My PhD project, looking at materials suitable for 3D printing buildings using swarming aerial robots, began with investigating low density, expanding polyurethane foam ‘LD40’ (a) manufactured by the company Isothane. Now it is the turn of high density polyurethane foams, and the studies have used Reprocell 300 (b) and Reprocell 500 (c), commercially available foams manufactured by the same company.
While LD40 is established in the construction industry as an insulating material, the higher density foams are not readily associated with construction. Reprocell 300 is usually found as a substitute for timber in prop and set design along with applications such as balustrades and mantelpieces, whereas Reprocell 500 is primarily used for deep sea buoyancy applications.
Blocks of foam created so that the materials may be tested in line with the British standards, have been primarily made by hand mixing on the high density foams, such as the compressive strength test specimens shown in the figure.
Through trial and error, I arrived at a recipe for making the high density specimens. Firstly, I heat the two liquid components (one resin, one hardening agent) to 30°C, pour together and hand-mix for 90 seconds. The creamy, viscous liquid then turns a darker brown and becomes much less viscous as the mixed liquid heats up and the polymerisation process begins. I then stir again until the 150 second mark, at which point the material expands. A few more careful stirs then follow until I withdraw and the material quickly hardens, becoming solid at 180 seconds and feeling like a block of concrete.
This recipe has produced specimens which exceed the density stated in the manufacturer’s literature, most notably with the Reprocell 500 (specimens average 685 kg/m3). Compressive tests on the 500 have shown strength in excess of 30 MPa, astonishingly competitive with concrete. Flexural tests have also shown strengths indicating the material is competitive with the lower range of timbers. Reprocell 300 has around a third of the compressive strength of the 500.
Reprocell 500 has therefore shown that it has potential to be a structural material. The downside of denser specimens of course is that the material does not expand quite so freely so I am having to use more of the liquid components!
Making 500 specimens with my little syringe device outlined in the previous post has proved to be challenging. Despite experimenting with longer tubing and multiple static mixers, the material is being deposited in its creamy viscous state, and then the darker / hotter / thinner / runnier phase is occurring on the surface of the mat after deposition, leading to lateral spreading.
After the current round of tests are complete, several options will be investigated, most notably whether a catalyst may be applied to speed up the reaction time and the investigation of whether particles can be added to the material to favourably alter its rheological properties – these may include clay nanoparticles, graphene, carbon fibres, or ways to combine the lower density foam with the higher density foam.
Barrie Dams 19/01/2017