Polylactic Acid (PLA): How to select the right grade?

Polylactide (PLA) — Exploring Chemistry and Properties

Polylactide or PLA (also known as polylactic acid or lactic acid polymer) is a versatile commercial biodegradable thermoplastic. It is based on lactic acid.

Lactic acid (LA or 2-hydroxypropionic acid) is the most widely occurring hydroxycarboxylic optically active acid. This chiral molecule exists as two enantiomers – L- and D-lactic acid. Lactic acid monomers can be produced from 100% renewable resources, like corn and sugarbeets.
Stereoisomers of Lactic Acid

The Enantiomers of Lactic Acid (Source: ResearchGate)
PLA has different stereoisomers, such as:

Poly-L-lactide (PLLA)
Poly-D-lactide (PDLA), and
Poly-DL-lactide (PDLLA)

Polylactide has been able to replace conventional petroleum-based thermoplastics. This is due to the excellent combination of properties it possesses.

Polylactide is one of the most promising biopolymers used today and has a large number of applications. This includes medical and healthcare, packaging, automotive applications, etc.

Unveiling the properties of PLA

PLA is a bio-based, biodegradable, and biocompatible polymer. It has proved itself to be a promising alternative to petroleum-based polymers. Its properties are on par with widely used polymers like polyethylene terephthalate (PET), polyvinyl chloride (PVC), etc. The high-performance PLA grades are an excellent replacement for polystyrene (PS), polypropylene (PP), and acrylonitrile butadiene styrene (ABS) in demanding applications.

Molecular Structure of Polylactic Acid (PLA, polylactide) Bioplastic

The commercial viability of PLA is sometimes limited by its high production costs compared to its petroleum-based counterparts. Optimization of LA and PLA production processes, and with an increase in PLA demand, a reduction in its price can be achieved.

Here are some of the key properties of PLA:

Crystallinity — Most of the commercial L-PLA products are semi-crystalline polymers. It is desirable that PLA should have some crystalline content to benefit the quality of the finished product.

Melting and glass transition temperature — They have a high melting point of 180 °C and a glass transition temperature (Tg) in the range of 55 - 60 °C.

Strength — PLA is a high-strength and high-modulus thermoplastic with a good appearance. It has high stiffness and strength, comparable to polystyrene (PS) at room temperature.

Processability — PLA can be easily processed using common techniques like injection molding, extrusion, blow molding, and 3D printing. This makes it versatile for a wide range of manufacturing processes.

Energy consumption — Less energy is required in the production of PLA as compared to other plastics. They have better thermal processing.

Further development of composites, nanocomposites, and biocomposites is expanding the properties and potential applications of PLA.

Strategies to improve PLA properties

PLA properties can be altered or improved by the use of additives and developing polymer blends. Some of the examples are:

Plasticization
Lactide monomer is an excellent plasticizer for PLA, however, it tends to migrate to the PLA surface. Other plasticizers such as citrate esters & low-Mw PEG have shown only modest improvements in toughness. This is accompanied by a dramatic loss in tensile stress at break and tensile modulus.

Mineral fillers
They provide substantial improvement in the impact toughness of PLA. For example, ppt CaCO₃ at 30% loading.

Impact modifiers
They can improve PLA properties. However, adding them will compromise the compostability of the PLA.

Polymer blending
PLA/PCL blends are also degradable polyester. Due to its low Tg, it exhibits rubbery characteristics with an elongation at break of approximately 600%. This makes it an ideal candidate for toughening polylactide. PLA blends with PHA have shown significant improvement in impact toughness. They exhibit a modest decrease in modulus and strength. They do not compromise the bio-based content and compostability of PLA.

The development of PLA nanocomposites using nanoscale fillets represents a better alternative to traditional composites. Due to their high surface area, improved matrix adhesion, and aspect ratio these nanofillers (colloidal silica, clay platelet) offer enormous advantages over traditional macro- or micro-particles (e.g., talc, glass, and carbon fibers). For example, PLA-clay nanocomposites show improved mechanical, barrier, optical, and thermal properties.

Life Cycle of PLA and Its Production Pathways

What are the major phases in the life cycle of PLA?

The life cycle of polylactic acid (PLA) polymers can be divided into four main stages:

Feedstock production — PLA is made from renewable resources such as cornstarch or sugarcane. These crops are grown, harvested, and processed to produce lactic acid, the monomer from which PLA is made.

PLA production — Lactic acid is polymerized to form PLA. This process involves a series of chemical reactions that combine lactic acid molecules to form long chains. The properties of the PLA polymer can be controlled by varying the polymerization conditions.

PLA processing — PLA is a versatile material that can be processed into a variety of products. For example, films, sheets, fibers, and molded parts. The processing method depends on the desired product properties.

PLA end-of-life — PLA can undergo landfilling, composting, anaerobic digestion, or recycling.
- Landfiling - PLA is a relatively stable polymer and can persist in a landfill for hundreds of years.
- Composting - Here, the PLA is broken down into compost, which can be used to improve soil quality.
- Anaerobic digestion - PLA can be digested anaerobically along with other organic wastes. The biogas can be used to generate electricity or heat.
- Recycling - PLA can be recycled into its monomer components, which can then be used to make new PLA products.

Looking for PLA grades to ease your end-of-life concerns? We got you covered here:

Major pathways of PLA production

Polylactic acid is an aliphatic polyester that can be obtained using different routes:

➤ Direct polycondensational reaction (DPR)
It usually leads to low molecular weight polymers. They can then be converted to higher molecular weight polymers by the addition of chain coupling agents.

➤ Ring-opening polymerization (ROP)
PLA is produced by the formation of lactide monomer at first. The formed lactide is then put through ROP usually using metal alkoxides as catalysts. This results in high molecular weight polyester – PLA.

➤ Azeotropic dehydrative condensation (ADC)
Organic solvents are introduced into the reaction mixture to ease the removal of water. As a result, it produces a higher molecular weight product.
PLA Synthesis Process

Process of PLA Synthesis (Source: Royal Society of Chemistry)
Currently, the first two methods are the most used techniques for industrial production. ROP currently dominates as the process of choice for industrial PLA production. This is due to low time consumption and a high molecular weight final product. This makes it probably the most used and viable method to produce PLA. High temperatures and low pressure must be still used to achieve the final product.

However, new methods such as polymerization using microwave irradiation and ultrasonic sonochemistry could lead to faster and cheaper production of PLA.

Processing Methods and Conditions for PLA Grades

PLA can be easily processed like other thermoplastics through conventional processing techniques. These include injection molding, film extrusion, blow molding, thermoforming, fiber spinning, etc., to yield molded parts, films, or fibers.

Requirements for PLA processing by injection molding

PLA resins can be successfully dried using the standard drying systems. Advised conditions for standard desiccant-based column dryers are:

A pre-drying of 2 to 4 hours at 45 °C up to 90 °C
A moisture level lower than 250 ppm will help keep the melt viscosity stable over time at elevated temperatures
Typical desiccant dryer regeneration temperatures exceed the melting point of PLA resins
To prevent issues with pellet bridging, sticking, or melting, the drying system should be verified. This ensures adequate temperature control during operation and regeneration cycles since valve leakage is common in many systems.

Colorants and slip agents can be added as a masterbatch at 15 - 30 wt% in PLA. This is done by dry blending with the neat resin in the required amount and adding the blend to the injection molder. The addition of colorants has been successfully done using liquid injection technology as well. Since PLA is not compatible with most incumbent materials, it is important that all additive masterbatches use PLA as a carrier.

Polylactide injection molding grades are compatible with the use of hot runner systems. Typical conditions for injection molding are as follows:

Conditions	Values
Adapter temperature	185 - 200 °C
Dew point	(-)40 - (-)35 °C
Die temperature	185 - 200 °C
Drying temperature	45 - 100 °C
Feed temperature	165 - 185 °C
Melt temperature	154.4 - 243.3 °C
Mold temperature	10 - 105 °C
Nozzle temperature	171.1 - 220 °C
Back pressure	0.345 - 1.724 MPa
Injection pressure	55.16 - 137.9 MPa
Moisture content	0.01 - 0.025%
Screw speed	20 - 200 rpm
Drying air flow rate	14.16 l/pm

Plate-out of lactide can occur over time if injection speeds are too low, and/or mold temperature is too cold. Another concern is that PLA shear thins slower and to a lesser extent than resins like PS, PE, and PP. Because of this, the filling of the mold is a concern, especially for thin-walled products like drinking cups. It is possible to overcome this issue by finding the right melt temperature and injection speed necessary to fill the part.

Usually, one will have to raise the melt temperature, which can have an adverse effect on the cooling time of the part while in the mold.

PLA fiber melt spinning grade processing

Polylactide fiber melt spinning grades are designed for extrusion into mechanically drawn staple fibers. This is done by using conventional fiber spinning and drawing equipment. They can be used as a low-melt binder polymer in a sheath-core configuration.

General-purpose screws with L/D ratios of 24:1 to 30:1 and 3:1 compression ratios are recommended
Typical melt spinning temperatures are 220 - 240 °C.
The recommended moisture content to prevent viscosity degradation and potential loss of properties is < 0.005% (50 ppm).
Typical drying conditions are 8 to 12 hours at 40 °C - 50 °C.

Like PET, the polylactide fiber melt spinning grades require either high filament velocity or drawing and controlled heat setting to control shrinkage. In-line drying capabilities are essential to process PLA injection stretch blow molding grades.

PLA heat seal layer processing

Polylactide (PLA) heat seal layer processing is a technique used to improve the heat sealability of PLA films. They can be coextruded with other PLA resin to form a sealant layer for biaxially oriented PLA film. It requires drying prior to processing and in-line drying. PLA polymers can be processed on conventional extruders.

Several process conditions to comply with for heat seal layer processing of PLAs are given as follows:

Conditions	Values
Moisture content	< 0.025% (250 ppm)
Drying	4 hours at 11 °F (45 °C)
L/D ratio Compression ratio	24:1 to 30:1 2:1 to 3:1
Melt temperature	210 °C
Feed section	180 °C
Compression section	190 °C
Metering section	200 °C
Die	190 °C

Screw cooling capabilities through the feed section are necessary to prevent the resin from sticking to the screw root. Smooth barrels are recommended. This grade is suitable only as a heat seal layer in a coextrusion process. Processing polylactide as a monolayer film is not recommended.

Processing requirements for PLA high heat films

Polylactide extrusion grade can be converted into a biaxially oriented film at up to 150 °C (300 °F). PLA resins can be successfully dried using standard drying systems. They can also be processed on conventional cast tenter equipment.

Conditions	Values
Drying	4 hours at 80 °C (175 °F)
Moisture level	< 250 ppm (0.025%)
L/D ratio Compression ratio	24:1 to 30:1 2.5:1 to 3:1
Melt temperature	200 - 220 °C
Feed section	180 °C
Compression section	190 °C
Metering section	200 °C
Die	200 °C

Polylactide spun-bond processing

Polylactide spun-bond grades process on conventional spun-bond equipment.

General-purpose screws with L/D ratios of 24:1 to 30:1 and 3:1 compression ratios are recommended.
Typical melt spinning temperatures are 250 - 300 °C.
The recommended moisture content to prevent viscosity degradation and potential loss of properties is <0.01%.
Typical drying conditions are 2 - 4 hours at 50 - 60 °C.

PLA filaments for 3D printing

3D Printing using PLA filaments is a promising way to produce complex biomedical devices by computer design. This process opens new developments using patient-speciﬁc anatomical data. It can also be used in a wide range of industrial and architectural applications. PLA printing was found feasible for such applications mainly by using direct or indirect 3D printing and fused deposition modeling technologies.

Browse PLA grades compatible with various processing methods:

Commonly available physical forms of PLA

Several different forms of PLA are available in the market today. The specific form that is used for a particular application depends on the desired properties of the final product. Commonly available forms of PLA are as follows:

Pellets
PLA pellets are small, cylindrical pieces of PLA. They can be melted and molded into complex shapes. This makes it suitable for injection molding, extrusion, and thermoforming.

Filament
PLA filament is a thin strand of PLA. It can be extruded into a strand that can be deposited layer by layer to create a three-dimensional object. Thus, it is commonly used in 3D printing.

Film
PLA film is a thin sheet of PLA. It is transparent, flexible, and acts as a barrier to moisture and gases. Thus it is commonly used for packaging.

Sheet
A PLA sheet is a thicker sheet of PLA. It can be heated and molded into a variety of shapes and thus is commonly used for thermoforming and other applications.

Fiber
PLA fiber is a long, thin strand of PLA. The soft, strong, and wrinkle-resistant properties of PLA fibers make them suitable for use in textiles and other applications.

Foam
PLA foam is a lightweight, porous material. Its lightweight, shock-absorbing, and insulating nature makes it suitable packaging.

Explore different physical forms of PLA:

Assessing the Pros and Cons of PLA

The versatile range of benefits

PLA is a versatile material with a range of benefits such as:

Biodegradability — PLA is compostable and can break down in industrial composting facilities. This reduces environmental impact and waste in landfills.

Renewable origin — PLA is derived from renewable resources, such as corn. This makes it a more sustainable option compared to petroleum-based plastics.

Lower carbon footprint — Lesser greenhouse gases are generated by PLA production as compared to traditional plastics. This reduces its contribution to climate change. View all low carbon footprint PLA grades.

Non-toxic — They are usually safe and non-toxic, making them suitable for food and medical applications.

Transparency and gloss — They have a clear and glossy appearance. This makes them suitable for products where aesthetics are important.

Ease of processing — PLA is easy to process in various manufacturing techniques. For example, injection molding, extrusion, and 3D printing.

Biocompatibility — Their compatibility makes them suitable for use in medical devices and implants. They do not cause adverse reactions in the body.

With growing trends in sustainability in the market, more innovative and sustainable products made from PLA can be seen.

Addressing the challenges of PLA

It has a lower glass transition temperature (Tg ~ 55 °C)
It has poor ductility, low impact strength, and brittleness. This limits its use as compared to other thermoplastics such as ABS
It has a low crystallization rate and processing that results mainly in amorphous products
As compared to PET (aromatic polyester), PLA is much more susceptible to chemical and biological hydrolysis
It is thermally unstable and has poor gas barrier performance
It has low flexibility and requires long mold cycles
It is relatively hydrophobic and has a slow degradation rate

Major areas of PLA application

✔ Packaging: PLA is used to make a variety of food and non-food packaging products, such as cups, lids, utensils, straws, and bags.
✔ 3D printing: PLA is used in 3D printing for a variety of objects, like prototypes, toys, models, and even medical implants.
✔ Textiles: PLA can be spun into fibers to create textiles for clothing, home furnishings, and other applications.
✔ Medical devices: PLA is used to make a variety of medical devices, such as sutures, stents, and implants.
✔ Other applications: PLA is also used in various products, like disposable cutlery, compostable bags, agricultural mulch films, etc.

Explore all PLA grades for various applications as per your need:

Regulatory Compliance and Labels

Some of the specific regulations and labels that apply to PLA are as follows:

Food and Drug Administration (FDA) — PLA is approved for food contact by the FDA. Thus, PLA can be used to make containers and packaging that will come into contact with food. However, PLA products must be labeled with the PLA content of the product and its source.

European Food Safety Authority (EFSA) — PLA is also approved for food contact by the EFSA. The EFSA has established a maximum allowable concentration (MAC) for PLA in food. The MAC is the maximum amount of PLA that can be safely present in food.

American Society for Testing and Materials (ASTM) — ASTM has developed many standards for PLA. These include standards for the manufacture, testing, and labeling of PLA products.

International Organization for Standardization (ISO) — It develops and publishes voluntary consensus standards. ISO has developed a number of standards for PLA, including standards for the manufacture, testing, and labeling of PLA products.

Apart from the above regulatory standards, a number of voluntary certifications are also applicable for PLA. These include DIN CERTCO certification, TÜV SÜD certification, UL Environment certification, etc.

Browse all PLA grades compliant with various regulatory standards:

Explore All Polylactide (PLA) Grades

View a wide range of polylactide grades (PLA, PLA alloy) available in the market today, analyze technical data of each product, get technical assistance, or request samples.

Key Properties

	Property	POLYLACTIDE	PLA MELT SPINNING	PLA HEAT SEAL LAYER	PLA HIGH HEAT FILMS	INJECTION MOLDED POLYLACTIDE	SPUNBOND POLYLACTIDE
Mechanical
Flexural Modulus, Gpa	-	-	-	-	3.8	-
Elongation at Break, %	5 - 7	-	-	179 - 181	2 - 3	-
Strength at Break (Tensile), MPa	52 - 54	-	-	102.2 - 104.2	-	-
Strength at Yield (Tensile), MPa	59 - 61	-	-	-	48 - 52	-
Young's Modulus, GPa	3.4 - 3.6	-	-	3.3 - 3.5	3.5 - 3.6	-
Optical
Gloss, %	-	-	89 - 91	89 - 91	-	-
Haze, %	-	-	2	2 - 2.2	-	-
Physical
Biodegradable	-	Yes	Yes	Yes	Yes	Yes
Biobased Content, %	-	100	100	100	100	100
Density, g/cm³	1.23 - 1.25	1.23 - 1.25	1.23 - 1.25	1.23 - 1.25	1.24 - 1.26	1.23 - 1.25
Glass Transition Temperature, °C	-	55 - 65	52 - 58	-	55 - 60	55 - 60
Shrinkage, %	-	-	-	-	0.3 - 0.5	-
Service Temperature
HDT @0.46 Mpa (67 psi), °C	-	-	-	-	53 - 56	-
Thermal
Coefficient of Linear Thermal Expansion x 10^-5, /°C	-	-	-	-	8.5	-
Thermal Insulation, W/m.K	-	-	-	-	0.11 - 0.195	-

Key Applications

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