Polylactic Acid (PLA)

Polylactic Acid (PLA) - The Biodegradable and Compostable Truth

Polylactic Acid (PLA) is one of the main types of bio-plastics available in today’s market. Although it was first synthesized by DuPont scientist Wallace Carothers in 1932, it wasn’t commonly used until the late 20th century. 

Today, PLA is primarily used for food containers, textiles, biomedical devices, and the agricultural industry. Some examples of items made from PLA include food-grade film wrapping, eco-friendly clothing fibers, sutures, and seeding trays.

Discover everything you need to know about PLA, its manufacturing processes, its benefits and drawbacks, and the truth about its biodegradability and compostability.

What is PLA?

Polylactic Acid (PLA), also known as polylactide, is a type of thermoplastic monomer manufactured from natural, renewable resources. Most PLA used globally is manufactured from corn starch and produced in countries with large corn processing facilities, such as the United States.

For instance, one of the world’s largest PLA factories is the NatureWorks plant, located in Blair, Nebraska, one of the heartlands of American corn. The plant has an estimated yearly output of 150,000 metric tons.

PLA is one of the world’s most widely produced bioplastics, with the second-highest yearly production volumes behind thermoplastic starch. It has similar properties to many traditional plastics produced with petroleums, such as polypropylene (PP), polyethylene (PE), and polystyrene. PLA is also notable for its compatibility with existing industrial processes and equipment, such as PET blow molding, PP injection molding, or polystyrene extrusion.

Table of Properties

Density 1.00 to 3.41 g/cm³

Melting point 64-220°C (147-428°F)

Tensile strength, yield 8 to 103 MPa (1,160 to 14,900 psi)

Rockwell hardness 104-118

Solubility in water Non-soluble (0 mg/mL)

Flammability (UL 94 rating) HB (slow-burning) V-0 (burning stops within 10 seconds on vertical test)

How is it Made?

Manufacturing polylactic acid is a three-stage process: sugar extraction, fermentation, and polymerization. The resulting material is formed into pellets, powder, sheeting, or filament, which can be processed and transformed into finished products in other factories.

Sugar Extraction

A PLA plant starts with harvested natural crops, such as corn, sugarcane, or cassava. The first stage of the PLA manufacturing process is to process these crops and extract the sugars they contain.

PLA manufacturing facilities can use two essential ingredients to make polylactic acid: glucose or dextrose. Dextrose is typically extracted from corn and cassava, whereas glucose is commonly derived from sugarcane.

Fermentation

After extracting sugar from the natural crops, it is mixed with water, vitamins, and nutrients. The mixture is then inoculated with a specially grown strain of Lactobacillus bacteria, such as L. delbrueckii bulgaricus. 

The resulting inoculated medium is stored in large bioreactors at a specific temperature ranging from 40 to 45°C and a particular level of acidity (pH 5.4-6.4). These conditions promote the growth of these bacteria, encouraging them to consume the sugars and convert them into lactic acid.

The fermentation process typically takes 2 to 4 days, depending on the strain, conditions, and type of sugar used. Once the fermentation is complete, the PLA plant separates and purifies the lactic acid from the solution using calcium hydroxide.

Polymerization

Once extracted and purified, the lactic acid is ready for the polymerization process. Polymerization is a chemical reaction linking molecules of the same type together to form a single compound (a polymer). Polymerized lactic acid molecules form polylactic acid.

Most PLA plants today use the ring-opening polymerization (ROP) process due to its reliability and ease of scaling. It involves the following steps:

  • Formation of lactide. Lactic acid is inserted into a prepolymer reactor, where it is oligomerized (reformed into short chains) and depolymerized to create an intermediate material called lactide.

  • Lactide purification. This process results in crude lactide, which is then purified through distillation or crystallization and stored in a controlled environment to prevent it from absorbing moisture.

  • Repolymerization. Once ready, the purified lactide is inserted into a polymerization reactor, where the ring-opening polymerization process is carried out. The result is molten PLA, which can then be transformed into a base form.

  • Rendering. Molten PLA is stabilized, cooled, and molded into a particular form, depending on its intended use. Common forms include pellets, powder, solid sheeting, film, or filament. For example, PLA pellets are commonly used as the raw material to produce blow-molded drink bottles, whereas PLA filament is typically destined for 3D printing.

Benefits and Drawbacks of PLA

As a bioplastic, polylactic acid offers many benefits. It is a sustainable material with numerous practical applications for the plastic industry. However, it also presents many drawbacks, such as cost, durability, and food supply challenges.

Benefits of PLA:

  • Made from renewable resources. The raw materials required to produce PLA are commonly available crops like cornstarch, sugarcane, and cassava. PLA production helps reduce dependency on non-renewable resources like petroleum.

  • Recyclable. PLA can be mechanically or chemically recycled, provided it is handled by facilities adapted to PLA recycling; which are rare, so basically, PLA is not recyclable.

  • Versatile. PLA can replicate the properties and applications of numerous traditional plastics, such as polyethylene, PET, PP, polystyrene, and PVC.

  • Food-safe. PLA is generally safe for use in food packaging and containers. It does not contain nor leach any toxic substances.

Drawbacks of PLA:

  • Limited durability. Less durable grades of PLA have a low melting point and start deforming under temperatures as low as 60°C (140°F).

  • Competes with the food supply industry. PLA requires food crops to produce, meaning it can impact the food market by reducing the supply of corn, cassava, or sugarcane.

  • High cost. Although the cost of PLA production has steadily decreased over time, it remains more expensive than petroleum-based alternatives.

Is PLA Biodegradable and Compostable?

One of the most commonly cited benefits of using PLA is its biodegradability and compostability. While it can break down into water, biomass, and carbon dioxide and meet industrial compostability standards such as ASTM-D6400, it requires specific conditions to start degrading.

Most PLA materials require a high-temperature environment to fully biodegrade and compost within the industry-standard 12-week period. The required temperature ranges from 55-70°C (131-158°F), depending on the PLA grade. The decomposition process also requires recycling and composting centers specifically equipped to sort and handle PLA. 

So, is PLA compostable? Yes, but it depends on access to specific facilities. For instance, PLA will not biodegrade in common, home-based composting solutions because they do not reach the necessary temperatures. Without access to the correct composting centers, PLA may require at least 80 years, up to hundreds of years, to biodegrade naturally.

A New Option

Traditional plastics fortified with modern biodegradable plastic additives, such as Pristine®, can biodegrade and compost without the need for special facilities. The biodegradation process follows the ASTM-D5511, D6691, and D5338 protocols and can start in standard landfills, traditional composters, and even marine environments. 

Pristine® is Your Partner in Biodegradable Plastic Innovation

While PLA and bioplastics like it have advantages in promoting sustainability, they are no longer the only solution available. Pristine® LLC is a cost-effective solution that promotes the rapid biodegradation and decomposition of traditional plastic materials such as PP, HDPE, LDPE, PVC, PET, polyester, and polycarbonate. 

Contact us to learn more about Pristine® plastic additive and how it ensures efficient biodegradation of traditional plastic in standard landfills, compost sites, and marine environments.

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