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Polylactic Acid
10.12
Polylactic Acid
R Hagen, Uhde Inventa-Fischer GmbH, Berlin, Germany © 2012 Elsevier B.V. All rights reserved.
10.12.1 10.12.2 10.12.3 10.12.4 10.12.5 10.12.6 10.12.7 10.12.8 References
Introduction Nondepleting Properties of PLA Market Potential of PLA Process Routes to PLA Processing of PLA Properties of PLA Perspective LA as Raw Material of PLA
10.12.1 Introduction Polylactide or polylactic acid1,2 (PLA) is a synthetic, aliphatic polyester from lactic acid (LA) (Figure 1). For industrial appli cations, such as fibers, films, and bottles, the chain length n should be between 700 and 1400. This is significantly higher than with partially aromatic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), where n is between 100 and 200. Therefore, the requirements on both raw material purity and technical effort are much higher. PLA is made from LA. LA can frequently be found in plants and animals as a by-product or intermediate product of meta bolism. Therefore, LA and the hydrolytic degradation products of PLA are nontoxic by its nature. After hydrolysis, PLA is biologically degradable by common microorganisms. Only in an industrial composting facility are the high temperature (60 °C) and humidity required for the hydrolysis achieved. At temperatures below its glass transition point (e.g., 55 °C, depending on comonomer content), PLA is as stable as PET or PBT.
10.12.2 Nondepleting Properties of PLA LA can be industrially produced from a number of starch- or sugar-containing agricultural products (Figure 2). Competition between human food, industrial LA, and PLA production is not to be expected: For example, using PLA as substitute for 5% of the German packaging plastics consump tion requires only 0.5% (sugar beet) to 1.25% (wheat) of the agricultural area available (Figure 3). At the same time, approximately 30% of the available area lies fallow mainly for economic reasons. Research is in progress on processes and microorganisms that produce LA from cellulose coming from agricultural residues such as maize stalks or straw. Several recycling methods can be applied to waste PLA (Figure 4). Composting allows only moderate benefits. In future, sorting, purification of PLA waste, and refeeding into the polymerization plant would seem to be the most attractive ways of recovery. PLA – like other biopolymers – is often criticized for its requirement for need of process energy from fossil resources. Even if this is the case at present, 1 kg of PLA represents less Polymer Science: A Comprehensive Reference, Volume 10
231 231 231 231 232 232 232 234 235
energy equivalents than 1 kg of polymers from petrochemical feedstock (Figure 5). Consequently, PLA producers can also reap financial benefits by trading CO2 emission certificates (Figure 6). If process energy is supplied by biomass, for example, biogas, the fossil energy required for 1 kg PLA can be cut by half, thus duplicating the benefits from trading CO2 emission certificates. Additionally, significant potential exists for saving process energy by improving LA and polymerization technologies.
10.12.3 Market Potential of PLA The first market place for PLA is Europe, with Italy, Germany, the Netherlands, the United Kingdom, and France in the front line, followed by Japan, the United States, and South Korea. In Germany and France, the governments have issued specific regulations to give preference to renewable and biodegradable polymers. As PLA is a rather young polymer, many companies are developing applications. These are mainly focused on the field of packaging fresh food, but other applications are emer ging. Processing PLA is possible on standard polymer equipment for the various process techniques. A problem for most of the companies is the availability of the PLA polymer. Only NatureWorks LLC, a US-based produ cer, which has a 140 000 t per annum facility, is going to be operated on full capacity in 2012. Besides NatureWorks, there are some suppliers (Table 1) offering smaller capacities. Another hindrance for the success of PLA is that the big retailers do not want to be dependent on one supplier. So many converters and brand owners are waiting for a second big supplier, which then rapidly increases demand for PLA.
10.12.4 Process Routes to PLA Several process routes have been developed or are practiced on an industrial scale: ring-opening polymerization (ROP), direct polycondensation in high boiling solvents (DP-S), and direct polymerization in bulk followed by chain extension with reac tive additives. ROP is the route that delivers by far the highest proportion of PLA chips available on the market. The other routes produce
doi:10.1016/B978-0-444-53349-4.00269-7
231
232
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
CH3 n HO
C C OH
10.12.6 Properties of PLA
CH3 H
C C OH
O
H O
PLA is a crystal clear, transparent material when amorphous that becomes hazier the higher the crystallinity. Crystallized material is opaque. When producing lactide, meso-lactide is formed as a by-product. Separation from the L-lactide takes special efforts in the purification step. When polymerizing L-lactide with some meso-lactide, a copolymer is formed. Increasing meso-lactide leads to decreasing crystallinity. With more than 10–15% meso-lactide, the polymer is amorphous. Most of the physical and thermal PLA properties deteriorate at increasing meso-lactide content. However, some applications require moderate meso-lactide content or even amorphous PLA. By varying the meso concentration, the properties of the poly mer can be adjusted for specific applications. One of the reasons for the limited consumption of PLA up to now is the low thermal resistance. The Tg (glass transition temperature) is about 55 °C depending on comonomer con tent to a small extent (see Table 2). Methods of improving thermal resistance are to prepare a stereocomplex (sc-PLA) or a stereoblock copolymer (sbc-PLA). Melting point and heat distortion temperature (HDT) will increase significantly. Improving the thermal properties can extend the applications of PLA considerably in the future. There are also various additives that improve the properties of PLA with respect to impact strength, melt viscosity, HDT, crystallinity, and so on.
(n –1) H2O
H O n
Lactic acid
PLA
Figure 1 From lactic acid to polylactide.
only minor amounts or did not get past the pilot scale. Figure 7 depicts the steps of an ROP process, starting from LA. In the first part, lactide is formed, which – after fine purification – is converted by ROP to PLA. Pure lactide is offered in limited quantities by only one producer. Table 1 shows PLA producers, trade names, and the routes used.
10.12.5 Processing of PLA A major advantage of PLA is the possibility of processing the polymer on common process equipment. In particular, the converters of polyolefins do not require a change to other process equipment. They only need to change the handling of granulate. It is very important to dry the polymer before pro cessing, otherwise it will degrade. Water and high temperatures (up to 240 °C) facilitate fast degradation. PLA is a polymer that can be processed by • • • • • • •
injection molding, sheet extrusion, sheet blow molding, thermoforming, injection stretch blow molding, fiber spinning, and nonwoven spinning, spun bonding.
10.12.7 Perspective PLA combines all prerequisites of sustainability with important properties of well-established polymers. Applications have already been found in many niches of packaging and textile
H2O & CO2 ~ 9.54 t ha–1 sucrose Sugar beet ~ 0.14 ha
~1370 kg Sucrose Compost
~ 6.3 t ha–1 starch Maize ~ 0.21 ha
~ 5.3 t ha–1 starch Wheat ~ 0.25 ha
~1300 kg Lactic acid
~1000 kg PLA
~1370 kg Glucose Recycling ~1300 kg Starch
Figure 2 Raw materials and consumption. Germany, average 2004–2007.
‘Green Energy’
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
233
Consumption of polymers: c. 16.1 million tons in 2007 33% for packaging: c. 5.3 million tons
(5%)
Substitution potential at least (based on current application range) Market potential ca. 265 000 t Agricultural area necessary: 37 100 ha sugar beets = 10% of acreage used for sugar beets in Germany = 0.2% of total acreage in Germany
• Industrial composting − Most attractive method of disposal based on public acceptance − No recovery of material and energy • Mechanical recycling − Loss of product properties cannot be recovered − ‘Downcycling’ • Burning (energy recycling) − Recovers ‘green energy’ • Chemical recycling − Back into polymerization − Collecting and sorting to be solved yet Figure 4 Methods of PLA recycling.
Total fossil energy (GJ t–1 plastic)
140
(kg CO2eq kg–1)
Figure 3 Packaging and acreage. Source: Wirtschaftsdaten und Grafiken, VKE e.V. Internet version, 16.10.2008.
9 8 7 6 5 4 3 2 1 0
120 100
HDPE
PET
PLA
Figure 6 CO2 emissions by PLA vs. polymers from fossil feedstock ‘cradle to gate’. Reproduced from Patel, M.; Narayan, R. In Natural Fibers, Biopolymers and Biocomposites; Mohanty, A.; Misra, M.; Drzal, L., Eds.; Taylor & Francis: Boca Raton, 2005.3
Table 1
Fossil fuel Fossil raw material
PA 6
PLA producers, production capacity
Producer
Trade name
Route
Capacity (t yr−1)
Country
Ingeo Lacea
ROP DP-S
140 000 500
United States Japan
40
Nature Works Mitsui Chemicals Teijin Hisun Toray
Biofront Revode Ecodea
ROP ROP ROP
1000 5000 5000
20
Futerro
Futerro
ROP
1500
Japan PR China Republic of Korea Belgium
80 60
0 PA 6
HDPE
PET
PLA
Figure 5 Consumption of fossil resources by PLA vs. polymers from
fossil feedstock ‘cradle to gate’. Reproduced from Patel, M.; Narayan, R.
In Natural Fibers, Biopolymers and Biocomposites; Mohanty, A.;
Misra, M.; Drzal, L., Eds.; Taylor & Francis: Boca Raton, 2005.3
products. Within those niches, fast growth of consumption is expected to continue depending on the availability of PLA polymer. High research activity is dedicated to overcome typical weaknesses of PLA – low impact strength and low HDT – and
State: September 2008; no claim to completeness.
to develop tailor-made PLA grades in order to serve special applications. These activities will conquer new niches for PLA and will help to increase PLA consumption at high velocity. Other growth factors are the availability and prices of raw oil, agricultural products, and production plants and technology. Within the foreseeable future, PLA will not become a com modity polymer such as polyethylene (PE), polypropylene (PP), and polystyrene (PS) – this is considered to be an advan tage both for PLA producers and converters. However, this could change in the long term.
234
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
Lactic acid Water to hydrolysis
Evaporation/distillation Concentrated lactic acid
Water, lactic acid
Precondensation Purge
Oligomers Prepolymer Formation of cyclic dimer Crude lactide Lactide purification Highly purified lactide Ring opening polymerization
Dilactide
Polylactide with monomer Demonomerization/stabilization Polylactide Figure 7 Steps of a PLA process with ring-opening polymerization.
Table 2
Properties of PLA types
Type
Tm ( °C)
Tg ( °C)
σn
Eb (%)
PLLA PL/DLA sc-PLA sbc-PLA
160–180 – 220–230 185–195
55–65 55 60 55
45–55 MPa – – –
3–5 50–200 3–5 5–10
Tm, melting temperature; Tg, glass transition temperature; Eb, elongation at break; and σn, tensile strength at break.
10.12.8 LA as Raw Material of PLA Polymer grade LA must be optically pure (> 99% L-LA), ‘heat stable’ (no discoloration after heating, i.e., no residual sugars and amino acids), free of metal ions (alkali, earth alkali, heavy metals), and free of organic acids other than LA. It is traded with a water content of approximately 10% in order to prevent it from freezing during transport and storage. Any D-LA impurities would be converted to meso-lactide in the lactide formation process. Therefore, pure L-LA is required as raw material in order to prepare crystalline grades of PLA. Living organisms produce pure L-LA. Industrial production of LA is exclusively based on agricultural products and biological processes. Racemic LA prepared by chemical synthesis would deliver a mixture of L-, D-, and meso-lactide, which after polymerization would deliver amorphous PLA only. This may change in future if stereospecific polymerization catalysts reach industrial maturity, allowing the preparation of sbc of PLA from racemic lactide.
Presently, LA is produced from sucrose or glucose by fer mentation. Sucrose is produced from sugar beets or sugarcane depending on climate zone. Glucose is prepared from maize (see Figure 2). Several other starch-containing products could be used alternatively, such as wheat or cassava. Selection cri teria are availability, glucose price and purity, and use of side products such as edible oil or nutrients. Fermentation is done under anaerobic conditions; no gas eous by-products are formed. Sucrose or glucose is congested in large agitated vessels by bacteria or yeasts. Appropriate bacteria are Lactobacillus paracasei, Bacillus coagulans, Pediococcus, among others. Industrially strains are bred or are genetically modified in order to meet high conversion, low residual sugar, high broth concentration, productivity, and optical purity (low D-content). Nutrients have to be added at the lowest possible limit as noncongested residues would deteriorate the LA purity. Bacteria need pH to be controlled at around neutral value. This implicates that LA must be neutralized during fermentation by alkaline reagents. The fermentation process is operated as repeated batch or fully continuous. Yeasts are more tolerant against an acidic environment; they offer the opportunity to run at lower pH, saving neutralizing agent and producing less by-product. The type of the neutralization reagent determines the mode of recovery and purification of the LA formed. Figure 8 shows three methods as examples. The most common industrial pro cess I uses calcium hydroxide. Calcium lactate is formed from which LA is recovered by adding an equivalent amount of sulfuric acid after fermentation. A large amount of wet gypsum is formed as a by-product. The raw LA is purified by ion exchange and activated carbon treatment. Concentrated LA is produced by multiple stage water evaporation. The final short-path distillation transfers the LA completely into vapor phase. Recondensation delivers LA with the required purity.
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
Ca(OH)2
H2SO4
Fermentation/ filtration
Gypsum precipitation & filtration
Ion exchange A-Carbon
235
Evaporation
Distillation
I
Bipolar electrodialysis
Ion exchange (polishing)
Evaporation
II
Simulated moving bed chromatography
Ion exchange A-carbon
Evaporation
III
CaSO4 (land filling or construction) NaOH
Fermentation/ filtration
Monopolar electrodialysis
NH4OH
H2SO4
Fermentation/ filtration
Ammonium lactate conversion
(NH4)2SO4 (Fertilizer)
Figure 8 Lactic acid technologies.
The gypsum from fermentation may cause problems: Some countries do not allow it to be dumped in landfills, sometimes it is not accepted by the manufacturers of construction materials. For that reason, other LA recovery methods have been developed. Method II of Figure 8 uses electrodialysis in the LA recovery. Sodium hydroxide neutralizes LA during fermentation. Sodium lactate is split into LA and NaOH in the bipolar electrodialysis, NaOH being recycled into the fermentation. No by-product is formed in this process. Part of the excess bacterial mass can be recovered as a nutrient. Method III of Figure 8 requires aqueous ammonia as neu tralizing reagent forming ammonium lactate. After separating it from the biomass, the ammonium salt is converted with sulfu ric acid into LA. The dissolved ammonium sulfate is separated from LA in a recoverable chromatographic column operated as a simulated moving bed. Ammonium sulfate can be used as a fertilizer in agriculture.
The price of LA is determined first by the cost of glucose or sucrose, next by energy. Nutrients are another significant cost factor. LA is available only in limited quantities. A typical industrial scale PLA production plant of 60 000 MT yr−1 would require 80 000 MT yr−1 of polymer grade LA (100%). This amount is more than 50% of the LA presently available on the world market. It is clear from these figures that every industrial scale PLA plant needs its own LA plant.
References 1. Haas, T.; Kircher, M.; Köhler, T.; et al. In: White Biotechnology; Höfer, R. Ed.; Sustainable Solutions for Modern Economies, RSC Publishers: Cambridge, UK, 2009. 2. Auras, R.; Loong-Tak, L.; Selke, S. E. M.; Tsuji, H. Eds. Poly(Lactic Acid): Synthesis, Structures, Properties, Processing, and Applications, John Wiley & Sons: Hoboken, NJ, 2010. 3. Patel, M.; Narayan, R. In Natural Fibers, Biopolymers and Biocomposites; Mohanty, A.; Misra, M.; Drzal, L., Eds.; Taylor & Francis: Boca Raton, 2005.
236
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
Biographical Sketch Rainer Hagen studied chemical engineering at the University of Erlangen and holds a doctoral degree from the Technical University of Berlin. At Uhde Inventa-Fischer and its predecessors, he was responsible for development and design of polymerization processes, process modeling, and simulation. He is product manager of the PLAneo® – PLA – process of Uhde InventaFischer.
Polylactic Acid
R Hagen, Uhde Inventa-Fischer GmbH, Berlin, Germany © 2012 Elsevier B.V. All rights reserved.
10.12.1 10.12.2 10.12.3 10.12.4 10.12.5 10.12.6 10.12.7 10.12.8 References
Introduction Nondepleting Properties of PLA Market Potential of PLA Process Routes to PLA Processing of PLA Properties of PLA Perspective LA as Raw Material of PLA
10.12.1 Introduction Polylactide or polylactic acid1,2 (PLA) is a synthetic, aliphatic polyester from lactic acid (LA) (Figure 1). For industrial appli cations, such as fibers, films, and bottles, the chain length n should be between 700 and 1400. This is significantly higher than with partially aromatic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), where n is between 100 and 200. Therefore, the requirements on both raw material purity and technical effort are much higher. PLA is made from LA. LA can frequently be found in plants and animals as a by-product or intermediate product of meta bolism. Therefore, LA and the hydrolytic degradation products of PLA are nontoxic by its nature. After hydrolysis, PLA is biologically degradable by common microorganisms. Only in an industrial composting facility are the high temperature (60 °C) and humidity required for the hydrolysis achieved. At temperatures below its glass transition point (e.g., 55 °C, depending on comonomer content), PLA is as stable as PET or PBT.
10.12.2 Nondepleting Properties of PLA LA can be industrially produced from a number of starch- or sugar-containing agricultural products (Figure 2). Competition between human food, industrial LA, and PLA production is not to be expected: For example, using PLA as substitute for 5% of the German packaging plastics consump tion requires only 0.5% (sugar beet) to 1.25% (wheat) of the agricultural area available (Figure 3). At the same time, approximately 30% of the available area lies fallow mainly for economic reasons. Research is in progress on processes and microorganisms that produce LA from cellulose coming from agricultural residues such as maize stalks or straw. Several recycling methods can be applied to waste PLA (Figure 4). Composting allows only moderate benefits. In future, sorting, purification of PLA waste, and refeeding into the polymerization plant would seem to be the most attractive ways of recovery. PLA – like other biopolymers – is often criticized for its requirement for need of process energy from fossil resources. Even if this is the case at present, 1 kg of PLA represents less Polymer Science: A Comprehensive Reference, Volume 10
231 231 231 231 232 232 232 234 235
energy equivalents than 1 kg of polymers from petrochemical feedstock (Figure 5). Consequently, PLA producers can also reap financial benefits by trading CO2 emission certificates (Figure 6). If process energy is supplied by biomass, for example, biogas, the fossil energy required for 1 kg PLA can be cut by half, thus duplicating the benefits from trading CO2 emission certificates. Additionally, significant potential exists for saving process energy by improving LA and polymerization technologies.
10.12.3 Market Potential of PLA The first market place for PLA is Europe, with Italy, Germany, the Netherlands, the United Kingdom, and France in the front line, followed by Japan, the United States, and South Korea. In Germany and France, the governments have issued specific regulations to give preference to renewable and biodegradable polymers. As PLA is a rather young polymer, many companies are developing applications. These are mainly focused on the field of packaging fresh food, but other applications are emer ging. Processing PLA is possible on standard polymer equipment for the various process techniques. A problem for most of the companies is the availability of the PLA polymer. Only NatureWorks LLC, a US-based produ cer, which has a 140 000 t per annum facility, is going to be operated on full capacity in 2012. Besides NatureWorks, there are some suppliers (Table 1) offering smaller capacities. Another hindrance for the success of PLA is that the big retailers do not want to be dependent on one supplier. So many converters and brand owners are waiting for a second big supplier, which then rapidly increases demand for PLA.
10.12.4 Process Routes to PLA Several process routes have been developed or are practiced on an industrial scale: ring-opening polymerization (ROP), direct polycondensation in high boiling solvents (DP-S), and direct polymerization in bulk followed by chain extension with reac tive additives. ROP is the route that delivers by far the highest proportion of PLA chips available on the market. The other routes produce
doi:10.1016/B978-0-444-53349-4.00269-7
231
232
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
CH3 n HO
C C OH
10.12.6 Properties of PLA
CH3 H
C C OH
O
H O
PLA is a crystal clear, transparent material when amorphous that becomes hazier the higher the crystallinity. Crystallized material is opaque. When producing lactide, meso-lactide is formed as a by-product. Separation from the L-lactide takes special efforts in the purification step. When polymerizing L-lactide with some meso-lactide, a copolymer is formed. Increasing meso-lactide leads to decreasing crystallinity. With more than 10–15% meso-lactide, the polymer is amorphous. Most of the physical and thermal PLA properties deteriorate at increasing meso-lactide content. However, some applications require moderate meso-lactide content or even amorphous PLA. By varying the meso concentration, the properties of the poly mer can be adjusted for specific applications. One of the reasons for the limited consumption of PLA up to now is the low thermal resistance. The Tg (glass transition temperature) is about 55 °C depending on comonomer con tent to a small extent (see Table 2). Methods of improving thermal resistance are to prepare a stereocomplex (sc-PLA) or a stereoblock copolymer (sbc-PLA). Melting point and heat distortion temperature (HDT) will increase significantly. Improving the thermal properties can extend the applications of PLA considerably in the future. There are also various additives that improve the properties of PLA with respect to impact strength, melt viscosity, HDT, crystallinity, and so on.
(n –1) H2O
H O n
Lactic acid
PLA
Figure 1 From lactic acid to polylactide.
only minor amounts or did not get past the pilot scale. Figure 7 depicts the steps of an ROP process, starting from LA. In the first part, lactide is formed, which – after fine purification – is converted by ROP to PLA. Pure lactide is offered in limited quantities by only one producer. Table 1 shows PLA producers, trade names, and the routes used.
10.12.5 Processing of PLA A major advantage of PLA is the possibility of processing the polymer on common process equipment. In particular, the converters of polyolefins do not require a change to other process equipment. They only need to change the handling of granulate. It is very important to dry the polymer before pro cessing, otherwise it will degrade. Water and high temperatures (up to 240 °C) facilitate fast degradation. PLA is a polymer that can be processed by • • • • • • •
injection molding, sheet extrusion, sheet blow molding, thermoforming, injection stretch blow molding, fiber spinning, and nonwoven spinning, spun bonding.
10.12.7 Perspective PLA combines all prerequisites of sustainability with important properties of well-established polymers. Applications have already been found in many niches of packaging and textile
H2O & CO2 ~ 9.54 t ha–1 sucrose Sugar beet ~ 0.14 ha
~1370 kg Sucrose Compost
~ 6.3 t ha–1 starch Maize ~ 0.21 ha
~ 5.3 t ha–1 starch Wheat ~ 0.25 ha
~1300 kg Lactic acid
~1000 kg PLA
~1370 kg Glucose Recycling ~1300 kg Starch
Figure 2 Raw materials and consumption. Germany, average 2004–2007.
‘Green Energy’
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
233
Consumption of polymers: c. 16.1 million tons in 2007 33% for packaging: c. 5.3 million tons
(5%)
Substitution potential at least (based on current application range) Market potential ca. 265 000 t Agricultural area necessary: 37 100 ha sugar beets = 10% of acreage used for sugar beets in Germany = 0.2% of total acreage in Germany
• Industrial composting − Most attractive method of disposal based on public acceptance − No recovery of material and energy • Mechanical recycling − Loss of product properties cannot be recovered − ‘Downcycling’ • Burning (energy recycling) − Recovers ‘green energy’ • Chemical recycling − Back into polymerization − Collecting and sorting to be solved yet Figure 4 Methods of PLA recycling.
Total fossil energy (GJ t–1 plastic)
140
(kg CO2eq kg–1)
Figure 3 Packaging and acreage. Source: Wirtschaftsdaten und Grafiken, VKE e.V. Internet version, 16.10.2008.
9 8 7 6 5 4 3 2 1 0
120 100
HDPE
PET
PLA
Figure 6 CO2 emissions by PLA vs. polymers from fossil feedstock ‘cradle to gate’. Reproduced from Patel, M.; Narayan, R. In Natural Fibers, Biopolymers and Biocomposites; Mohanty, A.; Misra, M.; Drzal, L., Eds.; Taylor & Francis: Boca Raton, 2005.3
Table 1
Fossil fuel Fossil raw material
PA 6
PLA producers, production capacity
Producer
Trade name
Route
Capacity (t yr−1)
Country
Ingeo Lacea
ROP DP-S
140 000 500
United States Japan
40
Nature Works Mitsui Chemicals Teijin Hisun Toray
Biofront Revode Ecodea
ROP ROP ROP
1000 5000 5000
20
Futerro
Futerro
ROP
1500
Japan PR China Republic of Korea Belgium
80 60
0 PA 6
HDPE
PET
PLA
Figure 5 Consumption of fossil resources by PLA vs. polymers from
fossil feedstock ‘cradle to gate’. Reproduced from Patel, M.; Narayan, R.
In Natural Fibers, Biopolymers and Biocomposites; Mohanty, A.;
Misra, M.; Drzal, L., Eds.; Taylor & Francis: Boca Raton, 2005.3
products. Within those niches, fast growth of consumption is expected to continue depending on the availability of PLA polymer. High research activity is dedicated to overcome typical weaknesses of PLA – low impact strength and low HDT – and
State: September 2008; no claim to completeness.
to develop tailor-made PLA grades in order to serve special applications. These activities will conquer new niches for PLA and will help to increase PLA consumption at high velocity. Other growth factors are the availability and prices of raw oil, agricultural products, and production plants and technology. Within the foreseeable future, PLA will not become a com modity polymer such as polyethylene (PE), polypropylene (PP), and polystyrene (PS) – this is considered to be an advan tage both for PLA producers and converters. However, this could change in the long term.
234
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
Lactic acid Water to hydrolysis
Evaporation/distillation Concentrated lactic acid
Water, lactic acid
Precondensation Purge
Oligomers Prepolymer Formation of cyclic dimer Crude lactide Lactide purification Highly purified lactide Ring opening polymerization
Dilactide
Polylactide with monomer Demonomerization/stabilization Polylactide Figure 7 Steps of a PLA process with ring-opening polymerization.
Table 2
Properties of PLA types
Type
Tm ( °C)
Tg ( °C)
σn
Eb (%)
PLLA PL/DLA sc-PLA sbc-PLA
160–180 – 220–230 185–195
55–65 55 60 55
45–55 MPa – – –
3–5 50–200 3–5 5–10
Tm, melting temperature; Tg, glass transition temperature; Eb, elongation at break; and σn, tensile strength at break.
10.12.8 LA as Raw Material of PLA Polymer grade LA must be optically pure (> 99% L-LA), ‘heat stable’ (no discoloration after heating, i.e., no residual sugars and amino acids), free of metal ions (alkali, earth alkali, heavy metals), and free of organic acids other than LA. It is traded with a water content of approximately 10% in order to prevent it from freezing during transport and storage. Any D-LA impurities would be converted to meso-lactide in the lactide formation process. Therefore, pure L-LA is required as raw material in order to prepare crystalline grades of PLA. Living organisms produce pure L-LA. Industrial production of LA is exclusively based on agricultural products and biological processes. Racemic LA prepared by chemical synthesis would deliver a mixture of L-, D-, and meso-lactide, which after polymerization would deliver amorphous PLA only. This may change in future if stereospecific polymerization catalysts reach industrial maturity, allowing the preparation of sbc of PLA from racemic lactide.
Presently, LA is produced from sucrose or glucose by fer mentation. Sucrose is produced from sugar beets or sugarcane depending on climate zone. Glucose is prepared from maize (see Figure 2). Several other starch-containing products could be used alternatively, such as wheat or cassava. Selection cri teria are availability, glucose price and purity, and use of side products such as edible oil or nutrients. Fermentation is done under anaerobic conditions; no gas eous by-products are formed. Sucrose or glucose is congested in large agitated vessels by bacteria or yeasts. Appropriate bacteria are Lactobacillus paracasei, Bacillus coagulans, Pediococcus, among others. Industrially strains are bred or are genetically modified in order to meet high conversion, low residual sugar, high broth concentration, productivity, and optical purity (low D-content). Nutrients have to be added at the lowest possible limit as noncongested residues would deteriorate the LA purity. Bacteria need pH to be controlled at around neutral value. This implicates that LA must be neutralized during fermentation by alkaline reagents. The fermentation process is operated as repeated batch or fully continuous. Yeasts are more tolerant against an acidic environment; they offer the opportunity to run at lower pH, saving neutralizing agent and producing less by-product. The type of the neutralization reagent determines the mode of recovery and purification of the LA formed. Figure 8 shows three methods as examples. The most common industrial pro cess I uses calcium hydroxide. Calcium lactate is formed from which LA is recovered by adding an equivalent amount of sulfuric acid after fermentation. A large amount of wet gypsum is formed as a by-product. The raw LA is purified by ion exchange and activated carbon treatment. Concentrated LA is produced by multiple stage water evaporation. The final short-path distillation transfers the LA completely into vapor phase. Recondensation delivers LA with the required purity.
Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
Ca(OH)2
H2SO4
Fermentation/ filtration
Gypsum precipitation & filtration
Ion exchange A-Carbon
235
Evaporation
Distillation
I
Bipolar electrodialysis
Ion exchange (polishing)
Evaporation
II
Simulated moving bed chromatography
Ion exchange A-carbon
Evaporation
III
CaSO4 (land filling or construction) NaOH
Fermentation/ filtration
Monopolar electrodialysis
NH4OH
H2SO4
Fermentation/ filtration
Ammonium lactate conversion
(NH4)2SO4 (Fertilizer)
Figure 8 Lactic acid technologies.
The gypsum from fermentation may cause problems: Some countries do not allow it to be dumped in landfills, sometimes it is not accepted by the manufacturers of construction materials. For that reason, other LA recovery methods have been developed. Method II of Figure 8 uses electrodialysis in the LA recovery. Sodium hydroxide neutralizes LA during fermentation. Sodium lactate is split into LA and NaOH in the bipolar electrodialysis, NaOH being recycled into the fermentation. No by-product is formed in this process. Part of the excess bacterial mass can be recovered as a nutrient. Method III of Figure 8 requires aqueous ammonia as neu tralizing reagent forming ammonium lactate. After separating it from the biomass, the ammonium salt is converted with sulfu ric acid into LA. The dissolved ammonium sulfate is separated from LA in a recoverable chromatographic column operated as a simulated moving bed. Ammonium sulfate can be used as a fertilizer in agriculture.
The price of LA is determined first by the cost of glucose or sucrose, next by energy. Nutrients are another significant cost factor. LA is available only in limited quantities. A typical industrial scale PLA production plant of 60 000 MT yr−1 would require 80 000 MT yr−1 of polymer grade LA (100%). This amount is more than 50% of the LA presently available on the world market. It is clear from these figures that every industrial scale PLA plant needs its own LA plant.
References 1. Haas, T.; Kircher, M.; Köhler, T.; et al. In: White Biotechnology; Höfer, R. Ed.; Sustainable Solutions for Modern Economies, RSC Publishers: Cambridge, UK, 2009. 2. Auras, R.; Loong-Tak, L.; Selke, S. E. M.; Tsuji, H. Eds. Poly(Lactic Acid): Synthesis, Structures, Properties, Processing, and Applications, John Wiley & Sons: Hoboken, NJ, 2010. 3. Patel, M.; Narayan, R. In Natural Fibers, Biopolymers and Biocomposites; Mohanty, A.; Misra, M.; Drzal, L., Eds.; Taylor & Francis: Boca Raton, 2005.
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Carbohydrate-Based Polymer Building Blocks and Biopolymers | Polylactic Acid
Biographical Sketch Rainer Hagen studied chemical engineering at the University of Erlangen and holds a doctoral degree from the Technical University of Berlin. At Uhde Inventa-Fischer and its predecessors, he was responsible for development and design of polymerization processes, process modeling, and simulation. He is product manager of the PLAneo® – PLA – process of Uhde InventaFischer.