Thermal Oxidative Stability of Heat-Stabilised Polyamide 66 by Differential Scanning Calorimetry

The thermal stability of heat-stabilised polyamide 66 in an oxidative environment is evaluated by DSC. The oxidative stability of the polyamide decreases as a result of repeated injection moulding. The results also indicate that the presence of glass fibres in the polyamide has a negative influence on the oxidative stability. Both isothermal and dynamic DSC measurements seem to be useful tools for assessing the stability of polyamides and there is a relationship between data determined using both procedure

Modeling of multiple cracking and decohesion of a thin film on a polymer substrate

Thin brittle films on polymer substrates are finding increasing use as gas barriers for example in the medical and food packaging industries and also for the next generation of ultra-light displays based on flexible polymer substrates. In order to determine the durability of the barrier under thermal and mechanical loads, test procedures and corresponding data reduction methods are needed to feed the analysis models. One of the tests frequently employed for this kind of multi-layer material systems is the fragmentation test, whose designation comes from the progressively denser pattern of parallel cracks developing when the specimen is loaded under uniaxial tension. From the crack-density versus strain data obtained, a critical strain for crack growth and an assessment of the adhesion of the coating to substrate can be obtained. However, no accepted data reduction methods exist to extract material properties from the test or inversely, successfully predict the crack density as a function of a set of material properties without fitting parameters. In an earlier paper, the authors presented a finite element based analysis methodology to determine the fracture toughness of both the coating and the interface from the fragmentation data. In the simulations, the plastic constitutive behavior of the substrate and the debonding of the coating from the substrate were explicitly included, the latter by use of a cohesive zone model. In this paper an extension of this methodology is presented that enables crack-density evolution with strain to be predicted. The results presented comprise comparisons with experiments to validate the methodology and the influence of (i) coating toughness, (ii) interface toughness and (iii) coating thickness on crack density versus strain

Reaction-Induced Phase Separation in Poly(vinyl acetate)/ Polyester Blend

In blends of unsaturated polyester (UP), poly (vinyl acetate) (PVAc), and styrene, a reaction-induced phase separation occurs upon curing that is due to the crosslinking between styrene and the UP molecules. The evolution of the morphology was observed by optical microscopy on a heated stage. Light transmission was used in parallel to precisely detect the onset of phase separation and the formation of microvoids. Using Fourier transform IR spectroscopy in the same conditions, the conversions at phase separation and at microvoiding were evaluated. Phase separation occurs at a very low degree of conversion and microvoiding develops at around 60% of conversion. The final morphology of the blend was investigated by scanning electron microscopy. The relative influences of the cure temperature, the concentration in PVAc, and the molecular weight of PVAc were investigated. It was confirmed that the early stages of the reaction at high temperature determine the final morphology of the blends

Unsaturated flow in compressible fibre preforms

The isothermal infiltration of compressible fibre preforms was modelled taking into account the gradual saturation of the porous medium and the fibre tows. Similarities between this and drainage/imbibition processes, led to the adaptation of a finite-element code originally developed for soil mechanics. The dual scale of the porous medium was accounted for by considering delayed radial flow into the fibre tows. The chosen case study materials were polypropylene transversally infiltrating glass fibre mats. Local preform strain and stress profiles, as well as micro and macro-saturation, and local matrix pressure profiles were obtained. The influence of numerical and material parameters is discussed in relation to experimentally observed phenomena

Tailoring of the practical adhesion between polyethylene and galvanised steel

Abstract—The practical adhesion of maleic anhydride grafted polyethylene (MAH-PE) to galvanised steel was studied using 3-point flexure tests, before and after hydrothermal ageing. Before bonding, the electro-galvanised steel was treated with γ -aminopropyltriethoxysilane (γ -APS). The influence of the silane coating thickness and deposition pH on the practical adhesion of MAH-PE to steel was investigated. FT-IR spectroscopy and microscopy enabled to gain understanding of the interphase formation between the silane and the metal substrate. It was found that, at the natural pH of the γ -APS, Zn ions dissolved in the silane coating with subsequent formation of crystals. This interphase could be held responsible for the better durability of the bonds than for silane coatings applied at quasi-neutral pH, for which dissolution of Zn ions was not observed

Calculation of adhesive and cohesive fracture toughness of a thin brittle coating on a polymer substrate

Determination of fracture parameters for brittle coatings with a sub-micron thickness is not a straightforward task. Since direct evaluation through testing with for instance a double cantilever beam or compact tension tests is hardly applicable due to the extreme thinness of the coating, methods such as the fragmentation test are used. When a structure with a brittle coating on a soft substrate is strained, the coating develops a crack pattern with parallel cracks perpendicular to the loading direction. The crack density (number of cracks per unit length) increases with strain up to a saturation value. Analytical formulas to model the fragmentation process exist but are limited to elastic materials. In this work finite element simulations are applied in order to deduce the adhesive and cohesive fracture properties of the interface and coating respectively from experimental data. The simulations include both the plastic behaviour of the substrate and debonding of the coating from the substrate, the latter achieved by application of a cohesive zone model. The main conclusion is that the plastic dissipation within the substrate must be correctly accounted for to get realistic interfacial and coating fracture toughness values

Vibration Frequency Control of a Polymer Beam Using Embedded Shape-Memory-Alloy Fibres

The possibility to actively change the natural vibration frequencies of a composite beam by electrically activating a series of embedded shape-memory-alloy fibres is investigated. A model system composed of an epoxy matrix with prestrained shape-memory-alloy fibres is used. The natural frequencies of vibration of the composite are measured in a clamped beam configuration. When electrically heated, the fibres undergo a reverse martensite to austenite transformation. Since this transformation is restrained by the constraints of both the matrix and the clamping device, a recovery force is generated. This force produces an increase of the natural frequency of vibration of the whole composite beam. Vibration frequency changes of more than 50% are obtained. The glass-to-rubber transition of the matrix affects the reversibility of the effect. The role of the matrix thermal expansion is discussed

Passive vibration damping in an alpine ski by integration of shape memory alloys

With exception of the wooden core, all the components of a modern alpine ski exhibit a decreasing vibration damping capacity with decreasing temperature. Although the damping effect of the wooden core is prevailing, the extreme hardness of snow under freezing conditions, at temperatures as low as -30°C, favours the appearance of vibrations which tend to make the skis more difficult to control. Therefore, the aim is to obtain a simple adaptive system to enhance vibrational damping at low temperatures, without spoiling the overall characteristics of the ski at higher temperatures. Shape memory alloys (SMA), in their low-temperature martensitic phase, exhibit a damping capacity which is orders of magnitude higher than in the standard materials of a ski. Transition from the SMA's high-temperature and low-damping austenitic phase to the high-damping martensitic phase is achieved through a reversible thermally induced martensitic phase transformation (MPT). In this project, CuZnAl plates have been integrated into the complex sandwich structure of an alpine ski. Three point bending and differential scanning calorimetry (DSC) have been used to characterise the materials and their effects on the damping behaviour. In order to ensure the most efficient damping through shear deformations, an excellent adherence between the SMA plates and other materials has to be guaranteed by means of a surface treatment including sand blasting and chemical etching. The damping effect also depends on the location of the SMA plates within the ski. First tests on modified full-size alpine skis have already illustrated the improvements in damping capacity achieved with the integration of CuZnAl elements