If you were ever looking to replace glass with a transparent plastic, you may have turned to polycarbonate or PMMA – materials that perhaps get the job done, but lack the strength of a metal.
Researchers from the University of Warwick and Queen Mary University of London devised a method for creating transparent polyethylene sheets that have a strength greater than aluminum – at a fraction of the weight.
The processing technique supports protective glazing for display screens, windscreens, and visors in the automotive and aerospace industry.
High-density polyethylene, or HDPE, is a versatile material used in everything from hard hats to plastic bags. Research led by Professor Ton Peijs at the University of Warwick and Professor Cees Bastiaansen at Queen Mary University of London have found a way to strengthen HDPE’s mechanical properties, without using additives.
The duo took HDPE polyethylene and drew out, or stretched, the sheets at a range of temperatures below the plastic’s melting temperature. By tuning the drawing temperature, Peijs and Bastiaansen could achieve a transparency of 90% in the visible range. The best balance between strength and transparency was achieved at drawing temperatures between 90 and 110 degrees centigrade. (Read the research:“Glass-like transparent high strength polyethylene films by tuning drawing temperature.”)
Traditionally, drawn polyethylene material maintains an opaque appearance due to defects and voids introduced by the drawing process, limiting applications where both mechanical properties and optical transparency are required.
The highly transparent films from Peijs and Bastiaansen 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 polycarbonate and PMMA plastics.
Aluminum, by contrast, has a Young’s Modulus of 69 GPa, and aerospace-grade aluminum alloy features tensile strengths up to around 500 MPa.
Additionally, polyethylene's density is less than 1000 kg/m3. Aluminum has a density of approximately 2700 kg/m3, demonstrating that the transparent polymer films have a higher strength per weight than the metal.
Prof. Pejs spoke with Tech Briefs about what’s possible when you can give common plastic an aluminum-like strength.
Tech Briefs: If you can give polythene film an aluminum type strength, what are the most exciting application possibilities to you?
Professor Ton Peijs: Applications for high-performance polyethylene films or fibers in construction materials are in composite laminates which can withstand high impact or blast loads, such as in protective panels or helmets. When combined with other fibers like carbon in more structural lightweight applications, the material can be used in sports equipment, as well as the aerospace or automotive industry.
Normally these high-strength polyethylene films or fibers, however, are not transparent.
If we can introduce transparency while maintaining high mechanical properties, these films can be of interest for impact-resistant glazing for buildings, transparent visors, or displays.
Tech Briefs: Why would someone want to strengthen plastic films, and what were the traditional options?
Professor Peijs: Normal polymer products are typically produced by extrusion or injection molding technologies. The polymer molecules or chains in these products are mostly randomly organized, a bit like cooked spaghetti would look like on a plate. The mechanical properties of such plastic products are typically low compared to metals because the properties originate from interaction forces between the polymer chains. These interaction forces are mostly weak intermolecular interactions, such as Van der Waals forces, meaning that plastics are typically soft materials.
If we, however, align the polymer chains in one direction – typically through a stretching process – we can create a very strong and stiff polymer film. Now, upon loading, we no longer load the weak interactions between the chains but the much stronger interactions within the polymer chain.
So, basically through processing, we have converted the microstructure of a polymer – resembling cooked spaghetti – into a microstructure resembling aligned (uncooked) spaghetti. Using such an approach, the tensile strength in the case of, for example, a very simple and cheap plastic like polyethylene can be increased by a factor of 50 or more.
Tech Briefs: How does your technique improve on those options?
Professor Peijs: Until now, none of these oriented polyethylene fibers and films were transparent. What we have done is added transparency as an additional functionality by optimizing the processing conditions.
Tech Briefs: What is it about the polymer structure and configuration that offers this kind of strength?
Professor Peijs: Through the stretching step, we can convert a randomly oriented polymer microstructure into a highly oriented structure. Besides that, we also need to use a polymer grade that has a relatively high molecular weight, one that consists of long polymer chains. Because polymer chains have a finite length, tensile strength will increase if there are fewer chain ends, as these chain ends act like defects.
Tech Briefs: And what inspired you and your team to try this kind of structure?
Professor Peijs: Oriented plastics is in our blood. My colleague Cees Bastiaansen and I have been “academically raised” by Profs. Piet Lemstra and Paul Smith, co-inventors of DSM’s high strength polyethylene fiber Dyneema®, so we are always thinking along the lines of improving a polymer materials property by orientation.
Tech Briefs: What lesson did you learn from this experience? Anything that can inspire those in our audience who work with polymers?
Professor Peijs: Mainly, that even in the case of the simplest and cheapest plastic like polyethene – which is used in numerous everyday products – one can still introduce novel functionalities through a better understanding of manufacturing processes.
What do you think? Where do you see this high-strength plastic being used? Share your comments and questions below.