Determination of Effective Diffusion Coefficients of Nitrogen in Extruded Polystyrene Foam by Gravimetric Sorption

A macrogravimctric sorption technique has been employed to obtain time-dependent weight gain data for thin-sliced closed-cell extruded polystyrene foam samples Effective diffusion thickness and diffusion time were used to compute a scaled age parameter The weighing apparatus operated in an isolated chamber filled with pure nitrogen gas The effective diffusion coefficient of the gas in the foam structure was computed from the slope of the weight ratio, defined as the instan tancous weight gain to equilibrium weight gain, with the thickness-scaled age pa ramctcr. Effective solubility coefficient was computed from the film solubility coeffi cicnt, foam to polymer ratio and environmental conditions. The Geometric Factor, computed from the ratio of effective to film permeability, was about four times the value based on cellular dimensions

The Reference Period for Predicting Long-Term Thermal Performance of Thermal Insulating Foams

With the advent of thermal insulating foams manufactured with alternative blowing agents, the need for estimating their long-term thermal performance became evident. Two predictive techniques (slicing and scaling [1,2] or slicing and modelling [3,41]) are now considered for different foam products. However, the selection of the reference period for which the thermal performance is evaluated has not received sufficient attention.

In this paper, the limit of economic considerations for aging processes is assumed to be fifteen to twenty years. The analysis indicates that for a wide range of thermal insulation products and thickness, the same reference time can be used. The reference time denotes the period of aging when the measured thermal resistivity (inverse of thermal conductivity) represents the timeaveraged thermal resistivity over the considered service life.

This paper postulates that the five year reference period should be used for all thermal insulations with thermal performance varying during the service time. This estimate is based on economic analysis and modelling techniques applied to five different cellular plastic insulations and analyzed in three Canadian climates using two heating fuels for comparison. The cost of construction of the wall assemblies and the cost of insulation alternatives, as well as all economic assumptions such as future interest rates, inflation rate and fuel escalation rate, were the same as provided by the provincial authorities for development of the 1995 Canadian Energy Code.

Modeling of Diffusion in Closed Cell Polymeric Foams

Closed-cell foams made of polymers have the lowest thermal conductivity of any currently available insulation material other than vacuum insulation systems. The increase of foam conductivity with age occurs as air diffuses into the foam while the blowing agent diffuses out, thus modifying the cell gas composition. Also, the change in cell gas composition influences the dimensional stability of the foams. To predict the long term aging behavior and dimensional stability of these foams, the diffusion characteristics of the different components need to be known. Several models exist in the literature which describe diffusion in foams. The most popular of these models are reviewed, and effective diffusivities predicted from one model are compared with experimental data. An unsteady state model is then proposed and solved numerically using a finite difference scheme. The numerical solution algorithm is developed to efficiently solve the large number of coupled equations resulting from this model. The uptake curves predicted from both the unsteady-state model and a discrete model (Bart and Du Cauze De Nazelle, 1993) are compared with available experimental data for the polystyrene-nitrogen system. From the analysis of uptake curves generated for different numbers of cells, the effective diffusivity of the PS/N2 system is predicted. Also, the effect of initial cell gas composition and cell size on both the long term aging profile and dimensional stability of polyurethane foam is considered. The proposed model can easily be extended to include the influence of blowing agent concentration on diffusivity in the polymer phase and the isotherm describing the distribution of blowing agent between the gas and polymer phases.

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.