Mechanical properties and structural characteristics of dynamically photocrosslinked PP/EPDM blends

Mechanical Properties and Structural Characteristics of Dynamically Photocrosslinked PP/EPDM Blends*

WEIZHI WANG, QIANGHUA WU, and BAOJUN QU**

Department of Polymer Science and Engineering University of Science and Technology of China

230026, Hefei Anhui, P, R. China

The mechanical properties and crystal morphological structures of dynamically photocrosslinked polypropylene (PP) /ethylene-propylene-diene terpolymer (EPDM) blends have been studied by mechanical tests, wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Dynamically photocrosslinking of PP/EPDM blends can improve the mechanical properties considerably, especially the notched Izod impact strength at low temperature. Data obtained from mechanical tests show that the notched Izod impact strength of a dynamically photocrosslinked sample with 30% EPDM at -20°C is about six times that of an uncrosslinked sample with the same EPDM component. The results from the WAXD, SEM, and DSC measurements reveal the enhanced mechanism of impact strength for the dynamically photocrosslinked PP/EPDM blends as follows: (i) the f3-type crystal structure of PP is formed and the interplanar distance of a-type crystal increases slightly with an increase in the EPDM component: (ii) the droplet size of the EPDM phase in the photocrosslinked PP/EPDM blends is obviously reduced and the droplet number is increased with an increase in the EPDM component during the dynamical photocrosslinking process: (iii) the graft copolymer of PP-g-EPDM is formed at the interface between PP and EPDM components. All the above changes from the crystal morphological structures are favorable for increasing the compatibility and enhancing the toughness of PP/EPDM blends at low temperatures.

INTRODUCTION studied for many years (10-24). Recently, there have olypropylene (PP) as a thermoplastic is widely used P in many fields, such as building materials, furni-

ture, automobiles, the toy industry, and so on. How- ever, its main disadvantage is its poor impact resist- ance, especially at low temperatures. Therefore, PP is usually modified with elastomers such as ethylene- propylene-diene terpolymer (EPDM) to improve its impact strength (1-4). In recent years, many research- ers have studied dynamic vulcanization of PP/EPDM blends by peroxide or phenolic systems (5-9) in order to increase the impact strength, at low temperatures. But these methods have some disadvantages, such as the short half-life of the thermal initiator at the high processing temperatures of PP, and the dimculty of controlling the vulcanization process. Photoinitiated crosslinking of polymeric materials has been extensively

'Supported by the National Natural Science Foundation of China (No. 50073022). "Corresponding author. Fax: +86-55 1-3607245. E-mail address: qi>bi@ustc,edu.cn 1B.J. Qu).

been significant breakthroughs in photoinitiated crosslinking of polyethylene either in basic research (1 0 - 17) or industrial applications (18-201. Zamotaev et al. (2 1) have reported in photoinitiated crosslinking of PP. Hillborn et aL (22-24) have studied the photocrosslinking of EPDM systematically. But no work has been done on the dynamical photocrosslinking of PP/EPDM blends in the presence of a photoinitiator. The dynamical photocrosslinking initiated by W light may over- come the above shortcomings in the dynamic vulcanization of PP/EPDM blends initiated by a peroxide system. The decomposition of the photoinitiator does not occur at the high processing temperature of PP. At the same time, the photocrosslinking process can be easily controlled by UV irradiation intensity and time. In the present study, we describe the dynamical photocrosslinking of PP/EPDM blends and investigate the mechanical properties and crystal morphological structures of dynamical photocrosslinked thermoplastic elastomer in order to clarify the relationship between structure and properties.

1798 POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11

Dynamically Photocrosslinked PP/EPDM Blends

EXPERIMENTAL

Materials

PP (F401, MFR = 2.7) was supplied by Yangzi Petro- chemical Co. Ltd., China. EPDM 4045 (60 wt% ethylene, 3 wt% 5-ethylidene-2-norbomene) was supplied by Mitsui Petrochemical Co. Ltd., Japan. Photoinitia- tor benzil dimethyl ketal (BDK) and hindered amine photostabilizer (Tinuvin 144) were obtained from the Ciba-Geigy Corporation, Switzerland. Crosslinking agent triallyl isocyanurate (TAIC) was obtained from Anhui Institute of Chemical Engineering. All chemi- cals were used as received without any purification.

Sample Preparation

PP was first mixed with a given amount of EPDM, 1 wt% BDK and 4 wt% TAIC (based on the EPDM content) for 7 min at 180°C using a double roller mixer. Then, the PP/EPDM blends were exposed to a 2 kW Philips HPM 15 lamp for 90 s during the mechanical mixing. The dynamically photocrosslinked PP/EPDM blends were continuously mixed for 5 min and then pressed into sheets of suitable thickness using a hot press with 25 MPa at 200°C. The molded sheets were cut into desired sample sizes for detailed characteriza- tion. The samples with different PP and EPDM com- positions are marked as PP,E, and PP,EP,, for uncrosslinked and dynamically photocrosslinked blends, respectively, where m and n indicate the contents of PP and EPDM in the blends, as shown in Table 1.

Measurements

The gel content of a sample was determined by ex- tracting the photocrosslinked sample (w~) in the basket for 48 h with boiling xylene stabilized by 0.2 wt% (based on the solvent content) antioxidant Tinuvin 144 and with N, bubbling to prevent oxidation. The dynamically photocrosslinked sample was cut into thin slices and put into a basket made of 200-mesh

stainless steel net. The solvent was renewed after the first 24 h of extraction. After the extraction, the basket was washed with acetone. After drying in a vac- uum desiccator at about 70°C to constant weight, the insoluble residue (W2) was weighed. The average gel content (wt%) in the test was calculated as the value of w2/wI. Usually, five samples were analyzed to de- termine the average gel content for a given set of experimental conditions.

The tensile properties were measured with a Univer- sal Testing Machine (DCS5000, Shimadzu, Japan) at 2 5 2 2°C. The crosshead speed was 25 mm/min. Dumbbell-shaped specimens were prepared according to ASTM D412-87. The notched Izod impact strength was measured by an Izod impact tester (made in Chengde, China). The sample size of the rectangular specimens was 80 X 10 X 4 mm3 with a 45" V-shaped notch (tip radius 0.25 mm, depth 2 mm). The average value from five specimens was used for the data plot.

Wide-angle X-ray difiaction (WAXD) measurements were recorded at room temperature with a D/Max-rA rotating anode X-ray diffractometer (Rigaku Electrical Machine Company, Japan) equipped with a CuKa tube and Ni filter. The diffraction patterns were determined over a range of diffraction angle 20 = 10" - 40" at 40 kV and 50 mA.

Micrographs were taken by a scanning electron mi- croscope (SEM) (Hitalchi X560 scanning electron mi- croanalyzer, Japan). The samples were previously etched by cyclohexarte and coated with a conductive gold layer. The differential scanning calorimeter (DSC) data were obtained in nitrogen atmosphere at a heat- ing rate of 10"C/min using a CDR-4P differential scanning calorimeter (Shanghai Balance Instrument Co., China). The degree of crystallization of PP in the PP/EPDM blends was evaluated from the relative ratio of the values of fusion heat of the blend to the fusion heat of PP (AHI,p = 209 J/g) (25). The melt flow rate (MFR) was measured at 230°C under a load of 2.16 Kg according to ASTM D- 1238.

Table 1. Mechanical Properties, Melt, Flow Rate, and Gel Content of the IUncrosslinked and Dynamically Photocrosslinked PPEPDM Samples.

Sample Gel Tensile Ultimate Notched lrod No. and M FR Content Strength Elongation Impact Strength Code* (dl0 min) ("/I (MPa) ("/I (kJ/m2, 25°C)

1 PP,,k,, 2 PP80E20

4 PP,oE, PP70E30

PP50E50

3.3 I 2.5 I 2.2 I 1.7 I 1.6 I

34.0 31.5 22.5 17.0 13.5 11.0

1140 21 0 650 500 230 160

2.3 10.8 17.2 31.2 34.7 36.6

6 PPwEPl0 3.8 7 PPfJOEP20 1.8

PP70EP30 1.7

10 PPSoEP5o 0.5 9 PPWEP40 1.2

12.7 18.8 26.4 28.9 32.4

33.0 25.0 21.2 18.8 17.4

50 200 130 100 90

11.0 48.3 52.6 51.4 49.0

~~ ~ ~

'PPWE,, means 90% PP and 10 EPDM in an uncrosslinked sample, PP, EP,, means 90% PP and 10 EPDM in a dynamically photocro!;slinked sample, and so on

POLYMER ENGINEERING AND SCIENCE, NOVEMBER2003, Vol. 43, No. 11 1799

Weizhi Wang, Qianghua Wu, and Baojun Qu

RESULTS AND DISCUSSION

Mechanical Properties of Dpafdcdy Photocrosslinked PP/EPDM Blends

The mechanical properties of uncrosslinked and various dynamically photocrosslinked PP/EPDM samples are summarized in Table 1 together with data on the melt flow rate and gel content. It can be seen that the MFR values of uncrosslinked samples No. 1 to No. 5 and photocrosslinked PP/EPDM samples No. 6 to No. 10 decrease with an increasing amount of the EPDM component, especially for the photocrosslinked samples, which means that dynamical photocrosslinking can increase the viscosity of the blends because of the formation of crosslinking networks. The data obtained from gel content measurements show that the effective crosslinking degree in PP / EPDM increases with increasing EPDM component in the blends, which indicates that dynamical photocrosslinking mainly takes place in the EPDM phase. The tensile strengths (TS) of the uncrosslinked samples (No. 1-5) decrease with increasing content of EPDM from 31.5 to 11 MPa. in comparison with the 34 MPa of a control sample No. 0 without EPDM. However, the TS values of the dynamically photo-crosslinked samples (No. 6-10) are much higher than those of the corresponding uncrosslinked samples (No. 1-5). This is because the formation of the crosslinking network increases the TS and decreases the ultimate elongation, as shown in the gel contents of Table 1. The values of notched Izod impact strength (NIIS) at room temperature are also listed in Table 1. I t can be seen that control sample No. 0 has a very low NIIS value, 2.3 kJ/m2. The blend samples with EPDM have very

high NIIS values of 10.8 to 52.6 kJ/m'. Apparently, the NIIS values from dynamically photocrosslinked samples No. 6-10 are much higher than those of the simple blending samples No. 1-5.

Figure 1 shows the changes of notched lzod impact strength of uncrosslinked and photocrosslinked PP/EPDM samples measured at low temperature, -20°C. with EPDM content. It can be seen that very low impact strength values, less than 10 kJ/m2 at low temperature, are observed for the uncrosslinked samples with less than 30 wt% EPDM and for the photocrosslinked samples with less than 20 wtYo EPDM. Since the Tg of PP in this work is about - 1O"C, the PP matrix at -20°C is brittle in the glassy state. Gener- ally, the impact strength of PP/EPDM blends has a critical value for the EPDM component. When the EPDM content reaches this critical value, the impact strength increases dramatically. In the present study, we found that the critical values are 40 wt% and 30 wt% for the uncrosslinked and dynamically photocrosslinked samples, respectively. The impact strength of the photocrosslinked sample with 30% EPDM increases sharply to 68 kJ/m2, whereas that of the uncrosslinked sample with the same ratio of EPDM is only 12 kJ/m2, or about one-sixth that of the former. The impact strength of an uncrosslinked sample with 40% EPDM can increase to 60 kJ/m2, meaning that dynamical photocrosslinking can improve the impact strength of PP/EPDM blends signifi- cantly at low temperatures.

There are two main causes of the enhanced toughness by dynamical photocrosslinking. First, the rubber domains act as stress concentration sites for the dissipation of shock or impact energy by controlling and

90 J

/O--O

80 - -0- photocrosslink -a- uncrosslink

70 -

~ l/!a 60 -

50 -

40 - 30 - 20 - 10 / 8 0 I I I I

0 10 20 30 40 50

EPDM content (%) Fig. I . Changes in the notched Izod impact strength of uncrosslinked and dynamically photocrosslinked PP/EPDM samples, measured at - Z O T , us. EPDM content.



promoting matrix deformation. The addition of rubber into PP leads to relaxation of the stress concentration and suppresses the formation of the matrix crazes or deformation. Second, the discrete crosslinked rubber particles greatly decrease the possibility of rubber co- hesion into bulky particles during the mechanical and photocrosslinking mixing. As a result, the sites for dissipation of shock or impact energy are greatly increased in the dynamically photocrosslinked PP/ EPDM blends.

Crystal Structures of PP in the PPIEPDM Blends WAXD spectra of uncrosslinked PP/EPDM samples

with different EPDM contents are shown in Fig. 2a The 28 peaks at 14.1", 16.8", 18.5" and 21.2" due to the 110, 040, 130 and the overlapping 131, 041 and 111 planes are characteristic of a-type crystal stmc-

and 2b shows that new peak at 16.2" appears in the dynamically photocrosslinked PP/EPDM samples. This new peak has been assigned to the 6-type crystal structure of PP. I t has been reported (26) that the p-type crystal structure can enhance the impact strength of PP.

The values of interplanar distance (d value) obtained from the corresponding WAXD spectra of the uncrosslinked and photocrosslinked PP/EPDM samples are listed in Table 2. The data show that the d values increase slightly with increasing EPDM content in both uncrosslinked and photocrosslinked samples because of the rubber particles inserted into PP spherulites, as reported in the literature (1).

Morphological Structures of EPDM in the PPIEPDM Blends

tures of PP. Figure 2b shows the WAXD spectra of dynamically photocrosslinked PP/EPDM samples with the same EPDM contents. A comparison of Figs. 2a

SEM micrographs of uncrosslinked blend samples No. 1 to No. 3 are shown Figs. 3a, 3b, and 3c. The black voids visible in these micrographs are due to the

10 15 20 25 30

Fg. 2. X-ray dmactwn patterns. (A] uncrosslinked PP/EPDM blends; [B] dynamically photocrosslinked PPIEPDM blends.

I\ 16.2" B

10 15 20 25 30

20 (Degree)

POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11 iaoi

Weizhi W a q , Qianghua W u , and Baojun Q u

Table 2. Interplanar Distance of Uncrosslinked and Dynamically Photocrosslinked PPEPDM Blends With Different EPDM Contents.

PP9OElO PPEOE20 PP70E30 PP60E40 PP50E50

Reflection no(') d (A) 20(') d(A) 28(") d(A) 2W) d ( 4 2 W ) d(A)

110 14.08 6.29 14.06 6.30 14.05 6.30 14.03 6.31 13.99 6.33 040 17.09 5.19 16.85 5.26 16.84 5.27 16.77 5.29 16.77 5.29 130 18.66 4.76 18.53 4.79 18.51 4.79 18.48 4.80 18.49 4.80 111 21.68 4.10 21.65 4.11 21.22 4.19 21.19 4.19 21.14 4.21

PP90EPlO PPEOEP20 PP70EP30 PP60EP40 PP50EP5.0

2W" d [A) 2N") d (h 2 W ) d ( h 2W) d(A) 2'3(") d(A) ~ ~~~ ~

110 14.06 6.30 14.06 6.30 14.04 6.31 13.99 6.33 13.98 6.34 040 17.09 5.19 16.87 5.26 16.85 5.26 16.84 5.26 16.77 5.29

18.64 4.76 18.55 4.78 18.53 4.79 18.49 4.80 18.52 4.79 130 21.33 4.17 21.19 4.19 21.22 4.19 21.15 4.21 111 21.79 4.08

EPDM droplets that were etched by cyclohexane. The small spherical droplets of sample No. 1 with 10 wtYo EPDM are dispersed in the continuous PP matrix (Fig. 3a). The droplets become larger and non-spherical when the EPDM content increases to 20 wt% (Fig. 3b). When the EPDM content further increases to 30 wt%, the droplets are elongated to semi-continuous nets (Fig. 3c) because of the coalescence of very small EPDM droplets during mechanical shearing. Appar- ently, PP in the blends is in a continuous phase, while the EPDM component for the uncrosslinked samples (No. 1-3) changes from the dispersed phase gradually into the interconnected phase with increasing EPDM content.

SEM micrographs of the dynamically photocrosslinked samples No. 6 to No. 8 are presented in Ftgs. 3 d , 3e , and 3f. No apparent big black voids are observed, since the crosslinked EPDM droplets cannot be etched by cyclohexane because of the gelled network nature. The numbers of EPDM droplets in the dynamically photocrosslinked blends increase, and their average sizes are smaller than those in the corresponding uncrosslinked blends. This is because the crosslinked EPDM droplets are easily broken into smaller ones and dispersed homogeneously into PP under the condition of high mixing torque.

The changes in the morphological structures after the dynamical photocrosslinking of PP/EPDM blends reveal that the crosslinked EPDM droplets seem to have better compatibility with PP. This might be due to the formation of graft copolymers at the interface of PP and EPDM during the dynamical photocrosslinking, which can increase the interfacial adhesion and compatibility of PP/EPDM blends.

Formation of PP-gEPDM Graft Copolymer

Figure 4 shows the DSC thermograms of photocrosslinked PP/EPDM blends. A small melting peak (Tmz) at 163°C is observed in most DSC traces of photocrosslinked blends except for a main melting peak (Tml). This small melting peak (Tm2) cannot be attributed to the formation of photocrosslinked EPDM or

TAIC homopolymers because they are not crystalliz- able, as is evident from the DSC traces in Fig. 5. Therefore, the small melting peak can be attributed to the graft copolymer of PP and EPDM (PP-g-EPDM) at the interface. The very small amount of PP-g-EPDM grafted copolymer can enhance the mechanical properties of PP/EPDM blends, especially for the impact strength at low temperature. Inoue (27) also reported the same results.

The melting behavior and crystallinity of uncrosslinked and dynamically photocrosslinked PP/EPDM blends determined by the DSC method are listed in Table 3. It can be seen that the heat of fusion, the melting point, and the crystallinity of both sets of samples decrease with increasing EPDM content, in con- trast to the values of the control sample PP,,. The decreases in the heat of fusion and melting point are due to the decrease of crystallinity in the blends. The decrease in crystallinity is due to the fact that the noncrystallizable component EPDM inhibits the crystal growth of PP and thus leads to the formation of small and unperfected PP crystals. However, the degree of crystallinity obtained from the dynamically photocrosslinked samples is a little higher than those obtained from the corresponding uncrosslinked samples with the same EPDM content, which might be due to the formation of P-type crystals and crystallines interface of PP-g-EPDM copolymer. The T,, melting points in the dynamically photocrosslinked samples give positive evidence of PP-g-EPDM graft copolymer.

CONCLUSIONS

The dynamical photocrosslinking method can apparently improve the mechanical properties of PP/EPDM blends, such as the notched Izod impact strength. The WAXD data show that the formation of the P-type crystal structure of PP and the extenuation of the a-type crystal structure of PP partly enhance the impact strength of dynamically photocrosslinked PP/ EPDM thermoplastic elastomers. SEM micrographs demonstrate that the decrease in droplet size and the increase in droplet number of the EPDM phase caused



Fg. 3. Scanning electron micrographs offracture surfaces of uncrosslinked and photocrosslidced PP/EPDM samples obtainedfrorn Izod impact testing at -20°C. a. PP,,E,,; b, PPsoEzo; C, PP,,E30 for the uncrosslinked samples: d, PP,EP,,: e, PP8,EPz0:f, PP,,EP,, for the dynamically photocrosslinked samples.

POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11 1803

Weizhi Wang, Qianghua Wu, and Baojun Qu

t u .- E b 5 s w

150 155 160 165 170 175 180 160 170 180

Temperature (“C) Temperature (“C) Fig. 4. DSC thermograms of the photocrosslinked PPIEPDM blends.

Frg. 5. DSC thermograms of pure PP. photocrosslinked TAlC with 1 wt% BDK, and photocrosslinked EPDM with 4 wt% TAIC (irradiation time: 90 s).

Table 3. Melting Behavior and Crystallinity of the Uncrosslinked and Dynamically Photocrosslinked PPEPDM Blends. -~ ~ ~ ~~ ~

Blend Sample Heat of Fusion Melting Temperature (“C) Crystallinity Systems Code (AH), Jls T,1 T, (”/)

~ ~ ~~ ~

Uncrosslinked pp,oo Blends PPSOE,O

PP80E20 PP,OE,O PP60E40 PP50E50

Dynamically PP90EPlO Photocrosslinked PP80EP*O

PP,oEP,o PP5OEPE.O

Blends’ P70E ‘30

84.1 75.8 63.2 56.4 54.1 52.6 77.3 70.0 66.7 58.0 55.5

169.1 167.2 166.4 166.8

165.5 168.7 167.0 166.8 166.4 166.1

165.8

I I I I I I

164.0 164.0 163.8 163.5 163.2

40.2 36.3 30.2 27.0 25.9 25.2 37.0 33.5 31.9 27.8 26.6

‘UV-irradiation time was 90 s

by the mechanical shearing and photocrosslinking strengthen the interfacial adhesion and increase the sites for dissipation of shock and impact energy in the dynamically photocrosslinked PP/EPDM blends. The DSC results give positive evidence of the formation of the graft copolymer of PP-g-EPDM, which greatly en- hances the compatibility of PP and EPDM and thus increases the impact strength of PP/EPDM blends at low temperatures.

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Mechanical properties and structural characteristics of dynamically photocrosslinked PP/EPDM blends

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