polymers 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 polymer- ization 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 through- out 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.