(PP)/Waste EPDM Thermoplastic Elastomers Using Ultrasonically Aided Twin-Screw Extrusion

polymers

Article Manufacturing Polypropylene (PP)/Waste EPDM Thermoplastic Elastomers Using Ultrasonically Aided Twin-Screw Extrusion

Hui Dong, Jing Zhong and Avraam I. Isayev *

Department of Polymer Engineering, The University of Akron, Akron, OH 44325-0301, USA; [email protected] (H.D.); [email protected] (J.Z.) * Correspondence: [email protected]

Abstract: The compounding of waste EPDM from postindustrial scrap with polypropylene (PP) is a possible way to manufacture thermoplastic elastomers to solve a signiﬁcant environmental problem. Accordingly, the present study considers the one-step (OS), two-step (TS), and dynamic revulcanization (DR) compounding methods for the manufacturing of PP/EPDM blends at different ratios of components with the aid of an ultrasonic twin-screw extruder (TSE) at various ultrasonic amplitudes. In the OS method, PP and waste EPDM particles were directly compounded using TSE with and without ultrasonic treatment. In the TS and DR methods, the waste EPDM particles were fed into the TSE and devulcanized without and with ultrasonic treatment. Then, in the TS method the devulcanized EPDM was compounded with PP using TSE without the imposition of ultrasound. In the DR method, the devulcanized EPDM after compounding with curatives was mixed with PP and dynamically revulcanized without the imposition of ultrasound in TSE. The die pressure during compounding was recorded and correlated with the rheological properties of compounds. The mechanical properties of the PP/EPDM blends obtained in the OS and TS methods did not show any improvement with ultrasonic treatment. In the DR method, all the PP/EPDM blends showed a signiﬁcant increase in the tensile strength and elongation with ultrasonic amplitude and a slight decrease in the Young’s modulus. In particular, a tensile strength of 30 MPa and an elongation at break of 400% were achieved at an ultrasonic amplitude of 13 µm for the PP/EPDM blend at a ratio Citation: Dong, H.; Zhong, J.; Isayev, of 75/25. The complex viscosity, storage, and loss moduli of dynamically revulcanized PP/EPDM A.I. Manufacturing Polypropylene (PP)/Waste EPDM Thermoplastic blends increased with the ultrasonic amplitude while the loss tangent decreased. At the same time, Elastomers Using Ultrasonically the results for the blends obtained by the OS and TS methods showed an opposite trend in the Aided Twin-Screw Extrusion. dynamic property behavior with the ultrasonic amplitude. Optical micrographs indicated that the Polymers 2021, 13, 259. https:// blends obtained by the DR method at an ultrasonic treatment at 13 µm showed the lowest sizes of doi.org/10.3390/polym13020259 dispersed revulcanized EPDM particles in the PP matrix, leading to the excellent performance of these thermoplastic elastomers. Received: 19 December 2020 Accepted: 9 January 2021 Keywords: ultrasound; extrusion; rheology; polypropylene; EPDM; elastomer Published: 14 January 2021

Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- 1. Introduction ms in published maps and institutional affiliations. The manufacturing of thermoplastic elastomers (TPEs) based on blends of plastics with rubbers is cost-effective and practically valuable. TPEs are materials that exhibit both thermoplastic and elastomeric properties. TPEs have advantages such as ease of processing as well as the possibility to tailor properties by changing the ratio of the components. Copyright: © 2021 by the authors. Li- They also display disadvantages, including softening and losing their rubbery behavior at censee MDPI, Basel, Switzerland. elevated temperatures [1]. This article is an open access article Many TPEs are obtained by the dynamic vulcanization. During this process, elastomer distributed under the terms and con- particles are vulcanized and dispersed in the thermoplastic matrix during vulcanization. ditions of the Creative Commons At- The properties of the blends are improved, with a reduction in the size of the elastomer tribution (CC BY) license (https:// particles. The process was first introduced in 1962 [2]. The first TPEs based on a crosslinked creativecommons.org/licenses/by/ rubber-thermoplastic composition were derived in 1973 by partially crosslinking the EPDM 4.0/).

Polymers 2021, 13, 259. https://doi.org/10.3390/polym13020259 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 259 2 of 19

with peroxide in a PP matrix [3]. Important enhancements in the properties of these blends were achieved in 1978 by fully vulcanizing the rubber phase while maintaining the processability of the thermoplastics [4]. In 1982, these blends were further improved by the use of phenolic resin as a curative, improving the rubber-like properties and the processing features [5]. Possible applications of thermoplastic elastomers and thermoplastic vulcanizates are ﬂexible diaphragms, bumpers, seals, plugs, and wire and cable insulation. Due to these applications, research on novel PP/EPDM thermoplastic vulcanizates remains active nowadays, as is evident from the recent publications on the topic [6–10]. At the beginning of this century, a novel method was proposed to enhance the properties of plastic/plastic, plastic/rubber, and rubber/rubber blends by means of ultrasonic compatibilization [11–14]. It was found that during mixing with the aid of ultrasound, the compatibilization and interfacial adhesion between diverse phases are improved. Research on the in situ compatibilization of PP/EPDM blends using ultrasound-aided extrusion was also conducted [15]. The mechanical properties of the compression-molded samples of TPEs made from ultrasonically treated PP/EPDM were improved due to the formation of a copolymer during ultrasonic treatment. Ultrasound was also successfully used to prepare the TPE consisting of PP and virgin EPDM [16]. In another study [17], maleic anhydride grafted PP was compounded with waste EPDM powder of sizes between 5 and 20 µm using a twin screw extruder with an ultrasonic horn at the exit. The study showed that ultrasonic treatment led to improved performance properties. However, random co-PP/waste EPDM blends showed the deterioration of the properties after ultrasonic treatment. Therefore, it is important to search new way to develop a process for manufacturing TPEs from blends of PP/waste EPDM powder. In the current study, this was accomplished by using the ultrasonic twin screw extruder (TSE) recently developed in our laboratory, in which the ultrasonic horn was installed in the barrel. The effect of ultrasonic treatment on the mechanical and rheological properties of the PP/waste EPDM blends prepared using several methods was studied using this ultrasonic TSE. A method to obtain TPEs based on PP/waste EPDM powder with a good mechanical performance was proposed.

2. Experimental 2.1. Materials The PP used for preparing PP/waste EPDM blends was metallocene-based polypropylene PP3825 supplied by the Exxon Chemical Company. Its melt ﬂow rate is 32g/10min and its molecular weight is 144,800 [18]. Waste EPDM rubber from the post-industrial scrap of 40 meshes (MD-184-EPDM) manufactured by Lehigh Technologies, Inc. (Tucker, GA, USA), was used.

2.1.1. Ultrasonic Twin-Screw Extruder Devulcanization and compounding was carried out using an ultrasonic co-rotating TSE (Prism USALAB 16, Thermo Electron Co., Waltham, MA, USA). A schematic of this ultrasonic TSE is shown in Figure1. Two screws with a diameter of 16 mm were used to convey and mix the EPDM rubber with PP. Ultrasonic waves with a frequency of 40 kHz were applied to the EPDM rubber by a horn mounted in the barrel. The horn has a 28 mm × 28 mm square cross-section and was connected to a booster and a converter. The converter was driven by a Branson 2000bdc power supply (Branson Ultrasonic Co., Danbury, CT, USA). The gap between the horn and screws was 2.5 mm, and the volume of the ultrasonic treatment zone was 1.9 cm3. The circular die of the extruder had a diameter of 4 mm and a length of 11 mm. The ultrasonic horn was cooled by water at 45 ◦C coming from a thermostat (GP-100, NESLAB Instruments Inc., Newington, NH, USA). Additionally, the converter was cooled by compressed air. Figure2 shows the schematics of two screw designs Figure2a,b. The screw design (Figure2a) contained conveying and kneading elements and cylindrical elements in the ultrasonic treatment zone. The screw design (Figure2b) was a mixing design containing conveying and kneading elements without the ultrasonic treatment zone. Polymers 2021, 13, x FOR PEER REVIEW 3 of 19 Polymers 2021, 13, x FOR PEER REVIEW 3 of 19

CT, USA). The gap between the horn and screws was 2.5 mm, and the volume of the ul- CT, USA). The gaptrasonic between treatment the horn zone and wasscrews 1.9 wascm3 . 2.5The mm, circular and thedie volumeof the extruder of the ul- had a diameter of 4 3 trasonic treatmentmm zone and was a length 1.9 cm of. The11 mm. circular The ultrasonicdie of the hornextruder was hadcooled a diameter by water of at 445 °C coming from mm and a length ofa 11thermostat mm. The (GP-100,ultrasonic NESLAB horn was Instruments cooled by water Inc., atNewington, 45 °C coming NH, from USA). Additionally, a thermostat (GP-100,the converter NESLAB was Instruments cooled by Inc., compressed Newington, air. FigureNH, USA). 2 shows Additionally, the schematics of two screw the converter was designscooled byFigure compressed 2a,b. The air. screw Figure design 2 shows (Figur thee 2a) schematics contained of convey two screwing and kneading ele- designs Figure 2a,b.ments The andscrew cylindrical design (Figur elementse 2a) incontained the ultrason conveyic treatmenting and kneading zone. The ele- screw design (Figure ments and cylindrical elements in the ultrasonic treatment zone. The screw design (Figure Polymers 2021, 13, 259 2b) was a mixing design containing conveying and kneading elements without the3 of ultra- 19 2b) was a mixing designsonic treatment containing zone. conveyin g and kneading elements without the ultrasonic treatment zone.

FigureFigure 1. 1. SchematicSchematic of of the the ultrasonic ultrasonic twin twin screw screw extruder. extruder. Figure 1. Schematic of the ultrasonic twin screw extruder.

FigureFigure 2. Schematics 2. Schematics of theof the ultrasonic ultrasonic (a) and(a) and mixing mixing (b) ( screws.b) screws. The The flow flow direction direction is from is from left left to right. to right. Figure 2. Schematics of the ultrasonic (a) and mixing (b) screws. The flow direction is from left to right. 2.1.2.2.1.2. Preparation Preparation of PP/Wasteof PP/Waste EPDM EPDM TPEs TPEs 2.1.2. Preparation of PP/Waste EPDM TPEs ThreeThree different different methods methods to manufactureto manufacture PP/waste PP/waste EPDM EPDM thermoplastic thermoplastic elastomers elastomers Three differentwerewere used.methods used. The Theto first manufacture first method method is PP calledis /wastecalled the EPDMthe one-step one-step thermoplastic (OS) (OS) method. method. elastomers In thisIn this method, method, the the PP PP were used. Thepellets firstpellets method were were fed is fed intocalled into the the the hopper on hoppere-step of theof(OS) the TSE method. TSE using using aIn feeder athis feeder method, (K-Tron (K-Tron the Soder, Soder,PP Houston, Houston, TX, TX, pellets were fedUSA), intoUSA), the and andhopper the the waste wasteof the EPDM TSEEPDM wasusing was also a alsofeeder fed fed into (K-Tron into the the same Soder, same hopper Houston,hopper using using TX, a high-precision a high-precision USA), and the twin-screwwastetwin-screw EPDM feeder feederwas (MT-2,also (MT-2, fed Brabender intoBrabender the Technologiesame Techno hopperlogie GmbH using GmbH &a Co.high-precision& Co. KG, KG, Duisburg, Duisburg, Germany). Germany). twin-screw feederTheThe (MT-2, screw screw Brabender design design shown shown Techno in in Figurelogie Figure GmbH2a 2a was was & used Co. used KG, without without Duisburg, and and with Germany).with ultrasonic ultrasonic treatment treatment at The screw designat amplitudesamplitudesshown in Figure of 7.5, 2a 10, was and used 13 µμwithoutm. The andtotal total with flow flow ultrasonic rate rate was was 8 8treatment g/min. g/min. The Theat screw screw speed speed was amplitudes of 7.5,was 10, 200 and rpm, 13 andμm. theThe zonetotal temperaturesflow rate was from8 g/min. the The entrance screw of speed the extruderwas to the die 200 rpm, and the zone temperatures◦ from the entrance of the extruder to the die were 200 rpm, and thewere 150/180/190/190/190/190zone 150/180/190/190/190/190 temperatures from °C.the TheentranceC. extrudates The extrudatesof the extruderfrom from extruders to extruders the diewere were were cooled, cooled, dried, dried, and 150/180/190/190/190/190andpelletized pelletized °C. usingThe using extrudatesa apelletizer pelletizer from (Scheer (Scheer extruders Bay Company,Company, were cooled, BayBay City, City,dried, MI, MI, and USA). USA). The The second second pelletized usingmethod amethod pelletizer is calledis called (Scheer the the two-step Baytwo-step Company, (TS) (TS) method. method. Bay InCity, thisIn this MI, method, method, USA). waste Thewaste EPDMsecond EPDM was was fed fed into into an an method is calledextruder the two-step equipped (TS) withmethod. the screwIn this design method, depicted waste EPDM in Figure was2a fed at a into flow an rate of 8 g/min and devulcanized without and with ultrasonic treatment at amplitudes of 7.5, 10, and 13 µm. Then, PP/devulcanized EPDM blends of 75/25, 50/50, and 25/75 compositions were physically mixed and fed to the TSE using the screw design depicted in Figure2b at a flow rate of 8 g/min without the ultrasonic treatment. The screw speed and zone temperatures were same as in the OS method. The third method is called the dynamic revulcanization (DR) method. In this method, waste EPDM was fed into the extruder using the screw design depicted in Figure2a at a flow rate of 8 g/min and devulcanized without and with ultrasonic treatment at amplitudes of 7.5, 10, and 13 µm. After extrusion, the devulcanized EPDM rubber was compounded with curatives using a two-roll mill Polymers 2021, 13, 259 4 of 19

(Reliable Rubber & Plastic Machinery Co., North Bergen, NJ, USA) with a gap size of 5 mm. A rotor speed of 20 rpm and a cooling water temperature of 40 ◦C were used. Then, it was crushed into particles using a grinder (Weima). The compounding recipe was as follows: 100 phr devulcanized EPDM rubber, 1 phr sulfur, 2 phr zinc oxide, 1 phr stearic acid, 0.75 phr TMTD and 0.375 phr MBT. All the curatives were supplied by Akrochem Corporation (Akron, OH, USA). Finally, PP/devulcanized EPDM blends of 75/25, 50/50, and 25/75 compositions were physically mixed and then fed to the TSE using the screw design depicted in Figure2b at a ﬂow rate of 8 g/min without the imposition of ultrasound. The screw speed and zone temperatures were same as in the OS method. The extrudates were then cooled, dried, and ﬁnally pelletized using a pelletizer (Scheer Bay Company, Bay City, MI, USA). The size of these particles was about 2 mm.

2.2. Characterization Methods 2.2.1. Tensile Test PP/EPDM tensile bars at ratios of 75/25 and 50/50 were prepared using a mini-jet injection molding machine (DSM Research B.V). A cylinder temperature of 200 ◦C and a mold temperature of 60 ◦C at a pressure of 40 MPa were used. The PP/EPDM blends at a ratio of 25/75 could not be prepared by injection molding due to their high viscosity. Thus, tensile samples of these PP/EPDM blends were prepared by a compression molding press (Carver Inc., Wabash, IN) at 180 ◦C under a pressure of 13.8 MPa with a mold of dimensions 90 mm × 60 mm × 1.5 mm. The tensile tests of pure PP and PP/EPDM blends were conducted by an Instron Ten- sile Tester 5567 at room temperature at an elongation rate of 50 mm/min. An extensometer was not used. The tests were operated according to the ASTM D638 standard.

2.2.2. Rheological Tests A small amplitude oscillatory shear (SAOS) test of the pure PP and PP/EPDM blends at ratios of 75/25 and 50/50 at a temperature of 180 ◦C was conducted by using a stress- controlled Discover Hybrid Rheometer (DHR-2, TA Instruments, New Castle, DE, USA) equipped with 25 mm parallel plates. Specimens of pure PP and PP/EPDM blends were molded at 180 ◦C under a pressure of 25 MPa for 5 min using a compression molding press (Carver Inc.). The samples had a diameter of 25 mm and thickness of 2 mm. For PP/EPDM blends at a ratio of 75/25, the frequency sweep was in the range of 0.1 to 200 rad/s at a stress amplitude from 4 to 40 Pa. It was conﬁrmed that the stress amplitudes were within the linear region. The rheological properties of the PP/EPDM blends at ratio of 25/75 cannot be tested by DHR-2 due to the wall slip. The rheological properties of these blends were measured using the Advanced Polymer Analyzer (APA 2000, Alpha Technologies, Akron, OH, USA) at 180 ◦C and a strain amplitude of 4.2% within a frequency range from 0.01 to 200 rad/s.

2.2.3. Morphological Tests The sizes of the original EPDM powder and the phase morphology of the PP/EPDM blends were observed using an optical transmission microscopy (Laborlux 12 POL S, Leitz Ltd., Midland, ON, Canada). Thin ﬁlms of about 20 µm were cut using a microtome (Model 820, Reicher-Jung GmbH, Nussloch, Germany) from an injection molded dumbell sample. The images were captured by a camera and then analyzed using the ImageJ software.

Ltd., Midland, ON, Canada). Thin films of about 20 μm were cut using a microtome (Model 820, Reicher-Jung GmbH, Nussloch, Germany) from an injection molded dumbell sample. The images were captured by a camera and then analyzed using the ImageJ software.

3. Results and Discussion 3.1. Die Pressure The pressure before the die during the twin-screw extrusion of PP/waste EPDM blends from TSE as a function of the ultrasonic amplitude at various PP/EPDM ratios is presented Figure 3a–c for the OS, TS, and DR methods, respectively. For the OS and TS Polymers 2021, 13, 259 methods, the die pressure decreased with the increasing concentration of PP due5 of 19 to the lower viscosity of PP than that of EPDM rubber. The die pressure also reduced with an increase in the ultrasonic amplitude, indicating the better processability of the melts under ultrasonican increase treatment. in the ultrasonic The decrease amplitude, in the indicating die pressure the better with processability amplitude in of the the OS melts method wasunder due to ultrasonic the degradation treatment. of The PP and decrease the devulcanization in the die pressure of withEPDM amplitude caused by in the OSacoustic cavitation.method wasFor duethe toTS the method degradation Figure of 3b, PP the and devulcanization the devulcanization of waste of EPDM EPDM caused led byto a re- the acoustic cavitation. For the TS method Figure3b, the devulcanization of waste EPDM duction in the complex viscosity, which caused a reduction in the die pressure. However, led to a reduction in the complex viscosity, which caused a reduction in the die pressure. forHowever, the DR method for the shown DR method in Figure shown 3c, in the Figure die 3pressurec, the die increased pressure increasedwith an increase with an in the ultrasonicincrease amplitude. in the ultrasonic During amplitude. the dynamic During revulcanization, the dynamic revulcanization, the chemical the network chemical gener- atednetwork by the generated curing of by the the EPDM curing phase of the EPDM in the phase blends in theled blendsto the ledhigher to the complex higher complex viscosity of theviscosity blends, ofasthe shown blends, below. as shown below.

FigureFigure 3. Die 3. pressureDie pressure as asa function a function of of the the ultrasonic amplitude amplitude of theof the PP/EPDM PP/EPDM blends blends prepared prepared by the OSby (athe), TS OS (b ),(a and), TS (b), and DRDR ( (cc)) methods. methods. DieDie pressure pressure for for pure pu PPre PP is also is also indicated. indicated.

3.2.3.2. Dynamic Dynamic Properties Properties Figure4 shows the complex viscosity (Figure4a), storage (Figure4b) and loss (Figure4c) moduli, and tangent loss (Figure4d) as a function of the frequency at various ultrasonic amplitudes for pure PP, devulcanized EPDM, and PP/EPDM blends at ratios of 75/25, 50/50, and 25/75 from TSE using the OS method. Firstly, the complex viscosity of the blends increased tremendously with the increasing EPDM concentration. According to the earlier study [11], this occurs due to the higher viscosity of EPDM rubber than that of PP, leading to the higher viscosity of PP/EPDM blends. This is also in accordance with the higher die pressure of blends, as shown in Figure3a. Secondly, it is seen that the complex viscosity of PP decreased the with increasing ultrasonic amplitude due to the degradation of PP. Moreover, it can be seen that the complex viscosity of PP/EPDM blends decreased with the increasing ultrasonic amplitude at different weight ratios of PP/EPDM blends. The complex viscosity was affected by two factors: the devulcanization of EPDM and the degradation of Polymers 2021, 13, x FOR PEER REVIEW 6 of 19

Figure 4 shows the complex viscosity (Figure 4a), storage (Figure 4b) and loss (Figure 4c) moduli, and tangent loss (Figure 4d) as a function of the frequency at various ultrasonic amplitudes for pure PP, devulcanized EPDM, and PP/EPDM blends at ratios of 75/25, 50/50, and 25/75 from TSE using the OS method. Firstly, the complex viscosity of the blends increased tremendously with the increasing EPDM concentration. According to the earlier study [11], this occurs due to the higher viscosity of EPDM rubber than that of PP, leading to the higher viscosity of PP/EPDM blends. This is also in accordance with the higher die pressure of blends, as shown in Figure 3a. Secondly, it is seen that the complex viscosity of PP decreased the with increasing ultrasonic amplitude due to the degradation of PP. Moreover, it can be seen that the complex viscosity of PP/EPDM blends decreased with the increasing ultrasonic amplitude at different weight ratios of PP/EPDM Polymers 2021, 13, 259 blends. The complex viscosity was affected by two factors: the devulcanization6 of 19EPDM and the degradation of PP under ultrasonic treatment. Specifically, the complex viscosity of ultrasonically treated PP/EPDM blends at a ratio of 75/25 and at an amplitude of 13 μm decreasedPP under significantly. ultrasonic treatment. It was even Specifically, lower thethan complex the complex viscosity viscosity of ultrasonically of the untreated treated PP. ThePP/EPDM storage (Figure blends at 4b) a ratio and of loss 75/25 (Figure and at 4c) an amplitudemoduli decreased of 13 µm decreaseddramatically significantly. at an ampli- tudeIt was of 13 even μm lower at all thanratios the of complex PP/EPDM viscosity blends. of The the untreated loss tangent PP. The(Figure storage 4d) (Figure increased4b) with ultrasonicand loss (Figureamplitude,4c) moduli with decreasedthe highest dramatically value being at an at amplitude an amplitude of 13 µ ofm at13 all μm. ratios These of ob- servationsPP/EPDM were blends. due The to the loss fact tangent that (Figurethe high4d)er increased ultrasonic with amplitude ultrasonic caused amplitude, a higher with level µ of thedevulcanization highest value being of EPDM at an amplitudeand more ofdegradation 13 m. These of observations PP. Both the were ultrasonic due to the devulcani- fact that the higher ultrasonic amplitude caused a higher level of devulcanization of EPDM and zation of EPDM and PP degradation are considered to be induced by acoustic cavitation more degradation of PP. Both the ultrasonic devulcanization of EPDM and PP degradation [19,20].are considered to be induced by acoustic cavitation [19,20].

FigureFigure 4. Complex 4. Complex viscosity viscosity (a), ( astorage), storage modulus modulus ( (bb),), loss loss modulus ( c(),c), and and loss loss tangent tangent (d) ( asd) aas function a function of the of frequency the frequency for thefor pure the pure PP, devulcanized PP, devulcanized EPDM, EPDM, and and PP PP/EPDM/EPDM blends blends preparedprepared by by the the OS OS method. method.

Figure5 depicts the complex viscosity (Figure5a), storage (Figure5b) and loss Figure 5 depicts the complex viscosity (Figure 5a), storage (Figure 5b) and loss (Fig- (Figure5c) moduli, and tangent loss (Figure5d) as a function of the frequency at vari- ureous 5c) ultrasonic moduli, amplitudesand tangent for loss pure (Figure the PP, pure5d) as EPDM, a function and PP/EPDM of the frequency blends of theat various ratios ultrasonicof 75/25, amplitudes 50/50, and for 25/75 pure from the TSE PP, using pure the EPDM, TS method. and PP/EPDM Compared blends to the OS of method,the ratios of 75/25,the complex50/50, and viscosity 25/75 offrom the TSE blends using increased the TS tremendously method. Compared with the to increasing the OS method, EPDM the concentration due to the higher viscosity of EPDM rubber than that of pure PP, leading to the higher viscosity of PP/EPDM blends. This is also in accordance with the die pressure shown in Figure3b. Additionally, it can be seen that the complex viscosity of the PP/EPDM blends decreased with the increasing ultrasonic amplitude at various ratios of PP/EPDM blends. However, at this time the complex viscosity of the blends is just dependent on the extent of the devulcanization level of EPDM, because PP was not treated by ultrasound. Besides this, the storage (Figure5b) and loss (Figure5c) moduli decreased dramatically at an amplitude of 13 µm. The loss tangent (Figure5d) shows the highest value at an amplitude of 13 µm. These observations are due to the fact that, at such a high ultrasonic Polymers 2021, 13, x FOR PEER REVIEW 7 of 19

complex viscosity of the blends increased tremendously with the increasing EPDM concentration due to the higher viscosity of EPDM rubber than that of pure PP, leading to the higher viscosity of PP/EPDM blends. This is also in accordance with the die pressure shown in Figure 3b. Additionally, it can be seen that the complex viscosity of the PP/EPDM blends decreased with the increasing ultrasonic amplitude at various ratios of PP/EPDM blends. However, at this time the complex viscosity of the blends is just de- Polymers 2021, 13, 259 pendent on the extent of the devulcanization level of EPDM, because PP was not7 of treated 19 by ultrasound. Besides this, the storage (Figure 5b) and loss (Figure 5c) moduli decreased dramatically at an amplitude of 13 μm. The loss tangent (Figure 5d) shows the highest valueamplitude, at an amplitude more devulcanization of 13 μm. These of EPDM observations occurs, leadingare due to to a the lower fact value that, ofat thesuch gel a high ultrasonicfraction ofamplitude, devulcanized more EPDM devulcanization rubber [16,17]. of EPDM occurs, leading to a lower value of the gel fraction of devulcanized EPDM rubber [16,17].

FigureFigure 5. Complex 5. Complex viscosity viscosity (a),( astorage), storage moduli moduli (b (b),), loss loss moduli moduli ((cc),), andand loss loss tangent tangent (d )(d as) aas function a function of the of frequencythe frequency for for the purethe purePP, devulcanized PP, devulcanized EPDM, EPDM, and and PP/EP PP/EPDMDM blends blends prepared prepared byby the the TS TS method. method.

FigureFigure 6 6shows shows the complexcomplex viscosity viscosity (Figure (Figure6a), storage6a), storage (Figure (Figure6b) and 6 lossb) and(Figure loss6 (Figurec) 6c)moduli, moduli, and and tangent tangent loss loss (Figure (Figure6d) as a6 functiond) as a function of the frequency of the atfrequency various ultrasonic at various am- ultra- plitudes for the pure PP, devulcanized EPDM, and PP/EPDM blends of the ratios of 75/25, sonic amplitudes for the pure PP, devulcanized EPDM, and PP/EPDM blends of the ratios 50/50, and 25/75 from TSE using the DR method. In this case, the complex viscosity of the of 75/25,blends 50/50, still increased and 25/75 with from the increasing TSE using EPDM the DR concentration, method. In because this case, the the higher complex viscosity viscosity ofof EPDM the blends rubber comparedstill increased to that ofwith PP causedthe increasing the higher viscosityEPDM concentration, of the PP/EPDM because blends. the higherHowever, viscosity unlike of the EPDM OS and rubber TS methods, compared at all differentto that of ratios PP caused of the PP/EPDM the higher blends viscosity the of thecomplex PP/EPDM viscosity blends. increased However, with amplitudesunlike the atOS 10 an andd 13TSµ methods,m. The storage at all (Figure different6b) and ratios of theloss PP/EPDM (Figure6 c)blends moduli the also complex increased viscosity dramatically increased at an ultrasonic with amplitudes amplitude of at 13 10µ m.and The 13 μm. Theloss storage tangent (Figure (Figure 66b)d) hadand the loss lowest (Figure value 6c) at anmoduli ultrasonic also amplitude increased of dramatically 13 µm. According at an ul- trasonicto a previous amplitude study of [11 13], molecularμm. The loss transformations, tangent (Figure such as6d) the had formation the lowest of a copolymer,value at an ul- might occur during the dynamic revulcanization in the extrusion simultaneously with the trasonic amplitude of 13 μm. According to a previous study [11], molecular transfor- generation of the dense chemical network in the ultrasonically treated EPDM phase, leading mations,to an increase such as in the the formation viscosity. of Additionally, a copolyme ther, might compatibilization occur during of PP/EPDMthe dynamic and revulcan- the izationgood in dispersion the extrusion of EPDM simultaneously particles in the with blends the might generation lead to of an the increase dense in chemical viscosity [network15]. The latter is caused by the fact that in the DR method the EPDM particles were cured during the extrusion, preventing their agglomeration due to their high viscosity and poor flowability. This results in the good dispersion of dynamically revulcanized EPDM particles in the PP phase. Therefore, the relative contribution of these effects defines the viscosity of these blends. These effects are strongest at an amplitude of 13 µm due to the higher Polymers 2021, 13, x FOR PEER REVIEW 8 of 19

in the ultrasonically treated EPDM phase, leading to an increase in the viscosity. Addi- tionally, the compatibilization of PP/EPDM and the good dispersion of EPDM particles in the blends might lead to an increase in viscosity [15]. The latter is caused by the fact that in the DR method the EPDM particles were cured during the extrusion, preventing their Polymers 2021, 13, 259 agglomeration due to their high viscosity and poor flowability. This results in8 ofthe 19 good dispersion of dynamically revulcanized EPDM particles in the PP phase. Therefore, the relative contribution of these effects defines the viscosity of these blends. These effects are strongestlevel of devulcanizationat an amplitude of of EPDM 13 μm rubber, due asto reflectedthe higher by thelevel reduced of devulcanization storage modulus of ofEPDM rubber,devulcanized as reflected EPDM by rubber the reduced at the high storage amplitude. modulus of devulcanized EPDM rubber at the high amplitude.

FigureFigure 6. Complex 6. Complex viscosity viscosity (a), (a storage), storage modulus modulus ( b),), lossloss modulus modulus (c ),(c and), and loss loss tangent tangent (d) as (d a) function as a function of the frequencyof the frequency at at variousvarious ultrasonic ultrasonic amplitudes amplitudes for the purepure PP, PP, devulcanized devulcanized EPDM, EPDM, and and PP/EPDM PP/EPDM blends blends prepared prepared by the by DR the method. DR method.

FiguresFigures 7–97–9 showshow thethe complexcomplex viscosity viscosity (Figures (Figure7a,8 7a,a and Figure9a), storage 8a and (Figures Figure7b, 9a),8b storage and9b), and loss (Figures7c,8c and9c) moduli and loss tangent (Figures7d,8d and9d) (Figure 7b, Figure 8b and Figure 9b), and loss (Figure 7c, Figure 8c and Figure 9c) moduli as a function of the frequency for PP/EPDM blends of ratios of 75/25, 50/50, and 25/75 andat loss the ultrasonictangent (Figure amplitudes 7d, ofFigure 0 µm 8d (without and Figure ultrasonic 9d) treatment)as a function and 13of µthem fromfrequency TSE for PP/EPDMusing the blends OS, TS, of and ratios DR methods, of 75/25, respectively. 50/50, and Firstly,25/75 at anthe ultrasonic ultrasonic amplitude amplitudes of 0 µ ofm 0 μm (withoutthe complex ultrasonic viscosity, treatment) storage, and and loss 13 moduliμm from of PP/EPDM TSE using blends the OS, using TS, the and DR DR method methods, respectively.was only slightly Firstly, higher at an than ultrasonic those of amplitude PP/EPDM blendsof 0 μm using the thecomplex TS method. viscosity, The loss storage, andtangent loss moduli of PP/EPDM of PP/EPDM blends using blends the DRusing method the DR was method lower than was that only of PP/EPDM slightly higher blends than thoseusing of thePP/EPDM TS method. blends It can using be explained the TS method that the. chemicalThe loss networktangent generatedof PP/EPDM by curing blends us- ingin the the DR EPDM method phase was of the lower dynamically than that revulcanized of PP/EPDM PP/EPDM blends blendsusing obtainedthe TS method. from the It can DR method led to the better compatibilization between PP and EPDM rubber, resulting in be explained that the chemical network generated by curing in the EPDM phase of the a higher modulus and complex viscosity and a lower loss tangent. The complex viscosity, dynamicallystorage, and revulcanized loss moduli of PP/EPDM PP/EPDM blends blends usingobtained the OS from method the DR were method lower than led thoseto the bet- terof compatibilization the PP/EPDM blends between using thePP DRand and EPDM TS methods. rubber, The resulting loss tangent in a ofhigher the PP/EPDM modulus and complexblends viscosity using the and OS method a lower was loss higher tangent. than The that complex of the PP/EPDM viscosity, blends storage, using and the loss DR mod- uliand of PP/EPDM TS methods. blends Based onusing the previousthe OS method study [14 were], this lower can be explainedthan those by of the the fact PP/EPDM that blendscompatibilization using the DR between and TS PP methods. and EPDM The rubber loss tangent using the of DR the and PP/EPDM TS methods blends might using be the OSbetter method than was that inhigher the OS than method, that resultingof the PP/ in theEPDM higher blends moduli using and viscositythe DR ofand PP/EPDM TS methods. Basedblends on usingthe previous the DR and study TS methods. [14], this can be explained by the fact that compatibilization

Polymers 2021, 13, x FOR PEER REVIEW 9 of 19 Polymers 2021, 13, x FOR PEER REVIEW 9 of 19

Polymers 2021, 13, 259 9 of 19 between PP and EPDM rubber using the DR and TS methods might be better than that in thethe OSOS method,method, resultingresulting inin thethe higherhigher modulimoduli andand viscosityviscosity ofof PP/EPDMPP/EPDM blendsblends usingusing thethe DRDR andand TSTS methods.methods.

FigureFigure 7. 7.Complex Complex viscosity viscosity ( a(a),),), storage storagestorage modulus modulusmodulus ( b((b),),), loss lossloss modulus modulusmodulus ( c((c),),), and andand loss lossloss tangent tangenttangent ( d((d))) as asas a aa function functionfunction of ofof the thethe frequency frequencyfrequency at theat the ultrasonic ultrasonic amplitudes amplitudes of 0 ofµ m0 μ (withoutm (without ultrasonic ultrasonic treatment) treatment) and and 13 µ13m μ form for the the PP/EPDM PP/EPDM 75/25 75/25 blends blends prepared prepared by theby OS,the TS,OS, andTS, and DR methods.DR methods.

Figure 8. Complex viscosity (a), storage moduli (b), loss moduli (c), and loss tangent (d) as a function of the frequency at the ultrasonic amplitudes of 0 µm (without ultrasonic treatment) and 13 µm for the PP/EPDM 50/50 blends prepared by

the OS, TS, and DR methods. Polymers 2021, 13, x FOR PEER REVIEW 10 of 19

Figure 8. Complex viscosity (a), storage moduli (b), loss moduli (c), and loss tangent (d) as a function of the frequency at thePolymers ultrasonic2021, 13amplitudes, 259 of 0 μm (without ultrasonic treatment) and 13 μm for the PP/EPDM 50/50 blends prepared10 of 19 by the OS, TS, and DR methods.

Figure 9.Figure Complex 9. Complex viscosity viscosity (a), (storagea), storage moduli moduli (b (b),), loss loss moduli ( c(),c), and and loss loss tangent tangent (d) as(d a) functionas a function of the frequencyof the frequency at at the ultrasonicthe ultrasonic amplitudes amplitudes of 0 ofμm 0 µ (withoutm (without ultrasonic ultrasonic treatment) and and 13 13µm μ form thefor PP/EPDMthe PP/EPDM 25/75 25/75 blends blends prepared prepared by by the OS, theTS, OS, and TS, DR and methods. DR methods. Secondly, at an amplitude of 13 µm the complex viscosity, storage, and loss moduli ofSecondly, the PP/EPDM at an blends amplitude using the of DR13 μ methodm the werecomplex higher viscosity, than those storage, of the PP/EPDM and loss moduli of theblends PP/EPDM using the blends TS method. using Thethe lossDR tangentmethod of were the PP/EPDM higher than blends those using of thetheDR PP/EPDM blendsmethod using was the lower TS than method. that of theThe PP/EPDM loss tang blendsent of using the thePP/EPDM TS method. blends Again, using based the DR methodon the was previous lower study than [that15] it of can the be PP/EPDM explained thatblends molecular using transformationsthe TS method. such Again, as based the formation of a copolymer might occur during the dynamic revulcanization in the on the previous study [15] it can be explained that molecular transformations such as the extrusion simultaneously with the generation of a dense chemical network in the curing of formationthe EPDM of phasea copolymer in PP/EPDM might blends occur using during the DR the method. dynamic Additionally, revulcanization a higher in level the extru- sionof simultaneously compatibilization with between the generation PP and EPDM of rubbera dense using chemical the DR network method might in the lead curing to of the EPDMan increase phase inin thePP/EPDM modulus andblends viscosity using of the PP/EPDM DR method. blends inAdditionally, comparison with a higher the TS level of compatibilizationmethod. Morphology between study, PP shown and later,EPDM shows rubber that theusing dispersion the DR of method EPDM particles might lead in to an increasethe PP/EPDM in the modulus blends using and the viscosity DR method of wasPP/EPDM much better blends than in that comparison of the PP/EPDM with the TS blends using the TS method. This may also contribute to the higher moduli and viscosity method.in the caseMorphology of PP/EPDM study, blends shown in the DRlater, method. shows The that complex the dispersion viscosity, storage, of EPDM and lossparticles in the moduliPP/EPDM of the blends PP/EPDM using blends the DR using method the TS methodwas much were better higher than and thethat loss of tangent the PP/EPDM blendswas using lower thanthe TS those method. of the PP/EPDMThis may blendsalso contribute using the OSto the method. higher This moduli is due toand the viscosity in thedegradation case of PP/EPDM of PP in the blends PP/EPDM in the blends DR prepared method. by The the OScomplex method, viscosity, but the degradation storage, and loss modulidid not of occurthe PP/EPDM in the TS method blends because using PP the was TS not method treated bywere the ultrasound.higher and the loss tangent was3.3. lower Mechanical than those Properties of the PP/EPDM blends using the OS method. This is due to the degradationFigure of 10 PP shows in the the PP/EPDM tensile stress–strain blends prepared curves of by PP/EPDM the OS method, blends at but ratios the of degrada- tion75/25 did not (Figure occur 10 a),in 50/50the TS (Figure method 10b), beca anduse 25/75 PP (Figurewas not 10 c)treated manufactured by the ultrasound. at different ultrasonic amplitudes by the OS method. Figure 11 indicates the elongation at break 3.3. Mechanical Properties

Polymers 2021, 13, x FOR PEER REVIEW 11 of 19

Figure 10 shows the tensile stress–strain curves of PP/EPDM blends at ratios of 75/25 (Figure 10a), 50/50 (Figure 10b), and 25/75 (Figure 10c) manufactured at different ultra- Polymers 2021, 13, 259 sonic amplitudes by the OS method. Figure 11 indicates the elongation at break11 of 19 (Figure 11a), tensile strength (Figure 11b), and Young’s modulus (Figure 11c) as a function of the ultrasonic amplitude for PP/EPDM blends of different ratios obtained by the OS method. The(Figure Young’s 11a), modulus tensile strength at different (Figure ratios11b), and of Young’sPP/EPDM modulus was (Figurenot affected 11c) as by a function an increase in the ofultrasonic the ultrasonic amplitude. amplitude The for elongation PP/EPDM blendsat break of differentand tensile ratios strength obtained decreased by the OS with an method. The Young’s modulus at different ratios of PP/EPDM was not affected by an increase in the ultrasonic amplitude at different ratios of PP/EPDM, with the lowest value increase in the ultrasonic amplitude. The elongation at break and tensile strength decreased beingwith at anan increase amplitude in the of ultrasonic 13 μm. This amplitude is because at different the complex ratios of PP/EPDM,viscosity of with the thePP/EPDM blendslowest was value lowest being at at an an amplitude of of 13 13µm. μm This (Figure is because 4) due the complex to the coalescence viscosity of the of EPDM particlesPP/EPDM in the blends PP phase was lowest during at an the amplitude extrusion. of 13 Theµm (Figurecoalescence4) due to of the EPDM coalescence particles of led to agglomeratesEPDM particles of EPDM in the PP phase phase in during the PP, the extrusion.as supported The coalescence by the morphology of EPDM particles study shown later.led The to agglomeratesagglomeration of EPDM led to phasethe poor in the dispersion PP, as supported of the EPDM by the phase morphology in the studyPP, resulting shown later. The agglomeration led to the poor dispersion of the EPDM phase in the PP, in poorresulting mechanical in poor mechanicalproperties. properties. In an earlier In an study earlier [17], study PP/waste [17], PP/waste EPDM EPDM at a ratio at a of 25/75 wasratio compounded of 25/75 was in compounded TSE with the in TSEaid withof ultras the aidonic of ultrasonicwaves. The waves. mechanical The mechanical properties of the propertiesultrasonically of the ultrasonicallytreated PP/waste treated EPDM PP/waste decreased EPDM decreased due to duethe tolow the chemical low chemical interaction at theinteraction interface at and the interface the poor and adhesion the poor adhesionbetween betweenthe waste the EPDM waste EPDM and PP. and PP.

Figure 10.Figure Stress–strain 10. Stress–strain curves curves of PP/EPDM of PP/EPDM 75/25 75/25 (a), ( a50/50), 50/50 (b), (b ),and and 25/75 25/75 (c (c) )blends blends prepared at at different different ultrasonic ultrasonic am- plitudesamplitudes by the OS by method. the OS method.

Polymers 2021, 13, 259 12 of 19 Polymers 2021, 13, x FOR PEER REVIEW 12 of 19

Figure 11. Elongation at break ( a), tensile strength ( b), and Young’s modulus (c) as a function of thethe ultrasonic amplitude for various ratios of the PP/EPDM bl blendsends prepared prepared by by the the OS OS method. method.

Figure 1212 shows shows the the tensile tensile stress–strain stress–strain curves curves of PP/EPDM of PP/EPDM blends blends at ratios at ratios of 75/25 of (Figure75/25 (Figure 12a), 50/50 12a), (Figure 50/50 (Figure 12b), and 12b), 25/75 and (Figure 25/75 (Figure 12c) manufactured 12c) manufactured at different at different ultrasonicultrasonic amplitudes amplitudes by the by TS the method. TS method. Figure Figure13 indicates 13 indicates the elongation the elongation at break at (Figure break 13a),(Figure tensile 13a) ,strength tensile strength (Figure (Figure13b), and 13 b),Young’s and Young’s modulus modulus (Figure (Figure 13c) as 13 a c)function as a function of the ultrasonicof the ultrasonic amplitude amplitude for PP/EPDM for PP/EPDM blends blends of different of different ratios. ratios. As indicated As indicated in Figure in Figure 13, the13, themodulus modulus was wasunaffected unaffected by an by increase an increase in the in ultrasonic the ultrasonic amplitude amplitude at different at different ratios ofratios PP/EPDM. of PP/EPDM. The elongation The elongation at break at and break tens andile tensilestrength strength were decreased were decreased for all the for ul- all trasonicallythe ultrasonically treated treated samples. samples. This is This because is because the complex the complex viscosity viscosity of the PP/EPDM of the PP/EPDM blends decreasedblends decreased for the treated for the treatedsamples, samples, as shown as shownin Figure in 5, Figure due 5to, duethe coalescence to the coalescence of EPDM of particlesEPDM particles in the PP in during the PP the during extrusion. the extrusion. The coalescence The coalescence of the EPDM of the particles EPDM led particles to the agglomerationled to the agglomeration of EPDM, as of shown EPDM, later. as shown The ag later.glomeration The agglomeration led to the poor led dispersion to the poor of dispersion of EPDM particles in the PP/EPDM blends, resulting in their poor mechanical EPDM particles in the PP/EPDM blends, resulting in their poor mechanical properties. In properties. In earlier study,16 PP was compounded with ultrasonically treated waste EPDM earlier study,16 PP was compounded with ultrasonically treated waste EPDM rubber at a rubber at a ratio of 25/75. The size of original waste EPDM powder used was much smaller ratio of 25/75. The size of original waste EPDM powder used was much smaller (5 to 20 (5 to 20 µm) than that in the present study (150 to 250 µm). The smaller size of EPDM μm) than that in the present study (150 to 250 μm). The smaller size of EPDM particles in particles in the early study led to the good dispersion of EPDM particles in the PP phase, the early study led to the good dispersion of EPDM particles in the PP phase, resulting in resulting in the higher elongation at break and higher tensile strength. However, in an early the higher elongation at break and higher tensile strength. However, in an early study [17] study [17] the elongation at break of the PP/waste EPDM blends decreased from 300% to the elongation at break of the PP/waste EPDM blends decreased from 300% to 150% and 150% and the tensile strength decreased from 12 to 5 MPa with the increasing ultrasonic the tensile strength decreased from 12 to 5 MPa with the increasing ultrasonic amplitude. amplitude. This was explained to be due to the low chemical interaction at the interface This was explained to be due to the low chemical interaction at the interface and the poor and the poor adhesion between waste EPDM and PP. In the present study, the elongation adhesionat break of between the PP/waste waste EPDMEPDM blendsand PP. at In a ratio the present of 25/75 study, decreased the elongation from 130% at to 60%break and of thethe PP/waste tensile strength EPDM decreasedblends at a from ratio 7 of to 25/75 4 MPa decreased with the from increasing 130% to ultrasonic 60% and amplitudethe tensile strengthdue to the decreased agglomeration from 7 of to EPDM 4 MPa particles with the in increasing the blend. ultrasonic amplitude due to the agglomeration of EPDM particles in the blend.

PolymersPolymers 20212021, 13,, 13x FOR, 259 PEER REVIEW 13 of13 19 of 19

Polymers 2021, 13, x FOR PEER REVIEW 13 of 19

FigureFigure 12. 12.Stress–strainStress–strain curves curves of PP/EPDM of PP/EPDM 75/25 75/25 (a), 50/50 (a), 50/50(b), and (b ),25/75 and ( 25/75c) blends (c) blendsprepared prepared at different at different ultrasonic ultrasonic am- plitudesFigure by the 12. TS Stress–strain method. curves of PP/EPDM 75/25 (a), 50/50 (b), and 25/75 (c) blends prepared at different ultrasonic am- amplitudesplitudes by by the the TS TS method. method.

FigureFigure 13. 13.Elongation Elongation at at break break (a (),a), tensile tensile strength strength ((bb),), andand Young’sYoung’s modulus ( (cc)) as as a a function function of of ultrasonic ultrasonic amplitude amplitude for for FigurePP/EPDM 13. Elongation blends at of break different (a), ratiostensile prepared strength by (b the), and TS method.Young’s modulus (c) as a function of ultrasonic amplitude for PP/EPDM blends of different ratios prepared by the TS method. PP/EPDM blends of different ratios prepared by the TS method.

PolymersPolymers 20212021, 13, ,13 x ,FOR 259 PEER REVIEW 14 of14 19 of 19

FigureFigure 14 14shows shows the thetensile tensile stress–strain stress–strain curves curves of PP/EPDM of PP/EPDM blends blendsat ratios at of ratios 75/25 of (Figure75/25 14a), (Figure 50/50 14 a),(Figure 50/50 14b), (Figure and 14 25/75b), and (Figure 25/75 14c) (Figure manufactured 14c) manufactured at different at different ultrasonicultrasonic amplitudes amplitudes by the DR by method. the DR method.Figure 15 Figure indicates 15 indicatesthe elongation the elongation at break (Figure at break 15a),(Figure tensile 15 a)strength, tensile (Figure strength 15b), (Figure and 15Young’sb), and modulus Young’s modulus(Figure 15c) (Figure as a 15functionc) as a functionof the ultrasonicof the ultrasonic amplitude amplitude for PP/EPDM for PP/EPDM blends of di blendsfferent of ratios. different The ratios. elongation The elongationat break of at PP/EPDMbreak of blends PP/EPDM at different blends atratios different increase ratiosd increasedwith increasing with increasing ultrasonic ultrasonic amplitude. am- Clearly,plitude. this Clearly, increase this in increasethe elongation in the at elongation break in atPP/EPDM break in blends PP/EPDM obtained blends in the obtained DR methodin the is DR an method indication is an of indicationtheir increased of their elasticity. increased Additionally, elasticity. Additionally,the tensile strength the tensile of PP/EPDMstrength blends of PP/EPDM at ratios blends of 75/25 at and ratios 50/50 of 75/25 increased and with 50/50 the increased increasing with ultrasonic the increasing amplitude.ultrasonic Based amplitude. on the previous Based on study the previous [16], molecular study [ 16transformations], molecular transformations such as the for- such mationas the of formation a copolymer of a and copolymer the generation and the of generation the dense of chemical the dense network chemical in the network ultrason- in the icallyultrasonically treated EPDM treated phase EPDM occur phase during occur the duringdynamic the revulcanization dynamic revulcanization in the extrusion in the pro- extru- cess.sion It process.is believed It isthat believed the copolymer that the copolymerwas formed was at the formed interface at the of interfacetwo phases, of tworeducing phases, thereducing interfacial the tension interfacial between tension the between two components the two components in blends and in improving blends and their improving me- chanicaltheir mechanical properties. properties.This compatibilization This compatibilization between PP and between EPDM PP rubber and EPDM led to the rubber good led dispersionto the good and dispersion smaller sizes and of smaller EPDM sizes particles of EPDM in the particles PP continuous in the PP phase continuous during phasedy- namicduring revulcanization, dynamic revulcanization, as shown below as by shown the morphological below by the morphologicalstudy. This is because study. in This the is DRbecause method in the the DREPDM method particles the EPDM were particlescured during were curedthe extrusion, during the preventing extrusion, their preventing ag- glomerationtheir agglomeration due to their due high to their viscosity high viscosityand poor and flowability. poor ﬂowability. This resulted This resultedin the good in the dispersiongood dispersion of EPDM of particles EPDM particles in the PP in phase. the PP Additionally, phase. Additionally, for PP/EPDM for PP/EPDM blends at blendsa ratio at of a25/75, ratio the of 25/75, tensile the strength tensile and strength Young’s and mo Young’sdulus slightly modulus decreased, slightly decreased, while the elonga- while the tionelongation at break slightly at break increased slightly increased with the withincrea thesing increasing ultrasonic ultrasonic amplitude. amplitude. This is due This to isthe due lowerto the gel lower fraction gel of fraction devulcanized of devulcanized EPDM rubber EPDM at rubber an amplitude at an amplitude of 13 μm. of 13 µm.

FigureFigure 14. 14. Stress–strainStress–strain curves curves of PP/EPDM of PP/EPDM 75/25 75/25 (a), 50/50 (a), 50/50 (b), and (b), 25/75 and 25/75(c) blends (c) blends prepared prepared at different at different ultrasonic ultrasonic am- plitudesamplitudes by the by DR the method. DR method.

Polymers 2021,, 1313,, 259x FOR PEER REVIEW 1515 of of 19 Polymers 2021, 13, x FOR PEER REVIEW 15 of 19

Figure 15. Elongation at break (a), tensile strength (b), and Young’s modulusmodulus ((c)) asas aa functionfunction ofof ultrasonicultrasonic amplitudeamplitude for PP/EPDMFigure 15. blends Elongation of various at break ratios (a), prepared tensile strength by the DR(b), method.and Young’s modulus (c) as a function of ultrasonic amplitude for PP/EPDMPP/EPDM blendsblends ofof variousvarious ratiosratios preparedprepared byby thethe DRDR method.method. 3.4. Morphology of Blends 3.4.3.4. MorphologyMorphology ofof BlendsBlends Figure 16 shows the micrograph of original 40-mesh waste EPDM powder as ob- Figure 16 shows the micrograph of original 40-mesh waste EPDM powder as obtained tainedFigure by the 16 optical shows transmission the micrograph microscopy of origin. Ital indicates40-mesh thatwaste diameters EPDM powder of the EPDM as ob- by the optical transmission microscopy. It indicates that diameters of the EPDM powder powdertained byrange the fromoptical 100 transmission to 250 μm. microscopy. It indicates that diameters of the EPDM rangepowder from range 100 from to 250 100µm. to 250 μm.

Figure 16. Optical micrographs of the original EPDM powder. FigureFigure 16.16.Optical Optical micrographsmicrographs ofof the the original original EPDM EPDM powder. powder. Figure 17 shows the SEM micrographs of the 75/25 PP/EPDM blends obtained using the OSFigureFigure method 17 17 shows showswithout the the (Figure SEM SEM micrographs micrographs 17a) and with of of theul thetrasonic 75/25 75/25 PP/EPDM PP/EPDMtreatment blendsatblends an amplitude obtainedobtained using ofusing 13 the OS method without (Figure 17a) and with ultrasonic treatment at an amplitude of 13 µm μthem (FigureOS method 17b). without Firstly, it(Figure can be 17a) observed and with from ul Figuretrasonic 17 treatment that the particles at an amplitude in blends of are 13 (Figure 17b). Firstly, it can be observed from Figure 17 that the particles in blends are much muchμm (Figure smaller 17b). than Firstly, those of it thecan originalbe observed EPDM from powder. Figure This 17 that is because the particles of the infact blends that the are smallermuch smaller than those than of those the original of the original EPDM powder.EPDM powder. This is becauseThis is because of the fact of thatthe fact the that EPDM the

Polymers 2021, 13, 259 16 of 19

rubber was devulcanized during the ultrasonic extrusion. It is noteworthy that there are some agglomerates of EPDM particles in the 75/25 PP/EPDM blend treated at an amplitude of 13 µm. This is due to the fact that, at such a high ultrasonic amplitudes, devulcanized EPDM particles have a lower viscosity, leading to their coalescence (agglomeration) during the extrusion. The formation of the agglomerates led to poor mechanical properties, as Polymers 2021, 13, x wasFOR PEER shown REVIEW earlier for these blends. The similar effect is observed for the 75/25 PP/EPDM 16 of 19 blend obtained using the TS method without treatment and with ultrasonic treatment at an amplitude of 13 µm. This is seen in Figure 18a,b. The optical micrographs of the 75/25 PP/EPDM blends obtained using the DR method without treatment (Figure 18a) and with EPDM rubber was devulcanized during the ultrasonic extrusion. It is noteworthy that ultrasonic treatment at an amplitude of 13 µm (Figure 18b) are presented in Figure 19. One there are some agglomerates of EPDM particles in the 75/25 PP/EPDM blend treated at an can observe in Figure 19b that the EPDM particles in the blends obtained at an amplitude of amplitude of 13 μm. This is due to the fact that, at such a high ultrasonic amplitudes, 13 µm exhibit much smaller sizes and a better dispersion. This is because in the DR method devulcanized EPDM particles have a lower viscosity, leading to their coalescence (ag- the EPDM particles were dynamically revulcanized during the extrusion, preventing their glomeration) during the extrusion. The formation of the agglomerates led to poor mechan- agglomeration due to their high viscosity and poor ﬂowability. The latter leads to a smaller ical properties, as was shown earlier for these blends. The similar effect is observed for size of EPDM particles and their good dispersion in the PP matrix. These are reasons why the best improvementthe 75/25 in PP/EPDM the mechanical blend propertiesobtained using of the the PP/EPDM TS method blends without was treatment obtained and with ul- using the DR method.trasonic treatment at an amplitude of 13 μm. This is seen in Figure 18a,b. The optical mi- The distributioncrographs of of EPDM the 75/25 particles PP/EPDM sizes blends in the obtained PP/EPDM using 75/25 the blendsDR method obtained without treatment by the OS, TS,(Figure and DR 18a) methods and with without ultrasonic treatment treatment and at with an amplitude ultrasonic of treatment 13 μm (Figure at an 18b) are pre- amplitude of 13sentedµm isin given Figure in 19. Figure One 20 can. It observe can be seen in Figure that the 19b blend that the prepared EPDM using particles the in the blends OS method exhibitsobtained the at largest an amplitude number of of EPDM 13 μm particlesexhibit much of sizes smaller of over sizes 100 andµm, a while better the dispersion. This blend preparedis usingbecause the in DR the method DR method exhibits the the EPDM lowest particles number were of large dynamically particles. revulcanized In fact, during the blend obtainedthe extrusion, by the DR preventing method at their an amplitudeagglomeration of 13 dueµm to does their not high contain viscosity EPDM and poor flowa- particles overbility. 100 µ Them. Additionally,latter leads to ita smaller contains size the of largest EPDM number particles of and EPDM their particlesgood dispersion in the with sizes lessPP than matrix. 10 µ m.These The are analysis reasons presented why the inbest Figure impr 20ovement correlates in the well mechanical with the properties of rheological andthe mechanical PP/EPDM propertiesblends was discussed obtained earlier.using the DR method.

Figure 17. Optical micrographs of PP/EPDM 75/25 blends without (a) and with ultrasonic treatment Figure 17. Optical micrographs of PP/EPDM 75/25 blends without (a) and with ultrasonic treatment at an amplitude of 13 at an amplitude of 13 µm (b) prepared by the OS method (right micrographs were treated by the μm (b) prepared by the OS method (right micrographs were treated by the ImageJ software). ImageJ software).

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Polymers 2021, 13, x FOR PEER REVIEW 17 of 19

Figure 18. Optical micrographs of PP/EPDM 75/25 blends without (a) and with ultrasonic treatment Figure 18. Opticalat an amplitudemicrographs of 13of PP/Eµm (PDMb) prepared 75/25 blends by the without TS method (a) and (right with micrographs ultrasonic treatm wereent treated at an by amplitude the of 13 μFigurem (b) 18.prepared Optical by micrographs the TS method of PP/E (rightPDM microgra 75/25 blendsphs were without treated (a) byand the with ImageJ ultrasonic software). treatm ent at an amplitude of 13 μm (b) preparedImageJ by software).the TS method (right micrographs were treated by the ImageJ software).

FigureFigure 19. OpticalFigure micrographs 19. Optical micrographs ofof PP/EPP/EPDMPDM of 75/2575/25 PP/EPDM blends blends 75/25without without blends ( a(a) )and and without with with ultrasonic (ultrasonica) and with treatm treatm ultrasonicentent at at an treatment an amplitude amplitude of of13 13 μμmm ((bb)) preparedpreparedat an by amplitude the DR methodmethod of 13 µ m (Right(Right (b) prepared microgramicrographs byphs the were were DR treated treated method by by (Rightthe the ImageJ ImageJ micrographs software). software). were treated by the ImageJ software).

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The distribution of EPDM particles sizes in the PP/EPDM 75/25 blends obtained by the OS, TS, and DR methods without treatment and with ultrasonic treatment at an amplitude of 13 μm is given in Figure 20. It can be seen that the blend prepared using the OS method exhibits the largest number of EPDM particles of sizes of over 100 μm, while the blend prepared using the DR method exhibits the lowest number of large particles. In fact, the blend obtained by the DR method at an amplitude of 13 μm does not contain EPDM particles over 100 μm. Additionally, it contains the largest number of EPDM particles with sizes less than 10 μm. The analysis presented in Figure 20 correlates well with the rheo- Polymers 2021, 13, 259 logical and mechanical properties discussed earlier. 18 of 19

400 <10 μm 10−50 μm 50−100 μm 300 >100 μm

200

100 Number of EPDM particles

0 OS-0 µm OS-13 µm TS-0 µm TS-13 µm DR-0 µm DR-13 µm

Figure 20. Analysis of the EPDM particle number in PP/EPDM 75/25 blends using different methods. Figure 20. Analysis of the EPDM particle number in PP/EPDM 75/25 blends using different methods.4. Conclusions Compounding PP with waste EPDM using ultrasonically aided extrusion was carried 4. Conclusions out. The effect of the PP/waste EPDM ratio, compounding method, and ultrasonic ampli- tudeCompounding were investigated. PP with The waste one-step EPDM (OS), using two-step ultrasonically (TS), and aided dynamic extrusion revulcanization was carried out.(DR) The methods effect wereof the applied PP/waste using EPDM an ultrasonic ratio, compounding TSE at ultrasonic method, amplitudes and ultrasonic varying ampli- from tude0 to 13 wereµm. investigated. PP and waste The EPDM one-step were compounded (OS), two-step at ratios(TS), and of 75/25, dynamic 50/50, revulcanization and 25/75. In (DR)the OS methods method, were PP and applied waste using EPDM an particles ultrasonic were TSE compounded at ultrasonic in the amplitudes extruder with varying and fromwithout 0 to ultrasonic 13 μm. PP treatment. and waste In theEPDM TS andwere DR compounded methods, the at waste ratios EPDM of 75/25, particles 50/50, were and 25/75.fed into In thethe extruderOS method, and PP devulcanized and waste EPDM without particles and with were ultrasonic compounded treatment. in the extruder Then, in withthe TS and method without devulcanized ultrasonic treatment. EPDM was In compounded the TS and DR with methods, PP in the the extruder waste withoutEPDM par- the ticlesimposition were fed of ultrasound. into the extruder In the DRand method, devulcan devulcanizedized without EPDM and with after ultrasonic compounding treatment. with Then,curatives in the was TS mixed method with devulcanized PP and dynamically EPDM was revulcanized compounded in the with extruder PP in the without extruder the withoutimposition the ofimposition ultrasound. of ultrasound. It was found In that the inDR the method, DR method, devulcanized the mechanical EPDM propertiesafter com- poundingof the PP/EPDM with curatives blends increased was mixed with with the increasePP and indynamically the ultrasonic revulcanized amplitude duein the to theex- truderformation without of copolymer, the imposition the compatibilization of ultrasound. between It was found PP and that EPDM in the rubber, DR andmethod, the good the mechanicaldispersion of properties revulcanized of the EPDM PP/EPDM particles blends in the increased PP matrix with phase. the In increase particular, in thethe tensileultra- sonicstrength amplitude of 30 MPa due and to the the formation elongation of at copo breaklymer, of 400% the compatibilization were achieved at anbetween amplitude PP and of EPDM13 µm forrubber, the PP/EPDM and the good blend dispersion at ratio of of 75/25. revulcanized However, EPDM in the particles OS and in TS the methods, PP matrix the phase.mechanical In particular, properties the of thetensile PP/EPDM strength blends of 30 decreasedMPa and withthe elongation an increase at in break the ultrasonic of 400% wereamplitude achieved due at to an the amplitude coalescence of of 13 devulcanized μm for the PP/EPDM EPDM particles, blend at leading ratio of to 75/25. the creation How- ever,of agglomerates in the OS and of EPDM TS methods, particles the in the mechanical PP phase. properties These ﬁndings of the are PP/EPDM supported blends by optical de- creasedmicroscopy with studies an increase of various in the blends. ultrasonic In the amplitude DR method, due theto the complex coalescence viscosity of devulcan- increases izedwith EPDM increasing particles, ultrasonic leading amplitude to the creation due to of the agglomerates presence ofthe of EPDM dense networkparticles ofin EPDMthe PP phase.phase inThese the PP/EPDM findings are blends suppo andrted the by good optical dispersion microscopy of EPDM studies particles of various in the blends. PP phase. In theIn the DR OS method, method, the the complex complex viscosity viscosity increase decreasess with with increasing the ultrasonic ultrasonic amplitude amplitude due due to tothe the degradation presence of of the PP dense and thenetwork devulcanization of EPDM phase of EPDM in the particles. PP/EPDM In blends the TS and method, the good the dispersioncomplex viscosity of EPDM decreases particles with in the the PP ultrasonic phase. In amplitude the OS method, due to thethe devulcanizationcomplex viscosity of EPDM particles.

Author Contributions: Conceptualization, A.I.I.; Writing—original draft, H.D. and J.Z. All authors

have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Data are in the authors possession. Polymers 2021, 13, 259 19 of 19

Acknowledgments: Authors wish to express their appreciation to the Akrochem Corporation for providing curing ingredients and the Lehigh Technologies, Inc., for supplying the EPDM powder. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. De, S.K.; Bhowmick, A.K. Thermoplastic Elastomers from Rubber-Plastic Blends; Ellis Horwood: Washington DC, USA, 1990. 2. Gessler, A.M.; Haslett, W.H. Process for Preparing a Vulcanized Blend of Crystalline Polypropylene and Chlorinated Butyl Rubber. U.S. Patent 3,037,954, 5 June 1962. 3. Fischer, W.K.; Conn, W. Thermoplastic blend of partially cured monoolefin copolymer rubber and polyolefin plastic. U.S. Patent 3,758,643, 4 June 1973. 4. Coran, A.Y.; Das, B.; Patel, R.P. Thermoplastic vulcanizates of olefin rubber and polyolefin resin. U.S. Patent 4,130,535, 19 December 1978. 5. Abdou-Sabet, S.; Fath, M.A. Thermoplastic elastomeric blends of olefin rubber and polyolefin resin. U.S. Patent 4,311,628, 19 January 1982. 6. Tasdemir, M.; Topsakaloglu, M.; Caneba, G.T. Dynamically vulcanized EPDM (Ethylene-Propylene-Diene Polymer)/PP (Polypropylene) polymer blend: Performance index (PI). J. Polym. Mater. 2010, 27, 337–349. 7. Wu, H.G.; Ning, N.Y.; Zhang, L.Q.; Tian, H.C.; Wu, Y.P.; Tian, M. Effect of additives on the morphology evolution of EPDM/PP TPVs during dynamic vulcanization in a twin-screw extruder. J. Polym. Res. 2013, 20, 266. [CrossRef] 8. Uthaipan, N.; Junhasavasdikul, B.; Nakason, C.; Thitithammawong, A. Prediction models for the key mechanical properties of EPDM/PP blends as affected by processing parameters and their correlation with stress relaxation and phase morphologies. Polym. Adv. Technol. 2015, 26, 970–977. [CrossRef] 9. Shi, M.; Zhang, D.; Zhu, J.; Shi, Y.; Sun, J.; Yubi Ji, Y. Morphology, crystallization, and thermal and mechanical properties of dynamic vulcanization EPDM/PP thermoplastic elastomer. J. Thermoplast. Compos. Mater. 2019, 32, 922–935. [CrossRef] 10. Banerjee, S.S.; Bhowmick, A.K. An effective strategy to develop nanostructured morphology and enhanced physico-mechanical properties of PP/EPDM thermoplastic elastomers. J. Mater. Sci. 2016, 51, 6722–6734. [CrossRef] 11. Chen, G.; Guo, S.; Li, H. Ultrasonic improvement of the compatibility and rheological behavior of high density polyethy- lene/polystyrene blends. J. Appl. Polym. Sci. 2002, 86, 23–32. [CrossRef] 12. Kim, H.; Lee, J.W. Effect of ultrasonic wave on the degradation of polypropylene melt and morphology of its blend with polystyrene. Polymer 2002, 43, 2585–2589. [CrossRef] 13. Kim, H.; Ryu, J.G.; Lee, J.W. Evolution of phase morphology and in-situ compatibilization of polymer blends during ultrasound- assisted melt mixing. Korea-Aust. Rheol. J. 2002, 14, 121–128. 14. Isayev, A.; Hong, C. Ultrasound assisted continuous process for making polymer blends and copolymers. U.S. Patent 6,528,554, 4 March 2003. 15. Feng, W.; Isayev, A.I. In situ compatibilization of PP/EPDM blends during ultrasound aided extrusion. Polymer 2004, 45, 1207–1216. [CrossRef] 16. Feng, W.; Isayev, A.I. In situ ultrasonic compatibilization of unvulcanized and dynamically vulcanized PP/EPDM blends. Polym. Eng. Sci. 2004, 44, 2019–2028. [CrossRef] 17. Kim, J.K.; Lee, S.H.; Hwang, S.H. Study on the thermoplastic vulcanizate using ultrasonically treated rubber powder. J. Appl. Polym. Sci. 2003, 90, 2503–2507. [CrossRef] 18. Isayev, A.I.; Churdpunt, Y.; Guo, X. Comparative Study of Ziegler-Natta and Metallocene Based Polypropylenes in Injection Molding: Simulation and Experiment. Int. Polym. Process. 2000, 15, 72–82. [CrossRef] 19. Isayev, A.I.; Yushanov, S.P.; Chen, J. Ultrasonic devulcanization of rubber vulcanizates. I. Process model. J. Appl. Polym. Sci. 1996, 59, 803–813. [CrossRef] 20. Isayev, A.I.; Yushanov, S.P.; Chen, J. Ultrasonic devulcanization of rubber vulcanizates. II. Simulation and Experiment. J. Appl. Polym. Sci. 1996, 59, 815–824. [CrossRef]