PET is formed via the condensation of ethylene glycol (ethane-1,2-diol, aka EG) and terephthalic acid (1,4-benezenedicarbyoxylic acid, aka BDC). In the process, a proton (H, a hydrogen) is lost from the carboxylic acid (COOH -> COO) and a hydroxyl is lost from the alcohol (R-OH -> R). The H and OH combine to form a water (hence "condensation") and leave, and an ester bond (RCOO-R') is formed. Amide bonds (RCONR'), the backbone proteins and materials like Kevlar, spider silk, and Nylon, are similar, but less flexible and harder to break.
This condensation reaction (and others) is reversible. If you heat PET in boiling water, you can begin to depolymerize the polymer into its constituent monomers by having a water molecule consumed in the reaction (RCOOR' + H2O -> RCOOH + HO-R').
However, PET is a rigid plastic that acts as a barrier material (that's why it's used for bottles and packaging), and has a high glass transition temperature (the temperature at which polymer chain mobility becomes broadly possible -- the polymer is in equilibrium, while it is kinetically trapped below the Tg). This inhibits the breakdown of PET, particularly at low temperature.
Hence, the sentence from the abstract:
> With a high ratio of aromatic terephthalate units—which reduce chain mobility—PET is a polyester that is extremely difficult to hydrolyse.
Aromatic rings are just that -- rings. They don't flex a lot and can't undergo a lot of thermal motion, can't change conformation, etc.
> Here we describe an improved PET hydrolase that ultimately achieves, over 10 hours, a minimum of 90 per cent PET depolymerization into monomers, with a productivity of 16.7 grams of terephthalate per litre per hour (200 grams per kilogram of PET suspension, with an enzyme concentration of 3 milligrams per gram of PET).
> We also show that biologically recycled PET exhibiting the same properties as petrochemical PET can be produced from enzymatically depolymerized PET waste, before being processed into bottles, thereby contributing towards the concept of a circular PET economy.
So, they depolymerize the PET, an subsequently repolymerize the terephthalic acids to produce new PET. I haven't clicked through to find if they recycle the EG as well, though.
Poly(ethylene terephthalate) (PET) is the most abundant polyester plastic, with almost 70 million tons manufactured annually worldwide for use in textiles and packaging3. The main recycling process for PET, via thermomechanical means, results in a loss of mechanical properties4. Consequently, de novo synthesis is preferred and PET waste continues to accumulate. With a high ratio of aromatic terephthalate units—which reduce chain mobility—PET is a polyester that is extremely difficult to hydrolyse5. Several PET hydrolase enzymes have been reported, but show limited productivity6,7. Here we describe an improved PET hydrolase that ultimately achieves, over 10 hours, a minimum of 90 per cent PET depolymerization into monomers, with a productivity of 16.7 grams of terephthalate per litre per hour (200 grams per kilogram of PET suspension, with an enzyme concentration of 3 milligrams per gram of PET).
Basically, plastics are polymers (many units) of repeating building block monomers (single unit). The single unit is terephthalate, a " dimethyl-ester that is a major starting material for polyester fibers and coatings." So you're breaking down the already formed polymers into the building blocks that can be redeployed.
I think that it decomposes the plastic back into the monomers that it is made up of. At which point it could be polymerized again to create new plastic.
