Polylactic acid
Encyclopedia
Poly or polylactide (PLA) is a thermoplastic
aliphatic polyester
derived from renewable resource
s, such as corn starch (in the United States), tapioca
products (roots, chips or starch mostly in Asia) or sugarcane
s (in the rest of world). It can biodegrade under certain conditions, such as the presence of oxygen, and is difficult to recycle.
The name "polylactic acid" is to be used with caution, not complying to standard nomenclatures (such as IUPAC) and potentially leading to ambiguity (PLA is not a polyacid (polyelectrolyte), but rather a polyester).
from corn starch or cane sugar. However, lactic acid cannot be directly polymerized to a useful product, because each polymerization reaction generates one molecule of water, the presence of which degrades the forming polymer chain to the point that only very low molecular weights are observed. Instead, two lactic acid molecules undergo a single esterfication and then catalytically cyclized to make a cyclic dilactate ester
. Although dimerization also generates water, it can be separated prior to polymerization due to a significant drop in polarity. PLA of high molecular weight is produced from the dilactate ester by ring-opening polymerization using most commonly a stannous octoate catalyst, but for laboratory demonstrations tin(II) chloride
is often employed. This mechanism does not generate additional water, and hence, a wide range of molecular weights is accessible.
Polymerization of a racemic mixture of L- and D-lactides usually leads to the synthesis of poly-DL-lactide (PDLLA) which is amorphous. Use of stereospecific catalysts can lead to heterotactic PLA which has been found to show crystallinity. The degree of crystallinity, and hence many important properties, is largely controlled by the ratio of D to L enantiomers used, and to a lesser extent on the type of catalyst used.
nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide (PLLA) is the product resulting from polymerization of L,L-lactide (also known as L-lactide). PLLA has a crystallinity
of around 37%, a glass transition temperature between 60-65 °C
, a melting temperature
between 173-178 °C and a tensile modulus between 2.7-16 GPa. However, heat resistant PLA can withstand temperatures of 110C (230F).
PLA has similar mechanical properties to PETE
polymer, but has a significantly lower maximum continuous use temperature.
Polylactic acid can be processed like most thermoplastics into fiber
(for example using conventional melt spinning
processes) and film. The melting temperature of PLLA can be increased 40-50 °C and its heat deflection temperature can be increased from approximately 60°C to up to 190 °C by physically blending the polymer with PDLA (poly-D-lactide). PDLA and PLLA form a highly regular stereocomplex with increased crystallinity. The temperature stability is maximised when a 50:50 blend is used, but even at lower concentrations of 3-10% of PDLA, there is still a substantial improvement. In the latter case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Biodegradation of PDLA is slower than for PLA due to the higher crystallinity of PDLA. PDLA has the useful property of being optically transparent.
There is also poly(L-lactide-co-D,L-lactide) (PLDLLA) – used as PLDLLA/TCP scaffolds for bone engineering.
PLA is currently used in a number of biomedical applications, such as sutures, stents, dialysis
media and drug delivery devices. The total degradation time of PLA is a few years.
It is also being evaluated as a material for tissue engineering
.
Because it is biodegradable, it can also be employed in the preparation of bioplastic
, useful for producing loose-fill packaging, compost bags, food packaging, and disposable tableware. In the form of fibers and non-woven textiles, PLA also has many potential uses, for example as upholstery
, disposable garments, awning
s, feminine hygiene products, and diapers.
PLA has been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersome
s.
PLA is a sustainable alternative to petrochemical
-derived products, since the lactides from which it is ultimately produced can be derived from the fermentation of agricultural by-products such as corn starch or other carbohydrate-rich substances like maize
, sugar
or wheat
. PLA can be an alternative to high-impact polystyrene by using as much as 1 wt% non-PLA due to creating co-polymers which can strengthen PLA plastic .
PLA is more expensive than many petroleum-derived commodity plastics, but its price has been falling as production increases. The demand for corn is growing, both due to the use of corn for bioethanol and for corn-dependent commodities, including PLA.
PLA has also been developed in the United Kingdom to serve as sandwich packaging.
PLA has also been used in France to serve as the binder in Isonat Nat’isol, a hemp fiber building insulation.
PLA is used for biodegradable and compostable disposable cups for cold beverages, the lining in cups for hot beverages, deli containers and clamshells for food packaging. Researchers have improved the glass transition temperature of PLA to use with hot liquids up to 110C (230F).
PLA is being used as a replacement for nylon in "silken" teabags.
The pharmaceutical industry uses oligolactic acid (OLA), a shorter polymer of lactic acid, in pressurized metered dose inhalers (pMDIs) as a surfactant. Studies have shown that on standing, pMDIs without OLA will yield a much higher dose than intended as compared to those containing OLA which delivers a fairly constant dosage.
PLA is used as a feedstock material in 3D printers
such as Reprap and Makerbot.
was the primary producer of PLA (bioplastic
) in the United States.
Other companies involved in PLA manufacturing are PURAC Biomaterials (The Netherlands) and several Chinese manufacturers. The primary producer of PDLLA is PURAC, a wholly owned subsidiary of [CSM] located in the Netherlands.
Galactic and Total Petrochemicals
operate a joint-venture, Futerro, that is developing a second generation of polylactic acid product. This project includes the building of a PLA pilot plant of 1500 tonne
s/year in Belgium.
The Korean research center KAIST
has announced that they have found a way to produce PLA using bio-engineered Escherichia coli
.
Since 2009, Purac has developed a unique business model by starting production of Lactides (D and L, the monomers for PLA production, in their Spanish production plant with a capacity of several thousands of tons, while in March 2010 the construction has started of a 75.000 tons lactide plant at their production site in Thailand (Rayong Province). Purac has developed the technology to polymerize these lactides with Sulzer, a Swiss Engineering company. Purac collaborates with various PLA production partners to develop production scale and new markets for PLA. Thanks to the availability of D-lactide, Purac partners will be able to use stereo-complex technologies to produce new PLA grades with heat-stability (HDT) up to 180 degrees C, enabling use in higher value application area's. In a tripartite collaboration between Purac, Sulzer and Synbra solutions were developed to allow Synbra to start production of PLA and subsequently E-PLA, an attractive biodegradable and/or biobased alternative to EPS-foam in avariety of application areas. This collaboration was awarded by Frost and Sullivan with the Innovation of the Year Award in 2008 citing "This unique offering will significantly reduce the process and product development time enabling faster and more reliable market entry for PLA producers".
, a highly purified lactic acid is extracted and can be considered as raw material for the manufacturing of virgin PLA with no loss of original properties (cradle-to-cradle recycling).
Currently, SPI Resin identification code
7 is applicable. In 2007, a State Senate bill in California (SB 898) proposed the marking of PLA with a new "0" code. However, this part of the bill was removed before passage.
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
aliphatic polyester
Polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
derived from renewable resource
Renewable resource
A renewable resource is a natural resource with the ability of being replaced through biological or other natural processes and replenished with the passage of time...
s, such as corn starch (in the United States), tapioca
Tapioca
Tapioca is a starch extracted Manihot esculenta. This species, native to the Amazon, Brazil, Colombia, Venezuela, Cuba, Puerto Rico, Haiti, the Dominican Republic, Honduras, and most of the West Indies, is now cultivated worldwide and has many names, including cassava, manioc, aipim,...
products (roots, chips or starch mostly in Asia) or sugarcane
Sugarcane
Sugarcane refers to any of six to 37 species of tall perennial grasses of the genus Saccharum . Native to the warm temperate to tropical regions of South Asia, they have stout, jointed, fibrous stalks that are rich in sugar, and measure two to six metres tall...
s (in the rest of world). It can biodegrade under certain conditions, such as the presence of oxygen, and is difficult to recycle.
The name "polylactic acid" is to be used with caution, not complying to standard nomenclatures (such as IUPAC) and potentially leading to ambiguity (PLA is not a polyacid (polyelectrolyte), but rather a polyester).
Synthesis
Bacterial fermentation is used to produce lactic acidLactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
from corn starch or cane sugar. However, lactic acid cannot be directly polymerized to a useful product, because each polymerization reaction generates one molecule of water, the presence of which degrades the forming polymer chain to the point that only very low molecular weights are observed. Instead, two lactic acid molecules undergo a single esterfication and then catalytically cyclized to make a cyclic dilactate ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
. Although dimerization also generates water, it can be separated prior to polymerization due to a significant drop in polarity. PLA of high molecular weight is produced from the dilactate ester by ring-opening polymerization using most commonly a stannous octoate catalyst, but for laboratory demonstrations tin(II) chloride
Tin(II) chloride
Tin chloride is a white crystalline solid with the formula 2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent , and in electrolytic baths for tin-plating...
is often employed. This mechanism does not generate additional water, and hence, a wide range of molecular weights is accessible.
Polymerization of a racemic mixture of L- and D-lactides usually leads to the synthesis of poly-DL-lactide (PDLLA) which is amorphous. Use of stereospecific catalysts can lead to heterotactic PLA which has been found to show crystallinity. The degree of crystallinity, and hence many important properties, is largely controlled by the ratio of D to L enantiomers used, and to a lesser extent on the type of catalyst used.
Chemical and physical properties
Due to the chiralChirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide (PLLA) is the product resulting from polymerization of L,L-lactide (also known as L-lactide). PLLA has a crystallinity
Crystallinity
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In a gas, the relative positions of the atoms or...
of around 37%, a glass transition temperature between 60-65 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
, a melting temperature
Melting temperature
Melting temperature may refer to:* Melting point, the temperature at which a substance changes from solid to liquid state.* DNA melting temperature, the temperature at which a DNA double helix dissociates into single strands....
between 173-178 °C and a tensile modulus between 2.7-16 GPa. However, heat resistant PLA can withstand temperatures of 110C (230F).
PLA has similar mechanical properties to PETE
Pete
Pete is a diminutive from Peter. It may refer to:-Athletics:* The Pete, Petersen Events Center, athletics complex and basketball arena on the campus of the University of Pittsburgh* Pete the Penguin, mascot of Youngstown State University...
polymer, but has a significantly lower maximum continuous use temperature.
Polylactic acid can be processed like most thermoplastics into fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
(for example using conventional melt spinning
Melt spinning
Melt spinning is a technique used for rapid cooling of liquids. A wheel is cooled internally, usually by water or liquid nitrogen, and rotated. A thin stream of liquid is then dripped onto the wheel and cooled, causing rapid solidification. This technique is used to develop materials that require...
processes) and film. The melting temperature of PLLA can be increased 40-50 °C and its heat deflection temperature can be increased from approximately 60°C to up to 190 °C by physically blending the polymer with PDLA (poly-D-lactide). PDLA and PLLA form a highly regular stereocomplex with increased crystallinity. The temperature stability is maximised when a 50:50 blend is used, but even at lower concentrations of 3-10% of PDLA, there is still a substantial improvement. In the latter case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Biodegradation of PDLA is slower than for PLA due to the higher crystallinity of PDLA. PDLA has the useful property of being optically transparent.
There is also poly(L-lactide-co-D,L-lactide) (PLDLLA) – used as PLDLLA/TCP scaffolds for bone engineering.
Applications
Stereocomplex blends of PDLA and PLLA have a wide range of applications, such as woven shirts (ironability), microwavable trays, hot-fill applications and even engineering plastics (in this case, the stereocomplex is blended with a rubber-like polymer such as ABS). Such blends also have good form-stability and visual transparency, making them useful for low-end packaging applications. Progress in biotechnology has resulted in the development of commercial production of the D enantiomer form, something that was not possible until recently.PLA is currently used in a number of biomedical applications, such as sutures, stents, dialysis
Dialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...
media and drug delivery devices. The total degradation time of PLA is a few years.
It is also being evaluated as a material for tissue engineering
Tissue engineering
Tissue engineering is the use of a combination of cells, engineering and materials methods, and suitable biochemical and physio-chemical factors to improve or replace biological functions...
.
Because it is biodegradable, it can also be employed in the preparation of bioplastic
Bioplastic
Bioplastics are a form of plastics derived from renewable biomass sources, such as vegetable fats and oils, corn starch, pea starch, or microbiota, rather than fossil-fuel plastics which are derived from petroleum...
, useful for producing loose-fill packaging, compost bags, food packaging, and disposable tableware. In the form of fibers and non-woven textiles, PLA also has many potential uses, for example as upholstery
Upholstery
Upholstery is the work of providing furniture, especially seats, with padding, springs, webbing, and fabric or leather covers. The word upholstery comes from the Middle English word upholder, which referred to a tradesman who held up his goods. The term is equally applicable to domestic,...
, disposable garments, awning
Awning
An awning or overhang is a secondary covering attached to the exterior wall of a building. It is typically composed of canvas woven of acrylic, cotton or polyester yarn, or vinyl laminated to polyester fabric that is stretched tightly over a light structure of aluminium, iron or steel, possibly...
s, feminine hygiene products, and diapers.
PLA has been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersome
Polymersome
In biotechnology, polymersomes are a class of artificial vesicles, tiny hollow spheres that enclose a solution. Polymersomes are made using amphiphilic synthetic block copolymers to form the vesicle membrane, and have radii ranging from 50 nm to 5 µm or more...
s.
PLA is a sustainable alternative to petrochemical
Petrochemical
Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane....
-derived products, since the lactides from which it is ultimately produced can be derived from the fermentation of agricultural by-products such as corn starch or other carbohydrate-rich substances like maize
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
, sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
or wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
. PLA can be an alternative to high-impact polystyrene by using as much as 1 wt% non-PLA due to creating co-polymers which can strengthen PLA plastic .
PLA is more expensive than many petroleum-derived commodity plastics, but its price has been falling as production increases. The demand for corn is growing, both due to the use of corn for bioethanol and for corn-dependent commodities, including PLA.
PLA has also been developed in the United Kingdom to serve as sandwich packaging.
PLA has also been used in France to serve as the binder in Isonat Nat’isol, a hemp fiber building insulation.
PLA is used for biodegradable and compostable disposable cups for cold beverages, the lining in cups for hot beverages, deli containers and clamshells for food packaging. Researchers have improved the glass transition temperature of PLA to use with hot liquids up to 110C (230F).
PLA is being used as a replacement for nylon in "silken" teabags.
The pharmaceutical industry uses oligolactic acid (OLA), a shorter polymer of lactic acid, in pressurized metered dose inhalers (pMDIs) as a surfactant. Studies have shown that on standing, pMDIs without OLA will yield a much higher dose than intended as compared to those containing OLA which delivers a fairly constant dosage.
PLA is used as a feedstock material in 3D printers
3D printing
3D printing is a form of additive manufacturing technology where a three dimensional object is created by laying down successive layers of material. 3D printers are generally faster, more affordable, and easier to use than other additive manufacturing technologies. However, the term 3D printing is...
such as Reprap and Makerbot.
Production
As of Jun 2010, NatureWorksNatureWorks LLC
NatureWorks LLC is a subsidiary of Cargill. NatureWorks produces a family of polymers derived entirely from annually renewable resources - namely plants - designed to compete with petroleum-based plastic packaging materials, consumer goods and fiber products...
was the primary producer of PLA (bioplastic
Bioplastic
Bioplastics are a form of plastics derived from renewable biomass sources, such as vegetable fats and oils, corn starch, pea starch, or microbiota, rather than fossil-fuel plastics which are derived from petroleum...
) in the United States.
Other companies involved in PLA manufacturing are PURAC Biomaterials (The Netherlands) and several Chinese manufacturers. The primary producer of PDLLA is PURAC, a wholly owned subsidiary of [CSM] located in the Netherlands.
Galactic and Total Petrochemicals
Total S.A.
Total S.A. is a French multinational oil company and one of the six "Supermajor" oil companies in the world.Its businesses cover the entire oil and gas chain, from crude oil and natural gas exploration and production to power generation, transportation, refining, petroleum product marketing, and...
operate a joint-venture, Futerro, that is developing a second generation of polylactic acid product. This project includes the building of a PLA pilot plant of 1500 tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
s/year in Belgium.
The Korean research center KAIST
KAIST
KAIST , is located in Daedeok Innopolis, Daejeon, South Korea. KAIST was established by the Korean government in 1971 as the nation's first research oriented science and engineering institution. The QS-The Times World University Rankings in the year of 2009 placed KAIST 69th overall and 21st in...
has announced that they have found a way to produce PLA using bio-engineered Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
.
Since 2009, Purac has developed a unique business model by starting production of Lactides (D and L, the monomers for PLA production, in their Spanish production plant with a capacity of several thousands of tons, while in March 2010 the construction has started of a 75.000 tons lactide plant at their production site in Thailand (Rayong Province). Purac has developed the technology to polymerize these lactides with Sulzer, a Swiss Engineering company. Purac collaborates with various PLA production partners to develop production scale and new markets for PLA. Thanks to the availability of D-lactide, Purac partners will be able to use stereo-complex technologies to produce new PLA grades with heat-stability (HDT) up to 180 degrees C, enabling use in higher value application area's. In a tripartite collaboration between Purac, Sulzer and Synbra solutions were developed to allow Synbra to start production of PLA and subsequently E-PLA, an attractive biodegradable and/or biobased alternative to EPS-foam in avariety of application areas. This collaboration was awarded by Frost and Sullivan with the Innovation of the Year Award in 2008 citing "This unique offering will significantly reduce the process and product development time enabling faster and more reliable market entry for PLA producers".
Recycling
In Belgium, Galactic started the first pilot unit to chemically recycle PLA (Loopla). Unlike mechanical recycling, waste material can hold various contaminants. Through thermal depolymerizationThermal depolymerization
Thermal depolymerization is a depolymerization process using hydrous pyrolysis for the reduction of complex organic materials into light crude oil. It mimics the natural geological processes thought to be involved in the production of fossil fuels...
, a highly purified lactic acid is extracted and can be considered as raw material for the manufacturing of virgin PLA with no loss of original properties (cradle-to-cradle recycling).
Recycling code
Resin identification code
The SPI resin identification coding system is a set of symbols placed on plastics to identify the polymer type. It was developed by the Society of the Plastics Industry in 1988, and is used internationally....
7 is applicable. In 2007, a State Senate bill in California (SB 898) proposed the marking of PLA with a new "0" code. However, this part of the bill was removed before passage.
See also
- PolymerizationPolymerizationIn polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
- PLA filmPLA filmPLA film is an advanced type of packaging for application to the containers of soft drinks and dairy products. It can be a film overwrap and is heat-shrinkable...
- CellophaneCellophaneCellophane is a thin, transparent sheet made of regenerated cellulose. Its low permeability to air, oils, greases, bacteria and water makes it useful for food packaging...
- Metallic glass
- Plastarch materialPlastarch materialPlastarch Material is a biodegradable, thermoplastic resin. It is composed of starch combined with several other biodegradable materials. The starch is modified in order to obtain heat-resistant properties, making PSM one of few bioplastics capable of withstanding high temperatures...
- PolycaprolactonePolycaprolactonePolycaprolactone is a biodegradable polyester with a low melting point of around 60°C and a glass transition temperature of about −60°C. PCL is prepared by ring opening polymerization of ε-caprolactone using a catalyst such as stannous octoate. Recently a wide range of catalysts for the ring...
- PolyglycolidePolyglycolidePolyglycolide or Polyglycolic acid is a biodegradable, thermoplastic polymer and the simplest linear, aliphatic polyester. It can be prepared starting from glycolic acid by means of polycondensation or ring-opening polymerization. PGA has been known since 1954 as a tough fiber-forming polymer...
- Poly-3-hydroxybutyrate
- ZeinZeinZein is a class of prolamine protein found in maize. It is usually manufactured as a powder from corn gluten meal.-Commercial uses:Zein is one of the best understood plant proteins and has a variety of industrial and food uses...