Foam Core Materials for High Performance Sandwich Constructions: Preparation, Testing and Properties

Doktorsavhandling, 1998

The basic task for foams used as core materials in sandwich constructions is to separate the skin layers in order to enhance stiffness with little contribution to the total weight. However, modern design in sandwich technology require that core materials contribute to the perfor-mance with specific mechanical and physical properties. In this thesis preparation and characterisation of different foamed structures as well as their application in sandwich constructions are discussed. The foams prepared are based on two different polymer matrix systems, either an Inter Penetrating Polymer Network (IPN) from PVC entangled by a poly-urea/amide/imide network or polyethersulfone (PES). Based on the IPN system two new materials were developed, a ductile foam core and a foam containing waste material. The IPN system was also used to demonstrate a new concept where different foam cores, with different material characteristics, were fused together into one single gradient material. The densities for all foam core materials were in the range of 40-200 kg/m3. The ductile foam core material was characterised by energy absorption capability and shock mitigation properties twice as high as that of standard material. This was achieved by obtaining a two fold increase in elongation in combination with withheld moduli and strengths relative the reference. The addition of 2-8% waste material was shown to give a 7-8°C increase in Tg as well as increased strengths and moduli. Furthermore, it was found that the procedure for the pre-paration of the waste material, sanding or grinding, influence material properties significantly. Foam prepared using sanded waste material was superior to foam made using ground waste. The gradient core concept demonstrated that it is possible to join different foam core materials keeping the their nominal density and maintaining an overlapping continuos cell structure. It was also shown that it was possible to combine and utilise extreme specific material charac-teristics from the individual core materials in the composite by choosing a proper design. In the preparation of both the ductile material and the foam cores filled with waste materials, it was found that rheological phenomena controls the final cell morphology. High viscosity during nucleation and growth of the polymer matrix promotes the generation of thicker cell walls and decreased cell size, which are beneficial to improved strength and moduli. In the preparation of low density rigid foam core materials using polyethersulfone as matrix material, three different expansion techniques were investigated: mechanical counter pressure, gas counter pressure and micro wave expansion. It was demonstrated that the gas counter pressure method was superior to the other two with respect to processability and formation of foam with a more optimal cell structure. Furthermore the expanded PES-material was found to posses high temperature stability as well as extremely good behaviour in flammability tests. Low amounts of smoke, heat and toxic fumes are generated when the material burns.