PLA is a type of polyester made from fermented plant starch from corn, cassava, maize, sugarcane or sugar beet pulp. The sugar in these renewable materials are fermented and turned into lactic acid, when is then made into polylactic acid, or PLA.
There is more detailed information on PLA production methods below.
The material properties of PLA makes it suitable for the manufacture of plastic film, bottles and biodegradable medical devices, including screws, pins, plates and rods that are designed to biodegrade within 6 to 12 months).
PLA can be used as a shrink-wrap material since it constricts under heat. This ease of melting also makes polylactic acid suitable for 3D printing applications.
However, many types of PLA have a low glass transition temperature, making them unsuitable for making plastic cups designed to hold hot liquids.
PLA production uses 65% less energy than producing conventional plastics and generates 68% fewer greenhouse gases and contains no toxins. It can be also remain environmentally friendly should the correct end-of-life scenario be followed.
However, the rate of degradation is very slow in ambient temperatures, with a 2017 study showing that there was no degradation seen in over a year of the material being submerged in seawater at 25°C.
However, PLA can be degraded by hydrolysis, thermal degradation or photodegradation:
- Hydrolysis: The molecular weight is reduced by cleaving the ester groups of the main chain
- Thermal Degradation: This process leads to the appearance of different compounds, such as linear and cyclic oligomers or lighter molecules with different lactide and Mw
- Photodegradation: UV radiation causes degradation, particularly where PLA is exposed to sunlight
There are currently four common end-of-life scenarios for PLA:
1. Recycling
This is either chemical or mechanical. Waste material can hold contaminants, but ployactic acid can be chemically recycled using thermal depolymerisation or hydrolysis to create a monomer that can then be manufactured into virgin PLA. PLA can also be chemically recycled using transesterification to create methyl lactate.
2. Composting
Industrial composting conditions allow for chemical hydrolysis followed by microbial digestion to degrade the PLA.
3. Incineration
End-of-life PLA can be incinerated, creating 19.5 MJ/kg (8,368 btu/lb) of energy and leaving no residue.
4. Landfill
While PLA can go to landfill, this is the least environmentally friendly option, due to the slow degradation rates of the material in ambient temperatures.
Due to the nature of lactic acid, there are several distinct forms of polyactide. These include poly-L-lactide (PLLA) which comes from the polymerization of L,L-lactide (also known as L-lactide).
In addition, while PLA can be produced from different biomass materials, such as corn starch or sugar cane, it can also be enhanced by adding other materials to provide different properties. This is particularly true with PLA filaments where the additional materials allow 3D printed PLA to be used in different ways.
There are many different PLA blends available, although adding materials to PLA can make 3D printing more difficult and even reduce the properties of PLA. Using blends can also mean that you need to alter the temperature required to melt the material while printing.
1. Wood Filaments
PLA is mixed with woods such as bamboo, cedar, coconut wood, cork, pine, or walnut. This can, for example, be used to give PLA printed furniture a natural-looking appearance.
2. Metal Filaments
Mixing PLA with metals such as brass, bronze, copper, iron and steel can make printed parts stronger and glossy.
3. Other Filaments
PLA can also be mixed with other materials and substances, including carbon fibre, conductive carbon and even beer or coffee (to add a scent to printed items). PLA filaments can also be given colour-changing properties.
PLA is soluble in solvents including dioxane, hot benzene, and tetrahydrofuran. The physical and mechanical properties differ according to the exact type of polymer, ranging from an amorphous glassy polymer to a semi or highly crystalline polymer with a glass transition of 60–65 °C, a melting temperature 130-180 °C, and a tensile modulus of 2.7–16 GPa.
Heat resistant PLA can withstand temperatures of 110 °C, and the melting temperature can be increased by 40–50 °C and the heat deflection temperature can be increased from around 60 °C to as much as 190 °C by physically blending the polymer with PDLA (poly-D-lactide).
Annealing, adding nucleating agents or forming composites with other materials can all change the mechanical properties of PLA. However, the basic mechanical properties of PLA range between those of polystyrene and PET, with similar properties to PET but a lower maximum continuous use temperature.
The high surface energy of PLA makes it ideal for 3D printing. PLA can also be solvent welded using dichloromethane, while acetone softens the surface of the material, making it sticky without dissolving it so it can be welded to another PLA surface. Ethylacetate can be used as an organic solvent, dissolving PLA and making it a good solution for removing PLA printing supports or cleaning 3D printing extruder heads. Propylene carbonate and pyridine can also be used as a solvent, but are less favourable than ethylacetate and propylene carbonate, being less safe in the first instance and emitting a distinct bad fish odour in the second.
Here are the general properties of PLA:
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Property
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Value
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Heat Deflection Temperature (HDT)
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126 °F (52 °C)
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Density
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1.24 g/cm³
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Tensile Strength
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50 MPa
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Flexural Strength
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80 MPa
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Impact Strength (Unnotched) IZOD (J/m)
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96.1
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Shrink Rate
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0.37-0.41% (0.0037-0.0041 in/in)
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PLA provides several advantages over other materials, including:
- Environmentally friendly (if disposed of correctly)
- Easy to 3D print
- Safe for use in applications such as food containers and medical devices
- Comes with a wide range of composite and colour options to provide different properties and appearances
- Can be solvent welded (such as with dichloromethane)
There are, however, some disadvantages with using PLA, including:
- Low heat resistance
- Comparatively low strength
- Machine processing can be difficult
There are several industrial ways to produce usable PLA with a high molecular rate. Lactic acid and the cyclic di-ester, lactide are the two main monomers used for this.
The most common method of creating PLA is ring-opening polymerisation of lactide with various metal catalysts (typically tin octoate) either in a solution or as a suspension. The metal-catalysed reaction tends to lead to recemisation of the PLA, which reduces stereoregularity when compared to the biomass starting material.
It is also possible to produce PLA through the direct condensation of lactic acid monomers. This process is carried out at temperatures under 200 °C, at which point an entropically favoured lactide monomer is generated. This process generates water equivalent to each esterification step. The water needs to be removed either by using a vacuum or through azeotropic distillation to promote polycondensation and attain a high molecular rate. Even higher molecular rates can be achieved by crystallising the crude polymer from the melt. This concentrates carbolyxic acid and alcohol end groups in the amorphous region of the solid polymer, reacting to achieve molecular weights of 128–152 kDa.
By polymerising a racemic mixture of L- and D-lactides, it is possible to synthesise the amorphous poly-DL-lactide (PDLLA). Stereospecific catalysts can lead to heterotactic PLA, that has been known to show crystallinity. The degree of this crystallinity is controlled by the ratio of D to L enantiomers that are used, as well as by the type of catalyst that is used. The five-membered cyclic compound lactic acid O-carboxyanhydride (lac-OCA) has also been used in academic surroundings instead of lactic acid and lactide. This compound doesn’t produce water as a co-product and is more reactive than lactide. PLA has also been directly biosynthesised while lactic acid has also been contacted with a zeolite, creating a one-step process that takes place at a temperature that is around 100 °C lower.
PLA has a number of common uses, including for medical and food purposes. It is also widely used as a 3D printing feedstock for desktop fused filament fabrication 3D printers. PLA is popular for 3D printing as it can easily be sanded, painted or post processed. A user friendly material, this plastic works with low extrusion temperatures and there is no need for a heated bed, printer chamber or reinforced nozzle. Another benefit is that PLA behaves better than many tougher plastics and also doesn’t release fumes or bad odours. Storage is easy and it can be produced in a variety of colours and as the base for a range of composites with additional properties (see above).
Because PLA can degrade into lactic acid, it can be used for medical implants such as anchors, screws, plates, pins, rods or as a mesh. It breaks down in between 6 months and 2 years, depending on the exact type of material used. This means that these products can gradually transfer a load from a PLA support structure to the body as it heals.
PLA, created with injection moulding, casting or by being spun, is also used as a decomposable packaging material, film or for cups and bags. It is used for compost bags, food packaging, disposable tableware, and loose fill packaging. As a fibre or nonwoven fabric, PLA is used for upholstery, disposable clothing, feminine hygiene products and nappies.
Made from a recyclable and renewable resource, PLA has a lot of positives for the future, plus with rising oil prices, a corn-based plastic has financial benefits too. For all of these positives the low melting point of PLA compared to plastics like PET means that it has not been picked up for as many applications as of yet.
The cost of PLA production has also reduced over the decades, but care needs to be taken to decompose this material, which needs special composting in facilities that can heat the material to 140°C degrees for ten days. However, while this requires a plant to achieve, it is by far more preferable to sending used PLA to landfill, where it is estimated it would take as long as 100 to 1,000 years to break down.
While PLA is not quite a miracle substance, the lack of fossil fuels and lower air pollution in production mean it certainly has a place in the future of materials.
Used in a variety of applications, PLA has many advantages over other plastics – including environmentally. Widely used for 3D printing and able to be used as part of a composite, PLA is also used in the food and medical industries.