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Why Does PLA Smell Sweet? The Science Explained

If you have tried a hand at 3D printing and have watched your printer melt and extrude PLA, you might have noticed that it has a distinct scent. It’s sweet and pleasant, very much unlike burning plastic. But why so? 

Polylactic acid (PLA) smells sweet because of its organic composition. It is a biopolymer produced from natural materials like corn. Thus, it does not contain harmful chemicals that typically release foul odors. 

If you are interested in learning more about PLA and the science behind PLA’s smell, dig into this article to learn more. 

What Makes PLA Smell Sweet?

Now, to truly understand the science behind PLA’s smell, we need first to understand what PLA is. 

A polymer is a chain of repeating molecules, with each molecule unit referred to as a monomer. These chains take on the properties of their monomers and how these units were linked together. 

Polylactic acid (PLA) is an agricultural biopolymer. Its monomers are derived from lactic acid, which is fermented from natural materials like sugar beets, sugar cane, or corn. Thus, it has a sweet smell because of its organic composition. 

How Is PLA Synthesized?

Now, you might be curious how a sugar cane or corn can be turned into PLA. There are common ways to fabricate PLA: ring-opening polymerization and direct polymerization, followed by direct additives.  

Fermentation

PLA synthesis often begins with the production of lactic acid through fermentation. While synthetic lactic acid can be produced in laboratories, the fermentation route is preferred to ensure optical purity. 

Fermentation is also more eco-friendly, with fewer carbon emissions, and has the potential to make use of agricultural residues (although this part is still being researched). It also uses different microbes, and thus, there is little participation in chemicals. 

After fermentation, lactic acid is purified further, then turned into lactide before proceeding into ring-opening or polycondensation. 

Ring-Opening Polymerization

Ring-opening polymerization is the standard method used to produce high-molecular-weight PLA. After fermentation, lactic acid fermentation may be followed by cyclic dimer (lactide) preparation. 

The cyclic or ring structure may be opened in the presence of a catalyst. Metal is often used as a catalyst. 

More purification steps may be needed, but this method is still the more practical way to synthesize heavy PLA chains. 

Polycondensation

Polycondensation was actually the first way in which PLA was produced. However, it is not the practical approach if a high-molecular-weight PLA is the target end product. This process requires long reaction times and solvents. 

If, however, a high-molecular-weight PLA is needed to be made using polycondensation, then chain extension may also be done. 

Now, these are just a few ways by which PLA is produced. Some methods may not need to use chemicals, while others may not. 

However, generally, PLA production uses fewer chemicals and has natural materials as the source. Hence it is non-toxic and smells sweet. 

PLA Emits a VOC: Lactide 

Another thing that may also contribute to the smell of PLA is the volatile organic compounds it emits. But, first of all, what exactly is a VOC? 

What Is a VOC?

You might have heard of VOC and how they can be carcinogens or harmful chemicals. While a negative connotation comes with the term, not all compounds are harmful. 

Volatile organic compounds (VOCs) are gases emitted by other products. Characteristically, they have low boiling points, hence volatile, and can often be detected through our sense of smell. For PLA, the main VOC is lactide, which is non-toxic.

Smell of Lactide

As I have mentioned, VOCs may contribute to the odor. Most sources state that lactide is odorless, but a source said it has a pleasant smell. Either way, this may explain why the sweet smell of organic sugars dominates PLA. 

VOC Emission Levels of PLA

Now, if you work a lot with PLA, for instance, through 3D printing, you may be concerned about its VOC levels. Although PLA and lactide are organic and non-toxic, it is still essential to be cautious. 

According to a study, compared to other 3D printer filaments like ABS, PLA emits much fewer VOCs and ultrafine particles. It also does not emit any concerning VOCs since its main VOC, lactide, is not on the list of harmful compounds. 

Thus, there is an increasing interest in PLA as its non-toxic and biodegradable nature presents many potential applications. Still, despite the consensus that PLA and its fumes are safe to work with, it is recommended to keep the place well-ventilated. 

Applications of PLA 

The biodegradable, non-toxic, and organic nature of PLA has made it a preferable material for many applications. Thus, researchers are looking more at improving it and how common agricultural wastes, like corn stalks, can be used for its production. 

But, to focus more on the matter, here are some typical and practical applications of PLA: 

  • Packaging: One of its more popular purposes is as a packaging material. Being biodegradable and non-toxic, it is an emerging choice for companies that want to cut back on greenhouse emissions while keeping their products safe. 
  • Automobile and industry: It is also used in the fields of automobiles, electronics, and medicine. For cars, it can be used to make structural parts. The applications are still few in electronics due to limitations, like brittleness, but the material is being combined with nanotechnology to improve this. 
  • Medicine: PLA’s non-toxicity has made it an attractive material of choice in medicine. It is used for bone plates, drug delivery, and medical devices. It is even used for tissue engineering. 
  • 3D Printing: Another quite common application of PLA is also 3D printing. It is the preferred filament of choice for many beginners as it is easy to work with, does not need a heated bed, and comes in various colors. 

Final Thoughts 

PLA is made of natural and organic materials like corn and sugarcane; hence, it has a sweet aroma. But aside from being a bonus perk for PLA handlers, its sweet smell may also be seen as a sign that we can shift towards more eco-friendly materials.