Last week we described our experimental apparatus (the hydraulic press) and a technique for characterizing materials. This week we will allow The Hydraulic Press Channel to perform our experiments (crush various materials with a press that has holes cut in the top), and we will analyze the results.
Several materials were extruded through holes of uniform size via hydraulic press. In this article we will limit our analysis to the orange mystery material, and the crayons. The experiment can be viewed below.
With enough pressure, both materials passed through the holes. After passing through the holes, the extruded crayons took on a long, cylindrical shape while the orange mystery substance fractured dramatically and exploded through the holes. The extruded crayon cylinders were observed to be very fragile and could not be bent without fracturing. The shreds of the orange material that passed through the holes flowed and stick to each other almost instantly, eventually reforming into one continuous medium.
The first observation we can make is how the material behaved before and after being “processed” by the press. The crayons had a well defined shape before they are pressed, and came out as long cylinders that were brittle and could not recombine. This is one of the ways we would expect a solid to behave.
The orange material appears to fill the cup of the press before it is processed. Liquids tend to flow and take on the shape of their vessel. After the orange material passes through the holes of the press, it appears that the individual bits begin to reform into a single mass – they flow together and combine, much like a very thick liquid would.
The second observation we can make is the way each material passed through the holes of the press. The crayons appeared to flow through the holes smoothly but the orange mystery material seemed to shatter and shoot through the holes. Based solely on this observation, one may guess the orange material is a solid, while the crayons are a liquid.
These two sets of observations appear to be at odds with each other. The crayons begin solid, then (without altering temperature) flow through the holes. On the other hand, the orange material initially takes on the shape of its vessel and melts back into itself after processing, but fractures like a solid as it passes through the holes. What gives?
Crayons are obviously solid, right? So let’s start from that assumption. Solids that are brittle will fracture. Less brittle solids can “elastically deform” under small stresses, meaning they will return to their original shape when the stress is removed (a traditional bow uses this property to fire an arrow). Under large enough stress, these less brittle materials may snap (like a twig), destroying the material in the process.
Alternatively it may deform permanently, but remain structurally undamaged. The latter is a property of metal and is something your local blacksmith takes advantage of.
Note that this is not the same as melting. “Plastic” deformation as described above, requires a large stress. However if one were to melt the steel, it would flow with the smallest of stresses. This technique can be used for casting objects with a mold.
Eh-hem… The One Ring wasn’t forged in Mordor, it was actually cast.
Based on our observations, we know that the crayons aren’t returning to their original shape after being pushed through the holes so the deformation isn’t elastic. Additionally, the crayons are not fracturing and being crushed into a powder like what might happen to a rock under the press; the crayons are not undergoing a destructive process. This suggests the molecules that make up our crayons are able to slip past each other when under enough stress, but are large and dense enough such that they do not flow under normal circumstances.
It turns out crayons are mostly made from paraffin wax, which is a collection of large hydrocarbons between 20 and 40 carbons long. A hydrocarbon is a term for a molecule that consists of some number of carbon atoms linked in a straight line, surrounded by hydrogen bonds. A model of a typical hydrocarbon is shown below.
Gasoline, kerosene and diesel are also made of hydrocarbons, though with fewer carbons. These shorter hydrocarbons are liquids because it is easier for smaller molecules to move around each other when they are together in bulk. As we deal with larger and larger molecules randomly assembled in bulk, they become jammed and are no longer able to move past each other as easily. However, with enough stress, these big long hydrocarbons can be forced to align, allowing the molecules to slip past each other easier. Therefore, under higher stress, we see the crayons plastically deform and flow.
Now let’s consider our orange material. We will call our orange material a liquid since, when left alone, it appears to flow like a high viscosity fluid. This means if two pieces of the orange mystery material are left in contact, they will eventually become one. However, as it gets pushed through the holes it begins to act like a solid. It breaks and fractures, then is ejected through the holes. Afterwards, we see that it begins to flow together once more.
Where pushing the crayons through the holes of the press caused the molecules to align and slip, it appears this same processing technique causes the molecules in the orange mystery sample to jam, tangle, and act solid.
It turns out the orange mystery material is a polymer very similar to Silly Putty. Polymers, like hydrocarbons, can be thought of strings of atoms bonded together. However, polymers like Silly Putty are typically much longer than the hydrocarbons that make up waxes, and much more flexible. This means that polymers can act solid as the individual molecules jam and tangle together, but are flexible such that, given enough time, they can wiggle themselves past each other and flow like a liquid.
A common analogy is to consider the molecules that make up Silly Putty to be spaghetti noodles in a pot. If you grab one noodle and slowly pull it out of the pot, it will slip past the other noodles, no problem. But if you grab a single noodle and try to pull it out of the pot quickly, you may pull the rest of the noodles out with you or snap the noodle. Pulling the noodle slowly allows it to flow like a liquid. Pull the noodle quickly, and you have a solid mass of noodles.
Pushing the orange material through the small holes of the press with such high force is like grabbing a few noodles and ripping them out of the bowl. The orange material will not flow nicely though the whole and instead fracture. Leaving the fractured bits alone for some time allows them to flow back together.
By crushing these materials and observing the carnage, it is possible to make some well informed guesses about what these materials are made of. Knowing what a material is made of and how it behaves in model experiments can allow material scientists to select the correct materials for specific applications, which is an important task for chemical companies that supply plastic and rubber materials to manufacturers. This type of science is also very important in the cosmetic industry and (my favorite), food science.