J. L. Ruiz-Herrero

The Effect of Processing on the Structure and Properties of Crosslinked Closed Cell Polyethylene Foams

This paper analyses the relationships between production, structure and properties of a collection of crosslinked closed cell polyethylene foams. Foams with similar densities produced from a similar base polymer and manufactured using three different technologies (high pressure nitrogen solution process, compression moulding and semicontinuous processing) have been characterised. On the one hand, several foam characteristics such as density, cell diameter, cell wall thickness, cell shape, fraction of mass in the edges, gel content, crystallinity and melting temperature have been measured and related to the way in which the foams were produced. On the other hand, three important physical properties such as thermal conductivity, Youngs modulus and thermal expansion have been measured analysing the experimental results in terms of the previously cited foam characteristics. The results have shown that the production route used to manufacture the foam strongly influences the foam structure and as a consequence the main physical properties.

Gas Diffusion in Polyolefin Foams during Creep Tests. Effect on Impact Behaviour and Recovery after Creep

A method to obtain the effective diffusion coefficient of the gas contained in closed cell polyolefin foams under static loading is presented. This property is obtained from pressure decrease with time using an analytical solution of the diffusion equation. The effect of density, type of base polymer, crosslinking and sample size on the diffusion coefficient is analysed. It has been shown that the impact behaviour of low density closed cell polyethylene based foams deteriorates after compressive creep periods and that this reduction of the cushion capabilities is directly related with the diffusion coefficient of the foams. Moreover, the recovery of the foams after creep showed a peculiar non-homogeneous behaviour, which has been analysed. Gas diffusion during creep is the main responsible for this particular behaviour.

Prediction of cushion curves for closed cell polyethylene-based foams. Part I. Modelling

The possibility of prediction of the cushioning performance of closed cell foams under compressive impacts has been analyzed, using simple theoretical models. Several corrections have been introduced into the models previously proposed by Burgess. These corrections take into account the variation of gas volume in the foam with density, they incorporate the effect of the falling weight into the calculations, and finally they consider the base polymer effect. Even though the predictability of the model is not complete it provides a useful tool from the practical viewpoint.

Prediction of Cushion Curves for Closed Cell Polyethylene-Based Foams. Part II: Experimental

In this paper the compressive impact behaviour of a collection of low density polyethylene (LDPE) and polyethylene-co-vinyl acetate (EVA) based foams with different densities, during purely compressive impacts, has been studied. The experimental cushion curves for these materials were compared with several theoretical models. The results have shown that the simplest models, adiabatic and isothermal, are able to fit the experimental data and to predict qualitatively the evolution of the minima in the cushion curves as a function of foam thickness and falling height. In addition, it has been shown that the impact behaviour of closed cell polyethylene based foams deteriorates after compressive creepperiods. This fact should be considered in the design of real cushion systems.

Characterization of Polyethylene Foams under Compressive Impact Loading

It has long been recognized that the mechanical behaviour of materials under conditions of rapid loading and impact differs significantly from that under static load application [1].These differences are specially important for those materials as polymeric foams used as low energy impact absorbing materials[2]. An optimum energy absorbing material needs to dissipate the kinetic energy of the impact while keeping the force on it below some limit, thus resulting in a no-dangerous deceleration of the protected object[3]. The mechanical properties at room temperature of six polyethylene foams with closed cells and different densities have been evaluated in purely compressive impact loading conditions. The energy absorption characteristics have been evaluated through different parameters as the peak of deceleration, the load transmitted, the maximum strain and the impact time. The peak of deceleration is used to obtain the cushion diagrams at five different heights, useful to design energy absorption structures.