Ton Peijs from the University of Warwick. Photo: University of Warwick.
Ton Peijs from the University of Warwick. Photo: University of Warwick.

A research team led by Ton Peijs at the University of Warwick and Cees Bastiaansen at Queen Mary University of London, both in the UK, has devised a processing technique that can create transparent polythene film that is stronger than aluminium but a fraction of the weight. This film could find use in glazing, windscreens, visors and displays.

In a paper on this work in Polymer, the authors show that, by carefully selecting the type of polythene and tuning the temperature, they can create a lightweight transparent material with significant strength and a resilience approaching, and in some ways exceeding, that of metals.

Previously, anyone looking to replace heavy and often brittle glasses with a transparent plastic have looked at conventional transparent plastics like polycarbonate (PC) and poly(methylmethacrylate) (PMMA). Unfortunately, both these plastics possess a relatively unsatisfactory mechanical performance compared with an engineering material like aluminium.

On the other hand, there are methods for creating high strength plastic films, such as hot-drawing of high-density polyethylene (HDPE), that can compete or even out-perform traditional engineering materials like metals.

"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 team member Yunyin Lin, a PhD student at Queen Mary University of London.

The problem is that drawn polythene materials normally have an opaque appearance due to defects and voids introduced by the drawing process, limiting applications where both mechanical properties and optical transparency are required.

Some success has recently been achieved by using highly specific additives in hot-drawn HDPE materials to produce 90% transparency while still conferring high strength. Now, the research team led by Peijs and Bastiaansen has developed a new post-manufacturing technique for HDPE that endows strength and resilience while preserving transparency, without using any additives.

The researchers took HDPE polythene sheets and drew them out at a range of temperatures below the melting point of HDPE. By tuning the drawing temperature, they found they could achieve a transparency of 90% in the visible range. The best balance between strength and transparency was achieved at drawing temperatures between 90°C and 110°C.

"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 Peijs.

The highly transparent films possess a maximum resilience, or Young's Modulus, of 27GPa and a maximum tensile strength of 800MPa along the drawing direction; these are both more than 10 times higher than those of PC and PMMA plastics. For comparison, aluminium has a Young's Modulus of 69GPa and aerospace-grade aluminium alloy can have tensile strengths up to around 500MPa. But polythene has a density of less than 1000kg/m3, while aluminium has a density of around 2700kg/m3, meaning that on a weight basis these high strength transparent polymer films can outperform such metal materials.

"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," said Peijs. "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."

This story is adapted from material from the University of Warwick, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.