Properties of cellular polymeric materials in relation to their structure

The first part of the work is devoted to the development of a method for predicting the mechanical response of honeycomb panels made by polymeric material via a continuous process. After experimental validation of the predictive ability of the three dimensional finite element model, the mechanical response of several geometries was analysed in order to optimize the honeycomb structure for the use as acoustic barrier for traffic noise control, taking into account both the mechanical and acoustical requirements. Mechanical and acoustical performances of polymeric foams were studied in dependence to the constituent material and to the structure of the cells and its change under load. Indeed the static deformation may affect the microstructure and the constituent material properties, thus modifying significantly the foam behaviour. It was shown that the time-temperature equivalence is applicable to predict the dynamic mechanical (DMA) response at frequencies not directly accessible, and that the effect of static pre-strain and of temperature are decoupled. The obtained data were used to simulate the acoustic behaviour of a car part made by a layer of polyurethane foam sandwiched between two steel sheets. The prediction was in accordance with the measured acoustic performances over a wide range of frequencies, significantly wider than that directly accessible in DMA experiments, and gave better results than the prediction carried out using material properties measured at a single frequency only. The acoustical and mechanical behaviour of foams is also affected by the presence of open porosity. This was investigated in a study on a novel open-cell polyethylene foam (PEOC) produced at CellMat laboratory (Universidad de Valladolid, Spain) through a well-controlled production route. This material displays a peculiar structure characterized by almost closed cell connected by small holes. Crushing the foam up to 90% of its original thickness allows to obtain a material with different structure and mechanical response (PEOC90). Results suggested that changing the microstructure can be a very effective way to control, and enhance, its sound absorption characteristic. The very same foams showed interesting response to quasi-static and cyclical mechanical stimuli. In fact, unlike other open cell foams, such as PU flexible foam, PEOC compression behaviour is significantly affected by the rate of the applied compressive strain. Extending the observations of the studies performed on liquid-filled polyurethane foams present in the literature, this peculiar behaviour was attributed to the stress contribution arising from gas flow through the holes interconnecting cells, and a simple model based on Darcy’s law (flow through a porous medium) was able to describe the evolution of stress with strain at different strain rates. DMA characterization performed at fixed frequency (1Hz) and several static strains evidences the differences among the studied materials, which can be related to their structure. The comparison between PEOC and PEOC90 seems to confirm that the changes in the structure have an effect not only on the modulus, as expected, but also on the loss factor, which decreases with increasing holes size in cells walls, while static deformation does not seem to affect significantly the loss factor. In the case of closed cell foams, the observed reduction of loss factor with increasing static strain might be attributed to the contribution of air pressure to E’. Finally, the mechanical properties in relation to anisotropic microstructure of polypropylene– based medium density (180 kg/m3) foams were studied. A non-standard compression moulding technology, called improved compression moulding (ICM), was used to achieve anisotropy of cellular structure and a fine control on the final foam density. Four different processing pressures (0.5; 1.5; 4; 8 MPa) and two different formulations (pure PP and PP+nanoclays) were employed to prepare a total of eight different foams. The influence of processing condition and of the addition of nanoclays on the foaming process was studied by means of microstructure and mechanical characterisations. Changing the process parameters allowed obtaining different morphologies, while maintaining constant foam density. Microstructure characterization, through image analysis, revealed an overall cell orientation in the expansion direction and a dependence of structure morphology on the process conditions and formulations.Nanoclay charged foams prepared at the lowest pressure conditions displayed high structure anisotropy and bimodal cell size distribution. More homogeneous cell size distribution was obtained in pure PP foams and in foams produced at higher pressure. A correlation between cell anisotropy and cell size was observed and a different trend for each material was observed. In particular, it was noted that smaller cells of nanocomposite PP foams tend to have rounded shape. Structure anisotropy was reflected by mechanical properties: all materials displayed higher stiffness in the expansion direction with a ratio between the modulus measured in the expansion to transversal direction ranging from 1.5 to 3. A part of the study was devoted to assess the applicability of existing models in order to predict the dependence of mechanical behaviour on the anisotropy ratio of the foams microstructure. The simple model based on rectangular cell proposed by Gibson and Ashby, in spite of the complexity of the structure of the studied foams, can describe their behaviour in acceptable way.

La prima parte del lavoro è dedicata allo sviluppo di un metodo di predizione della risposta meccanica di un pannello alveolare in materiale polimerico prodotto mediante tecnologia di produzione continua. La seconda parte dell studio riguarda gli espansi polimerici e lo studio della loro risposta acustica e meccanica. In particolare si è studiato la dipendenza della risposta del materiale in dipendenza dal polimero base e dalla deformazione statica imposta sulla struttura cellulare. Si mostra che il principio di equivalenza tempo-temperatura può essere impiegato per determinare la risposta del materiale a frequenze non direttamente accessibili sperimentalmente. L'influenza della presenza di porosità aperte sulle performance acustiche è stato studiato nel corso di uno studio in cui una nuova schiuma polietilene con celle parzialmente aperte è stata messa a confronto con altri espansi di simile densità. La medesima schiuma mostra inoltre un comportamento a compressione fortemente dipendente dalla velocità di sollecitazione. Questo comportamento è stato correlato con il flusso di gas attraverso le porosità del materiale. Infine si sono studiate schiume di polipropilene e polipropilene+nanoargilla. Si sono cercate correlazioni tra le condizioni di processo, la microstruttura e le proprietà meccaniche, evidenziando che la presenza di nanocariche favorisce la nucleazione di bolle e influenza il grado di anisotropia della microstruttura, che può essere a sua volta correlata con l'anisotropia delle proprietà meccaniche mediante le relazioni proposte in letteratura.