Biodegradable Polymers for the Production of Nets for Agricultural Product Packaging

materials

Article Biodegradable Polymers for the Production of Nets for Agricultural Product Packaging

Francesco Paolo La Mantia 1,2,* , Manuela Ceraulo 2, Paolo Testa 1 and Marco Morreale 3

1 Department of Engineering, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy; [email protected] 2 Consorzio INSTM, Via Giusti 9, 50121 Firenze, Italy; [email protected] 3 Faculty of Engineering and Architecture, Kore University of Enna, Cittadella Universitaria, 94100 Enna, Italy; [email protected] * Correspondence: [email protected]

Abstract: It is well known that the need for more environmentally friendly materials concerns, among other fields, the food packaging industry. This regards also, for instance, nets used for agricultural product (e.g., citrus fruits, potatoes) packaging. These nets are typically manufactured by film blowing technique, with subsequent slicing of the films and cold drawing of the obtained strips, made from traditional, non-biodegradable polymer systems. In this work, two biodegradable polymer systems were characterized from rheological, processability, and mechanical points of view, in order to evaluate their suitability to replace polyethylene-based polymer systems typically used for agricultural product net manufacturing. Furthermore, laboratory simulation of the above-mentioned processing operation paths was performed. The results indicated a good potential for biodegradable polymer systems to replace polyethylene-based systems for agricultural product packaging.

Keywords: biodegradable polymer; fruit packaging; fibers; films; elongational flow

Citation: La Mantia, F.P.; Ceraulo, M.; 1. Introduction Testa, P.; Morreale, M. Biodegradable Nets for packaging of fruits and vegetables are usually made from blown films of Polymers for the Production of Nets high-density polyethylene (HDPE) or blends of polyethylenes (PEs) cut into small slices, for Agricultural Product Packaging. and then cold drawn. To date, the production of polymer films used for this application Materials 2021, 14, 323. https:// relies on polyolefins, either neat or blended, and the main mechanical properties of interest doi.org/10.3390/ma14020323 in such application are rigidity and elongation at break. Furthermore, a fundamental

Received: 1 December 2020 property for this application is weldability, which cannot be easily obtained by only using Accepted: 6 January 2021 HDPE; this issue is overcome by blending it with a low-density polyethylene (LDPE) or Published: 9 January 2021 metallocene (M-PE), which make the processing much easier. Therefore, in order to find suitable candidates for replacing polyolefins in these applications, the research should be Publisher’s Note: MDPI stays neu- focused on filmability properties, as well as the ability to be efficiently cold drawn and tral with regard to jurisdictional clai- to give rise to slices with a good rigidity and deformability. At the same time, the large ms in published maps and institutio- use of plastics in the packaging industry has been increasing the environmental concern; nal affiliations. therefore, the interest in the use of biodegradable polymers to replace conventional polymers, in order to reduce the environmental impact caused by inappropriate disposal, has grown [1]. Poly(lactic acid) (PLA) is a thermoplastic, biodegradable, and biocompatible polymer that can be produced from renewable resources [2]. PLA is known to have low Copyright: © 2021 by the authors. Li- toughness at room temperature and low melting resistance compared to conventional censee MDPI, Basel, Switzerland. polymers, with these factors actually hindering its application in large-scale processes, such This article is an open access article as industrial blown film extrusion, blowing, and foaming, where stability to the melt is a distributed under the terms and conditions of the Creative Commons At- fundamental factor [3,4]. The melting resistance of PLA must be, therefore, improved in tribution (CC BY) license (https:// order to widen its processing frame and therefore its field of application. Copolymeriza- creativecommons.org/licenses/by/ tion, the addition of plasticizers, and blending with other polymers are some of the main 4.0/). ways to achieve improvements in the properties of PLA [5]. PLA blends with polymers

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such as poly(butylene-succinate-co-adipate) (PBSA) [6], poly(butylene succinate) (PBS) [7], poly(butyl acrylate) (PBA) [8], and poly(butylene-adipate-co-terephthalate) (PBAT) [9,10] have been studied in order to improve the mechanical properties of PLA, mainly to increase its toughness. In particular, PBAT is a very flexible biodegradable polymer, despite being somewhat underestimated, due to its lower rigidity. Blends of these polymers can give good properties for agricultural and for packaging applications [11,12]. In spite of this, these polymers also have a significant disadvantage—the production cost limits their large-scale application. In order to overcome the issue, biodegradable blends can be prepared with low-cost reinforcements, such as natural fibers and common minerals, thus reducing the final cost and positively influencing the properties of the matrix, especially with regard to tensile strength and Young’s modulus, while deformability may be reduced by increasing the mineral filler content [13]. Thus, nanofillers have also been investigated in place of traditional microfillers. With specific concern to the production of biodegradable fibers, La Mantia et al. [14] studied the effect of cold drawing on the mechanical properties. It is not easy to obtain polymeric fibers with adequate mechanical properties and tailor them to the specific application. The main ways to “tailor” the mechanical properties of a given biodegradable polymer fiber are based on crystallinity and orientation control. However, the crystallinity can be modified only marginally during the processing, while the orientation of the macromolecules can be controlled, both during hot and cold drawing. Although some data are available on the effects of orientation on the main properties [15], only a few papers have addressed the effects of drawing on biodegradable fibers [16]. In general, it can be stated that the orientation leads to a significant increase in the elastic modulus and the tensile strength (with a subsequent decrease in the elongation at break) along the stretching direction, although this applies to semi-crystalline polymers; amor- phous polymers and blends can experience a transition from brittle to ductile behavior, induced by orientation [17,18], with a subsequent increase in the elongation at break as the orientation increases. As regards the effect of cold drawing on the properties of polymeric fibers, the information available in the literature is generally focused on traditional non-biodegradable polymers. Cold drawing was found to significantly improve the elastic modulus, tensile strength, and thermomechanical strength of the fibers compared to hot spun fibers. The elastic modulus showed higher rates of increase in biodegradable systems upon increasing the drawing ratio. Another promising synthetic biopolymer class, designed to replace conventional plastic in numerous applications, is Mater-Bi, a commercial biodegradable polymer based on biodegradable polyesters from renewable production sources. Mater-Bi has found important commercial applications as it shows interesting mechanical properties, thermal stability, processability, and biodegradability [19,20]. In this work, two biodegradable polymer systems were characterized from rheological, processability, and mechanical points of view to evaluate if biodegradable polymers can be suitable for preparing nets for fruits and vegetables, thus replacing traditional, non- biodegradable PE-based nets. As already stated, these nets are prepared by a combined process, where film blowing leads to the production of oriented films, which are then subjected to slicing (or, in other words, films are cut longitudinally in order obtaining numerous, thick slices) and then to cold drawing of the slices, thus obtaining filaments that are directly arranged to form the nets. For the sake of clarity, a flow diagram of the process is shown as Scheme1. In particular, laboratory simulation was also performed by simulating these two processing operations paths, i.e., film blowing and cold drawing, and by evaluating the obtained results in comparison with the results obtained, under the same experimental procedure, with a typical polymer industrially used for this application. Moreover, the final results were also compared to the mechanical properties of the industrially processed polymer. MaterialsMaterials2021 2021, 14, ,14 323, x FOR PEER REVIEW 3 of3 10of 10

SchemeScheme 1. 1.Flow Flow diagram diagram of of the the net net manufacturing manufacturing process. process.

2.2. Materials Materials and and Methods Methods 2.1. Materials and Processing 2.1. Materials and Processing The materials used in these works were an Ecovio F23B1 sample, i.e., a blend of The materials used in these works were an Ecovio F23B1 sample, i.e., a blend of PBAT with a small amount of PLA and inert, insoluble particles [21] (produced by BASF, PBAT with a small amount of PLA and inert, insoluble particles [21] (produced by BASF, Ludwigshafen, Germany); a MaterBi EF51L sample (Novamont, Novara, Italy), i.e., an Ludwigshafen, Germany); a MaterBi EF51L sample (Novamont, Novara, Italy), i.e., an extrusion grade with proprietary composition, based on biodegradable aliphatic and extrusion grade with proprietary composition, based on biodegradable aliphatic and al- aliphatic/aromatic polyesters [22]; a HDPE sample commercialized by REPSOL (Madrid, iphatic/aromatic polyesters [22]; a HDPE sample commercialized by REPSOL (Madrid, Spain) as Alcudia HDPE R4805D1; and an ethylene-hexene metallocene copolymer (mPE) ENABLESpain) 2005HHas Alcudia from HDPE ExxonMobil R4805D1; Chemical and an (Houston, ethylene-hexene TX, USA), metallocene containing technologi- copolymer cal(mPE) adjuvants ENABLE and thermal2005HH stabilizing from ExxonMobil additives. Chemical (Houston, TXs, USA), containing technologicalActually, a blendadjuvants of HDPE and thermal and mPE stabilizing (80/20) is additives. used for the preparation of the films to produceActually, the nets; a therefore,blend of HDPE the 80/20 and HDPE/mPEmPE (80/20) is blend used wasfor the considered preparation as the of referencethe films to materialproduce to the be nets; compared therefore, to the the biodegradable 80/20 HDPE/m polymers.PE blend was This considered HDPE/mPE as the blend reference was preparedmaterial using to be a compared co-rotating to twin-screw the biodegra extruderdable withpolymers. a length-to This HDPE/mPE diameter ratio blend (L/D) was equalprepared to 35 (OMC,using a Saronno, co-rotating Italy), twin-screw with a 170–180–190–200–210–220–230 extruder with a length-to diameter◦C temperature ratio (L/D) profileequal and to 35 220 (OMC, rpm screwSaronno, speed. Italy), The with extruded a 170–180–190–200–210–220–230 materials were then pelletized °C temperature and used forprofile the preparation and 220 rpm of specimensscrew speed. to beThe subjected extruded to materials mechanical were and then rheological pelletized tests, and after used cuttingfor the them preparation from compression-molded of specimens to be sheets subjecte preparedd to mechanical using a Carver and rheological (Wabash, IN, tests, USA) after laboratorycutting them press. from Ecovio compression-molded and MaterBi were sheet alwayss prepared conditioned using in a vacuum Carver oven(Wabash, before IN, high-temperatureUSA) laboratory processing. press. Ecovio and MaterBi were always conditioned in vacuum oven beforeThe high-temperature film blowing operation processing. on HDPE/mPE, PLA/PBAT (Ecovio), and MaterBi was performedThe film using blowing a Brabender operation (Duisburg, on HDPE/mPE, Germany) PLA/PBAT equipment, (Ecovio), on the basis and ofMaterBi a single- was screwperformed extruder using (D = a 19 Brabender mm, L/D =(Duisburg, 25) and a filmGermany) blowing equipment, unit, operating on the at thebasis following of a sin- conditionsgle-screw (Table extruder1): (D = 19 mm, L/D = 25) and a film blowing unit, operating at the following conditions (Table 1): Table 1. Investigated systems and processing conditions. Table 1. Investigated systems and processing conditions. Polymer System Temperature Profile (◦C) Screw Speed (rpm) Draw Ratio, DR Blow-Up Ratio, BUR Polymer System Temperature Profile (°C) Screw Speed (rpm) Draw Ratio, DR Blow-Up Ratio, BUR HDPE/mPE 170–190–210–230–230–230HDPE/mPE 170–190–210–230–230–230 100 100 3 3 2 2 MaterBi 120–130–140–150–160–170MaterBi 120–130–140–150–160–170 80 80 3 3 2 2 Ecovio 150–160–170–170–170–170Ecovio 150–160–170–170–170–170 80 80 3 3 2 2 Finally, approximately 80 μm thick films were prepared, from which samples for µ subsequentFinally, approximately characterizations 80 werem thick cut off. films Furt werehermore, prepared, in order from to better which simulate, samples at for the subsequentlaboratory characterizationsscale, the industrial were process, cut off. we Furthermore, prepared filaments in order by to bettercold-drawing simulate, of atthe the laboratory scale, the industrial process, we prepared filaments by cold-drawing of blown-extruded films (2 mm wide, 5 cm long strips, cut off the blown-extruded films; the blown-extruded films (2 mm wide, 5 cm long strips, cut off the blown-extruded films; clamping length was set at 3 cm), which were then subjected to cold (i.e., room temper- clamping length was set at 3 cm), which were then subjected to cold (i.e., room temperature) ature) drawing treatment at different draw ratios (DR = 2 and 3) by means of an Instron drawing treatment at different draw ratios (DR = 2 and 3) by means of an Instron (Norwood, (Norwood, MA, USA) 3365 universal machine at 10 mm/min deformation speed. MA, USA) 3365 universal machine at 10 mm/min deformation speed.

2.2.2.2. Characterization Characterization SamplesSamples were were subjected subjected to to rheological rheological characterization characterization under under shear shear ﬂow, flow, using using both both a rotationala rotational plate–plate plate–plate rheometer rheometer (Ares (Ares G2, TA G2, Instruments, TA Instruments, New Castle, New Castle, DE, USA; DE, dynamic USA; dy- frequencynamic frequency sweep tests sweep at 0.1–100tests at 0.1–100 rad/s frequency rad/s frequency range, range, strain strain = 5%) = and 5%) aand capillary a capil- rheometerlary rheometer (Rheologic (Rheologic 1000, 1000 Ceast,, Ceast, Pianezza, Pianezza, Italy; Italy; capillary capillary size size L = L 4 = cm, 4 cm, D =D 1= mm);1 mm); non-isothermalnon-isothermal elongational elongational ﬂow flow characterization characterization was was performed performed on on the the same same apparatus apparatus byby means means of aof dedicated a dedicated drawing drawing unit, locatedunit, located at the exitat the of theexit capillary of the die.capillary Melt strengthdie. Melt

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strength (MS) and breaking stretching ration (BSR) were then calculated, according to the procedures described elsewhere [23]. Mechanical (tensile) characterization on samples coming from the compression molded sheets and blown films and on filaments coming from cold drawing of the blown Materials 2021, 14, 323 films was carried out using a Instron (Norwood, MA, USA) mod. 3365 universal ma- 4 of 10 chine, with a 3 cm clamping length, a first crosshead speed of 1 mm/min (until 3 min), and then a second crosshead speed increase to 100 mm/min, up to specimen failure.

3. Results and(MS) Discussion and breaking stretching ration (BSR) were then calculated, according to the procedures The extrusiondescribed profile elsewhere and in particular [23]. the temperature at the die reported in Table 1 for the conventionalMechanical polymer (tensile) is the extrusio characterizationn profile usually on samples adopted coming in the from industrial the compression molded extrusion forsheets this material and blown and filmswas adopte and ond filamentsin this work. coming The extrusion from cold temperature drawing of the blown films profiles for the two biodegradable polymers were chosen on the basis of the flow curves, was carried out using a Instron (Norwood, MA, USA) mod. 3365 universal machine, with in particular on the basis of the flow curve at the typical extrusion velocity gradient range a 3 cm clamping length, a first crosshead speed of 1 mm/min (until 3 min), and then a in an extruder, which is usually in the range of 100–300 s−1. In Figuresecond 1, the crosshead flow curves speed measured increase from to rheological 100 mm/min, tests upin a torotational specimen rheom- failure. eter (complex viscosity, η*, vs. frequency) and in a capillary viscometer (viscosity, η, vs. shear rate) 3.for Results all the materials, and Discussion at the three different temperatures (230 °C for the conventional polymerThe blend, extrusion 170 °C profile for the and two in biodegradable particular the systems), temperature are shown at the (data die reported in Table1 reproducibility:for the ±5%). conventional polymer is the extrusion profile usually adopted in the industrial As is wellextrusion evident, for the this superposition material and between was adoptedthe curves in from this the work. rotational The extrusionrhe- temperature ometer andprofiles the capillary for the viscometer two biodegradable occurred for polymers the conventional were chosen blend on theand basisfor the of the flow curves, in MaterBi, butparticular the Cox–Merz on the rule basis did of not the seem flow to curve be followed at the typical by the extrusion Ecovio; this velocity is in gradient range in agreement anwith extruder, other studies which on is heterogene usually inous, the rangemulti-phase of 100–300 systems s− containing1. solid particles [24]. In Figure1, the flow curves measured from rheological tests in a rotational rheome- It can be observed that the highest levels of viscosity were shown by the HDPE/mPE ter (complex viscosity, η*, vs. frequency) and in a capillary viscometer (viscosity, η, vs. blend and the lowest (on average) by the Ecovio. In the higher shear rate range, the ◦ curves tendedshear to approach rate) for each all the other materials, (in particular, at the with three regard different to those temperatures obtained at the (230 C for the con- ◦ capillary rheometer),ventional and polymer this meant blend, that170 the extrudabilityC for the two of the biodegradable three samples was systems), sim- are shown (data ilar. reproducibility: ±5%).

Figure 1. Flow curvesFigure of 1. theFlow three curves polymers of the three at different polymers temperatures. at different temperatures. Data were taken Data both were in taken a rotational both in a rheometer (open rotational rheometer (open points) and in a capillary viscometer (full points). points) and in a capillary viscometer (full points).

As is well evident, the superposition between the curves from the rotational rheometer and the capillary viscometer occurred for the conventional blend and for the MaterBi, but

the Cox–Merz rule did not seem to be followed by the Ecovio; this is in agreement with other studies on heterogeneous, multi-phase systems containing solid particles [24]. It can be observed that the highest levels of viscosity were shown by the HDPE/mPE blend and the lowest (on average) by the Ecovio. In the higher shear rate range, the curves tended to approach each other (in particular, with regard to those obtained at the capillary rheometer), and this meant that the extrudability of the three samples was similar. The filmability of the polymers in a typical film blowing process is controlled by the behavior under non-isothermal elongational flow. For this reason, the melt strength (MS) and breaking stretching ratio (BSR) were measured at the same temperatures of the rheological tests, which is reported in Figures2 and3 (data reproducibility: ±7%). Materials 2021, 14, x FOR PEER REVIEW 5 of 10 Materials 2021, 14, x FOR PEER REVIEW 5 of 10

The filmability of the polymers in a typical film blowing process is controlled by the The filmability of the polymers in a typical film blowing process is controlled by the Materials 2021, 14, 323behavior under non-isothermal elongational flow. For this reason, the melt strength (MS) 5 of 10 behaviorand breaking under stretching non-isothermal ratio (BSR) elongational were meas flow.ured For at this the resameason, temperatures the melt strength of the (MS) rhe- andological breaking tests, stretchingwhich is reported ratio (BSR) in Figures were meas 2 andured 3 (data at the reproducibility: same temperatures ± 7%). of the rheological tests, which is reported in Figures 2 and 3 (data reproducibility: ± 7%).

Figure 2. MeltFigure strength 2. Melt of thestrength three of polymers the three as polymers a function as of a thefunc apparenttion of the shear apparent rate. Temperaturesshear rate. Temperatures and symbols are as in Figure 2. Melt strength of the three polymers as a function of the apparent shear rate. Temperatures Figure1. and symbols are as in Figure 1. and symbols are as in Figure 1.

Figure 3. Breaking stretching ratio of the three polymers as a function of the apparent shear rate. Figure 3. BreakingFigureTemperatures stretching3. Breaking and ratio symbolsstretching of the are three ratio as polymersin of Figure the three as1. a polymers function of as the a function apparent of shear the rate.apparent Temperatures shear rate. and symbols Temperatures and symbols are as in Figure 1. are as in Figure1. The BSR values showed that the conventional polymer system was the less stretchable Themelt, BSR while values theThe twoshowed BSR biodegradable values that the showed conventi systems thatonal the presented conventionalpolymer higher system polymer values. was systemthe The less combined wasstretch- the less stretchable ableresults melt, of MSwhile andmelt, the BSR whiletwo clearly biodegradable the two gave biodegradable evidence systems that systemspresented also the presented filmability higher values. higher of these The values. twocombined Thebiode- combined results results of MS andof MS BSR and clearly BSR clearlygave evidence gave evidence that also that the also filmability the ﬁlmability of these of two these biode- two biodegradable polymer systems was at least similar or even better than that of the conventional polymer blend.

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The results from the tensile tests, elastic modulus (E), tensile strength (TS), and elongation at break (EB), measured on isotropic (i.e., compression molded) specimens, are shown in Table2 below.

Table 2. Tensile properties of isotropic, compression-molded samples.

Sample E (MPa) TS (MPa) EB (%) HDPE/mPE 285 ± 5 17.2 ± 0.3 743 ± 10 MaterBi 110 ± 7 21.5 ± 0.6 617 ± 19 Ecovio 98 ± 1.5 9.2 ± 0.5 633 ± 14

Both of the biodegradable systems showed lower rigidity (on the basis of the elastic modulus values) than the HDPE/mPE blend, while they can be considered competitive in terms of ductility; with regard to the tensile strength, the MaterBi sample was even better than the HDPE/mPE blend, while Ecovio’s values were lower. The same mechanical properties on the anisotropic samples (blown ﬁlms) are shown in Table3 for MD and TD specimens.

Table 3. Tensile properties in machine direction (MD) and transverse direction (TD) of the anisotropic (ﬁlm) samples.

Sample MD TD E (MPa) TS (MPa) EB (%) E (MPa) TS (MPa) EB (%) HDPE/mPE 393 ± 8 24.3 ± 0.7 480 ± 17 293 ± 6 19.1 ± 0.6 583 ± 20 Ecovio 212 ± 7 27.1 ± 1 385 ± 20 119 ± 6 17.3 ± 0.8 495 ± 23 MaterBi 327 ± 8 33 ± 2 319 ± 19 269 ± 6 27.7 ± 1.8 469 ± 21

According to the experimental results, the films were oriented in both directions at different levels as the elastic modulus and the tensile strength were higher than the same values of the isotropic sheet and the elongation at break was lower. Due to the processing conditions, the orientation was more relevant in the machine direction. With regard to the elastic modulus in MD, there were significant differences between the two biodegradable systems and the non-biodegradable system. On the other hand, the tensile strength (in both directions) was competitive or even higher; MaterBi showed higher values than Ecovio. As for the elongation at break, the biodegradable systems showed lower values than the conventional material. These comments can be made also with concern to the TD direction. It is worth mentioning, however, that the following drawing action, meant to produce the final filaments for the net manufacturing, was performed on the strips along the machine direction. Table4 reports the mechanical properties of Ecovio and MaterBi cold-drawn filaments at increasing draw ratios, compared to the same properties measured for a standard filament, cut from a commercial net for fruit packaging. First, it can be observed that the filaments of both of the two biodegradable materials strongly improved their stiffness and tensile strength upon increasing the draw ratio; on the other hand, the elongation at break decreased. All of these results were in agreement with the expectations, since the cold drawing increased the orientation degree of the macromolecules and reduced their reciprocal slipping ability. However, the elongation still kept values higher than that of the standard filament and, at the same time, the improvement of elastic modulus and tensile strength allowed us to reduce the gap with the standard filaments. Moreover, the values of the elastic modulus of Mater-Bi, at this draw ratio, became fairly similar to the conventional filament. Materials 2021, 14, 323 7 of 10 Materials 2021, 14, x FOR PEER REVIEW 7 of 10

Table 4. Tensile properties of the biodegradable polymer filaments, at increasing draw ratios, Table 4. Tensilecompared properties to the of reference,the biodegradable industrially pol manufactured,ymer filaments, non-biodegradable at increasing draw polymerratios, filament. compared to the reference, industrially manufactured, non-biodegradable polymer filament. System DR E (MPa) TS (MPa) EB (%) System DR E (MPa) TS (MPa) EB (%) ± ± ± 1 1 212 ± 2127 27.17 ± 1 27.1 385 ±1 20 385 20 Ecovio 2 461 ± 20 119 ± 4 233 ± 19 Ecovio 2 3 461 ± 81620 ± 13 119 ± 4 163 233± ±3 19 100 ± 29 3 816 ± 13 163 ± 3 100 ± 29 1 327 ± 8 33 ± 2 319 ± 19 1 327 ± 8 33 ± 2 319 ± 19 MaterBi 2 734 ± 29 85 ± 6 239 ± 16 MaterBi 2 3 734 ± 153129 ± 37 85 ± 6 154 239± ±5 16 167 ± 17 3 1531 ± 37 154 ± 5 167 ± 17 Reference industrially manufactured filament – 1904 ± 100 176 ± 10 89 ± 9 Reference industrially manufactured filament – 1904 ± 100 176 ± 10 89 ± 9

In order to Inbetter order assess to better the actual assess effect the actual of increasing effect of increasingthe draw ratio, the draw and thus ratio, the and thus the orientation orientationdegree, of cold-drawn degree, of cold-drawn filaments, we filaments, calculated we and calculated plotted and the plotteddimensionless the dimensionless tensile propertiestensile (i.e., properties the ratio (i.e., between the ratio the between actual average the actual value average of that value property of that and property the and the average valueaverage of the value same of theproperty same property measured measured at DR at= 1, DR which = 1, which is that is thatof the of thecorre- corresponding sponding stripstrip obtained obtained from from th thee blown blown film), film), as as shown shown in inFigures Figures 4–6.4–6 It. Itcan can be be observed observed that the that the mechanicalmechanical properties properties at atDR DR = 1 = were 1 were slightly slightly higher higher in in the the MaterBi MaterBi sample. sample. The The sole effect sole effect of cold drawingdrawing thethe filamentfilament significantlysignificantlyimproved improved the the properties properties of of both; both; moreover, it moreover, itappeared appeared that, that, at at higher higher DRs, DRs, the the orientation orientation was was slightly slightly more more efficient efficient in in the MaterBi the MaterBisample. sample. As As regards regards EB, EB, as as already already stated, stated, this this decreased decreased upon upon increasing increasing the the orientation, orientation,but but thethe decreasedecrease waswas proportionally lower in in the the MaterBi. MaterBi. The The overall overall results results allowed allowed us usto state to state that that the theorientation orientation significantly significantly improved improved the mechanical the mechanical properties properties with with primaryprimary importance importance for fruit for fruit net netapplication, application, i.e., i.e., modulus modulus and and tensile tensile strength; strength; moreover, moreover, the reduction in the elongation at breakbreak (and(and thus,thus, deformability)deformability) isis aa beneficialbenefi- result in cial result inthis this context. context.

Figure 4. FigureDimensionless 4. Dimensionless elastic modulus elastic modulusas a function as a of function the draw of theratio. draw ratio.

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Figure 5. Dimensionless tensile strength as a function of the draw ratio. Figure 5. DimensionlessFigure 5. Dimensionless tensile strength tensile as strengtha function as of a functionthe draw of ratio. the draw ratio.

Figure 6. Dimensionless elongation at break as a function of the draw ratio. Figure 6. FigureDimensionless 6. Dimensionless elongation elongation at break as at a break function as a functionof the draw ofthe ratio. draw ratio. 4. Conclusions 4. Conclusions4. Conclusions In this work, two biodegradable polymer systems were compared to a traditional, In this work,In two this biodegradable work, two biodegradable polymer systems polymer were systems compared were to compared a traditional, to a traditional, non-biodegradable polyethylene-based blend, in view of possible application for the non-biodegradablenon-biodegradable polyethylene-based polyethylene-based blend, in view blend, of in possible view of application possible application for the for the pro- production of nets for the food packaging industry, particularly for the packaging of production ofduction nets for of netsthe food for the packaging food packaging industry, industry, particularly particularly for the for packaging the packaging of of fruits fruits and vegetables. fruits and vegetables.and vegetables. The industrial process was accurately simulated on the laboratory scale via film The industrialThe process industrial was processaccurately was simulated accurately on simulated the laboratory on the scale laboratory via film scale via film blowing, cutting of the film into thin strips and cold drawing to obtain drawn filaments. blowing, cuttingblowing, of the cutting film into of thethin film strips into and thin cold strips drawing and cold to obtain drawing drawn to obtain filaments. drawn filaments. The investigated systems were subjected to rheological and mechanical characterization. The investigatedThe investigatedsystems were systems subjected were to subjectedrheological to and rheological mechanical and characterization. mechanical characterization. From the rheological characterization, we assessed that the processability of the bi- From the rheological characterization, we assessed that the processability of the biodegradable systems was, on average, comparable to that of the traditional, odegradable systems was, on average, comparable to that of the traditional,

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From the rheological characterization, we assessed that the processability of the biodegradable systems was, on average, comparable to that of the traditional, non-biodegradable blend; the same applied to the non-isothermal elongational flow parameters (related to filmability). Mechanical characterization on compression-molded samples showed that MaterBi’s rigidity was significantly lower than that of HDPE/mPE, while the tensile strength was slightly higher. Tests on the MD films showed that both of the biodegradable polymers (and, in particular, the MaterBi sample) can be considered as alternatives for the reference non–biodegradable films. The tests on the drawn filaments demonstrated that the elastic modulus and the tensile strength can be dramatically improved by increasing the draw ratio.

Author Contributions: Conceptualization, F.P.L.M.; methodology, F.P.L.M.; validation, F.P.L.M., M.C., P.T.; formal analysis, F.P.L.M., M.M., M.C., P.T.; investigation, P.T., M.C., F.P.L.M.; resources, F.P.L.M.; data curation, F.P.L.M., P.T., M.M.; writing—original draft preparation, M.M., F.P.L.M.; writing—review and editing, M.M., F.P.L.M.; visualization, M.M., P.T.; supervision, F.P.L.M.; project administration, F.P.L.M. All authors have read and agreed to the published version of the manuscript. Funding: This work has been financially supported by INSTM-INDPA004. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest.

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