Although it technically is possible to recycle Plexiglass, it's a complex and inefficient process, so it generally just isn't done. A simple new technique, however, is claimed to break the plastic down into all of its building blocks for near-complete recycling.
Plexiglass (aka polymethyl methacrylate or PMMA) is made up of long polymer chains, which in turn consist of a series of linked monomer molecules. Because these chains are very uniform in structure, they have no specific "points of attack" at which they can be easily broken apart. This means the monomers stay locked together, making them unavailable for recycling.
That said, polymer chains such as those in Plexiglass can be broken via a process of thermal decomposition known as pyrolysis. This involves heating the material to a temperature of at least 400 ºC (752 ºF) in the absence of oxygen.
Needless to say, reaching and maintaining those kinds of temperatures requires a lot of energy. Additionally, the recovered monomers still need to be purified via an involved and expensive procedure. As a result, pyrolysis simply isn't feasible for the large-scale recycling of Plexiglass.
Another option that has been explored involves attaching heat-sensitive detachable monomers to the ends of long polymer chains. As those special monomers detach when heated, they trigger the entire chain to fall apart, resulting in recovered monomer yields of up to 90%.
Polymers utilizing this technology couldn't be manufactured using existing machinery, however, plus they lack thermal stability. Additionally, the depolymerization process is hampered if additives such as dyes are present in the plastic.
That brings us to the new technique, which is being developed by Prof. Athina Anastasaki and colleagues at ETH Zurich university in Switzerland.
The process involves adding a chlorinated dichlorobenzene solvent to crushed Plexiglass, heating the mixture to between 90 and 150 ºC (194 and 302 ºF), then exposing it to ultraviolet light. That light causes a chlorine radical to split off from the solvent, which triggers the complete breakdown of the polymer chain.
All of the freed up monomers can then be purified using "straightforward distillation processes," ultimately taking the form of virgin-grade building blocks for the production of new Plexiglass. And importantly, even when additives are present in the Plexiglass that is being recycled, the process still boasts yields of 94 to 98%.
The scientists are now looking into methods of performing the process without the environmentally-unfriendly solvent, skipping straight to the chlorine radical. A paper on the research was recently published in the journal Science.
Source: ETH Zurich
