10 Compostable Polymer Materials: Definitions, Structures, and Methods of Preparation Ewa Rudnik OUTLINE 10.1 Biodegradable Polymers from Renewable Resources 192 10.1.1 Poly(lactic acid)dPLA 192 10.1.2 PolyhydroxyalkanoatesdPHA 195 10.1.3 Thermoplastic StarchdTPS 198 10.2 Other Compostable Polymers from Renewable Resources 201 10.2.1 Cellulose 201 10.2.2 Chitosan 201 10.2.3 Proteins 202 10.3 Biodegradable Polymers from Petrochemical Sources 203 10.3.1 Aliphatic Polyesters and Copolyesters 203 10.3.2 Aromatic Polyesters and Copolyesters 204 10.3.3 Poly(caprolactone)dPCL 205 10.3.4 Poly(esteramide)dPEA 206 10.3.5 Poly(vinyl alcohol)dPVA 207 10.3.6 Blends 208 References 209 “Biodegradable polymers” or “compostable polymers” were first commercially introduced in the 1980s. These first-generation biodegradable products were made from a conventional polymer, usually polyolefin (e.g., polyethylene) mixed together with starch or some other organic substance. When starch was eaten by microorganisms, the products were broken down, leaving small fragments of polyolefins. In 1994 Narayan et al. wrote: “The U.S. biode- gradables industry fumbled at the beginning by introducing starch filled (6e15%) polyolefins as true biodegradable materials. These at best were only biodisintegradable and not completely biodegrad- able. Data showed that only the surface starch bio- degraded, leaving behind a recalcitrant polyethylene material” [1]. The situation confused consumers and govern- ment regulators, and put into question the biode- gradable plastics market for some years. Since then the confusion or misunderstanding appeared about what was and what was not biodegradable and/or compostable. Additionally, no scientifically based test methods or standards existed to support claims made by plastics manufacturers for the “biodegradability” or “compostability” of their products. More recently, international and national stan- dards bodies, i.e., International Organization for Standardization (ISO), American Society for Testing and Materials (ASTM), Japanese Standards Associ- ation (JIS), and European Organization for Stan- dardization (EN), have developed definitions related to the degradation of plastics. Nowadays, ISO and ASTM standards exist describing in detail the purposes of “biodegradable” and “compostable.” The ASTM D6400 standard establishes the requirements for the labeling of materials and prod- ucts, including packaging made from plastics, as “compostable in municipal and industrial compost- ing facilities” (Table 10.1). ISO 17088 specifies test methods and require- ments to determine and label plastic products and products made from plastics that are designed to be recovered through aerobic composting. It particularly establishes the requirements for labeling of materials and products, including packaging made from plastics, as “compostable,” “compostable in munic- ipal and industrial composting facilities,” and “biodegradable during composting.” Ebnesajjad: Handbook of Biopolymers and Biodegradable Plastics. http://dx.doi.org/10.1016/B978-1-4557-2834-3.00010-0 Ó 2013 Elsevier Inc. All rights reserved. Adapted from a chapter in: Rudnik, Compostable Polymer Materials (2008). 189
Transcript
10 Compostable Polymer Materials: Definitions,Structures, and Methods of Preparation
Ewa Rudnik
E
�
O U T L I N E
10.1 Biodegradable Polymers from RenewableResources 19
“Biodegradable polymers” or “compostablepolymers” were first commercially introduced in the1980s. These first-generation biodegradable productswere made from a conventional polymer, usuallypolyolefin (e.g., polyethylene) mixed together withstarch or some other organic substance. Whenstarch was eaten by microorganisms, the productswere broken down, leaving small fragments ofpolyolefins.
In 1994 Narayan et al. wrote: “The U.S. biode-gradables industry fumbled at the beginning byintroducing starch filled (6e15%) polyolefins as truebiodegradable materials. These at best were onlybiodisintegradable and not completely biodegrad-able. Data showed that only the surface starch bio-degraded, leaving behind a recalcitrant polyethylenematerial” [1].
The situation confused consumers and govern-ment regulators, and put into question the biode-gradable plastics market for some years. Sincethen the confusion or misunderstanding appearedabout what was and what was not biodegradableand/or compostable. Additionally, no scientificallybased test methods or standards existed to supportclaims made by plastics manufacturers for the
//d
, C
“biodegradability” or “compostability” of theirproducts.
More recently, international and national stan-dards bodies, i.e., International Organization forStandardization (ISO), American Society for Testingand Materials (ASTM), Japanese Standards Associ-ation (JIS), and European Organization for Stan-dardization (EN), have developed definitions relatedto the degradation of plastics. Nowadays, ISO andASTM standards exist describing in detail thepurposes of “biodegradable” and “compostable.”
The ASTM D6400 standard establishes therequirements for the labeling of materials and prod-ucts, including packaging made from plastics, as“compostable in municipal and industrial compost-ing facilities” (Table 10.1).
ISO 17088 specifies test methods and require-ments to determine and label plastic products andproducts made from plastics that are designed to berecovered through aerobic composting. It particularlyestablishes the requirements for labeling of materialsand products, including packaging made fromplastics, as “compostable,” “compostable in munic-ipal and industrial composting facilities,” and“biodegradable during composting.”
Table 10.2 Definitions of Compostability Accordingto ISO 17088 [3]
Compostable PlasticsA plastic that undergoes degradation bybiological processes during composting toyield CO2, water, inorganic compounds, andbiomass at a rate consistent with other knowncompostable materials and leaves no visible,distinguishable, or toxic residue.
CompostingThe autothermic and thermophilic biologicaldecomposition of biowaste (organic waste) inthe presence of oxygen and under controlledconditions by the action of micro- andmacroorganisms in order to produce compost.
CompostOrganic soil conditioner obtained bybiodegradation of a mixture consistingprincipally of vegetable residues, occasionallywith other organic material and having a limitedmineral content.
DisintegrationThe physical breakdown of a material into verysmall fragments.
Table 10.1 Definitions of Compostability Accordingto ASTM D6400 [2]
Compostable PlasticA plastic that undergoes degradation bybiological processes during composting to yieldcarbon dioxide, water, inorganic compounds,and biomass at a rate consistent with otherknown compostable materials and leaves novisually distinguishable or toxic residues.
CompostingA managed process that controls the biologicaldecomposition and transformation ofbiodegradable materials into a humus-likesubstance called compost: the aerobicmesophilic and thermophilic degradation oforganic matter to make compost, thetransformation of biologically decomposablematerial through a controlled process ofbiooxidation that proceeds through mesophilicand thermophilic phases and results in theproduction of carbon dioxide, water, minerals,and stabilized organic matter (compost orhumus). Composting uses a natural process tostabilize mixed decomposable organic materialrecovered from municipal solid waste, yardtrimmings, biosolids (digested sewage sludge),certain industrial residues, and commercialresidues.
Degradable PlasticA plastic designed to undergo a significantchange in its chemical structure under specifiedenvironmental conditions, resulting in a loss ofsome properties that may be measured bystandard test methods appropriate to the plasticand the application in a period of time thatdetermines its classification.
190 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
The definition of “compostable plastic” proposedin ISO 17088 is identical to that given in the ASTMD6400 standard (Table 10.2).
In spite of its very large use (and abuse), the term“biodegradable” is not helpful because it is notinformative. The term does not convey any informa-tion about the specific environment where thebiodegradation is supposed to take place, the rate thatwill regulate the process (fast, slow), and the extent ofbiodegradation (partial or total conversion into CO2).
The definition of “biodegradable” has beenassessed during the past decade. Some examples ofdefinitions of “biodegradable plastic” are given in thefollowing.
ASTM definition [2]: “a degradable plastic inwhich the degradation results from the action ofnaturally occurring microorganisms such as bacteria,fungi, and algae.”
ISO and CEN definition [4]: “degradable plasticin which degradation results in lower molecularweight fragments produced by the action of naturallyoccurring microorganisms such as bacteria, fungi andalgae.”
According to ISO definition [4] degradable plasticmeans “A plastic designed to undergo a significantchange in its chemical structure under specific envi-ronmental conditions resulting in a loss of someproperties that may vary as measured by standardtest methods appropriate to the plastic and theapplication in a period of time that determines itsclassification.”
Japanese Biodegradable Polymers Society(BPS) defines biodegradable plastics (called Green-Pla) as plastics that can be used as conventionalplastics, while on disposal they decompose to waterand carbon dioxide by the action of microorganismscommonly existing in the natural environment [5].
Most of the definitions of biodegradation are basedon the same concept: the action of microorganisms
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 191
on the material and its conversion into carbon dioxideor methane and water.
A plastic can be degradable without being biode-gradable, i.e., it might disintegrate into pieces or evenan invisible powder, but not be assimilated bymicroorganisms. A plastic can be degradable andeven biodegradable without being compostable, i.e.,it might biodegrade at a rate that is too slow to becalled compostable [6].
The difference between biodegradable and compo-stable polymers lies in additional requirements relatedto the latter. Besides biodegradation into carbondioxide, water, inorganic compounds, and biomass,compostable polymers must fulfill other criteria suchas compatibility with the composting process, nonegative effect on quality of compost and a degradationrate consistentwith other knowncompostingmaterials.
It is noteworthy that compostable plastics area priori designed for a given method of safe disposal,i.e., composting. This means that after their usefullife they will biodegrade in a composting process.The idea of compostable polymers is in agreementwith life-cycle thinking.
To summarize, the requirements a material mustsatisfy to be termed “compostable” include miner-alization (i.e., biodegradation to carbon dioxide,water, and biomass), disintegration into a compostingsystem, and completion of its biodegradation duringthe end-use of the compost, which, moreover, mustmeet relevant quality criteria, e.g., no ecotoxicity.The satisfaction of requirements should be proved bystandardized test methods.
Compostable polymers can be divided accordingto source of origin or method of their preparation(Fig. 10.1).
COMPOSTABLE POLYMER MATERIALS
Methods ofpreparation
Petrochemical RenewableOrigin
Conventional synthesis
Modification ofbiomass products
Biotechnology
Blending
Figure 10.1 Classification of compostable polymers.
On the basis of origin, compostable polymers arederived from renewable and petrochemical resources.
Biodegradable polymers from renewableresources include the following:
3. preparation directly from biomass, e.g.,plantsdstarch;
4. blending, e.g., starchePCL blends.
A method based on blending of biodegradablepolymers is very often used in order to improve theproperties of compostable polymer materials or todecrease their cost. The various polymers used areboth renewable and of petrochemical origin. Nova-mont’s Mater-Bi is an example of such a material.
The molecular structure of PLA is schematicallypresented in Fig. 10.2. PLA, linear aliphatic ther-moplastic polyester, is prepared from lactic acid.Lactic acid (2-hydroxy propionic acid) is one of thesimplest chiral molecules and exists as two stereo-isomers, L- and D-lactic acid (Fig. 10.3).
Lactic acid is the most widely occurring carbox-ylic acid in nature [7]. It was discovered by theSwedish chemist Scheele in 1780 as a sour compo-nent of milk, and was first produced commercially byCharles E. Avery at Littleton, Massachusetts, USA,in 1881. Lactic acid can be manufactured by chem-ical synthesis or carbohydrate fermentation. First,lactic acid was petrochemically derived [8]. Thecommercial process for chemical synthesis is basedon lactonitrile (CH3CHOHCN) obtained from
HO C C O H
H O
CH3n
Figure 10.2 Structure of poly(lactic acid).
H
C
HO
CH3
COOH
H3C
C
HO
H
COOH
L (+) Lactic acid D (–) Lactic acid
Figure 10.3 Stereoforms of lactic acid.
acetaldehyde (CH3CHO) and hydrogen cyanide(HCN). After recovery and purification by distilla-tion, lactonitrile is then hydrolyzed to lactic acid[7,8]. Lactic acid produced by the petrochemicalroute exists as a racemic (optically inactive) mixtureof D and L forms. Though chemical synthesisproduces a racemic mixture, stereospecific lactic acidcan be made by carbohydrate fermentation depend-ing on the strain being used.
Lactic acid-based polymers are prepared by poly-condensation, ring-opening polymerization (ROP),and other methods (chain extension, grafting). High-molecular-weight PLA is generally produced by theROP of the lactide monomer. The conversion of lac-tide to high-molecular-weight polylactide is achievedcommercially by two routes. Recently, Cargill Dowused a solvent-free process and a novel distillationprocess to produce a range of PLA polymers. Theprocess consists of three separate and distinct stepsthat lead to the production of lactic acid, lactide, andPLA high polymer [8] (Fig. 10.4).
Each of the process steps is free of organicsolventdwater is used in fermentation while moltenlactide and polymer serve as the reaction media inmonomer and polymer production. The essentialnovelty of the process lies in the ability to go fromlactic acid to a low-molecular-weight PLA, followedby controlled depolymerization to produce the cyclicdimer, commonly referred to as lactide. An organo-metallic catalyst, e.g., tin octanoate, is used toenhance the rate and selectivity of the intramolecularcyclization reaction [9]. This lactide is maintained inliquid form and purified by distillation. CatalyticROP of the lactide intermediate results in the
HOOH
O
H CH3
Condensation– H2O
O
O
H CH3 nPolymer Mn ~5000
Lactic acid
Depolymerization O
O
CH3
O
H3C
O
Lactide
Ring openingpolymerization
Solvent free
High molecular weight PLA
O
O
H CH3 n
Figure 10.4 Manufacturing route to poly(lactic acid)
according to the Cargill Dow process.
n
OOH
OH
O
CH2HO
Starch
Enzyme hydrolysisCargill Inc.
+H2O
OOH
OH
HO
CH2HO
OH
Dextrose (glucose)
Fermentation(GMO free)
NatureWorks LLC
Lactic acid (99.5% L)
H2OOH
O
H CH3
Figure 10.5 Cargill route to lactic
acid.
Lactic acid
HOOH
CH3
O
Azeotropicdehydrative
condensation–H2O
HOO
OOH
CH3
O CH3
O CH3
On
High molecular weight PLAMw > 100 000
Figure 10.6 Manufacturing route to poly(lactic acid)
according to the Mitsui process.
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 193
production of PLA with controlled molecularweights. The process is continuous with no necessityto separate the intermediate lactide.
Lactic acid used in the preparation of PLA isderived from annually renewable resources. CargillDow uses sugar from maize as feedstock, due to itslow cost and abundance, but it is envisaged to uselocal plant sources containing starch or sugar, such aswheat, sugar beets, or agricultural waste (Fig. 10.5).
The ROP of lactic acid monomers is catalyzed bycompounds of transition metals: tin, aluminium, lead,zinc, bismuth, iron, and yttrium. A collection of morethan 100 catalysts for PLA synthesis was reviewed[10,11]. The catalysts used mainly consist of metalpowders, Lewis acids, Lewis bases, organometalliccompounds and different salts of metals. However,organometallic compounds are very effective in thesynthesis of high-molecular-weight PLA particularlyalkali metals and metal halides, oxides, carboxylates,and alkoxides.
In contrast, Mitsui Toatsu (presently MitsuiChemicals) utilizes a solvent-based process, in whicha high-molecular-weight PLA is produced by directcondensation using azeotropic distillation to removethe water of condensation continuously (Fig. 10.6).
The synthesis of PLA through polycondensationof the lactic acid monomer gave an average molec-ular weight lower than 1.6 � 104, whereas ROP oflactides gave average molecular weights rangingfrom 2 � 104 to 6.8 � 104 [7].
Purac, producer of lactic acid, developed thetechnology of formation of stereocomplex PLA ina solid status by melt-blending PLLA and PDLA
through a transesterification process using a catalyst[9]. The PLLA and PDLA polymers originate fromseparately polymerized L-lactide and D-lactide.
Copolymerization and blending of PLA has beenextensively investigated as a useful route to obtaina product with a particular combination of desirableproperties. Other ring formed monomers are alsoincorporated into the lactic acid-based polymer byROP [7,12]. The most utilized comonomers areglycolide (1,4-dioxane-2,5-dione), 3-caprolactone(2-oxepanone), g-valerolactone (2-pyranone), 1,5-dioxepane-2-one, and trimethylene carbonate (1,3-dioxan-2-one). Examples of repeating units ofcomonomers are given in Table 10.3.
Lactic acid-based polyesters could also beproduced by enzymatic catalysis. For example,lipase-catalyzed ROP of cyclic lactides is applicablefor the synthesis of PLA [12e14].
The polymers derived from lactic acid by thepolycondensation route are generally referred to aspoly(lactic acid) and the ones prepared from lactideby ROP as polylactide [15]. Both types are generallyreferred to as PLA.
Table 10.3 Repeating Units of the Most Common Lactic Acid Comonomers
Name Lactones
O C R
O
Structure where R
Poly(glycolide)
O
O
O
O
H
H
CH2
Poly(lactide)
O
O
O
O
CH3
H3C
CH
CH3
Poly(d-valerolactone)
O
O
(CH2)4
Poly(3-caprolactone)
O
O
(CH2)5
Poly(b-hydroxybutyrate) OO CH3 CH2 CH
CH3
Poly(b-hydroxyvalerate) OO C2H5 CH2 CH
C2H5
Poly(1,5-dioxepane-2-one)
O
O
(CH2)2 (CH2)2O
Poly(trimethylene carbonate)
O O
O
O (CH2)3
194 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
Table 10.4 Commercially Available PLA Polymers
Trade Name Supplier Origin Website
Lacea Mitsui Chemicals Japan www.mitsui-chem.co.jp/e
Lacty Shimadzu Japan www.shimadzu.co.jp
NatureWorks Cargill Dow USA www.NatureWorksLLC.com
Hycail Hycail b.v. The Netherlands www.hycail.com
Biofront Teijin Japan http://www.teijin.co.jp/english
PLA Zhejiang Hisun Biomaterials China http://hisunpla.en.gongchang.com/
PURAC1 Purac Biochem The Netherlands http://www.purac.com
1Partnership between PURAC, Sulzer, and Synbra. PURAC provides lactide and Synbra polymerizes the lactide into PLA, using PLA
technology that was jointly developed by PURAC and Sulzer. Synbra processes the polymer into expanded PLA foam [9].
CC
HH3C
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 195
Table 10.4 lists the commercially available PLApolymers.
CH3O
n
Figure 10.8 Repeating unit of PHB.
10.1.2 PolyhydroxyalkanoatesdPHA
Figure 10.7 shows the generic formula for PHAs,where x is 1 for all commercially relevant polymersand R can be hydrogen or hydrocarbon chains of upto C15 in length.
Polyhydroxyalkanoates (PHA) are polyesters ofvarious hydroxyalkanoates that are synthesized bymany Gram-positive and Gram-negative bacteriafrom at least 75 different bacteria [16]. These poly-mers are accumulated intracellularly to levels as highas 90% of the cell dry weight under conditions ofnutrient stress and act as a carbon and energy reserve.
In 1920s French bacteriologist Lemoigne discov-ered aliphatic polyesterdpoly(3-hydroxybutyrate)(PHB) as a granular component in bacterial cells[17]. PHB is the reserve polymer found in manytypes of bacteria, which can grow in a wide variety ofnatural environments and which have the ability toproduce and polymerize the monomer [R]-3-hydroxybutyric acid. The repeating unit of PHB hasa chiral center (Fig. 10.8) and the polymer is opticallyactive.
HO C (CH2)x C O H
H
RO
n
Figure 10.7 Structure of polyhydroxyalkanoates.
It was determined by Stanier, Wilkinson, andcoworkers that PHB granules in bacteria serve as anintracellular food and energy reserve [17]. PHBpolymer is produced by the cell in response toa nutrient limitation in the environment in order toprevent starvation if an essential element becomesunavailable [17]. It is consumed when no externalcarbon source is available.
Since the discovery of the simple PHB homo-polymer by Lemoigne in the mid-1920s, a family ofover 100 different aliphatic polyesters of the samegeneral structure has been discovered. PHB is only theparent member of a family of natural polyesters havingthe same three-carbon backbone structure but differingin the type of alkyl group at the b or 3 position [17].These polymers are referred to in general as poly-hydroxyalkanoates (PHAs) and have the same config-uration for the chiral center at the 3 position, which isvery important both for their physical properties and forthe activities of the enzymes involved in their biosyn-thesis and biodegradation. PHAs are also namedbacterial polyesters since they are produced inside thecells of bacteria.
Awide range of PHA homopolymers, copolymers,and terpolymers have been produced, in most cases atthe laboratory scale. Bacteria that are used for theproduction of PHAs can be divided into two groupsbased on the culture conditions required for PHA
196 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
synthesis [18]. The first group of bacteria requires thelimitation of an essential nutrient such as nitrogen,phosphorous, magnesium, or sulfur for the synthesisof PHA from an excess carbon source. The followingbacteria are included in this group: Alcaligeneseutrophus, Protomonas extorquens, and Protomonasoleovorans. The second group of bacteria, whichincludes Alcaligenes latus, a mutant strain ofAzotobacter vinelandii, and recombinant Escherichiacoli, do not require nutrient limitation for PHAsynthesis and can accumulate polymer duringgrowth.
PHAs exist as discrete inclusions that are typically0.2 � 0.5 mm in diameter localized in the cellcytoplasm [18]. The molecular weight of PHAsranges from 2 � 105 to 3 � 106, depending on themicroorganism and the growth conditions.
Today, PHAs are separated into three classes: shortchain-length PHA (scl-PHA, carbon numbers ofmonomers ranging from C3 to C5), medium chain-length PHA (mcl-PHA, C6eC14), and long chain-length PHA (lcl-PHA, >C14). The main members ofthe PHA family are the homopolymers PHB, whichhas the generic formula in Fig. 10.7 with R ¼1(methyl), and poly(3-hydroxyvalerate) (PHV), withthe generic formula with R ¼ 2 (ethyl). PHAs con-taining 3-hydroxy acids have a chiral center andhence are optically active (Table 10.5).
Mcl-PHAs were first discovered in 1983 whenPseudomonas oleovorans was grown in octane [19].Since then many fluorescent Pseudomonas specieshave been used for their production. To date morethan 150 units of mcl-PHA monomers have beenproduced by culturing various Pseudomonas strainson different carbon substrates [x]. The versatility ofPseudomonas species in using a range of carbon
Table 10.5 Polyhydroxyalkanoates Family
PHA 3
P(3HB) d
P(3HV) d
P(3HB-co-3HV) (Biopol�)1 d
P(3HB-co-3HHx) (Kaneka)2, (Nodax�)3 d
P(3HB-co-3HO) (Nodax�) d
P(3HB-co-3HOd) (Nodax�) d
1Patent held by Metabolix, Inc. 2Kaneka holds the paten
patents.
sources and low substrate specificity of the mcl-PHAsynthase, the key enzyme involved in the polymeri-zation of medium chain-length hydroxyacyl coen-zyme A (CoA) into mcl-PHA, is responsible for thediversity in mcl-PHA monomers. Pseudomonasspecies can be grown on both structurally related andunrelated carbon sources for producing PHAs.Structurally related carbon sources such as alkanes,alkenes, and aldehydes produce precursor substratesthat exhibit structures related to the constituents ofthe mcl-PHAs. Mcl-PHAs are more structurallydiverse than scl-PHAs and hence can be more readilytailored for specific applications. Studies on mcl-PHAs are still somewhat limited to P(3HO) and itscopolymers and P(3HB-co-3HHx), which are avail-able in large quantities.
Copolymers of PHAs vary in the type andproportion of monomers, and are typically random insequence. PHBV is made up of a random arrange-ment of the monomers R 5 ¼ 1 and R ¼ 2. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH)consists of the monomers R ¼ 1 (methyl) and R ¼ 3(propyl). The Nodax� family of copolymers arepoly(3-hydroxybutyrate-co-3-hydroxyalkanoate)swith copolymer content varying from 3 to 15 mol%and chain length from C7 up to C19 [20].
Large-scale commercial production of PHAs usesfermentation technologies. A generic process forPHA produced by bacterial fermentation consists ofthree basic steps: fermentation, isolation and purifi-cation, and blending and palletizing [20]. Subsequentto inoculation and small-scale fermentation, a largefermentation vessel is filled with mineral mediumand inoculated with seed ferment (containing themicrobe or bacteria). The carbon source is fed atvarious rates until it is completely consumed and cell
-Hydroxy Acids With Side Chain R
CH3
CH2 CH3
CH3 and dCH2CH3
CH3 and dCH2 CH2 CH3
CH3 and dCH2 CH2 CH2 CH2CH3
CH3 and d(CH2)14 CH3
t on chemical composition. 3P&G holds processing and application
C
O
H3C CoAS
acetyl-CoA
3-ketothiolase ( phbA gene)CoASH
C
O
H3C CH3
CS CoA
Oacetoacetyl-CoA
NADPH + H+
NADP+
CHH3C CH3
CS CoA
OOH(R)-3-hydroxybutyryl-CoA
PHB synthase ( phbC gene)CoASH
CH2
CO
CHH2C
CO
CH
O CH3 O CH3
n
PHB
acetoacetyl-CoA( phbA gene) reductase
Figure 10.9 PHB synthesis in Ralstonia eutropha.
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 197
growth and PHA accumulation is complete. Currentcarbon sources for producing PHA are carbohydrates(glucose, fructose, sucrose); alcohols (methanol,glycerol); alkanes (hexane to dodecane); and organicacids (butyrate upward). In the United States, the rawmaterial source is chiefly corn steep liquor; in theEurope beet sugar predominates. The total fermen-tation step typically takes 38e48 h. To isolate andpurify PHA, the cells are concentrated, dried, andextracted with hot solvent. The residual cell debris isremoved from the solvent containing dissolved PHAby a solideliquid separation process. The PHA isthen precipitated by addition of a nonsolvent andrecovered by the solideliquid separation process.PHA is washed with solvent to enhance the qualityand dried under vacuum and moderate temperatures(in certain cases where high purity product is notneeded, solvent extraction may not be required). Thesolvents are distilled and recycled separately. Theneat polymer is typically preformed into pellets withor without other polymer ingredients [20].
PHAs are produced from a wide variety ofsubstrates such as renewable resources (sucrose,starch, cellulose, triacylglycerols), fossil resources(methane, mineral oil, lignite, hard coal), by-products (molasses, whey, glycerol), chemicals(propionic acid, 4-hydroxybutyric acid), and carbondioxide [16].
As the major cost in the production of PHA is themedium, efforts are focused on finding cheap media.Extensive studies to select cheap sources forfermentation include media containing molasses,corn steep liquor, whey, wheat and rice bran, starchand starchy wastewaters, effluents from olive milland palm olive mill, activated sludge, and swinewaste [21,22].
The microorganisms of choice for the industrialproduction of PHA varies depending on factors thatinclude the cell’s ability to utilize an inexpensivecarbon source, the cost of the medium, the growthrate, the polymers synthesis rate, the quality andquantity of PHAs, and the cost of downstreamprocesses [22]. Although more than 300 differentmicroorganisms synthesize PHAs, only a few-such asCupriavidus necator (formerly known as Ralstoniaeutropha or Alcaligenes eutrophus), Alcaligeneslatus, Azotobacter vinelandii, Pseudomonas oleo-vorans, Paracoccus denitrificans, Protomonasextorquens, and recombinant Escherichia coli-areable to produce sufficient PHA for large-scaleproduction [22].
There are different approaches and pathways forthe synthesis of PHAs. Zimm et al. [23] distinguishedfour biosynthetic approaches to produce PHA: invitro via PHA-polymerase catalyzed polymerization,and in vivo with batch, fed-batch, and continuous(chemostat) cultures.
The biosynthetic pathway of P(3HB) in Alcali-genes eutrophus (now renamed Ralstonia eutropha)consists of three enzymatic reactions catalyzed bythree different enzymes [16,18] (Fig. 10.9).
The first reaction consists of the condensation oftwo acetyl-CoA molecules into acetoacetyl-CoA byb-ketoacyl-CoA thiolase. The second reaction is thereduction of acetoacetyl-CoA to (R)-3-hydrox-ybutyryl-CoA by an NADPH-dependent acetoacetyl-CoA dehydrogenase. Lastly, the (R)-3-hydrox-ybutyryl-co-A monomers are polymerized into PHBby P(3HB) polymerase.
Homopolymer PHB is a brittle, crystalline ther-moplastic and undergoes thermal decomposition justat its melting point, thus making processing difficultand limiting its commercial usefulness. Therefore,extensive efforts have been directed toward synthesisof copolymers that have better properties than PHB.Zeneca (formerly Imperial Chemical Industries(ICI)) has developed the PHB copolymer PHBValsoknown as Biopol, which is less stiff and less brittle
O
CH2OH
O
198 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
than homo-polymer PHB. The ratio of HB to HVmonomer can be varied by changing the glucose topropionic acid ratio. By increasing the ratio of HV toHB, the melting temperatures are lower andmechanical properties are improved. In 1996 Zenecasold its Biopol business to Monsanto, and then in2001 Metabolix acquired Monsanto Biopol tech-nology. Recently, Metabolix began work on a $15million program, supported by the US Department ofEnergy, to produce PHAs in high yield from nativeAmerican prairie grass. In 2006 Metabolix and ADMestablished a joint venture, Telles, to sell PHA-basedbioplastics under trade name Mirel in the UnitedStates, Europe, and other countries. Mirel isa product of corn sugar fermentation with proprietarygenetically engineered bacteria.
Another company, Procter & Gamble, has directedefforts into development and commercialization ofa variety of PHA copolymers under the name Nodax.The Nodax� family of copolymers are poly(3-hydrox-ybutyrate-co-3-hydroxyalkanoate)s with a copolymercontent varying from 3 to 15 mol% and chain lengthfrom C7 up to C19 [20]. In 2003 Procter & Gamblelicensed recovery and processing routes for PHAs to theJapanese company Kaneka Corporation. The compa-nies have a joint agreement to commercialize theNodaxfamily of PHAs, made from corn or sugar beet andvegetable oils.
Commercially available PHAs are given in Table10.6.
OH
OH
n
Figure 10.10 General structure of starch.
10.1.3 Thermoplastic StarchdTPS
Starch, the storage polysaccharide of cereals,legumes and tubers, is a renewable and widely
available raw material, being the end product ofphotosynthesis. Starch is composed of a mixture oftwo substances, an essentially linear polysaccharide,amylose, and a highly branched polysaccharide,amylopectin (Figs 10.10 and 10.11).
Both forms of starch are polymers of a-D-glucose.The ratio of both forms varies according to thebotanical origin of the starch. Natural starchescontain 15e30% amylose and 85e70% amylopectin[24]. Both amylose and amylopectin have a distribu-tion of sizes with different average numbers (degreeof polymerization) of glucose residues. The averagenumber of glucose residues for amylose can varyfrom 250 to 5000, and the average number of glucoseresidues for amylopectin can vary from 10,000 to100,000.
Amylose is a relatively long, linear a-glucancontaining around 99% (1/4)-a- and 1% (1/6)-a-linkages [25]. Amylose has a molecular weight ofapproximately 1 � 105 to 1 � 106, a degree ofpolymerization (DP) by number (DPn) of 324e4920with around 9e20 branch points equivalent to 3e11chains per molecule. Amylopectin is a much largerthan amylose with a molecular weight of 1 � 107to 1� 109 and a heavily branched structure built fromabout 95% (1/4)-a- and 5% (1/6)-a-linkages.The DPn is typically within the range 9600e15,900.
Origin Website
USA www.metabolix.com
tabolix) USA www.mirelplastics.com
&G Japan www.nodax.com
Gas Japan www.mgc.co.jp
Germany www.biomer.de
China www.tienan-enmat.com
trial S.A. Brasil www.biocycle.com.br
O
OH
OH
CH2OH
OH
O
HO
OH
OH
CH2OH
O
O
OH
OH
CH2OH
O
n
(a)
(b)
OOHO
HOH
CH2OH
H H H
OO
HO
HOH
CH2
H H H
OO
HO
HOH
CH2OH
H H H
O
H
H
H
O
O
HO
H
OH
CH2OH
HH
H
O
H
- (1.6) - (1.4)
Figure 10.11 Schematic structure of (a) amylose
and (b) amylopectin.
(a)
(b)
(c)
Figure 10.12 Optical micrographs of starch
granules: (a) potato, (b) wheat, and (c) maize.
Table 10.7 Diameter and Gelatinization Tempera-ture of Starch Granules [26]
SourceMeanDiameter,mm
GelatinizationTemperature,�C
Corn 15 62e71
Wheat 20e22 53e64
Rice 5 65e73
White potato 33 62e68
Sweet potato 25e50 82e83
Tapioca 20 59e70
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 199
The size, shape, and morphology of the starchgranules are characteristic of the particular botanicalsource (Fig. 10.12). Starch granules, typicallyranging in size from 2 to 30 mm, depending on theplant origin, are partially crystalline and insoluble incold water (Table 10.7).
The conventional processing of starch, includingfood processing and processing to produce pastes,thickeners and adhesives, is in the presence of heatand excess water [27].
In 1980s a breakthrough occurred by processingstarch at approximately its natural water content(15%) in a closed volume at temperatures above100 �C. Using conventional injection molding,glassy, amorphous, TPS polymers (Tg 60 �C) wereobtained with moduli similar to those of poly-propylene and high-density polyethylene.
TPS can be produced from native starch usinga swelling or plasticizing agent while applying a drystarch in compound extruders without adding water.When starch with a water content higher than 5% isplastified or pasted under pressure and temperature,a destructured starch is always formed. In theproduction procedure of TPS, the mainly water-freeraw material is homogenized and melted in anextrusion process with a plastifing material. Severalplasticizers have been studied, including water,glycerol, sorbitol, glycol, poly(ethylene glycol), urea,glucose, maltose, as well as melt-flow accelerators,
200 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
such as lecithin, glycerol monostearate, and calciumstearate [28].
The glass temperature of starch-containing mate-rials is a function of plasticizer content. Dependingon the processing conditions and plasticizer content,thermomechanical processing of granular starch withthe aid of plasticizers and melt-flow acceleratorsgives a complex starch plastic material. This iscomposed of residual swollen granular starch,partially melted, deformed and disrupted granules,completely molten starch, and recrystallized starch.The degree of disruption and melting of the variousgranular starches is regulated by the plasticizercontent and by the processing parameters (shearstress, melt viscosity, and temperature).
The starch destructurization is defined as a partialfragmentation of the crystalline structure within thepolysaccharides. By the transformation of nativestarch materials to highly amorphous thermoplastics,the compounded TPS formulation is remeltable andextrusion or injection molding is processable byrenewing the energy input. Native starches can bedestructurized within co-rotating twin screw extrudersystems by a controlled feeding of suitable destruc-turization additives (water, glycerol) in combinationwith defined operating parameters [29].
Various mature technologies for processingconventional polymers, such as film/sheet extrusion,foaming extrusion, injection and compressionmolding, and casting, as well as new techniques likereactive extrusion, have been used to produce starch-based polymers [30]. However, as the processing ofstarch is much more difficult than conventionalpolymers, modifications to traditional processingtechniques, carefully controlled processing condi-tions and the judicious use of additives have beenused to overcome the various challenges presented in
Table 10.8 Commercially Available Starch-Based Polym
Trade Name Structure Supplier
Solanyl Starch based Rodenburg Biopolymers
Bioplast TPS TPS Biotem
EverCorn Starch based Japan Corn Starch
Plantic Starch based Plantic Technologies
Biopar Starch based BIOP BiopolymerTechnologies AG
Placorn Starch based Nihon Shokuhin Kako
the processing of starch-based polymers. Theachievements in this area are reviewed in Ref. [30].The processing of starch is much more complicatedand difficult to control than for conventional poly-mers, due to the unsatisfactory processing propertiesas a result of its unique phase transitions, highviscosity, water evaporation, fast retrogradation, etc.However, with proper formulation development andsuitable processing conditions, many of these chal-lenges can be overcome. Formulation developmentsinclude the following:
� Adding appropriate plasticizers;
� Adding appropriate lubricants;
� Using modified starch in which the hydroxylshave been replaced with ester and ether groups(e.g., carboxymethyl starch and hydroxypropy-lated starch);
� Blending starch with a hydrophobic polymer(e.g., PLA, PCL, or cellulose) in the presenceof an appropriate compatibilizer (oftena starch-graft-copolymer grafted with the hydro-phobic polymer);
� Using copolymers of starch-graft-hydrophobicpolymer, such as starch-graft-PLA, starch-graft-PCL, etc;
� Blending starch with a nanoclay to form starchnanocomposites.
Blends or composites materials have beenproduced by the processing of starch with biode-gradable polymers such as PCL, PLA, PVA, PHBV,and PEA. The most common are Mater-Bi fromNovamont and Ecostar from National Starch.Commercially available starch-based polymers arelisted in Table 10.8.
ers
Origin Website
The Netherlands www.biopolymers.nl
Germany www.biotec.de
Japan www.japan-cornstarch.com
Australia www.plantic.com.au
Germany www.biopag.de
Japan www.nisshoku.co.jp
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 201
10.2 Other Compostable Polymersfrom Renewable Resources
10.2.1 Cellulose
Cellulose, the most abundant organic compoundon earth, is the major structural component of the cellwall of higher plants [24]. It is a major component ofcotton (95%), flax (80%), jute (60e70%), and wood(40e50%). Cellulose pulps can be obtained frommany agricultural by-products such as sugarcane,sorghum bagasse, corn stalks, and straws of rye,wheat, oats, and rice.
Cellulose is a polydisperse linear polysaccharideconsisting of b-1,4-glycosidic linked D-glucose units(so-called anhydroglucose unit) (Fig. 10.13).
The consequence of the supra-molecular structureof cellulose is its insolubility in water, as well as incommon organic liquids [24,31]. Poor solubility incommon solvents is one of the reasons why celluloseis converted to its cellulose esters. Another reason isthat cellulose is not melt-processible, because itdecomposes before it undergoes melt flow [32].
Cellulose esters have been commercially impor-tant polymers for nearly a century, and have founda variety of applications, including solvent-bornecoatings, separation, medical and controlled release
applications as well as composites and laminates, andplastics.
The most common cellulose esters comprisecellulose acetate (CA), cellulose acetate propionate(CAP), and cellulose acetate butyrate (CAB). Theyare thermoplastic materials produced through ester-ification of cellulose. Different raw materials such ascotton, recycled paper, wood cellulose, and sugar-cane are used to make the cellulose ester biopolymersin powder form. Bioceta, plasticized celluloseacetate, is prepared from cotton flakes and wood pulpthrough an esterification process with acetic anhy-dride. Cellulose acetate propionate (CAP) andcellulose acetate butyrate (CAB) are mixed estersproduced by treating cellulose with appropriate acidsand anhydrides in the presence of sulfuric acid.
Cellulose-based polymers are given in Table 10.9.
10.2.2 Chitosan
Chitin (poly(N-acetyl-D-glucosamine)) representsthe second most abundant polysaccharide aftercellulose. It is found in the exoskeleton of crusta-ceans and insects and in the cell wall of fungi andmicroorganisms [33]. Arthropod shells (exoskele-tons), the most easily accessible sources of chitin,contain 20e50% of chitin on a dry basis. Wastes ofseafood processing industries are used for thecommercial production of chitin.
The structure of chitin is essentially the structureof cellulose, with the hydroxyl group at C-2 of theD-glucopyranose residue substituted with an N-ace-tylamino group [24] (Fig. 10.14).
Chitosan, poly-b(1,4)-2-amino-2-deoxy-D-gluco-pyranose, is the deacetylated product of chitin(Fig. 10.15).
able
Origin Website
(formerlyialties-UCB)
UK www.innoviafilms.com
USA www.eastman.com
Italy www.mazzucchelli1849.it
Germany www.albis.com
OHO
AcNH OH
OH
OHO
AcNHO
OH
OHO
AcNHO
OH
OHO
AcNHO
OH
HO
nchitin
Figure 10.14 Schematic structure of chitin.
OHO
NH2 OH
OH
OHO
AcNHO
OH
OHO
NH2O
OH
OHO
NH2O
OH
HO
n
where m is <<< nchitosan
Figure 10.15 Schematic structure of chitosan.
N C C
H H
ORn
Figure 10.16 Schematic structure of proteins.
202 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
Chitosan is composed of glucosamine (2-amino-2-deoxy-glucopyranose) and N-acetyl glucosamine(2-acetamido-2-deoxy-glucopyranose) linked in ab (1,4)-manner; the glucosamine to N-acetylglucosamine ratio being referred to as the degree ofdeacetylation [34]. Depending on the source andpreparation procedure, its molecular weight mayrange from 300 to over 1000 kD with degrees ofdeacetylation from 30 to 95%.
Chitosan is obtained on an industrial scale by thealkaline deacetylation of chitin [33,34]. The maincommercial sources of chitin are shells of shellfish(mainly crabs, shrimps, lobsters, and krills) as wastesof the seafood processing industry. Basically, theprocess consists of deproteinization with a diluteNaOH solution, demineralization with a dilute HClsolution and decoloration of the raw shell material.Chitin is obtained as an almost colorless to off-whitepowdery material. Chitosan is produced by deacety-lating chitin using 40e50% aqueous alkali at100e160 �C for a few hours. The resultant chitosanhas a degree of deacetylation up to 0.95.
Chitosan has been found to be nontoxic, biode-gradable, biofunctional, biocompatible, and was
Table 10.10 Chitosan Producers in E
Structure Supplier Origin
Chitosan France Chitine Franc
Chitosan Nova Matrix Norwa
Chitosan Primex Icelan
Chitosan Heppe GmbH Germ
reported by several researchers to have strong anti-microbial and antifungal activities. Thus, chitosan-based films have attracted serious attention in foodpreservation and packaging technology. The poten-tial of chitosan as ingredient for active bio-basedfilms production and the different methods used forchitosan-based films preparation and their perspec-tives in the modern food packaging technology aresummarized in Ref. [35].
There are many producers of chitin and chitosanworldwide; Table 10.10 gives producers found inEurope.
10.2.3 Proteins
A protein is considered to be a random copolymerof amino acids. A generic protein monomeric unit isgiven in Fig. 10.16, where R represents the side chainof an amino acid. Proteins can be divided intoproteins from plant origin (e.g., gluten, soy, pea, andpotato) and proteins from animal origin (e.g.,collagen (gelatin), casein, silk, keratin, whey).
Potential candidates for use in the fabrication ofbiodegradable films include soy proteins, wheatgluten, corn proteins, myofibrillar proteins from fish,and pea proteins [36,37]. Proteins are considered asstructured heteropolymers [37]. Two classes ofproteins can be distinguished, globular or pseudo-globular proteins such as globulins or gliadins andfibrous or “polymerized” proteins such as collagen orglutenins.
Gluten is a mixture of monomeric proteins (glia-dins) and polymerized proteins (glutenins) linkedthrough intermolecular disulfide bridges. Gluten isthe main storage protein in wheat. In general, gluten-based plastics require the addition of plasticizer
urope
Website
e www.france-chitine.com
y www.novamatrix.biz
d www.primex.is
any www.biolog-heppe.de
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 203
agents. Hydrophilic compounds (water, polyols,oligosaccharides) and lipidic compounds (waxes,oils, fatty acids, monoglycerols) are used as proteinplasticizersdthe most frequently used is glycerol[38,39]. Plasticizers decrease the protein interactionsand increase polymer chain mobility and intermo-lecular spacing, decreasing also the glass transitiontemperature of proteins.
Soy protein-based plastics are another group ofbiodegradable, environmentally friendly polymermaterials from an abundantly renewable resource[40e42]. There are several types of soybean productsthat can potentially be utilized for engineeringstructural applications [40].
Two processes are currently used to prepareprotein-based films: the wet method (“casting”),which involves the solubilization of protein anda plasticizer in a solvent followed by the formation ofa protein network on evaporation of the solvent; andthe dry method, which is based on thermoplasticcharacteristics of proteins and combines the use ofpressure and heat to plasticize protein chains [36,43].Dehulled soybean, after solvent defatting and mealgrinding, becomes a fat-free, low-fiber soy flour(48.5% protein). The soy flour, after leaching out ofthe water/alcohol soluble sugars, is termed soyprotein concentrate (above 65% protein). The soyprotein concentrate, if it is further extracted by alkaliand reprecipitated by acidification, becomes thepurest commercially available soy protein isolate(above 90% protein).
Vegetable and animal proteins have been used inmany nonfood applications, but despite the potential,protein-based plastics have not yet made significantprogress in commercialization at a large scale.
Aliphatic polyesters are the representatives ofsynthetic biodegradable polymers.
Synthetic biodegradable polyesters are generallymade by the polycondensation method and raw mate-rials are obtained from petrochemical feed stocks.Aliphatic polyesters such as poly(butylene succinate)and poly(3-caprolactone) are commercially produced.Besides these aliphatic polyesters, various types ofsynthetic biodegradable polymers have been designed[44]. They are, e.g., poly(ester amide)s, poly(ester
carbonate)s, poly(ester urethane)s, etc. Most of themare still at a premature stage.
The traditional way of synthesizing polyesters hasbeen by polycondensation using diols and a diacid(or an acid derivative), or from a hydroxy acid[44,45].
Polycondensation can be applicable for a varietyof combinations of diols and diacids, but it requires,in general, higher temperature and longer reactiontime to obtain high-molecular-weight polymers. Inaddition, this method suffers from such shortcomingsas the need for removal of reaction by-products anda precise stoichiometric balance between reactiveacid and hydroxy groups. The ROP of lactones,cyclic diesters (lactides and glycolides), is an alter-native method, which can be carried out under milderconditions to produce high-molecular-weight poly-mers in a shorter time. Furthermore, recent progressin catalysts has enabled the production of polyestersof controlled chain lengths.
Recently, enzyme-catalyzed polymer synthesishas been established as another approach to biode-gradable polymer preparation [46e48].
10.3.1 Aliphatic Polyestersand Copolyesters
One of the most promising polymers in this familyis poly(butylene succinate) (PBS), which is chemi-cally synthesized by the polycondensation of 1,4-butanediol with succinic acid (Fig. 10.17). High-molecular-weight PBS is generally prepared bya coupling reaction of relatively low-molecular-weight PBS in the presence of hexamethylene dii-socyanate as a chain extender.
Bionolle is produced through the poly-condensation reaction of glycols such as ethyleneglycol and butanediol-1,4, and aliphatic dicarboxylicacids such as succinic and adipic acid used as prin-cipal raw materials [49]. Aliphatic polyesters,trademarked “Bionolle,” such as polybutylenesuccinate (1000 series), polybutylene succinate adi-pate copolymer (3000 series), and polyethylenesuccinates (6000 series), with high molecularweights ranging from several tens of thousands toseveral hundreds of thousands, were invented in 1990and produced through the polycondensation reactionof glycols with aliphatic dicarboxylic acids andothers.
Commercially available aliphatic polyesters andcopolyesters are given in Table 10.11.
O (CH2)4 O C (CH2)2 C
OO
n
Poly(butylene succinate) PBS
O
O
(CH2)4
(CH2)2 (CH2)2
(CH2)4O
O
C
C
(CH2)2 C O O C (CH2)4 C
OOOO
y
Poly(butylene succinate adipate) PBSA
C
OO
n
Poly(ethylene succinate) PES
Poly(ethylene succinate adipate) PESA
x
O (CH2)2 (CH2)2O C (CH2)2 C O O C (CH2)4 C
OOOO
yx
Figure 10.17 Aliphatic polyesters and
copolyesters.
204 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
10.3.2 Aromatic Polyestersand Copolyesters
While the biological susceptibility of manyaliphatic polyesters has been known for many years,aromatic polyesters such as polyethylene tere-phthalate (PET) or polybutylene terephthalate areregarded as nonbiodegradable [50]. To improve theuse properties of aliphatic polyesters, an attempt wasmade to combine the biodegradability of aliphaticpolyesters with the good material performance ofaromatic polyesters in novel aliphaticearomaticcopolyesters (Fig. 10.18).
Using standard polycondensation techniques,copolyesters with molar masses in a range necessaryfor technical application were obtained [51,52]. Thebest results with regard to the use properties wereachieved with a combination of 1,4-butanediol,adipic acid, and terephthalic acids.
Commercially available aromatic copolyesters aregiven in Table 10.12.
Poly(trimethylene terephthalate) (PTT) is a lineararomatic polyester produced by polycondensation of1,3-propanediol (trimethylene glycol or PDO) witheither purified terephthalic acid (PTA) or tri-methylene terephthalate (Fig. 10.19).
While both these monomersdthe diacid and thediol componentdare conventionally derived frompetrochemical feedstocks, DuPont, Tate & Lyle,and Genencor have recently succeeded in intro-ducing PDO using an aerobic bioprocess withglucose from corn starch as the feedstock, openingthe way for bulk production of PTT from abio-based monomer.
The natural fermentation pathway to PDOinvolves two steps: yeast first ferments glucose toglycerol, then bacteria ferment this to PDO. In thebioprocess developed by DuPont, dextrose derivedfrom wet-milled corn is metabolized by geneti-cally engineered Escherichia coli bacteria andconverted within the organism directly to PDO viaan aerobic respiration pathway (Fig. 10.20). ThePDO is then separated from the fermentation brothby filtration, and concentrated by evaporation,followed by purification and distillation. The PDOis then fed to the polymerization plant. PTT can beproduced by transesterification of dimethyl tere-phthalate (DMT) with PDO, or by the esterifica-tion route, starting with PTA and PDO (Fig. 10.21)[20]. The polymerization can be a continuousprocess and is similar to the production of PET. Inthe first stage of polymerization, low-molecular-
Table 10.11 Commercially Available Aliphatic Polyesters and Copolyesters
Mitsubishi Chemical Japan http://www.dia-chem.co.jp/en/products/gspla/index.html
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 205
weight polyester is produced in the presence ofexcess PDO, with water of esterification (in thecase of PTA) or methanol (in the case of DMT)being removed. In the second stage, poly-condensation, chain growth occurs by removal ofPDO and remaining water/methanol. As chaintermination can occur at any time (due to thepresence of a monofunctional acid or hydroxylcompound), both monomers must be very pure. Asthe reaction proceeds, removal of traces of PDObecomes increasingly difficult. This is compen-sated for by having a series of reactors operatingunder progressively higher temperatures and lower
O O OC (CH2)4 C C C
OOOO
m n o(CH2)4
Figure 10.18 Aromatic copolyesters.
pressures. In a final step, highly viscous moltenpolymer is blended with additives in a static mixerand then pelletized.
PCL was one of the earliest polymers synthe-sized by the Carothers group in the early 1930s.Poly-(3-caprolactone) is a linear polyester manu-factured by ROP of a seven-membered lactone, 3-caprolactone (Figs 10.22 and 10.23). Catalysts suchas stannous octoate are used to catalyze the poly-merization and low-molecular-weight alcohols canbe used to control the molecular weight of thepolymer.
Anionic, cationic, coordination, or radical poly-merization routes are all applicable [53,54].Recently, enzymatic catalyzed polymerization of
HO(CH2)3O O C C O (CH2)3 H
O O
n
Figure 10.19 Schematic structure of PTT.
Table 10.12 Commercially Available Aromatic Copolyesters
206 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
3-caprolactone has been reported [47]. It is a semi-crystalline polymer with a degree of crystallinityaround 50%. It has a rather low glass transitiontemperature (�60 �C) and melting point (61 �C).
PCL was recognized as a biodegradable andnontoxic material, and a promising candidate forcontrolled release applications, especially for long-term drug delivery. The superior rheological andviscoelastic properties over many of its aliphaticpolyester counterparts renders PCL easy to manu-facture and manipulate into a large range of
O
H
H
OH
OHH
OHHO
H
HOCH2
H
glucose
Figure 10.20 Biotechnological route to
1,3-propanediol.
implants and devices. The application of PCL asa biomaterial over the last two decades focusing onmedical devices, drug delivery, and tissue engi-neering was reviewed in Ref. [55]. PCL may becopolymerized with many other lactones, such asglycolide, lactide, d-valerolactone, 3-decalactone,poly(ethylene oxide), and alkyl-substituted 3-capro-lactone. Blends of PCL with other biodegradablepolymers such as PHB, PLA, and starch have beenprepared. Commercially available PCLs are listed inTable 10.14.
10.3.4 Poly(esteramide)dPEA
Polyesteramide BAK 1095 is based on capro-lactam (Nylon 6), butanediol, and adipic acid; BAK2195 is based on adipic acid and hexamethylene-diamine (Nylon 6,6) and adipic acid with butanedioland diethylene glycol as ester components [56]. The
HC
CH2 OH
OH
CH2 OH H2C
CH2
H2C OH
OH
E. coli (GM)Enzymatic conversions
glycerol 1,3-propanediol
Table 10.14 Commercially Available PCL Polymers
Trade Name Supplier Origin Website
Tone Union Carbide USA www.unioncarbide.com
CAPA Perstorp UK www.perstorp.com
Placcel Daicel Chemical Indus. Japan www.daicel.co.jp/english/kinouhin/category/capro.html
CH2
CH2
CH2 OH
1,3-Propanediol(PDO)
+ HO C C OH
OO
Purified terephthalic acid (PTA)- Water- PDO
HO CH2 CH2 CH2 O C
O
C O CH2 CH2 CH2 OH
O
nPoly(trimethylene terephthalate) (PTT)
O C C O
OO
+ H3C CH3
- Methanol- PDO
Dimethyl terephthalate (DMT)
OH
Figure 10.21 Manufacturing routes to PTT.
Table 10.13 PTT Polymers
Trade Name Supplier Origin Website
Sorona� DuPont USA www.dupont.com
Corterra� Shell Canada www.shellchemicals.com
PermaStat RTP USA www.rtpcompany.com
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 207
production process is solvent and halogen free.Commercially available PEAs are detailed inTable 10.15 and the structure of PEAs is shown inFig. 10.24.
10.3.5 Poly(vinyl alcohol)dPVA
Poly(vinyl alcohol) (PVA) (Fig. 10.25) is thelargest volume water-soluble polymer producedtoday. PVA is not produced by direct polymerization
O (CH2)5 C
O
n
Figure 10.22 Structure of PCL.
of the corresponding monomer, since vinyl alcoholtends to convert spontaneously into the -enol form ofacetaldehyde, driven by thermodynamic reasons andwith extremely limited kinetic control [57]. PVA isattained instead from the parent homopolymerpoly(vinyl acetate) (PVAc). The polymerization ofvinyl acetate occurs via a free-radical mechanism,usually in an alcoholic solution (methanol, ethanol)
O
O
ROPO
O
n
Figure 10.23 Schematic route to PCL.
CH2 CH
OH
Figure 10.25 Schematic structure of PVA.
Table 10.15 Commercially Available PEAs
Tradename Supplier Origin Website
BAK1 Bayer AG Germany www.bayer.com
1In 2002 the production was suspended.
C (CH2)4 C O (CH2)4 O C (CH2)5 NH
O OO
n
Figure 10.24 Schematic structure of PEAs.
OH
O+
catalyst
O
initiatorO O
poly(vinyl acetate)
CH3OHcatalyst
OH
+
O
O
poly(vinyl alcohol)
Figure 10.26 Manufacturing route to PVA.
208 HANDBOOK OF BIOPOLYMERS AND BIODEGRADABLE PLASTICS
although for some specific applications a suspensionpolymerization can be used. The scheme for indus-trial production of PVA is given in Fig. 10.26.
PVA is produced on an industrial scale by hydro-lysis (methanolysis) of PVAc, often in a one-potreactor. Different grades of PVA are obtaineddepending upon the degree of hydrolysis (HD).Polymerization reactions can be carried out in batch
Table 10.16 PVA Producers
Trade Name Supplier
Mowiol Clariant GmbH
Erkol Erkol SA
Sloviol Novacky
Polyvinol Vinavil SpA
Elvanol DuPont
Cevol Celanep
Airvol Air Products
KurarayPoval
Kuraray Co. Ltd
Unitika Poval Unitika Ltd
Gohsenol Nippon GohseidThe Nippon SyntheticChemical Industry Co. Ltd
Hapol Hap Heng
or in continuous processes, the latter being usedmostly for large-scale production. In the continuousindustrial process, the free-radical polymerization ofvinyl acetate is followed by alkaline alcoholysis ofPVAc. The molecular weight of PVAc is usuallycontrolled by establishing the appropriate residencetime in the polymerization reactor, vinyl acetate feedrate, solvent (methanol) amount, radical initiatorconcentration, and polymerization temperature.
The main producers of PVA are given in Table10.16.
10.3.6 Blends
One of the strategies adopted in producing com-postable polymer materials is blending of biode-gradable polymers. Blending is a common practice in
Origin Website
Germany www.cepd.clarinet.com
Spain www.erkol.com
Slovakia www.nchz.sk
Italy www.mpaei.it/it/vinavil/home.htm
USA www.dupont.com/industrial-polymers/elvanol/index.html
USA www.celanesechemicals.com
USA
Japan www.kuraray.co.jp/en
Japan www.unitika.co.jp/e/home_e2.htm
Japan www.nippongohsei.com/gohsenol/index.htm
China
Table 10.17 Commercially Available Blends
Trade Name Supplier Origin Website
Mater-Bi Novamont Italy www.novamont.com
Ecostar National Starch USA www.nationalstarch.com
Ecofoam National Starch USA www.nationalstarch.com
Ecoflex (blends of Ecoflex and PLA) BASF Germany www.bioplastics.basf.com
Fasal (cellulose based) Austel 1 IFA Austria www.austel.at
Cereplast Cereplast, Inc. USA www.cereplast.com
COMPOSTABLE POLYMER MATERIALS: DEFINITIONS, STRUCTURES, AND METHODS OF PREPARATION 209
polymer science to improve unsatisfactory physicalproperties of the existing polymer or to decrease cost.By varying the composition and processing ofblends, it is possible to manipulate properties. Theleading compostable blends are starch-based mate-rials. The aim is to combine the low cost of starchwith higher cost polymers having better physicalproperties. An example of such material is Mater-Bimanufactured by Novamont [58]. Mater-Bi isprepared by blending starch with other biodegradablepolymers in an extruder in the presence of water orplasticizer. The following three main classes ofMater-Bi are commercially available (see also Table10.17):
� Class ZdTPS and PCL;
� Class YdTPS and cellulose derivatives;
� Class VdTPS more than 85%.
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