial interaction is dictated by the surface chemistry of the carbon fibres and the composition of the matrix. The composition of poly(vinylidene fluoride) (PVDF) was modified by the addition of maleic anhydride grafted PVDF. The surface properties of the various matrix formulations were characterised by contact angle and electrokinetic measurements. Carbon fibres were modified by industrial electrochemical oxidation and oxidation in nitric acid, or the use of a traditional epoxy-sizing of industrially oxidised fibres. The surface composition, morphology and wetting behaviour of the carbon fibres was characterised. The interaction between modified PVDF and the carbon fibres was studied by direct contact angle measurements between PVDF melt on single carbon fibres and by single fibre pull-out tests. The best wetting and adhesion behaviour was achieved between PVDF containing 5 ppm grafted maleic anhydride (MAH) and epoxy-sized carbon fibres. The addition of MAH-grafted PVDF to the unmodified PVDF caused the apparent interfacial shear strength to increase by 184%. The apparent interfacial shear strength of this fibrematrix combination allowed for the utilisation of 100% of the yield tensile strength of PVDF.
Mechanical and microstructural characterisation of multifunctional structural power composites
(2013)
Although the inherent anisotropy of polymer composites has presented daunting technical challenges, these materials now offer engineers considerable opportunities for efficient structural design. More recently, the advent of multifunctional composites which can fulfill more than one role within a system has attracted considerable interest, providing designers with exciting opportunities to innovate. Of particular interest here are structural power composites, which simultaneously carry mechanical load whilst storing/delivering electrical energy. Although the development of these composites is highly challenging, often with conflicting constituent requirements, the STORAGE consortium has had considerable success in the development of these materials for automotive applications.
The focus of this paper is structural supercapacitors, the basic architecture of a single cell of which is shown in Fig. 1. This entails two carbon fibre woven lamina (electrodes) which sandwich a glass fibre woven lamina (separator), all of which is embedded within a multifunctional matrix (electrolyte). This architecture has been the focus of the research to date, leading to components such as that shown in Fig.1 having been fabricated. This paper reports on the mechanical properties and microstructures of the different reinforcement and matrix combinations for structural supercapacitors.
Electrolyte systems that can carry mechanical load while allowing for high levels of ionic conductivity are an important prerequisite for structural power storage devices. Introduction of structural power storage into the variety of consumer products will allow saving in weight and volume. Moreover, using a supercapacitor/battery system in hybrid electric vehicles (HEV), the supercapacitor part will extend the battery lifetime by protecting it from the high peak currents. To successfully produce structural power storage requires the development
of multifunctional electrolytes where one has to simultaneously maximize mechanical properties and ionic conductivity.
Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria (Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(l-lactic acid) was quantified using the single fiber pullout test.
Structural supercapacitor electrolytes based on bicontinuous ionic liquid-epoxy resin systems
(2013)
'Structural electrolytes' retain the desirable mechanical characteristics of structural (epoxy) resins whilst introducing sufficient ionic conductivity to operate as electrolytes in electrochemical devices. Here, a series of ionic liquid–epoxy resin composites were prepared to identify the optimum system microstructure required to achieve a high level of multifunctionality. The ionic conductivity, mechanical properties, thermal stability and morphology of the cured epoxy based structural electrolytes were studied as a function of phase composition for three fully formulated high performance structural epoxy systems. At only 30 wt% of structural resin and 70 wt% of ionic liquid based electrolyte, stiff monolithic plaques with thicknesses of 2–3 mm were obtained with a room temperature ionic conductivity of 0.8 mS cm-1 and a Young's modulus of 0.2 GPa. This promising performance can be attributed to a long characteristic length scale spinodal microstructure, suggesting routes to further optimisation in the future.