New processing technique can make transparent polythene as strong as aluminium, with implications for several industrial sectors
Science fiction fans, of which we know there are many among Engineer readers, will doubtless remember the scene in Star Trek 4 (the one with the whales, and undoubtedly the best Star Trek film featuring the original cast) in which indomitable engineer Montgomery Scott had to teach a metals supplier the secret of making 23rd century “transparent aluminium”, so that he could construct tanks to hold and transport two humpback whales in the hold of a starship.
While transparent aluminium itself remains wholly fictional (although transparent aluminium-based ceramics do exist) a collaboration between polymer engineers Prof Ton Peijis at the Warwick Manufacturing Group and Prof Cees Bastiaansen of Queen Mary, University of London has devised a method of making transparent polythene sheets that have tensile strength greater than aluminium. The material could be used for glazing in automotive and aerospace applications, high-strength display screens and other applications where impact resistant transparent windows might be needed.
Peijis and Bastiaansen explain in a paper in the journal Polymer that they have tuned the process of drawing sheets of polymer from a solid in such a way that the strength of the material is enhanced whilst transparency is not compromised. Previous attempts to do this resulted in material which is strong but opaque.
Engineers have been trying for many years to develop a material that can replace glass in vehicle glazing applications. The problem is that glass is heavy and brittle, and its weight contributes to increasing the fuel consumption of vehicles. Transparent polymers are the obvious choice, because of their lower density, but the most likely candidates, polycarbonate and poly(methylmethacrylate) have very inferior mechanical properties; notably, they scratch and fog easily.
Peijis and Bastiaansen claim that their new technique, which works by orienting the strands of polymer molecules within the sheet, enhances the mechanical properties of the cheaper and more plentiful polymer high-density polyethylene (HDPE) to the point where it can not only compete with glass, but outperform traditional engineering materials such as metals. HDPE is most familiar as a material from which buckets are made.
Drawing is a technique with a very long history in polymer engineering. It simply involves pulling the material under tension, generally at an elevated temperature to allow the molecules to be reoriented. It is generally used to make fibres.
“The microstructure of polymers before drawing very much resembles that of a bowl of cooked spaghetti or noodles, while after stretching or drawing the molecules become aligned in a way similar to that of uncooked spaghetti, meaning that they can carry more load” explained Yunyin Lin, a PhD student in Professors Peijs and Bastiaansen’s team.
But the technique is not perfect. Generally, when drawing polyethylene, defects and voids in the bulk structure are introduced which destroy the polymer’s transparency and also reduce its strength. The Warwick and Queen Mary researchers started with HDPE sheets and drew them out at a temperature below the polymer’s melting point. They found that by drawing at temperatures between 90 and 110°C they achieved the best balance between strength and transparency. “We expect greater polymer chain mobility at these high drawing temperatures to be responsible for creating fewer defects in the drawn films, resulting in less light scattering by defects and therefore a higher clarity,” said Prof Peijs.
The highly transparent drawn HDPE films possessed a Young’s modulus of 27GPa (a measure of how much they stretch under tension) and the maximum tensile strength of 800MPa along the drawing direction, both of which are 10 times higher than those of PA and PMMA plastics. Aluminium, meanwhile, has Young’s modulus of 69GPa and aerospace grades have tensile strengths up to 500MPa. However, the density of HDPE is less than 1000kg/m³ while aluminium is 2700kg/m³ and glass is around 2500kg/m³. The transparent aluminium-based ceramics have densities approaching 4000kg/m³.
“Our results showed that a wide processing window ranging from 90°C to 110°C can be used to tailor the required balance between optical and mechanical performance. It is anticipated that these lightweight, low-cost, highly transparent, high strength and high stiffness HDPE films can be used in laminates and laminated composites, replacing or strengthening traditional inorganic or polymeric glass for applications in automotive glazing, buildings, windshields, visors, displays etc,” said Prof Peijs. Whether it could contain humpback whales and a large volume of seawater onboard a starship remains to be seen.