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Biodegradable Polymer, Biopolymer, Agro-polymer, Bioplastic, Biomaterial, Compostable Packaging

 
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- Bioplastics -

Biodegradable polyesters (PLA, PHA, PCL ...)

 

See also, biodegradable polymers or agro-polymers (starch, )

Figure 2 shows the chemical structures of various biodegradable polyesters (bioplastics). Table 1 shows main polyesters, which are commercially available. Besides, physical and mechanical properties of some commercial polyesters are given in Table 2.

Bioplastic

 

                                               Trade Name      Company

Agro-resources based polyesters:

               Polyhydroxyalkanoate (PHA)
                                        (PHB, PHBV)            Tirel                     Metabolix/ADM (USA)
                                        (PHB, PHBV)            Enmat                                Tianan (China)
                                        (PHB, PHBV)           Biocycle                          Copersucar (Brazil)
                                        (PHB, PHBV)           Biomer L                         Biomer (Germany)
                     (PHBHx, PHBO, PHBOd)            Nodax                       Procter & Gamble (USA)*

              Poly(lactic acid) (PLA)                  Natureworks                   Cargill (USA-Japan)
                                                                            Lacty                              Shimadzu (Japan)
                                                                           Lacea                        Mitsui Chemicals (Japan)
                                                                           Heplon                             Chronopol (USA)
                                                                           CPLA                    Dainippon Ink Chem. (Japan) 
                                                                            Futerro                     Total/Galactic (Belgium)
                                                                            PLA                              Galactic (Belgium)
                                                                          Biofront                            Teijin (Netherland)
                                                                            L- PLA                             Purac (Netherland)
                                                                           PLA                       Zhejiang Hisun Biomaterials (China)

 

Petroleum-based polyesters:

              Polycaprolactone (PCL)                     CAPA                             Perstorp (UK)
                                                                             Tone                           Dow chemical (USA)*
                                                                          Celgreen                              Daicel (Japan)

             
              Polyesteramide (PEA)                        BAK                             Bayer (Germany)*

             
              Aliphatic copolyesters (PBSA ...)  Bionolle                    Showa Highpolymer (Japan)
                                                                            EnPol                        Ire Chemical ltd (Korea)
                                                                         Skygreen                       SK Chemicals (Korea)
                                                                         PBS                    Anqing Hexing Chemical Co (China)

    
              Aromatic copolyesters (PBAT...)  Eastar Bio                   Eastman Chemical (USA)*
                                                                           Ecoflex                            BASF (Germany)
                                                                           Biomax                               Dupont(USA)
                                                                          Origo-Bi                            Novamont (Italy)

(*) The production of these polyesters has been stopped or sold.

Table 1: Main commercial biopolyesters

 

 

PLA

Dow-Cargill
(NatureWorks)

PHBV

Monsanto
(Biopol D400G)
HV=7 mol%

PCL

Solway
(CAPA 680
)

PEA

Bayer
(BAK 1095)

PBSA

Showa
(Bionolle
3000)

PBAT

Eastman
(eastar bio 14766)

Density.

1.25

1.25

1.11

1.07

1.23

1.21

Melting point, in °C (DSC)

152

153

65

112

114

110-115

Glass transition, in °C (DSC)

58

5

-61

-29

-45

-30

Cristallinity (in %)

0-1

51

67

33

41

20-35

Modulus, in MPa (NFT 51-035)

2050

900

190

262

249

52

Elongation at break, in %
(NFT 51-035)

9

15

>500

420

>500

>500

Tensile stress at break or max.,
in MPa (NFT 51-035)

-

-

14

17

19

9

Biodegradation*
Mineralization in %

100

100

100

100

90

100

Water permeability
WVTR at 25 °C (g/m2/day)

172

21

177

680

330

550

Surface tension**  (g) in mN/m.
gd (Dispersive component)
gp (Polar component)

50
37
13

-
-
-

51
41
11

59
37
22

56
43
14

53
43
11

(*) At 60 days in controlled composting according to ASTM 5336.
(**) Determinations from contact angles measurements of probes liquids

Table 2: Biopolyesters Properties

 

Agro-resources-based polyesters

        • Poly(lactic acid)

Lactic acid can be produced in different ways: chemical or biological i.e., by fermentation of carbohydrate from lactobacillus [11]. The enantiomeric monomers (D and L) are polycondensed via its cyclic dimmer (lactide) by ring-opening polymerisation (ROP) to a high molecular weight polymer [10, 12-13].
Compared to the others biodegradable polyesters, PLA is the product that at the present has one of the highest potential due to its availability on the market and its low price [14-16]. For instance, Cargill-Dow has developed processes that use corn and other feedstock to produce different PLA grades (NatureWorks®) [18]. For this company, the estimated production in 2006 is 50-70 KTons. Actually, it is the highest production of biodegradable polyester. In 2007, its price is less than 3€/Kg. Different companies such as Mitsui Chemicals (Japan), Galactic (Belgium), Treofan (Netherland) or Dainippon Ink Chemicals (Japan) produce smaller PLAs outputs.
Properties of PLA are highly related to the ratio between the two mesoforms D and L. Commercially available, we can find 100% L-PLA which present a high cristallinity (C-PLA) and copolymers of poly(L-lactic acid) and poly(D,L-lactic acid) which are rather amorphous (A-PLA) [17-19]. PLA can shows crystalline polymorphism [20] which can lead to different melting peaks [21] with a main transition at 152°C for the D,L-PLA (see Table 2). Furthermore, PLA can be plasticized using oligomeric lactic acid (OLA), citrate ester [22] or low molecular weight polyethylene-glycol (PEG) [21, 23]. The effect of plasticization increases the chains mobility and then favours the PLA organization and cristallization. We obtain, after plasticization, a cristallinity ranging between 20 and 30%.
PLA presents a medium water and oxygen permeability level [24] comparable to polystyrene [25]. These different properties associated with its tunability and its availability favour its actual developments in different packaging applications (trays, cups, bottle, films …) [15, 18].
McCarthy (1999) [26] showed that A-PLA presents a soil degradation rate much slower compared to PBSA. PLA is presumed to be biodegradable although the role of hydrolysis vs. enzymatic depolymerization in this process remains open to debate [27]. Composting conditions are found only in industrial units with a high temperature (above 50°C) and a high relative humidity (RH) to promote chain hydrolysis. But, according to Tuominen et al. (2002) [28], PLA biodegradation does not exhibit any eco-toxicological effect.

        • Polyhydroxyalkanoates

Polyhydroxyalkanoates can be produced in different ways, chemically or biologically, by fermentation from feedstock. This family comprises mainly a homopolymer, polyhydroxybutyrate (PHB), and different copolyesters, polyhydroxybutyrate co-hydroxyalkanoates such as polyhydroxybutyrate co-hydroxyvalerates (PHBV) (see Figure 2), or polyhydroxybutyrate co-hydroxyhexanoate (PHBHx), polyhydroxybutyrate co-hydroxyoctonoate (PHBO) and polyhydroxybutyrate co-hydroxyoctadecanoate (PHB0d).
PHB is a natural polymer. It is an intracellular storage product of bacteria and algae. After fermentation, PHB can be obtained by solvent extraction. Recently, Monsanto has developed genetically modification of plants to make them produce small quantities of PHB [18].
PHB is a highly crystalline polyester (80%) with a high melting point, Tf=173-180°C (Monsanto data), compared to the other biodegradable polyesters. Glass transition temperature (Tg) is around 5°C. The homopolymer shows a narrow window for the process condition. To ease the transformation, PHB can be plasticized with citrate ester, but the corresponding copolymer (PHBV) is more adapted for the process.
PHBV can be produced by bacterial fermentation (e.g., with alcaligens eutrophus) [29] of bioproducts such as glucose containing propionic or valeric acid, followed by an extraction step and purification of the polymer. PHBV can be also produced from butyrolactone and valerolactone with an oligomeric aluminoxane catalyst [30]. According to the synthesis, we obtain different structures, isotactic with random stereosequences for the bacterial copolyesters and with partially stereoregular block for the synthetic copolyesters.
A large range of bacterial copolymer grades had been industrially produced by Monsanto under the Biopol® trade mark, with HV contents reaching 20%. The production was stopped at the end of 1999. Metabolix bought Biopol® assets in 2001. Currently, different small companies produce bacterial PHBV, e.g., Copersucar-Biocycle (PHB Industrial-Brazil) produces PHBV (HV=12 %) 45% crystalline, from sugar cane molasses [31]. Recently, Procter and Gamble has begun to develop a large range of polyhydroxybutyrate co-hydroxyalkanoates (PHBHx, PHBO, PHBOd). Industrial production is not planned.
Figure 2 and Table 2 give respectively the chemical structure and the properties of some PHBV. Material properties can be tailored by varying the HV content. An increase of the HV content induces an increase of the impact strength and a decrease of:
- melting temperature and glass transition [32],
- crystallinity [33],
- water permeability [33],
- tensile strength [34]
Besides, PHBV properties can evolve when plasticization occurs, e.g., with citrate ester (triacetin) [34-35]. The polyhydroxyalkanoates as the PLAs are sensitive to the process conditions. Under extrusion, we obtain a rapid diminution of the viscosity and the molecular weight due to macromolecular linkage by increasing the shear level, the temperature and/or the residential time [36].
The kinetic of enzymatic degradation is variable according to the cristallinity, the structure, [5, 31] and then, to the processing history [37]. Bacterial copolyesters are more biodegradable than synthetic copolyesters [38].

 

    Petroleum-based polyesters

A large number of biodegradable polyesters are based on petroleum resources obtained chemically, from synthetic monomers [12-18]. We can distinguish (see Table 1), according to the chemical structures (see Figure 2), polycaprolactones, polyesteramides, aliphatic or aromatic copolyesters. All these polyesters are soft at room temperature.

        • Polycaprolactone

Poly(e-caprolactone) is obtained by ROP (Ring Opening Polymerisation) of e-caprolactone in the presence of aluminium isopropoxide [12-13, 38]. PCL is widely used as a PVC solid plasticizer or for polyurethane applications. But, it finds also some application based on its biodegradable character in domains such as controlled release of drugs, soft compostable packaging, … Different commercial grades are produced by Solvay (CAPA®) or by Union Carbide (Tone®).
Figure 2 and Table 2 give respectively the chemical structure and the properties of this polyester. PCL shows a very low Tg (-61°C) and a low melting point (65°C), which could be a handicap in some applications. Therefore, PCL is generally blended [27, 30, 39-40] or modified (e.g., copolymerisation, crosslink [41]).
Tokiwa and Suzuki (1977) [42] have discussed the hydrolysis of PCL and biodegradation by fungi. They have shown that PCL can be easily enzymatically degraded. According to Bastioli (1998) [27], the biodegradability can be clearly claimed but the homoplymer hydrolysis rate is very low. The presence of starch can significantly increase the biodegradation rate of PCL [39].

        • Polyesteramide

Polyesteramide was industrially obtained from the statistical copolycondensation of polyamide (PA 6 or PA 6-6) monomers and adipic acid [18, 43]. Bayer had developed different commercial grades under BAK® trademark but their productions stopped in 2001. Figure 2 and Table 2 show, respectively, the chemical structure and the properties of this polyester. It is the polyester, which presents the highest polar component, and then it shows good compatibility with other polar products, e.g., starchy compounds. On the other hand, it presents the highest water permeability (see Table 2).
Currently, the environmental impact of this copolymer is still open to discussion. Fritz (1999) [44] had shown that this biodegradable polyester presented after composting a negative eco-toxicological impact but more recently, Bruns et al. (2001) [45] have infirmed these results. These authors discussed Fritz’s experiments and more precisely the composting methods used.

        • Aliphatic copolyesters

A large number of aliphatic copolyesters are biodegradable copolymers based on petroleum resources. They are obtained by the combination of diols such as: 1,2-ethanediol, 1,3-propanediol or 1,4-butadenediol, and dicarboxylic acid: adipic, sebacic or succinic acid. Showa Highpolymer (Japan) has developed a large range of polybutylene succinate (PBS) obtained by polycondensation of 1,4-butanediol and succinic acid. Polybutylene succinate/adipate (PBSA), presented on Figure 2, is obtained by addition of adipic acid. These copolymers are commercialised under the Bionolle® trade mark [18]. Table 2 shows the properties of such a terpolymer. Ire chemical (Korea) commercialises exactly the same kind of copolyesters under EnPol® trade mark. Skygreen®, a product from SK Chemicals (Korea) is obtained by polycondensation of 1,2-ethanediol, 1,4-butadenediol with succinic and adipic acids [46]. Nippon Shokubai (Japan) also commercialises an aliphatic copolyester with Lunare SE® trademark. These copolyesters properties depend on the structure [47] i.e., the combination of diols and diacids used.
These products biodegradability depends also on the structure. The addition of adipic acid, which decreases the cristallinity [48] tends to increase the compost biodegradation [49]. According to Ratto et al. (1999) [50], the biodegradation results demonstrate that although PBSA is inherently biodegradable, the addition of starch filler significantly improve the rate of degradation.

        • Aromatic copolyesters

Compared to totally aliphatic copolyesters, aromatic copolyesters are often based on terephtalic diacid. Figure 2 and Table 2 show respectively the chemical structure and the properties of such products (e.g., see Eastar Bio® from Eastman, which production stopped in 2002). Besides, BASF and DuPont commercialise aromatic copolyesters with Ecoflex® [18] and Biomax® trade marks, respectively. Biomax® shows a high terephtalic acid content which modifies some properties such as the melting temperature (200°C). But, according to Muller et al. (1998) [47], an increase of terephthalic acid content tends to decrease the degradation rate while aromatic like aliphatic copolyesters degrade totally in microorganisms environment (compost). Ecoflex® biodegradation has been analysed by Witt et al. (2001) [51], they concluded that there is no indication for an environmental risk (ecotoxicity) when aliphatic-aromatic copolyesters of the Ecoflex-type are introduced into composting processes.

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