The Crystallization Behavior of Isotactic ... - ACS Publications

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Industrial & Engineering Chemistry Research

The crystallization behavior of isotactic polypropylene induced by a novel anti-nucleating agent and its inhibition mechanism of nucleation Shicheng Zhao,

†

Xin Yu,

†

Hanzhang Gong,

†

Zhong Xin,*,

†

Yaoqi Shi,

‡

Shuai

Zhou† †

Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, State-Key

Laboratory of Chemical Engineering, Department of Product Engineering, East China University of Science and Technology, Shanghai, 200237, People’s Republic of China ‡

Shanghai Key Laboratory of Catalysis Technology for Polyolefins, Shanghai

Research Institute of Chemical Industry, Shanghai, People’s Republic of China

Abstract: The addition of foreign substances is well known to be able to improve the crystallization of isotactic polypropylene (iPP) following a nucleation-promoting mechanism. In the present work, however, we found that zinc salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo

[2.2.1]

hept-5-ene-2,

3-dicarboxylic

acid

(HCHD-Zn)

unexpectedly decreased the crystallization temperature (Tcp) and crystallization rate of iPP, generating remarkable anti-nucleation effects. The results of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) showed Tcp of iPP decreased more than 4 oC and nucleation densities of iPP decreased nearly 70% upon the addition of the “anti-nucleating agent” HCHD-Zn. Isothermal crystallization kinetics indicated HCHD-Zn lowered the crystallization kinetic constant k of iPP but had similar Avrami exponent with neat iPP, which revealed HCHD-Zn hindered the

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crystallization of iPP but did not change the type of nucleation and growth geometries of iPP. We proposed a mechanism of anti-nucleation of iPP upon the addition of HCHD-Zn from a structural perspective. Hydrogens on the tertiary carbons in the melt of iPP may react with the chlorine in the HCHD-Zn particles dispersed within. The removal of hydrochloride left a 3D network of iPP chains cross-linked by HCHD-Zn particles. Therefore, the restricted movement of polymer chains inhibited the nucleation of iPP. Key words: Polypropylene, Anti-nucleating agent, Crystallization behavior, Inhibition mechanism Introduction Semicrystalline polymers, especially isotactic polypropylene (iPP), are widely used materials because of its low cost, easy processing, good chemical resistance, etc. Due to the intrinsic semicrystalline character, iPP is usually processed from their molten state and subjected to crystallization process. Therefore, crystallization behavior is a very important because it controls the polymer’s micro-structure formation and, thereby, strongly influences the final product’s properties. 1-4 Many approaches have been developed to improve or adjust the crystallization behavior of iPP to improve the physical and mechanical properties. 5-17 For example, optimizing crystallization condition

5-8

(e.g. temperature, pressure, cooling rate, flow,

shear, etc), adding foreign substances

9-17

(additives or fillers). Among these,

introducing foreign substances is the most convenient and effective method to improve the crystallization properties of iPP. Extensive research work has focused on


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the effect of foreign substances on the crystallization behavior of iPP and demonstrated that the addition of foreign substances can increase the crystallization temperature and accelerate the crystallization rate of iPP by acting as “nucleating agent” (heterogeneous nucleation). 10-17 As previously stated, foreign substances are commonly believed to serve as nucleating agent to improve the crystallization of iPP. In recent studies, however, we found that zinc salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid (abbreviated as HCHD-Zn in this article) unexpectedly decreased the crystallization temperature and crystallization rate of iPP, generating remarkable anti-nucleation effects. We defined the additives with anti-nucleation effects as “anti-nucleating agents”. Heretofore, to the best of our knowledge, no publications have reported the anti-nucleating agents of iPP and their anti-nucleation effects. As far as we are concerned, this interesting finding has not only theoretical value but realistic significance. The investigation of the negative effects of foreign materials on polymer crystallization can not only enrich the theory of polymer crystallization but deepen the understanding of the positive effects (nucleation effects) of foreign materials during polymer crystallization. In some processes (e.g. foaming process) where a low crystallization temperature or rate of iPP may be required, anti-nucleating additives can be applied to meet the demand. Generally, the foaming process comprises three major steps: nucleation, bubble growth, and stabilization. For semicrystalline polymers, crystallization behavior is a critical factor in the foaming process due to the polymer melt solidifies at the moment of crystallization, which results in “freeze” of



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the foam structure at the crystallization temperature. Therefore, low crystallization temperature will be benefit to extend the time of bubble growth and make sure the bubble can be fully expanded in some cases. In the present work, we proposed the concept of “anti-nucleating agents” of iPP and focused on their anti-nucleation effects. A novel anti-nucleating agent, zinc salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid, was found, and its anti-nucleation effects were characterized by differential scanning calorimeter (DSC) and polarized optical microscopy (POM). Isothermal crystallization kinetics was calculated to further reveal the anti-nucleation effects and mechanism of HCHD-Zn during iPP crystallization. Finally, a mechanism of anti-nucleation of iPP upon the addition of HCHD-Zn was proposed from a structural perspective, which was verified by investigating the viscoelastic properties of neat iPP and iPP/HCHD-Zn. Experimental Section Materials The isotactic polypropylene (iPP) (trade name T30S, Mw=450000, Mn=128000, MWD=3.5) used in this study was kindly provided by Jiujiang Petrochemical Corporation (China). 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic anhydride (HCHD) and bicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic anhydride (HHHD) were obtained from Xiya Reagent Research Center (China) and Alfa Chemical Co. Ltd (China), respectively. Synthesis of HCHD-Zn and its H-substituted counterpart (HHHD-Zn)


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Zinc salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid (HCHD-Zn) was synthesized through a double replacement reaction of sodium salt of 1, 4, 5, 6, 7, 7-hexachlorobicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid and zinc chloride (scheme 1a). HCHD (0.054 mol, 20g) and sodium hydroxide (0.108 mol, 4.32g) were dissolved in water (200 mL) and stirred at 70 °C for 1 h. Subsequently, zinc chloride (0.054 mol, 7.36g) was added and then maintained at 70 °C for an additional 2 h. The white solid was collected after filtrated and rewashing in water (yield= 88.2%). Its H-substituted counterpart, zinc salt of bicyclo [2.2.1] hept-5-ene-2, 3-dicarboxylic acid (HHHD-Zn), was prepared by similar procedure to that described for HCHD-Zn using HHHD (0.054 mol, 8.86g) instead of HCHD (yield= 95.8%; scheme 1b). The FT-IR spectra was recorded on a Nicolet iS10 FT-IR spectrometer (the United States) using KBr pellets in the range of 4000-500 cm−1. Elemental analysis was performed on a Elementar vario EL Ⅲ elemental analyzer (Germany). Calc. for HCHD-Zn: C 19.76, H 0.44; Found for HCHD-Zn: C 19.82, H 0.52. IR data (ν/cm-1): 3449, 1626 [ν(COO-)as)], 1395 [ν(COO-)s)], 1279, 1235, 1176, 1066. Calc. for HHHD-Zn: C 36.93, H 2.73; Found for HHHD-Zn: C 36.91, H 2.68. IR data (ν/cm-1): 3440, 3081, 2991, 2958, 2868, 1597 [ν(COO-)as)], 1537 [ν(COO-)as)], 1438 [ν (COO-)s)], 1417 [ν(COO-)s)], 1335, 1307, 1273, 1255, 1093. The IR spectra of HCHD-Zn and HHHD-Zn were presented in Figure 1. The result of carbon and hydrogen elemental analyses and infrared spectroscopy of HCHD-Zn and its H-substituted counterpart (HHHD-Zn) indicated that their simple molecular structures correspond to ZnOOCC7H2Cl6COO and ZnOOCC7H8COO, respectively. Their



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melting points are over 300 oC by melting point detector. Cl Cl Cl

Cl Cl Cl CO

Cl Cl

NaOH, water, 70 oC

Cl COONa

O

CO

Cl

1h

Cl Cl

COONa

ZnCl2, water, 70 oC

2h

(a) White precipitate

Colorless solution

CO CO

COONa

NaOH, water, 70 oC

O

1h

COONa

ZnOOCC7H2Cl6COO

ZnCl2, water, 70 oC

2h

Colorless solution

ZnOOCC7H8COO

(b)

White precipitate

Scheme 1. Synthetic routes of (a) HCHD-Zn and (b) HHHD-Zn.

Figure 1. FTIR spectra of HCHD-Zn and HHHD-Zn.

Sample Preparation iPP and HCHD-Zn powders were dry-blended by high-speed mixer for 5 min. Then the mixtures were extruded by a corotating twin screw extruder (SJSH-30, Nanjing Rubber and Plastic Machinery Plant Co., Ltd.) through a strand die and pelletized. The pellets were molded into standard test specimens by an injection-molding machine (CJ-80E, Guangzhou Zhende Plastics Machinery Plant Co., Ltd.). The sample of iPP with 0.2wt % HCHD-Zn was denoted as iPP/HCHD-Zn. The neat iPP sample was prepared by the same method for comparison. Differential scanning calorimetry (DSC)


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DSC (Diamond, Perkin-Elmer) was carried out to study the crystallization behavior and kinetics. Temperature was calibrated before the measurements by using indium as a standard medium. The crystallization peak temperature (Tcp) was determined from the crystallization curves. Measurements were performed with the sample of 3~5 mg at a standard heating and cooling rate of 10 oC /min under nitrogen starting from 50 to 200 oC, and the samples were held at 200 oC for 5 min to erase any thermal and mechanical history. Isothermal crystallization kinetics: The first step in the thermal treatment typically involved annealing at 200 °C for 5 min to erase earlier thermo- and mechanical histories. Sequently, the samples were rapidly cooled to desired crystallization temperature and maintained at that temperature for isothermal crystallization. The isothermal crystallization exotherms were recorded at the temperature of 132, 130, 128, 126 and 124 oC, respectively. Polarized optical microscopy (POM) Polarized optical microscopy (POM) observations were performed on the specimens with an Olympus microscope (BX51, Japan) and an Olympus camera (DP70). A Linkam (THMS600, Britain) hot stage was used to control the experimental temperature. The extruded sample were placed between two microscopy slides, melted and pressed at 200 oC for 5 min to erase previous thermal and mechanical history, and then rapidly cooled to a predetermined crystallization temperature. The samples were kept isothermal until the crystallization process



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completed, and meanwhile, photographs were taken at different time. Rheological measurement The rheological properties were measured using a rotational rheometer, MCR 101 (Anton Paar, Austria). All measurements were conducted using oscillatory shear flow with 25 mm diameter parallel plates (gap distance of 1 mm) at a temperature of 200 o

C under an extra dry nitrogen atmosphere. Strain sweep tests were performed to

determine that the 1% strain was in the linear viscoelastic range. Frequency sweep tests were conducted over the angular frequency range of 0.01~100 rad/s within the linear 1% strain region. Theory of isothermal crystallization The isothermal crystallization kinetics of polymer can be analyzed by evaluating its degree of crystalline conversion as a function of time at a constant temperature. The variation of the crystallinity is related to the ratio of the heat generated at time t to the heat generated at infinite time according to Equation (1). X (t ) = ∫ t0 ( dH / dt ) dt / ∫ ∞0 ( dH / dt )dt

(1)

Where dH / dt is the rate of heat evolution. Development of relative crystallinity can be analyzed using the Avrami equation. 18-19 X (t ) / X ( ∞ ) = 1 − exp( − kt n )

(2)

This equation can be changed to ln(1 − X (t ) / X( ∞ )) = − kt n

log[− ln(1 − X (t ) / X (∞))] = n log t + log k

(3a) (3b)

Where n is the Avrami exponent, whose value depends on the mechanism of


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nucleation and on the form of crystal growth, k is the crystallization rate constant containing the nucleation and the growth parameters. From a graphic representation of log[ − ln(1 − X (t ) / X (∞ ))] versus log t , the values of n (slope of the straight line)

and k (intersection with the y-axis) can be calculated. However, the value of k is often obtained from Equation (4). 20 n k = ln 2 / t1/2

(4)

Where t1/2 is the half-time of crystallization, which is taken from the experimental data.

Results and discussion Non-isothermal crystallization behaviors The non-isothermal crystallization behavior of neat iPP and iPP/HCHD-Zn at different cooling rates was studied by DSC. As shown in Figure 2a and 2b, the peak crystallization temperatures (Tcp) of iPP/HCHD-Zn are lower than that of neat iPP at the cooling rate range from 2.5 to 100 oC /min. For the purpose of clarity, Tcp was plotted as a function of cooling rates (Figure 2c). When cooling rate is below 10 o

C/min, the difference of Tcp ( Δ Tcp ) of neat iPP and iPP/HCHD-Zn gradually

increases with increasing cooling rates, whereas, within a relatively high cooling rates range (10-100 oC/min), ΔTcp is almost unchanged, keeping about 4 oC. The result indicates that HCHD-Zn hinders the crystallization of iPP, generating the remarkable anti-nucleation effects, which acts as “anti-nucleating agent”.



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Figure 2. Non-isothermal crystallization curves of (a) neat iPP and (b) iPP with HCHD-Zn and (c) the peak temperatures of crystallization at different cooling rates.

Generally, the crystallization behavior of polymer could be strongly affected by the concentration of additives. Therefore, the effect of the concentration of HCHD-Zn on the crystallization peak temperature of iPP was investigated from 0.05 wt% to 1 wt%. As show in Figure 3, the crystallization peak temperature gradually decreased with an increasing concentration of HCHD-Zn (

The Crystallization Behavior of Isotactic ... - ACS Publications

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