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An overview of bio-based polymers for packaging materials

Yuanfeng Pana,*, Madjid Farmahini-Farahanib, Perry O’Hearnb , Huining Xiaob and Helen Ocampob

a) 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.cn

ABSTRACT

Synthetic 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 environmental problems. Thus, there is a tendency to substitute such polymers with natural polymers and copolymers that are easily bio-degraded and less likely 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 reviews the present state-of-the-art biodegradable polymers made from renewable resources and discusses the main features of their properties and design.

Keywords: bio-based polymers; environmental aspects; packaging materials; poly(lactic acid); cellulose.

1. INTRODUCTION

Synthetic polymers are considered to be indispensable tomodern sciences and technology. They have become a major component of our modern world with their wide range of applications in a variety of fields such as packaging, agriculture, food, consumer products, medical appliances, building materials, industry, aerospace materials for their durability, low cost, and resistance to degradation. However, many polymer applications such as those used in packaging can cause real environmental damage, and take decades to hundreds of years to breakdown and decompose.1 Over 60% of post-consumer plastics waste is produced by households, most of it as single use packaging.2

The effects of polymers on the environment can be significant. Worldwide statistics show that 43% of marine mammal species, 86% of sea turtle species, and 44% of seabird species are susceptible to ingesting marine plastic debris.3 The extent of the environmental effect through polymer exposure goes beyond the death of these species, subsequently releasing these polymers back into their natural ecosystem.4

Not only has it been a major problem for sea life but also the high waste management cost. Each year approximately 140 million tons of synthetic polymers are produced, an estimated 20-25% of which ends up in municipal landfills.5 The cost of collecting and managing landfill sites under proper environmental regulation can place a strain on the budget of many municipalities. Recycling is one avenue to eliminate waste and for most polymer-based materials this can be achieved. However, the general costs of the economics associated with transporting and processing the recycled material directly outweighs producing new

polymer based products. Therefore, an alternative resolution is the use of biodegradable plastics.

Biodegradable plastics are defined as plastics with similar properties to conventional plastics, but will decompose after disposal to the environment by the activity of microorganisms to produce carbon dioxide (CO2) and water (H2O).2 The aim of this emerging developing field is to change the nature of polymer products and minimize the environmental impact. For such a change to be viable, the materials are required to maintain their desired properties but be able to be broken down into their constituent components once disposed of.

Biobased polymers may be classified into three main categories based on their origin and production: (1) Polymers produced by classical chemical synthesis using renewable biobased monomers, for example Polylactic acid and biopolyester polymerized from lactic acid monomers, are produced via fermentation of carbohydrate feedstock. (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 cellulose are in progress.

The various groups emerging from the aforementioned categories are illustrated in figure 1. Naturally occurring substances may prove useful for food packaging due to their partial to complete biodegradable properties. However, natural polymers are expensive and exhibit poor mechanical properties, therefore it is necessary to develop various structural configurations to achieve a desired product.7

REVIEW PAPER

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Fig.1 Classification of main biodegradable polymers6

In examining the biodegradation of polymers an

important distinction must be made between degradation and biodegradation. Generally, materials that are exposed to environmental conditions which include weathering, aging, and/or burying will undergo mechanical, thermal and chemical transformations. These conditions contribute to the change of polymeric structure and properties, and can be a trigger or useful factors to initiate the biodegradation process. Instances of compression, tension, shear, and other forces can contribute to the mechanical degradation of a material. These factors do not play a prevailing role in the whole biodegradation process but can stimulate or sustain it.7

Various biodegradable polymers show promise in being used as a means of packaging and some are already being used. For instance, cellophane is the most commonly cellulose-based biopolymer for various food packaging.8, 9 Starch-based polymers, such as amylose, hydroxylpropylated starch, and dextrin, tend to swell and deform when exposed to moisture. Other starch-based polymers of interest are polylactide, polyhydroxyalkanoate (PHA), polyhydroxybuterate (PHB), and a copolymer of PHB and valeric acid (PHB/V).10 However, poor associated mechanical properties, high hydrophilicity, and limited ability to be processed limit their application.11

2. PHYSICAL CHARACTERISTICS OF PACKAGING

Food packaging and other consumer goods make up 30% percent of all polymer based packaging consumed today.12 Food packaging is designed to prevent food degradation and deterioration, allowing for the product to maintain and retain the beneficial effects of processing, by ensuring extended shelf-life, and maintaining or increasing the quality and safety of food sold to the consumer. To accomplish this broad task, packaging provides protection from three major classes of external influences: chemical, biological, and physical.10

Chemical protection minimizes compositional changes triggered by environmental influences. Plastics, glass, and metals provide an almost absolute barrier to chemical and other environmental agents, but few packages are purely glass or metal since these forms of packing require another material as a mean of closure to facilitate both filling and emptying. Plastic packaging offers a large range of barrier properties but has a higher permeability than that of glass or metal. Biological protection provides a barrier to microorganisms, insects, rodents, and other animals, thereby preventing disease and spoilage. In addition, biological barriers maintain conditions to control ripening and aging. Biological barriers also provide a host of other functions including, but not limited to preventing product

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tampering, preventing odor transmission, and maintaining the internal environment of the package. Physical protection shields food from mechanical damage such as cushioning against the shock and vibration encountered during distribution.10 Physical protection is often a secondary level of protection to ensure the quality of the product.

Polymers based packaging materials are affected by the chemical structure, molecular weight, crystallinity of the polymer and processing conditions. The physical characteristics required in packaging are generally determined by the items being packaged and the environments in which the packages will be stored, whether the product will be refrigerated or dry stored. Food packaging demands have stricter requirements than most other consumer products to ensure food safety and regulations instituted by law.10 There is a growing movement for food packing that is more environmentally friendly due to the rising concern about environmental impact, but the strict requirements make it a challenge. Thus, there is research is being conducted on the possible biodegradable polymer substitutes for food packaging.3

Various approaches are currently being investigated as to possible polymers that may be utilized to design adequate environmentally friendly packaging. Consumer power coupled with the dramatic rise in pre-packaged disposable meals, means that food manufacturers and packaging industry have begun to focus their attention on necessary changes to meet the consumer demand.3 Polymers formed from chemical synthesis exhibit some traits that are promising, which can easily decompose and be mineralized by microorganisms. Another emerging possible avenue is the use of starch based polymers derived from various crops. 3. BIO-BASED POLYMERS FOR PACKAGING 3.1 Poly (lactic acid)

Poly (lactic acid), PLA (Fig.2) is considered a typical biodegradable synthetic semi-crystalline polyester commercially obtained by ring opening polymerization of lactones, or by poly condensation of hydroxy-carbonic acids. PLA is a biodegradable material with exceptional mechanical and optical properties. Lactic acid, the monomer of PLA, is a chiral molecule existing as two stereoisomers, L- and D-lactic acid which can be produced by biological or chemical synthesis.13

HOO

OOH

CH3

O

O

O

CH3

CH3 n Fig.2 Chemical structure of PLA

Lactic acid is obtained through biological synthesis

by fermentation of carbohydrates from lactic bacteria

belonging mainly to the genus Lactobacillus, or fungi.14 This fermentative process requires a bacterial strain, a carbon source (carbohydrates), a nitrogen source (e.g. yeast extract, peptides), and mineral elements to allow the bacterial growth and the production of lactic acid. The lactic acid formed, exists almost exclusively as L-lactic acid and leads to poly(L-lactic acid) (PLLA) with low molecular weight by poly-condensation eaction. However, Moon et al. 15 proposed an alternative solution to obtain higher molecular weight PLLA by poly-condensation route.

The chemical reactions, leading to the formation of a cyclic dimer, lactide, as an intermediate step for the production of PLA, could produce long macromolecular chains with L- and D-lactic acid monomers via a ring-opening polymerization (ROP). The proportions and the sequencing of L- and D-lactic acid units of the two enantiomers generated can effectively be altered, allowing an advantageous control over the final properties of PLA.15, 16 Currently, due to its large availability on the market and its relatively low price, 17 PLA is one of the promising bio-polyesters for packaging applications.

The steric shielding effect, provided by the hydrophobic property of the methyl side groups, makes PLA more resistant to hydrolysis than polyglycolide (PGA). A typical glass transition temperature, elongation at break and tensile strength for representative commercial PLA is 63.8 °C, 30.7% and 32.22 MPa respectively.18 Regulation of the physical properties and biodegradability of PLA can be achieved by employing a hydroxy acid comonomer component or by racemization of D- and L- isomers.19 A semi-crystalline polymer (PLLA) (crystallinity about 37%) is obtained from L-lactide whereas poly (DL-lactide) (PDLLA) is an amorphous polymer.20 Their mechanical properties are different as are their degradation times.21 PLLA is a hard, transparent polymer with an elongation at break of 85%-105% and a tensile strength of 45-70 MPa. It has a melting point of 170-180°C and a glass transition temperature (Tg) of 53°C.22 PDLLA has a glass transition temperature around 55°C but no melting point. It also shows a much lower tensile strength.23

PLA can be plasticized with oligomeric acid, citrate ester or low molecular polyethylene glycol24 to improve the chain mobility and to favor its crystallization. High molecular weight PLAs are obtained through ring opening polymerization. This route also allows the control of the final properties of PLA by adjusting the proportions of the two enantiomers.25 Other routes include melt/solid state polymerization,26 solution polymerization or chain extension reaction.27 High molecular weight PLA has better mechanical properties.28 Different companies commercialize PLA with various ratios of D/L lactide. Trade names and

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suppliers of different grades of PLA are listed in Table 1.

The rate of degradation of PLA depends on the degree of crystallinity. The degradation rate of PLLA is very low compared to PGA, therefore, some copolymers of lactide and glycolide have been investigated as bio-resorbable implant materials.29 The

biodegradability of PLA can also be enhanced by grafting. The graft copolymerization of L-lactide onto chitosan was carried out by ring opening polymerization using a tin catalyst. The melting transition temperature and thermal stability of graft polymers increase with increasing grafting percentages.

Table 1 Trade names and suppliers of PLA

Trade name Company Country NatureWorks® Cargill Dow USA

Galacid® Galactic Belgium Lacea® Mitsui Chem. Japan Lacty® Shimadzu Japan

Heplon® Chronopol USA CPLA® Dainippon Ink Chem. Japan

Eco plastic® Toyota Japan Treofan® Treofan Netherlands PDLA® Purac Netherlands

Ecoloju® Mitsubishi Japan Biomer® L Biomer Germany

As the lactide content increases, the degradation of the graft polymer decreases.30

The eco-friendly characteristics of PLA have led to a recent upsurge in its application as a polymer used to coat paper products. The relatively high resistance of PLA films to water vapor is among a variety of factors attributing to its extensive use in paper coating. The permeability of PLA nanocomposites to water vapor decreased by 74% [26.0 g/(m2 day)].31

Song et al32 mentioned that paperboard coated with PLA could be used as a substitute for PE-coated paperboard to manufacture 1-way paper cups or containers for high moisture foods such as beverage cartons and ice cream containers. PLA film does not exhibit improved adhesion to paper in the direct coating extruding process. Therefore, to achieve optimum results and sufficient adhesion, the polylactic acid must be applied at the highest possible temperature. The inherent brittleness of PLA causes leaks and cracks whereby the coating does not endure the creasing or bending and extending, which are essential to producing plate or mold form products from PLA. 3.2 Poly (3-hydroxybutyrate) and Poly (3-hydroxybutyrate-co-3-hydroxyvalerate)

Bio-polyesters are natural macromolecules from bacterial sources. These polymers are still expensive but have increasing applications due to their environmentally friendly nature. The simplest and most common poly-β-hydroxyl alkanoate (PHA) is poly- β-hydroxybutyrate (PHB). PHB (Fig.3) and its copolymers Poly 3-hydroxybutyrate-co-3 -hydroxyvalerate (PH BV), (Fig.4) is a biodegradable

semicrystalline polymer that can be produced by bacteria from biomass through natural biosynthesis.

Fig.3 Chemical structure of PHB

Fig.4 Chemical structure of PHBV

PHB is synthesized as a storage material by a large

number of resting bacteria. The amount of PHB in bacteria is normally in the range of 1% to 30% of their dry weight. Under controlled fermentation conditions, some Azotobacter and Alcaligenes ssp. can aggregate polymer up to 90% of their dry mass.33, 34

There are two constraints in using PHB: a narrow processability window and a relatively low-impact resistance because of its high degree of crystallinity. These drawbacks have hindered the utilization of PHB as a common plastic. The blending of PHB with other polymers can offer opportunities to extend and explore their many useful and interesting properties and to alter its undesirable properties. For instance, PHB has been blended with poly (ethylene oxide), poly (vinyl butyral), poly (vinyl acetate), poly (vinylphenol), cellulose acetate butyrate, chitin and chitosan.35

Unique properties of this group of polymers include excellent chemical resistance, heat resistance, and rigidity. These are similar to isotactic polypropylene (PP), which has the most noteworthy water vapor resistance of PHB in comparison to the existing

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biopolymers in the market. In light of these qualities, these biodegradable polymers may very well come to be used in various applications in the near future, particularly for packaging applications.36

Multicomponent polymeric systems containing PHB have been obtained by two methods. The first is a radical polymerization of an acrylic polymer in the presence of PHB. The second relies on melt mixing PCL with PHB. Peroxide is used in both processes to form inter-grafted species responsible for compatibilization.37 These methods have been considered as reactive blending. Besides the bacterial synthetic approach, other chemical methods for the production of PHB have been developed, such as the ring opening polymerization of β-butyrolactone.38 Different structures are obtained according to the synthesis route. An isotactic polymer with random stereo sequences is obtained via a bacterial process while a polymer with a partially stereo regular block is obtained via chemical synthesis. Applications that have been developed from PHB and related materials (e.g. Biopol) can be found in the areas of cover packaging, hygienic, agricultural, and biomedical products. Recent application developments are based on medium-chain length PHAs, ranging from high solid alkyd-like paints to pressure sensitive adhesives, biodegradable cheese coatings and biodegradable rubbers. Technically, the prospects for PHAs are exceptionally promising. At the point when the cost of these materials can be further reduced, the use of bio-polyester applications will likewise become economically attractive.39

PHBV copolymers have recently received significant scientific attention due to their biodegradability, and because their properties can be easily controlled by the content of hydroxy valerate (HV). PHBV copolymers with different content of hydroxyvalerate have been used recently as matrices in eco-composites,40 where different natural fibers were applied as reinforcement. 41 Recent research has focused on the use of alternative substrates, novel extraction methods, genetically enhanced species and mixed cultures with a view to making PHAs more commercially attractive.42, 43 The classical microbiology and modern molecular biology have been brought together to interpret the intricacies of PHA metabolism for production purposes and for the unraveling of the natural role of PHA.44 It is easier to develop a commercially attractive recombinant process for large scale production of PHAs. PHA synthase (PhaC) enzymes, which catalyze the polymerization of 3-hydroxyacyl-CoA monomers to P (3HA)s, were subjected to various forms of protein engineering to improve the enzyme activity or substrate specificity.45 The use of mixed cultures and renewable sources obtained from waste organic carbon can substantially decrease the cost of PHA and increase their market potential.46

3.3 Cellulose

The biodegradable backbone of polymers from renewable resources has led to their growing importance as potential substitutes for petrochemicals in different fields. Cellulose (Fig.5) is the most abundant, sustainable, compostable, biodegradable and reusable organic material on earth, and has many applications. Plant cellulose is predominantly believed to be among the richest natural polymers on earth.

Fig.5 Chemical structure of cellulose

Cellulose is a natural polymer, a long chain consists of sugar, β-D-glucose.47 The inter-chain hydrogen bonds in the crystalline regions are strong, giving the resultant fiber good strength and insolubility in most solvents. They also prevent cellulose from melting (non-thermoplastic). In the less-ordered regions, the chains are further apart and more available for hydrogen bonding with other molecules, such as water. Most cellulose structures can absorb large quantities of water (hygroscopic). Thus, cellulose swells but does not dissolve in water.48

Paper and board material have excellent mechanical properties as packaging material, however, the gas and water vapor permeability are often very high for many food application.49-51 To alleviate this, paper–based materials have been coated or modified with a thin layer of synthetic plastic which has provided the materials with the required gas property and water resistanc.52, 53 Surface modification was also used to render paper antimicrobia.54, 55 3.4 Starch

Starch, an omnipresent bio-material, is one of the most abundant and inexpensive polysaccharide sources which have the unique characteristic of biodegradability and to easily dissolve in water. Starch is produced by agricultural plants in the form of granules, which are hydrophilic. Starch is mainly extracted from potatoes, corn, wheat and rice. It is composed of amylose (poly-α-1,4-D-glucopyranoside), a linear and crystalline polymer and amylopectin (poly-α-1,4-Dglucopyranoside and α-1,6-D -glucopyranoside), a branched and amorphous polymer (Fig.6). The relative amounts and molar masses of amylose and amylopectin vary with the starch source, yielding to materials of different mechanical properties and biodegradability.56, 57 As the amylose content of starch increases, the elongation and strength also increase.

O

OH

OHHOH

H

O

OH

H

H

H

H

OOC

C

H

OH

ORH

H

OH

H OHOH

OH

OH

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Fig.6 Chemical structure of starch

Starch, which has low stability under stress, has different proportions of amylose and amylopectin ranging from about 10–20% amylase and 80–90% amylopectin depending on the source. Amylose is soluble in water and forms a helical structure. The relative amounts and molar masses of amylose and amylopectin vary with the starch source, yielding to materials of different mechanical properties and biodegradability.56, 57 The stability of starch under stress is not high.

Starch is often used as a biodegradation additive. The major role of starch has been found to provide higher oxygen permeability as it is consumed by microorganisms.58 Starch components are more rapidly degraded by microorganisms than the synthetic polymers in the blends. Biodegradable packaging materials may be divided into starch-filled and starch-based polymers. The starch content of starch-filled polymers is typically under 15% by weight. Starch is used as a natural filler in conventional plastics, especially polyolefins such as starch/high-density polyethylene. These films bio-deteriorate on exposure to microbial environments.59-61 Starch-based polymers contain relatively higher percentages of starch (more than 40%), blended with synthetic components. It makes them attractive for biodegradability and several blends have been prepared and tested for packaging applications. The range of mechanical properties of these polymers misses the mark concerning the required properties, yet improvements have been made by blending, copolymerization, grafting and cross-linking.59, 62

Starch can be plasticized through destructuration in the presence of specified amounts of water, heat and then extruded63 to be used as a thermoplastic. The most common plasticizers are polyols as glycerol.64 When used, polyols may induce a recrystallization reaction called retrogradation. The properties of the extruded starch depend on the water content and relative humidity.65 Thermoplastic starch (TPS) has a high sensitivity to humidity. Thermal properties of TPS have been shown to be more influenced by the content of water than the starch molecular weight.66 TPS thus obtained is almost amorphous. Biodegradation of starch is achieved via hydrolysis at the acetyl link by enzymes.67, 68 The α-1, 4 link is attacked by amylases while glucosidases attack the α-1, 6 link. The degradation products are non-toxic.

4 CONCLUSIONS AND PERSPECTIVES

Biodegradable polymer development is still in its early stages; at present, it covers a small portion of the current packaging market worldwide. Given the rise of environmental and health concerns, biodegradable polymers have been the topic of many studies, and of growing attention over the last two decades. The development of these polymers is a significant contribution to feasible advancements in view of the wider range of disposal options at lower environmental impacts. The changing attitude of societies and government legislations are becoming a further incentive to the development of biodegradable products.

The increased utilization of biomass as an energy source and raw materials is necessary in the long term due to the limited crude oil and natural gas resources available. However, polyolefins can present the same oxo-biodegradability as biopolymers, and they are more economical and effective during use. Therefore, some of them remain the materials of choice for packaging application.

At present, only a few groups of the aforementioned biopolymers are of market importance. The high cost associate with their development limits their profitability and competition in the market. The future of each biopolymer is dependent not only on its competitiveness but also on consumer affordability. The future outlook for development in the field of biopolymers materials is promising.

Biopolymers fulfill the environmental concerns but have some limitations in terms of economics and performance, such as thermal resistance, barrier, and mechanical properties. For value-added products, smart and responsive molecules are often introduced into the polymers for active and intelligent packaging, giving the ability to identify the properties of the products inside the package (quality, shelf-life, microbiological safety) and nutritional values, in the case of food packaging. To improve the properties of biodegradable polymers, the physical blending of different morphologies and physical characteristics are being examined, these include active packaging technology and natural fiber reinforcements. The use of biodegradable polymers is expected to increase in almost all areas where practicality is favored by reason. With the development of new technologies on biodegradable polymers, the reduction in cost is directly related to the increase in mass production. As environmental concerns continue to arise it is clear that biopolymer has a promising future.

ACKNOWLEDGMENTS

Financial support from the NSERC Strategic Network—Innovative Green Wood Fibre Product (Canada),

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and Natural Science Foundation of China (Grant No. 21466005) are gratefully acknowledged.

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