e-Polymers 2004, no. 021. ISSN 1618-7229

http://www.e-polymers.org

Plasticizing of isotactic polypropylene upon addition of hydrocarbon oils Koh-hei Nitta 1 *, Hidetaka Ando 2, Takuo Asami 2,3 1 Department of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa, 920-8667, Japan; Fax +81-76-234-4829; nitta@t.kanazawa-u.ac.jp 2 School of Materials Science, JAIST, 923-1292, Japan; Fax +81-761-51-1625 3 Material Science Laboratory, Mitsui Chemicals Inc., 580-32 Nagaura, Sodegaura, Chiba, 299-0265, Japan; Fax +81-43-64-2443; Takuo.Asami@mitsui-chem.co.jp (Received: February 10, 2004; published: April 22, 2004)

Abstract: The effect of the addition of hydrocarbon oil on the mechanical behaviour of isotactic polypropylenes (iPPs) was examined. It was found that the oil mole-cules are completely dissolved in the amorphous region of iPP so that the blending lowers the glass transition temperature, Tg, of iPP. As a result, Young’s modulus of iPP/oil blends is dominated by the difference between the measurement temper-ature and Tg (∆Tg

= T - Tg), independent of the oil content. The elongation at break is proportional to ∆Tg, while the strength at break increases linearly with increasing tie-molecule fraction (which increases with decreasing oil content), being inde-pendent of ∆Tg.

Introduction The glass transition temperature Tg is one of the most important characteristics of polymeric materials. The control of Tg broadens significantly their service temperature range and improves their mechanical toughness. The addition of low-molecular-weight diluents to amorphous polymers is a well-established technology in the industry for the improvement of mechanical properties [1]. The diluents, commonly referred to as a plasticizers, can effect a large depression in Tg. In plastics industry, brittle polymers such as poly(vinyl chloride) and polystyrenes are often plasticized as commercial grades [2,3]. Isotactic polypropylene (iPP) resins have been extensively used in various products such as automotive parts, furniture and containers. However, the application of iPP as an engineering plastic is limited owing to its poor impact toughness, in particular at lower temperatures, because of its relatively high Tg. It has been identified in plastics industry that hydrocarbon oils are miscible with the amorphous polypropylene component and the addition of the diluent oils to iPP materials depresses their Tg. However, little has been done to investigate the additional effects of the diluents on the morphology, mechanical properties and their interrelation [3]. The purpose of this study is to develop a fundamental understanding of the additive effect of miscible diluent oils on semicrystalline iPP materials. The outcome of this work is a new type 1

of isotactic polypropylenes with much improved low-temperature characteristics without sacrifice in performance at higher temperatures. Experimental part Materials Isotactic polypropylene (iPP) used in this study was commercial grade supplied by Mitsui Chem. Inc. The molecular weight of the iPP was Mw = 160 000 and Mw/Mn = 2 to 3, as determined by gel permeation chromatography (Waters 150C). Hydrocarbon oil of molecular weight 500 - 800 was added to the iPP, with various oil contents of 2, 5, 9, 18, and 27 wt.-%. The samples were melt-pressed in a laboratory hot press at 463 K and 45 t. Film sheets with a thickness of about 500 µm were prepared by rapid quenching from the melt into 283 K. Sample code for iPP/oil blends used in this study is iPPO and the end numeral of the sample code indicates the wt.-% content of oil: e.g., iPPO-18 is the iPP/oil blend containing 17.7 wt.-% oil. Molecular characteristics of the samples are summarized in Tab. 1. Tab. 1. Molecular and structural characteristics of samples

Sample Oil content in wt.-%

Tm /K Tg

/K ρ /kg·m-3 χv Lp /nm

iPP 0 438.7 277.0 904.9 0.622 15.2 iPPO-2 2.4 438.0 273.9 905.7 0.626 15.4 iPPO-5 4.8 437.5 266.9 905.7 0.618 15.9 iPPO-9 9.1 437.2 258.1 904.6 0.592 16.9 iPPO-18 17.7 434.9 237.9 901.5 0.528 19.1 iPPO-27 26.5 433.1 223.9 900.1 0.475 21.9

Characterization Differential scanning calorimetry (DSC) measurements were carried out using a Mettler DSC 820 to examine the melting behaviour. Samples of 10 mg, which were cut from the sheets, were heated from room temperature to 473 K at a scanning rate of 20 K/min under nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) measurements were carried out with a Rigaku RU-200 diffractometer with Ni-filtered Cu-Kα radiation (40 kV, 100 mA, λ = 0.154 nm) under a sample-camera distance of 90 or 100 mm. The diffraction patterns of blend sheets showed that the iPP crystal form of all blend sheets was monoclinic. Densities of the sheets were determined by a floatation method. A binary medium prepared from various ratios of distilled water and ethyl alcohol was used. The crystallinity of these polymer films can be determined using their density data:

χv = (ρ − ρa) / ( ρc - ρa) (1)

where χv is the degree of crystallinity (volume fraction), and ρ is the density of the sample, ρa is the density of the amorphous region and ρc is the density of the crystal.

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The density of the monoclinic unit cell, 936 kg/m3, was used as ρc and the value of ρa was taken to be 854 kg/m3 according to the literature [4,5]. Small angle X-ray scattering (SAXS) measurements were performed with a point-focusing optics and a one-dimensional position-sensitive proportional counter (PSPC) with an effective length of 10 cm. The Cu-Kα radiation supplied by a RINT2500 (Rigaku) operating at 40 kV and 200 mA was used throughout. From the data of the volume fraction of crystals χv and SAXS long period Lp, the lamellar crystal thickness Lc and amorphous layer thickness La can be determined from the following relation-ships:

Lc = χv Lp La = (1 - χv) Lp (2)

The growth rate of spherulite radius was quantitatively estimated. The radius of a spherulite was measured on a polarized optical microscope (POM) as a function of time under an isothermal crystallization process. The polarized optical microscope (Olympus, B201) fitted with an automated hot stage was used. The hot stage (Mettler Toledo, FP82HT) could be held at a steady temperature to ± 0.2 K by a proportional controller. The film samples were sandwiched between a microscope slide and a cover glass, heated to 503 K, and kept at this temperature for 10 min to melt completely the crystallites. Then the samples were rapidly quenched to a given crystallization temperature Tc (408 K) and allowed to crystallize isothermally. Measurements Dynamic mechanical properties were investigated using a dynamic mechanical analyser (Rheology Co., Ltd. DVE-V4) on sample specimens of the following dimensions: length 20 mm, width 3 mm, and thickness about 500 µm. The temper-ature dependences of storage modulus E’ and loss modulus E” were measured between 253 and 443 K at a constant frequency of 10 Hz and a heating rate of 2 K/min. The uniaxial tensile behaviour was investigated using a Shimadzu AGS-5kN. Dumbbell-shaped sample specimens were cut, in which the gauge length was 2 mm. The tensile strain was calculated from the ratio of the increment of the length between clamps to the initial gauge length. The tensile stress was determined by dividing the tensile load by the initial cross section. The strain-stress curves were measured in a temperature range from 223 to 298 K and a cross-head speed of 1 mm/min.

Results and discussion The mechanical relaxation spectra at 10 Hz of iPP and iPP/oil blends are shown in the form of storage modulus and loss modulus in Fig. 1. In these dynamic mechanical spectra, there are two relaxation processes assigned as α and β in the order of decreasing temperature. The α-processes, ascribed to the relaxation of crystalline phase, are observed in the temperature range from 350 to 450 K for all samples. The temperature location was almost independent of iPP content, but the magnitude of the dynamic modulus reduced with increasing oil content. In addition, the melting point did not change much by the addition of oil (see Tab.1). These results demon-strate that the addition of oil did not much affect the crystalline state or completeness in crystal packing of iPP.

3

105

106

107

108

109

1010

100 200 300 400 500

E' &

E" /

Pa

Temperature / K

10Hz

E''

β

E'

α

iPPiPPO - 2iPPO - 5iPPO - 9iPPO - 18iPPO - 27

Fig. 1. Dynamic mechanical spectra of iPP and iPP/oil blends. The frequency used in this study was 10 Hz

0

2

4

6

8

10

12

0 10 20

Rel

axat

ion

Stre

ngth

Oil-content / wt%30

Fig. 2. Relaxation strength (change in storage modulus during relaxation divided by storage modulus after relaxation) of iPP and iPP/oil blends with various oil fractions

The β-relaxations of iPP and iPP/oil blends are observed in the temperature range from 250 to 300 K. The β-relaxation is ascribed to the glass transition of the iPP amorphous region. As shown in Fig. 1, the addition of oil drastically depresses the temperature of the β-relaxation and broadens the β-relaxation peak reflecting the enhancement of molecular mobility or dynamic flexibility of amorphous iPP. In addition, the relaxation strength, which was estimated from the jump in the storage modulus E’ in the β-relaxation, was also found to almost linearly increase with increasing oil contents (see Fig. 2). This result implies that the glass relaxation process of the blends occurs co-operatively and that hydrocarbon oil acts as a

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plasticizer diluent for the amorphous iPP component. At higher contents of the oil plasticizer the β loss peaks seem to be multiple and a broadening in the distribution of relaxation times is observed. This is indicative of the emergence of another separate miscellaneous phase in addition to the existing two-phase system of crystalline and amorphous regions due to the excessive addition of oil. As shown in Fig. 3a, the glass temperatures of blends were found to decrease with increasing mass fraction of oil. Taking the Tg value of the diluent oil to be 208 K as measured by DSC and correcting for the amorphous fraction in the iPP as determined by density data, it is shown in Fig. 3b that the agreement between the experimental values of Tg of iPP/oil blends and the theoretical ones evaluated by the Fox equation [6] is very good, suggesting that only the amorphous component of iPP is plasticized and the crystalline fraction and crystalline morphology is not much affected. This suggests no specific interaction between iPP segments and oil molecules.

200

220

240

260

280

300

0 10 20

Tg /

K

Oil-content / wt%

(a)

30

200

220

240

260

280

300

0 20 40 60 80 10

Tg /

K

Oil fraction in amor0

phous iPP / wt%

(b)

Fig. 3. (a) Tg of iPP with various oil fractions. (b) Comparison between measured and calculated Tg of the amorphous fraction of iPP with various oil fractions. The solid line is obtained from the Fox equation Although the addition of oil lowers the density or bulk crystallinity, the additional effects of oil on lamellar morphology such as crystalline lamellar thickness and amorphous layer thickness are much different from each other. Fig. 4 shows that as the oil content increases, the crystalline lamellar thickness almost remains constant while the amorphous layer thickens. This also indicates that the majority of the diluent is incorporated into the amorphous phase in iPP. This is consistent with the result that the hydrocarbon oil is miscible with the amorphous iPP and plasticizes only the amorphous component of iPP. The same result was also seen for miscible blends of iPP and α-olefin copolymers [7]. In particular for quenched iPP samples, a smectic phase in which molecular mobility is somewhat hindered acts as a rigid amorphous phase [8,9]. For this reason, a three-phase model based on fully crystalline, mobile and rigid amorphous phases provides a more precise morphological view. The linear relation between the oil fraction and the relaxation strength as shown in Fig. 2 suggests that the oil molecules devitrify the rigid amorphous region and/or are mixed with the amorphous PP chains. However, the depression of Tg of iPP due to the addition of oil is thought to be

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caused not by the devitrification of the rigid amorphous region but by amorphous molecules whose mobility is enhanced by co-operative motion with oil molecules. For convenience the two-phase model was employed in this work as a useful first-order description of the morphology.

0

5

10

15

0 10 20

Lc &

La

/nm

Oil-content / wt30

%

Lc

La

Fig. 4. Additional effect of oil on the thickness of crystalline lamellae Lc and of the amorphous layer La of iPP

0

5

10

15

0 1 2 3 4 5 6

Time / min

iPPO-18

iPP

Sph

erul

ite ra

dius

in µ

m

Fig. 5. Growth process of spherulites in an isothermal state (at 404 K) for iPP and iPPO-18

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The isothermal growth rate of spherulites at a constant temperature of 408 K was investigated using an optical microscope with a hot stage. Fig. 5 shows the time variation in the spherulite radius during isothermal crystallization for iPP and iPPO-18. As shown in the figure, spherulite radii of the samples increase linearly with time over the experimental range, giving the growth rate, which can be determined by the slope of the lines in Fig. 5. It was found that the growth rate of iPP is decreased by the addition of the oil. Considering that the crystallization temperature employed in this work is much higher than Tg and is in the range that the free energy for the formation of a critical sized nucleus dominates the overall growth rate, this result is attributed to the following reasons: the oil molecules act as a diluent for iPP in the molten state and blending of the diluent leads to a higher free energy for the formation of nuclei. This also strongly suggests that the hydrocarbon oil is miscible with the amorphous region of iPP. The same result was also seen for miscible blends of iPP and α-olefin copolymers [10].

The stress-strain curves measured at room temperature are shown in Fig. 6 and Young’s modulus and toughness, which were estimated from these curves, are plotted against the oil content (see Fig. 7). Young’s modulus linearly decreases with increasing oil content whereas the toughness or elongation at break has a maximum around 10 wt.-% of the oil content. As seen in Fig. 6, the strength at break appears to monotonously decrease with increasing oil content. This strongly demonstrates that the blending of the hydrocarbon oil softens the iPP material and plays the role of a plasticizer for iPP. This is because of the incorporation of the oil molecules into the amorphous region of iPP.

0

10

20

30

40

50

0 0.1 0.2 0.3 0.4 0.5 0.6

iPPiPPO-2iPPO-5iPPO-9iPPO-18iPPO-27

Stre

ss /

MP

a

Strain Fig. 6. Stress-strain curves of iPP/oil blends measured at 298 K and under a constant elongation rate of 1 mm/min

107

108

109

0

5

10

15

0 10 20 30

Toug

hnes

s / M

Jm-3

Youn

g's

mod

ulus

/ Pa

Oil-content / % Fig. 7. Young’s modulus and toughness plotted against the oil content. Young’s modulus was estimated from the initial slope of the stress-strain curves in Fig. 6 and the toughness was estimated from the area under the stress-strain curves from the origin to the fracture point

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Fig. 8. Temperature dependence of stress-strain curves of iPPO-9 and iPPO-27. In the figure, ∆Tg is given as T - Tg The temperature dependence of stress-strain curves is exemplified in Fig. 8. In the figures, ∆Tg means the temperature difference between the measurement temper-ature T and the glass transition temperature Tg. The strength decreases and the elongation at break increases with increasing ∆Tg. Fig. 9 exemplifies Young’s modulus measured at ∆Tg = 0 K and 23.1 K plotted against the oil content. Then Young’s modulus is independent of the content of oil and is affected by the temper-ature difference from each Tg only. When plotting Young’s modulus vs. ∆Tg for iPP and iPP/oil blends as shown in Fig. 10, all data fall onto a common line, which is given by the following relation:

E = Eg exp(-β ∆Tg) (3)

where β is 0.0254 K-1. This is the proof that oil molecules act as a plasticizer for iPP materials and Young’s modulus is dominantly affected by the molecular mobility of the amorphous region. This is due to the fact that the deformation in the initial strain region concentrates on the amorphous region since the modulus of the amorphous region is much lower than that of the crystalline lamellae. Consequently the value of ∆Tg becomes the dominant factor for Young’s modulus of these iPP/oil blends. Incor-poration of oil molecules into the amorphous layer of iPP enhances the molecular mobility of the amorphous phase.

The elongation at break is plotted against the value of ∆Tg for all samples in Fig. 11. The elongation at break is linearly proportional to ∆Tg, suggesting that the elongation process is essentially based on the nature of the amorphous regions. Similar results were obtained for a series of ethylene-propylene random copolymers with various components [11]. We can find a critical point at ∆Tg = -20 K at which the elongation at break becomes zero. The failure process will be drastically changed below -20 K from Tg, at which point an intrinsic failure transition such as ductile-brittle transition may take place, being independent of the content of oil. The temperature depend-ence of the failure process is dominated by the rates of conformational rearrange-ments of stretched molecular chains taking place during tensile deformation. The slope of the line in Fig. 11 is greatly reduced at higher oil contents (see Fig. 12). 8

108

109

1010

0 10

Youn

g's

mod

ulus

/ Pa

Oil-cont

‡ ™Tg = 0K

Fig. 9. Young’s modulus vs. oil content measur

107

108

109

0 20 40 60 80 100

iPPiPPO-2iPPO-5iPPO-9iPPO-18iPPO-27

Youn

g's

mod

ulus

/ Pa

‡ K

Elon

gatio

n at

bre

ak

Fig. 10. Young’s modulus plotted against ∆Tg for a set of iPP/oil blends

Figag

™Tg / ∆Tg / K

0.000

0.002

0.004

0.006

0.008

0.010

0 10 20

ε b / ∆

Tg /

K-1

Oil-content / %30

Figco

9

e

™Tg = 23K

e

a

n

‡

20 30

nt / wt% d at ∆Tg = 0 and 23.1 K

0

0.2

0.4

0.6

0.8

-50 -25 0 25 50 75 100

iPPiPPO-2iPPO-5iPPO-9iPPO-18iPPO-27

‡ ™ K. 11. Elongation at break plotted inst ∆Tg for a set of iPP/oil blends

Tg / ∆Tg / K

. 12. Slope of the line in Fig. 10 vs. oil tent

0

10

20

30

40

50

0 0.1 0.2 0.3

Stre

ngth

at b

reak

/ M

Pa

Tie molecule fraction Fig. 13. Strength at break plotted against the tie-molecule fraction, which is estimated from the Huang-Brown equation [8]

On the other hand, the strength at break was found to be independent of ∆Tg. As shown in Fig. 13, the strength at break depends on the tie molecule fraction, which was evaluated as the probability that a single molecule connects two adjacent crystalline lamellae in the amorphous phase [12]. As a result, the tie molecule fraction decreases with the addition of oil. The linear relationship between the tie molecule fraction and the strength demonstrates that tie molecules support the external force required for the fragmentation of crystalline parts, which takes place on the yielding or failure process of these samples [13,14]. As described before the addition of oil did not affect the crystalline parts so that the strength of these parts of the blends is independent of the oil content and almost the same as that of pure PP. In addition, the external force pulls tie molecules taut in the ultimate region and the taut tie molecules act as a stress-transmitter. Consequently, a decrease in the tie-molecule fraction can be considered to lead directly to a decrease in the ultimate stress.

Acknowledgement: The authors wish to acknowledge Mitsui Chemicals, Inc. for financial support and the permission to publish this paper. The authors also thank to Y. Itou and M. Inaba of Mitsui Chemicals Inc. and H. Kan-no and the researchers of Mitsui Chemical Analysis & Consulting Service Inc. for their experimental support.

[1] Kurtz, S. S.; Sweely, J. S.; Staut, W. J.; in “Plasticisers Technology“, Burns, P. F., editor; Reinhold, N. Y. 1965. [2] Baur, H. E.; Warren,P. C.; J. Macromol. Sci., Phys. 1981, B20, 381. [3] Ellul, M. D.; Rubber Chem. Tech. 1997, 71, 244. [4] Jones, A. T.; Cobbold A. J.; J. Polym. Sci. 1968, B6, 539. [5] Samuels, R. J.; Yee, R. Y.; J. Polym. Sci. A2 1972, 10, 385. [6] Fox, T. G.; Bull. Am. Phys. Soc. 1956, 1, 123.

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[7] Yamaguchi, M.; Nitta, K.; Miyata, H.; Masuda, T.; J. Appl. Polym. Sci. 1996, 63, 467. [8] Cohen, Y.; Saraf, R. F.; Polymer 2001, 42, 5865. [9] VanderHart, D. L.; Snyder, C. R.; Macromolecules 2003, 36, 4813. [10] Yamaguchi, M.; Miyata, H.; Nitta, K.; J. Polym. Sci., Polym. Phys. 1997, 35, 953. [11] Shin, Y. W.; Uozumi, T.; Terano, M.; Nitta, K.; Polymer 2001, 42, 9611. [12] Huang, Y. L.; Brown, N.; J. Polym. Sci., Polym. Phys. 1991, 29, 129. [13] Nitta, K.; Takayanagi, M.; J. Macromol. Sci., Phys. 2003, B42, 107. [14] Takayanagi, M.; Nitta, K.; Kojima, O.; J. Macromol. Sci., Phys. 2003, B42, 1049.

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