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In the paper the effects of keratin and polyvinyl alcohol additives on the stress relaxation course in thermoplastic starch film were presented. The 3-parameter ...
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ScienceDirect Agriculture and Agricultural Science Procedia 7 (2015) 80 – 86

Farm Machinery and Processes Management in Sustainable Agriculture, 7th International Scientific Symposium

Studies on stress relaxation process in biodegradable starch film Krzysztof Gołackia, Zbigniew Stropeka*, Paweł Kołodzieja, Bożena Gładyszewskab, Andrzej Rejakc, Leszek Mościckic, Marek Borygaa a

Department of Mechanical Engineering and Automatics, University of Life Sciences in Lublin, Głęboka 28, 20-612 Lublin, Poland b Department of Physics, University of Life Sciences in Lublin, Akademicka 13, 20-950 Lublin, Poland c Department of Food Process Engineering, University of Life Sciences in Lublin, Doświadczalna 44, 20-280 Lublin, Poland

Abstract In the paper the effects of keratin and polyvinyl alcohol additives on the stress relaxation course in thermoplastic starch film were presented. The 3-parameter Zener model was used to describe the behavior of film under loading taking into account sample sizes and initial deformation velocity. The effects of air relative humidity, initial deformation velocity and the way of sample cutting on model parameters were also determined. The increase of keratin content resulted in the decrease of E0 parameter, whereas the increase of polyvinyl alcohol caused its increase.

© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder underresponsibility responsibilityofofthe theCentre Centrewallon wallonde deRecherches Recherchesagronomiques agronomiques(CRA-W) (CRA-W). Peer-review Keywords: stress relaxation; thermoplastic starch; keratin; polyvinyl alcohol; Zener model.

1. Introduction On account of its properties thermoplastic starch film is a material which can replace synthetic packaging in the future (Hangoard et al., 2001; Nafchi et al., 2013). Modified starch is a plant polymer easily available, obtained from potato, maize and rice. It is characterized by complete biodegradability, easy availability of produce and also low production cost. (Kyrikou and Briassoulis, 2007; Siwek et al., 2010). Unlike petrochemical origin materials, starch does not cause environmental pollution and disease risk growth. These qualities encourage producers to develop environment and health friendly packaging technology (Boryniec et al., 2004; Zhao et al., 2005; Iovino et al., 2008). Unfortunately, there are some drawbacks related to starch properties such as: susceptibility to moisture absorption, x x

Corresponding author. Tel.: +48 81 5319755 E-mail address: [email protected]

2210-7843 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Centre wallon de Recherches agronomiques (CRA-W) doi:10.1016/j.aaspro.2015.12.038

Krzysztof Gołacki et al. / Agriculture and Agricultural Science Procedia 7 (2015) 80 – 86

unsatisfactory mechanical properties and fast aging, which reduce the range of their full applications (Martin et al., 2001; Chen and Lai, 2008; Da Róz et al., 2011). To obtain the specified level of mechanical properties and their stability in time during production, plasticizers (Moore et al., 2006; Jiang et al., 2006) or other supplementary additives (Mao et al., 2000; Majdzadeh-Ardakani and Nazari, 2010) are used. There are also applied different kinds of starch (de Graaf et al., 2003; Janssen and Mościcki, 2009) and various production engineering is tested (Xie et al., 2009; Ashogbon and Akintayo, 2014). Using additives improves mechanical properties determined by such parameters as: modulus of elasticity, critical stress and strain as well as puncture resistance (Petersen et al., 2001; Follain et al., 2005; Gołacki et al., 2014). Apart from elasticity and plasticity the film is characterized by viscoelastic properties, which are connected with the energy dissipation processes (Stropek et al., 2014). Susceptibility to impact loading is another parameter which is not only studied for biological origin materials (Stropek and Gołacki, 2013, 2015, 2016) but also polymers (Kołodziej et al., 2014). The aim of this paper was determination of the effects of keratin and polyvinyl alcohol additives on the stress relaxation course in thermoplastic starch film. The 3-parameter Zener model was used to describe the behaviour of film under loading taking into account sample sizes and initial deformation velocity. The effects of air relative humidity, initial deformation velocity and the way of sample cutting on model parameters were also determined. 2. Material and method 2.1. Material Thermoplastic starch was produced with the use of: potato starch, plant glycerol, polyvinyl alcohol with the molecular mass of 72000 g/mol and keratin with 87.5% dry substance. The film was prepared in two stages. In the first stage the mixtures of potato starch, glycerol and keratin or polyvinyl alcohol were made. From those mixtures the starch pellet was obtained in the TS-45 single-thread worm extruder. The labels and compositions of mixtures are presented in Table 1. The second stage consisted in producing a film sleeve from starch pellet by means of the blowing method. Table 1. Samples composition, % by mass. Sample label

Potato starch

Glycerol

Polyvinyl alcohol

sga 1

78

20

2

sga 2

75

20

5

sga 3

70

20

10

sgk 1

79.5

20

0.5

sgk 2

79

20

1

sgk 3

78.5

20

1.5

sgak 3

76

20

3

Keratin

1

2.2. Method The samples were cut on specially designed knife-edge die of 80x150 mm sizes and put into plastic bags allowing the air to be suck off in order to delay the aging process of film. Relaxation stress tests were carried out with the Instron 8872 universal testing machine. The films with different contents of keratin, polyvinyl alcohol and mixture of keratin and polyvinyl alcohol were tested. The samples were put between the specially developed gripping jaws.

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The samples were characterized by different moisture contents because they were stored under three various conditions with the air relative humidity 50%, 91% and 99%. The first level of humidity was obtained under the environmental conditions and the other two by placing the film samples in the exsiccators with the water solution of table salt at the specified concentrations. The humidity of 91% corresponded to the solution with 5-mole concentration, and humidity of 99% - 0.3 mole. The samples sealed to obtain the humidity corresponding to the solution concentration were suspended for 24 h from special frames in the exsiccators. The stress relaxation tests were carried out in three stages. Before each test the sample thickness was measured by means of micrometer caliper with the accuracy of 0.01 mm. The first stage consisted in testing the two film compositions sga1 and sgk1 at two initial deformation velocities of 6 and 60 mm/min. At first tests for the two additional deformation velocities of 600 and 3000 mm/min were planned to be carried out. However, it was impossible due to samples tearing. The samples were tensed to the force value of 30 N and next permanent deformation was maintained. The force response value changes were recorded for 50 s. 10 repetitions were made for each velocity. The second stage consisted in checking effects of sample cutting way. The samples were cut parallel and perpendicularly to the film blowing direction. In this case the two film compositions sgk3 and sga3 were tested at the initial deformation velocity of 60 mm/min. The samples were tensed to the force value of 30 N and next the permanent deformation was maintained. The force response value changes were recorded for 50 s. 6 repetitions were made for each direction of sample cutting. The third stage consisted in carrying out stress relaxation test for the samples with different moisture contents. The five film compositions sgak3, sga2, sga3, sgk2, sgk3 were tested at the environmental humidity amounted to 50%, 91% and 99%. The samples were tensed to the force value of 10 N and at the initial deformation velocity of 60 mm/min. Next the permanent deformation was maintained. The force response value changes were recorded for 50 s. 3 repetitions were made for each humidity. 2.3. The modelling of experimental courses The 3-parameter Zener model was used to describe the behaviour of film under loading. The sample sizes and deformation velocity were taken into account in the initial stage of the test (Chen and Fridley, 1972; Gołacki and Stropek, 2001). E § S ˜ v tm ·  KE1 ˜ t t m  1 ˜ t m  t K1 ¨ F (t ) ( E  E1 ˜ e ) ˜ dt ¸ ˜ e 1 ¨ l ³0 0 ¸ © ¹

(1)

where: S is the cross section area, v is the initial deformation velocity, l is the sample length, tm - is the increasing deformation time and t is the time counted from the beginning of sample deformation. Formula (1) is applied in the second stage of the test in which the constant deformation was maintained (t > tm). It takes into account also the relaxation stress which already occurred during the increasing deformation. The force response courses obtained as a result of the experiment were approximated by the formula:

F (t )

A0  A1 ˜ e D1 ˜ t t m

(2)

in which A0, A1, and α1 are the parameters. The nonlinear minimization Quasi-Newton method was applied to determine the values of those parameters. 33 measuring points were used to approximate the experimental course in the following way: the first 10 points every 0.01 s, the next 10 every 0.1 s, the successive 9 every 1 s and the last 4 every 10 s. From the comparison of formulae (1) and (2) there were determined the E0, E1 and K1 model parameters (Chen and Chen, 1986) described with equations (3-5).

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E0

A0 ˜ l S ˜ v ˜ tm

E1

A1 ˜ l ˜ D1 S ˜ v ˜ 1  e D1 ˜t m



(4)

K1

A1 ˜ l S ˜ v ˜ 1  e D1 ˜t m



(5)

(3)





3. 3. Results Figures 1 and 2 show the relationship between the E0 modulus of elasticity as well as t1 time constant and air relative humidity for different film compositions respectively. The time constant t1 is the quotient of K1 and E1. 140

120

E0 (MPa)

100

80

60

40

20

0

50

91

99

SGA3 SGA2 SGK2 SGK3 SGAK3

Air relative humidity (%)

Fig. 1. Relationship between the E0 parameter of Zener model and the air relative humidity for different film compositions

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4.0 3.5

Time constant 1t(s)

3.0 2.5 2.0 1.5 1.0 0.5 0.0

50

91

99

SGA3 SGA2 SGK2 SGK3 SGAK3

Air relative humidity (%)

Fig. 2. Relationship between the t1 parameter of Zener model and the air relative humidity for different film compositions

Fig. 1 shows the effect of polyvinyl alcohol and keratin additives on the value changes of E0 parameter. The increase of polyvinyl alcohol content up to 10% resulted in higher values of E0 parameter for air relative humidity of 50% and 91%. At the air humidity of 99% statistically significant differences between the mean values of E0 parameter for different polyvinyl alcohol content films were not found. The increase of keratin content from 1% to 1.5% caused the decrease of E0 parameter for the whole air humidity in the films where keratin was the only additive. As follows from Fig. 2 both keratin and polyvinyl alcohol content do not cause changes of time constant for different air relative humidity values. While determining the effect of the initial deformation velocity on the parameters of Zener model, statistically significant differences between the mean values of E0 parameter for different film compositions were not found. The initial deformation velocity had the effect only on t1 time constant as shown in Fig. 3.

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12

Time constant 1t (s)

10

8

6

4

2

0

6

60

SGA1 SGK1

Initial deformation velocity (mm/min) Fig. 3. Relationship between the t1 time constant and the initial deformation velocity

The t1 time constant decreased with the increasing initial deformation velocity for both tested film compositions. Time constant is a measure of stress loss in material during the test. The way of cutting sample had no effect on model parameters. The sample cutting parallel and perpendicularly to the direction of film blowing did not induce statistically significant differences between the mean values of Zener model parameters for both tested film compositions. The 3-parameter Zener model applied for description of the film behaviour under loading was consistent with the experimental curves. It was composed of free term E0 equation 1 or A0 equation 2 presenting stabilized stress level at the end of the test and viscoelastic terms (E1 and K1 parameters in equation 1 or A1 and D1 in equation 2) describing stress relaxation. 4. Conclusions The increase of keratin content resulted in the decrease of E0 parameter, whereas the increase of polyvinyl alcohol caused its increase. The values of E0 parameter for different contents of keratin and polyvinyl alcohol pointed out to large differences at the air relative humidity amounting to 50%. Those differences disappeared at the air relative humidity of 99%. The initial deformation velocity had the effect only on the t1 time constant. The time constant decreased with the increasing initial deformation velocity for the tested film compositions. The 3-parameter viscoelastic Zener model described the stress loss processes in the material during the test in a satisfactory way. The research results do not allow explaining their relation to the structure of the film material. There was no statistically significant effect of additives on t1 time constant. References Ashogbon, A.O., Akintayo, E.T., 2014. Recent trend in the physical and chemical modification of starches from different botanical sources: A review. Starch/Stärke 66, 41-57. Boryniec, S., Ślusarczyk, C., Żakowska, Z., Stobińska, H., 2004. Biodegradacja folii z polietylenu modyfikowanego skrobią. Badanie zmian struktury nadcząsteczkowej polietylenu Polimery 49, 424-431. Chen, C., Lai, L., 2008. Mechanical and water vapor barrier properties of tapioca starch/decolorized hsian-tsao leaf gum films in the presence of plasticizer. Food Hydrocolloids 22, 1584-1595.

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