Project Details Structure and properties of linear and crosslinked structural PVC foams

Project No.:
2003-038-4-400
Start Date:
01 July 2005
End Date:
31 December 2011
Division Name:
Polymer Division
Division No.:
400

Objective

To investigate the effect of crosslinking on the structure and properties of rigid PVC foams. The establishment of the influence of thickness and density on the structure and properties of rigid PVC foams.Important properties which should be determined are the shear- and elastic- modulus dependence upon temperature and frequency; fractural mechanical behavior including fatigue crack growth depending on temperature and load rate; compression behavior in air and water; thermal behaviour by DSC. Morphological investigations should contribute to the understanding of the micromechanical deformation behaviour.

Description

Rigid foam cores are of high importance for structural applications, such as bumpers, ship building, roof covering and crash belts. Important properties are high strength and good stiffness to weight ratio, Also, good thermal insulation, self-extinguishing, good fire behaviour and very low water absorption without outgassing are important.Despite the wide range of applications there is a need for a better understanding of the influence of the morphological foam structure on the relevant thermal and mechanical properties. To explore structure properties relationships of polymer foams, two types of commercial PVC foams with a crosslinked and linear matrix in a density range between 50 and 140 kg/m3 and a thickness range between 5 and 40 mm will be investigated. The determination of DMA properties is well established for compact materials. Hoewever, for foams, the method of load introduction affects the results significantly. By comparing the results from different equipment under tension, torsion and compression the differences should be better understood and the influence of crosslinking, thickness and density should be clarified. The reproducibility of the modulus and the transition temperatures will be specifically investigated.Under compression, yield strength, and the stiffness behaviour below and above the yield point will be investigated as a function of the loading rate and temperature. By making measurements in water the effect of the air inclusions will be examined. The relationship between modulus and density will be established and the modulus of the compact material will be extrapolated from this.By fractural mechanical investigations the KIC from CT-specimens as well as the essential work of fracture will be determined. Finally the question will be answered whether fractural mechanics is a suitable method for the characterisation of polymer foams The investigation will cover thermal- and mechanical properties as well as fracture mechanical and fatigue properties in relationship of density and cell size. First materials have been delivered in July 2000, more material for additional contributions is available on request.

Progress

February ’09 – The investigations of the compression behaviour of these foams led to several results. Scanning electron microscopy was used to study the morphology of linear and cross-linked foams and their heterogeneous foam deformation. The compression tests illustrated the influence of foam density and cross-linking. The elastic collapse of cells during compression was linked to strain-softening of the corresponding bulk material.

Furthermore, a novel approach was developed in order to transfer cell morphological data obtained by computer tomography (CT) to realistic finite element (FE) meshes for numerical simulations. This approach allows to take into account the statistical character of the foam morphology and can be applied to predict the mechanical properties of foams. First, a CT analysis was performed to determine the size distribution of the polymer foams using a non-destructive technique. The CT information was applied to build FE meshes using a tessellation of modified Kelvin cell units or truncated octahedra of various cell sizes. Then FE simulations were performed using these meshes in order to predict the compressive behaviour of polymer foams.

The results of this project are summarized in two manuscripts:

1. G.T. Lim, F. Ramsteiner and V. Altstädt, Understanding the Compressive Behaviour of Linear and Cross-Linked Poly(vinyl chloride) Foams, Journal of Cellular Plastics September 2009 45:419-439; doi: 10.1177/0021955X09105372 

2. F. Fischer, G.T. Lim, U.A. Handge, and V. Altstädt, Numerical Simulation of Mechanical Properties of Cellular Materials using Computed Tomography Analysis, Journal of Cellular Plastics September 2009 45: 441-460, doi:10.1177/0021955X09339340

project completed