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Poly-lactic Acid (PLA) – Eco-friendly plastic

what-is-pla

Today, polymers and materials used for food packaging consist of a variety of petrochemical‐based polymers, metals, glass, paper, and board, or combinations hereof. The durability and degradability of packaging materials are 2 contradictory subjects; the 1st is desirable for packaging stability and protection for its contents during shelf life and the 2nd for its rapid degradation in the environment.

Advantages of petrochemical‐based polymers, which encouraged industries to use them are:

(a) low cost and high‐speed production;

(b) high mechanical performance;

(c) good barrier properties; and

(d) good heat sealability.

On the other hand, several disadvantages include:

(a) declining oil and gas resources;

(b) increasing oil and gas prices during recent decades;

(c) environmental concerns for their degradation or incineration and global warming;

(d) uneconomical costs and cross‐contaminations in their recycling; and

(e) consumer toxicity risks about their monomers or oligomers migrating to edible materials.

Mechanical recycling (segregated plastics, mixed plastics), biological recycling (sewage, compost, soil), and energy recovery (incineration, pyrolysis) are 3 alternative ways for plastics waste management, with each having some advantages and disadvantages as to economical, processing, and technological aspects.

Polylactic Acid (PLA) is different than most thermoplastic polymers in that it is derived from renewable resources like corn starch or sugar cane. Most plastics, by contrast, are derived from the distillation and polymerization of nonrenewable petroleum reserves. Plastics that are derived from biomass (e.g. PLA) are known as “bioplastics.”

Polylactic Acid is biodegradable and has characteristics similar to polypropylene (PP), polyethylene (PE), or polystyrene (PS). It can be produced from already existing manufacturing equipment (those designed and originally used for petrochemical industry plastics). This makes it relatively cost efficient to produce. Accordingly, PLA has the second largest production volume of any bioplastic (the most common typically cited as thermoplastic starch).

How PLA is Made?

PLA is a polyester (polymer containing the ester group) made with two possible monomers or building blocks: lactic acid, and lactide. Lactic acid can be produced by the bacterial fermentation of a carbohydrate source under controlled conditions. In the industrial scale production of lactic acid, the carbohydrate source of choice can be corn starch, cassava roots, or sugarcane, making the process sustainable and renewable.

Production of PLA by the direct condensation of lactic acid is possible. However, this process usually results in the less-desired low-density PLA. To produce high-density PLA, the lactic acid is heated in the presence of an acid catalyst to form cyclic lactide. In the presence of metal catalysts, lactide undergoes a ring-opening polymerization process to form high-density PLA.

Research is ongoing to come up with even more eco-friendly and cheaper methods of producing PLA. In addition the agricultural produce itself, crop residue such as stems, straw, husks, and leaves, can be processed and used as alternative carbohydrate sources. Residue that cannot be fermented can be used as a heat source to lessen the use of fossil fuel-derived hydrocarbons.

Properties of PLA
Characteristics Unit Amount Reference
Physical:
 Molecular weight g/mol 66000 Garlotta (2001)
 Specific gravity 1.27  
 Solid density g/cm3 1.2515  
 Melt density g/cm3 1.0727  
 Glass transition temperature °C 55 Mehta and others (2005)
 Melting temperature °C 165  
 Specific heat (Cp) J/Kg °C    
  190 °C   2060  
  100 °C   1955  
  55 °C   1590  
 Thermal conductivity W/m °C    
  190 °C   0.195  
  109 °C   0.197  
  48 °C   0.111 http://www.natureworksllc.com (technical data sheet)
Optical:
 UV light transmission:     Auras and others
  190 to 220 nm   <5%  (2004)
  225 to 250 nm   85%  
  >300 nm   95%  
 Visible light transmission   95%  
Color
  L*   90.64 ± 0.21  
  a*   −0.99 ± 0.01  
  b*   −0.50 ± 0.04  
Mechanical:
 Tensile strength MPa 59  
 Elongation at break % 7.0  
 Elastic modulus MPa 3500  
 Shear modulus MPa 1287 http://www.natureworks.com (technical data sheet)
 Poissons ratio 0.3600  
 Yield strength MPa 70  
 Flexural strength MPa 106  
 Unnotched izod impact J/m 195  
 Notch izod impact J/m 26  
 Rockwell hardness HR 88  
 Heat deflection temp. °C 55  
 Vicat penetration °C 59  
 Ultimate tensile strength MPa 73  
 Percent of elongation % 11.3  
 Young’s modulus MPa 1280  
Applicable processes with PLA Material

PLA can be processed via extrusion, injection molding, casting, blown film, thermoforming, and fiber spinning to form useful and versatile products. It is typically available in the market as thin films for thermoforming, plastic pellets for injection molding, or 3D printable filaments. A variety of colors of PLA is available.

Sturdier versions of PLA have been developed by mixing different PLA isomers, resulting in higher melting temperature (higher by 40 to 50 Celsius) and increased mechanical strength. This improved version has seen a wide array of applications, such as microwavable containers and engineering plastics. Short-term applications such as food containers, water bottles, and disposable cutlery are a popular use for PLA. A PLA film shrinks upon heating, making it a desirable material for shrink wrap.

Remarks from firms that use PLA as material for food packaging indicate the preference for PLA due to increased aesthetics, better printability, good resistance to grease and oils, and reduced issues in taste and odor transfer.

PLA is widely used in the medical field due to its ability to degrade into non-toxic lactic acid. Medical implants such as screws, rods, pins and mesh have been made using PLA. Inside the patient’s body, these implants completely break down in 6 months to 2 years, eliminating the need for further surgery.

PLA can be extruded into thin fibers with significant mechanical strength. These PLA fibers have been used to manufacture casual sports apparel, upholstery material, hygiene products, and diapers.

PLA is one of the two plastics most commonly used in 3D printing (the other one being Acrylonitrile Butadiene Styrene, or ABS). Specifically, PLA is widely used in fused filament fabrication 3D printing, where PLA solids are encased in plaster-like mouldings to form moulds that can be filled with molten metal. This is a technique known as “lost PLA casting”

Advantages of PLA

One of the major advantages of PLA is its biodegradable nature and the sustainable process by which it is made, making it the environmentally friendly choice of plastic. Under the right circumstances, PLA can break down into its natural elements in less than a month in contrast to the centuries it will take for traditional plastics to decompose. PLA is especially suitable in short lifespan applications such as in water bottles and food containers.

The process by which PLA Is made is also more environment-friendly. In addition to using renewable raw materials, emission of greenhouse gases during production is also lower. Because carbon dioxide is consumed during the growth of corn, the net greenhouse gas emission of the overall PLA production process can even be considered negative. Ongoing studies on the use of alternative carbohydrate sources, such as agricultural and household wastes, even suggest that PLA production can lead to a decrease in overall solid waste.

PLA is a thermoplastic, meaning it will turn into a liquid in its melting point of 150 to 160 Celsius. A nifty feature of thermoplastics is that they can be heated, set upon cooling, and reheated again to form other shapes without any degradation. In contrast, a thermosetting plastic (such as epoxy or melamine) can only be heated and molded once, but the resulting product can no longer be reheated as it will just burn. This property of PLA makes it a desirable material for recycling.

PLA can be broken down to its original monomers by a thermal de-polymerization process or by hydrolysis. The resulting monomer solution can be purified and used for subsequent PLA production without any loss of quality. Should a material made from PLA be incinerated, no toxic fumes will be generated.

Disadvantages of PLA

The ease with which PLA melts makes it a material that is easy to work with. However, this also renders PLA unsuitable for high temperature applications such as containers made for holding hot liquids. A material made from PLA may even show signs of getting soft or deforming on a hot summer day.

PLA is still considered inferior to polyethylene terephthalate (PET) for long-term food storage applications due to permeability issues with PLA. PLA packaging material has been found to be more permeable to moisture and oxygen compared to other plastics, which may result in faster food spoilage. Applications where toughness and impact resistance are critical may also be inappropriate for the more brittle PLA.

Posted in Material Science

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