Poly(lactic acid) ( PLA): Synthesis, Properties & Applications

Poly(lactic acid) (PLA)

Poly(lactic acid) (PLA) is a biodegradable and bioactive thermoplastic aliphatic polyester derived from renewable resources such as corn starch, cassava roots, or sugarcane. It is one of the most promising biopolymers due to its compostability, biocompatibility, and mechanical properties comparable to conventional petroleum-based plastics like PET and PS.

PLA belongs to the family of polyesters and is produced through the polymerization of lactic acid or its cyclic dimer, lactide. It exists in three stereoisomeric forms: poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and poly(DL-lactide) (PDLLA), which significantly influence its crystallinity and mechanical performance.

Synthesis of PLA

PLA is a biodegradable thermoplastic polyester synthesized from renewable biomass like corn starch and sugarcane.

PLA is primarily synthesized by two methods: direct polycondensation of lactic acid or ring-opening polymerization (ROP) of lactide. The ROP method is industrially preferred due to higher molecular weight and better control over stereochemistry.

• Direct polycondensation: Lactic acid monomers condense under high temperature and low pressure to form PLA, often resulting in low molecular weight polymers.
• Ring-opening polymerization (ROP) of lactide: Lactic acid is converted into cyclic lactide dimers, which are then polymerized using metal catalysts to form high molecular weight PLA with better mechanical properties.

The polymer exists in different stereoisomeric forms such as PLLA, PDLA, and PDLLA, allowing tuning of crystallinity and physical properties.

1. Production of Lactic Acid

Lactic acid is produced via bacterial fermentation of carbohydrates:

C₆H₁₂O₆ → 2 CH₃CH(OH)COOH
(Glucose → 2 Lactic Acid)

2. Formation of Lactide

Lactic acid undergoes dehydration and cyclization to form lactide (cyclic dimer):

2 CH₃CH(OH)COOH → C₆H₈O₄ + 2 H₂O
(2 Lactic Acid → Lactide + Water)

3. Ring-Opening Polymerization (ROP)

Lactide undergoes ROP in the presence of a catalyst (typically stannous octoate, Sn(Oct)₂):

n C₆H₈O₄ → [–O–CH(CH₃)–CO–]_n
(n Lactide → Poly(lactic acid))

Poly(lactic acid) – From Biomass to Bioplastic

PLA – From Biomass to Bioplastic: Corn → Lactic Acid → Lactide → Biodegradable PLA Products

Properties of PLA

• Biodegradable and compostable thermoplastic polyester
• Derived entirely from renewable resources
• Good mechanical strength and transparency
• Processable by extrusion, injection molding, blow molding, and 3D printing
• Thermal properties depend on stereochemistry and crystallinity

Property	Value	Comparison
Density	1.24–1.26 g/cm³	Lower than PET (1.38 g/cm³)
Glass Transition Temperature (Tg)	55–65 °C	Similar to PS
Melting Temperature (Tm)	150–180 °C (PLLA)	Lower than PET (260 °C)
Tensile Strength	50–70 MPa	Comparable to PET
Young’s Modulus	3.0–4.0 GPa	Stiffer than LDPE
Elongation at Break	2–10%	Brittle compared to PP
Biodegradability	Compostable (ASTM D6400)	Degrades in 3–6 months under industrial composting

Applications of PLA

1. Packaging

• Food packaging (trays, cups, films)
• Compostable bags and wraps
• Rigid containers (e.g., yogurt cups)

2. 3D Printing

PLA is the most widely used filament in fused filament fabrication (FFF) due to low warping and ease of printing.

3. Biomedical Applications

• Absorbable sutures (e.g., Vicryl)
• Drug delivery systems (microspheres, implants)
• Tissue engineering scaffolds
• Bone fixation devices (screws, plates)

4. Textiles & Fibers

• Non-woven fabrics (agricultural mulch, diapers)
• Apparel and home textiles (Ingeo™ fibers by NatureWorks)

5. Consumer Goods

• Disposable cutlery, plates
• Electronics casings
• Compostable phone cases

Market Insight: Global PLA market was valued at ~\$100.8 billion in 2023 and is projected to reach \$102.5 billion by 2030 (CAGR ~18%).

Environmental Impact & Sustainability

PLA offers significant environmental advantages over fossil-based plastics:

• Renewable feedstock – Reduces dependence on petroleum
• Lower carbon footprint – ~1.8 kg CO₂/kg PLA vs. 3.2 kg CO₂/kg PET
• Industrial compostability – Fully degrades in 3–6 months at 58 °C and high humidity
• Closed-loop potential – Can be chemically recycled back to lactic acid

Caution: PLA does not biodegrade in home compost or natural environments within a reasonable timeframe. It requires industrial composting facilities.

Challenges

• High production cost compared to PP/PS
• Limited heat resistance
• Slow crystallization rate
• Competition for food crops (corn)

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