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Rheological, mechanical and thermal properties of propylene-ethylene block copolymer/polyalphaolefins blends.

INTRODUCTION

Isotactic polypropylene (iPP) is widely used in many applications because of its outstanding properties such as high heat resistance, high mechanical strength, easy processability, and low price. However, its application for engineering thermoplastic is limited because of its low impact toughness, especially at low temperatures. To improve the impact strength of iPP, various types of elastomers such as ethylene-propylene-diene rubber (EPDM) [1-4], ethylene-propylene rubber [5, 6], styrene-butadiene-styrene copolymer [7] and styrene-ethylene-butadiene-styrene copolymer [8] have been blended.

On the other hand, polyalphaolefins (APAOs) are low molecular weight polypropylenes or propylene copolymers with either ethylene or 1-butene with predominantly atactic molecular structure. There are many types of APAOs depending on the molecular weight, comonomer type, and contents, but the main characteristics of APAOs are low molecular weight and low crystallinity. So, they give easy processing, good adhesion to a varieties of substrates, good miscibility with solvents, oligomers and polymers, low temperature flexibility as well as toughening propylene-based polymers. Studies using APAOs as polymer modifier are sparse [9-12]. Instead, APAO has mostly been applied for hot melt adhesive, and little has been evolved for toughening propylene-based polymers [13]. Since APAOs are propylene-based low crystalline polymers, high compatibility with polypropylene is expected to provide the blend with well balanced mechanical strength and toughness. The effects should depend on the molecular weight, comonomer content and type of APAOs, and the type of polypropylene, i.e., propylene-ethylene block copolymer (PPB), propylene-ethylene random copolymer, and polypropylene homo polymer.

In this work, we investigated the effects of molecular weight, comonomer type, and content of the APAOs on the rheological, mechanical, and thermal properties of PPB/APAO blends.

EXPERIMENTAL

Materials

PPB, SB9108 which was manufactured by Korea Petrochemical Ind. Co., was used as the base material. PPB was produced by using two slurry reactor processes. Low molecular weight homo polypropylene was polymerized in the first reactor, and then, it was fed into the second reactor to be polymerized with propylene/ethylene block tail. The molar ratio of propylene/ethylene and the weight content of block segment in the second reactor were controlled about 1/0.25 and 9%.

UBETA[C.sup.R] (Ube, Japan) APAOs were polymerized with propylene or copolymerized with either propylene and ethylene or propylene and 1-butene. Each APAO grade was selected according to the comonomer type and content and molecular weight. The samples used in this study were all commercially available grades.

Characterization of Materials

[.sup.13.C]-NMR spectra were recorded on a Varian Unity Inova-300 nuclear magnetic resonance spectroscopy for the analysis of comonomer types and contents of all samples, and the content of ethylene-propylene block (EP). The molecular weight was determined by gel permeation chromatography (GPC, alliance GPCV 2000, Waters). Tetrahydrofuran (THF) was used as carrier solvent, and the calibration curve was established by using standard polystyrene. In the case of PPB, it was firstly dissolved in boiling decalin for 2 h, and then was annealed at 25[degrees]C, 1 h before separation by quantitative filter paper to measure both molecular weights of filtered (PP) and nonfiltered (mostly EP block) composition. All the characteristics are listed in Table 1.

Preparation of Polymer Blends

Each APAOs was shattered to pieces by low temperature pulverator (NRX2, Shimpo Ind., Japan) at 0[degrees]C and then mixed with PPB. Blends were done by Ikegai 45-mm corotating twin screw extruder (PCM-45, Japan) at 220 rpm and 190[degrees]C. To avoid ambiguity, mixing conditions were kept the same for the all blends.

Rheological Properties

Rheological properties were measured by Advanced Rheometrics Expansion System (ARES, Rheometrics) with a 25-mm parallel plate fixture. Dynamic frequency sweep tests were performed in a nitrogen atmosphere at constant strain of 15% and oscillatory angular frequency ranging between 0.1 and 400 rad/s, at 170[degrees]C.

Thermal Properties

Thermal properties were investigated by differential scanning calorimetry (DSC, Perkin-Elmer Pyris II). Specimens were heated to 200[degrees]C at 10[degrees]C/min, kept for 1 min and then cooled down to 30[degrees]C at 10[degrees]C/min, to measure crystallization temperature. Samples were reheated with the same heating condition to determine melting temperature.

Mechanical Properties

Injection molded specimens were made with a Nissei 35oz injection molding machine at 200[degrees]C of cylinder temperature and 40[degrees]C of mold temperature. These specimens prepared with the same procedure were also used to analyze rheological, mechanical, and thermal properties. Tensile properties and flexural modulus were measured with a Universal Test Machine (UTM, Instron 4302, USA) following ASTM D 638 and D 790. Hardness was measured with a Shore hardness tester (Shore-D type, Toyoseiki, Japan) following ASTM D 2240 and Izod impact strength was measured with a Izod impact tester (Toyoseiki Seisa-kusho, Japan) following ASTM D 256.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Rheological Properties

Complex viscosities ([eta]*) of PPB/APAO blends are shown in Figs. 1 and 2. The [eta]* of all samples show shear thinning behavior regardless of comonomer type and blend composition. The [eta]* decreases with increasing APAO content caused by blending of very low [eta]* of APAO (dotted line in Figs. 1 and 2), and the effect of comonomer type is most pronounced with 1-butene (B78), followed by homo (H18), and ethylene (E58, Fig. 1) at similar molecular weight and [eta]*. For APAOs with ethylene comonomer, less decrease in [eta]* is obtained with more ethylene content (Fig. 2).

Viscosity-composition curves of the blends are shown in Fig. 3. Utracki and Kanial [14] divided viscosity-composition curve of blends into three types compared with linear additivity line of the simple mixing rule: positive deviation blend (PDB); negative deviation blend; and positive-negative deviation blend.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Our [eta]* versus blend composition curves show strong PDB at all frequencies. Thornton et al. [15] found similar behavior for polystyrene/polymethyl methacrylate blends, and they suggested that the system can be described in terms of a continuous two phase model where the macromolecules of one polymer are physically entrapped in the phase of the other constituent.

Regarding the effect of comonomer type, PDB is most pronounced with E58 (ethylene), followed by H18 (homo) and B78 (1-butene) at all frequencies (just showed the data at 0.1 rad/s in Fig. 3), an order opposite to the viscosity decrease. That is, comonomer giving rise to low viscosity decrease produces great PDB. Exactly the same tendency is also found in Fig. 3 where high ethylene content APAO gives great PDB at all frequencies. From the above results, it is suggested that PP molecules are entrapped by APAO, and ethylene component of APAO is embedded with ethylene-propylene block component of PPB, so blending of ethylene comonomer type, especially more ethylene content of APAO shows less decrease in [eta]* and more PDB behavior.

[FIGURE 5 OMITTED]

The Cole-Cole plots of PPB/APAO blends are shown in Fig. 4, where semi-circular shapes are observed for all blend compositions. Wisniewski et al. [16] observed the same result in homopolymer and miscible polymer blends.

[FIGURE 6 OMITTED]

Thermal Properties

Thermal properties of the blends are given in Table 2. The shape of melting peak is shown in Fig. 5, where melting peak ([T.sub.m]) is splitted into two which will be denoted by [T.sub.ml] (low temperature peak) and [T.sub.mh] (high temperature peak) at some compositions (E38 and E58 at 30 and 50 wt%, all at 50 wt%), and there were no remarkable melting peaks around 90-140[degrees]C. [T.sub.ml] decreases and [T.sub.mh] increases with increasing APAO content (Fig. 6) and the order of decrease is B78 > homo H18 > E28 > E38 > E58 which is consistent with the decrease of [eta]* as mentioned earlier. E38 and E58 having relatively high ethylene content show splitted melting peaks at lower APAO content (30 wt%) than other ones (50%). According to Iwakura [17], high density polyethylene/ethylene-octene copolymer blends show two melting peaks because of the cocrystallization of the partially miscible mutual ethylene domains. Splitted melting should imply cocrystallization between APAO and propylene-ethylene block component of PPB, and this is remarkable in ethylene comonomer types of APAOs.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Recrystallization temperature ([T.sub.c]) of the blends decreases with increasing APAO contents (Fig. 7), and it also depends on comonomer type and content, and the order is almost the same with the order of [T.sub.m].

Crystallinity (Table 2) is calculated based on 209 J/g of heat of fusion for perfect crystalline polypropylene [18]. Crystallinity of the blends decreases with increasing APAO content (Fig. 8). Although the difference is not big, decrease in crystallinity also depends on comonomer type and content with an order of B78 > H18 > E38 > E58.

The decrease in crystallinity is due to the dilution effect of APAOs because there is no change in size and shape of crystallite with the addition of APAO (Fig. 9).

Mechanical Properties

Mechanical properties of the blends are given in Table 3. Stiffness of the blends decreases with the addition of APAO to the PP block copolymer, which can be explained by the replacement of the crystalline component of PPB with relatively high amorphous one of APAO, lowering the crystallinity of blends compared with virgin PP.

The tensile strength decreases with increasing APAO content, with a negative deviation from the linear additivity except at 10 wt% of APAO (Fig. 10). On the other hand, homo APAO (H18) shows positive deviation at all compositions.

The flexural modulus also decreases with increasing APAO contents, showing negative deviation except at 10 wt% for all types of APAO (figure not shown).

Hardness of the blends also decreases with increasing APAO contents, but it shows positive deviation (figure not shown). This implies that hardness is mainly affected by the surface stiffness of PPB matrix in blends. All blends exhibit positive inflection point at 10 wt% APAO which implies better mechanical miscibility [8] than those of the other compositions.

The elongation at break increases linearly with increasing APAO content up to 30 wt%, and then leveled off (figure not shown). Low molecular weight APAO (E33) showed less increase than other ones. The increase in elongation is caused by the decreased crystallinity of PP with increasing APAO content.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The Izod impact strength increases slightly up to 30 wt% of APAO (Fig. 11), and then dramatically increases except homo APAO (H18). High molecular weight APAO (E38) exhibits great increase in impact strength than low molecular weight one (E33) at almost same comonomer content. Similar results have also been reported for PP/EPDM blends where a critical content of 30% gave a dramatic increase of impact strength [19], and then it was leveled off above 40 wt%. In our experiment, the critical value was also about 30 wt%, but it was not leveled off up to 50 wt%. The reason of continuous increase in impact strength may be suggested that APAO relaxes stress concentration and suppresses the formation of the matrix crazes or deformation.

CONCLUSIONS

We investigated the effects of molecular weight, comonomer types and contents and blend compositions on the rheological, thermal, and mechanical properties of propylene-ethylene block copolymer (PPB)/polyalphaolefins (APAOs) blends.

It was found that ethylene copolymerized APAO, especially having higher comonomer content one has better miscibility with PPB than the other ones from the result of less decrease of viscosity and melting temperature and strong PDB in viscosity-composition curve. Therefore, it can be concluded that PP molecules are entrapped by APAO, and the ethylene segments of APAO are embedded in ethylene propylene block of PPB, so high ethylene content of APAO shows less decrease in [eta]*, [T.sub.m], [T.sub.c], and more PDB in PPB/APAO blends.

Addition of APAO to PPB decreased stiffness, but stiffness-composition curve showed positive inflection point at 10 wt% of APAO which implies better mechanical miscibility than those of the other compositions.

Impact strength of the blends increased dramatically above 30 wt% of APAO, especially with high comonomer content, so it is concluded that APAO can successfully be used as a modifier of PP block copolymer.

REFERENCES

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7. V. Choudhary, H.S. Varma, and I.K. Varma, Polymer, 32, 2534 (1991).

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9. I.O. Hong, W.K. Kim, and H.J. Kang, Polymer, 24, 513 (2000).

10. M.T. Thakker, J.F. Galindo, and D. Jani, U.S. Patent 006,080,818A (2000).

11. S.W. Tsui and A.F. Johnson, WO 02/36704 Al (2002).

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13. T. Iwao, M. Takahami, and E. Ikuo, EP 0527589 Al (2003).

14. L.A. Utracki and M.R. Kanial, Polym. Eng. Sci., 22(2), 96 (1982).

15. B.A. Thornton, R.G Villasenor, and B. Maxwell, J. Appl. Polym. Sci., 25, 653 (1980).

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17. K. Iwakura, Plastics, Japan, 41(9), 26 (1990).

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19. W. Wang, Q. Wu, and B. Qu, Polym. Eng. Sci., 43, 1798 (2003).

H.S. Ha, (1) M.C. Kang, (2) H.H. Cho, (3) B.K. Kim (4)

(1) Korea Petrochemical Ind. Co., LTD., Ulsan, Korea

(2) National Core Research Center, Pusan National University, Busan 609-735, Korea

(3) Department of Organic Materials Science and Engineering, Pusan National University, Busan 609-735, Korea

(4) Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Korea

Correspondence to: B.K. Kim; e-mail: bkkim@pnu.edu

Contract grant sponsors: Ministry of Science & Technology, Korea Science & Engineering Foundation; contract grant number (NCRC): R15-2006-022-01001-0.
TABLE 1. The characteristics of test materials.

 Comonomer
Sample code Grade name Type Content (wt%)

H18 UBE 2180 -- --
E28 UBE 2280 Ethylene 4.3
E33 UBE 2330 Ethylene 7.8
E38 UBE 2385 Ethylene 7.4
E58 UBE 2585 Ethylene 13.0
B78 UBE 2780 1-Butene 32.0
PPB KPIC SB9108 Ethylene 4.5 (overall)
 EP composition 9.1

Sample code [M.sub.n] ([10.sup.4] g/mol) [M.sub.w] ([10.sup.4] g/mol)

H18 0.80 3.68
E28 0.81 3.62
E33 0.40 1.81
E38 0.84 3.62
E58 0.87 3.55
B78 0.90 4.08
PPB 4.4 (PP) 29.4 (PP)
 20.9 (EP) 98.7 (EP)

TABLE 2. The thermal properties of all blends.

 APAO
Sample contents [T.sub.mh] [T.sub.ml] [DELTA][H.sub.f]
code (%) ([degrees]C) ([degrees]C) (J/g)

PPB 0 -- 163.7 109.3
H18 10 -- 161.5 100.9
 30 -- 159.2 85.6
 50 163.2 157.0 71.6
 100 157.5 138.0 32.8
E28 10 -- 161.9 103.3
 30 -- 159.3 86.9
 50 163.5 157.3 70.2
 100 149.5 128.1 23.3
E33 10 -- 161.1 106.0
 30 -- 159.2 86.8
 50 163.2 157.0 69.1
 100 142.2 127.6 14.5
E38 10 -- 162.2 103.9
 30 163.1 159.8 85.1
 50 163.5 158.2 69.4
 100 141.5 123.3 17.2
E58 10 -- 162.4 103.6
 30 163.2 160.5 88.8
 50 163.5 159.0 71.6
 100 -- 129.2 11.4
B78 10 -- 161.4 100.7
 30 -- 158.9 84.3
 50 162.8 156.5 62.9
 100 -- 90.5 24.5

 APAO
Sample contents [T.sub.c] Crystallinity
code (%) ([degrees]C) (%)

PPB 0 117.8 52.3
H18 10 115.2 48.3
 30 114.2 41.0
 50 112.2 32.5
 100 98.9 16.0
E28 10 116.0 49.4
 30 114.5 41.6
 50 112.5 33.6
 100 86.5 11.4
E33 10 115.9 50.7
 30 114.2 41.5
 50 113.2 33.1
 100 68.5 7.1
E38 10 116.4 49.7
 30 114.5 40.7
 50 113.2 33.2
 100 72.2 8.4
E58 10 117.2 49.6
 30 114.7 42.5
 50 113.5 34.3
 100 68.2 5.6
B78 10 114.9 48.2
 30 114.2 40.3
 50 111.9 30.1
 100 53.2 12

TABLE 3. The mechanical properties of all blend compositions.

Sample APAO Hardness Izod impact strength
code contents (%) (Shore D) ([kg.sub.f] cm/cm)

PPB 0 72 6.8
H18 10 70.0 6.5
 30 64.5 7.6
 40 61.0 8.3
 50 57.0 9.3
 100 27.2 NB
E28 10 70.5 6.7
 30 62.0 8.9
 40 57.5 11.0
 50 52.5 16.3
 100 17.0 NB
E33 10 70.5 7.2
 30 60.0 8.2
 40 55.5 12.2
 50 50.0 19.5
 100 -- NB
E38 10 70.0 7.6
 30 60.0 10.2
 40 55.0 23.8
 50 50.5 47.0
 100 5.0 NB
E58 10 68.5 8.0
 30 60.0 14.0
 40 54.5 25.3
 50 50.0 44.7
 100 4.0 NB
B78 10 68.5 7.6
 30 62.0 9.3
 40 55.0 20.1
 50 45.0 41.9
 100 2.0 NB

 Tensile strength
 Flexural modulus Yield point
Sample APAO ([kg.sub.f]/ ([kg.sub.f]/
code contents (%) [cm.sup.2]) [cm.sup.2]) Elongation (%)

PPB 0 11,100 270 670
H18 10 11,000 270 710
 30 7300 200 870
 40 -- -- --
 50 4500 150 870
 100 500 24 65
E28 10 10,400 265 720
 30 5500 180 870
 40 -- -- --
 50 2400 130 870
 100 100 13 65
E33 10 10,200 250 720
 30 5500 175 860
 40 -- -- --
 50 2100 115 860
 100 -- 8 55
E38 10 10,400 260 720
 30 5500 175 870
 40 -- -- --
 50 2200 120 860
 100 -- 8 50
E58 10 10,200 250 690
 30 5600 175 830
 40 -- -- --
 50 2200 110 840
 100 -- 4 80
B78 10 10,100 245 710
 30 5900 175 860
 40 -- -- --
 50 1500 85 860
 100 -- 3 220
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Author:Ha, H.S.; Kang, M.C.; Cho, H.H.; Kim, B.K.
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:9SOUT
Date:Nov 1, 2007
Words:3132
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