Info

Time (days)

FIGURE 5.10 Time variation of the CO2 production of PVA/OR blends in compost soil degradation test.

Time Profile Degradation Pva

Time (days)

FIGURE 5.11 Time variation of the extent of mineralization of PVA/OR blends in compost soil degradation test.

Time (days)

FIGURE 5.11 Time variation of the extent of mineralization of PVA/OR blends in compost soil degradation test.

a recent study on PVA/starch plastic degradation in activated sludge, it was observed that PVA degradation occurred only if PVA-degrading bacterium or enzyme were added to the sludge. This behavior was attributed both to the rare presence of PVA bacteria in the natural environment and to the long period of time necessary to enrich them to the level where they could exhibit PVA-degrading activity.78 It has also been reported that the addition of PVA in starch-glycerol blends lowered both the rate and extent of blend degradation.76 Interestingly, about the same CO2 production was recorded for PORSt and ORSt, thus indicating that, in this hybrid composite, the PVA presence has no negative effect on the degradation of the natural components. Thus in investigating the supposition that the presence of a certain amount of natural polymers in blends with synthetic polymers can promote the degradation of the synthetic component, lignocellulosic fillers appeared particularly promising.79

Figure 5.11 reports the time variation of the extent of mineralization of the investigated samples evaluated as the percentage of the theoretical CO2 production. After 20 days of experiment, the blend (PORSt) reached a 31% mineralization in comparison with 54% mineralization for the fillers (ORSt). These values confirmed the rapid degradation of the natural components (St and OR) and the not-yet-initiated degradation of the synthetic polymer fraction.

5.2.3 Hybrid Composites by Injection Molding and Foaming

Hybrid composites with PVA and starch as continuous matrices were prepared with corn fiber (CF). CF is an industrial name given to the pericarp fraction of the corn kernel that is a coproduct of ethanol production by wet-milling technology. CF contains pericarp as well as starch and protein from the endosperm. The wet CF

TABLE 5.9

Composition of Biobased Mixtures Consisting of PVA and CF Processed by Injection Molding

TABLE 5.9

Composition of Biobased Mixtures Consisting of PVA and CF Processed by Injection Molding

PVA

Corn Fiber

Starch

Glycerol

Pentaerythritol

PEG

Sample

(%)

(%)

(%)

(%)

(%)

(%)

PCF1

42

26

0

21

11

0

PCF2

40

25

0

20

10

5

PCFStl

36

23

9

18

9

5

PCFSt2

33

21

17

17

8

4

PCFSt3

29

32

14

14

7

4

Note: P = poly(vinyl alcohol) (PVA); CF = corn fibers; St = starch; PEG = poly(ethylene glycol).

Note: P = poly(vinyl alcohol) (PVA); CF = corn fibers; St = starch; PEG = poly(ethylene glycol).

(60% moisture) is sold at about $15 per ton. Dried and ground at 10 mesh, CF is sold at about $50 per ton, with its main use being in animal feeds. The CF used in our experiments had a composition of 1% fat, 14% protein, 25.5% starch, 59% lignocellulosic component, and 0.5% ash. Injection-molded specimens were prepared to test glycerol, pentaerythritol, and polyethylene glycol 2000 as suitable plasticizers.48 Composition of the prepared samples is reported in Table 5.9. Compounding was performed at 160 to 170°C, thus avoiding fiber degradation during processing. PCF1 produced a composite that was cohesive and flexible. In the second mixture (PCF2), a limited amount of PEG (5%) was introduced to further lower the viscosity of the melt.

Mechanical properties of the produced items are reported in Table 5.10. Samples were tested after storing at 23 °C and 50% relative humidity (RH) for 7 days and after 1 year.

Due to the greater amount of plasticizer in this composite, percent elongation at break (EB) increased in PCF2 compared with PCF1, with a concomitant decrease in ultimate tensile strength (UTS) and Young's modulus (YM). Only small changes in tensile properties were observed for composites after 1 year when compared with the composites stored for 7 days under the same conditions. In composites containing no starch (PCF1 and PCF2), changes in EB were not significant. UTS and YM

TABLE 5.10

Mechanical Properties of Injection-Molded Composites at Different Aging

TABLE 5.10

Mechanical Properties of Injection-Molded Composites at Different Aging

Mechanical Property

Aging

PCF1

PCF2

PCFSt1

PCFSt2

PCFSt3

EB (%)

7 days

599

645

396

297

101

EB (%)

365 days

613

619

415

356

95

UTS (MPa)

7 days

11.7

7.1

8.3

8.0

7.8

UTS (MPa)

365 days

9.5

7.2

6.2

6.4

8.6

YM (MPa)

7 days

52.0

27.5

94.2

112.0

122.2

YM (MPa)

365 days

34.2

38.6

65.0

84.7

183.3

Note: EB = elongation at break; UTS = ultimate tensile strength; YM = Young's modulus.

Note: EB = elongation at break; UTS = ultimate tensile strength; YM = Young's modulus.

decreased moderately for PCF1 and were almost the same (UTS) or moderately increased (YM) for PCF2.

The plasticizer proportion as in PCF2 was used for the rest of the composites. Corn starch was introduced in the formulation and the amount of CF was progressively increased up to a value of 32% in the mixture. As the starch and CF volume fraction increased, elongation at break decreased, while the modulus generally increased. Interestingly, UTS was not significantly affected by increasing CF. Also, composites made with 32% fibers and only 29% PVA resulted in cohesive extrusions.

For samples PCFStl and PCFSt2, prepared respectively with 9 and 17% starch, and approximately the same fiber:PVA weight ratio as in composite PCF2, EB was slightly increased with storage, and UTS and YM were decreased. These changes are probably an indication that water was absorbed during storage. Composite PCFSt3, which contained a higher ratio of fibers to PVA, had an increase in UTS and YM and a decrease in EB. This indicates that this composite was getting stiffer and less flexible with age. Composites tested after 1 year of storage had tensile properties similar to composites tested after 7 days of storage. Thus the prepared materials appear extremely promising for several practical applications.

In consideration of the positive results obtained by injection-molding processing of polymeric matrices with corn fibers, further investigations were done to evaluate the effect of using corn fibers in starch-based foam trays. Foam food containers contribute in large scale to the amount of plastics in municipal solid waste streams. These are mainly produced by expanded polystyrene (EPS) or coated paperboard. In recent years, several efforts have been devoted to the production of similar items based on polymers from renewable resources such as starch.8081

Potato starch, CF, magnesium stearate, and PVA, respectively 88% (P88) and 98% (P98) hydrolysis degree, were first mixed using a kitchen mixer with a wire whisk attachment. For PVA-free batter and with less then 50-weight part of fibers, gum arabic (1% by weight of starch) was added to prevent starch settling (Table 5.11). Water was added to reach the required total solids content.82

TABLE 5.11

Composition of the Biobased Batter Used To Produce Trays by Foaming Technique

TABLE 5.11

Composition of the Biobased Batter Used To Produce Trays by Foaming Technique

St

PVA

CF

Arabic Gum

Magn

esium Stearate

Batter

(%)

(%)

(%)

(%)

(%)

St

96

0

0

1

3

StCF50

65

0

31

1

3

StCF100

49

0

49

0

2

StCF150

39

0

59

0

2

StP

80

16

0

1

3

StPCF50

58

11

29

0

2

StPCF100

45

8

45

0

2

StPCF150

36

7

55

0

2

Note: St =

starch, CF =

corn

fibers, P =

poly(vinyl alcohol) (PVA).

FIGURE 5.12 Dependence of a) Maximum Force (MF) and b) Deformation at Maximum Force (MFD) as a function of Corn Fibers (CF) content.

Foam trays were prepared using a lab model-baking machine essentially consisting of two heated steel molds, the top of which can be hydraulically lowered to mate with the bottom half for a set amount of time. Dimensions of the mold were 220-mm long, 135-mm wide, 20-mm deep, and 3-mm plate separation. Baking temperature was set at 200°C. Baking time was the minimum required to avoid soft or bubbled trays and varied in the range of about 120 to 180 sec.

Mechanical tests were performed on the trays, after conditioning for 1 week at 23°C and 50% RH, by using an Instron Model 4201 Universal Testing Machine equipped with a cylindrical probe (80-mm diameter). The probe was lowered onto the tray until a load of 0.5 N was reached and then lowered at 30 mm/min. Parameters calculated were the maximum force (MF) and deformation to MF (MFD).

Increasing fiber content resulted in lowered MFD and MF values, indicating that the fibers' irregular shape was not allowing a strengthening effect (Figure 5.12). PVA presence improved MFD and MF values, especially for P98, and the resulting

Pva Sem Electron
FIGURE 5.13 Scanning electron micrographs (SEM) of a) foams based on potato starch (St), b) foams based on potato starch/Corn fibers (StCF100), c) foams based on potato starch/PVA (SW), d) foams based on potato starch/PVA/corn fibers (StPCF100).

trays were still cohesive, even with a high content of fibers in the composition (45%). Moreover, if 88% hydrolyzed PVA is added as a powder grade, its addition can be performed in the dry mixture, thus avoiding the time- and energy-consuming step of PVA dissolution in hot water that was adopted in previous procedures.80

Distilled water (100 ml) was poured in trays with fibers and trays without fibers. After 30 min, trays based on potato starch or strengthened by PVA easily softened because of water addition. In contrast, trays containing a high percentage of fibers (45%) and PVA (8%), such as StPCF100, softened but remained cohesive. This effect is attributed to the disposition of the fibers in the external part of the trays. Thus the foam trays have a dense outer skin with large, thin-walled channels comprising the core as a result of the faster drying of the paste placed nearer to the mold, as shown in the SEM (scanning electron microscope) micrographs reported in Figure 5.13. We can conclude that the introduction of corn fibers improves resistance to moisture and water effects due to the fibers' disposition on the external sides of the tray.

5.2.4 Biodegradation of Poly(Vinyl Alcohol)

The environmental fate of water-soluble poly(vinyl alcohol) (PVA) has been primarily investigated due to its large utilization in textile and paper industries that generate considerable amounts of wastewaters contaminated by PVA. In 1936, it was observed that PVA was susceptible of sustaining ultimate biodegradation when submitted to the action of Fusarium lini.83 Afterward, the nature of PVA as a truly biodegradable synthetic polymer was repeatedly and intensively assessed.

Most of the PVA-degrading microorganisms have been identified as aerobic bacteria belonging to Pseudomonas, Alcaligenes, and Bacillus genera. Some species degrade and assimilate PVA axenically, although symbiotic association exhibiting complex cross-feeding processes is a rather common feature of PVA biodegradation.84-87

Incubation time (days)

FIGURE 5.14 Time profiles of mineralization of PVA-based blow films, 98% hydrolized PVA (PVA98), and cellulose in an aqueous medium in the presence of paper mill sewage sludge.

Nevertheless, the extensive biodegradation of PVA is accomplished almost exclusively by specific degrading microorganisms whose occurrence in the environment appears to be uncommon and, in most cases, strictly associated with PVA-contam-inated environments.8889 Correspondingly, limited mineralization of PVA-based blown film was recorded during respirometric biodegradation tests in an aqueous medium inoculated with municipal sewage sludge.90 In contrast, in the presence of the sewage sludge collected from a paper mill wastewater treatment plant, the mineralization extent of PVA and PVA-based blown film was comparable with that of cellulose, although in a longer incubation time (Figure 5.14).90 This behavior demonstrates that the selective pressure exerted by the constant presence of large amounts of PVA in wastewater from paper factories is effective in establishing a microbial consortia able to degrade and assimilate PVA. The enrichment procedure of paper mill sewage sludge, as obtained by repeated sequential transfers of this microbial inoculum in the presence of PVA as sole carbon and energy source, induced a significant acceleration in the degradation rate, with a substantial increase of the extent of PVA mineralization. On the other hand, the acclimation of the microbial strains to PVA led to an almost total abatement of cellulose assimilation (Figure 5.15), thus confirming the high specificity of PVA-degrading microorganisms.90

Molecular weight, degree of hydrolysis (HD), at least in the 80 to 100% range, and content of head-to-head units does not appear to greatly affect the enzymatic random endocleavage of PVA macromolecules in aqueous media.91-93 However, earlier investigations are suggesting that some structural characteristics of PVA, such as the hydrophobic character (associated with the residual acetyl group content), may considerably influence the activity of different PVA-degrading enzymes.909495 Accordingly, no major differences in the extent of mineralization in the presence of an acclimated microbial inoculum have been observed among three commercial PVA

Time Profile Degradation Pva

Incubation time (days)

FIGURE 5.14 Time profiles of mineralization of PVA-based blow films, 98% hydrolized PVA (PVA98), and cellulose in an aqueous medium in the presence of paper mill sewage sludge.

20 H

20 H

mf''

l"

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