(PLA/PHB) Bioblends

materials

Article Processability and Mechanical Properties of Thermoplastic Polylactide/Polyhydroxybutyrate (PLA/PHB) Bioblends

Olga Olejnik 1, Anna Masek 1,* and Jakub Zawadziłło 2,3

1 Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, ul. Stefanowskiego 12/16, 90-924 Lodz, Poland; [email protected] 2 Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeze˙ Wyspianskiego 27, 50-370 Wroclaw, Poland; [email protected] 3 Institute of Polymer Materials, Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden, Germany * Correspondence: [email protected]

Abstract: This work considers the application of eco-friendly, biodegradable materials based on polylactide (PLA) and polyhydroxybutyrate (PHB), instead of conventional polymeric materials, in order to prevent further environmental endangerment by accumulation of synthetic petro-materials. This new approach to the topic is focused on analyzing the processing properties of blends without incorporating any additives that could have a harmful impact on human organisms, including the endocrine system. The main aim of the research was to ﬁnd the best PLA/PHB ratio to obtain materials with desirable mechanical, processing and application properties. Therefore, two-component polymer blends were prepared by mixing different mass ratios of PLA and PHB (100/0, 50/10, 50/20, 40/30, 50/50, 30/40, 20/50, 10/50 and 0/100 mass ratio) using an extrusion process. The prepared Citation: Olejnik, O.; Masek, A.; blends were analyzed in terms of thermal and mechanical properties as well as miscibility and surface Zawadziłło, J. Processability and characteristics. Taking into account the test results, the PLA/PHB blend with a 50/10 ratio turned Mechanical Properties of Thermoplastic out to be most suitable in terms of mechanical and processing properties. This blend has the potential Polylactide/Polyhydroxybutyrate to become a bio-based and simultaneously biodegradable material safe for human health dedicated (PLA/PHB) Bioblends. Materials 2021, for the packaging industry. 14, 898. https://doi.org/10.3390/ ma14040898 Keywords: polylactide; polyhydroxybutyrate; blend; bioplastic

Academic Editors: Valentina Beghetto and Anthony Waas 1. Introduction Received: 22 December 2020 Biopolyesters, including polyhydroxybutyrate and polylactide, are of great interest to Accepted: 8 February 2021 researchers [1–5]. These polymers are polyesters that are synthesized using bio-resources Published: 14 February 2021 such as sugar or plant oil. On the one hand, the whole polymerization process can be conducted via enzymatic reactions giving rise to the polymers. On the other hand, biosynthesis Publisher’s Note: MDPI stays neutral may lead to the creation of substrates, which are used in a further, non-biological process with regard to jurisdictional claims in to synthesize biopolyester [6,7]. Biopolyesters are biodegradable and biocompatible, which published maps and institutional affil- iations. makes them very attractive from the point of view of the packaging industry [8] as well as the medical industry [9]. They can be used as packaging material [10] or as a drug delivery system [11]. In addition, biopolyesters such as polylactide or polyhydroxybutyrate can be successfully used in 3D printing technology [12]. As far as the packaging and medical industries are concerned, the processing and Copyright: © 2021 by the authors. modification of biopolyesters should be carried out in a way that does not alter their Licensee MDPI, Basel, Switzerland. biodegradability and biocompatibility [13]. Various modifiers, including fillers, antioxi- This article is an open access article dants, and processing aids, can be harmful to the environment, which makes the resultant distributed under the terms and conditions of the Creative Commons “bio” composite dangerous as well. This is why the selection of additives that are not harm- Attribution (CC BY) license (https:// ful to the environment is of great interest to scientists and engineers [14]. Biopolyesters creativecommons.org/licenses/by/ can also be modified via polymer–polymer blending. This route allows the creation of 4.0/). composites with unique properties. Moreover, this technique is much easier, faster and

Materials 2021, 14, 898. https://doi.org/10.3390/ma14040898 https://www.mdpi.com/journal/materials Materials 2021, 14, 898 2 of 12

cheaper than other polymeric material creation methods, such as copolymerization [15]. Polylactide can be blended with both petropolymers [16,17] and biopolymers [18–20]. Un- fortunately, it is very often essential to use polymer blend compatibilizers due to chemical and structural differences as well as incompatibility related to the surface energy or viscosity of components [21]. It is also noteworthy that, in many cases, the miscibility of selected polymers, including biopolyesters, may depend on their molecular mass [22]. Moreover, blends based on biopolymers are susceptible to hydrolysis, which occurs especially during high-temperature processing in the presence of moisture [23]. Because of this phenomenon, it is essential to achieve process repeatability, for example by processing window broad- ening. Nevertheless, blending polylactide with other biopolyesters leads to the creation of composite materials that are environmentally friendly [24]. Thus, it is still necessary to overcome problems and create novel, pro-ecological materials with desirable properties. Both polylactide (PLA) and polyhydroxybutyrate (PHB) are bio-based and biodegradable polyesters and are characterized by good biocompatibility, biodegradability and sustainability. These polymers can be compared with conventionally used plastics in terms of the thermal and mechanical properties [22]. Therefore, polyhydroxybutyrate and polylactide can be used as transparent films in the food packaging industry because of their biodegradability and biocompatibility, which are extremely necessary nowadays [23]. PLA and PHB are also suitable for the production of disposable cutlery [24] and biomedical implants [25]. Separately, PLA and PHB are characterized by poor processing [26], but such properties of their blends have not been interpreted. Moreover, these polymers are brittle at room temperature. Nevertheless, blending polylactide with polyhydroxybutyrate, which is a highly crystalline biopolyester, results in new materials with interesting physical, thermal and mechanical properties in comparison to neat polylactide (PLA) [27]. Blending polylactide with polyhydroxybutyrate is not a new idea [28,29]. On the basis of information available in the literature, mixing PLA with 25 wt% of PHB provided a reinforcement effect and better mechanical properties in comparison to neat PLA, which was observed by Zhang et al. [15]. Furthermore, some interactions between both polymers were also noticed. Crystallization behavior of poly(L-lactic acid) affected by the addition of a small amount of poly(3-hydroxybutyrate) was researched by Hu et al. [30]. The impact of different molecular weight PHB on the PLA matrix was also detected. Nevertheless, most of the research concerns poly(3-hydroxybutyrate), and there is lack of information about commercially available PHB containing 12 mol% 4-hydroxybutyrate, which could be of interest to both scientists and producers. Most of the research concerning PLA/PHB blends is focused on testing films, and there is no information about thicker forms of specimens that could be dedicated to stiffer packaging, including boxes. The processability of such blends is also insufficiently described. Furthermore, the best mass ratio of PLA and PHB to obtain a blend with the most satisfactory properties has not been reported. Thus, we decided to create PLA/PHB blends from commercially available pellets in order to obtain materials with desirable mechanical, processing and application properties that can be dedicated to environmentally friendly packaging.

2. Materials and Methods 2.1. Reagents The ﬁrst component of the blend was polylactide (PLA), IngeoTM 2003D (provided by NatureWorks LLC, Minnetonka, MN, USA) with a glass transition temperature of 328–333 K, peak melt temperature of 418–433 K and with a melt ﬂow rate of 6 g/10 min (conditions: temperature 483 K, nominal load 2.16 kg). The density of this material amounts to 1.24 g/cm3. The second ingredient was polyhydroxybutyrate (PHB), which belongs to the polyhydroxyalkanoate group of polymers (PHA) and was obtained from Simag Holdings LTD (Hong Kong, China). This material, containing 12 mol% 4-hydroxybutyrate, is characterized by an average molecular weight of 520 kDa and a density of 1.25 g/cm3. Materials 2021, 14, 898 3 of 12

The melt ﬂow rate is 18 (conditions: temperature 443 K, nominal load 2.16 kg), and the moisture content is 0.05%.

2.2. Samples Preparation The selected PLA and PHB pellets were first dried at 343 K in a vacuum oven for 24 h. Then, PLA/PHB mixtures were prepared by manually pre-mixing selected pellets in a laboratory beaker according to the composition presented in Table1. The prepared mixture was processed using a single-screw laboratory extruder (Zamak Mercator, Skawina, Poland) equipped with a 25 mm screw diameter and characterized by a L/D ratio of 24. The temperature of the extrusion process amounted to 453 K, the screw rotation speed was about 40 rpm, and pressure equaled 17 atm. The flat tape of blends was formed using a special flat slit extrusion head. The estimated output of the process amounted to 8 kg/h. The formed flat tape was cooled in the air at room temperature. Finally, the tape was cut into smaller samples, which were 150 mm long, 25 mm wide and 1 mm thick.

Table 1. Composition of two-component polymer blends based on polylactide and polyhydroxybutyrate.

Ingredient Polymer Blend (Mass Ratio) Polylactide (PLA) 100 - 50 50 40 50 30 20 10 Polyhydroxybutyrate (PHB) - 100 10 20 30 50 40 50 50

2.3. Measurement of Mixing Energy The investigation of mixing energy and torque was carried out in a Brabender micromixer in conjunction with the WinMix program (version 3.0.0) [31]. The experiment consisted in ﬁlling the Brabender mixing chamber (Brabender GmbH & Co. KG, Duisburg, Germany) with a 50 g mixture of pure polylactide (PLA) and polyhydroxybutyrate (PHB) granulate in selected ratios. The temperature of the micromixer was about 453 K. After ﬁlling the mixer, torque as a function of time was recorded by the WinMix program. The mixing energy was calculated from the resistance shown by the mixed granules. This investigation not only measured mixing energy but also determined the role of each component, including plasticizing properties, by comparing torque and mixing energy at the selected temperature.

2.4. Differential Scanning Calorimetry Differential Scanning Calorimetry (DSC) was utilized to determine the glass transition temperature, the temperature of cold crystallization and the melting point of the polymeric blends. The research was performed with a DSC1 device provided by Mettler Toledo (TA 2920, TA Instruments, Greifensee, Switzerland). One of the aluminum pans was filled with about 5–7 mg of the sample, and the second one acted as a reference. The analysis was conducted using the following parameters: the first cycle (heating from 273 K to 473 K) was performed under a dynamic flow of argon at a rate of 50 mL/min for 15 min, and the second cycle was performed at 473 K under a dynamic flow of argon at a rate of 50 mL/min for 10 min. The third cycle (cooling from 473 K to 273 K) was performed under a dynamic flow of argon at a rate of 50 mL/min for 15 min, and the fourth cycle (heating from 273 K to 623 K) was performed under a dynamic flow of air at a rate of 50 mL/min. The heating rate amounted to 20 K/min. The temperatures of the phase transitions of the samples and their specific heat were determined for the different compositions on the basis of the DSC curves.

2.5. Melt Flow Rate Determination The melt ﬂow rate is one of the most important parameters in terms of polymer processing, including injection molding, extrusion, thermoforming or printing. The measurement was conducted according to ISO 1133D with the use of a MeltFloWon plus melt index tester (CEAST, Planegg, Germany). The measurement was performed by utilizing Materials 2021, 14, 898 4 of 12

typical parameters used for polylactide research, including a temperature of 463 K and a mass of 2.16 kg.

2.6. Mechanical Properties The measurements were carried out using a Zwick 1435 test machine (DEGUMA- SCHÜTZ GmbH, Geisa, Germany) in accordance with PN-ISO 527-1/-2, using ﬂat strip test pieces with a width of 25 mm and a length of 80 mm. The parameters of maximum tensile strength (TFmax), tensile strength at break (TS), elongation at break (Eb) and elongation at maximum strength (EFmax) were obtained. The measurements were carried out at a rate of 50 mm/min and initial force of 0.1 N.

2.7. Dynamic Mechanical Analysis Dynamic mechanical analysis is one of the most useful techniques for characterizing materials to obtain parameters that are relevant from the processing and end-use points of view. An Ares G2 rheometer provided by TA Instruments (New Castle, DE, USA) was used. The measurements were carried out as a function of rotor rotation frequency from 0 to 625 rad/s at a constant temperature of 453 K, and the storage and loss moduli, viscosity and damping characteristics were determined. The test pieces were discs with a diameter of 20 mm and thickness of 1 mm.

2.8. Surface Characterization of Polymer Blends The surface character is crucial in terms of the resistance of biopolymers to different solvents and their compatibility with other materials or living organisms. Information about polar and dispersive components of surface energy can be obtained from suitable calculations. To calculate surface energy and its components, the Owens–Wendt–Rabel– Kaelble (OWRK) method was used. In this technique, the contact angle was calculated using a goniometer model OCA15EC (DataPhysics Instruments GmbH, Filderstadt, Ger- many) and three liquids with different polarity: water, diiodomethane and ethylene glycol. The volume of a drop was about 1.2 µL, and the speed of dosing was 2 µL/s. The surface energy was calculated using the Young equation:

γS = γSL + γL cosθ (1)

where γS is the surface energy of solid phase, γSL is the surface energy at the solid–liquid interphase and γL is the surface energy of liquid phase.

3. Results 3.1. Measurement of Mixing Energy According to the results presented in Figure1, the mixing energy of pure PLA amounted to 0.69 kNm/g and diminished in proportion to the polyhydroxybutyrate (PHB) content (%) in a blend. As can be seen, pure polylactide is stiffer and shows greater resistance to mixing than polyhydroxybutyrate (PHB). The mixing energy of pure PLA is about 6 times higher than that of pure PHB. Polyhydroxybutyrate (PHB) makes polylactide PLA easier to process, which was observable by the lower mixing energy after adding the PHB to the PLA. The higher the amount of PHB, the lower the mixing energy. Materials 2021, 14, 898 5 of 12

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0.7 Neat PLA 0.70.6 Neat PLA 0.60.5

0.50.4

0.40.3 Neat PHB 0.3 0.2 Neat PHB 0.2

Mixing energy [kNm / g] Mixing energy [kNm / 0.1

Mixing energy [kNm / g] Mixing energy [kNm / 0.1 0 20 40 60 80 100 0 20 40 60 80 100 PHB content [%] PHB content [%] Figure 1. Mixing energy as a function of PHB content (%) in a blend. FigureFigure 1. Mixing 1. Mixing energy energy as asa function a function of ofPHB PHB content content (% (%)) in ina blend. a blend. 3.2. Differential Scanning Calorimetry (DSC) Analysis 3.2. Differential Scanning Calorimetry (DSC) Analysis 3.2. DifferentialDifferential Scanning scanning Calorimetry calorimetry (DSC) providesAnalysis information about temperatures of glass transition,DifferentialDifferential cold scanning crystallization scanning calorimetry calorimetry, and provides melting provides information point. information Moreover, about about temperatures this temperatures measurement of ofglass glass can be transition,usefultransition, for c olddetermining cold crystallization crystallization, the ,compatibility and and melting melting point.of point. polymer Moreover, Moreover, blends. this thisIt measurement can measurement be observed can can bethat be the useful for determining the compatibility of polymer blends. It can be observed that the usefulphase for transition determining temperature the compatibility of composites of polymer depends blends. on Itthe can intermolecular be observed that interactions the phase transition temperature of composites depends on the intermolecular interactions phasebetween transition the chains temperature of these of polymers. composites In thedepends case ofon compatible the intermolecular materials, interactions the interaction betweenbetween the the chains chains of ofthese these polymers. polymers. In Inthe the case case of ofcompatible compatible materials, materials, the the interaction interaction forcesforces werewere stronger,stronger and, and a single a single temperature temperature for a specific for a specific phase transition phase transition was observed. was ob- forcesserved were. If astronger composite, and consist a singleed of temperature incompatible for components, a specific phase two transition temperatures was ob-for glass servedIf a composite. If a composite consisted consist of incompatibleed of incompatible components, components, two temperatures two temperatures for glass for transition, glass transition, crystallization and melting point were visible in the DSC curve. The DSC transition,crystallization crystallization and melting and point melting were visiblepoint were in the visible DSC curve. in the The DSC DSC curve. curves The of selected DSC curves of selected PLA/PHB blends are presented in Figure 2. According to this figure, curvesPLA/PHB of selected blends PLA/PHB are presented blends in are Figure presented2. According in Figure to this2. According figure, PLA/PHB to this figure, blends PLA/PHB blends were characterized by two visible melting peaks. The first peak charac- PLA/PHBwere characterized blends were by characte two visiblerized meltingby two visible peaks. melting The first peaks. peak The characterized first peak charac- one type teriof crystalzed one melting, type of probably crystal melting corresponding, probably to PLA,corresponding while the second to PLA one, while possibly the second related one terized one type of crystal melting, probably corresponding to PLA, while the second one to PHB crystal melting, which was also detected by Zhang et al. [15]. Nevertheless, the possiblypossibly related related to toPHB PHB crystal crystal melt melting,ing which, which was wasalso alsodetected detected by Zhang by Zhang et al. et[15]. al. [15]. second peak of every blend was responsible for whole material melting, and this parameter Nevertheless,Nevertheless, the the secon second peakd peak of every of every blend blend was was responsible responsible for whole for whole material material melting melting, , is noted in Table2 as melting point (T m). m andand this this parameter parameter is notedis noted in Tablein Table 2 as 2 melting as melting point point (Tm). (T ).

DT DTm= = mPHB PHB

20 W/g 20

PLA/PHB PLA/PHB 50/10 50/10 PLA/PHB PLA/PHB 40/30 40/30 PLA/PHB PLA/PHB 50/50 50/50 PLA/PHB PLA/PHB 30/40 30/40 PLA/PHB PLA/PHB 20/50 20/50 PLA/PHB PLA/PHB 10/50 10/50 PLA PLA PHB

Heat flow [a.u.] PHB Heat flow [a.u.] DTCC= DTCC=

DTm = DTPLA = Egzo up Egzo mPLA Egzo up Egzo 300 350 400 450 500 550 300 350 400 450 500 550 T [K] T [K]

Figure 2. DSC curves of polylactide (PLA), polyhydroxybutyrate (PHB) and their blends. FigureFigure 2. DSC curves ofof polylactidepolylactide (PLA), (PLA) polyhydroxybutyrate, polyhydroxybutyrate (PHB) (PHB and) and their their blends. blends.

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According to Figure2 and Table2, blends with an equal or higher polyhydroxybutyrate (PHB) content had lower glass transition (Tg) temperatures than materials with a higher amount of polylactide (PLA). This parameter decreased for these blends in a linear way from 333 K to 325 K as a function of polyhydroxybutyrate (PHB) content. The addition of smaller amounts of polyhydroxybutyrate (PHB) to polylactide (PLA) caused an irregular drop in this parameter. Nevertheless, it can be assumed that polyhydroxybutyrate (PHB) acts as plasticizer, in relation to polylactide, because every blend containing this biopolyester had lower Tg in comparison to pure polylactide. Furthermore, only one glass transition was observed in the DSC.

Table 2. Composition of two-component polymer blends based on polylactide (PLA) and polyhydroxybutyrate (PHB).

Sample PHB Content [%] Tg [K] ∆Hcc [J/g] Tcc [K] ∆Hm [J/g] Tm [K] PLA 0 335 14.6 400 15.6 429 PHB 100 317 15.4 355 33.9 439 PLA/PHB 50/10 17 332 23.1 394 19.8 426 PLA/PHB 40/30 43 334 22.4 385 23.2 435 PLA/PHB 50/50 50 331 22.4 379 26.0 436 PLA/PHB 30/40 57 329 19.5 374 24.8 438 PLA/PHB 20/50 71 327 18.4 370 28.8 438 PLA/PHB 10/50 83 325 17.6 366 30.4 438

NOTE: where Tg—glass transition, ∆Hcc—cold crystallization enthalpy, Tcc—temperature of cold crystallization, ∆Hm—enthalpy of melting, Tm—melting point.

According to Figure2 and Table2, the temperature of cold crystallization (T cc), simi- larly to glass transition temperature (Tg), decreased with the increase in polyhydroxybutyrate content. This parameter changed in a linear way. The blend containing the lowest amount of polyhydroxybutyrate (PHB) had the highest value of about 394 K, which was lower than that of pure PLA by about 6 degrees. The blend with the highest PHB content had the lowest cold crystallization temperature of about 366 K, which was higher than the value for pure PHB by about 13 degrees. This means that the higher the PHB content, the faster the blend crystalizes at lower temperatures. The enthalpies of blend phase changes also show that the addition of PHB led to an increase in the crystal phase, which was also observed by Hu et al. [30]. Blending a small amount of polyhydroxybutyrate (PHB) with polylactide (PLA) caused a decrease in melting point from 429 K to 426 K in comparison to pure PLA, which can be seen in Figure2 and Table2. Polymer blends of 43% or higher PHB content revealed a higher melting point than pure polylactide. Only a small addition of polyhydroxybutyrate can be useful for reducing the melting point, which corresponds to the material processing temperature and acts as a plasticizer.

3.3. Melt Flow Rate Determination Measurement of melt ﬂow rate also conﬁrms the plasticizing effect of polyhydroxybutyrate addition (Figure3a,b). The relationship between melt volume rate (MVR) and the amount of polyhydroxybutyrate was exponential. It ranged from 10 to 302 cm3/10 min, and higher values of MVR were related to the polymer blends with greater amounts of polyhydroxybutyrate. Viscosity of the blends also depends on the polyhydroxybutyrate content, and the higher the PHB content, the lower the viscosity. This dependence also took an exponential form. Materials 2021, 14, 898 7 of 12

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Figure 3. Dependence of melt volume flow rate (cm3/10 min) (a) and viscosity (b) on polyhydroxybutyrate content (%) in PLA/PHB blend.

3.4. Mechanical Properties FigureFigure 3. Dependence 3. Dependence of melt of melt volume volumeAccording flow flow rate rate to(cm (cm Figure33/10/10 min min) 4) ,( (aathe)) and addition viscosityviscosity ( b(of)b on) ona polyhydroxybutyrate smallpolyhydroxybutyr amount of contentate PHB content (%) resulted in (%) in in the im- PLA/PHBPLA/PHB blend. blend. provement in mechanical properties. Similar observations were noted by Zhang et al. [15], wherein the enhancement was explained as a fine dispersion of PHB crystals in the PLA 3.4.3.4. Mechanical Mechanical Properties Properties matrixAccording such that to the Figure crystals4, the additionacted as of a afiller. small Too amount many of PHB PHB resulted crystals in thecanno im-t be well dis- persedprovementAccording, and in this mechanicalto Figurecauses properties.4 , deteriorationthe addition Similar of in observations a mechanicalsmall amount were properties. noted of PHB byZhang resulted The et al. maximumin [15 the], im- tensile provement in mechanical properties. Similar observations were noted by Zhang et al. [15], strengthwherein (T theFmax enhancement) in prepared wasexplained blends depend as a fine dispersioned on their of PHBPLA/PHB crystals incomposition the PLA ma- and signifi- wherein the enhancement was explained as a fine dispersion of PHB crystals in the PLA cantlytrix such diminishe that the crystalsd as a actedfunction as a filler. of polyhydroxybutyrate Too many PHB crystals cannot content, be well but dispersed, it should be noted matrixand thissuch causes that the deterioration crystals acted in mechanical as a filler. properties. Too many The PHB maximum crystals tensilecannot strength be well dis- that the correlation was not perfectly linear (Figure 5). The highest value was about 47 persed(TFmax, and) in preparedthis causes blends deterioration depended on in their mechanical PLA/PHB properties. composition The and maximum significantly tensile MPa for the PLA/PHB 50/10 blend, and the lowest was 26 MPa for PLA/PHB 10/50. The strengthdiminished (TFmax as) in a functionprepared of blends polyhydroxybutyrate depended on content,their PLA/PHB but it should composition be noted that and the signifi- cantlyelongationcorrelation diminishe at was maximumd not as perfectly a function strength linear of (Figurepolyhydroxybutyrate (EFmax5).) Thedoes highest not depend value content, was significantly about but 47it should MPa on for be theblend noted composi- thattionPLA/PHB the, and correlation the 50/10 values blend, was of not and this perfectly the parameter lowest linear was 26range (Figure MPad for from 5 PLA/PHB). The 2.68% highes 10/50. tot 3.84%.value The elongation was There about was 47 a slight at maximum strength (E ) does not depend significantly on blend composition, and the MPalinear for decrease the PLA/PHB in this 50/10 Fmaxparameter blend, andas a the function lowest ofwas PHB 26 MPacontent. for PLA/PHB 10/50. The values of this parameter ranged from 2.68% to 3.84%. There was a slight linear decrease in elongationthis parameter at maximum as a function strength of PHB (EFmax content.) does not depend significantly on blend composition, and the values of this parameter ranged from 2.68% to 3.84%. There was a slight linear decrease in this parameter as a function of PHB content. 55 8

TFmax 50 EFmax 7 55 8

45 TFmax 50 6 EFmax 7 40 45 [%] [MPa] 6 5 35 40 Fmax

[%]

Fmax 4 E [MPa] 5 T 3530

Fmax

Fmax 4 E 3 T 3025

2520 3 2 0 20 40 60 80 100 20 2 0 20 PHB40 content60 80 [%] 100 PHB content [%] FigureFigure 4. 4. DependenceDependence ofof maximummaximum tensile tensile strength strength (TFmax (T)Fmax and) elongationand elongation at ultimate at ultimate strength strength (EFmax(EFmax) on) on PHB PHB content content inin the the PLA/PHB PLA/PHB blend. blend. Figure 4. Dependence of maximum tensile strength (TFmax) and elongation at ultimate strength (EFmax) on PHB content in the PLA/PHB blend.

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50 TS 50 Eb 40 40

30 30

Eb [%]

TS [MPa] 20 20

10 10

0 0 0 20 40 60 80 100

PHB content [%] FigureFigure 5. 5. DependenceDependence of tensileof tensile strength strength at break at (TS) break and elongation (TS) and at breakelongation (Eb) on PHBat break content (Eb) on PHB con- tentin in the the PLA/PHB PLA/PHB blend. blend. The dependence of polyhydroxybutyrate content on tensile strength at break (TS) lookedThe similar dependence to maximum of tensilepolyhydroxybutyrate strength (TFmax). According content to Figure on5 ,tensile the higher strength the at break (TS) PHB content, the lower the TS value. The correlation between the 30/40, 50/50 and 40/30 looked similar to maximum tensile strength (TFmax). According to Figure 5, the higher the blends was the most interesting because a decrease in this parameter from 32 MPa to 7 MPa PHBwas content, observed. the The elongationlower the at breakTS value. also depends The correlation on PLA/PHB between composition the and 30/40, rises 50/50 and 40/30 blendsin an exponentialwas the most way as interesting a function of PHB because content. a decrease in this parameter from 32 MPa to 7 MPa was observed. The elongation at break also depends on PLA/PHB composition and 3.5. Dynamic Mechanical Analysis (DMA) rises inThe an values exponential of storage way and loss as modulia function changed of PHB in an irregular content. way (Figure6). The behavior and dynamic properties of material based on PLA with PHB cannot be predicted 3.5by. Dynamic the content Mechanical of a component Analysis in the blend.(DMA) The PLA/PHB 50/20 blend was the only material that revealed an evident intersection of G’ and G” in the range of the investigated angularThe values frequency. of Instorage contrast and to the loss other moduli blends, blendschange ofd 50/10, in an 40/30 irregular and 30/40 way (Figure 6). The behaviorPLA/PHB and were dynamic close to having properties an intersection of material in the deﬁnite based range on PLA of angular with frequency. PHB cannot be predicted by Thethe lowest content value of of a loss component angle was for in the the PLA/PHB blend. 50/20 The blend,PLA/PHB which contained 50/20 blend about was the only ma- 28% of polyhydroxybutyrate (PHB) (Figure6b). A higher value of tg δ was found for the terialPLA/PHB that revealed 30/40 blend an containing evident about intersection 57% of PHB, of but G’ the and lower G” PHB in content the range of about of the investigated angular43% resulted frequency in higher. valuesIn contrast of G”/G’. to The the dynamic other viscosity blends, results blends for polymer of 50/10, blends 40/30 and 30/40 PLA/PHBdiffered from were the close static ones.to having The highest an intersection values of dynamic in the viscosity definite were range obtained of for angular frequency. the PLA/PHB 50/20 blend and the lowest ones for the PLA/PHB 10/50 blend (Figure7a). TheNo lowest closer correlation value of between loss angle this parameter was for and the PHB PLA/PHB content was 50/20 found. blend, which contained about 28% of polyhydroxybutyrate (PHB) (Figure 6b). A higher value of tgδ was found for the PLA/PHB 30/40 blend containing about 57% of PHB, but the lower PHB content of about 43% resulted in higher values of G”/G’. The dynamic viscosity results for polymer blends differed from the static ones. The highest values of dynamic viscosity were obtained for the PLA/PHB 50/20 blend and the lowest ones for the PLA/PHB 10/50 blend (Figure 7a). No closer correlation between this parameter and PHB content was found.

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FigureFigureFigure 6. Storage 6. 6.Storage Storage and andloss and loss moduliloss moduli moduli as asa as function a a function function of of ofangular angular angular frequencyfrequency frequency for forfor the: the (a:: ) ((a 30/40a)) 30/40 30/40 PLA/PHB PLA/PHB PLA/PHB blend, blend blend (,b ()b, 50/20) .(50/20b).50/20 PLA/PHB PLA/PHB PLA/PHB blend,blend, blend,(c) 50/10 (c(c)) 50/10 50/10PLA/PHB PLA/PHBPLA/PHB blend, blend, blend, (d) 40/30( (dd)) 40/30 40/30 PLA/PHB PLA/PHB PLA/PHB blend. blend. blend.

Figure 7. Viscosity of PLA/PHB blends (a) and loss tangent coefficient (tgδ) (b) as a function of angular frequency. FigureFigure 7. Viscosity 7. Viscosity of PLA/PHB of PLA/PHB3.6. Surfaceblends blends (Caharacterization) (anda) and loss loss tangent tangent of P olymercoefficient coefﬁcient Blends (tg(tgδ δ)() b(b) as) as a functiona function of angularof angular frequency. frequency. The contact angle results were useful for the calculation of surface energy (SE) and 3.6.3.6. itsSurface components Surface Characterization Characterization (Figure 8). of ofI tP Polymercanolymer be noted BlendsBlends that there was no visible correlation between surfaceTheThe contact energy contact angleand angle polyhydroxybutyrate results results were were useful content forfor the the calculationnor calculation the polar of or surfaceof dispersivesurface energy energy component (SE) (SE) and and its componentsitsof componentsSE. Nevertheless, (Figure (Figure almost 88).). I Itt can every bebe result notednoted show that that thereed there that was thewas no specimens no visible visible correlation had correlation a hydrophobic between between surfacesurfacecharacter. energy energy Only and and the polyhydroxybutyrate polyhydroxybutyrate40/30 PLA/PHB blend content contentseemed nor tonor thebe the le polarss polar hydrophobic or dispersiveor dispersive in componentcomparison component of SE. Nevertheless, almost every result showed that the specimens had a hydrophobic of SE.to theNevertheless, materials. almost every result showed that the specimens had a hydrophobic character. Only the 40/30 PLA/PHB blend seemed to be less hydrophobic in comparison character.to the materials. Only the 40/30 PLA/PHB blend seemed to be less hydrophobic in comparison to the materials.

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Figure 8.8. Dependence of surface energy and and it itss components components on on polyhydroxybutyrate polyhydroxybutyrate content content in in a a polymerpolymer blend.blend.

4. Conclusions On the basis of the measurement results,results, itit cancan bebe notednoted thatthat torquetorque asas wellwell asas mixingmixing energy decrease with with higher higher content content of ofpolyhydroxybutyrate polyhydroxybutyrate (PHB) (PHB) in the in thePLA/PHB PLA/PHB mix- mixture.ture. Therefore, Therefore, in such in such a blend, a blend, polyhydroxybutyrate polyhydroxybutyrate facilitates facilitates the theprocessability processability of the of thepolylactide polylactide matrix matrix.. In Inspite spite of ofthe the higher higher melting melting point point of of polyhydroxybutyrate polyhydroxybutyrate (PHB), whichwhich waswas detecteddetected by by Differential Differential Scanning Scanning Calorimetry Calorimetry (DSC), (DSC), the the addition addition of PHB of PHB to the to blendthe blend causes causes an increase an increase in melt in volumemelt volume flow rate,flow which rate, which also facilitates also facilitates material material processing. pro- Thecessing addition. The addition of polyhydroxybutyrate of polyhydroxybutyrate (PHB) to(PHB) polylactide to polylactide causes causes a decrease a decrease in glass in transitionglass transition temperature temperature (Tg). The (Tg). measurement The measurement of mechanical of mechanical properties properties reveals reveal that tensiles that strengthtensile strength at break at (TS) break and (TS) maximum and maximum tensile strength tensile (TstrengthFmax) diminish (TFmax) diminish as a function as a offunction (PHB) content,of (PHB) butcontent a small, but amount a small ofamount PHB leadsof PH toB leads material to material enhancement. enhancement On the. On other thehand, other elongation at break (E ) rises with the polyhydroxybutyrate (PHB) content but is lower hand, elongation at breakb (Eb) rises with the polyhydroxybutyrate (PHB) content but is inlower comparison in comparison to neat to PLA. neat DynamicPLA. Dynamic Mechanical Mechanical Analysis Analysis (DMA) (DMA) results results showed showed that PLA/PHBthat PLA/PHB blends blends are shear-thinning are shear-thinning fluids, fl regardlessuids, regardless of their of composition. their composition. Only the Only 50/20 the PLA/PHB50/20 PLA/PHB blend blend had G”/G’had G’’/G’ lower lower than than 1 in 1 the in frequencythe frequency of 500–625 of 500– rad/s.625 rad/s. Moreover, Moreo- thever, contactthe contact angle angle measurements measurements provided provided information information about about the hydrophobicitythe hydrophobicity of the of blends. Polylactide is a stiffer material than polyhydroxybutyrate; thus, blends prepared the blends. Polylactide is a stiffer material than polyhydroxybutyrate; thus, blends pre- from these two polymers have intermediate properties, including mixing energy, glass pared from these two polymers have intermediate properties, including mixing energy, transition temperature and viscosity or melt flow rate. Polyhydroxybutyrate facilitates the glass transition temperature and viscosity or melt flow rate. Polyhydroxybutyrate facili- processability of polylactide, so these blends could become novel materials in the packaging tates the processability of polylactide, so these blends could become novel materials in the sector. Based on the test results, the PLA/PHB blend with a 50/10 ratio seems to be the most packaging sector. Based on the test results, the PLA/PHB blend with a 50/10 ratio seems appropriate because of its mechanical and processing properties. These results support to be the most appropriate because of its mechanical and processing properties. These the idea that thermodynamic mixing and energetics of the polymer–polymer interface results support the idea that thermodynamic mixing and energetics of the polymer–poly- are critical design parameters for creating highly pro-ecological polymeric materials. The mer interface are critical design parameters for creating highly pro-ecological polymeric presented blend has the potential for replacing and being safer than non-biodegradable materials. The presented blend has the potential for replacing and being safer than non- materials, which contain additives harmful to human beings. biodegradable materials, which contain additives harmful to human beings. Author Contributions: Methodology and materials, O.O., J.Z. and A.M.; writing—original draft preparation,Author Contribution review, O.O.,s: Methodology A.M. and J.Z.; and supervision, materials, O.O., manager J.Z. and of NCBR A.M.; project,writing— A.M.;original conceptual- draft ization,preparation formal, review analysis,, O.O., A.M., A. M. O.O. and All J.Z. authors; supervision have, read manager and agreedof NCBR to theproject published, A.M.; conceptu- version of

thealization manuscript., formal analysis, A.M., O.O. All authors have read and agreed to the published version of the manuscript. Funding: This study was supported by the National Centre for Research and Development (NCBR) Funding: This study was supported by the National Centre for Research and Development project: LIDER/32/0139/L-7/15/NCBR/2016.g. (NCBR) project: LIDER/32/0139/L-7/15/NCBR/2016.g.

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Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing is not applicable to this article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Tábi, T. The application of the synergistic effect between the crystal structure of poly(lactic acid) (PLA) and the presence of ethylene vinyl acetate copolymer (EVA) to produce highly ductile PLA/EVA blends. J. Therm. Anal. Calorim. 2019, 138, 1287–1297. [CrossRef] 2. Katiyar, V. Non-isothermal degradation kinetics of P.LA-functionalized gum (fG) biocomposite with dicumyl peroxide (DCP). J. Therm. Anal. Calorim. 2019, 138, 195–210. 3. Kelnar, I.; Kratochvíl, J.; Kaprálková, L. Crystallization and thermal properties of melt-drawn PCL/PLA microfibrillar composites. J. Therm. Anal. Calorim. 2016, 124, 799–805. [CrossRef] 4. Luyt, A.; Kelnar, I. Effect of blend ratio and nanofiller localization on the thermal degradation of graphite nanoplatelets-modified PLA/PCL. J. Therm. Anal. Calorim. 2018, 136, 2373–2382. [CrossRef] 5. Bubeck, R.A.; Merrington, A.; Dumitrascu, A.; Smith, P.B. Thermal analyses of poly(lactic acid) PLA and micro-ground paper blends. J. Therm. Anal. Calorim. 2018, 131, 309–316. [CrossRef] 6. Penczek, S.; Petrula, J.; Lewiński,P. Polimery z odnawialnych surowców, polimery biodegradowalne. Polimery 2013, 58, 833–958. 7. Jayasekara, R.; Harding, I.H.; Bowater, I.C.; Lonergan, G. Biodegradability of a Selected Range of Polymers and Polymer Blends and Standard Methods for Assessment of Biodegradation. J. Polym. Environ. 2005, 13, 231–251. [CrossRef] 8. Wang, S.; Chen, W.; Xiang, H.; Yang, J.; Zhou, Z.; Zhu, M. Modification and Potential Application of Short-Chain-Length Polyhydroxyalkanoate (SCL-PHA). Polymers 2016, 8, 273. [CrossRef] 9. Wu, Q.; Wang, Y.; Chen, G.-Q. Medical Application of Microbial Biopolyesters Polyhydroxyalkanoates. Artif. Cells Blood Substit. Biotechnol. 2009, 37, 1–12. [CrossRef] 10. Sorrentino, A.; Gorrasi, G.; Vittoria, V. Potential perspectives of bio-nanocomposites for food packaging applications. Trends Food Sci. Technol. 2007, 18, 84–95. [CrossRef] 11. Joshi, J.R.; Patel, R.P. Role of biodegradable polymers in drug delivery. Int. J. Curr. Pharm. Res. 2012, 4, 74–81. 12. Chiulan, I.; Frone, A.N.; Brandabur, C.; Panaitescu, D.M. Recent Advances in 3D Printing of Aliphatic Polyesters. Bioengineering 2017, 5, 2. [CrossRef][PubMed] 13. Nocita, D.; Forte, G.; Drakopoulos, S.X.; Visco, A.; Gianporcaro, A.; Ronca, S. Processing and characterization of bio-polyester reactive blends: From thermoplastic blends to cross-linked networks. Polymer 2017, 132, 252–263. [CrossRef] 14. Malinconico, M.; Cerruti, P.; Santagata, G.; Immirzi, B. Natural Polymers and Additives in Commodity and Specialty Applications: A Challenge for the Chemistry of Future. Macromol. Symp. 2014, 337, 124–133. [CrossRef] 15. Zhang, M.; Thomas, N.L. Blending polylactic acid with polyhydroxybutyrate; the effect on thermal, mechanical, and biodegradation properties. Adv. Polym. Technol. 2011, 30, 67–79. [CrossRef] 16. Topkanlo, H.A.; Ahmandi, Z.; Taromi, F.A. An in-depth study on crystallization kinetics of PET/PLA blends. Iran Polym. J. 2017, 27, 13–22. [CrossRef] 17. Ploypetchara, N.; Suppakul, P.; Atong, D.; Pechyen, C. Blend of polypropylene/poly(lactic acid) for medical packaging application: Physicochemical, thermal, mechanical, and barrier properties. Energy Procedia 2014, 56, 201–210. [CrossRef] 18. Feng F, Ye LStructu and Property of Polylactide/Polyamide Blends. J. Macromol. Sci. B 2010, 49, 1117–1127. [CrossRef] 19. Al-Itry, R.; Lamnawar, K.; Maazouz, A. Biopolymer Blends Based on Poly (lactic acid): Shear and Elongation Rheol- ogy/Structure/Blowing Process Relationships. Polymers 2015, 7, 939–962. [CrossRef] 20. Tănse, E.E.; Popa, M.E.; Râpa, M.; Popa, O. Preparation and characterization of biopolymer blends based on polyvinyl alcohol and starch. Rom. Biotechnol. Lett. 2015, 20, 10306–10315. 21. Imre, B.; Pukánszky, B. Compatibilization in bio-based and biodegradable polymer blends. Eur. Polym. J. 2013, 49, 1215–1233. [CrossRef] 22. Abdelwahab, M.A.; Flynn, A.; Chiou, B.S.; Imam, S.; Orts, W. Thermal, mechanical and morphological characterization of plasticized PLA-PHB blends. Polym. Degrad. Stabil. 2012, 97, 1822–1828. [CrossRef] 23. Hu, Y.; Daoud, W.A.; Cheuk, K.K.L.; Lin, C.S.K. Newly Developed Techniques on Polycondensation, Ring-Opening Polymeriza- tion and Polymer Modification: Focus on Poly(Lactic Acid). Materials 2016, 9, 133. [CrossRef] 24. Sangeetha, V.H.; Deka, H.; Varghese, T.O.; Nayak, S.K. State of the Art and Future Prospectives of Poly(Lactic Acid) Based Blends and Composites. Polym. Compos. 2016, 39, 1–21. [CrossRef] 25. Farah, S.; Anderson, D.G.; Langer, R. Physical and mechanical properties of PLA, and their functions in widespread applications— A comprehensive review. Adv. Drug Deliv. Rev. 2016, 107, 367–392. [CrossRef][PubMed] 26. Tang, X.Z.; Kumar, P.; Alvi, S.; Sandeep, K.P. Recent Advances in Biopolymers and Biopolymer-Based Nanocomposites for Food Packaging Materials. Crit. Rev. Food Sci. 2012, 52, 426–442. [CrossRef][PubMed] Materials 2021, 14, 898 12 of 12

27. Armentano, I.; Fortunati, E.; Burgos, N.; Dominici, F.; Luzi, F.; Fiori, S.; Jiménez, A.; Yoon, K.; Ahn, J.; Kang, S.; et al. Processing and characterization of plasticized PLA/PHB blends for biodegradable multiphase systems. eXPRESS Polym. Lett. 2015, 9, 583–596. [CrossRef] 28. Arrieta, M.P.; Samper, M.D.; Aldas, M.; López, J. On the Use of PLA-PHB Blends for Sustainable Food Packaging Applications. Materials 2017, 10, 1008. [CrossRef] 29. Plavec, R.; Hlaváˇciková, S.; Omaníková, L.; Feranc, J.; Vanovˇcanová, Z.; Tomanová, K.; Boˇckaj,J.; Kruželák, J.; Medlenová, E.; Gálisová, I.; et al. Recycling possibilities of bioplastics based on PLA/PHB blends. Polym. Test. 2020, 92, 106880. 30. Park, J.W.; Doi, Y.; Iwata, T. Uniaxial Drawing and Mechanical Properties of Poly[(R)-3-hydroxybutyrate]/Poly(L-lactic acid) Blends. Biomacromolecules 2004, 5, 1557–1566. [CrossRef][PubMed] 31. WinMix, version 3.0.0; Software for Measuring Mixing Process; Brabender®GmbH & Co. KG: Duisburg, Germany, 2017.