Compostable Packaging

Compostable Packaging

1 Introduction

Composting is considered to be a good way of recycling because it is “nature's own way” [1]. A definition of compostable plastics, as “a plastic that undergoes degradation by biological processes during composting to yield carbon dioxide, water, inorganic compounds, and biomass at a rate consistent with other known compostable materials and leaves no visually distinguishable or toxic residues”, is given in ASTM D400 and ISO/DIS 17088 [1]. The difference between biodegradable and compostable plastics is that there are established requirements on time taken to break down at a set temperature for compostable plastics [2].

Due to increasing fossil fuel prices and concern for the environmental impact of traditional plastics, recently there has been a great increase in the development and manufacturing of compostable or biodegradable plastics recently [2]. s suggest that the municipal packaging waste was 78.81 million tons produced by the USA in 2003, and was 56.3 million tons in Europe in 2005 and 3.3 million tons in Australia in 2004 [3]. Plastics are the major materials for packaging. Although recycling is encouraged as a good way to reduce packaging wastes, it is impractical for those which have already been contaminated [3]. Therefore, composting is a promising alternative. The worldwide manufacturing capacity for bioplastics was 10 000 tons in 1995, while the global demand was estimated to be more than 100 000 tons by 2011 [2, 4].

In this review, five typical categories of compostable plastics are studied on their basic definition, physical and thermal properties, methods of preparation, biodegradion mechanism and biodegradability.

2 Compostable polymer materials

2.1 PLA

2.1.1 Definition, structure and preparing

Lactic acid occurs naturally in animals and foods such as yogurt and beer [5]. As a chiral molecule, it has two stereo isomers, and the structures are shown in 2. It can be manufactured commercially by chemical synthesis or carbohydrate fermentation [1, 5]. Plants containing sugar or starch such as wheat, sugar beet, maize or agricultural waste can be used as feed stocks [1].

PLA is usually prepared by polycondensation, ring-opening polymerization, chain extention and grafting [1]. Products produced by polycondensation of lactic acid monomers have a generally lower molecular weight, which is lower than 1.6×104, whereas products produced by ring-opening polymerization of lactides have a generally higher molecular weight, which ranges from 2×104 to 6.8×104 [6]. The lactides used in the ring-opening polymerization can be generated by depolymerization of low-molecular-weight polylactic acid [1]. Today, more and more PLA copolymers and blends have been investigated for a combination of desirable properties [1].

2.1.2 Properties and application

PLA polymers have a good combination of properties comparable to those of conventional thermoplastics. However, the molecular weight, macromoleclar structure and the degree of crystallization, which are determined by the reaction conditions in the polymerization process, vary a lot to give products that can meet different requirements. [1]

Melting temperature and crystallinity are affected by the ratio of L- to D-lactide. Pure poly(L-lactide) is a semicrystalline polymer. The glass transition temperature (Tg) of it is about 55℃ and the melting temperature (Tm) is about 180℃. With an incorporation of D-lactide, both Tm and the degree of crystallinity will decrease. [1] Most commercial products of PLA are amorphous polymers.

PLA polymers have good mechanical properties such as stiffness and strength, but like other amorphous polymers such as polystyrene, have poor impact strength. PLA has good electrical properties in terms of insulation like crosslinked polyethylene (XLPE). As a preferable packaging material, PLA also has advantages of deadfold and twist retention, low temperature heat sealability, and flavour and grease resistance. By orientation, blending or copolymerization, the toughness of PLA can also be considerably improved. [1]

As PLA polymers can provide good properties at a relatively low price, they have been developed into many commodity plastics [7]. The main applications include packaging, fibres and fabrics for textiles and non-wovens, components for electric appliances and electronics, and for automobile interiors, carpets, and some others [1].

2.1.3 Biodegradation

According to Auras [8],PLA degrades in two stages. In the first step, hydrolysis, random non-enymatic chain scission, of the ester groups occurs and the average molecular weight is reduced ( 3). This step can be accelerated by acids or bases and is also affected by temperature and moisture levels. In the second step, the lower molecular weight lactic acid oligomers are metabolized by microorganisms, yielding carbon dioxide, water and biomass.

It is reported that PLA cannot degrade in typical garden compost, due to its resistance to attack by microorganisms in soil or sewage under ambient conditions [1]. The first stage of degradation must proceed at elevated temperatures (above 58℃) for the microorganism to digest lower molecular weight lactic acid and water soluble compounds [1]. 4 presents a typical degradation curve of PLA under a composting condition at 60℃.

2.2 Cellulose

2.2.1 Definition, structure and preparing

Cellulose is the most abundant organic compound on earth [1]. It is the major component of the cell wall of higher plants [1]. Cellulose used in industry comes from wood and dry pulp from many agricultural byproducts such as sugarcane and corn stalks [9]. 5 presents schematically the structure of cellulose. Due to such structure, it has very poor solubility in common solvents [1]. It is also not melt-processible and esterification is conducted with acid to improve its behaviour [9].

There are three common cellulose esters, which are cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB).

Bioceta, plasticized cellulose acetate (CA), is produced from cotton flakes and wood pulp through esterification with acetic anhydride [1]. CAP is mixed esters prepared by reaction of cellulose, acetic and propionic acids and anhydrides in the presence of sulphuric acid. CAB is mixed esters produced by reaction of cellulose, acetic and butyric acids and anhydrides in the presence of sulphuric acid. [10] Other properties, especially water resistance, can be improved by introducing some functionalities such as phthalate and succinate [11]. The structures of CAB-SU160 made by Eastman Chemical Company are shown in 5. The increasing level of acid-treatment or plasticizers added make grades of cellulose materials vary. More plasticizer added results in increase of flow and toughness but decrease of creep resistance [10].

2.2.2 Properties and application

Cellulose esters are amorphous thermoplastics. They are renowned for toughness, clarity and chemical resistance [12]. CA has a high vapour permeability and limited heat resistance, poor electrical insulation and dimensional stability, and its water absorption rate is also high compared to PVC. CAP and CAB are slightly softer, lighter and flow more easily, and have lower heat distortion temperatures. Some other cellulose derivatives have attained good water proof property. [10] Table 4 shows some properties of two commercial cellulose esters.

Table 4 Properties of cellulose esters [1]

Cellulose acetate propionate Albis CAP CP800 (10% plasticizer)

Cellulose acetate butyrate Albis CAB B900 (10% plasticizer)

Physical properties

Density (g/cm3)

1.21

1.19

Water absorption at 24 hours

1.6

1.4

Mechanical properties

Tensile strength at yield (MPa)

31.7

28.3

Elongation at break (%)

30

30

Flexural modulus (MPa)

1240

1170

Flexural strength (MPa)

41.4

37.2

Thermal properties

Vicat softening point (℃)

102

104

Cellulose esters are important industrial coating materials. It is also widely used in composites as binder, filler, and laminate layers. Other application areas include thin films, containers, handles, spectacle frames, optical applications, automotive applications, toys, writing instruments, and electric insulation films, lights and casings. [1,10] One company currently producing compostable cellulose-based products films for packaging application in the UK is Innovia Films [13].

2.2.3 Biodegradation

Microorganisms can produce a series of enzymes with different specificities to degrade cellulose. There are two types of cellulases, endoglucanases (endo-1,4-β-glucanases) (EGs) and cellobiohydrolases (exo-1,4-β-glucanases) (CBHs), that hydrolyse the β-1,4-glycosidic linkage of cellulose [1]. It was found that the biodegradability of cellulose acetate (CA) was affected profoundly by moisture conditions, degree of substitution (DS) and substituent size

PHA

[1, 14]. The CA DS-1.7 film with moisture contents of 60, 50 and 40% disappeared in 6, 16 and 30 days, respectively. Smaller substituent size can give tolerance of higher DS [1].

2.3 PHA

2.3.1 Definition, structure and preparing

Polyhydroxyalkanoates (PHA) are produced by many gram-positive and gram-negative bacteria as “intercellular reserve materials” when they are fed carbon sources under conditions of nutrients stress [3]. The generic structure of PHAs is shown in 7, where R can be hydrogen or hydrocarbon chains of up to C 15 in length.

When x=1 and R=CH3, the polymer is poly(3-hydroxy butyrate) (PHB). Homopolymer PHB is a brittle and crystalline thermoplastic. It decomposes just at its melting point. Thus extensive researches have been carried out on its copolymers for better properties [1].

PHB and the family of natural polyesters having the same three-carbon backbones are very important members of PHAs for their physical properties and the activities of the enzymes involved [1].

PHAs are produced from many substrates, including renewable resources (sucrose, starch, cellulose, triacylglycerols), fossil resources (methane, mineral oil, lignite, hard coal), byproducts (molasses, whey, glycerol), chemicals (propionic acid, 4-hydroxybutyric acid) and carbon dioxide [15]. Synthesis of PHAs can happen both in vitro via PHA-polymerase catalysed polymerization, and in vivo with batch, fed-batch, and continuous (chemostat) cultures [16].

2.3.2 Properties and application

The mechanical properties of PHAs vary considerably from hard to elastic due to different composition of monomer units [1]. It is often loosely compared to polypropylene which has similar melting point [1]. Table 5 presents a comparison of mechanical properties of PHAs and polypropylene.

Table 5 comparison of mechanical properties of PHAs and polypropylene [1]

Polymer

Copolymer content

Melting temperature,


Young modulus, GPa

Tensile strength, MPa

Elongation at break,

%

PP

-

170

1.7

34.5

400

P(3HB)

-

179

3.5

40

5

P(3HB-co-3HV)

3 mol% 3HV

170

2.9

38

-

P(3HB-co-3HV)

25 mol% 3HV

137

0.7

30

-

P(3HB-co-4HB)

3 mol% 4HV

166

-

28

45

P(3HB-co-4HB)

90 mol% 4HV

50

100

65

1080

P(4HB)

-

53

149

104

1000

P(3HB-co-3HHx)

52

-

20

850

The property that distinguishes PHBs from most current bioplastics is that it is water insoluble and relatively resistant to hydrolytic degradation. However, homo PHBs are stiff, brittle, and thermo unstable as it will degrade just above the Tm. [1]

PHAs were initially used in packaging films, containers and paper coating. Now applications also include housewares, electrical and electronics, adhesive and soil stabilization, paints,

 

TPS

appliances, and automotive. [16]

2.3.3 Biodegradation

PHAs degrade under exposure to soil, compost, or marine sediment [16]. They are broken down by enzymes secreted by microorganisms into molecular building blocks called hydroxyacids, but quite resistant to moisture [16]. Erosion happens selectively from the surface, where polymer chains undergo initially endo-scissions (randomly throughout the chain) and then by exo-scissions (from the chain ends) degradation [1].

PHA have been reported to be compostable even at a maximum temperature of around 60 ℃ with moisture levels at 55% and at temperatures as low as 6 ℃ in a aquatic environment within 254 days [14]. PHA is also “home compostable”.

2.4 TPS

2.4.1 Definition, structure and preparing

Starch is the end product of photosynthesis as an energy storage which exists in large quantity in cereals, legumes and tubers [1].

It is cemicrystalline polymers composed of linear polysaccharide-amylose and highly branched poloysccharide-amylopectin [17]. Amyloses are built up by repeating α-D-glucopyraosyl units which contain 99% (1→4)-α and 1% (1→6)-α-linkages [18]. Amylopectins are built up by (1→4)-α linked backbones and 5% (1→6)-α linked branches [17]. Natural starches contain 15-30% amylase and 85-70% amylopectin [1].

Starch based thermoplastics are blends or composites produced by introducing biodegradable polymers such as PLA, PVA, poly(ε-carprolactone) and polyesteramide. One of the most popular of preparing them is to apply dry star and a swelling or plasticizing agent in compound extruders in the presence of heat without water. [1]

Plasticizers can be water, glucose, maltose, glycerol urea or glycol. Melt-flow accelerators used include lecithin, glycerol, monostearate and calcium stearate. When the water content exceeds 5%, a de-structural starch is always fomed under pressure and temperature. [1]

2.4.2 Properties and application

Thermoplastic starch (TPS) are generally good oxygen barrier and not electrically chargeable [10]. The properties of TPS are mainly determined by the type and amount of plasticizer. It was reported that shorter glycols were more effective than high molecular weight ones for plasticizing starch. [19]

The Tg is always higher with smaller amount of plasticizers. The mechanical properties of both starch and starch-based plastics are dependent on plasticizers. With the decrease of Tg, modulus and tensile strength decrease and elongation increases. [1] Table 6 shows properties of some thermoplastic starch.

Table 6 Properties of thermoplastic starch [1]

Potato thermoplastic starch

Wheat thermoplastic starch*

Density (g/cm3)

1.34-1.39

Tensile strength at yield (MPa)

22

1.4-21.4

Elongation at yield (%)

3

3-104

Tensile modulus (MPa)

1020

11-1144

Glass transition temperature (℃)

(-)20-43

α-transition (DMTA) (℃)

1-63

*Properties after equilibrium at 23℃ and 50%, 6 weeks; glycerol to starch ratio: 0.135-0.538; water content: 16-0 wt%.

TPS is originally water soluble, not easy to process and not very strong and tough. However, the disadvantages have been modified by blending graft copolymerization or esterification. [10]

Starch based polymers are mainly used in packaging which includes films & bags, foams for loose-fill packaging, and mouldable products. They are also used in hot melt adhesives and laminated papers. [1, 10] A commercially available biopolymer packaging material based on thermoplastic starch is Mater-Bi produced by Novamont [20].

2.4.3 Biodegradation

TPS degrades rapidly in soil and water [10]. it was found to be so easily biodegradable that the experiment conditions and the types of substrates merely had an influence on the Biodegradable polymers from petrochemical sources evolution of the carbon repartitioned [21].In another assaying, a complete minerlization of starch was observed after 44 days. On the other hand, the average biodegradation of cellulose was 96.5% after 47 days [22].

2.5 Biodegradable polymers from petrochemical sources

2.5.1 Definition, structure and preparing

Aliphatic polyesters and aromatic-aliphatic copolyesters are representatives of biodegradable polymers synthesized from petrochemical feed stocks. Poly(butylenes succinate), poly(ε-caprolactone), poly(butylenes succinate adipate), poly(ethylene succinate) and poly(ethylene succinate adipate) are now commercially available aliphatic polyesters ( 9). Among them, Poly(butylenes succinate), which is a promising polymer, is chemically synthesized by polycondensation of 1,4-butanediol and succinic acid. However, aliphatic polyesters have the weakness of biological susceptibility. Therefore, aromatic polyesters, which are regarded as non-biodegradable, are introduced to form aliphatic-aromatic copolyesters with both good biodegradability and mechanical performance. 10 shows a structure of aliphatic-aromatic copolyester. Successful products with satisfying properties have been manufactured by combining 1,4-butanediol, adipic acid and terephthalic acids. [1]

The traditional method of preparation, polycondensation, suffers from drawbacks such as the need of removing reaction by-products, high temperature and long reaction time for high molecular weight polymers. As a result, the ring-opening polymerization is a preferable alternative when high molecular weight polymers produced under lower temperature and in a shorter time are required. Recent developments have made chain lengths controllable by employing catalysts. Enzyme-catalysed polymer synthesis has also been developed as a novel method of preparation. [1]

2.5.2 Properties and application

Poly(butylenes succinate) (PBS) is mentioned as a promising commercially available aliphatic polyester, because it is easy to process, and has good thermal and chemical resistance. As a result, it can be made into injection moulded products and also textiles as filaments and yarns. [1]

One of the major manufacturers of aliphatic polyesters and copolyesters is Showa Highpolymer, whose products are named under Bionolle. They are white crystalline thermoplastics. The melting points are around 100˚C and the glass transition temperatures are more than 10 degrees below 0˚C. They are relatively weaker but tougher than PLA. Table 7 shows some properties of Bionolle. It was reported that Tg, Tm, crystallinity, melt viscosity and spherulite growth rate decreased when the degree of the ethyl and n-octyl chain branches increased. [23]

Table 7 Properties of typical grades of Bionolle [23]

Property

PBSU

#1000

PBSU

#2000

PBSU

#3000

PESU

#6000

LDPE

F082

HDPE

5110

PP 210

MFR190˚C(g/10 min)

1.5

4.0

28

3.5

0.8

11

3.0*

Density(g/cm3)

1.26

1.25

1.23

1.32

0.92

0.95

0.90

Melting point (˚C)

114

104

96

104

110

129

163

Glass transition temperature(˚C)

-32

-39

-45

-10

-120

-120

-5

Yielding strength (Pa)

0.336

0.270

0.192

0.209

0.100

0.285

0.330

Elongation (%)

560

710

807

200

700

300

415

Stiffness 103(kg/cm3)

5.6

4.2

3.3

5.9

1.8

12.0

13.5

Izod impact strength** (kg-cm/cm) 20˚C

30

36

>40

10

>40

4

22

Combustion heat (cal/g)

5550

5640

5720

4490

>11000

>11000

>11000

*MFR was measured at 230˚C.

**Izod impact strength was measured with notched samples.

Bionolle polyesters and copolyesters are used to produce mulch film, cutlery, containers, packaging film, bags and “flushable hygiene products” [23].

As representatives of aliphatic- aromatic copolyesters, both poly(trimethylene terephthalte) (PTT) and poly(butylenes adipate-co-terephthalate) (PBAT) have advantages of improved thermal stability and mechanical properties compared to biodegradable aliphatic polyesters, and good processability. [1]

Typical properties of PTT include high crystallinity, hardness, strength, and toughness.

Commercial PBAT under the tradename Ecoflex (BASF), which contains approximately 42-45 mol% terephthalic acid, was reported to have a Tg at -30˚C, a Tm at 110-115˚C. The physical and mechanical properties of Ecoflex are similar to those of LDPE. [1] It stands out for flexibility and good tensile strength especially suitable for film applications [24]. It is also grease, moisture and temperature variation resistant. Modification is allowed to give it a wide variety of applications ranging from packaging to filament or coating of other biodegradable materials such as starch, PLA, PHA and cellulose [25].

2.5.3 Biodegradation

There are three general conditions for degradation mechanisms. The first one is biocorrosion, which is usually an undesirable process that can change material parameters such as tensile strength and flexibility, or the colour. The second one is in vivo degradation, of which the degradation mechanisms are often abiotic, e.g. non-enzymically catalysed hydrolysis. The third one is biodegradation in the environment. Plastics materials are usually not water soluble and have a molar mass which is too high for transporting through cell membranes into microbes. Therefore, the polymer chains are first cleaved into by enzymes excreted from microorganisms. Then the shorter and water soluble intermediates are dissolved into the mediums, assimilated into the microbial cells and metabolized there to produce final products. [26]

Perspectives

Pure aromatic polyesters were reported to be resistant to any hydrolytic degradation, except for under drastic chemical treatments, e.g. sulfuric acid at 150℃ [27]. They are regarded as not biodegradable at a reasonable degradation rate [26]. Aliphatic polyesters and copolymers such as Bionolle are biodegradable in compost, in moist soil, in fresh water with activated sludge and in sea water [23]. However, aliphatic-aromatic copolyesters with aromatic acid under a maximum content are completely biodegradable. For PBAT, it was reported that the terephthalic acid content of approximately 50-60% was the limit for biodegradability [1]. One the commercially available PBAT, Ecoflex was observed to be able to break down within a few weeks without leaving toxic residues under certain conditions which were referred to in ASTM D 6400 [29].

Individual strains isolated and identified from compost were studied to depolymerise PBAT by Kleeberg and co-workers [26]. 30 out of 61 isolates were able to attack a PBAT (40 mol% terephthalic acid, 60 mol% adipic acid). The two most active strains were identified as Thermonospora fusca.

2.6 Perspectives

Compostable polymer materials are stated to be more environmentally friendly than traditional polymers. Although they have some drawbacks such as recycling issues, high cost, the long-term perspective of them are still very promising, considering that they are under developing stage while the technologies for traditional polymers have been relatively mature.

The main advantage of compostable bioplastics over petrochemical-based polymers is less environmental impact. That's due to lower energy consumption in manufacture, lower greenhouse gas emission, and not neglectable benefit of end-of-life value by composting.

The first barrier for developing compostable polymer materials is the issue of high price.

Table 8 shows the price of some compostable plastics in 2002. Compared to conventional plastics such as LDPE and PS (prices ca. 1 USD/kg), the prices of compostable polymer materials are too high according to Table 8. However, the costs in general have been largely reduced since 1998. The decrease of costs not only depends on the optimism of processing, but also depends on the scaling up of production. [1]

Table 8 Evolution of price of compostable polymer materials [1]

Polymer

2002 price, EUR/kg

PHB

20

PHA

10-20

PLA

2.2-3.4

Bionolle (aliphatic polyesters based on succinic acid)

3.5

Ecoflex (aliphatic-aromatic polyesters)

3.1 (2004)

Mater-Bi (blends of starch)

2.5-3

TPS

1, 0.2-0.5

PET

1-1.5 (2004)

PE

0.8 (2004)

The second problem is about disposal and recycle of compostable plastics. It was reported that starch based bioplastics can degrade during recycling and discolour the recyclate if mixed with other plastics [2]. Some composting and recycling manager argued that it was problematic to sort compostable material from conventional ones, e.g. PLA from PET and HDPE, although technology allows [4]. Therefore, new collecting system and registrations are required.

Another barrier is caused by some legislation that introduces taxes or charges on plastic bags. However, there is also some other legislation that promotes compostable polymer materials. The German Packaging Ordinance now gives special provision for certified compostable packaging which is exempt from DSD (Duales System Deutschland) recycling fee until 2012. Any plastic products labeled “compostable”, “biodegradable” and “degradable” must meet current standards such as ASTM in USA, EN 13432:2000 in Europe, or DIN V54900 in Germany. [1]

The demand for compostable polymer materials is estimated to keep increasing. In Europe, forecasts of bio-based plastics have been made to be 500-1000 kt in 2010 by the European Climate Change Programme, and to be 2000-4000 kt, half of which would consist of compostable plastics in 2010 by the International Biodegradable Plastics Society. In Japan, estimates on biodegradable polymer consumption were made be 200 kt in 2010 and 1500 kt in 2015 by Japanese Biodegradable Plastics Society. The market of biodegradable polymer in the USA will also grow to 67.8 million tonnes in 2010 according to research by Frost & Sullivian firm. New applications are being innovated into automotive, computer and consumer electronic markets. [1]

To conclude, the potential of compostable polymer materials is promising, but it depends on the reduction of their price and the increase of fossil oil price, manufacture optimization, legislative support, development of composting facilities and public awareness.

References

[1] E. Rudnik, “Compostable polymer materials,” Elsevier Ltd., 2008.

[2] D. Clayton, U. Mines, “Bioplastics: degradable, renewable, compostable,” C-Tech Innovation Ltd., 2007.

[3] G. Kale, T. Kijchavengkul, R. Auras, M. Rubino, S.E. Selke, and S.P. Singh, "Compostability of bioplastic packaging materials: an overview," Macromolecular bioscience, vol. 7, issue 3, 2007.

[4] “Bigging up bioplastics packaging,” The Packaging Professional, vol. 32, No. 6, IOM Communications Ltd, 2009.

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[9] R. Smith, “Biodegradable polymers for industrial application,” Woodhead, 2005.

[10] N. Thomas, Lecture 10 handout of “Industrial polymers”, Loughborough University, 2009.

[11] K. Edgar, "Advances in cellulose ester performance and application," Progress in Polymer Science, vol. 26, 2001.

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[13] http://www.innoviafilms.com/ accessed on 22/01/2010.

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[19] A.L. Da Roz, et al., “The effect of plasticizers on thermoplastic starch compositions obtained by melt processing,” Carbohydrate Polymers, vol.63, issue 3, 2006.

[20] http://www.materbi.com/ accessed on 22/01/2010.

[21] “Biodegradation study of a starch and poly(lactic acid) co-extruded material in liquid composting and inert mineral media,” International Biodeterioration & Biodegradation, vol. 50, issue 1, 2002.

[22] F. Degli-Innocenti, M. Tosin, C. Bastioli, “Evaluation of the biodegradation of starch and cellulose under controlled composting conditions,” Journal of polymers and the environment, vol.6, No.4, 1998.

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