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.
I always though that scene one of the best in the whole canon, especially when Scotty picks up the mouse and uses it as a microphone. However the premise, needing strong transparent walls for a tank in a spaceship so you could see the whales in the tank was a bit spurious, surely steel would have actually been lighter and stronger in that context. Why did the crew have to see the whales for the 5 minute voyage from San Francisico 20th Century to San Francisco 24th Century? Surely having some air space in the tank for the whales to breath would have been more sensible.
Using the draw of PE molecules to promote strength in PE and other olefins is well known and my PhD was in the SCORIM process, this story and the connection to the film has taken me back through the years to when the film was released. Lovely to see the phenomenon still being applied and expanded and to products that were un-imagined (even by the writers of Star Trek) at the time.
I could never figure out why the tank had to be transparent either. George and Gracie deserved some privacy after being gawped at for years in the aquarium.
Ah – but perhaps it wasn’t for the benefit of the human crew? Perhaps it was so that the whales would not suffer catastrophic claustrophobia ? and at least see out of the tank even though effectively in an acoustic straight jacket? – and cinematic-ally it would look better on screen to see happy whales rather than a steel tank !
Otherwise what an exiting development !! – one awaits to consider form-ability ,thermal stability and fatigue characteristics but if say these can be used to replace vehicle windscreens, it could proves a step change in fuel efficiency, one wonders what other applications it could be applied to? Roll on industrial development and well done to Warwick & Queen Mary groups ! ( with commiserations to Warwick, I understand the founding member of Warwick Industrial whose name I am ashamed to say i cant remember other than he was Indian died recently? )
It is impressive that the transparency has been achieved – though I would like to see more information about the surface hardness (scratchability).
The use of thermoplastics in laminate (and fibre composites) improves the toughness – compared to conventional composite and, in this form could be used for additive layer manufacture. (Also thermoplastics are easily recyclable – compared to traditional composites).
It would be interesting to see how new windscreens would be made with thin (tough) glass layers (say Gorilla Glass) and thick plastic layers (an inversion of current practice) and how much of the (roughly) 100 pounds weight of glass could be replaced (as the stiffness of the windshield, for example, needs to be maintained – as it has a mechanical function, in a car see https://drivinglife.net/windshield-protect-structure-vehicle/)
Many years ago (1966) I recall visiting the ICI Melinex plant: Ardere, Scotland. This was polyester film (headed for photographic and sound recording applications: but I recall well seeing the ‘double draw process. The ‘as-extruded’ material was positioned in a modified “textile” stentor (used in ‘ my’ trade to ‘set’ fabrics) and then stretched/drawn both lengthwise and width ways at the same time. This gave the unique properties required. This present R&D sounds very like what I saw all those years ago.
Ah, laddies! First you have to have a starship. Then you get to have voyages.
Nevertheless, if you now combine this new HDPE drawn membrane, with a very thin layer of aluminum atoms, and another drawn layer oriented perpendicular to the first set of strands, what will you have? Try it and find out! Maybe a new pair of polarizing sunglasses?
Very true. Even when glass adheres tenaciously to polyurethane sealing and fastening resin, the very best bond is achieved through adding a rough ceramic trace around the windshield, because it is indeed what gives the vehicle roof pilars their structural strenght… but unless special ways to adhere the PE, it would be quite difficult to retain a windshield in the frame.
I must admit I had not known about the edge bonding you mention; interesting.
As the PE has a significantly lower modulus than the glass that would reduce its stiffness; however I was wondering if a sandwich laminate material (much like aluminium composites – such as GLARE) would retain that stiffness with using thin layers of strong glass (Willow or Gorilla glass).
Alternately the idea that Mercedes (I think) had for using double glazing (i.e. an air gap between the transparent layers might help bring the stiffness back up – as well as providing better impact energy absorption ; blow/vacuum forming such shapes in sheet HDPE would be easier than glass.
Whatever became of the idea of polarizing headlight glass ‘left’ and windscreen glass ‘right’ so that drivers would see their own ‘beam’ fine: but simply circles of light coming towards them.
Is this just an old man joining one and one and making 3: or was this actually possible?
About 20 years ago I had to replace a windshield in one of my cars because a crack appeared at the bottom and was progressing daily towards the upper part. The glass was included in the insurance but the windshield shop selected by the insurers was lousy… they used a Butyl rubber sealer like those used in old times, like before 1980. Fortunately, one companion at work had seen a TV program called “20-20”, where the presenter Barbara Walthers demonstrated several tragic accidents where the car´s top was smashed because both the front and back windshields were NOT properly adhered with hight strenght Polyurethane adhesive sealer. (One of the best brands of this polyurethane is “SIKA” from Germany). Thanks to that, I was able to protest and the insurance company had to order the reinstallation at another shop, which not only used the proper polyurethane, but used a “primer” that attacks the ceramic trace around the windshield to promote the correct bonding. Later on, I found that the Repair shop manual for that car notes that “windshield and back glass are a “STRUCTURAL” part of the vehicle, so that the glass replacement must be done by a professional”.