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Morphological, Rheological, and Mechanical Properties of Polyamide 6/Polypropylene Blends Compatibilized by Electron-Beam Irradiation in the Presence of a Reactive Agent Boo Young Shin 1, *, Man Ho Ha 2 and Do Hung Han 1 1 2

*

School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Korea; [email protected] R & D Center, Korea Petrochemical Limited, Ulsan 44785, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-53-810-2511; Fax: +82-53-810-4631

Academic Editor: Volker Altstädt Received: 30 March 2016; Accepted: 2 May 2016; Published: 6 May 2016

Abstract: An immiscible polyamide 6 (PA6)/polypropylene (PP) blend was compatibilized by electron-beam irradiation in the presence of reactive agent. Glycidyl methacrylate (GMA) was chosen as a reactive agent for interfacial cross-copolymerization between dispersed PP and continuous PA6 phases initiated by electron-beam irradiation. The PA6/PP (80/20) mixture containing GMA was prepared using a twin-screw extruder, and then exposed to an electron-beam at various doses at room temperature to produce compatibilized PA6/PP blends. The morphological, rheological, and mechanical properties of blends produced were investigated. Morphology analysis revealed that the diameter of PP particles dispersed in PA6 matrix was decreased with increased irradiation dose and interfacial adhesion increased due to high surface area of treated PP particles. Complex viscosities (η*) and storage moduli (G’) of blends increased with increasing irradiation dose and were higher than those of PA6 and PP. The complex viscosity of the blend irradiated at 200 kGy was 64 and 8 times higher than PA6 and PP, respectively. The elongation at break of blend irradiated less than 100 kGy was about twice that of PA6. Electron beam treatment improved the compatibility at the interface between PA6 and PP matrix in the presence of GMA. Keywords: compatibilization; PA6/PP blend; electron-beam irradiation; morphology; rheological properties; mechanical properties; glycidyl methacrylate (GMA)

1. Introduction Polyamide 6 (PA6), commercially known as nylon 6, is a crystalline engineering thermoplastic which is tough, strong, and abrasion resistant and has a high melting temperature [1–3]. However, its high cost, low dimensional stability due water absorption, and low melt viscosity limits its utilization in specific applications [3]. For these reasons, PA6 can be blend with other thermoplastic as PP for improving desired properties, because PP is a high-volume cheap thermoplastic and shows lower water absorption, and higher melt viscosity. Unfortunately, PA6 and PP are immiscible due to their structural and polarity differences [1,4], and, thus, several compatibilization strategies have been introduced [5–7]. Some research studies were conducted over recent years on the effects of irradiation on the properties of polymer blends [8–10], expecting cross-copolymerization (such as grafting or crosslinking) at the interface between the continuous and dispersed phases by using high-energy radiation without any reactive agent. However, to facilitate radiation-initiated cross-copolymerization effectively at the interface, a reactive agent is needed to be added [11,12]. Because electron-beam exposure process is usually performed at ambient temperature and immiscible blends have a gap at interface, which might be needed to fill before Materials 2016, 9, 342; doi:10.3390/ma9050342

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Materials process. 2016, 9, 342 To the best of our knowledge, no previous study is reported describing 2 of 11 the irradiation compatibilization of a PA6/PP blend by electron-beam-initiated interfacial cross-copolymerization in ambient temperature and immiscible blends have a gap at interface, which might be needed to fill the presence of a reactive agent. before irradiation process. To the best of our knowledge, no previous study is reported describing In this study, we compatibilized a PA6/PP blend using an electron-beam irradiation in the the compatibilization of a PA6/PP blend by electron-beam-initiated interfacial cross-copolymerization in presence of reactive agent. Monomeric GMA was chosen as a reactive agent for interfacial the presence of a reactive agent. cross-copolymerization because it has two reactive sites, namely, an epoxy functional In this study, we compatibilized a PA6/PP blend using an electron-beam irradiation group in the and a double bond. The epoxy group reacts easily with other functional groups during melt mixing [13–15] presence of reactive agent. Monomeric GMA was chosen as a reactive agent for interfacial and the double bond can bebecause easily opened byreactive a radical species. due togroup low molecular cross-copolymerization it has two sites, namely,Furthermore, an epoxy functional and a double The group reacts easily with other functional during melt mixing [13–15] weight, it canbond. easily beepoxy diffused to the interface during melt mixinggroups [16]. The morphological, rheological, and the double bond can opened by a were radical species. Furthermore, due low molecular and mechanical properties of be theeasily obtained blends measured and analyzed to to determine the effects weight, it can easily be diffused to the interface during melt mixing [16]. The morphological, of this proposed compatibilization strategy.

rheological, and mechanical properties of the obtained blends were measured and analyzed to determine effects of this proposed compatibilization strategy. 2. Results andthe Discussion 2. Results and Discussion 2.1. Morphology

The cryofractured surface of the non-irradiated blend (Figure 1a) showed irregular and large 2.1. Morphology imbeddedThe PPcryofractured particles with clear boundaries between the dispersed (PP) and continuous (PA6) surface of the non-irradiated blend (Figure 1a) showed irregular and large phases. In Figure 1a, SEM images showed largest PP particles (diameters in non-irradiated imbedded PP particles with clear boundaries between the dispersed (PP)ě30 andµm) continuous (PA6) sample. However, irradiation at 20 kGy markedly reduced the diameters of dispersed PP particles phases. In Figure 1a, SEM images showed largest PP particles (diameters ≥30 μm) in non-irradiated and weakened boundaries (Figure 1b).kGy This trend continued further withofhigh irradiation doses and sample. However, irradiation at 20 markedly reduced the diameters dispersed PP particles and weakened boundaries 1b). This trend with at high doses1c). and This finally these boundaries were(Figure not observed in thecontinued sample further irradiated 200irradiation kGy (Figure finally these boundaries observeddue in the sample irradiated at 200 kGytension (Figure between 1c). This PA6 observed decrease in particlewere sizenot is believed to the reduction in interfacial observed decreaseAccordingly, in particle size believedthat due electron-beam to the reductionirradiation in interfacialintension betweenofPA6 and PP components. weissuggest the presence reactive and PP components. Accordingly, we suggest that electron-beam irradiation in the presence of agent GMA induced cross-copolymerization due to grafting and crosslinking at the interface between reactive agent GMA induced cross-copolymerization due to grafting and crosslinking at the interface dispersed PP and PA6 continuous phases [10]. between dispersed PP and PA6 continuous phases [10].

Figure 1. SEM images of cryofractured surfaces of PA6/PP (80/20) blends irradiated at 0 kGy (a); Figure 1. SEM images of cryofractured surfaces of PA6/PP (80/20) blends irradiated at 0 kGy (a); 20 kGy (b); and 200 kGy (c). 20 kGy (b); and 200 kGy (c).


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Tensile provide better insight of interfacial behaviors which, generally not observed Tensile fractures fracturescan can provide better insight of interfacial behaviors which, generally not on cryofracture surfaces. Therefore, we observed tensile fractured surface of blends irradiated at 0 kGy, observed on cryofracture surfaces. Therefore, we observed tensile fractured surface of blends 20 kGy, andat200 kGy, as in Figure 2. In non-irradiated sample,2.large spherical particles (diameter irradiated 0 kGy, 20shown kGy, and 200 kGy, as shown in Figure In non-irradiated sample, large about 30 µm) were observed imbedded in elongated holes, which presumably were formed by spherical particles (diameter about 30 μm) were observed imbedded in elongated holes, pulled which out PP particles. spherical natureout of these PP particles lack ofofinterfacial presumably wereThe formed by pulled PP particles. The indicates spherical anature these PP adhesion particles and a failure to of transfer tensile stress across interface. On the other hand, sample irradiatedOn at indicates a lack interfacial adhesion and a the failure to transfer tensile stress across the interface. 20 kGy showed elongated PP microfibrils, indicating sufficient interfacial adhesion to transfer tensile the other hand, sample irradiated at 20 kGy showed elongated PP microfibrils, indicating sufficient stress. Sample produced at 200 kGy stillstress. showed microfibrils with at greater diameter than that of sample interfacial adhesion to transfer tensile Sample produced 200 kGy still showed microfibrils irradiated 20 kGy due to reduced elongation of blend. These morphological results showed that with greater diameter than that of sample irradiated 20 kGy due to reduced elongation of blend. the compatibility of PA6/PP blend was greatly enhanced, which presumably was due to interfacial These morphological results showed that the compatibility of PA6/PP blend was greatly enhanced, cross-copolymerization an electron-beam radiation process in theby presence of reactive which presumably wasinduced due to by interfacial cross-copolymerization induced an electron-beam agent GMA. radiation process in the presence of reactive agent GMA.

Figure 2. SEM images of the tensile fractured surfaces of PA6/PP (80/20) blends irradiated at 0 kGy (a); Figure 2. SEM images of the tensile fractured surfaces of PA6/PP (80/20) blends irradiated at 0 kGy 20 (b); (b); andand 200 200 kGykGy (c). (c). (a);kGy 20 kGy

2.2. Mechanisms of PA6/PP Compatibilization 2.2. Mechanisms of PA6/PP Compatibilization Here, we propose reaction mechanisms for the electron-beam-initiated cross-copolymerization Here, we propose reaction mechanisms for the electron-beam-initiated cross-copolymerization with GMA as a reactive agent at the interface. Scheme 1 estimates ring opening (temperature with GMA as a reactive agent at the interface. Scheme 1 estimates ring opening (temperature dependent) reaction occurring between functional group of GMA and PA6 during the melt mixing, dependent) reaction occurring between functional group of GMA and PA6 during the melt mixing, whereby the epoxy group of GMA reacts with primary amine and/or carboxylic acid end groups of whereby the epoxy group of GMA reacts with primary amine and/or carboxylic acid end groups of PA6 to form linear PA6−GMA [14,17,18]. In addition, the epoxy group of GMA can react with the PA6 to form linear PA6´GMA [14,17,18]. In addition, the epoxy group of GMA can react with the secondary amine of the amide to form GMA grafted PA6 (PA6−g−GMA) polymeric chains secondary amine of the amide to form GMA grafted PA6 (PA6´g´GMA) polymeric chains (branching). (branching).

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Scheme 1. 1. Schematics ofof the ring opening reaction occurring between the GMA GMA and PA6 PA6 during melt Scheme the ring opening reaction occurring between the and melt 1. Schematics Schematicsof the ring opening reaction occurring between the GMA and during PA6 during mixing. melt mixing. mixing.

Scheme 22 involved involved formation formation of of macroradicals, macroradicals, namely namely PA6•, PA6•, PP•, PP•, PA6−GMA•, PA6−GMA•, and Scheme Scheme 2 involved formation of macroradicals, namely PA6‚, PP‚, PA6´GMA‚, and and PA6−g−GMA•. The The PA6• PA6• and and PP• PP• represent represent radicals radicals formed formed by by C−H C−H cleavage cleavage at at the the arbitrary arbitrary carbon carbon PA6−g−GMA•. PA6´g´GMA‚. The PA6‚ and PP‚ represent radicals formed by C´H cleavage at the arbitrary atom sites sites of of PA6 PA6 and and hydrogen hydrogen abstraction abstraction at at the the quaternary quaternary carbon carbon atom atom sites sites of of PP PP [19]. The The atom carbon atom sites of PA6 and hydrogen abstraction at the quaternary carbon atom sites of[19]. PP [19]. PA6−GMA• and and PA6−g−GMA• PA6−g−GMA• seems seems to to be produced produced through through opening opening the double double bonds in in the PA6−GMA• The PA6´GMA‚ and PA6´g´GMA‚ seemsbe to be produced through openingthe the doublebonds bonds in the the PA6−GMA and PA6−g−GMA by hydrogen radical (H•), as shown in Scheme 2. PA6−GMA and PA6−g−GMA by hydrogen radical (H•), as shown in Scheme 2. PA6´GMA and PA6´g´GMA by hydrogen radical (H‚), as shown in Scheme 2.

Scheme 2. 2. Electron-beam Electron-beam initiation initiation schemes schemes of of the the PA6, PA6, PP, PP, PA6−GMA, PA6−GMA, and and PA6−g−GMA. PA6−g−GMA. Scheme Scheme 2. Electron-beam initiation schemes of the PA6, PP, PA6´GMA, and PA6´g´GMA.

Scheme 33 estimates estimates cross-copolymerization cross-copolymerization reactions reactions (grafting (grafting and and crosslinking) crosslinking) under under active active Scheme Scheme 3 estimates cross-copolymerization reactions (grafting and crosslinking) under active radical centers centers at at the the PA6−PP PA6−PP interface interface to to produce produce hybrid hybrid branched branched macromolecules macromolecules or or network network radical radical centers at the PA6´PP interface to produce hybrid branched macromolecules or network structures [20]. Grafting would involve PA6−GMA• and PP•, while crosslinking would involve structures [20]. Grafting would involve PA6−GMA• and PP•, while crosslinking would involve structures [20]. and Grafting would involve PA6´GMA‚ and PP‚, while crosslinking would involve PA6−g−GMA• PP• and both are likely to increase interfacial adhesion between PA6 and PP PP PA6−g−GMA• and PP• and both are likely to increase interfacial adhesion between PA6 and phases. According According to to these these schemes, schemes, the the GMA GMA seems seems to to be be very very important important agent agent for for cross-copolymerization cross-copolymerization phases. between PA6 and PP polymeric chains. between PA6 and PP polymeric chains.


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PA6´g´GMA‚ and PP‚ and both are likely to increase interfacial adhesion between PA6 and PP phases. According to these schemes, the GMA seems to be very important agent for cross-copolymerization between PA69,and Materials 2016, 342 PP polymeric chains. 5 of 11

Scheme 3. 3. Suggested Suggested mechanisms mechanisms of of interfacial interfacial cross-copolymerization. cross-copolymerization. Scheme

These cross-copolymerization reactions at interface would probably reduce the dispersed PP These cross-copolymerization reactions at interface would probably reduce the dispersed PP sizes by reducing the interfacial tension and stabilize the morphology by preventing the coagulation sizes by reducing the interfacial tension and stabilize the morphology by preventing the coagulation of dispersed particles, and greatly increase interfacial adhesion, as indicated by the morphological of dispersed particles, and greatly increase interfacial adhesion, as indicated by the morphological analysis of the treated samples [1]. analysis of the treated samples [1]. 2.3. Rheological Properties 2.3. Rheological Properties The rheological properties of polymers are highly dependent on molecular weights and The rheological properties of polymers are highly dependent on molecular weights and structures, structures, such as, branching and crosslinking [21–23]. Figure 3 shows the complex viscosities of such as, branching and crosslinking [21–23]. Figure 3 shows the complex viscosities of PA6, PP, PA6, PP, and PA6/PP blends irradiated at 0 kGy, 10 kGy, 20 kGy, 50 kGy, 100 kGy, and 200 kGy. PA6 and PA6/PP blends irradiated at 0 kGy, 10 kGy, 20 kGy, 50 kGy, 100 kGy, and 200 kGy. PA6 exhibited Newtonian flow behavior over the evaluated frequency range having a low melt viscosity exhibited Newtonian flow behavior over the evaluated frequency range having a low melt viscosity of around 300 Pa∙s. On the other hand, PP had higher complex viscosities than PA6 and showed weak of around 300 Pa¨s. On the other hand, PP had higher complex viscosities than PA6 and showed shear-thinning behavior. Remarkably, all PA6/PP blends had very higher complex viscosities than weak shear-thinning behavior. Remarkably, all PA6/PP blends had very higher complex viscosities PA6 or PP and showed a significant shear-thinning behavior at all observed frequencies. At frequency than PA6 or PP and showed a significant shear-thinning behavior at all observed frequencies. At of 0.1 rad/s, PA6/PP blends had 15~64 and 2~8 times higher complex viscosities than PA6 and PP, frequency of 0.1 rad/s, PA6/PP blends had 15~64 and 2~8 times higher complex viscosities than PA6 respectively. In addition, the dependency of complex viscosity on the frequency increased. The and PP, respectively. In addition, the dependency of complex viscosity on the frequency increased. increase in the complex viscosity and non-Newtonian behavior of PA6/PP blends on increasing The increase in the complex viscosity and non-Newtonian behavior of PA6/PP blends on increasing irradiation dose suggested increased interfacial adhesion. irradiation dose suggested increased interfacial adhesion.

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Figure of PA6/PP Figure 3. Complex viscosities (80/20) atdifferent different dosages as function of Figure3. 3.Complex Complexviscosities viscositiesof ofPA6/PP PA6/PP(80/20) (80/20)blends blendsirradiated irradiatedat differentdosages dosagesas asaaafunction functionof of ˝ frequency at 235 °C. frequency C. frequencyat at235 235 °C.

Figure 44 shows aa plot of complex viscosity versus irradiation dose at frequencies of 0.1 rad/s, Figure 4 shows a plotof of complex complexviscosity viscosityversus versusirradiation irradiationdose doseat atfrequencies frequenciesof of0.1 0.1rad/s, rad/s, Figure shows plot 11 rad/s, 10 rad/s, and 100 rad/s. As shown in Figure 4, complex viscosity was observed to increase rad/s, 10 10 rad/s, rad/s, and rad/s. As complex viscosity viscosity was was observed observed to to increase increase 1 rad/s, and 100 100 rad/s. As shown shown in in Figure Figure 4, 4, complex markedly at 0.1 rad/s on increasing irradiation dose, but complex viscosities at 11rad/s, 10 rad/s, and markedly at 0.1 rad/s on increasing irradiation dose, but complex viscosities at rad/s, 10 rad/s, and markedly at 0.1 rad/s on increasing irradiation dose, but complex viscosities at 1 rad/s, 10 rad/s, 100 rad/s increased at lower rate on increasing irradiation dose. According to the literature [24], 100 rad/s increased at lower rate on increasing irradiation dose. According to the literature [24], and 100 rad/s increased at lower rate on increasing irradiation dose. According to the literature [24], rheological frequencies represent interfacial properties of polymer blends due to rheologicalproperties propertiesat atlower lower due to rheological properties at lower frequencies frequencies represent representinterfacial interfacialproperties propertiesofofpolymer polymerblends blends due relatively long relaxation time of droplet shape. The above results indicate that compatibility of blend relatively long relaxation time of droplet shape. The above results indicate that compatibility of blend to relatively long relaxation time of droplet shape. The above results indicate that compatibility of was by irradiation in presence of GMA and that compatibility was was improved improved by electron-beam electron-beam irradiation in the the presence ofof GMA was blend was improved by electron-beam irradiation in the presence GMAand andthat that compatibility compatibility was dependent on irradiation dose. dependent on irradiation dose. dependent on irradiation dose.

Figure viscosities of (80/20) blends as of dose Figure 4. 4. Complex Complex of PA6/PP PA6/PP (80/20) blends asaa function function of irradiation irradiation dose at at different different Figure 4. Complexviscosities viscosities of PA6/PP (80/20) blends as a function of irradiation dose at frequencies. frequencies. different frequencies.

Figure Figure 55 shows shows changes changes in in the the storage storage moduli moduli of of PA6, PA6, PP, PP, and and of of PA6/PP PA6/PP blends blends irradiated irradiated at at Figure 5 shows changes in the 5, storage moduli of PA6, PP,blends and ofwere PA6/PP blends irradiated at different doses. As shown by Figure the storage moduli of all higher than those different doses. As shown by Figure 5, the storage moduli of all blends were higher than thoseof ofPA6 PA6 different doses. As shown by Figure 5, the storageFurthermore, moduli of all blends moduli were higher than those of and and PP PP over over the the entire entire frequency frequency range range [24,25]. [24,25]. Furthermore, storage storage moduli of of PA6/PP PA6/PP blends blends PA6 and PP over the entire frequency range [24,25]. Furthermore, storage moduli of PA6/PP blends increased increased with with increasing increasing irradiation irradiation dose dose and and the the storage storage modulus modulus curves curves of of blends blends converged converged at at increased with increasing irradiation dose and the The storage modulus curvespure of blendsare converged at the the the same same plateau plateau modulus modulus at at higher higher frequencies. frequencies. The storage storage moduli moduli of of pure PA6 PA6 are very very low low at at aa same plateau modulus at higher frequencies. The storage moduli of pure PA6 are very low at a lower lower lowerfrequency frequencyrange, range,resulting resultingin influctuation fluctuationdue dueto tothe thedetecting detectinglimit limitof ofARES. ARES. frequency range, resulting in fluctuation due to the detecting limit of ARES.


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Figure 5. Storage moduli of of PA6/PP PA6/PP (80/20) Figure 5. Storage moduli (80/20)blends blendsirradiated irradiatedat at different different dosages dosages as as aa function function of of Figure 5. Storage moduli of PA6/PP (80/20) blends irradiated at different dosages as a function of ˝ frequency frequency at at 235 235 °C. C. frequency at 235 °C.

Modified Cole-Cole plots, log storage modulus (G′) log loss modulus (G″), are useful for 1 ) vs. Modified Cole-Cole plots, log modulus (G vs. log log loss loss modulus modulus (G″), (G”), are are useful useful for for Modified Cole-Cole plots, log storage storage modulus (G′) analyzing structural change of polymer molecule, such as vs. branching and crosslinking [21–23]. The analyzing structural change change of of polymer molecule, such branching crosslinking [21–23]. The analyzing structural polymer molecule, such as as branching, branching and and [21–23]. The effects of molecular weight, molecular weight distribution, and crosslinking crosslinking on modified effects of molecular weight, molecular weight distribution, branching, and crosslinking on modified effects of molecular weight, molecular weight and crosslinking on modified Cole-Cole plot for various polymers have been distribution, investigated branching, both experimentally and theoretically. In Cole-Cole plot for various various polymers polymers have have been been investigated investigated both both experimentally experimentally and and theoretically. theoretically. In In Cole-Cole plot for general, for linear polymers, the plot is not dependent on molecular weight but dependent on chain general, for linear polymers, the plot is not dependent on molecular weight but dependent on chain general, forand linear polymers,and the slightly plot is not dependent molecular weight but dependent on chain branching crosslinking dependent onon molecular weight distribution. As shown in branching and crosslinking and slightly dependent on molecular weight distribution. As shown in branching and crosslinking and slightly dependent on molecular weight distribution. As shown in Figure 6, the plots of blends were not coincident, presumably due to structural difference. Figure Figure 6, 6, the the plots plots of of blends blends were were not not coincident, coincident, presumably presumably due due to to structural structural difference. difference.

Figure 6. Plots of storage modulus (G’) against loss modulus (G”) for virgin PA6, PP, and PA6/PP Figure 6. 6. Plots of (G’) against PA6, PP, PP, and and PA6/PP PA6/PP Figure Plotsirradiated of storage storageatmodulus modulus against loss loss modulus modulus (G”) (G”) for for virgin virgin PA6, (80/20) blends different (G’) dosages. (80/20) blends (80/20) blends irradiated irradiated at at different different dosages. dosages.

These rheological results supported the morphological analysis results and suggested reactions. These rheological resultsofsupported theproperties morphological analysis results and suggested reactions. In addition, the dependence rheological on irradiation dose presumably suggests that These rheological results supported the morphological analysis results and suggested reactions. In addition, the dependence of rheological properties on irradiation dose presumably suggests that the compatibility of PA6/PP blends depends on irradiation dose. In addition, the dependence of rheological properties on irradiation dose presumably suggests that the the compatibility of PA6/PP blends depends on However,ofthese rheological results could beirradiation affected bydose. gel formation of constituent. Therefore, compatibility PA6/PP blends depends on irradiation dose. However, these rheological results could be affected bythe gel formation of constituent. Therefore, we extracted blend by dissolving PA6 in formic acid. Then, component of residue must be However, these rheological results could be affected by gelmain formation of constituent. Therefore, we extracted blend by dissolving PA6 in formic acid. Then, the main component of residue must be PP and the blend minorby component might be gel acid. of PA6 and induced by electron-beam we extracted dissolving PA6 in formic Then, thePA6-co-PP main component of residue must be PP PP and the minor component might be gel ofitPA6 and PA6-co-PP induced by and electron-beam irradiation in the presence ofmight GMA. was difficult to separate gel of PA6 PA6-co-PP and the minor component beUnfortunately, gel of PA6 and PA6-co-PP induced by electron-beam irradiation irradiation in the presence of GMA. Unfortunately, it was difficult to separate gel of PA6 and PA6-co-PP from main component of PP. As listed in Table 1, there was no significant increase of residue with in the presence of GMA. Unfortunately, it was difficult to separate gel of PA6 and PA6-co-PP from from main component of PP. As listed in Table 1, there was no significant increase of residue with increasing irradiation which indicates that thenogel content increase of PA6 is Inincreasing previous main component of PP.dose, As listed in Table 1, there was significant of negligible. residue with increasing irradiation dose, from which40~150 indicates that gel content [3] of and PA6 PA6 is negligible. In studies, neat PA6which irradiated had the 0% of gelPA6 content irradiated at previous 200 kGy irradiation dose, indicates that thekGy gel content is negligible. In previous studies, neat studies, neat PA6 irradiated from 40~150 kGy had 0% gel content [3] and PA6 irradiated at complex 200 kGy in the presence of GMA showed only 1.3% of gel content [18]. Accordingly, the increase in PA6 irradiated from 40~150 kGy had 0% gel content [3] and PA6 irradiated at 200 kGy in the presence in the presence of GMAobserved showed only 1.3% of gel content [18]. increase in complex viscosity and modulus on increasing irradiation doseAccordingly, was not duethe to PA6 crosslinking by viscosity and modulus observed on increasing irradiation dose was not due to PA6 crosslinking electron-beam irradiation but rather due to the cross-copolymerization at the interface, as shown by in electron-beam irradiation but rather due to the cross-copolymerization at the interface, as shown in Scheme 3. Scheme 3.


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of GMA showed only 1.3% of gel content [18]. Accordingly, the increase in complex viscosity and modulus observed on increasing irradiation dose was not due to PA6 crosslinking by electron-beam irradiation but rather due to the cross-copolymerization at the interface, as shown in Scheme 3. Table 1. Thermal properties of PA6, PP, and blends, and extraction result of blends. Sample

T m1 (˝ C)

Xc (%) of PA6 in blend

T m2 (°C)

Xc (%) of PA6 in Blend

Residue (%)

PA6 PP Blend irradiated at 0 kGy Blend irradiated at 10 kGy Blend irradiated at 20 kGy Blend irradiated at 50 kGy Blend irradiated at 100 kGy Blend irradiated at 200 kGy

– 164 157 156 155 155 153 151

– 33.1 31.7 39.5 39.4 26.3 26.4 27.0

216 – 216 216 216 217 215 214

32.0 – 28.5 28.2 24.6 27.8 27.1 27.5

– – 20.1 20.5 20.8 20.3 20.5 21.8

Tm1 and Tm2 are the melting temperatures of PP and PA6 phases in the blend respectively.

2.4. Mechanical Properties Mechanical performance is an essential parameter for practical applications in the plastic industry and is dependent on9, 342 improved compatibility of polymer blends. The changes in elongation Materials 2016, 8 of 11 at break of PA6/PP blends irradiated at different doses are shown in Figure 7. PA6/PP blends irradiated at 2.4. Mechanical Properties less than 100 kGy, including non-irradiated blends, elongation at break values of PA6/PP blends Mechanical performance is an essential parameter for practical applications in the plastic were observed twice than that of pure PA6. In contrast, elongation at break values of PA6/PP industry and is dependent on improved compatibility of polymer blends. The changes in elongation blends irradiated 100 and 200 kGy were reduced to that PA6; in spite markedly at break of PA6/PP blends irradiated at different dosesof arepure shown in Figure 7. of PA6/PP blends increased compatibility as evidenced inkGy, the including results non-irradiated of morphological investigation. strengths at irradiated at less than 100 blends, elongation at break The valuestensile of PA6/PP blends blends were observed twice than that of pure PA6. In contrast, elongation at break values PA6/PP break PA6/PP approximately increased with increasing irradiation doseofdue to increased blends irradiated 100 and 200 kGy were reduced to that of pure PA6; in spite of markedly increased compatibility, as shown in Figure 8. Mechanical properties might be also affected by crystallinity and compatibility as evidenced in the results of morphological investigation. The tensile strengths at gel contentbreak of constituents inapproximately blend. As shown in Table 1, melting temperature andtocrystallinity of PA6 PA6/PP blends increased with increasing irradiation dose due increased were affected very little by irradiation dose; however, the crystallinity of PP increased up to 20 kGy and compatibility, as shown in Figure 8. Mechanical properties might be also affected by crystallinity and gel content of constituents in blend. As shown in Table 1, melting temperature and crystallinity of then decreased, which might be due to both the degradation and crosslinking of molecule [11]. The PA6 were affected very little by irradiation dose; however, the crystallinity of PP increased up to ˝ melting temperature of PP in the blend decreased with increasing irradiation dose from 164 C (pure 20 kGy and then decreased, which might be due to both the degradation and crosslinking of molecule ˝C PP) to 151[11]. (200 kGy) to degradation. Inblend addition, change gel content of PA6 with The meltingdue temperature of PP in the decreased with in increasing irradiation dose fromirradiation dose was already discussed the increase in elongation of blends compared with 164 °C (pure PP) to 151above °C (200and, kGy)therefore, due to degradation. In addition, change in gel content of PA6 dose was already discussedof above and, therefore, thethe increase in elongation of blends of blends PA6 mightwith be irradiation due to increased compatibility blend. However, decrease in elongation might be due to increased compatibility of blend. However, the decrease in irradiated compared over 100with kGyPA6 may be caused by degradation of PP, not by formation crosslinking or change elongation of blends irradiated over 100 kGy may be caused by degradation of PP, not by formation in crystallinity of PA6 [11]. crosslinking or change in crystallinity of PA6 [11].

Figure 7. Effect of irradiation dose on the elongation at break of PA6/PP (80/20) blends.



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8. Effect of irradiation dose thetensile tensilestrength strength at (80/20) blends. FigureFigure 8. Effect of irradiation dose onon the at break breakofofPA6/PP PA6/PP (80/20) blends.

3. Experimental 3.1. Materials Polyamide 6 (Domamid® 24) with a density of 1.14 g/cm3 was obtained from DOMO Caproleum GmbH (Premnitz, Germany). Glycidyl methacrylate (GMA) and formic acid were provided by Sigma-Aldrich (Milwaukee, WI, USA). Polypropylene (YUHWA POLYPRO® 4017M) with a density of 0.9 g/cm3 and melt flow index (MFI) of 14 g/(10 min) at 230 ˝ C and 2.16 kg was obtained from Korea Petrochemical Inc. Co. Ltd. (Ulsan, Korea). 3.2. Melt Mixing of PA6, PP, and GMA The blend ratio 80/20 (weight percent) PA6 to PP was chosen and the amount of GMA was fixed at 3 parts per hundred resin (phr) based on the total mass of PA6 and PP. PA6, PP, and GMA were mixed in a plastic bag before being extruded in a twin-screw co-rotating extruder (SM PLATEK Co. Ltd., TEK 30MHS, Ansan, Korea). Screw diameter was 31.6 mm with 40:1 L/D ratio. The extruder was operated at 200 rpm with a constant feed rate of 20 kg/h. The barrel and die temperatures were set at 200~240 ˝ C and 245 ˝ C, respectively. The extrudate was immediately cooled in chilled water and cut into pellets of a diameter less than 1 mm. Then, the pellets were dried for 24 h at 80 ˝ C prior to the electron-beam irradiation. 3.3. Electron-Beam Irradiation Obtained pellets were irradiated using a commercial electron-beam accelerator (ELV-0.5, BINP, Novosibirsk, Russia) with a maximum beam current of 40 mA and a beam energy range of 0.5–0.7 MeV under a nitrogen atmosphere at room temperature. The irradiation doses were 10 kGy, 20 kGy, 50 kGy, 100 kGy and 200 kGy, which were controlled by varying beam current from 0.5 to 10 mA and a conveyor speed from 1 to 2 m/min. The irradiation doses were measured using film dosimeters (B3 WINdose Dosimetry, GEX Co., Centennial, CO, USA) and a dosimeter (GENESYS 20, Thermo SCIENTIFIC Co., Waltham, MA, USA). Acceleration voltage was 0.7 MeV and the effective penetration depth was about 2 mm for a substrate of density 1 g/cm3 [26,27]. The irradiated samples were dried in an oven at 80 ˝ C for 12 h to eliminate residual radicals. 3.4. Characterization The morphologies of the blends were examined by observing cryogenic and tensile fracture surfaces using a scanning electron microscope (SEM, Hitachi model s-4200, Tokyo, Japan). Rheological properties were measured using an ARES (Advanced Rheometric Expansion System: Rheometric


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Scientific Co. Ltd., New Castle, DE, USA) rotational rheometer. The equipment was run in parallel plate configuration at 235 ˝ C and a strain of 2% in the angular frequency range of 0.1 to 100 rad/s. Mechanical properties of the blends were determined using INSTRON 4464 tensile tester (INSTRON, Norwood, MA, USA). Tests were performed on tensile bars (type II) that were compression molded according to the KS M3600 test method using a hot press (Model 3851-O, Carver Inc., Wabash, IN, USA) at a set temperature of 240 ˝ C and a molding pressure of 14 MPa. The experiment was performed at room temperature with a gauge speed of 10 mm/min and a gauge length of 35 mm. All tests were performed in four fold and only the average value was reported. Thermal properties were determined using differential scanning calorimetry (DSC; TA INSTRUMENTS Q200, New Castle, DE, USA). Samples were heated from room temperature to 250 ˝ C at a rate of 20 ˝ C/min and maintained at 250 ˝ C for 3 min to remove the thermal history. Subsequently, they were quenched to ´30 ˝ C then reheated to 250 ˝ C at 10 ˝ C/min under a nitrogen atmosphere. The degree of crystallinity, Xc , was calculated as: H f ,i {φi Xc p%q “ ˆ 100 (1) H of ,i where H f ,i and H of ,i are the enthalpies (J/g) of fusion of the blend and 100% crystal components, respectively. φi is the mass fraction of component in the blend. H of of PA6 and PP are 188.1 J/g [3] and 209 J/g [28], respectively. The prepared PA6/PP blends irradiated at the doses of 0 kGy, 10 kGy, 20 kGy, 50 kGy, 100 kGy and 200 kGy were separated by extraction in formic acid for 48 h using Soxhlet extractor. The formic acid is a good solvent for PA6 but a non-solvent for PP. The % of residue of extracted blend was calculated as follows: Residue (%) = Wr /Wo ˆ 100,

(2)

where Wr is the weight of residue after extraction with boiling formic acid Wo is the weight of specimen before extraction. 4. Conclusions In summary we studied the effect of electron-beam exposure in the presence of reactive agent on the compatibility of PA6/PP (80/20) blend, in which interfacial cross-copolymerization was induced. Microscopic studies of cryofractured and tensile fractured sample morphologies revealed that dispersed particle size of PP decreased and interfacial adhesion increased on increasing irradiation dose. At a frequency of 0.1 rad/s, complex viscosities and storage moduli of all PA6/PP blends were 15~64 and 2~8 times greater than those of pure PA6 and PP, respectively. In addition, elongation at break values of PA6/PP blends were observed about twice than that of PA6 when irradiated at 0 kGy, 10 kGy, 20 kGy and 50 kGy. The observed morphological improvement and enhanced rheological and mechanical properties of irradiated PA6/PP blends supported the formation of PA6-co-PP copolymer at the interface, wherein GMA acted as a reactive agent. Furthermore, these findings show that the compatibility of PA6/PP (80/20) blend containing a reactive agent can be improved by exposure to electron-beam irradiation. Acknowledgments: This research was supported by the Yeungnam University research grants in 2015. Author Contributions: All authors contributed to this study. Boo Young Shin designed the research and wrote this paper. Man Ho Ha performed the experiment. Boo Young Shin and Do Hung Han and analyzed the data and discussed the experiment. Conflicts of Interest: The authors declare no conflict of interest.

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