Resume

Imperial College London

Project Title: Polyimide Cellular Materials for High Altitude Vehicles
Advisor:Dr. R. Byron Pipes

Introduction

This project was done over the course of 8 weeks from June 20th-August 12th, 2005, as part of the Summer Research Program with Imperial College London. It was concerned with the growth and mechanical testing of foams produced from TEEK-L powders of various particle sizes, as part of a long term project on using such foams in airships. Four different powders were provided for testing, of particle sizes 0-106, 106-150, 150-180, and 180-300 microns. A 1 kW microwave and an oven capable of temperatures to over 300°C were used for growing foam samples, and a metallic mould was constructed to grow flat plates of foam. Mechanical testing was also performed on the plates of foam, and images taken of all foams grown to compare the cell sizes produced.

Project Objectives

  • Construction of Metallic Moulds which the foams would eventually be grown in.
  • Growing of foams in a microwave, performed in test tubes.
  • Growing of foams in an oven, performed in test tubes.
  • Growing of foams in an oven performed in the Metallic Moulds constructed in Part 1, using optimal temperature conditions as found in Part 3.
  • Mechanical testing of foams grown in Part 4.

Approach

The moulds were made out of 3000 Series Aluminium, and consisted of several pieces which were slotted together and held together with spring clamps. It was lined with Teflon Film so the foams would not stick to the mould, as TFE spray was ineffective in this regard.

The Microwave used had a maximum power of 1kW, which proved to be too small, and it was estimated that one of 10kW power was required. This lack in power resulted in a structure similar to friable balloons being formed instead of a foam, which are weak, non-immidised cells. However these cells can be inflated during subsequent heating cycles, if a higher power microwave were to be used. It was found that 20 minutes was the optimum time for the cells to expand to their maximum extent.

Initially foams were grown in test tubes in order to establish the optimum parameters for the heating cycle. Two types of heating cycles were attempted – a 3 stage cycle involving an hour of growth at 100, 200, and 300C respectively, and a single stage cycle where the sample was held at a single temperature.

The Foams were then grown in the flat sheet moulds which had been constructed. 2g of powder was used as the standard mass for these tests, and a cardboard mould was constructed in order to ensure there was an even distribution of powder. It was also found that by actually wrapping the powder itself in a Teflon sheet of the size to which the foam was expected to grow, resulted in the optimum uniformity of cell growth.

Samples of sheet foam grown in the metallic moulds underwent tensile testing, with plastic shields placed over the grips to ensure that they would not bite into the foam and tear it before it had reached the maximum tensile strength. This machine had an error of ±1.47N.

Images were taken of all samples produced, to observe the cells produced and their uniformity.

Findings

A microwave of 1kW power was insufficient even at prolonged heating times to produce a foam from any of the particle sizes tested. A 10 kW microwave would most likely grow foams and allow for an alternative method of foam production besides the oven. However a non-metallic mould would have to be made in order to grow flat plates of foam inside the microwave, to avoid the metal arcing and sparking.

A Programmable Oven which was able to reach temperatures of 300C would be most useful, as it would allow experiments to be conducted with a varying heating rate, and if it was of sufficient size, the larger of the two moulds could also be used to make flat foams.

From the Mechanical Data obtained it is clear that there is some variation in properties with average particle size, however several samples of each type must be made and tested in order to fully ascertain this.

The presence of voids and creases were the primary aids to crack propagation and subsequent failure of the foam. These can be minimised by ensuring even powder distribution within the mould and making the teflon sheets as flat as possible, respectively.

Finding some kind of lubricant which could be used to ensure that the foams did not stick to the mould would also be a priority, as it would negate the problems with creases in the foams caused by drooping teflon sheets. Teflon gel would be one such possibility.


The Metallic Mold.
Fracture Surface.

Contact me: alexander.gibson@ic.ac.uk