Reactive Extrusion of Maleic-Anhydride-Grafted Polypropylene by Torque Rheometer and Its Application As Compatibilizer

Article Reactive Extrusion of Maleic-Anhydride-Grafted Polypropylene by Torque Rheometer and Its Application as Compatibilizer

Asra Tariq 1 , Nasir M. Ahmad 1,*, Muhammad Asad Abbas 1 , M Fayzan Shakir 1 , Zubair Khaliq 2 , Sikandar Raﬁq 3, Zulﬁqar Ali 4 and Abdelhamid Elaissari 5

1 Polymer Research Lab., School of Chemical and Materials Engineering, National University of Science and Technology, Islamabad 44000, Pakistan; [email protected] (A.T.); [email protected] (M.A.A.); [email protected] (M.F.S.) 2 Department of Polymer Engineering, National Textile University, Faisalabad 37610, Pakistan; [email protected] 3 Department of Chemical, Polymer and Material Engineering, University of Engineering and Technology, Kala Shah Kaku Campus 54890, Pakistan; [email protected] 4 Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus 45550, Pakistan; zulﬁ[email protected] 5 Univ Lyon, University Claude Bernard Lyon-1, CNRS, LAGEPP-UMR 5007, F-69622 Lyon, France; [email protected] * Correspondence: [email protected]

Abstract: This study is based upon the functionalization of polypropylene (PP) by radical polymerization to optimize its properties by influencing its molecular weight. Grafting of PP was done at different concentrations of maleic anhydride (MAH) and benzoyl peroxide (BPO). The effect on Citation: Tariq, A.; Ahmad, N.M.; viscosity during and after the reaction was studied by torque rheometer and melt flow index. Results Abbas, M.A.; Shakir, MF.; Khaliq, Z.; showed that a higher concentration of BPO led to excessive side-chain reactions. At a high percentage Rafiq, S.; Ali, Z.; Elaissari, A. Reactive of grafting, lower molecular weight product was produced, which was analyzed by viscosity change Extrusion of Maleic-Anhydride- during and after the reaction. Percentage crystallinity increased by grafting due to the shorter chains, Grafted Polypropylene by Torque which consequently led to an improvement in the chain’s packing. Prepared Maleic anhydride grafted Rheometer and Its Application as polypropylene (MAH-g-PP) enhanced interactions in PP-PET blends caused a partially homogeneous Compatibilizer. Polymers 2021, 13, 495. https://doi.org/10.3390/ blend with less voids. polym13040495 Keywords: functionalization; grafting; polypropylene; reactive extrusion; torque analysis; ther-

Academic Editor: Dagmar R. D’hooge mal analysis Received: 15 August 2020 Accepted: 14 September 2020 Published: 5 February 2021 1. Introduction Publisher’s Note: MDPI stays neutral Polypropylene (PP) is the second-largest consumable polymer due to its better me- with regard to jurisdictional claims in chanical properties, flexibility, transparency, low cost, ease in processability, and high published maps and institutional affil- chemical and moisture resistance [1]. However, the major drawbacks of PP are high ther- iations. mal expansion coefficient, poor bonding properties, and susceptibility to oxidation [2]. The blending of PP with another polymer with improved thermal characteristics may be more favorable to optimize its properties. PP is a highly nonpolar polymer, so it has limited compatibility with polar polymers. A stabilized homogenous mixture can be achieved by Copyright: © 2021 by the authors. generating compatibility among the polymeric blends that will lower the interfacial ten- Licensee MDPI, Basel, Switzerland. sion [3]. However, the functionalization of PP will alter its characteristics due to structural This article is an open access article changes [4–6]. distributed under the terms and Various researchers have investigated the grafting of maleic anhydride (MAH) on conditions of the Creative Commons polymer chains to make it compatible with polar polymers [4–15]. MAH imparts carbonyl Attribution (CC BY) license (https:// functional groups on the backbone of PP and makes it compatible with polymers [7,16]. creativecommons.org/licenses/by/ The solution process was initially used for the grafting of MAH on PP, and high reaction 4.0/).

Polymers 2021, 13, 495. https://doi.org/10.3390/polym13040495 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 495 2 of 17

yields were achieved [7,8,17]. However, the solution process has limited applications due to the involvement of solvent. MAH was grafted on PP by melt technique by reactive extrusion at varying MAH concentrations, type and amount of initiator, and processing conditions [4,6,7,11,18,19]. In the grafting of MAH on PP chains at high-temperature, peroxide forms free radicals on the PP chain, thus withdrawing hydrogen atoms while imparting radicals on the PP chain. The MAH ring attaches to the PP chain bearing a radical on it; thus, the grafting process continues until termination [6]. Benzoyl peroxide (BPO) has been widely used as initiator to start the reaction for MAH functionalization on polymer chains and was considered suitable for bulk polymerization reactions [20]. Control of the reaction is very important to avoid excessive chain scission and gel formations, which eventually alter the rheological characteristics of PP. Previously, work has been successfully carried out on grafted MAH on HDPE by different peroxides in the presence of electron donor additives (dimethyl sulfoxide, dimethylacetamide, tri(nonylphenyl) phosphate) using torque of reaction. The formation of gel or crosslinking was controlled by optimizing the peroxide amount and its type [21]. The effect of MAH and dicumyl peroxide (DCP) as initiators on chain scission has also been reported [22]. Chain scission was initially high, which was observed by a decrease in the mixing torque value that eventually stabilizes as the reaction proceeds. The addition of monomers during MAH grafting on PP chains has also been investigated [23–26]. The grafting of MAH on PP was conducted by adding styrene as co-monomer, and a high grafting yield was obtained at styrene to MAH 1:1 proportion. Due to an equal number of monomer ratio being employed, no side reaction occurred [26]. During the melt grafting of MAH on PP, homo-polymerization of MAH by the attack of the peroxide initiator did not appear at 180 ◦C to 190 ◦C temperature [12]. Despite extensive research, the effect of MAH grafting on the structure of PP during and after complete reaction is yet not clear, and this area continues to attract research attention [4]. Furthermore, the occurrence and effect of side reactions during MAH grafting on PP have not been investigated in detail. In consideration of the above, it would be vital from the perspective of both product and process development, to explore in detail the effect of important parameters such as MAH and initiator concentration on the extent of grafting and rheology during the course of reactive extrusion. The effect of MAH and BPO concentrations on the polymers chains’ behavior throughout the reaction during the reactive extrusion process is discussed in this study. In addition, after-grafting changes in the physical properties of PP are not clear in the literature. Hence, the effect of grafting MAH on PP crystallinity, melting temperature, and melt ﬂow index is evaluated. Apart from torque rheometer, Fourier-transform infrared spectroscopy (FTIR), melt ﬂow index (MFI) and differential scanning calorimetry (DSC) techniques were utilized in this study. To check the effect of Maleic anhydride grafted polypropylene MAH-g-PP as a compatibilizer, grafted PP was added in polyethylene terephthalate (PET) blends at varying concentrations. The effect of this compatibilizer on morphology was analyzed by SEM. Dynamic mechanical analysis (DMA) was done to analyze the thermomechanical behavior of compatibilized blends.

2. Experimental 2.1. Materials and Methods MAH (>99% pure) with a density of 1.314 g/cm3 and BPO (99% pure) were purchased from Sigma-Aldrich St. Louis, MO, USA. Isotactic PP (Mn =∼ 123.7 Kg/mol) was purchased from LCY Chemicals Corp, Kaohsiung, Taiwan. BPO was used as initiator for the grafting of PP with MAH. The density of PP was 0.908 g/cm3. The melt ﬂow index (MFI) was 3.297 g/10 min at 190 ◦C. Commercial grade acetone was used as solvent for MAH and BPO. The solvents and materials were used without further puriﬁcation. Film grade PET was provided by Gatron Industries limited Karachi, Pakistan. Polymers 2021, 13, 495 3 of 17

2.2. Chemical Reaction and Reactive Extrusion Process Table1 indicates the number of samples with varying concentrations of MAH and BPO in parts per hundred (phr). The samples were named as PM1~PM5 (varying concentration of MAH) and PB1~PB5 (varying concentration of BPO). MAH and BPO were dissolved in acetone and stirred for 10 min at room temperature. PP pellets were added, and the mixture was placed for at least 3 days at room temperature to allow evaporation of acetone. BPO and MAH adhered homogenously onto the surface of PP pellets after the evaporation of acetone. HAAKE Rheomix OS Lab internal mixer (Thermoﬁsher, Dreieich, Germany) was utilized for free radical reaction of PP and MAH using BPO. The functionalization of PP was carried out by a reactive extrusion process in an internal engineering mixer system.

Table 1. Experimental design by varying Maleic Anhydride (MAH) at constant Benzoyl Peroxide (BPO), and varying BPO at constant MAH.

Sample Sample MAH (phr) BPO (phr) MAH (phr) BPO (phr) Name Name PM1 0.05 0.4 PB1 0.15 0.2 PM2 0.10 0.4 PB2 0.15 0.3 PM3 0.15 0.4 PB3 0.15 0.4 PM4 0.20 0.4 PB4 0.15 0.45 PM5 0.25 0.4 PB5 0.15 0.5

The barrel of the mixer was preheated at 160 ◦C until the temperature was stabilized. 40 g of MAH- and BPO-coated PP pellets were added in two equal parts by weight. The extrusion process was carried out at 160 ◦C with a screw speed of 60 rpm for 10 min. The change in torque was recorded for at least 10 min.

2.3. Mechanical Blending of PET and PP with MAH-g-PP Blends of MAH-g-PP and pure PP with PET were prepared by varying composition in an internal mixer by melt blending. PET was added ﬁrst then PP and MAH-g-PP were added. Blending was done at 270 ◦C, 70 rpm for 10 min. Compositions details are detailed in Table2. Films of PET and PP blends were fabricated by the compression molding machine at 200 ◦C temperature and 2000 psi pressure.

Table 2. Composition details of Polypropylene (PP) /Polyethylene terephthalate (PET) PP/PET blends.

Samples Film Grade PET (%) Isotactic PP (%) MAH-g-PP (%) CB 1 60% 39% PP 1% CB 2 60% 37.5% PP 2.5% CB 3 60% 35% PP 5% CB 4 60% 40% PP - CB 5 100% - - CB 6 - 100% -

2.4. Process and Physical Characteristics HAAKE Rheomix lab internal mixer was used at a deﬁned speed (shear rate) and time, and PP ﬂow behavior was recorded as torque value. Rotors’ rpm was 60 at 160 ◦C and a 10-min reaction was conducted. If viscosity increases inside the reaction chamber, the system gains more energy to maintain the speed of rotors, which generates a signal recorded by a transducer. The value of torque was obtained from the attached transducer of the internal mixer. The torque throughout the reaction was continuously monitored, and variation in values was studied. FTIR was done on PP grafted samples. The spectrum was recorded by a Bruker instrument (Fremont, CA, USA) Model Alpha. PP grafted samples were thermally characterized in a Perkin Elmer DSC 4000 (Waltham, MA, USA), by heating Polymers 2020, 12, x FOR PEER REVIEW 4 of 17

MB, Italy). The analysis was carried out 3 times for each reactive extruded sample, and the average value was calculated. Scanning electron microscopy (SEM) Joel JSM 6490A (Peabody, Massachusetts, USA) was performed to study the morphology of blends. Dynamic mechanical analysis (DMA) (TA Instruments, New Castle, Delaware, USA) of all blends in comparison of pure PET and PP was done according to ASTM E1640-13 in bending mode with a dual cantilever. Dual cantilever bending mode was used owing to the delicate films. The sample was run at 5 °C/min and 1 Hz frequency under nitrogen.

3. Results and Discussion Polymers 2021, 13, 495 4 of 17 3.1. Reactive Extrusion Process

Polymers 2020, 12, xWhen FOR PEER the REVIEW mixing process started, the variation in temperature and torque from4 theof 17 experiment ◦ was recorded5–8 mgto analyze of the samplethe effect at 10of theC/min free radi undercal polymer a nitrogen reaction (N2) atmosphere on the melt fromviscosity ambient of PP. ◦ MB, Italy).Figure The analysis 1a,btemperature describes was carried tothe 200 outtorque C.3 times The variation MFI for each of with PP reactive grafted reaction extruded samples time by was sample, varying measured and BPO the in and accordanceaverage MAH content, with ASTM 1238 under the weight of 2.16 kg at 190 ◦C temperature in a Noselab ATS Plastometer value was calculated.respectively. Scanning electron microscopy (SEM) Joel JSM 6490A (Peabody, Massachusetts, (Nova Milanese MB, Italy). The analysis was carried out 3 times for each reactive extruded USA) was performed to study the morphology of blends. Dynamic mechanical analysis (DMA) (TA sample, and the average value was calculated. Scanning electron microscopy (SEM) model Instruments, New Castle, Delaware, USA) of all blends in comparison of pure PET and PP was done Joel JSM 6490A (Peabody, MA, USA) was done to analyze the blends’ morphology. Dynamic according to ASTM E1640‐13 in bending mode with a dual cantilever. Dual cantilever bending mode mechanical analysis (DMA) by TA Instruments, New Castle, DE, USA of all the prepared was used owing to blendsthe delicate in contrast films. ofThe pure sample PP and was PET run was at 5 performed °C/min and using 1 Hz ASTM frequency E1640-13 under in bending nitrogen. mode with a dual cantilever. Dual cantilever bending mode was used because of the fragile nature of samples. The sample was run under nitrogen atmosphere at 5 ◦C/min and 1 Hz 3. Results and Discussionfrequency.

3.1. Reactive Extrusion3. Process Results and Discussion 3.1. Reactive Extrusion Process When the mixing process started, the variation in temperature and torque from the experiment was recorded to analyzeWhen the effect the mixingof the free process radical started, polymer the variationreaction on in temperaturethe melt viscosity and torque of PP. from the experiment was recorded to analyze the effect of the free radical polymer reaction on Figure 1a,b describes the torque variation with reaction time by varying BPO and MAH content, the melt viscosity of PP. Figure1a,b describes the torque variation with reaction time by respectively. varying BPO and MAH content, respectively.

(a) (b)

Figure 1. Torque variation of PP with reaction time by altering (a) BPO concentration and (b) MAH concentration.

The recorded instantaneous torque (ᴛ) is correlated with the viscosity (𝜂) of the material in the reaction at temperature T for time t according to the below-mentioned Equations(1)–(3) [27].

𝜏 (1) 𝜂= 𝛾

where:

𝜂=dynamic viscosity (Pa·s) 𝜏=shear stress(a) (N/cm2) (b) Figure 1. a b FigureTorque𝛾= 1. Torque variationshear ratevariation of PP(sec-1) with of PP reaction with reaction time by alteringtime by (altering) BPO concentration(a) BPO concentration and ( ) MAH and (b concentration.) MAH concentration. The recorded instantaneous torque (ᴛ) is correlated with the viscosity (η) of the 𝜏= material in the reaction at temperature2𝜋𝑅T for𝐿 time t according to the below-mentioned(2) The recorded instantaneous torque ᴛ is correlated with the viscosity 𝜂 of the material in the Equations (1)–(3) [27]. reaction at temperature T for time t according to the below‐mentionedτ Equations(1)–(3) [27]. η = (1) γ

where: 𝜏 (1) 𝜂 η = dynamic viscosity (Pa·s) 𝛾 τ = shear stress (N/cm2) where: γ = shear rate (sec-1) 𝜂dynamic viscosity (Pa∙s) = 𝜏shear stress (N/cm2) τ 2 (2) 2πRs L 𝛾 shear rate (sec‐1) 2ωR2R2 γ = c s (3) ᴛ 2 2 2 X (Rc − Rs ) 𝜏 (2) 2𝜋𝑅 𝐿

Polymers 2020, 12, x FOR PEER REVIEW 4 of 17

3. Results and Discussion

3.1. Reactive Extrusion Process When the mixing process started, the variation in temperature and torque from the experiment was recorded to analyze the effect of the free radical polymer reaction on the melt viscosity of PP. Figure 1a,b describes the torque variation with reaction time by varying BPO and MAH content, respectively.

(a) (b)

Figure 1. Torque variation of PP with reaction time by altering (a) BPO concentration and (b) MAH concentration.

The recorded instantaneous torque (ᴛ) is correlated with the viscosity (𝜂) of the material in the reaction at temperature T for time t according to the below-mentioned Equations(1)–(3) [27].

𝜏 (1) 𝜂= 𝛾 where: 𝜂=dynamic viscosity Polymers(Pa·s) 2021, 13, 495 5 of 17 𝜏=shear stress (N/cm2)

𝛾=shear rate (sec-1) where: ᴛ = torque (Nm) 𝜏= (2) 2𝜋𝑅L=𝐿 effective spindle length (m) Rs = spindle radius (m) Rc = container radius (m) ω = rotational speed (radians/s) X = radial location During the reaction time, all quantities are constant except torque. Hence,

(t,T) ∝ η(t, T) (4)

In all samples, initial high filling peaks appear due to the addition of solid PP (MAH and BPO coated) pellets into the mixer. Torque on the curve shows continuous variation. This unbroken disturbance reflects the feeding and molten accumulation of PP. A high value of torque is visible during the initial 2–4 min due to friction, high viscosity, and surface melting of PP pellets [28]. It is observed that during the initial few minutes, torque highly fluctuates because of radical reaction on PP chains. This results in an increase in viscosity due to a number of radicals formation and MAH molecules. With the passage of time, the fusion of material takes place along with chain breakage and decrease in the molecular weight by grafting of the MAH functional group on PP [2,3]. The molecular weight of PP is possibly decreased due to chain scission in the free radical polymerization process [2]. By reduction in molecular weight, the viscosity decreased. Hence, the possible effects produce a change in torque. The experimental observations indicate that the value of torque stabilizes at the end of the reaction after all the material seems to completely melt. Equilibrium is achieved between shear heating and constant chamber temperature, which results in stable torque value [28]. The increase in the amount of MAH or BPO reduces the time required to reach the steady-state torque value. However, the effect on reaction time by varying the MAH concentration is more visible as shown in Figure1b. High BPO concentration (Figure1a) and increasing MAH percentage at constant BPO (Figure1b) led towards chain scission and crosslinking reaction. The change in torque value remained highly visible (Figure1b) when the amount of MAH is increased, which shows that high concentration of MAH leads towards more grafting reaction and chain scission [6]. However, high loadings of BPO led to excessive side-chain reactions such as crosslinking [21]. The stabilized value of torque in both the cases at the ending of the reaction time indicates that the chemical reaction did not move towards excessive crosslinking or complete degradation of PP [28]. The stable torque value was higher than the initial torque value due to slight crosslinking in chains. A possible reaction mechanism [22] during the grafting of MAH on PP by using BPO as initiator is given in Figure2. The radical is produced either as phenyl radical or benzoyl radical. These radicals attack on the main chain of carbon in polypropylene and extract an hydrogen atom, leaving a radical on the PP chain. On this produced active site, the ring of maleic anhydride is attached. Crosslinking density in PP by adding MAH and BPO was calculated using the difference between initial and final torque values [21]. An increase in the value of crosslinked density shows that more complex structures are formed as well as gel formation. The gel formation is not required and reduces the grafting percentage. Figure3 explains the trends in crosslinking density by varying MAH and BPO concentration. It is clearly visible that when BPO amount is increased at constant MAH, overall, the crosslinking density increases due to a large number of free radicals that attack on the PP chain and produce many complex structures and gel structures. However, at constant BPO, initially at 1.0 phr MAH, crosslinking density increases due to side chain reactions; however, after this concentration, free radicals possessed more available sites for attack, and the reaction moved towards grafting [21]. Polymers 2020, 12, x FOR PEER REVIEW 6 of 17

Polymers 2021, 13, 495 6 of 17

Polymers 2020, 12, x FOR PEER REVIEW 6 of 17

Figure 2. (a) Initiation reaction for the grafting by free radical polymerization. (b) Grafting of MAH ring on the backbone of PP.

Crosslinking density in PP by adding MAH and BPO was calculated using the difference between initial and final torque values [21]. An increase in the value of crosslinked density shows that more complex structures are formed as well as gel formation. The gel formation is not required and reduces the grafting percentage. Figure 3 explains the trends in crosslinking density by varying MAH and BPO concentration. It is clearly visible that when BPO amount is increased at constant MAH, overall, the crosslinking density increases due to a large number of free radicals that attack on the PP chain and produce many complex structures and gel structures. However, at constant BPO, initially at 1.0 phr MAH, crosslinking density increases due to side chain reactions; however, after this concentration, free radicals Figure 2. (a) Initiation reaction for the grafting by free radical polymerization. (b) Grafting of MAH ring on the backbone of PP. possessedFigure 2. more(a) Initiation available reaction sites for for attack,the grafting and the by reactionfree radical moved polymerization. towards grafting (b) Grafting [21]. of MAH ring on the backbone of PP.

Crosslinking density in PP by adding MAH and BPO was calculated using the difference between initial and final torque values [21]. An increase in the value of crosslinked density shows that more complex structures are formed as well as gel formation. The gel formation is not required and reduces the grafting percentage. Figure 3 explains the trends in crosslinking density by varying MAH and BPO concentration. It is clearly visible that when BPO amount is increased at constant MAH, overall, the crosslinking density increases due to a large number of free radicals that attack on the PP chain and produce many complex structures and gel structures. However, at constant BPO, initially at 1.0 phr MAH, crosslinking density increases due to side chain reactions; however, after this concentration, free radicals possessed more available sites for attack, and the reaction moved towards grafting [21].

Figure 3. Effect of MAH and BPO concentration on the crosslinking density during the reaction. Figure 3. Effect of MAH and BPO concentration on the crosslinking density during the reaction. 3.2. Carbonyl Index of Grafted PP

FTIR spectra of pure polypropylene (PP) and highest MAH-grafted PP (PM4) are shown in Figure4. In both spectra, peaks are observed at 2950 cm −1 due to the asym- −1 metric stretching in the methyl group (-CH3). At 2915 cm peak is due the asymmetric −1 stretching in the methylene group (-CH2-), 2870 cm for -CH3 symmetric stretching, and −1 2840 cm for -CH2-symmetric stretching. Bending peaks of -CH2- and –CH3 are observed at 1455 cm−1 and 1370 cm−1, respectively. In all grafted samples, two peaks appeared at 1750 cm−1 for the carbonyl (C=O) group of the ﬁve-membered ring anhydride and the C=C

Figure 3. Effect of MAH and BPO concentration on the crosslinking density during the reaction.

Polymers 2020, 12, x FOR PEER REVIEW 7 of 17

3.2. Carbonyl Index of Grafted PP FTIR spectra of pure PP and highest MAH‐grafted PP (PM4) are shown in Figure 4. In both spectra, peaks are observed at 2950 cm−1 for the asymmetric stretching of the methyl group (‐CH3). At 2915 cm−1 for the asymmetric stretching of the methylene group (‐CH2‐), 2870 cm−1 for ‐CH3 symmetric stretching, and 2840 cm−1 for ‐CH2‐symmetric stretching. Bending peaks of ‐CH2‐ and –CH3 are Polymers 2021 13 , , 495 observed at 1455 cm−1 and 1370 cm−1, respectively. In all grafted samples, two peaks appeared at7 of 1750 17 cm−1 for the carbonyl (C=O) group of the five‐membered ring anhydride and the C=C peak at 1655 cm−1. From the FTIR spectra of all PP grafted samples, the carbonyl index (CI) was calculated using Equation (5)peak [5]. at 1655 cm−1. From the FTIR spectra of all PP grafted samples, the carbonyl index (CI) was calculated using Equation (5) [5]. A CI AA (5) CI = 1750 (5) A1455 −1 where 𝐴 is the area of absorbance peak at 1750 cm , which is characteristic of the carbonyl −1 functionalwhere groupA from1750 isfive the‐membered area of absorbance cyclic anhydrides; peak at 1750 𝐴 cm is ,the that area is characteristicof absorbance peak peak of at 1455cm−1, thewhich carbonyl is characteristic functional of group the CH from2 and ﬁve-membered is proportional cyclic to anhydrides;the concentrationA1455 ofis thePP. area Table of 3 −1 displays theabsorbance CI values peak for atthe 1455 prepared cm , whichsamples. is characteristic A higher CI of value the CH of 2MAHand is‐grafted proportional PP indicates to the higher graftingconcentration [5]. of PP. Table3 displays the CI values for the prepared samples. A higher CI value of MAH-grafted PP indicates higher grafting [5].

Figure 4. Comparison of the spectra of pure polypropylene (PP) and highest maleic anhydride Figure(MAH) 4. Comparison grafted PP of obtained Fourier by Transform Fourier Transform Infrared Spectroscopy Infrared Spectroscopy (FTIR ) spectra (FTIR). of pure PP and highest MAH grafted PP. Table 3. Carbonyl Index (CI) values of all grafted samples. Table 3. Carbonyl Index (CI) values of all grafted samples. Standard Standard Sample Sample Sample CI CI Value Deviation Sample CI CI Value Deviation Name Standard Deviation (+/−) Name Standard Deviation (+/−) Name Value (±) Name Value (±) PM1 PM10.24 0.240.012 0.012PB1 PB1 0.25 0.250.0125 0.0125 PM2 PM20.37 0.370.0185 0.0185PB2 PB2 0.27 0.270.0135 0.0135 PM3 PM30.38 0.380.019 0.019PB3 PB3 0.38 0.380.019 0.019 PM4 PM40.41 0.410.0205 0.0205PB4 PB4 0.36 0.360.018 0.018 PM5 PM50.40 0.400.02 0.02PB5 PB5 0.36 0.360.018 0.018

The value of CI is directly proportional to the percentage of grafting; hence, the sample that shows a high carbonyl index is exhibiting high grafting percentage. Table3 shows that

PM4 has a high CI value and hence high % grafting. Polymers 2020, 12, x FOR PEER REVIEW 8 of 17

Polymers 2021, 13, 495 The value of CI is directly proportional to the percentage of grafting; hence, the sample that8 of 17 shows a high carbonyl index is exhibiting high grafting percentage. Table 3 shows that PM4 has a high CI value and hence high % grafting. 3.3. Crystallinity and Melting Temperature of Grafted Samples 3.3. Crystallinity and Melting Temperature of Grafted Samples The values of heat of fusion obtained by DSC measurements for pure PP and grafted The valuesPP of samples heat of werefusion utilized obtained to by calculate DSC measurements the percentage for crystallinity pure PP and by grafted Equation PP (6)samples [6].

were utilized to calculate the percentage crystallinity by Equation (6)∗ [6]. ∆Hf % Crystallinity =∗ (6) ∆H ∆H100 % Crystallinity f (6) ∆H where ∆H∗ is the heat of fusion of grafted PP and ∆H100 is the fusion heat for a theoretical where ∆𝐻∗ is the heat off fusion of grafted PP and ∆𝐻 is the heatf of fusion for hypothetically 100% 100% crystalline PP [6]. 207 J/g is the value used for the enthalpy of fusion for 100% crystalline PP [6]. The value for the enthalpy of fusion of 100% crystalline PP used is 207 J/g [29]. A crystalline PP obtained from literature [29]. A comparison of the DSC thermograms of comparison ofprocessed the DSC thermograms samples by reactive of processed extrusion samples with varyingby reactive MAH extrusion content with is shown varying in FigureMAH 5. content is shown in Figure 5.

Figure 5. Differential scanning calorimetry (DSC) thermograms for MAH-grafted PP samples for Figure 5. Differential scanning calorimetry (DSC) thermograms for MAH‐grafted PP samples for varying MAH contents. varying MAH contents. All grafted samples showed a high ∆H . On the base of ∆H , the calculated % ∆𝐻 f ∆𝐻 f All graftedcrystallinity samples showed is shown a againsthigh different. On the MAH base and of BPO, the content calculated for grafted % crystallinity PP in Figure is6 . shown against differentIt is observed MAH and that BPO in content all processed for grafted samples, PP in the Figure percentage 6. of crystallinity is much higher. This behavior is due to the addition of BPO and MAH, which causes degradation of the PP chains into shorter chains [5]. Chain scission caused a reduction in molecular weight and further reduced entanglement in chains. This reduction enabled a rise in the degree of order of PP chains and hence caused an increase in overall crystallinity [4,5]. When BPO content increased, chains scission increased due to a high level of degradation; thus, the percentage crystallinity remained high for all samples [5]. However, after increasing the MAH concentration at 1.5 phr, percentage crystallinity is noted to be reduced. The reason for this slight fall is MAH attachment on chains, which reduced chain packing [30]. Figure7 shows the variation in melting temperature ( Tm) by altering MAH and BPO amounts. The structural changes are caused by the addition of MAH on PP chains, which slightly inﬂuenced Tm. Variation in MAH at constant BPO and in BPO at constant MAH ﬁrst initiated the reduction of Tm in processed samples, followed by a sudden rise.

Polymers 2020, 12, x FOR PEER REVIEW 9 of 17

Polymers 2021, 13, 495 9 of 17 Polymers 2020, 12, x FOR PEER REVIEW 9 of 17

Figure 6. Effect on percentage crystallinity for grafted samples by varying MAH and BPO content.

It is observed that in all processed samples, the percentage crystallinity is much higher. This behavior is due to the addition of BPO and MAH, which causes degradation of the PP chains into shorter chains [5]. Chain scission caused a reduction in molecular weight and further reduced entanglement in chains. This reduction enabled a rise in the degree of order of PP chains and hence caused an increase in overall crystallinity [4,5]. When BPO content increased, chains scission increased due to a high level of degradation; thus, the percentage crystallinity remained high for all samples [5]. However, after increasing the MAH concentration at 1.5 phr, percentage crystallinity is noted to be reduced. The reason for this slight fall is MAH attachment on chains, which reduced chain packing [30].

Figure 7 shows the variation in melting temperature (T by altering MAH and BPO amounts. The structural changes are caused by the addition of MAH on PP chains, which slightly influenced T. Variation in MAH at constant BPO and in BPO at constant MAH first initiated the reduction of Figure 6. T in processed samples,Effect followed on percentage by a sudden crystallinity rise. for grafted samples by varying MAH and BPO content. Figure 6. Effect on percentage crystallinity for grafted samples by varying MAH and BPO content.

Figure 7 shows the variation in melting temperature (T by altering MAH and BPO amounts. The structural changes are caused by the addition of MAH on PP chains, which slightly influenced

T. Variation in MAH at constant BPO and in BPO at constant MAH first initiated the reduction of T in processed samples, followed by a sudden rise.

Figure 7. Effect on melting temperature (Tm) for grafted samples by varying MAH and BPO contents. Figure 7. Effect on melting temperature (Tm) for grafted samples by varying MAH and BPO contents.

The fall in Tm at low concentrations of MAH and BPO is caused by the chains breakage and formation of lower molecular weight chains. However, at higher concentrations of MAH and BPO, complex molecular structures started to appear; thus, Tm increased [4,5].

3.4. Melt Flow Index (MFI) MFI values for the functionalized PP samples are given in Table4.

Figure 7. Effect on melting temperature (Tm) for grafted samples by varying MAH and BPO contents.

Polymers 2020, 12, x FOR PEER REVIEW 10 of 17

The fall in Tm at low concentrations of MAH and BPO is caused by the chains breakage and formation of lower molecular weight chains. However, at higher concentrations of MAH and BPO,

Polymers 2021, 13, 495 complex molecular structures started to appear; thus, 𝑇 increased [4,5]. 10 of 17

3.4. Melt Flow Index (MFI)

MFI valuesTable 4.forMelt the ﬂow functionalized index for processed PP samples samples are and given pure in PP. Table 4.

MFI (g/10 min) Standard MFI (g/10 min) Standard Sample Table 4. Melt flow index for processed samplesSample and pure PP. at 2.16 kg and Deviation at 2.16 kg and Deviation Name Name Sample MFI (g/10min) at190 2.16◦C Standard(± ) Sample MFI (g/10 min)190 ◦ Cat Standard(±) Name PPkg and 190 °C 3.297 Deviation 0.16485 (±) Name 2.16 kg and 190 °C Deviation (±) PP PM13.297 6.0110.16485 0.30055 PB1 11.84 0.592 PM1 PM26.011 9.2560.30055 0.4628 PB1 PB211.84 13.99 0.592 0.6995 PM2 PM39.256 14.280.4628 0.664 PB2 PB313.99 15.97 0.6995 0.7985 PM3 PM414.28 8.9010.664 0.44505 PB3 PB415.97 12.70 0.7985 0.635 PM4 PM58.901 8.6040.44505 0.4302 PB4 PB512.70 8.380 0.635 0.419 PM5 8.604 0.4302 PB5 8.380 0.419 Figure8 displays the variation in MFI by altering MAH and BPO amounts. When Figureadding 8 displays low the content variation of MAH in MFI and by BPO, altering the MFI MAH value and increases BPO amounts. remarkably. When adding By adding low content of MAHMAH andand BPO, BPO, chains the MFI scission value occurs increases due remarkably. to termination By by adding disproportionation MAH and BPO, and chains chain scission occurstransfer, due andto termination due to the presenceby chain oftransfer possible and side disproportionation, reactions. It can be and inferred due to that the apresence combi- of possiblenation side reactions. reaction for It termination can be inferred is less probablethat a combination than chain transfer reaction or for disproportionation termination is less [5]. probable thanShorter chain chains transfer with or low disproportionation molecular weight caused[5]. Shorter a high chains ﬂow rate.with As low the molecular concentration weight of caused highMAH flow and rate. BPO As was the further concentration increased, of complex MAH and molecules BPO was started further to form, increased, which resultedcomplex molecules instarted lower to MFI form, values which [5 ].resulted in lower MFI values [5].

Figure 8. Melt ﬂow index of functionalized samples by varying MAH and BPO. Figure 8. Melt flow index of functionalized samples by varying MAH and BPO. 3.5. Morphology by Scanning Electron Microscopy (SEM) 3.5. Morphology Figuresby Scanning9 and Electron 10 display Microscopy the SEM (SEM) images of pure polyethylene terephthalate (PET), pure polypropylene (PP) and compatibilized and un-compatibilized PP/PET blends, re- Figures 9 and 10 display the SEM images of pure PP and PET and compatibilized and un‐ spectively. compatibilized PP/PET blends, respectively.

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(a) (b) (a) (b) Figure 9. Scanning Electron MicroscopyFigure (SEM) 9. SEM images images of of (a )(a pure) pure polyethylene PET and (b Terephthalate) pure PP. (PET) and (b) pure polypropylene (PP). Figure 9. SEM images of (a) pure PET and (b) pure PP.

Figure 10.FigureScanning 10. ElectronSEM images Microscopy of (a (SEM)) PP/PET images and of ( (ba)) PP/PETMaleic Anhydride and (b) Maleic grafted Anhydride Polypropylene/ grafted Polypropy- Figure 10. SEM images of (a) PP/PET and (b) Maleic Anhydride grafted Polypropylene/ lene/ Polypropylene/Polypropylene/ Polyethylene Polyethylene Terephthalate Terephthalate (MAH-g-PP/PP/PET) (MAH‐g‐PP/PP/PET) blends blends with with composition composition 40/60 40/60 and and 2.5/37.5/60, Polypropylene/ Polyethylene Terephthalate (MAH‐g‐PP/PP/PET) blends with composition 40/60 and respectively.2.5/37.5/60, respectively. 2.5/37.5/60, respectively. Figure 9 shows that pureFigure PET9 shows and PP that possess pure completely PP and PET uniform possess microstructures totally uniform and microstructures that there and are noFigure phase 9 showsboundaries. that pure there However, PET are and no phase thePP possessblends boundaries. completelyof PP and However, uniformPET showed the microstructures blends phase of PP boundaries. and and PET that showed thereThe phase presenceare no phase of MAH boundaries.‐g‐boundaries.PP in PETHowever, and The presencePP the blends blends of promoted MAH-g-PP of PP and the in PET PETformation showed and PP of blends phase much promoted boundaries.finer dispersed the The formation of morphology,presence of MAHuniformity,‐gvery‐PP in fineand PET and much and dispersed betterPP blends morphology,adhesion promoted (Figure better the 10a) adhesion,formation than un andof‐ compatibilizedmuch partial finer uniformity dispersed blends (Figure 10a) (Figuremorphology, 10b). Thisuniformity, increasethan an and in un-compatibilized finenessmuch better of the adhesion blend blend resulted (Figure(Figure 10 in10a) b).the This thanreduction increase un‐compatibilized of in voids fineness and ofblendshence the prepared reduced(Figure 10b).the passage This increase blendsof small showedin molecules fineness lesser ofthrough voids the blend and the hence filmsresulted decreasedand indecreased the the reduction passing its permeability of smallvoids moleculesand for waterhence across the molecules.reduced the The passage blendfilms of having small and molecules overallno MAH reduced‐ gthrough‐PP itsshowed waterthe films permeability.phase and separation decreased The blendand its permeability voids having due no MAH-g-PPto for lack water of showed phase separation and voids due to lack of compatibility between PET molecules and PP compatibilitymolecules. The between blend PEThaving molecules no MAH and‐g PP‐PP chains showed [31–33]. phase Figure separation 11 shows and the voids comparison due to oflack PET of chains [31–33]. Figure 11 shows the comparison of PET and PP blends by changing the andcompatibility PP blends between by varying PET the molecules concentration and PP of chains MAH ‐[31–33].g‐PP. Figure 11 shows the comparison of PET and PP blends by varyingconcentration the concentration of MAH-g-PP. of MAH‐g‐PP. In Figure 11, two blends having different compatibilizer concentration are compared, and the SEM image of 5% MAH-g-PP shows less voids that are possibly owing to the presence of uneven physical interactions between the PET and PP chains. In the blends of 2.5% and 5% MAH-g-PP, agglomerates started to appear, which shows the immiscibility between PP and PET molecules in some regions of the blends. PET molecules began to adhere with same type of molecules because of its higher possibility of interactions among the similar kind of molecules to overall stabilize the system [34].

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(a) (b)

Figure 11. SEM images of (a) MAH‐g‐PP/PP/PET and (b) MAH‐g‐PP/PP/PET blends with composition 2.5/37.5/60 (anda) 5/35/60, respectively. (b) Figure 11.FigureScanning 11. Electron SEM images Microscopy of (a (SEM)) MAH images‐g‐PP/PP/PET of (a) MAH-g-PP/PP/PET and (b) MAH‐g and‐PP/PP/PET (b) MAH-g-PP/PP/PET blends with blends with compositioncompositionIn Figure 2.5/37.5/60 11, 2.5/37.5/60 two andblends and 5/35/60, 5/35/60,having respectively. respectively.different compatibilizer concentration are compared, and the SEM image of 5% compatibilizer shows some voids that are probably due to uneven physical interactionsIn Figure between 11, two the blends Fracturetwo havingpolymers. analysis different In of the all compatibilizer theblends prepared of 2.5% blendsconcentration and were 5% compatibilizer, carried are compared, out by taking there and SEMtheare images on the edges of films shown in Figure 12. The spherical shaped beads present in the SEMagglomerates, image of which 5% compatibilizer indicates immiscibility shows some between voids PET that and are PP probably in some dueareas to of uneven the blends. physical PET figure show the PP material, and PET is the main matrix material. In uncompatibilized moleculesinteractions started between to jointhe twowith polymers.each other In due the toblends greater of interactions2.5% and 5% between compatibilizer, the same theretypes are of blend, it can be seen that the spherical beads are becoming debonded from the main moleculesagglomerates, to stabilize which indicatesthe system immiscibility [34]. between PET and PP in some areas of the blends. PET matrix that is PET and this is because of the lack of adhesion on interface. Many holes are Fracture analysis of blends was carried out using SEM images shown in Figure 12. The spherical molecules started tovisible join with in un-compatibilized each other due to samples greater dueinteractions to the drawing between of the weakly same adhered types of molecules moleculesshaped beads to stabilize belongand to the the less system PP physical polymer, [34]. interactions. and the main Morphology matrix is PET. of compatibilized These spherical blend beads of are PP clearly and PET blends seen Fractureto be debonded analysisshowed offrom blends the smaller wasmatrix carried bead material size out using as due compare SEMto the images to lack uncompatibilized ofshown interfacial in Figure adhesion blend 12. The owing in spherical an to un the‐ greater compatibilizedshaped beads belong blend.physical to Many the PP holes interactions polymer, are clearly and present. the visible main Spherical in matrix un‐compatibilized shapedis PET. These beads samplesspherical of PP polymer owing beads to noware the clearly appeared pull‐ to be outseen of to these be debondedweakly adheredattached from polymers.the to thematrix matrix Functionalized material material due which PP/PETto the is PET lack blends’ by of developing interfacial morphology bridges. adhesion showed The in a appeared smalleran un‐ physical beadcompatibilized size due to blend. greaterinteractions Many interaction. holes are are Spherical due clearly to dipole-dipole visible shaped in beads un‐ attractioncompatibilized now seemed forces tosamples among be adhered owing the carbonyl to to the the matrix pull group‐ of PET byout forming of these weaklybridges. adhered andThese the interactions polymers. maleic anhydride Functionalized are due groupto dipole PP/PET grafted‐dipole blends’ on attractions PP chains morphology between present showed asPET’s compatibilizer. acarbonyl smaller There groupbead size and due the tomaleic greaterare anhydride planeinteraction. surface group Spherical and in PP. fibrils In shaped the extensions fractured beads on now surface the seemed fractured of compatibilized to be side adhered of compatibilized blends, to the matrix fibrils blends that byextension forming as bridges. well as showplaneThese thesurfaceinteractions compatibilized fracture are weredue PP/PET to analyzed, dipole blend‐dipole so is it moderately isattractions illustrated ductile between that the material PET’s functionalized [carbonyl32]. An optimum groupPP/PET and is themoderately maleicconcentration anhydride ductile ingroup ofnature MAH-g-PP in PP. [32]. In theAn used fractured optimum as compatibilizer surface amount of compatibilizedof provided compatibilizer partial blends, homogeneous provided fibrils blend extensionhomogeneous as well PET/PP as ofplane blend PP/PET surface with with fewerfracture lesser phase phasewere separations. analyzed, separations. so it is illustrated that the functionalized PP/PET is moderately ductile in nature [32]. An optimum amount of compatibilizer provided homogeneous PET/PP blend with fewer phase separations.

(a) (b)

Figure 12.FigureScanning 12. Electron SEM images Microscopy of fracture (SEM) analysis images of of 60% fracture PET analysis (a) 5% ofMAH 60%‐g PET‐PP/PET (a) 5% blends MAH-g-PP/PET and (b) un ‐ blends and (b) un-compatibilizedcompatibilized PP/PET. PP/PET. (a) (b) Figure 12. SEM images of fracture analysis of 60% PET (a) 5% MAH‐g‐PP/PET blends and (b) un‐ compatibilized PP/PET.

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3.6. Dynamic Mechanical Analysis 3.6. Dynamic Mechanical Analysis Figure 13 shows the temperature dependence on the tan delta of PET/PP compatibilized blends Figure 13 shows the temperature dependence on the tan delta of PP/PET compatibi- in comparison to pure PP and pure PET. lized blends as compare to pure PP and pure PET.

Figure 13. Variations in the tan delta by increasing temperature for prepared samples analyzed by Figure 13. Variations in tan delta with temperature for all samples measured by Dynamic Mechanical Dynamic Mechanical Analyzer (DMA) dual cantilever in bending mode. Analyzer (DMA) dual cantilever in bending mode. A sharp prominent peak appeared at around 85 ◦C in the graph of pure PET that Pure PET showed a sharp prominent peak at about 85 °C, which is the Tg of PET and is almost represents the Tg of pure PET and it is nearly the same as the data obtained from the equal to the data obtainedanalysis from DSC. of DSC. Pure Two PP smallshowed shoulders two small are shoulders visible in at the −15 graph °C and of 70 pure °C. PPAt − at15− 15 ◦C and ◦ ◦ °C, which is the Tg of isotactic70 C. At polypropylene,−15 C, which β transitions is the Tg of occur isotactic in PP. polypropylene, At higher temperature,β transitions 70 occur°C, in PP. At α transition occurred, whichhigher is temperature,related to the PP 70 crystalline◦C, α transition fracture occurred, [35]. However which pure is related PET has to greater the PP crystalline area under the peak infracture comparison [35]. Howeverwith PP; hence, pure PET PET showed has a higher larger ability area under to dissipate the peak energy in comparison on with application of load in purecomparison PP curve; with hence, pure PET PP. has PP a higherchains abilityshowed to dissipateelasticity energyin structure, on application which of load in indicates more load storagecontrast ability with purethan PP.dissipation. The chains From of PP the displayed compatibilized elasticity blends, in material the partial structure, which miscibility of the componentsindicates was more clear load as the storage peak started ability to than merge. dissipation. The low‐ Thetemperature partial miscibility peak in all of both the blends started to movecomponents towards the in high PET/PP‐temperature blends ispeak. evident This as behavior the peak is started due to tothe merge. addition In all of blends, the peak at low-temperature is started to shift near the high-temperature peak. This behavior compatibilizer in PET and PP, which allowed physical interaction between the components and the is because of the presence of MAH-g-PP in PET and PP blends, which allowed physical formation of partially homogenous blends. The peak area also increased in all blends in comparison to interaction between the components and the partially homogenous blends’ formation. The pure PP, which indicated good impact bearing properties [36–38]. Figure 14 explains the effect of area under the peak also enhanced in all prepared blends as compared to pure PP, which temperature on the storagerevealed modulus high of impact compatibilized bearing properties blends in comparison of blends [36 to–38 pure]. Figure PET and 14 clariﬁespure PP. the effect of temperature on the value of storage modulus of prepared compatibilized blends in contrast to pure PP and pure PET. In pure PET, a lowest value of storage modulus can be seen and which was noted to be regularly decreased by increasing temperature. It was observed that within this temperature range, PET did not show melting but only a slight softening by increasing temperature, while crystal melting was observed in long-range temperature. Furthermore, PP exhibited a higher value of storage modulus as compare to pure PET; however, a sudden decrease in the value of storage modulus of PP at 0 ◦C appeared proceeded with a high variation in G0 due to β transition in PP. By adding compatibilizer in the PET/PP blend, the value of storage modulus rose in contrast to both pure PP and PET. This increase in storage modulus indicates a rise in the stiffness of the polymer due to a restriction in the segmental motion. This raise indicates a high physical interaction and improved compatibility between PP and PET chains. By increasing the concentration of MAH-g-PP, the storage was not pronounced owing to the presence of uneven physical interaction, which eventually led to improper PET/PP phase adhesion. The presence of MAH-g-PP decreased the transition region in the blend of PP and PET. It is worth to notice that with

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3.6. Dynamic Mechanical Analysis Figure 13 shows the temperature dependence on the tan delta of PET/PP compatibilized blends in comparison to pure PP and pure PET.

Figure 13. Variations in tan delta with temperature for all samples measured by Dynamic Mechanical Analyzer (DMA) dual cantilever in bending mode.

Pure PET showed a sharp prominent peak at about 85 °C, which is the Tg of PET and is almost equal to the data obtained from DSC. Pure PP showed two small shoulders at −15 °C and 70 °C. At −15 °C, which is the Tg of isotactic polypropylene, β transitions occur in PP. At higher temperature, 70 °C, α transition occurred, which is related to the PP crystalline fracture [35]. However pure PET has greater Polymers 2021, 13,area 495 under the peak in comparison with PP; hence, PET has a higher ability to dissipate energy on 14 of 17 application of load in comparison with pure PP. PP chains showed elasticity in structure, which indicates more load storage ability than dissipation. From the compatibilized blends, the partial miscibility of the components was clear as the peak started to merge. The low‐temperature peak in all blends started to themove rise towards in temperature, the high‐temperature the prepared peak. samples This behavior appeared is due to obtainto the addition a small differenceof in the compatibilizer in storagePET and modulus PP, which value, allowed and physical curves interaction started tobetween come the close. components With increasing and the temperature, formation of partiallypolymer homogenous chains startedblends. toThe move, peak area and also softness increased in samples in all blends appeared. in comparison It is thus to concluded that pure PP, which indicatedthe compatibilizer good impact increased bearing theproperties value of[36–38]. storage Figure modulus 14 explains of the the blend, effect which of approaches temperature on theto storage high stiffness modulus in of samplescompatibilized due to blends the annealing in comparison of the to pure ﬁlms PET at room and pure temperature PP. [36,37].

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Figure 14. Variations in storage modulus with temperature for all samples measured by DMA dual cantilever in bending mode.

Pure PET showed a minimum value of storage modulus, and this value was observed to regularly decrease with temperature. In this temperature range, PET did not melt, with a slight softening with increasing temperature, while crystal melting was observed in long‐range temperature. Furthermore, PP showed a higher storage modulus than pure PET; however, PP showed a sudden fall in storage modulus at 0 °C, and a high variation in G′ with temperature was noted due to β transition in PP. By the addition of MAH‐g‐PP in the PET/PP blend, storage modulus increased in comparison to both pure PET and pure PP. This increase in storage modulus indicates a rise in the stiffness of the polymer due to a restriction in the segmental motion. This raise indicates a high interaction and enhanced compatibility between PET and PP. By increasing the amount of

compatibilizer, storage was not pronounced due to uneven physical interaction, which eventually

led to improper phaseFigure adhesion. 14. Variations The compatibilizer in storage modulus reduced by increasing the transition temperature region in for PET prepared and PP samples measured blend. It is worth bynoticing DMA that dual with cantilever the increase in bending in temperature, mode. all samples started to achieve less difference in the value of storage modulus, and curves came close. With increasing temperature, chains started to move, andFigure softness 15 presents in samples the appeared. change It in is thus loss concluded modulus that by increasingthe compatibilizer temperature for the enhanced the storagecompatibilized modulus of the blends blend, which of PET/PP leads towards in contrast high stiffness to pure in PP samples and pure due to PET. the It displays the annealing of the filmsenergy at room dissipation temperature for prepared[36,37]. samples. The value of loss modulus and storage reduces Figure 15 presentsdue to the the variation smaller in force loss neededmodulus for with deforming temperature the for sample. PET/PP Initially, compatibilized all samples hinder the blends in comparisonsegmental to pure motionPET and of pure molecules, PP. It shows but withthe energy a rise dissipation in temperature, for all the samples. molecular The motion of this ◦ value of storage andtype loss is activated.modulus decreases From ﬁgure due to the the T gsmallerof PET force is 82 requiredC that for is nearly deforming the samethe value found ◦ sample. All samples,from initially, DSC analysis.resist molecular The T gsegmentalof pure PP motion, appeared but with at − increasing15 C. By temperature, forming a compatibilized ◦ these kinds of molecularblend ofmotion PET andare activated. PP, in CB1, PET the has T ga ofTg PPof 82 slightly °C, which shifted is almost to about the 10sameC, that and the Tg of PET was obtained fromwas DSC unnoticeable. analysis. Pure PP It can showed be inferred its Tg at − that15 °C. CB1 By has making improved a compatibilized compatibility blend, between PP and in CB1, the Tg of PPPET slightly components. moved to CB2 about and 10 CB3°C, and also the presented Tg of PET an was increase undetectable. in the It value can be of Tg of PP; and ◦ deduced that CB1 thesehas high two compatibility compatibilized between blends PET also and displayedPP phases. theCB2 T andg of CB3 PET also at 90 showedC temperature an [36,37]. increase in the Tg of PP; on the other hand, these two blends also showed the Tg of PET at 90 °C [36,37].

Figure 15. VariationsFigure in 15. lossVariations modulus in with the valuetemperature of loss for modulus all samples with measured increasing by temperature DMA dual for prepared samples cantilever in bendingmeasured mode. by DMA dual cantilever in bending mode.

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4. Conclusions In this study, grafting of PP by MAH was carried out using a torque rheometer. Variations in PP structure during the reaction and after grafting were studied by torque evolution and flow behavior. It was found that the highest percentage of grafting was achieved at 0.2 phr MAH and 0.4 phr BPO, since increasing the amount of MAH and BPO from the said values started side-chain reactions and crosslinking. However, at high grafting percentage, the molecular weight decreased, and lower viscosity at high flow rate was observed. This decrease in viscosity is due to chain scission in the free radical polymerization reaction. The high amount of BPO favors more side-chain reactions, which is why the amount of BPO should be controlled to less than 0.4 phr. Chains breakage caused an increase in percentage crystallinity, which was found by heat of fusion of MAH-g-PP samples. Grafting on PP chains also showed a slight change in melting temperature (1 ◦C to 3 ◦C) analyzed by DSC thermograms owing to chains breakage. The study showed that free radical polymerization yielded a high grafting percentage at the expense of molecular weight. Side reactions occurred that caused structural changes that eventually effected the flow behavior of PP. MAH-g-PP provided excellent compatibilization for synthesizing homogeneous PET and PP 60/40 ratio. However, with an increase in the amount of MAH- g-PP greater than 1% in 60/40 PET and PP ratio, agglomeration started to appear, reducing the compatibility between the phases.

Author Contributions: Conceptualization, A.T. and N.M.A.; Methodology, A.T.; Software, A.T. and M.F.S.; Validation, N.M.A. and A.E.; Formal Analysis, A.T.; Investigation, A.T.; Resources, N.M.A. and Z.A.; Data Curation, A.T.; Writing-Original Draft Preparation, A.T.; Writing-Review & Editing, A.T. and S.R.; Visualization, A.T. and Z.K.; Supervision, N.M.A.; Project Administration, M.A.A.; Funding Acquisition, N.M.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Higher Education Commission Pakistan under NRPU Project no. 3526 and 6020. Acknowledgments: This research was assisted by the Department of Polymer Engineering, National Textile University, Faisalabad, Pakistan, and the Department of Chemical Engineering, COMSATS University Islamabad CUI, Lahore Campus, Pakistan, by providing lab facilities. We would also like to express our thanks to the School of Chemical and Materials Engineering, National University of Science and Technology, Islamabad, Pakistan, for assisting during the course of this research. Nasir M. Ahmad acknowledges the support of HEC NRPU Project no. 6020. Conﬂicts of Interest: The authors have no conﬂict of interest.

Abbreviations

PP Polypropylene MAH Maleic anhydride MAH-g-PP Maleic anhydride grafted polypropylene BPO Benzoyl peroxide Phr Parts per hundred DCP Dicumyl peroxide DSC Differential scanning calorimetry HDPE High density polyethylene SEM Scanning electron microscope DMA Dynamic mechanical analyzer Mn Number average molecular weight PET Polyethylene terephthalate CI Carbonyl index Tm Melting temperature FTIR Fourier-transform infrared spectroscopy Tg Glass transition temperature Polymers 2021, 13, 495 16 of 17

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