You’d be hard-pressed to walk down nearly any aisle of a modern food store without coming across something made of plastic. From jars of peanut butter to bottles of soda, along with the trays that hold cookies firmly in place to prevent breakage or let a meal go directly from freezer to microwave, food is often in very close contact with a plastic that is specifically engineered for the job: polyethylene terephthalate, or PET.
For makers of non-food objects, PET and more importantly its derivative, PETG, also happen to have excellent properties that make them the superior choice for 3D-printing filament for some applications. Here’s a look at the chemistry of polyester resins, and how just one slight change can turn a synthetic fiber into a rather useful 3D-printing filament.
Not Just for Clothes
Like many plastics with practical applications, PETG is a copolymer. The homopolymer upon which it is built is PET, or polyethylene terephthalate. From the polyester family of polymers, PET was first patented in 1941 by a pair of British chemists, John Whinfield and James Dickson. Like many others, they were looking for synthetic fibers like nylon, which had made a big splash when introduced by DuPont a few years prior.
Whinfield and Dickson found that a condensation reaction between the organic acid terephthalic acid, a compound originally isolated from turpentine, and the diol ethylene glycol, which is the main component of automotive antifreeze. They found that the monomers would link together into long chains, producing a substance that could be drawn into fine fibers and made into yarn. Wartime secrecy laws kept their invention, dubbed Terylene, under wraps until 1946.
Today, PET is produced by other processes. The DMT method uses dimethyl terephthalic acid, which is just terephthalic acid with two methyl groups attached. When ethylene glycol is reacted with DMT at high temperatures and under basic conditions, a transesterification reaction occurs, linking the long chains of DMT together with a small fragment of the ethylene glycol. This reaction generates methanol, which needs to be removed for the polymerization reaction to continue.
As versatile as PET is, it’s not without its weaknesses. While it’s very well suited for the manufacture of synthetic fibers, it doesn’t perform well in applications where other thermoplastics excel, like extrusion or injection molding. That’s where PETG comes in. The “G” stands for “glycol modified,” which is a somewhat confusing nomenclature. Many sources seem to think this means that glycol is added to the polymerization reaction, but as we’ve seen, ethylene glycol is already part of the polymerization reaction. Glycol modification refers to the fact that some of the ethylene glycol in the growing chain is replaced with another monomer, resulting in a copolymer with different properties than the homopolymer.
In the case of PETG, the comonomer is another diol, cyclohexane dimethanol (CHDM). This molecule is much larger than the compact ethylene glycol, but undergoes transesterification in much the same way as the smaller molecule. The effect of adding CHDM is that the distance between the terephthalic acid residues is increased, making it harder for neighboring polymer chains to nestle together. This results in a water-clear plastic with a lower melting temperature than PET that can be molded and extruded.
Best of Both Worlds
These properties make PETG and other PET copolymers extremely useful for commercial products. For the home gamer, PETG is a common choice for 3D-printing filament, and basically combines the best properties of ABS and PLA into a filament that easy to work with. It has greater strength and better flexibility than PLA, and its low shrinkage, great layer adhesion, and tenacious bed adhesion makes it less likely to warp or delaminate during printing. One nice feature compared to both PLA and ABS is that PETG doesn’t really smell like much while it’s printing. So if you’re sick of fumes that make your shop smell like an organic chemistry lab, PETG might be worth a try.
PETG also wins over PLA when it comes to environmental factors. PETG is resistant to many solvents, and stands up much better to wind, rain, and UV exposure than PLA, making it a great choice for outdoor applications. Unpigmented PETG is also translucent when printed, which can lend an aesthetic to prints that other filaments can’t. And PETG is considered a food-safe plastic, but you’ll want to take other factors into account before using it in contact with food.
PETG isn’t perfect, of course. It is more flexible than either ABS or PLA, which may be a problem for some applications. It also has a tendency to shatter suddenly when stressed beyond its limits, as opposed to yielding gradually. And as you might expect from a polymer that descended from textiles, PETG is subject to stringing while printing. That’s easily remedied with a blast from a heat gun, but it’s still something to keep in mind if you’re looking for form over function.
If you haven’t given PETG a try in a 3D printer, you should.