An Investigation and Comparison of the Blending of LDPE and PP with Different Intrinsic Viscosities of PET

Article An Investigation and Comparison of the Blending of LDPE and PP with Different Intrinsic Viscosities of PET

Shi-Chang Chen, Li-Hao Zhang, Guo Zhang, Guo-Cai Zhong, Jian Li, Xian-Ming Zhang * and Wen-Xing Chen

National Engineering Laboratory for Textile Fiber Materials and Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, China; [email protected] (S.-C.C.); [email protected] (L.-H.Z.); [email protected] (G.Z.); [email protected] (G.-C.Z.); [email protected] (J.L.); [email protected] (W.-X.C.) * Correspondence: [email protected]; Tel.: +86-0571-8684-3623

Received: 5 December 2017; Accepted: 1 February 2018; Published: 4 February 2018

Abstract: The blending of aliphatic polyolefins and aromatic poly(ethylene terephthalate) (PET) based on different intrinsic viscosities (IV) was conducted in a torque rheometer. The comparison of blend components in terms of low density polythene (LDPE) and polypropylene (PP) in blending with PET was investigated, and the effects of the IV and proportion of PET on polymer blends are discussed in detail. Polymer blends with or without compatibilizer were examined by using a differential scanning calorimeter, thermogravimetric analyzer, rotary rheometer, field-emission scanning electron microscopy and a universal testing machine. It was found that the blending led to an increase in processability and a decrease in thermal stability for blends. The morphological analysis revealed that the incompatibility of blends was aggravated by a higher IV of PET, while this situation could be improved by the addition of compatibilizer. Results showed that there was an opposite effect for the tensile strength and the elongation at break of the polymer blend in the presence of a compatibilizer, wherein the influence of IV of PET was complicated.

Keywords: polymer blend; poly(ethylene terephthalate); intrinsic viscosity; polyoleﬁn; compatibilizer

1. Introduction Polythene (PE) and polypropylene (PP), as representatives of aliphatic polyolefins, and poly(ethylene terephthalate) (PET), as a representative of aromatic polymers, are the three most common commercialized synthetic polymers, owing to their excellent overall performance and affordable price. The yield of the three polymers in China reached more than 14, 20, 40 million tons in 2016, respectively. PET can be categorized into three types with different intrinsic viscosities (IV) or molecular weights according to the application field: fiber grade, bottle grade and industrial yarn, whose IV are about 0.65 dL/g, 0.85 dL/g and 1.0 dL/g and whose corresponding aims are to be applied in the field of textiles, containers bottles and belts, respectively [1]. Occasionally, these three polymers can be used simultaneously in one product, for example, in beverage bottles. It also can be seen that recycled PET from different sources is basically a mixture of PETs with different properties [2]. The blending of polyolefins and PET can be an alternative to improve cost-effectiveness for industrial applications [3]. However, the thermodynamically immiscible nature of blend roots with large differences in their polarities, which leads to insufficient interfacial adhesion between blend components, result in poor mechanical properties [4–6]. Therefore, the polyolefin/PET compatibilization has already attracted a great deal of attention in the past decades.

Polymers 2018, 10, 147; doi:10.3390/polym10020147 www.mdpi.com/journal/polymers Polymers 2018, 10, 147 2 of 14

Generally, the compatibilization of two immiscible polymers can be achieved by implementing a suitable grafting modification of polyolefin chains with reactive groups. The modification is achieved by chemical reactions between polar functional groups and polymer chains in melting blending [7]. By enhancing the interactions between grafted functional groups of polyolefin and PET end groups, the phase dispersion and adhesion at the interface can be improved [8]. For example, the functional groups like maleic anhydride (MAH) or glycidyl methacrylate (GMA) can be easily grafted onto polyolefins, and advantageously react with the carboxyl and hydroxyl end groups of PET. This method presents an excellent compatibilization effect with respect to chemical interactions between polyolefin and PET [9,10]. However, the conversion of functional groups grafted onto polyolefins is usually insufficient even though some residual active ingredients get involved in crosslinking reactions [11]. More importantly, it is not difficult to imagine that this route goes against the large-scale application due to its high cost. From this perspective, the grafted functional groups of polyolefins can be used as compatibilizers, with a small amount added into the PET/polyolefin blend during extrusion processes, then the desirable compatibilization effect can be obtained. The compatibilizer is concentrated at the interface of two phases in the blending system, and the co-crystallization/molecular chains’ entanglement between the polymer component of the compatibilizer and polymer matrix would be generated to facilitate the interfacial adhesion between two immiscible polymers. Consequently, two immiscible blending components are effectively compatible by forming a dispersed structure in one unity [12–14]. In the previous literature, GMA, MAH, acrylic acid (AA), ethylene-vinyl acetate copolymer (EVA), and maleimide (MI) are the common functional groups that can be either grafted onto polyolefins or copolymerized into compatibilizer. Recently, Jazani et al. [15] improved the mechanical properties of bottle grade PET by grafting GMA onto PP with the aid of styrene (St) comonomer, and found that the increase of impact strength was attributed to the enhanced interaction between epoxy groups of GMA and carboxyl end groups of PET, as well as the hydrogen bonds formed between PP-g-GMA and PET. In yet another approach, a compatibilizer composed of styrene–ethylene–butylene–styrene (SEBS) grafted with MAH was used as the compatibilizer in the blending of PP and PET spun fibers [16]. Results showed that the compatibilizer significantly affected the elongation at break of the blend, while no relevant effect could be observed in the anisotropically-oriented fibers. Using PP-g-AA as a compatibilizer, a good interaction between the PP and PET surfaces was achieved as PET islands was deformed in micro-fibers after hot stretching [17]. In most published works, PP is the dominant continuous phase in the blend system of polyolefins/PET. There is little research focusing on PE/PET blends, let alone on the compatibilizer, by grafting the functional groups onto PE. Lei et al. [18] studied the compatibility between PET and HDPE using PE-g-MAH, SEBS-g-MAH and E–GMA, respectively. It was found that the compatibilizing effectiveness of E–GMA was the best. Raffa et al. [19] indicated the chemical reactions among polymer and additives had a significant effect on the ultimate melt rheology and mechanical properties of recycled PET/polyolefin blends. In most cases, a variety of nanomaterials as a third component are added into two immiscible polymers to improve the compatibilizing effect [20]. As a matter of fact, the functional groups grafted onto polyolefins have been commercialized and applied gradually, such as PP-g-MAH and PE-g-MAH, which offer the best convenience for designing polymer blend systems. Based on the successful surface functionalization of polypropylene/polystyrene and their application in polymer blends, as described in our previous work [7,21–24], the motivation has turned to developing possible industrialization polymer blends by exploring the relationships between the structure and properties in the processable polymers. Consequently, this work concentrates on the comparative investigation of the blending of PP and LDPE with different IV of PET using commercialized polyolefin-g-MAH as a compatibilizer. A systematic performance investigation of the polymer blends with or without compatibilizer was performed to elucidate the processable correlation between several major polymer commodities. Polymers 2018, 10, 147 3 of 14

2. Materials and Methods

2.1. Materials Three poly(ethylene terephthalate) (PET) chips with different intrinsic viscosities (IV) of 0.7 dL/g, 0.85 dL/g and 1.0 dL/g, were purchased from Zhejiang Guxiandao Polyester Dope Dyed Yarn Co., Ltd. (Shaoxing, China). The moisture and carboxyl end group content of the PET chips are also listed as Table1. LDPE (F401) and PP (DNDB7441) were purchased from Sinopec Yangzi Petrochemical Co., Ltd. (Nanjing, China), LDPE-g-MAH and PP-g-MAH were purchased from Fine-blend Compatibilizer Jiangsu Co., Ltd. (Nantong, China).

Table 1. The properties of the PET chips.

PET [η] (dL/g) [COOH] (mol/t) H2O (wt %) PET1 0.70 22 0.35 PET2 0.85 18 0.26 PET3 1.00 15 0.10

In order to decrease the hydrolytic decomposition, the PET chips were dried at 130 ◦C for 9 h in a vacuum oven before being blended into either LDPE or PP. The polymer blends were processed in a RM-200C torque rheometer (Hapro, Haerbin, China). The processing temperatures were set to 260 ◦C for all blends in the rheometer, and the processing time was set to 5~10 min and the rotor speed was set to 50 r/min. The total weight of polymer blend was no more than 50 g. The weight ratio of the components in the blending are listed in Table2. The compatibilizer usually accounts for ﬁve percent of the total weight of the two polymers (5 wt %). The blank control experiments were also carried out in the absence of compatibilizer.

Table 2. The weight ratio of the components in the polymer blends.

No. PET1 PET2 PET3 PP PP-g-MAH LDPE LDPE-g-MAH 1 20 80 2 20 80 3 20 80 4 30 70 5 30 70 6 30 70 7 20 80 5 8 20 80 5 9 20 80 5 10 30 70 5 11 30 70 5 12 30 70 5 13 20 80 14 20 80 15 20 80 16 30 70 17 30 70 18 30 70 19 20 80 5 20 20 80 5 21 20 80 5 22 30 70 5 23 30 70 5 24 30 70 5 Polymers 2018, 10, 147 4 of 14

2.2. Measurements The thermal behavior of the blends was investigated using differential scanning calorimeter (DSC1, Mettler Toledo, Schwerzenbach, Switzerland) and thermogravimetric analyzer (TGA, Mettler Toledo, Schwerzenbach, Switzerland) under nitrogen atmosphere. The drying samples were initially heated from 25 to 300 ◦C at a rate of 50 ◦C/min. After that, the samples were kept at the final temperature for 3 min to eliminate the thermal history, and then cooled to 50 ◦C at a rate of 20 ◦C/min to determine the crystallization temperature (Tc) and before being kept for 3 min. Finally, the samples were heated ◦ ◦ to 300 C at a rate of 10 C/min to determine the melt point (Tm). About 5 mg of the samples was heated from 25 to 600 ◦C at a rate of 10 ◦C/min with a nitrogen flow rate of 45 mL/min to determine the decomposition temperature (thermal gravimetric rate 5%, T5%). The rheological properties of the polymer blends as functions of angular frequency were investigated via plate-plate geometry (plate radius = 25 mm; gap = 1 mm) using a stress-controlled rheometer (MCR 301, Anton Paar, Graz, Austria) at 260 ◦C. The measurements of the granules and sheet samples were performed in air atmospheres. The loaded frequency sweep was undirectional from the angular frequency of ω = 500 1/s to 0.1 1/s. The banded samples of polymers were prepared from a blending system so that the morphological characteristics of the polymer blends could be examined using an ULTRATM 55 field-emission scanning electron microscopy (SEM Ultra 55, Zeiss, Jena, Germany) at an acceleration voltage of 1 kV. The samples were cooled in liquid nitrogen for 15 min and then taken out quickly to be brittle fractured. The fracture surface of PET was etched by using the mixed solution of phenol and tetrachloroethane (1/1 (wt %)), then the fracture surface was gilded because of the poor electrical conductivity. To test the mechanical property of the polymer blends, the round rod samples with diameter of 1.5 mm were stretched on a universal testing machine (Instron 3367, Instron, Canton, USA) at speed of 10 mm/min. Generally, the effective stretch length was about 2 mm, and then the tensile strength and elongation at break could be achieved.

3. Results and Discussion

3.1. Thermal Properties Figure1 shows differential scanning calorimeter (DSC) scans of polymers and polyolefin/PET blends. The crystallization could be observed in the cooling scan and the melting process was recorded in the subsequent heating scans. The evident cold crystallization peaks (Tc1) of the polymer blends, observed in Figure1a, was between polyolefins and PET, while the second cold crystallization peaks (Tc2) was almost invisible in most cases. In contrast, there were two melting peaks in the heating DSC scans, which indicated the melting point of the polyolefin component (Tm1) and PET component (Tm2). Noticeably, the Tc1, Tm1 and Tm2 for all polyolefin/PET blends were listed in Table3. It also could be seen that blending caused a gradual increase in the crystallization temperature (Tc1) from LDPE to LDPE/PET1, to LDPE/LDPET-g-MAH/PET, however, the Tc1 of compatibilized PP/PET1 blends was lower than that of incompatible PP/PET1 blends. This might be ascribed to the plasticizing effect of compatibilizer on the PP/PET blend and the facilitation of crystallization [25]. On the other hand, it is well known that the regularity of the LDPE chain is better than PP, which increases the crystallization temperature of blending system. Generally, the Tc1 of incompatible LDPE/PET (Figure1b) increased slightly, yet the Tm1 and Tm2 presented a gradual decline either with the increase of the content of PET or with the increase of the IV of the PET, but there was less significant difference among PP/PET blends. When the compatibilizer was added to two immiscible polymers (Figure1c), the Tc1 of LDPE/PET blends presented a minor depression, whereas the changing trends for the PP/PET blends is just opposite. It is worth stressing that the difference in the Tc1, Tm1 and Tm2 of the polymer blends was quite inconspicuous, and that the thermal behaviors need further verification. This is perhaps explained by the fact that both the content and property of PET have no significant influence on the cold crystallization performance of Polymers 2018, 10, 147 5 of 14 Polymers 2018, 10, x FOR PEER REVIEW 5 of 14 polyolefins/PET.crystallizationThese performance allow of us polyolefins/PET. to conduct a good These deal allow of us the to blending conduct a ofgood polyolefin deal of the with blending PET from differentof polyolefin sources. with PET from different sources.

Figure 1. Cooling (solid line) and heating (short dot line) DSC scans of polymers and polymer blends, Figure 1. Cooling (solid line) and heating (short dot line) DSC scans of polymers and polymer (a) polymers and blends; (b) incompatible polyolefin/PET blends; (c) compatibilized polyolefin/PET blends, (a) polymers and blends; (b) incompatible polyoleﬁn/PET blends; (c) compatibilized blends. polyoleﬁn/PET blends. Table 3. The crystallization temperature and melting point of polymer blends. Table 3. The crystallization temperature and melting point of polymer blends. Samples Tc1 (°C) Tm1 (°C) Tm2 (°C) LDPE 91.09◦ 110.12 ◦ — ◦ Samples Tc1 ( C) Tm1 ( C) Tm2 ( C) PP 102.51 165.33 — LDPEPET1 167.69 91.09 — 110.12 254.33 — LDPE/PET1-80/20PP 102.5191.85 110.68 165.33 255.67 — PET1 167.69 — 254.33 LDPE/PET2-80/20 92.96 110.58 254.81 LDPE/PET1-80/20 91.85 110.68 255.67 LDPE/PET3-80/20 93.43 110.23 253.32 LDPE/PET2-80/20 92.96 110.58 254.81 LDPE/PET3-80/20LDPE/PET1-70/30 93.4392.49 110.77 110.23 2557.5 253.32 LDPE/PET1-70/30LDPE/PET2-70/30 92.4993.44 109.68 110.77 254.34 2557.5 LDPE/PET2-70/30LDPE/PET3-70/30 93.4494.44 109.86 109.68 252.88 254.34 LDPE/PET3-70/30PP/PET1-80/20 119.61 94.44 163.31 109.86 255.28 252.88 PP/PET1-80/20PP/PET2-80/20 119.61119.14 162.96 163.31 255.25 255.28 PP/PET2-80/20PP/PET3-80/20 119.14119.21 163.33 162.96 254.87 255.25 PP/PET3-80/20PP/PET1-70/30 119.21119.13 163.70 163.33 255.75 254.87 PP/PET1-70/30PP/PET2-70/30 119.13118.67 162.96 163.70 254.83 255.75 PP/PET2-70/30 118.67 162.96 254.83 PP/PET3-70/30 118.65 164.42 254.81 PP/PET3-70/30 118.65 164.42 254.81 LDPE/PET1-80/20/5LDPE/PET1-80/20/5 93.61 110.63 110.63 255.18 255.18 LDPE/PET2-80/20/5LDPE/PET2-80/20/5 91.39 111.66 111.66 255.96 255.96 LDPE/PET3-80/20/5LDPE/PET3-80/20/5 92.13 111.13 111.13 253.88 253.88 LDPE/PET1-70/30/5LDPE/PET1-70/30/5 91.78 111.40 111.40 256.22 256.22 LDPE/PET2-70/30/5LDPE/PET2-70/30/5 92.32 111.63 111.63 254.93 254.93 LDPE/PET3-70/30/5LDPE/PET3-70/30/5 91.23 111.66 111.66 254.66 254.66 PP/PET1-80/20/5PP/PET1-80/20/5 114.91 162.62 162.62 255.96 255.96 PP/PET2-80/20/5PP/PET2-80/20/5 115.61 162.86 162.86 254.93 254.93 PP/PET3-80/20/5 115.07 163.39 254.66 PP/PET3-80/20/5 115.07 163.39 254.66 PP/PET1-70/30/5 114.36 162.09 255.44 PP/PET1-70/30/5 114.36 162.09 255.44 PP/PET2-70/30/5 115.45 162.87 255.71 PP/PET3-70/30/5PP/PET2-70/30/5 116.00115.45 162.87 162.09 255.71 253.88 PP/PET3-70/30/5 116.00 162.09 253.88

TheThe comparison comparison of of thermogravimetric thermogravimetric (TG) (TG) and and derivative derivative thermogravimetric thermogravimetric (DTG) (DTG)scans of scans of polymerpolymer with with polymerpolymer blendsblends is reported in in Figure Figure 2.2 .PET PET presented presented decomposition decomposition at lower at lower temperatures than LDPE and PP, as a result of the relatively poor stability of the ester group in PET chains. A small difference in the decomposition rate could be seen between PP and its blends based on the TG and DTG curves. Nevertheless, it was clear that the LDPE/PET blend showed worse Polymers 2018, 10, x FOR PEER REVIEW 6 of 14 Polymers 2018, 10, 147 6 of 14 temperatures than LDPE and PP, as a result of the relatively poor stability of the ester group in PET chains. A small difference in the decomposition rate could be seen between PP and its blends based thermalon the stability TG and than DTG that curves. of LDPE Nevertheless, and that it thewas compatibilizersclear that the LDPE/PET promote blend the thermalshowed worse stability of twothermal immiscible stability polymers. than that This of LDPE was and mainly that attributedthe compatibilizers to the improvement promote the thermal of compatibility stability of two with the immiscible polymers. This was mainly attributed to the improvement of compatibility with the addition of grafting maleic anhydride; the interface adhesion was enhanced by the chemical reactions addition of grafting maleic anhydride; the interface adhesion was enhanced by the chemical reactions betweenbetween the the grafted grafted functional functional groups groups ofof polyoleﬁnpolyolefin and and PET PET end end groups groups [26]. [26 ].

Figure 2. Comparison of TG (a) and DTG (b) curves of polymers and polymer blends. Figure 2. Comparison of TG (a) and DTG (b) curves of polymers and polymer blends. Figures 3 and 4 show the effect of the content and IV of PET on the thermal stability of incompatibleFigures3 and and4 show compatibilized the effect of polyolefin/PET the content and blends, IV of respectively. PET on the thermal The DTG stability curves ofof incompatiblepolymer and compatibilizedblends presented polyoleﬁn/PET asymmetric peaks, blends, which respectively. indicated the The whole DTG curvesdecomposition of polymer of the blends different presented components. Accordingly, the decomposition temperature at thermal gravimetric rate of 5% (T5%) asymmetric peaks, which indicated the whole decomposition of the different components. Accordingly, and the corresponding temperature of the maximum decomposition rate (Tcmax) of all polymer blends the decomposition temperature at thermal gravimetric rate of 5% (T ) and the corresponding are also listed in Table 4. Regarding LDPE/PET blends without a compatibilizer5% (Figure 3), the temperaturedecomposition of the temperature maximum decomposition was slightly increased rate (Tcmax with) ofthe all increase polymer in blendsthe IV areof PET, also which listed inwas Table 4. Regardingvalidated LDPE/PET by the temperature blends of without maximum a compatibilizer decomposition rate (Figure as shown3), the in the decomposition DTG graphs and temperature Table was4, slightly but the increaseddifference was with unremarkable the increase under in the the IV co ofnditions PET, which of more was PET validated content in bythe the blends. temperature On of maximumthe contrary, decomposition it almost stayed rate at a asconstant shown thermal in the gravimetric DTG graphs rate andfor PP/PET Table4 blends, but the in a difference ratio of 80 was unremarkableto 20 as theunder IV of PET the increased, conditions while of more it was PET a little content better in later the blends.with more On PET the content contrary, in the it almost blends. stayed at a constantWhen the thermal compatibilizer gravimetric was rateadded for to PP/PET the blends blends (Figure in a ratio4), the of 80thermal to 20 asstability the IV was of PET slightly increased, whileincreased. it was a As little already better mentioned, later with the more interface PET coul contentd be enhanced. in the blends. It was When also noticed the compatibilizer that both the was maximum decomposition rate and the corresponding temperature of the compatibilized added to the blends (Figure4), the thermal stability was slightly increased. As already mentioned, the polyolefin/PET blends at a ratio of 70 to 30 were lower than that of 80/20 PP/PET blends in view of interfacethe poorer could thermal be enhanced. stability It of was PET. also Furthermore, noticed that the both polymer the maximum blends with decomposition a higher IV of rate PET and the correspondinggenerally suggested temperature better ofthermal the compatibilized stability and higher polyoleﬁn/PET value of T5% and blends Tcmax. atThese a ratio results of 70are to easy 30 were lowerto thanunderstand that of if 80/20 it is remembered PP/PET blends that PET in wi viewth a ofhigher the poorer IV has a thermal longer polymer stability chain. of PET. Furthermore, the polymer blends with a higher IV of PET generally suggested better thermal stability and higher value of T5% and Tcmax. These results are easy to understand if it is remembered that PET with a higher IV has a longer polymer chain.

Table 4. The decomposition temperature (T5%) and the corresponding temperature of the maximum decomposition rate (Tcmax) of polymer blends.

0/100 80/20 70/30 80/20/5 70/30/5 100/0 Blend T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax (◦C) (◦C) (◦C) (◦C) (◦C) (◦C) (◦C) (◦C) (◦C) (◦C) (◦C) (◦C) LDPE/PET1 410.8 444.3 419.7 473.4 420.5 478.7 419.6 482.8 419.4 481.7 437.5 477.7 LDPE/PET2 407.5 445.7 421.3 475.9 416.3 476.0 423.2 481.7 420.3 480.9 - - LDPE/PET3 407.2 447.7 422.5 480.3 416.4 475.8 423.6 480.8 419.5 479.2 - - PP/PET1 410.8 444.3 419.5 463.5 420.3 463.3 416.3 465.9 419.7 465.7 420.8 461.3 PP/PET2 407.5 445.7 417.1 463.4 414.3 461.4 421.1 463.3 411.5 465.0 - - PP/PET3 407.2 447.7 420.7 463.2 412.1 460.5 417.5 462.9 409.9 461.8 - - Polymers 2018, 10, 147 7 of 14 Polymers 2018, 10, x FOR PEER REVIEW 7 of 14 Polymers 2018, 10, x FOR PEER REVIEW 7 of 14

Figure 3. TG (a) and DTG (b) curves of incompatible polymer blends. Figure 3. TG ( a)) and DTG ( b) curves of incompatible polymer blends.

Figure 4. TG (a) and DTG (b) curves of compatibilized polymer blends. Figure 4. TG (a) and DTG (b) curves of compatibilized polymer blends. Figure 4. TG (a) and DTG (b) curves of compatibilized polymer blends. Table 4. The decomposition temperature (T5%) and the corresponding temperature of the maximum Table 4. The decomposition temperature (T5%) and the corresponding temperature of the maximum 3.2. Analysisdecomposition of Rheological rate (Tcmax Behavior) of polymer blends. decomposition rate (Tcmax) of polymer blends. Figure5 shows0/100 the complex80/20 viscosity as a70/30 function of frequency80/20/5 at 26070/30/5◦C for polymers100/0 and 0/100 80/20 70/30 80/20/5 70/30/5 100/0 Blend T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax polymerBlend blends. The rheological curves of polymer blends showed similar trends with the neat T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax T5% Tcmax T5% ◦Tcmax constituents of(°C) polymers. (°C) Considering(°C) (°C) that(°C) the Tm(°C)of PET(°C) was approximately(°C) (°C) (°C) equal to(°C) 260 (°C)C, all (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) (°C) LDPE/PET1 410.8 444.3 419.7 473.4 420.5 478.7 419.6 482.8 419.4 481.7 437.5 477.7 theLDPE/PET1 rheological 410.8 measurements 444.3 419.7 of polymers473.4 420.5 and polymer478.7 blends419.6 were482.8 conducted419.4 481.7 in air without437.5 477.7 inert LDPE/PET2 407.5 445.7 421.3 475.9 416.3 476.0 423.2 481.7 420.3 480.9 - - gasLDPE/PET2 protection, 407.5 which 445.7 was consistent421.3 475.9 with 416. the3 blending476.0 423.2 process. 481.7 Three 420.3 kinds 480.9 of PET displayed - - a LDPE/PET3 407.2 447.7 422.5 480.3 416.4 475.8 423.6 480.8 419.5 479.2 - - pseudo-NewtonianLDPE/PET3 407.2 behavior447.7 422.5 for the480.3 whole 416. frequency4 475.8 range.423.6 The480.8 complex 419.5 viscosity479.2 signiﬁcantly - - PP/PET1 410.8 444.3 419.5 463.5 420.3 463.3 416.3 465.9 419.7 465.7 420.8 461.3 PP/PET1 410.8 444.3 419.5 463.5 420.3 463.3 416.3 465.9 419.7 465.7 420.8 461.3 increasedPP/PET2 with 407.5 the IV445.7 of PET 417.1 due to463.4 the longer 414.3 polymer 461.4 chain421.1 [1].463.3 Both the411.5 polyoleﬁns 465.0 and - polymer - PP/PET2 407.5 445.7 417.1 463.4 414.3 461.4 421.1 463.3 411.5 465.0 - - blendsPP/PET3 presented 407.2 remarkable 447.7 420.7 pseudoplastic 463.2 412. behavior,1 460.5 and417.5 LDPE 462.9 and its409.9 blends 461.8 revealed - a stronger - shearPP/PET3 thinning 407.2 effect than447.7 PP420.7 and its463.2 blends 412. because1 460.5 of the 417.5 better chain462.9 mobility.409.9 461.8 - - 3.2. AnalysisCompared of Rheological with LDPE, Behavior the incompatible LDPE/PET blend showed a higher viscosity at low 3.2.frequency Analysis and of Rheological a lower viscosity Behavior at high frequency. However, the complex viscosity of compatibilized Figure 5 shows the complex viscosity as a function of frequency at 260 °C for polymers and LDPE/PETFigure blend5 shows was the always complex lower viscosity than that as of a LDPE function except of frequency at an extremely at 260 low °C frequency, for polymers and alsoand polymer blends. The rheological curves of polymer blends showed similar trends with the neat polymerlower than blends. that of The incompatible rheological LDPE/PET curves of blendpolyme withinr blends the wholeshowed frequency similar range.trends However,with the onceneat constituents of polymers. Considering that the Tm of PET was approximately equal to 260 °C, all the constituentsthe compatibilizer of polymers. was formally Considering incorporated that the into Tm of the PET system, was approximately the viscosity of equal the system to 260 decreased,°C, all the rheological measurements of polymers and polymer blends were conducted in air without inert gas whichrheological was duemeasurements to the role ofof thepolymers compatibilizer and polymer in reducing blends were the dispersity conducted and in inducingair without interaction inert gas protection, which was consistent with the blending process. Three kinds of PET displayed a pseudo- protection,at the interface which between was consistent blend components, with the blending eventually process. increasing Three thekinds mobility of PET of displayed the compatibilized a pseudo- Newtonian behavior for the whole frequency range. The complex viscosity significantly increased Newtoniansystem [27, 28behavior]. PP showed for the thewhole lowest frequency viscosity range. at low The frequency, complex andviscosity the compatibilized significantly increased PP/PET with the IV of PET due to the longer polymer chain [1]. Both the polyolefins and polymer blends withblend the presented IV of PET intermediate due to the behavior longer polymer between ch theain incompatible [1]. Both the blend polyolefins and pure and PP. polymer As mentioned blends presented remarkable pseudoplastic behavior, and LDPE and its blends revealed a stronger shear presented remarkable pseudoplastic behavior, and LDPE and its blends revealed a stronger shear thinning effect than PP and its blends because of the better chain mobility. thinning effect than PP and its blends because of the better chain mobility.

Polymers 2018, 10, x FOR PEER REVIEW 8 of 14

Polymers 2018, 10, 147 8 of 14 above, the carbonyl group in PET could interact with the pendant group in the PP chain so that the blending system gets less viscous, whereas the compatibilizer played a role of softening agent [29]. Figure6 shows the complex viscosity of incompatible polymer blends. It can be seen that the viscosity of both the LDPE/PET and PP/PET blends present a slight increase as the IV of PET increases. This is probably due toFigure the fact 5. Complex that the viscosity higher IVforof polymers PET has and alonger polymer polymer blends. chain so that the PET itself possesses a higher viscosity. However, the blends with a higher content of PET did not exhibit a higherCompared viscosity with but displayed LDPE, the a slightincompatible decline. LDPE/P This suggestedET blend that showed the PET a dispersedhigher viscosity in polyolefins at low frequency and a lower viscosity at high frequency. However, the complex viscosity of compatibilized hadPolymers some 2018 lubrication, 10, x FOR PEER effect REVIEW in a way due to the presence of flexible chain segments in PET. 8 of 14 LDPE/PET blend was always lower than that of LDPE except at an extremely low frequency, and also lower than that of incompatible LDPE/PET blend within the whole frequency range. However, once the compatibilizer was formally incorporated into the system, the viscosity of the system decreased, which was due to the role of the compatibilizer in reducing the dispersity and inducing interaction at the interface between blend components, eventually increasing the mobility of the compatibilized system [27,28]. PP showed the lowest viscosity at low frequency, and the compatibilized PP/PET blend presented intermediate behavior between the incompatible blend and pure PP. As mentioned above, the carbonyl group in PET could interact with the pendant group in the PP chain so that the blending system gets less viscous, whereas the compatibilizer played a role of softening agent [29]. Figure 6 shows the complex viscosity of incompatible polymer blends. It can be seen that the viscosity of both the LDPE/PET and PP/PET blends present a slight increase as the IV of PET increases. This is probably due to the fact that the higher IV of PET has a longer polymer chain so that the PET itself possesses a higher viscosity. However, the blends with a higher content of PET did not exhibit a higher viscosity but displayed a slight decline. This suggested that the PET dispersed in polyolefins had some lubrication effect in a way due to the presence of flexible chain segments in Figure 5. Complex viscosity for polymers and polymer blends. PET. Figure 5. Complex viscosity for polymers and polymer blends. Compared with LDPE, the incompatible LDPE/PET blend showed a higher viscosity at low frequency and a lower viscosity at high frequency. However, the complex viscosity of compatibilized LDPE/PET blend was always lower than that of LDPE except at an extremely low frequency, and also lower than that of incompatible LDPE/PET blend within the whole frequency range. However, once the compatibilizer was formally incorporated into the system, the viscosity of the system decreased, which was due to the role of the compatibilizer in reducing the dispersity and inducing interaction at the interface between blend components, eventually increasing the mobility of the compatibilized system [27,28]. PP showed the lowest viscosity at low frequency, and the compatibilized PP/PET blend presented intermediate behavior between the incompatible blend and pure PP. As mentioned above, the carbonyl group in PET could interact with the pendant group in the PP chain so that the blending system gets less viscous, whereas the compatibilizer played a role of softening agent [29]. Figure 6 shows the complex viscosity of incompatible polymer blends. It can be seen that the viscosity of both the LDPE/PET and PP/PET blends present a slight increase as the IV of PET Figure 6. Complex viscosity for polymer blends with different IV of PET without compatibilizer. increases.Figure This 6. isComplex probably viscosity due to for the polymer fact that blends the with higher different IV of IV PET of PET has without a longer compatibilizer. polymer chain so that the PET itself possesses a higher viscosity. However, the blends with a higher content of PET did Figure 7 shows the complex viscosity for polymer blends with different IV of PET in the presence not exhibit a higher viscosity but displayed a slight decline. This suggested that the PET dispersed in of compatibilizer.Figure7 shows It the was complex in good viscosity agreement for polymer with the blends viscosity with sequence different IVof incompatible of PET in the presencepolymer polyolefins had some lubrication effect in a way due to the presence of flexible chain segments in blendsof compatibilizer. with different It was IV in of good PET. agreement Moreover, with it was the noticed viscosity that sequence the viscosity of incompatible of blends polymer with a higher blends PET. withcontent different of PET IV ofwas PET. significant Moreover, increased. it was noticed Several that thestudies viscosity have of shown blends withevident a higher results content that polyolefin/PETof PET was significant blends are increased. completely Several immiscible. studies By have adding shown compatibilizer evident results into the that blending polyolefin/PET system, blends are completely immiscible. By adding compatibilizer into the blending system, the interaction between the compatibilizer and blend components led to a reduction of interfacial tension and the suppression of coalescence [3,30–32]. Hence, it could be inferred that with the increase of PET content, the compatibilizer provided increasingly favorable support for an enhanced interaction between blend components.

Figure 6. Complex viscosity for polymer blends with different IV of PET without compatibilizer.

Figure 7 shows the complex viscosity for polymer blends with different IV of PET in the presence of compatibilizer. It was in good agreement with the viscosity sequence of incompatible polymer blends with different IV of PET. Moreover, it was noticed that the viscosity of blends with a higher content of PET was significant increased. Several studies have shown evident results that polyolefin/PET blends are completely immiscible. By adding compatibilizer into the blending system,

Polymers 2018, 10, x FOR PEER REVIEW 9 of 14 Polymers 2018, 10, x FOR PEER REVIEW 9 of 14 the interaction between the compatibilizer and blend components led to a reduction of interfacial tensionthe interaction and the suppressionbetween the ofcompatibilizer coalescence [3,30–32]. and blend He componentsnce, it could beled inferred to a reduction that with of the interfacial increase tension and the suppression of coalescence [3,30–32]. Hence, it could be inferred that with the increase Polymersof PET2018 content,, 10, 147 the compatibilizer provided increasingly favorable support for an enhanced9 of 14 interactionof PET content, between the blend compatibilizer components. provided increasingly favorable support for an enhanced interaction between blend components.

Figure 7. Complex viscosity for polymer blends with different IV of PET in the presence of Figure 7. Complex viscosity for polymer blends with different IV of PET in the presence compatibilizer.Figure 7. Complex viscosity for polymer blends with different IV of PET in the presence of of compatibilizer. compatibilizer. 3.3. Morphological Characterization of Polymer Blends 3.3.3.3. Morphological Characterization ofof PolymerPolymer BlendsBlends Figure 8 shows the morphology of incompatible and compatibilized polyolefin/PET1 polymer Figure8 shows the morphology of incompatible and compatibilized polyolefin/PET1 polymer blends.Figure It can 8 showsbe seen the that morphology the blending of incompatibleof two immiscible and compatibilized components results polyolefin/PET1 in an entirely polymer clear blends. It can be seen that the blending of two immiscible components results in an entirely clear phase phaseblends. interface. It can be The seen cross that profile the blending of LDPE/PET of two was immiscible a brittle fracturecomponents and PETresults dispersed in an entirely unevenly clear in interface. The cross profile of LDPE/PET was a brittle fracture and PET dispersed unevenly in LDPE LDPEphase phaseinterface. in the The form cross of profile particles. of LDPE/PET In yet another was way,a brittle the fracture large size and of PETthe circulardispersed hole unevenly indicated in phase in the form of particles. In yet another way, the large size of the circular hole indicated that the thatLDPE the phase PET-dispersing in the form phase of particles. appeared In yet in theanother PP continuous way, the large phase, size resulting of the circular in a typical hole sea-island indicated PET-dispersing phase appeared in the PP continuous phase, resulting in a typical sea-island structure. structure.that the PET-dispersing Figure 8 also phaseshows appeared the scanning in the elec PP troncontinuous microscopy phase, (SEM) resulting micrography in a typical of sea-island different Figure8 also shows the scanning electron microscopy (SEM) micrography of different amounts of amountsstructure. of Figure PET with 8 also or showswithout the compatibilizer. scanning elec Withtron microscopythe increase (SEM)of PET mi content,crography the holesof different of the PET with or without compatibilizer. With the increase of PET content, the holes of the LDPE/PET LDPE/PETamounts of and PET PP/PET with or blends without were compatibilizer. increased in quan Withtity, the size increase and dispersion of PET content, uniformity. the holes The blends of the and PP/PET blends were increased in quantity, size and dispersion uniformity. The blends with a withLDPE/PET a higher and content PP/PET of blends PET were were more increased likely in to quan be assembledtity, size and in thedispersion blending uniformity. process, resulting The blends in higher content of PET were more likely to be assembled in the blending process, resulting in the thewith enlargement a higher content of the of PETdispersed were morephase likely size. to Compared be assembled to incompatible in the blending polymer process, blends, resulting it was in enlargement of the dispersed phase size. Compared to incompatible polymer blends, it was observed observedthe enlargement that compatibilized of the dispersed LDPE/P phaseET blends size. had Compared a better dispersityto incompatible of holes polymer appearing blends, in the it cross was that compatibilized LDPE/PET blends had a better dispersity of holes appearing in the cross profile, the profile,observed the that fracture compatibilized surface was LDPE/P no longerET blends clear had but a presentedbetter dispersity a significant of holes layeredappearing hippocampal in the cross fracture surface was no longer clear but presented a significant layered hippocampal structure, and the structure,profile, the and fracture the circular surface hole was in noLDPE/PET longer clear blends but with presented PP-g-MAH a significant tended to belayered dispersed hippocampal uniformly circular hole in LDPE/PET blends with PP-g-MAH tended to be dispersed uniformly with diminishing withstructure, diminishing and the size. circular All these hole confirmed in LDPE/PET that blendsthe compatibilizer with PP-g-MAH had indeed tended improved to be dispersed the compatibility uniformly size. All these confirmed that the compatibilizer had indeed improved the compatibility between betweenwith diminishing polyolefins size. andAll thesePET. confirmed As mentioned that the above compatibilizer in the analysishad indeed of improvedrheological the behavior, compatibility the polyolefins and PET. As mentioned above in the analysis of rheological behavior, the compatibilizer compatibilizerbetween polyolefins enhanced and the PET. interaction As mentioned between theabove blend in componentsthe analysis which of rheological decreased thebehavior, interfacial the enhanced the interaction between the blend components which decreased the interfacial tension and tensioncompatibilizer and suppressed enhanced the the coalescence interaction [3,30–32]. between the blend components which decreased the interfacial suppressedtension and thesuppressed coalescence the coalescence [3,30–32]. [3,30–32].

Figure 8. SEM micrographs of incompatible (a,b,e,f) and compatibilized (c,d,g,h) polymer blends. Polymers 2018, 10, x FOR PEER REVIEW 10 of 14 Polymers 2018, 10, x FOR PEER REVIEW 10 of 14 Polymers 2018, 10, 147 10 of 14 Figure 8. SEM micrographs of incompatible (a, b, e, f) and compatibilized (c, d, g, h) polymer blends. Figure 8. SEM micrographs of incompatible (a, b, e, f) and compatibilized (c, d, g, h) polymer blends. The effect of the addition of different IV of PET on the blending system with or without The effect of the addition of different IV of PET on the blending system with or without compatibilizer was visualized by morphological characterization, as shown in Figures9 and 10, compatibilizer was visualized by morphological characterization, as shown in Figures 9 and 10, respectively. In Figure9, the dispersed phase in PP/PET blends was getting more and more clear respectively. In Figure 9, the dispersed phase in PP/PET blends was getting more and more clear with with the increase in the IV of PET; by contrast, the phase separation of the two blend components the increase in the IV of PET; by contrast, the phase separation of the two blend components was was attenuated gradually, so that a rough interface and more crack propagation could be seen in attenuated gradually, so that a rough interface and more crack propagation could be seen in PP/PET PP/PET blends. This could be explained in that it was more difﬁcult to diffuse into the polyoleﬁn blends. This could be explained in that it was more difficult to diffuse into the polyolefin phase for a phase for a longer polymer chain of PET, as a consequence, the thinner interface layer suggested poor longer polymer chain of PET, as a consequence, the thinner interface layer suggested poor compatibility. With the addition of compatibilizer (Figure 10), the PET dispersed uniformly and the compatibility. With the addition of compatibilizer (Figure 10), the PET dispersed uniformly and the interface coalescence was strengthened. Not surprisingly, in this situation, the effect of the IV of PET interface coalescence was strengthened. Not surprisingly, in this situation, the effect of the IV of PET on the dispersed phase size was weakened. on the dispersed phase size was weakened.

Figure 9. SEM micrographs of LDPE/PET (a, b, c) and PP/PET (d, e, f) blends with different IV of PET Figure 9. SEMSEM micrographs micrographs of of LDPE/PET LDPE/PET (a, ( ab,– cc)) andand PP/PET (d, (d –e,f )f) blends blends with withdifferent different IVIV ofof PET without compatibilizer (80/20). without compatibilizercompatibilizer (80/20).(80/20).

Figure 10. SEM micrographs of LDPE/PET (a, b, c) and PP/PET (d, e, f) blends with different IV of Figure 10. SEM micrographsmicrographs ofof LDPE/PETLDPE/PET ((aa,– cb,) andc) and PP/PET PP/PET (d –(fd,) blendse, f) blends with different with different IV of PET IV inof PET in the presence of compatibilizer (80/20/5). thePET presence in the presence of compatibilizer of compatibilizer (80/20/5). (80/20/5).

3.4. Mechanical Properties of Polymers before and after Blending

Polymers 2018, 10, 147 11 of 14

3.4. Mechanical Properties of Polymers before and after Blending Tables5 and6 show the elongation at break and the tensile strength of different polymer blends, respectively. The pure PET exhibited a decrease of elongation at break and an increase of tensile strength with the increase of IV. The polymer blends with a higher content of PET had a relatively lower elongation at break. As an immiscible polyolefin/PET blend, the interphase adhesiveness of two blending components was insufficient, which resulted in an evident interphase division, and therefore, worse interphase adhesiveness existed in polymer blends with higher content of PET. Without compatibilizer, the elongation at break of both LDPE/PET blends and PP/PET blends declined with the increase of the IV of PET. Yet it increased remarkably after the compatibilizer was added to polyolefin/PET blends, especially for the highest IV of PET3 in blends. With longer polymer chains and higher steric effects, the PET with a higher enlarged interphase between the polyolefins and the PET, showed a decrease of elongation at break. The compatibilized polymer blends were expected to have a higher elongation at break because of the increased interphase adhesiveness and enhanced interaction. In this case, PET3 with better mechanical properties had a positive effect on the elongation at break of blends. This is different from the PE/PET blend with organoclay as a compatibilizer, as there may be a possible degradation of the clay surfactant during melt compounding [33]. It was noted that the elongation at break of the LDPE blending system was higher than that of the PP blending system, which was consistent with the observation of the rheological behavior which found that LDPE/PET blends had the benefit of higher viscosity. In addition, the elongation at break of the polyolefin/PET blend increased with the increase of PET dispersed phase content, which was in good agreement with the results of the research by Tavanaie et al. [34].

Table 5. The elongation at break of polymer blends.

Blend 0/100 (%) 80/20 (%) 70/30 (%) 80/20/5 (%) 70/30/5 (%) 100/0 (%) LDPE/PET1 8.55 51.13 25.51 67.45 37.52 223.45 LDPE/PET2 8.45 44.67 23.50 78.25 41.47 - LDPE/PET3 6.41 44.16 22.21 148.54 58.97 - PP/PET1 8.55 15.1 11.7 38.5 30.5 428 PP/PET2 8.45 14.7 11.5 34.8 28.0 - PP/PET3 6.41 13.7 10.2 31.3 24.3 -

Table 6. The tensile strength of polymer blends.

Blend 0/100 MPa 80/20 MPa 70/30 MPa 80/20/5 MPa 70/30/5 MPa 100/0 MPa LDPE/PET1 26.0 16.71 23.52 14.32 18.25 12.31 LDPE/PET2 28.8 15.62 17.25 13.88 12.74 - LDPE/PET3 37.5 14.45 17.01 12.90 12.24 - PP/PET1 26.0 17.1 24.2 11.3 12.5 35.1 PP/PET2 28.8 20.6 25.0 11.9 12.41 - PP/PET3 37.5 24.4 25.2 12.0 12.2 -

The variation of the tensile strength of polymer blends with PET content was largely different from the elongation at break, in that it was somewhat confused. In such a situation, the polymer blends were brittle fractured to subdue the tenacity of the materials. With the increase of the IV of PET, the tensile strength of the LDPE/PET blends was decreased, while it was gradually increased for PP/PET blends. According to the SEM micrographs, the interphase adhesiveness of incompatible LDPE/PET blends was relatively weaker than that of PP/PET blends. However, with the addition of compatibilizer, LDPE/PET blends showed better tensile properties since the shorter chain segment of LDPE-g-MAH was more likely to diffuse successfully into two blend components. It was also noticed that the tensile strength of all polymer blends generally declined, and the difference of tensile strength caused by either changing the IV of PET or changing the PET content also decreased. Therefore, it can Polymers 2018, 10, 147 12 of 14 be stated that the compatibilizer played a role in increasing interphase adhesiveness to prevent brittle fracture, thus increasing the mechanical properties of polymer blends.

4. Conclusions The blending of LDPE and PP with different intrinsic viscosities (IV) of PET was performed in the absence or presence of compatibilizer. The impact of the blending components on the thermal, rheological, morphological and mechanical properties was discussed in detail, and the following conclusions were drawn from the discussion: (1) The blending leads to a higher crystallization temperature of the polyolefin component, yet the melting peak both of the polyolefin component and of the PET component in the blends are little different from their pure polymer. While the thermal stability of polymer blends is generally better than PET but worse than LDPE, and blending of PP with PET had no significant effect on the thermal stability of PP. The thermal performance of polyolefin/PET was slightly decreased by either the increase of the IV of PET or the decrease of PET content. It was found that the compatibilizer can improve the thermal stability of the blending system by enhancing the interface adhesion. (2) The complex viscosity of incompatible polymer blends is slightly higher than that of compatibilized blends. The results show that it is increased with the increase of the IV of PET, yet decreased with the increase of PET content increasing in blends. The complex viscosity of the blend with compatibilizer can be similarly increased by either increasing the IV of PET or increasing the PET content in blend. (3) PET disperses nonuniformly in LDPE in the form of granules; the cross-section of the incompatible blend shows brittle fracture. The number of holes in the blend increases with the increase of PET content. In contrast, the cross-section tends to be indistinct in the presence of LDPE-g-MAH as a compatibilizer. As the IV of the PET increases in the polymer blends, PET can be easily gathered to enlarge the scale of the dispersed phase, resulting in worse nonuniform dispersity, while it can be improved by adding compatibilizer. (4) It was demonstrated that with the increase of the IV of PET, the elongation at break of the polymer blend decreases slightly and the tensile strength of the LDPE/PET blend also decreased accordingly, while the tensile strength of the PP/PET blend gradually increased. It is worth noting that the higher content of PET in the blend results in a smaller elongation at break and lower tensile strength. When the compatibilizer was added to the blends, an increase of elongation at break and a decrease of tensile strength could be seen simultaneously, and the difference of tensile strength caused by either changing the IV of PET or changing the PET content declined.

Acknowledgments: Gratitude is expressed to the National Key Research and Development Program of China (No. 2016YFB0303000) and Zhejiang Provincial Natural Science Foundation of China (No. LQ18E030011), Science Foundation of Zhejiang Sci-Tech University (ZSTU) (No. 17012072-Y, 2017YBZX01) for their financial support. Author Contributions: Xian-Ming Zhang and Wen-Xing Chen conceived and designed the experiments; Guo-Cai Zhong and Jian Li performed the blending experiments and thermal analysis testing; Li-Hao Zhang conducted the SEM, rheology and mechanical properties measurement; Shi-Chang Chen and Guo Zhang contributed reagents/materials/analysis tools; Shi-Chang Chen analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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