Rheological, morphological and mechanical investigations on ethylene octene copolymer toughened polypropylene prepared by continuous electron induced reactive processing

Abstract

We prepared ethylene octene copolymer (EOC) toughened polypropylene (PP) by continuous electron induced reactive processing (EIReP) as well as investigated its rheological, morphological and mechanical properties. A time temperature superposition (TTS) method was utilized in order to investigate the specific thermo-rheological behavior of these PP/EOC blends. At a low concentration of EOC, a reduced blend viscosity was found. In contrast, the blend viscosity increases at higher concentrations of EOC in comparison to virgin PP. The differences in the molecular architecture of EOC and PP as well as the specific interfacial properties of the PP/EOC blend lead to this behavior. In addition, EIReP modified samples showed lower viscosity than their non-modified counterparts. This reduction of the absolute value of complex viscosity was related to the degradation of the PP phase. In EIReP modified samples an enhancement of the storage modulus at low frequencies was attributed to grafting at the interface and crosslinking of the dispersed EOC phases. All investigations of the thermo-rheological properties showed that TTS holds for the EIReP modified and non-modified blends. Using SEM micrographs, the effect of EIReP and blend ratio on the microstructure of toughened PP was studied. Finally, we discussed the origins of the differences of EOC toughened PP in tensile and impact experiments as well as proposed a micro-mechanism based on the investigation of thermal properties and considering molecular effects of EIReP on polymers.

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1. Introduction

Polypropylene (PP) is one of the most versatile commodity polymers because of its exceptional properties including excellent chemical and moisture resistance, good mechanical properties, good processability, low density and relatively low price level.^1,2 In addition to traditional applications in packaging, food and the pipe industry, PP is increasingly used as a functional material in the electrical and automotive industries.^3,4 These applications as an engineering plastic are seriously limited by its low impact resistance.⁵ Blending of PP with rubber is one of the most promising routes to improve its impact properties. There exist various reports on improvement of impact properties of PP by blending with ethylene propylene rubber (EPR), ethylene propylene diene rubber (EPDM) and styrene ethylene butylene styrene (SEBS) copolymer.^2,6 Recent developments in catalytic methods allowed the design of a new family of thermoplastic elastomers, EOC, which is a copolymer of ethylene and octene.⁷ EOC is a soft rubbery material. It provides enhanced impact properties after its blending with PP.⁵ EOC possesses great advantages over other rubbers like EPDM or natural rubber (NR). Due to its availability in the form of pellets, it can be easily used in melt processing to prepare final parts.⁸ In addition, the presence of long and short chain branches leads to a good melt processability.⁹ Moreover, PP/EOC blends showed higher impact strength in comparison to PP/EPDM or PP/NR.¹⁰ Despite these advantages, the application of EOC at higher temperatures is restricted due to its relative low melting temperature.⁸ In addition, the weak interfacial adhesion between EOC phases and PP matrix in EOC-toughened PP¹¹ can negatively influence the final properties of the blend.¹² Consequently, a modification of EOC and its interface with PP in PP/EOC blends is required. These shortcomings can be reduced by crosslinking of rubber phase and interfacial grafting. Generally, there are two routes of chemical modifications: (1) the use of chemical agents or (2) the exposure to high energy radiation.^8,13 High energy electrons belong to the second group and are used in a wide spectrum of applications such as polymerization and crosslinking as well as degradation and functionalization of polymers.¹⁴ In contrast to chemical agents like peroxide or silane, the generation of chemical reactive species by high energy electrons is free of low molecular unreacted products. Nevertheless, electron beam (EB) treatments in the solid state and/or below the glass transition temperature might lead to trapped radicals. Consequently, there is the possibility of post treatment alteration of polymer properties. EB treatment during melt mixing will avoid this drawback¹⁵ as well as enables the modification of the whole polymer volume at high efficiency.¹⁶ This novel processing method is called electron induced reactive processing (EIReP).

Lugão et al. indicate that the irradiation of PP by high energy electrons or gamma rays in air and nitrogen atmospheres leads to the degradation of PP. This effect is more pronounced for an irradiation of PP in air atmosphere.¹⁷ On the other hand, Li et al. showed that the major effect of irradiation of EOC is crosslinking. In the case of nitrogen atmosphere, an increased crosslinking efficiency of EOC was observed.¹⁸ The use of chemical agents for the modification of PP and EOC leads to the same structural changes, e.g. to degradation and crosslinking of PP and EOC, respectively. Further, it was found that mechanical and fatigue properties of chemically modified and unmodified PP/EOC blends are superior in comparison to those of modified and unmodified PP/EPDM blends. The increased properties of PP/EOC were related to the unique molecular architecture of EOC and the enhanced interfacial interaction of PP and EOC as compared to PP and EPDM.¹⁹ In addition, our research team prepared high performance PP/EOC thermoplastic vulcanisates (TPV) by tailored processing conditions of discontinuous electron induced reactive processing.^15,20

Although numerous reports are available on different aspects of properties of PP/EOC blends and effects of EB treatment on its components, there is no report on the preparation of EOC toughened PP by continuous electron induced reactive processing. Taking into account our previous studies on PP/EOC blends and TPVs, we used the EIReP pilot plant for the preparation of EOC toughened PP and investigated their rheological, morphological and mechanical properties. Finally, we correlated the properties with microstructure alterations.

2. Experimental

2.1. Materials

Polypropylene (PP HG 455FB; M_n = 84 800 g mol⁻¹, polydispersity = 2.64, MFI (230 °C/2.16 kg) = 27 g/10 min, Borealis AG, Australia) and ethylene octene copolymer (EOC Engage 8100; M_n = 86 600 g mol⁻¹, polydispersity = 1.58, MFI (190 °C/2.16 kg) = 2.16 g/10 min, octene comonomer content = 23 mol%, Dow Chemical, USA) were used as received.

2.2. Pilot plant for continuous electron induced reactive processing

Scheme 1 shows the pilot plant for continuous EIReP. It includes three processing steps; (1) non-reactive processing by a twin screw extruder (TSE DSE18, Leistritz, Germany), (2) electron induced reactive processing in a high viscosity reactor (IPF, Dresden, Germany) coupled to an electron accelerator (ELV-2 BINP, Novosibirsk, Russia) and (3) pelletizing step including water-bath and granulator (Collin, Germany).

Scheme 1 Pilot plant for electron induced reactive processing.

2.3. Sample preparation

EOC toughened PP with EOC contents of 2.5, 5, 7.5 and 10 wt% were exposed to dose values of 0, 3, 6 and 12 kGy by this novel continuous EIReP. The non-reactive blending was performed in a twin screw extruder at a temperature of 180 °C and a screw speed of 120 rpm. Afterwards, the molten blends were irradiated across its passage through the high viscosity reactor at a temperature of 180 °C and under a nitrogen gas stream in order to reduce oxidative reactions. Finally, the chemically modified molten strand passed the water bath and the granulator in order to form a solid strand and pellets, respectively. The prepared samples coded as Ea/b kGy where a and b stand for wt% of EOC and dose, respectively. In addition, original PP, EOC and PP/EOC blends were processed without EIReP modification in order prepare reference materials with the same processing history.

2.4. Characterization

2.4.1. Gel measurements. The gel content of PP, EOC and PP/EOC blend with 92.5/7.5 weight ratio (E7.5) were measured by extraction in boiling xylene (140 °C, 24 h). After extraction, all samples were dried in a vacuum oven and weighed. The gel content (X_g) in percentage was calculated according to:

X_g = (m₁/m₀) × 100

where m₀ and m₁ are the sample weight before and after extraction, respectively.

2.4.2. Rheological studies. Dynamic rheometry in the molten state was carried out with an ARES rheometer (Rheometric Scientific, Inc.) equipped with parallel plate geometry (diameter = 25 mm, gap = 1 mm). The samples were prepared as discs of 25 mm diameter and 2 mm thickness by compression molding. All measurements were done in a dry nitrogen atmosphere to reduce oxidative degradation. Dynamic strain sweep measurements performed at a strain of 10% safely fall within the linear viscoelastic limit of all samples. Finally, dynamic frequency sweep experiments were performed in the frequency range of 0.06–100 rad s⁻¹. All the measurements were carried out at a temperature of 180 °C. The obtained data were precisely reproducible.

2.4.3. Morphological studies. Scanning electron microscopy (SEM; NEON 40 EsB, Carl Zeiss, Oberkochen, Germany) was employed to characterize the morphology of the prepared samples. After cryo-fracturing in liquid nitrogen at −160 °C, an extruded strand of each sample was microtomed to obtain a smooth surface which was then sputter coated with 3 nm platinum prior to the SEM analysis.

2.4.4. Thermal analysis. Thermal properties of specimens were studied by differential scanning calorimetry (DSC; TA-Instrument Q1000, USA). A sample of about 5 mg was scanned in a cycle of heating–cooling–heating from −80 °C to 240 °C at 10 K min⁻¹ in standard mode. The samples were held at 190 °C for 0.5 min to erase the thermal history.

2.4.5. Mechanical characterization. The tensile test was performed on a tensile tester (Zwick/Roell, Germany) according to the ISO 527-2 standard at ambient temperature with crosshead speeds of 1 mm min⁻¹ (at initial stages of experiment) and 50 mm min⁻¹ (at high strains) using an applied force of 1000 N. For each sample average values of five measurements were reported. Charpy impact tests were performed on the samples to study the effect of blend ratio and EIReP on impact resistance. In Charpy impact test, a notched bar of polymer is struck by a pendulum and the dissipated energy in fracture is calculated. The test was performed at room temperature according to ISO 179 standard using an impact tester (Zwick/Roell, Germany). The pendulum impact velocity was 2.9 m s⁻¹. The average values of five measurements were reported. Standard samples for tensile and impact tests were prepared by injection molding operated at 180–210 °C from hopper to nozzle at a pressure of 100 bars. The mold temperature was about 40 °C.

3. Results and discussion

3.1. Gel content

The results of gel content experiment for PP, EOC and E7.5 have been shown in Fig. 1. It was observed that irradiated PP do not left any gel during gel content measurements which is due to its more tendency to degradation rather than crosslinking.¹⁷ On the other hand, in the case of EOC and E7.5, irrespective of small amounts of formed gel at low dosages of EB, generally it was found that these samples tend to create crosslinked structures and left gel rather than degradation. Also, it has been reported that at low EB doses when the predominant phenomenon is creation of long chain branches in EOC chains, the amount of crosslinked structures is small so that in some cases the gel content is immeasurable.²⁰

Fig. 1 Gel content for PP, EOC and PP/EOC blend.

3.2. Rheology

Fig. 2 shows the storage modulus and absolute value of complex viscosity of non-modified PP/ECO blends at different blend ratios and their pure components as a function of angular frequency in a log–log plot. It is clearly seen that viscosity and elasticity of EOC are considerably higher than those of PP. Although the polydispersity of PP is higher than EOC, EOC starts its non-Newtonian behavior at lower frequencies as compared to PP. In addition, EOC shows more shear-thinning behavior in the non-Newtonian region. Compared to PP, the higher pseudoplastic behavior of EOC stems from its molecular architecture. EOC shows more shear-thinning behavior due to disentanglement of its long chain branches under large shear frequencies. In the region of low frequencies, these entanglements donate more viscosity. These are typical characters of polymers with long chain branches.^21,22

Fig. 2 Storage modulus and absolute value of complex viscosity of non-irradiated blends and their associated components versus angular frequency (T = 180 °C).

By taking glance at viscosity of blends, it is unexpectedly seen that the adding of EOC into PP up to 5 wt% reduces the absolute value of complex viscosity of the blend in the whole range of frequency. With further increase of EOC content, an inverse behavior is observed, i.e., the curve of |η*| of blend locates between those of its components. Generally, the material functions of a typical blend are controlled by both interfacial properties and viscoelastic properties of its components. In the case of these PP/EOC blends, a specific interfacial phenomenon leads to this unusual behavior. This behavior has been termed as “ball bearing effect”. It demonstrates that the second phase of the blend is highly viscous and there is hardly any considerable interaction between both components. Mackay et al.²³ reported that the addition of highly crosslinked spherical polystyrene nanoparticles decreases the viscosity of polystyrene solution due to the free volume change. Further, the reduction in viscosity of poly(2-hydroxyethyl methacrylate) was related to a decreased number of physical entanglements between linear polymers²⁴ as a consequence of adding hyperbranched polyester. In the present system, it seems that low interfacial interaction among the blend phases prevails over high viscosity of EOC at low concentrations of EOC. Consequently, the viscosity curves of E2.5/0 kGy and E5/0 kGy are lower than the viscosity curve of neat PP. At higher concentrations of EOC, the high viscosity of EOC outweighs over the reducing effect of interlayer slippage on viscosity.²⁵ Consequently, the viscosity curves of E7.5/0 kGy and E10/0 kGy lie above the viscosity curve of neat PP. At higher frequencies, the viscosity curves of PP/EOC blends slightly converge to that of neat PP since the absolute value of complex viscosity is mainly determined by that of matrix at higher frequencies.

Due to its linear structure, neat PP shows typical liquid-like behavior, i.e. G′ ∝ ω², in the terminal zone where only the longest relaxation times contribute to the viscoelastic behavior.²⁶ On the other hand, samples including neat EOC and PP/EOC blends show non-terminal behavior. This type of behavior, i.e. reduction in the slope of G′, suggests that there should be a longer relaxation mechanism. In the case of EOC, this behavior can be ascribed to entangled long chain branches of EOC and associated longer time required for their relaxation. In the case of blends, shape relaxation of dispersed phase is another main reason which assist in deviation of slope of G′ despite the longer relaxation time of long chain branches of EOC.

Fig. 3 shows the effect of dose on linear viscoelastic properties of PP/EOC blends. The absolute value of complex viscosity substantially decreases with increasing dose values within the whole range of frequency. As briefly mentioned in the introduction and also results of gel content measurement, EB treatment of PP mainly leads to chain scission and subsequent reduction of its molecular weight. On the other hand, the major effect of EB treatment on EOC is the crosslinking and long chain branching of its macromolecules. Finally, the absolute value of complex viscosity of EIReP modified PP/EOC blends decreases as a function of dose due to the low concentration of EOC in toughened PP.

Fig. 3 Effect of irradiation dose on viscoelastic behavior of (a) E5/b kGy and (b) E10/b kGy samples (T = 180 °C).

In comparison to the absolute value of complex viscosity, the storage modulus is a more sensitive material function to trace morphological and interfacial related phenomenon in polymeric blends. It is generally seen that as the dose increases, G′ decreases in the whole range of frequency, while there are slight tendency to upwards in its values at lower frequency ranges. These behaviors are due to the fact that generally created shorter chains reduce the elasticity, while at terminal zone longer chains which were formed as a result of crosslinking and long chain branching in EOC phase and also in the interface of PP and EOC,^15,20 because of having more relaxation times contribute to some reduction in the slope of G′. Increase in G′ at low frequency ranges has also been attributed to the strong contribution of the interfacial tension by Kock et al.²⁷

Time temperature superposition (TTS) is a valuable tool for the description of both linear and even nonlinear viscoelastic properties over a broad range of times or frequencies by performing measurements at different temperatures and then shifting of data to a reference temperature.²⁸ In order to investigate the applicability of TTS for rheological data of PP/EOC blends and for the effect of EIReP on thermo-rheological complexity of this blend, linear viscoelastic properties of E5/0 kGy, E5/6 kGy and E5/12 kGy were measured at three different temperatures of 190, 200 and 220 °C. A material is termed as “thermo-rheologically simple” if the obtained data at different temperatures can superpose and constitute a comprehensive diagram (master curve) after shifting to a reference temperature. From molecular standpoint, this happens if all of the contributing relaxation or retardations mechanisms have the same dependency on temperature.²⁹ In this method, the measured temperature specific data are shifted to a reference temperature using temperature dependent horizontal (a_T) and vertical (b_T) shift factors. A master curve can be established by these shift factors if the TTS is valid. Using the following eqn (1) these shift factors can be related to each other:

where T₀ and T are reference temperature and temperature of measurement, respectively and η₀ is zero shear viscosity. Since the vertical shift factor (b_T) is relatively independent of temperature and close to unity, the horizontal shift factor (a_T) at temperature T can be approximated by:²⁹

In addition, the zero-shear viscosity (η₀) in a frequency sweep experiment can be calculated by fitting of absolute value of complex viscosity |η*| data with the Cross's equation:

where n is a shear-thinning index and λ is the relaxation time. The reciprocal of λ is called the critical shear rate and accounts for the onset of shear-thinning region.³⁰

Taking the reference temperature and the vertical shift factor equal to 180 °C and unity, respectively, as well as by calculation of horizontal shift factor for each temperature using absolute value of complex viscosity data and eqn (2) and (3) and finally by implementation of TTS concept for storage modulus, loss modulus and absolute value of complex viscosity according to the following eqn (4)–(6), the obtained data for non-modified and EIReP modified 95/5 PP/EOC blends were shifted to 180 °C and the results are shown in Fig. 4a and b.

b_TG′(T, a_Tω) = G′(T₀, ω) (4)

b_TG′′(T, a_Tω) = G′′(T₀, ω) (5)

Fig. 4 Master curves of (a) storage and loss modulus and (b) complex viscosity along with (c) van Gurp–Palmen plot.

It is seen that data obtained at different temperatures have been superposed to an acceptable level and formed a master curve. In the case of non-modified blends, all the data are precisely superposed on each other. This result is in contradiction to the results obtained from the applicability of TTS for metallocene polyethylenes where the presence of low levels of long chain branches resulted in thermo-rheological complexity and hence failure of TTS.³¹ It should be mentioned that the observed superposition may be due to the selected experimental range, i.e., if in the present case experiments are performed at lower frequencies the long chain branches may show their effect on appearance of thermo-rheological complexity, as well. Furthermore, it is seen that the absolute value of complex viscosity and loss modulus curves have been completely superposed. In the case of EIReP modified blends, there is a small deviation in lower frequency ranges of storage modulus curves. A possible reason for this deviation could be the formation or splitting of chemical couplings during EIReP, i.e. crosslinking and long chain branching of EOC and PP EOC grafting at the interface as well as degradation of PP. The resulting molecules have their specific relaxation/retardation mechanism dependency on temperature which might differ from that of virgin molecules in the PP/EOC blend. Although visually all superpositions are deemed to be acceptable, the applicability of TTS is checked in a more methodological manner. A plot of loss angle (δ) versus log(G*) known as van Gurp–Palmen plot, as a simple and direct method, is used to verify the TTS in polymer blends. In this plot the influence of shifting along the frequency is eliminated. Therefore, the isothermal plot should coincide if TTS holds.³² By examining this criterion for each sample in Fig. 4c, it is clearly seen that all the data obtained in different temperatures coincide on each other and hence TTS holds for them.

3.3. Morphology

Fig. 5 represents SEM images of non-modified PP/EOC blends at three different compositions. In all the three compositions, the EOC minor phase has been dispersed uniformly within the PP matrix. Although particle size is increased by increasing of EOC content, it remains at submicron level for all compositions. From morphological standpoint, an uniform distribution of EOC phases within PP matrix, a lower polydispersity of EOC and submicron size EOC phases are three microstructural characteristics leading to best toughening performance.^33,34 As it is seen from Fig. 5, all these microstructural characteristics have been successfully attained. In order to investigate the effect of EIReP on the microstructure of blends, the SEM images of EIReP modified 90/10 PP/EOC blends at two different doses along with non-modified counterpart, as a reference material, have been shown in Fig. 6. It is seen that the above mentioned morphological features has been achieved in the case of EIReP modified blends, as well. Despite some differences resulted from the surface preparation in microtoming process, it is unexpectedly observed that EIReP do not change the size of EOC phase or homogeneity level within the blends. During EIReP, the viscosity of PP decreases due to chain scission of PP. In contrast, the viscosity of EOC increases since EOC tends to crosslinking reactions. Consequently, one may expect increased size of disperse phase due to increased viscosity mismatch. This can be explained in the following way. The concentration of the second phase is much lower compared to that of matrix. In addition, EIReP leads to grafting at PP/EOC interface. Therefore, it seems that prior to coalescence of dispersed phases, EIReP acts as a tool to fix the blend morphology and prevents subsequent coalescence of dispersed phase. Furthermore, it is expected that the morphology of EIReP modified blends will be preserved in the subsequent processing steps like injection molding since crosslinking and grafting reactions take place within the EOC phase and at the PP/EOC interface, respectively. The stabilization of morphology in irradiated blends of PP/EPDM has been proved by Gisbergen and coworkers.³⁵ Freezing of morphology by means of electron induced reactions has a great potential from industrial perspectives since it is a tool to tailor the desired morphology in accordance to the final application of material in the compounding and/or masterbatch production step.

Fig. 5 SEM micrographs of PP/EOC blends in three different blend ratios.

Fig. 6 SEM micrographs of non-irradiated and irradiated 90/10 PP/EOC blends at different doses.

3.4. Mechanical properties

Table 1 summarizes the results of tensile tests and Charpy impact experiments. In addition, the Fig. 7 and 8 show the relative values of tensile and impact properties for non-modified and EIReP modified blends with respect to neat PP as reference material. Generally, all the samples show ductile behavior since they have large values of elongation at break (Table 1). Further, they represent yield behavior and finally fail in subsequent stress hardening. It is observed that in EIReP modified and non-modified blends the E-modulus decreases as the concentration of rubber phase increases (Fig. 7a). This behavior can be attributed to the rubbery nature of EOC with low modulus. At low concentrations of EOC in PP matrix, there is no significant effect of EIReP on the values of E modulus within the experimental uncertainty. It seems that at higher concentrations of rubber, increase in modulus of this phase as result of branching and grafting reactions prevails over its decreasing effect on modulus. This observation is in accordance with the reports of Nunes³⁶ and Nielsen.³⁷ It has been observed that modulus of polymer at temperatures higher than its glass transition is increased as a result of crosslinking, on the other hand reduction of molecular mass has marginal effect on modulus. Furthermore, the similar trend is observable in the yield strength values, since modulus and tensile strength are interconnected concepts (Fig. 7b).

Table 1 Mechanical properties of non-modified and EIReP modified PP/EOC blends

Sample	E∥ (MPa)	σy (MPa)	εb (%)	σb (MPa)	Impact (kJ m^−2)
PP	1179 ± 40	26.7 ± 0.3	552 ± 6	32.7 ± 1.0	2.9 ± 0.4
E2.5/0 kGy	1175 ± 21	26.7 ± 0.6	528 ± 43	31.0 ± 0.7	4.7 ± 0.3
E2.5/3 kGy	1081 ± 60	26.0 ± 0.2	577 ± 39	31.4 ± 0.4	4.9 ± 0.8
E2.5/6 kGy	1080 ± 23	26.3 ± 0.4	617 ± 20	32.3 ± 0.8	4.6 ± 0.8
E5/0 kGy	1035 ± 57	25.4 ± 0.6	560 ± 34	30.7 ± 0.6	6.2 ± 0.7
E5/6 kGy	1056 ± 46	25.6 ± 0.4	625 ± 32	31.5 ± 0.8	6.7 ± 0.6
E5/12 kGy	977 ± 61	25.1 ± 0.5	630 ± 16	31.0 ± 1.0	7.0 ± 0.6
E10/0 kGy	881 ± 21	22.3 ± 0.3	585 ± 6	29.2 ± 0.2	8.2 ± 0.7
E10/6 kGy	924 ± 62	23.2 ± 1.2	640 ± 12	31.0 ± 1.0	8.9 ± 0.5
E10/12 kGy	921 ± 42	22.9 ± 0.6	656 ± 15	31.3 ± 1.0	9.7 ± 1.0

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Fig. 7 Tensile properties of non-modified and EIReP modified PP/EOC blends.

Fig. 8 Charpy impact strength of PP/EOC blends exposed to different doses of electron beam.

The elongation at break values of non-modified and EIReP modified samples show marginal increment toward enhancement in concentration of EOC phase (Fig. 7c). There is a considerable increment of elongation at break values after EIReP. It seems that in EIReP modified blends entanglements between chains have been intensified and hence these samples can sustain larger amounts of strain. The results of failure strengths show a similar tendency for the effect of EIReP as the results of elongation at break. However, the strength at break reduces with increased EOC content (Fig. 7d) which is related to the less modulus of second phase.

Since the toughness describes the capability of energy absorption before breaking,³⁶ we can expect a good correlation between the results of elongation at break and impact experiments. Fig. 8 shows the normalized values of impact strength with respect to neat PP. As expected, the blending of EOC into PP matrix enhances the impact strength of the blend (Fig. 8 black line). There is a significant additional enhancement of the impact strength by EIReP (Fig. 8 blue line). Consequently, we focused on further investigations in order to identify the mechanism for this improvement of impact properties by EIReP of PP/EOC blends. Rubber toughening of polymers is based on three main mechanisms: (i) multiple shear yielding, (ii) multiple crazing and (iii) network yielding. The former mechanism is dominant in the temperatures above the glass transition temperature (T_g) of the matrix, the second one occurs at temperatures below T_g and the last one happens when thermoplastic particles are embedded into a rubber network.³³ According to the used blend ratios, the last mechanism is not valid for EOC toughened PP prepared. In addition, we have to note that T_g is a rate dependent parameter. Consequently, it is more accurate to consider the T_g of the matrix as criteria in relation to the experimental conditions of the impact experiment in order to make decision about the toughening mechanism. The T_g of PP equals to about 0 °C and is close to the temperature of impact experiment. Therefore, mechanisms (i) and (ii) of rubber toughening of polymers are dominant in EIReP modified and non-modified PP/EOC blends. As a first hypothesis it comes to mind that differences in the degree of crystallinity (1) and/or size of crystallites (2) as well as a change in glass transition temperature of matrix (3) may have an effect on impact strength. Consequently, we studied DSC thermograms of 90/10 PP/EOC blends at different dose values (Fig. 9). The glass transition temperature of PP matrix in E10/0 kGy, E10/6 kGy and E10/12 kGy samples equal to −1.8, −4.0 and −5.6 °C, respectively (Fig. 9a). In addition, we calculated the degree of crystallinity of these blends from Fig. 9b taking into account a crystallization enthalpy of 209 g⁻¹ (ref. 38) or the fully crystalline PP. The degree of crystallinity of PP/EOC blends is 44.85, 45.18 and 43.55%, respectively. Moreover, we calculated the relative crystallinity with respect to the temperature using the following eqn (7):

where H is heat flow, T_o is onset crystallization temperature and T_e stands for end crystallization temperature.³⁹ Fig. 9c shows the results.

Fig. 9 Thermal properties of 90/10 PP/EOC exposed to different doses of electron beam: (a) melting, (b) cooling and (c) relative crystallinity.

By comparing T_g, the degree of crystallinity and the slope of relative crystallinity, it is seen that differences in T_g and crystallinity are small. Further, the slope of relative crystallinity which is a kinetic parameter attributed to growth of crystallites,⁴⁰ is equal for these samples. Therefore, the results of DSC (T_g, crystallinity and relative crystallinity) indicate that there is no substantial difference between thermal properties, crystallinity and crystal structures of blends. Hence, the differences in impact properties between non-modified and EIReP modified PP/EOC blends are not related to these parameters and therefore the proposed (1) to (3) hypotheses fail.

A further reason leading to enhanced impact strength of EIReP modified PP/EOC blends is a change of interface structure by EIReP. Rubber domains act as stress concentrators for absorption and dissipation of impact energies. When the stress around the rubber particle overcomes yield stress of matrix, shear yielding mechanism of fracture becomes predominant over destructive crack and craze propagation. Consequently, enhanced toughness is achieved.⁴¹ In order to promote shear yielding in the matrix, it is important that stress concentration fields developed from EOC phases interact effectively with each other in PP matrix. If the interface between rubber phase and matrix gets weak it will debond during impact and in that case not only stress will relieved, but also void and flows will be propagated. A kind of continuous stress concentration zone will be developed in the matrix leading to subsequent shear yielding and energy absorption in sample if a sufficient interaction is attained. Consequently, the toughness dramatically enhances. In the case of non-modified PP/EOC blends, the development of stress concentration zones and subsequent shear yielding are not occurring due to a weak interface. In EIReP modified PP/EOC blends, there is no debonding during struck and therefore shear yielding can occur in the matrix and enhances the toughness due to grafting between EOC and PP chains at the interface. From molecular point of view, the properties of interphase differ considerably from bulk properties of blend due to increased compatibility and miscibility of EOC and PP chains. One of these properties is the glass transition temperature which according to Fox's law or Gordon–Taylor's equation³⁹ and compatibility level between molecules of interphase will attain a value between T_g of matrix and T_g of EOC (about −50 °C). The reduction of glass transition temperature of the interphase could be another factor for dominating of shear yielding mechanism over crazing and crack propagation. Furthermore, it was reported by Bucknall⁴² that during impact, rubber particles are subjected to very large levels of tensile strains, in this situation branching, grafting and crosslinking of rubber particles itself can improve impact properties of samples by fibrillation and sustaining higher levels of strain. The inverse effect may be seen⁴³ if the degree of crosslinks proceeds a moderate value. A schematic representation for these explanations are shown in Scheme 2.

Scheme 2 Performance of non-modified and EIReP modified PP/EOC blends during impact test.

4. Conclusion

In continuation of our previous studies on EIReP of PP/EOC TPVs, we used a pilot plant for continuous EIReP for commercial preparation of EOC toughened PP in the present study. The processing conditions were set based on our previous experiences in batch processing of these materials. A series of PP/EOC blends were prepared by this novel continuous EIReP pilot plant at different dose values. The effect of blend ratio and dose on prepared samples was analyzed in terms of rheological, morphological and mechanical properties. We found that EOC, despite its high viscosity, in lower concentration leads to a reduction of the blend viscosity, while at higher concentrations it enhances viscosity. This behavior was explained by differences in molecular architecture of blend components and interfacial properties of the blend. Further, EIReP modified PP/EOC blends showed much less viscosity than their non-modified counterparts which is beneficial from processing point of view in processing techniques like injection molding or melt spinning. The reduction of viscosity was related to the degradation of PP phase. The enhancement of elasticity at low frequency ranges was attributed to grafting at the interface and long chain branching of dispersed EOC phase. In addition, the thermo-rheological complexity of EOC toughened PP was investigated. Surprisingly, it was found that TTS holds for EIReP modified and non-modified PP/EOC blends. SEM micrographs indicated that the resulted morphology is completely desirable to use in rubber toughening applications. Further, EIReP can be implemented as a tool to stabilize blend morphologies. Mechanical properties investigations showed that EIReP improves tensile and impact properties of EOC toughened PP. In the case of impact strength, we proposed and analyzed several reasons based on thermal properties investigations. Finally, we propose a micro-mechanism in order to explain differences in impact strength of EIReP modified and non-modified PP/EOC blends.

Acknowledgements

The EOC toughened PP was developed within the Collaborative Research Centre SFB 639 “Textile-reinforced composite components for function-integrating multi-material design in complex lightweight applications”. Consequently, the authors acknowledge the German Science Foundation DFG for funding this work. Finally, the authors would like to thank Sabine Krause and Holger Scheibner from Leibniz-Institut für Polymerforschung Dresden e.V. for rheological studies and thermal analysis as well as mechanical measurements.

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