polymers based on renewable raw materials – part ii

Polymers Based on RenewableRaw Materials – Part II

S. Jovanoviæ,a* J. V. DÞunuzoviæ,b and Ý. Stojanoviæc

� Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11 120 Belgrade, Serbia� Institute of Chemistry, Technology and Metallurgy (ICTM) – Center of Chemistry,

University of Belgrade, Studentski trg 12–16, 11 000 Belgrade, Serbia� Henkel Serbia, AG General Industry, Henkel Adhesive Technologies,

Bulevar Osloboðenja 383, 11 040 Belgrade, Serbia

A short review of biopolymers based on starch (starch derivatives, thermoplastic starch), ligninand hemicelluloses, chitin (chitosan) and products obtained by degradation of starch and otherpolysaccharides and sugars (poly(lactic acid), poly(hydroxyalkanoates)), as well as some of theirbasic properties and application area, are given in this part. The problem of environmental andeconomic feasibility of biopolymers based on renewable raw materials and their competitive-ness with polymers based on fossil raw materials is discussed. Also pointed out are the prob-lems that appear due to the increasing use of agricultural land for the production of rawmaterials for the chemical industry and energy, instead for the production of food for humansand animals. The optimistic assessments of experts considering the development perspectivesof biopolymers based on renewable raw materials in the next ten years have also been pointedout.

At the end of the paper, the success of a team of researchers gathered around the experts from thecompany Bayer is indicated. They were the first in the world to develop a catalyst by which theymanaged to effectively activate CO� and incorporate it into polyols, used for the synthesis ofpolyurethanes in semi-industrial scale. By applying this process, for the first time a pollutant willbe used as a basic raw material for the synthesis of organic compounds, which will have significantconsequences on the development of the chemical industry, and therefore the production ofpolymers.

Key words: Biopolymers, renewable raw materials, properties, application

Introduction

Raw materials containing carbon are essential to thechemical industry for the production of organic substances.In the past fifty years, this demand was fulfilled mostly byfossil raw materials (oil, natural gas and coal), while onlydeveloped countries used around 10 % of renewable rawmaterials of plant and animal origin.

The constantly growing prices of fossil raw materials, par-ticularly of oil and natural gas, as well as the harmful effectsof their use on the environment have prompted growinginterest in the use of renewable raw materials of plant andanimal origin for the production of polymers and otherchemicals. Literature describes numerous natural macro-molecular substances – renewable raw materials, whichmay be transformed directly or after appropriate physical,chemical or biochemical modifications into biodegradableor non-biodegradable polymers and low molecular weightchemicals.

Apart from being used for the production of food forhumans and animals, as well as chemicals and polymers in

the chemical industry, renewable raw materials, i.e. bio-mass, is increasingly being used for the production ofenergy. Because of the competition in the use of agricul-tural land, a significant increase in food prices hasoccurred. Taking into account the rapid growth of theglobal population, and the associated need to increasefood production, researchers around the world havealready started to question the thesis that the amount ofbiomass produced per year is sufficient to meet the needsof producers of food, chemical products and energy, andwhether it is possible in the foreseeable future to com-pletely replace fossil raw materials in the chemical industrywith renewable raw materials. Bearing in mind that the useof renewable raw materials for the production of polymericmaterials is economically and environmentally the mostacceptable, due to their possible reuse and recycling, it isexpected that their production from renewable raw mate-rials will prevail compared to the production of energy,especially as there are other alternative procedures forenergy production.

Part I of this paper indicates the basic definitions of bio-polymers, the raw materials for their production, as well asdescribes their application areas, existing production ca-pacities, and the properties of cellulose from wood andannual plants, as the most important renewable raw ma-

S. JOVANOVIÆ et al.: Polymers Based on Renewable Raw Materials – Part II, Kem. Ind. 62 (9-10) 315–326 (2013) 315

KUI – 19/2013Received September 14, 2012Accepted December 11, 2012

�Corresponding author: Prof. dr. Slobodan Jovanoviæe-mail: [email protected]

terial for the chemical industry.� Part II describes onlyselected examples of renewable raw materials and biode-gradable polymers based on these raw materials, whichalready have their place in the market of polymer materialsor have a realistic chance of finding a place in the market inthe near future. The environmental and economic feasibil-ity of the application of biodegradable polymers, and theimpact of using biomass for biopolymer production onfood production are discussed, as well as the perspectivesof production development and application of biodegrad-able biopolymers based on renewable raw materials.

The most commonly used biopolymers basedon renewable raw materials

Starch and starch derivatives

Green plants such as potato, maize (corn), rice, and cas-sava are raw materials for production of biopolymers. Themain component of these plants is starch. Starch is consid-ered a potential polymer to be used in biodegradablematerials because of its low cost, availability, and becauseit is produced from renewable resources. Since it is not athermoplastic material, it has been used to produce blendswith non-biodegradable polymers, biodegradable materi-als, and in injection molding.�

Starch is a mixture of two D-glucan homopolymers, com-posed of �-D-glucopyranosyl units (AGU), namely, amyloseand amylopectin (Fig. 1). These two parts of starch have adifferent degree of branching. Amylose is essentially a li-near polymer consisting of AGU, although some (1-6)-lin-kages are thought to exist at about every 180–400 units.�Amylopectin is a highly branched form of amylose, con-sisting of (1-4)-linked AGU and a significant portion of1,4,6-tri-O-substituted residues acting as (1-6)-linkagebranch points. Amylose and amylopectin are uniformly dis-tributed inside the granule. Amylose constitutes amorphousregion and amylopectin constitutes crystalline region.

Molecular weight of amylose depends on the starch origin.Generally, amylose extracted from cereals (degree of poly-merisation, DP, of about 1000) has lower molecular weightthan others (DP of about 3300). Molecular weight of amy-lose is connected with the degree of branching: cerealamylose has 3–5 branches per macromolecule, and amy-lose from sweet potato has about 10 branches per macro-

molecule.� Amylopectin is a hyperbranched component ofstarch. Average DP of the branches depends on the starchtype, and range from 19 to 31. Molecular weight of amy-lopectin is much higher than that of amylose, and has avalue of about 50 to 500 million, making it one of the big-gest macromolecules.

The amylose/amylopectin ratio varies depending on thesource of the starch and its maturity. Starch isolated fromgranules contains 20 – 30 % of amylose and 70 – 80 % ofamylopectin. Some cultivated plant varieties have pureamylopectin starch without amylose, known as waxystarches. Waxy starches have less retrogradation, resultingin a more stable paste. High amylose starch (amylomaize) iscultivated for use because of its gel strength, and as a resis-tant starch (a starch that resists digestion) in food products.Beside amylose and amylopectin, there are different quan-tities of lipids, proteins, phosphates, etc. in granules.

In the plants, starch is organised in discrete particles – gran-ules, whose size, shape, morphology, composition andsupramolecular structure depend on the botanical source.The diameters of the granules generally range from lessthan 1 �m to more than 100 �m, and shapes can be regu-lar (e.g., spherical, ovoid, or angular) or quite irregular.�

Today, the main commercially refined starches are corn-flour, tapioca, wheat and potato starch. Worldwide, drystarch production amounts to about 67 million tons.� Cur-rent annual production of primary starch sources is esti-mated to be 46.1 million tons of maize, 9.1 million tons ofcassava, 5.15 million tons of wheat, and 2.45 million tonsof potato. Starch and its derivatives are utilised in severalindustrial applications due to their low cost, availability,and ability to impart a broad range of functional propertiesto food and nonfood products.

In nonfood applications, the most traditional use of starchhas been in the paper industry (about 10 million tons peryear). Recently, with the increasing cost of cellulose pulp,paper producers have begun to seek lower-cost fillers.These fillers often require specially modified starches tobind them to the pulp. In addition to the use of starches inthe paper and textile industries and for production of newbiodegradable materials, starches can also be used in thefuel market. With new trends in the biofuel and bioethanolmarket, there has been increasing interest in some starchycrops.

316 S. JOVANOVIÆ et al.: Polymers Based on Renewable Raw Materials – Part II, Kem. Ind. 62 (9-10) 315–326 (2013)

F i g. 1 – Schematic diagram of amylose (left) and amylopectin (right) with a branch point at the O6 positionS l i k a 1 – Shematski dijagram amiloze (lijevo) i amilopektina (desno) s grananjem u poloÞaju O6

Modification of starch

Starches are modified chemically or physically or both toaccentuate their positive characteristics, diminish their un-desirable qualities or add new attributes. Common limita-tions associated with native normal starches are excessiveviscosity at low solids content (difficulty in handling, lack ofbody), high susceptibility to retrogradation (gel opacity,syneresis, and lack of freeze–thaw stability) and lack ofprocess tolerance.

Chemical modification

Chemical modification, which is the most common in-dustrial means of enhancing starch properties, requiresreaction or treatment of starch with chemical reagents tointroduce new chemical substituent groups, to influencethe molecular scission or promote oxidation or molecularrearrangements. Chemical modification of starch is basedupon hydroxyl group chemistry and a modified starch canbe defined as one whose hydroxyl groups have beenaltered by chemical reaction (e.g., by oxidation, esterifica-tion, etherification, crosslinking, etc.). The physicochemi-cal properties of such modified starch are largely depend-ent on the degree of substitution (DS), which is defined asthe average number of substituted hydroxyl groups perAGU. The maximum DS value for a single AGU is 3. Obvi-ously, this value is reduced in the case of amylopectin,since a branched AGU has a maximum theoretical DSvalue of 2.

Products that are partially depolymerised are called “con-verted starches”. There are three basic processes for ma-king converted starches:

– using acid,– using an oxidant in an alkaline system, and– by application of heat.

Acid-modified starches are obtained by adding a mineralacid (hydrochloric or sulphuric) in stirred slurry of starchgranules at temperature below the gelatinization tempera-ture of the starch. Oxidised starches are often produced bytreatment of starch with:

– a solution of sodium hypochlorite with introducing car-bonyl groups or– hydrogen peroxide and copper(II) ions without introduc-ing carbonyl groups or– ammonium persulphate to depolymerise starches oxida-tively for paper sizing and coating processes.

Ethers and esters of starch are produced by reactingstarches with monofunctional reagents. Both esters andethers are made in the same general way, using two-stepreactions. The first step represents the activation of starch.In the past, organic solvent was used as reaction medium,while activator was pyridine or similar nucleophile. Re-cently, water and/or alcohol have been used as reactionmedium and hydroxide as activator. An alkaline pH con-verts some hydroxyl groups into alkoxide ions for participa-tion in nucleophilic substitution reactions.��

Manufactured monostarch ethers include hydroxyethyl, hy-droxypropyl, carboxymethyl, and cationic starches. Reac-tion with ethylene oxide produces hydroxyethyl starch,

reaction with propylene oxide produces hydroxypropylderivatives, and reaction with sodium monochloroacetateproduces sodium carboxymethyl starch.� The productsgenerally have the same properties as the monosubstitutedstarch esters, the difference being that ethers are stable toacids and bases whereas esters are not.

As implied, cationization imparts a positive charge to thestarch chains via derivatisation with reagents containingimino, amino, ammonium, sulphonium or phosphoniummoieties. From a commercial perspective, the most in-dustrially significant and commonly utilised reagent is(3-chloro-2-hydroxypropyl)trimethylammonium chloride(CHPTMA).�� Cationic starch derivatives are typicallyemployed as wet end and surface sizing and coating add-itives in the paper industry for enhancement of sheetstrength and retention of fines, as well as industrial floccu-lants in sludge dewatering and mineral mining operati-ons.�� Values of DS for cationic starch derivatives in paper-making applications generally range from 0.02 to 0.07,while those for flocculent applications are significantlyhigher (DS � 0.2 – 0.7). Cationic starches are very efficientflocculants for removal of Hg��, Cu�� and Zn��, with thosecationic derivatives possessing primary amino groupsexhibiting the highest sorption efficiency

Starch crosslinking reactions are employed to strengthenthe structure of swollen granules upon gelatinisation,enhancing the resistance to viscosity breakdown as a resultof mechanical shear, acid conditions or high temperature.Crosslinked starch is mostly used in food application, butcrosslinked carboxymethyl starch can be used as super-absorber.��

Physical modification – thermoplastic starch (TPS)

Thermomechanical deformation of starch produces ther-moplastic starch or destructurised starch. Destructurisedstarch is described as a material obtained by thermoplasticmelt formation, by heating starch and water, with moisturemass fraction in the range from 5 to 40 %, in a closed vol-ume.�� The starch is heated above the melting and glasstransition temperatures (t�) of its components so that theyundergo an endothermic transition. As a consequence, amelting and disordering of the molecular structure of thestarch granule takes place, so that a destructurised starch isobtained. The applicability of destructurised starch is lim-ited because of the degradation of starch due to water lossat elevated temperatures. Hence, the materials can only beprocessed by the addition of water, other plasticizers ormelt flow accelerators after the granular disruption step. Inorder to overcome these problems, starch is mixed with anappropriated additive or plasticizing agent, such as glyceroland sorbitol, under conditions that yield a thermoplasticstarch melt after thermomechanical transformations.��

In general, starch films have high tensile strength, but arebrittle and exhibit almost no elongation at break. For in-stance, at 15 % water content, extruded starch has poormechanical properties and is inadequate for film applica-tions (initial tensile strength 20–30 MPa and elongation atbreak 10–15 %). Moreover, these properties are affectedby relative humidity and thus by the correspondingchanges in t�. At low relative humidity, there are problemswith brittleness, whereas at high relative humidity, with


softening. In addition to moisture sensitivity, TPS materialschange their time-dependent mechanical properties (alsocalled “postprocessing aging”). In order to overcome theselimitations, different strategies are often adopted, such as:– chemically modified starch in formulations,– suitable plasticizers,– association of biodegradable, renewable, or syntheticpolymers to thermoplastic starch, and– surface modifications of starch plastics.

Starch esterification by acetylation has been shown to bean efficient chemical modification for obtaining TPS-basedmaterials with reinforced hydrophobicity and higher ther-mal stability. Better properties are achieved with DS higherthan 1.7. The properties of starches modified by chemicalderivatisation are also some options for TPS materials, i.e.they depend greatly on DS and the length of the intro-duced alkyl chain, the higher the DS and the longer the ali-phatic chain the better are the hydrophobic properties andblends with crosslinking modified starch by hydroxylgroups and sodium trimetaphosphate.

Starch has also been widely blended with biodegradablepolymers, including polycaprolactone, poly(lactic acid),polyhydroxybutyrate and poly(hydroxyester ether).��Thermoplastic starch blended with thermoplastic polyes-ters, such as polycaprolactone, is the most popular basefilm. Commercially, TPS is used alone, mainly in solublecompostable foams, such as loose fillers and other ex-panded items as a replacement for polystyrene. These filmsare biodegradable, compostable, and fulfil the require-ments of different application fields, such as packaging andagriculture.

Lignin and hemicellulose

Beside cellulose and hemicellulose, lignin is the third basiccomponent of wood. Lignin represents a tri-dimensionalbranched and crosslinked copolymer of three basicphenol-propane derivatives, which are formed by biosyn-thesis in wood and which formulas are given in Fig. 2.

F i g. 2 – Structural formulas of three basic comonomersof lignin

S l i k a 2 – Strukturne formule tri osnovna komonomera lignina

Lignin is synthesised by dehydrated copolymerisation ofthree comonomers presented in Fig. 2. This reaction iscatalysed by peroxidase bonded at the surface of cellulosefibres, and due to that, synthesised lignin is placedbetween cellulose fibres as well. Lignin represents a matrixin which cellulose fibres are distributed. This structure issimilar to the structure of polymers reinforced with glassfibres and brings excellent mechanical properties of wood.

Lignin has a quite complex structure, which mostlydepends on the plant in which it is synthesised and themethod used for extraction from the plant. Fig. 3 shows themodel of the lignin structure obtained from coniferoustrees.

The average mass fraction of lignin in wood is around 18 –30 %. It has been estimated that plants synthesize around20 · 109 tons of lignin per year. During chemical extractionof cellulose from wood, around 50 million tons of ligninper year are obtained as a side product. At the moment,the largest amount of lignin is burned directly in factoriesfor production of cellulose fibres, in order to obtain heatenergy and for the regeneration of chemicals used in theextraction of cellulose from wood. A small amount of ligninis extracted from the parent solution, and after purificationit is obtained as powder and represents an amorphous,highly branched, and still partially crosslinked polymermaterial with t� of about 160 °C. Lignin is hydrophobicthermoplastic polymer material, resistant to pressure butnot to UV light. After adequate modification, lignin can beused: as material for production of conductive plates andother elements used in electronics and electrotechnics,heat and sound isolators, vanillin, UV – absorbers in creamsfor sun protection, additives used to improve cement qual-ity, as components for production of aminoplast, pheno-plast and polyurethanes, as tool for dispersion in biocideconfectioning, as viscosity regulator of scum in the exploi-tation of oil wells, as adhesive during the briquetting ofcoal, etc.

The term hemicellulose represents polysaccharides of plantorigin, which can be dissolved in diluted solutions of alkalisand cannot degrade under the effect of �-amylase. Theseare macromolecular substances, composed of a largenumber of monosaccharide such as hexose and uronicacids and different pentoses. The most frequent amongthem are xylans. The basic structure of hemicellulose iscomposed of �-(1,4) bonded chains of xylose. Besides that,xylans are substituted with molecules of arabinose andmethyl glucuronic acid, which are non-regularly distrib-uted along the macromolecular chains, thus contributing tothe fact that hemicellulose has no crystal structure.


F i g. 3 – Chemical structure of lignin obtained from coniferoustrees

S l i k a 3 – Kemijska struktura lignina iz drveta èetinjaèe

Up to twenty years ago, studies on the application of he-micellulose as biopolymer were insignificant. The reasonbeing that a strong degradation of hemicellulose occursduring wood processing using sulphite or sulphateprocesses, leading to the formation of a large number oflow molecular weight products (hexose, pentose, formicacid, acetic acid, some hydroxyl carbonic acid, resins,waxes, fatty acids, turpentine, etc.). Until now, theprocesses for extracting and purification of the aforemen-tioned substances, as well as procedures for mutual fer-mentation of hexose and pentose were developed in orderto obtain alcohol (ethanol, butanol, isopropanol and poly-ols such as arabitol, glycol, xylitol), organic acids (formic,acetic, butyric and lactic acid) and gases (methane andhydrogen).��

Chitin and chitosan

Chitin is a biopolymer – polysaccharide, different from cel-lulose only in one thing: the –OH group on C2 atom of glu-cose is substituted with N-acetylamino group. As shown inFig. 4, chitin represents poly(�-(1�4)-N-acetyl-2-amino-2--deoxyglucopyranose). It is formed in the shell of shellfish(crayfish and crabs and other marine animals), and insects,and can be found in the cell walls of some fungi.

F i g. 4 – Schematic representation of macromolecular seg-ments of chitin and enzymatic transformation ofchitin into chitosan

S l i k a 4 – Shematski prikaz segmenata makromolekula hitina ienzimska transformacija hitina u hitozan

In the shell of shellfishes, together with calcium carbonateand some proteins, between 40 and 60 % of chitin can bebuilt. Chitin is one of the most widespread polysaccharidesin the world.�� Chitin is mostly extracted from the shell ofcrayfish and crabs, firstly by processing with cold HCl (5 %solution) in order to dissolve calcium carbonate, and thenthe obtained powder is processed with boiled NaOH solu-tion (w = 4 %) in order to remove proteins. After bleach-ing, chitin is obtained as a white powder, insoluble inwater, diluted acids, bases, and in organic solvents. Themolar weight of chitin is between 1 and 2 � 10 g mol��. It

can be dissolved in concentrated formic acid and methanesulphonic acid. Under the influence of concentrated solu-tion of inorganic acids, chitin degrades on acetic acid andD-glucosamine. So far, chitin has limited application in thepharmaceutical and cosmetic industries and medicine.

When chitin is processed with 40 % NaOH solution athigher temperatures or with certain enzymes at room tem-perature (see Fig. 4), the deacetylation of chitin occurs.After processing with alkalis chitin has 40 % or less N-ac-etylamino groups, and is called chitosan. Chitosan is themost commonly used derivative of chitin.

Chitosan is soluble in diluted solutions of inorganic acids inwhich it acts as polycation. The molar weight of chitosan isusually between 104 and 105 g mol��. Chitosan has a moresignificant application in the pharmaceutical and cosmeticindustries and medicine than chitin. In addition, it is usedas flocculant in water purification and as ion-exchanger,which can selectively extract heavy metals from solutionscontaining magnesium, calcium, potassium, and sodium.

Biopolymers obtained by biosynthesisof renewable low molecular weightand macromolecular raw materials

Glycerol, glucose, sugars, as well as products of hydrolysisof starch, cellulose and other polysaccharides are used asraw materials for the microbiological synthesis of a largenumber of monomers and polymers, some of which arehere mentioned.

Poly(lactic acid) (PLA)

Lactic acid is a natural product that can be found in milk,sauerkraut, in human muscles, etc., but it can also bechemically synthesised from fossil raw materials. The lacticacid molecule has one asymmetric C-atom and is thereforeoptically active. Depending on the synthesis conditionsand selected microorganisms, two optical isomers of lacticacid can be formed: L-lactic acid and D-lactic acid, asshown in Fig. 5.

F i g. 5 – Molecular structure of L (left) and D (right) lactic acidS l i k a 5 – Molekularna struktura L- (lijevo) i D-mlijeène kiseline

(desno)

The largest amount of lactic acid which people use today isobtained as a side product during glucose fermentation,which is usually obtained in the same process by enzymaticdegradation of starch.� Poly(lactic acid) is a polyester whichis not formed in nature, but e.g. by polycondensation oflactic acid (Fig. 6a) or by polymerisation of lactide by thering-opening process (Fig. 6b). Direct polycondensation oflactic acid is used very rarely due to the reaction difficultiesas well as those in obtaining a stable product.

Poly(lactic acid) is produced in several stages in industry. Inthe first stage, poly(lactic acid) of low molecular weightwith degree of polymerisation e.g. up to DP = 3000 (Fig.6a) is obtained by polycondensation of monomer – lactic


acid. The water formed during this reaction – (n – 1) H�Ocan be removed from the reaction mixture by evaporation.The obtained poly(lactic acid) is then depolymerised athigh temperature. Since the water is already removed fromthe reaction mixture by evaporation, the molecules of thelactic acid cannot be formed again. Instead, two monomerresidues bond into ring-shaped-lactide dimmer. The mole-cules of lactide in the presence of catalyst (powder of tin,Sn(Oct)�) are bonded in macromolecules of polylactide(PLA) by ring-opening process (Fig. 6b).

The starting substance for the synthesis of poly(lactic acid)is lactic acid, which appears as two optical isomers (Fig. 5).According to this, during dimmer formation three forms oflactic acid dimmers can be formed: L-lactide, D,L-lactide ormeso lactide and D-lactide, as shown in Fig. 7.

The properties of PLA, which are important for its process-ing and application, depend on the structure and composi-tion of macromolecular chains. The ratio between L and D

lactic acid also has special significant.For example, during polymerisation ofonly L-lactide, isotactic polymer isobtained. During copolymerisation ofL-lactide and D,L-lactide (meso lac-tide), an amorphous product with aquite lower melting point than that ofhomopolymer and lower rate of crys-tallisation, is obtained. When PLLAand PDLA are mixed, the macro-molecular chains of these copolymersform racemic crystals with a meltingpoint of 190 – 240 °C. By mixingPDLA of low molecular weight withPLLA, the formed crystals act as nucle-uses for PLLA crystallisation andincrease its rate of crystallisation,which is very important for reducingproduction time e.g. using the injec-tion technique, and for increasingshape stability at high temperature.��Depending on the applied conditions

of polycondensation and polymerisation, the obtained PLAcan have molar weight between 40 000 and 30 0000 gmol��. Since all properties of PLA, beside macromolecularcomposition, also depend on molecular weight, the selec-tion of polymerisation conditions can have a considerableinfluence on its properties. Additionally, lactic acid can becopolymerised with a large number of monomers, leadingto copolymers that can biodegrade very fast, but also tonon-biodegradable copolymers. PLA mixes very well withsome polymers, allowing preparation of blends with quitedifferent properties, thus contributing to their increasedapplications, especially for production of packaging.

Thanks to the combination of properties such as biode-gradability and biocompatibility, PLA is applied in medi-cine for production of surgical sutures and implants. It isalso used in pharmacy. In recent years, large efforts havebeen invested in improving the properties of PLA blends,so that they can be used as technical polymers.�� PLA canbe shaped and processed applying all the techniques thatare used for the production of different subjects from syn-thetic thermoplastic polymers. For the sake of comparison,Table 1 lists only some selected properties of PHB, PP,PLLA and PDLA. (The first large collection of data on theproperties of biodegradable polymers based on renewablevs. fossil raw materials can be found at: www.materialdata-center.com (16. 11. 2010.).

Poly(hydroxyalkanoates) (PHA)

Poly(hydroxyalkanoates) represent a group of biodegrad-able aliphatic polyesters, which are obtained using ade-quate bacteria placed on groundwork containing e.g. glu-cose, saharose, starch or palm oil. The procedure for thebiosynthesis of poly(hydroxyalkanoates) based on glycerol,the side product formed during production of biodieselfrom plants oil, has also been developed.�� The propertiesof poly(hydroxyalkanoates) can be varied by changing thedegree of polymerisation, by changing side groups markedwith R, and by copolymerisation of different monomers orby blending with other biodegradable polymers. Fig. 8gives the general formula of polyalkanoates.


F i g. 6 – Two ways for the synthesis of poly(lactic acid), i.e. polylactideS l i k a 6 – Dva puta sinteze poli(mlijeène kiseline), odnosno polilaktida

F i g. 7 – Isomers of lactic acid and resulting lactide dimersS l i k a 7 – Izomeri mlijeène kiseline i rezultirajuæi laktid dimeri

From the large number of possible PHA, the widest appli-cation have homopolymers of poly(3-hydroxybutyric acid)– PHB (R = –CH�) and poly(3-hydroxyvalerianic acid) –PHV (R = –C�H), as well as copolymers of these two acids.Some plants can use bacteria Alcaligenes eutrophus andsynthesize homopolymer of hydroxybutyric acid and itscopolymer with �-hydroxyvalerianic acid and deposit as areserve food. It is interesting to mention that the mass ofthis reserve food (synthesised copolymer) can be even90 % of the dry mass of bacteria. PHB represents linear(Fig. 8), highly crystalline polyester of 3-hydroxybutyricacid (CH�-CH(OH)-CH�-COOH), with a degree of crystalli-sation up to 74 %. The physical properties of PHB are simi-lar to the properties of PP (see Table 1), with only one dif-ference – PHB is slightly stiffer and more brittle.

T a b l e 1 – Some properties of PHB, PP, PLLA and PDLA

T a b l i c a 1 – Neka svojstva polimera PHB, PP, PLLA i PDLA

Polymer

Polimert� / °C t� / °C

Tensilestrength / MPa

Vlaènaèvrstoæa / MPa

Bendingmodulus / MPa

Savojnimodul / MPa

Elongationat break / %

Prekidnoistezanje / %

PHB 4 179 40 3500 5PP –10 170 38 1700 400

PLLA 59 170 48 3250 2PDLA 53 165 35 2250 5

PHB is resistant to fats, oils and water. It is permeable togases like O�, CO� and water vapour. In the presence ofcertain microorganisms and water, PHB can biologicallydegrade completely. PHB is not only biodegradable, it isalso biocompatible. Because of these two properties, PHB,just like PLA, has significant application in medicine andthe pharmaceutical industry. Lately, PHB is used moreoften for the production of packaging, especially for foodpackaging. It is also used in agriculture and the paper in-dustry. For the aforementioned applications, PHB can beshaped and processed applying all the techniques usuallyused to manufacture products from synthetic thermoplasticpolymers.

Mechanical and thermal properties of PHB can be changedsignificantly by adding softeners of low molecular weight(glycerol, citric acid), and by copolymerisation with otherbiodegradable monomers, such as 3-hydroxyvalerianic acid.The increase in the content of 3-hydroxyvalerianic acid inthe copolymer decreases melting point and degree of crys-

tallinity, and increases impact resistance. Blends of PHBwith other biodegradable polymers have higher flexibilityand tensile strength.

Besides PLA, PHB and PHV, special attention has beendevoted to the investigation of synthesis and properties ofsome microbial polysaccharides such as: xanthan,�� dex-tran,�� and pullulan� which have practical application, andhave been available on the market for a long time.�

Polyurethanes are obtained by the reaction of polyadditionof multivalent alcohol (polyols) and di- or poly-isocyanate.Polyols and diisocyanates can be chemically or biochemi-cally obtained from plant oils and saccharides or poly-saccharides, and glycerol is obtained as a side product inbioethanol production. Polyurethanes synthesised usingthese raw materials and filled with natural fibres can beused to obtain products containing 82 % of renewable rawmaterials mass. The production of polyurethanes in theworld is around 8 million tons per year.

Some biopolymers based on renewableraw materials which have their placein the market of polymer materials

Among the natural biopolymers, caoutchouc – cis-1,4-poly(isoprene), which is extracted from latex secreted by theplant Hevea brasiliensis, has a special place. Beside thelarge number of synthetic caoutchouc, around 40 % ofmass of natural caoutchouc is used for the production oftires for transport vehicles and other articles made fromrubber. Natural caoutchouc is one of the rare productsbased on renewable raw materials, which has for yearsbeen competitive by price and properties in relation to syn-thetic caoutchouc.

Colophony can be obtained in a similar manner as naturalcaoutchouc, only from conifer wood. This product has avery complex composition and is obtained from numerouslow molecular weight substances, and as side product ob-tained during extraction of cellulose fibres from coniferwood. Around one-million tons of colophony is producedin the world and used directly or after the preparation of itsderivatives and mixing with other polymers, mostly as in-dustrial adhesive in different areas.��

Proteins of animal origin such as casein, gelatine, collagenand albumin, and proteins of plant origin – proteins fromsoybean, maize and other cereal, for years have been usedsuccessfully as adhesives, especially as adhesive for labels,and some of them as thickeners for food products.��

Is the use of biopolymers environmentallyand economically acceptable?

About 100 million tons of polymer material based on fossilraw materials is used per year in the world for packagingproduction. The majority of these polymers ends uptogether with communal waste in landfills. To reduce theamount of landfill waste, and due to environmental andeconomic reasons, methods have been developed for thereuse of certain components of communal waste. Forexample, prior to their disposal at a landfill, polymer mate-rials are separated from other types of waste and recycled


F i g. 8 – General formula of polyalkanoatesS l i k a 8 – Opæa formula polialkanoata

into “new” materials, raw materials or are burned in appro-priate ovens together with other combustible types ofwaste to obtain energy, and only the residue remainingafter combustion are disposed at landfills.

After a short or long period of use, biopolymers based onrenewable raw materials and polymers based on fossil rawmaterials end up as waste. Biopolymers based on renew-able raw materials, which are disposed of as waste can,under the right conditions, be reused by applying the threementioned methods developed for polymers based on fos-sil raw materials. However, biopolymers based on renew-able raw materials, which are proven to be biodegradable,can be further recycled using biological processes, such ascomposting (aerobic degradation) or fermentation (anaero-bic degradation) and converted into useful products suchas humus and/or methane.

The fact that, due to human negligence, a large amount ofmunicipal waste, in which packaging made from polymershas a significant share, is casually thrown away on land andat sea, irritates environmental societies, which have in sev-eral countries more than once driven the initiative to pro-hibit the use of polymers based on fossil raw materials forpackaging production, particularly for packaging of foodproducts, in order to reduce environmental pollution(“Litter-Problems”). One such initiative was realised in Italyat the end of 2010, leading to the prohibition of using bagsmade from polymer materials based on fossil raw materialsfor carrying bought goods, particularly food products. Eco-logical societies and a number of uninformed politiciansrequired that for the production of food packaging, bio-polymers based on renewable raw materials that are bio-degradable, compostable and in “harmony with nature”,i.e. real ecological materials should be used.

Ecological materials represent materials that require lessraw material and energy for their production, applicationand recycling, and which pollute the environment less thanother materials. Environmental and economic feasibility ofapplying polymer materials cannot be correctly evaluatedonly on the basis of data on the origin of the raw materialsused for production, and its biodegradability. This questioncan be answered only by creating an ecological balancesheet for selected biopolymers based on renewable raw

materials, and a conventional polymer based on fossil rawmaterials (or any other material), which is used e.g. to pro-duce packaging for the same product. Fig. 9 shows theorder of operations used in creating an ecological balancesheet by the DIN EN ISO 14040 – 44 standard.

As shown in Fig. 9, when setting an ecological balancesheet for some materials, it is necessary first to define thegoals and limits of balancing and then to develop theenergy and material balance for each “phase of life” ofproducts starting from raw materials, preparation of materi-als, manufacturing products which have some use value,their use, disposal to landfill or recycling. After that, it isnecessary to determine the environmental impact of wastegases, wastewater, solid waste and waste produced duringtransport of raw materials and products. Finally, the proc-essing of the results concerning the set of goals and theirinterpretation should be made. Ecological balances of thetwo materials, made by the same procedure, allow deter-mining which material is ecological material and thatshould be done only for one purpose of the material. Inaddition, development of an ecological balance sheetallows proper comparison of different methods used forthe production of the same product, and thus the optimalselection of the process for its manufacturing, use, andrecycling.�

For most applications, biopolymers based on renewableraw materials have better ecological balance than all othermaterials used for the same purpose. This is above allattributed to the fact that renewable raw materials (partiallyCO� neutral) are used for their production. Thus, it is notpossible to make the average estimation that the applica-tion of biopolymers based on renewable raw materials isenvironmentally justified in all cases. For example, in astudy considering the application of biodegradable poly-mers, made in 2008 in Austria, the results of the ecologicalbalance sheet for application of cups for single and multi-ple use were presented.�� Cups made from conventionalpolymers, such as polypropylene and poly(ethylene tere-phthalate), multilayer cardboard and biodegradable bio-polymer – PLA were analysed. The result of this study wasthat multiple-use cups made from all used materials haveless impact on the environment than single-use cups. Theenvironmental pollution made by using single-use cupsprepared from PLA is comparable with single-use cups pre-pared from PET, but compared to multilayer cardboardcups, the study suggested that PLA cups lead to higherenvironmental pollution. This example shows that biopoly-mers may not always have better ecological propertiescompared to synthetic non-biodegradable polymers.

When an economic balance sheet is developed in parallelwith the ecological balance sheet, taking into account thecosts of the raw materials and each operation in the prepa-ration of materials and products, their use, recycling anddisposal at landfills, the economic balance sheet for prod-ucts made from different materials is obtained. So far, poly-mers based on fossil raw materials have a better economicbalance sheet than biopolymers based on renewable rawmaterials, therefore biopolymers based on renewable rawmaterials are still used in areas for which the biodegradabil-ity is one of the basic properties, which ensures their appli-cation.


F i g. 9 – Schematic of creating an ecological balance sheetS l i k a 9 – Shema izrade ekološke bilance

When it comes to biodegradable biopolymers based onrenewable raw materials, it must be clearly noted that theirapplication instead of conventional synthetic polymerscannot solve the problem of the presence of packaging andother waste in nature (“Litter-Problems”), resulting fromuncontrolled disposal of used packaging in inappropriateplaces in the environment. Above all, the problem lies inthe fact that citizens are poorly informed and thus have anunacceptable attitude toward the materials they use, aswell as in the inadequate relationship between the stateand waste, and the behaviour of their citizens.

In some countries, the application of bio-photo and bio-oxo degradable polymers instead of conventional polymersbased on fossil raw materials for the production of bags forfood products is advertised. These polymers cannot obtainthe DIN EN ISO 13432 certificate, considering their abilityto be composted.�� Using these polymers, the oppositeeffect than expected is achieved. When trying to compostthese polymers, the environment is additionally pollutedby introduction into the ground of active chemicals andfragments of molecular chains such as polyolefins, forwhich no data on their compatibility with nature are avail-able and at the same time, the energy that these polymerscontain is irreversibly wasted.

Researchers in the world have given great attention to thedevelopment and application of biopolymers based onrenewable raw materials, but not in order to solve theproblem of waste in the world (“Litter-Problems”), but pri-marily to reduce the consumption of deficient fossil rawmaterials and CO� emissions into the atmosphere, i.e. tocontribute to sustainable development.

The production of food or biopolymersand bioenergy?

According to most forecasts, it is expected that around 9.5billion people will live on the Earth in 2050. Therefore, itwill be necessary to increase food production by approxi-mately 70 % in the future, and provide suitable agriculturalland. This will certainly lead to a conflict of interestbetween food producers, manufacturers of raw materialsfor the chemical industry and producers of renewableresources for energy production.

The use of agricultural land in the world will be illustratedusing the example of Germany.�� In this highly developedcountry, 500 000 hectares of agricultural land were used in1998 for the production of biomass for the chemical in-dustry and energy sources, and in 2011 for this purpose2 282 500 hectares were used. Land area of 1 966 000hectares were used for the production of biomass as anenergy source (solid fuels, bioethanol, biodiesel and bio-gas), and only 316 500 hectares were used to obtain rawmaterials for the chemical industry, of which one part isused for the production of biopolymers. The specified areais about 11 % of agricultural land. Experts in Germany haveestimated that up to 20 % of agricultural land could beused for intensive production of biomass, without negativeinfluence on satisfying the demands of German citizens forfood products. The situation is similar in other developedcountries, which has led to a reduction in the supply offood in the world market, and an increase in its price. In

many undeveloped countries, people are unable to pro-duce enough food for themselves and have not sufficientmoney to buy more expensive food. Therefore, the WorldBank has already announced the possibility of an outbreakof social unrest in 33 undeveloped countries in the world,if the food prices continue to increase in the world mar-kets. Taking all this into account, one question arises: Is itethical that land provided for food production for humansand animals is used for other purposes, and does that con-tribute to sustainable development?��

This problem could be alleviated if people start to use landfor biomass production, which is still not used to producefood for humans and animals. There are around 5.0 billionhectares of land in the world that can be used as agricul-tural land. For now, 1445 million hectares are cultivatedand used to produce food for people (260 million hec-tares), food for animals (1030 hectares), for bioenergy (55million hectares) and for production of materials (100 mil-lion hectares). The remaining 3.55 billion hectares are pas-tures for wild animals, and located mainly in remote areasthat lacked adequate infrastructure, and in many cases lacksufficient water for intensive production of biomass. There-fore, the manufacturers of biomass as an energy source andraw material for the chemical industry prefer to use landthat has already been cultivated and used for food produc-tion.

Within Interpak Fair, held in Düsseldorf from 12 to 18 May2011, the congress entitled Save Food was also held, organ-ised by the World Organization for nutrition of the UnitedNations (FAO). At this meeting, the results of studies onglobal food losses and wastage of food in the world are alsopresented among others works. The study was compiled bythe Institute for Food and Biotechnology from Sweden onbehalf of the UN.

One of the results of this study is that approximately onethird of the food produced for human consumption is lostor wasted (not used for nutrition) in the world, which annu-ally represents about 1.2 billion tons of food. This suggeststhat one third of the resources used for food production islost, and that one third of the CO� released during the pro-duction of food pollutes the atmosphere unnecessarily. Ithas been noted that the foodstuffs are lost in the entirechain, starting from agricultural production to householdconsumption. In developed countries, the losses are thegreatest in the first stage of the chain, and the smallest byend-users. The reason for the loss of food in undevelopedcountries is the lack of resources and adequate techniquesto harvest, pack, transport, store, and cool agriculturalproducts, as well as the lack of proper infrastructure andmarketing systems. In developed countries, food losses arethe greatest at the beginning (of agricultural production)and the end of the chain, where about 30 % of preparedfood are thrown despite the fact that in many cases it is stillusable. According to the study, the food losses in Europeand USA per capita are between 95 and 115 kg per year.Wasted food never reaches the market and thus affects itsdestabilization. At the meeting, it was also possible to hearthe opinion that a significant reduction in food losses indeveloping countries could be achieved by abolition ofcertain bureaucratic regulations, especially by improvingthe awareness of citizens as members of the consumersociety.


The conflict of interest between producers of food, bio-energy sources, and raw materials for the chemical industrywith a population, especially in undeveloped countries,could also be avoided or at least mitigated if developedcountries begin to follow the already signed documentsconcerning sustainable development, especially the socialcomponents of sustainable development.

Prospects of biopolymers basedon renewable raw materials

According to expert opinion, biopolymers based on re-newable raw materials could replace about 70 % of syn-thetic polymers based on fossil raw materials in packagingproduction, and 10 % in other areas of application. Bearingin mind that the trend of development in the world is di-rected at reducing consumption of materials and energysources based on fossil raw materials, it can be concludedthat the future of polymer materials based on renewableraw materials is provided. For now, for the production ofpackaging from polymer materials, less than 1 % of bio-polymers based on renewable raw materials are used, andtheir price is about two to four times higher than the priceof packaging made from polymers based on fossil rawmaterials. In the polymer materials market, their place canfind only materials with better or at least equal propertiesand price as already existing conventional polymer materi-als. How soon will biopolymers based on renewable rawmaterials be able to substitute polymers based on fossil rawmaterials depends on how fast researchers will improvetheir performances, increase efficiency and production ca-pacities, and thus contribute to reducing the cost of bio-polymers based on renewable raw materials.

In the last ten years, more attention has been paid to bio-renewable raw materials in the world. For example, theEuropean Union and other developed countries are invest-ing significant resources in so-called bio-refineries, inwhich new methods of separation and purification of allbiomass components will be developed and used, as wellas procedures for their use, primarily as a sources of rawmaterials for the chemical industry, food production forpeople and animals, and energy sources.�� This will allowfull use of biomass e.g. from wood, annual plants, algae,etc., which would surely result in a smaller price of bio-polymers and increase their competitiveness on the mar-ket. It is interesting to mention that, also participating in therealization of these projects, beside researchers from uni-versities and research institutes, are many experts fromlarge chemical companies involved in the production ofpolymers based on fossil raw materials as their main activi-ties (Dow Chemicals, BASF, Bayer, Breskem, Solvay, DSM,Evonik Degussa GmbH, Cereplast, etc.), which will cer-tainly contribute to shortening the time required to transferresearch results to production.

Major investment of recourses in the research and devel-opment of polymers based on biomass has already begunto yield results. For example:

– Cereplast has developed several biopolymers obtaineddirectly from natural algae and announced their produc-tion for the end of 2011.

– Bio-oriented foils with improved properties made fromPLA have been developed.– Sulzer Chemtech has, together with collaborators, devel-oped an innovative process and started production of PLA,which is stable at temperatures higher by about 50 °C thanthe standard types of PLA.�

In December 2011, Ceresana published the a study of themarket of biopolymers based on renewable raw materials,which describes in detail all biopolymers based on re-newable raw materials, as well as the known technologiesfor their production. It also contains relevant data for about87 companies that produce biopolymers based on renew-able raw materials, as well as estimates of production grow-th in the next 10 years for all 16 biopolymers found in themarket today.�

Table 1 from the first part of this study,�� presents theproduction of biodegradable and non-biodegradable bio-polymers based on renewable raw materials during 2011,and a production forecast for 2016. The following Table(Table 2) shows the world production capacities for bio-polymers based on renewable raw materials in 2011, and aproduction forecast for 2016, sorted by region.��

T a b l e 2 – Built production capacities for biopolymersbased on renewable raw materials in the world in 2011, and fore-cast for 2016 by region��

T a b l i c a 2 – Izgraðeni kapaciteti za proizvodnju biopolimerana bazi obnovljivih sirovina u svijetu tijekom 2011. godine i prog-noza za 2016. godinu po regijama��

RegionRegija

Capacity in 2011Kapacitet u 2011.

Forecast for 2016Prognoza za 2016.

AsiaAzija

34.6 % 46.3 %

South AmericaJuÞna Amerika

32.8 % 45.1 %

EuropeEuropa

18.5 % 4.9 %

North AmericaSjeverna Amerika

13.7 % 3.5 %

AustraliaAustralija

0.4 % 0.2 %

Total amount ofbiopolymers / tUkupna kolièinabiopolimera / t

1 167 000 5 779 000

The data presented in Table 2 are very interesting, bearingin mind that most studies and developments of biopoly-mers have been carried out by scientific institutions inEurope, North America and Japan, and that only around32 % of this type of polymer was produced in Europe andNorth America in 2011, and it is predicted that this sharewill be reduced to only 8.4 % in 2016. This means that therole of Europe and North America in the production of thisrelatively new group of polymeric materials will be mar-ginalised.

One carbon atom and two oxygen atoms are very tightlybounded in a molecule of carbon dioxide. A number of


papers describing the attempts to activate CO� and incor-porate it into organic substances under acceptable condi-tions have been published in the literature.�� Until twoyears ago, only Mother Nature could, from CO�, water,some enzymes and solar energy, synthesize polysaccha-rides and a range of low molecular weight compounds thatare all biodegradable. However, in 2010, the experts ofBayer, University of Aachen, the Center for Catalysis inAachen, and RWE Power, producers of electric and ther-mal energy based on coal, synthesised a catalyst based onzinc, with which it was possible to activate and incorporateCO� into organic compounds, e.g. polyols, and still usethem for the synthesis of polymers such as polyurethanes.At the beginning of 2011, the experts of these institutionscommissioned a semi-industrial unit in the Chemical Parkin Leverkusen, in which they checked the newly devel-oped catalytic process of using CO� to obtain polyols, theirincorporation into polyurethanes and properties of ob-tained polyurethanes. If the trial run is successful, it isexpected that by 2015 the first factory for the synthesis ofpolyurethanes based on CO� obtained from burning fossilfuels will be started up. From a major pollutant, CO� willbecome an excellent raw material and will be included inthe closed cycle of matter in nature, owing to this disco-very.�

List of symbols and abbreviationsPopis simbola i kratica

AGU – �-D-glucopyranosyl unit– �-D-glukopiranozilna jedinica

CHPTMA – (3-chloro-2-hydroxypropyl)trimethylammoniumchloride

– (2-hidroksi-3-klorpropil)trimetilamonijev klorid

PDLA – poly(D-lactic acid)– poli(D-mlijeèna kiselina)

PET – poly(ethylene terephthalate)– poli(etilen-tereftalat)

PHB – poly(3-hydroxybutyric acid)– poli(3-hidroksibutanska kiselina)

PHV – poly(3-hydroxyvalerianic acid)– poli(3-hidroksipentanska kiselina)

PLA – poly(lactic acid)– poli(mlijeèna kiselina)

PLLA – poly(L-lactic acid)– poli(L-mlijeèna kiselina)

PP – polypropylene– polipropilen

TPS – thermoplastic starch– termoplastièni škrob

DP – degree of polymerisation– stupanj polimerizacije

DS – degree of substitution– stupanj supstituiranosti

t� – glass transition temperatures, °C– staklište, °C

t� – melting temperature, °C– talište, °C

w – mass fraction, %– maseni udjel, %

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SAÝETAK

Polimeri na bazi obnovljivih sirovina – II. dioS. Jovanoviæ,�� J. V. DÞunuzoviæ� i Ý. Stojanoviæ�

U okviru ovoga dijela rada dan je kratak prikaz biopolimera na bazi škroba (derivati škroba, ter-moplastièni škrob), lignina i hemiceluloze, hitina (hitozan) i na bazi proizvoda razgradnješkroba, drugih polisaharida i šeæera (poli(mlijeèna kiselina), polihidroksialkanoati), kao i nekanjihova osnovna svojstva i podruèje primjene. Razmatran je problem ekološke i ekonomskeprihvatljivosti biopolimera na bazi obnovljivih sirovina i njihove konkurentnosti s polimerima nabazi fosilnih sirovina. Ukazano je i na probleme do kojih veæ dolazi zbog sve veæe upotrebe po-ljoprivrednog zemljišta za proizvodnju sirovina za kemijsku industriju i energente, umjesto zaproizvodnju hrane za ljude i Þivotinje. Ukazano je takoðer na optimistiène procjene struènjakao perspektivi razvoja proizvodnje biopolimera na bazi obnovljivih sirovina.

Na kraju rada je ukazano i na uspjeh tima istraÞivaèa okupljenih oko struènjaka tvrtke Bayer,koji su prvi u svijetu razvili katalizator, pomoæu kojeg su uspjeli uèinkovito aktivirati CO� i ugra-diti ga u poliole, koje upotrebljavaju za sintezu poliuretana u poluindrustrijskim razmjerima.Ovim postupkom æe prvi put jedan zagaðivaè biti upotrijebljen kao osnovna sirovina za sintezuorganskih spojeva, što æe imati nesagledive posljedice na razvoj kemijske industrije, a samim timi proizvodnje polimera.

� Tehnološko-metalurški fakultet, Univerzitet u Beogradu, Prispjelo 14. rujna 2012.Karnegijeva 4, 11 120 Beograd, Srbija Prihvaæeno 11. prosinca 2012.

� Institut za hemiju, tehnologiju i metalurgiju (IHTM)– Centar za hemiju, Univerzitet u Beogradu,Studentski trg 12–16, 11 000 Beograd, Srbija

� Henkel Srbija, AG General Industry, Henkel Adhesive Technologies,Bulevar Osloboðenja 383, 11 040 Beograd, Srbija

polymers based on renewable raw materials – part ii

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