What do you think of when you hear the word plastic? Flexible? Recyclable? Strong?
We typically associate material characteristics with their applications. Plastics used in safety helmets have very good impact resistance but those used for plastic bottles are chemically resistant. Even with the wide range of plastics available, recyclable plastics are typically softer than their harder non-recyclable counterparts. Wouldn’t it be great if we could pick and choose the properties we want in a material; for example, a durable plastic that’s also recyclable?
How can you change the performance of a material? By changing its structure.
Damien Montarnal and his colleagues describe in Science (DOI: 10.1126/science.1212648) a new epoxy that it is moldable and recyclable. This is an exciting discovery because conventional epoxy (i.e. thermosetting polymers) cannot be reprocessed after they are formed.
Plastics are classified as either thermosets or thermoplastics, depending on how they react to temperature. Chemical and structural changes occur when plastics are cured (hardened). Thermosets permanently harden by chemical reactions, forming one giant molecule, and won’t soften or melt when reheated. Thermosets are crosslinked, meaning adjacent polymer chains are covalently bonded to each other at specific positions. The bonds anchor the polymer chains together to resist chain movement at high temperatures.
Thermoplastics on the other hand soften when heated and harden when cooled. The softening, hardening processes are reversible and repeatable because the bonding between polymer chains changes at high temperatures, allowing the chains to move. The bonding differences are why thermosets are difficult to recycle and don’t melt, but can be used at higher temperatures and are more chemically inert than thermoplastics.
Montarnal and his colleagues created a new epoxy with exchangeable crosslinks. The exchangeable crosslinks in the new polymer allow it to be reshaped when heated because unlike conventional thermosets, there are no restrictions on which polymer chains can bond together.
Only heat is required to activate the exchange, allowing the epoxy to be recycled and reshaped repeatedly.
The team says another advantage of their epoxy is that it behaves like silica glass when heated. This means the plastic can be shaped without using a mold and only local heating is required, which is similar to how glassblowers sculpt their pieces. Thus broken epoxy pieces can be repaired or recycled, simply by heating them. In the future, this material could be an alternative to the conventional thermosets that are already used in the aircraft industry.
Montarnal and his colleagues have created a new plastic that is recyclable, remoldable, and stable at high temperatures. Their work has shown that the properties of conventional polymers can be improved by altering the bonding structure. Material properties are simply guidelines. You can create a material with the desired combination of properties with the right techniques.
Montarnal D, Capelot M, Tournilhac F, & Leibler L (2011). Silica-like malleable materials from permanent organic networks. Science (New York, N.Y.), 334 (6058), 965-8 PMID: 22096195