An Overview Of Bio-based Polymers For Packaging Materials
Journal of Bioresources and Bioproducts. 2016, 1(3):106-113Peer-ReviewedREVIEW PAPERAn overview of bio-based polymers for packaging materialsYuanfeng Pana,*, Madjid Farmahini-Farahanib, Perry O’Hearnb , Huining Xiaob and Helen Ocampoba) School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004 China.b) Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, E3B 5A3 Canada.*Corresponding author: panyf@gxu.edu.cnABSTRACTSynthetic polymers are the most widely used materials for packaging because of their ease of processing, low cost, and low density.However, many of these materials are not easily recyclable and are difficult to degrade completely in nature, creating environmentalproblems. Thus, there is a tendency to substitute such polymers with natural polymers and copolymers that are easily bio-degraded and lesslikely to cause environmental pollution. There has been a greater interest in poly-lactic acid (PLA), poly-hydroxyalkanoates (PHAs),cellulose and starch based polymers and copolymers as the emerging biodegradable material candidates for the future. This paper reviewsthe present state-of-the-art biodegradable polymers made from renewable resources and discusses the main features of their properties anddesign.Keywords: bio-based polymers; environmental aspects; packaging materials; poly(lactic acid); cellulose.1. INTRODUCTIONSynthetic polymers are considered to be indispensable tomodern sciences and technology. They have become amajor component of our modern world with their widerange of applications in a variety of fields such aspackaging, agriculture, food, consumer products, medicalappliances, building materials, industry, aerospace materialsfor their durability, low cost, and resistance to degradation.However, many polymer applications such as those used inpackaging can cause real environmental damage, and takedecades to hundreds of years to breakdown anddecompose.1 Over 60% of post-consumer plastics waste isproduced by households, most of it as single usepackaging.2The effects of polymers on the environment can besignificant. Worldwide statistics show that 43% of marinemammal species, 86% of sea turtle species, and 44% ofseabird species are susceptible to ingesting marine plasticdebris.3 The extent of the environmental effect throughpolymer exposure goes beyond the death of these species,subsequently releasing these polymers back into theirnatural ecosystem.4Not only has it been a major problem for sea life but alsothe high waste management cost. Each year approximately140 million tons of synthetic polymers are produced, anestimated 20-25% of which ends up in municipal landfills.5The cost of collecting and managing landfill sites underproper environmental regulation can place a strain on thebudget of many municipalities. Recycling is one avenue toeliminate waste and for most polymer-based materials thiscan be achieved. However, the general costs of theeconomics associated with transporting and processing therecycled material directly outweighs producing newwww.Bioresources-Bioproducts.compolymer based products. Therefore, an alternativeresolution is the use of biodegradable plastics.Biodegradable plastics are defined as plastics withsimilar properties to conventional plastics, but willdecompose after disposal to the environment by the activityof microorganisms to produce carbon dioxide (CO2) andwater (H2O).2 The aim of this emerging developing field isto change the nature of polymer products and minimize theenvironmental impact. For such a change to be viable, thematerials are required to maintain their desired propertiesbut be able to be broken down into their constituentcomponents once disposed of.Biobased polymers may be classified into three maincategories based on their origin and production: (1)Polymers produced by classical chemical synthesis usingrenewable biobased monomers, for example Polylactic acidand biopolyester polymerized from lactic acid monomers,are produced via fermentation of carbohydrate feedstock.(2) Polymers directly extracted/removed from biomass, forexample, polysaccharides such as starch and cellulose, andproteins like casein and gluten. (3) Polymers produced bymicroorganisms or genetically modified bacteria. To date,this group of bio-based polymers consists mainly ofpolyhydroxyalkonoates, but developments with bacterialcellulose are in progress.The various groups emerging from the aforementionedcategories are illustrated in figure 1. Naturally occurringsubstances may prove useful for food packaging due to theirpartial to complete biodegradable properties. However,natural polymers are expensive and exhibit poor mechanicalproperties, therefore it is necessary to develop variousstructural configurations to achieve a desired product.7106
Journal of Bioresources and Bioproducts. 2016, 1(3):106-113Peer-ReviewedFig.1 Classification of main biodegradable polymers6In examining the biodegradation of polymers animportant distinction must be made between degradationand biodegradation. Generally, materials that are exposed toenvironmental conditions which include weathering, aging,and/or burying will undergo mechanical, thermal andchemical transformations. These conditions contribute tothe change of polymeric structure and properties, and canbe a trigger or useful factors to initiate the biodegradationprocess. Instances of compression, tension, shear, and otherforces can contribute to the mechanical degradation of amaterial. These factors do not play a prevailing role in thewhole biodegradation process but can stimulate or sustainit.7Various biodegradable polymers show promise in beingused as a means of packaging and some are already beingused. For instance, cellophane is the most commonlycellulose-based biopolymer for various food packaging.8, ated starch, and dextrin, tend to swell anddeform when exposed to moisture. Other starch-basedpolymers of interest are polylactide, polyhydroxyalkanoate(PHA), polyhydroxybuterate (PHB), and a copolymer ofPHB and valeric acid (PHB/V).10 However, poor associatedmechanical properties, high hydrophilicity, and limitedability to be processed limit their application.11www.Bioresources-Bioproducts.com2. PHYSICAL CHARACTERISTICS OF PACKAGINGFood packaging and other consumer goods make up 30%percent of all polymer based packaging consumed today.12Food packaging is designed to prevent food degradationand deterioration, allowing for the product to maintain andretain the beneficial effects of processing, by ensuringextended shelf-life, and maintaining or increasing thequality and safety of food sold to the consumer. Toaccomplish this broad task, packaging provides protectionfrom three major classes of external influences: chemical,biological, and physical.10Chemical protection minimizes compositional changestriggered by environmental influences. Plastics, glass, andmetals provide an almost absolute barrier to chemical andother environmental agents, but few packages are purelyglass or metal since these forms of packing require anothermaterial as a mean of closure to facilitate both filling andemptying. Plastic packaging offers a large range of barrierproperties but has a higher permeability than that of glass ormetal. Biological protection provides a barrier tomicroorganisms, insects, rodents, and other animals,thereby preventing disease and spoilage. In addition,biological barriers maintain conditions to control ripeningand aging. Biological barriers also provide a host of otherfunctions including, but not limited to preventing product107
Journal of Bioresources and Bioproducts. 2016, 1(3):106-113tampering, preventing odor transmission, and maintainingthe internal environment of the package. Physical protectionshields food from mechanical damage such as cushioningagainst the shock and vibration encountered duringdistribution.10 Physical protection is often a secondary levelof protection to ensure the quality of the product.Polymers based packaging materials are affected by thechemical structure, molecular weight, crystallinity of thepolymer and processing conditions. The physicalcharacteristics required in packaging are generallydetermined by the items being packaged and theenvironments in which the packages will be stored, whetherthe product will be refrigerated or dry stored. Foodpackaging demands have stricter requirements than mostother consumer products to ensure food safety andregulations instituted by law.10 There is a growingmovement for food packing that is more environmentallyfriendly due to the rising concern about environmentalimpact, but the strict requirements make it a challenge.Thus, there is research is being conducted on the possiblebiodegradable polymer substitutes for food packaging.3Various approaches are currently being investigated as topossible polymers that may be utilized to design adequateenvironmentally friendly packaging. Consumer powercoupled with the dramatic rise in pre-packaged disposablemeals, means that food manufacturers and packagingindustry have begun to focus their attention on necessarychanges to meet the consumer demand.3 Polymers formedfrom chemical synthesis exhibit some traits that arepromising, which can easily decompose and be mineralizedby microorganisms. Another emerging possible avenue isthe use of starch based polymers derived from variouscrops.3. BIO-BASED POLYMERS FOR PACKAGING3.1 Poly (lactic acid)Poly (lactic acid), PLA (Fig.2) is considered atypical biodegradable synthetic semi-crystallinepolyester commercially obtained by ring openingpolymerization of lactones, or by poly condensation ofhydroxy-carbonic acids. PLA is a biodegradablematerial with exceptional mechanical and opticalproperties. Lactic acid, the monomer of PLA, is a chiralmolecule existing as two stereoisomers, L- and D-lacticacid which can be produced by biological or chemicalsynthesis.13CH3CH3OOOHHOOOCH3nOFig.2 Chemical structure of PLALactic acid is obtained through biological synthesisby fermentation of carbohydrates from lactic wedbelonging mainly to the genus Lactobacillus, or fungi.14This fermentative process requires a bacterial strain, acarbon source (carbohydrates), a nitrogen source (e.g.yeast extract, peptides), and mineral elements to allowthe bacterial growth and the production of lactic acid.The lactic acid formed, exists almost exclusively asL-lactic acid and leads to poly(L-lactic acid) (PLLA)with low molecular weight by poly-condensationeaction. However, Moon et al. 15 proposed analternative solution to obtain higher molecular weightPLLA by poly-condensation route.The chemical reactions, leading to the formation of acyclic dimer, lactide, as an intermediate step for theproductionofPLA,couldproducelongmacromolecular chains with L- and D-lactic acidmonomers via a ring-opening polymerization (ROP).The proportions and the sequencing of L- and D-lacticacid units of the two enantiomers generated caneffectively be altered, allowing an advantageous controlover the final properties of PLA.15, 16 Currently, due toits large availability on the market and its relatively lowprice, 17 PLA is one of the promising bio-polyesters forpackaging applications.The steric shielding effect, provided by thehydrophobic property of the methyl side groups, makesPLA more resistant to hydrolysis than polyglycolide(PGA). A typical glass transition temperature,elongation at break and tensile strength forrepresentative commercial PLA is 63.8 C, 30.7% and32.22 MPa respectively.18 Regulation of the physicalproperties and biodegradability of PLA can be achievedby employing a hydroxy acid comonomer componentor by racemization of D- and L- isomers.19 Asemi-crystalline polymer (PLLA) (crystallinity about37%) is obtained from L-lactide whereas poly(DL-lactide) (PDLLA) is an amorphous polymer.20Their mechanical properties are different as are theirdegradation times.21 PLLA is a hard, transparentpolymer with an elongation at break of 85%-105% anda tensile strength of 45-70 MPa. It has a melting pointof 170-180 C and a glass transition temperature (Tg) of53 C.22 PDLLA has a glass transition temperaturearound 55 C but no melting point. It also shows amuch lower tensile strength.23PLA can be plasticized with oligomeric acid, citrateester or low molecular polyethylene glycol24 to improvethe chain mobility and to favor its crystallization. Highmolecular weight PLAs are obtained through ringopening polymerization. This route also allows thecontrol of the final properties of PLA by adjusting theproportions of the two enantiomers.25 Other routesinclude melt/solid state polymerization,26 solutionpolymerization or chain extension reaction.27 Highmolecular weight PLA has better mechanicalproperties.28 Different companies commercialize PLAwith various ratios of D/L lactide. Trade names and108
Journal of Bioresources and Bioproducts. 2016, 1(3):106-113suppliers of different grades of PLA are listed in Table1.The rate of degradation of PLA depends on thedegree of crystallinity. The degradation rate of PLLA isvery low compared to PGA, therefore, somecopolymers of lactide and glycolide have beeninvestigated as bio-resorbable implant materials.29 ThePeer-Reviewedbiodegradability of PLA can also be enhanced bygrafting. The graft copolymerization of L-lactide ontochitosan was carried out by ring openingpolymerization using a tin catalyst. The meltingtransition temperature and thermal stability of graftpolymers increase with increasing grafting percentages.Table 1 Trade names and suppliers of PLATrade nameNatureWorks Galacid Lacea Lacty Heplon CPLA Eco plastic Treofan PDLA Ecoloju Biomer LCompanyCargill DowGalacticMitsui Chem.ShimadzuChronopolDainippon Ink Chem.ToyotaTreofanPuracMitsubishiBiomerAs the lactide content increases, the degradation of thegraft polymer decreases.30The eco-friendly characteristics of PLA have led to arecent upsurge in its application as a polymer used tocoat paper products. The relatively high resistance ofPLA films to water vapor is among a variety of factorsattributing to its extensive use in paper coating. Thepermeability of PLA nanocomposites to water vapordecreased by 74% [26.0 g/(m2 day)].31Song et al32 mentioned that paperboard coated withPLA could be used as a substitute for PE-coatedpaperboard to manufacture 1-way paper cups orcontainers for high moisture foods such as beveragecartons and ice cream containers. PLA film does notexhibit improved adhesion to paper in the directcoating extruding process. Therefore, to achieveoptimum results and sufficient adhesion, the polylacticacid must be applied at the highest possibletemperature. The inherent brittleness of PLA causesleaks and cracks whereby the coating does not endurethe creasing or bending and extending, which areessential to producing plate or mold form productsfrom PLA.3.2 Poly (3-hydroxybutyrate) and lyesters are natural macromolecules frombacterial sources. These polymers are still expensivebut have increasing applications due to theirenvironmentally friendly nature. The simplest and mostcommon poly-β-hydroxyl alkanoate (PHA) is polyβ-hydroxybutyrate (PHB). PHB (Fig.3) and erate (PH BV), (Fig.4) is a erlandsJapanGermanysemicrystalline polymer that can be produced bybacteria from biomass through natural biosynthesis.Fig.3 Chemical structure of PHBFig.4 Chemical structure of PHBVPHB is synthesized as a storage material by a largenumber of resting bacteria. The amount of PHB inbacteria is normally in the range of 1% to 30% of theirdry weight. Under controlled fermentation conditions,some Azotobacter and Alcaligenes ssp. can aggregatepolymer up to 90% of their dry mass.33, 34There are two constraints in using PHB: a narrowprocessability window and a relatively low-impactresistance because of its high degree of crystallinity.These drawbacks have hindered the utilization of PHBas a common plastic. The blending of PHB with otherpolymers can offer opportunities to extend and exploretheir many useful and interesting properties and to alterits undesirable properties. For instance, PHB has beenblended with poly (ethylene oxide), poly (vinyl butyral),poly (vinyl acetate), poly (vinylphenol), celluloseacetate butyrate, chitin and chitosan.35Unique properties of this group of polymers includeexcellent chemical resistance, heat resistance, andrigidity. These are similar to isotactic polypropylene(PP), which has the most noteworthy water vaporresistance of PHB in comparison to the existing109
Journal of Bioresources and Bioproducts. 2016, 1(3):106-113biopolymers in the market. In light of these qualities,these biodegradable polymers may very well come tobe used in various applications in the near future,particularly for packaging applications.36Multicomponent polymeric systems containing PHBhave been obtained by two methods. The first is aradical polymerization of an acrylic polymer in thepresence of PHB. The second relies on melt mixingPCL with PHB. Peroxide is used in both processes ization.37 These methods have beenconsidered as reactive blending. Besides the bacterialsynthetic approach, other chemical methods for theproduction of PHB have been developed, such as thering opening polymerization of β-butyrolactone.38Different structures are obtained according to thesynthesis route. An isotactic polymer with randomstereo sequences is obtained via a bacterial processwhile a polymer with a partially stereo regular block isobtained via chemical synthesis. Applications that havebeen developed from PHB and related materials (e.g.Biopol) can be found in the areas of cover packaging,hygienic, agricultural, and biomedical products. Recentapplication developments are based on medium-chainlength PHAs, ranging from high solid alkyd-like paintsto pressure sensitive adhesives, biodegradable cheesecoatings and biodegradable rubbers. Technically, theprospects for PHAs are exceptionally promising. At thepoint when the cost of these materials can be furtherreduced, the use of bio-polyester applications willlikewise become economically attractive.39PHBV copolymers have recently received significantscientific attention due to their biodegradability, andbecause their properties can be easily controlled by thecontent of hydroxy valerate (HV). PHBV copolymerswith different content of hydroxyvalerate have beenused recently as matrices in eco-composites,40 wheredifferent natural fibers were applied as reinforcement.41Recent research has focused on the use of alternativesubstrates, novel extraction methods, geneticallyenhanced species and mixed cultures with a view tomaking PHAs more commercially attractive.42, 43 Theclassical microbiology and modern molecular biologyhave been brought together to interpret the intricaciesof PHA metabolism for production purposes and forthe unraveling of the natural role of PHA.44 It is easierto develop a commercially attractive recombinantprocess for large scale production of PHAs. PHAsynthase (PhaC) enzymes, which catalyze thepolymerization of 3-hydroxyacyl-CoA monomers to P(3HA)s, were subjected to various forms of proteinengineering to improve the enzyme activity or substratespecificity.45 The use of mixed cultures and renewablesources obtained from waste organic carbon cansubstantially decrease the cost of PHA and increasetheir market eviewed3.3 CelluloseThe biodegradable backbone of polymers fromrenewable resources has led to their growingimportance as potential substitutes for petrochemicalsin different fields. Cellulose (Fig.5) is the mostabundant, sustainable, compostable, biodegradable andreusable organic material on earth, and has
(2) Polymers directly extracted/removed from biomass, for example, polysaccharides such as starch and cellulose, and proteins like casein and gluten. (3) Polymers produced by microorganisms or genetically modified bacteria. To date, this group of bio-based polymers consists mainly of polyhydroxyalkonoates, but developments with bacterial
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