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, …read more

Source:: Hackaday