Scientists replacing heavy and often brittle glasses with a transparent plastic previously used conventional transparent plastics like polycarbonate (PC) and poly(methylmethacrylate)(PMMA), both of which possess relatively unsatisfactory mechanical performance compared to an engineering material like aluminum. Current methods of creating high-strength plastic films such as hot-drawing of high-density polyethylene (HDPE) can lead to materials that can compete or even outperform traditional engineering materials like metals.
A processing technique was developed that can create transparent polythene film that can be as strong as aluminum at a fraction of the weight that could be used use in glazing, windscreens, visors, and displays in ways that add strength and resilience while reducing weight. After carefully selecting the type of polythene and tuning the temperature during the creation of oriented polythene films, a balance was created that produced a highly useful and lightweight transparent material with a significant strength and resilience approaching, and in some ways exceeding that of metals.
A new post-manufacturing technique for HDPE endows strength and resilience while preserving transparency without using additives. The HDPE polythene sheets were drawn out at a range of temperatures below the melting temperature of HDPE. By tuning the drawing temperature, researchers could achieve a transparency of 90% in the visible range; however, the best balance between strength and transparency was achieved at drawing temperatures between 90 and 110 °C. Greater polymer chain mobility observed at the high drawing temperatures is responsible for creating fewer defects in the drawn films, resulting in less light scattering by defects and therefore a higher clarity.
The highly transparent films possess a maximum resilience or Young's Modulus of 27 GPa and a maximum tensile strength of 800 MPa along the drawing direction, both of which are more than 10 times higher than those of PC and PMMA plastics. For comparison, aluminum has a Young's Modulus of 69 GPa and aerospace-grade aluminum alloy can have tensile strengths up to around 500 MPa. Polythene, however, has a density of less than 1000 kg/m3 while aluminum has a density of around 2700 kg/m3, meaning that on weight basis, these high-strength transparent polymer films can outperform such metals.
The 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.