Thermal Insulation and Mechanical Properties of Polylactic Acid (PLA) at Different Processing Conditions

Article Thermal Insulation and Mechanical Properties of Polylactic Acid (PLA) at Diﬀerent Processing Conditions

Mohamed Saeed Barkhad 1, Basim Abu-Jdayil 1,* , Abdel Hamid I. Mourad 2 and Muhammad Z. Iqbal 1

1 Chemical and Petroleum Engineering Department, United Arab Emirates University, Al Ain 15551, Abu Dhabi, UAE; [email protected] (M.S.B.); [email protected] (M.Z.I.) 2 Mechanical Engineering Department, United Arab Emirates University, Al Ain 15551, Abu Dhabi, UAE; [email protected] * Correspondence: [email protected]; Tel.: +971-3-7135317; Fax: +971-3-7624262

Received: 15 August 2020; Accepted: 3 September 2020; Published: 14 September 2020

Abstract: This work aims to provide an extensive evaluation on the use of polylactic acid (PLA) as a green, biodegradable thermal insulation material. The PLA was processed by melt extrusion followed by compression molding and then subjected to diﬀerent annealing conditions. Afterwards, the thermal insulation properties and structural capacity of the PLA were characterized. Increasing the annealing time of PLA in the range of 0–24 h led to a considerable increase in the degree of crystallization, which had a direct impact on the thermal conductivity, density, and glass transition temperature. The thermal conductivity of PLA increased from 0.0643 W/(m K) for quickly-cooled · samples to 0.0904 W/(m K) for the samples annealed for 24 h, while the glass transition temperature · increased by approximately 11.33% to reach 59.0 ◦C. Moreover, the annealing process substantially improved the compressive strength and rigidity of the PLA and reduced its ductility. The results revealed that annealing PLA for 1–3 h at 90 ◦C produces an optimum thermal insulation material. The low thermal conductivity (0.0798–0.0865 W/(m K)), low density (~1233 kg/m3), very low water · retention (<0.19%) and high compressive strength (97.2–98.7 MPa) in this annealing time range are very promising to introduce PLA as a green insulation material.

Keywords: polylactic acid; biopolymer; green thermal insulator; thermal properties; water retention; mechanical properties

1. Introduction Biodegradable materials have attracted much attention from researchers due to the pressing need to reduce enduring waste worldwide. Among the biodegradable materials, biodegradable polymers have received the most attention [1]. They are classiﬁed into three groups based on their sources: natural, semi-synthetic, and synthetic. Most biodegradable polymers are obtained from renewable sources such as starch. Aliphatic and mixed aliphatic/aromatic polyesters or polysaccharides (cellulose and its derivatives) are the most widely studied biodegradable polymers [2]. With depleting fossil-based raw materials for conventional polymers, biodegradable polymers provide an alternative with improved functional properties [3]. In addition, the biodegradability feature oﬀers a real solution of municipal waste management issue. Polylactic acid (PLA) is a natural biodegradable polymer derived from renewable raw materials such as corn, starch, and sugarcane [4,5]. PLA is an enantiomeric polyester, which includes poly (L-lactic acid) and poly (D-lactic acid), as shown in Figure1[ 6]. Comparatively high D content (>20%)

Polymers 2020, 12, 2091; doi:10.3390/polym12092091 www.mdpi.com/journal/polymers Polymers 2020, 12, 2091 2 of 16 Polymers 2020, 12, x FOR PEER REVIEW 2 of 16 can yield an entirelyentirely amorphousamorphous polymer,polymer, whereaswhereas lowerlower DD contentcontent ((<2%)<2%) leads to highly crystalline material [[7].7].

(a) (b)

Figure 1. Chemical structure of (a) L-lacticL-lactic acid and (b) D-lacticD-lactic acid.

Furthermore, PLA PLA is a is thermoplastic, a thermoplastic, high-modulus, high-modulus, high-strength high-strength polymer withpolymer large withproduction large ratesproduction and various rates and applications. various applications. PLA is easily PLA processable is easily processable on standard on plastic standard equipment plastic equipment into films, fibersinto films, or molded fibers partsor molded [5]. However, parts [5]. despite However, numerous despite positive numerous features, positive PLA features, is brittle andPLA rigid is brittle with lowand plastic rigid with deformation low plastic (~3%) deformation [8]. The inherent (~3%) [8]. brittleness The inherent shortens brittleness PLA use shortens in implementations PLA use in whichimplementations require plastic which deformation require plastic at higher deformation levels of stress at higher (e.g., fracturelevels of fixation stress (e.g., plates fracture and screws) fixatio [9n]. platesPLA and exhibits screws) a [9]. slow degradation rate which is critical in biomedical applications [10] as well as in the disposalPLA exhibits process a slow of consumer degradation commodities. rate which The is critical degradation in biomedical of PLA is applications a function of [10] its molecularas well as weightin the disposal and its distribution, process of consumer crystallinity, commodities. morphology, The stereoisomeric degradation content of PLA and is a di functionffusion rate of its of watermolecular [11]. Nevertheless,weight and its PLA distribution, is relatively crystallinity, hydrophobic morphology,and lacks reactive stereoisomeric side-chain groups content on and its surfacediffusion [5 ,rate9]. of water [11]. Nevertheless, PLA is relatively hydrophobic and lacks reactive side-chain groupsThe on mechanical its surface properties [5,9]. of a polymer are a complex function of a microstructure, such as chain relaxation,The mechanical degree of properties crystallization of a polymer and orientation are a complex [12]. Heatfunction treatment of a microstructure (annealing), improvessuch as chain the mechanicalrelaxation, degree characteristics of crystallization of a polymer. and orientation Annealing [12]. improves Heat treatment heat and chemical(annealing) resistance, improves glass the transitionmechanical and characteristics tensile strength of a inpolymer. polymers Annealing [7,13]. Most improves likely, heat annealing and chemical improves resistance, a material’s glass performancetransition and by tensile refining strength its crystallinity in polymers [7]. However, [7,13]. Most annealing likely, annealing may not alter improves melting a material’s and glass transitionperformance temperatures by refining when its crystallinity the molecular [7]. weight However, is high annealing enough [may5,14 ].not alter melting and glass transitionThe microstructural temperatures when orientation the mole andcular annealing weight are is high considered enough e ff[5,ective14]. techniques for enhancing the mechanicalThe microstructural performance orientation of PLA. and The annealing orientation are improves considered and effective increases techniques toughness for and enhancing strength whereasthe mechanical annealing performance improves theof PLA. chain The unwinding orientation and improves crystallinity and in increases PLA [12]. toughness Temperature, and shearing,strength stretching,whereas annealing and other processingimproves theconditions chain influenceunwinding the and resulting crystallinity formation in of PLA the crystal [12]. Temperature, structure [15]. PLAshearing, exhibits stretching, three crystalline and other forms:processingα, β conditions, and γ, and influence a new crystalthe resulting modification formation of αofdenoted the cryst asal αstr0-formucture (disorder [15]. PLAα )[exhibits15,16]. three The α crystalline0-form appears forms: by α, annealing β, and γ, PLAand belowa new 120crystal◦C[ modification16]. of α denotedTakayama as α′-form et al. (disorder [17] examined α) [15,16 the]. The mechanical α′-form appears properties by annealing of PLA/poly-epsilon-caprolactone PLA below 120 °C [16]. (PLATakayama/PCL) and etPLA al./ poly-epsilon-caprolactone [17] examined the mechanical/Lysine properties triisocyanate of PLA/ (PLAp/olyPCL-epsilon/LTI) blends.-caprolactone These blends(PLA/PCL) face annealingand PLA/p processes.oly-epsilon The-caprolactone/Lysine annealing impact ontriisocyanate the PLA/PCL (PLA/PCL/LTI) blend led to anblends. increase These in theblends flexural face strengthannealing and process moduluses. The of itannealing because ofimpact the crystallization on the PLA/PCL of the blend PLA led phase to an by increase annealing in asthe well flexural as the strength PLA/PCL and/LTI modulus blend whichof it because experienced of the polymerizationcrystallization of besides the PLA annealing. phase by Inannealing general, theas well annealing as the PLA/PCL/LTI process leads toblend strengthening which experienced the microstructure, polymerization which beside consequentlys annealing. improves In general, the mechanicalthe annealing properties. process leads to strengthening the microstructure, which consequently improves the mechanicalTabi et al.properties. [18] reported effects of annealing PLA on crystallinity induced in neat, processed, and injectionTabi molded et al. [18] resins. reported They effects reported of annealing that annealing PLA injectionon crystallinity molding induced grade PLAin neat, in the processed, temperature and rangeinjection of 100–140 molded◦ C resins. for 10 min They resulting reported inhigh that crystallinity annealing (up injection to 40%). molding Annealing grade PLA forPLA 10 inmin the in thetemperature 100–140 ◦ Crange temperature of 100–140 range °C achievedfor 10 mi veryn resul highting crystallinity in high crystallinity (40%) in an (up injection-molded to 40%). Annealing grade PLA.PLA for Moreover, 10 min completein the 100 annealing–140 °C temperature disappeared range the cold achieved crystallization very high exotherm crystallinity in PLA. (40%) in an injection-molded grade PLA. Moreover, complete annealing disappeared the cold crystallization exotherm in PLA.

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Weir et al. [19] investigated the effects of the annealing, molecular weight and morphology of poly-L-lactide (PLLA) on its degradability. The annealing increased crystallinity to 43% and 40% in compression-molded and extruded PLLA, respectively. Consequently, increased crystallinity decreased extension at the break and tensile strength whereas the Young’s modulus was improved. Hassan et al. [20] added PLA to starch/cellulose composite foams to enhance their water barrier property and other properties. Different proportions of PLA were added to the mixture, and their thermo-mechanical and water absorption properties were analyzed. They found that PLA has a considerable impact in water barrier properties, flexural strength, and stiffness. Furthermore, the addition of PLA to the mixture increased the peak thermal degradation temperature resulting in the formation of a more stable structure. PLA is the most extensively researched and utilized biodegradable aliphatic polyester in human history [5]. Due to its merits, PLA is a leading biomaterial for numerous applications in industry replacing conventional petrochemical-based polymers. Owing to its low thermal conductivity and hydrophobic nature, PLA can work as a potential thermal-insulating material to replace conventional fossil-based insulating materials. However, there is a scarcity of data on PLA as insulation material in the published literature to date. This work primarily focuses on the effects of processing conditions on the thermal insulation behavior of melt-processed PLA and its construction capacity. The impact of annealing conditions is reported on various thermal and mechanical properties as well as the morphology of PLA.

2. Materials and Methods

2.1. Materials Poly(lactic acid) (PLA) (commercial grade 4032D; weight-average molar weight = 1 105 g/mol; × speciﬁc gravity = 1.24, and Tm = 155–170 ◦C) was supplied by Zhejiang Zhongfu Industrial Ltd. and imported from Nature Works LLC (Hangzhou, China) in granule form with a ratio of L-lactide to D-lactide from 24:1 to 32:1 (manufacturer data). PLA was used in this study without any further modiﬁcations.

2.2. PLA Samples Preparation and Thermal Treatment

PLA was dried for two hours under vacuum at 90 ◦C followed by drying in a desiccator for one hour prior to processing due its hydrolytic susceptibility at higher temperatures. After that, an internal mixer mini-twin conical screw extruder (MiniLab Haake Rheomex CTW5, Dreieich, Germany) was used to melt the PLA pellets. PLA (5 g) was melt blended in a twin screw microcompounder (MiniLab Haake Rheomex CTW5, Dreieich, Germany) at 190 ◦C and 140 rpm for 3 min. The extruded PLA compression molded in hot press (Carver, AUTOFOUR/3015) in three heating-pressure cycles: (1) a force of 0.5 tons at 180 ◦C for 16 min, (2) a 0.52 ton force at 185 ◦C for 10 min, and (3) 3 tons of force at 100 ◦C for 3:30 min. The molded samples were thermally annealed in a convection oven (explained later). Figure2 shows the molded sample for the thermal conductivity (a) and cylindrical sample (b) used for additional tests. Polymers 2020, 12, x FOR PEER REVIEW 4 of 16

1-PLA 1 h 3-PLA 3 h 10-PLA 10 h 17-PLA 17 h 24-PLA 24 h

2.3. Scanning Electron Microscopy (SEM) A JEOL-JCM 5000 NeoScope SEM (JEOL, Tokyo, Japan) was used to investigate the microstructure in processed PLA. The samples were sputter coated with gold for 3 min to remove the electrostatic charges in order to attain the maximum magnification of textural and morphological features. The SEM images are reported at various magnifications.

2.4. Thermal Conductivity The thermal conductivity was measured using the Lasercomp FOX-200 thermal conductivity tester. Samples were shaped in specially designed molds with dimensions of 110 × 110 × 3 mm3 Polymers(Figure2020 2a), 12 and, 2091 thermal conductivity was measured using ASTM C1045-07 [23]. The results4 of are 16 reported as the average of three measurements.

(a) (b)

Figure 2. DimensionsDimensions of prepared prepared specimens specimens:: ( a)) the the t thermalhermal conductivity conductivity specimen specimen;; and ( b) the the w waterater retention and compression strength specimen.specimen.

2.5. DifferentialThermal annealing Scanning wasCalorimetry conducted in a convection oven at 95 ◦C. Since annealing between melting (Tm) and the glass transition temperature (Tg) is more effective [21,22], annealing was performed at Thermal properties of PLA were studied using a differential scanning calorimeter (DSC 25—TA 95 ◦C (PLA Tm = 155–170 ◦C and Tg ~ 60 ◦C). The annealing was conducted for a range of annealing Instruments, TA Instruments, New Castle, DE, USA). An approximately 5–7 mg sample was heated times of 0.5, 1, 3, 10, 17, and 24 h. One sample was annealed “Fast” by cooling from hot press under a 50 mL/min nitrogen flow rate. The DSC results are reported as two cases: Case A simulates temperature to room temperature by flowing air over the samples in a fume-hood. Sample codes as a the oven-based annealing of PLA whereas in Case B, the effects of the fast cooling rate on PLA thermal function of annealing time interval are provided in Table1. properties are reported. The temperature protocols followed in these two cases are detailed below: Case A: Samples were quickly heatedTable 1. toSamples 200 °C abbreviation.at 40 °C/min and kept isothermally at 200 °C for 5 min to eliminate the thermal history. Subsequently, the molten samples were cooled to 95 °C at 5 °C/min and isothermally annealed for the Annealing following Processing time intervals: Interval 0 min (fast cooling/without stopping at 95 °C), 30 min, 0-PLAand 1, 3, 10, 17, and 24 h. This step Fast is a simulation of the external annealing process. Following complete0.5-PLA annealing, the samples 0.5 were h cooled to 20 °C at 5 °C/min and 1-PLA 1 h subsequently heated from 20 °C to 200 °C at 2 °C/min to evaluate the thermal characteristics (glass 3-PLA 3 h transition temperature and10-PLA degree of crystallinity) as a function 10 h of annealing time interval (see Figure 3a). 17-PLA 17 h Case B: Samples were24-PLA quickly heated to 200 °C at 40 24°C/min h and kept isothermally at 200 °C for 5 min to eliminate thermal history (Figure 3b). The samples were rapidly cooled to 95 °C at 80 °C/min to2.3. avoid Scanning the Electrondevelopment Microscopy of premature (SEM) crystallinity before reaching 95 °C. The samples were annealed at 95 °C for 0, 5, 10, 20, 30, 40, 50, or 60 min. Followed by annealing, the samples were cooled A JEOL-JCM 5000 NeoScope SEM (JEOL, Tokyo, Japan) was used to investigate the microstructure in processed PLA. The samples were sputter coated with gold for 3 min to remove the electrostatic charges in order to attain the maximum magnification of textural and morphological features. The SEM images are reported at various magnifications.

2.4. Thermal Conductivity The thermal conductivity was measured using the Lasercomp FOX-200 thermal conductivity tester. Samples were shaped in specially designed molds with dimensions of 110 110 3 mm3 (Figure2a) × × and thermal conductivity was measured using ASTM C1045-07 [23]. The results are reported as the average of three measurements.

2.5. Differential Scanning Calorimetry Thermal properties of PLA were studied using a differential scanning calorimeter (DSC 25—TA Instruments, TA Instruments, New Castle, DE, USA). An approximately 5–7 mg sample was heated under a 50 mL/min nitrogen flow rate. The DSC results are reported as two cases: Case A simulates Polymers 2020, 12, 2091 5 of 16 the oven-based annealing of PLA whereas in Case B, the effects of the fast cooling rate on PLA thermal properties are reported. The temperature protocols followed in these two cases are detailed below: Case A: Samples were quickly heated to 200 ◦C at 40 ◦C/min and kept isothermally at 200 ◦C for 5 min to eliminate the thermal history. Subsequently, the molten samples were cooled to 95 ◦C at 5 ◦C/min and isothermally annealed for the following time intervals: 0 min (fast cooling/without stopping at 95 ◦C), 30 min, and 1, 3, 10, 17, and 24 h. This step is a simulation of the external annealing Polymers 2020, 12, x FOR PEER REVIEW 5 of 16 process. Following complete annealing, the samples were cooled to 20 ◦C at 5 ◦C/min and subsequently heated from 20 C to 200 C at 2 C/min to evaluate the thermal characteristics (glass transition to 20 °C at 80 °C/min◦ and ◦subsequently◦ heated from 20 °C to 200 °C at 10 °C/min to evaluate the temperature and degree of crystallinity) as a function of annealing time interval (see Figure3a). thermal properties.

(a)

(b)

Figure 3. DDiifferentialﬀerential scanning calorimeter (DSC)(DSC) heating–coolingheating–cooling cycle for (a)) Case A and (b) Case B.

2.6. CompressionCase B: Samples Test were quickly heated to 200 ◦C at 40 ◦C/min and kept isothermally at 200 ◦C for 5 min to eliminate thermal history (Figure3b). The samples were rapidly cooled to 95 ◦C at The compression test was conducted following ASTM D695-15 using a universal testing 80 ◦C/min to avoid the development of premature crystallinity before reaching 95 ◦C. The samples instrument (MTS model MH/20) with 100 kN load cell and with an overhead speed of 1.3 mm/min at were annealed at 95 ◦C for 0, 5, 10, 20, 30, 40, 50, or 60 min. Followed by annealing, the samples were room temperature. The compressive modulus, compressive strength, and elongation at break were reported. Cylindrical specimens of a length of 25.7 mm and diameter of 12.3 mm (Figure 2b) were compressed between the fixed (lower) and movable (upper) plates. The loading was continued until either the value of the load decreased by 10% of the maximum value or the specimen fracture occurred; otherwise, the experiment was interrupted manually when a specific contraction value was reached. Figure 4 shows the specimen after compression: the left image represents a buckled sample while the right one shows a fractured sample.

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cooled to 20 ◦C at 80 ◦C/min and subsequently heated from 20 ◦C to 200 ◦C at 10 ◦C/min to evaluate the thermal properties.

2.6. Compression Test The compression test was conducted following ASTM D695-15 using a universal testing instrument (MTS model MH/20) with 100 kN load cell and with an overhead speed of 1.3 mm/min at room temperature. The compressive modulus, compressive strength, and elongation at break were reported. Cylindrical specimens of a length of 25.7 mm and diameter of 12.3 mm (Figure2b) were compressed between the ﬁxed (lower) and movable (upper) plates. The loading was continued until either the value of the load decreased by 10% of the maximum value or the specimen fracture occurred; otherwise, the experiment was interrupted manually when a speciﬁc contraction value was reached. Figure4 shows the specimen after compression: the left image represents a buckled sample while the right one shows a fractured sample. Polymers 2020, 12, x FOR PEER REVIEW 6 of 16

Figure 4. SamplesSamples after after the the compression compression test. test. On the On left: the buckled left: buckled sample.sample. On the right: On thefractured right: fracturedsample. sample.

2.7. Water R Retentionetention For thethe waterwater retentionretention test test following following ASTM ASTM D570-98 D570-98 [24 [24],], two two cylindrical cylindrical specimens specimens (L: 12.3(L: mm12.3 andmm D:and 25.7 D: mm)25.7 mm) (Figure (Figure2b) were 2b) preparedwere prepared for 0-PLA, for 0- 3-PLA,PLA, 3- andPLA, 24-PLA. and 24 The-PLA. samples The samples were dried were indried an ovenin an atoven 80 ◦atC 80 for °C four for hoursfour hours followed followed by drying by drying in a desiccatorin a desiccator until until a constant a constant weight weight was attainedwas attained (called (called initial initial weight weight (Winitial ()). The)). samplesThe samples were were immersed immersed in distilled in distilled water water at 25 at◦C. 25 The °C. specimensThe specimens were were removed remo withinved within the first the 24first h, pressed24 h, pressed dry with dry awith cloth a andcloth weighed and weighed up to sevenup to seven times (Intervals:times (Intervals: 3, 3, 3, 3, 3, 3, 4, 4,3, and3, 4, 44 h)., and Within 4 h). theWithin second the 24 second h, each 24 specimen h, each specimen was weighed was upweighed to five timesup to (Intervals:five times (Intervals: 5, 5, 5, 5, and 5, 5, 4 h).5, 5, Afterwards, and 4 h). Afterwards, the specimens the werespecimens weighed were once weighed every threeonce every days. Totalthree waterdays. Total absorbed water was absorbed calculated was using calculated Equation using (1): Equation (1): − Wf inal Winitial (%) = − × 100 (1) WR % 100 (1) Winitial × where WR(%) is the total water absorbed, and represents the weight after immersing in where WR(%) is the total water absorbed, and W represents the weight after immersing in distilled distilled water for a specific time. Three samplesf inalwere tested for each type and averaged results were water for a specific time. Three samples were tested for each type and averaged results were reported. reported. 2.8. Density 2.8. Density The volume and the mass as a function of annealing intervals were measured up to four significant digitsThe to calculate volume the and sample the mass density. as a The function reported of valuesannealing are averages intervals of were three measured repetitions. up to four significant digits to calculate the sample density. The reported values are averages of three repetitions.

3. Results and Discussion

3.1. Thermal Properties of Processed PLA

3.1.1. Case A The effect of the annealing time on the thermal behavior of processed PLA is shown in Figure 5.

The DSC probes several properties of PLA including the glass transition temperature (), melting

temperature ( ), crystallization temperature ( ), enthalpy of crystallization (∆ ), degree of crystallinity (), and melting enthalpy (∆). Table 2 lists the thermal properties extracted from the DSC profiles of the processed PLA.

Generally, increasing the annealing time of PLA increases . The fast cooling sample exhibited = 53 °C whereas = 59 °C was observed in 24 PLA samples. The values of measured for neat PLA in this study were within the range reported in the literature [5,6]. In general, in PLA

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3.Polymers Results 2020 and, 12, x Discussion FOR PEER REVIEW 7 of 16

3.1. Thermal Properties of Processed PLA depends on D-lactate contents and the molecular weight. Accordingly, PLAs exhibit lower (up to 3.1.1.60 °C) Case compared A to petroleum-based polyesters, with as high as 80 °C [14,25]. In addition, a cold crystallization exotherm was observed in the 0-PLA sample (Figure 5) at = 89.9 °C and ∆ = 19.2 J/g. TheThe disappearance eﬀect of the annealing of the cold time crystallization on the thermal peak behavior from annealed of processed PLA samples PLA is (0.5 shown-PLA in– Figure24-PLA)5. Theindicates DSC that probes no more several crystalline properties structures of PLA includingare generated the glassduring transition the heating temperature cycle. There (Tg is), a melting strong temperature (Tm), crystallization temperature (Tc), enthalpy of crystallization (∆Hc), degree of relationship between the annealing time and ∆, as when the annealing time increases, the ∆ crystallinitydecreases. In ( Xaddition,c), and melting almost enthalpythere is no (∆ exothermicHm). Table2 peak lists thethat thermal could be properties identified extracted at PLAs’ from samples the DSC proﬁles of the processed PLA. that have ~ 40% [18,26].

Figure 5. EffectEﬀect of the the annealing time on the DSC profile proﬁle of processed polylacticpolylactic acid (PLA).(PLA).

The annealingTable time 2. E ﬀ showedect of annealing no significant time on the effect DSC proﬁle on the of meltin the processedg temperature PLA. as reported elsewhereInterval [27]. (h) The peakTg (◦ C) melting T temperaturec (◦C) T slightlym (◦C) increased∆Hc (J/g) as a ∆ functionHm (J/g) of annealingXc (%) time interval from 168.4 for 0-PLA to 169.7 °C for 24-PLA. However, increasing the annealing interval Fast (0) 53 89.9 168.4 19.2 50.6 33.8 increased ∆0.5, which was 57.6 calculated as 50.6 J/g,168.7 50.3 J/g, 54.5 J/g, and 60.348.8 J/g for 0-PLA, 52.5 3-PLA, 10- PLA, and 1 17-PLA, respectively 56. The increased 168.6 ∆ further indicates 47.7 a change in 51.3 degree of crystallization3 with annealing. 56.4 The degree of crystallization 168.5 (%) was calculated 50.3 by Equatio 54.1n (2): 10 57.6 168.8 54.5 58.6 17 56.6 168.3∆ − ∆ 60.3 64.8 %Crystallinity = = (2) 24 59 169.7∆ 54.3 58.4 where ∆ is the experimental enthalpy of melting, ∆ is the enthalpy of cold crystallization if observed,Generally, and ∆ increasing is the the theoretical annealing melti timeng of enthalpy PLA increases of 100%Tg. crystalline The fast cooling PLA (93sample J/g [28]). exhibited The Tdegreeg = 53 of◦ Ccrystallization whereas Tg =for59 all◦ Csamples was observed is reported in 24in Table PLA samples.2. The degree The of values crystallization of Tg measured improved for neatsubstantially PLA in thiswith study the increasing were within annealing the range time. reported The calculated in the literature values [5, 6were]. In 33.8, general, 51.3,T 54.1,g in PLA58.6 dependsand 64.8% on for D-lactate 0-PLA, 1 contents-PLA, 3-PL andA, the 10- molecularPLA, and 17 weight.-PLA, respectively. Accordingly, The PLAs 24- exhibitPLA exhibited lower T ga (uplower to 60crystallization◦C) compared percentage to petroleum-based indicating that polyesters, PLA might with haveTg as reached high as crystallization 80 ◦C[14,25]. saturation In addition, between a cold crystallization17 and 24 h of exothermannealing. was observed in the 0-PLA sample (Figure5) at Tc = 89.9 ◦C and ∆Hc = 19.2 J/g. The disappearance of the cold crystallization peak from annealed PLA samples (0.5-PLA–24-PLA) indicates that noTable more 2. crystalline Effect of annealing structures time are on generated the DSC profile during of thethe heatingprocessed cycle. PLA. There is a strong

Interval (h) (°C) (°C) (°C) ∆ (J/g) ∆ (J/g) (%) Fast (0) 53 89.9 168.4 19.2 50.6 33.8 0.5 57.6 168.7 48.8 52.5

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relationship between the annealing time and ∆Hc, as when the annealing time increases, the ∆Hc decreases. In addition, almost there is no exothermic peak that could be identified at PLAs’ samples that have Xc ~ 40% [18,26]. The annealing time showed no significant effect on the melting temperature as reported elsewhere [27]. The peak melting temperature slightly increased as a function of annealing time interval from 168.4 for 0-PLA to 169.7 ◦C for 24-PLA. However, increasing the annealing interval increased ∆Hm, which was calculated as 50.6 J/g, 50.3 J/g, 54.5 J/g, and 60.3 J/g for 0-PLA, 3-PLA, 10-PLA, and 17-PLA, respectively. The increased ∆Hm further indicates a change in degree of crystallization with annealing. The degree of crystallization (%) was calculated by Equation (2):

∆Hm ∆Hc %Crystallinity = Xc = − (2) ∆Hm∞ wherePolymers ∆20H20m, 1is2, x the FOR experimental PEER REVIEW enthalpy of melting, ∆Hc is the enthalpy of cold crystallization8 of 16 if observed, and ∆Hm∞ is the theoretical melting enthalpy of 100% crystalline PLA (93 J/g [28]). The degree of crystallization1 for all56samples is reported168.6 in Table2. The degree47.7 of crystallization 51.3 improved substantially with3 the increasing56.4 annealing time.168.5 The calculated Xc values50.3 were 33.8%,54.1 51.3%, 54.1%, 58.6% and 64.8%10 for 0-PLA, 1-PLA,57.6 3-PLA, 10-PLA,168.8 and 17-PLA, respectively.54.5 The 24-PLA58.6 exhibited a lower crystallization17 percentage56.6 indicating that168.3 PLA might have reached60.3 crystallization 64.8 saturation between 17 and 2424 h of annealing.59 169.7 54.3 58.4 Figure6 shows the degree of crystallization of the processed PLA and its measured density. With Figure 6 shows the degree of crystallization of the processed PLA and its measured density. an increasing annealing time interval, the density increase was attributed to the structural compactness With an increasing annealing time interval, the density increase was attributed to the structural which consequently increased the degree of crystallization. It is noteworthy, however, that the density compactness which consequently increased the degree of crystallization. It is noteworthy, however, diﬀerence in 0-PLA and 17-PLA was not signiﬁcant. The fast cooling in 0-PLA might not have resulted that the density difference in 0-PLA and 17-PLA was not significant. The fast cooling in 0-PLA might in a completely amorphous structure. not have resulted in a completely amorphous structure.

Crystallinity (%) Crystallinity 30

10 1228 1230 1232 1234 1236 1238 1240 Density (kg/m3)

Figure 6. Degree of crystallinity versus sam samplesples density.

3.1.2. Case B

Case B involves the eeffectsffects of fastfast coolingcooling onon thethe degreedegree ofof crystallizationcrystallization (Xc) and the thermalthermal properties ofof PLA before reaching thethe annealing temperaturetemperature (95(95 ◦°C).C). AA cooling rate of 80 ◦°C/minC/min was applied toto avoidavoid anyany prematurepremature crystallinity crystallinity before before the the annealing annealing temperature temperature (Figure (Figure3b). 3b). The The e ff effectect of annealingof annealing time time (0, 5,(0, 10, 5, 10, 20, 30,20, 40,30, 5040, and 50 and 60 min) 60 min on) the onPLA’s the PLA’s thermal thermal profile profile is shown is shown in Figure in Figure7 and

7 and the thermal parameters are enlisted in Table 3. The decreased slightly with the increasing annealing interval contradicting the behavior observed in Case A. In general, the observed was in the range of 49.1–55.1 °C [14,25]. Comparing with the annealing time, the fast cooling in Case B reduced . Unlike in Case A, Figure 7 shows three peaks of crystallization at 101, 99, and 94.3 °C for a 0, 5, and 10 min annealing time, respectively. The appearance of these peaks indicated that these samples still have the capacity to produce crystalline structures during the heating cycle of the DSC.

Polymers 2020, 12, 2091 9 of 16

the thermal parameters are enlisted in Table3. The Tg decreased slightly with the increasing annealing interval contradicting the behavior observed in Case A. In general, the observed Tg was in the range of 49.1–55.1 ◦C[14,25]. Comparing Tg with the annealing time, the fast cooling in Case B reduced Tg. Unlike in Case A, Figure7 shows three peaks of crystallization at 101, 99, and 94.3 ◦C for a 0, 5, and 10 min annealing time, respectively. The appearance of these peaks indicated that these samples still

Polymershave the 2020 capacity, 12, x FOR to PEER produce REVIEW crystalline structures during the heating cycle of the DSC. 9 of 16

FigureFigure 7. 7. EffectEﬀect of of annealing annealing time time on on the the DSC DSC profile proﬁle of of the the processed processed PLA PLA (Case (Case B) B)..

Table 3. DSC results for the PLA in one hour of the annealing process at 95 C (Case B). Table 3. DSC results for the PLA in one hour of the annealing process at 95 °C◦ (Case B). T T T T ∆H ∆H X Interval (min) g c m1 m2 ∆H (J/g) m1 m2 c ( C) ( C) ( C) ( C) c (J/g) (J/g) (%) Interval (min) ◦ ◦ ◦ ◦∆ (J/g) ∆ (J/g) ∆ (J/g) Fast (0)(°C) 55.1 (°C) 101(°C) 168.6(°C) 32.5 36 38.7 (%) Fast (0)5 55.1 54.3 101 99168.6 167.6 32.5 30.436 41.4 44.5 38.7 10 54 94.3 152 167 3.5 2.5 43.6 49.6 5 2054.3 55 99 167.6 152.5 165.830.4 41.4 3.8 40.9 48.1 44.5 10 3054 54.1 94.3 152 152.8167 1653.5 2.5 5.3 36.343.6 44.7 49.6 20 4055 52.1 152.5 153.1165.8 164.2 3.8 54.7 40.9 58.8 48.1 50 50.8 153.2 163.1 55.4 59.6 30 6054.1 49.1 152.8 153.3165 162.1 5.3 55 36.3 59.1 44.7 40 52.1 153.1 164.2 54.7 58.8 Moreover,50 a shoulder50.8 to the153.2 melting 163.1 point was observed for annealing55.4 over 5 min (Figure59.67) attributed60 to the presence49.1 of two153.3 crystalline 162.1 structures with annealing55 [15,16]. With increasing59.1 annealingMoreover, time, a two shoulder melting to peaks the melting (Tm1 and pointTm2 )was appeared observed closer for to annealing each other over (Table 5 min3). The (Figure degree 7) attributedof crystallization to the increased presence ofwith two increasing crystalline annealing structures time with following annealing the Case[15,16 A]. trendWith and increasing similar to that previously reported by Angela et al. [29]. However, fast cooling in Case B produced a more annealing time, two melting peaks ( and ) appeared closer to each other (Table 3). The degree ofcrystalline crystallization PLA at increased the same with annealing increasing time. annealing time following the Case A trend and similar to that previously reported by Angela et al. [29]. However, fast cooling in Case B produced a more 3.2. Thermal Conductivity crystalline PLA at the same annealing time. Figures8 and9 show the e ﬀect of the annealing time on the thermal conductivity of processed PLA 3.2.in the Thermal temperature Conductivity range of 5–50 ◦C. A signiﬁcant impact of the annealing process was observed on the thermal conductivity of PLA. For example, the thermal conductivity increased from 0.06426 W/(m K) Figures 8 and 9 show the effect of the annealing time on the thermal conductivity of processed· for the fast cooled PLA to 0.09044 W/(m K) for the 24 h-annealed samples. Increasing annealing time PLA in the temperature range of 5–50 °C.· A significant impact of the annealing process was observed on the thermal conductivity of PLA. For example, the thermal conductivity increased from 0.06426 W⁄(m ∙ K) for the fast cooled PLA to 0.09044 W⁄(m ∙ K) for the 24 h-annealed samples. Increasing annealing time interval increased the thermal conductivity. Increasing annealing time might have allowed spherulites to form comfortably and crystallites to grow in a more orderly fashion, which helped the fast heat transfer due to compact structure [22]. Additionally, the increasing degree of crystallization of PLA might have reduced gaps between the chains and air voids resulting in increased thermal conductivity. The blue code in Figure 9 indicates the low thermal conductivity region where thermal conductivity is comparable to conventional insulators [30]. Since less thermal conductivity is desired,

Polymers 2020, 12, 2091 10 of 16 interval increased the thermal conductivity. Increasing annealing time might have allowed spherulites Polymers 2020, 12, x FOR PEER REVIEW 10 of 16 to formPolymers comfortably 2020, 12, x FOR and PEER crystallites REVIEW to grow in a more orderly fashion, which helped the10 fastof 16 heat transfer due to compact structure [22]. Additionally, the increasing degree of crystallization of PLA the samplesthe samples annealed annealed for for one one hour hour showedshowed promisingpromising low low thermal thermal conductivities conductivities (0.074–0.086 (0.074–0.086 might have reduced gaps between the chains and air voids resulting in increased thermal conductivity. W⁄(mW(m∙K∙⁄K)), indicating)), indicating that that they they can can act act as as thermal thermal insulators.insulators .

0.110.11 ))

)) 0.10 0.10𝐾 . . 𝑚

∕( 0.09 ∕(

0.09𝑊

0.08 0.08 0.07 0.07 0.06 0.06 0.05 Thermal conductivity ( conductivity Thermal 0.05

Thermal conductivity Thermal conductivity ( 0.04 5 101520253035404550 0.04 Temperature (oC) 5 10 15 20 25 30 35 40 45 50 24 hr 3 hrTemperature1 hr (oC) Fast cooling

Figure24 8. hr Thermal conductivity3 hr for PLA1 at hrdifferent annealingFast times.cooling

Figure 8. Thermal conductivity for PLA at different diﬀerent annealing times.

0.05 0.06 0.10 0.07

) 0.08 K . 0.09

m 0.05

/ 0.09 0.10

W 0.06

(

0.10

y 0.07

i v 0.08

i 0.08

t c 0.09

0.09u 0.10

o 0.07

0.08a m

r 0.06 20

e )

h 15 r T h ( 0.07 0.05 e 10 im T 310 g 5 n 300 li 0.06 a 20 T 290 e emp 0 n eratu 280 n 15 Thermal conductivity (W/m.K) conductivity Thermal re (K) A 0.05 10 Figure 9. Dependence of the PLA thermal conductivity on the temperature and annealing time. Figure 9. Dependence310 of the PLA thermal conductivity on the temperature and annealing time. 5 300 290 Temperature (K) 0 280 Annealing Time (hr)

Figure 9. Dependence of the PLA thermal conductivity on the temperature and annealing time.

Polymers 2020, 12, 2091 11 of 16 Polymers 2020, 12, x FOR PEER REVIEW 11 of 16

3.3. WaterThe blue Retention code in Figure9 indicates the low thermal conductivity region where thermal conductivity is comparable to conventional insulators [30]. Since less thermal conductivity is desired, the samples In the selection of thermal insulation materials, water retention is a pertinent physical property annealed for one hour showed promising low thermal conductivities (0.074–0.086 W/(m K)), indicating dictating the performance of thermal insulation materials. Figure 10 presents the effect· of annealing that they can act as thermal insulators. time on the water retention (WR%) of processed PLA as a function of immersion time (hours). The 3.3.WR% Water increased Retention with increasing time without reaching an equilibrium value within 8 days of the immersion period. Furthermore, the 𝑊𝑅(%) calculated from Eq. 1 increased with the annealing time In the selection of thermal insulation materials, water retention is a pertinent physical property (24 h > 3 h > fast cooling). dictating the performance of thermal insulation materials. Figure 10 presents the eﬀect of annealing It is difficult to compare the water retention results from this study with the published literature time on the water retention (WR%) of processed PLA as a function of immersion time (hours). The due to the scarcity of published data. The processed PLA exhibited a very low WR% (<0.35%), which WR% increased with increasing time without reaching an equilibrium value within 8 days of the is less than polyesters and polyester-based composites [31], and various types of polyurethane (1–6% immersion period. Furthermore, the WR(%) calculated from Equation (1) increased with the annealing after 24 h) [32] proposed for thermal insulation applications. The low WR% of PLA is attributed to its time (24 h 3 h fast cooling). hydrophobic> characteristics> [33] which make it a potential candidate for thermal insulation.

0.4

0.35

0.3

0.25

0.2

0.15

0.1 Water retention (WR%) retention Water

0.05

0 0 50 100 150 200 Time (hrs)

Fast cooling 3 hrs 24 hrs

Figure 10. WaterWater retention results for the processed PLA samples.

3.4. MechanicalIt is diﬃcult and to Morphological compare the waterCharacteristics retention results from this study with the published literature due to the scarcity of published data. The processed PLA exhibited a very low WR%(<0.35%), which The mechanical performance of semi-crystalline polymers depends on multiple factors is less than polyesters and polyester-based composites [31], and various types of polyurethane (1–6% including the degree of crystallinity. The stress–strain behavior of processed PLA is shown in Figure after 24 h) [32] proposed for thermal insulation applications. The low WR% of PLA is attributed to its 11a. The effect of the annealing process on the compression properties of PLA is presented in Figure hydrophobic characteristics [33] which make it a potential candidate for thermal insulation. 11a–d. 3.4. Mechanical and Morphological Characteristics The mechanical performance of semi-crystalline polymers depends on multiple factors including the degree of crystallinity. The stress–strain behavior of processed PLA is shown in Figure 11a. The eﬀect of the annealing process on the compression properties of PLA is presented in Figure 11a–d.

Polymers 2020, 12, 2091 12 of 16 Polymers 2020, 12, x FOR PEER REVIEW 12 of 16

(a) (b)

140 120 100 80 60 40 20 Stress (MPa) Stress 0

Annealing time

Compression strength Yield Point

Modulus (GPa) 0.5 0.1 0.0 0 Strain (mm/mm)

Annealing time Annealing time

Figure 11. EffectEﬀect of of the the PLA PLA annealing annealing time time on on ( (aa)) the the stress–strain stress–strain curve, curve, ( (bb)) the the compressive compressive strength, strength, and yield point, ( c) the compressive modulus and (d) the compressivecompressive strain.strain.

Increasing the annealing time improved improved the comp compressiveressive strength, compressive yield point, and the compressioncompression modulusmodulus of of the the processed processed PLA. PLA. The The compressive compressive strength strength of theof the fast-cooled fast-cooled PLA PLA was was58 MPa, 58 MPa, which which increased increased by 70% by 70% and and 84% 84% to reach to reach 98.7 98.7 MPa MPa and and 108 MPa108 MPa upon upon annealing annealing for 3 for and 3 and24 h, 24 respectively. h, respectively. This behaviorThis behavior can be can attributed be attribut to theed to shaping the shaping of the spherulitesof the spherulites and crystals and crystals during duringthe annealing the annealing period. period. Therefore, Therefore, increasing increasingXc is likely 𝑋 is to likely strengthen to strengthen the structure the structure of the polymer, of the polymer,resulting inresulting the increase in the in increase the compressive in the compressive strength and strength compressive and compressive modulus [17 ].modulus The compressive [17]. The compressivemodulus increased modulus with increased the annealing with timethe annealin interval,g reaching time interval, a maximum reachi forng 24-PLAa maximum (Figure for 11 24-PLAc). The (Figure24 h-annealed 11c). The PLA 24 exhibited h-annealed a 73% PLA increment exhibited compared a 73% increment to the neat compared PLA, from to the 1700 neat MPa PLA, to ~3000 from MPa.1700 MPa Figureto ~3000 11 MPa.d shows that the maximum strain decreased with the annealing time interval. A maximumFigure strain 11d shows decrease that was the noticed maximum for the strain 24-PLA decreased where the with maximum the annealing strain reduced time interval. from 0.58 A maximumfor 0-PLA strain to ~0.095 decrease for 24-PLA. was noti Theced annealedfor the 24-PLA PLA sampleswhere the gradually maximum lost strain their reduced ability tofrom absorb 0.58 forenergy 0-PLA to fracture to ~0.095 with for the 24-PLA. increasing The annealingannealed timePLA interval.samples Moreover,gradually thelost decreased their ability fracture to absorb strain energyalso indicates to fracture the formation with the ofincreasing a more compactannealing structure time interval. with annealing, Moreover, in the agreement decreased with fracture above strainresults. also The indicates length of the the formation annealing processof a more plays compact a pivotal structure role in rearranging with annealing, the PLA’s in agreement microstructure with aboveas reﬂected results. by The the mechanicallength of the properties. annealing Theprocess annealing plays processa pivotal reduces role in the rearranging degree of the molecular PLA’s microstructurerandomization as and reflected hence, improvesby the mechanical the mechanical properties. modulus The andannealing overall process strength reduces [22]. Overall, the degree the ofPLA molecular annealed randomization from 1 to 3 h and produces hence, high improves compressive the mechanical strength modulus PLA (97.2–98.6 and overall MPa) strength with a high[22]. Overall,degree of the ductility PLA annealed (strain ~ 0.37–0.18%).from 1 to 3 h The produces compressive high compressive strength and strength modulus PLA were (97.2–98.6 high compared MPa) with a high degree of ductility (strain ~ 0.37–0.18%). The compressive strength and modulus were

Polymers 2020, 12, x FOR PEER REVIEW 13 of 16 Polymers 2020, 12, 2091 13 of 16 high compared to the polyester and its composites (strength: 19–103 MPa and modulus: 264–1370 MPa) [34] used for insulation applications. to the polyester and its composites (strength: 19–103 MPa and modulus: 264–1370 MPa) [34] used for The SEM micrographs of the annealed PLA shown in Figure 12 support the observed mechanical insulation applications. and thermal behavior. Figure 12A1 and Figure 12A2 show the morphology of 0-PLA at 10 μm and 2 The SEM micrographs of the annealed PLA shown in Figure 12 support the observed mechanical μm, respectively. However, it can be described as ductile, while Figure 12B1 and Figure 12B2 exhibit and thermal behavior. Figure 12A1,A2 show the morphology of 0-PLA at 10 µm and 2 µm, respectively. the morphology of 24-PLA at 10 μm and 2 μm, respectively, which can be recognized as brittle. The However, it can be described as ductile, while Figure 12B1,B2 exhibit the morphology of 24-PLA at comparison between the 0-PLA and the 24-PLA samples revealed that the ductile deformation of the 10 µm and 2 µm, respectively, which can be recognized as brittle. The comparison between the 0-PLA 0-PLA is suppressed by the annealing and the microstructure of 24-PLA is more rigid than that of 0- and the 24-PLA samples revealed that the ductile deformation of the 0-PLA is suppressed by the PLA. The voids and cavities between the polymer segments were reduced significantly upon annealing and the microstructure of 24-PLA is more rigid than that of 0-PLA. The voids and cavities annealing for 24 h. between the polymer segments were reduced signiﬁcantly upon annealing for 24 h.

(a1) (a2)

(b1) (b2)

Figure 12. SEM micrograph for the PLA specimensspecimens at different different resolutions: ( (a1a1)) 0-PLA (low (low zoom), (a2) 0-PLA (high zoom),zoom), ((b1)) 24-PLA24-PLA (low(low zoom),zoom), andand ((b2b2)) 24-PLA24-PLA (high(high zoom).zoom). 4. Conclusions 4. Conclusions In this study, the effect of the annealing conditions on the thermal insulation and mechanical In this study, the effect of the annealing conditions on the thermal insulation and mechanical properties of the biopolymer PLA processed with a melt extruder and shaped in a compression molding properties of the biopolymer PLA processed with a melt extruder and shaped in a compression system was reported. Increasing the annealing time in the range of 0–24 h led to a significant increase molding system was reported. Increasing the annealing time in the range of 0–24 h led to a significant in the degree of crystallization, which had a direct effect on the thermal conductivity, density, and glass increase in the degree of crystallization, which had a direct effect on the thermal conductivity, transition temperature. The thermal conductivity of PLA at 25 C increased from 0.06426 W/(m K)) density, and glass transition temperature. The thermal conductivity◦ of PLA at 25 °C increased from· for the fast-cooled samples (0-PLA) to 0.09044 W/(m K)) for the samples annealed for 24 h (24-PLA). 0.06426 W(m·K)⁄ ) for the fast-cooled samples (0-PLA)· to 0.09044 W(m·K)⁄ ) for the samples The glass transition temperature increased approximately by 11.33% between the 0-PLA and the annealed for 24 h (24-PLA). The glass transition temperature increased approximately by 11.33% 24-PLA samples to 59 C. Moreover, the annealing process improved the compressive strength and between the 0-PLA and◦ the 24-PLA samples to 59 °C. Moreover, the annealing process improved the

PolymersPolymers 20202020, 12, x12 FOR, 2091 PEER REVIEW 14 of14 16 of 16 compressive strength and rigidity of PLA substantially, by more than 80%, and reduced its ductility. Fromrigidity a thermal of PLA insulation substantially, perspective, by more than the 80%,low andthermal reduced conductivity, its ductility. low From density, a thermal and insulation high compressiveperspective, strength the low are thermal required. conductivity, A brief compar low density,ison andbetween high compressivethe aforementioned strength properties are required. is A presentedbrief comparison in Figure 13 between and shows the aforementionedthat the annealing properties process of is PLA presented for 1–3 in h Figureproduces 13 andan optimum shows that thermalthe annealing insulation process material. of PLA The forPLA 1–3 thermal h produces conductivity an optimum values thermal (0.0798–0.0865 insulation W(m·K) material.⁄ The), and PLA thermal conductivitykg⁄ m values (0.0798–0.0865 W/(m K)), and density (~1233 kg/m3) in this annealing density (~1233 ) in this annealing time range· are very promising and comparable with traditionaltime range and are commercial very promising heat insulators, and comparable while withthe corresponding traditional and compressive commercial strength heat insulators, (97.2–98.7 while MPa)the is corresponding much higher compressive than that of strength traditional (97.2–98.7 thermal MPa) insulators is much and higher comparable than that ofwith traditional construction thermal materials.insulators and comparable with construction materials.

0.000813 110 105 ) 0.000812 100 /kg 3 95 m 0.000811 90 0.00081 85 80 0.000809 75 70 0.000808 Specific volume ( Specific volume

65 strength (MPa) Compressive 0.000807 60 0 4 8 12 16 20 24 Annealing time (hr) Specific volume Thermal resistance Compressive strength

FigureFigure 13. 13.ThermalThermal resistance, resistance, specific speciﬁc volume, volume, and and the thecompressive compressive strength strength of PLA. of PLA.

AuthorAuthor Contributions: Contributions: M.S.B.:M.S.B.: Data curation;curation; Formal Formal analysis; analysis; Investigation; Investigation; Writing—original Writing—original draft preparation. draft preparation.B.A.-J.: Conceptualization; B.A.-J.: Conceptualization; Funding acquisition; Funding Investigation; acquisition; Methodology; Investigation; Project Methodology; administration; Resources;Project administration;Supervision; Writing—reviewResources; Supervision; & editing. Writing—review A.H.I.M.: Investigation; & editing. Resources; A.H.I.M.: Supervision; Investigation; Writing—review Resources; & Supervision;editing. M.Z.I.: Writing—review Investigation; & Methodology;editing. M.Z.I.: Resources; Investigation; Writing—review Methodology; & editing.Resources; All Writing—review authors have read & and agreed to the published version of the manuscript. editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by The Emirates Center for Energy and Environment Research—UAE Funding:University This (Project research # 31R163).was funded by The Emirates Center for Energy and Environment Research—UAE University (Project # 31R163). Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Tan, L.; Yu, X.; Wan, P.; Yang, K. Biodegradable Materials for Bone Repairs: A Review. J. Mater. Sci. Technol. 1. Tan,2013 L.; ,Yu,29, X.; 503–513. Wan, P.; [CrossRef Yang, K.] Biodegradable Materials for Bone Repairs: A Review. J. Mater. Sci. Technol. 2. 2013Laycock,, 29, 503–513, B.; Nikolić,M.A.; doi:10.1016/j.jmst.2013.03.002. Colwell, J.M.; Gauthier, E.; Halley, P.J.; Bottle, S.E.; George, G. Lifetime prediction 2. Laycock,of biodegradable B.; Nikolić polymers., M.A.; Colwell,Prog. Polym. J.M.; Sci.Gauthier,2017, 71E.;, 144–189. Halley, [P.J.;CrossRef Bottle,] S.E.; George, G. Lifetime 3. predictionJohansson, of C.; Bras,biodegradable J.; Mondragon, I.;polymers. Nechita, P.;Prog. Plackett, Polym. D.; Simon, Sci. P.;Aucejo, 2017 S., Renewable71, 144–189, fibers and doi:10.1016/j.progpolymsci.2017.02.004.bio-based materials for packaging applications—A review of recent developments. BioResources 2012, 7, 3. Johansson,2506–2552. C.; Bras, [CrossRef J.; Mondragon,] I.; Nechita, P.; Plackett, D.; Simon, P.; Aucejo, S. Renewable fibers and 4. bio-basedGuan, X.materialsFabrication for ofpackaging Poly-Lactic applicat Acid (PLA)ions—A Composite review Films of recent and Their developments. Degradation BioResources Properties; University 2012, 7, of 2506–2552.Toledo: Toledo, Spain, 2012. 4. 5. Guan,Farah, X. Fabrication S.; Anderson, of Poly-Lactic D.G.; Langer, Acid (PLA) R. Physical Composite and Films mechanical and Their properties Degradation of PLA, Properties and their; University functions of in Toledo:widespread Toledo, applications—ASpain, 2012. comprehensive review. Adv. Drug Deliv. Rev. 2016, 107, 367–392. [CrossRef] 5. 6. Farah,Gupta, S.; Anderson, B.; Revagade, D.G.; N.; Langer, Hilborn, R. J. Physical Poly(lactic and acid) me fiber:chanical An properties overview. Prog.of PLA, Polym. and Sci.their2007 functions, 32, 455–482. in widespread[CrossRef ]applications—A comprehensive review. Adv. Drug Deliv. Rev. 2016, 107, 367–392, doi:10.1016/j.addr.2016.06.012.

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