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Polylactic acid · polyhydroxybutyrate · thermoplastic starch · biodegradable polymer blends · modification · me-chanical properties

Influence of structure of the blend based on polylactic acid, polyhydroxybutyrate and thermoplastic starch (PLA/TPS/PHB) on morphology and mechanical proper-ties was studied. Optimal composition of the blend was determined to ensure good dispersion of the particular compo-nents in the polymer matrix which resulted in acceptable properties of materials for selected applications. In order to obtain a complex description of the studied system, the design of experi-ment (DoE) was used as an experimen-tal strategy to explore the possible inter-actions between components of the blend. Mechanical properties were measured 24 h after sample (film) prep-aration and also after the specific stor-age time:up to1 month to study the effect of storage time on the mechanical properties.

Vielkomponenten Verschnitt aus Poly-2-Hydroxypropion-säure, Polyhydroxybutyrat und thermoplastischer StärkePoly-2-Hydroxypropionsäure · Polyhyd-roxybutyrat · Thermoplastische Stärke · bioabbaubare Polymerverschnitte · Mo-difizierung · mechanische Eigenschaften

Es wurde der Einfluss der Struktur des Verschnitts aus Poly-2-Hydroxypropion-säure, Polyhydroxybutyrat und thermo-plastischer Stärke (PLA/TPS/PHB) auf die Morphologie und auf die mechanischen Eigenschaften untersucht. Die optimale Zusammensetzung des Verschnitts wurde ermittelt, um eine gute Dispersion der einzelnen Komponenten in der Poly-mermatrix sicherzustellen, was in akzep-tablen Eigenschaften der Materialien für ausgewählte Anwendungen resultiert. Um eine komplexe Beschreibung der untersuchten Systeme zu erhalten, wurde als experimentelle Strategie die statisti-schen Versuchsplanung (DoE) für die Untersuchung möglicher Wechselwir-kungen zwischen den Verschnittkompo-nenten eingesetzt. Mechanische Eigen-schaften wurden 24 Stunden nach der Probenpräparation (Film) und auch nach definierter Lagerungszeiten von bis zu einem Monat gemessen, um den Effekt der Lagerzeit auf die mechanischen Eigenschaften zu studieren.

Figures and Tables: By a kind approval of the authors.

IntroductionCurrently, biodegradable polymers have raised great interest as the decreasing reserve of fossil raw materials and also common use of petrol-based polymer materials have great impact on the envi-ronment. Therefore, application of poly-mers, highly resistant to degradation, results in a large amount of municipal and industrial plastic waste which is then being stored for many years in na-ture or in landfills. Material recycling is only partial solution of full ecological application of polymers, including poly-mer waste elimination. Biodegradable polymers based on renewable raw mate-rials seem to be the best solution of this serious ecological problem including polymer waste processing.1 Biodegrada-ble materials have many special suitable properties for some application but pro-cessing properties, brittleness and sensi-bility to water and air moisture limit their application on an industrial scale.2

Polylactic acid (PLA) is a biodegrada-ble polyester with high strength and high modulus. The current method for PLA preparation is more ecological than the other processes with regards to the emissions of carbon dioxide, i.e. the syn-thesis of monomer (lactic acid) from glycerol using cascade bio- and chemoca-talysis.3 In addition, if D,L-PLA is synthe-sized, amorphous product with improved toughness is obtained; but again, the elongation at break (εb) does not achieve values higher than tens of percents in this case. Also, the effect of physical age-ing of PLA causes rapid decrease of PLA flexibility in a very short time period (several hours) after the melt processing of PLA regardless of the PLA synthesis method.4

Starch is usually used in biodegrada-ble polymer blends to improve biodegra-dability as well as to reduce the price of the final material. On the other hand, it decreases the mechanical properties of the blends. To improve starch dispersion in polymer matrix and physical proper-ties of the blend, thermoplastic starch (TPS) is applied. TPS is usually prepared by mixing of native starch with plasticiz-

ers in a twin-screw extruder at elevated temperatures. The mixture of glycerol and water is usually used as the plasti-cizer5.

Polyhydroxybutyrate (PHB) is a biode-gradable thermoplastic polymer with a strong potential for industrial purposes due to its relatively high crystallinity (50–70 %), excellent gas barrier and physical properties similar to those of polypropylene, but its thermal stability during the melt processing is very low and similarly to PLA, high fragility of PHB limits its applications in thin wall prod-ucts like packaging films. The processa-bility and mechanical properties of PHB can be improved by its combination with other polymers in the polymer blends.

Multicomponent Blends based on Polylactic Acid, Polyhydroxybuty-rate and thermoplastic Starch

AuthorsZuzana Vanovčanová, KatarínaTomanová, Jozef Feranc, JánBočkaj, Roderik Plavec, LeonaOmaníková, Ivana Gálisová,Patrik Jurkovič, Patrik Macúrik,Elena Medlenová, Pavol Alexy,Bratislava, Slovak Republic,Přemysl Menčík, Radek Přikryl,Brno, Czech Republic, Jan SalačZlín, Czech Republic Corresponding Author: Zuzana VanovčanováSlovak University of Technology in BratislavaFaculty of Chemical and Food TechnologyInstitute of Natural and Synthetic PolymersRadlinského 9812 37 Bratislava, Slovak RepublicE-Mail: zuzana.vanovcanova@gmail.com

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Combination of PHA, especially PHB with TPS is rather problematic due to high sensitivity of PHB to degradation via the alcoholysis degradation mechanism in the presence of alcoholic OH groups. Polyols which are usually used as starch plasticizers cause strong degradation of PHB. Janigova et. al.6 found that degra-dation of PHB is strongly accelerated by presence of glycerol which is stronger prodegradant than water. Glycerol leads to much faster alcoholysis of ester bounds in the main chain of PHB than in case of hydrolysis by water. According to the work7, also ethylene glycol is a stronger prodegradant than glycerol7. Therefore, blending of TPS with the PHB containing polymeric matrix needs to find an effective modification which will be able to protect PHB polymer against alcoholysis degradation. According to patent application8, the addition of an-hydrides during the blend preparation leads to improved dispersion of TPS as well as to better mechanical properties.

In this work, blends based on PLA and PHB containing plasticized starch were prepared using a twin-screw extruder. The phthalic anhydride was used as pro-tective modifier. The prepared pellets were used to prepare films using a sin-gle-screw extruder. The objective of the present work was to study the influence of concentration of the main compo-nents in the composition on mechanical properties of PLA/PHB blends containing plasticized starch. The method of the design of experiment was chosen in or-der to obtain a comprehensive descrip-tion of the studied system.

Experimental

Materials PLA (PLA 4042D; product of Nature Works, USA): m.p. = 150–162°C; M = 120000 g.mol−1; MFI = 8 g per 10 min (190 °C, 2.19 kg). Starch (Meritena 100; product of Amylum, Slovakia): M = 692 g.mol−1; pH: 6,7; particle size: 5-20 µm; surface area: 1,79 m2/g . PHB (product of Biomer, Germany): M = 290000 g.mol−1; Tm = 175°C; MFI = 15-30 g per 10 min (tested at 190°C; 2,16 kg loading). Glyc-

erol (product of HCI Slovakia): M = 92.09 g.mol−1; ρ = 1.26 g.cm−3; refractive index: 1,449-1,455 (20°C). Acetyltributylcitrate (ATBC, product of Jungbunzlauer, Ger-many): viscosity η (25°C) = 32,7 mPa.s, ρ = 1.048 g.cm−3, Tm = -59°C. Phthalic an-hydride (PHAN) (Provided by Chemolak Smolenice, Slovakia): M = 148 g/mol; ρ = 1,53 g.cm-3.

Preparation of blendsBlends of biodegradable polymers were prepared using a laboratory twin-screw extruder with the L/D ratio of 40 and the screw diameter of 16 mm. Temperature profile of the extrusion in the direction from hopper to head was set to 160–175–190–190–190–190–190–185–180–170 °C and the extrusion speed was 100 min−1. Dry blend of starch, glycerol and phthalic anhydride as modifier was pre-pared in the laboratory high speed mixer at laboratory temperature. All compo-nents for individual blends were fed into blending twin screw extruder. The ex-truded melt in strand form was cooled by air and granulated into small pellets.

Preparation of filmsChill-roll technology was used for film preparation. A laboratory Brabender sin-gle-screw extruder was used for film ex-trusion at the temperature profile (from hopper to head): 160–170–180–180°C and the extrusion speed of 20 min−1. Screw diameter was 19 mm, and the L/D ratio was 25. Cross section of the flat die

1 DoE conditionsfactor/coded level - 1,682 -1 0 1 1,682

PHB/PLA x1 0,089 0,168 0,286 0,403 0,483starch/(PLA+PHB) x2 0,150 0,221 0,325 0,429 0,500ATBC/(PLA+PHB) x3 0,176 0,228 0,303 0,377 0,429

2 Composition of blends of DoE (g)sample PLA PHB starch ATBC1 1004.4 169.2 259.3 267.12 836.7 336.9 259.3 267.13 878.2 148,0 440.3 233.5

4 731.6 294.6 440.3 233.55 910.2 153.3 235.0 401.56 758.2 305.3 235.0 401.57 805.4 135.7 403.8 355.28 670.9 270.2 403.8 355.29 959.5 85.0 339.5 316,010 704.5 340.0 339.5 316,011 910.4 260,0 175.6 354.112 733.6 209.5 471.6 285.313 880.7 251.5 368,0 199.814 754.1 215.4 315.1 415.515 812.5 232.0 339.5 316,016 812.5 232.0 339.5 316,017 812.5 232.0 339.5 316,018 812.5 232.0 339.5 316,019 812.5 232.0 339.5 316,020 812.5 232.0 339.5 316,0

3 Measured tensile strength at break (DoE)

samplecoded factor coordinates σb [MPa] σb [MPa] σb [MPa] σb [MPa]

x1 x2 x3 24 hours 1 week 2 weeks 1 month1 -1 -1 -1 15,8 13,4 14,3 15,22 1 -1 -1 14,7 15,1 15,3 14,9

3 -1 1 -1 16,0 14,6 12,9 12,64 1 1 -1 16,1 16,7 16,5 18,15 -1 -1 1 10,5 9,8 9,4 9,46 1 -1 1 10,2 9,6 9,9 10,27 -1 1 1 15,0 10,9 10,1 11,48 1 1 1 11,0 11,4 9,7 12,09 -1,682 0 0 15,6 15,5 15,5 15,010 1,682 0 0 15,2 13,7 13,3 14,011 0 -1,682 0 11,1 12,4 11,9 13,712 0 1,682 0 15,3 15,1 15,1 14,713 0 0 -1,682 15,8 13,6 10,3 12,014 0 0 1,682 12,2 11,0 10,9 12,515 0 0 0 14,5 14,7 14,1 13,916 0 0 0 13,8 13,4 13,6 14,217 0 0 0 14,2 13,9 13,9 13,918 0 0 0 15,3 17,8 15,9 15,219 0 0 0 13,7 14,3 14,8 13,820 0 0 0 14,4 13,8 14,2 13,4

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had the width of 70 mm and the height of 0.55 mm. The polymer melt was ex-truded on the surface of a chill-roll chilled with water at 15°C.

Mechanical properties measurement Tensile strength at break (σb) and elon-gation at break (εb) were measured ac-cording to standard STN ISO 527 using a

Zwick machine at the cross-head speed of 50 mm min−1. Test specimens had sha-pe of the strips, 15 mm wide and 100 mm long. Working distance at extensio-meter was 30 mm. Mechanical proper-ties were measured 24 hours after the film preparation and also after one week, two weeks and one month of storage at 23°C and 50% of RH to determine the storage time influence.

Results and discussionTo protect PHB against to alcoholysis deg-radation, it is necessary to add an effec-tive modifier to the blend. Based on re-sults in work8, the phthalic anhydride was used in all prepared blends. Effect of main components concentration on the blend properties was studied using meth-odology of design of experiment (DoE) while the 3-factor 5-level DoE was used for experimental area description. The re-gression equations obtained from DoE evaluation were used for optimisation procedure. The following weight ratios were chosen as factors of experiment:

𝑥𝑥1 = 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃

𝑥𝑥2 = 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠ℎ

𝑃𝑃𝑃𝑃𝑃𝑃 + 𝑃𝑃𝑃𝑃𝑃𝑃 𝑥𝑥3 =

𝑃𝑃𝐴𝐴𝑃𝑃𝐴𝐴𝑃𝑃𝑃𝑃𝑃𝑃 + 𝑃𝑃𝑃𝑃𝑃𝑃

where PHB is the weight of polyhydroxy-butyrate, PLA is the weight of polylactic acid, starch is the weight of corn starch without any additives, ATBC is the weight of acetyltributylcitrate.

The condition of Doe listed in table 1 were calculated according to desired concentration ranges of the blend com-ponents:

PLA from 40 to 60 wt%PHB from 5 to 20 wt%Starch from 10 to 30 wt%

For each blend glycerol (plasticizer for starch) and phthalic anhydride (modifier) were used in constant relative amount as follow:

weight ratio Glycerol/Starch was kept 40/60weight ratio PHAN/(Starch+Glycerol) was kept 1/100Composition of all blends prepared in DoE is listed in the table 2. All blends and films with composition according to table 2 were prepared. Me-chanical properties are listed in table 3 and 4.

The results were further processed by analysis of variance (ANOVA) and by re-gression analysis. The following regressi-

4 Measured elongation at break (DoE)

samplecoded factor coordinates εb [%] εb [%] εb [%] εb [%]

x1 x2 x3 24 hours 1 week 2 weeks 1 month1 -1 -1 -1 280 257 210 1172 1 -1 -1 242 247 208 149

3 -1 1 -1 300 237 185 1054 1 1 -1 280 254 224 1875 -1 -1 1 305 159 118 756 1 -1 1 231 92 76 417 -1 1 1 306 140 114 558 1 1 1 217 69 85 239 -1,682 0 0 290 270 235 16110 1,682 0 0 263 200 165 3711 0 -1,682 0 257 140 77 8512 0 1,682 0 285 237 205 14613 0 0 -1,682 280 184 94 1914 0 0 1,682 268 128 109 5515 0 0 0 304 225 196 7916 0 0 0 287 227 204 13717 0 0 0 277 206 178 8218 0 0 0 303 303 223 12519 0 0 0 291 247 218 9620 0 0 0 282 235 201 64

5 Table of ANOVA for tensile strength at break, 24 hours after the film preparations f s^2 F Fcrit

s1 52,261 3 17,420 47,495 5,409s2 9,742 6 1,624 4,427 4,950SR 9,754 10 0,975 2,659 4,735

SE 1,834 5 0,367 1SLF 7,920 5 1,584 4,318 5,050

sb0 = +/- 0,24 bk0 = 0,63sbi = +/- 0,16 bki = 0,42sbii = +/- 0,16 bkii = 0,41sbij = +/- 0,21 bkij = 0,55SLF = +/- 1,06se = +/- 0,61

6 Regression analysis for tensile strength at break (σb)Parameter σb (1d) σb (1w) σb (2w) σb (1m)

coef. bc coef. bc coef. bc coef. bcb0 14,334 0,635 14,655 1,667 14,441 0,859 14,071 0,616b1 -0,439

0,421

0,0871,106

0,071

0,570

0,366

0,409b2 1,0245 0,745 0,420 0,443b3 -1,608 -1,645 -1,385 -1,235b12 -0,318

0,550

0,12

1,445

0,214

0,744

0,701

0,534b13 -0,403 -0,428 -0,566 -0,479b23 0,488 0,018 0,071 0,391b11 0,261

1,410

-0,177

1,078

-0,112

0,555

0,028

0,398b22 -0,511 -0,485 -0,435 -0,085b33 -0,221 -0,997 -1,457 -0,777

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on model was applied for the used type of DoE.

2

1

1

110 i

k

iiiij

k

i

k

ijiji

k

ii xbxbxbbY ∑∑∑∑

=

−

+==

+++=

where b0, bi, bij, bii are regression coeffi-cients, k is the number of factors (in our case k = 3).

Standard tables of ANOVA were calcu-lated for all measured parameters. Table 5 shows ANOVA table for tensile strength at break for films measured 24 hours af-ter film preparation where:

Fig. 1: Dependence of σb on starch and PHB content in PLA/TPS/PHB blends, measured one day after film extrusion (concentra-tion of ATBC: 15%, x3 = -1,682).

1

7 Tab.7 Regression analysis for elongation at break (σb)Parameter εb (1d) εb (1w) εb (2w) εb (1m)

koef. bc koef. bc koef. bc koef. bcb0 290,813 11,738 241,032 35,130 203,136 16,803 96,566 29,699b1 -19,6725

7,79-18,280

23,313-11,127

11,151

-11,704

19,709b2 6,811 8,011 15,527 6,639b3 -4,613 -46,159 -29,89 -22,246b12 0,273

10,176

2,909

30,454

6,804

14,567

6,549

25,746b13 -13,065 -18,079 -13,711 -22,469b23 -8,908 -3,621 1,891 -8,014b11 -5,711

7,587

-3,80

22,706

0,504

10,861

3,128

19,196b22 -7,548 -20,383 -20,280 8,978b33 -6,372 -31,699 -34,262 -18,942

Fig. 2: Dependence of σb on starch and PHB content in PLA/TPS/PHB blends, measured one day after film extrusion (concentra-tion of ATBC: 23%, x3 = 0).

2

Fig. 3: Dependence of σb on starch and PHB content in PLA/TPS/PHB blends, measured one day after film extrusion (concentra-tion of ATBC: 30%, x3 = 1,682).

3

Fig. 4: Dependence of εb on ATBC content (factor x3) and PHB content (factor x1) in PLA/TPS/PHB blends, measured one day after film extrusion.

4

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s1 – the variability of the linear part of the equation

s2 – the variability of the non-linear part of the equation

SR – a residual variabilitySE – an experimental variabilitySLF – variability correspond to the lack of

fit sb0, sbi, sbiiasbij - standard deviations of regression coefficients

bc0, bci, bciia bcij - critical values of regres-sion coefficients (significance level 0,05)Fcrit – critical values of F criteria (signifi-

cance level 0,05)s – the sum of squaresf – the number of degrees of freedom s^2 – the variance

The same analysis was done for all meas-ured properties. Results of regression

analysis for all evaluated properties are shown in the tables 6 and 7. The coeffi-cients typed in bold (tab. 6 and 7) are statistically significant at a significance level of 0.05%.

Dependencies of tensile properties of PLA/PHB/TPS flat films measured 24 hours after the preparation are shown in Figs. 1-4 in form of 3D diagrams. The ten-sile strength at break (σb) was influenced by all three factors without mutual inter-actions. The tensile strength was increa-sed and passed the maximum depending on the starch content of the blend. Tensile strength at break was simultaneously de-creased by increasing concentration of both PHB and ATBC (Fig. 1-3). All coordi-nates are in the coded values.

Based on the the obtained results, we can see that it is possible to achieve ten-sile strength at break at the level of 18 MPa by a suitable combination of the composition of the mixture. The elonga-tion at break depends only on the PLA / PHB ratio (factor x1) and on the concent-ration of the plasticizer in the blend (fac-tor x3). The increasing PHB concentration caused decreasing elongation at break. The concentration of ATBC affects this property only in relation to the PHB con-centration (only interaction x1/x3 is sig-nificant). By increasing the concentration of PHB in the PLA/TPS blend, it appears that the plasticizer is reallocated bet-ween these two polymers, which must necessarily affect the properties of the mixed material where the two polymers form the matrix. The change in starch concentration in the blend does not have a statistically significant effect on PLA/PHB/TPS foil flexibility. Based on this in-vestigation it can be assumed that TPS is sufficiently deformable in the domain during the tensile test and TPS domains do not act as significant stress concen-

Fig. 5: Dependence of σb on ATBC content (factor x3 in coded coordinates), measured one week after film extrusion.

5

Fig. 6: Dependence of εb on ATBC content (factor x3 in coded coordinates), measured one week after film extrusion.

6

Fig. 7: Dependence of σb on ATBC content (factor x3 in coded co-ordinates), measured two weeks after film extrusion

7

Fig. 8: Dependence of εb on ATBC content (factor x3) and starch content (factor x2) in PLA/TPS/PHB blends, measured two weeks after film extrusion.

8

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trators. The total flexibility of the materi-al is given only by the flexibility of the polymer PLA/PHB matrix.

Dependence of elongation at break (εb) of PLA/PHB/TPS flat films measured 24 hours after the preparation is shown in Fig. 4.

Different situation was observed in mechanical properties measured one week after film preparation. The proper-ties of the films depended significantly only on the concentration of ATBC (factor x3) one week after film extrusion. Tensile strength at break was linearly decreased with increasing content of plasticizer and elongation at break was passed over the maximum. The simple dependencies of the two monitored parameters on the ratio x3 in the coded coordinates are shown in the figures 5 and 6.

The tensile strength was slightly de-creased with the storage time and the elongation at break reaches the maxi-mum in the area around the coded level x3 = -1 which corresponds to the real concentration of ATBC in polymer matrix PLA/PHB of 18,5%.

After two weeks of storage time tensi-le strength at break (Fig. 7) depends only on concentration of plasticizer but elon-gation at break (Fig.8) depends on factor x2 and x3 (starch and ATBC content).

It was shown, that the mechanical pro-perties depend not only on the compositi-on of the blend but also on the storage time. Changes occur mainly in extremely

low or extremely high concentration ran-ges of these components. This is particu-larly evident from fig. 4, 6 and 8, where the elongation at break was decreased after one or two weeks of storage at the edges of the experimental space and re-mains at approximately 200% in the cen-tre of the experimental space. It can be assumed, that these effects can be con-nected to liquid additives (ATBC, glycerol) re-distribution between polymers (PLA, PHB and starch) during the storage time. The physical ageing can also take a role in mechanical properties changes.

Regression equations calculated based on experimental data evaluation (DoE) were used to determine an optimal blend composition. Based on the DoE, information about the effect of observed factors on the mechanical properties and their changes during short-term storage was obtained. In addition, a system of equations was obtained which describes the dependence of the properties on the composition of the blend. Using the al-gorithm of Solver routine from Microsoft Excel, two optimized blends were simu-lated and calculated as follows:

OPTσ – optimization criterion - the maximum tensile strength at break without any limitsOPTε – optimization criterion - the maxi-mum value of the elongation at break at tensile strength at break after two weeks of storage of at least 14 MPa.

Based on required limits given above, the coded levels of each factor were cal-

culated (tab. 8) using Solver from MS Ex-cel. The calculated composition of the blends in the coded coordinates of the factors was recalculated to weight com-position (tab. 9). The blends were then prepared under the same conditions as the blends for the DoE and mechanical properties were measured. Comparison of measured and calculated values of tensile properties is shown on Fig. 9-12.

Based on the obtained results it can be concluded, that the mathematical models adequately describe the experi-mentally obtained values of mechanical properties, which is demonstrated by close values of the measured mechanical properties with the values calculated with respect to the experimental variabi-lity (Figs. 9, 10). Tensile strength at break (blend OPTσ) measured one month after extrusion is almost the same as the value measured one day after preparing of film and achieves 15 MPa (Fig. 9). In case of the blend OPTσ the measured value of elongation at break is even slightly high-er than the calculated value one month after film storage (Fig. 10).

The measured tensile strength values (blend OPTε are very close to the calcula-ted and even one month after the extrusi-on the strength is 15 MPa (Fig. 11). This corresponds to the minimum tensile strength at break criterion given in the optimisation procedure (14 MPa). The fig. 12 shows the measured and calculated values of elongation at break. The value (εb) was slightly decreased due to the storage time. The elongation at break va-

Fig. 9: Measured and calculated values of tensile strength at break for the optimized blend OPTσ, from one day to one month after extrusion.

9

9 Composition of optimized blends in gramssample PLA starch GL water PHB ATBCOPTσ 150,37 83,57 33,43 3,34 69,36 46,70OPTε 160,20 81,64 32,66 3,27 59,8 48,35

8 Coded factor levels calculated based on optimisation processsample factor/coded level

x1 x2 x3OPTσ 1,500 0,532 -1,201OPTε 0,749 0,443 -1,104

Fig. 10: Measured and calculated values of elongation at break for the optimized blend OPTσ, from one day to one month after extrusion.

10

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Fig. 11: Measured and calculated values of tensile strength at break for the optimized blend OPTε, from one day to one month after extrusion.

11

Fig. 12: Measured and calculated values of elongation at break for the optimized blend OPTε, from one day to one month after extrusion.

12

lue did not fall below the calculated value, not even one month after the film prepa-ration. Vice versa, the measured value is even slightly higher and reaches 170%.

ConclusionBased on the obtained results, it can be concluded that an addition of PHB influ-ences the properties of biodegradable PLA/TPS polymer blends. Results of DoE show that there is an optimal composi-tion within which maximal elongation at break can be reached. The blends exhibit-ed sufficient tensile strength and elonga-tion at break also at higher starch content. Values of elongation at break remain at approximately 200% in the centre of the applied experimental space. Therefore, the materials can be considered suitable for packaging applications in terms of their flexibility. With the addition of PHB, acceptable value of tensile strenght can be achieved, also in case of higher starch content. The effective modification using phthalic anhydride is able to protect PHB polymer against alcoholysis degradation

due to the action of glycerol. The DoE re-gression equations were used for optimi-sation of blend composition with respect to the required mechanical properties. The mathematical models describing the real dependencies of mechanical proper-ties on blend composition are sufficiently accurate. Therefore, it will be possible to calculate the composition of the mixtures with respect to the required properties for individual application with sufficient precision. The presented combination of polymers represents a biodegradable ma-terial based fully on renewable polymeric raw materials providing suitable mechan-ical properties including flexibility of the thin wall products. Blending of PHB with PLA in combination with plasticized starch increases the application potential of re-newable biodegradable thermoplastics on an industrial scale.

Acknowledgement This work was supported by the Slovak Research and Development Agency (pro-ject no. APVV-14-0301 and APVV-14-0175).

References[1] Vroman I., Tighzert L.: Biodegradable poly-

mers. Mat. 2. 307-344. DOI: 10.3390/ma 2020307. 2009.

[2] Niaounakis M.: Biopolymers: Applications and trends. Oxford, UK: W. A. Publishing. 2015.

[3] Morales M. et al.: Environmental and econo-mic assessment of lactic acid production from glycerol using cascade bio- and chemo-catalysis. Energy and Env. Sc. 8. 558-567. DOI: 10.1039/c4ee03352c. 2015.

[4] Ren J.: Biodegradable poly (lactic acid): Synthesis, modification, processing and ap-plications. Heidelberg. Germany: Springer. 2011.

[5] Nafchi A. M. et al.,. 65. 61-72. DOI: 10.1002/star.201200201.2013.

[6] Janigova I. et. al., Polymer degradation and stability, 77, 35, 2002.

[7] Špitálsky Z. et al.,Polymer degradation and stability, 91, 856, 2006

[8] Alexy et. al.: A biodegradable polymer blend and method for its preparation, PCT patent application No. PCT/SK2017/050009, 2018.

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