Effect of a natural polyphenolic extract on the properties of a biodegradable starch-based polymer

Effect of a natural polyphenolic extract on the properties of a biodegradable starch-based polymer

Polymer Degradation and Stability 96 (2011) 839e846 Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: ww...

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Polymer Degradation and Stability 96 (2011) 839e846

Contents lists available at ScienceDirect

Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

Effect of a natural polyphenolic extract on the properties of a biodegradable starch-based polymer P. Cerruti a,1, G. Santagata a,1, G. Gomez d’Ayala a, V. Ambrogi b, *, C. Carfagna a, M. Malinconico a, P. Persico a a b

Institute of Polymer Chemistry and Technology (ICTP-CNR), via Campi Flegrei 34, 80078 Pozzuoli (Na), Italy Department of Materials and Production Engineering, University of Napoli “Federico II”, p.le Tecchio 80, 80125 Napoli, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 November 2010 Received in revised form 31 January 2011 Accepted 4 February 2011 Available online 13 February 2011

A polyphenol-containing extract from winery bio-waste (EP) has been used as an additive for a starchbased polymer (Mater-Bi). EP was used to tailor Mater-Bi properties, thus avoiding the use of synthetic polymer additives. It was found that EP was able to efficiently modulate the processing, mechanical, thermal and biodegradation properties. The observed decrease in melt viscosity showed that EP could improve productivity in polymer processing. Owing to the plasticizing activity of the additive, larger values of elongation at break were found. Moreover, the Mater-Bi crosslinking, which occurs upon thermal aging, was delayed in the presence of EP. Finally, the bio-disintegration rate of doped Mater-Bi decreased, thus suggesting that EP acted as an antimicrobial agent by interfering with the bio-digestion of the polymer films. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Starch Thermal oxidation Natural antioxidants Bio-disintegration Polyphenols Processing aid

1. Introduction Renewable biodegradable polymers represent a valid alternative to traditional oil-derived polymers in packaging and agricultural applications, to reduce environmental impact and to develop eco-sustainable cost-competitive products [1]. When disposed in bioactive environments, biopolymers are degraded by the enzymatic action of microorganisms, such as bacteria, fungi and algae, and converted into biomass, CO2, CH4, water and other natural substances [2]. Depending on their origin, biodegradable polymers can be synthetic or natural. The latter, which are referred to as biopolymers, include polysaccharides, proteins and polyesters produced by microorganisms [3,4], whereas poly(vinyl alcohol) and polyesters are the most representative among the synthetic biodegradable polymers [5]. The use and industrial development of biodegradable polymers is limited because of their poor chemicalephysical properties, unsuitable mechanical performances and difficult processability [6]. Therefore, research has focused growing attention on biodegradable polymer systems, such as composites and blends, in which

* Corresponding author. Tel.: þ39 0817682511; fax: þ39 0817682404. E-mail address: [email protected] (V. Ambrogi). 1 These authors equally contributed to this work. 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.02.003

the biodegradable polymeric matrix is associated with reinforcing fibers [7], organic and inorganic additives [8] and degradable or non-degradable polymers [9]. Among biopolymers, starch is one of the most widely investigated, as it is widely available and easily modified to get a thermoplastic polymer [10]; nevertheless, due to the hydrophilic nature responsible for fast degradation via hydrolysis, thermoplastic starch applications are limited [6]. To overcome this experimental drawback, starch is generally modified by blending with synthetic polymers, such as polyesters or vinyl alcohol copolymers [10]. This approach has been adopted by Novamont under the MatereBi trademark [11]. Target markets of Mater-Bi include packaging materials, disposable cutlery, consumer goods and agricultural tools [12]. The increasing demand to improve Mater-Bi properties, often unsuitable for specific commercial goals, has led to different strategies of investigation. To get inherent improvements in processability and chemicalephysical performance, Mater-Bi-based composites have been prepared by using different cellulose fibers [13,14]. Enhancement of mechanical properties, decrease in water sorption, and modulation of biodegradation kinetics have been observed due to the fiber presence [14,15]. Furthermore, the use of suitable additives could provide upgrading of processing and manufacturing performance, and enhancement of physico-chemical properties. As a matter of fact,

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thermoplastic polymers rely on specific additives to obtain products that fulfill the market needs [16]. Several categories of natural and synthetic additives are available, such as processing aids, plasticizers, stabilizers and antibacterial additives [17]. Natural additives obtained from wastes or by-products of agrofood industry deserve particular attention. Nowadays, huge amounts of bio-wastes are produced by agri-food processing industry, and their disposal has a significant environmental impact [18]. To improve eco-sustainable and cost-effective waste management, several attempts have been made to convert by-products into bio-fuels, compost and feedstock [19]. Moreover, large amounts of by-products generated during crop growing, processing and storage contain highly helpful, biologically active substances, whose extraction provides high added-value compounds [20]. The winemaking industry, for example, produces solid by-products, such as skin, seed, pomace and stems that represent about the 30 wt.% of the grapes used [21]. Most of these by-products are sources of antioxidant, antibacterial, anti-inflammatory compounds, such as polyphenols, stilbenes, flavonoids, etc., whose activity is well known in the pharmaceutical, cosmetic and food industry [22]. Some components of these extracts are chemical analogs of synthetic stabilizers widely used in plastic industry. To the best of our knowledge, the scientific literature reports only few studies dealing with the evaluation of the effects of these natural extracts when added in polymeric matrices as antioxidants [23,24] and plasticizers [25e27]. In this paper, an extract from winery waste (named EP) has been used as a natural additive for Mater-Bi. The use of a natural processing by-product can be attractive to tailor Mater-Bi properties in a cost-effective manner, to further improve its performance in agricultural and packaging applications. The effect of EP on processability, thermal stability, mechanical properties and biodegradability of Mater-Bi has been evaluated and discussed. 2. Experimental 2.1. Materials The commercially available thermoplastic starch-based polymer used in this work was Mater-Bi grade CF03A kindly supplied by Novamont, Italy. This polymer is a biodegradable thermoplastic material made with corn starch and a biodegradable copolyester of proprietary technology. The copolyester is based on diacids and a glycol that are obtained from renewable and non-renewable resources. From infrared spectroscopy analysis (not reported), absorption bands due to benzene ring vibrations (1019 and 873 cm1) were detected along with peaks associated to alkyl chain motions, thus suggesting that this polymer grade was based on an aliphaticearomatic copolyester [28]. The grape-based additive (EP) was supplied by CTAEX (Spain). It was obtained from Cabernet pomace through hydroalcoholic extraction at room temperature using a 70:30 (v/v) ethanol:water mixture as a solvent. The total phenolic content (as quercetin) was found to be 4.3  1.2 wt.% with respect to the dry EP. It is well known that the phenolic amount of grape pomace changes according to cultivar and solvent used in the extraction [29]. Besides phenolic compounds, the dry grape pomace may contain also carbohydrates (especially pectins, up to 30 wt.%), lipids (but hydroalcoholic extraction did not extract all of them), some proteins (less than 3 wt.%), and carboxylic acids, in particular tartaric acid, which can be found in amounts as much as 5e7 wt.% [30e32]. 2.2. Film preparation A total of 4.0 g of EP was dissolved in methanol (16% w/v), then 100 g of polymer granules were added. The resulting mixture was

kept at 50  C under mixing to allow solvent evaporation. Prior to processing, the doped pellets were dried under vacuum for 24 h at 60  C. Polymer films (average thickness 30 mm) were obtained using a Collin Teachline E20T single screw extruder equipped with a Collin BL50T film blowing unit. The temperature profile was as follows (from hopper to die): 150, 160, 165, 150, 150  C. Due to the different melt viscosity, the processing schedule adopted for the undoped Mater-Bi was 150, 160, 180, 170, 170  C. Two film samples were prepared, namely neat Mater-Bi (MB), and Mater-Bi containing 4 wt.% of EP (MB4). 2.3. Accelerated aging The films were thermo-oxidized at 70  C in a forced air oven (Vittadini model Plus) up to 1000 h. Aged samples were collected at different times and their chemical and physical properties were evaluated. 2.4. Melt flow and rheological properties Apparent viscosity of both plain and doped unprocessed MaterBi pellets was measured by means of a Bohlin Instruments RH7 capillary rheometer equipped with a 1 mm-diameter die. Tests were performed at 150 and 180  C on samples dried for 24 h at 60  C under vacuum. The effect of EP on the polymer melt flow behavior was assessed by Melt Flow Index (MFI) determination using a CEAST Junior 7023 MFI apparatus. MFI values were obtained according to ASTM D1238 standard test method at 190  C, using a 2.16 kg weight. 2.5. Fourier Transform Infrared (FTIR) spectroscopy FTIR spectroscopy on polymer films was carried out by means of a Nicolet Nexus spectrometer. Spectra were recorded in transmission mode as an average of 32 scans in the range 4000e400 cm1, with a resolution of 4 cm1. The hydroxyl concentration was measured as the ratio of areas under the absorbance bands in the range 3650e3000 cm1 and 3000e2720 cm1. Prior to measurements, the samples were kept in a desiccator over dry silica gel for 7 days. 2.6. Thermogravimetric Analysis (TGA) TGA measurements were carried out by means of a Perkin Elmer Pyris Diamond TG-DTA. Samples were heated at 10  C/min, from 30 to 600  C in nitrogen (flow rate 100 ml/min). To eliminate small amounts of water absorbed, a 30-min isothermal treatment at 90  C was carried out prior to the heating run. 2.7. Dynamic Mechanical Analysis (DMA) A TA Instruments DMA Q800 in the tensile mode was used to study the dynamic mechanical behavior of the specimens. Rectangular specimens of approximately 15  1 mm, 6.0  0.5 mm, 30  7 mm thick (depending on take-up speed) were analyzed. The test was run with oscillation frequency of 1 Hz at a heating rate of 5  C/min from 70e130  C under nitrogen. The initial static force was 1 N. The oscillation amplitude was 20 mm. Glass transition temperatures (Tg) were taken as the maximum values of tand curves. 2.8. Mechanical properties Tensile tests were performed using a Instron model 5564 dynamometer equipped with a 1 kN load cell, according to ASTM D882-02 standard test method at 23  2  C, 45  5% RH, with a 50 mm/min clamp separation rate.

P. Cerruti et al. / Polymer Degradation and Stability 96 (2011) 839e846

2.9. Bio-disintegration tests The bio-disintegration degree of the Mater-Bi-based samples was evaluated according to ISO 20200:2004 test method. The plastic films were inserted into testing frames, vacuum-dried at 40 C for 72 h, then buried in a solid composting material and subjected to aerobic degradation at 58  C. The bio-disintegration experiments were carried out in duplicate. The composting process was monitored by visual inspection of the buried films: samples were collected and photographed periodically. The bio-disintegration extent was evaluated by means of an image analysis technique, and expressed as a percentage of the area of bio-disintegrated material, with respect to the initial sample area [12]. 3. Results and discussion 3.1. Processing behavior

Shear viscosity, Pa s

Rheological characterization was carried out to get further insight on the effect of EP on the processability of Mater-Bi. Indeed, during the preparation of samples in presence of EP it was necessary to change the temperature profile to achieve optimal blown film processing conditions. Moreover, the pressure measured at the extruder die was consistently lower. Hence, it was clear that the grape extract affected the melt flow behavior of Mater-Bi. Viscosity of plain and doped unprocessed Mater-Bi pellets was measured through capillary viscometry. Apparent viscosity data are shown in Fig. 1. Both Mater-Bi-based melts displayed a decrease of viscosity (h) with increasing shear rate ðg_ Þ. This trend is commonly referred to as shear thinning behavior and it is described for numerous thermoplastic polymers [33], as well as for starch-based systems [8]. It is ascribed to the gradual loosening of polymer intermolecular interactions. The presence of EP was responsible for lower viscosity values over the entire range of explored shear rates, the difference being higher at lower g_ (from 50 to 500 s1). Moreover, increasing temperatures led to a reduction in viscosity, as the samples were more prone to flow. The dependence of the apparent viscosity on the shear rate has been expressed as: h ¼ K g_ n1 where h is the apparent viscosity, K is the consistency, g_ is the shear rate, n is the power law index [34].

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Table 1 lists the power law parameters logK and n obtained by least squares analysis of the data in Fig. 1. From Table 1, it can be inferred that K decreased in presence of EP, and also decreased with increasing temperatures, at the same additive weight content. It is likely that EP acts as a plasticizer. EP strongly interacts with polymer chains, weakening their mutual forces, thus facilitating chain slip under pressure. The tendency of plasticizers to suppress consistency coefficients in starch formulations has been reported in literature [35]. Larger n values were observed for MB4 at 180  C, accounting for a less pseudoplastic and more Newtonian polymer melt. That is, polymers with high n values have lower entanglement density and are easier to disentangle under shear [35,36]. These effects are particularly significant to determine the processing features of Mater-Bi-based systems and also relevant to the evaluation of rheology-dependent parameters aimed to scaling up the extrusion process, from a lab level to a high output equipment. In this context, MFI measurements were performed to get further insight on the industrial applicability of EP. MFI values were found to be 0.47 and 1.56 g/10 min for MB and MB4, respectively, indicating that EP could be suitable to increase Mater-Bi processing output. 3.2. Aging behavior 3.2.1. FTIR spectroscopy In Fig. 2a the FTIR spectra of MB at different aging times in the range between 4000 and 2400 cm1 are shown. Two main absorption bands are visible in this range, namely the eOH (3800e3000 cm1) and eCHe (3000e2800 cm1) bond stretching, respectively [37]. It can be noticed that the eCHe absorption remained constant upon aging, while variations of the area related to eOH bonds were observed. More specifically, a reduction up to t ¼ 504 h was followed by an increase of the band area at longer aging times. This trend is more evident in Fig. 2b, in which the ratios between the peak areas of eOH and alkyl stretching vibrations are plotted as a function of the aging time. This tendency may be explained in terms of two opposed phenomena occurring in the course of the aging process: hydroxyl consumption due to their condensation reaction and eOH groups formation associated to thermal oxidation. That is, during aging, owing to the high temperature it is likely that the hydroxyl groups of pyranose rings in the starch fraction undergo condensation, resulting in water removal, and consequent ether linkage formation [38,39]:

1000

100

MB 180 °C MB4 180 °C MB 150 °C MB4 150 °C 100

1000

10000 -1

Shear rate, s

Fig. 1. Melt viscosity versus shear rate for MB and MB4 at 180  C and 150  C. Lines correspond to the theoretical linear fits.

In a recent work Zhang et al. [40] reported an NMR spectroscopy study on thermally treated starch. Condensation between hydroxyl groups, forming ether links, was detected when starch-based samples were heated at temperatures around 300  C. It is believable that the same reaction occurred on MB and MB4 samples after a prolonged aging time even if at moderately elevated temperatures.

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Table 1 Consistency index (logK) and power law exponent (n) for MB and MB4 melts. Sample

MB MB4

Consistency logK (Pa sn)

Power law index n

150  C

180  C

150  C

180  C

4.44 4.01

4.03 3.43

0.395 0.502

0.465 0.601

However, for longer times (t > 504 h) the thermo-oxidative process becomes significant and the formation of new eOH groups is expected to take place in the starch fraction, along with the hydrolysis of polyester moieties [41]:

A similar trend was observed for MB4, although in this case the rate of eOH consumption was markedly lower between 240 and 504 h. This suggests that EP inhibited reactivity of eOH groups, affecting the crosslinking process to a slight degree.

Absorbance, a.u.

2

a

t=0h t = 240 h t = 368 h t = 504 h t = 744 h t = 1000 h

1

0 4000

3800

3600

3400

3200

3000

2800

2600

2400

-1

Wavenumber, cm 3.2

3.2.2. Thermogravimetric analysis TGA was carried out on unaged MB and MB4, as well as on samples thermally oxidized for different times. Fig. 3 shows the thermogravimetric curves of unaged MB and MB4 along with the thermogram of EP. From the analysis of the thermograms, it can be noticed that degradation of MB was composed of two weight-loss steps. The first decomposition step, occurring from about 280 to 340  C, is likely due to the degradation of the starch fraction, accounting for approximately 20% weight loss [42]. The inset in Fig. 3 shows that for MB4 the mass loss in the first stage was faster, probably due to the simultaneous volatilization of the pomace extract. This was

confirmed by the examination of the TGA curve of EP, which showed a weight loss of about 50% in the corresponding temperature range. At higher temperatures (>350  C), the thermal decomposition of the polyester portion could be observed [5,39]. In this range of weight loss, MB and MB4 displayed a similar behavior and the influence of the additive on the thermal stability was negligible. Fig. 4 reports weight loss and derivative (DTG) curves of MB (a) and MB4 (b) thermo-oxidized for different times (t ¼ 0, 240, 504, 1000 h). In Table 2, the temperatures of 5% weight loss (T5%), the maximum decomposition rate temperature for both degradation steps (Tmax1 and Tmax2), and the char yield at 600  C (Char600) are shown. It can be noticed that the aged MB samples (Fig. 4a) were more thermally stable than the untreated ones in the first weight-loss stage. The increased stability can be due to starch crosslinking, as already reported for Mater-Bi-based systems [39]. In the DTG curves, starch degradation showed a complex peak with a shoulder between 280 and 340  C. This pattern was also observed by other authors [42], and may be due to the different degradation rate of amylose and amylopectin. At a slightly higher temperature (T z 350  C), a small peak was detectable, attributed either to

b

100

100

Weight, %

96

80

2.8

Weight, %

Area 3300 /Area 2900

3.0

2.6 2.4 2.2

MB MB4 200

80 220

600

800

1000

1200

Time, h Fig. 2. a) FTIR spectra (4000e2400 cm1) of MB samples aged at 70  C; b) Ratio of eOH to eCH stretching peak areas as a function of the aging times for MB and MB4 films.

240

0 100

260

280

300

320

Temperature, °C

40

20 400

88

84

60

2.0 0

92

MB MB4 EP 200

300

400

500

600

700

Temperature, °C Fig. 3. Thermogravimetric curves for neat EP and unaged MB and MB4.

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a

0,24

Weight, %

-1,0

40

t=0h t = 240 h t = 504 h t = 1000 h

20

0 150

200

-0,5

Derivative Weight, wt.%/°C

80

60

t=0h t = 504 h t = 1000 h

-1,5

0,20

a

0,16

tanδ

100

843

0,12 0,08 0,04

250

300

350

400

450

0,0 550

500

0,00 -80

-60

-40

-20

Temperature, °C

20

40

60

80

100

Temperature, °C 0,24

100

b

t=0h t = 504 h t = 1000 h

-1,5

0,20

-1,0

40 -0,5

t=0h t = 240 h t = 504 h t = 1000 h

20

0 150

200

250

b

0,16

tanδ

60

Derivative Weight, wt.%/°C

80

Weight, %

0

0,12 0,08 0,04

300

350

400

450

500

0,0 550

0,00 -80

-60

-40

-20

Temperature, °C

0

20

40

60

80

100

Temperature, °C

Fig. 4. Thermogravimetric and DTG curves for a) MB, and b) MB4 at different aging times.

Fig. 5. tand versus temperature for a) MB, and b) MB4 at different aging times.

degradation of the compatibilising agent usually present in MaterBi formulations, or to interpenetrating networks formed by starch with the synthetic polymer [43]. In the second main degradation stage, related to polyester pyrolysis, there were no significant differences between aged and unaged samples. A similar degradation behavior was observed for MB4 (Fig. 4b), however, owing to the presence of EP, a more complex pattern of starch decomposition was found. From Table 2, it is also worth to notice that the temperature of maximum degradation rate of starch (Tmax1) in the first 504 h increased up to 320  C, then dropped to 315  C after 1000 h. On the other hand, Tmax2 was not affected by the thermal aging. These data are in agreement with FTIR spectroscopy results, confirming that crosslinking of the starch fraction in the early

period of aging slightly improves the thermal resistance of MaterBi. On the other hand, thermal oxidation effects are prevailing for prolonged aging times. As for MB4, the thermal treatment did not influence Tmax1 and Tmax2 values, probably indicating that the crosslinking process was restrained in presence of EP. 3.2.3. Dynamic mechanical and mechanical properties Fig. 5 reports the temperature dependence of the tangent of the phase angle (tand) at 1 Hz for MB (a) and MB4 (b). By first examining the behavior of MB (Fig. 5a), two distinct peaks were detected indicating that polyester and starch fractions are present in Mater-Bi as separated phases. The first peak, at about 20  C, is related to the glass transition of the polyester portion, while the molecular relaxation of the starch component is

Table 2 Temperature of 5% weight loss (T5%), maximum decomposition rate temperatures (Tmax1 and Tmax2), and char yield at 600  C (Char600) for MB and MB4.

Table 3 Glass transition temperatures (Tg), tensile strength (s) and elongation at break (e) for MB and MB4 at different aging times.

Aging time

MB T5% ( C)

Tmax1 ( C)

Tmax2 ( C)

Char600 (%)

T5% ( C)

Tmax1 ( C)

Tmax2 ( C)

Char600 (%)

Aging MB time (h) Tg1 ( C)

0 240 504 1000

289 303 301 305

313 317 320 315

410 409 410 408

4.5 9.1 7.3 8.0

262 296 297 299

318 317 319 319

407 410 407 410

5.2 6.3 6.6 5.8

0 240 504 1000

MB4

18 e 17 20

MB4 Tg2 s (MPa) ( C) 64 e 82 71

13.6 13.7 15.9 15.9

   

1.7 0.3 2.0 1.7

e (%) 518 368 410 416

   

55 15 67 36

Tg1 ( C)

Tg2 s (MPa) ( C)

20 e 19 20

66 e 68 62

13.2 13.5 14.6 14.6

   

1.3 2.7 0.9 2.3

3 (%) 589 387 426 413

   

38 27 10 33

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visible at higher temperatures [44]. Table 3 lists the glass transition temperatures of polyester (Tg1) and starch (Tg2) for MB and MB4 at different aging times. The aging process significantly affected Tg of starch in MB. In fact, a sharp increase in Tg2 was noticeable after 504 h. This finding could be ascribed to starch crosslinking, which retards molecular relaxations, leading to a more rigid system. At t ¼ 1000 h, Tg2 decreased to 71  C, as an indication of partial disruption of the starch network caused by thermal oxidation. As for the synthetic thermoplastic portion, a small decrease in Tg1 was noticed after 1000 h aging, probably due to a reduction of the molecular weight as a consequence of the degradation process. In the case of MB4, the increase in Tg2 after 504 h was less significant than that observed for MB. This suggests that EP may interfere with starch crosslinking, as also shown by FTIR and TGA results. As a consequence of prolonged aging times, Tg2 dropped to a value (63  C) lower than that of the unaged MB4. Furthermore, it can be seen the Tg1 did not change upon aging, suggesting that EP could improve the thermo-oxidative stability of the polyester component. By comparing Tg1 of unaged MB and MB4, it can be inferred that EP acts as a plasticizing agent since MB4 showed a slightly lower Tg. This hypothesis is supported by the results of tensile tests, which show that elongation at break (3) for unaged MB4 was about 14% higher than that of MB (Table 3). As far as the tensile strength (s) is concerned, the presence of EP did not affect the mechanical performance of Mater-Bi. The effect of the thermal oxidation on MB and MB4 was also evaluated. Elongation at break was influenced by the aging treatment, and a significant drop was observed for both samples at 240 h, as a consequence of the partial crosslinking of starch. Prolonged aging did not bring about any further detriment of elongation at break values since the polyester fraction, responsible for Mater-Bi ductility, seemed to withstand thermal accelerated treatment. It is also remarkable that after 1000 h aging plain and doped samples showed comparable elongation at break values. Most probably, in the case of MB4 the effect of plasticization is lost due to the partial volatilization of EP, as mentioned in the Section 3.2.2.

Tensile strength was slightly dependent on the thermal oxidation times for both the investigated samples. Similar results were found by Tzankova Dintcheva and La Mantia [45] on photooxidized Mater-Bi-based samples. The development of a crosslinked structure caused the polymer to become stiffer due to the reduced mobility of chains, and a modest increase in strength for both systems resulted from 240 h onwards. Moreover, because of the plasticizing activity of EP, the tensile strength values of MB4 were found to be lower throughout the aging process.

3.3. Effect of EP on the bio-disintegration rate In a bioactive environment, polymer degradation occurs by material fragmentation and subsequent mineralization. The action of heat and moisture as well as the enzymatic activity of microorganisms shorten and weaken the polymer chains [2]. In particular, as for biodegradable polyester-based systems, molecular chain breakdown proceeds by random hydrolytic chain scission of ester linkages [46]. Hydrolytic reactions are involved also in starch biodegradation. In order to evaluate the modulating effect of EP on Mater-Bi degradation, bio-disintegration tests were carried out according to ISO 20200:2004 standard test method. Fig. 6 shows the evolution of degradation of MB and MB4 films buried under solid composting material. It can be observed that both samples achieved complete bio-disintegration within 30 days, thus well before 90 days that is the period by the end of which plastic materials are considered disintegrable, according to ISO 20200:2004. The bio-disintegration rate was evaluated by means of an image analysis technique, and expressed as a percentage of the area of bio-disintegrated material, with respect to the initial sample area (Table 4). It could be observed that after a 15-day composting period MB4 was degraded to a slight extent (18%), whereas MB exhibited a considerable consumption of the area exposed to soil (61%). MB was completely bio-disintegrated after 22 days, when the compost was sieved and no residual fragments were recovered. The same

Fig. 6. Evolution of degradation of MB and MB4 films buried under solid composting material.

P. Cerruti et al. / Polymer Degradation and Stability 96 (2011) 839e846 Table 4 Evolution of degradation of Mater-Bi films expressed as amount of bio-disintegrated material. Bio-disintegration time (days)

Bio-disintegrated material (% area) MB

MB4

1 15 22 31

0 61 99 99

0 18 84 96

results were evident for MB4 after 31 days, indicating that biodegradation was efficiently retarded by the presence of EP. It is likely that EP affects the activity of the microorganisms present in the composting environment. As a matter of fact, pomace extracts were demonstrated to exert antimicrobial action on important pathogenic bacteria [29]. Moreover, the phenolic antioxidant resveratrol, contained in grapes, has been known to possess antifungal properties [47]. Therefore, EP probably has a broad-spectrum antibiotic activity which may interfere with microbial digestion of polymer films, slowing down the rate of Mater-Bi bio-disintegration. 4. Conclusions A polyphenol-containing extract (EP) derived from winery waste was used as a natural additive in Mater-Bi. EP was found to cause a decrease in melt viscosity, thus suggesting that it could act as a Mater-Bi processing aid. As shown by thermal and viscoelastic characterization, EP affected the crosslinking which occurred in the starch fraction upon thermal aging. Furthermore, tensile tests indicated that EP exerted a plasticizing effect on Mater-Bi. Also the polymer biodegradability was influenced by the additive, as neat Mater-Bi bio-disintegrated earlier than the loaded polymer. Overall, the use of a natural agricultural processing by-product was shown to be attractive due to its ability to tailor Mater-Bi properties in a cost-effective manner. Acknowledgments Dr. Maurizio Tosin (Novamont, Italy) is gratefully acknowledged for supplying Mater-Bi and for performing the bio-disintegration tests. Dr. Gianni Gasperoni (CTAEX, Badajoz, Spain) is thanked for supplying the pomace extract. This work was supported by the MIUR-CNR Project “Agroalimentare, ambiente e salute”. References [1] Wu R-L, Wang X-L, Li F, Li H- Z, Wang Y-Z. Green composite films prepared from cellulose, starch and lignin in room-temperature ionic liquid. Biores Technol 2009;100:2569e74. [2] Mohee R, Unmar GD, Mudhoo A, Khadoo P. Biodegradability of biodegradable/ degradable plastic materials under aerobic and anaerobic conditions. Waste Manage 2008;28:1624e9. [3] Godbole S, Gote S, Latkar M, Chakrabarti T. Preparation and characterization of biodegradable poly-3-hydroxybutyrateestarch blend films. Biores Technol 2005;86:33e7. [4] Briassoulis D. An overview on the mechanical behaviour of biodegradable agricultural films. J Polym Environ 2004;12:65e81. [5] Puglia D, Tomassucci A, Kenny JM. Processing, properties and stability of biodegradable composites based on Mater-Bi and cellulose fibres. Polym Adv Technol 2003;14:749e56. [6] Rosa MF, Chiou B, Medeiros ES, Wood DF, Williams TG, Mattoso LHC, et al. Effect of fibre treatments on tensile and thermal properties of starch/ethylene vinyl alcohol copolymers/coir biocomposites. Biores Technol 2009;100:5196e202. [7] Morreale M, Scaffaro R, Maio A, La Mantia FP. Mechanical behaviour of Mater-Bi/ wood flour composites: a statistical approach. Composites A 2008;39:1537e46. [8] Ma XF, Yu JG, Ma YB. Urea and formamide as a mixed plasticizer for thermoplastic wheat flour. Carbohydr Polym 2005;60:111e6.

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