Poly(vinyl alcohol)–collagen hydrolysate thermoplastic blends: II. Water penetration and biodegradability of melt extruded films

Polymer Testing 22 (2003) 811–818 www.elsevier.com/locate/polytest

Material properties

Poly(vinyl alcohol)–collagen hydrolysate thermoplastic blends: II. Water penetration and biodegradability of melt extruded films P. Alexy a,∗, D. Bakosˇ a, G. Crkonˇova´ a, Z. Krama´rova´ a, J. Hoffmann c, M. Julinova´ c, E. Chiellini b, P. Cinelli b a

Department of Plastics and Rubber, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinske´ho 9, 812 37 Bratislava, Slovakia b Department of Chemistry and Industrial Chemistry, University of Pisa, via Risorgimento 35, 56126 Pisa, Italy c Department of Environmental Technology and Chemistry, Faculty of Technology, Tomas Bata University in Zlin, Nam. TGM 275, 76272 Zlin, Czech Republic Received 14 November 2002; accepted 25 January 2003

Abstract Water solubility of polyvinyl alcohol (PVA) is related to degree of hydrolysis, molecular weight and modification during blending in the presence of other processing additives. In the present paper the effect of collagen hydrolysate (CH), an abundant waste product of the leather industry, and glycerol on PVA water sensitivity has been investigated. This study is a continuation of the previous research on experimental design optimisation of PVA-collagen hydrolysate blends (P. Alexy, D. Bakosˇ, S. Hanzelova´, L. Kukolı´kova´, J. Kupec, K. Charva´tova´, E. Chiellini, P. Cinelli, Polymer Testing 2003, 22 doi:10.1016/S0142-9418(03)00016-3). CH content affects water penetration into the prepared blown films, affecting therefore their solubility. An increasing content of CH in PVA based blends shortens the time to the first disruption of the film after immersing in water, restraining the negative effect of glycerol on solubility. Water penetration into film is influenced by both added components—glycerol and CH, and mutual effects depend upon their proportional amounts in the blends. Pure PVA film presented limited biodegradation at low temperature (5 °C). The CH addition in the blend significantly increases biodegradation rate at that temperature. PVA/CH blends properties are of practical relevance for applications as hospital laundry bags and containers of water-soluble substances, such as chemical agents for treatment of waste and potable water, fertilizers, washing agents, sanitary products, etc.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyvinyl alcohol; Collagen hydrolysate; Water penetration; Biodegradability; Blends

1. Introduction Polyvinyl alcohol (PVA) as a water-soluble synthetic polymer has been produced for many industrial applications. It is used as emulsifier, colloid stabiliser, sizing

∗ Corresponding author. Tel.: +1-421-2-59325-536; fax: +1421-2-52493-198. E-mail address: [email protected] (P. Alexy).

agent, coating in the textile industry, adhesive, and house building industry [2–4]. After use, depending upon the application, PVA is generally discarded in waste water treatment plants or into a solid waste disposal system. In the future, water-soluble polymers, hydrogels, and watersoluble packaging will be designed to completely biodegrade in suitable disposal infrastructures such as wastewater treatment facilities or composting plants. The urgency for meeting this goal is higher than that needed for water insoluble plastic waste, often receiving a great

0142-9418/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-9418(03)00015-1

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deal of attention, due to the difficulty of recovering water-soluble polymers as feedstock for mechanical or chemical recycling. Indeed, any option that is viable for water insoluble plastics waste is not really a convenient option for water-soluble polymers. This change will require, however, a major technical breakthrough if the current polymers are to be replaced on a cost/performance basis and not simply with higher cost biodegradable alternatives [5]. PVA solubility in water depends on both degree of hydrolysis and degree of polymerisation with a maximum at 88% hydrolysis, and higher solubility for lower degree of polymerisation. It has been observed that water-soluble polymers such as polyethylene glycol, dextran, polyvinyl pirrolidone, and gelatine increase solubility of final PVA products [6]. The changes in molecular structure of the basic polymer are strongly influenced by changes of temperature, pH, or solvent composition [7–10]. PVA degradation in aqueous and soil environments has been extensively studied [11–16]. A strong dependence of PVA degradation on the presence of the natural microbial populations and degradation conditions has been assessed [17,18]. Thus, PVA degradation proceeds quite slowly in an unadapted environment and at low temperature [19]. PVA has been blended with natural polymers because of its hydrophilic characteristics that allows for good compatibility [20–24]. Natural polymers allow for cost reduction of the final items and are supposed to speed up the degradation process. PVA biodegradability has been investigated in the presence of natural polymers such as starch, chitin, gelatine, sugar cane bagasse and other lignocellulosic fillers [25–31]. Natural polymers speed up degradation of the blends and composites based on PVA, even if the synthetic polymer degradation appears very limited. The present study is connected and interlocked with the recent study on experimental design optimisation of PVA-collagen hydrolysate blends [1]. It is also related to a previous paper dealing with the evaluation of basic processing properties of PVA blends containing collagen hydrolysate and glycerol as minor component [32]. Manufacture of these blends allows for the utilization of collagen hydrolysate (CH), a proteinaceous material produced by enzymatic hydrolysis of solid waste generated down stream to chrome tanning (leather shavings) in the presence of organic amines, thus contributing to solving a worldwide recognized disposal problem of the tannery industry [33,34]. The use of CH is also supposed to be valuable as a natural fertilizer in agriculture applications due to the relatively high nitrogen content (⬇13%) and, moreover, it may act as a promoter for blend degradation based on water-soluble continuous matrices. CH is readily degradable both in an aqueous environment and with standard activated sludge at a rate higher than in soil, and showed a positive effect on

PVA/CH blend biodegradation in anaerobic conditions [1]. In the present paper an investigation on water sensitivity as a function of glycerol and CH content, as well as biodegradability in an aerobic, liquid medium are reported.

2. Experimental 2.1. Materials Polyvinyl alcohol (PVA), white powder SLOVIOL P 88-08 with 4% solution viscosity 8 mPas at 20 °C and 88 mol% hydrolysis degree, was from NCHZ Nova´ ky, Slovakia. Collagen hydrolysate (CH) is made from waste from tanned leather by enzymatic hydrolysis with subsequent removal of chromium compounds at KORTRAN, Hra´ dek Nad Nisou, Czech Republic. Glycerol (GL) from HCI, Slovakia, was used as a plasticizer. Talc (Wessalite, DEGUSSA, Germany) was used as an antiblock agent and Stearin III (SETUZA, Czech Republic) as an additive. 2.2. Preparation of extruded films In contrast to the previous study where we used plasticized blends and extruded tapes [1], extruded films were used for the study of water penetration and biodegradability. Polymer blends for extrusion with powder ingredients PVA, CH, talc and Stearin III were used from the previous study [1]. The films were prepared by blowing in a Brabender laboratory extruder operating under the following conditions: extruder diameter, 19 mm; L/D ratio, 25; compression ratio 1:3; thermal profile from the feeding zone to head: 210–220–230–200 °C; speed, 35 rpm. 2.3. Evaluation of water penetration The first attack of water and total film dissolution were evaluated by gripping films of identical thickness within two rectangular frames and immersing them into water. Time to the first damage of film (FD) and time to total dissolution of film (TD) were measured with continuous stirring at the selected water temperature. 2.4. The method of experimental design and optimisation The method of experimental design (DOE) was used for a statistical evaluation of measurements the same as in the previous study [1] with the chosen DOE factors: x1 ⫽

CH GL ,x ⫽ PVAL 2 CH ⫹ PVAL

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The scale of the factors was designed as follows: the content of CH in the blend was planned in the interval from 14–30 wt%, and GL content also in the interval from 11–23 wt%. The conditions of DOE are given in Ref. [1], and the compositions of individual blends for this study are shown in Table 1.

was thus 0.2 g l⫺1 (approximately 0.1 g l⫺1 DOC). Carbon dioxide evolution and DOC were determined at least daily.

2.5. Biodegradability evaluation

Results of water penetration measurements of prepared films, the time to the onset of film disintegration (FD) and time to total dissolution (TD) at the selected temperatures of 5, 10 and 20 °C are listed in Table 2. Results from statistical and regression analysis of these characteristics are reported in Table 3. The DOE method allowed to describe the experimental space and to construct the response surfaces for the followed-up properties. Response surface of the time to the FD of films at 5 °C (Fig. 1) demonstrates strong interaction of both factors. This interaction is manifested by substantial increase of FD values of films at 5 °C. Interestingly, higher concentrations of glycerol (GL) result in a negative effect on this parameter. The addition of CH abates this negative effect. It is interesting to compare the values of FD (5 °C) for PVA film without CH (dashed plane) with FD (5 °C) for films containing CH (response surface). PVA/CH films show values several times lower than pure PVA films, with the exception of the experimental area at low CH and high GL concentrations. This parameter very clearly demonstrates a strong positive effect of CH on water penetration into the PVA/CH blend films. Increasing temperature of dissolution from 5–10 °C decreases the FD values (Fig. 2). The effect of the factor

Biodegradation of polymer films were followed by means of CO2-production (according to ASTM method [35] and substrate removal (Zahn–Wellens test [36]. The application of the ASTM method is also described in more detail in our previous work [37]. In both these tests the samples under study were the sole source of organic carbon and energy in a cultivating aqueous medium. The carbon dioxide produced was absorbed in a solution of hydroxide and subsequently determined by titration. Dissolved organic carbon (DOC) was determined on a carbon analyzer Shimadzu 5000. The basic conditions of the tests were as follows: Total volume in individual reactors was 2500 ml. Inoculation was performed using activated ‘unadapted’ or ‘adapted’ sludge in a dose of approximately 0.5 g l⫺1. ‘Unadapted’ inoculum: freshly withdrawn activated sludge from municipal wastewater treatment plant was filtered, decanted with tap water, centrifuged, resuspended in mineral medium and continuously aerated for not less than 24 h prior to test. ‘Adapted’ inoculum obtained from a previous PVAL biodegradation test was centrifuged and resuspended in fresh mineral medium. Samples (0.5 g) were dosed following dissolution in 100 ml mineral medium, actual concentration in the reactor

3. Results and discussion

Table 1 Compositions of the individual blends based on poly(vinyl alcohol) (PVA) and collagen hydrolysate (CH) Sample

1 2 3 4 5 6 7 8 9 10 11 12 13 O14 P15

Coded levels of factors

Real levels of factors

Compositions of the samples [wt%]

x1

x2

x1

x2

PVAL

CH

GL

S III

Talc

⫺1 1 ⫺1 1 ⫺1.414 1.414 0 0 0 0 0 0 0

-1 -1 1 1 0 0 ⫺1.414 1.414 0 0 0 0 0

0.1989 0.3735 0.1989 0.3735 0.1628 0.4096 0.2862 0.2862 0.2862 0.2862 0.2862 0.2862 0.2862 0.5399

0.1549 0.1549 0.2751 0.2751 0.215 0.215 0.13 0.3 0.215 0.215 0.215 0.215 0.215 0.2056

72 63 65 57 70 58 68 59 64 64 64 64 64 61.1 81.5

14 23 13 21 11 23 19 17 18 18 18 18 18 20.4 –

13 13 21 21 17 17 11 23 17 17 17 17 17 18.5 18.5

0.3 0.3 0.2 0.2 0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.3 0.31 0.31

0.3 0.3 0.2 0.2 0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.3 0.31 0.31

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Table 2 The results of measurements of the onset time to disintegration (FD) and the time to total dissolution (TD) of polyvinyl alcohol (PVA) and polyvinyl alcohol/collagen hydrolysate (PVA/CH) films Sample

FD (5 °C) [s]

FD (10 °C) [s]

FD (20 °C) [s]

TD (5 °C) [min] TD (10 °C) [min] TD (20 °C) [min]

1 2 3 4 5 6 7 8 9 10 11 12 13

4.50 2.38 28.57 4.30 24.30 8.39 1.83 6.36 7.64 9.59 7.97 3.74 4.70

4.47 2.32 7.74 8.47 1.45 13.05 1.53 9.57 8.94 3.39 4.42 2.01 3.88

1.71 0.74 1.73 9.24 8.06 3.53 3.62 8.30 0.99 2.95 1.07 0.93 2.46

4.39 6.40 2.35 4.92 2.37 5.84 5.74 3.51 4.22 4.55 4.72 5.67 5.20

4.95 6.27 2.32 5.91 2.55 6.12 4.20 5.43 4.08 5.99 4.73 6.01 5.35

4.86 6.41 2.60 5.92 2.63 9.21 6.25 3.99 5.34 5.41 4.38 5.78 5.42

FD (5, 10, 20 °C), the time to the first destruction at the temperature 5, 10 and 20 °C; TD (5, 10, 20 °C), the time to the total dissolution of films at the temperature 5, 10 and 20 °C. Table 3 The results from statistical and regression analysis of the onset time to disintegration (FD) and the time to total dissolution (TD) of polyvinyl alcohol (PVA) and polyvinyl alcohol/collagen hydrolysate (PVA/CH) films Coefficient FD (5 °C) [s]

b0 b1 b2 b12 b11 b22

bi

bc

6.73 ⴚ6.11 4.05 ⴚ5.54 4.74 ⫺1.39

3.02 2.38 3.37 2.56

FD (10 °C) [s]

FD(20 °C) [s]

TD (5 °C) [min] TD (10 °C) [min]

TD (20 °C) [min]

bi

bc

bi

bc

bi

bi

3.26

1.68 0.016 1.89 2.12 1.43 1.51

1.18

4.61 1.88 2.60 0.72 1.16 0.31

2.58 3.64 2.76

0.94 1.32 1.00

4.87 1.19 ⫺0.83 0.14 ⫺0.35 ⫺0.086

bc 0.71 0.56 0.79 0.60

bi 5.23 1.24 ⫺0.16 0.57 ⫺0.38 ⫺0.14

bc 1.03 0.81 1.15 0.87

5.26 1.77 ⫺0.74 0.44 0.18 ⫺0.21

bc 0.65 0.51 0.73 0.55

Note: The regression coefficients statistically significant are printed in bold. bc, the critical value of the coefficient on 95% probability level.

PVA/CH is not significant in this case (10 °C) and the concentration of GL only slightly prolongs the time to FD. However, even at this temperature PVA/CH films in all experimental areas give values significantly lower than those observed in CH free film (dashed plane). Increasing temperature from 10–20 °C slightly changed time to destruction. Interaction between the two factors is observed also in this case (Fig. 3). The effect of the CH/PVA factor is reflected in the decreasing of time to FD at low GL content in the blend. The total range of values for time to the first damage at 20 °C is comparable with the values recorded at 10 °C and, again, practically all response surfaces are below the plane recorded for pure PVA film. Time to FD for the analyzed films gave an indication

of the rate of water penetration into the structure of the relevant blends. Therefore, this parameter is important for film applications, implying a rapid disintegration of the structure to let loose a packed substance in the film after immersing into water—e.g. for packaging of powdered pesticides, detergents, etc. The observed effects of GL in prolonging the FD time can be induced by the expected strong interactions between PVA and GL via hydrogen bonds. At low temperature (5 °C) the kinetic energy levels are not high enough for such strong interactions. The CH addition compensates the negative effect of GL, by competing through possible chemical interactions of CH with both PVA and GL. These effects are partially suppressed at higher temperatures due to sufficient kinetic energy being available for breaking

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Fig. 1. The response surface for the onset time to disintegration (FD) at the temperature 5 °C, for polyvinyl alcohol/collagen hydrolysate (PVA/CH) films. Dashed plane represents values of this parameter for collagen-free polyvinyl alcohol (PVA) films.

Fig. 2. The response surface for onset time to disintegration (FD), at the temperature 10 °C, for polyvinyl alcohol/collagen hydrolysate (PVA/CH) films. Dashed plane represents values of this parameter for collagen-free polyvinyl alcohol (PVA) films.

hydrogen bonds. Therefore, in order to increase sensitivity of films to water at low temperatures and eliminate the negative effect of GL, it is necessary to increase the content of CH in the film. From the DOE evaluation, the interaction of both factors is substantially lower at higher temperatures. The effect of both factors on TD times is less significant compared to FD. The response surfaces are plotted in Figs 4, 5 and 6. In these cases, we can see that the presence of CH extends TD, whereas GL shortens this time. No significant interactions between the two factors evaluated in DOE are observed. The plane for pure PVA film without CH is not drawn in these figures, because all pure PVA films were characterized by much longer TD times. It can be concluded that the complex examin-

815

Fig. 3. The response surface for onset time to disintegration (FD), at the temperature 20 °C, for polyvinyl alcohol/collagen hydrolysate (PVA/CH) films. Dashed plane represents values of this parameter for collagen-free polyvinyl alcohol (PVA) films.

Fig. 4. The response surface for the time to total dissolution (TD) at 5 °C, for polyvinyl alcohol/collagen hydrolysate (PVA/CH) films.

ation of both parameters shows that CH positively influences penetration of water into PVA films by significantly shortening the FD time of films and improving the TD times. Water-soluble films for packaging of substances used for swimming pool and waste water treatment is one possible application of PVA/CH films. The presence of CH enhancing water solubility of the blend films can improve biodegradability of PVA films and have a positive influence on inoculum adaptation. In this respect, the investigation of the effect of inoculum adaptation and water temperature on biodegradability of PVA film in comparison to PVA/CH blends film has been undertaken. The experiments were carried out on samples O14 and P15 (Table 1). The composition of sample O14 was determined in the optimization process and taking into consideration criteria based on good processability and

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Fig. 5. The response surface the time to total dissolution (TD) at 10 °C for polyvinyl alcohol/collagen hydrolysate (PVA/CH) films.

Fig. 6. The response surface for the time to total dissolution (TD) at 20 °C for polyvinyl alcohol/collagen hydrolysate (PVA/CH) films.

good mechanical properties. The composition of P15 film is the same as O14, apart from the absence of CH. The aerobic biodegradation experiment was carried out on typical films by using two types of inoculum: a fresh inoculum ‘unadapted’ and an ‘adapted’ inoculum. Fig. 7 shows the comparison of biodegradation of both types of PVA films in the presence of unadopted inoculum. A higher propensity to biodegradation of the PVA/CH blend is evident when the unadopted inoculum was used. The lag phase of pure PVA is shorter and biodegradation runs faster, mainly in the first stage of biodegradation. When the adopted inoculum was used, the differences in aerobic biodegradation of PVA and PVA/CH practically disappeared and the biodegradation kinetic profile was similar for both samples (Fig. 8). With respect to the experiment carried out in the presence of the unadopted

Fig. 7. Biodegradation of polyvinyl alcohol/collagen hydrolysate (PVA/CH) film containing 20.3 %wt of CH (sample O14) and PVA film CH free (sample P15) in fresh unacclimated inoculum.

Fig. 8. Biodegradation of polyvinyl alcohol/collagen hydrolysate (PVA/CH) film containing 20.3 %wt of CH (sample O14) and PVA film CH free (sample P15) in acclimated inoculum.

inoculum, both samples biodegraded only in one step and at a faster rate. The effect of temperature on the biodegradation was tested only in experiments carried out in the presence of the adopted inoculum (22 and 5 °C). Respirometric activity monitored by released CO2 was inadequate at low temperature, presumably due to the activity of nitrification bacteria. Therefore, biodegradation was evaluated according to organic carbon depletion during incubation. The dependencies of organic carbon depletion on incubation time for both samples O14 and P15 are shown in Fig. 9. No differences in biodegradation of both PVA/CH and PVA at 20 °C were observed by using the adopted inoculum, whereas biodegradation at the low temperature of

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Acknowledgements This research was supported by grant VEGA No. 2/7044/20.

References

Fig. 9. Biodegradation expressed as dependency of organic carbon elimination on incubation time polyvinyl alcohol/collagen hydrolysate (PVA/CH) film containing 20.3 %wt of CH (sample O14) at 22 and 5 °C and PVA film CH free (sample P15) at 5 °C.

5 °C appears to be influenced by CH presence. Thus, at such low temperature biodegradation of CH free PVA film was extremely low, contrary to what was observed for biodegradation of PVA/CH films.

4. Conclusion The results obtained from the study of solution properties of the blends and films prepared from PVA blended with collagen hydrolysate (CH) show strong effects of CH on water resistance and penetration into the blend films. These effects are influenced by the presence of glycerol used as plasticizer of thermoplastic blends. Especially, CH affects solubility of prepared films at low temperatures compensating the negative effect of glycerol on solubility. Generally, the values of the both evaluated parameters [time to the onset film damage (FD) and time to total dissolution (TD)] of films prepared with CH show better and more uniform solubility in comparison to CH free films. No differences in biodegradation of PVA and PVA/CH blends at 20 °C were observed when an adopted inoculum was used. On the contrary, at lower temperature (5 °C) the biodegradation level of CH-free PVA films was much lower than that detected for the PVA/CH blend film. These aspects play an important role in applications implying packaging of water-soluble substances, such as chemical agents for treating of water, washing agents, sanitary products, and agrochemicals, etc.

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